RuII, IrIII and OsII mesoionic carbene complexes: Efficient catalysts for transfer hydrogenation of selected functionalities

Article abstract of DOI:10.1039/c6dt01324d

Pyridine-appended triazolylidene donors have been recently used as ligands in various homogeneous catalytic processes. We present here a new pyrimidine substituted triazolium salt which was prepared and used in the coordination to Ru^II and Os^II. The new triazolium salt and the obtained complexes were characterized by multinuclear NMR spectroscopy. The molecular composition of the mentioned compounds was confirmed by positive ion electrospray ionization (ESI+) mass spectra. The new pyrimidyl containing complexes, as well as the related pyridyl-triazolylidene containing complexes, were applied in transfer hydrogenation reactions of carbonyls, alkenes, imines and nitroarenes. The pyrimidyl containing complexes reveal an over-all better activity in comparison to their pyridine bearing analogues. The studies of electronic effects of the ligands, as well as mechanistic insights for the reduction of nitrobenzene with three selected precatalysts are presented.

Full text of DOI:10.1039/c6dt01324d

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RuII, IrIII and OsII mesoionic carbene complexes:
efficient catalysts for transfer hydrogenation of
selected functionalities†‡
Cite this: DOI: 10.1039/c6dt01324d
Aljoša Bolje,a Stephan Hohloch,b Janez Košmrlj*a and Biprajit Sarkar*b
Pyridine-appended triazolylidene donors have been recently used as ligands in various homogeneous
catalytic processes. We present here a new pyrimidine substituted triazolium salt which was prepared and
used in the coordination to RuII and OsII. The new triazolium salt and the obtained complexes were
characterized by multinuclear NMR spectroscopy. The molecular composition of the mentioned com-
pounds was confirmed by positive ion electrospray ionization (ESI+) mass spectra. The new pyrimidyl
containing complexes, as well as the related pyridyl-triazolylidene containing complexes, were applied in
transfer hydrogenation reactions of carbonyls, alkenes, imines and nitroarenes. The pyrimidyl containing
complexes reveal an over-all better activity in comparison to their pyridine bearing analogues. The studies
of electronic effects of the ligands, as well as mechanistic insights for the reduction of nitrobenzene with
three selected precatalysts are presented.
Received 6th April 2016,
Accepted 7th July 2016
DOI: 10.1039/c6dt01324d
www.rsc.org/dalton
alternatives to widely used phosphines in many catalytic
processes. Today, a vast number of transformations are known
Introduction
The transfer hydrogenation reaction promoted by metal- to proceed only with NHC-based catalysts.2 1,2,3-Triazol-5-
complexes is currently one of the most commonly investigated ylidenes are a prominent subclass of NHCs possessing a
homogeneous hydrogenation processes.1 This reaction is mesoionic carbene (MIC) structure. Due to a lower heteroatom
highly appealing for several reasons. It does not require hazar- stabilization and mesoionic character, MICs are among one of
dous gaseous hydrogen and pressurized reactors, and it the best σ-donors among the NHCs.3 The highly efficient and
employs cheap and sustainable solvents as the source of hydro- modular nature of click reaction that is generally used to access
gen, to name just a few. The wide range of functional groups 1,2,3-triazol-5-ylidenes makes the design and preparation of
that can be reduced by transfer hydrogenation is noteworthy. diverse libraries of these ligands extremely easy.4 Complexes
These include carbonyl, olefin, imine, nitrile, and nitro com- bearing triazolylidenes are increasingly used in homogeneous
pounds. The need to develop simple and sustainable reaction metal-based catalysis, as well as for photophysical and bio-
conditions that require only low catalyst loadings is currently chemical purposes.4d Many catalytic systems of this type have
emerging in developing new catalysts and catalytic systems.
been studied, showing comparable or even better efficiency in
In the last two decades N-heterocyclic carbenes (NHCs) comparison to the majority of their NHC analogues.5 Metal
have been developed as a powerful class of ligands. Despite complexes of MIC ligands have been used as catalysts among
being used in nearly every field of modern chemical sciences, others in olefin metathesis,6 oxidations,7 reductions,8 click reac-
without a doubt NHCs have most dramatically affected organo- tions,9 and C–C/C–N bond formation reactions.10
metallic chemistry and catalysis. Owing to their strong σ-
Recently, three series of complexes consisting of pyridine
donation ability, air and moisture stability, high structural appended triazolylidenes as ligands with ruthenium(II),
variability, as well as low toxicity, they have become viable iridium(III) and osmium(II) were reported and they were suc-
cessfully used in transfer hydrogenation of selected carbonyl
substrates (Scheme 1).11 All complexes have shown remarkable
activity even at very low catalyst loadings with several different
substrates investigated. The ruthenium(II) complexes were also
used in oxidation of primary and secondary alcohols.7d
Encouraged by these results, we were prompted to test the
activity of these complexes also for transfer hydrogenation of
substrates bearing other functional groups. In particular, we
aFaculty of Chemistry and Chemical Technology, University of Ljubljana, Večna pot
113, SI-1000 Ljubljana, Slovenia
bInstitut für Chemie und Biochemie, Anorganische Chemie, Freie Universität Berlin,
Fabeckstraße 34-36, D-14195 Berlin, Germany. E-mail: biprajit.sarkar@fu-berlin.de
†Dedicated to Prof. Bernhard Lippert on the occasion of his 70th birthday.
‡Electronic supplementary information (ESI) available. See DOI:
10.1039/c6dt01324d
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Scheme 2 The synthesis of triazolium salt 2.
tive route to generate only NTriazole methylated pyridyl-tri-
azolium salts.13 Some of these salts have already been successfully
used to generate ruthenium(II), osmium(II) and iridium(III)
complexes.7d,11 Inspired by these reports, we designed and pre-
pared an analogous derivative, that has a pyrimidine ring
attached directly to the triazole nitrogen atom N1 (Scheme 2).
