Modulating the electronics of orthometalated RuII-NHC complexes via substitution patterns or NHC donors: Studies towards the impacts in catalysis

Article abstract of DOI:10.1016/j.jorganchem.2021.122008

The effect of electronic modulations on a series of C?C orthometalated Ru^II-NHC complexes 2-10 was explored in (de)hydrogenation reaction. The electronic tweaking of the complexes was achieved either by varying the carbene donors (ImNHC vs 1,2,

Full text of DOI:10.1016/j.jorganchem.2021.122008

Journal of Organometallic Chemistry 951 (2021) 122008
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Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Modulating the electronics of orthometalated RuII-NHC complexes via
substitution patterns or NHC donors: Studies towards the impacts in
catalysis
Praseetha Mathoor Illam1, Vivek Kumar Singh1, Priya, Arnab Rit∗
Department of Chemistry, Indian Institute of Technology Madras, Chennai 600036, India
a r t i c l e i n f o
a b s t r a c t
The effect of electronic modulations on a series of C˄C orthometalated RuII-NHC complexes 2-10 was ex-
plored in (de)hydrogenation reaction. The electronic tweaking of the complexes was achieved either by
varying the carbene donors (ImNHC vs 1,2,4-TzNHC) or the nature and position of the substituents on the
N-phenyl wingtip of the NHC ligand. All the synthesized new complexes were fully characterized by us-
ing NMR analyses, mass spectrometry as well as X-ray crystallography. The electronic nature of the com-
plexes was ascertained from the electrochemical analyses in addition to 13 C{1H} NMR studies. Catalytic
activities of the complexes were investigated in the acceptorless dehydrogenation of 1-phenyl-1-propanol
to propiophenone, and we observed that RuII-NHC complexes containing ImNHC donors outperformed
their TzNHC analogues. Further, the RuII-ImNHC precatalysts with substituents at the para-position with
respect to the imidazolium moiety exhibited an activity trend of electron deficient complexes being more
active than that of the electron rich complexes. Whereas the analogous precatalysts with the substituents
at meta-position exhibited different activity trend (electron rich complex is superior compared to the
electron deficient complexes). Additionally, the complexes were also tested in the transfer hydrogenation
of acetophenone and surprisingly, in contrast to dehydrogenation reaction, the meta-substituents had no
effect on the transfer hydrogenation activities of the complexes.
Article history:
Received 28 May 2021
Revised 12 July 2021
Accepted 27 July 2021
Available online 30 July 2021
Keywords:
N-heterocyclic carbene
ruthenium
orthometalation
electronic tuning
acceptorless dehydrogenation
transfer hydrogenation
© 2021 Elsevier B.V. All rights reserved.
1. Introduction
tronic modifications of ancillary NHC ligands in the catalytic per-
formances of their transition metal complexes. In this line, Valerga
After the seminal report of isolable N-heterocyclic carbene
(NHC) by Arduengo in 1991 [1], the organometallic chemistry in-
volving NHCs has become an exciting area of research. Soon after
this, NHCs have gained a privileged position among the ancillary
ligands due to better σ-donor properties as compared to the clas-
sical ligands like amines and trisubstituted phosphines [2a,2b,2c].
Owing to the relatively easy stereoelectronic tunability and the
stronger M-NHC bonds, the NHC-transition metal complexes have
been widely used as efficient homogeneous catalysts for diverse
organic transformations [3]. Among various NHC ligands, gener-
ally the bidentate and tridentate variants are getting more at-
tention than their monodentate analogues possibly due to their
ability to improve the stability and efficiency imparted to their
metal complexes because of the chelate effect [4]. In addition, sev-
eral attempts have been made to study the impact of stereoelec-
et al. in 2012 have reported the picolyl-functionalized orthometa-
lated RuII-NHC complexes for the N-alkylation of amines as well as
the transfer hydrogenation of carbonyl and imine moieties [5]. The
stereoelectronic modifications in these complexes were achieved
by changing the backbone and the wingtip of ancillary NHC lig-
ands. Further, the electronic difference between the imidazolyli-
dene and 1,2,4-triazolylidene donors was successfully showcased
by Choudhury et al. in the oxidative cleavage of alkenes where
the electron deficient 1,2,4-triazolylidene based precatalyst outper-
formed its imidazolylidene analogue [6a]. Recently, the same group
has reported various stereoelectronically modifiable IrIII-complexes
of bidentate ImNHC ligands for the hydride transfer reaction and
it was established that the yaw angle, bite angle, and the electron
density at the metal centers imparted by the ligands significantly
influence their catalytic performances [6b]. Along the same line,
a recent report from our group described the impact of ancillary
NHC ligand modifications, in terms of electronics as well as sterics,
on the catalytic activity of the corresponding RuII-NHC complexes
in transfer hydrogenation reaction [7a]. We have also demonstrated
a similar kind of tuning in the heteroditopic NHC supported RuII-
∗
Correspondence author.
E-mail address: arnabrit@iitm.ac.in (A. Rit).
1
These authors contributed equally.
https://doi.org/10.1016/j.jorganchem.2021.122008
0022-328X/© 2021 Elsevier B.V. All rights reserved.

P.M. Illam, V.K. Singh, Priya et al.
Journal of Organometallic Chemistry 951 (2021) 122008
Scheme 1. a) Synthesis of various azolium salts, 1a-j via Ullmann coupling protocol followed by quaternization of the azole rings. b) Molecular structure of [1i]+ with 60%
probability level. Bromide counterion and the hydrogen atoms except H2 and H11 are omitted for clarity. The N-ethyl group is shown in capped stick.
Table 1
complexes and the impact of these modulations in their catalytic
13 C{1H} NMR chemical shifts of the CNHC and CPhenyl carbons in complexes 2-10.
activities [4b,7b]. Inspired by these results, we aimed to study the
effect of such type of tuning in RuII-NHC based catalyst systems in
other functional group transformations. In this line, a series of or-
thometalated RuII-NHC complexes were synthesized by varying the
NHC donors and the substituents (including their position) at the
N-phenyl wingtip group for studying their activity in the acceptor-
less dehydrogenation of secondary alcohol, a crucial step in various
C-C and C-N bond forming reactions via borrowing hydrogen (BH)
pathway [2d,3b], to explore the impact of the ancillary NHC ligand
modulations. Additionally, the catalytic behavior of the complexes
was also examined in transfer hydrogenation of carbonyl function-
ality.
Entry
Complex
CNHC chemical shift (ppm)
CPhenyl chemical shift (ppm)
1
2
3
4
5
6
7
8
9
2
187.0
186.5
185.4
188.0
189.2
187.2
187.5
187.6
189.3
162.4
162.1
155.7
163.4
164.7
157.6
170.1
181.1
159.6
3
4
5
6
7
8
9
10
evidenced from their 1H NMR spectra where the most downfield
shifted N-CH-N protons were absent (see supporting information).
