Synthesis, structure and catalytic activities for the hydrogen transfer reactions of the ruthenium(II) carbonyl chloride complexes with pyridine-2,6-diimine

Article abstract of DOI:10.1016/j.inoche.2009.07.004

Two Ru(II) carbonyl chloride complexes with pyridine-2,6-diimine, (L)Ru(CO)Cl₂ (L = 2,6-diacetylpyridinebis(2,4,6-trimethylanil), 1; L = 2,6-diacetylpyridinebis(2,6-diisopropylanil), 2) have been synthesized and characterized. The structure of

Full text of DOI:10.1016/j.inoche.2009.07.004

Inorganic Chemistry Communications 12 (2009) 887–889
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Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
Synthesis, structure and catalytic activities for the hydrogen transfer reactions of
the ruthenium(II) carbonyl chloride complexes with pyridine-2,6-diimine
*
Xiang-Yong Lu, Hui-Jun Xu, Xue-Tai Chen
State Key Laboratory of Coordination Chemistry, Nanjing National Laboratory of Microstructures, School of Chemistry and Chemical Engineering, Nanjing University,
Nanjing 210093, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Two Ru(II) carbonyl chloride complexes with pyridine-2,6-diimine, (L)Ru(CO)Cl2 (L = 2,6-diacetylpyri-
dinebis(2,4,6-trimethylanil), 1; L = 2,6-diacetylpyridinebis(2,6-diisopropylanil), 2) have been synthesized
and characterized. The structure of compound 1 has been determined by X-ray crystallography, which
show the distorted octahedral geometry around Ru(II). Complexes 1 and 2 have been proved to be active
in the transfer hydrogenation of acetophenone by propan-2-ol.
Received 17 May 2009
Accepted 3 July 2009
Available online 7 July 2009
Keywords:
Ó 2009 Elsevier B.V. All rights reserved.
Ruthenium complex
Crystal structure
Transfer hydrogenation
Pyridine-2,6-diimine
Transfer hydrogenation (TH) of ketone catalyzed by metal com-
plex is one of the most important route to reduce ketone into alco-
hol, which has been extensively investigated [1]. A great deal of
ruthenium complexes have found to be efficient catalyst precur-
sors in the TH of ketones [1]. Recently several ruthenium(II) com-
plexes with N3 pincer ligands such as 2,6-bis(pyrazolyl)pyridine
have been used as effective catalysts for this organic transforma-
tion [2].
Pyridine-2,6-diimine has attracted renewed attention since the
discovery of the highly effective polymerization catalysts based on
the iron and cobalt pyridine-2,6-diimine complexes [3]. Many me-
tal complexes with pyridine-2,6-diimine have been prepared and
very unusual ligand-based reactivities have been observed, which
might be related to the novel catalytic performances of the metal
complexes. It is surprising to find that ruthenium complexes with
this interesting ligand have only been scarcely investigated [4,5],
among which only few have been reported to be effective homoge-
neous catalysts in epoxidation reaction [5a], the cyclopanation of
styrene with ethyl diazoacetate [5b], and transfer hydrogenation
(TH) of ketone [5c,d]. Some ruthenium complexes with (L1)Ru-
(L2)Cl2 (L1 = pyridine-2,6-diimine, L2 = PPh3, N-donor) have been
demonstrated to be active in the TH of ketones reactions [5c,d].
Ruthenium(II) carbonyl halide complexes with bi- or tri-dentate
nitrogen-containing heterocycles such as 2,20:60,200-terpyridine
have attracted great attention [6]. To our knowledge, ruthenium(II)
carbonyl chloride complex have not been reported. Herein, we
report the synthesis of the Ru(II) carbonyl chloride complexes with
pyridine-2,6-diimine and their catalytic activities in TH of ketones
reactions.
Pyridine-2,6-diimine ligands [3a] and the metal precursor
[Ru(CO)2Cl2]n [7] were prepared according to the reported litera-
tures. (L)Ru(CO)Cl2 (L = 2,6-diacetylpyridine bis(2,4,6-trimethyla-
nil), 1; L = 2,6-diacetylpyridine bis(2,6-diisopropylanil), 2) were
obtained by the reaction of [Ru(CO)2Cl2]n with L [8] and were char-
acterized by 1H NMR, ESI-MS, IR and elemental analysis [9]. Only
one set of 1H NMR signals were observed for the coordinated pyr-
idine-2,6-diimine ligand in 1 and 2 suggested the molecule of 1
and 2 are symmetry related. The strong absorption bands at 2000
and 2013 cmꢀ1 observed in 1 and 2, respectively, can be assigned
to the stretching vibration of the carbonyl groups.
Single-crystal X-ray structural analysis of compound 1 revealed
ꢀ
that this compound crystallizes in triclinic space group P1 (No. 2)
[10]. The asymmetric unit contains two crystallographically non-
equivalent molecules (A, B) of 1 and one cocrystallized ether mol-
ecule. The structural parameters of these two molecules are almost
the same, with only small differences in bond distances and bond
angles. Therefore, only the structure of one molecule of 1 (A) is dis-
cussed here. The molecular structure of complex 1 is shown in
Fig. 1 with the selected bond lengths and bond angles listed in
the caption. The geometry around the Ru(II) ion is an octahedron,
in which the equatorial plane (N1/N2/N3/C1) is formed by three
N atoms (N1, N2 and N3) from the pyridine-2,6-diimine ligand
and one carbonyl C atom (C1). The axial positions are occupied
by two Cl atoms (Cl1 and Cl2). The coordination octahedron is
rather distorted with the N1–Ru1–N3 angle of 155.21(18)°.
This angle is considerably smaller than the ideal angle of 180°,
which is comparable to those reported for pyridine-2,6-diimine
* Corresponding author. Fax: +86 25 83314502.
E-mail address: xtchen@netra.nju.edu.cn (X.-T. Chen).
1387-7003/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.inoche.2009.07.004

