Synthesis and reactivity of early-late heterobimetallic complexes displaying a η2-tantalum-alkylidene to palladium interaction

Tantalaalkylidene compounds, CHRTaCl3L2 (R=tBu or CMe2Ph, L=THF or 1/2dimethoxyethane) mixed with the cyclopalladated dimer [Pd(2-C6H4CH2NMe2)(μ-Cl)]2, 1, afford good yields of heterodimetallic complexes [Pd(2-C6H4CH2NMe2)(μ-Cl)(μ-CHR=TaCl3L], 3a, 3b, in which the TaC unit is η2-interacting with the palladium atom, while a chloride ligand is bridging the tantalum and the palladium atoms. These compounds are fairly stable in air in the solid state and also in solution at RT. The interaction of the TaC unit with Pd in these bimetallic compounds is weak as shown by the ready formation of [Pd(2-C6H4CH2NMe2)PyCl] and CHRTaCl3Py2 upon treatement with pyridine. Compounds analogous to 3a, b can also be obtained with 12 electrons tantalum complexes. Thus treating the same cyclopalladated dimer 1 with CHRTa(OAr)3 (OAr=2,6-diisopropylphenyloxy) led to a much more stable though electron deficient species: [Pd(2-C6H4CH2NMe2)(μ-Cl)(μ-CHtBu=Ta(OAr)3], 3c. Substitution in 3a of one chloride ion by an alkyl group occurred at the tantalum metal via reaction with ZnR2 (R=CH2CMe2Ph) leading to [Pd(2-C6H4CH2NMe2)(μ-Cl)(μ-CHtBu=TaCl3(CH2CMe2Ph)], 4 for which there is no free rotation around the new TaC bond and in which one of the methylene protons is strongly interacting with the palladium centre. This compound is believed to mimic an intermediate to the formation of tantalacarbyne derivative, which was obtained earlier via reaction of the uncomplexed tantalacarbene compound with dialkylzinc compounds.