Polypyrrole-functionalized ruthenium carbene catalysts as efficient heterogeneous systems for olefin epoxidation

Article abstract of DOI:10.1039/c4dt00698d

New Ru complexes containing the bpea-pyr ligand (bpea-pyr stands for N,N-bis(pyridin-2-ylmethyl)-3-(1H-pyrrol-1-yl)propan-1-amine), with the formula [RuCl₂(bpea-pyr)(dmso)] (isomeric complexes 2a and 2b) or [Ru(CN-Me)(bpea-pyr)X)]ⁿ⁺ (CN-Me = 3-methyl-1-(pyridin-2-yl)-1H- imidazol-3-ium-2-ide; X = Cl, 3, or X = H₂O, 4), have been prepared and fully characterized. Complexes 3 and 4 have been anchored onto an electrode surface through electropolymerization of the attached pyrrole group, yielding stable polypyrrole films. The electrochemical behaviour of 4, which displays a bielectronic Ru(IV/II) redox pair in solution, is dramatically affected by the electropolymerization process leading to the occurrence of two monoelectronic Ru(IV/III) and Ru(III/II) redox pairs in the heterogeneous system. A carbon felt modified electrode containing complex 4 (C-felt/poly-4) has been evaluated as a heterogeneous catalyst in the epoxidation of various olefin substrates using PhI(OAc)₂ as an oxidant, displaying TON values of several thousands in all cases and good selectivity for the epoxide product.

Full text of DOI:10.1039/c4dt00698d

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Polypyrrole-functionalized ruthenium carbene
catalysts as efficient heterogeneous systems for
olefin epoxidation†
Cite this: Dalton Trans., 2014, 43,
9916
a,b
a
c
b
Mohamed Dakkach, Xavier Fontrodona, Teodor Parella, Ahmed Atlamsani,
a
a
Isabel Romero* and Montserrat Rodríguez*
New Ru complexes containing the bpea-pyr ligand (bpea-pyr stands for N,N-bis(pyridin-2-ylmethyl)-3-
(
2
1H-pyrrol-1-yl)propan-1-amine), with the formula [RuCl (bpea-pyr)(dmso)] (isomeric complexes 2a and
n+
2
b) or [Ru(CN-Me)(bpea-pyr)X)] (CN-Me = 3-methyl-1-(pyridin-2-yl)-1H-imidazol-3-ium-2-ide; X =
2
Cl, 3, or X = H O, 4), have been prepared and fully characterized. Complexes 3 and 4 have been anchored
onto an electrode surface through electropolymerization of the attached pyrrole group, yielding stable
polypyrrole films. The electrochemical behaviour of 4, which displays a bielectronic Ru(IV/II) redox pair in
solution, is dramatically affected by the electropolymerization process leading to the occurrence of two
monoelectronic Ru(IV/III) and Ru(III/II) redox pairs in the heterogeneous system. A carbon felt modified
electrode containing complex 4 (C-felt/poly-4) has been evaluated as a heterogeneous catalyst in the
Received 7th March 2014,
Accepted 30th April 2014
DOI: 10.1039/c4dt00698d
2
epoxidation of various olefin substrates using PhI(OAc) as an oxidant, displaying TON values of several
www.rsc.org/dalton
thousands in all cases and good selectivity for the epoxide product.
Although homogeneous catalysts based on transition
metals usually display good performance in oxidation reac-
Introduction
Heterogeneous catalysis is of unquestionable importance in tions, degradation of the complexes takes place to a consider-
chemical industry since more than 80% of the described pro- able extent through pathways such as auto-oxidation or
1
cedures involve a heterogeneous catalytic step. One of the oligomerization, in some cases through formation of oxo
remarkable advantages of heterogeneous catalysis is the easy bridges. To circumvent such drawbacks some authors have
separation of the catalyst from the reaction mixture, thus described the immobilization of the catalyst within organic
4
avoiding the use of purification techniques which might often polymeric matrices, a strategy which avoids or minimizes the
be arduous and increase the overall cost of the process. Cata- contact between molecules of the catalyst and consequently
lyst self-deactivation pathways are also generally minimized, leads to a much lower degree of auto-deactivation. Among the
thus increasing the catalyst performance. In this framework, different techniques described in the literature, the electro-
new strategies towards the heterogenization of classical homo- polymerization onto an electrode surface is one of the most
2
,3
5,6
geneous catalysts are being developed.
developed and involves the inclusion of redox-active func-
tional groups in the catalyst structure which allows, upon oxi-
dation or reduction, the formation of a polymeric film on the
surface, leading to a modified electrode. A wide diversity of
modified electrodes has been described, which have been used
a
Departament de Química i Serveis Tècnics de Recerca, Universitat de Girona,
Campus de Montilivi, E-17071 Girona, Spain. E-mail: montse.rodriguez@udg.edu,
marisa.romero@udg.edu
as heterogeneous catalysts for
a
variety of chemical
7
–10
reactions.
Their main advantages, besides the ones men-
b
Laboratoire des Matériaux et Systèmes Interfaciaux, Département de Chimie,
tioned above for heterogeneous systems, are the possibility of
carrying out electrocatalysis through the electrode itself and
Faculté des Sciences, B.P.: 2121, 93000 Tétouan, Morocco
c
Departament de Química i Servei de RMN Universitat Autònoma de Barcelona,
1
1
the low amount of catalysts generally used.
