Triazolylidene-Iridium Complexes with a Pendant Pyridyl Group for Cooperative Metal–Ligand Induced Catalytic Dehydrogenation of Amines

Article abstract of DOI:10.1002/chem.201700676

Two iridium(III) complexes containing a C,N-bidentate pyridyl-triazolylidene ligand were prepared that are structurally very similar but differ in their pendant substituent. Whereas complex 1 contains a non-coordinating pyridyl unit, complex 2 has a phenyl group on the triazolylidene substituent. The presence of the basic pyridyl unit has distinct effects on the catalytic activity of the complex in the oxidative dehydrogenation of benzylic amines, inducing generally higher rates, higher selectivity towards formation of imines versus secondary amines, and notable quantities of tertiary amines when compared to the phenyl-functionalized analogue. The role of the pyridyl functionality has been elucidated from a set of stoichiometric experiments, which demonstrate hydrogen bonding between the pendant pyridyl unit and the amine protons of the substrate. Such N_pyr???H?N interactions are demonstrated by X-ray diffraction analysis, ¹H NMR, and IR spectroscopy, and suggest a pathway of substrate bond-activation that involves concerted substrate binding through the Lewis acidic iridium center and the Lewis basic pyridyl site appended to the triazolylidene ligand, in agreement with ligand–metal cooperative substrate activation.

Full text of DOI:10.1002/chem.201700676

A Journal of
Accepted Article
Title: Triazolylidene iridum complexes with a pending pyridyl group for
cooperative metal-ligand induced catalytic dehydrogenation of
amines
Authors: Marta Valencia, Ana Pereira, Helge Müller-Bunz, Tomas R
Belderrain, Pedro J Perez, and Martin Albrecht
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Triazolylidene iridum complexes with a pending pyridyl group for cooperative metal-
ligand induced catalytic dehydrogenation of amines
a,b
b,c
b
c
c
Marta Valencia, Ana Pereira, Helge Müller-Bunz, Tomás R. Belderraín, Pedro J. Pérez,
a,b
Martin Albrecht*
a
Department of Chemistry and Biochemistry, University of Bern, Freiestrasse 3, CH-3012
Bern, Switzerland. E-mail: martin.albrecht@dcb.unibe.ch
b
School of Chemistry, University College Dublin, Belfield, Dublin 4, Ireland
c
Laboratorio de Catálisis Homogénea, Unidad Asociada al CSIC, CIQSO- Centro de
Investigación en Química Sostenible and Departamento de Química. Universidad de Huelva.
1007 Huelva (Spain)
2
Abstract
Two iridium(III) complexes containing a C,N-bidentate pyridyl-triazolylidene ligand were
prepared that are structurally very similar but differ in their pending substituent. While complex
1
contains a non-coordinating pyridyl unit, complex 2 has a phenyl group as triazolylidene
substituent. The presence of the basic pyridyl unit has distinct effects on the catalytic activity
of the complex in the oxidative dehydrogenation of benzylic amines, inducing generally higher
rates, higher selectivity towards formation of imines vs secondary amines, and notable
quantities of tertiary amines when compared to the phenyl-functionalized analogue. The role of
the pyridyl functionality has been elucidated in a set of stoichiometric experiments, which
demonstrate hydrogen bonding between the pending pyridyl unit and the amine protons of the
1
substrate. Such Npyr…H–N interactions are demonstrated by X-ray diffraction analysis, H
NMR and IR spectroscopy and suggest a pathway of substrate bond activation that involves
concerted substrate binding through the Lewis acidic iridium center and the Lewis basic pyridyl
site appended to the triazolylidene ligand, in agreement with ligand-metal cooperative substrate
activation.
ꢀ
1
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Introduction
Secondary amines and imines are valuable reactive intermediates with wide use for the
synthesis of pharmaceutical and biological compounds, as well as industrially employed
[
1]
chemicals. Typically, the synthesis of secondary amines is accomplished by condensation of
a primary amine with a suitable carbonyl compound, followed by imine reduction. Although
[
2,3]
amine alkylation with alcohols has received much attention in recent years,
amine
In most cases, self-condensation of amines is
carried out in the presence of heterogeneous catalytic metallic systems, such as gold or
[
4,5,6,7]
dehydrogenation is much less developed.
[
8]
2 2
palladium, and in the presence of an oxidant, e.g. CeO or O . Only a few examples are known
in the literature where homogeneous noble metal catalysts have been used in amine cross-
coupling reaction, mainly with ruthenium and iridium. These reactions normally require high
[9]
temperature or the presence of an additive such as a base.
Key steps in amine dehydrogenation and coupling are the activation of the substrate N–H bond
and the subsequent b hydrogen elimination to form the dehydrogenated imine intermediate.
[
10]
While the latter is a classic reaction trajectory brought about by transition metals, the N–H
bond activation is typically imparted by Lewis bases. Hence, a plausible catalyst design entails
a metal center with a labile ligand and a Lewis basic site in close mutual proximity, yet sterically
constrained to avoid formation of a stable Lewis acid-base pair. This concept of frustrated
Lewis pairs (FLPs) has been widely exploited with main group metals for the activation of
[
11]
dihydrogen and other substrates,
and is conceptually related with metal-ligand
[
12]
bifunctionality.
Here we have designed a triazolylidene ligand that upon metal coordination forms a complex
comprised of a chelate-stabilized late transition metal for dehydrogenation, and a pending
pyridyl unit for N–H bond activation through hydrogen bonding. The underlying concept of
this design is the strain imposed by the central five-membered heterocycle, which arranges the
two pyridyl units in a wide angle that allows only one pyridyl unit to bind to the metal center.
While in a monodentate triazolylidene complex, the metal carbene bond is dissecting the
carbene N–C–C angle essentially evenly (θ = 0°, A in Fig. 1) metal chelation of pyridyl-
[
13]
triazolylidene ligands induces a large yaw angle (θ around 18º), which places the metal center
far away from the second pyridyl unit (B in Fig 1). The yaw angle of this ligand is bigger than
that observed in chelating bisimidazolylidene complexes (θ < 14º for palladium
[
14]
[15]
coordination), or in a chelating bispyridyl-imidazolylidene complex (θ = 13.5º). The large
[
13,16]
yaw angle of the pyridyl-triazolylidene system B
is therefore expected to prevent
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2
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10.1002/chem.201700676
[
17]
coordination of the second pyridyl site, and to provide a basic entity for N–H activation in
close proximity to the reactive metal center, while the metal coordinated to the triazolylidene
[18]
can complete the dehydrogenation process via hydrogen elimination. Triazolylidenes,
a
[
19]
subclass of N-heterocyclic carbenes (NHCs),
have shown excellent catalytic activity in
[
20]
[9e,16d,f,21]
oxidative transformations such as water oxidation, alcohol dehydrogenation,
and
[22]
CH bond activation.
θ ≈ 0
θ ≠ 0
M
M
FG
N
R
R
N
N
N N
N N
A
B
ꢀ
Figure 1. Schematic illustration of the position of the metal center in monodentate triazolylidene complexes (A)
and in C,N-bidentate chelating systems (B), and the difference in yaw angle q.
Results and Discussion
1
. Synthesis of triazolium salts and triazolylidene iridium(III) complexes.
[
16,20]
Based on our previous experience in chelating triazolylidene iridium complexes
and in
order to design this new type of functionalized triazolylidene ligand we have chosen as starting
point the bis(pyridyl)triazolium salt L1, to study the capacity of a free pyridyl unit to cooperate
actively in a catalytic process. The analogous ligand precursor L2 lacks the second, potentially
non-coordinating pyridyl functionality (Scheme 1) and will serve as model to evaluate the
relevance of the pending pyridyl functionality. These two ligand precursors were prepared by
[23]
methylation of the corresponding pyridyl-triazole compound with 1.2 equiv. of MeOTf.
Precursor L2 was obtained in excellent yield (96%), while methylation of the
bis(pyridyl)triazole was less selective and therefore, L1 with exclusively methylated triazolium
salt and non-methylated pyridine groups was isolated in a moderate 28% yield after purification.
1
Both salts displayed a characteristic low-field CHtrz proton singlet in the H NMR spectra (δ
H
9
3
.68 and 9.04 for L1 and L2, respectively, CDCl solution).
ꢀ
3
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Scheme 1. Synthesis of iridium complexes 1 and 2 and ORTEP representation of both complexes (50% probability
–
level; hydrogen atoms, solvent molecules, non-coordinated OTf anions, and the second independent molecule in
the unit cell of 1 omitted for clarity).
The synthesis of the triazolylidene iridium(III) complexes 1 and 2 was carried out via Ag
mediated proton abstraction and in situ transmetallation with [Ir(Cp*)Cl in a one pot
procedure (Scheme 1). Complexes 1 and 2 are air stable yellow solids, which were conveniently
2
O-
2 2
]
[
24]
purified by column chromatography and isolated in 96% and 95% yield, respectively. Both
1
13
1
were characterized by elemental analysis, H and C{ H} NMR spectroscopy and X-ray
diffraction analysis. We note that the synthesis of complex 1 was highly chemoselective and
chelation only involved the electron-richer pyridyl group linked to the the triazolylidene
[
16b]
nitrogen
as indicated by spectroscopic analysis of a sample from the crude reaction mixture.
