Ruthenium (II) quinoline-azoimine complex: Synthesis, crystalline structures spectroelectrochemistry and catalytic properties

Article abstract of DOI:10.1016/j.molstruc.2020.128327

An octahedral ruthenium(II) complex with the general formula [Ru^II(Y)(bpy)Cl](ClO₄) {Y= C₆H₅N=C(COCH₃)-N=NC₉H₆N and bpy = 2,2′-bipyrdine} (1) was synthesized. The new ligand (Y) is coordinated to ruthenium via quinoline-N, imine-N and azo-N atoms. Both the novel complex 1 and the new ligand H₂Y were structurally characterized by X-ray crystallography, spectroscopic (IR, UV–Vis and NMR spectroscopy) and electrochemical (cyclic voltammetry) techniques. The bonding in 1 and its published skeletal isomer [Ru^II(L1)(bpy)Cl](PF₆) {L1 = C₆H₅N=N-C(COCH₃) = NC₉H₆N5} (1A) has been analyzed using molecular orbital theory. The novel tridentate ligand (Y) stabilizes the Ru(II) oxidation state showing the Ru(III/II) couple at 1.05 V vs. Cp₂Fe/Cp₂Fe⁺. The potential use of 1 as a catalyst for hydrogenation of α, β-unsaturated aldehyde has been investigated. UV/Vis and IR-spectroelectrochemistry on complex 1 were performed.

Full text of DOI:10.1016/j.molstruc.2020.128327

Journal of Molecular Structure 1217 (2020) 128327
Contents lists available at ScienceDirect
Journal of Molecular Structure
journal homepage: http://www.elsevier.com/locate/molstruc
Ruthenium (II) quinoline-azoimine complex: Synthesis, crystalline
structures spectroelectrochemistry and catalytic properties
,
,
Mousa Al-Noaimi a *, Firas F. Awwadi b **, Ayman Hammoudeh c,
Obadah S. Abdel-Rahman d, Manal I. Alwahsh b
a Department of Chemistry, Hashemite University, P.O.Box 150459, Zarqa, 13115, Jordan
b Department of Chemistry, The University of Jordan, Amman, 11942, Jordan
c Department of Chemistry, Yarmouk University, P.O. Box 566, Irbid, Jordan
d Department of Chemistry, College of Science, King Faisal University, PO Box 400, Hofuf, 31982, Saudi Arabia
a r t i c l e i n f o
a b s t r a c t
An octahedral ruthenium(II) complex with the general formula [RuII(Y)(bpy)Cl](ClO4) {Y¼
C6H5N¼C(COCH3)-N¼NC9H6N and bpy ¼ 2,20-bipyrdine} (1) was synthesized. The new ligand (Y) is
coordinated to ruthenium via quinoline-N, imine-N and azo-N atoms. Both the novel complex 1 and the
new ligand H2Y were structurally characterized by X-ray crystallography, spectroscopic (IR, UVeVis and
NMR spectroscopy) and electrochemical (cyclic voltammetry) techniques. The bonding in 1 and its
published skeletal isomer [RuII(L1)(bpy)Cl](PF6) {L1 ¼ C6H5N¼N-C(COCH3) ¼ NC9H6N5} (1A) has been
analyzed using molecular orbital theory. The novel tridentate ligand (Y) stabilizes the Ru(II) oxidation
state showing the Ru(III/II) couple at 1.05 V vs. Cp2Fe/Cp2Feþ. The potential use of 1 as a catalyst for
Article history:
Received 14 March 2020
Received in revised form
20 April 2020
Accepted 23 April 2020
Available online 5 May 2020
Keywords:
Ruthenium
Quinoline -azoimine
Skeletal isomer
Spectroelctrochemistry
DFT calculation
hydrogenation of
a
,
b-unsaturated aldehyde has been investigated. UV/Vis and IR-
spectroelectrochemistry on complex 1 were performed.
© 2020 Elsevier B.V. All rights reserved.
CRedit author statement
1. Introduction
Mousa Al-Noaimi: Conceptualization, Methodology, Funding
acquisition, Formal analysis, Writing - original draft, Writing - re-
view & editing, Approval of the version of the manuscript to be
published. Firas F. Awwadi: Conceptualization, Methodology,
Funding acquisition, Formal analysis, Writing - original draft,
Writing - review & editing, Approval of the version of the manu-
script to be published. Ayman Hammoudeh: Conceptualization,
Methodology, Funding acquisition, Formal analysis, Writing -
original draft, Writing - review & editing, Approval of the version of
the manuscript to be published. Obadah S. Abdel-Rahman: Funding
acquisition, Formal analysis, Approval of the version of the manu-
script to be published. Manal I. Alwahsh: Funding acquisition,
Formal analysis, Approval of the version of the manuscript to be
published
Pincer ligands (mer-tridentate ligand) are widely used in coor-
dination chemistry because of their high thermal stability,
chelating effect, high tunability, ability to coordinate a large variety
of metals, and their application as homogeneous catalysis [1].
Moreover, the multiple synthetic approaches to tune the steric and
electronic properties of these complexes to achieve the targeted
applications have were applied, via the modification of the central
coordinating atom, the lateral ones, and the linker between them
[2]. A wide variety of NNO [3], NNP [4], NNS [5] and NHC-based
pincer complexes [6] have found numerous applications in homo-
geneous catalysis and supramolecular chemistry. NNN pincer
chelate ligand such as terpyridine derivatives play a fundamental
role in inorganic chemistry [7].
8-Aminoquinoline and its derivatives have received much
attention due to their medicinal properties [8,9]. They have been
also used as strongly fluorescent ligands, chelates for heavy metals
[10e12] and conducting co-polymers [13e16]. Because of the
interesting properties of these materials, various synthetic routes
have been developed to functionalize 8-Aminoquinoline by react-
ing it with carbonyl derivatives [17,18] or hydrazonylchloride [19] to
* Corresponding author.
** Corresponding author.
E-mail address: manoaimi@hu.edu.jo (M. Al-Noaimi).
https://doi.org/10.1016/j.molstruc.2020.128327
0022-2860/© 2020 Elsevier B.V. All rights reserved.

2
M. Al-Noaimi et al. / Journal of Molecular Structure 1217 (2020) 128327
(DMSO,
d
ppm):25.0 (COCH3), 188.7 COCH3, M.p is 130 ꢂC.
produce pincer ligands analogous to terpyridine derivatives.
