Synthesis, structure, and proton-transfer reactions of bronsted acidic pyridylpyrazole complexes of ruthenium

Article abstract of DOI:10.1246/bcsj.20100331

A reaction of [{CpRu(μ₃-Cl)}₄] (Cp: η⁵-C5(CH3)5) with 5-phenyl-3-(2-pyridyl)-1H-pyrazole (PhpypzH) afforded a chelating pyrazole complex [CpRuCl(PhpypzH)] (2) bearing an NH proton at the β-position to the metal. Treatment

Full text of DOI:10.1246/bcsj.20100331

© 2011 The Chemical Society of Japan
Bull. Chem. Soc. Jpn. Vol. 84, No. 3, 251–258 (2011)
251
BCSJ Award Article
Synthesis, Structure, and Proton-Transfer Reactions of Brønsted Acidic
Pyridylpyrazole Complexes of Ruthenium
Yohei Kashiwame, Megumi Watanabe, Kenjirou Araki, Shigeki Kuwata,* and Takao Ikariya*
Department of Applied Chemistry, Graduate School of Science and Engineering, Tokyo Institute of Technology,
O-okayama, Meguro-ku, Tokyo 152-8552
Received November 25, 2010; E-mail: skuwata@apc.titech.ac.jp, tikariya@apc.titech.ac.jp
A reaction of [{Cp*Ru(®3-Cl)}4] (Cp*: ©5-C5(CH3)5) with 5-phenyl-3-(2-pyridyl)-1H-pyrazole (PhpypzH) afforded
a chelating pyrazole complex [Cp*RuCl(PhpypzH)] (2) bearing an NH proton at the ¢-position to the metal. Treatment of
2 with an equimolar amount of silver nitrite followed by anion exchange with KOTf (OTf: OSO2CF3) gave the
pyrazolato-nitrosyl complex [Cp*Ru(NO)(Phpypz)](OTf) (3) through a proton shift from the pyrazole ligand to the nitrite
ion. In contrast, simple ligand exchange took place in the reaction of silver nitrite and [Cp*RuCl(bipy)] (bipy: 2,2¤-
bipyridine) without the protic chelate ligand to give the nitro complex [Cp*Ru(NO2-¬N)(bipy)] (4). Protonation of 4 with
triflic acid led to the formation of the nitrosyl complex [Cp*Ru(NO)(bipy)](OTf)2 (5). The pyrazolato complex 3 was also
obtained by the reaction of [Cp*Ru(NO)Cl2] with PhpypzH and KOTf in water. Reversible protonation of 3 resulted in
the formation of the dicationic pyrazole complex [Cp*Ru(NO)(PhpypzH)](OTf)2 (6). The detailed structures of 2-4 and 6
have been determined by X-ray crystallography.
– H+
Pyrazoles have been used as versatile ligands for a variety of
–
coordination compounds ranging from bioinorganic models1
and antitumor agents2 to materials with efficient luminescent
properties.3 The Brønsted basic imino nitrogen in pyrazoles
allows them to be ·-donors toward metal centers as in other
N-heterocycles typified by pyridines.4,5 Provided that the NH
group is not masked with substituents, such pyrazoles are
amphiprotic molecules with a Brønsted acidic NH group in
addition to the basic imino nitrogen. Deprotonation of the
protic pyrazoles affords pyrazolide anions (Scheme 1), which
bind to metals with more diversified manners including the ®-
¬N,¬N¤ coordination in dinucleating ligands.4-6 The acidity and
hydrogen-bond-donating ability of protic pyrazoles are increas-
ed upon coordination,5 and consequently most protic pyrazole
complexes are associated with intra- and intermolecular hydro-
gen bonds. Some poly(protic pyrazole) complexes are known
as anion hosts through multiple hydrogen bonds.4,7 Deproto-
nation of coordinated protic pyrazoles with enhanced acidity is
also intriguing, because it should cause significant electronic
perturbation to the properties of the pyrazole complexes.
In the (pyrazole)ruthenium complexes [RuCl(PzH)(bipy)2]+
(PzH: a protic pyrazole, bipy: 2,2¤-bipyridine), for example,
the two MLCT absorptions are red-shifted by 60 and 30 nm
upon treatment with a base.8 Lam and co-workers created a
molecular “pivot-hinge” composed of two square-planar (protic
pyrazole)platinum units, in which the two units are flipped in
response to deprotonation and reprotonation of the NH group in
the ligated pyrazole.9 Satake and co-workers demonstrated that
N N
N N
H+
H
protic pyrazole
pyrazolide anion
M+
– H+
H+
N N
N N
M
pyrazolato complex
M+
H
pyrazole complex
Scheme 1. Reversible deprotonation of uncoordinated and
ligated protic pyrazole.
