magnetic frameworks

8/12/2019 Magnetic frameworks.

1/28

This article was published as part of the

2009 Metalorganic frameworks issue

Reviewing the latest developments across the interdisciplinary area of

metalorganic frameworks from an academic and industrial perspective

Guest Editors Jeffrey Long and Omar Yaghi

Please take a look at the issue 5table of contentsto access

the other reviews.

View Article Online / Journal Homepage / Table of Contents for this issue
http://www.rsc.org/Publishing/Journals/CS/article.asp?JournalCode=CS&SubYear=2009&Issue=5&type=Issuehttp://www.rsc.org/Publishing/Journals/CS/article.asp?JournalCode=CS&SubYear=2009&Issue=5&type=Issuehttp://pubs.rsc.org/en/journals/journal/CS?issueid=CS038005http://pubs.rsc.org/en/journals/journal/CShttp://dx.doi.org/10.1039/b804757jhttp://www.rsc.org/publishing/journals/CS/Index.asphttp://www.rsc.org/Publishing/Journals/CS/article.asp?JournalCode=CS&SubYear=2009&Issue=5&type=Issue


2/28

Magnetic metalorganic frameworksw

Mohamedally Kurmoo

Received 14th November 2008

First published as an Advance Article on the web 24th February 2009

DOI: 10.1039/b804757j

The purpose of this critical review is to give a representative and comprehensive overview

of the arising developments in the field of magnetic metalorganic frameworks, in particular those

containing cobalt(II). We examine the diversity of magnetic exchange interactions between

nearest-neighbour moment carriers, covering from dimers to oligomers and discuss their

implications in infinite chains, layers and networks, having a variety of topologies. We

progress to the different forms of short-range magnetic ordering, giving rise to

single-molecule-magnets and single-chain-magnets, to long-range ordering of two- and

three-dimensional networks (323 references).

1. Introduction

One of the many attributes of MetalOrganic Frameworks(MOF) is magnetism.1,2 It can be implemented by incorporating

magnetic moment carriers such as paramagnetic metals or

open-shell organic ligands or both.3,4 This, in itself, is not

enough to render a material magnetic as magnetism is a

cooperative phenomenon and thus requires some kind of

exchange between the moment carriers.5 Therefore, the connec-

tion between moment carriers at distances within interacting

range is required. This is often achieved in MOFs. Magnetism

can also be introduced as guests while the framework remains

non-magnetic. So far the majority of magnetic frameworks are

those containing paramagnetic metal centres and in particular,

the first row transition metals (V, Cr, Mn, Fe, Co, Ni and Cu).

These metals, which may exist in different oxidation states,allow variation of the two important parameters, spin quantum

number and magnetic anisotropy. The potential of such a range

of variables opens many challenges in a field without frontiers.

The readers of this collection of works will notice that many

of the compounds reported today are classified in different

categories belonging to the general theme Framework, such

as MOF (MetalOrganic Framework), IRMOF (isoreticular

MetalOrganic Framework), MOP (MetalOrganic

Polyhedra), ZIF (Zeolitic Imidazole Framework), PMOF

(Porous MetalOrganic Framework), CP (Coordination Poly-

mers), PCP (Porous Coordination Polymers), MMOF

(Microporous MetalOrganic Framework) and more.613

However, there is not one where magnetic is mentioned, for

example MMOF (Magnetic MetalOrganic Framework). This

is, most likely, due to the fact that the field of magnetism is

much older and there is already an existing classification,

where the classes are Itinerant Magnets, Mineral Magnets,

Organic Magnets, Molecular Magnets and some less

common ones.1416 Within this classification, Magnetic MOF

are well described by Molecular Magnets. They are both

branches of coordination chemistry where metals are bound

in a solid by coordination bonds to organic ligands. From the

structural point of view, there is a great interest in finding a

logical way to develop a catalogue and thus the development

proposed by OKeefe17,18 which consequently brings the

notion of Yaghis Secondary-Building-Units1921 where the

atoms in a crystal are replaced by a cluster and the organic

connections between them make the bonds. Consequently,

many simple and basic crystal structures, for example NaCl,

CsCl, CdCl2, rutile, diamond, quartz and more,22,23 were

realised which happens to be the way magnetic materials were

classified as the structure defines the exchange pathways.5,14

Interestingly, as the inorganic nodes get bigger and organic

linkers get longer, and of different geometry and multitopicity,

a variety of highly stable lightweight materials has been

synthesised.2426 Some of these materials have spaces between

the nodes containing solvents that can be emptied and refilled

without loss of the mosaicity of the crystals. As a result of the

porosity there is a major interest in this area for different

applications; among them that of storing fuels and unwanted

gases is of utmost importance.2732

It is to be noted that the present word Framework is in a

sense related to Cooperative which has been in use for many

years and is still used today.33,34 The word Cooperative has

several meanings; in our context it principally refers to inter-

action of the valence electrons of neighbouring atoms in a

Dr Mohamedally Kurmoo is now a permanent researcher of the

CNRS (France), based at Universitede Strasbourg, following a

PhD at University College London, and research assistant

positions at the Inorganic Chemistry Laboratory (Oxford)

and Clarendon Laboratory (Oxford). He was Assistant

Director of the Davy-Faraday Research Laboratory at the

Royal Institution of Great Britain (London) for five years. He

has held several invited research and professorial positions in

France, Italy, United Kingdom, Japan and the USA. He has

published over 300 papers on the synthesis, characterisation and

studies of the properties of metalorganic materials, in

particular, those exhibiting multi-properties (structural,

electrical, optical, magnetic and porosity).

Laboratoire de Chimie de Coordination Organique, CNRS-UMR7140,Universitede Strasbourg, 4 rue Blaise Pascal, 67000, StrasbourgCedex, France. E-mail: [email protected]

w Part of the metalorganic frameworks themed issue.

This journal is c The Royal Society of Chemistry 2009 Chem. Soc. Rev., 2009, 38, 13531379 | 1353

CRITICAL REVIEW www.rsc.org/csr | Chemical Society ReviewsView Article Online
http://dx.doi.org/10.1039/b804757jhttp://dx.doi.org/10.1039/b804757j


3/28

molecule or solid. The consequence of these interactions is a

wide range of cooperative phenomena such as electrical

conductivity, magnetism and optical properties. In principle,

going from atoms to SBUs does not change these cooperative

properties and therefore, most of the electrical and optical

ground states are basically the same. However, this is not

always true for the case of magnetism and in this review, we

will develop the progress made in the last ten years which

highlights the fact. As the progress of the field is exponential, it

is very difficult to cover all the cases in the literature.

Therefore, we have made one selection; that is to present only

magnetic materials of cobalt and where necessary mention

some relevant materials having other magnetic centres or

combination.

This review, therefore, proceeds as follows: we first discuss the

reasons behind our choice for cobalt highlighting its magnetic

properties as a monomer, then develop our understanding of the

exchange between nearest neighbours, from experimental results,

of its different modes of connection and their consequence in

simple dimers, through trimers, tetramers to oligomers. Finally,

we present how long-range magnetic ordering develops in linear

chains, layers and 3D-networks. We also present recent develop-

ments such as single-molecule-magnetism, single-chain-magnetism

as well as the effects of solvents on the magnetic ground states in

some porous magnets.3537 We conclude with a general view of

where we are coming from and where we are going.

There are several reviews, in addition to the other chapters

of this volume, covering the designs, synthetic approaches,

structures and physical properties of these materials.3855

Therefore, we will not go into already published details but

will keep this chapter to the relevant parts in view of

developing the structuremagnetic property relationships to

provide a platform for further development of interesting

magnetic materials, both of academic and industrial interests.

Magnetic interaction between nearest neighbour moment

carriers as presented in this review is derived from several

structural modes which may result in different signs and

values. The general view of the strength of the interaction,

based on experimental observations and in some cases on

theory, is that it decreases with the number of intervening

bonded atoms between the moment carriers. For example, the

critical ordering temperatures for the three magnetic metals of

the first row transition metals range between 560 and 1380 K,

while for oxides (one-atom bridge) of the same metals it does

not exceed 900 K and for compounds with two-atom bridges

(e.g., cyanide) it is below 350 K. On increasing the number of

atoms in the bridge to three (e.g., dicyanamide and azide) the

transition temperatures do not get higher than 50 K.56,57

When the connection is through four-atom bridges, no long-

range magnetic ordering is observed above 2 K. Furthermore,

the transition temperature decreases in the order 3D 4 2D4 1D.

We have to stress that the above correlation applies to systems

which has all pairwise exchange pathways equivalent. For the

present review we will adopt the definition of magnetic

dimensionality by considering only bridges of less than four

atoms, although bridges of four and more atoms become

important at low temperatures through dipolar interactions

and especially when the ordered SBU (may it be a cluster, a

chain or a layer) has a large resultant correlated moment.

Common to the chemistry of the frameworks, especially for

cobalt, is the presence of water, hydroxide and carboxylate

that form the connections between the magnetic centres. These

are the main examples that will be considered here, though

some examples may contain in addition bipyrimidine,

pyrazole, triazole and tetrazole that provide CoNNCo

bridges.

2. Cobalt

Cobalt, translated from its German name Kobald chosen by

its discoverer Georg Brandt, meaning goblin or evil spirit, is

absolute gold for scientists interested in structural chemistry

and magnetism. One of the interesting structural aspects of

working with cobalt in contrast to nickel, iron and manganese

is the range of geometries, octahedral, tetrahedral, square-

pyramidal, trigonal-bipyramid and square-planar, that are

stable.58 The order of occurrence in the literature for the

known MOFs is as listed. We note that the coordination

geometry is purely a consequence of serendipity and is not a

designable parameter. As for magnetism it provides the highest

magneto-crystalline anisotropy which results in record

magnetic hardness.

