Carbene transfer reactivities of nickel(II)-N-heterocyclic carbene complexes and their applications in the synthesis of metal-NHC complexes

Article abstract of DOI:10.1021/om200881s

We describe a new synthetic procedure for transition-metal N-heterocyclic carbene complexes (NHCs). A number of Pd(II), Pt(II), Co(III), and Ru(II) complexes containing functionalized NHCs have been obtained in good to excellent yields using Ni^II-NHC complexes as carbine transfer reagents. Ni^II-NHC complexes are easily prepared from the direct reactions of the corresponding imidazolium salts with commercially available Raney nickel powder. The byproduct of transmetalation reactions, NiCl₂, can be easily removed by simple filtration. This study offers a new, economical, and practical synthetic method for metal-NHC complexes.

Full text of DOI:10.1021/om200881s

Article
pubs.acs.org/Organometallics
Carbene Transfer Reactivities of Nickel(II)−N-Heterocyclic
Carbene Complexes and Their Applications in the Synthesis of
Metal−NHC Complexes
Bo Liu,† Xiaolong Liu,† Chao Chen,† Congyan Chen,† and Wanzhi Chen*,†,‡
†Department of Chemistry, Zhejiang University, Xixi Campus, Hangzhou 310028, People’s Republic of China
‡State Key Laboratory of Elemento-organic Chemistry, Nankai University, Tianjin, People’s Republic of China
S
* Supporting Information
ABSTRACT: We describe a new synthetic procedure for
transition-metal N-heterocyclic carbene complexes (NHCs).
A number of Pd(II), Pt(II), Co(III), and Ru(II) complexes
containing functionalized NHCs have been obtained in good
to excellent yields using NiII−NHC complexes as carbene
transfer reagents. NiII−NHC complexes are easily prepared from the direct reactions of the corresponding imidazolium salts with
commercially available Raney nickel powder. The byproduct of transmetalation reactions, NiCl2, can be easily removed by simple
filtration. This study offers a new, economical, and practical synthetic method for metal−NHC complexes.
The poor tolerance to base-sensitive N substituents sig-
nificantly restricts its application. A convenient route
involves the reactions of imidazolium salts with basic metal
acetates, alkoxides, amides, etc. (method B5). The limitation of
method B comes from the moisture sensitivity of these metal
precursors and their availability. The in situ deprotonation
reaction of an imidazolium salt with a transition-metal pre-
cursor in the presence of a base (K2CO3, NEt3, etc.) is also
often used (method C).6 Metal−NHC complexes may also be
obtained by thermal decomposition of carbene adducts of
alcohol,7 chloroform,8 pentafluorobenzene,8,9 CO2,10 CS2,10b
cyanide,11 and phosphenium12 in the presence of a suitable
metal precursor (method D). Method D suffers from the
availability of the carbene precursors or the use of strong bases
and toxic reagents. Oxidative addition of the C2−X bond of
imidazolium salts (X = H) or 2-haloimidazolium salts (X = Cl, I)
with moisture-sensitive zerovalent metal complexes is also
applicable (method E).13 In comparison with the aforemen-
tioned methods, NHC transfer from one metal center to
another represents a practical approach with the obvious
advantage of broad tolerance toward imidazolium salts bearing
various N substituents (method F).14 The readily available
AgI−NHC complex is the most commonly used NHC transfer
reagent. A great number of metal−NHC complexes have been
obtained via transmetalation from AgI−NHCs.15 However,
AgI−NHC complexes are somewhat light-sensitive, and silver
occasionally induces degradation of the imidazolium precur-
sors.16 Other metal−NHC complexes involving W, Mn, Cu,
Au, and Ni have been scarcely employed as carbene sources.17
We have shown that metal−NHCs can be prepared from
imidazolium salts and metal powders in air (method G).18
INTRODUCTION
■
Due to the unique properties of N-heterocyclic carbenes
(NHCs), transition-metal complexes of NHCs are playing an
increasingly important role in the fields of materials, medicine,
and especially in catalysis.1 Since the first application of NHCs
in homogeneous catalysis was reported by Herrmann et al.,2
numerous and ever-increasing uses of NHCs in catalysis have
been described. To a large extent, extensive use, especially in
industrial applications, of these metal−NHC catalysts depends
on the ease of their preparation.3 The known synthetic routes
to metal−NHC complexes can be summarized in Scheme 1.
Scheme 1. Synthetic Routes to Metal−NHC Complexes
Generally, metal−NHCs can be prepared from nucleophilic
reactions of free carbenes and the appropriate metal precursors
(method A).4 However, the generation of a free carbene inter-
mediate often requires a strong base and handling unstable carbene.
Received: September 20, 2011
Published: December 29, 2011
© 2011 American Chemical Society
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Table 1. Preparation of NiII−NHC Complexes
Alternatively, they can be electrochemically obtained using
imidazolium salts as the carbene sources and the metal plate
as the sacrificial anode (method H).19 These two routes are
suitable for first-row transition metals. As a continuation of our
organometallic chemistry of NHCs,18−20 we now report a new,
practical, and convenient synthetic procedure for PdII−, PtII−,
CoIII−, and RuII−NHC complexes using readily available NiII−
NHC complexes as the carbene sources.
RESULTS AND DISCUSSION
■
Preparation of NiII−NHC Complexes. The transmeta-
lation reaction is commonly used to prepare NiII−NHC
complexes.21 A more convenient route is the reaction of an
imidazolium salt with commercially available nickel powder in
air,18 shown in Scheme 2.
