Dinuclear ruthenium complexes containing the Hpbl ligand: Synthesis, characterization, linkage isomerism, and epoxidation catalysis

Article abstract of DOI:10.1021/ic501483s

Three dinucleating Ru-Cl complexes containing the hexadentate dinucleating ligand [1,1′-(4-methyl-1H-pyrazole-3,5-diyl)bis(1-(pyridin-2-yl)ethanol)] (Hpbl) and the meridional 2,2′:6′,2″-terpyridine ligand (trpy) have been prepared and isolated. These comp

Full text of DOI:10.1021/ic501483s

Article
pubs.acs.org/IC
Dinuclear Ruthenium Complexes Containing the Hpbl Ligand:
Synthesis, Characterization, Linkage Isomerism, and Epoxidation
Catalysis
Laia Francas,†,‡ Rosa María Gonzalez-Gil,‡ Daniel Moyano,‡ Jordi Benet-Buchholz,† Jordi García-Anton,‡
̀
́
́
Lluís Escriche,*,‡ Antoni Llobet,*,†,‡ and Xavier Sala*,‡
†Institute of Chemical Research of Catalonia (ICIQ), Av. Països Catalans 16, E-43007 Tarragona, Spain
‡Departament de Química, Universitat Auton
oma de Barcelona, Cerdanyola del Valles, 08193 Barcelona, Spain
̀ ̀
S
* Supporting Information
ABSTRACT: Three dinucleating Ru−Cl complexes containing the hexadentate
dinucleating ligand [1,1′-(4-methyl-1H-pyrazole-3,5-diyl)bis(1-(pyridin-2-yl)ethanol)]
(Hpbl) and the meridional 2,2′:6′,2″-terpyridine ligand (trpy) have been prepared and
isolated. These complexes include {[RuCl(trpy)]2(μ-pbl-κ-N3O)}+ (1a+), {[RuCl-
(trpy)]2(μ-Hpbl-κ-N3O)}2+ (1b2+), and {[RuCl(trpy)]2(μ-Hpbl-κ-N2O2)}2+ (1c2+) and
were characterized by analytic and spectroscopic techniques. In addition, complexes 1b2+
and 1c2+ were characterized in the solid state by monocrystal X-ray diffraction analysis.
The coordination versatility of the Hpbl ligand allows the presence of multiple isomers
that can be obtained depending on the Ru oxidation state and were thoroughly
characterized by electrochemical techniques, namely, cyclic voltammetry and coulometry.
Finally, 1a+ and its recently reported mononuclear analogue, in-[RuCl(Hpbl)(trpy)]+,
have been tested as catalysts for epoxidation of cis-β-methylstyrene.
Chart 1. Drawing of the Ligands Used in This Work (right)
and Dinucleating Bridging Ligands Previously Employed To
Induce Metal Cooperation (left)
INTRODUCTION
■
Redox catalysts play a prominent role in the selective
preparation of versatile intermediates for organic synthesis, as
is the case, for instance, in the regio- and/or stereoselective
transformation of double bonds into epoxides.1 The epoxides
are extremely useful molecules for both the chemical industry
(particularly in polymer manufacture)2 and the synthesis of fine
chemicals, such as pharmaceuticals, food additives, or flavor and
fragrances.3 On the other hand, water oxidation redox catalysis
is one of the major topics that need to be dominated in order to
come up with new energy conversion schemes that allow the
transition from fossil to solar fuels.4
Mono- and dinuclear Ru complexes based on RuII−OH2/
RuIVO motifs, achievable via proton-coupled electron
transfer (PCET) processes,5 have been thoroughly studied as
redox catalysts. A number of oxidative transformations
including the ones mentioned above have been described. We
recently contributed to this chemistry6 focusing our attention at
the influence of metal cooperation over catalysis.7 For this
purpose, several dinucleating ligands (Chart 1) have been used
as bridges between two Ru metal centers, and the catalytic
properties of the resulting complexes have been studied for
reactions of water oxidation and alkene epoxidation.8
A key issue for this type of chemistry is the need to cycle
among different oxidation states, such as, for instance, Ru(II)
and Ru(V), while keeping a stable coordination environment.9
In order to come up with ligands that can stabilize low and high
oxidation states we recently reported a new dinucleating
bridging ligand with N/O ambidentate motifs (Hpbl, Chart 1)
and its mononuclear Ru complexes. We have shown that when
oxidation state III is reached it undergoes N → O linkage
Received: June 20, 2014
Published: September 25, 2014
© 2014 American Chemical Society
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off, washed with water and diethyl ether, and vacuum dried. Yield: 75
mg (27%) for the reflux reaction; 100 mg (36%) for the microwave
reaction. ESI-HRMS (MeOH): m/z = 1205.0500 ([M]+). CV
(MeOH/TBAH) E1/2 = 0.07 V; E1/2 = 0.77 V. OCP: 0.1 V. Anal.
Calcd for C48H40Cl2F6N10O2PRu2: C, 47.77; H, 3.34; N, 11.61. Found:
C, 47.52; H, 3.45; N, 11.37. UV−vis (MeOH) [λmax, nm (ε, M−1
cm−1)]: 276 (26 822), 283 (25 083), 320 (28 868), 379 (8637), 516
(5148), 585 (5956).
isomerization, drastically reducing redox potentials of high
oxidation states.10
Herein, we report the coordination chemistry and linkage
isomerization processes of dinuclear Ru−Hpbl complexes of
general formula {[RuCl(trpy)]2(μ-pbl)}n+, together with their
catalytic properties with regard to epoxidation of cis-β-
methylstyrene. In addition, the epoxidation capacity of their
mononuclear Ru−Hpbl counterparts is also reported and
compared to related complexes previously reported in the
literature (Chart 1).