Further, the presence of four aromatic protons instead of five sug-
gests that the metal is also attached to the phenyl ring via C-H
activation to form ruthenacycles. Additionally, orthometalation re-
sults in the loss of symmetry around the Ru center which was sup-
ported by the splitting of p-cymene aromatic protons (four instead
of two doublets) in the region of 5.30–5.70 ppm as well as the
splitting of the N-CH2 signals of the ethyl moiety (quartet in the
azolium salts) into two multiplets [7a,8b]. To our surprise, all our
attempts for the metalation of 1g to synthesize analogous com-
plex under various conditions ended up with the formation of a
nearly 1:1 mixture of two orthometalated complexes (11a and 11b,
scheme 3) as evidenced from the 1H NMR spectrum of the isolated
product (Fig. 1).
2. Results and discussion
2.1. Synthesis of various imidazolium and triazolium salts,1a-j
To begin with our plan of synthesizing stereoelectronically tun-
able RuII-NHC complexes, various imidazolium and triazolium salts
were isolated in good yields (73-89%, scheme 1). To have better
insight into the electronic modulations, position (para-, 1b-e and
meta-, 1f-i) of the substituents at the N-phenyl ring with respect
to the imidazolium moiety as well as the NHC-moieties (imida-
zolium, 1a vs triazolium, 1j) were altered. The azolium salts 1a,
1c, and 1d were synthesized following the previous procedure [7a].
For the synthesis of these azolium salts, initially, the respective
aryl halides were coupled with imidazole or 1,2,4-triazole via Ull-
mann coupling protocol to provide the corresponding compounds
A-J which on treatment with ethyl bromide in acetonitrile under
reflux condition delivered the respective azolium salts 1a-j as off-
white to yellow solids. Interestingly, the imidazolium salts with
electron donating substituents (1b-c and 1f-g) were found to be
hygroscopic whereas the triazolium salt 1j and other imidazolium
salts with electron withdrawing substituents 1d-e and 1h-i were
isolated as air and moisture stable solids. All these azolium salts,
the precursors for NHC donors, were thoroughly characterized via
NMR spectroscopy (see supporting information), mass spectrome-
try as well as by X-ray crystallography for 1i. The presence of the
downfield shifted N-CH-N protons and the characteristic quartets
and triplets for the N-ethyl moieties in 1a-j in their 1H NMR spec-
tra when compared to their parent compounds A-J confirmed the
formation of the azolium salts.
Further, the ESI-MS spectrum of the isolated product
displays only
a
single peak for the positive ion [M+Na]+
(M = C22H27N2ORuBr for 11a/b) with m/z = 541.0238 which
is in good agreement with the calculated value (m/z = 541.0244
with the isotopic pattern matching exactly, Fig. 2). This observation
reinforces the conclusion obtained from 1H NMR spectrum that
the metalation of 1g results in the formation of two regioisomeric
products 11a and 11b.
It is to be noted that the p-OMe substituted imidazolium salt
1c resulted in the formation of the corresponding orthometalated
complex 4 as both the ortho-phenyl protons with respect to the
NHC moiety are equivalent. Whereas, the presence of m-OMe sub-
stituent somehow makes the non-equivalent ortho-C-H bonds rea-
sonably electron rich and accessible for orthometalation, as sup-
ported by the experimental results (Fig. 1) in contrast to other
meta-substituents.
2.2. Synthesis of the orthometalated RuII-NHC complexes 2-10
The coordination of the RuII-center to the ligands in a chelat-
After having the required azolium salts in our hand, we targeted
the synthesis of the corresponding RuII-NHC complexes. In this di-
rection, a series of orthometalated RuII-NHC complexes with p-Me
(3) and p-NO2 (6) as well as the meta-substituted complexes (7-9)
were synthesized along with a 1,2,4-triazolylidene analogue 10 as
shown in scheme 2.
ing (CNHC∧Cphenyl
)
fashion is further confirmed from 13C{1H}
NMR spectra which show the corresponding metal bound carbene
(CNHC) signals as well as the CPhenyl signals in their usually ob-
served range [7a,8] as shown in Table 1. In line with previous
study [7a], the new orthometalated complexes with substituents
at the para-position, 3 and 5 also exhibited the similar trend for
their 13C{1H} chemical shift values (Table 1, entry 2 and 4). The
RuII-bound CNHC and CPhenyl chemical shift values were found to
All the complexes were isolated as air stable yellow solids in
good yields (69-80%). Formation of the complexes was primarily
2

P.M. Illam, V.K. Singh, Priya et al.
Journal of Organometallic Chemistry 951 (2021) 122008
Scheme 2. a) Synthesis of the orthometalated RuII-NHC complexes 2-10. b) Molecular structures of complexes 8 (left) and 10 (right) with 40% and 60% probability levels,
respectively. Hydrogen atoms are omitted for clarity. N-Ethyl and p-cymene moieties are shown in capped stick.
Scheme 3. Attempted synthesis of the orthometalated RuII-NHC complexes of 1g.
be upfield shifted for complex 3 whereas those of the complex 5
were found to be downfield shifted than that of the unsubstituted
complex 2 and this supports the difference in electronic nature of
the NHC ligands in these complexes [7c].
onance is more downfield shifted in case of m-NO2 substitution
(complex 9) compared to its p-NO2 analogue 6 (entry 8 vs 5). This
can be justified based on the fact that the substituents at the meta-
position with respect to ImNHC are actually in the para-position
of the orthometalated carbon and thus, the CPhenyl chemical shift
is most affected by the electron- withdrawing or donating nature
of the substituents. Finally, due to the presence of an extra elec-
tronegative N-atom in 1,2,4-triazolylidene than that in the imida-
zolylidene, [6a,9] the complex 10 exhibited the most downfield
shifted CNHC resonance among all the complexes listed in Table 1.
The molecular structures of the complexes 8 and 10 were un-
ambiguously confirmed by the X-ray diffraction studies of the sin-
gle crystals obtained by the slow diffusion of n-pentane into the
saturated DCM solutions of the complexes. The molecular struc-
tures clearly show that the NHC ligand coordinate to the RuII cen-
ter in chelated fashion as concluded from their multinuclear NMR
analyses. The Ru-CNHC bond lengths are found to be shorter than
the Ru-CPhenyl lengths for both the complexes 8 and 10 possibly be-
cause of slight back donation from the RuII center to the NHC moi-
ety [10]. Further, the RuII-CNHC bond length of the complex 10 is
slightly longer than the imidazolylidene complexes [2.033 vs 2.010
(for 2) and 2.020 (for 8)] and the same is also observed for the Ru-
CPhenyl bond length [2.089 vs 2.066 (for 2) and 2.051 (for 8)]. As a
In contrast to this observation, the meta-substituents at the N-
phenyl ring with respect to the imidazolylidene ring has different
influence on the 13C{1H} NMR chemical shift values of the com-
plexes 7-9 (Table 1). The substituents have insignificant effect on
the CNHC resonances, however, they have substantial influence on
the CPhenyl resonances (entry 6-8 vs 1). The Me-substituted com-
plex 7 has the most upfield shifted phenylic carbon signal whereas
the NO2-substituted complex 9 has the most downfield shifted
phenylic carbon signal and the CF3-substituted complex 8 has the
corresponding carbon resonance value in between that observed
for the complexes 7 and 9. So, the substituents at the para-position
influence both the CNHC and CPhenyl 13C{1H} NMR resonances (en-
try 2-5) in their orthometalated RuII-complexes. In contrast, the
substituents at the meta-position although do not influence the
respective CNHC chemical shift values, the CPhenyl resonance shifts
substantially from their non-substituted analogue (entry 6-8 vs 1).