Metalation of the triazole heterocycle was indicated by the disappearance of the low-field CHtrz
1
proton resonance of the ligand precursors in the H NMR spectra. Additionally, the spectrum
of complex 1 showed two sets of four aromatic proton resonances, indicating the presence of
two inequivalent pyridyl units and hence a single isomer. The set of four aromatic proton
resonances of the chelating pyridyl group in complex 1 appears at almost identical frequency
as those of complex 2. Unambiguous assignment of the pyridyl protons in complex 1 was
1
1
accomplished by H– H correlation spectroscopy, and by establishing Nuclear Overhauser
3
Effects between the pyridyl C –H proton of the non-metalated pyridyl unit (δ
H
8.06) and the
1
3
triazolylidene N–CH
3
group (δ
H
4.49). The C NMR spectra showed the carbene carbon
resonance at 153.3 and 151.8 ppm for complex 1 and 2 respectively. The carbene chemical
shifts together with the aromatic carbon resonances of the Cp* ligand at 91.9 and 91.7 ppm for
ꢀ
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complexes 1 and 2, respectively, suggest that the Lewis acid character of these iridium metal
centers is similar.
Complexes 1 and 2 were crystallized by slow diffusion of Et
2
O into a solution of the complex
in CH Cl . Both complexes adopt a three-legged piano-stool-type molecular geometry,
2
2
comprised of a N,C-bidentate chelating pyridyl-triazolylidene ligand, while the second pyridyl
moiety in 1 is non-coordinating. Selective bonding of the more electron-rich pyridyl ring
attached to the triazolylidene nitrogen as surmised from solution analysis is therefore confirmed
in the solid state. Such behavior was previously observed in cyclometalation reactions when
two phenyl substituents were present as triazolylidene wingtip groups, and C–H bond activation
[
16b]
exclusively involved the more electron-rich N-bound phenyl group.
The metal environment
is very similar in complexes 1 and 2 and bond lengths and angles are essentially identical and
[
19e]
within the expected range.
The yaw angle in complexes 1 and 2 is also identical (θ = 15.4º).
The pendant pyridyl group in complex 1 features the nitrogen in pseudo anti orientation to the
iridium center, presumably due to electrostatic repulsion of the nitrogen lone pair and the
electron density at the iridium-bound chloride.
Table 1. Selected bond lengths (Å) and angles (°) for complexes 1 and 2.
1
2
Ir–Ctrz
2.024(2)
2.127(2)
2.023(2)
2.129(2)
Ir–Npyridyl
Ir–Cl
2.4079(7) 2.3904(6)
C
trz–Ir–Npyridyl
77.36(8)
88.14(6)
85.27(5)
113.7(1)
144.4(2)
15.4
77.28(8)
85.88(6)
87.62(5)
113.8(6)
118.4(1)
15.4
Cl–Ir–Ctrz
Cl–Ir–Npyridyl
N
trz−Ctrz−M
C
trz−Ctrz−M
θ
2
. Catalytic Experiments with protic substrates.
The activity of related triazolylidene iridium complexes as catalyst precursors for the oxidation
[
20,21]
of oxygen-containing protic substrates such as H
2
O or alcohols
prompted us to investigate
the ability of complexes 1 and 2 to catalyze the related dehydrogenation of amines. In particular
we aimed at probing the bifunctional character of the triazolylidene iridium system 1 due to the
presence of a basic pyridyl group as compared to the inert phenyl substituent in complex 2.
ꢀ
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The dehydrogenation of benzylamine and related substrates was evaluated with complexes 1
and 2 under conditions similar to those reported for the oxidation of alcohols with related
monofunctional triazolylidene complexes, i.e. catalyst loading of 5 mol% in 1,2-
[
16,21,25]
dichlorobenzene at 150 °C and in the absence of an oxidant or a base.
Under these
conditions, both complexes 1 and 2 are active catalysts and converted primary amines into a
mixture of secondary imines, secondary amines, and tertiary amines in different proportions.
The results of the catalytic experiments are compiled in Table 2.
a
Table 2. Oxidative coupling of amines
b
c
c
c
Entry Cat
Amine
Conversion (%)
Imine
2º Amine
3º Amine
y
2
3
4
1
2
1
2
97
93
57
40
65
52
28
21
31
47
4
1
d
43
29
19
d
60
5
6
7
8
9
1
2
1
2
1
2
1
2
95
97
95
96
93
76
95
65
50
55
63
53
70
55
86
68
28
21
26
35
29
45
13
32
22
4
11
12
0
1
0
1
2
0
1
1
1
0
1
1
3
4
1
2
80
80
42
44
57
56
0
0
[
a] Conditions: amine (0.2 mmol), [Ir] (5 mol% of catalyst), 1,2-dichlorobenzene (1,2-DCB; 2 mL), 150 °C. [b]
1
Determined by H NMR spectroscopic analysis with hexamethylbenzene as internal standard. [c] Ratio of products
given in percentage. [d] Secondary amine as substrate.
Both complexes 1 and 2 produce a catalytically active species that reaches excellent
conversions after 24 h. Analysis of the products showed a mixture comprised of the benzylidene
imine, dibenzylamine, and tribenzylamine (entries 1, 2). The latter was observed only in traces
in catalytic runs with 2 (ca. 1%), and was a bit more pronounced when the pyridyl-containing
complex 1 was used as precursor. The ratio of imine vs secondary amine was about 2:1 when
starting with complex 1, while it was close to 1:1 with the phenyl-substituted triazolylidene
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iridium complex 2. The enhanced selectivity of complex 1 towards imine formation suggests
that H is not captured in the metal coordination sphere, therefore hampering hydrogenation of
2
the imine to the secondary amine. In contrast, complex 2 is more efficient in storing hydrogen
and in re-hydrogenating the substrate in what is often referred to as a “borrowing hydrogen”
[
7,26]
process.
The low selectivity observed for complexes 1 and 2 contrasts previous work with
[
9b,e]
related triazolylidene ruthenium complexes and iridium imidazolylidene complexes,
which
afforded imine products selectively. Accordingly, both complexes 1 and 2 are better re-
hydrogenation catalysts than the previously investigated systems.
The formation of tertiary amines indicates that the dehydrogenated benzylimine is activated,
since secondary amines are not usually forming tertiary amines easily. However, tertiary amines
[
9b]
were reported to be formed from N-methylaniline and benzylalcohol.
Hence, it is
conceivable that the tertiary amines observed with complex 1 are formed via the formation of
benzaldehyde from benzylimine in the presence of fortuitous water. In order to further
investigate the reactivity of the secondary amine, dibenzylamine was used as a substrate for
dehydrogenation (entries 3,4). With complex 1 under standard reaction conditions, indeed
nearly 30% tribenzyl amine was obtained, and about equal amounts of dehydrogenated imine.
The conversion was lower with complex 2, suggesting that the pyridyl group had a moderately
rate-enhancing effect.
The trends deduced from benzylamine conversion were also observed with different substrates
(Table 2, entries 5–14). Generally, the pyridyl group in complex 1 induces a slightly higher
catalytic dehydrogenation activity than the phenyl group in complex 2. With activating
electron-donating methoxy groups as aromatic substituents on the substrate, the higher activity
resulted in substantially higher ratios of tertiary amine products (entries 5,6). The same trend,
albeit weaker, was also observed with less donating methyl substituents (entries 7,8). When the
substrate contains electron-withdrawing chloro-substituents, only complex 1 induced full
conversion while the catalyst precursor lacking the pyridyl unit gave only about 70%
conversion within 24 h (entries 9–12). In addition to the enhanced activity, in all cases the imine
to secondary amine ratio is higher with complex 1 than with complex 2.