{[RuCl(trpy)]2(μ-pbl-κ-N3O)}+, 1aH+. This complex was obtained in
situ by adding ascorbic acid to a MeOD solution of 1a+. 1H NMR (500
3
MHz, MeOD + ascorbic acid): δ = 10.01 (d, 1H1, J1−2 = 5.62 Hz),
8.60−8.40 (m, 8Htrpy), 8.26 (d, 1Htrpy), 8.17 (t, 1Htrpy), 8.08 (t,
3
2Htrpy), 8.00−7.88 (m, 7Htrpy,2,8), 7.80 (t, 1H), 7.74 (d, 1H4, J4−3
=
EXPERIMENTAL SECTION
5.50 Hz), 7.66 (d, 1Htrpy), 7.56 (t, 1Htrpy), 7.50 (ddd, 1H9, 3J9−8 = 8.50
■
4
5
Hz, J9−10 = 7.30 Hz, J9−11 = 1.44 Hz), 7.40 (t, 1Htrpy), 7.27 (ddd,
1H3, 3J3−2 = 7.70 Hz, 4J3−4 = 5.50 Hz, 5J3−1 = 1.10 Hz), 7.25 (d, 1H11,
3J11−10 = 5.80 Hz), 6.61 (ddd, 1H10, 3J10−9 = 7.30 Hz, 4J10−11 = 5.80 Hz,
5J10−8 = 1.50 Hz), 2.14 (s, 3H6), 1.67 (s, 3H5), 1.44 (s, 3H7). UV−vis
(MeOH) [λmax, nm (ε, M−1 cm−1)]: 280 (26 692), 282 (27 401), 320
(33 910), 379 (9798), 489 (6321), 527 (6166).
Materials. All reagents used in the present work were obtained
from Sigma-Aldrich Chemical Co. and used without further
purification. Reagent-grade organic solvents were obtained from
SDS. RuCl3·3H2O was supplied by Alfa Aesar and used as received.
Instrumentation and Measurements. UV−vis spectroscopy was
performed by a HP8453 spectrometer using 1 cm quartz cells. NMR
spectroscopy was performed on a Bruker DPX 250 MHz, DPX 360
MHz, or a DPX 400 MHz spectrometer. Gas chromatography (GC)
was performed in an Agilent 6890N with a mass-selective detector
with ionization by electronic impact or in an Agilent 6890 GC with a
flame ionization detector (FID) detector using a HP5 column.
Electrospray ionization mass spectrometry (ESI-MS) experiments
were carried out on a microTOF-Q system from Bruker Daltonics at
{[RuCl(trpy)]2(μ-Hpbl-κ-N3O)}(PF6)2, 1b(PF6)2. A sample of 50 mg
(0.04 mmol) of 1a(PF6) was dissolved in 50 mL of MeOH. The
mixture was left stirring at room temperature for 1 week. Then,
saturated aqueous NH4PF6 (0.2 mL) and water (0.5 mL) were added
to the solution, and the volume was reduced on a rotary evaporator.
The solution was cooled at −33 °C overnight. A brown precipitate
appeared, which was filtered off and washed with cold methanol and
dried with diethyl ether. Yield: 35 mg (65%). ESI-MS (MeOH): m/z =
531.5 ([M − 2PF6]2+). CV (MeOH/TBAH): E1/2 = 0.3 V; E1/2 = 0.77
V. OCP: 0.465 V. Anal. Calcd for C48H41Cl2F12N10O2P2Ru2: C, 42.61;
H, 3.05; N, 10.35. Found: C, 42.47; H, 3.04; N, 10.23. UV−vis
(MeOH) [λmax, nm (ε, M−1 cm−1)]: 275 (34 171), 281 (32 372), 318
(30 894), 377 (7216), 475 (5088), 513 (4062).
́
̀ ̀
isi Quimica of the Universitat Autonoma de
the Servei d’Anal
Barcelona (SAQ-UAB).
Cyclic voltammetry (CV) experiments were performed on PAR283
potentiostat, IJ-Cambria HI-660 potentiostat, or BioLogic potentio-
stat/galvanostat and EC-Lab software using a three-electrode cell. A
glassy carbon electrode (3 mm diameter) was used as working
electrode, platinum wire as auxiliary electrode, and SSCE as a reference
electrode. Working electrodes were polished with 0.05 μm alumina
paste washed with distilled water and acetone before each measure-
ment. E1/2 values reported in this work were estimated from CV
experiments as the average of the oxidative and reductive peak
potentials (Ep,a + Ep,c)/2. All electrochemical measurements are
performed in MeOH with 0.1 M n-Bu4N+PF6− (TBAH) as supporting
electrolyte.
{[RuCl(trpy)]2(μ-Hpbl-κ-N2O2)}(PF6)2·MeOH, 1c(PF6)2·MeOH. A
sample of 50 mg (0.040 mmol) of 1a(PF6) and 24.02 mg (0.044
mmol) of Ce(NO3)6(NH4)2 was dissolved in 50 mL of methanol. The
mixture was stirred at room temperature for 4 days under nitrogen
atmosphere. Then, the mixture was filtered, and the solvent was
evaporated and replaced by CH2Cl2. The insoluble portion was filtered
off, and the filtered solution was washed with 20 mL of water plus 20
mL of a saturated NaCl solution. The organic fraction was then dried
with anhydrous Na2SO4, filtered, and evaporated to dryness. The
residue was redissolved in methanol, and 0.2 mL of a saturated
aqueous solution of NH4PF6 was added together with 0.5 mL of water.
