5 years ago

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

Light Excitation of a Bismuth Iodide Complex Initiates I–I Bond Formation Reactions of Relevance to Solar Energy Conversion
Ke Hu, Gerald J. Meyer, Andrew B. Maurer
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–max = 640 nm), and an η1 species [(η1-I2)BiI4]3–max = 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.

Publisher URL: http://dx.doi.org/10.1021/jacs.7b01793

DOI: 10.1021/jacs.7b01793

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