New olefin metathesis catalyst bearing N-mesitylimidazole and nitrate ligands – Synthesis, activity, and performance in aqueous media

Article abstract of DOI:10.1016/j.jorganchem.2019.06.018

A new 18-electron ruthenium complex, where ruthenium catalytic center is coordinated with the N-mesitylimidazole and nitrate ligands, as well as o-isopropoxystyrene moiety, is reported. The synthesis and detailed characterization of the Ru complex, togeth

Full text of DOI:10.1016/j.jorganchem.2019.06.018

Accepted Manuscript
New olefin metathesis catalyst bearing N-mesitylimidazole and nitrate ligands –
Synthesis, activity, and performance in aqueous media
Marta Malinowska, Mariana Kozlowska, Agnieszka Hryniewicka, Jacek W. Morzycki
PII:
S0022-328X(19)30257-8
DOI:
https://doi.org/10.1016/j.jorganchem.2019.06.018
JOM 20830
Reference:
To appear in: Journal of Organometallic Chemistry
Received Date: 10 May 2019
Revised Date: 10 June 2019
Accepted Date: 14 June 2019
Please cite this article as: M. Malinowska, M. Kozlowska, A. Hryniewicka, J.W. Morzycki, New
olefin metathesis catalyst bearing N-mesitylimidazole and nitrate ligands – Synthesis, activity,
and performance in aqueous media, Journal of Organometallic Chemistry (2019), doi: https://
doi.org/10.1016/j.jorganchem.2019.06.018.
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ACCEPTED MANUSCRIPT
New olefin metathesis catalyst bearing N-mesitylimidazole and
nitrate ligands – synthesis, activity, and performance in aqueous
media
Marta Malinowska*1, Mariana Kozlowska2 , Agnieszka Hryniewicka1
and Jacek W. Morzycki1
1Institute of Chemistry, University of Białystok, Ciołkowskiego Street 1K, 15-245 Białystok, Poland
2Institute of Nanotechnology, Karlsruhe Institute of Technology, Hermann-von-Helmholtz-Platz 1,
76344 Eggenstein-Leopoldshafen, Germany
Email: Marta Malinowska – marta.terpilowska@gmail.com
* Corresponding author
Abstract
A new 18-electron ruthenium complex, where ruthenium catalytic center is coordinated with
the N-mesitylimidazole and nitrate ligands, as well as o-isopropoxystyrene moiety, is
reported. The synthesis and detailed characterization of the Ru complex, together with density
functional theory calculations (DFT), are presented. The complex is air- and moisture-stable,
although has weak catalytical activity in the model metathesis reactions. However, its activity
increases upon the addition of an aqueous HCl 1M solution. Activated Ru complex
successfully promotes metathesis in organic solvents as well as in water, enabling efficient
performance (even up to 100%) of the catalyst under environment-friendly conditions. The
activation mechanism of the reported catalyst is supported by time-dependent DFT
calculations and ab initio molecular dynamics simulations.
Keywords
ruthenium catalyst; DFT calculations; mechanism of activation; metathesis
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1. Introduction
Olefin metathesis is widely known as a useful and powerful tool in modern organic chemistry.
It allows to construct of carbon-carbon double bonds in different compounds, e.g.
macromolecules [1,2] or biologically active compounds [3–5], making their synthesis easier
and more efficient. Despite the progress of metathesis methodology, no universal catalyst,
effective in all types of reactions has been designed yet. Though there are a lot of known and
effective ruthenium catalysts, e.g., 1 - 8, shown in Figure 1, in most cases, they have some
limitations, including lack of reactivity during the synthesis of tetrasubstituted olefins and
poor Z/E stereocontrol. Despite the success achieved by Grubbs et al. [6–8], as well as
Hoveyda and Schrock [9], in obtaining a Z-selective catalyst, there is still a need to develop
stable, easy-to-use complexes. In the face of the expectations of modern green chemistry,
there are ongoing studies on the improvement of the metathesis catalysts and their adaptation
to new challenges. Following the trend of environment-friendly chemical transformations,
new catalysts that can be efficiently used in the reactions carried out in aqueous media are
produced [10] and are still sought after.
Another class of the metathesis catalysts is represented by the latent complexes, which have
been developed in recent years [11–15]. They were found to be especially attractive in
metathesis polymerization reaction [16], where high control over the ROMP processes
significantly improves the synthesis. The latent catalysts can be activated either by chemical
or physical factors. One of the most frequently used methods of their activation is thermal
[17–21] as well as UV-light [22–25] initiation. At the same time, activation using Brönsted
acids has been reported recently [26–28]. Here, the most widely used additive is HCl [29–32],
but also TSMCl [11], TFA [33] or even Cu(I) salts [34,35] are known to be effective. In
general, the role of Brönsted acid (i.e. HCl) is to induce the anionic ligand exchange for more
active chloride ligand.
