Novel neutral phosphinite bridged dinuclear ruthenium(II) arene complexes and their catalytic use in transfer hydrogenation of aromatic ketones: X-ray structure of a new Schiff base, N3,N3′-di-2-hydroxybenzylidene-[2,2′]bipyridinyl-3,3′-diamine

Article abstract of DOI:10.1016/j.molcata.2010.04.010

A novel Schiff base N3,N3′-di-2-hydroxybenzylidene-[2,2′]bipyridinyl-3,3′-diamine, 1 was synthesized from condensation of salicylaldehyde with 3,3′-diamino-2,2′-bipyridine. Reaction of 1 with two equivalents of PPh₂Cl in the presence of Et₃N proceeds in toluene to give N3,N3′-di-2-(diphenylphosphino)benzylidene-[2,2′]bipyridinyl-3,3′-diamine, 2 in quantitative yield. Ruthenium(II) dimers [Ru(η⁶-arene)(μ-Cl)Cl]₂ readily react with phosphinite ligand [(Ph₂PO)₂-C₂₄H₁₆N₄], 2 in toluene at room temperature, to afford the neutral derivatives [C₂₄H₁₆N₄{OPPh₂-Ru(η⁶-arene)Cl₂}₂] {arene: benzene 3, p-cymene, 4}. All the complexes were fully characterized by analytical and spectroscopic methods. ³¹P-{¹H} NMR, ¹H-¹³C HETCOR or ¹H-¹H COSY correlation experiments were used to confirm the spectral assignments. Molecular structure of the Schiff base, 1 was also determined by X-ray single crystal diffraction study. The catalytic activity of complexes 3 and 4 in the transfer hydrogenation of acetophenone derivatives was tested. Stable ruthenium(II)-phosphinite complexes were found to be efficient catalysts in the transfer hydrogenation of aromatic ketones in excellent conversions up to 99% (up to 530 per hour) in the presence of iso-PrOH/KOH.

Full text of DOI:10.1016/j.molcata.2010.04.010

Journal of Molecular Catalysis A: Chemical 326 (2010) 75–81
Contents lists available at ScienceDirect
Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Novel neutral phosphinite bridged dinuclear ruthenium(II) arene complexes and
their catalytic use in transfer hydrogenation of aromatic ketones: X-ray structure
of a new Schiff base,
N3,N3ꢀ-di-2-hydroxybenzylidene-[2,2ꢀ]bipyridinyl-3,3ꢀ-diamine
Murat Aydemira,∗, Feyyaz Durapa, Akın Baysala, Nermin Merica, Ays¸ egül Buldag˘ a, Bahattin Gümgüma,
Saim Özkarb, Leyla Tatar Yıldırımc
a Dicle University, Department of Chemistry, TR-21280 Diyarbakır, Turkey
b Middle East Technical University, Department of Chemistry, TR-06531 Ankara, Turkey
c Hacettepe University, Department of Engineering Physics, Beytepe, TR-06800 Ankara, Turkey
a r t i c l e i n f o
a b s t r a c t
A novel Schiff base N3,N3ꢀ-di-2-hydroxybenzylidene-[2,2ꢀ]bipyridinyl-3,3ꢀ-diamine, 1 was synthe-
sized from condensation of salicylaldehyde with 3,3ꢀ-diamino-2,2ꢀ-bipyridine. Reaction of 1 with
two equivalents of PPh2Cl in the presence of Et3N proceeds in toluene to give N3,N3ꢀ-di-2-
(diphenylphosphino)benzylidene-[2,2ꢀ]bipyridinyl-3,3ꢀ-diamine, 2 in quantitative yield. Ruthenium(II)
dimers [Ru(␩6-arene)(␮-Cl)Cl]2 readily react with phosphinite ligand [(Ph2PO)2-C24H16N4], 2 in toluene
at room temperature, to afford the neutral derivatives [C24H16N4{OPPh2–Ru(␩6-arene)Cl2}2] {arene: ben-
zene 3, p-cymene, 4}. All the complexes were fully characterized by analytical and spectroscopic methods.
Article history:
Received 27 January 2010
Received in revised form 13 April 2010
Accepted 21 April 2010
Available online 29 April 2010
Keywords:
Schiff base
Bis(phosphinite)
Transfer hydrogenation
Ruthenium
31P–{ H} NMR, 1H–13C HETCOR or 1H–1H COSY correlation experiments were used to confirm the spec-
1
tral assignments. Molecular structure of the Schiff base, 1 was also determined by X-ray single crystal
diffraction study. The catalytic activity of complexes 3 and 4 in the transfer hydrogenation of acetophe-
none derivatives was tested. Stable ruthenium(II)–phosphinite complexes were found to be efficient
catalysts in the transfer hydrogenation of aromatic ketones in excellent conversions up to 99% (up to 530
per hour) in the presence of iso-PrOH/KOH.
Catalysis
X-ray
© 2010 Elsevier B.V. All rights reserved.
1. Introduction
laldehyde) with aliphatic or aromatic mono- or diamines. Schiff
base pyridine, bipyridine, terpyridine, phenanthroline, naphtyri-
One of the most challenging research areas in both material sci-
ence and supramolecular chemistry is the design and synthesis
of multifunctional compounds with desirable properties such as
luminescence, redox, transport of information and catalytic activ-
ity [1,2]. Ligands bearing one or more chelating 2,2ꢀ-bipyridine
or 1,10-phenanthroline units have attracted considerable interest
as building blocks in the preparation of supramolecular architec-
tures in combination with a wide range of transition metals [3,4].
Schiff bases are important class of ligands, and such ligands have
been extensively studied in coordination chemistry mainly due
to their facile synthesis and their applications in many biolog-
ical aspects and in catalysis [5–7]. Schiff base ligands are most
often obtained from o-hydroxy aromatic aldehydes (e.g. salicy-
dine and pyridine–pyridazine ligands exhibit a rich coordination
chemistry towards transition metal complexes providing unusual
coordination modes [8,9].
The chemistry of aminophosphines and phosphinites has also
been intensively explored in recent years [10]. These compounds
are extremely attractive as potential ligands since various struc-
tural modifications are accessible via simple P–N and P–O bond
formation [11]. Many modified phosphine ligands and a vari-
ety of chiral aminophosphine–phosphinite ligands have important
applications in organometallic chemistry and catalysis, giving
selective catalysts for hydroformylation, hydrosilylation and asym-
metric transfer hydrogenation [12–15]. While much effort has
been devoted to the synthesis of aminophosphines and their metal
complexes, similar studies on the analogous bis(phosphinites) are
less extensive [16,17], even though some of their complexes have
proved to be efficient catalysts [18,19].
