Electron deficient organoiron(II) complexes of amidinates and betha-diketiminates Sciarone, Timotheus
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2005
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Sciarone, T. (2005). Electron deficient organoiron(II) complexes of amidinates and betha-diketiminates. s.n.
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5 Iron(II) (2-pyridyl)- β -diketimine complexes
In the preceding chapters, monoanionic nitrogen-based ligands were used to stabilise iron(II) alkyls. The choice for anionic ligands has allowed the isolation of very electron deficient Fe(II) monoalkyls. Nevertheless, in spite of their electronic unsaturation, the neutral monoalkyls proved to be inactive for polymerisation of ethene. Thus far, the pyridine-2,6-diimine (PDI) ligand provides the only platform giving significant polymerisation activities in combination with iron (see Section 1.2).1 The PDI ligand has been modified extensively to study structure-activity relations.
Most structural variations have involved changes in the aryls on the imine nitrogens, the central donor atom and the imine donors (for an overview, see section 1.2).
These studies demonstrate that in order to achieve good activities in Fe-catalysed polymerisation, the PDI ligand allows only limited possibilities for variation. In general, changes in the aryl groups on the imines influence the molecular weight of the polymer, but any modification of the basic pyridine-diimine skeleton leads to significant loss of activity.
Successful variations have inherited from the PDI ligand the meridional arrrangement of the donor atoms. This appears to be a key feature for the ligand and suggests that conjugation in the extended π-system of the PDI ligand may be an important factor for activity. Alkylation experiments with PDI complexes of metals other than Fe have shown that the PDI ligand is able to accommodate electrons upon formal reduction of the metal centre.2-7 The redox-activity of the ligand can also be related to the conjugated nature of the PDI skeleton.
To test the importance of the spatial distribution of the donors, a new ligand was developed which places the donor atoms in a facial coordination environment. This podal ligand, PyBDKH (Figure 5.1), is based on a pyridyl-functionalised β-diketimine.
Facial coordination is expected on the basis of the sp3-hybridisation of the central carbon atom. Important characteristics of the PDI ligand like the ligating functionalities and the sterically demanding aryls on the imine nitrogens are conserved in the PyBDKH ligand. Its electronic properties however, are expected to be drastically different due to the fact that conjugation between the pyridine and imine donors is absent in PyBDKH. Due to the anticipated facial coordination, the steric shielding provided by the aryl groups is also spatially different from the steric protection in the meridional PDI metal complexes.
N N N1 2 H
3 4 5 6
X N
Fe N N Ar
H
N N
N Fe
X X Ar
5.1 Neutral ligands with tris(ketimino)methane structures
The tris(ketimino)methane structure in PyBDKH is found in several tris(N- heterocycle)methane ligands (heterocycle = pyridin-2-yl, N-methylimidazol-2-yl).8 With Fe(II), this type of tripodal ligands usually forms six-coordinated 2:1 complexes [L2Fe]2+ (Figure 5.2).9 The sterically demanding aryl substituents in PyBDKH are expected to disfavour the formation of 2:1 complexes.
Certain octahedral Fe(II) complexes supported by N4-ligands incorporating β- diketimine moieties are known to insert coordinated acetonitrile ligands to form tris(ketimino)methane structures (Figure 5.3). 1011-13
N N N
H
N N
N H
Fe
2 PF6-
Figure 5.2
5.2 Ligand synthesis
In principle, target ligand PyBDKH can be envisaged as the coupling product of a pyridyl moiety and a sterically hindered β-diketimine. Alkylation of the corresponding β-diketimine at the central carbon atom was recently reported by reaction of β- diketiminate anions with alkyl halides.14,15 The steric bulk on the imines protects the nitrogen atoms from side reactions. Nucleophilic substitution of 2-bromopyridine with nucleophiles containing a β-diiminate skeleton also has some precedent in literature.
Thus, tris(2-pyridyl)methane was synthesised by Osuch and Levine by reaction of 2- picolyllithium with 2-bromopyridine.16 Byers et al. have prepared a tripodal ligand by addition of 2-bromopyridine to deprotonated bis(N-methylimidazol-2-yl)methane.8 However, synthesis of PyBDKH by nucleophilic aromatic substitution of 2- bromopyridine by the lithiated β-diketiminate was unsuccessful. Only starting materials were recovered (Scheme 5.1). Apparently, the β-diketiminate anion is a weaker nucleophile than those employed in the two literature examples mentioned above. This may be related to the fact that in those cases the β-diimine skeleton contains two heterocyclic aromatic rings, whereas the methylene group in the β- diketimine connects two isolated imines. In the anions of the former class, the
N N
N Fe C NH CH3
H
N
N C CH3 2+
N
N N
N Fe
N C CH3
N C CH3
2+
cat. base
cat. base
N N
N N
Fe N C CH3
N C CH3
2+
N N
N N
H
H N N
Fe
2+
Figure 5.3
An alternative route to PyBDKH is based on introduction of the two imine functions in separate steps17 and incorporation of the pyridyl moiety prior to imine formation. The full synthesis route is outlined in Scheme 5.2.
In the first step, 2-methylpicoline is acylated by N,N-dimethylacetamide to give 1-(2- pyridyl)acetone,18,19 which is subsequently condensed with one equivalent of 2,6- diisopropylaniline to the corresponding monoimine. This reaction yields a mixture of the monoimine and the tautomeric enamine. Upon deprotonation both products give the same azaallyl anion which is coupled with the imidoylchloride.17 The latter is obtained from 2,6-diisopropylphenylacetamide and phosphorus pentachloride.20 The overall yield of PyBDKH is ca. 36 %.