The ligands used in the development of MOF are usually

those having oxygen (oxide (rarely found for cobalt), water,

hydroxide, alkoxide, alcohol and carboxylate) or nitrogen

(amine, pyridine, azide and azole) coordinating atoms and in

few cases, sulfur (thiolate). Water or alcohol can contribute

one (terminal) or two (one-atom bridge) bonds to the metals,

while hydroxide or alkoxide can contribute from one

(terminal) up to three (one-atom bridge) bonds. In contrast,

carboxylate can contribute from one up to four bonds to the

metals, adopting combinations ofsyn and anticonfigurations

that have some consequences on the magnetic exchange

between nearest neighbours. Ancillary ligands with nitrogen

can contribute one bond per pyridine nitrogen atom and one

or two per azide nitrogen atom to the metals. The connection

can be a two-atom bridge as in the case of pyrazole, triazole,

tetrazole and bipyrimidine. For these donor atoms the

ligand-field remains weak favouring the spin state, S = 3/2

(t2g5eg

2(Oh) or eg

4t2g

3(Td)).

As found in nature, cobalt-containing MOFs can have a

wide range of colours which makes it easy, in many cases, to

identify different phases.59 This may be one of the reasons why

cobalt frameworks are also popular with magneto-chemists.

The most common colours are, a very light pink for octahedral

coordinated ones and an intense blue for tetrahedral

coordination. For distorted octahedra, the pink colour can

intensify to orange, dark red, purple and violet depending on

the ligand and the type of distortions. For compounds having

both tetrahedral and octahedral coordinated ions, they are

usually green to blue depending on the distortions and the

ratio of cobalt atoms in the two coordination geometries.

However, some unusual colours such as colourless, gray and

green have been observed for octahedral cobalt ions.6062

Structural polymorphism is occasionally encountered in

MOFs.6366 In most cases, different condensations of salt

are formed when one works at different temperatures

in the hydrothermal synthesis.6770 For example, in a

1354 | Chem. Soc. Rev., 2009, 38, 13531379 This journal is c The Royal Society of Chemistry 2009

View Article Online
http://dx.doi.org/10.1039/b804757j


4/28

neutral or slightly acid medium the hydrogen-bonded

network [Co(H2O)6](dicarboxylate) is formed at temperatures

below 90 1C, and it transforms into the one-dimensional

Co(H2O)4(dicarboxylate) for temperatures between 100 and

120 1C and to a two-dimensional Co(H2O)2(dicarboxylate) at

a slightly higher temperature.7173 By increasing the alkalinity

of the medium, condensed hydroxide containing networks are

often obtained. There is a general tendency to form clusters at

lower temperature, where the clusters increase in size to chains

and ladders through layers and finally 3D-frameworks at

higher temperatures. This progression of structural condensa-

tion provides a whole gamut of clusters that are very

important in the understanding of the magnetism of MOFs;

and consequently, our aim for this review is to develop our

understanding from that of a simple metal centre to a dimer,

progressively through the oligomers to infinite 1D, 2D and 3D

structures.

Cobalt, as a metal, has the highest Curie temperature among

the three ferromagnets of the first-row transition metals and is

also the most anisotropic magnetically. In its oxidised form the

divalent (d7) and tetravalent (d5) cations are paramagnetic but

the trivalent (d6) is generally diamagnetic. In most MOFs of

cobalt the oxidation state is two and therefore, we will only

consider this ionic state in what follows. In octahedral coordi-

nation geometry, the ground state is a 4T1gsuch that if one uses

the spin-only formula for S = 3/2 and a g-value of 2, the

effective moment is expected to be 3.87 cm3 K mol1. However,

the observed values are usually higher (4.75.2 mB) due to the

unquenched orbital-moment as a consequence of spinorbit

coupling (l=B170 cm1

= 118 K).58

The spinorbit coupling

lifts the twelve-fold degeneracy of the ground state and gives

rise to a doublet, a quartet and a sextet of Kramers levels. The

lowest level being the doublet. Upon cooling, the depopulation

of the upper levels causes the effective magnetic moment to be

reduced even in the absence of magnetic exchange and Mabbs

and Machin analysed the temperature dependence of the

magnetic susceptibility and arrived at an artificial Weiss

constant of20 K for an isolated Co(II).74 The temperature

dependence of the magnetic susceptibility has frequently been

simulated using the equation of Mabbs and Machin or

modifications thereof to include axial distortion of the six-

coordinated Co(II) ion by taking into account the energy gap

between the singlet 4A2and doublet4E levels resulting from the

splitting of the orbital triplet 4T1g ground state under an axial

distortion.75 For the low temperature state, Abraham and Price

have found an effectiveS= 1/2 and a gav= 13/3 describe best

the EPR spectra of several Co(II) salts.7678 Consequently, the

isothermal magnetization deviates from that of an expected

Brillouin function for an isotropic S= 3/2 ion with a saturation

value (gS) of 2.32.4mB. For tetrahedral Co(II), the ground state

term is 4

A2 and the effect of spinorbit is not so large. The

observed range of effective magnetic moment is 4.54.8 mB,

gav = 2.4 and S= 3/2.7987 For non-interacting tetrahedral

Co(II), the Weiss constant is close to zero. For a trigonal-

bipyramid and square-pyramid, the behaviour is intermediate

between those of octahedral and tetrahedral coordinated

Co(II).88 Square-planar Co(II) systems, which are very rare in

MOF, adopt the low-spin state (S= 1/2). The average g-value

is slightly elevated from the free-electron value due to a small

orbital contribution resulting in an effective moment range of

2.22.7mB.89

3. Magnetism of metalorganic frameworks

There exist several reviews dealing with the different aspects of

magnetism.90105 In addition to the general reviews, some

cover special areas such as (a) simple intuitive syntheticapproaches to designing magnets, (b) the effect of distance,

(c) symmetry relations, (d) and particular ligands ( e.g.cyanide,

oxalate, azideetc.). Furthermore, there are some more physics

oriented reviews covering special issues such as frustration,

low-dimensionality and more. These reviews provide a very

good basis of magnetism in molecular systems from which we

can draw much in understanding those of MOFs. We shall

start with a single centre and develop through to more com-

plex clusters and to infinite 1D, 2D and 3D networks.

3.1 Monomers

Monomeric compounds of cobalt with carboxylate ligands

are rarely found when the synthesis is carried out hydro-

thermally.63,106,107 An example of one which is closest to a

perfect octahedral geometry is [Co(H2O)6](H2pm) where pm is

1,2,4,5-benzenetetracarboxylate (Fig. 1 (I)) which has the

hexaaquo-cobalt hydrogen-bonded to the pyromellitate to

form a 3D network.63

The susceptibility is as described above

with Curie constant (C = 2.95 cm3 K mol1 Co) corres-

ponding to an effective moment of 4.86 mBand Weiss constant,

Y = 19.9(6) K. The isothermal magnetization saturates at

2.3 mB. This set of values is observed for a series of

one-dimensional chain compounds of the general formula,

Co(H2O)4(dicarboxylate) where the linear dicarboxylate

(fumarate, succinate, 1,4-cyclohexanedicarboxylate,

hexanoate, and others) forms the bridge between the

Co(H2O)4 planar units.108 These chains can be helical as in

the case of 2,20-biphenyldicarboxylate but with surprisingly a

larger Weiss constant of 51 K.109 However, when the

cobalt environment is heavily distorted from an octahedron

these values can show major deviations. As an example,

Co2(cis-1,4-chdc)2(dpa)2 where chdc is cyclohexanedicarboxylate

and dpa is 2,2 0-dipyridylamine (Fig. 1 (II)),110 which has

bond lengths in the range 2.0882.301 A for CoO and

2.0502.084 A for CoN. The major distortions are generated

by the presence of chelating carboxylate groups which resulted

in a range of angles of 59.6144.91for OCoO and 93.6159.91

for OCoN. Analysis of the magnetic susceptibility gives

C = 2.98 cm3 K mol1 Co and Y = 1.4(2) K. For an

almost tetrahedral Co(II), as found in Co2(2,20-bpdc)2(dpa)2

where bpdc is biphenyldicarboxylate (Fig. 1 (III)),110 where

the cobalt ions are coordinated by two oxygen atoms and two

nitrogen atoms,C= 2.47 cm3 K mol1 Co and Y= 1.2(2) K.

The conclusion from these simple cases is that for tetrahedral

system we usually find the expected values while for octahedral

systems one has to consider the distortions from a regular

octahedron before commenting on the Weiss constants. To

our knowledge, examples of isolated Co-carboxylate in

penta- or square-planar coordination are rather rare in the

field of MOF.


View Article Online


5/28

3.2 Dimers

By modelling the temperature dependence of the susceptibility of

dimers and trimers it is usually possible to extract the value of the

exchange interaction between the nearest neighbours, which can

in turn be very useful in our understanding of the magnetism of

more extended systems. However, such analyses for cobalt have

not been very extensive due to the change of regime, consequent

of the effect of spinorbit coupling, in the range of temperature

where experiments are performed. While most analyses are

superficial, some elaborate calculations have been performed

by Ostrovsky et al. and Palii et al. for specific examples.111114

Such calculations will be very informative and are required for

the different modes of connection that are known. These

connections can be just a simple corner-sharing with one bridging

oxygen atom as for Co2(2-pzc)(H2O)(VO3)3 (Fig. 2 (I)),115

2-pzc = 2-pyrazinecarboxylate, and Co5(bpc)2(H2O)1212H2O

(Fig. 2 (III)),116 bpc = benzenepentacarboxylate, or an edge-

sharing through oxygen bridges as for Co2(hypa)2(4,40-bpy)

(Fig. 2 (IV)),117 hypa = hydroxyphenylacetate. A corner sharing

and two carboxylate bridges are observed for an elegant porous

system, Co2(nicotinate) (Fig. 2 (II)),118 where the lattice water

molecules can be removed and other solvent inserted without

damaging the crystals. For the last four examples, the estimation

of the exchange constant have been obtained by simple

CurieWeiss fit of the high-temperature data or using equations

for dimers derived from an isotropic Hamiltonian. Many authors

have used the argument of Goodenough and Kanamori to

discuss the types of exchange interaction (J) where a CoOCo0

angle of less than 971 is assumed positive (ferromagnetic) and

anything above is negative (antiferromagnetic).119121 This simple

rule appears to operate with success for the case of cobalt(II). As

the examples in Fig. 2 show that modes I and II are antiferro-

magnetic while all the edge-sharing modes III and IV are

ferromagnetic. The latter has been confirmed by neutron

studies on compounds, Co2(OH)PO4, NaCo2(H3O2)(MoO4)2(Fig. 2 (V)) and Co2(pm), displaying this type of connection

(see later).122124 When the bridge is a carboxylate, that is

CoOCOCo, most, if not all, of the reported exchange

interactions are antiferromagnetic. However, the absolute values

can vary depending on the modes of connection (synsyn,

synantior antianti), the angles and also on the distance. One

Fig. 2 Structures of binuclear units showing the different modes of

connection via one-atom (O) and three-atom (OCO) bridges.

