Synthesis and electrochemistry of organometallic cobaltadithiaazulenes

Reaction of tropolone or hinokitiol with phosphorus pentasulfide (P(2)S(5)) directly gives the sulfurized precursor [PS(2)(SST)](2) or [PS(2)(SSH)](2) (SST = dithiotropolonato or SSH = dithiohinokitiolato). The resulting [PS(2)(SST)](2) or [PS(2)(SSH)](2) is further reacted with [CpCoI(2)(CO)] (Cp = η(5)-cyclopentadienyl) to form the organometallic [CpCo(I)(SST)] (1) or [CpCo(I)(SSH)] (2), respectively. 1 and 2 have a cobaltadithiaazulene ring containing one cobalt and two sulfur atoms in the five-membered ring of azulene. Although X-ray structure analysis of 1 reveals the iodide-coordinated structure, 1 becomes the iodide-free complex [CpCo(SST)](+) (4(+)) in solution. Electrochemical studies of 4(+) by CV and spectroelectrochemical measurements (ESR, UV-vis-NIR) in solution are carried out. 4(+) is stepwise reduced by 2e(-) to form the stable neutral radical (4(•)) and unstable anion (4(-)). It is proposed that the anion 4(-) undergoes dimerization to afford the dimer (6(2-)) by anion radical coupling at the 5 or 7 position in the seven-membered ring of the cobaltadithiaazulene, since the similar anion radical coupling of a reduced azulene has been reported. Electrochemical reoxidation of 6(2-) slowly undergoes monomerization, giving the original monomer 4(•). DFT calculation of 4(+) explains that there is a delocalized lowest unoccupied molecular orbital (LUMO) in the whole molecule, and that of radical 4(•) has a delocalized singly occupied molecular orbital (SOMO). In these CpCo-SST (or SSH) complexes, there could be metal/ligand electron transfer since the SST (or SSH) ligand is potentially redox active. The spin density distribution of 4(-) obtained by the DFT method supports the mechanism of the anion radical coupling at the 5 or 7 position in the seven-membered ring.