Early Transition Metal Permethylpentalene Complexes for the

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Early Transition Metal Permethylpentalene Complexes for the Polymerization of Ethylene F. Mark Chadwick,† Robert T. Cooper,† Andrew E. Ashley,‡ Jean-Charles Buffet,† and Dermot M. O’Hare*,† †

Chemistry Research Laboratory, Oxford University, Mansfield Road, Oxford, OX1 3TA, U.K. Department of Chemistry, Imperial College London, South Kensington, London SW7 2AZ, U.K.

‡

S Supporting Information *

ABSTRACT: A series of group 4 permethylpentalene− cyclopentadiene mixed sandwich complexes have been synthesized including the mono- and bis-cyclopentadienyl derivatives Pn*MCp2−xClx (Pn* = η8-C8Me6, M = Ti, Zr, Hf, x = 0, 1) and the monopentamethylcyclopentadienyl species Pn*MCp*Cl (Pn* = η8-C8Me6, M = Ti, Zr, and Hf). The compounds were all characterized via a range of techniques including single crystal X-ray diffraction allowing for comparison of trends descending the group as well as the effect of permethylation of the cyclopentadienyl moiety. The complexes have been tested for their efficacy as catalysts for the solution polymerization of ethylene, and Pn*ZrCpCl was found to give the highest activity of 6993 kg mol−1 h−1 bar−1 at 80 °C with a [MAO]0/[Zr]0 ratio of 1000 and 2 bar pressure of ethylene gas in hexane.

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INTRODUCTION Pentalene (Pn; C8H6) has intrigued physical, organic, and inorganic chemists for decades due to its illusive nature. Initially proposed by Robinson et al.,1 it and its substituted analogues have been tantalizing targets for synthesis. The neutral antiaromatic bicyclic system can be thought of as two ring fused cyclopentadienyl units, or as a cyclooctatetraene with a 1,5-transannular bond. Indeed, it was the connection made to the latter that led Katz et al. to propose that the two electron reduction of the organic framework would lead to a 10 π electron aromatic dianion, which would make an exciting ligand precursor in organometallic synthesis.2,3 Of the multitudes of coordination modes, particular interest is drawn toward the fascinating η8-binding mode, where the pentalene framework disrupts aromaticity to wrap itself around a metal center: in this instance pentalene can be thought of as two ring-fused cyclopentadienyl (Cp−; C5H5−) moieties, however with two fewer valence electrons (8 vs 10).4 In 1997 both the Cloke and Jonas groups, almost simultaneously, established the η8-bonding mode for pentalene complexes (throughout this paper Pn/Pn* refers exclusively to the η8 binding mode).5−7 The work of Cloke et al. focused on PnRTaCl3 [R = 1,4-(SiMe3)2] initially (though the methyl derivatives were subsequently swiftly synthesized), whereas Jonas’s work was with the group 4 metals. Treatment of CpTiCl2 with the pentalene dianion furnished the paramagnetic PnTiCp; reaction of this species with half an equivalent of 1,2dichloroethane resulted in the facile oxidation of the Ti3+ center, affording the 18 electron species PnTiCpCl.7 The Zr derivative was synthesized directly from the combination of ZrCp2Cl2 and Li2Pn, with reaction of a further equivalent of the pentalene synthon giving the intriguing Pn2Zr species; © XXXX American Chemical Society

unfortunately no structural characterization for this compound was obtained. The report however details the only structurally characterized η8-Pn complexes of group 4 metals before investigations in the area by O’Hare et al. and permethylated analogues were targeted for comparison. The progression of organometallic pentalene chemistry has been hindered over the years mainly due to the difficulty in synthesis of starting materials: pathways either require complex equipment or are plagued with low yields and low purity.8 However, pentalene complexes have shown some fascinating reactivity; for example Cloke et al. have recently synthesized a Ti double sandwich (μ:η5:η5-PnR)Ti2 [R = 1,4-(SiiPr3)2] which can reductively deoxygenate CO2 to form a dicarbonyl complex.9 In 2005 Ashley and co-workers devised the homogeneous, high-yielding synthesis of suitable precursors for permethylpentalene (Pn*; C8Me6).10,11 In subsequent synthesis this carbocycle has demonstrated the η8 binding mode with f-block elements,12,13 and recently a set of group 4 “Pn*MCl2” synthons has been reported.14 Herein we report the utilization of these synthons in the formation of a series of group 4 permethylpentalene cyclopentadienyl (and pentamethylcyclopentadienyl) mixed sandwich compounds and their performance as potential catalysts for the polymerization of αolefins. Synthesis and Characterization of Pn*MCpCl (M = Ti, Zr, Hf; 1M). Synthesis of 1Ti. The reaction of [Pn*Ti(μCl)Cl]214 with 2 equiv of NaCp in THF at −78 °C afforded a red solution upon warming to room temperature (Scheme 1). Subsequent workup and cooling of a hexane solution produced Received: May 6, 2014

