Mono- and dinuclear nickel complexes with phosphino-, phosphinito-, and phosphonitopyridine ligands: Synthesis, structures, and catalytic oligomerization of ethylene

The P,N-type ligands 2-[(diphenylphosphino)methyllpyridine (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 NiCl2 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 CH2CH2,, spacer, with 14 and 16, which have a isosteric CH2-O spacer. Reactions of the phosphinitopyridine ligands 14 and 16 and of the phosphonitopyridine 15 with [NiX2(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 oligornerization of ethylene, with AlEtCl2 or MAO as cocatalyst, in order to evaluate the influence of the stereoelectronic properties of the phosphorus substituents. With only 6 equiv of AlEtCl2, as cocatalyst and 4 x 10(-5) Mol precatalyst, complex 18 was the most active, with turnover frequencies (TOF) up to 91 200 C2H4/(mol Ni center dot h), and 20 with 2 equiv of AlEtCl2 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 C2H4/(mol Ni center dot 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%).