Selective hydrogenation of the C{double bond, long}O bond of ketones using Ni(0) complexes with a chelating bisphosphine

Article abstract of DOI:10.1016/j.molcata.2009.05.026

The nickel complexes [(dippe)Ni(η²-O,C-benzophenone)] (2), [(dippe)Ni(η²-O,C-4-methylbenzophenone)] (3), [(dippe)Ni(η²-O,C-acetophenone)] (4), [(dippe)Ni(η²-O,C-acetone)] (5), [(dippe)Ni(η²-O,C-fluore

Full text of DOI:10.1016/j.molcata.2009.05.026

Journal of Molecular Catalysis A: Chemical 309 (2009) 1–11
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Editor choice paper
Selective hydrogenation of the C O bond of ketones using Ni(0) complexes with a
chelating bisphosphine
Areli Flores-Gaspara, Paulina Pinedo-Gonzáleza, Marco G. Crestania, Miguel Mun˜oz-Hernándezb,
David Morales-Moralesc, Brian A. Warsopd, William D. Jonesd, Juventino J. Garcíaa,∗
a Facultad de Química, Universidad Nacional Autónoma de México, México, D.F. 04510, Mexico
b Centro de Investigaciones Químicas, Universidad Autónoma del Estado de Morelos, Cuernavaca, Morelos 62210, Mexico
c Instituto de Química, Universidad Nacional Autónoma de México, México, D.F. 04510, Mexico
d Department of Chemistry, University of Rochester, Rochester, NY, 14627, USA
a r t i c l e i n f o
a b s t r a c t
The nickel complexes [(dippe)Ni(␩2-O,C-benzophenone)] (2), [(dippe)Ni(␩2-O,C-4-methylben-
zophenone)] (3), [(dippe)Ni(␩2-O,C-acetophenone)] (4), [(dippe)Ni(␩2-O,C-acetone)] (5), [(dippe)Ni(␩2-
O,C-fluorenone)] (6), [(dippe)Ni(␩2-O,C-di(2-pyridyl) ketone)] (7a) [(dippe)Ni(␬2-N,N-di(2-pyridyl)
ketone)] (7b), [(dippe)Ni(␬2-O,O-2,2ꢀ-pyridil)] (8), [(dippe)Ni(␬2-O,O-benzil)] (9a), and [((dippe)Ni)2(␩2-
O,C-benzil)] (9b) were prepared by the reaction of [(dippe)Ni(␮-H)]2 (1) with the corresponding ketone
or 1,2-diketone at room temperature. The structures of compounds 2, 6, 9a and 9b were confirmed by
X-ray crystallography. The selective hydrogenation of the two types of substrates was undertaken using
H2, giving high conversions to the corresponding reduction products, either alcohols or alkanes. Tunable
reaction conditions to promote the partial or total hydrogenation (hydrogenolysis) of the substrates are
described.
Article history:
Received 7 March 2009
Received in revised form 28 May 2009
Accepted 31 May 2009
Available online 10 June 2009
Keywords:
Ketone
Hydrogenation
Nickel
Diphosphine
Homogeneous catalysis
© 2009 Elsevier B.V. All rights reserved.
1. Introduction
tions if molecular hydrogen is used as the reducing agent, whereas a
second set can be described as transfer hydrogenations whenever an
A variety of organic substrates containing the CO group are
important in synthesis since they can be transformed into many
other different functional groups [1]. Partial and total reduction of
the carbonyl group serve as important examples of these trans-
formations as they give the corresponding alcohols or alkanes, but
preparative methods that may drive these reactions selectively have
been seldom optimized in the past. In the first case, the partial
reduction has been observed to take place only when an excess
of a conventional reducing agent such as LiAlH4, NaBH4, Al(OPri)3,
or EtSiH are used and the reaction is followed by a hydrolysis [2].
In the second case, Raney-nickel [3] as well as Wolff-Kishner and
Clemmensen reductions [4] are typically used, although the toler-
ance to other functional groups in the target molecules is an issue
that remains challenging.
organic compound is used as the hydrogen source. A seminal con-
tribution to the latter set was made by Noyori and co-workers who
developed a homogeneous asymmetric transfer hydrogenation sys-
tem employing complexes of the type [RuCl2(P–P)(1,2-diamine)],
where (P–P) is a chiral bisphosphine [6]. Currently, ruthenium is
still the most widely used metal for this reaction [7], but sim-
ilar reactivities have also been observed with other compounds
using iridium [8] and rhodium [9]. Shvo’s ruthenium catalyst
has been used in a broad range of hydrogen transfer reductions,
including carbonyl reductions [10]. Recently, Baratta et al. reported
the use of osmium complexes with the formula, [OsCl2P2(Pyme)]
(P = phosphine, Pyme = 1-(pyridin-2-yl)methanamine)), as active
catalysts for the reduction of ketones in basic alcohol media [11],
and an effective iron-based catalyst was reported by Casey for the
selective hydrogenation of aldehydes and ketones under mild con-
ditions [12]. The use of nickel in homogeneous hydrogenation of
ketones is relatively scarce; however, heterogeneous examples are
rather common, but not always well understood [13]. With the aim
of developing inexpensive and efficient catalysts, we herein disclose
the use of bisphosphine-nickel(0) complexes which are active cata-
lysts for the homogeneous hydrogenation of mono- and diketones,
giving their corresponding alcohols or alkanes in high conversions
and tunable conditions.
Industrially, large-scale processes that involve the catalytic
hydrogenation of numerous organic functions are operated world-
wide [5]. The metal catalysts and the processes in which they
participate can be classified depending on the hydrogen source that
is used. A first set of reactions can be described as direct hydrogena-
∗
Corresponding author. Tel.: +52 55 56223514; fax: +52 55 56162010.
E-mail address: juvent@servidor.unam.mx (J.J. García).
1381-1169/$ – see front matter © 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2009.05.026

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A. Flores-Gaspar et al. / Journal of Molecular Catalysis A: Chemical 309 (2009) 1–11
Scheme 1. Formation of [(dippe)Ni(␩2-C,O-ketone)] complexes 2–6.
