Simple synthesis of CpNi(NHC)Cl complexes (Cp = cyclopentadienyl; NHC = N-heterocyclic carbene)

Article abstract of DOI:10.1021/om0501879

The reaction of saturated and unsaturated imidazolium salts with nickelocene in refluxing THF results in the formation of NHC complexes of general formula CpNi(NHC)Cl (NHC = SIMes (2), IPr (3), SIPr (4)). This protonation of Cp₂Ni was also test

Full text of DOI:10.1021/om0501879

3442
Organometallics 2005, 24, 3442-3447
Simple Synthesis of CpNi(NHC)Cl Complexes (Cp )
Cyclopentadienyl; NHC ) N-Heterocyclic Carbene)
Roy A. Kelly, III, Natalie M. Scott, Silvia D´ıez-Gonza´lez, Edwin D. Stevens, and
Steven P. Nolan*
Department of Chemistry, University of New Orleans, New Orleans, Louisiana 70148
Received March 14, 2005
The reaction of saturated and unsaturated imidazolium salts with nickelocene in refluxing
THF results in the formation of NHC complexes of general formula CpNi(NHC)Cl (NHC )
SIMes (2), IPr (3), SIPr (4)). This protonation of Cp2Ni was also tested using phosphon-
ium salts, and the reaction of nickelocene with triethylphosphonium chloride leads to
CpNi(PEt3)Cl (5). All compounds were characterized by NMR and X-ray crystallography.
The catalytic activity of the NHC compounds was tested in aryl amination (Buchwald-
Hartwig reaction) and in aryl halide dehalogenation reactions.
Introduction
these current methods require an additional step,
increasing the cost and generating waste.
N-heterocyclic carbenes (NHCs) have become a very
important class of ligand in organometallic chemistry.1
Oftentimes compared to tertiary phosphines because of
similarities in bonding and transition-metal (TM) cata-
lyst activity, the NHCs possess properties that render
them more desirable for a number of catalytic applica-
tions.2 A minor hurdle hindering a more widespread use
of NHCs as ligands is the need to generate them from
their respective imidazolium salt, thus either requiring
an additional isolation step or, alternatively, forming
them in situ. The most common method of affixing a
NHC onto a metal center is synthesizing the free
carbene by action of a base on the imidazolium salts
and subsequently reacting the NHC with a metal
complex.3 A method gaining popularity in recent reports
is the transmetalation of the NHC from a silver complex
that can be synthesized by reacting Ag2O with the
azolium chloride.4 This transmetalation has been espe-
cially useful with NHCs that are difficult to isolate or
that are somewhat unstable as a free carbene. Both of
We have recently taken advantage of a Ni system
((NHC)Ni(CO)x; x ) 2, 3) to quantify the steric and
electronic parameters characterizing a number of NHCs.5
Recent reports of Ni-NHC complexes capable of medi-
ating organic transformations have drawn attention to
this low-cost organometallic system. Louie and co-
workers have shown that a Ni-NHC system success-
fully facilitated the cycloaddition of alkynes with either
isocyanates or carbon dioxide at a higher rate than when
Ni-tertiary phosphine systems were employed.6 Fort
has had success with Ni-catalyzed aryl amination and
transfer hydrogenation using NHCs as ancillary lig-
ands.7,8 Mori has recently reported on the stereoselective
synthesis of (Z)-allylsilanes from dienes and aldehydes
mediated by a Ni/NHC system,9 and a similar system
involving alkynes10 has been used by Montgomery. A
most recent report by Jamison uses an IPr/Ni(0) system
to effect the enantioselective and regioselective coupling
of allenes, aldehydes, and silanes.11 While this paper
was in preparation, very recent work from the Snieckus
group came to our attention where CpNi(IMes)Cl
(IMes ) 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene)
was used as precatalyst in a cross-coupling of aryl
Grignards with sulfamates.12 Since a general synthetic
route has not yet been demonstrated leading to these
CpNi(NHC)Cl catalysts, we felt that in this context a
simple route to a large number of compounds in this
family would be a welcome contribution.
* To whom correspondence should be addressed. E-mail: snolan@
uno.edu.
(1) (a) Herrmann, W. A.; Ko¨cher, C. Angew. Chem., Int. Ed. 1997,
36, 2163-2187. (b) Bourissou, D.; Guerret, O.; Gabbai, F. P.; Bertrand,
G. Chem. Rev. 2000, 100, 39-91. (c) Weskamp, T.; Bo¨hm, V. P. W.;
Herrmann, W. A. J. Organomet. Chem. 2000, 600, 12-22.