∗
Corresponding author.
E-mail addresses: aydemir4921@hotmail.com, aydemir@dicle.edu.tr
Le Lagadec and co-workers [20] have reported the first crys-
tallographic determination of a bimetallic bridged structure using
(M. Aydemir).
1381-1169/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2010.04.010

76
M. Aydemir et al. / Journal of Molecular Catalysis A: Chemical 326 (2010) 75–81
this sort of ligands. Furthermore, a related structure has been
invoked by Balakrishna et al. [21] for the formation of the
ruthenium complex where the ligand also serves as bridge
between two metal centers, however no crystallographic evidence
was presented. Another related structure has been reported by
Osborn and co-workers [22], where the phosphinite ligand PONOP
[C6H3N-2,6-(CH2OPPh2)2] behaves as bridging ligand between
two platinum centers, forming a large 20 membered diplati-
namacrocycle. Most recently [23] a similar structure has been
determined for a BINAP-based phosphinite ligand BINAPO, it
has been noted that compounds bearing binaphtyl-based ligands
bridging two metal centers are very rare. Based on these attrac-
tive studies, our research group has reported the synthesis of
bridging ligands [24] and their corresponding bridged dinuclear
Ru(II) complexes [C10H6N2{NHPPh2–Ru(␩6-p-cymene)Cl2}2] and
[C10H6N2{OPPh2–Ru(␩6-p-cymene)Cl2}2] and their catalytic eval-
uation in the transfer hydrogenation of ketones [25]. Furthermore
the well-known ability of (␩6-p-arene)Ru(II) species to act as effi-
cient catalysts in hydrogen transfer reactions between alcohols and
ketones [26,27] prompted us to study the catalytic activity of the
analogous derivatives.
Ketones are the most common unsaturated substrates used in
organic synthesis. Extensive efforts have been devoted to their
reduction into secondary alcohols especially via hydrogenation
[28]. Transfer hydrogenation of ketones by propan-2-ol is conve-
nient in large-scale synthesis since there is no need to employ a
high hydrogen pressure or to use hazardous reducing agents [29].
As a part of our interest in designing new ligand systems with
different spacers to control the electronic attributes at phosphorus
centers and to explore their coordination chemistry, in this paper,
we report (i) the ready synthesis of a novel Schiff base N3,N3ꢀ-di-
2-hydroxybenzylidene-[2,2ꢀ]bipyridinyl-3,3ꢀ-diamine and its cor-
responding bridged phosphinite ligand, (ii) the synthesis and full
characterization of two bridged half-sandwiches dinuclear ruthe-
nium(II) complexes [C24H16N4{OPPh2–Ru(␩6-benzene)Cl2}2], 3
and [C24H16N4{OPPh2–Ru(␩6-p-cymene)Cl2}2], 4 and (iii) their
subsequent application in transfer hydrogenation of the acetophe-
none derivatives.
siloxane) (30 m × 0.32 mm × 0.25 ␮m). The GC parameters were as
follows for transfer hydrogenation of ketones; initial temperature,
110 ◦C; initial time, 1 min; solvent delay, 4.48 min; temperature
ramp, 80 ◦C/min; final temperature, 200 ◦C; final time, 21.13 min;
injector port temperature, 200 ◦C; detector temperature, 200 ◦C,
injection volume, 2.0 ␮L.
2.3. Synthesis
2.3.1. Synthesis of
N3,N3ꢀ-di-2-hydroxybenzylidene-[2,2ꢀ]bipyridinyl-3,3ꢀ-diamine,
1
Salicylaldehyde (350 mg, 2.87 mmol) was added to solution
of 3,3ꢀ-diamino-2,2ꢀ-bipyridine (250 mg, 1.35 mmol) in 20 mL of
methanol. The solution was heated under reflux for 10 h. On cool-
ing to the room temperature an orange powder was precipitated
out which was out collected by vacuum filtration. Recrystallization
of the crude product from ethanol gave the orange crystals of the
Schiff base ligand (yield 380 mg, 72%), m.p. 207.5–208.5 ◦C. 1H NMR
(400.1 MHz, in CDCl3) ı = 12.10 (s, 2H, OH), 8.73 (d, 2H, J = 4.4 Hz, H-
6), 8.50 (s, 2H, HC N), 7.64 (d, 2H, J = 8.0 Hz, H-4), 7.50 (dd, 2H,
J = 4.4 and 8.0 Hz, H-5), 7.32 (dd, 2H, J = 7.6 and 8.0 Hz, H-13), 7.10
(d, 2H, J = 7.2 Hz, H-11), 6.87 (d, 2H, J = 8.0 Hz, H-14), 6.82 (dd, 2H,
J = 7.2 and 7.6 Hz, H-12); 13C NMR (100.6 MHz, in CDCl3): ı = 164.71
(HC N), 148.06 (C-6), 133.67 (C-13), 132.55 (C-11), 125.97 (C-4),
124.58 (C-5), 119.20 (C-12), 117.10 (C-14), 160.75, 151.70, 143.49,
119.05 (quaternary carbons), assignment was based on the 1H–13
HETCOR and 1H–1H COSY spectra; IR (KBr): ꢀ = 1622 (C N) cm−1
C
;
C24H18N4O2 (mw: 394.4 g/mol): calcd. C 73.08, H 4.60, N 14.20;
found C 72.96, H 4.54, N 14.15.
2.3.2. Synthesis of N3,N3ꢀ-di-2-
(diphenylphosphinite)benzylidene-[2,2ꢀ]bipyridinyl-3,3ꢀ-diamine,
2
Chlorodiphenylphosphine (60 mg, 0.254 mmol) was added
to
a
stirred solution of N3,N3ꢀ-di-2-hydroxybenzylidene-
[2,2ꢀ]bipyridinyl-3,3ꢀ-diamine,
1
(50 mg, 0.127 mmol) and
triethylamine (26 mg, 0.254 mmol) in toluene (40 mL) at 0 ◦C
with vigorous stirring. The mixture was stirred at room tem-
perature for 4 h and the white precipitate (triethylammonium
chloride) was filtered off under argon and dried in vacuo to produce
a white viscous oily compound 2 (yield: 96.8 mg, 99%). 31P NMR
(162.0 MHz, CDCl3): ı = 117.05 (s) ppm.
