N,N′-dialkylation catalyzed by bimetallic iridium complexes containing a saturated bis-N-heterocyclic carbene (NHC) ligand

Article abstract of DOI:10.1021/om300800c

Reaction of the bis-aminophosphinimine [m-C₆H ₄(HNCH₂CH₂N=PPh₃)] with W(CO) ₆ afforded [m-C₆H₄{(CNCH₂CH ₂NH)W(CO)₅}₂] (2), which underwent N-alkylation with benzyl bromide to yield [m-C₆H₄{(CNCH ₂CH₂NCH₂Ph)W(CO)₅}₂] (3). A carbene transfer reaction from W(0) to Ir(I) proceeded smoothly via the reaction of 3 with [Ir(COD)Cl]₂ under mild conditions to give the diiridium(I) carbene complex [m-C₆H₄{(CNCH ₂CH₂NH)Ir(CO)₂Cl}₂] (4). Ligand substitution of 4 with an excess of PPh₃ produced the phosphine complex 5. All complexes have been characterized by spectroscopic and elemental analyses. Complexes 2 and 5 were further confirmed by X-ray diffraction studies. Complex 4 is an efficient catalyst for the reductive N,N′-dialkylation of phenylenediamines with alcohols. The mechanistic pathway of the catalysis involving the possible synergistic effect between two metal centers is discussed.

Full text of DOI:10.1021/om300800c

Article
pubs.acs.org/Organometallics
N,N′-Dialkylation Catalyzed by Bimetallic Iridium Complexes
Containing a Saturated Bis-N-Heterocyclic Carbene (NHC) Ligand
Hsin-Ya Kuo, Yi-Hong Liu, Shie-Ming Peng, and Shiuh-Tzung Liu*
Department of Chemistry, National Taiwan University, Taipei 106, Taiwan
S
* Supporting Information
ABSTRACT: Reaction of the bis-aminophosphinimine [m-
C6H4(HNCH2CH2NPPh3)] with W(CO)6 afforded [m-
C6H4{(CNCH2CH2NH)W(CO)5}2] (2), which underwent
N-alkylation with benzyl bromide to yield [m-
C6H4{(CNCH2CH2NCH2Ph)W(CO)5}2] (3). A carbene
transfer reaction from W(0) to Ir(I) proceeded smoothly via
the reaction of 3 with [Ir(COD)Cl]2 under mild conditions to
give the diiridium(I) carbene complex [m-
C6H4{(CNCH2CH2NH)Ir(CO)2Cl}2] (4). Ligand substitution of 4 with an excess of PPh3 produced the phosphine complex
5. All complexes have been characterized by spectroscopic and elemental analyses. Complexes 2 and 5 were further confirmed by
X-ray diffraction studies. Complex 4 is an efficient catalyst for the reductive N,N′-dialkylation of phenylenediamines with
alcohols. The mechanistic pathway of the catalysis involving the possible synergistic effect between two metal centers is
discussed.
developed a synthetic approach to build a bis(saturated NHC)
framework via a double cyclization of nucleophilic attacks of
iminophosphoranes toward tungsten carbonyls followed by a
carbene transfer reaction from tungsten to iridium.13
Accordingly, the synthetic approach leading to the desired
complexes is outlined in Scheme 1. Compound 1 was prepared
via the reductive amination of m-isophthalaldehyde with
H2N(CH2)2N3. Treatment of 1 with triphenylphosphine
followed by hexacarbonyltungsten readily gave the desired
bis-tungsten carbonyl complex 2. Deprotonation of 2 by
sodium hydride and then alkylation with benzyl bromide
yielded 3. Transfer of carbonyl ligands and carbene from
tungsten to iridium proceeded by the reaction of 3 with excess
[Ir(COD)Cl]2 at ambient temperature for 72 h. The desired
diiridium complex 4 was obtained as a yellow solid in 60%
yield. Ligand substitution of 4 with excess triphenylphosphine
provided the substitution product 5 in high yield.
It is worth mentioning that we did not observe any formation
of mononuclear tungsten bis-carbene complex 6. Scheme 2
summarizes the reaction pathway of 1 with phosphine and
W(CO)6. Reaction of the bis-azido compound 1 with
phosphine provides the corresponding iminophosphorane 7,
and one of the imino-phosphorane functionalities undergoes
nucleophilic attack on the metal carbonyl accompanied by the
elimination of phosphine oxide to form the isocyanide metal
complex 8.13a Intramolecular cyclization of 8 followed by
proton transfer yields the mono-carbene complex 9 and the
other imino-phosphorane functionality proceeds similarly,
resulting in the formation of complex 2. Apparently, the
INTRODUCTION
■
Alkylated amines are important chemicals for many uses such as
synthetic intermediates, pharmaceuticals, agrochemicals, and
bulk chemicals. A variety of synthetic approaches leading to the
desired amines are available.1 However, the selective N-
alkylation of amines ought to be the most direct and useful
methodology.1,2 In this context, transition-metal-catalyzed N-
alkylation of amines with alcohols by a borrowing-hydrogen
strategy affords a concise route for C−N bond formation and
has received much attention.2−11 In this approach, the alcohol
is initially oxidized to the corresponding carbonyl compound
accompanied by the generation of metal hydride. Subsequently,
the carbonyl compound reacts with amines to form imines,
which are then reduced to amines by the pregenerated
hydrides. Indeed, there are numerous reports concerning the
alkylation of monoamine molecules. However, few concern the
N,N′-dialkylation of diamino compounds such as benzenedia-
mines.4a
In terms of catalysts for the borrowing-hydrogen N-
alkylation of amines with alcohols, mononuclear ruthenium
and iridium complexes2−10 and a few other metal ions4f,8,11 are
frequently employed for this purpose. However, the use of
bimetallic complexes as precatalysts in this kind of reductive
amination has been less frequently reported.11c Here we
describe the synthesis of diiridium carbene complexes and their
uses as catalysts for the N,N′-dialkylation of diamines.