Scheme 2. Synthesis of NiII−NHC Complexes
Using the above procedure, [Ni(L1)3](PF6)2 (1) was ob-
tained in ca. 40% yield from [HL1](PF6) (L1 = 3-methyl-
1-(pyridin-2-yl)imidazolylidene) and an excess of nickel powder
(200 mesh) in acetonitrile at 70 °C for 2 days. Freshly prepared
Raney nickel powder is more reactive, and the yield could be
increased to 60%. Similarly, 2−7 were obtained in good to
excellent yields (Table 1). Complexes 2 and 6 could even be
obtained in nearly quantitative yields based on the amount of
imidazolium salts (entries 2 and 6). These reactions took place
without removal of moisture and oxygen, and the excess nickel
powder was removed via simple filtration. Compounds 1−4
and 7 have been obtained via other preparative procedures.22
The structures of 5 and 6 were determined by X-ray diffraction
analysis (see the Supporting Information).
Carbene Transfer from Ni(II) to Pd(II). With well-defined
NiII−NHC complexes in hand, we studied the possibility of
transmetalation from nickel(II) to palladium(II). The trans-
metalation reactions between 1−7 and Pd(COD)Cl2 were
carried out in acetonitrile at 70 °C, giving 8−14 in good to
excellent yields (Table 2). The initiation of the reaction was
easily monitored by the observation of green NiCl2 precipitate,
which could be simply removed after the reaction.
Using this procedure, reaction of 1 with 1 equiv of Pd-
(COD)Cl2 did not give its isostructural palladium complex, but
[Pd(L1)2(CH3CN)](PF6)2 (8) was obtained in 63% yield. Nickel
complexes 2, 5, and 6 reacted with 2 equiv of Pd(COD)Cl2,
affording 9, 12, and 13 in more than 90% yields. Complex 10
was obtained in 73% yield from 3 and Pd(COD)Cl2. Complex
10 is isostructural with 3, and the cis configuration of 10 was
confirmed by NOE measurements. Similarly, 11 and 14 were
obtained in 69% and 82% yields from 4 and 7, respectively.
[Pd(η3-C3H5)Cl]2 could also be used as the palladium source.
For example, 13 was obtained in 50% yield when [Pd(η3-
C3H5)Cl]2 was used. Complexes 8−11 and 14 have already
been prepared utilizing other synthetic procedures.22b,d,23 The
structures of 12 and 13 were further determined by X-ray
diffraction studies (see the Supporting Information).
a
Reaction conditions: imidazolium salt 0.20 mmol, Raney nickel
b
2 mmol, CH3CN 3 mL, 70 °C, 2 days. Isolated yields are calculated
on the basis of the imidazolium salts used.
Complex 2 reacted with 2 equiv of Pt(COD)Cl2 in acetonitrile
at 70 °C, affording 15 in 90% yield. Treatment of 3 with
1 equiv of Pt(COD)Cl2 afforded a mixture of cis and trans
1
isomers 16 in 63% yield. In its H NMR spectrum, two sets of
resonance signals were observed due to two isomers. The cis
and trans isomers could be distinguished by NOE measure-
ments. When 5 was treated with 2 equiv of Pt(COD)Cl2, 17
was isolated in 91% yield as the sole product. Similarly, the
PtII−NHC complex 18 could be obtained by treatment of 6
1
with 2 equiv of Pt(COD)Cl2 in 94% yield. H NMR spectrum
of 18 showed one set of resonance signals of the corresponding
ligand. The 13C NMR spectrum of 18 exhibited a resonance
signal at 147.0 ppm ascribed to the carbenic carbon atom.20c,24
The molecular structure of 18 is given in the Supporting
Information.
Treatment of 1 with 1 equiv of anhydrous CoCl2 in
acetonitrile at 70 °C afforded Co(III) complex 19 as a yellow
solid in 31% yield. The structure of 19 was determined by
elemental analysis. Unfortunately, a Co−C resonance signal in
13C NMR spectrum of 19 was not observed. This phenomenon
has also been found in our previous study and others’ work.25
Complex 20 was obtained as a yellow solid via treatment of
Carbene Transfer from Ni(II) to Pt(II), Co(III), and
Ru(II). The present procedure was also used to prepare Pt,
Co, and Ru complexes, and the results are given in Table 3.
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a
Table 2. Synthesis of PdII−NHC Complexes
a
Reaction conditions: Ni−NHC 0.05 mmol, Pd(COD)Cl2 0.05 mmol for 8, 10, 11, and 14 or 0.10 mmol for 9, 12, and 13, CH3CN 2 mL, 70 °C,
b
24 h. [Pd(η3-C3H5)Cl]2 was used instead of Pd(COD)Cl2.
a
Table 3. Carbene Transfer from Ni(II) to Pt(II), Co(III), and Ru(II)
a
Reaction conditions: Ni−NHC 0.05 mmol, MLnClm 0.05 mmol for 16 and 19−22 or 0.10 mmol for 15, 17, and 18, CH3CN 2 mL, 70 °C, 24 h,
MLnClm = Pt(COD)Cl2, CoCl2, [Ru(p-cycmene)Cl2]2. Yields quoted are isolated yields.
6 with 1 equiv of CoCl2 in 56% yield. The 13C resonance of
carbenic carbon in 20 appears at 182.2 ppm, similar to case for
the known CoIII−NHC complexes.25a,26 Reactions of 3 and 4
with [Ru(p-cymene)Cl2]2 in acetonitrile at 70 °C led to the
isolation of 21 and 22 in yields of 94% and 71%, respectively.
Complex 21 could also be obtained via in situ transmetalation
of the corresponding AgI−NHC with 1/2 equiv of [Ru(p-
cymene)Cl2]2 in acetonitrile. The structures of 20 and 21 were
finally determined by X-ray diffraction (see the Supporting
Information).
