Amine-functionalized mesoporous silica as a support for on-demand

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Amine-functionalized mesoporous silica as a support for on-demand release of copper in the A3-coupling reaction: ultra-low concentration catalysis and confinement effect Julio Cesar S. Terra, Audrey Moores, and Flávia Cristina Camilo Moura ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.9b00576 • Publication Date (Web): 08 Apr 2019 Downloaded from http://pubs.acs.org on April 8, 2019

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ACS Sustainable Chemistry & Engineering

Amine-functionalized mesoporous silica as a support for on-demand release of copper in the A3-coupling reaction: ultra-low concentration catalysis and confinement effect Julio C. S. Terra†, Audrey Moores†,*, Flavia C. C. Moura#,* † Centre for Green Chemistry and Catalysis, Department of Chemistry, McGill University, 801 Sherbrooke Street West, Montreal, QC, H3A 0B8, Canada. *[email protected] # Departamento de Química, ICEx, Universidade Federal de Minas Gerais, Av. Antônio Carlos, 6627, Pampulha, Belo Horizonte, MG, 31270-901, Brazil. *[email protected]

Keywords MCM-41, supported catalyst, confinement effect, A3-coupling.

Abstract

ACS Paragon Plus Environment

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The study of catalysts activity at ultra-low concentration is a

prime

importance

for

the

development

of

more

sustainable

catalytic processes. In this work, we designed mesoporous MCM-41 silica

with

covalent

functionalization

with

amine

groups

at

different levels of coverage. This material as used as a support to immobilize small quantities of Cu (I) species to be used as a catalyst in the A3-coupling reaction. The support design allowed the controlled release of the catalytically active species, as well

as

its

scavenging

after

reaction.

This

system

achieved

excellent catalytic performance, leading to 95% yield of the desired propargylamine within 2 hours of reaction at 100ºC under microwave conditions, using only 0.02 mol% of catalyst, the lowest catalyst amount ever reported for this reaction and high TON (4750) and TOF (2375 h-1). An interesting effect was noticed, where the catalyst on the support yielded improved reaction rates as compared to unsupported catalysts in solution, under the same concentration conditions. We discuss the possibility of a confinement of metal species inside the mesoporous structure of the support, which made the

immobilized

system

more

effective

than

the

homogenous

counterpart. The present study pushes the limits of A3-coupling reaction

conditions

and

the

potential

of

supported

metal

catalysis. Introduction


2

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Propargylamines are versatile compounds, frequently used as intermediates for building nitrogen-containing heterocycles1 such as natural products and biologically-active molecules.2 They can be

atom-economically

obtained

from

a

one-pot

three-component

condensation between an aldehyde, an amine and an alkyne, known as A3-coupling.3 In this Mannich-type reaction, the alkyne C-H bond needs

to

be

activated

for

the

process

to

happen,

hence

the

importance of an adequate catalyst choice.4 This reaction has been extensively studied in the context of homogeneous catalysis, since

the 2000s and the seminal work of

the Li group with the use of inorganic salts of various metals, including systems

gold,3 were

silver5

developed

and to

copper.6–9 improve

Thereafter,

the

different

process,

including

transition metal complexes as homogeneous catalysts.10–16 Despite the high efficiency of the homogenous version of this reaction, difficulties in reusability and product purification led to

the

development

of

heterogeneous

catalysts,17

18

with

a

particular focus on copper systems, thanks to the abundance and low cost of this metal.19,20 Copper nanoparticles, for instance, have been used in their suspended form21–23 and on several different supports such as montmorillonite24, graphene25, carbon nanotubes26 and activated carbon.27 Another common strategy is the grafting of copper

ions

on

functionalized

surfaces,

enabling

means

to


3


Page 4 of 45

hetereogenize such species. This is achieved by covalently binding organic ligands on the surface of selected organic or inorganic supports, which will then coordinate to the metal species.

Many

examples can be found in the literature using complex multidentate ligands

to

graft

metal

species

onto

solid

supports,

mainly

polymers,28–32 silica beads33–35 and mesoporous silica.36–39 Although

heterogeneous

catalysts

are

meant

to

avoid

the

presence of metal species in the final product, many do typically suffer

from

leaching

issues.40

Copper

can

be

toxic

beyond

a

threshold concentration and its content in products is strictly regulated.41,42 This significant drawback causes many pharmaceutical processes to necessitate the use of expensive scavengers column. But

leaching

also

leads

to

mechanistic

uncertainties.

