Unveiling the role of ancillary ligands in acceptorless benzyl alcohol dehydrogenation and etherification mediated by mesoionic carbene iridium complexes

Article abstract of DOI:10.1039/c7dt04109h

We synthesized a set of triazolylidene iridium(iii) complexes [IrCp?(C^N)L]ⁿ⁺ (Cp? = pentamethylcyclopentadienyl, C^N = C,N-bidentate coordinating pyridyl-triazolylidene) containing different neutral or anionic ancillary ligands L and evaluated their impact on the catalytic activity in alcohol conversion. We demonstrate that these ancillary ligands have a strong influence on the catalytic selectivity and direct whether the iridium center preferentially catalyzes either the dehydrogenation or the dehydration of benzyl alcohol. Ligand exchange experiments provide a direct correlation of ligand lability with catalytic activity and selectivity. These results underline the relevance of ancillary ligands and provide a rational approach to tailor the catalytic activity of the iridium center towards aldehyde formation (loss of H₂) or etherification (elimination of H₂O).

Full text of DOI:10.1039/c7dt04109h

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Unveiling the role of ancillary ligands in acceptorless benzyl alcohol dehydrogenation and
etherification mediated by mesoionic carbene iridium complexes
Angela Vivancos,a,b Ana Petronilho,b Joao Cardoso,b Helge Müller-Bunzb and
Martin Albrecht *,a,b
a
Departement für Chemie und Biochemie, Universität Bern, Freiestrasse 3, CH-3012 Bern, Switzerland.
b
School of Chemistry & Chemical Biology, University College Dublin, Belfield, Dublin 4, Ireland.
E-mail: martin.albrecht@dcb.unibe.ch
Abstract
We synthesized a set of triazolylidene iridium(III) complexes [IrCp*(C^N)L]n+ (Cp* =
pentamethylcyclopentadienyl, C^N = C,N-bidentate coordinating pyridyl-triazolylidene)
containing different neutral or anionic ancillary ligands L and evaluated their impact on the
catalytic activity in alcohol conversion. We demonstrate that these ancillary ligands have a strong
influence on the catalytic selectivity and direct whether the iridium center preferentially catalyzes
either the dehydrogenation or the dehydration of benzyl alcohol. Ligand exchange experiments
provide a direct correlation of ligand lability with catalytic activity and selectivity. These results
underline the relevance of ancillary ligands and provide a rational approach to tailor the catalytic
activity of the iridium center towards aldehyde formation (loss of H2) or etherification
(elimination of H2O).
1

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Introduction
In contrast to reductive transformations such as (transfer) hydrogenations, which are
mechanistically very well understood, comparably little is known about oxidative catalysis.1 This
lack of understanding may be, in parts, due to the oxidation sensitivity of many privileged ligand
classes such as phosphines. With the emergence of N-heterocyclic carbenes2 as powerful ligands,3
a class of strong donors has become available that is generally inert to oxidative degradation.4
Consequently, a range of transition metal complexes containing N-heterocyclic carbenes have
been demonstrated to have remarkable activity as catalysts for oxidation reactions,5 such as the
dehydrogenation of alcohols to yield ketones or aldehydes.6 This reaction usually requires the use
of a base to facilitate H+ displacement, and generally yields the corresponding carbonyl
derivatives selectively. In the absence of a base, this process is often slow and requires high
catalyst loading.6 Similar transformations have been investigated with amines.7 With specific
complexes, dehydration reactions of alcohols display altered selectivity and afforded ethers from
the coupling of two equivalents of alcohol and elimination of H2O (dehydration pathway) as
opposed to dehydrogenation.8
We have recently reported an iridium complex containing a C,C-bidentate chelating
triazolylidene-pyridylidene ligand that mediates both the dehydrogenation and dehydration of
alcohols without the use of an external base.8b With a C,N-bidentate triazolylidene-pyridyl ligand,
the addition of silanes affects the process and enhances the selectivity towards the formation of
ether products from a variety of ketones, and also from alcohols.9 However, the selectivity was
strongly dependent on the ancillary ligand and the iridium chloride complex demonstrated
diverging reactivity from the complex containing an aqua ligand. We also noted that the iridium-
bound water molecule can be easily replaced by acetonitrile, while the chloride derivative does
not undergo exchange with acetonitrile but exchanges partly with water when solubilized in
aqueous solvents.10
Based on these observations, we were hypothesizing that the ancillary ligand in these
complexes plays a distinct role in controlling the reactivity of the iridium center. We therefore
targeted a set of triazolylidene pyridyl iridium complexes containing different neutral or anionic
ancillary ligands to evaluate their impact on the catalytic activity. Here we demonstrate that these
ancillary ligands strongly influence whether the iridium center catalyzes dehydrogenation or
dehydration of benzyl alcohol. Ligand exchange experiments provide a direct correlation with
catalytic activity and selectivity. These results underline the relevance of ancillary ligands and
provide a rational approach to tailor the catalytic activity of the metal center to either
dehydrogenation (aldehyde formation) or dehydration and etherification.
2

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Results and Discussion
Synthesis of complexes with different ancillary ligands. The synthesis of complexes 1 and 2
was previously reported.4c The dicationic complex 2 was prepared by chloride abstraction from 1
with silver triflate in the presence of water. The same methodology was employed for the
synthesis of the new complexes 3 and 4. Compound 3 was obtained in good yields (~80%) from
1 and AgOTf in CH2Cl2 that was saturated with gaseous NH3,11 or alternatively by using an excess
of ammonium hydroxide solution. No side products were observed in the later procedure such as
a potential hydroxo complex, or the aqua complex 2. This high selectivity for ammonia
coordination suggests a stronger Ir–NH3 bond in comparison to Ir–OH2. Similarly, AgOTf-
mediated chloride abstraction in the presence of aniline afforded complex 4, which was obtained
as a yellow powder in moderate yields of 65%. When performing the reaction with AgOTf in
MeCN, complex 5 was obtained.4c
Scheme 1. Iridium complexes with variable ancillary ligands.