Fig. 1 Structures containing (I) regula r octahedr a:

[Co(H2O)6](H2pm), (II) distorted octahedra: Co2(cis-1,4-chdc)2(dpa)2and (III) tetrahedra: Co2(2,2

0-bpdc)2(dpa)2.


View Article Online


6/28

exception to this, Kitagawa et al. reported a ferromagnetic

interaction (Y = +8.3 K; C= 4.39 cm3 K mol1 Co) for a

dimeric unit bridged by two synanticarboxylato bridges in the

bilayered polymer, Co(3,4-pydc)(H2O)2H2O (Fig. 2 (VI)), pydc

= pyridinedicarboxylate.125 In the case of one, two or three

OCO bridges, the exchange interactions are usually weak but for

the four OCO bridges, as found in the paddlewheel dimers, it can

be very large. A CurieWeiss fit of the susceptibility data of

Co2(2,5-Br2bdc)2(py)2 (Fig. 2 (VII)) gives Y = 218 K while

using an isotropic dimer model for S= 3/2, we estimate J to

be 68 K. Ouahabet al.used a BleaneyBowersS= 1/2 model

instead and found a singlettriplet gap of 420 cm1 (292 K) with

a fittedg= 4.78.126,127 To explain the observation, they propose

a molecular-orbital scheme, different to that of Cotton et al.,

suggesting the formation of a weaks-bond (dz2).128,129 Using

hydrothermal synthesis with the chiral D-(+)-camphoric acid,

Zeng et al. obtained a chiral two-dimensional (4,4) network of

binuclear paddlewheels, Mx

M02x(D-cam)2(1,4-dimb), which

are then connected by 1,4-di(1-imidazolylmethyl)benzene

(1,4-dimb).130

They were able to make solid solutions of CoNi

covering the whole range. Surprisingly all the compounds show

long-range antiferromagnetic ordering between 7 and 23 K as

well as weak ferromagnetism, spin-flop, and glassy behaviour

that result from the randomness of the mixed metal pairs,

magnetic anisotropy of the metallic cations, and possibly anti-

symmetric exchange. Using the same camphoric acid and

Co(OAc)2 but this time carrying out the reaction using

ionothermal conditions in 1-ethyl-3-methylimidazolium bromide

(EMImBr), the chiral (EMIm)[Co2(D-cam)2(OAc)] was obtained

where the (4,4) 2D-network of paddlewheels are bridged by

acetate (OCO) bridges to give a 3D-framework with the cations

sitting in the cavities.131 No magnetic properties were reported.

Oxalate is another ligand that can be considered as a bridge

through two OCO connections, but it differs electronically by

the presence of the CQC bond which increase the interaction

within a pair due to the p-orbitals (Fig. 2 (VIII)). Different

groups have reported estimations ofJand all are in agreement

for a weak antiferromagnetic interaction.132134

3.3 Trimers

Similar types of connection as for dimers are found within

trimers. Magnetic measurements are available for some and

similar approach to analysing the data has been reported. The

elaborate analysis by Calvo-Pe rezet al.considering only a trimer

where the connection between cobalt atoms is via one oxygen

and two OCO bridges and ignoring the connection between

trimers for the chain compound Co3(m-OOCCF3)4(m-H2O)2)-

(OOCCF3)2(H2O)2(C4H8O2)2C4H8O2 suggests that Jis weakly

antiferromagnetic. 135 For the similar trimer Co3(PhCOO)6(py)2(Fig. 3 (I)) no magnetic data are given.

136For Co3(phcinna)6(quin)2

where phcinna = phenylcinnamate and quin = quinoline,

which has three OCO bridges between cobalt, Oka et al.

estimates a ferromagnetic J by analysing the temperature

and field dependence of the magnetization.137 On the other

hand, Yuet al. found antiferromagnetic coupling for a similar

trimer in Co(tatb)2(H2O)2(dma3H2O), tatb = 40,400-s-triazine-

2,4,6-triyltribenzoate, dma = dimethylacetamide.138 Parket al.

made Co3(sda)3(dmf)2(Fig. 3 (II)) where the stilbenedicarboxylate

(sda) bridges trimeric units into hexagonal layers.139 Each trimeric

unit consists of one octahedral cobalt atom at the centre

symmetrically connected to two tetrahedra through three

OCO bridges each. The Curie constant of 3.12 cm3 K mol1

Co was reported to be independent of the applied field

(100 and 1000 Oe) but the Weiss constant is reported to

be slightly dependent on field (31.32 K for H = 100 Oe

and 24.69 K for H= 1000 Oe) but negative and higher than

that of a monomer, suggesting that the nearest-neighbour

interaction is antiferromagnetic. Paramagnetism is

reported for a trimer where the connections are through

one oxygen and one carboxylate.

140,141

Unfortunately, nomagnetic data are available for the two edge-sharing trimers,

Co3(H2O)6(succinate)3 (Fig. 3 (III)) and Na[Co3(1,3,5-btc)2(m3-OH)-

(m2-H2O)4(H2O)7]1.5H2O (Fig. 3 (IV)), btc = benzenetri-

carboxylate,142,143 where one may expect a ferromagnetic J

between nearest-neighbours.

Using two different carboxylate containing ligands, 3-(1H-

benzimidazol-2-yl)propanoate (pa) and isonicotinate (ina), Yao

et al.prepared a 2D network, Co3(OH)2(pa)2(ina)2(Fig. 3 (V)),

consisting of Co(II) TdOhTd trimers.144 Although the trimers

are connected through multiple-atom bridges, the compound is

reported to display long-range antiferromagnetic ordering at

20 K. From field- and frequency-dependence ac-susceptibility

measurements, the authors conclude that the ground state is acanted-antiferromagnet deriving from DzyaloshinskiMoriya

interaction in addition to a single-molecule magnetic

behaviour.145,146

3.4 Tetramers

As the number of moment carriers within a cluster increases,

the magnetism becomes more interesting due to the possibility

of obtaining systems behaving as single-molecule-magnets

given that cobalt(II) has an important single-ion anisotropy

(D) due to the presence of three unpaired electrons in the

d-shell. A positive D is a requirement for the observation of

Fig. 3 Structures of trinuclear units showing the different modes of

connection via one-atom (O) and three-atom (OCO) bridges.


View Article Online


7/28

single-molecule-magnetic properties and its absolute value is

related to the barrier for magnetization reversal.35

Powell et al. made the rhombus tetramer,

Co4(m3-OH)2(H2O)6(ntp)22H2O (Fig. 4 (I)) where

ntp = nitrilotripropionate, which consists of an edge-sharing

cobalt dimer connected by the apices of two octahedra at the

hydroxide sites.147 The magnetic properties reveal competing

antiferromagnetic and ferromagnetic interactions between

the four Co(II) ions and below 1 K a long-range Ne el

state is established. Liu et al. reported a network,

Co4(1,3,5-btc)2(L3)2(OH)2(H2O)4.5H2O, L

3 = 1,10-(1,4-butanediyl)-

bis(imidazole), consisting of two types of rhombus tetramers

(Fig. 4 (II)) composed of edge-sharing polyhedra.148 In one

type an edge-sharing octahedral cobalt pair connects to the

edges of two trigonal-bipyramidal cobalt centres. The other

consists of four cobalt octahedra sharing edges. This

compound is found to display dominant antiferromagnetic

interaction (Y = 89 K) and exhibits a very high Curie

constant per cobalt of 6.2 cm3 K mol1, which is twice the

expected value. In a slightly different structured tetramer,

Co4(OH)2(3,4-pbc)3(Fig. 4 (III)), 3,4-pbc = 3-pyrid-4-ylbenzoate,

consisting of an edge-sharing octahedral cobalt pair connected

to the apices of two trigonal-bipyramidal cobalt.149 A

CurieWeiss fit of the temperature dependence of the

susceptibility gives a Weiss constant of again 89 K and a

normal Curie constant of 3.5 cm3 K mol1 Co.

The 3D network of pyromellitate bridged tetramers

Co4(OH)2(H2O)2(C4H11N2)2(pm)1.5H2O,150 consisting of an

edge-shared octahedral dimer connected via the apices to two

square-pyramids also displays dominant antiferromagnetic

coupling with features indicating the presence of ferromagnetic

coupling. Co4(2,5-pydc)4(bpe), pydc = pyridinedicarboxylate,

bpe = bipyridylethane, an eight-connected self-penetrating net

based on a cross-linked a-Po subnet, has tetrameric nodes based

on OCO bridged dimers.151 It behaves as antiferromagnetically

coupled paramagnets (Y= 61 K and C= 1.48 cm3 K mol1

of Co which is only half the expected value). Ma and Zhou

synthesised a highly symmetric network, H2[Co4O(tatb)8/3]

(Fig. 4 (IV)) where tatb = 4,40,400-s-triazine-2,4,6-triyltribenzoate,

exhibiting high gas absorption capacity and consisting of a

beautiful Co4(m4-O) tetramer.152

Each cobalt atom adopts a

square-pyramid geometry and sits at the corners of a perfect

square with the appearance of a St Johns cross. If the

exchange between pairs is antiferromagnetic, such an arrange-

ment will be frustrated magnetically and consequently, the

absence of magnetism of this compound will be very

interesting indeed.