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Carbene Transfer from Ni(II) to Ni(II). The present
procedure is also useful for the carbene transfer from one nickel
to another. As shown in Scheme 3, reactions of 2 or 5 with
Scheme 5. Carbene Transfer Process
Scheme 3. Carbene Transfer from Ni(II) to Ni(II)
using imidazolium salts and Raney nickel powder in excellent
yields. NiII−NHCs are very stable to air, moisture, and light and
thus are suitable carbene sources. The carbene transfer reac-
tions from Ni(II) to other metal centers take place under mild
conditions in air and do not require any expensive reagents and
handling of air-sensitive compounds. Moreover, the reactions
also tell us that the fast association and dissociation of Ni2+ ion
and free NHC is possible and the Ni−C bond is kinetically
active, although we have not obtained any spectroscopic
evidence at present. Further research aiming at studying the
kinetics of the carbene transfer reactions as well as utilizing
these metal−NHC complexes in catalytic organic reactions are
currently in progress.
2 equiv of Ni(PPh3)2Cl2 at 70 °C readily afforded 23 and 24.
Complex 23 has also been obtained from the corresponding
AgI−NHC and Ni(PPh3)2Cl2.22b As expected, complex 24
could also react with Pd(COD)Cl2 and Pt(COD)Cl2, giving 12
and 17, respectively. Heating the mixture of 2 or 5 in the
EXPERIMENTAL SECTION
■
All chemicals were obtained from commercial suppliers and used
without further purification. [HL1](PF6),29 [HL2](PF6),22b [HL3]-
(PF6),30 [HL4](PF6),23c [HL5](PF6),31 and [H2L7](PF6)2
were
22d
prepared according to known procedures. The synthetic procedure of
n
presence of an excess of anhydrous KCl, LiCl, or Bu4NCl at
[HL6](PF6) is given in the Supporting Information. The elemental
analyses were performed on a Flash EA 1112 instrument. H and 13C
1
70 °C for 24 h did not afford 23 and 24, which illustrated that
23 and 24 did not result from the thermal decomposition of
2 and 5.
NMR spectra were recorded on a Bruker Avance-400 (400 MHz)
spectrometer. Chemical shifts (δ) are expressed in ppm downfield from
TMS at δ = 0 ppm, and coupling constants (J) are expressed in Hz.
General Procedure for the Preparation of NiII−NHC
Complexes. NiII−NHCs 1−7 were synthesized by the following
route: a solution of the imidazolium salt (0.20 mmol) in 3 mL of
CH3CN was treated with an excess of Raney nickel powder (120 mg).
The mixture was allowed to react at 70 °C for 2 days in air. After it was
cooled to room temperature, the solution was filtered through Celite.
Then the filtrate was concentrated to ca. 1 mL. The compounds were
obtained by adding diethyl ether to the filtrate.
In Situ Carbene Transfer Reactions. The metal−NHC
complexes could also be prepared from imidazolium salts
without isolation of the NiII−NHC complexes. For example, as
shown in Scheme 4, heating the mixture of [HL5](PF6) and an
excess of Raney nickel powder in acetonitrile at 80 °C for 24 h
and subsequent addition of 1 equiv of M(COD)Cl2 (M = Pd,
Pt) afforded 12 and 17 in more than 80% yields after a simple
workup.
It is generally believed that metal−carbene bonds are strong
and inert. Dissociation of a NHC ligand or its substitution by
an incoming ligand has been scarcely observed in Ni−NHC
complexes.27,28 We noted that 1 decomposed slowly in CD3CN
at 75 °C to give the corresponding imidazolium and unidenti-
fied inorganic nickel species, demonstrating the lability of the
Ni−NHC bond. An equilibrium between Ni species and a
free carbene in solution may exist.27a The formation of more
stable M−C bonds might be the driving force for the reactions.
The formation of insoluble NiCl2 in acetonitrile obviously
promoted the dissociation of the Ni−C bond and the carbene
transfer process (Scheme 5).
[Ni(L1)3](PF6)2 (1). This complex was prepared from [HL1](PF6)
(61 mg, 0.20 mmol) and Raney nickel (120 mg). Yield: 33 mg (60%),
1
red-brown powder. H NMR (400 MHz, DMSO-d6): δ 8.47 (s, 1H),
8.16 (br, 2H), 8.07 (s, 1H), 7.99 (m, 3H), 7.72 (br, 4H), 7.64 (s, 1H),
7.60 (s, 1H), 7.54 (s, 1H), 7.50 (s, 1H), 7.36 (s, 1H), 7.32 (s, 1H),
7.24 (s, 1H), 3.69 (s, 3H, CH3), 3.34 (s, 3H, CH3), 3.32 (s, 3H, CH3).
Anal. Calcd for C27H27F12N9NiP2: C, 39.25; H, 3.29; N, 15.26. Found:
C, 39.42; H, 3.28; N, 14.86. No satisfactory 13C NMR spectrum of 1
was obtained.
[Ni(L2)2](PF6)2 (2).22b This complex was prepared from [HL2](PF6)
(90 mg, 0.20 mmol) and Raney nickel (120 mg). Yield: 87 mg (93%),
brown powder. Anal. Calcd for C40H39F12N9NiP2 ([Ni(L2)2]-
(PF6)2·CH3CN): C, 48.31; H, 3.95; N, 12.68. Found: C, 48.59; H,
3.95; N, 12.31. The compound is paramagnetic, and thus no NMR
spectrum was obtained.