Cerdanyola del Vallès, E-08193 Barcelona, Spain
†
Electronic supplementary information (ESI) available: Crystal data for the X-ray
Polypyrrole has become the most studied conducting
structures of complexes 2a and 2b; possible diastereoisomers for complex 3; polymer in the last few decades due to its stability and high
NMR spectra (1D and 2D) of all complexes described in this work; cyclic voltam-
mograms of complexes 3, 4 and C/poly-3; linear regression of E1/2 values vs. pH
for the cyclic voltammograms of complex 4. CCDC 990320 and 990321. For ESI
and crystallographic data in CIF or other electronic format see DOI: 10.1039/
c4dt00698d
12
electrical conductivity. The use of polypyrrole as an organic
matrix for catalyst heterogenization usually encompasses the
presence of a covalently bonded pyrrole unit in the catalyst
structure that, after electrochemical polymerization, allows the
9916 | Dalton Trans., 2014, 43, 9916–9923
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Scheme 1 Ligands used in this work.
anchoring of the catalyst onto the polypyrrole backbone at the
electrode surface. These heterogeneous systems have been
6
,13
widely used in oxidative catalysis
and, in the particular case
of olefin epoxidation, most of the polypyrrole-based catalysts
1
4
contain Mn-porphyrin entities. However, despite that ruthe-
nium complexes have been broadly studied in homogeneous Scheme 2 Synthesis of complexes 2–4.
1
5,16
epoxidation catalysis,
the polypyrrole-based heterogeneous
Ru systems applied to oxidative catalysis deal mainly with
1
7
18
alcohol or water oxidation.
In this work we describe the synthesis of a family of Ru
complexes containing the bpea-pyr and CN-Me ligands
(Scheme 1) together with their anchoring onto electrode sur-
faces through oxidative polymerization of the pendant pyrrole
group of the bpea-pyr ligand. One of the Ru-OH complexes syn-
2
thesized has been tested in olefin epoxidation and constitutes,
to the best of our knowledge, the first Ru-polypyrrole system
successfully applied to heterogeneous epoxidation catalysis.
Results and discussion
Synthesis, structure and spectroscopic properties
Fig. 1 ORTEP plots (ellipsoids at 50% probability) for the structures of
complexes 2a (left) and 2b (right).
The synthetic strategy followed for the preparation of the com-
plexes is outlined in Scheme 2. The addition of the tridentate
bpea-pyr ligand to [RuCl (dmso) ] leads to the displacement of
2
4
three of the four dmso ligands, yielding either complex 2a or with the dmso ligand coordinated through its S atom. The meri-
b that can be obtained separately in their respective pure dional disposition of the ligand in 2a does not induce any
2
form by simply controlling the extent of the reaction. The trans remarkable differences between the structural parameters of the
or cis notation used refers to the relative position of the two two isomers, which are similar to other Ru complexes contain-
1
9,20
chloro ligands in isomers 2a and 2b, and also to the position ing bpea-type ligands previously described in the literature.
−
of the monodentate (Cl or H O) ligand with regard to the ali-
The addition of the bidentate carbene ligand CN-Me to a
2
phatic N atom of the bpea-pyr ligand in 3 and 4. On the other solution of either complex 2a or 2b leads to the formation of
II
+
hand, mer or fac indicates the meridional or facial disposition the chloro complex [Ru Cl(CN-Me)(bpea-pyr)] , 3. The coordi-
of the tridentate ligand in each case. The flexibility of the bpea- nation of the CN-Me ligand could potentially lead to a set of
19
pyr ligand, as is the case of the related ligand bpea, permits seven possible isomers including those presenting the bpea-
its coordination to a metal center either in a mer or fac fashion, pyr ligand in a mer or fac fashion (see ESI† for a schematic rep-
the former being a kinetically stable arrangement of the ligand resentation of the different isomers). However, complex 3 is
which is relatively uncommon in octahedral environments.
obtained exclusively in the trans,fac form, which is probably
The X-ray diffraction structure of the two isomeric com- favoured by the occurrence of H-bonding interactions between
plexes 2a and 2b has been solved and the corresponding the monodentate chloro ligand and the H atoms in the 2-posi-
ORTEP plots are shown in Fig. 1, whereas the crystallographic tion of the pyridyl rings of the bpea-pyr ligand (H1 and H12,
data and the main bond distances and angles are given in the see Fig. 2 for a numbering scheme), as has already been dis-
2
0a,b
ESI.† Both complexes present a distorted octahedral geometry cussed for analogous complexes.
On the other hand, the
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dπ(Ru) levels and consequently to higher energy MLCT tran-
sitions in the case of the aquocomplex.
Redox properties
All complexes have been characterized electrochemically
through CV experiments and display chemically and electro-
chemically reversible waves. The cyclic voltammogram of chlor-
ocomplex 3 displays a Ru(III/II) redox pair at E1/2 = 0.67 V vs.
SSCE (see ESI†), which is in the range of other Ru chloro-
2
0a,22
complexes with N- or C-donor ancillary ligands.
The redox properties of the aquocomplex 4 have been inves-
tigated in dichloromethane and also in aqueous media. The
II
electrochemical behaviour of Ru -aquocomplexes is generally
1
pH-dependent due to the simultaneous exchange of electrons
and protons during the redox processes as indicated in
eqn (1):
Fig. 2 H-NMR spectrum of complex
3 together with numbering
scheme.
