Oxidation of phosphines containing two or three tetrathiafulvalene (TTF) or o-dimethyl-TTF moieties. Evidence for formation of radical polycations

Abstract

Stepwise oxidation of tetrathiafulvalenyl(diphenyl)phosphine (1), 4′,5′-dimethyltetrathiafulvalen-4-yl(diphenyl)phosphine (1a), bis(tetrathiafulvalenyl)phenylphosphine (2), bis(4′,5′-dimethylt-tetrathiafulvalen-4-yl)phenylphosphine (2a), tris(tetrathiafulvalenyl)phosphine (3) and tris(4′, 5′-dimethyltetrathiafulvalen-4-yl)phosphine (3a) was followed by EPR and ENDOR spectroscopy, and cyclic voltammetry. Each of the tetrathiafulvalene (TTF) or 4,5-dimethyltetrathiafulvalene (o-DMTTF) moieties in 1–3a donates two electrons, so that exhaustive oxidation at potentials below 1 V (vs. SCE) leads to the dications 1²⁺ or 1a²⁺, the tetracations 2⁴⁺ or 2a⁴⁺ and the hexacations 3⁶⁺ or 3a⁶⁺. In 2, 2a, 3 and 3a, the electrons are removed one-by-one from different TTF or o-DMTTF moieties. In the initially formed radical cations 2˙⁺, 2a˙⁺, 3˙⁺ and 3a˙⁺, the electron hole is delocalized over all TTF or o-DMTTF moieties, two in 2˙⁺ or 2a˙⁺ and three in 3˙⁺ or 3a˙⁺. On the other hand, in paramagnetic species produced by further oxidation and also giving rise to well defined EPR and ENDOR spectra, the unpaired electron appears on the hyperfine time-scale as localized in only one donor moiety. These species are supposed to be the radical trications 2˙³⁺ or 2a˙³⁺ and the radical pentacations 3˙⁵⁺ or 3a˙⁵⁺. The intermediately occurring dication 2²⁺ or 2a²⁺ and 3²⁺ or 3a²⁺, as well as the tetracations 3⁴⁺ or 3a⁴⁺, should have a triplet ground-state, while the trications 3³⁺ or 3a³⁺ are expected to be quartet species.