trinuclear mixed-valent mnii/mniv/mnii complexes — structure and magnetic behavior

Trinuclear Mixed-Valent MnII/MnIV/MnII Complexes � Structure andMagnetic Behavior

Maria Alexioua, Curtis M. Zaleskib, Catherine Dendrinou-Samaraa, Jeff Kampfb, Dimitris P. Kessissogloua,*,and Vincent L. Pecorarob,*a Thessaloniki / Greece, Department of Chemistry, Aristotle University of Thessalonikib Ann Arbor, MI / USA, Department of Chemistry, University of Michigan

Received July 18th, 2003.

Dedicated to Professor Bernt Krebs on the Occasion of his 65th Birthday

Abstract. Examples of mixed-valent manganese compounds with2e� difference between the oxidation states of the metal ions arerare. Herein we report the synthesis, crystal structure, spectroscopicand magnetic studies of a series of MnII-MnIV-MnII trinuclear com-pounds of the general composition MnIIMnIVMnII(pko)4-(CH3O)2(X)2[X� Cl� (1), NCO�(2), NCS� (3)]. These moleculescan be prepared by adding the ketonoxime ligand in the presenceof sodium hydroxide and the analogous pseudohalide salt toMnCl2·4H2O in methanol and with exposure to air. The crystalstructures for the three compounds are quite similar with one sixcoordinate MnIV ion in an MnO6 coordination environment andtwo six coordinate MnII ions having an N5O or N4OCl coordi-nation environment. Four nitrogen atoms come from two ketonoxi-

Dreikernige, gemischt-valente MnII/MnIV/MnII-Komplexe � Struktur und magnetischeEigenschaften

Inhaltsübersicht. Beispiele von gemischt-valenten Mangan-Verbin-dungen mit 2e�-Differenz zwischen den Oxidationsstufen des Me-tallions sind selten. Wir berichten hier über die Synthese, Kristall-struktur, spektroskopische und magnetische Untersuchungen aneiner Reihe von dreikernigen MnII-MnIV-MnII-Verbindungen derallgemeinen Zusammensetzung MnIIMnIVMnII(pko)4(CH3O)2-(X)4[X� Cl� (1), NCO� (2), NCS� (3)]. Diese Verbindungen kön-nen aus MnCl2·4H2O bzw. den Pseudohalogeniden und den Keton-oxim-Liganden in Gegenwart von Natriumhydroxid in Methanolan Luft synthetisiert werden. Die Kristallstrukturen der drei Ver-bindungen sind sehr ähnlich mit einem sechsfach koordiniertenMnIV-Ion in einer MnO6-Umgebung und zwei sechsfach koordi-nierten MnII-Ionen mit einer N5O- bzw. N4OCl-Koordination. Vier

Introduction

Professor Bernt Krebs has given significant contributionsranging from structural characterization of metal sulfideclusters [1] to bioinorganic models of phosphatases [2] andX-ray crystallographic characterization of enzymes such ascatechol oxidase [2a, 3]. Recently, Professor Krebs has pub-lished important reactivity models for the Mn catalase [4].Therefore, we feel that it is appropriate to dedicate this con-

2348 2003 WILEY-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim DOI: 10.1002/zaac.200300246 Z. Anorg. Allg. Chem. 2003, 629, 2348�2355

mato ligands, an oxygen atom from a methoxide bridge and achloro or nitrogen atom from the halide or pseudohalide terminalligand. Each pko� ligand acts as a tridentate chelating agent byusing a pyridine, and an oximato-nitrogen, bound to the MnII ionand the oximato oxygen, coordinated to the MnIV central ion. Theother nitrogen atom of the pko� ligand remains uncoordinated.The magnetic parameters obtained from the fitting procedure areJ12 � �7.8 cm�1 and gMnIV � 2.07 for 1, J12 � � 8.1 cm�1 andgMnIV � 2.08 for 2, and J12 � �6.1 cm�1 and gMnIV � 2.09 for 3which indicate a ferromagnetic behavior. The ground states for allthe compounds are S�13/2.

