THE LIVELY CHEMISTRY OF THE DI-MU-METHYLENE-BIS(PENTAMETHYLCYCLOPENTADIENYL-RHODIUM) COMPLEXES, ANALOGS OF SURFACE-REACTIONS IN HETEROGENEOUS CATALYSIS

The chemistry of the di-mu-methylene-bis(pentamethylcyclopentadienly-rhodium) complexes is reviewed. The complex [((eta(5)-C(5)Me(5))RhCl2)(2)] (1a) reacted with MeLi to give, after oxidative work-up, blood-red cis-[((eta(5)-C(5)Me(5))Rh(mu-CH2))(2)(Me)(2)](2) 2. This has the two rhodiums in the +4 oxidation state (d(5)), and linked by a metal-metal bond (2.620 Angstrom). Trans-2 was formed on isomerisation of cis-2 in the presence of Lewis acids, or by direct reaction of 1a with Al(2)Me(6), followed by dehydrogenation with acetone. The Ph-methyls in [((eta(5)-C(5)Me(5))Rh(mu-CH2))(2)(Me)(2)] were readily replaced under acidic conditions (HX) to give [((eta(5)-C(5)Me(5))Rh(mu-CH2))(2)(R)(2)] (X = Cl, Br or I); these latter complexes reacted with a variety of RMgX to give [((eta(5)-C(5)Me(5))Rh(mu-CH2))(2)(R)(2)] (R = alkyl, Ph, vinyl, etc.). Trans-2 also reacted with HBF4 in the presence of L to give first [((eta(5)-C(5)Me(5))Rh(mu-CH2))(2)(Me)(L)](+) and then [((eta(5)-C(5)Me(5))Rh(mu-CH2))(2)(L)(2)](2+) (L = MeCN, CO, etc.). The ((eta(5)-C(5)Me(5))Rh(mu-CH2))(2), core is rather kinetically inert and also forms a variety of complexes with oxy-ligands, both cis-, e.g. [((eta(5)-C(5)Me(5))Rh(mu-CH2))(2)(mu-OAC)](+) and trans-, such as [((eta(5)-C(5)Me(5))Rh( mu-CH2))(2)(H2O)(2)](2+). The complexes [((eta(5)-C(5)Me(5))Rh(mu-CH2))(2)(R)L](+) (R = Me or aryl) in the presence of CO, or [((eta(5)-C(5)Me(5))Rh(mu-CH2))(2)(R)(2)] (R = Me, Ph or CO(2)Me) in the presence of mild oxidants, readily yield the C-C-C coupled products RCH=CH2. The mechanisms of these couplings have been elucidated by detailed labelling studies: they are more complex than expected, but allow direct analogies to be drawn to C-C couplings that occur during Fischer-Tropsch reactions on rhodium surfaces.