2. Experimental
2.1. Materials and methods
Unless otherwise stated, all reactions were carried out under
an atmosphere of argon using conventional Schlenk glassware, sol-
vents were dried using established procedures and distilled under
argon immediately prior to use. Analytical grade and deuterated
solvents were purchased from Merck. PPh2Cl, salicylaldehyde and
2-chloro-3-nitropyridine are purchased from Fluka and were used
as received. The starting materials 3,3ꢀ-diamino-2,2ꢀ-bipyridine
[30,31], [Ru(␩6-p-cymene)(␮-Cl)Cl]2 [32,33], [Ru(␩6-benzene)(␮-
Cl)Cl]2 [34] were prepared according to the literature procedures.
The IR spectra were recorded on a Mattson 1000 ATI UNICAM
FT-IR spectrometer as KBr pellets. 1H (400.1 MHz), 13C NMR
(100.6 MHz) and 31P NMR spectra (162.0 MHz) were recorded on
a Bruker Avance 400 spectrometer, with ı referenced to exter-
nal TMS and 85% H3PO4. Elemental analysis was carried out
on a Fisons EA 1108 CHNS-O instrument. Melting points were
recorded by Gallenkamp Model apparatus with open capillar-
ies.
2.3.3. Synthesis of [C24H16N4{OPPh2–Ru(ꢁ6-benzene)Cl2}2], 3
To a solution of [(␩6-benzene)RuCl2]2 (63.5 mg, 0.127 mmol)
in toluene, a solution (toluene, 25 mL) of [(Ph2PO)2-C24H16N4], 2
(96.8 mg, 0.127 mmol) was added. The resulting reaction mixture
was allowed to proceed under stirring at room temperature for 3 h.
After this time, the orange solution was filtered through Celite to
remove a small amount of insoluble material before reducing the
volume to 0.5 mL and addition of petroleum ether (10 mL) to pre-
cipitate an orange solid that was isolated by filtration and dried
in vacuo. Following recrystallization from diethylether/CH2Cl2, a
red powder was obtained (yield 135 mg, 84%), m.p. 182–184 ◦C. 1
H
NMR (400.1 MHz, in CDCl3) ı = 8.94 (d, 2H, J = 4.5 Hz, H-6), 8.36 (br,
2H, HC N), 6.90–8.05 (m, 12H, protons of phenyls), 5.97 (s, 12H,
protons of Ru-C6H6); 13C NMR (100.6 MHz, in CDCl3): ı = 165.47
(HC N), 147.12 (C-6), 133.96 (C-13), 132.80 (d, J = 38.0 Hz, i-carbons
of P(C6H5)2), 132.53 (C-11), 131.75 (d, J = 12.07 Hz, o-carbons of
P(C6H5)2), 129.67 (s, p-carbons of P(C6H5)2), 128.41 (d, J = 8.0 Hz,
m-carbons of P(C6H5)2), 124.26 (C-4), 122.68 (C-5), 119.81 (C-12),
117.70 (C-14), 161.30, 147.88, 143.12, 119.40 (quaternary car-
bons), 88.12 (carbon of Ru-C6H6), assignment was based on the
1H–13C HETCOR and 1H–1H COSY spectra; 31P NMR (162 MHz, in
CDCl3): ı = 116.99 (s); IR (KBr): ꢀ = 906 (P–O), 1432 (P–Ph), 1619
2.2. GC analyses
GC analyses were performed on a HP 6890N Gas Chromatograph
equipped with capillary column (5% biphenyl, 95% dimethyl-

M. Aydemir et al. / Journal of Molecular Catalysis A: Chemical 326 (2010) 75–81
77
(C N) cm−1; C60H48N4O2P2Ru2Cl4 (mw: 1262.6 g/mol): calcd. C
57.08, H 3.83, N 4.44; found C 56.92, H 3.80, N 4.39.
Table 1
Crystal data and results of structure refinement for C24H18N4O2.
Formula
(C12H9N2O)2
Formula weight [g/mol]
Temperature [K]
Wavelength [Å]
Crystal system, space group
Unit cell dimensions: [Å,◦]
394.42
293(2)
1.54184
Monoclinic, C2/c (no.15)
a = 21.004(2), b = 8.435(2),
c = 12.989(4), ˇ = 121.94(2)
1952.8(8)
4/1.342
0.711
2.3.4. Synthesis of [C24H16N4{OPPh2–Ru(ꢁ6-p-cymene)Cl2}2], 4
To a solution of [(␩6-p-cymene)RuCl2]2 (77.8 mg, 0.127 mmol)
in toluene, a solution (toluene, 25 mL) of (96.8 mg, 0.127 mmol)
[(Ph2PO)2-C24H16N4], 2 was added. The resulting reaction mixture
was allowed to proceed under stirring at room temperature for 3 h.
After this time, the orange solution was filtered through Celite to
remove a small amount of insoluble material before reducing the
volume to 0.5 mL and addition of petroleum ether (10 mL) to pre-
cipitate an orange solid that was isolated by filtration and dried
in vacuo. Following recrystallization from diethylether/CH2Cl2, a
red powder was obtained (yield 152 mg, 87%), m.p. 189–191 ◦C.
