Pyridazine-based N-heterocyclic carbene complexes and ruthenium-catalyzed oxidation reaction of alkenes

Article abstract of DOI:10.1021/om300644h

[Ru₂Cl(L)(CH₃CN)₄](PF₆) ₃ (1, L = 3,6-bis(N-(pyridylmethyl)imidazolylidenyl)pyridazine), [Cu₃L^a₃](PF₆)₃ (2, L ^a = 3-(N-(pyridylmethyl)imidazolylidenyl)-6-(N-(pyridylmethyl) imidazolylonyl)pyridazine), [Pd₂(allyl)₂L](PF ₆)₂ (3), [Pd₂(allyl)₂L ^b₂](PF₆)₂ (4, L^b = N-pyridylmethylimidazole), and [NiL^c₂](PF ₆)₂ (5, L^c = 3-(N-(pyridylmethyl) imidazolylidenyl)-6-methoxylpyridazine) have been synthesized and fully characterized by NMR spectroscopy, elemental analysis, and X-ray diffraction analysis. In complex 1, ligand L binds to two Ru(II) centers, forming a well-behaved Ru₂(L)Cl plane with a five-membered metallocyclic ring. Complex 2 is trinuclear, containing a triangular Cu₃ unit bonded together by three 3-(N-(pyridylmethyl)imidazolylidenyl)-6-(N-(pyridylmethyl) imidazolylonyl)pyridazine, where one imidazolylidene was oxidized into imidazolone. Deprotonation reaction with Ag₂O in CH₃CN and CH₃OH resulted in C-N cleavage of the imidazolium salt, and subsequent reaction with [Pd(allyl)Cl]₂ and Ni(PPh₃) ₂Cl₂ gave 4 and 5, respectively. Dinuclear Ru(II)-NHC complex 1 exhibits excellent catalytic activity for the oxidation of alkenes into diketones.

Full text of DOI:10.1021/om300644h

Article
pubs.acs.org/Organometallics
Pyridazine-Based N‑Heterocyclic Carbene Complexes and
Ruthenium-Catalyzed Oxidation Reaction of Alkenes
Xiaolong Liu† 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 300071, People's Republic of China
S
* Supporting Information
ABSTRACT: [Ru2Cl(L)(CH3CN)4](PF6)3 (1, L = 3,6-bis-
(N-(pyridylmethyl)imidazolylidenyl)pyridazine), [Cu3La ]-
3
(PF6)3 (2, La = 3-(N-(pyridylmethyl)imidazolylidenyl)-6-(N-
(pyridylmethyl)imidazolylonyl)pyridazine), [Pd2(allyl)2L]-
(PF6)2 (3), [Pd2(allyl)2Lb ](PF6)2 (4, Lb = N-pyridylmethy-
2
limidazole), and [NiLc2](PF6)2 (5, Lc = 3-(N-(pyridylmethyl)-
imidazolylidenyl)-6-methoxylpyridazine) have been synthe-
sized and fully characterized by NMR spectroscopy,
elemental analysis, and X-ray diffraction analysis. In complex
1, ligand L binds to two Ru(II) centers, forming a well-
behaved Ru2(L)Cl plane with a five-membered metallocyclic
ring. Complex 2 is trinuclear, containing a triangular Cu3 unit
bonded together by three 3-(N-(pyridylmethyl)-
imidazolylidenyl)-6-(N-(pyridylmethyl)imidazolylonyl)-
pyridazine, where one imidazolylidene was oxidized into
imidazolone. Deprotonation reaction with Ag2O in CH3CN and CH3OH resulted in C−N cleavage of the imidazolium salt,
and subsequent reaction with [Pd(allyl)Cl]2 and Ni(PPh3)2Cl2 gave 4 and 5, respectively. Dinuclear Ru(II)-NHC complex 1
exhibits excellent catalytic activity for the oxidation of alkenes into diketones.
complexes with bischelating,12 pincer,13 tripodal,14 or bridging
INTRODUCTION
■
NHC ligands15 have been synthesized and showed great catalytic
Pyridazine-based transition metal complexes have received
activities in C−C coupling,13b,16 hydrogenation,13d,17 and
considerable attention in coordination chemistry because of
hydrosilylation reactions.12d,18 Nevertheless, the pyridazine/
their great structural diversity.1 In many cases, the pyridazine-
NHC hybrid ligands are relatively less investigated.19 3,6-
derived ligands are capable of incorporating two metal ions in
Diimidazolium-substituted pyridazine ligand precursors (B)
close proximity via their diazine fragments in the presence of
have been used to prepare Ag(I) and Hg(II) complexes.19a,b,e
peripheral donating groups that are bonded to the diazine
Recently, our group has reported a series of dinuclear
moiety.2 Because of interesting structural, electrochemical, and
palladacyclic complexes derived from B, and C−N cleavage
catalytic properties of the dinuclear transition metal complexes,
much effort has been devoted to developing the architecture of
was observed.19e As an extension of our studies on functionalized
NHCs,8e,10c,11a herein we present the synthesis and character-
pyridazine-derived ligands.3 N donors were commonly intro-
duced to the systems in the form of nitrogen heterocyclic groups4
ization of a modified pyridazine-functionalized imidazolium salt
and Schiff bases.5 For instance, 3,6-dipyridylpyridazine (A in
(C) in which two pyridine groups are attached to the imidazole
rings as two additional coordination sites. The double-pincer
Figure 1) has shown to form predominantly dinuclear complex-
es.2e,4b
ligand showed remarkable coordination diversity.
