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)(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