Ruthenium(II) supported by phosphine-functionalized N-heterocyclic carbene ligands as catalysts for the transfer hydrogenation of ketones

Article abstract of DOI:10.1016/j.inoche.2013.09.046

We have prepared and characterized two ruthenium(II) complexes supported by phosphine-functionalized N-heterocyclic (NHC) ligands. One of the complexes (2a) underwent ortho-metalation of the N-phenyl moiety giving rise to a tridentate PC_NHCC

Full text of DOI:10.1016/j.inoche.2013.09.046

Inorganic Chemistry Communications 37 (2013) 138–143
Contents lists available at ScienceDirect
Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
Ruthenium(II) supported by phosphine-functionalized N-heterocyclic
carbene ligands as catalysts for the transfer hydrogenation of ketones
Matthew E. Humphries , Wiktoria H. Pecak , Sallie A. Hohenboken , Samuel R. Alvarado a,2,
a
a,1
a
b
a,
Dale C. Swenson , Gregory J. Domski ⁎
Augustana College, Rock Island, IL 61201, USA
a
b
The University of Iowa, Department of Chemistry, Iowa City, IA 52242-1294, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 1 July 2013
Accepted 23 September 2013
Available online 1 October 2013
We have prepared and characterized two ruthenium(II) complexes supported by phosphine-functionalized
N-heterocyclic (NHC) ligands. One of the complexes (2a) underwent ortho-metalation of the N-phenyl moiety
−
giving rise to a tridentate PCNHC
C
coordinating ligand whereas 2b bears an N-mesityl group in order to prevent
C–H activation of the aryl ring thereby enforcing a PCNHC bidentate binding mode. Both 2a and 2b were shown to
t
catalyze transfer hydrogenation of ketones at 82 °C in 2-propanol in the presence of KO Bu albeit with vastly dif-
Keywords:
ferent catalytic activities. Catalytic transfer hydrogenation by 2b was shown to proceed at room temperature and
in air using unpurified 2-propanol as solvent and hydrogen donor. Time studies revealed unique kinetic profiles
for the two precatalysts; this may shed light on the difference in their catalytic activities.
© 2013 Elsevier B.V. All rights reserved.
N-heterocyclic carbene
Transfer hydrogenation
Ruthenium(II) complexes
Ortho-metalation
Arduengo's discovery demonstrating that N-heterocyclic carbenes
NHCs) are isolable using common air- and moisture-free techniques
1] led early pioneers like Herrmann [2–5], Enders [6,7], Dixneuf,
functionalized NHCs have been reported and their efficacy as transfer
hydrogenation catalysts has been explored. Thus far ruthenium com-
plexes supported by chelating NHC ligands bearing nitrogen [16–27],
anionic carbon [28,29], oxygen [30,31], phosphorous [32,33], arene
[34–36], and alkene [30] donors have been used as transfer hydrogena-
tion catalysts. Compared to other donor-functionalized NHC complexes,
phosphine-functionalized NHC ruthenium(II) complexes are under-
developed. To our knowledge, there have been only two reports of
transfer hydrogenation catalyzed by ruthenium complexes of
phosphine-functionalized chelating NHCs. Chiu and Lee [32] have re-
(
[
Çetinkaya [8], Nolan [9], and Grubbs [10,11] to employ NHCs as ligands
supporting catalytically competent transition metal centers during the
mid to late 1990s. Since this time the number of catalytic applications
using metal–NHC complexes has increased rapidly. Due to their strong
σ-donating ability, NHCs effectively stabilize numerous transition metal
centers [12]. However, several common decomposition pathways exist
for metal–NHC complexes; this limits the ability of monodentate
NHC ligands to effectively stabilize metal centers at the high temper-
atures employed in some catalytic applications. Chelating, donor-
functionalized NHCs have been developed to circumvent decomposition
and have expanded the utility of metal–NHC catalysts for use in higher
temperature applications [13].
ported on the synthesis and catalytic behavior of tridentate PCNHC
P
complexes of ruthenium(II), and Miranda-Soto and co-workers
[33] have reported on a ruthenium(II) cyclopentadienyl complex
supported by a phosphine-functionalized NHC bearing an N–H moiety.
