Synthesis and characterization of Ru(arene) complexes of bispyrazolylazines: Catalytic hydrogen transfer of ketones

Article abstract of DOI:10.1016/j.ica.2009.04.011

The bis(pyrazol-1-yl)azine ligands 2,3-bis(pyrazol-1-yl)quinoxaline (bpzqnx), 2,3-bis(pyrazol-1-yl)pyrazine (bpzprz) and 3,6-bis(3,5-dimethylpyrazol-1-yl)pyridazine (bpz*pdz) were prepared by the reaction of pyrazolate salts and the corresponding azine di

Full text of DOI:10.1016/j.ica.2009.04.011

Inorganica Chimica Acta 362 (2009) 4486–4492
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Synthesis and characterization of Ru(arene) complexes of bispyrazolylazines:
Catalytic hydrogen transfer of ketones q
a
b
M. Laura Soriano a, Félix A. Jalón a, , Blanca R. Manzano , Miguel Maestro
*
a Departamento de Química Inorgánica, Orgánica y Bioquímica, Facultad de Químicas-IRICA, UCLM, Avda. Camilo J. Cela 10, 13071 Ciudad Real, Spain
b Departamento de Química Fundamental, Universidade da Coruña, 15071 A Coruña, Spain
a r t i c l e i n f o
a b s t r a c t
Article history:
The bis(pyrazol-1-yl)azine ligands 2,3-bis(pyrazol-1-yl)quinoxaline (bpzqnx), 2,3-bis(pyrazol-1-yl)pyra-
zine (bpzprz) and 3,6-bis(3,5-dimethylpyrazol-1-yl)pyridazine (bpz*pdz) were prepared by the reaction
of pyrazolate salts and the corresponding azine dichloride derivatives. The reaction of these ligands with
Ru(arene) precursors led to the mononuclear complexes [RuCl(arene)(L)]BPh4 (arene = p-cymene,
L = bpzqnx, 1, bpzprz, 5, bpz*pdz, 7; arene = C6H6, L = bpzqnx, 2, bpzprz, 6, bpz*pdz, 8) with the N-donor
ligand coordinated in a bidentate chelate way. In general, the ligands coordinate through one pyrazole
ring and the azine, except in the cases of 1 and 2 where the two pyrazolyl rings are coordinated to the
metal in a symmetrical way. When the reactions between the ruthenium precursors and bpzqnx are car-
ried out in MeOH, the complexes [RuCl(arene)(OMepzqnx)]BPh4 with partially methanolyzed ligands are
isolated (arene = p-cymene, 3; C6H6, 4). In this process a methoxy group has replaced one of the pyrazole
groups in the ligand. The X-ray structures of 6 and 7 have been determined. These compounds have a
three-legged piano-stool structure with cations and anions packed through weak interactions. Complexes
1–8 are active in ketone hydrogenation transfer processes even in the absence of base.
Received 29 January 2009
Received in revised form 1 April 2009
Accepted 16 April 2009
Available online 23 April 2009
Dedicated to Prof. Jerry Trofimenko
Keywords:
Ruthenium
Pyrazole
Catalysis
Hydrogen transfer
N-donor ligands
Arene derivatives
Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction
2. Experimental
Ru(arene) derivatives constitute a family of interesting precur-
sors in a variety of catalytic processes [1]. Noyori and other authors
have designed very active systems with basic centres (N-donor
mainly) in the asymmetric catalytic reduction of ketones using
alcohols as the hydrogen source [2]. This situation has raised inter-
est in the preparation of systems bearing N-donor centres that
could participate in one or more steps of these hydrogenation pro-
cesses [3]. However, the use of arene ruthenium derivatives with
heterocyclic auxiliary ligands as polypyridines in catalytic hydro-
genation processes has not been explored very much [4]. Our expe-
rience in the preparation of Ru(arene) complexes with N-donor
ligands and their use in hydrogen transfer reactions [5] inspired
the work described here, which focuses on the study of the effect
that uncoordinated basic centres could have in hydrogen transfer
catalytic processes. With this aim in mind, a family of bis(pyra-
zol-1-yl)azines, which are easily prepared, have been chosen. The
preparation of mononuclear [RuCl(arene)(L)]BPh4 derivatives was
carried out and these compounds were used as precursors in
hydrogen transfer reaction processes with and without a base as
co-catalyst.
2.1. General procedures
All manipulations were carried out under an atmosphere of dry
oxygen-free nitrogen using standard Schlenk techniques. Elemen-
tal analyses were performed with a Carlo Erba Instruments EA
1108 CHNS/O microanalyzer. IR spectra were recorded on a Shima-
dzu IRPRESTIGE-21 spectrophotometer deposing a pure solid sam-
ple over an ATR device (4000–700 cmꢀ1 range). FAB+ mass spectra
(position of the peaks in Da) were recorded with a VG Biotech
Quattro Spectrometer. NMR spectra were recorded at room tem-
perature (25 °C) on Varian Unity Inova-400 (400 MHz for 1H;
100.6 MHz for 13C) and Varian Inova FT-500 (500 MHz for 1H;
125 MHz for 13C) spectrometers. 1H shifts (ppm) were recorded
using the residual proton signal of the solvent as an internal stan-
dard (see numbering of protons and carbons in Scheme 1). For the
acquisition of the COSY, g-HMBC and g-HMQC spectra the standard
VARIAN pulse sequences were used (VNMR 6.1 C software). The fol-
lowing parameters were used for COSY: acquisition time 0.214 s,
pulse width 10 ls, relaxation delay 1 s, 16 scans, 512 increments.
For the g-HMBC and g-HMQC the spectra were acquired using
7996-Hz (1H) and 25133.5-Hz (13C) widths; 16 transients of 2048
data points were collected for each of the 128 increments. The
nOe difference spectra were recorded with 5000 Hz, acquisition
time 3.27 s, pulse width 90°, relaxation delay 4 s, and irradiation
power 5–10 dB. All the 13C{1H} NMR resonances are singlets.
q
In memoriam of our friend and master Jerry Trofimenko.
