Transition metal complexes of a bis(carbene) ligand featuring 1,2,4-triazolin-5-ylidene donors: structural diversity and catalytic applications

Article abstract of DOI:10.1039/d0dt02142c

Dialkylation of the 1,3-bis(1,2,4-triazol-1-yl)benzene with ethyl bromide results in the formation of [L-H₂]Br₂which, upon salt metathesis with NH₄PF₆, readily yields the bis(triazolium) salt [L-H₂](PF₆)₂with non-coordinating counterions. [L-H₂](PF₆)₂and Ag₂O react in a 1?:?1 ratio to yield a binuclear Ag^I-tetracarbene complex of the composition [(L)₂Ag₂](PF₆)₂which undergoes a facile transmetalation reaction with [Cu(SMe₂)Br] to deliver the corresponding Cu^I-NHC complex [(L)₂Cu₂](PF₆)₂. In contrast, the [L-H₂]Br₂reacts with [Ir(Cp*)Cl₂]₂to generate a doubly C-H activated Ir^III-NHC complex5. Similarly, the triazolinylidene donor supported diorthometalated Ru^II-complex6is also obtained. Complexes5and6represent the first examples of a stable diorthometalated binuclear Ir^III/Ru^II-complex supported by 1,2,4-triazolin-5-ylidene donors. The synthesized Ir^III-NHC complex5is found to be more effective than its Ru^II-analogue (6) for the reduction of a range of alkenes/alkynesviathe transfer hydrogenation strategy. Conversely, Ru^II-complex6is identified as an efficient catalyst (0.01 mol% loading) for the β-alkylation of a wide range of secondary alcohols using primary alcohols as alkylating partnersviaa borrowing hydrogen strategy.

Full text of DOI:10.1039/d0dt02142c

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14 March 2017
Pages 3073-3412
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DOI: 10.1039/D0DT02142C
Transition Metal Complexes of a Bis(carbene) Ligand Featuring
1,2,4-Triazolin-5-ylidene Donors: Structural Diversity and
Catalytic Applications
Vivek Kumar Singh, S. N. R. Donthireddy, Praseetha Mathoor Illam, and Arnab Rit*
Department of Chemistry, Indian Institute of Technology Madras, Chennai 600036, India.
*To whom correspondence should be addressed. E-mail: arnabrit@iitm.ac.in
Abstract:
Dialkylation of the 1,3-bis(1,2,4-triazol-1-yl)benzene with ethyl bromide resulted in the formation
of [L-H2]Br2 which upon salt metathesis with NH4PF6 readily yielded the bis(triazolium) salt [L-
H2](PF6)2 with non-coordinating counterions. [L-H2](PF6)2 and Ag2O react in 1:1 ratio to yield a
binuclear AgI-tetracarbene complex of composition [(L)2Ag2](PF6)2 which undergoes facile
transmetalation reaction with [Cu(SMe2)Br] to deliver the corresponding CuI-NHC complex
[(L)2Cu2](PF6)2. In contrast, the [L-H2]Br2 reacts with [Ir(Cp*)Cl2]2 to generate a doubly C-H
activated IrIII-NHC complex 5. Similarly, the triazolinylidene donor supported diorthometalated
RuII-complex 6 was also obtained. Complexes 5 and 6 represent the first examples of stable
diorthometalated binuclear IrIII/RuII-complex, respectively supported by 1,2,4-triazolin-5-ylidene
donors. The synthesized IrIII-NHC complex 5 was found to be more effective than their RuII-
analogue (6) for the reduction of a range of alkenes/alkynes via the transfer hydrogenation strategy.
Conversely, the RuII-complex 6 was identified as an efficient catalyst (0.01 mol% loading) for the
β-alkylation of a wide range of secondary alcohols using primary alcohols as alkylating partners
via borrowing hydrogen strategy.
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Introduction
During the past few decades, N-heterocyclic carbenes (NHCs) have become very popular in
organometallic chemistry especially, in catalysis.1 The ease of synthesis along with the tuning of
their stereoelectronic properties coupled with strong σ-donor ability have made them an elite class
of ligands.1b,2 Although NHC ligands based on the imidazole heterocycles (ImNHCs) are the most
extensively studied ones,1a,3 click-derived triazole containing analogues (tzNHCs), commonly
known as mesoionic carbenes (MICs), became popular in the carbene chemistry after their
captivating reactivity and applications introduced by Albrecht et al..4a-e Among triazole based
NHCs, 1,2,3-triazolin-5-ylidenes are the most common ones and have gained the attention of the
researchers. On contrary, transition metal chemistry of the 1,2,4-triazolin-5-ylidene donor,
introduced to the mainstream research by Enders and coworkers,4f,g containing systems are rather
unexplored although they might lead to interesting systems because of its weak -donating and
strong -accepting properties as compared to the most studied ImNHC moiety.5
Cyclometalated, in particular, orthometalated complexes are gaining researchers’ attention mostly
due to their increased stability as compared to their non-chelated variants.6 However, the reported
orthometalated late transition metal-NHC complexes containing IrIII and RuII metals mostly are of
mononuclear type.6 Only recently, related multinuclear iridium-NHC complexes7a-e have been
highlighted for their augmented catalytic efficiency, but, still the multinuclear ruthenium-NHC
complexes are rare in the literature.7f Multiple azolium units installed at a phenyl ring featuring
various substitution patterns are the common structural motifs in this area. In general, 1,4-
phenylene-bridged bis(azolium) salts tend to form diorthometalated complexes via double C-H
activation (Scheme 1a-b), whereas, 1,3-phenylene-bridged ones are more prevailed to produce
mononuclear pincer-type complexes because of the inherent difference in their geometry.8 In 2014,
Sarkar et al. have reported the stable diorthometalated, resulting 5-membered iridacycles, IrIII-
complexes of 1,4-phenyl-substituted bis(1,2,3-triazole or 1,2,3-triazolin-5-ylidene) containing
ligands with a C^N or C^C binding modes (Scheme 1b).7a It should be noted that the formation of
a mixture of mono- and dinuclear orthometalated RuII-NHC complexes was observed for the
related bis-imidazolium ligand.7f Later in 2016, Graiff’s group has reported another doubly
orthometalated binuclear IrIII-complex of 1,4-phenyl-substituted ImNHC based ligand system,
however, featuring six-membered iridium-containing chelate rings (Scheme 1a).7b
2

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Scheme 1. Overview of the reported orthometalated binuclear bis-NHC complexes and the present
work.
