Ancillary ligands switch the activity of Ru–NHC-based oxidation precatalysts

Article abstract of DOI:10.1016/j.ica.2019.119195

Herein we demonstrate how the inner-sphere coordinating ligands switch the activity of Ru–NHC-based oxidation precatalysts in the oxidative conversion of olefins to carbonyl compounds, with the help of a series of systematically varied imidazolydene-NHC (Im-NHC) and triazolydene-NHC (Tz-NHC)-based ruthenium(II)-complexes. It is shown that the catalytic activity of the para-cymene-containing precatalysts varies in the order of [(Tz-NHC)Ru(para-cymene)Cl]⁺ > [(Im-NHC)Ru(para-cymene)Cl]⁺, while the order of activity of the MeCN-containing precatalysts is found to be reversed, i.e., [(Im-NHC)Ru(MeCN)₄]²⁺ > [(Tz-NHC)Ru(MeCN)₄]²⁺. Along with the electronic influence of the NHC ligands, the effect of the lability of the para-cymene and MeCN ligands, and the overall charge of the complexes might be attributed toward such a switching of catalytic activity. This finding led to develop a new precatalyst with improved activity which was further utilized in selective oxidation of a series of styrene substrates containing other oxidation-sensitive functionalities.

Full text of DOI:10.1016/j.ica.2019.119195

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Research paper
Ancillary Ligands Switch the Activity of Ru–NHC-based Oxidation Precata-
lysts
Suraj K. Gupta, Tanmoy Mandal, Tejaswinee Gangber, Vivek Singh, Joyanta
Choudhury
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https://doi.org/10.1016/j.ica.2019.119195
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Please cite this article as: S.K. Gupta, T. Mandal, T. Gangber, V. Singh, J. Choudhury, Ancillary Ligands Switch
the Activity of Ru–NHC-based Oxidation Precatalysts, Inorganica Chimica Acta (2019), doi: https://doi.org/
10.1016/j.ica.2019.119195
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Ancillary Ligands Switch the Activity of Ru–NHC-based Oxidation
Precatalysts
Suraj K. Gupta, Tanmoy Mandal, Tejaswinee Gangber, Vivek Singh and Joyanta Choudhury*
Organometallics & Smart Materials Laboratory, Department of Chemistry, Indian Institute of Science
Education and Research Bhopal, Bhopal 462 066 (India)
Corresponding author: Joyanta Choudhury (joyanta@iiserb.ac.in). Address: Organometallics & Smart
Materials Laboratory, Department of Chemistry, Indian Institute of Science Education and Research
Bhopal, Bhopal 462 066 (India)
Dedication: This work is dedicated to Professor Goutam Kumar Lahiri on the occasion of his 60th
birthday.
Abstract
Herein we demonstrate how the inner-sphere coordinating ligands switch the activity of Ru–NHC-based
oxidation precatalysts in the oxidative conversion of olefins to carbonyl compounds, with the help of a
series of systematically varied imidazolydene-NHC (Im-NHC) and triazolydene-NHC (Tz-NHC)-based
ruthenium(II)-complexes. It is shown that the catalytic activity of the para-cymene-containing
precatalysts varies in the order of [(Tz-NHC)Ru(para-cymene)Cl]+ > [(Im-NHC)Ru(para-cymene)Cl]+,
while the order of activity of the MeCN-containing precatalysts is found to be reversed, i.e., [(Im-
NHC)Ru(MeCN)4]2+ > [(Tz-NHC)Ru(MeCN)4]2+. Along with the electronic influence of the NHC
ligands, the effect of the lability of the para-cymene and MeCN ligands, and the overall charge of the
complexes might be attributed toward such a switching of catalytic activity. This finding led to develop a
new precatalyst with improved activity which was further utilized in selective oxidation of a series of
styrene substrates containing other oxidation-sensitive functionalities.
Graphical Abstract
Keywords
N-Heterocyclic carbene (NHC) / Ruthenium / Oxidation / Alkenes / Sodium Periodate

