University of Groningen

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

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Iron(II) β-diketiminate complexes

93

4 Iron(II) β-diketiminate complexes†† The sterically hindered amidinate ligands employed in the previous chapters frequently showed coordinative lability. Especially, ligand redistribution leading to Fe(II) bis(amidinate) complexes frustrated the isolation of electronically unsaturated (12 VE) alkyl species [(amidinate)FeR]. Nevertheless, these monoalkyls can be stabilised by an additional Lewis base. Such stabilised (14 VE) Fe(II) monoalkyls may contain either an additional coligand (pyridine in Chapter 2) or a donor functionality linked to the amidinate ligand (dimethylaminoethyl in Chapter 3). However, the introduction of the dimethylaminoethyl functionality has restored the ability of the ligand to coordinate in a bridging fashion, resulting in dimeric structures. In order to obtain mononuclear, electronically unsaturated Fe(II) monoalkyls without the need for an additional Lewis basic donor, a non-bridging and coordinatively inert ligand is required. This chapter describes the preparation of Fe(II) monoalkyls supported by a β-diketiminate ligand which fulfils both requirements.

4.1 β-Diketiminate ligands Just like amidinates are the nitrogen analogues of carboxylates, β-diketiminates [RC{C(R’)NR’’}2]

– (Figure 4.1B), are the nitrogen analogues of β-diketonates [RC{C(R’)O}2]

– (Figure 4.1A). The parent β-diketimines can be prepared by condensation of β-diketones with two equivalents of amine.1,2 Introduction of the monoanionic ligand on metals is most often accomplished by salt metathesis, although amine or hydrocarbon elimination routes have also been applied.3

The 6-membered chelates obtained with β-diketiminates result in very stable complexes with main group metals, transition metals, lanthanides and actinides.4 Besides the most commonly found κ2(N,N’) coordination mode (Figure 4.2A), κ1(C3),

5,6 κ3(N,C3,N)7 and η5-coordination8-16 (Figure 4.2B–D) have been reported for

β-diketiminates. Recently, a U(III) complex featuring the ligand in an η3-(N,C,C’)-1-azaallyl bonding mode was structurally characterised.17

†† Parts of this chapter have been published: Sciarone, T. J. J.; Meetsma, A.; Hessen, B.;

Teuben, J. H. Chem. Commun. 2002, 1580-1581.

NM

N

R'R'

R

R'' R''O

MO

R' R'

R

A B

Figure 4.1 β-diketonate chelate ring (A), β-diketiminate chelate ring (B)

Chapter 4

94

As a result of the more convergent orientation of the nitrogen lone pairs, β-diketiminate ligands have a lower tendency to form bridged structures compared to amidinates. Although bridged structures are possible for β-diketiminates,16,18-20 they are less common than for amidinates. Due to the 6-membered chelate ring, β-diketiminate complexes generally have much larger bite angles than chelating amidinates.

The imine nitrogen atoms offer the possibility to introduce steric protection close to the metal centre by use of sterically demanding R’’ substituents. The well-known 2,6-diisopropyl(phenyl) group (R’’ = 2,6-iPr2C6H3) is commonly used to serve this purpose. β-Diketiminates carrying these sterically encumbering aryl groups generally form mono(ligand) metal complexes, with the exception of some alkaline-earth metals and lanthanides.15,21-23 Bis(β-diketiminate) complexes of transition metals are known for ligands with smaller substituents on the nitrogen atoms.2,3,7,8,10,24

The strong steric shielding of β-diketiminates with 2,6-disubsituted aryl groups on the nitrogen atoms has been recognised by many groups as a means of stabilising transition metal complexes with very low coordination numbers and consequently low electron counts e.g. in structural models for metalloproteins.25-35 The same properties have allowed the isolation of main group element complexes in unusually low oxidation states and the kinetic stabilisation of double bonds between such elements.36-44 The shielding of low coordinate, electron deficient metal centres is also important in many homogeneous catalytic transformations. Not surprisingly, sterically encumbered β-diketiminate ligands have found application as ancillary ligands in polymerisation catalysts. Insertion polymerisation of olefins is catalysed by β-diketiminate complexes of divalent Ni and trivalent Sc, Ti, V and Cr.1,5,7,24,45-50 The ligands have also been employed in Zn(II)-catalysed alternating copolymerisation of epoxides and carbon dioxide to polycarbonates.51-55 Stereospecific polymerisation of lactide can be effected by divalent Zn and Mg β-diketiminate complexes.56,57 Recently, polymerisation of methyl methacrylate promoted by Yb(II) β-diketiminate complexes was reported.58-60 Rh(I) complexes catalyse the hydrogenation of olefins.61 Cu(I) β-diketiminates have been employed as catalysts for alkene cyclopropanation and aziridination.62,63. Intramolecular hydroamination is catalysed by Ca(II) and cationic Sc(III) β-diketiminate complexes.64,65

4.2 Iron(II) monoalkyl complexes stabilised by a β-diketiminate ligand

As β-diketiminates are in general coordinatively stable, non-bridging ligands with a low tendency to form bis(ligand) complexes, this type of ligand is attractive for stabilisation of electron deficient Fe alkyls. The well-known β-diketiminate [HC{C(Me)NAr}2]

– ([BDK]–, Ar = 2,6-iPr2C6H3) carries bulky aryls on the nitrogen

NM

N

R'R'

R

R'' R'' N N

R'R'

M R

R'' R''

R'N

N

M

R''

R''

R'

R R'N

N

M

R''

R''

R'

R

A B C D

Figure 4.2 Different coordination modes observed in β-diketiminate complexes

Iron(II) β-diketiminate complexes

95

atoms which offer steric protection in order to preclude formation of dimers or bis(ligand) complexes. In addition, the metal centre is shielded from other co-ligands to ensure low electron count.

Holland et al. recently reported the synthesis of the ferrate complex [(BDK)Fe(µ-Cl)2Li(THF)2] from Li[BDK]

66 and FeCl2 in THF.67 This ferrate was chosen as starting

material for the preparation of Fe(II) alkyls supported by the β-diketimiminate ligand. The ferrate complex was generated in situ and reacted with benzylmagnesium bromide or (trimethylsilyl)methyllithium in THF producing the base-free, monoalkyl derivatives [(BDK)FeCH2R] 4.1 (R = Ph) and 4.2 (R = SiMe3), respectively. Red (4.1) or orange (4.2) crystals of the highly air-sensitive monoalkyls were obtained by recrystallisation from pentane.

Even though alkyl complexes 4.1 and 4.2 are formally electronically unsaturated (12 VE), the Fe(II) centres do not retain a coordinating THF molecule from the reaction solvent. The observed three-coordination (vide infra) is relatively rare for Fe(II),68-70 but several examples with β-diketiminate ligands have been reported in recent years. The group of Holland has obtained the three-coordinated chloride complex [{HC{C(tBu)NAr}2}FeCl] (Ar = 2,6-iPr2C6H3).

