Bis(phenoxy-imine) ruthenium(II) carbonyl complexes: syntheses, structures and their catalytic activities for conversion of aldehydes to nitriles

Article abstract of DOI:10.1080/00958972.2020.1795646

Three new ruthenium(II) carbonyl complexes that bear phenoxy-imine chelate ligands, [RN = CH(C₆H₄O)]₂Ru(CO)₂ (1: R = 2,6-Me₂C₆H₃; 2: R = 4-OMeC₆H₄; 3: R = 4-ClC₆H₄) have been synthesized. These mononuclear ruthenium(II) complexes were fully characterized by elemental analysis, FT?IR, and ¹H and ¹³C NMR. Furthermore, the molecular structures of 1-3 were determined by X-ray crystal diffraction analysis. In the presence of NaHCO₃ as the base, the Ru carbonyl complexes showed moderate to good catalytic activities for the conversion of aldehydes to nitriles via the dehydration of aldoximes.

Full text of DOI:10.1080/00958972.2020.1795646

Journal of Coordination Chemistry
ISSN: (Print) (Online) Journal homepage: https://www.tandfonline.com/loi/gcoo20
Bis(phenoxy-imine) ruthenium(II) carbonyl
complexes: syntheses, structures and their
catalytic activities for conversion of aldehydes to
nitriles
Dong-Zhu Ma , Ying Li & Zhi-Qiang Hao
To cite this article: Dong-Zhu Ma , Ying Li & Zhi-Qiang Hao (2020): Bis(phenoxy-imine)
ruthenium(II) carbonyl complexes: syntheses, structures and their catalytic activities for conversion
of aldehydes to nitriles, Journal of Coordination Chemistry, DOI: 10.1080/00958972.2020.1795646
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JOURNAL OF COORDINATION CHEMISTRY
https://doi.org/10.1080/00958972.2020.1795646
Bis(phenoxy-imine) ruthenium(II) carbonyl complexes:
syntheses, structures and their catalytic activities for
conversion of aldehydes to nitriles
Dong-Zhu Maa, Ying Lib and Zhi-Qiang Haob
aDepartment of Environment and Chemical Engineering, Hebei College of Industry and Technology,
b
Shijiazhuang, China; College of Chemistry and Material Science, Hebei Normal University,
Shijiazhuang, China
ABSTRACT
ARTICLE HISTORY
Received 28 April 2020
Accepted 25 June 2020
Three new ruthenium(II) carbonyl complexes that bear phenoxy-
imine chelate ligands, [RN¼ CH(C6H4O)]2Ru(CO)2 (1: R ¼ 2,6-
Me2C6H3; 2: R ¼ 4-OMeC6H4; 3: R ¼ 4-ClC6H4) have been synthe-
sized. These mononuclear ruthenium(II) complexes were fully
characterized by elemental analysis, FT–IR, and 1H and 13C NMR.
Furthermore, the molecular structures of 1-3 were determined by
X-ray crystal diffraction analysis. In the presence of NaHCO3 as the
base, the Ru carbonyl complexes showed moderate to good cata-
lytic activities for the conversion of aldehydes to nitriles via the
dehydration of aldoximes.
KEYWORDS
Phenoxy-imine ligands;
ruthenium carbonyl
complexes; nitriles;
dehydration; cata-
lytic conversion
1. Introduction
Nitriles are one of the most important class of compounds that are widely used as
intermediates for the synthesis of agrochemicals, pharmaceuticals and biologically
active compounds [1–3]. The most well-known procedures for preparation of nitriles
include the reaction of aqueous ammonia with alcohols and the dehydration of
amides with the aid of dehydrating agents (such as acetonitrile, and silane) [4–8].
However, the efficiency of these common methods is limited owing to the high activa-
tion energy barrier and the formation of a large amount of waste, which is often haz-
ardous to the environment. Thus, the development of more sustainable and efficient
approaches for their production is always in demand.
CONTACT Zhi-Qiang Hao
haozhiqiang1001@163.com
College of Chemistry and Material Science, Hebei
Normal University, Shijiazhuang 050024, China
Supplemental data for this article is available online at https://doi.org/10.1080/00958972.2020.1795646.
ß 2020 Informa UK Limited, trading as Taylor & Francis Group