* Corresponding author. Tel.: +34 926295300; fax: +34 926295318.
E-mail address: Felix.Jalon@uclm.es (F.A. Jalón).
0020-1693/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2009.04.011

M.L. Soriano et al. / Inorganica Chimica Acta 362 (2009) 4486–4492
4487
BPh4
6
7
BPh4
R
8
5
4a
8a
R'
N
N
[Ru]
Ru
N
N
2
Cl
N
N
or
3
N
MeO
S
N
5'
Cl
N
N
N
N
N
NaBPh4
N
4'
3'
N
N
Ru
S= MeOH, R = Me, R' = iPr, 3
S= MeOH, R = R' = H, 4
bpzqnx
R
R'
S = CH2Cl2, R = Me, R' = iPr, 1
S = CH2Cl2, R = R' = H, 2
3
5
2
R
BPh4
1
4
6
5
6
[Ru]
N
N
N
N
3
N
2
R'
Ru
3'
Cl
S
N
N
N
N
N
5''
NaBPh4
N
N
5'
4''
4'
3''
bpzprz
i
S = CH2Cl2, R = Me, R' = Pr, 5
S = MeOH, R = R' = H, 6
BPh4
R'
Cl
[Ru]
N
N
N
R
Ru
N
N
5''
3'
N
N
N
N
CH2Cl2
4''
N
N
6
3
4'
NaBPh4
N
5'
3''
4
5
bpz*pdz
[Ru] = [Ru(p-cymene)Cl2]2
i
R = Me, R' = Pr, 7
R = R' = H, 8
or [Ru(C6H6)Cl2(CH3CN)]
Scheme 1.
P
2.2. Solvents and reagents
w||Fo|2 ꢀ |Fc|2|2, where w = [
r
2(I) + (0.0355P)2 + 9.1687P]ꢀ1 for 6
and w = [
r
2(I) + (0.0327P)2 + 5.8723P]ꢀ1 for 7, and P = (|Fo|2 +
Solvents were supplied by SDS (reagent grade) and distilled
from the appropriate drying agents and degassed before use. Start-
ing materials [RuCl2(p-cymene)]2 [6] and [RuCl2(C6H6)]2) [7] were
prepared according to literature procedures. NaBPh4, acetophe-
none and benzophenone were purchased from Aldrich.
2|Fc|2)/3. The values of f, f 0 and f 00 were taken from International
Tables of X-ray Crystallography [9]. All hydrogen atoms were com-
puted and refined using a riding model. For compound 6, the final R
(on F) factor was 0.0354, wR (on |F|2) = 0.0969 and goodness of fit =
1.243 for all observed reflections. The number of refined parame-
ters was 442. Max. shift/esd = 0.002, Mean shift/esd = 0.00. Max.
and Min. peaks in final difference synthesis were 0.871 and
ꢀ0.592 eÅꢀ3, respectively. For compound 7, the final R (on F) factor
was 0.0343, wR (on |F|2) = 0.079 and goodness of fit = 1.021 for all
observed reflections. The number of refined parameters was 514.
Max. shift/esd = 0.001, Mean shift/esd = 0.00. Max. and Min. peaks
2.3. Structure solution and refinement
Suitable single crystals of the title compounds for X-ray study
were grown from the vapour transfer of pentane over a methanol
solution of 6 or the slow evaporation of a methanol solution of 7.
Crystal data and refinement are summarized in Table S1 for com-
pound 6 and in Table S2 for compound 7. A yellow prism
(0.33 ꢁ 0.29 ꢁ 0.08 mm) of compound 6 and a yellow irregular
block (0.33 ꢁ 0.23 ꢁ 0.20 mm) of compound 7 were selected and
mounted on a Bruker SMART-CCD area diffractometer. Unit cell
parameters were determined from 1271 frames of intensity data
in final difference synthesis were 1.517 and ꢀ1.092 eÅꢀ3
,
respectively.
2.4. Preparation of ligands and complexes
2.4.1. Synthesis of 2,3-bis(pyrazol-1-yl)quinoxaline (bpzqnx)
covering 0.3° in
x
over a hemisphere of the reciprocal space by
A solution of pyrazole (729.1 mg, 10.71 mmol) in 10 ml of THF
was added dropwise to suspension of NaH (257.2 mg,
combination of three exposure sets, and refined by the least-
a
squares method. Intensities were collected with graphite mono-
10.71 mmol) in 10 ml of THF. After 2 h of reaction at room temper-
ature the solution was filtered and 2,3-dichloroquinoxaline
(0.710 g, 3.57 mmol) was added to the solution. The mixture was
refluxed during 4 h and then the solvent was removed under vac-
uum. The product was extracted with CH2Cl2 (3 ꢁ 10 ml), then the
solvent was evaporated and finally the solid was washed with 2 ml
of ethylacetate to remove the excess of pyrazole. Yield: 0.655 g
(70%). Anal. Calc. for C14H10N6: C, 64.10; H, 3.84; N, 32.04. Found:
C, 63.94; H, 3.83; N, 31.92%. IR (Nujol, cmꢀ1): 1554 (m), 1526
(m). 1H NMR (CDCl3) d (ppm): 8.08 (m, 2H, H5,8), 7.77 (m, 2H,
chromatized Mo Ka radiation (k = 0.71073 Å) using the x/2h
scan-technique. A total of 40 592 reflections for 6 were measured
in the range 1.21 6 h 6 26.37 and 26 018 reflections for 7 were
measured in the range 1.68 6 h 6 26.37. Lorentz-polarization and
absorption corrections were made.
