Dinuclear Cp* Cobalt Complexes of the 1,2,4,5-Benzenetetrathiolate Bischelating Ligand

Abstract

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)I 2]. 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 ( t )BuOK in tetrahydrofuran, or from benzo[ d]-1,3-dithiolane-2-one ( 7) and ( t )BuOK 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 CH 2Cl 2 show reversible Co (III) to Co (II) 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, = 13 000 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 (*+).