A

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that PnZrCpCl is purified by crystallization from THF, and in this solvent LiCp is very soluble; by dint of permethylation the solubility of pure 1Zr was found to be considerably enhanced in all solvents tried, and even more so in the presence of LiCp, which complicated any differential solubility characteristics. An alternative synthetic pathway was sought, utilizing the recently synthesized zirconium and hafnium permethylpentalene synthons [Pn*M(μ-Cl)3/2)]2(μ-Cl)2Li(THF)x(Et2O)y (M = Zr or Hf).14 Reaction of these species with 2 equiv of NaCp in Et2O yielded Pn*MCpCl (1M, Scheme 1), which were crystallized in drastically improved yields by slow cooling of hexane solutions (83% for both congoners). 1 H and 13C NMR Spectroscopy of 1M. The room temperature 1H NMR spectrum of 1M reveals four sharp singlets in a 5:6:6:6 intensity ratio (δ = 5.38, 2.19, 1.64, and 1.51 ppm for 1Ti; δ = 5.65, 2.11, 1.76, and 1.65 ppm for 1Zr; δ = 5.56, 2.10, 1.94, and 1.75 ppm for 1Hf), and one may conclude that the solution phase structure of the molecule is of Cs symmetry, as observed in the solid state (Figure 1). The Pn* resonances are split into three sets of equal intensity, due to the reduced symmetry of the molecule. A mirror plane is present, which bisects the molecule through the Pn* and Cp ligands, and runs along the M(1)−Cl(1) bond, resulting in two sets of independent nonwingtip methyl (NWT-Me) protons (C9/C14 and C11/C12 in Figure 1). The wingtip methyl (WT-Me) (C10 and C13 in Figure 1) proton resonance occurs at higher field to the remaining two Pn* resonances, which have the absolute configuration shown as determined by 2D NOESY NMR. A similar splitting is observed in Jonas’s nonmethylated example.7 The resonances corresponding to the Cp moiety (5.38/5.65 ppm for 1Ti and 1Zr respectively) are shifted somewhat upfield relative to PnTiCpCl and PnZrCpCl (5.76/ 6.03 ppm), reflecting the inductive donation of the methyl groups on the pentalene rings. The 13C NMR spectrum displays nine resonances, the three non-ring-junction quaternary carbon atoms for 1Ti being shifted downfield from those reported for the unsubstituted analogue, whereas the bridgehead atoms are almost unchanged and nonequivalent; intriguingly the non-ring-junction quaternary carbon resonances are shifted upfield in the zirconium congener. 2D NMR experiments (HSQC, HMBC, NOESY) allowed for the definitive assignment of all the resonances for all the samples. Variable temperature studies were undertaken in order to investigate whether coalescence of the two NWT-Me environments could be achieved (C11/C12 and C9/C14). The coalescence of said resonances is postulated to arise from Pn* ring-whizzing, a phenomenon readily observed in Cp and Cp* complexes, with rotation along the metal−bridgehead bond axis, rendering all NWT-Me groups equivalent on the NMR time scale. However, although a slight broadening of the NWT-Me resonances was observed, full coalescence of the resonance was not resolved at temperatures up to 373 K. X-ray Crystallographic Analysis of 1M. All three congeners of 1M are isoelectronic and isostructural with Jonas’s analogous nonmethylated examples PnMClCp (M = Ti, Zr).7 1M crystallize in the P21/c space group with a single independent molecule in the asymmetric unit. They are 18 electron complexes and possess a distorted pseudotetrahedral arrangement of ligands about their respective metal centers.7 This is exemplified by the Cpcent−Ti(1)−Cl(1) and average Pn*cent− Ti(1)−Cl(1) angles (102.0 and 107.8° respectively), which are almost identical to those of PnTiClCp (103.7 and 107.5°

Scheme 1. Synthesis of Pn*MCpCl (1M; M = Ti, Zr, Hf)

a red microcrystalline solid in excellent yield (83%), which was identified as Pn*TiCpCl (1Ti). Single crystals suitable for X-ray diffraction were grown by slow cooling of a hexane solution to −35 °C, and the structure is shown in Figure 1.

Figure 1. Solid-state molecular structure of 1Hf (1Ti and 1Zr are isostructural). H atoms omitted for clarity, thermal ellipsoids at 50% probability.

Synthesis of 1Zr and 1Hf. Initial attempts to synthesize the zirconium derivative of 1 followed a parallel route to Jonas’s unsubstituted analogue.7 However, reaction of ZrCp2Cl2 with 1 equiv of Li2Pn*(TMEDA)x in THF at −78 °C, followed by standard workup procedures, failed to furnish the desired product, as determined by 1H NMR and mass spectrometry. Changing the solvent to benzene in this instance led to immediate reaction, and the product Pn*ZrCpCl (1Zr) was isolated as a yellow microcrystalline substance following highvacuum sublimation and recrystallization from toluene, albeit in poor yield (12%). Single crystals suitable for X-ray diffraction were grown by slow cooling of a solution in the same solvent to −35 °C. The purification of 1Zr necessitates a sublimation procedure due to the equimolar amounts of LiCp byproduct, which proved inseparable by bulk crystallization. Jonas reports B

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respectively) and similarly for Zr [Cpcent−Zr(1)−Cl(1), 103.3° for Pn*, 105.2° for Pn; Pn*/Pncent−Zr(1)−Cl(1), 109.5° for Pn*, 109.4° for Pn].7 The fold angle (defined as the angle by which the ligand is found to deviate from planarity)14 of 1Ti (31.0°) is 2° less than that reported for PnTiClCp (33.0°). This smaller value may be attributed to the increased donor capability of the Pn* ligand relative to Pn,7,15 since the distortion from planarity is thought to arise from the shortening of M−C wingtip contacts that alleviate a lack of electron density on a metal center [as demonstrated in the series PnRTaMeyX3−y (R = 1,4-(SiiPr3)2; y = 0, x = I; y = 1-3 X = Cl)].11 It can be posited that a less extreme fold is required for Pn* to balance any electron deficiency at Ti. Similarly 1Zr follows the same trend with the Pn* fold angle being 30.7° (the fold angle for PnZrCpCl is reported to be 33°).11 The average Ti(1)−Pn*cent distance (1.995 Å) is shorter than the Ti(1)−Cpcent distance (2.063 Å), which is possibly a reflectance of the higher electron count on the ligand available for donation to the metal (eight compared to five); the same ordering is observed for PnTiClCp (1.970 and 2.057 Å respectively). The Ti(1)−Cl(1) bond [2.5068(8) Å] is unusually long compared with TiCp2Cl2 (2.364 Å)16 and TiCp*2Cl2 (2.349 Å)17 but is comparable to Jonas’s mixed sandwich analogue PnTiCpCl [2.512(1) Å] and is thought to be the result of steric congestion around the Ti center. Similarly the Zr atom lies closer to the centroids of the Pn* than the centroid of the Cp (2.07 Å vs 2.22 Å), and the Zr−Cl bond is unusually long compared with ZrCp2Cl2 (2.46 Å) and ZrCp*2Cl2 (2.45 Å),18 but is similar to other pentalene−Zr complexes PnZrCpCl (2.54 Å) and the half-sandwich PnZr(THF)2Cl2 (2.58 Å).7 The Hf−Cpcent distance is comparable to other bis(cyclopentadiene)hafnium dichloride compounds in the literature: HfCpCp*Cl2, HfCp*2Cl2·CaCp*2, and HfCp2Cl2 [average 2.20 Å compared to 2.190(3) Å for 1Hf].19−21 The average Hf−C distance for the Cp ring is the same as that seen for HfCp4 (2.50 Å).22 The Hf−Cl distance is however considerably longer than in these compounds and more comparable to the known Zr Pn compounds: Pn*ZrCpCl, PnZrCpCl, and ZrPnCl2·2THF.7 1Hf represents only the second structurally characterized Hf containing pentalene compound, the other being the corresponding starting material.14 The bond lengths in 1Hf are extremely similar to 1Zr, with the trend being that 1Hf generally shows a slight shortening of M−X distances [e.g., M− Pn*cent is 2.090 Å for 1Hf whereas it is 2.110 Å for 1Zr, or M−Cl distance is 2.517(2) Å for 1Hf and is 2.548(1) Å for 1Zr]; the same trend is seen in the respective starting materials.14 The fold angle in 1Hf is very similar to that of the dimeric starting material, and the M−Pn*cent distances are also comparable (30.9° and 2.089 Å for 1Hf, 30.2° and 2.085 Å for the starting material). This is unsurprising bearing in mind that both complexes are 18 electron and demonstrates that the relative steric demand of three bridging chlorides is approximately the same as that of a Cp ring within the coordination sphere. The M−C(Pn*) lengths can be partitioned into three categories with the shortest contacts to the ring junction [1Ti (2.15 Å), 1Zr (2.27 Å), 1Hf (2.25 Å)], the longest to the wingtip atoms [1Ti (2.54 Å), 1Zr (2.62 Å), 1Hf (2.61 Å)], and the remainder intermediate [1Ti (2.41 Å), 1Zr (2.49 Å), 1Hf (2.48 Å)]; this pattern is repeated in the other structurally characterized η8-pentalene species.8 There is significant deviation between the average M−C(1/7) [1Ti (2.555 Å), 1Zr