Keywords: Manganese; Mixed-valence state; Trinuclear complexes

Stickstoffatome werden von zwei Ketooximato-Liganden geliefert,ein Sauerstoffatom von einer Methoxid-Brücke und ein Chlor- bzw.Stickstoffatom von dem endständigen Halogenid- bzw. Pseudoha-logenid-Liganden. Jeder pko�-Ligand ist ein dreizähniger Chelatli-gand durch ein Pyridin- und ein Oximato-Stickstoffatom am MnII-Ion und dem Oximato-Sauerstoffatom, das am zentralen MnIV-Ionkoordiniert ist. Das andere Stickstoffatom des pko�-Liganden istungebunden. Die gefitteten magnetischen Parameter sind J12 �

�7.8 cm�1 and gMnIV � 2.07 for 1, J12 � � 8.1 cm�1 and gMnIV �

2.08 for 2, and J12 � �6.1 cm�1 and gMnIV � 2.09 for 3, welchesferromagnetisches Verhalten zeigt. Der Grundzustand für alle Ver-bindungen ist S�13/2.

tribution in the area of Mn bioinorganic chemistry to Pro-fessor Krebs in recognition of his 65th birthday.

Manganese is an essential element in many biologicalprocesses for which two functional activities can be ident-ified: MnII can act as a Lewis acid hydrolytic catalyst, andin higher oxidation states MnIII and MnIV act as oxidativecatalysts. In most Mn redox enzymes [5, 6], Mn exists inthe 2�, 3� or 4� oxidation states. Important redox activemanganese enzymes include the manganese-containing ri-

Trinuclear Mixed-Valent MnII/MnIV/MnII Complexes

bonucleotide reductase [7, 8], the Manganese ThiosulfateOxidase [9], the mitochondrial SOD [10] the ManganesePeroxidase (MnP) [11, 12] the manganese catalases [13, 14]and a tetranuclear cluster in the Oxygen Evolving Complex(OEC) [15�19]. Among them, the most important processin Nature, occurs in the OEC of Photosystem II (PSII)which is responsible for the conversion of water to dioxy-gen. It has become accepted that the water oxidation cata-lytic site in this enzyme requires four manganese ions thatare bridged together by water-derived O2� and OH� li-gands. The manganese coordination sphere is believed to bedominated by O and N donors from available amino acidside chains or oxygen atoms derived from water [20]. Thecrystal structure of the OEC confirmed the presence of fourmanganese atoms, yet a clear image of the manganese clus-ter still eludes us [18, 19]. In addition, EXAFS data [21�27]indicate the presence of at least two �2.7 A Mn···Mn inter-actions in addition to an �3.3 A interaction. Thus, thequestion whether the catalytic center is a tetranuclear 1:3or 2:2 cluster still remains.

While cycling through the catalytic water oxidation reac-tion, the OEC forms five resolved oxidation levels called Sstates [29]. S0 is the most reduced state and S4, from whichdioxygen is liberated, is the most oxidized. The availabledata strongly suggest that a polynuclear cluster is respon-sible for the observed EPR signals of S0 (g � 2 multiline),the “active form” of S1 (g � 4.8 in parallel polarization)and S2 (g � 2 multiline or g � 4.1 with fine structure) oxi-dation states of the OEC [29�31]. Interpretation of theseEPR spectra, in conjunction with XANES spectroscopy[29], provides the best determination of the Mn oxidationlevels in each S state. Based on these studies, an ensembleof MnIIMnIIIMnIV

2 has been proposed as the oxidationstates for the S0 manganese cluster [32]. This assignmentis important in two ways. First, we can extract importantmechanistic information for the reaction since S0 is theproduct state of the reaction. Second, having three differentoxidation states of the same element in a simple cluster ishighly unusual suggesting that the cluster ligand environ-ment must be somewhat asymmetric.

Synthetic model compounds containing three differentmetal oxidation states are extremely rare. Even low nu-

* Prof. Vincent L. PecoraroDepartment of ChemistryUniversity of Michigan,Ann Arbor, MI48109-1055 / USAFAX: �[email protected]* Dimitris P. KessissoglouDepartment of ChemistryAristotle University of ThessalonikiThessaloniki, 54124 / GreeceFAX:�30-2310-997738E-mail: [email protected]

Supporting information for this article is available on theWWW under http://www.wiley-vch.de/home/zaac or from theauthor

Z. Anorg. Allg. Chem. 2003, 629, 2348�2355 zaac.wiley-vch.de 2003 WILEY-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim 2349

clearity Mn compounds containg both MnII and MnIV haveseen only a few reports [33�35]. Previous efforts have beencentered on the preparation and characterization of trinu-clear mixed-valent MnIIIMnIIMnIII(Schiff-base)2(carboxyl-ato)4(Solvent)2 complexes with predominantly oxygen-do-nor ligands, in order to understand the fundamental coordi-nation, structure, and magneto-chemistry of complexes po-tentially relevant to the active site of the OEC [36�42].These complexes are valence trapped as evidenced by thelong central octahedral MnII to heteroatom bond lengthsthat is flanked by two Jahn-Teller elongated octahedral orsquare-pyramidal MnIII ions and give 19-lines EPR signals[42]. Recently, we have reported [36] the first example of amixed-valent MnIIMnIVMnII(pko)4(CH3O)2(SCN)2 trinu-clear compound that could be considered as a valence iso-mer to MnIIIMnIIMnIII series compounds.