Following the synthesis and characterisation of the

[M4(cit)4] cubane moiety (Fig. 4 (V)) in the series

[C(NH2)3]8[MII4(cit)4]8H2O (cit = citrate, M = Mg, Mn,

Fe, Co, Ni, Zn),153,154

the two independent groups performed

extended studies of the ac and dc magnetic properties covering

the milliKelvin (mK) temperature region, field dependence,

field-sweep dependence and frequency dependence.155,156 They

arrived at the same conclusion of a SMM behaviour for the

cobalt complex with the characteristic blocking temperature of

3 K, DE/kB= 21 K, andt0= 8 107 s. Due to the very fast

quantum tunnelling of the magnetization, the hysteresis loops

collapse at zero-field. Murray et al. also showed that the

related octanuclear cluster [Co8(cit)4(H2O)12]24H2O,156

containing a [Co4(cit)4] cubane moiety, behaves likewise

(DE/kB = 20.5 K, t0 = 1.0 107 s). Furthermore, for the two

parent compounds, [(NMe4)3Na{Co4(cit)4[Co(H2O)5]2}]11H2O,

[(NMe4)4{Co4(cit)4[Co(H2O)5]2}]16H2O, and their partially de-

hydrated congeners, [(NMe4)3Na{Co4(cit)4[Co(H2O)5]2}]7H2O,

[(NMe4)4{Co4(cit)4[Co(H2O)5]2}]6H2O, having the Co4O4cubane structure connected by two Co(H2O)5 units, Murrie

et al.found SMM behaviour in their dc- and ac-susceptibility

study.153 The fully solvated samples exhibit a blocking

temperature of 6 K with onset of a frequency dependence

but without a peak in the temperature dependence of the

imaginary ac-susceptibility (w00) above 1.8 K. In contrast,

for the partially dehydrated samples, well-resolved peaks for

w00 are observed from which the gap and relaxation time (t0)

were estimated from the Arrhenius fits: DE/kB = 26 K and

t0 = 8.2 109 s for [(NMe4)3Na{Co4(cit)4[Co(H2O)5]2}]

7H2O and DE/kB = 3 2 K a n d t0 = 2.1 109 s for

[(NMe4)4{Co4(cit)4[Co(H2O)5]2}]6H2O. The change in co-

ordination geometry upon dehydration is suggested to be

responsible for the difference in behaviour.

Fig. 4 Structures of tetranuclear units showing the different modes of

connection via one-atom (O) and three-atom (OCO) bridges and

face-sharing within Co4O4 cubanes.


View Article Online


8/28

Xianget al.found another compound containing the Co4O4citrate cubane unit in {KCo3(cit)(Hcit)(H2O)28H2O}8(Fig. 4 (VII)) which is obtained as violet prismatic crystals

by the hydrothermal reaction of a suspension of cobalt

acetate, citric acid and KOH in ethanolaqueous solution at

120 1C.157 The structure is highly symmetric and is described

as anatase with an assembly analogous to polyoxometalate.

The key feature is the similar cubane units described above

where each cubane is connected to six neighbours via six

tetrahedral cobalt units; the latter connects four cubanes.

The overall structure was demonstrated to be porous by gas

sorption measurements. The Weiss constant is +1.4 K

suggesting ferromagnetic interaction dominates within the

cluster. Considerable field dependence is observed in the

dc-magnetization below 20 K which is interpreted as being due

to canting with a remanent magnetization of only 92 cm3 G mol1

as expected for a very small canting angle of 0.131. Frequency-

dependent ac-susceptibility is observed below 5 K. The

dehydrated compound behaves in a similar way with almost

the same characteristics.

The Co4O4 cubane was found in [Co12(bm)12(NO3)-

(OAc)6(EtOH)6](NO3)5(Fig. 4 (VIII)), where bm = benzimidazole

methanol caps the cubane partially and the coordinating NO 3sits in the center of three cubanes while the acetate connects

the cubanes on the periphery to give a Co12 cluster.158 It

displays SMM behaviour below 3 K where the Argand-

diagram suggests a simple one-barrier model. The isothermal

magnetization at 2 K reaches full saturation in 70 kOe. The

compounds containing the Co4O4 cube appear to be the

smallest true SMM to be realised though Yao et al. hinted

to a trimer having such properties.144

Chiang et al. found a fusion of two Co4O4 cubanes at one

vertex to give Co7(m3-OH)8 (Fig. 4 (VI)) clusters which are

bridged by six oxalate and six piperazines to form a

highly symmetric (space group R3) 3D-framework

Co7(m3-OH)8(ox)3(pip)3.159 Analysis of the high-temperature

susceptibility gaveC= 3.14 cm3 K mol1 Co andY= 2.76 K.

The latter indicates a nearest-neighbour ferromagnetic

interaction between cobalt atoms within the cluster. Due to

antiferromagnetic coupling between these ferromagnetic

clusters the ground state is an overall antiferromagnet

below TN of 26 K. Isothermal magnetization at different

temperatures are linear with field and that at 2 K suggests

a spin-flop about 60 kOe. The isostructural nickel analogue

is likewise composed of ferromagnetic clusters (defined by two

exchange pathways J1 = 40 K an d J2 = 0 K) ,160 which

are ordered antiparallel to one another to give a Ne el state

below 17 K.

3.5 Pentamers

Pentamers are rare; a search of the literature found only one

example, Co5(OH)2(pm)2(H2O)4xH2O.161 It consists of

edge-sharing trimers with apical connection to two octahedral

cobalt atoms on the peripheral ends of the trimer. These

pentamers are linked into frameworks by the benzenetetra-

carboxylate. The link involves bridges consisting of the

carboxylate and the benzene backbone. No magnetic

measurements are reported for this compound.

3.6 Hexamers

Co6(OH)2(bpc)2(H2O)6 (Fig. 5 (I)), bpc = 1,2,3,4,5-benzene-

pentacarboxylate, consists of a linear edge-sharing tetramer

connected to two octahedral cobalt atoms on the peripheral

ends of the tetramer.116 The links between hexamers involve

only carboxylate connections. Extensive magnetic measure-

ments are reported for this compound demonstrating a long-

range antiferromagnetic ordering below 5 K. The Curieconstant is 3.35 cm3 K mol1 Co and the Weiss constant

is 30 K. The saturation magnetization at 2 K of 1.08 mBis far

from the expected result of 4.6 mB. These results indicate a

ferrimagnetic order within the linear hexamer and weak anti-

ferromagnetic coupling between them to assure a long-range

ordering to a Ne el state. This weak coupling can be overcame

by fields higher than the metamagnetic critical field of 4 kOe to

realise a ferrimagnet where four ferromagnetically coupled

cobalt atoms are antiferromagnetically coupled to the

remaining two. From the phase diagram, Sevov et al.

established an ordering temperature of 4 K.116

Using 2-phenylcinnamic acid the hexamer complex,

Co6(OH)2(phcinna)10(Fig. 5 (II)), phcinna =a-phenylcinnamate,has been isolated and it consists of two edge-sharing octahedra

and four tetrahedral units linked through m-carboxylate

ligands and m-hydroxide ions.162 Antiferromagnetic coupling

between Co ions is suggested by the temperature dependence

of the susceptibility while the isothermal magnetization at 2 K

indicates anS= 3 ground-state. Assuming the octahedral pair

is ferromagnetically coupled, the moments of the four

tetrahedra must be antiparallel resulting to a saturation of

6 mB. Co6(OH)2(PhCOO)10(PhCOOH)43PhCH3 (Fig. 5 (III))

Fig. 5 Structures of different hexanuclear units.


View Article Online


9/28

contains a closely related hexamer but the magnetic properties

are not reported.136

As a tripod ligand (phosphonomethyl)iminodiacetate was

found to coordinate via the oxygen atoms of the carboxylate

and phosphite end as well as by the amine nitrogen atoms to

cobalt(II) to form {K2[CoO3PCH2N(CH2CO2)2]}6xH2O

(Fig. 5 (IV)) by a solvothermal reaction in methanol.163,164

While the carboxylate groups and the nitrogen block the

cobalt atoms from propagating into a 3D-polymeric structure,

the phosphite group connects penta-coordinated (CoO4N)

into a highly symmetric ring. These are then packed into a

3D network by hydrogen bonding with the lattice water

molecules and by ionic bonds with the potassium atoms.

Analysis of the magnetic susceptibility above 100 K,

taken in 100 Oe, gave a Weiss constant of 14 K and

considerable field dependence below 20 K, consistent with

canted-antiferromagnetism, as confirmed by the linear

dependence of the isothermal magnetization at 5 K and the

presence of a small hysteresis loops. At first sight, the presence

of a long-range ordering for such a ring is quite surprising but

if the moments within the rings are ferromagnetically coupled

a weak dipolar interaction between the rings can result in an

ordered state. It is further suggested by the authors that the

canting is derived predominantly by the single-ion anisotropy

due to the absence of an inversion centre relating the cobalt

atoms by symmetry.

Reaction of another tripod ligand, 2-amino-2-methyl-1,3-

propanediol (ampdH2), with Co(OAc)2 in methanol affords

the mixed-valent hexamer Co6(H2O)(MeOH)(OAc)6(ampd)4(Fig. 5 (V)) compound (four divalent and two trivalent)

formed of a segment of a brucite layer.165

At high tempera-

tures the magnetic susceptibility data are dominated by the

single-ion properties of high-spin Co(II) centres with distorted-

octahedral coordination geometries. Below 30 K ferro-

magnetic coupling was found using theoretical analysis in

terms of an anisotropic exchange model, and inelastic neutron

scattering data.