Ruthenium complexes built from these ligands have been the foci
for many research groups [20] as they can be used as hydrogena-
tion, oxidation and polymerization catalysts [21e23].
2.2.2. Preparation and characterization of ((1Z)-2-oxo-N-quinolin-
8-ylpropanehydrazonoyl chloride (H2Y)
Flexible polydentate azoimine (N¼N-C¼N-) ligands can be
prepared by reacting hydrazonylchloride and amine derivatives
[19,24e27]. Previously, we reported the synthesis of [RuII(L1)(bpy)
Cl](PF6) { L1 ¼ C6H5N¼N-C(COCH3) ¼ NC9H6N} (1A) where L1 is
azoimineequinoline pincer tridentate ligand which was prepared
by reacting hydrazonylchloride with 8-aminoquinoline [19]. Here-
in, a new isomer of (H2L) ligand namely quinoline -azoimine (H2Y),
was prepared by reacting 8-aminoquinoline hydrazonylchloride
with aniline. A new octahedral ruthenium(II) complex having the
general molecular formula [RuII(Y)(bpy)Cl](ClO4) (1) {bpy ¼ 2,20-
bipyrdine and the new ligand (Y¼ C6H5N¼C(COCH3)-N¼HNC9H6N)
has been synthesized to compare its structure and reactivity with
the related complex 1A reported previously (Scheme 1) [19]. The
characterization of the complex was accomplished by X-ray
diffraction as well as by a variety of spectroscopic techniques (IR,
UVeVis, 1H, 13C NMR, UV/Vis and IR-spectroelectrochemistry), in
addition to cyclic voltammetric measurements. Furthermore, the
synthesized complex has been tested as a catalyst for the hydro-
genation of cinnamaldehyde in isopropanol in presence of KOH as a
base.
To
a
solution
of
(1Z)-2-oxo-N-
quinoline
-8-
ylpropanehydrazonoyl chloride (4.94 g, 20 mmol) in 10.0 mL ab-
solute ethanol, (1.84 g, 20 mmol) aniline and (2.4 g, 24 mmol)
triethylamine were added. The mixture was refluxed for 2 h, a
yellow solid was formed upon cooling to room temperature. The
solid was recrystallized from ethanol yielding (3.6 g, 59%). Anal.
Calc. for C18H16N4O: C, 71.04; H, 5.30; N, 18.41, Found: C,71.21; H,
5.20; N, 18.31%. UVeVis in acetonitrile: lmax (εmax/Mꢀ1cmꢀ1): 368
(8.5 ꢁ 104), 276 (4.88 ꢁ 104), 254 (6.3 ꢁ 104). IR:
(C¼N) 1580,
n
n
(C¼O) 1660. 1H NMR (DMSO,
d ppm): 9.42 (s, 1H, NH, 8.47 (s, 1H,
NH), 8.10 (d, 1H, H6), 7.76 (d, 1H, H1), 7.55 (t, 1H, H5), 7.3 (m, 5H, H7,
H8, H2), 6.95 (m, 2H, H9, H4), 6.79 (d, 1H, H3), 2.71 (s, 3H, COCH3).
M.p is 154 ꢂC.
2.2.3. Preparation of [Ru(bpy)(Y)Cl]ClO4(1)
(261 mg, 1.0 mmol) ruthenium trichloride trihydrate and
(0.304 g, 1.0 mmol) of (H2Y) were dissolved in 100 mL absolute
ethanol. The mixture was refluxed for 2 h, then 1.0 mmol, 0.156 g) of
2,20-bipyridine was added. The mixture was heated for an addi-
tional 1 h before an excess amount of LiCl (500 mg, 11.8 mmol) was
added. The volume was reduced to 10 mL and then an excess of
saturated aqueous solution of NH4ClO4 was added to precipitate the
crude product. The crude product was filtered, washed with H2O,
diethylether, then dissolved in 20 mL (2:1) dichloromethane/
acetonitrile and purified by chromatography on grade (III) alumina.
The first yellow band, which corresponds to the ligand, was eluted
with dichloromethane. The red band, which corresponds to the
complex, is eluted with (1:1) dichloromethane: acetonitrile. Yield:
(0.313 g, 45%). Anal. Calc. for C28H22N6O5Cl2Ru: C, 48.42; H, 3.19; N,
12.10%. Found: C, 42.41; H, 3.42; N, 12.15%. UVeVis in dichloro-
methane: lmax in nm (εmax/Mꢀ1cmꢀ1): 242 (6.5 ꢁ 104), 300
(4.8 ꢁ 104), 360 (2.4 ꢁ 104), 410 (2.2 ꢁ 104), 502 (1.6 ꢁ 104). IR
2. Experimental
2.1. Chemicals and reagents
Ruthenium trichloride trihydrate, aniline, 3-chloro-pentane-
2,4-dione, sodium nitrite, pyridine, 8-aminoquinoline and trie-
thylamine were purchased from Aldrich and used without further
purification. All solvents: diethyl ether (anhydrous, 99.0%),
dichloromethane, acetone and absolute ethanol were purchased
from TEDIA. 8-Aminoquinoline hydrazonylchloride was prepared
according to published procedure [27].
(cmꢀ1):
n
(N¼N) 1473,
n
(C¼N) 1605,
n
(C¼O) 1707. 1H NMR (DMSO,
2.2. Synthesis
d
ppm): 9.56 (t, 2H, H8), 8.57 (t, 1H, H9), 8.42 (d, 2H, H7), 8.18 (t, 1H,
H5), 8.04 (t, 1H, H2), 7.95 (m, 2H, H1, H10), 7.68 (t, 1H, H11), 7.44 (m,
1H, H12) 7.20 (m, 2H, H13, H3), 7.20 (d, 1H, H6), 7.09 (t, 1H, H15),
6.99 (m, 2H, H16, H14), 2.77 (s, 3H, COCH3).
2.2.1. ((1Z)-2-oxo-N-quinolin-8-ylpropanehydrazonoyl chloride
Yield. (3.2 g, 52%). Anal. Calc. for C12H10N3OCl: C, 58.19; H, 4.07;
N, 16.97. Found: C,58.21; H, 4.01; N, 16.71%. UVeVis in acetonitrile:
lmax (εmax/Mꢀ1cmꢀ1): 368 (8.09 ꢁ 104), 276 (4.69 ꢁ 104), 254
(6.24 ꢁ 104). IR:
n
(C¼O) 1668,
n
(C¼N) 1577, 1H NMR (DMSO,
d
ppm):
2.3. Instrumentation
10.53 (s, 1H, NH), 8.85 (d, 1H, H1), 8.10 (d, 1H, H6), 7.73 (d, 1H, H4),
7.55 (t, 1H, H5), 7.47 (m, 2H, H3, H2), 2.61 (s, 3H, COCH3). 13C NMR
1H NMR (300 MHz) was measured on a Bruker Avance III 300
Scheme 1. Structure of 1 and its published skeletal isomer 1A.

