A Recoverable Ruthenium Aqua Complex Supported on Silica Particles: An Efficient Epoxidation Catalyst

A Journal of

Accepted Article

Title: A recoverable ruthenium aqua complex supported onto silica

particles: an efficient epoxidation catalyst

Authors: M.Isabel Romero, Ingrid Ferrer, Xavier Fontrodona, Anna

Roig, and Montserrat Rodríguez

This manuscript has been accepted after peer review and appears as an

Accepted Article online prior to editing, proofing, and formal publication

of the final Version of Record (VoR). This work is currently citable by

using the Digital Object Identifier (DOI) given below. The VoR will be

published online in Early View as soon as possible and may be different

to this Accepted Article as a result of editing. Readers should obtain

the VoR from the journal website shown below when it is published

to ensure accuracy of information. The authors are responsible for the

content of this Accepted Article.

To be cited as: Chem. Eur. J. 10.1002/chem.201604463

Link to VoR: http://dx.doi.org/10.1002/chem.201604463

Supported by

10.1002/chem.201604463

Chemistry - A European Journal

A recoverable ruthenium aqua complex supported onto silica

particles: an efficient epoxidation catalyst

Íngrid Ferrer,^[a]Xavier Fontrodona,^[a]Anna Roig,^[b]Montserrat Rodríguez*^[a]and Isabel

Romero*^[a]

^[a]Departament de Química and Serveis Tècnics de Recerca, Universitat de Girona, Campus de Montilivi, E-

17003 Girona, Spain.

^[b]Institut de Ciència de Materials de Barcelona, ICMAB (CSIC), Campus UAB, 08193 Bellaterra, Spain

1

10.1002/chem.201604463

Chemistry - A European Journal

Abstract

The preparation and characterization of new complexes with a phosphonated trpy ligand (trpy-

P-Et) and a bidentate pyridylpyrazole (pypz-Me) ligand, with formula [Ru^II(trpy-P-Et)(pypz-

Me)X]ⁿ⁺(X = Cl, n= 1, 2; X=H₂O, n=2, 3) is described, together with the anchoring of 3 onto two

types of supports: mesoporous silica particles (SP) and silica coated magnetic particles (MSP).

The aqua complex 3 is easily obtained through reflux of 2 in water and displays a bielectronic

Ru(IV/II) redox process. It has been anchored onto SP and MSP supports through two different

synthetic strategies, yielding the heterogeneous systems SP@3 and MSP@3 that have been

fully characterized by IR, UV-vis, SEM, CV and DPV. Catalytic olefin epoxidation has been

tested with the molecular complex 3 and the SP@3 and MSP@3 heterogeneous counterparts,

including the reuse of the heterogeneous systems. The MSP@3 material can be easily

recovered by a magnet facilitating their reusability.

KEYWORDS: ruthenium / aqua complexes / surface functionalization / epoxidation / heterogeneous

catalysis.

2

10.1002/chem.201604463

Chemistry - A European Journal

Introduction

Olefin epoxidation has received considerable interest from both academics and industry since

epoxides play an important role as intermediates and building blocks in organic chemistry and

materials science.¹During the last decades, a series of homogeneous catalysts were identified

to be active in this reaction and, among them, we have developed efficient polypyridyl Ru-OH₂

catalysts for epoxidation reactions.²These Ru-aqua complexes can easily lose protons and

electrons and reach reactive, higher oxidation state Ru^IV=O species, and as a consequence

they have been widely used as redox catalysts for the oxidation of both organic and inorganic

species. In addition, the reaction mechanisms for the oxidation of several substrates by Ru^IV=O

have been thoroughly described together with the establishment and optimization of catalytic

processes.³The redox potentials, and therefore the performance of the polypyridyl Ru-OH₂

catalysts can be tuned through the electronic and steric effects of the auxiliary polypyridyl

ligands coordinated to the Ru metal center. The main advantages of homogenous catalysis are

selectivity and easy tuning of properties of catalysts through the modification of ligands;

however, from the perspective of sustainability and industrial applicability, homogeneous

catalysts are often expensive, create additional waste and their regeneration is difficult. In this

context, covalent grafting of homogeneous catalysts onto an insoluble porous solid support with

high surface, as mesoporous silica, ⁴is a good approach where the catalyst can be recovered

by simple filtration.⁵Also, the combination of different materials to create multifunctional

systems is a good alternative to obtain recyclable and efficient catalysts. This is the case of

nano o micro core-shell magnetic particles where the active sites are mainly distributed on the

“outer” surface of the support. These particles have been recently subject of extensive research,

since such materials combine the unique magnetic properties of the core together with the

possibility to further functionalize the surface; in addition the application of an external magnetic

field enables the removal of the particles in a simple way.⁶

Few examples of well-defined molecular epoxidation catalysts supported onto the previously

mentioned supports have been described in the literature.⁷In this context, the design of new

catalysts anchored to different supports for alkene oxidation is an important challenge in the

development of sustainable methodologies.

In the present paper we describe the synthesis and characterization of new ruthenium

compounds with general formula [Ru^II(trpy-P-Et)(pypz-Me)X]ⁿ⁺,where trpy-P-Et and pypz-Me are

N-donor ligands (see Scheme 1), and X = Cl, H₂O. The ruthenium aqua complex has been

anchored onto two different supports via covalent bonds and the anchored Ru-aqua compound

does not modify its coordination and electronic properties upon heterogenization. We also report

the good performance of the Ru complex as catalyst for olefin epoxidation reaction in

homogeneous and heterogeneous phase, along with the reutilization of the corresponding

supported catalysts and their recovery by a magnet when magnetic particles are used as

anchoring support. The stereospecific epoxidation of cis-epoxide observed in homogeneous

phase is consistent with the occurrence of a bielectronic transfer process in the catalyst.

3

10.1002/chem.201604463

Chemistry - A European Journal

Scheme 1. Ligands structures and synthetic strategy for the preparation of complexes 2 and 3.

trpy-P-Et, L1

pypz-Me, L3

Results and discussion

Synthesis, Structure and Redox Characterization

Molecular Complexes

The synthetic strategy followed for the preparation of Ru^IIcomplexes 2 and 3, containing the

trpy-P-Et L1, and pypz-Me L3, ligands, is outlined in Scheme 1. The trichlorido Ru complex

[Ru^IIICl₃(trpy-P-Et)], 1 (previously obtained by reaction of equimolar amounts of RuCl₃·2.38H₂O

and the trpy-P-Et ligand, L1, in dry methanol at reflux for 3 h^7a), was treated with the didentate

ligand pypz-Me, L3, in presence of NEt₃, producing the substitution of two chlorido ligands on 1.

After addition of a saturated NH₄PF₆aqueous solution the corresponding chlorido complex 2

was obtained as a 1:1 mixture of trans and cis isomers, due to the no symmetric nature of the

pypz-Me ligand (the nomenclature cis or trans refers to the relative position of the monodentate

ligand, Cl or H₂O, with regard to the pyrazole ring of the pypz-Me ligand, see Figure 1). Further

addition of saturated aqueous solution of NH₄PF₆to the mother liquor lead to the precipitation of

a little fraction of the almost isomerically pure cis-[Ru^IICl(trpy-P-Et)(pypz-Me)](PF₆) complex. All

attempts to separate the isomers were unsuccessful. The aqua complex [Ru^II(trpy-P-Et)(pypz-

Me)(OH₂)](PF₆)₂, 3, is easily obtained by dissolving complex 2 in water under reflux and the

complex is also isolated as a mixture of cis-3 and trans-3 isomers. It is worth mentioning here

4

10.1002/chem.201604463

Chemistry - A European Journal

that this behaviour is different to that shown by the analogous complex [Ru(trpy)(pypz-

Me)(OH₂)]^{2+ 8}where addition of AgNO₃to the chlorido complex precursor is necessary to

produce the exchange of Cl by aqua ligand. Probably, the presence of the phosphonate group

on the trpy-P-Et ligand in complex 2 produces different electronic effects that make the Cl ligand

more labile, as can be found in the literature for other substituted trpy ligands.⁹

Crystal structures of trans- and cis-2 have been solved jointly by X-ray diffraction analysis from

a single crystal where the two isomers crystallize together in an equimolar ratio. Figure 1

displays their molecular structure whereas the main crystallographic data and selected bond

distances and angles can be found in the Supplementary Information section (Tables S1 and

S2).

