Triazolylidene Iridium Complexes for Highly Efficient and Versatile Transfer Hydrogenation of C=O, C=N, and C=C Bonds and for Acceptorless Alcohol Oxidation

Article abstract of DOI:10.1021/acs.inorgchem.7b01707

A set of iridium(I) and iridium(III) complexes is reported with triazolylidene ligands that contain pendant benzoxazole, thiazole, and methyl ether groups as potentially chelating donor sites. The bonding mode of these groups was identified by NMR spectroscopy and X-ray structure analysis. The complexes were evaluated as catalyst precursors in transfer hydrogenation and in acceptorless alcohol oxidation. High-valent iridium(III) complexes were identified as the most active precursors for the oxidative alcohol dehydrogenation, while a low-valent iridium(I) complex with a methyl ether functionality was most active in reductive transfer hydrogenation. This catalyst precursor is highly versatile and efficiently hydrogenates ketones, aldehydes, imines, allylic alcohols, and most notably also unpolarized olefins, a notoriously difficult substrate for transfer hydrogenation. Turnover frequencies up to 260 h^-1 were recorded for olefin hydrogenation, whereas hydrogen transfer to ketones and aldehydes reached maximum turnover frequencies greater than 2000 h^-1. Mechanistic investigations using a combination of isotope labeling experiments, kinetic isotope effect measurements, and Hammett parameter correlations indicate that the turnover-limiting step is hydride transfer from the metal to the substrate in transfer hydrogenation, while in alcohol dehydrogenation, the limiting step is substrate coordination to the metal center.

Full text of DOI:10.1021/acs.inorgchem.7b01707

Article
pubs.acs.org/IC
Triazolylidene Iridium Complexes for Highly Efficient and Versatile
Transfer Hydrogenation of CO, CN, and CC Bonds and for
Acceptorless Alcohol Oxidation
†
,‡
‡
,†
,‡
uez*,†
Zahra Mazloomi, Rene
†
́
Pretorius, Oscar Pam
̀
ies,* Martin Albrecht,* and Montserrat Dieg
́
Departament de Química Física i Inorgan
̀
ica, Universitat Rovira i Virgili, C/Marcel·lí Domingo, 1, 43007 Tarragona, Spain
Department of Chemistry and Biochemistry, University of Bern, Freiestrasse 3, 3012 Bern, Switzerland
S Supporting Information
‡
*
ABSTRACT: A set of iridium(I) and iridium(III) complexes is reported
with triazolylidene ligands that contain pendant benzoxazole, thiazole, and
methyl ether groups as potentially chelating donor sites. The bonding
mode of these groups was identified by NMR spectroscopy and X-ray
structure analysis. The complexes were evaluated as catalyst precursors in
transfer hydrogenation and in acceptorless alcohol oxidation. High-valent
iridium(III) complexes were identified as the most active precursors for
the oxidative alcohol dehydrogenation, while a low-valent iridium(I)
complex with a methyl ether functionality was most active in reductive
transfer hydrogenation. This catalyst precursor is highly versatile and
efficiently hydrogenates ketones, aldehydes, imines, allylic alcohols, and
most notably also unpolarized olefins, a notoriously difficult substrate for
transfer hydrogenation. Turnover frequencies up to 260 h− were
recorded for olefin hydrogenation, whereas hydrogen transfer to ketones −
1
1
and aldehydes reached maximum turnover frequencies greater than 2000 h . Mechanistic investigations using a combination of
isotope labeling experiments, kinetic isotope effect measurements, and Hammett parameter correlations indicate that the
turnover-limiting step is hydride transfer from the metal to the substrate in transfer hydrogenation, while in alcohol
dehydrogenation, the limiting step is substrate coordination to the metal center.
1
. INTRODUCTION
libraries containing more than 300 systematic structural
variations were synthesized from inexpensive reagents (e.g.,
carbohydrates) and containing mainly oxazoline, oxazole, and
thiazole moieties as the nitrogen functionality.
In the last two decades, N-heterocyclic carbenes (NHCs) have
Sustainable production is one of the important challenges our
society is currently facing, and catalysis is undoubtedly a key
enabling technology in this process. Small amounts of catalyst
allow large quantities of compounds to be transformed in fewer
reaction steps and with fewer side products than in uncatalyzed
approaches. Research toward improved activity and selectivity of
a catalytic system is therefore at the core of sustainable processes
for the reduction of costs, waste, and time. The performance of
catalytic reactions depends, to a large extent, on the appropriate
selection of ligands, which provides an electronically and
sterically well defined scaffold for homogeneous catalysts. Within
this context, thousands of homo- and heterodonor ligands have
been developed, mainly P- and N-containing ligands with either
C or C symmetry, although only a few of them have a general
scope. Among these, bidentate heterodonor ligands (P-N, P-S,
P-P′, etc.) with different and electronically diverging donor sites
have made some of the most fundamental contributions to the
development of catalysis. Specifically, P-N bidentate ligands
containing oxazoline, oxazole, or thiazole N-donor sites have
played a key role in catalysis. Our group has contributed to
this development by furnishing improved generations of modular
heterodonor P-X ligand libraries, obtained from readily available
3c−e
emerged as a class of powerful ligands for promoting catalytic
4
activity. Owing to their strong σ-donor ability, air stability, and
low toxicity, NHCs have initially been considered as practical
4
,5
alternatives to the more commonly used phosphines, even
though their electronic and steric properties are distinct. Because
of these unique features, exploring new classes of NHCs has been
an attractive target of organometallic chemistry. 1,2,3-Triazoly-
lidene (trz) ligands are a relevant subclass of NHCs possessing a
6
mesoionic carbene (MIC) structure on 1,3-disubstitution. Work
7
8
by us and others has demonstrated the great potential of this
2
1
9
1
ligand class in catalysis, which has spurred further development.
The triazole framework imparts increased σ-donor properties in
comparison to classical Arduengo-type NHCs, and the
mesoionic character enhances the ability to stabilize different
metal oxidation states. In addition, the ligand precursors are
accessible via efficient and functional-group-tolerant “click”
2
1c,2
Received: July 5, 2017
3
and easy to handle starting materials. Among them, P-N ligand
©
XXXX American Chemical Society
A
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Figure 1. Benzoxazole-, thiazole-, and ether-functionalized triazolylidene ligand precursors L1H·OTf, L2H·OTf, and L3H·BF , respectively, and their
4
Ir(I)/Ir(III) complexes 1−7 used in this work.
Scheme 1. Synthesis of Triazolium-Benzoxazole/Thiazole Salts L1H·OTf and L2H·OTf
6
c,10
chemistry.
These features facilitate the synthesis of donor-
2. RESULTS AND DISCUSSION
.1. Synthesis and Characterization of Ligands and
Complexes. The benzoxazole-/thiazole-triazolium salts L1H·
OTf and L2H·OTf used as ligand precursors were prepared via
copper-catalyzed [3 + 2] cycloaddition of methyl azide and the
corresponding functionalized trimethylsilyl-protected alkyne,
followed by methylation of the triazole at the N3 position
functionalized MIC ligands with different steric and electronic6
properties in the quest to maximize catalytic performance.
Despite these prospects, the development of heterodonor-
containing trz ligands has predominantly focused on pyridyl
units, while other heteroatom donor groups have not been
2
9e,i,11
explored extensively.
Inspired by the potential of trz-based carbene complexes and
the success of oxazole-/thiazole-containing ligands in homoge-
neous catalysis, we became interested in combining these two
scaffolds in a single bidentate ligand and in exploring its potential
in catalysis. Herein, the first examples of mixed oxazole-/thiazole-
trz ligands L1H·OTf and L2H·OTf and their iridium(I) and
iridium(III) complexes 1−4 are reported and compared with
analogues 5−7 containing an ether-functionalized trz ligand
(
Scheme 1). This procedure yielded the desired benzoxazole-
triazolium salt L1H·OTf as the sole product when starting from
the TMS-protected alkynyl benzoxazole but gave a mixture of a
thiazole-triazolium salt (desired ligand L2H.OTf) and a triazole-
thiazolium salt (10) when the thiazole-triazole 9 was used as the
starting material (ca. 1:3 ratio). The unselective methylation of
the thiazole-triazole 9 is attributed to the similar pK values of the
a
11d
4
two heterocycles, while the basicity of the benzoxazole in 8 is
substantially lower than that of triazole and hence leads to the
exclusive formation of L1H·OTf. Pure salts L2H·OTf and 10
were obtained after separation by column chromatography. The
formation of ligand precursors L1H·OTf and L2H·OTf was
confirmed by 1H and C NMR spectroscopy and mass
spectrometry. For ligand L1H·OTf single crystals suitable for
X-ray diffraction analysis were obtained and diffraction analysis
further confirmed its formation (see the Supporting Informa-
derived from L3H·BF
(Figure 1).