The structures were solved by direct methods using the SHELXS
computer program [8] and refined by the full-matrix least-squares
method with the SHELX97 computer program [8], using 6786 reflec-
tions for 6 and 8471 reflections for 7. The function minimized was

4488
M.L. Soriano et al. / Inorganica Chimica Acta 362 (2009) 4486–4492
0
0
H6,7), 6.52 (dd, 2H, H4 ), 7.63 (d, J = 1.5 Hz, 2H, H3 ), 7.97 (d,
and THF, partially soluble in chloroform and insoluble in diethyl-
ether and alkanes. Yield: 78.4 mg (80%). Anal. Calc. for
C46H44BClN4ORu: C, 67.69; H, 5.43; N, 6.86. Found: C, 67.59; H,
5.35; N, 6.81%. IR (ATR, cmꢀ1): 3065 (s), 3002 (s), 1578 (m), 1526
(m), 1478 (s), 1044 (s). FAB+ MS (NBA): m/z (%) = 497 (100) [M+–
(BPh4)]. 1H NMR (CDCl3) d (ppm): 8.82 (d, J = 3.4, 1H, H5’), 8.33
(d, J = 8.6 Hz, 1H, H5), 8.05 (d, J = 8.3 Hz, 1H, H8), 7.90, 7.81 (2d,
J ꢂ 7.3 Hz, 2H, H6,7), 7.65 (d, J = 2.0, 1H, H3’), 6.62 (t, 1H, H4’), 5.30
(d, J = 6.4 Hz, 1H, Hc2ym), 5.24 (d, J = 5.9 Hz, 1H, H6cym), 5.16 (d,
J = 6.4 Hz, 1H, H5cym), 5.02 (d, J = 6.3 Hz, 1H, Hc3ym), 4.36 (s, 3H,
MeO), 2.31 (m, J = 6.8 Hz, 1H, CHMe2), 1.84 (s, 3H, Mecym), 0.96,
0.90 (2d, J = 6.8 Hz, 6H, CHMe2 cym). 13C{1H} NMR (CDCl3) d (ppm):
0
J = 3.4 Hz, 2H, H5’). 13C{1H} NMR (CD3Cl): 142.8 (C3 ), 140.6 (C2,3),
0
0
140.1 (C4a,8a), 131.2 (C6,7), 130.4 (C5 ), 128.8 (C5,8), 108.3 (C4 ).
2.4.2. Synthesis of 2,3-bis(pyrazol-1-yl)pyrazine (bpzprz)
The synthesis of bpzprz is similar to that of bpzqnx. The
amounts were as follows: pyrazole (959.6 mg, 14.10 mmol), NaH
(338.2 mg, 14.10 mmol) and 2,3-dichloropyrazine (631.9 g,
4.24 mmol). The mixture was refluxed during 24 h and then the
solvent was removed under vacuum. The product was extracted
with toluene (3 ꢁ 10 ml), then the solvent was removed and finally
the solid was washed (3 ꢁ 2 ml) with ethylacetate to remove the
pyrazole in excess. Yield: 0.540 g (60%). Anal. Calc. for C14H10N6:
C, 56.60; H, 3.80; N, 39.60. Found: C, 56.47; H, 3.72; N, 39.10%. IR
(Nujol, cmꢀ1): 1584 (m), 1526 (m). 1H NMR (acetone-d6) d
(ppm): 8.64 (s, 2H, H5,6), 8.14 (dd, J = 2.2 Hz, 2H, H5’), 7.55 (d,
150.4 (C3’), 138.9 (C5’), 132.2 (C8), 130.4 (C5), 128.5, 128.3 (C6,7),
2,5
113.8 (C4’), 87.7 (Cc6ym), 85.3 (Cc3ym), 84.2 (C ), 56.6 (OMe), 34.4
cym
(CHMe2 cym), 22.7, 21.8 (2C, CHMe2 cym), 18.8 (Me cym).
0
0
J = 1.8 Hz, 2H, H3 ), 6.47 (dd, 2H, H4 ). 13C{1H} NMR (CDCl3): 142.8
2.4.6. Synthesis of [RuCl(benzene)(OMepzqnx)](BPh4)ꢃ(MeOH)2 (4)
The synthesis of 4 is similar to that of 1 but it was carried out in
MeOH as solvent. The amounts were as follows: bpzqnx (77.7 mg,
0.30 mmol) in 20 ml of methanol, [RuCl(C6H6)(MeCN)] (86.2 mg,
0.30 mmol) and NaBPh4 (101.3 mg, 0.30 mmol). The resulting
product is a yellow-brownish solid and it is purified by recrystalli-
zation in acetone/pentane. 4 is soluble in acetone and methanol,
partially soluble in chloroform and insoluble in diethylether and
alkanes. Yield: 187.9 mg (76%). Anal. Calc. for C44H44BClN4O3Ru:
C, 64.12; H, 5.38; N, 6.80. Found: C, 64.19; H, 5.15; N, 6.85%. IR
(ATR, cmꢀ1): 3003 (s), 1577 (m), 1525 (m), 1479 (s), 1043 (s).
FAB+ MS (NBA): m/z (%) = 441 (100) [M+–(BPh4)]. 1H NMR (CDCl3)
d (ppm): 8.80 (d, J = 2.9, 1H, H5’), 8.24 (d, J = 8.6 Hz, 1H, H5), 8.08
(d, J = 8.3 Hz, 1H, H8), 7.90, 7.80 (2d, J = 7.1, 7.8 Hz, 2H, H6,7), 7.71
(d, J = 2.0, 1H, H3’), 6.65 (t, 1H, H4’), 5.17 (s, 6H, C6H6), 4.35 (s, 3H,
MeO). 13C{1H} NMR (CDCl3) d (ppm): 150.0 (C3’), 138.1 (C5’),
128.7, 128.0 (C5,8), 132.3, 130.5 (C6,7), 113.6 (C4’), 86.9 (C6H6),
50.3 (OMe).
0
0
0
(C3 ), 141.5 (C5,6), 130.3 (C5 ), 108.4 (C4 ).
