Synthesis and structure of ruthenium(IV) complexes featuring N-heterocyclic ligands with an N-H group as the hydrogen-bond donor: Hydrogen interactions in solution and in the solid state

Article abstract of DOI:10.1021/ic200547k

The synthesis and characterization of novel ruthenium(IV) complexes [Ru(η³:η³-C₁₀H₁₆)Cl ₂L] [L = 3-methylpyrazole (2b), 3,5-dimethylpyrazole (2c), 3-methyl-5- phenylpyrazole (2d), 2-(1H-pyrazol-5-yl)phenol (2e), 6-azauracile (3), and 1H-indazol-3-ol (4)] are reported. Complex 2e is converted to the chelated complex [Ru(η³:η³-C₁₀H ₁₆)Cl(k²-N, O-2-(1H-pyrazol-3-yl)phenoxy)] (5) by treatment with an excess of NaOH. All of the ligands feature N-H, O-H, or CdO as the potential hydrogen-bonding group. The structures of complexes 2a-2c, 2e, 3, and 5 in the solid state have been determined by X-ray diffraction. Complexes 2a-2c and 3, which contain the pyrazole N-H group, exhibit intra- and intermolecular hydrogen bonds with chloride ligands [N-H... Cl distances (A): intramolecular, 2.30-2.78; intermolecular, 2.59-2.77]. Complexes 2e and 3 bearing respectively O-H and CdO groups also feature N-H...O interactions [intramolecular (2e), 2.27 A; intermolecular (3), 2.00 A]. Chelated complex 5, lacking the O-H group, only shows an intramolecular N-H...Cl hydrogen bonding of 2.42 A. The structure of complex 3, which turns out to be a dimer in the solid state through a double intermolecular N-H...O hydrogen bonding, has also been investigated in solution (CD₂Cl₂) by NMR diffusion studies. Diffusion-ordered spectroscopy experiments reveal an equilibrium between monomer and dimer species in solution whose extension depends on the temperature, concentration, and coordinating properties of the solvent. Preliminary catalytic studies show that complex 3 is highly active in the redox isomerization of the allylic alcohols in an aqueous medium under very mild reaction conditions (35 °C) and in the absence of a base.

Full text of DOI:10.1021/ic200547k

ARTICLE
pubs.acs.org/IC
Synthesis and Structure of Ruthenium(IV) Complexes Featuring
N-Heterocyclic Ligands with an NꢀH Group as the Hydrogen-Bond
Donor: Hydrogen Interactions in Solution and in the Solid State
Josefina Díez,† Josꢀe Gimeno,*,† Isabel Merino,‡ Eduardo Rubio,*,†,‡ and Francisco J. Suꢀarez†
†Departamento de Química Orgꢀanica e Inorgꢀanica, Instituto Universitario de Química Organometꢀalica “Enrique Moles”
(Unidad Asociada al CSIC), Universidad de Oviedo, 33071 Oviedo, Spain
‡Unidad de Resonancia Magnꢀetica Nuclear, Servicios Científico-Tꢀecnicos de la Universidad de Oviedo, 33071 Oviedo, Spain
S Supporting Information
b
ABSTRACT: The synthesis and characterization of novel
ruthenium(IV) complexes [Ru(η3:η3-C10H16)Cl2L] [L =
3-methylpyrazole (2b), 3,5-dimethylpyrazole (2c), 3-methyl-5-
phenylpyrazole (2d), 2-(1H-pyrazol-5-yl)phenol (2e), 6-azaura-
cile (3), and 1H-indazol-3-ol (4)] are reported. Complex 2e is
converted to the chelated complex [Ru(η3:η3-C10H16)Cl(k2-N,
O-2-(1H-pyrazol-3-yl)phenoxy)] (5) by treatment with an excess
of NaOH. All of the ligands feature NꢀH, OꢀH, or CdO as the potential hydrogen-bonding group. The structures of complexes
2aꢀ2c, 2e, 3, and 5 in the solid state have been determined by X-ray diffraction. Complexes 2aꢀ2c and 3, which contain the pyrazole
NꢀH group, exhibit intra- and intermolecular hydrogen bonds with chloride ligands [NꢀH Cl distances (Å): intramolecular,
3 3 3
2.30ꢀ2.78; intermolecular, 2.59ꢀ2.77]. Complexes 2e and 3 bearing respectively OꢀH and CdO groups also feature NꢀH
O
3 3 3
interactions [intramolecular (2e), 2.27 Å; intermolecular (3), 2.00 Å]. Chelated complex 5, lacking the OꢀH group, only shows an
intramolecular NꢀH Cl hydrogen bonding of 2.42 Å. The structure of complex 3, which turns out to be a dimer in the solid state
3 3 3
through a double intermolecular NꢀH O hydrogen bonding, has also been investigated in solution (CD2Cl2) by NMR diffusion
3 3 3
studies. Diffusion-ordered spectroscopy experiments reveal an equilibrium between monomer and dimer species in solution whose
extension depends on the temperature, concentration, and coordinating properties of the solvent. Preliminary catalytic studies show that
complex 3 is highly active in the redox isomerization of the allylic alcohols in an aqueous medium under very mild reaction conditions
(35 °C) and in the absence of a base.
’ INTRODUCTION
complexes, providing relevant examples with unusual properties
in several fields.4 The development of new materials and com-
plexes that behave as sensors, chromophores, and medicines,
among other applications, has recently been described.5
Since the first reports on the existence of the hydrogen bond
around a century ago, there has been a growing interest in this
type of interaction, which plays a key role in numerous chemical
systems ranging from inorganic and organic to biological chem-
istry.1 The abilities of proteins to fold into stable three-dimen-
sional structures and to link small and large molecules such as the
double-helical structures of DNA and RNA macromolecules are
among the most important manifestations. Hydrogen bonding is
also responsible for the remarkable selectivity and acceleration
rate observed in ubiquitous enzymatic processes of biological
interest. On the other hand, its utilization as an activation force
has become a powerful tool in modern organocatalysis.2
Metalloenzymes, which combine the properties of metals and
hydrogen bonds, are also very common natural catalysts of
important biological processes whose active sites generally
exhibit many hydrogen interactions.3 Because natural processes
exploit these properties to accomplish very important chemical
transformations, the design of metal complexes bearing hydro-
gen-bonding groups for a practical use is a goal of current special
interest. In this regard, new types of these interactions have
emerged as crucial structural and functional features in metal
In order to mimic the metalloenzyme activity in natural
systems, there has been a growing interest to enhance the metal
catalysts and substrate binding via the promotion of hydrogen
bonding. The key point of this favorable catalytic activity stems
from the cooperative effects provided by the bifunctional char-
acter of molecular catalysts, which may act as both acidic and
basic active sites in metalloenzymes.6 The most genuine catalytic
transformations of this type, in terms of high selectivity and rate
enhancement, are concerned with transition states characterized
by “outer-sphere” hydrogen bonding featuring low-energy bar-
riers responsible for high catalytic performances and enantios-
electivities (Figure 1).7 Typical examples showing this type of
interaction8,9 are involved in the hydrogenation of polar func-
tional groups such as aldehydes, ketones, or imines catalyzed
by semisandwich ruthenium(II) hydrides [Noyori's (A)7a and
Received: January 18, 2011
Published: May 10, 2011
r
2011 American Chemical Society
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|

Inorganic Chemistry
ARTICLE
Chart 2
intramolecular hydrogen bonding in the solid state are reported.
The results of NMR studies using variable-temperature and
diffusion-ordered spectroscopy (DOSY) experiments for a mod-
el complex show that these interactions also exist in solution.
Preliminary catalytic studies on the redox isomerization of allylic
alcohols in water using the uracile derivative I as a catalyst are also
described, revealing a high activity under very mild reaction
conditions (35 °C) and in the absence of a base.
Figure 1. Examples of catalytic species featuring “outer-sphere” hydro-
gen bonding.
’ EXPERIMENTAL SECTION
The manipulations were performed under an atmosphere of dry
nitrogen using vacuum-line and standard Schlenk techniques. Solvents
were dried by standard methods and distilled under nitrogen before use.
All reagents were obtained from commercial suppliers and used without
further purification with the exception of compounds [{Ru(η3:η3-
C10H16)(μ-Cl)Cl}2] (1)14 and [Ru(η3:η3-C10H16)Cl2(pyrazole)]
(2a),15 which were prepared by following the methods reported in the
literature. IR spectra were recorded on a Perkin-Elmer 1720-XFT
spectrometer. The conductivities were measured at room temperature,
in ca. 10ꢀ3 mol dmꢀ3 water solutions, with a Jenway PCM3 conducti-
meter. C, H, and N analyses were carried out with a Perkin-Elmer 2400
microanalyzer. NMR spectra were recorded on a Bruker DPX-300
instrument at 300 MHz (1H) or 75.4 MHz (13C) using SiMe4 as the
standard. DEPT experiments have been carried out for all of the
compounds reported. The numbering for the protons and carbons of
the 2,7-dimethylocta-2,6-diene-1,8-diyl skeleton is as follows:
Chart 1
Shvo’s (B)7bꢀd catalysts] as well as iridium(III) complexes (C
and D) proposed in the catalyzed asymmetric CꢀC and CꢀN
bond formation.7l
Water often plays a prominent role in the active sites of
metalloenzymes via the formation of weak noncovalent hydro-
gen bonding. However, examples in which these interactions
enhance the catalytic activity of transition-metal complexes in
aqueous media have remained elusive. Recently, ruthenium-
catalyzed regioselective and highly efficient procedures in the
hydration of alkynes10 and nitriles11 to give aldehydes and
amides, respectively, have been reported.12 Significantly, an
intermediate aquaruthenium(II) complex displaying an intramo-
lecular N H hydrogen bonding of pendant k-P bidentate P,N-
3 3 3
imidazolylphosphane ligands with water has been isolated (E,
Chart 1).10b This fact is assumed to be responsible for the
observed high catalytic efficiency.