3.7 Heptamers

It is interesting to highlight the four examples of heptamers

though they do not strictly belong to our selection; that is

containing a carboxylate group. The first is an interesting

cubane of cobalt [Co(H2O)6Co8(L1)12]X6nH2O (X = NO3,

n= 12; X = HCO3,n= 24; HL1 = 4,6-bis(2-pyridyl)-1,3,5-

triazin-2-ol) and inside the cube is a Co(H2O)6 hydrogen-

bonded to the alcohol group of the ligand.166 The unit is a

fragment of an NbO structure. Though there is a three-atom

bridge between cobalt pair in the cube, the magnetic properties

are close to that of isolated paramagnetic cobalt(II) centres.

The other three examples are all cyclic wheels of six cobalt

atoms on the perimeter and one in the centre (Fig. 6 ( I)). This

cyclic wheel is a fragment of a brucite layer where each

octahedron shares three edges with its neighbours. They differ

in the valencies of the cobalt atoms and in their disposition

within the cluster. The first contains seven divalent cobalt

atoms, while the second has four divalent ones (one at the

centre and three symmetrically positioned on the perimeter)

and three trivalent ones (on the perimeter). The third has three

divalent cobalt atoms (defining the diameter) and two pairs of

trivalent ones on each side of it.

The first example, [Co7(hdeo)6(N3)6](ClO4)22H2O contains azide

and oxo bridged Co(II) (Hhdeo = 2-hydroxy[1,2-di(pyridin-2-yl)]-

ethane-1-one).167,168 Magnetic data were theoretically fitted

and intramolecular ferromagnetic couplings between nearest

neighbours were found. It shows a blocking temperature of

ca. 3 K with clear frequency dependence of both components

of the ac-susceptibility in fields of zero and 600 Oe, suggesting

SMM behaviour.

The same structural unit is found in [CoII4CoIII

3(HL)6-

(NO3)3(H2O)3]2+ by using the tripod ligand H3L = H2NC-

(CH2OH)3,169 but this time the complex is a mixed-valent salt.

In contrast to azido bridges in the above compound, this

species has oxygen atom bridges. In this electronic configuration

only the Co(II) is the moment carrier and they are arranged in

a star of three corners and one centre. The complex is reported

to be an exchange-biased single-molecule magnet. The

blocking temperature is similar to that of the fully reduced

system.

Extending the tripod ligand to 2-amino-2-methyl-1,3-

propanediol (H2ampd) Ferguson et al. obtained the hepta-

nuclear ring structure when the reaction with Co(OAc)2 was

performed solvothermally in methanol.169 The cluster is

connected into chain by a cobalt(II) to give the overall formula

Co8(H2O)2(OAc)7(ampd)6, where the heptanuclear cluster

contains four divalent and three trivalent cobalt ions. The

magnetic data at high temperatures were analysed showing

dominant single-ion properties of high-spin Co(II) centres with

distorted-octahedral coordination geometries, while the

low-temperature data suggest ferromagnetic exchange within

the heptanuclear unit and negligible interactions along the

chain between the hepta- and mononuclear fragments.

Despite the presence of negative magnetic anisotropy of the

Co(II), a requisite for the observation of SMM behaviour,

Luneau et al. observed a lack of such behaviour for the

mixed-valent heptanuclear wheel, CoII3CoIII

4(L)6(MeO)6,

LH2 = 1,1,1-trifluoro-7-hydroxy-4-methyl-5-azahept-3-en-2-one.170

Structural study showed that the three paramagnetic ions are

in a line along the diameter of the wheel. The temperature- and

field-dependence magnetizations evidence ferromagnetic inter-

actions within the compound. Using ab initio calculations

including spinorbit coupling to simulate the susceptibility

and magnetization, good agreements were obtained for

nearest-neighbour and next-nearest-neighbour exchange para-

meters (1 and 3.8 K, respectively). Facile inter-valence

Fig. 6 Structures of two heptanuclear units.


View Article Online


10/28

electron-transfer appears to contribute to the strong ferro-

magnetic interaction between distant cobalt ions. The absence

of single-molecule-magnet behaviour is associated with large

tunnelling rates between states in the presence of dipolar

transverse magnetic fields in the crystal.

KCo7(OH)3(1,3-bdc)6(H2O)412H2O (1,3-bdc = isophthalate),

is another framework formed of a CoOCo connected Co7cluster and these MOPs are connected into a high-symmetry

(R3c) system by the isophthalate.171 Water molecules occupy

the space within the lattice and single-crystal-to-single-crystal

transformation is demonstrated after removal of the water

providing a void space (10 A radius and 4.5 A aperture) of

22% of the total volume. The Co7 cluster (Fig. 6 (II)) has an

interesting topology where all the cobalt adopt octahedral

geometry in which four of them are edge-sharing forming a

plane and the other three are vertex-sharing with them as well as

between themselves. Each cluster has six neighbours. Analysis

of the magnetic susceptibility gave a Curie constant of

2.93 cm3 K mol1 Co and Weiss constant of49 K. A plot of

the effective momentvs. temperature indicates a ferrimagnetic

behaviour. A possible coupling mechanism for a ferrimagnetic

arrangement is presented. Interestingly, ac-susceptibilities at

different frequencies indicate a single-molecule-magnet

behaviour with a blocking at around 6 K. The dehydrated

compound behaves as a paramagnet.

So far, the experimental results have established some

general trends in the pairwise magnetic interaction between

nearest-neighbour cobalt(II), whereby one-oxygen atom

bridges (vertex-sharing) generate antiferromagnetic inter-

action while two-oxygen bridges (edge-sharing) give rise to

ferromagnetic ones. In addition, it is observed that one, two or

three-atom bridges through carboxylate are weak interactions

and can be either ferromagnetic as in the case when the mode

of coordination is synanti or antiferromagnetic if they are

synsyn. In contrast, when there are four OCO bridges as in

a paddlewheel complex, the interaction can be large and

antiferromagnetic if the structure is regular but it can become

ferromagnetic if there are severe distortions.

4. Chains

Developing through to chains and ladders we observe the same

connections between the cobalt centres and therefore the

general trends, remarked above, for the nearest-neighbour

magnetic exchange interactions can be considered valid. As

for clusters where well-separated ones with a resultant moment

can behave as single-molecule-magnet, for chains single-chain-

magnetism has been observed as a result of the high

anisotropy of cobalt and consequently, realising one-dimensional

Ising systems following Glauber dynamics.172 We will follow

as above by describing the results from the simpler to the more

complex systems, as there are many fascinating examples of

chain compounds in the literature.

The doubly carboxylate bridged chain of five-coordinated

cobalt in Co(2,5-dibromoterephthalate)py (Fig. 7 (I)) is obtained

hydrothermally from cobalt chloride and dibromoterephthalic

acid neutralised with pyridine.126 It behaves as a paramagnet

with C = 2.76 cm3 K mol1 Co and Y = 14.9(6) K.

A similar conclusion was obtained for the single carboxylate

bridged chain in Co2(mpda)(H2O)6H2O (Fig. 7 (II)),

mpda = 1,2,3,4-benzenetetracarboxylate.173 The saturation

magnetization at 2 K is 2.35 mB. Similarly, a simple chain of

octahedra is formed by a combination of a one-atom (O)

and a three-atom (OCO) bridge which are separated by very

bulky ligand in the compound, Co2(2,5-diphenylterephthalate) 2-

(H2O)2(Fig. 7 (III)).126 The susceptibility (C= 3.12 cm3 K mol1

Co and Y = 28.2 K) suggests weak antiferromagnetic

coupling within the chain. A similar conclusion was reported

for {[Co(dpyo)(1,4-bdc)(H2O)2][Co(H2O)6](1,4-bdc)H2O}

(Fig. 7 (IV)), dpyo = 4,4 0-bipyridyl N,N0-oxide, by Manna

et al.174 A closely related chain was observed for the trimesate

containing compound K[Co3(1,3,5-btc)(1,3,5-Hbtc)2]5H2O

(Fig. 7 (V)) where the octahedra were found to be more

regular.175 The CurieWeiss fit of the susceptibility data gave

C= 3.1 cm3 K mol1 Co andY= 0 K; indicating the nearest-

neighbour interaction may be ferromagnetic. However,

replacing the single-atom oxygen bridge by that of a nitrogen

atom from azide, as has been reported by Gao et al. for

((CH3)2NH2)[M(N3)2(HCOO)],176

M = Co or Fe, the isolated

chain is obtained. These compounds exhibit metamagnetism

because of the strong intrachain ferromagnetic and weak

interchain antiferromagnetic couplings. The latter may be

due to pp interaction between the azides of neighbouring

Fig. 7 Structures of single-strand chains with different modes of

connection.


View Article Online


11/28

chains. Nataraj et al. reported a low-dimensional magnetic

behaviour for the edge- and vertex-sharing chain of cobalt,

(piperazinium)[Co4(HPO3)2(ox)3].177

Two simple chain compounds of formula, Co(PhCOO)2, are

formed with the benzene ring of the benzoate isolating the

chains magnetically from each other.136,178 They are composed

of alternating octahedra and tetrahedra cobalt centres bridged

by one oxygen atom and one carboxylate (Fig. 7 (VI)). The

chain is therefore zigzag in nature. While the orthorhombic

(Pcab) modification is reported to be a long-range ordered

ferromagnet with a Curie temperature of 3.7 K, the monoclinic

(C2/c) modification remains paramagnetic to very low

temperature and become a SCM below 0.6 K. Both have very

anisotropic susceptibilities. For the former the easy axis is not

parallel to the chain axis but for the latter it is parallel.

However, there is no isothermal magnetization of the ortho-

rhombic phase to justify the ferromagnetism. If the moments

within the chain are antiferromagnetically aligned and all the

chains are oriented in the same direction, a ferrimagnet will

be most likely due to the difference in g-values. There seems to

be no structural difference within the chain to explain the

difference in magnetism.