Highlights


A series of imidazolydene-NHC (Im-NHC) and triazolydene-NHC (Tz-NHC)-based
ruthenium(II)-complexes are utilized as precatalysts for oxidation of alkenes and alkynes
Catalytic activity of the para-cymene-containing precatalysts varies in the order of [(Tz-
NHC)Ru(para-cymene)Cl]+ > [(Im-NHC)Ru(para-cymene)Cl]+, while the order of activity of
the MeCN-containing [(NHC)Ru(MeCN)4]2+-type precatalysts is found to be reversed
Variable electronic effect of the NHC ligands plays a crucial role in controlling the activity order

1. Introduction
Designing an organometallic catalyst for selective and efficient oxidative conversion of unsaturated
hydrocarbons to the valuable oxygenated derivatives is an important research area in organometallic
chemistry and catalysis [1]. In case of oxidation of olefins to aldehydes, traditional strong oxidants are
used in (over)stoichiometric amount [2]. Additionally, there are a few alternative catalytic systems based
on OsO4–Oxone/H2O2/NaIO4/tBuOOH and RuCl3/NaIO4 for this transformation although activity,
selectivity and other issues are compromised to a great extent [3]. Along this line, effective and selective
catalysts based on well-defined Ru-complexes are increasingly becoming popular during recent time [4].
In general, the advantage of using metal-complexes lies in the fact that the knowledge of the effect of
coordinated ligands on the catalytic steps helps to improve the catalytic efficiency, selectivity and
robustness by modifying their electronic and/or steric properties [5]. Our group has been actively
involved in the development of metal–NHC-based robust catalytic systems for efficient oxidative
functionalization of not only aliphatic and aromatic C–H bonds, but also of carbon-carbon multiple bonds
[6]. We rationalized that the NHC ligands would provide the required stability to the reactive high-valent
metal complex intermediates and also impart the resistance toward decomposition under strongly
oxidizing and acidic conditions [6f]. Recently, we disclosed that the rate of (NHC)RuII(para-cymene)-
catalyzed oxidative cleavage of carbon-carbon multiple bond to the corresponding carbonyl functionality
can be accelerated through electronic modulation from a remote spectator RuII(terpy)2 unit installed
within the catalyst backbone [6e]. We also demonstrated that the efficiency of this oxidative catalysis can
be tuned by modifying the ancillary NHC ligand backbone of the (NHC)RuII(para-cymene) precatalysts
(Fig. 1a,b) [6d]. Controlled mechanistic studies on the proposed catalytic steps (Fig. 1c) revealed that the
reversible dissociation of the para-cymene ligand from the precatalysts was slow and responsible for
controlling the rate of the reaction. It was observed that the triazolydene-NHC (Tz-NHC) based
ruthenium complex, [(Tz-NHC)Ru(para-cymene)Cl]+ (2-Cym), being electron deficient system, releases
para-cymene relatively faster as compared to imidazolydene NHC (Im-NHC) based ruthenium complex
[(Im-NHC)Ru(para-cymene)Cl]+ (1-Cym) and hence was more active than the later (Fig. 1d). These
results motivated us to investigate the [(NHC)Ru(MeCN)4]2+-type precatalysts (1-MeCN when NHC =
Im‒NHC, and 2-MeCN when NHC = Tz‒NHC), wherein both the para-cymene and Cl ligands are
removed from the Ru coordination-sphere and replaced with loosely-bound acetonitrile ligands. The aim
was to probe the effect of eliminating the ligand dissociation barrier (which would have been created by
bound para-cymene as well as Cl) on the catalytic activity. In the present study, we report the above-
mentioned investigation and reveal an interesting behavior of the ancillary NHC ligands (Im‒NHC and
Tz‒NHC) leading to a switching catalytic activity (Fig. 1d). To this end, we also show that how this
finding led to develop a new precatalyst with improved catalytic efficiency which was further utilized in
selective oxidation of a series of styrene substrates containing other oxidation-sensitive functionalities.