The starting triazole 1 was prepared by copper-catalyzed cyclo-
addition of phenylacetylene with 2-azidopyrimidine (tetrazolo
[1,5-a]pyrimidine) as reported previously.14 The methylation
into 2 was achieved by mixing the triazole with MeOTf in dry
dichloromethane at 0 °C for 24 h (Scheme 2). Triazolium salt 2
was isolated as a triflate salt in 91% yield. In analogy to
1-(2-pyridyl)-1,2,3-triazoles,13 1-(2-pyrimidyl)-1,2,3-triazole
1
could be selectively alkylated with MeOTf at triazole nitrogen
N3 with no need of pyrimidine ring protection.
Next, the coordination ability of triazolium salt 2 towards
Ru(II), Ir(III) and Os(II) was examined. To this end, we chose the
already established transmetalation protocol via the intermedi-
ate carbenic silver(I) complex, shown in Scheme 3.15
Scheme 1 The RuII, OsII and IrIII pyridine-appended MIC complexes
(up),11 and RuII and OsII pyrimidine-appended MIC complexes (bottom)
used in this work.
The triazolium salt 2 was stirred with Ag2O and in dry aceto-
nitrile at room temperature for 4 days (Scheme 3). The for-
mation of complex Ag-2 was suggested by the disappearance of
were interested in different carbonyl, olefin, imine, and
nitroarene substrates.
1
the triazolium proton H-5 from the H NMR spectral analysis
In addition to the pyridine-appended MICs, we decided to
design novel MICs that are functionalized with a pyrimidine
ring. The starting 1-(2-pyrimidyl)-functionalized triazolium
salt was prepared in excellent yields and purity and used to
construct the corresponding Ru(II) and Os(II) complexes
(Scheme 1). The coordination was achieved by a transmetalla-
tion protocol and the corresponding complexes were character-
ized by 1H, 13C and 15N NMR spectroscopy and high resolution
mass spectrometry. Unfortunately, the preparation of the Ir(III)
analogue failed. The two pyrimidine-appended MIC complexes
of Ru(II) and Os(II) and the previously reported7d,11 pyridine-
appended MICs of Ru(II), Ir(III), and Os(II), shown in Scheme 1,
were used in transfer hydrogenation of the above mentioned
functionalities. This is also the first report on pyrimidine-
appended triazolium salts, the corresponding MICs and their
transition metal complexes. Some mechanistic insights into
the reduction of nitrobenzene with the three selected
complexes are presented.
in DMSO-d6. Due to the general instability of such silver com-
plexes, we were unable to obtain more experimental data for
its structure. Subsequently, Ag-2 was subjected to transmetala-
tion without previous isolation employing [Ru(p-cym)Cl2]2 as a
ruthenium source (Scheme 3). The reaction progress was
monitored by TLC analysis. The pure complex Rupyrim was iso-
lated as a triflate salt after 2 hours by simple three-step fil-
tration–solvent evaporation–crystallization workup in 85%
yield (Table 1). For the preparation of Ospyrim, which involved
transmetalation with the OsII precursor [Os(p-cym)Cl2]2 the
reaction mixture had to be stirred for a prolonged time of
3 days. The complex Ospyrim was isolated in the pure form as a
PF6 salt in a moderate yield of 53% (Table 1). Unfortunately,
we were unable to prepare the IrIII analogue. After the addition
of [IrCp*Cl2]2 to the reaction mixture of the triazolium salt 2
Results and discussion
Ligand synthesis and coordination
In contrast to earlier reports of unselective methylation of
pyridyl-triazoles,12 some of us have recently reported a selec- Scheme 3 The ruthenation of 2 via transmetalation.
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Table 1 The synthesis of Rupyrim and Ospyrim complexes
Table 2 The transfer hydrogenation of benzaldehydea
Conv.b
(%)
Conv.b
(%)
Conv.b
(%)
Precatalyst
Precatalyst
Precatalyst
Precatalyst loading: 0.5 mol%
c
c
c
Ru1 c
>99
>99
>99
>99
Ir1 c
>99
>99
>99
Os1 c
>99
>99
>99
>99
Ru2 c
Ru3
Ir2 c
Ir3
Os2 c
Os3
Ospyrim
Entry
Time 1 (days)
Time 2 (h)
Product
Yielda (%)
Rupyrim
1
2
4
4
2
72
Rupyrim
Ospyrim
85
53
Precatalyst loading: 0.01 mol%
c
c
c
Ru1 c
>99
>99
>99
>99
Ir1 c
>99
>99
>99
Os1 c
>99
>99
>99
>99
Ru2 c
Ru3
Ir2 c
Ir3
Os2 c
Os3
Ospyrim
a Isolated pure product.
Rupyrim
a General reaction conditions: benzaldehyde (0.5 mmol), precatalyst,
KOH (0.085 mmol), iPrOH (4 mL), 100 °C for 3 h. b Conversion deter-
mined by 1H NMR spectroscopy using hexamethylbenzene as an
internal standard. c Data collected from ref. 11.
and Ag2O in acetonitrile, no pure product could be isolated.
Instead a mixture of products was isolated. The use of a base
such as Cs2CO3 or heating of the reaction mixture did not lead
to any improvement. Even though we could identify the
product in the mass spectrum of the crude reaction mixture
(see the ESI‡), attempts to purify it led to its decomposition.
Although it is not clear at this stage as to why the product
decomposes at the purification stage, we think that it does not
survive either the filtration through Celite, or the salt meta-
thesis with KPF6 that is necessary for its purification.
Table 3 The reduction of acetophenonea
Transfer hydrogenation catalysis
Conv.b
(%)
Conv.b
(%)
Conv.b
(%)
Precatalyst
Precatalyst
Precatalyst
After having the two complexes Rupyrim and Ospyrim prepared,
we examined their activity in transfer hydrogenation catalysis.
In this context we also tested some RuII, IrIII and OsII
complexes from our previous reports, which were already used
for carbonyl group reduction.11 As the model reaction we
examined the reduction of benzaldehyde to benzyl alcohol.
After testing at room temperature, 60 °C and 80 °C, we
observed that the best conversions are obtained at 100 °C.
Hence, all transfer hydrogenation reactions (other than
nitrobenzene) were carried out at 100 °C. The results for
benzaldehyde are presented in Table 2.
