university of groningen electron deficient …iron(ii) amidinate complexes 23 2 iron(ii) amidinate...

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Electron deficient organoiron(II) complexes of amidinates and betha-diketiminatesSciarone, Timotheus

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Iron(II) amidinate complexes

23

2 Iron(II) amidinate complexes As described in Chapter 1, iron-based olefin polymerisation catalysts can have many attractive features (low cost and toxicity of metal, proven high catalytic activity for ethene polymerisation). Nevertheless, the scope of the iron-based catalysts known to date is very limited, and little information is available on the actual active species. In the research presented in this thesis, attempts are made to prepare well-defined iron alkyl complexes that are either active in olefin polymerisation themselves, or can be activated to yield catalytically active species. This chapter describes a first approach to generate electron-deficient monoalkyl compounds of iron supported by amidinate ligands. The choice of an anionic ancillary ligand avoids complications related to the generally observed thermal instability of iron dialkyls, yet they theoretically should be able to yield species that are sufficiently electron-deficient to show olefin polymerisation activity. Iron complexes of a sterically hindered amidinate ligand are reported together with exploratory investigations of their reactivity towards alkylating agents.

2.1 Amidinate ligands

Amidinate anions [RC(NR’)(NR’’)]– are the nitrogen analogues of carboxylates [RCO2]

– (Figure 2.1) and, like these, can donate 4 electrons† to a metal. An essential difference between these two types of ligand is that in the amidinates the nitrogen atoms each carry an extra substituent, R’ and R”. These substituents offer the possibility of fine-tuning the electronic and steric properties of the ligand system.

In amidinate chemistry the substituents on nitrogen provide synthetic handles to influence the steric aspects of the ligand. The steric protection offered by large substituents R’ and R”, limits the number of coordination possibilities for amidinate metal complexes. Especially when monomeric complexes with a low electron count for the metal are to be stabilised, it is very important to apply amidinate ligands with sufficient steric demand. Although a strong electronic effect is also to be expected from the substituents on the nitrogen atoms, studies addressing this subject have not been published as the steric aspects of these substituents have received most attention. However, the electron donating properties of the ligand can also be controlled by the nature of the substituent on the central carbon atom. Thus, metal complexes can be rendered more electrophilic by amidinates carrying inductively electron withdrawing substituents on the amidinate carbon.1

† In some rare cases, donation from the amidinate π-system to the metal can occur. In those cases, the ligand should be considered as a 6 electron donor.44,123

R

OM

O

R

NM

N R''R'

A B

Figure 2.1

Chapter 2

24

2.1.1 Coordination chemistry Amidinate complexes of many transition metals, lanthanides, actinides and main group elements have been reported.2,3 In their metal complexes, amidinates show a wide variety of bonding modes (Figure 2.2). Monodentate (κ1) coordination (A) is sometimes reported, mainly for amidinates carrying sterically encumbering substituents on the nitrogen or carbon atoms.4-10 The chelating (κ2) bonding mode is most often found, either in a conjugated fashion (B) or localised as a combined amido-imino ligand (C).11-13

Due to the small size of the four-membered chelate ring, amidinates have relatively small bite-angles (N-M-N typically ≈ 63 – 65°). Because the orientation of the nitrogen lone pairs is not optimal for chelation, amidinates (like carboxylates) are also commonly observed as bridging ligands, forming so-called ‘A’-frame structures (D, E, F). The bridging coordination mode is found in a class of compounds called ‘lantern-complexes’ or ‘paddlewheel-complexes’ (Figure 2.3A), in which three or four amidinates bridge two metal centres. This type of complexes has received much attention because of their potential for metal-metal interaction.14-30 Bridging and chelating amidinate ligands can coexist within one complex (Figure 2.3B).22,31-33 In complexes containing bridging amidinates, each of the amidinate nitrogen atoms can coordinate to either one or two metal centres, leading to µ-κ2:κ1 (E) and µ-κ2:κ2 (F) coordination modes, respectively.22,32,34-42 One example has been reported of an amidinate bridging two metal centres by coordinating as a chelate to one metal, and in a π-allyl mode to the second (µ-1κ2N,N:2κ3N,C,N).43,44 In trinuclear species, more complicated bridging arrangements have been observed.45

N NR'

R

R''

M

N NR'

R

R''

M

N NR'

R

R''

M A B C

N NR'

R

R''

M M

N NR'

R

R''

M M

N NR'

R

R''

M M D E F

Figure 2.2 Frequently observed coordination modes of amidinates. A: monodentate (κ1), B: chelating delocalised (κ2), C: chelating localised (κ2), D: bridging (µ), E: bridging (µ-κ2:κ1), F: bridging (µ-κ2:κ2)


25

The substituents on the carbon and nitrogen atoms of the amidinate strongly influence the coordination properties of the amidinate ligand. Sterically demanding substituents, like tbutyl, on the central carbon22-27,46-51 push the substituents on the nitrogen atoms more towards the metal centre, and favour chelation over bridging coordination (Figure 2.4).26 Bridging may be anticipated if the substituent on the carbon atom is hydrogen, which explains the frequent use of formamidinates‡ in the synthesis of lantern complexes.52

2.1.2 Ligand synthesis and introduction on metals There is a large variety of synthesis routes (Scheme 2.1) to amidinates and this allows easy variation of the electronic and steric properties of these ligands. Addition of N-silylated lithium or magnesium amides to nitriles results in the formation of N-silyl-substituted amidinates (A).53-56 This route is limited to nitriles without α-hydrogens. Reaction of carbodiimides with organolithium reagents leads to symmetric amidinates (B).8,57,58 Condensation of carboxylic acids or amides with amines at 160 °C with polyphosphoric acid trimethylsilyl ester (PPSE) as dehydration agent directly affords symmetrical or unsymmetrical amidines (C).59 A highly versatile route to symmetrical as well as unsymmetrical amidines is the reaction of imidoyl chlorides with amines (D).60

‡ Amidinates are named after the substituent R on the central carbon: R= H: formamidinate, R = CH3: acetamidinate, R = C6H5: benzamidinate) etc.

N NR'

R

R'

N NR'

R

R'

M M

N NR'

R

R'

N NR'

R

R'N

N

R'

R

R'N NR'

R

R'

M M

N NR'

R

R'

N

N

R'

R

R'

A B

Figure 2.3 Lantern (paddlewheel) complex (A) and dinuclear complex (B) with combination of bridging and chelating amidinates

NN R'

R

R''

NN

R

R'' R'

A B Figure 2.4 R small (A), R large (B)

Chapter 2

26

Metal complexes of amidinate ligands can be prepared by various methods. Early reports describe the introduction of the ligand by heating metal carbonyls with amidines.14,61,62 Currently, salt metathesis is most often employed for amidinate introduction. Deprotonation of amidine metal complexes can be considered as a variant of this method.5,19,48,63 In some cases, metal acetates can be used as starting materials for salt metathesis or serve as an internal base to deprotonate amidines.64-

69 Thermolysis of a monoamidine GaCl3 complex has been reported to give the monoamidinate GaCl2 complex with elimination of hydrogen chloride.70 Alkane or amine elimination reactions of amidines with metal alkyls or amides directly afford the corresponding amidinate metal alkyl or amide complexes, respectively.70-73 Insertion of carbodiimides into metal-carbon or metal-hydrogen bonds has also been reported as a clean and direct route to amidinate transition metal complexes.74-76

NLi

TMS

R' R C N N NR' TMS

R

Li

+

A

R' N C N R'RLi + N NR' R'

R

Li B

RO

X

NH

O

R

R'

N NH

R

R' R'

N NH

R

R' R''

R''NH2

PPSE∆

RLiN NR' R''

R

Li

RLiN NR' R'

R

LiX = OH or halogen

2 R'NH2

PPSE∆

C

NH

O

R

R'RO

ClN Cl

R

R'PCl5R' NH2

N Cl

R

R'N N

H

R

R' R''

R'' NH2 RLiN NR' R''

R

Li D

Scheme 2.1 Synthesis of amidinates by: Addition of lithium amides to nitriles (A), Addition of alkyllithiums to carbodiimides (B), Dehydration of amides by PPSE (C) and Reaction of imidoyl chlorides with amines (D).