To further probe the relevance of the hydrogen borrowing process, catalytic studies were carried
out under inert conditions in a Young NMR tube in the presence of complex 1, and under strict
control of the atmosphere (Table 3). When focusing on modifications after full substrate
conversion, we note that hydrogen transfer continues. Thus, while no primary amine was
observed anymore in the reaction mixture after 24 h (entry 1), prolonging the reaction for a
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further 24 h continued to induce dehydrogenation of the secondary amine and increased the
ratio of both the imine and the tertiary amine (entry 2). These secondary dehydrogenation
processes are not dependent on the presence of oxygen from the air, and similar transformations
were also observed when the reaction mixture was degassed and kept under an argon
atmosphere for the second 24 h (entries 3–5). However, when the atmosphere after 24 h is
2
changed to H , rapid hydrogenation of the imine took place (entries 6,7). Therefore, these
experiments identify complex 1 as a highly active catalyst precursor for imine hydrogenation
under a reducing atmosphere. Interestingly, the slow formation of the tertiary amine is not
2
suppressed by a H atmosphere, indicating that dehydrogenation of the secondary amine is still
occurring, though the equilibrium is shifted from the imine to the amine side.
a
Table 3. Oxidative coupling of benzylamine under different conditions
c
b
b
b
Entry conversion (%)
gas
Air
Air
Air
Ar
time Ph–CH=NR
HNR
54
2
NR
0
3
comment
1
2
3
4
5
6
7
8
94
98
95
95
96
94
96
62
24 h
48h
46
63
61
60
59
7
0.5 mL (NMR tube)
32
5
0.5 mL (NMR tube)
24 h
26 h
48 h
26 h
48 h
24 h
38
1
2 mL (closed vial)
37
3
c)
Ar
25
17
4
c)
H
H
2
2
89
c)
1
84
15
0
c)
Air
54
46
1 mL (vial with no headspace)
[
a] Conditions: benzylamine (0.1 mM in 1,2-dichlorobenzene), 1 (5 mol%), 150 °C, conversion of benzylamine,
1
determined by H NMR spectroscopic analysis with hexamethylbenzene as internal standard; b) relative product
ratio in %; c) 0.5 mL product mixture from entry 3 was transferred to a pressure NMR tube and charged with the
indicated gas after 0 h and after 2 h times reported are cumulated (e.g entry 4 is 24 h as in entry 3 plus 2 h under
Ar).
The formation of amine and imine products observed in the absence of a hydrogen atmosphere
was consequently attributed to a partial degassing and loss of H
Accordingly, the H formed in the first step of the reaction during dehydrogenation of the
substrate is not stored in the metal coordination sphere but more likely as H in solution.
by NMR spectroscopy were prevented by the small quantities of H
2
from the reaction mixture.
2
2
Attempts to detect H
2
2
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formed and by the high reaction temperature which was used for the reaction. In agreement
with this model, a catalytic run in a closed vial without any headspace gave only incomplete
conversion after 24 h (62% vs 95% under standard conditions) and shifted the product
selectivity towards the secondary amine (imine/amine ratio from 2:1 to 1:1 entry 8). The
reduced catalytic turnover suggests that dehydrogenation of the substrate amine is disfavored,
in agreement with a hydrogen borrowing mechanism (Scheme 2). Accordingly, the release of
2
H to the atmosphere enhances the rate of dehydrogenation as the initial step. Moreover, the
selectivity change is commensurate with complex 1 being a catalyst for the hydrogenation of
imines when hydrogen is forced to remain in the reaction mixture (last step), as induced by the
2
increase of H in solution when the headspace is minimized.
ꢀ
Scheme 2. Amine alkylation by proton borrowing reaction.
Hydrogen borrowing is a widely accepted process in the homo-coupling of amines to imines
and subsequent reduction of the imine intermediates to secondary amines, leading to an overall
[27]
[9h,28]
alkylation of amines. Generally, a metal hydride has been inferred as active species,
which removes an acidic proton from the substrate either via a metal-bound hydride to form H
or via another basic ligand site to store a H equivalent within the catalytic structure. According
to the selectivity patterns observed with complex 1, storage of a H equivalent is not supported
in this case. Presumably, the basic pyridyl site in complex 1 assists the substrate deprotonation,
which facilitates then H release and hence a larger ratio of imine vs amine formation.
2
,
2
2
2
Alternatively, we hypothesized that the pendant pyridyl group may be involved in substrate
recognition and binding rather than proton abstraction. In order to explore such bonding in more
detail, several stoichiometric experiments were carried out.
3
. Stoichiometric Experiments
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Substrate bonding was investigated by the reaction of complex 1 with one equivalent of amine,
induced by chloride abstraction with stoichiometric amounts of AgOTf. To avoid
dehydrogenation, these reactions were performed at room temperature, initially using aniline
as a model substrate which has no b hydrogens and cannot dehydrogenate. Subsequently,
benzylamine was used as a real substrate. These reactions afforded complexes 3 and 4 in high
yields (87%) as air-stable, pale yellow solids (Scheme 3). For comparative purposes, complexes
5
and 6 were prepared. These complexes are analogues of 3 and 4, respectively, yet lacking the
pyridyl function and therefore allow the assessment the relevance of substrate recognition and
potential hydrogen bonding. The relevance of complex 4 as a catalyst precursor was
demonstrated by exposing this complex to benzylamine under catalytic conditions as described
in Table 2. Similar yields and selectives as complex 1 were obtained (Table S6), yet initial
activities were lower (Fig. S1).
When complexes 3 and 4 were treated with a second equivalent of AgOTf, the heterobimetallic
Ir/Ag complexes 7 and 8 were formed, in which the pendant pyridyl is coordinated to the
silver(I) center. Hence, complexes 3–6 provide insight into the role of the hydrogen bond
acceptor (pyridyl vs phenyl pending group), while complexes 7 and 8 contain a modified Lewis
acid coordinated to the pyridyl group and hence constitute a model with a modified ‘hydrogen
bond’ donor. Comparison of the different bonding situations was performed in the solid state
by X-ray diffraction, and in solution by NMR and IR spectroscopy.
Scheme 3
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Figure 2. ORTEP representation of complexes 3’, 4’ and 7 (50% probability level, most hydrogen atoms, solvent
–
–
4
molecules, and non-coordinated OTf and BPh anions omitted for clarity).
Crystals suitable for X-ray diffraction analysis were obtained for complexes 3’, 4’ and 7. For 3
–
–
and 4, counterion exchange from OTf to BPh
4
was essential to grow crystals of sufficiently
high quality. The molecular structures of all three complexes reveal the typical three-legged
[29]
piano stool geometry around iridium (Figure 2). In addition, complex 7 features a silver(I)
center in a distorted tetrahedral coordination geometry defined by two monodentate OTf
ligands, a coordinated water molecule and the pyridyl group that was non-coordinating in
complex 1.
Bond lengths and angles around iridium are unsurprising and very similar to the metric data of
complex 1 (Table 4). In all three complexes, the pyridyl group that is not bound to iridium is
interacting with Lewis acids. In complex 3’, the Naniline–H…Npyr distance is relatively long at
[
30]
3
.08 Å, suggesting only a weak hydrogen bond. The stabilizing nature of this interaction is
supported by the orientation of the heterocyclic pyridyl nitrogen, which is pointing towards the
amine proton, whereas an opposite orientation was observed in complex 1 (vide supra). In
complex 4’ with a coordinated benzylamine ligand, the NBn–H…Npyr distance is much shorter,
2
.27(3) Å, indicative of a relatively strong intramolecular hydrogen bond. A strong interaction
between the acidic amine proton and the basic pyridyl unit may also rationalize the slightly
shorter Ir−Ctrz bond in complex 4’ compared to complexes 3’ and 7, 2.016(2) vs 2.043(5) and
2
.037(2), respectively. No such NH…N interaction was observed in complex 7, and instead,
the pyridyl nitrogen is bound to the silver ion with a distance of 2.252(2) Å, coincidentially
similar to the H…N distance in complex 4’.
Table 4. Selected bond lengths (Å) and angles (in degrees) for complexes 3’, 4’ and 7.
3
’ (X = Haniline) 4’ (X = Hamine) 7 (X = Ag)
Ir–Ctrz
2.043(5)
2.130(5)
2.043(5)
2.016(2)
2.138(1)
2.147(2)
2.037(2)
2.125(2)
2.191(2)
Ir–Npyr
Ir–Namine
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C
trz–Ir–Npyr
76.6(2)
87.1(2)
88.2(2)
3.082(4)
113.8(3)
142.9(4)
76.28(7)
86.87(7)
94.62(6)
2.27(3)
77.75(9)
91.30(9)
83.15(8)
2.252(2)
113.2(2)
145.5(2)
N
N
N
N
amine–Ir–Ctrz
amine–Ir–Npyr
pyr…X
trz−Ctrz−M
115.3(1)
142.0(2)
C
trz−Ctrz−M
2 2
Hydrogen bonding was also observed in solution by NMR and IR spectroscopy (CH Cl ). For
1
example in the H NMR spectrum of complex 3, the pyridyl C6-H resonance appears at d
H
=
9
.06 (atom labeling scheme as in Fig. 3), at significantly lower field compared to the same
H
resonance in complex 1 (d = 8.73). Deshielding upon aniline coordination indicates a lower
electron density in the heterocycle, which is expected upon Lewis acid bonding. Principally, a
downfield shift may also be entailed by indirect coulombic effects, arising from chloride
abstraction and the associated increase of the formal charge at the iridium center from
monocationic in 1 to dicationic in 3. However, comparison with the phenyl substituted systems
(complex 2 vs. 5) strongly supports the notion that such electrostatic considerations are not a
dominant factor, as the phenyl proton resonances shift only marginally (<0.1 ppm) upon amine
coordination. (cf. Dd >0.3 ppm with the pyridyl homologue). The chemical shift of the pyridyl
–
C6–H proton does not vary upon exchange of the counterion from weakly coordinating OTf
–
to less coordinating BPh
bonding.
