Keto–enol tautomers of mixed-ligand ruthenium(II) complexes containing α-diamine and azoimine bearing alkyne group ligands

Article abstract of DOI:10.1016/j.ica.2016.04.029

Refluxing azoimine ligand containing terminal acetylene group (L = C₆H₅N[dbnd]NC(COCH₃)[dbnd]NC₆H₄C[tbnd]CH) with RuCl₃·3H₂O in ethanol resulted hydrating the terminal acetylene group to the corresponding enol form C₆H₅N[dbnd]N[sbnd]C(COCH₃)[dbnd]NC₆H₄C(OH)[dbnd]CH₂(L1), ketone form C₆H₅N[dbnd]NC(COCH₃)[dbnd]NC₆H₄COCH₃(L2) and vinyl chloride: C₆H₅N[dbnd]N[dbnd]C(COCH₃)[dbnd]N[sbnd]C₆H₄C(Cl)[dbnd]CH₂(L3) via Markovnikov selectivity. Ruthenium complexes of the later ligands and α-diamines of the formula trans-[Ru(N-N)(Y)Cl₂] (Y = L1, N-N = 4,4′-dimethoxy-2,2′-bipyridine; dmeb (1), 1,10-phenanthroline; phen (2), 3,4,7,8-tetramethyl-1,10-phenanthroline; tmphen (3), Y = L2, N-N = dmeb (4), phen (5), tmphen (6)) were synthesized from L, N-N ligands and RuCl₃·3H₂O. Complexes 1–6 were characterized by spectroscopic (IR, UV–Vis,¹H,¹³C NMR, DEPT-135) and electrochemistry techniques. The crystal structures of trans-[Ru(tmphen)(L1)Cl₂] (3) and trans-[Ru(bpy)(L3)Cl₂] (7) were determined and found to have distorted octahedral geometry. The catalytic activity of 3 towards the hydration of cinnamaldehyde is reported.

Full text of DOI:10.1016/j.ica.2016.04.029

Accepted Manuscript
Keto-enol Tautomers of Mixed-Ligand Ruthenium(II) Complexes Containing
α-Diamine and Azoimine Bearing Alkyne Group Ligands
Mousa Al-Noaimi, Ayman Hammoudeh, Mohammad El-khateeb, Firas F.
Awwadi, Deeb Taher, Ahmad Mansi, Obadah S. Abdel-Rahman
PII:
S0020-1693(16)30195-5
http://dx.doi.org/10.1016/j.ica.2016.04.029
ICA 17013
DOI:
Reference:
Inorganica Chimica Acta
To appear in:
Received Date:
Revised Date:
Accepted Date:
16 February 2016
12 April 2016
15 April 2016
Please cite this article as: M. Al-Noaimi, A. Hammoudeh, M. El-khateeb, F.F. Awwadi, D. Taher, A. Mansi, O.S.
Abdel-Rahman, Keto-enol Tautomers of Mixed-Ligand Ruthenium(II) Complexes Containing α-Diamine and
Azoimine Bearing Alkyne Group Ligands, Inorganica Chimica Acta (2016), doi: http://dx.doi.org/10.1016/j.ica.
2016.04.029
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Keto-enol Tautomers of Mixed-Ligand Ruthenium(II) Complexes
Containing α-Diamine and Azoimine Bearing Alkyne Group Ligands
Mousa Al-Noaimia*, Ayman Hammoudehb, Mohammad El-khateebc, Firas F.
Awwadid, Deeb Taherd, Ahmad Mansia, Obadah S. Abdel-Rahmane
aDepartment of Chemistry, Hashemite University, P.O. Box 150459, Zarqa-13115-Jordan
bDepartment of Chemistry, Yarmouk University, P.O. Box 566, Irbid, Jordan
cDepartment of Chemistry, Jordan University of Science and Technology, Irbid 22110, Jordan
dDepartment of Chemistry, The University of Jordan, Amman 11942, Jordan
eFachbereich Chemie der Universität Konstanz, Universitätstraße 10, D-78457 Konstanz,
Germany
1

Abstract
Refluxing
azoimine
ligand
containing
terminal
acetylene
group
(L=
≡CH) with RuCl
C6H5N=NC(COCH3)=NC6H4C 3.3H2O in ethanol resulted hydrating the
terminal
acetylene
group
to
the
corresponding
enol
form
C6H5N=N-
C(COCH3)=NC6H4C(OH)=CH2 (L1), ketone form C6H5N=NC(COCH3)=NC6H4COCH3 (L2)
and vinyl chloride: C6H5N=N=C(COCH3)=N–C6H4C(Cl)=CH2 (L3) via Markovnikov
selectivity. Ruthenium complexes of the later ligands and α-diamines of the formula trans-
[Ru(N-N)(Y)Cl2] (Y = L1, N-N = 4,4'-dimethoxy-2,2'-bipyridine; dmeb (1), 1,10-
phenanthroline; phen (2), 3,4,7,8-tetramethyl-1,10-phenanthroline; tmphen (3), Y = L2, N-N =
dmeb (4), phen (5), tmphen (6)) were synthesized from L, N-N ligands and RuCl3.3H2O.
1
Complexes 1-6 were characterized by spectroscopic (IR, UV–Vis, H-, 13C-NMR, DEPT-135)
and electrochemistry techniques. The crystal structures of trans-[Ru(tmphen)(L1)Cl2] (3) and
trans-[Ru(bpy)(L3)Cl2] (7) were determined and found to have distorted octahedral geometry.
The catalytic activity of 3 towards the hydration of cinnamaldehyde is reported.
Key words: Ruthenium, Catalytic Hydration, Keto-enol tautomers, Spectroelectrochemistry,
Electrochemistry.
---------------------------------------------------------------------------------------------------------------
*Corresponding authors. Fax: +962 (5) 3826613.
E–mail address: manoaimi@hu.edu.jo (M. Al-Noaimi)
2