1H NMR (400.1 MHz, CDCl3) ı = 9.02 (d, 2H, J = 4.6 Hz, H-6), 8.24
(br, 2H, HC N), 6.80–8.15 (m, 12H, protons of phenyls), 5.38
(d, 4H, J = 6.40 Hz, aromatic hydrogen of p-cymene), 5.31 (d, 4H,
J = 6.40 Hz, aromatic hydrogen of p-cymene), 2.55 (m, 2H, –CH–
of p-cymene), 1.71 (s, 6H, CH3-Ph of p-cymene), 0.81 (d, 12H,
J = 6.40 Hz (CH3)2CHPh of p-cymene); 13C NMR (100.6 MHz, CDCl3):
ı = 164.75 (HC N), 145.97 (C-6), 135.75 (d, J = 45.00 Hz, i-carbons
of P(C6H5)2), 133.67 (C-13), 132.06 (d, J = 11.26 Hz, o-carbons of
P(C6H5)2), 132.58 (C-11), 131.14 (s, p-carbons of P(C6H5)2), 128.11
(d, J = 10.0 Hz, m-carbons of P(C6H5)2), 124.60 (C-4), 122.63 (C-
5), 119.28 (C-12), 117.05 (C-14), 160.72, 147.99, 143.28, 119.20
(quaternary carbons), 111.22, 96.18 (quaternary carbons of p-
cymene), 92.58, 87.14 (aromatic carbons of p-cymene), 30.04
(–CH– of p-cymene), 21.18 ((CH3)2CHPh of p-cymene), 17.05
Cell volume [Å3]
Z/Calculated density [g/cm3]
Absorption coefficient [mm−1
F(0 0 0)
]
824
Crystal size [mm]
0.4 × 0.4 × 0.4
ꢃ-range for data collection [◦]
Limiting indices
4.96–74.22
−22 ≤ h ≤ 26, −10 ≤ k ≤ 0,
−16 ≤ l ≤ 0
Reflections collected/unique
Max. and min. transmission
Refinement method
2023/1940 [Rint = 0.0329]
0.790 and 0.752
Full-matrix least-squares
on F2
Data/parameters
1940/140
1.193
R1 = 0.1264, wR2 = 0.2328
R1 = 0.1454, wR2 = 0.2532
1.17, −0.95
Goodness-of-fit on F2
Final R indices [I > 2ꢄ(I)]
R indices (all data)
Largest diff. peak and hole [e/Å3]
Additional material available from Cambridge Crystallographic Data Center as depo-
sition No: CCDC 633281 comprises H-atom coordinates, thermal parameters, and
remaining bond lengths and angles.
Table 2
(CH3Ph of p-cymene), assignment was based on the 1H–13
C
Selected bond lengths (Å) and bond angles (◦).
HETCOR and 1H–1H COSY spectra; 31P NMR (162 MHz, CDCl3):
ı = 118.05 (s); IR (KBr): ꢀ = 906 (P–O), 1465 (P–Ph), 1619 (C N)
cm−1. C68H64N4O2P2Ru2Cl4 (mw: 1375.4 g/mol): calcd. C 59.38, H
4.69, N 4.07; found C 59.25, H 4.64, N 4.03.
N2–C9
O–C1
1.342(3)
1.358(4)
1.279(3)
1.407(3)
1.325(3)
1.445(3)
1.495(4)
1.00(8)
N2–C9–C9a
N2–C10–C11
O–C1–C2
115.7(2)
123.5(2)
119.7(3)
121.4(3)
117.2(2)
126.6(2)
122.2(2)
116.4(2)
120.8(2)
119.4(2)
121.9(2)
N1–C7
N1–C8
N2–C10
C7–C6
C9–C9a
O–H1
O–C1–C6
C10–N2–C9
C12–C8–N1
C7–N1–C8
C9–C8–N1
C8–C9–C9a
C5–C6–C7
C1–C6–C7
2.4. Transfer hydrogenation of ketones
Typical procedure for the catalytic hydrogen transfer reaction:
under inert atmosphere a solution of ruthenium catalyst precur-
sors [C24H16N4{OPPh2–Ru(␩6-benzene)Cl2}2], 3 (0.005 mmol) or
[C24H16N4{OPPh2–Ru(␩6-p-cymene)Cl2}2], 4 (0.005 mmol), KOH
(0.025 mmol) and the corresponding ketone (0.5 mmol) in degassed
iso-PrOH (5 mL) were refluxed for 15 min for 3 and 4. After this
time a sample of the reaction mixture is taken off, diluted with
acetone and immediately analyzed by gas chromatography. Con-
version obtained is related to the residual unreacted ketone.
N1–C7–C6
N2–C9–C8
122.2(2)
123.5(2)
a
−x, y, 1/2 −z.
2.5. X-ray diffraction structure analysis
The single crystal X-ray diffraction studies of N3,N3ꢀ-di-2-
hydroxybenzylidene-[2,2ꢀ]bipyridinyl-3,3ꢀ-diamine were carried
out using Enraf-Nonius CAD-4 single X-ray diffractometer with
Cu K␣ (ꢂ = 1.54184 Å) radiation. The lattice parameters and their
estimated standard deviations were determined from a least-
squares refinement of 25 centered reflections in the range of
21.80 ≤ ꢃ ≤ 43.10◦ using CAD-4 express [35]. Data reduction was
carried out using X-CAD4 [36]. The structure was solved by the
direct method and refined by the full-matrix least-square tech-
nique using the programs SHELXS97 and SHELXL97 [37].
Hydrogen atom (H1) of the hydroxyl group is taken from a dif-
ference Fourier map and refined, while the other hydrogen atoms
were placed with Uiso(H) = 1.2 Ueq(C). For all non-hydrogen atoms
anisotropic displacement parameters were refined. Results of the
X-ray structure determination are presented in Tables 1 and 2 and
Fig. 1.
Fig. 1. Molecular structure and atomic numbering scheme of the title compound
(30% probability displacement ellipsoids). H atoms are shown as small circles of
arbitrary radii (i: −x, y, −z + 1/2).

78
M. Aydemir et al. / Journal of Molecular Catalysis A: Chemical 326 (2010) 75–81
Scheme 1. Synthesis of the N3,N3ꢀ-di-2-hydroxybenzylidene-[2,2ꢀ]bipyridinyl-3,3ꢀ-diamine, 1.
benzene)Cl2}2], 3 was isolated as indicated by a singlet in the 31P
3. Results and discussion
NMR spectra at (ı) 116.99 ppm, in line with the values previously
observed for similar compounds [41]. It is significant to note that
31P NMR signals of ligand and complex do not differ significantly
[42]. Analysis by 1H NMR reveals this compound to be diamag-
netic, exhibiting signals corresponding to the aromatic rings for 3
at 8.05–6.90 ppm. The 1H NMR spectrum of complex 3 displays the
(CH N) resonance as a broad signal at 8.36 ppm (2H) and the C6H6
protons as a singlet 5.97 ppm (12H). In the 13C NMR spectrum of 3,
the imine carbon signal was observed at 165.47 ppm and the C6H6
carbon resonance was occurred at 88.12 (s) ppm. Furthermore in
3.1. Synthesis of the compounds
The
reaction
in
of
salicylaldehyde
methanol yields
and
the
3,3ꢀ-diamino-
Schiff base
2,2ꢀ-bipyridine
N3,N3ꢀ-di-2-hydroxybenzylidene-[2,2ꢀ]bipyridinyl-3,3ꢀ-diamine
(Scheme 1). This new Schiff base was characterized by elemental
analysis, IR, and multinuclear NMR spectroscopies and its structure
was elucidated by X-ray crystallography.
