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 (C 5 Me 5 )M(C 2 H 4 ) 2 (M = Rh, Co) to give the bimetallic species (C 5 Me 5 ) 2 M 2 (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 η 4 -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 (C 5 Me 5 ) fragments was calculated to be Δ G ⧧ (350 K) = 16.8 kcal/mol and Δ G ⧧ (248 K) = 11.4 kcal/mol for the rhodium and cobalt complexes, respectively. (C 5 Me 5 ) 2 Rh 2 (2,2‘-biphenyl) reacted with CO to give (C 5 Me 5 )Rh(CO) 2 and (C 5 Me 5 )Rh(CO)(2,2‘-biphenyl), while the cobalt analog gave (C 5 Me 5 )Co(CO) 2 and fluorenone. (C 5 Me 5 ) 2 Co 2 (2,2‘-biphenyl) gave (C 5 Me 5 )Co(PMe 3 ) 2 and (C 5 Me 5 )Co(PMe 3 )(2,2‘-biphenyl) when reacted with PMe 3, as determined by X-ray crystallography of the latter complex. Both complexes were found to be resistant to hydrogenation.