Synthesis, Reactivity, and Computational Studies of the Cationic

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Organometallics 2004, 23, 4444-4461

Synthesis, Reactivity, and Computational Studies of the Cationic Tungsten Methyl Complex [W(NPh)(N2Npy)Me]+ and Related Compounds (N2Npy ) MeC(2-C5H4N)(CH2NSiMe3)2) Benjamin D. Ward,† Gavin Orde,† Eric Clot,*,‡ Andrew R. Cowley,† Lutz H. Gade,*,§ and Philip Mountford*,† Inorganic Chemistry Laboratory, University of Oxford, South Parks Road, Oxford OX1 3QR, U.K., LSDSMS (UMR 5636), cc 14, Universite´ Montpellier 2, 34095 Montpellier Cedex 5, France, and Anorganisch-Chemisches Institut, Universita¨ t Heidelberg, Im Neuenheimer Feld 270, 69120 Heidelberg, Germany Received April 26, 2004

Reaction of the dimethyl complex W(NPh)(N2Npy)Me2 (1) (N2Npy ) MeC(2-C5H4N)(CH2NSiMe3)2) with either BArF3 or [Ph3C][BArF4] (ArF ) C6F5) gave quantitative conversion to the monomethyl cation [W(NPh)(N2Npy)Me]+ (2+). In contrast, reaction of 1 with [PhMe2NH][BArF4] gave [W(NPh)(HN2Npy)Me2][BArF4] (3-BArF4) by protonation of one of the amido nitrogen atoms of N2Npy. Reaction of cationic 2+ with MeCN or THF gave the labile adducts [W(NPh)(N2Npy)Me(L)]+ (L ) MeCN (4+) or THF (5+)). For comparison the neutral tantalum derivatives Ta(NtBu)(N2Npy)R (R ) Me (6) or η1-allyl (7)) were synthesized by reaction of Ta(NtBu)(N2Npy)Cl(py) with MeLi or (allyl)MgCl. Compound 6, valence isoelectronic with 2+, was crystallographically characterized. Although both 2+ and 6 possess trigonal bipyramidal geometries at the metal, the methyl ligand in 2+ lies in the equatorial plane (with NPh trans to pyridyl), whereas in 6 the opposite arrangement of methyl and imido ligands is found. Reaction of 1 with 0.5 equiv of BArF3 gave the fluxional Me-bridged cation [{W(NPh)(N2Npy)Me}2(µ-Me)]+ (8+); 8+ was also formed by direct reaction of 1 with 2+. The methyl cation 2+ underwent facile methyl group exchange with Cp2ZrMe2 and ZnMe2 as established by spin saturation transfer and deuterium labeling studies. Although a stable intermediate was not spectroscopically observed for either reaction, for the latter case a likely adduct was identified by DFT calculations on a model system and features coordination of Zn to the imido nitrogen and a Zn-Me‚‚‚W interaction. Reaction of 2+ with AlMe3 formed [W{MeC(2-C5H4NAlMe)(CHNSiMe3)(CH2NSiMe3)}(µ-NPh)Me2]+ (9+) and CH4 by deprotonation of a CH2 linkage of N2Npy. DFT (B3PW91) calculations on model systems of the type M(NR){HC(2-C5H4N)(CH2SiH3)2}(X) (X ) Cl, Me) showed that there is an unambiguous electronic preference for the imido ligand to lie trans to the pyridyl nitrogen. This geometry allows optimal π-donation from the imido and the amido nitrogen atoms. Inclusion of the steric bulk of the SiMe3 groups and the R group (Ph or tBu) on the imido ligand through ONIOM(B3PW91:UFF) calculations showed that the underlying electronic preference for the imido ligand to be trans to pyridyl can be reversed because of increased steric repulsions between the imido and amido N-substituents in this isomer. These cause a misdirection of the amido lone pair π-donation, which in turn destabilizes the metal-imido ligand π-bonding.

Introduction We have been developing1-10 the chemistry of early transition metal imido11-24 complexes supported by the diamido-pyridine ligand MeC(2-C5H4N)(CH2NSiMe3)2 (N2Npy)25 and its homologues . Examples of the different classes of compounds are summarized in Chart 1. For group 4 imido compounds (Chart 1, types I and II) the combination of the π-donor amide and hemilabile py* Corresponding authors. E-mail: [email protected]; [email protected]; [email protected]. † University of Oxford. ‡ Universite ´ Montpellier 2. § Universita ¨ t Heidelberg.

ridyl functions of N2Npy have provided (for titanium in particular) an excellent platform for developing coupling reactions of the MdNR linkage with a wide range of (1) Pugh, S. M.; Blake, A. J.; Gade, L. H.; Mountford, P. Inorg. Chem. 2001, 40, 3992. (2) Blake, A. J.; Collier, P. E.; Gade, L. H.; Mountford, P.; Pugh, S. M.; Schubart, M.; Skinner, M. E. G.; Tro¨sch, D. J. M. Inorg. Chem. 2001, 40, 870. (3) Ward, B. D.; Dubberley, S. R.; Gade, L. H.; Mountford, P. Inorg. Chem. 2003, 42, 4961. (4) Pugh, S. M.; Tro¨sch, D. J. M.; Wilson, D. J.; Bashall, A.; Cloke, F. G. N.; Gade, L. H.; Hitchcock, P. B.; McPartlin, M.; Nixon, J. F.; Mountford, P. Organometallics 2000, 19, 3205. (5) Bashall, A.; Collier, P. E.; Gade, L. H.; McPartlin, M.; Mountford, P.; Pugh, S. M.; Radojevic, S.; Schubart, M.; Scowen, I. J.; Tro¨sch, D. J. M. Organometallics 2000, 19, 4784.

10.1021/om049701l CCC: $27.50 © 2004 American Chemical Society Publication on Web 08/20/2004

Cationic Tungsten Methyl Complex Chart 1. Previously Reported Transition Metal Imido Compounds Containing the N2NPy Ligand

unsaturated substrates. The corresponding five- and sixcoordinate group 5 and 6 imido compounds (Chart 1, types III-IVa) with N2Npy have not shown comparable MdNR bond reactivity. This is consistent with the general observation of increasing metal-ligand multiplebond stability on going from group 4 to the middle (6) Bashall, A.; Collier, P. E.; Gade, L. H.; McPartlin, M.; Mountford, P.; Troesch, D. J. M. Chem. Commun. 1998, 2555. (7) Pugh, S. M.; Tro¨sch, D. J. M.; Skinner, M. E. G.; Gade, L. H.; Mountford, P. Organometallics 2001, 20, 3531. (8) Tro¨sch, D. J. M.; Collier, P. E.; Bashall, A.; Gade, L. H.; McPartlin, M.; Mountford, P.; Radojevic, S. Organometallics 2001, 20, 3308. (9) Ward, B. D.; Clot, E.; Dubberley, S. R.; Gade, L. H.; Mountford, P. Chem. Commun. 2002, 2618. (10) Gade, L. H.; Mountford, P. Coord. Chem. Rev. 2001, 216-217, 65. (11) Gade, L. H. Chem. Commun. 2000, 173 (Feature Article). (12) Wigley, D. E. Prog. Inorg. Chem. 1994, 42, 239. (13) Nugent, W. A.; Mayer, J. M. Metal-Ligand Multiple Bonds; Wiley-Interscience: New York, 1988. (14) Schrock, R. R. Acc. Chem. Res. 1990, 23, 158. (15) Woo, L. K. Chem. Rev. 1993, 93, 1125. (16) Dubois, M. K. Chem. Rev. 1989, 89, 1. (17) Trnka, T. M.; Parkin, G. Polyhedron 1997, 16, 1031. (18) Bottomley, F.; Sutin, L. Adv. Organomet. Chem. 1988, 28, 339. (19) Dehnicke, K.; Stra¨hle, J. Angew. Chem., Int. Ed. Engl. 1992, 31, 955. (20) Sharp, P. J. Chem. Soc., Dalton Trans. 2000, 2647. (21) Nugent, W. A.; Haymore, B. L. Coord. Chem. Rev. 1980, 31, 123. (22) Chisholm, M. H.; Rothwell, I. P. Comprehensive Coordination Chemistry; Pergamon Press: Oxford, 1987.

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transition elements.12 Notwithstanding this general observation, Legzdins26,27 for example has shown that certain tungsten imides undergo cycloaddition reactions with isocyanates, while Jensen and Børve have suggested on the basis of DFT calculations that bis(imido)chromium alkyl cations might undergo cycloaddition reactions with ethylene at a CrdNR linkage.28,29 On the other hand, the diamide-supported imido molybdenum and tungsten complexes studied comprehensively by Boncella show no apparent reactions at the MdNR bonds.30-46 We recently described the synthesis and structure of the six-coordinate imidotungsten dimethyl compound W(NPh)(N2Npy)Me2 (1) (type III in Chart 1) and its isotopomers W(NPh)(N2Npy)(CD3)2 (1-d6) and W(NPh)(N2Npy)(13CH3)2 (1-13C2).3 Neither 1 nor its dichloride analogue W(NPh)(N2Npy)Cl2 shows any well-defined reactivity with unsaturated substrates. However, we speculated that methide abstraction from 1 might yield a reactive, five-coordinate cation. Part of this work has recently been communicated.9 In this contribution we describe in full the synthesis, selected reactions, and computational studies of cationic d0 tungsten methyl compounds and other related ML5 compounds. Cationic tungsten alkyl complexes such as [Cp*WMe4]+ 47,48 and [Cp2WMe(MeCN)]+ 49,50 have been described previously, (23) Cundari, T. R. Chem. Rev. 2000, 100, 807. (24) Eikey, R. A.; Abu-Omar, M. M. Coord. Chem. Rev. 2003, 243, 83. (25) Friedrich, S.; Schubart, M.; Gade, L. H.; Scowen, I. J.; Edwards, A. J.; McPartlin, M. Chem. Ber./Recl. 1997, 130, 1751. (26) Legzdins, P.; Phillips, E. C.; Rettig, S. J.; Trotter, J.; Veltheer, J. E.; Yee, V. C. Organometallics 1992, 11, 3104. (27) Legzdins, P.; Rettig, S. J.; Ross, K. J. Organometallics 1994, 13, 569. (28) Jensen, V. R.; Borve, K. J. Chem. Commun. 2002, 542. (29) Jensen, V. R.; Borve, K. J. Organometallics 2001, 20, 616. (30) VanderLende, D. D.; Abboud, K. A.; Boncella, J. M. Organometallics 1994, 13, 3378. (31) Wang, S. S.; Abboud, K. A.; Boncella, J. M. J. Am. Chem. Soc. 1997, 119, 11990. (32) Boncella, J. M.; Wang, S. S.; VanderLende, D. D.; Huff, R. L.; Abboud, K. A.; Vaughn, W. M. J. Organomet. Chem. 1997, 530, 59. (33) Huff, R. L.; Wang, S. S.; Abboud, K. A.; Boncella, J. M. Organometallics 1997, 16, 1779. (34) Wang, S. S.; VanderLende, D. D.; Abboud, K. A.; Boncella, J. M. Organometallics 1998, 17, 2628. (35) Ortiz, C. G.; Abboud, K. A.; Boncella, J. M. Organometallics 1999, 18, 4253. (36) Boncella, J. M.; Wang, S. S.; VanderLende, D. D. J. Organomet. Chem. 1999, 591, 8. (37) Cameron, T. M.; Ortiz, C. G.; Abboud, K. A.; Boncella, J. M.; Baker, R. T.; Scott, B. L. Chem. Commun. 2000, 573. (38) Mills, R. C.; Abboud, K. A.; Boncella, J. M. Organometallics 2000, 19, 2953. (39) Ortiz, C. G.; Abboud, K. A.; Cameron, T. M.; Boncella, J. M. Chem. Commun. 2001, 247. (40) Cameron, T. M.; Abboud, K. A.; Boncella, J. M. Chem. Commun. 2001, 1224. (41) Cameron, T. M.; Ortiz, C. G.; Ghiviriga, I.; Abboud, K. A.; Boncella, J. M. Organometallics 2001, 20, 2032. (42) Cameron, T. M.; Ghiviriga, I.; Abboud, K. A.; Boncella, J. M. Organometallics 2001, 20, 4378. (43) Mills, R. C.; Wang, S. Y. S.; Abboud, K. A.; Boncella, J. M. Inorg. Chem. 2001, 40, 5077. (44) Mills, R. C.; Abboud, K. A.; Boncella, J. M. Chem. Commun. 2001, 1506. (45) Cameron, T. M.; Ortiz, C. G.; Ghiviriga, I.; Abboud, K. A.; Boncella, J. M. J. Am. Chem. Soc. 2002, 124, 922. (46) Cameron, T. M.; Gamble, A. S.; Abboud, K. A.; Boncella, J. M. Chem. Commun. 2002, 1148. (47) Liu, A. H.; Murray, R. C.; Dewan, J. C.; Santarsiero, B. D.; Schrock, R. R. J. Am. Chem. Soc. 1987, 109, 4282. (48) Maus, D. C.; Copie, V.; Sun, B.; Griffiths, J. M.; Griffin, R. G.; Luo, S.; Schrock, R. R.; Liu, A. H.; Seidel, S. W.; Davis, W. M.; Grohmann, A. J. Am. Chem. Soc. 1996, 118, 5665. (49) Carmichael, A. J.; McCamley, A. J. Chem. Soc., Dalton Trans. 1995, 3125.