1. Introduction
The addition of water to the triple bond of non-activated alkynes, in presence of
electrophilic catalysts, results in the formation of carbonyl derivatives [1-3]. Historically,
mercuric ions in dilute acidic conditions were used as catalysts for the synthesis of ketones
from terminal and internal alkynes [3]. Due to the environmental concerns of mercury, other
transition-metal-complexes such as Au(I) [1,2], Ru(III) [4], Rh(II) [5], Pd(II) [6], Os(II) [7]
and other metals [8, 9] have been used. For instance, when phenylacetylene and p-toluene
sulfonic acid were reacted, in dichloromethane in the presence of catalytic amount of FeCl3,
acetophenone was selectively obtained in a good yield in addition to 2-vinyl chlride as a by-
product [9]. Ru(II) complexes, in the presence of appropriate auxiliary phosphine ligands,
have efficiently catalyzed the anti-Markovnikov addition of oxygen to terminal alkynes,
yielding mainly aldehydes [10].
As part of our continuing interest in the ruthenium azoimine chemistry [11-15], we
synthesized the terminal acetylene azoimine ligand C6H5NH-N=C(COCH3)NHC6H4C≡CH
(L) [15]. When L reacted with RuCl3.3H2O, the terminal acetylene group of the ligand (L)
was catalytically hydrated via Markovnikov addition to the corresponding enol (L1) and
keto (L2) forms. Two mixed-ligand complexes, trans-[Ru(bpy)(L1)Cl2] and
[Ru(bpy)(L2)Cl2], were prepared and characterized [18]. In this work, mixed-ligand
ruthenium complexes with α-diamines (N-N = 1,10-phenanthroline (phen), 4,4'-dimethoxy-
2,2'-bipyridine (dmeb), 3,4,7,8-tetramethyl-1,10-phenanthroline (tmphen)) having the
general formula trans-[Ru(N-N)(Y)Cl2] (1-6) (Y =L1, L2) were prepared to study the effect
of the N-N ligands on the electronic properties of the ruthenium center. The effect of the
substituents on the N-N ligands will be monitored by the shift of the Ru(III/II) couple and
the low energy metal-ligand charge transfer for these complexes. Complex 3 will be
investigated as a catalyst in the hydrogenation of cinnamaldehyde.
3

2. Experimental
2.1. Materials
Ruthenium trichloride trihydrate (RuCl3.3H2O), 1,10-phenanthroline (phen), 4,4'-
dimethoxy-2,2'-bipyridine (dmeb), 3,4,7,8-tetramethyl-1,10-phenanthroline (tmphen),
tetrabutylammonium hexafluorophosphate (TBAPF6) were purchased from Aldrich. C6H5–
NHN=C(COCH3)NHC6H4C≡CH (L) was prepared following the reported procedure [15].
2.2. General Procedure for the Preparation of trans-[Ru(N-N)(L)(Cl2)](1-6);
RuCl3.3H2O (0.27 g, 1.0 mmol) and the ligand L (0.25 g, 1.0 mmol) were dissolved in 100
mL of absolute ethanol. The mixture was refluxed for 1 h after which 1.0 mmol of (N-N)
ligand was added to the solution. The reaction mixture was refluxed for additional 2 h then
the solvent was removed under reduced pressure. The volume of the reaction mixture was
reduced to ca. 20 mL and then diethyl ether was added to precipitate the product. The
product was dissolved in a minimum volume of CH2Cl2 and subjected to chromatographic
separation on a silica gel column (60–120 mesh). Red bands of the [Ru(bpy)(L3)Cl2] (7),
trans-[Ru(N-N)(L1)Cl2] (1-3) and trans-[Ru(N-N)(L2)Cl2] (4-6) were obtained by
gradually increase the amount of acetone as mobile phase from the mixture of 3:1 (V:V)
of acetone/hexane to pure acetone. Upon slow evaporation of the solvent a few red crystals
of complexes 3 and 7 were found. Complex 7 was obtained only in a trace amount and
characterized only by x-ray structure determination.
2.2.1.trans-[Ru(dmeb)(L1)Cl2] (1): Yield: 0.17
g
(25%). Anal. Calcd. for
C29H27Cl2N5O4Ru: C, 51.11; H, 3.99; N, 10.28. Found: C, 51.16; H, 4.05; N, 10.34. UV-
Vis in CH2Cl2 [λmax/nm (εmax/ M-1cm-1)]: 256 (26.13×103), 313 (16.18×103), 389 (6.53×103),
1
505 (4.70×103). IR (KBr, cm-1): 1449 (v N=N), 1590 (v C=N), 1691(v C=O). H-NMR (300
4