2.4.3. Synthesis of [RuCl(p-cymene)(bpzqnx)](BPh4) (1)
To a suspension of [RuCl2(p-cymene)]2 (68.8 mg, 0.11 mmol) in
20 ml of dry dichloromethane, bpzqnx (59.0 mg, 0.22 mmol) was
added. The mixture was stirred at room temperature during 12 h
and then NaBPh4 (77.2 mg, 0.22 mmol) was added. After an hour
of stirring the solvent was evaporated and the product was purified
by extraction in dichloromethane (3 ꢁ 5 ml). The resulting product
is soluble in acetone, dichloromethane and THF and insoluble in
diethylether and alkanes. Yield: 159.3 mg (85%). This product can
be recrystallised in dichloromethane/hexane. Anal. Calc. for
C48H44BClN6Ru: C, 69.69; H, 4.50; N, 9.38. Found: C, 69.99; H,
4.34; N, 9.21%. IR (ATR, cmꢀ1): 3047 (s), 2984 (s), 1576 (m), 1555
(m), 1520 (m), 1493 (s), 1031 (s). FAB+ MS (NBA): m/z (%) = 533
(100) [M+–(BPh4)]. 1H NMR (CDCl3) d (ppm): 8.19 (m, J = 2.9 Hz,
2H, H6,7), 8.02 (d, J = 2.4 Hz, 2H, H3’), 7.95 (m, J = 2.9, 2H, H5,8),
7.90 (d, J = 2.9, 2H, H5’), 6.52 (dd, 2H, H4’), 4.98 (d, J = 5.9 Hz, 2H,
2,6
3,5
H
), 4.31 (d, J = 6.4 Hz, 2H, H ), 2.24 (m, J = 6.4 Hz, 1H, CHMe2
2.4.7. Synthesis of [RuCl(p-cymene)(bpzprz)](BPh4) (5)
cym
cym
cym), 1.26 (s, 3H, Mecym), 1.14 (d, J = 7.3 Hz, 6H, CHMe2 cym).
13C{1H} NMR (CDCl3) d (ppm): 148.2 (2C, C3’), 140.9 (C4a,8a), 139
(C2,3), 138.6 (2C, C5’), 134 (C5,8), 129.6 (C6,7), 111.0 (2C, C4’), 104.2
The synthesis of 5 is similar to that of 1. The amounts were as
follows: 2,3-bis(pyrazol-1-yl)pyrazine (32.4 mg, 0.15 mmol),
[RuCl2(p-cymene)]2 (46.8 mg, 0.08 mmol) and NaBPh4 (52.4 mg,
0.15 mmol) in 15 ml of dichloromethane. The yellow product is
purified by recrystallization in a mixture of dichloromethane/pen-
tane. 5 is soluble in acetone, dichloromethane and chloroform and
insoluble in diethylether and alkanes. Yield: 90.3 mg (75%). Anal.
Calc. for C44H42BClN6Ru: C, 65.88; H, 5.28; N, 10.48. Found: C,
65.83; H, 5.35; N, 10.56%. IR (ATR, cmꢀ1): 3036 (s), 2965 (s),
1580 (m), 1535 (m), 1520 (m), 1472 (s), 1030 (s). FAB+ MS
(NBA): m/z (%) = 483 (100) [M+–(BPh4)]. 1H NMR (acetone-d6) d
(ppm): 9.47 (d, 1H, H6), 8.78 (dd, J = 2.2, 0.5 Hz, 1H, H3’), 8.72 (d,
J = 2.9 Hz, 1H, H5), 8.39 (dd, J = 2.7, 1.7 Hz, 1H, H5’’), 7.96 (dd,
2,6
3,5
(C1cym), 102.0 (Cc4ym), 86.9 (C ), 82.4 (C ), 30.9 (CHMe2 cym),
cym
cym
22.4 (CHMe2 cym), 18.3 (Mecym).
2.4.4. Synthesis of [RuCl(benzene)(bpzqnx)](BPh4) (2)
The synthesis of 2 is similar to that of 1. The amounts were as
follows: bpzqnx (67.7 mg, 0.26 mmol), [RuCl(C6H6)(MeCN)]
(75.1 mg, 0.26 mmol) and NaBPh4 (88.3 mg, 0.26 mmol). The
resulting product is a yellow solid and is purified by recrystalliza-
tion in acetone/hexane. 2 is soluble in acetone and chloroform and
insoluble in diethylether and alkanes. Yield: 169.7 mg (82%). Anal.
Calc. for C44H36BClN6Ru: C, 66.38; H, 4.56; N, 10.56. Found: C,
66.39; H, 4.42; N, 10.75%. IR (ATR, cmꢀ1): 3053 (s), 1567 (m),
1545 (m), 1529 (m), 1490 (s), 1030 (s). FAB+ MS (NBA): m/z
(%) = 477 (100) [M+–(BPh4)]. 1H NMR (CDCl3) d (ppm): 8.63 (d,
J = 2.6, 2H, H5’), 8.39 (m, J = 3.3 Hz, 2H, H6,7), 8.29 (d, J = 2.2, 2H,
H3’), 8.23 (m, J = 3.3 Hz, 2H, H5,8), 6.33 (dd, 2H, H4’), 5.77 (s, 6H,
benzene). 13C{1H} NMR (CDCl3) d (ppm): 147.2 (2C, C3’), 140.1
(C4a,8a), 138.3 (C2,3), 137.5 (2C, C5’), 133.2 (C5,8), 130.6 (C6,7),
110.5 (2C, C4’), 87.0 (C6H6).
J = 2.2, 1.7 Hz, 1H, H3’’), 7.17 (dd, J = 2.9, 0.5 Hz, 1H, H5’), 6.84 (dd,
2,6
1H, H4’), 6.75 (dd, 1H, H4’’), 6.27, 6.17 (d, J = 6.1 Hz, 2H, H ),
cym
3,5
6.04, 5.95 (d, J = 6.1 Hz, 2H, H ), 2.86 (m, J = 6.8 Hz, 1H, CHMe2
cym
cym), 2.25 (s, 3H, Mecym), 1.22, 1.17 (2d, J = 6.8 Hz, 6H, CHMe2 cym).