Further, the CPhenyl resonance is more upfield shifted if the Me-
substituent is at the meta-position (complex 7) rather than at the
para-position (complex 3; entry 6 vs 2). Similarly, the CPhenyl res-
3

P.M. Illam, V.K. Singh, Priya et al.
Journal of Organometallic Chemistry 951 (2021) 122008
Fig. 1. 1H NMR spectrum (CDCl3) of the product obtained from the reaction of 1g with [Ru(p-cymene)Cl2]2.
Fig. 2. ESI-MS spectrum of the product obtained from the reaction of 1g with [Ru(p-cymene)Cl2]2 showing the peak at m/z = [M+Na]+, M = C22H27N2ORuBr.
measure of the steric profile of chelating ligands, the yaw and bite
angles at the NHC/aryl moieties of complexes were calculated and
found to be 10.60° and 77.41°, respectively for the complex 8 while
10.99° and 76.92°, respectively for the complex 10.
More insight into the electronic nature of the synthesized com-
plexes was obtained from their electrochemical analyses and the
relevant DPV diagram is presented in Fig. 3. When the potential
values of the newly synthesized para-substituted complexes 3 and
6 were compared with other para-substituted complexes 4 and 5
[7a], we observed that the electron density around the Ru center
in complex 3 is lower than the analogous OMe substituted com-
plex 4 (E1/2 = 82.4 mV vs 72.0 mV) whereas the complex 6 with
NO2 substituent is more electron deficient than that of the CF3-
substituted counterpart 5 (E1/2 = 271.3 mV vs 224.5 mV) in line
with the higher -R effect of the NO2-group.
The potential values obtained for the meta-substituted com-
plexes reveal that the complex 7 having a Me-substituent has
the highest electron density around the metal center (E1/2 = 60.1
mV) which is also clear from the CPhenyl 13C{1H} NMR chemi-
cal shift (Table 1) whereas the NO2-substituted complex 9 is the
least electron rich (E1/2 = 285.2 mV) among the complexes ana-
lyzed. The RuII-center in the triazolylidene complex 10 is relatively
more electron rich than the complexes with electron withdraw-
ing substituents (CF3, NO2) either at the para- or meta- positions.
All these observations imply that our simple strategy of NHC lig-
and modulation by varying the substitution has considerable effect
4

P.M. Illam, V.K. Singh, Priya et al.
Journal of Organometallic Chemistry 951 (2021) 122008
Fig. 3. DPV plot of the selected complexes. The measurements of all the RuII-NHC complexes (1 mM) were performed in degassed CH2Cl2 using Bu4NPF6 (0.1 M) as support-
ing electrolyte with three electrode configurations: counter electrode, Pt wire; working electrode, Glassy carbon; reference electrode, Ag/AgNO3 at sweep rate of 100 mV/sec.
Ferrocene (E1/2, Fc/Fc+ = 0.269 volts vs. Ag/Ag+) was used as an external calibration standard for all the measurements. The potential values obtained are tabulated in the
right-hand side.
Table 2
plex 2 still provided good conversion (87%, entry 1) but the activ-
Acceptorless dehydrogenation of 1-phenyl-1-propanol using different or-
thometalated RuII-NHC complexesa.
ity of the complex 10 was further reduced (34% conversion, entry
2). To move forward, we selected the RuII-imidazolylidene com-
Entry
Precatalyst
Loading (mol%)
Conversionb (%)
plexes with varying substituents at different positions of the N-
phenyl ring (complexes 3-9). When the para-substituted complexes
(3-6) were tested, the electron deficient complexes 5 and 6 per-
formed better (97-100% conversion) than their electron rich vari-
ants (complexes 3 and 4, 77-84%). We then checked the effect of
electronic tuning by selecting the complexes with substituents at
meta-positions (complexes 7-9) and we observed a different reac-
tivity pattern for the complexes.
1
2
0.5 (0.3)
0.5 (0.3)
0.3
97 (87)
49 (34)
84
2
10
3
3
4
4
0.3
77
5
5
0.3
100
97
6
6
0.3
7
7
0.3
98
8
8
0.3
74
9
9
0.3
80
The most electron rich RuII-complex 7 outperformed the analo-
gous electron deficient complexes 8 and 9 for the conversion of 1-
phenyl-1-propanol to propiophenone (entry 7, 98% vs 74-80%, en-
tries 8-9). A reaction using only [Ru(p-cymene)Cl2]2 (0.3 mol%) was
also carried out which resulted in 36% conversion under our stan-
dard reaction conditions (entry 10) which shows that our carbene
ligand is highly beneficial for improving the catalytic efficiency of
the RuII-center. It is important to mention that the complexes 5-7
are found to be better precatalysts compared to the previously re-
ported RuII-NHC complexes for the dehydrogenation of secondary
alcohols [13a,13b,13c]. In order to understand the different reac-
tivity patterns of the para- (3-6) and meta-substituted complexes
7-9, possibly in terms of the mechanism involved, we carried out
some controlled experiments. Addition of excess arenes (p-cymene,
mesitylene and 1,3,5-trimethoxybenzene) did not alter the catalytic
outcome of reactions catalyzed by the representative complexes
6 and 7 which possibly rules out the irreversible release of p-
cymene during the catalytic process [6a,13d,13e]. Further, the mer-
cury dropping test did not have any influence on the reaction out-
come which suggests that the catalytic reaction proceeds in homo-
geneous fashion [13d,14]. To have better idea about the reaction
mechanism, we have monitored the reaction progress using NMR
spectroscopy. The 1H NMR spectrum of the reaction mixture after
stirring the m-methyl substituted complex 7, KOtBu and 1-phenyl-
1-propanol for 3 h is shown in the Fig. 4. The spectrum suggests
that the secondary alcohol undergoes dehydrogenation under the
generation of a RuII-di-hydride intermediate, possibly by cleaving
the orthometalation, as indicated by the two upfield shifted reso-
nances of nearly equal intensity at -10.01 and -9.92 ppm. Further,
the single multiplet for the methylene protons of N-ethyl wingtip
group of the NHC ligand at 4.31-4.40 ppm suggests the dissociation
of the Ru-CPhenyl bond which would make the methylene protons
non-diastereotopic.
10
[Ru(p-cymene)Cl2]2
0.3
36
a
General conditions: 1-phenyl-1-propanol (0.5 mmol), complex (0.3-
0.5 mol%), KOtBu (0.2 equiv.), 12 h. bDetermined by GC-MS with respect
to 1-phenyl-1-propanol.
in tuning the electronic properties of their metal complexes. Fur-
ther, the above described electrochemical analyses reinforce that
the substituents at the meta-position is more effective than the
para-substitution (entry 2 vs 5 and 4 vs 7, Table 2) in electronic
modulation of their orthometalated complexes which are also sup-
ported by the M-CPhenyl 13C{1H} NMR resonances (Table 1).