MHz, CDCl3): 8.08 (2H, d, H3), 7.73 (3H, m, H4, H5), 7.58 (3H, m, dmeb), 7.54 (1H, d,
H1), 7.40 (2H, d, H2), 6.93 (1H, d, dmeb), 6.5 (2H, m, dmeb), 5.93 (1H, s, =CHa), 5.67
(1H, s, =CHb), 4.70 (1H, s, C(OH)=C), 3.93 (3H, s, OCH3), 3.92 (3H, s, OCH3), 2.84 (3H,
s, COCH3). 13C-NMR (300 MHz, CDCl3): 189.1(C=O), 135.3 (C(OH)=C), 113.0 (=CH2),
56.17 (OCH3), 56.14 (OCH3) , 30.2, (COCH3).
2.2.2. trans-[Ru(phen)(L1)Cl2] (2): Yield: 0.141 g (22%). Anal. Calcd. for
C29H23Cl2N5O2Ru: C, 53.96; H, 3.59; N, 10.85. Found: C, 53.85; H, 3.62; N, 10.90. UV-Vis
in CH2Cl2 [λmax/nm (εmax/ M-1cm-1)]: 316 (26.65×103), 379 (15.72×103), 510 (12.09×103).
IR (KBr, cm-1): 1448 (v N=N), 1591 (v C=N), 1701 (v C=O). 1H-NMR (300 MHz, CDCl3):
8.34 (1H, d, phen), 8.32 (1H, d, phen ), 8.18 (3H, m, H3, phen), 7.97 (1H, d, phen), 7.87
(2H, d, H1), 7.79 (1H, t, H5), 7.71 (2H, d, H2), 7.61 (2H, t, H4), 7.58 (4H, m, phen), 5.94
(1H, s, =CHa), 5.71 (1H, s, =CHb), 4.77 (1H, s, C(OH)=C) 2.52 (3H, s, COCH3). 13C-NMR
(300 MHz, CDCl3): 189.0 (C=O), 135.7 (C(OH)=C113.2 (=CH2), 31.70 (COCH3).
2.2.3. trans-[Ru(tmphen)(L1)Cl2] (3): Yield: 0.092 g (13.1%). Elem. Anal. Calcd. For
C33H31Cl2N5O2Ru: C, 56.49; H, 4.45; N, 9.98. Found: C, 56.44; H, 4.40; N, 9.96. UV-Vis in
CH2Cl2 [λmax/nm (εmax/ M-1cm-1)]: 271 (28.4×103), 321 (6.65×103), 381 (4.23×103), 510
1
(2.95×103). IR (KBr, cm-1): 1439 (v N=N), 1611(v C=N), 1673 (v C=O). H-NMR (300
MHz, CDCl3): 8.11 (2H, d, H3), 8.01 (2H, d, H1), 7.81 (3H, m, H4, H5), 7.67 (1H, s,
tmphen), 7.60 (4H, m, H2, tmphen), 7.17 (1H, s, tmphen), 5.93 (1H, s, =CHa), 5.70 (1H, s,
=CHb), 4.70 (1H, s, C(OH)=C), 2.87 (3H, s, COCH3), 2.60 (6H, s , tmphen-CH3), 2.10 (3H,
13
s , tmphen-CH3), 2.10 (3H, s , tmphen-CH3), C-NMR (300 MHz, CDCl3): (C(OH)=C),
113.18 (=CH2), 30.3 (COCH3) , 18.10 (tmphen-CH3), 18.08 (tmphen-CH3), 14.63 (tmphen-
CH3), 14.55 (tmphen-CH3).
5

2.2.4. trans-[Ru(dmeb)(L2)Cl2] (4): Yield: 0.242 g (36%). Elem. Anal. Calcd. for
C29H27Cl2N5O4Ru: C, 51.11; H, 3.99; N, 10.28. Found: C, 51.02; H, 3.84; N, 10.26. UV-Vis
in CH2Cl2 [λmax/nm (εmax/ M-1cm-1)]: 267 (26.0×103), 369 (5.3×103), 502 (0.50×103). IR
1
(KBr, cm-1): 1455 (v N=N), 1627 (v C=N), 1672 (v C=O), 1692 (v C=O). H-NMR (300
MHz, CDCl3): 8.11 (2H, d, H3), 7.74 (3H, m, H4, H5), 7.42 (3H, m, dmeb), 6.98 (1H, d,
H1), 6.54 (2H, d, H2), 6.42 (1H, d, dmeb), 6.4 (2H, m, dmeb), 4.04 (6H, s, OCH3), 2.84
(3H, s, COCH3), 2.74 (3H, s, COCH3). 13C-NMR (300 MHz, CDCl3): 195.4 (C=O), 189.22
(C=O), 56.40 (OCH3), 56.38 (OCH3), 30.2 (COCH3), 27.3 (COCH3).
2.2.5. trans-[Ru(phen)(L2)Cl2] (5): Yield: 0.199 g (30.8%). Elem. Anal. Calcd. for
C29H23Cl2N5O2Ru: C, 53.96; H, 3.59; N, 10.85. Found: C, 53.79; H, 3.66; N, 10.94. UV-
Vis in CH2Cl2 [λmax/nm (εmax/ M-1cm-1)]: 318 (26.4×103), 380 (14.82×103), 512 (12.35×103).
IR (KBr, cm-1): 1449 (v N=N), 1588 (v C=N), 1678 (v C=O), 1700 (v C=O). 1H-NMR (300
MHz, CDCl3): 8.33 (1H, d, phen), 8.32 (1H, d, phen ), 8.16 (3H, m, H3, phen), 7.97 (1H, d,
phen), 7.88 (2H, d, H1), 7.80 (1H, t, H5), 7.70 (2H, d, H2), 7.60(2H, t, H4), 7.54 (4H, m,
phen), 2.88 (3H, s, COCH3), 2.74 (3H, s, COCH3). 13C-NMR (300 MHz, CDCl3): 198.4
(C=O), 192.3 (C=O), 29.2 (COCH3), 26.8 (COCH3).
2.2.6. trans-[Ru(tmphen)(L2)Cl2] (6): Yield: 0.243 g (34.5%). Elem. Anal. Calcd. for
C33H31Cl2N5O2Ru: C, 56.49; H, 4.45; N, 9.98. Found: C, 56.39; H, 4.41; N, 9.88. UV-Vis in
CH2Cl2 [λmax/nm (εmax/ M-1 cm-1)]: 271 (28.48×103), 323 (6.04×103), 383 (3.96×103), 506
(3.08×103). IR (KBr, cm-1): 1444 (v N=N), 1617 (v C=N), 1699 (v C=O), 1704 (v C=O). 1H-
NMR (300 MHz, CDCl3): 8.20 (2H, d, H3), 8.12 (2H, d, H2), 8.02 (2H, m, tmphen), 7.81
(1H, t, H5), 7.70 (3H, m , H1, tmphen), 7.61 (2H, t, H4), 7.11 (1H, d, tmphen), 2.88 (3H, s,
6