a cationic (³-allyl)palladium-pyrazole complex is an excellent
precatalyst for cyclopropanation of ketene silyl acetals with
allylic acetates.10 However, chemical transformations promoted
by reversible proton transfer from pyrazole complexes still
remain unexplored11 in spite of increasing research activities on
protic pyrazole complexes.2,4-16 In addition, detailed structural
comparison between pyrazole complexes and the conjugate
base, ¬N-pyrazolato (also termed pyrazolido or pyrazolyl)
complexes, has rarely been performed.12
We have demonstrated that late transition metal bifunctional
complexes with chelating primary amine ligands catalyze both
reductive and oxidative transformations as well as C-C bond-
Published on the web February 11, 2011; doi:10.1246/bcsj.20100331

252
Bull. Chem. Soc. Jpn. Vol. 84, No. 3 (2011) BCSJ AWARD ARTICLE
1
38%). H NMR (CD3CN): ¤ 1.67 (s, 15H, Cp*), 7.27 (s, 1H,
Ph
aryl), 7.39-7.51 (m, 4H, aryl), 7.89 (m, 2H, aryl), 8.13 (m, 2H,
aryl), 9.06 (m, 1H, aryl), 12.76 (br, 1H, NH). Found: C, 58.82;
H, 5.76; N, 7.60%. Calcd for C26H30ClN3O0.5Ru: C, 59.03; H,
5.72; N, 7.94%.
Ir
Ir
base
N
Cl
N
N
N
N
Ph2NH·HCl
NH
Synthesis of [Cp*Ru(NO)(Phpypz)](OTf) (3). From 2:
A mixture of 2 (111.0 mg, 0.210 mmol) and AgNO2 (36.0 mg,
0.234 mmol) in CH3CN (5 mL) was stirred for 4 h at room
temperature. The resultant suspension was filtered, and KOTf
(212.0 mg, 1.13 mmol) was added to the filtrate. After stirring
for 3 h, the mixture was evaporated to dryness and dissolved
in CH2Cl2 (4 mL). The solution was washed with water
(3 mL © 2). Removal of the solvent from the separated organic
layer afforded 3 (93.6 mg, 0.147 mmol, 70%). 1H NMR
(CD3CN): ¤ 1.86 (s, 15H, Cp*), 7.32 (m, 1H, aryl), 7.44 (m,
4H, aryl), 7.94 (m, 2H, aryl), 8.05, 8.19, 8.38 (m, 1H each,
aryl). IR (KBr): 1799 cm¹1 (¯NO). Found: C, 47.16; H, 3.97; N,
8.81%. Calcd for C25H25F3N4O4RuS: C, 47.24; H, 3.96; N,
8.81%.
Ir
Ph
Ph
1
H2
H
N
cat 1/base
(5 mol%)
N
toluene, 50 °C
Ph
Ph
Ph
Ph
Scheme 2. Reversible dehydrochlorination of protic pyr-
azole complex 1 and intramolecular hydroamination of
½-aminoalkene catalyzed by 1.
forming reactions.17 The notable catalytic performance results
from the cooperation of the metal center and the neighboring
protic amine ligand at the ¡-position to the metal. We thus
envisioned that the protic pyrazole complexes bearing an NH
group at the ¢-position to the metal would also serve as such
metal-ligand bifunctional catalysts. As part of our research
program focusing on ¢-protic cooperating ligands,18,19 we have
recently demonstrated that the half-sandwich iridium complex
1 bearing C-N chelate protic pyrazole ligand undergoes
reversible dehydrochlorination and catalyzes intramolecular
hydroamination of unactivated alkenes (Scheme 2).20 We
describe here the synthesis, structures, and proton-transfer
reactions of the isoelectronic N-N chelate pyrazole complexes
of ruthenium.
From [Cp*Ru(NO)Cl2]: A mixture of [Cp*Ru(NO)Cl2]
(54.9 mg, 0.163 mmol) and PhpypzH (36.1 mg, 0.163 mmol) in
H2O (5 mL) was stirred for 15 h at 50 °C. To the resultant
solution was added KOTf (153.0 mg, 0.813 mmol). After
stirring the mixture for 3 h at room temperature, the resultant
brown solution was extracted with CH2Cl2 (3 mL © 6).
Evaporation of the extract and recrystallization from CH3CN-
diethyl ether (7 mL/20 mL) afforded red crystals of 3 (76.5 mg,
0.120 mmol, 74%).
Synthesis of [Cp*Ru(NO2-¬N)(bipy)]¢CH2Cl2 (4¢CH2Cl2).