Complexes 3 and 4 were characterized by NMR spectroscopy. The pyridyl moiety of both
3 and 4 shows only slight changes when compared to the chloride derivative 1. Most diagnostic
is the shift of the ortho pyridyl signal to lower field with respect to 1 (dH = 9.01 in 1 vs dH = 9.16
and 9.31 ppm for complexes 3 and 4, respectively (acetone–d6 solution). A similar downfield
shift was previously noted for complex 2 (dH = 9.32 ppm), indicative of stronger pyridyl
coordination to the metal center and therefore a direct consequence of the change of the ancillary
ligand from an anionic chloride to a neutral amine. The spectrum of complex 3 further shows a
signal at dH = 4.10 for the coordinated NH3 ligand. The broad nature of this resonance suggests
1
either hindered rotation about the Ir–N bond, or ligand exchange rates that are close to the H
NMR frequency. The resonance is much less shielded in this iridium(III) complex than when
ammonia is bound to other iridium centers.12 In complex 4, the NH2 protons of the aniline ligand
appear as two distinct AB doublets at 7.39 and 7.27 ppm (2JHH = 10 Hz), a direct consequence of
the chirality at iridium. In the 13C{1H} NMR spectra, the iridium-bound Ctrz nucleus appears at dC
= 153.5 for complex 3 and 155.9 ppm for 4, well within the range of triazolylidene resonances of
similar compounds.13
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Single crystals suitable for X-ray diffraction analysis of complexes 3 and 4 were obtained
from slow evaporation of either a CH2Cl2 solution of 3 or by diffusion of Et2O into a CH2Cl2
solution of complex 4. The molecular structures confirm the expected ligand connectivity with
the typical piano-stool geometry of iridium featuring a N,C-bidentate chelating pyridyl-carbene
ligand and the amine ligand as the three legs (Fig. 1). Complex 3 crystallized with an additional
molecule of water, which is supported by hydrogen bonding with both the NH3 ligand and the
triflate counterion (Fig. 1a). The coordination of NH3 and not H2O is another indication that the
Ir–NH3 bond is stronger than the Ir–OH2 bond. The Ir-C1 bond distance is 2.029(3) Å, identical
within esds to that found for related complexes such as the aqua derivative 2 (Ir-C1 in 2 is
2.030(2) Å).4c These identical values indicate that the Ir-triazolylidene bond is not significantly
affected by the substitution of the ancillary ligand. Interestingly, the Ir-triazolylidene bond in the
chloride complex 1 is marginally shorter (2.017(2) Å) than that in the dicationic complexes 2 and
3. Such a decreased bond length does not reflect the reduced charge of the metal center
(monocationic in 1 vs dicationic in 2 and 3). The Ir-N(5) bond distance to the NH3 ligand is
2.127(2) Å, slightly shorter than the iridium- pyridyl bond length (Ir-N(4) = 2.140(2) Å), and
also shorter than in a related phosphine complex.11
In the single crystals of complex 4, the carbene ligand is completely disordered over the
two coordination sites of iridium occupied by the chelate. These two configurations represent the
enantiomers of complex 4 due to invertion of chirality at iridium (Fig. 1b, 1c). While both
enantiomers of 3 co-crystallized in a defined translational order (as indicated by the
centrosymmetric point group P21/c), the enantiomers of complex 4 are completely disordered.
This disorder demonstrates the steric similarity of the pyridyl fragment and the dimethyl-
substituted triazolylidene heterocycle. Hence, this specific triazolylidene is an excellent structural
substitute of pyridine, as previously noted for ruthenium complexes.14 Since the disorder is well-
resolved, refinement converges with high reliability and bond lengths and angles are well-defined.
Accordingly, the Ir-Ctrz bond distance Ir-C1a 1.990(6) Å and Ir-C1b 1.994(7) Å are smaller and
outside the 3s range when compared to the corresponding bond lengths in complexes 2 (2.029(3)
Å) and 3 (2.030(2) Å; Table 1).15 This alteration may in part be due to a p interaction between the
pyridyl heterocycle and the phenyl group of the aniline ligand. In compensation for the relatively
short Ir–Ctrz bond, the Ir-Npyr bond (Ir-N4a 2.156(5) Å, Ir-N4b 2.174(6) Å) is stretched in both
enantiomers when compared with the structures of 1 and 3, and is similar to the bond length
observed for the iridium-aniline bond, Ir-N5 2.179(2) Å.
4

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a)
b)
c)
Ir
Ir
N5
Ir
N5
N4
C1
N4
N4
C1
C1
Figure 1. ORTEP representation (a) of the complex cations of 3 with the co-crystallized H2O
molecule and (b,c) of both configurations of complex 4; all ellipsoids at 50% probability level,
hydrogen atoms, and non-coordinating OTf– anions omitted for clarity.
Table 1. Selected bond lengths (Ǻ) and angles (º) for complexes 3 and 4.
1b
3
4
2.017(2)
2.129(2)
2.420
1.816
76.08
2.029(3)
2.140(2)
2.127(2)
1.810
1.990(6)/1.994(7)
2.156(5)/2.174(6)
2.179(2)
Ir-Ctrz
Ir-Npy
Ir-L
Ir-Cg a
Ctrz-Ir-Cpy
1.804
76.24(9)
77.2(2)/75.6(3)
a Cg refers to the centroid position of Cp*. b Data taken from ref. 4c.