COCH3), 2.76 (3H, s, COCH3), 2.62 (6H, s , tmphen-CH3), 2.12 (3H, s , tmphen-CH3), 2.05
13
(3H, s , tmphen-CH3). C- NMR (300 MHz, CDCl3): 197.14 (C=O) , 189.07 (C=O), 30.2
(COCH3), 26.9 (COCH3), 18.12 (tmphen-CH3), 18.06 (tmphen-CH3), 14.66 (tmphen-CH3),
14.57 (tmphen-CH3).
2.3. Instrementation
NMR spectra were obtained using a Bruker Avance 300 spectrometer. Microanalytical data
were collected on an Eurovector E.A.3000 instrument using thin sample-tubes. Infrared
spectra were recorded on JASCO 420. UV–Vis spectral studies were performed using a
TIDAS Fiberorptic diode array spectrometer. Cyclic voltammetric measurements were
carried out using a BAS CV-27 voltammograph.. A platinum wire working electrode, a
platinum wire auxiliary electrode and Ag wire reference electrode were used in a standard
three-electrode configuration. Tetrabutylammonium hexafluorophosphate (0.1 M) was used
as the supporting electrolyte; the scan rate used was 0.1V s-1 in acetonitrile under N2
atmosphere. Referencing was done with the addition of 1 mg of ferrocene [16, 17]. The
spectroelectrochemistry of complex 3 was investigated as a representative using an optically
transparent thin layer electrode (OTTLE) cell [18].
7

2.4. Crystallography
Details of crystal analysis, data collection and structure refinement data for complexes 3
and 7 are given in Table 1. Crystals mounting was done on glass fibers with epoxy cement.
Single crystal data collections were performed on Xcalibur/Oxford Diffractometer using Mo
tube (λ = 0.71073 Å) as X-ray source. CrysAlis Pro software was used for data collection,
absorption correction and data reduction to give SHELX-format-hkl files [19]. The structures were
solved and refined using SHELXTL program package [20]. In the crystal structure of complex 7,
two restraints were applied; the anisotropic displacement parameters of the atoms C16 and
C17 in the direction of C16-C17 bond are restrained to be equal, similarly, these parameters
of atoms C16 and C13 are restrained to be equal along C16-C13 bond.
2.5. Procedure for Catalytic Hydrogenation of Cinnamaldehyde
The liquid-phase hydrogenation of cinnamaldehyde (1:1 cis-to-trans mixture) was
performed in a stainless steel hydrogenator (Parr-4842) equipped with a Waltow-945 unit to
control the temperature of reactions and the stirring rate. The reaction solution, prepared by
dissolving the desired amounts of 3 (0.040 g), potassium hydroxide (0.047) and
cinnamaldehyde (0.80 g) in 100 ml 2-propanol, is placed in the reaction vessel and the
hydrogenator is tightly closed. The reactor temperature is adjusted at 86 oC before admitting
the hydrogen gas at 3 bars. Samples from the reaction solution are taken at various time
intervals and analyzed by gas chromatography (Agilent, FID, 30-m crosslinked FAFF
capillary column).
8

3. Results and Discussion
3.1. Synthesis
Treatment of azoimine ligand has a terminal acetylene group (L: Ph–NH-N=C(COCH3)-
NH–PhC≡CH) with RuCl3.3H2O in refluxing absolute ethanol offered a catalytic hydration
of the triple bond via Markovnikov addition to corresponding enol (L1: Ph–NH-
N=C(COCH3)-NH–PhC(OH)=CH2) and ketone (L2: Ph–NH-N=C(COCH3)-NH–
Ph(COCH3) in addition to trace amount of the vinyl chloride (L3: Ph–NH-N=C(COCH3)-
NH–PhC(Cl)=CH2) (Scheme1). The proposed mechanism for the hydration of acetylene
involves the dissociation of one chloride ligand from starting Ru(III)Cl3.nH2O complex,
generating Ru(III)Cl2.nH2O active species. The catalytic hydration starts by the activation
of the acetylene group through Ru(III)-π interactions [21]. Enol and keto form may have
obtained by the hydration of the coordinated alkyne by addition of one water molecule to
the internal position of the triple bond [22, 23]. The vinylchloride complex is believed to
have been generated by the nucleophilic attack of the dissociated chloride ligand on the
activated acetylene group.
COCH3
OH
N
RuCl3.nH2O
Ethanol
Cl
R
N
H
N
H
+
R-COCH3
+
R
L1
L2
L3
L
= R
Scheme1: catalytic hydration products
Mixed-ligand ruthenium complexes built from α-diamine ((N-N) where N-N= 1,10-
phenanthroline (phen), 4,4'-dimethoxy-2,2'-bipyridine (dmeb), 1,3,4,7,8-tetramethyl-1,10-
phenanthroline (tmphen)] having a general formula of trans-[Ru(N-N)(Y)Cl2] (1-6) (where
Y =L1, N-N= phen (1), dmeb (2), tmphen (3) and Y=L2, N-N= phen (4), dmeb (5), tmphen
9