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commercial 1st generation catalysts
PCy3
Cl
Ru
PCy3
Cl
Ru
PCy3
Cl
Ru
Ph
Cl
Cl
Cl
PCy3
PCy3
O
1
3
2
commercial 2nd generation catalysts
Mes
N
N Mes
Cl
Mes
N
N Mes
Cl
Ru
O
Ru
Cl
Cl
PCy3
4
5
catalysts bearing N-donor ligands
PCy3
Cl
Mes
O
N
N Mes
N
PCy3
Cl
O
N
Ru
N
O
O
Ru
Cl
O
Ru
N
N
N
Ph
PCy3
N
O
7
8
6
Figure 1: Examples of ruthenium olefin metathesis catalysts.
It also facilitates dissociation of a suitable ligand allowing a latent metathesis catalyst to enter
the catalytic cycle [36]. In the case of tert-butoxycarbonyl-aminobenzylidene catalyst 8, HCl
plays a dual role: it activates the chelate ring by protonation of the nitrogen atom, and it
prevents the re-coordination of PCy3 [15].
To analyze the microscopic properties of the synthesized catalysts and to understand the
underlying mechanisms of their activation, quantum mechanical calculations are most
frequently used and have become an integral part of the characteristics of new ruthenium
complexes. Over the last year, there were many publications in which calculations confirmed
the results of experiments. Here, Density Functional Theory (DFT) calculations have emerged
as a valuable predictive tool to explain many aspects of Ru-catalyzed olefin metathesis.
Jawiczuk et al. described the synthesis and DFT calculations of cationic carbenes [37],
whereas Grela et al. proposed using DFT as reliable and accurate guidance in the fast and
inexpensive design of new metathesis catalysts [38]. In another report from the group of
Grela, DFT was used to optimize the geometry of new catalysts and the results led to a
topographic steric map and calculation of percent ‘buried’ volume values for each quadrant
around the metal center [39]. DFT calculations were also used to understand the influence of
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different ligand structures on the activity of a second-generation ruthenium olefin metathesis
catalysts [40]. Many efforts have also been made to validate computational protocols.
Kulkarni et al. assessed the performance of the DFT and Møller–Plesset second-order
perturbation theory (MP2) to predict structural parameters in Ru complexes [41]. Zhao et al.
discussed selected applications and validations of the Minnesota density functionals, focusing
on catalysis [42,43]. Benitez et al. have used DFT with B3LYP and M06 functions to show
factors responsible for diastereo- and enantioselectivity effects in the metathesis reaction [44].
Poater et al. used DFT calculations to rationalize the Z-selective catalyst mechanistic pathway
[45] as well as to investigate the activation steps in various Ru-catalysts for olefins
metathesis. Poater’s group also proposed a computational protocol (M06/TZVP//BP86/SVP)
for the Ru-based olefin metathesis catalysts [46]. The proposed method works with high
precision, offering a computationally effective tool for rationalizing experimentally available
data, as well as a prediction of the behavior of the catalyst.
In this paper, we report a novel protocol to obtain a new metathesis catalyst with the
promising application in environment-friendly catalysis after acid activation. We synthesized
the Ru complex containing N-mesitylimidazolium and nitrate ligands from Hoveyda 1st
generation catalyst (2). The compound shows weak reactivity in model reactions, but after
activation with aqueous 1M HCl solution, the conversions increased to 92% (CH2Cl2, 40 °C)
and to 100% (water, 40 °C). We studied the application profile of the catalyst in ring-closing
metathesis (RCM), enyne cyclization and cross-metathesis (CM) reactions. In addition, we
investigated in detail RCM of diethyl diallylmalonate in organic solvents, as well as in
aqueous media. To gain insight into the activation mechanism of the new Ru catalyst, we
performed the density functional theory calculations and ab initio molecular dynamics
simulations. It should be underlined that the new catalyst can be obtained in efficient and
straightforward two-step synthesis, it is easy to handle and may operate in aqueous media.
2. Results and discussion
The new latent catalyst was synthesized in two-steps: i) preparation of imidazolium nitrate; ii)
reaction of the nitrate with base and catalyst 2. The obtained new catalyst was characterized
by NMR, IR, mass spectrometry and X-ray crystal analysis what is described in detail in the
following subsections.
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2.1 Synthesis
2.1.1 Synthesis of imidazolium nitrate
An imidazolium-based salt used in this study was prepared from N-mesitylimidazole (9) [47]
by reaction with acetyl nitrate, generated in situ from fuming nitric acid and acetic anhydride.
The reaction was carried out overnight and the volatile material was evaporated under a
stream of nitrogen to obtain N-mesylimidazolium nitrate (10) in good yield (78%, Scheme 1).
100% HNO3
(CH3CO)2O
N
NH
N
N
Mes
Mes
NO3
9
10 - 78%
Scheme 1: Synthesis of imidazolium nitrate.
2.1.2 Synthesis of new catalyst 11
The new catalyst was synthesized from the commercial complex 2 in the reaction with salt 10
and t-BuOK by stirring two hours in dry toluene at room temperature. The product was
isolated by column chromatography as an air-stable crystalline green solid in 55% yield
(Scheme 2). The possible mechanism the formation of the catalyst is shown on Scheme 3.
