Tetranuclear copper(ii) cubane complexes derived from self-assembled 1,3-dimethyl-5-(o-phenolate-azo)-6-aminouracil: structures, non-covalent interactions and magnetic property

Abstract

Tetranuclear copper(II) complexes are of paramount importance in structural biology, and they are potential materials for magnetism and catalysis. To develop such a system, a new 6-aminoazouracil ligand, 1,3-dimethyl-5-(o-phenolate-azo)-6-aminouracil (H₂L, 1) with a N_uN_aO (N_u, uracil-N and N_a, azo-N) chromophore was synthesized and used to generate a noble discrete doubly opened Cu₄O₄ cubane-like cluster ([CuL]₄·2H₂O, 2.2H₂O) for studying magnetism. The coordination environment of Cu(II) is distorted square planar linked through phenolate-μ₂-O bridges. The ligand crystallizes in the monoclinic space group P12₁/c1 and the complex is in the tetragonal I4₁/a space group with an S₄ symmetry. The ligand has two dissociable hydrogen atoms in the solution with pK₁ 4.91 (hydrazone proton) and pK₂ 9.68 (phenolic proton). In the solid state, the ligand exhibits displaced stacking (energy, −69.5 and −77.3 kJ mol⁻¹ for the molecules A and B, respectively) and tetrel bonding interactions (energy, −43.8 kJ mol⁻¹). In 2.2H₂O, the symmetrical units are stacked to show weak noncovalent interactions. The magnetic property of 2.2H₂O was investigated based on the cubane [Cu₄O₄] core and discussed in detail, resulting in the exchange coupling parameter [(J₁ (short Cu⋯Cu distances) = −110.1(1) cm⁻¹, J₂ (long Cu⋯Cu distances) = −27.1(2) cm⁻¹)] that indicate a strong antiferromagnetic interaction between tetranuclear copper(II) ions through μ-phenolate linkers, which is a result of the interaction of d_x²_−y² orbitals in the square plane. The EPR study is concomitant with the results of magnetism. Thus, it could be a potential material in the field of antiferromagnetic spintronics as well.