[Ni(L3)2](PF6)2 (3). This complex was prepared from [HL3](PF6)
CONCLUSION
■
(64 mg, 0.20 mmol) and Raney nickel (120 mg). Yield: 48 mg (69%),
1
We have described a convenient method for the synthesis of
metal−NHC complexes using NiII−NHC complexes as the
carbene sources. NiII−NHC complexes can be easily prepared
pale yellow powder. H NMR (400 MHz, DMSO-d6): δ 8.14 (t, J =
7.2 Hz, 2H, 4-PyH), 7.99 (d, J = 5.6 Hz, 2H, 6-PyH), 7.91 (d, J = 8.0 Hz,
2H, 3-PyH), 7.70 (s, 2H, NCHCHN), 7.48 (t, J = 6.4 Hz, 2H,
Scheme 4. In Situ Carbene Transfer from Ni(II) to Pd(II) and Pt(II)
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5-PyH), 7.37 (s, 2H, NCHCHN), 6.79 (d, J = 14.8 Hz, 2H, CH2), 5.81
(d, J = 15.2 Hz, 2H, CH2), 3.17 (s, 6H, CH3). 13C NMR (100 MHz,
DMSO-d6): δ 157.64 (Ni−C), 155.09, 153.05, 141.42, 125.47, 125.31,
124.94, 123.58, 53.82, 36.08. Anal. Calcd for C20H22F12N6NiP2: C,
34.56; H, 3.19; N, 12.09. Found: C, 34.82; H, 3.19; N, 11.82. The cis
configuration of 3 was confirmed by NOE measurement.
1.78 (m, 2H, NCH2CH2CH2CH3), 1.33 (m, 2H, NCH2CH2CH2CH3),
0.92 (t, J = 6.8 Hz, 3H, NCH2CH2CH2CH3). Anal. Calcd for
C19H18ClF6N4PPd: C, 38.73; H, 3.08; N, 9.51. Found: C, 39.04; H,
3.10; N, 9.25.
[Pd(L3)2](PF6)2 (10). This complex was synthesized from 3 (35 mg,
0.05 mmol) and Pd(COD)Cl2 (14 mg, 0.05 mmol). Yield: 27 mg
(73%), white powder. 1H NMR (400 MHz, DMSO-d6): δ 8.29 (d, J =
5.2 Hz, 2H, 6-PyH), 8.24 (t, J = 7.6 Hz, 2H, 4-PyH), 7.96 (d, J = 7.6 Hz,
2H, 3-PyH), 7.76 (d, J = 1.6 Hz, 2H, NCHCHN), 7.62 (t, J = 6.4 Hz, 2H,
5-PyH), 7.49 (d, J = 1.6 Hz, 2H, NCHCHN), 6.17 (d, J = 14.8 Hz, 2H,
NCH2), 5.80 (d, J = 15.6 Hz, 2H, NCH2), 3.31 (s, 6H, NCH3). 13C
NMR (100 MHz, DMSO-d6): δ 156.05 (Pd−C), 154.28, 152.88,
142.00, 126.28, 125.91, 124.35, 123.64, 54.76, 37.04. Anal. Calcd for
C20H22F12N6P2Pd: C, 32.34; H, 2.99; N, 11.31. Found: C, 32.29; H,
3.10; N, 11.66.
[Ni(L4)2](PF6)2 (4).22a This complex was prepared from [HL4]-
(PF6) (77 mg, 0.20 mmol) and Raney nickel (120 mg). Yield: 63 mg
1
(77%), light yellow powder. H NMR (400 MHz, DMSO-d6): δ 8.97
(d, J = 4.8 Hz, 4H, 4,6-pyrimidinyl H), 8.32 (d, J = 5.2 Hz, 2H,
6-pyridyl H), 8.21 (t, J = 7.6 Hz, 2H, 4-pyridyl H), 8.04 (d, J = 7.6 Hz,
2H, 3-pyridyl H), 7.62−7.52 (m, 8H, NCHCHN + 5-pyridyl H +
5-pyrimidinyl H), 6.84 (d, J = 15.2 Hz, 2H, CH2), 6.04 (d, J = 14.8 Hz,
2H, CH2). Anal. Calcd for C26H22F12N10NiP2: C, 37.94; H, 2.69; N,
17.02. Found: C, 38.02; H, 2.69; N, 16.56.
[Pd(L4)2](PF6)2 (11).23c This complex was synthesized from 4 (42 mg,
0.05 mmol) and Pd(COD)Cl2 (14 mg, 0.05 mmol). Yield: 30 mg
(69%), white powder. 1H NMR (400 MHz, DMSO-d6): δ 8.86 (d, J =
4.8 Hz, 4H, 4,6-pyrimidinyl H), 8.53 (d, J = 5.6 Hz, 2H, 6-pyridyl H),
8.30 (t, J = 7.6 Hz, 2H, 4-pyridyl H), 8.08 (d, J = 7.6 Hz, 2H, 3-pyridyl
H), 7.72−7.56 (m, 8H, NCHCHN + 5-pyridyl H + 5-pyrimidinyl H),
6.23 (d, J = 14.4 Hz, 2H, NCH2), 6.00 (d, J = 14.8 Hz, 2H, NCH2).
Anal. Calcd for C26H22F12N10P2Pd: C, 35.86; H, 2.55; N, 16.08.
Found: C, 35.79; H, 2.59; N, 15.77.
[Pd(L5)Cl](PF6) (12). This complex was synthesized from 5 (43 mg,
0.05 mmol) and Pd(COD)Cl2 (29 mg, 0.10 mmol). Yield: 49 mg
(92%), white powder. 1H NMR (400 MHz, DMSO-d6): δ 9.43 (d, J =
5.6 Hz, 2H, 6-PyH), 8.24 (t, J = 7.6 Hz, 2H, 4-PyH), 7.89 (d, J =
7.6 Hz, 2H, 3-PyH), 7.70 (t, J = 7.2 Hz, 2H, 5-PyH), 7.64 (s, 2H,
NCHCHN), 5.72 (s, 4H, NCH2). 13C NMR (100 MHz, DMSO-d6):
δ 156.20 (Pd−C), 153.03, 149.19, 141.70, 127.01, 125.71, 122.13,
53.95. Anal. Calcd for C15H14ClF6N4PPd: C, 33.54; H, 2.63; N, 10.43.
Found: C, 33.67; H, 2.65; N, 10.25.