A mixture of [Cp*RuCl(bipy)] (296.5 mg, 0.6929 mmol) and
AgNO2 (111.0 mg, 0.721 mmol) in THF (35 mL) was stirred at
room temperature for 12 h. The reaction mixture was filtered,
and the filtrate was evaporated to dryness. The resultant dark
brown powder was recrystallized from CH2Cl2-hexane
(15 mL/50 mL) to afford 4¢CH2Cl2 (245.9 mg, 0.4698 mmol,
Experimental
1
General.
All manipulations were performed under an
68%) as red crystals. H NMR (C6D6): ¤ 1.58 (s, 15H, Cp*),
atmosphere of argon using standard Schlenk technique unless
otherwise specified. Solvents were dried by refluxing over
sodium benzophenone ketyl (THF, diethyl ether, and hexane),
P2O5 (acetonitrile), CaH2 (dichloromethane), and distilled
before use. Water and 1,2-dichloroethane were degassed by
bubbling with argon. 5-Phenyl-3-(2-pyridyl)-1H-pyrazole
(PhpypzH),13 [{Cp*Ru(®3-Cl)}4],21 [Cp*Ru(NO)Cl2],22 and
[Cp*RuCl(bipy)]23 were prepared according to the literature.
1H NMR spectra (300.40 and 399.78 MHz) were obtained on a
JEOL JNM-LA300 and JNM-ECX400 spectrometer. Infrared
spectra were recorded on a JASCO FT/IR-610 spectrometer.
Elemental analyses were performed by the Analytical Facility
at the Research Laboratory of Resources Utilization, Tokyo
Institute of Technology or on a Perkin-Elmer 2400II CHN
analyzer.
Synthesis of [Cp*RuCl(PhpypzH)]¢0.5THF (2¢0.5THF).
To a solution of [{Cp*Ru(®3-Cl)}4] (170.6 mg, 0.1569 mmol)
in CH3CN (16 mL) was added PhpypzH (139.2 mg, 0.6291
mmol), and the solution was stirred for 16 h at room temper-
ature. After removal of the solvent under reduced pressure, the
resulting solid was recrystallized from THF-hexane (3 mL/
18 mL) to give dark brown crystals. The crystals were rinsed
with acetone and dried in vacuo (126.9 mg, 0.2399 mmol,
6.58 (m, 2H, aryl), 6.83 (m, 2H, aryl), 7.10 (m, 2H, aryl), 9.12
(m, 2H, aryl). IR (KBr): 1321, 1280 cm (¯NO). The presence
¹1
of solvated dichloromethane was confirmed by 1H NMR
spectroscopy and a preliminary X-ray analysis. Satisfactory
analytical data could not be obtained by partial loss of the
solvated molecule. Found: C, 49.26; H, 4.89; N, 8.29%. Calcd
for C21H25Cl2N3O2Ru: C, 48.19; H, 4.81; N, 8.03%. Single
crystals of 4 suitable for X-ray analysis were obtained as a
monohydrate by recrystallization from wet 1,2-dichloroethane-
hexane.
Protonation of 4. To a solution of 4¢CH2Cl2 (34.1 mg,
0.0651 mmol) in THF (5 mL) was added triflic acid (=trifluoro-
methanesulfonic acid; 16.0 ¯L, 0.18 mmol) at ¹78 °C. The
mixture was warmed to room temperature over the course of
12 h. After removal of the solvent in vacuo, the resultant yellow
powder was extracted with CH3CN (5 mL). Recrystallization of
the extract from CH3CN-diethyl ether (2 mL/20 mL) afforded
[Cp*Ru(NO)(bipy)](OTf)2 (5; 36.5 mg, 0.0507 mmol, 78%)24
as yellow microcrystals.
Synthesis of [Cp*Ru(NO)(PhpypzH)](OTf)2¢CH3CN
(6¢CH3CN). From 3: To a solution of 3 (26.5 mg, 0.0417
mmol) in CH2Cl2 (4 mL) was added triflic acid (3.7 ¯L, 0.042
mmol) at ¹78 °C, and the mixture was slowly warmed to room