13C{1H} NMR (acetone-d6): 149.0 (C3’), 147.5 (C6), 144.0 (C3’’),
142.0 (C5), 140.0 (C2), 138.2 (C3), 135.0 (C5’), 132.6 (C5’’), 112.4
(C4’), 110.0 (C4’’), 107.1 (Cc1ym), 103.9 (C4cym), 86.3, 85.5 (C ), 85.2,
2,6
cym
3,5
84.6 (C ), 31.2 (CHMe2 cym), 21.7, 21.6 (2C, CHMe2 cym), 18.1
cym
(Mecym).
2.4.5. Synthesis of [RuCl(p-cymene)(OMepzqnx)](BPh4) (3)
2.4.8. Synthesis of [RuCl(benzene)(bpzprz)](BPh4)ꢃ(MeOH)2 (6)
The synthesis of 6 is similar to that of 1 but it was carried out in
MeOH as solvent. The amounts were as follows: 2,3-bis(pyrazol-1-
yl)pyrazine (60.9 mg, 0.3 mmol), [RuCl(C6H6)(MeCN)] (83.6 mg,
0.3 mmol), NaBPh4 (98.2 mg, 0.3 mmol) in 10 ml of methanol.
The yellow product is soluble in acetone and methanol, partially
soluble in dichloromethane and insoluble in diethylether and
The synthesis of 3 is similar to that of 1 but was carried out in
MeOH as solvent. The amounts were as follows: [RuCl2(p-cym-
ene)]2 (37.7 mg, 0.06 mmol) in 15 ml of methanol, bpzqnx
(32.3 mg, 0.12 mmol) and NaBPh4 (42.1 mg, 0.12 mmol). The
resulting product is a yellow-brownish solid and it was purified
by recrystallization in acetone/hexane. 3 is soluble in acetone

M.L. Soriano et al. / Inorganica Chimica Acta 362 (2009) 4486–4492
4489
alkanes. Yield: 175.0 mg (72%). Single crystals were obtained by
recrystallization of the product in a mixture of MeOH/pentane.
Anal. Calc. for C42H42BClN6O2Ru: C, 62.27; H, 5.23; N, 10.37. Found:
C, 62.26; H, 5.38; N, 10.83%. IR (ATR, cmꢀ1): 3053 (s), 1593 (m),
1575 (m), 1526 (s), 1474 (s), 1034 (s). FAB+ MS (NBA): m/z
(%) = 427 (100) [M+–(BPh4)]. 1H NMR (acetone-d6) d (ppm): 9.60
(d, 1H, H6), 8.84 (dd, J = 2.2, 0.5 Hz, 1H, H3’), 8.75 (d, J = 3.2 Hz,
1H, H5), 8.42 (dd, J = 2.7, 1.7 Hz, 1H, H5’’), 7.98 (t, J = 1.7 Hz, 1H,
10 ml of 2-propanol for 24 h. 2-Propanol was removed under vac-
uum and an aliquot of the remaining product was analysed by 1H
NMR in CDCl3. The yield of the process was calculated considering
the relative integrals of ketone and alcohol.
3. Results and discussion
3.1. Synthesis of ligands and complexes
H
3’’), 7.14 (dd, J = 3.2, 0.5 Hz, 1H, H5’), 6.85 (dd, 1H, H4’), 6.79 (dd,
1H, H4’’), 6.29 (s, 6H, C6H6). 13C{1H} NMR (acetone-d6): 149.2
(C3’), 148.0 (C6), 144.0 (C3’’), 141.8 (C5), 140.1 (C2), 138.2 (C3),
134.9 (C5’), 132.8 (C5’’), 112.4 (C4’), 109.9 (C4’’), 87.6 (C6H6).
The bis(pyrazol-1-yl)azine ligands used in this work (see
Scheme 1) were prepared from the corresponding sodium pyrazo-
late (obtained in THF after the reaction of NaH with pyrazole or
3,5-dimethylpyrazole), which was reacted with the respective
dichloroazine.
2.4.9. Synthesis of [RuCl(p-cymene)(bpz*pdz)](BPh4)ꢃ(CH2Cl2) (7)
The synthesis of 7 is similar to that of 1. The amounts were as fol-
The ligands 2,3-bis(pyrazol-1-yl)quinoxaline, bpzqnx, and 2,3-
bis(pyrazol-1-yl)pyrazine, bpzprz, were prepared in this study for
the first time. The derivative 3,6-bis(3,5dimethylpyrazol-1-yl)pyr-
idazine, bpz*pdz, has been described previously and prepared by
different methods [10]. We used a method similar to that described
by Thompson et al. [10c]. This compound has been used as a ligand
with Cu(I) [11], Rh(I) [12], Cu(II) and Co(II) [10c,13]. The similar li-
gand 3,6-bis(pyrazol-1-yl)pyridazine was also previously prepared
by a similar method [14] and used as a ligand in Cu coordination
chemistry [13b,15].
All of these ligands can be considered as ditopic and suitable for
a bisbidentate chelate coordination mode with potential coordina-
tion of the N-pyrazole and N-azine atoms. The ligands differ in the
position of the pyrazole ring in the azine, with this ring being 1,2 in
bpzqnx and bpzprz and 1,4 in bpz*pdz.
The cationic complexes 1, 2, 5, 7 and 8 were easily prepared by
the reaction of the respective ligands and [Ru(p-cymene)Cl2]2 or
[Ru(C6H6)Cl2(CH3CN)] in CH2Cl2 in the presence of NaBPh4 (Scheme
1). When the reactions of the Ru precursors with bpzqnx were car-
ried out in MeOH as the solvent, complexes 3 and 4 were obtained
as a consequence of the partial alcoholysis of the ligand bpzqnx.