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Scheme 1. Synthesis and Trapping of Cationic Tungsten Methyl Compounds

and it was thought that comparisons between the diamido-imido-supported cationic methyl compounds reported herein and these earlier cyclopentadienylsupported systems might provide new insights into their reactive potential. Results and Discussion Synthesis of a Tungsten Monomethyl Cation and its Lewis Base Adducts. Much is now understood about the methods of synthesis and reactivity of organometallic alkyl cations, especially in the context of olefin polymerization catalysis.51 Typical reagents used in conjuction with neutral dialkyl precursors are the perfluorophenyl borane and borates BArF3, [Ph3C][BArF4], and [PhMe2NH][BArF4] (ArF ) C6F5),52-54 which we employed in the studies reported herein. Reaction of W(NPh)(N2Npy)Me2 (1) with BArF3 in CD2Cl2 afforded [W(NPh)(N2Npy)Me][MeBArF3] (2-MeBArF3) in quantitative yield, as determined by 1H NMR spectroscopy (Scheme 1). [W(NPh)(N2Npy)Me][BArF4] (2-BArF4) was likewise prepared from 1 and [Ph3C][BArF4]. The 1H and 13C NMR spectra of the cation [W(NPh)(N2Npy)Me]+ (2+) in 2-BArF4 and 2-MeBArF3 are identical at both ambient temperature and -90 °C (500.0 and 125.7 MHz for 1H and 13C, respectively), implying that it is identical in both instances and that anion coordination is not significant in CD2Cl2. For 2-MeBArF3 the difference between the meta- and para19F resonances for the [MeBArF ]- anion was 2.6 ppm, 3 (50) Jernakoff, P.; Fox, J. R.; Hayes, J. C.; Lee, S.; Foxman, B. M.; Cooper, N. J. Organometallics 1995, 14, 4493. (51) Gibson, V. C.; Spitzmesser, S. K. Chem. Rev. 2003, 103, 283. (52) Bochmann, M.; Lancaster, S. J. J. Organomet. Chem. 1992, 434, C1. (53) Bochmann, M. Angew. Chem., Int. Ed. Engl. 1992, 31, 1181. (54) Yang, X.; Stern, C. L.; Marks, T. J. J. Am. Chem. Soc. 1991, 113, 3623.

also suggesting that there is negligible coordination,55 unlike the situation found with more electron-deficient cations for which M‚‚‚MeBArF3 (M ) cationic metal center) interactions may be observed.52,54,56,57 Cation 2+ is unstable at ambient temperature in CD2Cl2 solution, having a half-life of approximately 2 h. Owing to this instability, it cannot be isolated in pure form (unless it is as a Lewis base adduct; see below). Attempts to study 2-BArF4 and 2-MeBArF3 in the comparatively inert solvent C6D5Br were unsuccessful since the products form oily solvent clathrates. However, spectroscopic and reactivity studies can easily be carried out on samples of 2+ prepared in situ, and reaction products can be isolated.58 We chose to develop the chemistry of 2-MeBArF3 since this avoids the need to remove the coproduct Ph3CMe formed in the reaction of 1 with [Ph3C][BArF4]. In reactions where both anions were employed, no difference in reactivity was observed. Interestingly, dimethyltungstenocene (Cp2WMe2) also reacts with [CPh3]+,59 but in this case one-electron oxidation to unstable d1 [Cp2WMe2]+ occurs, and at room temperature loss of a hydrogen radical leads (ultimately) to the d2 cationic olefin-hydride [Cp2WH(η-C2H4)]+, most probably via the d0 methylidene [Cp2W(CH2)Me]+.60 The W(VI) d0 compound 1 does not have access to this metalcentered redox chemistry, and this probably accounts for its relatively “simple” behavior with [Ph3C][BArF4].61 (55) Horton, A. D. Organometallics 1996, 15, 2675. (56) Bochmann, M.; Lancaster, S. J. Angew. Chem., Int. Ed. Engl. 1994, 33, 1634. (57) Bochmann, M.; Lancaster, S. J.; Hursthouse, M. B.; Malik, K. M. A. Organometallics 1994, 13, 2235. (58) Ward, B. D.; Orde, G.; Clot, E.; Cowley, A. R.; Gade, L. H.; Mountford, P. Manuscript in preparation. (59) Benfield, F. W. S.; Green, M. L. H. J. Chem. Soc., Dalton Trans. 1974, 1324. (60) Hayes, J. C.; Cooper, J. C. J. Am. Chem. Soc. 1982, 104, 5570. (61) Harlan, C. J.; Fujita, E.; Norton, J. R. J. Am. Chem. Soc. 1999, 121, 7274.

Cationic Tungsten Methyl Complex

It is relevant at this point to note that the synthesis of the tetramethyltungsten cation [Cp*WMe4]+ also relies on redox chemistry, namely, the one-electron oxidation of Cp*WMe4 with [Cp2Fe][PF6].47 As mentioned, 2-BArF4 and 2-MeBArF3 were characterized by NMR spectroscopy. The 1H and 13C NMR spectra of 2+ feature single SiMe3 resonances indicating Cs symmetry. A 1H NOE (nuclear Overhauser effect) difference experiment conclusively placed the methyl ligand cis to the pyridyl group, consistent with the trigonal bipyramidal geometry (with W-Me in the equatorial plane) proposed for 2+ in Scheme 1. The methyl ligand 1H and 13C resonances feature characteristic 183W (I ) 1/2, 14.5% abundance) satellites [2J(WH) ) 6.3 Hz, 1J(WC) ) 90 Hz] of ca. 15% of the total signal intensity, consistent with a terminal (nonbridging) coordination mode. The normal value of 1J(CH) (126 Hz) in this ligand indicates that R-agostic interactions are not present in this formally 18 valence electron cation. We note that DFT calculations have suggested that the singlet state of the 16 valence electron cation Cp2WMe+ has an R-agostic structure.62 ONIOM(B3PW91:UFF) calculations on 2+ (see below and Figure S1 in the Supporting Information) find that in this cation all W-C-H angles are equivalent (110.0°, 112.1°, and 110.9°) and within the normal ranges. The values of the C-H bond distances (1.098, 1.097, and 1.090 Å) also speak in favor of the absence of an R-agostic interaction in 2+. Certain early transition metal alkyls52 react with [PhMe2NH][BArF4] via formal protonolysis to yield cations that may be PhMe2N base-stabilized. However (Scheme 1), reaction of 1 with [PhMe2NH][BArF4] in CH2Cl2 gave rise to the N2Npy-protonated dialkyl product [W(NPh)(HN2Npy)Me2][BArF4] (3-BArF4), which was isolated in 76% yield. The side product, PhMe2N, could not be separated from 3-BArF4 by recrystallization. The solution NMR specta of 3-BArF4 are consistent with free PhMe2N and [BArF4]-, and a C1-symmetrical 3+ cation possessing inequivalent methyl ligands. These clearly remain coordinated to the tungsten, as inferred from the presence of 183W satellites [1J(WC) ) 66 Hz for Me trans to the amido nitrogen of HN2Npy and 107 Hz for the Me trans to the amino (protonated) nitrogen; for 1 1J(WC) ) 71 Hz3]. That 1 is protonated at an amido nitrogen is consistent with it being a “π-loaded” (apparently 20 valence electron) compound with an amide nitrogen atoms-centered nonbonding orbital as its HOMO (see below for a discussion of the DFT electronic structure of 1). Again, it is interesting to compare the reaction of d0 1 with proton sources and those of d2 dimethyltungstenocene. Reaction of Cp2WMe259 with HBF4‚Et2O 49 or [NH4][PF6]50 results in the elimination of CH4 (presumably49 via initial protonation at the metal forming d0 [Cp2WMe2(H)]+). The arising transient 16 valence electron monomethyl cation can be trapped with acetonitrile as [Cp2W(Me)(MeCN)]+. The difference in reactivity of d0 (π-loaded) 1 and d2 Cp2WMe2 with H+ sources stems from the presence of a ligand-based HOMO (lone pair) in the former and a metal-centered lone pair in the latter case. (62) Green, J. C.; Jardine, C. J. J. Chem. Soc,. Dalton Trans. 2001, 274.


Both MeCN and THF react with in-situ-prepared 2+MeBArF3, forming [W(NPh)(N2Npy)Me(L)][MeBArF3] (L ) MeCN 4-MeBArF3 or THF 5-MeBArF3) in 70 and 54% yield, respectively (Scheme 1). For solid 4-MeBArF3 the ν(CtN) stretch of the coordinated MeCN is observed at 2362 cm-1 (KBr), consistent with previously reported values63 and significantly shifted from that of unbound MeCN (ν(CtN) ) 2254 cm-1). The ambient temperature solution 1H NMR spectra of [W(NPh)(N2Npy)Me(L)]+ are consistent with fully dissociated [W(NPh)(N2Npy)Me]+ (2+) and the respective L. At low temperature, however, the spectra are consistent with C1 symmetrical 4+ and 5+ cations (Scheme 1) and feature two SiMe3 resonances and characteristic multiplets for the inequivalent CH2 linkages of N2Npy. Overall, the NMR data suggest that there is a temperature-dependent equilibrium between 4+ or 5+ and separated 2+ and the respective ligands in solution. At low temperatures the equilibrium favors 4+ and 5+, and the dynamic behavior of 4+ and 5+ at lower temperatures is reminiscent of that of the neutral group 5 compounds M(NR)(N2Npy)Cl(py) (M ) Nb or Ta; R ) tBu or 2,6-C H iPr ), which exist in dissociative dynamic 6 3 2 equilibria with M(NR)(N2Npy)Cl and pyridine.1 The activation parameters for the dissociation of the MeCN in 4+ have been determined by line width analysis of the variable-temperature 1H NMR spectra recorded at 2 °C intervals between -80 and -70 °C. The line widths obtained at each temperature were corrected by subtracting the natural line width obtained from the low-temperature limit at -90 °C. These values were used to calculate the observed rate constant (kobs) at each temperature.64 Chemical rate constants (kchem) were calculated according to kchem ) 2kobs65 and used to calculate the activation parameters by means of Eyring plots. The values for the activation parameters are ∆Hq ) 91.8 ( 1.5 kJ mol-1 and ∆Sq ) 252 ( 9 J mol-1 K-1. The value obtained for ∆Hq is comparable to that reported (87.6 ( 2.3 kJ mol-1) for the related tantalum compound Ta(NtBu)(N2Npy)Cl(py).1 The value of ∆Sq is consistent with a dissociative mechanism, although the value is somewhat larger than that (99 ( 10 J mol-1 K-1) associated with pyridine dissociation for Ta(NtBu)(N2Npy)Cl(py) in the same solvent.1 This is possibly due to rearrangement of second coordination sphere solvent molecules (and the anion) during the dissociation of the MeCN, which probably display a greater degree of ordering owing to the cationic nature of 4+. Synthesis of Neutral Tantalum Monomethyl and η1-Allyl Complexes. On the basis of literature precedent for neutral group 5 compounds of the type III in Chart 11,7 there is little surprise concerning the geometries proposed for the cation [W(NPh)(N2Npy)Me(L)]+ in Scheme 1 (phenylimide trans to pyridyl). However, the isomer found for base-free [W(NPh)(N2Npy)Me]+ (2+) itself (methyl in equatorial plane, phenylimide trans to pyridyl) was by no means a foregone conclusion. As shown in Chart 1, three types of five-coordinate M(N2Npy)(NR)X (X ) anionic or neutral ligand) imido compounds [I (X ) neutral) and IV/IVa (X ) monoan(63) Nakamoto, K. Infrared and Raman Spectra of Inorganic and Coordination Compounds. Part B, 5th ed.; Wiley: New York, 1997. (64) Sandstro¨m, J. Dynamic NMR Spectroscopy; Academic Press: New York, 1992. (65) Green, M. L. H.; Wong, L.-L.; Sella, A. Organometallics 1992, 11, 2660.

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Ward et al. Table 1. Selected Bond Lengths (Å) and Angles (deg) for [Ta(NtBu)(N2Npy)Me] (6) and for the Calculated Structure at the ONIOM(B3PW91:UFF) Level (6transoniom) 6

Figure 1. Displacement ellipsoid plot of [Ta(NtBu)(N2Npy)Me] (6) (25% probability). H atoms omitted for clarity.

ionic)] have been reported by us.10 Later in this article we explore computationally the relative energies and preferences for the two basic geometries of the type IV/ IVa, namely, whether it is the imide or the monoanionic ligand that occupies an equatorial or axial position. Only aryloxide and chloride derivatives of the type IV/IVa have been reported to date. These ligands are arguably different from methyl. We therefore made the valence electronic (neutral) tantalum analogue Ta(NtBu)(N2Npy)Me (6). For the purposes of comparison, the allyl compound Ta(NtBu)(N2Npy)(η1-C3H5) (7) was also prepared. Originally, we attempted to prepare a phenylimido compound Ta(NPh)(N2Npy)Me in order to make as close a comparison as possible with 2+. However, despite considerable effort, the required precursor Ta(NPh)(N2Npy)Cl(py) was not accessible. Moreover, we have previously shown that the 2,6-diisopropylphenylimido homologue Ta(N-2,6-C6H3iPr2)(N2Npy)Cl(py) does not react cleanly with alkylating reagents,7 whereas the tert-butylimide Ta(NtBu)(N2Npy)Cl(py) reacts cleanly with LiCH2SiMe3 to form Ta(NtBu)(N2Npy)(CH2SiMe3). This compound is an oil that exists as a mixture of isomers containing either κ2- or κ3-bound N2Npy ligands in solution. Reaction of Ta(NtBu)(N2Npy)Cl(py) with MeLi in THF yielded the pale yellow methyl compound Ta(NtBu)(N2Npy)Me (6) in good yield (eq 1). Attempts to prepare a niobium analogue of 6 from Nb(NtBu)(N2Npy)Cl(py)1 and MeLi, MeMgBr, or Me2Zn (no reaction) were unsuccessful.

Ta(NtBu)(N

The molecular structure of 2Npy)Me (6) is shown in Figure 1. Selected bond lengths and angles

6transoniom

Ta(1)-N(1) Ta(1)-N(2) Ta(1)-N(3) Ta(1)-N(4)

1.783(3) 2.010(3) 2.008(3) 2.363(3)

1.791 2.028 2.025 2.420

N(1)-Ta(1)-N(2) N(2)-Ta(1)-N(3) N(1)-Ta(1)-N(3) N(3)-Ta(1)-C(20) N(4)-Ta(1)-C(20) Ta(1)-N(1)-C(1) Ta(1)-N(2)-C(11) Ta(1)-N(2)-Si(1) C(11)-N(2)-Si(1) Ta(1)-N(3)-C(12) Ta(1)-N(3)-Si(2) C(12)-N(3)-Si(2)

122.72(13) 114.32(12) 118.52(13) 98.04(14) 167.79(14) 171.7(3) 104.2(2) 138.85(18) 116.2(2) 111.2(2) 129.72(16) 117.8(2)

120.5 112.0 121.5 96.8 166.2 170.3 104.6 135.8 117.7 113.6 126.3 118.8

are listed in Table 1 along with those for the ONIOM(B3PW91:UFF) calculated structure (for further discussion see below). Compound 6 exhibits a distorted trigonal bipyramidal geometry with the N2Npy ligand coordinating to the tantalum center in a fac manner and the imido ligand lying cis to the pyridyl group in the equatorial position (cf. that found for Nb(NtBu)(N2Npy)Cl, IVa, Chart 1). The bond lengths for the central coordination sphere are all consistent with values reported previously.66,67 The angle subtended at N(1) (171.7(3)°) is consistent with the imido ligand acting as an approximately linear, four-electron donor. The sums of the angles subtended at the amido nitrogens (359.25(20)° and 358.72(20)° for N(2) and N(3), respectively) show that both are approximately trigonal planar and consequently that the amide nitrogen atoms can in principle donate up to three electrons each to the metal center. Formally, compound 6, like 2+, possesses an 18 valence electron count. The NMR data (C6D6) for 6 are consistent with the solid state geometry being maintained in solution. Unambiguous NOE enhancements of both the N2Npy ortho-pyridyl (δ 8.99 ppm) and TaMe (δ 0.73 ppm) resonances were observed on irradiation at the tert-butyl resonance frequency (δ 1.70 ppm), conclusively placing the imido ligand in the equatorial plane. It is interesting that compound 6 is prepared from Ta(NtBu)(N2Npy)Cl(py) in the presence of (displaced) pyridine, but forms a five-coordinate pyridine-free complex. In contrast [W(NPh)(N2Npy)Me]+ forms isolable Lewis base adducts (albeit labile in solution), probably owing to the greater charge at the metal center. We speculated that an allyl complex might not be as entropically likely to favor a five-coordinate alkyl analogous to 6 or 2+ and might instead form a six-coordinate species of the type III (Chart 1), somewhat analogous to the niobium benzamidinate complex Nb(NtBu)(N2Npy){PhC(NSiMe3)2}.7 Thus, reaction of Ta(NtBu)(N2Npy)Cl(py) with C3H5MgCl in THF afforded Ta(NtBu)(N2Npy)(η1-C3H5) (7) as an orange powder in 56% isolated yield. On the basis of low-temperature 1H and 13C NMR data (66) Allen, F. H.; Kennard, O. Chem. Des. Autom. News 1993, 8, 1 and 31. (67) Fletcher, D. A.; McMeeking, R. F.; Parkin, D. J. Chem. Inf. Comput. Sci. 1996, 36, 746.