Y. Kashiwame et al.
Bull. Chem. Soc. Jpn. Vol. 84, No. 3 (2011)
253
Table 1. X-ray Crystallographic Data for 2¢0.5THF, 3, 4¢H2O, and 6¢CH3CN
2¢0.5THF
3
4¢H2O
6¢CH3CN
Formula
Fw
Space group
Crystal color
Crystal dimension/mm3
a/¡
b/¡
c/¡
¡/°
¢/°
£ /°
V/¡3
Z
C26H30ClN3O0.50Ru
C25H25F3N4O4RuS
635.62
P21/c (No. 14)
orange
0.6 © 0.2 © 0.1
7.719(2)
23.019(7)
14.602(5)
90
93.428(4)
90
2589.9(14)
4
C20H25N3O3Ru
456.51
P1 (No. 2)
C28H29F6N5O7RuS2
826.75
P21/n (No. 14)
orange
0.3 © 0.2 © 0.2
13.845(8)
16.479(8)
16.432(10)
90
119.331(7)
90
3268(3)
4
529.07
ꢀ
ꢀ
P1 (No. 2)
dark brown
0.4 © 0.3 © 0.2
11.467(4)
12.894(5)
17.482(8)
82.633(16)
77.049(15)
75.449(16)
2431.1(17)
4
dark brown
0.5 © 0.4 © 0.3
8.952(3)
10.518(4)
11.184(4)
88.587(10)
89.850(12)
67.918(6)
975.5(6)
2
T/K
193
1.445
7.75
193
1.630
7.47
193
1.554
8.29
93
1.680
6.95
¹3
D
calcd/g cm
¹1
®(Mo K¡)/cm
Reflections measured
No. of unique reflections
Transmission factors
No. of variables
Rint
22488
10581
0.771-0.856
574
0.037
23256
5861
0.796-0.928
368
0.088
8611
4197
0.712-0.780
196
0.022
29342
7454
0.544-0.870
507
0.106
R1 [I > 2·(I)]
0.065
0.043
0.050
0.069
wR2 (all data)
0.168
0.136
0.136
0.192
Goodness-of-fit
Residual electron density/e ¡
1.002
1.35, ¹1.01
1.000
0.79, ¹0.72
1.004
1.52, ¹0.97
1.006
2.66, ¹1.50
¹3
temperature with stirring. After 18 h, the mixture was evapo-
rated to dryness. Subsequent recrystallization from CH3CN-
diethyl ether (1 mL/15 mL) afforded 6¢CH3CN as orange
effects and for absorption. Details of crystal and data collection
parameters are summarized in Table 1.
Structure solution and refinements were carried out by using
the CrystalStructure program package.25 The heavy-atom
positions were determined by a Patterson method program
(DIRDIF99 PATTY;26 for 2¢0.5THF) or direct methods
program (SIR92;27 for 3, 4¢H2O, and 6¢CH3CN) and remain-
ing non-hydrogen atoms were found by subsequent Fourier
syntheses. The non-hydrogen atoms were refined anisotropi-
cally by full-matrix least-squares techniques based on F2.
Because both of the two crystallographically independent Cp*
groups in 2¢0.5THF were severely disordered, these groups
were included in the refinements as follows. The methyl carbon
atoms in one Cp* group (C(6)-C(10)) in 2¢0.5THF were
refined with isotropic thermal parameters. The other Cp* group
was located at two disordered positions with 70% and 30%
occupancies, and the minor component was treated as a rigid
group with isotropic thermal parameters. The C and O atoms in
the solvating THF molecule in 2¢0.5THF were refined with
fixed isotropic thermal parameters. For 4¢H2O, the Cp* ligand
was located at two disordered position with 50% occupancies
and refined isotropically as rigid groups. For 6¢CH3CN, the
triflate anion uninvolved in the hydrogen bond with the cation
was located at two disordered positions with 55% and 45%
occupancies. The hydrogen atoms in the solvated water
molecule in 4¢H2O were found in the difference Fourier map
and all the other hydrogen atoms except for those in the
disordered Cp* groups were placed at calculated positions;
these hydrogen atoms were included in the refinements with a
riding model. Crystallographic data have been deposited with
1
crystals (29.2 mg, 0.0353 mmol, 85%). H NMR (CD3CN): ¤
1.88 (s, 15H, Cp*), 7.60 (m, 3H, aryl), 7.70 (s, 1H, aryl), 7.76
(m, 1H, aryl), 7.89 (m, 2H, aryl), 8.39 (m, 2H, aryl), 8.56
¹1
(m, 1H, aryl), 13.27 (s, 1H, NH). IR (KBr): 1828 cm (¯NO).
Found: C, 40.60; H, 3.58; N, 8.33%. Calcd for C28H29F6N5O7-
RuS2: C, 40.68; H, 3.54; N, 8.47%.
From [Cp*Ru(NO)Cl2]: A mixture of [Cp*Ru(NO)Cl2]
(55.9 mg, 0.166 mmol) and AgOTf (85.2 mg, 0.332 mmol) in
CH3CN (5 mL) was stirred for 3 h at room temperature. The
reaction mixture was filtered, and PhpypzH (36.7 mg,
0.166 mmol) was added to the filtrate. After stirring for 16 h
at room temperature, the mixture was filtered. Evaporation of
the filtrate and recrystallization from CH3CN-diethyl ether
afforded 6¢CH3CN as orange solid (88.6 mg, 0.107 mmol,
64%).