(2.471 Å), 1Hf (2.546 Å)] and M−C(3/5) [1Ti (2.493 Å), 1Zr (2.347 Å), 1Hf (2.423 Å)] distances, resulting in a tilting of the Pn* moiety away from the Cl atom. A similar canting of the ligand is also seen in PnTiClCp (2.433 and 2.321 Å respectively). Synthesis and Characterization of Pn*MCp2 (M = Ti, Zr, Hf; 2M). Synthesis of 2M. The family of monocyclopentadienyl Pn* derivatives for all the group 4 congeners having been synthesized, it seemed appropriate to attempt the synthesis of a bis-cyclopentadienyl Pn* derivative. Such a complex would be expected to be extremely sterically crowded and electronically saturated, and this, in turn, may lead to interesting structural features. The reported synthetic pathway to Jonas’s analogue of 1Zr, PnZrClCp, involved the direct combination of Cp2ZrCl2 and Li2Pn, whereby a cyclopentadienyl ligand is substituted in preference to a chloride.7 The alternative product PnZrCp2, produced by substitution of both chloride ligands, was later postulated as being too sterically saturated to form under the conditions.8 Preliminary NMR scale investigations involved sonication of [Pn*Ti(μ-Cl)Cl]2 with excess NaCp, which produced a novel Pn* species containing two Cp resonances that significantly differed from that observed for 1Ti. Attempts at scaling up the reaction with simple stirring over a range of different temperatures, for prolonged reaction times, and in a variety of ethereal, aromatic, and aliphatic hydrocarbon solvents failed to replicate the reactivity exhibited on the NMR scale; the only product identified was 1Ti and an intractable pyrophoric solid, presumed to be unreacted NaCp. It was subsequently noted that the preliminary NMR scale reactions required sonication, in order to solubilize the ionic NaCp, in the nonpolar C6D6 medium. The synthesis of Pn*Ti(η5-Cp)(η1Cp) (2Ti) is detailed in Scheme 2; reaction of [Pn*Ti(μScheme 2. Synthesis of Pn*MCp2 (2M; M = Ti, Zr, Hf)

Cl)Cl]2 with 5 equiv of NaCp was found to form the target compound only upon prolonged sonication in benzene, which afforded a red solution. Subsequent workup and cooling of a saturated hexane solution produced a red microcrystalline solid in good yield (62%), which was identified as 2Ti by elemental analysis and X-ray crystallography. The other congeners were synthesized by a parallel route from their respective starting materials. The complete conversion to 2Zr was found to occur (as judged by 1H NMR spectroscopy), which was in contrast to the case for Hf, which is formed in an approximate 6:1 ratio of 2Hf and 1Hf even with extended sonication. Similar solubilities C

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canting in the opposite direction to 1Ti, which is reasoned by the replacement of Cl with the sterically cumbersome η1-Cp ligand. The three examples of the Zr(η1-Cp) fragment [Zr(η1CpR)(η5-CpR)3 (R = H, Me) and Zr(η1-CpR)(η5-CpR)2Cl (R = SiMe3)] have Zr-(η1-Cp) distances of 2.442, 2.513, and 2.396 Å respectively [vs 2.476 (2) Å for 2Zr],21,26,27 the considerably shorter of the latter being due to the electron deficient metal center (16 e− vs 18 e− for 2Zr). However, both the former species are 20 electron complexes: the nonmethylated example has a slightly shorter Zr−(η1-Cp) distance than that found in 2Zr presumably due to the larger steric bulk of the Pn* pushing the η1-Cp ring out; a slight increase in the steric demand of the Cp rings in the methylated example gives a longer Zr−(η1-Cp) bond than in 2Zr, demonstrating the steric effect a single methyl group can have in this crowded system. The M−(η5-Cpcent) distance in 2Zr [2.215(2) Å] is shorter than that in the other 20 electron examples above (2.294−2.349 Å for the nonmethylated example and 2.303−2.353 Å for the methylated example) and more comparable to the 16 electron compound Zr(η1-CpR)(η5-CpR)2Cl (R = SiMe3) (2.203−2.226 Å), implying that factors other than sterics also play a role. Therefore, initial DFT calculations were undertaken to investigate the observed bond lengths. These showed that the HOMO is primarily a bonding orbital between the metal center, the Pn* π-orbitals, and the η5-Cp ring with an antibonding interaction between the metal and the η1-Cp ring (Figure 3). The population of this orbital explains the relative differences in these structural parameters.