Fig. 1. (a) A general scheme for “(NHC)Ru”-catalyzed oxidation of alkenes and alkynes to carbonyl
compounds; (b) design of the precatalysts used in this study; (c) suggestive catalytic steps; (d) key finding
of the present study.
2. Experimental
2.1. General methods and materials
1H and 13C{1H} NMR spectra were recorded on Bruker AVANCE III 400 and 500 MHz NMR
spectrometers at room temperature unless mentioned otherwise. Chemical shifts (δ) are expressed in ppm
1
using the residual proton resonance of the solvent as an internal standard (CHCl3: δ = 7.26 ppm for H
spectra, 77.2 ppm for 13C{1H} spectra; CH3CN: δ = 1.94 ppm for 1H spectra, 118.3 ppm and 1.3 ppm for
1
13C{1H} spectra). All coupling constants (J) are expressed in hertz (Hz) and only given for H-1H
couplings unless mentioned otherwise. The following abbreviations were used to indicate multiplicity: s
(singlet), d (doublet), t (triplet), q (quartet), dd (doublet of doublet), dt (doublet of triplet), m (multiplet).
ESI mass spectroscopy was performed on a Bruker microTOF QII spectrometer. GCMS analysis was

performed on Agilent 7890A GC/5975C MS system. The electrochemical measurements (differential
pulse voltammetry, DPV) were carried out using a CHI 620E Electrochemical Analyzer at room
temperature. Dry solvents and reagents were obtained from commercial suppliers and used without
further purification. Deuterated solvents were purchased from Aldrich. All the alkenes and alkynes are
purchased from Aldrich. The products were previously reported and the identity of the products was
1
verified by GC and GCMS with known samples. H NMR spectroscopy was also used to match the
products with the known samples. 1-Cym and 2-Cym were previously reported by our group and
synthesized via the reported procedure [6d].
2.2. Synthesis of complexes
Synthesis of 1-MeCN. Bromide salt of 1‒Cym was converted to hexafluorophosphate salt by anion
metathesis [7]. Hexafluorophosphate salt of 1‒Cym (96 mg, 0.15 mmol) and silver triflate (46 mg, 0.18
mmol) were taken together in a Schlenk tube. After addition of 5 mL of acetonitrile, reaction mixture was
refluxed for 60 h under dark conditions. This reaction mixture was filtered through a Celite plug. The
filtrate was reduced to a minimum volume of 0.5 mL. Addition of 10 mL of diethylether resulted into a
solid product. Crystallization (MeCN/Et2O) of this solid product resulted into the analytically pure