M. Al-Noaimi et al. / Journal of Molecular Structure 1217 (2020) 128327
3
spectrometer at room temperature. Elemental analyses were
measured with a Eurovector E.A. 3000 instrument. IR spectra were
collected on a Bruker Vertex FT-IR instrument. UVeVis/NIR spectra
were recorded on a Cary 5000 UVeViseNIR spectrometer. Catalytic
activity was investigated using SCION 456-GC. Electrochemical
measurements were performed in dichloromethane (Aldrich, HPLC
grade) using Nova 2.11 workstation. Pt disk (2 mm) was thereby
used as a working electrode with a Pt wire as a counter electrode
and Ag wire as a pseudo-reference electrode. The temperature was
spectroscopy and X-ray diffraction. The ATR spectrum of H2Y
showed intense absorption bands at 3240 cmꢀ1, 1688 cmꢀ1 and
1570 cmꢀ1 which are assigned to the stretching frequencies of the
two NH groups, C¼O of the acetyl group, and the C¼N group,
respectively. In addition to the phenyl protons, the 1H NMR spec-
trum of H2Y showed a sharp singlet at 2.6 ppm assigned for the
acetyl H3CCO group. The two resolved singlet signals at
d
¼ 9.03 ppm and at
d
¼ 7.44 ppm with integral ratio of 1:1 are
assigned to the two NH groups. In contrast to the NMR results, the
solid ATR spectrum of H2Y showed only one characteristic, intense
absorption band for the two NH groups. It seems that the two NH
stretching frequencies are very close in energy and cannot be
discriminated and resolved in the solid-state ATR, most presumably
because of the broadening of the NH bands. There is only one
structural isomer for H2Y ligand which is confirmed by the pres-
ence of only one signal for each hydrogen in the 1H NMR spectrum.
[RuII(Y)(bpy)Cl](ClO4) (1) was prepared according to a previ-
ously published procedure [19] by the stepwise addition of equi-
molar amounts of H2Y, RuCl3 and bpy into boiling ethanol. The
counter ion PFꢀ6 was here replaced by ClOꢀ4 to get a good-quality
single crystal suitable for x-ray diffraction analysis (Scheme 1).
The coordination sphere around the ruthenium ion consists of a
chloride ligand, three nitrogens from Y (azo-N, imine-N0 and qui-
nolone-N00) and two bpy nitrogen donor atoms. During the reaction,
ruthenium(III) has undergone a one-electron reduction and the
ligand H2Y is oxidized to azoimine (Y). Also, ethanol presumably
acts both as a solvent and as a reducing agent for Ru(III). No N-H
bands is observed in the IR spectrum for complex 1 confirming the
loss of the two dissociable protons of the NH groups. The complex 1
has a sharp absorption bands at 1667 cmꢀ1, 1606 cmꢀ1 and
1505 cmꢀ1 which are assigned to stretching frequencies for the
C¼O, C¼N, and N¼N groups, respectively. In the 1H NMR spectrum,
the two signals corresponding to NH group of the azoimine are also
absent supporting the full oxidation to azoimine moiety. The aro-
matic region of H2Yand 1 showed several resonance multiplets due
to the aromatic protons of the phenyl rings of H2Y and bpy ligands.
controlled (at 25.0
0.1 ꢂC) by a Haake D8-G refrigerator. The
ferrocene to ferrocenium oxidation was taken as internal reference
[28,29]. UV/Vis and IR-spectroelectrochemistry of complex 1 were
analyzed using an optically transparent thin layer electrochemistry
(OTTLE) cell [30]. The potential was controlled by the Nova 2.11
workstation. At any given potential, the system allowed to reach
equilibrium (i ~0 A) before the spectrum was taken.
2.4. Crystallography
A suitable crystal of complex 1 and its ligand H2Y was mounted
on a glass fiber and the data was collected using enhanced Mo ra-
diation,
l
¼ 0.71073 Å and Xcalibur/Oxford Diffractometer. CrysAlis
Pro software was used for data collection, absorption correction
and data reduction to give SHELX-format-hkl files [31]. The struc-
ture was solved using SHELXTL program package [32].
2.5. Computational methods
Full geometry optimization of 1 was carried out using density
functional theory (DFT) at the B3LYP level [33]. All calculations
were carried out using the GAUSSIAN 09 and Gauss-View program
[34]. For C, H, Cl, N and O the cc-pvdz basis sets was assigned, while
for Ru, the LanL2DZ basis set with effective core potential was
employed [35]. Vertical electronic excitations based on B3LYP
optimized geometries were computed using the time-dependent
density functional theory (TD-DFT) formalism [36e38] in
dichloromethane using conductor-like polarizable continuum
model (CPCM) [36e38]. GaussSum was used to calculate the frac-
tional contributions of various groups to each molecular orbital
[39].
3.2. Crystal structures
Crystal structure data and selected bond lengths and angles for
the ligand H2Y as well as [Ru(Y)(bpy)Cl](ClO4) (1) are presented in
Table 1 and Table 2, respectively. ORTEP drawing of complex 1 and
H2Y are shown in Fig. 1 and Fig. 2, respectively. H2Y crystallized in
the triclinic system while complex 1 crystallized in the monoclinic
space group P 21/c. The bond length for N(12)-N(11) is 1.343(2) Å in
H2Y. The bond length for N(12)-C(13) is 1.295(2) Å which is shorter
than N(16)-C(13) (1.386(2)Å). The azo group is coplanar with the
quinoline ring, the deviation of N11 and N12 atoms from the mean
plane of the quinoline ring is 0.10 Å and 0.29 Å, it is stabilized by the
intramolecular N11-H/N1 hydrogen bonding interactions. The
parameters of this interactions are 2.67 Å, 2.33 Å and 103.9ꢂ. This
2.6. Catalytic hydrogenation of cinnamaldehyde
Complex 1 was tested as catalyst for the hydrogenation of cin-
namaldehyde (mixture of 1:1 cis-to-trans) in a stainless-steel
reactor (Parr-4842) equipped with a Waltow-945 unit to control
the temperature of reaction and the stirring rate. In a typical
experiment, the vessel was loaded with 0.040 g of Ru complex,
(0.047 g) potassium hydroxide, (0.80 g) cinnamaldehyde and
100 mL 2-Propanol. The mixture was placed in the reactor and then
tightly closed. The reactor temperature was adjusted to 86 ꢂC
before, hydrogen gas was added and adjusted to 4 bars. The course
of the reactions was monitored by SCION 456-GC. various time
intervals (SCION, FID, 30-m crosslinked FAFF capillary column).