Et
Previous Reports
N
N
N
N
Ir
N
N
Ir
Cl
N
Me
N
H
Cl
R
Ir
Cl
N
R
N
Ir
H
Cl
N
N
Cl
Ir
Me
Cl
Ir
N
R = 2,6-diisopropylphenyl
N
N
Et
a
b
c
2PF6
Present Work
N
N
N
N
N
N
N
N
N
N
N
R
N
N
R
R
R
M
M
M
M
R
arene
R
arene
X
M = IrIII/RuII
X
N
N
N
N
N
M = AgI/CuI
Conversely, the only diorthometalated IrIII-NHC metal complex based on a 1,3-diaryl-substitution
pattern was reported by Danopoulos et al. by employing a ligand precursor where two imidazolium
units are installed at a pyridine rather than a phenyl platform (Scheme 1c). However, it should be
noted that the relatively reactive IrI-precursor [Ir(cod)Cl]2 was utilized for this purpose.9a It is
worth mentioning that the RuII-based such complexes with double C-H activation are not known
in the literature to the extent of our knowledge. Further, the chemistry of poly-NHC ligand
possessing 1,2,4-triazolin-5-ylidene donors is rather unexplored.5e,f Therefore, we became
interested in developing the chemistry of bis(NHC) ligands incorporating 1,2,4-triazolin-5-ylidene
donors. Herein, we present the coordination behavior of a 1,3-disubstituted phenylene-based
bis(1,2,4-triazolium) salt towards the coinage metals (AgI and CuI) as well as the catalytically
relevant RuII- and IrIII-metal centers. Coinage metals resulted in the formation of binuclear
tetracarbene complexes of general formula [(L)2M2](PF6)2, whereas, the RuII- and IrIII-metal
centers gave access to the 1,2,4-triazolin-5-ylidene donor supported doubly orthometalated
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binuclear complexes. Iridium-complexes are reported to be effective as catalysts in the reduction
of unsaturated functional groups, whereas in recent years, the RuII-complexes are also securing
their position as efficient catalyst systems in various important organic transformations.5a,6a,b
Keeping this in mind, we also studied the catalytic efficiency of the synthesized binuclear IrIII- and
RuII-NHC complexes which unveiled that the IrIII-NHC complex 5 is more effective than its RuII-
analogue, 6 for the catalytic reduction of alkenes/alkynes via the transfer hydrogenation strategy.
Further, the RuII-complex 6 was identified as an efficient catalyst for the β-alkylation of diverse
secondary alcohols using primary alcohols.
Results and Discussion
Synthesis of the Bis(triazolium) Salts 1 and 2. The 1,3-bis(1,2,4-triazol-1-yl)benzene (A) was
synthesized via coupling of the 1,3-dibromobenzene with 1,2,4-triazole by following a modified
literature procedure.10a Further alkylation of A with ethyl bromide in DMF gave the ethylated
compound [L-H2]Br2 (1) in 75% yield as a white solid (Scheme 2a). Presence of the characteristic
downfield shifted triazolium proton signals at δ = 11.32/9.64 ppm along with the N-ethyl protons
at 4.42 ppm (quartet for the CH2) and 1.60 ppm (triplet for the CH3) confirms the quaternization
of the triazole ring (Fig. S1). Exchange of the bromide counterions in [L-H2]Br2 with the non-
coordinating PF6 ions via salt metathesis using NH4PF6 in methanol provided the [L-H2](PF6)2 in
an excellent yield of 94% (Scheme 2a). This exchange of counterions is supported by the 19F and
1
31P NMR studies and the change in chemical shift of the triazolium H NMR signals to  =
11.12/9.56 ppm. This shift is possibly due to lesser hydrogen bonding interaction of the PF6
counterions with the triazolium protons as compared to bromide ions.9b Formation of the
triazolium salts is supported by the ESI mass spectra (positive ions) of [L-H2]Br2 and [L-H2](PF6)2
displaying peaks at m/z = 452.9827 for the monopositive fragment [M+Na]+ (calculated m/z =
452.9837) and at m/z = 415.1573 for the [M-PF6]+ fragment (calculated m/z = 415.1235),
respectively. Single crystal X-ray diffraction studies of a crystal of [L-H2](PF6)2 (Scheme 2b)
confirmed the expected structures of the compounds. Both the triazolium salts were found to be
air/moisture stable and are readily soluble in polar organic solvents like DMF/DMSO but insoluble
in solvents such as DCM, THF, and diethyl ether.
Scheme 2. (a) Synthesis of the bis(triazolium) salts [L-H2]Br2/(PF6)2. (b) Molecular structure of
the [L-H2](PF6)2 with displacement parameters at 40% probability level. The PF6 counterions and
4

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the hydrogen atoms except H3 and H12 are omitted for clarity. N-ethyl groups are shown in capped
stick.
a)
2Br
EtBr
N
N
N
N
N
N
N
N
DMF, 90 °C
N
N
N
N
A
Et
Et
L
1
[ -H2]Br2;
MeOH, RT
NH4PF6
b)
2PF6
N
N
N
N
N
N
Et
Et
L
2
[ -H2](PF6)2;
Synthesis of the Binuclear AgI- and CuI-NHC Complexes 3-4. Poly-NHC ligand systems are
well known for the generation of supramolecular architectures with coinage metals. Further, AgI-
NHC complexes are known to undergo facile transmetalation reactions to yield various interesting
structures containing other transition metal ions.8a,11 To obtain a new type of AgI-NHC based
architectures, we attempted the metalation of 2 having non-coordinating PF6 counterions with
Ag2O (which plays the dual role of the base as well as metal source) to give the corresponding
AgI-NHC complex. We chose the triazolium salt with PF6 rather than a bromide counterion simply
because of the fact that (a) presence of a coordinating bromide ion typically leads to complex
structures instead of an expected structure based on the ligand architecture employed and (b)
absence of coordinating anion leads to the formation of exclusively NHC-based architectures.12a,b
Accordingly, a binuclear tetracarbene AgI-complex 3 was isolated from the reaction of 2 with
Ag2O as an air-stable but light-sensitive white solid in an excellent yield of 81% (Scheme 3a).
Absence of the most acidic triazolium proton signal at  = 11.12 ppm and the appearance of the
13
13C{1H} NMR signal as two doublets in the range of  = 183.0–181.4 ppm due to C−107/109Ag
coupling are consistent with the formation of a AgI-NHC complex (Fig. S8). The appearance of a
1
single H NMR resonance at  = 8.60 ppm for both the triazole backbone protons signifies the
formation of a symmetrical complex (Fig. S7). Formation of a binuclear AgI-tetracarbene complex
of composition [(L)2Ag2](PF6)2 is evidenced by the ESI mass spectrometric analysis (positive ions)
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which exhibits the most intense peak at m/z = 897.0680 for the corresponding mono-positive
fragment [M−PF6]+ (calculated m/z = 897.0619) with isotopic patterns in perfect agreement with
the calculated patterns (Fig. S11).
Scheme 3. (a) Synthesis of the binuclear NHC-bridged AgI- and CuI-complexes 3 and 4. (i) Ag2O,
CH3CN, 70 °C, 36 h. (ii) [Cu(SMe2)Br], CH3CN, RT, 14 h. (b) Molecular structures of 3 (left) and
4 (right) with displacement parameters at the 40% probability level. Counterions and hydrogen
atoms are omitted for clarity. N-ethyl groups are shown in capped stick. Some important bond
lengths (Å) and bond angles (): Ag1-C8 2.090(3), Ag1*-C1 2.098(3), C8-Ag1-C1* 174.55(12);
Cu1-C1 1.887(11), Cu1*-C14 1.874(12); C1-Cu1-C14* 174.0(4).
a)
2PF6
2PF6
2PF6
N
N
N
Et
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
Et
Et
Et
Et
Et
Et
(i)
(ii)
Cu
Cu
Ag
Ag
Et
Et
N
N
N
N
N
N
N
N
N
N
N
Et
N
L
[ -H2](PF6)2
L
3
[( )2Ag2](PF6)2;
L
4
[( )2Cu2](PF6)2;
b)
The structure of the complex was confirmed by X-ray diffraction studies with a single crystal
obtained via slow diffusion of diethyl ether into a saturated solution of the complex 3 in acetonitrile
(Scheme 3b). The crystallographic data show that the complex 3 crystallized in monoclinic space
group P21/c. Each silver atom is connected to two triazolinylidene donors from two different
ligands in a bridging manner. The geometry around the AgI ion slightly deviates from linearity
with C8−Ag1−C1* angle of 174.55(12) and the observed Ag−CNHC bond distances
(2.090(3)−2.098(3) Å) fall in the range reported for analogous Ag−NHC complexes.8a,12c It is also
clear from the crystal structure that the bridging phenyl rings are coplanar to each other with
centroid to centroid separation of 4.463 Å and the Ag−Ag distance is found to be 7.218(5) Å.