Figure 1. Ortep plots and labelling schemes for a) trans-2 and b) cis-2.

a)

Cl1

C17

O2

C13

N3

C16

C12

O1

C14

C9

C6

C10

N2

C20

C21

C11

C15

N4

C8

C3

P1

C22

C7

C4

O3

C5

C1

N1

C18

N6

C24

C23

C25

C2

C28

N5

C19

C26

C27

b)

O1

C16

P1

C17

O2

Cl1

C13

C9

C14

C10

C12

N3

C8

C11

C6

C5

C15

N4

C28

N5

O3

C7

C4

N2

C18

N1

C1

N6’

C25

C27

C19

C20

C3

C2

C24

C23

C21

C26

C22

In both cases, the Ru metal center adopts an octahedral distorted type of coordination where

the trpy-P-Et ligand is bonded in a meridional manner and the pypz-Me ligand acts in a

didentate fashion. The sixth coordination site is occupied by the chlorido ligand. All bond

distances and angles are within the expected values for this type of complexes,^7a,8,10although

5

10.1002/chem.201604463

Chemistry - A European Journal

the diffraction data does not allow a precise discussion of structural parameters, in particular

those corresponding to the pypz-Me ligand; the data only allows its refinement as a single

molecule with disordered positions at the area of atoms C20-C28. The N(1)-Ru-N(2) and N(2)-

Ru-N(3) angles are 80º and 81.1º respectively, showing the geometrical restrictions imposed by

the tridentate trpy-P-Et ligand, which is considered to define the equatorial plane of the

structure; as a consequence of this, the other two equatorial angles, N(1)-Ru-N(4) and N(3)-Ru-

N(4), are larger than the 90º expected for an ideal octahedral geometry.

The IR spectra for both complexes 2 and 3 show vibrations around 1200, 1000 and 800 cm^-1

that can be assigned to _P=O, _P-Oand _P-Cstretching modes, respectively. In the case of 3, it

can be observed a band around 3000 cm^-1that can be assigned to _O-Hstretching of the aqua

ligand. The one-dimensional (1D) and two-dimensional (2D) NMR spectra of both complexes

were registered in d₆-acetone and are presented in Supplementary Information (Figures S1-S3).

The resonances found are consistent with the structures obtained in the solid state for 2. ¹H-

NMR shows two sets of signals, one in the aromatic region corresponding to the nitrogen

ligands and the other in the aliphatic region assigned to the ethyl groups bound to phosphonate

and the methyl group of de pypz-Me ligand. It is worth mentioning the difference observed in the

chemical shift of H20 for the two isomers of compound 2; in the case of trans isomer this signal

is found at 10.16 ppm whereas for the cis isomer it appears at 8.17 ppm. This difference is due

to the deshielding effect of the spatially close Cl ligand in the case of the trans isomer.

The UV-Vis spectra for both complexes 2 and 3 registered in CH₂Cl₂are displayed in Figure S4.

The complexes exhibit ligand based -* bands below 350 nm and relatively intense bands

above 350 nm mainly due to d(Ru)-*(L) MLCT transitions.¹¹In the case of the Ru-Cl complex

the MLCT bands are shifted to longer wavelengths with regard to the corresponding Ru-OH₂

complex because of the relative destabilization of the d (Ru) levels provoked by the anionic

chlorido ligand.^12-14

The redox properties of complexes 2 and 3 have been determined by cyclic voltammetry (CV)

and differential pulse voltammetry (DPV) experiments (see Figures S5 and S6). Chlorido

complex 2 (Figure S5a) exhibits a quasireversible monoelectronic Ru(III/II) redox wave at E_1/2

around 0.93 V versus SCE; the analogous complex [RuCl(trpy)(pypz-Me)]^{+ 8}reaches the Ru(III)

oxidation state at lower potential (0.80 V), evidencing the higher electron-withdrawing capacity

of the trpy-P-Et ligand with regard to trpy, that provokes a destabilization of the Ru(III) oxidation

state in complex 2. On the other hand, the CV of the aqua complex 3 (Figure S5b) exhibits a

quasireversible wave at E_1/2around 0.5 V vs SCE in aqueous phosphate buffer at pH= 7

corresponding, as we could confirm through Pourbaix diagrams (see below), to the bielectronic

Ru(IV/II) redox couple of the two cis and trans-3 isomers, which display closely similar potential

values. Differential pulse voltammetry (DPV, Figure S6) obtained at pH = 5.5 allows

differentiating these two redox processes at E_1/2= 0.51 and 0.61 V that, by comparison with the

analogous complex [Ru(trpy)(pypz-Me)(OH₂)]²⁺,⁸can be assigned respectively to the trans-3

and cis-3 isomers. Figure 2 shows the Pourbaix diagrams for both trans- and cis-3 complexes,

with a unique pH-dependent redox process throughout the whole pH range with a change in the

slope value (from approximately 58 to 30 mV/pH unit) at pH around 7.6 for trans-3 and 9.6 for

cis-3, a behavior that is generally indicative of the occurrence of a two-electron (IV/II) redox

process.^2b,cThe changes in the slope throughout the whole pH range correspond to the pKa

values of Ru(II) or Ru(III) species and are indicated by vertical lines in the Pourbaix diagrams.

Electrochemical and thermodynamic (pKa values) data corresponding to trans- and cis-3 are

gathered in Table 1 together with analogous information for similar complexes described in the

literature.

6

10.1002/chem.201604463

Chemistry - A European Journal

Table 1. pK_aand electrochemical data (pH = 7, E_1/2in V vs SCE) for aqua complexes described in this

work and others for purposes of comparison.

E_1/2

(III/II) (IV/III)

E_1/2

E

E

Entry

Compound

pK_a(II) pK_a(III)

ref.

(IV/II)^b

(IV/II)^a

1

2

3

4

5

6

7

trans-3

cis-3

0.42^c

0.52^c

<0

0.42

0.52

0.48

0.495

0.55

0.66

0.65

7.6

9.6

1.1

1.5

this work

trans-[Ru(trpy)(pypz-Me)(OH₂)]²⁺0.39

0.57

0.52

180

50

10.1

10.75

9.75

0.95

1.65

1.21

8

cis-[Ru(trpy)(pypz-Me)(OH₂)]²⁺

trans-[Ru(trpy)(pyrpy-O)(OH₂)]⁺

[Ru(trpy-P)(bpm)(OH₂)]²⁺

[Ru(trpy)(bpy)(OH₂)]²⁺

0.47

0.55^c

0.66^c

0.57

<0

8

<0

7a

15

0.74

0.17

9.7

1.7

^aE = E_1/2(IV/III) - E_1/2(III/II) in mV. ^bAverage value calculated according to: E(IV/II) = [E_1/2(IV/III) + E_1/2(III/II)]/2, in V. ^c

E_1/2(IV/II) in V.

A first look at Table 1 shows that both isomers of complex 3 (entries 1 and 2) present Ru(III/II)

potential values slightly higher than the analogous cis- and trans-[Ru(trpy)(pypz-Me)(OH₂)]²⁺

complexes (entries 3 and 4), which is in accordance with the electron-withdrawing character of

the phosphonate group of the trpy-P-Et ligand, as we have noted above. Another complex also

containing this modified trpy ligand (entry 6) displays the highest Ru(III/II) potential value for the

set of complexes showed in Table 1. Nevertheless, the E_1/2values are not only governed by the

electronic properties of the ligands but also by the geometry of the complex. Thus, when

comparing for instance entries 1 and 2 or 3 and 4, we can see that coordination of a relatively

good σ-donor (or less π-acceptor) ligand in a trans fashion with respect to the aqua ligand, as in

the case for the trans isomers, lowers E_1/2(III/II) with respect to the analogous cis isomers hence

stabilizing the high oxidation state species.¹⁶However, in complexes displaying two

monoelectronic processes (entries 3 and 4), the effect on the Ru(IV/III) wave seems to be

reversed and, consequently, the difference between Ru(IV/III) and Ru(III/II) E values (E, see

Table 1) is increased for the trans geometry. Thus, the occurrence of a bielectronic process

seems to arise from a specific balance between the geometry and the -donor and -acceptor

properties of the ancillary ligands.¹⁷

7

10.1002/chem.201604463

Chemistry - A European Journal

Figure 2. Pourbaix diagram of a) trans-3 and b) cis-3. The pH-potential regions of stability for the different

oxidation states and their dominant proton compositions are indicated.

a)

1

pKa_Ru(III)

0.9

Ru^III-OH₂

0.8

Ru^IV=O

0.7

0.6

0.5

pKa_Ru(II)

0.4

Ru^II-OH₂

0.3

Ru^II-OH

0.2

0.1

0

2

4

6

8

10

12

pH

b)

1

0.9

0.8

0.7

0.6

0.5

0.4

0.3

0.2

0.1

0

pKa_Ru(III)

Ru^III-OH₂

Ru^IV=O

pKa_Ru(II)

Ru^II-OH₂

Ru^II-OH

0

2

4

6

8

10

12

pH

8

10.1002/chem.201604463

Chemistry - A European Journal

Finally, regarding the pKa values for the Ru(II) species, the trans- and cis-3 isomers display

lower pKa values than the corresponding [Ru(trpy)(pypz-Me)(OH₂)]²⁺isomers (7.6 and 9.6 vs.

10.1 and 10.75 for trans and cis, see entries 1-4). This higher acidic character for 3 is again in

accordance with the enhanced electron-withdrawing nature of the trpy-P-Et ligand when

compared to trpy.