Scheme 2: Preparation of complex 11.
Scheme 3: The proposed mechanism of the catalyst formation.
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2.1.3 Characterization of new catalyst 11
The Ru complex 11 was characterized by NMR, IR, mass spectrometry and X-ray crystal
1
analysis. The H NMR spectrum of the catalyst 11 shows the characteristic signal for
ruthenium carbene complexes at 19.51 ppm. There is also a singlet from the proton at C2 of
the imidazole ring and signals derived from the phosphine ligand. The crystal structure of
complex 11 is depicted in Figure 2.
Figure 2. The X-ray molecular structure of catalyst 11. Hydrogen atoms were omitted for
clarity (thermal ellipsoids are shown at the 50% probability level) a) single molecule b) dimer
form of catalyst 11.
We estimated that the coordination geometry of the ruthenium center is triclinic: the
ruthenium-oxygen coordination of the isopropoxy group is replaced by ruthenium-nitrogen
coordination of N-mesitylimidazolium ligand. Additionally, one of the chlorine atoms is
exchanged by bidentate nitrate ligand. The angle between the nitrogen atom of the imidazole
ring and the bulky o-isopropoxystyrene ligand is 88°, and the nitrate group occupies an
opposite position to the phosphine (Figure 2). The X-ray analysis indicates that the complex
11 is prone to form dimers in its crystalline form.
In addition, the calculated molecular mass of catalyst 11 (813.2975) was not compatible with
the observed ionic mass obtained from the MS-ESI spectrum (m/z found 761.3271). The
difference in m/z value is triggered by the interaction between nitrate ligand and imidazolium
ring, proceeding via leaving of Cl and OH radical, what is already known in the
literature [48].
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2.2 Performance of new Ru catalyst
The catalytic activity of the new catalyst was investigated in model RCM, CM, and enyne
reactions and compared with that of the commercial 1st generation catalysts (1 - 3), as well as
2nd generation catalysts (4, 5).
2.2.1 The reaction rate of the ring-closing metathesis (RCM)
The RCM reaction of diethyl diallylmalonate (12, Scheme 4) was carried out in toluene (60
°C) or water (40 °C with SDS – sodium dodecylsulfate). The RCM reaction rate with catalyst
1
11 was followed by H NMR and compared with that attained using complex 1, which is
shown in Figure 3. Complex 11 initiates RCM significantly faster when reaction occurs in
water in the presence of the surfactant than in toluene, despite lower reaction temperature.
Scheme 4. Model RCM reaction of 12.
100
90
80
70
60
50
40
30
20
10
0
0
50
100
1 (water) + SDS
Time [min]
11 (toluene)
11 (water) + SDS
Figure 3. RCM of diallylmalonate 12 with 5 mol% catalyst 11 or 1 at 40 °C (water) /60 °C
(toluene). The conversion was determined by 1H NMR spectroscopy.
2.2.2 Metathesis activity of complex 11
Initial attempts to study the activity of 11 towards RCM under different conditions with and
without an additive were focused on the cyclization of diethyl diallylmalonate (12). The
reactions were carried out in such solvents as CH2Cl2, toluene, and water at room temperature,
40 °C or 60 °C (Table 1). The catalyst has very weak activity at room temperature in CH2Cl2
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(Table 1, entry 1), even at 60 °C in toluene the conversion was only 58% (Table 1, entry 3).
The addition of CuCl, TMSCl (trimethylsilyl chloride) or the aqueous solution of 1M HCl
(10:1, nadditive:ncatalyst each) causes the increase of obtained yields. Diethyl diallylmalonate (12)
is fully converted into 13 after 2 h of reaction carried out with 5 mol% of catalyst 11 in water
at 40 °C with the addition of 1M HCl (Table 1, entry 13). After changing the solvent into
CH2Cl2, the conversion of 92% is achieved (Table 1, entry 8). The conversion decreases to
81% in the presence of 1 mol% 11 after 3 h of reaction at 60 °C when the cyclization is
performed in toluene (Table 1, entry 12). As it is discussed further, the activation results from
the conversion of the 18-electron complex into the corresponding 14-electron benzylidene
complex (Scheme 5).
Table 1. RCM of 12 with 11 with or without an additivea).
Entry
1.
Solvent
CH2Cl2
CH2Cl2
Toluene
Water
Temp.
25 °C
40 °C
60 °C
40 °C
40 °C
40 °C
40 °C
40 °C
40 °C
60 °C
60 °C
40 °C
Time
2 h
%mol 11
Additiveb) Yieldc)
1
5
5
5
5
5
5
5
1
1
1
5
-
6.5%
24%
58%
32%
10%
66%
92%
72%
15%
66%
81%
100%
2.
160 h
2 h
-
3.
-
4.d)
16 h
0.5 h
16 h
2 h
-
6.
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
Toluene
Toluene
Water
CuCl
CuCl
HCl
TMSCl
HCl
HCl
HCl
HCl
7.
8.
9.
2 h
10.
11.
12.