Scheme 2. Formation of [(dippe)Ni(␩2-C,O-di-2-pyridyl ketone)] (7a) and [(dippe)Ni(␬2-N,N-di-2-pyridyl ketone)] (7b).
2. Results and discussion
complete crystallographic and structure refinement data are
summarized in Table 1. The geometry around the Ni center cor-
responds to square planar and shows a considerable lengthening
of the ␩2-coordinated carbonyl group (1.343(2) Å) vs. that of the
free ligand (1.223 Å) [17]. The NiCO plane is nearly coplanar with
the NiP2 plane (4.2◦). The almost equidistant Ni–P1 and Ni–P2
bond lengths suggest that the Ni(dippe) fragment is symmetrically
bonded. Similarly, an X-ray structure of fluorenone adduct 6 shows
a lengthened C–O bond to 1.326(5) Å, compared with 1.222 Å in
the free ketone (Fig. 2) [18], and the angle between the NiP2 and
NiCO planes is 3.0◦. A closely related structure has been reported
by Hillhouse and coworkers [19].
2.1. Reactivity of [(dippe)Ni(ꢀ-H)]2 1 with monoketones
As has been reported previously, the nickel(I) dimer 1 can be
used to cleave the C–C [14], C–S [15], and C–CN [16] bonds of a
variety of substrates, following reductive elimination of H2 and
in situ generation of the reactive nickel(0) moiety, [(dippe)Ni0]. 1
has been found to react with monoketones at room temperature in
toluene-d8 and THF-d8 solutions, allowing the formation of stable
␩2-C,O complexes with formula [(dippe)Ni(␩2-C,O-ketone)] (2–6).
Scheme 1 summarizes the reactions that take place.
Complexes 2–6 typically display two doublets in the range
60–72 ppm with P–P coupling constants in the range of 66–77 Hz
in their 31P{ H} NMR spectra. The presence of the doublets is
1
characteristic of two non-equivalent phosphorus atoms coordi-
nated to a metal center, the magnitude of the coupling constants
being indicative of JP–P through a Ni(0) center, i.e. [(P–P)Ni(␩2-
2
1
C,O-ketone)]. The 13C{ H} NMR spectra of the same complexes
exhibited a doublet of doublets in the range ı 82–87 for the carbon
in the coordinated C O bond, which appeared shifted to high-field
when compared with the corresponding carbonyl signal in the free
ketones (observed as a singlet in the region of ı 195–206). The
changes in chemical shifts and multiplicities are consistent with
coordination of the carbonyl moiety to nickel(0). It is worth not-
ing that a different reaction outcome was observed in the reaction
of 1 with di(2-pyridyl) ketone, which yielded two different prod-
ucts, the expected [(dippe)Ni(␩2-O,C-di(2-pyridyl) ketone)] (70%)
7a, and the unexpected [(dippe)Ni(␬2-N,N-di(2-pyridyl) ketone)]
(30%) 7b. The latter compound corresponds to a bis-N,N chelate
1
complex whose structure is illustrated in Scheme 2. In the 31P{ H}
NMR spectrum compound 7a displayed two doublets at ı 63.9 and
2
72.4 with JP–P = 57.6 Hz.16 7b exhibited a broad singlet centered
at ı 116.7 arising from the equivalence in the phosphorus atoms
coordinated to the nickel(0) center.
X-ray quality crystals for complex 2 were grown from a con-
centrated solution in benzene at room temperature. The ORTEP
representation of this compound is presented in Fig. 1 and the
Fig. 1. ORTEP representation of 2 showing thermal ellipsoids at the 30% probability
level. Selected bond distances (Å): O1–C1 (1.343(2)), Ni1–O1 (1.8549(11)), Ni1–C1
(1.9734(16)), Ni1–P1 (2.1722(5)), Ni1–P2 (2.1461(5)). Selected bond angles (deg):
O1–Ni1–C1 (40.94(6)), O1–Ni1–P1 (109.42(4)), C1–Ni–P1 (150.15(5)), O1–Ni–P2
(159.21(4)), C1–Ni–P2 (118.28(5)), P1–Ni–P2 (91.267(18)).

A. Flores-Gaspar et al. / Journal of Molecular Catalysis A: Chemical 309 (2009) 1–11
3
Table 1
Summary of crystallographic data for compounds 2, 6, 9a and 9b.
Empirical formula
2
6
9a
9b
P2C27H42NiO
C27H40NiOP2
C28 H42 Ni O2 P2
C42H74 Ni2O2P4
Formula weight
Temperature
Wavelength
Crystal system
Space group
503.26
193(2) K
0.71073 Å3
Trigonal
P3(2)
501.24
193(2) K
0.71073 Å
Orthorhombic
Pbca
531.27
173(2) K
0.71073 Å
Monoclinic
C2/c
852.31
100(2) K
0.71073 Å
Monoclinic
Cc
Unit cell dimen.
