Molybdenum alkylidene complexes with linked cycloheptatrienyl–phosphane ligands for potential use in olefin metathesis

The synthesis of cycloheptatrienyl molybdenum complexes incorporating the linked cycloheptatrienyl–phosphane ligand [2-(diisopropylphosphanyl)phenyl]cycloheptatrienyl, o-iPr2P-C6H4-C7H6, is described. Reaction of the 17-electron dibromide [(o-iPr2PC6H4-η7-C7H6)MoBr2(P-Mo)] (1) with NaBH4 furnishes a tetrahydroborate complex, which can be directly converted into the acetate complex [(o-iPr2PC6H4-η7-C7H6)Mo(η2-O2CCH3)(P-Mo)] (2). The acetate ligand can be cleaved upon treatment with diazoalkanes, N2CHR, in the presence of Me3SiCl affording the molybdenum alkylidenes [(o-iPr2PC6H4-η7-C7H6)MoCl(CHR)(P-Mo)] (4a, R=Ph; 4b, R=SiMe3). Complexes 4 might serve as a source for the preparation of potential olefin metathesis catalysts of the type [(o-iPr2PC6H4-η7-C7H6)Mo(CHR)(P-Mo)]+ by halide abstraction. CH2 insertion into the metal–carbon bond of the diphenylacetylene complex 5 resulted in the formation of the cationic η3-vinylcarbene complex 6, which can be regarded as an alkene adduct of a 16-electron complex of just that kind. In addition, the X-ray crystal structures of 2 and 6 are reported.