(6)) were achieved by the stepwise addition of equimolar amounts of (L) and N-N ligands to
RuCl3.3H2O in absolute ethanol (Scheme 2).
Scheme 2: Synthesis of trans-[Ru(N-N)(Y)Cl2] (1-6)
In the carbonyl region of the IR spectra, complexes 1-3 showed an intense band in the range
of 1673-1701 cm-1 corresponding to the acetyl group while complexes 4-6 exhibited two
bands (1672-1699 and 1792-1704 cm-1) for the two acetyl groups types. Complexes 1-3
were confirmed by the appearance of two doublets for the vinyl protons (Ha and Hb) at δ
5.93-5.94 and 5.67-5.716 ppm, one exchangeable singlet for the hydroxyl proton (4.70-4.77
ppm) and one negative signal for the vinyl carbon at 113.0-113.2 ppm in their 13C-NMR
1
(distortionless enhancement by polarization transfer (DEPT-135)) spectra. In the H NMR
spectra of complexes 4-6, two CH3 singlets in the ranges of 2.84-2.88 ppm and 2.74-2.76
ppm are observed. The carbons of these methyl groups are presented in the ranges of 29.2-
10

13
30.2 and 26.8-27.3 ppm in the C NMR spectra. In addition, two signals in the ranges of
198.4-194.4 and 189.1-189.3 ppm corresponding to the carbonyl carbons are also shown.
3.2. Crystal Structures
Single crystals of the complexes 3 and 7 are obtained by slow diffusion of hexane into a
dichloromethane solution of the complexes. Selected bond distances and bond angles are
given in Table 2. The ORTEP drawings with numbering schemes of these complexes are
shown in Figure 1 and Figure 2, respectively. The geometry around the Ru atom of
complex 3 and 7 occupies a pseudo octahedral coordination which consists of two trans
chloride ligands and four nitrogen donor atoms. For complex 3, all the atoms in the side-
chain C(17), C(16) and O(2) are coplanar. The bond length C(17)–C(16) of 1.434(17) Å is
comparable to distinct C–C bonds in asymmetric enolized 1,3-diones [24-26]. The C13-C16
bond length of 1.50 Å, is a typical distance for C-C single bond. Furthermore, the dihedral
angle of 26.81 between the enol plane and the plan of the benzene ring attached to the
azoimine skeleton indicates minimal conjugation between the two groups. The enol O(2)-
C(16) distance is 1.478(17) Å while the ketone O(1)-C(8) bond distance is 1.176(11) Å. The
bite-angles for the five membered ring for coordinated L1 N(4)-Ru(1)-N(5), 76.9(3)º and
imine ligand tmphen N(1)-Ru(1)-N(3), are almost equivalent.
The average Ru–N(azo) and Ru–N(methine) distances of the azomethine ligand of complex
7 is 1.983 Å. The Ru–N(bpy) bond length of 2.127 Å which is slightly shorter than the RuII-
bpy bond in trans-[Ru(bpy)(L2)Cl2] (average, 2.147 Å) [15]. The atoms in the side-chain
C(17), C(16) and Cl(3) are coplanar. The C(17)–C(16) bond length in complex 7 is 1.314(9)
Å is shorter than the corresponding bond in complex 3. Furthermore, the dihedral angle
between the vinyl chloride plane and the phenyl ring attached to the azoimine skeleton is
19.0° indicating minimal conjugation between the two groups.
11

The Ru–N(tmphen) bond for in complex 3 (2.1165 Å) are slightly shorter than the Ru–
N(bpy) bonds (2.127 Å) in 7 revealing that there is no significant difference in the strength
of the coordination for the two ligands. The average Ru–Cl bond lengths (for 3: 2.358, 7:
2.3619 Å) are comparable to those reported for similar systems (2.389–2.401 Å) [11-15].
3.3. Electrochemistry
The Ru(III/II) couple of complexes 1-6 was tested by cyclic voltammetric measurements
and the results are presented in Table 3. The cyclic voltammogram of complex 3 is shown
in Figure 3 and exhibits a reversible oxidative response around 0.60 V vs. Cp2Fe0/+ which
has been assigned to Ru(III/II) oxidation. The irreversible couple at negative potential is
assigned to the ligand (0/-1) reduction which, however is less anodic than the corresponding
peak of the keto complex 6.
Table 3 shows that the half-wave potential of the phenanthroline complexes was shifted
anodically by ~ 40 mV compared to those observed in the bipyridine complexes. This may
indicates that the donor ability of phenanthroline is slightly weaker than that of bipyridine.
The Ru(III/II) couple is slightly affected by changing the substituent on the phenanthroline
and 2,2'-bipyridine ligands and is shifted to more negative potentials upon replacing the
hydrogen atoms by donating methyl group (Table 3).
12

3.4 Electronic Structure
As a representative example, Figure 4 shows the UV–Vis spectrum of complex 3 in
dichloromethane. The lowest energy band at 512 nm was assigned to metal ligand charge
transfer (MLCT). The high intensity and energy band at 284 nm may be attributed to a
ligand-to-ligand charge transfer LLCT (π-π* (phenyl ring) and n-π* (azomethine (C=N))
transitions. Figure 4 shows the stepwise oxidation of complex 3 in dichloromethane to 3+,
there is a decrease in the intensity of the MLCT centered at 512 nm and a grow of two new
bands at 450 nm and 650 nm. The latter band may result from the Cl- (pπ)→Ru(III)(dπ)
ligand to metal charge transfer (LMCT), while the former may assigned to L1(π) →
Ru(III)(dπ) LMCT.
Stepwise reduction at negative potentials for complex 3 in dichloromethane to 3- causes the
the azoimine-targeted MLCT absorption band to disappear (Figure 5) and a very intense
band in the UV and NIR-region to appear. The intense band in the NIR region is assigned to
L-(π)→Ru(dπ*) LMCT while the bands around 450 and 350 nm may have resulted from
LLCT.
3.5. Hydrogenation of Cinnamaldehyde (CALD)
Cinnamaldehyde has been selected as a model substrate for hydrogenation. Scheme 3
represents the various hydrogenation routes that CALD may undergo [27]. The
hydrogenation of the C=O bond yields the highly desirable cinnamyl alcohol (CALC) while
that of the C=C bond produces the less desirable hydrocinnamaldehyde (HCALD).
Consecutive hydrogenation of CALC and HCALD gives phenyl propanol (PP). Complex 3
was tested as a representative example with respect to the catalytic activity of the Ru(II)
complexes for the hydrogenation of cinnamaldehyde in the liquid phase.
13

