UV-Vis-IR Thin-Layer Spectroelectrochemical Studies of Hexakis(aryl isocyanide)chromium Complexes: In Situ Generation and Characterization of Four Oxidation States

Spectroelectrochemistry in the UV-vis and IR spectral regions has been used to characterize the Cr(0), Cr(I), Cr(II), and Cr(III) analogues of a series of chromium hexakis(aryl isocyanide) complexes. Controlled-potential bulk electrolyses in an improved thin-layer spectroelectrochemical cell were monitored with UV-vis and FT-IR spectrometers in separate experiments. All electrolyses proceeded isosbestically and were chemically reversible. IR data show that each sequential oxidation results in higher vcnstretching frequencies and lower absolute integrated intensities for the v_CN absorption. This is related to the decrease in dπ-π*_CN back-bonding in the more oxidized forms. Rough correlations between the electron-releasing ability of the aryl substituent and both the v_CN stretching frequency and the absolute integrated intensities of the Cr(III) complexes are observed. This is attributed to varying degrees of σ cn_R → Cr donation, which become apparent only when the degree of dπ-π*_CN back-bonding is small. UV-vis experiments show that for all compounds studied the ligand-to-metal charge-transfer (LMCT) band energy decreases upon successive oxidation, while the energy of the MLCT band increases. This is in accord with the increased oxidation strength of the central chromium ion. A correlation can also be made between the electron-releasing abilities of the aryl substituents and the energies of the LMCT electronic transitions in the Cr(II) and the Cr(III) complexes. This reflects the greater σ_CNR→ Cr donation of aryl isocyanides with electron-donating substituents and is consistent with the trend in the absolute integrated intensities of Cr(III) species observed in IR experiments.