þ
ꢀ
þ
ꢀ
ꢀ
1H ;ꢀ1e
ꢀ1H ;ꢀ1e
bpea-pyr ligand adopts a facial disposition in complex 3 even
when the CN-Me coordination is carried out using the mer
isomer 2a as the starting product, again evidencing that fac is
the thermodynamically preferred coordination geometry.
Finally, the aquocomplex 4 is easily obtained by reaction of
complex 3 with a silver salt in aqueous media, retaining the
former trans,fac geometry.
II
III
IV
ꢀ
ꢀꢀꢀꢀꢀ*
ꢀꢀꢀꢀꢀꢀ*
L5Ru ꢀOH2 )ꢀꢀꢀꢀꢀꢀ L5Ru ꢀOH )ꢀꢀꢀꢀꢀꢀ L5Ru vO ð1Þ
þ
ꢀ
þ
ꢀ
þ1H ;þ1e
þ1H ; þ1e
The reversibility of these redox processes constitutes the
basis for the use of this kind of complexes in redox catalysis.
In most cases, the two monoelectronic redox processes indi-
cated in eqn (1), corresponding to the Ru(III/II) and Ru(IV/III)
redox couples, can be observed in cyclic voltammetry.
However, in the case of complex 4 a unique redox wave is dis-
played throughout a wide pH range in aqueous media (Fig. 4).
The dependence of the E1/2 values vs. pH accomplishes the
Nernst equation with a slope value of ca. 58 mV, corresponding
to the transfer of an equivalent number of protons and elec-
trons (see ESI†), which would be in accordance with the simul-
taneous transfer of two protons and two electrons as indicated
in eqn (2):
All the complexes have been thoroughly characterized
through mono- and bidimensional NMR experiments, allowing
us to unambiguously assign all the resonances (see the Experi-
1
mental section). Fig. 2 displays the H-NMR spectra of
complex 3, together with the numbering scheme used.
The UV-vis spectra of complexes 3 and 4 recorded in
dichloromethane are shown in Fig. 3. In the range between
2
50 and 350 nm the complexes display a set of π–π* intraligand
absorptions whereas at higher wavelengths dπ–π* MLCT
2
1
þ
ꢀ
absorptions can be observed (see the Experimental section).
ꢀ2H ;ꢀ2e
II
IV
ꢀ
ꢀꢀꢀꢀꢀ*
L5Ru ꢀOH2 )ꢀꢀꢀꢀꢀꢀ L5Ru vO
ð2Þ
The replacement of a chloro ligand by aquo in 4 produces a
hypsochromic shift of the maximum wavelength originated by
þ
ꢀ
þ2H ;þ2e
the lower π-donor capacity of H O when compared to the
2
anionic Cl− ligand, thus leading to the stabilization of the
Fig. 3 UV-vis spectra of complexes 3 and 4 in dichloromethane (con-
centration 1 × 10 M).
Fig. 4 Cyclic voltammetries of complex 4 at different pH values in
aqueous solution (scan rate 100 mV s ).
−4
−1
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This behaviour is also consistent with the bielectronic wave
2
+
displayed by the analogous complex [Ru(CN-Me)(bpea)(H O)]
(
2
2
3
where bpea is N,N′-bis(2-pyridylmethyl)ethylamine) which
contains practically identical ligands and consequently similar
electronic effects are expected to be exerted at the metal center.
On the basis of the electrochemical properties displayed by
other previously reported Ru complexes, carbenic ligands seem
to provide a balance between σ-donor and π-acceptor properties
that can reduce to a considerable extent the difference between
the Ru(III/II) and the Ru(IV/III) E1/2 values leading in some
22a,24
cases to bielectronic Ru(IV/II) processes.
This behaviour
may play an important role in the selectivity obtained when
using these complexes as catalysts in epoxidation reactions as
will be discussed later. On the other hand, the cyclic voltamme-
try of the aquocomplex 4 performed in dichloromethane also
displays a single, reversible wave (see ESI†) with a difference
between the anodic and cathodic peak potentials (Ep,a − Ep,c
)
which is half of that obtained from the CV of the analogous
chlorocomplex 3 recorded under the same conditions, a fact
indicative of the transfer of two electrons in the case of 4.
Heterogenization of the complexes
The anchoring of complexes 3 and 4 onto an electrode surface
to generate the corresponding modified electrodes C/poly-3
and C/poly-4 was first carried out using a glassy carbon elec-
trode (3 mm diameter), through successive scans of potentials
between 0 and 1.3 V from a 1 mM solution of the corres-
ponding complex in dichloromethane. Fig. 5A displays the vol-
tammograms obtained in the course of the polymerization for
Fig. 5 (A) Growth of a C/poly-4 film during the first 20 consecutive CV
cycles on a glassy carbon disk electrode (3 mm diameter) in 1 mM of 4
1
in 0.1 M TBAH, CH
2
Cl
2
(υ = 100 mV s− ). (B) Five voltammetric cycles for
the aquocomplex 4, where the intensity of the metal-centred the C/poly-4 film coated electrode upon transferring it to a blank elec-
−1
2 2
trolyte solution of 0.1 M TBAH in CH Cl (υ = 100 mV s ). Final amount
redox process (found at around 0.95 V) increases continuously,
indicating the deposition of the catalyst on the surface (the CV
obtained in the analogous experiment performed with the
chlorocomplex 3 are shown in the ESI†).
of anchored complex = 7.70 × 10− mol cm
10
−2
.
