Structural, spectroscopic and electrochemical studies of nickel(II) “sandwich” complexes with ligands featuring tethered 1,4,7-triazacyclononane macrocycles

Abstract

A structural and electrochemical study of the nickel(II) complexes, [Ni(tacn)₂](ClO₄)₂ (1), [NiL^eth](ClO₄)₂ (2), [NiL^ox](ClO₄)₂·H₂O (3), [Ni₂L^dur](ClO₄)₄·2H₂O (4), [Ni₃L^dur(H₂O)₃](ClO₄)₆·9H₂O (5) and [Ni₂L^isomes(H₂O)₃](ClO₄)₄·7H₂O (6) [tacn = 1,4,7-triazacyclononane, L^eth = 1,2-bis(1,4,7-triazacyclonon-1-yl)ethane, L^ox = 1,2-bis(1,4,7-triazacyclonon-1-ylmethyl)benzene, L^dur = 1,2,4,5-tetrakis(1,4,7-triazacyclonon-1-ylmethyl)benzene, L^isomes = 1,2,3-tris(1,4,7-triazacyclonon-1-ylmethyl)benzene] is reported. Single-crystal X-ray crystallography has established that 2 and 6 feature nickel(II) centres in a distorted octahedral geometry, which are sandwiched between pairs of tacn rings. In 6, a second nickel(II) centre coordinates the third tacn ring of L^isomes and three water molecules complete the distorted octahedral coordination sphere. For the sandwiched Ni(II) centres, steric contraints introduced by the ethane and ortho-oriented tacn rings in L^isomes cause significant distortions to the Ni(II) coordination spheres. Cyclic, square-wave and near steady-state voltammetric studies on 1–6 indicate that the sandwiched nickel(II) centres may be reversibly oxidised to the nickel(III) state, with the potential for the nickel(II)/(III) couple showing a dependence on the nature of the tether linking the sandwiching tacn rings. Complex 4 undergoes oxidation in two overlapping one-electron processes [i.e., Ni(II)Ni(II) → Ni(III)Ni(II) → Ni(III)Ni(III)], indicating that the two nickel centres within the complex behave in an essentially independent fashion.