Stoichiometric and catalytic reactions of thermally stable nickel(0) NHC complexes

Article abstract of DOI:10.1021/om300045t

Although there are many organic reactions that are catalyzed by either Ni⁰ or Pd⁰ complexes, in comparison with the case for Pd⁰ there has been significantly less work studying coordinatively unsaturated Ni⁰ complexes. Here, we develop a simple synthetic route for preparing a number of thermally stable NHC-supported Ni⁰ hexadiene complexes in good yield. We examine the stoichiometric reactivity of one of these species and demonstrate that the coordinated hexadiene moiety is labile and can be replaced with a variety of different ligands, including CO, phosphines, isonitriles, and olefins. In addition, we show that the Ni ⁰ hexadiene complexes are relatively rare examples of homogeneous first-row transition-metal catalysts for the hydrogenation of olefins.

Full text of DOI:10.1021/om300045t

Communication
pubs.acs.org/Organometallics
Stoichiometric and Catalytic Reactions of Thermally Stable Nickel(0)
NHC Complexes
Jianguo Wu, John W. Faller, Nilay Hazari,* and Timothy J. Schmeier
Department of Chemistry, Yale University, P.O. Box 208107, New Haven, Connecticut 06520, United States
S
* Supporting Information
ABSTRACT: Although there are many organic reactions that
are catalyzed by either Ni0 or Pd0 complexes, in comparison
with the case for Pd0 there has been significantly less work
studying coordinatively unsaturated Ni0 complexes. Here, we
develop a simple synthetic route for preparing a number of
thermally stable NHC-supported Ni0 hexadiene complexes in good yield. We examine the stoichiometric reactivity of one of
these species and demonstrate that the coordinated hexadiene moiety is labile and can be replaced with a variety of different
ligands, including CO, phosphines, isonitriles, and olefins. In addition, we show that the Ni0 hexadiene complexes are relatively
rare examples of homogeneous first-row transition-metal catalysts for the hydrogenation of olefins.
here are numerous organic reactions which are catalyzed
by either zerovalent Ni or Pd complexes.1 However,
Scheme 1
T
although Ni0 is more accessible than Pd0, there have been
many more investigations into the properties of coordinatively
unsaturated Pd0 species.2 One of the major reasons for this
difference is the relative paucity of suitable Ni0 precursors. The
most frequently used Ni0 source for both stoichiometric and
catalytic reactions is almost certainly commercially available
Ni(COD)2, which is relatively difficult to prepare3 and is both
air and thermally unstable. Although Ni(COD)2 is an effective
Ni0 source, it would be useful to have more potential Ni0
synthons. Porschke and co-workers have demonstrated that
̈
direct reduction of NiII complexes in the presence of a ligand of
interest can generate Ni0 complexes, but the resulting Ni0 com-
plexes have rarely been used as synthons.4 Similarly, precur-
sors of the type Ni(t,t,t-cdt) (t,t,t-cdt = trans,trans,trans-1,5,
9-cyclododecatriene),3a Ni(c,c,c-cdt) (c,c,c-cdt = cis,cis,cis-1,5,
6
9-cyclododecatriene),5 and Ni(C2H4)3 are not commercially
available and are difficult to handle, which limits their utility. In
contrast, complexes of the type Ni(NHC)2 have been described
and are quite stable, but ligand substitution in these species is
difficult, which also limits their use as Ni0 precursors.7
In Pd0 chemistry, coordinated chelating linear dienes are easy
unstable intermediates. We describe the stoichiometric reac-
tivity of one of these thermally stable Ni0 species and show
that the hexadiene ligand can be easily replaced, so that our
new complexes can act as synthons for a variety of other Ni0
complexes containing a single NHC ligand. Furthermore,
we demonstrate that the Ni0 hexadiene complexes are rare
examples of homogeneous Ni catalysts for olefin hydro-
genation.
to substitute and Porschke and co-workers have prepared
̈
related Ni0 complexes of the type Ni2(1,6-diene)3,8 as well as
Ni0 phosphine complexes supported by nonchelating olefin9
and alkyne ligands.10 Although these complexes show promise
as precursors for a range of Ni0 complexes, they are not easy to
access.8,11 Recently, we described the unusually stable IPr-
supported Ni0 1,5-hexadiene complex 1, as a decomposition
product of thermally unstable (η1-allyl)(η3-allyl)Ni(IPr).12
Previously, related phosphine and bis(imine) supported com-
plexes were prepared following a similar route (Scheme 1), but
these are highly unstable at room temperature and are un-
suitable as Ni0 synthons.9a,13 Here, we report a new and
straightforward route to prepare a variety of NHC-supported
Ni0 hexadiene complexes, which does not involve any thermally
Our new synthesis of NHC-supported Ni0 hexadiene
complexes (1−4) starts from commercially available anhydrous
NiCl2 (eq 1). Reaction of NiCl2 with the appropriate NHC in
Received: January 19, 2012
Published: January 27, 2012
© 2012 American Chemical Society
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Communication
THF, followed by treatment with allyl Grignard, generates
compounds 1−4 in yields greater than 65% in all cases.