During polymerization of 4, a new wave with low intensity described above but on a preparative scale using carbon felt
appears at ca. 0.65–0.70 V that is discussed next. After 20 (1 × 1 × 0.5 cm) as the working electrode instead of a glassy
cycles, the modified C/poly-4 electrode is transferred to a carbon disk (the modified electrode is termed in this case
blank electrolyte solution and five further cycles are recorded C-felt/poly-4). The results obtained in the oxidation of the
(
Fig. 5B). It is striking to see that, once the complex is anchored olefin substrates tested are shown in Table 1, where the
onto the electrode surface, it displays two clearly differentiated amount of supported Ru-catalyst 4 is indicated in each case.
redox processes of equal intensity that can be assigned to the
As can be observed, the heterogeneous system shows a
monoelectronic Ru(IV/III) and Ru(III/II) redox pairs, at E1/2 highly remarkable catalytic activity with turnover numbers of
values of 1.04 V and 0.66 V respectively. Thus, the polypyrrole several thousands in all cases. The selectivity for the epoxide
environment dramatically affects the redox behavior of the product is excellent for both aromatic and aliphatic olefins
complex avoiding the occurrence of the bielectronic process except for styrene (entry 1), which also displays the lowest con-
observed in homogeneous solution. On the other hand, the version degree among the substrates tested. In this case, the
polymer generated at the surface of the electrode displays high occurrence of two monoelectronic redox processes in the
stability with good maintenance of the intensity of the signals anchored Ru-catalyst will probably favour the generation of
despite the low level of cross-linking expected for a compound radical intermediates which can drive the reaction towards
5c,11a,14a
with only one pyrrole group per monomer unit.
substrate polymerization instead of epoxidation. This hypo-
thesis would also explain the formation of a certain amount of
trans epoxide when oxidizing cis-β-methylstyrene (entry 3), as
Catalytic activity
The catalytic performance of the heterogeneous poly-4 system the cis→trans isomerization prior to epoxide ring closure must
was tested in the epoxidation of some olefins using iodosyl- involve a radical intermediate, where the C–C bond rotation is
2
5
benzene diacetate as the oxidant in dichloromethane. The het- allowed. At this point, it is interesting to briefly mention the
erogeneous catalysts were set following the procedure results obtained in olefin epoxidation using the analogous
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Table 1 Epoxidation of different olefins using the heterogeneous C-felt/poly-4 system (Conv. and Sel. stand for conversion and selectivity values
−4
−4
respectively). Conditions: substrate (1.25 × 10 mol), oxidant PhI(OAc)
2 2 2
(1.25 × 10 mol), CH Cl (5 mL), 25 °C, 32 h. The amount of supported cata-
lyst is indicated in each case. Conversions and yields are evaluated by GC analysis using biphenyl as the internal standard
a
Substrate
Product
Mols of 4
Conv. (%)
9
Sel. (%)
TON
2683
9
1
2
4.07 × 10−
16
3.4 × 10−
9
24
92
82
8823
1.72 × 10−
2.59 × 10−
9
9
40
b
29 024
16 891
3
4
35
94
85
5.79 × 10−
2.22 × 10−
9
9
18
26
3874
5
6
c
89
14 712
a
b
c
Calculated as [epoxide yield/substrate conversion] × 100. Epoxide 96% cis/4% trans. Epoxidation exclusively at the cyclohexene ring.
2
+
II
[Ru(CN-Me)(bpea)(H
2
O)] complex (which displays a Ru(IV/II)
Synthesis of trans,mer-[Ru Cl
2
(bpea-pyr)(dmso)], 2a. 0.14 g
bielectronic wave) as a homogeneous catalyst under analogous (0.412 mmol) of bpea-pyr and 0.2 g (0.412 mmol) of
23
2 4
conditions, with a selectivity for the styrene epoxide product [RuCl (dmso) ] were dissolved in 20 mL of absolute ethanol.
higher than 93% and with the cis-β-methylstyrene epoxide The solution was kept under reflux for 45 min leading to the
obtained quantitatively in its cis form, though with much lower formation of an orange precipitate. After completing the reac-
TON values. Consequently, despite the occurrence of the two tion the volume of the solution was reduced under reduced
monoelectronic processes in the polypyrrole-anchored complex, pressure and the precipitate was filtered, washed with cold
the overall catalytic activity of the heterogeneous C-felt/poly-4 ethanol and diethyl ether and dried in vacuo. Yield: 0.155 g
system is excellent, with TON values among the highest reported (67.6%). Elem. Anal. found (calc.) for C21
2 4 1 1 1
H28Cl N O Ru S : C,
for this type of process either in the homogeneous or hetero- 45.21 (45.32); N, 10.18 (10.06); H, 5.43 (5.07); S, 5.59 (5.76).
1
geneous phase and with remarkably good selectivity values.