Cinnamaldehyde (CALD)
Hydrocinnamaldehyde (HCALD)
O
O
II
I
OH
OH
III
IV
Phenyl propanol (PP)
Cinnamyl alcohol (CALC)
Scheme 3: Cinnamaldehyde hydrogenation.
The progress of cinnamaldehyde hydrogenation using complex 3 as a catalyst is depicted in
Figure 6. The ratio of substrate (CALD): co-catalyst (NaOCH3): catalyst (3) is 68:22:1. The
reaction was conducted at 86 oC under hydrogen pressure of 4 atm. In the first 120 min. of
the reaction, CALD concentration changes very slowly. This is the stage where the actual
catalyst is being produced. The hydrogenation of CALD follows thereafter a first order
kinetics with a rate constant of 1.56×10-3 min-1. The formation of CALC obeys also first-
order kinetics with a rate constant of 4.00×10-4 min-1. There is no indication that CALC
undergoes consecutive hydrogenation to PP, on contrary to HCALD whose concentration
goes through a maximum, as expected for intermediates, after 500 min. of reaction. The
catalytic efficiency of complex 3 is however very small and the turn-over-frequency (TOF,
number of substrate molecules reacted per catalytic site per time) after 550 min. of reaction
is 3 h-1. This TOF is very small compared to that observed for Ru(bipy)(C6H5N=N–
C(COCH3)=NC6H4SPh)Cl2 that contains a thiophenyl group instead of the enol
functionality in the azoimine ligand for which a TOF value of 42 h-1 was observed at a
substrate: catalyst ratio of 84:1 [28]. In addition to the low activity (expressed by
conversion) of complex 3 (Table 4), the selectivity to CALC is also rather small (~33-37%).
14

The TOF for the formation of CALC is thus only 1 h-1 compared to 40 h-1 observed for the
other catalysts reported for which the CALC selectivity reached 95%.
Conclusion
The reaction of azoimine ligand bearing a terminal acetylene group, L= Ph–NH-
N=C(COCH3)-NH–C6H4C≡CH, with RuCl3.3H2O in absolute ethanol resulted in the
catalytic hydration of the terminal acetylene group via Markovnikov selectivity to the
corresponding enol (L1) , ketone (L2) in addition to trace amount of the vinyl chloride (L3).
Mixed-ligand ruthenium complexes having the general formula trans-[Ru(N-N)(Y)Cl2] (1-
6) (Y = L1, N-N= 4,4'-dimethoxy-2,2'-bipyridine; dmeb (1), 1,10-phenanthroline; phen (2),
3,4,7,8-tetramethyl-1,10-phenanthroline; tmphen (3) and Y = L2, N-N= dmeb (4),; phen (5),
tmphen (6)) were synthesized. The crystal structures of trans-[Ru(tmphen)(L1)Cl2] (3) and
trans-[Ru(bpy)(L3)Cl2] (7) were determined. The UV-Vis spectra of 3, 3+ and 3- in
dichloromethane has been obtained and discussed.
Acknowledgements
M. Al-Noaimi would like to thank the Hashemite University (Jordan) for research support.
Appendix A. Supplementary data
CCDC 1439361 (for 3) and 1439360 (for 7) contain the supplementary crystallographic
data for the two complexes. These data can be obtained free of charge via
http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge Crystallographic
Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or e-
mail: deposit@ccdc.cam.ac.uk.
15

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11925.
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10. T. Suzuki, M. Tokunaga, Y. Wakatsuki, Org. Lett. 3 (2001) 735.
11. M. Al-Noaimi, M. El-khateeb, S. Haddad, M. Sunjuk, R. Crutchley, Polyhedron 27
(2008) 3239
12. M. Al-Noaimi, R.J. Crutchley, M. AlDamen, A. Rawashdeh, M.A. Khanfar, K. Seppelt,
Polyhedron 30 (2011) 2075.
13. M. Al-Noaimi, M.A. AlDamen, Inorg. Chim. Acta 387 (2012) 45.
14. M. Al-Noaimi, M. El-khateeb, I. Warad, S.F. Haddad, Inorg. Chim. Acta 400 (2013) 20.
15. M. Al-Noaimi, F.F. Awwadi, A. Mansi, S. Abdel-Rahman, A. Hammoudeh, I. Warad,
Spectrochim. Acta, Part A: Mol. Biomol. Spec. 135 (2015) 828.
16. M. Krejcik, M. Danek, F. Hartl, J. Electroanal. Chem. 317 (1991) 179.
17. C. Nataro, A.N. Campbell, M.A. Ferguson, C.D. Incarvito, A.L. Rheingold, J.
Organomet. Chem. 673 (2003) 47.
18. T. Gennett, D.F. Milner, M.J. Weaver, J. Phys. Chem. 89 (1985) 2787.
19. CrysAlisPro, Agilent Technologies, Version 1.171.35.11 (Release 16–05–2011
CrysAlis171.NET) (Compiled May 16 2011, 17:55:39).
20. SHELXTL (XCIF, XL, XP, XPREP, XS), version 6.10, Bruker AXS Inc.: Madison, WI,
2002.
16

