Synthesis, Reactivity, and Computational Studies of the
Cationic Tungsten Methyl Complex [W(NPh)(N2Npy)Me]+
and Related Compounds (N2Npy =
MeC(2-C5H4N)(CH2NSiMe3)2)
posted on 2004-09-13, 00:00authored byBenjamin D. Ward, Gavin Orde, Eric Clot, Andrew R. Cowley, Lutz H. Gade, Philip Mountford
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.