Synthesis, Structure, and Reactivity of Group VI Metal Complexes

Mar 8, 2013 - (9) (a) Krueger, S. T.; Poli, R.; Rheingold, A. L.; Staley, D. L. Inorg. Chem. 1989, 28, 4599. (b) Krueger, S. T.; Owens, B. E.; Poli, R...
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Synthesis, Structure, and Reactivity of Group VI Metal Complexes Bearing Group IV Metallocenyldiphosphine Moieties and a Pentamethylcyclopentadienyl Ligand Takamasa Miyazaki, Yoshiaki Tanabe, Masahiro Yuki, Yoshihiro Miyake, and Yoshiaki Nishibayashi* Institute of Engineering Innovation, School of Engineering, The University of Tokyo, Yayoi, Bunkyo-ku, Tokyo 113-8656, Japan S Supporting Information *

ABSTRACT: A series of group VI metal complexes bearing a group IV metallocenyldiphosphine moiety and a pentamethylcyclopentadienyl ligand have been prepared and structurally characterized. Reactions of trinuclear complexes with KC8 give the corresponding neutral chloride-bridged complexes. Further reaction of the resulting chloride-bridged complex with NaBArF4 gives the corresponding monocationic chloride-bridged complex. On the other hand, reactions of the neutral chloride-bridged complexes with n-BuLi give the corresponding chloride-bridged alkyl complexes, where the group IV metallocene(III) moiety and the coordinatively unsaturated group VI metal(II) center are in close proximity. Spectroscopic and crystallographic studies of the reaction of a chloride-bridged alkyl complex with CO or a protic acid indicate that further disproportionation takes place at the alkyl zirconocene(III) moiety.

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INTRODUCTION The unique properties of multimetallic complexes such as electronic interaction between metal centers and cooperative activation of substrates by each of the metal centers may develop novel reactions.1 In particular, heterobimetallic complexes where the different metal centers are in close proximity seem to offer fascinating opportunities to realize unique molecular transformations. Although unique stoichiometric reactions using heterobimetallic complexes have been widely reported, relatively few catalytic reactions using heterobimetallic complexes as catalysts have been reported so far.2 Recently, we have reported the unique reactivity of group VIII metal complexes bearing group IV metallocenyldiphosphine moieties toward catalytic dehydrogenation of amine− boranes, where cooperative activation of amine−boranes by group IV and group VIII metal centers is essential for efficient catalysis.3 In addition, we have also reported the catalytic propargylic substitution reactions of propargylic alcohols catalyzed by ruthenium complexes bearing group IV metallocenyldiphosphine moieties, where the cooperative participation of the group IV metal chloride and ruthenium− allenylidene moieties is essential for the catalysis.4 In light of our continuing research interest in the synthesis and application of heterobimetallic complexes bearing metallocenyldiphosphine moieties, we initiated an effort to prepare group VI metal complexes bearing group IV metallocenyldiphosphine moieties. Several research groups have previously reported the preparation of a series of group VI metal carbonyl complexes bearing group IV metallocenyldiphosphine moieties.5 However, most of their detailed molecular structures © 2013 American Chemical Society

have not been confirmed crystallographically. Moreover, most of their coordination sites on the group VI metal centers are occupied by carbonyl ligands, which blocks reactive sites to realize the bifunctional reactivity derived from the heterobimetallic moiety such as cooperative activation of substrates. Herein, we wish to report the preparation and molecular structures of a series of group VI metal complexes bearing group IV metallocenyldiphosphine moieties and pentamethylcyclopentadienyl ligand as well as their unique redox behavior.

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RESULTS AND DISCUSSION Preparation and Characterization of Group VI Metal Complexes Bearing Group IV Metallocenyldiphosphine Moieties. At first, we prepared trinuclear group VI metal complexes bearing group IV metallocenyldiphosphine moieties. Treatment of a dimeric group VI metal tetrachloride complexes bearing a pentamethylcyclopentadienyl ligand, [Cp*M1Cl4]2 (Cp* = η5-C5Me5, M1 = Mo, W), with 1 equiv of the group IV metallocenyldiphosphines [(η5-C5H4PEt2)2MCl2] (M = Zr, Hf)3,4 in CH2Cl2 at room temperature for 1 h gave the trinuclear complexes [ ZrCl 2 {(μ-η 5 :η 1 -C 5 H 4 PEt 2 )(MoCl 4 Cp*)} 2 ] (1a), [HfCl 2 {(μ-η 5 :η 1 -C 5 H 4 PEt 2 )(MoCl 4 Cp*)} 2 ] (1b), and [ZrCl 2 {(μ-η 5 :η 1 -C 5 H 4 PEt 2 )(WCl4Cp*)}2] (1c) in 99%, 85%, and 95% yields, respectively (Scheme 1). The molecular structures of these paramagnetic complexes were characterized by X-ray crystallographic Received: January 31, 2013 Published: March 8, 2013 2007

