The P,N-type ligands 2-[(diphenylphosphino)methyl]pyridine ( 12 ), 2-[2-(diphenylphosphino)ethyl]pyridine ( 13 ), 2-methyloxy(diphenylphosphino)pyridine ( 14 ), 2-methyloxy(dibenzyl-1,2-oxaphosphorino)pyridine ( 15 ), and 2-methyloxy(di- tert -butylphosphino)pyridine ( 16 ) have been prepared in good yields, and 12 and 13 have been used to synthesize Ni(II) complexes of formula [Ni(P,N)Cl 2 ], 17 (P,N = 12 ) and 18 (P,N = 13 ), by reaction with NiCl 2 in methanol. The crystal structure of 18 has been determined by X-ray diffraction to be dinuclear with a distorted square-base pyramidal geometry around the Ni(II) centers. To examine the possible influence of the nature of the spacer link between the P and N donor atoms, we compared ligand 13, with a CH 2 CH 2 spacer, with 14 and 16, which have a isosteric CH 2 -O spacer. Reactions of the phosphinitopyridine ligands 14 and 16 and of the phosphonitopyridine 15 with [NiX 2 (DME)] (X = Cl or Br) afforded the complexes [Ni(P,N)Cl 2 ] 20 (X = Cl; P,N = 14 ), 21 (X = Br; P,N = 14 ), 22 (X = Cl; P,N = 16 ), and 23 (X = Cl; P,N = 15 ), respectively. The mononuclear structure of complex 22 has been established by X-ray diffraction and showed a distorted tetrahedral geometry around the metal center. Complexes 17, 18, and 20 − 22 have been tested as precatalysts in the oligomerization of ethylene, with AlEtCl 2 or MAO as cocatalyst, in order to evaluate the influence of the stereoelectronic properties of the phosphorus substituents. With only 6 equiv of AlEtCl 2 as cocatalyst and 4 × 10 −5 mol precatalyst, complex 18 was the most active, with turnover frequencies (TOF) up to 91 200 C 2 H 4 /(mol Ni·h), and 20 with 2 equiv of AlEtCl 2 showed the highest selectivities for ethylene dimers (up to 97%) and in 1-butene (up to 72%). When only 10 −5 mol precatalyst was used, the TOF values went up to 207 600 for 18 and 150 100 for 20 . With only 25 equiv of MAO as cocatalyst, complex 18 was again the most active, with TOF values up to 20 600 C 2 H 4 /(mol Ni·h). Despite the high selectivity for C 4 olefins of 17, 18, 20, and 21 (up to 93% for 20 ), 22 presented the best selectivities for 1-butene (up to 73%) with MAO as cocatalyst, and its high reactivity for the reinsertion of 1-butene resulted in 2-ethyl-1-butene being the main product of the catalytic reaction (up to 91%).
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Kermagoret et al. (2007) studied this question.