planarity is reinforced by the extended delocalization of the
p-
system of the quinoline ring to include the azo group. The N(12)-
N(11) and C(10)-N(11) bond lengths are 1.343 Å and 1.387 Å,
respectively, which are shorter than the corresponding single
bonds and longer the corresponding double bonds. The N16-H/O5
hydrogen bond interactions connect the molecular units of the
ligand to form dimer structure (Fig. S1, top), the hydrogen bonding
interaction parameters are 2.96 Å, 2.19 Å and 148.6ꢂ, respectively.
3. Results and discussion
3.1. Synthesis
The new quinolineeazoimine ligand (H2Y¼ C6H5NH-
C(COCH3) ¼ N-NHC9H6N) was prepared by reacting ((1Z)-2-oxo-N-
quinoline-8-ylpropanehydrazonoyl chloride, aniline and triethyl-
amine in refluxing ethanol (Scheme 2).
C-H/N(p) interactions connect the dimers to form a layer struc-
ture in the bc crystallographic plane (Fig. S1, bottom). The param-
eters of these interactions are listed in Table 3. For Complex 1, The
ligand H2Y is coordinated to ruthenium as a tridentate N, N0, N00-
donor and in meridional fashion; the bpy ligand is in cis orientation.
Pure H2Y was obtained by recrystallization from ethanol. The
structure of H2Y was confirmed by elemental analysis, 1H NMR

4
M. Al-Noaimi et al. / Journal of Molecular Structure 1217 (2020) 128327
Scheme 2. Synthesis of [Ru(Y)(bpy)Cl](ClO4).
Table 1
Crystal data and structure refinement for Ligand (H2Y) and complex 1.
Ligand H2Y
Complex 1
Empirical formula
Formula weight
Temperature
Wavelength
Crystal system
Space group
C18H16N4O
304.35
293(2) K
0.71073 Å
Triclinic
C29 H24 Cl4 N6 O5 Ru
779.41
293(2) K
0.71073 Å
Monoclinic
P 21/c
P ꢀ1
Unit cell dimensions
a ¼ 7.3903(6) Å,
a
b
¼ 105.595(8)ꢂ
¼ 99.047(8)ꢂ
a
¼ 105.595(8)ꢂ.
a ¼ 8.2795 Å,
a
¼ 90
b ¼ 10.3828(9)Å,
b ¼ 32.318(2) Å,
b
¼ 96.263(6)ꢂ.
c ¼ 11.2688(11) Å,
800.34(12) Å3
2
g
¼ 100.194(7).
c ¼ 12.0898(10) Å,
2591.8(2) Å3
4
g
¼ 90ꢂ.
Volume
Z
Density (calculated)
Absorption coefficient
F(000)
Crystal size
Theta range for data collection
Index ranges
1.263 Mg/m3
0.082 mmꢀ1
320
1.610 Mg/m3
0.868 mmꢀ1
1568
0.42 ꢁ 0.32 ꢁ 0.25 mm3
0.4220 ꢁ 0.3315 ꢁ 0.1377 mm3
3.14e25.00ꢂ..
2.91e25.00ꢂ
ꢀ7 ꢃ h ꢃ 8, ꢀ12 ꢃ k ꢃ 12, ꢀ13 ꢃ l ꢃ 13
ꢀ9 ꢃ h ꢃ 9, ꢀ38 ꢃ k ꢃ 28,
ꢀ14ꢃ l ꢃ 14
Reflections collected
5727
17111
Independent reflections
Completeness to theta ¼ 26.30ꢂ
Absorption correction
Max. and min. transmission
Refinement method
2820 [R(int) ¼ 0.0215]
5659 [R(int) ¼ 0.0423]
99.8%
99.8%
Semi-empirical from equivalents
1.00000 and 0.91278
Full-matrix least-squares on F2
2820/0/208
Semi-empirical from equivalents
1.00000 and 0.62877
Full-matrix least-squares on F2
5659/0/406
Data/restraints/parameters
Goodness-of-fit on F2
1.040
1.203
Final R indices [I > 2sigma(I)]
R indices (all data)
Largest diff. peak and hole
R1 ¼ 0.0526, wR2 ¼ 0.1217
R1 ¼ 0.0820, wR2 ¼ 0.1485
0.216 and ꢀ0.181 eÅꢀ3
R1 ¼ 0.0788, wR2 ¼ 0.1616
R1 ¼ 0.0997, wR2 ¼ 0.1712
0.586 and ꢀ0.696 e.Åꢀ3
R1 ¼ S||Fo| - |Fc||/S|Fo|; wR2 ¼ {S[ w(F2o - Fc2)2]/S [w(Fo2)2]}1/2
.

M. Al-Noaimi et al. / Journal of Molecular Structure 1217 (2020) 128327
5
Table 2
Selected bond lengths and angles for complex 1, and its oxidized (1þ) as well as reduced (1-) forms.
