Synthesis, reactivity, and computational studies of the cationic tungsten methyl complex [W(NPh)(N2Npy)Me]+ and related compounds (N2Npy = MeC(2-C5H4N)(CH2NSiMe3)2)

Reaction of the dimethyl complex W(NPh)(N(2)Npy)Me-2 (1) (N2Npy = MeC(2-C5H4N)(CH2NSiMe3)(2)) with either BAr3F or [Ph3C] [BAr4F] (Ar-F = C6F5) gave quantitative conversion to the monomethyl cation [W(NPh)(N2Npy)Me](+) (2+). In contrast, reaction of 1 with [PhMe2NH] [BAr4F] gave [W(NPh)(HN2Npy)Me-2] [BAr4F] (3-BAr4F) 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((NBu)-Bu-t)(N2Npy)R (R = Me (6) or eta(1)-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 BAr3F gave the fluxional Me-bridged cation [{W(NPh)(N2Npy)Me}(2)(mu-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-(MeW)-W-... interaction. Reaction of 2(+) with AlMe3 formed [W{MeC(2-C5H4NAlMe)(CHNSiMe3)(CH2NSiMe3)}(mu-NPh)Me-2](+) (9(+)) and CH4 by deprotonation of a CH2 linkage of N(2)Npy. 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 g-donation from the imido and the amido nitrogen atoms. Inclusion of the steric bulk of the SiMe3 groups and the R group (Ph or Bu-t) 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 pi-donation, which in turn destabilizes the metal-imido ligand pi-bonding.