RESULTS AND DISCUSSION
■
Preaparation and Characterization of Bis-NHC-Iridium
Complexes. Few bis(NHC) carbene complexes with a
phenylene unit as a spacer have been reported,12 but all have
an unsaturated N-heterocyclic carbene moiety. In this work, we
Received: August 20, 2012
Published: October 8, 2012
© 2012 American Chemical Society
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Scheme 1. Preparation of Bimetallic Iridium Complexes
Scheme 2. Reaction Pathway of 1 with Phosphine Followed by W(CO)6
iminophosphorane moiety in 9 does not undergo the
intramolecular nucleophilic attack on the carbonyl ligand in
the molecule to yield 6. Presumably, the strong σ-donating
ability of the NHC ligand on the tungsten metal readily causes
an increase in the strength of M−C back bonding, which
reduces the electrophilic character of that carbon center.13a
All tungsten and iridium complexes, which are air-stable,
were isolated as solids; complexes 2 and 5 were even obtained
in crystalline form. All complexes were characterized by IR,
NMR, and elemental analysis, and selected spectral data are
collected in Table 1. The presence of two CO bands in the IR
spectra of both 2 and 3 are consistent with the existence of a
W(CO)5 fragment. Similarly, the IR spectrum of 4 also
indicates the presence of a “Ir(CO)2” unit in the molecule. The
coordination of the NHC ligands to the metal centers is
confirmed by the appearance of a downfield shift in the 13C
NMR spectra for the carbenic carbon (Table 1). These shifts
are in the range of 198−206 ppm, typical for the coordination
of the saturated NHC toward these metal ions.13c FAB mass
spectra of 2 and 4 show molecular ion peaks at m/z 890.0042
([M]+: C24H18O10N4W2) and 991.0980 ([M + H]+:
Table 1. Selected Spectral Data for Carbene Complexes
IR
(νCO
a
a
b
complex
13C shift, Ccarbene−M 13C shift, Ccarbonyl−M
)
2 (M = W) 205.7
3 (M = W) 208.8
4 (M = Ir) 198.6
201.6 (trans), 198.2
(cis)
2063, 1897
2061, 1898
2062, 1977
1996
201.5 (trans), 198.4
(cis)
181.9 (trans), 168.5
(cis)
5 (M = Ir) 203.8 (t, JP−C = 13.2) 183.9 (t, JP−C = 14.7)
a
In units of ppm with J in Hz; in CD2Cl2 except complex 5 in CDCl3.
b
IR in CH2Cl2, in units of cm−1.
C32H31O4N4Cl2Ir2), respectively, indicating the formation of
dimetallic species. For complex 5, the trans relationship of two
phosphine ligands is supported by the 13C NMR spectrum, in
which the carbenic and carbonyl carbon signals appear as a
triplet splitting and the phosphorus−carbon coupling constants
for JP−C(carbene) and JP−C(carbonyl) are 13.2 and 14.7 Hz,
respectively. Furthermore, detailed coordination configurations
of 2 and 5 were confirmed by X-ray crystal structure analysis.
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Crystallography. Figure 1 displays a perspective view of
the bis-tungsten carbene complex 2, whose selected bond
distances of W(1)−C(1) (1.996(4) Å) and W(1)−C(20)
(1.980(3) Å) are due to the trans influence of the NHC donors.
The distance between two tungsten centers is 6.953 Å. Both
tungsten atoms are arranged in an “anti” conformation.12a
However, the CH2 spacer hydrogen atoms showed a single
shift, indicating free rotation along these C−C bonds at
ambient temperature.
Slow evaporation of a chloroform/hexane solution of 5
yielded bright yellow crystals with the composition 5·6CHCl3
that were suitable for X-ray analysis. An ORTEP plot of 5 is
given in Figure 2. Both iridium centers have square-planar
geometry with two phosphine ligands coordinating toward each
metal center in a trans fashion. All bond distances and bond
angles (Table 3) are in the normal range as compared to those
Table 3. Selected Bond Distances (Å) and Bond Angles
(deg) for 5
Ir(1)−C(1)
Ir(1)−C(2)
Ir(1)−P(1)
P(1)−Ir(1)−P(2)
C(1)−Ir(1)−C(2)
P(1)−Ir(1)−C(1)
1.857(5)
2.060(4)
2.3253(9)
174.7(2)
178.21(4)
89.34(11)
Ir(1)−P(2)
C(1)−O(1)
2.3108(9)
1.149(5)
Figure 1. ORTEP plot of 2 (drawn with 30% probability ellipsoids).
C(2)−Ir(1)−P(1)
Ir(1)−C(1)−O(1)
90.20(10)
177.2(4)
Table 2. Selected Bond Distances (Å) and Bond Angles
(deg) for 2
of the related species. The Ir−Ccarbene bond length (2.060(4) Å)
lies in the same range as for other reported Ir-(NHC)
complexes.13c As expected, the Ir(1)−C(1) distance (1.857(5)
Å), trans to the carbene donor, is shorter than those of normal
M−Ccarbonyl, again due to the trans influence. The crystal
structure of 5 reveals the orientation of both iridium ions in a
syn conformation.12a,f However, the two metal centers are
situated away from the phenylene unit, probably due to steric
repulsion. In fact, the Ir(1)···Ir(1A) distance is 11.671 Å,
indicating that the two metal centers are far away from each
other.
