A heterometallic (Fe6Na8) cage-like silsesquioxane: Synthesis, structure, spin glass behavior and high catalytic activity

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A Heterometallic (Fe Na ) Cage-like Silsesquioxane: Synthesis,

6

8

Structure, Spin Glass Behavior and High Catalytic Activity

Received 00th January 20xx,

Accepted 00th January 20xx

ab

a

ac

Alexey N. Bilyachenko,* Mikhail M. Levitsky, Alexey I. Yalymov, Alexander A. Korlyukov, Anna

a

de

a

f

V. Vologzhanina, Yuriy N. Kozlov, Lidia S. Shul’pina, Dmytro S. Nesterov, Armando J. L.

Pombeiro, Frédéric Lamaty,* Xavier Bantreil, Amandine Fetre, Diyang Liu, Jean Martinez,

DOI: 10.1039/x0xx00000x

f

g

h

i

www.rsc.org/

Jérôme Long, Joulia Larionova, Yannick Guari, Alexander L. Trigub, Yan V. Zubavichus, Igor E.

a

de

Golub, Oleg A. Filippov, Elena S. Shubina, and Georgiy B. Shul’pin*

6 8 7 8

The exotic “Asian Lantern” heterometallic cage silsesquioxane [(PhSiO1.5)20(FeO1.5) (NaO0.5) (n-BuOH)9.6(C H )] (I) was

obtained and characterized by X-ray diffraction, EXAFS, topological analyses and DFT calculation. The magnetic properties

investigations revealed that it shows an unusual spin glass-like behavior induced by a particular triangular arrangement of

Fe(III) ions. Cyclohexane and other alkanes as well as benzene can be oxidized to the corresponding alkyl hydroperoxides

and phenol, respectively, by hydrogen peroxide in air in the presence of catalytic amounts of complex I and nitric acid. The

1

6

O

18

O

I-catalyzed reaction of cyclohexane, c-C

6

H

12, with H

2

in an atmosphere of

2

gave a mixture of labeled and non-

1

6

16

18

labeled cyclohexyl hydroperoxides, c-C H11- O- OH and c-C H11- O- OH, respectively, with O incorporation level of ca.

6

1

2 %. Compound I also revealed high efficiency in the oxidative amidation of alcohols into amides: in the presence of

-1

complex I, only 500 ppm of iron allowed to reach TON and TOF values of 1660 and 92 h .

1

b,

4

5

widely applied for nanoparticle assembling process,

2

g,

Introduction

including several cases of chiral compound aggregation.

Also, some data are available concerning gas adsorption of

6

such clusters. Usually, the design of these architectures is

Iron-oxo compounds attract significant attention from

scientific teams worldwide due to their ability to form unique

1

high clusters (up to 168 iron atoms). Such complexes are

performed by using a self–assembly reaction through the

assistance of different types of ligands including oxygen-

and/or nitrogen based ones, e.g. amino-alcohol or alkoxide

relevant to the field of contemporary molecular magnetism as

2

Single Molecule Magnets (SMMs), or spin glasses exhibiting a

3

slow dynamic of the magnetisation. Additionally, they are

1

d, 7

8

1e, 9

containing ligands,

oximes or carboxylates.

In this line

of thought, it is reasonable to suggest that siloxane ligands,

being known for their structural flexibility and readiness to

1

0

a.

complexation to metal ions, may be regarded as a versatile

tool for the design and synthesis of desired iron-containing

compound. Indeed, several examples of such complexes have

Nesmeyanov Institute of Organoelement Compounds, Russian Academy of

Sciences, Vavilov Str., 28, 119991 Moscow, Russia. E-mail:

bilyachenko@ineos.ac.ru

People's Friendship University of Russia, Miklukho-Maklay Str., 6, 117198

b.

1

already been reported.

Moscow, Russia.

Pirogov Russian National Research Medical University, Ostrovitianov str., 1,

c.

In this article we present an easy approach to the synthesis of

the heteronuclear sodium, iron-oxo cluster in the

1

17997 Moscow, Russia

d.

e.

f.

Semenov Institute of Chemical Physics, Russian Academy of Sciences, ul. Kosygina,

dom 4, 119991 Moscow, Russia. E-mail: shulpin@chph.ras.ru

Plekhanov Russian University of Economics, Stremyannyi pereulok, dom 36,

n

silsesquioxane ([RSiO1,5] ) framework (compound I). A detailed

discussion of its structural features is provided. This compound

also presents unusual magnetic properties, caused by a

particular triangular arrangement of Fe(III) ions in the crystal

structure that induced an appearance of a short range

magnetic ordering. A special emphasis is given on the study of

the catalytic properties of this compound. Note that transition

1

17997 Moscow, Russia

Centro de Química Estrutural, Instituto Superior Técnico, Universidade de Lisboa,

Avenida Rovisco Pais, 1049-001 Lisboa, Portugal

g.

Institut des Biomolécules Max Mousseron (IBMM), UMR 5247, CNRS, Université

de Montpellier, ENSCM, Site Triolet, Place Eugène Bataillon, 34095 Montpellier

cedex 5, France. E-mail: frederic.lamaty@univ-montp2.fr

ICGM - UMR5253- Equipe IMNO, Université de Montpellier, Place Eugène

Bataillon - CC 1502 34095 Montpellier CEDEX 5, France

National Research Center ‘‘Kurchatov Institute’’, pl. Akad. Kurchatova, dom 1,

h.

i.

1

2

13

metal complexes, and in particular, iron-based compounds,

1

23098 Moscow, Russia

are powerful catalysts in the oxidation of saturated and

aromatic hydrocarbons with peroxides. Oxygen-activating

Electronic Supplementary Information (ESI) available: [details of any

supplementary information available should be included here]. See proteins and especially enzymes containing polynuclear iron

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sites attract a great deal of interest. For example, some molecules along with the siloxanolate species, and a risk of the

enzymes with di-iron sites are capable of activating C–H metal chlorides hydrolysis cannot be excluded (Scheme 2). As a

1

4

bonds. Methane monooxygenase (MMO) converts alkanes result, an assembling of a target metallasilsesquioxane in a

into the corresponding alcohols. The crucial step of the alkane high yield is not probable.

conversion in the active site containing a few bridges can be

attributed to hydrogen atom abstraction from an alkane

molecule by a radical-like polynuclear iron(IV) and iron(III)

1

species. It is noteworthy that soluble MMO from M.

5

1

6

capsulatus (Bath) oxidizes regioselectively normal pentane,

hexane, and heptane to afford predominately (in the case of n-

1

7

heptane even exclusively) 2-alcohols.

Polynuclear iron

complexes containing chelating ligands can serve as models for Figure 1. Simplified mechanism of metallasilsesquioxane formation: purpose reaction

1

8,19

and undesirable hydrolysis

hydroxylating enzyme centers.

Usually polydentate

nitrogen-containing ligands have been used in these

complexes, and either molecular oxygen or hydrogen peroxide

have been employed as oxidant but the efficiency of these

Taking in mind this reasoning, we performed the synthesis of

the desired iron containing silsesquioxane compound adding

no water to the reaction mixture (Scheme 1). We assumed

that amount of water which present in the nonabsolute

alcohol would be enough to provide the formation of essential

cyclic siloxanolate synthones, with a negligible extent of

systems is rarely high. Compound

I containing a polynuclear

iron core with chelating oxo-ligands is active in the alkane

oxygenation and can be considered as a model of oxidizing

enzymes, an “inorganic oxygenase”. This work is in direct line

with previous reports, since some of us demonstrated that

polynuclear copper(II) silsesquioxanes catalyze oxidation of

3

hydrolysis of FeCl . We have implemented the interaction of

phenyltriethoxysilane with sodium hydroxide in ethanol

followed by the addition of dry iron(III) chloride. Finally, just

before the filtration, some amount of n-butanol and toluene

was added to the reaction mixture to precipitate the NaCl side

product. The resulting good yield of the crystallized product

2

0

21

2 2

alcohols, benzene and alkanes with H O and tert-BuOOH.

Results and discussion

(~60%) confirms the correctness of the suggested approach.

Synthesis of compound I.

The simple synthesis of

siloxanolate [PhSi(O)ONa]

phenyltriethoxysilane) and anhydrous FeCl

I involves an interaction of sodium

Structure of Compound I.

Compound represents an unusual Asian Lantern-like type of

n

(formed

in

situ

from

I

3

in the ethanol

molecular geometry (Figure 2, left), where silsesquioxane

skeleton is formed by the following types of fragments – two

solution according to Scheme 1. The reaction was followed by

the crystallization of filtrate, which allowed to isolate the

2 5

cyclic [PhSiO ] and one acyclic [(PhSi)10O21] ligands (Figure 2,

single crystals of the cage-like Fe(III),Na-silsesquioxane I (in the

right).

yield of 60%). X-ray diffraction analysis was applied to establish

the crystal structure of this compound.

Scheme 1. Synthesis of the Fe(III),Na phenylsilsesquioxane I

Noteworthy

that

a

classical

approach

to

a

metallasilsesquioxanes design through the use of alkoxysilanes

implies a stage of silane partial hydrolysis in order to convert

the functional SiOR groups into the reactive Si-OH fragments.

