Green synthesis of graphene oxide (GO)-anchored Pd/Cu bimetallic nanoparticles using Ocimum sanctum as bio-reductant: an efficient heterogeneous catalyst for the Sonogashira cross-coupling reaction.

To explore the synergism between two metal centers we have synthesized graphene oxide (GO) supported Pd/Cu@GO, Pd@GO and Cu@GO nanoparticles through bio-reduction of Pd(NO3)2 and CuSO4·5H2O using Tulsi (Ocimum sanctum) leaf extract as the reducing and stabilizing agent. The graphene oxide (GO) was obtained by oxidation of graphite following a simplified Hummer's method. The as-prepared nanomaterials have been extensively characterized by FTIR, powder X-ray diffraction (PXRD), HRTEM, TEM-EDS, XPS, ICP-AES and BET surface area measurement techniques. The morphological study of Pd/Cu@GO revealed that crystalline bimetallic alloy type particles were dispersed on the GO layer. The activity of Pd@GO, Cu@GO and Pd/Cu@GO as catalysts for the Sonogashira cross-coupling reaction have been investigated and it was found that the Pd/Cu@GO nanostructure showed highly superior catalytic activity over its monometallic counterparts, substantiating the cooperative influence of the two metals. The inter-atom Pd/Cu transmetalation between surfaces was thought to be responsible for its synergistic activity. The catalyst showed higher selectivity towards coupling of aryl iodides with both aliphatic and aryl alkynes resulting in moderate to excellent isolated yield of the desired products (45-99%). The products have been characterized by GC-MS and 1H-NMR spectroscopic techniques and compared with authentic samples. The Pd/Cu@GO catalyst could be easily isolated from the reaction products and reused for up to at least ten successive runs effectively.

An immobilized symmetrical bis-(NHC) palladium complex as a highly efficient and recyclable Suzuki-Miyaura catalyst in aerobic aqueous media.

A bis-N-heterocyclic carbene (NHC) functionalized with chloropropyltriethoxysilane over silica was used to synthesize a well-defined immobilized palladium complex. This complex is a suitable and easily retrievable catalyst for Suzuki-Miyaura cross-coupling reactions under mild aqueous conditions. Excellent yields and conversions were obtained with low palladium loadings (down to 0.03 mol% Pd). The catalyst can be reused up to six cycles without the loss of its activity. The catalytic role of the NHC complex of palladium during coupling between bromobenzene and phenylboronic acid was investigated theoretically. The oxidative addition step is predicted to be endothermic in agreement with the experimental conditions.