From symmetrical to unsymmetrical bimetallic nickel complexes bearing aryl-linked iminopyridines; synthesis, structures and ethylene polymerisation studies

Both symmetrical and unsymmetrical tetramethylphenyl-linked iminopyridines, 1,4-{(2-C5H4N)RCN}2-2,3,5,6-Me4C6 [R = H (L1a), Me (L1b)] and 1-{(2-C5H4N)HCN}-4-{(2-C5H4N)MeCN}-2,3,5,6-Me4C6 (L1c), have been prepared in good yield using straightforward condensation strategies. The molecular structures of L1a and L1c reveal the adjacent imino and pyridyl nitrogen atoms to adopt transoid configurations. Interaction of L1x with two equivalents of NiX2 [NiX2 = (DME)NiBr2 (DME = 1,2-dimethoxyethane), NiCl2] in n-BuOH at elevated temperature affords the paramagnetic bimetallic complexes, [(L1x)Ni2X4] [L1x = L1a, X = Br (1a); L1x = L1b, X = Br (1b); L1x = L1c, X = Br (1c); L1x = L1a, X = Cl (1d)] in moderate to good yield. Adduct formation results on treatment of bromide-containing 1a–1c with DMF (dimethylformamide) to yield dicationic [(L1x)Ni2Br2(DMF)6]Br2 [L1x = L1a (2a), L1b (2b), L1c (2c)], while with chloride-containing 1d the neutral species [(L1a)Ni2Cl4(DMF)4] (3) is obtained. Activation of 1a–1d and 2c with excess methylaluminoxane (MAO) generates active ethylene polymerisation catalysts (1b/MAO > 1c/MAO > 1a/MAO ∼ 1d/MAO > 2c/MAO) affording mixtures of waxes and low molecular weight solid polyethylene. Multinuclear NMR and GC analysis of the waxy components reveal methyl branched materials that contain mostly internal unsaturation along with low levels of α-olefins. Broad molecular weight distributions are observed for all the polymers obtained, with that from 1b/MAO leading to the highest molecular weight. Single crystal X-ray diffraction studies have been performed on L1a, L1c, 2a–2c and 3.

Graphical abstractBoth symmetrical and unsymmetrical aryl-linked iminopyridines have been prepared and their bimetallic nickel(II) halide complexes synthesised; crystallisation from DMF results in adduct formation (either as salts or neutral complexes). All dinickel systems on treatment with MAO are active for the polymerisation of ethylene affording low molecular weight branched polyethylene.Figure optionsDownload full-size imageDownload as PowerPoint slide