Synthesis, crystal structure, electrochemical and kinetic studies on a cyano-bridged CuII–NiII–CuII hetero-trinuclear complex

A cyano-bridged hetero-trinuclear complex, [Cu(HL)]2[Ni(CN)4] (2) (H2L = 3,9-dimethyl-4,8-diazaundec-3,8-diene-2,10-dione-dioxime) has been synthesized and characterized by single crystal X-ray structure determination. The square planar [Cu(HL)]+ cation (1), otherwise reluctant to bind axially, is converted to a square pyramidal geometry through axial ligation by the cyano group of [Ni(CN)4]2− ion in di-trans fashion. Electrochemical studies in MeCN give (i) E1/2(CuII/I) = −0.52 V and −0.55 V and (ii) E1/2(CuIII/II) = 1.15 and 1.23 V versus Ag/AgCl for complexes 1 and 2 respectively, whereas in aqueous solution, the corresponding E1/2(CuII/I) appear at −0.61 V and −0.58 V. The E1/2(CuIII/CuII) appears at 1.04 V for 2 only; no such process is encountered for the complex 1 at potential ⩽1.20 V. In addition to these there is a redox process, occurring in aqueous solution only, for the copper(II) bound ligand oxidation at 0.63 and 0.60 V versus Ag/AgCl for the complex 1 and 2 respectively. Both the complexes undergo acid catalyzed decomposition with a second-order dependence in H+. It is found that complex 2 decomposes at a rate ∼2 times faster than the complex 1. The kinetic studies on the periodate oxidation reveal that both HIO4 and IO4- are reacting species for the oxidation of 1, while HIO4 is the sole reacting species for the complex 2. Again, complex 2 reacts faster than 1. An inner-sphere mechanism has been invoked for both the complexes based on large Q, the association constant values.

A cyano-bridged hetero-trinuclear complex, [Cu(HL)]2[Ni(CN)4] (2) has been synthesized and characterized structurally. Electrochemical and chemical redox properties of [Cu(HL)]+ (1) as well as 2 demonstrate that there is a distinct difference in behavior of 2 over 1 due to the presence of [Ni(CN)4]− in 2.Figure optionsDownload as PowerPoint slide