Synthesis of Complexes [Ru(η3:η3-C10H16)Cl2L] [L =
3-Methylpyrazole (2b), 3,5-Dimethylpyrazole (2c), 3-Meth-
yl-5-phenylpyrazole (2d), 2-(1H-Pyrazol-3-yl)phenol (2e),
6-Azauracil (3), and 2-Indazolinone (4)]. The corresponding
ligand (0.64 mmol) was added, at room temperature, to a solution of
complex 1 (0.200 g; 0.32 mmol) in 20 mL of dichloromethane. After the
mixture was stirred for 10 min, the solvent was removed under vacuum
and the resulting orange solid residue was washed with hexane (3 ꢁ
10 mL) and dried in vacuo. 2b. Yield: 76% (191 mg). Anal. Calcd for
RuC14H22Cl2N2: C, 43.08; H, 5.68; N, 7.18. Found: C, 42.91; H, 5.59;
N, 7.15. IR (KBr, cmꢀ1): ν 605 (m), 684 (s), 781 (mf), 952 (m), 1109
(s), 1282 (s), 1385 (s), 1463 (m), 1563 (m), 2913 (m), 3281 (s). 1H
NMR (CD2Cl2): δ 2.37 (s, 6H, CH3), 2.40 (s, 3H, CH3 L), 2.47 (m, 2H,
H4 and H6), 3.09 (m, 2H, H5 and H7), 4.34 (s, 2H, H2 and H10), 4.48 (s,
With these precedents in mind and in the context of our
ongoing interest in the study of ruthenium-catalyzed reactions,
we have devoted special attention to efficient catalytic transfor-
mations in aqueous media.13 In this regard, we are engaged in the
synthesis of new active catalysts with structural features enabling
hydrogen interactions to be used as potential bifunctional
catalysts in aqueous media. Herein, we describe the synthesis
and characterization of new ruthenium(IV) complexes bearing
N-heterocyclic ligands functionalized with uncoordinated CdO,
NꢀH, and OꢀH groups (FꢀI, Chart 2), which are prone to
form hydrogen bonds. Molecular structures determined by X-ray
diffraction studies showing unequivocal information on inter- and
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ARTICLE
2H, H1 and H9), 5.18 (m, 2H, H3 and H8), 6.27 (s, 1H, H4 L), 8.16 (s,
1H, H3 L), 11.74 (s, 1H, NH). 13C{1H} NMR (CD2Cl2): δ 11.0 (s,
CH3), 20.9 (s, CH3), 36.7 (s, C4 and C5), 76.9 (s, C1 and C8), 94.5 (s, C3
and C6), 106.8 (s, C4 L), 130.0 (s, C2 and C7), 141.8 (s, C5 L), 143.2 (s,
C3 L). 2c. Yield: 57% (148 mg). Anal. Calcd for RuC15H24Cl2N2: C,
44.56; H, 5.98; N, 6.93. Found: C, 45.05; H, 5.86; N, 7.00. IR
(KBr, cmꢀ1): ν 660 (m), 803 (s), 955 (m), 1021 (s), 1149 (m), 1283
(s), 1390 (m), 1570 (vs), 2907 (m), 3312 (s). 1H NMR (CD2Cl2): δ
2.26 (s, 3H, CH3 in C3 L), 2.35 (m, 2H, H4 and H6), 2.37 (s, 6H, CH3),
2.43 (s, 3H, CH3 in C5 L), 3.00 (m, 2H, H5 and H7), 4.49 (s, 2H, H2 and
H10), 4.88 (s, 2H, H1 and H9), 5.15 (m, 2H, H3 and H8), 5.95 (s, 1H, H4
L), 11.21 (s, 1H, NH). 13C{1H} NMR (CD2Cl2): δ 10.7 (s, CH3 in C3
L), 15.7 (s, CH3 in C5 L), 20.9 (s, 2CH3), 36.3 (s, C4 and C5), 76.5 (s, C1
and C8), 95.0 (s, C3 and C6), 108.7 (s, C4 L), 130.7 (s, C2 and C7), 141.4
(s, C5 L), 157.9 (s, C3 L). 2d. Yield: 86% (260 mg). Anal. Calcd for
RuC20H26Cl2N2: C, 51.50; H, 5.62; N, 6.01. Found: C, 51.43; H, 5.88;
N, 6.04. IR (KBr, cmꢀ1): ν 693 (s), 763 (vs), 1022 (vs), 1294 (s), 1382
(s), 1467 (s), 1496 (s), 1564 (m), 2909 (m), 3222 (s). 1H (CD2Cl2): δ
2.35 (m, 2H, H4 and H6), 2.40 (s, 6H, 2CH3), 2.61 (s, 3H, CH3 in C3 L),
3.02 (m, 2H, H5 and H7), 4.59 (s, 2H, H2 and H10), 4.93 (s, 2H, H1 and
H9), 5.21 (m, 2H, H3 and H8), 6.45 (s, 1H, H4 L), 7.35ꢀ7.52 (m, 5H,
Ph), 12.04 (s, 1H, NH). 13C{1H} NMR (CD2Cl2): δ 15.8 (s, CH3 in C3
L), 21.1 (s, 2CH3), 36.4 (s, C4 and C5), 76.5 (s, C1 and C8), 95.1 (s, C3
and C6), 106.6 (s, C4 L), 125.3 (s, Co Ph), 128.3 (s, Cipso Ph), 129.1 (s,
Cp Ph), 129.2 (s, Cm Ph), 131.2 (s, C2 and C7), 144.3 (s, C5 L), 158.8 (s,
C3 L). 2e. Yield: 60% (180 mg). Anal. Calcd for RuC19H24Cl2N2O: C,
48.72; H, 5.16; N, 5.98. Found: C, 48.51; H, 5.70; N, 5.9. IR
(KBr, cmꢀ1): ν 743 (vs), 779 (s), 957 (s), 1083 (vs), 1116 (s), 1243
(s), 1258 (s), 1345 (m), 1383 (m), 1444 (s), 1490 (s), 1552 (m), 1594
(m), 1612 (m), 2973 (m), 3202 (s). 1H NMR (CD2Cl2): δ 2.33 (s, 6H,
2CH3), 2.43 (m, 2H, H4 and H6), 3.08 (m, 2H, H5 and H7), 4.35 (s, 2H,
H2 and H10), 4.50 (s, 2H, H1 and H9), 5.17 (m, 2H, H3 and H8), 5.97 (s,
1H, OH), 6.82 (s, 1H, H4 L), 6.90 (d, JHH = 8.1 Hz, 1H, H6 Ph), 7.03 (t,
0.2 mmol) in dichloromethane was treated with an excess of NaOH.
After 2 h, the solution was filtered off through Celite and the product
purified by column chromatography (silica gel with dichloromethane as
the eluent). Yield: 35% (32 mg). Anal. Calcd for RuC19H23ClN2O: C,
52.83; H, 5.37; N, 6.49. Found; C, 52.65; H, 5.32; N, 6.45. IR
(KBr, cmꢀ1): ν 594 (w), 659 (w), 765 (s), 849 (w), 1128 (m), 1315
(vs), 1440 (s), 1488 (s), 1521 (w), 1549 (w), 1595 (s), 2909 (w), 3137
(w), 3250 (vs). 1H NMR (CD2Cl2): δ 2.02 and 2.37 (s, 6, 2CH3), 2.69
(m, 2H, H4 and H6), 3.26 (m, 2H, H5 and H7), 3.22 and 3.93 (s, 2H, H2
and H10), 4,25 and 4.48 (s, 2H, H1 and H9), 4.31 and 4.92 (m, 2H, H3
and H8), 6.52 (m, 1H, H5 Ph), 6.56 (m, 1H, H3 Ph), 6.82 (m, 1H, H4 L),
6.92 (m, 1H, H4 Ph), 7.45 (m, 1H, H6 Ph), 7.80 (m, 1H, H5 L), 12.13 (s,
1H, NH). 13C{1H} NMR (CD2Cl2): δ 18.3 and 20.5 (s, 2CH3), 36.3
and 36.9 (s, C4 and C5), 75.0 and 19.4 (s, C1 and C8), 98.6 and 103.5 (s,
C3 and C6), 102.9 (s, C4 L), 113.9 (s, C5 Ph), 116.5 (s, C2 Ph), 124.3 (s,
C3 Ph), 126.5 and 128.0 (s, C2 and C7), 127.7 (s, C6 Ph), 129.5 (s, C4
Ph), 131.3 (s, C5 L), 131.4 (s, C3 L), 149.6 (s, C1 Ph). Method b: A
solution of 2-(1H-pyrazol-3-yl)phenol (32 mg, 0.2 mmol) was treated
with an excess of NaOH. After 1 h of agitation, complex 1 (61.6 mg, 0.1
mmol) was added to the solution. After 2 h, the solution was filtered off
through Celite and the product purified by column chromatography
(silica gel with dichloromethane as the eluent). Yield: 41% (38 mg).
