Synthesis and Electrochemistry of Organometallic Cobaltadithiaazulenes

Abstract

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.

Coordination Programming of Two-Dimensional Metal Complex Frameworks

“…The electronic communication between the π orbital of a dithiolate ligand and the d orbital of a metal ion produces quasi-aromaticity, the delocalization of π electrons over the metalladithiolene ring. The unsaturated metal center results in redox activity, − photofunction, and chemical reactivity, − providing an interesting motif for CONASHs. π-Conjugated multinuclear metalladithiolene systems allow very strong electronic communication among the metal centers, and their electronic structures can be easily controlled by changing the metal centers.…”

Long-Range Intramolecular Electronic Communication in a Trinuclear Ruthenium Tropolonate Complex

“…In the former view, Htrop and its derivatives have been mainly investigated in terms of the antioxidant properties such as the inhibition of various oxidases. − In the latter sense, Htrop derivatives have attracted attention as a framework for organic redox systems, − ever since Nozoe proposed tropoquinone (cyclohepta-3,6-diene-1,2,5-trione) as an oxidized form of 5-hydroxytropolone . The redox chemistry of metal tropolonate complexes has been also investigated, − including the study of Htrop derivatives as redox-active ligands, − while ruthenium tropolonate complexes have not been fully investigated. Even a basic cyclic voltammetry (CV) study of [Ru­(trop) 3 ] ( 1 ) had not been reported until our recent report .…”