Deprotonation of 6 to Give 3. A mixture of 6¢CH3CN
(29.1 mg, 0.0352 mmol) and K2CO3 (4.9 mg, 0.035 mmol) in
CH2Cl2 (2.5 mL) was stirred for 17 h at room temperature. The
reaction mixture was filtered, and the filtrate was evaporated to
dryness. Recrystallization from CH3CN-diethyl ether (1 mL/
15 mL) afforded red crystals of 3 (20.0 mg, 0.0315 mmol,
89%).
X-ray Diffraction Studies. Diffraction experiments were
performed on a Rigaku Saturn CCD area detector with graphite
monochromated Mo K¡ radiation (- = 0.71070 ¡). Single
crystals suitable for X-ray analyses were mounted on glass
fibers. Intensity data were corrected for Lorentz-polarization

254
Bull. Chem. Soc. Jpn. Vol. 84, No. 3 (2011) BCSJ AWARD ARTICLE
Table 2. Selected Interatomic Distances (¡) and Angles (°)
for 2¢0.5THF
[{Cp*Ru(µ3-Cl)}4] +
Ru
N
NH
Ph
2, 38%
N
Cl
Molecule A
Molecule B
2.4888(17)
N
N
CH3CN
rt, 16 h
NH
Ph
Ru(1)-Cl(1)
Ru(1)-N(2)
Ru(1)-N(3)
N(1)-N(2)
N(1)-C(11)
C(11)-C(12)
C(12)-C(13)
N(2)-C(13)
N(1)£Cl(2)
H(1)£Cl(2)
2.4728(17) Ru(2)-Cl(2)
2.114(3)
2.136(4)
1.349(5)
1.365(6)
1.376(7)
1.412(6)
1.336(6)
3.264(4)
2.37
Ru(2)-N(5)
Ru(2)-N(6)
N(4)-N(5)
N(4)-C(35)
C(35)-C(36)
C(36)-C(37)
N(5)-C(37)
N(4)£Cl(1)
H(27)£Cl(1)
2.105(3)
2.139(4)
1.365(5)
1.361(6)
1.372(6)
1.405(7)
1.333(6)
3.197(4)
2.30
(PhpypzH)
1 : 4
Scheme 3. Synthesis of protic pyrazole complex 2.
N(2)-Ru(1)-N(3) 74.55(14) N(5)-Ru(2)-N(6) 74.54(15)
Ru(1)-N(2)-N(1) 136.1(3)
Ru(1)-N(2)-C(13) 116.9(2)
N(1)-N(2)-C(13) 106.4(3)
N(2)-N(1)-C(11) 111.2(3)
N(1)-C(11)-C(12) 106.9(3)
C(11)-C(12)-C(13) 105.5(4)
N(2)-C(13)-C(12) 110.1(4)
Ru(2)-N(5)-N(4) 135.1(3)
Ru(2)-N(5)-C(37) 117.9(3)
N(4)-N(5)-C(37) 106.1(3)
N(5)-N(4)-C(35) 110.2(3)
N(4)-C(35)-C(36) 107.8(4)
C(35)-C(36)-C(37) 105.2(4)
N(5)-C(37)-C(36) 110.6(4)
PF6
Ru
Figure 1. Crystal structure of 2¢0.5THF with thermal
ellipsoids at the 30% probability level. A pair of two
crystallographically independent molecules is shown.
Hydrogen atoms except for the NH protons, the minor
component of the disordered Cp* ligand, and the solvated
THF molecule are omitted for clarity.
Ru
N
N
Cl
N
Cl
N
NH
NH
7
8
the Cambridge Crystallographic Data Centre: Deposition
numbers CCDC-802114, CCDC-802115, CCDC-802116, and
CCDC-802117 for compounds 2¢0.5THF, 3, 4¢H2O, and
6¢CH3CN, respectively. Copies of the data can be obtained
free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.
html (or from the Cambridge Crystallographic Data Centre, 12,
Union Road, Cambridge, CB2 1EZ, U.K.; Fax: +44 1223
336033; e-mail: deposit@ccdc.cam.ac.uk).
Chart 1.
