Synthesis and structure of the tantalum trimethyl complex [P2N2]TaMe3 and its conversion to the tantalum methylidene species [P2N2]Ta=CH2(Me) ([P2N2] = PhP(CH2SiMe2NSiMe2CH2)2PPh)

A tantalum(V) trimethyl complex and its photolysis product, a relatively stable tantalum(V) methyl methylidene, were prepared using the macrocyclic bis(amido-phosphine) PhP(CH2SiMe2NSiMe2CH2)(2)PPh, abbreviated [P2N2], as an ancillary ligand. The methylidene precursor [P2N2]TaMe3 is generated by the reaction of [P2N2]Li-2. C4H8O2 (C4H8O2 = 1,4-dioxane) with TaMe3Cl2. The solid-state X-ray molecular structure demonstrates that the geometry of [P2N2]TaMe3 is a seven-coordinate capped trigonal prism, and NMR studies confirm this is also true in solution at low temperature. Photolysis with a UV source generates [P2N2]Ta=CH2(Me), with the elimination of methane. The X-ray solid-state molecular structure of [P2N2]Ta=CH2(Me) is intermediate between octahedral and trigonal-prismatic geometry. Variable-temperature NMR studies demonstrate that the double-bonded methylidene moiety rotates with a barrier of 33.5 +/- 0.6 kJ/mol, in contrast to the larger barrier estimated for the previously characterized 18-electron complex (eta(5)-C5H5)(2)Ta=CH2(Me). Reaction of [P2N2]TaMe3 with Ph3C+BF4- or PhNMe2H+B(C6F5)(4)(-) produced {[P2N2]TaMe2}+BF4- and {[P2N2]TaMe2}B+(C6F5)(4)(-), respectively. Attempts to deprotonate {[P2N2]TaMe2}(+) did not provide a chemical route to [P2N2]Ta=CH2(Me) and instead generated either [P2N2]TaMe2F or the product of deprotonation of the [P2N2] ligand methylene backbone, Ta(Me-2)[(CHSiMe2NSiMe2CH2P(Ph)CH2SiMe2NSiMe2CH2PPh)], depending on the base and counteranion used.