Heterogeneous Complexes

The two synthetic strategies followed for the immobilization of [Ru^II(trpy-P)(pypz-Me)(OH₂)]²⁺, 3,

are shown in Scheme 2. Both strategies are based in the formation of hybrid materials in which

the organic and inorganic parts are linked by a chemical bond. A covalent binding is formed

between the phosphonate-modified terpyridine ligands (trpy-P-Et, L1) and (trpy-P-H, L2) and

silica particles, leading to the modification of the surface of silica supports. Two different SiO₂

supports were used: commercial silica mesoporous particles (SP) and silica-coated -Fe₂O₃

magnetic particles (MSP), with mean diameters of aprox. 200 nm in both cases, and surface

area of 886 m²/g and 25 m²/g, respectively. Mass fraction of -Fe₂O₃in MSP is 7.2% as

determined by ICP-MS; these values are summarized in Table S3.

Scheme 2. Synthetic strategies for the immobilization of Ru complex 3. L3 represents the bidentate pypz-

Me ligand and grey spheres represent the silica particles.

9

10.1002/chem.201604463

Chemistry - A European Journal

Following the strategy I, (Scheme 2, 1a/b) terpyridine phosphonate-coated silica particles were

synthesized by grafting (trpy-P-Et, L1) or (trpy-P-H, L2) ligands onto silica materials.

Diethylphosphonate L1 was mixed with SP or MSP in toluene at reflux overnight, in order to

achieve the hydrolysis of the ethyl groups, whereas L2 was mixed with SP or MSP in water

overnight at room temperature. Removal of the ethyl groups upon grafting in the case of ligand

L1 is confirmed through NMR spectroscopy (see Figure S8). In both cases the immobilized

terpyridine ligands were obtained after centrifugation. The corresponding ligand-coated silica

particles were then mixed with RuCl₃in methanol overnight to afford [RuCl₃(trpy-P)]-coated,

deep brown silica particles that were washed, centrifuged and dried in a hot air oven. In parallel,

a sample of naked silica particles (SP) was treated with RuCl₃following the same procedure but

a whitish silica was recovered and thus a significant degree of direct deposition of

uncoordinated Ru at the surface can be ruled out. Finally, the bidentate pypz-Me ligand, L3,

was coordinated to [RuCl₃(trpy-P)]-coated silica particles by reflux in methanol using NEt₃as

reducing agent, leading to the final Ru(II) aqua complex 3 attached to the corresponding

support. This heterogeneous system was centrifuged, washed and dried in a hot air oven. It is

important to note here that the final immobilized compound was the aqua complex 3 instead of

the expected chlorido complex 2. This is also the case for strategy II (see below) and is

10

10.1002/chem.201604463

Chemistry - A European Journal

probably due to the lability of chlorido ligand that, in presence of non-anhydrous solvents, leads

to the exchange of Cl by the aqua ligand.

Strategy II consists in the reaction of molecular chlorido complex 2 with SP particles as is shown

in Scheme 2(2). A mixture of trans- and cis-[RuCl(trpy-P-Et)(pypz-Me)](PF₆), 2, was dissolved in

toluene under reflux (with addition of acetone after 4 hours of reflux to further solubilize complex

2, see experimental section), in presence of SP support. After 20 hours, the hydrolysis of the

ester groups was produced and these conditions also lead to the hydrolysis of the chlorido

ligand, generating the corresponding aqua complex 3 anchored to the support. An alternative

anchoring protocol using a mixture of toluene/acetone as refluxing solvent from the beginning of

the process was tested; however, under these conditions the exchange of chlorido ligand by

aqua ligand took place before the hydrolysis of the ester groups causing the anchorage of the

complex through the aqua ligand position preferentially, leading to a blockage of the labile

position needed for the catalytic activity. To the best of our knowledge, this is the first example

of an aqua ruthenium complex immobilized and fully characterized (see below) onto silica

supports including magnetic particles since in previous works an aqua complex is generated in

situ by dissolving in water the Ru-Cl precursor supported onto Fe₃O₄nanoparticles.^7a

The ruthenium loading in the corresponding supports were measured by inductively coupled

plasma-atomic emission spectrometry (ICP-AES) and the values are shown in Table S4. As it

can be observed strategy 1a and 1b (starting from the ethyl or the acid trpy-P ligand

respectively, see scheme 2) were tested using MSP support, being the strategy 1a the most

effective probably due to the harsher reaction conditions used, necessary to hydrolyze the ethyl

groups of the phosphonate ligand. Strategy 1b was used with the SP support yielding a high

amount of Ru anchored, and this support was also tested using strategy II (direct anchoring of

the molecular chlorido complex). A lower amount of Ru was anchored through this strategy

when compared to strategy 1b, probably due to the incomplete hydrolysis of the ester groups.

Scanning electron microscopy (SEM) was performed on the two supports after functionalization,

and the images obtained are displayed in Figure 3.

Figure 3. SEM images of a) SP and b) MSP after the anchoring of the Ru complex.

MSP@3

SP@3

1

1m

1 m

As can be observed, all supports remain practically intact even after harsh synthetic conditions

in some cases, maintaining their particle size and morphology after the grafting process. Figure

11

10.1002/chem.201604463

Chemistry - A European Journal

S7 shows the SEM images obtained for supports SP and MSP before anchoring the complex,

and full characterization of MSP particles can be found elsewhere¹⁸. Thermogravimetric profiles

have been registered for the naked SP support and for the two (SP and MSP) supports after

anchoring the Ru complex, and the results obtained are shown in Figure 4. In all cases, the

supports present an initial weight loss at temperatures below 150˚C, corresponding to the

evaporation of adsorbed solvent. At temperatures above 300˚C, the weight loss for the three

functionalized supports corresponds to the release of the Ru complex. For SP the weight loss

was 7.7%, whereas it was 3.9% for MSP (both prepared through strategy 1b).

Figure 4. Thermogravimetric profiles of SP (blue), SP@3 (green), MSP@3 (grey).

The spectroscopic (IR and UV-vis) and electrochemical characterization of the anchored

catalyst 3 has been performed on SP functionalized systems. Figure 5a displays the IR spectra

for SP and SP@3 and the inset shows the detail of the bands found around 2900-3000 cm^-1that

can be assigned to _N-Hand _C-Hstretching modes of the supported trpy-P ligand in SP@3. On

the other hand, difference IR spectra are provided in the supporting information (Figure S9)

together with the IR spectrum of the molecular complex 3. As can be observed in Figure S9a, a

shift of the vibrations around 800 cm^-1to higher wavenumber values take place upon grafting of

the complex onto SP particles, presumably indicating a modification on the phosphonate group.

The UV-Vis spectra for SP@3 obtained through strategies 1b and 2 have been registered on a

suspension of the heterogeneous support in dichloromethane and are displayed in Figure 5b

together with the UV-vis spectrum corresponding to the molecular aqua complex 3. The

anchored complexes exhibit ligand based -* bands below 350 nm and relatively intense

bands above 350 nm mainly due to d(Ru)-*(L) MLCT transitions¹¹that are similar to those

obtained for the homogeneous complex 3; therefore, we can assert that the anchored 3

complex is in both cases analogous to the molecular species, and its electronic properties have

not been substantially modified.

12

10.1002/chem.201604463

Chemistry - A European Journal

Figure 5. a) IR spectra of SP (orange) and SP@3 (blue); b) UV-vis spectra for SP@3 obtained through

strategy 1b (red), 2 (blue) and homogeneous complex 3 (dotted grey).

a)

100

90

100

99

80

98

70

97

96

60

95

3200 3100 3000 2900 2800 2700

50

wavenumber (cm^‐1)

40

4000 3500 3000 2500 2000 1500 1000

500

wavenumber (cm^‐1)

b)

1,8

1,6

1,4

1,2

1,0

0,8

0,6

0,4

0,2

0,0

250

300

350

400

450

500

550

600

λ(nm)

The redox properties of the functionalized particles have been investigated using cyclic

voltammetry (CV) and differential pulse voltammetry (DPV) experiments by settling a layer of

the heterogeneous functionalized particles on the working electrode surface. Figure 6 displays

13

10.1002/chem.201604463

Chemistry - A European Journal

the differential pulse voltammetry obtained for SP@3 together with that of the homogeneous

complex 3 for purposes of comparison, and the cyclic voltammetries of SP@3 registered in

MeOH and in aqueous phosphate buffer are shown in the supporting information (Figure S10).

The value of E_1/2=0.56V obtained for SP@3 is similar to the average of the potential values

obtained for the two isomers cis and trans of complex 3 in solution. In the case of MSP@3 this

value was E_1/2=0.60V. The DPV experiment performed on the 3-modified support does not allow

the distinction of the two waves for the cis- and trans- isomers of the complex though an

isomeric mixture is expected to be formed in the heterogeneous supports as suggested by the

UV-vis spectrum obtained.

Figure 6. Differential Pulse Voltammetry of SP@3 (red) and homogeneous complex 3 (dotted grey) in

aqueous phosphate buffer at pH=5.5.