Presumably, the reaction proceeds through an intermediate of
the type (η1-allyl)(η3-allyl)Ni(NHC). Importantly, our syn-
thesis does not involve the isolation of any thermally unstable
intermediates and requires only two steps, which can be
performed in a single pot. The new complexes 2−4 were fully
characterized and are stable for more than 24 h in refluxing
toluene. Using our synthetic method, it was also possible to
prepare the PiPr3-supported Ni complex (hexadiene)Ni(PiPr3)
(5). In contrast to the case for other phosphine-supported
species,9a,13 5 could be heated to 60 °C before decomposition
occurred.
labile and can be replaced with a number of different ligands,
including phosphines, CO, isonitriles, and olefins (Scheme 2).
Scheme 2
X-ray-quality crystals of 2 and 3 were grown from saturated
solutions of pentane at −35 °C (Figure 1). In the case of
Reaction of 1 with CO gives the coordinatively saturated Ni0
complex Ni(CO)3(IPr) (6). In contrast, reaction of 1 with
tBuNC or 1 equiv of the bidentate phosphine dppe (dppe =
1,2-bis(diphenylphosphino)ethane) gives the three-coordinate
Ni0 complexes Ni(tBuNC)2(IPr) (7) and Ni(dppe)(IPr) (8),
respectively, although surprisingly no reaction is observed
between 1 and 2,2′-bipyridine. When 1 is treated with 1 equiv
of more electron rich bidentate phosphines, such as dmpe
(dmpe = 1,2-bis(dimethylphosphino)ethane) and dppp
(dppp = 1,2-bis(diphenylphosphino)propane), both the hexa-
diene and IPr ligands are substituted and the Ni0 complexes
Ni(dmpe)2 and Ni(dppp)2 are formed, along with 0.5 equiv of
starting material. Addition of excess dmpe or dppe results only
in the formation of Ni(dmpe)2 or Ni(dppp)2. The monoden-
tate phosphine PEt3 gives similar reactivity, and only Ni(PEt3)4
was isolated from reactions between 1 and excess PEt3.
Figure 1. (a) X-ray structure of 2 (hydrogen atoms omitted for clarity).
Selected bond lengths (Å) and angles (deg): Ni1−C1 = 2.051(5), Ni1−
C2 = 2.071(9), Ni1−C5 = 2.065(9), Ni1−C6 = 2.039(5), Ni1−C7 =
1.938(3), C1−C2 = 1.472(11), C2−C3 = 1.479(14), C3−C4 =
1.527(15), C4−C5 = 1.412(14), C5−C6 = 1.357(9); C1−Ni1−C2 =
41.8(3), C1−Ni1−C5 = 116.9(3), C1−Ni1−C6 = 155.33(19), C1−
Ni1−C7 = 106.78(18). (b) X-ray structure of 3 (hydrogen atoms
omitted for clarity). Selected bond lengths (Å) and angles (deg) for one
independent molecule in the unit cell: Ni1−C1 = 2.010(4), Ni1−C2 =
1.998(4), Ni1−C5 = 2.003(4), Ni1−C6 = 2.027(4), Ni1−C7 =
1.912(4), C1−C2 = 1.399(5), C2−C3 = 1.518(5), C3−C4 = 1.528(5),
C4−C5 = 1.493(6), C5−C6 = 1.393(5); C1−Ni1−C2 = 40.84(15),
C1−Ni1−C5 = 117.69(15), C1−Ni1−C6 = 155.64(15), C1−Ni1−
C7 = 99.03(14).