[(arene)Ru] complex 714 as well as the closely related protic N-
heterocyclic carbene complex 8 (Chart 1),18 though the hydro-
gen-bonded dimers of 1 and 7 are not C2-symmetric but
centrosymmetric. The Ru-N(pyrazole) distance of 2.110 ¡
(mean) is comparable with that in the isoelectronic complex 7
(2.0511(17) ¡).14
Results and Discussion
Synthesis and Structure of Pyrazolato-Nitrosyl Com-
plex 3. Although half-sandwich complexes with chelating
protic pyrazole ligands such as 2 are not unprecedented,14,15,20
little is known about their proton-donating ability. We therefore
examined dehydrative conversion of nitrite ion to nitrosyl
ligand on 2, which would be triggered by proton migration
from the chelate pyrazole ligand in 2 to nitrite ion.18 Treatment
of the pyrazole-chlorido complex 2 with an equimolar amount
of silver nitrite followed by anion exchange with KOTf
resulted in the formation of the pyrazolato-nitrosyl complex
[Cp*Ru(NO)(Phpypz)](OTf) (3) as shown in Scheme 4. The IR
spectrum of 3 exhibits a strong absorption assignable to an NO
stretching band at 1799 cm¹1, indicating the formation of a
linear nitrosyl ligand. The detailed structure of 3 has been
determined by X-ray crystallography (Figure 2a); selected
bond distances and angles are listed in Table 3. The nitrosyl
ligand is essentially linear with the Ru-N-O angle of
164.3(3)°. In contrast to the pyrazole complex 2, the uncoordi-
nated nitrogen atom in the chelate (N(1)) is far from the
Synthesis and Structure of Pyrazole-Chlorido Complex 2.
The protic pyrazole complex of ruthenium [Cp*RuCl-
(PhpypzH)] (2) was obtained by the stoichiometric reaction
of [{Cp*Ru(®3-Cl)}4] with PhpypzH in acetonitrile, as shown
1
in Scheme 3. The H NMR spectrum of 2 exhibits a low-field
broad singlet assignable to the pyrazole proton. The Brønsted
acidity of this proton was inferred from a facile H-D exchange
with D2O in a 1H NMR experiment. Figure 1 depicts the
molecular structure of 2 determined by a single-crystal X-ray
structural analysis; selected bond distances and angles are listed
in Table 2. Complex 2 has a three-legged piano-stool structure,
in which the Brønsted acidic NH group and Brønsted basic
chlorido ligand point to totally different directions with
averaged intramolecular NH£Cl distances of 4.21 ¡. As a
consequence, 2 forms a hydrogen-bonded dimer in the solid
state with an approximate C2-symmetry as shown in Figure 1.
Similar intermolecular hydrogen bonds are observed for the
isoelectronic C-N chelate iridium complex 120 and cationic

Y. Kashiwame et al.
Bull. Chem. Soc. Jpn. Vol. 84, No. 3 (2011)
255
OTf
1) H2O
N
1) CH3CN
rt, 4 h
50 °C
Ru
N
Ru
N
15 h
N
Cl
N
Ru
Cl
+ AgNO2
O
N
+
N
2) KOTf
(70%)
2) KOTf
(74%)
N
Cl
O
NH
N
NH
Ph
Ph
Ph
1 : 1
1 : 1
2
3
K2CO3 (1 equiv)
CH2Cl2, rt
17 h
TfOH (1 equiv)
CH2Cl2
–78 °C to rt
(89%)
(85%)
(OTf)2
1) AgOTf (2 equiv)
CH3CN, rt, 3 h
Ru
N
N
N
Ru
O
2) PhpypzH (1 equiv)
CH3CN, rt, 16 h
N
Cl
O
NH
Cl
Ph
(64%)
6
Scheme 4. Synthesis and reversible protonation of pyrazolato-nitrosyl complex 3.
Table 3. Selected Interatomic Distances (¡) and Angles (°)
for 3 and 6¢CH3CN
3
6¢CH3CN
Ru(1)-N(2)
Ru(1)-N(3)
Ru(1)-N(4)
N(4)-O(1)
N(1)-N(2)
N(1)-C(11)
C(11)-C(12)
C(12)-C(13)
2.044(3)
2.136(3)
1.775(3)
1.162(4)
1.350(4)
1.347(5)
1.416(5)
1.377(5)
1.356(5)
®
2.086(6)
2.139(6)
1.802(4)
1.145(6)
1.353(6)
1.368(9)
1.398(9)
1.380(9)
1.341(9)
2.810(7)
1.92
N(2)-C(13)
N(1)£O(2)
H(1)£O(2)
®
N(2)-Ru(1)-N(3)
Ru(1)-N(2)-N(1)
Ru(1)-N(2)-C(13)
N(1)-N(2)-C(13)
N(2)-N(1)-C(11)
N(1)-C(11)-C(12)
C(11)-C(12)-C(13)
N(2)-C(13)-C(12)
Ru(1)-N(4)-O(1)
76.03(13)
130.1(2)
119.0(2)
110.8(3)
106.0(3)
110.6(3)
104.1(3)
108.5(3)
164.3(3)
75.3(2)
133.4(5)
118.5(3)
106.5(5)
110.5(5)
106.5(5)
105.7(6)
110.8(5)
161.6(7)
hydrogen-bond acceptors, with the shortest N(1)£O(triflate)
distance of 4.541(5) ¡. Loss of the NH proton is further
1
evidenced by the H NMR spectrum of 3, which displays no
NH signals. The reaction would involve the initial formation
of the nitro-pyrazole complex [Cp*Ru(NO2-¬N)(PhpypzH)],
which undergoes proton transfer from the pyrazole ligand to
the nitro ligand to give 3, as observed in protonation and
protonolysis to nitro complexes with external Brønsted acid.28
Figure 2. Crystal structures of 3 (a) and 6¢CH3CN (b) with
thermal ellipsoids at the 30% probability level. Hydrogen
atoms except for the NH proton as well as the minor
components of the disordered triflate anion are omitted for
clarity.