4,0E‐06

3,0E‐06

I(A)

2,0E‐06

1,0E‐06

0,0E+00

0,2

0,4

0,6

E(V)

0,8

1

The redox properties of Ru-aqua complexes are pH dependent due to the simultaneous

exchange of protons and electrons during the redox processes, and this dependence can be

plotted as E vs. pH, or Pourbaix, diagrams. The Pourbaix diagram obtained for a SP@3 sample

prepared through strategy I (1b) is displayed in Figure 7 and it is similar to that obtained when

the functionalized support is prepared by following strategy II (Figure S11). In both cases, the

change of E_1/2value with pH displayed by the heterogeneous SP@3 systems confirm that the

species anchored is indeed the aqua complex since a parallel behaviour to the homogeneous

phase is observed (see Figure 2); a single wave was observed throughout the whole the pH

range, which can tentatively be assigned to a Ru(IV/II) 2-electron transition of the supported

complex. The values shown correspond to an average of the data for cis- and trans- isomers.

14

10.1002/chem.201604463

Chemistry - A European Journal

Figure 7. Pourbaix diagram of SP@3 obtained through strategy 1b. The pH-potential regions of

stability for the various oxidation states and their dominant proton compositions are indicated.

1

pKa_Ru(III)

0.9

Ru^III-OH₂

0.8

Ru^IV=O

0.7

0.6

0.5

pKa_Ru(II)

0.4

Ru^II-OH₂

0.3

Ru^II-OH

0.2

0.1

0

2

4

6

8

10

12

pH

Catalytic epoxidation of alkenes

The catalytic activity of an isomeric trans- and cis- mixture of aqua complex 3 was checked in

the epoxidation of cis--methylstyrene in dichloromethane, using PhI(OAc)₂as oxidant. No

epoxidation occurred in the absence of catalyst or in the presence of naked silica supports. The

catalytic results are shown in Table 2 together with those corresponding to previously reported

complexes for comparison purposes.

Table 2. Catalytic epoxidation of cis--methylstyrene by Ru aqua complexes using PhI(OAc)₂as

oxidant.^a

Conversion Selectivity

% of

cis-epoxide

ref.

Entry

Compound

(%)

(%)^b

1

2

3

4

3

>99

70

>99

98

This work

trans-[Ru(trpy)(pypz-Me)(OH₂)]²⁺

cis-[Ru(trpy)(pypz-Me)(OH₂)]²⁺

[Ru(trpy-P)(bpm)(OH₂)]²⁺

90

99

86

68

8

>99

97

>99

7a

^aConditions: complex (1.25 mol), substrate (125 mol), PhI(OAc)₂(250 mol), CH₂Cl₂(2.5 mL), 25ºC, 24 h,

biphenyl (125 mol) as internal standard.

^bSelectivity for epoxide, (Yield/Conversion)x100.

15

10.1002/chem.201604463

Chemistry - A European Journal

An interesting reaction in epoxidation catalysis is the stereoselective epoxidation of cis-

alkenes,^19-24in which the well-known and efficient Mn-salen complexes ²⁵and others²⁶generally

give mixtures of cis and trans-epoxides. The cis→trans isomerization in epoxidation processes

involving cis alkenes is a common phenomenon due to the higher thermodynamic stability of the

trans epoxides. To undergo such isomerization, a rotation around the C-C bond is required

involving the presence of a relatively long-lived free substrate radical during the oxygen transfer

process.²⁷

As we can observe in Table 2, the mixture of isomers 3 presents high levels of conversion and

selectivity for the epoxide (with benzaldehyde identified as main by-product), comparable to

other complexes described in the literature.^7a,8The corresponding cis-epoxide was detected as

the sole product, with no formation of the trans-epoxide. This result indicates that, if an alkene-

localized radical species was formed as intermediate, the closure of the ring would be much

faster than the C-C rotation at the radical intermediate. However, a potential O-atom transfer

concerted mechanism cannot be ruled out, and this would be in turn consistent with the low

stability of the Ru(III) oxidation state that favors 2e^-versus 1e^-transfer process.²⁸

We have also evaluated the heterogeneous catalytic activity of the SP@3 anchored

aquacomplex in the epoxidation of a diversity of alkenes in dichloromethane and using

PhI(OAc)₂as oxidant (peracetic acid was also tested but the conversion values were low). All

substrates were tested under analogous conditions (catalyst:substrate:oxidant, 1:100:200). No

epoxidation occurred in the absence of catalyst.

A preliminary essay of the catalytic activity was performed using two different samples of the

catalyst SP@3, obtained through strategies 1b and 2 respectively, to check the influence of the

synthetic strategy on the catalytic performance. The substrate tested was styrene and in both

cases the conversion and selectivity values obtained were similar. This result seems to indicate

that the two heterogeneous catalytic materials are similar, in consistency with the UV-vis and

electrochemical characterization of the systems obtained through both strategies (see above).

Then, SP@3 prepared through 1b was used for the evaluation of the catalytic oxidation of

further substrates.

First of all, the reaction time for the epoxidation of different substrates was optimized and the

conversion values registered at different times are collected in Table S5. Almost total

conversion was attained in 6 h reaction for aromatic substrates (styrene and cis--

methylstyrene) whereas for the aliphatic 4-vinylcyclohexene substrate longer reaction times

were needed. Thus, 6 hours was chosen as the reaction time for all the essays. The conversion

and selectivity values obtained for the epoxidation of four different alkenes using SP@3 as

catalyst are gathered in table 3. In most cases, high conversions and moderate selectivity

values are obtained except for styrene which displays a high selectivity value. It is also

remarkable that the percentage of cis epoxide in the epoxidation of cis--methylstyrene (91%) is

still high but the process is not stereospecific in contrast with the quantitative generation of the

cis epoxide in the case of the homogeneous counterpart. This indicates that the silica support

plays a certain role in the catalytic pathway, perhaps inducing a more significant occurrence of a

radical pathway mechanism in parallel to the oxo-transfer mechanism. The support could also

lead to slight modifications of the electronic and/or structural parameters of the active species

that could slower the ring closure step in a putative Ru-coordinated radical intermediate, thus

allowing the cistrans C-C bond rotation to a higher extent.²⁷

16

10.1002/chem.201604463

Chemistry - A European Journal

Table 3. Catalytic epoxidation of selected alkenes by SP@3. Conditions: SP@3 (1 mol Ru),

substrate (100 mol), PhI(OAc)₂(200 mol), CH₂Cl₂(2.5 mL), 25ºC, 6h, biphenyl (100 mol) as

internal standard.

Substrate

Conversion (%)

Selectivity (%)^a

92(58)^b

79(40)^b

94

58^c

95^d

43^d

34(100/0)^e

25

^aSelectivity for epoxide: (Yield/Conversion)x100. ^bIn parentheses, conversion and selectivity values obtained using

MSP@3.^c91% of cis-epoxide was obtained. ^dAnalysis performed after 15h. ^eIn parentheses, ratio [ring epoxide/vinyl

epoxide].

The performance of the heterogeneous catalyst was also tested with aliphatic substrates; with

4-vinylcyclohexene the process was regiospecific towards the ring epoxidation, and with 1-

octene modest performance was obtained in agreement with the lower reactivity towards non-

activated monosubstituted alkenes. The MSP@3 catalyst was also evaluated in the epoxidation

of styrene showing moderate conversion and selectivity values (Table 3). The performance of

our heterogeneous catalysts are comparable to that displayed by other supported Ru

complexes^7a,29though the SP@3 system reaches almost complete conversion for most

substrates in shorter reaction times. The lower performance in the case of aliphatic substrates

such as 1-octene is commonly described also for heterogeneous catalysts based on other

metals,^7calthough in the case of some heterogeneous molybdenum catalysts the conversion

and selectivity values are remarkable in relatively short reaction times.^7b

Based in the above results, we have also investigated the recyclability of the SP@3 and

MSP@3 heterogeneous catalytic systems in CH₂Cl₂. In most cases, the catalyst maintains a

good performance through up to four runs. In the epoxidation of styrene by SP@3, a reaction

time of 6 h was initially set for each run as in the previous essays. However, the conversion

value decreased dramatically after the first run (Figure 8a). Then, a similar experiment (Figure

8b) was performed keeping each of the catalytic runs throughout 15 h and the conversion

values were improved, indicating that the low conversion values obtained in the first case (6h

per run) were in part a consequence of slow kinetics in heterogeneous phase. However, a

gradual decrease in the conversion values was observed throughout successive runs when kept

for either 6 or 15 hours and thus a progressive deactivation of the catalyst is presumably taking

place. We have also considered the possibility of catalyst leaching, which can similarly lead to a

decrease of heterogeneous catalytic activity throughout the successive runs and can also cause

difficulty to unravel whether the activity arises from the heterogeneous or homogeneous

(leached) catalyst. Thus, the amount of Ru complex was measured through ICP-AES in the

reaction solvent for a whole set of reuses in the case of styrene and the rest of substrates tested

(see below). In all cases, the amount of Ru present in the final solution was under the detection

limit of the technique (1.8-2% of the initial Ru anchored). We can then assert that leaching of Ru

17

10.1002/chem.201604463

Chemistry - A European Journal

is negligible for the heterogeneous systems described and then the decrease in activity is

seemingly due to catalyst deactivation. On the other hand, as can be observed in Figure 8b,

selectivity values above 75% were achieved for the styrene oxide product in all runs.