As alternatives to the traditional phosphine and nitrogen
EXPERIMENTAL SECTION
ligands, N-heterocyclic carbenes (NHCs) have gained increasing
importance due to their unique properties.6 Compared to the
widely used phosphine complexes, the NHC complexes have
shown remarkable stability toward air, moisture, and heat
because of their strong σ-donating ability.7 Additionally, the N-
bonded functional groups are easily modified, and various
functional groups such as pyridine,8 pyrimidine,9 pyrazole,10 and
phenanthroline11 have been introduced to the NHC systems, as
such kinds of donors are versatile for the assemblage of
multinuclear complexes. A large number of transition metal
■
All chemicals were obtained from commercial suppliers and used
without further purification. 3,6-Di(1H-imidazol-1-yl)pyridazine (see
Supporting Information) and Ni(PPh3)2Cl2 were prepared according to
the known procedure.20 Elemental analyses were performed on a Flash
1
EA1112 instrument. H and 13C NMR spectra were recorded on a
Bruker Avance-400 (400 MHz) spectrometer. Chemical shifts (δ) are
Received: July 10, 2012
Published: August 31, 2012
© 2012 American Chemical Society
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Article
Figure 1. Pyridazine-containing ligand A and ligand precursors B and C.
expressed in ppm downfield to TMS at δ = 0 ppm, and coupling
constants (J) are expressed in Hz.
DMSO-d6): δ 8.97 (d, J = 4.8 Hz, o-C5H4N, 2H), 8.74 (s, C4N2H2, 2H),
8.26 (d, J = 1.2 Hz, NCHCHN, 2H), 8.17 (t, J = 7.8 Hz, p-C6H4N, 2H),
7.97 (d, J = 1.6 Hz, NCHCHN, 2H), 7.88 (d, J = 8.0 Hz, m-C6H4N, 2H),
7.64 (t, J = 6.4 Hz, m-C6H4N, 2H), 5.76−5.56 (m, C3H5 and CH2, 6H),
4.52 (d, J = 8.0 Hz, C3H5, 2H), 3.75 (d, J = 13.2 Hz, C3H5, 2H), 3.37 (br,
C3H5, 2H), 2.56 (br, C3H5, 2H). 13C NMR (100 MHz, DMSO-d6): δ
177.5 (Pd−Ccarbene), 154.9, 154.8, 153.0, 140.2, 126.1, 125.7, 125.2,
124.4, 120.7, 120.2, 74.4, 54.9, 51.0.
Synthesis of H2L·(PF6)2 (L = 3,6-bis(N-(pyridylmethyl)-
imidazolylidenyl)pyridazine). A solution of 3,6-di(1H-imidazol-1-
yl)pyridazine (4.24 g, 20 mmol) and 2-(chloromethyl)pyridine (12.8 g,
100 mmol) in 50 mL of DMF was stirred at 120 °C for 12 h. The
resulting white solid was collected by filtration and dissolved in 100 mL
of water. Subsequent addition of an aqueous solution of NH4PF6 (8.15
g, 50 mmol) to the solution afforded a white precipitate, which was
collected by filtration and dried. Yield: 8.75, 64%. Anal. Calcd for
C22H20F12N8P2: C, 38.50; H, 2.94; N, 16.33. Found: C, 38.23; H, 2.91;
Synthesis of [Pd2(allyl)2Lb ](PF6)2 (4, Lb = N-pyridylmethyli-
2
midazole). A solution of H2L·(PF6)2 (137 mg, 0.20 mmol) in 6 mL of
CH3CN/CH3OH (1:1) was treated with Ag2O (48 mg, 0.20 mmol) at
50 °C. After 5 h, Ag2O completely disappeared, and [Pd(allyl)Cl]2 (74
mg, 0.20 mmol) was added to the solution. After it was stirred for 12 h at
50 °C, the solution was filtered. The filtrate was then concentrated to ca.
2 mL. Addition of Et2O (20 mL) to the filtrate afforded a white
precipitate, which was collected and washed with Et2O. Yield: 62 mg,
69%. Anal. Calcd for C24H28F12N6P2Pd2: C, 31.91; H, 3.12; N, 9.30.
Found: C, 32.20; H, 3.09; N, 9.13. 1H NMR (400 MHz, DMSO-d6): δ
8.83 (s, 2H), 8.19 (s, 2H), 8.05 (s, 2H), 7.63 (m, 4H), 2.27 (s, 2H), 6.48
(br, C3H5, 2H), 6.00 (m, 2H), 5.44 (s, CH2, 4H), 4.26 (br, C3H5, 4H),
3.43 (br, C3H5, 4H). 13C NMR (100 MHz, DMSO-d6): δ 155.3, 151.4,
141.0, 139.4, 128.7, 125.8, 124.9, 120.4, 119.1, 83.7, 63.0, 52.5.