The volume was then reduced on a rotary evaporator until a
precipitate appeared, and the remaining solution was cooled at −33 °C
overnight. The brown solid formed was filtered off, washed with cold
methanol, and dried with diethyl ether and under vacuum. Yield: 11
mg (20%). CV (MeOH/TBAH) E1/2 = 0.30 V. OCP: 0.468 V. Anal.
Calcd for C49H44Cl2F12N10O3P2Ru2: C, 42.53; H, 3.20; N, 10.12.
Found: C, 42.47; H, 3.04; N, 10.23. UV−vis (MeOH) [λmax, nm (ε,
M−1 cm−1)]: 317 (24 000), 328 (21 284), 411 (5028), 444 (4616).
ESI-MS (MeOH): m/z = 531.0 ([M − 2PF6]2+).
X-ray Crystal Structure Determination. Crystals of both
complexes 1b(PF6)2 and 1c(PF6)2 were obtained by slow diffusion
of diethyl ether into an acetone solution containing complex 1a(PF6)
at room temperature and in the presence of atmospheric oxygen. The
measured crystals were prepared under inert conditions immersed in
perfluoropolyether as protecting oil for manipulation.
Data Collection. Crystal structure determinations for 1b(PF6)2 and
1c(PF6)2 were carried out using a Bruker-Nonius diffractometer
equipped with an APEX II 4K CCD area detector, a FR591 rotating
anode with Mo Kα radiation, Montel mirrors as monochromator, a
Kappa 4-axis goniometer, and an Oxford Cryosystems low-temper-
ature device Cryostream 700 plus (T = −173 °C). Full-sphere data
collection was used with ω and φ scans. Programs used: Data
collection with APEX-2,12 data reduction with Bruker Saint,13 and
absorption correction with SADABS.14
Epoxidation catalytic experiments were performed as follows. A
solution of 1.25 × 10−3 mmol of dinuclear catalyst (or 2.50 × 10−3
mmol of mononuclear), 1.60 g (5.0 mmol) of (diacetoxyiodo)benzene
(PhI(OAc)2), 1 mmol of biphenyl, and 90 μL (5.0 mmol) of water
were dissolved in 1 mL of 1,2-dichloroethane (DCE) and allowed to
stir for 2 h. Then 2.5 mmol of substrate were added, reaching a final
volume of approximately 1.4 mL. Aliquots were taken every 5, 10, 15,
20, 25, 30 min or until the reaction was completed. Each aliquot was
filtered through a Pasteur pipet filled with Celite and rinsed with
diethyl ether. The filtrate was analyzed by GC and GC-MS.
Preparation. The starting complex [RuCl3(trpy)]11 and the Hpbl
ligand [1,1′-(4-methyl-1H-pyrazole-3,5-diyl)bis(1-(pyridin-2-yl)-
ethanol)]10 were prepared as described in the literature. All synthetic
manipulations were routinely performed under nitrogen atmosphere
using Schlenck tubes and vacuum line techniques.
{[RuCl(trpy)]2(μ-pbl-κ-N3O)}(PF6), 1a(PF6). A sample of 200 mg
(0.454 mmol) of [RuCl3(trpy)] and 61 mg (1.439 mmol) of LiCl was
dissolved in 20 mL of dry MeOH containing 250 μL (1.816 mmol) of
NEt3. The mixture was stirred at room temperature (RT) for 20 min,
and then 74 mg (0.228 mmol) of Hpbl in 2.4 mL of a 0.29 M MeONa
solution (0.685 mmol) were added. The mixture was refluxed under
stirring for 24 h. Alternatively, the reaction can also be performed in a
microwave reactor by carrying out five 10 min heating cycles (75 °C/
300 W) with 5 min of equilibration time between them. The mixture
was filtered, and 2 mL of a saturated NH4PF6 aqueous solution and 5
mL of water were added to the filtrate. Then, the volume was reduced
on a rotary evaporator until a precipitate appeared, which was filtered
off and dried. The obtained solid was partially dissolved in hot CH2Cl2,
and the solution was cooled down. The remaining solid was filtered
Structure Solution and Refinement. Crystal structure solution was
achieved using direct methods as implemented in SHELXTL15 and
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a
Scheme 1. Synthetic Strategy for Preparation of the Complexes Described in This Work
a
The terpyridine ligand is represented with three N atoms linked by arcs. Color code for Ru: orange for oxidation state II, and fuchsia for oxidation
state III. This color code is used from now on in all schemes.
Scheme 2. Structure of All Possible Isomeric Species for Dinuclear RuII−Cl Complexes Bearing the Hpbl and trpy Ligands
visualized using the program XP. Missing atoms were subsequently
RESULTS AND DISCUSSION
■
located from difference Fourier synthesis and added to the atom list.
1. Synthesis and Structure. The dinuclear complex
Least-squares refinement on F2 using all measured intensities was
1a(PF6) is prepared using [RuCl3(trpy)] as metal precursor
carried out using the program SHELXTL. All non-hydrogen atoms
in combination with the Hpbl ligand (Scheme 1) under inert
conditions in a 2:1 [RuCl3(trpy)]:Hpbl molar ratio with excess
of MeONa as a base. Either reflux or microwave heating in
MeOH can be used, giving similar results with moderate yields
(27% for conventional reflux and 36% for microwave).
Complex 1a(PF6) is then washed with hot CH2Cl2.
There are a number of potential Ru−Cl dinuclear complexes
that can be obtained given the chelating, dinucleating
were refined including anisotropic displacement parameters.