A consequent condensation of the silanol groups leads to the

in situ formation of the siloxanolate ligands. Then, these

compounds were used for an aggregation of the

metallasilsesquioxanes by the interaction with the metal

Figure 2. Structure of I in crystal (left). Structure of silsesquioxane ligands in I (right).

These ligands coordinate in the complex way six iron(III) ions

15−

or, more exactly, two trinuclear units [Fe

3

O

12

]

(Figure 3).

Structure of latter deserves an additional discussion. Such

trinuclear unit is closely related to the widely employed

2

chlorides. Unfortunately, a condensation of the silanol

groups of RSi(OH) ONa incites a formation of a water

2

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III

Fe

3

(μ

3

-O) clusters. For instance, we can cite the family of Figure 4. Fourier transform of Fe K-edge EXAFS spectrum for I: experiment (black line)

III

23

3 3 2 6 3 2

(μ -O)(μ-O CR) L ], where L = H O, MeOH, Py were used

and best fit (red line)

[

Fe

for construction of discrete complexes, L = 1,5-dicyanamine

2

4

2

was used for the assembly of 1-periodic chains or L = O CR,

2

-

MoO

lattices.

4

were employed for the synthesis of 2-periodic

, 25

One can propose that substitution of bidentate-

9

bridge L ligands with a monodentate bridge of polydentate

cluster anions should cause distortion of the (μ -oxo)-tri-

3

iron(III) cluster. Several structures containing trinuclear iron-

oxo units as a part of the structure have already been

2

reported. Six compounds (including I), matching the search

6

requirements, are listed and discussed in Table S1, ESI.

Figure 5. Experimental Fe K-edge XANES spectra for sample I. Pre-edge region is shown

in the inset

The XANES spectra (Figure 5) were used to investigate the

symmetry of the metal sites and the oxidation state of metal

ion, which is of crucial significance for discussion of the

magnetic properties of the compound (see below). Energy

scale was calibrated using Fe foil which spectrum was

measured simultaneously with the sample. According to a

XANES study of a row of minerals containing iron made by

Figure 3. The triangular iron-oxo fragment similar for the six structures

In turn, sodium ions of compound I are mostly located outside

[28]

Wilke et al., a pre-edge peak of Fe K-edge XANES spectra

can be used to investigate oxidation state of iron atoms. For

the cage skeleton but one of them, namely Na1, is

encapsulated by inner void of the complex. To our knowledge,

3

+

Fe samples the energy position of this peak is in the range

between 7113 and 7116 eV. The pre-edge peak position

this

the

only

known

example

of

cage-like

metallasilsesquioxane, encapsulating a positively charged

moiety. While instances of neutral (water, pyrazine) and

3

+

(~7115 eV) for sample I points that iron atom is in the Fe

-

negatively charged (OH , Cl ) species “capture” are well known

-

oxidation state.

2

7

in the chemistry of cage metallasiloxanes, the cage

a new style of encapsulation (Figure S3, ESI).

I

presents

Magnetic Properties

The magnetic properties of

magnetometer working in the temperature range 1.8 – 350 K

up to 7 T (Figure 6). The room temperature value of T is equal

to 30.70 cm ·K·mol . This value is higher than the theoretical

I were investigated using a SQUID

XAS investigations

χ

Parameters of local surrounding of iron atoms in

I

retrieved

3

‒1

from Fe K-edge EXAFS are fully consistent with X-ray (Figure 4).

First shell of oxygen atoms for nonequivalent iron atoms were

fitted by the three distances 1.94 Å (1.89-1.93 Å), 2.09 Å (1.98-

3

‒1

value of 26.25 cm ·K·mol expected for six non-interacting

Fe(III) ions (S = 5/2 and g = 2.00). This discrepancy most likely

originates from the occurrence of ferromagnetic interactions

between the spin carriers that are still operative at room

temperature. This scenario is confirmed by the temperature

2.04 Å) and 2.26 Å (2.14-2.19 Å). Values in parentheses

correspond to distance ranges from X-ray. The comparison of

the radii for other atomic shells is presented in Table S2, ESI.

dependence of the χT product, which reveals an increase up to

a maximum for a temperature of 145 K before decreasing to

3

‒1

reach the value of 8.72 cm ·K·mol at 1.8 K. Consequently, the

increase of can be ascribed to the presence of

χ

T

ferromagnetic interactions between three Fe(III) ions of a

triangle unit shown in Figure 7, while the decrease may be

inputted to antiferromagnetic interactions between the two

triangles constituting the cluster (see Figure 2) and/or the

occurrence of zero-field splitting. The field dependence of the

magnetization performed at 1.8 K yields to a value of 9.81

ꢀ

B

at 7 T without evidence of a saturation indicating the presence

of a magnetic anisotropy.

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time

τ

m

. For sample

I, the Arrhenius law fitting of the blocking

temperature obtained from the

observation times =1/2πν (with

oscillating field) gives the Ueff and the

K and 7.55×10-13 s, respectively. The obtained

slightly lower than those expected for pure superparamagnetic

χ

" maxima for different

being the frequency of the

values equal to 164.49

value is

3

2

1

5

0

5

0

5

0

5

0

τ

ν

10

9

8

7

6

5

4

3

2

1

0

τ

0

τ

0

-8

-12

30

systems (10 –10 s). This is confirmed by the Mydosh

parameter value of = 0.090 ( log

log min)) which indicates the amplitude of the out-of-phase

M

ϕ

= (Tmax – Tmin)/(Tmax

×

ν -

max

ν

T

χ

peaks’ shift with frequency. This value is slightly lower than the

0

20000

40000

60000

H

/ Oe

value observed for classical superparamagnets (> 0.1).

0

50

100

150

200

250

300

T

/ K

Figure 6. Temperature dependence of χT under a 1000 Oe DC field performed for

sample I. Inset: Field dependence of the magnetisation at 1.8 K.

Figure 8. Temperature dependence of the in-phase

susceptibilities

(χ') and out-of-phase (χ")

Secondly, the short range magnetic ordering may also be

induced by a spin-glass like behavior, caused by the presence

3

1

of strong dipolar interactions between the magnetic clusters.

However, since the later are largely separated in the crystal by

bulky ligands (the shortest distance between iron ions in

neighboring cages is 14.54 Ǻ), this phenomenon may be not

dominant and then ruled out.

Figure 7. Representation of triangular Fe units for I

At 1.8 K, a clear opening of the hysteresis loop can be

observed with a small coercive field of 40 Oe (Figure S4, ESI)

suggesting the presence of a blocking of the magnetization at

low temperature. In order to study in detail the origin of this

phenomenon, alternative currents (AC) measurements were

performed. The temperature dependence of the AC

Thirdly, the observed dynamic behavior may be attributed to

the intrinsic spin-glass behavior relative to a single cluster,

which may be caused by intrinsic volume or surface spin

frustration. In order to confirm this, the temperature

dependence of the AC susceptibilities (at the frequency of 100

Hz) was measured as a function of an applied DC magnetic

susceptibility, its in-phase

components, measured in zero DC magnetic field in the 1 to

500 Hz frequency range shows a series of frequency

dependent peaks (Figure 8). At 1 Hz, the maximum of '' is

(χ') and out-of-phase (χ'')

field (Figure 10). Upon increasing DC field, the peaks of χ"

component shift to lower temperatures and completely

1

vanished for the fields larger than 600 Oe, highlighting a

χ

disappearance of a short-range magnetic ordering. The

2/3

observed at 6.4 K; it progressively shifts to higher

temperature maximum of χ" decreases linearly with H which

corresponds to the Almeida-Thouless line, confirming an

temperatures as the frequency increases with an increase in its

intensity. Correspondingly, the maximum of the

χ

' peak shifts

32

evidence of a spin glass-like behaviour. An extrapolation at H

to higher temperature with frequency, but the peak’s intensity

decreases. Such behavior is frequently interpreted as a

signature of a short range magnetic ordering arising from the

slow dynamic of the magnetization.

=

0 Oe gives the freezing temperature at 9.6 K.

Such spin-glass behavior may be explained by a particular

arrangement of six Fe(III) ions in the crystal structure of

I.

Indeed, they are situated in the corner of two

crystallographically independent equilateral triangles (Figure

In general, the slow dynamic of individual clusters is usually

assigned to the presence of a single large magnetic moment. In

this respect, three magnetic behaviors may be identified,

based on the type and the magnitude of inter- or intra-cluster

interactions. Firstly, magnetically non-interacting molecules

may be described as non-interacting magnetic moments that

7

) with the shortest iron - iron distance ranging from 3.138 to

.392 A. The triangular topology is usually considered as an

3

archetype of the geometrically frustrated spins, which may

induce an appearance of a spin glass-like behavior of isolated

molecules as we previously observed in triangular Co-based

2

9

give rise to an appearance of the superparamagnetism. In

this case, the relaxation time, , obeys an Arrhenius law,

exp(Ueff/k T) (where Ueff is the average energy barrier for the

reversal of the magnetization, is the attempt time and k is

the Boltzmann constant) and becomes equal to the measuring

10h

metallasilsesquioxane system. Note that this phenomenon

τ

τ =

has also been observed in small nanoparticles with surface or

33

volume spin frustration, or for geometrically frustrated

34

garnet-type structures.

