Oxidative addition of 1,2,5,6-Tetrathiocins to Co(I): A Re-Examination of Crown Ether Functionalized Benzene Dithiolate Cobalt(III) Complexes

Abstract

Oxidative addition of the 4′,4″,5′,5″-crown-functionalized dibenzo-1,2,5,6-tetrathiocins, [(OCH2CH2)nOC6H2S2]2 (1, n = 4; 2, n = 5), to the Co(I) complex CpCo(CO)2 under microwave irradiation in toluene affords 16e– Co(III) dithiolate complexes CpCo{S2C6H2O(CH2CH2O)n} 3 (n = 4) and 4 (n = 5) in 71–75% recovered yield. Complexes 3 and 4 were characterized by X-ray diffraction. Compound 3 was found to be polymorphic, crystallizing as the 16e– monomer, 3, or the 18e– dimer, (3)2, depending upon reaction conditions. In contrast to previous literature which suggested formation of 1:1 complexes between 3 and s-block cations, reaction of 3 with the s-block metal salts M[BPh4] (M = Na, K, Rb, and Cs) formed the 2:1 complexes [{CpCo(S2C6H2O(CH2CH2O)4)}2M][BPh4] (5a–5d, respectively) whose structures were determined by X-ray diffraction and revealed that the alkali metal ion is sandwiched by two crown ether substituents in all cases. UV/vis titration studies of complex 3 with NaBPh4 were consistent with formation of the 2:1 complex as the dominant species in solution. Mass spectrometry studies on 5a–5d revealed the presence of both [(3)2M]+ and [(3)M]+ ions (M = Na, K, Rb, and Cs). Electrochemical studies on 3 revealed an irreversible 1e– oxidation attributed to a ligand-based process based on DFT calculations. Chemical oxidation of 3 afforded two dimeric structures [{CpCo(S2C6H2O(CH2CH2O)4)}2][X]2 (6a, X = OTf; 6b, X = BF4) in which dithiolate ligand oxidation leads to complexes containing a disulfide bond, consistent with electrochemical and computational studies.