As

an

example, Moaddeli and coworkers38 used functionalized mesoporous silica with a super paramagnetic core to immobilize copper and successfully catalyzed the formation of propargylamines. Leaching studies evidenced that 38% of the initial copper content in the catalyst was lost after one cycle (~811 ppm metal/product) and 65% after three reaction cycles, along with moderate decrease in the catalytic

activities.

polymer-supported

In

copper

another species

study, to

Rus

catalyze

et the

coll.28

used

A3-coupling

reaction. The authors reported a decrease of about 15% in the copper content of the catalyst (~350 ppm metal/product) after


4

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running the reaction for one cycle. These leached copper species often have the ability to homogeneously catalyze the process as well. In fact, many catalysts, although designed as heterogeneous, actually worked as providers of active – yet often uncredited – homogeneous species,43 especially given the fact that A3-coupling reactions copper.

can

proceed

Another

with

impressively

Cu-catalyzed

reaction

small

quantities

relying

on

of

alkyne

activation, the Huisgen condensation, has been shown by the Yamada and Uozumi group44 to work with copper levels as low as 4.5 ppm using

a

self-assembled

Following the same

polymeric

imidazole

copper

principle, some groups have

catalyst.

been

able

to

significantly lower the catalytic charge for A3-coupling. For example, copper-impregnated magnetite has been used to effectively catalyze

A3-coupling

at

a

0.1

mol%

copper

load45

(~230

ppm

metal/product) and montmorinollite-supported copper nanoparticles have been able to catalyze the process at a catalyst load as low as 0.05 mol%24 (~120 ppm metal/product). Yet at very low charge, any leaching will quickly cause the loss of a significant portion of the heterogenized phase of copper. Catalytic

systems

where

dynamic

exchanges

between

nanoparticles and soluble species play a role in activity have been known and studied for several decades, but remain an active area

of

research

due

to

the

difficulties

to

unambiguously


5


Page 6 of 45

characterize the active species. Suzuki-Miyaura and Mizoroki-Heck are excellent examples, where soluble Pd(II) and Pd (0) species are the active catalytic species which may exist in equilibrium with nanoparticulate systems.46 An interesting strategy developed by Crudden and coworkers40 is the use of thiol-functionalized mesoporous silica to deliver and subsequently trap catalytically active Pd(II), relying on the high affinity between sulfur and palladium atoms. Mesoporous

silica

materials

are

attractive

and

highly

versatile because these highly porous inorganic networks feature large surface areas and hydroxylated surfaces which are prone to functionalization.47,48 The easily tunable functionalization of such materials

has

chromatographic

given

them

stationary

diverse

applications,

phases49

immobilization of different molecules

to 50,51

ranging

supports

for

from the

and metal species for

both environmental remediation52,53 and catalytic interests.35,40 In particular, amine-grafted mesoporous silica have been extensively researched.54–58 Amine grafted mesoporous silica are particularly attractive for the adsorption of copper species, as rationalized in terms of Pearson’s hard-soft acid-base theory. For instance, Lam et al.59 have previously reported the high affinity of silver to

thiol-functionalized

materials,

whereas

adsorbed on amine-functionalized supports.

copper

was

better

Similar strategies


6

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have been reported for the A3-coupling reaction

33–39

using large

ligands with multiple complexation sites. Such ligands are great in

immobilizing

exchanges

the

between

metal

the

species,

but

heterogeneous

also

and

prevent

homogeneous

dynamic media,

potentially leading to catalytic systems that suffer from poor diffusion. Based on this state of the art, we took the appraoch of embracing the role of leaching in the heterogeneous catalyzed A3 coupling

and

designed

a

system

able

to

deliver

on-demand

homogeneous copper species and also recover them at the end of the process. Herein, we report the use of aminopropyl-functionalized MCM-41 as a multifunctional support, which not only provides copper for catalyzing the A3-coupling reaction inside its own pores, but also scavenges eventually leached metal species from the resulting propargylamine. In addition, we discovered that mesopores were able to confine species and provided nanoenvironments enhancing catalytic activity as compared to the homogenous blank. This allowed the use of unprecedently low catalyst loads. The use of simple ligands and low metal loading on the support ensures the ability of the ions to be freed and furtherly trapped by the highly available adsorption sites, providing an interesting system that works

on

the

frontier

between

homogeneous

and

heterogeneous

catalysis.