(2) (a) Hill, J. E.; Nile, T. A. J. Organomet. Chem. 1977, 137, 293-
300. (b) Lappert, M. F.; Maskell, R. K. J. Organomet. Chem. 1984, 264,
217-228. (c) Gardiner, M. G.; Herrmann, W. A.; Reisinger, C. P.;
Schwarz, J.; Spiegler, M. J. Organomet. Chem. 1999, 572, 239-247.
(d) Hillier, A. C.; Lee, H. M.; Stevens, E. D.; Nolan, S. P. Organome-
tallics 2001, 20, 4246-4252. (e) Lee, S.; Beare, N. A.; Hartwig, J. F. J.
Am. Chem. Soc. 2001, 123, 8410-8411. (f) Grasa, G. A.; Viciu, M. S.;
Huang, J. K.; Zhang, C. M.; Trudell, M. L.; Nolan, S. P. Organometallics
2002, 21, 2866-2873. (g) Jackstell, R.; Frisch, A.; Beller, M.; Ro¨ttger,
D.; Malaun, M.; Bildstein, B. J. Mol. Catal. A: Chem. 2002, 185, 105-
112. (h) Muehlhofer, M.; Strassner, T.; Herrmann, W. A. Angew. Chem.,
Int. Ed. 2002, 41, 1745-1747.
The first transition-metal-NHC complexes were not
formed by the use of a free carbene, as they were
(5) Dorta, R.; Stevens, E. D.; Scott, N. M.; Costabile, C.; Cavallo,
L.; Hoff, C. D.; Nolan, S. P. J. Am. Chem. Soc. 2005, 127, 2485-2495.
(6) Duong, H. A.; Cross, M. J.; Louie, J. J. Am. Chem. Soc. 2004,
126, 11438-11439.
(7) Omar-Amrani, R.; Thomas, A.; Brenner, E.; Schneider, R.; Fort,
Y. Org. Lett. 2003, 5, 2311-2314.
(8) Kuhl, S.; Schneider, R.; Fort, Y. Organometallics 2003, 22, 4184-
4186.
(9) Sawaki, R.; Sato, Y.; Mori, M. Org. Lett. 2004, 6, 1131-1133.
(10) Mahandru, G. M.; Liu, G.; Montgomery, J. J. Am. Chem. Soc.
2004, 126, 3698-3699.
(11) Ng, S.-S.; Jamison, T. J. Am. Chem. Soc. 2005, 127, 7320-7321.
(12) Macklin, T. K.; Snieckus, V. Private communication.
(3) (a) Herrmann, W. A.; Elison, M.; Fischer, J.; Kocher, C.; Artus,
G. R. J. Chem. Eur. J. 1996, 2, 772-780. (b) Douthwaite, R. E.;
Haussinger, D.; Green, M. L. H.; Silcock, P. J.; Gomes, P. T.; Martins,
A. M.; Danopoulos, A. A. Organometallics 1999, 18, 4584-4590. (c)
Scholl, M.; Trnka, T. M.; Morgan, J. P.; Grubbs, R. H. Tetrahedron
Lett. 1999, 40, 2247-2250. (d) Jafarpour, L.; Nolan, S. P. Organome-
tallics 2000, 19, 2055-2057.
(4) Wang, H. M. J.; Lin, I. J. B. Organometallics 1998, 17, 972-
975.
10.1021/om0501879 CCC: $30.25 © 2005 American Chemical Society
Publication on Web 05/26/2005

CpNi(NHC)Cl Complexes
Organometallics, Vol. 24, No. 14, 2005 3443
Scheme 1. Synthetic Route to CpNi(NHC)Cl
Complexes
ylidene), CpNi(IPr)Cl (3; IPr ) 1,3-bis(2,6-diisopropyl-
phenyl)imidazol-2-ylidene), and CpNi(SIPr)Cl (4; SIPr
) 1,3-bis(2,6-diisopropylphenyl)-4,5-dihydroimidazol-2-
ylidene) were synthesized as shown in Scheme 1. Initial
support for the proposed structure was derived from
both 1H and 13C NMR, which showed one carbene ligand
and one η5-bonded cyclopentadienyl ligand. The NMR
spectroscopy was also consistent with the reported data
for the IMes derivative.17a Elemental analysis confirmed
the expected product composition.
Attempts to synthesize in a similar manner the
ItBu, ICy, and IAd nickel complexes failed. All three
bear alkyl substituents (tert-butyl, cyclohexyl, and ada-
mantyl, respectively) directly on the imidazolium nitro-
gens, rendering the C-H imidazolium bond stronger
and possibly more difficult to activate.