We set out to explore ruthenium(II) complexes supported by bidentate
The transfer hydrogenation of aldehydes, ketones, and imines is an
industrially relevant reaction often conducted in refluxing 2-propanol
or formic acid using ruthenium-based catalysts [14,15]. Under these
conditions the ability of chelating, donor-functionalized NHCs to stabi-
lize metal centers while preventing decomposition is quite valuable.
Examples of ruthenium complexes supported by chelating, donor-
phosphine-functionalized NHCs bearing N-aryl groups and unexpectedly
−
generated a ruthenium(II) complex supported by a tridentate PCNHC
C
ligand as a result of intramolecular C–H activation of the N-phenyl
group (2a, Scheme 1). In order to explore the effects of ortho-metalation
on catalytic transfer hydrogenation activity, we prepared the N-mesityl
analogue (2b) and evaluated both 2a and 2b as catalysts for the transfer
hydrogenation of several ketones.
The phosphine-functionalized bisimidazolium salts (1a,b) were
prepared according to the method reported by Zhou and co-workers
[37] with one minor modification; in the coupling reaction between
o-(diphenylphosphino)benzyl chloride and 1-arylimidazoles, N,N-
dimethylformamide was used as the solvent and the reaction was
heated to 90 °C. In our hands, this modification allowed us to avoid
the complicating factors introduced when ethanol was used as
⁎
Corresponding author. Tel.: +1 309 794 3482; fax: +1 309 794 8399.
E-mail address: gregdomski@augustana.edu (G.J. Domski).
Present address: University of Illinois at Chicago, Department of Chemistry, Chicago, IL
1
6
0607, USA.
2
Present address: Department of Chemistry, Iowa State University and Ames
Laboratory, Ames, Iowa 50011, USA.
1387-7003/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.inoche.2013.09.046

M.E. Humphries et al. / Inorganic Chemistry Communications 37 (2013) 138–143
139
Scheme 1. Preparation of 2a and 2b.
solvent. Specifically, ethanol acts as a competitive nucleophile for o-
diphenylphosphino)benzyl chloride thereby leading to an undesired
Table 1
(
Crystal data and structure refinement for 2a.
side product and requiring chromatographic purification of the desired
imidazolium salt [38]. The ruthenium(II) complexes were prepared via
Empirical formula
31 3 2 2
C H24Cl N O PRu
Formula weight
Temperature [K]
Wavelength [Å]
Crystal system, space group
Unit cell dimensions
694.91
210(2)
0.71073
transmetalation from the Ag–NHC complex to [Ru(CO)
3 2 2
Cl ] dimer
(Scheme 1.) The orthometalated N-phenyl complex (2a) [39] furnished
a disappointing 5.4% yield while the N-mesityl complex (2b) [40] was
isolated as a crystalline solid in 49.9% yield. The low yield of 2a may
be due to its higher solubility in the crystallization solvent system
since crude yields were higher (ca. 70%).
Triclinic, P−1
a = 8.1600(9) Å α = 100.109(5)°
b = 12.1940(13) Å β = 93.560(5)° c
=
14.9713(16) Å
γ = 93.469(5)°
1459.9(3)
2, 1.581
0.899
700
Single crystals suitable for X-ray diffraction studies of both 2a and 2b
were grown via vapor diffusion of diethyl ether into a saturated dichloro-
methane solution of the product isolated after column chromatography.