Co(4,40-bpdc)(H2O)2H2O, 4,40-bpdc = 4,40-biphenyldi-

carboxylate, is the only one of the MOFs to have a linear

edge-sharing chain (Fig. 7 (VII)).62 The connection at the edge

is via two oxygen atoms of water molecules while one of the

carboxylate group bridges nearest-neighbour cobalt and the

other is hydrogen-bonded to the coordinating water molecules

of adjacent chains. The nearest-neighbour interaction was

found to be ferromagnetic (Y = +26 K) and an antiferro-

magnetic ordering is observed at 5.5 K which was confirmed

by heat capacity measurements.179 A metamagnetic transition

is observed at 2 K with a critical field of 500 Oe. This kind of

chain, but without the bridging carboxylate, is found in mineral

derivatives such as natrochalcite, NaCo2(H3O2)(MoO4)2,

and adamite, Co2(OH)PO4,122,123 where neutron diffraction

studies have shown that the moments are indeed parallel to the

chain axis in both cases and the long-range ordering is

antiferromagnetic due to the presence of antiferromagnetic

exchange pathways between chains.

There are three known compounds with zigzag chain of

edge-sharing octahedra, namely M(1,4-napdc) (Fig. 7 (VIII))

(M = Co or Mn; 1,4-napdc = napthalene-1,4-dicarboxylate),170

M2(dobdc)(H2O)28H2O (Fig. 7 (IX)) (M = Co, Ni or Zn;

dobdc = 2,5-dioxyterephthalate)181,182 and their dehydrated

forms (Fig. 7 (X)) and M2(pm) (Fig. 7 (XI)) (M = Co, Mn or

Fe; pm = 1,2,4,5-benzenetetracarboxylate).63,183,184 The first

consists of pair-wise zigzag chains which are well separated by

the bulky naphthalene and for the second the zigzag chains

have a periodicity of three octahedra and are also separated

form each other by the benzene ring. In contrast, the third has

a dimer periodicity and the chains are connected by OCO

bridges to give a 2D-network which are separated by the

benzene. These slight differences, though subtle, exert major

differences in the magnetic properties. For Co(1,4-napdc) and

Co2(pm) the CurieWeiss fit of the high-temperature data gave

positive Weiss constants of +27 and +16 K, respectively. On

the other hand that of Co2(dobdc)(H2O)28H2O is negative

(6 K). All these values suggest that the exchanges within the

chains in each of these compounds are ferromagnetic.

However, due to transverse antiferromagnetic coupling

between them, long-range antiferromagnetic order sets in at

5.5, 8 and 16 K, respectively. However, in each case a field high

enough to overcome the transverse coupling energy reverses all

the moments to a ferromagnetic state and the compounds then

exhibit hysteresis. The critical metamagnetic fields are

quite different, 250 Oe for Co(1,4-napdc), 430 kOe for

Co2(dobdc)(H2O)28H2O and 1400 Oe for Co2(pm). The large

variation in critical field does not scale with the interchain

distance as one may expect. Most unexpectedly, the

ac-susceptibility of Co(1,4-napdc) and Co2(pm) shows the

presence of both the real and the imaginary components. A

spontaneous magnetization was confirmed for Co2(pm) by

zero-field and field-cooled measurements in a field of 1 Oe

which demonstrated the presence of a canted state at 13 K

below the collinear Ne el state at 16 K (Fig. 8). The lack of such

measurements for the other two compounds limits a full

comparison. Heat capacity measurements for Co2(pm) esti-

mates that 60% of the entropy is found below the Ne el

temperature and the effective spin of the cobalt is 1/2.

Unpublished neutron diffraction data of Co2(pm) in zero field

show that the moments are perpendicular to the chain and the

nearest-neighbour chains connected by OCO bridges point

also in the same direction while those of adjacent layers are

antiparallel.124 However, in a field of 40 kOe all the moments

Fig. 8 ZFCFC magnetization (top) and isothermal magnetization at

different temperatures in the three regions of the phase diagram

(bottom) of Co2(pm).


View Article Online


12/28

are parallel to one another and points perpendicular to the

chain axis. This is as expected from the observed saturation

moment of a ferromagnet. No LRO has been found for the

isostructural Mn(1,4-napdc) while Ne el transitions are

observed for Mn2(pm) (18 K) and Fe2(pm) (26 K). There are

no magnetic data for Ni2(dobdc)(H2O)28H2O. It is also to

be noted that Co2(dobdc)(H2O)28H2O can be reversibly

dehydrated/rehydrated without loss of crystallinity and gases

such as hydrogen and carbon dioxide have been loaded in the

channels. Again it will be of interest to study the dehydrated

Co2(dobdc) which now has the cobalt in square-pyramidal

coordination geometry and also the absence of any exchanges

through hydrogen-bonds.

Co2Na(4-cpa)2(m3-OH)(H2O) (Fig. 7 (XII)) (4-cpa = 4-carboxy-

phenoxyacetate) was obtained hydrothermally and it is

constructed of zigzag chains of edge-sharing octahedra.

These chains are separated by the sodium cations and the

organic ligands.185 The analysis of the high temperature

dc-susceptibility data gave C = 3.61 cm3 K mol1 Co and

Y= 56 K. At low temperature blocking of the moments was

observed which gave a clear ZFCFC bifurcation very close to

the blocking temperature of 4 K and also the magnetization

attained almost the saturation value. Field dependent suscept-

ibility is encountered well above the blocking temperature. A

minimum is wT was interpreted as due to ferrimagnetic

behaviour. Most interestingly, the ac-susceptibility at different

frequencies clearly exhibits slow relaxation effect. The

relaxation time was obtained from the Arrhenius law

t(T) = t0exp(2DE/kBT) with t0 = 2.8 1010 s and

DE/kB = 106 K. A constricted hysteresis loop is observed at

2 K with coercive field of ca. 6 kOe and a saturation

magnetization at 70 kOe of 2.88 mB consistent with a non-

compensated resultant moment within the chain.

Kumagaiet al. observed chains of mixed edge- and vertex-

sharing octahedra within the isostructural compound

M3(1,3,5-chtc)2(m-H2O)2(H2O)25H2O (Fig. 7 (XIII)) (M =

Co, Ni), where 1,3,5-chtc is the flexible cis,cis-1,3,5-cyclo-

hexanetricarboxylate.186 The chains consist of trinuclear motifs

of pseudo-octahedra linked throughm-OH2and m-carboxylate.

The temperature dependence of the magnetic susceptibilities

(Fig. 9) indicates dominant antiferromagnetic interaction within

the chain of the cobalt complex (C = 2.9 cm3 K mol1 Co;

Y= 27 K) tending towards a ferrimagnetic alignment at low

temperatures due to the uncompensated moment with two of

one kind and one of the other. For the nickel complex the

interaction is ferromagnetic (C= 1.35 cm3 K mol1 Co;Y= 6

K) with a possible antiferromagnetic order below 5 K.

Zheng et al. obtained a layered structure consisting of

magnetically isolated chains of paddlewheels connected

by 1,2-cyclohexanedicarboxylate in CoII(trans-1,2-chdc)

(Fig. 7 (XIV)) where the dimers are connected by edge-sharing

viathe oxygen of the carboxylate.187 This mode of connection

is classified as flat-ribbon for the copper carboxylate

derivatives.128,129 The inter-dimer CoOCo angles are ca.

1001 and the octahedra are clearly distorted with two short,

two intermediate and two elongated CoO distances. These

distortions are known to affect the magnetic exchange within

the dimers. Most interestingly CoII(trans-1,2-chdc) was

found to display a single-chain-magnetic behaviour at low

temperatures. The susceptibility data above 30 K are

unexpected, Curie constant of C = 2.83 cm3 K mol1 Co

and Weiss constant of +15.87 K, fit well to a ferromagneti-

cally coupled alternating chain (S = 3/2) with intrachain

J1 = +11.51 K, J2 = +3.95 K and a mean-field inter-chain

J0 = 0.01 K. Considerable anisotropy was shown by

the measurement of the susceptibility on a single crystal to

demonstrate the presence of an Ising one-dimensional ferro-

magnetic chain. Consequently, a SCM behaviour was realized

as demonstrated by the frequency-dependent ac-susceptibility

around the blocking temperature of 6 K. From the Arrhenius

plot, two regimes were identified where the relaxation times

are 5.1 1011 and 5.5 108 s for the high- and low-

temperature regions, respectively, and two different corres-

ponding barriers (DE/kB = 80.9 and DE/kB = 50.2 K). A

hysteresis loop with a constriction at zero-field was observed at

several temperatures.

More complexities in the chain-structures are brought about

by the different modes of connection and these have sometimes

been described as ribbons or strips. The following

section details some of these compounds. A 3D-network

Co3(bime)2(m3-OH)2(HO-BDC)2 (Fig. 10 (XV)), bime = 1,2-

bis(imidazol-1-yl)ethane, HO-BDC = 5-hydroxyisophthalate,

consisting of chains of alternate edge-sharing octahedra and

edge-sharing five-coordinated cobalt dimers separated by the

ligands.188 This fully edge-sharing chain behaves as a ferri-

magnet. From ac-susceptibility measurements using a range of

oscillating frequencies substantial dependence was observed

demonstrating a single-chain-magnetic behaviour. The

hysteresis loop is quite wide (22 kOe at 1.77 K) and the

saturation magnetization of 2.35 mB is consistent with

one cobalt resultant moment in a ferrimagnetic state.