27

2.1.3 Applications The good possibilities for steric and electronic ‘fine-tuning’ of amidinates (section 2.1.2) can account for their popularity in coordination chemistry and catalysis. Amidinate ligands are used as ancillary ligands in different homogeneous catalytic transformations. Tetravalent and divalent Sn and Ge derivatives catalyse cyclotrimerisation and cyclodimerisation of isocyanates.77 Y(III) amidinates have been employed in catalytic dimerisation of terminal alkynes.78 Group 4 amidinate complexes promote isomerisation of α-olefins.79 In polymerisation catalysis, amidinates often appear as a replacement for cyclopentadienyl ligands. Amidinate complexes of Cu(I) and Cu(II) can catalyse the living polymerisation of carbodiimides.80 Y(III) and Fe(III) amidinate alkoxide complexes have been reported to catalyse the polymerisation of cyclic esters.81,82 Amidinate group 3 and 4 dihalides and dialkyls have been found to be efficient catalysts for polymerisation of ethene, α-olefins, nonconjugated dienes and vinylcyclohexane, in some cases even with high stereoregularity.83-96 Ethene oligomerisation or polymerisation can also be catalysed by Ta(V) and V(III) complexes with amidinate ancillary ligands.1,97 Amidinate complexes of the main group metal aluminium have been observed to initiate ethene polymerisation as well.98 The use of amidinate ancillary ligands for olefin polymerisation catalyst precursors has been described in several patents.99,100 Recently, chiral amidinate ligands have been employed in asymmetric hydrozirconation.100,101

Homogeneous catalysis is not the only field of application for amidinate ligands. Also in materials science complexes containing the amidinate ligand offer attractive possibilities for materials synthesis. Amidinate complexes of divalent Mg and trivalent Al and Ga are precursors for nitride semiconductor materials since their coordination sphere contains only nitrogen (minimising carbide formation).32,70,102 Transition metal complexes of certain amidinates are also well-suited for use as precursors for atomic layer deposition (ALD).31,103 Control of the volatility of the complexes is crucial for these purposes and this can be adjusted by variation the amidinate substituents. The strong tendency of amidinate ligands to bridge two metal centres, often in structures containing metal-metal bonds has been employed to form diruthenium(III, III) lantern complexes with axial alkynyl ligands. These have been prepared with the objective of constructing oligo-metalla-yne ‘molecular wires’ based on Ru2 compounds.51,104,105 Cotton and coworkers have studied the electronic communication between two linked Mo2

4+ units, in which each dimetallic unit is bridged by amidinate ligands.106-110

2.1.4 Sterically hindered amidinates From the previous sections it is clear that the coordinative properties of amidinate ligands can conveniently be fine-tuned by variation of the substituents on the central carbon and the two nitrogen atoms. This aspect makes this type of ligand an attractive candidate for stabilisation of electron deficient Fe(II) monoalkyls. In order to ensure electronic unsaturation, coordination numbers need to be limited. This can be achieved by employing amidinate ligands with sterically demanding substituents.

Several groups have reported amidinates with steric protection originating from bulky terphenyl groups attached to the central carbon atom (Figure 2.5A).7-9,71,72,111,112 In this way, bowl-shaped amidinates are obtained, which, by virtue of the large substituent on carbon, do not act as bridging ligands. This type of amidinate usually forms chelate complexes with transition metals and main group elements, although

Chapter 2

28

sometimes the extreme steric bulk leads to monodentate coordination in alkali metal complexes.7-9

The two nitrogen atoms of the amidinate ligand also provide an opportunity to introduce steric shielding. This strategy has the advantage that steric bulk is closer to the metal as it is located on the coordinating atoms. 2,6-Disubstituted aryl groups are particularly effective as these substituents shield the positions above and below the coordination plane due to the orthogonality of the aryl rings with respect to the amidinate NCN plane. In this respect, the nitrogen substitution serves the same purpose as in the pyridine-2,6-diimine ligand employed by Brookhart and Gibson.113 Amidinates with 2,6-diisopropylphenyl groups on the nitrogen atoms are known to form predominantly mononuclear complexes. Even the formamidinate [HC(N-2,6-iPr2C6H3)2]

– coordinates in a chelating fashion to monovalent alkali metals and divalent alkaline earths.114,115 With the exception of the very recently reported bis(amidinate) magnesium complex [{4-Me-PhC(N-2,6-iPr2C6H3)2}2Mg],116 the steric bulk provided by the aryl groups usually ensures formation of mono(amidinate) complexes. Thus, Coles et al. have reported the aluminium(III) dialkyl [tBuC(N-2,6-iPr2C6H3)2AlMe2].

58 Mono(amidinate) Al(III) and Y(III) complexes were obtained with [4-Me-PhC(N-2,6-iPr2C6H3)2]

–.81,116 The related benzamidinate [PhC(N-2,6-iPr2C6H3)2]

– ([Dipp2BAM]–) (Figure 2.5B) has been employed by Bambirra et al. for stabilisation of Ln(III) dialkyls [{Dipp2BAM}Ln(THF)n(CH2SiMe3)2] (n = 1,2; Ln = Sc, Y, lanthanide).86,117 The nickel monoalkyl complex [{Dipp2BAM}Ni(CH2SiMe3)(py)] has been described in the patent literature.100

The benzamidinate [Dipp2BAM]– seems promising for stabilisation of electron deficient iron alkyl complexes of the type [(amidinate)FeR]. The steric protection provided by the large substituents on the nitrogen atoms enforces the formation of mononuclear complexes and at the same time ensures low electron count. Furthermore, the parent amidine [Dipp2BAM]H is conveniently prepared by reaction of 2,6-diisopropylaniline with the imidoyl chloride prepared from 2,6-diisopropylphenylbenzanilide (Scheme 2.1D).86,118

N N RR iPriPr

N N

PhiPr

iPr

iPr

iPr A B [Dipp2BAM]–

Figure 2.5 Sterically hindered amidinate ligands with protective groups attached to the central carbon atom (A) and to the nitrogen atoms (B).


29

2.2 Complexation of [Dipp2BAM]– to Fe(II)

In principle, the target compounds [{Dipp2BAM}FeR] would be accessible by an alkane elimination reaction between FeR2 and the amidine [Dipp2BAM]H. Bambirra has successfully applied this method in the synthesis of the monoamidinate Y(III) complexes [{Dipp2BAM}Y(THF)n(CH2SiMe3)2].

86 Unfortunately, suitable Fe(II) dialkyl precursors are not available. Non-stabilised (homoleptic) Fe(II) dialkyls suffer from low thermal stability, while Fe(II) dialkyls stabilised by Lewis bases often contain strong-field ligands, which are not easily displaced (c.f. Section 1.6). Therefore, a stepwise process in which introduction of the amidinate ligand is followed by alkylation using salt metathesis routes seems a more attractive route towards monoalkyl iron complexes supported by the [Dipp2BAM]– ligand.

The lithium salt Li[Dipp2BAM] was conveniently generated in situ by reaction of the amidine with n-BuLi in THF. Addition of Li[Dipp2BAM] at 0 °C to a slurry of FeCl2 in the same solvent gave the ferrate complex [{Dipp2BAM}FeCl(µ-Cl)Li(THF)3] (2.1, Scheme 2.2). Pentane extraction, followed by recrystallisation from THF/hexanes afforded pale yellow crystals. Yields (20 – 70%) were critically dependent on the workup conditions. Extraction and crystallisation must be conducted rapidly and at low temperatures as the product is unstable in aliphatic solvents (see section 2.3).

The molecular structure of 2.1 was established by X-ray diffraction. The asymmetric unit contains one ferrate complex and an uncoordinated THF molecule, which is readily lost from the lattice in vacuo or upon drying in a nitrogen stream. The crystal structure of 2.1 is shown in Figure 2.6 and selected interatomic distances and bond angles are given in Table 2.1.

The iron centre is pseudotetrahedrally coordinated by a chelating amidinate ligand and two chlorides. One of the chlorides bridges to a lithium ion, which is solvated by three THF molecules. The aryl rings make angles of 69.4(2) and 80.3(2)° with the coordination plane. The phenyl ring is tilted 45.6(2)° with respect to the same plane.

Ph

N N ArAr

Fe

ClCl

(THF)3Li

Ph

N N ArAr

Li

+ FeCl2THF

Ar = 2,6-iPr2C6H3 2.1

Scheme 2.2

Chapter 2

30

Figure 2.6 Molecular structure of 2.1. Hydrogen atoms and cocrystallised THF molecules are omitted for clarity


31

The iron chloride distance of the bridging chloride is significantly longer than that of the terminal chloride, as expected. The approximately tetrahedral coordination of lithium shows no geometrical irregularities. The iron nitrogen distances are identical within experimental error, indicating that coordination of the ligand is symmetric. The bite angle (N1–Fe–N2) of the ligand equals 63.78(11)°, which is within the range usually observed for amidinates. Delocalisation within the amidinate NCN frame is evidenced by the equal CN bond lengths, although the nitrogen atoms show some deviation from planarity (ΣLN1 = 358.3°, ΣLN2 = 356.5°).