7


Page 8 of 45

Results and discussion Catalyst support characterization The catalyst synthesis process is summarized on Figure 1 and detailed in the experimental section. The methodology for making MCM-41 has been previously reported by us (the Moura Group60) and is an adaptation of the Grün method,61 which is a modification of the original method reported by Kresge and Beck in 1992.62,63 The modification of silica with silane coupling agents was inspired by the method reported by the Resasco Group,64 which is carried out in

room

temperature,

simpler

than

reflux

methods

previously

reported in the literature.40,59 Post-synthetic functionalization was chosen to assure the shape and size homogeneity of the pores, but there are other methods where the organosilane reagent is added during the synthesis of the material and co-condensed within the inorganic

network.

The

co-condensation

method

gives

a

more

homogeneous distribution of the organic groups through the surface of the support, but it compromises the regularity of the porous structure.48

MCM-41

was

functionalized

with

APTMS

[(3-

aminopropyl)trimethoxysilane] in different APTMS to MCM-41 ratios X (X = 1.5, 3.0, 4.5 and 6.0 mmol.g-1), leading to four different amine grafted materials labeled NH2_X/MCM-41, using a slightly modified reported method.64


8

Page 9 of 45

SCHEME OF MCM-41 FUNCTIONALIZATION AND COPPER IMMOBILIZATION O SUPPORT FUNCTIONALIZATION AND COPPER IMMOBILIZATION Si OH

O

Si

Si

CuI

Si O O

O Si

O

O Si

H2N APTMS/MCM-41 1.5 mmol.g-1

O Si

O

Si O O O

MCM-41

O

NH2

(H3CO)3Si

O

Si O

CuI 0.4

H 2N

MCM-41

O

Si O O O

OO Si OH OH H2N Si O O Si OH O Si OH Si OH H3CO SiOO OCH H3CO SiSi 3OH OH OO APTMS/MCM-41: APTES/MCM-41: Si OH 1.5, 3.0, 4.5Oand 6.0 mmol.g-1

H 2N

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Cu +

H2N O

NH2

mmol.L-1

24h, RT

Si

O

NH

NH 2 2

Cu+ O

Si O O

(from NH-MCM-41_1.5)

(1.5, 3.0, 4.5, 6.0)

O O

NH 2

Cu-NH-MCM-41

NH-MCM-41

O

O Si

NH2/MCM-41

Si O

NH2

O Si O

O Si O

Si

O

Cu/NH2/MCM-41

Pore size around 3 nm, structures not drawn in scale.

Figure 1. Scheme of catalyst synthetic procedure, which consists of support functionalization and copper immobilization.

Initially,

functionalization

of

the

materials

investigated by solid state nuclear magnetic resonance (NMR). Cross-Polarization spectra

were

Magic

registered

Angle for

Spinning

both

NMR

starting

(29Si and

CPMAS

was 29Si

NMR)

functionalized

silica materials (Figure 2).


9


NH2_6.0/MCM-41 NH-MCM-41_6.0

5

NH2_4.5/MCM-41 NH-MCM-41_4.5

4

NH2_3.0/MCM-41 NH-MCM-41_3.0

3

T3

T2

NH2_1.5/MCM-41 NH-MCM-41_1.5

-25

-30

Q4

Q2 -35

-40

-45

-50

-55

-60

2

Q3

MCM-41 MCM-41

-20

Page 10 of 45

-65

-70 -75 f1 (ppm)

-80

-85

-90

-95

-100

-105

-110

-115

1

-120

-125

 (ppm)

T2 O

O OH Si

NH2

T3 O

Q2

O O Si

NH2

OH Si O OH O

Q3

Q4

OH Si O O O

O Si O O O

Geminal silanol Free Silanol [SiO2(OH)2] [SiO3OH]

Siloxane [SiO4]

Figure 2. Top: 29Si CPMAS NMR spectra for parent MCM-41 and functionalized silica materials NH2_X/MCM-41 (X=1.5, 3.0, 4.5 and 6.0). Below: List of functionalities assigned to 29Si CPMAS NMR peaks observed for these materials. [O] fragments represent oxygen atoms connected to the silica matrix via O-Si bonds.