product. Yield: = 100 mg (85%). 1H NMR (500 MHz, CD3CN, 300K): δ = 8.88 (d, J = 5.6 Hz, 1H), 8.18
(d, J = 2.3 Hz, 1H), 8.16 – 8.11 (m, 1H), 7.92 (d, J = 8.3 Hz, 1H), 7.70 – 7.60 (m, 5H), 7.53 (d, J = 2.2
Hz, 1H), 7.48 (t, J = 6.5 Hz, 1H), 2.15 (s, 6H), 1.96 (s, 3H), 1.90 (s, 3H). 13C{1H} NMR (125 MHz,
CD3CN, 300K): δ = 190.5, 156.2, 154.4, 142.1, 140.5, 131.6, 131.1, 128.9, 127.6, 126.3, 126.1, 125.1,
19
124.0, 113.3, 4.6, 4.5, 0.4 ppm. F NMR (471 MHz, CD3CN, 300K): δ = -79.30 (s, OTf), -72.88 (d, J =
706.5, PF6) ppm. 31P NMR (202 MHz, CD3CN, 300K): δ = -144.62 ppm. Anal. Found: C, 34.88; H, 2.87;
N, 12.54. Calcd for C23H23N7RuPSO3F9·0.5H2O: C, 34.94; H, 3.04; N, 12.41.
Synthesis of 2-MeCN. This complex was synthesized by following the same procedure as used for the
synthesis of 1‒MeCN. Yield: = 88 mg (75%). 1H NMR (500 MHz, CD3CN, 300K): δ = 9.42 (s, 1H), 8.92
(d, J = 5.6 Hz, 1H), 8.25 – 8.18 (m, 1H), 8.09 (d, J = 8.3 Hz, 1H), 7.78 – 7.73 (m, 2H), 7.73 – 7.65 (m,
13
3H), 7.59 (t, J = 6.6 Hz, 1H), 2.15 (s, 6H), 1.96 (s, 3H), 1.96 (s, 3H) ppm. C{1H} NMR (125 MHz,
CD3CN, 300K): δ = 190.5, 154.3, 153.0, 142.0, 140.6, 139.8, 131.5, 130.5, 127.9, 126.5, 126.1, 125.2,
124.8, 123.5, 120.9, 113.9, 4.2, 4.0 ppm. 19F NMR (471 MHz, CD3CN, 300K): δ = -79.28 (s, OTf), -72.88
31
(d, J = 706.4, PF6) ppm. P NMR (202 MHz, CD3CN, 300K): δ = -144.63 ppm. Anal. Found: C, 32.82;
H, 2.88; N, 14.24. Calcd for C22H22N8RuPSO3F9·H2O: C, 33.00; H, 3.00; N, 14.00.
Synthesis of 1-Cl. 1-MeCN (78.0 mg, 0.1 mmol and tetrabutylammonium chloride (139 mg, 0.5 mmol)
were mixed in 5 mL of acetone-ether solvent mixture (2:1) in a round bottom flask and stirred for 30
minutes at room temperature, which resulted in a large amount of precipitate. This precipitate was filtered
and the solid compound was washed with diethyl ether. The solid compound was recrystallized in
1
acetonitrile/diethyl ether solvent mixture to get the desired product. Yield = 41 mg (80%). H NMR (500
MHz, CD3CN, 300 K) δ 9.67 (dd, J = 5.7, 0.9 Hz, 1H), 8.38 (d, J = 2.4 Hz, 1H), 8.06 (ddd, J = 18.6, 12.6,
4.8 Hz, 2H), 7.68 (tt, J = 6.0, 3.1 Hz, 4H), 7.64 – 7.60 (m, 1H), 7.51 (d, J = 2.4 Hz, 1H), 7.44 (ddd, J =
13
7.2, 5.8, 1.4 Hz, 1H). C{1H} NMR (126 MHz, CD3CN, 300 K) δ 193.1, 156.2, 154.0, 140.5, 140.2,
130.6, 130.4, 128.4, 126.6, 124.8, 123.9, 111.9. HRMS (ESI, positive ion) m/z = 398.9957 (calculated for
[C16H14N4ClRu]+ = 398.9946), 417.0043 (calculated for [C16H16N4OClRu]+ = 417.0051),. 440.0209
(calculated for [C18H17N5ClRu]+ = 440.0212).