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Scheme 1. Preparation of Trinuclear Complexes

analyses, which clearly demonstrated that two Cp*MCl4 (M = Mo, W) moieties are coordinated to the two different phosphorus atoms of the group IV metallocenyldiphosphines to form elongated quasi-linear trinuclear structures.6 Next, we investigated the reduction of the trinuclear complexes 1a−c (Scheme 2). When 1a was reacted with 4

Figure 1. ORTEP drawing of 2a. Hydrogen atoms are omitted for clarity. Selected interatomic distances (Å) and angles (deg): Zr(1)···Mo(1), 5.2526(6); Zr(1)−Cl(1), 2.4358(11); Zr(1)−Cl(2), 2.4416(13); Mo(1)−Cl(3), 2.4284(10); Mo(1)−Cl(4), 2.4316(11); Mo(1)−P(1), 2.5104(11); Mo(1)−P(2), 2.5073(10); Cl(1)−Zr(1)− Cl(2), 95.84(4); Cl(3)−Mo(1)−Cl(4), 150.74(3); P(1)−Mo(1)− P(2), 93.83(3).

Scheme 2. Reduction of Trinuclear Complexes with KC8

C5H4PEt2)(μ-Cl)MoClCp*] (3a), [HfCl(μ-η5:η1-C5H4PEt2)(μ-Cl)MoClCp*] (3b), and [ZrCl(μ-η5:η1-C5H4PEt2)(μ-Cl)WClCp*] (3c) in 65%, 79%, and 63% yields, respectively (Scheme 2). The 1H NMR spectrum of 3a exhibits a set of four cyclopentadienyl resonances (δ 6.68, 5.57, 5.27, 4.95, 2H each) and a set of two ethyl resonances (δ 2.54 (2H), 1.28 (6H), for CH2; 1.16 (6H), 0.90 (6H) for CH3), while the 31P{1H} NMR spectrum shows only one singlet resonance at δ 26.1. These spectroscopic features are comparable to those of [ZrCl(μη5:η1-C5H4PPh2)(μ-Cl)Mo(CO)3],5e suggesting that 3a has a Cs symmetry where two cyclopentadienylphosphines bridging the ZrCl2 and Cp*MoCl moieties lie tilted to each other. The molecular structure of 3a was confirmed by X-ray crystallography. As shown in Figure 2, the interatomic distance between Zr and Mo atoms (3.4532(3) Å) is too long to form a metal− metal single bond, where one chloride ligand bridges between Zr and Mo atoms. The configuration around the Mo atom of 3a adopts a typical four-legged piano-stool coordination geometry. It is interesting to note that the bite angle of the diphosphine moiety of 3a (133.88(4)°) is much larger than that of 2a (93.83(3)°). As for 3a, the bridging chloride ligand places the Zr and Mo atoms in close proximity, which may contribute to the increase of the bite angle of the diphosphine moiety in comparison to that of 2a having no bridging chloride ligand. The diamagnetic nature and the absence of a direct bonding interaction between the two metal centers of 3a indicate that the formal oxidation state of the heterobimetallic moiety is Zr(IV)/Mo(II). The spectroscopic and structural features6 of 3b,c are comparable to those of 3a. Further reaction of 3a with 1 equiv of NaBArF4 (ArF = 3,5(CF3)2C6H3) in C6H5F at room temperature for 12 h gave the cationic chloride-bridged heterobimetallic complex [Zr(μ-η5:η1C5H4PEt2)2(μ-Cl)2MoCp*]BArF4 (4a) in 40% yield (Scheme 3). The 31P{1H} NMR spectrum shows only one singlet resonance at δ 23.6, suggesting that 4a has Cs symmetry. The molecular structure of 4a was characterized by X-ray crystallography. As shown in Figure 3, the distance between the Zr and Mo atoms of 4a (3.0271(9) Å) is between the values of the sum of the metal atom radii of the Zr and Mo atoms