In addition to synthetic and structural aspects, we also present
the application of these complexes in transfer hydrogenation
catalysis and acceptorless alcohol oxidation. Mechanistic insights
were obtained through isotope labeling studies and correlation of
Hammett parameters with reaction rates. Specifically, the most
active catalysts are not only active for a broad range of classic
substrates (ketones, aldehydes, imines) but also catalyze the
transfer hydrogenation of unpolarized di- and trisubstituted
olefins.
13
tion). The triazolium-ether salt L3H·BF was prepared for
4
B
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Scheme 2. Synthesis of Benzoxazole/Thiazole/Ether-Appended 1,2,3-Triazol-5-ylidene Iridium Complexes 1−7
1
13
a
Table 1. Selected H and C NMR Signals (ppm) for Complexes 1−5 and Ligands L1H·X−L3H·X
13C
compound
C-5
C-4
O−CN
CHcod
1H CHcod
1
176.7
133.3
154.4
83.3; 82.9
4.62 (bs); 4.51 (bs)
2.88 (bs); 2.8b (bs)
5
2.1; 51.7
2
3
4
5
161.1
152.0
158.4
169.0
136.4
135.5
145.7
146.7
154.5
151.9
158.4
81.3; 79.8
1.3; 51.1
4.55 (m); 4.44 (m)
5
2.68−2.82 (m)
L1H·OTf
L2H·OTf
L3H·BF4
132.3
131.0
146.9
132.6
136.9
130.0
151.2
150.3
1
1d
a
Unless otherwise noted, all resonances are singlets (bs, broad singlet; m, multiplet).
comparative purposes according to previously reported
procedures.
coordination of the 1,2,3-triazolylidene group was confirmed
1
1d
by the disappearance of the triazolium H-5 proton resonance in
1
Iridium complexes 1−7 were obtained by a one-pot reaction
from the corresponding triazolium salts L1H·X−L3H·X.
Reaction with Ag O and Me NCl yielded the desired Ag-carbene
the H NMR spectra and by the large downfield shift of the
13
iridium-bound C-5 carbon signal in the C NMR spectra (Table
1), which is more pronounced in the iridium(I) complexes than
in the iridium(III) analogues. In the [IrCl(L)(cod)] complexes 1
and 5, monodentate ligand coordination through the trz group
was also confirmed by the distinct resonance frequency of all four
methinic groups of the cyclooctadiene ligand, which are
diastereotopic due to (unresolved) planar chirality of the
complexes. Two sets of signals at significantly different chemical
2
4
intermediates, and subsequent in situ transmetalation with either
[Ir(μ-Cl)(cod)]2 or [Ir(Cp*)Cl2]2 gave complexes 1−7
(Scheme 2). Note that, under these reaction conditions, we
were unable to prepare the iridium(I) analogue of complex 1
containing a thiazole instead of a benzoxazole donor group.
When the triazolium-benzoxazole salt was used, metalation with
the iridium(III) precursor compound [IrCl (Cp*)] produced a
shifts were observed for the methine protons around δ 4.5 and
2
2
H
3
:1 mixture of complexes 2 and 3 with bidentate and
2.8 ppm each, characteristic of methinic protons trans to chloro
12
monodentate coordination of the triazolylidene ligand, respec-
tively. Complexes 2 and 3 were both obtained in pure form after
separation by column chromatography. The Ir(III) complexes 6
and 7 were synthesized as previously described.
All complexes were obtained as air-stable solids and were fully
and trans to carbene ligands, respectively. Likewise, four
distinct resonances appeared for the methine carbons (ca. 80 and
13
50 ppm in the C NMR spectrum; Table 1).
11d
The ability of ligand L1 to bind in a mono- or bidentate
coordination mode to the iridium(III) center was easily
1
13
13
characterized by H and C NMR spectroscopy, elemental
confirmed by C NMR spectroscopy. Thus, the signal of the
analysis, mass spectrometry, and X-ray diffraction. Metal
quaternary O−CN nucleus was shifted downfield to 154.5
C
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Figure 2. ORTEP plots of the Ir(I) and Ir(III) complexes 1−5 (50% probability ellipsoids; H atoms and noncoordinating anions omitted for clarity).
Table 2. Selected Bond Distances (Å) and Angles (deg) for Complexes 1−5
1
2
3
4
5
Ir−Cl
2.3587(16)
2.3949(11)
2.4150(4)
2.4260(9)
2.3635(11)
2
.4205(5)
Ir−C5
Ir−N4
C5−C4
C5−Ir−N4
Cl−Ir−N4
C5−Ir−Cl
2.035(5)
1.399(8)
2.027(4)
2.159(4)
1.380(6)
76.07(16)
86.38(11)
84.97(13)
2.046(2)
2.022(4)
2.141(3)
1.387(5)
75.56(13)
90.35(8)
88.69(10)
2.063(4)
1.396(6)
1.401(3)
91.8 (2)
87.87(5)
91.39(12)
8
8.06(6)
Ir−C5−C4
Ir−C5−N1
132.9(4)
126.1(4)
117.8(3)
140.9(4)
131.76(14)
126.92(14)
117.2(3)
140.5(3)
135.7(3)
121.3(3)
ppm when the ligand was coordinated in a bidentate fashion as in
complex 2, while it appeared at 151.9 ppm in complex 3 with a
monodentate triazolylidene, similar to the chemical shift of the
ligand precursor.
2.2. Ir-Catalyzed Transfer Hydrogenation Reactions.
Catalytic transfer hydrogenation (TH) of unsaturated bonds is
currently one of the most investigated hydrogenation reactions.
It is a sustainable, efficient, and mild method that is operationally
simpler and significantly safer than direct hydrogenation, which
Suitable crystals for X-ray diffraction analysis were obtained for
1
3
16
complexes 1−5. The molecular structures confirm the
proposed connectivity patterns and show the expected square-
planar and piano-stool arrangements for Ir(I) and Ir(III)
complexes, respectively (Figure 2). Comparison of the different
structures reveals interesting details. For example, the Ir(III)
complexes 2 and 3 both contain the benzoxazole-triazolylidene
ligand L1, though a slight contraction of the Ir−C5 and Ir−Cl
bonds is noted upon chelation in complex 2, which can be
attributed to the cationic nature of the complex when the ligand
is coordinated in a bidentate fashion (Table 2). The C5−Ir bond
is longer in the iridium(I) complex 5 than in the iridium(III)
complexes (cf. 2.063(4) vs 2.035(5) Å in complexes 1 and 5).
Bidentate coordination of L1 is also responsible for the smaller
Ir−C5−C4 angle in complex 2 in comparison to 3. This
distortion is compensated by the increase of the Ir−C5−N1
uses molecular hydrogen. Ru, Rh, and Ir catalysts bearing NHC
17
ligands have been widely used for TH, with results comparable
to those of Noyori-type catalysts for some substrates. For other
substrates, such as heteroaromatic ketones, imines, and alkenes,
selectivity and turnover frequencies are not optimal yet for TH to
be competitive with conventional hydrogenation. For these
reasons, efforts have been directed toward extending the range of
applicable substrates through improved ligand design. So far,
only a few triazolylidene-type MIC complexes have been
reported for transfer hydrogenation, and their substrate scope
9
i,11c,18
is generally rather narrow.
In addition, most procedures
19
involve substantial amounts of base (typically 10 mol %). The
development of catalytic systems that require the least possible
amount of base is of great importance because the base is able to
promote the metal-free reduction of the substrates and it is
corrosive, which hampers the industrial implementation of this
methodology.
20
14
angle, producing yaw angles of 11.6(2)° in both complexes
Table 2). These yaw angles are smaller than those in related
(
15
pyridyl-trz bidentate complexes. The two chelate structures 2
and 4 have very similar bond metrics about the iridium center,
and despite the different heterocyclic ligand (benzoxazole vs
thiazole), the Ir−N4 bond lengths barely differ in those two
complexes. The C5−Ir−N4 bite angles are essentially identical
and relatively acute for both complexes (76°; cf. Table 2) and are
very similar to those measured for related pyridyl-triazolylidene
2.2.1. Screening of Catalyst Precursors. In a first set of
experiments the catalytic properties of the Ir(I) and Ir(III)
complexes 1−7 were evaluated in the transfer hydrogenation of
benzophenone S1 as a model substrate, using 2-propanol as both
i
hydrogen donor and solvent and NaO Pr as a base and activator.