Synthesis of 2-Cl. This complex was synthesized by following the similar procedure as described for the
1
synthesis of complex 1-Cl. Yield = 47 mg (75%). H NMR (500 MHz, CD3CN, 300 K) δ 10.23 (s, 1H),
9.64 (d, J = 5.7 Hz, 1H), 8.54 (d, J = 8.3 Hz, 1H), 8.11 (t, J = 7.9 Hz, 1H), 7.79 (m, 2H), 7.70 – 7.61 (m,
3H), 7.53 – 7.46 (m, 1H). (ESI, positive ion) m/z = 399.9978 (calculated for [C15H13N5ClRu]+ =
399.9898), 418.0083 (calculated for [C15H15N5OClRu]+ = 418.0004), 441.0258 (calculated for
[C17H16N6ClRu]+ = 441.0164).
2.3. Single crystal X-ray diffraction analyses
Single crystals of 1–MeCN, 2–MeCN, 1-Cl and 2-Cl suitable for X-ray diffraction studies were grown
by diffusion of diethyl ether into the concentrated acetonitrile solution of the respected complexes. Data
collection were carried out on a Bruker SMART APEX II CCD diffractometer with graphite
monochromated Mo Kα (λ = 0.71073 Å) radiation at low temperature. The hydrogen atoms were placed
in calculated positions and refined using a riding model with isotropic displacement parameters. The data
quality of 2-Cl was poor and the anion, PF6, was found to be distorted. Attempts to grow better quality
crystal of 2-Cl resulted into a similar issue. Full crystallographic data (CIFs) of 1‒MeCN (CCDC
1441845), 2‒MeCN (CCDC 1441847), 1-Cl (CCDC 1875517) and 2-Cl (CCDC 1875518). can be
obtained free of charge from The Cambridge Crystallographic data Center via
www.ccdc.cam.ac.uk/data_request/cif.
2.4. Electrochemical studies
The electrochemical studies were carried out with three-electrode-configuration. Working electrode: Pt
disk (1 mm diameter); counter electrode: a Pt wire; reference electrode: saturated calomel electrode, SCE.
All the samples were prepared in dry deoxygenated acetonitrile. 0.1 M solution of [NBu4]PF6 solution
was used as the supporting electrolyte. Ferrocene (E1/2, Fc/Fc+ = 0.37 volts vs. SCE) was used as an
external calibration standard for all the experiments.
2.5. General procedure for the catalytic studies
Substrate (0.4 mmol) in 1 mL of acetone and catalyst (0.5 mol%) were taken in a round bottom flask. 2
mL of acetone and 2 mL of H2O were added to it. NaIO4 (213 mg, 1.0 mmol) was dissolved in 1 mL of
H2O and transferred to the reaction mixture. The reaction mixture was stirred at room temperature for ~15
min‒240 min. After this time, Na2SO3 (2.0 mmol) was added to the reaction mixture followed by addition
of 2 mL of DCM and 3 mL of H2O. The reaction mixture was further stirred for 10 min. A GC standard
(ethylbenzene or mesitylene or stilbene or acetophenone) was added as a reference and the reaction
mixture was again stirred for 5 min. It was then transferred to a separating funnel with the help of 3 mL of
H2O and 8 mL of DCM. The organic layer was separated and aqueous layer was again extracted with 5
mL of DCM (2 times). The combined organic layer was washed with 20 mL of brine solution. Products
and unreacted substrates were analyzed by GCMS. The yields were calculated by GC analyses.
3. Results and discussion
The [(NHC)Ru(MeCN)4]2+-type complexes 1‒MeCN and 2‒MeCN were synthesized in good yields
(85% and 75% respectively) by heating the corresponding precursors 1-Cym and 2-Cym respectively in
MeCN solvent in the presence of AgOTf under dark (Fig. 2a, see Supplementary Information for details)
[8]. These complexes were fully characterized by NMR spectroscopic and electron-spray ionization mass