The 1H NMR spectrum of PyBDKH in CDCl3 shows only a single septet for the isopropyl methine protons and a resonance at 13.4 ppm with an integral of 1H. The
1H NMR spectrum suggests that the ligand is fully present as the enamine tautomer.
N N
Ar Ar Li
THF PhLi
THF
Ar Ar
N N N H N
Br
N N
Ar Ar H
Scheme 5.1
N H
N N
Ar Ar
N
1. nBuLi
2. CH3CONMe2 N
O ArNH2
O ArHN
PCl5
Cl ArN
N Ar N
Cl ArN 1. nBuLi 2.
N Ar N
N Ar NH
Ar = 2,6-iPr2C6H3
Scheme 5.2
This is confirmed in the 13C NMR spectrum, which exhibits a resonance for the central pyridyl-substituted carbon at 104.5 ppm, indicating sp2-hybridisation. The absence of a proton from this carbon atom was confirmed by an APT-NMR experiment.
5.3 Complexation to iron(II)
Complexation of PyBDKH to ferrous chloride takes place readily in THF, yielding [(κ3-
PyBDKH)FeCl2] (5.1). The product is modestly soluble in THF or dichloromethane.
Yellow crystals, obtained from concentrated solutions in these solvents, incorporate solvent molecules which are readily lost in vacuo or upon drying in a nitrogen stream.
The resulting yellow powder is stable in air.
The molecular structure of 5.1 was established by an X-ray diffraction study (Figure 5.5). Selected bond lengths and bond angles are given in Table 5.1.
N H
N N
Ar Ar N N
Ar Ar
N
H Ar = 2,6-iPr2C6H3
Figure 5.4 Imine-enamine tautomerism of PyBDKH
+ FeCl2 THF RT
Ar Ar
N N N H
Ar Ar N N
N H
Fe Cl Cl
Ar = 2,6-iPr2C6H3 5.1
Scheme 5.3
The asymmetric unit contains one iron complex and two dichloromethane solvent molecules. The X-ray structure confirms the tridentate coordination mode of the ligand. The six-membered diketimine chelate ring adopts a boat conformation. The aryl rings make angles of 87.0(3)° and 83.0(3)° with the planar N=C–Me-moieties.
The geometry around Fe is best described as a distorted trigonal bipyramid with Cl1 and the pyridine nitrogen occupying the apical positions (N3–Fe–Cl1 = 169.61(5)°).
The equatorial plane is defined by N1, N2 and Cl2, the sum of the angles around Fe equalling 356.84°.
Figure 5.5 Molecular structure of 5.1. Hydrogen atoms and cocrystallised CH2Cl2 molecules have been omitted for clarity.
The Fe-N bond lengths range from 2.163(2) (Fe–N1) to 2.282(2)° (Fe–N3) which is longer than the average Fe–N distance in [(HC(2-py)3)2Fe][NO3]2 (1.949(3) Å), probably reflecting the dicationic nature of the latter complex.9 Inspection of the bond lengths of the diketimine chelate ring in 5.1 shows localised C=N bonds (1.277 Å) and normal C–C bond lengths (1.521(3) – 1.526(3) Å). These distances are comparable to those in the β-diketimine complex [{H2C(CMeNAr)2}NiBr2] (Ar = 2,6- iPr2C6H3).21 The N1–Fe–N2 diketimine bite angle of 85.97(7)° is considerably smaller than 120° for an ideal trigonal bipyramid due to the geometrical constraints of the ligand. This value is also substantially lower than the 93.7(2)° reported for the bidentate Ni(II) β-diketimine complex.21 Dichloride 5.1 can be regarded as the facial analogue of the meridional complex [(PDI)FeCl2]. In contrast to the distorted trigonal bipyramidal coordination geometry observed for 5.1, the meridional coordination of the PDI ligand results in a distorted square pyramidal geometry with one of the chlorides in the apical position.22 When compared to 5.1, [(PDI)FeCl2] shows slightly longer imine-iron bonds (2.238(4) and 2.250(4) Å) whereas the pyridine-iron bond (2.088(4) Å) is much shorter than in 5.1.
Table 5.1 Selected interatomic distances and angles for 5.1.
Distances (Å)
Fe-Cl1 2.3247(7) N1-C13 1.277(3)
Fe-Cl2 2.3306(7) N2-C16 1.277(3)
Fe-N1 2.163(2) C13-C15 1.521(3)
Fe-N2 2.196(2) C15-C16 1.526(3)
Fe-N3 2.282(2) C15-C30 1.524(3)
Angles (°)
Cl1-Fe-Cl2 93.71(2) C13-C15-C16 117.1(2) Cl1-Fe-N1 104.23(6) C13-C15-C30 108.00(19) Cl1-Fe-N2 91.68(6) C16-C15-C30 108.48(19) Cl1-Fe-N3 169.61(5) Fe-N1-C1 121.34(15) Cl2-Fe-N1 123.07(6) Fe-N1-C13 119.81(16) Cl2-Fe-N2 147.80(6) C1-N1-C13 118.3(2) Cl2-Fe-N3 88.71(5) Fe-N2-C16 118.86(16) N1-Fe-N2 85.97(7) Fe-N2-C18 121.68(15) N1-Fe-N3 82.74(7) C16-N2-C18 118.1(2) N2-Fe-N3 81.01(7)
Magnetic susceptibility measurements using a SQuID magnetometer indicate a high- spin configuration (µeff. = 5.2 µB, µs.o. = 4.9 for S = 2) for 5.1 at room temperature.