This reactivity can be attributed to the electrophilic character of
the heterocyclic ring of the quinoxaline. The concomitant presence
of alkoxy and heterocyclic groups as substituents in quinoxaline
derivatives is uncommon [16], although (3-heterocycle)(4-alk-
oxy)quinoxaline systems have been described as herbicides [17]
and other active pharmaceuticals drugs [18]. A different type of
behaviour was observed for the ligand bpzprz because in this case,
although the reaction with [Ru(C6H6)Cl2(CH3CN)] was performed
in MeOH, complex 6 was obtained and the alcoholysis of the ligand
was not observed. All of these complexes were isolated in good
yields, sometimes as solvate species, and the microanalysis data
support their purity.
lows:
3,6-bis(3,5-dimethylpyrazol-1-yl)pyridazine
(44.5 mg,
0.17 mmol) in 30 ml of dichloromethane, [RuCl2(p-cymene)]2
(50.8 mg, 0.08 mmol) and NaBPh4 (56.8 mg, 0.17 mmol). The prod-
uct was isolated as a solvate with one molecule of CH2Cl2. The yellow
product is soluble in acetone, dichloromethane and methanol and
insoluble in diethylether and alkanes. Yield: 136.2 mg (85%). Single
crystals were obtained by slow evaporation of a methanol solution
of the product. Anal. Calc. for C49H52BCl3N6Ru: C, 62.40; H, 5.56; N,
8.91. Found: C, 62.60; H, 5.49; N, 9.09%. IR (ATR, cmꢀ1): 3053 (s),
3034 (s), 2968 (s), 1591 (m), 1578 (m), 1549 (m), 1479 (s), 1028
(s). FAB+ MS (NBA): m/z (%) = 539 (100) [M+-(BPh4)]. 1H NMR (CDCl3)
d (ppm): 8.02 (d, 1H, H5), 7.12 (d, J = 9.4 Hz, 1H, H4), 6.12 (s, 1H, H4’’),
6.10 (s, 1H, H4’), 5.52, 5.50 (2d, J = 6.2 Hz, 2H, H ), 5.33, 5.29 (2d,
3,5
cym
J = 6.1 Hz, 2H, H ), 2.68 (s, 1H, Me5’’), 2.46 (s, 1H, Me3’), 2.55 (m,
2,6
cym
1H, CHMe2 cym), 2.28 (s, 1H, Me3’’), 2.15 (s, 3H, Mecym), 2.02 (s, 1H,
Me5’), 1.04, 1.03 (2d, J = 6.8 Hz, 6H, CHMe2 cym). 13C{1H} NMR (CDCl3)
d (ppm): 157.4 (C3’), 154.8 (C5), 153.6 (C3’’), 149.5 (C4), 145.7 (C5’),
141.8 (C5’’), 120.1 (C6), 114.6 (C4’), 112.3 (C4’’), 107.3 (Cc1ym), 104.0
(C4cym), 86.9, 84.6 (C ), 84.3 (C ), 31.5 (CHMe2 cym), 22.7, 22.6
2,6
3,5
cym
cym
(2C, CHMe2 cym), 16.4 (Me3’), 15.5 (Me5’’), 14.6 (Me5’), 14.1 (Me3’’),
19.3 (Me cym).
2.4.10. Synthesis of [RuCl(benzene)(bpz*pdz)](BPh4)ꢃ(CH2Cl2)2 (8)
The synthesis of 8 is similar to that of 1. The amounts were as
follows: 3,6-bis(3,5-dimethylpyrazol-1-yl)pyridazine (52.7 mg,
0.2 mmol) in 20 ml of dichloromethane, [RuCl(C6H6)(MeCN)]
(57.2 mg, 0.2 mmol), NaBPh4 (67.22 mg, 0.2 mmol). The product
was isolated as a solvate with two molecules of CH2Cl2. The yellow
product is soluble in acetone and methanol, partially soluble in
dichloromethane and insoluble in diethylether and alkanes. Yield:
172.7 mg (89%). Anal. Calc. for C46H46BCl5N6Ru: C, 56.94; H, 4.77;
N, 8.65. Found: C, 56.82; H, 4.73; N, 8.63%. IR (ATR, cmꢀ1): 3059
(s), 2980 (s), 1576 (m), 1553 (m), 1479 (s), 1030 (s). FAB+ MS
(NBA): m/z (%) = 483 (100) [M+–(BPh4)]. 1H NMR (CDCl3) d (ppm):
8.08 (d, J = 9.8 Hz, 1H, H5), 7.14 (d, J = 9.8 Hz, 1H, H4), 6.13 (s, 1H,
3.2. Structural characterization
H
4’’), 6.11 (s, 1H, H4’), 5.29 (s, 6H, C6H6), 2.66 (s, 1H, Me5’’), 2.39
(s, 1H, Me3’), 2.28 (s, 1H, Me3’’), 2.16 (s, 1H, Me5’). 13C{1H} NMR
(CDCl3) d (ppm): 157.4 (C3’), 154.7 (C5), 153.6 (C3’’), 149.3 (C4),
145.1 (C5’), 142.0 (C5’’), 120.0 (C6), 114.6 (C4’), 112.5 (C4’’), 86.9
(C6H6), 16.2 (Me3’), 15.5 (Me5’’), 14.7 (Me5’), 14.0 (Me3’’).
The new complexes were analysed by IR, 1H and 13C{1H} NMR,
mass spectrometry and, for compounds
diffraction.
6 and 7, by X-ray
The C@C and C@N stretching vibrations of the heterocycles ap-
pear in the 1500–1600 cmꢀ1 region. In general, these bands are
shifted to higher frequencies with respect to those in the free li-
gands as a result of the coordination, and sometimes a higher num-
2.5. Catalysis experiments
Hydrogen transfer experiments for the hydrogenation of aceto-
phenone or benzophenone were carried out using 2-propanol as
the solvent and hydrogen source at the temperature of the reflux-
ing solvent (82 °C). Each run was repeated three times to assure the
reproducibility.
