Heteroleptic Cu(ii) saccharin complexes: Intriguing coordination modes and properties

Heteroleptic Cu(ii) compounds containing 4,4′-bis(hydroxymethyl)-2,2′-bipyridine (bipy-OH), tropolonate (Trop) and saccharinate (Sac) have been synthesized and characterized. Different synthetic procedures have been followed for the preparation of a neutral Sac [(bipy-OH)Cu(Trop)Sac], 2, selectively isolated in its blue anhydrous 2a and green mono-hydrated 2b crystalline solid-state forms. The structural characterization of 2 has revealed that the Sac ion is bound to the Cu(ii) centre through one of the oxygen atoms of the sulphonyl group, a rare coordination herein proposed to be the thermodynamically preferred one through computational analysis. Investigation to rationalize the hydrogen bonding motifs introduced by the Sac ion in 2 is conducted using hydrogen bond propensity calculations, which are successful in predicting both the formation of its mono-hydrate solid state form and its behaviour in a biological environment. Moreover, both of the solid state forms of 2 are found emissive in the NIR region, in the crystalline solid state at room temperature and in acetonitrile solution at 77 K, with no emission in solution. Finally, computational analysis is carried out showing that 2 when exposed to model molecules mimicking biological targets undergoes ligand exchange reactions with N- and S-donor ligands, with the detached Sac ion interacting via hydrogen bonds with the bipy-OH moiety of the copper derivatives.