Consistently, the room temperature 1H NMR spectrum of 5.1 in CD2Cl2 (Figure 5.6) shows paramagnetically shifted resonances between +110 and –45 ppm. The spectrum exhibits a complicated pattern and the number of signals is much higher than expected from the nearly Cs-symmetric solid state structure (15 signals expected for Cs-symmetry). Heating 5.1 in tetrachloroethane-d2 solution, results in gradual coalescence of some signals, suggesting that the high number of resonances at room temperature results from a dynamic process that is slow on the NMR timescale. At 120 °C, the fast-exchange limit is still not reached and the complexity of the spectrum precludes unambiguous assignment of the resonances.
Dichloride 5.1 was tested as precursor for ethene polymerisation in toluene with MAO cocatalyst (Al/Fe = 500, T = 50 °C, PC2H4 = 10 bar). No significant ethene uptake was observed. In the absence of ethene, 5.1 reacts with MAO solution in toluene to give a black precipitate, suggesting reduction to Fe(0). The complex does not react with carbon monoxide (ca. 1 bar) in THF-d8 solution at room temperature.
5.4 Alkylation
Alkylation of 5.1 was attempted with two equivalents of LiCH2SiMe3 in THF. After workup and recrystallisation from pentane, the monoalkyl [{PyBDK}FeCH2SiMe3] (5.2) was obtained in 64% yield as orange crystals.
ppm -40
-20 0
20 40
60 80
100 120
Figure 5.6 1H NMR spectrum of 5.1 (500 MHz, CD2Cl2, RT)
Monoalkyl 5.2 clearly results from deprotonation of the central α-proton by the strongly basic alkylating agent. A separate NMR tube experiment in C6D6 confirmed the formation of tetramethylsilane. If only one equivalent of LiCH2SiMe3 was used, tetramethylsilane was also formed in the NMR tube, with a concomitant colour change from yellow to red. This experiment suggests that proton abstraction takes place prior to alkylation. Deprotonation converts the neutral (2-pyridyl)-β-diketimine into a monoanionic (2-pyridyl)-β-diketiminate ligand. The resulting sp2-hybridisation of C15 precludes further coordination of the pyridyl moiety as it is directed away from the metal centre. Alkylation of the β-diketiminate complex is eventually effected by the second equivalent of LiCH2SiMe3.
The formation of a β-diketiminate iron monoalkyl can explain why 5.1 shows no activity in ethene polymerisation. Reaction of 5.1 with MAO or free AlMe3 is likely to yield [{PyBDK}FeMe] as the initial product. From Chapter 4 it has become clear that β- diketiminate-supported Fe(II) monoalkyls are not active in olefin polymerisation.
The molecular structure of 5.2 was elucidated by X-ray diffraction (Figure 5.7).
Pertinent bond lengths and bond angles are listed in Table 5.2.
2 LiCH2SiMe3 Ar THF
Ar
N N N H
Fe Cl Cl
+ SiMe4
N N
Ar Ar
Fe
SiMe3 N
5.1 5.2
Ar = 2,6-iPr2C6H3
Scheme 5.4
The asymmetric unit consists of one molecule of the iron alkyl complex. The iron centre is chelated by the two β-diketiminate nitrogens, leaving the pyridyl group uncoordinated. The pyridyl ring makes an angle of 83.311(19)° with the β- diketiminate chelate ring. The structure of 5.2 is very similar to that of the corresponding β-diketiminate monoalkyl [(BDK)FeCH2SiMe3] (4.2) with comparable bond distances and angles. The largest differences with 4.2 is found in the orientation of the alkyl ligand in the binding pocket of the β-diketiminate ligand, which is more symmetric than in 4.2 as shown by the smaller difference in CH2–Fe–N angles (5.3° for 5.2 vs. 18.87° for 4.2)
Figure 5.7 Molecular structure of 5.2. Hydrogen atoms have been omitted for clarity.
The 1H NMR spectrum of complex 5.2 in C6D6 exhibits 15 paramagnetically shifted resonances in the spectral window from –250 ppm to +250 ppm. As has been reported for other β-diketiminate iron monoalkyls,23 the α-protons of the (trimethylsilyl)methyl group in 5.2 are not observed, probably due to extreme line broadening. All other resonances can be assigned (Table 5.3) on the basis of their integrals and by comparison to the 1H NMR spectrum of [(BDK)FeCH2SiMe3] (4.2).
Table 5.2 Selected interatomic distances and angles for 5.2.