6

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Notably the triazolinylidene rings are not oriented perpendicular to the central phenyl ring, but
rather inclined towards the phenyl ring (torsion angle of 37.0-44.63) resulting in a shorter inter-
centroid separation of the phenyl rings.
Transmetalation of AgI-NHC complexes is recognized as a preferred route to obtain various
transition metal-NHC complexes than their direct synthesis from the precursor azolium salts.10b,12b
After getting access to AgI-NHC complex, we tried the transmetalation strategy to realize other
coinage metal complexes and accordingly, treatment of the AgI-NHC complex 3 with two
equivalents of [Cu(SMe2)Br] at room temperature in acetonitrile smoothly afforded a CuI-NHC
complex 4 in good yield of 82%. The complex was isolated as pale yellow air and moisture
sensitive solid in contrast to the parent AgI-NHC complex. Formation of the complex 4 was
ascertained by the absence of the characteristic 13C{1H} NMR resonance of the AgI-bound carbene
carbon at  = 183.0–181.4 ppm for the complex 3 and the appearance of a new distinct NHC signal
at  = 180.6 ppm corresponding to the CuI–coordinated carbenic carbon atom. Other 1H (Fig. S12)
and 13C-NMR (Fig. S13) resonances for the complex 4 do not show much deviation from the parent
Ag-complex. The structure of the complex 4 was established by the X-ray diffraction studies
(Scheme 3b) which reveals that the binuclear nature of the parent AgI–NHC complex 3 is still
maintained. Similar transmetalation reactions of polynuclear carbene complexes by retaining the
overall structure of the complex have been observed before.8a,12a The Cu−Cu separation in 4 was
observed to be similar to the corresponding Ag−Ag distance in 3 (7.224(2) vs 7.218(5) Å), whereas,
the Cu−CNHC bond lengths are found to be shorter (1.874(12)-1.887(11) Å) than the observed
Ag−CNHC bond distances in complex 3 (2.090(3)−2.098(3) Å) as expected from the difference in
size between the CuI and AgI ions. As observed in the case of AgI–NHC complex, the phenyl rings
are oriented in a co-planar fashion, however, the separation between the centroids of the phenyl
rings was found to be 3.980 Å which is 0.5 Å less than seen in 3. This can be explained by the
shorter bond length of CuI-NHC than AgI-NHC and lower torsion angle of the triazolinylidene
ring with respect to the phenylene ring in the complex 4 (35.65−43.47) than in 3 (37.0−44.63).
The geometry around the Cu(I)−center is slightly distorted from the expected linearity with an
C1−Cu1−C14* angle of 174.0(5).
Synthesis of the Doubly Orthometalated IrIII- and RuII-NHC Complexes 5-6. After being able
to synthesize the binuclear AgI- and CuI-NHC complexes which are structurally important and
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thus, no catalytic experiments were performed with them, we then turned our attention towards
catalytically relevant IrIII- and RuII-complexes obtained from [L-H2]Br2. Taking into consideration
that the triazolium salt [L-H2]Br2 may also provide the corresponding pincer type complexes via
activation of the C5-triazolium and one of the central phenyl ring protons, we initially carried out
reactions of [L-H2]Br2 with 0.5 equiv. of [Ir(Cp*)Cl2]2 under various conditions. However, in all
of our attempts, we always observed the formation of a dinuclear IrIII-NHC complex as the main
product and mononuclear IrIII-complex was not detected. Finally, the IrIII-complex 5 was obtained
in 80% yield as an air stable solid by reacting [L-H2]Br2 with one equiv. of [Ir(Cp*)Cl2]2 in
presence of Cs2CO3 as base in acetonitrile (Scheme 4a). It is to be noted that the related sterically
more relaxed 1,4-subtituted bis-imidazolium salt yielded only the mononuclear orthometalated
iridium(III)-complex even in the presence of one or more equiv. of [Ir(Cp*)Cl2]2.12d The complex
5 was fully characterized by NMR spectroscopy, mass spectrometry, and elemental analysis as
1
well as by the X-ray crystallographic studies. The H NMR spectrum of the complex reveals
resonances of the dicarbene ligand at upfield shifted regions as compared to the parent triazolium
salt [L-H2]Br2. Formation of a IrIII-NHC complex is revealed by the absence of the highly
downfield shifted 1H NMR signal at  = 11.32 ppm (the C5-triazolium protons in [L-H2]Br2) and
the appearance of a new signal at  = 165.9 ppm, characteristics of an IrIII-NHC signal,6c,12d,13a in
the 13C{1H} NMR of the complex (Fig. S17). Additionally, 1H NMR spectrum (Fig. S16) features
three distinct aromatic signals in 1:1:2 ratio corresponding to inequivalent aromatic protons of the
phenylene bridge and two equivalent triazole C3-protons, respectively. Further, the presence of
only two phenyl ring protons and a single type of triazolinylidene proton suggest the possibility of
a symmetrical complex formation via double orthometalation which is backed by the previous
observations that an iridium-center activates the aromatic protons in close vicinity.7a,13
Additionally, diastereotopic nature of the N-CH2 protons of the ethyl moiety as reflected by two
sets of quartets in the aliphatic region ( = 4.25-4.38 ppm) further supports the orthometalation.
As per our expectation, orthometalation is also evidenced by the distinct IrIII-coordinated ortho-
Cphenyl signal at  = 137.4 ppm (established by 2D correlation NMR spectroscopy). The observed
chemical shift for the iridium-CPhenyl carbon atoms is in the range of structurally related
diorthometalated complexes,12d,13a however, upfield shifted from the monoorthometalated
complexes.6c,12d Relying on previous literatures, we propose that the doubly orthometalated
complex 5 has been formed via triazolinylidene assisted concerted-metalation-deprotonation
8

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(CMD) pathway.6b,c,14a-c Generation of the potential isomers of complex 5 resulting due to the
different orientations of the Cp* rings with respect to the central phenylene ring was not observed
by NMR spectroscopy.
Scheme 4. (a) Synthesis of the diorthometalated IrIII-complex 5. (i) [Ir(Cp*)Cl2]2, Cs2CO3,
CH3CN, reflux, 30 h. (b) Molecular structure of 5 with displacement parameters at 40% probability
level. Metric parameters of one out of four molecules in the asymmetric units are shown here and
they are within the experimental errors to the parameters observed for other molecules. The N-
ethyl groups and Cp* are shown in capped stick. Some important bond lengths (Å) and bond angles
() are mentioned: C22-Ir2 2.056(14), C24-Ir1 2.047(13), C27-Ir2 1.973(17), C31-Ir1 2.001(17),
Ir-Br 2.5293(18)-2.5339(18); C27-Ir2-C22 76.9(6), C31-Ir1-C24 77.1(6).