Figure 8. Conversion and selectivity values obtained throughout a number of consecutive reuses of

catalyst SP@3 in the epoxidation of styrene: a) 6 h per run; b) 15 h per run.

a)

b)

The recyclability of the catalytic SP@3 system has been also investigated on other substrates

as cis--methylstyrene, 4-vinylcyclohexene and 1-octene and the results are displayed in

Figures S12-S14. These results evidence that the catalyst maintains good conversion values for

cis--methylstyrene and 4-vinylcyclohexene substrates for 3-4 runs whereas the less activated

1-octene displays much lower activity. The overall turnover numbers are 172 for cis--

18

10.1002/chem.201604463

Chemistry - A European Journal

methylstyrene, 108 for 4-vinylcyclohexene and 12 for 1-octene. The selectivity for the epoxide is

also well maintained except for this latest substrate, with a dramatic drop of selectivity at the

second run and practically null conversion at the fourth run. In the case of cis--methylstyrene

the stereoselectivity for the cis-epoxide is maintained around 80-90% through all the runs For 4-

vinylcycloxexene the epoxidation takes place exclusively at the ring position in all runs.

The reusability of the MSP@3 system has been tested in the epoxidation of styrene and the

results obtained are displayed in Figure 9a, which shows that the overall values of conversion

and selectivity are moderate and are clearly lower than the ones obtained for this substrate

using catalyst SP@3 under the same reaction conditions. The origin of this difference could

arise from the large difference in the surface area values between the two supports (886 m²/g

for SP and only 25 m²/g for MSP, see Table S1), and the amount of ruthenium anchored onto

these supports (1·10^-4mmol/g_silicafor SP@3 and 2,96·10^-4mmol/g_silicafor MSP@3), that would

entail that molecules of anchored catalyst were placed much closer to each other at the MSP

support, due to the higher concentration of complex 3 onto the surface of the MSP when

compared to SP support (4·10^-6mmol/m²for MSP vs 0,33·10^-6mmol/m²for SP). This fact could

hinder the approach of the substrate to the active site or alternatively favor the catalyst self-

deactivation in the MSP@3. However, their magnetic core allows us to attract and remove them

from the reaction medium by a magnet, facilitating the reusability of the material as we can see

in Figure 9b.

19

10.1002/chem.201604463

Chemistry - A European Journal

Figure 9. a) Conversion and selectivity values obtained throughout a number of consecutive reuses of

catalyst MSP@3 in the epoxidation of styrene. b) Photographs of the CH₂Cl₂dispersion of MSP@3

without and with a magnet.

a)

b)

Conclusions

We have synthesized and fully characterized two new complexes, [Ru^IICl(trpy-P-Et)(pypz-

Me)](PF₆), 2, and [Ru^II(trpy-P-Et)(pypz-Me)(OH₂)](PF₆)₂, 3, both containing the trpy-P-Et ligand

which is used as linker to anchor the molecular aqua complex 3 to silica supports. Unlike other

complexes found in the literature, the molecular aqua complex was easily obtained after reflux

of chlorido complex in water, evidencing the influence of electronic factors arising from the

phosphonate group of the electron-withdrawing trpy-P-Et ligand. The aqua complex 3 was

tested in the epoxidation of cis--methylstyrene showing high levels of conversion and

specificity for the cis-epoxide. This behavior is in accordance with the occurrence of a

bielectronic (IV/II) redox process displayed by the catalyst 3 that could allow the occurrence of a

concerted oxo-transfer mechanism. The aqua complex 3 was easily and successfully

immobilized onto two silica supports (including magnetic particles) using two different synthetic

strategies that allowed either i) the anchoring of the trpy-P-Et ligand onto the surface of the

supports and the subsequent preparation of compounds in situ or ii) the immobilization of the

previously synthesized molecular ruthenium complex onto the supports. The characterization of

these hybrid materials corroborated that the aqua ruthenium compound, anchored through the

different strategies, doesn’t modify significantly its coordination and electronic properties with

20

10.1002/chem.201604463

Chemistry - A European Journal

respect to the molecular counterpart. To the best our knowledge, our aqua complex is the first

one anchored onto silica supports that has been isolated and fully characterized. The

heterogeneous SP@3 and MSP@3 systems were effective in the epoxidation of a variety of

olefin substrates being selective in the epoxidation of cis--methylstyrene with high levels of

conversion and stereoselectivity for the cis epoxide. System MSP@3 showed moderate

conversion and selectivity values in the epoxidation of styrene with regard to the values

displayed by the analogous SP@3 catalyst, manifesting a different effect of the SP and MSP

supports on the catalytic performance, probably as consequence of the higher grafting density

of complex on the MSP supports, that could hinder the approach of the substrate to the active

site or alternatively favour the catalyst self-deactivation. However, they can be recovered

through a simple magnetic separation avoiding the centrifugation process.

The reutilization of SP@3 system in the epoxidation of alkenes has been evaluated

demonstrating the effective recyclability of the catalytic system up to 3 or 4 successive runs in

all cases, keeping high conversion and selectivity values in the epoxidation of styrene through

all the runs. Moderate values were obtained for the rest of the alkenes tested. The epoxidation

of cis--methylstyrene is not stereospecific in the heterogeneous catalytic processes but high

steroeselectivity values were obtained that were maintained through the different reuses.

Experimental Section

Materials

All reagents used in the present work and silica mesoporous particles (SP) were obtained from Sigma-

Aldrich and were used without further purification. Reagent grade organic solvents were obtained from

SDS and high purity de-ionized water was obtained by passing distilled water through a nano-pure Mili-Q

water purification system. Diethyl 2,2’:6’,2’’-terpyridine-4’-phosphonate (trpy-P-Et, L1) and diethyl

2,2’:6’,2’’-terpyridine-4’-phosphonic acid (trpy-P-H, L2) ligands were supplied by HetCat and were directly

used without further purification. RuCl₃·2.38H₂O was supplied by Johnson and Matthey Ltd. and was

used as received.

Instrumentation and Measurements

IR spectra were recorded on an ATR MK-II Golden Gate Single Reflection. UV-Vis spectroscopy was

performed on a Cary 50 Scan (Varian) UV-Vis spectrophotometer with 1 cm quartz cells. Cyclic

voltammetric (CV) and Differential Pulse Voltammetry (DPV) experiments were performed in an IJ-

Cambria IH-660 potentiostat using a three electrode cell. Glassy carbon electrode (3 mm diameter) from

BAS was used as working electrode, platinum wire as auxiliary and Ag/AgCl as the reference electrode.

All cyclic voltammograms presented in this work were recorded under nitrogen atmosphere. The

complexes were dissolved in solvents containing the necessary amount of n-Bu₄NH⁺PF₆(TBAH) as

-

supporting electrolyte to yield a 0.1 M ionic strength solution. All E_1/2values reported in this work were

estimated from cyclic voltammetric experiments as the average of the oxidative and reductive peak

potentials (E_pa+E_pc)/2, or directly from DPV. Unless explicitly mentioned the concentration of the

complexes was approximately 1mM. The NMR spectroscopy was performed on a Bruker DPX 300 and

400 MHz. Samples were run in CD₂Cl₂or d₃-acetonitrile with internal references (residual protons and/or

tetramethylsilane). Elemental 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 Finnigan, using acetonitrile as a mobile phase. SEM images were recorded using a SEM QUANTA

FEI 200FEG-ESEM. Gas chromatography experiments were performed by capillary GC, using a GC-2010

Gas Chromatograph from Shimadzu, equipped with an Astec CHIRALDEX G-TA Column (30 m x 0.25

mm diameter) incorporating a FID detector. All the product analyses in the catalytic experiments were

21

10.1002/chem.201604463

Chemistry - A European Journal

performed by means of GC using biphenyl as internal standard. For metal content determination a

sequential inductively coupled plasma atomic emission spectrometer (ICP-AES, Liberty Series II, Varian,

Australia) in radial configuration and a quadrupole-based inductively coupled plasma mass spectrometer

system (ICP-MS, Agilent 7500c, Agilent Technologies, Tokyo, Japan) were used. This latest instrument is

equipped with an octapole collision reaction cell. However, in this work, the collision reaction cell acts only

as an ion focusing lens because it was not filled with any pressurized gas. Prior to measurements,

samples were digested with HCl:HNO₃. Thermogravimetric analysis (TGA) was performed under N2

atmosphere with a 10ºC min-1 heating rate from 30ºC to 700ºC. The BET specific surface area (Sa) was

determined by N2 adsorption-desorption measurements at 77 K using an ASAP 2000 (Micrometrics

Instrument Corporation, USA) after degasification at 120ºC under vacuum for 24h.