2.3. Catalytic acceptorless dehydrogenation of 1-phenyl-1-propanol
After having the well-characterized RuII-NHC complexes in our
hand, we intended to study the effect of their electronic tuning
in their catalytic activity. For that, the acceptorless dehydrogena-
tion of 1-phenyl-1-propanol was chosen as the model reaction. The
method of acceptorless dehydrogenation has gained immense at-
tention in recent time as it is a green and atom economic way
to convert the generally less reactive alcohols into more reactive
carbonyl functions under the release of only molecular hydrogen
[3b,11]. Importantly, it is one of the intermediate steps of the C-C
and C-N bond forming reactions via borrowing hydrogen strategy
[12].
Initially, we studied the catalytic efficacy of the basic imida-
zolylidene and triazolylidene complexes 2 and 10 by using KOtBu
(20 mol%) as the base in toluene solvent (Table 2). When the re-
action was carried out with 0.5 mol% catalyst loading for 12 h, we
observed that the complex 2 converted the 1-phenyl-1-propanol
to propiophenone near quantitatively (97%, entry 1), whereas the
complex 10 exhibited poor activity (entry 2, 49% conversion).
When the lower catalyst loading of 0.3 mol% was employed, com-
5

P.M. Illam, V.K. Singh, Priya et al.
Journal of Organometallic Chemistry 951 (2021) 122008
Fig. 4. In situ 1H NMR spectrum of the reaction mixture (CD3CN) for the reaction of complex 7, KOtBu and 1-phenyl-1-propanol in toluene after 3 h. The triplet (∗)
corresponds to the reactant Ph(CH)OH(CH2CH3).
tion of a monohydride intermediate B under the release of pro-
piophenone. This monohydride species might be reactive and thus
may further react with 1-phenyl-1-propanol to generate a RuII-di-
hydride species (C). Further, the release of dihydrogen (see Figure
S33, supporting information) from the hydride intermediate C/B
via reaction with 1-phenyl-1-propanol continue the catalytic cycle
through A. Involvement of the RuII-mono-hydride intermediate in
the catalytic cycle was supported by the in situ 1H NMR analysis of
the reaction performed with the complex 5 (see Figure S32, sup-
porting information).
2.4. Catalytic transfer hydrogenation of acetophenone
After studying the catalytic activity of the synthesized RuII-NHC
complexes in acceptorless dehydrogenation process, we further in-
vestigated their activity in the reverse reaction of hydrogenation.
For that, para- and meta-methyl substituted complexes 3 and 7
(keeping in mind our previous experience that electron-rich com-
plexes perform better in hydride transfer reaction) [7a] as well
as the basic ImNHC and TzNHC complexes 2 and 10, respectively
were selected and the hydrogenation of acetophenone was chosen
as the model reaction under the transfer hydrogenation conditions
of 10 mol% KOtBu in isopropanol (as solvent as well as hydride
source). The catalytic outcomes with various complexes are shown
in Table 3.
Scheme 4. Plausible reaction pathway for the acceptorless dehydrogenation of 1-
phenyl-1-propanol.
In addition, the four symmetrical doublets corresponding to the
p-cymene aromatic protons observed for the orthometalated com-
plex 7 collapsed in the region 5.47-5.76 ppm which also supports
the cleavage of the orthometalation [15]. Based on these observa-
tions, a plausible mechanistic pathway for the acceptorless dehy-
drogenation process has been proposed in scheme 4. At first, the
base promoted deprotonation of the 1-phenyl-1-propanol followed
by reaction with the RuII-NHC precatalysts generates the interme-
diate A, which upon β-hydride elimination results in the forma-
From the entry 1 and 4, it is clear that the unsubstituted com-
plexes 2 and 10 perform similarly providing 66% (0.1 mol% load-
ing) to ∼85% (0.25 mol% loading) conversion to 1-phenylethanol.
Further, with the low catalyst loading of 0.1 mol%, we observed a
substantial difference in the activities between the methyl substi-
tuted (both para- and meta-) complexes 3 and 7 (entry 2-3) and
the parent unsubstituted complexes (entry 1 and 4). This might be
accredited to the electron richness of the RuII-center in the com-
6

P.M. Illam, V.K. Singh, Priya et al.
Journal of Organometallic Chemistry 951 (2021) 122008
Table 3
chemicals were purchased from the commercial sources and used
as received without further purification.
Transfer hydrogenation of acetophenone using complexes 2, 3, 7, and 10a.
Entry
Precatalyst
Loading (mol%)
Conversion (%)b
4.2. X-ray crystallography
1
2
3
4
2
0.25/ 0.1
0.1
86/ 66
83
3
7
0.25 / 0.1
0.25 / 0.1
87/ 85
84/ 66
X-ray data were collected on a Bruker APEX-II CCD diffrac-
10
tometer equipped with graphite-monochromated Mo Kα radiation
˚
a
(λ = 0.71073 A) [17a]. Crystal was fixed at the tip of a glass fiber
General conditions: acetophenone (1 mmol), complex (0.25-0.1 mol%),
KOtBu (0.1 equiv.), 1 h. bDetermined by GC-MS with respect to acetophe-
none.
loop, and after mounting on the goniometer head, it was opti-
cally centered. The APEXII and APEXII-SAINT program were used
for the data collection and unit cell determination, respectively
[17a]. Processing of the raw frame data was performed using SAINT
[17a,17b]. The structures were solved by SHELXT 2014/5 methods
[17c] and refined against F2 using all reflections with the SHELXL-
2014/7 (WinGX) program [17c,17d]. Non-hydrogen atoms were re-
fined anisotropically. All hydrogen atoms were placed at the cal-
culated positions. The crystallographic figures were generated by
using Mercury 4.2.0 programme.
plexes 3 and 7 as compared to the parent complexes 2 and 10
(Fig. 3 and Table 1). On the other hand, no considerable difference
in the catalytic activities between the Me-substituted complexes 3
(with p-Me) and 7 (with m-Me) was noted (entry 2 vs 3). It is ev-
ident from these observations that the substituent position (meta-
vs para-) has minimal effect on the transfer hydrogenation activity.
