Palladium(ii) complexes supported by PBP and POCOP pincer ligands: a comparison of their structure, properties and catalytic activity

Abstract

A Pd(II) chloride complex supported by a Yamashita–Nozaki PBP pincer ligand, [C₆H₄-1,2-(NCH₂P^tBu₂)₂B]PdCl (1a), was synthesized. The structure, properties and catalytic activity of complex 1a were compared with those of the corresponding POCOP pincer complex [C₆H₃-2,6-(OP^tBu₂)₂]PdCl (2a). It was found that the Pd centre in complex 1a is more electron rich and easier to be oxidized than that in complex 2a; complex 1a is a much better catalyst for Suzuki–Miyaura cross-coupling reactions than complex 2a. Starting from complexes 1a and 2a, two series of Pd(II) pincer complexes bearing a SH, BH₄, CS, CSe or N₃ covalent ligand, [C₆H₄-1,2-(NCH₂P^tBu₂)₂B]PdY (Y = SH, 1b; BH₄; 1c; CS, 1d; CSe, 1e; and N₃, 1f) and [C₆H₃-2,6-(OP^tBu₂)₂]PdY (Y = SH, 2b; BH₄, 2c; CS, 2d; CSe, 2e; and 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 CS, CSe and N₃ ligands and the coordination mode of the BH₄ ligand are significantly different in these two types of palladium pincer complexes.