W(1)−C(1)
W(1)−C(2)
W(1)−C(3)
W(1)−C(4)
W(1)−C(5)
W(1)−C(6)
C(1)−O(1)
C(6)−W(1)−C(1)
C(6)−W(1)−C(2)
C(3)−W(1)−C(5)
1.996(4)
2.037(4)
2.053(4)
2.046(4)
2.031(4)
2.245(3)
1.151(4)
179.1(1)
95.1(1)
W(2)−C(20)
W(2)−C(21)
W(2)−C(22)
W(2)−C(23)
W(2)−C(24)
W(2)−C(19)
C(20)−O(6)
C(19)−W(2)−C(20)
C(19)−W(2)−C(21)
C(22)−W(2)−C(24)
1.980(3)
2.044(3)
2.034(4)
2.040(4)
2.043(4)
2.244(3)
1.160(4)
176.7(1)
86.6(1)
175.6(1)
170.4(1)
Catalysis. It is well-documented that iridium complexes are
frequently used as precatalysts for N-alkylation of amines with
alcohols.2−10 In addition, the introduction of a NHC ligand to
the metal center would enhance its catalytic activity.14 Thus,
with these diiridium carbene complexes in hand, their catalytic
activities toward dialkylation of diamines with alcohols were
distances and angles are reported in Table 2. As expected from
the spectral analysis of 2, both metal centers are in the same
coordination mode, surrounded by five carbonyl ligands and by
the carbene ligand in an octahedral geometry. All bond
distances and bond angles lie within normal ranges. The shorter
Figure 2. ORTEP plot of the cationic part of 5 (drawn with 30% probability ellipsoids; labels for the phenyl group are omitted for clarity).
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investigated. In order to create an environmentally benign
process, the catalytic reactions investigated in this work do not
use any organic solvent.
products in very good yields (Table 5, entries 4 and 7). Instead
of N-alkylation, reaction of o-phenylenediamine with benzyl
alcohol gave 2-phenyl-1H-benzo[d]imidazole as the exclusive
product under the catalytic conditions described for other
alkylations (Table 5, entry 8). The formation of the cyclized
product is due to the intramolecular attack of an amino group
at the imine functionality (Scheme 3).
Interestingly, the yield of N,N′-dialkylation of o-phenyl-
enediamine with benzyl alcohol increased as the reaction
proceeded under an ambient pressure of molecular hydrogen
(Table 5, entry 9). Presumably, the hydrogen atmosphere slows
down the reductive elimination of hydrides from the metal
center, which then facilitates the reduction of imine. Therefore,
the intramolecular attack of amine at imine, which leads to the
formation of benzo[d]imidazole, is suppressed.15 In the
presence of an H2 atmosphere, the reaction between o-
phenylenediamine with various alcohols under the standard
catalytic conditions provided the N,N′-dialkylation in excellent
yields (Table 5, entries 9 and 10).
To obtain information on the catalytic system, we first
examined the amination of m-phenylenediamine with benzyl
alcohol catalyzed by 4 in the presence of various bases. In a
typical experiment, a mixture of m-phenylenediamine (0.3
mmol), benzyl alcohol (1.2 mmol), complex 4 (3 × 10−3
mmol), base (0.15 mmol), and molecular sieves (0.3 g) was
heated in an oil bath at 120 °C for 24 h. In this reaction, two
major products (10a,b) were found (eq 1). Other partial
monoamination products were not observed by the NMR
spectroscopic determination. The results are summarized in
Table 4. It appears that the bases affect the selectivity of
Next we further ran a series of trial reactions of p-
phenylenediamine with PhCH2OH in the presence of various
iridium complexes to compare the selectivity of these catalysts
(eq 2). Table 6 summarizes the results of amination catalyzed
Table 4. N,N′-Dialkylation of m-Phenylenediamine with
a
Benzyl Alcohol Catalyzed by 4
c
product (%)
b
entry
base
temp (°C) conversn (%)
10a
10b
d
1
2
3
4
5
6
7
8
9
DBU
120
120
120
120
120
120
120
120
100
120
100
85
trace
50
90
KOH
KOtBu
30
100
6
56
41
LiOH
4
trace
trace
32
K2CO3
100
100
55
85
Cs2CO3
Et3N
44
trace
92
48
CsOH·H2O
CsOH·H2O
CsOH·H2O
100
87
trace
35
40
e
10
100
90
trace
a
Reaction conditions: m-phenylenediamine (0.3 mol), benzyl alcohol
by various iridium catalysts. On the basis of the 100%
conversion and product analysis, all iridium complexes have
excellent activity in the oxidation of benzyl alcohol into the
benzaldehyde, which subsequently reacts with p-phenylenedi-
amine to give the corresponding imine under the catalytic
conditions described above. Among these iridium catalysts,
complex 4 proves to have the best activity in the reduction of
imine to give the desired product 11a, whereas mononuclear
iridium complexes show moderate activity, providing mostly
the partial reduction product 11b. It is noted that the
phosphine-substituted complexes, complex 5 and [Ir(COD)-
Cl]2/PPh3, gave 11b as the major product, so was complex 5.
To further validate the activity of the diiridium system 4, the
product distribution of dialkylation of p-C6H4(NH2)2 with
benzyl alcohol catalyzed by complexes 4 and [Ir(COD)Cl]2
was monitored by NMR spectroscopy, and the results are
summarized in Figure 3. The direct N,N′-dialkylation catalyzed
by diiridium complex 4 proceeded smoothly, particularly with a
very low concentration of 11b during the reaction course,
except at the initial stage. Compound 11b is the partial
reduction product, which is the essential intermediate for the
final product 11a. Apparently, 11b was readily reduced in the
bimetallic catalyst system 4. On the other hand, the catalytic
system of [Ir(COD)Cl]2 provided 11b as the major product
accompanied by a minor amount of the desired dialkylation
product 11a. This observation indicates that the diiridium
catalytic system 4 is highly efficient and, more importantly,
demonstrates enhanced activity in comparison to the
(1.2 mmol), complex 4 (3 × 10−3 mmol), base (0.15 mmol), and
molecular sieves (0.3 g) for 24 h. Based on the consumption of
b
c
d
diamine. NMR yields. DBU = 1,8-diazabicyclo[5.4.0]undec-7-ene.
e
p-Phenylenediamine (0.5 mol) as the substrate.
product dramatically. A number of bases do catalyze the
reactions with excellent conversions but different selectivities.