The hexadiene ligand can also be replaced with olefins. Reac-
tion between 1 and 1 atm of ethylene or 2 equiv of electron-
poor dimethyl fumarate gives the unusual three-coordinate
bis(olefin) species 9 or 10, respectively. In the case of the
reaction with ethylene, there is no evidence to suggest that the
tris(ethylene) complex Ni(C2H4)3 was formed.6 When 1 is
treated with the chelating olefin allyl ether or dimethylvinyl silyl
ether (dvse), the coordinatively unsaturated Ni species 11 or 12
is formed. Compounds 11 and 12 were characterized by X-ray
crystallography (Figure 2). Although crystallographically
characterized Pd0 species with allyl ether ligands are known,15
to the best of our knowledge 11 is the first crystallographically
characterized example of a Ni0 species with an allyl ether ligand
and 12 is a rare example with a dvse ligand.16 The overall
geometries of 11 and 12 are similar to those described for 2 and
3. The Ni−C bond lengths in 11 and 12 are slightly shorter
than those in 2 and 3, consistent with allyl ether and dvse being
better ligands than 1,5-hexadiene.
compound 2, there is significant disorder in the hexadiene ligand,
due to up and down conformations of the methylene groups,
relative to the Ni coordination plane. Nevertheless, it is clear that
complexes 2 and 3 have similar geometries, and the solid-state
structures confirm a pseudo-three-coordinate trigonal-planar
geometry around Ni, with moderate back-donation from the
coordinated olefins to the metal center.12 The new structures are
analogous to the previously reported structure of 1 and represent
rare examples of structurally characterized transition-metal com-
plexes containing 1,5-hexadiene ligands.12 The imidazole rings of
the NHC ligands are perpendicular to the metal coordination
plane; therefore, the angles between the planes are 62.1° in 2 and
57.9° in 3. The Ni−NHC bond lengths are significantly longer
(Ni1−C7 = 1.938(3) Å in 2 and Ni1−C7 = 1.912(4) in 3) than
those reported by Arduengo and co-workers7a in a complex of the
type Ni(NHC)2 and comparable with those reported by Hill-
house et al. in a two-coordinate Ni(II) complex containing a
single NHC ligand.14
All of the above substitution reactions demonstrate that 1
can be used as an easily accessible precursor for the preparation
of a range of different Ni0 complexes, including several species
In order to probe the reactivity of the hexadiene complexes,
compound 1 was used as a model. The hexadiene ligand is
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Organometallics
Communication
Ni benzene adduct, similar to that observed by Porschke and
̈
co-workers.9c It also indicated that it may be possible to use 1
as a catalyst for olefin hydrogenation.
We screened complexes 1−4 as catalysts for the hydro-
genation of cyclopentene, along with the related PiPr3 (5)- and
PPh3-supported Ni hexadiene complexes (Table 1). Although
Table 1. Catalyst Screening for the Hydrogenation of
Cyclopentene
Figure 2. (a) X-ray structure of 11, one of three independent
structures in the unit cell (hydrogen atoms and isopropyl groups of the
IPr ligand omitted for clarity). Selected bond lengths (Å) and angles
(deg) for one independent molecule in the unit cell: Ni1−C1 =
1.964(10), Ni1−C2 = 1.989(10), Ni1−C5 = 1.982(9), Ni1−
C6 = 1.962(10), Ni1−C19 = 1.910(8), C1−C2 = 1.418(13), C2−
C3 = 1.464(11), C4−C5 = 1.487(12), C5−C6 = 1.413(13); C1−
Ni1−C2 = 42.0(4), C1−Ni1−C5 = 125.0(4), C1−Ni1−C6 =
166.4(4), C1−Ni1−C19 = 94.0(4). (b) X-ray structure of 12
(hydrogen atoms and isopropyl groups of IPr ligand omitted for
clarity). Selected bond lengths (Å) and angles (deg) for one
independent molecule in the unit cell: Ni1−C1 = 1.987(4), Ni1−C2
= 2.013(3), Ni1−C3 = 2.009(3), Ni1−C4 = 1.989(4), Ni1−C9 =
1.906(3), C1−C2 = 1.392(5), C2−Si1 = 1.833(3), Si1−O1 =
1.636(3), O1−Si2 = 1.635(2), Si2−C3 = 1.838(4), C3−C4 =
1.390(5); C1−Ni1−C2 = 40.72(13), C1−Ni1−C3 = 132.04(13),
C1−Ni1−C4 = 172.68(14), C1−Ni1−C9 = 90.42(12).
a
cat.
time (h)
conversn (%)
(hexadiene)Ni(IPr) (1)
(hexadiene)Ni(SIPr) (2)
(hexadiene)Ni(IMes) (3)
(hexadiene)Ni(SIPrMes) (4)
(hexadiene)Ni(PiPr3) (5)
(hexadiene)Ni(PPh3)
23
23
23
23
23
23
23
23
62
98
95
76
54
0
(C2H4)2Ni(IPr) (9)
35
41
b
Ni(COD)2
a
Average of two runs measured by 1H NMR spectroscopy using
b
an internal standard. A heterogeneous catalyst is formed when
Ni(COD)2 is used as the catalyst. The conditions for catalytic
reactions were cyclopentene (8.0 mg, 0.118 mmol), catalyst (0.012
mmol), ferrocene (0.059 mmol, internal standard), and 1 atm H2 in
0.4 mL of C6D6 at 50 °C.
which are coordinatively unsaturated. Prior to this work some
of these coordinatively unsaturated species could only be
prepared using metal-vapor synthesis, and most required the
generation of thermally unstable intermediates, which makes
our route a significant improvement.16a Furthermore, com-
pound 1 also undergoes facile oxidative addition with allyl
chloride or 2-methylallyl chloride to form compounds 13 and
14, respectively. This suggests that it may be possible to utilize
hexadiene-supported species as precatalysts for reactions which
utilize Ni0/NiII catalytic cycles.
