Arene-ruthenium(II) and -iridium(III) complexes with "click"- based pyridyl-triazoles, bis-triazoles, and chelating abnormal carbenes: Applications in catalytic transfer hydrogenation of nitrobenzene

Article abstract of DOI:10.1021/om4009185

The complexes [(Cym)Ru(L)Cl]PF₆, 2-4, and [Cp*Ir(L)Cl] PF₆, 6-8 (Cym = p-cymene, Cp* = pentamethylcyclopentadienyl), with L = "click"-derived pyridyl-triazol, bis-triazole, or bis-abnormal carbene, were synthesized and spectroscopically characterized. Structural elucidation of the complexes shows a half-sandwich, piano-stool type of coordination around the metal centers and a delocalized situation within the triazolylidene rings. All the complexes were tested for their catalytic efficiency in the transfer hydrogenation of nitrobenzenes, and the results were compared with their 2,2′-bipyridine (bpy) Ru counterpart 1 and Ir counterpart 5. Remarkably, the nature of the final catalytic product is strongly dependent on the chosen metal center, with aniline being preferentially formed with the Ru complexes and azobenzenes with the Ir complexes. Judicious selection of catalyst and reaction conditions also facilitates the isolation of azoxybenzene. To the best of our knowledge, this is a rare example of a homogeneous catalytic synthesis of azobenzene from nitrobenzene. The influence of ligand substitution, metal substitution, and temperature variation on catalytic activity and selectivity has been investigated, whereby a systematic variation of the ligands from bpy, to pyridyl-triazole, to bis-triazole, to bis-abnormal carbene has been carried out. We also present a mechanistic investigation for this transformation with the aim of understanding reaction behavior.

Full text of DOI:10.1021/om4009185

Article
pubs.acs.org/Organometallics
Arene−Ruthenium(II) and −Iridium(III) Complexes with “Click”-Based
Pyridyl-triazoles, Bis-triazoles, and Chelating Abnormal Carbenes:
Applications in Catalytic Transfer Hydrogenation of Nitrobenzene
Stephan Hohloch, Lisa Suntrup, and Biprajit Sarkar*
Institut fur Chemie und Biochemie, Anorganische Chemie, Freie Universitat Berlin, Fabeckstraße 34-36, D-14195, Berlin, Germany
̈
̈
S
* Supporting Information
ABSTRACT: The complexes [(Cym)Ru(L)Cl]PF6, 2−4, and [Cp*Ir(L)Cl]PF6, 6−8
(Cym = p-cymene, Cp* = pentamethylcyclopentadienyl), with L = “click”-derived pyridyl-
triazol, bis-triazole, or bis-abnormal carbene, were synthesized and spectroscopically
characterized. Structural elucidation of the complexes shows a half-sandwich, piano-stool
type of coordination around the metal centers and a delocalized situation within the
triazolylidene rings. All the complexes were tested for their catalytic efficiency in the
transfer hydrogenation of nitrobenzenes, and the results were compared with their 2,2′-
bipyridine (bpy) Ru counterpart 1 and Ir counterpart 5. Remarkably, the nature of the final
catalytic product is strongly dependent on the chosen metal center, with aniline being
preferentially formed with the Ru complexes and azobenzenes with the Ir complexes.
Judicious selection of catalyst and reaction conditions also facilitates the isolation of
azoxybenzene. To the best of our knowledge, this is a rare example of a homogeneous
catalytic synthesis of azobenzene from nitrobenzene. The influence of ligand substitution,
metal substitution, and temperature variation on catalytic activity and selectivity has been investigated, whereby a systematic
variation of the ligands from bpy, to pyridyl-triazole, to bis-triazole, to bis-abnormal carbene has been carried out. We also
present a mechanistic investigation for this transformation with the aim of understanding reaction behavior.
being present on potentially chelating bis-triazolylidenes.14 Out
INTRODUCTION
■
of those, only one was actually shown to function as a chelating
The Cu(I)-catalyzed [3+2] cycloaddition reaction between
azides and alkynes, the best “click” reaction reported to date,
has turned into a power house for synthetic chemists.1 The
resulting 1,2,3-triazoles in various forms have found extensive
use as ligands in recent years in coordination and organo-
metallic chemistry.2 Metal complexes derived from such ligands
have been utilized for investigating electron transfer3 and
magnetic properties,4 as well as in homogeneous catalysis.5 The
ease of methylation of the triazoles, and their subsequent
deprotonation to generate triazolylidene-based abnormal N-
heterocyclic carbenes (aNHC),6 has opened up another vibrant
field of research.7 These ligands, which have also been labeled
as mesoionic carbenes (MIC),8 have mainly found use in
homogeneous catalysis in the last years.9 N-Heterocyclic
carbenes (NHC) occupy a special place in the toolbox of
chemists dealing with homogeneous catalysis, and the potent
nature of the metal complexes of these ligands has been
reported for a variety of catalytic chemical transformation.10
Although NHCs have primarily been utilized as ligands in
homogeneous catalysis, other fascinating properties of these
compounds have also been emerging in recent years,11 with
their redox-active behavior being one of them.12
ligand.14a
Anilines and azobenzenes are important precursors and
intermediates for various industrially useful chemicals.15 Hence,
benign and catalytic ways of generating anilines is an important
chemical goal. Catalytic transfer hydrogenation provides an
excellent method for the hydrogenation of multiple bonds.16
One advantage of this method over the direct use of H2 for
hydrogenation reactions is the ability of transfer hydrogenation
reactions to function without the need for gases at high
pressures. The elucidation of the mechanism of catalytic action
is the best way to improve catalytic activity and, hence, develop
better catalysts. Inspired by a recent report on the catalytic
activity of Cym-ruthenium complexes with polypyridine ligands
(Cym = p-cymene) in the transfer hydrogenation of nitroarenes
to anilines,17 and by our own interest in developing a
systematic correlation between bipyridine (bpy, L1), pyridyl-
triazole, bis-triazole, and bis-triazolylidene ligands,3d−f we
ventured into the present project.