67 Reduction of this compound under N2 affords the dinitrogen complex [(HC{C(tBu)NAr}2)FeNNFe(HC{C(tBu)NAr}2)].

71 Alkylation of the parent monochloride gives the monoalkyls [{HC{C(tBu)NAr}2}FeR] (R = Me, Et, CH2tBu, iPr, tBu).

72 Gibson et al. have isolated the bulky alkoxide [{HC{C(tBu)NAr}2)FeOtBu].

59

Panda et al. have shown that the sterically less demanding BDK ligand can also be employed to stabilise three-coordinated Fe(II) in [(BDK)Fe{N(SiMe3}2] which contains a bulky amide coligand. Compounds 4.1 and 4.2 are the first three-coordinated Fe(II) alkyls supported by the BDK ligand.3 Their isolation underlines that tBu groups on the β-diketiminate backbone are not strictly required for obtaining three-coordinated Fe(II). Apparently the protection provided by the o-aryl substituents in BDK is sufficient to limit the coordination number to three in 4.1 and 4.2. Concurrently with this work,73 the Holland group also reported other alkyl analogues [(BDK)FeR], (R = Et, iBu),74 and in a following paper, the series was expanded to R = Me, n-C3H7, n-C4H9, CH2CH2CF3, Cy, CHMePh and CH2tBu.

75

The molecular structures of 4.1 and 4.2 were established by X-ray diffraction (Figure 4.3). The asymmetric unit for 4.1 contains one disordered pentane molecule and two independent molecules of the iron complex. As the two molecules are structurally nearly identical, only one will be discussed here. For 4.2, the asymmetric unit

1.)n-BuLi2.) FeCl2THF, R.T.

N NAr Ar

Fe

R

RCH2MTHF, RT

N NAr Ar

Fe

Cl Cl

LiTHF THF

N NAr Ar

H

M = MgBr, 4.1 R = Ph M = Li, 4.2 R = SiMe3

Scheme 4.1

Chapter 4

96

consists of one molecule of the iron complex. Metric parameters for the structures are summarised in Table 4.1.

A

B

Figure 4.3 Molecular structure of 4.1 (A) and 4.2 (B), hydrogen atoms and cocrystallised pentane (4.1) have been omitted for clarity.

Iron(II) β-diketiminate complexes

97

Both X-ray structures show iron centres coordinated in a trigonal planar fashion, the sum of the angles around iron equalling 360° and 359.26° for 4.1 and 4.2, respectively. The iron centre is located slightly out of the least-squares plane through the diketiminate NCCCN backbone atoms (0.268(1) Å in 4.1 and 0.012(1) Å in 4.2). The iron-carbon bonds (2.0414(18) Å in 4.1 and 2.0222(18) Å in 4.2) lie well in the range of Fe–C distances observed for three-coordinate β-diketiminate iron alkyls.74

The β-diketiminate ligand coordinates in the normal κ2(N,N) chelating mode and the aryl nitrogen substituents are oriented roughly orthogonal to the coordination plane.76 The N–Fe–N bite angles of ca. 94° are common for for Fe(II) β-diketiminate complexes of BDK for which values between 92.77(7) and 93.44(9)° have been reported.3,67,74 The β-diketiminate ligand is somewhat ‘opened’ by widening the C113–C115–C116 angle to ca. 129° (120° would be expected for the sp2-hybridised

Table 4.1 Selected interatomic distances and angles for 4.1 and 4.2.

4.1 4.2

Distances (Å)

Fe–C30 2.0414(18) 2.0222(18)

Fe-N1 1.9831(11) 1.9915(13)

Fe1-N2 1.9807(11) 1.9926(12)

C13-C15 1.405(2) 1.401(2)

C15-C16 1.401(2) 1.399(2)

N2-C16 1.3405(18) 1.334(2)

N1-C13 1.3321(17) 1.336(2)

Angles (°)

Fe1-C30-C31 113.18(11)

Fe-C30-Si 123.48(10)

N1-Fe1-N2 94.35(5) 94.01(5)

C30-Fe-N1 131.81(6) 142.06(6)

C30-Fe-N2 133.84(6) 123.19(6)

C13-C15-C16 128.97(13) 129.14(15)

Fe-N1-C13 124.38(9) 124.34(10)

Fe-N1-C6 115.54(8) 116.67(10)

C6-N1-C13 120.05(11) 118.85(13)

Fe-N2-C16 123.69(10) 125.01(10)

Fe-N2-C18 117.97(9) 114.74(10)

C18-N2-C16 118.32(12) 119.90(12)

Chapter 4

98

C15). Consequently, the two CN vectors are not parallel. The almost equal C–C and C–N bond lengths within the ligand backbone indicate a large degree of delocalisation in the six-membered chelate ring. The diketiminate nitrogens are planar (ΣLN = 359.65° – 359.98°), indicating sp

2-hybridisation. The Fe–N–Cimine angles however, are restricted to 124° (av.) by the chelate ring. These angles are approximately bisected by the N–Cipso-vectors, which results in Fe–N–Cipso angles 116° (av.) which are slightly smaller than the Cimine–N–Cipso angles 119° (av. ).

Despite the formal electron deficiency of the three-coordinate iron benzyl complex, no interaction between the arene part of the benzyl group and the iron atom is present in 4.1. The Fe–Cipso distance equals 2.9609(16) Å and Fe–CH2–Cipso-angle = 113.18(11)° indicating that the aromatic ring of the benzyl group is bent away from the metal. An even larger angle is found in 4.2 where Fe–CH2–Si = 123.48(10)°. The Fe–CH2–Cipso/Si angles in 4.1 and 4.2 are enlarged to minimise steric interactions with the sterically demanding BDK ligand. Steric hindrance of the (trimethylsilyl)methyl group in the small binding pocket of the β-diketiminate ligand is also expressed in the unsymmetric Cα–Fe–N angles, which differ by 19° (ca 2° difference in 4.1). The situation in 4.1 and 4.2 parallels the absence of secondary interactions in the monoalkyls [(BDK)FeR] reported by Vela et al.74,75

4.3 Magnetism and 1H NMR spectroscopy The paramagnetic complexes 4.1 and 4.2 both have a solution magnetic moment of 5.6 µB (Evans’ method,

77,78 C6H6), most consistent with a high-spin d6-configuration

(4 unpaired electrons plus an orbital contribution). For the Fe(II) monoalkyls [(BDK)FeR] (R = iBu, Et) solution magnetic moments of 6.0 and 5.6 µB have been reported.74 Fe(II) monoamido complexes supported by the BDK ligand display solution magnetic moments ranging from 4.9 – 5.1 µB.

3 Substantially lower values are found by Bart et al. for four-coordinate Fe(II) dialkyl complexes supported by diimine and diamine ligands, which display solution magnetic moments in the range 4.6 – 4.9 µB.

