Aromatic versus heteroradialene character in extended thiophloroglucinol ligands and their trinuclear nickel(II) complexes.

Extended phloroglucinol ligands and complexes are best described as nonaromatic heteroradialenes. Herein, the electronic structures of extended thiophloroglucinol ligands and their Ni-3(II) complexes are evaluated by comparison to their phloroglucinol analogs by means of NMR, FTIR, UV/Vis, and structural parameters. To provide a full set of compounds for this comparison of S versus O substitution, a new triplesalen ligand, its Ni-3(II) complex, and a new thiophloroglucinol were synthesized. H-1 and N-15 NMR chemical shifts and coupling constants prove that the thiophloroglucinol ligands exist as the N-protonated and not the O-protonated tautomer. C-13 and N-15 NMR chemical shifts and structural parameters further demonstrated that the extended thiophloroglucinol ligands must be described with a predominant thione-enamine (heteroradialene) character despite the participation of a C=S double bond. In the Ni-3(II) complexes, this heteroradialene character is reduced but still predominant.