In the following we present metal complexes with the ligands
L1−L4 (Figure 1) coordinated to [Ru(Cym)Cl] (1−4) and to
[Ir(Cp*)Cl] (5−8, Cp* = pentamethylcyclopentadienyl). 1
and 5 are literature-reported compounds and have been
resynthesized here for comparison purposes.18a,b Structural
The most extensively used bis-chelating “click” ligand in the
literature has been the pyridyl-triazole ones,2c,d with reports on
the bis-triazole ligands being rather limited.3f,13 On the
triazolylidene front, almost all efforts have concentrated on
the monodentate triazolylidenes,7 with only a handful of reports
Received: September 13, 2013
Published: November 21, 2013
© 2013 American Chemical Society
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and purity of the ligands were established by 1H and 13C NMR
1
spectroscopy and elemental analysis. In their H NMR spectra,
the ligands L2, L3, and L4[BF4]2 display a characteristic singlet
for the C−H proton of the triazole or triazolium rings at low
fields (see Experimental Section).
The complexes 2, 3, 6, and 7 were synthesized by the
reactions of the chloro-bridged dimeric metal precursors with
the corresponding nitrogen-donating ligands either in acetone
(for the Ru complexes) or in methanol (for the Ir complexes).
Salt metathesis with Bu4NPF6 followed by washing with
methanol and diethyl ether delivered the complexes in excellent
yields (Schemes 1 and 2). The complexes were characterized by
NMR spectroscopy, mass spectrometry, and elemental analysis
(see Experimental Section). The complexes 4 and 8, with the
chelating aNHC ligand L4, were synthesized via the trans-
metalation route with Ag2O. The silver complex was first
generated by the reaction of L4[BF4]2 with Ag2O in the
presence of a chloride source in the dark. The thus generated
silver complex was reacted with the chloro-bridged dimeric
metal precursors. Salt metathesis with KPF6 resulted in the
generation of the metal complexes in good yields (Scheme 2).
The complexes were characterized by 1H and 13C NMR
spectroscopy, elemental analyses, and mass spectrometry.
Disappearance of the C−H signal of the triazolium ring in
Figure 1. Ligands used in this work.
characterization of the ligands and the complexes is presented.
The use of these complexes as (pre)catalysts for the transfer
hydrogenation of nitroarenes in the presence of PrOH is
presented. The influence of changing the metal atom (Ru(II) vs
Ir(III)), the ligand donor properties, and the reaction
conditions on the catalytic efficiency and product distribution
is probed. Mechanistic insights into the catalytic reactions are
provided below as well.
i
1
the H NMR spectrum of the complexes was a first indication
RESULTS AND DISCUSSION
of the formation of the carbene complexes. The molecular peak
■
−
for the cationic part of the complexes (without PF6 ) was
Synthesis and Characterization of Ligands and
Complexes. The ligands L2−L4 were chosen with the same
substituent, 2,6-diisopropylphenyl (dipp), on the triazole rings.
This was done to keep all electronic factors other than ligand
donor atoms the same in the ligands. Furthermore, the bulky
dipp substituents are expected to provide steric protection at
the metal center during catalysis. The ligand L2 was reported by
us previously.3f,5e For the present work, we used a new
synthetic route, developed in our group, that uses Cu(I)
complexes of aNHCs as catalysts for the “click” reaction (see
Experimental Section).9j This route delivered L2 in excellent
yields. The ligand L3 has been reported before by Bertrand et
al.14 We have synthesized it via a route published by Fletcher et
al. for the synthesis of bis-triazole ligands.13b This route uses
1,4-bis(trimethylsilyl)-1,3-butadiyne as the dialkyne source.
Methylation of L3 with Meerwein’s salt delivered L4[BF4]2 in
excellent yields, which served as a precursor for the generation
of the chelating aNHC L4 (see discussion below). The identity
observed in the ESI spectrum. The carbene-C signal for 4 and 8
was observed at 179.2 and 154.5 ppm, respectively, in their 13C
NMR spectrum. The signals corresponding to the Cym group
in 4 were broadened at room temperature, possibly due to the
formation of rotamers and fast interconversion between them.
On heating the sample to 80 °C, sharp “normal” NMR signals
were observed for 4, indicating the existence of only one
rotamer at that temperature.
X-ray Crystal Structures of Ligands and Complexes.
Single crystals of L3 suitable for X-ray diffraction were grown by
slow evaporation of a concentrated diethyl ether solution at
room temperature. L3 crystallizes in the monoclinic P2(1)/n
space group (Table S1). The bond lengths within the triazole
rings in L3 are consistent with previous reports3d−f and exhibit a
central short N1−N3 bond with a distance of 1.305(1) Å
flanked by two longer bonds with bond lengths of N3−N5 =
1.352(2) Å and N1−C3 = 1.362(2) Å. The same situation is
Scheme 1. Synthesis of Complexes with Nitrogen-Donor Ligands
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Scheme 2. Synthesis of Complexes with aNHC-Donor Ligands
Figure 2. ORTEP plot of L3 (left) and L4[OTf]2 (right). All ellipsoids are drawn at 50% probability. Hydrogen atoms (except for C−H of the
triazolium ring in L4[OTf]2) and counterions are omitted for clarity.
observed in the second triazole ring of L3 as well (Table S2).
The two triazole rings in L3 are oriented anti to each other
(Figure 2) and are almost coplanar, with a dihedral angle of
9.01(7)° between them. The dipp substituents on the triazole
rings in L3 are perpendicular to the triazole rings, showing
dihedral angles of 89.89(6)° and 88.47(7)° with respect to the
triazole planes.
the complexes display the expected half-sandwich three-legged
piano-stool type of coordination around the metal centers
(Figure 3). The Cym ligand in 2−4 is bound to the Ru(II)
center in a η6 mode, and the Cp* ligand in 6−8 is coordinated
to the Ir(III) center in a η5 mode. The Ru−N and Ru−Cl
distances in 2 and 3 and the Ir−N and Ir−Cl distances in 6 and
7 are in the expected range (Table S2).13c,e The bond
localization within the triazole ring, as observed in L3, is
maintained in the complexes 2, 3, 6, and 7. For the complexes 2
and 6 with mixed-donor ligands, the Ru1−N1 and Ir1−N1
distances to the triazole N-donor of 2.072(2) and 2.071(2) Å,
respectively, are shorter than the Ru−N2 and Ir−N2 distances
of 2.113(2) and 2.122(2) Å, respectively, to the pyridine N-
donors. The Ru−N1(triazole) and Ir1−N1(triazole) distances
in the complexes 3 and 7 with bis-triazole ligands are however
longer than the corresponding metal-N(triazole) distances in 2
and 6 (Table S2). Hence, the shorter metal−N(triazole)
distances compared to the metal−N(pyridine) distances in 2
and 6 are probably a result of the geometrical conformation of
the mixed-donor ligands and not necessarily related to better
donor properties of the triazole-N donors, as compared to the
pyridine-N donors. Coordination of metal centers to the
triazole rings in 2, 3, 6, and 7 leads to an opening up of the
angles C4−N2−N4 and C3−N1−N3. This is apparent in the
larger values of these angles in the metal complexes, as
compared to the free ligand L3.
