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 Odouble bond; length as m-dashRe–Cl unit. In contrast, for ReOCl(PPh3)2(O–O), the position trans to the Redouble bond; length as m-dashO 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 dxz and dyz 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 dxz and dyz 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.