4
, suggesting that anion effects are not interfering with hydrogen
Figure 3. Carbon and hydrogen numbering scheme for the non-coordinated pyridyl unit in complexes 3 and 4 ([Ir]
2
+
=
[Ir(Cp*] ).
Benzylamine coordination and formation of complex 4 induced a similar shift as observed upon
aniline bonding, with the diagnostic resonance of the pyridyl C6–H in complex 4 resonating at
d
H
= 9.01. The downfield shift of this resonance is slightly more pronounced when the Lewis
+
acid is exchanged from a benzylamine proton to a Ag ion, with d
H
= 9.09 and 9.08 in complexes
7
and 8, respectively. Hence, the deshielding of this proton supports bonding of a Lewis acidic
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Chemistry - A European Journal
10.1002/chem.201700676
site in all complexes 3, 4, 7, and 8 and corroborates intramolecular hydrogen bonding in
complexes 3 and 4 also in solution.
The resonances due to the NH
and appeared as broad signals around d
observed at d ~ 5.4 for the benzylamine protons in complexes 6 and 8, in which the
benzylamine NH
2
unit were not diagnostic in the aniline complexes 3, 5, and 7
H
~ 7.5 (Figure S3). A similar resonance pattern was
H
2
unit is not hydrogen bonded either due to the lack of hydrogen bond acceptor
+
(
in 6) or due to competition of a different Lewis acid (Ag in complex 8; Figure 4). Complex 4
in contrast features two distinct resonances at d
H 2
= 5.5 and 5.3 for the NH protons. The
diastereotopic nature of the two NH protons in complex 4 indicates a locked configuration at
[
31]
the amine nitrogen, in full agreement with hydrogen bonding in this complex.
1
Figure 4. Section of the H NMR spectrum (400 MHz, acetone-d
6
, 298 K) of complexes 4, 6 and 8 featuring the
2 2
NH and CH proton resonances of the benzylamine ligand.
1
3
The C NMR spectra showed only minor differences upon amine coordination for the pyridyl
unit and the Cp* ligand, generally to lower field (<2 ppm). For example, Cp* resonances shifted
downfield from d
C
C
= 91.8(±0.1) ppm in complexes 1 and 2 to d = 93.4(±0.1) ppm upon aniline
coordination and to d
C
= 93.2(±0.1) ppm upon benzylamine coordination, likely because the
iridium metal center is formally dicationic in complexes 3−8 while it is monocationic in
complexes 1 and 2. Remarkably, the carbene resonance is insensitive to this change and appears
at d
C
= 153.2(±0.1) ppm in the pyridyl complexes 1, 3, and 7 containing an aniline ligand. The
chemical shift is affected, however, by the type of amine; the Ctrz–Ir resonance of the aniline
C
complexes appears at lower field than that of the benzylamine complexes 4 and 8 (d = 150.7
ppm).
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Hydrogen bonding was further investigated by IR spectroscopy of complexes 1–8 as the
amineH…Npyr interactions are IR sensitive. Figure 5a shows superimposed spectra of the aniline
N
–
1
complexes 3, 5, and 7 in the 4000–2000 cm range. When comparing the spectrum of complex
with the spectra of amine-free complexes 1, 2 and its deuterated analogue 3−D , which was
prepared by deuteration of the aniline protons of 3 in methanol-d , the strong absorptions at nNH
3235 and 3124 cm for complexes 3, 5 and 7 can unambiguously be assigned to the
asymmetric and symmetric N–H stretching bands, respectively (Fig. 4a). Just slightly different
3
2
4
–
1
=
–
1
energies were noted for the benzylamine series (nNH = 3233 and 3150 cm Fig. 5b). These
bands are significantly lower than the N–H stretch vibrations of free aniline (nNH = 3429 and
–
1 [32]
[33]
3
352 cm ) and its derivatives such as p-methylaniline or m-methylaniline, Likewise, the
NH stretching vibrations of free benzylamine were reported at higher energy, nNH = 3372 and
–
1 [34]
3
303 cm .
–
1
Figure 5. Infrared spectra of the complexes in the 4000–2000 cm range; a) spectra of complexes 1, 2, 3, 3–D
and 7 (aniline series); b) spectra of complexes 1, 2, 4, 4–D , 6, and 8 (benzylamine series).
2
, 5
2
Upon metal coordination, the N–H stretch vibrations typically experience a bathochromic shift
–
1
of around 100 cm . Extensive vibrational studies with transition metals of the first row,
[
32,35]
cadmium, and mercury, confirmed this trend.
For example aniline coordinated to copper
–
1 [36]
–
features N–H vibrational bands at nNH = 3301 and 3234 cm . They vary by less than 10 cm
1
[33,37]
when using methylated or chelating aniline derivatives.
Directly relevant to our studies,
several iridium complexes containing a coordinated amine ligand were reported. For example,
–
1
Bergman and coworkers assigned the bands at nNH = 3282 and 3239 cm to the N–H stretch of
–
1
an Ir–NH
2
tBu complex, and nNH = 3266 and 3197 cm for an iridium complex containing a
[38]
sterically hindered aniline ligand with iPr substituents in ortho position. Similarly, Ikariya
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10.1002/chem.201700676
–
1
and coworkers observed bands in the 3325–3045 cm range for several amino iridium and
[
39]
ruthenium complexes, hence corroborating our assignments. In addition, the lower energy
observed in our system is indicative for efficient hydrogen bonding.
2
The corresponding deuterated complex 3−D shows the expected shift of these stretching modes
–
1
to lower frequency (nND around 2410 and 2315 cm ). The nNH/nND ratio is ideally 1.369 for
[
40]
[41]
free molecules, yet significantly lower in hydrogen-bonded systems. For example, this
[
42]
[43]
ratio drops to 1.355 for H
For complex 3, the nNH/nND ratio is 1.34(1), which is smaller than the ratio measured in free
aniline (1.36(2) for Ph−NH vs Ph−ND ) and also the ratio reported for water (see above), hence
corroborating the presence of a moderate NH…N hydrogen bond.
A similar evaluation of the benzylamine complex 4 (nNH = 3233 and 3150 cm ) and its
2 2
O, and as far as 0.96 for strongly hydrogen-bonding HCrO .
2
2
[
40]
–
1
–
1
analogue containing a dideuterated benzylamine ligand 4−D
a nNH/nND ratio of 1.35(1). An identical ratio was observed for the free benzylamine (Bn−NH
vs Bn−ND ), which suggests a NH…Npyr hydrogen bond strength similar to that of the aniline
2
(nND = 2400 and 2320 cm ) gave
2
2
complex 3. While X-ray analysis and NMR spectroscopy indicate a stronger hydrogen bond
interaction for the benzylamine complex 4 than for aniline, the IR analyses may be
compromised by the relatively large error, presumably due to the broadness and splitting of the
absorption bands.
Conclusions
We have developed a pair of triazolylidene iridium complexes that catalyze the oxidative
coupling of benzylic amines under base- and oxidant-free conditions. In addition, these
complexes are catalytically active both in the dehydrogenation of benzylic secondary amines
and the inverse reaction, i.e. the hydrogenation of imines. Such dual reactivity may be of interest
for amine racemization, e.g. for dynamic kinetic resolution, as well as for the reversible storage
and release of dihydrogen. Activity and selectivity of the catalyst is affected by the availability
of a pendant pyridyl site that can act as Lewis base for the stabilization of substrates through
hydrogen bonding. Such metal-ligand cooperation for substrate conversion has been evidenced
by a series of stoichiometric experiments. Optimization of these effects will be greatly
facilitated by the synthetic versatility of the click reaction to generate the triazole ligand scaffold
which allows to introduce a variety of functional groups. The confinement of a redox−active
Lewis acidic metal site and a Lewis basic imine unit within an active site is reminiscent of the
concept of frustrated Lewis pairs, and constitutes an appealing concept for the activation of
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10.1002/chem.201700676
protic substrates and for designing new generations of catalysts for de- and re-hydrogenation
of potential H carriers. Further optimization will therefore include modifications in the ligand
2
as well as the substrate to access an ideal Lewis acid base match.
Experimental Section
[
23]
1
2 2
-(2-pyridyl)-4-(2-pyridyl)triazole, 1-(2-pyridyl)-4-(2-phenyl)triazole, and[Cp*IrCl ] were
[44]
prepared according to literature procedures. Solvents were dried by passage through solvent
purification columns. All other reagents were purchased from commercial sources and used
without further purification. All reactions involving iridium were carried out under strict
1
exclusion of air using Schlenk-tube techniques under an atmosphere of dry nitrogen. All H and
1
3
C NMR spectra were recorded on Varian spectrometers operating at 400, 500 or 600 MHz
1
(
H NMR). Chemical shifts (δ) are given in ppm and referenced to residual solvent resonances.