Time−conversion profiles indicate that all complexes are active
immediately without an observable induction period (section SI-
3 in the Supporting Information). The results are summarized in
15
chelate complexes (76°).
D
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>1100 h− with only 0.1 mol % catalyst loading identify complex
1
Table 3 and indicate that the activity is highly dependent on the
nature of the catalyst precursor. Thus, Ir(I) precursors 1 and 5
5 as one of the most active triazolylidene-based catalysts known
18
to date, though these TOFs are about 2 orders of magnitude
lower than those of the fastest transfer hydrogenation catalysts
Table 3. Transfer Hydrogenation of Benzophenone S1 Using
a
24
Complexes 1−7
reported.
2.2.2. Scope and Limitations of Transfer Hydrogenation
with Complex 5. Encouraged by these initial results, we
investigated complex 5 in the reduction of a broad range of
substrates bearing other unsaturated groups such as ketones
(S1−S17), aldehydes (S18−S25), imines (S26, S27), α,β-
unsaturated ketones (S28−S30), allylic alcohols (S31, S32), and
even olefins (S33, S44). In all cases, the reactions were
performed using 0.5 mol % of catalyst precursor 5 and 5 mol
i
TOF/h−1
b
c
entry
Cat./mol %
NaO Pr/mol %
yield/% (min)
1
2
3
4
5
6
7
8
9
1 (1)
2 (1)
3 (1)
4 (1)
5 (1)
6 (1)
7 (1)
5 (1)
5 (1)
5 (1)
5 (0.5)
5 (0.25)
5 (0.1)
5
5
250
12
84 (60)
8 (60)
5
24
29 (60)
58 (60)
97 (30)
62 (60)
54 (60)
97 (30)
97 (400)
96 (720)
96 (30)
85 (60)
98 (150)
%
of base.
5
60
We first investigated the reduction of 17 ketones with different
5
780
96
steric and electronic properties, including challenging hetero-
aromatic ketones (S1−S17; entries 1−17, Table 4). The results
indicate that the catalytic activity depends on both the steric and
electronic properties of the substrate, with TOFs as high as 2100
5
5
72
10
2.5
1
840
48
−
1
h
for the reduction of the less sterically demanding
1
1
1
1
0
1
2
3
24
cyclohexenone S2 and electron-poor 4-(trifluoromethyl)-
acetophenone S8 (entries 2 and 8). The activity toward
reduction of aryl ketones decreased with increasing steric
demand of the alkyl substituents (S1, S3−S5; cf. entry 5 vs
entry 3). However, the catalytic activity was hardly affected by
steric factors on the aryl moiety of the substrate. Interestingly,
ortho-substituted aryl ketones were slightly more reactive than
the nonsubstituted analogues (entry 3 vs entry 11). When
comparing several para-substituted aryl ketones (S3, S6−S8), we
note that the catalytic activity increases with enhanced electron-
withdrawing character of the para substituent (entries 6−8; see
also section 2.2.3 for a mechanistic discussion). Finally, the
transfer hydrogenation of meta-substituted aryl ketones (S9,
S10) provided high activities similar to S3 (Table 4, entry 3 vs 9
and 10).
5
990
820
1100
5
5
a
Reaction conditions: 0.4 M benzophenone S1 in 2-propanol, catalyst
i
b
precursor (0.1−1 mol %), NaO Pr (1−10 mol %). TOF in mol of S1
−
1
c
1
(
(mol of Cat.) h) measured after 5 min. Yield measured by H
NMR spectroscopy using mesitylene as internal standard (the time
required to reach the indicated yield is given in parentheses).
provided much higher activity than Ir(III) analogues 2−4 and 6
and 7, respectively (i.e. entry 1 vs entries 2−4 and entry 5 vs
entries 6 and 7). A plausible explanation is that the higher
electron density at the low-valent metal center facilitates the
formation of the metal hydride species. Generally, complexes 5−
7
containing the ether-functionalized triazolylidene ligand L3
induce higher activity in comparison to complexes 1−4 with
The scope was then extended to heteroaromatic ketones,
which are more challenging substrates because coordination of
the heteroatom to the metal often reduces the activity of the
catalyst drastically. Therefore, catalytic systems able to reduce
this substrate class under transfer hydrogenation conditions are
21
benzoxazole or thiazole donor groups. This difference was
tentatively attributed to the higher propensity of the benzoxazole
and thiazole moieties to coordinate to iridium in comparison to
22
the methyl ether unit in L3. It should be noted that the activity
profiles and turnover frequencies of complexes 2 and 3 with the
benzoxazole as a (potentially) chelating group differ substan-
tially, suggesting that the catalytically active species are different
and depend on whether the initial species contains a chelating
triazolylidene ligand or not.
25
relatively rare. To the best of our knowledge, only one study
has reported the reduction of 2-acetylpyridine using a cationic
26
Rh(III) complex containing a bis-trz ligand. When complex 5
was used, all evaluated heteroaromatic ketones (S12−S17; Table
4, entries 12−17) were reduced with very high conversion and
−1
Further optimization of base and catalyst loading was
performed with complex 5, which exhibited the highest activity
TOFs up to 1400 h . The activity was strongly dependent on the
type of heteroaromatic moiety and on its substitution pattern.
For example, furyl-based ketones (S16, S17) induced slightly
23
under the initial reaction conditions. The amount of base had a
−1
substantial effect on activity (Table 3, entries 5 and 8−10):
higher activities (TOFs up to 1500 h ) than the phenyl-derived
substrates (entries 16 and 17 vs entry 3). They also induced
higher activities in comparison to the pyridyl-based substrates
S12−S14, which we attribute to the lower ability of the furyl
group to coordinate to the metal center. The catalytic activity was
i
lowering the amount of NaO Pr from 5 to 2.5 mol % decreased
the catalytic performance markedly, as indicated by a more than
1
0-fold lower turnover frequency and by an increase in the
reaction times by more than 1 order of magnitude to reach
completion (entry 9 vs entry 5). Increasing the concentration of
the base did not improve the activity substantially (initial TOF =
−1
lowest toward the thienyl-based ketone S15 (TOF = 100 h ;
entry 15). The reduction of pyridyl-based ketones (S12−S14;
entries 12−14) showed a marked dependence on the
heteroaromatic ring substitution pattern and was most efficient
with 3- and 4-acetylpyridines (S12 and S13) yet significantly
−1
780 vs 840 h , entries 5 and 8), and therefore, 5 mol % of
i
NaO Pr was used in further reactions. The amount of catalyst
precursor had little effect on the activity, and initial rates were
−1
−1
only moderately varying (TOF = 780−1100 h ), yet time to
completion increased with lower catalyst loading (Table 3,
entries 5 and 11−13). Evaluation of the TOF vs catalyst
concentration suggests a linear dependence, indicating that the
reaction is first order in catalyst. It is worth noting that TOFs
slower for 2-acetylpyridine S14 (TOF = 260 h ). Such a trend is
not unexpected when it is considered that both the substrate and
the product of 2-acetylpyridine reduction are potentially
chelating ligands that may deactivate the catalyst through
competitive coordination to the active site. Such an interaction
E
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a
Table 4. Transfer Hydrogenation of Ketones, Aldehydes, and Imines Using Ir Complex 5
a
i
b
−1
Reaction conditions: 0.4 M substrate in 2-propanol, 5 (0.5 mol %), NaO Pr (5 mol %), reflux. TOF in mol of substrate ((mol of 5) h) measured
c
1
after 5 min. Yield measured by H NMR spectroscopy using mesitylene as internal standard or by GC using dodecane as internal standard.
is obviously more effective for S14 than in the substrates
containing the acyl substituent in a meta or para position.
Complex 5 is also a highly efficient catalyst precursor for the
TH of several aldehydes (Table 4, entries 18−25). In general
complex 5 converts aldehydes significantly faster in comparison
to ketones and TOFs are 50−100% higher when substrates with
identical substitution patterns are compared. The activity follows
the same trend as observed for the reduction of ketones, although
the electronic effects are less pronounced. The efficient
conversion of aldehydes is remarkable, as commercial grade
aldehydes have been used here, which tend to undergo
Cannizzarro reactions with many other TH catalysts. No
such products from partial oxidation were detected when
complex 5 was used as catalyst precursor.
The high activity of this complex prompted us to evaluate its
activity for the transfer hydrogenation of less reactive substrates.
