Exploratory studies on coordination chemistry of a redox-active bridging ligand: synthesis, properties and solid state structures of the complexes

The explorative coordination chemistry of the bridging ligand TTF-PPB is presented. Its strong binding ability to Co(II) and then to Ni(II) or Cu(II) in the presence of hexafluoroacetylacetonate (hfac−), forming new mono- and dinuclear complexes 1–3, is described. X-ray crystallographic studies have been conducted in the case of the free ligand TTF-PPB as well as its complexes [Co(TTF-PPB)(hfac)2] (1) and [Co(hfac)2(μ-TTF-PPB)Ni(hfac)2] (2). Each metal ion is bonded to two bidentate hfac− anions through their oxygen atoms and two nitrogen atoms of the PPB moiety with a distorted octahedral coordination geometry. Specifically, nitrogen donor atoms of TTF-PPB adopt a cis-coordination but not in the equatorial plane, which is quite rare. Electronic absorption, photoinduced intraligand charge transfer (1ILCT), and electrochemical behaviour of 1–3 have been investigated. UV-Vis spectroscopy shows very strong bands in the UV region consistent with ligand centred π–π* transitions and an intense broad band in the visible region corresponding to a spin-allowed π–π* 1ILCT transition. Upon coordination, the 1ILCT band is bathochromically shifted by 3100, 6100 and 5900 cm−1 on going from 1 to 3. The electrochemical studies reveal that all of them undergo two reversible oxidation and one reversible reduction processes, ascribed to the successive oxidations of the TTF moiety and the reduction of the PPB unit, respectively.