equiv of KC8 in THF at room temperature for 24 h, the heterobimetallic complex [ZrCl 2 (μ-η 5 :η 1 -C 5 H 4 PEt 2 ) 2 MoCl2Cp*] (2a) was obtained in 26% yield together with the formation of [Cp*MoCl2]2 as a byproduct.7 The value of the magnetic moment of 2a is 1.67 μB, which is almost the same as the expected spin-only value of 1.73 μB corresponding to a doublet (S = 1/2) spin system.8 The molecular structure of 2a was confirmed by X-ray crystallographic analysis. As depicted in Figure 1, the Zr and Mo atoms are bridged by a set of cyclopentadienylphosphines with an interatomic distance of 5.2526(6) Å between the two metals, which is greater than the value of the Zr···Mo separations (4.8524(3) Å) found in the analogous Zr−Mo heterobimetallic complex [ZrCl2(μ-η5:η1-C5H4PPh2)2Mo(CO)4],5a ruling out any direct bonding interaction between the two metal atoms. The configuration around the Mo atom adopts a four-legged piano-stool coordination geometry, a typical structure of mononuclear Mo complexes bearing phosphine and pentamethylcyclopentadienyl ligands [Cp*MoX2(PR3)].9 The paramagnetic nature as well as no direct bonding interaction between the two metal centers of 2a indicate that the formal oxidation state of the heterobimetallic moiety is Zr(IV)/ Mo(III). On the other hand, reactions of 1a−c with a larger amount (6 equiv) of KC8 in THF at room temperature for 24 h gave the chloride-bridged heterobimetallic complexes [ZrCl(μ-η5:η12008

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Figure 3. ORTEP drawing of 4a. Hydrogen atoms and the BArF4 anion are omitted for clarity. Selected interatomic distances (Å) and angles (deg): Zr(1)···Mo(1), 3.0271(9); Zr(1)−Cl(1), 2.5792(12); Zr(1)−Cl(2), 2.6059(12); Mo(1)−Cl(1), 2.4656(12); Mo(1)−Cl(2), 2.4968(12); Mo(1)−P(1), 2.5847(13); Mo(1)−P(2), 2.5924(13); Cl(1)−Zr(1)−Cl(2), 103.39(4); Zr(1)−Cl(1)−Mo(1), 73.71(4); Zr(1)−Cl(2)−Mo(1), 72.74(4); Cl(1)−Mo(1)−Cl(2), 110.15(5); P(1)−Mo(1)−P(2), 140.08(5).

Figure 2. ORTEP drawing of 3a. Hydrogen atoms are omitted for clarity. Selected interatomic distances (Å) and angles (deg): Zr(1)···Mo(1), 3.4532(3); Zr(1)−Cl(1), 2.5503(9); Zr(1)−Cl(2), 2.5584(14); Mo(1)−Cl(1), 2.4925(13); Mo(1)−Cl(3), 2.5131(12); Mo(1)−P(1), 2.4943(13); Mo(1)−P(2), 2.4932(15); Cl(1)−Zr(1)− Cl(2), 80.07(4); Cl(1)−Mo(1)−Cl(3), 135.04(4); P(1)−Mo(1)− P(2), 133.88(4).

On the other hand, the reaction of 3a with 2 equiv of nBuLi in toluene at room temperature for 12 h gave the chloridebridged heterobimetallic alkyl complex [ZrnBu(μ-η 5:η 1 C5H4PEt2)(μ-Cl)MoCp*] (5a) in 39% yield (Scheme 3). The value of the magnetic moment of 5a (1.42 μB) indicates the paramagnetic nature of 5a, corresponding to a doublet (S = 1 /2) spin system. This result strongly suggests that one-electron reduction occurred during the conversion of 3a into 5a. The molecular structure of 5a was unequivocally determined by Xray crystallography. As shown in Figure 4, the interatomic

Scheme 3. Preparation of 4a and 5

(2.92 Å) and the sum of their covalent radii (3.29 Å), suggesting a direct bonding interaction between the Zr and Mo atoms.10 However, the formation of a metal−metal bond in 4a seems impossible, due to the existence of two bridging chloride ligands in 4a, which fulfill not only the configuration around the Mo center as the typical four-legged piano-stool coordination geometry but also the coordinatively saturated Mo center in 4a. We consider that two bridging chloride ligands in 4a force the Zr and Mo atoms into such a close approach. The bite angle of the diphosphine moiety of 4a (140.08(5)°) is increased in comparison with that of 3a (133.88(4)°), which presumably is due to the existence of two bridging chloride ligands in 4a.