a = 10.1065(4) Å
b = 10.1065(4) Å
c = 22.9752(12) Å
˛ = 90◦
a = 9.3568(5) Å
b = 17.1854(9) Å
c = 32.1948(16) Å
˛ = 90◦
a = 27.766(3) Å
b = 15.1846(16) Å
c = 22.811(2) Å
˛ = 90◦
a = 15.063(3) Å
b = 17.403(3) Å
c = 17.427(3) Å
˛ = 90◦
ˇ = 90◦
ˇ = 90◦
ˇ = 120.496(2)◦
ꢁ = 90◦
ˇ = 106.155(3)◦
ꢁ = 90◦
ꢁ = 120◦
ꢁ = 90◦
Volume
Z
Density (calculated)
Absorption coefficient
F(000)
2032.32(16) Å3
3
5176.9(5) Å3
8
8287.0(15) Å3
12
4387.9(14) Å3
4
1.234 Mg/m3
0.850 mm−1
810
1.286 Mg/m3
0.890 mm−1
2144
1.277 Mg/m3
0.840 mm−1
3408
1.290 Mg/m3
1.037 mm−1
1832
Crystal size
ꢂ Range for data collection
Index ranges
0.20 mm × 0.26 mm × 0.28 mm
2.33–28.29◦
−13 ≤ h ≤ 7, −11 ≤ k ≤ 13,
−30 ≤ l ≤ 30
13,360
0.28 mm × 0.24 mm × 0.06 mm
0.152 mm × 0.132 mm × 0.082 mm 0.18 mm × 0.14 mm × 0.11 mm
2.37–28.31◦
1.64–25.50◦
1.83–25.03◦
−12 ≤ h ≤ 12, −22 ≤ k ≤ 22,
−42 ≤ l ≤ 42
59,661
6420 [R(int) = 0.0816]
99.6%
−33 ≤ h ≤ 33, −18 ≤ k ≤ 18,
−17 ≤ h ≤ 17, −20 ≤ k ≤ 20,
−27 ≤ l ≤ 27
−20 ≤ l ≤ 20
Reflections collected
Independent reflections
Completeness to theta
Refinement method
39,094
20,872
6371 [R(int) = 0.0172]
97.5%
7669 [R(int) = 0.0836]
99.3%
7732 [R(int) = 0.0545]
99.8%
Full-matrix least-squares
Full-matrix least-squares
Full-matrix least-Squares on
Full-matrix least-Squares on
on F2
on F2
F2
F2
Data/restraints/parameters
Goodness-of-fit on F2
6371/1/288
1.035
6420/0/287
1.253
7669/0/459
1.017
7732/2/398
0.861
Final R indices [I > 2ꢃ(I)]
R indices (all data)
Largest diff. peak and hole
R1 = 0.0243, wR2 = 0.0547
R1 = 0.0262, wR2 = 0.0554
0.221 and −0.168 e Å−3
R1 = 0.0887, wR2 = 0.1637
R1 = 0.1149, wR2 = 0.1729
0.829 and −0.941 e Å−3
R1 = 0.0466, wR2 = 0.0987
R1 = 0.0744, wR2 = 0.1088
0.473 and −0.284 e Å−3
R1 = 0.0616, wR2 = 0.1534
R1 = 0.0705, wR2 = 0.1622
2.139 and −0.605 e Å−3
14%). A minute amount of the nickel complex [(dippe)Ni(␬2-O,O-
2,2ꢀ-pyridil)] (8) was also detected (ı 79.8, s, 1%). Use of 2 equiv.
of 2,2ꢀ-pyridil resulted in the formation of 8 as the major product
(Scheme 3), the structure of this complex is proposed in analogy to
complex 9a (vide infra).
2.2. Reaction of [(dippe)Ni(ꢀ-H)]2 1 with 1,2-diketones
The stoichiometric reaction of 2,2ꢀ-pyridil with 1 yielded Ni(CO)4
as the main product (caution, poisonous gas!). The latter was
obtained as a white crystalline powder at −78 ◦C. Its presence was
accompanied by formation of decomposition products from com-
In sharp contrast to 2,2ꢀ-pyridil, the equimolecular reaction of 1
with benzil yielded the complexes, [(dippe)Ni(␬2-O,O-benzil)] (9a)
and [((dippe)Ni)2(␩2-C,O-benzil)] (9b) in a (1:1) ratio (Scheme 4).
No decomposition or formation of Ni(CO)4 was observed. The lat-
ter reaction is very susceptible to the stoichiometry employed, as
the use of 2 equiv. of benzil resulted in the selective formation of
9a over 9b. A singlet for the diphosphine ligand was observed at ı
1
pound 1, as indicated in the 31P{ H} NMR spectrum of the crude
reaction mixture. The additional by-products present were char-
acterized as [Ni(dippe)2] (ı 52.5, s, 82%) and free dippe (ı 7.3, s,
1
79.2 in the 31P{ H} NMR spectrum for 9a. The ␬2-coordinated car-
bonyls were located as a triplet (3JC–P = 3 Hz) at ı 145.2 by 13C{ H}
1
NMR spectroscopy. If an excess of 1 (1.5 equiv.) is used, the forma-
1
tion of 9b was favored and a multiplet at ı 60.6 in the 31P{ H} NMR
spectrum was observed instead of the singlet described above for
9a. The equivalent ␩2-coordinated carbonyls in 9b were located at
1
ı 93.6 in the 13C{ H} NMR spectrum. Compound 9b can be com-
pletely converted to 9a by heating at 120 ◦C for 2 days in benzene-d6
(Scheme 4).
Green crystals of 9a suitable for X-ray diffraction were grown
from a concentrated benzene solution at room temperature. The
crystallographic and structure refinement data for this compound
are also summarized in Table 1. The structure of 9a shows a
square planar geometry around the nickel(II) center as a result of
formation of the ␬2-O,O chelate (Fig. 3), with trans O–Ni–P geome-
tries of O1–Ni1–P2 (176.35(7)◦) and O2–Ni1–P1 (174.03(7)◦). The
NiP2 plane and the Ni–O1–O2 plane lie at an angle of 4.5◦. The
C1–O1 (1.369(3) Å) and C2–O2 (1.363(3) Å) bond lengths appeared
considerably longer than that of the same bond in the free lig-
and (1.21(2) Å),20 due to reduction by the metal, along with the
shortening of C1–C2 bond (1.364 vs 1.525 for the free ligand), both
indicative of an alkenediolate moiety coordinated to the metal
center.
Fig. 2. ORTEP representation of 6 showing thermal ellipsoids at the 30% probability
level. Selected bond distances (Å): O1–C14 (1.326(5)), Ni1–O1 (1.869(3)), Ni1–C1
(1.973(4)), Ni1–P1 (2.1436(11)), Ni1–P2 (2.1789(11)). Selected bond angles (deg):
O1–Ni1–C1 (40.26(14)), O1–Ni1–P1 (156.36(9)), C1–Ni1–P1 (116.17(12)), O1–Ni–P2
(111.83(9)), C1–Ni–P2 (152.03(12)), P1–Ni–P2 (91.62(4)).

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A. Flores-Gaspar et al. / Journal of Molecular Catalysis A: Chemical 309 (2009) 1–11
Scheme 3. Formation of [(dippe)Ni(␩2-C,O-ketone)] complexes.