Staying with chains of edge-sharing octahedra,

Co3(OH)2(C4O4)3H2O (Fig. 10 (XVI)) contains a strip

segment of a brucite layer.189,190 These are connected to each

other by the squarate dianion that provide four-atom bridges

(OCCO) between the cobalt. Due to the sharing of three edges

at each hydroxide centre the chain is strictly flat. The 3D

structure contains channels occupied by three water molecules

per formula unit and these can be removed and reinserted

repeatedly without affecting the framework as has been shown

Fig. 9 Temperature dependence of wT for M3(1,3,5-chtc)2-

(m-H2O)2(H2O)25H2O, M = Co (blue), Ni (red).186


View Article Online


13/28

by in situ single-crystal crystallography (Fig. 11). Analysis of

the susceptibilities of the hydrated, dehydrated and

rehydrated forms gave C= 3.32 0.02 cm3 K mol1 Co and

Y = +0.2, +2.7 and 0.0 K, respectively, suggesting ferro-

magnetically coupled cobalt within the chain. The virgin sample

orders as a linear antiferromagnet at 8 K and a canting is

observed at 6 K associated with the bifurcation seen in the

ZFCFC measurements. Upon dehydration the sample orders

as a ferromagnet at 8 K (Fig. 12). Ac-susceptibilities and heat

capacity confirm the magnetic ordering. The original magnetic

state is regained upon rehydration. Heat capacity confirms the

LRO and the lack of frequency-dependent ac-susceptibility

eliminates the possibility of SCM behaviour. The nickel

analogue is an antiferromagnet below 7.4 K.

Among the most abundant of linear chains within MOFs is

the edge- and vertex-sharing strip of a [110] layer of a rutile

(Fig. 10 (XVIIXIX)) having the general formula,

M3(OH)2(dicarboxylate)2(H2O)4nH2O. It is found for several

metals (Co, Ni, Mn and Zn) and is bridged by several

dicarboxylate, including fumarate, both cis- and trans-1,4-

cyclohexanedicarboxylate and 4,40-biphenyldicarbarboxy-

late.108,125,191194 For nickel compounds all the sites adopt

octahedral coordination but for cobalt the edge-sharing pair

can be either octahedral or square-pyramidal while for zinc

they are tetrahedral. The simplest case is when the cobalt

coordination are all octahedral thus the chain is very regular,

though there is a slight bend in the chains due to the m3-OH.

The dicarboxylate connects these chains into three-dimensional

structures and in some cases water molecules occupy the space

(Fig. 13). The magnetism in most cases, cobalt and nickel, is

dominated by antiferromagnetic interaction while the ferro-

magnetic interaction between the edge-sharing pairs are not

detected in the CurieWeiss analyses. For cobalt and nickel

the chains are ferrimagnetic and this is seen as a prominent

minimum in thewT vs.Tplot. However, at temperatures below

the minima the moments are saturated at a certain tempera-

ture, for example for the fumarate it is 13 K (Co) and 6 K (Ni)

and for cis-1,4-chdc (Fig. 10 (XVII)) it is 10.2 K (Co) and

2.1 K (Ni). The magnetic behaviour of the nickel compounds

behaves in an expected way as for classical ferrimagnets but

that of the cobalt compounds does not. For example the

ZFCFC measurements of the cobalt fumarate compound

shows a weak spontaneous magnetization at the Ne el

transition while that of 1,4-chdc behaves in a similar way as

that of Co2(pm) discussed above, where the collinear state is

established at 10.2 K and a canting takes place at 9.3 K.193,63

Temperature and field dependence magnetization measure-

ments on powders are quite confusing while those on one

aligned single crystal along the three crystallographic axes

suggest a magnetic easy-axis perpendicular to the

Fig. 11 Structure of Co3(OH)2(C4O4)3H2O showing the squarate

connected brucite strips with the hydrogen-bonded water molecules

within the channels (yellow).

Fig. 12 In situfield-cooling magnetization of Co3(OH)2(C4O4)3H2O

in its virgin, dehydrated and rehydrated forms.

Fig. 10 Structures of multiple-strand chains with different modes of

connection.


View Article Online


14/28

crystallographic chain-axis and very unusual metamagnetic

behaviour along the easy axis whereas normal ferromagnetic-

like behaviour is shown perpendicular to it (Fig. 14). Further

work is in progress to elucidate the origin of this character,

which has never been encountered in the field of magnetism.

Another interesting aspect of this structure is that the water

molecules can be removed upon heating and in the case of the

nickel compound it was found to reversibly transformed from

a ferrimagnet at TC of 2.1 K for the virgin sample to a

ferromagnet at TC of 4.5 K (Fig. 15).192 Tong et al. made the

corresponding compound containing 3,4-pyridyldicarboxylate,

Co3(OH)2(3,4-pydc)2(H2O)2 (Fig. 10 (XVIII)), where the

structure is analogous except for the coordination of the

pyridine to the cobalt taking the position of one water

molecule.125 There is no space to accommodate any solvent.

The magnetic behaviour was described as a ferrimagnet with

TCofca.15 K. The hysteresis loop for a powdered sample was

constricted at zero field and may be a result of different

orientations of the crystallites.

Co3(OH)2(btca)23.7H2O and its dehydrated phase

Co3(OH)2(btca)2 (Fig. 10 (XIX)), btca = benzotriazole-5-

carboxylate, were reported to have the sra network

topology (analogous to that of SrAl2).195 It contains chains

where edge-sharing pairs of two square-pyramids are

vertex-sharing with an octahedral cobalt and the hydroxide

functioning as am3-bridge as in the examples above. The Weiss

constant is quite high (58 K) but negative suggesting the

antiferromagnetic exchange dominates over the ferromagnetic

one. The hydrated phase is reported to order magnetically at 8 K

as a ferrimagnet and single-chain-magnet-like behavior is observed

by ac-susceptibility while the dehydrated phase shows field-

induced metamagnetism from a compensated antiferromagnet

(TN= 4.5 K) to a ferrimagnet. The coexistence of long-range

ferrimagnetism and single-chain-magnetism appears to be

contradictory, as the latter is not supposed to be ordered at all.

However, the frequency dependence of the ac-susceptibility

takes place below the ordering temperature of 8 K and so

it is most likely due to domain effects rather than SCM

characteristics.

Pink needle crystals of Co4(phcinna)6(OH)2(H2O)42H2O

(Fig. 10 (XX)), phcinna = a-phenylcinnamate, shows the

formation of chains consisting of alternate orthogonal edge-

sharing pairs where the adjacent pairs are connected at the

vertices by a hydroxide and carboxylate oxygen atoms.196 It is

a diagonal strip of a [110] rutile layer. These chains are not

connected by bonds and the minimum distance between the

centre of one chain and its neighbours is 14 A . It exhibits a

long-range antiferromagnetic ordering at 5.5 K. Due to the

isolation of the chain with its neighbours a very small field of

25 Oe is enough to reverse the moments. However, the

moment at saturation is only a third of that expected if the

moments of all the carriers were aligned parallel to the field.

Co3(2,4-pydc)2(m3-OH)25H2O and Co3(2,4-pydc)2(m3-OH)2-

(H2O)7H2O (Fig. 10 (XXI)), 2,4-pydc = pyridine-2,4-

dicarboxylate, have been hydrothermally synthesized and

shown to have 3D frameworks with hydroxide-bridged metal

chains.197 The chain is constructed by vertex-sharing quad-

rangles formed via edge-sharing triangles. Magnetic studies

show that the former exhibits spin-canted antiferromagnetism

and a field-induced spin-flop transition while the latter behaves

as a normal antiferromagnet. The magnetic properties are

largely unchanged by removal of the water molecules from

Fig. 14 Temperature dependence of the susceptibility and isothermal

magnetization of Co3(OH)2(cis-1,4-chdc)2(H2O)42H2O along its three

crystallographic axes. See Fig. 13 for referencing the orientations.

Fig. 13 Structure of Co3(OH)2(cis-1,4-chdc)2(H2O)42H2O showing

the connected rutile strips with the hydrogen-bonded water molecules

within the channels (yellow).


View Article Online


15/28

the channels. Li et al. prepared Co3(2,4-pydc)2(m3-CH3O)0.5-

(m3-OH)1.5(H2O)4.5H2O and solved its crystal structure. It

contains a 3D-structure with a modified arrangement of the

octahedra within the chains.198 There are tetrameric edge-

sharing units connected at the vertex with an edge-sharing

dimer. It is to be noted that part of the hydroxide is replaced

by methoxide. If the reaction is carried out in ethanol the fully

hydroxide substituted complex is obtained. The magnetic

properties indicate ordering as an antiferromagnet below

6 K. A slightly more complex chain structure is found in

Co5(m3-OH)2(pm)2(bpp) (Fig. 10 (XXII)), bpp is 1,3-bis(4-

pyridyl)propane, where three octahedral cobalt centres share

edges and these are then bridged by two five-coordinated

cobalt centres.199 These chains are bridged by the pyromellitate

and the bending bpp ligand to give a 3D-framework. In an

applied field less than 3.5 kOe, it behaves as a compensated

antiferromagnet and in fields exceeding the metamagnetic

critical field of 4 kOe a ferrimagnetic alignment is found. The

saturation moment amounts to that of one cobalt (2.4 mB). It

suggests that the ferrimagnetism is due to the three octahedral

cobalt atoms having their moments parallel while they are

antiparallel to those of the two five-coordinated cobalt centres. The

Ne el temperature is 12 K. Cheetham et al. obtained a cobalt

succinate, Co7(H2O)3(OH)6(C4H4O4)47H2O (Fig. 10 (XXIII)),

having a hexamer cluster bridged by a monomer forming a

chain. No magnetic properties of the complexes are given.200

The exchange interaction between two paramagnetic centres

through an oxalate bridged has been of much interest in the past

and several theoretical and experimental studies have been

performed to evaluate its magnitude and sign. Many chains and

layers containing such bridges are known to display long-range

magnetic ordering. This remains a matter for debate amongst

experimentalists and theoreticians given that one-dimensional

magnetic chain should not show long-range magnetic ordering

at finite temperature.201,202 There are two types of chains that can

be formed with cobalt octahedra and oxalate bridges while there

are none made of tetrahedra. The first type is a linear one with the

oxalate bridges in trans-position and the other is zigzag with

oxalate in cis-position.174,203217 A whole gamut of ligands is

known to bond to the remaining two sites of the octahedron.