Distances (Å)
Fe-N1 1.971(3) N1-C13 1.331(5)
Fe-N2 1.973(3) N2-C16 1.342(5)
Fe-C35 2.012(5) C13-C15 1.403(5)
C15-C16 1.399(5)
Angles (°)
N1-Fe-N2 92.00(14) C1-N1-Fe 113.9(3) C35-Fe-N1 136.83(19) C1-N1-C13 119.3(4) C35-Fe-N2 130.53(19) Fe-N1-C13 126.7(3) Fe-C35-Si 121.0(3) C18-N2-Fe 112.8(2) C13-C15-C16 127.2(3) C18-N2-C16 121.4(3) C13-C15-C30 116.9(3) Fe-N2-C16 125.8(3) C16-C15-C30 115.9(3)
ppm -120
-100 -80 -60 -40 -20 0 20 40 60 80
* *
The four pyridyl signals at 82.1, 42.6, 39.6 and 30.3 ppm are easily recognised by their narrow line widths and 1H integrals. The narrow line widths for the pyridyl resonances are consistent with their relatively large distance to the iron atom with H6
of the pyridyl ring experiencing the strongest influence from the paramagnetic metal centre. The resonance at 59.4 ppm (9H) of the trimethylsilyl protons overlaps with the ligand γ-methyl signal. The positions of the β-diketiminate ligand resonances are similar to those observed for 4.2.
The isopropyl methyl and methine and the aryl meta protons show the double number of signals compared to 4.2, indicating a Cs-symmetric solution structure at room temperature. This suggests hindered rotation of the pyridyl substituent. Indeed, heating a solution of 5.2 in toluene-d8 leads to coalesence of these signals at 110
°C. Reversibility was confirmed by recooling the solution to ambient temperature.
This type of fluxionality has precedent in the hindered rotation of the aryl substituent Ar’ in the Ni(II) bis(β-diketiminate) complexes [{Ar’C(CHNPh)2}2Ni] (Ar’ = 2-Me-C6H4, naphthyl)17
Like other monoalkyl Fe(II) β-diketiminate complexes, 5.2 is thermally robust. In benzene-d6 or toluene-d8 solution, the complex is stable for weeks at ambient temperature. Only after heating at 110 °C for 3 days, the complex shows minor decomposition, releasing SiMe4.
Monoalkyl 5.2 proved to be inert towards ethene (ca. 1 bar) in C6D6 solution. This observation is consistent with the lack of reactivity observed for β-diketiminate Fe(II) monoalkyls without β-hydrogens.23,24 In view of the reluctance of β-diketiminate monoalkyls to polymerise ethene in the presence of MAO cocatalyst (see Chapter 4), no further polymerisation tests were conducted with 5.2.
Table 5.3 Comparison of 1H NMR shifts (ppm) of 5.2 and 4.2 (500 MHz, C6D6, 298 K).
γ-CH3 m-HAr p-HAr iPr-CH iPr-CH3 iPr-CH3
5.2
4.2
57.5
74.7
-10.2 -10.7 -9.0
-81.0
-68.5
-133.6 -143.6 -126.9
-12.2 -14.0 -14.5
-98.6 -101.1 -100.4
Si(CH3)3 Py-H Py-H Py-H Py-H
5.2 4.2
59.3 56.1
30.3 -
39.6 -
42.5 -
82.1 -
5.5 Complexation attempts with a (2-pyridyl)- β -diketimine ligand without enolisable protons
Ligand deprotonation upon treatment of [(PyBDKH)FeCl2] with alkylating agents can be circumvented by replacing the hydrogen on the central central α-carbon atom of
PyBDKH by a methyl group. The methylated ligand PyBDKMe (Figure 5.9) was prepared analogously to PyBDKH (Scheme 5.2), starting from 2-ethylpyridine.
In sharp contrast to the preparation of 5.1, the methylated ligand PyBDKMe does not complexate FeCl2 in THF under the same conditions. Even when the reaction mixture was refluxed in THF for 8 hrs or in 1,2-dimethoxyethane for 24 hrs, the ligand was recovered almost quantitatively. In an attempt to facilitate complexation by using an Fe(II) source with readily displaceable ligands, PyBDKMe was refluxed with [Fe(MeCN)2][OTf]2 in acetonitrile. This attempt did not lead to complexation either, again leaving only starting material.
The reluctance of PyBDKMe towards Fe(II) complexation is in sharp contrast to the instantaneous complexation observed for PyBDKH. Possibly, coordination of
PyBDKMe to the metal centre is unfavourable due to repulsive interactions between the three methyl groups which are brought in close proximity in the configuration required for terdentate coordination. However, the neutral β-diketimine ArNC(Me)CH2C(Me)NAr (BDKH) also fails to react with FeCl2 under the same conditions, suggesting that steric factors are not the only reason for the difference in reactivity of PyBDKH and PyBDKMe towards FeCl2. A speculative explanation for the difference in coordination behaviour between the PyBDKH and PyBDKMe may be the ability of the former to coordinate to Fe(II) initially in a bidentate fashion, as the zwitterionic tautomer. A subsequent proton shift with concomitant coordination of the pyridyl function would then give 5.1 (Scheme 5.5).
Ar Ar
N N
CH3
N
Ar = 2,6-iPr2C6H3
PyBDKMe
Figure 5.9
5.6 Concluding remarks
The neutral, tridentate pyridyl-functionalised β-diketimine ligand PyBDKH forms a five coordinate iron dichloride complex in which the ligand coordinates in a facial manner.