The use of DBU as the base provided the formation of 10b as
the major product. On the other hand, the use of K2CO3 or
CsOH as the base gave the desired diamine 10a in excellent
yields. Other bases for the reactions show poor results or less
selectivity. It is worth mentioning that the reaction proceeded
more slowly and less selectively at a lower reaction temperature
(Table 4, entry 9). Similarly, the reaction of p-phenylenedi-
amine with benzyl alcohol yielded the desired N,N′-dialkylation
product in 90% yield under similar reaction conditions with the
use of CsOH as the base (Table 4, entry 10).
Encouraged by the promising results, we further looked at
extending the above methods to other phenylenediamines and
various alcohols (Table 5). A variety of substituted benzyl
alcohols were subjected to the N,N′-dialkylation with m- or p-
phenylenediamines (Table 5, entries 1−7). In all cases, we
observed the double N-alkylation in good isolated yields, except
for o-methylbenzyl alcohol (Table 5, entry 3), presumably due
to steric reasons. Reactions of simple alkyl alcohols such as
ethanol with phenylenediamines under the same catalytic
conditions also provided the corresponding N,N′-dialkylating
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a
Table 5. N,N′-Dialkylation of Phenylenediamines with Various Alcohols
a
Reaction conditions: diamine (0.3 mmol), alcohol (1.2 mmol), complex 4 (3 × 10−3 mmol), CsOH·H2O (0.15 mmol), and molecular sieves (0.3 g)
b
c
at 120 °C for 24 h. Isolated yields. Under an H2 atmosphere.
Scheme 3. Pathway for the Formation of 2-Phenyl-1H-
benzo[d]imidazole
synergistic or cooperative effect between two metal centers, the
coordination of two nitrogen donors of substrates toward two
metal centers is proposed for intermediates in this reaction.
Certainly, a kinetic study of the reaction is another alternative
for establishing this cooperative effect, which is currently under
investigation.
SUMMARY
■
Table 6. Results of Amination Catalyzed by Various Iridium
Complexes
In this work, we have prepared and characterized diiridium
NHC complexes. Further utilization of these types of carbene
complexes for N,N′-dialkylation of phenylenediamine with
alcohols was investigated. It appears that complex 4 is an
excellent catalyst for direct N,N′-dialkylation, presumably due
to the cooperative effect between the two metal centers in the
complex. More studies involving the cooperative effect between
metal centers in 4 for other catalyses and theoretical
considerations are currently in progress.
a
b
yield (%)
entry
Ir complex
complex 4
conversn (%)
11a
11b
1
2
3
4
5
6
100
100
100
97
90
4
trace
94
complex 5
c
[(IBn)Ir(CO)2Cl]
[Ir(COD)Cl]2
50
23
8
49
70
d
[Ir(COD)Cl]2/PPh3
[Ir(COD)Cl]2/TMEDA
100
99
90
e
35
60
EXPERIMENTAL SECTION
■
a
Reaction conditions: p-phenylenediamine (0.3 mmol), benzyl alcohol
General Information. All reaction and manipulation steps were
performed under a dry nitrogen atmosphere. Tetrahydrofuran was
distilled under nitrogen from sodium benzophenone ketyl. Dichloro-
methane was dried over CaH2 and distilled under nitrogen. Other
chemicals and solvents were of analytical grade and were used after
degassing. Nuclear magnetic resonance spectra were recorded in
CDCl3 on a Bruker AVANCE 400 spectrometer. Chemical shifts are
(1.2 mmol), complex (3 × 10−3 mmol), CsOH·H2O (0.15 mmol), and
b
c
molecular sieves (0.3 g) at 120 °C for 24 h. NMR yields. IBn = 1,3-
d
e
dibenzylimidazolin-2-ylidene. PPh3 (0.01 mmol). TMEDA =
tetramethylethylenediamine (5 × 10−3 mmol).
1
monoiridium complexes. A possible explanation is that the
presence of a second metal in 4 might produce a synergistic or
cooperative effect between two metal centers, which is absent in
a mononuclear system. In fact, the mass spectrum of the
catalytic reaction mixture showed a peak corresponding to the
species I (ESI-MS C50H51Ir2N6O3 m/z found 1167.76 (100%),
calcd 1167.28) (Scheme 4). Thus, a plausible catalytic cycle for
this dialkylation is proposed in Scheme 4. Basically, the reaction
pathway is based on the general reductive amination of amines
with alcohols catalyzed by metal ions. Due to the possible
given in parts per million relative to Me4Si for H and 13C NMR and
relative to 85% H3PO4 for 31P NMR. Infrared spectra were measured
on a Varian 640-IR spectrometer.
1,3-Bis[(2-azidorthylamino)methyl]benzene (1). A mixture of
H2NCH2CH2N3 (2.06 g, 24 mmol) and isophthalaldehyde (0.79 g, 6
mmol) in anhydrous methanol (24 mL) was stirred at ice-cold
temperature for 10 min. Then, NaBH4 (1.36 g, 36 mmol) was added
slowly under a nitrogen atmosphere. The resulting mixture was stirred
at room temperature for 2 h. Water (20 mL) was added slowly to
quench the excess hydrides. The reaction mixture was extracted with
ethyl acetate (20 mL × 2), and the extracts were dried over MgSO4.
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Figure 3. Product distribution during the alkylation of p-C6H4(NH2)2 with C6H5CH2OH: (a) complex 4 as catalyst; (b) [Ir(COD)Cl]2 as catalyst.
a
Reaction conditions applied to this study are similar to those described in footnote of Table 6.