888
X.-Y. Lu et al. / Inorganic Chemistry Communications 12 (2009) 887–889
Fig. 1. The molecular structure of 1, showing 30% probability displacement ellipsoids and partial atom labeling scheme. For clarity, hydrogen atoms are omitted. Selected
bond lenghs (Å) and bond angles (°): Ru1–C1 = 1.969(6); Ru1–N1 = 2.108(5); Ru1–N2 = 1.996(5); Ru1–N3 = 2.104(4); Ru1–Cl1 = 2.4054(18); Ru1–Cl2 = 2.414(2); O1–
C1 = 1.066(10); Cl1–Ru1–Cl2 = 177.96(7); N1–Ru1–N3 = 155.21(18); N2–Ru1–C1 = 172.5(2).
metal complexes [4–5]. The N2–Ru1–C1 and Cl1–Ru1–Cl2 angles
are 172.50(2)° and 177.96(7)°, respectively. The Ru1–N1 and
Ru1–N3 bond lengths are 2.108(5) and 2.104(4) Å, respectively,
100
90
which are similar to the reported values for other Ru(II) pyri-
dine-2,6-diimine complexes [4]. The Ru–CO bond of 1.969(6) Å is
somewhat longer than the reported value for [RuCl2(terpy)(CO)]
(1.882(5) Å) [6b]. The five-membered chelate rings formed by
atoms Ru1/N2/C20/C27/N3 and Ru1/N2/C24/C25/N1 are planar
and the maximum deviations from their planes are 0.050 (2) and
0.092 (2) Å, respectively, for atom N2. These two chelate rings
are nearly coplanar with a dihedral angle of 2.21(5)°.
Piers et al. have demonstrated that similar ruthenium(II) car-
bonyl bromide complex with the CNC pincer ligand pyridine-
2,6-bis(N-heterocyclic carbene) is efficient in the transfer hydro-
genation of ketone [11]. This stimulated us to test the catalytic
activities of 1 and 2 for transfer hydrogenation of ketones. The
classical transfer hydrogenation of acetophenone by propan-2-ol
was chosen as a model reaction (see Scheme 1) [12]. The results
depicted in Fig. 2 indicated that complexes 1 and 2 are active cat-
alysts, leading to nearly quantitative conversions (yield ꢁ 95%) of
acetophenone into 1-phenylethanol after 2.4 and 4.1 h, respec-
tively. Complex 1 shows better performance than complex 2,
suggesting that the steric requirement has an influence on the
catalytic activity. Complex 1 adopts the trans(Cl) configuration
revealed by the single-crystal X-ray diffraction while the detailed
80
70
60
50
40
30
20
10
0
1
2
0
50
100
150
Time (min)
200
250
300
Fig. 2. Conversion vs. reaction time of the catalytic transfer hydrogenation of
acetophenone using 1 and 2 as catalyst precursors.
Acknowledgment
This work was supported by National Basic Research Program of
China (Nos. 2006CB806104 and 2007CB925102).
structure of
2 is unknown. Therefore the different catalytic
performance between 1 and 2 might be due to the difference
configuration.
Appendix A. Supplementary material
In summary, two new ruthenium(II) carbonyl chloride com-
plexes 1 and 2 with pyridine-2,6-diimine have been synthesized
and characterized. The catalytic test indicated they are efficient
for the transfer hydrogenation of acetophenone.
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.inoche.2009.07.004.
O
OH
OH
O
0.2mol% catalyst
4.8mol%NaOH
+
+
Scheme 1.

X.-Y. Lu et al. / Inorganic Chemistry Communications 12 (2009) 887–889
889
[8] Synthesis of complex 1: ligand L1 (0.23 g, 0.59 mmol) was dissolved in
absolute ethanol (10 mL), which was added to an ethanol solution (15 mL) of
[Ru(CO)2Cl2]n (0.13 g, 0.59 mmol). The mixture was refluxed at 80 °C for 2.5 h.
After cooling, a dark brown microcrystalline solid was collected, and washed
with cooled absolute ethanol and then dried at air. Yield: 71%. Complex 2 was
prepared by the similar procedure. Yield: 51%.
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