The full details of the X-ray diffraction studies have been reported previ-
ously by Domski and co-workers [41,42]. Crystal data, refinement, and
data collection details are presented in Tables 1 (2a) and 2 (2b). Selected
bond length and bond angle data are presented in Tables 3 (2a) and 4
(2b). The unit cell of both compounds includes one dicholoromethane
molecule of crystallization. Both compounds adopt distorted octahedral
coordination geometry in the solid state (Figs. 1; 2a and 2; 2b). For 2a
the bond angles of the cis-substituents at ruthenium range from 77.19°
to 95.70°. For 2b the bond angles of the cis-substituents at ruthenium
Volume [Å3]
3
Z, Calculated density [mg/m ]
Absorption coefficient [mm 1]
−
F(000)
Crystal size [mm]
θ range for data collection [°]
Limiting indices
0.17 x 0.08 x 0.03
2.77 to 27.84
−10 ≤ h ≤ 10, −16 ≤ k ≤ 15,
−
19 ≤ l ≤ 19
Reflections collected/unique
Completeness to θ = 27.84
Absorption correction
Max. and min. transmission
Refinement method
24,026/6906 [Rint = 0.0481]
99.5 %
Semi-empirical from equivalents
0.9735 and 0.8622
Full-matrix least-squares on F
6906/22/383
1.009
2
Data/restraints/parameters
Goodness-of-fit on F
2
Final R indices [I N 2σ (I)]
R indices (all data)
R
R
1
= 0.0417, wR
= 0.0773, wR
2
= 0.0742
= 0.0863
1
2
Largest diff. peak and hole [e/Å]
0.656 and −0.644
Table 3
Selected bond lengths (Å) and angles (°) for 2a.
Bond lengths (Å)
Bond angles (°)
Ru(1)–C(1)
Ru(1)–C(2)
Ru(1)–C(3)
Ru(1)–C(11)
Ru(1)–P(1)
Ru(1)–Cl(1)
1.858(4)
1.933(4)
2.066(3)
2.129(3)
2.4267(8)
2.4735(8)
C(1)-Ru(1)–C(2)
C(1)–Ru(1)–C(3)
C(1)–Ru(1)–C(11)
C(2)–Ru(1)–C(11)
C(3)–Ru(1)–C(11)
C(1)–Ru(1)–P(1)
C(2)–Ru(1)–P(1)
C(3)–Ru(1)–P(1)
C(2)–Ru(1)–Cl(1)
C(3)–Ru(1)–Cl(1)
C(11)–Ru(1)–Cl(1)
P(1)–Ru(1)–Cl(1)
91.18(14)
87.44(13)
89.85(12)
93.56(13)
77.77(12)
95.28(10)
93.05(10)
95.72(9)
92.23(10)
88.50(8)
85.40(8)
89.08(3)
Table 2
Crystal data and structure refinement for 2b.
Empirical formula
34 4 2 2
C H31Cl N O PRu
Formula weight
Temperature [K]
Wavelength [Å]
Crystal system, space group
Unit cell dimensions
773.45
190(2)
0.71073
Monoclinic, C 2/c
a = 22.539(3) Å α = 90°
b = 16.4065(17) Å β = 111.004(5)°
c = 19.852(2) Å γ = 90°
6853.2(13)
8, 1.499
0.849
Volume [Å3]
3
Z, Calculated density [mg/m ]
Absorption coefficient [mm−1]
Table 4
Selected bond lengths (Å) and angles (°) for 2b.
F(000)
3136
Crystal size [mm]
θ range for data collection [°]
Limiting indices
0.21 × 0.20 × 0.19
2.97 to 27.50°
Bond lengths (Å)
Bond angles (°)
−29 ≤ h ≤ 29, −20 ≤ k ≤ 21,
Ru(1)–C(1)
Ru(1)–C(2)
Ru(1)–C(3)
Ru(1)–P(1)
Ru(1)–Cl(1)
Ru(1)–Cl(2)
1.849(3)
1.934(3)
2.071(3)
2.4325(8)
2.4515(7)
2.4529(8)
C(1)–Ru(1)–C(2)
C(1)–Ru(1)–C(3)
C(2)–Ru(1)–C(3)
C(1)–Ru(1)–P(1)
C(3)–Ru(1)–P(1)
C(2)–Ru(1)–Cl(1)
C(3)–Ru(1)–Cl(1)
P(1)–Ru(1)–Cl(1)
C(1)–Ru(1)–Cl(2)
C(2)–Ru(1)–Cl(2)
P(1)–Ru(1)–Cl(2)
Cl(1)–Ru(1)–Cl(2)
88.15(13)
92.46(11)
97.13(11)
89.86(9)
96.87(7)
85.46(9)
84.81(7)
97.20(3)
93.92(9)
82.72(9)
83.52(3)
88.80(3)
−
25 ≤ l ≤ 25
Reflections collected/unique
Completeness to θ = 27.50
Absorption correction
Max. and min. transmission
Refinement method
53,042/7865 [Rint = 0.0433]
99.9 %
Semi-empirical from equivalents
0.8553 and 0.8418
Full-matrix least-squares on F
7865/0/402
1.077
2
Data/restraints/parameters
2
Goodness-of-fit on F
Final R indices [I N 2σ (I)]
R indices (all data)
Largest diff. peak and hole [e/Å]
R
R
1
= 0.0378, wR
= 0.0568, wR
2
= 0.0860
= 0.0966
1
2
0.983 and −0.994

140
M.E. Humphries et al. / Inorganic Chemistry Communications 37 (2013) 138–143
Fig. 1. ORTEP diagram of 2a; thermal ellipsoids are shown at the 50% probability level. Hydrogen atoms and dichloromethane molecule of crystallization omitted for clarity.