2Br
a)
Et
b)
N
N
N
N
N
N
N
N
N
(i)
Et
Et
Ir
Ir
Br
Br
N
N
N
Et
5
L
[ -H2]Br2
The ESI mass spectrometric analysis (positive ions) provides additional support for the formation
of a diorthometalated iridium-NHC complex by showing the most intense peak at m/z = 1001.2025
for the fragment [M-Br]+ (calculated m/z = 1001.2052) with isotopic patterns matching properly
with the calculated one. To unequivocally confirm the structure of the complex, X-ray
crystallographic stuides were performed with a crystal obtained via slow diffusion of diethyl ether
into a saturated DCM solution of the complex at ambient temperature. The crystal structure
confirmed the formation of a doubly orthometalated binuclear IrIII-NHC metal complex 5 (Scheme
4b) as concluded from the NMR and mass spectral analysis. The complex 5 features a typical
three-legged piano stool geometry around the metal center and the three legs are represented by
the phenylic carbon, bromide ion, and the triazolinylidene carbene carbon. Diorthometalation
results in the formation of two 5-membered iridacycles which are bridged by a phenylene spacer
and the Ir-Br moieties are observed to be in the thermodynamically more preferred trans-
orientation with respect to this essentially planar metal-ligand skeleton [torsion angles are 0.60
(C31-N2-C25-C24) and 0.20 (C27-N4-C21-C22)]. The bite angles of the ligands in the
iridacycles are noted to be 76.9(6)-77.1(6) which fall well in the region reported for the IrIII-
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complexes containing 5-membered CNHC^CPh chelate rings.6c,7a The Ir-Cphenyl bonds (2.047(13)-
2.056(14) Å) are slightly longer than the Ir-CNHC bonds (1.973(17)-2.001(17) Å) and this is
probably due to -back donation from iridium to the reasonably good -acceptor triazolinylidene
moiety.5a,b The intramolecular separation between the iridium center is about 6.2 Å and the
separation between the iridium center and the Cp* ring centroid measures 1.853-1.857 Å. When
compared to the parent triazolium cation, the conversion to a coordinated NHC results in shrinking
of the N-C5-N bond angles (6-7) and increase of the N-C5 bond distances (d  0.049 Å) in line
with the previous findings and DFT-calculations.12d-f
Scheme 5. Synthesis of the diorthometalated binuclear RuII-NHC complex 6. (i) [Ru(p-
cymene)Cl2]2, Cs2CO3, THF, 70 C, 24 h.
2Br
Et
N
N
N
N
N
N
N
N
N
Et
Et
(i)
Ru
Ru
Br
Br
N
N
N
Et
6
L
[ -H2]Br2
After the success in accessing the binuclear diorthometalated iridium complex 5, we turned our
attention towards the synthesis of similar doubly orthometalated complex based on the RuII-center.
Accordingly, the triazolium salt [L-H2]Br2 was reacted with [Ru(p-cymene)Cl2]2 using Cs2CO3 as
base at 70 C in THF and to our delight, after the reaction a yellow colored compound was isolated
whose spectroscopic data correspond to a diorthometalated RuII-NHC complex similar to that of
the iridium-case described before (Scheme 5). However, contrary to the iridium-complex (5), the
isolated RuII-NHC complex was observed to be air and moisture sensitive. This may be due to the
presence of two RuII-Caryl bonds at the same phenylene ring which are prone to hydrolysis
especially in presence of a catalytic acidic impurity and similar hydrolytic decomposition of the
orthometalated RuII-NHC has been observed previously.7f The most acidic triazolium proton signal
observed in [L-H2]Br2 at  = 11.32 ppm was, as expected, missing in the 1H NMR of the complex
6 (Fig. S18). The presence of the distinctive metal bound carbene carbon signal at  = 187.3 ppm,
within the usual range for RuII-complexes containing CNHC^CPh chelate rings,6b,7f,12g in the 13C{1H}
NMR spectrum (Fig. S19) supports the formation of a RuII-NHC complex. Furthermore, four
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aromatic protons, two phenylene and the triazolinylidene backbone protons, appear in 1:1:2 ratio
as observed for the iridium-complex 5. These observations along with the presence of a single
RuII-bound phenyl carbon signal at  = 155.6 ppm (ascertained via 2D correlation NMR
spectroscopy) in the expected region6b,7f,12g reinforce the formation of a rarely observed
symmetrically diorthometalated binuclear RuII-NHC complex.7f Symmetrical diorthometalation is
further supported by the single set of diastereotopic N-CH2 protons and the presence of only one
set of four (a set of two doublets of each two protons intensity should be observed in the absence
of orthometalation) distinct doublets in the region of  = 5.45-5.81 ppm corresponding to the
aromatic protons of p-cymene groups. Mass spectrometric analysis (positive ions) of the complex
6 establishes its formation by displaying the most intense peak at m/z = 858.1009 corresponding
to the mono-positive fragment [M-Br+CH3CN]+ (calculated m/z = 858.1019) with isotopic patterns
matching exactly with the calculated one (Fig. S20). Unfortunately, even after several attempts we
could not obtain the single crystals of 6 suitable for X-ray crystallographic analysis due to
decomposition of the complex in the chlorinated solvent over time. To the best of our knowledge,
5 and 6 are the first examples of diorthometalated binuclear IrIII/RuII–complex, respectively with
1,2,4-triazolin-5-ylidene donors.
IrIII-NHC Complex 5 Catalyzed Transfer Hydrogenation Reaction. Hydrogenation of the
organic molecules containing unsaturated -systems is an important and fundamental catalytic
process.15a Reducing the unsaturated bonds through hydrogen gas is a perilous process whereas
transfer hydrogenation which uses inorganic or organic hydrogen source makes it a competent,
sustainable, and mild method that is operationally simpler and safer than the direct hydrogenation
with dihydrogen.15b Possibly because of that, catalytic reduction of the unsaturated bonds via the
transfer hydrogenation (TH) strategy is currently one of the most investigated hydrogenation
pathways. Transfer hydrogenation of non-polarized substrates such as olefins and alkynes are less
studied as compared to the polarized substrates like ketones, imines, and α,β-unsaturated
carbonyls. In general, this method involves various hydrogen sources such as formates, amine-
borane adducts, and alcohols.15b-f Although various transition metal-NHC complexes have shown
improved catalytic activities for the reduction of polarized C=O and C=N bonds via transfer
hydrogenation strategy,15b available reports on the NHC or MIC ligand based catalytic systems for
the transfer hydrogenation of more challenging alkenes/alkynes are relatively less16,17 possibly due
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to inadequate polarization of the C=C and CC bonds. Nevertheless, the Peris and Mata et al. have
described that the iridium complexes of NHC/aNHC based chelating ligands (2.5 mol% loading)
may act as catalysts to reduce olefins/phenylacetylene under transfer hydrogenation strategy
however, with poor efficiency.17d Later on, the Elsevier and Sarkar group have independently
shown that related iridium and ruthenium complexes of chelating ligands featuring NHC and/or
MIC donors (1 mol% loading) are able to transfer hydrogenate olefins more effectively however,
with narrow substrate scope.17a,c Very recently, the groups of Diéguez and Wang have separately
reported the effective transfer hydrogenation of variety of alkenes using an NCP-iridium pincer
complex and triazolinylidene iridium complex, respectively.17b,18
After isolating the well characterized air and moisture stable IrIII-NHC complex in decent amount,
we moved forward to study its catalytic efficacy. Initially, we tried the transfer hydrogenation of
styrene as an exemplary reaction using the complex 5, under standard transfer hydrogenation
conditions (2-propanol as both solvent and hydrogen source with base under reflux condition)
using substrate/catalyst/base in the ratio of 1/0.003/0.2. To our delight, quantitative formation of
ethylbenzene was achieved in 6 h and among all the different bases tested, NaOtBu provided the
best result. We also studied the catalytic efficiency of the RuII-NHC complex 6 and observed that
it is less effective (only 43% conversion of styrene was observed under the similar conditions
employed for IrIII-NHC complex 5).