of these species led to cocrystallization in 64% yield (2Zr 86% yield). X-ray Crystallographic Analysis of 2M. Single crystals of 2M suitable for X-ray diffraction were grown by recrystallization from cooling a saturated hot (60 °C) hexane solution to −35 °C. All congeners are isostructural and crystallize in the P1̅ space group, with one independent molecule in the asymmetric unit. They possess two distinctly different Cp coordination modes, η1 and η5, giving a total valence electron count of 18 for each complex, isoelectronic with 1M. A diagram of the molecular structure is shown in Figure 2. Examples of

Figure 2. Solid-state molecular structure of 2Zr. Thermal ellipsoids at 50% probability, H atoms omitted for clarity.

crystallographically characterized M(η1-Cp) (M = Ti, Zr, or Hf) moieties are extremely rare when compared to other Cp binding modes, with 2Ti and 2Zr being the fourth known examples and 2Hf the third documented example. As expected 2Zr and 2Hf have extremely similar bond distances with the slight increase in fold angle seen for 2Hf (31.8° vs 30.9°) attributed to the more diffuse 5d orbitals allowing for better overlap with the carbocyclic framework which compensates for the deviation from planarity and loss in aromaticity. The geometry around the metal center is distorted pseudotetrahedral analogous to 1M. The Ti(1)−C(20) bond length [2.439(3) Å] is substantially longer than that of the 16 electron complex Ti(η5-Cp)2(η1-Cp)2 [2.332(2) Å]23 and even more so than CpTi(NH t Bu)(μ-η 1 :η 1 -NNMe 2 )(μ-η 2 :η 1 NNMe2)TiCp(η1-Cp) [2.293(3) Å].24 This parallels the unusually long Ti(1)−Cl(1) bond in 1Ti and PnTiClCp,7 which may be due to the extreme steric crowding at the Ti center, preventing a closer approach of the (η1-Cp) moiety. The monohapto-cyclopentadienyl moiety is planar in all congeners of 2M; for example in 2Ti no carbon atom deviates from the mean plane by more than 0.023 Å and the C−C bond lengths vary systematically around the ring in a manner consistent with a bond-alternant diene structure [starting at the carbon atom bound to Ti: 1.445(5), 1.374(6), 1.422(8), 1.365(6), and 1.456(5) Å]. This bonding pattern and the bond lengths are consistent with those seen for other structurally characterized Ti−(η1-Cp) complexes.23−25 In a similar manner to 1Ti, there are two distinct sets of Ti−C(NWT) distances. Here the average Ti(1)−C(1/7) (2.466 Å) is longer than Ti(1)−C(3/5) (2.361 Å), meaning that the Pn* moiety is

Figure 3. HOMO of 2Zr. Zr atom is rendered in sky blue, C atoms are rendered in gray, H atoms are rendered in white. Molecular orbital occupancy: Zr dxy 3.1%; C(1) pz 4.5%, C(3) pz 3.9%; C(5) pz 4.4%; C(7) pz 4.4%; C(21) py 21.7%; C(24) py 21.5%. Isosurface limits: ±0.014.

The only examples of structurally characterized (η1-Cp) Hf complexes are Hf(η5-Cp)2(η1-Cp)2 and Hf(η5-CpR)2(η1-CpR) Cl (R = Me).27,28 The Hf−(η1-Cp) distances for these species (2.387 and 2.349 Å respectively) are somewhat shorter than those for 2Hf [2.441(4) Å], and the average Hf−(η5-Cpcent) distances are somewhat longer [2.199 and 2.195 Å for the two literature examples, 2.186(3) Å for 2Hf]. It is thought that the same argument regarding the nature of the HOMO stands for 2Hf, but due to the complications arising from relativistic considerations DFT calculations were not undertaken. D

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Cp ↔ η5-Cp) were observed. This is in stark contrast to the highly fluxional TiCp4,31 which is observed to interconvert, even at low temperatures. The reasoning for this observation was attributed to TiCp4 being a 16 electron species and thus possessing an empty valence-shell orbital, which could stabilize a transition state for the interconversion. This is not available to 2Ti, being an 18 electron complex and therefore unable to facilitate a low energy pathway for the transition, making interconversion of the Cp’s energetically unviable. Unfortunately for 2Zr and 2Hf it was not possible to reach the slow exchange limit as in the case for 2Ti (and as a consequence kinetic parameters for this process could not be determined for comparison). Initial cooling led to all four of the NWT-Me and Cp resonances sharpening and the most downfield resonance (attributed to the η1-bound Cp ring) beginning to broaden and eventually flattening fully into the baseline. In the case of 2Hf two new peaks begin to appear (at δ = 3.66 and 6.35 ppm, integral ratio 1:2), and, by comparison with 2Ti, it is hypothesized that a third resonance of integral ratio 2 was under the solvent peak at approximately 7 ppm. It was found that, on warming 2Zr and 2Hf, coalescence of their NWT-Me resonances into a single broad peak and the broadening of the two Cp resonances into the baseline occurred (at 314 K for 2Zr and at 356 K for 2Hf). Upon further heating of 2Zr a new broad peak of integral 10 emerged at 5.39 ppm, which was attributed to all the Cp protons (indicating that all the Cp environments had become equivalent on the NMR time scale) and a resonance of integral 12 began to emerge at approximately 2 ppm (showing that the NWT-Me groups were now equivalent on the NMR time scale). This indicates the presence of two possible fluxional processes: first η1↔η5 interconversion must be occurring (necessary for all Cp protons to be equivalent on the NMR time scale). This process would also lead to the loss of symmetry of the NWT-Me groups of the Pn* framework, however it is also possible for the Pn* moiety to undergo a “ring-whizzing” process along the axis of the metal/bridgehead bond. It is likely that both processes are present in the high temperature limit, however there is no way to tell by NMR. In the case of 2Hf the broadening occurs at a higher temperature, and therefore it was not possible to reach the high-temperature limit. Synthesis and Characterization of Pn*MCp*Cl (M = Zr, Hf; 3M). Synthesis of 3M. The comparison between Jonas’s nonmethylated Pn compounds and the permethylated Pn* analogues reported herein has shown somewhat the effect of methylation of a ligand. To investigate this further it seemed appropriate to attempt the synthesis of Pn*MCp*Cl (3M). A number of attempts to synthesize Pn*TiCp*Cl proved unsuccessful with reduction of the metal center occurring in all cases. This occurrence has been seen previously in group 4 Pn* complexes. The starting materials “Pn*MCl2” (M = Zr or Hf) could both be made from Li2Pn*(TMEDA)x whereas to make the Ti analogue, [Pn*TiCl(μ-Cl)]2, the softer Pn* synthon [(SnMe3)2Pn*] must be used, reflecting the relative ease of oxidation.14 Combination of half an equivalent of the relevant starting material with 1 equiv of KCp* in toluene resulted in a lightening in color of the two solutions (Scheme 3). Subsequent workup and crystallization from saturated toluene solutions cooled to −35 °C led to the formation of yellow or orange plates (for 3Zr and 3Hf respectively) in moderate to good yields (59% and 68%).