The amidinate NCN angle (112.0(3)°) as well as the Cam.–N–Fe angles (av. 92.2°) are markedly contracted with respect to the ideal 120° for sp2-hybridised atoms. These contractions are required to accommodate the small bite angle of the ligand and are compensated for by a slight increase in the C14–C13–N angles (av. 124.0°) and considerably larger Fe–N–Cipso-Ar angles (av. 140.3°).

Table 2.1 Selected interatomic distances and angles for 2.1.

Distances (Å)

Fe-Cl1 2.2778(12) O22-Li 1.730(11)

Fe-Cl2 2.2443(13) O33-Li 2.189(14)

Fe-N1 2.088(4) O21-Li 2.149(14)

Fe-N2 2.093(3) O1-Li 1.919(13)

N1-C13 1.327(6) O34-Li 1.696(13)

N2-C13 1.337(5) Cl1-Li 2.333(11)

Angles (°)

Cl1-Fe-Cl2 118.46(5) Cl1-Li-O1 114.1(5)

Cl1-Fe-N1 114.46(10) Cl1-Li-O22 112.5(6)

Cl1-Fe-N2 111.60(8) Cl1-Li-O33 101.8(5)

Cl2-Fe-N1 114.94(10) Cl1-Li-O21 97.3(5)

Cl2-Fe-N2 121.86(9) Cl1-Li-O34 123.9(7)

N1-Fe-N2 63.76(14) O1-Li-O22 111.0(7)

Fe-Cl1-Li 109.1(3) O1-Li-O33 116.4(6)

Fe-N1-C1 141.3(3) O1-Li-O21 105.8(6)

Fe-N1-C13 92.4(3) O1-Li-O34 105.7(7)

C1-N1-C13 124.6(4) N1-C13-N2 112.0(3)

Fe-N2-C13 91.9(2) N1-C13-C14 124.6(4)

Fe-N2-C20 139.3(2) N2-C13-C14 123.4(4)

C13-N2-C20 125.3(3)

Chapter 2

32

Compound 2.1 has a room temperature magnetic moment µeff. = 5.4 µB in benzene (Evans method119,120). Although this value is somewhat higher than the spin-only value for a high-spin d6 complex (µS.O. = 4.9 µB), it is most consistent with 4 unpaired electrons in combination with some orbital contribution.121 The paramagnetism of 2.1 is also evident from its 1H NMR spectrum in C6D6 (Figure 2.7). Seven broad singlets are observed between +30 and –30 ppm in a 300 ppm frequency window. The broadness of some of the resonances renders integration inaccurate. This complicates full assignment of the spectrum, but some tentative assignments can be made. The spectrum displays four large signals integrating for approximately 12H each. The upfield peak at –17.9 ppm (356 Hz) can be assigned to one of the diastereotopic methyls of the isopropyl groups. The resonance at 8.9 ppm (169 Hz) is probably due to the other diasterotopic iPr-Me group. The two signals at 2.7 and 6.3 ppm have similar linewidths (65 and 90 Hz, respectively). Therefore, assignment of these peaks to the β- and α-protons of the coordinated THF molecules seems reasonable. The four remaining resonances at 23.1, 18.7 and 13.2 ppm could not be assigned unequivocally.


33

In THF-d8, a different 1H NMR spectrum is obtained for the iron complex and resonances for free THF are noted, suggesting that the THF molecules in 2.1 exchange with the deuterated solvent. The spectrum in THF-d8 is more readily interpretable, displaying 9 lines with smaller widths compared to C6D6. Characteristic signals for the iPr-Me protons are found at 5.5 and –12.9 ppm (12H each). The lines at 19.0 (2H) and 15.6 ppm (4H) are attributed to the meta protons of the phenyl and aryl rings, respectively. Resonances for the para protons are observed at 2.3 ppm (1H, p-HPh) and –22.5 ppm (2H, p-HAr). The broad lines at 14.6 (4H) and 11.8 (2H) ppm are probably due to the iPr-CH and the o-HPh protons, since these are relatively close to the paramagnetic Fe(II) ion. The presence of only two iPr-Me resonances suggests a C2v-symmetric molecule in solution. Assuming a tetrahedral coordination geometry around Fe, the apparent C2v-symmetry can be accounted for by fast site

ppm -25 -20 -15 -10 -5 0 5 10 15 20 25

*

*

A

ppm -20 -15 -10 -5 0 5 10 15 20

* *

THF THF

B

Figure 2.7 1H NMR spectra of 2.1. A: 300 MHz, C6D6, RT. Asterisks indicate C6D5H and residual hexane. B: 500 MHz, THF-d8, RT. Asterisks indicate free amidine and residual hexane.

Chapter 2

34

exchange of the Li(THF)3 moiety (Scheme 2.3). The exchange process is fast on the NMR timescale down to –50 °C. Details of this process are presently unclear. It could proceed via ion pair formation, with [(Dipp2BAM)FeCl2][Li(THF)4] as intermediate, via (possibly THF-induced) reversible dissociation of (THF)3LiCl, or via loss of THF, with [(Dipp2BAM)Fe(µ-Cl2)Li(THF)2] as intermediate. The fast exchange observed in THF solution would seem to make the latter possibility less likely.

2.3 Thermal stability of monoamidinate 2.1

As noted in the previous paragraph, success in the preparation of 2.1 depends strongly on the workup conditions. In some experiments, the pentane extract turned green and yields of 2.1 were diminished. Therefore, the thermal stability of isolated 2.1 was investigated. Solutions in aromatic hydrocarbons or alkane solvents are not stable at room temperature, slowly turning green in the course of several hours with concomitant precipitation of a white solid. The green product was identified by 1H NMR spectroscopy as the bis(amidinate) complex [Fe(Dipp2BAM)2] (7.3) which could be isolated in 48% yield (based on Fe). The precipitate was not characterised, but presumably consists of LiCl and FeCl2. The formulation of 7.3 was confirmed by its independent synthesis from FeCl2 and two equivalents of Li[Dipp2BAM] (Scheme 2.4). The direct synthesis and full characterisation of this compound will be discussed in Chapter 7.

Ph

N N ArAr

Fe

ClCl

(THF)3Li

THF-d8

Ph

N N ArAr

Fe

ClCl

Li(THF)3

Scheme 2.3 Site exchange of Li(THF)3 moiety in 2.1


35

There are several literature examples in which the synthesis of mono(amidinate) transition metal complexes is accompanied by formation of the corresponding bis(amidinates), regardless of the stoichiometry employed. Stewart et al. obtained inseparable mixtures of the mono- and disubstituted niobium complexes upon treatment of [(py)2Nb(NtBu)Cl3] with one equiv. of Li[PhC(NSiMe3)2].

122 Even sterically hindered amidinates have yielded bis(amidinate) complexes of Cr(II) – Ni(II) as the only isolable products.112 Amidinate complexes of transition metals have also proved to be susceptible to ligand transfer to main group metals.123

The problem of amidinate redistribution can sometimes be circumvented by employing additional chelating donor ligands. Thus, Lee et al. isolated the mono(amidinate) complex [{PhC(NAr)(NSiMe3)}FeCl(κ2-TMEDA)] (Ar = 2,6-Me2C6H3).

12 The stabilising role of donor solvents was also illustrated by Cotton et al. They obtained only the bis(amidinate) [{HC(NAr)2}2Cr] and unreacted CrCl2 from the 1:1 reaction of the lithium amidinate and the metal salt in toluene, whereas the dinuclear 1:1 complex [{µ-HC(NAr)2}(µ-Cl)Cr(THF)]2 (Ar = 2,6-Me2C6H3) was formed in THF.66 Consistently, solutions of 2.1 in THF-d8 show no significant decomposition after heating for 24 h. at 100 °C. The reason for this significant stabilisation of 2.1 by THF is presently unclear. It may prevent a process that is initiated by THF loss, or stabilise the mono(amidinate) species in the form of an ion pair, as mentioned above as a possible intermediate in the symmetrisation of the complex in THF solution.