In this report, we present the crystal structures and spec-troscopic and magnetic studies of a series of MnIIMnIV-MnII(pko)4(CH3O)2(X)2 [X � Cl� (1), NCO� (2), SCN�

(3)] compounds which are rare examples of mixed-valentMnII-MnIV compounds with a 2e� difference between theoxidation states of the metal ions [33, 34]. The compound3 has been previously reported as a communication [33].

Results and Discussion

Synthesis

The mixed valent trinuclear compounds of the general com-position MnIIMnIVMnII(pko)2(X)4[X � Cl� (1), NCO� (2),SCN� (3)] can be prepared by adding the ketonoxime ligandin the presence of sodium hydroxide and the analogouspseudohalide salt to MnCl2·4H2O in methanol and withexposure to air (Scheme 1).

Scheme 1

The mixed valence trinuclear compounds are dark browncrystalline solids that appear to be air and moisture stable.The compound 1 is insoluble in all solvents tested, 2 is sol-uble in DMF and DMSO and 3 is soluble in DMF, CH2Cl2,THF and CHCl3 but insoluble in water. Complexes 2 and3 are not electrolytes in DMF.

Description of the Structures

ORTEP diagrams of complexes 1 and 2 are shown as Fig-ures 1 and 2. Bond distances and angles for the coordi-

M. Alexiou, C. M. Zaleski, C. Dendrinou-Samara, J. Kampf, D. P. Kessissoglou, V. L. Pecoraro

Figure 1 ORTEP diagram of 1 at 60 % probability.

nation spheres are listed in Table 1. The description of thestructure refers to compound 2 which is comparable to 1and 3. Each MnII ion (Mn2) is six coordinate having anN4OCl (1) or N5O (2) coordination environment. Four ni-trogen atoms come from two pko� ligands, an oxygen atomfrom the methoxide bridge, and a chloro or nitrogen atomfrom the halide or pseudohalide terminal ligand. In 2, theterminal unidentate pseudohalide ligand is bound to theMnII ions through a nitrogen atom. Each pko� ligand actsas a tridentate chelating agent by using a pyridine, N(3),and an oximato-nitrogen, N(1), bound to the MnII ion andthe oximato oxygen atom, O(1), coordinated to the MnIV

central ion (Mn1). Meanwhile the other nitrogen atom ofthe pko� ligand, N(2), remains uncoordinated. The ter-minal MnII ions are coordinated to five nitrogen atoms andan oxygen atom with bond distances varying from2.076(3) A to 2.299(3) A, and the Mn-N/Oavg � 2.103 A issimilar with other trinuclear MnIIMnIIMnII compounds[41, 43, 44], e.g. MnIIMnIIMnII (pybim)2(AcO)6 with Mn-N(O)avg r � 2.168 A [41]. The bond distances fully supporta valence trapped 2� oxidation state of the terminal ions.

The bond distances around the MnIV ion (Mn1) areMn(1)-O(3)methoxy � 1.880(2) A, Mn(1)-O(2)oximato �1.920(2) A and Mn(1)-O(3)avg � 1.915 A, and they are simi-lar with the analogous trinuclear MnII-MnIV-MnII com-pound reported [33]. The average Mn(1)-O distance is1.905 A which is consistent with the MnIV oxidation level.Other mononuclear MnIV compounds have slightly shorteraverage distances � MnIV(saladhp)2 , Mn-Oav � 1.889 A[36, 37] and [MnIV(hib)3]2�, Mn-Oav � 1.841 A [45]. In ad-dition, the lack of a Jahn-Teller distortion on Mn(1) atomis strong support for the assignment of a 4� rather a 3�oxidation state. The central Mn(1) and the terminal Mn(2)ions are bridged by the methoxide oxygen and two oxamatogroups from the pko� ligands resulting in a 3.397 AMn(1)·······Mn(2) separation with a 114.31(10)° Mn(1)-O(3)-Mn(2) angle.

2003 WILEY-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim zaac.wiley-vch.de Z. Anorg. Allg. Chem. 2003, 629, 2348�23552350

Figure 2 ORTEP diagram of 2 at 60 % probability.