Figure 4. ORTEP drawing of 5a. Hydrogen atoms are omitted for clarity. Selected interatomic distances (Å) and angles (deg): Zr(1)···Mo(1), 3.3858(3); Zr(1)−Cl(1), 2.5111(5); Zr(1)−C(29), 2.3266(17); C(29)−C(30), 1.552(3); C(30)−C(31), 1.531(3); C(31)−C(32), 1.519(3); Mo(1)−Cl(1), 2.4438(4); Mo(1)−P(1), 2.4421(4); Mo(1)−P(2), 2.4156(5); Cl(1)−Zr(1)−C(29), 78.28(5); Zr(1)−C(29)−C(30), 123.03(11); C(29)−C(30)−C(31), 112.13(14); C(30)−C(31)−C(32), 114.83(15); Zr(1)−Cl(1)− Mo(1), 86.197(14); P(1)−Mo(1)−P(2), 94.640(14). 2009

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chloride ligand on the group IV metal atom of B with 1 equiv of BuLi proceeds to form 5 as the final product. Furthermore, we have investigated the reactivity of the newly synthesized heterobimetallic complex 5a. Treatment of 5a with 1 atm of CO in hexane at room temperature for 16 h gave [ZrnBu(μ-η5:η1-C5H4PEt2)2(μ-Cl)MoClCp*] (6a) in 19% yield as the only identifiable product, which was confirmed by the NMR spectra and X-ray crystallography6 (Scheme 5). The

distance between the Zr and Mo atoms (3.3858(3) Å) is larger than the sum of the covalent radii of the Zr and Mo atoms (3.29 Å),10 suggesting that no bonding interaction exists between the two metal atoms. In contrast to the structures of 2a−4a, the configuration around the Mo center adopts a threelegged piano-stool coordination geometry, which is typical for a coordinatively unsaturated 16-electron mononuclear Mo(II) complex bearing diphosphine and pentamethylcyclopentadienyl ligands.11 The bite angle of the diphosphine moiety of 5a (94.640(14)°) is significantly decreased in comparison to those of 3a (133.88(4)°) and 4a (140.08(5)°) because of the dynamic configurational change around the Mo center. The nbutyl ligand on the Zr atom is opposite to the coordinatively unsaturated Mo atom. In addition, the structural parameters related to the n-butyl ligand indicate no interaction between the n-butyl ligand and both Zr and Mo atoms. These structural data suggest that the formal oxidation state of the heterobimetallic moiety of 5a is Zr(III)/Mo(II). It is noteworthy that examples of the isolation of monoalkyl zirconocene(III) complexes are quite limited,12 because most of them readily decompose via β-hydrogen elimination of their alkyl ligand.13 To the best of our knowledge, 5a is the first successful example of the isolation and structural characterization of an n-butyl zirconocene(III) complex. We consider that coordination of the chloride ligand of the adjacent Cp*MoII moiety to the Zr(III) atom of 5a precludes the nbutyl zirconocene(III) moiety from undergoing degradation. Reactions of 3b,c with 2 equiv of nBuLi also gave the corresponding paramagnetic complexes [Hf nBu(μ-η 5 :η 1 C 5 H 4 PEt 2 )(μ-Cl)MoCp*] (5b) and [Zr n Bu(μ-η 5 :η 1 C5H4PEt2)(μ-Cl)WCp*] (5c) in 26% and 32% yields, respectively. The molecular structures of these complexes were unambiguously characterized by X-ray crystallography.6 It was revealed that they have also coordinatively unsaturated 16electron group VI metal moieties analogous to that of 5a. On the basis of these results, a plausible reaction pathway for the formation of 5 is shown in Scheme 4. At first, the reaction

n

Scheme 5. Reaction of 5a with CO

diamagnetic feature and the typical four-legged piano-stool coordination geometry around the Mo center of 6a are similar to those of 3a, suggesting that the formal oxidation state of the heterobimetallic moiety of 6a is Zr(IV)/Mo(II). A plausible reaction pathway is shown in Scheme 6. At first, CO coordinates to the Zr atom to give the Zr(III) Scheme 6. Plausible Reaction Pathway for the Formation of 6a

Scheme 4. Plausible Reaction Pathway for the Formation of 5 monocarbonyl complex C with the dissociation of the bridging chloride ligand from the Zr atom.16 Then, the zirconocene(III) moiety of C disproportionates to give 6a together with the formation of the corresponding Mo(I) monocarbonyl complex D.17 It is well-known that the dimer of zirconocene(III) monohalide readily disproportionates at room temperature under an atmospheric pressure of CO to give the corresponding zirconocene(IV) dihalide and zirconocene(II) dicarbonyl complexes.18 In this case, the zirconocene(III) monocarbonyl moiety of C disproportionates with the Cp*MoII monochloride moiety to give 6a and D. The IR and mass spectra of the crude reaction mixture indicate the formation of the monocarbonyl adduct C and the dicarbonyl adduct D during the reaction, which supports the plausible reaction pathway shown in Scheme 6.19 It is noteworthy that no formation of 6a was observed at all when the reaction was carried out under an atmospheric pressure of Ar, indicating that the bridging chloride ligand in 5a works to stabilize the reactive zirconocene(III) moiety and prevents it from disproportionation. On the other hand, when 5a was reacted with 1 equiv of [H(Et2O)2]BArF4 in C6H5F at room temperature for 16 h