Ligand exchange experiments. While in aqueous solution, complex 1 exists in an ionic
strength-dependent equilibrium with the corresponding solvento complex 2,10 no such solvolysis
was observed in MeCN and complex 1 remained unchanged. However, dissolving complex 2 in
MeCN led to rapid ligand exchange and formation of complex 5, which is stable and does not
transform back to the aqua complex even when dissolved in water. This behaviour highlights the
preferred binding of MeCN over H2O in these triazolylidene iridium(III) complexes.
Therefore, the Ir–L bond strength was qualitatively evaluated for NH3 and aniline. To this
end, the ammonia complex 3 was dissolved in acetone-d6 and exposed to an excess (20 mol
equiv.) of competitive ligands H2O, MeCN, and aniline, respectively (Scheme 2). Similarly, the
aniline complex 4 was exposed to H2O, NCMe, and NH3 (as NH4OH solution in H2O). NMR
monitoring of these mixtures indicated that complex 3 remained intact and no traces of the other
complexes were observed, irrespective of the added ligand. Likewise, complex 4 remained
unaltered in the presence of H2O or NCMe, however, addition of NH4OH induced immediate
displacement of the coordinated aniline for NH3, indicated by the diagnostic signal at dH = 4.10
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for the coordinated NH3 protons. Ammonia coordination was also visually noticed by the
instantaneous color change of the solution from dark to pale yellow upon addition of the ammonia
solution. According to these experiments, the Ir–L bond strength decreases in the sequence of L
= NH3 > aniline > MeCN > H2O.
Scheme 2. Evaluation of the coordination ability of neutral ancillary ligands to iridium.
Catalytic acceptorless oxidation and ether formation. Iridium complexes 1 and 2 have
shown excellent catalytic activity in oxidation reactions, which is imparted by the unique features
of mesoionic triazolylidene ligands,13,16 especially in combination with a chelating pyridyl
substituent.4c,17 For example, we recently reported the base-free etherification of benzyl alcohol
induced by an iridium complex containing a triazolylidene ligand with a cyclometalated pyridyl
fragment.8b Such etherification occurs through a dehydration pathway, while competitive
dehydrogenation results in the formation of benzaldehyde (Scheme 3).8a Mechanistic
investigations suggest that substrate coordination is turnover limiting. This step is obviously
dependent on the type of ancillary ligand, and modulation of this ligand therefore offers an
opportunity to tailor the catalytic activity. Based on these precedents, we tested the activity of
different iridium complexes in this reaction. Etherification of benzyl alcohol was used as standard
reaction, as the formation of the ether is slow and easily monitored.
Scheme 3. Benzyl alcohol oxidation catalyzed by iridium NHC complexes.
When benzyl alcohol was heated to 150 ºC in the presence of 2 mol% of complex 3 in
1,2-dichlorobenzene, formation of benzaldehyde and dibenzyl ether was observed (Figure 2;
Table S1). At early stages of the reaction, benzaldehyde is produced selectively, though after 30
min, minor quantities (6%) of the symmetric dibenzyl ether were detected. After 4 h, 82%
6

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conversion was noted, though the selectivity eroded considerably and the benzaldehyde vs
dibenzyl ether ratio was just about 2:1 (57% vs 25% spectroscopic yield). This ratio remained
essentially constant up to high conversions. Substrate turnover rates gradually slowed down and
97% conversion required up to 24 h.
Figure 2. Oxidation of benzyl alcohol catalyzed by iridium complex 3 () produces initially only
benzaldehyde (dehydrogenation), while prolonged reaction times lead to increasing formation of
dibenzyl ether (dehydration); this change in selectivity presumably arises from a gradual
modification of the catalytically active species over time (2 mol% catalyst loading, 150 °C, in
1,2-DCB; conversions determined by 1H NMR spectroscopic analysis in CDCl3 using
hexamethylbenzene as internal standard; note that the % given for the symmetric ether refers to
the % of alcohol converted to ether).
Both the catalytic activity as well as the etherification vs dehydrogenation selectivity were
strongly affected by the catalyst loading. When four parallel reactions were performed in the
presence of 1%, 2%, 3%, and 5% catalyst 3, respectively, dehydrogenation was favored for higher
catalyst loadings. For example, runs with 5 mol% catalyst displayed a 9:1 selectivity towards
benzaldehyde formation after 2 h, while an identical run with less catalyst loading showed a
gradual decrease to 6:1 (3 mol%), to 3:1 (2 mol%) to eventually provide only a 2:1 selectivity at
1 mol% loading (Figure 3). This trend is even more pronounced after longer reaction times when
almost full conversion was reached even with 1 mol% of complex 3 (lower than 1:1 ratio with
51% ether vs 42% aldehyde).
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Figure 3. BnOH oxidation catalyzed by iridium complex 3 at different catalyst loadings (2h (left)
and 20h (right), 150 ºC, 1,2-DCB). Conversions determined by 1H NMR spectroscopic analysis
in CDCl3 using hexamethylbenzene as internal standard. Note that the % given for the symmetric
ether refers to the % of alcohol converted in ether.
A few more conclusions can be extracted from these measurements. Firstly, the selectivity
towards benzaldehyde is very high at the beginning of the reaction, yet ether formation is
preferred at later stages (cf 2 and 20 h selectivity with 1 mol% loading). Therefore, ether formation
is dependent on the reaction time, and not on a specific stage of conversion. Secondly, dehydration
and dehydrogenation are two distinct catalytic cycles, with irreversible product formation. This
is best demonstrated by the consistent product ratio at 5 mol% catalyst loading after 2 h and 20 h.