1
N, 16.12. H NMR (400 MHz, DMSO-d6): δ 10.47 (s, NCHN, 2H),
8.81 (s, C4N2H2, 2H), 8.69 (m, NCHCHN, 2H), 8.58 (d, J = 4.4 Hz, o-
C5H4N, 2H), 8.17 (m, NCHCHN, 2H), 7.94 (m, p-C6H4N, 2H), 7.61
(d, J = 8.0 Hz, m-C6H4N, 2H), 7.44 (m, m-C6H4N, 2H), 5.77 (s, CH2,
4H). 13C NMR (100 MHz, DMSO-d6): δ 153.3, 152.0, 150.0, 138.0,
137., 125.2, 124.3, 124.0, 123.2, 120.4, 54.2.
Synthesis of [Ru2Cl(L)(CH3CN)4](PF6)3, 1. A solution of
H2L·(PF6)2 (137 mg, 0.20 mmol) in 6 mL of acetonitrile was treated
with Ag2O (48 mg, 0.20 mmol) at 50 °C. After 5 h, Ag2O disappeared
completely, and [Ru(p-cymene)Cl2]2 (122 mg, 0.20 mmol) was added
to the solution. After it was stirred for 12 h at room temperature, the
solution was filtered. The filtrate was then concentrated to ca. 2 mL.
Addition of Et2O (20 mL) to the filtrate afforded a red precipitate, which
was collected and washed with Et2O. Yield: 63 mg, 26%. Anal. Calcd for
C30H30ClF18N12P3Ru2·0.5CH3CN: C, 29.75; H, 2.54; N, 13.99. Found:
C, 29.93; H, 2.53; N, 13.67. 1H NMR (400 MHz, DMSO-d6): δ 9.88 (d,
J = 4.8 Hz, o-C5H4N, 2H), 8.78 (s, C4N2H2, 2H), 8.61 (d, J = 2.4 Hz,
NCHCHN, 2H), 8.14 (t, J = 7.8 Hz, p-C6H4N, 2H), 7.98 (d, J = 2.0 Hz,
NCHCHN, 2H), 7.85 (d, J = 7.6 Hz, m-C6H4N, 2H), 7.70 (t, J = 6.4 Hz,
m-C6H4N, 2H), 6.13 (s, CH2, 4H), 2.06 (s, CH3CN, 12H). 13C NMR
(100 MHz, DMSO-d6): δ 194.8 (Ru−Ccarbene), 156.6, 155.3, 155.0,
138.7, 126.9, 125.4, 124.3, 124.1, 119.0, 118.1, 53.6, 3.0.
Synthesis of [NiLc ](PF6)2 (5, Lc = 3-(N-(pyridylmethyl)-
2
imidazolylidenyl)-6-methoxylpyridazine). A solution of
H2L·(PF6)2 (137 mg, 0.20 mmol) in 6 mL of CH3CN/CH3OH (1:1)
was treated with Ag2O (48 mg, 0.20 mmol) at 50 °C. After 5 h, Ag2O
completely disappeared, and Ni(PPh3)2Cl2 (131 mg, 0.20 mmol) was
added to the solution. After it was stirred for 12 h at 50 °C, the filtrate
was concentrated to ca. 2 mL. Addition of Et2O (20 mL) to the filtrate
afforded a yellow precipitate, which was collected and washed with Et2O.
Yield: 71 mg, 81%. Anal. Calcd for C28H26F12N10NiO2P2·0.5CH3CN: C,
38.54; H, 3.07; N, 16.27. Found: C, 38.71; H, 3.13; N, 16.08. 1H NMR
(400 MHz, DMSO-d6): δ 8.30−8.19 (m, 6H), 7.97 (d, J = 7.6 Hz, 2H),
7.56 (m, 8H), 6.73 (d, J = 15.2 Hz, CH2, 2H), 5.93 (d, J = 14.8 Hz, CH2,
2H), 4.20 (s, CH3, 6H). 13C NMR (100 MHz, DMSO-d6): δ 165.0 (Ni−
Ccarbene), 159.2, 154.1, 152.9, 149.4, 141.4, 137.0, 125.5, 125.2, 123.9,
121.9, 119.8, 55.3, 54.0.
