One-Electron Oxidation of ReCp(CO)2L (L = PPh3, η2-2-Butene, η-Diphenylacetylene): Electrochemical, Spectroscopic, and Computational Studies of the Electronic Properties and Dimerization Tendencies of 17-Electron Rhenium Complexes

Abstract

Electrochemical oxidation of ReCp(CO)2L (Cp = η5-C5H5; L = PPh3 (2), η2-2-C2Me2H2 (3), η-C2Ph2 (4)) has been studied in CH2Cl2/[NBu4][B(C6F5)4]. All three complexes undergo quasi-Nernstian one-electron oxidations with E1/2 values (vs ferrocene) of 0.49 V (2/2+), 0.45 V (3/3+), and 0.15 V (4/4+). A second reversible one-electron oxidation is observed for 4 at 0.97 V. The radical cation 2+ efficiently forms the Re–Re-bonded dimer dication 222+, which was isolated after chemical oxidation of 2 by [thianthrene][B(C6F5)4]. The 17-electron complex 3+ shows no tendency to dimerize. On the basis of the E1/2 value of 3 and expected potential shifts when replacing a methyl group by a hydrogen, a ligand electronic parameter, EL, of 0.38 was calculated for π-bonded ethylene, which is much lower than the existing literature value of 0.76. IR, ESR, and UV–vis spectra were recorded for 4+. DFT calculations were performed on the neutral complexes, the monocations 2+, 3+, and 4+, and the dimer dication 222+. The SOMO of 2+ is similar to that of the parent piano-stool complex [ReCp(CO)3]+ (1+) in being metal-based (55%) and directionally disposed to formation of a metal–metal bond with another radical cation. A Re–Re bond distance of 3.315 Å was calculated for the resulting dimer 222+. Although the SOMO of 3+ is also predominantly (60%) metal-based, the orbital lacks the geometry necessary to form a metal–metal-bonded dimer. The HOMO of 4 is highly delocalized, having only about one-third metal character and two-thirds coming from the Cp and diphenylactylene ligands. One-electron oxidation of 4 brings about a significant change in electronic structure, with the SOMO of 4+ seeing increased contributions from diphenylacetylene and the two carbonyls (14%), along with a decreased metal contribution (27%). The diphenylacetylene contributions are large enough to justify π-C2Ph2 being designated as a “non-innocent” ligand in this system.