For example, imines are known to be a challenging class of
substrates, as imine coordination to the catalyst is often too stable
27
F
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Table 5. Reduction of α,β-Unsaturated Ketones and Allylic Alcohols Using Ir Complex 5 under Transfer Hydrogenation
a
Conditions
a
i
b
1
Reaction conditions: 0.4 M substrate in 2-propanol, 5 (0.5 mol %), NaO Pr (5 mol %), reflux. Yield measured by H NMR spectroscopy using
mesitylene as internal standard or by GC using dodecane as internal standard.
and, therefore, higher catalyst loading and much longer reaction
times are usually required for efficient conversion. Interestingly,
the transfer hydrogenation of aldimine S26 proceeded smoothly1
under our mild reaction conditions, reaching a TOF of 770 h−
Scheme 3. Deuterium Labeling Experiments of Allylic Alcohol
S31 using Ir Catalyst Precursor 5
a
(
Table 4, entry 26). Ketimine S27 was also converted, though
−
1
with a significantly lower TOF (30 h ), which may be
rationalized by the steric demand of this substrate (cf. faster
conversion of aldehyde vs ketone; see above). It should be noted
that, for both substrates, the reduction proceeded cleanly and
provided the desired amines as the exclusive products according
a
The percentage of incorporation of deuterium atoms is shown in
parentheses. Deuterium incorporation due to isomerization is shown
in red.
1
to H NMR spectroscopy.
Hence, catalyst precursor 5 is able to reduce a wide range of
ketones, including the most challenging heteroaromatic ones,
aldehydes and aldimines, with high activities (TOFs up to 2100
transfer hydrogenation of olefins. Olefins are challenging targets
because they lack the polarity of ketones and other CE double
bonds and therefore are usually poorly converted in TH. So far
only a few catalytic systems containing NHC or MIC ligands
have been reported for olefin transfer hydrogenation, and the
−1
h ). Although there are some metal carbene species, mainly Ru
NHC complexes that are able to promote these transformations
2
4d,28
11c,17a,10,18d,30
more efficiently,
our results compete favorably with those
substrate scope is typically very limited.
work by us and others suggests that NHC iridium and
Previous
9
i,11c,18a,c,d,f
17a
17d
for known trz iridium catalytic systems.
In addition to imine hydrogenation, complex 5 was also
evaluated in the reduction of α,β-unsaturated ketones and allylic
alcohols (Table 5). Full reduction to the corresponding saturated
alcohols was observed within 1 h.
ruthenium complexes efficiently transfer hydrogenate a small
18d
selection of olefins. More recently the groups of Elsevier and
11c
Sarkar have shown the reduction of cyclooctene using NHC
iridium and MIC ruthenium and iridium complexes. However,
long reaction times were required (20 h for 50% conversion for
the NHC system, 24 h for complete conversion with the MIC
ruthenium system), as well as significant amounts of catalyst
loading (1 mol %) and base (up to 20 mol %). Complex 5, in
contrast, induces the efficient transfer hydrogenation of a broad
range of olefins under milder reaction conditions (4−6 h for
essentially quantitative conversion at 0.5 mol % iridium and 5
mol % base; Table 6). We note that the catalytic performance is
relatively insensitive to the olefin substitution pattern and to the
geometry of the double bond, and activities are similarly high for
a range of linear mono- and disubstituted olefins, including (E)-
as well as cyclic (Z)-olefins with TOFs up to 260 h− (entries 1−
11). Generally, aryl substituents increase the rate in comparison
to alkyl substituents (cf. entry 3 vs entry 5 or entry 4 vs entry 7).
We also found that the steric effects influence the turnover
frequency considerably. For example, transfer hydrogenation of
1,1-disubstituted olefins S41−S43 proceeds at gradually lower
rates upon increasing the steric demand of the alkyl substituent
(entries 9−11). Similarly, the reduction of the sterically even
The high activity of complex 5 toward substrates S28−S32
compares well with the activity toward simple ketones,
suggesting that the reaction proceeds via a tandem isomer-
ization/transfer hydrogenation reaction rather than via the direct
reduction of the ketone and the olefin as observed by Elsevier et
al. using similar Ir(I) complexes containing chelating 1,2,3-
18d
triazolylidene-NHC ligands.
To support this mechanistic
model, we performed the transfer deuterogenation of α-
vinylbenzyl alcohol S31 using 2-propanol-d as a deuterium
8
source and sodium isopropoxide-d as base (Scheme 3). The
7
formed product contained deuterium not only at the expected
position from double-bond deuterogenation but also at the allylic
position, which corroborates a competing isomerization path-
1
2
9
way. A rapid isomerization was also supported by mass
spectrometric analysis of the corresponding deuterated products,
which revealed species with more than two deuterium atoms
incorporated.
On the basis of the successful conversion of enones and other
challenging substrates, we expanded the substrate scope to the
G
DOI: 10.1021/acs.inorgchem.7b01707
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Inorganic Chemistry
Article
a
Table 6. Transfer Hydrogenation of Olefins Using Ir Complex 5
a
i
b
−1
Reaction conditions: 0.4 M substrate in 2-propanol, 5 (0.5 mol %), NaO Pr (5 mol %), reflux. TOF in mol of substrate ((mol of 5) h) measured
c
1
after 10 min. Yield measured by H NMR spectroscopy using mesitylene as internal standard or by GC using dodecane as internal standard.
more hindered trisubstituted olefin S44 was accomplished with
the lowest activity of the series and required 12 h to reach
synthetically useful conversions. However, it is noteworthy that
final conversions are high and that complex 5 is one of the rare
catalyst precursors that is capable of efficiently reducing such a
broad range of unpolarized and highly substituted olefins.
In an attempt to assess the relative rate of olefin hydrogenation
vs alkene double bond isomerization, parallel catalytic runs were
performed using trans-β-methylstyrene S37 and allylbenzene
ization. According to the deuterium incorporation at the terminal
position, this isomerization involves an iridium deuteride species
rather than an intramolecular H transfer. Furthermore, time-
dependent GC and 1H NMR analyses indicate that the
isomerization is faster than the hydrogenation. Thus, after 10
min the allylbenzene S38 was fully isomerized to the internal
olefin S37 under TH conditions, as indicated by GC analysis (see
31
section SI-8 in the Supporting Information). These results
confirmed that the reduction of allylbenzene proceeds via a
tandem isomerization/transfer hydrogenation reaction similar to
that discussed for α,β-unsaturated ketones and allylic alcohols
S38 as substrates and 2-propanol-d as the hydrogen source
8
(Scheme 4). For both substrates, deuterium incorporation took
(
see above).
.2.3. Mechanistic Insights. Initial mechanistic investigations
Scheme 4. Deuterium Labeling Experiments of Olefins S37
and S38 Using Ir Catalyst Precursor 5
2
a
of the Ir-catalyzed transfer hydrogenation with complex 5
included the racemization of the monodeuterated (S)-1-
phenylethanol-d ((S)-12) in the presence of acetophenone
(
S3) (Scheme 5). This simple experiment has been previously
used to elucidate the nature of the metal hydride species
32
responsible for the catalytic activity. Thus, a selective carbon to
carbon hydrogen transfer is indicative of a metal monohydride
species responsible for the catalytic activity, while a nonselective
hydrogen transfer (involving both oxygen to carbon and carbon
to carbon H transfer) reveals the involvement of a metal
a
The percentage of incorporation of deuterium atoms is shown in
16,32
dihydride species.
When the reaction was catalyzed with
parentheses. Deuterium incorporation due to isomerization is shown
in red.
complex 5, a high degree of retention of deuterium in the α
position (>95%) was observed after full racemization. This result
indicates that a monohydride Ir−H species is formed as the
catalytically active species, in agreement with previous
place at the olefinic as well as at the allylic position, and the ratios
are equal within measuring errors, which suggests fast isomer-
33
mechanistic studies on related NHC iridium(I) complexes.
Scheme 5. Deuterium Content in 1-Phenylethanol-d (rac-12) after Racemization of (S)-1-Phenylethanol-d Using Ir Catalyst
Precursor 5
H
DOI: 10.1021/acs.inorgchem.7b01707
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Inorganic Chemistry
Article
Scheme 6. Kinetic Isotope Effects of the Transfer Hydrogenation of Ketones (Acetophenone (S3) as an Example) and Olefins
(Styrene (S39) as an Example) Using Iridium Complex 5
Further mechanistic insights into the rate-determining step of
the reaction were obtained by studying the kinetic isotope effects
KIE) of the transfer hydrogenation of acetophenone (S3) and
the drawbacks of oxidant-free dehydrogenation of alcohols is that
the product might remain in the coordination sphere and be
subsequently transformed into undesired ethers and acetals.