The 1H NMR spectrum of the Schiff base exhibits a characteristic
signal at 8.50 ppm for the –CH N– group and a signal at 12.07 ppm
for the phenolic proton which is highly deshielded indicating an
intramolecularhydrogenbonding between thephenolicOHandthe
imine N atom. This could be verified by structural analysis (Fig. 1).
The IR spectrum also shows a broad absorption band at 3680 cm−1
for the O–H stretching which is broadened and shifted towards
lower frequency due to the hydrogen bonding. The IR spectrum
1
the 13C–{ H} NMR spectrum of 3, J(31P–13C) coupling constants of
the carbons of the phenyl rings was observed, which is consistent
with the literature values [43]. The structure of the 3 was further
confirmed by IR spectroscopy and microanalysis, and found to be in
good agreement with the theoretical values (for details see Section
2).
The reaction of [Ru(␩6-p-cymene)(␮-Cl)Cl]2 with one equiv-
alent of [(Ph2PO)2-C24H16N4], 2 affords only the corresponding
monodendate [C24H16N4{OPPh2–Ru(␩6-p-cymene)Cl2}2] 4, as the
main product. Complexation reaction was straightforward, with
coordination to ruthenium being carried out at room temper-
ature. The initial color change, i.e. from clear orange to deep
red [44], attributed to the dimer cleavage most probably by the
bis(phosphinite) ligand. The 31P NMR spectrum is quite consis-
tent with the structure [45] of 4 showing a single resonance at ı
118.05 ppm. Furthermore, 1H NMR spectral data of 4 is consistent
with the structure proposed. The signals consisting of two doublets
centered at 5.38 ppm and 5.31 ppm are due to the presence of the
aromatic protons in the p-cymene group, this information is com-
plemented by the presence of signals at 2.55 ppm and 0.81 ppm due
to the CH and CH3 of the iso-propyl groups of the p-cymene moiety.
Finally, a signal due to the presence of the methyl in the p-cymene
group is observed at 1.71 ppm.
of 1 also shows a strong absorption band at 1622 cm−1 for the C
stretching, indicating the presence of azomethine group.
N
A prismatic yellow crystal of 1 (C12H9N2O)2 was obtained from
recrystallization solutions and its structure was resolved by X-ray
crystallography. Fig 1 shows the molecular structure of the com-
pound 1 with ellipsoids, 30% probability.
As a result of ongoing research program, we synthesized a novel
modified bridging bidentate ligand. The reaction of the N3,N3ꢀ-di-
2-hydroxybenzylidene-[2,2ꢀ]bipyridinyl-3,3ꢀ-diamine, 1 with two
equivalents of chlorodiphenylphosphine in the presence of tri-
ethylamine in toluene affords phosphinite, [(Ph2PO)2-C24H16N4],
2 (Scheme 2). The 31P NMR of 2 exhibits a singlet resonance due to
phosphinite at 117.05 ppm in the spectrum, indicative of both phos-
phorus being equivalent as a result of the symmetry (C2 symmetry)
of the molecule.
Solution of the ligand in CDCl3, prepared under anaerobic con-
dition, is unstable and decomposes gradually to give oxide and the
hydrolysis product diphenylphosphinous acid, Ph2P(O)H [38,39].
Furthermore, the 31P NMR spectrum also displays formation of
PPh2PPh2 and P(O)Ph2PPh2, as indicated by signals at about ı
−16.0 ppm as singlet and ı 36.8 ppm and ı −21.0 ppm as doublets
1
In the 13C–{ H} NMR spectra of 4, J(31P–13C) coupling constants
of the carbons of the phenyl rings were observed (for details see
Section 2), which are consistent with the literature values [46–48].
1
The most relevant signals of 13C–{ H} NMR spectra of complex
4 are those corresponding to p-cymene ligands. Carbon atoms of
the arene rings in p-cymene ligands are observed as two singlets
at 92.58 ppm and 87.14 ppm in compound 4. The structural com-
position of the complex was also confirmed by IR and elemental
analysis.
1
with J(PP) 220 Hz [40]. Because the [(Ph2PO)2-C24H16N4], 2 is not
stable enough in solution the ruthenium(II) complexes 3 and 4 were
synthesized in situ. Reactions of [(Ph2PO)2-C24H16N4], 2 with metal
precursors [Ru(␩6-benzene)(␮-Cl)Cl]2 or [Ru(␩6-p-cymene)(␮-
Cl)Cl]2 are depicted in Scheme 2. The reaction of stoichiometric
amounts of [Ru(␩6-benzene)(␮-Cl)Cl]2 and [(Ph2PO)2-C24H16N4],
2 affords the complex [C24H16N4{OPPh2–Ru(␩6-benzene)Cl2}2], 3
in good yield as a dark red microcrystalline powder. Ligand 2 was
expected to cleave the [Ru(␩6-p-cymene)Cl2]2 dimer to give the
corresponding complex 3 via monohapto coordination of the phos-
phinite group. Analysis by 31P NMR exhibits a unique signal in
the spectrum, indicative of both phosphorus being equivalent as a
result of the symmetry of the complex. [C24H16N4{OPPh2–Ru(␩6-
3.2. Catalytic transfer hydrogenation of acetophenone derivatives
The excellent catalytic performance and the higher structural
permutability of phosphinite based transition metal complexes
[49–51] prompted us to develop new Ru(II) complexes with well-
shaped ligands. We paid particular attention to arene ligands
[52], because (i) the spectator ligands automatically occupy three

M. Aydemir et al. / Journal of Molecular Catalysis A: Chemical 326 (2010) 75–81
79
Scheme 2. Synthesis of the [(Ph2PO)2-C24H16N4], 2, [C24H16N4{OPPh2–Ru(␩6-benzene)Cl2}2], 3 and [C24H16N4{OPPh2–Ru(␩6-p-cymene)Cl2}2], 4. (i) 2 equiv. Ph2PCl, 2 equiv.
