Mixed-Metal Hydrocarbyl Complexes Involving the Rh/Mo and Rh/W Metal Combinations

Reaction of [RhMo(H)(CO) 4 (dppm) 2 ] (dppm = Ph 2 PCH 2 PPh 2 ) with a number of alkynes, RC⋮CR‘ (R = R‘ = C(O)OCH 3, CF 3; R = H, R‘ = C(O)CH 3, C(O)OCH 3 ) yields the alkenyl complexes [RhMo(C(R‘) C(H)R)(CO) 4 (dppm) 2 ], which in all cases have an electron-withdrawing substituent on the α-carbon. A series of analogous complexes [RhMo(R)(CO) 4 (dppm) 2 ] (R = CHCH 2, CH 3, CH 2 CHCH 2, CH 2 C 6 H 5, C 6 H 5 ) are prepared by the reaction of [RhMoCl(CO) 4 (dppm) 2 ] and the appropriate Grignard reagent. The analogous Rh/W complex (R = CHCH 2 ) was prepared from [RhWCl(CO) 4 (dppm) 2 ] and vinylmagnesium chloride. In all cases the hydrocarbyl fragment is η 1 -bound to Rh, with the four carbonyl ligands primarily bound to the group 6 metal; two carbonyls assume a semibridging arrangement. The reaction of both vinyl complexes (Rh/Mo and Rh/W) with HBF 4 at −80 °C yields the respective ethylidene products [RhM( C(H)CH 3 )(CO) 4 (dppm) 2 ][BF 4 ] by protonation at the β-carbon of the vinyl group. Upon warming, these carbenes transform to the ethylene adducts [RhM(C 2 H 4 )(CO) 4 (dppm) 2 ][BF 4 ]. Owing to uncertainties concerning the structure of the halide precursor [RhMoCl(CO) 4 (dppm) 2 ], its structure was determined by X-ray techniques and the chloro ligand was found to bridge the metals giving a square planar coordination at Rh and an octahedral coordination at Mo. One carbonyl on Mo assumes a weak semibridging interaction with Rh.