Light Excitation of a Bismuth Iodide Complex Initiates I-I Bond Formation Reactions of Relevance to Solar Energy Conversion

The titration of iodide into acetonitrile solutions of BiI3 resulted in the formation of [BiI6](3-). Ligand-to-metal charge transfer (LMCT) excitation of [BiI6](3-) yielded a transient species assigned as the diiodide anion I-2(center dot-) directly ligated to Bi, [Bi(I-2(center dot-))I-x](n). With 20 ns time resolution, transient absorption measurements revealed the appearance of two species assigned on the analysis of the iodine molecular orbitals as an eta(2) ligated I-2(center dot-), [(eta I-2-(2))BiI4](3-) (lambda(max) = 640 nm), and an eta(1) species [(eta(1)-I-2)BiI4](3-) (lambda(max) = 750 nm). The rapid appearance of this intermediate was attributed to intramolecular I-I bond formation. The [(eta(2)-I-2)BiI4](3-) subsequently reacted with 1 equiv of iodide to yield [(eta(1)-I-2)BiI5](4-). Interestingly, [(eta(1)-I-2)BiI5](4-) decayed to ground state products with a first-order rate constant of k = 2 x 10(3) s(-1) Under the same experimental conditions, I-2(center dot-) in CH3CN rapidly disproportionates with a tremendous loss of free energy, Delta G degrees = -2.6 eV. The finding that metal ligation inhibits this energy wasting reaction is of direct relevance to solar energy conversion. The photochemistry itself provides a rare example of one electron oxidized halide species coordinated to a metal ion of possible relevance to reductive elimination/oxidation addition reaction chemistry of transition metal catalysts.