Palladium complexes with simple iminopyridines as catalysts for polyketone synthesis

Abstract

Four iminopyridines (N–N′) differing in the nature of the substituents on the iminic carbon and on the ortho positions of the aryl ring (H or CH₃) on the iminic nitrogen were used for the synthesis of neutral and monocationic palladium(II) complexes of general formulae [Pd(CH₃)Cl(N–N′)] and [Pd(CH₃)(NCCH₃)(N–N′)][PF₆]. The detailed NMR characterization in solution highlighted that: (i) for both series of complexes, the Pd–CH₃ signal is progressively shifted to a lower frequency on increasing the number of methyl groups on the ligand skeleton; (ii) for the neutral derivatives, the chemical shift of the ¹⁵N NMR signals, determined through {¹H,¹⁵N}-HMBC spectra, is significantly affected by the coordination to palladium; (iii) the coordination induced shift (CIS) of the nitrogen atom trans to the CH₃ ligand is smaller than the other. The structure in the solid state for the neutral derivatives with all the four ligands was solved, pointing out that: (iv) the Pd–C bond distance increases with the basicity of the nitrogen-donor ligand; (v) the Pd–N bond distance correlates well with the CIS value. The combining of the solution and solid state structural features allows stating that: (vi) the Pd–CH₃ singlet is a good probe for the electron donor capability of the ligand; (vii) the CIS value might be used as a probe for the strength of the Pd–N bond. All monocationic complexes generated active catalysts for the CO/vinyl arene copolymerization, leading to prevailingly syndiotactic polyketones. The catalyst performances, both in terms of catalyst productivity and polymer molecular weight, correlate well with the precatalyst structural features.