Orange crystals of complex 9b were grown from a concentrated
constant at 2 mol%. The results obtained for this series of exper-
iments are summarized in Table 3. Due to the relatively high
temperatures used in some of the catalytic runs, selected experi-
ments were performed using the mercury drop test to confirm the
homogeneity of the system (see experimental section). No signif-
icant differences in conversions were observed, and the color of
the solutions was always yellow at the end of each experiment. No
black residues characteristic of metallic nickel or nanoparticles [21]
of such metal were observed.
solution of benzene at room temperature and were also charac-
terized by X-ray diffraction. The crystallographic and structure
refinement data for 9b are summarized in Table 1. The NiP2 and
NiCO planes are nearly coplanar (5.1–5.8◦), consistent with square
planar coordination as in 2 and 6 (Fig. 4). The O1–C15–C22–O2
(179.4◦) and C16–C15–C22–C23 (176.9◦) torsion angles indicate that
both benzene moieties lie essentially in the same plane. Each car-
bonyl group appeared coordinated to a [(dippe)Ni0] fragment and
thus, exhibited a mutually anti conformation. Also, they displayed
similar C–O bond lengths (C15–O1, 1.347(8) Å; C22–O2, 1.357(8) Å)
which are longer than those in benzil (1.21(2) Å) [20], although
smaller than those encountered in 9a. This lengthening is indicative
of depleting of C–O bonding character as a result of back-donation
from the [(dippe)Ni0] moieties.
Hydrogenation of benzophenone at 250 ◦C under 60 psi of H2
resulted in formation of a small amount of diphenylmethane (25%)
after 72 h (Table 3, entry1). Its yield was increased to 72% by
heating the mixture to 270 ◦C (entries 2 and 3). Hydrogenation
of acetophenone at 250 ◦C, resulted in almost complete forma-
tion of ethylbenzene as the sole product (98%), after 96 h (entry
4). The apparently greater reactivity observed for acetophenone
can be attributed to the smaller steric hindrance of the methyl
substituent as opposed to the phenyl rings. Reduction of 4-
methylbenzophenone at 250 ◦C for 96 h or 270 ◦C for 72 h resulted
in formation of tolylphenylmethane in 25 and 65% yield (entries 5
and 6 respectively), exactly as observed for benzophenone.
The presence of a heteroatom in the phenyl substituent as in the
case of di(2-pyridyl) ketone apparently lowers the energy barrier
required to promote hydrogenation of the carbonyl functionality.
The efficient hydrogenation of di(2-pyridyl) ketone was achieved at
a much lower temperature (150 ◦C, entries 7–10) than that required
for the benzophenone analogs. This result suggests that the rate of
hydrogenation of di(2-pyridyl) ketone may be affected by the pres-
ence of the pyridine ring which is known to coordinate to nickel(0)
competitively with the carbonyl (see Scheme 2). The steric hin-
drance limitation that operates in the case of benzophenone is
probably not a major issue for di(2-pyridyl) ketone. The presence
of an adjacent carbonyl in a benzophenone related molecule such
as benzil also resulted in a higher reactivity which also permitted
hydrogenation at 150 ◦C (entries 11–13). Analysis of the product dis-
tributions (vide infra) suggests that the carbonyls acquire distinct
electrophilic character in the molecule when only one of the two is
coordinated to nickel(0), enabling partial or even total hydrogena-
tion to be effected in just one of the two carbonyls (Scheme 5).
In the case of di(2-pyridyl) ketone, hydrogenation at 150 ◦C using
60 psi of H2 yielded both the alcohol, di-pyridin-2-yl-methanol
(86%), and the alkane, di-pyridin-2-yl methane (14%), after 12 h
(entry 7). The ratio of products was maintained even after heating
the vessel for 96 h (entry 8). A greater H2 pressure (120 psi) showed
only a slight increase in the corresponding alkane (35%) after 96 h
2.3. Hydrogenation experiments
2.3.1. Stoichiometric reactions
The hydrogenations of benzophenone, di(2-pyridyl) ketone and
benzil were performed in either toluene or THF solvent using
1 equiv. of 1 and 2 equiv. of mono-ketone, or 1 equiv. of 1 and 1 equiv.
of the 1,2-diketone, under hydrogen (60 psi). The results obtained
are summarized in Table 2.
As can be seen in Table 2, formation of the corresponding
alcohol (partial hydrogenation product) or alkane (total hydrogena-
tion product) depends on the reaction conditions. In particular,
for benzophenone formation of diphenylmethanol was observed
at 150 ◦C, although in low conversion (entry 1). Upon increas-
ing the temperature to 250 ◦C, total reduction of the C O bond
was achieved, resulting in formation of diphenylmethane (entry
2). Using di(2-pyridyl) ketone, formation of the partial reduction
product di-pyridin-2-yl-methanol (88%, entry 3) was observed after
12 h, longer reaction time (close to 100 h) allow the formation of the
corresponding alkane (98%, entry 4). In the case of benzil (entry 5),
heating to 150 ◦C for 12 h gave a mixture of products, whereas only
2-phenylacetophenone (100% yield) was observed after 48 h (entry
6; see also Scheme 5 vide infra). A thermal stability test at the dif-
ferent temperatures used was performed, and none of the ketone
complexes exhibited any significant decomposition in the absence
of H2.
2.3.2. Catalytic hydrogenation reactions
Experiments were performed using a variety of reaction con-
ditions, maintaining the concentration of the catalyst precursor 1

A. Flores-Gaspar et al. / Journal of Molecular Catalysis A: Chemical 309 (2009) 1–11
5
Fig. 3. ORTEP representation of 9a showing thermal ellipsoids at the 50% prob-
ability level. Selected bond distances (Å): C1–O1 (1.369(3)), C2–O2 (1.363(3)),
Ni1–O1 (1.832(2)), Ni1–O2 (1.851(2)), Ni1–P1 (2.1357(9)), Ni1–P2 (2.1508(9)),
C1–C2 (1.364(4)). Selected bond angles (deg): O1–Ni1–P2 (176.35(7)), O2–Ni1–P1
(174.03(7)), P1–Ni1–P2 (89.38(4)), O1–Ni–O2 (87.62(9)).
