Labile Rh(II) species resulting from oxygen or/and Cu(II) oxidation of Rh(I) complexes

Abstract

Among the catalysts used for promoting homogeneous oxidations of olefins with molecular oxygen, rhodium complexes seem to be especially efficient. Indeed, several groups have recently reported on the ability of some rhodium complexes to catalyze the oxidation of terminal olefins via a direct activation of molecular oxygen [1]. The coordination of both the olefin and O 2 on the same metal center has been postulated. However, the proposed mechanisms are still largely speculative, especially as far as the metal oxidation level is concerned. We describe here the EPR characterization of unstable Rh(II) species (d 7) resulting from a direct oxidation of Rh(I) complexes by molecular oxygen at room temperature. Moreover, the same Rh(II) intermediates are also formed by electron transfer from Cu(II) salts in conditions typical of olefin oxidation. Typically, when O 2 is bubbled through a HAcEtOH solution of a Rh(I) complex (no EPR signal), the colour turns purple and the build up of an unstable paramagnetic Rh(II) complex can be monitored by EPR. The presence of the same complex is also evidenced after addition of an equimolecular amount of Cu(II) salt to the Rh(I) solution, in the absence of oxygen. In both cases, the concentration of the paramagnetic species decreases with time. ▪ The main features of the EPR spectrum of the purple solution at 77 K are seen in Fig. 1. When the spectrum is measured at a higher temperature (up to 250 K), the symmetry of the paramagnetic species remains unchanged. However at temperatures higher than 170 K, the Rh-hyperfine structure disappears and eventually the spectrum of the Figure (measured at 170 K) is observed. The EPR spectrum is attributed to a Rh(II) complex, the formation of which is totally reproducible. On the basis of experimental and theoretical results published in the case of EPR spectra of d 7 ‘low spin’ elements, we propose a C 2v symmetry for the Rh(II) complex, the unpaired electron being principally located in a d x 2−y 2 orbital. This electron is largely delocalized on the only chloride ion of the complex, on the symmetry axis.