79

For of 4.1, magnetic susceptibility data were also collected on crystalline material using a SQuID magnetometer in the temperature range of 5 – 300 K. Figure 4.4 displays the temperature dependence of the reciprocal magnetic susceptibility and the effective magnetic moment. The observed reciprocal susceptibility obeys the Curie-Weiss law in the temperature range 50 – 300 K with θ = 2.1 K. The magnetic moment in the solid state at 300 K has a value of 4.4 µB, which is substantially lower than the solution magnetic moment (vide supra). The value obtained from the SQuID data is between the spin-only moments for S = 3/2 (3.87 µB) and S = 2 (4.90 µB). We do not have a satisfactory explanation for the low magnetic moment for 4.1 in the solid state. Holland has noted difficulties in reproduction of temperature dependence of magnetic moments of similar compounds using a SQuID magnetometer. This was attributed to alignment of the crystallites by the applied magnetic field.72

Iron(II) β-diketiminate complexes

99

The presence of unpaired electrons in 4.1 and 4.2 results in 1H NMR spectra consisting of paramagnetically shifted, broad resonances in the frequency window of +150 to –150 ppm (Figure 4.5). The β-diketiminate resonances can be assigned by comparison to literature 1H NMR data of three-coordinated Fe(II) monoalkyls of the same ligand (Table 4.2).74,75

Resonances from the alkyl ligands are observed at 45.2 (2H), 40.0 (2H) and –44.3 (1H) ppm for 4.1 and at 56.1 (9H) ppm for 4.2. The three alkyl signals in the spectrum of 4.1 are assigned to the meta, ortho and para protons, respectively of the benzyl moiety on basis of line widths and integrals. The para resonance is characterised by its 1H integral and small line width (∆ν½ = 61 Hz). The meta and ortho signals do not differ much in chemical shift, but are distinguished by their different line widths (∆ν½ = 120 Hz and 2.0 kHz), the broadest signal probably belonging to the ortho protons,

0

20

40

60

80

100

120

140

0 50 100 150 200 250 300

T (K)

χ-1

(mo

l.cm

-3)

0,00

0,50

1,00

1,50

2,00

2,50

3,00

3,50

4,00

4,50

5,00

µef

f. (µ

B)

= (8

χT)1

/2

Figure 4.4 Temperature dependence of reciprocal magnetic susceptibility and effective

magnetic moment for 4.1. Solid line represents best fit using parameters given in the text.

Table 4.2 Comparison of 1H NMR ligand shifts (ppm), approximate integral in parentheses.

Compound α-H γ-CH3 m-CH p-CH iPr-CH iPr-CH3 iPr-CH3

[(BDK)FeiBu]a 130(1) 70(6) –12(4) –74(2) –132(4) –18(12) –115(12)

[(BDK)FeEt]a 130(1) 69(6) –12(4) –76(2) –123(4)c –20(12) –123(12)c

[(BDK)FeCH2Ph] (4.1)b

117(1) 55(6) –8.7(4) –76.3(2) –119(4) –18(12) –112(12)

[(BDK)FeCH2SiMe3] (4.2)b

116(1) 75(6) –9.0(4) –68.5(2) –127(4) –15(12) –100(12)

a C6D6, 294 K.74 b C6D6, 298 K.

c overlapping resonances.

Chapter 4

100

which are closest to the paramagnetic centre. For 4.2, the 9H resonance (δ = 56.1 ppm) is unequivocally assigned to the trimethylsilyl protons. The methylene resonance from the alkyl moieties is not observed in both cases, which is consistent with the absence of alkyl α-CH signals in the spectra of the β-diketiminate iron monoalkyls reported by Holland’s group.72

4.4 Exploratory reactivity studies

4.4.1 Thermal stability Solutions and solid samples of monoalkyls 4.1 and 4.2 are highly air-sensitive turning black immediately upon exposure to air. Under inert atmosphere, however, the

ppm -80 -60 -40 -20 0 20 40 60 80 100 120 140 -100 -120

A

ppm -120 -80 -60 -40 -20 0 20 40 60 80 100 120 140 -100

B

Figure 4.5 1H NMR spectra (500 MHz, C6D6, RT) of 4.1 (A) and 4.2 (B). Diamagnetic resonances between 0 and 10 ppm originate from pentane, C6D5H and free β-diketimine.

Iron(II) β-diketiminate complexes

101

complexes are thermally very robust. Solutions of 4.1 or 4.2 in C6D6 are stable at 80 °C for at least 7 days. In toluene-d8, the monoalkyls are stable as well. No significant decomposition is detected after heating for 8 days at 110 °C.

The observed thermostability is consistent with the results of Holland et al for their β-diketiminate Fe(II) monoalkyls. Remarkably, even the ethyl complex [{HC(C(tBu)NAr)2}FeEt] (Ar = 2,6-iPr2C6H3) was found to be stable.

76,80 The β-diketiminate monoalkyls most likely owe their thermal robustness to a combination of steric protection and the high spin state (S = 2) of the iron centre.

4.4.2 Coordination of Lewis bases Possessing only 12 valence electrons, the monoalkyls [(BDK)FeCH2R] possess a free valence orbital, even with four unpaired electrons, and should be able to coordinate additional Lewis bases. Monobenzyl 4.1 forms the monoadduct [(BDK)Fe(CH2Ph)(py)] (4.3) upon reaction with pyridine. The product crystallises from toluene as deep red crystals of composition 4.3·PhMe.

The X-ray structure (Figure 4.6) shows a pseudotetrahedral coordination of the iron centre. As expected, the metal-ligand distances are slightly longer than in the three-coordinated parent complex 4.1. The Fe–Npy bond length is longer than the Fe–NBDK distances and is comparable to the corresponding Fe–NtBu-py distance (2.124(2) Å) in the hydride [{HC(C(tBu)NAr)2}Fe(H)(4-tBu-py)].

81

Figure 4.6 Molecular structure of 4.3, hydrogen atoms and cocrystallised toluene molecule have been omitted for clarity.

Chapter 4

102

Apart from the slightly unequal Fe–NBDK distances, the interatomic distances and angles within the β-diketiminate ligand are unexceptional, as is the bite angle N1–Fe–N2. The Fe–CH2–Cipso angle in 4.3 is ca. 6° larger than in base-free 4.1. Again this is probably not due to an α-agostic interaction but rather the result of increased steric crowding around the metal centre in the four-coordinated complex. When compared to the pyridine adduct of the monoamidinate Fe alkyl (2.3) described in Chapter 2, 4.3 displays a considerably smaller angle between the alkyl and the pyridine ligand (N–Fe–C = 102.78(6) for 4.3, 113.63(9) for 2.3). This difference may be accounted for by the more confined coordination pocket of the β-diketiminate ligand compared to the more open amidinate ligand. The different steric demands of the alkyls however, may also contribute.

The 1H NMR spectrum of 4.3 in toluene-d8 at room temperature features extremely broad, overlapping resonances. The line widths and consequent peak overlap are much greater than for the parent compound 4.1 under similar conditions, suggesting that the molecule shows fluxional behaviour in solution. Cooling to –50 °C reveals 16 well-separated signals, while 18 resonances would be expected from the solid state structure. The missing resonances are probably those of the o-pyridine and the benzyl methylene resonances, as these protons are close to the paramagnetic centre.

At 110 °C, 12 of the 14 resonances expected for a time-averaged C2v-symmetric molecule, are visible in the spectrum. Possibly some resonances do not appear due to extreme line broadening. Unfortunately, assignment of the resonances was not successful. The temperature dependence of the spectrum can be accounted for by fast site exchange of the Lewis base (Scheme 4.2). Recooling the sample to room temperature restores the original spectrum, confirming reversibility of the process.