Table 1 Selected Bond distances /A and angles /° of MnIIMnIV-MnII(pko)4(CH3O)2(Cl)2 (1), MnIIMnIVMnII(pko)4(CH3O)2-(OCN)2 (2), and MnIIMnIVMnII(pko)4(CH3O)2(SCN)2·CH3OH (3)

1 2 3 [33]

Mn(1)-O(1)oximato 1.924(2) 1.944(2) 1.922(2)Mn(1)-O(2)oximato 1.924(2) 1.920(2) 1.933(2)Mn(1)-O(3)methoxy 1.888(2) 1.880(2) 1.881(2)Mn(2)-N(1)oximato 2.254(3) 2.159(2) 2.259(3)Mn(2)-N(3)pyridyl 2.293(3) 2.299(3) 2.291(3)Mn(2)-N(4)oximato 2.324(3) 2.286(3) 2.241(2)Mn(2)-N(6)pyridyl 2.292(3) 2.283(3) 2.280(3)Mn(2)-O(3)methoxy 2.155(2) 2.159(2) 2.150(2)Mn(2)-N(7)thiocyanato � � 2.119(3)Mn(2)-N(7)cyanato � 2.076(3) �Mn(2)-Cl 2.399(2) � �S(1)-C(23) � � 1.624(4)N(7)-C(23) � 1.153(4) 1.157(4)Mn(1)···Mn(2) 3.406 3.397 3.372O(4)-C(23) � 1.189(4) �

1 2 3

O(3)-Mn(2)-N(3) 132.99(10) 141.16(9) 144.92(9)N(1)-Mn(2)-N(6) 119.41(11) 132.29(9) 136.15(10)N(3)-Mn(2)-N(4) 126.55(11) 116.27(9) 109.43(9)O(3)-Mn(2)-N(6) 134.80(10) 126.64(9) 123.44(10)N(1)-Mn(2)-Cl 143.73(8) � �N(4)-Mn(2)-Cl 134.97(8) � �N(1)-Mn(2)-N(7) � 131.11(12) 123.38(10)N(4)-Mn(2)-N(7) � 145.50(12) 154.16(11)N(7)-C(23)-S(1) � � 179.1(3)N(7)-C(23)-O(4) � 179.2(4) �O(3)-Mn(1)-O(2) 90.83(10) 91.36(9) 92.68(8)O(3)-Mn(1)-O(1) 91.38(10) 90.94(9) 90.41(9)O(1)-Mn(1)-O(2) 92.28(10) 89.39(9) 91.09(8)Mn(1)-O(3)-Mn(2) 114.64(11) 114.31(10) 113.39(10)

Though these complexes are very similar in terms of co-ordination spheres and ligand arrangement, there is a sig-nificant distortion of bond angles around the Mn(2) centersin each compound (Figure 3, Table 1). In 1 the N(1)-Mn(2)-Cl(1) bond angle is 143.73(8)° and the corresponding anglesin 2 and 3 [N(1)-Mn(2)-N(7)] are 131.11(12)° and


Figure 3 ORTEP of the first coordination sphere of the Mn2 atomin each structure. Notice how the angles distort around the Mn2in each structure. a) 1 at 60 % probability b) 2 at 60 % probabilityc) 3 at 50 % probability.

Figure 4 ORTEP view down the pseudo 3-fold axis in each struc-ture. N1, O3, and N6 comprise the upper plane (dashed line), whileN3, N6, Cl1 or N7 comprise the lower plane (solid line). a) 1 at60 % probability b) 2 at 60 % probability c) 3 at 50 % probability.

123.38(10)°, respectively. In addition, the N(4)-Mn(2)-Cl(1)angle in 1 is 134.97(8)°, while the equivalent angles in 2and 3 [N(4)-Mn(2)-N(7)] are 145.50(12)o and 154.16(11)°,respectively. For 2 the torsion angle about C(23)-N(7)-Mn(2)-O(3) is �32.7°, while in 3 the equivalent torsionangle is �164.5°. These angles clearly show how the coordi-nation about the Mn(2) can be distorted simply by chang-ing the nature of the bound anion.