Synthesis of [Cu3La ](PF6)3, (2, La = 3-(N-(pyridylmethyl)-
imidazolylidenyl)-6-3(N-(pyridylmethyl)imidazolylonyl)-
pyridazine). A solution of H2L·(PF6)2 (137 mg, 0.20 mmol) in 6 mL of
acetonitrile was treated with Ag2O (48 mg, 0.20 mmol) at 50 °C. After 5
h, Ag2O disappeared completely, and an excess of copper powder (0.2 g)
was added to the solution. After it was stirred for 12 h at 50 °C, the
solution was filtered to remove the excess copper powder. The filtrate
was then concentrated to ca. 2 mL. Addition of Et2O (20 mL) to the
filtrate afforded a yellow precipitate, which was collected and washed
with Et2O. Yield: 56 mg, 45%. Anal. Calcd for C66H54Cu3F18N24O3P3: C,
42.69; H, 2.93; N, 18.10. Found: C, 42.88; H, 2.73; N, 17.92. 1H NMR
(400 MHz, DMSO-d6): δ 8.90 (d, J = 9.2 Hz, C6H4N, 3H), 8.61 (d, J =
2.4 Hz, NCHCHN, 3H), 8.46 (s, 3H), 8.37 (m, 3H), 8.08 (t, J = 7.6 Hz,
3H), 7.91 (s, 3H), 7.86 (m, 3H), 7.76 (d, J = 7.6 Hz, 3H), 7.35 (m, 9H),
6.98 (m, 6H), 6.83 (d, J = 2.8 Hz, NCHCHN, 3H), 5.40 (s, CH2, 6H),
5.16 (d, J = 16.8 Hz, CH2, 3H), 4.86 (d, J = 16.0 Hz, CH2, 3H). 13C
NMR (100 MHz, DMSO-d6): δ 167.7 (Cu−Ccarbene), 156.1 (CO),
152.2, 152.1, 152.0, 150.8, 149.2, 148.5, 140.2, 137.1, 126.3, 125.1, 124.7,
123.8, 123.2, 123.0, 122.6, 121.3, 115.3, 107.4, 53.8, 48.2. IR (KBr):
ν(CO) 1702 cm−1.
General Procedure for Preparations of Diketones. In a glass tube, a
mixture of 1 (6 mg, 0.005 mmol), alkene (0.5 mmol), and NaI (23 mg,
0.15 mmol) in 3.0 mL of CH3CN was stirred. To the solution was added
TBHP (1 mL, 65% solution in water) via a syringe at room temperature.
The stirring was continued at room temperature for 10 min. The
reaction was quenched by adding a saturated Na2SO3 solution and then
extracted with dichloromethane. The combined organic layers were
dried with anhydrous magnesium sulfate. After filtration, the solvent was
removed under vacuum, and the residue was purified by column
chromatograph on silica gel (ethyl acetate/petroleum ether) to afford
the desired product.
X-ray Structural Determination. Single-crystal X-ray diffraction
data were collected at 298(2) K on a Siemens Smart/CCD area-detector
for 1 and 2 or an Oxford Diffraction Gemini A Ultra diffractometer for
3−5 with Mo Kα radiation (λ = 0.71073 Å) by using an ω−2θ scan
mode. Unit-cell dimensions were obtained with least-squares refine-
ment. Data collection and reduction were performed using SMART and
SAINT or Oxford Diffraction CrysAlisPro software.21 The structures
were solved by direct methods, and the non-hydrogen atoms were
subjected to anisotropic refinement by full-matrix least-squares on F2
using the SHELXTXL package.22 Hydrogen atom positions for all of the
structures were calculated and allowed to ride on their respective C
atoms with C−H distances of 0.93−0.97 Å and Uiso(H) = 1.2−
1.5Ueq(C). Disordered solvents in the lattice for 2 could not be modeled
successfully and were removed from their reflection data with
Synthesis of [Pd2(allyl)2L](PF6)2, 3. A solution of H2L·(PF6)2 (137
mg, 0.20 mmol) in 6 mL of acetonitrile was treated with Ag2O (48 mg,
0.20 mmol) at 50 °C. After 5 h, Ag2O disappeared completely, and
[Pd(allyl)Cl]2 (74 mg, 0.20 mmol) was added to the solution. After it
was stirred for 12 h at 50 °C, the solution was filtered. The filtrate was
then concentrated to ca. 2 mL. Addition of Et2O (20 mL) to the filtrate
afforded a white precipitate, which was collected and washed with Et2O.