Exchanging the counterion from Cl to BF4 or PF6
leads to no reaction of either aryl- or alkylimidazolium
salts and recovery of nickelocene. We believe that the
strong Ni-Cl bond formed in the reaction with imida-
zolium chlorides must act as a significant thermody-
namic driving force leading to product.
considered unstable at the time. O¨ fele13 and Wanzlick14
both use the direct addition of imidazolium salts to
metal precursors. Since Arduengo15 showed the feasibil-
ity of generating a stable free carbene, the use of direct
addition of the azolium salt to a transition metal has
seen limited use. The reaction of an imidazolium salt
and Pd(OAc)2 has been shown to form Pd-NHC com-
plexes,16 and there have been a few reports of Ni-NHC
complexes made in this fashion.17 One report describes
the use of Ni(indenyl)2 and an alkyl-substituted imida-
zolium salt, but the generality of the approach was not
expanded upon.17c There has only been one example
reported so far that makes use of a simple assembly
protocol involving IMes‚HCl and Cp2Ni leading to
CpNi(IMes)Cl (1) in one pot.17a Since recent publications
have focused on catalytically active Ni-NHC complexes
(see above), a facile synthetic method of preparation
leading to such complexes seemed desirable. We there-
fore undertook a detailed study probing the generality
of the one-pot synthetic approach from nickelocene.
We also wondered if the protocol could be extended
to trialkylphosphine complexes using a phosphonium
salt as the phosphine source. CpNi(PR3)Cl (PR3
)
tertiary phosphine) complexes have been known for
quite some time.19 The preferred synthetic route to
these complexes involved a disproportionation reaction
where a mixture of nickelocene and Ni(PR3)2Cl2 is
refluxed until an equilibrium is reached. Yields are
generally very low, and workup is not always straight-
forward. The handling of trialkylphosphines is also a
possible complication, as these are extremely air-sensi-
tive compounds. In attempting this one-pot synthesis
with phosphonium salts20 of trialkylphosphines, we
targeted a solution that would address these two
shortcomings.
Results and Discussion
The saturated NHC complexes do exhibit enhanced
reactivity in certain instances.18 For this reason, it was
of interest to test whether SIMes‚HCl could be reacted
directly with nickelocene. Interestingly, this appears to
be a fairly straightforward reaction, as a solution of
nickelocene and the saturated imidazolium chloride
upon being refluxed in THF gave the desired product.
Reaction completion is apparent by a change of color
from dark green to red. This protocol also proved valid
for the more sterically encumbered IPr and its saturated
congener, SIPr (see Scheme 1). The isolated products
are all stable in solution as well as being air-stable in
the solid state. Complexes CpNi(SIMes)Cl (2; SIMes )
1,3-bis(2,4,6-trimethylphenyl)-4,5-dihydroimidazol-2-
Our first attempts involved the bulky [PtBu3]HCl. The
reaction with nickelocene and the salt did not lead to
formation of the desired product. In light of the very
bulky nature of PtBu3 and of the small size of Ni, we
reasoned that steric congestion might be at the origin
of this failure. Use of a less sterically demanding but
electronically equivalent tertiary phosphine, triethyl-
phosphine (PEt3), was then tested. The synthesis re-
quires the quaternization of the phosphine, but this is
easily achieved, since the air-sensitive triethylphosphine
can be straightforwardly treated with an anhydrous HCl
solution in dioxane to yield the air-stable phosphonium
salt. A solution of nickelocene and triethylphosphonium
chloride was stirred at room temperature until the char-
acteristic green to red color change occurred after 2 h.
This procedure afforded a moderate yield (32%) of
CpNi(PEt)3Cl (5) (eq 1). The identity of 5 was confirmed
(13) O¨ fele, K. J. Organomet. Chem. 1968, 12, P42-P43.
(14) Wanzlick, H. W.; Schonherr, H. J. Angew. Chem., Int. Ed. Engl.
1968, 7, 141-142.
(15) Arduengo, A. J., III; Harlow, R. L.; Kline, M. J. Am. Chem. Soc.
1991, 113, 361-363.
(16) For some examples see: (a) Herrmann, W. A.; Reisinger,
C.-P.; Spiegler, M. J. Organomet. Chem. 1998, 557, 93-96. (b) Andrus,
M. B.; Song, C.; Zhang, J. Org. Lett. 2002, 4, 2079-2082. (c) Andrus,
M. B.; Song, C. Org. Lett. 2001, 3, 3761-3764. (d) Baker, M. V.;
Skelton, B. W.; White, A. H.; Williams, C. C. Dalton 2001, 2, 111-
120.
(17) (a) Abernethy, C. D.; Cowley, A. H.; Jones, R. A. J. Organomet.
Chem. 2000, 596, 3-5. (b) Shukla, P.; Johnson, J. A.; Vidovic, D.;
Cowley, A. H.; Abernethy, C. D. Chem. Commun. 2004, 360-361. (c)
Sun, H. M.; Shao, Q.; Hu, D. M.; Li, W. F.; Shen, Q.; Zhang, Y.
Organometallics 2005, 24, 331-334.