Crystallographic Data Collection and Structure Determination

Measurement of the crystals were performed on a Bruker Smart Apex CCD diffractometer using

graphite-monochromated Mo Kα radiation (λ = 0.71073Å) from an X-Ray tube. Data collection, Smart V.

5.631 (BrukerAXS 1997-02); data reduction, Saint+ Version 6.36A (Bruker AXS 2001); absorption

correction, SADABS version 2.10 (Bruker AXS 2001) and structure solution and refinement, structure

solution and refinement, SHELXL-2013 (Sheldrick, 2013). The crystallographic data as well as details of

the structure solution and refinement procedures are reported in supplementary information. CCDC

1505432 contains the supplementary crystallographic data for this paper. These data can be obtained

free of charge from The Cambridge Crystallographic Data Centre via

www.ccdc.cam.ac.uk/products/csd/request/

Preparations

[RuCl₃(trpy-P-Et)] 1 complex^7aand 2-(1-methyl-3-pyrazolyl)pyridine³⁰(pypz-Me, L3) ligand were prepared

according to literature procedures. Maghemite-silica composite nanoparticles (MSP) with mean diameters

of aprox. 2 m and 200 nm, respectively, were synthesized in our laboratory by hydrolysis and

polycondensation of tetramethylorthosilicate (TMOS, 98%, Sigma-Aldrich) and high temperature

supercritical drying, according to the procedures described elsewhere (Sol-gel route to direct formation of

silica aerogel microparticles using supercritical solvents).³¹Supercritical fluid assisted one-pot synthesis

of biocompatible core(-Fe₂O₃)@shell(SiO₂) nanoparticles as high relaxivity T₂-contrast agents for

Magnetic Resonance Imaging.¹⁸

Electrochemical experiments were performed under N₂atmosphere with degassed solvents. All

spectroscopic, electrochemical and synthetic experiments were performed in the absence of light unless

explicitly mentioned.

Synthesis of the complexes

Trans- and cis-[RuCl(trpy-P-Et)(pypz-Me)](PF₆), trans- and cis-2. A sample of 1 (0.15 g, 0.26 mmol)

was added to a 100 mL round bottomed flask containing a solution of LiCl (0.022 g, 0.52 mmol) dissolved

in 40 mL of EtOH/H₂O (3:1) under magnetic stirring. Then, NEt₃(0.06 mL, 0.52 mmol) was added and the

reaction mixture was stirred at room temperature for 30 min. Afterwards, pypz-Me, L3, (0.041 g, 0.26

mmol) was added and the mixture was heated at reflux for 3h. The hot solution was then filtered off in a

frit and the volume was reduced in a rotary evaporator. After addition of a saturated aqueous solution of

NH₄PF₆a precipitate formed which was filtered off and washed with water. The solid obtained in this

manner was a mixture approximately 1:1 of complexes trans-2 and cis-2. Yield: 111 mg (61%). Anal.

Found (Calc.) for 2·H₂O: C, 40.5 (40.6); H, 3.32 (3.77); N, 9.81 (10.1). IR (ν, cm^-1): 3050 (ν (=C-H)), 1598

(ν (C=C)), 1439 (ν (C=N)), 1243 (ν (P=O)), 1015 (ν (P-O-C)), 840-752 (ν (P-C)). E_1/2(CH₂Cl₂+ 0.1M

TBAH) = 0.93 V vs. SCE. UV-Vis (CH₂Cl₂) [λ_max, nm (ε, M^-1cm^-1)]: 282 (28207), 324 (23103), 394 (7658),

512 (8615).

For trans-2, ¹H-NMR (acetone- d₆, 400 MHz): δ 1.42 (t, 3H, H17, J_17-16= 7 Hz), 1.44 (t, 3H, H19, J_19-18= 7

Hz), 3.02 (s, 3H, H28), 4.34 (m, 4H, H16, H18), 7.24 (d, 1H, H26, J_26-27= 2.8 Hz), 7.51 (ddd, 2H, H2, H14,

J_2-1

=

J_14-15= 5.6 Hz; J_2-3

=

J_14-13= 7.5 Hz; J_2-4

=

J_14-12= 1.1 Hz), 7.58 (d, 1H, H27, J_27-26= 2.8 Hz), 7.85 (d,

22

10.1002/chem.201604463

Chemistry - A European Journal

2H, H1, H15, J_1-2

8.06 (ddd, 2H, H3, H13, J_3-2

=

J_15-14= 5.6 Hz), 7.91 (ddd, 1H, H21, J_21-20= 5.6 Hz; J_21-22= 7.2 Hz; J_21-23= 1.3Hz),

J_13-14= 7.7 Hz; J_3-4J_13-12= 5.2 Hz; J_3-1= 1.4 Hz), 8.35 (dt, 1H, H22, J_22-21

=

J_22-23= 7.2 Hz; J_22-20= 1.5 Hz), 8.53 (d, 1H, H23, J_23-22= 7.2 Hz), 8.84 (d, 1H, H4, J_4-3= 5.2 Hz), 8.86 (d,

1H, H12, J_12-13= 5.2 Hz), 8.87 (s, 1H, H7), 8.90 (s, 1H, H9), 10.16 (d, 1H, H20, J_20-21= 5.6 Hz). ¹³C-NMR

(acetone- d₆): δ 16.8 (C17, C19), 38.3 (C28), 63.9 (C16, C18), 105.6 (C26), 123.1 (C23), 124.5 (C4, C7),

124.6 (C9), 125.1 (C12), 125.6 (C21), 128.8 (C2, C14), 137.2 (C27), 137.9 (C3, C13), 138.3 (C8, C22),

153.4 (C1, C15), 153.8 (C24, C25), 153.9 (C20), 159.7 (C5), 160.6 (C11), 160.8 (C6), 161.6 (C10).

1

For cis-2, H-NMR (acetone-d₆, 400 MHz): δ 1.42 (t, 3H, H17, J_17-16= 7.1 Hz), 1.44 (t, 3H, H19, J_19-18

=

7.1 Hz), 4.34 (m, 4H, H16, H18), 4.73 (s, 3H, H28), 6.91 (ddd, 1H, H22, J_22-21= 7.8 Hz; J_22-23= 5.8 Hz; J_22-

₂₀= 1.3 Hz), 7.37 (d, 1H, H23, _23-22= 5.8 Hz), 7.53 (ddd, 2H, H2, H14, J_2-1

8 Hz; J_2-4J_14-12= 1.1 Hz), 7.61 (d, 1H, H26, J_26-27= 2.8 Hz), 7.72 (dt, 1H, H21, J_21-20

J_21-23= 1.3Hz), 8.03 (ddd, 2H, H3, H13, J_3-2J_13-14= 8 Hz; J_3-4J_13-12= 5.2 Hz; J_3-1= 1.4 Hz), 8.10 (d,

2H, H1, H15, J_1-2

=

J_14-15= 6.1 Hz; J_2-3

= J_14-13=

=

J_21-22= 7.8 Hz;

=

J_15-14= 6.1 Hz), 8.17 (d, 1H, H20, J_20-21= 7.8 Hz), 8.47 (d, 1H, H27) J_27-26= 2.8 Hz,

8.81 (s, 1H, H7), 8.82 (d, 1H, H4, J_4-3= 5.2 Hz), 8.83 (d, 1H, H12, J_12-13= 5.2 Hz), 8.85 (s, 1H, H9). ¹³C-

NMR (acetone- d₆): δ 16.8 (C17, C19), 42.0 (C28), 63.9 (C16, C18), 106.2 (C26), 122.6 (C20), 124.4

(C7), 124.5 (C4), 125.0 (C12, C22), 125.1 (C9), 128.8 (C2, C14), 137.0 (C21), 138.0 (C3, C13), 138.1

(C8, C27), 151.2 (C24), 152.7 (C23, C25), 154.3 (C1, C15), 159.5 (C5), 159.5 (C11), 161.4 (C6), 161.5

(C10). For the NMR assignments we use the same labeling scheme as for the X-ray structures (Figure 1).

trans and cis-[Ru(trpy-P-Et)(pypz-Me)(OH₂)](PF₆)₂, trans- and cis-3. A 0.03 g (0.036 mmol) sample of

trans and cis-2 was dissolved in 40 ml of water, the resulting solution was heated at reflux for 18h. After

reduction of the volume in a rotary evaporator a saturated aqueous solution of NH₄PF₆was added. The

precipitate formed was filtered off and washed several times with cold water. The solid obtained in this

manner was a mixture of complexes trans-3 and cis-3. Yield: 16 mg (42%). Anal. Found (Calc.) for

3·1.5(C₂H₅)₂O: C, 39.1 (38.9); H, 4.1 (4.4); N, 8.0 (8.0). IR (ν, cm^-1): 3081 (ν (=C-H)), 2918 (ν (O-H)),

1602 (ν (C=C)), 1441 (ν (C=N)), 1227 (ν (P=O)), 1015 (ν (P-O-C)), 840-753 (ν (P-C)). UV-Vis (CH₂Cl₂)

[λ_max,nm (ε, M^-1cm^-1)]: 278 (45478), 320 (36314), 382 (7540), 480 (10418).