τ

0

B

τ

0

B

4

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OOH

OH

PPh

3

0.15

0.10

0.05

0.00

10

9

0

2

5

1

2

4

6

Oe

5 Oe

0 Oe

00 Oe

8

2 2 2 2

O and O O )

(¹⁸

7

oxidant: H

preꢀcatalyst: compound I

coꢀcatalyst: nitric acid (HNO₃)

6

x

a

5

m

1000 Oe

T

4

χ

3

OH

2

5

10

15

20

25

0

10

20

2/3

/ Oe

30

40

T

/ K

H

Scheme 2. Oxidation of cyclohexane or benzene catalyzed by complex I

Figure 9. Left: Temperature dependence of out-of-phase susceptibilities (

χ

") for various

" as a

DC magnetic fields. Right: Field dependence of the temperature maximum of

function of field (Almeida-Thouless line).

χ

We studied kinetics of the cyclohexane oxidation. Products of

the hydrocarbon oxidation were quantified by GC. If the direct

injection of a reaction sample (of the alkane oxidation with O₂

Theoretical (DFT) Calculations.

Full geometry optimization of Na

structure of

with removed solvent shell, I _QM) was provided amounts of cyclohexanol and cyclohexanone, the reduction of

with Gaussian09 program package using the two-layer the sample with PPh

8 6 120 100

Fe Si20C H O

2 2

43 (X-ray or/and H O ) into the chromatograph gave comparable

I

3

5

3

prior to GC analysis (for this method

3

6

40

ONIOM approach. High layer includes Si–O–Fe framework developed earlier by one of us, see Refs. ) led to the

with Na atoms, and was described by unrestricted open shell noticeable predominance of the alcohol. This comparison

approach (M06 functional with spin-state-corrected s6- clearly indicates that cyclohexyl hydroperoxide was formed as

3

7

3

8

3

1G(d) basis set for Fe atom and 6-31G(d,p) for other atoms). the main primary product. The formation of cyclohexyl

Ph groups attached to Si atoms were in low layer described hydroperoxide and its reduction to cyclohexanol with PPh is

3

9

with semi-empirical PM6 method. Optimized structure of

QM slightly differs from crystal structure.

I

depicted in Scheme 2. This method gives very valuable

information on the existence or non-existence of alkyl

_

The optimized geometry reproduced quite well the siloxane peroxides in the reaction mixture and allows us to estimate

core (the difference between the calculated and concentrations of the three products (ketone, alcohol and alkyl

experimentally observed for Fe–O(Fe) bonds and Si–O(Si) are hydroperoxide) formed in the reaction. In our kinetic studies,

less than 0.06 Å) while other bonds is slightly overestimated for precise determination of oxygenate concentrations, data

r[Fe–O(Si)] and r[Si–O(Fe)] but not more than 0.13 Å. The exclusively obtained after reduction of the sample with PPh

3

quantum chemical calculation reveals six non-interacting Fe were usually used. We will operate in this paper with the initial

atoms with Muliken atomic spin density in range 1.2–1.5 when rate of the formation of a sum of cyclohexyl hydroperoxide,

total multiplicity of molecule was 11.

cyclohexanol and cyclohexanone which is determined as the

sum of cyclohexanol and cyclohexanone after reduction with

PPh

3 0 0

. This rate W = (d[products]/dt) is equal to the initial rate

Table 1. Geometry parameters (distances, Å) of crystal structure and

optimized ONIOMꢀmodel of I.

of the formation of oxygenates.

Crystal of I

ONIOMꢀmodel of I

1.592–1.709

1.563–1.701

1.770–2.061

1.920–2.246

1.849–2.011

3

.7. Selectivity Parameters for the Catalytic Oxidation of

2

Hydrocarbons.

Si–ꢁ ꢀO(Si)

1.614–1.666

1.582–1.610

1.893–1.995

2.040–2.377

1.910–2.024

To get an insight into the nature of oxidizing species, we

measured the selectivity parameters in oxidations of certain

linear, branched and cyclic saturated hydrocarbons. The

regioselectivity parameter determined for the oxidation, by

the system under consideration, of n-heptane is relatively low,

i.e. C(1) : C(2) : C(3) : C(4) ≈ 1 : 7.1 : 7.0 : 6.5. These values are

close to the parameters determined previously for systems

which oxidize alkanes with the participation of hydroxyl

2

Si–ꢁ ꢀO(Fe)

2

Fe–ꢁ ꢀO(Si)

3

Fe–ꢁ ꢀO(Si)(Fe)

3

Fe–ꢁ ꢀO(Fe)

2

2 2

Catalytic Oxidation of Hydrocarbons with H O .

4

1

radicals. As in the case of linear alkanes, the bond-selectivity

We have found that cyclohexane and other alkanes as well as

benzene can be oxidized in acetonitrile solution to the

corresponding alkyl hydroperoxides and phenol, respectively,

by hydrogen peroxide in air in the presence of catalytic

0

parameter (1 : 2 : 3 ) in the oxidation of methylcyclohexane

catalyzed by (1:7.1: 22.2; see Figure S5) is close to the

corresponding values found for the systems oxidizing alkanes

I

4

1b, 41c

with hydroxyl radicals.

,2-dimethylcyclohexanes proceeds non-stereoselectively

because the trans/cis ratios of isomeric alcohols (after

reduction with PPh ) were close to unity, namely 0.9. Thus, on

the basis of the selectivity parameters, we can assume that the

The oxidation of cis- and trans-

amounts of complex I and nitric acid, as shown in Scheme 2.

1

The reaction proceeds in acetonitrile solution under mild

conditions (typically at 50 °C). The alkyl hydroperoxide is

relatively stable in the solution, and can be easily reduced by

3

PPh to the corresponding alcohol.

alkane oxidation reactions catalyzed by compound

via the formation of hydroxyl radicals.

I proceed

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Figure 11. Graph A: Dependence of the initial rate of oxygenate (a sum cyclohexanol+

cyclohexanone) formation on initial concentration of in the oxidation of

cyclohexane (0.46 M) with hydrogen peroxide (50% aqueous) catalyzed by compound

(0.05M) in MeCN at 30 °C. Concentrations of cyclohexanone

and cyclohexanol were determined after reduction of the aliquots with solid PPh . Pink

curve “a” is a calculated curve (see the text). Graph B: linearization of the curve

W

0

I

3

.8. Kinetics of the Catalytic Hydrocarbon Oxygenation.

Stirring a solution of cyclohexane, CyH, in acetonitrile at 30 °C in the presence of HNO

with 50% aqueous hydrogen peroxide in the presence of

compound and nitric acid leads to oxygenation of the alkane presented in Graph A.

Figure 10, curves 1). The reaction is noteciably slower in the Dependences of initial oxidation rate on the concentration of

I

3

I

(

3

co-catalyst HNO as well as of added water are presented in

absence of an acid promoter. Trifluoroacetic acid also

accelerates the oxidation, although less efficiently (Figure 10,

curves 2).

Figure 12. The role of added nitric or trifluoroacetic acid is not

completely clear. We can assume that protons from these acids

promotes splitting certain bonds in the precatalyst 1. This leads to

facilitation of approaching oxidant and substrate to the metal

reaction centres. We realize that the full structure of precatalyst 1

can be more or less substantially modified under the action of the

0.14

0.12

0.10

0.08

1a

2 2

acid as well in the presence of H O .

2a

in the presence

of HNO

3

0.06

8

B

[

RH]

10³

(s)

0

.04

.02

2b

W

0

4

0

1b

0

1

2

3

4

5

6

0

0.5

1.0

Time (h)

[cyclohexane]₀(M)

15

0.06

1

0

5

0

.05

.04

.03

.02

A

3

a

0

in the presence

COOH

of CF

3

0

0.2 0.4

0.6 0.8 1.0

[

0

cyclohexane] (M)

3b

0.01

Figure 12. Dependences of the initial rate of oxygenate formation W

concentration of added HNO (Graph A) and H O (Graph B) in the oxidation of

cyclohexane (0.46 M) with hydrogen peroxide (50% aqueous; 1.5 M) catalyzed by

compound (2 × 10-4 M) and HNO (0.05M) in MeCN at 30 °C. Concentrations of

cyclohexanone and cyclohexanol were determined after reduction of the

aliquots with solid PPh₃

0

on

0

3

2

0

5

10

15

20

25

30

Time (h)

I

3

Figure 10. Accumulation of cyclohexanol (curves a) and cyclohexanone (curve b) in the

-

4

cyclohexane oxidation with hydrogen peroxide catalyzed by

presence of acids (0.05 M): HNO (curves 1 and 2) and CF COOH (curves 3). Solvent was

acetonitrile (total volume of the reaction solution was 5 mL); 30 °C. Other conditions:

Cyclohexane, 0.46 M; H (50% aqueous), 1.5 M. Amounts of cyclohexanone and

cyclohexanol were determined after reduction of the aliquots with solid PPh . Curves 2

are the same as curves 1 but the concentrations have been determined without

I

(2 × 10 M) in the

.

