C-C activation in biphenylene. Synthesis, structure, and reactivity of (C5Me5)(2)M-2(2,2'-biphenyl) (M=Rh, Co)

Carbon-carbon bond cleavage of biphenylene was achieved with (C5Me5)M(C2H4)(2) (M = Rh, Co) to give the bimetallic species (C5Me5)(2)M-2(2,2'-biphenyl). The isomorphous solid state X-ray structures of these complexes showed the biphenyl substrate to be sigma-coordinated to one metal and eta(4)-bound to the other. These complexes display dynamic behavior, interchanging the sigma- and pi-bonding. Variable-temperature NMR studies were performed on both binuclear species, and the energy barrier for interconversion of the two (C5Me5) fragments was calculated to be Delta G(double dagger) (350 K) = 16.8 kcal/mol and Delta G(double dagger) (248 K) = 11.4 kcal/mol for the rhodium and cobalt complexes, respectively. (C5Me5)(2)Rh-2(2,2'-biphenyl) reacted with CO to give (C5Me5)Rh(CO)(2) and (C5Me5)Rh(CO)(2,2'-biphenyl), while the cobalt analog gave (C5Me5)Co(CO)(2) and fluorenone. (C5Me5)(2)Co-2(2,2'-biphenyl) gave (C5Me5)Co(PMe3)(2) and (C5Me5)Co(PMe3)(2,2'-biphenyl) when reacted with PMe3, as determined by X-ray crystallography of the latter complex. Both complexes were found to be resistant to hydrogenation.