With these optimized reaction conditions, we explored the substrate scope of the present catalytic
system using a small library of electronically and sterically varied alkenes and the results obtained
are summarized in Table 1. The p-substituted styrene derivatives were efficiently hydrogenated to
the corresponding alkanes (9b-d). Among them, styrenes containing electron-donating substituent
were reduced faster whereas, the electron withdrawing group (halide) substituted styrenes were
comparatively less reactive towards the reduction process (9b vs 9c-e). The activity of ortho-
chlorostyrene was comparable with that of the corresponding para-substituted styrene (9c vs 9e).
2-Vinylnaphthalene was converted to 2-ethylnaphthalene quantitatively with a slightly higher
catalyst loading of 0.5 mol% in 24 h (9f). In order to probe the effectivity of our catalytic system
towards internal alkenes, cyclooctene was chosen as model substrate and excellent yield was
realized in 24 h (9h). In line with the previous literature reports,17b alkyl substituted olefins undergo
slower hydrogenation than that of the aryl substituted olefins (9g-h vs 9a). Further, reduction of
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the sterically hindered α-methyl styrene needs longer reaction duration along with higher catalyst
loading to obtain the product 9i in decent yield.
Table 1. Transfer hydrogenation of alkenes and alkynesa
5
Cat.
R
R
or
iPrOH, NatBuO
reflux
or
R
R
R
R
9
Cl
R
b
9f
; 99% (24 h)
9e
; 91% (12 h)
9a
R = H, ; 98% (6 h)
9b
= Me, ; 93% (6 h)
9c
= Cl, ; 99% (12 h)
b
9d
= Br, ; 92% (24 h)
b
c
9h
; 94% (24 h)
9i
; 61% (24 h)
Ph
n-hexene
4
4
4
4
d
Ph
9g
; 100% (24 h)
Ph
Ph
(64%) +
(24 h)c,e
f
9j
9j'
(36%) 9k
9k'
(34%)
(66%) +
(24 h)c
aGeneral conditions: alkenes or alkynes (0.4 mmol), complex 5 (0.3 mol%), NatBuO (20 mol%),
iPrOH (4 mL), reflux temperature; yields were determined by GC-MS using mesitylene as internal
b
c
standard. Catalyst (0.5 mol%). Catalyst (1 mol%). dHexene (57%) was observed as mixture of
isomers. eCalculated based on 1H NMR. fAnd other isomers of dodecene.
To probe the versatility of our present catalytic system, we have also carried out the reduction of
selected alkynes using the same strategy. At the outset, diphenylacetylene was transfer
hydrogenated under optimized conditions using the complex 5 (1 mol%) and quantitative reduction
of diphenylacetylene was observed. However, the 1H NMR analysis of the reaction mixture shows
the presence of a mixture of semi-reduced trans-stilbene (9j, 64%) and completely reduced
diphenylethane (9j’, rest 36%). Further, the dialkyl-substituted acetylene (6-dodecyne) also
undergoes quantitative reaction. In this case, GC-MS analysis of the reaction mixture shows the
formation of the completely hydrogenated dodecane (9k’) in 34% along with the presence of a
mixture of semi hydrogenated isomeric dodecenes (9k). This type of isomerization of the obtained
alkene product has also been observed previously.16a,b
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Cross coupling of secondary and primary alcohols via borrowing hydrogen strategy.
Development of atom-efficient methods to construct C-C bonds to access long chain/branched
alcohols is highly desirable as the produced alcohols are useful as biofuels which are potent
alternatives to the conventional fossil fuels.19a Notably, borrowing hydrogen (BH) or hydrogen-
autotransfer strategy for the alkylation of ketones/alcohols and amines has evolved to be a
demanding process because of the atom economy and greener nature as it generates water as sole
byproduct.20 In this context, several transition metal catalysts as well as some metal free systems
have been reported for the realization of higher order alcohols via BH strategy.19d,20a-e,21 β-
Alkylation of secondary alcohols using primary alcohols has now become a very important tool
and greener alternative to realize long chain alcohols as it avoids the use of expensive alkyl halides
or other toxic electrophiles and the use of renewable and non-hazardous alcohols as alkylating
agents makes this process versatile with high practical applicability. Despite the significant
advancements, most of the reports have their own limitations such as use of high catalyst loading,
higher amount of base, and elevated temperatures along with longer reaction duration, which hence
reduces the efficiency of the catalytic system.
Table 2. Optimization studies for the β-alkylation reactiona
OH
OH
O
6
Cat. (0.01 mol%)
OH
Toluene, 120 C
10a
10a'
Entry Catalyst Base (equiv.) Time % Conv.
Product
10a : 10a’
1
2
3
4
5
6
7
6
6
6
6
6
6
5
KtBuO (0.1)
KtBuO (0.1)
KOH (0.1)
12 h
24 h
24 h
24 h
24 h
12 h
24 h
66
79 : 21
100
100
55
80 : 20
72 : 28
64 : 36
91 : 9
Cs2CO3 (0.1)
KtBuO (0.2)
KtBuO (0.2)
KtBuO (0.2)
99
99
93 : 7
100
89 : 11
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aGeneral conditions: 1-phenylethanol (0.5 mmol), benzyl alcohol (0.55 mmol), catalyst (0.01
mol%), and toluene (1 mL). Base equivalent is w.r.t. 1-phenylethanol. Conversion and product
ratio were determined by GC-MS analysis based on 1-phenylethanol using mesitylene as internal
standard.
In 2016, Kundu et al. has reported 2,2’-bipyridine based effective bifunctional RuII-catalysts for
the β-alkylation of secondary alcohols utilizing primary alcohols using low catalyst (0.1 mol%)
but high base (0.5 equiv.) loading.19b Same group has recently observed that the introduction of
NHC-donor in the supporting ligand leads to a very efficient RuII-based catalyst system (achieving
very high turnover number) for the same process although still demand relatively high base loading
(0.4 equiv.).19c Inspired by this progress, we focused towards the β-alkylation of secondary
alcohols utilizing primary alcohols as alkylating agents using our doubly orthometalated RuII-NHC
complex with lower catalyst and base loadings.
