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 com plexes showed the biphenyl substrate to be sigma-coordinated to one me tal and eta(4)-bound to the other. These complexes display dynamic beh avior, interchanging the sigma- and pi-bonding. Variable-temperature N MR studies were performed on both binuclear species, and the energy ba rrier for interconversion of the two (C5Me5) fragments was calculated to be Delta G(double dagger) (350 K) = 16.8 kcal/mol and Delta G(doubl e 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 an alog gave (C5Me5)Co(CO)(2) and fluorenone. (C5Me5)(2)Co-2(2,2'-bipheny l) gave (C5Me5)Co(PMe3)(2) and (C5Me5)Co(PMe3)(2,2'-biphenyl) when rea cted with PMe3, as determined by X-ray crystallography of the latter c omplex. Both complexes were found to be resistant to hydrogenation.