Organometallics, Vol. 23, No. 19, 2004 4449 Table 2. Selected Geometrical Parameters (distances in Å and angles in deg) for the Experimental Complex W(NPh)(N2NPy)Me2 (1)3 and the Calculated Geometries for W(NPh)(N2Nqm)Me2 (1qm) and W(NPh)(N2NPy)Me2 (1oniom) at the DFT(B3PW91) and ONIOM(B3PW91:UFF) Levels, Respectivelya 1

Figure 2. Optimized geometry (B3PW91) of the model system W(NPh)(N2Nqm)Me2 (1qm).

and NOE experiments, the ground state structure shown in eq 1 with a monohapto allyl positioned trans to the pyridyl group was unambiguously assigned. On warming to 293 K, however, the allyl protons coalesced to form an apparent quintet (integration 1 H) and a broad multiplet (4 H) consistent with the well-established68 η1 T η3 T η1 (or η3 T η1 T η3 for ground states with an η3-bound allyl) dynamic equilibria found for certain allyl compounds. It is not known whether the inferred short-lived Ta(NtBu)(N2Npy)(η3-C3H5) intermediate has the imido group cis or trans to pyridyl (cf. type III in Chart 1). Computational Study of W(NPh)(N2Npy)Me2. The dimethyl complex W(NPh)(N2Npy)Me2 (1) has previously been characterized by X-ray diffraction3 and can serve as a reference to calibrate our computational methodology. In all the DFT calculations (B3PW91), the N2Npy ligand has been modeled by HC(2-C5H4N)(CH2NSiH3)2 (abbreviated as N2Nqm), whereas in the ONIOM(B3PW91: UFF) calculations the real ligand N2Npy has been modeled with the “missing” Me groups (attached to apical C and Si) treated at the MM (molecular mechanics) level (UFF model). The imido group NPh has been explicitly considered and treated at the DFT level, whereas the imido group NtBu has been modeled by NCH3 in the DFT calculations, and the missing Me groups have been treated at the MM level in the ONIOM calculations. The optimization of the geometry at the B3PW91 level of the model system W(NPh)(N2Nqm)Me2 (1qm) yielded a structure in very good agreement with the experimental results (Figure 2 and Table 2). Although the geometry optimization was carried out without any symmetry constraint, 1qm exhibits a mirror plane bisecting the two methyl groups and the two amido groups, respectively. A simpler model, where Ph on the imido ligand was replaced by H, yielded very similar results (Table 2), albeit with a significantly shorter W(1)-N(1) distance (1.737 vs 1.751 Å for 1qm) and, consequently, a longer W(1)-N(4) bond length (2.410 vs 2.388 Å for 1qm). As the actual π-donation from the (68) Abrams, M. B.; Yoder, J. C.; Loeber, C.; Day, M. W.; Bercaw, J. E. Organometallics 1999, 18, 1389.

1qm

1oniom

W(1)-N(1) W(1)-N(2) W(1)-N(3) W(1)-N(4) W(1)-C(7) W(1)-C(8)

1.745(2) 1.997(2) 2.042(2) 2.336(2) 2.250(2) 2.228(2)

1.751 (1.737) 2.036 (2.038) 2.037 (2.035) 2.388 (2.410) 2.219 (2.217) 2.218 (2.217)

1.751 2.029 2.045 2.414 2.225 2.225

C(7)-W(1)-C(8) N(1)-W(1)-N(4) N(2)-W(1)-N(3) N(3)-W(1)-C(7) N(2)-W(1)-C(8) W(1)-N(1)-C(1) N(3)-W(1)-N(4)-C(9) N(2)-W(1)-N(4)-C(9) C(7)-W(1)-N(3)-Si(2) C(8)-W(1)-N(2)-Si(1)

87.1(1) 165.9(1) 88.6(1) 86.4(1) 92.0(1) 164.8(2) 131.5 -138.9 -53.5 85.7

90.3 (90.3) 167.3 (167.8) 86.3 (86.5) 88.1 (88.0) 88.1 (87.6) 173.7 (174.0) 136.0 (136.4) -136.5 (-136.0) -62.4 (-64.8) 63.5 (63.1)

88.5 163.3 86.3 89.9 87.1 166.5 133.5 -138.9 -55.4 74.6

a For 1 qm, the values in parenthesis correspond to the model system W(NH)(N2Nqm)Me2. For the numbering scheme of the atoms see Figure 2.

imido ligand plays a central role in modulating the electronic structure of these complexes (vide infra), the more elaborate model NPh was used in all the calculations. The pseudo-Cs symmetry in 1qm is not present in the experimental complex 1, where the pyridine ring is somewhat twisted from a symmetrical position with respect to the equatorial ligands (see dihedral angles N(2)-W(1)-N(4)-C(9) and N(3)-W(1)-N(4)-C(9) in Table 2). Additionally, the W-imido linkage is less linear in 1 than in 1qm (W(1)-N(1)-C(1) ) 164.8(2)° and 173.7°, respectively), and the orientation (with respect to the equatorial plane) of the planes defined by the three atoms bonded to the amido nitrogen atoms (Nam) is different for the two Nam (see dihedral angles C(7)-W(1)-N(3)-Si(2) and C(8)-W(1)-N(2)-Si(1), Table 2). In absolute terms, there is a difference of ca. 30° in 1 (53.5° vs 85.7°), whereas this asymmetry is absent in 1qm (62.4° vs 63.5°). In the real system 1 there are thus some deformations, certainly induced by the bulky SiMe3 groups, that the calculations on the simplified model 1qm failed to reproduce. An optimization of the geometry of W(NPh)(N2Npy)Me2 (1oniom) was therefore carried out at the ONIOM(B3PW91:UFF) level. The calculated structure of 1oniom is in excellent agreement with that of 1, even in its more subtle aspects (Table 2). The asymmetry in the orientation of the pyridine ring and the W(1)-N(1)-C(1) angle are faithfully reproduced. Also the description of the relative orientation of the planes defined by the trigonal amido groups has improved with a difference in absolute value of ca. 20° (see dihedral angles C(7)-W(1)-N(3)Si(2) and C(8)-W(1)-N(2)-Si(1), Table 2). The orientation of these planes will prove to be of great importance, as it influences strongly the π-donating ability of the amido nitrogen atoms. Hence the plane formed by the three substituents of N(2) is almost perpendicular to the equatorial plane (C(8)-W(1)-N(2)-Si(1) ) 85.7° in 1 and 74.6° in 1oniom), implying that the 2p lone pair on

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Table 3. Selected M-N Bond Distances (Å) for the Complexes M(NR)(N2Nqm)X and Relative Energy, ∆E (kJ mol-1), between the cis and trans Isomersa M-N(1) M ) W+, R ) Ph, X ) Me M ) W+, R ) Me, X ) Me M ) W+, R ) Ph, X ) Cl M ) Ta, R ) Me, X ) Me M ) Ta, R ) Me, X ) Cl

M-N(2)

M-N(3)

∆E

cis

trans

cis

trans

cis

trans

11.2 15.2 15.6 11.4 20.8

1.747 1.731 1.746 1.783 1.779

1.750 1.731 1.754 1.788 1.784

1.928 1.929 1.917 2.025 2.005

1.913 1.920 1.914 2.031 2.025

1.928 1.929 1.917 2.025 2.005

1.936 1.932 1.938 2.029 2.025

a A positive value for ∆E implies that the cis isomer is more stable than the trans one. The numbering scheme shown in Figure 2 has been followed. W complexes are monocationic, and Ta complexes are neutral.

N(2) is almost in the equatorial plane. This π-donation seems to be more efficient, as indicated by the shorter W(1)-N(2) distance compared to W(1)-N(3) (2.029 and 2.045 Å for 1oniom, 1.997(2) and 2.042(2) Å for 1, respectively). In the case of the W(1)-N(3) bond length, the agreement between the ONIOM value and the experimental value is perfect, and an excellent agreement is also observed for the dihedral angle C(7)-W(1)N(3)-Si(2). In the case of the W(1)-N(2) bond, the calculated value is ca. 0.03 Å longer than the experimental one, but the calculated dihedral angle C(8)W(1)-N(2)-Si(1) is ca. 10° less than the experimental one. The 2p lone pair on N(2) is thus less well aligned with the equatorial plane in 1oniom compared to that in 1, leading to less efficient π-donation. The HOMO of 1 computed at the DFT(B3PW91) level on the geometry of 1oniom is represented in Figure S2 of the Supporting Information and clearly shows it to be essentially an amido nonbonding lone pair, thus preventing this complex from reaching the 20-electron configuration. The HOMO is more heavily weighted on N(3), as this atom is less involved in the π-donation to W. The shape of the HOMO also explains the result of the reaction of 1 with proton donors with protonation at one amido nitrogen atom. These results (1qm vs 1oniom) illustrate how weak interactions between the ligands can alter the geometry around the metal center that would otherwise have resulted from consideration only of the main electronic bonding interaction.69 Such subtle effects prove to have critical importance in setting the imido ligand site preferences in 2+ and 6 (see below). The comparative study of 1 with the calculated models 1qm and 1oniom shows that our methodology is accurate in describing the main bonding properties around the metal center (1qm), as well as taking into account the interligand steric interactions (1oniom). Geometry of M(NR)(N2Nqm)(X): Electronic Factors. The complexes [W(NPh)(N2Npy)Me]+ (2+) and Ta(NtBu)(N2Npy)Me (6) are isoelectronic, but they differ in the ligand occupying the apical position trans to pyridine, namely, NPh in 2+ and Me in 6. Electronic factors could be at the origin of the difference in geometries, as the W complex is cationic, while the Ta system is neutral. For these π-loaded, d0 systems, the energy and the spatial extension of the metal d orbitals certainly has an influence on the preferred geometry. The model used in the DFT calculations for 2+ is [W(NPh)(N2Nqm)Me]+, and the geometry is that of a (69) Clot, E.; Eisenstein, O.; Dube´, T.; Faller, J. W.; Crabtree, R. H. Organometallics 2002, 21, 575.

trigonal bipyramid with apical pyridyl and equatorial amido groups. The isomer with Me in the equatorial plane (cis to pyridyl) is denoted 2cisqm, while the other isomer (Me trans to pyridyl) is denoted 2transqm. In all complexes M(NR)(N2Nqm)(X) this notation will be used, i.e., with cis or trans referring to the position of the monoanionic X (Me, Cl) ligand with respect to the pyridyl group. In agreement with the experimental observations, 2cisqm is computed to be more stable than 2transqm by 11.2 kJ mol-1 (Table 3). In the case of the neutral system Ta(NMe)(N2Nqm)Me, the cis isomer 6cisqm is also computed to be more stable (by 11.4 kJ mol-1) than the trans isomer 6transqm (Table 3). To make as close as possible a comparison between the two metals, the complex [W(NMe)(N2Nqm)Me]+ was evaluated and the energy difference between the cis and trans isomers amounts to 15.2 kJ.mol-1 (cis isomer more stable). Due to the smaller radius of the metal in the cationic complexes, bond lengths to W are significantly shorter than those to Ta (Table 3). NMe is expected to be a better π-donor group than NPh because no lone pair on the imido nitrogen atom can be stabilized through conjugation with the ring. This is confirmed by the shorter W-N(1) bond distance obtained for [W(NMe)(N2Nqm)Me]+ (1.731 vs 1.747 Å for 2cisqm). However, the amido nitrogen atoms are also π-donors and the electronic structure of both the cis and trans isomers result from a competition between the π-donation from the amido and imido ligands (see below). To probe this competition, we have also considered the systems [W(NPh)(N2Nqm)Cl]+ and Ta(NMe)(N2Nqm)Cl, namely, where the Me ligand has been substituted by Cl. In both cases, the cis isomer is still the more stable and the energy difference between the cis and the trans isomers has increased (from 11.2 to 15.6 kJ mol-1 for W and from 11.4 to 20.8 kJ mol-1 for Ta). Substitution of Me by Cl resulted mainly in a significant shortening of the M-Namide (M ) W, Ta) distances in the cis isomer (Table 3). These results all establish that there is a clear electronic preference for the imido ligand to be trans to pyridyl in these systems. This preference can be traced to the π-donating properties of NR and to a more efficient donation when NR is apical (see below). However, donation from the amido groups should not be neglected and the actual magnitude depends on the nature of the X ligand in the equatorial plane with weak σ-donor-promoting π-donation. Electronic Structure of [W(NH)(NH2)2(py)Me]+. To study in more detail the π-bonding pattern in these ML5 d0 complexes, DFT(B3PW91) calculations on ideal-



Figure 3. (a) The four idealized geometries of [W(NH)(NH2)2(py)Me]+ with their relative energies (kJ mol-1). (b) In- and out-of-phase combinations of amido nitrogen 2p lone orbitals (viewed in projection into the equatorial plane, apical groups omitted).