Attempts at crystallization of L4[BF4]2 unfortunately did not
result in suitable single crystals. Hence we resorted to anion
exchange and generated the triflate salt L4[OTf]2 by using a
reported synthetic route.14a Gratifyingly, slow diffusion of
diethyl ether in a concentrated solution of dichloromethane
resulted in the formation of suitable single crystals of L4[OTf]2.
L4[OTf]2 crystallizes in the monoclinic space group P2(1)/n
(Table S1). Methylation of the triazole rings leads to a more
delocalized situation within the triazolium rings as compared to
the neutral triazole rings in L3. This can be seen from the N1−
N3 and N3−N5 bond distances in L4[BF4]2 which are 1.321(8)
and 1.319(8) Å, respectively; the corresponding distances in L3
are 1.305(1) and 1.352(2) Å, respectively (Table S2).
Furthermore, the triazolium rings are now no longer coplanar,
as seen from the dihedral angle of 50.9(3)° between them. The
dipp substituents are almost perpendicular to the triazolium
rings, exhibiting a dihedral angles of 79.3(3)° and 84.4(3)°,
respectively.
Single crystals of the complexes 2−4 and 6−8 as solvates
were obtained for X-ray diffraction studies (see Experimental
Section). The crystallographic details are given in Table S1. All
In 4 and 8, the Ru and Ir centers are bonded through the
triazolylidene C atoms of the ligand L4. The Ru1−C1 distance
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Figure 3. ORTEP plots of 2, 3 (top), 4, 6 (middle), and 7, 8 (bottom). All ellipsoids are drawn at 50% probability. Hydrogen atoms, counterions,
and solvent molecules are omitted for clarity.
of 2.059(6) Å in 4 and the Ir1−C1 distance of 2.081(6) Å in 8
are in the expected range for distances between Ru or Ir centers
and NHC or aNHC carbon atoms.7b,c Deprotonation of C1
and coordination of the metal centers through the C1 and C2
atoms of L4 (Figure 3) leads to a shrinking of the respective
angles around the C1 and C2 atoms. This is apparent in the
smaller C3−C1−N5 and C4−C2−N6 angles in 4 and 8 as
compared to the ligand precursors L3 and L4[BF4]2. Just as in
the free ligands, the dipp substituents are perpendicular to the
triazole or triazolylidene rings in all the ruthenium and iridium
complexes. The distances between the centroid of Cym and the
Ru center are 1.677(1), 1.670(1), and 1.727(1) Å respectively
for 2, 3, and 4. The corresponding distance between the
centroid of Cp* and the Ir center are 1.771(1), 1.765(1), and
1.841(1) Å for 6, 7, and 8, respectively. Hence it is seen that for
the complexes 4 and 8, with the aNHC ligands, the metal
centers are farther away from the arene rings as compared to
the complexes with the nitrogen-donor ligands. This is most
likely due to the larger steric repulsion between the dipp
substituents and the arene rings in 4 and 8, where the dipp
substituents are closer to the metal center as compared to 2, 3
or 6, 7 (Figure 3).
Catalytic Transfer Hydrogenation of Nitrobenzene.
Anilines and azobenzenes are valuable chemicals for basic
research and for the industry.15 Anilines appear as inter-
mediates for various chemicals that are useful for the
pharmaceutical industry.15a,b Recently, an efficient catalytic
route for the transfer hydrogenation of nitroarenes to anilines,
by using Ru-Cym in combination with polypyridine ligands as
catalyst, was reported.17 Certain hints at the reaction
mechanism were also presented in that report. Inspired by
that work, we wanted to check the influence of the variation of
ligand donicity on the catalytic efficiency of the corresponding
metal complexes. The complexes presented above, with a
systematic variation of the “click”-derived ligands, present an
ideal platform for testing such effects. Development of efficient
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Scheme 3. Conversion of Nitrobenzene to Various Products Using the Conditions Shown in Tables 1 and 2
catalysts for the synthesis of useful chemicals and the
understanding of the reaction mechanisms of such catalytic
processes are highly important goals in contemporary
chemistry.
Ru(II) complex 2 as a (pre)catalyst, increasing the temperature
from 80 °C to 100 °C resulted in an increase in aniline
formation from 50% of the total products to 56% of the total
products (Scheme 3 and Table 2). The increase in the amount
of aniline formation at higher temperatures corresponded with
a decrease in the amount of azoxybenzene formation.
Increasing the amount of the (pre)catalyst to 5 mol % resulted
in a drastic improvement in catalytic activity. The best results
were obtained with 5 mol % of 2 at 100 °C. Under those
conditions, a complete conversion of nitrobenzene to aniline
was observed (Table 2). The use of excess amounts of ligands
was not necessary for high catalytic activity. For the Ir(III)
complex 7, an increase in the temperature from 80 °C to 100
°C by keeping catalyst loading at 1.5 mol % resulted in an
increase in the amount of aniline formation (Table 2). The
increase in the amount of aniline came at the expense of
azobenzene and azoxybenzene, whose formation was negligible
at higher temperatures. Increasing the amount of catalytic
loading to 5 mol % by setting the temperature at 80 °C showed
more conversion to aniline at the expense of azoxybenzene as
compared to 1.5 mol % catalyst loading at the same
temperature (Table 2). On using 5 mol % of 7 at 100 °C,
the amount of aniline formed was seen to increase at the
expense of azobenzene (Table 2). Hence while using 7 as a
(pre)catalyst, an increase in temperature and catalytic loading
leads to a decrease in the formation of azoxybenzene and an
increase in the formation of aniline and azobenzene by various
amounts.