2
D.-Z. MA ET AL.
To overcome some of the above economic and environmental related problems,
the synthetic method of direct conversion of primary amines to nitriles via dehydro-
genation has received increased attention [9–11]. In 2016, Szymczak and co-workers
reported the synthesis of nitriles through acceptorless dehydrogenation of amines cat-
alyzed by an N,N,N-pincer ruthenium(II) complex [12]. Analogously, the aerobic oxida-
tive dehydrogenative reaction of primary amines to nitriles using RuII/TEMPO/O2
catalytic system has also been developed [13]. But, these reactions are usually carried
out at high temperature (ꢀ130 ꢁC) and long reaction times (ꢀ24 h) were also needed.
Another alternative approach for the synthesis of nitriles is dehydration of aldoximes
[14, 15]; the dehydration of aldoximes generated in situ from aldehydes and hydroxyl-
amine hydrochloride is an efficient protocol [16, 17], which can eliminate the pre-prep-
aration of aldoximes and the formation of water as the only by-product. For example,
Enthaler et al. reported the zinc-catalyzed conversion of aldehydes and hydroxylamine
hydrochloride to nitriles with high activity [18], and a variety of substituted benzalde-
hyde molecules could be converted to their corresponding nitriles in good to excellent
yields. The straightforward dehydration of aldoximes to the nitriles is still rare since
the rearrangement of aldoximes into primary amides also occurs during the reaction
[19, 20]. Hence, investigation of the synthesis of nitriles via dehydration of aldoximes
catalyzed by new transition metal catalysts is highly desired.
Recently, transition metal carbonyl complexes, especially ruthenium carbonyl com-
plexes, have attracted much attention because of their vital applications in the prepar-
ation of nitriles [13, 21]. Nagashima and co-workers reported an acenaphthene-based
triruthenium carbonyl cluster for dehydration of primary carboxamides to nitriles using
hydrosilane as dehydrating agent [22]. More recently, we utilized N,O-bidentate lig-
and-supported trinuclear ruthenium carbonyl complexes to catalyze aerobic oxidation
of primary benzylic amines to amides and nitriles with high activity and selectivity
[23]. However, the reactions catalyzed by mononuclear Ru carbonyl complexes and
the conversion of aldehydes to nitriles occurring through dehydration of aldoximes
are still rare. Herein, we report the synthesis and characterization of mononuclear
ruthenium(II) carbonyl complexes containing phenoxy-imine ligands. Furthermore,
these Ru complexes have been employed as catalysts for catalytic synthesis of nitriles
via the reaction of aldehydes with hydroxylamine hydrochloride.
2. Experimental
2.1. General considerations
All manipulations were carried out using standard high-vacuum-line techniques.
Solvents were distilled from appropriate drying agents under N2 before use.
Hydroxylamine hydrochloride, aniline and its derivatives were purchased from Boka
Chemical Co. Other reagents were purchased from Aldrich and used without purifica-
tion. 1H- and 13C-NMR spectra were recorded on a Bruker AV III-500 instrument in
CDCl3 and the chemical shifts are referenced to the residual protio impurity of the
deuterated solvent. IR spectra were recorded as KBr disks on a Thermo Fisher iS50
spectrometer. Elemental analyses were performed with a VarioEL III analyzer. Phenoxy-