Figure 4. The σ-only MO diagram (metal d orbitals only) for an idealized ML5 tbp system of Cs symmetry.

ized Cs systems [W(NH)(NH2)2(py)Me]+ have been carried out (Figure 3a). These simplified models allow the basic orientation preferences of the amido and imido ligands to be analyzed in both a computationally efficient way and in the absence of constraints imposed by the fac-chelating nature of N2Npy and its model systems. At each point (Figure 3a) the geometry was frozen to values typical of these systems (see Supporting Information, Figures S3 and S4), and single-point calculations followed by a natural bonding orbital (NBO) analysis were performed. Local trigonal planar geometries at the amido nitrogen atoms were conserved, and their orientation with respect to the equatorial plane is defined by the value of the dihedral angle R ) Leq-WNamide-H (Leq ) Me for the cis isomers; Leq ) NH for the trans isomers). For R ) 0, the NH2 amido groups are in the equatorial plane and the Namide 2p lone pairs are perpendicular to that plane. For R ) 90 the reversed situation is obtained with the Namide 2p lone pairs now lying in the equatorial plane. Figure 4 shows the σ-only MO diagram (metal d orbitals only) for an ideal ML5 tbp system of Cs symmetry (σh lies in the xz plane for the coordinate system chosen).70 When it is in the apical position, π-donation from the imido ligand is more efficient because it can use the two lowest unoccupied d orbitals 1a′ and 1a′′ (using the Nimide 2px and 2py orbitals). For R ) 90, the amido lone pair combination a′(R)90) (Figure 3b) can be

stabilized by π-donation to 2a′, whereas the a′′(R ) 90) combination remains essentially nonbonding. When it is in the equatorial position, π-donation from the imido ligand is less efficient, because, along with 1a′, the less stable 2a′′ orbital is involved instead of 1a′′. To recover some stabilization through π-donation to 1a′′, the amide hydrogens must be parallel to the equatorial plane (i.e., so as to use the a′′(R ) 0) SALC (Figure 3b)). However, a′(R) 0) then competes with the imido ligand N 2pz orbital for donation to the 1a′ metal acceptor orbital, and overall, the π-donating pattern is less efficient and leads to a less stable isomer. In agreement with the above qualitative analysis, Wcis90 is the more stable system, featuring an apical imido group and amido NH2 planes perpendicular to the equatorial plane (Figure 3). Rotation of the amido planes forming Wcis00 introduces a large destabilization (142.9 kJ mol-1). The situation with the imido group in the equatorial position is less favorable, but the isomer Wtrans00 is notably more stable than Wtrans90 (56.5 and 108.6 kJ mol-1, respectively), consistent with the above qualitative analysis. In Figure 5, the frontier orbitals of Wcis90 and Wtrans00 are schematically represented on the same energy scale (see Supporting Information, Figures S3 and S4, for a more accurate representation of these MOs and for their energies). The almost equal energy (-0.46936 vs -0.46412 au) between φ2,c (Wcis90) and φ1,t (Wtrans00) implies that the π-donating power of the (two) amido and imido nitrogen atoms (donation into dyz) is of comparable magnitude in this system. Competition between the imido and amido N atoms for the same d orbitals (φ2,t and φ3,t) and donation into a higher d orbital (φ4,t, analogous to 2a′′ in Figure 4) lead to a less efficient π-bonding pattern in Wtrans00. The a′(R)90) amido lone pair linear combination donates into the σ*(W-Me) orbital, and this explains why substitution of the good σ-donor Me by the weaker Cl in [W(NPh)(N2Nqm)Cl]+ (Table 3) allows for increased π-donation from the amido group because the σ*(W-Cl) orbital is lower in energy. The donation into 2a′ of ML5 generates the HOMO (φ5,c) of Wcis90, as illustrated below because of the destabilizing mixing with σ(W-Me). The shape of this HOMO easily explains the reactivity

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Figure 5. Schematic MO diagram of the frontier orbitals for Wcis90 and Wtrans00. The scale of the energies of the orbitals is globally preserved.

of 2+ with unsaturated molecules, which we will report elsewhere.58

Table 4. NPA Charges (Q) for Selected Atoms and Energies (au) of the Imido π-Bonds (π1 and π2) and the Amido Lone Pairs (LP) for [W(NH)(NH2)2(py)Me]+

Q(W) Q(Nimido) Q(Namido)

There is therefore not a single factor to explain the stability of Wcis90 with respect to Wtrans00. However, the π-donation from the imido ligand is an important aspect, and the higher energy of Wtrans00 results from the competition of π-donation from both amido and imido nitrogen orbitals into the same d orbital (giving φ2,t and φ3,t) and also from the donation in the high lying 2a′′ orbital of the ML5 complex (φ4,t). This is confirmed by the NBO analysis (Table 4), where the natural population analysis (NPA) charges indicate that the W center is more electron rich in Wcis90. This is traced to the increased donation from the imido, as illustrated by the lower negative charge on Nimido. Interestingly, the NPA charges of the amido nitrogen atoms Namido are comparable in both complexes. The NBO procedure

π1 π2 LP

Wcis90

Wtrans00

NPA Charges 1.653 -0.664 -1.082

1.705 -0.806 -1.060

Natural Bond Orbital Energies -0.4512 -0.443 -0.4505 -0.436 -0.379 -0.4139

allows one also to estimate the energy of the natural bond orbitals, and results in Table 4 show that the two π-bonds of the imido linkage are at lower energy in Wcis90 than in Wtrans00, still in agreement with more efficient donation of the imido ligand in the cis isomer. No W-amido π-bond was found with the NBO search, but the energy of the lone pairs is indicative of the slightly larger donation in the trans isomer (more stable lone pairs, see Table 4). Geometry of M(NR)(N2Npy)(X): Steric Factors. Solely on the basis of electronic factors, the geometrical


Figure 6. Evolution of the energy (kJ mol-1) of [W(NH)(NH2)2(py)Me]+ as a function of R ) Leq-W-Namido-H (Leq ) Me, cis; Leq ) NH, trans). Energies are expressed relative to that of Wcis90.

preference in d0 M(NR)(N2Npy)(X) is always to have NR in the apical position trans to pyridyl. However, the orientation of the amido planes is also of great importance. To illustrate this influence, the energy of several conformers of the idealized Cs system [W(NH)(NH2)2(py)Me]+ has been computed. As above, the dihedral angle R is defined as Leq-W-Namido-H (Leq ) Me for the cis isomer; Leq ) NH for the trans isomer), and the evolution of the energies of WcisR and WtransR is shown in Figure 6. The two curves cross for a critical R value of Rc = 50° with the cis isomer being more stable for R > 50°. The curve for WcisR exhibits a larger variation in energy upon rotation of the NH2 group since from R ) 90° to R ) 0° competition with NH for π-donation in both the lower 1a′ and 1a′′ orbitals (Figure 4) is switched on, leading to an important destabilization. If the value of the R angle is lowered below 50°, then the thermodynamic preference for the cis isomer can be altered to a thermodynamic preference for the trans isomer. In the real cis isomer (e.g., 2+), for R ) 90° the amido N-SiMe3 substituents get close to the substituents on the apical imido group (Ph or tBu), and therefore adverse steric repulsions will develop, preventing the R angle from reaching its optimal value (based on electronic preferences). Geometry optimizations of M(NR)(N2Npy)(X) at the ONIOM(B3PW91:UFF) level have been carried out. In the case of [W(NPh)(N2Npy)Me]+, the cis isomer 2cisoniom is still more stable than the trans (2transoniom) but by only 0.9 kJ mol-1 (see Table 5). In the real systems there is no longer Cs symmetry and the value of the R angle associated with each Namido atom is different (defined as R2 for N(2) and R3 for N(3)). For 2cisoniom both values are larger than the critical dihedral angle, Rc, and the cis isomer is still more stable than the trans (Table 5). The steric repulsion between SiMe3 and Ph is not strong enough to distort significantly the electronically preferred geometry. When Ph is substituted by tBu, greater repulsion between SiMe3 and NtBu is expected, and this is confirmed by the ONIOM calculations where the trans isomer becomes now the more stable species (Table 5: M ) W+ or Ta, R ) tBu, X ) Me). The preference for the trans geometry observed experimentally for 6 is thus reproduced by the ONIOM calculations with an energy


difference of 6.7 kJ.mol-1 in favor of 6transoniom with respect to 6cisoniom, whereas 6cisqm was 11.4 kJ mol-1 more stable than 6transqm. The calculated structure for 6transoniom is in excellent agreement with the experimental data for 6 (see Table 1). Comparison of the DFT R2 and R3 values with those of the ONIOM calculations clearly shows that inclusion of the steric factors has resulted in a lowering (in absolute value) of the R values for both cis and trans isomers (Table 5). This lowering is due to the steric repulsion exerted on SiMe3 by the apical ligand (NR or X). The lowering of the R values does not have the same consequences for each of the isomers. The cis geometry is destabilized, whereas the trans arrangement is stabilized (Figure 6). This explains why the electronic preference for the cis isomer can be reversed with appropriate choice of the ligands. As expected, the influence of the NR ligand is more pronounced and the exact identity of the real (experimental) R group may decide which isomer is ultimately obtained. With the bulky tBu group, the steric repulsions are large and the trans isomer is obtained (6). For the less bulky Ph group, the steric interactions are not strong enough to overcome the electronic preference for the cis isomer (2+). Formation of Binuclear Complexes and Intermolecular Methyl Group Exchange Reactions. A number of homo- and hetero-bimetallic complexes containing a central {M2Me3}+ unit have been formed in cases where a dimethyl compound was treated with 0.5 equiv of BArF3 or [Ph3C][BArF4].52,56,71-78 Of particular relevance to our work are Schrock’s reports of the group 4 dimethyls M(N2MesNpy)Me2 and binuclear trimethyl cations derived therefrom (M ) Zr or Hf; N2MesNpy ) MeC(2-C5H4N)(CH2NMes)2 where Mes ) 2,4,6-C6H2Me3).78,79 The methyl ligands in the trigonal bipyramidal compounds M(N2MesNpy)Me2 appear to undergo axialequatorial site exchange exclusively via an intermolecular mechanism involving methyl-bridged M2(N2MesNpy)Me2(µ-Me)2 intermediates.79 Furthermore, treatment of M(N2MesNpy)Me2 with either 1.0 or 0.5 equiv of [Ph3C][BArF4] yields predominantly the bridging methyl cations [M2(N2MesNpy)(µ-Me)3]+ according to NMR data.78 The Lewis acidic group 4 metal in [M(N2MesNpy)Me]+ is sufficient to trap the “other half” of the M(N2MesNpy)Me2 dimethyl starting compound. This is evidently not the case for the reaction of W(NPh)(N2Npy)Me2 (1) with 1 equiv of MeBArF3 or [Ph3C]+, which forms quantitatively [W(NPh)(N2Npy)Me]+ (2+), a formally 18 valence electron species. (70) Albright, T. A.; Burdett, J. K.; Whangbo, M.-H. Orbital Interactions in Chemistry; Wiley-Interscience: New York, 1985. (71) Doerrer, L. H.; Green, M. L. H.; Ha¨ussinger, D.; Sassmannshausen, J. J. Chem. Soc., Dalton Trans. 1999, 2111. (72) Bochmann, M.; Green, M. L. H.; Powell, A. K.; Sassmannshausen, J.; Triller, M. U.; Wocadlo, S. J. Chem. Soc., Dalton Trans. 1999, 43. (73) Chen, Y.; Stern, C. L.; Yang, S.; Marks, T. J. J. Am. Chem. Soc. 1996, 118, 12451. (74) Lancaster, S. J.; Bochmann, M. J. Organomet. Chem. 2002, 654, 221. (75) Zhang, S.; Piers, W. E. Organometallics 2001, 20, 2088. (76) Mehrkhodavandi, P.; Bonitatebus, P. J., Jr.; Schrock, R. R. J. Am. Chem. Soc. 2000, 122, 7841. (77) Hayes, P. G.; Piers, W. E.; McDonald, R. J. Am. Chem. Soc. 2002, 124, 2132. (78) Mehrkhodavandi, P.; Schrock, R. R.; Pryor, L. L. Organometallics 2003, 22, 4569. (79) Mehrkhodavandi, P.; Schrock, R. R.; Bonitatebus, P. J., Jr. Organometallics 2002, 21, 5785.

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Table 5. Energy Differences (kJ mol-1) between the trans and the cis Isomers of M(NR)(N2Npy)(X) (M ) W+or Ta) Calculated at the DFT(B3PW91) Level (∆Eqm) and at the ONIOM(B3PW91:UFF) Level (∆Eoniom)a cis isomer M ) W+, R ) Ph, X ) Me M ) W+, R ) tBu, X ) Me M ) W+, R ) Ph, X ) Cl M ) Ta, R ) tBu, X ) Me M ) Ta, R ) tBu, X ) Cl

trans isomer

∆Eqm

∆Eoniom

R2

R3

R2

R3

11.2 15.2 15.6 11.4 20.8

0.9 -6.2 1.6 -6.7 -7.6

-54.0(-68.4) -39.6(-75.0) -60.2(-70.7) -53.8(-81.7) -66.2(-85.4)

71.3(75.4) 58.1(75.0) 74.1(79.9) 55.6(81.7) 34.6(85.5)

-8.4(-19.4) -10.1(-19.1) -7.9(-26.0) -6.8(-28.0) -40.7(-30.2)

22.4(33.1) 20.9(24.1) 31.6(41.6) 19.6(27.5) 40.8(30.2)

a A positive value for ∆E qm or ∆Eoniom implies that the cis isomer is more stable than the trans one. Dihedral angles (R, deg) Leq-WNamido-Si (Leq ) X, cis; Leq ) NR, trans) indicate the orientation of the amido NR2 planes with respect to the equatorial plane. The angles R2 and R3 correspond to N(2) and N(3), respectively. The values in parentheses are the DFT(B3PW91) calculated values on M(NR)(N2Nqm)(X).