Et3N, toluene, 0 ◦C; (ii) 1 equiv. [Ru(␩6-benzene)(␮-Cl)Cl]2, toluene, rt; (iii) 1 equiv. [Ru(␩6-p-cymene)(␮-Cl)Cl]2, toluene, rt.
adjacent coordination sites of Ru in an octahedral coordination
environment, leaving three facial sites for other functions, (ii) arene
ligands that are relatively weak electron donors may provide a
unique reactivity on the metallic center, and (iii) the substitution
pattern on the ring is flexible. Complexes 3 and 4 were tested
as catalysts in transfer hydrogenation of aromatic ketones in an
iso-PrOH solution. iso-PrOH is the conventional hydrogen source
having favorable properties; it is stable, easy to handle (b.p. 82 ◦C),
non-toxic, environmentally friendly, inexpensive and many organic
compounds to be catalytically reduced are soluble in it. Moreover,
the acetone product is readily removable [53,54].
In a preliminary study, complexes 3 and 4 were used as precat-
alysts, iso-PrOH/KOH as the reducing system, and acetophenone as
the model substrate. The results collected from the catalytic test
reactions are listed in Table 3. At room temperature no apprecia-
ble formation of 1-phenylethanol was observed (Table 3, Entries
1 and 2). As can be inferred from the Table 3 (Entries 3 and 4)
the precatalysts as well as the presence of KOH are necessary to
observe appreciable conversions. The base facilitates the forma-
tion of ruthenium alkoxide by abstracting proton of the alcohol
and subsequently alkoxide undergoes ␤-elimination to give ruthe-
nium hydride, which is an active species in this reaction. This
is the mechanism proposed by several workers on the studies
of ruthenium-catalyzed transfer hydrogenation reaction by metal
hydride intermediates [55–58]. In addition, performing the reac-
tion in air and water slowed down the reaction but did not affect
the conversion of the product. As shown in Table 3, increasing
the substrate-to-catalyst ratio does not damage the conversions of
the product in most cases. Remarkably, the transfer hydrogenation
of acetophenone could be achieved to 99% yield even when the
substrate-to-catalyst ratio reached 1000:1, though an increase in
the reaction time (Table 3). However, when the reaction tempera-
ture was increased to 82 ◦C smooth reduction of acetophenone into
1-phenylethanol occurred, with conversion ranging from 96.5% to
97.2% after 15 min for the reaction starting with 3 and 4 (Table 4,
Entries 1 and 2). A model reaction using acetophenone indicated
that various structural parameters including the alkyl substituents
on the arene ligands did not markedly and straightforwardly affect
the rate of the reaction. The catalytic activities of 3 and 4 are mea-
sured as TOF values of 523 h−1 and 534 h−1, respectively (Table 3,
Entries 13 and 14). These values are obtained when conversion is
less than 43.6% or 44.5%, respectively. Results obtained from opti-
mization studies indicate clearly that both complexes are active
and efficient catalysts leading to nearly quantitative conversions,
with no significant difference between the catalytic activities.
Following the optimization studies, we also examined the trans-
fer hydrogenations of aromatic ketones using complexes 3 and
4. These results are presented in Table 4. The fourth column
of Table 4 illustrates some conversions of the reduction per-
formed in a 0.1 M 2-propanol solution containing 3 or 4 and KOH
(ketone:Ru:KOH = 100:1:5).
The catalytic reduction of acetophenone derivatives was all
tested with the conditions optimized for acetophenone. Complexes
3 and 4 showed very high activity for the most of the ketones.
The introduction of electron withdrawing substituents, such as
F, Cl and Br to the para position of the aryl ring of the ketone
decreased the electron density of the C O bond so that the activ-
ity was improved giving rise to easier hydrogenation [59,60]. In
addition, the catalytic efficiency was seen not to be dependent
on the arene ligand. That is, the catalytic activity remains unaf-
fected in the presence of free arenes. As expected, in accord with
the catalytic activity shown by half-sandwich ruthenium(II) com-

80
M. Aydemir et al. / Journal of Molecular Catalysis A: Chemical 326 (2010) 75–81
Table 3
Transfer hydrogenation of acetophenone with iso-PrOH catalyzed by [C24H16N4{OPPh2–Ru(␩6-benzene)Cl2}2], 3 and [C24H16N4{OPPh2–Ru(␩6-p-cymene)Cl2}2], 4.
.
b
Entry
Catalyst
S/C/NaOH
100:1:5
Time
Conversion (%)a
TOF (h−1
)
1
2
3
4
5
6
7
8
9
10
11
12
13
24
3c
4c
3d
4d
3e
4e
3f
1 h
1 h
1 h
1 h
1 h
<1
<1
<5
<5
–
–
–
100:1:5
100:1
100:1
–
100:1:5
100:1:5
100:1:5
100:1:5
500:1:5
500:1:5
1000:1:5
1000:1:5
100:1:5
100:1:5
98.0
98.3
98.2
99.6
97.2
98.3
96.9
97.8
43.6
44.5
98
98
196
199
97
98
32
33
523
534
1 h
30 min
30 min
1 h
1 h
3 h
3 h
5 min
5 min
4f
3g
4g
3h
4h
3i
4i
a
Determined by GC (three independent catalytic experiments).
b
c
d
e
f
Referred at the reaction time indicated in column; TOF = (mol product/mol Ru(II)Cat.) × h−1
At room temperature: acetophenone/Ru/KOH, 100:1:5.
Refluxing in iso-PrOH: acetophenone/Ru/KOH, 100:1, in the absence of base.
Added 0.1 mL of H2O.
.
Refluxing the reaction in air.
g
h
i
Refluxing in iso-PrOH: acetophenone/Ru/KOH, 500:1:5.
Refluxing in iso-PrOH: acetophenone/Ru/KOH, 1000:1:5.
Refluxing in iso-PrOH: acetophenone/Ru/KOH, 100:1:5.
Table 4
Transfer hydrogenation results for substituted acetophenones with the catalyst systems prepared from [(Ph2PO)2-C24H16N4], 2 and [Ru(␩6-benzene)Cl2]2, 3 and [Ru(␩6-p-
cymene)Cl2]2, 4a.
.
c
Entry
R
Time (min)
Conversion (%)b
TOF (h−1
)
[C24H16N4{OPPh2–Ru(benzene)Cl2}2], 3
1
2
3
4
5
6
H
15
15
15
15
15
15
96.5
99.4
98.0
97.2
94.6
92.8
386
398
392
389
378
371
4-F
4-C1
4-Br
2-MeO
4-MeO
[C24H16N4{OPPh2–Ru(p-cymene)Cl2}2],4
1
2
3
4
5
6
H
15
15
15
15
15
15
97.2
99.9
97.5
97.3
93.6
92.4
389
400
390
389
374
370
4-F
4-C1
4-Br
2-MeO
4-MeO
a
b
c
Catalyst (0.005 mmol), substrate (0.5 mmol), iso-PrOH (5 mL), KOH (0.025 mmol%), 82 ◦C, 15 min for 3 and 4, the concentration of acetophenone derivatives is 0.1 M.
