Spectroelectrochemistry and Dimerization Equilibria of Chloro(terpyridine)platinum(II). Nature of the Reduced Complexes

Abstract

[Pt(tpy)Cl]+ (tpy is 2,2‘:6‘,2‘‘-terpyridine) undergoes reversible one-electron reductions in 0.1 M TBAH/DMF (TBAH is tetrabutylammonium hexafluorophosphate) at E° ‘+/0 = −0.74 V and E° ‘0/- = −1.30 V (vs AgCl (1.0 M KCl)/Ag). The first reduction couple is substantially positive of those observed for other M(II)−tpy complexes (for example, E° ‘0/- = −1.36 V for [Zn(tpy)Cl2]), a finding that suggests there is coupling between the empty 6pz orbital of square planar Pt(II) and the π* orbital of tpy, stabilizing the (π*)1 radical state. The dimerization constants of both [Pt(tpy)Cl]+ and [Pt(tpy)Cl] in 0.1 M TBAH/DMF were determined spectroelectrochemically and found to be 8(1) × 10 and 10(4) × 10 M-1, respectively. On the basis of variable-concentration studies, a species observed at an intermediate level of reduction is formulated as the mixed-valence dimer [(Pt(tpy)Cl)2]+, with Kmix = [(Pt(tpy)Cl)2+]/([Pt(tpy)Cl+][Pt(tpy)Cl]) = 18(4) × 10 M-1. Analysis of variable-temperature EPR spectra indicates that the first reduction is ligand-centered (2B2) with substantial contributions from Pt(II) 5dyz (4−6%) and 6pz (3−4%). The second reduction is tentatively assigned as metal-centered; 5dx2-y2 is the likely acceptor orbital.