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The synthesis, characterization, and reactivity of nickel 2-pyridylphosphine complexes Le Page, Matthew Derek

Abstract

Numerous nickel(0) and nickel(II) 2-pyridylphosphine complexes were synthesized as they are potentially water-soluble, and may be of use in catalysis (e.g., in olefin hydration). Four different coordination modes of 2-pyridylphosphines were observed: coordination through the phosphorus only (P), through the phosphorus and one pyridyl group in a chelating fashion (P,N), through the phosphorus and one pyridyl group in a bridging fashion (µ-P,N), and through all three pyridyl groups in a chelating fashion (N,N',N"). Isolated complexes were characterized in general by NMR, IR and UV-Vis spectroscopies, mass spectrometry, elemental analysis, conductivity, magnetic susceptibility, and melting point, while seven complexes and one compound (used as a ligand) were also characterized by X-ray crystallography. Complexes synthesized in situ were characterized by 31P{1H} NMR. The water-soluble, diamagnetic, P-coordinated NiX2(P-P) complexes 1a-5a, 1b-5b (X = Cl (1), Br (2), I (3), NCS (4), NO3 (5); P-P = dpype (l,2-bis[bis(2-pyridyl)phosphino]ethane) (a) and dpypcp (l,2-bis[bis(2-pyridyl)phosphino]cyclopentane) (b)) and Ni(CO)2(P-P) (20a-b) were obtained by ligand substitution of NiX2(PPh3)2 or Ni(CO)2(PPh3)2 with P-P; X-ray crystal structures were obtained for five of the Ni(II) and one of the Ni(0) species. In aqueous media, the Ni(II) species form [Ni(H2O)2(P-P)]2+, and the Ni(0) species form N-atom protonated [Ni(CO)2(2H-P-P)]2+, as shown by comparison of in situ NMR data to those of the isolated species [Ni(H20)2(P-P)](PF6)2 (6a-b) and [Ni(CO)2(2H-P-P)](PF6)2. The novel [Ni(H2O)2- (dppe)](PF6)2 (13) (dppe = Ph2P(CH2)2PPh2) was also prepared. The known, water-soluble, paramagnetic, NiX2(PNx)2 species 9a-9c, 1Oa-1Oc (X = Cl (9), Br (10); PNX = PPh3-xpyx, where x = 1 (a), 2 (b), 3 (c), and py = 2-pyridyl) were briefly examined and proposed to form either [Ni(H2O)2(PNx)2]2+ or NiX2(nH-PNx)2n+ in water. Factors affecting the in situ yields of the known Ni(CO)2(PNx)2 species (19a-c), prepared via ligand substitution of Ni(CO)2(PPh3)2 by PNX, were examined. The known, tetrahedral, P,N-coordinated complex NiCl2(PN3) (7), uniquely prepared from Ni(CO)2(PPh3)2 in CH2Cl2 via a photolytically-induced chlorine-abstraction process, converted over 15 d to the octahedral, water-soluble, N,N',N"-coordinated complex [Ni(PN3)2]Cl2 (8) whose X-ray crystal structure was obtained. A species tentatively characterized as the P,N-coordinated dimer [Ni2(CO)(µ-PN2)2]Cl4|(19d) was prepared from Ni(CO)2(PPh3)2 and PN2 in CH2Cl2. The air-sensitive, P-coordinated, tertiary Ni(0) phosphine complexes Ni(PR3)2(dppe) (21a-d), Ni(PR3)2(dpype) (22a-d), Ni(PR3)2(dpypcp) (23a-d) (PR3 = PPh3 (a), PN1 (b), PN2 (c), PN3 (d)), Ni(P-P)2 (P-P = dpypcp (24a), dppe (24b), dpype (24c)), Ni(P-P)(P'-P') (P-P/P'-P' = dppe/dpype (25a), dppe/dpypcp (25b), dpype/dpypcp (25c)), and the known Ni(PR3)4 species (PR3 = PPh3 (26a), PN1 (26b), PN2 (26c), PN3 (26d)) were prepared from Ni(1,5-COD)2. Reactions of Ni(PN3)4 and Ni(dpypcp)2 with C2H4, H2O, O2, or Cl2 were not delineated, but reaction of several of the Ni(0) complexes with CH3I yielded the trans-iodo(methyl) species. The monomethyl phosphonium salts [(CH3)(PN3)]I, [(CH3)(dppe)]I, [(CH3)(dpype)]I, [(CH3)(dpypcp)]I, and [(CH3)(PPh3)]I, and the bismethyl phosphonium salts [(CH3)2(dppe)]I2, [(CH3)2(dpype)]I2, and [(CH3)2(dpypcp)]I2 were also prepared as some of these species were observed in situ in the formation of the Ni(II) trans-iodo(methyl) complexes. NiCl2(dppe), Ni(CO)2(PPh3)2, and complexes 1a, 2b, 7, 8, 10b, and 20a-b were not effective as catalysts for hydration of maleic acid in aqueous media, but some catalytic isomerization to fumaric acid (up to 11%) occurred after 48 h at 100°C. NiCl2(PN2)2 was an effective catalyst for the water-gas-shift reaction (WGSR) in aqueous, alkaline EtOH, with a turnover frequency for H2/CO2 production of 27 h-1 obtained at 100°C with 40 atm CO, in the range reported for typical homogeneous WGSR catalysts. NiCl2(PPh3)2, NiBr2(PPh3)2, Nil2(dppe), NiCl2 • 6H2O, anhyd. NiBr2, Nil2 • 6H2O, 3a-b, 10a, and 10c were effective precursor catalysts for the transfer hydrogenation of cyclohexanone in refluxing, alkaline i-PrOH, with the activity of NiBr2 and Nil2 • 6H2O being ~ 3 and ~ 5 times that of the 2-pyridylphosphine and phenylphosphine complexes, respectively. Optimization of reaction conditions (~ 1.5 M cyclohexanone, ~ 5 mM NiBr2, ~ 0.5 M NaOH) resulted in 100% conversion to cyclohexanol after 30 min refluxing. Kinetic dependences on NiBr2 , NaOH, cyclohexanone, and i-PrOH were determined, as well as the effect of various additives (acetone, cyclohexanol, water, H2 , halides, and PPh3); discussion of a plausible mechanism is presented. The catalyst was shown to be homogeneous, with activity decreasing over four cycles. Transfer hydrogenation of 1-octene, 2-butanone, and 2-pentanone by NiBr2 proceeded at useful rates (i.e., ~ 99% conversion after 1/2 to 48 h), while hydrogenation of cyclohex-2-ene-1-one, nitrobenzene, and 4-nitrobenzaldehyde was also observed. The hydrogenation of acetophenone and 1-heptanal proceeded at rates similar to the Ni-free, "base-only" system. Experiments with 2,4-pentanedione and 2,5-hexanedione yielded sodium acetylacetonate and an aldol condensation product, respectively. No hydrogenation was observed for trans-2-octene, cyclooctene, 1,5- cyclooctadiene, benzene, acetonitrile, benzonitrile, propionic acid or 3-buten-2-one; selective hydrogenation of 1-octene over trans-octene in a 1:1 mixture of the two was noted. [Scientific formulae used in this abstract could not be reproduced.]

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