3

The curve of dependence of W

cyclohexane is approaching a plateau at [CyH]

3, Graph A). The rate dependence on initial concentration of

hydrogen peroxide is linear at [H < 1.6 М (Figure 14).

0

on the initial concentration of

0

> 0.4 M (Figure

2 2

O

1

3

2 2 0

O ]

40

3

reduction of the aliquots with solid PPh (for this method, see Refs.[ ]).

1

2

B

We studied the dependence of the initial rate W

cyclohexane oxidation on the concentration of catalyst

(

has an S-shape.

0

of

8

4

0

I

Figure 11). It can be clearly seen that the dependence curve

2

1

0

5

0

1

2

[H O]added (M)

25

0

5

2

0

5

A

1

a

5

0

1

0

5

0

0.02 0.04 0.06 0.08 0.10 0.12

HNO ] (M)

[

3

0

1

Figure 13. Dependences of oxidation rate W

0

on initial concentration of cyclohexane

Graph A) in the oxidation of cyclohexane with H (50% aqueous). Conditions: [

= 0.05 M, solvent MeCN, 30 °C. Concentrations of the products

. Graph B:

linearization of dependence presented by Graph A using coordinates [cyclohexane]

0

1

2

3

4

5

6

(

O

2 2

0

I] = 2

× 10⁴(M)

–4

[I]

0

×

(

3 0

10 M, [HNO ]

cyclohexanol and cyclohexanone) were measured after reduction with PPh

3

[I]

0

1

0²

(M¹^/²s⁻¹^/²

)

0

–

3

2

W

0

[

0

RH] /W

0

.

1

0

B

0

5

10

15

6

|

J. Name., 2012, 00, 1-3

₍_M1/2 _s−1/2

)

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3

10

×

W

0

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Figure 15. Incorporation of the labelled oxygen into cyclohexanol and cyclohexanone

15

(top) and triphenylphosphine oxide (bottom) in the course of the cyclohexane

3 0

oxidations with subsequent reduction of the reaction sample with PPh . Conditions: [I]

1

0

5

–4

18

=

2 × 10 M; [HNO

bar. The yields and isotopic abundances were measured after reduction of the reaction

samples with PPh

3 0 2 2 0 0 2

] = 0.05 M; [H O ] = 1.5 M; [cyclohexane] = 0.46 M; 30 °C; O 1

3

0

1

The origin of the non-labelled ( O) portion of cyclohexanol is

6

0

0.4

0.8 1.2 1.6 2.0

(M)

1

6

the O-water, present in the reaction mixture, and O-

16

2 2 0

[H O ]

Figure 14. Dependences of oxidation rate

W

0

on initial concentration of hydrogen

(50% aqueous).

= 0.05 M, solvent MeCN, 30 °C. Concentrations of

the products (cyclohexanol and cyclohexanone) were measured after reduction with

PPh

hydrogen peroxide. One may expect the incorporation of the

peroxide in the oxidation of cyclohexane (0.46 M) with

H O

2 2

18

O

isotope into the formed triphenylphosphine oxide

) via the following reaction scheme where the alkyl

hydroperoxide is reduced to alcohol by phosphine:

–

4

0 3 0

Conditions: [I] = 2 × 10 M, [HNO ]

(O=PPh

3

.

1

8

18

Cy– O OH + PPh

3 3

→ Cy– OH + O=PPh

3

.9. Experiments on the Alkane Oxidation in the Presence of Simultaneously the reduction of remaining non-labeled

1

8

18

O and H₂O.

16

3

hydrogen peroxide gives O=PPh :

2

1

6

16

The experiments with isotopically labeled species are useful H O OH + PPh → H OH + O=PPh₃

3

mechanistic probes to determine the reaction mechanisms. It The catalytic system based on

I

demonstrates a gradual

1

8

was shown previously that cycloalkane oxidation, catalysed by accumulation of O=PPh reaching a 12% incorporation level

3

1

8

non-heme iron species, in an O atmosphere results in a wide after 3 h reaction time (Figure 16, bottom graph).

2

1

8

range of incorporation percentages, depending on the reaction The mode of dependence of the O incorporation into the

4

1a

mechanism. Based on the results taken from the literature cyclohexanone is different from that obtained for cyclohexanol

and on our recent studies on incorporation of labeled (Figures 15, 16). Although the maximum percentage of labeled

2

1, 41a,b,c

oxygen

similar study for the catalyst

The accumulation of labeled oxygenated products with time levels at 1 h reaction time. This effect can be explained by a

in the cyclohexane oxidation, we performed a ketone exceeds 40% (Figure 17), the absolute concentration of

I.

this compound is quite low and, moreover, falls to nearly zero

–4

18

was studied for conditions [I]₀= 2 × 10 M and 30 °C under the rapid oxygen exchange of O-labeled ketone with excess of

18

atmosphere of O (if all O is conventionally considered as

16

2

O-water present in the reaction mixture. It is necessary to

4

1d, 41e

dissolved in the reaction mixture its concentration is 0.21 M). mention that recently Talsi, Bryliakov and coworkers

The yields and isotopic abundances were measured after demonstrated that the blank process which proceeds in acidic

1

8

reduction of the reaction samples with PPh . The highest medium ([c-C H =O] = 0.25 M, [HNO ] = 0.05 M, [H O] =

3

6

10

16

0

3

2

1

8

18

degree (14%) of O incorporation into the cyclohexanol 2.37 M and [H₂O] = 0.08 M) in CH CN gives 82% of

O

3

formed from CyOOH and PPh was observed at the beginning incorporation into c-C H =O measured after 10 and 20 min

3

6 10

of the reaction (Figure 15, top graph), then reaching a plateau time. Unfortunately, this side process does not allow us to

1

estimate correctly the amount of cyclohexanone- O formed in

8

at 11% level.

This degree is considerably lower than those observed the catalytic oxidation of cyclohexane.

4

1a

previously for complexes of osmium (68%)

and copper

2

50%) under similar reaction conditions. Lower incorporation

1

0.07

0.06

(

1

of O can be due to relatively low activity of

8

I

at the studied

16

18

OH

0

.05

conditions with the overall yield of products based on the

cyclohexane of ca. 14%.

+

0.04

0.03

5

4

3

2

1

0

18

O

OH

0.02

0.01

0.00

18

OH

0.0

0.5

1.0

1.5

2.0

2.5

3.0

Time (h)

0

.008

.007

.006

18

16

O

0

.0

0.5

1.0

1.5

2.0

2.5

3.0

+

0

Time /h

1

4

2

0

8

6

4

2

0

0.005

0.004

0.003

0.002

0.001

0.000

18

O

18

O

P

0.0

0.5

1.0

1.5

Time (h)

2.0

2.5

3.0

0

.0

0.5

1.0

1.5

2.0

2.5

3.0

Time (h)

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Figure 16. Kinetic curves of accumulation with time of cyclohexanol (top) and similar, showing by-products typical for hydroxyl radical

16

18

cyclohexanone (bottom) containing both partially labeled (the sum O+ O) and

oxidation of cyclohexane (Figure 18).

18

completely O-labeled oxygenates. For conditions of the experiments, see caption to

For the case of

2

O test, the incorporation of labeled oxygen

Figure 16. The yields and isotopic abundances were measured after reduction of the

into the 1,3- and 1,4-cyclohexanediols was found to be ca. at

3

reaction samples with PPh .

1

8

9

% level (Figure 18, Table 2). This is close to a level of

, no incorporation of incorporation into the main product, cyclohexanol. Doubly

labeled oxygen into cyclohexanol was observed when the labeled diols were not detected. The amount of 1,2-

O

1

8

In contrast to the reaction under

O

2

1

6

16

18

reaction with the H

presence of 2.15 M of H

exhibit the highest level of O content of 44% which is content of labeled oxygen in hydroxycyclohexanones was

2 2 2

O / O combination is carried out in the cyclohexanediol was insufficient for reliable estimate of its O

1

8

2

O. Cyclohexanone was found to incorporation levels. An interesting observation is that the

1

8

1

8

constant in time (Figure 17). This indicates that labeled ketone found to be at nearly zero level (0, 3 and 2% of O for 2-, 3-

1

8

c-C

between cyclohexanone and H

.5 ratio). Triphenylphosphine oxide (O=PPh

6

H10= O comes most probably from the exchange reaction and 4-hydroxycyclohexanones, respectively).