At first, the reaction of 1-phenylethanol with benzyl alcohol was chosen as benchmark alkylation
reaction to probe the potential of the present catalytic system. By employing RuII-precatalyst 6
(0.01 mol%), various bases were screened (Table 2) and observed that KtBuO (20 mol%) at 120
C for 24 h results in essentially quantitative conversion of 1-phenylethanol with 93% selectivity
to the 1,3-diphenylpropan-1-ol (entry 5). Under identical reaction conditions, analogous IrIII-
complex 5 showed similar performance (entry 7) and hence we decided to proceed with the
reasonably inexpensive RuII-precatalyst 6. Further optimizations disclose that the reaction is
complete within 12 h maintaining the high selectivity (entry 6) which provide the 1,3-
diphenylpropan-1-ol in 88% isolated yield. With these optimized reaction conditions, we explored
the scope of the β-alkylation reaction with a diverse range of alcohols using the complex 6 as
precatalyst (Table 3). First, 1-phenylethanol was reacted with structurally and electronically
different primary alcohols under the optimized reaction conditions. Irrespective of the electronic
nature of the substituted (both electron-donating and withdrawing) benzyl alcohols, this catalytic
system afforded good to excellent conversions to the corresponding long chain alcohols with high
selectivity (entries 2-4). Further, 4-OMe/Cl substituted benzyl alcohols exclusively provided the
corresponding β-alkylated alcohols, which could be isolated in good to excellent yields (entries 3-
4). The activity of 3-chlorobenzyl alcohol was alike with that of the corresponding 4-chlorobenzyl
alcohol (entry 5 vs 4).
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Table 3. Scope for the synthesis of β-alkylated alcoholsa
OH
O
OH
6
Cat. (0.01 mol%)
R
R
R
OH
KtBuO, Toluene
10
10'
12 h, 120 C
Product
10 : 10'
Entry
Secondary alcohol
Primary alcohol
Conversion (%)
1
2
R = H
99
98
93:7 (88c)
OH
OH
R = Me
96:4
R
100:0 (86c)
100:0 (74c)
92
90
3
4
R = OMe
R = Cl
OH
Cl
OH
93
93:7 (76c)
5
Me
OH
92:8 (80c)
OH
OH
6
90
OH
OH
94:6 (84c)
93b
7
OH
99b,d
95b,d
8
9
X = O
X = S
90:10
94:6
X
OH
O
95:5 (91c)
OH
100
10
O
OH
95:5 (86c)
81:19
11
95
99
R = Me
OH
12
13
14
R = OMe
R = Cl
91:9
R
99
99
92:5e (84c)
R = Br
OH
OH
100:0 (94c)
95:5
98
15
16
OH
63b,f
OH
aGeneral conditions: secondary alcohol (0.5 mmol), primary alcohol (0.55 mmol), catalyst 6 (0.01
mol%), KtBuO (0.1 mmol), and toluene (1 mL). Conversion/product selectivity was determined
by GC-MS/1H NMR analysis w.r.t. secondary alcohol (mesitylene/1,3,5-trimethoxybenzene was
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b
c
d
e
used as internal standard). 0.1 mol% catalyst was used. Isolated yields of 10. 16 h. 2%
debromination of product alcohol was observed. f24 h.
The sterically hindered ortho-methylbenzyl alcohol can also be utilized to provide the desired
product in good yield (entry 6). Next, we targeted the more challenging aliphatic alcohol, 1-
hexanol which also afforded the corresponding β-alkylated product in excellent yield when 0.1
mol% catalyst loading was used (entry 7). The heteroaromatic alcohols such as 2-furanmethanol
and 2-thiophenemethanol also act as good alkylating agents and provided the corresponding
products in excellent yields (entries 8-9). Notably, piperonyl alcohol smoothly coupled with 1-
phenylethanol to deliver the corresponding long-chain secondary alcohol in 91% isolated yield
(entry 10).
The scope of the reaction was further extended using various secondary alcohols keeping benzyl
alcohol same. As seen before for primary alcohols, electron donating (-Me, -OMe) and electron
withdrawing groups (-Cl, -Br) containing 1-phenylethanol also efficiently underwent alkylation
with benzyl alcohol to afford the corresponding secondary alcohols in excellent yields with high
selectivity (entries 11-14). Gratifyingly, this catalyst system offers excellent tolerance towards the
aryl-halide bonds (entries 4-5 and 13-14). Further, the (naphthalen-2-yl)ethanol was also
successfully coupled to benzyl alcohol and delivered the product in excellent isolated yield of 94%
(entry 15). The alkylation of 2-heptanol with benzyl alcohol also provided the desired product with
very high selectivity, however in moderate yield (entry 16). Overall, the present system under 0.01
mol% catalyst loading along with lower base loading shows similar activity to the most active
Ru(II)-based systems with reasonable substrate scope.19b,c However, a direct comparison with the
previous reports may not be appropriate due to incompatible catalytic system and reaction
conditions. Due to the presence of two ruthenium centers in the precatalyst 6, it may be tricky to
study the reaction mechanism involving metal centers. However, we have studied the time-
dependent product distribution of the β-alkylation of 1-phenylethanol with benzyl alcohol under
the optimized reaction conditions (Fig. S21). The reaction profile shows that the formation of 1,3-
diphenylpropan-1-ol (10a) increases steadily over time which reaches maximum towards the end
of the reaction. Whereas, the amount of 1,3-diphenylpropan-1-one (10a’) remains nominal
throughout the course of the reaction. These observations establish that the present catalyst system
is highly selective towards the end-alcohol rather than ketone product.
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Conclusion
In summary, we have developed a 1,2,4-triazolin-5-ylidene donor based biscarbene ligand with a
phenylene spacer which in presence of coinage metal ions having linear geometry yields the
binuclear tetracarbene complexes of general formula [(L)2M2](PF6)2 (M = AgI, CuI). Whereas,
reactions of [L-H2]Br2 with [Ir(Cp*)Cl2]2 and [Ru(p-cymene)Cl2]2 under suitable conditions result
in the formation of doubly orthometalated IrIII- and RuII-NHC complexes 5 and 6, respectively.
Complexes 5 and 6 represent the first examples of 1,2,4-triazolin-5-ylidene donor supported stable
diorthometalated binuclear IrIII/RuII-complexes. When applied as catalyst, the IrIII-NHC complex
5 was detected to be more active than 6 for the transfer hydrogenation of a variety of alkenes and
alkynes. Further, the RuII-NHC complex 6 was identified as an efficient catalyst for the β-
alkylation of diverse secondary alcohols via cross coupling with primary alcohols under low
catalyst (0.01 mol%) loading.
Experimental Section
General considerations. All manipulations were performed under argon atmosphere using either
standard Schlenk line or Glove box techniques. Glassware were dried at 130 C in an oven
overnight before use. The solvents used for the synthesis were dried, distilled, and degassed by
standard methods and stored over 4 Å molecular sieves. NMR measurements were performed
using Bruker 400 and 500 MHz FT-NMR spectrometers. The chemical shifts in the 1H NMR
1
spectra were referenced to the residual proton signals of the deuterated solvents (DMSO-d6, H
1
2.50 ppm and 13C{1H} 39.52 ppm; CD3CN, H 1.94 ppm and 13C{1H} 1.32 and 118.26 ppm;
CDCl3, 1H 7.26 ppm and 13C{1H} 77.16 ppm) and reported relative to tetramethylsilane. Coupling
constants are expressed in Hz. ESI-MS spectra were recorded with a Micromass Q-TOF Mass
spectrometer or an Agilent 6545A Q-TOF Mass spectrometer and the elemental analysis were
performed using PerkinElmer 24000 instrument. The starting materials [Ru(p-cymene)Cl2]2,
[Ir(Cp*)Cl2]2, and [Cu(SMe2)Br] were prepared according to the literature procedures.22a-c All
other chemicals were purchased from commercial sources and used as received without further
purification. Secondary and primary alcohols were synthesized following reported procedure.22d
1,3-bis(1,2,4-triazol-1-yl)benzene (A). The compound A was prepared by following a modified
reported procedure.10a 1,3-dibromobenzene (4.0 g, 16.96 mmol), 1,2,4-triazole (2.908 g, 42.10
mmol), K2CO3 (5.88 g, 42.59 mmol), CuO (0.162 g, 2.06 mmol), and DMSO (25 mL) were taken
in a predried schlenk tube under inert atmosphere and heated at 150 C for 48 h. After that, the
reaction mixture was cooled to room temperature and extracted with DCM several times. The
combined DCM layer was washed with brine solution and dried over MgSO4. Finally, the pure
compound was obtained as off-white solid via column chromatographic separation using
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CH2Cl2/MeOH as eluent in 81% yield (2.93 g, 13.80 mmol). Formation of the product is confirmed
by correlating the spectral data with previous literature data.
