A novel di-compartmental bis-(2-hydroxyisophtalamide) macrocyclic ligand and its mononuclear Cu(ii) and Ni(ii) complexes

Abstract

The synthesis and characterisation of the new di-compartmental bis-(2-hydroxyisophtalamide) macrocyclic pro-ligand, LH₆, which comprises two phenol-diamide units linked by ethylene bridges, is herein reported, together with its corresponding di-phenolate salt, [NBu₄]₂[LH₄]. The three macrocyclic compounds, [LH₄(OMe)₂] (protected ligand), LH₆ and [LH₄][NBu₄]₂ were fully characterised including X-ray crystallography for [LH₄(OMe)₂] and [NBu₄]₂[LH₄]. The results of solid-state and solution studies have indicated that the macrocycle can adopt specific conformations, which are influenced by H-bonding interactions as well as the deviation of the amide carbonyl relative to the phenol plane. LH₆ reacts with M^II(acetate)₂·(H₂O)₆ (M = Ni, Cu) in a 1 : 1 ratio in the presence of 4 eq of [NBu₄](OH) in methanol to afford the dianionic [M(LH₂)]²⁻ complexes, 1²⁻ and 2²⁻, respectively. The X-ray crystallography, EPR, NMR and UV-vis spectroscopic data, combined with DFT calculations, indicate that 1²⁻ and 2²⁻ are unique unsymmetrical square planar mononuclear complexes that are intramolecularly H-bonded. Thus, one macrocyclic compartment contains a M^II–N₂O₂ centre resulting from the tetra-anionic di-phenolato di-amidato ligation; the other compartment possesses two protonated amide N–H groups that are H-bonded the coordinated phenolate O atoms. This represents a unique example in which a phenolate is both coordinated and intramolecularly H-bonded. This H-bonding appears unusually strong as revealed by N(H/D) exchange experiments; and may be responsible for the stability of the mononuclear complex, and the difficulty in isolating the corresponding dinuclear complex [M₂(L)]²⁻.