H-NMR (600 MHz, CD Cl ) δ (ppm): 8.76 (d, 2 H, H1, H12,
2 2
Attempts to reuse the C-felt/poly-4 catalyst in the second
J1–2 and 12–11 = 6.45 Hz), 7.62 (pt, 2 H, H3, H10, J3–2 and 10–11 =
run resulted in very low degree of conversion. We considered 6.75 Hz, J10–9 and 3–4 = 7.45 Hz), 7.24 (pt, 2 H, H2, H11,
then the possibility of catalyst leaching from the electrode J2–1 and 11–12 = 6.45 Hz, J2–3 and 11–10 = 6.75 Hz), 7.20 (d, 2 H,
surface, but no traces of ruthenium were found by ESI-MS and H4, H9, J9–10 and 4–3 = 7.45 Hz), 6.56 (t, 2 H, H16, H19,
ICP-AES spectrometry carried out on the residual solution after
the end of the catalysis. Finally, no epoxidation was detected J17–16,19 and 18–16,19 = 2 Hz), 5.79 (d, 2 H, H6b, H7a, J6b–6a and 7a–7b
when using a bare carbon felt electrode (without the Ru 14.3 Hz), 4.28 (d, 2 H, H6a, H7b, J6a–6b and 7b–7a = 14.3 Hz), 3.75
J16–17,18 and 19–17,18 = 2 Hz), 6.05 (t, 2 H, H17, H18,
=
complex) as the heterogeneous catalyst.
(t, 2 H, H15, J15–14 = 6.8 Hz), 3.52 (s, 6 H, H20, H21), 3.06 (m,
1
3
2
H, H13), 2.03 (m, 2 H, H14). C-NMR (151 MHz, CD Cl ,
2 2
25 °C) δ (ppm): 165.1 (C5, C8), 156.4 (C1, C12), 135.9 (C3,
C10), 123.7 (C2, C11), 121.4 (C4, C9), 120.3 (C16, C19), 108.2
C17, C18), 63.8 (C6, C7), 56.7 (C213), 47.0 (C15), 45.9 (C20,
C21), 27.2 (C14). NOEs: H7b–H13 and H14; H7b–H9;
H12–H20. For the NMR signal assignment we have used the
(
Experimental procedures
Synthesis of ligands and complexes
The ligands N,N-bis(pyridin-2-ylmethyl)-3-(1H-pyrrol-1-yl)- same numbering scheme described for the X-ray structure
2
6
−1
propan-1-amine (bpea-pyr) and 3-methyl-1-(pyridin-2-yl)-1H- shown in Fig. 1. IR (νmax, cm ): 3095 (w), 2944 (w), 1440 (s),
2
7
imidazol-3-ium bromide (HCN-Me), as well as the complex 1076 (s), 1004 (m), 892 (m). E1/2 (III/II), (CH
2
2
Cl + 0.1 M TBAH
II
28
[
Ru Cl (dmso) ], 1, have been prepared following synthetic (tetrabutylammonium hexafluorophosphate)): 0.58 V vs. SSCE.
2
4
2 2
Cl
): λmax, nm (ε, M− cm ) 254 (7775), 362 (2614),
1
−1
methods previously described in the literature. All the manipu- UV-vis (CH
lations have been systematically performed under a nitrogen 428 (4994). ESI-MS: m/z (relative intensity, assignment) = 521
−
+
atmosphere using Schlenk techniques.
(100%, [M − Cl ] ).
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II
2
Synthesis of cis,fac-[Ru Cl (bpea-pyr)(dmso)], 2b. This com- 6.5 Hz, J11–10 = 7.7 Hz), 7.18 (td, 1H, H21, J21–22 and 21–20 =
pound was obtained through a procedure analogous to the one 6.5 Hz), 6.39 (t, 2 H, H16, H19, J16–17,18 and 19–17,18 = 2.2 Hz),
described for complex 2a but by prolonging the reflux up to 5.93 (t, 2 H, H17, H18, J17–16,19 and 18–16,19 = 2.2 Hz), 4.51 (d,
1
2 hours. Yield: 0.125 g (54.5%). Elem. Anal. found (calc.) for 1 H, H7a, J7a–7b = 16.6 Hz), 4.37 (d, 1 H, H6b, J6b–6a = 16.6 Hz),
C H Cl N O Ru S : C, 44.86 (45.32); N 9.96 (10.06); H 5.06 4.10 (d, 1 H, H7b, J7b–7a = 16.6 Hz), 4.08 (d, 1 H, H6a, J6a–6b
=
2
1
28
2
4
1
1 1