H and 13C NMR Spectroscopy of 2M . The room temperature 1H NMR spectra of all congeners of 2M reveal five resonances in an intensity ratio 5:5:6:6:6. Similar to 1M, 2M are all of Cs symmetry in solution as in the solid state, with a mirror plane bisecting both the Pn* and Cp ligands, running along the M−C(20) bond. This leads to two sets of independent NWT-Me protons as for 1M, with the WT-Me resonance again occurring at higher field to the remaining two. The sharp resonance of integral value five in 2Ti (δ = 4.55 ppm; C7D8) is assigned to the η5-Cp moiety, which is shifted considerably to higher field compared to 1Ti (δ = 5.38 ppm; C6D6), postulated as the result of replacing the highly electron withdrawing Cl group with the inductively donating η1-Cp fragment (a similar effect is seen with the other congeners). The remaining broad resonance in 2Ti (δ = 5.90 ppm, ν1/2 = 36.1 Hz; C7D8) is attributed to the η1-Cp and indicates the presence of fluxional behavior; VT 1H NMR measurements were conducted to investigate its origin. Both Cp resonances for 2Zr and 2Hf were broad at room temperature (Figure 4). 1

Figure 4. A stack plot of the variable temperature 1H NMR spectra of 2Zr (173−353 K, increasing in increments of 10 K up the page).

The occurrence of fluxional behavior for η1-Cp complexes is known for a number of metal complexes;29−31 it was Cotton and co-workers who first produced a qualitative argument to suggest that line shape analysis of the 1H NMR spectra could be instrumental in determining the mechanism operating.30 It was proposed that if 1,2-shifts of the metal atom around the ring were predominant, the line corresponding to HA should broaden more rapidly with increasing temperature than that corresponding to HB; 1,3-shifts would produce the opposite effect. On cooling 2Ti, it was possible to freeze out the η1-Cp rotation into three separate resonances, characteristic of allylic and vinylic proton environments. Upon warming, the resonance at δ = 5.38 ppm (C7D8) was found to collapse more rapidly, and implementing a multitude of NMR techniques, the chemical shift was assigned as HA, indicating that a 1,2-sigmatropic shift was operating. The data was modeled, and a regression analysis (R2 = 0.9934) between the temperatures 215 and 300 K permitted the extraction of thermodynamic parameters for the exchange mechanism; these were as follows: ΔG⧧ = 47 ± 4 kJ mol−1, ΔH⧧ = 29 ± 2 kJ mol−1, and ΔS⧧ = −65 ± 8 kJ mol−1. Upon heating 2Ti, the initially broad resonance at room temperature sharpened to a line width comparable to that of the η5-Cp, but no coalescence and, hence, interconversion (η1E

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X-ray Crystallographic Analysis of 3M and Comparison of the 1M, 2M, 3M Series. Both 3Zr and 3Hf are isostructural and crystallize in the P21/n space group with a single molecule in the asymmetric unit, a model of which is shown in Figure 5. As seen before for 1M and 2M, the Zr and Hf analogues have extremely similar unit cell dimensions and structural parameters. The eight complexes synthesized above make a comprehensive set of group 4 Pn* mixed sandwich compounds. By comparing and contrasting their structural parameters, the effect of permethylation of the Cp moiety and descending the group will be displayed. A table of the pertinent bond lengths is presented in Table 1. A couple of trends can be identified from Table 1. Intriguingly, the Ti and Hf species have almost identical fold angles. Ti4+ and Hf4+ have different ionic radii (0.68 Å for Ti and 0.78 Å for Hf),32 and as such one might expect the Ti complexes to have a greater fold angle compared to Hf. However, due to the more diffuse nature of Hf atomic orbitals, there is expected to be greater orbital overlap with the Pn* ligand, compensating for the loss in aromaticity. It is down to this interplay of factors that Ti and Hf have similar fold angles. Zr represents the middle ground, being larger than Ti (ionic radius of Zr4+ is 0.79 Å),32 but having less diffuse orbitals than Hf (and therefore poorer orbital overlap) leading to smaller fold angles for the Zr congeners. Comparing the mono- and bis-Cp complexes we see that the structural parameters are, in general, extremely similar, with only a slight increase in fold angle for the bis-Cp derivatives of each metal (approximately 0.7°). Comparing the Cp and Cp* complexes (1M and 3M) generally reveals a slight lengthening of both the M−Pn*cent and M−Cp/Cp*cent distances. This may be due to the increased steric bulk of the Cp* ligand forcing the bulky Pn* ligand further out to minimize the steric interaction. Solution Ethylene Polymerization Using 1M, 2M, and 3M (M = Zr and Hf). Complexes 1M, 2M, and 3M (where M = Zr or Hf) were tested for homogeneous ethylene polymerization. Each polymerization data point corresponds to at least two sets of polymerizations. Initial testing was carried out with 1 mg catalyst loading, an initial [MAO]0/[Zr]0 ratio of 1000, and 2 bar pressure of ethylene gas in hexane. However, under these conditions, effective stirring of the polymerization reaction with 1M and 2M proved difficult, with a quantity of polyethylene forming quickly and seizing the reaction mixture. However, 3M demonstrated lower activities (156.5 kg mol−1 h−1 bar−1 for the zirconium congener and 78.3 kg mol−1 h−1 bar−1 for the hafnium species). Due to this, further in-depth investigations with 3M were not undertaken. It is proposed that the active site in 3M is too sterically crowded compared to 1M and 2M for efficient catalysis

Scheme 3. Synthesis of Pn*MCp*Cl (3M; M = Zr and Hf)

Figure 5. Solid-state molecular structure of 3Hf. Thermal ellipsoids at 50% probability, H atoms omitted for clarity.