(ESI-MS) spectrometric methods (details provided in Supplementary Information). The proposed
structures of 1‒MeCN and 2‒MeCN based on the above characterization techniques were unambiguously
confirmed by X‒ray diffraction studies (Fig. 2b). Both 1‒MeCN and 2‒MeCN showed distorted
octahedral geometries around the ruthenium centre with the CNHC–RuII–Cpyridine bite angle of 78.22(17)˚
and 79.20(2)˚ respectively, and CNHC–RuII bond length of 1.994(4) and 2.004(7) Å respectively. The Ru–
NMeCN bond trans to the NHC ligand was found to be larger than the other Ru–NMeCN bonds, due to strong
trans influence of NHC ligand [8,9]. 1‒MeCN and 2‒MeCN showed RuII/III redox potential values of
1.512 V and 1.692 V vs SCE respectively (Fig. 3). Notably these peaks were neither due to ligand
oxidation nor solvent oxidation (see Supplementary Information for control analysis). The potential for
2‒MeCN was found to be anodically shifted by ~180 mV compared to that for 1‒MeCN. This shift may
be attributed to the inherent poor σ‒donor and better π‒acceptor properties of Tz‒NHC compared to
Im‒NHC [10]. However, the RuII/III redox potential values of both 1‒MeCN and 2‒MeCN were found to
be anodically shifted compared to their para‒cymene counter-partners 1-Cym (1.392 V vs SCE) and 2-
Cym (1.484 V vs SCE) respectively [6d]. This may be due to the fact that the former complexes are
dicationic and the latter are monocationic. The extra positive charge may have caused the shifting of the
oxidation potential toward more anodic.
Fig. 2. (a) Synthesis of the precatalysts 1-MeCN and 2-MeCN; (b) molecular structures of 1-MeCN and
2-MeCN (H atoms and counterions have been omitted for clarity). Selected bond lengths (Å) and bond
angles (°): 1-MeCN: Ru(1)-C(1) = 1.994(4), Ru(1)-N(3) = 2.068(4), Ru(1)-N(4) = 2.027(4), Ru(1)-N(5) =
2.116(4), Ru(1)-N(6) = 2.036(4), Ru(1)-N(7) = 2.030(4), C(1)-Ru(1)-N(3) = 78.22(17), N(2)-C(1)-N(1) =
104.2(4). 2-MeCN: Ru(1)-C(1) = 2.004(7), Ru(1)-N(1) = 2.068(5), N(3)-N(4) = 1.391(8), Ru(1)-N(5) =

2.026(5), Ru(1)-N(6) = 2.027(6), Ru(1)-N(7) = 2.113(5), Ru(1)N-N(8) = 2.026(5), C(1)-Ru(1)-N(1) =
79.2(2), N(4)-C(1)-N(2) = 102.2(6).
Fig. 3. Differential pulse voltammetric (DPV) plots of 1-MeCN and 2-MeCN recorded in CH3CN.
After successful synthesis and characterization, the solvato complexes 1‒MeCN and 2‒MeCN were
utilized as precatalysts (0.5 mol% loading) for selective oxidation of various alkenes and alkynes to the
corresponding aldehydes and ketones (Table 1). The catalytic activity of 1‒MeCN containing the Im-
NHC ligand was found to be an order of magnitude higher than that of 2‒MeCN containing the Tz-NHC
ligand. This activity order is in stark contrast to our previous findings with the corresponding para-
cymene-containing precatalysts 1-Cym and 2-Cym, wherein the order was 2‒Cym>1‒Cym [6d]. This
contrasting order of activity (2‒Cym>1‒Cym but 1‒MeCN>2‒MeCN) was further verified by the
kinetic profiles of a model reaction for the oxidation of 4-methystyrene to 4-methylbenzaldehyde as
shown in Fig. 4. Apparently, the change in reactivity seemed to be related to a probable contrasting
influence of the two NHC ligands imparted to the structurally different two sets of precatalysts. However,
the precatalysts under consideration are different in terms of the lability of the para-cymene and MeCN
ligands, charge and solubility. The effect of such factors on the change in reactivity could not be ruled out
at this stage.
Nevertheless, to derive additional kinetic information for the present MeCN-based precatalysts, the initial
rate studies were carried out for the 1-MeCN and 2-MeCN-catalyzed reactions, by varying the
concentration of the precatalyst, substrate and oxidant one by one. The kinetic data showed that for both
cases, the rate of reaction is dependent on the concentration of the RuII‒precatalysts (first order), oxidant
NaIO4 (first order), whereas it is independent of the concentration of the alkene substrate (zero order) (see
Supplementary Information for details). This fact implied that the interaction of the
[(NHC)Ru(MeCN)4]2+- precatalysts and NaIO4 might be involved as the major rate-influencing factor. In
such a scenario, the formation of the high-valent Ru-oxo species (probably cis-dioxo) from the reaction of
[(NHC)Ru(MeCN)4]2+ species and NaIO4, would be favored by electron-donating Im-NHC ligand in 1-
MeCN via electronic stabilization of the high-valent Ru (e.g., RuVI). This effect should have favored the
observed activity order of 1-MeCN>2-MeCN.

