Photoinduced One-Electron Reduction of Alkyl Halides by Dirhodium(II,II) Tetraformamidinates and a Related Complex with Visible Light

Various substituted dirhodium tetraformamidinate complexes, Rh2(R-form)4 (R = p -CF3, p -Cl, p -OCH3, m -OCH3; form = N,N‘-diphenylformamidinate), and the new complex Rh2(tpgu)4 (tpgu = 1,2,3-triphenylguanidinate) have been investigated as potential agents for the photoremediation of saturated halogenated aliphatic compounds, RX (R = alkyl group). The synthesis and characterization of the complexes is reported, and the crystal structure of Rh2(tpgu)4 is presented. The lowest energy transition of the complexes is observed at ∼870 nm and the complexes react with alkyl chlorides and alkyl bromides under low energy irradiation (λirr ≥ 795 nm), but not when kept in the dark. The metal-containing product of the photochemical reaction with RX (X = Cl, Br) is the corresponding mixed-valent Rh2(II,III)X (X = Cl, Br) complex, and the crystal structure of Rh2(p -OCH3-form)4Cl generated photochemically from the reaction of the corresponding Rh2(II,II) complex in CHCl3 is presented. In addition, the product resulting from the dimerization of the alkyl fragment, R2, is also formed during the reaction of each dirhodium complex with RX. A comparison of the dependence of the relative reaction rates on the reduction potentials of the alkyl halides and their C−X bond dissociation energies are consistent with an outer-sphere mechanism. In addition, the relative reaction rates of the metal complexes with CCl4 decrease with the oxidation potential of the dirhodium compounds. The mechanism of the observed reactivity is discussed and compared to related systems.