Isomeric separation in donor–acceptor systems of Pd(II) and Pt(II) and a combined structural, electrochemical and spectroelectrochemical study

Abstract

Compounds of the form [(pap)M(Q2−)] (pap = phenylazopyridine; Q = 3,5-di-tert- butyl-benzoquinone, M = Pd, 1a and 1b, M = Pt, 2a and 2b; Q = 4-tert-butyl- benzoquinone, M = Pd, 3a and 3b; M = Pt, 4a and 4b) were synthesized in a one- pot reaction. The geometrical isomers, which are possible because of the built in asymmetry of these ligands, have been separated by using different temperatures and variable solubility. Structural characterization of 1b shows that the metal centers are in a square planar environment, the pap ligand is in the unreduced neutral state and the quinones are in the doubly reduced, Q2−catecholate form. Cyclic voltammetric measurements on the complexes display two one-electron oxidations and two one-electron reductions. EPR and vis-NIR spectra of the one-electron oxidized forms of the complexes indicate that the first oxidation takes place on the Q2− ligands to produce a metal bound semiquinone (Q˙−) radical. Reduction takes place on the pap ligand, generating metal bound pap˙− as seen from the 14N (I = 1) coupling in their EPR spectrum. All the complexes in their [(pap)M(Q2−)] neutral forms show strong absorptions in the NIR region which are largely LLCT (ligand to ligand charge transfer) in origin. These NIR bands can be tuned over a wide energy range by varying the metal center as well as the Q ligand. In addition, the intensity of NIR bands can be switched on and off by a simple electron transfer at relatively low potentials. DFT studies were used to corroborate these findings.