H and 13C NMR Spectroscopy of 3M. The 1H NMR spectra of 3M consists of four resonances in a 6:6:15:6 intensity ratio (δ = 2.08, 1.95, 1.77, and 1.67 ppm for 3Zr and δ = 2.20, 2.11, 1.80, and 1.66 ppm for 3Hf respectively), demonstrating 3M to be Cs symmetric. The 13C NMR shows the expected 10 resonances, eight from the Pn* moiety and two from the Cp* ring. This implies that (in a manner analogous to 1M and 2M) there is a mirror plane running through the metal atom and the bridgehead bond in solution, giving two NWT-Me environments. Upon heating no change in peak shape is seen, demonstrating that the energetic barrier to rotation of the Pn* unit is higher than the NMR time scale at temperatures up to 100 °C. This is due to the increased steric bulk of Cp* vs Cp, demonstrating that 3M has a higher energetic barrier to ring rotation than 1M. 1

Table 1. A Selection of Structural Parameters of 1M, 2M, and 3M

Pn*TiCpCl Pn*TiCp2 Pn*ZrCpCl Pn*ZrCp2 Pn*ZrCp*Cl Pn*HfCpCl Pn*HfCp2 Pn*HfCp*Cl

fold angle, deg

M−Cl or M−(η1-Cp) (Å)

30.9(2) 31.6(2) 30.3(3) 30.9(2) 29.3(2) 30.9(6) 31.8(4) 30.3(3)

2.506(1) 2.439(3) 2.548(1) 2.476(2) 2.526(1) 2.517(2) 2.441(4) 2.501(9)

M−Pn*cent (Å) 1.997(2) 1987(2) 2.109(2) 2.119(1) 2.121(1) 2.089(3) 2.097(2) 2.101(2)

F

1.992(2) 2.009(2) 2.109(2) 2.107(1) 2.135(1) 2.090(3) 2.081(2) 2.117(2)

M−Cpcent/M−Cp*cent (Å) 2.063(2) 2.054(3) 2.219(2) 2.215(2) 2.253(1) 2.190(3) 2.186(3) 2.186(3)

dx.doi.org/10.1021/om5004754 | Organometallics XXXX, XXX, XXX−XXX

Organometallics

Article

(in 20 °C increments). The results are presented in Figure 7. Even at only slightly elevated (40 °C) temperatures, both 1Zr

to occur. To date detailed studies of the mechanism of the initiation of polymerization have not been undertaken, however it is believed that a similar mechanism to those seen with trisCp rare earths may be in effect. It is proposed that MAO may abstract a cyclopentadienyl ring, allowing for coordination of ethylene and subsequent insertion into the remaining M−Cp bond.33 Experiments to ascertain whether this is the case (e.g., limited addition of ethylene and subsequent analysis of the polymer end groups) are considered beyond the scope of this work. An alternative mechanism could involve the lowering of the hapticity of the Pn* allowing for methylation by MAO and subsequent coordination and insertion of ethylene, however there is no precedent for this in the literature. Further polymerization runs were carried out with 1M, 2M, and 3M at lower catalytic loading (0.1 mg) with an initial [MAO]0/[Zr]0 ratio of 1000. At this concentration effective stirring still could not be maintained for the complete hour, however using a 30 min run an accurate and reproducible activity could be elucidated (Figure 6) [4176 (1Zr), 1009 (1Hf),

Figure 7. Ethylene polymerization activities using 1Zr (red square) and 2Zr (blue diamond) as initiators. Polymerization conditions: 0.1 mg catalyst loading, [MAO]0/[Zr]0 = 1000, 30 min, 2 bar pressure ethylene gas).

and 2Zr still retain very high catalytic activity (2779 and 4035 kg mol−1 h−1 bar−1 respectively). Complex 1Zr displays a significant drop in activity below 60 °C, and 2Zr shows a considerable increase in activity at 80 °C (5952 kg mol−1 h−1 bar−1). It is therefore proposed that the optimum temperature for 1Zr is approximately 70 °C, whereas 2Zr has a higher optimum temperature at 80 °C. Above these temperatures we see a sharp drop in the activity of the complexes, as is expected as the rate of deactivation increases. As expected, an increase in temperature of polymerization led to decreasing molecular weights (from around 700,000 kg mol−1 at 40 °C to below 100,000 kg mol−1 at 100 °C), and the polydispersities were generally low (Figure 8). Molecular weights for 2M were generally higher than those when 1M was used (e.g., at 40 °C, Mw were 764,532 and 697,259 kg mol−1 respectively). The polydispersities at 100 °C are very low (Mw/Mn of 2.77 for 1M and 2.79 for 2M), Figure 9.

Figure 6. Ethylene polymerization activities using 1M, 2M, and 3M (M = Zr and Hf) as initiators. Polymerization conditions: 0.1 mg catalyst loading, [MAO]0/[Zr]0 = 1000, 80 °C, 30 min, 2 bar pressure ethylene gas).

4194 (2Zr), and 247.1 kg mol−1 h−1 bar−1 (2Hf)]. As is often the case for group 4 catalysts the hafnium congeners produced lower yields of polyethylene and (despite the higher molecular weight) had lower activities than the zirconium congeners.34−36 The activities of the zirconium catalysts are very high on the Gibson scale,37 and as such these two were further optimized, with the highest recorded activity being 6993 kg mol−1 h−1 bar−1 (using complex 2Zr after 15 min at 80 °C). As expected, use of a lower ratio of [MAO]0/[Zr]0 caused a drop in activity from 4176 to 2530 kg mol−1 h−1 bar−1 for 1Zr and from 4194 to 2921 kg mol−1 h−1 bar−1 for 2Zr. The molecular weights, Mw, of the synthesized polyethylene vary from 171,150 to 399,343 kg mol−1, and the polydispersities, Mw/Mn, are between 2.76 and 4.92. An increase of the initial ratio of [MAO]0/[Zr]0 from 300 to 1000 using 1Zr led to increasing molecular weights and similar polydispersities (Mw of 201,708 and 224,185 kg mol−1 and Mw/Mn of 3.17 and 3.56 respectively). A change from zirconium to hafnium, 2Zr to 2Hf, shows a decrease in molecular weight and an increase in polydispersity (Mw of 202,902 and 171,150 kg mol−1 and Mw/ Mn of 2.76 and 4.92 respectively). Variation of the temperature of polymerization using complexes 1Zr and 2Zr has been investigated. Polymerization experiments were therefore undertaken between 40 and 100 °C