1

8

18

2

O (at the [H

2

O]

0

/[CyH]

0

=

One can select two general pathways (Figure 19, a and b)

4

3

) was found to towards cyclohexanediols and another two (c and d) towards

1

8

18

contain ca. 2.5% of O. Assuming that the only way for oxygen hydroxycyclohexanones. Since the incorporation of O into

to appear in the O=PPh is the oxidation of PPh with the the diols almost equals to that for cyclohexanol one may

oxygen-containing peroxo species, such as cyclohexyl assume that most of diols are formed through the route a,

3

1

8

O

2

hydroperoxide Cy–OOH, one may suppose that a small portion whereas route b is unfavourable due to depletion of the

of Cy–OOH formed in the catalytic oxidation of cyclohexane amount at reaction time > 2 h. Hence, it can be concluded that

1

8

contains O.

the route d will always give unlabeled hydroxycyclohexanones

1

8

(no free

2

O available). Pathway c will give unlabeled product

50

40

30

20

10

0

due to the oxygen exchange with excess of unlabeled water

16

1

8

O

2

H O, and this will not depend on the isotopic composition of

the introduced ketone group. Considering the contributions of

both routes c and d to be nearly equal, at least twice less

1

8

content of O in hydroxycyclohexanones in comparison with

cyclohexanediols may be expected. This assumption is in

1

8

O

P

1

8

agreement with the observed level of O incorporation into

the hydroxycyclohexanones (Table 2). Besides, the 2-

1

hydroxycyclohexanone was found to be 100% O-free. The

8

0

.0

0.5

1.0

1.5

2.0

2.5

3.0

18

effect of lowered O incorporation into this by-product was

Time (h)

2

already observed by some of us in the copper catalysed

1

8

cyclohexane oxidation under the atmosphere of

O

2

1

8

16

0.018

0.016

0.014

0.012

0.010

0.008

0.006

0.004

0.002

0.000

O

+

18

O

0

.0

0.5

1.0

1.5

2.0

2.5

3.0

Time (h)

Figure 17. Incorporation of labeled oxygen into cyclohexanone and

triphenylphosphine oxide (top) in the course of the cyclohexane oxidations.

–

4

Conditions: [I]

0.48 M; 30 °C; [H

cyclohexanone containing both partially labeled (the sum

0

= 2 × 10 M; [HNO

3

]

0

= 0.05 M; [H

2 2 0 0

O ] = 0.91 M; [cyclohexane]

1

8

=

2 0

O] = 2.15 M. Kinetic curves of accumulation with time of

1

6

18

O+ O) and

18

completely Oꢀlabeled oxygenates are shown in the bottom graph. The yields

and isotopic abundances were measured after reduction of the reaction samples

3

with PPh .

Figure 18. Oxidation of cyclohexane by the “

I

–HNO

atmosphere (bottom). Chromatograms of the reaction samples

taken after 180 min time and reduced with PPh show over-oxidation products. The

3 2 2

–H O ” system in the presence of

18

2 2

H O (top) or O

3

With cyclohexyl hydroperoxide as the main reaction product

one may expect the formation of isomeric cyclohexane

peaks of main products (cyclohexanol and cyclohexanone) appear at 8.1 and 8.3 min,

respectively, and they are omitted for clarity. Numbers (%) above peaks indicate the

18

dihydroperoxides, c-C

6

H10(OOH)

as the main over-oxidation content of O in the corresponding products. Incorporation of O into the

2

products which would produce 1,2-, 1,3-, and 1,4- cyclohexanediols and hydroxycyclohexanones is given in Table 2

.

cyclohexanediols upon reduction with PPh

1

3

.

The over-

8

18

O and H O catalytic tests are

oxidation patterns for both

2 2

8

|

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labeled 1,2-cyclohexanol under the conditions of the

experiment.

ꢀ4

18

OH

8.0x10

6.0x10

4.0x10

2.0x10

16

1

6

OH

14

ꢀ

4

+

12

10

8

1

6

OH

1

6

18

OH

ꢀ4

16OH

6

18

OH

16

ꢀ4

OH

4

2

0.0

0

0.0

0.5

1.0

1.5

2.0

2.5

3.0

Time (h)

Figure 20. Kinetic curves of accumulation with time of 1,2-cyclohexananediol

16

18

containing both partially labeled (the sum O+ O) and completely O-labeled

oxygenates (left Y-axis). Incorporation of the labeled oxygen into the 1,2-

cyclohexanediol in the course of the cyclohexane oxidations is also shown (right Y-axis).

Figure 19. Main processes which could lead to different isotopic isomers of

cyclohexanediols and hydroxycyclohexanones (shown in the form of

corresponding hydroperoxides; pathways a–d) in the oxidation of cyclohexane

–4

Conditions: [

I

]

0

18

= 2 × 10 M; [HNO

3

]

0

= 0.05 M; [H

M; 30 °C; [H₂O]₀= 2.15 M. The yields and isotopic abundances were measured after

reduction of the reaction samples with PPh

2 2 0 0

O ] = 0.91 M; [cyclohexane] = 0.48

1

8

under

does not involve

subsequent decomposition of hydroperoxide to form ketone. Route a results in a

O

2

. Here [Ox] means oxidation to form a ketone, either in a way which

O

2

participation, or via hydroxyl radical attack with the

3

.

labeled cyclohexanediol, while both routes

hydroxycyclohexanone (see the text).

a and d do not form labeled

The accumulation of labeled phenol in the oxidation of

1

8

benzene in the presence of H

reaction conditions as for cyclohexane oxidation, except of

lower amount of the oxidant [H = 0.7 M (Figure 21). The

2

O was studied using the same

1

8

2 2 0

O ]

Table 2. Content of O (%) in cyclohexanediols and hydroxycyclohexanones

(

H

main byꢀproducts) produced in the oxidation of cyclohexane by the “I–HNO

3

–

attack of benzene by hydroxyl radical proceeds in a different

manner compared to the case of saturated hydrocarbons, the

18

2

O

2

” system in the presence of H

2

O or in a

O

2

atmosphere

OH

phenol oxygen appears from HO• but not from dioxygen or

41c

Source

OH

O

1

8

OH

water.

However, oxidation of benzene using catalyst I

1

of

O

HO

8

resulted in ca. 2.5% incorporation of O into the phenol (the

overall yield based on the benzene is 7% after 3 h reaction

HO

OH

18

3

time). Close amounts of O were found in O=PPh indicating

the presence of some mechanisms involving exchange with

water oxygen.

1

8

O

2

–

8

10

0

3

0

2

1

8

H

2

O

15

42

36

46

3.0

18

OH

2

.5

1

8

The content of O in the by-products of reaction in the

18

2.0

1.5

presence of H₂O reveals expected levels if one considers that

8

i) alcohols (oxidation products) are O-free and ii) ketones get

1

8

18

H₂O.

18

O

from

the

oxygen

exchange

with

O

P

1.0

Hydroxycyclohexanones were found to contain notable levels

1

8

0.5

0.0

of O. Comparison of mass-spectra of hydroxycyclohexanones

ESI, Figures S6-S8) taken within the current work with the

(

previously obtained spectra (where labeling of hydroxyl groups

was assumed) shows some differences leading to the

conclusion that in the present case the labeled oxygen is

located at the ketone group, in agreement with the postulated

mechanisms.

0

.0

0.5

1.0

1.5

2.0

2.5

3.0

Time (h)

ꢀ

3

4

0.04

0.03

0.02

0.01

0.00

1.5x10

18

16

OH

1.2x10

1

8

+

Surprisingly, the reaction in the presence of H₂O gave some

portion of labeled 1,2-cyclohexahediol whereas incorporation

9.0x10

6.0x10

1

8

of O was observed neither in cyclohexanol nor in 1,3- or 1,4-

cyclohexanediols. The accumulation of labeled 1,2-diol is

1

8

OH

1

8

shown in Figure 20. As can be seen incorporation of O into

the 1,2-diol reaches 15% at 3 h reaction time. Blank test has

ꢀ4

3.0x10

been performed ([cis-1,2-diol] = 0.02 M, [HNO ] = 0.05 M,

3

0.0

1

H₂O] = 2.6 M in CH CN, at room temperature) and showed

8

[

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3

Time (h)

no labeled diol formation. However, we still cannot exclude

the presence of some non-catalytic mechanisms leading to

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Figure 21. Incorporation of the labeled oxygen into phenol and triphenylphosphine

–

4

oxide (top) in the course of the benzene oxidations. Conditions: [

I

]

0

= 2 × 10 M;

= 2.15 M. Bottom

Graph: kinetic curves of accumulation with time of phenol containing both partially

a

18

Table 3. Optimization of the reaction conditions

[

3 0 2 2 0 0 2 0

HNO ] = 0.05 M; [H O ] = 0.7 M; [benzene] = 0.5 M; 20 °C; [H O]

En

try

Mol% of Fe

Equiv.

2a

Base

(equiv.)

Yield

(%)

TON/TOF

‒1 b

16

18

labeled (the sum O+ O) and completely O-labeled oxygenates. The yields and

(h )

isotopic abundances were measured after reduction of the reaction samples with PPh

3

1

c

5

1.5

CaCO

3

(1)

73

Catalytic oxidative amidation of alcohols

2

3

4

5

6

7

8

0.05

0.01

0.05

1.5

2

CaCO

None

68

65

77

50

Oxidation of alcohols into amides has attracted considerable

4

attention since the pioneering results on ruthenium and

2

d

(1)

4

3

rhodium catalysis. These borrowing hydrogen approaches

allowed to circumvent the drawbacks associated with classical

amide bond synthesis involving carboxylic acids, amines and

coupling agents. Toxicity of the reagents as well as solvent

used for this kind of coupling prompted chemists to search for

alternative routes. Since ruthenium and rhodium are highly

expensive metals, cheap transition metals such as Mn, Zn, Cu

and Fe were investigated to perform amide formation in the

2

(0.5) 81

22

1620/90

8.6/2.2

2

e

d

10

1.3

CaCO

3

86

4

2 2

(FeCl .4H O)

presence of TBHP (tert-butylhydroperoxide). We already

2

0c

a

.

reported

that copper silsesquioxanes gave outstanding

Reaction conditions: (α)ꢀmethylbenzylamine HCl (0.5 mmol), benzyl alcohol

‒1

(

0.75 mmol), CaCO

3 2

99% pure (0.5 mmol), TBHP (70% in H O, 4 equiv., 2.0

b

results in this kind of reaction, allowing to go down to 100 ppm

in copper, and reach TON and TOF values up to 9400 and 392

mmol), CH CN (1 mL), 80 °C, 18 h. TON = (mmol of product)/((mmol of Fe);

3

c

TOF = (mmol of product)/(mmol of Fe)/(reaction time). Reaction performed in 4

-1

d

e

[44b]

h , respectively. As a continuing study and since iron salts h. CaCO₃99.995% pure was used. Result from Ref.