Yield: 171 mg, 87%. Anal. Calcd for C28H28F12N8P2Pd2: C, 34.34; H,
2.88; N, 11.44. Found: C, 34.17; H, 2.79; N, 11.29. 1H NMR (400 MHz,
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Table 1. Summary of the Crystallographic Data for 1−5
[Ru2Cl(L)(CH3CN)4](PF6)3, 1
[Cu3La3](PF6)3, 2
[Pd2(allyl)2L](PF6)2, 3 [Pd2(allyl)2Lb ](PF6)2, 4
[NiLc2](PF6)2, 5
2
formula
fw
C32H33ClF18N13P3Ru2
1272.21
C66H54Cu3 F18N24O3P3
1856.86
C28H28F12N8P2Pd2
979.32
C28H34F12N8P2Pd2
985.37
C30H29F12N11NiO2P2
924.29
cryst syst
space group
a, Å
monoclinic
P21/n
cubic
orthorhombic
Pbcn
monoclinic
P21/c
monoclinic
P21/c
P213
10.8610(15)
12.0263(19)
36.135(3)
90
20.2458(19)
20.2458(19)
20.2458(19)
90
14.4781(8)
15.6757(8)
15.4845(8)
90
8.9307(2)
14.1680(3)
14.3697(3)
90
20.0236(9)
13.0568(6)
15.1251(10)
90
b, Å
c, Å
α, deg
β, deg
γ, deg
V, Å3
Z
92.3010(10)
90
90
90
100.477(2)
90
97.998(5)
90
90
90
4716.0(11)
4
8298.6(13)
4
3514.3(3)
4
1787.89(7)
2
3915.9(4)
4
D
calcd, Mg/m3
1.792
1.486
1.851
1.830
1.568
reflns collected
24 118
39 544
14 479
8935
22 692
reflns indep, Rint
goodness-of-fit on F2
R1, wR2 (I > 2σ(I))
R1, wR2 (all data)
8292, 0.0633
1.065
4876, 0.1440
0.922
3133, 0.0301
1.110
3143, 0.0210
1.043
6895, 0.0498
1.037
0.0543, 0.1060
0.1047, 0.1331
0.0660, 0.1680
0.0920, 0.1823
0.0734, 0.2021
0.0945, 0.2233
0.0382, 0.1005
0.0447, 0.1064
0.0658, 0.1806
0.0899, 0.2032
Scheme 1. Synthesis of the Imidazolium Salt, [Ru2Cl(L)(CH3CN)4](PF6)3 (1), [Cu3La3](PF6)3 (2), and [Pd2(allyl)2L](PF6)2 (3)
SQUEEZE.23 Further details of the structural analysis are summarized in
Table 1.
heating 3,6-di(1H-imidazol-1-yl)pyridazine and 2-
(chloromethyl)pyridine in DMF and subsequent anion exchange
reaction in water (Scheme 1). The imidazolium salt was isolated
as a white solid in 64% yield.
RESULTS AND DISCUSSION
■
Synthesis and Characterization of [Ru2Cl(L)(CH3CN)4]-
(PF6)3 (1). Reaction of the imidazolium salt H2L·(PF6)2 (L = 3,6-
bis(N-(pyridylmethyl)imidazolylidenyl)pyridazine) with Ag2O
in CH3CN at 50 °C afforded a colorless solution of [Ag6(L)4]-
Synthesis of the Imidazolium Salt. The pyridazine-
functionalized bis(NHC) ligand precursor (H2L·(PF6)2) was
reported by us in 2007, but it was obtained in very poor yield
(4.5%).19c In this work, the imidazolium salt was prepared by
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Figure 4. Thermal ellipsoids of [Pd2(allyl)2L](PF6)2 (3) shown at the
30% probability level with hydrogen atoms omitted for clarity. Selected
bond distances (Å) and angles (deg): Pd(1)−C(4) 2.022(9), Pd(1)−
C(14) 2.083(11), Pd(1)−C(13) 2.088(10), Pd(1)−N(3) 2.101(8),
Pd(1)−C(15) 2.168(12), C(4)−Pd(1)−C(14) 138.3(4), C(4)−
Pd(1)−C(13) 102.7(4), C(14)−Pd(1)−C(13) 35.7(4), C(4)−
Pd(1)−N(3) 87.9(3), C(14)−Pd(1)−N(3) 133.9(4), C(13)−
Pd(1)−N(3) 168.4(3), C(4)−Pd(1)−C(15) 170.1(4), C(14)−
Pd(1)−C(15) 32.7(4), C(13)−Pd(1)−C(15) 67.8(4), N(3)−Pd(1)−
C(15) 101.3(4). Symmetry code: #1 −x, y, −z−1/2.
Figure 2. Thermal ellipsoids of [Ru2Cl(L)(CH3CN)4](PF6)3 (1)
shown at the 30% probability level with hydrogen atoms omitted for
clarity. Selected bond distances (Å) and angles (deg): Ru(1)−C(5)
1.900(7), Ru(1)−N(9) 2.020(6), Ru(1)−N(10) 2.036(6), Ru(1)−
N(1) 2.053(5), Ru(1)−N(5) 2.110(6), Ru(1)−Cl(1) 2.5101(18),
Ru(2)−C(14) 1.885(7), Ru(2)−N(12) 2.033(6), Ru(2)−N(11)
2.033(7), Ru(2)−N(2) 2.067(5), Ru(2)−N(8) 2.117(6), Ru(2)−
Cl(1) 2.5044(18), C(5)−Ru(1)−Cl(1) 172.6(2), C(14)−Ru(2)−
Cl(1) 173.1(2), Ru(2)−Cl(1)−Ru(1) 98.01(6).
double-pincer fashion, forming a well-behaved Ru2(L)Cl plane,
whereas four acetonitrile molecules occupy the axial positions.