1
(
5.07); S 5.71 (5.76). H-NMR (600 MHz, CD
2
Cl
2
) δ (ppm): 9.73 16.6 Hz), 3.70 (m, 2 H, H15), 3.40 (s, 3H, H27), 2.32 (m, 1 H,
(
7
d, 1 H, H1, J1–2 = 6.5 Hz), 9.21 (d, 1 H, H12, J12–11 = 5.5 Hz), H13a), 2.10 (m, 1 H, H14a), 2.05 (m, 1 H, H14b), 2.00 (m, 1 H,
1
3
.63 (td, 1 H, H3, J3–2 and 3–4 = 7.8 Hz, J3–1 = 1.6 Hz), 7.47 (td, H13b). C-NMR (151 MHz, d -acetone, 25 °C) δ (ppm): 203.4
6
1
H, H10, J10–11 and 10–9 = 7.8 Hz, J10–12 = 1.5 Hz), 7.26 (pt, 1 H, (C28), 161.8 (C8), 159.9 (C5), 155.2 (C24), 152.0 (C12), 151.5
H2, J2–1 = 6.5 Hz, J2–3 = 7.8 Hz), 7.22 (d, 1 H, H4, J4–3 = 7.8 Hz), (C20), 149.9 (C1), 137.0 (C22), 136.8 (C3), 135.1 (C10), 125.0
.08 (d, 1 H, H9, J9–10 = 7.8 Hz), 7.06 (pt, 1 H, H11, J11–12 = 5.5 (C26), 124.0 (C2), 123.4 (C11), 121.7 (C4), 121.1 (C21), 120.7
7
Hz, J11–10 = 7.8 Hz), 6.75 (t, 2 H, H16, H19, J16–17,18 and 19–17,18
=
(C9), 120.1 (C16, C19), 116.2 (C26), 110.7 (C25), 108.1 (C17,
2
4
1
1
.1 Hz), 6.15 (t, 2 H, H17, H18, J17–16,19 and 18–16,19 = 2.1 Hz), C18), 68.4 (C7), 66.5 (C6), 65.0 (C13), 46.0 (C15), 35.2 (C27),
.52 (d, 1 H, H7a, J7a–7b = 10.5 Hz), 4.49 (d, 1 H, H6b, J6b–6a
5.9 Hz), 4.15 (m, 1 H, H13a), 4.00 (d, 1 H, H7b, J7b–7a
=
=
24.6 (C14). NOEs: H9–H7b; H27–H7a; H6b–H4; H25–H23;
H20–H1. For the NMR signal assignment we have used the
−
1
0.5 Hz), 3.99 (m, 2 H, H15), 3.98 (m, 1 H, H13b), 3.92 (d, 1 H, numbering scheme shown in Fig. 2. IR (νmax, cm ): 1614 (m),
H6a, J6a–6b = 15.9 Hz), 3.52 (s, 3 H, H20), 3.03 (s, 3 H, H21), 1492 (w), 1093 (m), 836 (s), 738 (m), 555 (m). E1/2 (III/II) (CH Cl
2
2
1
3
2
.23 (m, 2 H, H14). C-NMR (151 MHz, CD
2
Cl
2
, 25 °C) + 0.1 M TBAH): 0.67 V vs. SSCE. UV-vis (CH
2
Cl
2
): λmax, nm
δ (ppm): 164.4 (C8), 160.4 (C5), 154.2 (C12), 152.5 (C1), 137.0 (ε, M− cm ) 246 (8631), 270 (5809), 373 (5046). ESI-MS: m/z
1
−1
−
+
(C3), 135.4 (C10), 124.1 (C2), 124.0 (C9), 120.9 (C16, C19), (relative intensity, assignment) = 602.1 (100%, [M − PF ] ).
6
II
1
20.6 (C4), 120.6 (C11), 108.8 (C17, C18), 69.5 (C7), 68.3 (C6),
Synthesis of trans,fac-[Ru (CN-Me)(bpea-pyr)OH
2 6 2
](PF ) ,
6
2.9 (C13), 47.8 (C15), 45.1 (C20), 44.7 (C21), 26.2 (C14). NOEs: 4·6H
2
O. To a solution of 3 (0.025 g, 0.033 mmol) in 15 mL of
H12–H21; H18–H19. For the NMR signal assignment we have H O were added 1.5 equivalents (0.0085 g, 0.05 mmol) of
2
used the same numbering scheme described for the X-ray AgNO
3
. The solution was kept under reflux for 3 h. After
structure shown in Fig. 1. IR (νmax, cm ): 3089 (w), 2950 (w), cooling in an ice bath, the precipitated AgCl was filtered
477 (m), 1068 (s), 1004 (s), 892 (m), 757 (s). E1/2 (III/II), through celite and 1 mL of saturated aqueous NH PF solution
−
1
1
4
6
(
CH
2
Cl
2
+ 0.1 M TBAH): 0.57 V vs. SSCE. UV-vis (CH
2
Cl
2
): λmax
,
was added to the filtrate. The solution was concentrated under
−
1
−1
nm (ε, M cm ) 251 (6770), 371 (4531). ESI-MS: m/z (relative reduced pressure until precipitation. The green solid obtained
intensity, assignment) = 521 (100%, [M − Cl ] ).