H2Y
Complex
1
1-
1þ
Bond lengths (Å) (experimental)
Bond lengths (Å)(experimental, calculated)
Bond lengths (Å)(calculated)
N(12)-N(11)
N(16)-C(13)
N(12)-C(13)
O(5)-C(14)
C(13)-C(14)
1.343(2)
1.386(2)
1.295(2)
1.220(2)
1.476(3)
Ru(1)-N(11)
Ru(1)-N(16)
Ru(1)-N(34)
Ru(1)-N(1)
Ru(1)-N(23)
Ru(1)-Cl(2)
N(11)-N(12)
N(16)-C(13)
1.889(6), 1.932
1.972
2.079
2.089
2.155
2.145
2.448
1.331
1.351
1.972
2.090
2.127
2.141
2.172
2.337
1.289
1.332
2.008(5), 2.056
2.068(6), 2.118
2.084(5), 2.157
2.117(6), 2.180
2.3559(19), 2.402
1.322(7), 1.297
1.319(9), 1.328
Bond angles (ꢂ)
Bond angles (ꢂ) (experimental, calculated)
Bond angles (ꢂ) (calculated)
C(13)-N(12)-N(11)
C(13)-N(16)-C(17)
N(12)-N(11)-C(10)
N(12)-N(11)-H(11A)
O(5)-C(14)-C(15)
O(5)-C(14)-C(13
N(12)-C(13)-C(14)
N(16)-C(13)-C(14)
N(12)-N(11)-C(10)
N(12)-C(13)-N(16)
118.67(16)
124.91(17)
119.26(16)
120.4
121.40(18)
119.48(18)
115.65(17)
117.77(17)
119.26(16)
126.50(18)
N(11)-Ru(1)-N(16)
N(11)-Ru(1)-N(34)
N(16)-Ru(1)-N(34)
N(11)-Ru(1)-N(1)
N(16)-Ru(1)-N(1)
N(1)-Ru(1)- N(34)
N(11)-Ru(1)-N(23)
N(16)-Ru(1)-N(23)
N(34)-Ru(1)-N(23)
N(1)-Ru(1)-N(23)
77.8(2), 76.7
76.7
98.1
92.6
79.7
156.4
92.5
174.9
101.6
77.0
75.9
101.9
86.8
79.9
154.3
90.0
177.3
106.1
76.5
98.2(2), 101.0
89.4(2), 93.8
81.9(2), 80.9
159.4(2), 157.6
90.3(2), 90.9
175.2(2), 176.4
99.5(2), 101.3
77.7(3), 76.0
100.5(2), 101.1
102.0
97.8
Fig. 1. ORTEP of complex 1. Thermal ellipsoids of ORTEP plots are reported at 30%
probability.
The average bond length for RueN(bpy) is 2.093 Å. The azomethine
ligand is known to interact strongly with the Ru(II) center via the
dpep* interactions [24e27]. This has been reflected in the average
Fig. 2. ORTEP of ligand (H2Y). Thermal ellipsoids of ORTEP plots are reported at 30%
probability.
Ru(1)-N(11) (Azo) bond length (1.889(6)Å) and the Ru(1)-
N(16)(methine) bond length (2.008(5)Å). The shortening in the
bond length for Rueazoimine fragment compared to that of
Table 3
C-H/N(p
) interaction distances (Å) and angles (ꢂ) in the ligand (H2Y).
RueN(1)(pyridine) (2.093 Å) indicates that the MeL
p interaction is
Interaction
C21-H/N1(
p
)
C15-H/N1(p)
localized in the MeAzo fragment [19,24e27]. The bond angles in
the five membered ring described by the coordination of an azoi-
mine (N(11)-Ru(1)-N(16)), iminepyridine N(11)-Ru(1)-N(1) and
C/N
H/N
C-H/N
3.498
2.694
145.3
3.787
2.867
160.7
o
o
bpy N(34)-Ru(1)-N(23) are 77.8(2)o, 81.9(2)
respectively.
and 77.7(3)
,
The supramolecularity of the complex is mainly developed
based on the non-classical C-H…A hydrogen bonding interactions
respectively. Subsequently, the C-H…A hydrogen bonding in-
teractions link the ClOꢀ4 anion, dichloromethane and the cationic
molecules of the complex to form the final three-dimensional
structure (Fig. S2, bottom).
For 1, the difference between the (Ru-N11 and Ru-N16) bond
distances is significant (0.12 Å). Whereas, the average of (Ru-N11
and Ru-N16) is 1.949 Å which is equal, within the experimental
(A ¼ O or Cl) and C¼O/C
hydrogen bonding interactions are listed in Table 4. Furthermore,
C4-H/Cl1 hydrogen bonding and C¼O/C interactions connect
the cationic complex molecules to form a chain structure parallel to
p interactions. The parameters of the
p
the a-axis (Fig. S2, top). The C¼O…C
p interaction lengths are
2.99 Å, 4.02 Å and 142.4ꢂ for C33/O5, C14/O5 and C14 ¼ O5/C33,

6
M. Al-Noaimi et al. / Journal of Molecular Structure 1217 (2020) 128327
Table 4
1A (1.313(4) Å) [19]). Aslo the Ru-N(azo) bond distances is shorter
than the Ru-N(imine) in the two complexes (Table F1). However,
the difference in the bond distances is quite large in 1 is 0.12 Å
whereas it is significantly small in 1A (0.02 Å. Theoretical calcula-
tions were used to rationalize the differences in bond distances in
the coordination sphere in complex (1) and (1A). The two complex-
cations were optimized using the crystal structure coordinate as
starting geometry. The difference between the optimized (Ru-N11
and Ru-N16(imine) bond distances is 0.12 Å, whereas the corre-
sponding value in 1A is 0.03 Å, which agrees with the values ob-
tained from the crystal structures (Table 5). The percent
distribution of the frontier orbitals in the cationic complexes was
carried out using GaussSum program [39] (Table 6, Fig. 3). A closer
look at the LUMO orbitals of the two cationic complexes shows two
interesting observations; (a) For both complexes the LUMO is
constructed from the azoimineequinoline (L and Y) Ligand while
the for the HOMOs, the first three highest energy orbitals are Ru in
character. The significant contribution from Ru(II) to the LUMO can
C-H…A Interactions distances (Å) and angles (ꢂ) in the ligand (H2Y) where A ¼ Cl or
O.
C-H…A
C…A
H…A
C-H…A
C4-H/Cl1
C2-H/O3
C31-H/O1
C15-H/Cl4
C1-H/O2
3.58
3.36
3.19
3.72
3.55
2.78
2.67
2.48
2.94
2.68
144.1
131.5
133.0
138.6
148.6
Table 5
RueN and N¼N bond distances (Å) for 1 and 1A.
Complex
1
1A
Optimized
Experimental
Optimized
Experimental
RueN(azo)
RueN(imine)
N¼N
1.932
2.055
1.297
1.889
2.008
1.322
2.035
2.008
1.299
1.945
1.960
1.313
be correlated to the significant
p-back donation from the metal
d orbital to azoimine-qinoline ligands (Table 6, Fig. 3) [40]. (b) in
the two complexes, not any part of the LUMO is located on the
bipyridine ligand c) The LUMO is mainly azo in character, but in 1A
it is extended over the quinoline which is trans to the azo group.
