Structural Elucidation, Aggregation, and Dynamic Behaviour of N,N,N,N‐Copper(I) Schiff Base Complexes in Solid and in Solution: A Combined NMR, X‐ray Spectroscopic and Crystallographic Investigation

Abstract

A series of Cu(I) complexes of bidentate or tetradentate Schiff base ligands bearing either 1‐ H‐imidazole or pyridine moieties were synthesized. The complexes were studied by a combination of NMR and X‐ray spectroscopic techniques. The differences between the imidazole‐ and pyridine‐based ligands were examined by ¹H, ¹³C and ¹⁵N NMR spectroscopy. The magnitude of the ¹⁵N _imine coordination shifts was found to be strongly affected by the nature of the heterocycle in the complexes. These trends showed good correlation with the obtained Cu−N _imine bond lengths from single‐crystal X‐ray diffraction measurements. Variable‐temperature NMR experiments, in combination with diffusion ordered spectroscopy (DOSY) revealed that one of the complexes underwent a temperature‐dependent interconversion between a monomer, a dimer and a higher aggregate. The complexes bearing tetradentate imidazole ligands were further studied using Cu K‐edge XAS and VtC XES, where DFT‐assisted assignment of spectral features suggested that these complexes may form polynuclear oligomers in solid state. Additionally, the Cu(II) analogue of one of the complexes was incorporated into a metal‐organic framework (MOF) as a way to obtain discrete, mononuclear complexes in the solid state.

Abstract

A series of N,N,N,N Cu(I) Schiff base complexes was studied by NMR and X‐ray spectroscopic techniques. In solution, the aggregation behaviour was investigated by DOSY and variable‐temperature NMR. ¹⁵N NMR studies shed light on the Cu−N interactions in the complexes. XAS and XES, supported by DFT calculations, highlighted the complexes’ tendencies to form polynuclear species in the solid state.