3. Conclusion
4.3. Syntheses and characterizations
In summary, the catalytic behavior of various orthometalated
RuII-NHC complexes, possessing different electronic natures, in ac-
ceptorless dehydrogenation reaction of 1-phenyl-1-propanol was
investigated. The electronic modifications in the complexes were
achieved by varying the carbene donors from ImNHC (2-9) to 1,2,4-
TzNHC (10) as well as the substituents and their position on the N-
phenyl wingtip. The electrochemical analysis revealed the shuttle
electronic variations among the complexes and the complex 7 with
m-Me-substituent has the maximum electron density at the RuII-
center amongst the analyzed complexes. The effect of this elec-
tronic modulations was showcased in their catalytic performances
in acceptorless dehydrogenation of 1-phenyl-1-propanol. The imi-
dazolylidene complexes 2-9 were noted to be more active than the
triazolylidene complex 10. Among the imidazolylidene complexes,
the reactivity pattern is significantly influenced by the position and
nature of the substituents. We have also noted that under our re-
action condition, this electronic modulation among the analogous
meta- and para-substituted complexes has less effect on their ac-
tivities in hydride transfer reaction. We believe that this study will
help in designing and developing efficient catalytic systems for var-
ious fundamental and useful functional group transformations in
synthetic chemistry.
4.3.1. Synthesis of 3-ethyl-1-(p-tolyl)-1H-imidazol-3-ium bromide
(1b). To a pressure tube, compound B (0.684 g, 4.32 mmol),
bromoethane (4.079 g, 37.43 mmol) and 6 mL of dry CH3CN were
added. The reaction mixture was then stirred at 90°C for 24 h. Af-
ter that, the reaction mixture was allowed to attain room temper-
ature. After decanting the solvent, the precipitated solid product
was washed with diethyl ether and dried in vacuo. The product
was obtained as colorless hygroscopic solid. Yield: 901 mg (3.36
mmol, 78.0%). 1H NMR (400 MHz, CDCl3) δ 10.84 (s, 1H), 7.76 (s,
1H), 7.69 (s, 1H), 7.63 (d, J = 8.1 Hz, 2H), 7.31 (d, J = 8.1 Hz, 2H),
4.63 (q, J = 7.3 Hz, 2H), 2.37 (s, 3H), 1.63 (t, J = 7.3 Hz, 3H) ppm.
13C{1H} NMR (101 MHz, CDCl3) δ 140.6, 135.4, 132.1, 131.0, 123.1,
121.7, 120.9, 45.7, 21.2, 15.9 ppm.
4.3.2. Synthesis of 3-ethyl-1-(4-nitrophenyl)-1H-imidazol-3-ium
bromide
(1e). A pressure tube was charged with compound E (0.204
gram, 1.075 mmol), bromoethane (1.02 gram, 9.31 mmol) and 6
mL of dry CH3CN. This mixture was then allowed to stir at 90°C
for 24 h. After cooling, the solvent was decanted and the obtained
solid was washed with diethyl ether before drying in vacuo to get
an air stable yellow powder. Yield: 243 mg (0.812 mmol, 76.0%).
1H NMR (400 MHz, DMSO-d6) δ 10.11 (s, 1H), 8.50 (m, 3H), 8.14
(d, J = 7.8 Hz, 3H), 4.33 (q, J = 7.2 Hz, 2H), 1.53 (t, J = 7.2 Hz,
3H) ppm. 13C{1H} NMR (101 MHz, DMSO-d6) δ 147.5, 139.4, 136.1,
125.6, 123.4, 122.9, 121.0, 45.0, 14.8 ppm.
4. Experimental section
4.1. Materials and method
All manipulations were performed under argon atmosphere us-
ing either standard Schlenk line or Glove box techniques un-
less otherwise stated. Glassware was 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
4.3.3. Synthesis of 3-ethyl-1-(m-tolyl)-1H-imidazol-3-ium bromide
(1f). To a pressure tube compound F (500 mg, 3.16 mmol) and
bromoethane (2.98 g, 27.37 mmol) and 6 mL of dry CH3CN were
added. The reaction mixture was stirred at 90°C for 24 h. A sim-
ilar work up procedure to the previous case was followed to ob-
tain the product as an off-white hygroscopic solid. Yield: 662 mg
(2.47 mmol, 78.0%). 1H NMR (500 MHz, CDCl3) δ 10.90 (s, 1H), 7.75
(s, 1H), 7.69 (s, 1H), 7.58 (s, 1H), 7.55 (d, J = 8.1 Hz, 1H), 7.41 (t,
J = 8.1 Hz, 1H), 7.29 (d, J = 7.6 Hz, 1H), 4.65 (q, J = 7.8, 2H), 2.43 (s,
3H), 1.64 (t, J = 7.8 Hz, 3H) ppm. 13C{1H} NMR (126 MHz, CDCl3)
δ 141.3, 135.9, 134.5, 131.2, 131.1, 130.5, 122.8, 122.5, 120.7, 119.0,
45.9, 21.4, 15.9 ppm.
˚
over 4 A molecular sieves. NMR measurements were performed
on Bruker 400 and 500 MHz FT-NMR spectrometers. The chem-
ical shifts in the 1H NMR spectra were referenced to the resid-
ual proton signals of the deuterated solvents (CDCl3, 1H 7.26 ppm
and 13C{1H} 77.16 ppm; DMSO-d6, 1H 2.50 ppm and 13C{1H} 39.52
ppm; CD3CN, 1H 1.94 ppm and 13C{1H} 1.32 and 118.26 ppm) and
reported relative to TMS. Coupling constants are expressed in Hz.
ESI-MS spectra were recorded with an Agilent 6545A Q-TOF Mass
spectrometer and the electrochemical measurements were car-
ried out with Metrohm autolab potentiostat/galvanostat MAC90009
instrument. The 3/4-substituted phenyl imidazoles, phenyl-1,2,4-
triazole, [Ru(p-cymene)Cl2]2 and 1-phenyl-1-propanol were syn-
thesized according to the literature procedures [4b,7a,16]. All other
4.3.4. Synthesis of 3-ethyl-1-(3-methoxyphenyl)-1H-imidazol-3-ium
bromide
(1g). It was synthesized by following a similar procedure men-
tioned above using compound G (500 mg, 2.87 mmol) and bro-
moethane (2.5 g, 23 mmol) and obtained as hygroscopic, off-white
7

P.M. Illam, V.K. Singh, Priya et al.
Journal of Organometallic Chemistry 951 (2021) 122008
solid. Yield: 624 mg (2.10 mmol, 73.0%). 1H NMR (400 MHz, CDCl3)
δ 11.02 (s, 1H), 7.79 (s, 2H), 7.44 (s, 1H), 7.41 (d, J = 8.2 Hz, 1H),
7.29 (m, 1H), 7.02 (d, J = 8.3 Hz, 1H), 4.63 (q, J = 7.2 Hz, 2H),
3.94 (s, 3H), 1.67 (t, J = 7.2 Hz, 3H) ppm. 13C{1H} NMR (101 MHz,
CDCl3) δ 161.2, 135.9, 135.5, 131.3, 122.7, 120.8, 116.8, 113.3, 107.3,
56.6, 45.8, 15.9 ppm.
