ZnO–Co₃O₄ nanocomposite as efficient heterogeneous catalyst for the ultrasound-assisted facile multicomponent synthesis of benzo[g]chromene derivatives

A highly remarkable characteristic of ZnO–Co₃O₄ nanocomposite (NC) has been explored as a heterogeneous catalyst for synthesizing organic heterocyclic compounds. An array of benzo[g]chromene (BgC) derivatives have been synthesized using ZnO–Co₃O₄ NC via ultrasound-assisted multicomponent reaction of various aromatic aldehydes, malononitrile, and 2-hydroxy-1,4-naphthoquinone in ethanol as solvent at room temperature. The NC was easy to synthesize using a simple chemical co-precipitation method, which exhibited brilliant catalytic activity with excellent product yields and reusability for up to six runs. These are small particles compared to ZnO NPs due to the surface modification by the Co⁺² and Co⁺³ ions, thus giving a large surface area (23.979 m²/g), making it an efficient catalyst. They have a substantially high negative zeta potential value (−35.03 mV) arising from the surface charges of the catalyst, which are directly related to the catalytic phenomenon, thus behaving as a Lewis base. Therefore, the greenest reaction condition has been realized for the synthesis of catalyst as well as BgCs due to its environmental friendliness, high efficiency, cost-effectiveness, and ease of operation.

A facile protocol is demonstrated using ZnO–Co₃O₄ NC as an effective heterogeneous catalyst for the one-pot multicomponent synthesis of benzo[g]chromenes (BgCs) from the reaction of aromatic aldehydes 1, malononitrile 2, and 2-hydroxy-1,4-naphthoquinone 3 in ethanol under ultrasonication producing excellent yield in short reaction time (15 min). ZnO–Co₃O₄ NC is synthesized via the chemical co-precipitation method.