X-ray Crystal Structure Determination of Complexes
2aꢀ2c, 2e, 3, and 5. The most relevant crystal and refinement data
are collected in Table 1.
In all cases, crystals suitable for X-ray diffraction analysis were
obtained by the slow diffusion of hexane into a saturated solution of
the complexes in dichloromethane.
Data collection was performed on an Oxford Diffraction Xcalibur
Nova single-crystal diffractometer, using Cu KR radiation (λ = 1.5418 Å;
2aꢀ2c and 5). Images were collected at a 75 mm fixed crystalꢀdetector
distance, using the oscillation method, with 1° oscillation and variable
exposure time per image (3ꢀ50 s). The data collection strategy was
calculated with the program CrysAlis Pro CCD.16 Data reduction and cell
refinement were performed with the program CrysAlis Pro RED.16
An empirical absorption correction was applied using the SCALE3
ABSPACK algorithm, as implemented in theprogram CrysAlis Pro RED.16
For 2e and 3, diffraction data were recorded on a Nonius Kappa CCD
single-crystal diffractometer, using Mo KR radiation (λ = 0.710 73 Å).
Images were collected at a 35 mm fixed crystalꢀdetector distance, using
the oscillation method, with 1° oscillation and 50ꢀ100 s exposure time
per image. The data collection strategy was calculated with the program
Collect.17 Data reduction and cell refinement were performed with the
programs HKL Denzo and Scalepack.18 A semiempirical absorption
correction was applied using the program SORTAV.19 The software
package WINGX20 was used for space group determination, structure
solution, and refinement. The structures for complexes 2b, 2c, 2e, and 3
were solved by Patterson interpretation and phase expansion using
DIRDIF.21 For 2a and 5, the structures were solved by direct methods
using SIR92.22 In the asymmetric unit of the crystal of 2a, 1.5 molecules
were found (2aI and 0.52aII). In the molecule 2aII, the atoms C19
and N4 are disordered in two positions with an occupancy factor of
ca. 0.5. Isotropic least-squares refinement on F2 using SHELXL9723 was
performed.
J
HH = 7.5 Hz, 1H, H4 Ph), 7.23 (t, JHH = 7.8 Hz, 1H, H5 Ph), 7.65 (d, JHH
= 7.6 Hz, 1H, H3 Ph), 7.27 (s, 1H, H3 L), 12.85 (s, 1H, NH). 13C{1H}
NMR (CD2Cl2): δ 21.0 (s, 2CH3), 36.7 (s, C4 and C5), 77.0 (s, C1 and
C8), 94.7 (s, C3 and C6), 104.3 (s, C4 L), 115.3 (s, C2 Ph), 116.6 (s, C6
Ph), 121.4 (s, C4 Ph), 127.6 (s, C3 Ph), 130.2 (s, C5 Ph), 129.9 (s, C2 and
C7), 142.5 (s, C3 L), 142.7 (s, C5 L), 152.3 (s, C1 Ph).
3. Yield: 76% (204 mg). Anal. Calcd for RuC13H19Cl2N3O2: C,
37.06; H, 4.55; N, 9.97. Found: C, 37.16; H, 4.45; N, 9.92. IR
(KBr, cmꢀ1): ν 570 (s), 787 (m), 857 (m), 1021 (m), 1108 (m),
1383 (m), 1444 (s), 1609 (m), 1685 (vs), 1726 (vs), 3086 (s), 3430 (s).
1H NMR (CD2Cl2): δ 2.41 (s, 6H, 2CH3), 2.43 (m, 2H, H4 and H6),
3.09 (m, 2H, H5 and H7), 4.35 (s, 2H, H2 and H10), 4.86 (s, 2H, H1 and
H9), 5.36 (m, 2H, H3 and H8), 8.65 (s, 1H, CH, L), 9.10 and 11.74 (s,
2NH). 13C{1H} NMR (CD2Cl2): δ 21.0 (s, 2CH3), 36.3 (s, C4 and C5),
77.5 (s, C1 and C8), 95.0 (s, C3 and C6), 133.4 (s, C2 and C7), 143.1 (s,
C5 L), 146.5 (s, C4 L), 154.3 (s, C2 L). Conductivity (water, 20 °C): 177
Ω
ꢀ1 cm2 molꢀ1
4. Yield: 43% (122 mg). Anal. Calcd for RuC17H22Cl2N2O: C, 46.16;
.
H, 5.01; N, 6.33. Found: C, 46.80; H, 4.93; N, 6.25. IR (KBr, cmꢀ1): ν
633 (m), 745 (s), 950 (m), 1352 (m), 1624 (s), 3295 (vs), 3446 (vs). 1H
NMR (CD2Cl2): δ 2.39 (s, 6H, 2CH3), 2.45 (m, 2H, H4 and H6), 3.10
(m, 2H, H5 and H7), 4.35 and 4.66 (s, 2H, H2 and H10), 4.90 and 5.27 (s,
2H, H1 and H9), 5.05 (m, 2H, H3 and H8), 7.10 (m, 1H, H5 L), 7.31 (m,
1H, H7 L), 7.41 (m, 1H, H6 L), 7.65 (m, 1H, H4 L), 10.12 and 10.64
(2H, NH and OH). 13C{1H} NMR (CD2Cl2): δ 21.1 (s, 2CH3), 36.7 (s,
C4 and C5), 74.8 and 76.6 (s, C1 and C8), 95.5 and 97.2 (s, C3 and C6),
109.1 (s, C7 L), 113.6 (s, C3a L), 119.5 (s, C5 L), 120.2 (s, C4 L), 129.1 (s,
C6 L), 131.5 and 131.9 (s, C2 and C7), 142.8 (s, C7a L), 163.0 (s, C2 L).
Synthesis of Complex [Ru(η3:η3-C10H16)Cl(j-N,O-2-(1H-
pyrazol-3-yl)phenoxy)] (5). Method a: A solution of complex
[Ru(η3:η3-C10H16)Cl2(k-N-2-(1H-pyrazol-3-yl)phenol)] (2e; 100 mg,
During the final stages of refinement, all of the positional parameters
and anisotropic temperature factors of all of the non-H atoms were
refined. For 2a, 2e, and 3, the H atoms were geometrically located and
their coordinates were refined by riding on their parent atoms. For 2b,
2e, and 5 (as well as H4N, H19, H1a, H1b, H10a, and H10b for 2a and
H1a, H1b, H10a, and H10b for 2e and 3), the coordinates of the H
atoms were found from different Fourier maps and included in a
refinement with isotropic parameters.
The function minimized was ([∑w(Fo ꢀ Fc2)/∑w(Fo )]1/2, where
2
2
w = 1/[σ2(Fo ) þ (aP)2 þ bP] (a and b values are collected in Table 1)
2
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ARTICLE
Table 1. Crystallographic Data for Compounds of 2aꢀ2c, 2e, 3, and 5
2a
2b
2c
2e
3
5
chemical formula
C
13H20Cl2N2Ru
C14H22Cl2N2Ru
390.31
293(2)
1.5418
Pna21
C15H24Cl2N2Ru
404.33
C19H24Cl2N2ORu
486.37
C13H19Cl2N3O2Ru
421.28
C19H23ClN2ORu
431.91
293(2)
1.5418
Pbca
fw
376.28
293(2)
1.5418
Fdd2
T, K
293(2)
1.5418
293(2)
0.7107
293(2)
1.5418
wavelength, Å
space group
a, Å
P21/c
P21/c
P21/c
33.2559(8)
24.4116(6)
11.1014(2)
90
7.6952(1)
22.8324(3)
9.0117(1)
90
9.1513(1)
22.9768(2)
7.8822(1)
90
16.2255(8)
8.3163(4)
14.5735(4)
90
12.3933(7)
8.5805(4)
15.8735(10)
90
8.8541(1)
13.8896(1)
29.3729(2)
90
b, Å
c, Å
R, deg
β, deg
γ, deg
Z
90
90
93.313(1)
90
91.335(3)
90
95.632(3)
90
90
90
90
90
24
4
4
4
4
8
V, Å3
9012.4(3)
1.664
1583.35(3)
1.637
1654.60(3)
1.623
1965.96(15)
1.582
1679.85(16)
1.666
3612.28(5)
1.588
F
calcd, g cmꢀ3
μ, mmꢀ1
11.586
1.019
11.014
1.083
10.562
1.079
1.258
8.441
GOF on F2
1.067
0.866
0.956
1.035
weight function (a, b)
R1a [I > 2σ(I)]
wR2a [I > 2σ(I)]
R1a (all data)
wR2a (all data)
F(000)
0.0195, 8.5210
0.0426
0.1151
0.0443
0.1169
4560
0.0689, 0.0
0.0329
0.0904
0.0343
0.0915
792
0.0691, 0.0677
0.0347
0.0474, 0.0
0.0375
0.0541, 0.0
0.0336
0.0355, 1.3321
0.0235
0.0613
0.0255
0.0629
1760
0.0920
0.1009
0.0922
0.0386
0.0645
0.0494
0.0949
0.1068
0.1119
824
952
848
a R1 = ∑(|Fo| ꢀ |Fc|)/∑|Fo|; wR2 = {∑[w(Fo ꢀ Fc2)2]/∑[w(Fo ) ]}1/2
.
