A novel 1,3,5-triaminocyclohexane-based tripodal ligand forms a unique tetra(pyrazolate)-bridged tricopper(ii) core: solution equilibrium, structure and catecholase activity

Abstract

Copper(II) complexes of a polydentate tripodal ligand L × 3HCl (L = N,N′,N′′-tris(5-pyrazolylmethyl)-cis,cis-1,3,5-triaminocyclohexane) were characterized in both solution and solid states. Combined evaluation of potentiometric, UV-VIS, and EPR data indicated the formation of two mononuclear (CuHL, CuL) and three trinuclear (Cu₃H_−xL₂, x = 2, 3, 4) complexes. The high stability and spectroscopic properties of the CuL species indicate a coordination of two pyrazole rings in addition to the three secondary amino groups of L in a square pyramidal geometry. In parallel with the formation of trinuclear species, intense charge transfer bands appear at around 400–500 nm, which indicate the formation of pyrazolate-bridged complexes. The crystal structure of [Cu₃H₋₄L₂](ClO₄)₂·5H₂O (1) reveals the formation of a unique trinuclear complex that features a tetra(pyrazolate)-bridged linear tricopper(II) core. The Cu⋯Cu interatomic distances are around 3.8 Å. The two peripheral copper(II) ions have a slightly distorted square pyramidal geometry. The four pyrazole rings bound to the peripheral copper(II) ions are deprotonated and create a flattened tetrahedral environment for the central copper(II), i.e. the formation of the trinuclear complexes is under the allosteric control of the two peripheral copper(II) ions. The triply deprotonated trinuclear complex is an efficient catechol oxidase mimic with a surprisingly low pH optimum at pH = 5.6. Since the mononuclear CuL species is not able to promote the oxidation of 3,5-di-tert-butylcatechol, we assume that the central copper(II) ion of the trinuclear complex with an unsaturated coordination sphere has a fundamental role in the binding and oxidation of the substrate. The experimental and structural details were further elaborated by a series of hybrid density functional theory calculations that support the presence of an antiferromagnetically coupled ground state. However, the magnitude and the pattern of spin coupling are dependent on the composition of the functionals. The optimized theoretical structures highlight the role of the crystal packing effects in inducing asymmetry between the two peripheral copper(II) sites.