of the group IV metallocene dichloride moiety of 3 with nBuLi proceeds to give the group IV metal(II) butene complex A and butane via the formation of the corresponding dibutyl group IV metallocene complex.13,14 Then, immediate comproportionation of A with the remaining 3 affords the chloride-bridged complex B together with the formation of butene. Indeed, other groups have already reported the comproportionation of in situ generated Cp2ZrII with the zirconocene(IV) derivatives to give the corresponding Zr(III) species.15 Finally, substitution of the 2010

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under an atmospheric pressure of Ar, the diamagnetic 4a was obtained as the sole identifiable product in 50% yield (Scheme 7). The formation of 4a clearly indicates the dissociation of the n-butyl ligand on the Zr atom of 5a as butane during the reaction.

Article

EXPERIMENTAL SECTION

General Methods. 1H NMR (270 MHz) and 31P NMR (109 MHz) spectra were recorded on a JEOL Excalibur 270 spectrometer in suitable solvents. 31P NMR chemical shifts were quoted relative to an external standard of 85% H3PO4. Elemental analyses were performed at the Microanalytical Laboratory of The University of Tokyo or on an Exeter Analytical CE-440 Elemental Analyzer. IR spectra were recorded on a JASCO FT/IR 4100 Fourier Transform infrared spectrophotometer. Mass spectra were measured on a JEOL Accu TOF JMS-T100LP mass spectrometer. Magnetic moments were measured by the Evans method.20 All reactions were carried out under a dry nitrogen atmosphere or in an argon-filled glovebox. Solvents were dried by general methods and degassed before use. [Cp*MCl4]2 (M = Mo, W),21 NaBArF4,22 [(η5-C5H4PEt2)2MCl2] (M = Zr, Hf),3,4 and [H(Et2O)2]BArF423 were prepared according to the literature procedures. nBuLi was used neat with the removal of hexane from a commercial hexane solution of nBuLi (1.60 M). Other reagents were purchased commercially and used as received. Preparation of [ZrCl2{(μ-η5:η1-C5H4PEt2)(MoCl4Cp*)}2] (1a). To a solution of [(η5-C5H4PEt2)2ZrCl2] (764 mg, 1.63 mmol) in CH2Cl2 (30 mL) was added [Cp*MoCl4]2 (1.20 g, 1.61 mmol), and the mixture was stirred at room temperature for 1 h. Then, the solvent was removed and the resulting brown residue was washed with hexane (10 mL × 2) to afford 1a as a brown solid (1.95 g, 1.61 mmol, quantitative). Effluorescent brown plates of 1a·0.375CH2Cl2 suitable for X-ray crystallography were obtained by layering hexane onto a CH2Cl2 solution of 1a. Anal. Calcd for C38.375H58.75Cl10.75Mo2P2Zr (1a·0.375CH2Cl2): C, 36.98; H, 4.75. Found: C, 36.51; H, 4.79. Preparation of [HfCl2{(μ-η5:η1-C5H4PEt2)(MoCl4Cp*)}2] (1b). To a solution of [(η5-C5H4PEt2)2HfCl2] (169 mg, 0.304 mmol) in CH2Cl2 (6 mL) was added [Cp*MoCl4]2 (226 mg, 0.303 mmol), and the mixture was stirred at room temperature for 1 h. Then, the solvent was removed and the resulting brown residue was washed with hexane (5 mL × 3) to afford 1b as a brown solid (336 mg, 85% isolated yield). Effluorescent brown plates of 1b·0.25CH2Cl2 suitable for X-ray crystallography were obtained by layering hexane onto a CH2Cl2 solution of 1b. Anal. Calcd for C 3 8 . 2 5 H 5 8 . 5 Cl 1 0 . 5 HfMoP 2 (1b·0.25CH2Cl2): C, 34.73; H, 4.46. Found: C, 34.35; H, 4.31. Preparation of [ZrCl2{(μ-η5:η1-C5H4PEt2)(WCl4Cp*)}2] (1c). To a solution of [(η5-C5H4PEt2)2ZrCl2] (469 mg, 1.00 mmol) in CH2Cl2 (20 mL) was added [Cp*WCl4]2 (922 mg, 1.00 mmol), and the mixture was stirred at room temperature for 1 h. Then, the solvent was removed and the resulting brown residue was washed with hexane (15 mL × 2) to afford 1c as a brown solid (1.32 g, 95% isolated yield). Effluorescent brown platelets of 1c·0.5CH2Cl2 suitable for X-ray crystallography were obtained by layering hexane onto a CH2Cl2 solution of 1c. Anal. Calcd for C38.5H59Cl11P2W2Zr (1c·0.5CH2Cl2): C, 32.28; H, 4.15. Found: C, 32.32; H, 4.01. Preparation of [ZrCl2(μ-η5:η1-C5H4PEt2)2MoCl2Cp*] (2a). To a slurry of KC8 (54.8 mg, 0.405 mmol) in toluene (4 mL) was added 1a (121 mg, 0.100 mmol), and the mixture was stirred at room temperature for 24 h. The resulting reaction mixture was filtered through a pad of Celite, and solvent was removed from the filtrate in vacuo. The resulting brown residue was recrystallized from benzene− hexane to afford 2a as brown platelets (19.9 mg, 0.0258 mmol, 26% isolated yield). Magnetic moment (Evans): μeff = 1.67 μB at 299 K. Anal. Calcd for C28H43Cl4MoP2Zr: C, 43.64; H, 5.62. Found: C, 43.51; H, 5.28. Preparation of [ZrCl(μ-η5:η1-C5H4PEt2)(μ-Cl)MoClCp*] (3a). To a slurry of KC8 (809 mg, 5.98 mmol) in THF (30 mL) was added 1a (1.21 g, 0.997 mmol), and the mixture was stirred at room temperature for 24 h. The resulting reaction mixture was filtered through a pad of Celite, and solvent was removed from the filtrate in vacuo. The resulting reddish brown residue was recrystallized from benzene−hexane to afford 3a·0.5C6H6 as orange platelets (501 mg, 0.647 mmol, 65% isolated yield). 1H NMR (C6D6): δ 6.68 (br, 2H, C5H4), 5.57 (br, 2H, C5H4), 5.27 (br 2H, C5H4), 4.95 (br, 2H, C5H4), 2.61−2.47 (m, 2H, PCH2), 1.71 (s, 15H, Cp*), 1.44−1.22 (m, 6H, PCH2), 1.26 (dt, 3JHP = 13.8 Hz, 3JHH = 7.3 Hz, 6H, CH2Me), 0.90 (dt,