For testing the complexes for their catalytic ability in the
transfer hydrogenation of nitrobenzene, an initial screening was
carried out with 1.5 mol % of the complexes and 0.5 equivalent
of KOH in 2-propanol at 80 °C (Scheme 3). The catalytic
screening showed us that the final products of catalysis were
strongly dependent on the metal center present in the catalyst
(Ru or Ir). Whereas the use of the Ru(II) complexes 1−4 as
(pre)catalysts delivered various amounts of aniline and
azoxybenzene as the final products, the Ir(III) complexes 5−
8 delivered a mixture of aniline, azobenzene, and azoxybenzene
as the final products (with various amounts of unreacted
nitrobenzene, Table 1). Under identical conditions, and in the
Table 1. Product Distribution on Using the Various
a
(Pre)catalysts for the Reaction Shown in Scheme 3
nitrobenzene (recovered)
aniline
azobenzene
azoxybenzene
1
2
3
4
5
6
7
8
46
16
34
37
0
29
50
30
26
21
22
9
0
2
25
32
36
37
38
36
23
67
0
0
41
40
68
3
2
0
21
9
a
Having established the best conditions for catalytic perform-
ance and having observed the differences in product
distribution under various temperatures and reaction con-
ditions, we next turned to the mechanism of action of this
catalytic process. At this point, a couple of published works
were extremely useful for our interpretation. The first of these
is the work on the transfer hydrogenation of nitrobenzene with
Ru-Cym complexes of polypyridines as mentioned above.17
The second paper deals with the metal-induced conversion of
nitrosobenzene to azoxybenzene.19 This contribution is of
particular relevance to us, as we observe the formation of
azoxybenzene under certain conditions with our catalysts. On
the basis of this knowledge, a catalytic mechanism as shown in
Scheme 4 can be formulated.
Reaction conditions: 1.5 mol % catalyst loading, 0.5 equiv of KOH,
iPrOH, 80 °C, 24 h.
absence of a catalyst, 70% unreacted nitrobenzene was
recovered, and aniline, azobenzene, and azoxybenzene were
formed in amounts of 6%, 7%, and 17%, respectively. From this
initial screening, it was clear that the Ru(II) complex 2 is the
best catalyst for converting nitrobenzene to aniline, and the
Ir(III) complex 7 is the best catalyst for producing azobenzene
from nitrobenzene. Under the same conditions complex 8
delivered the best yield for azoxybenzene. Hence the rest of the
investigations were carried out with 2 and 7 as (pre)catalysts.
We were next interested in finding the optimum reaction
conditions for the functioning of our best (pre)catalysts. In
order to do this, both the temperature of the reaction and the
catalyst loading were varied. While using 1.5 mol % of the
On the basis of previous literature reports, it can be assumed
that the metal complexes presented here would get converted
a
Table 2. Product Distribution on Using 2 or 7 as (Pre)catalysts under Various Conditions for the Reaction Shown in Scheme 3
catalyst
loading (mol %)
temp (°C)
nitro-benzene (recovered)
aniline
azobenzene
azoxybenzene
2
2
2
7
7
7
1.5
5
100
80
23
10
0
56
75
2
0
19
15
0
5
100
100
80
100
50
0
1.5
5
0
49
74
55
1
0
26
0
5
100
0
45
0
a
i
Reaction conditions: 0.5 equiv of KOH, PrOH, 24 h.
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Scheme 4. Postulated Mechanism of the Catalysts in the
Transfer Hydrogenation of Nitrobenzene
Scheme 5. Conversion of Nitrosobenzene to Various
Products by Using 2 or 7 as (Pre)catalysts
a
Table 3. Product Distribution on Using 2 or 7 as
(Pre)catalysts under Various Conditions for the Reaction
Shown in Scheme 5
loading (mol
%)
temp
(°C)
catalyst
aniline azobenzene azoxybenzene
2
7
2
7
1.5
1.5
5
80
80
18
17
93
44
23
83
0
59
0
100
80
7
5
56
0
and nitrosobenzene as a substrate, a conversion to 44% aniline
and 56% azobenzene was observed with 5 mol % of the catalyst
at 100 °C (Table 3). These results are identical to the catalytic
results obtained while starting from nitrobenzene, hence
pointing to the participation of nitrosobenzene in the catalytic
cycle.
On using azobenzene as a substrate, the Ru(II) precatalyst 2
delivered a quantitative conversion to aniline with a 5 mol %
catalyst loading at 100 °C (Scheme 6, Table 4). The Ir(III)
complex 7 on the other hand showed no conversion of
azobenzene to any products with 1.5 mol % catalyst loading at
80 °C. On increasing the catalyst loading to 5 mol %, at a
temperature of 80 °C a 40% conversion to aniline was
observed, with the rest of azobenzene remaining unreacted.
These results match well with the results obtained while
starting with nitrobenzene as a substrate, thus also indicating
the involvement of azobenzene in the catalytic cycle.
In order to determine the relevance of path A or B with the
Ru- and Ir-containing catalysts, a time-dependence of the
product formation under catalytic conditions was carried out
with 2 and 8 with nitrobenzene as the substrate. By using 5 mol
% of 2 and a temperature of 80 °C, a conversion to about 70%
aniline is observed. Additionally, the formation of a small
amount of azoxybenzene is also seen (Figure 4). While the
amount of azoxybenzene formed remains constant after a
period of about 15 h, the increase in the formation of aniline
correlates well with the decrease in the amount of nitro-
benzene. No amount of azobenzene was detected during this
conversion. These observations clearly establish the preference
of path A (Scheme 4) for aniline formation with the ruthenium
complex 2.
On using 5 mol % of 8 at 100 °C, a time-dependence of
product formation as shown in Figure 5 is observed. As can be
seen from Figure 5, we followed the reaction for about 8 days.
The first product obtained is azoxybenzene. At the same time
small amounts of azobenzene and aniline are already detectable.