JOURNAL OF COORDINATION CHEMISTRY
3
imine ligands RN ¼ CHC6H4OH (L1H: R ¼ 2,6-Me2C6H3; L2H: R ¼ 4-OMeC6H4; L3H: R ¼ 4-
ClC6H4) were synthesized according to the literature methods [24, 25].
2.2. Synthesis of bis((E)-2-(((2,6-
dimethylphenyl)imino)methyl)phenoxy)dicarbonyl ruthenium(II) Ru(j2-N,O-
L1)2(CO)2 (1)
A solution of 2,6-Me2C6H3N ¼ CHC6H4OH L1H (0.53 g, 2.35 mmol) and Ru3(CO)12
(0.30 g, 0.47 mmol) in 20 mL of toluene was heated at reflux for 8 h under an argon
atmosphere. The mixture was allowed to cool to room temperature and the solvent
was evaporated under vacuum. The residue was chromatographed in an Al2O3 column
with petroleum ether and ethyl acetate as eluent to give 1 as a green powder (0.15 g,
22%). Anal. Calc. for C32H28N2O4Ru (%): C, 63.5; H, 4.66; N, 4.6. Found: C, 63.4; H, 4.8;
N, 4.7; 1H NMR (CDCl3, 500 MHz): d 2.22 (s, 6H, CH3), 2.69 (s, 6H, CH3), 6.55 (t, 2H,
J ¼ 8.5 Hz, Ar-H), 6.69 (d, 2H, J ¼ 8.5 Hz, Ar-H), 7.09-7.14 (m, 6H, Ar-H), 7.20-7.24 (m, 4H,
Ar-H), 7.77 (s, 2H, N ¼ CH); 13C NMR (CDCl3, 125 MHz): d 18.5 (CH3), 18.9 (CH3), 114.4
(ArC), 120.5 (ArC), 122.3 (ArC), 126.7 (ArC), 128.6 (ArC), 129.1 (ArC), 129.4 (ArC), 131.7
(ArC), 135.6 (ArC), 135.7 (ArC), 155.1 (ArC), 166.7 (ArC), 168.4 (N ¼ CH), 197.3 (CO); IR
(tCO, KBr, cmꢂ1): 2043 (s), 1970 (s).
2.3. Synthesis of bis(E)-2-(((4-methoxyphenyl)imino)methyl)phenoxy)dicarbonyl
ruthenium(II) Ru(j2-N,O-L2)2(CO)2 (2)
Using
a procedure similar to that described above for 1, reaction of 4-
OMeC6H4N ¼ CHC6H4OH L2H (0.54 g, 2.35 mmol) and Ru3(CO)12 (0.30 g, 0.47 mmol) in
20 mL of toluene gave 2 as an orange powder (0.13 g, 18%). Anal. Calc. for
1
C30H24N2O6Ru (%): C, 59.1; H, 3.9; N, 4.6. Found: C, 59.2; H, 3.8; N, 4.4; H NMR (CDCl3,
500 MHz): d 3.85 (s, 6H, OCH3), 6.59 (t, 2H, J ¼ 7.5 Hz, Ar-H), 6.93 (d, 4H, J ¼ 8.0 Hz, Ar-
H), 7.04 (d, 2H, J ¼ 7.5 Hz, Ar-H), 7.24 (d, 2H, J ¼ 7.5 Hz, Ar-H), 7.36 (t, 2H, J ¼ 7.5 Hz, Ar-
H), 7.42 (d, 4H, J ¼ 8.0 Hz, Ar-H), 8.06 (s, 2H, N ¼ CH); 13C NMR (CDCl3, 125 MHz): d 55.5
(OCH3), 114.0 (ArC), 115.0 (ArC), 119.9 (ArC), 122.3 (ArC), 123.8 (ArC), 135.5 (ArC), 135.8
(ArC), 152.2 (ArC), 158.3 (ArC), 166.5 (ArC), 166.9 (N ¼ CH), 195.5 (CO); IR (tCO, KBr,
cmꢂ1): 2039 (s), 1969 (s).
2.4. Synthesis of bis(E)-2-(((4-chlorophenyl)imino)methyl)phenoxy)dicarbonyl
ruthenium(II) Ru(j2-N,O-L3)2(CO)2 (3)
Using
a procedure similar to that described above for 1, reaction of 4-
ClC6H4N ¼ CHC6H4OH L3H (0.54 g, 2.35 mmol) and Ru3(CO)12 (0.30 g, 0.47 mmol) in
20 mL of toluene gave 3 as an orange powder (0.12 g, 17%). Anal. Calc. for
C28H18Cl2N2O4Ru (%): C, 54.4; H, 2.9; N, 4.5. Found: C, 54.2; H, 2.8; N, 4.4; 1H NMR
(CDCl3, 500 MHz): d 6.59-6.62 (m, 2H, Ar-H), 6.95 (d, 2H, J ¼ 8.5 Hz, Ar-H), 7.20 (dd, 2H,
J1 ¼ 2.0 Hz, J2 ¼ 8.0 Hz, Ar-H), 7.32-7.39 (m, 10H, Ar-H), 7.98 (s, 2H, N ¼ CH); 13C NMR
(CDCl3, 125 MHz): d 115.2 (ArC), 119.5 (ArC), 122.4 (ArC), 124.2 (ArC), 129.3 (ArC), 132.8