13.d)
2 h
2 h
3 h
2 h
a)
b)
c)
Conditions: solvent (0.1 M), temperature, time, cat. 11;
Determined by 1H NMR; d) C = 0.17 M
nadditive:ncatalyst = 10:1
The obtained results are very promising, especially the ability to react at low temperatures,
which prevents unexpected thermal decomposition of the catalyst. Such high-temperature
decompositions have been described by Grubbs et al. [49 - 51]. They have also been
theoretically calculated by Cavallo and Nolan et al. [45, 52, 53]. It should be emphasized that
metathesis carried out in primary alcohol solutions at elevated temperatures and prolonged
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reaction times may lead to the decomposition of ruthenium catalysts to a monohydride species
[54, 55]. The application of mild reaction conditions may protect the catalyst against these
undesired transformations.
2.2.3 Metathesis in water with surfactants
The comparative study of catalysts 11 and commercial 1-5 in the RCM of 12 was carried out
in water in the presence of surfactant (SDS - sodium dodecylsulfate, Tween 40 –
polyoxyethylene sorbitane monopalmitate) and is presented in Table 2. The complex 11
shows high activity (88% yield) in the presence of SDS as the surfactant in a ratio
[substrate]/[SDS] = 5, comparable to that of commercial compound 1 (Table 2, entry 1 and 2).
The increase in reaction rate is probably due to the formation of micelles. Unfortunately,
Tween 40 has a less significant impact on the reaction yields (Table 2, entry 7).
Table 2. Comparative study of catalysts in RCM of 12 in watera).
Entry
1.
Catalyst Surfactantb) Yieldc)
SDS
88%
79%
83%
93%
100%
100%
28%
67%
81%
98%
100%
100%
11
1
2.
SDS
3.
SDS
2
4.
SDS
3
5.
SDS
4
6.
SDS
5
7.
Tween 40
Tween 40
Tween 40
Tween 40
Tween 40
Tween 40
11
1
8.
9.
2
10.
11.
12.
3
4
5
a)
b)
Conditions: water (C = 0.17M), 40 °C, 2 h, 5 mol% [Ru];
c) Determined by 1H NMR
0.2 equiv. (20 mol%);
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2.2.4 Activity of complex 11 in model RCM, enyne and CM reactions
Catalysts 11 and 1 (5 mol%) were screened in a selected set of RCM, enyne and CM
reactions. In all reactions with 11 aqueous 1M solution of HCl was added (Tables 3 and 4).
Diethyl diallylmalonate (12) was again subjected to cyclization in water to achieve full
conversion using 11, whereas the activity of 1 was two times lower (Table 3, entry 1). Diethyl
methylallylmalonate (14) is more challenging in RCM than 12 due to steric effects, and only
18% yield was obtained for catalyst 11 (Table 3, entry 2). The RCM reactions of diethyl
dimethylallylmalonate, leading to tetrasubstituted olefin using both complexes, failed. In the
enyne cycloisomerization of 16 catalyzed by 1 in CH2Cl2 quantitative conversion was
observed (Table 3, entry 3), whereas new complex 11 reacted with lower efficiency (53%). In
the reaction conducted in water, again twice higher catalytic activity of 1 was observed as
compared to 11 (Table 3, entry 4).
Table 3. Comparative investigation of catalysts 1 and 11 in RCM.
Entry Substrate
Product
Conditions
Catalyst
Yieldb)
100%
40 °C, 3 h,
11a)
COOEt
EtOOC
1.
2.
3.
4.
water, 0.17 M
5 mol% [Ru]
53%
18%
1
12
11a)
COOEt
EtOOC
40 °C, 3 h,
CH2Cl2, 0.1 M
5 mol% [Ru]
14
94%
53%
1
40 °C, 20 h,
11a)
CH2Cl2, 0.1 M
5 mol% [Ru]
100%
36%
76%
1
11a)
40 °C, 3 h,
water, 0.17 M
5 mol% [Ru]
1
a) Catalyst 11 was used with 50 mol% HCl(aq) (1M); b) Determined by 1H NMR
Next challenging reaction was the CM of allylbenzene (18) and cis-1,4-diacetoxy-2-butene
(19). In this transformation, 11 gave only 12% conversion, which was significantly lower than
that attained for 1 (Table 4, entry 1). The homodimerization reactions were carried out at a
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relatively high substrate concentration (3M). In homometathesis of 18, catalyst 11 showed
lower reactivity than 1 (Table 4, entry 2), but better Z-selectivity (45% of isomer Z). Finally,
the activity of catalyst 11 was investigated for the homodimerization reaction of methyl 10-
undecenoate (22) substrate. Comparative conversions for both catalysts were obtained.
However, in the case of reported here complex 11, relatively more Z-isomer was formed
(Table 4, entry 3).
Table 4. Comparative investigation of catalysts 1 and 11 in CM reactions.
Entry Substrate(s)
1.
Product
Conditions
Catalyst Yieldb)
11a)
1
12% (21% Z)
40 °C, 20 h,
CH2Cl2, 0.1M,
5 mol% [Ru]
43% (22% Z)
36% (45% Z)
11a)
35 °C, 20 h
THF, 3M
2.