Precatalyst loading: 0.5 mol%
c
c
c
Ru1 c
>99
>99
>99
>99
Ir1 c
88
71
92
Os1 c
76
88
65
70
Ru2 c
Ir2 c
Ir3
Os2 c
Ru3
Os3
Rupyrim
Ospyrim
Precatalyst loading: 0.01 mol%
c
Ru1 c
Ru2 c
Ru3
80
89
60
94
Rupyrim
a General reaction conditions: acetophenone (0.5 mmol), precatalyst,
KOH (0.085 mmol), iPrOH (4 mL), 100 °C for 3 h. b Conversion deter-
mined by 1H NMR spectroscopy using hexamethylbenzene as an
internal standard. c Data collected from ref. 11.
The reduction of benzaldehyde under the applied reaction
conditions proceeds smoothly with all tested complexes.
Applying 0.5 mol% of the precatalysts, as well as with only
0.01 mol%, the reduction occurred with full conversion of the
starting material, reflecting in a TON of about 10 000. This is has shown the best activity amongst all tested precatalysts.
comparable to known ruthenium(II)–NHC complexes16 and in With 0.5 mol% of the Ru2 precatalyst, we were able to isolate
line with the activity of their pyridyl-MIC analogues.11 To inves- the product in 85% yield (conversion >99%). Prompted by very
tigate further the catalytic performance of the Rupyrim and good overall results with acetophenone we decided to continue
Ospyrim, acetophenone was used as a substrate. Ketones in with a more sterically hindered substrate, benzophenone. The
general are known to be more challenging substrates than results are collected in Table 4.
aldehydes for the reduction. The results are outlined in
Table 3.
Benzophenone was fully converted into diphenylmethanol
with the Rupyrim precatalyst at 0.5 mol%. The same complex
Whereas the reduction of acetophenone with the ruthe- gave an excellent 93% conversion also at 0.1 mol% precatalyst
nium complex Rupyrim at the precatalyst loadings of 0.5 mol% loading, which is the best performance for all tested com-
gave full conversion of acetophenone to 1-phenylethanol, the pounds. With 0.1 mol% of the Ru1 precatalyst, we were able to
osmium complex Ospyrim gave only 70% of the reduced sub- isolate the product in 60% yield (conversion 69%, Table 4). On
strate. By using 0.01 mol%, the pyrimidine analogue Rupyrim the other hand Ospyrim gave only 72% conversion by employing
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Table 4 The reduction of benzophenonea
Table 6 The reduction of cyclooctenea
Conv.b
(%)
Conv.b
(%)
Conv.b
(%)
Conv.b
(%)
Conv.b
(%)
Conv.b
(%)
Precatalyst
Precatalyst
Precatalyst
Precatalyst
Precatalyst
Precatalyst
Precatalyst loading: 1.0 mol%
Precatalyst loading: 0.5 mol%
Ru1
Ru2
Ru3
Rupyrim
>99
>99
>99
>99
Ir1
Ir2
Ir3
73
60
80
Os1
Os2
Os3
Ospyrim
84
93
59
89
c
c
c
Ru1 c
>99
>99
>99
>99
Ir1 c
65
55
85
Os1 c
68
88
25
72
Ru2 c
Ir2 c
Ir3
Os2 c
Os3
Ospyrim
Ru3
Rupyrim
Precatalyst loading: 0.5 mol%
Ru1
Ru2
Ru3
Precatalyst loading: 0.1 mol%
79
94
72
87
c
Ru1 c
Ru2 c
Ru3
69
90
59
93
Rupyrim
Rupyrim
a General reaction conditions: cyclooctene (0.5 mmol), precatalyst,
KOH (0.085 mmol), iPrOH (4 mL), 100 °C for 24 h. b Conversion deter-
mined by 1H NMR spectroscopy using hexamethylbenzene as an
internal standard.
a General reaction conditions: benzophenone (0.5 mmol), precatalyst,
KOH (0.085 mmol), iPrOH (4 mL), 100 °C for 3 h. b Conversion deter-
mined by 1H NMR spectroscopy using hexamethylbenzene as an
internal standard. c Data collected from ref. 11.
of the carbonyl group prompted us to test other functional
0.5 mol% of the precatalyst. This result is in line with other groups. We started with olefin substrates: cyclooctene, methyl-
Os(II) complexes Os1–3. To finish, we conducted the reduction styrene and trans-stilbene. The results for the reduction of
of cyclohexanone as a representative of an aliphatic substrate. cyclooctene are presented in Table 6.
The results are summarized in Table 5.
For the reduction of cyclooctene, we used similar reaction
The two pyrimidyl-MIC complexes Rupyrim and Ospyrim have conditions as mentioned above. A mixture of cyclooctene, the
shown the best activity for the transfer hydrogenation of cyclo- selected precatalyst and KOH in iPrOH was stirred under an
hexanone into cyclohexanol. Employing either 0.5 mol% or inert atmosphere at 100 °C for 24 h. By using 1.0 mol% of
0.1 mol% of the precatalyst resulted in an excellent overall ruthenium-based precatalysts we observed full conversions
activity. The Rupyrim gave a very good 78% conversion also by into cyclooctane with all four tested complexes. Also with the
using only 0.05 mol% of the precatalyst. Good results of the iridium and osmium analogues, the conversions were very
complexes Rupyrim and Ospyrim in the transfer hydrogenation good. Scaling down to 0.5 mol% of Ru(II) complexes, we could
still reach excellent conversions, better than those described
for already known similar systems.17 The best performance
was seen with Ru2, bearing the 4-methoxyphenyl donor group
at the triazole, which is in line with the results for the
Table 5 The reduction of cyclohexanonea
reduction of carbonyls. Excellent performance in the reduction
of cyclooctene encouraged us to try other such substrates. We
took trans-β-methylstyrene and applied the same reaction
conditions as for cyclooctene. The results are summarized in
Table 7.
Conv.c
(%)
Conv.c
(%)
Conv.c
(%)
In comparison to cyclooctene, trans-β-methylstyrene proved
to be a more challenging substrate. With the exception of the
Os(II) representatives, the complexes still performed very well.
