Synthesis, structural characterisation and studies of octanuclear copper complexes of pyridonate and carboxylate ligands

Abstract

A series of octanuclear copper(II) complexes has been prepared which features carboxylate and 2-pyridonate ligands. These compounds have the general formula [Cu₈(O)₂(O₂CR)₄L₈](where R = Me, Ph or CF₃, and L = 6-chloro-, 6-bromo- or 6-methyl-2-pyridonate). X-Ray structural analyses of the acetate complexes indicate that the compounds contain an edge-sharing Cu₆O₂ bitetrahedral core, surrounded by two [CuL₄] units. The Cu ⋯ Cu contacts within the core vary from 2.92 to 3.58 Å, depending on the ligand bridging the metal–metal vector. Two routes to these complexes have been investigated, one involving reaction of preformed copper pyridonate complexes with copper carboxylates, the second involving reaction of copper carboxylates with molten HL. Differential scanning calorimetry and thermogravimetric analysis studies of the latter route show it proceeds via formation of copper pyridonate complexes. The FAB mass spectral results show interesting fragmentation patterns, consistent with the observed structures, and NMR spectra of the octanuclear complexes show paramagnetically shifted resonances due to both the pyridonate and carboxylate ligands. Magnetic studies of [Cu₈(O)₂(O₂CMe)₄L₈](L = 6-chloro- or 6-bromo-2-pyridonate) show largely antiferromagnetic exchange coupling within the Cu₆O₂ core. The structure of the complexes and the magnitude of the couplings lead to spin-frustrated systems, with subtle differences between the two complexes studied.