59% (35% Z)
31% (54% Z)
1
5 mol% [Ru]
11a)
35 °C, 20 h
THF, 3M
3.
33% (50% Z)
1
5 mol% [Ru]
a)
b)
Catalyst 11 was used with 50 mol% HCl(aq) (1 M); %Z determined by 1H NMR, isolated
yields after silica gel chromatography.
2.3 Activation mechanism
It can be seen from Table 1 that the product yield with catalyst 11 increases up to 92% and
100% upon the addition of HCl additive in CH2Cl2 and water, respectively. At the same time,
we have noticed visual changes of the complex 11 after its HCl activation: the synthesized
compound is green but acquires brown color in its active form (Figure 4) that suggests a
1
change of the chemical composition of the catalyst upon the addition of HCl. The H NMR
spectrum in deuterated chloroform in the presence of traces of acid shows additional carbene
proton signals at 20.32 ppm and 18.79 ppm. It can, therefore, be inferred that a new Ru=CH
species forms during the spectrum registration.
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Figure 4. The color change of the catalyst 11 solution in CH2Cl2 a) the inactive form b) the
active form with the addition of 1M aqueous HCl solution.
According to the literature [13], there are several potential pathways of activation of complex
11 by HCl. One possibility (path a) is that the acid protonates both imidazole and nitrate
ligands, exchanging them for chloride to form a 14-electron benzylidene complex A (Scheme
5). In the path b only the imidazole ligand undergoes protonation, while the complex B with
the nitrate ligand acts as an active catalyst.
PCy3
N
O
NH
Cl
Cl
PCy3
Cl
Ru
O
N
O
Ru
O
N
+ 2 HCl
O
Cl
path a
N
O
OH
N
+ HCl
path b
A
O
PCy3
Cl
Ru
O
O
N
NH
O
N
Cl
O
B
Scheme 5. Potential pathways of activation of complex 11 by HCl.
To estimate the reason of the catalyst color change and to find the active form of the latent
complex 11, theoretical calculations, including density functional theory (DFT), time-
dependent DFT (TD-DFT) and ab initio molecular dynamics (MD), were performed. Using
the determined X-ray structure of the complex 11 (see Figure 2), the gas-phase geometry
optimizations of the dimer and monomer forms of the complex 11, and also of complexes A
and B (see Scheme 5), were performed using B3LYP functional with def2-SVP basis set as
described in the section “Theoretical calculations”. No significant structural differences
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between DFT optimized structures and their equivalent in the crystal were detected (see
Supporting Information). The optimized structures were used in further investigations.
Due to the fact that it was not possible to measure the UV-vis spectra of the latent and active
forms of 11, we calculated the absorption spectra of all reactants with the TD-DFT approach.
The first twenty singlet-singlet excitations of the monomer form of 11, A and B, together with
the first forty singlet-singlet excitations of the dimer form of 11, are depicted in Figure 5.
Though the strongest excitations are localized in the UV region, several close-to-dark states in
the visible region were detected (see the zoomed region in Figure 5).
Figure 5. Absorption spectra of monomer and dimer form of complex 11, complexes A and
B, as given in Scheme 5, obtained from the time-dependent DFT calculations with B3LYP
functional and def2-SVP basis set.
The transition orbitals and electron densities upon the most relevant excitations on the spectra
are shown in Figure S1 (see Supporting Information). Spectral changes in the visible region of
all examined compounds clearly indicate that only the dimer form of complex 11, as known
from X-ray analysis, is green. The dimer of complex 11 absorbs in the violet and red regions
and does not absorb green wavelength localized at ca. 500-570 nm, therefore, it is green.
Upon the addition of HCl, the dimer of 11 transforms into the monomer form, which
immediately undergoes protonation and activation with the simultaneous change of the color
to brown. In Figure 5, there are no absorption bands in the red region of complexes A and B,
but new bands in the green and close-to-green regions appear, what confirms the correctness
of the assumptions presented in Scheme 5. The lack of the absorption bands at 670 nm and the
dark states in the green region are characteristic to the monomer form of 11 (marked in red in
Figure 5), confirming the latent form of 11 as a dimer and change of the color upon dimer
decay. Among the spectral changes there is also a 14 nm red-shift at 374 nm in the monomer
of 11 in comparison to its dimer form.
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Figure 6. Dimer form of the catalyst 11 with visualized intermolecular noncovalent
interaction (NCI) regions, i.e., π-interactions and C-H···O hydrogen bonds, from different
perspectives (a, b, c). The depicted NCI surfaces correspond to the reduced density gradient of
0.4 a.u [57, 58]. Intermolecular and intramolecular C-H···O H-bonds are marked with black
and pink dashed lines in a), respectively.
