Oxo-rhenium(V) complexes containing bis(diphenylphosphino)ethane and catecholate ligands

Bis(diphenylphosphino)ethane (dppe) complexes of the type ReOCl(dppe)(O-O), where O-O = catecholate or tetrachlorocatecholate dianion, were prepared by reacting ReOCl₃(dppe) with the catechol ligand in the presence of NEt₃. X-ray diffraction on the tetrachlorocatecholate compound showed that the molecule adopts a distorted octahedral structure, in which the dppe ligand and the bidentate catecholate lie in the equatorial plane, perpendicular to the ORe-Cl unit. In contrast, for ReOCl(PPh₃)₂(O-O), the position trans to the ReO bond is occupied by a catecholate oxygen, whereas the two PPh₃ ligands are trans to one another in the equatorial plane. The UV-Vis absorption spectrum of ReO(OMe)(dppe)(oxalate) is similar to those of ReO(OR)X₂(dppe) compounds, showing two weak bands for the spin-allowed d-d transitions from the filled interaxial d orbital in the xy plane into the inequivalent metal d_xz and d_yz orbitals, respectively. For the catecholate complexes, the spectra are dominated by charge-transfer transitions from the HOMO π orbital of the catecholate ligand into the d_xz and d_yz orbitals. Both the singlet-singlet and the singlet-triplet transitions are generally observed. No information could be obtained on the weaker d-d transitions for the catecholate compounds.