2.4 Alkylation attempts with monoamidinate 2.1

The generation of electron deficient iron alkyls [(Dipp2BAM)FeR] was investigated by 1H NMR spectroscopy in THF-d8 using alkylating agents devoid of β-hydrogen atoms. Reactions of 2.1 with equimolar amounts of KCH2Ph or LiCH2EMe3 (E = C, Si) were accompanied by colour changes from pale yellow via red to dark yellow. The 1H NMR spectra showed the formation of new paramagnetic species, presumably the alkyls [(Dipp2BAM)FeR(THF)]. The paramagnetic 1H NMR spectra were not readily interpretable, but some resonances with characteristic integrals (iPr-CH3, tBu and SiMe3 groups) could be assigned. In all cases, the iPr-methyl protons show only two resonances (integrating for 12H each), which suggests fast exchange of coordinated THF. If THF-d8 was removed under reduced pressure and replaced by cyclohexane-d12, the 1H NMR spectra predominantly showed resonances characteristic of

Ph

N N ArAr

Li

+ FeCl2

THFRT–2 LiCl2

Ph

N N ArAr

Fe

ClCl

(THF)3Li

Ph

N N ArAr

Fe

Ph

NNAr Ar∆PhMe or hexane– 2 LiCl– FeCl2– 3 THF

2

Ar = 2,6-iPr2C6H3 2.1 7.3

Scheme 2.4

Chapter 2

36

bis(amidinate) complex 7.3 with disappearance of the resonances belonging to the initially observed alkyl species. In an experiment employing LiCH2SiMe3 as the alkylating agent, the alkyl coupling product Me3SiCH2CH2SiMe3 was detected (GC-MS) in the cyclohexane-d12 solution. Apparently, the iron alkyls decompose in the absence of a donor solvent and the formation of alkyl coupling products suggests radical pathways or reductive elimination from FeR2 species resulting from ligand redistribution. The formation of 7.3 is consistent with the lability of the amidinate ligand in 2.1 observed upon thermolysis in non-coordinating solvents.

On a preparative scale, alkylation of 2.1 was attempted using the more sterically demanding LiCH(SiMe3)2. However, reaction of 2.1 with the lithium alkyl in THF, followed by pentane extraction yielded bis(amidinate) Fe complex 7.3 as the only isolable product. The isolation of 7.3 again demonstrates that the intermediate alkyl species are only stable in the presence of a Lewis base. Amidinate ligand scrambling upon alkylation has precedent in the attempted methylation of the Ti(IV) complex [{PhC(NSiMe3)2}(µ-Cl)TiCl2]2. Instead of the targeted monoamidinate complex [{PhC(NSiMe3)2}TiMe3], the bis(amidinate) [{PhC(NSiMe3)2}2TiMe2] was isolated. The authors propose that the desired monoamidinate titanium trimethyl is formed initially, but disproportionates into the observed product and unstable TiMe4.

124

2.5 Alkylation in the presence of carbon monoxide

The iron alkyl species generated by reaction of alkyllithiums with 2.1 in THF may be trapped as the corresponding CO adducts (or possibly as acyl derivatives following from insertion125). If excess CO is present, complete saturation with CO usually takes place, giving stable 18 VE products. Therefore, CO was admitted directly after reaction of 2.1 with LiCH(SiMe3)2. Red crystals could be isolated by slow diffusion of pentane into the THF solution. The acquisition of 1H NMR spectra was frustrated by the presence of metallic iron in the red THF-d8 solution, possibly as a result of photolysis of the product. The spectra show broad, overlapping resonances between +10 and 0 ppm. Although the structure of the complex in solution could not be established, the chemical shift range is consistent with a diamagnetic product. Due to light-sensitivity and easy loss of cocrystallised THF, no satisfactory elemental analysis could be obtained. The compound was characterised by IR spectroscopy and X-ray diffraction.

The X-ray structure revealed that the product is not an iron-alkyl or iron-acyl species. Instead, the product is the dinuclear carbamoyl bisferrate complex [Li(THF)2]2[{µ-C(O)N(Ar)C(Ph)N(Ar)-1κ2C,N:2κO}Fe(CO)2(µ-CO-1κC:2κO)]2 (2.2). The carbamoyl moiety results from CO-insertion into an Fe–N bond (Scheme 2.5). The Fe valence shell is completed by three additional carbon monoxide ligands. In the 18 valence electron ferrate anion {[C(O)N(Ar)C(Ph)N(Ar)]Fe(CO)3}

– the formal oxidation state of the metal is Fe(0). The two ferrate anions are bridged by two Li(THF)2 cations that coordinate to the carbamoyl oxygen of one iron and to one of the equatorial carbonyls of the second iron centre. The molecular structure of 2.2 is shown in Figure 2.8 and selected interatomic distances and bond angles are presented in Table 2.2.


37

Whereas the formation of carbamoyl chelates from Re(I), Mn(I), Mo(II) and Ru(II) carbonyl complexes and alkali amidinates is a well-known reaction,126-129 their formation by insertion of carbon monoxide into the nitrogen metal bond of an amidinate complex is less common. Hagadorn et al. observed the insertion of CO into the amidinate complex [Cp{FcC(NCy)2}Fe(CO)] to give the carbamoyl complex [Cp{N(Cy)C(Fc)N(Cy)C(O)}Fe(CO)] (Cy = c-C6H11, Fc = (C5H4)Fe(C5H5)).

130 In addition, a carbamoyl W(II) complex has been obtained by reaction of amines with [Tp’W(CO)3(NCMe)][BF4] (Tp’= hydrotris(3,5-dimethylpyrazolyl)borate)).131

Ph

N N ArAr

Fe

ClCl

(THF)3Li

1) LiCH(SiMe3)22) COTHF

Fe

NN

Ph

Ar Ar

OC O

CO

CO(THF)2Li

Fe

N N

Ph

ArAr

OCO

CO

CO Li(THF)2

2.1 2.2 Ar = 2,6-iPr2C6H3

Scheme 2.5

Chapter 2

38

The unit cell contains one diiron compound with a non-crystallographic inversion centre and two non-coordinated THF molecules. Each iron in 2.2 forms the centre of a distorted trigonal bipyramid with the carbamoyl carbon and one carbonyl occupying the apical positions (C132–Fe–C134 = 173.37(17)°). The equatorial plane is defined by the carbamoyl nitrogen (N11) and the two other carbonyls, the sum of their angles with the iron centre being 358°. The lithium ions are at the centres of distorted tetrahedrons with angles varying from 99.7(4)° (O21–Li11–O44) to 123.7(4)° (O41–Li42–O61).

Figure 2.8 Molecular structure of 2.2. Hydrogen atoms, THF-carbon atoms and cocrystallised THF molecules have been omitted for clarity.


39


Distances (Å)

Fe11-N11 1.997(3) Fe41-N41 1.9960(6)

Fe11-C132 1.928(3) Fe41-C432 1.935(4)

Fe11-C133 1.767(4) Fe41-C433 1.768(4)

Fe11-C134 1.798(4) Fe41-C434 1.798(4)

Fe11-C135 1.725(4) Fe41-C435 1.720(4)

N11-C113 1.299(4) N41-C413 1.301(3)

N12-C113 1.383(4) N42-C413 1.381(3)

N12-C132 1.443(4) N42-C432 1.442(4)

O11-C132 1.244(4) O41-C432 1.246(4)

O14-C135 1.185(5) O44-C435 1.179(4)

O13-C134 1.155(5) O43-C434 1.156(4)

O12-C133 1.160(6) O42-C433 1.168(4)

O11-Li11 1.832(7) O41-Li42 1.850(6)

O14-Li42 2.005(8) O44-Li11 1.949(11)

Angles (°)

N11-Fe11-C132 81.02(13) N41-Fe41-C432 81.04(11)

N11-Fe11-C133 114.86(17) N41-Fe41-C433 115.63(13)

N11-Fe11-C134 95.11(15) N41-Fe41-C434 94.77(12)

N11-Fe11-C135 132.58(17) N41-Fe41-C435 131.48(17)

C132-Fe11-C133 89.17(17) C432-Fe41-C433 89.19(18)

C132-Fe11-C134 173.37(17) C432-Fe41-C434 173.02(18)

C132-Fe11-C135 87.93(16) C432-Fe41-C435 88.50(18)

C133-Fe11-C134 97.35(19) C433-Fe41-C434 97.68(19)

C133-Fe11-C135 110.9(2) C433-Fe41-C435 111.4(2)

C134-Fe11-C135 90.80(18) C434-Fe41-C435 90.15(18)

Fe11-N11-C11 123.4(2) Fe41-N41-C41 123.25(3)

Fe11-N11-C113 116.1(2) Fe41-N41-C413 116.01(14)