41
(
styrene (S39) (Scheme 6). Several experiments were carried out
Recent work with iridium triazolylidene based complexes
revealed that monometallic species favor the etherification of
the carbonyl product, while dimetallic species suppress this
using isopropyl alcohol, isopropyl alcohol-d , and isopropyl
7
alcohol-d as solvent and with sodium tert-butoxide instead of
8
42
sodium isopropoxide as a base to avoid potential base-mediated
H/D scrambling. Reaction monitoring by GC revealed a
significant KIE for the hydrogen transfer reaction of both
transformation and form the ketone selectively.
2.3.1. Dehydrogenation of Benzyl Alcohol: Catalyst
Precursor Screening. In a first set of experiments, benzyl alcohol
(S45) was used as the benchmark substrate to study the
effectiveness of the iridium(I) and iridium(III) complexes 1−7 in
acceptorless alcohol oxidation. For comparison, these complexes
were evaluated under the conditions optimized in previous
substrates when the carbinol position was deuterated (kCHOH
/
kCDOH ≈ kCHOH/kCDOD ≈ 4), though the deuteration vs
protonation of the oxygen did not alter the rate. Likewise,
modulation of the solvent from isopropyl alcohol-d to isopropyl
7
34
42
alcohol-d did not affect the rate (kCDOD/kCDOH ≈ 1). These
studies of Ir MIC complexes. Reactions were therefore carried
8
results clearly indicate that the hydride transfer is the rate-
limiting step for both substrate types, while the second process
out without base, using a catalyst loading of 5 mol % in 1,2-
dichlorobenzene at elevated temperatures (150 °C). Benzalde-
hyde formation was monitored over time (see the Supporting
Information), and conversions after 12 and 72 h are compiled in
Table 7. For all complexes, the time−conversion profiles showed
no significant activation time. Interestingly, only the formation of
(proton transfer) is comparably fast.
To further distinguish whether hydrogen transfer from the
isopropyl alcohol donor to the metal or the transfer from the
metal hydride to the substrate is turnover-limiting, the catalytic
conversion of differently para substituted acetophenones (S3, S7,
1
the desired benzaldehyde was observed in the H NMR
35
42
S8) was compared against the Hammett σ values of these
substrates (see Figure SI-4 in the Supporting Information). The
positive ρ value (ρ = 0.52) indicates that the electron density
increases in the rate-determining step. In combination with the
substantial KIE, these results (i.e., the fact that the substrate
affects the rate as well as the buildup of negative charge according
to a Hammett parameter correlation) lend strong support to the
transfer of the hydride from iridium to the substrate as turnover-
limiting step.
spectrum, while no etherification was noted. As observed in
the transfer hydrogenation reaction, the activity is highly
dependent on the catalyst precursor (Table 7). As expected for
oxidation reactions and in contrast to reductive TH, high-valent
Table 7. Acceptorless Oxidation of Benzyl Alcohol (S45)
a
Using Complexes 1−7
2.3. Ir-Catalyzed Acceptorless Alcohol Oxidation. Since
transfer hydrogenation inherently involves the dehydrogenation
of a donor system such as isopropyl alcohol, we were interested
to investigate the activity of the iridium complexes 1−7 in the
dehydrogenation of alcohols. Various procedures are available for
the oxidation of alcohols, ranging from the use of high-valent oxo
metal species, Oppenauer-type oxidation, and the use of
b
yield/%
entry
Cat.
12 h
72 h
1
2
3
4
5
6
7
1
2
3
4
5
6
7
30
33
18
28
30
54
36
33
99
34
94
38
60
55
36
hypervalent iodine among others to biomimetic and enzymatic
approaches using more benign oxidants such as hydrogen
37
peroxide and dioxygen. Acceptorless oxidation of alcohols with
liberation of H has been less studied, although it is very attractive
2
in terms of atom economy, waste reduction, and reversible
3
8
a
hydrogen storage. Research in this area has rapidly moved from
Reaction conditions: 0.1 M benzyl alcohol (S45) in 1,2-
dichlorobenzene, catalyst precursor (5 mol %). Yield measured by
H NMR spectroscopy using hexamethylbenzene as internal standard.
39
b
phosphine ligands to NHC ligands, including triazolylidene-
9h,j,18,40
1
based catalysts of ruthenium(II) and iridium(III).
One of
I
DOI: 10.1021/acs.inorgchem.7b01707
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Inorganic Chemistry
Article
a
Table 8. Base-Free Dehydrogenation of Alcohols Using Ir Complexes 2 and 4
a
b
1
Reaction conditions: 0.1 M substrate in 1,2-dichlorobenzene, Ir catalyst precursor (5 mol %). Yield measured by H NMR spectroscopy using
c
1
mesitylene as internal standard or by GC using dodecane as internal standard. Selectivity determined by H NMR spectroscopy.
Ir(III) complexes induced much higher activity in comparison to
the Ir(I) analogues (entry 1 vs entries 2−4; entry 5 vs entries 6
and 7). Analysis of the catalytic profiles reveals that all systems
deactivate, except for complexes 2 and 4 (entries 2 and 4 vs
entires 3, 6, and 7). Complex 6 with a pendant ether group shows
reduced the selectivity considerably (entry 5 vs entries 3 and 7).
The formation of the undesired corresponding ethers is lower
when catalyst precursor 4 is used instead of 2, suggesting that
intrinsic properties of the catalyst structure contribute to
controlling the selectivity (entries 4 and 8 vs entries 3 and 7).
The conversion rate is strongly substrate dependent, is generally
higher for secondary in comparison to primary alcohols (entries
−1
the highest initial activity (TOF = 12 h in comparison to
ini
−1
TOF = 4 h for complexes 2 and 4), though catalytic activity
ini
ceased after about 12 h (ca. 60% conversion), while complexes 2
and 4 turn over continuously to full substrate conversion. The
enhanced stability of complexes 2 and 4 may be attributed to the
stronger coordination of the benzoxazole and thiazole moieties
in comparison to the methyl ether in complex 6 (cf. Scheme 2).
In contrast to transfer hydrogenation, acceptorless alcohol
oxidation benefits from a chelating triazolylidene ligand.
9
−12 vs entries 1−8), and is enhanced by electron-donating
groups in the para position (cf. entry 3 vs entry 5 and entry 10 vs
entry 11; see below for mechanistic implications).
Mechanistic insights were obtained from isotope labeling
experiments. Kinetic isotope effects (KIE) of the dehydrogen-
ation reaction were determined by conducting experiments using
1
-phenylethanol (CHOH), monodeuterated 1-phenylethanol-d
2
.3.2. Dehydrogenation of Other Primary and Secondary
(
CDOH), and bis-deuterated 1-phenylethanol-d2 (CDOD;
Alcohols: Mechanistic Insights. Encouraged by the high
selectivity induced by the iridium(III) complexes 2 and 4, their
efficiency was assessed with a range of benzylic alcohols S45−
S48 and secondary alcohols S49−S52 (Table 8). Generally,
conversions were high and good to excellent selectivity was
achieved. While the substituents of the substrate do not
significantly affect the selectivity for secondary alcohols (entries
Scheme 7). The reactions were monitored over time by GC
analysis. In contrast to the transfer hydrogenation reaction, only
very small KIEs were obtained (kCHOH/kCDOH ≈ kCHOH/kCDOD
≈
Scheme 7. Kinetic Isotope Effects of the Dehydrogenation of
Alcohols Using Ir Catalyst Precursor 2
9
8
−12), a dependence was noted for benzylic alcohols (entries 1−
). Thus, while the electron-poor benzylic alcohol S47 was
converted almost exclusively to the desired aldehyde, methyl
groups in a meta or para position (substrates S46 and S48)
J
DOI: 10.1021/acs.inorgchem.7b01707
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Inorganic Chemistry
Article
3
2b
50
1
.1; kCDOD/kCDOH ≈ 1). These low values indicate that neither
phenylethan-1-d-1-ol,
and propan-d -2-ol were prepared as
7
previously described. All other reagents and substrates were
commercially available and were used as received. H (400 MHz),
C{ H} (101 MHz), and F NMR (377 MHz) spectra were recorded
on a Bruker NMR spectrometer at room temperature (298 K).
