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 I2•– directly ligated to Bi, [Bi(I2•–)Ix]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 η2 ligated I2•–, [(η2-I2)BiI4]3– (λmax = 640 nm), and an η1 species [(η1-I2)BiI4]3– (λmax = 750 nm). The rapid appearance of this intermediate was attributed to intramolecular I–I bond formation. The [(η2-I2)BiI4]3– subsequently reacted with 1 equiv of iodide to yield [(η1-I2)BiI5]4–. Interestingly, [(η1-I2)BiI5]4– decayed to ground state products with a first-order rate constant of k = 2 × 103 s–1. Under the same experimental conditions, I2•– in CH3CN rapidly disproportionates with a tremendous loss of free energy, ΔGo = −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.