Hence, the aldehyde is not an intermediate for the formation of the ether product. Based on these
considerations, we surmise that a catalytically active species is formed initially that is closely
related to complex 3, for example a fragile iridium hydride,18 which catalyzes the
dehydrogenation and formation of the aldehyde. A gradual transformation of this species is then
supposed to provide a different catalyst with a so far elusive structure that promotes the formation
of ether rather than dehydrogenation.
Further support for such a process was obtained from experiments aimed at testing the
stability of the catalytically active species derived from complex 3 upon repetitive additions of
substrate. Sequential addition of 20 equivalents of benzyl alcohol every 24 h revealed essentially
quantitative conversions for the first four batches, and a reduced 65% conversion for the fifth
batch, pointing towards some catalyst deactivation after a few days (Table 2). The continued
activity of the catalyst for about 100 h at 150 ºC indicates a high robustness of the active species.
In agreement with experiments with variable catalyst loading, the selectivity of the catalyst
changes upon prolonged activity. While the product mixture demonstrated a 4:1 selectivity
towards the benzaldehyde vs dibenzyl ether for the conversion of the first batch of benzaldehyde
(entry 1), the mixture is equal after the second run (entry 2), and then gradually changes to
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predominant ether formation (entries 3–5). When analysing the selectivity for each added batch
of substrate (Table S2), we note an inversed preference towards the dibenzyl ether for all
subsequent batches (1:4 aldehyde/ether ratio for second, third, and fourth batch).19 This preference
for dibenzyl ether formation is in agreement with the gradual transformation of the iridium species
from a dehydrogenation catalyst to a dehydration catalyst, corroborating the conclusions drawn
from modulation of catalyst loadings (cf Figure 3). This second catalyst is exceptionally stable
and remains active at 150 ºC for several days.
Table 2. Catalyst reuse and stability during consecutive additions of benzyl alcohol.a
Conversion
(%)b
Ratio
BnCHO/Bn2O
Entry
TON
1
2
3
4
5
0.4 mmol
+0.4 mmol
+0.4 mmol
+0.4 mmol
+0.4 mmol
100
100
100
99
82
50
39
34
37
18
50
61
65
54
20
40
60
79
92
4.5
1.0
0.6
0.7
0.7
91
a
Conditions: benzyl alcohol (41 µL, 0.4 mmol), complex 3 (16 mg, 0.02 mmol, 5 mol%), 1,2-
dichlorobenzene (2 mL), 150 ºC, sealed tube; 24 h interval before addition of the next portion of substrate.
b It refers to total conversion. Determined by 1H NMR spectroscopic analysis. Note that the % given for
the symmetric ether refers to the % of alcohol converted in ether.
The role of the ancillary ligand. We previously reported that in these triazolylidene
iridium complexes the ancillary ligand plays a pivotal role in imparting catalytic activity.9 While
the chloride complex 1 was inactive in silane-induced etherification of alcohols, the
corresponding cationic aqua complex 2 shows excellent activity. We were therefore interested to
evaluate the impact of various ancillary ligands on the catalytic activity and selectivity of the
triazolylidene iridium complex and have monitored the catalytic conversion of benzyl alcohol in
the presence of complexes 2, 3 and 4, as well as their deprotonated analogues 6, 7 and 8 (Figure
4).
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Figure 4. Monocationic complexes 6–8 obtained from deprotonation of complexes 2, 3 and 4,
respectively.
The results of this catalyst screening are compiled in Table 3. Inspection of these results
evidences that the different ancillary ligands affect the outcome of the catalysis significantly, both
in terms of activity and selectivity. Comparison of complexes 2–4 (entries 4–12) shows that the
aqua complex 2 is more active (78% conversion after 0.5 h) than the aniline complex 4 (62%
conversion), while the ammonia complex 3 is the slowest of this series (46% conversion). A
similar trend was observed when comparing the conversions after 6 h (97%, 91%, and 86% for 2,
4, and 3, respectively). In contrast, the parent chloride complex 1 is considerably less active and
reaches only 25% conversion after 0.5 h, and 70% after 24 h (entries 1–3). Likewise, solvent
changes were not productive. A reaction with complex 2 in acetone at 60 °C gave an appreciable
50% conversion after 24 h, while reactions in H2O at 100 °C led to only 7% conversion within
the same time span.
Most importantly, the three complexes with neutral ancillary ligands have not only
distinct activities, but also diverging selectivity. While the aqua complex 2 predominantly forms
the dibenzyl ether product (benzaldehyde/ether ratio is 0.4; entries 4,5), the ammonia complex 3
displays a marked preference for benzaldehyde formation (initial benzaldehyde/ether ratio is 6.7).
As discussed above, this ratio erodes with increasing reaction time and converges to about 2 at
higher conversions. The aniline complex 4 does not show any change of ratio throughout the
reaction and forms the ether and the aldehyde in equal amounts (entries 10–12).
Hence while the catalytic activity decreases along the series 2 (L = OH2) > 4 (PhNH2) >
3 (NH3), the selectivity towards dehydrogenation and benzaldehyde formation increases in the
same order. This sequence is consistent with the above observation that for complex 3, slower
conversions (as induced for example by lower catalyst loadings) enhances the preference towards
benzaldehyde formation. Moreover, the sequence correlates with the bond strength deduced from
ligand substitution experiments (see above). Independent of the underlying mechanism, these
results highlight the pivotal role of the ancillary ligand to control activity and selectivity of the
iridium center.
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Table 3. BnOH oxidation catalyzed by iridium complexes 2-4 and 6-8; influence of the ancillary
ligands.a
Conv.
(%)
Ratio
BnCHO/Bn2O
Entry
[Ir]
X or L
Time
1
2
1
2
Cl
OH2
NH3
0.5 h
6 h
--
3.7
3.2
0.4
0.3
n.d.