After the detailed analysis of the transition orbitals upon the singlet excitations (listed in
Table S1-S4, see Supporting Information), we have found that the local environment of the
complex 11 in its dimer form is different than in the monomer form. This influences the
electron density localization on the frontier molecular orbitals, changing the respective orbital
energies of the highest occupied molecular orbital (HOMO) and the lowest unoccupied
molecular orbital (LUMO), inducing the mentioned shift. The energy of HOMO orbital of
complex 11 changes from -4.976 eV in a monomer form to -4.985 eV in a dimer form of this
complex, while LUMO energy changes from -1.779 eV to -1.946 eV. As it is shown in Figure
S2 (see Supporting Information), the electron density transitions upon absorption occur from
the metal center, imidazole and chloride ligands to o-isopropoxystyrene. In the dimer of
complex 11, there is close positioning of N-mesitylimidazolium ligand of one monomer to o-
isopropoxystyrene of the other monomer and C-H···π interactions between these aromatic
rings (see dark yellow surfaces in Figure 6). These interactions together with the set of other
intermolecular π-interactions and C-H···O hydrogen bonds (marked with black dashed lines
in Figure 6a) influence both the shift and the absorption bands in the red region. These
intermolecular C-H···O hydrogen bonds are of the H···O distance of 2.31 Å and CHO angle
of 128º, therefore, can be classified as weak H-bonds [56]. The qualitative analysis of the
mentioned interactions was made using the noncovalent interaction (NCI) index utilizing
NCIPLOT code (v.3.0) [57, 58]. The obtained NCI surfaces are marked in dark yellow in
Figure 6. The binding energy of the NCI in the dimer of 11 was calculated as 38.1 kcal·mol-1
using the supermolecular approach. It indicates strong intermolecular interactions in the
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dimer, therefore its high structural stability, differences in the absorption spectrum, and lower
catalytic activity, specifically in the aqueous solutions. This form is catalytically less active
because the metal active center is blocked for the desired reaction due to the hydrophobic
nature of the outer dimer coating (see Figure 6). As a result, firstly, the dimer of 11 should be
broken to enable further catalyst activation due to the protonation. Upon the addition of HCl,
all mentioned transitions occur easier; therefore, the significant increase in the activity of 11 is
observed. This was also observed in ab initio MD simulations of the dimer of complex 11
with ten HCl molecules. Even if the time of the MD simulations was short (2 ps), we captured
the formation of temporary interactions via Cl-H···O and Cl-H···N bridges (see snapshot 1, 3,
4 in Figure S2 in Supporting Information), which participate during the protonation of nitrate
and imidazole ligands.
Moreover, we monitored the bond breaking between both Ru-nitrate and Ru-imidazole, as
depicted in snapshot 3 and 4 in Figure S2 (see Supporting Information), respectively. Owing
-
to the fact that NO3 ligand is smaller and more labile than N-mesitylimidazolium ligand, and
the energy of its coordination to Ru is lower by 8.9 kcal·mol-1 than that of imidazole:
46.1 kcal·mol-1 and 55.0 kcal·mol-1 for nitrate and imidazole, respectively, we assume that
path a in Scheme 5 is more probable than path b at 0 K. Additionally, the coordination of the
active center with chloride ligand in the complex A is 14.3 kcal·mol-1 energetically more
favorable than for the formation of the nitrate containing complex B.
To take into account the impact of all reagents during the activation of complex 11, we
calculated the reaction energy of proposed reaction paths (as shown in Scheme 5). The total
free energy of the reaction in the path a and b, calculated with thermodynamic functions
obtained from the vibrational spectra (see Table S5 in Supporting Information) using hybrid
B3LYP functional with def2-SVP basis set, is +16.5 kcal·mol-1 and +15.9 kcal·mol-1 at room
temperature and +16.2 kcal·mol-1 and +15.5 kcal·mol-1 at 40 ºC, respectively. To estimate the
impact of the DFT functional used in the calculated energy differences, we have performed
similar calculations using meta-GGA functional, i.e., TPSS functional [59], which has been
shown before to give reliable values of reaction barriers [60]. Calculations with TPSS
functional (see Table S6 in Supporting Information) confirm small free energy differences
between reactions proceeding according to the path a and b: free energies are in the range of
+12.5 – +14.8 kcal·mol-1 with the difference between two pathways of 1.6 kcal·mol-1. Here,
the same as in the case of B3LYP functional, path b has lower reaction energy difference.
This suggests that reactions according to the path a and path b both take place at room
temperature (energy barriers are below 20 kcal·mol-1), but energy differences, i.e., 0.65-0.67
15

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kcal·mol-1 and 1.58-1.61 kcal·mol-1, obtained with B3LYP and TPSS functionals,
respectively, are lower than the accuracy of the DFT method. Therefore, they should be
treated only qualitatively. More precise methods, like CCSD(T), should be used in this case,
but taking into account amount of electrons of the examined molecular complexes and not
realistically feasible calculation of force constants used in free energy calculations, we are
limited in usage of this sophisticated method.
Based on the DFT calculated data, both reaction pathways in Scheme 5 are possible, but
owing to higher coordination affinity of chloride ligand in comparison to the nitrate ligand,
complex B is prone to change to complex A, where the latter is more stable complex.
