Aggregation of mononuclear copper complexes by metal-oxidation-induced ligand deprotonation

The coordination chemistry of the tetrapodal-pentadentate ligand L, which has a mixed N3O2 donor set, has been studied in the presence of copper(I) sources. The metal-to-ligand stoichiometry of the mononuclear copper(I) complexes resulting from 1:1 metal/ligand mixtures is markedly dependent on the character of the counter ion, giving 1:1 ([Cu(L)Cl]) and 1:2 complexes ([Cu(L)2](PF6)), with coordinating chloride and non-coordinating hexafluorophosphate, respectively. 1H NMR and UV-Vis spectroscopy and single crystal X-ray structure analysis reveal tetrahedral coordination in both cases, with the “soft” copper centre engaging only two pyridine donors out of the N3O2 donor set. Upon chemical oxidation of [Cu(L)2](PF6) with dioxygen or 3,5-di-tert-butyl-quinone, the same binuclear copper(II) complex [Cu2(L-H)2](PF6)2 is selectively formed, indicating metal-oxidation induced deprotonation of one ligand, extrusion of the second ligand and subsequent fusion of the coordination spheres via alkoxide bridges. Elongation along a Jahn-Teller active molecular axis in [Cu2(L-H)2]2+ is affected by the nature of the counter ion, giving N2O3 and N3O2 coordination patterns in the hexafluorophosphate and in the chloride salt, respectively. In addition to the oxidised complex, isolated in close-to-quantitative yield, catechol is identified as the only reduction product, indicating a 2e−/2H+ reduction of the quinone. Electron transfer from copper(I) to the chemical oxidant is suggested to steer the formation of base equivalents from O2 and Q, which then drive ligand deprotonation and complex nucleation.