Nonetheless, our catalysts have shown similar or better per-
formance in this reduction than the previously reported
examples.17 The ligand effect trends were similar to the pre-
vious observations. For the Ru(II) complexes the electron-
donating substituents at the triazole ring worked better,
whereas the Ir(III) complexes preferred electron withdrawing
groups.11 Furthermore, we investigated the transfer hydrogen-
ation of trans-stilbene, a sterically more hindered alkene. The
results from Table 8 confirm the reduction of trans-stilbene
into 1,2-diphenylethane as more demanding for our catalytic
system, which has been also reported for other such systems.17
Precatalyst
Precatalyst
Precatalyst
Precatalyst loading: 0.5 mol%
d
d
d
Ru1 d
>99
>99
>99
>99
Ir1 d
Ir2 d
Ir3
86
75
92
Os1 d
>99
>99
91
Ru2 d
Ru3
Os2 d
Os3
Rupyrim
Ospyrim
>99
Precatalyst loading: 0.1 mol%
d
d
Ru1 d
Ru2 d
Ru3
79
94
Os1 d
62
76
50
94
Os2 d
Os3
72
Rupyrim
>99, 78b
Ospyrim
a General reaction conditions: cyclohexanone (0.5 mmol), precatalyst,
KOH (0.085 mmol), iPrOH (4 mL), 100 °C for 3 h. b Precatalyst
(0.05 mol%). c Conversion determined by 1H NMR spectroscopy using
hexamethylbenzene as an internal standard. d Data collected from
ref. 11.
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Table 7 The transfer hydrogenation of trans-β-methylstyrenea
Table 9 The transfer hydrogenation of N-benzylideneanilinea
Conv.b
(%)
Conv.b
(%)
Conv.b
(%)
Conv.b
(%)
Conv.b
(%)
Conv.b
(%)
Precatalyst
Precatalyst
Precatalyst
Precatalyst
Precatalyst
Precatalyst
Precatalyst loading: 1.0 mol%
Precatalyst loading: 1.0 mol%
Ru1
65
75
56
78
Ir1
Ir2
Ir3
62
58
67
Os1
Os2
Os3
Ospyrim
8
6
4
0
Ru1
>99
>99
>99
>99
Ir1
Ir2
Ir3
50
23
17
Os1
Os2
Os3
Ospyrim
19
18
17
30
Ru2
Ru2
Ru3
Ru3
Rupyrim
Rupyrim
Precatalyst loading: 0.5 mol%
Precatalyst loading: 0.5 mol%
Ru1
Ir1
Os1
45
25
4
Ru1
Ru2
Ru3
Rupyrim
71
87
60
90
a General reaction conditions: trans-β-methylstyrene (0.5 mmol), pre-
catalyst, KOH (0.085 mmol), iPrOH (4 mL), 100 °C for 24 h. a General reaction conditions: N-benzylideneaniline (0.5 mmol), pre-
b Conversion determined by 1H NMR spectroscopy using 4-bromoben- catalyst, K2CO3 (0.025 mmol), iPrOH (4 mL), 100 °C for 18 h.
zaldehyde as an internal standard.
b Conversion determined by 1H NMR spectroscopy using hexamethyl-
benzene as an internal standard.
Table 8 The transfer hydrogenation of trans-stilbenea
Table 10 Cyclohexenone reductiona
Conv.b
(%)
Conv.b
(%)
Conv.b
(%)
Precatalyst
Precatalyst
Precatalyst
Conv.b
(%)
Conv.b
(%)
Conv.b
(%)
Precatalyst loading: 1.0 mol%
Precatalyst
Precatalyst
Precatalyst
Ru1
Ru2
Ru3
Rupyrim
30
41
28
13
Ir1
Ir2
Ir3
9
11
13
Os1
Os2
Os3
Ospyrim
15
16
4
Ru1
Ru2
Ru3
Rupyrim
0/100
8/92
0/100
0/100
Ir1
Ir2
Ir3
21/79
31/69
42/58
Os1
Os2
Os3
Ospyrim
67/33
83/17
64/36
30/70
8
a General reaction conditions: trans-stilbene (0.5 mmol), precatalyst,
KOH (0.085 mmol), iPrOH (4 mL), 100 °C for 24 h. b Conversion deter-
mined by 1H NMR spectroscopy using hexamethylbenzene as an
internal standard.
a Reaction conditions: cyclohexenone (0.5 mmol), catalyst (0.5 mol%),
KOH (0.1 mmol), isopropanol (4 mL), 100 °C, 20 h reflux. b Conversion
determined by 1H NMR spectroscopy using hexamethylbenzene as an
internal standard; >99% conversion of starting meterials: ketone/alco-
hol product ratio.
To continue we took an imine substrate and selected
N-benzylideneaniline.18 We employed both K2CO3 as well as displayed relatively high selectivity towards selective reduction
KOH as bases and stirred the reaction mixture for 18 h at of the CvC bond over the CvO bond.
100 °C. Both bases delivered similar conversions. The results
are collected in Table 9.