Comments to the Structures. Compound 1b(PF6)2 crystallizes
−
with 1 molecule of the metal complex, two PF6 anions, 2 acetone
molecules, and 0.5 molecule of water in the asymmetric unit. One of
the acetone molecules is disordered in two positions with a ratio of
53:47. The water molecule is also disordered (50:50) and shared with
the neighboring unit cell. Compound 1c(PF6)2 crystallizes with 1
−
molecule of the metal complex, 2 PF6 anions, and 2.5 molecules of
hexadentate nature of the Hpbl ligand and assuming that the
water in the asymmetric unit. One of the PF6− anions is disordered in
trpy ligands remain always bonded to the Ru metal center.
two positions (ratio 60:40). Water molecules are highly disordered
with an occupancy of 1.00:0.75:0.50:0.125:0.125. As a consequence of
this, the hydrogen atoms of the water molecules could not be precisely
localized (B alert in the CheckCIF report).
Scheme 2 presents a drawing of all these potential isomers at
oxidation state II, classified as a function of the N/O pbl−
chelating atoms, where pbl− ligand can have different degrees of
protonation.
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1
Figure 1. H NMR spectra for complex 1aH+ in MeOD generated from addition of ascorbic acid to 1a+ and numbering scheme.
Figure 2. ORTEP plots (thermal ellipsoids at 50% probability) for X-ray crystal structures of the cationic part of the complexes: 1b2+ (left) and 1c2+
(right). Hydrogen atoms have been omitted for clarity. Color codes: C, black; Cl, green; N, dark blue; O, red; Ru, light blue.
From a geometrical perspective it is interesting to realize that
species C, D, and F (Scheme 2) contain a C2 rotation axis that
bisects the pyrazolyl moiety of the pbl− ligand, whereas species
A, B, and E have no symmetry elements except for the identity,
“E”. This is important since analytic techniques such as NMR
allow us to discern between symmetric and nonsymmetric
the NMR is consistent with a nonsymmetric complex
containing two terpyridines and one Hpbl ligand. Accordingly,
the symmetric species (C, D, and F, Scheme 2) are discarded.
For Ru−Hbpp complexes we have a substantial number of
both mononuclear and dinuclear complexes7a,9a,b,10,16 that have
1
been thoroughly characterized by H NMR spectroscopy. For
1
species. The H NMR spectrum of the dinuclear complex 1a+
these complexes the trans influence of the Ru−Cl group
induces the presence of a pseudotriplet at 6−7 ppm due to the
pyridyl group of the Hbpp ligand. The fact that in the present
case we only find one triplet in this region suggests that one
pyridyl of pbl− is coordinated whereas the other one is not. The
noncoordinated Hpbl pyridyl group at low pH is protonated,
and this produces the appearance of a doublet above 10.0 ppm
due to the CH alpha to the pyridyl N atom. The Ru−Cl trans
to the pyridyl group of the pbl− ligand is confirmed by the
absence of a second resonance in this region. If the Ru−Cl were
presents broad resonances (see Figure S1 in the Supporting
Information), suggesting the paramagnetic nature of the
Ru(II)−Ru(III) complex. This is further confirmed by the
open-circuit potential situated above the first oxidation wave
(see below). Addition of a reducing agent such as ascorbic acid
to a NMR tube containing this paramagnetic complex generates
1
the diamagnetic 1aH+ complex, whose H NMR spectrum is
presented in Figures 1 and S2 in the Supporting Information.
On the basis of the number of resonances and their integration,
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trans to the N-pyrazolyl moiety then the Ru−Cl group would
interact with the H alpha to the coordinated N-pyridylic group
of the pbl− ligand that would be strongly shifted downfield
toward the 9.5−10 ppm region. In addition, given the
substitutionally inert nature of Ru(II) in this type of
complexes,10 the ligand arrangement found in the X-ray
structure of 1b2+ (see Figure 2) further confirms the trans
disposition of the Ru−Cl group with regard to the N-pyridylic
group of the bpl− ligand. All this is also in agreement with the
experimental potentials found in the CV which indicate a N5
type of coordination environment for one of the Ru metal
centers and a ON4(Pz) for the second one, as will be detailed
later on. This discards species B and E as potential candidates.
Finally and in addition to the previous arguments, electro-
chemistry allows discriminating between species A and E. A
wave at 0.78 V associated with a Ru first coordination sphere
environment of 5 N-pyridyl and Cl appears in the cyclic
voltammetry, which allows us to assign this complex to species
A since species E would have a much lower redox potential
given the alkoxo coordination of its Ru centers (vide infra)
(Table 1).10 Furthermore, the complex has been characterized
by UV−vis and MS techniques (See Figures S7, S8, and S10 in
the Supporting Information).