The general procedure was as follows: A solution of the ketone
(1 mmol), KOH (0.2 ml of a 0.2 M solution in 2-propanol) and the
corresponding catalyst (0.002 mmol) was heated under reflux in
ber of signals are observed. The m(CH) and d(CH) vibrations are also
observed for the heterocycles and the BPh4ꢀ counteranion (see Sec-
tion 2 for details).
The 1H and 13C{1H}-NMR resonances were assigned on the basis
of literature information and taking into account the information
from 1H–1H-COSY and heteronuclear g-HMBC and g-HMQC spec-
tra. As one example, the corresponding spectra for complex 8 can
be found in the Supplementary material (Figs. S2–S5). The 1H

4490
M.L. Soriano et al. / Inorganica Chimica Acta 362 (2009) 4486–4492
and 13C{1H} NMR spectra are indicative of the symmetry of the
complexes. For example, the spectra of 1 and 2 show the reso-
nances for only one type of pyrazole ring whereas two distinct pyr-
azoles are observed for the rest of the complexes due to the
existence of one uncoordinated ring. In general, this uncoordinated
pyrazole exhibits resonances with chemical shifts comparable to
those of the free ligands, while a shift is usually observed in the sig-
nals of the coordinated pyrazole and azine heterocycles as a conse-
quence of the coordination to the metal. The resonances of the p-
cymene ring also reflect the symmetry of the complex. In complex
1, two resonances are observed for the aromatic CH groups,
whereas the rest of the complexes (3, 5 and 7) present four reso-
shown in Fig. 1. A selection of bond distances and angles is com-
piled in Table 1 and the crystallographic information is included
in the Supplementary material.
Both compounds crystallize in the monoclinic system with
P2(1)/c space groups. The two complexes have a three-legged pia-
no-stool structure with centroid(p-cymene)–Ru–Cl and centroid(p-
cymene)–Ru–N angles not far from 130°. The N-donor ligands are
coordinated in a bidentate chelate manner through one nitrogen
atom of a pyrazole and other of the azine. As expected, one of
the pyrazoles is not coordinated to the Ru centre. In 7 the dihedral
angle between the coordinated heterocycles is quite small (9.52°)
and that of the pyridazine and the uncoordinated pyrazole rings
is even lower (8.94°), indicating an approximate coplanar disposi-
tion of the heterocycles, which facilitates the electronic delocaliza-
tion. The uncoordinated pyrazole is oriented in such a way that the
expected repulsion between the lone electron pairs of the nitrogen
atoms (pyrazole and azine) is avoided. This coplanarity is not ob-
served in 6. In this complex the two coordinated rings are nearly
coplanar (as in 7) with a dihedral angle of 1.39°, but the value
for this angle is 62.94° for the uncoordinated pyrazole and the pyr-
azine rings. The out-of-plane orientation of the uncoordinated pyr-
azole is very probably due to the steric repulsion between the two
pyrazoles in a contiguous position of the azine. The Ru–C, Ru–N
and Ru–Cl distances in both derivatives are similar to those in com-
parable complexes [20]. The Ru–N bond distances follow the ex-
pected order considering the pKa values of the heterocycles: Ru–
Npyrazine > Ru–Npyridazine > Ru–Npyrazole. Unexpectedly, the Ru–Npz*
(pz* = 3,5-dimethylpyrazole) distance is longer than that of the
Ru–Npyrazole, probably due to steric repulsion between Me3’ and
i
nances. The Pr methyl groups also reflect this asymmetry and ap-
pear as two doublets in the asymmetric complexes and one
doublet in 1. In complexes 1 and 5, the nOe effect observed be-
tween the H3’ pyrazole resonances and the methyl resonances of
i
the Pr group was conclusive for the differentiation of the H3’ and
H5’ protons. For 5 an nOe is also observed between the methyl
(iPr) resonances and H6 of the pyridazine. The observed differences
in the coupling constants of the pyrazole protons (J4’5’ > J4’3’) were
also used to differentiate the H3’ and H5’ resonances [19]. NOe stud-
ies also proved important in the assignment of the p-cymene reso-
nances. The benzene ring gives rise to a singlet in the 5–6 ppm
region and the MeO group appears around 4.30 ppm.
Mass spectra (FAB+) were obtained for all of the complexes and
in all cases the peak corresponding to the cation complex was ob-
served (M–BPh4).
3.3. X-ray diffraction
i
the Pr group of the p-cymene ligand.
The molecular structures of complexes 6 and 7 were deter-
mined by X-ray diffraction. In the analysed samples solvate mole-
cules were not found. The corresponding ORTEP diagrams are
Cations and counteranions are packed in the crystal through
weak interactions between one another. On the one hand, there
are CH–
p
interactions between CH bonds of the p-cymene, Me
6
7
Fig. 1. ORTEP representation (ellipsoids at a 30% level) and labelling for the cations of complexes 6 and 7. Hydrogen atoms are omitted for clarity.
Table 1
Selected bond lengths (Å) and angles (°) for complexes 6 and 7.