To conclude, we tested the transfer hydrogenation of the
nitro group. Recently, an efficient transfer hydrogenation cata-
With 1.0 mol% precatalyst loading the ruthenium-based lytic system for the reduction of nitroarenes into anilines was
precatalysts converted the N-benzylideneaniline into N-benzyl- reported and was based on Ru(II) and Ir(III) MIC precatalysts.8a
aniline fully. Also by using 0.5 mol% the same complexes gave We carried out an initial screening with nitrobenzene as a sub-
an excellent overall conversion. The Rupyrim performed best, strate, applying 1.5 mol% of the precatalysts and 0.5 equiv. of
but also the Ru2 showed excellent activity, confirming the KOH in iPrOH at 80 °C.8a Azobenzene and azoxybenzene were
ligand effects for the Ru(II) complex series. On the other hand expected as the side products. The results are presented in
the Ir(III) and Os(II) precatalysts were less active for the Table 11. The screening showed a catalyst dependent conver-
reduction of this type of substrates. The activity of our com- sion, as well as product distribution. In contrast to the pre-
plexes is comparable to the previously reported ruthenium viously tested functional groups, the Ir(III) precatalysts were the
catalyst in the transfer hydrogenation of N-benzylidene- most active of all the tested complexes, affording aniline as the
aniline.18 We also tested the reduction of cyclohexenone as an major product. The Ru(II) complexes gave good conversion into
example of an α,β-unsaturated carbonyl. Whereas conversions aniline and azoxybenzene with only minute amounts of azo-
were high for all tested precatalysts (Table 10), only Os2 benzene. Once again the pyrimidine analogue Rupyrim showed
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Table 11 The transfer hydrogenation of nitrobenzenea
Precatalyst
Anilineb
Starting nitrob
Azob
Azoxyb
Precatalyst loading: 1.5 mol%
Ru1
Ru2
Ru3
Rupyrim
Ir1
50
56
32
71
93
85
>99
50
83
50
80
10
26
40
0
0
0
0
31
0
35
0
10
4
0
12
7
11
0
0
3
0
3
30
14
28
17
0
4
0
19
14
15
17
Ir2
Ir3
Os1
Os2
Os3
Ospyrim
Precatalyst loading: 5.0 mol%
Ru1
Ru2
Ru3
Rupyrim
Ir1
>99
>99
>99
>99
97
0
0
0
0
0
0
0
36
10
40
0
0
0
0
0
3
7
0
0
0
0
0
0
0
0
0
0
0
0
0
3
0
0
Ir2
93
Ir3
Os1
>99
64
Os2
Os3
87
60
Ospyrim
>99
a General reaction conditions: nitrobenzene (0.5 mmol), precatalyst,
KOH (0.21 mmol), iPrOH (4 mL), 80 °C for 24 h. b Conversion in %
determined by GC using hexadecane as an internal standard.
Scheme 4 The postulated mechanism for the transfer hydrogenation
of nitrobenzene.8a,19
superior activity. The Os(II) complexes performed similar to
the Ru(II) analogues, giving slightly higher conversions. The
best performance was seen for Os2, where aniline was the
main product.
chosen precatalysts, we carried out the reduction of nitro-
benzene under catalytic conditions with 1.5 mol% loading and
analysed aliquots over time. The resulting data are represented
in Fig. 1. With ruthenium complex Ru1 54% conversion into
aniline was observed after 32 h at 80 °C. While the amount of
nitrobenzene decreased over time, the amounts of azoxyben-
zene and aniline increased. The formation of aniline, as well
as azoxybenzene correlated well with the consumption of the
starting nitrobenzene. Only minor amounts of azobenzene were
traced after 32 h. The results point to pathway B. The iridium
complex Ir3 gave 84% conversion to aniline after 24 h with the
As was previously observed,8a when employing 5.0 mol% of
the precatalysts, we observed a drastic improvement in cata-
lytic activity. The best results were seen with all four Ru(II)
complexes, giving full conversion of nitrobenzene into aniline.
The Ir(III) analogues performed very similarly. On the other
hand, the Os(II) complexes were less active, except for Ospyrim
,
which yielded aniline quantitatively.
Mechanistic studies
Recently, a mechanistic study for a similar catalytic system was amount of azoxybenzene less than 18%. The consumption of
reported8a where a two-pathway mechanism was postulated nitrobenzene correlates very well with the formation of aniline.
based on two previous literature reports.19 The mechanism is These results indicate more preference for path A. With Os2
outlined in Scheme 4. The metal complex is converted into 74% conversion into aniline was observed with no azobenzene
hydrides “[M]H2” in basic iPrOH. Next, nitrosobenzene is as a side product. The amount of aniline correlates very well
formed, which can be reduced into aniline through two with the consumption of nitrobenzene. The conversion into
different pathways. Route A proceeds through N-phenylhydroxy- azoxybenzene was below 8% and quite constant after 24 h. This
amine giving aniline directly. In contrast, path B involves an indicates the preference for pathway A.
azoxybenzene intermediate, which gets converted to azo-
For the transfer hydrogenation of the nitro functional
benzene before being reduced to aniline. To determine the group we chose four additional substrates: 2-, 3- and 4-nitro-
reaction pathways for each of the complex families, we ran a anisole, as well as 1-chloro-2-nitrobenzene. We selected Os2 and
time-dependent screening of the product formation. With the Ir3 as the precatalysts and conducted the transfer hydrogen-
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Table 12 The transfer hydrogenation reduction of four selected nitro-
arenesa
Precatalyst
Aminob
Nitrob
Azob
Azoxyb
2-Nitroanisole
Ir3
Os2
65
54
35
46
0
0
0
0
3-Nitroanisole
Ir3
Os2
100
75
0
25
0
0
0
0
4-Nitroanisole
Ir3
Os2
54
18
46
82
0
0
0
0
1-Chloro-2-nitrobenzene
Ir3
100
58
0
42
0
0
0
0
Os2
a General reaction conditions: nitro substrate (0.5 mmol), precatalyst
(1.5 mol%), KOH (0.21 mmol), iPrOH (4 mL), 80 °C for 24 h.
b Conversion in % determined by GC using hexadecane as an internal
standard.
were detected. For the reduction of cyclohexene and for the
reduction of nitrobenzene, we performed the Hg-poisoning
test by adding Hg during the catalysis. The addition of Hg had
no effect on the yield or the rate of formation of the catalytic
products. Hence, we believe that catalysis in the present case is
likely homogeneous.
Conclusions
The scope of the pyridine-functionalized triazolylidene com-
plexes of ruthenium, iridium and osmium as precatalysts for
transfer hydrogenation was investigated. We have also pre-
sented the synthesis of the 1-(2-pyrimidyl)-functionalized tri-
azolium salt and used it as a precursor for the carbenic ligand
for coordination to ruthenium(II) and osmium(II). This is the
first report of a pyrimidine-appended triazolium salt and also
the first examples of ruthenium and osmium complexes with
such MIC ligands. All tested complexes are very efficient (pre-)
catalysts for the reduction of several functional groups such as
carbonyls, alkenes, imines and also nitroarenes. We demon-
strated that in general the additional nitrogen atom of the pyri-
midine ring is beneficial in terms of catalytic activity. The two
pyrimidine-functionalized complexes have shown a better con-
Fig. 1 Time-dependence screening for the selected complexes Ru1, Ir3
and Os2.
ation with 1.5 mol% precatalyst loading. Each of the four sub-
strates was stirred with 0.5 equiv. of KOH in iPrOH at 80 °C for version in comparison to pyridine analogues, except for the
24 h (Table 12). 2-Nitroanisole was converted with both tested
reduction of CvC double bond substrates. The ruthenium
complexes into o-anisidine as the only product. The conver-
and osmium complexes having triazolylidene ligands substi-
sions were 65% and 54%, respectively. Both complexes cata- tuted with an electron-donating group performed better,
lysed the reduction of 3-nitroanisole into m-anisidine very well.
