Correlations Between Structure, Magnetism, and Polymerization Activity of Paramagnetic Benzylchromium Complexes

Reaction of [{Cp*Cr( μ ‐Cl)} 2 ] with benzyllithium yielded [{Cp*Cr‐( μ ‐Bz)} 2 ] ( 1 ), a Cr II alkyl complex with bridging benzyl groups and a CrCr bond, which rearranged at room temperature to [Cp*Cr( η 1 ‐Bz)( μ ‐ η 3 : η 6 ‐Bz)CrCp*] ( 2 ), a mixed valent (Cr I /Cr III ) complex with a benzyl ligand coordinating as an η 6 ‐arene ligand to one chromium and as an η 3 ‐allyl ligand to the other one. Upon heating, 2 reductively eliminated bibenzyl and yielded [(Cp*Cr) 2 ( μ ‐ η 6 : η 6 ‐Bz‐Bz)] ( 3 ), in which two 17‐electron sandwich complexes are linked by an ethano bridge. The crystal structures of 2 and 3 have been determined. Magnetic susceptibility and saturation magnetization measurements showed 2 to be ferromagnetically coupled (ground state S = 2, J = + 23(1) cm −1 ). Of the three isomers, only 2 catalyzed the polymerization of ethylene, presumably because it contains a Cr III alkyl.