(18) For some examples see: (a) Viciu, M. S.; Navarro, O.; Germa-
neau, R. F.; Kelly, R. A.; Sommer, W.; Marion, N.; Stevens, E. D.;
Cavallo, L.; Nolan, S. P. Organometallics 2004, 23, 1629-1635. (b)
Viciu, M. S.; Germaneau, R. F.; Nolan, S. P. Org. Lett. 2002, 4, 2053-
2056 and references therein.

3444 Organometallics, Vol. 24, No. 14, 2005
Kelly et al.
Figure 1. Ball and stick structures of CpNi(SIMes)Cl (2) (left) and CpNi(IPr)Cl (3) (right). Most hydrogen atoms have
been omitted for clarity.
Figure 2. Ball and stick structures of CpNi(SIPr)Cl (4) (left) and CpNi(PEt3)Cl (5) (right). Most hydrogen atoms have
been omitted for clarity.
Table 1. Selected Bond Lengths (Å) and Angles (deg) for CpNi(SIMes)Cl (2), CpNi(IPr)Cl (3), CpNi(SIPr)Cl
(4), CpNi(PEt3)Cl (5), and CpNi(IMes)Cl (1)
2a
3
4
5
1b
Ni-Lc
1.85(2), 1.89(2)
2.199(5), 2.183(6)
1.77(6), 1.76(7)
133.5(8), 134.4(8)
127.4(8), 127.1(9)
98.1(6), 99.4(6)
1.8748(11)
2.1876(3)
1.80(2)
134.8(8)
131.0(8)
93.86(3)
1.8752(16)
2.1795(5)
1.768(8)
137.5(3)
130.4(3)
92.06(5)
2.1505(3)
2.1871(3)
1.753(8)
136.2(3)
131.3(2)
92.386(11)
1.917(9)
2.185(2)
1.760(7)
132.4(2)
129.2(2)
98.2(2)
Ni-Cl
Ni-Cp(c)
L-Ni-Cp(c)
Cl-Ni-Cp(c)
L-Ni-Cl
a Two independent molecules are present in the asymmetric unit. b Values taken from ref 17a. c L ) SIMes, IPr, SIPr, PEt3.
by comparison with the previously reported literature
spectroscopic data.19 To the best of our knowledge, this
is the first report of the isolation of a metal-phosphine
complex through the use of a phosphonium salt. This
method may well be general for a variety of phosphine
ligands and is especially applicable to systems contain-
ing larger metal centers, where steric crowding about
the metal coordination sphere is not a concern.
To unequivocally determine the identity of 2-5, single
crystals were obtained from slow cooling of a warm
saturated toluene solution of the respective compounds.
The molecular structures of the complexes of type
CpNi(L)Cl (L ) SIMes (2), IPr (3), SIPr (4), PEt3 (5))
are shown as ball-and-stick diagrams in Figure 1 (2 and
3) and Figure 2 (4 and 5), and relevant bond lengths
and angles are provided in Table 1. In the case of 2,
poor-quality crystals yielded a higher R value than for
complexes 3-5. Nevertheless, the skeletal arrangement
is correct, with the asymmetric unit comprising of two
independent molecules with their bond distances and
angles lying close to those obtained for 3 and 4. In all
cases the coordination geometry can be described as
trigonal planar (sum of bond angles using the Cp ring
centroid ∼360.0°) comprised of a η5-Cp ligand, a carbene
(or phosphine in the case of 5), and a chloride ion. For
complexes 2-4, the Ni-carbene bond lengths lie in the
range 1.85-1.89 Å and are similar to those reported for
[Ni(Cp)(NHC)2]+ (1.883(2) Å; NHC ) tetramethylimi-
dazol-2-ylidene)21 and (Cp)2Ni(NHC) (1.885(4) Å; NHC
)
1,3-bis(2,6-dimethyl-4-bromophenylimidazol-2-yli-
dene)21 but are somewhat shorter than that reported
for 1 (1.917(9) Å).17a The complexes containing the
saturated NHC ligands (compounds 2 and 4) show
slightly shorter bond distances than their corresponding
unsaturated analogues (Table 1); however, these differ-
ences are not significant. The Ni-Cl distances are
similar to those of 1 (2.185(2) Å). As also observed for
1,17a the aryl substituents on the NHC ligand are
(19) Young, G. B.; Black, S. I. Polyhedron 1989, 8, 585-588.
(20) Netherton, M. R.; Fu, G. C. Org. Lett. 2001, 3, 4295-4298.
(21) Abernethy, C. D.; Clyburne, J. A. C.; Cowley, A. H.; Jones, R.