4.4.2. Synthesis of complex 6
Using the imidazolium salt 1e (100 mg, 0.32 mmol), the gen-
eral procedure was followed and the complex 6 was isolated as an
orange solid. Yield 132 mg (0.25 mmol, 78.0%). 1H NMR (400 MHz,
CDCl3) δ 8.94 (s, 1H), 7.87 (d, J = 8.4 Hz, 1H), 7.41 (s, 1H), 7.12
(d, J = 8.6 Hz, 1H), 7.09 (s, 1H), 5.66 (d, J = 5.8 Hz, 1H), 5.62 (d,
J = 5.5 Hz, 1H), 5.55 (d, J = 5.8 Hz, 1H), 5.39 (s, J = 5.8 Hz, 1H),
4.59 (dd, J = 14.0, 7.0 Hz, 1H), 4.48 (dd, J = 13.7, 7.1 Hz, 1H), 2.24
(m, 1H), 2.18 (s, 3H), 1.65 (t, J = 7.3 Hz, 3H), 0.91 (d, J = 6.8 Hz,
3H), 0.75 (d, J = 6.8 Hz, 3H) ppm. 13C{1H} NMR (101 MHz, CDCl3)
δ 189.2, 164.7, 150.9, 143.8, 136.3, 121.1, 119.4, 115.3, 110.5, 105.3,
101.4, 93.2, 90.8, 88.6, 84.5, 46.0, 31.3, 23.1, 21.9, 19.7, 16.7 ppm.
4.3.5. Synthesis of
3-ethyl-1-(3-(trifluoromethyl)phenyl)-1H-imidazol-3-ium bromide
(1h). Using compound
H (500 mg, 2.35 mmol) and bro-
moethane (2.0 mg, 19 mmol) following the similar procedure for
1g, the imidazolium salt 1h was isolated as air stable white solid.
Yield: 672 mg (2.09 mmol, 89.0%). 1H NMR (400 MHz, CDCl3) δ
11.06 (s, 1H), 8.21 (s, 1H), 8.03 (s, 2H), 7.96 (s, 1H), 7.70 (d, J = 4.1
Hz, 2H), 4.59 (q, J = 7.3 Hz, 2H), 1.63 (t, J = 7.3 Hz, 3H) ppm.
13C{1H} NMR (101 MHz, CDCl3) δ 135.9, 135.0, 132.9, 132.5, 131.6,
126.9, 125.9, 123.6, 121.1, 118.9, 45.9, 15.7 ppm.
4.4.3. Synthesis of complex 7
Using the imidazolium salt 1f (100 mg, 0.374 mmol), the gen-
eral procedure was followed and the complex 7 was isolated as
a yellow solid. Yield 148 mg (0.29 mmol, 79.0%). 1H NMR (500
MHz, CDCl3) δ 7.95 (d, J = 7.5 Hz, Ar-H, 1H), 7.34 (s, 1H), 7.00 (d,
J = 2.0 Hz, Imidazole-H, 1H), 6.90 (s, 1H), 6.79 (d, J = 7.5 Hz, Ar-H,
1H), 5.56 (d, J = 6.0 Hz, p-cymene−Ph,1H), 5.52 (d, J = 6.5 Hz, p-
cymene−Ph, 1H), 5.39 (d, J = 6.0 Hz, p-cymene−Ph, 1H), 5.30 (d,
J = 6.5 Hz, p-cymene−Ph, 1H), 4.58 (dq, J = 14.7, 7.4 Hz, NCH2CH3,
1H), 4.48 (dq, J = 14.6, 7.3 Hz, NCH2CH3, 1H), 2.33 (s, Ph-Me, 3H),
2.26 (sept, J = 6.9 Hz, p-cymene−iPr, 1H), 2.12 (s, p-cymene−Me,
3H), 1.62 (t, J = 7.4 Hz, NCH2CH3, 3H), 0.93 (d, J = 6.9 Hz, p-
cymene−iPr, 3H), 0.77 (d, J = 6.9 Hz, p-cymene−iPr, 3H) ppm.
13C{1H} NMR (126 MHz, CDCl3) δ 187.2, 157.6, 145.4, 141.6, 131.7,
125.5, 119.6, 114.5, 112.2, 103.0, 100.2, 92.4, 89.9, 87.4, 83.7, 45.7,
31.2, 23.2, 21.8, 21.2, 19.7, 16.8 ppm.
4.3.6. Synthesis of 3-ethyl-1-(3-nitrophenyl)-1H-imidazol-3-ium
bromide
(1i). Using compound I (500 mg, 2.64 mmol) and bromoethane
(2.3 g, 21 mmol) following the similar procedure for 1g, the imi-
dazolium salt 1i was isolated as air stable pale yellow solid. Yield:
590 mg (1.98 mmol, 75.0%). 1H NMR (500 MHz, DMSO-d6) δ 10.02
(s, 1H), 8.73 (s, 1H), 8.47 (s, 1H), 8.43 (d, J = 8.1 Hz, 1H), 8.29
(d, J = 7.3 Hz, 1H), 8.12 (s, 1H), 7.98 (t, J = 8.2 Hz, 1H), 4.32
(q, J = 7.2 Hz, 2H), 1.53 (t, J = 7.3 Hz, 3H) ppm. 13C{1H} NMR
(126 MHz, DMSO-d6) δ 148.4, 136.1, 135.6, 131.7, 128.4, 124.3, 123.1,
121.3, 117.4, 44.9, 14.8 ppm.
4.4.4. Synthesis of complex 8
4.3.7. Synthesis of 4-ethyl-1-phenyl-1H-1,2,4-triazol-4-ium bromide
(1j). Using compound J (600 mg, 4.13 mmol) and bromoethane
(3.6 g, 33 mmol) following the similar procedure for 1g, the imida-
zolium salt 1j was isolated as air stable white solid. Yield: 808 mg
(3.18 mmol, 77%). 1H NMR (400 MHz, CDCl3) δ 12.17 (s, 1H), 9.54
(s, 1H), 8.02 (d, J = 7.7 Hz, 2H), 7.51 (q, J = 9.9, 8.3 Hz, 3H), 4.76
(q, J = 7.4 Hz, 2H), 1.72 (t, J = 7.4 Hz, 3H) ppm. 13C{1H} NMR (126
MHz, CDCl3) δ 144.3, 140.9, 140.8, 134.9, 131.0, 130.4, 120.6, 44.8,
15.9 ppm.
Using the imidazolium salt 1h (100 mg, 0.311 mmol), the gen-
eral procedure was followed and the complex 8 was isolated as a
dark yellow solid. Yield: 140 mg (0.25 mmol, 80.0%). 1H NMR (500
MHz, CDCl3) δ 8.21 (d, J = 7.7 Hz, Ar-H, 1H), 7.41 (s, 1H), 7.24 (s,
1H), 7.17 (d, J = 7.7 Hz, Ar-H, 1H), 7.06 (s, 1H), 5.58 (m, J = 4.6
Hz, p-cymene−Ph, 2H), 5.46 (d, J = 6.0 Hz, p-cymene−Ph, 1H),
5.36 (d, J = 5.9 Hz, p-cymene−Ph, 1H), 4.58 (dq, J = 14.5, 7.4 Hz,
NCH2CH3, 1H), 4.48 (dq, J = 14.2, 7.2 Hz, NCH2CH3, 1H), 2.24 (m,
J = 6.9 Hz, p-cymene−iPr, 1H), 2.14 (s, p-cymene−Me, 3H), 1.63 (t,
J = 7.3 Hz, NCH2CH3, 3H), 0.92 (d, J = 6.9 Hz, p-cymene−iPr, 3H),
0.76 (d, J = 6.8 Hz, p-cymene−iPr, 3H) ppm. 13C{1H} NMR (126
MHz, CDCl3) δ 187.5, 170.1, 145.7, 142.0, 120.6 (q, J = 3.5 Hz), 120.4,
114.8, 107.2 (q, J = 3.5 Hz), 104.3, 101.2, 92.7, 90.6, 88.2, 84.4, 45.8,
31.2, 23.1, 21.8, 19.7, 16.8 ppm. 19 F {1H}NMR (471 MHz, CDCl3) δ
-61.35 (s) ppm.
