Electrochemical oxidation of [Cr(CO) 4(tmp)] to the low-spin Cr(I) species [Cr(CO) 4(tmp)] + (tmp=3,4,7,8-tetramethyl-1,10-phenanthroline): an IR, UV–Vis, and EPR spectroelectrochemical and DFT computational study of the accompanying changes in molecular and electronic structure

Abstract

The complexes [Cr(CO) 4(phen)] and [Cr(CO) 4(tmp)] (phen=1,10-phenanthroline, tmp=3,4,7,8-Me 4-phen) are electrochemically oxidized at −0.01 and −0.05 V versus Fc/Fc + to the corresponding cations. The oxidation is electrochemically reversible and chemically nearly reversible at room temperature and a scan rate of 1000 mV s −1. The cation [Cr(CO) 4(tmp)] + is chemically stable at 193 K but undergoes a complete loss of CO ligands at higher temperatures. It was characterized by EPR, IR, and UV–Vis spectroelectrochemistry at 193 K and its structure and bonding have been investigated by density functional theory (DFT) calculations. It follows that [Cr(CO) 4(tmp)] + can be described as a Cr(I) complex with a low-spin d 5 electron configuration. The EPR spectrum shows a g-anisotropy ( g ‖– g ⊥) of 0.1, which is characteristic of a metal-centered radical. Oxidation is accompanied by a large shift of electron density toward the Cr atom, manifested by nearly a 100 cm −1 shift of the ν(CO) IR bands to higher frequencies on oxidation. Moreover, the IR spectrum of [Cr(CO) 4(tmp)] + suggests that the stretching force constants of the axial and equatorial CO ligands become nearly equal upon oxidation and their interaction strengthens. DFT calculations have revealed that [Cr(CO) 4(tmp)] + has three low-lying electronic states, which are very close in energy. Each of these states can be the electronic ground state. They differ in the symmetry of the singly occupied MO, which, in each case, has a predominant Cr 3d character. While the spin density is localized mostly on the Cr atom, the charge on Cr is nearly the same in the neutral and in the cationic complexes. The charges on the CO ligands and, to a lesser extent, on tmp become more positive upon oxidation, due to electron-density redistribution. Lengthening of the CrC bonds and shortening of CO and CrN bonds are the main calculated structural changes caused by the oxidation of [Cr(CO) 4(tmp)]. Oxidation of the analogous W complexes is chemically irreversible on the cyclic voltammetry time scale at room temperature, reflecting the high reactivity of [W(CO) 4(tmp)] + and [W(CO) 4(phen)] +.