Fig. 4. ORTEP representation of 9b showing thermal ellipsoids at the 50% prob-
ability level. Selected bond distances (Å): C15–O1 (1.37(8)), Ni1–C15 (1.999(6)),
Ni1–O1 (1.872(5)), Ni1–P2 (2.127(19)), Ni1–P1 (2.171(2)), C22–O2 (1.357(8)),
Ni2–C22 (1.983(7)), Ni2–O2 (1.848(5)), Ni2–P4 (2.168(2)), Ni1–P3 (2.124(2)),
Ni2–P4 (2.168(2)), Ni2–O2 (1.848(5)), Ni2–C22 (1.983(7)), C22–O2 (1.357(8)).
Selected bond angles (deg): P1–Ni1–P2 (92.02(8)), O1–Ni1–C15 (40.6(2)), O1–Ni1-
P1 (111.16(18)), C15–Ni1–P2 (115.93(18)), P1–Ni1–O1 (156.31(17)), P3–Ni2–P4
(91.42(8)), O2–Ni2–C22 (41.3(2)), O2–Ni2–P4 (111.58(18)), C22–Ni2–P3 (115.4(2)),
C22–Ni2–P4 (152.5(2)), O2–Ni2–P3 (156.67(17)).
(entry 9). Heating the mixture for a longer time (216 h) significantly
improved the yield of di-pyridin-2-yl methane (70%, entry 10).
In the case of benzil, catalytic hydrogenation at 150 ◦C resulted
in formation of 2-phenylacetophenone in low yield (2%) after
96 h. An increase of H2 pressure to 250 psi resulted in complete
conversion of this substrate, producing a mixture of three hydro-
genation products (benzoin, 60%; 2-phenylacetophenone, 17%; and
hydrobenzoin, 23%; entry 12). An increase in the H2 pressure and
reaction time showed little variation in the composition of products
other than small changes in their relative ratios. This latter reaction
demonstrates that both 2-phenylacetophenone and hydrobenzoin
are produced in successive stages of the hydrogenation process
starting from benzoin (Scheme 5).
In light of the distribution of products that were obtained in
the case of benzil hydrogenation, it is possible that a reversible
␤-H elimination route competing with the overall hydrogenation
process to form hydrobenzoin might be operating. This possibil-
ity was examined by exploring the separate reactivity of benzoin
and hydrobenzoin in the presence of 1. In the case of benzoin, the
formation of two nickel(0) complexes, not isolated, were observed
after mixing in benzene-d6 at room temperature and after stirring
Scheme 4. Reactions of 1 with benzil.

6
A. Flores-Gaspar et al. / Journal of Molecular Catalysis A: Chemical 309 (2009) 1–11
Table 2
Stoichiometric hydrogenation of ketones using 1 as precursor.
Entry
T (◦C)
Time (h)
Substrate
Product(s)e
1a,b
150
96
2a,b
250
150
96
12
3a,d
4a,d
150
150
150
96
12
48
5c,d
6c,d
a
Ketone/precursor = 2/1.
THF as solvent (15 mL).
b
c
1,2-Diketone/precursor = 1:1.
Toluene as solvent (15 mL).
The reactions were carried out using a Parr reactor. All conversions were determined by GC–MS and 1H NMR.
d
e
Scheme 5. Benzil hydrogenation.

A. Flores-Gaspar et al. / Journal of Molecular Catalysis A: Chemical 309 (2009) 1–11
7
Table 3
Catalytic hydrogenation of ketones using 2% 1 as catalyst precursor.
Entry
Substrate
T (◦C)
P (psi)
Time (h)
72
Product(s)e
1a,b
250
60
2a,b
3a,b
4a,b
270
270
250
60
60
60
72
96
96
5a,b
250
270
60
60
96
72
6a,b
7a,d
8a,d
9a,d
150
150
150
60
60
12
96
96
120
10a,d
150
150
120
60
216
96
11c,d

8
A. Flores-Gaspar et al. / Journal of Molecular Catalysis A: Chemical 309 (2009) 1–11
Table 3 (Continued)
12c,d
150
250
96
13c,d
150
480
120
a
2% mol of catalytic precursor.
THF as solvent (15 mL).
4% mol of catalytic precursor.
Toluene as solvent (15 mL).
b
c
d
e
The reactions were carried out using a Parr reactor. All yields were quantified by GC–MS and 1H NMR.
1
for 4d at room temperature: 31P{ H} NMR: ı 69.4 (d, J = 62.6 Hz);
3. Conclusions
66.13 (d, J = 62.6 Hz) and ı 68.9 (d, J = 68.1 Hz); 63.9 (d, J = 68.1 Hz).
The current report shows an active system for the selective
A complete transformation into 9a was observed to take place after
3 days while heating the mixture at 140 ◦C. Scheme 6 summarizes
these reactions.
A similar examination of potential secondary reactions was
performed using a benzene-d6 solution of hydrobenzoin in the
presence of 1. Reaction took place after heating at 140 ◦C, resulting
ultimately in the formation of 9a within a period of 2 d (Scheme 7),
and also confirmed the hypothesis of a competing ␤-H elimination
process taking place in the hydrobenzoin produced in the catalytic
hydrogenation process.
hydrogenation of ketones with the use of a cheap metal. Conver-
sions were found to be high in general, producing either the alcohol
or corresponding alkane. The selectivity for these can be tuned in
certain cases, which makes the hydrogenation process synthetically
attractive. In the case of diketones such as benzil, a ␤-H elimination
process was found to play an important role competing with the
hydrogenation route, thereby rendering the benzil hydrogenation a
product-inhibited process. Studies are underway to probe the appli-
cation of related type of catalysts to the ones disclosed herein for the
enantioselective hydrogenation of ketones while using asymmetric
diphosphine ligands.