C−H nor O−H bond making or breaking is a turnover-limiting
process. Comparison of the activity of complex 2 in the
conversion of different para-substituted benzyl alcohols (S45−
S47) against their Hammett σ values (see Figure SI-6 in the
Supporting Information) provides a slope with a negative ρ value
1
13
1
19
35
1
13
Chemical shifts are relative to that of SiMe ( H and C) as internal
4
1
13
standard. H and C NMR assignments were made on the basis of
(ρ = −0.61), indicating a decrease of electron density in the
1
1
1
13
1
13
H− H gCOSY, H− C gHSQC, and H− C gHMBC experiments.
transition state. A plausible mechanistic model may therefore
include the coordination of the substrate alcohol to the iridium
center as the turnover-limiting step. The substitution of a metal-
bound chloride by an alcohol is expected to be faster if the
alcohol is electron-rich, which rationalizes the observed
Hammett correlation. Such a proposal is in line with a reduced
charge density in the substrate upon coordination, and it also
concurs with the measured KIEs, as no CH or OH bond cleavage
is involved. According to this model, solvolysis of the iridium
complex and displacement of the Ir−Cl bond by an Ir−O(H)R
species is slow and prevents the catalyst from turning over more
quickly. Rate-limiting alcohol coordination also corroborates the
observed dependence of the catalytic rate on the ligand structure,
as neutral 6 is expected to undergo solvolysis faster in
comparison to the cationic chelate complexes 2 and 4. Similar
trends were also noted in related alcohol dehydrogenation with
Elemental analyses were performed by the University of Bern
Microanalytic Laboratory using a Thermo Scientific Flash 2000
CHNS-O elemental analyzer. High-resolution mass spectrometry
(HRMS) was carried out with a Thermo Scientific LTQ Orbitrap XL
instrument (ESI-TOF).
4
.2. General Procedure for the Preparation of Triazoles 8 and
9
. A suspension of MeI (0.21 mL, 3.3 mmol) and NaN (644 mg, 9.9
3
mmol) in H O/THF (14 mL; 1/1 v/v) was stirred at room temperature
2
for 48 h. CuSO ·5H O (49.5 mg, 0.198 mmol), sodium ascorbate (393
4
2
mg, 1.98 mmol), and the corresponding functionalized trimethylsilyl-
protected alkyne (3.96 mmol) were added subsequently, and the
mixture was stirred at 70 °C for 36 h. All volatiles were removed under
reduced pressure, and the residue was suspended in CH Cl (30 mL)
2
2
and washed with water (2 × 50 mL) and brine (2 × 50 mL). After drying
over MgSO , activated carbon was added to the solution and stirred for
0 min. The suspension was filtered through Celite and eluted with
CH Cl (20 mL), and the filtrate was evaporated to dryness, yielding the
4
3
2
2
9j
triazole as an off-white powder.
triazolylidene ruthenium(II) complexes.
4
4
.3. Synthesis of 2-(1,3-Dimethyl-1H-1,2,3λ -triazol-4-yl)-
benzo[d]oxazole Trifluoromethanesulfonate (L1H·OTf). A sus-
pension of 2-(1-methyl-1H-1,2,3-triazol-4-yl)benzo[d]oxazole (400 mg,
3
. CONCLUSIONS
2
.00 mmol) and MeOTf (249 μL, 2.20 mmol) in CH Cl (6 mL) was
We have disclosed efficient and selective hydrogen transfer
catalysts that contain a triazolylidene iridium scaffold. Catalytic
activity is strongly dependent on the iridium oxidation state and
on the donor functionality linked to the triazolylidene ligand.
Low-valent iridium(I) complexes perform better in transfer
hydrogenation, while high-valent iridium(III) complexes show
higher activity for acceptorless alcohol oxidation, especially when
the iridium(III) center is stabilized by strongly chelating
benzoxazole and thiazole substituents. In contrast, transfer
hydrogenation is most efficient when a pendant methyl ether
functionality is linked to the triazolylidene ligand, providing
excellent activity also to less reactive substrates such as imines
and even apolar olefins. These results underline the relevance of
ligand tailoring for specific transformations and demonstrate that
different ligand design principles are required for transfer
hydrogenation catalysis and for alcohol oxidation, including
stabilization of low- vs high-valent iridium complexes and weak
vs strong chelation. Even though it may be intuitive that low-
valent complexes show increased activity in reduction (TH) and,
vice versa, high-valent catalyst precursors in oxidation catalysis,
the distinct performance is remarkable, as the overall catalytic
process is the same hydrogen transfer from/to a substrate.
Principally, the microscopic steps of the reaction are therefore
expected to be highly related. However, mechanistic inves-
tigations reveal completely different rate-limiting steps for the
2 2
stirred at −12 °C for 20 h. Et O (20 mL) was added, and a suspension
formed. All volatiles were evaporated under reduced pressure. The
residue was washed with Et O (3 × 10 mL) to afford L1H·OTf as a
white solid (400 mg, 55%).
2
2
4
4
.4. Synthesis of 2-(1,3-Dimethyl-1H-1,2,3λ -triazol-4-yl)-
thiazole Trifluoromethanesulfonate (L2H·OTf). According to the
same procedure as for L1H·OTf, 2-(1-methyl-1H-1,2,3-triazol-4-
yl)thiazole (597.43 mg, 3.30 mmol) and MeOTf (411 μL, 3.63
mmol) in CH Cl (15 mL) gave a mixture of L2H·OTf and compound
2
2
1
1
0 in an approximately 1:3 ratio ( H NMR spectroscopy). Purification
by column chromatography (SiO , CH Cl /MeOH gradient 20/1 to
2
2
2
pure MeOH) yielded L2H·OTf in the first fraction (120 mg, 11%) and
0 in the second fraction (240 mg, 33%) as pale yellow waxy solids.
.5. Synthesis of [IrCl(L1)(cod)] (1). A suspension of L1H·OTf
107 mg, 0.30 mmol), Me NCl (33 mg, 0.30 mmol), and Ag O (140 mg,
1
4
(
4
2
0.6 mmol) in MeCN (8 mL) was stirred protected from light for 16 h.
The suspension was then filtered through Celite, and the volatiles were
removed under reduced pressure. The residue was suspended in CH
Cl
2
2
(8 mL), and [Ir(cod)Cl)] (100 mg, 0.15 mmol) was added. The
2
reaction mixture was stirred for 1 h protected from light and then filtered
over Celite, and all volatiles were removed under reduced pressure. The
residue was triturated with pentane, yielding product 1 as a light brown
solid (90 mg, 55%). H NMR (400 MHz, CD Cl ): δ 7.87−7.72 (m, 2H,
H ), 7.46 (m, 2H, H ), 4.62 (bs, 1H, CH ), 4.51 (bs, 1H, CH ),
1
2
2
Ph
Ph
cod
cod
4
1
.49 (s, 3H, NCH ), 4.43 (s, 3H, NCH ), 2.88 (bs, 1H, CHcod), 2.80 (bs,
3
3
H, CHcod), 2.34 (bs, 1H, CH2 co d1 ), 2.19 (bs, 3H, CH2 cod), 1.76 (bs, 2H,
3
1
CH2 cod), 1.58 (bs, 2H, CH2 cod). C{ H} NMR (101 MHz, CD Cl ): δ
2
2
4
3
two processes, emphasizing the relevance of sophisticated
catalyst design. Moreover, the guidelines deduced from this work
may also apply to further optimization of related catalysts for
hydrogen transfer reactions.
176.7 (Ctrz−Ir), 154.4 (O−CN), 150.7 (CPh), 141.7 (CPh), 133.3
(Ctrz−C), 126.6 (CPh−H), 125.5 (CPh−H), 120.6 (CPh−H), 111.4
(
(
C −H), 83.3 (CH ), 82.9 (CHcod), 52.1 (CH ), 51.7 (CHcod), 42.4
Ph
cod
cod
NCH ), 40.3 (NCH ), 34.3 (CH2 cod), 33.7 (CH2 cod), 30.2 (CH2 cod).
3
3
+
HRMS (ESI+): m/z found 515.1417 [M − Cl] (calcd for
C H IrN O, 515.1410). Anal. Calcd for C H ClIrN O: C, 41.49;
H, 4.03; N, 10.19. Found: C, 41.13; H, 3.87; N, 9.89. Suitable crystals for
X-ray diffraction were obtained by slow diffusion of pentane into a
CH Cl solution of 1.