The solid state structure for the cationic parts of 1b2+ and
1c2+ are very similar to related low-spin d6 Ru(II) and low-spin
d5 Ru(III) complexes previously published in the literature
showing pseudo-octahedral geometry (Figure 2, Table 2).6−9
Table 2. Crystallographic Data for Complexes [1b(PF6)2]·
(C3H6O)2·(H2O)0.5 and [1c(PF6)2]·(H2O)2.5
[1b](PF6)2·(C3H6O)2·
compound
formula
solvent
(H2O)0.5
[1c](PF6)2·(H2O)2.5
C54H54Cl2F12N10O4.5P2Ru2
1/2H2O + 2acetone
C48H44Cl2F12N10O4.5P2Ru2
2.5H2O
detected
fw
1478.05
1395.91
cryst size
(mm3)
0.50 × 0.20 × 0.05
0.20 × 0.10 × 0.05
cryst color
temp (K)
cryst syst
space group
a (Å)
brown
100
red
100
triclinic
triclinic
P1
̅
P1
̅
12.5295(6)
14.6459(7)
18.2104(9)
107.284(2)
109.045(2)
96.360(2)
2935.2(2)
2
13.2322(9)
15.2695(10)
15.8750(10)
70.027(2)
68.822(2)
89.430(2)
2787.4(3)
2
b (Å)
c (Å)
α (deg)
β (deg)
γ (deg)
V (Å3)
Table 1. Electrochemical Data for Mononuclear10 and
a
Dinuclear Hpbl Complexes
Z
ρ (g/cm3)
1.672
1.663
E1/2 vs SSCE in
V Ru(III/II)
b
entry
complex
μ (mm−1
)
0.754
0.789
b
10
1
[Ru−N5−H2O]2+
0.72
0.24
0.38
0.78
0.07
0.78
0.3
θmax (deg)
36.60
26.38
b
10
10
2
[Ru−ON4(Pz)−H2O]2+
[Ru−ON4(Py)−H2O]2+
no. of reflns
measd
101 293
24 599
b
3
c
[Cl−(Pz)N4O−Ru--Ru−N5−Cl]+, 1a+
[Cl−(Pz)N4O−Ru--Ru−N5−Cl]+, 1a+
[Cl−(Py)N4O−Ru--Ru−N5−Cl]2+, 1b2+
[Cl−(Py)N4O−Ru--Ru−N5−Cl]2+, 1b2+
[Cl−(Py)N4O−Ru--Ru−ON4(Py)−Cl]2+, 1c2+
no. of unique
reflns obsd
20 024 [Rint = 0.0696]
8054 [Rint = 0.0853]
4
c
5
abs corr
SADABS
0.871/1.000
834
SADABS
0.703/0.962
845
c
6
trans. min/max
parameters
c
7
c
8
0.3
R1/wR2 [I >
2σ(I)]
0.0415/0.0980
0.0568/0.1537
a
Bold font indicates the metal center undergoing the redox process.
Complexes are labeled indicating the atoms at the first Ru
coordination sphere. The trpy ligands are thus represented by N3
and the pbl− ligand either by N2 or by one NO (N(Py) when
coordinated by the pyridyl group or N(Pz) by the pyrazolyl). Finally,
R1/wR2 [all
data]
0.0758/0.1186
1.037
0.0873/0.1796
1.036
goodness-of-fit
(F2)
b
the H2O or Cl ligands are explicitly written. In 0.1 M of triflic acid
peak/hole (e/
Å3)
1.235/−0.977
1.373/−1.002
c
aqueous solution. In 0.1 M of TBAH in MeOH.
Complex 1a+ slowly isomerizes into 1b2+ in MeOH at room
temperature (1 week), substituting the initial coordinated
pyrazolyl moiety from pbl− by the free pyridyl on the alkoxo
side and generating a RuIII−ON4(Py) moiety (Scheme 1). An
X-ray structure of this new complex is presented in Figure 2
that will be discussed below. The UV−vis spectrum, the CV,
and the MS spectrum are shown in Supporting Information
Figures S4, S8, and S11, respectively.
Addition of 1 equiv of Ce(IV) to a methanolic solution of the
mixed-valence RuIII−RuII complex 1a+ at room temperature
generates the dinuclear complex 1c2+ where both Ru centers are
now at oxidation state III,III. The first coordination sphere of
the latter is RuIII−ON4(Py), which is with alkoxo coordination,
with the two pyridyls from the pbl− ligands also coordinated. In
addition, the N atoms of the pyrazolate moiety are not involved
in the metal coordination as can be seen in the crystal structure
in Figure 2 and also in a drawing in Scheme 1 (see Figures S5,
S9, and S12 in the Supporting Information for further
characterization).
For complex 1b2+ it is interesting to realize that the pyrazolyl-
coordinating side generates a 6-membered ring metallacycle
with the Ru metal center, whereas the other metal center with
no pyrazolyl coordination generates a 5-membered ring. This
can be an important factor in the isomerization driving force.17
In addition, this new coordination environment produces a
large separation between the two Cl ligands that are now
situated at 6.8 Å. This is important from a reactivity perspective
because it will preclude an intramolecular interaction between
potential active sites derived from their substitution. The latter
is even more strongly manifested in the crystal structure of 1c2+,
where now the two Ru−Cl bonds are situated trans to one
another with a Cl−Cl distance of 12.7 Å.
2. Redox Chemistry and Linkage Isomerism. The
electrochemical behavior of the dinuclear complex 1a+ has also
been studied by cyclic voltammetry in MeOH, and the potential
complexes generated and their generation routes are mapped in
Scheme 3.
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Scheme 3. Schematic Representation of the Different Linkage Isomerization Processes Taking Place for 1a+ and Its 1e−
a
Oxidized and Reduced Species
a
Here the terpyridine ligand is represented by “T”, and its axial coordination positions are not shown for clarity purposes.
In order to facilitate the tracking of the different species
generated electrochemically we are using a nomenclature that
only shows the first coordination sphere. In this nomenclature
the Cl ligands are not shown since they are always coordinated
as are the trpy ligands that are displayed with three N. The
The equation numbering is in accordance with Scheme 3. In
other two positions for the octahedral coordination of the Ru
metal center are occupied by the pbl− ligand. They can be
either the alkoxo moiety labeled O or the pyridyl labeled as
N(Py) or the pyrazolyl labeled as N(Pz). In the case where
both pyridyl and pyrazolyl groups are coordinated, they are
labeled with two N. For instance, 1a+ is labeled as N5−RuII--
RuIII−ON4(Pz), as indicated in Scheme 3.
addition, the Ru metal undergoing a change in oxidation state is
marked in bold. This assignment is based on related
mononuclear complexes that contain exactly the same first
coordination environment for the Ru center (Table 1). In
addition, the one-electron nature of the process at E1/2 = 0.07 V
was corroborated by bulk electrolysis (see Supporting
Information Figure S3).
