EPR study of the oxidation reaction of nickel(0) phosphine complexes with Lewis and Brønsted acids

Syntheses and properties of molecular nickel(II) hydride, methyl, and nickel(I) complexes supported by trimethylphosphane and (2-diphenylphosphanyl)thiophenolato and -naphtholato ligands

“…A similar disproportionation of the nickel hydride(II) NiH(CH 3-COO)(PPh 3 ) 2 giving a nickel(I) complex is known for the system Ni(PPh 3 ) 4 /CH 3 COOH [8]. Despite a great number of publications devoted to nickel complex catalysts, the question of the structure of the active species remains open.…”

“…Sparingly soluble 6 and 7 were obtained in a similar way (Eqs. (8) and (9)). The orange crystals of 4, the red needles of 5, the red microcrystals of 6, and orange prismatic crystals are stable in air for at least two weeks, as after that time characteristic infrared bands d(NiCH 3 ) at 1184 cm À1 (4), at 1158 cm À1 (5), at 1157 cm À1 (6), and at 1219 cm À1 (7) and in the 1 H NMR at 20°C doublets for NiCH 3 groups (4: -0.37 ppm, 3 J(PH) = 8 Hz; 5: -0.33 ppm, 3 J(PH) = 9 Hz; The present studies allow a conclusion of the most reasonable ways to the alkyl and hydride nickel(II) complexes with thiophenolato and thionaphtholato ligands.…”

“…When the temperature was carefully controlled dihydrogen began evolving at À10°C and a green solid of Ni(P^pS) 2 precipitated. Filtering a decomposed solution of 1 affords microcrystalline Ni(P^pS) 2 and a red solution containing free trimethylphosphine that on cooling yields red crystals of the composition Ni(P^pS)(PMe 3 ) 2 (8). These are air-sensitive and under argon decompose above -10°C.…”

“…These assumptions are based on the classic works of Tolman [16,17] and others concerned with NMR identification of Ni(II) hydride model complexes [3][4][5][6]. On the other hand, according to the EPR data, the process of formation of the nickel complex catalysts includes the generation and stabilization of Ni(I) complexes, independent of the metal valence state in the initial compound [8][9][10][11][12].…”

“…Others have considered the mechanism of how cationic nickel complexes arise from Ni(PPh 3 ) 4 and boron trifluoride etherate [8]. Symproportionation of phosphine complexes of Ni(0) and Ni(II) is regarded to be the key step.…”