−
+
was filtered, washed with diethylether and pentane and dried
II
Synthesis of trans,fac-[Ru Cl(CN-Me)(bpea-pyr)](PF
6
), 3·0.8 in vacuo. Yield: 0.020 g (69.3%). Elem. Anal. found (calc.) for
CH
2
Cl
2
. A sample of complex 2a or 2b (0.050 g, 0.099 mmol)
28 33 12 7 1 2 1 2
C H F N O P Ru ·6H O: C 34.40 (34.22); N 9.83 (9.98);
1
together with LiCl (0.081 g, 1.98 mmol) was dissolved in H 3.33 (4.62). H-NMR (600 MHz, d -acetone/10% D O, 25 °C)
6
2
2
0 mL of diethylene glycol and the solution was stirred at δ (ppm): 9.05 (d, 1 H, H1, J1–2 = 5.5 Hz), 8.85 (d, 1 H, H12,
room temperature for 30 min. Afterwards the mixture was pro- 12–11 = 6.5 Hz), 8.49 (d, 1 H, H25, J25–26 = 2.2 Hz), 8.35 (d, 1 H,
J
gressively warmed and, when the temperature reached 60 °C, a H20, J20–21 = 5.8 Hz), 8.21 (d, 1 H, H23, J23–22 = 7.6 Hz), 8.12
solution of [HCN-Me]Br pre-ligand (0.016 g, 0.099 mmol) and (td, 1 H, H22, J22–21 and 22–23 = 7.6 Hz, J22–20 = 1.6 Hz), 7.99 (td,
NEt
added. The mixture was kept at 150 °C overnight. After J10–11 and 10–9 = 7.7 Hz, J10–12 = 1.6 Hz), 7.62 (m, 2 H, H2, H4),
cooling, 40 mL of H O and 2 mL of a saturated aqueous 7.59 (d, 1 H, H26, J26–25 = 2.2 Hz), 7.46 (d, 1 H, H9, J9–10
NH PF solution were added. A yellowish precipitate was 7.7 Hz), 7.39 (pt, 1 H, H11, J11–12 = 6.5 Hz, J11–10 = 7.7 Hz), 7.26
formed which was filtered and recrystallized from CH Cl – (pt, 1 H, H21, J21–22 = 7.6 Hz, J21–20 = 5.8 Hz), 6.37 (t, 2 H, H16,
3
(0.04 mL, 0.297 mmol) in 2 mL of diethylene glycol was 1 H, H3, J3–2 and 3–4 = 7.6 Hz, J3–1 = 1.6 Hz), 7.81 (td, 1 H, H10,
2
=
4
6
2
2
pentane 1 : 1 (v : v). The final precipitate was filtered, washed H19, J16–17,18 and 19–17,18 = 2.0 Hz), 5.92 (t, 2 H, H17, H18,
with diethylether and pentane and dried in vacuo. Yield: 17–16,19 and 18–16,19 = 2.0 Hz), 4.54 (d, 1 H, H7a, J7a–7b
.040 mg (54.1%). Elem. Anal. found (calc.) for 16.6 Hz), 4.42 (d, 1 H, H6b, J6b–6a = 16.6 Hz), 4.11 (d, 1 H, H7b,
Ru ·0.8CH Cl : C, 42.55 (42.44); N, 11.86 7b–7a = 16.6 Hz), 4.09 (d, 1 H, H6a, J6a–6b = 16.6 Hz), 3.67 (m, 2 H,
12.03); H 4.27 (4.03). H-NMR (600 MHz, d -acetone, 25 °C) H15), 3.53 (s, 3 H, H27), 2.14 (td, 1 H, H13a, J13a–14 = 3.8 Hz,
δ (ppm): 9.59 (ddd, 1 H, H1, J1–2 = 5.5 Hz, J1–3 = 1.4 Hz, J1–4 J13a–13b = 11.9 Hz), 2.00 (m, 1 H, H14a), 1.97 (m, 1 H, H14b),
.7 Hz), 9.49 (ddd, 1 H, H12, J12–11 = 6.5 Hz, J12–10 = 1.4 Hz, 1.90 (td, 1 H, H13b, J13b–14 = 4.6 Hz, J13b–13a = 11.9 Hz).
J
=
0
C
(
28
H
31Cl
1
F
6
N
7
P
1
1
2
2
J
1
6
=
0
1
3
J12–9 = 0.7 Hz), 8.37 (d, 1 H, H25, J25–26 = 2.2 Hz), 8.15 (ddd,
C-NMR (151 MHz, d
6 2
-acetone/10% D O, 25 °C) δ (ppm):
1
H, H20, J20–21 = 6.5 Hz, J20–22 = 1.4 Hz, J20–23 = 0.7 Hz), 8.07 199.4 (C28), 162.4 (C8), 160.5 (C5), 156.3 (C24), 153.4 (C12),
(
dt, 1 H, H23, J23–22 = 8.2 Hz, J23–21 = 1.0 Hz), 7.96 (ddd, 1 H, 150.4 (C20), 149.4 (C1), 139.9 (C22), 138.7 (C3), 137.2 (C10),
H22, J22–23 = 8.2 Hz, J22–21 = 6.5 Hz, J22–20 = 1.4 Hz), 7.89 (td, 126.8 (C26), 125.5 (C2), 125.0 (C11), 123.4 (C4), 122.9 (C21),
H, H3, J3–2 and 3–4 = 7.7 Hz, J3–1 = 1.4 Hz), 7.70 (td, 1 H, H10, 122.3 (C9), 120.9 (C16, C19), 118.1 (C26), 112.5 (C25), 108.9
10–11 and 10–9 = 7.7 Hz, J10–12 = 1.4 Hz), 7.52 (d, 1 H, H4, J4–3 (C17, C18), 69.7 (C7), 68.4 (C6), 66.2 (C13), 46.7 (C15), 36.4
.7 Hz), 7.50 (m, 1 H, H2), 7.47 (d, 1 H, H26, J26–25 = 2.2 Hz), (C27), 25.2 (C14). NOEs: H9–H7b; H27–H12; H6a–H20; H6b–
1
J
7
7
=
.36 (d, 1H, H9, J9–10 = 7.7 Hz), 7.26 (pt, 1 H, H11, J11–12
=
H4; H1–H20; H23–H25. For the NMR signal assignment we
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have used the numbering scheme shown in Fig. 2. IR (νmax
,
trode is afterwards transferred to a clean electrolyte solution and
−1
cm ): 3662 (w), 1616 (m), 1488 (m), 1091 (m), 831 (s), 555 (m). five further cycles are recorded. The amount of complex immobi-
1/2 (IV/II), (phosphate buffer pH = 7): 0.32 V vs. SSCE. E1/2 lized has been determined through the Faraday’s law from the
E
(
(
(
2 2 2 2
IV/II), CH Cl : 0.95 V vs. SSCE. UV-vis (CH Cl ): λmax, nm overall charge of the Ru(III/II) anodic peak in each case.