2
2 2
with σ2(Fo ) from counting statistics and P = [max(Fo , 0) þ 2Fc2]/3.
Atomic scattering factors were taken from the International Tables for
X-ray Crystallography.24 Geometrical calculations were made with
PARST.25 The crystallographic plots were made with PLATON26 and
ORTEP-3 for Windows.27
delay pulse program (ledbpgps2s in Bruker software) with spinning of the
sample to avoid convection influence.32 The diffusion time (D20) and
gradient duration (P30) were previously optimized with the ledbpgp2s1d
sequence for each measurement to get 1ꢀ5% of the residual signal with
maximum strength while observing a progressive decay of the signal
intensities. For a typical 1H DOSY experiment, P30 = 0.9ꢀ1.0 ms and
D20 = 120 ms, an eddy current delay (Te) of 5 ms, and a spoil gradient
(P19) of 0.8 ms were used. The pulse gradients were increased from 2 to
95% of the maximum strength in a linear ramp through 16 steps of 16K
data points each. The size of the flame ionization detector was 16K, the
spectral width was set to AQ = 1.5 s, the number of scans were 16ꢀ32,
and a relaxation delay D1 = 1 s was used; the direct dimension was zero-
filled to 32K, Fourier-transformed, and phase-corrected. The diffusion
dimension was zero-filled to 128 and processed with the standard Bruker
DOSY algorithm. The diffusion coefficients were measured in a loga-
rithmic scale, and the accuracy of the reported values is (0.01. NMR
Data at 183 K for Complex 3. 1H NMR (600.15 MHz, CD2Cl2, 183 K):
δ 11.75 (bs, 1H, NH-2), 11.23 (bs, 1H, NH-4), 8.62 (s, 1H, CH), 5.30
(m, 1H, internal allylic), 5.24 (m, 1H, internal allylic), 4.89 (s, 1H,
terminal allylic), 4.73 (s, 1H, terminal allylic), 4.33 (s, 1H, terminal
allylic), 4.22 (s, 1H, terminal allylic), 3.06 (m, 2H, aliphatic CH2), 2.36
(m, 2H, aliphatic CH2), 2.30 (s, 6H, CH3). 13C{1H} NMR (150.91
MHz, acetone-d6, 183 K): δ 155.3 (C, C-5), 148.2 (C, C-3), 143.7 (CH,
C-6), 133.3 (C, allylic), 96.3 (CH, allylic), 94.6 (CH, allylic), 77.1 (CH2,
allylic), 36.6 (CH2, aliphatic), 36.4 (CH2, aliphatic), 21.9 (CH3), 21.8
(CH3). NMR Data in the Presence of DMSO-d6 at 298 K. 1H NMR
(600.15 MHz, CD2Cl2, 298 K): δ 11.70 (bs, 1H, NH-2), 11.26 (bs, 1H,
NH-4), 7.27 (d, JHH = 1.6 Hz, 1H, CH-6), 5.13 (m, 2H, internal allylic),
4.72 (s, 2H, terminal allylic), 3.99 (s, 2H, terminal allylic), 3.20 (m, 2H,
aliphatic CH2), 2.57 (m, 2H, aliphatic CH2), 2.32 (s, 6H, CH3).
13C{1H} NMR (150.91 MHz, acetone-d6, 298 K): δ 157.3 (C, C-5),
149.1 (C, C-3), 135.4 (CH, C-6), 129.6 (C, allylic), 102.3 (CH, allylic),
75.3 (CH2, allylic), 35.5 (CH2, aliphatic), 20.5 (CH3).
2
2
Theoretical Calculations. The theoretical calculations were per-
formed using the program package Gaussian0328 at the density func-
tional theory (DFT) level by means of the hybrid B3LYP functional.29 In
all geometry optimizations, Pople’s 6-31G(d) split-valence basis set was
used for N, C, H, O, and Cl elements and LANL2DZ,30 for Ru which
combines quasi-relativistic effective core potentials with a valence
double-basis set. Frequency calculations were performed to determine
whether the optimized geometries were minima on the potential energy
surface. The energy in the solvent was estimated by the application
of single-point calculations using the polarizable continuum model
(PCM) with standard UA0 solvation spheres and the extended basis
set 6-311þþG(d,p) on the optimized stationary points in CH2Cl2
(ε = 8.93).31
NMR Studies. The NMR spectra were recorded on a Bruker AV 600
1
spectrometer operating at 600.15 and 150.91 MHz for H and 13C,
respectively, using a 5 mm PATXI-1H/D-13C/15N inverse probe with a
z-gradient coil or on a Bruker AV 400 spectrometer operating at 400.13
and 100.61 MHz for 1H and 13C, respectively, using a 5 mm TBO-1H/X-
BB/31P/D direct probe with a z-gradient coil. All of the experiments
were acquired and processed with TOPSPIN 1.3 Bruker NMR software.
The two-dimensional NMR experiments were recorded on the AV 600
spectrometer using gradient-enhanced versions of the pulse sequences.
1
The H DOSY experiments were performed on the AV400 spectro-
meter. The gradient unit of the spectrometer produced magnetic-field
pulsed gradients in the z axis of 53.5 G cmꢀ1. The gradient strength was
calibrated using a D2O sample to obtain a diffusion coefficient of 1.90 ꢁ
10ꢀ9 m2 sꢀ1 for HDO. During the DOSY experiments, the temperature
was set to 298 K and maintained with an air flow of 400 L hꢀ1. The
experiments were acquired with the bipolar longitudinal eddy current
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Inorganic Chemistry
ARTICLE
Scheme 1
Scheme 2
’ RESULTS AND DISCUSSION
With the aim of providing ruthenium(IV) complexes featuring
hydrogen bonding via NꢀH groups, we have selected the
pyrazole derivatives 6-azauracil and 1,2-dihydro-indazol-3-one
as typical heterocycles acting as two-electron N-donor ligands.
The presence of additional functionalities such as OH and CdO
groups can supply complementary effects. The use of dimer 1 as
the starting material for the direct synthesis of the target
complexes stems from the following: (i) it is a readily and/or
commercially available material;33 (ii) it is an excellent precursor
of the unsaturated 16-electron fragment trans-[Ru(η3:η3-C10H16)-
Cl2], which enables the binding of two-electron N ligands;34
(iii) the resulting complexes provide a trans-RuCl2 disposition,
which is prone to generating favorable intra- and intermolecular
Cl H hydrogen bonding.
6-diene-1,8-diyl group in the NMR spectra are in accordance
with the formation of a simple equatorial adduct with C2 sym-
metry in which the halves of the bis(allyl) ligand are in equivalent
environments.34 In addition, the spectra display the expected
signals for the nitrogen ligands (see the Experimental Section for
3 3 3
Synthesis of [Ru(η3:η3-C10H16)Cl2L] [L = Pyrazole (2a),15
3-Methylpyrazole (2b), 3,5-Dimethylpyrazole (2c), 3-Meth-
yl-5-phenylpyrazole (2d), 2-(1H-Pyrazol-5-yl)phenol (2e),
and 6-Azauracil (3)]. The treatment of 1 with 2 equiv of the
appropriate pyrazole derivative, 2-(1H-pyrazol-3-yl)phenol or
6-azauracil, in dichloromethane or dichloromethane/methanol
at room temperature affords the corresponding mononuclear
complexes 2aꢀ2e and 3. All complexes have been isolated
(43ꢀ86%) as air-stable orange solids (Scheme 1).
1
details). Significantly, IR and H NMR spectra are consistent
with the k-N coordination of pyrazole ligands and 6-azauracil,
i.e., ν(NꢀH) absorption band (KBr) at 3222ꢀ3312 (2bꢀ2e)
and 3430 (3) cmꢀ1; δ NꢀH resonance at 11.21ꢀ12.85
(2bꢀ2e) and 11.74 and 9.10 (3). Further NMR data of complex
3 prove the existence of dimeric association in solution through
intermolecular hydrogen bonding (see below).