A. J. Am. Chem. Soc. 1999, 121, 2329-2330.

CpNi(NHC)Cl Complexes
Organometallics, Vol. 24, No. 14, 2005 3445
Table 2. Dehalogenation of p-Bromotoluene with
Table 3. Aryl Amination Catalyzed with
CpNi(NHC)Cl Complexes as Precatalysts
CpNi(NHC)Cl Complexes as Precatalysts
yielda (%)
conversiona (%)
time CpNi-
CpNi-
CpNi-
CpNi-
solvent,
CpNi-
CpNi-
CpNi-
CpNi-
entry
R
X
(h) (IMes)Cl (SIMes)Cl (IPr)Cl (SIPr)Cl
T (°C)
(IPr)Cl
(SIPr)Cl
(IMes)Cl
(SIMes)Cl
1
2
3
4
p-Me Cl 20
20b
32b
98
20b
34b
92
45b
75
90
76
48b
82
98
80
THF, 65
p-dioxane, 105
24
30
29
31
40
40
37
23
p-CN Cl
p-Me Br
7
7
a Conversions were determined by GC and are averages of two
runs.
o-Me Br 20
49
50
a Isolated yields. b Conversion determined by 1H NMR of the
crude product.
twisted (by 34.7(8) and 33.34(70)° for 2, 37.91(5)° for 3,
and 31.12(0.08)° for 4), resulting in a favorable steric
arrangement with the metal center. In addition the two
sp3 carbons in the SIMes and SIPr complexes show tor-
sion angles lower than the known value for free SIMes
(13.4°), suggesting that some restriction in rotation is
present (1.6(2)° for 2 and 6.8(3)° for 4). For complex 5,
the nickel-phosphorus (2.1505(3) Å) and nickel-
chloride (2.1871(3) Å) distances lie within the ranges
reported for other three-coordinate nickel complexes.22-25
Catalytic Activity. The reactivity of these
(NHC)Ni(II) complexes was tested in aryl dehalogena-
tion and aryl amination reactions. Dehalogenation of
aryl halides is an important reaction in organic chem-
istry as well as in the design of environmentally benign
industrial processes, due to the high toxicity of these
types of compounds.26 Metal-catalyzed hydrogenation,27
Grignard reagents,28 and the use of Raney Ni-Al alloy
under basic conditions29 are some of the most widely
studied methods of dehalogenation. Fort et al.30 recently
reported on the reduction of aryl halides catalyzed by
nickel(II)/imidazolium chloride in the presence of a
â-hydrogen-containing alkoxide. Conditions similar to
those reported by Fort were tested with our CpNi-
(NHC)Cl complexes. The results of these catalytic tests
are presented in Table 2. Moderate toluene formation
was observed by GC after 4 h of reaction when bromo-
toluene was subjected to these conditions. No further
conversion was observed after longer reaction times. The
yields are similar for all nickel complexes, but the
fastest reaction rates were observed with CpNi(IMes)-
Cl (1): 36% conversion after 1 h in THF at 65 °C. With
CpNi(IPr)Cl (3), for example, only 14% of the dehalo-
genated product was obtained under the same condi-
tions. Similar results were observed under reflux in both
THF and dioxane, despite the difference in reaction
temperature. These results are somewhat puzzling, but
the activation step in this transformation may require
reduction of Ni(II) to Ni(0) prior to oxidatively adding
the aryl halide. The ease with which this reduction is
accomplished (or lack thereof) may be at the origin of
the poor catalytic performance in dehalogenation using
this system.
Carbon-nitrogen bond-forming reactions have been
most widely studied with palladium complexes bearing
sterically demanding ligands such as phosphines31 and
N-heterocyclic carbenes.32 Much less attention has been
paid to nickel-catalyzed aryl amination, even though it
is an attractive alternative to the more costly palladium
derivatives. Different nickel-based catalysts have shown
their utility for this transformation: Ni(COD)2 (COD
) cyclooctadiene) associated with 1,1′-(bis(diphenylphos-
phino)ferrocene (DPPF) or 1,10-phenanthroline,33 het-
erogeneous Ni(0)/C-DPPF,34 and Ni(0)-2,2′-bipyri-
dine.35 Fort et al.36 developed the amination of aryl
chlorides based on a nickel/N-heterocyclic carbene sys-
tem. The Ni(II)/SIPr‚HCl precatalyst in the presence of
NaOtBu was found to be efficient in the arylation of
secondary cyclic or acyclic amines and anilines.
To achieve aryl amination reactions, we first tested
the conditions reported by Fort. Unfortunately, no
conversion was observed by GC. Some optimization
studies were then carried out and revealed that 2 equiv
of KOtBu in dioxane at 105 °C provided the best
catalytic system. Under these conditions, p-chloroben-
zonitrile and p-bromotoluene yielded the corresponding
coupling products with morpholine in good yields (Table
3). Lower conversions were observed using o-bromotolu-
ene. It is noteworthy that no difference in reactivity was
observed between nickel complexes in the coupling of
morpholine and p-bromotoluene, but more challenging
aryl halides showed that CpNi(IPr)Cl (3) and CpNi-
(22) Ballester, L.; Pe´rez, S.; Gutie´rrez, A.; Perpin˜an, M. F.; Guti-
e´rrez-Puebla, E.; Monge, A.; Ruiz, C. J. Organomet. Chem. 1991, 414,
411-420.
