Microwave-Assisted C-C and C-S Couplings Catalysed by Organometallic Pd-SCS or Coordination Ni-SNS Pincer Complexes

A family of SCS and SNS pincer compounds of the type [PdCl{C6H3-2,6-(CH2SR)(2)}] {R = tBu (3a), sBu (3b), iBu (3c)} and [NiCl2{C5H3N-2,6-(CH2SR)(2)}] {R = tBu (4a), sBu (4b)} have been prepared. Among these, complexes 3b, 3c, 4a-4c are reported for the first time. Dimeric compounds such as [NiCl{C5H3N-2,6-(CH2SiBu)(2)}mu-Cl](2) (4c) were found in the solid state for the nickel complexes with lower steric hindrance exhibiting octahedral metal centers, whereas other nickel structures such as [NiCl2{C5H3N-2,6-(CH(2)StBu)(2)}-(iPrOH)] (4d) could also expand their coordination number by coordinating to solvents. The single crystal X-ray diffraction results for 4a, 4c and 4d are presented. The catalytic activity of the six compounds was studied in C-C and C-S cross-coupling reactions under conventional heating and under microwave irradiation conditions. The palladium catalysts enabled good to excellent conversions in Suzuki-Miyaura couplings of p-substituted halobenzenes with phenylboronic acid. Comparable yields resulted from application of the nickel complexes in the thioetherification of iodobenzene with different disulfides. The fast increase in reaction temperature associated with microwave irradiation, in combination with the robust pincer catalysts, allowed for quantitative conversions in only minutes.