Synthesis, spectroscopic characterizations, biological activity, DNA-binding investigation combined with DFT studies of new proton-transfer complexes of 2,4-diaminopyrimidine with 2,6-dichloro-4-nitrophenol and 3,5-dinitrosalicylic acid

We investigate spectroscopically and theoretically proton-transfer (PT) complexes associated with hydrogen bonding formed between 2,6-dichloro-4-nitrophenol (DCNP) or 3,5-dinitrosalicylic acid (DNS) as proton donors and 2,4-diaminopyrimidine (DAPY) as proton acceptor in solution (EtOH and CH3CN) and also both complexes were isolated in the solid-state. The stoichiometry of the titled complexes is determined by using Job’s and photometric titration methods to be 1:1 in both solvents. The formation constant and molar extinction coefficient are determined by applying the modified (1:1) Benesi-Hildebrand equation. Interestingly, the DAPY-DCNP complex is more stable than the DAPY-DNS complex in the studied solvents. The physical and thermodynamic parameters of the formed PT complexes are also determined. The solid PT complexes were synthesized and analyzed using different analytical methods, including elemental analysis, Fourier transforms infrared, nuclear magnetic resonance and mass spectroscopies, as well as powder X-ray diffraction. The donor-acceptor interactions were visualized based on N+H⋯O− type bonding. Free DAPY and its PT complexes have excellent antimicrobial activity against various bacteria and fungi. The calf-thymus–DNA binding and molecular docking with the formed complexes were investigated. The structures of the DAPY-DCNP and DAPY-DNS complexes are calculated by using density functional theory at the B3LYP level with 6-31+G(d,p) basis sets. The electronic properties and the ultraviolet–visible spectra of DAPY-DCNP and DAPY-DNS are also discussed. The longest-wavelength electronic transition in DAPY-DCNP is predicted to be an internal electronic transition, whereas, in the DAPY-DNS system, it is a charge-transfer-based transition.