Scheme 4. Catalytic Pathway for the Dialkylation
Upon concentration, the desired compound 1 was obtained as a yellow
liquid (1.15 g, 70%): IR (CH2Cl2) 2099 cm−1 (νNN); 1H NMR (400
MHz, d6-acetone) δ 2.81 (t, J = 5.7 Hz, 4H, CH2N3), 3.38 (t, J = 5.7
Hz, 4H, NCH2), 3.80 (s, 4H, ArCH2N), 7.23−7.25 (br, 3H, ArH),
7.37 (s, 1H, ArH); 13C NMR (100 MHz, CDCl3) δ 140.1, 128.6,
127.7, 126.8, 53.5, 51.4, 47.9; ESI-HRMS for [M + H]+ (C12H19N8)
calcd 275.1733, found 275.1723. Anal. Calcd for C12H18N8: C, 52.54;
H, 6.61; N, 40.85. Found: C, 52.99; H, 6.81; N, 40.79.
(3.23 g, 11.5 mmol) in anhydrous toluene (100 mL) was added a
solution of 1 (1.47 g, 5.3 mmol) in anhydrous toluene (50 mL) by a
flux needle under a nitrogen atmosphere. After the mixture was stirred
at room temperature for 6 h, a solution of W(CO)6 (9.80 g, 17.5
mmol) in THF (150 mL) was added. The resulting mixture was stirred
at room temperature for 72 h. After removal of solvents, the residue
was chromatographed on silica gel with CH2Cl2/hexane (1/3) as
eluent, and a yellow band fraction was collected. Upon concentration,
complex 2 was obtained as a yellow solid (4.5 g, 60%), which was
recrystallized from CH2Cl2/hexane to give a clear yellow crystalline
[(1,3-Phenylene)bis(methylene)]bis(4,5-dihydroimidazol-2-
ylidene)bis[pentacarbonyltungsten(0)] (2). To a solution of PPh3
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5.19 (m, 8H, ArCH2), 7.37−7.64 (m, 14H, ArH); 13C NMR (100
MHz, CD2Cl2) δ 198.6 (Ir-C), 181.9 (CO), 168.5 (CO), 136.2, 135.3,
129.1, 128.7, 128.3, 127.8, 127.2, 126,9, 54.4, 54.3, 48.6, 48.5; FAB-
HRMS for [M + H]+ (C32H31O4N4Cl2Ir2) calcd 991.0981, found
991.0980. Anal. Calcd for C32H30O4N4Cl2Ir2: C, 38.82; H, 3.05; N,
5.66. Found: C, 38.47; H, 3.39; N, 5.28.
Table 7. Crystal Data for 2 and 5·6CHCl3
2
5
formula
fw
C24H18N4O10W2
890.12
C102H90F12Ir2N4O2P6·6CHCl3
2918.21
cryst syst
space group
a, Å
triclinic
monoclinic
P21/m
[(1,3-Phenylene)bis(methylene)]bis(1-benzyl-4,5-dihydroi-
midazol-2-ylidene)bis[trans-carbonylbis(triphenylphosphine)-
iridium(I)] Bis(hexafluorophosphate) (5). A mixture of 4 (60 mg,
0.06 mmol) and PPh3 (64.4 mg, 0.24 mmol) in chloroform (20 mL)
was stirred for 1 h. Upon removal of solvents, a solution of KPF6 (25.6
mg, 0.14 mmol) in acetonitrile (20 mL) was added to the residue with
stirring at room temperature. The reaction mixture was crystallized
from a CHCl3/hexane solution to give a bright yellow crystalline solid
(70%): IR (CH2Cl2) 1996 cm−1 (νCO); 31P (161.9 MHz, CDCl3) δ
19.2 (s), −144.1 (sept, JP−F = 712.4 Hz); 1H NMR (400 MHz,
CDCl3) δ 2.62 (t, J = 9.6 Hz, 4H, NCH2CH2N), 2.82 (t, J = 9.6 Hz,
4H, NCH2CH2N), 3.72 (s, 4H, ArCH2N), 3.82 (s, 4H, ArCH2N), 5.80
(d, J = 7.6 Hz, 2H, ArH), 5.99 (s, 1H, ArH), 6.12 (t, J = 7.6 Hz, 2H,
ArH), 6.34 (d, J = 7.6 Hz, 4H, ArH), 6.91 (t, J = 7.6 Hz, 4H, ArH),
7.11 (t, J = 7.5 Hz, 2H, ArH), 7.49−7.54 (m, 60H, ArH); 13C NMR
P1
̅
10.76500(10)
11.5015(2)
12.5253(2)
88.7600(10)
65.414(2)
13.6471(2)
25.3253(3)
17.3337(2)
90
b, Å
c, Å
α, deg
β, deg
γ, deg
V, Å3; Z
99.9070(10)
90
84.1140(10)
1402.39(4); 2
5901.49(13); 2
1.642
d(calcd), Mg/m3 2.108
F(000)
836
2892
cryst size, mm3
0.20 × 0.15 × 0.10
32 037
0.20 × 0.15 × 0.10
36 585
no. of rflns
collected
no. of indep rflns 6421 (Rint = 0.0386)
36 585 (Rint = 0.0000)
4.28 to 27.49°
(100 MHz, CDCl3) δ 203.8 (t, JP−C = 13.2 Hz, Ir-C), 183.9 (t, JP−C
=
θ range, deg
2.87 to 27.50°
14.7 Hz, CO), 134.0, 133.9, 133.1, 132.0, 131.8, 131.5, 130.5, 129.3,
129.2, 129.1, 128.9, 128.7, 128.6, 128.3, 54.8, 54.4, 48.5, 48.2; HRMS-
ESI for [M]2+ (C102H90O2N4P4Ir2) calcd m/2 956.2631
(C102H90O2N4P4Ir2), found 956.2662.
refinement
full-matrix least squares on F2
method
goodness of fit on 1.013
0.963
F2
General Procedure for the Catalytic Dialkylation of
Diamines. A mixture of phenylenediamine (0.3 mmol), alcohol (1.2
mmol), iridium complex 4 (3 mg, 1 mol %), CsOH (25.6 mg, 0.15
mmol), and molecular sieves (0.3 g) in a reaction tube was flushed
with nitrogen gas. The reaction mixture was heated for 24 h. After the
reaction, water and ethyl acetate were added. The organic extract was
separated, dried, and concentrated. The desired product was purified
by chromatography with CH2Cl2/EtOAc 30/1−10/1 as eluent.
