Blended Pyrrole–Pyrazole Units as Versatile Ligands for Metal Complexes: Current Status

ABSTRACT Pyrrole and pyrazole are two distinct five‐membered heterocycles bearing one and two sp 2 ‐hybridized nitrogen atoms within the ring, respectively, that feature different basic nature of the molecules in their neutral forms. Integrating these two heterocycles mainly via C2 pyrrole and N1 pyrazole atoms gives rise to a versatile ligand system which offers both anionic and neutral σ‐donor sites in a single skeleton for metal ion coordination. Besides, the hydrogen bonding ability of two N‐donor centers of the blended frameworks becomes contrasting. Given this, the preparation of several pyrrole–pyrazole hybrid ligands in the forms of mono(pyrrole)–mono(pyrazole), mono(pyrrole)–bis(pyrazole), mono(pyrrole)–tetrakis(pyrazole), and bis(pyrrole)–bis(pyrazole) polychelators is explored in this short review. In addition, the utilization of both neutral donor and anionic donor nitrogen atoms of the chelators in the presence of a base or only neutral donor sites of the chelators in the absence of a base for the formation of metal complexes are minutely discussed. Apart from the variation in coordination modes of many transition metal complexes, the pyrrole‐π‐donation of ligand backbone, catalytic properties, fluxional behavior, steric bulk of the ligands on the structural formation, and hemilability of a few metal complexes are included. Finally, a few hints of research endeavors are outlined for future purview.