1

For trans-3, H-NMR (acetone-, 400 MHz): δ 1.45 (m, 6H, H17, H19), 3.08 (s, 3H, H28), 4.39 (m, 4H,

H16, H18), 7.28 (d, 1H, H26), 7.65 (m, 3H, H2, H14, H27), 8.03 (ddd, 1H, H21), 8.07 (d, 2H, H1, H15),

8.19 (m, 2H, H3, H13), 8.46 (dt, 1H, H22), 8.64 (d, 1H, H23), 8.96 (m, H4), 8.99 (dd, 1H, H12), 9.03 (s,

1H, H7), 9.06 (s, 1H, H9), 9.68 (d, 1H, H20). E_1/2(IV/II), phosphate buffer pH = 7.12: 0.42 V vs. SCE.

1

For cis-3, H-NMR (acetone- d₆, 400 MHz): δ 1.45 (m, 6H, H17, H19), 4.39 (m, 4H, H16, H18), 4.64 (s,

3H, H28), 6.94 (ddd, 1H, H22), 7.40 (d, 1H, H23), 7.65 (m, 3H, H2, H14, H26), 7.76 (dt, 1H, H21), 8.19

(m, 4H, H1, H3, H13, H15), 8.29 (ddd, 1H, H20), 8.56 (d, 1H, H27), 8.95 (dd, 1H, H4), 8.96 (m, H12), 8.97

(s, 1H, H7), 9.00 (s, 1H, H9). E_1/2(IV/II), phosphate buffer pH = 7.12: 0.52 V vs. SCE.

Preparation of the heterogeneous systems

Two synthetic strategies have been followed:

Strategy I

I.1) Preparation of ligand-functionalized silica

.

For ligand L1: a sample of MSP (0.1 g) was added to a solution of trpy-P-Et, L1, (0.05 g, 0.14 mmol) in

toluene (10 ml) and the suspension was refluxed overnight. The resulting support, (trpy-P)-modified silica,

was centrifuged, washed with acetone (2 x 10 ml) and dried in a hot air oven at 110°.

For ligand L2: A sample of SP or MSP (0.1 g) was added to a solution of trpy-P-H, L2, (0.05 g, 0.16

mmol) in water (10 ml). The suspension was stirred overnight. The resulting support, (trpy-P)-modified

silica, was centrifuged, washed with water (2 x 10 ml) and dried in a hot air oven at 110°.

I.2) Preparation of complex-functionalized silica.

A 0.1 g sample of organically modified silica (SP@(trpy-P) or MSP@(trpy-P) was added to a solution of

RuCl₃·2.38H₂O (0.040 g, 0.16 mmol) in methanol. The mixture was stirred overnight. The resulting solid

(SP@[RuCl₃(trpy-P)] or MSP@[RuCl₃(trpy-P)]) was centrifuged, washed with methanol (2 x 10 ml) and

dried in a hot air oven at 110°C.

A 0.1 g sample of ruthenium-functionalized silica (SP@[RuCl₃(trpy-P)] or MSP@[RuCl₃(trpy-P)]) was

placed in a round-bottomed flask together with 6 ml of methanol. Then, LiCl (0.014 g, 0.32 mmol) and

23

10.1002/chem.201604463

Chemistry - A European Journal

triethylamine (44.6 μl, 0.32 mmol) were added under nitrogen atmosphere and the mixture was stirred at

room temperature for 30 min. After this time, pypz-Me (L3) ligand (0.026 g, 0.16 mmol) dissolved in 4 ml

of methanol was added and the mixture was heated at reflux for 2 h. The resulting modified silica

(SP@[Ru(trpy-P)(pypz-Me)(OH₂)]²⁺, SP@3, or MSP@[Ru(trpy-P)(pypz-Me)(OH₂)]²⁺, MSP@3) was

centrifuged, washed with methanol (2 x 10 ml) and dried in a hot air oven at 110°C. For SP@3: E_1/2(IV/II),

phosphate buffer pH = 5.5: 0.56 V vs SCE. UV-Vis (CH₂C_l2) [λ_max, nm]: 276, 316, 378, 484. For MSP@3:

E_1/2(IV/II), phosphate buffer pH = 5.5: 0.60 V vs SCE.

Strategy II

SP (0.1 g) was added to a solution of 2 (0.05 g, 0.06 mmol) in 9 ml of toluene. The mixture was stirred

and heated at reflux for 4 h. afterwards, 1 ml of acetone was added and the solution was refluxed for 20h.

The resulting product was centrifuged, washed with acetone (2 x 10 ml) and dried in a hot air oven at

110°C. UV-Vis (CH₂Cl₂) [λ_max, nm]: 280, 328, 386, 490. E_1/2(IV/II), phosphate buffer pH = 7.1: 0.55 V vs.

SCE.

Catalytic studies

Alkene epoxidation

Homogeneous System

The ruthenium catalyst (1.25 μmol), alkene (125 μmol) and PhI(OAc)₂(250 μmol) were stirred at room

temperature in CH₂Cl₂(2.5 ml) for 24 h. After addition of an internal standard, the sample was filtered

through a basic alumina plug and quantified by gas chromatographic (GC) analysis based on calibration

curves for each substrate and epoxide.

Heterogeneous System

Alkene (100 μmol) and PhI(OAc)₂(200 μmol) were dissolved in 2.5 ml of CH₂Cl₂together with 1 μmol of

SP or MSP-supported ruthenium catalyst. The amount of heterogenized catalyst was calculated taking

into account the functionalization of SPs and MSPs (mmol Ru·g^-1) in each case. For experiments using

SP-based system, the resulting solution was centrifuged while for MSP-based system a magnet was used

for the separation of the catalytic material from the reaction medium. The catalytic heterogeneous

systems were washed twice with CH₂Cl₂and were used in a subsequent catalytic run. For leaching

calculations, the resulting solution after several reuses was centrifuged and filtered through celite in order

to avoid particles in the ICP analysis.

Acknowledgements

This research has been financed by AGAUR (Generalitat de Catalunya, projects 2014-SGR-149 and

2014-SGR-213), by MINECO (project CTQ2015-66143-P) and by Universitat de Girona (project

MPCUdG2016/048). IF thanks UdG for a predoctoral grant. Dr. M. Iglesias from the Chemistry

Department of UdG is gratefully acknowledged for her help with the ICP quantification measurements.

24

10.1002/chem.201604463

Chemistry - A European Journal

References

[1] a) S. Y. Ko, A. W. M. Lee, S. Masamune, L. A. Reed, K. B. Sharpless, F. J. Walker, Science 1983, 220, 949–951; b)

K. C. Nicolaou, N. Winssinger, J. Pastor, S. Ninkovic, F. Sarabia, Y. He, D. Vourloumis, Z. Yang, T. Li, P.

Giannakakou, E. Hamel, Nature 1997, 387, 268–272; c) S. D. Gagnon in Encyclopedia of Polymer Science,

Engineering, 2nd ed. (Eds: H. F. Mark, N. M. Bikales, C. G. Overberger, G. Menges, J. I. Kroschwitz), Wiley-VCH:

Weinheim, 1994, pp. 275–307; d) D. J. Darensbourg, R. M. Mackiewicz, A. L. Phelps, D. R. Billodeaux, Acc.

Chem. Res. 2004, 37, 836–844; e) E. N. Jacobsen, Catalytic Asymmetric Synthesis (Ed: I. Ojima), VCH: New York,

1993, p. 229; f) G. De Faveri, G. Ilyashenko, M. Watkinson, Chem. Soc. Rev. 2011, 40, 1722–1760.

[2] a) I. Serrano, M. I. López, Í. Ferrer, A. Poater, T. Parella, X. Fontrodona, M. Solà, A. Llobet, M. Rodríguez, I.

Romero, Inorg. Chem. 2011, 50, 6044–6054; b) E. Masllorens, M. Rodríguez, I. Romero, A. Roglans, T. Parella, J.

Benet-Buchholz, M. Poyatos, A. Llobet, J. Am. Chem. Soc. 2006, 128, 5306–5307; c) M. Dakkach, X. Fontrodona,

T. Parella, A. Atlamsani, I. Romero, M. Rodríguez, Adv. Synth. Catal. 2011, 353, 231–238; d) J. Mola, C. Dinoi, X.

Sala, M. Rodríguez, I. Romero, T. Parella, X. Fontrodona, A. Llobet, Dalton Trans. 2011, 40, 3640–3646; e) M.

Dakkach, A. Atlamsani, T. Parella, X. Fontrodona, I. Romero, M. Rodríguez, Inorg. Chem. 2013, 52, 5077–5087.