Scheme 7. Reaction of 5a with [H(Et2O)2[BArF4]]

A plausible reaction pathway is also shown in Scheme 8. At first, protonation of the Zr(III) atom and subsequent reductive elimination of butane proceed to give the monocationic Zr(III) complex E. Then, disproportionation of the zirconocene moiety of E with the Cp*MoII monochloride moiety occurs to give 4a, which is a reaction pathway similar to that shown in Scheme 6.18 Unfortunately, we could not confirm the formation of the corresponding one-electron-reduced complex F, probably due to subsequent decomposition. Scheme 8. Plausible Reaction Pathway for the Formation of 4a from 5a

In summary, we have prepared a series of novel heterobimetallic group VI metal complexes bearing group IV metallocenyldiphosphine moieties and a pentamethylcyclopentadienyl ligand. Their molecular structures were unambiguously characterized by X-ray analyses. Isolation of 5 provides unprecedented examples of n-butyl zirconocene(III) and hafnocene(III) complexes stabilized by the adjacent chloride ligand of Cp*MII (M = Mo, W) moieties. Investigation of their reactivity revealed that the zirconocene(III) moiety of 5a readily disproportionates with the Cp*MoII monochloride moiety of 5a to give the corresponding heterobimetallic complexes such as 4a and 6a under specific reaction conditions. The disproportionation can be considered as part of the unique reactivity derived from the heterobimetallic moiety of the group VI metal complexes bearing a group IV metallocenyldiphosphine and the pentamethylcyclopetadienyl ligand, because both Zr and Mo centers participate in redox reactions. Further investigations of the reactivity of these heterobimetallic complexes are currently under way. 2011

