Dinuclear Cp* cobalt complexes of the 1,2,4,5-benzenetetrathiolate bischelating ligand

The dinuclear Cp*Co dithiolene complex [Cp*Co(btt)CoCp*] (1) is prepared in high yield from the reaction of the bis(dibutyltin) complex of 1,2,4,5-benzenetetrathiolate (btt) with 2 equiv of [Cp*Co(CO)12]. Mononuclear complexes are also obtained from 1,2,4,5-tetrakis(isopropylthio)benzene (2) and sodium in pyridine, from benzo[1,2-d;4,5-d']bis(1,3-dithiolane-2,6-dione) (3) and (BuOK)-Bu-t in tetrahydrofuran, or from benzo[d]-1,3-dithiolane-2-one (7) and (BuOK)-Bu-t to afford respectively 4a, 4b, and [Cp*Co(bdt)] (6), while [Cp*Co(dmit)] (8) is obtained by literature methods. The X-ray structures of the dinuclear complex 1 and the mononuclear complexes 4a and 6 were determined. They are all characterized by Cp*... btt face-to-face intermolecular interactions, leading to a recurrent 4-fold symmetry motif. The cyclic voltammograms of the [Cp*Co(dithiolene)] complexes performed in CH2Cl2 show reversible Co-III to Coil reduction but irreversible oxidation waves. The large potential difference between the two reduction waves of the bimetallic complex 1 (269 mV) indicates a stable mixed-valence Co-III-Co-II state for the reduced [Cp*Co(btt)CoCp*](-) anion. Upon trimethyl phosphite addition, the mono-P(OMe)(3) adduct [1.P(OMe)(3)] exhibits a red shift of the low-energy absorption band to the IR region (856 nm, epsilon = 13000 M-1.cm(-1)), while [8.P(OMe)(3)] exhibits a 150 nm blue shift. The stability constants of these P(OMe)(3) adducts were determined from UV-vis spectroscopic titration experiments, with, for example, log(K/mol(-1)-dm(3)) values of 3.1 and 0.52 for the mono- and bis-adduct of 1, respectively. The electrochemical investigation of 1 and 8 in excess phosphite shows a strong current enhancement upon oxidation, attributable to the catalytic generation of the radical cation P(OMe)(3)(center dot+).