Preparation and electronic properties of rhenium(V) complexes with bis(diphenyl phosphino)ethane

Complexes of bis(diphenylphosphino)ethane (dppe) of the types ReOX3(dppe) and ReO(OR)X-2(dppe) (with X= Cl or Br, and R = Me, Et, Pr, Ph, cyclohexyl (Cy), or -CH2CH2OH) were prepared to evaluate the influence of ligand changes on the low-energy d-d transitions in these Re(V) low-spin d(2) systems. Arylimido compounds Re(NR)Cl-3(dppe) (with R = Ph and p-ClC6H4) were also obtained. X-ray diffraction studies on ReO(OPr)Cl-2(dppe), ReO(OPh)Br-2(dppe), and ReO(OCy)Cl-2(dppe) confirmed the presence of the trans O=Re-OR unit and showed that the structural characteristics of the Re-O-R segment are not affected by the presence of a bulky cyclohexyl substituent or an aromatic phenyl group. The structure of the arylimido complex Re(p-ClC6H4N)Cl-3(dppe) was also determined. The electronic absorption spectra of the ReOX3(dppe) compounds include two low-energy components at similar to11 500 and similar to16 000 cm(-1), assigned to the two spin-allowed d-d transitions expected for these low-symmetry systems. Substitution of the oxo ligand by an arylimido group has little effect on the lower-energy component, but moves the two components closer together. Replacing the halogen trans to the Re=O bond by an alkoxo group shifts the whole system to higher energies. These variations were found to correlate well with the energies of the frontier orbitals determined from DFT calculations. While attempting to prepare ReOCl3(dppe) from ReOCl3(PPh3)(2), the Re(III) compound fac-ReCl3(PPh3){Ph2PC2H4PPh2(O)} was obtained, in which one end of dppe had become a phosphine oxide. Upon standing in DMSO, this compound gave the octahedral compound ReCl4{Ph2PC2H4PPh2(O)}, in which the formation of a chelate ring involving a phosphine and a phosphine oxide was ascertained by X-ray diffraction.