256
Bull. Chem. Soc. Jpn. Vol. 84, No. 3 (2011) BCSJ AWARD ARTICLE
(OTf)2
AgNO2
(1 equiv)
TfOH
(3 equiv)
O
Ru
N
Ru
N
Ru
N
N
N
N
Cl
N
N
O
THF, rt
12 h
–78 °C to rt
O
4, 68%
Scheme 5. Synthesis and protonation of nitro complex 4.
5, 78%
Table 4. Selected Interatomic Distances (¡) and Angles (°)
for 4¢H2O
(a) Distances
Ru(1)-N(1)
Ru(1)-N(2)
Ru(1)-N(3)
N(1)-O(1)
N(1)-O(2)
2.074(3)
2.111(3)
2.106(4)
1.248(4)
1.253(4)
O(1)£O(3)
O(1)£H(9)
O(2)£O(3)*
O(2)£H(10)*
2.870(4)
2.06
2.932(6)
2.21
(b) Angles
Figure 3. Crystal structure of 4¢H2O with thermal ellip-
soids at the 30% probability level. Only one of the two
conformers of the disordered Cp* ligand is shown.
Hydrogen atoms except for those in the solvated water
molecule are omitted for clarity. Asterisks denote atomic
position determined by crystallograhic inversion center.
N(2)-Ru(1)-N(3) 75.77(13) Ru(1)-N(1)-O(2) 121.0(2)
Ru(1)-N(1)-O(1) 123.0(2) O(1)-N(1)-O(2) 116.0(3)
cooperation of the metal center and the neighboring pyrazole
ligand that supplies a proton.
Reversible Protonation of Pyrazolato Complex 3 to Give
Pyrazole Complex 6. The pyrazolato complex 3 proved to
undergo protonation at the uncoordinated ¢-nitrogen atom in
the pyrazole ring. When 3 was treated with an equimolar
amount of triflic acid, the dicationic pyrazole-nitrosyl complex
The pyrazolato-nitrosyl complex 3 was also obtained by the
ligand substitution of the nitrosyl complex [Cp*Ru(NO)Cl2]
with PhpypzH in water (Scheme 4). Notably, the pyrazole
proton is lost spontaneously under this synthetic condition,
suggesting the strong Brønsted acidity of the dicationic species
[Cp*Ru(NO)(PhpypzH)]2+, which may be formed transiently.
To gain more insight into the role of the protic pyrazole
ligand in 2 in the conversion of 2 to 3, we carried out the
reaction of the bipyridine complex [Cp*RuCl(bipy)] without a
Brønsted acidic proton in the chelate ligand under similar
conditions.29 The reaction led to the formation of the nitro
complex 4 as shown in Scheme 5. Although a number of nitro
complexes have been known so far, organometallic nitro
complexes are rather scarce.28,30 The IR spectrum of 4 exhibits
the NO stretching bands at 1321 and 1280 cm¹1, indicating the
coordination of nitrite ion.28,31 An X-ray analysis of 4¢H2O
established that the nitrite ion binds to the metal through the
nitrogen atom rather than the oxygen atom (Figure 3 and
Table 4). The metric parameters around the nitro ligand are
unexceptional,31 and the two N-O distances in the nitro ligand
are almost identical to each other. The nitro oxygen atoms are
linked with the solvated water molecule by a hydrogen bond to
form a dimeric structure of 4 in the crystal, suggesting that
these oxygen atoms have proton-accepting ability. In fact, the
nitro ligand in 4 undergoes facile protonation and protonolysis
to afford the dicationic nitrosyl complex 524 in good yield. The
sequential transformations shown in Scheme 5 are consistent
with the transient formation of the nitro-pyrazole complex
[Cp*Ru(NO2-¬N)(PhpypzH)] in the reaction of the pyrazole-
chlorido complex 2 and nitrite. Dehydrative conversion of
nitrite ion under neutral conditions is thus achieved by the
1
6 was obtained as shown in Scheme 4. The H NMR spectrum
of 6 displays the NH resonance at 13.27 ppm, which dis-
appeared upon treatment with D2O. The ¯(NO) value of
1828 cm¹1 observed for 6 is higher than that for 3 (1799 cm¹1),
being consistent with the reduced back donation to the nitrosyl
ligand in the dicationic complex 6. Furthermore, the structure
of 6 has been confirmed by an X-ray analysis (vide infra).