4. Experimental
4.1. General methods
Unless otherwise noted, all manipulations were performed
using standard Schlenk techniques in an inert-gas/vacuum
double manifold or under argon atmosphere in an MBraun
glovebox (<1 ppm H2O and O2). Benzophenone, acetophenone,
4-methylbenzophenone, di-2-pyridyl ketone, benzil and 2,2ꢀ-
pyridil-ketone were purchased from Aldrich and were used as
received. The nickel(I) dimer, [(dippe)Ni(␮-H)]2 (1), was prepared
from an n-hexanes slurry of [(dippe)NiCl2] [22] using Super-hydride
(LiHBEt3), similar to the literature procedure.15a Solvents were pur-
chased from J.T. Baker (reagent grade) and were additionally dried
and distilled from sodium/benzophenone ketyl. n-Hexanes and
ethyl acetate for chromatographic separation of reduction products
were used without further purification. NMR spectra of complexes
and products were recorded at room temperature on a 300 MHz
Varian Unity spectrometer unless otherwise noted. Deuterated
Scheme 6. Reactivity of benzoin with 1.
Scheme 7. Reactivity of hydrobenzoin with 1.

A. Flores-Gaspar et al. / Journal of Molecular Catalysis A: Chemical 309 (2009) 1–11
9
solvents for NMR experiments were purchased from Cambridge Iso-
tope Laboratories and were stored over 3 Å molecular sieves in the
glovebox for at least 24 h before use. All complexes were handled
under argon using thin wall (0.38 mm) WILMAD NMR tubes with J
tophenone (0.028 g, 0.156 mmol) in toluene-d8 solution (1 mL)
yielded [(dippe)Ni(␩2-O,C-acetophenone)] (4). Anal. Calcd. (%). for
C
22H40NiOP2: C, 59.89; H, 9.13. Found: C, 59.72; H, 9.14. 92%Yield.
NMR in toluene-d8 1H, ı 7.7 (m, 2H, CH), 6.8–7.2 (m, 3H, CH), 1.9 (s,
1
Young valves. 1H and 13C{ H} chemical shifts (ı, ppm) are reported
3H, CH3), 1.8–1.4 (m, 4H, CH), 1.3–0.8 (m, 4H, CH2), 0.8–0.4 (m, 24H,
1
3
3
relative to either the residual protiated solvent or deuterated car-
CH3). 13C{ H}, ı 153.6 (d, JC–P (trans) = 4.6, JC–P (cis) = 1.9, C), 133.0
(s, CH), 82.2 (d, C), 25.9 (d, JC–P = 22.1 Hz, CH), 24.5 (d, JC–P = 24 Hz,
CH), 22.9–22.3 (m, CH2), 20.4 (s, CH3), 19.2 (s, CH3), 18.1 (s, CH3).
1
bon resonances of the solvent, respectively. 31P{ H} NMR chemical
shifts (ı, ppm) are reported relative to external 85% H3PO4. 1H and
1
1
13C{ H} NMR spectra of the reduction products were obtained in
31P{ H} ı 68.4 (d, 2JP–P = 70.4 Hz), 64.1 (d, 2JP–P = 70.4 Hz).
CDCl3. Elemental analyses were carried out by USAI-UNAM using
an EA 1108 FISONS Instruments analyzer, reproducible elemental
analyses could not be obtained due to few samples high inestability
(9a and 9b). A Bruker APEX CCD diffractometer with monochrom-
atized Mo K␣ radiation (ꢄ = 0.71073 Å) was used for X-ray structure
determinations.
4.5. Preparation of [(dippe)Ni(ꢅ2-O,C-acetone)] (5)
Reaction of 1 (0.050 g, 0.078 mmol) with an excess of acetone
(20 mL) yielded [(dippe)Ni(␩2-O,C-acetone)] (5). The reagents were
mixed at room temperature, adding 1 to acetone, and the excess
of acetone was eliminated by high vacuum evaporation at −78 ◦C.
Slow evaporation of solvent at r.t. in the dry box allowed the crystal-
lization of a waxy residue which was further dried under vacuum.
Re-crystallization from hexanes of the remaining residue at −30 ◦C
gave an analytically pure solid. Anal. Calcd. (%). for C17 H37NiOP2: C,
53.43; H, 9.75. Found: C, 53.31; H, 9.77. 85%Yield. NMR (toluene-
d8): 1H, ı 1.5 (dd, 6H, CH3), 1.6–0.8 (m, 8H, CH and CH2), 0.8-0.4
Hydrogenation experiments were conducted in a 300 mL stain-
less steel Parr Series 4560 Bench Top Mini Reactor or alternatively
in a 100 mL stainless steel Parr Series 4590 Micro Bench Top reactor
vessel. The mixtures were charged into the vessels in the glovebox.
The respective nickel(0) catalysts were prepared in situ from 1. All
hydrogen used in this work was supplied by Praxair in high purity
grade (99.998%). The hydrogenation products were quantified by 1H
NMR spectroscopy. Product identification was made by direct com-
1
(m, 24H, CH3). 13C{ H}, ı 82.7 (d, 2JC–P = 29.6), 31.5 (d, CH3), 25.9 (d,
parison of their 1H and 13C{ H} NMR spectra and melting points
JC–P = 22.1 Hz, CH), 24.5 (d, JC–P = 24 Hz, CH), 22.9–22.3 (m, CH2), 20.4
1
1
with commercially available materials.
(s, CH3), 19.2 (s, CH3), 18.1 (s, CH3). 31P{ H}, ı 71.9 (d, 2JP–P = 66 Hz),
61.4 (d, 2JP–P = 66 Hz).
4.2. Preparation of [(dippe)Ni(ꢅ2-O,C-benzophenone)] (2)
4.6. Preparation of [(dippe)Ni(ꢅ2-O,C-fluorenone)] (6)
The reaction of dark red 1 (0.050 g, 0.078 mmol) with benzophe-
none (0.028 g, 0.156 mmol) in 1 mL toluene-d8 yielded monocoor-
dinated nickel(0) complex [(dippe)Ni(␩2-O,C-benzophenone)] (2).
Immediate effervescence due to reductive elimination of H2 was
observed after mixing. The resulting solution was analyzed by NMR
spectroscopy: 1H, ı 8.1 (d, 4H, CH), 7.1 (m, 2H, CH), 7.2 (m, 2H, CH),
1.9–1.4 (m, 4H, CH), 1.3–0.8 (m, 4H, CH2), 0.8–0.4 (m, 24H, CH3).