Reaction of W(NPh)(N2Npy)Me2 (1) with 0.5 equiv of BArF3 in CD2Cl2 at room temperature forms the products shown in eq 2 as deduced from the variabletemperature 1H and 13C NMR spectra of these mixtures, as well as of those formed in analogous experiments starting from 1-d6 or 1-13C2. Exactly the same mixtures are obtained when preformed 2+ (or its 2H- or 13Cenriched isotopomer) is treated with 1 (or one of its isotopomers), or when [Ph3C][BArF4] is used in place of BArF3. The mixtures are comparatively stable in solution (in comparison with 2+), with a half-life of approximately 24 h at ambient temperature. Treatment of these mixtures with a further 0.5 equiv of BArF3 cleanly forms [W(NPh)(N2Npy)Me]+ (2+). A second, minor species is also present at this stage (and in the intermediate mixture, eq 2). We tentatively formulate this as the trans-methyl cation 2a+ on the basis of its NMR spectra and the overall stoichiometry of the reaction. This species is the valence isoelectronic analogue of structurally characterized Ta(NtBu)(N2Npy)Me (6). Once formed, the ratio of 2+ to 2a+ (ca. 5:1) does not change significantly. The mixture gradually decomposes in CD2Cl2, as is the case for pure 2+. Note that when 2+ is formed by treating 1 with 1.0 equiv of BArF3 or [CPh3]+ all at once (Scheme 1), the second isomer 2a+ is not formed in any appreciable amount. However, 2a+ is observed to appear by a small amount in the 1H NMR spectra as 2+ decomposes. We believe either that the isomerization of 2+ to 2a+ is inherently slow with a rate comparable to that of decomposition or (and) that a species formed in the decomposition reaction helps accelerate the isomerization process. The major product in eq 2 is assigned as the methylbridged binuclear cation [{W(NPh)(N2Npy)Me}2(µ-Me)]+

(8+), which is highly fluxional at room temperature and still gives rise to broad resonances at 193 K. However, two methyl ligand environments in a 1:2 ratio can be determined through use of various isotopomers, and these are assigned to bridging (δ 1H 1.1, 13C 36.5 ppm) and terminal (δ 2H 0.1, 13C 38.5 ppm) sites. The molar ratio of 8+ to 2a+ is ca. 2:1 (i.e., 4W:1W centers). A monomethyl-bridged dimeric structure for 8+ is favored over a triply bridged (three µ-Me ligands) alternative since the latter would require a rather crowded sevencoordinate tungsten. Assignment of 8+ as an imidobridged compound is disfavored since this would account for neither the methyl ligand exchange reactions observed with added 1 (see below) nor the overall Cs symmetry of 8+ deduced from the NMR spectra (only one type of N2Npy ligand environment). Broadened resonances for nontrapped 1 are also clearly visible in the NMR spectra. Compound 1 is always present in equal amounts to 2a+. From the variable-temperature spectra and use of isotopomers it can be deduced that the methyl groups of binuclear cation 8+ and dimethyl 1 are in exchange on the NMR and chemical time scales. In contrast, the resonances for 2a+ remain sharp at all temperatures between 193 and 293 K, and so this species does not exchange with the other components on the NMR time scale. However, the addition of 13C-enriched dimethyl (1-13C2) to a preformed 1:1:2 (molar ratio) mixture of 1, 2a+, and 8+ resulted in the statistical scrambling of the 13CH3 groups between all three compounds (but not into the [MeBArF3]- anion). This shows that 2a+ is able to interact with one or both of the other tungsten species on a chemical time scale. The formation of 2a+ appears to depend on its participation in the fluxional process involving 8+ since it does not form in significant quantities via a simple intramolecular isomerization of 2+. Although the details of this process are not entirely clear, there appear to be parallels with Schrock’s suggestion that the axial-equatorial methyl ligand interconversion in five-coordinate M(N2MesNpy)Me2 proceeds via an intermolecular mechanism involving a methylbridged binuclear intermediate. An attempt to use 1 to catalyze the conversion of preformed 2+ to an equilibrium mixture of 2+ and 2a+ was attempted. However, although resonances for 2a+ were clearly seen, the mixture decomposed over the course of several hours (as in the case for 2+ alone), presumably since insufficient 1 was now present to stabilize 2+ as 8+ (these mixtures have much longer lifetimes, as noted above). While the cis-methyl isomer 2+ traps dimethyl 1 as 8+, the trans-methyl isomer 2a+ does not interact as



Scheme 2. Reactions of 2+ with Cp2ZrMe2, ZnMe2, and AlMe3

readily with 1. This can be tentatively attributed to the nature of the six-coordinate intermediate 8a+ (see below) that would probably be formed in this case. In this species one W center has the imido ligand cis to pyridyl and trans to an NSiMe3 group. This is apparently an unfavorable isomer of six-coordinate compounds of the type III (Chart 1), which invariably have the imido ligands trans to pyridyl, presumably in order to avoid having strong trans influence π-donor amide and imide ligands placed strictly trans to each other.

Furthermore, the observation that monomethyl cation 2a+ does not form an identifiable adduct with dimethyl

1 is consistent with the separate species being present in equal amounts in the mixtures formed in eq 2. Assuming thermodynamic control, the ratio of 2+ and 2a+ is ca. 2:1 (the former being trapped by unreacted 1 forming 8+). Although the trapping of 2+ by 1 could perturb this observed ratio, the fluxional (weakly bridged) nature of 8+ suggests that this effect is probably quite small. Therefore the energies of 2+ and 2a+ in solution are probably rather similar, but there is a kinetic barrier to their interconversion (probably via 8+). This conclusion is in agreement with the computational studies described above, with the computed gas phase energy difference between 2+ and 2a+ being only 0.9 kJ mol-1 (Table 5) at the ONIOM(B3PW91:UFF) level. Reactions of [W(NPh)(N2Npy)Me]+ (2+) with Selected Metal Methyl Compounds. As seen, 2+ undergoes interesting interactions with the parent dimethyl 1. We therefore examined its behavior with certain other methyl compounds, specifically Cp2ZrMe2, ZnMe2, and AlMe3 (Scheme 2). Methyl-bridged cations have long been associated with zirconocene systems,52,56,71-74,80

4456


and interactions between methyl cations and AlMe3 have also received attention.56,81 Addition of 2+ (as the [MeBArF3]- salt) in CD2Cl2 to Cp2ZrMe2 and analysis by 1H NMR spectroscopy showed no evidence of reaction or interaction, as no broadening of the resonances were observed, even at -90 °C. However, although no evidence of any interaction between 2+and Cp2ZrMe2 was directly observed by NMR spectroscopy, reaction of [W(NPh)(N2Npy)CD3]+ (2+-d3) with Cp2ZrMe2 showed a statistical scrambling of the CD3 group over the WMe and ZrMe2 and into the anion sites, thus showing that these methyl ligands can exchange on a chemical time scale. This in turn implies that a short-lived intermediate such as 2+-Cp2ZrMe2 and (subsequently) 1 and [Cp2ZrMe]+ (or its adduct with [MeBArF3]- 54) must be formed even though they are not observed by NMR spectroscopy (Scheme 2). Reaction of 2+ with Cp2Zr(CD3)2 also leads to deuterium incorporation into the WMe site, as well as into the [MeBArF3]- anion (not observed in the reactions of 2+ with 1 or ZnMe2, see below). This is to be expected since it is known that [Cp2ZrMe]+ forms an adduct with [MeBArF3]- through which methyl group exchange may proceed.52,54,56,57 On standing, a second Cp-containing species is formed in the mixtures of 2+ and Cp2ZrMe2. This was unambiguously identified as Cp2Zr(Me)Cl by comparison with an independently prepared sample.82 In our hands, pure samples of Cp2ZrMe2 on their own also decompose in CD2Cl2 to afford Cp2Zr(Me)Cl, but only after many weeks. We propose that the exchange reaction between 2+ and Cp2ZrMe2 indirectly activates the Cp2ZrMe2 to reaction with the CD2Cl2 solvent (presumably via the cationic monomethyl intermediate [Cp2ZrMe]+), accounting for the appearance of Cp2Zr(Me)Cl. Treatment of 2+ (as the [MeBArF3]- salt) in CD2Cl2 with ZnMe2 gives an 1H NMR spectrum in which the resonances attributable to the WMe and ZnMe ligands are broadened. On cooling to -90 °C, these resonances become broader still, but no new resonances were observed. At room temperature, spin saturation transfer experiments indicate that the WMe and ZnMe methyl environments are in exchange. Reaction of 2+-d3 with ZnMe2 showed that the methyl groups scramble to a statistical distribution of CD3 or CH3 within 5 min. No incorporation of CH3 into the [D3CBArF3]- anion (formed alongside 2+-d3 from 1-d6 and BArF3) was observed. These observations imply that a transient and/or weak adduct of the type 2+-ZnMe2 is formed between 2+ and ZnMe2 (Scheme 2). Although several different structures could be proposed for this adduct, DFT computational studies (see below) strongly support the structure shown for 2+-ZnMe2 in Scheme 2. It is likely that such an adduct could be an intermediate on the pathway to WMe/ZnMe group exchange, although a transition state for this process could not be identified by DFT calculations. Tricoordinate zinc methyl cations were recently reported by Bochmann,83 and in this context we note (80) Zhou, J.; Lancaster, S. J.; Walker, D. A.; Beck, S.; ThorntonPett, M.; Bochmann, M. J. Am. Chem. Soc. 2001, 123, 223. (81) Bochmann, M.; Lancaster, S. J. J. Organomet. Chem. 1995, 497, 55. (82) Stuhldreier, T.; Keul, H.; Ho¨cker, H.; Englert, U. Organometallics 2000, 19, 5231. (83) Hannant, M. D.; Schormann, M.; Bochmann, M. J. Chem. Soc., Dalton Trans. 2002, 4071.

Ward et al.

also Bochmann’s other recent report of intermolecular methyl group exchange between neutral and cationic zinc alkyls.84 As indicated, the reaction of 2+ with ZnMe2 was studied computationally using the model system [W(NH)(N2Nqm)Me]+, 2Hqm, and ZnMe2. Several starting geometries for an adduct between 2Hqm and ZnMe2 were considered (Figure 7a), but geometry optimizations yielded only one structure, namely, 2Hqm-ZnMe2 (Figure 7b). This structure corresponds to an adduct between N and Zn (Zn-N ) 2.335 Å) with one methyl group on Zn interacting with W (W‚‚‚Me ) 2.649 Å). The energy of this adduct, with respect to separated 2Hqm and ZnMe2, is +22.4 kJ mol-1, consistent with it not being observed experimentally. Despite many attempts, we were not able to locate a transition state structure corresponding to the methyl exchange between W and Zn. However, our results show that the imido group is involved in the exchange process, as it allows for the adduct 2Hqm-ZnMe2 to be formed at a low energy cost. This adduct brings one methyl group of ZnMe2 in close contact to W, a necessary condition to observe exchange. With the observation of methyl exchange reactions discussed above, it was thought that [W(NPh)(N2Npy)Me]+ (2+) might undergo a similar exchange process with AlMe3, especially since analogous reactions have been well documented in the literature.56,85-88 However, on reaction with AlMe3, the organometallic cation in 2-MeBArF3 undergoes N2Npy ligand degradation, with deprotonation occurring at one of the methylene groups to afford [W{MeC(2-C5H4NAlMe)(CHNSiMe3)(CH2NSiMe3)}(µ-NPh)Me2][MeBArF3] (9-MeBArF3) in 64% isolated yield (Scheme 2). The structure proposed in Scheme 2 is based on the best fit interpretation of all NMR data including a ROESY (rotating-frame nuclear Overhauser enhancement spectroscopy) spectrum and comparative experiments with 2H- and 13C-labeled isotopomers. For example, the reaction of [W(NPh)(N2Npy)13CH3]+ (2+-13C) with AlMe3 shows that the labeled methyl group appears only in the methyl environment giving the signal at 1.29 ppm (but then scrambles over all other methyl ligand sites over 48 h). Thus the CH4 eliminated on forming 9+ arises exclusively from one of the AlMe3 methyl groups. NMR tube scale experiments between [W(NPh)(N2Npy)CD3]+ (2+d3) and AlMe3 did not result in depletion of deuterium in the CD3 ligand, as would be the case if the reaction proceeded via initial deprotonation of the W-CD3 unit (forming CDH3) and subsequent net transfer of a proton from a N2Npy methylene group. The presence of two tungsten-bound methyl groups was confirmed by 183W satellites in the NMR spectra. Satellites (J ) 47 Hz) were also observed in the 13C{1H} NMR spectrum for the resonance attributed to the imine methine carbon (formally a CH2 of N2Npy). The 1J(CH) coupling constant (84) Walker, D. A.; Woodman, T. J.; Hughes, D. L.; Bochmann, M. Organometallics 2001, 20, 3772. (85) Klimpel, M. G.; Eppinger, J.; Sirsch, P.; Scherer, W.; Anwander, R. Organometallics 2002, 21, 4021. (86) Lieber, S.; Prosenc, M. H.; Brintzinger, H. H. Organometallics 2000, 19, 377. (87) Holton, J.; Lappert, M. F.; Ballard, D. G. H.; Pearce, R.; Atwood, J. L.; Hunter, W. E. J. Chem. Soc., Dalton Trans. 1979, 45. (88) Holton, J.; Lappert, M. F.; Ballard, D. G. H.; Pearce, R.; Atwood, J. L.; Hunter, W. E. J. Chem. Soc., Dalton Trans. 1979, 54.



Figure 7. (a) The three initial starting geometries used in the minimization of the adduct (2H-ZnMe2) between [W(NH)(N2Nqm)Me]+ (2Hqm) and ZnMe2. (b) Minimized final DFT geometry of the adduct 2H-ZnMe2.

of 165 Hz for the imine CH is consistent with an intermediate hybridization between sp2 and sp3, but since 9+ is diamagnetic, we propose that a metallacyclopropane structure is more appropriate in this case. Among the other NOE interactions found was one between the N2Npy pyridyl H6 and the Al-Me, clearly placing the pyridyl donor on aluminum and not tungsten. Conclusions A detailed experimental and computational study of neutral and cationic five-coordinate complexes of the type M(NR)(N2Npy)X has been carried out. The formation of [W(NPh)(N2Npy)Me]+ (2+) from W(NPh)(N2Npy)Me2 (1) and either BArF3 or [CPh3]+ proceeds smoothly without redox side-reactions, whereas reaction of 1 with [PhMe2NH][BArF4] results in protonation of the N2Npy ligand. These reactions can be attributed to the π electron-rich nature of d0 1. The different geometries of 2+ and [Ta(NtBu)(N2Npy)Me] (6) are analyzed in detail through DFT and ONIOM calculations. The site preferences for the imido and methyl ligands in 2+, 6, and related species are rationalized in terms of π-bonding preferences modified by steric influences that adjust the orientation of the amido nitrogen π-donor lone pairs. Methyl cation 2+ undergoes methyl ligand exchange with 1, Cp2ZrMe2, and ZnMe2; the adduct with 1 is

oberved by NMR spectroscopy, whereas that with ZnMe2 is deduced from DFT calculations and appears to involve the participation of the imido nitrogen. Overall the chemistry of 1 and 2+ is attributed to the unique combination of the fac coordinating N2Npy diamidodonor ligand and the ancillary imido ligand. Experimental Section General Methods and Instrumentation. All manipulations were carried out using standard Schlenk line or drybox techniques under an atmosphere of argon or of dinitrogen. Solvents were predried over activated 4 Å molecular sieves and were refluxed over potassium (tetrahydrofuran, hexanes, benzene), sodium/potassium alloy (pentane, diethyl ether), or calcium hydride (dichloromethane, acetonitrile) under a dinitrogen atmosphere and collected by distillation. CD2Cl2 was dried over phosphorus pentoxide, distilled under reduced pressure, and stored under dinitrogen in Teflon valve ampules. NMR samples were prepared under dinitrogen in 5 mm Wilmad 507-PP tubes fitted with J. Young Teflon valves. 1H, 13 C{1H}, 13C, 19F, and 11B{1H} NMR spectra were recorded on Varian Mercury-VX 300 and Varian Unity Plus 500 spectrometers. 1H and 13C assignments were confirmed when necessary with the use of DEPT-135, DEPT-90, and two-dimensional 1H1 H and 13C-1H NMR experiments. 1H and 13C spectra were referenced internally to residual protio-solvent (1H) or solvent (13C) resonances and are reported relative to tetramethylsilane (δ ) 0 ppm). 19F and 11B spectra were referenced externally to CFCl3 (19F) and BF3‚Et2O (11B). Chemical shifts are quoted in