In contrast to the corresponding PDI complex, in which the nitrogen atoms are arranged in a meridional fashion, the PyBDKH complex is not a catalyst for olefin polymerisation. A fair comparison between the geometric and electronic properties of Fe(II) complexes of PDI and PyBDKH is precluded, however, by the reactivity of the
PyBDKH ligand towards strongly Brønsted basic alkylating agents. These convert the neutral terdentate ligand into a monoanionic bidentate β-diketiminate ligand through deprotonation of the acidic proton on the central carbon atom of the ligand. Replacing the enolisable proton by a methyl group results in problematic complexation, which has thus far not permitted isolation of the potentially more relevant facial analogue [(PyBDKMe)FeCl2] of the meridional [(PDI)FeCl2] complex.
5.7 Experimental
General considerations See section 2.8.
Starting materials
Anhydrous FeCl2 was synthesised according to a literature procedure.25 2-Picoline (98% Acros), 2,6-diisopropylaniline (98% Acros) were used without further purification.
Preparation of PyBDKH
Pyridin-2-yl-propan-2-one18,19 To a solution of 2-picoline (9.3 g, 100 mmol) in THF (50 mL) was added at -60 °C nBuLi (40 mL, 2.5 M in hexane). The temperature was raised to room temperature. The mixture was cooled to –60 °C again, then N,N- dimethylacetamide (9.3 mL, 100 mmol) in 20 mL of diethylether was slowly added.
The temperature was raised to 20 °C, then water (40 mL) was added with care, followed by 6.2 mL of concentrated aqueous HCl. The organic layer was separated.
The water layer was extracted with chloroform (3 x 20 mL). The extracts were combined with the organic layer and this was dried (Na2SO4) and concentrated. The concentrate was distilled to give 9.8 g of crude product (bp. ca. 60 °C, 1 torr), containing 16 mol% dimethylacetamide according to 1H NMR. Yield 66%.
Ar Ar
N N N H
Fe Cl Cl H N
N N
Ar Ar
H N
N N
Ar Ar Fe
Cl Cl FeCl2
N
N N
Ar Ar H
Scheme 5.5
(1-Methyl-2-pyridin-2-ylethylidene)-2,6-diisopropylphenylamine. Crude pyridin-2-yl- propan-2-one (9.8 g, ca 66 mmol) and diisopropylaniline (12.5 mL, 66 mmol) were dissolved in 125 mL of toluene. p-Toluenesulfonic acid (0.5 g) was added and the mixture was refluxed for 1 hour. A Dean-Stark trap was used to remove water. The mixture was washed with dilute NaHCO3, and once with brine, dried (Na2SO4) and concentrated to give a slowly solidifying oil. Crystallisation from 50 mL of ethanol yielded 14.2 g (72%) of a yellow product, mp 55–65 °C, which was a mixture of imine (a) and enamine (b). Ratio a:b = 1:2. The compound should be stored at –25 °C. IR (nujol) ν~= 1627, 1588, 1544 cm–1. 1H NMR (300 MHz, CDCl3, RT) δ = 8.55 (d, J = 5 Hz, py-H), 8.26 (d, J = 5 Hz, py-H), 7.63 (m), 7.44–6.99 (m), 6.87 (d, J = 7 Hz), 6.72 (t, J
= 6 Hz), 5.12 (s, b), 3.99 (s, 2H, a), 3.23 (m, 2H, b), 2.70 (m, 2H, a), 1.69 (s, 3H, ArN=C–CH3 a), 1.68 (s, 3H, ArN=C–CH3 b), 1.20 and 1.12 (2 x d, J = 7 Hz, 6H, iPr-Me b), 1.09 and 1.02 (2 x d, J = 7 Hz, 6H, iPr-Me a) ppm. 13C NMR-APT (75 MHz, CDCl3, RT) δ = 166.8, 157.8, 155.3, 148.4, (–)146.9, 145.0, (–)144.6, 143.4, (–)133.9, 133.5, (–)132.8, (–)124.7, (–)121.3, (–)120.9, (–)120.7, 120.5, (–)120.3, (–)119.2, (–)119.0, (–)118.1, (–)113.5, 95.5, (–)90.5, 48.1, (–)25.9, (–)25.5, (–)25.4, (–)22.4, (–)21.3, (–
)20.7, (–)20.5, (–)20.1, (–)17.9, (–)17.7 ppm.
N-(2,6-diisopropylphenyl)acetanilide20 was prepared following a slightly modified literature procedure for benzanilide.26 To a solution of sodium hydroxide (1.3 g, 32.5 mmol) in 5 mL of water was added CH2Cl2 (10 mL) and 2,6-diisopropylaniline (4.7 mL, 25 mmol). The mixture was cooled to –10 °C and acetyl chloride (2.1 mL, 30 mmol) was slowly added. Then, the temperature was raised to 20 °C. The organic layer was washed once with 5% aqueous NaHCO3, once with water, once with 2N HCl, and once with brine, then dried (Na2SO4), and concentrated, to give 5.4.g (99%) of product as a white solid. The solid was used as such.
N-(2,6-Diisopropylphenyl)acetimidoyl chloride. A mixture of phosphorus pentachloride (3.26 g, 15.7 mmol), toluene (5 mL) and crude N-(2,6- diisopropylphenyl)acetanilide (3.42 g, 15.7 mmol) was heated at 50 to 60 °C for 15 minutes. Then the solvent and POCl3 were removed in vacuum (0.5 torr.). The remaining colourless oil was used as such. 1H NMR (300 MHz, CDCl3, RT) δ = 7.17 (m, 3H, Ar–H), 2.75 (septet, J = 6.9 Hz, 2H, iPr–CH), 2.63 (s, 3H, N=C–CH3), 2.61 (s, 3H), 1.17 (d, J = 6.9 Hz, 12H, iPr-Me) ppm.