Spectral Data of Dialkylation Products. N,N′-Dibenzyl-p-
phenylenediamine.16 1H NMR (400 MHz, CDCl3): δ 3.43 (br, 1H,
−NH−), 4.25 (s, 4H, −ArCH2N−), 6.57 (s, 4H, ArH), 7.25−7.28 (m,
2H, ArH), 7.31−7.38 (m, 8H, ArH). 13C NMR (100 MHz, CDCl3): δ
140.2, 139.4, 128.1, 127.2, 126.6, 114.3, 49.6.
R indices (I >
2σ(I))
R1 = 0.0225, wR2 =
R1 = 0.0573, wR2 = 0.1342
R1 = 0.0995, wR2 = 0.1501
0.0528
R indices (all
data)
R1 = 0.0279, wR2 =
0.0561
solid: IR (CH2Cl2) 2063, 1897 cm−1 (νCO); H NMR (400 MHz,
CD2Cl2) δ 3.36 (t, J = 10.4 Hz, 4H, NCH2CH2N), 3.64 (t, J = 10.4 Hz,
4H, NCH2CH2N), 4.89 (s, 4H, ArCH2N), 5.94 (s, 2H, −NH−), 7.16
(s, 1H, ArH), 7.25 (d, 2H, J = 7.6 Hz, ArH), 7.41 (t, 1H, J = 7.6 Hz,
ArH); 13C NMR (100 MHz, CD2Cl2) δ 205.7 (W−C), 201.6 (trans-
CO), 198.2 (t, J(13C−183W) = 63 Hz, cis-CO), 136.9, 129.4, 127.5,
125.7, 55.8, 47.6, 45.6; FAB-HRMS for [M]+ (C24H18O10N4W2) calcd
890.0048, found 890.0042. Anal. Calcd for C24H18N4O10W2: C, 32.38;
H, 2.04; N, 6.29. Found: C, 32.70 ; H, 2.24; N, 6.83.
1
N,N′-Bis(p-methoxybenzyl)-p-phenylenediamine.4a 1H NMR
(400 MHz, CDCl3): δ 3.77 (s, 6H, −OCH3−), 4.17 (s, 4H,
−ArCH2N-), 6.56 (s, 4H, ArH), 6.85 (d, J = 8.8 Hz, 4H, ArH), 7.25
(d, J = 8.8 Hz, 4H, ArH). 13C NMR (100 MHz, CDCl3): δ 158.5,
140.5, 131.6, 128.8, 114.7, 113.8, 55.3, 49.1. ESI-HRMS for [M + H]+:
calcd 349.1911 (C22H25N2O2), found 349.1909.
[(1,3-Phenylene)bis(methylene)]bis(1-benzyl-4,5-dihydroi-
midazol-2-ylidene)bis[pentacarbonyltungsten(0)] (3). Sodium
hydride (2.19 g, 60 mmol) in a Schlenk tube was washed with
anhydrous hexane (10 mL × 3) under a nitrogen atmosphere. A
solution of 3 (4.05 g, 4.5 mmol) in anhydrous THF (70 mL) was
added to the tube with stirring. Benzyl bromide (4.44 mL, 36 mmol)
was then added to the mixture, and stirring was continued at room
temperature for 36 h. Water (10 mL) was slowly added to quench the
excess NaH, and the reaction mixture was extracted with ethyl acetate
(30 mL × 2). The organic extracts were dried and concentrated. The
crude product was washed with hexane to remove the unreacted
benzyl bromide. Complex 3 was obtained as a yellow solid (3.9 g,
N,N′-Bis(o-methylbenzyl)-p-phenylenediamine. Yellow solid. 1H
NMR (400 MHz, CDCl3): δ 3.37 (s, 6H, −CH3−), 4.21 (s, 4H,
−ArCH2N−), 6.59 (s, 4H, ArH), 7.15−7.19 (m, 6H, ArH), 7.33 (d, J
= 6.0 Hz, 2H, ArH). 13C NMR (100 MHz, CDCl3): δ 140.4, 137.1,
135.8, 129.9, 127.9, 126.8, 125.7, 114.2, 47.7, 19.3. ESI-HRMS for [M
+ H]+: alcd 317.2012 (C22H25N2), found 317.2016.
N,N′-Diethyl-p-phenylenediamine.17 1H NMR (400 MHz,
CDCl3): δ 1.22 (t, J = 6.8 Hz, 6H, CH3−), 3.10 (q, J = 6.8 Hz, 4H,
−CH2N−), 6.55 (s, 4H, ArH). 13C NMR (100 MHz, CDCl3): δ 140.4,
114.4, 39.9, 15.5. ESI-HRMS for [M + H]+ calcd 165.1392
(C10H17N2), found 165.1391.