The shortening in the Ru(II)- azo bond in 1A relative to that in 1 can
be rationalized by the amount of the back donation or MLCT from
Ru(II) to the azo group and by the trans-effect, as well. The atom
error, to the corresponding value (1.953 Å) in its skeletal isomer
[Ru(L1)(bpy)Cl](PF6) (1A)(Scheme 1). However, it is interesting to
note that the bond lengths RueN(Azo ((1.889(6) (Å) for complex 1
are significantly shorter than the corresponding lengths of its re-
ported skeletal isomer 1A (RueN(azo) ¼ 1.945 (3) Å and N¼N bond
distance (1.322(7) Å) in is longer than the corresponding distance in
Table 6
DFT energies and % composition of selected molecular orbitals (LUMO (L) and HOMO (H)) for 1 and its published skeletal isomer 1A [19] expressed in terms of composing
fragments.
M.O
eV
1
Ru
1
Azo
1
Ph
1
8-Aminoqu.
bpy
1
Cl
1
1A
1A
1A
1A
1
1A
1A
1A
L þ3
L þ2
L þ1
L
ꢀ4.48
ꢀ5.03
ꢀ5.29
ꢀ6.21
ꢀ8.48
ꢀ8.7
ꢀ4.46
ꢀ4.83
ꢀ5.32
ꢀ6.15
ꢀ8.4
2
2
4
23
36
30
30
5
2
2
4
21
48
36
38
38
0
18
0
47
15
16
14
3
1
10
0
49
9
10
10
12
0
1
0
3
22
34
1
0
2
0
12
1
8
11
10
2
79
0
22
7
3
1
86
1
14
12
3
20
23
96
0
95
2
2
4
95
1
95
1
4
3
0
0
0
3
17
12
47
0
0
0
0
3
25
40
15
12
H
H-1
H-2
H-3
ꢀ8.77
ꢀ9.11
ꢀ9.18
ꢀ8.96
ꢀ9.23
5
0
3
1
6
5
90
Fig. 3. Isodensity plot for HOMOs and LUMOs of Complex 1 and complex 1A.

M. Al-Noaimi et al. / Journal of Molecular Structure 1217 (2020) 128327
7
Fig. 4. Cyclic voltammogram of complex 1 vs. Cp2Fe0/þ (TBAHF, 0.1 M, dichloromethane, 25 ꢂC). Inset shows the Ru(II/III) couple at different scan rates.
trans to the azo group in 1 is the bipyridyl nitrogen N23 (Fig. 3),
while, it is the quinoline nitrogen in 1A. So the shortening of Ru-
N11(Azo) (1.889(6) Å) and the elongation of Ru-N34(azo)
(2.117(6) Å) in 1A can be explained as follows: In IA, there is
some back donation from Ru(II) to the trans-quinoline which re-
duces the back donation to the azo group, while in 1, the azo is trans
to bpy and so the back donation is localized in the Ru(II)-azo bond
[40].
lowest unoccupied and highest occupied molecular orbitals have
been placed in Table 7. Moreover, the isodensity plots for the HO-
MOs and LUMOs orbitals are shown in Fig. 3. DFT results of 1 show
that the HOMO to HOMO-3 are mainly Ru and azoimine (Y) in
character. The LUMO, LUMOþ2 and LUMOþ3 molecular orbital are
mainly azoimine in a character. The LUMO has 24% metal d orbital
Ru character suggesting significant back donation (Table 4) [40].
Computation of 30 excited states of complex 1 allowed the
interpretation of the experimental spectra for the complexes in the
300e800 nm range (Table 8). The theoretical absorption spectrum
was simulated using GaussSum software [39]. Both the experi-
mental UVeVis spectrum of complex 1 reported in acetonitrile and
its simulated absorption spectrum (inset Fig. 5) were in acceptable
agreement. The intense absorption band at 300 (350 calculated) is
results mainly from HOMO-10 (Y in character)/LUMO (Y in
character) (44%), thus this band is assigned as ligand-to-ligand
3.3. Electrochemistry
The cyclic voltammogram for complex 1 is shown in Fig. 4. Inset
Fig. 4 shows the oxidation peak (1.05 V) at different scan rates, the
small change of peak potentials with scan rates meaning that the it
is quasi-reversible oxidation process. Since the HOMO of 1 has a
metal contribution of about 36%, the couple at 1.05 V has been
assigned to Ru(III/II). On the other hand, the ligand azo moiety
contributes by about of 47% to the LUMO; thus the reduction cou-
ples at ꢀ1.15 and ꢀ1.75 V vs. Cp2Fe0/þ (cathodic wave peak max-
ima) can be assigned to L(0/-) and L(ꢀ/ꢀ2). The anodic shift for
Ru(III/II) and the cathodic shift for L(0/-) in [Ru(Y)(bpy)Cl](ClO4) (1)
compared to that in [Ru(L1)(bpy)Cl](PF6) (1A) (Ru (III/II) ¼ 0.98 V
charge transfer LLCT (Y(p))/Y(p*)) (Table 8). The band centered
at 370 (374 calculated) results from HOMO-1 (mainly Ru and bpy)
/Lþ1(bpy in character) (49%), thus this band is assigned to met-
aleligand-to-ligand charge transfer (Ru(dp), L(bpy))/ bpy (p*))
MLLCT [41e43]. The band centered at 410 (411 nm calculated) re-
sults from HOMO (Ru and Y in character) /Lþ2 (bpy in character)
(72%) thus this band is assigned as metaleligand-to-ligand charge
and L(0/-) ¼ ꢀ1.30 V) suggests that the ligand Y is a better
p-
transfer (Ru(dp), Y(p))/bpy(p*)) MLLCT. The band centered at 500
acceptor ligand compared to the previously prepared azomine
bidentate ligand L1 [19].
(527 calculated) results from HOMO-2/LUMO (69%) thus this
band is assigned as metal-Ligand charge transfer (Ru(d
MLCT.
p)/Y(p*))
3.4. Electronic structure and DFT calculations
UV/Vis spectroelectrochemical measurements were performed
on complex 1 in acetonitrile in order to obtain the electronic ab-
sorption spectra for 1þ (Fig. 6). Upon oxidation there is a complete
disappearance of the MLLCT bands at 374 and 430 nm and the
appearance of a new band near 650 nm. The oxidation of 1 is
reversible with greater than 95% recovery of the Ru-(II) complex
spectrum.