The approximate coordination around each Mn(2) is trig-onal prismatic, but again the distortions vary depending onthe bound anion (Figure 4). The planes of reference formeasuring the twist angle from the crystallographic dataare a plane comprising of N(1), N(4), and O(3) and anotherplane comprising of N(3), N(4), and X [X � Cl(1) for 1and N(7) for 2 and 3]. For 1 the estimated twist angle be-tween the planes is 3.0°, practically a true trigonal prism.However, upon changing the bound anion to �OCN in 2this angle increases to 5.9°, and in 3 the most staggeredarrangement is seen with a twist angle of 9.5°. For referencean octahedron has a twist angle of 30°. Again it is clearthat a change in the bound anion has a dramatic effect onthe geometry about the Mn(2) atom. The ultimate conse-quence of these distortions, which may be relevant to themagnetic exchange for the compounds, is that theMn·······Mn separation decreases by 0.04 A across the series.However, because MnII is a high spin d5 ion, with a spheri-cally symmetric d shell, one would not expect that thesegeometric perturbations would have much impact on themagnetic exchange in the system.


Figure 5 Variable-temperature magnetic susceptibility data for 1.Curve A is the χM plot and curve B is the µeff plot. The solid lineindicates the fit to the data as described in the text.

Figure 6 Variable-temperature magnetic susceptibility data for 2.Curve A is the χM plot and curve B is the µeff plot. The solid lineindicates the fit to the data as described in the text.

Magnetic behavior

We collected the variable-temperature magnetic suscepti-bility and variable-field magnetic susceptibility data for 1,2 and 3 to discern the type of magnetic exchange interac-tions for these structural isomers. The magnetic data for3 has been reported previously [33]. Variable-temperaturemeasurements indicate that there is a small ferromagneticinteraction between the MnII and MnIV centers for bothstructures 1 and 2 (Figures 5 and 6). For 1 the value of µeff

increases as the temperature was lowered from 300 K(µeff � 9.79) to 40 K (µeff � 12.16). A similar trend is exhib-ited by complex 2 (at 300 K µeff � 9.95 and at 40 K µeff �12.40). A spin-only value for magnetically coupled 2 MnII

and one MnIV is µeff � 13.96. The MnIIIMnIIMnIII valence-isomers compounds showed weak antiferromagnetic ex-change interactions (J� �6 cm�1) with an S � 3/2 groundstate, whereas the tetranuclear MnII

3MnIV exhibited bothantiferromagnetic and ferromagnetic exchange [34]. By vir-


tue of crystallographic criteria the isotropic HeisenbergHamiltonian for the complexes 1, 2 and 3 is given by eq.(1),

H � �J1(SMnII1

· SMnIV � SMnII2

· SMnIV) � J2(SMnII1

· SMnII2) (1)

(having an arrangement SMnII1- SMnIV- SMnII

2) where

SMnII1

� SMnII2

� 5/2 and SMnIV � 3/2. Because of the largedistance between the terminal MnII ions (6.744�6.812 A)we can exclude this interaction and, therefore, the majorexchange interaction is expected between the terminal MnII

and the central MnIV. This assumption is also based on anextended study for the valence-isomer III-II-III compounds[40�42] where it has been proved that the role of the J2 wasunimportant in the fitting procedure. This equation thensimplifies the Hamiltonian to

H � �J(SMnII1

· SMnIV � SMnII2

· SMnIV) (2)

The exchange energies are given by

E(S, S�) � �J/2 · S(S�1) � J/2 · S�(S��1) (3)

where S� � SMnII1

� SMnII2

and S � S� � SMnIV. For thevalence-isomer III-II-III compounds, it has been assumedeither different [40] g values for MnIII and MnII in the Zee-man Hamiltonian and have added first and second ordercontribution to the Van Vleck equation or isotropic g-val-ues in the Zeeman term, giving satisfactory magneticmodels [42]. In this model we set the g-value for MnII as2.0 and varied the g-value for the MnIV during the fittingprocess. The gS,S� factor is given in eq. (4),

gS,S� �gMnII[S(S�1)�S�(S��1) SMnIV(SMnIV�1)]

2S(S�1)� (4)

�gMnIV[S(S�1) S�(S��1)�SMnIV(SMnIV�1)]

2S(S�1)

From here the Van Vleck formula can be calculated, andthe final expression of the susceptibility is expressed as

χ � (Nβ / 3kT) (5)

�2SMnII

S��0�

S��SMnIV

S��S��SMnIV�gSS�

2 S(S�1)(2S�1)exp[�E(S,S��1)/kT]

�2SMnII

S��0�

S��SMnIV

S��S��SMnIV�(2S�1)exp[�E(S,S�)/kT]

where N is Avogadro’s number, β is the Bohr magneton, kis the Boltzmann constant, and T is temperature. The mag-netic parameters obtained from the fitting procedure areJ12 � �7.8 cm�1 and gMnIV � 2.07 for (1), J12 � �8.1 cm�1

and gMnIV � 2.08 for (2), and J12 � �6.1 cm�1 and gMnIV �2.09 for (3). These J values further support that a ferromag-netic interaction exist between the MnII and MnIV centerswithin the complex. Based on these data, we can concludethat the small changes in the Mn·······Mn separation do notcorrelate with the observed ferromagnetic coupling values.


