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

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Abstract
Reaction of [{Cp*Cr(μ‐Cl)}2] with benzyllithium yielded [{Cp*Cr‐(μ‐Bz)}2] (1), a CrII alkyl complex with bridging benzyl groups and a CrCr bond, which rearranged at room temperature to [Cp*Cr(η1‐Bz)(μ‐η3:η6‐Bz)CrCp*] (2), a mixed valent (CrI/CrIII) complex with a benzyl ligand coordinating as an η6‐arene ligand to one chromium and as an η3‐allyl ligand to the other one. Upon heating, 2 reductively eliminated bibenzyl and yielded [(Cp*Cr)2(μ‐η6:η6‐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 CrIII alkyl.

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