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.