Ir3 gave full conversion into the amino product. The reduction
whereas for the iridium complexes the effects were the oppo-
site: complexes with electron-withdrawing groups are superior
of 4-nitroanisole into p-anisidine proceeded with both com- catalysts. In the final part we have shown some mechanistic
plexes, however, giving lower conversions. By using the
complex Ir3, 1-chloro-2-nitrobenzene was fully converted into
studies for the reduction of nitrobenzene with three selected
precatalysts, one for each metal (ruthenium, iridium and
2-chloroaniline. In contrast, Os2 gave only 58% of the aniline osmium). The Cp*–Ir-containing pre-catalysts presented here
derivative after 24 h. Overall, the Ir(III) complex Ir3 was more
deliver better conversions for the conversion of CvO groups
and comparable conversion for the reduction of CvC com-
active in the transfer hydrogenation catalysis of the selected
nitro substrates. Surprisingly no azo- or azoxy-side products pared to the reported Cp*–Ir complexes with NHC ligands.2l
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On the other hand, the Cp*–Ir-containing complexes deliver Melting points were determined on a Kofler micro hot stage
more aniline through the reduction of nitrobenzene compared instrument. The reactions were monitored by TLC on
to related complexes.8a These results are important to under- TLC-CARD silica gel, 220–440 mesh. 1H NMR spectra of benzyl
stand the behaviour of the complexes during the catalysis. The alcohol, 1-phenylethanol, diphenylmethanol, cyclohexanol and
results also show the potential of the tested complexes with cyclooctane were compared with those of the authentic
chelating pyridyl- and pyrimidyl-triazolylidenes in the transfer samples (Tables 2–6). Propylbenzene20 (Table 7), 1,2-diphenyl-
hydrogenation of several functionalities, such as the carbonyl ethane21 (Table 8) and N-benzylaniline22 (Table 9) were identi-
group, CvC double bond, imine and also the nitro group. The fied by comparison of their 1H NMR spectral data with the
newly synthesized pyrimidyl-triazolylidenes might be useful literature reports. GC-MS analysis was performed on a Varian
ligands for generating both homo- and heterodinuclear Saturn 2100C (column: Varian factory four capillary column
complexes.
VF-5 ms; temperature: 50 to 250 °C, heating rate 20 K min−1).
The calibration was done as follows: we collected the GC ana-
lyses of all the products, using pure samples. We obtained ret.
times and also factors for calculating the areas of the GC
peaks into conversions, percent yields. Ret. times (min): 6.22
(aniline), 7.36 (nitrobenzene), 11.21 (azobenzene), 12.42
Experimental section
Materials and physical methods
The reagents and solvents were used as obtained from com- (azoxybenzene); 5.63 (2-anisidine), 6.86 (2-nitroanisole); 5.29
mercial sources (Sigma-Aldrich, Fluka, Alfa Aesar). Acetonitrile (2-chloroaniline), 6.17 (2-chloronitrobenzene); 6.11 (3-anisidine),
was dried over 3 Å molecular sieves, and dichloromethane over 6.73 (3-nitroanisole); 5.96 (4-anisidine), 7.22 (4-nitroanisole).
4 Å molecular sieves. For methylation reactions dry glassware Catalytic reactions were carried out using Schlenk tubes and a
and solvents were used. Triazole 1 was prepared as described pressure-vessel. All catalytic reactions were performed at least in
in the literature.14 NMR spectra were recorded with a Bruker duplicate.
Avance 300, Bruker Avance III 500 (Ljubljana) and Jeol ECS 400
(Berlin) spectrometers. Proton and carbon spectra were refer-
enced to Si(CH3)4 as the internal standard. Some 13C chemical
shifts were determined relative to the 13C signal of the solvent
DMSO-d6 (39.5 ppm). 19F NMR and 31P NMR spectra were
referenced to CCl3F and 85% phosphoric acid, respectively, as
external standards at δ = 0. The nitrogen chemical shifts were
extracted from 1H–15N HMBC spectra with 20 Hz digital resolu-
tion in the indirect dimension. 15N chemical shifts are
rounded to integer numbers because of the digital resolution
limits of the experiment. The reported 15N chemical shifts
Synthesis of the ligand
Pyrimidyl-triazole 1 in dry CH2Cl2 was purged with dry argon
and stirred on an ice-bath at 0 °C for a few minutes. MeOTf
was added via syringe and the reaction mixture was stirred for
30 min at 0 °C and then 24 h at room temperature. The reac-
tion mixture was concentrated using a rotary evaporator and
the residue was column chromatographed on silica gel using a
mixture of CH2Cl2 and MeOH (10 : 1) as the eluent (Rf ≈ 0.3).
Re-crystallization from EtOAc–light petroleum afforded analyti-
cally pure triazolium salts 2OTf. Quantities used in the above
procedure, analytical and spectral data of 2OTf are given
below.