Azobenzene production then immediately increases as soon as
most of the nitrobenzene is consumed. At the same time the
amount of azoxybenzene decreases again and is almost fully
consumed as soon as azobenzene reaches its top concentration
in the solution. When no more azoxybenzene is available for
reduction and the concentration of azobenzene has reached its
top value, the production of aniline increases dramatically at the
expense of the amount of azobenzene. These observations
a
“H2” refers to a transfer hydrogenation process.20
to hydrides “[M]H2” in the presence of KOH in 2-propanol
(Scheme 4).17,21 A hydride signal at −14.46 ppm was observed
for 7 by performing the reaction under catalytic conditions
without substrate (Figure S1). These hydrides are the starting
points in the catalytic cycle. Insertion of nitrobenzene into the
M−H bond and the elimination of H2O (with participation of
2-propanol) results in the formation of nitrosobenzene. Once
formed, the nitrosobenzene can follow two different pathways
to deliver aniline. Whereas path A proceeds through N-
phenylhydroxyamine and its direct conversion to aniline, path B
proceeds through an azoxybenzene intermediate, which gets
converted to azobenzene, before being finally converted to
aniline (Scheme 4). In order to test this mechanism, we carried
out additional catalytic experiments starting with two different
substrates, nitrosobenzene and azobenzene, which appear as
intermediates in the catalytic cycle. On starting with nitro-
sobenzene as a substrate and using 1.5 mol % of the Ru(II)
complex 2 as a (pre)catalyst at 80 °C, a mixture of aniline,
azobenzene, and azoxybenzene was obtained as products, with
the major amount (59%) of the products being azoxybenzene
(Scheme 5, Table 3). On increasingly the catalyst loading to 5
mol % and the temperature to 100 °C, a 93% conversion to
aniline was observed, with the rest of the 7% being
azoxybenzene. On using the Ir(III) complex 7 as a (pre)catalyst
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Scheme 6. Conversion of Azobenzene to Various Products by Using 2 or 7 as (Pre)catalysts
Table 4. Product Distribution on Using 2 or 7 as
(Pre)catalysts under Various Conditions for the Reaction
Shown in Scheme 6
CONCLUSIONS
■
In conclusion, we have presented here a total of six new
complexes of Ru(II) and Ir(III) with “click”-derived ligands. All
of the complexes have been structurally characterized. In
addition, we have compared the catalytic properties of those
complexes with their bpy analogues. The donor properties of
the ligands have been varied through the symmetric chelating
bpy-N donors, to asymmetric chelating pyridyl-triazole-N
donors, to symmetric bis-triazole-N donors, and finally to
symmetric bis-aNHC-C donors. In doing so, we have also
presented the crystal structures of 4 and 8, which are only the
second examples of metal complexes with a bidentate aNHC
ligand of the triazolylidene type acting as a chelate to a metal
center.14a Catalytic transfer hydrogenation of nitrobenzene with
these complexes as (pre)catalysts presents some interesting
trends. The catalysts with the bpy ligands delivered the worst
results. Gratifyingly, the preferential reduction of nitrobenzene
to either aniline, or azobenzene, or azoxybenzene could be
controlled by varying the temperature, catalyst loading, the type
of metal center, or the nature of the ligands. Thus, complex 8,
with a bis-aNHC ligand, delivers the largest amount of
azoxybenzene on catalytic reduction of nitrobenzene. The
Ru(II) complexes with the triazole-N donor containing ligands
at high catalytic loading and high temperatures show a
complete reduction of nitrobenzene to aniline. The Ir(III)
complexes with the nitrogen-donor ligands on the other hand
display a propensity to preferentially deliver azobenzene as the
reduction product of nitrobenzene. Mechanistic investigations
have been used to provide a rationale for this product
distribution.
loading (mol
%)
temp
(°C)
catalyst
aniline azobenzene azoxybenzene
2
7
2
7
1.5
1.5
5
80
80
36
0
64
100
1
0
0
0
0
100
80
99
40
5
60
Figure 4. Time-dependence of product formation starting from
nitrobenzene with 5 mol % of 2 at 80 °C.
This is a rare case where the reduction product of
nitrobenzene can be controlled by tuning the catalyst and
reaction conditions. We also note that out of all the recent
mechanisms discussed in the literature for the catalytic
reduction of nitrobenzene under homogeneous conditions,17,20
ours is the one where the role and formation of nitrosobenzene,
N-phenylhydroxyamine, azoxybenzene, azobenzene, and aniline
have been discussed and probed with relevant experiments. Our
results here show the power of the “click” method in generating
new ligands for organometallic chemistry and homogeneous
catalysis. Furthermore, we have also shown the beauty of ligand
variation on generating product selectivity in catalysis. In view
of the importance of aniline, azobenzene, and azoxybenzene as
useful chemical products, it will be intriguing to see if selectivity
of the catalysts can be further improved by rational ligand
tuning. Work in that direction is currently being pursued in our
laboratories.
Figure 5. Time-dependence of product formation starting from
nitrobenzene with 5 mol % of 8 at 100 °C.
clearly indicate the preference of path B (Scheme 4) for the
iridium complex 8.
Some generalizations can be made from the various control
reactions that have been carried out. The Ru(II) complexes
(with triazole donors as ligands) are highly efficient in
catalytically reducing nitrobenzene to aniline, whereas the
Ir(III) complexes are efficient in generating azobenzene. Lower
catalyst loading and comparatively lower temperatures are the
best conditions for generating azoxybenzene, with the bis-
aNHC-containing Ir(III) complex 8 being the most efficient
catalyst for generating azoxybenzene (Table 1). The preference
for path A or B in the catalytic cycle (Scheme 4) seems to
depend on the rate of direct conversion of N-phenylhydroxy-
amine to aniline. The Ru(II) catalysts seem to react
preferentially through path A. For the Ir(III) catalysts, the
direct conversion of N-phenylhydroxyamine to aniline is less
efficient, and hence path B predominates with the Ir(III)
catalysts with azobenzene being the main product of nitro-
benzene reduction in those cases.
EXPERIMENTAL SECTION
■
Materials and Physical Methods. [RuCymCl2]2 and [IrCp*Cl2]2
are commercially available. All commercially available reagents were
used as supplied. The solvents used for metal complex synthesis were
dried and distilled under argon and degassed by common techniques
prior to use. 1H and 13C NMR spectra were recorded on a Bruker AC
250 spectrometer or a Jeol ECS 400 spectrometer. Elemental analyses
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were performed by the Perkin-Elmer Analyzer 240 and a Elementar
Vario EL III. Mass spectrometry was performed on an Agilent 6210
ESI-TOF. GC-MS analysis was performed on a Varian Saturn 2100C
(column: Varian factory four capillary column VF-5 ms; method: 50 to
250 °C, heating rate 20 K/min).
and the corresponding ligand (2 equiv, 0.2 mmol) were mixed in 15
mL of solvent (acetone for the ruthenium(II) complexes, methanol for
the iridium(III) complexes) and stirred under an inert gas atmosphere
overnight. Afterward the solvents were removed under high vacuum,
and methanol (5 mL) and tetrabutylammonium hexafluorophosphate
(4 equiv, 0.4 mmol, 155 mg) were added. The mixture was stirred until
yellow solids started to precipitate. The yellow solids were collected by
filtration and washed several times with methanol first, followed by
diethyl ether, and dried under air to give the desired product in good
to moderate yields from 75% to 90%.