Purity of compounds is checked by NMR and GC (three independent catalytic experiments), yields are based on methyl aryl ketone.
TOF = (mol product/mol Cat.) × h−1
.
plexes bearing heterodifunctional P,N-ligands [61,62], P,O-ligands
[63], and NHCs ligands [64,65], complexes 3 and 4 are more active
in the catalytic transfer hydrogenations of aromatic ketones lead-
ing to nearly quantitative formation of corresponding alcohols
derivatives after 15 min. In regard the comparative catalytic per-
formance with respect to analogous complexes (bridged dinuclear
Ru(II)–phosphinite complexes), results indicate that the O–P link-
ages possibly can stabilize a catalytic transition state [66,67]. The
catalytic performance shown by both of these complexes is higher
than that of recently reported for the related half-sandwich com-
plexes [68].
4. Conclusions
This report describes the preparing and characterization of two
new binuclear Ru(II) complexes based monodendate phosphinite
ligand and investigating their use in transfer hydrogenation of
ketones. We found that these complexes are efficient homoge-

M. Aydemir et al. / Journal of Molecular Catalysis A: Chemical 326 (2010) 75–81
81
neous catalytic systems that can be readily implemented and lead
to secondary alcohols from good to excellent yields. Furthermore,
the influence of arene ring in the catalytic transfer hydrogenation
of aromatic ketones was investigated and it was seen that their
catalytic activities were very similar. The procedure is simple and
efficient towards various aryl ketones. Further studies of other tran-
sition metal complexes of this ligand are in progress and future
investigations are aiming at the development of an asymmetric
version of this catalysis.
[30] S. Kanoktanaporn, J.A. Hugh-MacBride, J. Chem. Soc., Perkin Trans. 1 (1978)
1126–1131.
[31] L. Kaczmarek, P. Nontha-Namirski, Acta Pol. Pharm. XXXVI (1979) 629–634.
[32] M.A. Bennet, A.K. Smith, J. Chem. Soc., Dalton Trans. (1974) 233–241.
[33] M.A. Bennet, T.N. Huang, T.W. Matheson, A.K. Smith, Inorg. Synth. 7 (1982)
74–78.
[34] R.A. Zelonka, M.C. Baird, J. Organomet. Chem. 35 (1972) C43.
[35] Enraf-Nonius, CAD-4 Express Software. Version 1.1, Enraf-Nonius, Delft,
Netherlands, 1993.
[36] K. Harms, S. Wocadlo, XCAD-4. Program for Processing CAD-4 Diffractometer
Data, University of Marburg, Germany, 1995.
[37] G.M. Sheldrick, SHELXS97 and SHELXL97. Program for Crystal Structure Solu-
tion and Refinement, University of Göttingen, Germany, 1997.
[38] Z. Fei, R. Scopelliti, P.J. Dyson, Inorg. Chem. 42 (2003) 2125–2130.
[39] S. Berger, S. Braun, H.-O. Kolinowski, NMR-Spektroskopie von Nichtmetallen
Band 3, 31P NMR-Spektroskopie, Georg Theime Verlag, Stuttgart, New York,
1993.
[40] Z. Fei, R. Scopelliti, P.J. Dyson, Eur. J. Inorg. Chem. (2004) 530–537.
[41] R. Tribo, S. Munoz, J. Pons, R. Yanez, A. Alvarez-Larena, J.F. Piniella, J. Ros, J.
Organomet. Chem. 690 (2005) 4072–4079.
Acknowledgements
Partial support of this work by Turkish Academy of Sciences and
Dicle University (Project Number: DÜAPK 05-FF-27) is gratefully
acknowledged.
[42] I.D. Kostas, B.R. Steele, A. Trezis, S.V. Amosova, Tetrahedron 59 (2003)
3467–3473.
[43] F. Majoumo, P. Lönnecke, O. Kühl, E. Hey-Hawkins, Z. Anorg. Allg. Chem. 630
(2004) 305–308.
[44] A.M. Maj, K.M. Pietrusiewicz, I. Suisse, F. Agbossou, A. Mortreux, Tetrahedron:
Asymmetry 10 (1999) 831–835.
[45] P. Le Gendre, M. Offenbecher, C. Bruneau, P.H. Dixneuf, Tetrahedron: Asymme-
try 9 (1998) 2279–2284.
[46] D. Morales-Morales, R. Redon, C. Yung, C.M. Jensen, Chem. Commun. (2000)
1619–1620.
[47] E. Lindler, M. Mohr, C. Nachtigal, R. Favzi, G. Henkel, J. Organomet. Chem. 595
(2000) 166–177.
[48] O. Akba, F. Durap, M. Aydemir, A. Baysal, B. Gümgüm, S. Özkar, J. Organomet.
Chem. 694 (5) (2009) 731–736.
[49] M. Alvarez, N. Lugan, R. Mathieu, J. Chem. Soc., Dalton Trans. (1994) 2755–2760.
[50] H. Yang, M. Alvarez, N. Lugan, R. Mathieu, J. Chem. Soc., Chem. Commun. (1995)
1721–1722.
[51] G. Venkatachalam, R. Ramesh, Inorg. Chem. Commun. 8 (2005) 1009–1013.
[52] E.L. Muetterties, J.R. Bleeke, E.J. Wucherer, T.A. Albright, Chem. Rev. 82 (1982)
499–525.
[53] R. Noyori, S. Hashıguchi, Acc. Chem. Res. 30 (1997) 97–102.
[54] M. Yig˘it, B. Yig˘it, I. Özdemir, E. C¸ etinkaya, B. C¸ etinkaya, Appl. Organomet. Chem.
20 (2006) 322–327.
[55] M.J. Palmer, M. Will, Tetrahedron: Asymmetry 10 (1999) 2045–2061.
[56] E.P. Kelson, P.P. Phengsy, J. Chem. Soc., Dalton Trans. (2000) 4023–4024.
[57] H. Zhang, B.C. Yang, Y.Y. Li, Z.R. Donga, J.X. Gao, H. Nakamura, K. Murata, T.