Signal assignments are based on heteronuclear correlation and heteronuclear multiple bond
correlation experiments. Elemental analysis was performed by the microanalytical laboratories
of University College Dublin and the University of Bern. FT-IR spectra were recorded on a
Jasco FT-IR 4700 spectrometer with attenuated total reflectance (ATR) sampling technique at
room temperature.
Synthesis of L1. A suspension of 1-(2-pyridyl)-4-(2-pyridyl)triazole (700 mg, 2.74 mmol) and
2 2
MeOTf (361 µL, 3.29 mmol) in dry CH Cl (15 mL) was stirred at 0 ºC for 30 min and then at
r.t for 2 h. The solvent was evaporated under reduced pressure. The residue was precipitated
1
and washed with Et
2
O. Analysis of the crude product by H NMR spectroscopy showed a
mixture of two products (triazolium vs pyridinium ca. 1:3). The crude mixture was purified by
gradient flash chromatography (SiO
2
, CH
2
Cl
2
/MeOH 40:1 to 20:1). L1 was eluted as the first
, 400 MHz, 298 K): d 9.68 (s, 1H, Htrz),
1
fraction (white solid, 300 mg, 28%). H NMR (CDCl
3
8
.79 (d, JHH = 4.8 Hz, 1H, Hpy), 8.67 (dd, JHH = 4.8, 0.9 Hz, 1H, Hpy), 8.27 (d, JHH = 8.0 Hz,
H, Hpy), 8.26 (d, JHH = 7.7 Hz, 1H, Hpy), 8.12 (td, JHH = 8.0, 1.6 Hz, 1H, Hpy), 8.01 (td, JHH
.7, 1.5 Hz, 1H, Hpy), 7.64 (dd, JHH = 8.0, 4.8 Hz, 1H, Hpy), 7.53 (dd, JHH = 7.7, 4.8 Hz, 1H,
1
=
7
1
3
H
3 3
py), 4.79 (s, 3H, NCH ). C NMR (CDCl , 100.6 MHz, 298 K): d 150.1, 149.6 (2 x CHarom),
1
1
4
46.7, 142.8 (2 x Cipso), 142.3 (Ctrz), 140.6, 138.6, 127.3, 126.3 (4 x CHarom), 126.2 (CHtrz),
–
1
26.0, 115.8 (2 x CHarom), 41.9 (NCH
3 12 3 5 3
). Anal. Calcd. For C14H F N O S (387.34 g mol ): C,
3.41; H, 3.12; N, 18.08 %. Found: C, 43.69; H, 3.04; N, 17.72 %.
Synthesis of L2. A suspension of 1-(2-pyridyl)-4-(2-phenyl)triazole (649 mg, 2.92 mmol) and
2 2
MeOTf (384 µL, 3.50 mmol) in dry CH Cl (5 mL) was stirred at 0 ºC for 30 min and
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Chemistry - A European Journal
10.1002/chem.201700676
subsequently at r.t for 1 h. The solvent was evaporated under reduced pressure. The residue was
1
precipitated and washed with Et
2
O to obtain L2 as a white solid. (1.082 g, 96%). H NMR
(CDCl
3
, 400 MHz, 298 K): d 9.04 (s, 1H, Htrz), 8.60 (d, JHH = 4.0 Hz, 1H, Hpy), 8.23 (d, JHH
=
8
7
.1 Hz, 1H, Hpy), 8.09 (td, JHH = 8.1, 1.7 Hz, 1H, Hpy), 7.74 (dd, JHH = 7.8, 1.5 Hz, 2H, HPh),
1
3
3 3
.68–7.56 (m, 4H, Hpy + HPh), 4.43 (s, 3H, NCH ). C NMR (CDCl , 100.6 MHz, 298 K):
d 149.3 (CH), 146.7 (Cpy), 144.5 (Ctrz), 140.6 (CHpy), 132.3 (CHPh), 129.9 (CHPh), 129.8 (CHPh),
1
27.2 (CHpy), 124.5 (CHtrz), 121.8 (CPh), 115.7 (CHpy), 39.5 (NCH
3
). Anal. Calcd. For
–
1
C
15 13 3 4 3
H F N O S (386.35 g mol ): C, 46.63; H, 3.39; N, 14.75 %. Found: C, 46.23; H, 3.10; N,
1
4.43 %.
Synthesis of 1. A mixture of L1 (150 mg, 0.39 mmol), Ag
IrCl Cp*] (193 mg, 0.24 mmol) in dry CH Cl (15 mL) was stirred at room temperature for
8 h. After filtration through Celite and solvent evaporation, the yellowish residue was purified
2
O (112 mg, 0.48 mmol) and
[
2
2
2
2
4
by gradient flash chromatography (SiO
2
, CH
2
Cl
2
/MeOH 40:1 to 20:1). The product was isolated
–
1
1
as a yellow solid (280 mg, 96%). IR (cm ): n (py) 2915 (w). H NMR (CDCl
3
, 400 MHz, 298
K): d = 8.87 (ddd, JHH = 4.8, 1.7, 0.9 Hz, 1H, Hpy), 8.73 (dd, JHH = 5.7, 0.6 Hz, 1H, Hpy), 8.38
dd, JHH = 8.2, 0.6 Hz, 1H, Hpy), 8.30 (ddd, JHH = 8.2, 7.4, 1.4 Hz, 1H, Hpy), 8.06 (d, JHH = 7.7
Hz, 1H, Hpy), 7.93 (td, JHH = 7.7, 1.7 Hz, 1H, Hpy), 7.80 (ddd, JHH = 7.4, 5.7, 1.4 Hz, 1H, Hpy),
(
1
3
7
.54 (ddd, JHH = 7.7, 4.8, 1.2 Hz, 1H, Hpy), 4.49 (s, 3H, NCH
NMR (CD Cl , 100.6 MHz, 298 K): d = 153.3 (s, Ir–Ctrz), 151.7, 150.5 (2 x CHpy), 150.1 (Cpy),
46.1 (Cpy), 143.9 (Ctrz), 142.7, 137.7, 128.0, 127.3, 125.4, 115.1 (6 x CHpy), 91.9 (CCp*), 39.8
3 3
), 1.55 (s, 15H, Cp–CH ). C
2
2
1
–
1
(NCH
3
), 9.23 (Cp–CH
3 3 5 3
). Anal. Calcd. For C24H26ClF IrN O S (749.22 g mol ): C, 38.47; H,
3
.50; N, 9.35 %. Found: C, 38.30; H, 3.30; N, 9.07 %.
Synthesis of 2. A mixture of L2 (200 mg, 0.52 mmol), Ag
IrCl Cp*] (258 mg, 0.32 mmol) in dry CH Cl (15 mL) was stirred at room temperature for
6 h. After filtration through Celite and solvent evaporation, the yellowish residue was purified
2
O (150 mg, 0.65 mmol) and
[
2
2
2
2
3
by flash chromatography (SiO
2
, CH
2
Cl
2
/MeOH 20:1). The product was isolated as a yellow
–
1
1
solid (420 mg, 95%). IR (cm ): n (py) 2915 (w). H NMR (CDCl
3
, 500 MHz, 298 K): d = 8.68
(d, JHH = 5.6 Hz, 1H, Hpy), 8.30 (d, JHH = 8.3 Hz, 1H, Hpy), 8.18 (ddd, JHH = 8.3, 7.3, 1.2, 2H,
Hpy), 7.91–7.79 (m, 2H, HPh), 7.63 (ddd, JHH = 7.3, 5.6, 1.2 Hz, 1H, Hpy), 7.61–7.54 (m, 3H,
1
3
H
3 3 3
Ph), 4.29 (s, 3H, NCH ), 1.52 (s, 15H, Cp–CH ). C NMR (CDCl , 125.8 MHz, 298 K): d =
1
51.9 (Ir–Ctrz), 151.0 (Cpy), 150.9 (CHpy), 146.1 (Ctrz), 141.9 (CHpy), 131.5 (CHPh), 131.0
(
CHPh), 129.4 (CHPh), 126.8 (CHpy), 126.1 (CPh), 114.9 (CHpy), 91.7 (CCp*), 38.6 (NCH ), 9.1
3
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–
1
(
Cp–CH
. Found: C, 39.87; H, 3.53; N, 7.21 %.
Synthesis of 3. A mixture of 1 (100 mg, 0.134 mmol), AgOTf (38 mg, 0.15 mmol) and aniline
13.4 µl, 0.146 mmol) in dry CH Cl (5 mL) was stirred at room temperature for 18 h, protected
from light. After filtration through Celite, the brown solution was concentrated to about 0.5 mL.