21. J. Halpern, B.R. James, A.L. W. Kemp, J. Am. Chem. Soc. 83(1961) 4097.
22. I.K. Meier, J.A. Marsella, J. Mol. Cat. 78 (1993) 31.
23. C.M. Krauter, A.S.K. Hashmi, M. Pernpointner. Chem. Cat. Chem. 2 (2010) 1226
24. W. Bell, J.A. Crayston, C. Glidewell, M.A. Mazid, M.B. Hursthouse, J. Organomet.
Chem. 434 (1992) 115.
25. A. Mostad, U. Pedersen, P.B. Rasmussen, S. Lawesson, Acta Chem. Scand. 37b (1983)
901.
26. L.A.M. Baxter, A.J. Blake, G.A. Heath, T.A. Stephenson, Acta Cryst. C, Crystal Struct.
Comm. 46 (1990) 508.
27. I. Warad, M. Al-Noaimi, O.S. Abdel-Rahman, M. AlDamen, B. Hammouti, T.B.
Hadda, Polyhedron 63 (2013)182.
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58.
17

Table 1. Crystal data and structure refinement parameters for 3 and 7.
3
7
Empirical formula
Formula weight
Temperature
C33H31Cl2N5O2Ru
701.60
C27H22Cl3N5ORu
639.92
293(2) K
293(2) K
0.71073 Å
Triclinic
P-1
Wavelength
0.71073 Å
Monoclinic
P 1 21/n 1
Crystal system
Space group
Unit cell dimensions
a=13.6384(14),b=15.8352(11)Å a=8.8814(7) Å, b=12.1926(10)Å,.
c=15.8432(16)Å c=13.8803(13)Å,
α= 90, β= 06.362(12)°, γ= 90° α=80.964(7),β=77.056(7),γ= 72.185(7)°
3
3
Volume
3283.0(5) Å
4
1388.3(2) Å
2
Z
3
3
1.531 Mg/m
Density (calculated)
Absorption coefficient
F(000)
1.419 Mg/m
-1
-1
0.677 mm
1432
0.883 mm
644
2.97 to 25.00°.
3.03 to 26.30°
Theta range for data
collection
Index ranges
-16≤h≤15, -18≤k≤16, -18≤l≤18
15471
-11≤h≤9, -15≤k≤10, -17≤l≤17
10061
Reflections collected
Independent reflections
5769 [R(int) = 0.0731]
5632 [R(int) = 0.0533]
99.9 %
Completeness to theta = 99.8 %
26.30°
Absorption correction
Max. and min.
Semi-empirical from equivalents Semi-empirical from equivalents
1.0 and 0.98819
1.00000 and 0.75059
transmission
2
2
Refinement method
Full-matrix least-squares on F
/ 5769 / 0 / 388
Full-matrix least-squares on F
5632 / 2 / 334
Data
/
restraints
parameters
2
Goodness-of-fit on F
1.036
0.993
Final
R
indices R1 = 0.0787, wR2 = 0.1578
R1 = 0.0654, wR2 = 0.1058
[I>2sigma(I)]
R indices (all data)
R1 = 0.1553, wR2 = 0.2013
R1 = 0.1393, wR2 = 0.1368
-3
-3
Largest diff. peak and 1.000 and -0.433 e.Å
hole
0.448 and -0.638 e.Å
2
2 2
R1 = Σ||Fo| - |Fc||/Σ|Fo|; wR2 = {Σ[ w(Fo - Fc2)2]/ Σ [w(Fo ) ]}1/2
18

Table 2. Selected bond Lengths and angles for complexes 3 and 7
3
7
Bond lengths( Å )
Ru(1)-N(1)
Ru(1)-N(3)
Ru(1)-N(5)
Ru(1)-N(4)
Ru(1)-Cl(2)
Ru(1)-Cl(1)
O(1)-C(8)
1.941(6)
2.006(6)
2.117(7)
2.166(6)
2.352(2)
2.369(2)
1.176(11)
0.8200
Ru(1)-N(1)
Ru(1)-N(3)
Ru(1)-N(5)
Ru(1)-N(4)
Ru(1)-Cl(1)
Ru(1)-Cl(2)
N(1)-N(2)
1.954(5)
2.012(5)
2.103(5)
2.151(5)
2.3545(17)
2.3694(16)
1.320(6)
1.445(8)
1.341(8)
1.314(9)
1.745(8)
O(2)-H(2C)
C(17)-C(16)
O(2)-C(16)
N(1)-N(2)
N(1)-C(1)
1.434(17)
1.478(17)
1.316(8)
1.317(10)
N(5)-C(27)
C(16)-C(17)
C(16)-Cl(3)
N(3)-C(7)
Bond angles (°)
N(1)-Ru(1)-N(3)
N(1)-Ru(1)-N(5)
N(3)-Ru(1)-N(5)
N(1)-Ru(1)-N(4)
N(3)-Ru(1)-N(4)
N(5)-Ru(1)-N(4)
N(1)-Ru(1)-Cl(2)
N(3)-Ru(1)-Cl(2)
N(5)-Ru(1)-Cl(2)
N(4)-Ru(1)-Cl(2)
N(1)-Ru(1)-Cl(1)
N(3)-Ru(1)-Cl(1)
N(5)-Ru(1)-Cl(1)
N(4)-Ru(1)-Cl(1)
76.4(3)
N(1)-Ru(1)-N(3) 75.8(2)
N(1)-Ru(1)-N(5) 102.8(2)
N(3)-Ru(1)-N(5) 175.7(2)
N(1)-Ru(1)-N(4) 175.26(19)
N(3)-Ru(1)-N(4) 105.7(2)
N(5)-Ru(1)-N(4) 76.00(19)
N(1)-Ru(1)-Cl(1) 94.56(15)
N(3)-Ru(1)-Cl(1) 89.95(14)
N(5)-Ru(1)-Cl(1) 86.09(14)
N(4)-Ru(1)-Cl(1) 89.95(13)
N(1)-Ru(1)-Cl(2) 92.32(15)
N(3)-Ru(1)-Cl(2) 93.16(15)
N(5)-Ru(1)-Cl(2) 90.97(14)
N(4)-Ru(1)-Cl(2) 83.14(13)
Cl(1)-Ru(1)-Cl(2) 172.97(6)
102.1(3)
172.5(3)
175.3(2)
105.3(3)
76.9(3)
93.75(19)
89.0(2)
83.74(18)
90.72(17)
91.65(19)
93.0(2)
94.32(18)
83.87(18)
Cl(2)-Ru(1)-Cl(1) 174.54(8)
O(1)-C(8)-C(7) 119.8(10)
19