Figure 8. Molecular weights and polydispersities of the polyethylene synthesized at increasing temperature using 1Zr (black square) and 2Zr (red circle). Polydispersities are represented in parentheses. Polymerization conditions: 0.1 mg catalyst loading, [MAO]0/[Zr]0 = 1000, 30 min, 2 bar pressure ethylene gas). G

dx.doi.org/10.1021/om5004754 | Organometallics XXXX, XXX, XXX−XXX

Organometallics

Article

solvents were stored in potassium mirrored ampules. THF was distilled from a purple sodium/benzophenone ketyl indicator and stored under nitrogen in oven-dried ampules. Diethyl ether was distilled from Na/K and stored in potassium mirrored ampules. Deuterated NMR solvents were purchased from Goss Scientific and were dried over a potassium mirror and freeze−thaw−degassed three times. A typical crystal was mounted on a MiTeGen Micromounts using perfluoropolyether oil and cooled rapidly to 150 K in a stream of nitrogen gas using an Oxford Cryosystems Cryostream unit.38 Data were collected with an Enraf-Nonius Kappa CCD diffractometer, using graphite-monochromated Mo Kα radiation (λ = 0.71073 Å). Raw frame data were reduced using the DENZO-SMN package.39 Intensity data were corrected using multiscan method with SCALEPACK (within DENZO-SMN). The structures were solved using direct methods with SIR9240 and refined using full-matrix least-squares refinement on all F2 data using the CRYSTALS program suite.41,42 In general distances and angles were calculated using the full covariance matrix. Dihedral angles were calculated using PLATON.43 All DFT calculations were performed with the ORCA program package.44 The geometry optimizations of the complexes and singlepoint calculations on the optimized geometries were carried out at the B3LYP level of DFT.45−47 This hybrid functional often gives better results for transition metal compounds than pure gradient-corrected functionals, especially with regard to metal−ligand covalency.48 For the geometry optimizations the all-electron Gaussian basis sets were those developed by the Ahlrichs group.49−51 Triple-ζ quality basis sets def2-TZVP with one set of polarization functions on the metals and on the atoms directly coordinated to the metal center were used. For the carbon and hydrogen atoms, slightly smaller polarized split-valence def2-SV(P) basis sets were used, that were of double-ζ quality in the valence region and contained a polarizing set of d-functions on the non-hydrogen atoms. Auxiliary basis sets were chosen to match the orbital basis.52−54 The RIJCOSX15 approximation was used to accelerate the calculations.55−57 Orbitals were generated with the program Chimera.58 Elemental analyses were carried out by Stephen Boyer of London Metropolitan University. IR spectra were recorded on a Nicolet iS5 ThermoScientific spectrometer (range 4000−400 cm−1, resolution 1 cm−1) as KBr disks. Samples were prepared in the glovebox, mixed and ground with anhydrous KBr, and then pressed into disks using an inhouse purpose built press and holder, and the spectra were then recorded immediately. EI-MS were recorded by Colin Sparrow of the Chemistry Research Laboratory, University of Oxford. Solution phase NMR samples were prepared in a glovebox under a nitrogen atmosphere and sealed in 5 mm Young type concentric stopcocks. NMR spectra were obtained either on a Varian Mercury VX-Works 300 MHz spectrometer or a Varian Venus 300 MHz spectrometer. [Pn*Ti(μ-Cl)Cl]2,14 [Pn*Zr(μ-Cl)3/2]2(μ-Cl)2Li(THF)x(Et2O)y,14 [Pn*Hf(μ-Cl)3/2]2(μ-Cl)2Li(THF)x(Et2O)y,14 NaCp,59 and KCp*60 were all prepared via published procedures. Synthesis of 1Ti. To a Schlenk charged with [Pn*Ti(μ-Cl)Cl]2 (100 mg, 0.164 mmol) and NaCp (29 mg, 0.328 mmol) was added −78 °C THF, and the mixture was stirred for 30 min. On warming to room temperature the reaction mixture passed through light orange and then red in color and was stirred for a further hour. Solvent was removed under reduced pressure and subjected to a dynamic vacuum ( 2σ(I)]. CCDC 1000521. Crystal data of 1Hf: HfC19H23Cl, monoclinic (P21/c), a = 13.0761(6) Å, b = 15.3535(6) Å, c = 8.6382(4) Å, β = 108.8040(18)°, V = 1641.68(13) Å3, Z = 4, λ = 0.71073 Å, T = 150(2) K, μ = 6.51 mm−1, Dcal = 1.883 Mg m−3, 3666 independent reflections, [R(int) = 0.035]; R1 = 0.036, wR2 = 0.101 [I > 2σ(I)]. CCDC 1000522. Crystal data of 2Ti: TiC24H28, triclinic (P1)̅ , a = 8.5580(2) Å, b = 8.8507(2) Å, c = 14.4309(4) Å, α = 76.3148(12)°, β = 87.3980(12)°, γ = 62.8115(12)°, V = 942.19(4) Å3, Z = 2, λ = 0.71073 Å, T = 150(2) K, μ = 0.46 mm−1, Dcal = 1.281 Mg m−3, 4256 independent reflections, [R(int) = 0.050]; R1 = 0.056, wR2 = 0.144 [I > 2σ(I)]. CCDC 1000523. Crystal data of 2Zr: ZrC24H28, triclinic (P1̅), a = 75.8081(7) Å, b = 808783(1) Å, c = 14.3868(2) Å, α = 75.8081(7)°, β = 86.3927(7)°, λ = 62.3999(7), V = 967.39(2) Å3, Z = 2, λ = 0.71073 Å, T = 150(2) K, μ = 0.57 mm−1, Dcal = 1.400 Mg m−3, 4411 independent reflections, [R(int) = 0.013]; R1 = 0.027, wR2 = 0.068 [I > 2σ(I)]. CCDC 1000524. Crystal data of 2Hf: HfC24H28, Triclinic (P1̅), a = 8.7793(2) Å, b = 8.8597(2) Å, c = 14.3991(4) Å, α = 75.8512(11)°, β = 86.5774(10)°, γ = 62.3409(12)° V = 960.02(4) Å3, Z = 2, λ = 0.71073 Å, T = 150(2) K, μ = 5.44 mm−1, Dcal = 1.712 Mg m−3, 4342 independent reflections,