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cooled to −35 °C to afford 5c as brown blocks (16.5 mg, 0.0204 mmol, 32% isolated yield) suitable for X-ray crystallography. Anal. Calcd for C32H52ClP2WZr: C, 47.50; H, 6.48. Found: C, 47.28; H, 6.53. Reaction of 5a with CO. A solution of 5a (25 mg, 0.0347 mmol) in hexane (3 mL) was placed under an atmospheric pressure of CO at room temperature for 16 h. After the solvent was removed, the resulting brown residue was recrystallized from toluene−hexane to afford [Zr n Bu(μ-η 5 :η 1 -C 5 H 4 PEt 2 ) 2 (μ-Cl)MoClCp*]·0.5C 6 H 14 (6a·0.5C6H14) as brown plates (5.2 mg, 0.00650 mmol, 19% crude yield) suitable for X-ray crystallography. In this reaction, simultaneous formation of the corresponding carbonyl adducts hampered the isolation of 6a in pure form. 1H NMR (C6D6): δ 6.17 (br, 2H, C5H4), 5.34 (br, 2H, C5H4), 5.29 (br 2H, C5H4), 4.86 (br, 2H, C5H4), 2.70− 2.53 (m, 2H, PCH2), 1.99−1.85 (m, 2H, PCH2), 1.71 (br, 3H, ZrnBu), 1.51 (s, 15H, Cp*), 1.42−1.23 (m, 14H, CH2Me, PCH2, and ZrnBu), 1.18 (t, 3JHH = 7.2 Hz, 2H, ZrnBu), 1.02 (dt, 3JHP = 10.8 Hz, 3JHH = 7.4 Hz, 6H, CH2Me). 31P{1H} NMR (C6D6): δ 31.2 (s). Reaction of 5a with [H(Et2O)2]BArF4. To a solution of 5a (45.8 mg, 0.0635 mmol) in C6H5F (3 mL) was added [H(Et2O)2]BArF4 (64.3 mg, 0.0635 mmol) at room temperature, and the mixture was stirred at room temperature for 16 h. Then, layering hexane onto the resulting reaction mixture gave 4a (49.7 mg, 0.0318 mmol, 50% isolated yield) as yellow-brown microcrystalline solids.