80%): IR (CH2Cl2) 2061, 1898 cm−1 (νCO); H NMR (400 MHz,
1
CD2Cl2) δ 3.32 (s, 8H, NCH2CH2N), 5.04 (s, 4H, ArCH2), 5.05 (s,
4H, ArCH2), 7.26−7.40 (m, 14H, ArH); 13C NMR (100 MHz,
CD2Cl2) δ 208.8 (W-C), 201.5 (trans-CO), 198.4 (t, J(13C−183W) =
63 Hz, cis-CO), 137.6, 136.5, 129.6, 129.2, 128.3, 127.9, 126.8, 57.5,
48.7; FAB-HRMS for [M]+ (C38H30O10N4W2) calcd 1070.0994, found
1070.0981. Anal. Calcd for C38H30N4O10W2: C, 42.64; H, 2.83; N,
5.23. Found: C, 42.80; H, 2.98; N, 5.64.
N,N′-Dibenzyl-m-phenylenediamine.16 1H NMR (400 MHz,
CDCl3): δ 4.26 (s, 4H, −ArCH2N−), 5.92 (s, 1H, ArH), 6.06 (dd, J
= 8.1 Hz, 2.0 Hz, 2H, ArH), 6.97 (t, J = 8.0 Hz, 1H, ArH), 7.24−7.35
(m, 10H, ArH). 13C NMR (100 MHz, CDCl3): δ 148.8, 139.1, 129.6,
128.2, 127.1, 126.7, 102.8, 97.0, 48.5.
[(1,3-Phenylene)bis(methylene)]bis(1-benzyl-4,5-dihydroi-
midazol-2-ylidene)bis[dicarbonyliridium(I) chloride] (4). A
mixture of 3 (100 mg, 0.08 mmol) and [Ir(COD)Cl]2 (156 mg,
0.24 mmol) in CH2Cl2 (10 mL) was stirred at room temperature
under a nitrogen atmosphere for 72 h. The reaction mixture was
filtered through Celite, and the filtrate was concentrated. The residue
was chromatographed on silica gel with acetone/hexane (1/4) as
eluent. A yellow band fraction was collected to give complex 4 as a
N,N′-Bis(p-methoxybenzyl)-m-phenylenediamine.4a 1H NMR
(400 MHz, CDCl3): δ 3.86 (s, 6H, OCH3), 4.22 (s, 4H,
−ArCH2N−), 5.92 (s, 1H, ArH), 6.07 (dd, J = 8.0 Hz, 2.4 Hz, 2H,
ArH), 6.85 (d, J = 8.8 Hz, 4H, ArH), 6.99 (t, J = 8.0 Hz, 1H, ArH),
7.27 (d, J = 8.8 Hz, 4H, ArH). 13C NMR (100 MHz, CDCl3): δ 158.1,
148.8, 131.2, 129.5, 128.4, 113.6, 102.8, 96.9, 55.4, 47.9. ESI-HRMS
for [M + H]+: calcd 349.1916 (C22H25N2O2), found 349.1912.
1
1
yellow solid (66 mg, 60%): IR (CH2Cl2) 2062, 1977 cm−1 (νCO); H
NMR (400 MHz, CD2Cl2) δ 3.49−3.61 (m, 8H, NCH2CH2N), 5.04−
N,N′-Diethyl-m-phenylenediamine. Purple solid. H NMR (400
MHz, CDCl3): δ 1.24 (t, J = 7.2 Hz, 6H, CH3−), 3.13 (t, J = 7.2 Hz,
7254
dx.doi.org/10.1021/om300800c | Organometallics 2012, 31, 7248−7255

Organometallics
Article
4H, −CH2N−), 5.88 (s, 1H, ArH), 6.00 (d, J = 7.6 Hz, 2H, ArH), 6.96
(t, J = 7.6 Hz, 1H, ArH). 13C NMR (100 MHz, CDCl3): δ 149.4,
129.7, 102.7, 96.9, 38.6, 15.1. ESI-HRMS for [M + H]+: calcd
165.1392 (C10H17N2), found 165.1390.
(5) (a) Bahn, S.; Imm, S.; Mevius, K.; Neubert, L.; Tillack, A.;
Williams, J. M. J.; Beller, M. Chem. Eur. J. 2010, 16, 3590. (b) Imm, S.;
̈
Bahn, S.; Neubert, L.; Neumann, H.; Beller, M. Angew. Chem., Int. Ed.
̈
2010, 49, 8126 and references therein.
2-Phenyl-1H-benzimidazole.18 1H NMR (400 MHz, DMSO-d6): δ
7.22 (d, J = 6.8 Hz, 2H), 7.57−7.49 (m, 4H), 7.67 (d, J = 6.8 Hz, 1H),
8.19 (d, J = 7.6 Hz, 2H), 12.88 (s, 1H, −NH−).
(6) (a) Kawahara, R.; Fujita, K.-i.; Yamaguchi, R. Adv. Synth. Catal.
2011, 353, 1161. (b) Kawahara, R.; Fujita, K.-i.; Yamaguchi, R. J. Am.
Chem. Soc. 2010, 132, 15108. (c) Zhu, M. W.; Fujita, K.-i.; Yamaguchi,
R Org. Lett. 2010, 12, 1336 and references therein.
(7) (a) Watson, A. J. A.; Maxwell, A. C.; Williams, J. M. J. J. Org.
Chem. 2011, 76, 2328. (b) Saidi, O.; Blacker, A. J.; Lamb, G. W.;
Marsden, S. P.; Taylor, J. E.; Williams, J. M. J. Org. Process Res. Dev.
2010, 14, 1046. (c) Saidi, O.; Blacker, A. J.; Farah, M. M.; Marsden, S.
P.; Williams, J. M. J. Chem. Commun. 2010, 46, 1541 and references
therein.
N,N′-Dibenzyl-o-phenylenediamine.16 1H NMR (400 MHz,
CDCl3): δ 4.32 (s, 4H, −ArCH2N−), 6.71−6.74 (m, 2H), 6.78−
6.81 (m, 2H), 7.26−7.40 (m, 10H). 13C NMR (100 MHz, CDCl3): δ
138.9, 136.7, 128.2, 127.4, 126.8, 119.1, 111.7, 48.9.
N,N′-Bis(p-methoxybenzyl)-o-phenylenediamine.4a Yellow solid.