1
were extracted from H–15N HMBC spectra (with 20 Hz digital
resolution in the indirect dimension) with respect to external
90% CH3NO2 in CDCl3 and converted to the δ 15N (liq. NH3) =
0 ppm scale using the relationship: δ 15N (CH3NO2) = δ 15N
(liq. NH3) + 380.5 ppm. Chemical shifts are given on the
δ scale (ppm). Coupling constants (J) are given in hertz. The
multiplicities are indicated as follows: s (singlet), d (doublet),
t (triplet), q (quartet), sept (septet), m (multiplet) and br
(broadened). Assignments of proton, carbon and nitrogen
resonances were performed by standard 2D NMR techniques
3-Methyl-4-phenyl-1-(pyrimidin-2-yl)-1H-1,2,3-triazolium tri-
(1H–1H COSY, 1H–13C HSQC, 1H–13C HMBC, and 1H–15N fluoromethanesulphonate (2OTf). Triazole
1 (223.2 mg,
HMBC). The numbering used for the assignment of NMR 1.00 mmol), CH2Cl2 (3 mL), MeOTf (180.4 mg, 1.10 mmol,
signals is as follows: 1,2,3-triazole ring, simple figures; pyri- 0.12 mL). White solid (350.7 g, 91%). Rf (CH2Cl2 : MeOH =
dine ring, primed figures; phenyl ring, double-primed figures 10 : 1) = 0.3. Mp 154–156 °C. IR: 3119, 3093, 2972, 1612, 1589,
(C-1″ is attached to the 1,2,3-triazole ring). An Agilent 6224 1424, 1399, 1264, 1228, 1145, 1031, 830, 772, 762, 697,
1
time-of-flight (TOF) mass spectrometer equipped with
a
635 cm−1. H NMR (500 MHz, DMSO-d6): δ = 4.48 (s, 3H, CH3-
double orthogonal electrospray source at atmospheric pressure trz), 7.72–7.70 (m, 3H, H-3″, H-5″, H-4″), 7.87 (dd, J = 7.5,
ionization (ESI) coupled to an Agilent 1260 HLPC and an 2.0 Hz, 2H, H-2″, H-6″), 7.99 (t, J = 4.8 Hz, 1H, H-5′), 9.22 (d,
Agilent 6210 ESI-TOF spectrometer were used for recording J = 4.9 Hz, 2H, H-6′, H-4′), 10.04 (s, 1H, H-5). 13C NMR
HRMS spectra. Elemental analysis was performed with a (126 MHz, DMSO-d6): δ = 39.8 (CH3-trz), 122.3 (C-1″), 124.4
Perkin-Elmer 2400 Series II CHNS/O Analyzer. IR spectra were (C-5′), 127.2 (C-5), 129.4 (C-3″, C-5″), 129.6 (C-2″, C-6″), 131.7
obtained with a Perkin-Elmer Spectrum 100, equipped with a (C-4″), 143.5 (C-4), 152.2 (C-2′), 160.5 (C-6′, C-4′). 15N NMR
Specac Golden Gate Diamond ATR as a solid sample support. (51 MHz, DMSO-d6): δ = 245 (N-3), 255 (N-1), 270 (N-1′, N-3′),
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340 (N-2). 19F NMR (471 MHz, DMSO-d6): δ = −77.8 (s, 3F, in vacuo. The residue was dissolved in methanol (10 mL), KPF6
OTf). HRMS (ESI+): calcd for C13H12N5 [M]+ 238.1087, found was added (10 equiv.) and the mixture was stirred for
+
238.1088. Anal. calcd for C14H12F3N5O3S: C 43.41, H 3.12, 10 minutes. Then water was added (100 mL). An orange pre-
N 18.08; found: C 43.12, H 3.29, N 18.51.
cipitate was collected after 10 minutes of stirring by filtration
and dried at room temperature. Pure OspyrimPF6 was obtained
and fully characterized. For quantities used, and analytical
and spectral data of OspyrimPF6, see below.
Synthesis of complexes
Ruthenium complex Rupyrim. A mixture of a triazolium salt
2OTf and Ag2O in acetonitrile was purged with argon and
stirred in the absence of light at room temperature for 4 days.
After the addition of [Ru(η6-p-cym)Cl2]2 the stirring was contin-
ued for 2 hours. The reaction mixture was filtered through
Celite. The solvent was removed in vacuo and the residue was
crystallized from chloroform (1–2 mL) with slow addition of
hexane to give the pure Rupyrim. For quantities used, and
analytical and spectral data of RupyrimPF6, see below.
3-Methyl-4-phenyl-1-(pyrimidin-2-yl)-1H-1,2,3-triazolium
chloro(p-cymene)osmium(II) hexafluorophosphate(V) (OspyrimPF6).
Triazolium salt 2OTf (77.4 mg, 0.20 mmol), Ag2O (162.4 mg,
0.70 mmol), KCl (149.2 mg, 2.0 mmol), MeCN (10 mL),
[Os(p-cym)Cl2]2 (79.1 mg, 0.10 mmol). Red-brown solid
(70.3 mg, 53%). 1H NMR (400 MHz, CD2Cl2, 25 °C, TMS):
δ = 0.90 (d, J = 7.0 Hz, 3H, (CH3)2CH), 0.93 (d, J = 6.9 Hz, 3H,
(CH3)2CH), 2.06 (s, 3H, CH3Cym), 2.32 (septet, J = 6.9 Hz,
(CH3)2CH), 4.24 (s, 3H, CH3–N-3), 5.11 (d, J = 5.7 Hz, 1H,
ArHCym), 5.44 (d, J = 5.7 Hz, 1H, ArHCym), 5.51 (bs, 2H,
ArHCym), 7.63 (bs, 6H, H-5′, H-2″, H-3″, H-4″, H-5″, H-6″), 8.93
(dd, J = 4.8, 2.0 Hz, 1H, H-4′ or H-6′), 9.40 (dd, J = 5.8, 2.0 Hz,
1H, H-6′ or H-4′). 13C NMR (100 MHz, CD2Cl2, 25 °C, TMS): δ =
17.8 (CH3Cym), 20.8 ((CH3)2CH), 22.0 ((CH3)2CH), 30.4
((CH3)2CH), 38.1 (CH3–N-3), 73.0 (ArCym), 76.4 (ArCym), 78.7
(ArCym), 81.3 (ArCym), 89.7 (ArCym), 103.5 (ArCym), 122.7 (C-5′),
124.1 (C-3″, C-5″), 124.8 (C-1″), 129.0 (C-2″, C-6″), 129.6, 130.8
(C-4″), 146.5 (C-4), 150.6 (C-2′), 159.6 (C-4′ or C-6′), 165.0 (C-6′
or C-4′). 15N NMR (41 MHz CD2Cl2): δ = 196 (N-1′ or N-3′), 244
(N-3), 336 (N-2). 19F NMR (377 MHz, CD2Cl2): δ = −72.7 (d, J =
712 Hz, 6F, PF6). 31P NMR (162 MHz, CD2Cl2): δ = −144.5 (sept,
J = 712 Hz, 1P, PF6). HRMS (ESI+): calcd for C24H26ClN4Os+
[M]+ = 598.1413, found 598.1422.