For the former (Fig. 16 (XXIV)) the ligands are monodentate

(water, dipyridylethane, dipyridylethylene, 4,40-bipyridine,

4,40-bipyridine N-oxide, piperazine, aminopyridine) and for the

latter (Fig. 16 (XXV)) they are usually chelating (2,2 0-dipyridyl-

amine, 1,3-propanediol, 3-hydroxypyridine, isoquinoline,

adenine, purine and oxalate). All the known compounds show

antiferromagnetic exchange interaction between nearest neigh-

bours which is characterized by a broad hump in the

temperature dependence of the susceptibility. In several cases

the latter has been modelled. The diaquo compound was the

first to be studied and was found by magnetization, EPR and

heat capacity measurements as well as neutron scattering to be

a collinear antiferromagnet below 6.3 K. It was modelled to a

one-dimensional Ising magnet where the intrachain nearest-

neighbour exchange is 30 K and the ratio of the interchain to

intrachain is 0.003. The other linear chain with aminopyridine is a

canted antiferromagnet at 8 K. In contrast to the linear chains, there

are more examples of canted antiferromagnets for the zigzag

compounds,211216 for example those with 1,3-propanediol

(10.6 K), 4,40-bipy (13 K), 2,20-dipyridylamine (7.5 K) and oxalate

(Fig. 16 (XXVI)) in K2Co(ox)2 (37 K). The high transition

temperature for the latter may be due to the ionic-bonded network

formed by the potassium and the non-coordinated oxygen

atoms of the oxalate. It is interesting to note the observation of

an SCM behaviour for the alternate mixed-metal complex

[(C12H24O6)K][(C12H24O6)(FC6H4NH3)][Co(H2O)2Cr(ox)3]2where the bulky ions and crown ether sits in between layers

consisting of hydrogen-bonded zigzag chains.217

Co4(pico)4(4,40-bpy)3(H2O)22H2O (Fig. 16 (XXVII)), pico

= hydroxypicolinate, is closely related to the oxalato-bridged

chain discussed above except that one of the arms is a five-

membered ring chelate as opposed to four-membered.218 The

chains are bridged by the 4,4 0-bpy into two types of layers

which are interpenetrated to give the 3D-structure. The chains

are magnetically isolated from each other. The magnetic

properties are similar to those of Co2(pm) where a very sharp

drop in susceptibility is observed at 3.5 K due to collinear

antiferromagnetic ordering.63 Although the canting has not

been observed, the ac-susceptibility suggests there is a

spontaneous magnetization. The isothermal magnetization

establishes a metamagnetic critical field of 160 Oe and shows

Fig. 15 Temperature dependence of the susceptibility of Ni3(OH)2-

(cis-1,4-chdc)2(H2O)42H2O in its virgin, dehydrated and rehydratedforms.189


View Article Online


16/28

a superposition of hysteresis loops analogous to that ofCo2(pm) where the saturation magnetization is not reached

even in 70 kOe at 2 K. Using hydroxyphosphonoacetic acid

(pa) neutralized with ethylenediamine (enH2)[Co2(pa)2(H2O)2]

was obtained and it contains a similar bridged chain which are

then connected by the PO3 unit into layers. It behaves as a

canted-antiferromagnet below 3.3 K.219

5. Ladders

Well-isolated ladder structures in the MOF family are very

rare. There are three known systems, Na2Co2(ox)3(H2O)2(Fig. 17 (I)), Co

3(2,5-pydc)

2(m

3-OH)

2(OH

2)2

(Fig. 17 (II)),

pydc = pyridinedicarboxylate, and Co(C8H8O4) (Fig. 17 (III)),

that can be classified in this category.220222 Magnetic studies

are available for only the first two. Na2Co2(ox)3(H2O)2 has

one oxalate forming the rung and a OCO bridge for the legs.

These two-leg ladders are separated by sodium cations.

Co3(2,5-pydc)2(m3-OH)2(OH2)2 has MOM legs and OCO

rungs. The legs are closely related to the Co3(OH)2rutile strip

presented above except for the fact that two of the octahedra

are now replaced by distorted five-coordinated cobalt atoms.

All the carboxylate oxygen and the pyridine nitrogen atoms of

the 2,5-pyridinedicarboxylate are involved in the coordination

to the cobalt atoms. For the third example, Co(C8H8O4), the

carboxylate groups of 4-cyclohexene-1,2-dicarboxylate forms

the OCO connections of the legs and rungs of the ladder.

The first reported susceptibility of Na2Co2(ox)3(H2O)2exhibits a broad maximum (21 K) which was associated with

short-range correlations, and a phase transition to a long-

range ordered Ne el state at 9 K.220 Above TN the data are

fitted to several models which suggest the intra-ladder

couplings are of the order 2.83.5 K and the inter-ladder

coupling is approximately 0.14 K. Later magnetic measurements

eliminate the presence of long-range magnetic ordering

(Fig. 18) and specially those on aligned single crystals were

used to fit the low-temperature data to a S= 1/2 spin-ladder

model giving a gap of 16 K for H8 and 13 K for H>.223,224

There was no anomaly in the ZFCFC measurements and the

two superpose for the whole temperature range (2300 K). A

broad hump atB10 K was observed in the heat-capacity data

which was fitted using the same model to give a gap of 20 K.

Doping the crystals with 10% Zn the broad maximum in the

susceptibility is still present while a very clear bifurcation at

10 K in the ZFCFC magnetization in 100 Oe was interpreted

as being due to spin-glass behaviour.225 The analogous nickel

and iron compounds are also known and their magnetic

properties reported.

The magnetism of Co3(2,5-pydc)2(m3-OH)2(OH2)2 is more

complex but has some similarity to that of the rutile Co3(OH)2chains discussed above.

221A long-range antiferromagnetic

state with a small canting is developed at 30 K in low applied

fields which corresponds to antiferromagnetically coupled

ferrimagnetic chains. In the absence of a neutron diffraction

study one cannot be certain if the antiferromagnetism is

between individual legs within a ladder or between ladders.

Several steps were observed in the isothermal magnetization

measured on a polycrystalline sample and has been described

as multiple bistability. At fields higher than the metamagnetic

Fig. 17 Structures of three known ladders.

Fig. 16 Structures of oxalate-bridged chains and one with hydroxy-

picolinate bridge.


View Article Online


17/28

critical field of 15 kOe, the moments are reversed. For such a

high critical field compared to those described above it is quite

likely that the reversal of moment is taking place within a

ladder. From the saturation value of 3 mB mol1, a ferri-

magnetic state is the final state. All the behaviour is similar tothe rutile chains described earlier. The interesting point is that

the transition temperature is 23 times that of single-strand

chains.

6. Layers

The most condensed of the layers in terms of MOM con-

nections is that of brucite. There are no example of a purely

brucite layer in the MOFs. It does exist for Co(OH)2 and for

two others, Co2(OH)3NO3which contains a coordinated NO3in

the place of one OH and Co5

(OH)6

(SO4

)2

(H2O)

4(Fig. 19 (I))

which is pillared by OSO3Co(H2O)4O3SO at the missing

OH sites.226231 The interlayer distances are therefore

increased from 4.7 A for Co(OH)2to 6.95 A for Co2(OH)3NO3and 10.3 A for Co5(OH)6(SO4)2(H2O)4. Modification of the

brucite layer is observed with periodic vacancies in the layer

where each vacant position is then replaced by tetrahedral

units above and below the layer. The number and disposition

of the tetrahedra vary from case to case. Only four such

compounds, Co5(OH)8(trans-1,4-chdc)4H2O (Fig. 19 (II)),232

[Co7(OH)12(H2O)2](C2H4(SO3)2) (Fig. 19 (III)),233 and

Co8(OH)12(SO4)2(diamine)nH2O (Fig. 19 (IV)),234,235 where

n = 3 for ethylenediamine or 1 for DABCO (C6N2H12) have

been fully characterised by single-crystal X-ray diffraction,

while a large number are inferred to be isostructural from

XRPD. Co5(OH)8(trans-1,4-chdc)4H2O has adjacent layers

bridged by the dicarboxylatevia the apices of the tetrahedral

units and similarly for Co8(OH)12(SO4)2(diamine)nH2O by

the diamines. XRPD of both compounds show they can lose

water of crystallization without loss of crystallinity, and for

the former in-situ single-crystal diffraction revealed the

complete structure of the fully dehydrated compound, and

its reversibility to the original state after exposure to air.232 In

contrast, [Co7(OH)12(H2O)2](C2H4(SO3)2) has coordinated

water while the disulfonate sits in the gallery and is hydrogen

bonded to the layers. It is worth noting that the ratio of

tetrahedral to octahedral units in the structure may be related

to the length of the sulfonate.

The magnetic properties of these layers are quite varied and

the reason for the difference is logical. First, Co(OH)2 and

Co2(OH)3NO3are metamagnets with Ne el transitions of 10 1 K

and critical field at 2 K of less than 1500 Oe.227229,236,237 The

interpretation is that the cobalt atoms within the layers are

ferromagnetically coupled and the layers are then anti-

ferromagnetically coupled. A weak critical field reverses the

moments to the ferromagnetic state as judged by the satura-

tion magnetization approaching 5 mB. On the other hand all

the studies, magnetization at ambient and elevated pressures,

heat capacity and neutron scattering, indicate that

Co5(OH)6(SO4)2(H2O)4 has the characteristics of a pure ferro-

magnet at all fields and the Curie temperature is 14 K.230,231

Neutron diffraction analysis suggests that the moments of the

cobalt atoms are in the plane of the layers (Fig. 19 (I)). Further

analysis confirms the xy nature of the magnetic system.

Fig. 19 Structures of structurally characterised layered cobalt

hydroxides.

Fig. 18 Temperature dependence of the magnet

magnetic frameworks

Documents