(23) Coleman, J. M.; Dahl, L. F. J. Am. Chem. Soc. 1967, 89, 542-
552.
(24) Klani, W.; Schmidt, K.; Bockmann, A.; Brauer, D. J.; Wilke, J.;
Lueken, H.; Elserhans, Inorg. Chem. 1986, 25, 4125-4130.
(25) Koenig, H.; Menu, M. J.; Dartiguenave, M.; Dartiguenave, Y.;
Klein, H. F. J. Am. Chem. Soc. 1990, 112, 5351-5353.
(26) Hudlicky, M. In Comprehensive Organic Synthesis; Trost, B.
M., Fleming, I., Eds.; Pergamon: Oxford, U.K., 1991; Vol. 8, p 895.
(27) (a) Ordonez, S.; Diez, F. V.; Sastre, H. Appl. Catal. B: Environ.
2001, 31, 163-171. (b) Esteros, Y.; Salagre, P.; Medina, F.; Sueiras,
J. E.; Tichit, D.; Coq, B. Appl. Catal. B: Environ. 2001, 32, 25-35.
(28) Hara, R.; Sato, K.; Sun, W.-H.; Takahashi, T. Chem. Commun.
1999, 845-846.
(31) (a) Huang, X.; Anderson, K. W.; Zim, D.; Jiang, L.; Klapars,
A.; Buchwald, S. L. J. Am. Chem. Soc. 2003, 125, 6653-6655. (b)
Hartwig, J. F.; Kawatsura, M.; Hauck, S. I.; Shaughnessy, K. H.;
Alcazar-Roman, L. M. J. Org. Chem. 1999, 64, 5575-5580.
(32) (a) Viciu, M. S.; Kissling, R. M.; Stevens, E. D.; Nolan, S. P.
Org. Lett. 2002, 4, 2229-2231. (b) Stauffer, S. R.; Lee, S.; Stambuli,
J. P.; Hauck, S. I.; Hartwig, J. F. Org. Lett. 2000, 2, 1423-1426.
(33) Wolfe, J. P.; Buchwald, S. L. J. Am. Chem. Soc. 1997, 119,
6054-6058.
(34) (a) Lipshutz, B. H.; Ueda, H. Angew. Chem., Int. Ed. 2000, 39,
4492-4494. (b) Lipshutz, B. H. Adv. Synth. Catal. 2001, 343, 313-
326.
(35) (a) Brenner, E.; Fort, Y. Tetrahedron Lett. 1998, 39, 5359-5362.
(b) Brenner, E.; Schneider, R.; Fort, Y. Tetrahedron 1999, 55, 12829-
12842.
(29) Liu, G.-B.; Tsukinoki, T.; Kanda, T.; Mitoma, Y.; Tashiro, M.
Tetrahedron Lett. 1998, 39, 5591-5994.
(30) Desmarets, C.; Kuhl, S.; Scheneider, R.; Fort, Y. Organometal-
lics 2002, 21, 1554-1559.
(36) (a) Gradel, B.; Brenner, E.; Schneider, R.; Fort, Y. Tetrahedron
Lett. 2001, 42, 5689-5692. (b) Desmarets, C.; Schneider, R.; Fort, Y.
J. Org. Chem. 2002, 67, 3029-3036.

3446 Organometallics, Vol. 24, No. 14, 2005
Kelly et al.
CpNi(SIPr)Cl (4). A solution of nickelocene (2.0 g, 10.6
mmol) in tetrahydrofuran (100 mL) was added to 1,3-bis(2,6-
diisopropylphenyl)-4,5-dihydroimidazolium chloride (4.69 g,
11.0 mmol). The mixture was refluxed overnight. During the
first 1 h of reflux, the color of the solution changed from dark
green to dark red. The solvent was removed under vacuum,
and the resulting red residue was extracted with hot (100 °C)
toluene (100 mL). The solution was filtered and reduced in
volume to 25 mL. When the solution stood for 12 h at ambient
temperature, large red crystals of the product formed. These
were collected by filtration and washed with pentane (25 mL),
leading to 4.7 g (81% yield) of the title compound. 1H NMR
(CDCl3, 400 MHz, δ): 1.23 (d, 12H, CH3); 1.47 (s, 12H, CH3);
3.31 (m, 4H, CH(CH3)2); 3.99 (s, 4H, CH2CH2); 4.48 (s, 5H,
C5H5); 7.35 (d, 4H, m-H); 7.49 (t, 2H, p-H). 13C NMR (CDCl3,
100.6 MHz, δ): 203.18 (s, N-C-N), 147.74 (s, SIPr C), 137.52
(s, SIPr C), 129.49 (s, SIPr C), 124.58 (s, SIPr C), 92.70 (s,
C5H5), 53.57 (s, NCH2CH2N), 28.79 (s, SIPr CH), 26.89 (s, SIPr
CH3), 23.52 (s, SIPr CH3). Anal. Calcd for C32H43N2ClNi: C,
69.90; H, 7.88; N, 5.09; Cl, 6.45. Found: C, 69.85; H, 8.09; N,
5.07; Cl, 6.72.