3-(2-Pyridin-2-yl)pentane-2,4-dione-bis-(2,6-diisopropylphenyl) imine (PyBDKH). To a cold (–30 °C) solution of (1-methyl-2-pyridin-2-ylethylidene)-2,6-diisopropylphenyl amine (4.41 g, 15 mmol) in ether (40 mL) was slowly added 2.5 M nBuLi (6 mL). The temperature was raised to 20 °C, then the mixture was cooled again to -30 °C. A solution of N-(2,6-diisopropylphenyl)acetimidoyl chloride (prepared from the anilide, see above, ca 15 mmol) in 10 mL of ether was added. The mixture was refluxed for 1 hour, then cooled again. A second portion of nBuLi (3 mL) was added. After 1 hour of reflux, the previous procedure was repeated with 1.5 mL of nBuLi. After standing for 1 night, water was carefully added and the ether layer was separated, washed with brine, dried (Na2SO4) and concentrated to give 7.5 g of a solid. After crystallisation
(s, 6H, Ar-H), 3.16 (m, 4H, iPr-CH), 1.45 (s, 6H, γ-Me), 1.15 (d, J = 7 Hz, 12H, iPr-Me), 1.10 (d, J = 7 Hz, 12H, iPr-Me) ppm. 13C NMR-APT (75 MHz, CDCl3, RT) δ = 159.0, 158.7, (–)147.0, 140.0, 137.9, (–)133.8, (–)125.3, (–)122.6, (–)120.6, (–)118.5, 104.5, (–
)25.7, (–)21.8, (–)20.9, (–)16.9 ppm. EI+-MS: 494 [M–H]+. Exact mass (C34H45N3) calc.
495.36133, found 495.36065.
Preparation of PyBDKMe
3-Pyridin-2-yl-butan-2-one.18 To a solution of 2-ethylpyridine (10.7 g, 100 mmol) in THF (50 mL) was added at –60 °C nBuLi (40 mL, 2.5 M in hexanes). The temperature was raised to room temperature. The mixture was cooled to –60 °C again, then dimethylacetamide (9.3 mL, 100 mmol) in 20 mL of ether was slowly added. The temperature was raised to 20 °C, then water (40 mL) was added with care, followed by 6.2 mL of concentrated aqueous HCl. The organic layer was separated. The water layer was extracted with chloroform (3 x 20 mL). The extracts were combined with the organic layer and this was dried (Na2SO4) and concentrated.
The concentrate was distilled to give 4.0 g (27%) of product (bp, ca. 60 °C, 1 Torr, reported27 96 – 97 °C, 7.1 Torr).
Note. A second fraction (bp. ca. 140 °C) was found during distillation. This seems to be formed from reaction of Me2NCOCH2Li with the product.
2,6-Diisopropylphenyl)-(1-methyl-2-pyridin-2-yl-propylidene)-amine. Crude 3-pyridin- 2-yl-butan-2-one (4.0 g, ca 27 mmol) and diisopropylaniline (6.25 mL, 33 mmol) were dissolved in 100 mL of toluene. p-Toluenesulfonic acid (0.25 g) was added and the mixture was refluxed for 2 hour. A Dean-Stark trap was used to remove water. The mixture was washed with dilute NaHCO3 , with brine, dried (Na2SO4) and concentrated. The concentrate was heated at 120 °C (0.2 Torr) in a Kugelrohr apparatus to remove excess amine and remained starting material. The residue, 5.75 g (70%, oil) was used as such in the next step. 1H NMR (300 MHz, CDCl3, RT) δ = 8.52 (d, 1H, py-H), 7.62 (2xt, 1H, py-H), 7.36 (d, 1H, py-H), 7.15–7.12 (m, 1H, py-H), 7.11–6.96 (m, 3H, Ar-H), 4.06 (q, J = 7 Hz, 1H, py-C(H)CH3), 2.60 (m, 2H, iPr-CH), 1.63 (d, J = 7 Hz, 3H, py-C(H)CH3)), 1.57 (s, 3H, ArN=C–CH3), 1.09, (d, J = 6 Hz, 3H, iPr- Me), 1.04 (d, J = 6 Hz, 3H, iPr-Me), 1.01 (d, J = 6 Hz, 3H, iPr-Me), 0.96 (d, J = 6 Hz, 3H, iPr-Me) ppm.