4

4a,b

44c,d

have been proven by us

efficiently this tandem oxidation/amidation reaction,

compound was evaluated in this reaction.

To compare with our previous reports using FeCl

the following study was performed taking into account the

quantity of iron involved, keeping in mind that complex

contains 6 iron atoms. In the presence of 5 mol% of iron, TBHP

and others

to perform

In order to prove the usefulness of these new conditions,

several amines were reacted with benzyl alcohol 2a in the

4a,b presence of only 500 ppm of iron (Table 4). Delightfully, even

I

4

2

.4H O,

though isolated yields were not always better than with 10

.

mol% of FeCl

2

4H

2

O, TON and TOF values were found much

I

higher, thus proving the robustness of iron cage compound I.

.

Primary amines 3a-c reacted correctly giving corresponding

amides 4aa-ac in 72-81% yield (Table 4, entries 1-3). However,

highly hindered tert-butyl amine was moderately converted

(

4 equiv.) and calcium carbonate, (

α)-methylbenzylamine HCl

3a

was converted into corresponding benzamide 4aa in 73%

yield after 4 h (Table 3, entry 1). Decreasing catalyst loading

was found possible with a concomitant increase of the

reaction time to 18 hours. Thus, a loading of 0.05 mol% in iron

was enough to obtain 68% of 4aa (Table 3, entry 2). At this

loading, the purity of the base is of importance since metallic

contaminants could catalyze the amidation reaction. Contrary

into 4ad (Table 4, entry 4). Nevertheless, TON and TOF values

.

were 135- and 30-fold higher than with FeCl

2

4H

2

O,

respectively. Secondary amines 3e-g were also benzoylated

efficiently, with yields up to 83% and TON and TOF values up

-1

to 1660 and 92 h , respectively (Table 4, entries 5-7).

2

0c

to what was observed with copper cages, the purity of

CaCO , 99.995% or >99% pure, had little influence on the

3

outcome of the reaction (Table 3, entries 2 and 3). Eventually,

an increase in the stoichiometry of benzyl alcohol 2a allowed

to reach 77% yield (Table 3, entry 4). Decreasing even more

the catalyst loading to 100 ppm in iron was detrimental to the

reaction (Table 3, entry 5). Finally, reducing the amount of Table 4. Variation of the amine at low catalyst loading^a

base to 0.5 equivalent allowed to slightly increase the isolated

Entry

Product

I (0.05 mol% in Fe)

FeCl

2 2

.4H O

d

yield to 81% (Table 3, entry 6). A control experiment

demonstrated that CaCO was required to obtain good yield in

this reaction (Table 3, entry 7). As a comparison, the optimized

catalytic conditions obtained in the presence of precatalyst

(10 mol%)

3

b,c

Yield

81

TON/TOF

Yield

86

TON/TOF

I

1

‒

1

1620/90

8.6/2.2

allowed to reach TON and TOF values of 1620 and 90 h ,

.

respectively, while FeCl

2

4H

2

O permitted TON and TOF values

‒

of solely 8.6 and 2.2 h , respectively (Table 3, entries 6 and 8)

1

2

74

1480/82

76

7.6/1.9

1

0

|

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ARTICLE

7 8

BuOH)9.6(C H )] I in a high yield. Specificity of the product I

3

72

40

1440/80

800/44

89

59

8.9/2.2

5.9/1.5

composition (two triiron(III) clusters, “embraced” by cyclic and

acyclic silsesquioxane ligands) provokes the formation of a

remarkable Asian Lantern-like molecular architecture.

The structure of compound I was studied by single crystal X-

4

5

ray, EXAFS, topological analysis and DFT calculations.

Significant distortion of iron-containing topologically identical

2M3-1 clusters due to the interaction with polydentate ligand

is found. This fact causes the appearance of a spin-frustrated

system and spin glass behaviour. Taking into account that the

chains are straight, it is not surprising that these are packed

parallel with bups-to-hollow mutual disposition of

metallasilsesquioxane cluster in the hexagonal lattice fashion.

54

65

1080/60

1300/72

66

6.6/1.7

Compound I can be considered as an inorganic model of alkane

oxygenases. Alkanes and benzene can be oxidized in

acetonitrile solution to the corresponding alkyl hydroperoxides

and phenol, respectively, by hydrogen peroxide in air in the

e

6

7

a

n.a.

presence of catalytic amounts of complex I and nitric acid. The

regioselectivity parameters and a kinetic study of the oxidation

led to the conclusion that alkanes are transformed into the

corresponding oxygenates with the participation of hydroxyl

radicals. To understand the peculiarities of the catalytic

mechanisms the reactions have been performed under the

83

1660/92

81

8.1/2

1

8

2

atmosphere of labeled dioxygen ( O ) or in the presence of

1

8

18

21, 41c

O-labelled water (H

2

O). From previous studies,

for the

catalytic system oxidizing with hydroxyl radicals, ca. 50%

18

.

Reaction conditions: amine HCl (0.5 mmol), benzyl alcohol (1.0 mmol),

CaCO 99% pure (0.25 mmol), TBHP (70% in H O, 4 equiv., 2 mmol), I (0.05

mol% in Fe), CH CN (1 mL), 80 °C, 18 h. Isolated yields are given.

TON=(mmol of product)/(mmol of Fe). TOF=TON/(reaction time); given in h .

1

incorporation of O from the

8

2

O into the cyclohexyl

3

2

b

3

hydroperoxide was expected. However, in the case of I-

18

c

ꢀ1

catalyzed oxidation of cyclohexane under

2

O only 12% of

d

[44b, e]

Obtained from lit.

n.a. = not available.

18

labeled cyclohexanol was observed. The effect of lowered

O

level may be due to the lower activity of the present catalytic

In parallel to this study in batch conditions,

I

was evaluated system. This level fully corresponds to the observed amount of

3

under microwave-activated conditions, which were reported labelled O=PPh of ca. 15%.

to give, at that time, the highest TON and TOF values for the The experiments on cyclohexane oxidation performed in the

4

4a

18

oxidative amidation using TBHP as oxidant (Scheme 3).

Interestingly, under the exact same conditions, precatalyst

presence of O-labeled water revealed no O incorporation

into the main product, cyclohexyl hydroperoxide, and

I

.

outperformed FeCl

yield. Such result proves the robustness of

stabilizing effect of the silsesquioxane ligands around the iron known to possess fast oxygen exchange with water. The

2

4H

2

O (51% yield), giving amide 4aa in 71% corresponding cyclohexanol. These results point to the

I

, resulting from a absence of high-valent metal oxo species, many of which are

1

8

18

2

O

centers. Nevertheless, the reported catalytic system investigation of by-products patterns for both

O

2

and H

.

FeCl

in 84%.

2 2

4H O/Caffeine was still found superior to I

, yielding 4aa experiments showed few anomalies, namely the presence of

1

8

O-free 1,5-pentanediol in both cases (alcohols are expected

18

1

to contain O from

8

18

) and 15% of O incorporation into

O

2

1

,2-cyclohexanediol in case of experiment in the presence of

1

8

H

2

O. Moreover, in the last case 1,3- and 1,4-cyclohexanediols

1

8

were found to be O-free and blank experiment revealed no

oxygen exchange between 1,2-cyclohexanediol and water. We

assume that these observations may indicate the presence of

minor reaction mechanisms, which proceed without the

participation of free radicals. Such conclusion is also supported

by the detection of small amounts of labeled phenol in the

2 2

Scheme 3. Comparison of I and FeCl .4H O under microwave activation

oxidation of benzene catalyzed by

Complex was also evaluated in the oxidative amidation of

benzylic alcohols. The peculiar properties of allowed to work

at 500 ppm in iron. Hence, secondary and tertiary amides were

I.

Conclusions

I

Simple one-pot procedure of alkaline condensation of

I

^t^rⁱ^e^t^h^o^x^y^p^h^eⁿ^y^l^sⁱ^l^aⁿ^e^f^o^l^l^o^w^e^d^b^yⁱⁿ^t^e^r^a^c^tⁱ^oⁿ^wⁱ^t^h^F^e^C^l3

-1

allowed

us

to

obtain

cage-like

sodium,iron(III)

obtained with TON and TOF values as high as 1660 and 92 h ,

phenylsilsesquioxane

^[⁽^P^h^Sⁱ^O1 ) (FeO ) (NaO ) (n-

.5 20

1.5 6

0.5 8

respectively. The effectiveness of

I

was also highlighted under

J. Name., 2013, 00, 1-3 | 11

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Journal Name

microwave-activated conditions, giving better results than

FeCl .4H O alone. Those encouraging results demonstrated

2

that development of new iron silsesquioxane structures could

lead to even more active precatalysts.