4
D.-Z. MA ET AL.
Table 1. Crystal data and summary of X-ray data collection for 1–3.
Complex
1
2
3
Empirical formula
C32H28N2O4Ru
605.63
Monoclinic
P21/c
30.079(3)
16.7215(17)
11.3597(12)
90
92.1640(10)
90
5709.5(10)
8
1.409
0.588
2480
C30H24N2O6Ru
609.58
Monoclinic
P21/c
12.7601(12)
12.0830(11)
20.8669(18)
90
123.198(3)
90
2692.2(4)
4
1.504
0.629
1240
C28H18Cl2N2O4Ru
618.41
Monoclinic
P2/n
8.8469(5)
12.7402(7)
12.3050(7)
90
109.4820(10)
90
1307.51(13)
2
1.571
0.841
620
Formula weight
Crystal system
Space group
a (Å)
b (Å)
c (Å)
a (ꢁ)
b (ꢁ)
c (ꢁ)
V (Å3)
Z
Dcalc (g cmꢂ3
)
l (mmꢂ1
F(000)
)
h range (ꢁ)
2.29-25.02
28125
10053
0.0547
1.066
2.55-25.02
13102
4744
0.0908
1.059
2.374-28.325
17707
3204
0.0190
Collected reflections
Unique reflections
R(int)
Goodness-of-fit on F2
Final R indices [I > 2r(I)]
1.037
R1 ¼ 0.0484,
wR2 ¼ 0.0933
R1 ¼ 0.0946,
wR2 ¼ 0.1180
0.548 and ꢂ0.771
R1 ¼ 0.0511,
wR2 ¼ 0.1044
R1 ¼ 0.0888,
wR2 ¼ 0.1196
1.047 and ꢂ0.567
R1 ¼ 0.0253,
wR2 ¼ 0.0786
R1 ¼ 0.0283,
wR2 ¼ 0.0874
0.976 and ꢂ0.680
R indices (all data)
Largest diff. peak and hole (e Åꢂ3
)
(ArC), 136.0 (ArC), 136.0 (ArC), 156.7 (ArC), 166.8 (ArC), 167.1 (N ¼ CH), 195.2 (CO); IR
(tCO, KBr, cmꢂ1): 2046 (s), 1972 (s).
2.5. General procedure for catalytic conversion of aldehydes to nitriles
In a 25 mL Schlenk tube, a mixture of benzaldehyde (0.106 g, 1.0 mmol), NH2OHꢃHCl
(0.07 g, 1.0 mmol), 1 (0.006 g, 0.01 mmol), toluene (5 mL) and NaHCO3 (0.084 g,
1.0 mmol) was stirred at 110 ꢁC for 10 h under a nitrogen atmosphere. The reaction
mixture was cooled to room temperature and then concentrated under reduced pres-
sure. The residue was purified by flash silica gel column chromatography using ethyl
acetate/petroleum ether to afford the pure product of benzonitrile.
2.6. X-ray crystallography
Single crystals of 1–3 suitable for X-ray diffraction were obtained by crystallization
from CH2Cl2/n-hexane solution. Data collections were performed on a Bruker AXS
SMART 1000 CCD diffractometer using graphite monochromated Mo Ka radiation (u/x
scan, k ¼ 0.71073 Å). The structures were solved by direct methods and refined by full-
matrix least-squares. All calculations were done using the SHELXL program system. All
non-hydrogen atoms were refined anisotropically and hydrogen atoms were intro-
duced into calculated positions with the displacement factors of the host carbon
atoms. CIF data were deposited as CCDC 1538115 (1), 1523387 (2) and 1504155 (3),
respectively. A summary of crystal data and X-ray data collection for 1-3 is listed in
Table 1.