6.7
2.2
1.8
1.1
1.0
0.9
-
25
80
92
78
97
n.d.
46
86
25
63
70
22
24
n.d.
40
59
<1
17
22
56
73
n.d.
6
3
24h
0.5 h
6 h
4
5
6
24h
0.5 h
6 h
7
3
8
27
9
24h
0.5 h
6 h
97
62
62
33
35
29
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
4
NH2Ph
OH
91
98
16
22
24
45
47
16
22
24
46
51
<1
<1
<1
24h
0.5 h
6 h
6 b
6 c
7 c
8 c
-
24h
0.5 h
6 h
OH
50
71
86
38
71
45
59
67
38
58
5
9.0
4.9
3.5
--
12
19
<1
13
24h
0.5 h
6 h
NH2
4.5
24h
0.5 h
6 h
85
54
67
46
18
8
3.7
5.7
NHPh
82
70
12
5.8
24h
88
77
11
7.0
a Conditions: Iridium catalyst (0.008 mmol, 2 mol%), benzyl alcohol (40 µL, 0.4 mmol), 1,2-DCB (2 mL),
150 ºC, sealed tube. Note that the % given for the symmetric ether refers to the % of alcohol converted
in ether.
b Complex 6 (0.008 mmol) or complex 2 (0.008 mmol) + LiHMDS (0.008 mmol).
c Iridium catalyst 2, 3 or 4 (0.008 mmol) + KOtBu (0.008 mmol).
The activities of complexes 2–4 were compared to those of the analogous complexes 6–
8 containing the corresponding deprotonated ligands. Complex 6 with an ancillary hydroxyl
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ligand, was synthesized from the corresponding aqua derivative 2 in CH2Cl2 using 1 molequiv.
of lithium bis(trimethylsilyl)amide (LiHMDS), exploiting the relatively high aciditiy of the
iridium-coordinated water ligand (pKa = 8.3).10 The hydroxyl complex 6 was isolated as an
orange solid in moderate yields (51%). Compound 6 is highly hygroscopic, and probably for this
reason attempts to obtain microanalysis were unsuccessful. A catalytic test using complex 6 gave
results that were identical to those of runs in which complex 6 was generated in situ from complex
2 and 1 molequiv. LiHMDS (entries 13–15), and therefore, further catalytic experiments were
performed by deprotonating the neutral ligand in situ. Of note, however, deprotonation with
LHMDS afforded a catalytic species that was less active than when KOtBu was used as a base
(entries 16–18). The activity difference is significant, e.g. 50 vs 18% conversion after 0.5 h, and
almost quantitative formation after 24 h when KOtBu was used, while the catalyst generated with
LHMDS reached a modest 24% conversion. Independent of the deprotonation method, complex
6 is considerably less active than the analogous protonated aqua complex 2 (entries 4–6).
Remarkably, the selectivity was inverted, and while the neutral aqua complex produced
predominantly the ether product, the hydroxyl complex 6 greatly favored formation of
benzaldehyde, especially at early reaction stages (benzyldehyde/ether ratio is 9 after 0.5 h and at
50% conversion, entry 16).
The same deprotonation methodology was employed for generating complexes 7 and 8
in situ from 1 molequiv. KOtBu and complex 3 and 4, respectively. With these complexes, the
activity is lower when compared to the performance of their parent complexes 3 and 4 with neutral
ancillary ligands, though the difference is less pronounced than for the OH2/OH– pair 2 and 6.
With all complexes 6–8, final conversions did not exceed 90%, while all the parent complexes 2–
4 with neutral ligands converted benzyl alcohol in >97%. Like the hydroxyl complex 6,
complexes 7 and 8 produced predominantly benzaldehyde, with only small quantities of dibenzyl
ether formed throughout the entire reaction time (benzaldehyde/ether ratio is about 5). This
selectivity is more pronounced than that of the ammonia complex 3, and contrasts with the 1:1
unselective reaction of the aniline complex 4. These observations suggest that the presence of a
neutral ancillary ligand facilitates the etherification process while anionic ligands are more
selective towards the dehydrogenation pathway and the formation of benzaldehyde. The easy de-
coordination of neutral ancillary ligands may be key for the higher catalytic activity of complexes
2–4, while anionic ligands may provide easier access to a transient hydride as the critical species
for substrate dehydrogenation.
Decoordination was probed by exposing complex 2 to 20 molequiv. benzyl alcohol in an
NMR experiment (0.05 M acetone solution). The spectrum clearly revealed the disappearance of
the signal for coordinated water at dH = 2.79-3.37, suggesting a ligand substitution from H2O to
ROH. In contrast, the aniline complex 4 does not undergo any spectroscopic change when
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exposed to benzyl alcohol, arguably due to a stronger Ir–Naniline bond, which is in agreement with
the lower catalytic activity of this complex compared to the aqua complex 2.
Conclusions
The iridium complexes 3 and 4 with ammonia and aniline ancillary ligands are easily accessible
via chloride abstraction from the parent chloride complex 1. Competition experiments revealed
that the NH3 ligand is more strongly bound to iridium than acetonitrile, aniline or water and does
not undergo exchange with the former. Complex 3 was demonstrated to be an effective and robust
catalyst for both etherification and dehydrogenation of benzyl alcohol. The selectivity of the
catalysis is highly dependent on catalyst concentration, favoring dehydrogenation over
etherification at lower catalyst loadings and vice versa. The catalytic process is highly dependent
on the ancillary ligands in terms of rates, and selectivity. In general, the lability of the neutral
ligands (H2O > aniline > NH3) enhances catalytic activity and as a consequence, complexes with
neutral ancillary ligands are more efficient catalyst than those with a deprotonated monoanionic
ancillary ligand. While the complexes with anionic ancillary ligands are less active catalysts, they
show better selectivity towards dehydrogenation (aldehyde formation) as opposed to dehydration
(ether formation). The choice of ancillary ligand and catalyst concentration therefore allows to
rationally switch the selectivity of the reaction to either obtain the ether product or the aldehyde.