Table 4.3 Selected interatomic distances and angles for 4.3.

Distances (Å)

Fe–C35 2.107(2) C13-C15 1.414(3)

Fe-N1 2.0253(16) C15-C16 1.421(3)

Fe-N2 2.0393(15) N2-C16 1.340(2)

Fe-N3 2.1154(16) N1-C13 1.344(2)

Angles (°)

N1-Fe-N2 92.47(6) C13-C15-C16 129.02(17)

N2-Fe-N3 101.08(6) Fe-N1-C13 124.97(12)

N1-Fe-N3 102.78(6) Fe-N1-C1 114.84(11)

C35-Fe-N1 126.19(7) C1-N1-C13 120.14(15)

C35-Fe-N2 121.85(7) Fe-N2-C16 124.12(12)

C35-Fe-N3 108.54(7) Fe-N2-C18 115.28(11)

Fe1-C35-C36 119.18(13) C18-N2-C16 120.60(15)

Iron(II) β-diketiminate complexes

103

Site exchange of pyridine ligands has been reported for [{HC(CMeNAr)2}NiMe(2,4-lutidine)] (Ar = 2,6-Me2C6H3).

82 The 1H NMR spectrum of the Fe(II) hydride [{HC(CtBuNAr)2}FeH(4-tBu-py)] (Ar = 2,6-iPr2C6H3) suggests that the pyridine adduct is in equilibrium with the dimer of the base-free hydride complex.81

The weaker Lewis base THF also forms adducts with the monoalkyls 4.1 and 4.2. For 4.2 in THF-d8 solution, eight resonances are observed (average C2v-symmetry), consistent with fast site exchange of the solvent in the tetrahedral adduct at room temperature. Addition of a drop of THF-d8 to a solution of 4.1 in C6D6, induces a colour change from red to dark orange. In the 1H NMR spectrum 10 signals are observed between +70 and –100 ppm ((Table 4.4 cf. +120 to –80 ppm for 4.1 in neat C6D6). The appearance of only 10 resonances in the

1H NMR spectrum of 4.1·THF-d8 again indicates fast site exchange of the THF-d8 ligand.

In neat THF-d8 solution, site exchange for 4.1·THF-d8 is fast on the NMR timescale down to –50 °C as evidenced by the observation of ten resonances between +40 and –60 ppm. Evidently, coordination of THF is appreciably weaker than pyrdine coordination. The weak coordination of THF is consistent with the isolation of the monoalkyls (originally generated in THF) in base-free form after recrystallisation from pentane. Holland has found that, the THF adduct [(BDK)FeCl(THF)] is in equilibrium with the base-free monochloride [(BDK)FeCl],83 analogously to the corresponding Ni(II) complexes.84

4.4.3 Reactivity towards ethene In spite of the fact that the Fe(II) alkyls fulfil the basic requirements for catalytic polymerisation activity (electronic unsaturation and the presence of a metal carbon bond), none of them is active as such, i.e. in the absence of a cocatalyst. No ethene

N NAr Ar

Fe

Phpy

N NAr Ar

Fe

Ph py

4.3 4.3

Scheme 4.2

Table 4.4 1H NMR (500 MHz, 298 K) resonances of 4.1 (shifts in ppm).

Solvent α-H γ-CH3 m-HAr p-HAr Ipr-CH iPr-CH3

iPr-CH3

o-HBz m-HBz p-HBz

C6D6 117 55.4 –8.7 –76.3 –119 –17.7 –112 40.0 45.2 –44.3

C6D6/THF-d8

68.0 18.0 –1.61 –64.8 –93.0 –11.2 –82.7 20.3 42.3 –45.5

THF-d8 –27.9 –55.7 12.0 –43.6 –41.2 1.2 –25.4 –19.2 36.2 –50.1

Chapter 4

104

polymerisation was observed in vacuum line experiments with 4.1 in toluene-d8. 1H NMR spectroscopy did not show the formation of an ethene adduct. Only resonances of 4.1 and free ethene were observed. When a toluene solution of 4.1 was subjected to 10 bar ethene pressure in an autoclave, no ethene uptake was noted. Alkyl complex 4.2 was not tested under these conditions in view of its chemical similarity to 4.1. Monobenzyl complex 4.1 was also tested in an autoclave in the presence of MAO cocatalyst (Al/Fe = 500, PC2H4 = 10 bar, T = 50 °C and 100 °C). Again, significant ethene uptake was not observed.

In contrast to 4.1 and 4.2, which are devoid of β-hydrogens, related complexes that do contain β-hydrogens are reactive towards olefins. In an attempt to prepare the tertiary butyl complex [(BDK)FetBu], Holland et al. isolated the isobutyl derivative. It was established that the target t-butyl derivative is isomerised through reversible β-H-elimination followed by reinsertion of isobutene into a Fe–H intermediate.74 Apparently, olefin insertion into the Fe–C bond of the alkyls is impossible, but insertion into Fe–H species takes place readily in these systems. Consistently, the isobutyl complex [(BDK)FeCH2C(H)Me2] reacts with ethene to give the ethyl analogue with concomitant elimination of isobutene.74 This reaction is reminiscent of chain transfer in olefin polymerisation catalysts. The monoalkyls also transfer a β-hydrogen to C=N and C=O double bonds, yielding amido and alkoxy complexes.75 Closely related reactivity has been observed for the monoalkyls [(PDI)CoR].85 The difference in reactivity can be understood in terms of an intermediate hydride complex, resulting from β-H elimination. This hydride is susceptible to insertion of unsaturated molecules whereas insertion into the Fe–C bond of 4.1 of 4.2 is unfavourable.

4.5 Attempted generation of cationic iron(III) complexes [(BDK)FeR]+

It is well-known that many olefin polymerisation catalysts are only active in a cationic form while the corresponding neutral alkyl precursors are inert. However, in situ activation of the reported iron alkyls, e.g. by MAO does not result in catalytically active systems either. Therefore, it is relevant to investigate if synthesis of discrete cationic iron alkyls can be realised. These cationic alkyls would have two free orbitals available (10 V.E.), even in an S = 2 spin configuration.

4.5.1 Synthesis of a Fe(III) β-diketiminate complex An attractive method to generate monoalkyl cations [(BDK)FeR]+ would be alkyl abstraction from the neutral Fe(III) dialkyl precursors [(BDK)FeR2]. Since homoleptic iron trialkyls are unknown, alkane elimination is not a viable synthesis route for the preparation of Fe(III) dialkyls [(BDK)FeR2]. Therefore, the target dialkyls must be prepared by alkylation of the corresponding halide complexes. The Fe(III) dichloride [(BDK)FeCl2] (4.4) was prepared by a metathetical reaction of FeCl3 and Li[BDK] in THF (Scheme 4.3). The product was obtained as purple crystals after recrystallisation from hexane. Dichloride 4.4 is thermally stable in C6D6 solution. No noticable decomposition is observed after 24 hrs. at 373 K.