It is interesting to note that the formation of complex 2e
proceeds through tautomerization of the ligand-free 2-(1H-
pyrazol-3-yl)phenol into the coordinated 2-(1H-pyrazol-5-yl)-
phenol (Scheme 2). This is consistent with the well-known
tautomerism of diazoles via intermolecular proton transfer,35
although intramolecular tautomerization via the ruthenium
complex bearing a 2-(1H-pyrazol-3-yl)phenol ligand cannot be
ruled out. The structure of 2e in the solid state determined by
X-ray diffraction (see below) confirms the presence of the 2-(1H-
pyrazol-5-yl)phenol ligand.
Compounds 2aꢀ2e and 3 are soluble in dichloromethane and
acetone and insoluble in hexane and diethyl ether. In contrast to
2aꢀ2e, complex 3 is soluble in water. Although conductivity
measurements of acetone solutions show that there are no
electrolytes, the molar conductivity found for 3 in water is 177
Ω
ꢀ1 cm2 molꢀ1, a value that is among those expected for 1:1 or
2:1 electrolytes (118ꢀ131 and 235ꢀ273 Ωꢀ1 cm2 molꢀ1, res-
pectively). This seems to indicate that waterꢀchloride exchange
processes leading to an equilibrium containing cationic aqua
species of the type [Ru(η3:η3-C10H16)Cl(6-azauracile)(H2O)]þ
and [Ru(η3:η3-C10H16)(6-azauracile)(H2O)2]2þ may be formed.
Unfortunately, all attempts to isolate these species have been
unsuccessful. Interestingly, the aqueous solution is acidic with a
pH value of 4.0, indicating that proton dissociation either from
the coordinated water molecule or the uracile NꢀH group can
also take place. Spectroscopic data (IR and NMR spectroscopy)
and elemental analysis are in agreement with the proposed
formulations. In particular, 1H and 13C{1H} NMR spectra
(CD2Cl2) of complexes 2aꢀ2e and 3 show the signals for the
η3:η3-2,7-dimethylocta-2,6-diene-1,8-diyl ligand, which compare
well with those previously reported for analogous complexes
containing monodentate nitrogen ligands.34 It is worth noting
that proton and carbon resonances of the 2,7-dimethylocta-2,
Synthesis of [Ru(η3:η3-C10H16)Cl2(1H-indazol-3-ol)] (4).
By following an analogous synthetic procedure, complex 4 was
obtained by the reaction of 1 with 1,2-dihydroindazol-3-one
(Scheme 3). It has been isolated as an air-stable orange solid
(47%), which is soluble in chlorinated solvents and acetone and
insoluble in water, hexane, and diethyl ether.
Complex 4 has been characterized by elemental analysis and
spectroscopic methods (IR and 1H and 13C{1H} NMR). In parti-
cular, proton and carbon spectra reveal that the halves of the 2,7-
dimethylocta-2,6-diene-1,8-diyl ligand are now in an inequivalent
environment (i.e., a six-line pattern of 74.8, 76.6, 95.5, 97.2,
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Inorganic Chemistry
ARTICLE
Scheme 3
Scheme 4
different signals of the 2,7-dimethylocta-2,6-diene-1,8-diyl ligand,
as is expected for the loss of C2 symmetry. The X-ray crystal
structure of complex 5 (see below) confirms the formation of the
metalla-2-(1H-pyrazol-3-yl)phenoxy chelate ring.
X-ray Crystal Structures. The structures of complexes
2aꢀ2c, 2e, 3, and 5 have been determined by X-ray crystal-
lography in the solid state. Collection data are summarized in
Table 1, and selected bond distances and angles are shown in
Table 2. For complex 2a, two independent molecules were found
in the unit cell, so in this case, two sets of values are shown.
Drawings of the molecular structures are depicted in Figures 2ꢀ7.
Figures 8ꢀ12 provide views of the closest intermolecular hydrogen
bonds for complexes 2aꢀ2c, 2e, and 3.
131.5, and 131.9 ppm is observed for the skeletal allyl groups in
the 13C NMR spectrum). In addition, the IR spectrum (KBr)
shows the disappearance of the typical ν(CdO) absorption of
the free ligand at 1620 cmꢀ1, also displaying new ν(CdN) and
ν(OꢀH) absorptions at 1624 (w) and 3446 (vs and br) cmꢀ1
,
As a general trend, the geometry about the ruthenium(IV)
center can be described as a distorted trigonal bipyramid by
considering the allyl groups as monodentate ligands bound to
ruthenium through their centroids C*1 and C*2, showing the
expected η3:η3-coordination mode. The structures of complexes
2aꢀ2c, 2e, and 3 (Figures 2ꢀ6) feature the two chloride ligands
axially located in a formal trans disposition with slight deviations
of the Cl1ꢀRuꢀCl2 angle from ideal 180° [in the range of
170.31(8)ꢀ172.45(5)°]. Cl1ꢀRuꢀN1 and Cl2ꢀRuꢀN1
bonding angles along with bond distance values CꢀC and RuꢀC
[in the range of RuꢀC*1 = 1.9959(4)ꢀ2.0145(2) Å and
RuꢀC*2 = 1.9960(4)ꢀ2.0126(2) Å] compare well with those
observed in other structures containing the “Ru(η3:η3-C10H16)”
unit.36 All of the ligands are coordinated edge-on via the iminic N
atom, which resides in an equatorial position along with the allyl
groups. The RuꢀN bond length values are in the range of
2.142(6)ꢀ2.193(3) Å, which can be compared with those
reported in the literature.36 A significant feature is concerned
with the observed dihedral angle R formed by the corresponding
planes containing the RuꢀNꢀNꢀH skeleton of the heterocyclic
rings and the ClꢀRuꢀCl moiety. This angle arises from the
folded position of the plane containing either the pyrazole
(2aꢀ2c and 2e) or 6-azauracil (3) rings. With the exception of
the angle found in one of the independent molecules of 2a [e.g.,
1.4(12)°], the rest of the values are in the range of
11.5ꢀ36.7(2)°. The folding of the heterocyclic rings with respect
to the molecular plane appears to locate the NꢀH vector in such
a way that the intramolecular hydrogen bonding is maximized
(see below).
respectively. These data are consistent with the presence of the
iminic tautomer form of 1,2-dihydroindazol-3-one, namely, 1H-
indazol-3-ol (K, Scheme 4). This is probably promoted by the
coordination to ruthenium via the formation of intramolecular
Cl HꢀO and Cl HꢀN bonds.
3 3 3
3 3 3
Theoretical calculations of the stabilization energy of tauto-
meric forms of complex 4 have been analyzed by B3LYP DFT
studies. Optimized geometries of the complexes are depicted in
the Supporting Information. They have been obtained by using
bond lengths and angles from the X-ray data of analogous
complexes herein described (see below). Frequency calculations
for the optimized geometries have been performed and are
consistent with minima on the potential energy surface. It is
found that the proposed coordination of tautomer K stabilizes
the resulting complex 4 in 12.7 kcal molꢀ1 with respect to that of
1,2-dihydroindazol-3-one (J).
All attempts to obtain suitable crystals of 4 for X-ray diffraction
studies have been unsuccessful. Nevertheless, the optimized
geometries display the expected NꢀH/O-H Cl hydrogen
3 3 3
bonding (see the Supporting Information).
Synthesis of [Ru(η3:η3-C10H16)Cl(j2-N,O-2-(1H-pyrazol-3-
yl)phenoxy)] (5). The preparation of complex 2e containing the
N-coordinated ligand 2-(pyrazol-5-yl)phenol raises the question
of whether the expected deprotonation reaction of the hydroxyl
group could lead to a chelate complex (Scheme 2). Thus, the
treatment of 1 with an equivalent amount of 2-(1H-pyrazol-3-
yl)phenol in the presence of NaOH in dichloromethane leads to
the formation of complex 5, which is isolated after chromatographic
purification as an air-stable orange solid (41%). Alternatively, the
treatment of 2e with an excess of NaOH in dichloromethane also
leads to the formation of complex 5 (35%; Scheme 2).