[Ni(L5)2](PF6)2 (5). This complex was prepared from [HL5](PF6)
(80 mg, 0.20 mmol) and Raney nickel (120 mg). Yield: 72 mg (85%),
1
yellow powder. H NMR (400 MHz, CD3CN): δ 8.69 (s, 2H), 8.00
(br, 2H), 7.92 (s, 2H), 7.74 (br, 4H), 7.31 (m, 6H), 7.23 (d, J = 7.6 Hz,
2H), 7.03 (s, 2H), 6.44 (d, J = 15.2 Hz, 2H, CH2), 5.49 (d, J =
14.8 Hz, 2H, CH2), 5.01 (d, J = 15.2 Hz, 2H, CH2), 4.38 (d, J = 15.6 Hz,
2H, CH2). 13C NMR (100 MHz, CD3CN): δ 158.88 (Ni-C), 154.70,
154.26, 152.25, 149.73, 140.85, 137.60, 125.33, 124.02, 123.67, 123.24,
122.57, 54.60, 54.24. Anal. Calcd for C30H28F12N8NiP2: C, 42.43; H,
3.32; N, 13.19. Found: C, 42.34; H, 3.25; N, 12.98.
[Ni(L6)2](PF6)2 (6). This complex was prepared from [HL6](PF6)
(122 mg, 0.20 mmol) and Raney nickel (120 mg). Yield: 117 mg
(92%), yellow powder. Anal. Calcd for C54H48F12N16NiP2: C, 51.08;
H, 3.81; N, 17.65. Found: C, 51.00; H, 3.91; N, 17.70. No satisfactory
NMR spectra of 6 were obtained, although we repeatedly measured in
different deuterated solvents.
[Ni(L7)](PF6)2 (7).22d This complex was prepared from [H2L7]-
(PF6)2 (124 mg, 0.20 mmol) and Raney nickel (120 mg). Yield: 108 mg
1
12 was also obtained from 24 (49 mg, 0.10 mmol) and
Pd(COD)Cl2 (29 mg, 0.10 mmol). Yield: 51 mg (95%).
(80%), greenish yellow powder. H NMR (400 MHz, DMSO-d6):
δ 8.58 (d, J = 5.6 Hz, 2H, 6-PyH), 8.22 (d, J = 7.6 Hz, 2H, 4-PyH),
7.86 (m, 6H, NCHCHN + 3-PyH), 7.58 (d, J = 6.4 Hz, 2H, 5-PyH),
6.42 (s, 2H, NCH2N), 5.80 (s, 4H, NCH2C). Anal. Calcd for C19H18-
F12N6NiP2: C, 33.61; H, 2.67; N, 12.38. Found: C, 33.73; H, 2.71; N,
12.12.
General Procedure for Carbene Transfer from Ni(II) to Pd(II),
Pt(II), Co(III), Ru(II), and Ni(II). Complexes 8−24 were synthesized
via the following route: a solution of NiII−NHC (0.05 mmol) in 2 mL
of CH3CN was treated with the corresponding equivalent of
Pd(COD)Cl2, Pt(COD)Cl2, CoCl2, [Ru(p-cycmene)Cl2]2, or Ni-
(PPh3)2Cl2. The mixture was allowed to react at 70 °C for 24 h. The
solution was filtered through Celite to remove the precipitate of NiCl2.
Then the filtrate was concentrated to ca. 1 mL. The compounds were
obtained by adding diethyl ether to the filtrate.
[Pd(L1)2(CH3CN)](PF6)2 (8). This complex was synthesized from 1
(41 mg, 0.05 mmol) and Pd(COD)Cl2 (14 mg, 0.05 mmol). Yield:
24 mg (63%), white powder. 1H NMR (400 MHz, DMSO-d6): δ 8.97
(d, J = 4.8 Hz, 1H, PyH), 8.58 (br, 1H, PyH), 8.56 (s, 1H, imidazolylidene
H), 8.42 (t, J = 7.6 Hz, 1H, PyH), 8.40 (s, 1H, imidazolylidene H),
8.19 (d, J = 8.0 Hz, 1H, PyH), 8.18 (s, 1H, imidazolylidene H), 8.07 (t,
J = 7.8 Hz, 1H, PyH), 7.88 (s, 1H, PyH), 7.73 (t, J = 6.0 Hz, 1H, PyH),
7.57 (s, 1H, imidazolylidene H), 7.52 (t, J = 6.0 Hz, 1H, PyH), 4.06 (s,
3H, CH3), 2.98 (s, 3H, CH3), 2.07 (s, 3H, CH3CN). 13C NMR (100
MHz, DMSO-d6): δ 160.29 (Pd−C), 156.92 (Pd−C), 151.43, 150.39,
149.53, 148.18, 143.99, 140.03, 125.88, 125.83, 124.71, 124.11, 122.78,
118.43, 117.92, 117.56, 112.73, 39.00, 36.51, 1.51. Anal. Calcd for
C24H27F12N9P2Pd ([Pd(L1)2(CH3CN)](PF6)2·2CH3CN): C, 34.40;
H, 3.25; N, 15.05. Found: C, 34.57; H, 3.32; N, 14.87.
[Pd(L6)Cl](PF6) (13). This complex was synthesized from 6 (64 mg,
0.05 mmol) and Pd(COD)Cl2 (29 mg, 0.10 mmol). Yield: 71 mg
(95%), white powder. 1H NMR (400 MHz, DMSO-d6): δ 8.63 (s, 2H,
5-triazole H), 7.93 (q, J = 2.8 Hz, 2H, 4,7-benzimidazole H), 7.55 (q,
J = 2.8 Hz, 2H, 5,6-benzimidazole H), 7.41−7.38 (m, 10H, PhH), 5.89
(s, 4H, NCH2), 5.82 (s, 4H, NCH2). 13C NMR (100 MHz, DMSO-
d6): δ 157.85 (Pd−C), 139.94, 134.67, 132.50, 129.30, 129.13, 128.75,
125.74, 125.22, 112.21, 55.28, 40.96. Anal. Calcd for C28H25.5ClF6-
N8.5PPd ([Pd(L6)Cl](PF6)·0.5CH3CN): C, 43.79; H, 3.35; N, 15.50.
