Cationic−anionic complexes of Cu(II) and Co(II) with N-scorpionate ligand – structure, spectroscopy, and catecholase activity

We report structural and physicochemical characterization supported by quantum chemical studies of two novel copper(II) [CuLCl]₂[CuCl₄] (1) and cobalt(II) [CoLCl][CoL′Cl₃] (2) cationic−anionic complexes with N-scorpionate type ligand, N,N,N-tris(3,5-dimethylpyrazol-1-ylmethyl)amine (L), where L′ is 1-methylamine-3,5-dimethylpyrazole. The obtained complexes are the first reported examples of cationic−anionic coordination compounds tested for catecholase activity. Interestingly, only copper complex (1) shows catalytic activity in the oxidation of 3,5-di-tert-butylcatechol (3,5-DTBC), which turned out to be solvent dependent. Here, experimental UV-vis spectroscopy of 1 shows that essential features of the solid-state spectrum are maintained in DMSO and MeOH solvents. In contrast, the build-up of a new feature around 465 nm for 1 in CH₃CN was noted, along with negligible activity. According to quantum chemical calculations, this feature could be attributed to ligand-to-metal excitations within the [CuCl₄]²⁻ fragment disturbed by adjacent [CuLCl]⁺ species. The band shifts to lower energies compared to solid-state measurements as the two charged fragments get closer due to Coulomb interactions. In DMSO, the solvent molecule serves as an inert ligand in a [CuLCl]⁺ fragment and blocks the catalytic center, disturbing the formation of the [catalyst–substrate] complex and decreasing activity, while in MeOH, the solvent effectively stabilizes [CuCl₄]²⁻ via a H-bond network and the free coordination site is accessible, thus allowing a substrate molecule to bind. The critical advantage of the investigated complexes, in the context of their possible catalytic activity, was the fact that their usage would not introduce any unnecessary counterions.