10

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Unfunctionalized MCM-41 afforded a typical spectrum featuring the three possible chemical environments for Si atoms depicted in figure 2,65 labeled Q2, around -93 ppm, Q3, around -102 ppm and Q4

,

around -112 ppm (Q standing for 4 bound O atoms, quattuor in latin).66 Spectra of the functionalized materials featured not only Q signals, as parent MCM-41 does, but also T2, from -60 to -56 ppm, and T3 peaks, from -68 to -64 ppm (T standing for 3 bound O atoms), which reflect the introduction of Si atoms from the grafting of ATPMS. The presence of signals T2 and T3 was a firm evidence that organic groups are indeed covalently bonded to the support matrix rather than simply adsorbed onto its surface.67 Another feature noticeable in Figure 2 is the increase in the intensity of the Q4 signal

when

compared

to

the

intensity

of

Q3

peaks

for

functionalized materials. This is expected since Q3 are converted into T3 sites in the functionalized materials, while Q4 sites, lacking OH groups, are not prone to functionalization and remain unchanged.


11

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

MCM-41


11

O O Si O

2 1

3

22

3

Page 12 of 45

NH2 33

1

2

4

NH2_6.0/MCM-41 NH-MCM-41_6.0 N6.0

NH2 3

N4.5 NH2_4.5/MCM-41 NH-MCM-41_4.5

3

2 1

N3.0

4

NH2_3.0/MCM-41 NH-MCM-41_3.0

2

MCM-41

N1.5 1

NH-MCM-41_1.5 NH2_1.5/MCM-41 50

50

Si OOO

45

30

40

40

35

30

10

20

f1 (ppm)

25

20

15

10

MCM-41

 (ppm)

5

 (ppm)

Figure 3. 13C CPMAS NMR spectra for the four functionalized materials.

13C

CPMAS

NMR

further

confirmed

the

successful

functionalization of MCM-41 (Figure 3). The three major signals present for each material correspond to the three carbon atoms of the aminopropyl functionality. The peaks around 11 ppm, 20-30 ppm, and 44 ppm, correspond respectively to carbons 1, 2 and 3 as depicted on figure 3. The peaks were assigned in accordance with the

shielding

effect

provided

by

the

siloxane

and

amino

functionalities. The broader peak from carbon 2 could be attributed to conformational changes within the ligand. Signals around 16 ppm (peak 4) from materials NH2_1.5/MCM-41 and NH2_3.0/MCM-41 were attributed to adsorbed residual silane coupling agent, since such


12

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signals

were

not

found

in

the

parent

material

and

do

not

corroborate with previous literature.68 Interestingly, in both

29Si

and

13C

CPMAS NMR spectra, the

signals corresponding to propylsiloxane were consistently shifted downfield as the degree of functionalization increased, except for sample

NH2_6.0/MCM-41,

which

features

similar

displacement

to

NH2_4.5/MCM-41. This indicated that functionalization reached a plateau for APTMS initial loading of 4.5 mmol.g-1 (vide infra). Overall,

29Si

and

13C

CPMAS NMR confirmed the covalent grafting of

APTMS and the preservation of the propylamine structure. Following,

TGA

and

XPS

were

performed

to

quantify

the

effective grafting of the aminopropyl groups onto the supports (Table 1, further details in SI, sections 1 and 2). TGA allowed to access the degrees of functionalization, based on the premise that organic matter mass loss was coming from grafted propylamines and gave values of 1.7, 2.5, 3.3 and 3.2 mmol.g-1 for materials of theoretical functionalization 1.5, 3.0, 4.5 and 6.0, respectively. TGA confirmed complete loading at low charge, and then saturation of the loading at 3.3 mmol.g-1 at higher charge. As NMR results already suggested, functionalization reached a maximum for the initial APTMS load of 4.5 mmol.g-1, and plateaued thereafter. XPS functionalization degrees were obtained from Si/N atomic ratios and gave values of 2.3, 3.2, 4.2, and 4.0 mmol.g-1 for the initially added 1.5, 3.0, 4.5 and 6.0 mmol.g-1, respectively. Although this