Found: C,43.81; H, 3.46; N, 15.33.
13 was also obtained from 6 (64 mg, 0.05 mmol) and [Pd(η3-
C3H5)Cl]2 (18 mg, 0.05 mmol). Yield: 36 mg (50%).
[Pd(L7)](PF6)2 (14).22d This complex was synthesized from 7 (34 mg,
0.05 mmol) and Pd(COD)Cl2 (14 mg, 0.05 mmol). Yield: 30 mg
(82%), colorless powder. 1H NMR (400 MHz, DMSO-d6): δ 8.94 (d,
J = 5.2 Hz, 2H, 6-PyH), 8.32 (t, J = 7.6 Hz, 2H, 4-PyH), 7.97 (d, J =
7.6 Hz, 2H, 3-PyH), 7.85−7.80 (m, 6H, NCHCHN + 5-PyH), 6.50 (s,
2H, NCH2N), 5.74 (s, 4H, CCH2N). Anal. Calcd for C19H18F12-
N6P2Pd: C, 31.40; H, 2.50; N, 11.56. Found: C, 31.64; H, 2.50; N,
11.34.
[Pt(L2)Cl](PF6) (15).20c This complex was synthesized from 2 (47 mg,
0.05 mmol) and Pt(COD)Cl2 (38 mg, 0.10 mmol). Yield: 61 mg
(90%), yellow powder. 1H NMR (400 MHz, DMSO-d6): δ 8.98 (d, J =
8.8 Hz, 1H, phenanthroline H), 8.88 (d, J = 7.6 Hz, 1H, phenan-
throline H), 8.58 (d, J = 4.8 Hz, 1H, phenanthroline H), 8.47 (s, 1H,
NCHCHN), 8.31 (d, J = 8.4 Hz, 1H, phenanthroline H), 8.10 (br,
3H, phenanthroline H), 7.85 (s, 1H, NCHCHN), 4.25 (br, 2H,
NCH2CH2CH2CH3), 1.75 (m, 2H, NCH2CH2CH2CH3), 1.30 (m, 2H,
NCH2CH2CH2CH3), 0.90 (t, J = 7.2 Hz, 3H, NCH2CH2CH2CH3).
Anal. Calcd for C19H18ClF6N4PPt: C, 33.66; H, 2.68; N, 8.27. Found: C,
33.87; H, 2.69; N, 8.30.
[Pd(L2)Cl](PF6) (9).22b This complex was synthesized from 2 (47 mg,
0.05 mmol) and Pd(COD)Cl2 (28 mg, 0.10 mmol). Yield: 56 mg
(96%), yellow powder. 1H NMR (400 MHz, DMSO-d6): δ 9.05 (d, J =
8.8 Hz, 1H, phenanthroline H), 8.83 (d, J = 8.4 Hz, 1H, phenan-
throline H), 8.54 (s, 2H, NCHCHN + phenanthroline H), 8.39 (d, J =
8.8 Hz, 1H, phenanthroline H), 8.14 (s, 2H, phenanthroline H), 8.02
(dd, J = 8.0 Hz, J = 5.2 Hz, 1H, phenanthroline H), 7.82 (d, J = 2 Hz,
1H, NCHCHN), 4.36 (t, J = 6.8 Hz, 2H, NCH2CH2CH2CH3),
[Pt(L3)2](PF6)2 (16). This complex was synthesized from 3 (35 mg,
0.05 mmol) and Pt(COD)Cl2 (19 mg, 0.05 mmol). Yield: 26 mg
(63%, cis/trans = 0.8/1), white powder. 1H NMR (400 MHz, DMSO-
d6): δ 8.78 (d, J = 5.6 Hz, 2H, 6-PyHtrans), 8.39 (d, J = 5.6 Hz, 1.6H,
286
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Organometallics
Article
6-PyHcis), 8.28 (t, J = 7.6 Hz, 3.6H, 4-PyHtrans + 4-PyHcis), 8.02 (d, J =
8.4 Hz, 1.6H, 3-PyHcis), 7.99 (d, J = 8.0 Hz, 2H, 3-PyHtrans), 7.74 (s,
1.6H, NCHCHNcis), 7.71 (s, 2H, NCHCHNtrans), 7.64 (t, J = 6.4 Hz,
1.6H, 5-PyHcis), 7.55 (t, J = 6.4 Hz, 2H, 5-PyHtrans), 7.50 (s, 1.6H,
NCHCHNcis), 7.45 (s, 2H, NCHCHNtrans), 5.93 (d, J = 11.2 Hz, 1.6H,
CH2 cis), 5.90 (d, J = 11.2 Hz, 2H, CH2 trans), 5.75 (d, J = 16.4 Hz, 2H,
CH2 trans), 5.71 (d, J = 15.6 Hz, 1.6H, CH2 cis), 3.31 (s, 4.8H, CH3 cis),
3.17 (s, 6H, CH3 trans). 13C NMR (100 MHz, DMSO-d6): δ 164.62
(Pt−C), 156.95 (Pt−C), 155.41, 154.18, 153.17, 143.31, 142.27,
127.35, 127.28, 127.17, 126.92, 126.41, 124.10, 123.91, 123.02, 122.23,
54.63, 54.54, 36.67, 36.10. Anal. Calcd for C21H23.5F12N6.5P2Pt
([Pt(L3)2](PF6)2·0.5CH3CN): C, 29.60; H, 2.78; N,10.69. Found:
C, 29.56; H, 2.84; N, 10.36.