As repetitive cycles are carried out the presence of a new
wave at E1/2 = 0.30 V (Ep,a = 0.38 V, Ep,b = 0.17 V, ΔE = 210
mV) appears, which is associated with the linkage isomerization
that occurs when the N-pyrazolyl is replaced by the N-pyridyl
group once the Ru(III,III) species are reached, that is
Figure 3a shows the cyclic voltammograms obtained for 1a+
scanning anodically at 400 mV/s. The first scan shows the
presence of two quasireversible waves at E1/2 = 0.07 (Ep,a = 0.12
V, Ep,c = −0.03 V, ΔE = 150 mV) and 0.78 V (Ep,a = 0.84 V, Ep,c
= 0.73 V, ΔE = 110 mV), which are assigned to the following
redox couples
This newly generated species has one wave at 0.30 V as
indicated previously
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Figure 3. Cyclic voltammograms of complex 1a+ in 0.1 M TBAH in MeOH at different scan rates and scanning directions (red arrows). Black arrows
indicate the intensity increase or decrease: (a) scanning anodically at a scan rate of 400 mV/s with a previous equilibration time of 20 min at −0.30
V, (b) scanning cathodically at a scan rate of 200 mV/s and with an equilibration time of 45 min at 0.60 V, and (c) 50 mV/s scanning cathodically
after holding the potential at 1.10 V for 3 min.
and the second one that appears at the same potential as the
parent complex
Thus, at oxidation state III,III the isomerization from N-Pz to
N-Py is favored, whereas at oxidation state II,II the opposite
from N-Py to N-Pz is favored, which is a bit counterintuitive.
Another interesting feature of this system is observed when a
CV of 1a+ is performed after holding the potential at 1.10 V for
3 min as shown in Figure 3c. As it can be observed in the
cathodic scan that the wave at 0.78 V has completely
disappeared, and only one wave at 0.30 V is observed. This is
consistent with generation of species 1c2+, as corroborated with
a CV prepared from an authentic sample. Thus, the wave
observed now is due to
An authentic sample of 1b2+ prepared independently gives
exactly the same CV (see Figure S8 in the Supporting
Information).
The 230 mV difference in redox potentials from the N-Py to
N-Pz (entries 5 and 7 in Table 1) is a phenomenon that we
observed earlier for related mononuclear complexes as
displayed in Table 1 (compare entries 2 and 3) and is due to
the stronger σ-donation and less π-accepting character of the
pyrazolyl vs the pyridyl group.
After 10 scans complex 1a2+ is totally transformed into 1b3+,
as can be observed in Figure 3a, where the wave at 0.07 V has
totally disappeared. The reverse isomerization process occurs
upon scanning in the 0.60 to −0.30 V region as shown in
Figure 3b, that is by avoiding generation of oxidation state III
for the RuN5 moiety that as mentioned earlier is responsible for
the opposite process
Since the starting material is 1a+ and we indicated earlier that
after one-electron removal to form 1a2+ this species quickly
isomerizes to 1b3+, it seems reasonable that 1b3+ in turn also
isomerizes to 1c2+ but at a slower rate than the former
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The catalytic activity observed here for the binuclear 1a+ is
comparable to that of several mononuclear complexes earlier
described in the literature.6a,b,8b This is due to the fact that once
the high oxidation states are accessed the linkage isomerization
process can occur very fast, generating 1c2+ type of species.
This hampers both the electronic communication between the
two Ru centers (due to pyrazolyl decoordination) and the
potential through-space interaction of the active Ru−oxo
groups (see the X-ray structure of 1c2+) that would otherwise
confer a cooperative effect of the two Ru centers in complex
1a+.
A second run was carried out isolating the catalyst upon
addition of diethyl ether and repeating the same protocol. The
results obtained were basically the same based on stereo-
selectivity, but the TNs were about 10−15% lower. This
decrease is due to partial loss of catalyst during the operations
for the recovery process and to a certain extent to potentially
deactivation pathways. These results together with 100%
substrate conversion indicate that the catalyst stability is
relatively high.
A very interesting feature of these complexes is their capacity
to reach very high cis/trans-epoxide stereoselectivity, giving in
all cases more than 90% of the corresponding cis-epoxide (see
Table 3).
Finally, as can be deduced from Table 3, the velocity at which
the epoxidation reaction occurs is directly related to the
multianionic nature of the backbone dinucleating ligand. In this
respect the complex containing the μ-pyr-dc3− ligand has by far
the largest TOF.
3. Catalytic Activity. Complex 1a+ has been tested with
regard to its ability to oxidize cis-β-methylstyrene together with
related mononuclear complexes of formula [RuIICl(Hpbl)-
(trpy)]+,10 and the results are reported in Table 3. The table
Table 3. Catalytic Epoxidation of cis-β-Methylstyrene with
PhIO by Means of 1a+ and Related Mono- and Dinuclear
a
Complexes for Purposes of Comparison
[Epox], M cis/
trans selectivity
(%)
b
conv.
(%)
c
d
catalyst
TN
401
TOFi
39
ref
this
1a+
100
0.71 (94)
work
this
work
{[Ru(trpy)]2(μ−
OOCH3)(μ-
bpp)}2+
100
0.80 (93)
452
36
in-[RuCl(Hpbl)
100
100
92
0.74 (92)
0.81 (56)
1.32 (100)
499
458
22
this
work
(trpy)]+
in-[Ru(trpy)
20
this
work
(Hbpp)(H2O)]+
{[Ru(trpy)
(H2O)]2(μ-pdz-
dc)}2+
{[RuII(trpy)
(H2O)]2(μ-pyr-
dc)}+
1320
980
660
8a
100
0.98 (100)
9180 8c
a
Reaction conditions, cat. 1.0 mM for mononuclear and 0.5 mM for
CONCLUSIONS
dinuclear complexes/alkene 1.0 M/PhI(OAc)2 2.0 M/H2O 2.0 M/
■
biphenyl 0.4 M/1,2-dichloroethane up to a final volume of 1.4 mL.