JOURNAL OF COORDINATION CHEMISTRY
5
Scheme 1. Synthesis of ruthenium carbonyl complexes 1–3.
3. Results and discussion
3.1. Synthesis and characterization of ruthenium(II) carbonyl complexes
Thermal treatment of phenoxy-imine ligands RN ¼ CHC6H4OH (L1: R ¼ 2,6-Me2C6H3; L2:
R ¼ 4-OMeC6H4; L3: R ¼ 4-ClC6H4) with Ru3(CO)12 in a 5:1 molar ratio in refluxing tolu-
ene was performed to give the corresponding ruthenium(II) carbonyl complexes
[RN ¼ CH(C6H4O)]2Ru(CO)2 (1: R ¼ 2,6-Me2C6H3; 2: R ¼ 4-OMeC6H4; 3: R ¼ 4-ClC6H4)
(Scheme 1). On varying the ratio of ligand L1: Ru3(CO)12 from 3: 1 to 6:1, the yield of
1 increased gradually and reached the highest yield in the ratio of 5:1. Then 2 and 3
1
were synthesized in the same ratio. The H NMR spectra of ruthenium(II) complexes
1-3 shows singlets for their characteristic CH ¼ N protons at 7.77, 8.06 and 7.98 ppm,
respectively. It also indicates that the two molecules of ligands are in the same chem-
ical environment. The signals in the range 6.55 ꢀ 7.42 ppm can be attributed to their
aromatic protons. The –OH protons at 12.78 ppm in the free ligands disappeared in all
three complexes, suggesting that the phenoxy-imine ligands were successfully coordi-
nated to the ruthenium center. Their 13C NMR spectra displayed a singlet around
195 ppm assigned to the carbonyl signals. The representative 13C NMR of 2 is given in
Figure 1. In the IR spectra of 1-3, the strong absorption bands at 2040 cmꢂ1 and
1970 cmꢂ1 also indicate the existence of two metal carbonyls.
3.2. X-ray structural studies
Crystals of 1–3 suitable for X-ray crystallography were grown by slow diffusion of n-
hexane into CH2Cl2 solutions at ambient temperature. The molecular structures of 1–3
are depicted in Figures 2–4 and selected bond lengths and angles are given in Table
2. Complexes 1-3 crystallize in the monoclinic system with space group P21/c for 1
and 2 and P2/n for 3. X-ray diffraction analysis shows that all three ruthenium com-
plexes are mononuclear structures and have a six-coordinate distorted octahedral
geometry around the Ru metal center. The unit cell of 1 contains two crystallographic-
ally-independent molecules which possess similar connectivity and only one of them
shown in Figure 2. The structures of the two crystallographically-independent mole-
cules and the specific comparison can be seen in the Supporting Information. As seen
in Figure 2, the Ru ion was ligated by two molecules of phenoxy-imine ligands and
the two carbonyls are situated in the cis position. The bond length of Ru(1)-O(1)
[2.039(3) Å] is slightly shorter than the distance of Ru(1)-O(2) [2.048(3) Å] and also

6
D.-Z. MA ET AL.
Figure 1. 13C NMR spectrum of 2.
Figure 2. Molecular structure of 1 with thermal ellipsoids drawn at the 30% level. All hydrogens
are omitted for clarity.
shorter than the distances in complex Ru3(CO)8(C9H6NO)2 [2.12 Å and 2.18 Å] [26]. The
average Ru–N bond lengths are 2.098(4) Å and 2.092(4) Å, which are analogous to the
values of 2.083(4) Å and 2.085(4) Å in 2 and 2.0929(18) Å in 3. The Ru–C–O bond

JOURNAL OF COORDINATION CHEMISTRY
7
Figure 3. Molecular structure of 2 with thermal ellipsoids drawn at the 30% level. All hydrogens
are omitted for clarity.
Figure 4. Molecular structure of 3 with thermal ellipsoids drawn at the 30% level. All hydrogens
are omitted for clarity.
angles [176.22(18) Å and 177.73(18) Å] are in the range of normal Ru–C–O bonds for
ruthenium-carbonyl complexes [27, 28]. Complexes 2 and 3 display similar configura-
tions which are chelated by two N,O-bidentate ligands and two CO groups. The Ru–N
bond lengths of 2.083(4) Å and 2.085(4) Å for 2 and 2.092(4) Å for 3 are in agreement
with the Ru–N bond length found in the complex RuCl(j2–N,O–L–NO2)(CO)(PPh3)2
[2.113(3) Å] [29] and are relatively longer than the values [2.035(4) Å and 2.052(4) Å]
observed in complex [RuCl(PPh3)(salpn)]ꢃCH2Cl2 [30]. Complex 3 is symmetrical mol-
ecule in which the Ru–O, Ru–N lengths and Ru–C–O bond angles are similar to the
reported literatures [29, 31, 32]. As shown in Figure 5, the packing of 1–3 was con-
trolled by weak intermolecular C–H … O hydrogen bonding interactions, where the