Ikariya, J. Chem. Soc., Chem. Commun. (2003) 142–143.
[58] J.S. Chen, Y. Li, Z. Dong, B. Li, J. Gao, Tetrahedron Lett. 45 (2004) 8415–8418.
[59] J.W. Faller, A.R. Lavoie, Organometallics 20 (2001) 5245–5247.
[60] P. Pelagatti, M. Carcelli, F. Calbiani, C. Cassi, L. Elviri, C. Pelizzi, U. Rizzotti, D.
Rogolino, Organometallics 24 (2005) 5836–5844.
[61] C. Standfest-Hauser, C. Slugovc, K. Mereiter, R. Schmid, K. Kirchner, L. Xiao, W.
Weissensteiner, J. Chem. Soc., Dalton Trans. (2001) 2989–2995.
[62] A.M. Maj, K.M. Pietrusiewicz, I. Suisse, F. Agbossou, A. Mortreux, J. Organomet.
Chem. 626 (2001) 157–160.
References
[1] R. Ziessel, Coord. Chem. Rev. 216–217 (2001) 195–223.
[2] R. Ziessel, A. Harriman, J. Suffert, M.-T. Youinou, A. De Cian, J. Fischer, Angew.
Chem. Int. Ed. Engl. 36 (1997) 2509–2511.
[3] C. Kaes, A. Katz, M.W. Hosseini, Chem. Rev. 100 (2000) 3553–3590.
[4] N.C. Fletcher, J. Chem. Soc., Perkin Trans. 1 (2002) 1831–1842.
[5] M. Kidwai, V. Bansal, A. Saxena, S. Aerry, S. Mozumdar, Tetrahedron Lett. 47
(2006) 8049–8053.
[6] M. Salavati-Niasari, M. Hassani-Kabutarkhani, F. Davar, Catal. Commun. 7
(2006) 955–962.
[7] X.-H. Lu, Q.-H. Xia, H.-J. Zhan, H.-X. Yuan, C.-P. Ye, K.-X. Su, G. Xu, J. Mol. Catal.
A (2006) 62–69.
[8] G.G. Mohamed, Z.H. Abd-El Wahab, Spectrochim. Acta A 61 (2005) 1059–1068.
[9] G. Garcia-Friaza, A. Fernandez-Botello, J.M. Perez, M.J. Prieto, V. Morens, J. Inorg.
Biochem. 100 (2006) 1368–1377.
[10] M.D. Rudd, M.A. Creighton, J.A. Kautz, Polyhedron 23 (2004) 1923–1929.
[11] K.G. Gaw, M.B. Smith, J.W. Steed, J. Organomet. Chem. 664 (2002) 294–297.
[12] F. Agbossou, J.-F. Carpentier, F. Hopiot, I. Suisse, A. Mortreux, Coord. Chem. Rev.
178–180 (1998) 1615–1645.
[13] M.R. Zuburi, H.L. Milton, A.M.Z. Slawin, J.D. Woollins, Inorg. Chim. Acta 357
(2004) 243–249.
[14] H.J. Kreuzfelt, C. Döbler, J. Mol. Catal. A: Chem. 136 (1998) 105–110.
[15] F. Agbossou-Niedercorn, I. Suisse, Coord. Chem. Rev. 242 (2003) 145–158.
[16] M. Haiharasarma, C.H. Lake, C.L. Watkins, G.M. Gray, J. Organomet. Chem. 580
(1999) 328–338.
[17] P. Bergamini, V. Bertolasi, F. Milani, Eur. J. Inorg. Chem. (2004) 1277–1284.
[18] G.D. Cuny, S.L. Buchwald, J. Am. Chem. Soc. 115 (1993) 2066–2068.
[19] Z. Csok, G. Szalontai, G. Czira, L. Kollar, J. Organomet. Chem. 475 (1998) 23–29.
[20] R. Ceron-Camacho, V. Gomez-Benitez, R. Le Lagadec, D. Morales-Morales, R.A.
Toscano, J. Mol. Catal. A 247 (2006) 124–129.
[21] M.S. Balakrishna, P.P. George, S.M. Mobin, Polyhedron 24 (2005) 475–480.
[22] L. Barloy, G. Malaise, S. Ramdeehul, C. Newton, J.A. Osborn, N. Kyritsakas, Inorg.
Chem. 42 (2003) 2902–2907.
[23] T.J. Geldbach, A.B. Chaplin, K.D. Hanni, R. Scopelliti, P.J. Dyson, Organometallics
24 (2005) 4974–4980.
[24] A. Baysal, M. Aydemir, F. Durap, B. Gümgüm, S. Özkar, L. Tatar Yıldırım, Poly-
hedron 26 (2007) 3373–3378.
[25] M. Aydemir, A. Baysal, B. Gümgüm, J. Organomet. Chem. 693 (2008) 3810–3814.
[26] T. Naota, H. Takaya, S.-I. Murahashi, Chem. Rev. 98 (1998) 2599–2660.
[27] F. Fache, E. Schulz, M.L. Tommasino, M. Lemaire, Chem. Rev. 100 (2000)
2159–2231.
[28] V. Turcry, C. Pasquier, F. Agbossou-Niedercorn, C. R. Chim. 6 (2003) 179–184.
[29] H. Jiang, Q.D. Qiao, H. Gong, React. Kinet. Catal. Lett. 65 (1998) 193–197.
[63] A. Caballero, F.A. Jalon, B.R. Manzano, G. Espino, M. Perez-Manrique, A.
Mucientes, F.J. Poblete, M. Maestro, Organometallics 23 (2004) 5694–5706.
˙
[64] I. Özdemir, N. S¸ ahin, B. C¸ etinkaya, Monatsh. Chem. 138 (2007) 205–209.
˙
[65] I. Özdemir, S. Yas¸ ar, Trans. Met. Chem. 30 (2005) 831–835.
[66] K.-J. Haack, S. Hashiguchi, A. Fuji, T. Ikariya, R. Noyori, Angew. Chem. Int. Ed.
Engl. 36 (1997) 285–288.
[67] J.X. Gao, X.D. Yi, P.P. Xu, C.L. Tang, H.L. Wan, T. Ikariya, J. Organomet. Chem. 592
(1999) 290–295.
[68] P. Dani, T. Karlen, R.A. Gossage, S. Gladialli, F.G. Koten, Angew. Chem. Int. Ed.
Engl. 39 (2000) 743–745.