range from 82.72 to 97.20°. For both complexes the Ru–CO bond lengths
are inequivalent. In the structure of 2a the Ru–C1 bond is 0.072 Å shorter
than the Ru–C2 bond which is in agreement with the stronger trans-
influence of the NHC compared to the chloride ligand. For 2b the Ru–
C1 bond is 0.085 Å shorter than the Ru–C2 bond in agreement with the
stronger trans-influence of phosphines compared to chloride ligands.
The Ru–C11 bond distance (2.129 Å) in 2a is well within the range of
a Ru–C covalent bond. The Ru–C11 bond is retained in solution as
that the signal for the methylene protons appears as two doublets due
to the fact that these protons are diastereotopic. The H NMR spectrum
of 2b exhibits an identical signature for the methylene protons. Addition-
ally, the protons of the ortho-methyl groups on the mesityl ring resonate
at different frequencies suggesting that rotation about the N–CMes bond
is restricted in solution.
1
Both 2a and 2b catalyzed the transfer hydrogenation of acetophenone
at 82 °C with 2-propanol as solvent and hydrogen donor in the presence
13
t
evidenced by the doublet centered at 154.44 ppm in the C NMR spec-
of KO Bu [43]. Since 2b was isolated in a much higher yield than 2a, the
13
31
trum; the splitting of this signal is consistent with C – P coupling.
majority of catalytic trials were conducted using 2b; the results of these
trials are reported in Table 5. At loadings of 213:1 ([ketone]:[[2]b]) at
82 °C under a dry nitrogen atmosphere, acetophenone, cyclohexanone,
1
The H NMR spectrum of 2a is difficult to interpret due to extensive
overlapping of peaks in the aromatic region. One noteworthy feature is
Fig. 2. ORTEP diagram of 2b; thermal ellipsoids are shown at the 50% probability level. Hydrogen atoms and dichloromethane molecule of crystallization omitted for clarity.

M.E. Humphries et al. / Inorganic Chemistry Communications 37 (2013) 138–143
141
30
2000
Table 5
Results of catalytic trials with 2b.
1
1
1
1
800
600
400
200
a
b
c
25
20
Trial
Ketone
[ketone]:[Ru] Temperature Conversion
TOF
(
°C)
(%)
(h−1)
199
1
213:1
213:1
82
93.7
2
a
b
2
1
5
0
5
0
1000
800
d
2
82
90.5
192
1
600
400
200
0
e
3
1000:1
213:1
82
25
45.8
25.6
456
0
20
40
60
80
100
120
140
160
4
5
6
54.3
Time (min)
Fig. 4. Transfer hydrogenation of acetophenone to form 1-phenylethanol: turn over fre-
quency (TOF) vs. time for 2a and 2b.
213:1
213:1
82
82
96.7
61.3
206
130
complexes supported by phosphine-functionalized NHCs under similar
conditions [32,33].