[3] a) A. S. Hazari, A. Das, R. Ray, H. Agarwala, S. Maji, S. M. Mobin, G. K. Lahiri, Inorg. Chem. 2015, 54, 4998–

5012; b) J. R. Bryant, T. Matsuo, J. M. Mayer, Inorg. Chem. 2004, 43, 1587–1592; c) X. Sala, A. Poater, I. Romero,

M. Rodríguez, A. Llobet, X. Solans, T. Parella, T. M. Santos, Eur. J. Inorg. Chem. 2004, 2004, 612–618; d) W.-P.

Yip, W.-Y. Yu, N. Zhu, C.-M. Che, J. Am. Chem. Soc. 2005, 127, 14239–14249; e) R. Zong, R. P. Thummel, J. Am.

Chem. Soc. 2005, 127, 12802–12803.[

[4] a) C. D. Nunes, A. A. Valente, M. Pillinger, A. C. Fernandes, C. C. Romão, J. Rocha, I. S. Gonçalves, J. Mater.

Chem. 2002, 12, 1735–1742; b) M. Jia, A. Seifert, W. R. Thiel, Chem. Mater. 2003, 15, 2174–2180; c) A. Sakthivel,

J. Zhao, G. Raudaschl-Sieber, M. Hanzlik, A. S. T. Chiang, F. E. Kühn, Appl. Catal. A Gen. 2005, 281, 267–273.

[5] a) L. Yin, J. Liebscher, Chem. Rev. 2007, 107, 133–173.

[6] a) M. Shokouhimehr, Y. Piao, J. Kim, Y. Jang, T. Hyeon, Angew. Chemie 2007, 119, 7169–7173; b) M.

Shokouhimehr, Y. Piao, J. Kim, Y. Jang, T. Hyeon, Angew. Chem. Int. Ed. Engl. 2007, 46, 7039–7043; c) E. Ertem,

N. Murillo-Cremaes, R. P. Carney, A. Laromaine, E.-R. Janeček, A. Roig, F. Stellacci, Chem. Commun. 2016, 52,

5573–5576; d) S. Shylesh, V. Schünemann, W. Thiel, Angew. Chem. Int. Ed. 2010, 49, 3428 –3459; e) S. Shylesh,

L. Wang, W. R. Thiel, Adv. Synth. Catal. 2010, 352, 425- 432; f) K. S. M. Salih, P. Mamone, G. Dörr, T. O. Bauer,

A. Brodyanski, C. Wagner, M. Kopnarski, R. N. Klupp-Taylor, S. Demeshko, F. Meyer, V. Schünemann, S. Ernst,

L. J. Gooßen, W. R. Thiel, Chem. Mater. 2013, 25, 1430-1435; g) D. Dehe, L. Wang, M. K. Müller, G. Dörr, Z.

Zhou, R. N. Klupp-Taylor, Y. Sun, S. Ernst, M. Hartmann, M. Bauer, W. R. Thiel, ChemCatChem 2015, 7, 127-

136.

[7] a) L. Vaquer, P. Riente, X. Sala, S. Jansat, J. Benet-Buchholz, A. Llobet, M. A. Pericàs, Catal. Sci. Technol. 2013,

3, 706–714; b) S. Shylesh, J. Schweizer, S. Demeshko, V. Schünemann, S. Ernst, W. R. Thiel, Adv. Synth. Catal.

2009, 351, 1789–1795; c) S. Shylesh, M. Jia, W. R. Thiel, Eur. J. Inorg. Chem. 2010, 4395-4410.

[8] M. Dakkach, M. I. López, I. Romero, M. Rodríguez, A. Atlamsani, T. Parella, X. Fontrodona, A. Llobet, Inorg.

Chem. 2010, 49, 7072–7079.

[9] A. Rilak, I. Bratsos, E. Zangrando, J. Kljun, I. Turel, Ž. D. Bugarčić, E. Alessio, E. Inorg. Chem. 2014, 53, 6113–

6126.

[10] C. Sens, M. Rodríguez, I. Romero, A. Llobet, T. Parella, J. Benet-Buchholz. Inorg. Chem. 2003, 42, 8385–8394.

[11] V. Balzani, A. Juris, M. Venturi, S. Campagna, S. Serroni, Chem. Rev. 1996, 96, 759–834.

[12] E. T. Bell-Loncella, C. A. Bessel, Inorganica Chim. Acta 2000, 303, 199–205.

[13] R. Llanguri, J. J. Morris, W. C. Stanley, E. T. Bell-Loncella, M. Turner, W. J. Boyko, C. A. Bessel, Inorganica

Chim. Acta 2001, 315, 53–65.

[14] P. Passaniti, W. R. Browne, F. C. Lynch, D. Hughes, M. Nieuwenhuyzen, P. James, M. Maestri, J. G. Vos, J.

Chem. Soc. Dalton Trans. 2002, 1740.

[15] J. A. Gilbert, S. W. Gersten, T. J. Meyer, J. Am. Chem. Soc. 1982, 104, 6872–6873.

[16] R. A. Leising, K. J. Takeuchi, Inorg. Chem. 1987, 26, 4391–4393.

[17] A. B. P. Lever, Inorg. Chem. 1990, 29, 1271–1285.

[18] E. Taboada, R. Solanas, E. Rodríguez, R. Weissleder, A. Roig, Adv. Funct. Mater. 2009, 19, 2319–2324

[19] W. Mägerlein, C. Dreisbach, H. Hugl, M. K. Tse, M. Klawonn, S. Bhor, M. Beller, Catal. Today 2007, 121, 140–

150.

25

10.1002/chem.201604463

Chemistry - A European Journal

[20] M. K. Tse, S. Bhor, M. Klawonn, G. Anilkumar, H. Jiao, A. Spannenberg, C. Döbler, W. Mägerlein, H. Hugl, M.

Beller, Chem. Eur. J. 2006, 12, 1875–1888.

[21] J. Benet-Buchholz, P. Comba, A. Llobet, S. Roeser, P. Vadivelu, H. Wadepohl, S. Wiesner, Dalton Trans. 2009,

5910–5923.[

[22] G. A. Barf, R. A. Sheldon, J. Mol. Catal. A Chem. 1995, 102, 23–39.

[23] A. D. Chowdhury, A. Das, I. K, S. M. Mobin, G. K. Lahiri, Inorg. Chem. 2011, 50, 1775–1785.

[24] M. K. Tse, C. Döbler, S. Bhor, M. Klawonn, W. Mägerlein, H. Hugl, M. Beller, Angew. Chem. Int. Ed. Engl. 2004,

43, 5255–5260.

[25] a) W. Adam, K. J. Roschmann, C. R. Saha-Möller, D. Seebach, J. Am. Chem. Soc. 2002, 124, 5068–5073; b) S.-E.

Park, W. J. Song, Y. O. Ryu, M. H. Lim, R. Song, K. M. Kim, W. Nam, J. Inorg. Biochem. 2005, 99, 424–431.

[26] E. Manrique, A. Poater, X. Fontrodona, M. Solà, M. Rodríguez, I. Romero, Dalton Trans. 2015, 44, 17529–17543.

[27] M. Palucki, N. S. Finney, P. J. Pospisil, M. L. Güler, T. Ishida, E. N. Jacobsen, J. Am. Chem. Soc. 1998, 120, 948–

954.

[28] X.-Q. Yu, J.-S. Huang, W.-Y. Yu, C.-M. Che, J. Am. Chem. Soc. 2000, 122, 5337–5342.

[29] a) M. Dakkach, X. Fontrodona, T. Parella, A. Atlamsani, I. Romero, M. Rodríguez, Dalton Trans. 2014, 43, 9916-

9923; b) M. Tada, S. Muratsugu, M. Kinoshita, T. Sasaki, Y. Iwasawa, J. Am. Chem. Soc. 2010, 132, 713-724; c) M.

Tada, R. Coquet, J. Yoshida, M. Kinoshita, Y. Iwasawa, Angew. Chem. Int. Ed. 2007, 46, 7720-7723; d) X-Q. Yu, J-

S. Huang, W-Y- Yu, C-M. Che, J. Am. Chem. Soc. 2000, 122, 5537-5342.

[30] W. R. Thiel, M. Angstl, T. Priermeier, T. Chem. Ber. 1994, 127, 2373–2379.

[31] M. Moner-Girona, A. Roig, E. Molins, J. Llibre, J. Sol-Gel Sci. Technol. 2003, 26, 645–649.

26

10.1002/chem.201604463

Chemistry - A European Journal

FULL PAPER

A molecular ruthenium aqua complex

has been anchored onto silica

mesoporous and silica coated

magnetic particles, through two

synthetic strategies without a

Author(s), Corresponding Author(s)*

Page No. – Page No.

Title

significant modification of the

coordination and electronic properties

with respect to the molecular

counterpart. The catalytic results and

the reutilization of these hybrid

materials highlight their performance

in the epoxidation of alkenes.

27

Article Doi

A Recoverable Ruthenium Aqua Complex Supported on Silica Particles: An Efficient Epoxidation Catalyst

DOI: 10.1002/chem.201604463

Source and publish data:

Authors:

Article abstract of DOI:10.1002/chem.201604463

Full text of DOI:10.1002/chem.201604463

Products guided by the article

R&D Labs maybe for 106-86-5

Relevant to this article

Hot Product