Synthesis of Azo-Conjugated Metalladithiolenes and Their Photo- and Proton-Responsive Isomerization Reactions

A versatile synthetic method of azo-conjugated metalladithiolenes was developed, and new
complexes with various central metals and substituent groups were synthesized. Molecular structures of
the azo-conjugated metalladithiolenes of Ni(II), Pd(II), and Pt(II) with diphenylphosphinoethane as a co-ligand were determined by X-ray crystallography. While the energy of the reversible trans-to-cis
photoisomerization is considerably lower than that of azobenzene, the thermal stability of the cis form is
much higher than that of the organic azobenzene derivatives showing similar low-energy trans-to-cis
photoisomerization. A novel proton response of the azo group occurs, and the combination of photoisomerization and protonation leads to a novel proton-catalyzed cis-to-trans isomerization, the rate of which
correlates with the redox potential of the metalladithiolene moiety. The study including other azo-conjugated
metalladithiolenes has indicated that the protonation is a common feature for the azo-conjugated
metalladithiolenes, but trans-to-cis photoisomerization is strongly dependent on the electronic structure of
the trans form or a steric effect in the cis form.