Somewhat unexpectedly, given the high steric demand near the
carbonyl moiety, transfer hydrogenation of 2-methylacetophenone led
to the highest conversion among the acetophenones. It is possible that
the ortho-methyl group forces the ketone to adopt a conformation
that decreases the steric interactions between the ketone and the ligand
as the ketone approaches the Ru(II)-center. Substitution at the para-
position led to lower catalytic activity (entries 6 – 8); the electron do-
nating or withdrawing nature of the para-substituent did not seem to
have a significant effect on catalytic activity (entry 6 vs. 8).
Other noteworthy features of 2b's catalytic behavior include: its abil-
ity to catalyze the transfer hydrogenation of acetophenone at ambient
temperature (entry 4) and in air using unpurified 2-propanol with
only a minor loss of catalytic activity (entry 2). In the solid state and in
solution, 2b is stable for at least one week and likely longer. These two
observations combined suggest that 2b is a robust pre-catalyst for trans-
fer hydrogenation.
7
8
213:1
82
53.6
114
213:1
213:1
82
82
62.3
99.5
133
211
9
a
General conditions: 2b (16 μmol) was dissolved in 5.0 mL of dry, degassed 2-propanol
t
along with KO Bu (99 μmol) in an N
the reaction flask then the reaction mixture was heated to 82 °C with stirring. The reaction
duration in each case was 60 min. Determined via gas chromatography. mol alcohol/mole
Ru/h. This reaction was conducted using unpurified solvents in air. 16.0 mmol of ketone
2
atmosphere. The ketone (3.4 mmol) was added to
b
c
d
e
was used.
The percent conversion of acetophenone to 1-phenylethanol was
monitored over time for 2a and 2b; the results of the time studies are
summarized in Figs. 3 and 4. The data in Fig. 3 reveal that 2b initiates rap-
idly and that the reaction reaches 90% conversion within 20 min while
the activity of 2a under identical conditions is much lower. As expected
the TOF for 2b diminishes over time as acetophenone is consumed
and 2-methylacetophenone were converted to the corresponding
alcohols with ≥93.7% conversion in 60 min (entries 1, 5, and 9).
The turn over frequency (TOF) for the conversion of acetophenone to 1-
phenylethanol with 2b was at least 1.6 times higher than previously re-
ported examples of transfer hydrogenation catalyzed by ruthenium(II)
(Fig. 4). Interestingly, the TOF of 2a increases steadily from 20 min to
150 min suggesting that initiation of 2a is slow (Fig. 4). One potential
explanation for this observation is that the tridentate ligand of 2a exerts
extra steric demand at the active site thereby slowing initiation.
We have prepared and characterized two ruthenium(II) complexes
supported by phosphine-functionalized NHC ligands and have evaluated
their behavior as transfer hydrogenation catalysts. To our knowledge, this
is the first report of transfer hydrogenation catalyzed by a ruthenium(II)
complex supported by an ortho-metalated, phosphine-functionalized
NHC ligand. The ortho-metalated complex, 2a, showed low catalytic
activity and an increase in TOF over time suggesting slow initiation.
Notably, 2b showed much higher activity catalyzing the transfer hydro-
genation of several ketones to high conversion. Additionally, 2b was
shown to be a viable pre-catalyst at room temperature and in the pres-
ence of air using unpurified 2-propanol as the solvent and hydrogen
donor.
1
00
9
8
7
6
5
4
3
2
1
0
0
0
0
0
0
0
0
0
0
2a
2b
0
20
40
60
80
100
120
140
Acknowledgments
Time (min)
GJD thanks Augustana College and the American Chemical Society's
Petroleum Research Fund New Investigator Award program (PRF #
Fig. 3. Transfer hydrogenation of acetophenone to form 1-phenylethanol: % conversion vs.
time for 2a and 2b.

142
M.E. Humphries et al. / Inorganic Chemistry Communications 37 (2013) 138–143
5
1217-UNI3) for financial support. The authors thank Dr. Mary Ellen
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and catalytic activities of ruthenium(II) carbonyl chloride complexes containing
pyridine-functionalised N-heterocyclic carbenes, Dalton Trans. (2009) 7132–
Biggin for assistance with gas chromatography.