(

A)

The theoretical data were simulated using the photoelectron mean free

path λ(k), amplitude F(k) and phase shift (k).calculated ab initio using

PhSi(OEt)

3

i

was purchased from Sigma Aldrich and was used as received. program FEFF6. For the refined interatomic distances (R) the statistical

Experimental section

i

Ψ

i

48c

IR spectra were recorded on FTIR Shimadzu IR Prestigeꢀ21. IR spectrum error is 0.01–0.02 Å for the first coordination sphere.

in Nujol for solids and liquid solution in thin film were obtained using KBr

discs.

Computational Details. Full geometry optimizations were carried out with

the Gaussian09 (Revision C.01) package. Calculation provided by twoꢀ

Synthesis of Compound I. PhSi(OEt)

3

(4 g, 0.017 mol) and sodium layer ONIOM technique. The high core layer containing FeꢀOꢀSiꢀOꢀNa

hydroxide (0.865 g, 0.021 mol) were dissolved in 80 mL of EtOH. After skeleton was calculated by DFT/PBE0 functional with 6ꢀ31G(d,p) basis set.

complete dissolution of sodium hydroxide, the mixture was heated at reflux The medium layer consists from C groups attached to silicon atom.

6

H

5

for 2.5 h and then anhydrous iron(III) chloride (0.809 g, 0.005 mol) was Frequency calculations were performed for optimized complex in the gas

added. The resulting brickꢀcolored solution was additionally heated at phase and are reported without the use of scaling factors. The nature of all

reflux for 3 h and then cooled down to room temperature. Finally, a mixture the stationary points on the potential energy surfaces was confirmed by a

of 20 mL of nꢀbutanol/10 mL of toluene was added to the solution, then the vibrational analysis.

reaction mixture was filtered off. Formation of a crystalline product with

single crystals useful for Xꢀray diffraction analysis was observed in solution Magnetic Measurements. Magnetic susceptibility data were collected with

after approximately two weeks. After ceasing of the crystal fraction growth,

a Quantum Design MPMSꢀXL SQUID magnetometer working in the

the solution was decanted and the solid fraction was dried in vacuum. 1.65 temperature range 1.8 – 350 K with an applied magnetic field up to 7

g (60% yield in calculation for [(PhSiO1.5

colored product was obtained.

6 8

)20(FeO1.5) (NaO0.5) ]) of brickꢀ Tesla. The data were corrected for the sample holder (polyethylene film)

4

9

and the diamagnetic contributions calculated from the Pascal’s constants.

−

1

IR (Nujol): (PhSi): 1126 (s), 1104(sh) cm ; (Ph overtones) 1959, 1898, Catalytic Oxidation of Hydrocarbons. The reactions of alkanes were

−

1

−1

1

9

4

822, 1774 cm

ν

SiO(Si): 1068, 1038, 1024 cm ; νSiO(Fe): 997 (s), 978 (s), typically carried out in air in thermostated Pyrex cylindrical vessels with

−

1

−1

21 (sh) cm ; νFeO(Fe): 521, 514, 506 cm ; δSiO(Si): 467, 445 cm ; δSiO(Fe)

09, 396 cm . Elemental analysis: calcd. [(PhSiO1.5

:

vigorous stirring and using MeCN as solvent. Typically, catalyst I and the

−

1

)

20(FeO1.5

)

6

(NaO0.5

)

8

]: coꢀcatalyst (acid) were introduced into the reaction mixture in the form of

Fe, 10.12; Na, 5.56; Si, 16.97. Found: Fe, 9.97; Na, 5.49; Si, 16.78.

stock solutions in acetonitrile. The substrate (hydrocarbon) was then added

and the reaction started when hydrogen peroxide was introduced in one

X-Ray

Structure

Determination.

Single

crystals

2 2

of portion. (CAUTION. The combination of air or molecular oxygen and H O

C

165.8

H

204.8Fe

6

Na

8

O

52.6Si20 were red prisms. A suitable crystal was selected with organic compounds at elevated temperatures may be explosive!). The

and intensities of reflections were measured on a Bruker Smart 1000 CCD reactions after addition of nitromethane as a standard compound were

area detector diffractometer. The crystal was kept at 100.15 K during data analyzed by GC (instrument ‘HP 5890 – SerieꢀII’; fused silica capillary

4

5

collection. Using Olex2 the structure was solved with the XS structure columns column HewlettꢀPackard; the stationary phase was

4

6

solution program using Direct Methods and refined with the ShelXL

polyethyleneglycol: INNOWAX with parameters 25 m × 0.2 mm × 0.4 µm;

carrier gas was helium). Attribution of peaks was made by comparison with

chromatograms of authentic samples. Blank experiments with cyclohexane

showed that in the absence of catalyst I no products were formed.

47

refinement package using least squares minimisation.

Crystal Data for

monoclinic, space group P2

0.3021(11) Å, c = 32.2979(17) Å, β = 92.9200(10)°, V = 19664.1(18) Å , Experiments with Labeled

C

165.8

H

6

204.8Fe Na

8

O52.6Si20 (M =4120.10 g/mol):

1

/c (no. 14), a = 30.0277(16) Å, b =

3

18

2

O

2

and H

2

O. A PerkinꢀElmer Clarus 600

ꢀ1

3

Z = 4, T = 100.15 K, ꢁ(MoKα) = 0.647 mm , Dcalc = 1.392 g/cm , 147601 gas chromatograph, equipped with two capillary columns (SGE BPX5; 30

reflections measured (2.806° ≤ 2Θ ≤ 47.914°), 30628 unique (Rint

=

m × 0.32 mm × 25 µm), one having EIꢀMS (electron impact) and the other

one with FID detector, was used for analyses of the reaction mixtures. The

following GC conditions have been used: 50 °C (3 min), 50–120 °C (8

degrees per minute), 120–300 °C (35 °C per minute), 300 °C (4.11 min),

0

.1094, Rsigma = 0.0805) which were used in all calculations. The final R

1

2

was 0.0688 (I > 2σ(I)) and wR was 0.2094 (all data).

2

1 min total run time; 200 °C injector temperature. Helium was used as the

XAS experiments. The Fe Kꢀedge Xꢀray spectra for sample in the solid

state were collected at the beamline “Structural Materials Science” using

the equipment of “Kurchatov Synchrotron Radiation Source”

4

8a

carrier gas (1 mL per minute flow). All EI mass spectra were recorded with

7

CortecNet. Labeled water (97% of O) was purchased from Cambridge

Isotope Laboratories, Inc. For experiments with

catalytic reaction mixtures were frozen with liquid nitrogen, pumped and

filled with N a few times in order to remove air. The procedure was

repeated a few times, unfreezing the solutions under N atmosphere. Then

the mixtures were pumped again, vacuum pump turned off, Schlenk flasks

with vacuum inside were filled with

and heated up to 30 °C with a possibility of gas flow to compensate

excessive pressure. For experiments with H

volume was used (2.5 mL) and 0.1 mL of labeled water was added prior to

addition of the oxidant (hydrogen peroxide). The O and O compositions

of the oxygenated products were determined by the relative abundances of

mass peaks at m/z = 57/59 for cyclohexanol and m/z = M /M +2 molecular

ion peaks for other compounds.

18

0 eV energy. Labeled dioxygen (99% of O) was purchased from

1

8

(Moscow,Russia). The storage ring with an electron beam energy of 2.5

1

8

2

O , freshly prepared

GeV and a current of 80–100 mA was used as the source of radiation. All

the spectra were collected in the transmission mode using a Si (111)

channelꢀcut monochromator. The powder samples were spread over an

adhesive Kapton tape that was folded into several layers to achieve an

appropriate combination of total absorption and absorption edge jump.

EXAFS data (χexp(k)) were analyzed using the IFEFFIT data analysis

2

1

8

2

O using a syringe through a septa

4

8b

package. EXAFS data reduction used standard procedures for the preꢀ

edge subtraction and spline background removal. The radial pair

distribution functions around the Fe ions were obtained by the Fourier

18

2

O, a reduced reaction

16

18

2

transformation (FT) of the k ꢀweighted EXAFS functions χexp(k) over the

ꢀ1

ranges of photoelectron wave numbers k = 2.5–15.0 Å . The structural

i

parameters, including interatomic distances (R), coordination numbers (N)

+

i

2

and Debye–Waller factors (σ ), were found by the nonꢀlinear fit of

theoretical spectra (Formula (A)) to experimental ones.

1

2

|

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Catalytic oxidative amidation of alcohols.

Ghosh, Inorganic Chemistry Communications 2011, 14, 337-

42; (d) Y.-F. Zeng, X. Hu, L. Xue, S.-J. Liu, T.-L. Hu and X.-H.