C11-N11-C113 120.2(3) C41-N41-C413 120.27(14)

C113-N12-C120 121.9(3) C413-N42-C420 121.0(2)

C113-N12-C132 115.1(3) C413-N42-C432 115.3(2)

C120-N12-C132 118.4(3) C420-N42-C432 119.6(2)

N11-C113-N12 115.1(3) N41-C413-N42 115.3(2)

N11-C113-C114 126.3(3) N41-C413-C414 125.9(3)

N12-C113-C114 118.3(3) N42-C413-C414 118.6(3)

Chapter 2

40

The original amidinate NCN framework is no longer fully delocalised in the carbamoyl ligand. C113–N11 (1.299(4) Å) is significantly shorter than C113–N12 (1.383(4) Å), giving the former more double bond character. The carbamoyl ring however, is essentially planar, indicating at least some contribution of a zwitterionic resonance form (Figure 2.9).130

The lithium and iron centres are linearly bridged by carbon monoxide, which is unusual for CO, though not unprecedented. In cases where CO acts as a linearly bridging ligand, the most electropositive metal centre is usually bound to the oxygen atom.132-134 Compared to the terminal carbonyls, the bond lengths of the bridging CO’s are not significantly affected by coordination to the Li ions. The carbamoyl CO bond lengths (1.245(4) Å (av.)) are considerably longer and are comparable to those in Hagadorn’s CpFe(II) carbamoyl complex (C–O = 1.223(4) Å).130

In the IR spectrum, absorptions for the terminal carbonyls (νCO(term.)) are observed at 1883 and 1961 cm–1, while the bridging CO absorbs at a lower frequency (νµ-CO = 1814 cm–1). In Hagadorn’s carbamoyl complex, νCO(term.) is found at 1902 cm–1. The carbamoyl CO stretching absorption νC(O)N for 2.2 is observed at 1556 cm–1, which is at considerably lower frequency than in the aforementioned CpFe(II) derivative (νC(O)N = 1630 cm–1). The lower νC(O)N observed for 2.2 may be accounted for by coordination of the carbamoyl oxygen atom to Li.

The dinuclear carbamoyl ferrate 2.2 contains formally Fe(0). Apparently, reaction with the alkyllithum reagent has led to reduction of the Fe(II) ferrate 2.1. The stoichiometry of the formation of 2.2 is unclear. GC-MS analysis of the reaction mixture revealed the presence of bis(trimethylsilyl)methane, probably formed through hydrogen abstraction by bis(trimethylsilyl)methyl radicals. The coupling product of two HC(SiMe3)2 radicals, 1,1,2,2-tetrakis(trimethylsilyl)ethane, was not observed, suggesting that reductive elimination from a dialkyliron(II) species is probably not involved in the mechanism. The chromatogram also showed the presence of a compound with m/z = 186, which corresponds to bis(trimethylsilyl)ketene (Me3Si)2C=C=O.135 This ketene might be the β-H-elimination product of an iron acyl complex [Fe]–C(O)CH(SiMe3)2, which in turn could be formed by CO insertion into the Fe–C bond of an iron alkyl species (Scheme 2.6). Thus, GC-MS analysis offers some indirect evidence for the initial alkylation of 2.1. Other organic products observed in the mixture by GC-MS are free amidine and 2,6-diisopropylaniline (the latter resulting from ligand degradation).

FeN

N O

Ar

Ar

Ph

FeN

N O

Ar

Ar

Ph

FeN

N O

Ar

Ar

Ph

Figure 2.9


41

Ferrate 2.1 itself is unreactive towards CO. Solutions of 2.1 in THF-d8 or C6D6 were exposed to ca. 1 atm. CO for several days. In THF no changes in colour or in the 1H NMR spectrum were noted. In C6D6, a colour change from yellow to green was observed. 1H NMR spectroscopy confirmed the formation of bis(amidinate) complex 7.3 as the only new product. This experiment suggests that in the formation of 2.2, the process is initiated by the reaction of 2.1 with the alkyl lithium reagent.

2.6 Alkylation of 2.1 in the presence of pyridine

In situ alkylation experiments with 2.1 have shown that substitution of chloride for an alkyl ligand is possible, but that a Lewis base is required for preventing ligand redistribution leading to 7.3 (see section 2.3). Therefore, alkylation of 2.1 was studied in the presence of a stronger donor ligand than THF. In a one-pot reaction, ferrate 2.1 was generated in THF and reacted in situ with pyridine (2 equiv) followed by addition of LiCH2SiMe3. Extraction with pentane and crystallisation from the same solvent afforded orange crystals of [{Dipp2BAM}FeCH2SiMe3(py)] (2.3) in 38% isolated yield.

Figure 2.10 shows the solid state structure of 2.3 as determined by X-ray diffraction. The asymmetric unit contains two independent molecules of the iron complex. In view of their structural similarity, only one of these will be discussed. Pertinent bond lengths and bond angles for 2.3 are listed in Table 2.3.

[Fe]SiMe3

SiMe3

COSiMe3

SiMe3

O

[Fe]

β-H elim.

SiMe3

SiMe3

CO

[Fe] H

MW = 186.40

Scheme 2.6 Possible mechanism for the formation of bis(trimethylsilyl)ketene.

Chapter 2

42

Figure 2.10 Molecular structure of 2.3. One of the two independent molecules is shown. Hydrogen atoms have been omitted for clarity.


43

The iron centre in 2.3 is ligated in a distorted tetrahedral fashion with angles between the coordinating atoms in the range of 64° – 135°. The bite angle (N11–Fe–N12 = 63.62(7)°) is similar to 2.1 (63.76(14)°). The amidinate ligand is bound to Fe in the chelating mode, although slightly less symmetrically than in 2.1, as evidenced by small, but significant differences in the Fe–N and amidinate C–N distances (∆Fe–N ~ 0.04 Å, ∆C–N ~ 0.03 Å). This small tendency towards amido-imino coordination (Figure 2.2C) is consistent with the full planarity of N12 (ΣLN12 = 359.65°), while N11 is more distorted (ΣLN11 = 354.80°). The pyridine ligand is roughly orthogonal (84.58(11)°) to the Fe–N–C–N coordination plane, while the aryl groups and the phenyl ring are inclined at 66.15(10)°, 74.83(10)° (Ar) and 43.42(10)° (Ph) with respect to this plane.

Iron-carbon bond lengths in η1-bound organoiron complexes can vary from 1.67 to 2.20 Å, depending on the hybridisation and substitution pattern of the α-carbon.136 The Fe–C distance in 2.3 (2.049(2) Å) lies within this range and compares well with the four-coordinated bis(trimethylsilyl)methyl compounds [(LL)Fe(CH2SiMe3)2] (LL = chelating diamine or diimine, Fe–C = 2.042(3) – 2.0963(13) Å) reported by Bart et al.137

The Fe–C–Si angle (118.76(14)°) is relatively large for an sp3-hybridised carbon atom. For electron deficient early transition metal alkyls, large angles around the methylene carbon have been ascribed to α-agostic interactions between the metal centre and the methylene protons of the hydrocarbyl.138 In the case of 3.2 however, it is more likely that the increase in the angle relieves steric interactions between the trimethylsilyl group and the iPr substituents of the ligand. Steric crowding is also evident from the unsymmetric C–Fe–N angles (135.02(8) and 122.06(9)°).


Distances (Å)

Fe1–N11 2.0864(18) Fe1–C137 2.049(2)

Fe1–N12 2.1226(18) N11–C113 1.351(3)

Fe1–N13 2.1314(18) N12–C113 1.323(3)

Angles (°)

N11–Fe–N12 63.62(7) Fe1–N12–C113 91.67(13)

N11–Fe–N13 107.47(7) Fe1–N12–C120 141.52(14)

N12–Fe–N13 101.65(7) C113–N12–C120 126.46(18)

C137–Fe–N11 135.02(8) Fe1–N13–C132 118.43(15)

C137–Fe–N12 122.06(9) Fe1–N13–C136 124.14(15)

C137–Fe–N13 113.63(9) C132–N13–C136 117.02(19)

Fe1–N11–C11 140.06(13) N11–C113–N12 112.14(18)

Fe1–N11–C113 92.43(13) N11–C113–C114 122.25(19)

C11–N11–C113 122.31(18) N12–C113–C114 125.52(19)

Fe1–C137–Si1 118.76(14)

Chapter 2

44

Monoalkyl 2.3 has an effective magnetic moment µeff. = 5.5 µB in benzene at ambient temperature (Evans method119,120). Consistent with the high-spin nature of the compound, the 1H NMR spectrum consists of broad and paramagnetically shifted signals (Figure 2.11).