4.4. General procedure for the synthesis of orthometalated
RuII-complexes 2, 6 and 7-10
To a pressure tube equipped with a magnetic stirring bar,
azolium salt (1 equiv.), [Ru(p-cymene)Cl2]2 (0.5 equiv.), Cs2CO3 (2
equiv.), NaBr (3 equiv.) were added along with dry THF and stirred
for 24 h at 70°C. After cooling, the solvent was dried in high vacuo
and the residue was dissolved in dichloromethane. The obtained
dark brown suspension was then filtered through neutral alumina
to get a clear yellow solution which was further concentrated and
precipitated using n-hexane to get air and moisture stable yellow
to orange solids.
4.4.5. Synthesis of complex 9
Using the imidazolium salt 1i (100 mg, 0.335 mmol), the gen-
eral procedure was followed and the complex 8 was isolated as
a brown solid. Yield: 122 mg (0.23 mmol, 69.0%). 1H NMR (500
MHz, CDCl3) δ 8.27 (d, J = 8.2 Hz, Ar-H, 1H), 7.89 (d, J = 2.2
Hz, Imidazole-H, 1H), 7.82 (dd, J = 8.2, 2.2 Hz, 1H), 7.48 (d,
J = 2.1 Hz, 1H), 7.11 (d, J = 2.1 Hz, 1H), 5.64 (m, 2H), 5.52 (d,
J = 5.5 Hz, p-cymene−Ph,1H), 5.40 (d, J = 5.9 Hz, p-cymene−Ph,
1H), 4.57 (dq, J = 14.8, 7.4 Hz, NCH2CH3, 1H), 4.47 (dq, J = 14.6,
7.3 Hz, NCH2CH3,1H), 2.24 (m, p-cymene−iPr, 1H), 2.17 (s, p-
cymene−Me,3H), 1.65 (t, J = 7.4 Hz, NCH2CH3, 3H), 0.91 (d, J = 6.9
Hz, p-cymene−iPr, 3H), 0.75 (d, J = 6.9 Hz, p-cymene−iPr, 3H)
ppm. 13C{1H} NMR (126 MHz, CDCl3) δ 187.6, 181.1, 145.9, 144.4,
141.9, 120.9, 118.6, 115.1, 105.7, 105.2, 101.8, 93.31, 91.2, 89.0, 85.0,
45.9, 31.2, 23.1, 21.9, 19.7, 16.7 ppm.
4.4.1. Synthesis of complex 3
Using the imidazolium salt 1b (100 mg, 0.32 mmol), the general
procedure was followed and the complex 3 was isolated as a pale-
yellow solid. Yield 112 mg (0.22 mmol, 70.0%) 1H NMR (400 MHz,
CDCl3) δ 7.91 (s, 1H), 7.32 (s, 1H), 7.00 (s, 1H), 6.95 (d, J = 7.8 Hz,
1H), 6.73 (d, J = 7.7 Hz, 1H), 5.54 (dd, J = 9.9, 6.0 Hz, 2H), 5.41 (d,
J = 5.9 Hz, 1H), 5.33 (d, J = 5.8 Hz, 1H), 4.58 (dt, J = 14.0, 7.3 Hz,
1H), 4.47 (dd, J = 13.7, 7.2 Hz, 1H), 2.35 (s, 3H), 2.30–2.17 (m, 1H),
2.13 (s, 2H), 1.62 (t, J = 7.3 Hz, 3H), 0.91 (d, J = 6.9 Hz, 3H), 0.76 (d,
J = 6.9 Hz, 3H) ppm. 13C{1H} NMR (101 MHz, CDCl3) δ 186.5, 162.1,
143.3, 142.7, 133.4, 123.0, 119.6, 114.5, 110.6, 103.6, 99.8, 92.3, 89.9,
87.3, 84.0, 45.6, 31.2, 23.1, 21.9, 21.6, 19.7, 16.8 ppm.
4.4.6. Synthesis of complex 10
Using the imidazolium salt 1j (100 mg, 0.393 mmol), the gen-
eral procedure was followed and the complex 10 was isolated as a
8

P.M. Illam, V.K. Singh, Priya et al.
Journal of Organometallic Chemistry 951 (2021) 122008
dark yellow solid. Yield 132 mg (0.27 mmol, 69.0%). 1H NMR (500
MHz, CDCl3) δ 8.08 (m, 1H), 8.04 (s, 1H), 7.46 (m, 1H), 7.01 (m,
2H), 5.69 (d, J = 6.0 Hz, 1H), 5.62 (d, J = 5.9 Hz, 1H), 5.44 (d,
J = 6.0 Hz, 1H), 5.32 (d, J = 5.9 Hz, 1H), 4.59 (dq, J = 14.0, 7.5 Hz,
1H), 4.48 (dt, J = 14.0, 7.1 Hz, 1H), 2.32 (p, J = 6.9 Hz, 1H), 2.13 (s,
2H), 1.69 (t, J = 7.3 Hz, 3H), 0.95 (d, J = 6.9 Hz, 2H), 0.77 (d, J = 6.8
Hz, 2H) ppm. 13C{1H} NMR (126 MHz, CDCl3) δ 189.3, 159.6, 144.7,
141.6, 140.9, 125.4, 122.7, 112.8, 103.4, 102.0, 92.8, 90.4, 88.0, 83.6,
43.7, 31.3, 23.1, 21.9, 19.7, 16.5 ppm.
Supplementary materials
Supplementary material associated with this article can be
found, in the online version, at doi:10.1016/j.jorganchem.2021.
122008.
References
[1] A.J. Arduengo, R.L. Harlow, M. Kline, J. Am. Chem. Soc. 113 (1991) 361–363.
[2] (a) R.H. Crabtree, J. Organomet. Chem. 690 (2005) 5451–5457; (b) F.E. Hahn,
M.C. Jahnke, Angew. Chem. Int. Ed. 47 (2008) 3122–3172; (c) H.V. Huynh,
Chem. Rev. 118 (2018) 9457–9492; (d) T.T. Dang, B. Ramalingam, S.P. Shan,
A.M. Seayad, ACS Catal 3 (2013) 2536–2540.