Synthesis of Ligands. 1-(2,6-Diisopropylphenyl)-4-(2-pyridyl)-
1,2,3-triazole (L2). This ligand has been reported in the literature by
following a different synthetic route.3f,5e For the present work L2 was
synthesized following the standard click procedure by Hohloch et al.9j
2-Pyridylacetylene (1 equiv, 3 mmol, 309 mg) and 2,6-diisopropyl-
phenyl azide (1 equiv, 3 mmol, 609 mg) were mixed in a small vial,
and copper(I) iodide (0.01 equiv, 0.03 mmol, 6 mg), potassium tert-
butoxide (0.02 equiv, 0.06 mmol, 6 mg), and 3-methyl-1-(2-
(methylthio)phenyl)-4-phenyl-1,2,3-triazolium iodide (0.01 equiv,
0.03 mmol, 11 mg) were added. After a short time the mixture
evolved heat and solidified. The crude products were purified by flash
silica gel column chromatography using dichloromethane first,
followed by dichloromethane/methanol (9:1), giving the correspond-
Chloro(p-cymene)(2,2′-bipyridine)ruthenium(II) Hexafluoro-
phoshate (1). Even though this compound is literature known,18a
we synthesized it here following the general procedure stated above by
using [RuCymCl2]2 (60 mg) and L1 (31 mg). Yield: 89% (0.178
1
mmol, 102 mg). H NMR (400 MHz, (CD3)2CO; 25 °C, TMS): δ
(ppm) 9.60−9.56 (m, 2H); 8.61−8.56 (m, 2H); 8.32−8.27 (m, 2H);
7.81−7.76 (m, 2H); 6.21 (d, J = 6.4 Hz, 2H); 5.96 (d, J = 6.4 Hz, 2H);
2.75 (hept, J = 6.8 Hz, 1H); 2.28 (s, 3H); 1.06 (d, J = 6.8 Hz, 6H). MS
(ESI): m/z found 427.0543, calcd 427.0515 [C20H22N2Cl1Ru]+. Anal.
Calcd (%) for [C20H22N2Cl1RuP1F6]: C 42.00, H 3.88, N 4.90. Found:
C 42.21, H 3.49, N 4.68.
1
ing ligand as off-white powders in good yields of 90%. H NMR (400
MHz, CDCl3; 25 °C, TMS): δ (ppm) 8.62−8.58 (m, 1H); 8.32−8.28
(m, 2H); 8.22 (s, 1H); 7.84−7.78 (m, 1H); 7.52−7.48 (m, 1H); 7.31−
7.22 (m, 3H); 2.32 (hept, J = 7.2 Hz, 2H); 1.16−1.11 (m, 12H). 13C
NMR (100 MHz, CDCl3, 25 °C, TMS): δ (ppm) 150.3; 149.6; 148.2;
146.1; 137.1; 133.2; 131.0; 125.0; 123.9; 123.1; 120.5; 28.5; 24.3; 24.1.
MS (ESI): m/z found 329.1747, calcd 329.1737 [C19H22N4 + Na]+.
Anal. Calcd (%) for [C19H22N4]: C 74.48, H 7.24, N 18.29. Found: C
74.32, H 8.63, N 17.56. Mp: 112−114 °C.
Chloro(p-cymene)(1-(2,6-diisopropylphenyl)-4-(2-pyridyl)-1,2,3-
triazole)ruthenium(II) Hexafluorophoshate (2). Following the
general procedure and using [RuCymCl2]2 (60 mg) and L2 (61
mg). Yield: 84%, (0.168 mmol, 121 mg). 1H NMR (400 MHz,
CD2Cl2; 25 °C, TMS): δ (ppm) 9.24−9.21 (m, 1H); 8.42 (s, 1H);
8.11−8.05 (m, 1H); 8.02−7.98 (m, 1H); 7.65−7.58 (m, 2H); 7.42−
7.35 (m, 2H); 5.90−5.87 (m, 1H); 5.83−5.80 (m, 1H); 5.71−5.68 (m,
1H); 5.62−5.59 (m, 1H); 2.78 (hept, J = 7.2 Hz, 1H); 2.30−2.21 (m,
4H); 2.08 (hept, J = 7.2 Hz, 1H); 1.26−1.19 (m, 9H); 1.12−1.06 (m,
9H). MS (ESI): m/z found 577.1694, calcd 577.1670
[C29H36N4Cl1Ru]+. Anal. Calcd (%) for [C29H36N4Cl1Ru1P1F6]: C
48.24, H 5.02, N 7.76. Found: C 48.26, H 5.41, N 7.36. Dec: 255 °C.
Chloro(p-cymene)(1,1′-bis(2,6-diisophenyl)-4,4′-bis(1,2,3-
triazole))ruthenium(II) Hexafluorophoshate (3). Following the
general procedure and using [RuCymCl2]2 (60 mg) and L3 (91
mg). Yield: 79% (0.158 mmol, 138 mg). 1H NMR (250 MHz, CD2Cl2;
25 °C, TMS): δ (ppm) 8.44 (s, 2H); 7.70−7.60 (m, 2H); 7.47−7.38
(m, 4H); 5.93−5.88 (m, 2H); 5.68−5.64 (m, 2H); 3.00 (hept, J = 6.75
Hz, 1H); 2.43 (hept, J = 7 Hz, 2H); 2.17 (hept, J = 6.75 Hz, 2H);
1.32−1.25 (m, 18H); 1.22−1.12 (m, 12H). MS (ESI): m/z found
727.2832, calcd 727.2830 [C38H50N6Cl1Ru]+. Anal. Calcd (%) for
[C38H50N6Cl1RuP1F6]: C 52.32, H 5.78, N 9.63. Found: C 51.83, H
6.36, N 9.10. Dec: 248 °C.
Chloro(pentamethylcyclopentadienyl)(2,2′-bipyridine)iridium(III)
Hexafluorophoshate (5). Even though this compound is literature
known,18b we synthesized it here following the general procedure
stated above by using [IrCp*Cl2]2 (80 mg) and L1 (30 mg). Yield:
92% (0.184 mmol, 122 mg). 1H NMR (400 MHz, (CD3)2CO; 25 °C,
TMS): δ (ppm) 9.13−9.10 (m, 2H); 8.73−8.69 (m, 2H); 8.37−8.30
(m, 2H); 7.93−7.87 (m, 2H); 1.76 (s, 15H). MS (ESI): m/z found
519.1172, calcd 519.1167 [C20H23N2Cl1Ir]+. Anal. Calcd (%) for
[C20H23N2Cl1IrP1F6] C 36.17, H 3.49, N 4.22. Found: C 36.11, H
3.54, N 4.00.
