Structure and magnetic properties of tri- and hexa-nuclear hydroxo-bridged copper(ii) complexes formed by a trimacrocyclic derivative of 1,4,7-triazacyclononane

Abstract

Two hydroxo-bridged Cu(II) complexes of the trimacrocyclic ligand, 1,3,5-tris(1,4,7-triazacyclonon-1-ylmethyl)benzene (L^mes), have been prepared and characterized. The trinuclear complex, [Cu₃L^mes(μ-OH)₂(H₂O)₂](ClO₄)₄·3.2H₂O (1), formed when the pH of an aqueous solution of [Cu₃L^mes(H₂O)₆](ClO₄)₆·6H₂O, was adjusted to ca. 6. X-Ray structural analysis confirmed the presence of a binuclear [Cu₂(μ-OH)₂]²⁺ core and an isolated Cu(II) centre. The “roof-shaped” [Cu₂(μ-OH)₂]²⁺ core has a dihedral angle (δ) of 152° between the CuO₂ planes and exhibits a relatively short Cu⋯Cu distance of 2.9041(8) Å. An increase in pH to 9.5 generates the hexanuclear complex, [Cu₆(L^mes)₂(μ-OH₆](ClO₄)₆·2H₂O (2), by linking two [Cu₃L^mes(μ-OH)₂]⁴⁺ trinuclear units via two μ-hydroxo bridges. The structure of 2 features three [Cu₂(μ-OH)₂]²⁺ units, two with bent geometries, similar to that observed in 1 [δ = 153°, Cu⋯Cu = 2.8757(8) Å], and one with planar geometry [Cu⋯Cu = 2.961(1) Å]. A variable temperature magnetic susceptibility study on 1 has identified an S = 1/2 ground state, consistent with a system composed of an antiferromagnetically coupled Cu(II) pair (J = −24 cm⁻¹) and a magnetically isolated Cu(II) centre. Variable temperature Q-band EPR spectra confirmed this interpretation. Comparisons to the Q-band EPR spectra of the previously reported trinuclear complex, {[Cu₃L^mes(μ-OH)(μ₃-HPO₄)(H₂O)](PF₆)₃·3H₂O}_n, were made, the latter also shows a S = 1/2 ground state but with the unpaired electron delocalised between two Cu(II) ions. The susceptibility data for 2 were interpreted in terms of the presence of three independent Cu(II) binuclear units with weak antiferromagnetic coupling observed in both the bent and planar [Cu₂(μ-OH)₂]²⁺ cores (J = −61 and −29 cm⁻¹, respectively).