Three dinuclear Ru−Cl complexes containing the Hpbl and
trpy ligands, {[RuCl(trpy)]2(μ-pbl-κ-N3O)}+ (1a+), {[RuCl-
(trpy)]2(μ-Hpbl-κ-N3O)}2+ (1b2+), and {[RuCl(trpy)]2(μ-
Hpbl-κ-N2O2)}2+ (1c2+), have been prepared, isolated, and
characterized by analytic and spectroscopic techniques. The
coordination versatility of the Hpbl ligand allows the presence
of multiple isomers that can be obtained depending on the Ru
oxidation state and confers the system an extremely rich
chemistry with regard to the potential isomers that can be
obtained. In particular, fast N(Pz) → N(Py) (Hpbl acting from
κ-N3O) linkage isomerization is observed for the Hpbl ligand
for 1a2+ upon reaching oxidation state III,III. In sharp contrast,
the opposite reaction occurs when oxidation state II,II is
obtained. Finally, 1a+ and its recently reported mononuclear
analogue in-[RuCl(Hpbl)(trpy)]+ have been tested as catalysts
for epoxidation of cis-β-methylstyrene.
b
c
Cis/trans epoxide selectivity TN is the turnover number with regard
d
to epoxide formation. TOFi is the initial turnover frequency
expressed in epoxide cycles per hour.
also contains the performance of related complexes such as in-
[RuII(trpy)(Hbpp)(H2O)]+,16c {[RuII(trpy)]2(μ-CH3COO)(μ-
bpp)}2+,16b {[RuII(trpy)(H2O)]2(μ-pdz-dc)}2+,8a and
8c
{[RuII(trpy)(H2O)]2(μ-pyr-dc)}+ for comparison purposes.
As general methodology, catalytic reactions have been carried
out using a Cat:Subs:Ox ratio of 1:1000:2000 (mononuclear
catalysts) or 1:2000:4000 (dinuclear catalysts) using PhIO as
oxidant. The latter as well as formation of the corresponding
Ru−OH2 and its active higher oxidation state RuO species,
from Ru−Cl, is favored by allowing it to stir at room
temperature for 120 min before adding the corresponding
substrate. The product distribution of the catalytic reaction was
monitored by GC and GC-MS.
Milder and environmentally benign oxidants such as H2O2 or
tBuOOH are usually able to oxidize Ru(II) to Ru(III) but not
Ru(III) to Ru(IV) in this type of complexes. Since Ru(IV) is
the active species for the epoxidation reaction,8a,c no substrate
oxidation is observed using these peroxides.
ASSOCIATED CONTENT
* Supporting Information
■
S
Further spectroscopic (1D and 2D NMR) and spectroelec-
trochemical measurements for the reported complexes. This
material is available free of charge via the Internet at http://
pubs.acs.org.
In all cases the catalysts activity observed is very high, leading
to substrate transformations of 100%. The product distribution
observed is basically the epoxide and the products derived from
C−C bond scission such as benzaldehyde.
AUTHOR INFORMATION
■
Corresponding Authors
*E-mail: lluis.escriche@uab.cat.
*E-mail: allobet@iciq.es.
*E-mail: xavier.sala@uab.cat.
Notes
Complex 1a+ generates a 0.71 M concentration of cis-β-
methylstyrene oxide after 16 h. Similarly, its mononuclear Ru−
Hpbl counterpart, in-[RuCl(Hpbl)(trpy)]+, produces a final
epoxide concentration of 0.74 M after 20 h.
The authors declare no competing financial interest.
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Llobet, A. Inorg. Chem. 2011, 51, 320−327. (b) Sens, C.; Romero, I.;
Rodríguez, M.; Llobet, A.; Parella, T.; Benet-Buchholz, J. J. Am. Chem.
Soc. 2004, 126, 7798−7799. (c) Sens, C.; Rodríguez, M.; Romero, I.;
Llobet, A.; Parella, T.; Benet-Buchholz, J. Inorg. Chem. 2003, 42,
8385−8394.
(17) (a) Porterfield, W. W. Inorganic Chemistry: A Unified Approach,
2nd ed.; Academic Press, Inc.: San Diego, 1993. (b) Mackay, K. M.;
Mackay, R. A. Introduction to Modern Inorganic Chemistry, 4th ed.;
Blackie Academic & Professional: Glasgow, U.K., 1989.
ACKNOWLEDGMENTS
■
Support from MINECO (CTQ-2013-49075-R, CTQ2011-
26440, and CTQ2011-23156-C02-02) is gratefully acknowl-
edged. L.F. and R.M.G.-G. are grateful for the award of FI-UAB
and PIF doctoral grants from UAB, respectively.
REFERENCES
■
(1) Hauser, S. A.; Cokoja, M.; Kuhn, F. E. Catal. Sci. Technol. 2013, 3,
̈
552−561.
(2) Cavani, F.; Teles, J. H. ChemSusChem 2009, 2, 508.
(3) In Catalysts for fine chemical synthesis: Regio- and stereo-controlled
oxidations and reductions; Roberts, S. M., Whittall, J., Eds.; John Wiley
Sons, Ltd.: England, 2007; Vol. 5.