Addition of Et O (4 mL) induced precipitation of the product as a yellow solid, which was
washed with Et
3 3 4 3
). Anal. Calcd. For C25H27ClF IrN O S (748.24 g mol ): C, 40.13; H, 3.64; N, 7.49
%
(
2
2
2
2
O (3 x 4 mL) and subsequently dried in vacuo (light yellow solid, 112 mg,
–
1
1
8
7%). IR (cm ): n (NH) 3240, 3203, 3125, 3079 (w). H NMR (acetone-d
6
, 500 MHz, 298
K): d = 9.15 (d, JHH = 5.6 Hz, 1H, Hpy), 9.06 (ddd, JHH = 4.8, 1.7, 0.9 Hz, 1H, Hpy), 8.47 (ddd,
HH = 8.2, 7.5, 1.2 Hz, 1H, Hpy), 8.36 (d, JHH = 7.7 Hz, 1H, Hpy), 8.30 (td, JHH = 7.7, 1.7 Hz,
H, Hpy), 8.14 (dd, JHH = 8.2, 0.8 Hz, 1H, Hpy), 7.98 (ddd, JHH = 7.5, 5.6, 0.8 Hz, 1H, Hpy), 7.83
ddd, JHH = 7.7, 4.8, 1.2 Hz, 1H, Hpy), 7.48–7.34 (m, 2H, NH ), 6.92 (t, JHH = 7.4 Hz, 1H, HPh),
.87 (t, JHH = 7.4 Hz, 2H, HPh), 6.59 (d, JHH = 7.4 Hz, 2H, HPh), 4.53 (s, 3H, NCH ), 1.68 (s,
, 125.8 MHz, 298 K): d = 153.2 (CHpy), 153.2 (Ir–Ctrz),
J
1
(
2
6
1
1
1
3
1
3
3 6
5H, Cp–CH ). C NMR (acetone-d
52.0 (CHpy), 151.0 (Cpy), 146.2 (Cpy), 145.4 (Ctrz), 144.4 (CHpy), 140.4 (CPh), 139.4 (CHpy),
29.5 (CHPh), 129.4, 128.4, 126.8 (3 x CHpy), 125.9 (CHPh), 121.5 (CHPh), 115.6 (CHpy), 93.5
–
1
(
C
Cp*), 40.2 (NCH
8.95; H, 3.48; N, 8.79 %. Found: C, 38.78; H, 3.38; N, 8.41 %.
(90 mg, 0.26 mmol) in 0.5 mL of MeOH was added to
a solution of 3 (50 mg, 0.052 mmol) in 0.5 mL of MeOH. A pale yellow solid precipitated. The
3 3 33 6 6 6 2
), 9.1 (Cp–CH ). Anal. Calcd. For C31H F IrN O S (955.96 g mol ): C,
3
Synthesis of 3´. A solution of NaBPh
4
2
supernatant was removed and the solid was washed with MeOH (4 x 1 mL) and Et O (3 x 5
1
mL) and subsequently dried in vacuo (pale yellow solid, 32 mg, 47%). H NMR (acetone-d
6
,
4
1
00 MHz, 298 K): d = 9.18 (dd, JHH = 5.7, 0.7 Hz, 1H, Hpy), 9.04 (ddd, JHH = 4.8, 1.5, 1.1 Hz,
H, Hpy), 8.31 (ddd, JHH = 8.2, 7.5, 1.5 Hz, 1H, Hpy), 8.27–8.17 (m, 2H, Hpy), 8.06 (d, JHH = 8.2
Hz, 1H, Hpy), 7.89 (ddd, JHH = 7.5, 5.7, 1.5 Hz, 1H, Hpy), 7.79 (ddd, JHH = 7.1, 4.8, 1.6 Hz, 1H,
Hpy), 7.48–7.39 (br s, 2H, NH
2
), 7.38–7.29 (m, 16H, HBPh4), 6.98–6.84 (m, 16H, HBPh4 + 3H,
Haniline), 6.77 (t, JHH = 7.2 Hz, 8H, HBPh4), 6.57 (d, JHH = 7.9, Hz, 2H, Haniline), 4.43 (s, 3H,
1
3
3 3 6
NCH ), 1.65 (s, 15H, Cp–CH ). C NMR (acetone-d , 100.6 MHz, 298 K): d = 165.7 (q, JBC =
4
1
1
9.5 Hz, CBPh4), 153.5 (Ir–Ctrz), 153.1, 152.1 (2 x CHpy), 150.9 (Cpy), 146.0 (Cpy), 145.7 (Ctrz),
44.7 (CHpy), 140.0 (Caniline), 139.2 (CHpy), 137.0 (q, JBC = 1.7 Hz, CHBPh4), 129.8 (CHaniline),
29.6 (CHpy), 127.6 (CHaniline), 127.0, 126.4 (2 x CHpy), 126.0 (q, JBC = 2.7 Hz, CHBPh4), 122.3
3 3
(CHBPh4), 121.2 (CHaniline), 115.7 (CHpy), 93.6 (CCp*), 40.1 (NCH ), 9.1 (Cp–CH ). Anal. Calcd.
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Chemistry - A European Journal
10.1002/chem.201700676
–
1
73 2 6
For C77H B IrN (1296.28 g mol ): C, 71.34; H, 5.68; N, 6.48 %. Found: C, 71.14; H, 5.52;
N, 6.18 %.
Synthesis of 4. A mixture of 1 (150 mg, 0.20 mmol), AgOTf (56 mg, 0.22 mmol) and
benzylamine (24 µl, 0.22 mmol) in dry CH Cl (15 mL) was stirred at room temperature for 18
h. After filtration through Celite, the yellow solution was concentrated to about 1 mL. Addition
of Et O (50 mL) induced precipitation of the product as a pale yellow solid, which was washed
with Et
2
2
2
2
O (3 x 20 mL) and subsequently dried in vacuo (pale yellow solid, 167 mg, 87%). IR
–
1
1
(
cm ): n (NH) 3236, 3149 (w). H NMR (acetone-d
6
, 500 MHz, 298 K): d = 9.30 (ddd, JHH
=
5
.7, 1.4, 0.7 Hz, 1H, Hpy), 9.01 (ddd, JHH = 4.8, 1.7, 1.0 Hz, 1H, Hpy), 8.59 (ddd, JHH = 8.3, 7.3,
.4 Hz, 1H, Hpy), 8.53 (ddd, JHH = 8.3, 1.3, 0.7 Hz, 1H, Hpy), 8.21 (td, JHH = 7.7, 1.7 Hz, 1H,
py), 8.15 (dd, JHH = 7.7, 1.2 Hz, 1H, Hpy), 8.01 (ddd, JHH = 7.3, 5.7, 1.3 Hz, 1H, Hpy), 7.77
ddd, JHH = 7.7, 4.8, 1.2 Hz, 1H, Hpy), 7.36–6.98 (m, 5H, HPh), 5.59–5.49 (m, 1H, NH ), 5.33–
), 3.81–3.73 (m, 1H, CH ), 1.68
, 125.8 MHz, 298 K): d = 153.6 (CHpy), 152.1 (Cpy),
1
H
(
2
5
.23 (m, 1H, NH
2
), 4.53 (s, 3H, NCH
3
), 4.08–4.00 (m, 1H, CH
2
2
1
3
(s, 15H, Cp–CH
3
). C NMR (acetone-d
6
1
1
51.7 (CHpy), 150.7 (Ir–Ctrz), 146.5 (Ctrz), 146.3 (Cpy), 144.7, 139.2 (2 x CHpy), 139.0 (CPh),
29.5, 129.4 (2 x CHPh), 129.1 (CHpy), 128.9 (CHPh), 128.1, 126.6, 116.2 (3 x CHpy), 93.2
(
C
Cp*), 53.8 (CH
2
), 40.1 (NCH
g mol ): C, 39.62; H, 3.64; N, 8.66 %. Found: C, 39.78; H, 3.49; N, 8.32 %.
Synthesis of 4´. A solution of NaBPh (264 mg, 0.773 mmol) in 1 mL of MeOH was added to
3 3 35 6 6 6 2
), 8.98 (Cp–CH ). Anal. Calcd. For C32H F IrN O S (969.99
–
1
4
a solution of 4 (150 mg, 0.154 mmol) in MeOH (0.6 mL). A pale yellow solid precipitated. The
supernatant was removed and the solid was washed with copious amounts of MeOH (10 mL in
total) and Et
2
O (3 x 10 mL) and subsequently dried in vacuo. (pale yellow solid, 143 mg, 71%).
