On the electronic structure of [Pt(4,4 '-X-2-bipy)Cl-2](0/-/2-): an electrochemical and spectroscopic (UV/Vis, EPR, ENDOR) study

A series of complexes of general formula [Pt(4,4'-X-2-bipy)Cl-2] (bipy=2,2' -bipyridine; X=NH2, OEt, Me, H, Ph, Cl or CO2Me) has been prepared and thei r redox chemistry and UV/Vis spectroscopy examined. The half-wave potential of the first reduction process from cyclic voltammetry varies linearly wit h the Hammett parameter of X, and also with the MLCT maximum from UV/Vis sp ectroscopy. The first reduction processes of the complexes with X=OEt, Me, H, Ph, Cl or CO2Me are reversible and these complexes undergo a second quas i-reversible or irreversible reduction at potentials 580-760 mV more negati ve. The reduced, 17 e(-) species are characterised by EPR spectroscopy and the total platinum contribution (5d and 6p) to the SOMOs is calculated to b e only ca. 7-12%. The bulk of the unpaired electron density is carried in t he bipy ligand pi* system and orientation-selective H-1 ENDOR spectra of [P t(4,4'-X-2-bipy)Cl-2](-) (X=H or CO2Me) show that C5 and C5' carry the grea test spin density among the four ring C(H) positions. The second reduction product of [Pt(4,4'-(CO2Me)(2)-bipy)Cl-2] is EPR silent indicating spin-pai ring of the two reduction electrons in the same orbital in this particular complex.