CpNi(PEt3)Cl (5). A solution of nickelocene (0.2 g, 1.06
mmol) in tetrahydrofuran (20 mL) was added to triethylphos-
phonium chloride (0.18 g, 1.16 mmol). The mixture was stirred
at room temperature overnight. During the first 2 h, the color
of the solution changed from dark green to dark red. The
solvent was removed under vacuum, and the resulting residue
was extracted with dry pentane. The solution was filtered, and
the solvent was removed by vacuum to yield a red powder in
32% yield (178 mg). 1H NMR (C6D6, 400 MHz, δ): 0.89 (m,
9H, CH3); 1.13 (m, 6H, CH2); 5.04 (s, 5H, C5H5). 13C NMR
(CDCl3, 100.6 MHz, δ): 92.37 (s, C5H5); 16.92 (d, CH2); 8.50
(s, CH3). 31P NMR (C6D6, 161.9 MHz, δ): 30.17 (s, 1P, PEt3).
General Procedure for Dehalogenation of p-Bromo-
toluene. In an oven-dried vial fitted with a septum screw cap,
NaH (60 mg, 1.5 mmol, 3 equiv), CpNi(NHC)Cl (0.025 mmol,
5 mol %), and 1 mL of THF were loaded inside a glovebox.
Outside of the glovebox, the mixture was heated to 65 °C and
i-PrOH (0.155 mL, 3 equiv) was added. After 1 h of further
heating, p-bromotoluene was added and the reaction was
monitored by GC.
General Procedure for Arylation of Morpholine. In an
oven-dried vial fitted with a septum screw cap, KOtBu (0.112
g, 1 mmol, 2 equiv), CpNi(NHC)Cl (0.025 mmol, 5 mol %), and
1 mL of p-dioxane were loaded inside a dry glovebox. Outside
of the glovebox, the mixture was heated to 105 °C and
morpholine (65 µL, 1.5 equiv.) was added. After 30 min of
further heating, the aryl halide (0.5 mmol) was added and the
reaction was monitored by GC. After consumption of the
starting halide or no further conversion, the reaction mix-
ture was cooled to room temperature and adsorbed onto silica
gel. The crude reaction mixture was purified by silica gel
chromatography. All compounds described in Table 2 are
known in the literature and were characterized by comparing
their 1H and 13C NMR spectra to the previously reported
data: N-(4-methylphenyl)morpholine (Table 2, entries 1 and
3),37 N-(4-cyanophenyl)morpholine (Table 2, entry 2),38 and
N-(2-methylphenyl)morpholine (Table 2, entry 4).39
(SIPr)Cl (4) were the best catalyst precursors for this
transformation. Studies aimed at exploiting this system
in related catalytic transformations are ongoing.
Conclusions. We have prepared a series of catalyti-
cally active NHC-nickel compounds directly from a
relatively inexpensive and readily available metal pre-
cursor and imidazolium salts. We were also able to
synthesize a trialkylphosphine nickel complex through
the use of an air-stable salt of an otherwise pyrophoric
material. We plan to expand on the use of stable starting
materials in the development of catalytically active
compounds.
Experimental Section
General Considerations. All reactions were carried out
using standard Schlenk techniques under an atmosphere of
dry argon or in MBraun gloveboxes containing dry argon and
less than 1 ppm of oxygen. Solvents were distilled from
appropriate drying agents or were passed through an alumina
column in an MBraun solvent purification system prior to use.
Other anhydrous solvents were purchased from Aldrich and
degassed prior to use by purging with dry argon and were kept
over molecular sieves. Solvents for NMR spectroscopy were
degassed with argon and dried over molecular sieves. Aryl
halides and morpholine were used as received. Flash column
chromatography was performed on silica gel 60 (320-400
mesh). NMR spectra were collected on a 400 MHz Varian
Gemini spectrometer. Elemental analyses were performed by
Robertson Microlit Labs.