Figure 7 Variable-field magnetic susceptibility data for 1. The solidline represents the fit of the Brillouin function to the data with anS � 13/2 ground state.

Figure 8 Variable-field magnetic susceptibility data for 2. The solidline represents the fit of the Brillouin function to the data with anS � 13/2 ground state.

Isothermal magnetization measurements at 4.5 K from100 Gauss to 55,000 Gauss allowed for the ground statedetermination of each complex (Figures 7 and 8). The mo-lar magnetization values were fit using the Brillouin func-tion to determine the ground state of each complex withthe Brillouin function defined as

M�NgβS�2S�1

2Scoth �2S�1

2S·

gβSH

kT � �1

2Scoth � 1

2S·

gβSH

kT �� (6)

where S is the ground state spin, H is the applied field, gwas determined from the variable-temperature fitting, andthe other symbols have been defined above. Fitting of the


data for 1 and 2 both lead to a S � 13/2 ground state. Thismatches the previously reported ground state for 3.

In conclusion, we observe that a MnIIMnIVMnII arrange-ment of metal ions can be achieved by varying the halide orpseudohalide ligands. In all cases the complexes are weaklyferromagnetically coupled. One could not discern reductiveor oxidative electrochemical waves indicating that the struc-tures strongly localize the ions into discrete valences. Thesestructures demonstrate that a biological cluster rich in oxy-anions at one site and containing neutral nitrogen donorsat another site can allow a stable MnII/MnIV mixed valency.Compound 1 is particularly interesting in this respect sincechloride is an essential ion for photosynthetic water oxi-dation.

Experimental Section

The following abbreviations are used throughout the text: Hpko �

2,2��dipyridyl ketonoxime; dmf � dimethylformamide, HOAc �

acetic acid; H2sal � salicylic acid; H3saladhp � 1,3-dihydroxy-2-methyl-(salicylideneamino)propane; pybim � 2-(2-pyridyl)-benzi-midazol; thf�tetrahydrofuran.

Materials

The chemicals for the synthesis of the compounds were used aspurchased. Acetonitrile (CH3CN) was distilled from calcium hy-dride (CaH2) and CH3OH from Magnesium (Mg) and were storedover 3 A molecular sieves. Diethyl ether, anhydrous grade and abso-lute ethanol were used without any further purification. NaOCN,NaSCN and MnCl2·4H2O were purchased from Aldrich Co. Allchemicals and solvents were reagent grade.

Physical Measurements

Infrared spectra (400-4000 cm�1) were recorded on a Perkin ElmerFT-IR 1650 spectrometer with samples prepared as KBr pellets.U.V.-Vis spectra were recorded on a Shimadzu-160A dual beamspectrophotometer. C, H and N elemental analysis were performedon a Perkin-Elmer 240B elemental analyser, Mn was determinedby atomic absorption spectroscopy on a Perkin-Elmer 1100B spec-trophotometer. Variable temperature DC SQUID measurementswere collected on a Quantum Design MPMS Controller SQUIDsusceptometer (Model 1822) equipped with a 5 T superconductingmagnet over the temperature range of 5 to 300 K at a field of 5000Gauss. Powdered samples of the solid were used for data collection.Pascal’s constants were used to determine diamagnetic correctionfactors [46, 47]. The magnetic data were fit with the Van Vleckformula over the temperature range of 40 to 300 K. In the deri-vation of the formula, it was assumed that there was no interactionbetween the two MnII centers. The g value for MnII was fixed at2.0, and the g value for MnIV was allowed to vary. Variable fieldDC SQUID measurements were collected over the range of 100 to55,000 Gauss at 4.5 K. Electric conductance measurements werecarried out with a WTW model LF 530 conductivity outfit and atype C cell, which had a cell constant of 0.996. This represents amean value calibrated at 25 °C with potassium chloride. All tem-peratures were controlled with an accuracy of ± 0.1 °C using aHaake thermoelectric circulating system.