Table 1
Oxidation of carbon‒carbon multiple bonded substrates to carbonyl compounds.a
#
Substrate
Product
Yield (%)/Time(min)
1-MeCN
2-MeCN
1
2
3
75/80
81/90
70/40
15/80
18/90
15/40
4
5
6
7
78/90
78/60
73/90
75/120
17/90
13/60
16/90
52/120
8
81/15
95/15
96/15
56/15
65/15
86/15
9
10
11
12
65/120
68/40
14/120
38/40
13
14
78/90
70/90
35/90
40/90
a Reaction conditions: substrate, 0.4 mmol; NaIO4, 1.0 mmol; cat, 0.002 mmol; acetone:H2O (1:1), 6
mL; room temp. Yields were determined by GC.

50
40
30
20
10
0
2-Cym
1-Cym
1-MeCN
2-MeCN
40
30
20
10
0
0
20
40
60
80
0
20
40
60
80
Time (min)
Time (min)
Fig. 4. Reaction kinetics profile of (left) 1-Cym/2-Cym and (right) 1-MeCN/2-MeCN precatalysts for
the oxidation of 4-methylstyrene to 4-methylbenzaldehyde. Reaction conditions: 4-methylstyrene, 0.4
mmol; NaIO4, 1.0 mmol; mesitylene, 0.2 mmol, internal standard; cat, 0.002 mmol; acetone:H2O (1:1), 6
mL; room temp. Yields were determined by GC.
To further verify the hypothesis that the increase in the electron density at the ruthenium center of the
solvato complexes [(NHC)Ru(MeCN)4]2+, can increase the activity, two new solvato complexes, 1-Cl and
2-Cl of the type [(NHC)Ru(MeCN)3Cl]+ were prepared where one of the four MeCN ligands was
replaced with a chloride ligand (Fig. 5a). Molecular structures of both the complexes were confirmed by
single crystal X-ray diffraction (Fig. 5b) along with other spectroscopic characterization (see
Supplementary Information). The anionic π-donor Cl ligand and the less positive charge on the complex
should make Ru center relatively more electron-rich in complexes 1-Cl and 2-Cl as compared to 1-MeCN
and 2-MeCN respectively. The time vs yield profile for the catalytic oxidation of 4-methylstyrene was
derived with these new 1-Cl and 2-Cl precatalysts. Indeed, both 1-Cl and 2-Cl showed higher activity
relative to their MeCN analogs (Fig. 6). Notably, in this case also, the effect of overall charge of the
complexes might play additional role as well for the enhancement of the activity. Kinetic studies with 1-
Cl and 2-Cl showed similar rate-dependency profile as observed with 1-MeCN and 2-MeCN, that is, the
rate of the reaction is dependent on the concentration of the RuII‒precatalysts (first order), oxidant NaIO4
(first order), and it is independent of the concentration of alkene (zero order) (see Supplementary
Information for details), thus indicating similar nature of the rate-influencing step with these complexes
as well.

Fig 5. (a) Synthesis of the precatalysts 1-Cl and 2-Cl; (b) molecular structures of 1-Cl and 2-Cl. Selected
bond lengths (Å) and bond angles (°): 1-Cl: Ru(1)-C(6) = 1.962(4), Ru(1)-N(1) = 2.083(3), Ru(1)-N(4) =
2.050(5), Ru(1)-N(5) = 1.995(5), Ru(1)-N(6) = 2.027(4), Ru(1)-Cl(1) = 2.4997(12), C(6)-Ru(1)-N(1) =
78.03(16), C(6)-Ru(1)-Cl(1) = 172.68(13), N(6)-Ru(1)-N(1) = 177.14(16), N(5)-Ru(1)-N(4) = 178.50(2).
2-Cl: Ru(1)-C(6) = 1.968(7), Ru(1)-N(1) = 2.069(5), Ru(1)-N(4) = 2.018(6), Ru(1)-N(5) = 2.032(6),
Ru(1)-N(6) = 2.023(6), Ru(1)-Cl(1) = 2.4798(17), C(6)-Ru(1)-N(1) = 79.0(2), C(6)-Ru(1)-Cl(1) =
171.75(19), N(6)-Ru(1)-N(1) = 178.4(2), N(5)-Ru(1)-N(4) = 178.1(2).
80
1-MeCN
1-Cl
2-MeCN
60
2-Cl
40
20
0
0
20
40
60
80
Time (min)
Fig. 6. Comparison of the reaction kinetics profile of 1-MeCN/2-MeCN versus 1-Cl/2-Cl for the
oxidation of 4-methylstyrene to 4-methylbenzaldehyde. Reaction conditions: 4-methylstyrene, 0.4 mmol;
NaIO4, 1.0 mmol; mesitylene, 0.2 mmol, internal standard; cat, 0.002 mmol; acetone:H2O (1:1), 6 mL;
room temp. Yields were determined by GC.