3-Methyl-4-phenyl-1-(pyrimidin-2-yl)-1H-1,2,3-triazolium
chloro(p-cymene)ruthenium(II)
trifluoromethanesulphonate
(RupyrimOTf). Triazolium salt 2OTf (38.7 mg, 0.10 mmol), Ag2O
(34.5 mg, 0.15 mmol), acetonitrile (4 mL), time 1 = 4 days,
[Ru(η6-p-cym)Cl2]2 (30.6 mg, 0.05 mmol). Yellow solid (58.0 mg,
81%). Rf (CH2Cl2 : MeOH = 10 : 1) = 0.8. Mp 71–73 °C. IR: 2965,
1611, 1555, 1472, 1258, 1222, 1151, 1028, 803, 775, 710,
1
636 cm−1. H NMR (500 MHz, CDCl3): δ = 0.98 (d, J = 7.0 Hz,
3H, (CH3)2CH), 1.08 (d, J = 6.9 Hz, 3H, (CH3)2CH), 2.04 (s, 3H,
CH3Cym), 2.54 (septet, J = 6.9 Hz, 1H, (CH3)2CH), 4.34 (s, 3H,
CH3–N-3), 5.21 (dd, J = 6.2, 0.7 Hz, 1H, ArHCym), 5.58 (dd, J =
6.2, 0.8 Hz, 1H, ArHCym), 5.67 (t, J = 6.4 Hz, 2H, ArHCym),
7.72–7.71 (m, 3H, H-3″, H-4″, H-5″), 7.83–7.81 (m, 3H, H-5′,
H-2″, H-6″), 8.96 (dd, J = 4.8, 2.0 Hz, 1H, H-4′ or H-6′), 9.84 (dd,
J = 5.7, 2.0 Hz, 1H, H-6′ or H-4′). 13C NMR (126 MHz, CDCl3):
δ = 18.9 (CH3Cym), 21.7 ((CH3)2CH), 22.7 ((CH3)2CH), 31.2
((CH3)2CH), 38.7 (CH3–N-3), 83.5 (ArCym), 85.7 (ArCym), 88.3
(ArCym), 89.9 (ArCym), 104.2 (ArCym), 110.4 (ArCym), 123.3 (C-5′),
General procedure for the catalysed transfer hydrogenation
Carbonyl group. The selected substrate (0.5 mmol), KOH
125.9 (C-1″), 129.8 (C-3″, C-5″), 130.5 (C-2″, C-6″), 131.6 (C-4″), (0.085 mmol), iso-propanol (4 mL) and the chosen precatalyst
147.7 (C-4), 155.0 (C-2′), 160.0 (C-4′ or C-6′), 166.5 (C-6′ or C-4′), were mixed in a Schlenk tube under a nitrogen atmosphere.
170.7 (C-5). 15N NMR (51 MHz, DMSO-d6): δ = 218 (N-1′ or The reaction mixtures were stirred at 100 °C for 3 hours. The
N-3′), 242 (N-3), 268 (N-3′ or N-1′), 339 (N-2). 19F NMR solvent was then removed in vacuo and the residue was ana-
(471 MHz, DMSO-d6): δ = −78.3 (s, 3F, OTf). HRMS (ESI+): lysed by 1H NMR spectroscopy, using hexamethylbenzene as
calcd for C23H25ClN5Ru+ [M]+ = 508.0842, found 508.0834.
Osmium complex Ospyrim. Triazolium salt 2OTf was mixed versions indicated in Tables 2–5.
an internal standard, to identify products and determine con-
with Ag2O and KCl and stirred in dry acetonitrile under a nitro-
Olefin group. The selected substrate (0.5 mmol), KOH
gen atmosphere (in the absence of air), in the absence of light, (0.085 mmol), iso-propanol (4 mL) and the chosen precatalyst
at room temperature for 4 days. Later [Os(p-cym)Cl2]2 was were mixed in a Schlenk tube under a nitrogen atmosphere.
added and the mixture was stirred for an additional 3 days The reaction mixtures were stirred at 100 °C for 24 hours. The
under a nitrogen atmosphere, in the absence of light at room solvent was then removed in vacuo and the residue was ana-
1
temperature. The reaction mixture was filtered over Celite and lysed by H NMR spectroscopy, using hexamethylbenzene (for
washed with dichloromethane. All the solvents were removed trans-stilbene) or 4-bromobenzaldehyde (for trans-β-methyl-
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styrene and cyclooctene) as an internal standard, to identify
products and determine conversions indicated in Tables 6–8.
Imine group. N-Benzylideneaniline (0.5 mmol), K2CO3
(0.025 mmol), iso-propanol (4 mL) and the chosen precatalyst
were mixed in a Schlenk tube under a nitrogen atmosphere.
The reaction mixtures were stirred at 100 °C for 18 hours. The
solvent was then removed in vacuo and the residue was ana-
lysed by 1H NMR spectroscopy, using hexamethylbenzene as
an internal standard, to identify products and determine con-
versions indicated in Table 9.
Nitro group. The selected substrate (0.5 mmol), KOH
(0.21 mmol), iso-propanol (4 mL) and the chosen precatalyst
were mixed in a Schlenk tube under a nitrogen atmosphere.
The reaction mixtures were stirred at 80 °C for 24 hours. The
crude reaction mixture was diluted with ethyl acetate and sub-
jected to GC to identify products and determine conversion
indicated in Tables 10 and 11. Hexadecane was used as an
internal standard.
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Acknowledgements
The financial support from the Ministry of Education, Science
and Sport, Republic of Slovenia, the Slovenian Research
Agency (Grant P1-0230; Young Researcher Grant to A. B.), and
the Slovene Human Resources Development and Scholarship
Found, Public Call for Scholarships or Grants for the Research
Cooperation of Doctoral Students Abroad in 2012, No.: 11012-
16/2013 is acknowledged. We are grateful to the Fonds der
Chemischen Industrie (FCI) and the Freie Universität Berlin is
also kindly acknowledged for financial support.
Notes and references
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