3-Methyl-3-(2-pyridin-2-yl)pentane-2,4-dione-bis-(2,6-diisopropylphenyl) imine (PyBDKMe). To a cold (–30 °C) solution of 2,6-diisopropyl-phenyl)-(1-methyl-2-pyridin-
2-yl-propylidene)-amine (5.75 g, 19 mmol) in diethyl ether (40 mL) was slowly added 2.5 M nBuLi (7.5 mL). The temperature was raised to 20 °C, then the mixture was cooled again to –30 °C. A solution of N-(2,6-diisopropylphenyl)acetimidoyl chloride, prepared from the corresponding amide (4.07 g, 18.6 mmol) and PCl5 (3.88 g, 18.6 mmol) in 20 mL of ether was slowly added. After standing for 18 hours at room temperature, water was carefully added and the ether layer was separated, washed with brine, dried (Na2SO4) and concentrated to give 9.65 g (100%) of a tarry oil, which was probably a 12:3:1 mixture of syn/anti isomers. Ca. 8 g was crystallised from 40 mL of ethanol to give 5.5 g of the symmetrical (E,E)-isomer, mp 133 °C. 1H NMR (300 MHz, CDCl3, RT) δ = 8.57 (d, 2H, py-H), 7.68 (t, 1H, py-H), 7.49 (d, 1H, py- H), 7.18–6.97 (m, 7H, Ar-H + py-H), 3.18 (septet, 2H, iPr-CH), 2.75 (septet, 2H, iPr- CH), 2.08 (s, 3H, α-Me), 1.67 (s, 6H, γ-Me), 1.17 (d, J = 3 Hz, 6H, iPr-Me), 1.15 (d, J = 3 Hz, 6H, iPr-Me), 1.06 (d, J = 6 Hz, 6H, iPr-Me), 0.99 (d, J = 6 Hz, 6H, iPr-Me) ppm.
13C NMR-APT (75 MHz, CDCl3, RT) δ = 170.9, 159.4, (–)146.2, 144.0, 134.0, (–)133.6,
133.6, (–)120.5, (–)120.5, (–)120.2, (–)119.8, (–)119.4, 63.5, (–)25.2, (–)25.1, (–)21.5, (–)20.9, (–)20.7, (–)20.3, (–)19.3, (–)16.7 ppm. IR (KBr, nujol mull) ν~= 3061 (m), 1650 (s), 1584 (s), 1569 (m), 1439 (s), 1361 (s),1324 (m), 1293 (m), 1254 (m), 1232 (m), 1212 (m), 1191 (m), 1153 (w), 1086 (m), 1056 (m), 1042 (m), 1011 (w), 993 (m), 933 (w), 818 (m), 796 (m), 771 (s), 756 (s), 704 (w), 692 (w), 526 (w), 499 (w), 486 (w), 407 (w) cm–1. EI+-MS: 509 [M]+. Exact mass (C35H47N3) calc. 509.37698, found 509.37477.
Preparation of [(PyBDKH)FeCl2] (5.1)
A mixture of PyBDKH (5.50 g, 11.1 mmol) and anhydrous FeCl2 (1.4 g, 11.1 mmol) was stirred in THF (60 mL) for 4 hrs giving a dark yellow solution. A small amount of solid impurities was removed by filtration. The volume of the solution was reduced to ca. 50% and the solution was cooled to –25 °C, yielding yellow crystals. The crystalline material was filtered off and dried in vacuum during which the crystals disintegrated, leaving a bright yellow powder. Additional crops of spectroscopically identical material were obtained by further concentration of the filtrate and cooling to –25 °C. Combined isolated yield: 4.2 g, 61 % (based on PyBDKH). 1H NMR (500 MHz, CD2Cl2, RT) δ = 108.0 (1314 Hz), 58.0 (132 Hz), 57.0 (124 Hz), 51.3 (108 Hz), 48.0 (126 Hz), 46.2 (1125 Hz), 38.8 (207 Hz), 31.2 (898 Hz), 14.4 (91.7 Hz), 14.1 (82.4 Hz), 13.3 (78.0 Hz), 12.9 (88.9 Hz), 12.3 (119 Hz), 11.1 (119 Hz), 10.8 (80.8 Hz), 10.6 (81.4 Hz), 9.1 (204 Hz), 8.8 (78.6 Hz), 8.3 (234 Hz), 8.0 (48.9 Hz), 7.8 (67.0 Hz), 7.4 (97.4 Hz), 7.1 (131 Hz), 6.6 (55.4 Hz), 5.7 (76.2 Hz), 4.5 (454 Hz), 3.7 (59.4 Hz), 3.5 (59.9 Hz), 3.2 (65.8 Hz), 3.0 (66.4 Hz), 2.4 (73.2 Hz), 1.5 (78.3 Hz), 1.2 (188 Hz), 0.1 (47.3 Hz), –0.4 (77 Hz), –2.3 (151 Hz), –3.9 (160 Hz), –4.9 (219 Hz), –5.8 (145 Hz), –6.9 (93.5 Hz), –7.3 (100 Hz), –9.0 (104 Hz), –9.9 (240 Hz), –11.4 (224 Hz), –14.9 (375 Hz), –18.7 (239 Hz), –21.8 (74.1 Hz), –22.4 (75.8 Hz), –43.2 (2186 Hz) ppm. IR (KBr, nujol mull) ν~= 3058 (m), 1648 (s), 1592 (m), 1440 (s), 1422 (m), 1368 (s), 1337, 1326 (m), 1254 (w), 1210, 1173 (m), 1101, 1071, 1056, 1043, 1008, 829 (w), 789, 760 (m), 718, 562, 415 (w) cm–1. µeff. (SQuID, 298 K) = 5.2 µB. Anal.
C34H45N3FeCl2 (622.50) calcd. C 65.60, H 7.29, N 6.75, found C 65.43, H 7.26, N 6.59.