JHP = 11.3 Hz, 3JHH = 7.4 Hz, 6H, CH2Me). 31P{1H} NMR (C6D6): δ 26.1 (s). Anal. Calcd for C31H46Cl3MoP2Zr (3a·0.5C6H6): C, 45.75; H, 5.90. Found: C, 45.57; H, 5.51. Preparation of [HfCl(μ-η5:η1-C5H4PEt2)(μ-Cl)MoClCp*] (3b). To a slurry of KC8 (117 mg, 0.866 mmol) in THF (6 mL) was added 1b (1.21 g, 0.145 mmol), and the mixture was stirred at room temperature for 24 h. The resulting reaction mixture was filtered through a pad of Celite, and solvent was removed from the filtrate in vacuo. The resulting reddish brown residue was recrystallized from benzene−hexane to afford 3b·0.5C6H6 as effluorescent brown blocks (98.5 mg, 0.114 mmol, 79% isolated yield). 1H NMR (C6D6): δ 6.53 (br, 2H, C5H4), 5.57 (br, 2H, C5H4), 5.14 (br 2H, C5H4), 4.99 (br, 2H, C5H4), 2.65−2.51 (m, 2H, PCH2), 1.70 (s, 15H, Cp*), 1.46−1.20 (m, 6H, PCH2), 1.16 (dt, 3JHP = 14.0 Hz, 3JHH = 7.0 Hz, 6H, CH2Me), 0.79 (dt, 3JHP = 11.3 Hz, 3JHH = 7.2 Hz, 6H, CH2Me). 31P{1H} NMR (C6D6): δ 25.4 (s). Anal. Calcd for C31H46Cl3HfMoP2 (3b·0.33C6H6): C, 42.45; H, 5.35. Found: C, 42.19; H, 5.11. Preparation of [ZrCl(μ-η5:η1-C5H4PEt2)(μ-Cl)WClCp*] (3c). To a slurry of KC8 (143 mg, 1.06 mmol) in THF (7 mL) was added 1c (242 mg, 0.174 mmol), and the mixture was stirred at room temperature for 24 h. The resulting reaction mixture was filtered through a pad of Celite, and solvent was removed from the filtrate in vacuo. The resulting reddish brown residue was recrystallized from toluene−hexane to afford 3c·0.5C7H8 as brown blocks (94.4 mg, 0.109 mmol, 63% isolated yield). 1H NMR (C6D6): δ 6.64 (br, 2H, C5H4), 5.52 (br, 2H, C5H4), 5.18 (br 2H, C5H4), 4.65 (br, 2H, C5H4), 2.68− 2.52 (m, 2H, PCH2), 2.19 (br, 2H, PCH2), 1.76 (s, 15H, Cp*), 1.63− 1.43 (m, 2H, PCH2), 1.41−1.16 (8H, CH2Me and PCH2), 0.92 (dt, 3 JHP = 19.4 Hz, 3JHH = 7.4 Hz, 6H, CH2Me). 31P{1H} NMR (C6D6): δ −14.0 (s with 183W satellites, 1JPW = 288 Hz). Anal. Calcd for C31.5H47Cl3P2WZr (3c·0.5C7H8): C, 43.53; H, 5.45. Found: C, 43.12; H, 5.15. Preparation of [Zr(μ-η5:η1-C5H4PEt2)2(μ-Cl)2MoCp*]BArF4 (4a). To a solution of 3a·0.5C6H6 (36.8 mg, 0.0475 mmol) in C6H5F (2 mL) was added NaBArF4 (44.4 mg, 0.0501 mmol), and the mixture was stirred at room temperature for 12 h. Then, layering hexane onto the resulting reaction mixture gave 4a as orange blocks (29.4 mg, 0.0188 mmol, 40% isolated yield). 31P{1H} NMR (C6H5F): δ 23.6 (s). Anal. Calcd for C60H55BCl2F24MoP2Zr: C, 46.11; H, 3.55. Found: C, 45.93; H, 3.26. The 1H NMR spectrum of 4a could not be obtained, due to the rapid decomposition of 4a in representative deuterated solvents such as THF-d8 and CD2Cl2. Preparation of [ZrnBu(μ-η5:η1-C5H4PEt2)(μ-Cl)MoCp*] (5a). To a solution of 3a·0.5C6H6 (107 mg, 0.138 mmol) in toluene (5 mL) was added nBuLi (0.025 mL, 0.299 mmol) at room temperature, and the mixture was stirred at room temperature for 12 h. After the solvent was removed, the residue was extracted with hexane (5 mL × 3) and filtered through a pad of Celite. After solvent was removed from the filtrate, the resulting brown residue was dissolved in hexane and cooled to −35 °C to afford 5a as brown blocks (36.7 mg, 0.0508 mmol, 39% isolated yield) suitable for X-ray crystallography. Magnetic moment (Evans): μeff = 1.42 μB at 297 K. Anal. Calcd for C32H52ClMoP2Zr: C, 53.28; H, 7.27. Found: C, 52.91; H, 7.11. Preparation of [HfnBu(μ-η5:η1-C5H4PEt2)(μ-Cl)MoCp*] (5b). To a solution of 3b·0.5C6H6 (41.1 mg, 0.0477 mmol) in toluene (2 mL) was added nBuLi (0.009 mL, 0.108 mmol) at room temperature, and the mixture was stirred at room temperature for 12 h. After the solvent was removed, the residue was extracted with hexane (2 mL × 3) and filtered through a pad of Celite. After solvent was removed from the filtrate, the resulting brown residue was dissolved in hexane and cooled to −35 °C to afford 5b as brown blocks (10.1 mg, 0.0125 mmol, 26% isolated yield) suitable for X-ray crystallography. Anal. Calcd for C32H52ClHfMoP2: C, 47.53; H, 6.48. Found: C, 47.08; H, 6.42. Preparation of [ZrnBu(μ-η5:η1-C5H4PEt2)(μ-Cl)WCp*] (5c). To a solution of 3c·0.5C7H8 (55.5 mg, 0.0639 mmol) in toluene (2.5 mL) was added nBuLi (0.010 mL, 0.120 mmol) at room temperature, and the mixture was stirred at room temperature for 12 h. After the solvent was removed, the residue was extracted with hexane (2.5 mL × 3) and filtered through a pad of Celite. After solvent was removed from the filtrate, the resulting brown residue was dissolved in pentane and 3

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ASSOCIATED CONTENT

S Supporting Information *

CIF files, text, tables, and figures giving details of the X-ray study, structures, and crystallographic data for 1a·0.375CH2Cl2, 1b·0.25CH2Cl2, 1c·0.5CH2Cl2, 2a, 3a·0.5C6H6, 3b·0.5C6H6, 3c·0.5C7H8, 4a, 5a−c, and 6a·0.5C6H14. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail for Y.N.: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Funding Program for Next Generation World-Leading Researchers (GR025). T.M. is a recipient of the JSPS Predoctoral Fellowships for Young Scientists and acknowledges the Global COE Program for Chemistry Innovation. We also thank the Research Hub for Advanced Nano Characterization at The University of Tokyo for X-ray analysis.

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REFERENCES

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Organometallics

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