Antiferromagnetic cyclic tetrameric copper(ii) complex and 8-azaxanthine derived from 1,3-dimethyl-5-(5′-methyl-3′-isoxazolyl-azo)-6-aminouracil: structures, non-covalent interactions and magnetism

Abstract

The synthesis of uracil derivatives and their conversion to a number of potential compounds have drawn attention of researchers aiming to develop innovative materials. Therefore, a novel azo ligand, 1,3-dimethyl-5-(5′-methyl-3′-isoxazolyl-azo)-6-aminouracil (H₂L, 1), was synthesized from 1,3-dimethyl-6-aminouracil and 3-amino-5-methyl-isoxazole. Compound 1 was converted to an 8-azaxanthine derivative, 1,3-dimethyl-8-(5′-methyl-3′-isoxazolyl)azaxanthine (2), via its reaction with Cu(NO₃)₂·6H₂O in DMF solution. Alternatively, when 1 reacted with Cu(CH₃COO)₂·H₂O in the presence of NaN₃ in methanol, an antiferromagnetic Cu(II) complex (3) was formed, where Cu(II) is found to be coordinated with the in situ-generated novel ligand 1,3-dimethyl-5-(but-2-one-3-ene-4-amino-azo)-6-aminouracil (L¹) and azide ions. The structures of these compounds (1–3) were characterized using single crystal X-ray analysis. Complex 3 is a tetrameric assembly of Cu(II) with a nearly parallelogram shape. Its Cu(II) centers are interconnected through doubly end-on azide (μ_1,1-azide) bridges and doubly end-to-end azo (μ_1,2-azo) bridges. The theoretical study shows the presence of non-covalent interactions, including H-bonding and stacking interactions. Magnetic studies of 3, a nearly parallelogram tetranuclear Cu(II) unit with side lengths of 3.7839(6) Å and 4.7727(8) Å, revealed that it possesses strong antiferromagnetic interactions, as evidenced by its exchange coupling constant J₁ = −250 (1) cm⁻¹ through end-on azide bridging between the Cu(II) ions (Cu–N–Cu, 116°). Thus, the present study not only introduces a strong antiferromagnetic material, but also establishes a notable correlation between its structure and magnetic properties.