1,3-Bis(1-ethyl-triazolium-3-yl)benzene bromide [L-H2](Br)2 (1). An oven dried schlenk tube
was charged with compound A (2.0 g, 9.424 mmol) and ethyl bromide (13.97 mL, 188.48 mmol)
followed by DMF (20 mL). The resulting mixture was stirred at reflux temperature for 24 h. After
the completion of the reaction, mother liquor was decanted off and the precipitate obtained was
washed with diethyl ether (3 x 10 mL). Finally, after drying in high vacuum, the compound was
1
obtained as white air stable powder. Yield: 3.042 g (7.072 mmol, 75%). H NMR (500 MHz,
DMSO-d6)  11.32 (s, N-CH-NTrz, 2H), 9.63 (s, N-CH-NTrz, 2H), 8.59 (s, 1H), 8.24 (d, 3JH-H = 8.2
Hz, 2H), 8.06 (t, 3JH-H = 8.2 Hz, 1H), 4.42 (q, 3JH-H = 7.3 Hz, 4H, N-CH2), 1.60 (t, 3JH-H = 7.3 Hz,
6H, N-CH3) ppm. 13C{1H} NMR (101 MHz, DMSO-d6)  145.1, 142.4, 136.1, 132.2, 122.0, 112.8,
43.7, 14.2 ppm. MS (ESI, positive ions): m/z 452.9827 (calculated for [M + Na]+ 452.9837).
1,3-Bis(1-ethyl-triazolium-3-yl)benzene hexafluorophosphate [L-H2](PF6)2 (2). The bis-
triazolium dibromide salt, 1 (2.0 g, 4.64 mmol) was dissolved in a minimum amount of MeOH in
a schlenk tube. A solution of NH4PF6 (1.51 g, 9.29 mmol) in methanol was then added to it. The
resulting solution was stirred for 2 h at ambient tempearture. After the reaction, the precipitate
obtained was washed with diethyl ether (3 × 10 mL) and dried in high vacuum to provide the pure
compound as creamy white air stable solid. Yield: 2.44 g (4.36 mmol, 94%). 1H NMR (400 MHz,
DMSO-d6)  11.12 (s, N-CH-NTrz, 2H), 9.56 (s, N-CH-NTrz, 2H), 8.53 (s, 1H), 8.20 (d, 3JH-H = 8.2
Hz, 2H), 8.06 (t, 3JH-H = 8.2 Hz, 1H), 4.39 (q, 3JH-H = 7.3 Hz, 4H, N-CH2), 1.59 (t, 3JH-H = 7.3 Hz,
6H, N-CH3) ppm. 13C{1H} NMR (101 MHz, DMSO-d6)  145.1, 142.4, 136.2, 132.3, 122.0, 112.8,
43.8, 14.2 ppm. 19F NMR (471 MHz, DMSO-d6)  -70.18 ppm. 31P NMR (202 MHz, DMSO-d6)
 -144.22 ppm. MS (ESI, positive ions): m/z 415.1573 (calculated for [M-PF6]+ 415.1235).
Synthesis of the silver complex [(L)2Ag2](PF6)2 (3). Compound 2 (0.2 g, 0.357 mmol) and Ag2O
(0.1075 g, 0.464 mmol) were added to a predried schlenk tube under inert condition with the
exclusion of light. Dry acetonitrile (8 mL) was then added to the schlenk tube and heated for 36 h
in a preheated oil bath at 70 C. After the specific time, the reaction mixture was cooled to room
temperature and the suspension obtained was filtered through a small pad of celite to obtain a clear
solution. The filtrate was then concentrated and precipitated out using diethyl ether to get the
compound as a light sensitive white solid. Yield: 0.15 g, (0.144 mmol, 81%). 1H NMR (400 MHz,
CD3CN)  8.60 (s, N-CH-NTrz, 2H), 7.98 (s, 1H), 7.80 (d, 3JH-H = 8.1 Hz, 1H), 7.78 (d, 3JH-H = 8.1
Hz, 1H), 7.27 (t, 3JH-H = 8.1 Hz, 1H), 4.40 (q, 3JH-H = 7.2 Hz, 4H, N-CH2), 1.57 (t, 3JH-H = 7.2 Hz,
6H, N-CH3) ppm. 13C{1H} NMR (126 MHz, CD3CN)  183.0 (Ag-CNHC), 181.4 (Ag-CNHC), 144.8,
140.9, 131.1, 124.0, 118.7, 45.2, 16.5 ppm. 19F NMR (471 MHz, CD3CN) δ -70.11 ppm. 31P NMR
(202 MHz, CD3CN)  -144.20 ppm. MS (ESI, positive ions): m/z 897.0680 (calculated for [M-
PF6]+ 897.0619). Anal. Calcd. (%) for C28H32N12Ag2P2F12: C 32.26%, H 3.09%, N 16.13%; Found:
C 32.83%, H 2.48%, N 15.5%.
Synthesis of the copper complex [(L)2Cu2](PF6)2 (4). Complex 3 (0.120 g, 0.115 mmol),
[Cu(SMe2)Br] (0.071 g, 0.345 mmol) were added in predried schlenk tube under inert condition.
Then dry acetonitrile (8 mL) was added to the schlenk tube and stirred overnight at room
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temperature under the exclusion of light. The obtained light yellow color solution was filtered over
celite under argon atmosphere. The filtrate was concentrated and precipitated using dry diethyl
ether to get the compound as air and moisture sensitive yellow powder. Yield: 0.09 g, (0.094 mmol,
1
3
82%). H NMR (500 MHz, DMSO-d6)  9.41 (s, N-CH-NTrz, 2H), 9.18 (s, 1H), 8.44 (d, JH-H
=
7.5 Hz, 2H), 8.10 (t, 3JH-H = 7.5 Hz, 1H), 4.70 (q, 3JH-H = 6.8 Hz, 4H, N-CH2), 1.86 (t, 3JH-H = 6.8
Hz, 6H, N-CH3) ppm. 13C{1H} NMR (126 MHz, CD3CN)  180.6 (Cu-CNHC), 143.8, 140.7, 131.4,
130.8, 122.8, 44.6, 16.4 ppm. 19F NMR (471 MHz, CD3CN)  -72.84 ppm. 31P NMR (202 MHz,
CD3CN)  -144.64 ppm. Consistent CHN data could not be obtained due to the sensitive nature of
the compound.
Synthesis of the iridium triazolinylidene complex [(L)Ir2(Cp*)2Br2] (5). Compound 1 (0.054
g, 0.125 mmol), [Ir(Cp*)Cl2]2 (0.1 g, 0.125 mmol) and Cs2CO3 (0.188 g, 0.577 mmol) were added
in a predried schlenk tube under inert condition. Then dry acetonitrile (10 mL) was added to the
schlenk tube and stirred at 80 C for 30 h. After the specific reaction time, the reaction mixture
was allowed to cool to ambient temperature and all the volatiles were removed in high vacuum.