Each Ru(II) center is hexacoordinated in an octahedral sphere by
one carbon, four nitrogen atoms, and one chloride. Two
ruthenium centers are bridged by a pyridazine and a chloride,
forming a five-membered metallocyclic ring. The Ru−Cl−Ru
angle (98.01(6)°) is slightly smaller than the reported Ru−Cl−
Ru angle (99.67(6)°).3c The Ru−C distances are 1.900(7) and
1.885(7) Å, which are significantly shorter than those of reported
Ru-NHC complexes, which normally range from 1.91 to 2.10
Å.8e,11c,24 The Ru···Ru distance is 3.785 Å, slightly longer than
the value of a similar reported example (3.671 Å).3c
Synthesis and Characterization of [Cu3La ](PF6)3 (2). In
3
the past few years, a number of pyridazine-based dinuclear
Cu(II) complexes have been established that show interesting
magnetic and structural properties.2a−c,3a,5a However, very few
corresponding Cu(I) complexes have been reported.2d,e
Recently, direct synthesis of Cu-NHC complexes by using
copper powder has been reported by our group, and the method
was suitable for both Cu(I) and Cu(II) complexes.8d,10d
Interestingly, treatment of a solution of [Ag6(L)4](PF6)6 with
an excess of copper powder in air led to the isolation of
[Cu3La3](PF6)3 (2, La = 3-(N-(pyridylmethyl)imidazolylidenyl)-
6-(N-(pyridylmethyl)imidazolylonyl)pyridazine) as a yellow
crystalline solid, an unexpected trinuclear copper product other
than a dinuclear copper complex like 1. One NHC carbon has
been oxidized into a carbonyl group, which could be confirmed
by a strong absorption bond at 1702 cm−1 in its IR spectrum and
a singlet at 156.1 ppm in its 13C NMR spectrum. Such oxidation
has been previously observed in the preparation of copper
Figure 3. Thermal ellipsoids of [Cu3La3](PF6)3 (2) shown at the 30%
probability level with hydrogen atoms, uncoordinated pyridine, and
imidazolone rings omitted for clarity. Selected bond distances (Å) and
angles (deg): Cu(1)−Cu(1)#1 2.495(2), Cu(1)−Cu(1)#2 2.495(1),
Cu(1)−C(5)#1 2.046(7), Cu(1)−C(5) 2.072(7), Cu(1)−N(5)
2.101(6), Cu(1)−N(1) 2.142(6), C(14)−O(1) 1.232(11), C(5)#1−
Cu(1)−C(5) 164.6(3), C(5)#1−Cu(1)−N(5) 96.0(3), C(5)−Cu(1)−
N(5) 98.8(3), C(5)#1−Cu(1)−N(1) 100.8(2), C(5)−Cu(1)−N(1)
80.6(2), N(5)−Cu(1)−N(1) 100.0(2), Cu(1)#1−Cu(1)−Cu(1)#2
60.0. Symmetry code: #1 −y+1, z−1/2 −x+3/2; #2 −z+3/2, −x+1, y
+1/2.
complexes.25 The H NMR spectrum exhibits a singlet at 5.40
1
ppm ascribed to the CH2 groups between the imidazolone and
pyridine rings, whereas two doublets of AB pattern were
observed at 5.16 and 4.86 ppm, indicating that the two H atoms
of the CH2 groups between imidazolylidene and pyridine rings
are magnetically inequivalent.
The structure of trinuclear copper(I) complex 2 is depicted in
Figure 3. The cation consists of three Cu(I) ions and three La
ligands with a beautiful spiral divergence-shaped conformation.
Unlike 1, one-half of the ligand is coordinated to coppers in a
tridentate fashion by one carbon and two nitrogen atoms, and the
other half is left uncoordinated within which the carbenic carbon
(PF6)6.19c Subsequent addition of [Ru(p-cymene)Cl2]2 to the
solution gave [Ru2Cl(L)(CH3CN)4](PF6)3 (1) as a red solid in
26% yield. The 1H NMR spectrum of 1 exhibits two doublets at
8.61 and 7.98 ppm due to the imidazolidene backbone protons.
In its 13C NMR spectrum, the carbenic carbon resonance signal
was observed at 194.8 ppm as a singlet.
The details of the molecular structure of 1 were established by
an X-ray diffraction study. As shown in Figure 2, the cation
contains two Ru(II) ions, one L ligand, one chloride, and four
acetonitrile molecules. As expected, the ligand L coordinates in a
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Scheme 2. Synthesis of [Pd2(allyl)2Lb ](PF6)2 (4) and [NiLc2](PF6)2 (5)
2
Figure 5. Thermal ellipsoids of [Pd2(allyl)2Lb ](PF6)2 (3) are shown at
2
the 30% probability level with hydrogen atoms omitted for clarity.
Selected bond distances (Å) and angles (deg): Pd(1)−C(11) 2.104(6),
Pd(1)−N(3) 2.106(3), Pd(1)−C(10) 2.110(5), Pd(1)−C(12)
2.136(5), Pd(1)−N(1) 2.142(3), C(11)−Pd(1)−N(3) 133.2(2),
C(11)−Pd(1)−C(10) 38.5(2), N(3)−Pd(1)−C(10) 99.28(17),
C(11)−Pd(1)−C(12) 36.9(2), N(3)−Pd(1)−C(12) 167.86(18),
C(10)−Pd(1)−C(12) 68.8(2), C(11)−Pd(1)−N(1) 129.2(2),
N(3)−Pd(1)−N(1) 93.10(13), C(10)−Pd(1)−N(1) 167.39(17),
C(12)−Pd(1)−N(1) 98.91(17). Symmetry code: #1 −x+1, −y, −z+2.