19
22
4
19 22
4
4
. EXPERIMENTAL SECTION
4
.1. General Considerations. Grubbs columns were used for
solvent purification. Anhydrous isopropyl alcohol and 1,2-dichlor-
2
2
obenzene were used as commercially available. The compounds 2-
4.6. Synthesis of [IrCp*Cl(L1)]OTf (2) and [IrCp*Cl (L1)] (3). A
2
(
(trimethylsilyl)ethynyl)benzo[d]oxazole and 2-((trimethylsilyl)-
suspension of L1H·OTf (107 mg, 0.30 mmol), Me NCl (33 mg, 0.30
4
44
11d
11d
ethynyl)thiazole4, compound L3H·BF ; Ir complexes 6 and 7,
mmol), and Ag O (140 mg, 0.6 mmol) in MeCN (8 mL) was stirred
4
2
5
46
45
47
48
49
substrates S27, S31, S32, S42, S43, S44, and (S)-1-
protected from light for 16 h. The suspension was then filtered through
K
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Celite, and the volatiles were removed under reduced pressure. The
(C(CH ) ), 29.0 (C(CH ) ), 20.3 (CH CH ), 13.8 (CH CH ) HRMS
3
2
3
2
2
3
2
3
+
residue was suspended in CH Cl (8 mL), and [Ir(Cp*)Cl ] (90 mg,
(ESI+): m/z found 548.2008 [M + H] (calcd for C H ClIrN O,
2
2
2
2
19 34 3
+
0
.11 mmol) was added. The reaction mixture was stirred for 5 h
548.2014); found 512.2243 [M − Cl] (calcd for C H IrN O,
1
9
33
3
protected from light and then filtered over Celite, and all volatiles were
removed under reduced pressure to yield the crude products.
Purification by column chromatography (neutral Al O ; CH Cl /
512.2253). Anal. Calcd for C H ClIrN O: C, 41.71; H, 6.08; N, 7.68.
19 33 3
Found: C, 41.71; H, 6.10; N, 7.72. Suitable crystals for X-ray diffraction
were obtained by slow diffusion of pentane into a CH Cl solution of 5.
2
3
2
2
2
2
MeCN gradient, 3/1 to pure MeCN) yielded complex 2 in the first
fraction (70 mg, 32%) and complex 3 in the second fraction (25 mg,
4.9. Typical Procedure for the Transfer Hydrogenation. The
iridium complex (0.025 mmol) was dried under vacuum for 10 min.
i
i
i
1
4%) as yellow and orange solids, respectively.
Under argon, PrOH (2.5 mL), NaO Pr (0.1 M in PrOH, 0.25 mL, 0.025
mmol), and the corresponding internal standard (0.166 mmol;
1
4.6.1. [IrCp*Cl(L1)]OTf (2). H NMR (400 MHz, CD Cl ): δ 7.86−
2
2
1
7
.78 (m, 1H, H ), 7.77−7.70 (m, 1H, H ), 7.66−7.56 (m, 2H, H ),
mesitylene for H NMR or dodecane for GC analysis) were sequentially
Ph
Ph
Ph
4
.58 (s, 3H, NCH ), 4.37 (s, 3H, NCH ), 1.92 (s, 15H, Cp−CH ).
added and the mixture was stirred at reflux for 10 min. The reaction was
3
3
3
1
3
1
C{ H} NMR (101 MHz, CD Cl ): δ 161.1 (C −Ir), 160.4 (O−C
initiated by adding the substrate (0.5 mmol). Aliquots were taken and
2
2
trz
1
N), 152.0 (C ), 137.3 (C ), 136.3 (C −C), 128.1 (C −H), 127.4
were analyzed either by H NMR spectroscopy or by GC using the
Ph
Ph
trz
Ph
1
(
C −H), 121.2 (q, J = 321.27, CF ), 117.6 (C −H), 113.4 (C −
corresponding calibration curves.
Ph
C−F
3
Ph
Ph
19
H), 91.5 (C ), 40.6 (NCH ), 39.3 (NCH ), 10.4 (Cp−CH ). F NMR
4.10. Typical Procedure for Acceptorless Oxidation of
Alcohols. A mixture of substrate alcohol (0.1 mmol), hexamethylben-
zene (0.0165 mmol) as internal standard, and the iridium complex
Cp
3
3
3
(
377 MHz, CD Cl ): δ −78.92 (s). HRMS (ESI+): m/z found 577.1338
2
2
+
[
M − OTf] (calcd for C H ClIrN O, 577.1340). Anal. Calcd for
21
25
4
(
0.005 mmol) in 1,2-dichlorobenzene (1.0 mL) was heated under argon
C H ClF IrN O S: C, 36.39; H, 3.47; N, 7.72. Found: C, 36.43; H,
2
2
25
3
4
4
at 150 °C in a closed vial. Aliquots were taken at specified times, diluted
3
.58; N, 7.65. Suitable crystals for X-ray diffraction were obtained by
1
with CDCl , and analyzed by H NMR spectroscopy.
slow diffusion of pentane into a CH Cl solution of 2.
3
2
2
1
4.11. Crystal Structure Determination. Crystal data for all
compounds were collected on an Oxford Diffraction SuperNova area-
detector diffractometer using mirror optics monochromated Mo Kα
radiation (λ = 0.71073 Å) with Al filtering. Data reduction was
4
.6.2. [IrCp*Cl (L1)] (3). H NMR (400 MHz, CDCl ): δ 7.80−7.74
2
3
(
3
m, 1H, H ), 7.57−7.51 (m, 1H, H ), 7.42−7.34 (m, 2H, H ), 4.46 (s,
Ph
Ph
Ph
13 1
H, NCH ), 4.26 (s, 3H, NCH ), 1.74 (s, 15H, Cp−CH ). C{ H}
3
3
3
NMR (101 MHz, CDCl ): δ 152.0 (C −Ir), 151.9 (O−CN), 149.7
3
trz
5
1
performed using the CrysAlisPro program. The intensities were
corrected for Lorentz and polarization effects, and an absorption
correction based on the multiscan method using SCALE3 ABSPACK in
(
1
(
C ), 140.4 (C ), 135.5 (C −C), 125.1 (C −H), 123.9 (C −H),
Ph
Ph
trz
Ph
Ph
19.8 (C −H), 110.0 (C −H), 87.6 (C ), 40.5 (NCH ), 37.6
Ph
Ph
Cp
3
NCH ), 8.4 (Cp−CH ). HRMS (ESI+): m/z found 577.1341 [M −
3
3
50
+
CrysAlisPro was applied. The structure was solved by direct methods
Cl] (calcd for C H ClIrN O, 577.1341). Anal. Calcd for
2
1
25
4
52
using SHELXT, which revealed the positions of all non-hydrogen
C H Cl IrN O·H O: C, 40.00; H, 4.32; N, 8.89. Found: C, 39.51;
2
1
25
2
4
2
atoms of the title compound. The non-hydrogen atoms were refined
anisotropically. All H atoms were placed in geometrically calculated
positions and refined using a riding model. Refinement of the structure
H, 3.72; N, 8.47. Suitable crystals for X-ray diffraction were obtained by
slow diffusion of pentane into a CH Cl solution of 3.
2
2
4
.7. Synthesis of [IrCp*Cl(L2)]OTf (4). According to the procedure
2
was carried out on F using full-matrix least-squares procedures, which
described for complex 2, from L2H·OTf (100 mg, 0.30 mmol), Me NCl
33 mg, 0.30 mmol), and Ag O (140 mg, 0.6 mmol) in MeCN (8 mL)
4
2
2 2
minimized the function ∑w(Fo − F ) . The weighting scheme was
(
c
2
based on counting statistics and included a factor to downweight the
and [Ir(Cp*)Cl ] (90 mg, 0.11 mmol) in CH Cl (8 mL) complex 4
2
2
2
2
intense reflections. All calculations were performed using the SHELXL-
was obtained after purification by column chromatography as a yellow
5
3
1
3
2014/7 program. Further crystallographic details are compiled in the
Supporting Information. Crystallographic data for the structures of all
compounds reported in this paper have been deposited with the
Cambridge Crystallographic Data Centre (CCDC) as supplementary
publication numbers 1560417 (L1H·OTf), 1560418 (1), 1560415 (2),
1560420 (3), 1560419 (4), and 1560416 (5).
solid (70 mg, 37%). H NMR (400 MHz, CDCl ): δ 7.98 (d, J
= 3.5
3
3
H−H
3
Hz, 1H, Hthia), 7.75 (d, J
= 3.5 Hz, 1H, Hthia), 4.50 (s, 3H, NCH ),
H−H
13
1
4
.35 (s, 3H, NCH ), 1.86 (s, 15H, Cp−CH ). C{ H} NMR (101
3
3
MHz, CDCl ): δ 158.5 (C −Ir), 158.3 (S−CN), 145.7 (C −C),
3
trz
trz
1
1
40.7 (C −H), 122.9 (C −H), 120.7 (q, J = 320.06, CF ), 90.8
C−F
19
thia
thia
3
(
C ), 39.9 (NCH ), 38.7 (NCH ), 9.7 (Cp−CH . F NMR (377
Cp
3
3
3
MHz, CDCl ): δ −78.53 (s). HRMS (ESI+): m/z found 543.0961 [M −
3
+
OTf] (calcd for C H ClIrN S, 543.0950). Anal. Calcd for
ASSOCIATED CONTENT
Supporting Information
1
7
23
4
■
C H ClF IrN O S : C, 31.23; H, 3.35; N, 8.09; S, 9.26. Found: C,
1
8
23
3
4
3
2
*
S
3
1.41; H, 3.00; N, 7.77; S, 9.77. Suitable crystals for X-ray diffraction
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.7b01707.
were obtained by slow diffusion of pentane into a CH Cl solution of 4.
