Association complexes of Mg(II) and Co(II) with 1,3-dimethyl-5-(p-arsonic-phenylazo-)-6-aminouracil: Structures, non-covalent interactions, Solvatochromism and biological properties

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Single-crystal X-ray diffraction analysis of a new azouracil-arsonic compound, 1,3-dimethyl-5-( p -arsonic-phenylazo)-6-aminouracil (H 4 L, 1 ), revealed a nearly planar molecular structure, with a C4–N5–N4–C7 torsion angle of 177.5(3)°. Its monoanionic form generates association complexes with hydrated Mg(II), {[Mg(H 2 O) 6 ] 2+ [H 3 L] − 2 }.4H 2 O ( 2 ), and Co(II), {[Co(H 2 O) 6 ] 2+ [H 3 L] − 2 }.4H 2 O ( 3 ), as confirmed by single-crystal X-ray diffraction analysis. In 1 , the uracil-NH 2 group preferentially forms a six-membered intramolecular H-bond (HB) (N5—H3N, 2.019 Å), rather than a five-membered, involving the azo-N atom. Theoretical calculations further support this preference, showing that the six-membered H-bonded ring is energetically more stable than the five-membered counterpart, with stabilization energy differences of 24 kJ mol⁻¹ for the neutral form and 25 kJ mol⁻¹ for the anionic form. Non-covalent interaction (NCI) analysis further indicates weak-to-moderate intermolecular interactions between the O-H groups of the arsonic acid moiety (-AsO(OH) 2 ) and the adjacent carbonyl (–C=O) group of the uracil unit. In contrast, the crystal structures of 2 and 3 exhibit strong hydrogen bonding interactions between the arsonic acid groups and the coordinated water molecules of the hydrated metal cations, with calculated stabilization energies of 860 and 1076 kJ mol −1 , respectively. The compounds exhibit solvent-dependent spectral shifts that correlate with the hydrogen-bond donor (HBD) and hydrogen-bond acceptor (HBA) properties of the solvent, as validated by the Kamlet–Taft solvatochromic model. The pKₐ value of 1 was determined to be 5.03. The 1 and 3 exhibit antibacterial activity against Staphylococcus aureus (Gram-positive), Mycobacterium smegmatis (acid-fast), and Escherichia coli (Gram-negative), whereas, this activity is significantly diminished for the 2 . Compound 1 exhibits moderate antioxidant activity, showing 43.75% inhibition at 0.444 mM. Fluorescence quenching studies of BSA–compound interactions yielded Stern-Volmer constants (K SV ) of 3.044×10 4 M −1 for 1 , 3.733×10 4 M −1 for 2 and 6.825×10 4 M −1 for 3 . The corresponding binding constants are 1.398×10 6 M −1 (n = 1.38) for 1 , 6.967×10 3 M −1 (n = 0.85) for 2 , and 5.316×10 5 M −1 (n = 1.2) for 3 .