Therefore, we suggest that the active form of the catalyst 11, reported here, is complex A
(Scheme 5).
3. Conclusions
New metathesis catalyst with N-mesitylimidazolium and nitrate ligands was synthesized using
the efficient and straightforward procedure as well as it was characterized by NMR, IR, MS
and X-ray analyses. The catalytic activity of the Ru complex was tested under several reaction
conditions, where the new catalyst in some cases showed better activity than commercially
available 1st generation catalysts. The increase of yield up to 100% was observed in aqueous
media upon the addition of 1M HCl solution, which activates the catalyst. This activation
under mild conditions allows to avoid undesired side reactions. The metathesis reactions
carried out in aqueous media are environmentally friendly and advantageous from economical
point of view.
The activation mechanism was proposed and confirmed with the density functional theory
calculations and ab initio molecular dynamics simulations. It is based on the decay of the
stable dimer of complex 11 in acidic conditions with further protonation of both imidazole
and nitrate ligands by HCl, ligand exchange for chloride, and formation of an active 14-
electron benzylidene complex. The absorption spectra of inactive and active forms of the
catalyst were obtained with the time-dependent DFT calculations. A detailed analysis allowed
to propose the active form of the Ru complex and to explain the reason of the color changes
upon the activation of a catalyst. Free energy calculations, performed using B3LYP and TPSS
functionals, confirm the occurrence of the suggested activation pathways and the most
probable form of the active catalytic complex.
16

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4. Materials and methods
4.1 Synthesis
4.1.1 General remarks
Synthesis of catalyst 11 was carried out using standard Schlenk techniques under an
atmosphere of dry argon. CH2Cl2 was dried by distillation over CaH2, THF over
Na/benzophenone. Melting points were determined on an MP70 (Mettler Toledo) apparatus
1
and were uncorrected. H and 13C NMR spectra were recorded on a Bruker Avance II
spectrometer (400 and 100 MHz, respectively). Spectra are referenced relative to the chemical
shift (δ) of TMS. Mass spectra were obtained with Micromass LCT TOF and Accurate-Mass
Q-TOF LC/MS 6530 spectrometers. IR spectra were recorded on a Nicolet series II Magna-IR
550 FT-IR spectrometer. Column chromatography was performed on silica gel 230-400 mesh.
Catalysts 1 - 5 are commercially available. Substrates for testing catalysts in RCM reactions
were prepared by allylation of commercial diethyl malonate with allyl bromide and/or 3-
1
chloro-2-methylpropene according to Hensle [54]. Their purity was estimated by H NMR
spectroscopy and found to be at least 95%. Other chemicals are commercially available and
used as received. N-mesitylimidazole (9) was prepared according to the literature [47].
4.1.2 N-mesylimidazolium nitrate (10)
N-Mesitylimidazole 9 (250 mg, 1.34 mmol) was placed in a round bottom flask and cooled to
0 °C, next acetic anhydride (3.30 mL, 35 mmol) and fuming nitric acid (1.35 mL) were slowly
added. The mixture was stirred at room temperature for 24 h. After this time it was
concentrated to dryness in a stream of N2. The residue was dissolved in a small volume of
methylene chloride, and the product was precipitated with diethyl ether to give N-
mesylimidazolium nitrate (10) (261 mg, 78% yield) as a white solid; M.p. 180 °C (with
1
decomposition); IR (ATR) νmax 3132, 1573, 1537, 1461, 1389, 1375, 1317, 833 cm-1; H
NMR (CDCl3, 400 MHz) δH 9.08 (1H, s, C2-H), 8.85 (1H, s, CHimidazole), 7.15 (1H, s,
13
CHimidazole), 7.02 (2H, s, H-Ar), 2.34 (3H, s, p-CH3), 2.03 (6H, s, 2x o-CH3) ppm; C NMR
(CDCl3, 100 MHz) δC 141.0, 136.6, 134.3, 130.8, 129.7, 122.1, 121.8, 21.0, 17.2; m/z (ESI)
187 ([M-NO3]+, 100%); HRMS (ESI): found 187.1235 [C12H15N2]+, requires 187.1230.