The number of observed resonances (9) is smaller than expected (17) on the basis of the molecular structure of 2.3. The spectrum indicates a C2v-symmetric molecule in solution as judged from the presence of only two iPr-Me resonances (5.0 and –25.0 ppm, 12 H each) and one m-HAr resonance (11.6 ppm, 4H). The apparent C2v-symmetry of the molecule can be accounted for by fast site exchange of the pyridine molecule on the NMR timescale. Taking the fluxional process into account, 13 signals are expected for 2.3, leaving 4 resonances unobserved. This could be due to extreme line broadening for protons close to the paramagnetic centre (e.g. o-Hpy, Fe–CH2, o-HPh, iPr-CH). The SiMe3 proton resonances of the alkyl ligand (9H) are found at 35.6 ppm. The assignments of the 5 remaining peaks is ambiguous due to their similar integrals (1H or 2H).

Pyridine adduct 2.3 was tested for activity in ethene polymerisation (PC2H4 = 10 bar, T = 50 °C). With MAO cocatalyst (Al/Fe = 500), no significant ethene consumption was noted. The observed lack of activity for 2.3 contrasts with claims in the patent literature for related complexes. The benzamidinate complex [{PhC(N–2,4,6-Me3C6H2)2}FeCH2SiMe3(py)] is claimed in a patent as catalyst precursor for olefin polymerisation upon activation with aluminoxanes or a Lewis acid like B(C6F5)3, but characterisation data or activity figures are not provided.100 The reactivity of 2.3 towards B(C6F5)3 will be discussed in Chapter 6.

2.7 Concluding remarks

The approach of employing large aryl substituents on the nitrogen atoms of the amidinate ligand [Dipp2BAM]– has been successful in disfavouring bridging coordination. However, the steric protection has proved insufficient to prevent coordination of a second amidinate to the same iron centre. X-ray structures suggest this to originate mainly from the orientation of the 2,6-diisopropylphenyl groups; these are directed away from the metal instead of forming a concave binding pocket. The

ppm -30 -20 -10 0 10 20 30 40 50 60 70 80 90

*

*

Figure 2.11 1H NMR spectrum of 2.3 (500 MHz, C6D6, RT). Asterisks indicate C6D5H and free ligand.


45

formation of a bis(amidinate) complex constitutes a thermodynamic ‘sink’ and kinetic pathways to this complex are readily available in the absence of a Lewis base.

The instability of tricoordinate monoamidinate Fe(II) species with respect to ligand redistribution thus precludes isolation of base-free monoalkyls [(Dipp2BAM)FeR]. Ligand redistribution observed upon attempted alkylation of the chloro precursor seems to be accompanied by reduction to Fe(0). The ligand redistribution pathway can be blocked through coordination of a Lewis base to the [(Dipp2BAM)FeX] platform. Provided coordination of the base is strong enough, this allows isolation of a four-coordinate, 14 VE Lewis base adduct [(Dipp2BAM)FeR(L)].

2.8 Experimental

General considerations

All manipulations involving air-sensitive compounds were carried out under a dry dinitrogen atmosphere using standard Schlenk and drybox techniques. Diethyl ether and THF (99.9 % Aldrich) were percolated over a column of Al2O3 and stored under nitrogen. Toluene, hexane and pentane (99.9 % Aldrich) were percolated over a column packed with molecular sieves 4 Å (90 w%) and Al2O3 (10 w%) and stored under nitrogen. Deuterated solvents (Aldrich) were dried over Na/K alloy (C6D6, THF-d8) or used as received and stored under nitrogen.

Instrumentation

NMR-spectra were recorded on Varian Inova 500, VXR 300 and Varian Gemini 200 instruments. 1H chemical shifts are referenced to residual protons in deuterated solvents and are reported relative to tetramethylsilane. 1H NMR spectra of paramagnetic compounds were recorded with pulse widths of ca. 25° and acquisition times of ca. 200 ms using a window wide enough to place no peaks near the edge of the spectrum. If possible, concentrations of ca. 50 mg / 0.5 ml were used. IR-spectra were recorded on a Mattson 4020 Galaxy FT-IR spectrometer. GC-MS analyses were conducted using a HP 5973 mass-selective detector attached to a HP 6890 GC instrument. A JEOL JMS600 spectrometer was used for exact mass determinations. Elemental analyses were performed by the Microanalytical Department at the university of Groningen or by Mikroanalytisches Laboratorium H. Kolbe, Mülheim an der Ruhr, Germany. Reported values are the averages of two independent determinations. Magnetic susceptibility measurements were performed on crystalline samples (ca. 25 mg) using a Quantum Design MPMS-7 SQuID magnetometer in the department of Solid State Chemistry at the University of Groningen. Samples were contained in sealed NMR tubes which were inserted in a plastic straw. Variable temperature magnetisation data were collected at a field of 1000 G from 300 to 5 K with at least one data point every 5K. The data were corrected for diamagnetism using Pascal’s constants.139 Curie constants (θ) were determined by least-square fits

to the Curie-Weiss law θ

βχ−+=

T

SS

k

gN A )1(.

3

22

in temperature regions > 10θ. Effective

magnetic moments were calculated as Tmeff χµ 828.2. = so taking g = 2.00. Solution

effective magnetic moments were determined at 298 K in C6H6/C6D6 (0.375 mL / 0.125 mL) according to Evans method.119,120

Chapter 2

46

Starting materials

N,N’-bis(2,6-diisopropylphenyl)benzamidine,86 anhydrous FeCl2,140 FeCl2(THF)1.5,

141 LiCH2SiMe3

142, LiCH2CMe3143 and LiCH(SiMe3)2

144 were prepared according to literature procedures. All other chemicals are commercially available and were used without further purification.

Ligand synthesis

Preparation of N-(2,6-diisopropylphenyl)benzanilide. Benzoyl chloride (39 mL, 0.33 mol) was added to a vigorously stirred mixture of 10% aqueous NaOH (240 mL) and 2,6-diisopropylaniline (57.0 mL, 0.30 mol). After 60 min. the solid was collected on a glass filter and washed several times with water, and once with ethanol. The product was dried in vacuum and isolated as a white powder. Yield: 57.6 g (0.19 mol, 65%). 1H NMR (200 MHz, CDCl3, RT): δ = 7.92 (d, J = 6.6 Hz, 2H, ArH), 7.58-7.05 (m, 8H, ArH), 3.15 (sept, J = 6.8 Hz, 2H, iPr-CH), 1.22 (d, J = 6.8 Hz, 6H, iPr-CH3) ppm. IR (KBr): ν~ = 3302 (m), 3293 (m), 3270 (m), 2956 (m), 2923 (s), 2854 (m), 1640 (s), 1580 (w), 1515 (m), 1486 (m), 1463 (m), 1453 (m), 1380 (w), 1361 (w), 1291 (w) cm–

1.

Preparation of N-(2,6-diisopropylphenyl)benzimidoyl chloride. This compound was prepared according to a literature procedure from N-(2,6-diisopropylphenyl)benzanilide and thionyl chloride.145 Spectroscopic data have not been pubished before. 1H NMR (300 MHz, CDCl3, RT): δ = 8.26 (d, J = 5.2 Hz, 2H, ArH), 7.61-7.50 (m, 3H, ArH), 7.24 (s, 3H, ArH), 2.88 (sept, J = 4.4 Hz, 2H, iPr-CH), 1.27 (d, J = 4.4 Hz, 6H, iPr-CH3), 1.21 (d, J = 4.4 Hz, 6H, iPr-CH3) ppm. IR (KBr): ν~ = 2959 (s), 2926 (s), 1665 (s), 1582 (w), 1462 (m), 1451 (m), 1167 (m), 398 (m), 797 (w), 760 (m), 986 (m) cm–1.

