Antiferromagnetic Cyclic Tetrameric Copper(II) complex and 8-Azaxanthine derived from 1,3-Dimethyl-5-(5'-methyl-3'-isoxazolyl-azo)-6aminouracil: Structures, Non-Covalent Interactions and Magnetism

The synthesis of uracil derivatives and their conversion to a number of potential compounds have drawn attention of researchers aiming to develop innovative materials. A novel azo ligand,1,3-dimethyl-5-(5'-methyl-3'-isoxazolyl-azo)-6-aminouracil (H2L, 1) was therefore synthesized from 1,3-dimethyl-6-aminouracil and 3-amino-5-methyl-isoxazole. The compound 1 was converted to an 8-azaxanthine derivative, 1,3-dimethyl-8-(5'-methyl-3'-isoxazolyl)azaxanthine (2), while its reaction with Cu(NO3)2.6H2O in DMF solution. 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 coordinated with 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 the compounds (1 – 3) were characterized using single crystal X-ray analysis. The complex (3) is a tetrameric assembly of Cu(II) with a nearly parallelogram shape. The Cu(II) centers are interconnected through doubly end-on azide (μ1,1-azide) bridges and doubly end-to-end azo (μ1,2-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 evidenced by the exchange coupling constant J1 = -250 (1) cm−1 through end-on azide bridging between the Cu(II) ions (Cu-N-Cu, 1160). Thus, the studies not only introduce a strong antiferromagnetic material, but also establishes a notable correlation between its structure and magnetic properties.