Palladium(II) complexes supported by PBP and POCOP pincer ligands: comparison of structure, property and catalytic activity
A Pd(II) chloride complex supported by a Yamashita-Nozaki PBP pincer ligand, [C6H4-1,2-(NCH2PtBu2)2B]PdCl (1a), was synthesized. The structure, property and catalytic activity of complex 1a were compared with those of the corresponding POCOP pincer complex [C6H3-2,6-(OPtBu2)2]PdCl (2a). It was found that the Pd centre in complex 1a is more electron rich and easier to be oxidized than that of complex 2a; complex 1a is a much better catalyst for Suzuki-Miyaura cross-coupling reactions compared with complex 2a. Starting from complexes 1a and 2a, two series of Pd(II) pincer complexes bearing a SH, BH4, NCS, NCSe or N3 covalent ligand, [C6H4-1,2-(NCH2PtBu2)2B]PdY (Y = SH, 1b; BH4; 1c; NCS, 1d; NCSe, 1e; N3, 1f) and [C6H3-2,6-(OPtBu2)2]PdY (Y = SH, 2b; BH4, 2c; NCS, 2d; NCSe, 2e; N3, 2f), were synthesized and fully characterized. Single crystal X-ray diffraction analysis indicated that the Pd centre is less tightly chelated in PBP pincer complexes. The strong σ-donor ability of the PBP pincer ligand has little influence on the structure of the covalent ligand possessing both σ-donor and π-acceptor properties. However, the stretching vibrational frequencies of NCS, NCSe and N3 ligands and the coordination mode of BH4 ligand are significantly different in these two type of palladium pincer complexes.