Preparation of the compounds

(1) MnIIMnIVMnII(pko)4(CH3O)2(Cl)2: 1.2 mmol (0.240 g) of 2,2�-dipyridyl ketonoxime was added to a solution of 1.2 mmol (0.048 g)of sodium hydroxide in 50 mL of CH3OH. The resulting mixturewas stirred for 1 h, generating a pale yellow solution. 0.9 mmol(0.180 g) of MnCl2·4H2O dissolved in 50 mL of methanol were ad-ded with stirring. The reaction mixture was stirred for 1 h, afterwhich the solution was exposed to dioxygen by bubbling air intothe reaction mixture. The resulting black-red solution after 2 hrsstirring was reduced in volume (50 mL) and 20 mL of CH3CN wereadded. Black-brown crystals suitable for x-ray diffraction studieswere obtained by slow evaporation The crystalline product wascharacterized by elemental analysis with the formulaC46H38N12O6Cl2Mn3 (Fw � 1090.60) [MnIIMnIVMnII(pko)4-(CH3O)2(Cl)2]. Yield 60 %. Anal. Calcd.; C, 50.61; H, 3.48;N,15.40; Cl, 6.51; Mn, 15.12; Found: C, 50.50; H, 3.50;N, 14.95; Cl,6.50; Mn, 15.10 %. IR(KBr): νC�N: 1587 vs, νN�O:1466 vs cm�1; UV/Vis(nujol mulls): λmax: 360 nm(sh).

(2) MnIIMnIVMnII(pko)4(CH3O)2(OCN)2: 1.2 millimoles (0.24 g)of 2,2�-dipyridyl ketonoxime was added to a solution of 1.2 mmol(0.048 g) of sodium hydroxide and 0.6 mmol (0.40 g) of NaOCN in50 mL of CH3OH. The resulting mixture was stirred for 1 h, gener-ating a pale yellow solution. 0.9 mmol (0.180 g) of MnCl2·4H2Odissolved in 50 mL of methanol were added with stirring. The reac-tion mixture was stirred for 1 h, after which the solution was ex-posed to dioxygen by bubbling air into the reaction mixture. Theresulting black-red solution after 2 hrs stirring was reduced in vol-ume (50 mL) and 10 mL of CH2Cl2 were added. Black-brown crys-tals of complex suitable for X-ray diffraction studies were obtainedby slow evaporation of the mother liquid in a week. The crystallineproduct was characterized by elemental analysis with the formulaC48H38N14O8Mn3 (Fw � 1103) [MnIIMnIVMnII(pko)4-(CH3O)2(OCN)2]. Yield 65 %. Anal. Calcd.; C, 52.22; H, 3.44; N,17.76; Mn, 14.95; Found: C, 52.00; H, 3.50; N, 17.35; Mn, 14.50 %.IR(KBr): νO-C�N: 2198 vs, νC�N: 1592 vs, νN�O: 1460 vs; UV/Vis(nujol): λmax: 360 sh nm.

(3) Compound (3) was reported [33]. IR(KBr): νS-C�N: 2060 vs,νC�N: 1593 vs, νN�O: 1467 vs; UV/Vis(dmf): λmax(lg ε): 358 nm(20,300).

X-ray Crystal Structure Determination

Dark brown plates of (1) from a methanol/acetonitrile solution,black needles of (2) from a methanol solution, and brown needlesof (3) from a methanol/CH2Cl2 solution [33] were crystallized at 25deg. C. A crystal of dimensions 0.10 x 0.06 x 0.04 mm of (1), acrystal of dimensions 0.60 x 0.40 x 0.34 mm of (2) cut from a largerneedle and a crystal of dimensions 0.32 x 0.04 x 0.04 mm of (3)were mounted on a standard Bruker SMART CCD-based X-raydiffractometer equipped with a LT-2 low temperature device andnormal focus Mo-target X-ray tube (λ � 0.71073 A) operated at2000 W power (50 kV, 40 mA). The X-ray intensities were meas-ured at 133(2) K; the detector was placed at a distance 4.959 cmfrom the crystal. A total of 3030 frames for (1) and (2) and 2083for (3) were collected with a scan width of 0.3° in ω and phi withan exposure time of 75 s/frame of (1) and 60 s/frame of (2) and(3). The frames were integrated with the Bruker SAINT softwarepackage [48] with a narrow frame algorithm.