9-Fluorenone (38 mg, 0.021 mmol) and 25 mg 1 were combined
in 10 mL THF at r.t. The solvent was removed under vacuum and
the product 6 recrystallized from pentane. Anal. Calcd. (%) for
C
27H44NiOP2: C, 64.70; H, 8.04. Found: C, 63.97; H, 8.07. NMR
(toluene-d8): 1H, ı 0.27 (dd, 6H, CH3), 0.67 (dd, 6H CH3), 1.12 (m,
4H, CH), 1.37 (dd, 6 H, CH3), 1.44 (dd, 6 H, CH3), 1.56 (m, 2H), 2.18
1
3
3
1
13C{ H}, ı 149.4 (dd, JC–P (trans) = 5.4, JC–P (cis) = 1.5, C), 132.4 (s,
CH), 130.5 (s, CH), 128.3 (s, CH), 86.2 (d, 2JP–P = 22.2 Hz, C), 25.9 (d,
JC–P = 22.1 Hz, CH), 24.5 (d, JC–P = 24 Hz, CH), 22.9–22.3 (m, CH2), 20.4
(m, 2H), 7.11 (t, 2H), 7.17 (t, 2H), 7.66 (d, 2H), 7.68 (d, 2H). 31P{ H},
ı 72.48 (d, 2JP–P = 55 Hz), 81.40 (d, 2JP–P = 66 Hz).
1
(s, CH3), 19.2 (s, CH3), 18.1 (s, CH3). 31P{ H}, ı 71.9 (d, 2JP–P = 66 Hz),
4.7. Generation of [(dippe)Ni(ꢅ2-O,C-di(2-pyridyl) ketone)] (7a)
and [(dippe)Ni(ꢆ2-N,N-di(2-pyridyl) ketone)] (7b)
61.4 (d, 2JP–P = 66 Hz). Slow evaporation of toluene at r.t. in the dry
box allowed the crystallization of pure product. Anal. Calcd. (%). for
C27H42NiOP2: C, 64.43; H, 8.41. Found: C, 64.31; H, 8.44. 91%Yield.
Complex 1 (0.030 g, 0.046 mmol) was reacted with di(2-pyridyl)
ketone (0.017 g, 0.092 mmol). The reagents were mixed at room
temperature, adding the di(2-pyridyl) ketone to a dark red THF-d8
solution (1 mL) of 1. Anal. Calcd. (%) for C25H40N2NiOP2: C, 59.44;
H, 7.97; N, 5.54. Found: C, 57.84; H, 7.98; N 5.13. FAB+ 505.2. The
mixture was analyzed by NMR at room and low temperature. The
signals correspond to [(dippe)Ni(␩2-O,C-di(2-pyridyl) ketone)] (7a)
(70%) are: 1H (r. t.), ı 8.3 (m, 2H, CH), 8.0 (m, 2H, CH), 7.4 (m, 2H,
CH), 6.9 (m, 2H, CH), 2.1 (m, 4H, CH2), 1.4–0.4 (m, 28H, CH and
4.3. Preparation of [(dippe)Ni(ꢅ2-O,C-4-methyl-benzophenone)]
(3)
A toluene-d8 solution (1 mL) of 1 (0.050 g, 0.078 mmol) was
reacted with 4-methylbenzophenone (0.028 g, 0.156 mmol) at
room temperature, resulting in formation of the monocoordinated
nickel(0) complex [(dippe)Ni(␩2-O,C-4-methyl-benzophenone)]
(3). Effervescence resulting from reductive elimination of H2 was
observed immediately after mixing. Slow evaporation of toluene
at r.t. in the dry box allowed the crystallization of the pure prod-
uct. Anal. Calcd. (%). for C28H44NiOP2: C, 65.01; H, 8.57. Found: C,
64.96; H, 8.60. 92%Yield. NMR in toluene-d8: 1H, ı 8.1 (m, 4H, CH),
7.8–7.4 (m, 1H, CH), 7.4–6.8 (m, 4H, CH), 2.1 (s, 3H, CH3), 1.9–1.4
1
CH3). 13C{ H}, ı 168.0 (s, C), 135.0 (s, CH), 121.5 (s, CH), 118.6 (s,
1
CH), 86.5 (s, C), 22.5-17.5 (m, CH, CH2, CH3). 31P{ H}(r. t.): ı 72.4 (d,
2JP–P = 57.6 Hz), 63.9 (d, JP–P = 5.6 Hz) 31P{ H}(−90 ◦C): ı 80.1 (d,
2JP–P = 57.6 Hz), 68.6 (d, 2JP–P = 5.6 Hz). The signals of [(dippe)Ni(␬2-
N,N-di(2-pyridyl) ketone)] (7b) (30%) are: 1H (−90 ◦C), ı 9.4 (m, 2H,
CH), 8.7 (m, 2H, CH), 8.2 (m, 2H, CH), 7.2 (m, 2H, CH), 2.4–0.6 (m,
2
1
1
1
1
(m, 4H, CH), 1.3–0.8 (m, 4H, CH2), 0.8–0.4 (m, 24H, CH3). 13C{ H},
32H, CH, CH2 and CH3). 31P{ H}(r. t): ı 116.7 (s, br) 31P{ H}(−90 ◦C):
ı 149.7 (d, 3JC–P = 1.3 Hz), 146.0 (d, 3JC–P = 1.4 Hz), 143.0 (s, C), 130.0
(s, CH), 25.9 (d, JC–P = 22.1 Hz, CH), 24.5 (d, JC–P = 24 Hz, CH), 22.0 (s,
CH3), 22.9–22.3 (m, CH2), 20.4 (s, CH3), 19.2 (s, CH3), 18.1 (s, CH3).
ı 100.0 (s).
4.8. Preparation of [(dippe)Ni(ꢆ2-O,O-2,2ꢀ-pyridil)] (8)
1
31P{ H}, ı 71.7 (d, 2JP–P = 66.7 Hz), 61.7 (d, 2JP–P = 66.7 Hz).