4.1.3 Catalyst 11
A flame dried Schlenk flask with a magnetic stirring bar was charged under argon with
catalyst 2 (120 mg, 0.2 mmol), nitrate 10 (50 mg, 0.2 mmol) and potassium t-butoxide (22
17

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mg, 0.2 mmol). Next dry toluene (2 mL) was added. The reaction mixture turned green and
was stirred at room temperature for 2 h. After this time the crude product was purified by
column chromatography (CH2Cl2, then hexane – ethyl acetate v/v 1:2) to give catalyst 11 (89
mg, 55% yield) as a green solid; M.p. 163 °C; IR (ATR) νmax 2919, 1586, 1466, 1375, 1231,
1112 cm-1; 1H NMR (CDCl3, 400 MHz) δH 19.51 (1H, d, J = 10.7 Hz, Ru=CH), 9.13 (1H, d,
J = 6.8 Hz, C2-H), 7.74 (1H, ddd, J = 8.5, 7.0, 1.4 Hz, H-Ar), 7.65 (1H, s, H-Ar), 7.12 (1H, t,
J = 7.6 Hz, H-Ar), 6.94 (2H, d, J = 3.2 Hz, H-Ar), 6.87 (1H, d, J = 8.2 Hz, H-Ar), 6.78 (1H, s,
H-Ar), 6.71 (1H, s, H-Ar), 4.75 (1H, sep, J = 5.9 Hz, -O-CH(CH3)2), 2.32 and 2.04-1.11
(48H, m, 3x CH3-Mes + 3x Cy+(CH3)2CH-O-) ppm; 13C NMR (CDCl3, 100 MHz) δC 190.1,
171.1, 146.7, 146.6, 144.3, 139.9, 139.5, 135.7, 135.0, 133.6, 132.5, 129.1, 128.8, 122.4,
120.6, 114.0, 112.8, 69.9, 60.3, 35.7, 35.2, 34.9, 30.0, 29.0, 28.9, 28.8, 28.0, 27.9, 27.8, 27.8,
27.7, 27.7, 26.3, 22.4, 21.0, 17.4, 17.1, 17.0 ppm; m/z (ESI) 761 ([M-(Cl·, OH·)]+);
HRMS(ESI): found 761.3271 [C40H58N3O3P102Ru]+, requires 761.3259. Crystals suitable for
X-ray analysis were obtained by slow evaporation of a dichloromethane/hexane solution of
the catalyst for 10 h in the refrigerator.
4.1.4 X-ray crystallographic data
C40H59ClN3O4PRu, green plates of 0.3 × 0.25 × 0.04 mm; monoclinic space group triclinic
P1; a= 10.6421(3), b= 10.8897(3), c= 19.4405(6) Å; α= 78.883(3), β= 76.389(3), γ=
63.811(3)°; V= 1954.64(12)Å3; Z= 2; Dx = 1.382 Mg m−3; µ(Cu Kα) = 4.60 mm-1; 40133
reflections measured at SuperNova, Dual, Cu at home/near, Atlas diffractometer at 100K;
7660 reflections with I>2σ(I) were used for structure solution and refinement; data were
corrected for numerical absorption using program CrysAlis PRO, SHELXL2016/6 [61]: Tmin=
0.252, Tmax= 0.831; R= 0.042; wR= 0.114.
CCDC 1556596 contains the supplementary crystallographic data for this compound. These
data can be obtained free of charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
4.2 Theoretical calculations.
Geometry optimization was performed using B3LYP [62] and TPSS functionals [59] with
def2-SVP [62, 63] basis set (def2-ECP for Ru atom) Grimme D3 [64] dispersion correction
utilizing Turbomole V.7.1 [65]. All optimized structures were confirmed as local minima by
the vibrational analysis [66]. The reaction energy barriers were calculated using the values of
the total electronic energy correspondent to the fully optimized structures of reagents depicted
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in Scheme 5 and shown in Supporting Information. The free energy calculations, together
with other thermodynamic functions (Table S5-S6 in Supporting Information), were
calculated from vibrational spectra using freeh module with scaling factor of 0.9614 (B3LYP)
and 0.9914 (TPSS) in Turbomole V.7.1. The energy of coordination bond formation with
nitrate, imidazole and chloride ligands was calculated using the single point calculations of
the complex 11 with and without ligands. The absorption spectra were obtained using time-
dependent DFT with the same set-up as above. The discrete absorption peaks were broadened
with the natural peak broadening using the convolution with Lorentzian profile. The
visualization of the three-dimensional noncovalent interaction regions around the bond critical
points of the π-interactions between the monomers of 11 in its dimer (Figure 6) was made
using the reduced density gradient, originated from the electron density and its first derivative
[49,50]. The reduced density gradient was calculated using the promolecular densities of the
complex 11 with NCIPLOT code (Version 3.0) [57, 58]. Ab initio MD simulations were
performed using PBE [67] functional with def2-SVP basis set and Grimme D3 dispersion
correction. To optimize the computational set-up to investigate interactions between the dimer
of complex 11 with ten HCl molecules, multipole-accelerated-resolution-of-identity method
[68] was used and simulations for 2 ps at 800 K with the timestep of 8 a.u. using Nose-
Hoover [69, 70] thermostat were performed with Turbomole V.7.2.1. Three independent MD
production runs were independently analyzed.
Acknowledgments
The authors acknowledge financial support from the National Science Centre, Poland (UMO-
2015/17/N/ST5/03916). M. K. is grateful to M. Krstić for fruitful discussions. This work was
performed on the computational resource ForHLR II funded by the Ministry of Science,
Research and the Arts Baden-Württemberg and DFG ("Deutsche Forschungsgemeinschaft").
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1. New, unique 18-electron ruthenium air- and moisture-stable catalyst for
olefin metathesis was obtained
2. The complex upon activation with HClaq promotes reactions in organic
solvents and in water
3. The activation mechanism is supported by DFT calculations and ab initio
molecular dynamics simulations.