C−C Activation in Biphenylene. Synthesis, Structure, and Reactivity of (C5Me5)2M2(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)2M2(2,2‘-biphenyl). The isomorphous solid state X-ray structures of these complexes showed the biphenyl substrate to be σ-coordinated to one metal and η4-bound to the other. These complexes display dynamic behavior, interchanging the σ- and π-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 ΔG ⧧ (350 K) = 16.8 kcal/mol and ΔG ⧧ (248 K) = 11.4 kcal/mol for the rhodium and cobalt complexes, respectively. (C5Me5)2Rh2(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)2Co2(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.