The structure of complex 5 is shown in Figure 7 (selected
bonding parameters appear in the caption). The structure can
also be described as a distorted trigonal-bipyramidal geometry in
which the centroids C*1 and C*2 and N1 occupy the equatorial
sites and Cl1 and O1 the axial sites. Structural relevant para-
meters are related to the presence of the chelate six-membered
ring, which displays internal angles and bond distances with no
appreciable changes compared to its precursor 2e. The RuꢀO1
[2.0852(13) Å] andRuꢀCl1 [2.4139(5) Å] bond lengths compare
Compound 5 is soluble in chlorinated solvents and acetone
and insoluble in water, hexane, and diethyl ether. Elemental
1
analysis and spectroscopic methods (IR and H and 13C{1H}
NMR) of complex 5 are in accordance with the proposed formu-
lation and with the formation of the chelate ring. This is eviden-
ced in the IR (KBr) spectrum by the disappearance of the ν(OH)
absorption and in the 13C{1H} NMR spectrum, which shows 10
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Table 2. Main Bond Lengths (Å), Angles (deg), and Dihedral Angle r (deg) between the RuꢀCl and NꢀN Bonds in Complexes
2aꢀ2c, 2e, and 3
Cl1ꢀRuꢀ
RuꢀC*1a
RuꢀC*2a
RuꢀCl1
RuꢀCl2
RuꢀN1
Cl1ꢀRuꢀCl2
Cl1ꢀRuꢀN1
Cl2ꢀRuꢀN1
N1ꢀN2 (R)b
2ac
2.004(9),
2.001(9)
2.00(2)
2.005(9)
2.4092(17),
2.4193(17)
2.4323(11)
2.4337(6)
2.4193(1)
2.172(7),
2.181(9)
2.142(6)
2.193(3)
2.159(5)
2.231(4)
170.31(8),
170.96(11)
171.11(6)
173.15(3)
171.57(6)
172.45(5)
85.19(17),
85.48(5)
85.12(17)
21.3(6),
1.4(12)
39.9(5)
36.7(2)
24.3(3)
11.5(3)
2b
2c
2e
3
2.011(6)
2.4060(10)
2.4109(7)
2.4243(14)
2.4066(14)
85.19(12)
84.89(6)
85.94(12)
88.54(6)
2.0145(2)
2.0113(4)
1.9959(4)
2.0126(2)
2.0098(4)
1.9960(4)
2.4196(15)
85.21(13)
86.42(13)
2.3986(14)
85.48(12)
86.98(12)
a C*1 = centroid of the allylic groups C1, C2, and C4; C*2 = centroid of the allylic groups C7, C8, and C10. b Dihedral angle R between the “RuClN” and
ligand planes. c Two independent molecules in the unit cell.
Figure 4. ORTEP-type view of the structure of complex 2c, showing the
Figure 2. ORTEP-type view of the structure of complex 2aI, showing
crystallographic labeling scheme and the NꢀH Cl intramolecular
3 3 3
the crystallographic labeling scheme and NꢀH Cl intramolecular
hydrogen bond. Thermal ellipsoids are drawn at the 10% probability level.
3 3 3
hydrogen bond. Thermal ellipsoids are drawn at the 10% probability level.
Figure 5. ORTEP-type view of the structure of complex 2e, showing
the crystallographic labeling scheme and NꢀH Cl and NꢀH
O
3 3 3
3 3 3
intramolecular hydrogen bonds. Thermal ellipsoids are drawn at the
10% probability level. Selected bond distances (Å) and angles (deg):
N1ꢀC13 = 1.271(7), C13ꢀC14 = 1.500(8), C14ꢀC19 = 1.416(3),
C19ꢀO1 = 1.318(6); RuꢀN1ꢀC13 = 116.8(3), N1ꢀC13ꢀC14 =
117.0(5), C13ꢀC14ꢀC19 = 129.7(5), C14ꢀC19ꢀO1 = 117.9(5).
Figure 3. ORTEP-type view of the structure of complex 2b, showing
the crystallographic labeling scheme and the NꢀH Cl intramolecular
3 3 3
hydrogen bond. Thermal ellipsoids are drawn at the 10% probability level.
the axial position.15a,38 Distances between the NꢀH hydrogen
and the chloride are in the range 2.27(5)ꢀ2.78 Å and angles
N2ꢀHꢀCl1 lie in the range 102ꢀ138° (see Table 3). Despite
the fact that angles are rather small compared to those in optimal
hydrogen bonding (closer to 180°),39 these values along with the
observed dihedral angle R (Table 2) are consistent with the
existence of intramolecular hydrogen bonds.
well to those shown by the other ruthenium(IV) complexes
herein described and other complexes containing RuꢀO
alkoxides.37 The remaining structural parameters related to the
expected η3:η3-coordination mode of the diallyl group C10H16
are similar to those of the complexes described above.
Hydrogen Bonding. All molecular structures examined show
that the ligand NꢀH hydrogen (located on an equatorial site) is
involved in a hydrogen bond with one of the chloride ligands in
The unit cell packing diagrams of complexes 2aꢀ2c, 2e, and 3
shown in Figures 8ꢀ12 are particularly illustrative of the ability of
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ARTICLE
The packing diagram of the uracil complex 3 (Figure 12) is
exceptional because it shows a pairing of molecular units via two
intermolecular NꢀH OdC interactions. The distance and
3 3 3
angle of the NꢀH O contact of 2.00 Å and 162°, respectively,
3 3 3
allow it to be classified as “moderate” among those considered
most common in chemical systems.39
NMR Diffusion Studies. In order to investigate the existence
in solution of the intra- and intermolecular hydrogen bonds
detected in the analysis of the X-ray structures, we have carried
out a thorough NMR study for complex 3 as a model, including
structure elucidation, variable-temperature measurements, and
DOSY experiments.
1
The integrals of the room temperature H NMR spectrum
Figure 6. ORTEP-type view of the structure of complex 3, showing the
(CD2Cl2) of complex 3 account for 1:1 octadienediyl/6-azauracil
fragments, showing single resonances for all chemically equiva-
lent protons, as is also observed in the 13C NMR spectrum
(CD2Cl2). This feature indicates that the 6-azauracil ligand has
been incorporated into the equatorial position of the ruthenium
center and a free rotation about the RuꢀN bond, thus achieving
equivalent environments for the halves of the 2,7-dimethylocta-
dienediyl group. The proton signals for 6-azauracil appear as
three broad singlets at 11.74, 9.10, and 8.65 ppm. The HSQC
spectrum identifies the iminic proton at 8.65 ppm and sets two
more deshielded signals for the NH protons. Because their high
chemical shifts may be indicative of the participation of these NH
protons in some type of hydrogen bonding, we have carried out
several studies by varying the temperature, solvent, and concen-
tration in order to elucidate the bonding nature of these
exchangeable protons.
crystallographic labeling scheme and the NꢀH Cl intramolecular
3 3 3
hydrogen bond. Thermal ellipsoids are drawn at the 10% probability level.
First, we prepared several NMR samples with different con-
centrations (5, 12, 18, and 24 mM in CD2Cl2). Their 1H NMR
spectrum showed that the signal at 11.74 ppm remains unaffected
in all cases, while the signal at 9.10 ppm is shifted to lower fields
with increasing concentration. This concentration dependence
clearly points out to intermolecular hydrogen bonding for the
signal at 9.10 ppm and an intramolecular nature for the proton at
11.74 ppm. This assumption is also supported by the NOESY
spectrum of complex 3, which shows an exchange cross peak with
residual water for the NH proton at 9.10 ppm, which is not
detectable for the signal at 11.74 ppm. The unambiguous assign-
ment of the signal at 9.10 ppm for the NH-4 proton and the signal
at 11.74 ppm for the NH-2 through 2D NMR experiments at low
temperature confirms the existence of an intramolecular hydro-
gen bond between NH-2 and a Cl atom of the coordination
sphere of the metal.40
Figure 7. ORTEP-type view of the structure of complex 5, showing the
crystallographic labeling scheme and the NꢀH Cl intramolecular
3 3 3
hydrogen bond. Thermal ellipsoids are drawn at the 10% probability
level. Selected bond distances (Å) and angles (deg): Ru1ꢀC*1 =
1.99519(14), Ru1ꢀC*2 = 1.99289(14), Ru1ꢀCl1 = 2.4139(5),
Ru1ꢀO1 = 2.0852(13), Ru1ꢀN1 = 2.0982(16), N2ꢀH(N2) =
0.94(4); C*1ꢀRu1ꢀC*2 = 132.761(7), C*1ꢀRu1ꢀN1 = 114.26(5),
C*2ꢀRu1ꢀN1 = 112.87(5), C*1ꢀRu1ꢀO1 = 94.54(4), C*2ꢀRu1ꢀ
O1 = 85.65(4), N1ꢀRu1ꢀO1 = 86.03(6), C*1ꢀRu1ꢀCl1 = 95.381(16),
C*2ꢀRu1ꢀCl1 = 90.114(16), N1ꢀRu1ꢀCl1 = 86.86(5), O1ꢀRu1ꢀ
Cl1 = 172.64(4); N1ꢀC13 = 1.332(3), C13ꢀC14 = 1.458(3), C14ꢀC19 =
1.519(8), C19ꢀO1 = 1.322(2); RuꢀN1ꢀC13 = 123.31(15), N1ꢀC13ꢀ
C14 = 122.64(12), C13ꢀC14ꢀC19 = 123.25(19), C14ꢀC19ꢀO1 =
125.55(19), C19ꢀO1ꢀRu = 124.82(12), O1ꢀRuꢀN1 = 83.03(06)
(C*1 = centroid of the allylic groups C1, C2, and C4; C*2 = centroid of
the allylic groups C7, C8, and C10).