CpNi(SIMes)Cl (2). A solution of nickelocene (2.0 g, 10.6
mmol) in tetrahydrofuran (100 mL) was added to 1,3-bis(2,4,6-
trimethylphenyl)-4,5-dihydroimidazolium chloride (3.77 g, 11.0
mmol). The mixture was refluxed overnight. During the first
1 h of reflux, the color of the solution changed from dark green
to dark red. The solvent was removed under vacuum, and the
resulting red residue was extracted with hot (100 °C) toluene
(100 mL). The solution was filtered and reduced in volume to
25 mL. When the solution stood for 12 h at ambient temper-
ature, large red crystals of the product formed. These were
collected by filtration and washed with pentane (25 mL),
leading to the isolation of 4.3 g (87%) of the title compound.
1H NMR (CDCl3, 400 MHz, δ): 2.39 (s, 18H, CH3); 3.90 (s, 4H,
NCH2-CH2N); 4.54 (s, 5H, C5H5); 7.07 (s, 4H, m-H). 13C NMR
(CDCl3, 100.6 MHz, δ): 201.04 (s, N-C-N), 138.51 (s, SIMes
C), 137.11 (s, SIMes C), 129.70 (s, SIMes-C), 92.70 (s, C5H5),
51.21 (s, NCH2CH2N), 21.36 (s, SIMes CH3), 18.74 (s, SIMes
CH3). Anal. Calcd for C26H31N2ClNi: C, 67.06; H, 6.71; N, 6.02,
Cl, 7.61. Found: C, 67.25; H, 6.67; N, 5.98; Cl, 7.87.
CpNi(IPr)Cl (3). A solution of nickelocene (2.0 g, 10.6
mmol) in tetrahydrofuran (100 mL) was added to 1,3-bis(2,6-
diisopropylphenyl)imidazolium chloride (4.68 g, 11.0 mmol).
The mixture was refluxed for 2.5 h. During the first 30 min of
reflux, the color of the solution changed from dark green to
bright red. The solvent was removed under vacuum, and the
resulting red residue was extracted with hot (100 °C) toluene
(100 mL). The solution was filtered and reduced in volume to
25 mL. When the solution stood for 12 h at ambient temper-
ature, large red crystals of the product formed. These were
collected by filtration and washed with pentane (25 mL),
leading to 4.1 g (71% yield) of the title compound. 1H NMR
(CDCl3, 400 MHz, δ): 1.10 (d, 12H, CH3); 1.44 (d, 12H, CH3);
2.84 (m, 4H, CH(CH3)2); 4.51 (s, 5H, C5H5); 7.11 (s, 2H, NCH);
7.22 (d, 4H, m-H); 7.58 (t, 2H, p-H). 13C NMR (CDCl3, 100.6
MHz, δ): 169.31 (s, N-C-N), 146.37 (s, IPr C), 136.71 (s, IPr
C), 130.13 (s, IPr C), 125.51 (s, IPr C), 123.95 (s, NCHdCHN),
92.02 (s, C5H5), 28.61 (s, IPr CH), 26.11 (s, IPr CH3), 22.50 (s,
IPr CH3). Anal. Calcd for C32H41N2ClNi: C, 70.16; H, 7.54; N,
5.11; Cl, 6.47. Found: C, 69.99; H, 7.48; N, 4.99; Cl, 6.54.
Acknowledgment. The National Science Founda-
tion and the Louisiana Board of Regents (fellowship to
R.A.K.) are gratefully acknowledged for financial sup-
port of this work. S.D.-G. thanks the Education, Re-
search and Universities Department of the Basque
(37) Wolfe, J. P.; Buchwald, S. L. J. Org. Chem. 1996, 61, 1133-
1135.
(38) Wolfe, J. P.; Buchwald, S. L. J. Am. Chem. Soc. 1997, 119,
6054-6058.
(39) Hartwig, J. F.; Kawatsura, M.; Hauck, S. I.; Shaughnessy, K.
H.; Alcazar-Roman, L. M. J. Org. Chem. 1999, 64, 5575-5580.

CpNi(NHC)Cl Complexes
Organometallics, Vol. 24, No. 14, 2005 3447
Alternatively, the files CCDC 266059-266062 (compounds
2-5) contain the supplementary crystallographic data for this
paper. These data can be downloaded free of charge via
www.ccdc.cam.ac.uk/conts/retrieving.hmtl (or from the Cam-
bridge Crystallographic Data Centre, 12 Union Road, Cam-
bridge CB21EZ, U.K.; fax (+44) 1223-336-033; e-mail deposit@
ccdc.cam.ac.uk).
Government (Spain) for a postdoctoral fellowship. Lon-
za, FMC Lithium, and Eli Lilly and Co. are gratefully
acknowledged for their gifts of materials. Disclosure of
results prior to publication by Professor Victor Snieckus
is also acknowledged.
Supporting Information Available: X-ray crystallo-
graphic information for 2-5 (CIF files). This material is
available free of charge via the Internet at http://pubs.acs.org.
OM0501879