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 = CuII , NdIII , EuIII or TbIII ; 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 = NdIII , EuIII , TbIII ) complexes due to the exclusive binding of the P=O 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 P=O•••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 P=O group. This is attributed to the greater donor ability and steric accessibility of the P=O 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.