The electronic absorption spectrum of complex (1) was
measured using acetonitrile as a solvent (Fig. 5). The complex ex-
hibits an intense transition in the UV region (220e320 nm)
accompanied by a moderately intense band at longer wavelength
(320e550 nm). To assign these bands, TD-DFT calculations on the
optimized geometries of 1 was performed using GAUSSIAN 09
program [34]. The calculated bond lengths and angles are repro-
duced well in agreement with the X-ray determined structure
(Table 2). Relative percentages of atomic contributions to the
Theoretical calculations were performed on the oxidized form
(1þ); the calculations utilize both
a and b occupied molecular or-
bitals. Relative percentages of atomic contributions to the lowest

8
M. Al-Noaimi et al. / Journal of Molecular Structure 1217 (2020) 128327
Fig. 5. UVeVis of complex 1 in CH3CN. Inset shows simulated absorption spectrum of complex 1. (Black line) based on TD-DFT calculations, compared to excitation energies and
oscillator strengths.
unoccupied and highest occupied molecular orbitals have been
placed in Table S1. Moreover, the isodensity plots for the HOMOs
and LUMOs orbitals are shown in Fig. S3. Ru(III) does significantly
Table 7
Relative Percentages of Atomic Contributions to the Lowest Unoccupied and Highest
Occupied Molecular Orbitals (LUMO (L) and HOMO (H)) of 1 in acetonitrile.
contribute to LUMO of
b
b
-spin and HOMO-2, HOMO-4 of
-spin. HOMO to HOMO-3 of -spin and -spin and LUMO to
spin and (LUMO, LUMOþ2) are mainly azoimine (Y)
-spin and
a-spin and
Complex
1
eV
Ru
Cl
Azo
bpy
a
b
Lþ5
Lþ4
Lþ3
Lþ2
Lþ1
L
ꢀ1.49
ꢀ1.75
ꢀ1.8
3
5
3
1
0
0
0
0
0
2
13
3
32
6
2
95
69
24
16
80
1
3
7
3
7
LUMO þ1 of
a
26
72
82
17
73
33
46
23
53
96
61
in character. Chloride mainly contributes to HOMO-2 of
b
ꢀ2.61
ꢀ2.64
ꢀ3.93
ꢀ6.28
ꢀ6.6
HOMO-3 of a-spin.
3
The theoretical absorption spectrum for (1þ) was simulated
using GaussSum software [39] (inset Fig. 6, Table 8). The intense
absorption band at 650 nm (689 calculated) results mainly from
24
50
44
42
34
1
H
H-1
H-2
H-3
H-4
H-5
ꢀ6.84
ꢀ7.08
ꢀ7.3
HOMO(
and LUMO(
to-ligand charge transfer (Y(
550 nm (602 nm calculated) results from HOMO-2(
b
) and HOMO (
) (Y in character) thus this band is assigned as ligand-
)/Y( *)). The band centered at
) (Ru and
a
) which are (Y in character)/LUMOþ1(
b)
a
0
22
3
11
p
p
ꢀ7.36
6
a
Table 8
Computed excitation energies (nm), electronic transition configurations and oscillator strengths (f) for the optical transitions of complex 1, and its oxidized (1þ) as well as
reduced (1-) forms.
Complex
1
l
(nm)
Osc. Str.
Major contribs
528
451
412
374
369
352
1094
769
674
494
470
447
442
689
602
483
472
0.085
0.071
0.083
0.078
0.052
0.071
0.010
0.050
0.041
0.057
0.124
0.055
0.048
0.100
0.045
0.055
0.155
H-3/L (23%), H-2/L (74%)
H-5/L (87%)
H/Lþ2 (72%), H/Lþ6 (13%)
H-1/Lþ1 (53%), H-1/Lþ2 (30%)
H-9/L (55%), H-8/L (27%)
H-10/L (44%), H-1/Lþ6 (14%), H/Lþ3 (16%)
1-
H (
H (A) /Lþ1(A) (90%)
H-2( ) /L ( ) (70%), H-1(
H (
b) /L (b) (98%)
b
b
b
b
b
b
) /L (
) /L (
) /Lþ2(
) /L ( ) (23%), H-1(
) /Lþ5(
) /Lþ1(
) (20%), H-5(
) (13%), H-12(
) (16%), H-8( ) /L (
b
b
) (28%)
) (64%)
) (29%)
a
) /Lþ3(A) (16%), H-3(
H-1(
H-2(
H-2(
a
a
a
) /Lþ1(
a
) (53%), H (
) (44%), H-4(
) (12%), H (
b
) /L (
) /L (
a
a
b
a
b) (39%)
b
) /Lþ1(
b) (19%)
a
) (52%)
1þ
H (
a
) /L (
a
) (18%), H (
b
H-2(
H-5(
H-9(
a
a
b
) /L (
a
a
b
) /Lþ1(
) /L (
) (11%), H-6(
b
) (26%), H-4(
) (16%), H-9(
) /Lþ1
b
) /Lþ1(
) /L ( ) (23%)
) (19%), H-5(
b) (14%)
) /L (
) /L (
b
b
b
b
b
b
b
b
b
b
) /Lþ1(b) (14%)

M. Al-Noaimi et al. / Journal of Molecular Structure 1217 (2020) 128327
9
Fig. 6. UVeVis Spectroscopic changes on oxidation of complex 1 (CH3CN/NBu4PF6, r.t.). . Inset shows simulated absorption spectrum of complex 1þ. (Black line) based on TD-DFT
calculations, compared to excitation energies and oscillator strengths.
azoimine character)/LUMO(
in character) and HOMO-4(
) (Ru in character)/LUMOþ1(
character) thus this band is assigned as (Ru(d ), Y(
MLLCT [41e43]. The band centered at 450 nm (471 nm calculated)
results from HOMO-8 ( ) (Chloride in character)/LUMO( ) (Ru in
character) thus this band is assigned to Ligand Metal Charge
Transfer (LMCT).