7
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[39] Synthesis of 2a: Under
phenyl-3-(2-diphenylphosphinobenzyl)-1H-imidazol-3-ium chloride (1a) (0.36 g,
0.79 mmol) was allowed to react with Ag
O (0.19 g, 0.79 mmol) in dry, degassed di-
a nitrogen atmosphere in the absence of light, 1-
2-ylidene ligands, Tetrahedron Lett. 40 (1999) 2247–2250.
2
[12] W.A. Herrmann, N-heterocyclic carbenes: a new concept in organometallic catalysis,
Angew. Chem. Int. Ed. 41 (2002) 1290–1309.
chloromethane; 4 Å molecular sieves (approximately 0.5 g) were added to the mix-
ture at the beginning of the reaction. After 24 h, the reaction mixture was filtered
TM
[
13] A.T. Normand, K.J. Cavell, Donor-functionalised N-heterocyclic carbene complexes of
group 9 and 10 metals in catalysis: trends and directions, Eur. J. Inorg. Chem. 2008
through Celite under an atmosphere of nitrogen into a flask containing [Ru(CO)
3
Cl
dark. After stirring overnight, the reaction mixture was filtered through Celite
under an atmosphere of dry nitrogen and all volatiles were removed in vacuo. The
solid residue was purified via column chromatography (SiO , 40:1 CH Cl /MeOH)
2 2
] (0.21 g, 0.40 mmol); the reaction mixture was allowed to stir for 24 h in the
TM
(
2008) 2781–2800.
14] A.J. Blacker, Enantioselective transfer hydrogenation, in: C.J.E. Johannes, G. De Vries
Eds.), The handbook of homegenous hydrogenation, Wiley-VCH, Weinheim, 2007.
[
[
(
2
2
2
15] T. Ikariya, A.J. Blacker, Asymmetric transfer hydrogenation of ketones with bifunc-
tional transition metal-based molecular catalysts, Acc. Chem. Res. 40 (2007)
to furnish a yellow solid. Single crystals (0.025 g, 5.2%) were grown by dissolving
the crude product in the minimum amount of dry dichloromethane and allowing
1
300–1308.
diethyl ether to slowly evaporate, condense, and diffuse into the dichloromethane
1
[
[
[
[
[
16] Y. Cheng, J.-F. Sun, H.-L. Yang, H.-J. Xu, Y.-Z. Li, X.-T. Chen, Z.-L. Xue, Syntheses, struc-
tures, and catalytic properties of ruthenium(II) nitrosyl complexes with pyridine-
functionalized N-heterocyclic carbenes, Organometallics 28 (2009) 819–823.
17] W.B. Cross, C.G. Daly, Y. Boutadla, K. Singh, Variable coordination of amine
functionalised N-heterocyclic carbene ligands to Ru, Rh and Ir: C-H and N-H activa-
tion and catalytic transfer hydrogenation, Dalton Trans. 40 (2011) 9722–9730.
18] W.W.N. O, A.J. Lough, R.H. Morris, The hydrogenation of molecules with polar bonds
catalyzed by a ruthenium(II) complex bearing a chelating N-heterocyclic carbene
with a primary amine donor, Chem. Commun. 46 (2010) 8240–8242.
6
solution. H NMR (d -DMSO): δ 8.12 (d, J = 0.46 Hz, 1 H, imid-H), 7.79 (m, 2 H,
imid/Ar-H), 7.73 (m, 4 H, Ar-H), 7.57–7.47 (m, 10H, Ar-H), 7.27 (t, J = 1.93 Hz,
1 H), 7.05 (td, J = 0.37, 1.88, 1 H, Ar-H), 6.97 (t, J = 1.84 Hz, 1 H, Ar-H), 6.48
(t, J = 2.24 Hz, 1 H, Ar-H), 5.72 (s, 2 H, CH
2
Cl
N). C NMR (d
CO), 197.17 (d, J = 6.89 Hz, CO), 180.70 (d, J = 9.18 Hz, NCN), 154.44
(d, J = 12.25 Hz, metalated C ), 146.03, 140.34, 138.50, 138.37, 135.25, 135.14,
2
), 5.28 (d, J = 3.57 Hz, 1 H, CH
-DMSO): δ 199.37 (d, J = 7.65 Hz,
2
N),
13
5.10 (d, J = 3.57 Hz, 1 H, CH
2
6
6 4
H
134.57, 134.54, 134.21, 133.79, 133.43, 132.80, 132.45, 132.37, 132.29, 131.62,
131.57, 131.45, 129.88, 129.79, 129.28, 129.22, 129.13, 125.98, 125.92, 124.27,
31
1
19] F. Zeng, Z. Yu, Ruthenium(II) complexes bearing a pyridyl-supported pyrazolyl,
N-heterocyclic carbene (NNC) ligand and their catalytic activity in the transfer hy-
drogenation of ketones, Organometallics 27 (2008) 6025–6028.
