Spectroscopy and photoredox chemistry of ion-paired cobalt(III) am(m)ine complexes

Abstract

The cationic cobalt(III) am(m)ine complexes, Co(NH 3) 6 3+, Co(en) 3 3+ (en is ethylenediamine) and Co(NH 3) 5Br 2+, form ion pairs with the tetraphenylborate anion in methanol and 1:9 methanol/dichloromethane solutions. These ion pairs display a tetraphenylborate-to-metal charge transfer absorption band at ∼ 320 nm that tails into the visible wavelength region. Irradiation into this band causes highly efficient redox decomposition of the ion pair owing to the rapid thermal reactions of the primary photoproducts: the corresponding cobalt(II) complex and the tetraphenylboranyl radical. The photoredox quantum yield depends upon complex concentration, excitation wavelength, solvent polarity and concentration of dissolved oxygen. Incorporation of cobalt(III) am(m)ine tetraphenylborate complexes into thin films of an epoxide-functionalized polymer affords photosensitive materials that undergo crosslinking upon irradiation and subsequent heating.