4458


δ (ppm) and coupling constants in Hertz. Infrared spectra were prepared as KBr pellets or Nujol mulls between KBr or NaCl plates and were recorded on Perkin-Elmer 1600 and 1710 series FTIR spectrometers. Infrared data are quoted in wavenumbers (cm-1). Mass spectra were recorded by the mass spectrometry service of the University of Oxford Inorganic Chemistry Laboratory or Dyson Perrins Laboratory, and elemental analyses by the analytical services of the University of Oxford Inorganic Chemistry Laboratory. Literature Preparations. The compounds W(NPh)(N2Npy)Me2 (1), W(NPh)(N2Npy)(13CH3)2 (1-13C2), W(NPh)(N2Npy)(CD3)2 (1-d6),3 Ta(NtBu)(N2Npy)Cl(py),1 Cp2ZrMe2,89 and Cp2Zr(Cl)Me,82 were prepared according to published methods. The compound Cp2Zr(CD3)2 was prepared using a modified literature procedure.89 The compounds BArF3, [Ph3C][BArF4], and [HNMe2Ph][BArF4] were provided by DSM Research. All other compounds and reagents were purchased and used without further purification. Cp2Zr(CD3)2. Cp2ZrCl2 (500 mg, 1.71 mmol) was dissolved in Et2O (30 mL) and cooled to -78 °C for the dropwise addition of CD3MgI (3.42 mL of a 1.0 M solution in Et2O, 3.42 mmol). The reaction was allowed to warm to room temperature and stirred for a further 2 h before adding 1,4-dioxane to the reaction mixture (2 mL). The volatiles were removed under reduced pressure, and the resulting white solid was sublimed (80 °C/10-6 mbar) to afford Cp2Zr(CD3)2 as a white crystalline solid. Yield: 104 mg, 24%. [W(NPh)(N2Npy)Me][MeBArF3] (2-MeBArF3) (NMR tube scale). W(NPh)(N2Npy)Me2 (1) (40 mg, 0.07 mmol) was dissolved in CD2Cl2 (0.5 mL), and this solution was added to solid BArF3 (34 mg, 0.07 mmol). The mixture immediately turned brown. The solution was transferred to a 5 mm J. Young NMR tube, sealed, and analyzed by NMR spectroscopy or reacted in situ. Yield: 100% by 1H NMR. 1H NMR (CD2Cl2, 500.0 MHz, 293 K): 9.12 (1 H, dd, H,6 3J(H5H6) ) 5.3 Hz, 4J(H4H6) ) 0.5 Hz), 8.25 (1 H, td, H,4 3J(H3H4H5) ) 8.0 Hz, 4J(H4H6) ) 0.5 Hz), 7.77 (1 H, dd, H3, 3J(H3H4) ) 8.0 Hz, 4J(H3H5) ) 0.5 Hz), 7.68 (1 H, td, H5, 3J(H4H5H6) ) 8.0 Hz, 4J(H3H5) ) 0.5 Hz), 7.60 (2 H, td, m-C6H5, 3J ) 4.9 Hz, 4J ) 2.0 Hz), 7.32 (3 H, m, o-C6H5, p-C6H5), 4.29 (2 H, d, CHH, 2J ) 13.2 Hz), 3.99 (2 H, d, CHH, 2J ) 13.2 Hz), 1.72 (3 H, s, Me of N2Npy), 1.46 (3 H, s, WMe, 2J(WH) ) 6.3 Hz (15%)), 0.45 (3 H, br s, BMe), 0.17 (18 H, s, SiMe3) ppm. 13C NMR (CD2Cl2, 125.7 MHz, 293 K): 161.0 (C2), 154.3 (ipso-C6H5), 148.7 (C6F5, 1J(CF) ) 239 Hz), 146.5 (o-C6H5, 1J(CH) ) 183 Hz), 142.8 (C,6 1J(CH) ) 168 Hz), 137.8 (C6F5, 1J(CF) ) 251 Hz), 136.7 (C6F5, 1J(CF) ) 251 Hz), 129.5 (m-C6H5, 1J(CH) ) 162 Hz), 128.2 (p-C6H5, 1J(CH) ) 162 Hz), 125.1 (C,4 1J(CH) ) 170 Hz), 124.6 (C,5 1J(CH) ) 162 Hz), 120.8 (C3, 1J(CH) ) 167 Hz), 64.9 (CH2NSiMe3, 1J(CH) ) 144 Hz), 48.5 (C(CH2NSiMe3)2), 40.5 (WMe, 1J(CH) ) 126 Hz, 1 J(WC) ) 90 Hz (15%)), 22.0 (Me of N2Npy, 1J(CH) ) 131 Hz), 10.5 (BMe, br, 1J(CH) ) 118 Hz), 0.6 (SiMe3, 1J(CH) ) 120 Hz) ppm. 19F NMR (CD2Cl2, 282.2 MHz, 293 K): -133.4 (6 F, d, o-C6F5, 3J ) 19.8 Hz), -165.6 (6 F, app. t, p-C6F5, app. 3J ) 19.8 Hz), -168.1 (3 F, t, m-C6F5, 3J ) 22.7 Hz) ppm. 11B{1H} NMR (CD2Cl2, 160.4 MHz, 293 K): -14.7 (br s, MeBArF3) ppm. Elemental analysis was not obtained for this compound, which was insufficiently stable to be isolated. [W(NPh)(N2Npy)Me][BArF4] (2-BArF4) (NMR tube scale synthesis). A solution of W(NPh)(N2Npy)Me2 (1) (10 mg, 0.02 mmol) and [Ph3C][BArF4] (15 mg, 0.02 mmol) in CD2Cl2 was transferred to a 5 mm J. Young NMR tube. The 1H NMR spectrum after 15 min showed only resonances attributable to [W(NPh)(N2Npy)Me]+ (2+) and Ph3CMe. [W(NPh)(HN2Npy)Me2][BArF4] (3-BArF4). W(NPh)(N2Npy)Me2 (1) (102.4 mg, 0.166 mmol) was dissolved in CH2Cl2 and cooled to -78 °C. To this solution was added [PhMe2NH][BArF4] (133.1 mg, 0.166 mmol, 1 equiv) in CH2Cl2 dropwise, upon which the reaction immediately turned dark brown. The (89) Samuel, E.; Rausch, M. D. J. Am. Chem. Soc. 1973, 95, 6263.

Ward et al. reaction was allowed to warm to room temperature and stirred for 20 min before removing the volatiles under reduced pressure to yield the product as a brown solid. It was not possible to separate the product from the dimethylaniline byproduct by crystallization. Yield: 179 mg (76%). 1H NMR (CD2Cl2, 500.0 MHz, 293 K): 8.79 (1 H, d, H6, 3J(H5H6) ) 5.8 Hz), 8.18 (1 H, td, H4, 3J(H3H4H5) ) 7.8 Hz, 4J(H4H6) ) 1.7 Hz), 7.85 (1 H, d, H3, 3J(H3H4) ) 7.8 Hz), 7.65 (1 H, t, H5, 3 J(H4H5H6) ) 6.1 Hz), 7.60 (2 H, t, m-C6H5, 3J ) 7.6 Hz), 7.36 (2 H, d, o-C6H5, 3J ) 7.6 Hz), 7.31 (1 H, t, p-C6H5, 3J ) 7.6 Hz), 3.71 (1 H, d, CHH, 2J ) 16.1 Hz), 3.51 (1 H, d, CHH, 2J ) 16.1 Hz), 3.19 (1 H, dd, CHH, 2J ) 13.2 Hz, 3J ) 7.8 Hz), 2.61 (1 H, app. t, CHH, app. J ) 12.5 Hz), 2.16 (1 H, br t, NH, 3 J ) 9.8 Hz), 1.66 (3 H, s, Me of N2Npy), 1.03 (3 H, s, WMea, 2 J(WH) ) 9.5 Hz (15%)), 0.48 (3 H, s, WMeb, 2J(WH) ) 5.1 Hz (15%)), 0.32 (9 H, s, SiMe3), 0.25 (9 H, s, SiMe3) ppm. 13C NMR (CD2Cl2, 125.7 MHz, 293 K): 160.5 (C2), 153.8 (ipso-C6H5), 151.9 (C,6 1J(CH) ) 179 Hz), 148.4 (C6F5, 1J(CF) ) 238 Hz), 141.9 (C6H5, 1J(CH) ) 167 Hz), 138.5 (C6F5, 1J(CF) ) 250 Hz), 129.4 (C6H5, 1J(CH) ) 150 Hz), 125.4 (C,4 1J(CH) ) 170 Hz), 125.3 (C,5 1J(CH) ) 170 Hz), 122.4 (C6H5, 1J(CH) ) 167 Hz), 122.2 (C3, 1J(CH) ) 167 Hz), 115.5 (C6F5, 1J(CF) ) 250 Hz), 61.1 (CH2NSiMe3, 1J(CH) ) 139 Hz), 61.0 (CH2NSiMe3, 1J(CH) ) 139 Hz), 50.2 (C(CH2NSiMe3)2), 49.9 (WMeb, 1J(CH) ) 122 Hz, 1J(WC) ) 66 Hz (15%)), 43.8 (WMea, 1J(CH) ) 125 Hz, 1 J(WC) ) 107 Hz (15%)), 23.2 (Me of N2Npy, 1J(CH) ) 130 Hz), 1.2 (SiMe3, 1J(CH) ) 122 Hz), -0.1 (SiMe3, 1J(CH) ) 164 Hz) ppm. 19F NMR (CD2Cl2, 282.2 MHz, 293 K): -133.4 (8 F, s, o-C6F5), -169.9 (8 F, app. t, p-C6F5, app. 3J ) 20.1 Hz), -167.8 (4 F, t, m-C6F5, 3J ) 15.3 Hz) ppm. 11B{1H} NMR (CD2Cl2, 160.4 MHz, 293 K): -14.3 (br s, BArF4) ppm. IR (KBr plates, Nujol): 2726 (w), 1645 (w), 1604 (w), 1559 (w), 1510 (m), 1301 (w), 1261 (s), 1088 (s), 1021 (s), 980 (m), 896 (w), 800 (s), 756 (w), 723 (m), 685 (w), 661 (w), 574 (w), 473 (w) cm-1. Anal. Calcd for C45H52BF20N4Si2W‚C8H11N: C, 45.4; H, 4.5; N, 5.0. Found: C, 45.5; H, 4.0; N, 4.9. [W(NPh)(N2Npy)Me(NCMe)][MeBArF3] (4-MeBArF3). To a stirred solution of W(NPh)(N2Npy)Me2 (1) (200 mg, 0.32 mmol) in CH2Cl2 (20 mL) cooled to 0 °C was added a solution of BArF3 (166 mg, 0.32 mmol) in CH2Cl2 (20 mL). To the stirred reaction was added MeCN (1 mL). After stirring at room temperature for 1 h the solvent was removed under reduced pressure to afford 4-MeBArF3 as a golden brown solid. Yield: 266 mg (70%). 1H NMR (CD2Cl2, 500.0 MHz, 293 K): 8.85 (1 H, d, H6, 3J(H5H6) ) 5.9 Hz), 8.16 (1 H, td, H4, 3J(H3H4H5) ) 7.8 Hz, 4J(H4H6) ) 2.0 Hz), 7.75 (1 H, d, H3, 3J(H3H4) ) 8.3 Hz), 7.59-7.54 (3 H, m, H5, m-C6H5), 7.25 (3 H, m, o-C6H5, p-C6H5), 3.96 (2 H, d, CHH, 2J ) 14.0 Hz), 3.87 (2 H, d, CHH, 2 J ) 14.0 Hz), 2.18 (3 H, br s, NCMe), 1.66 (3 H, s, Me of N2Npy), 0.73 (3 H, s, WMe, 2J(WH) ) 5.6 Hz (15%)), 0.44 (3 H, br s, BMe), 0.13 (18 H, s, SiMe3) ppm. 1H NMR (CD2Cl2, 500.0 MHz, 183 K): 8.71 (1 H, d, H6, 3J(H5H6) ) 7.1 Hz), 8.10 (1 H, t, H,4 3J(H3H4H5) ) 7.5 Hz), 7.69 (1 H, d, H3, 3J(H3H4) ) 8.1 Hz), 7.53-7.49 (3 H, overlapping m, H5, m-C6H5), 7.20-7.17 (3 H, overlapping m, o-C6H5, p-C6H5), 4.09 (1 H, d, CHH, 2J ) 15.3 Hz), 3.97 (1 H, d, CHH, 2J ) 15.5 Hz), 3.51 (2 H, apparent t, v, apparent J ) 14.9 Hz), 2.28 (3 H, s, NCMe), 1.58 (3 H, s, Me of N2Npy), 0.43 (3 H, br s, WMe), 0.30 (3 H, br s, BMe), 0.08 (3 H, s, SiMe3), -0.04 (3 H, s, SiMe3) ppm. 13C{1H} NMR (CD2Cl2, 125,7 MHz, 183 K): 159.5 (C2), 152.9 (ipso-C6H5), 148.7 (C6), 147.0 (C6F5, 1J(CF) ) 239 Hz), 136.5 (C6F5, 1J(CF) ) 251 Hz), 135.4 (C6F5, 1J(CF) ) 251 Hz), 128.5 (C4), 127.1 (o-C6H5), 126.8 (m-C6H5), 124.3 (p-C6H5), 122.3 (C5), 119.7 (C3), 59.3 (CH2NSiMe3), 54.4 (CH2NSiMe3), 50.7 (C(CH2NSiMe3)2), 33.6 (WMe), 24.7 (Me of N2Npy), 8.5 (BMe), 0.27 (SiMe3), 0.17 (SiMe3) ppm. 19F NMR (CD2Cl2, 282.2 MHz, 183 K): -135.5 (6 F, d, o-C6F5, 3J ) 19.8 Hz), -166.8 (6 F, app. t, p-C6F5, app. 3J ) 19.8 Hz), -169.7 (3 F, t, m-C6F5, 3J ) 22.7 Hz) ppm. 11B{1H} NMR (CD2Cl2, 160.4 MHz, 183 K): -15.0 (br s, MeB(C6F5)3) ppm. IR (KBr pellet): 3352 (w), 3300 (w), 2958 (m), 2904 (m), 2850 (w), 2290 (m) (ν(CtN)), 1642 (m), 1606 (m),