Figure 6. Thermal ellipsoids of [NiLc2](PF6)2 (5) shown at the 30%
probability level with hydrogen atoms omitted for clarity. Selected bond
distances (Å) and angles (deg): Ni(1)−C(7) 1.857(4), Ni(1)−C(21)
1.859(4), Ni(1)−N(6) 1.936(4), Ni(1)−N(1) 1.955(3), C(13)−O(2)
1.331(7), C(14)−O(2) 1.432(9), C(27)−O(1) 1.343(6), C(28)−O(1)
1.428(8), C(7)−Ni(1)−C(21) 90.59(19), C(7)−Ni(1)−N(6)
177.26(18), C(21)−Ni(1)−N(6) 87.73(17), C(7)−Ni(1)−N(1)
87.18(16), C(21)−Ni(1)−N(1) 176.59(17), N(6)−Ni(1)−N(1)
94.60(15).
was oxidized to a carbonyl group. The Cu−Cu−Cu angles are all
60°, in accordance with a perfect equilateral triangle consisting of
three copper atoms. Each carbenic carbon is coordinated to two
Cu(I) centers, and the Cu(I)−C distances are 2.046(7) and
2.072(7) Å, which are comparable to those of the reported
Cu(I)-NHC examples (1.86−2.11 Å).7d,8d,14c,25,26 The Cu−Cu
bond length of 2.495 Å is within the normal range of those
previously observed for similar trinuclear Cu(I)-NHC complexes
(2.47−2.52 Å).25,26b The Cu−Npyridazine and Cu−Npyridine
distances are 2.142(6) and 2.101(6), respectively.
Synthesis and Characterization of [Pd2(allyl)2L](PF6)2
(3). The carbene transfer reaction of [Ag6(L)4](PF6)6 with
[Pd(allyl)Cl]2 afforded 3 as a white solid in 87% yield. The 1H
NMR spectrum shows a singlet at 8.74 ppm due to the pyridazine
backbone protons, showing no difference from that of 1 (8.78
ppm). In its 13C NMR spectrum, a singlet was observed at 177.5
ppm, ascribed to the NHC carbons, which is consistent with the
reported values in the range 206.3−149.5 ppm for Pd-NHC
complexes.27
with Ag2O proceeded in a mixed solvent of CH3CN and
CH3OH, C−N cleavage between the pyridazine and the
imidazole ring occurred due to nucleophilic substitution of one
N-pyridylmethylimidazole by a methoxyl group.19e When the
resulting solution was subsequently treated with [Pd(allyl)Cl]2,
complex 4 was obtained in 69% yield. Although many efforts
have been devoted to the isolation of the palladium complex
bearing Lc (Lc = 3-(N-(pyridylmethyl)imidazolylidenyl)-6-
methoxylpyridazine), unfortunately, we failed to isolate any Pd-
NHC species. However, when Ni(PPh3)2Cl2 was added to the
mixed solution after deprotonation reaction, complex 5
supported by Lc was isolated in 81% yield, which could be easily
confirmed by NMR spectroscopy, elemental analysis, and X-ray
diffraction analysis. The 1H resonances of 4 are all consistent with
its formulation. In its 1H NMR spectrum, the allyl group and the
The structure of 3 was further confirmed by X-ray diffraction
and is depicted in Figure 4. Each Pd(II) center is coordinated by
one carbenic carbon, one allyl group, and one pyridine group.
The NHC-pyridine unit binds to palladium in a bidentate fashion
with a bite angle of 87.9(3)°. The Pd−Ccarbene distances are
2.022(9) Å, showing no difference from those of the reported
examples.28
1
N-pyridylmethylimidazole could be easily identified. The H
NMR spectrum of 5 exhibits two doublets of AB pattern at 6.73
and 5.93 ppm, revealing that two protons of the CH2 linker are
magnetically inequivalent, and both show apparent downfield
shifts in comparison to those of 2 at 5.16 and 4.86 ppm. The
Synthesis and Characterization of [Pd2(allyl)2Lb ](PF6)2
2
(4) and [NiLc ](PF6)2 (5). As shown in Scheme 2, when the
2
deprotonation reaction of the imidazolium salt (H2L·(PF6)2)
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a
Table 2. Ruthenium-Catalyzed Oxidation of Alkenes to α-Diketones
a
Reaction conditions: alkene 0.5 mmol, Ru 0.005 mmol (1 mol %), NaI 0.15 mmol (30 mol %), TBHP 1 mL, CH3CN 3 mL, room temperature, 10
b
c
min. Isolated yield. Dialkene 0.5 mmol, Ru 0.01 mmol (2 mol %), NaI 0.30 mmol (60 mol %), TBHP 2 mL, CH3CN 6 mL.