, 500 MHz, 298 K): d = 9.24 (d, JHH = 5.7 Hz, 1H, Hpy), 9.01 (ddd, JHH
.8, 1.7, 0.9 Hz, 1H, Hpy), 8.51 (dd, JHH = 8.1, 1.4 Hz, 1H, Hpy), 8.48 (ddd, JHH = 8.1, 7.3, 1.3
Hz, 1H, Hpy), 8.19 (td, JHH = 7.7, 1.7 Hz, 1H, Hpy), 8.13 (d, JHH = 7.7 Hz, 1H, Hpy), 7.98 (ddd,
HH = 7.3, 5.7, 1.4 Hz, 1H, Hpy), 7.77 (ddd, JHH = 7.7, 4.8, 1.1 Hz, 1H, Hpy), 7.37–7.20 (m, 16H,
BPh4), 7.28–7.24 (m, 3H, HPh), 7.12–7.09 (m, 2H, HPh), 6.91 (t, JHH = 7.3 Hz, 16H, HBPh4), 6.77
t, JHH = 7.3 Hz, 8H, HBPh4), 5.52 (t, JHH = 11.0 Hz, 1H, NH ), 5.29 (t, JHH = 11.0 Hz, 1H, NH ),
), 4.16 (ddd, JHH = 14.8, 11.0, 3.3 Hz, 1H, CH ), 3.85 (ddd, JHH = 14.8, 11.0,
, 125.8 MHz, 298 K): d = 164.9
1
H NMR (acetone-d
6
=
4
J
H
(
2
2
4
3
.49 (s, 3H, NCH
3
2
1
3
.5 Hz, 1H, CH ), 1.67 (s, 15H, Cp–CH ). C NMR (acetone-d
2
3 6
(q, JBC = 49.5 Hz, 8 x CBPh4), 153.4, 152.1, 152.0 (3 x CHpy), 151.4 (Ir–Ctrz), 146.8 (Ctrz), 146.0,
1
45.1, 139.2 (3 x CHpy), 139.1 (CPh), 137.0 (q, JBC = 1.3 Hz, CHBPh4), 129.7 (CHPh), 129.4
(CHpy), 129.3 (s, CHPh), 129.1 (CHPh), 127.6, 126.9 (2 x CHpy), 126.0 (q, JBC = 2.8 Hz, CHBPh4),
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Chemistry - A European Journal
10.1002/chem.201700676
1
22.2 (CHBPh4), 116.2 (CHpy), 93.3 (CCp*), 53.9 (CH
2
), 40.1 (NCH
3 3
), 9.0 (Cp–CH ). Anal.
–
1
Calcd. For C78
H
75
B
2
IrN
6
(1310.31 g mol ): C, 71.50; H, 5.77; N, 6.41 %. Found: C, 71.32; H,
5
.63; N, 6.26 %.
Synthesis of 5. A mixture of 2 (100 mg, 0.134 mmol), AgOTf (38 mg, 0.15 mmol) and aniline
13.4 µl, 0.146 mmol) in dry CH Cl (5 mL) was stirred at room temperature for 18 h, protected
from light. After filtration through Celite, the brown solution was concentrated to about 0.5 mL.
Addition of die Et O (4 mL) induced precipitation of the product as an ivory solid, which was
washed with Et
(
2
2
2
2
O (3 x 20 mL) and subsequently dried in vacuo (ivory solid, 94 mg, 74%). IR
–
1
1
(
cm ): n (NH) 3227, 3202, 3132 (w). H NMR (acetone-d
6
, 400 MHz, 298 K): d = 9.00 (dd,
J
1
3
HH = 5.7, 1.0 Hz, 1H, Hpy), 8.44 (ddd, JHH = 8.2, 7.5, 1.0 Hz, 1H, Hpy), 8.11 (d, JHH = 8.2 Hz,
H, Hpy), 8.08–8.00 (m, 2H, HPh), 7.94 (ddd, JHH = 7.5, 5.7, 1.2 Hz, 1H, Hpy), 7.85–7.76 (m,
H, HPh), 7.48–7.33 (m, 2H, NH ), 6.96 (t, JHH = 7.3 Hz, 1H, Haniline), 6.90 (t, JHH = 7.3 Hz, 2H,
2
1
3
Haniline), 6.49 (d, JHH = 8.0 Hz, 2H, Haniline), 4.38 (s, 3H, NCH
3
), 1.66 (s, 15H, Cp–CH
3
). C
NMR (acetone-d
6
, 100.6 MHz, 298 K): d = 152.9 (CHpy), 151.3 (Ir–Ctrz), 151.1 (Cpy), 147.1
(C
trz), 144.4 (CHpy), 140.4 (Caniline), 132.3 (CHPh), 132.2 (CHPh), 130.5 (CHPh), 129.7 (CHaniline),
1
29.4 (CHpy), 126.3 (CPh), 125.9 (CHaniline), 121.4 (CHaniline), 115.5 (CHpy), 93.4 (CCp*), 39.4
–
1
(NCH
3
), 9.9 (Cp–CH
3 34 6 5 6 2 2 2
). Anal. Calcd. For C32H F IrN O S (954.98 g mol )·0.1 CH Cl : C,
4
0.02; H, 3.58; N, 7.27 %. Found: C, 39.69; H, 3.96; N, 7.08 %.
Synthesis of 6. A mixture of 2 (100 mg, 0.134 mmol), AgOTf (38 mg, 0.15 mmol) and
benzylamine (16 µl, 0.15 mmol) in dry CH Cl (10 mL) was stirred at room temperature for 18
h. After filtration through Celite, the yellow solution was concentrated to about 1 mL. Addition
of Et O (50 mL) induced precipitation of the product as a pale yellow solid, which was washed
with Et
2
2
2
2
O (3 x 20 mL) and subsequently dried in vacuo (pale yellow solid, 97 mg, 75%). IR
–
1
1
(
cm ): n (NH) 3239, 3165 (w). H NMR (acetone-d
6
, 400 MHz, 298 K): d = 9.29 (d, JHH = 5.6
Hz, 1H, Hpy), 8.60 (ddd, JHH = 8.0, 7.3, 1.3 Hz, 1H, Hpy), 8.54 (dd, JHH = 8.0, 1.1 Hz, 1H, Hpy),
8
.02 (ddd, JHH = 7.3, 5.6, 1.1 Hz, 1H, Hpy), 7.91–7.84 (m, 2H, HPh), 7.75–7.69 (m, 3H, HPh),
.26–7.19 (m, 5H, HBnNH2), 5.48–5.32 (m, 2H, NH ), 4.36 (s, 3H, NCH ), 3.86–3.63 (m, CH,
). C NMR (acetone-d , 100.6 MHz, 298 K): d = 153.4 (CHpy),
7
2
3
1
3
CH
2
), 1.67 (s, 15H, Cp–CH
3
6
1
52.3 (Cpy), 149.3 (Ir–Ctrz), 147.8 (Ctrz), 144.7 (CHpy), 138.7 (CBnNH2), 132.3 (CHPh), 132.0
(CHPh), 130.4 (CHPh), 129.7, 129.5 (2 x CHBnNH2), 129.1 (CHpy), 129.0 (CHBnNH2), 126.5 (CPh),
1
16.2 (CHpy), 93.1 (CCp*), 53.9 (CH
2
), 39.4 (NCH
3 3
), 8.9 (Cp–CH ). Anal. Calcd. For
–
1
C
33
H
36
F
6
IrN
5
O
6
S
2
(969.00 g mol )·0.2 Et O: C, 41.26; H, 3.89; N, 7.12 %. Found: C, 41.09;
2
H, 4.26; N, 7.18 %.
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Chemistry - A European Journal
10.1002/chem.201700676
Synthesis of 7. A mixture of 3 (20 mg, 0.021 mmol), AgOTf (6 mg, 0.023 mmol) and aniline
31 µl, 0.31 µl) in dry CH Cl (1 mL) was stirred at room temperature for 18 h. Addition of
O (4 mL) induced precipitation of the product as a yellow solid, which was washed with
O (3 x 4 mL) and subsequently dried in vacuo (ivory solid, 21 mg, 81%). Recrystallization
Cl /Et O was performed in a flask wrapped with aluminium foil to protect the
complex from light. IR (cm ): n (NH) 3235, 3201, 3124, 3077 (w). H NMR (acetone-d
(
2
2
Et
Et
2
2
from CH
2
2
2
–
1
1
6
, 400
MHz, 298 K): δ = 9.09 (d, JHH = 5.0 Hz, 2H, Hpy), 8.44 (m, 2H, Hpy), 8.35 (td, JHH = 7.5, 1.6
Hz, 1H, Hpy), 8.10 (d, JHH = 8.1 Hz, 1H, Hpy), 7.95 (ddd, JHH = 7.5, 5.0, 1.1 Hz, 1H, Hpy), 7.89
(ddd, JHH = 7.5, 5.0, 1.2 Hz, 1H, Hpy), 7.45–7.29 (m, 2H, NH
2
), 6.93 (t, JHH = 7.2 Hz, 1H,
Haniline), 6.87 (t, JHH = 7.4 Hz, 2H. Haniline), 6.56 (d, JHH = 7.8 Hz, 2H, Haniline), 4.51 (s, 3H,
1
3
3 3 6
NCH ), 1.68 (s, 15H, Cp–CH ). C NMR (acetone-d , 100.6 MHz, 298 K): d = 153.1 (CHpy),
1
1
1
C
53.1 (Ir–Ctrz), 152.7 (CHpy), 151.0 (Cpy), 146.2 (Cpy), 145.3 (Ctrz), 144.4 (CHpy), 140.4 (Caniline),
40.1 (CHpy), 129.6 (2 x CHaniline), 129.4 (CHpy), 129.1 (CHpy), 127.2 (CHpy), 125.9 (CHaniline),
21.5 (CHaniline), 115.6 (CHpy), 93.6 (CCp*), 40.2 (NCH
3
3
), 9.1 (Cp–CH ). Anal. Calcd. For
–
1
32 9 6 10 3
H35AgF IrN O S (1230.92 g mol ): C, 31.22; H, 2.87; N, 6.83. Found: C, 31.49; H, 2.97;
+
+
N, 6.94 %. HRMS (ESI , 27V): m/z: 807.1909; calculated for [M–Ag –H
07.1911.