Cationic Tungsten Methyl Complex 1512 (s), 1452 (s), 1382 (w), 1356 (w), 1254 (s), 1168 (w), 1086 (s), 1026 (m), 952 (s), 934 (m), 914 (m), 914 (m), 844 (s), 802 (m), 760 (m), 690 (w), 664 (w), 644 (w), 606 (w), 568 (w), 548 (w), 470 (w), 446 (w), 414 (w) cm-1. Anal. Calcd for C43H43BF15N5Si2W: C, 44.3; H, 3.7; N, 6.0. Found: C, 44.1; H, 3.8; N, 6.4. [W(NPh)(N2Npy)Me(THF)][MeBArF3] (5-MeBArF3). To a stirred solution of W(NPh)(N2Npy)Me2 (1) (200 mg, 0.32 mmol) in CH2Cl2 (20 mL) cooled to 0 °C was added a solution of BArF3 (166 mg, 0.32 mmol) in CH2Cl2 (20 mL). To the stirred reaction was added THF (1 mL). After stirring at room temperature for 1 h the solvent was removed under reduced pressure to afford 5-MeBArF3 as a golden brown solid. Yield: 210 mg (54%). 1H NMR (CD2Cl2, 500.0 MHz, 293 K): 8.96 (1 H, d, H6, 3 J(H5H6) ) 5.4 Hz), 8.24 (1 H, td, H4, 3J(H3H4H5) ) 7.8 Hz, 4 J(H4H6) ) 1.5 Hz), 7.78 (1 H, d, H3, 3J(H3H4) ) 7.8 Hz), 7.66 (1 H, td, H5, 3J(H4H5H6) ) 5.9 Hz, 4J(H3H5) ) 1.5 Hz), 7.59 (2 H, t, m-C6H5, 3J ) 7.8 Hz), 7.32-7.28 (3 H, overlapping m, o-C6H5, p-C6H5), 4.19 (2 H, d, CHH, 2J ) 13.2 Hz), 3.97 (2 H, d, CHH, 2J ) 13.2 Hz), 3.77 (4 H, br s, OCH2), 1.85 (4 H, br s, OCH2CH2), 1.71 (3 H, s, Me of N2Npy), 1.23 (3 H, s, WMe, 2 J(WH) ) 6.3 Hz (15%)), 0.43 (3 H, br s, BMe), 0.17 (18 H, s, SiMe3) ppm. 1H NMR (CD2Cl2, 500.0 MHz, 193 K): 8.43 (1 H, d, H6, 3J(H5H6) ) 5.9 Hz), 8.14 (1 H, t, H4, 3J(H3H4H5) ) 7.8 Hz), 7.73 (1 H, d, H3, 3J(H3H4) ) 8.3 Hz), 7.56-7.51 (3 H, m, H5, m-C6H5), 7.28 (2 H, d, o-C6H5, 3J ) 7.8 Hz), 7.17 (1 H, t, p-C6H5, 3J ) 7.3 Hz), 4.41 (1 H, br s, CHH), 4.20 (1 H, br s, OCH2), 4.17 (1 H, br s, OCH2), 3.97 (1 H, br s, OCH2), 3.94 (1 H, br s, OCH2), 3.76 (1 H, d, CHH, 2J ) 14.2 Hz), 3.51 (1 H, d, CHH, 2J ) 13.2 Hz), 3.44 (1 H, br s, CHH), 2.04 (1 H, br s, OCH2CH2), 1.96 (1 H, br s, OCH2CH2), 1.86 (1 H, br s, OCH2CH2), 1.60 (3 H, s, Me of N2Npy), 1.51 (1 H, br s, OCH2CH2), 0.48 (3 H, s, WMe), 0.29 (3 H, br s, BMe), 0.10 (9 H, s, SiMe3), 0.02 (9 H, s, SiMe3) ppm. 13C{1H} NMR (CD2Cl2, 125.7 MHz, 193 K): 160.2 (C2), 153.4 (ipso-C6H5), 147.8 (C6), 147.0 (C6F5, 1J(CF) ) 239 Hz), 136.5 (C6F5, 1J(CF) ) 251 Hz), 135.4 (C6F5 ) , 1J(CF) ) 251 Hz), 128.4 (C4), 128.1 (o-C6H5), 127.2 (m-C6H5), 124.8 (p-C6H5), 123.7 (C5), 119.8 (C3), 67.5 (OCH2), 63.8 (CH2NSiMe3), 58.9 (CH2NSiMe3), 51.1 (C(CH2NSiMe3)2), 34.5 (WMe), 25.1 (OCH2CH2), 23.5 (Me of N2Npy), 9.3 (BMe), 0.6 (SiMe3), 0.0 (SiMe3) ppm. 19F NMR data (CD2Cl2, 282.2 MHz, 193 K): -135.5 (6 F, d, o-C6F5, 3J ) 19.8 Hz), -166.8 (6 F, app. t, p-C6F5, app. 3J ) 19.8 Hz), -169.7 (3 F, t, m-C6F5, 3J ) 22.7 Hz) ppm. 11B{1H} NMR (CD2Cl2, 160.4 MHz, 193 K): -15.0 (br s, MeBArF3) ppm. IR (KBr pellet): 3064 (w), 2956 (m), 2902 (m), 2852 (w), 1640 (m), 1608 (m), 1510 (s), 1482 (s), 1452 (s), 1382 (w), 1356 (m), 1254 (s), 1168 (w), 1084 (s), 1022 (m), 966 (s), 952 (s), 934 (s), 916 (s), 844 (s), 802 (m), 784 (m), 760 (m), 736 (w), 688 (w), 660 (w), 646 (w), 632 (w), 606 (w), 568 (w), 452 (w), 402 (w) cm-1. Anal. Calcd for C45H48BF15N4OSi2W: C, 45.1; H, 4.0; N, 4.7. Found: C, 44.5; H, 4.0; N, 4.7. Ta(NtBu)(N2Npy)Me (6). To a stirred solution of Ta(NtBu)(N2Npy)Cl(py) (250 mg, 0.371 mmol) in THF was added MeLi (230 µL of a 1.6 M solution in Et2O, 0.371 mmol) at 0 °C. The yellow solution was allowed to warm to room temperature and stirred for 2 h. The solvent was removed under reduced pressure and the product extracted into pentanes. The solution was filtered and the volatiles were removed under reduced pressure to afford a brown powder. X-ray diffraction quality crystals were grown by slow evaporation of a solution in pentanes at room temperature. Yield: 160 mg (73%). 1H NMR (C6D6, 300.0 MHz, 293 K): 8.99 (1 H, m, H6), 6.97 (1 H, m, H4), 6.68 (1 H, m, H3), 6.47 (1 H, m, H5), 3.97 (2 H, d, CHH, 2J ) 12.7 Hz), 3.45 (2 H, d, CHH, 2J ) 12.7 Hz), 1.70 (9 H, s, t Bu), 0.95 (3 H, s, Me of N2Npy), 0.73 (3 H, s, TaMe), 0.08 (18 H, s, SiMe3) ppm. 13C{1H} NMR (C6D6, 75.5 MHz, 293 K): 160.5 (C2), 147.8 (C6), 137.3 (C4), 121.0 (C5), 120.6 (C3), 66.0 (CMe3), 62.6 (CH2), 44.9 (C(CH2NSiMe3)2), 35.1 (CMe3), 33.6 (TaMe), 23.8 (Me of N2Npy), 0.8 (SiMe3) ppm. IR data (KBr pellet): 3062 (w), 2958 (s), 2907 (s), 2860 (m), 2827 (m), 2358

Organometallics, Vol. 23, No. 19, 2004 4459 (w), 2342 (w), 1986 (w), 1957 (w), 1929 (w), 1868 (w), 1748 (w), 1716 (w), 1651 (w), 1601 (m), 1591 (w), 1571 (m), 1520 (w), 1505 (w), 1474 (m), 1456 (w), 1434 (m), 1402 (w), 1386 (w), 1364 (w), 1352 (m), 1271 (s), 1245 (s), 1210 (m), 1157 (w), 1149 (m), 1139 (w), 1088 (m), 1040 (s), 1019 (m), 1005 (w), 920 (s), 902 (s), 880 (m), 838 (s), 801 (m), 776 (m), 749 (m), 687 (m), 645 (w), 628 (w), 595 (m), 579 (w), 539 (w), 516 (w), 483 (s), 439 (w), 422 (w), 407 (w) cm-1. Anal. Calcd for C20H41N4Si2Ta: C, 41.8; H, 7.2; N, 9.8. Found: C, 41.2; H, 6.6; N, 9.2. High-resolution EI mass spectrum: m/z found (calcd for [M - Me]+, C19H38N4Si2Ta) ) 559.2127 (559.2115). Ta(NtBu)(N2Npy)(η1-C3H5) (7). To a suspension of Ta(NtBu)(N2Npy)Cl(py) (260.0 mg, 0.386 mmol) in benzene (25 mL), cooled to 7 °C, was added C3H5MgCl (227 µL of a 1.7 M solution in THF, 0.386 mmol). The solution was stirred at room temperature for 2.5 h. An excess of dioxane (1 mL) was added and the solution filtered. The filtrate was concentrated to dryness under reduced pressure, and the crude product was triturated with pentanes to afford an orange powder. Yield: 130 mg (56%). 1H NMR (C7D8, 500.0 MHz, 193 K): 9.01 (1 H, d, H6, 3J(H5H6) ) 4.9 Hz), 7.03 (1 H, m overlapping solvent, CH2CHCH2), 6.83 (1 H, m, H4), 6.35 (2 H, m overlapping, H3H5), 5.15 (1 H, m overlapping Htrans, Hcis to CH), 5.13 (1 H, d overlapping Hcis, Htrans to CH, 3J ) 15.6 Hz), 4.31 (2 H, d, CHH, 2J ) 12.7 Hz), 3.45 (2 H, d, CHH, 2J ) 12.7 Hz), 2.46 (2 H, d, TaCH2CHCH2, 3J ) 7.8 Hz), 1.75 (9 H, s, tBu), 0.79 (3 H, s, Me of N2Npy), 0.13 (18 H, s, SiMe3) ppm. 13C{1H} NMR (C7H8, 125.7 MHz, 193 K): 160.5 (C2), 147.5 (C6), 146.2 (TaCH2CHCH2), 137.3 (C4 overlapping solvent), 121.2 (C5), 120.2 (C3), 104.4 (TaCH2CHCH2), 66.3 (CMe3), 63.0 (CH2NSiMe3), 59.9 (TaCH2CHCH2), 44.0 (C(CH2NSiMe3)2), 34.8 (CMe3), 23.3 (Me of N2Npy), 0.7 (SiMe3) ppm. IR (KBr pellet): 4347 (w), 4115 (w), 3897 (w), 3868 (w), 3850 (w), 3836 (w), 3813 (w), 3793 (w), 3742 (w), 3720 (w), 3708 (w), 3687 (w), 3667 (w), 3646 (w), 3626 (w), 3584 (w), 3251 (w), 3099 (w), 3061 (m), 3022 (w), 2962 (s), 2918 (s), 2891 (s), 2821 (s), 2734 (w), 2690 (w), 2667 (w), 2361 (w), 2341 (w), 2105 (w), 2003 (w), 1931 (w), 1869 (w), 1852 (w), 1770 (w), 1600 (s), 1571 (m), 1515 (w), 1476 (s), 1454 (m), 1435 (m), 1416 (w), 1402 (w), 1388 (m), 1363 (w), 1353 (m), 1269 (s), 1246 (s), 1211 (m), 1181 (m), 1160 (w), 1139 (m), 1108 (w), 1087 (m), 1035 (s), 1005 (m), 993 (m), 975 (m), 953 (w), 903 (s), 882 (s), 839 (s), 801 (w), 778 (m), 749 (m), 727 (w), 700 (w), 683 (m), 643 (m), 630 (w), 597 (s), 564 (w), 537 (w), 525 (m), 490 (m), 430 (m), 410 (w) cm-1. Anal. Calcd for C22H43N4Si2Ta: C, 44.0; H, 7.2; N, 9.3. Found: C, 43.5; H, 7.2; N, 9.1. High-resolution EI mass spectrum: m/z found (calcd for [M]+, C22H43N4Si2Ta) ) 600.2510 (600.2506). Reaction of W(NPh)(N2Npy)Me2 (1) with 0.5 Equiv of BArF3 (NMR tube scale). W(NPh)(N2Npy)Me2 (1) (19 mg, 0.03 mmol) was dissolved in CD2Cl2 (0.6 mL), and this solution was added to solid BArF3 (8 mg, 0.02 mmol, 0.5 equiv). The reaction immediately turned brown. The solution was transferred to a 5 mm J. Young NMR tube and analyzed by NMR spectroscopy, which showed the presence of 1, 2a+, and 8. Data for [W(NPh)(N2Npy)(trans-Me)][MeBArF3] (2a+). 1 H NMR (CD2Cl2, 500.0 MHz, 293 K): 8.55 (1 H, d, H6, 3 J(H5H6) ) 5.4 Hz), 8.16 (1 H, apparent td, H,4 apparent 3 J(H3H4H5) ) 7.9 Hz, 4J(H4H6) ) 1.5 Hz), 7.73 (1 H, d, H3, 3 J(H3H4) ) 8.0 Hz), 7.53 (1 H, apparent t, H,5 apparent 3 J(H4H5H6) ) 8.3 Hz), 4.88 (2 H, d, CH2NSiMe3, 2J ) 13.6 Hz), 4.51 (2 H, d, CH2NSiMe3, 2J ) 13.6 Hz), 1.77 (3 H, s, Me of N2Npy), 1.07 (3 H, s, WMe), 0.07 (18H, s, SiMe3) ppm. Resonances for C6H5 were not observed due to overlap with the resonances of 2+. 13C{1H} NMR (CD2Cl2, 125.7 MHz, 293 K): 40.3 ppm (WMe, 1J(WC) ) 120 Hz (deduced from 13Clabeled tungsten-methyl isotopomer)) ppm (other resonances not clearly observed due to overlap and their broad nature). Data for [{W(NPh)(N2Npy)Me}2(µ-Me)][MeBArF3] (8MeBArF3). 1H NMR (CD2Cl2, 500.0 MHz, 293 K): 9.13 (2 H, br s, H6), 8.16 (2 H, td, H,4 3J(H3H4H5) ) 7.8 Hz, 4J(H4H6) ) 1.5 Hz), 7.72 (2 H, d, H3, 3J(H3H4) ) 8.3 Hz), 7.56 (2 H, t, H,5