ε, M cm−1) 245 (10 368), 268 (8380), 355 (5476). ESI-MS
−1
MeOH): m/z (relative intensity, assignment) = 586.1 (18%, Catalytic studies
−
− +
[M − H
2
O − 2PF
6
+ F ] ), 598.1 (100%, [M − H O + MeO −
2
Experiments have been performed in anhydrous dichloro-
methane at room temperature. In a typical run, a modified
C-felt/poly-4 electrode was immersed in a dichloromethane
solution (5 mL) containing the alkene (50 mM), the oxidant
−
+
− 2+
2
PF6 ] ), 283.5 (43%; [M − H O − 2PF ] ).
2
6
Instrumentation and measurements
FT-IR spectra were recorded on a Mattson-Galaxy Satellite
FT-IR spectrophotometer containing a MKII Golden Gate
Single Reflection ATR System. UV-vis spectroscopy was per-
formed on a Cary 50 Scan (Varian) UV-vis spectrophotometer
PhI(OAc) (50 mM) and biphenyl as the internal standard. The
2
mixture was stirred at room temperature for 32 h and the
epoxide formed was determined by gas chromatography (GC)
analysis using Shimadzu GC-17A gas chromatography appar-
atus with a TRA-5 column (30 m × 0.25 mm diameter) incor-
porating a flame ionization detector.
1
13
with 1 cm quartz cells. H and C-NMR spectroscopy was
carried out on a Bruker DPX 200 MHz or a Bruker 600 MHz.
Samples were run in d -acetone or CD Cl , with internal refer-
6
2
2
ences (residual solvent protons and/or tetramethylsilane). Elemen-
tal analyses were performed using a CHNS-O Elemental Analyser
EA-1108 from Fisons. ESI-MS experiments were performed on
a Navigator LC/MS chromatograph from Thermo Quest Finni-
gan, using acetonitrile as the mobile phase. Cyclic voltam-
metric (CV) experiments were performed with a IJ-Cambria
IH-660 potentiostat using a three electrode cell. Glassy carbon
electrodes (3 mm diameter) from BAS or carbon felt from
SOFACEL were used as working electrodes, platinum wire as
the auxiliary electrode and SSCE as the reference electrode. All
cyclic voltammograms presented in this work were recorded
under a nitrogen atmosphere. All E1/2 values were calculated as
Conclusions
We have prepared and characterized a new family of complexes
containing the bpea-pyr tridentate ligand together with Cl ,
dmso or pyridylcarbene (CN-Me) ligands. The compounds with
−
II
n+
the general formula [Ru (CN-Me)(bpea-pyr)(X)] (with X = Cl or
O, complexes 3 and 4) have been anchored onto an electrode
H
2
surface through repetitive voltammetric cycles, generating the
corresponding C/poly-3 and C/poly-4 modified electrodes,
which display a good stability despite the low level of cross-
linking expected from the presence of a unique pyrrole unit per
complex. Catalyst 4 has been anchored onto a carbon felt elec-
trode and tested in heterogeneous epoxidation catalysis using
iodosylbenzene diacetate as an oxidant, displaying excellent
TON values and good selectivity for the epoxide product. C-felt/
poly-4 constitutes, to the best of our knowledge, the first report
of a Ru-polypyrrole system in olefin epoxidation catalysis.
the average of the oxidative and reductive peak potentials (Epa
+
−1
Epc)/2 at a scan rate of 100 mV s . Unless explicitly mentioned,
the concentration of the complexes was approximately 1 mM. In
aqueous solution the pH was adjusted from 0–2 with HCl. Pot-
assium chloride was added to keep a minimum ionic strength
of 0.1 M. From pH 2–10, 0.1 M phosphate buffers were used,
2
and from pH 10–12 diluted, CO -free NaOH was used.
X-ray structure determination
Acknowledgements
Suitable crystals of 2a and 2b were obtained by slow diffusion
of pentane or diethylether into a solution of the complexes in This research was financed by MINECO of Spain through pro-
dichloromethane. Measurements of the crystals were per- jects CTQ2010-21532-C02-01, CTQ2012-32436 and CTQ2009-
formed on a Bruker Smart Apex CCD diffractometer using 08328. The authors acknowledge STR from UdG for technical
graphite-monochromated Mo Kα radiation (λ = 0.71073 Å) support. Johnson & Matthey Ltd is acknowledged for a
from an X-Ray tube. Data collection, Smart V. 5.631 (Bruker RuCl ·nH O loan. MD thanks AECID (MAEC) of Spain for the
3
2
AXS 1997–02); data reduction, Saint+ Version 6.36A (Bruker AXS allocation of a MAEC-AECID grant.
2
001); absorption correction, SADABS version 2.10 (Bruker AXS
2
001) and structure solution and refinement, SHELXTL
Version 6.14 (Bruker AXS 2000–2003). CCDC 990320 (2a) and
Notes and references
9
90321 (2b) contain the supplementary crystallographic data
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2
Cl
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+ 0.1 M TBAH. The
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Dalton Trans., 2014, 43, 9916–9923 | 9923