In a next step, we have studied the influence of the tempera-
1
ture. The H NMR spectrum of complex 3 (CD2Cl2) was
recorded for a range of temperatures from 298 to 183 K. The
analysis of the spectra showed that the signal for NH-4 (9.10
ppm) shifts to lower fields with decreasing temperature, while the
chemical shift for the NH-2 proton (11.74 ppm) remains unaffec-
ted (Figure 13). This behavior of the NH resonances is in
agreement with their respective exchangeable and nonexchange-
able character already detected in the variable concentration
study. Besides, there are other noticeable changes taking place
when the temperature is decreased: the allylic signals of the 2,7-
dimethyloctadienediyl group slowly broaden at 213 K and they
split into two sets of signals; on the other hand, the broad signals
for the iminic and NH-2 protons at 8.65 and 11.74 ppm become
sharp singlets. The compound was characterized at 183 K. At that
temperature, the 13C NMR spectrum also shows splitting for
some signals of the octadienediyl fragment: the aliphatic CH2,
these complexes to participate also in intermolecular contacts.
The distances between coordinated heterocyclic NꢀH and the
neighboring chloride ligands are somewhat similar to the in-
tramolecular ones [2.00ꢀ2.77 Å vs 2.27(5)ꢀ2.78 Å; Table 3]. In
contrast, values of the angles NꢀH Cl are greater (132ꢀ
3 3 3
198.7° vs 102ꢀ138°), supporting the existence of intermolecular
contacts. These interactions may play a significant role in the
bifunctional activation of substrates.
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Figure 8. Inter- and intramolecular hydrogen bonds observed in the packing structure of complex 2aI. All H atoms are excluded except those of the
pyrazole ring.
Figure 11. Inter- and intramolecular hydrogen bonds observed in the
packing structure of complex 2e. All H atoms are excluded except those
of the NꢀH and OꢀH groups.
Figure 9. Inter- and intramolecular hydrogen bonds observed in the
packing structure of complex 2b. All H atoms are excluded except those
of the pyrazole ring.
Figure 12. Inter- and intramolecular hydrogen bonds observed in the
packing structure of complex 3. All H atoms are excluded except those of
the NꢀH groups.
The above-discussed NMR experiments prove the existence of
an intramolecular hydrogen bond upon solution of complex 3 in
a poorly coordinating solvent such as CH2Cl2. However, because
the intermolecular hydrogen bonds could just as well involve
dimeric structures arising from hydrogen bonds between the uracil
groups of two molecules, we carried out several DOSY experiments
Figure 10. Inter- and intramolecular hydrogen bonds observed in the
packing structure of complex 2c. All H atoms are excluded except those
of the pyrazole ring.
to evaluate the size of the molecules present in solution.
DOSY41 is an extremely useful NMR technique capable of
the internal allylic CH, and the methyl groups. This new situation
indicates that at 183 K complex 3 is in the slow exchange zone,
with restricted rotation around the RuꢀN bond forced by the
intramolecular hydrogen bond. Complete assignment of the 1H
and 13C NMR spectra was achieved after inspection of the
HSQC, HMBC, and the ROESY spectra at 183 K: the ROESY
and HMBC cross peaks of the NH-2 and iminic protons led to
the unambiguous assignment of the two amino protons.
estimating the size of a molecule in solution from its diffusion
coefficient. It has been successfully applied in the identification
and study of hydrogen-bond interactions42 and chemical exchange.43
The measured diffusion coefficient of the molecule in the NMR
sample is a translational property, which the StokesꢀEinstein
equation44 relates to the hydrodynamic radius of the molecule in
solution. If the measurement is carried out in the presence of a
reference compound, then an inverse relationship between the
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Table 3. Hydrogen-Bondinga Contactsb in Complexes 2aꢀ2c, 2e, 3, and 5 (Å and deg)
intramolecular
intermolecular
DꢀH
A
H
A distance
DꢀH A angle
DꢀH
A
H
A distance
DꢀH A angle
3 3 3
3 3 3
2.56
3 3 3
3 3 3
3 3 3
3 3 3
2aI
2aII
2b
N2ꢀH Cl2
118
N2ꢀH Cl1i
2.59
132
3 3 3
3 3 3
N4ꢀH Cl3
2.67
102
3 3 3
N2ꢀH Cl1
2.78
114
N2ꢀH Cl1i
2.71
152
3 3 3
3 3 3
2c
N2ꢀH Cl1
2.55(4)
2.43(5)
2.27(5)
2.30
123(3)
122(5)
138(5)
137
N2ꢀH Cl1i
2.77(4)
2.45(5)
198.7(12)
174(5)
3 3 3
3 3 3
2e
N2ꢀH Cl1
O1ꢀH Cl2i
3 3 3
3 3 3
N2ꢀH O1
3 3 3
3
5
N2ꢀH Cl2
N3ꢀH O1i
2.00
162
3 3 3
3 3 3
N2ꢀH Cl1
2.42
124
3 3 3
a A hydrogen bond exists between DꢀH and an atom A if (1) it constitutes a local bond and (2) XꢀH acts as proton donor to A.39 b Weak hydrogen
bonding of the type CꢀH Cl is also detected but is not included in the table.
3 3 3
ratio of the diffusion coefficients and the hydrodynamic radii
(Da/Db = rHb/rHa) can be established. Considering that the two
molecules have a spherical shape, this relationship can be
extended to a molecular weight ratio45 between the two mol-
ecules: (Da/Db = MWb/MWa)1/3
.
The known complex [Ru(η3:η3-C10H16)Cl2(pyridine)]46 was
chosen as a reference compound for a monomer complex
because it is unable to form intermolecular hydrogen bonds.
The comparison was carried out between DOSY experiments
from two different but equally prepared NMR samples of
complex 3 and the ruthenium pyridine complex, respectively,
to avoid interactions between the two compounds in a mixed
sample. The measured diffusion coefficient for complex 3 (D =
1.17 ꢁ 10ꢀ9 m2 sꢀ1; log D = ꢀ8.93) is smaller that measured for
the pyridine complex (D = 1.44 ꢁ 10ꢀ9 m2 sꢀ1; log D = ꢀ8.84),47
denoting a larger molecule in solution than the pyridine complex.
The expected ratio between the diffusion coefficients of two
spherical molecules, one having twice the volume of the other, is
1.26: (MWb/MWa)1/3 = (2)1/3 = 1.26 = Da/Db. In this case, the
ratio between the diffusion coefficients of the complex 3 and that
of the pyridine complex (D[Ru]ꢀPy/D3 = 1.23) estimates for
complex 3 an intermediate molecular weight between the
monomer and that expected for a dimer complex formed by
the hydrogen-bond union of two monomers. This result ac-
counts for an equilibrium situation in solution between mono-
mer and dimer species, so an average diffusion coefficient pon-
dered by the extension of this dynamic process is measured.
Additional DOSY experiments were carried out in order to
gain more insight into this proposed intermolecular equilibrium.
1
Figure 13. Details of the variable-temperature H NMR experiments
for complex 3 (CD2Cl2).
(D = 1.29ꢀ1.58; log D = ꢀ8.89 to ꢀ8.80) with increasing
diffusion times.
The diffusing behavior of the NH-4 proton represents the
exchange of this labile proton with residual water in the deuter-
ated solvent, as has been previously reported for similar uracile
free molecules.48 On the other hand, poorly resolved signals in
the indirect dimension for the allylic terminal protons may be
indicative of the presence of a dynamic process not detected in
the time scale of these DOSY experiments.
An additional proof for the presence of intermolecular hydro-
gen bonds was obtained when the DOSY experiments were
performed on more diluted NMR samples of complex 3 (18, 12,
and 5 mM) with the same diffusion time (D20 = 120 ms). The
diffusion coefficient displayed by all of the protons in the molecule
increased with dilution: D = 1.17ꢀ1.35 (log D = ꢀ8.93 to ꢀ8.87)
for the CH protons and D = 1.32ꢀ1.58 (log D = ꢀ8.88 to ꢀ8.80)
for the NH-4 protons. In Figure 14, expansions of the DOSY
spectra of the 24 and 5 mM samples are compared. This result is in
good agreement with a larger presence of monomer species in
diluted solutions.
1
Thus, several H DOSY experiments were performed on a
24 mM sample of complex 3 in CD2Cl2 by varying the diffusion
time (50, 100, 200, 300, and 500 ms) of each measurement to
detect any dynamic process or exchange phenomenon occurring
in the molecule.43b The DOSY plots showed some differences
between the diffusion coefficients exhibited by the different
protons in the molecule (see Table 4). The CH resonances
afforded a diffusion coefficient D = 1.17 ꢁ 10ꢀ9 m2 sꢀ1 (log D =
ꢀ8.93) in all of the experiments with different diffusion times;
the diffusion coefficient for the NH-2 proton at 11.74 ppm was
slightly displaced, D = 1.20 ꢁ 10ꢀ9 m2 sꢀ1 (log D = ꢀ8.92), but
also independent of the diffusion time; however, the diffusion
coefficient displayed by the NH-4 proton at 9.10 ppm was a
bit larger than the coefficient of the other protons in all of
the measurements, and on the other hand, its value became larger
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)
Table 4. Diffusion Coefficient D (ꢁ10ꢀ9 m2 sꢀ1 a of Complex 3
diffusion time (D20, ms) in a 24 mM NMR sample
concentration of the NMR sample (mM) for a diffusion time D20 = 120 ms
50
100
200
300
500
24
18
12
5
CH
1.17
1.20
1.29
1.17
1.20
1.32
1.17
1.20
1.38
1.17
1.20
1.44
1.17
1.20
1.58
1.17
1.20
1.32
1.29
1.38
1.41
1.32
1.38
1.48
1.35
1.38
1.58
NH-2
NH-4
a The DOSY experiments were acquired with the bipolar longitudinal eddy current delay pulse program with spinning of the sample to avoid convection
influence. See the Experimental Section for details.