Finally, the precatalyst 1-Cl was further applied for selective oxidation of styryl double bond in presence
of several oxidation-sensitive functional groups (Fig. 7). For this purpose, some specific substrates were
tested as shown in Fig. 7. It is interesting to note that only styryl double bond was converted to the
corresponding aldehyde functionality under the given reaction conditions. Other oxidation-sensitive
functional groups such as aliphatic olefins – both internal and terminal, aliphatic primary alcohol,
benzylic C–H bonds and aromatic alkynes remained intact. All the oxidized products were isolated in
good yields as depicted in Fig. 7 (see Supplementary Information for details). Next, to examine the
stability and attachment of the Ru-bound NHC ligand backbone under the catalytic conditions, a reaction
of 4-methylstyrene with 1-MeCN (1-MeCN:NaIO4:4-methylstyrene = 1:42:17) was monitored by time-
1
1
dependent H NMR spectroscopy in CD3CN/D2O solvent mixture. The H NMR peaks related to the
bound NHC ligand of the complex remained intact with just ∼20% decrease of peak intensity only after
80 min. 4-Methylstyrene was found to be converted to 4-methylbenzaldehyde gradually with time
1
(Supplementary Information). Interestingly no diol or epoxide intermediates were detected in the H
NMR spectra, thus ruling out the alternate pathways to the carbon-carbon multiple bond cleavages for the
present system.
Fig. 7. 1-Cl catalyzed selective oxidation of styryl bond in the presence of other oxidation-sensitive
functionalities. Reaction conditions: substrate, 0.4 mmol; NaIO4, 1.0 mmol; cat, 0.002 mmol;
acetone:H2O (1:1), 6 mL; room temp. Yields represent isolated yields..

4. Conclusion
In summary, this study demonstrated that the influence of two electronically-different NHC ligands was
crucial to guide the choice of other labile co-ligands (para-cymene or MeCN) within the ruthenium
coordination sphere of the precatalysts for oxidative conversion of olefins to carbonyl compounds under
mild conditions. While the relatively less electron-donating triazolydene-NHC (Tz-NHC) ligand
enhances catalytic activity of “Ru(para-cymene)”-type precatalysts, the more electron-donor
imidazolydene (Im-NHC) ligand makes the “Ru(MeCN)3”-type precatalysts more active for the same
reaction. Along with the electronic influence of the NHC ligands, the effect of the lability of the para-
cymene and MeCN ligands, and the overall charge of the complexes might also be attributed toward such
interesting results. Finally, the [(Tz-NHC)Ru(MeCN)3Cl]+ precatalyst which was found to be best choice
among these precatalysts, was further utilized for selective oxidation of a series of styryl substrates to the
corresponding aldehydes without affecting the other oxidation-sensitive functionalities in the compounds.
Acknowledgements
J.C. sincerely thanks DST-SERB and IISER Bhopal for generous financial support. S.K.G. and T.M.
thank CSIR, India for doctoral fellowships. T.G. thanks DST-INSPIRE and V.S. thanks IISER Bhopal for
fellowships.
Appendix
Supplementary content
Supplementary information containing additional experimental details, characterization data, kinetics data
and additional figures are available.
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