Highly preorganized pyrazolate-bridged palladium(ii) and nickel(ii) complexes in bimetallic norbornenepolymerization

Abstract

New derivatives of pyrazolate-based binucleating ligands HL with appended imine functions have been synthesized to provide a versatile set of ligand systems with different backbone substituents both at the pyrazole-C⁴ and the imine-C (H, Me, Ph). These scaffolds have two adjacent coordination compartments akin to the α-diimine type. A series of binuclear palladium(II) complexes [LPd₂Cl₃] (1–4) and tetranuclear nickel(II) complexes [L₂Ni₄Br₆(solvent)₄] (5, 6) of the various ligands have been prepared and characterized, including X-ray structural analyses for two representative Pd and the two Ni complexes. Complexes 5 and 6 were found to contain an unusual central μ₄-bromide. Mononuclear nickel(II) complexes [L₂Ni] were detected as intermediates in the formation of the tetranuclear complexes and have been characterized by X-ray analyses in two cases (7, 8). The interconversion between 5′ and 7 has been investigated by UV/Vis spectroscopy and ESI mass spectrometry, and magnetic coupling in the [L₂Ni₄Br₆(solvent)₄] complexes has been studied (SQUID). Trans-coupling via the central μ₄-bromide is suggested to mediate significant antiferromagnetic interaction. The reactivity of such types of Pd and Ni complexes has been tested for the vinyl/addition polymerization of norbornene. In the presence of an excess of cocatalyst methylaluminoxane (MAO) the palladium complexes show high activity up to 5.9 × 10⁶ g_PNB mol_Pd⁻¹ h⁻¹ at 20 °C, while activities of the nickel systems are much lower, but strongly solvent dependent. Detailed studies on the dependence of activity on polymerization conditions such as molar ratios of catalyst and cocatalyst, temperature, reaction time and solvent were carried out. All obtained polynorbornenes (PNB) were noncrystalline and insoluble, but have high glass transition temperatures (T_g). Microstructures were analyzed by IR spectroscopy and solid state (CP/MAS) ¹³C NMR, revealing distinct patterns for the PNB produced by Ni- or Pd-catalysts. Structure/activity correlations deduced for the complexes with different ligand systems suggest that activities and polymer microstructures depend rather on the metal type than on ligand intricacies.