Iron(II) β-diketiminate complexes

105

The molecular structure of dichloride 4.4 was established by X-ray diffraction. The asymmetric unit consists of one molecule of the Fe(III) complex. The solid state structure of 4.4 is presented in Figure 4.7.‡‡ Selected bond lengths and angles are listed in Table 4.5.

‡‡ While this work was in progress, the synthesis of 4.3 (including IR, UV-vis data and X-ray structure determination) was published by Panda et al.3 While nearly identical unit cell parameters and IR absorptions were found, these authors report a green colour for crystals obtained from toluene.

1)n-BuLi2) FeCl3THF, RT N NAr Ar

Fe

Cl Cl

N NAr ArH

4.4

Scheme 4.3

Figure 4.7 Molecular structure of 4.4. Hydrogen atoms have been omitted for clarity.

Chapter 4

106

The X-ray structure shows the Fe(III) ion at the centre of a distorted tetrahedron. The angles between the ligating atoms deviate considerably from the ideal tetrahedral value, with the largest deviation caused by the N1-Fe-N2 bite angle of 95.66(10)°. The bite angle is slightly larger than in the Fe(II) dichloroferrate [(BDK)Fe(µ-Cl)2Li(THF)2] reported by Smith et al. (93.21(14)°), which reflects the higher oxidation state of the metal in 4.4. The difference in oxidation state is also expressed in the Fe–N and Fe–Cl bond lengths, which are about 0.1 Å shorter for the Fe(III) compound.67

Most striking is the out of-plane-coordination of the iron centre. The Fe(III) ion is located 0.298(1) Å out of the least squares plane through the chelate ring. This type of coordination has earlier been observed in β-diketiminate complexes of Sc(III), Al(III) and Mg(II).45,86,87 Following an argument presented by Hayes, this out-of-plane bonding is not the result of a trihapto coordination involving the C15 carbon (even though C15 resides 0.209(3) Å above the coordination plane), but is due to steric interactions between the N-aryl substituents and the two chlorides on the metal.88

4.5.2 Alkylation of Fe(III) β-diketiminate complex Preliminary ethene polymerisation tests with dichloride 4.4 with MAO cocatalyst (Al/Fe = 500, PC2H4 = 10 bar, T = 50 °C) showed no significant ethene uptake. Therefore, the alkylation of 4.4 was investigated in more detail. Reaction of 4.4 with two equivalents of PhCH2MgBr in diethylether results in a colour change from purple to yellow after addition of the first equivalent of the Grignard reagent, followed by a transition to red/brown upon addition of the second. After evaporation of the solvent, pentane extraction yields a bright orange solution. 1H NMR analysis of the extract showed the formation of the Fe(II) monobenzyl complex 4.1 (Scheme 4.4).

Table 4.5 Selected interatomic distances and angles for 4.4.

Distances (Å)

Fe-Cl1 2.2073(7) N1-C13 1.335(3)

Fe-Cl2 2.1812(8) C13-C15 1.414(5)

Fe-N1 1.975(3) C15-C16 1.389(6)

Fe-N2 1.946(2) N2-C16 1.337(3)

Angles (°)

Cl1-Fe-Cl2 117.11(3) Fe-N1-C13 116.7(2)

N1-Fe-N2 95.66(10) Fe-N1-C1 122.01(17)

Cl1-Fe-N1 106.65(6) C1-N1-C13 120.1(3)

Cl1-Fe-N2 107.25(7) Fe-N2-C16 116.0(2)

Cl2-Fe-N1 115.23(7) Fe-N2-C18 124.85(17)

Cl2-Fe-N2 112.60(7) C16-N2-C18 118.9(2)

Iron(II) β-diketiminate complexes

107

The coproducts bibenzyl, E-stilbene and toluene were detected in the extract by GC-MS analysis, indicating the involvement of free benzyl radicals. The initial colour change from purple to yellow suggests that reduction of Fe(III) to Fe(II) takes place prior to alkylation. The intermediate Fe(II) compound may well be the Et2O-analogue of the THF-solvate [Mg(THF)4][(BDK)Fe(µ-Cl)]2 reported by Smith et al.

67 Due to the intrinsic stability of the benzyl radical, reduction is often observed upon benzylation of transition metal complexes. Therefore, dialkylation was also attempted using LiCH2SiMe3 as the alkylating agent. Reaction of 4.4 with two equivalents of LiCH2SiMe3 in pentane resulted in an instantaneous colour change from purple to yellow with concomitant salt precipitation. After filtration and solvent removal, a yellow solid was obtained, which was identified by 1H NMR spectroscopy as the monoalkyl Fe(II) complex [(BDK)FeCH2SiMe3)] (4.2). The organic byproducts SiMe4 and Me3SiCH2CH2SiMe3 were detected by GC-MS analysis in a separate NMR tube experiment in C6D6. It seems likely that these products arise from free Me3SiCH2 radicals by hydrogen abstraction from the solvent or ligand and by radical recombination, respectively.

The rapid homolysis of the presumed Fe(III) alkyl intermediates contrasts with the corresponding Fe(II) monoalkyls which are stable towards homolysis. Reduction of Fe(III) to Fe(II) by alkyllithium, dialkylzinc or Grignard reagents has also been observed upon attempted alkylation of the Fe(III) guanidinate complex [{HN(iPr)C(NiPr)2}2FeCl].

89,90 In this context, it is noted that in the case of the iron pyridine-2,6-diketimine / MAO system for olefin polymerisation, nearly equivalent activities are observed for Fe(II) and Fe(III) precatalysts.91

4.5.3 Exploratory monooxidation experiments with the β-diketiminate Fe(II) monobenzyl complex

As alkylation of the β-diketiminate Fe(III) complex [(BDK)FeCl2] (4.4) gave reduction to the corresponding Fe(II) monoalkyls, the desired dialkyls [(BDK)FeR2] could not be obtained. Hence, preparation of Fe(II) monoalkyl cations by alkyl abstraction from the dialkyls is not possible. An alternative route towards Fe(III) alkyl cations seems direct oxidation of neutral Fe(II) alkyl complexes to the Fe(III) monoalkyl cations [(BDK)FeR]+. Oxidation of β-diketiminate-supported Fe(II) alkoxide and amide complexes by Ag(OSO2CF3) to the corresponding Fe(III) complexes was recently reported by Eckert et al.92

One-electron oxidation of the monoalkyls 4.1 and 4.2 was explored in NMR tube experiments. Reaction of 4.1 with Ag(OSO2CF3) in THF-d8 leads to immediate precipitation of a dark solid, presumably Ag(0), and the formation of a yellow solution. According to 1H NMR spectroscopy, no clean reaction takes place, but the major species formed shows ligand resonances in the frequency range typical for

N NAr Ar

Fe

Cl Cl

N NAr Ar

Fe

R

2 RCH2M

4.4 4.1 (M = MgBr, R = Ph) 4.2 (M = Li, R = SiMe3)

Scheme 4.4

Chapter 4

108

paramagnetic (BDK)Fe(II) complexes in THF-d8. GC-MS analysis shows formation of bibenzyl and toluene (not quantified). The byproducts are derived from free benzyl radicals, which suggests that initial oxidation of the iron centre is followed by rapid homolysis of the Fe–C bond.

