Synthesis, Characterization, Theoretical, and Antimicrobial Studies of Indenoquinoxalin-based Ligands and their reactions with CuBr(PPh3)3

Abstract 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(κS-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.