[Pt(L5)Cl](PF6) (17). This complex was synthesized from 5 (43 mg,
0.05 mmol) and Pt(COD)Cl2 (38 mg, 0.10 mmol). Yield: 57 mg
(91%), white powder. 1H NMR (400 MHz, DMSO-d6): δ 9.56 (d, J =
5.6 Hz, 2H, 6-PyH), 8.28 (t, J = 7.2 Hz, 2H, 4-PyH), 7.90 (d, J = 7.2 Hz,
2H, 3-PyH), 7.70 (t, J = 6.4 Hz, 2H, 5-PyH), 7.63 (s, 2H, NCHCHN),
5.58 (s, 4H, NCH2). 13C NMR (100 MHz, DMSO-d6): δ 155.97 (Pt−C),
153.09, 141.71, 139.21, 127.49, 126.38, 121.26, 53.68. Anal. Calcd
for C15H14ClF6N4PPt: C, 28.79; H, 2.25; N, 8.95. Found: C, 28.96; H,
2.21; N, 8.79.
for C23H24ClF6N6PRu: C, 41.48; H, 3.63; N, 12.62. Found: C, 41.49;
H, 3.63; N, 12.47.
[Ru(L4)(CH3CN)3](PF6)2 (22). This complex was synthesized from 4
(42 mg, 0.05 mmol) and [Ru(p-cycmene)Cl2]2 (15 mg, 0.05 mmol).
Yield: 34 mg (71%), bright yellow powder. 1H NMR (400 MHz,
DMSO-d6): δ 9.17 (d, J = 8.0 Hz, 1H), 9.07 (d, J = 5.2 Hz, 1H), 9.00
(d, J = 4.4 Hz, 1H), 8.36 (s, 1H, NCHCHN), 8.06 (t, J = 8.0 Hz, 1H),
7.91 (s, 1H, NCHCHN), 7.76 (d, J = 8.4 Hz, 1H), 7.66 (t, J = 5.2 Hz,
1H), 7.59 (t, J = 5.6 Hz, 1H), 6.02 (s, 2H, NCH2), 2.20 (s, 6H,
CH3CN), 2.07 (s, 3H, CH3CN). 13C NMR (100 MHz, DMSO-d6): δ
198.81 (Ru−C), 162.51, 159.38, 158.94, 156.77, 153.64, 138.13,
126.62, 124.22, 124.11, 119.03, 118.08, 53.19, 3.75, 3.40. Anal. Calcd
for C19H20F12N8P2Ru: C, 30.37; H, 2.68; N, 14.91. Found: C, 30.66;
H, 2.69; N, 14.95.
[Ni(L2)Cl](PF6) (23).22b This complex was synthesized from 2 (47 mg,
0.05 mmol) and Ni(PPh3)2Cl2 (66 mg, 0.10 mmol). Yield: 28 mg
1
(52%), orange powder. H NMR (400 MHz, acetone-d6): δ 9.04 (d,
J = 8.4 Hz, 1H, phenanthroline H), 8.86 (d, J = 8.4 Hz, 1H, phenan-
throline H), 8.74 (d, J = 4.8 Hz, 1H, phenanthroline H), 8.37 (d, J =
2.0 Hz, 1H, NCHCHN), 8.33 (d, J = 8.8 Hz, 1H, phenanthroline H),
8.20 (q, J = 8.8 Hz, 2H, phenanthroline H), 8.12 (dd, J = 4.8 Hz, J =
7.6 Hz, 1H, phenanthroline H), 7.72 (d, J = 2.0 Hz, 1H, NCHCHN),
4.50 (t, J = 7.2 Hz, 2H, NCH2CH2CH2CH3), 1.89 (m, 2H, NCH2-
CH2CH2CH3), 1.43 (m, 2H, NCH2CH2CH2CH3), 0.96 (t, J = 7.6 Hz,
3H, NCH2CH2CH2CH3). Anal. Calcd for C19H18ClF6N4NiP: C,
42.14; H, 3.35; N, 10.35. Found: C, 42.06; H, 3.30; N, 10.27.
[Ni(L5)Cl](PF6) (24). This complex was synthesized from 5 (43 mg,
0.05 mmol) and Ni(PPh3)2Cl2 (66 mg, 0.10 mmol). Yield: 10 mg
(20%), yellow powder. 1H NMR (400 MHz, acetone-d6): δ 8.13 (t, J =
7.6 Hz, 2H, 4-PyH), 8.11 (s, 1H, NCHCHN), 8.02 (m, 2H, PyH),
7.84−7.73 (m, 5H, PyH + NCHCHN), 5.79 (s, 4H, CH2). Anal. Calcd
for C15H14ClF6N4NiP: C, 36.81; H, 2.88; N, 11.45. Found: C, 36.94;
H, 2.82; N, 11.23. No satisfactory 13C NMR spectrum of 24 was
obtained.
General Procedure for in Situ Carbene Transfer from Ni(II)
to Pd(II) and Pt(II). A solution of imidazolium salt (0.10 mmol) in
3 mL of CH3CN was treated with an excess of Raney nickel powder
(100 mg). The mixture was allowed to react at 80 °C for 24 h in air.
Without any additional treatment, 0.10 mmol of Pd(COD)Cl2 or
Pt(COD)Cl2 was directly added to the reaction mixture. Then, the
mixture was allowed to react at 80 °C overnight. After it was cooled to
room temperature, the solution was filtered through Celite to remove
nickel powder and the precipitate of NiCl2. The filtrate was
concentrated to ca. 1 mL. The complexes were obtained by adding
diethyl ether to the filtrate. 12 and 17 were obtained in 84% and 87%
yields.
17 was also obtained from 24 (49 mg, 0.10 mmol) and
Pt(COD)Cl2 (38 mg, 0.10 mmol). Yield: 56 mg (90%).
