Bis(Tetrathiafulvalenes) with Mixed Phosphine and Silane Double Bridges

Abstract

Covalently linked bis(TTFs) (TTF = tetrathiafulvalene) are interesting electron donor precursors that allow the investigation of the extent of intramolecular electronic communication between the two redox active units and the access to mixed valence states. Here we describe the straightforward synthesis of the first rigid bis(TTFs) fused through six- or seven-membered-ring heterocycles containing two different heteroatomic bridges. The mixed phosphino/silane compound (PPh)(SiMe₂)(o-DMTTF)₂ 1 (o-DMTTF = ortho-dimethyl-tetrathiafulvalene) shows a distorted central phosphasilyline ring in the solid state and a planar and a moderately folded TTF. Compound 2 contains a seven member ring due to SiMe₂ and Me₂Si–SiMe₂ bridges and it crystallized in two polymorphic forms. The electrochemical study of 1 indicates sequential reversible oxidation into a mixed valence radical cation and then dication, with a peak-to-peak separation of 110 mV, thus suggesting through-bond electronic communication between the fused TTF units, as confirmed by EPR measurements of the radical cation 1^+•. Further, charge-transfer complex 1•TCNQ has been obtained as single crystals. The presence of a single phosphino bridge in 1 allowed the preparation of a mononuclear platinum(II) complex containing two such bis(TTF) ligands, trans located in the coordination sphere of the metal center.