Microwave assisted efficient protocol for the classic Ullmann homocoupling reaction using Cu–Mg–Al hydrotalcite catalysts

• Inexpensive and enviro-friendly hydrotalcites used for Ullmann homocoupling reaction.
• The developed protocol does not require any additional base or reducing agent.
• High yields of biphenyl were obtained in short reaction time under microwave heating.
• Isolated and easily reducible Cu2+ species were responsible for catalytic activity.
• Catalysts can be reused at least three cycles without loss activity.

Four hydrotalcite catalysts containing different amounts of Cu and Mg cations, with varying atomic ratios of Cu:Mg:Al in the brucite-like layers of 0:2:1, 0.5:1.5:1, 1:1:1 and 2:0:1, were prepared by co-precipitation method. The synthesized catalysts were tested for classic Ullmann homocoupling of iodobenzene to produce biphenyl without using any stoichiometric amounts of base or reducing agent. The Mg–Al hydrotalcite catalyst without Cu ions is inactive, but the doping of Cu ions (Cu/Al = 0.5) into the Mg–Al hydrotalcite framework led to 63% conversion of iodobenzene under microwave irradiation. Gradual increase of Cu content and concomitant decrease of Mg content resulted in an increase in the conversion of iodobenzene and selectivity to biphenyl. The physico-chemical properties of hydrotalcite catalysts were studied by chemical analysis, XRD, FTIR, N2-physisorption, SEM, XPS, ESR and TPR techniques to understand the state of Cu ions presented in the hydrotalcite framework. It appears that presence of easily reducible isolated Cu2+ ions is essential to obtain improved performance of the catalyst.

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