[Pt(L6)Cl](PF6) (18). This complex was synthesized from 6 (64 mg,
0.05 mmol) and Pt(COD)Cl2 (38 mg, 0.10 mmol). Yield: 78 mg
(94%), white powder. 1H NMR (400 MHz, DMSO-d6): δ 8.64 (s, 2H,
5-triazole H), 7.88 (q, J = 3.2 Hz, 2H, 4,7-benzimidazole H), 7.52 (q,
J = 3.2 Hz, 2H, 5,6-benzimidazole H), 7.43−7.35 (m, 10H, PhH), 5.83
(s, 4H, CH2), 5.79 (s, 4H, CH2). 13C NMR (100 MHz, DMSO-d6): δ
147.06 (Pt−C), 139.50, 134.54, 132.46, 129.32, 129.17, 128.76,
126.08, 124.96, 112.04, 55.43, 40.88. Anal. Calcd for C27H24-
ClF6N8PPt: C, 38.79; H, 2.89; N, 13.40. Found: C, 38.89; H, 2.95;
N, 13.28.
[Co(L1)3](PF6)3 (19). This complex was synthesized from 1 (42 mg,
0.05 mmol) and CoCl2 (7 mg, 0.05 mmol). Yield: 15 mg (31%),
yellow powder. 1H NMR (400 MHz, DMSO-d6): δ 9.44 (d, J = 5.6 Hz,
3H, 6-PyH), 8.64 (d, J = 1.2 Hz, 3H, NCHCHN), 8.43 (t, J = 8.0 Hz,
3H, 4-PyH), 8.28 (d, J = 8.4 Hz, 3H, 3-PyH), 7.69 (t, J = 6.6 Hz, 3H,
5-PyH), 7.66 (d, J = 1.6 Hz, 3H, NCHCHN), 2.80 (s, 9H, CH3). 13
C
NMR (100 MHz, DMSO-d6): δ 153.09, 143.30, 129.37, 123.73,
118.60, 113.36, 35.02. Anal. Calcd for C30H32.5CoF18N10O0.25P3
([Co(L1)3](PF6)3·CH3CN·0.25Et2O): C, 34.95; H, 3.18; N, 13.59.
Found: C, 35.26; H, 2.93; N, 13.53.
[Co(L6)2](PF6)3 (20). This complex was synthesized from 6 (64 mg,
0.05 mmol) and CoCl2 (7 mg, 0.05 mmol). Yield: 39 mg (56%), pale
1
yellow powder. H NMR (400 MHz, DMSO-d6): δ 8.53 (s, 4H, 5-
ASSOCIATED CONTENT
■
triazole H), 8.17 (q, J = 3.2 Hz, 4H, 4,7-benzimidazole H), 7.84 (q, J =
2.8 Hz, 4H, 5,6-benzimidazole H), 7.32 (t, J = 7.2 Hz, 4H, PhH), 7.21
(t, J = 7.2 Hz, 8H, PhH), 6.82 (d, J = 7.6 Hz, 8H, PhH), 5.39 (s, 8H,
CH2), 5.31 (s, 8H, CH2). 13C NMR (100 MHz, DMSO-d6): δ 182.17
(Co−C), 142.37, 134.23, 134.18, 129.14, 129.04, 128.22, 127.24,
125.60, 112.40, 55.18, 40.17. Anal. Calcd for C54H48CoF18N16P3: C,
45.84; H, 3.42; N, 15.84. Found: C, 45.83; H, 3.40; N, 15.74.
S
* Supporting Information
Text giving experimental details and figures, tables, and CIF
files giving structural drawings and crystallographic data for 5,
6, 12, 13, 18, 20, and 21. This material is available free of
charge via the Internet at http://pubs.acs.org.
[Ru(L2)Cl(CH3CN)2](PF6) (21). This complex was synthesized from 2
(47 mg, 0.05 mmol) and [Ru(p-cycmene)Cl2]21(31 mg, 0.05 mmol).
Yield: 62 mg (94%), reddish brown powder. H NMR (400 MHz,
DMSO-d6): δ 9.48 (d, J = 4.8 Hz, 1H, phenanthroline H), 8.76 (d, J =
8.8 Hz, 1H, phenanthroline H), 8.63 (d, J = 8.8 Hz, 1H, phenan-
throline H), 8.62 (d, J = 1.2 Hz, 1H, NCHCHN), 8.43 (d, J = 8.8 Hz,
1H, phenanthroline H), 8.20 (q, J = 8.8 Hz, 2H, phenanthroline H),
8.11 (dd, J = 4.8 Hz, J = 4.8 Hz, 1H, phenanthroline H), 7.74 (d, J =
2.0 Hz, 1H, NCHCHN), 4.46 (t, J = 7.2 Hz, 2H, NCH2CH2CH2CH3),
2.85 (s, 3H, CH3CN), 2.03 (s, 3H, CH3CN), 1.96 (m, 2H, NCH2-
CH2CH2CH3), 1.43 (m, 2H, NCH2CH2CH2CH3), 0.98 (t, J = 7.2 Hz,
3H, NCH2CH2CH2CH3). 13C NMR (100 MHz, DMSO-d6): δ
185.34 (Ru−C), 154.85, 153.26, 147.26, 146.96, 138.29, 138.02,
130.80, 127.72, 127.16, 127.05, 126.98, 126.26, 124.48, 124.08,
119.31, 111.55, 49.78, 32,48, 19.10, 13.68, 3.75, 3.07. Anal. Calcd
AUTHOR INFORMATION
■
Corresponding Author
*Tel and fax: (+) 86-571-88273314. E-mail: chenwzz@zju.edu.cn.
ACKNOWLEDGMENTS
■
We thank the National Natural Science Foundation of China
(No. 21072170) for financial support.
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