Synthesis, characterization, theoretical, and antimicrobial studies of indenoquinoxalin-based ligands and their reactions with CuBr(PPh3)3

A series of three ligands were derived from the condensation of indeno[1,2-b]quinoxalin-11-one and hydrazines (L1 from hydrazine, L2 from phenylhydrazine and L3 from thiosemicarbazide). The three ligands were stirred with CuBr(PPh3)(3) in dichloromethane; no reaction was observed except in the case of L3, providing a complex with the formula CuBr(PPh3)2(kappa(S)-L3) (Cu-L3). Crystal structure of the complex revealed that the central copper atom in the molecule is coordinated to two triphenylphosphine groups, a bromine atom and to the indeno[1,2-b]quinoxalin-11-ylidenecarbothioamidohydrazide (L3) ligand via S atom; forming a distorted tetrahedral geometry around it. In general, Cu-L3 complex has shorter Cu-P bond lengths compared to the parent complex which suggest less steric hindrance in Cu-L3. None of the three ligands were able to use the nitrogen atoms as donating atoms due to their highly engagement in hydrogen-bonding as observed in the crystal structure of Cu-L3 as well as the theoretical calculations. Moreover, theoretical calculation indicated that Cu-N interactions were weak with long distances beyond the typical bonding distances. Antimicrobial screening against a range of strains showed selective inhabitations of indeno[1,2-b]quinoxalin-11-one, L1 and L3 against S. aureus strain. Moreover, the introduction of the copper fragment to L3 did not cause any significant improvements to the antimicrobial properties of L3. (C) 2021 Elsevier B.V. All rights reserved.