2
2
4
.8. Synthesis of [IrCl(L3)(cod)] (5). Compound L3H·BF (150
4
mg, 0.50 mmol), Ag O (232 mg, 1.0 mmol), and NMe Cl (55 mg, 0.50
2
4
mmol) were suspended in MeCN (20 mL) and stirred for 21 h
protected from light. The reaction mixture was filtered over Celite, and
the volatiles were removed under reduced pressure. The residue was
dried in vacuo and then suspended in dry degassed CH Cl (15 mL).
NMR spectra of new triazolium salts L1H·OTf and L2H·
OTf and their precursors, characterization details of new
triazolium salts L1H·OTf and L2H·OTf and their
precursors, NMR spectra of Ir complexes 1−5, conversion
vs time plots of the TH of S1, effect of the catalyst
precursor, base loading, and catalysts loading, Hammett
plot showing the effect of substituents on the transfer
hydrogenation using catalytic system 5, proposed catalytic
cycle for the TH, conversion vs time plot of the
dehydrogenation of S45, effect of the catalyst precursors,
Hammett plot showing the effect of substituents on the
dehydrogenation using catalytic system 2, characterization
details and methods yielding determination of TH and
2
2
[
Ir(cod)Cl] (168 mg, 0.25 mmol) was added and the suspension stirred
2
for 2 h protected from light under a N atmosphere. The reaction
2
mixture was filtered through Celite, and the volatiles were removed
under reduced pressure. Purification by column chromatography (SiO2;
CH Cl /acetone gradient 20/1 to 10/1) yielded the title product as a
2
2
yellow oil which solidified upon drying thoroughly in vacuo and storage
1
at −20 °C (152 mg, 55%). H NMR (400 MHz, CDCl ): δ 4.90−4.71
3
(
m, 2H, NCH ), 4.60−4.50 (m, 1H, CH , 4.49−4.39 (m, 1H, CH ),
2
cod
cod
4
2
1
.14 (s, 3H, NCH ), 3.11 (s, 3H, OCH ), 2.83−2.69 (m, 2H, CHcod),
3
3
.29−2.10 (m, 5H, CH
+ CH CH ), 2.04 (s, 3H, C(CH ) ), 2.02−
2
cod
2
2
3 2
.91 (m, 1H, CH CH ), 1.86 (s, 3H, C(CH ) ), 1.75−1.60 (m, 2H,
1
2
2
3 2
3
dehydrogenation products, GCs, H NMR, and mass
CH CH ), 1.58−1.40 (m, 4H, CH ), 1.03 (t, J = 7.4 Hz, 3H,
2
3
2 cod
HH
13
1
spectra of transfer hydrogenation and transfer deuterina-
tion experiments of substrates S35 and S36, crystal data
and structure refinement for triazolium salt L1H·OTf and
iridium complexes 1−5, and an ORTEP representation
CH CH ) C{ H}(101 MHz, CDCl ): δ 169.0 (C −Ir), 146.7 (C −
2
3
3
trz
trz
C), 74.6 (CMe ), 81.4 (CH ), 79.8 (CH ), 55.7 (NCH ), 51.4
2
cod
cod
2
(
3
CHcod), 51.2 (OCH ), 51.1 (CH ), 38.8 (NCH ), 33.7 (CH2 cod),
3.4 (CH2 cod), 32.3 (NCH CH ), 30.1 (CH2 cod), 29.6 (CH2 cod), 29.3
2 2
3 cod 3
L
DOI: 10.1021/acs.inorgchem.7b01707
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
(
50% thermal ellipsoids, H atoms omitted for clarity) of
1578−1590. (e) Pam
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Phosphite/Phosphoroamidite-Containing Ligands for Asymmetric
Hydrogenation and C-X Bond-Forming Reactions: Ligand Libraries
with Exceptionally Wide Substrate Scope. Chem. Rec 2016, 16, 2460−
2481.
the triazolium-benzoxazole salt L1H·OTf (PDF)
Accession Codes
CCDC 1560415−1560420 contain the supplementary crystallo-
graphic data for this paper. These data can be obtained free of
charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
data_request@ccdc.cam.ac.uk, or by contacting The Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge CB2
(4) (a) Bourissou, D.; Guerret, O.; Gabbai, F. P.; Bertrand, G. Stable
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AUTHOR INFORMATION
Corresponding Authors
E-mail for O.P.: oscar.pamies@urv.cat.
M.A.: tel, (+41)316314644; e-mail, martin.albrecht@dcb.
unibe.ch.
M.D.: fax, (+34)977559563; tel, (+34)977558780; e-mail,
montserrat.dieguez@urv.cat .
■
1
́
*
Carbenes in Late Transition Metal Catalysis. Chem. Rev. 2009, 109,
3612−3676. (f) Hahn, F. E.; Jahnke, M. C. Heterocyclic Carbenes:
Synthesis and Coordination Chemistry. Angew. Chem., Int. Ed. 2008, 47,
3122−3172. (g) Peris, E. Smart N-Heterocyclic Carbene Ligands in
Catalysis. Chem. Rev. 2017, DOI: 10.1021/acs.chemrev.6b00695.
*
*
(h) Glorious, F. N-Heterocyclic Carbenes in Transition Metal Catalysis
ORCID
In Topics in Organometallic Chemistry; Springer: Berlin, 2007. (i) Cazin,
C. S. J. N-Heterocyclic Carbenes in Transition Metal Catalysis and
Martin Albrecht: 0000-0001-7403-2329
Montserrat Dieg
́
uez: 0000-0002-8450-0656
Organocatalysis; Springer, Berlin, 2011. (j) Díez-Gonzalez, S. N-
́
Heterocyclic Carbenes: from Laboratory Curiosities to Efficient Synthetic
Tools; RSC Publishing: Cambridge, U.K., 2011. (k) Huynh, H. V. The
Organometallic Chemistry of N-heterocyclic Carbenes; Wiley: Chichester,
U.K., 2017.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
(
5) (a) Hopkinson, M. N.; Richter, C.; Schedler, M.; Glorius, F. An
overview of N-heterocyclic carbenes. Nature 2014, 510, 485−496.
b) Crudden, C. M.; Allen, D. P. Stable Planar Six-π-Electron Six-
Financial support from the Spanish Ministry of Economy and
Competitiveness (CTQ2016-74878-P) and European Regional
Development Fund (AEI/FEDER, UE), the Catalan Govern-
ment (2014SGR670), the European Research Council (CoG
15653), the Swiss National Science Foundation (R’equip
06021_128724 and 206021_170755), the Irish Research
(
Membered N-Heterocyclic Carbenes with Tunable Electronic Proper-
ties. Coord. Chem. Rev. 2004, 248, 2247−2273. (c) Crabtree, R. H.
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Heterocyclic Carbenes. Coord. Chem. Rev. 2007, 251, 595−896.
6
2
(6) For reviews, see: (a) Donnelly, K. F.; Petronilho, A.; Albrecht, M.
Council (Ph.D fellowship to R.P.), and the ICREA Foundation
ICREA Academia award to M.D.) is gratefully acknowledged.
Application of 1,2,3-Triazolylidenes as Versatile NHC-type Ligands:
Synthesis, Properties, and Application in Catalysis and Beyond. Chem.
Commun. 2013, 49, 1145−1159. (b) Crabtree, R. H. Abnormal,
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(
We particularly thank COST Action CM1305 for enabling a
research visit of Z.M. We thank the group of Chemical
Crystallography of the University of Bern for solving the X-ray
structures of compounds L1H·OTf, 1, 2, and 5 and the X-ray
́
̀ ́
Diffraction Unit of the Institut Catala d’Investigacio Quimica
(
ICIQ) for solving the X-ray structures of compounds 3 and 4.
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Versatile Abnormal Carbene Ligands for Late Transition Metals. J. Am.
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