(4) (a) Sala, X.; Escriche, L.; Llobet, A. In Molecular Solar Fuels, 1st
ed.; Wydrzynski, T. J., Hillier, W., Eds.; Royal Society of Chemistry,
2012; Chapter 10, pp 273−288. (b) Berardi, S.; Drouet, S.; Francas, L.;
Gimbert-Surinach, C.; Guttentag, M.; Richmond, C.; Stoll, T.; Llobet,
A. Chem. Soc. Rev. 2014, DOI: 10.1039/C3CS60405E.
(5) (a) Meyer, T. J.; Huynh, M. H. V. Inorg. Chem. 2003, 42, 8140−
8160. (b) Dovletoglou, A.; Adeyemi, S. A.; Meyer, T. J. Inorg. Chem.
1996, 35, 4120−4127. (c) Suen, H. F.; Wilson, S. W.; Pomerantz, M.;
Walsh, J. L. Inorg. Chem. 1989, 28, 786−791. (d) Roecker, L.; Kutner,
W.; Gilbert, J. A.; Simmons, M.; Murray, R. W.; Meyer, T. J. Inorg.
Chem. 1985, 24, 3784−3791. (e) Takeuchi, K. J.; Thompson, M. S.;
Pipes, D. W.; Meyer, T. J. Inorg. Chem. 1984, 23, 1845−1851.
(f) Binstead, R. A.; Moyer, B. A.; Samuels, G. J.; Meyer, T. J. J. Am.
Chem. Soc. 1981, 103, 2897−2899. (g) Moyer, B. A.; Meyer, T. J.
Inorg. Chem. 1981, 20, 436−444. (h) Moyer, B. A.; Meyer, T. J. J. Am.
Chem. Soc. 1978, 100, 3601−3603.
(6) (a) Sala, X.; Santana, N.; Serrano, I.; Plantalech, E.; Romero, I.;
Rodríguez, M.; Llobet, A.; Jansat, S.; Gom
J. Inorg. Chem. 2007, 5207−5214. (b) Serrano, I.; Sala, X.; Plantalech,
E.; Rodríguez, M.; Romero, I.; Jansat, S.; Gomez, M.; Parella, T.;
́
ez, M.; Fontrodona, X.; Eur,
́
Stoeckli-Evans, H.; Solans, X.; Font-Bardia, M.; Vidjayacoumar, B.;
Llobet, A. Inorg. Chem. 2007, 46, 5381−5389. (c) Sala, X.; Poater, A.;
Romero, I.; Rodríguez, M.; Llobet, A.; Solans, X.; Parella, T.; Santos,
T. M. Eur. J. Inorg. Chem. 2004, 612−618.
(7) (a) Planas, N.; Christian, G. J.; Mas-Marza, E.; Sala, X.;
Fontrodona, X.; Maseras, F.; Llobet, A. Chem.Eur. J. 2010, 16,
́
7965−7968. (b) Aguilo, J.; Naeimi, A.; Bofill, R.; Mueller-Bunz, H.;
Llobet, A.; Escriche, L.; Sala, X.; Albrecht, M. New J. Chem. 2014, 38,
1980−1987.
(8) (a) Di Giovanni, C.; Vaquer, L.; Sala, X.; Benet-Buchholz, J.;
́
Llobet, A. Inorg. Chem. 2013, 52, 4335−4345. (b) García-Anton, J.;
Bofill, R.; Escriche, L.; Llobet, A.; Sala, X. Eur. J. Inorg. Chem. 2012,
4775−4789 and references therein. (c) Di Giovanni, C.; Poater, A.;
Benet-Buchholz, J.; Cavallo, L.; Sola,
2014, 20, 3898−3902.
(9) (a) Francas, L.; Sala, X.; Escudero-Adan
Escriche, L.; Llobet, A. Inorg. Chem. 2011, 50, 2771−2781. (b) Francas
̀
M.; Llobet, A. Chem.Eur. J.
̀
́
, E.; Benet-Buchholz, J.;
̀
,
L.; Sala, X.; Benet-Buchholz, J.; Escriche, L.; Llobet, A. ChemSusChem
2009, 2, 321−329. (c) Lopez, I.; Ertem, M. Z.; Maji, S.; Benet-
Buchholz, J.; Keidel, A.; Kuhlmann, U.; Hildebrandt, P.; Cramer, C. J.;
Batista, V. S.; Llobet, A. Angew. Chem., Int. Ed. 2014, 53, 205−209.
(10) Francas
̀ ́
, L.; Gonzalez-Gil, R. M.; Poater, A.; Fontrodona, X.;
García-Anton, J.; Sala, X.; Escriche, L.; Llobet, A. Inorg. Chem. 2014,
́
53, 8025−8035.
(11) Sullivan, B. P.; Calvert, J. M.; Meyer, T. J. Inorg. Chem. 1980, 19,
1404.
(12) Data collection with APEX II, version v2013.4-1; Bruker AXS
Inc.: Madison, WI, 2007.
(13) SAINT, version V8.30c; Bruker AXS Inc.: Madison, WI, 2007.
(14) Blessing. Acta Crystallogr. 1995, A51, 33−38. SADABS, V2012/
1; Bruker AXS Inc.: Madison, WI, 2001.
(15) Sheldrick, G. M. Acta Crystallogr. 2008, A64, 112−122.
SHELXTL, versions V6.12 and 6.14.
(16) (a) Roeser, S.; Ertem, M. Z.; Cady, C.; Lomoth, R.; Benet-
Buchholz, J.; Hammarstrom, L.; Sarkar, B.; Kaim, W.; Cramer, C. J.;
̈
10402
dx.doi.org/10.1021/ic501483s | Inorg. Chem. 2014, 53, 10394−10402