Synthesis, structure, magnetic properties and kinetics of formation of a cluster containing a {Cu₃(μ₃-OH)} core supported by a triazole-based ligand

<p>The trinuclear copper complex, [Cu₃(μ₃-OH)(CTMB)₃(NO₃)₂(CH₃CN)₂]·5CH₃CN·H₂O (<b>1</b>) {CTMB = cyclohexotriazole-3-(4-methoxybenzamide)}, has been prepared by mixing Cu(NO₃)₂·2.5H₂O and CHMBH {CHMBH = N,N′-cyclohexane-1,2-diylidene-bis(4-methoxybenzoylhydrazide)} in acetonitrile under ambient conditions. Compound <b>1</b> was characterized by IR and UV–visible spectroscopies as well as elemental analyses. X-ray crystallography shows that the cluster contains a {Cu₃(μ₃-OH)} core supported by three triazole-based Schiff base ligands. Each Cu is bound to the 2-N of one triazole ring and the 1-N of another. However, the coordination sphere of each Cu is different, one is five-coordinate and the other two are six-coordinate and bridged by a NO₃ group. The six-coordinate sites are different, one has a terminal NO₃ and the other a MeCN ligand. Magnetic measurements revealed the presence of isotropic and antisymmetric exchange between the copper(II) centers. The data were analyzed using the Hamiltonian containing isotropic exchange for an isosceles triangle together with antisymmetric exchange: <i>H</i> = –<i>J</i>₁(<i>S</i>₁<i>S</i>₂ + <i>S</i>₂<i>S</i>₃)−<i>J</i>₂<i>S</i>₁<i>S</i>₃ + <i>G</i>[<i>S</i>₁ × <i>S</i>₂ + <i>S</i>₂ × <i>S</i>₃ + <i>S</i>₃ × <i>S</i>₁]. Compound <b>1</b> exhibits strong antiferromagnetic coupling with <i>J</i>₁ = −180 and <i>J</i>₂ = −118 cm⁻¹ and antisymmetric exchange with <i>G</i>_z = 15 cm⁻¹. Stopped flow spectrophotometric studies show that the formation of <b>1</b> occurs in three distinct phases and the kinetics of each phase has been determined.