UV/Vis spectroelectrochemistry (Fig. 7) on the reduced species
1- was performed in acetonitrile in order to reduce Ru(II) to Ru(I).
a
) (Y in character), HOMO-5(
b
) (bpy
Theoretical calculations were performed to assign the experimental
bands. Relative percentages of atomic contributions to the lowest
unoccupied and highest occupied molecular orbitals have been
placed in Table S1. Moreover, the isodensity plots for the HOMOs
b
b
) (Y in
p
p))/bpy(p*))
b
b
and LUMOs orbitals are shown in Fig. S3. For
HOMO-3 are mainly Ru in character with a significant contribution
of azoimine ligand (Y). While
-spin of LUMOþ1 and LUMOþ3 is
mainly bpy, -spin is azoimine in character. Also, while -spin of
LUMO and LUMOþ2 is mainly bpy in character -spin is mainly
a and b spin, HOMO to
b
a
a
b
Fig. 7. UV/Vis/near-IR Spectroscopic changes on Reduction of complex 1 (CH3CN/NBu4PF6, r.t.). Inset shows simulated absorption spectrum of complex 1-. (Black line) based on TD-
DFT calculations, compared to excitation energies and oscillator strengths.

10
M. Al-Noaimi et al. / Journal of Molecular Structure 1217 (2020) 128327
Fig. 8. IR spectroscopic changes upon the oxidation of complex 1 (CH2Cl2/NBu4PF6).
Fig. 9. IR spectroscopic changes upon the full reduction of complex
1 CH2Cl2/
NBu4PF6).
azoimine in character.
TDDFT calculation was used to interpret the electronic spectrum
of the reduced species 1- (inset Fig. 7, Table 8). The band centered at
900 nm (1093 nm calculated) results from HOMO(
Ru(I)) /LUMO( ) (Y in character), thus this band is assigned as
(Ru(d ))/bpy( *)) (MLCT). The band centered at 675 nm (674
calculated) results from HOMO( ) (Y and Ru(I) in character)
/LUMOþ1( ) (Y in character), thus this band is assigned as
(Ru(d ), Y( ))/Y( *)) (MLLCT). The band centered at 465 nm (470
calculated) results from HOMO-1( ) and HOMO( ) (Ru(I) in char-
acter)/ LUMOþ1( ) and LUMOþ2( ) (Y in character) thus this
band is assigned as MLCT .
b) (mainly of
regio-selective hydrogenation possibilities of cinnamaldehyde.
In the hydrogenation of ketones and aldehydes homogeneously
catalyzed by Ru complexes the active catalyst form is believed to be
a hydride/dihydride Ru complex that forms under reaction condi-
tions [44]. The actual catalyst must thus be prepared before a
conversion of cinnamaldehyde is observed, which explains the in-
duction period in Fig. 10 at the begin of reaction. Although the
molar Ru: CALD ratio is relatively high (1: 68), complex 1 shows
only a rather low activity and moderate selectivity (Fig. 10). It took
14 h of reaction under indicated conditions for a conversion to
approach 70%. The percentage of cinnamyl alcohol (the desired
component) is thereby only 26% corresponding to a reaction
selectivity to cinnamyl alcohol of 37%. Maximum concentration of
hydrocinnamaldehyde (saturated aldehyde), which forms unde-
sirably in a parallel route, is reached in 9 h of reaction (HCALD
% ¼ 18%) but decreases thereafter due to consecutive hydrogena-
tion to phenylpropanol (PP). In a previous study, a related Ru(II)
complex, namely [Ru(PhN ¼ NC(COCH3) ¼ NPhSPh)(bpy)Cl2], was
reported to show a much higher activity in the hydrogenation of
cinnamaldehyde under similar conditions, reaching more than 90%
conversion in 4 h with a cinnamyl alcohol selectivity exceeding 90%
[45].
b
p
p
a
a
p
p
p
a
b
a
b
3.5. Infrared spectroelectrochemistry
IR spectroelectrochemistry is a powerful tool to estimate the
degree of charge delocalization between metal and -acidic ligand
p
upon oxidation/reduction processes. Fig. 8 and Fig. 9 show the IR
changes of complex 1 during the oxidation and reduction process,
respectively. The oxidation (Fig. 8) is reversible; however, the
reduction (Fig. 9) has a poor reversibility. Figs. 8 and 9 show the IR
spectrum of complex 1þ in dichloromethane (black line). The
complex shows intense vibrational bands at 1702 cmꢀ1, 1606 cmꢀ1
and 1590 cmꢀ1 which are assigned to the C¼O, C¼N, and N¼N
stretches, respectively. Upon oxidation of complex 1 to 1þ (Fig. 8),
the C¼O band is successively blue-shifted from 1702 cmꢀ1 to
1726 cmꢀ1. This blue shift for C¼O, which is conjugated with the
azo and imine moiety, is due to a decrease in back donation to p* of
4. Conclusion
non-innocent azoimine moiety upon oxidation. On the other hand,
there is only a slight shift with increasing intensity of the C¼N and
N¼N bonds. The reduction of the complex 1 to 1- (Fig. 9) showed an
increase in intensity and a slight red shift in energy for the C¼O
bond from 1702 cmꢀ1 to 1675 cmꢀ1. The red shift can be explained
by the reduction of the azo group which is conjugated to the C¼O
supported by the cyclic voltammetry and non-innocent behavior of
this ligand.
A new octahedral ruthenium(II) complex having the general
formula [RuII(Y)(bpy)Cl](ClO4) {Y¼ C6H5N¼C(COCH3)N¼NC9H6N
and bpy ¼ 2,20-bipyrdine} (1) was synthesized. The bonding in 1
and its published skeletal isomer [RuII(L1)(bpy)Cl](PF6)
{L1 ¼ C6H5N¼N-(COCH3)C¼NC9H6N} (1A) has been analyzed using
molecular orbital theory. Complex 1 is stabilized by the strong
p-
acceptor quinolineeazoimine ligand H2Y and showing the Ru(III/II)
couple at 1.05 V vs. Cp2Fe/Cp2Feþ. The UV/Vis and IR spectra for 1,
1þ, 1- was investigated by spectroelctrochemistry and their elec-
tronic bands and electronic structures was interpreted using DFT
and TDDFT calculations. Complex 1 has a moderate activity for the
3.6. Hydrogenation of cinnamaldhyde (CALD)
Cinnamaldehyde has been selected as a model substrate for
hydrogenation under mild condition. Scheme 3 shows the different
hydrogenation of a, b-unsaturated aldehyde.

M. Al-Noaimi et al. / Journal of Molecular Structure 1217 (2020) 128327
11
Scheme 3. Reaction pathways in the hydrogenation of cinnamaldhyde.
https://doi.org/10.1016/j.molstruc.2020.128327.
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Reactant: co-cat: cat ¼ 68 : 22: 1.
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Declaration of competing interest
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