The amidine [Dipp2BAM]H was prepared from N-(2,6-diisopropylphenyl)benzimidoyl chloride and 2,6-diisopropylaniline following a literature procedure.86

Preparation of [(Dipp2BAM)FeCl(µ-Cl)Li(THF)3] · THF (2.1)

A solution of n-butyllithium (0.92 mL, 2.5 M in hexanes, 2.3 mmol) was added dropwise to a solution of [Dipp2BAM]H (1.0 g, 2.3 mmol) in THF/pentane (10:90 v:v) over the course of 30 minutes. The solvents were removed in vacuo and pentane (20 mL) was added to the residue. All volatiles were removed again in vacuo. All manipulations in the following steps were performed at 0 °C. A solution of Li[Dipp2BAM] (1.02 g, 2.27 mmol) in THF (15 mL) was added to a suspension of FeCl2 (0.287 g, 2.27 mmol) in THF (10 mL) and the reaction mixture was stirred for 1 hour. The solvent was removed in vacuo and pentane (10 mL) was added to the residue. All volatiles were removed again in vacuo. The product was extracted with pentane (40 mL) until the pentane was colourless. The combined extracts were evaporated to dryness, leaving a yellow solid. The solid was washed with cold pentane (15 mL, 2x). The crude product was recrystallised from THF/hexane at –30 °C, yielding yellow crystals 1.22 g (1.59 mmol, 70%). µeff. (C6H6, 298 K) = 5.4 µB. 1H NMR (300 MHz, C6D6, RT): δ (∆ν½) = 23.1 (73 Hz, 2H), 18.7 (800), 13.2 (415), 8.9 (169 Hz, 12H), 6.3 (90 Hz, 12H), 2.7 (65 Hz, 12H), –17.9 (356 Hz, 12H) ppm (Hz). 1H NMR (500 MHz, THF-d8, RT): δ = 19.0 (23 Hz, 2H, m-HPh), 15.6 (32 Hz, 4H, m-HAr), 14.6 (849 Hz, 4H, iPr-CH), 11.8 (134 Hz, 2H, o-HPh), 5.5 (45 Hz, 12H, iPr-CH3), 3.6 (16 Hz, 12H, THF), 2.3 (20 Hz, 1H, p-HPh), 1.7 (13 Hz, 12H, THF), –12.9 (171 Hz, 12H, iPr-CH3), –22.5


47

(24 Hz, 2H, p-HAr) ppm. IR (nujol mull, KBr): ν~ = 1623 (w),1580 (m), 1326 (s), 1272 (s), 1253 (s), 1238 (s), 1183 (w), 1097 (w), 1045 (s), 959 (m), 935 (w), 919 (w), 893 (m), 803 (w), 784 (m), 766 (s), 696 (s) cm–1. Anal. C43H63Cl2FeLiN2O3 (789.68): calcd. C 65.40, H 8.04, N 3.55, Cl 8.98, Fe 7.07, Li 0.88 ; found C 64.89, H 8.04, N 3.57, Fe 6.98, Li 0.84.

Thermally induced disproportionation of 2.1

A suspension of 2.1.THF (0.30 g, 0.38 mmol) in pentane (40 mL) was stirred at room temperature for 48 hrs. During this period, the colour changed from yellow to dark green and salt precipitation was noted. The solution was filtered and concentration of the filtrate to ca. 5 mL afforded 7.3 as a dark green crystals. Yield 0.17 g (0.18 mmol, 48% based on Fe). Spectroscopic and analytic data for 7.3 are described in Chapter 7.

Reaction of 2.1 with LiCH2SiMe3

LiCH2SiMe3 (3.6 mg, 38 µmol) was added to a solution of 2.1 (30 mg, 38 µmol) in THF-d8 (0.5 mL). A colour change from pale yellow via red to dark yellow was observed. Spectroscopic data for the new species: 1H NMR (500 MHz, THF-d8, RT): δ = 19.3 (44 Hz, 1H), 16.9 (254 Hz, 9H, SiMe3), 13.4 (109 Hz, 4H), 10.5 (166 Hz, 1H), 6.8 (40 Hz, 5H), 6.6 (48 Hz, 2H), 5.5 (480 Hz, 12H, iPr-CH3), 3.6 (29 Hz, 16H, THF), 1.72 (24 Hz, 16H, THF), –12.4 (302 Hz,12H, iPr-CH3), –21.5 (45 Hz, 1H) ppm.

Reaction of 2.1 with LiCH2CMe3

LiCH2CMe3 (3.0 mg, 38 µmol) was added to a solution of 2.1 (30 mg, 38 µmol) in THF-d8 (0.5 mL). A colour change from pale yellow via red to dark yellow was observed. Spectroscopic data for the new species: 1H NMR (500 MHz, THF-d8, RT): δ = 43.0 (490 Hz, 9H, tBu), 20.7 (27 Hz, 2H), 14.4 (247 Hz, 2H), 13.0 (232 Hz, 4H), 5.5 (66 Hz, 12H, iPr-CH3), 3.6 (13 Hz, 12H, THF), 1.7 (13 Hz, 12H, THF), –12.7 (194 Hz, 2H), –14.9 (348 Hz, 12H, iPr-CH3), –21.3 (25 Hz, 2H) ppm.

Reaction of 2.1 with KCH2Ph

KCH2Ph (5.0 mg, 38 µmol) was added to a solution of 2.1 (30 mg, 38 µmol) in THF-d8 (0.5 mL). A colour change from pale yellow via red to dark yellow was observed. Precipitation of KCl was noted. Spectroscopic data for the new species: 1H NMR (500 MHz, THF-d8, RT): δ = 55.9 (1599 Hz), 35.5 (143 Hz), 31.8 (193 Hz), 29.9 (213 Hz), 21.9 (233 Hz), 20.5 (122 Hz), 19.4 (257 Hz), 16.9 (206 Hz), 15.7 (197 Hz), 14.7 (363 Hz), 13.9 (166 Hz), 12.4 (337 Hz), 5.4 (421 Hz), 3.6 (55 Hz, THF), 1.7 (53 Hz, THF), –1.7 (441 Hz), –5.4 (212 Hz), –6.3 (798 Hz), –11.0 (470 Hz), –13.2 (1317 Hz), –14.3 (264 Hz), –18.7 (994 Hz), –19.7 (269 Hz), –21.1 (114 Hz), –29.3 (667 Hz), –44.5 (539 Hz), –55.9 (126 Hz) ppm.

Chapter 2

48

Preparation of [{C(O)(2,6-iPr2Ph)NC(Ph)N(2,6-iPr2Ph)}Fe(CO)2-µ-((CO)Li(THF)2)]2 · 2THF (2.2)

A solution of n-butyllithium (0.46 mL, 2.5 M in hexanes, 2.3 mmol) was added dropwise to a solution of [Dipp2BAM]H (0.50 g, 1.13 mmol) in THF/pentane (10:90 v:v) in 30 minutes. The solvents were removed in vacuo and the residue was freed of residual THF by stirring the solid in pentane (20 mL) and subsequent removal of all volatiles. All manipulations in the following steps were performed at 0 °C. A solution of Li[Dipp2BAM] (1.02 g, 2.27 mmol) in THF (10 mL) was added to a suspension of FeCl2 (0.144 g, 1.13 mmol) in THF (5 mL) and the reaction mixture was stirred for 1 hour. A solution of LiCH(SiMe3)2 (0.188 g, 1.13 mmol) in THF (10 mL) was added dropwise to the reaction mixture in 30 minutes. After 30 minutes the reaction mixture was degassed and CO was admitted. After 3 hours, THF was removed in vacuo and the residue was extracted with pentane (30 mL, 3x) and hot hexane (30 mL, 1x). The alkane solvent was removed in vacuo and the crude product was dissolved in THF. The solution was concentrated and pentane was slowly diffused into the solution yielding dark red, block-shaped crystals (0.20 g, 0.12 mmol, 21 % based on FeCl2). IR (nujol mull, KBr): ν~ = 3058 (w), 1961 (s, C≡O), 1883 (s, C≡O), 1814 (s, C≡OLi), 1582 (w), 1556 (s, C=O), 1526 (w), 1493 (w), 1320 (w), 1262 (w), 1175 (w), 1103 (w), 1054 (m), 1009 (m), 913 (w), 834 (w), 805 (w), 784 (w), 747 (w), 711 (m), 696 (w), 679 (w), 631 (w), 587 (w), 551 (w), 538 (w), 477 (w) cm–1. 1H NMR analysis of the complex was frustrated by the formation of Fe(0) in the NMR tube, possibly due to photolysis. Broad, overlapping signals were observed in the diamagnetic region (+10–0 ppm). Satisfactory elemental analysis for 2.2 could not be obtained due to the light-sensitivity of the crystals and the fact that cocrystallised THF is easily lost from the lattice.