Such catalyst multi-tasking offers attractive opportunities for cascade transformations mediated
by one single catalyst.
Experimental
General. The metalation reaction was carried out under a nitrogen atmosphere using standard
Schlenk techniques and all the reagents and solvents were used as obtained from commercial
sources. Complexes 1 and 2 were synthesized as previously reported without modifications.4a
Microwave reactions were performed in a Biotage Initiator 2.0 reactor with high absorption.
Unless specified, NMR spectra were recorded at 25 ºC on Bruker spectrometers operating at 300
or 400 MHz (1H NMR) and 75 or 100 MHz (13C{1H} NMR) respectively. Chemical shifts (δ) are
expressed in ppm downfield from tetramethylsilane using the residual protonated solvent as an
internal standard. Assignments are based on homo- and hetero-nuclear shift correlation
spectroscopy. All the cationic complexes show a quartet around 120 ppm in the 13C NMR
spectrum due to the OTf counterion. Elemental analysis was performed on an Exeter Analytical
CE440 elemental analyzer, by the Microanalytical Laboratory at the University College Dublin,
Ireland.
Compound 3. Complex 1 (93 mg, 0.14 mmol) was dissolved in CH2Cl2 (10 mL) and AgOTf (38
mg, 0.15 mmol) and an ammonium hydroxide solution (15 M, 50µL, 0.75 mmol) and stirred for
2 hours. The mixture was filtered through Celite and the volume of the filtrate reduced to 1 mL.
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Upon addition of Et2O a precipitate formed, which was collected by filtration and dried in vacuo,
1
thus affording 3 as a pale yellow solid (88 mg, 79%). H NMR (CD2Cl2, 400 MHz): d = 8.88
(ddd, 5JHH = 0.8, 4JHH = 1.4, 3JHH = 5.7 Hz, 1H, Hpy), 7.90 (td, 4JHH = 1.4, 3JHH = 7.8 Hz, 1H, Hpy),
4
3
3
7.88 (ddd, 5JHH = 0.8, JHH = 1.4, JHH = 7.8 Hz, 1H, Hpy), 7.45 (ddd, 4JHH = 1.4, JHH = 5.7, 7.8
Hz, 1H, Hpy), 4.44, 4.30 (2 x s, 3H, NCH3), 3.61 (br s, 3H, NH3), 1.74 (s, 15H, CpCH3). 13C{1H}
NMR (CD2Cl2, 100 MHz): d = 153.8 (Cpy), 153.6 (Ctrz‒Ir), 149.4 (Cpy–Ctrz), 149.1 (Ctrz–Cpy),
140.1, 126.0, 121.6 (3 x Cpy–H), 90.8 (CCp), 40.0, 38.8 (2 x N–CH3), 8.6 (Cp–CH3). Anal. Calcd.
for C21H28F6IrN5O6S2: C: 30.88; H: 3.46; N: 8.57. Found: C: 30.48; H: 3.24; N: 8.21%.
Compound 4. Complex 1 (80 mg, 0.12 mmol) was dissolved in CH2Cl2 (10 mL) and AgOTf (33
mg, 0.13 mmol) was added, followed by aniline (12 µL, 0.13 mmol). The mixture was stirred for
16 h and filtered through Celite and the filtrate was concentrated to 1 mL. Upon addition of Et2O
a precipitate formed, which was isolated by filtration and dried in vacuo to yield complex 4 as a
1
3
dark yellow solid (65 mg, 62%). H NMR (CD2Cl2, 400 MHz): d = 9.10 (d, JHH = 5.8 Hz, 1H,
4
3
3
Hpy), 7.96 (td, JHH = 1.3, JHH = 7.8 Hz, 1H, Hpy), 7.69 (d, JHH = 7.8 Hz, 1H, Hpy), 7.50 (ddd,
4JHH = 1.3, 3JHH = 5.8, 7.8 Hz, 1H, Hpy), 6.96 (d, 2JHH = 10.3 Hz, 1H, NH2), 6.80 (t, 3JHH = 7.7 Hz,
1H, HPh), 6.72 (t, 3JHH = 7.7 Hz, 2H, HPh), 6.71 (d, 2JHH = 10.3 Hz, 1H, NH2), 6.19 (d, 3JHH = 7.7
Hz, 2H, HPh), 4.40, 4.19 (2 x s, 3H, N–CH3), 1.72 (s, 15H, Cp–CH3). 13C{1H} NMR (CD2Cl2, 100
MHz): d = 156.2 (Ctrz–Ir), 154.0 (Cpy–H), 149.2 (Ctrz–Cpy), 148.7 (Cpy‒Ctrz), 140.7 (Cpy–H), 140.2
(CPh–N), 128.0 (CPh–H), 126.8 (Cpy–H), 124.8 (CPh–H), 121.7 (Cpy–H), 120.0 (CPh–H), 91.8 (CCp),
40.4, 38.7 (2 x N–CH3), 8.7 (Cp–CH3). Anal. Calc. for C27H32F6IrN5O6S2: C: 36.32; H: 3.61; N:
7.84. Found: C: 36.17; H: 3.44; N: 7.64 %.
Ligand exchange experiments. Compound 2 was dissolved in CD3COCD3 (0.5 ml) in an NMR
tube and 20 mol equiv. of aniline, MeCN, or NH4OH (as a 25 wt. % in H2O) were added.