Synthesis of 8. A mixture of 4 (20 mg, 0.021 mmol), AgOTf (5.9 mg, 0.023 mmol) in dry
CH Cl (1 mL) was stirred at room temperature for 18 h protected against the light. After
solvent evaporation, the residue was dissolved in acetone (2 mL) and filtered through Celite.
The yellow solution was concentrated to about 0.5 mL. Addition of Et O (2 mL) induced
2 3 3 2
O–(CF SO ) ]
8
2
2
2
precipitation of a solid, which was washed with Et
2
O (3 x 2 mL) and subsequently dried in
–
1
1
vacuo (ivory solid, 14 mg, 54%). IR (cm ): n (NH) 3233, 3150 (w). H NMR (acetone-d
6
, 400
MHz, 298 K): d = 9.27 (d, JHH = 5.7 Hz, 1H, Hpy), 9.08 (ddd, JHH = 5.0, 1.6, 0.9 Hz, 1H, Hpy),
.63–8.56 (m, 1H, Hpy), 8.52 (dd, JHH = 7.8, 1.2 Hz, 1H, Hpy), 8.30 (td, JHH = 7.8, 1.6 Hz, 1H,
py), 8.17 (td, JHH = 7.8, 0.9 Hz, 1H, Hpy), 8.01 (ddd, JHH = 7.8, 5.7, 1.4 Hz, 1H, Hpy), 7.87
ddd, JHH = 7.8, 5.0, 1.2 Hz, 1H, Hpy), 7.25–7.19 (m, 5H, HBnNH2), 5.48–5.25 (m, 2H, NH ),
). C NMR
, 100.6 MHz, 298 K): d = 153.4, 152.9 (2 x CHpy), 152.1 (Cpy), 150.7 (Ir–Ctrz), 146.4
trz), 146.1 (Cpy), 144.7, 140.4 (2 x CHpy), 138.7 (CBnNH2), 129.6 (CHBnNH2), 129.4 (CHpy),
29.3, 128.9 (2 x CHBnNH2), 128.8, 127.2, 116.6 (3 x CHpy), 93.3 (CCp*), 53.8 (CH
8
H
(
2
1
3
4
3 2 3
.49 (s, 3H, NCH ), 4.02–3.85, 3.83–3.58 (2 x m, 1H, CH ), 1.68 (s, 15H, Cp–CH
(acetone-d
6
(C
1
2
), 40.1
–
1
(NCH
3 3 9 6 10 3
), 9.05 (Cp–CH ). Anal. Calcd. for C33H37AgF IrN O S (1244.95 g mol ): C, 31.84;
H, 3.00; N, 6.75 %. Found: C, 31.68; H, 3.03; N, 6.53 %.
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Chemistry - A European Journal
10.1002/chem.201700676
General Procedure for oxidative coupling of amines.
A mixture of amine (0.2 mmol) and the iridium complex (0.01 mmol) in 1,2- dichlorobenzene
1
(
2 mL) was heated at 150 ºC in a closed vial. The reaction mixture was analyzed by H NMR
spectroscopy using Hexamethylbenzene (0.033 mmol) as internal standard. The signals due to
reagents and products were identified according to commercially available samples
(benzylamine, 4-methoxybenzylamine, 4-methylbenzylamine, 4-chlorobenzylamine, 3-
chlorobenzylamine, α-methylbenzylamine, N-benzylidenbenzylamine, dibenzylamine,
tribenzylamine) or previously reported spectroscopic data (N-(4-chlorobenzylidene)-4-
chlorobenzylamine,
N-(3-chlorobenzylidene)-3-chlorobenzylamine,
p-methyl-N-(p-
[
9f]
methylbenzylidene)benzylamine, p-methoxy-N-(p-methoxybenzylidene)benzylamine, N-(α-
[
45a]
methylbenzylidene)-α-methylbenzylamine,
bis-(4-methoxybenzyl)amine,
bis-(4-
bis(1-
[
45b]
[45c]
[45d]
chlorobenzyl)amine,
bis(3-chlorobenzyl)amine, ꢀ bis(4-methylbenzyl)amine,
[
45e]
phenylethyl)amine,
tris(4-methoxybenzyl)amine, tris(4-methylbenzyl)amine, tris(4-
[
45f]
[45g]
chlorobenzyl)amine,
and tris(3-chlorobenzyl)amine.
Crystallographic analyses.
Crystal data for complexes 1, 2, 4´ and 7 were collected by using an Agilent Technologies (now
Rigaku) SuperNova A diffractometer fitted with an Atlas detector using Mo-Kα radiation
(0.71073 Å). A complete dataset was collected, assuming that the Friedel pairs are not
[
46]
equivalent. An analytical numeric absorption correction was performed.
Crystal data for
[47]
complex 3 was collected on an Oxford Diffraction SuperNova area-detector diffractometer
[
48]
using mirror optics monochromated Mo Kα radiation (λ = 0.71073 Å) and Al filtered. The
unit cell constants and an orientation matrix for data collection were obtained from a least-
squares refinement of the setting angles of reflections in the range 1.8° < θ < 27.9°. A total of
1
428 frames were collected using ω scans, with 20+20 seconds exposure time, a rotation angle
of 1.0° per frame, a crystal-detector distance of 65.0 mm, at T = 123(2) K. Data reduction was
[
47]
performed using the CrysAlisPro program. The intensities were corrected for Lorentz and
polarization effects, and a numerical absorption correction based on gaussian integration over
a multifaceted crystal model was applied. The structure of 1, 2, 4´ and 7 were solved by direct
methods using SHELXS-97 and refined by full-matrix least-squares fitting on F2 for all data
[
49]
using SHELXL-97. Hydrogen atoms were added at calculated positions and refined by using
a riding model. Their isotropic temperature factors were fixed to 1.2 times (1.5 times for methyl
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Chemistry - A European Journal
10.1002/chem.201700676
groups) the equivalent isotropic displacement parameters of the carbon atom the H-atom is
attached to. Anisotropic thermal displacement parameters were used for all nonhydrogen atoms.
[
50]
The structure of 3´ was solved by direct methods using SHELXT,
which revealed the
positions of all not disordered non-hydrogen 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 where each H-atom was assigned a fixed isotropic
displacement parameter with a value equal to 1.2Ueq of its parent atom (1.5Ueq for the methyl
2
groups). Refinement of the structure 3´ was carried out on F using full-matrix least-squares
2
2 2
procedures, which minimized the function Σw(F
o c
– F ) . The weighting scheme was based on
counting statistics and included a factor to downweight the intense reflections. All calculations
[
51]
were performed using the SHELXL-2014/7
program. The crystal contains one metal
complex with two co-crystalized tetraphenylborate anions and one dichloromethane molecule
in the asymmetric unit. There are more co-crystallized heavily disordered dichloromethane and
ether molecules present in the crystal which however could not modelled successfully. The
remaining electron density in the solvent accessible voids was therefore accounted for with the
[52]
PLATON
SQUEEZE procedure. Crystallographic details are compiled in Tables S1−S5.
CCDC No 1526895 (1), 1526897 (2), 1526899 (3´), 1526898 (4´), and 1526896 (7) contain the
supplementary crystallographic data for this paper. These data can be obtained free of charge
from the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
Supporting information available: Crystallographic details, and structure data in cif format;
comparative NMR and IR spectra.
Acknowledgment
We thank Dr. J. Hauser (Univ Bern) for solving the crystal structure of complex 3´. The authors
gratefully acknowledge financial support from the European Commission (ERC CoG 615653),
the Marie SklodowskaCurie Action for a fellowship to M.V. (grant 660929) as well as the Swiss
National Science Foundation (200021–162868 and 206021–128724). A.P. thanks MEC for a
FPU fellowship and COST CM1205 CARISMA for a STSM visiting fellowship.
Keywords
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23
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Chemistry - A European Journal
10.1002/chem.201700676
Iridium --- triazolylidene ---- ligand metal cooperativity --- hydrogen bonding --- amine
dehydrogenation
Entry for Table of Contents:
A pending pyridyl group in the C,N-bidentate
chelating carbene-pyridyl complex 1 facilitates
amine coordination to the iridium center and
enhances catalytic activity of the metal center in
amine dehydrogenation. Stoichiometric experiments
provide evidence for hydrogen bonding of the amine
substrate with the pending pyridyl unit.
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Chemistry - A European Journal
10.1002/chem.201700676
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