Similarly, oxidation of 4.1 with [Cp2Fe][BPh4] under the same conditions results in a colour change from red to dark yellow. The 1H NMR spectrum shows resonances characteristic of the paramagnetic species obtained earlier from silver triflate oxidation. In addition, the spectrum indicates the formation of ferrocene and bibenzyl. It seems likely that also in this experiment, the end product is an alkyl free cationic Fe(II) β-diketiminate complex rather than the target cationic Fe(III) monoalkyl. Since metal benzyl bonds are usually more susceptible to homolysis than other alkyls, (trimethylsilyl)methyl complex 4.2 was also subjected to oxidation with [Cp2Fe][BPh4]. Also this reaction leads to the same paramagnetic species with concomitant formation of ferrocene and the alkyl coupling product Me3SiCH2CH2SiMe3 as detected by GC-MS.

These preliminary investigations indicate that one electron oxidation of Fe(II) alkyls is not a suitable pathway to generate cationic iron alkyl species. The final product is not the desired Fe(III) monoalkyl cation but rather an Fe(II) cation resulting from Fe–C homolysis of an apparently unstable Fe(III) monoalkyl that has been formed initially. Triflate, tetraphenylborate, nor THF is strong enough a donor to stabilise the tentative Fe(III) alkyl cation. This situation for Fe(II) contrasts with the one-electron oxidation of Ti(III) dialkyl [(BDK)TiR2] (R = CH2tBu) by [Cp*2Fe][B(C6F5)4] to the Ti(IV) dialkyl cation [(BDK)TiR2][B(C6F5)4].

93

4.6 Concluding remarks The use of β-diketiminate ligand BDK allows the formation of stable Fe(II) complexes which can conveniently be alkylated without concomitant ligand redistribution. The steric protection by the diisopropylphenyl substituents on the nitrogens results in the isolation of very stable three-coordinated Fe(II) alkyls formally bearing only 12 valence electrons. Attempts to generate cationic Fe(II) alkyls stabilised by BDK were not successful. Dialkylation of [(BDK)FeCl2] was impeded by by reduction to the corresponding monoalkyls [(BDK)FeR]. One-electron oxidation of [(BDK)FeCH2Ph] led to homolysis of the Fe–C bond.

4.7 Experimental

FeArAr

NN

R

Ag[OSO2CF3]or[Cp2Fe][BPh4]THF-d8

THF-d8

FeArAr

NN

[OSO2CF3]-

or [BPh4]-

+ RCH2CH2R

R = Ph (4.1) R = SiMe3 (4.2) Ar = 2,6-iPr2C6H3

Scheme 4.5

Iron(II) β-diketiminate complexes

109

General considerations and instrumentation

See Section 2.8.

Starting materials

[BDK]H61, LiCH2SiMe3,94 [Cp2Fe][BPh4]

95 FeCl296 and FeCl2.(THF)1.5

97 were prepared by literature procedures. FeCl3 (Acros), acetylacetone and 2,6-diisopropylaniline were commercial products and were used without further purification.

Preparation of [(BDK)Fe(CH2Ph)] . 0.5 C5H12 (4.1)

n-BuLi (2.5 M in hexanes) (3.8 mL, 9.6 mmol) was added to a stirred solution of [BDK]H (4.0 g, 9.6 mmol) in THF (30 mL). The solution was stirred for 30 min. at room temperature. FeCl2 (1.21 g, 9.6 mmol) was added and the resulting yellow suspension was stirred for 18 hrs. PhCH2MgBr (1.43 M in Et2O) (6.7 mL, 9.6 mmol) was added. The solution turned red immediately and was stirred for 4 hrs. at room temperature. THF was evaporated under reduced pressure. Residual THF was removed by suspending the dark residue in pentane (30 mL, 3x) followed by removal of pentane in vacuo. The residue was extracted with pentane (100 mL, 4x). The extract was filtered, concentrated to ca. 75% and stored at –25 ˚C for 48 hrs. to give red crystals. Yield: 4.1 g (71%). 1H NMR (500 MHz, C6D6, RT,) δ (∆ν½, integral, assignment) 117.2 (787 Hz, 1H, α-H), 56.4 (349 Hz, 6H, γ-Me), 45.1 (115 Hz, H, m-HBz), 40.4 (1238 Hz, 2H, o-HBz), –8.7 (55 Hz, 4H, m-HAr), –17.6 (102 Hz, 12H, iPr-Me), –43.6 (60 Hz, 1H, p-HBz), –75.8 (77 Hz, 2H, p-HAr), –112.3 (651 Hz, 12H, iPr-Me), –119.4 (1302 Hz, 4H, iPr-CH) ppm. 1H NMR (500 MHz, THF-d8, RT) δ (∆ν½, integral, assignment) 36.2 (80 Hz, 2H, m-HPh), 12.0 (55 Hz, 4H, m-HAr), 1.2 (98, 12H, iPr-CH3), –19.2 (623, 2H, o-HPh), –25.4 (234, 12H, iPr-CH3), –27.9 (546, 1H, α-H), –41.2 (1212, 4H, iPr-CH), –43.6 (58.2, 2H, p-HAr), –50.1 (65.5, 1H, p-HPh), –55.7 (176, 6H, γ-Me). IR (KBr, nujol mull) ν~ = 3057, 3013, 1594(m), 1524(s), 1484(m), 1437, 1318, 1261(s), 1203, 1176, 1099, 1056, 1029, 1021, 992, 934(m), 859(w), 794, 758, 743, 695(m), 579, 521(w), 438(m) cm-1. µeff. (C6H6, 298 K) = 5.6 µB. Anal. C36H48N2Fe.0.5C5H12 (600.71): calcd. C 76.98, H 9.06, N 4.66, Fe 9.30, found C 77.47, H 9.31, N 4.74, Fe 9.00.

Preparation of [(BDK)FeCH2SiMe3] (4.2)

nBuLi (2.5 M in hexanes, 1.67 mL, 4.18 mmol) was added to a stirred solution of the β-diketimine (1.75 g, 4.18 mmol) in THF (20 mL). The pale yellow solution was stirred for 30 min. FeCl2(THF)1.5 (982 mg, 4.28 mmol) was added as a solid. The resulting yellow solution was stirred for 45 min. LiCH2SiMe3 (394 mg, 4.18 mmol) was added as a solid and a very dark solution was obtained. After 30 min. the solution had become dark red. After another 15 min. stirring, all THF was removed under reduced pressure. Any residual THF was removed by stirring the residue with pentane (25 mL) and subsequent evaporation of all volatiles (3x). Next, the residue was extracted with pentane (30 mL, 2x) to give an orange extract. The extract was concentrated to ca. 10 mL and cooled to –25 °C to afford 4.2 as yellow crystals. Yield: 624 mg (27%). 1H NMR (500 MHz, C6D6, RT) δ (∆ν½, integral, assignment) = 115.5 (720 Hz, 1H, α-CH), 74.7 (350 Hz, 6H, γ-CH3), 56.1 (396 Hz, 9H, Si(CH3)3), –9.0 (50 Hz, 4H, m-H), –14.5 (101 Hz, 12H, iPr-CH3), –68.5 (65 Hz, 2H, p-H), –100.4 (530 Hz, 12H, iPr-CH3), –126.9 (1766 Hz, 4H, iPr-CH) ppm. 1H NMR (500 MHz, THF-d8, RT) δ (∆ν½, integral,