Preparation of [{PhC(NAr)2}Fe(CH2SiMe3)(py)] (2.3)

N,N’-2,6-(diisopropylphenyl)benzamidine (780 mg, 1.77 mmol) was dissolved in THF (20 mL). n-BuLi (2.5 M in hexanes, 0.7 mL, 1.77 mmol) was added. The darkened solution was stirred for 30 min. and transferred to a dropping funnel. The Li-amidinate solution was added dropwise to a stirred suspension of FeCl2(THF)1.5 (416 mg, 1.77 mmol) in THF (10 mL). After 1 hr, pyridine (0.3 ml, 3.7 mmol) was added to the dark yellow solution. The colour of the solution became more intensely yellow in the course of 15 min. LiCH2SiMe3 (167 mg, 1.77mmol) was added, resulting in a dark yellow solution. After stirring for 15 min., all volatiles were pumped off. Residual THF/excess pyridine were removed by suspending the residue in hexanes and subsequent removal of all volatiles in vacuo (10 ml, 4x). The residue was then extracted with hexanes (30 ml, 3x), leaving an off-white salt. Cooling to –25 °C afforded orange crystals. Yield 440 mg (0.66 mmol, 38 %). µeff. (C6H6, 298 K) = 5.5 µB. 1H NMR (500 MHz, C6D6, RT) δ = 83.3 (1980 Hz, 2H), 35.6 (200 Hz, 9H, SiCH3), 31.8 (233 Hz, 2H), 27.5 (195 Hz, 2H), 26.0 (25 Hz, 2H), 11.6 (176 Hz, 4H, m-HAr), 5.0 (752 Hz, 12H, iPr-CH3), –19.9 (22 Hz, 2H, p-HAr), –25.0 (435 Hz, 12H, iPr-CH3) ppm. IR (nujol mull, KBr) ν~ = 3053 (m), 2962 (s), 2905 (s), 2893 (s), 1601 (w), 1579 (w), 1507 s), 1482 (s), 1454 (s), 1434 (s), 1401 (s), 1360 (s), 1316 (s), 1270 (m), 1251 (m), 1239 (s), 1214 (m), 1177 (w), 1153 (w), 1121 (w), 1098 (m), 1068 (w), 1053 (w), 1042 (w), 1027 (w), 1011 (w), 951 (w), 935 (w), 917 (w), 874 (s), 851 (m), 824 (m), 808 (m), 785 (m), 770 (m), 761 (m), 751 (m), 738 (m), 724 (m), 697 (s), 551 (w), 539


49

(m), 510 (m), 492 (w), 477 (w) cm–1. Anal. C40H55N3FeSi (661.83): calcd. C 72.59, H 8.38, N 6.35; found C 73.24, H 8.69, N 6.41.

Ethene polymerisation tests with 2.1 and 2.3

Toluene (99.9% Aldrich) used for polymerisation experiments was percolated over a column packed with molecular sieves 4Å (90 wt%) and Al2O3 (10 wt%) and freed of oxygen by passing it over BASF-R3-11 supported Cu-catalyst. A stainless steel 1L autoclave (Medimex), fully temperature and pressure controlled and equipped with solvent and catalyst injection systems, was preheated in vacuo for 45 min at 100 °C prior to use. The reactor was cooled to 50 C°, charged with 250 mL of toluene and pressurised with ethene (10 bar). Cocatalyst (PMAO solution in toluene, 4.9 w% Al (Al/Fe = 500)) was injected, followed by injection of the Fe complex in toluene (20 µmol in 5 mL). Temperature and pressure were monitored. After 30 min. the run was aborted by venting the reactor and addition of ethanol. Excess MAO was further destroyed by treatment of the toluene solution with ethanol/dilute hydrochloric acid. No significant amount of polymer was obtained.

X-ray structure determinations

2.1: Suitable crystals were obtained by slow cooling of a hexane/THF solution to –30 °C. The crystal was picked from the mother liquor and was covered with inert oil to avoid deterioration due to loss of solvent from the crystal lattice. Some atoms (C24, C32, C64) belonging to the coordinated THF molecules showed unrealistic displacement parameters when allowed to vary anisotropically (which is in line with the weak scattering power of the crystal investigated) suggesting dynamic disorder. (Dynamic means that the smeared electron density is due to fluctuations of the atomic positions within each unit cell). The most heavily disordered THF ligands (ring 2 : O2–C36..C39 and ring 3 : O3–C40..C43) have been described by two site occupancy factors each. The s.o.f. of the major fraction components of the disorder model refined to values of 0.533(6) and 0.518(6), respectively. No classic hydrogen bonds, no missed symmetry (MYSSYM), or solvent-accessible voids were detected by procedures implemented in PLATON.146,147

2.2: Suitable crystals were obtained by recrystallisation from THF. The electron density of C33 was disordered over two positions, indicating conformational disorder. The s.o.f. of the major fraction of the component of the twin model refined to a value of 0.696(11). Some other atoms (O31, C31, C32, C64) belonging to the coordinated THF molecules also showed unrealistic displacement parameters suggesting some degree of disorder, which is in line with the weak scattering power of the crystals investigated. No classic hydrogen bonds, no missed symmetry (MYSSYM), or solvent-accessible voids were detected by procedures implemented in PLATON.146,147

2.3: Suitable crystals were obtained by recrystallisation from pentane. Some atoms (C230 and C231) showed unrealistic displacement parameters when allowed to vary anisotropically, suggesting dynamic disorder. The smeared electron density for C230 and C231 has been described by two site occupancy factors with separately refined displacement parameters. The s.o.f. of the major fraction of the component of the disorder model refined to a value of 0.612(10). The hydrogen atom coordinates and isotropic displacement parameters belonging to the disordered C-atoms were

Chapter 2

50

allowed to ride on their carrier atoms with an isotropic displacement parameter related to the equivalent displacement parameter of their carrier atoms. No classic hydrogen bonds, no missed symmetry (MYSSYM), but potentially solvent-accessible areas (voids of 73.0 Å3 / unit cell) were detected by procedures implemented in PLATON.146,147

Crystal, collection and refinement data for 2.1, 2.2 and 2.3 are listed in Table 2.4.


51

Table 2.4 Crystal, collection and refinement data for complexes 2.1 – 2.3.

2.1 2.2 2.3

formula C43H63N2O3FeCl2Li.THF C86H110N4O12Fe2.2THF C40H55N3FeSi

fw 861.78 1661.63 661.82

cryst. dim. (mm) 0.22 x 0.21 x 0.20 0.35 x 0.27 x 0.15 0.50 x 0.34 x 0.24

colour, habit yellow, block red, irregular orange, prism

crystal system triclinic monoclinic monoclinic

space group, no.148 P 1 , 2 P21/c, 14 P21/c, 14

a (Å) 13.1932(7) 21.5290(1) 22.329(1)

b (Å) 13.4944(7) 10.7289(5) 17.831(1)

c (Å) 14.321(1) 39.956(2) 20.489(1)

α (°) 79.239(1) 90 90

β (°) 84.262(1) 91.712(1) 109.342(1)

γ (°) 72.039(1) 90 90

Z 2 4 4

V (Å3) 2380.3(2) 8994.1(8) 7697.2(7)

ρcalc (g/cm3) 1.202 1.227 1.142

θ range (°) 2.23 – 20.34 2.17 – 24.43 2.25 – 26.72

λ (Å) 0.71073 (Mo Kα) 0.71073 (Mo Kα) 0.71073 (Mo Kα)

T (K) 100(1) 110(1) 100(1)

data collect. time (h) 7.9 13.0 8.0

no. of meas. refl. 22682 63345 60961

no. of unique refl. 11506 15868 15636

µ (cm-1) 4.71 3.86 4.52

no. of parameters 634 1041 1250

weighting scheme: a,b[a] 0.0514, 0.0 0.0793, 16.9033 0.0509, 0.0148

R(F) for F0 ≥ 4σ(F0)[b] 0.0728 0.0699 0.0440

wR(F2)[c] 0.1628 0.1872 0.1057

res. el. dens. (e/Å3) –0.44, 0.58(8) –0.76, 0.86(7) –0.25, 0.46(6)

GoF[d] 0.984 1.027 1.018

[a] w = 1/[σ2(Fo

2) + (aP)2 + bP], P = [max(Fo2,0) + 2Fc

2] / 3 [b] R(F) = ∑ (||Fo| - |Fc||) / ∑ |Fo |, [c] wR(F2) = [∑ [w(Fo

2 - Fc2)2] / ∑ [w(Fo

2)2]]1/2, [d] GoF = [∑ [w(Fo

2 - Fc2)2] / (n-p)] ½, n = # refl., p = # param. refined.

Chapter 2

52

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university of groningen electron deficient …iron(ii) amidinate complexes 23 2 iron(ii) amidinate...

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