1,1′-Bis(2,6-diisophenyl)-4,4′-bis(1,2,3-triazole) (L3). An alternate
synthetic route for this ligand has been reported in the literature.14a
For the present work, L3 was synthesized using a standard procedure
reported by Fletcher et al.13b Copper sulfate pentahydrate (0.4 equiv,
1.2 mmol, 300 mg), sodium ascorbate (0.8 equiv, 2.4 mmol, 480 mg),
and potassium carbonate (2 equiv, 6 mmol, 830 mg) were mixed in a
flask. Afterward 30 mL of tert-butanol was added followed by 2,6-
diisopropylphenyl azide (2 equiv, 6 mmol, 1.220 g) and 1,4-
bis(trimethylsilyl)-1,3-butadiyne (1 equiv, 3 mmol, 582 mg). To the
mixture were added 30 mL of water and 3 mL of pyridine. The mixture
was capped and stirred at room temperature for up to 3 days. After the
reaction, the mixture was diluted with dichloromethane (150 mL) and
extracted with 5% ammonia solution in water five times (each washing
50 mL). The organic layers were separated and dried over sodium
sulfate (40 g), and the solvents were evaporated under reduced
pressure, leaving an off-white to brown product in excellent yields of
95% (0.285 mmol, 1.29 g). 1H NMR (250 MHz, CDCl3; 25 °C,
TMS): δ (ppm) 8.23 (s, 2H); 7.51 (t, J = 7.5 Hz, 2H); 7.31 (d, J =
7.75 Hz, 4H); 2.34 (hept, J = 6.75 Hz, 4H); 1.16 (d, J = 7.75 Hz,
24H). 13C NMR (60 MHz, CDCl3, 25 °C, TMS): δ (ppm) 146.1;
139.8; 132.9; 131.0; 123.9; 123.6; 28.4; 24.3; 24.1. MS (ESI): m/z
found 457.3066, calcd 457.3074 [C28H36N6 + H]+. Anal. Calcd (%) for
[C28H36N6]: C 73.65, H 7.95, N 18.40. Found: C 72.72, H 8.80, N
17.45. Mp: 266−268 °C.
1,1′-Bis(2,6-diisopropylphenyl)-3,3′-dimethyl-[4,4′-bis(1,2,3-tria-
zole)]-3,3′-diium Ditetrafluoroborate (L4[BF4]2). L3 (1 equiv, 1 mmol,
0.456 g) was dissolved in dichloromethane (15 mL) under an inert gas
atmosphere, and Meerwein’s salt (2.5 equiv, 2.5 mmol, 372 mg) was
added to the solution. The reaction mixture was then stirred for 3 days
at room temperature. After that, the crude mixture was slowly poured
into diethyl ether (200 mL), and the white precipitate was filtered off
and washed with diethyl ether several times. The product was obtained
as a white fluffy solid in a yield of 94% (0.94 mmol, 0.620 g). 1H NMR
(250 MHz, DMSO-d6; 25 °C, TMS): δ (ppm) 10.21 (s, 2H); 7.83−
7.74 (m, 2H); 7.64−7.58 (m, 4H); 4.51 (s, 6H); 2.31 (hept, J = 6.75
Hz, 4H); 1.23 (d, J = 675 Hz, 12H); 1.16 (d, J = 6.75 Hz, 12H). 13C
NMR (60 MHz, CDCl3, 25 °C, TMS): δ (ppm) 145.1; 134.2; 133.2;
130.2; 128.9; 125.0; 28.0; 23.9; 23.5. MS (ESI): m/z found 573.3480,
calcd 573.3500 [C30H42N6B1F4]+. Anal. Calcd (%) for [C30H42N6B2F8
+ 0.2CH2Cl2]: C 52.91, H 6.25, N 12.20. Found: C 52.69, H 6.35, N
12.29. Mp: 298−300 °C.
Chloro(pentamethylcyclopentadienyl)(1-(2,6-diisopropylphenyl)-
4-(2-pyridyl)-1,2,3-triazole)iridium(III) Hexafluorophoshate (6). Fol-
lowing the general procedure and using [IrCp*Cl2]2 (80 mg) and L2
1
(61 mg). Yield: 88% (0.176 mmol, 143 mg). H NMR (400 MHz,
(CD3)2CO; 25 °C, TMS): δ (ppm) 9.41 (s, 1H); 9.13−9.10 (m, 1H);
8.39−8.35 (m, 1H); 8.33−8.28 (m, 1H); 7.83−7.78 (m, 1H); 7.73−
7.67 (m, 1H); 7.56−7.49 (m, 1H); 2.43 (hept, J = 6.8 Hz, 1H); 2.26
(hept, J = 6.8 Hz, 1H); 1.83 (s, 15H). MS (ESI): m/z found 669.2335,
calcd 669.2341 [C29H37N4Cl1Ir]+. Anal. Calcd (%) for
[C29H37N4Cl1IrP1F6]: C 42.78, H 4.58, N 6.88. Found: C 42.87, H
4.96, N 6.76. Dec: 285 °C.
Chloro(1,1′-bis(2,6-diisophenyl)-4,4′-bis(1,2,3-triazole))iridium(III)
Hexafluorophoshate (7). Following the general procedure and using
[IrCp*Cl2]2 (80 mg) and L3 (91 mg). Yield: 96% (0.192 mmol, 185
1
Synthesis of Complexes with N∧N Ligands. General
Procedure. The corresponding metal dimer (1 equiv, 0.1 mmol)
mg). H NMR (250 MHz, CD2Cl2; 25 °C, TMS): δ (ppm) 8.59 (s,
2H); 7.70−7.61 (m, 2H); 7.47−7.37 (m, 4H); 2.40 (hept, J = 7 Hz,
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1H); 2.23 (hept, J = 7 Hz, 1H); 1.83 (s, 15H); 1.32−1.13 (m, 24H).
MS (ESI): m/z found 819.3475, calcd 819.3481 [C38H51N6Cl1Ir]+.
Anal. Calcd (%) for [C38H51N6Cl1IrP1F6]: C 47.32, H 5.33, N 8.71.
Found: C 46.82, H 5.68, N 8.43. Mp: 232−234 °C.
908392, 846889, and 911406 contain the cif files of L3, L4[OTf]2, 2, 3,
4, 6, 7, and 8, respectively. These data can be obtained free of charge
from www.ccdc.cam.ac.uk/data_requests/cif. The A-Alert in the
checkCIF file of L4[OTf]2 is related to the disorder of the triflate
anions.
Synthesis of Bis-carbene Complexes. General Procedure.
