Fluorescent detection of specific metal ions with organic ligands is an attractive subject in coordination chemistry. This account summarizes the evolution history of fluorescent metal sensor TQEN (N,N,N’,N’-tetrakis(2-quinolylmethyl)ethylenediamine), which is a TPEN (N,N,N’,N’-tetrakis(2-pyridylmethyl)ethylenediamine)-based hexadentate ligand responding to zinc and cadmium ions. The dim fluorescence intensity and insufficient Zn/Cd selectivity of TQEN have been effectively improved by changing the ligand structure. Mercury and pyrophosphate ions are also targeted by rational modification of the molecule.

Recent reports on utilization of polymeric cyano-bridged metal complexes, which have reticular structures utilizing M^C–CN–M^N coordination networks, as heterogeneous catalysts have been reviewed. Among the complexes, the most widely investigated ones contain zinc ions at M^N sites acting as Lewis acid, where polymerization of epoxide such as propylene oxide, esterification of fatty acids, and hydrolysis of glycerides are catalyzed. Another class of complexes containing redox active transition metals such as iron(II), cobalt(II) and copper(II) at M^N sites, catalyze oxidation and reduction reactions. Some complexes thermally oxidize benzene and styrene to phenol and styrene oxide with hydrogen peroxide, respectively. Additionally, some complexes can also photocatalyze Fenton reaction to decompose organic dyes with hydrogen peroxide, benzene oxidation to phenol with molecular oxygen, and water oxidation to evolve dioxygen together with a photosensitizer. All the complexes allow the improvements to achieve highly active catalysts based on atomic insights with various manners.