Iron(II) Complexes Based on π-Conjugated Terpyridine Ligands with Tetrathiafulvalene or Its Radical Analogue

Two tetrathiafulvalene (TTF) functionalized 2,2':6',2 ''-terpyridine derivatives, 4'-tetrathiafulvalene-2,2':6',2 ''-terpyridine (L-1) and 6,6 ''-dimethyl-4'-tetrathiafulvalene-2,2':6',2 ''-terpyridine (L-2), were synthesized and characterized. Based on L-1 and L-2, four electrochemically active TTF-containing iron(II) complexes, [Fe-II(L-1)(2)][ClO4](2) (1), [Fe-II(L1.+)(2)][ClO4](4) (2), [Fe-II(L-1)(2)][CF3SO3](2) (3) and [Fe-II(L-2)(2)][ClO4](2) (4), were successfully obtained. The preparation, spectroscopic and electrochemical properties of these new compounds as well as the crystal structures of complexes 1, 3 and 4 are described. Magnetic studies on 4 (with the ligand L-2) suggest that the Fe-II ion is in the high-spin state. Complex 2 was isolated as an interesting and stable open-shell substance, and the free spin is mainly associated with the TTF radicals, as indicated by EPR, UV/Vis spectra, electrochemical analysis, spectroelectrochemical measurements and XPS spectra. After the oxidation and the formation of the radical cation, the electrical conductivity of 2 is almost 3 orders of magnitude higher than that of 1. DFT and TDDFT calculations provided insight into interpreting the electronic properties of complex 1 and its oxidized states.