Then the resulting residue was extracted with DCM and filtered through a small pad of celite. The
clear yellow solution obtained was concentrated and precipitated using dry diethyl ether. The
compound was obtained as bright yellow-orange air and moisture stable powder. Yield: 0.108 g,
(0.1 mmol, 80%). 1H NMR (400 MHz, CDCl3)  8.05 (s, Ar-CH, 1H), 7.98 (s, N-CH-NTrz, 2H),
7.71 (s, Ar-CH, 1H), 4.38 (q, 3JH-H = 7.1 Hz, 2H), 4.25 (q, 3J H-H = 7.1 Hz, 2H), 1.86 (s, 30H), 1.60
(t, J = 7.1 Hz, 6H) ppm. 13C{1H} NMR (126 MHz, CDCl3)  165.9 (Ir-CNHC), 145.2, 140.3, 139.7,
137.4 (Ir-CPhenyl), 97.7, 91.6, 42.8, 16.6, 10.2 ppm. MS (ESI, positive ions): m/z 1001.2025
(calculated for [M-Br]+ 1001.2052). Anal. Calcd. (%) for C34H44N6Ir2Br2: C 37.77%, H 4.10%, N
7.78%; Found: C 37.51%, H 3.41%, N 7.97%.
Synthesis of ruthenium complex [(L)Ru2(p-cymene)2Br2], 6. Compound 1 (0.05 g, 0.116
mmol), [Ru(p-cymene)Cl2]2 (0.71 g, 0.116 mmol) and Cs2CO3 (0.189 g, 0.58 mmol) were added
to a predried schlenk tube under inert condition. Then dry THF (10 mL) was added to the schlenk
tube and the reaction mixture was stirred at 70 C for 24 h. After specific time, reaction mixture
was allowed to cool to room temperature and all the volatiles were removed in high vacuum. The
residue obtained was extracted with DCM and filtered through a small pad of neutral alumina to
obtain a clear brownish yellow solution. The filtrate was then concentrated and precipitated using
dry hexane to get a pale yellow powder. Yield: 0.036 g, (0.041 mmol, 35%). 1H NMR (500 MHz,
CDCl3)  8.74 (s, Ar-CH, 1H), 8.00 (s, N-CH-NTrz, 2H), 7.63 (s, Ar-CH, 1H), 5.81 (d, 3J H-H = 6.0
Hz, 2H), 5.59 (d, 3J H-H = 5.8 Hz, 2H), 5.40 (d, 3J H-H = 6.0 Hz, 2H), 5.35 (d, 3J H-H = 5.8 Hz, 2H),
4.63 (q 3J H-H = 13.7, 7.3 Hz, 2H), 4.48 (q, 3J H-H = 13.7, 7.3 Hz, 2H), 2.46 (m, 2H), 2.05 (s, 6H),
1.67 (t, J = 7.3 Hz, 6H), 1.00 (d, J = 6.8 Hz, 6H), 0.87 (d, J = 6.8 Hz, 6H) ppm. 13C{1H} NMR
(126 MHz, CDCl3)  187.3 (Ru-CNHC), 155.6 (Ru-CPhenyl), 152.8, 139.3, 138.7, 102.5, 99.6, 97.7,
91.9, 88.5, 86.2, 83.0, 42.4, 30.0, 22.1, 20.7, 18.5, 15.5 ppm. MS (ESI, positive ions): m/z 858.1009
(calculated for [M–Br+CH3CN]+ 858.1019).
General procedure for the transfer hydrogenation of alkenes/alkynes. An oven dried pressure
tube was charged with NaOtBu (0.08 mmol, 20 mol%) inside an argon filled glove box. To this,
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catalyst (0.0012 mmol, 0.3 mol%), alkene/alkyne (0.4 mmol), and isopropanol (4 mL) were added
using standard schlenk technique. The resulting mixture was refluxed for the specified time and
cooled to room temperature. Then a small portion of the reaction mixture was diluted with
methanol and an aliquot was analyzed by GC-MS (mesitylene was used as internal standard) to
obtain product ratio.
General procedure for the C-alkylation reaction. An ovendried Schlenk tube was charged with
the required amount of catalyst 6 stock solution in acetonitrile (0.01 mol%) and acetonitrile was
evaporated in vacuo. To this, KOtBu (0.1 mmol, 20 mol%), secondary alcohol (0.5 mmol), and
primary alcohol (0.55 mmol) followed by toluene (1 mL) were added using standard schlenk
technique. The resulting mixture was refluxed for the specified time and cooled to room
temperature. Then a small portion of the reaction mixture was diluted with methanol and an aliquot
was analyzed by GC-MS (mesitylene was used as internal standard) to obtain the product ratio.
For the isolation of the product, all the volatiles were removed after the completion of the reaction
and the obtained residue was purified via column chromatography using ethyl acetate and hexane
as eluent.
Details of Crystallographic data. X-ray data were collected on a Bruker AXS Kappa APEX-II
CCD diffractometer or a Bruker APEX-II CCD equipped with graphite-monochromated Mo Kα
radiation (λ = 0.71073 Å).23a Crystal was fixed at the tip of a glass fiber loop and after mounting
on the goniometer head, it was optically centered. The APEX2 and APEX2-SAINT/Bruker SAINT
programme were used for the data collection and unit cell determination, respectively. Processing
of the raw frame data was performed using SAINT/XPREP or Bruker SAINT.23b-c The structures
were solved by SHELXT-2014/4 or SHELXS-97 methods23d and refined against F2 using all
reflections with the SHELXL-2014/7 and SHELXL-2018/3 (WinGX) program.23e-f Non-hydrogen
atoms were refined anisotropically and all the hydrogen atoms were placed in calculated positions.
The crystal data (CCDC Nos. 1972778-1972781) and refinement details are summarized in table
S1 and S2 of the supporting information. The graphical representations were performed using the
program Mercury.23g
Conflicts of interest
There are no conflicts to declare.
Acknowledgments. We gratefully acknowledge the SERB and DST, India (Project No.
ECR/2016/001272 and DST/INSPIRE/04/2015/002219) for the financial support. V. K. S. and P.
M. I. thank IIT-Madras for HTRA fellowship and S.N.R.D. acknowledges UGC, India for JRF.
We are thankful to the Department of Chemistry and SAIF, IIT Madras for the instrumental
facility. We also thank Mr. V. Ramkumar for helping with the crystal structure solution.
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Supporting Information. Electronic supplementary information (ESI) available: Full
characterization data for the compounds 1-6 and the 1H NMR data of in situ experiments and the
isolated compounds from catalytic runs. CCDC 1972778-1972781.
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TOC Graphic:
The 1,2,4-triazolin-5-ylidene donor based bis(carbene) ligand provides binuclear tetracarbene
complexes of general formula [(L)2M2](PF6)2 with coinage metal ions (M = AgI, CuI), whereas,
formation of doubly orthometalated NHC complexes were observed in case of IrIII- and RuII-metal
ions. Further, the IrIII- and RuII-NHC complexes were detected to be efficient catalysts for the
transfer hydrogenation of alkenes/alkynes and β-alkylation of diverse secondary alcohols using
various primary alcohols (under low catalyst loading 0.01 mol%), respectively.
1