122.94, 116.43, 113.32, 55.46 (CH
(d -DMSO): δ 20.25 (1 P). Anal. Calcd for C30
3.48; N 4.03. Found: C, 53.28; H, 3.73; N, 4.00.
2
Cl
2
), 52.20 (d, J = 1.65, CH
2
N) P{ H}NMR
Cl : C, 53.58; H
6
H
22ClN PRu•CH
2
O
2
2
2
20] A. Sinha, P. Daw, S.M.W. Rahaman, B. Saha, J.K. Bera, A Ru(II)–N-heterocyclic
carbene (NHC) complex from metal–metal singly bonded diruthenium(I) pre-
cursor: synthesis, structure and catalytic evaluation, J. Organomet. Chem. 696
[40] Synthesis of 2b: In a nitrogen-filled glove box, a Schlenk flask was charged
with 1-mesityl-3-(2- diphenylphosphinobenzyl)-1H-imidazol-3-ium chloride (1b)
2
(0.76 g, 1.6 mmol), Ag O (0.37 g, 1.6 mmol), and 4 Å molecular sieves (ca. 0.5 g).
(
2011) 1248–1257.
The solids were suspended in dry, degassed dichloromethane and allowed to stir
TM
[
21] W.W.N. O, A.J. Lough, R.H. Morris, Transmetalation of a primary amino-functionalized
N-heterocyclic carbene ligand from an axially chiral square-planar nickel(II) complex
to a ruthenium(II) precatalyst for the transfer hydrogenation of ketones, Organome-
tallics 28 (2009) 6755–6761.
overnight in the dark. After 24 h, the reaction mixture was filtered through Celite
3 2 2
into a Schlenk flask that had been charged with [Ru(CO) Cl ] (0.41 g, 0.79 mmol)
under a nitrogen atmosphere. The reaction mixture was allowed to stir at room
temperature in the dark overnight. After 24 h, the reaction mixture was filtered
TM
[
[
[
[
[
22] M. Poyatos, J.A. Mata, E. Falomir, R.H. Crabtree, E. Peris, New ruthenium(II)
through Celite
and the volatiles were removed in vacuo to furnish a yellow
, 40:1 CH
/MeOH). Single crystals (0.545 g, 49.9%) of the title compound were obtained
by slow diffusion of diethyl ether onto a concentrated solution of the yellow powder
CNC-pincer bis(carbene) complexes: synthesis and catalytic activity, Organometallics
solid. The crude product was purified by column chromatography (SiO
Cl
2
2
22 (2003) 1110–1114.
2
23] M. Poyatos, A. Maisse-François, S. Bellemin-Laponnaz, E. Peris, L.H. Gade, Synthesis
and structural chemistry of arene-ruthenium half-sandwich complexes bearing an
oxazolinyl–carbene ligand, J. Organomet. Chem. 691 (2006) 2713–2720.
24] F.E. Fernández, M.C. Puerta, P. Valerga, Half-sandwich ruthenium(II) picolyl-NHC
complexes: synthesis, characterization, and catalytic activity in transfer hydrogena-
tion reactions, Organometallics 30 (2011) 5793–5802.
25] F.E. Fernández, M.C. Puerta, P. Valerga, Ruthenium(II) picolyl-NHC complexes: syn-
thesis, characterization, and catalytic activity in amine N-alkylation and transfer
hydrogenation reactions, Organometallics 31 (2012) 6868–6879.
