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Water-Soluble Nickel Catalysts:
From 2-Pyridylphosphine to “Ligand Free” Species
Matthew D. Le Page and Brian R. James*
Department of Chemistry, University of British Columbia, Vancouver,
British Columbia, V6T 1Z1, Canada
Interest remains high in development of homogeneous catalysis systems that are effective in
aqueous solution. Contributions from this laboratory have focussed on the use of the 2-
pyridylphosphine ligands PPh3-xpyx (or PNx, where x is the number of 2-pyridyl groups),
py2P(CH2)2Ppy2 (1, the analogue of diphos), and py2P(C5H8)Ppy2 (2, where C5H8 is a
cyclopentane backbone).1 Diamagnetic nickel complexes synthesized include the purely Pcoordinated
species: NiX2(P-P), Ni(CO)2(P-P), Ni(CO)2(PNx)2, Ni(PNx)4, Ni(P-P)2, and
Ni(P-P)2(PNx)2, where X = a monoanion, and P-P = 1 or 2. Isolated, paramagnetic
complexes include the P-coordinated NiX2(PNx)2, the P,N-coordinated NiCl2(PN3), and the
N,N’,N”-coordinated [Ni(PN3)2]Cl2. In aqueous media, species such as Ni(H2O)2(P-P)2+,
Ni(H2O)2(PNx)2
2+, and Ni(CO)(2H-P-P)2+ are formed, where 2H-P-P means diprotonation at
the N-atoms.
Representative complexes from those listed above have been tested in aqueous solution for
catalytic activity toward olefin hydration and the Water-Gas Shift Reaction (WGSR). The
only notable activity was that of NiCl2(PN2)2 for the WGSR, in which a turn-over frequency
for H2/CO2 production was ~30 h-1 at 100oC and 40 atm CO, in the range commonly
observed for homogeneous WGSR catalysts, including that noted for NiCl2(PMe3)2 in basic,
aqueous EtOH.2
During these studies, a report appeared on the use of a NiCl2(PPh3)2/NaOH/iPrOH system
for transfer hydrogenation of ketones and aldehydes.3 Our water-soluble Ni(II)
pyridylphosphines were of comparable activity, but some "blank tests" soon revealed that
the simple salts NiBr2 and NiI2 had much higher activity!
For example, 90% conversion of
cyclohexanone to cyclohexanol was attained after 1 h of refluxing in an alkaline iPrOH
solution containing NiBr2, whereas conversions of only 14 and 24% respectively, were
achieved with NiCl2(PPh3)2 and NiBr2(PPh3)2, respectively, under comparable conditions;
optimization of reaction conditions (~1.5 M substrate, 5 mM NiBr2, 0.5 M NaOH) resulted
in 100% conversion after 30 min refluxing. The simplicity of this system, which uses a
commercially available, inexpensive Ni(II) salt, makes it attractive for laboratory
hydrogenations without the need for H2.4 Of note, NiBr2(PPh3)2 in the alkaline medium
dissociates the PPh3 ligands, implying that in the reported work with the NiCl2(PPh3)2
system, the precursor catalyst may be simply NiCl2.
Studies with other acceptor substrates show that the NiBr2/NaOH/iPrOH system is effective
for catalytic transfer hydrogenation of other saturated ketones (97-100% in 24 - 48 h),
alkenes (e.g. n-octene to octane, 99% in 30 min), alpha, beta-unsaturated ketones (e.g. 2-
cyclohexen-1-one to cyclohexanol (71%) and cyclohexanone (2%) over 48 h), nitrobenzene
to exclusively aniline (19%, 24 h), and 4-nitro-benzaldehyde to a mixture of reduced
products (38%, 24 h).
[NiBr2, NaOH]
Acceptor (A) + Me2CH(OH)----->A(H)2 + Me2CO
reflux
No hydrogenation was observed for internal olefins such as trans-2-octene, and cyclooctene.
Kinetic studies on the hydrogenation of cyclohexanone, which appears to be a homogeneous
catalyzed system as judged by a mercury-addition test, reveal the following kinetic
dependences: 1st- to fractional-order on [NiBr2] with increasing concentration, 2nd order on
[NaOH], 1st-order on ketone (up to ~1.5 M), and 1st-order on iPrOH (up to ~2.0 M). Of
several plausible mechanisms, one involving hydride transfer from coordinated ispropoxide
to coordinated cyclohexanone, with a subsequent protonation to release the cyclohexanol,
satisfies the kinetics if the coordinated isopropoxide is associated within a cluster form of a
“Ni(OiPr)2” monomer. Spectroscopic data are being sought to support such a mechanism.
We thank NSERC of Canada for financial support.
References
1. N. Jones, K. S. MacFarlane, M. B. Smith, R. P. Schutte, S. J. Rettig and B. R. James,
Inorg.
Chem., 38 (1999) 3956.
2. P. Giannoccaro, E. Pannacciulli and G. Vasapollo, Inorg. Chim. Acta, 96 (1985) 179; R.
M.
Laine and E. J. Crawford, J. Mol. Catal., 44 (1988) 357.
3. S. Iyer and J. P. Varghese, J. Chem. Soc., Chem. Commun., (1995) 465.
4. M. D. Le Page and B. R. James, J. Chem. Soc., Chem. Commun., (2000) 1647.
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