Ni(II)-Complex Anchored onto Magnetically Separable Oxidized Single-Walled Carbon Nanohorn: A DFT-Supported Mechanistic Approach for Hydrogen-Borrowing Quinoxaline Synthesis

This work delivers a green, sustainable strategic provision for producing quinoxaline derivatives of pharmacological importance using a nickel-based functionalized single-walled carbon nanohorn catalyst (FNHDNi). Initially, two structurally characterized Ni-complexes (1 and 2) derived from two different nitro and bromo ligands (HL1 and HL2) are used as novel homogeneous catalysts for the production of quinoxaline derivatives. The involvement of the catalyst (complexes 1 and 2) in hydrogen-borrowing dehydrogenative coupling reaction between substituted diols and phenyl diamine generating 2,3-dimethylquinoxaline has been outlined in a probable mechanism and supported by density functional theory (DFT) calculations. Intriguingly, this catalyst provides a redox noninnocent ligand arm for storing the trapped hydrogen, avoiding metal hydride formation, and easily reverts back into the active species under experimental conditions. Afterward, to impart sustainability at a higher temperature and enhance the surface area and separability of the catalyst, the most potent thermal resilient complex 1 is modified to a heterogeneous catalyst by immobilizing onto a NiFe2O4-decorated single-walled carbon nanohorn via a linker l-dopa and named as FNHDNi. To ensure the amalgamation of the active catalyst with other heterogeneous components, an extensive characterization of the catalyst has been carried out using TEM, FT-IR analysis, powder XRD, magnetization, and TGA studies. A satisfactory yield in a shorter span of reaction time gained by using the FNHDNi catalyst ostensibly provides important insights into the synthesis of quinoxaline derivatives.