Chapter 4

110

assignment) = 51.4 (587 Hz, 1H, α-CH), 41.5 (300 Hz, 9H, Si(CH3)3), 17.0 (266 Hz, 6H, γ-CH3), 0.0 (27 Hz, 4H, m-H), –7.2 (87 Hz, 12H, iPr-CH3), –55.9 (56 Hz, 2H, p-H), –66.1 (382 Hz, 12H, iPr-CH3), –88.1 (1501 Hz, 4H, iPr-CH) ppm. IR (KBr, nujol mull) ν~ = 2909(s), 1526(s), 1317(s), 1289(m), 1252(m), 1239(s), 1175(m), 1099(m), 1023(w), 934(m), 917(m), 904(m), 850(m), 818(m), 794(m), 755(s), 743(w), 726(m), 498(w), 468(w), 435(w) cm-1. µeff. (C6H6, 298 K) = 5.6 µB. Anal. C33H52N2FeSi (560.72): calcd. C 70.69, H 9.35, N 5.00, found C 70.82, H 9.26, N 5.06.

Generation of 4.1 from 4.4 and PhCH2MgBr

A 1.43 M solution of PhCH2MgBr in Et2O (0.38 mL, 0.55 mm) was added to a stirred solution of 4.4 (150 mg, 0.28 mmol) in Et2O (20 mL). The solution changed colour from purple to red/brown via yellow. The solution was stirred at room temperature for 30 min. Et2O was removed under reduced pressure and the residue was stirred in pentane (20 mL) and evaporated to dryness to remove any residual Et2O (2x). The residue was extracted with pentane (20 mL, 2x) to give a bright orange extract, leaving a grey salt. Pentane was removed in vacuo giving a dark red oil. The iron complex was not purified by crystallisation but the oil was directly dissolved in C6D6 for identification of the products by 1H NMR spectroscopy and GC-MS analysis.

Reaction of 4.1 with THF-d8

A few drops of THF-d8 (excess) were added to a solution of 4.1 (20 mg, 33 µmol) in C6D6 (0.5 mL). The colour of the solution changed from red to dark orange.

1H NMR (500 MHz, C6D6, RT) δ (∆ν½, integral, assignment) = 68.0 (667 Hz, 1H, α-H), 42.3 (95 Hz, 2H, m-HBz), 20.3 (1071 Hz, 2H, o-HBz), 18.0 (323 Hz, 6H, γ-Me), –1.6 (47 Hz, 4H, m-HAr), –11.2 (89 Hz, 12H, iPr-Me), –45.5 (51 Hz, 1H, p-HBz), –64.8 (64 Hz, 2H, p-HAr), –82.7 (515 Hz, 12H, iPr-Me), –93.0 (1547 Hz, 4H, iPr-CH) ppm.

Reaction of 4.1 with Ag[OSO2CF3]

Fe benzyl complex 4.1 (20 mg, 33 µmol) was dissolved in ca. 0.5 mL THF-d8. Ag[OSO2CF3] (8.6 mg, 33 µmol) was added to this solution. A black solid precipitated from the yellow solution. 1H NMR (300 MHz, THF-d8, RT) data for major paramagnetic species: δ(∆ν½,) = 19.4 (60, 4H, m-HAr), 5.1 (146, 12H, iPr-Me), –10.1 (121, 12H, iPr-Me), –38.4 (77, 6H, γ-Me) ppm. 19F NMR (188 MHz, THF-d8, RT) δ(∆ν½,) = 7.0 (259 Hz), -10.5 (44 Hz) (integral ratio 66 : 34) ppm.

Reaction of 4.1 with [Cp2Fe][BPh4]

Ferrocinium tetraphenylborate (18.5 mg, 37 µmol) was added to a solution of 4.2 (22 mg, 37 µmol) in 0.5 mL THF-d8. A dark yellow solution was formed.

1H NMR (300 MHz, THF-d8, RT) data for major paramagnetic species δ = 21.2 (49, 4H, m-HAr), –8.6 (155, 12H, iPr-Me), –38.2 (49, 2H, p-HAr), –60.2 (131, 6H, γ-Me), –93.7 (345, 1H, α-H) ppm.

Reaction of 4.2 with [Cp2Fe][BPh4]

Iron(II) β-diketiminate complexes

111

Ferrocinium tetraphenylborate (42.4 mg, 83.8 µmol) was added to a solution of 4.2 (47 mg, 83.8 µmol) in 0.5 mL THF-d8. A dark green solution was formed.

1H NMR (300 MHz, THF-d8, RT) data for major paramagnetic species δ = 21.1 (99, 4H, m-HAr), –8.9 (211, 12H, iPr-Me), –38.6 (100, 2H, p-HAr), –60.8 (191, 6H, γ-Me), –94.4 (412, 1H, α-H) ppm.

Preparation of 4.2 from 4.4 and LiCH2SiMe3

A solution of LiCH2SiMe3 (100 mg, 1.07 mmol) in pentane (5 mL) was added to a stirred suspension of 4.4 (290 mg, 0.53 mmol) in pentane (10 mL). The reaction mixture was stirred at room temperature for 30 min. while the purple suspension turned yellow and a white salt precipitated. The finely divided LiCl was allowed to settle for 18 hrs at –25 °C and was filtered off. The residue was extracted once more with pentane (10 mL). The volume was concentrated to ca. 50% and crystallisation at –30 °C afforded 4.2 as yellow needles. Yield: 52 mg (19%).

Preparation of [(BDK)Fe(CH2Ph)(py)] |(4.3)

Monobenzyl (4.1) (200 mg, 0.35 mmol) was dissolved in toluene (5 ml). Pyridine (1 mL, 12 mmol) was allowed to diffuse into the solution. Dark red crystals of composition [(BDK)Fe(CH2Ph)(py)].toluene were formed in the toluene solution after 16 hours. The crystals (69 mg) were isolated by filtration and dried in vacuo. A second crop (75 mg) of spectroscopically identical crystals was obtained by concentration of the filtrate. Combined yield: 144 mg (0.20 mmol, 57 %). 1H NMR (500 MHz, PhMe-d8, –50 °C): δ (∆ν½, integral, assignment) = 58.2 (107 Hz, 1H, α-H), 50.9 (249 Hz, 2H, m-HPh), 48.1 (329 Hz, 1H, m-Hpy), 33.6 (292 Hz, 1H, m-Hpy), 27.5 (130 Hz, 2H, m-HAr), 21.1 (143 Hz, 2H, m-HAr), 14.6 (312 Hz, 6H, iPr-CH3), –2.8 (137 Hz, 6H, iPr-CH3), –8.9 (427 Hz, 6H, iPr-CH3), –18.6 (693 Hz, 6H, iPr-CH3), –24.7 (1784 Hz, 1H, o-Hpy), –41.3 (2345 Hz, 1H, o-Hpy), –60.3 (146 Hz, 2H, p-HAr), –76.2 (149 Hz, 1H, p-HPh), –103.1 (594 Hz, 1H, p-Hpy ), –125.7 (478 Hz, 6H, γ-CH3) ppm. IR (KBr, nujol) ν~ = 3104 (w), 3058, 3026 (m), 1925, 1862 (m), 1800 (w), 1591 (m), 1514 (s), 1454, 1314 (s), 1277 (m), 1254 (s), 1224, 1208 (m), 1173 (s), 1154, 1099, 1057, 1039, 1025, 1004 (m), 966 (s), 934 (m), 900, 883 (w), 845, 790(m), 751, 729, 699 (s), 628, 574 (w), 556, 528 (m), 464 (w), 426 (m) cm–1. µeff. (C6H6, 298 K) = 4.4 µB. Anal. C41H53N3Fe (643.74) calcd. C 76.50, H 8.30, N 6.53, found C 76.68, H 8.35, N 6.42. Although 1H NMR spectroscopy in C6D6 showed the presence of toluene in the crystals, elemental analysis was satisfactory for the solvent-free complex, suggesting slow loss of toluene from the crystal lattice.