1H NMR (400 MHz, CDCl3): δ 3.84 (s, 6H, OCH3), 4.27 (s, 4H,
−ArCH2N−), 6.75−6.77 (m, 2H), 6.83−6.85 (m, 2H), 6.92 (d, J = 8.8
Hz, 4H), 7.32 (d, J = 8.8 Hz, 4H). 13C NMR (100 MHz, CDCl3): δ
158.2, 136.6, 130.9, 128.6, 118.9, 113.6, 111.5, 55.4, 48.4. ESI-HRMS
for [M + H]+: calcd 349.1911 (C22H25N2O2), found 349.1921.
Crystallography. Crystals suitable for X-ray determination were
obtained for 2 and 5·6CHCl3 by recrystallization from dichloro-
methane/hexane and chloroform, respectively. Cell parameters were
determined by a Siemens SMART CCD diffractometer. The structure
was solved using the SHELXS-97 program19 and refined using the
SHELXL-97 program20 by full-matrix least-squares on F2 values.
Crystal data of these complexes are given in Table 7. Other
crystallographic data are deposited as Supporting Information.
(8) (a) Martinez-Asencio, A.; Ramon
2011, 67, 3140. (b) Martinez-Asencio, A.; Ramon
́
, D. J.; Yus, M. Tetrahedron
, D. J.; Yus, M.
́
Tetrahedron Lett. 2010, 51, 325 and references therein.
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Lett. 2010, 12, 4888. (b) Liu, Y.; Chen, W.; Feng, C.; Deng, G. Chem.
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̈
Organometallics 2008, 27, 571. (b) Gnanamgari, D.; Moores, A.;
Rajaseelan, E.; Crabtree, R. E. Organometallics 2007, 26, 1226.
(c) Cumpstey, I.; Agrawal, S.; Borbas, K. E.; Martin-Matute, B. Chem.
Commun. 2011, 47, 7827. (d) Lee, C.-C.; Chu, W.-Y.; Liu, Y.-H.; Peng,
S.-M.; Liu, S.-T. Eur. J. Inorg. Chem. 2011, 4801 and references therein.
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50, 3006. (b) Bertoli, M.; Choualeb, A.; Gusev, D. G.; Lough, A. J.;
Major, Q.; Moore, B. Dalton Trans. 2011, 40, 8941. (c) Zanardi, A.;
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ASSOCIATED CONTENT
* Supporting Information
■
S
Tables and CIF files providing crystallographic data, including
atomic positional parameters, bond distances and angles,
anisotropic thermal parameters, and calculated hydrogen
atom positions, for complexes 2 and 5. This material is
available free of charge via the Internet at http://pubs.acs.org.
́
(12) Selected examples: (a) Raynal, M.; Cazin, C. S. J.; Vallee, C.;
Olivier-Bourbigou, H.; Braunstein, P. Dalton Trans. 2009, 3824.
(b) Baker, M. V.; Brown, D. H.; Haque, R. A.; Skeltona, B. W.; White,
A. H. Dalton Trans. 2010, 39, 70. (c) Baker, M. V.; Brown, D. H.;
Haque, R. A.; Simpson, P. V.; Skelton, B. W.; White, A. H.; Williams,
C. C. Organometallics 2009, 28, 3793. (d) Willans, C. E.; Anderson, K.
M.; Paterson, M. J.; Junk, P. C.; Barbour, L. J.; Steed, J. W. Eur. J. Inorg.
Chem. 2009, 2835. (e) Baker, M. V.; Brown, D. H.; Hesler, V. J.;
Skelton, B. W.; White, A. H. Organometallics 2007, 26, 250. (f) Tulloch,
A. A. D.; Danopoulos, A. A.; Winston, S.; Kleinhenz, S.; Eastham, G.
Dalton Trans. 2000, 4499.
AUTHOR INFORMATION
Corresponding Author
*E-mail: stliu@ntu.edu.tw.
■
Notes
The authors declare no competing financial interest.
(13) (a) Liu, C.-Y.; Chen, D.-Y.; Lee, G.-H.; Peng, S.-M.; Liu, S.-T.
Organometallics 1996, 15, 1055. (b) Liu, S.-T.; Hsieh, T.-Y.; Lee, G.-
H.; Peng, S.-M. Organometallics 1998, 17, 993. (c) Chang, Y.-H.; Fu,
C.-F.; Liu, Y.-H.; Peng, S.-M.; Chen, J.-T.; Liu, S.-T. Dalton Trans.
2009, 861. (d) Liu, S.-T.; Reddy, K. R. Chem. Soc. Rev. 1999, 315.
(e) Hahn, F. E.; Jahnke, M. C. Angew. Chem., Int. Ed. 2008, 47, 3122.
(14) (a) Hanasaka, F.; Fujita, K.-i.; Yamaguchi, R. Organometallics
2004, 23, 1490. (b) Nair, V.; Menon, R. S.; Biju, A. T.; Sinu, C. R.;
Paul, R. R.; Jose, A.; Sreekumar, V. Chem. Soc. Rev. 2011, 40, 5336.
(c) Patil, N. T. Angew. Chem., Int. Ed. 2011, 50, 1759 and references
therein.
(15) Lee, C.-C.; Liu, S.-T. Chem. Commun. 2011, 47, 6981.
(16) Itoh, T.; Nagata, K.; Miyazaki, M.; Ishikawa, H.; Kurihara, A.;
Ohsawa, A. Tetrahedron 2004, 60, 6649.
(17) Doornbos, T.; Zuidema, G.; Strating, J. Org. Prep. Proced. Int.
1970, 2, 211.
ACKNOWLEDGMENTS
■
We thank the National Science Council for their financial
support (NSC100-2113-M-002-001-MY3). We also thank Miss
Yu Chang Chao for her generous support for ESI mass data
analysis.
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