Coordination-driven tautomerism in pyrrole-based Schiff-base ligands: Spectroscopic and crystallographic signatures.

• The tautomeric interconversions of pyrrole-based Schiff-base ligands are discussed. • Metal ion- and anion-coordination-driven tautomeric changes are presented. • Tautomerism alters the ligating behaviour in primary & secondary coordination spheres. • Two tautomers: imino-pyrrole & amine-azafulvene are described in solid and solution. • The detailed spectroscopic and crystallographic signatures are highlighted. Pyrrole-based Schiff-base ligands normally exist in two self-adaptable tautomeric forms: imino-pyrrole and amine-azafulvene. Upon metal ion or anion coordination, the imino-pyrrolic forms of the ligands sometimes undergo structural transformations to adopt amine-azafulvene forms and vice versa. The tautomerization alters the ligating behaviour of the molecules in both primary (anionic coordination versus dative coordination) and secondary (H-bond acceptor versus H-bond donor) coordination spheres, and helps in stabilizing a variety of metal ions with different oxidation states. This property of the ligands guides to understand the mechanisms of several enzymatic functions at the active sites. This review article highlights the coordination-driven tautomeric changes in various mono-, bis-, and tris-pyrrolic Schiff-base ligands. The detailed spectroscopic (1H NMR, IR) and crystallographic (single crystal XRD) signatures are also described in identifying the two tautomeric forms both in the solid and solution states. The coordination-induced structural transformations of two tautomeric forms (imino-pyrrole and amine-azafulvene) of pyrrole-based Schiff-base ligands which offer variable primary and secondary coordination spheres are discussed along with their spectroscopic and crystallographic signatures. [Display omitted] [ABSTRACT FROM AUTHOR]