Figure 14. (a) Details of the DOSY spectrum of a 24 mM NMR sample of complex 3 in CD2Cl2. (b) Details of the DOSY spectrum of a 5 mM NMR
sample of complex 3 in CD2Cl2.
Figure 15. (a) Details of the DOSY spectrum of complex 3 in CD2Cl2 without DMSO. (b) Details of the DOSY spectrum of complex 3 in CD2Cl2
with DMSO.
Taking advantage of the known tendency of dimethyl sulf-
oxide (DMSO) to break up hydrogen bonds,42a a limit situation
in which only monomer molecules would be present was forced
by adding a drop of DMSO-d6 to an NMR sample of complex 3 in
CD2Cl2. As depicted in Figure 15, the new measured diffusion
coefficient was larger that obtained without DMSO. The CH and
NH-2 protons afforded a diffusion coefficient D = 1.41 ꢁ 10ꢀ9
m2 sꢀ1 (log D = ꢀ8.85), whereas the coefficient exhibited by
the NH-4 proton was a bit larger, D = 1.51 ꢁ 10ꢀ9 m2 sꢀ1 (log
D = ꢀ8.82). When compared with the diffusion coefficient of the
Also, in the new 1H NMR spectrum at 298 K, it is noticeable
that the chemical shift for the NH-4 proton has been shifted from
9.10 to 11.23 ppm, indicating that this NH proton is now
solvated by DMSO molecules, preventing its participation in
intermolecular hydrogen bonds, while the signal for the NH-2
proton remains at 11.75 ppm. However, when variable-tempera-
ture 1H NMR experiments were recorded, it was observed that
both signals shifted to lower fields when the temperature was
lowered, showing the typical behavior of labile protons and
therefore providing proof that the addition of DMSO-d6 has
also broken the intramolecular hydrogen bond between the NH-
2 proton and the chlorine ligands. Accordingly, from 298 to 183
K, no splitting of the signals is found, indicating equivalent
environments for the halves of 2,7-dimethyloctadienediyl in the
reference rutheniumꢀpyridine complex (D[Ru]ꢀPy/D3-DMSO
=
1.44/1.41 = 1.023), the new diffusion coefficient measured in the
presence of DMSO would fit the expected value for a monomeric
species of complex 3.
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Table 5. Complex 3 Catalyzed Isomerization of 1-Octen-3-ol
into Octan-3-one a
ability of complex 3 to act as a bifunctional catalyst, favoring
interaction with the allylic alcohol.
’ CONCLUSIONS
Here we describe novel ruthenium(IV) complexes bearing
two-electron N-heterocyclic ligands such as pyrazole derivatives
(2bꢀ2e), 6-azauracil (3), and 1H-indazol-3-ol (4). The presence
of uncoordinated CdO, NꢀH, and OꢀH groups in these
ligands prone to form hydrogen bonds provides the appearance
in the new complexes of intra- and intermolecular hydrogen
interactions. Several ruthenium(IV) complexes of this type have
been characterized by X-ray diffraction in the solid state, showing
the nature of hydrogen bonding of the following type: (a)
intramolecular ones between pyrazole or indazole NꢀH and
chloride ligands; (b) intermolecular ones through NꢀH or
OꢀH and chloride ligands featuring large molecular arrays.
The packing diagram of the 2-(1H-pyrazol-3-yl)phenol complex
2e is significant in the series (Figure 11) because shows the ability
of the pyrazole NꢀH and chloride ligands to participate in
hydrogen bonding with the OꢀH not only intramolecular but
also intermolecular with the OꢀH of a neighboring molecule in
the crystal. This pattern of hydrogen interaction may model
those featured by a pyrazole bifunctional catalyst either with
hydroxylic solvents including water or for the activation of
analogous protic substrates.
T
yield TOF
(%)b (hꢀ1
)
entry solvent (°C) mol % 3 mol % KOtBu time
c
1
2
3
4
THF
THF
H2O
H2O
75
75
75
35
0.2
0.2
0.2
1
24 h
99
21
743
2970
33
0.4
40 min 99
10 min 99
3 h
99
a Reactions performed under a nitrogen atmosphere using 1 mmol of
1-octen-3-ol (0.2 M). b Yield of octan-3-one determined by gas chro-
matography. c Turnover frequencies [(mol of product/mol of
ruthenium)/time] were calculated at the time indicated in each case.
range of temperatures and denoting a free rotation about the
RuꢀN bond.
In conclusion, NMR characterization and diffusion studies
have proven the existence of intramolecular hydrogen bonds for
complex 3 in a poorly coordinating solvent (CD2Cl2). The ability
of the uracil moiety to establish also intermolecular hydrogen
bonds leads to an equilibrium between monomer and dimer
species in solution whose extension depends on the temperature,
concentration, and coordinating properties of the solvent, and
therefore the equilibrium situation can be modulated by varying
the reaction conditions.
The existence of intermolecular hydrogen bonds has also been
investigated in solution by means of NMR diffusion studies in the
uracil complex 3. Complex 3 is a dimer in the solid state based on
relatively short intermolecular NꢀH O hydrogen bonds
Catalytic Studies. Because it is presently well documented
that inter- and intramolecular hydrogen bonding are playing a
role in the activity of bifunctional catalysts,6,7 we believed it of
interest to check whether the existence of these types of inter-
actions in the above-described complexes would affect their
catalytic activities. Following our previous catalytic studies on
the isomerization of allylic alcohols catalyzed by ruthenium(IV)
complexes, isomerization of the allylic alcohol 1-octen-3-ol into
octan-3-one was tested using complex 3 as a reference case
(Table 5). In a model reaction, when a 0.2 M tetrahydrofuran
(THF) solution of 1-octen-3-ol was refluxed for 24 h with a
catalytic amount of 3 (0.2 mol %), a quantitative yield of octan-3-
one was obtained (entry 1). Although complex 3 is active in the
absence of a base as the cocatalyst, a higher activity is achieved in
the presence of 0.4 mol % KOtBu, reaching a quantitative
conversion in ca. 40 min (TOF = 743 hꢀ1, entry 2). This
turnover (TOF) value can be compared to those shown
by analogous mononuclear ruthenium(IV) catalysts, i.e., [Ru(η3:
η3-C10H16)Cl2(PiPr3)] (TOF = 750 hꢀ1) and [Ru(η3:η3-
C10H16)Cl2(NH2Ph)] (TOF = 500 hꢀ1).13d Taking advantage
of the solubility of complex 3 in water, we decided to perform the
catalytic study in an aqueous medium. Remarkably, the reaction
proceeds at a higher rate than those observed for THF even in the
absence of a base, reaching a quantitative conversion into octan-3-
one in only 10 min versus 24 h in THF. It is also worth mentioning
that the exceptional catalytic activity of complex 3 is maintained at
room temperature (entry 4; 99%; 3 h), a catalytic performance
reported for only a limited number of metal catalysts in water.49
Although no mechanistic studies have been carried out, this
enhanced catalytic activity is probably related with both the
presence of hydrogen bonding arising from both the NꢀH and
CdO groups of the uracile ligand and the formation of the aqua
complexes as intermediate active species. These facts enable the
3 3 3
(2.00 Å). DOSY experiments show that the ability of the NꢀH
and CdO groups to form intermolecular hydrogen bonds leads
to an equilibrium between monomer and dimer species in
solution whose extension depends on the temperature, concen-
tration, and coordinating properties of the solvent. Finally, it is
worth mentioning that preliminary studies have shown that
complex 3 containing the uracil ligand able to form intra- and
intermolecular hydrogen bonding is a highly efficient catalyst for
redox isomerization of 1-octen-3-ol into octan-3-one, a catalytic
activity that is among the most efficient reported to date. Further
experimental and theoretical work on the catalytic activity of
these types of complexes for the redox isomerization of allylic
alcohols in water focused on exploring the influence of hydrogen
bonding will be published elsewhere.
’ ASSOCIATED CONTENT
S
Supporting Information. Cartesian coordinates and total
b
electronic energies for the computed species J and K and an X-ray
crystallographic file in CIF format. This material is available free
of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: jgh@uniovi.es (J.G.), erubio@uniovi.es (E.R.).
’ ACKNOWLEDGMENT
Financial support from the Spanish MICINN (Projects
CTQ2006-08485/BQU, CTQ2010-14796/BQU, Consolider
Ingenio CSD2007-00006, and CTQ2007-61048) and Gobierno
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ARTICLE
ꢀ
del Principado de Asturias (FICYT Project IB08-036) is
acknowledged.
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