Complex 1 (0.030 g, 0.046 mmol) was reacted at room temper-
ature with 2,2ꢀ-pyridil (0.019 g, 0.092 mmol) in benzene-d6 (1 mL).
After mixing, an effervescence corresponding to H2 reductive elim-
ination and a black powder (metallic nickel) were observed. Cooling
the sample to −70 ◦C allowed the precipitation of Ni(CO)4. The
4.4. Preparation of [(dippe)Ni(ꢅ2-O,C-acetophenone)] (4)
Similar to the above described preparations for compounds
2
and 3, the reaction 1 (0.050 g, 0.078 mmol) with ace-

10
A. Flores-Gaspar et al. / Journal of Molecular Catalysis A: Chemical 309 (2009) 1–11
1
mixture was examined by NMR spectroscopy. The 31P{ H} shows
two signals, the first at ı 79.9 (s) corresponding to the complex
[(dippe)Ni(␬2-O,O-2,2ꢀ-pyridil)] (8), and the second for free dippe at
ı 7.3 (s). The formation of Ni(CO)4 was corroborated by comparison
of the corresponding IR spectra with an authentic sample.
ketone or 1,2-diketone (0.78 mmol): benzophenone (0.142 g),
acetophenone (0.094 g), 4-methylbenzophenone (0.153 g), di(2-
pyridyl) ketone (0.142 g) and benzil (0.164 g), dissolved in toluene
or THF (15 ml). The resulting solutions were heated with vigorous
stirring at the temperatures and times indicated in Table 3, after
which the vessels were cooled down and opened in a well vented
hood prior to work-up. Yellow solutions were typically recovered
from the vessels after reaction. These were filtered over celite and
analyzed by GC–MS following the above-described procedure.
4.9. Generation of [((dippe)Ni)2(ꢅ2-O,C-benzil)] (9a) and
[(dippe)Ni(ꢆ2-O,O-benzil)] (9b)
Complex 1 (0.030 g, 0.046 mmol) was dissolved in benzene-d6
(1 mL) at room temperature and benzil added (0.010 g, 0.046 mmol).
The crude mixture was examined by NMR spectroscopy, show-
4.13. Mercury drop experiments
ing the formation of two compounds by 31P{ H} spectroscopy.
1
Following the above described procedure; additionally adding
two drops of elemental Hg to the reaction mixture. After reaction
completion, the solution was filtered and analyzed by GC–MS: no
significant difference in conversion between these experiments and
those in the absence of mercury was observed, indicating that het-
erogeneous Ni(0) is not involved. The procedure was performed
in all the optimized conditions such as entries 5, 6 in Table 2 and
entries 3, 4, 6, 8, 10, 12, 13 in Table 3. Mercury was always visible at
the end of each experiment, immersed in the yellow solutions and
no black residues were observed.
A multiplet centered at ı 60.6 was assigned to [((dippe)Ni)2(␩2-
O,C-benzil)] (9a), and
a singlet at ı 79.2 was assigned to
[(dippe)Ni(␬2-O,O-benzil)] (9b) (benzene-d6). Complexes (9a) and
(9b) were separated by a short column (silica gel, hexane/THF
(80:20) as eluent) and were characterized separately. 9a Anal. Calcd.
(%) for C28H42NiO2P2: C, 63.3; H, 7.96. Found: C, 61.62; H, 7.80. FAB+
531. 31P{ H} NMR (THF-d8): ı 83.7 (s); 1H NMR (THF-d8) ı 7.4 (d,
1
4H, CH), 6.9 (t, 4H, CH), 6.8 (t, 2H, CH), 1.5 (dd, 12H, CH3), 2.2 (m,
4H, CH), 1.7 (d, 4H, CH2), 1.3 (dd, 12H, CH3); 13C{ H} NMR (THF-d8)
1
ı 145 (t, C, 3JC–P = 3 Hz), 142.5 (s, C), 127.9 (s, CH), 127.4 (s, CH), 123.6
(s, CH), 25.3 (d, CH), 20.9 (t, CH2, JC–P = 20 Hz), 19.4 (s, CH3), 18.7 (s,
CH3). 9b Anal. Calcd. (%) for C42H74 Ni2O2P4: C, 59.18; H, 8.74. Found:
Acknowledgments
C, 57.34; H, 8.55. FAB+(–Ni(dippe)) 530. 31P{ H} NMR (THF-d8): ı
1
We thank CONACyT (080606) and DGAPA-UNAM (IN-202907-
3) for their financial support to this work. We thank Dr. Alfredo
Toscano, Dr. Simón Hernández-Ortega and Dr. Alma Arévalo for
technical support. A.F.-G. also thanks CONACyT for a gradu-
ate studies grant. WDJ thanks the National Science Foundation
(Grant CHE-0717040 and REU program) for support, and Christine
Flaschenriem and Nicole Brunkan for the structures of 2 and 6.
60.6 (m); 1H NMR (THF-d8) ı 8.3 (d, 4H, CH), 6.9 (t, 4H, CH), 6.7 (t,
2H, CH), 1.9 (m, 8H, CH), 1.5–0.8 (m, 56H, CH2 and CH3); 13C{ H}
1
NMR (THF-d8) ı 150.9 (s, C), 129.1 (s, CH), 126.9 (s, CH), 122.2 (s,
CH), 93.6 (s (br), C), 25 (t, CH2), 22–18 (m, CH, CH3).
4.10. Reactivity of 1 with benzoin and hydrobenzoin
To a benzene-d6 (1 mL) solution of 1 (0.078 mmol, 0.050 g)
was added either benzoin (0.156 mmol, 0.033 g) or hydrobenzoin
(0.078 mmol, 0.017 g). The solutions were stirred in the glovebox
as H2 was vented. The resulting red-brown solutions were trans-
ferred into an NMR tube with a J. Young valve and analyzed by 1H
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.molcata.2009.05.026.
1
1
and 31P{ H} NMR spectroscopy. For Ni(0) intermediates at: 31P{ H}
NMR: ı 69.4 (d, J = 62.6 Hz); 66.13 (d, J = 62.6 Hz) and ı 68.9 (d,
J = 68.1 Hz); 63.9 (d, J = 68.1 Hz).
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1
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