Synthesis and Structure of the Tantalum Trimethyl Complex [P2N2]TaMe3 and Its Conversion to the Tantalum Methylidene Species [P2N2]TaCH2(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(CH 2 SiMe 2 NSiMe 2 CH 2 ) 2 PPh, abbreviated [P 2 N 2 ], as an ancillary ligand. The methylidene precursor [P 2 N 2 ]TaMe 3 is generated by the reaction of [P 2 N 2 ]Li 2 ·C 4 H 8 O 2 (C 4 H 8 O 2 = 1,4-dioxane) with TaMe 3 Cl 2 . The solid-state X-ray molecular structure demonstrates that the geometry of [P 2 N 2 ]TaMe 3 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 [P 2 N 2 ]Ta CH 2 (Me), with the elimination of methane. The X-ray solid-state molecular structure of [P 2 N 2 ]Ta CH 2 (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 (η 5 -C 5 H 5 ) 2 Ta CH 2 (Me). Reaction of [P 2 N 2 ]TaMe 3 with Ph 3 C + BF 4 - or PhNMe 2 H + B(C 6 F 5 ) 4 - produced {[P 2 N 2 ]TaMe 2 } + BF 4 - and {[P 2 N 2 ]TaMe 2 } + B(C 6 F 5 ) 4 -, respectively. Attempts to deprotonate {[P 2 N 2 ]TaMe 2 } + did not provide a chemical route to [P 2 N 2 ]Ta CH 2 (Me) and instead generated either [P 2 N 2 ]TaMe 2 F or the product of deprotonation of the [P 2 N 2 ] ligand methylene backbone, Ta(Me 2 )[(CHSiMe 2 NSiMe 2 CH 2 P(Ph)CH 2 SiMe 2 NSiMe 2 CH 2 PPh)], depending on the base and counteranion used.