4460


J(H4H5H6) ) 8.3 Hz), 7.30 (4 H, d, o-C6H5, 3J ) 7.3 Hz), 7.267.20 (6 H, overlapping m, m-C6H5, p-C6H5), 4.05 (4 H, br s, CHH), 3.87 (4 H, br s, CHH), 1.66 (6 H, s, Me of N2Npy), 0.44 (3 H, br s, BMe), 0.13 (36 H, br s, SiMe3) ppm. WMe resonances too broad to be observed at room temperature. 1H NMR (CD2Cl2, 500.0 MHz, 193 K): 9.12 (2 H, br s, H6), 8.19 (2 H, br s, H4), 7.71 (2 H, br s, H3), 7.55 (6 H, br s, H,5 m-C6H5), 7.26 (6 H, br s, o-C6H5, p-C6H5), 4.09 (4 H, br s, CHH), 3.88 (4 H, br s, CHH), 1.64 (6 H, br s, Me of N2Npy), 1.10 (3 H, br s, WMebridging), 0.33 (3 H, br s, BMe), 0.10 (6 H, br s, WMeterminal), 0.08 (36 H, br s, SiMe3) ppm. 13C{1H} NMR (CD2Cl2, 125.7 MHz, 193 K): 161.0 (C2), 154.8 (ipso-C6H5), 147.3 (C6F5, 1J(CF) ) 239 Hz), 147.2 (C6), 144.6 (o-C6H5), 136.7 (C6F5, 1J(CF) ) 251 Hz), 135.6 (C6F5, 1J(CF) ) 251 Hz), 128.9 (m-C6H5), 128.5 (p-C6H5), 127.8 (C4), 124.8 (C5), 123.3 (C3), 64.8 (CH2NSiMe3), 48.1 (C(CH2NSiMe3)2), 38.5 (WMeterminal), 36.5 (WMebridging), 21.0 (Me of N2Npy), 8.5 (BMe), 0.0 (SiMe3) ppm. 19F NMR (CD2Cl2, 282.2 MHz, 183 K): -135.5 (6 F, d, o-C6F5, 3J ) 19.8 Hz), -166.8 (6 F, app. t, p-C6F5, app. 3J ) 19.8 Hz), -169.7 (3 F, t, m-C6F5, 3J ) 22.7 Hz) ppm. 11B{1H} NMR (CD2Cl2, 160.4 MHz, 183 K): -15.0 (br s, MeBArF3) ppm. Elemental analysis was not obtained for this compound, which was insufficiently stable to be isolated. Reaction of [W(NPh)(N2Npy)Me][MeBArF3] (2-MeBArF3) with Cp2ZrMe2 (NMR tube scale). A solution of W(NPh)(N2Npy)Me2 (1) (16 mg, 0.03 mmol) in CD2Cl2 was added to solid BArF3 (13 mg, 0.03 mmol) and agitated for 30 s before adding the solution to solid Cp2ZrMe2 (7 mg, 0.03 mmol). The solution was transferred to a 5 mm J. Young NMR tube. A similar procedure was employed for the reactions of [W(NPh)(N2Npy)(13CH3)]+ with Cp2ZrMe2, [W(NPh)(N2Npy)(CD3)]+ with Cp2ZrMe2, and [W(NPh)(N2Npy)Me]+ with Cp2Zr(CD3)2. The reactions were analyzed by NMR spectroscopy. Reaction of [W(NPh)(N2Npy)Me][MeBArF3] (2-MeBArF3) with ZnMe2 (NMR tube scale). A solution of W(NPh)(N2Npy)Me2 (1) (14 mg, 0.02 mmol) in CD2Cl2 was added to solid BArF3 (12 mg, 0.02 mmol) and agitated for 30 s before adding ZnMe2 (12 µL of a 2.0 M solution in toluene, 0.02 mmol). The solution was transferred to a 5 mm J. Young NMR tube. The reaction was analyzed by 1H NMR spectroscopy. A similar procedure was employed for the reaction of [W(NPh)(N2Npy)(CD3)]+ with ZnMe2. [W{MeC(2-C5H4NAlMe)(CHNSiMe3)(CH2NSiMe3)}(µNPh)Me2][MeB(C6F5)3] (9-MeBArF3). To a stirred solution of W(NPh)(N2Npy)Me2 (1) (250 mg, 0.41 mmol) in CH2Cl2 (20 mL) cooled to 0 °C was added a solution of BArF3 (208 mg, 0.41 mmol) in CH2Cl2 (20 mL). To the stirred reaction was added a solution of trimethylaluminum (29 mg, 39 µL, 0.41 mmol) in CH2Cl2 (5 mL). After stirring at room temperature for 1 h the solvent was removed under reduced pressure to afford 9-MeBArF3 as an analytically pure, golden brown solid. Yield: 307 mg (64%). 1H NMR (CD2Cl2, 500.0 MHz, 293 K): 8.61 (1 H, dd, H6, 3J(H5H6) ) 5.7 Hz, 4J(H4H6) ) 1.5 Hz), 8.39 (1 H, td, H4, 3J(H3H4H5) ) 7.8 Hz, 4J(H4H6) ) 1.5 Hz), 7.86 (1 H, d, H3, 3J(H3H4) ) 8.2 Hz), 7.82 (1 H, ddd, H5, 3J(H5H6) ) 5.7 Hz, 3J(H4H5) ) 7.8 Hz, 4J(H3H5) ) 1.1 Hz), 7.45 (2 H, t, m-C6H5, 3J ) 8.3 Hz), 7.30 (3 H, m, o-C6H5, p-C6H5), 3.21 (1 H, d, CH, 4J ) 1.5 Hz), 2.83 (1 H, d, CHH, 2J ) 13.0 Hz), 2.70 (1 H, dd, CHH, 2J ) 13.0 Hz, 4J ) 1.5 Hz), 1.76 (3 H, s, WMe, 2 J(WH) ) 6.8 Hz (15%)), 1.68 (3 H, s, MeCCH2), 1.29 (3 H, s, WMe, 2J(WH) ) 7.5 Hz (15%)), 0.43 (3 H, br s, BMe), 0.22 (9 H, s, SiMe3), -0.13 (3 H, s, AlMe), -0.36 (9 H, s, SiMe3) ppm. 13 C NMR (CD2Cl2, 125.7 MHz, 293 K): 168.2 (C2), 153.4 (ipsoC6H5), 148.7 (C6F5, 1J(CF) ) 239 Hz), 145.6 (C,6 1J(CH) ) 167 Hz), 145.4 (C,4 1J(CH) ) 168 Hz), 137.8 (C6F5, 1J(CF) ) 251 Hz), 136.7 (C6F5, 1J(CF) ) 251 Hz), 129.1 (m-C6H5, 1J(CH) ) 157 Hz), 128.5 (p-C6H5, 1J(CH) ) 166 Hz), 126.8 (o-C6H5, 1 J(CH) ) 155 Hz), 125.5 (C5, 1J(CH) ) 166 Hz), 123.0 (C3, 1 J(CH) ) 167 Hz), 79.9 (CHNSiMe3, 1J(CH) ) 165 Hz, 1J(WC) ) 47 Hz (15%)), 60.5 (WMe, 1J(CH) ) 122 Hz, 1J(WC) ) 99 Hz (15%)), 57.7 (CH2NSiMe3, 1J(CH) ) 137 Hz), 44.9 (C(CH23

Ward et al. Table 6. X-ray Data Collection and Processing Parameters for [Ta(NtBu)(N2Npy)Me] (6) empirical formula fw temp/K wavelength/Å space group a/Å b/Å c/Å R/deg β/deg γ/deg V/Å3 Z d(calcd)/Mg‚m-3 abs coeff/mm-1 R indices R1, Rw [I>3σ(I)]a a

C20H41N4Si2Ta 574.69 150 0.71073 P1 h 9.5633(2) 10.1910(2) 15.0362(4) 104.2341(9) 100.1951(10) 105.6688(11) 1320.3 2 1.445 4.265 R1 ) 0.0261, Rw ) 0.0287

R1 ) ∑||Fo| - |Fc||/∑|Fo|; Rw ) x{∑w (|Fo| - |Fc|)2/∑(w|Fo|2}.

NSiMe3)), 37.0 (WMe, 1J(CH) ) 127 Hz, 1J(WC) ) 91 Hz (15%)), 22.6 (MeCCH2, 1J(CH) ) 128 Hz), 10.5 (BMe), 0.6 (SiMe3, 1J(CH) ) 119 Hz) ppm. 19F NMR (CD2Cl2, 470.4 MHz, 293 K): -135.0 (6 F, d, o-C6F5, 3J ) 21.0 Hz), -168.3 (6 F, t, p-C6F5, 3J ) 21.0 Hz), -171.0 (3 F, t, m-C6F5, 3J ) 18.1 Hz) ppm. 11B{1H} NMR (CD2Cl2, 160.4 MHz, 293 K): -14.7 (br s, MeBArF3) ppm. IR (KBr pellet): 3066 (w), 2956 (s), 1904 (s), 2852 (m), 2346 (w), 2324 (w), 1942 (w), 1868 (w), 1640 (s), 1614 (s), 1586 (m), 1570 (m), 1514 (s), 1384 (m), 1358 (s), 1214 (w), 1170 (w), 1090 (s), 1030 (s), 898 (s), 784 (w), 472 (w), 456 (w) cm-1. Anal. Calcd for C43H45AlBF15N4Si2W: C, 43.7; H, 3.8; N, 4.8; Cl, 0. Found: C, 43.4; H, 3.8; N, 5.1; Cl, 0. Crystal Structure Determination of Ta(NtBu)(N2Npy)Me (6). Crystal data collection and processing parameters are given in Table 6. Crystals of 6 were grown by slow evaporation of a pentane solution at room temperature. A single crystal was cut to give a fragment having dimensions approximately 0.08 × 0.18 × 0.22 mm. This was mounted on a glass fiber using perfluoropolyether oil and cooled rapidly to 150 K in a stream of cold N2 using an Oxford Cryosystems CRYOSTREAM unit. Diffraction data were measured using an Enraf-Nonius KappaCCD diffractometer. Intensity data were processed using the DENZO-SMN package.90 The structure was solved in the space group P1 h using the direct-methods program SIR92,91 which located all non-hydrogen atoms. Subsequent full-matrix least-squares refinement was carried out using the CRYSTALS program suite.92 Coordinates and anisotropic thermal parameters of all non-hydrogen atoms were refined. Hydrogen atoms were positioned geometrically after each cycle of refinement. A three-term Chebychev polynomial weighting scheme was applied. A full listing of atomic coordinates, bond lengths and angles, and displacement parameters for 3 have been deposited at the Cambridge Crystallographic Data Center. See Notice to Authors, Issue No. 1. Computational Details. All calculations were performed with the Gaussian 98 set of programs93 within the framework of hybrid DFT (B3PW91)94,95 on the model system M(NR){HC(2-C5H4N)(CH2SiH3)2}(X)+q (M ) W, R ) H, Ph, Me, X ) Me, q ) 1; M ) W, R ) Ph, X ) Cl, q ) 1; M ) Ta, R ) Me, X ) Me, Cl, q ) 0). The ONIOM calculations96 on the model system M(NR){MeC(2-C5H4N)(CH2SiMe3)2}(X)+q (M ) W, R ) Ph, t Bu, X ) Me, q ) 1; M ) W, R ) Ph, X ) Cl, q ) 1; M ) Ta, (90) Otwinowski, Z.; Minor, W. Processing of X-ray Diffraction Data Collected in Oscillation Mode, Methods in Enzymology; Academic Press: New York, 1997. (91) Altomare, A.; Cascarano, G.; Giacovazzo, G.; Guagliardi, A.; Burla, M. C.; Polidori, G.; Camalli, M. J. Appl. Crystallogr. 1994, 27, 435. (92) Watkin, D. J.; Prout, C. K.; Carruthers, J. R.; Betteridge, P. W.; Cooper, R. I. CRYSTALS, issue 11; Chemical Crystallography Laboratory: Oxford, UK, 2001.

Cationic Tungsten Methyl Complex R ) tBu, X ) Me, Cl, q ) 0) were performed with the QM part corresponding to M(NR){HC(2-C5H4N)(CH2SiH3)2}(X)+q (M ) W, R ) Ph, Me, X ) Me, q ) 1; M ) W, R ) Ph, X ) Cl, q ) 1; M ) Ta, R ) Me, X ) Me, Cl, q ) 0) treated at the B3PW91 level and the missing Me groups treated at the UFF level.97 The tungsten and tantalum atoms were represented by the relativistic effective core potential (RECP) from the Stuttgart group and the associated (8s7p5d)/[6s5p3d] basis set,98 augmented by an f polarization function (R ) 0.823, W; R ) 0.790, Ta)99 The chlorine and zinc atoms were represented by RECP from the Stuttgart group and the associated basis set,100 augmented by a d polarization function.101 A 6-31G(d,p) basis set102 was used for all the nitrogen atoms, the methyl (93) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. J. A.; Stratmann, R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels, A. D.; Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone, V.; Cossi, M.; Cammi, R.; Mennucci, B.; Pomelli, C.; Adamo, C.; Clifford, S.; Ochterski, J.; Petersson, G. A.; Ayala, P. Y.; Cui, Q.; Morokuma, K.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Cioslowski, J.; Ortiz, J. V.; Baboul, A. G.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Gomperts, R.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Gonzalez, C.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Andres, J. L.; Head-Gordon, M.; Replogle, E. S.; Pople, J. A. Gaussian 98, Revision A.11; Gaussian, Inc.: Pittsburgh, PA, 1998. (94) Becke, A. D. J. Chem. Phys. 1993, 98, 5648. (95) Perdew, J. P.; Wang, Y. Phys. Rev. B 1992, 82, 284. (96) Svensson, M.; Humbel, S.; Froese, R. D. J.; Matsubara, T.; Sieber, S.; Morokuma, K. J. J. Phys. Chem. 1996, 100, 19357. (97) Rappe´, A. K.; Casewitt, C. J.; Colwell, K. S.; Goddard, W. A.; Skiff, W. M. J. Am. Chem. Soc. 1992, 114, 10024. (98) Andrae, D.; Ha¨ussermann, U.; Dolg, M.; Stoll, H.; Preuss, H. Theor. Chim. Acta 1990, 77, 123. (99) Ehlers, A. W.; Bo¨hme, M.; Dapprich, S.; Gobbi, A.; Ho¨llwarth, A.; Jonas, V.; Ko¨hler, K. F.; Stegmann, R.; Veldkamp, A.; Frenking, G. Chem. Phys. Lett. 1993, 208, 111.

Organometallics, Vol. 23, No. 19, 2004 4461 group bonded to W, and the methyl groups on ZnMe2. The remaining atoms were represented by a 6-31G basis set. Full optimizations of geometry without any symmetry constraint were performed, followed by analytical computation of the Hessian matrix to confirm the nature of the located extrema as minima on the potential energy surface. The NBO and NPA analyses were performed using the method developed by Weinhold and co-workers.103

Acknowledgment. We thank the EPSRC, Leverhulme Trust and Royal Society (P.M.), British Council (P.M. and L.H.G.), the CNRS (L.H.G. and E.C.), and the Institut Universitaire de France (L.H.G.) for support. E.C. and P.M. thank the Royal Society of Chemistry for financial support (International Authors Travelling Grant). We thank Prof. J. D. Protasiewicz for helpful discussions. Supporting Information Available: X-ray crystallographic files in CIF format for the structure determination of [Ta(NtBu)(N2Npy)Me] (6); ONIOM(B3PW91:UFF) geometry of 2+ (Figure S1); HOMO of 1 (Figure S2); geometries, energies, and representation of the MO for the complexes Wcis90 (Figure S3) andWtrans00 (Figure S4). This material is available free of charge via the Internet at http://pubs.acs.org. OM049701L (100) Bergner, A.; Dolg, M.; Ku¨chle, W.; Stoll, H.; Preuss, H. Mol. Phys. 1993, 30, 1431. (101) Ho¨llwarth, A.; Bo¨hme, H.; Dapprich, S.; Ehlers, A. W.; Gobbi, A.; Jonas, V.; Ko¨hler, K. F.; Stagmann, R.; Frenking, G. Chem. Phys. Lett. 1993, 203, 237. (102) Hariharan, P. C.; Pople, J. A. Theor. Chim. Acta 1973, 28, 213. (103) Reed, A. E.; Curtiss, L. A.; Weinhold, F. Chem. Rev. 1988, 88, 899.

Synthesis, Reactivity, and Computational Studies of the Cationic

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