Structures, Redox and Spectroscopic Properties of PdII and PtII Complexes Containing an Azo Functionality

The complexes [PdCl2(pap)] (1) and [PtCl2(pap)] (2; pap = 2-phenylazopyridine) were synthesized by treating PdCl2 or K2PtCl4, respectively, with pap and characterized by 1H NMR spectroscopy and elemental analysis. Both these complexes, together with the previously reported complex [(az)Pd(μ-Cl)2Pd(az)] (3; az = azobenzene), were also characterized by X-ray crystallography. The structures of 1, 2, and 3 show a slightly elongated N–N azo double bond due to back-donation from the metal centers and a twisting of the uncoordinated part of the ligands with respect to the rest of the molecule. Cyclic voltammetry of 1, 2, 3, and the related complex [(az)Pt(μ-Cl)2Pt(az)] (4) shows reduction processes which are reversible for 1 and 2 but irreversible for 3 and 4. The first reduction of 1 and 2 leads to radical complexes, and EPR spectroscopy shows the spin to be predominantly located on the azo part of the complexes. The radical complex 2·– shows an unprecedented 195Pt hyperfine coupling constant and g anisotropy for such PtII radical species. The UV/Vis spectroscopy results for all the complexes are also discussed.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)