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.