Stability and structure of mono- and dinuclear Cu(ii), Ni(ii) and Zn(ii) complexes of pyrazole and triazole bridged bis-macrocycles

Abstract

The two ligands L¹ and L², consisting of two 1,4,7-triazacyclononane rings bridged by a pyrazole or a triazole group, form with Cu²⁺, Ni²⁺ and Zn²⁺ a series of mononuclear [MLH_n]^{(n + 2)+} (n = −3, −2, −1, 0, 1, 2) and dinuclear species [M₂LH_m]^{(m + 4)+} (m = 0, −1, −2, −3), the stabilities of which have been determined by potentiometric titrations. The triazole bridged ligand L¹ forms complexes which are more stable than those of the pyrazole bridged system L². It also shows a higher tendency to form dinuclear species compared to L². In addition two dinuclear species [Cu₂(L²H₋₁)(μ-Ph₂PO₂)](ClO₄)₂·MeCN and [Cu₄(L²H₋₁)₂(μ₄-PO₄)](ClO₄)₃·MeCN·H₂O have been isolated as solids and their X-ray structures have been determined. In the first complex we find a dinuclear Cu²⁺ species which is bridged by the pyrazolide group as well as by the exogenous diphenyl phosphinate. In the second case a μ₄-phosphate ion bridges two dinuclear units, resulting in a tetranuclear species.