Metal-coordinated azoaromatics: Strategies for sequential azo-reduction, isomerization and application potential

Reversible electron reservoir feature of redox non-innocent ligand is an important event from the broader perspectives of metal–ligand cooperativity in catalysis as well as in introducing the molecular bistability. In this context, facile reductions of the metal bound azo core ([-NN-]0 → [-NN-]−→[-NN-]2−) of azoheteroarenes such as 2,2′-azobis(pyridine), 2,2′-azobis(benzazole), azo-pincers as well as four-electron reduction mediated cleavage of the azo bond ([-NN-]0 + 4e−→2[=N-]2−) to form metal-imido fragment have been highlighted. Structural authentication of the redox states of azo core under selective coordination situations and their application towards aerobic alcohol oxidation and cycloaddition reactions have been reviewed. It also elaborates strategies towards multi-electron reductions and scission of azoaromatics via (i) metal-to-ligand charge transfer (MLCT) induced inner-sphere electron transfer, (ii) metathesis reaction pathway, where metal–metal bond breaking delivers electrons to the empty π*, σ* orbitals of the azo function, and (iii) synergism of metal with other reducing agent. An account for the synthetic methodologies of azoaromatics and its light triggered cis–trans isomerization including application potential to molecular materials and biological processes have been addressed.