Protonation of tetranuclear oxo(chloro)pyridinecopper(II) complexes by HBF 4 in nitrobenzene

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Tetrafluoroboric acid (HBF 4) and triethylamine (Et 3N) have been added consecutively to solutions of tetranuclear oxocopper(II) complexes (μ 4-O)py 3Cu 4Cl 4O ( Ia), (μ 4-O)py 4Cu 4Cl 4O ( Ib), (μ 4-O)py 3Cu 4Cl 6 ( IIa), (μ-O) 2N 4Cu 4Cl 4 ( IIIa) and (μ 4-O)N 4Cu 4Cl 6( IVa) in nitrobenzene at 25 °C to examine their proton basicities. Here, py is pyridine and N is N, N-diethylnicotinamide. The reactions were assigned from spectral and conductivity data. We conclude that the μ 4-oxo groups of IIa and IVa are reversibly monoprotonated by HBF 4 and are less basic than Et 3N. Further addition of HBF 4 apparently removes the central μ 4-oxo group of IIa and IVa as H 3O + and then protonates coordinated py and N. Complexes Ia, Ib and IIIa are irreversibly monoprotonated at their respective oxo groups, which are more basic than Et 3N. The protonation sequence for Ia consists of loss of its terminal oxo group Cu vO as H 3O +, followed by loss of coordinated py as pyH +. The sequence for Ib is the same except that py coordinated at the v site of Ib is protonated in preference to py at sites x. The μ 4-oxo group of Ia and Ib and the OH groups of diprotonated IIIa are less basic than their respective ligands py and N. Implications of the results on the mechanisms of copper-catalyzed phenolic oxidative coupling reactions are considered.