6
7
Bond lengths (Å)
Angles (°)
Bond lengths (Å)
Angles (°)
Ru(1)–N(1)
Ru(1)–N(3)
Ru(1)–C(13)
Ru(1)–C(11)
Ru(1)–C(12)
Ru(1)–C(16)
Ru(1)–C(15)
Ru(1)–C(14)
Ru(1)–Cl(1)
Ru(1)–Cymcent
2.029(3)
2.091(3)
2.175(3)
2.186(3)
2.203(3)
2.206(3)
2.237(3)
2.236(3)
2.4579(7)
1.461
N(1)–Ru(1)–N(3)
N(1)–Ru(1)–Cl(1)
N(3)–Ru(1)–Cl(1)
cymcent–Ru(1)–Cl(1)
cymcent–Ru(1)–N(3)
cymcent–Ru(1)–N(1)
N(1)–N(2)–C(4)–N(3)
N(6)–N(5)–C(7)–N(4)
74.27(10)
81.48(8)
82.34(7)
130.26
131.64
133.57
1.39
Ru(1)–N(3)
Ru(1)–N(1)
Ru(1)–C(16)
Ru(1)–C(15)
Ru(1)–C(17)
Ru(1)–C(19)
Ru(1)–C(20)
Ru(1)–C(18)
Ru(1)–Cl(1)
Ru(1)–cymCent
2.0652(19)
2.0796(19)
2.164(3)
2.192(2)
2.192(2)
2.205(2)
2.210(2)
2.226(2)
2.4028(7)
1.440
N(3)–Ru(1)–N(1)
N(3)–Ru(1)–Cl(1)
N(1)–Ru(1)–Cl(1)
cymcent–Ru(1)–Cl(1)
cymcent–Ru(1)–N(3)
cymcent–Ru(1)–N(1)
N(1)–N(2)–C(4)–N(3)
N(6)–N(5)–C(7)–N(4)
77.63(7)
86.66(6)
85.95(6)
128.20
130.16
132.22
9.52
62.94
8.94

M.L. Soriano et al. / Inorganica Chimica Acta 362 (2009) 4486–4492
4491
Fig. 2. Partial view of the crystal network of complex 6 down the c-axis, showing the alternate rows of cations (black) and anions (grey) and the penetration of one of the Ph
groups of the BPh4 into the alignment of the cations. Narrow lines represent the CH–
anion.
p interactions. Inset shows the CH–p interaction of H(6A) and one of the Ph rings of the
(pz) or phenyl (BPh4) groups and the neighbouring aromatic rings
[21]. An illustrative example of such contacts is the interaction in
complex 6 between H(6A) and one of the Ph rings of the counter-
anion, with a CH–centroid distance of 2.286 Å and a very similar
CH–plane distance of 2.262 Å. The CH–centroid angle is 160.11°
(See Fig. 2). Weak hydrogen bonds also exist between the chloride
atoms of the cations and Me(pz) groups in 7 and H(pz) and H(C6H6)
atoms in 6.
In the crystal network of 6 the formation of rows of cationic
molecules along the b axis is observed. The counteranions, which
are intercalated between these rows, penetrate the cationic align-
ments through one of the Ph groups (Fig. 2). Similar rows of cations
are formed in complex 7 along the a axis (see Fig. S1 in the Supple-
mentary Information) with the intercalation of the BPh4 anions.
benzophenone – although the difference is not marked. This sug-
gests that the size of substituents on the ketone is of little
importance.
In the reactions performed in the presence of base (KOH) the
derivatives with benzene generally show higher activity than those
containing p-cymene and of the nitrogenated ligands the following
order of activity was found: bpz*pdz > bpzprz > bpzqnx > OMe-
pzqnx, with poor results obtained for the complexes containing
the methoxy group in the ligand (3 and 4). The highest activity lev-
els were thus found with complexes 7 and 8.
It is remarkable that these complexes are active in the hydroge-
nation process in the absence of base. The addition of a base is usu-
ally necessary in transfer hydrogenations [22] except if a hydride,
which is usually unstable, is used as the starting material [23] or
some sort of activation of the precursor is previously necessary
[24]. In any case, for our complexes, lower activities are generally
observed without base although it is worth noting that for com-
plexes 3 and 4 (both containing OMe groups in the ligands) the
activity is higher in the absence of base and comparable results
to those obtained for other derivatives in the presence of base were
found. The best result was achieved with 3 and this also reflects the
general trend that p-cymene complexes are more active when base
is not added to the medium.
Although, the mechanism of these reactions was not studied in
the work described here, a prior step involving the formation of hy-
drides was thought to be necessary. It is reasonable that these hy-
drides can be formulated as [Ru(arene)H(L)]+. The partial
decoordination of the N-donor ligand is also necessary in order
to allow the coordination of the ketone. In such an unsaturated
intermediate, the transitory coordination of the OMe group is
probably possible and this could favour the deprotonation of the
alcohol, which in turn leads to the formation of the alkoxide in
the proximity of the metal and promotes the b-elimination and
the concomitant formation of the desired hydride. This proposed
role of the MeO group would be comparable to the role of the
C@O group of a cyclopentadienone experimentally determined by
Casey et al. [25] for the formation of Ru-hydrides in Shvo’s catalyst
[26].
3.4. Catalytic behaviour of complexes 1–8
Hydrogen transfer reactions of acetophenone and benzophe-
none, using 2-propanol as the hydrogen source, were carried out.
The reaction products were analysed by 1H NMR spectroscopy.
The experimental conditions are indicated in Section 2.5. Reactions
with or without the addition of KOH were explored for all of the iso-
lated halide precursors. A notable result is that the prior preparation
of a hydride derivative was not necessary to obtain the activities
shown in Table 2, even in the absence of base as a co-catalyst.
The activity of these complexes in the hydrogenation of ketones
is moderate, with better results obtained for acetophenone than for
Table 2
Yields and TON of the hydrogen transfer reactions of complexes 1–8 with
benzophenone and acetophenone (reaction conditions are given in experimental
Section 2.5).
Catalyst
With base
Yield (%)
Without base
Yield (%)
TON
TON
Benzophenone
1
2
3
4
5
6
7
8
14
23
14
7
49
92
100
74
70
115
70
9
2
45
10
74
22
36
25
57
25
370
110
180
125
285
125
35
245
460
500
370
Acknowledgements
We are grateful to the Spanish Ministerio de Ciencia e Innova-
cion MICINN (Project No. CTQ2008-03783) and the Consejería de
Educación y Ciencia of JCCM (PCI08-0054) for financial support.
Acetophenone
1
2
3
4
5
6
7
8
19
24
20
0
73
84
78
100
95
13
9
65
45
120
100
–
365
420
390
500
87
52
61
26
42
37
435
260
305
130
210
185
Appendix A. Supplementary material
CCDC 717640 and 717641 contain the supplementary crystallo-
graphic data for complexes 6 and 7. These data can be obtained
free of charge from The Cambridge Crystallographic Data Centre

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M.L. Soriano et al. / Inorganica Chimica Acta 362 (2009) 4486–4492
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