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 BiI 3 resulted in the formation of [BiI 6 ] 3– . Ligand-to-metal charge transfer (LMCT) excitation of [BiI 6 ] 3– yielded a transient species assigned as the diiodide anion I 2 •– directly ligated to Bi, [Bi(I 2 •– )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 η 2 ligated I 2 •–, [(η 2 -I 2 )BiI 4 ] 3– (λ max = 640 nm), and an η 1 species [(η 1 -I 2 )BiI 4 ] 3– (λ max = 750 nm). The rapid appearance of this intermediate was attributed to intramolecular I–I bond formation. The [(η 2 -I 2 )BiI 4 ] 3– subsequently reacted with 1 equiv of iodide to yield [(η 1 -I 2 )BiI 5 ] 4– . Interestingly, [(η 1 -I 2 )BiI 5 ] 4– decayed to ground state products with a first-order rate constant of k = 2 × 10 3 s –1 . Under the same experimental conditions, I 2 •– in CH 3 CN rapidly disproportionates with a tremendous loss of free energy, Δ G o = −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.