Azolium salt L4[BF4]2 (2 equiv, 0.2 mmol, 133 mg) was mixed
under an inert gas atmosphere with potassium chloride (20 equiv, 2
mmol, 148 mg) and silver(I) oxide (7 equiv, 0.0007 mol, 162 mg) and
dissolved in MeCN (10 mL). The mixture was stirred with exclusion
from light for 2 days at room temperature. Afterward the solution was
filtered through Celite, and the solvent was removed under high
vacuum to give a white solid. The solids were dissolved in DCM (15
mL), and the corresponding metal dimer (1 equiv, 0.1 mmol) was
added. The mixture was again stirred for 2 days excluded from light.
Afterward the mixture was filtered through Celite to remove silver
chloride formed in the reaction. The solvent was evaporated, the
yellow solids were then dissolved in acetone (3 mL), potassium
hexafluorophosphate (8 equiv, 0.8 mmol, 147 mg) was added, and the
resulting mixture was stirred for 10 min under air. Afterward water was
added (50 mL) slowly to guarantee slow precipitation of the desired
complexes. The yellow solids were collected by filtration, washed with
water several times, and dried under air to give the desired product in
good yields of 78% and higher.
General Procedure for Transfer Hydrogenation Catalysis.
Nitrobenzene (123 mg), nitrosobenzene (107 mg), and azobenzene
(182 mg) (each 1 mmol) were mixed under an inert gas atmosphere
with KOH (0.5 equiv, 0.5 mmol, 28 mg) and the corresponding
complex in a Schlenk tube, and the reaction mixture was dissolved in
i
dry PrOH (4 mL). The mixtures were heated at the assigned
temperature for 24 h, cooled, filtered through a small pad of silica
using iPrOH, and analyzed by GC-MS chromatography using
hexadecane as an internal standard.
General Procedure for Time-Dependent Transfer Hydro-
genation Experiments. Nitrobenzene (1 equiv, 61 mg, 0.5 mmol)
was mixed under a nitrogen atmosphere with KOH (0.5 equiv, 14 mg,
0.25 mmol) and the corresponding complex, and this was dissolved in
iPrOH (15 mL). The mixtures were stirred at the assigned temperature
for 2−8 days. After certain times small aliquots were taken out of the
reaction mixture to analyze the conversions by GC-MS using
hexadecane as an internal standard.
General Procedure for Catalyst-Free Transfer Hydrogena-
tions. Nitrobenzene (1 equiv, 61 mg, 0.5 mmol) was mixed under a
nitrogen atmosphere with KOH (0.5 equiv, 14 mg, 0.25 mmol), and
Chloro(p-cymene)(1,1′-bis(2,6-diisopropylphenyl)-3,3′-dimethyl-
4,4′-bis(1,2,3-triazol-5,5′-ylidene))ruthenium(II) Hexafluorophos-
phate (4). Following the general procedure stated above and using
i
this was dissolved in PrOH (4 mL). The mixtures were stirred at the
1
assigned temperature for 24 h. After certain times small aliquots were
taken out of the reaction mixture to analyze the conversions by GC-
MS using hexadecane as an internal standard.
[RuCymCl2]2 (60 mg). Yield: 78% (0.157 mmol, 141 mg). H NMR
(400 MHz, CD2Cl2; 25 °C, TMS): δ (ppm) 7.68−7.63 (m,2H); 7.47−
7.43 (m, 4H); 4.65 (s, 6H); 4.57−4.45 (m, 4H); 3.20 (hept, J = 6.8
Hz, 2H); 2.33 (hept, J = 6.8 Hz, 2H); 1.95 (s, br, 1H);1.67 (s, 3H);
1.36 (d, J = 6.8 Hz, 6H); 1.24 (d, J = 6.8 Hz, 6H); 1.19 (d, J = 6.8 Hz,
6H); 0.89 (d, J = 6.8 Hz, 6H); 0.74 (d, J = 6.8 Hz, 6H). 13C NMR
(100 MHz, CD2Cl2, 25 °C, TMS): δ (ppm) 179.2 (carbene-C), 147.2,
146.3, 139.4, 134.8, 132.0, 124.3, 123.8, 40.4, 31.3, 28.9, 28.0, 27.0,
25.5, 22.4, 21.6, 21.5, 18.6. MS (ESI): m/z found 755.3140, calcd
755.3144 [C4 0 H5 4 N6 Cl1 Ru]+ . Anal. Calcd (%) for
[C40H54N6Cl1Ru1P1F6·3H2O]: C 50.34, H 6.34, N 8.81. Found: C
50.93, H 6.27, N 8.68. Dec: 250 °C.
Chloro(pentamethylcyclopentadienyl)(1,1′-bis(2,6-diisopropyl-
phenyl)-3,3′-dimethyl-4,4′-bis(1,2,3-triazol-5,5′-ylidene))iridium(III)
Hexafluorophosphate (8). Following the general procedure stated
above and using [IrCp*Cl2]2 (80 mg). Yield: 84% (0.168 mmol, 167
mg). 1H NMR (400 MHz, CD2Cl2; 25 °C, TMS): δ (ppm) 7.60−7.54
(m, 2H); 7.41−7.36 (m, 4H); 4.70 (s, 6H); 3.35 (hept, J = 6.8 Hz,
2H); 2.35 (hept, J = 6.8 Hz, 2H); 1.29 (d, J = 6.8 Hz, 6H); 1.25−1.19
(m, 12H); 1.17 (s, 15H); 0.89 (d, J = 6.8 Hz, 6H). 13C NMR (100
MHz, CD2Cl2, 25 °C, TMS): δ (ppm) 154.5 (carbene-C); 147.2;
146.9; 142.1; 135.3; 131.9; 124.5; 124.1; 92.4; 40.6; 29.1; 27.7; 27.1;
25.3; 21.6; 21.5; 9.41. MS (ESI): m/z found 847.3789, calcd 847.3795
[C40H55N6Cl1Ir]+. Anal. Calcd (%) for [C40H55N6Cl1Ir1P1F6·H2O]: C
47.49, H 5.78, N 8.31. Found: C 47.23, H 5.73, N 7.76. Mp: 243−245
°C.
ASSOCIATED CONTENT
* Supporting Information
■
S
Cif files and tables giving the crystallographic details, bond
lengths, and bond angles for ligands and complexes. These data
can be obtained free of charge via the Internet at http://pubs.
acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: Biprajit.sarkar@fu-berlin.de.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
We are grateful to the Fonds der Chemischen Industrie for the
financial support of this project.
■
REFERENCES
■
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