Coordination compounds based on a phosphonic diamide-TEMPO diradical and CuII, NdIII, EuIII and TbIII ions: synthesis, magnetic properties and application in the aerobic oxidation of allylic and benzylic alcohols

Abstract

The synthesis and structure of a novel phenylphosphonic diamide bis-TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl) diradical (ppdATEMPO) and of four M(hfac)_n (hfac = hexafluoroacetylacetonate; M = Cu^II, Nd^III, Eu^III or Tb^III; n = 2, 3) complexes are reported herein. The X-ray diffraction studies revealed that the compounds consist of five-coordinated [Cu(ppdATEMPO)(hfac)₂] and eight-coordinated [Ln(ppdATEMPO)₂(hfac)₃] (Ln = Nd^III, Eu^III, Tb^III) complexes due to the exclusive binding of the PO group of the ligand to the metal ions. Two-dimensional (2D) supramolecular networks are formed through intermolecular N–O⋯H–N(P) hydrogen bonding interactions. In the case of the free diradical, the multilayer supramolecular structure is additionally supported by PO⋯H–N and N–O⋯H–C inter-chain interactions. A DFT study confirmed the exclusive coordination of ppdATEMPO to all metal cations investigated via the oxygen atom of the PO group. This is attributed to the greater donor ability and steric accessibility of the PO group in comparison with the two nitroxide moieties. The magnetic measurements of the complexes indicated the presence of weak intermolecular antiferromagnetic interactions. The behaviour of the copper complex as a catalysts in the aerobic oxidation of alcohols has been also investigated.