Photochemical generation of cationic lewis acids via deligation of metal-sandwich complexes

Photoinduced loss of one or both ligands from bis(arene) metal complexes provides an attractive route to cationic Lewis acids that can function as initiators for polymerization and crosslinking reactions. In this article, we examine the spectroscopy, photochemistry and photoinitiation activity of two such complexes: Fe(mes)₂²⁺ (mes is mesitylene) and Cr(tol)₂⁺ (tol is toluene). Acetonitrile solutions of Fe(mes)₂²⁺ undergo photochemical loss of both mesitylene rings to yield solvated Fe²⁺. The quantum efficiency of deligation ranges from 0.6 to 1.0 depending upon excitation wavelength, but is independent of complex concentration, solvent, oxygen concentration and excess counter-ion. Reaction appears to occur from both ligand field and ligand-to-metal charge transfer excited states. Low thermal stability, however, limits the use of Fe(mes)₂²⁺ as a practical photoinitiator. Deligation of both arene rings also occurs upon 366 nm irradiation of Cr(tol)₂⁺ in acetonitrile. The quantum efficiency for this process is 0.04 and is independent of oxygen concentration. Photoinitiated crosslinking and insolubilization occur in a thin film of COP (copolymer of glycidyl methacrylate and ethyl acrylate) containing Cr(tol)₂⁺.