N-heterocyclic carbene-palladium complex immobilized on Co-MOF 74 microrods as a highly selective catalyst for Suzuki–Miyaura cross-coupling

A new reusable, heterogeneous catalyst for Suzuki–Miyaura cross-coupling (SMC) was designed. For the synthesis of catalyst carrying Pd(II)-NHC complex as the active site, Co(II)-2,5-dihydroxyterephthalic acid based metal–organic framework microrods (Co-MOF 74 MRs) was selected as the support. Highly porous Co-MOF 74 microrods with a surface area of 407.6 m²/g was obtained by a hydrothermal protocol conducted at 100 °C, using Co(NO₃)₂⋅6H₂O and 2,5-dihydroxyterephthalic acid (DHTPA) as the metal salt and the organic linker with two hydroxyl functionalities. Co-MOF 74 microrods was particularly selected as the support since two hydroxyl functionalities allowed the generation of Pd(II)-N-heterocyclic carbene complex (Pd(II)-NHC) on their surface via a facile chemical route started by silanization. Hence, Pd(II)-NHC complex was obtained on Co-MOF 74 MRs via silanization of Co-MOF-74 using 3-chloropropyltrimethoxysilane, quaternization of bound chlorine moiety with 1-methylimidazole and the reaction of Pd(II) salt with the quaternary ammonium functionality obtained at the last stage. The effects of cross-coupling conditions such as Pd(II) loading, mass of catalyst, temperature, base type, and type of substituent on arylbenzene or phenylboronic acid on biphenyl (BP) formation yield were investigated with the synthesized catalyst. BP formation yields ranging between 90 and 100% w/w, indicating high selectivity in SMC reaction were achieved in most of the entries performed with different combinations of experimental conditions using Pd(II)-NHC@Co-MOF 74 as the heterogeneous catalyst. The cross-coupling reaction rates calculated at different reaction times demonstrated that the rate of obtained using Pd(II)-NHC@Co-MOF 74 as the catalyst was high, particularly in the first 10 min and the reaction largely completed in one hour with high biphenyl formation yields when the catalyst amount was satisfactorily high. No significant decrease was observed in BP formation yield over six consecutive catalytic runs. The selected MOF based support with multiple hydroxyl functionality and the silanization route proposed for generation of active site is also a promising configuration that can be evaluated for the synthesis of new heterogeneous catalysts for various organic reactions including SMC.