4.5. General procedure of the catalytic experiments
[3] (a) M.N. Hopkinson, C. Richter, M. Schedler, F. Glorius, Nature 510 (2014)
485–496; (b) M. Huang, J. Liu, Y. Li, X.-B. Lan, P. Su, C. Zhao, Z. Ke, Catal. To-
day 370 (2021) 114–141; (c) L.-A. Schaper, S.J. Hock, W.A. Herrmann, F.E. Kühn,
Angew. Chem. Int. Ed. 52 (2013) 270–289; (d) C.M. Crudden, D.P. Allen, Coord.
Chem. Rev. 248 (2004) 2247–2273.
[4] (a) E. Peris, R.H. Crabtree, Coord. Chem. Rev. 248 (2004) 2239–2246; (b)
S.N.R. Donthireddy, P.M. Illam, A. Rit, Inorg. Chem. 59 (2020) 1835–1847; (c)
D.A. Hey, R.M. Reich, W. Baratta, F.E. Kühn, Chem. Rev. 374 (2018) 114–132;
(d) H. Ohara, W.N.O. Wylie, A.J. Lough, R.H. Morris, Dalton Trans 41 (2012)
8797–8808.
4.5.1. Catalytic acceptorless dehydrogenation of 1-phenyl-1-propanol
Catalyst (0.5/0.3 mol%) from a stock solution, prepared by dis-
solving the required amount of the complex in acetonitrile, was
added to a predried Schlenk tube (25 mL) equipped with a mag-
netic stirring bar and the acetonitrile was then dried in high vacuo.
After that, KOtBu (0.2 equiv.) followed by 1-phenyl-1-propanol (0.5
mmol) and dry toluene were added. The reaction mixture was then
heated to reflux under open condition for the specified time. The
reaction mixture was then cooled, a 25 μL aliquot was then taken
and diluted with 1 mL of methanol to determine the conversion
using GC-MS.
[5] F.E. Fernández, M.C. Puerta, P. Valerga, Organometallics 31 (2012) 6868–6879.
[6] (a) S.K. Gupta, S.K. Sahoo, J. Choudhury, Organometallics 35 (2016) 2462–2466;
(b) S. Semwal, I. Mukkatt, R. Thenarukandiyil, J. Choudhury, Chem. Eur. J. 23
(2017) 13051–13057.
[7] (a) S. Bauri, S.N.R. Donthireddy, P.M. Illam, A. Rit, Inorg. Chem. 57 (2018)
14582–14593; (b) P.M. Illam, S.N.R. Donthireddy, S. Chakrabartty, A. Rit,
Organometallics 38 (2019) 2610–2623; (c) J. Soellner, I. Císarˇová, T. Strassner,
Organometallics 37 (2018) 4619–4629.
4.5.1. Catalytic transfer hydrogenation of acetophenone
Catalyst (0.25/0.1 mol%), KOtBu (0.1 equiv.) and acetophenone
(1 mmol) were added to an oven dried pressure tube (25 mL) and
heated to reflux for specified time. The reaction mixture was then
cooled, a 25 μL aliquot was then taken and diluted with 1 mL of
methanol to determine the conversion using GC-MS.
[8] (a) X. Xie, H.V. Huynh, Org. Chem. Front. 2 (2015) 1598–1603; (b) D. Schleicher,
H. Leopold, H. Borrmann, T. Strassner, Inorg. Chem. 56 (2017) 7217–7229.
[9] (a) S. Hitzel, C. Farber, C. Bruhn, U. Siemeling, Organometallics 33 (2014)
425–428; (b) S. Bauri, A. Mallik, A. Rit, Organometallics 39 (2020) 3362–3374.
[10] D. Schleicher, A. Tronnier, H. Leopold, H. Borrmann, T. Strassner, Dalton Trans
45 (2016) 3260–3263.
[11] (a) C. Gunanathan, D. Milstein, Science 314 (2013) 1229712; (b) M. Maji,
D. Panja, I. Borthakur, S. Kundu, Org. Chem. Front. 8 (2021) 2673–2709.
[12] (a) S.N.R. Donthireddy, C.S. Tiwari, S. Kumar, A. Rit, Asian J. Org. Chem. 10
(2021) 464–484; (b) V.K. Singh, S.N.R. Donthireddy, P.M. Illam, A. Rit, Dalton
Trans 49 (2020) 11958–11970; (c) S.N.R. Donthireddy, V.K. Pandey, A. Rit, J.
Org. Chem. 86 (2021) 6994–7001.
Declaration of Competing Interest
The authors declare that they have no known competing finan-
cial interests or personal relationships that could have appeared to
influence the work reported in this paper.
[13] (a) S. Shahane, C. Fischmeister, C. Bruneau, Catal. Sci.Technol.
2 (2012)
1425–1428; (b) W. Chang, X. Gong, S. Wang, L.–P. Xiao, G. Song, Org. Biomol.
Chem. 15 (2017) 3466–3471; (c) F.P. Malan, E. Singleton, P.H. van Rooyen,
M. Albrecht, M. Landman, Organometallics 38 (2019) 2624–2635; (d) J. De-
Pasquale, M. Kumar, M. Zeller, E.T. Papish, Organometallics 32 (2013) 966–979;
(e) X. Xie, H.V. Huynh, ACS Catal 5 (2015) 4143–4151.
Acknowledgments
[14] J.A. Widegren, R.G. Finke, J. Mol. Catal. A: Chem. 198 (2003) 317–341.
[15] (a) S. Burling, B.M. Paine, D. Nama, V.S. Brown, M.F. Mahon, T.J. Prior,
P.S. Pregosin, M.K. Whittlesey, J.M.J. Williams, J. Am. Chem. Soc. 129 (2007)
1987–1995; (b) K. Farrell, H. Müller-Bunz, M. Albrecht, Dalton Trans 45 (2016)
15859–15871; (c) L. Mercs, A. Neels, H. Stoeckil-Evans, M. Albrecht, Inorg.
Chem. 50 (2011) 8188–8196.
[16] M.A. Bennett, A.K. Smith, J. Chem. Soc., Dalton Trans. (1974) 233–241.
[17] (a) SADABS, v 2.05, Bruker AXS Inc., Madison, WI, 2003; (b) G.M. Sheldrick,
SAINT, version 8.37A, 2013 Bruker AXS Inc, WI; (c) G.M. Sheldrick, 2015 Acta
Crystallogr., Sect. A: Found. Adv., 71 3–8; (d) L.J. Farrugia, 2012 J. Appl. Crystal-
logr., 45 849–854.
We gratefully acknowledge DST and SERB, India (Project No.
DST/INSPIRE/04/2015/002219 and CRG/2020/000780, respectively)
for the financial support. P.M.I and V.K.S thank IIT Madras for the
research fellowship. We gratefully acknowledge the NMR and ESI-
MS (DST-FIST funded) facility at the Department of Chemistry, IIT
Madras, single-crystal X-ray diffractometer facility at the Depart-
ment of Chemistry and SAIF, IIT Madras, and Dr. K.R.R.’s laboratory
for the CV and DPV measurements. We sincerely thank Prof. F. E.
Hahn for the crystal measurements of 1i and 10.
9