Preparation of [(BDK)FeCl2] (4.4)

A 2.5 M solution of nBuLi in hexanes (1.13 mL, 2.83 mmol) was added to a solution of [BDK]H (1.17 g, 2.80 mmol) in 20 mL THF. The mixture was stirred at RT for 30 min. This solution was added to a solution of FeCl3 (455 mg, 2.81 mmol) in 20 mL THF. Immediately, a green solution was formed. After stirring for 1 hr at RT, THF was evaporated under reduced pressure. The residue was taken up in diethylether (30 mL, 3x) and evaporated to dryness to remove any residual THF. The residue was extracted with diethylether (40 mL, 4x) to give a purple extract, which was filtered and concentrated to ca 30 mL. Cooling to –25 °C yielded dark crystals. A second crop of

Chapter 4

112

spectroscopically identical material was obtained by further concentration of the filtrate and cooling to –25 °C. Total yield: 650 mg (43 %). 1H NMR (300 MHz, C6D6, RT) δ (∆ν½) 66.9 (805), 7.65 (1625), 6.51 (548), –57.3 (993), –62.8 (1510) ppm (Hz). IR (KBr, nujol mull) ν~ = 3059, 1879 1711(w), 1522(m), 1453, 1336, 1316, 1271, 1256, 1179, 1167(s), 1107, 1100, 1056, 1043, 1019(m), 976, 954(w), 934(s), 909, 863(m), 795, 758(s), 724, 708, 637, 622, 526(w), 407(m) cm-1. Anal. C29H41N2FeCl2 (544.39): calcd C 63.98, H 7.59, N 5.15, found C 63.49, H 7.68, N 5.05.

Ethene Polymerisation tests with 4.1 and 4.4

See Section 2.8 for experimental details. The experiments with 4.1 were performed both at 50 °C and 100 °C (with and without cocatalyst). In a separate experiment (50 °C), 4.1 was reacted with the MAO solution before injection. The latter protocol was also used for testing 4.4. In all cases, no significant ethene uptake or temperature effects were observed.

Crystal structure deteminations

4.1: Suitable crystals were obtained by recrystallisation from pentane. From the solution it was clear that both pentane solvent molecules were located over an inversion centre, implying disorder. A disorder model with a s.o.f. of .5 for both pentanes was used in the refinement. Positional and isotropic displacement parameters of all H atoms were refined freely, except those belonging to disordered pentanes. These hydrogen atoms were included in the final refinement riding on their carrier atoms with their positions calculated by using sp3 hybridization at the C-atom as appropriate with Uiso = c x Uequiv of their parent atom, where c = 1.2 for non-methyl hydrogen atoms and c = 1.5 for the methyl hydrogen atoms and where values Uequiv are related to the atoms to which the H atoms are bonded. The methyl-groups were refined as rigid groups, which were allowed to rotate freely. No classic hydrogen bonds, no missed symmetry (MISSYM) or solvent-accessible voids were detected by procedures implemented in PLATON.98,99

4.2: Suitable crystals were obtained by recrystallisation from pentane. No classic hydrogen bonds, no missed symmetry (MISSYM), but potential solvent-accessible area (voids of 29.2 Å3 / unit cell) were detected by procedures implemented in PLATON.98,99

4.3: Suitable crystals were obtained by crystallisation from toluene. No classic hydrogen bonds, no missed symmetry (MISSYM) or solvent-accessible voids were detected by procedures implemented in PLATON.98,99

4.4: Suitable crystals were obtained by recrystallisation from diethyl ether. No classic hydrogen bonds, no missed symmetry (MISSYM) or solvent-accessible voids were detected by procedures implemented in PLATON.98,99

Iron(II) β-diketiminate complexes

113

Table 4.6 Crystal, collection and refinement data for complexes 4.1 – 4.4.

4.1 4.2 4.3 4.4

formula 2C36H48N2Fe

·C5H12

C33H52N2FeSi C41H53N3Fe

·C7H8

C29H41N2FeCl2

fw 1201.37 560.72 735.88 544.39

cryst. dim. (mm) 0.45 x 0.40 x 0.12

0.28 x 0.21 x 0.12

0.45 x 0.38 x 0.065

0.22 x 0.19 x 0.11

colour, habit red, platelet orange, block red, platelet purple, block

crystal system monoclinic monoclinic Triclinic monoclinic

space group, no.100 P21/c, 14 P21/n, 14 P-1, 2 P21/n, 14

a (Å) 15.3319(9) 10.5957(5) 9.2518(8) 12.5407(8)

b (Å) 20.652(1) 21.292(1) 12.091(1) 19.581(1)

c (Å) 22.408(1) 14.7768(7 19.640(2) 13.1202(8)

α (°) 90 90 82.446(1) 90

β (°) 90.781(1) 97.218(1) 82.344(1) 117.184(1)

γ (°) 90 90 84.784(1) 90

Z 4 4 2 4

V (Å3) 7094.5(6) 3307.3(3) 2152.5(3) 2865.9(3)

ρcalc (g/cm3) 1.125 1.126 1.135 1.262

θ range (°) 2.38 – 29.75 2.36 – 28.28 2.55 – 26.37 2.72 – 26.02

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

T (K) 125(1) 100(1) 100(1) 100(1)

data collect. time (h) 7.95 8.0 13.0 13.0

no. of meas. refl. 44924 30277 17162 22276

no. of unique refl. 18149 8215 8581 5632

µ(Mo Kα) (cm-1) 4.52 5.14 3.85 7.32

no. of parameters 1181 539 713 471

weighting scheme: a,b[a]

0.0612, 0.8899 0.0568, 0.0 0.0431, 0.2791 0.0407, 0.0

R(F) for F0 ≥ 4σ(F0)[b] 0.0430 0.0392 0.0414 0.0447

wR(F2)[c] 0.1134 0.0997 0.0985 0.0989

res. el. dens. (e/Å3) –0.24, 0.52(6) –0.22, 0.43(6) –0.27, 0.34(5) –0.29, 0.58(8)

GoF[d] 1.022 1.024 1.029 1.004

[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 4

114

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