The integration of the data of (1) yielded a total of 25062 reflec-tions to a maximum 2θ value of 53.22° of which 4657 were indepen-


Table 2 a) Crystallographic data of MnIIMnIVMnII(pko)4(CH3O)2-(Cl)2 (1), MnIIMnIVMnII(pko)4(CH3O)2(OCN)2 (2), andMnIIMnIVMnII(pko)4(CH3O)2(SCN)2·CH3OH (3)

1 2 3 [33]

Formula C46H38Cl2Mn3N12O6 C48H38Mn3N14O8 C49H42Mn3N14O7S2

Space group P21/c P1 P1Crystal

system monoclinic triclinic triclinicM 1090.60 1103.74 1167.91a / A 12.5519(10) 8.7619(8) 10.5410(8)b / A 17.4919(14) 10.3791(9) 12.6576(10)c / A 10.3657(8) 13.8025(11) 13.5251(10)α / ° 90.0 99.313(4) 80.010(3)β / ° 105.665(3) 97.952(4) 68.366(3)γ / ° 90.0 107.760(3) 70.670(3)V / A3 2191.3(3) 1155.86(17) 1580.2(2)Z 2 1 1Dcalcd 1.653 1.586 1.227T / K 133(2) 123(2) 133(2)Refs

collect/unq 25062 / 4499 11716 / 5660 16328 / 6404Abs.co./

mm�1 1.040 0.880 0.710GOF 1.040 1.009 1.069wR2 b) 0.0993 0.1039 0.1331R1 c) 0.0466 0.0498 0.0482

a) Further details of the crystal structure investigation are available from theFachinformationszentrum Karlsruhe, D-76344 Eggenstein-Leopoldshafen(Germany), on quoting the depository number CSD-212765 (1) and CSD-212764 (2), the name of the authors, and citation of the paper. Crystallogra-phic data for the structures have been deposited with the Cambridge Crystal-lographic Data Centre. Copies of the data can be obtained free of charge onapplication to The Director, CCDC, 12 Union Road, Cambridge CB2 1EZ,UK (Fax: int. code �(1223)336-033; e-mail for inquiry: [email protected]; e-mail for deposition: [email protected].).

b) w �1

[σ2 � (F2o) � (a � P)2 � b � P]

and P2 �[max(F2

o, 0) � 2 � F2c]

3

c) R1 �Σ(� Fo � � � Fc �)

Σ(� Fo �), wR2 � Σ[w � (F2

o � F2c)2]

Σ[w � (F2o)2]

dent and 3112 were greater than 2σ(I). The final cell constants(Table 1) were based on the xyz centroids of 4400 reflections above10σ(I). Analysis of the data showed negligible decay during datacollection; the data were processed with SADABS and correctedfor absorption [49]. The structure was solved and refined with theBruker SHELXTL (version 5.10) software package [50], using thespace group P2(1)/c with Z � 2 for the formulaC46H38N12O6Cl2Mn3. All non-hydrogen atoms were refined aniso-tropically with the hydrogen placed in idealized positions. Full ma-trix least-squaresrefinement based on F2 converged at R1 � 0.0466and wR2 � 0.0993 [based on I > 2sigma(I)], R1 � 0.0827 andwR2 � 0.1137 for all data. Additional details are presented in Table1 and are given as supporting information as a CIF file.

The integration of the data of (2) yielded a total of 11716 reflec-tions to a maximum 2θ value of 56.68° of which 5660 were indepen-dent and 3723 were greater than 2σ(I). The final cell constants(Table 2) were based on the xyz centroids of 2897 reflections above10σ(I). Analysis of the data showed negligible decay during datacollection; the data were processed with SADABS and corrected forabsorption. The structure was solved and refined with the BrukerSHELXTL (version 5.10) software package, using the space groupP1 with Z � 1 for the formula C48H38N14O8Mn3. All non-hydrogenatoms were refined anisotropically with the hydrogen placed in ide-alized positions. The molecule lies on an inversion center in thecrystal structure. Both N-bound and O-bound models for the cyan-


ate were considered. The N-bond model is based on the modelgiving lower R-factors (by 0.03 for R1, 0.08 for wR2), more chemi-cally sensible displacement parameters and lower residual electrondensity. Full matrix least-squares refinement based on F2 convergedat R1 � 0.0498 and wR2 � 0.1039 [based on I > 2sigma(I)], R1 �

0.0958 and wR2 � 0.1210 for all data. Additional details are pre-sented in Table 2 and are given as Supporting Information as aCIF file.

Acknowledgements. The authors thank the agencies V.L.P (NIHGM39406), D.K (WG010 of COST action D21) for support ofthis research.

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trinuclear mixed-valent mnii/mniv/mnii complexes — structure and magnetic behavior

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