8
D.-Z. MA ET AL.
Table 2. Selected bond lengths (Å) and angles (ꢁ) for 1–3.
1
Ru(1)-C(31)
Ru(1)-O(1)
Ru(1)-N(1)
C(32)-Ru(1)-O(1)
C(31)-Ru(1)-O(1)
O(2)-Ru(1)-N(2)
C(31)-Ru(1)-N(1)
O(2)-Ru(1)-N(1)
1.872(7)
2.069(4)
2.160(4)
177.2(2)
93.0(2)
89.01(18)
97.3(2)
Ru(1)-C(32)
Ru(1)-O(2)
Ru(1)-N(2)
C(32)-Ru(1)-O(2)
O(2)-Ru(1)-O(1)
O(1)-Ru(1)-N(2)
N(2)-Ru(1)-N(1)
O(1)-Ru(1)-N(1)
1.866(7)
2.058(4)
2.095(5)
92.8(2)
84.98(16)
83.39(17)
166.92(18)
87.96(16)
80.45(17)
2
Ru(1)-O(1)
2.050(3)
Ru(1)-O(3)
2.054(3)
Ru(1)-N(1)
2.085(4)
Ru(1)-N(2)
2.083(4)
C(30)-Ru(1)-O(1)
C(29)-Ru(1)-O(3)
O(1)-Ru(1)-N(2)
C(30)-Ru(1)-N(1)
O(1)-Ru(1)-N(1)
173.98(18)
173.53(16)
84.54(13)
93.55(19)
90.77(15)
C(29)-Ru(1)-O(1)
O(1)-Ru(1)-O(3)
O(3)-Ru(1)-N(2)
N(2)-Ru(1)-N(1)
O(3)-Ru(1)-N(1)
91.99(19)
83.95(13)
90.93(14)
173.56(14)
84.18(13)
3
Ru(1)-O(2)
2.0552(13)
174.76(5)
88.79(6)
123.03(13)
173.03(7)
85.57(8)
Ru(1)-N(1)
2.0929(18)
95.65(7)
126.18(12)
121.03(13)
86.09(5)
C(1)-Ru(1)-O(2)
O(2)-Ru(1)-N(1)
C(8)-N(1)-Ru(1)
N(1i)-Ru(1)-N(1)
O(2)-Ru(1)-O(2i)
C(1)-Ru(1)-N(1)
C(2)-O(2)-Ru(1)
C(9)-N(1)-Ru(1)
O(2i)-Ru(1)-N(1)
O(2i)-Ru(1)-N(1i)
88.80(6)
Figure 5. Crystal packing of 1–3 showing intermolecular hydrogen bonds as dashed lines.

JOURNAL OF COORDINATION CHEMISTRY
9
Scheme 2. Catalytic conversion of aldehydes to corresponding nitriles.
Table 3. Screening of different solvents and bases for the synthesis of nitriles catalyzed by ruthe-
nium complex 1.a
Entry
Solvent
Base
Yieldb (%)
1
2
3
4
5
6
7
8
CH3CN
Toluene
C2H5OH
CH2Cl2
DMSO
NaHCO3
NaHCO3
NaHCO3
NaHCO3
NaHCO3
NaHCO3
Na2CO3
NaOH
70
79
60
57
14
46
65
51
54
62
37
DMF
Toluene
Toluene
Toluene
Toluene
Toluene
9
10
11
KOH
NaOAc
t-BuOK
aReaction conditions: benzaldehyde (1.0 mmol), NH2OHꢃHCl (1.0 mmol), base (1.0 mmol), catalyst 1 (1.0 mol%) and
solvent (5.0 mL).
bIsolated yield.
Table 4. Conversion of benzaldehyde to benzonitrile using 1–3.a
Entry
Catalyst
Temp. (ꢁC)
Time (h)
Yieldb (%)
1
2
3
4
5
6
7
8c
1
2
3
2
2
2
—
2
110
110
110
90
110
110
110
110
10
10
10
10
15
8
79
84
65
70
85
72
52
18
10
10
aReaction conditions: benzaldehyde (1.0 mmol), NH2OHꢃHCl (1.0 mmol), NaHCO3 (1.0 mmol), catalyst (1.0 mol %) and
toluene (5.0 mL).
bIsolated yield.
cWithout NaHCO3.
benzene ring protons are involved in intermolecular hydrogen bonding interactions
with the phenoxy oxygen atoms.
3.3. Catalytic conversion of aldehydes to nitriles
Ruthenium(II) carbonyl complexes 1–3 have been used as catalysts to catalyze conver-
sion of aromatic aldehydes to the corresponding nitriles in different solvents in the
presence of base (Scheme 2). Initial experiments were carried out to find the most
suitable solvent and base by means of benzaldehyde as model substrate and 1 as
catalyst. As shown in Table 3, ruthenium(II) complex 1 was found to be active toward
the transformation of benzaldehyde to benzonitrile in toluene, acetonitrile and ethanol
but showed less active in dichloromethane, DMSO and DMF (Table 3, entries 1-6). The
catalyst activity is strongly solvent-dependent and toluene is the best medium to give

