Robustness and stability of the l-carnosine copper(ii) dimer: changes in the copper coordination sphere upon heating/solvent substitution

Abstract

Coordination compounds of L-carnosine (H₂car) have been the subject of much research, as they have potential therapeutic applications due to their antioxidant and anti-inflammatory properties. This paper analyzes the coordination possibilities of L-carnosine in reactions with copper(II) under different conditions. Three solvates (1–3) of copper(II) coordination compounds with car ligands were prepared, their molecular and crystal structures determined, and their interconversion conditions described. The Cu(II) ion coordination polyhedron is square-pyramidal, with water/methanol molecules in apical positions [Cu₂(car)₂(solv)₂] (solv = water or methanol). All three compounds consist of discrete dimers in which two car ligands coordinate two Cu(II) ions. The methanol solvates transform to the most stable compound, [Cu₂(car)₂(H₂O)₂]·2H₂O, upon exposure to moist air. Upon heating, this compound loses both coordinated and solvent water molecules, and the copper coordination changes to square-planar, yet the dimer is stable up to 200 °C. When cooled in moist air, it readily converts to the original compound [Cu₂(car)₂(H₂O)₂]·2H₂O. An exhaustive temperature-dependent structural and spectroscopic solid-state analysis indicates the robustness and reactivity of the activated dimer without coordinated molecules (coordinatively unsaturated sites, CUS). Preliminary results indicate that the generation of CUS requires heating after which the pyridine-based linkers can be introduced by prolonged milling using polar aprotic dimethylformamide as a liquid. These findings present potential routes for utilizing the activated dimer in creating novel compounds.