Synthesis and Electrochemistry of Organometallic Cobaltadithiaazulenes

Reaction of tropolone or hinokitiol with phosphorus pentasulfide (P2S5) directly gives the sulfurized precursor [PS2(SST)]2 or [PS2(SSH)]2 (SST = dithiotropolonato or SSH = dithiohinokitiolato). The resulting [PS2(SST)]2 or [PS2(SSH)]2 is further reacted with [CpCoI2(CO)] (Cp = η5-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 (62–) 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 62– 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.