Compound 3 was dissolved in CD3COCD3 (0.5 ml) in an NMR tube and 20 mol equiv. of aniline,
H2O or MeCN were added. Compound 4 was dissolved in CD3COCD3 (0.5 ml) in an NMR tube
and 20 mol equiv. of H2O, MeCN, or NH4OH (as a 25 wt. % in H2O) were added. The samples
were analyzed by 1H NMR spectroscopy.
Typical procedure for catalytic runs. Benzyl alcohol (40 µL, 0.4 mmol) was added to a solution
of 1,2-dichlorobenzene (2 mL) containing the catalyst (8 µmol) and hexamethylbenzene (0.07
µmol) as internal standard. The vial was sealed and heated to 150 ºC. The reaction was monitored
by transferring aliquots (50 µL) of the reaction mixture to an NMR tube containing CDCl3 (0.5
mL) and subsequent 1H NMR spectroscopic analysis.
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General procedure for sequential additions of substrate. A mixture of complex 3 (16 mg, 0.02
mmol), 1,2-dichloromethane (2 mL) and benzyl alcohol (41 µL, 0.4 mmol) was heated for 20
hours at 150 ºC. An aliquot was taken for determining the conversion as detailed above, and a
fresh portion of benzyl alcohol was added. This procedure was repeated in intervals of 24 h.
Crystal structure determinations.
Crystal data for 3 and 4 were collected using a Rigaku (former Agilent Technologies) Oxford
Diffraction SuperNova A diffractometer fitted with an Atlas detector and using monochromated
Mo–Kα radiation (0.71073 Å). A complete dataset was collected, assuming that the Friedel pairs
are not equivalent. The structures were solved by direct methods using SHELXS-97 and refined
by full-matrix least squares fitting on F2 for all data using SHELXL-97.20 Hydrogen atoms were
added at calculated positions and refined by using a riding model. Anisotropic thermal
displacement parameters were used for all non-disordered nonhydrogen atoms. Further
crystallographic details are compiled in Tables S3 and S4. Crystallographic data (excluding
structure factors) for both complexes have been deposited with the Cambridge Crystallographic
Data Centre as supplementary publication no. CCDC 1583136 (3) and 1583137 (4).
ASSOCIATED CONTENT
Supporting Information: NMR spectra and crystallographic details of new complexes 3 and 4.
Catalytic results.
AUTHOR INFORMATION
Corresponding Author: *E-mail: martin.albrecht@dcb.unibe.ch
Notes: The authors declare no competing financial interest.
ACKNOWLEDGEMENTS
We acknowledge generous financial support from the European Research Council (CoG 615653)
and from the Swiss National Science Foundation (200021_162868, R'equip projects
206021_128724 and 206021_170755).
REFERENCES
15

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1. a) R. H. Crabtree, Chem. Rev. 2017, 117, 9228–9246; b) M. Nielsen, Hydrogen Production by
Homogeneous Catalysis: Alcohol Acceptorless Dehydrogenation. In Hydrogen Production and
Remediation of Carbon and Pollutants, eds. E. Lichtfouse, J. Schwarzbauer and D. Robert,
Springer, Heidelberg, 2015; c) Modern Oxidation Methods, ed. J.-E. Bäckvall, Wiley-VCH,
Weinheim, Germany, 2008; d) The Handbook of Homogeneous Hydrogenation, eds. J. G. De
Vries and C. J. Elsevier, Wiley-VCH, Weinheim, Germany, 2008.
2. For selected reviews on NHCs: a) D. Bourissou, O. Guerret, F. P. Gabbai and G. Bertrand, Chem.
Rev. 2000, 100, 39–92; b) A. J. Arduengo and G. Bertrand, Chem. Rev. 2009, 109, 3209–3210
(thematic issue); c) L. Mercs and M. Albrecht, Chem. Soc. Rev. 2010, 39, 1903–1912; d) M.
Melaimi, M. Soleilhavoup and G. Bertrand, Angew. Chem. Int. Ed. 2010, 49, 8810–8849; e) N-
Heterocyclic Carbenes: from Laboratory Curiosities to Efficient Synthetic Tools (2nd Edition), ed.
S. Díez-González, RSC Publishing, Cambridge, 2017; f) H. V. Huynh, The Organometallic
Chemistry of N-heterocyclic Carbenes, Wiley, Chichester, 2017.
3. a) W. A. Herrmann, Angew. Chem. Int. Ed. 2002, 41, 1290–1309; b) C. M. Crudden and D. P.
Allen, Coord. Chem. Rev. 2004, 248, 2247–2273; c) E. Peris and R. H. Crabtree, Coord. Chem.
Rev. 2004, 248, 2239–2246 (d) F. E. Hahn and M. C. Jahnke, Angew. Chem. Int. Ed. 2008, 47,
3122–3172; e) H. Jacobsen, A. Correa, A. Poater, C. Costabilie and L. Cavallo, Coord. Chem.
Rev. 2009, 253, 687–703; f) S. Díez-González, N. Marion and S. P. Nolan, Chem. Rev. 2009, 109,
3612–3676; g) E. Peris, Chem. Rev. 2017, in press (DOI: 10.1021/acs.chemrev.6b00695); h) N-
Heterocyclic Carbenes in Transition Metal Catalysis and Organocatalysis, ed. C. S. J. Cazin,
Springer, Berlin, 2011; i) N-Heterocyclic Carbenes in Transition Metal Catalysis, in Topics in
Organometallic Chemistry, ed. F. Glorius, Springer, Berlin, 2007.
4. a) D. G. H. Hetterscheid and J. N. H. Reek, Chem. Commun. 2011, 47, 2712-2714; b) R. H.
