Synthesis, crystal structure, and magnetic properties of an alkoxo-hydroxo-bridged octanuclear copper(II) complex showing chemically significant hydrogen-bonding interactions involving a metallamacrocyclic core

Abstract

A novel 16-member metallamacrocyclic octanuclear copper(II) complex of formulation [Cu₈L₄(OH)₄] (1) has been prepared from a reaction of [Cu₂L(O₂CMe)] and NaOH in methanol, where L is a pentadentate trianionic Schiff base ligand N,N'-(2-hydroxypropane-1,3-diyl)bis(salicylaldimine). The complex has been characterized by analytical, structural, and spectral methods. It crystallizes in the monoclinic space group C2/c with the following unit cell dimensions: a = 30.365(3) Å; b = 14.320(2) Å; c = 19.019(2) Å; β = 125.33(2)°; V = 6746.7(13) Å₃; Z = 4. A total of 4589 unique data with I > 2σ(I) were used to refine the structure to R1(F_o) = 0.0525 and wR2 = 0.1156. The structure consists of four binuclear {Cu₂L}⁺ units linked covalently by four hydroxide ligands to form an octanuclear core which is stabilized by strong hydrogen-bonding interactions involving the hydroxide ligands. Each binuclear unit has a pentadentate ligand L showing N₂O₃ coordination with an endogenous alkoxide bridging atom. The magnetic susceptibility data of 1, obtained in the temperature range 14-306 K, show the presence of antiferromagnetic exchange interactions between adjacent spin-½ Cu(II) ions. The µeff values are 1.54 and 0.26 μ_B (per copper) at 295 and 15 K, respectively. The magnetic data have been theoretically fitted using a Heisenberg spin-½ Hamiltonian with nearest-neighbor antiferromagnetic interactions. The spin coupling in the metallamacrocyclic ring has been modeled using four different coupling constants (J) on the basis of the structural parameters of the octanuclear core. The coupling constants obtained are J₁ = -318.8, J₂ = -293.3, J₃ = -111.6, and J₄ = -63.8 cm^-1. The theoretical modeling of the susceptibility data gives a higher magnitude of the antiferromagnetic interaction within the binuclear {Cu₂L}⁺ unit compared to those involving adjacent dimeric units.