Complex 6 was also obtained by the reaction of the chlorido
complex [Cp*Ru(NO)Cl2] with two equiv of silver triflate and
subsequent treatment with PhpypzH (Scheme 4), possibly
through the triflate complex [Cp*Ru(NO)(OTf)2].32 The
Brønsted acidity of the dicationic pyrazole complex 6 may be
estimated to be stronger than hydrochloric acid but weaker than
triflic acid, considering the isolation of the monocationic
pyrazolato complex 3 from the reaction without pretreatment
with silver triflate. As expected, the reaction of 6 with
potassium carbonate regenerated the pyrazolato complex 3 as
illustrated in Scheme 4.
The structure of the pyrazole complex 6 has been determined
by X-ray crystallography, as shown in Figure 2b and Table 3.
The short contact between the NH group and an oxygen atom
in one of the two triflate counter anion indicates the presence
of the hydrogen bond. Having acquired both pyrazolato and
pyrazole complexes 3 and 6 with exactly the same ancillary
ligand set, we compared the structures of 3 and 6 as well as
the chlorido complex 2 in detail (Table 3 and Figure 4). The
Ru(1)-N(2) distance in the pyrazolato complex 3 (2.044(3) ¡),

Y. Kashiwame et al.
Bull. Chem. Soc. Jpn. Vol. 84, No. 3 (2011)
257
Pyrazolato (3)
Pyrazole (6)
Pyrazole (2)
C(12)
C(12)
C(12)
1.377(5)
C(13)
1.416(5)
C(11)
1.380(9)
C(13)
1.398(9)
C(11)
1.409
C(13)
1.374
C(11)
1.356(5)
N(2)
1.347(5)
1.341(9)
1.368(9)
N(1)
1.335
1.363
N(1)
1.350(4)
1.353(6)
N(2)
1.357
C(12)
N(1)
N(2)
2.110
H
H
2.044(3)
C(12)
2.086(6)
C(12)
Ru
Ru
Ru
104.1(3)°
105.7(6)°
105.4
110.6(3)°
106.5(5)°
107.4
C(11)
C(11)
C(11)
C(13)
108.5(3)°
C(13)
C(13)
110.8(5)°
106.5(5)°
110.4
110.8(3)°
N(2)
N(1)
106.3
N(2)
106.0(3)°
110.5(5)°
110.7
N(1)
N(2)
N(1)
H
H
Ru
Ru
Ru
Figure 4. Bond distances (¡) and interior angles (deg) in the pyrazole ring of pyrazolato complex 3 and pyrazole complexes 6 and 2.
The values for 2 are averages of the two crystallographically independent molecules.
wherein the coordinated pyrazole is formally classified in an
anionic X-type ligand, is shorter than that in the dative (L-type)
pyrazole complexes 6 (2.086(6) ¡) and 2 (2.110 ¡, mean). The
double bonds in the pyrazole ring are delocalized in both
pyrazolato and pyrazole complexes; exceptionally, the C(11)-
C(12) bond in the pyrazolato complex 3 and the C(12)-C(13)
bond in the pyrazole-chlorido complex 2 are slightly longer
than the other bonds in agreement with their canonical
structures. The coordinated pyrazole nitrogen atom is almost
planar in both protonated and deprotonated forms with the angle
sum around the nitrogen atom of 358.4-359.9°. The N(1)-N(2)-
C(13) angle in the pyrazolato complex 3 (110.8(3)°) is mean-
ingfully larger than those in the pyrazole complexes 2 and 6
(106.3 (mean) and 106.5(5)°). On the contrary, the adjacent
Ru(1)-N(2)-N(1) and N(2)-N(1)-C(11) angles in 3 are smaller
than those in 2 and 6. Similar trend is observed in pyrazolato and
pyrazole complexes without the chelate arm12 as well as the
related protic N-heterocyclic carbene complex 8 and the
deprotonated form, imidazolyl complex.18 The dihedral angle
between the Cp* ligand and N-N chelate planes in the
pyrazolato complex 3 (50.0(2)°) is much larger than that in
the pyrazole complex 6 (29.1(3)°). The resultant short
N(1)£C(6) contact of 3.151(6) ¡ in the pyrazolato complex 3
may suggest the presence of an attractive CH£N interaction.33
This work was financially supported by a Grant-in-Aid for
Scientific Research on Priority Areas (No. 18065007, “Chem-
istry of Concerto Catalysis”) and Scientific Research (S)
(No. 22225004) from the Ministry of Education, Culture,
Sports, Science and Technology, Japan.
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