Crabtree, J. Organomet. Chem. 2014, 751, 174-180; c) J. A. Woods, R. Lalrempuia, A. Petronilho,
N. D. McDaniel, H. Müller-Bunz, M. Albrecht and S. Bernhard, Energy Environ. Sci. 2014, 7,
2316–2328; d) A. Petronilho, J. A. Woods, H. Müller-Bunz, S. Bernhard and M. Albrecht, Chem.
Eur. J. 2014, 20, 15775–15784; e) P. Daw, R. Petakamsetty, A. Sarbajna, S. Laha, R.
Ramapanicker and J. K. Bera, J. Am. Chem. Soc. 2014, 136, 13987-13990.
5. For selected examples see: a) A. Prades, E. Peris and M. Albrecht, Organometallics 2011, 30,
1162–1167; b) A. Bartoszewicz, R. Marcos, S. Sahoo, A. K. Inge, X. Zou and B. Martin-Matute,
Chem. Eur. J. 2012, 18, 14510–14519; c) J. Campos, L. S. Sharninghausen, M. G. Manas and R.
H. Crabtree, Inorg. Chem. 2015, 54, 5079–5084; d) V. Jimenez, J. Fernandez-Tornos, F. J.
Modrego, J. J. Perez-Torrente and L. A. Oro, Chem. Eur. J. 2015, 21, 17877–17889; e) H. Horvath,
G. Papp, R. Szabolcsi, A. Katho and F. Joo, ChemSusChem 2015, 8, 3036–3038.
6. a) D. Canseco-González and M. Albrecht, Dalton Trans. 2013, 42, 7424-7432; b) M. Delgado-
Rebollo, D. Canseco-Gonzalez, M. Hollering, H. Müller-Bunz and M. Albrecht, Dalton Trans.
2014, 43, 4462−4473; c) S. Hohloch, S. Hettmanczyk and B. Sarkar, Eur. J. Inorg. Chem. 2014,
3164–3171; d) A. Bolje, S. Hohloch, D. Urankar, A. Pevec, M. Gazvoda, B. Sarkar and J. Kosmrlj,
Organometallics 2014, 33, 2588–2598; e) K. F. Donnelly, C. Segarra, L.-X. Shao, R. Suen, H.
Müller-Bunz and M. Albrecht, Organometallics 2015, 34, 4076–4084; f) S. Sabater, H. Müller-
16

Page 17 of 18
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Bunz and M. Albrecht, Organometallics 2016, 35, 2256–2266; g) R. Pretorius, J. Olguín and M.
Albrecht, Inorg. Chem. 2017, 56, 12410–12420.
7. a) B. Bagh, A. M. McKinty, A. J. Lough and D. W. Stephan, Dalton Trans. 2014, 43, 12842–
12850; b) M. Valencia, A. Pereira, H. Müller-Bunz, T. R. Belderraín, P. Pérez and M. Albrecht,
Chem. Eur. J. 2017, 23, 8901–8911.
8. a) A. Prades, R. Corberán, M. Poyatos and E. Peris, Chem. Eur. J. 2008, 14, 11474–11479.b) M.
Valencia, H. Müller-Bunz, R. A. Gossage and M. Albrecht, Chem. Commun. J. 2016, 52, 3344–
3377.
9. A. Petronilho, A. Vivancos and M. Albrecht, Catal. Sci. Technol. 2017, in press
(10.1039/C7CY01832K).
10. A. Petronilho, A. Llobet and M. Albrecht, Inorg. Chem. 2014, 53, 12896–12901.
11. R. P. Hughes, J. M. Smith, C. D. Incarvito, K.-C. Lam, B. Rhatigan, A. L. Rheingold,
Organometallics 2002, 21, 2136–2144.
12. M. P. Betore, P. Garcia-Orduna, F. J. Lahoz, M. A. Casado, V. Polo and L. A. Oro, Eur. J. Inorg.
Chem. 2016, 5347–5355; The resonance is also at lower field than iridium(III) phosphine ammonia
complexes, see for example: M. A. Salomon, A.-K. Jungton and T Braun, Dalton Trans. 2009,
7669–7677.
13. K. F. Donnelly, A. Petronilho and M. Albrecht, Chem. Commun. 2013, 49, 1145–1159.
14. V. Leigh, W. Ghattas, R. Lalrempuia, H. Müller-Bunz, M. T. Pryce and M. Albrecht, M. Inorg.
Chem. 2013, 52, 5395–5402.
15. (Ir-C1+ 3s = 2.008 and Ir-C1b + 3s = 2.015).
16. (a) P. Mathew, A. Neels and M. Albrecht, J. Am. Chem. Soc. 2008, 130, 13534-13535; b) G.
Guisado-Barrios, J. Bouff ard, B. Donnadieu and G. Bertrand, Angew. Chem. Int. Ed. 2010, 49,
4759-4762; c) J. D. Crowley, A.-L. Lee and K. J. Kilpin, Aust. J. Chem. 2011, 64, 1118-1132.
17. (a) R. Lalrempuia, N. D. McDaniel, H. Müller-Bunz, S. Bernhard and M. Albrecht, Angew. Chem.
Int. Ed. 2010, 49, 9765-9768; b) I. Corbucci, A. Petronilho, H. Müller-Bunz, L. Rocchigiani, M.
Albrecht and A. Macchioni, ACS Catal. 2015, 5, 2714-2718.
18. (a) K. Riener, M. P. Höger, P. Gigler, F. E. Kühn, ACS Catal. 2012, 2, 613-621; b) P. Gigler, B.
Bechlars, W. A. Herrmann, F. E. Kühn, J. Am. Chem. Soc. 2011, 133, 1589-1596.
19. The selectivity of the fifth batch is less clear due to the substantial catalyst deactivation.
20. G. M. Sheldrick, Acta Crystallogr. Sect. A, 2008, 64, 112.
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82x47mm (300 x 300 DPI)