On the electronic structure of [Pt(4,4′-X2-bipy)Cl2]0/−/2−: an electrochemical and spectroscopic (UV/Vis, EPR, ENDOR) study

Abstract

A series of complexes of general formula [Pt(4,4′-X2-bipy)Cl2] (bipy = 2,2′-bipyridine; X = NH2, OEt, Me, H, Ph, Cl or CO2Me) has been prepared and their redox chemistry and UV/Vis spectroscopy examined. The half-wave potential of the first reduction process from cyclic voltammetry varies linearly with the Hammett parameter of X, and also with the MLCT maximum from UV/Vis spectroscopy. The first reduction processes of the complexes with X = OEt, Me, H, Ph, Cl or CO2Me are reversible and these complexes undergo a second quasi-reversible or irreversible reduction at potentials 580–760 mV more negative. The reduced, 17 e– species are characterised by EPR spectroscopy and the total platinum contribution (5d and 6p) to the SOMOs is calculated to be only ca. 7–12%. The bulk of the unpaired electron density is carried in the bipy ligand π* system and orientation-selective 1H ENDOR spectra of [Pt(4,4′-X2-bipy)Cl2]– (X = H or CO2Me) show that C5 and C5′ carry the greatest spin density among the four ring C(H) positions. The second reduction product of [Pt(4,4′-(CO2Me)2-bipy)Cl2] is EPR silent indicating spin-pairing of the two reduction electrons in the same orbital in this particular complex.