10
D.-Z. MA ET AL.
Table 5. Screening of substrates for nitriles synthesis catalyzed by ruthenium complex 2.a
Entry
1
Substrate
Product
Yieldb (%)
77
2
3
73
65
4
57
5
6
60
47
aReaction conditions: aldehyde (1.0 mmol), NH2OHꢃHCl (1.0 mmol), NaHCO3 (1.0 mmol), catalyst (1.0 mol %) and tolu-
ene (5.0 mL).
bIsolated yield.
benzonitrile in 79% yield. By varying the bases in catalytic reactions, the highest yield
of the desired product was achieved in the presence of NaHCO3 compared to the
others (Na2CO3, NaOH, KOH, Na2OAc and t-BuOK, 37-65% yields) (Table 3,
entries 7–10).
Complexes 1–3 were then investigated to optimize the best catalyst and the results
are summarized in Table 3. Among the different complexes, 2 having an electron-
donating group (viz. OMe) showed the highest activity, giving the corresponding ben-
zonitrile in an excellent yield of 84% (Table 4, entry 2). Analogously, 3 bearing a
chloro-substituted ligand displayed lower catalytic activity than 1 and 2 (Table 4, entry
3). When the reaction was carried out at 90 ꢁC, the yield of the desired product
dropped into 70% (Table 4, entry 4). Hence, a refluxing temperature of 110 ꢁC was
necessary to maintain the catalytic activity. Further extending the reaction time to
15 h did not significantly increase the yield of acetonitrile and the yield was decreased

JOURNAL OF COORDINATION CHEMISTRY
11
to 72% when the reaction was reduced to 8 h (Table 4, entries 5 and 6). The optimiz-
ing reaction time was determined to be 10 h. Finally, the control experiments showed
that 52% yield of the acetonitrile was obtained without the catalyst and just 18% was
achieved in the absence of NaHCO3 (Table 4, entries 7 and 8), indicating that both
catalyst and base are crucial to convert benzaldehyde to acetonitrile as well as elimin-
ate HCl and water.
To explore the scope of the present catalytic system, reactions of various aldehydes
were conducted using catalyst 2 under the optimized conditions. As shown in Table 5,
several substituted benzonitriles were successfully synthesized with moderate to good
isolated yields using the above optimized protocol. Electron-donating groups, such as
–CH3, –OCH3 and –OH (entries 1, 2 and 6), and electron-withdrawing substituents,
such as the –Cl, –Br and –NO2 (entries 3-5), on the benzaldehyde moiety altered the
reactions and the corresponding nitriles were obtained in the yields of 47-77%. The
substituted benzonitriles gave slightly lower yields compared with benzaldehyde, indi-
cating the present catalytic system was somewhat sensitive to the functional groups.
4. Conclusion
Mononuclear ruthenium(II) carbonyl complexes 1–3 bearing two phenoxy-imine
ligands have been synthesized and well characterized by elemental analysis, IR, NMR
spectroscopy and X-ray crystal diffraction analysis. In the presence of NaHCO3, these
ruthenium(II) complexes were employed as the catalysts for conversion of aldehydes
and NH2OHꢃHCl to the corresponding nitriles in moderate to good yields via the dehy-
dration of aldoximes. The present work offers an alternative route for synthesis of
nitriles in one pot catalyzed by transition metal carbonyl complexes.
Disclosure statement
No potential conflict of interest was reported by the authors.
Funding
The authors gratefully acknowledge financial support from the Hebei Natural Science
Foundation of China (grant nos. B2017205006 and B2019205087) and the Education
Department Foundation of Hebei Province (nos. ZD2018005 and QN2019036).
ORCID
Zhi-Qiang Hao
http://orcid.org/0000-0002-6554-803X
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