Interestingly, reaction of 1 with propyne did not give the
expected bis(propyne) complex. Instead, the propyne was sele-
ctively converted into 1,2,4-trimethylbenzene at room temper-
ature. In fact, 1 could catalytically perform this transformation
(eq 2). Although it has been previously demonstrated that Ni0
the activity was low, and a high catalyst loading was required,
1−5 were able to hydrogenate cyclopentene using only 1 atm
of H2. The PPh3-supported complex was completely inactive,
presumably due to its thermal instability, while the bis-
(ethylene) complex 9, which is relatively thermally stable,
gave limited activity. Although it is difficult to rigorously ex-
clude the possibility that Ni nanoparticles are responsible for
catalysis,18 no decrease in catalytic activity was observed when
the reaction was run in the presence of elemental Hg. In
addition, treatment of the reaction mixture at the end of a
catalytic run with ethylene resulted in the clean formation of 9
(>95% yield based on starting catalyst), suggesting that a
significant amount of Ni was still present in solution. After
filtration of the reaction mixture, the solution containing 9
could then be used for an additional catalytic reaction. In con-
trast to the apparent homogeneous hydrogenation performed
by 1−5, when Ni(COD)2 was used as a catalyst, under the
same reaction conditions, a black precipitate immediately
formed. This black precipitate was active for the hydrogena-
tion of cyclopentene, but no catalytic activity was obtained
when the solution was filtered, suggesting that Ni(COD)2
forms a heterogeneous catalyst.
The SIPr-supported species 2 was the most active catalyst for
hydrogenation and was utilized to investigate the substrate
scope (Table 2). Compound 2 can hydrogenate a variety of
cyclic olefins, although the catalyst is very slow for substrates
other than highly activated cyclopentene. Interestingly, the
linear olefin 1-octene can be hydrogenated, with no evidence
for isomerization products, which are common products from
the reaction of Ni species with alkenes.19 The disubstituted
alkenes isobutene and methylstyrene can also be hydrogenated
after extended reaction times. Homogeneous Ni catalysts for
hydrogenation are relatively rare, and there have only been a
small number of previous reports.20 In particular, Bouwman
catalysts can cyclotrimerize alkynes to form benzenes, this is a
rare example which starts from a well-defined Ni0 starting
material.17 Notably, no catalytic activity was observed when
Ni(COD)2 was used as the catalyst.
Treatment of 1 with H2 resulted in the elimination of hexane
and the formation of an unidentified Ni complex, which could
not be isolated (eq 3). Reaction of the unidentified Ni complex
with ethylene resulted in the formation of 9, which suggests
that the unidentified Ni complex may be an unstable NHC
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Communication
(4) Bach, I.; Porschke, K.-R.; Goddard, R.; Kopiske, C.; Krueger, C.;
̈
Table 2. Substrate Scope for Hydrogenation Using
Precatalyst 2
Rufinska, A.; Seevogel, K. Organometallics 1996, 15, 4959.
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a
substrate
time (h)
conversn (%)
(6) Fischer, K.; Jonas, K.; Wilke, G. Angew. Chem., Int. Ed. 1973, 12,
565.
cyclooctene
cyclohexene
cyclopentene
isobutene
92
68
23
64
57
27
88
93
98
58
94
93
98
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44
a
1
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Average of two runs measured by H NMR spectroscopy using an
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In conclusion, we have developed a straightforward synthesis
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synthon to prepare a variety of coordinatively unsaturated Ni0
species supported by phosphines, isonitriles, and olefins and
undergoes oxidative addition to form well-defined NiII
products. Furthermore, our Ni0 complexes are active catalysts
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ASSOCIATED CONTENT
* Supporting Information
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■
S
Text, tables, figures, and CIF files giving experimental details,
characterization data, and X-ray crystallographic information.
This material is available free of charge via the Internet at
http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: nilay.hazari@yale.edu.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
We thank Dr. Chris Incarvito for assistance with X-ray
crystallography.
■
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