3

Bu, Inorganic Chemistry 2012, 51, 9571-9573; (e) T.-Z. Zhang,

Z.-M. Zhang, Y. Lu and E.-B. Wang, Journal of Coordination

Chemistry 2012, 65, 48-54 and references cited therein.

(a) E. K. Brechin, Chemical Communications 2005, 5141-

Procedure at low catalyst loading. In

successively amine hydrochloride (0.5 mmol), CaCO

mmol), CH CN (1 mL), 1 (50 µL of a solution of 2.8 mg of I in 1 mL of

EtOAc), benzylic alcohol (104 µL, 1.0 mmol), and TBHP (70% in H O, 140

µL, 1.0 mmol). The mixture was stirred at 80°C for 2h and TBHP (70% in

O, 140 µL, 1.0 mmol) were again added to the mixture. After 16h at

0°C, the mixture was cooled to room temperature and 1N HCl and AcOEt

were added. The mixture was extracted twice with AcOEt, and the

combined organic phase was washed with saturated solution of NaHCO

brine, and concentrated under reduced pressure. To remove the excess of

benzylic alcohol, 80 mL of H O were added and evaporated under reduced

a

sealed tube were added

3

(25.0 mg, 0.25

3

2

5

153; (b) N. T. Madhu, J.-K. Tang, I. J. Hewitt, R. Clérac, W.

Wernsdorfer, J. v. Slageren, C. E. Anson and A. K. Powell,

Polyhedron 2005, 24, 2864-2869; (c) A. J. Tasiopoulos and S.

P. Perlepes, Dalton Transactions 2008, 5537-5555; (d) L.

Gregoli, C. Danieli, A. L. Barra, P. Neugebauer, G. Pellegrino,

G. Poneti, R. Sessoli and A. Cornia, Chemistry–A European

Journal 2009, 15, 6456-6467 and references cited therein; (e)

M. J. Rodriguez-Douton, A. Cornia, R. Sessoli, L. Sorace and

A.-L. Barra, Dalton Transactions 2010, 39, 5851-5859; (f) C.

Schlegel, E. Burzurí, F. Luis, F. Moro, M. Manoli, E. K. Brechin,

M. Murrie and J. van Slageren, Chemistry–A European

Journal 2010, 16, 10178-10185; g) Y.-Y. Zhu, X. Guo, C. Cui,

B.-W. Wang, Z.-M. Wang and S. Gao, Chemical

Communications 2011, 47, 8049-8051; (h) Y. Hou, X. Fang, K.

D. Kwon, L. J. Criscenti, D. Davis, T. Lambert and M. Nyman,

European Journal of Inorganic Chemistry, 2013, 1780-1787

and references cited therein; (i) Y.-Y. Zhu, C. Cui, K. Qian, J.

Yin, B.-W. Wang, Z.-M. Wang and S. Gao, Dalton Transactions

H

8

2

3

,

2

pressure. Crude product was then purified using silica gel chromatography

using gradients of cyclohexane/AcOEt to yield the pure compounds.

Procedure under microwave activation. In a 0.5ꢀ2.0 mL microwave reactor

were added

methylbenzylamine HCl (78.8 mg, 0.5 mmol, 1 equiv.). 1 mL of CH

benzyl alcohol (104 µL, 1.0 mmol, 2 equiv) and tertꢀbutylhydroperoxide

70% in H O, 140 µL, 2 equiv.) were then added. After 15 min of stirring at

30°C under microwave irradiation, another equiv. of tertꢀ

butylhydroperoxide (70% in H O, 140 µL) were added, and the mixture was

further stirred for 15 min at 130°C. Once again, 2 other equiv. of tertꢀ

butylhydroperoxide (70% in H O, 140 µL) were added and the mixture was

stirred for 15 min at 130 °C. After cooling to rt, HCl 1N (5 mL) and AcOEt

5 mL) were added. The mixture was extracted with AcOEt (2 × 5 mL), and

the combined organic phases were washed with saturated aqueous

NaHCO solution (10 mL), brine (10 mL), dried over magnesium sulfate

and concentrated under reduced pressure. H O (50 mL) was then added to

I

(13.9 mg, 0.025 mmol,

5

mol% in Fe), and (α)ꢀ

.

3

CN,

(

2

1

2

014, 43, 11897-11907.

2

3

4

X.-Y. Cao, J. W. Hubbard, J. Guerrero-Medina, A. J.

Hernandez-Maldonado, L. Mathivathanan, C. Rinaldi, Y.

Sanakis and R. G. Raptis, Dalton Transactions 2015, 44, 3399-

(

3409.

(a) M. Murugesu, R. Clérac, W. Wernsdorfer, C. E. Anson and

3

2

A. K. Powell, Angewandte Chemie International Edition 2005,

44, 6678-6682; (b) R. W. Saalfrank, I. Bernt, E. Uller and F.

Hampel, Angewandte Chemie 1997, 109, 2596-2599; (c) D.

Foguet-Albiol, K. A. Abboud, G. Christou, Chemical

Communications 2005, 4282-4284; d) G. Jiang, J. Bai, H. Xing,

evaporate remaining benzyl alcohol. The crude mixture was purified by

chromatography on silica gel using mixtures of cyclohexane and ethyl

acetate as eluent.

Y. Li and X. You, Crystal Growth & Design 2006, 6, 1264-1266.

Acknowledgements

5

a) X.-F. Ma, J.-L. Tian, W. Gu, S. Gao, S.-P. Yan and D.-Z. Liao,

Inorganic Chemistry Communications 2008, 11, 256-259; b)

Z.-M. Zhang, S. Yao, Y.-G. Li, R. Clérac, Y. Lu, Z.-M. Su and E.-

B. Wang, Journal of the American Chemical Society 2009,

This work has been supported by the Russian Foundation for

Basic Research (Grants 14-03-00713; 16-03-00206, 16-03-

00254), the French Embassy in Moscow (Programme André

1

31, 14600-14601; c) R. Singh, A. Banerjee, E. Colacio and K.

Mazon), the Balard Foundation, the University of Montpellier,

the Plateforme d'Analyse et de Caractérisation (PAC Balard),

the Centre National de la Recherche Scientifique (CNRS),

through the Groupe de Recherche International (GDRI)

Homogenous catalysis for sustainable development (CH2D),

the Fundação para a Ciência e Tecnologia (FCT), Portugal

K. Rajak, Inorganic Chemistry 2009, 48, 4753-4762.

Y.-L. Miao, J.-L. Liu, Z.-J. Lin, Y.-C. Ou, J.-D. Leng, M.-L. Tong,

Dalton Transactions 2010, 39, 4893-4902.

a) C. Papatriantafyllopoulou, C. M. Kizas, M. J. Manos, A.

Boudalis, Y. Sanakis and A. J. Tasiopoulos, Polyhedron 2013,

64, 218-230; b) A. Ferguson, L. H. Thomas and M. Murrie,

6

7

Polyhedron 2013, 52, 227-233.

a) M. Mohan and B. Paramhans, Croatica Chemica Acta

8

9

1

(projects

PTDC/QUIQUI/102150/2008,

PTDC/QUI-

1

981, 54, 173-182; b) W.-Q. Chen, Y.-M. Chen, T. Lei, W. Liu,

QUI/121526/2010, Project PTDC/QEQ-QIN/3967/2014 and

UID/QUI/0100/2013, fellowship SFRH/BPD/99533/2014), and

the “Science without Borders Program, Brazil–Russia”, CAPES

Y. Li, Inorganic Chemistry Communications 2012, 19, 4-9.

Y. Z. Zheng, M. L. Tong, W. Xue, W. X. Zhang, X. M. Chen, F.

Grandjean and G. J. Long, Angewandte Chemie International

Edition 2007, 46, 6076-6080.

0 (a) R. Murugavel, A. Voigt, M. G. Walawalkar and H. W.

Roesky, Chemical Reviews 1996, 96, 2205-2236; (b) L. King,

A. C. Sullivan, Coordination Chemistry Reviews 1999, 189, 19-

(grant A017-2013). L.S.S. and G.B.S. express their gratitude to

the FCT and Group 1 of Centro de Química Estrutural for

making it possible for them to stay at the Instituto Superior

Técnico, University of Lisbon, as invited scientists and to

perform a part of the present work (all the funding for the

invited scientist fellowship comes entirely from this Group).

5

Jancik and S. Singh, Chemistry-A European Journal 2004, 10

7; (c) H. W. Roesky, G. Anantharaman, V. Chandrasekhar, V.

,

4106-4114; (d) F. T. Edelmann, Metallasilsesquioxanes.

Synthetic and Structural Studies Wiley, 2007; (e) A. J. Ward,

A. F. Masters and T. Maschmeyer, in Applications of

Polyhedral Oligomeric Silsesquioxanes, Springer, 2011, pp.

135-166. (f) E. V. Beletskiy, Z. Shen, M. V. Riofski, X. Hou, ,

J.R. Gallagher, J. T. Miller, Y. Wu, H. H. Kung, M. C. Kung,

Chem. Commun. 2014, 50, 15699-15701; (g) F. Schax, E. Bill,

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J. Name., 2013, 00, 1-3 | 13

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ARTICLE

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