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Article
methyl group appears as a singlet at 4.20 ppm. Complex 5 shows
a typical resonance signal at 165.0 ppm for the NHC carbon in its
13C NMR spectrum, which is consistent with other Ni-NHC
complexes, ranging from 152.3 to 190.6 ppm.7g,8c,10c,15a,29
The solid-state structure of 4 determined by X-ray diffraction
analysis is shown in Figure 5. The cation is composed of two
palladium ions, two allyl groups, and two N-pyridylmethylimi-
dazole molecules. Two Pd(II) centers are linked by the two N-
pyridylmethylimidazole molecules, forming a 14-membered
metallocyclic ring. The N−Pd−N angle is 93.10(13)°. Two
imidazole rings are completely parallel to each other. The
distance between the two palladium atoms is 5.880 Å, much
shorter than that of 3 (7.881 Å). The Pd−Nimidazole and Pd−
higher catalytic activity in organic transformations.10c,d The high
activity of diruthenium complex 1 in oxidation of alkenes is
tentatively ascribed to the Ru···Ru cooperative effect. The
dinucleating ability of the pyridazine moiety, the strong donating
ability of carbenes, and the hexadentate structural property
greatly ensure the stability of 1 in oxidation state, which would
potentially increase its catalytic activity in oxidation reactions.
CONCLUSION
■
In summary, we have described the synthesis and structural
characterization of Ru(II), Cu(I), Pd(II), and Ni(II) complexes
containing pyridazine-based N-heterocyclic carbenes. The ligand
shows diversified reactivity toward different metal ions. A
diruthenium and dipalladium complexes bridged by 3,6-bis(N-
(pyridylmethyl)imidazolylidenyl)pyridazine were obtained
through carbene transfer reactions. However, the carbene
transfer reaction toward Cu(I) afforded an unexpected trinuclear
Cu(I)-NHC complex due to oxidation of a NHC to imidazolone.
C−N bond cleavage was observed, giving a dinuclear
palladacyclic complex and a mononuclear nickel complex
containing N-pyridylmethylimidazole and 3-(N-
(pyridylmethyl)imidazolylidenyl)-6-methoxylpyridazine, respec-
tively. Additionally, the dinuclear Ru(II) complex is a very
efficient catalyst for oxidation of alkenes.
Npyridine distances are 2.106(3) and 2.142(3) Å, respectively.
The molecular structure of 5 determined by X-ray diffraction is
depicted in Figure 6. Structural analysis of the cationic
[Ni(Lc)2]2+ reveals that the two chelates adopt a cis arrangement
around the Ni(II) center. Both NHC rings are nearly coplanar
with their trans-positioned pyridine rings, with small dihedral
angles of 4.48° and 6.40°, respectively. The Ni−C distances
(1.857(4) and 1.859(4) Å) are quite normal as compared to
many known tetracoordinate nickel(II)-NHC complex-
es.9b,15a,29a,30 The Ni−N distances are 1.936(4) and 1.955(3) Å.
Catalytic Properties. Ruthenium complexes of NHCs have
increasingly gained interest as catalysts for a number of important
reactions.6a,c,f Although plenty of ruthenium complexes of NHC
ligands have been reported to be active catalysts for olefin
metathesis,6d,f,7b,31 isomerization,32 polymerization,24a,33 hydro-
genation,34 and C−H activation reactions,35 the bimetallic Ru-
NHC complexes as catalysts for oxidation reactions of alkenes
have not been reported. Herein, we investigated the activity of 1
in oxidation of alkenes.
ASSOCIATED CONTENT
■
S
* Supporting Information
CIF files giving X-ray crystallographic data for 1−5, experimental
details of Heck coupling reactions, and electrochemical studies of
1. This material is available free of charge via the Internet at
http://pubs.acs.org.
With the bimetallic Pd-NHC complexes 3 and 4 in hand, we
first synthesized various alkenes through Heck coupling reaction
of aromatic halide and olefins under standard conditions (see
Supporting Information). The results shown in Table S1
illustrate that complex 3 is more active than 4, with excellent
catalytic activities for aryl bromides bearing electron-with-
drawing groups and aryl iodides. Considering the applications
of ruthenium complexes in oxidation reactions,3b−d,36 the activity
of 1 in oxidation of alkenes at low catalyst loadings was
investigated under very mild conditions. The results are
summarized in Table 2. The catalytic activity of 1 for the
oxidation reaction of (E)-1,2-diphenylethene was first examined.
The reaction could proceed smoothly at room temperature in
toluene under air, and benzil was obtained in 62% yield (entry 1).
The yield could be significantly increased to 92% by employing
CH3CN as solvent (entry 2). TBHP is crucial since no benzil was
detected when O2 or H2O2 was used as oxidant (entries 3, 4).
Various alkenes were then tested under the optimized
conditions. The reactions were completed at room temperature
within 10 min. Generally, alkenes bearing electron-donating
groups afforded higher yields than those with electron-
withdrawing groups (entries 5−8). It is worth mentioning that
the N-substituted indoles have also been successfully applied in
high yields (entries 10, 11). Moreover, heterocyclic alkenes, such
as those with thiophene and carbazole, can also be employed,
giving the target products in moderate yields (entries 12, 13).
Many efforts have been devoted to constructing cooperative
metal catalysts because such catalysts are believed to exhibit
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: chenwzz@zju.edu.cn. Tel & Fax: 0086-571-88273314.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was financially supported by Natural Science
Foundation of China (21072170).
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