Preparation of [(PyBDK)FeCH2SiMe3] (5.2)
[(PyBDKH)FeCl2] (5.1) (580 mg, 0.93 mmol) was suspended in 20 mL THF. The suspension was cooled to 0 °C and solid LiCH2SiMe3 (88 mg, 0.93 mmol) was added resulting in a clear red solution. After stirring at 0 °C for 30 min, a second equivalent (88 mg, 0.93 mmol) was added giving a slightly darker red solution, which was stirred at 0 °C for 1 hr. THF was removed under reduced pressure and the solid was freed of residual THF by stirring in pentane (10 mL, 2x) followed by evaporation of the volatiles in vacuo. The solid was extracted with pentane (20 mL, 5x) until a white salt remained. Some dark, insoluble impurities were removed from the greenish extract
12.2 (290 Hz, 6H, iPr-CH3), –14.0 (267 Hz, 6H, iPr-CH3), –81.0 (179 Hz, 2H, p-HAr), – 98.6 (669 Hz, 6H, iPr-CH3), –101.1 (918 Hz, 6H, iPr-CH3), -133.6 (2410 Hz, 2H, iPr- CH), –143.6 (2867 Hz, 2H, iPr-CH) ppm. 1H NMR (500 MHz, toluene-d8, 110 °C) δ = 55.2 (246 Hz, 1H, py-H), 41.3 (278 Hz, 9H, SiMe3), 35.3 (293 Hz, 6H, γ-CH3), 31.0 (92 Hz, 1H, Py-H), 28.4 (86 Hz, 1H, Py-H), 21.8 (66 Hz, 1H, Py-H), 0.0 (36 Hz, 4H, m-HAr), –7.6 (110 Hz, 12H, iPr-CH3), –57.1 (169 Hz, 2H, p-HAr), –64.0 (554 Hz, 12H iPr-CH3), –91.5 (4H, 2471 Hz, iPr-CH) ppm. IR (KBr, nujol mull) ν~= 3054 (m), 2901 (s), 1929, 1866 (w), 1582, 1561(m), 1529, 1455, 1329, 1314, 1253, 1239, 1179 (s), 1149, 1097, 1044, 1013 (m), 891, 851, 815, 802, 777 (s), 763, 730, 676, 654, 550, 531, 486, 447, 429 (m), 408 (w) cm–1. Anal. C38H55N3FeSi (637.81) calcd. C 71.56, H 8.69, N 6.59, found C 70.89, H 8.59, N 6.61.
Ethene polymerisation test with 5.1
Compound 5.1 was tested with MAO cocatalyst as described in Section 2.6, except for the use of 40 µmol Fe complex instead of 20 µmol. No significant ethene uptake or temperature effects were observed.
Crystal structure determinations
5.1: Suitable crystals were obtained by recrystallisation from dichloromethane. To avoid deterioration of the crystals due to loss of solvate molecules from the lattice, the crystal was covered with inert oil. No classic hydrogen bonds, no missed symmetry (MISSYM), or solvent-accessible voids were detected by procedures implemented in PLATON.28,29
5.2: Suitable crystals were obtained by recrystallisation from pentane. The refinement was complicated by a twin problem: during the refinement the wR2 value did not substantially drop when introducing anisotropic thermal parameters and also some residual peaks were observed in a subsequent difference Fourier synthesis. After introducing a twin matrix (for non-merohedral twins) as detected by PLATON28 (180°
rotation about the a-axis) with scale factors for the fractional contributions of the various twin components the remainder of the structure refined smoothly. The s.o.f.
of the major fraction of the component of the twin model refined to a value of 0.515(1). No classic hydrogen bonds, no missed symmetry (MISSYM), but potential solvent-accessible area (voids of 48.5 Å3 / unit cell) were detected by procedures implemented in PLATON.28,29
Table 5.4 Crystal, collection and refinement data for complexes 5.1 and 5.2.
5.1 5.2 Formula C34H45N3FeCl2.2CH2Cl2 C38H55N3FeSi
Fw 792.37 637.81
cryst. dim. (mm) 0.38 x 0.36 x 0.26 0.16 x 0.04 x 0.04 colour, habit yellow, block orange, needle crystal system monoclinic monoclinic space group, no.30 P21/n, 14 P21/n, 14
a (Å) 12.9314(7) 18.238(1)
b (Å) 15.4532(8) 11.1471(8)
c (Å) 19.978(1) 18.451(1)
α (°) 90 90
β (°) 95.327(1) 90.055(1)
γ (°) 90 90
Z 4 4
V (Å3) 3975.0(4) 3751.1(4)
ρcalc (g/cm3) 1.324 1.129
θ range (°) 2.23 – 26.90 2.41 – 25.02 λ (Å) 0.71073 (Mo Kα) 0.71073 (Mo Kα)
T (K) 100(1) 100(1)
data collect. time (h) 13.5 18.2 no. of meas. refl. 33084 32956 no. of unique refl. 8767 6630
µ (cm-1) 8.11 4.62
no. of parameters 427 402
weighting scheme: a,b[a] 0.0558, 3.1284 0.0457, 0.0
R(F) for F0≥ 4σ(F0)[b] 0.0458 0.0536
wR(F2)[c] 0.1198 0.1120
res. el. dens. (e/Å3) –0.68, 0.61(8) –0.1, 0.1(2)
GoF[d] 1.037 0.963
[a] w = 1/[σ2(Fo
2) + (aP)2 + bP], P = [max(Fo
2,0) + 2Fc 2] / 3
[b] R(F) = ∑(||Fo| - |Fc||) / ∑|Fo |,
[c] wR(F2) = [∑[w(Fo2 - Fc2)2] / ∑[w(Fo2)2]]1/2,
∑
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