Table 3: Electrochemical data for 1-6 in CH2Cl2/ TBAPF6 (0.1 mM); potentials are given
relative to the ferrocene/ferrocenium standard at v = 0.1 V/s
b
Complex
trans-[Ru(bpy)(L1)Cl2][18] 0.60
aRu(II/III)(V) Azo(0/-1)(V)
-1.07
-1.14
-1.18
-1.17
-1.06
-1.19
-1.11
-1.12
0.58
0.59
0.60
1
2
3
trans-[Ru(bpy)(L2)Cl2][18] 0.62
0.63
0.66
0.65
4
5
6
a reversible wave.
birreversible wave.
20

Table 4: The results of cinnamaldehyde hydrogenation using 3
time (min) HCALD% PP% CALD% CALC%
conv%
0
2.10
7.53
11.55
15.81
SCALC
-
64.41
59.80
63.16
64.56
%
0
0
0
100.00
97.90
92.47
88.45
84.19
0
60
0.59
2.58
3.47
4.24
0.16
0.44
0.78
1.36
1.35
4.50
7.30
10.20
120
180
240
21

Figure 1:.ORTEP drawing of 3.Thermal ellipsoids plots are reported at 30% probability.
22

Figure 2. ORTEP drawing of 7. Thermal ellipsoids plots are reported at 30%
probability.
23

Figure 3: Cyclic voltammogram of complex 1 vs. Cp2Fe0/+ (TBAPF6, 0.1M,
dichloromethane, 25˚C). Inset shows the Ru(II/III) at different scans rate.
24

2,5
2,0
1,5
1,0
0,5
0,0
200
300
400
500
600
700
800
Wavelength (nm)
Figure 4: Oxidation spectroelectrochemistry of complex 3 in dichloromethane
25

1,6
1,4
1,2
1,0
0,8
0,6
0,4
0,2
0,0
200
300
400
500
600
700
800
900
Wavelength (nm)
Figure 5: Reduction spectroelectrochemistry of complex 3 to 3- in dichloromethane
26

100
80
60
40
20
0
CALD%
HCALD%
CALC%
PP%
0
200
400
600
800
time (min)
Figure 6: The progress of the catalytic hydrogenation of CALD, Cinnamaldehyde:
0.6081 g in 100 mL i-propanol, 0.046 M; Co-catalyst: NaOCH3, 0.0810 g, 1.5×10-2 M;
Catalyst: complex 3, 0.0424 g, 6.8×10-4 M; Temperature: 86oC, Hydrogen pressure: 4
atm, reactant: co-cat : cat = 68 : 22 : 1
27

Refluxing
azoimine
ligand
(L=
C6H5N=NC(COCH3)=NC6H4C≡CH)
with
RuCl3.3H2O in ethanol resulted in catalytic
hydration of the terminal acetylene group to the
enol
form
C6H5N=N-
C(COCH3)=NC6H4C(OH)=CH2 (L1), the ketone
form C6H5N=NC(COCH3)=NC6H4COCH3 (L2)
and the vinyl chloride: C6H5N=N=C(COCH3)=N–
C6H4C(Cl)=CH2 (L3) via Markovnikov selectivity.
Ruthenium complexes of the formula trans-
[Ru(N-N)(Y)Cl2] (Y = L1, N-N = 4,4'-dimethoxy-
2,2'-bipyridine; dmeb (1), 1,10-phenanthroline;
phen (2), 3,4,7,8-tetramethyl-1,10-phenanthroline;
tmphen (3), Y = L2, N-N = dmeb (4), phen (5),
tmphen (6)) were made from L, N-N ligands and
RuCl3.3H2O. Complexes 1-6 were characterized
1
by spectroscopic techniques (IR, UV–Vis, H-,
13C-NMR, DEPT-135) and electrochemistry. The
crystal structures of trans-[Ru(tmphen)(L1)Cl2]
(3) and trans-[Ru(bpy)(L3)Cl2] (7) were
determined and found to have distorted octahedral
geometry. The catalytic activity of 3 towards the
hydration of cinnamaldehyde is described
28

Research highlights
•
•
Azoimine- ligand (L) with terminal acetylene.
Markovnikov hydration of the terminal acetylene in L to enol (L1), ketone (L2) and
the vinyl chloride (L3).
•
Mixed-ligand ruthenium complexes, trans-[Ru(N-N)(Y)Cl2] , N-N: α-diamine and Y:L1,
L2 and L3.
•
•
Crystal structures of the enol and vinyl chloride form.
A complex served as hydrogen transfer catalysts for cinnamaldehyde.
29