0.242 mmol) and NaCp (45 mg, 0.509 mmol) and was cooled to −196 °C before THF (50 mL) was added. The frozen mixture was allowed to warm to room temperature and stirred for a further hour before volatiles were removed in vacuo and subjected to a dynamic vacuum ( 2σ(I)]. CCDC 1000526. Crystal data of 3Hf: HfC24H33Cl, monoclinic (P21/n), a = 8.7602(1) Å, b = 15.9512(3) Å, c = 15.6731(3) Å, β = 101.6849(8)°, V = 2144.70(6) Å3, Z = 4, λ = 0.71073 Å, T = 150(2) K, μ = 4.99 mm−1, Dcal = 1.658 Mg m−3, 4895 independent reflections, [R(int) = 0.032]; R1 = 0.032, wR2 = 0.082 [I > 2σ(I)]. CCDC 1000527. Polymerization Technique. A stock solution of each catalyst was prepared in a glovebox prior to use by dissolving 10 mg of the given compound in 10 mL of toluene. A Rotaflo ampule containing a stirrer bar was charged with 1000 molar equiv of MAO and 49 mL of hexane. To this was added 1 mL of the stock solution of compound (for 0.1 mg catalyst loading 0.1 mL of the stock solution was used, with 50 mL of hexane). The ampule was quickly transferred onto a Schlenk line, degassed in vacuo for 5 min, and then heated to 80 °C with vigorous stirring. The ampule was then pressurized with ethylene gas (2 bar) for up to 1 h. The polymerization was quenched by exposing the ampule to vacuum. The polyethylene was collected on a frit in air and washed with pentane (3 × 50 mL) before drying in vacuo and being weighed. Gel permeation chromatography (GPC) data were collected by Supote Sibtute (SCG chemicals Ltd.) on a high temperature gel permeation chromatograph with an IR5 infrared detector (GPC-IR5). Samples were prepared by dissolution in 1,2,4-trichlorobenzene (TCB) containing 300 ppm of 3,5-di-tert-butyl-4-hydroxytoluene (BHT) at 160 °C for 90 min and then filtered with a 10 μm SS filter before being passed through a GPC column. The samples were run under a flow rate of 0.5 mL min−1 using TCB containing 300 ppm of BHT as mobile phase with 1 mg mL−1 BHT added as a flow rate marker. The GPC column and detector temperature were set at 145 and 160 °C respectively.

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ASSOCIATED CONTENT

S Supporting Information *

Crystallographic data in CIF format. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS J.-C.B. thanks SCG Chemicals, Ltd, Thailand, for funding and for GPC analyses. REFERENCES

(1) Armit, J. W.; Robinson, R. J. Chem. Soc. Trans. 1922, 121, 827. (2) Katz, T. J. J. Am. Chem. Soc. 1960, 82, 3784−3785. (3) Katz, T. J.; Rosenberger, M. J. Am. Chem. Soc. 1962, 84, 865−866. (4) Green, J. C.; Green, M. L. H.; Parkin, G. Chem. Commun. (Cambridge) 2012, 48, 11481−11503. (5) Abbasali, Q. A.; Cloke, F. G. N.; Hitchcock, P. B.; Joseph, S. C. P. Chem. Commun. 1997, 1541−1542. (6) Cloke, F. G. N.; Hitchcock, P. B.; Kuchta, M. C.; Morley-Smith, N. A. Polyhedron 2004, 23, 2625−2630. (7) Jonas, K.; Kolb, P.; Kollbach, G.; Gabor, B.; Mynott, R.; Angermund, K.; Heinemann, O.; Krüger, C. Angew. Chem., Int. Ed. Engl. 1997, 36, 1714−1718. (8) Summerscales, O. T.; Cloke, F. G. N. Coord. Chem. Rev. 2006, 250, 1122−1140. (9) Kilpatrick, A. F. R.; Cloke, F. G. N. Chem. Commun. (Cambridge) 2014, 50, 2769−2771. J

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(48) Neese, F.; Solomon, E. I. Magnetism: From Molecules to Materials; Miller, J. S., Drillon, M., Eds.; Wiley: New York, 2002; p 345. (49) Schafer, A.; Horn, H.; Ahlrichs, R. J. Chem. Phys. 1992, 97, 2571. (50) Schafer, A.; Huber, C.; Ahlrichs, R. J. Chem. Phys. 1994, 100, 5829. (51) Weigend, F.; Ahlrichs, R. Phys. Chem. Chem. Phys. 2005, 7, 3297−3305. (52) Eichkorn, K.; Treutler, O.; Ö hm, H.; Häser, M.; Ahlrichs, R. Chem. Phys. Lett. 1995, 242, 652−660. (53) Eichkorn, K.; Treutler, O.; Ö hm, H.; Häser, M.; Ahlrichs, R. Chem. Phys. Lett. 1995, 240, 283−290. (54) Eichkorn, K.; Weigend, F.; Treutler, O.; Ahlrichs, R. Theor. Chem. Acc. 1997, 97, 119−124. (55) Neese, F.; Wennmohs, F.; Hansen, A.; Becker, U. Chem. Phys. 2009, 356, 98−109. (56) Kossmann, S.; Neese, F. Chem. Phys. Lett. 2009, 481, 240−243. (57) Neese, F. J. Comput. Chem. 2003, 24, 1740−1747. (58) Pettersen, E. F.; Goddard, T. D.; Huang, C. C.; Couch, G. S.; Greenblatt, D. M.; Meng, E. C.; Ferrin, T. E. J. Comput. Chem. 2004, 25, 1605−1612. (59) Panda, T. K.; Gamer, M. T.; Roesky, P. W. Organometallics 2003, 22, 877−878. (60) Herrmann, W. A.; Salzer, A. Synthetic Methods of Organometallic and Inorganic Chemistry, 1st ed.; Herrmann, W. A., Ed.; Georg Thieme Verlag: New York, 1996.

K

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