1
6
isolated via column chromatography in dichloromethane. H NMR (d -DMSO): δ
7.99 (d, J = 1.8 Hz, 1 H, imid-H), 7.88 (m, 3 H, imid/Ar-H), 7.62 (pseudo-t,
J = 7.52 Hz, 1 H, Ar-H), 7.49–7.33 (m, 10 H, Ar-H), 7.18 (pseudo-t, J = 8.24 Hz,
1 H, Ar-H), 7.07 (s, 1 H, 2,4,6-Me
(d, J = 14.6 Hz, 1 H, CH N), 5.72 (s, 2 H, CH
1 H, CH N), 2.28 (s, 3 H, para-CH ), 1.95 (s, 3 H, ortho-CH
ortho-CH -DMSO): δ 195.27, 195.13 (d, J = 14 Hz, CO), 192.81,
3
C
6
H
2
), 7.03 (s, 1 H, 2,4,6-Me
Cl ), 5.06 (d, J = 14.6 Hz,
), 1.69 (s, 3 H,
3 6 2
C H ), 6.28
2
2
2
2
3
3
13
1
). C{ H} NMR (d
3 6
192.68(d, J = 13 Hz, CO), 166.73, 166.61, (d, J = 12 Hz, NCN), 143.04, 142.90,
140.19, 136.82, 136.22, 136.09 (2C), 134.02, 133.93, 133.90, 132.73, 131.67,
131.58, 131.51, 131.17, 131.08, 130.86, 130.81, 130.58, 129.81 (d, J = 5.36 Hz),
129.59 (2C, d, J = 6.88 Hz), 128.50 (2C, d, J = 9.95 Hz), 127.49, 127.08, 125.32,
26] X.-W. Li, F. Chen, W.-F. Xu, Y.-Z. Li, X.-T. Chen, Z.-L. Xue, Syntheses, structures and
catalytic activities of ruthenium (II) carbonyl iodide complexes with CNC-pincer
bis (carbene) ligand, Inorg. Chem. Commun. 14 (2011) 1673–1676.

M.E. Humphries et al. / Inorganic Chemistry Communications 37 (2013) 138–143
143
1
(
24.72, 55.46 (CH
ortho-CH ), 18.29 (ortho-CH
29Cl PRu•CH Cl
2
Cl
2
), 52.63 (d, CH
2
N, J = 11.48 Hz), 21.21 (para-CH
). P{ H}NMR (d -DMSO): δ 20.25 (1 P). Anal. Calcd
: C, 52.80; H, 4.04; N, 3.62. Found: C, 52.77; H, 4.22;
3
), 18.59
ruthenium(II) dichloromethane monosolvate, Acta Crystallogr.
Rep. Online 68 (2012) m1224–m1225.
E
Struct.
31
1
3
3
6
for C33
H
2 2
N
O
2
2
2
[43] General procedure for catalytic trials: Under an atmosphere of nitrogen, the catalyst
t
N, 3.56.
41] G.J. Domski, S.A. Hohenboken, D.C. Swenson, Rac-cis, cis-dicarbonyldichlorido
1-[2-(diphenylphosphanyl)benzyl]-3-mesityl-imidazol-2-yl-idene}ruthenium(II) di-
precursor (16 μmol) was added to a Schlenk flask with KO Bu (99 μmol) along with
[
[
5.0 mL of dry, degassed 2-propanol. To the reaction mixture was added 3.4 mmol of
ketone. The reaction mixture was heated to 82 °C with stirring for 60 min. After the
reaction period, the reaction mixture was filtered through a 0.45 μm syringe filter
under ambient conditions and immediately analyzed via gas chromatography to de-
termine the percent conversion.
{
chloromethane monosolvate, Acta Crystallogr. E Struct. Rep. Online 68 (2012) m1121.
42] G.J. Domski, W.H. Pecak, D.C. Swenson, Rac-cis-dicarbonylchlorido{1-[2-
1
2
(
diphenylphosphanyl-κP)benzyl]-3-(phenyl-κC )imidazol-2-yl-idene-κC }