13


Page 14 of 45

data follows the same trend as the data obtained by TGA, XPS systematically led to slightly higher anchoring rates. This can be understood by the fact that XPS is a surface technique, evaluating only the first few nm of the studied material. Hence it is known that grafting in mesoporous silica is usually more efficient in the outer surfaces and regions close to the pore openings.

Table 1. Degree of aminopropyl functionalization for materials NH2_X/MCM-41 (X=1.5, 3.0, 4.5 and 6.0) obtained by TGA and the XPS Si/N atomic ratios, in mmol of aminopropyl group per gram of MCM-41 and BET specific surface areas for pure MCM-41 and the four functionalized materials. Figures in parenthesis carry the calculation uncertainties.

Entry

1 2 3 4 5

Material

NH2_1.5/MCM41 NH2_3.0/MCM41 NH2_4.5/MCM41 NH2_6.0/MCM41 MCM-41

TGA effective

XPS effective

BET

degree of

degree of

Surface

functionalization functionalization Area (m2 (mmol.g-1)

(mmol.g-1)

g-1)

1,72(9)

2,3(3)

865

2,46(6)

3,2(2)

469

3,33(7)

4,1(7)

66

3,19(8)

3,9(6)

125

N/A

N/A

1271

In order to acquire information on the morphology and porosity of the materials, parent MCM-41 and aminopropyl-functionalized


14

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supports were characterized by N2-sorption and TEM (Figures S2 and S3). BET specific surface areas are shown in Table 1. As expected, specific surface area decreases as grafting load increases due to the possible obstruction of pores caused by the functionalization process.48

Samples

NH2_1.5/MCM-41

and

NH2_3.0/MCM-41

featured

relatively good specific surface areas of 865 and 469 m2 g-1 when compared to 1271 m2 g-1 for the parent MCM-41 (Table 1, entries 12 &5). We observed isotherm profiles (Figure S2) of the I(b) type, as proposed to Thommes et al.,69 associated with micropores or tiny mesopores, whose diameter lie just above the microporous threshold (2 nm). Transmission electron microscopy (TEM, Figure S3) confirms this point with pore sizes of about 3.3 nm. Materials NH-MCM41_4.5 and 6.0 were found to have surface areas as low as less than 10% of the surface of the original MCM-41 (Table 1, entries 3 and 4) and presented isotherm profiles of type II isotherm,69 typical of microporous or non-porous adsorbents, confirming that high functionalization degree leads to blocking most of pore entrances, essentially eliminating porous properties. Catalyst immobilization and catalytic activity study Once

we

have

established

that

the

materials

were

functionalized and learnt about their surface areas and porosity profiles, NH2_1.5/MCM-41 was chosen to be used to immobilize copper (I) species. The choice was made because such material has the highest surface area among the prepared supports and preserved


15


Page 16 of 45

most of its porous structure which is essential for improving diffusion

rates

featured

a

during

good

catalysis.

propylamine

The

chosen

loading,

material

essential

for

also Cu

stabilization. Therefore, that material was used to immobilize copper

species,

leading

to

the

catalyst

Cu

over

NH2/MCM-41

(Cu/NH2/MCM-41). NH2_1.5/MCM-41 was suspended in a CuI solution in acetonitrile and stirred for 24h. The amount of copper provided was chosen so that, in the case of adsorption on all the available metal, only half of the available sites would be occupied on the support, so that leached copper could be readsorbed onto the support after the reaction had been catalyzed. The previously white material turned light green upon Cu exposure and subsequent washing with acetonitrile. In order to evaluate the amount of immobilized copper in the hybrid material, it was digested in hot HNO3. The Cu content was then measured by ICP-OES analysis to be 0.0954 mmol g-1, which corresponds to occupation of about 10% of the available adsorption sites on the material (assuming each cation coordinates with two amine groups59). The

copper-loaded

materials

were

also

subjected

to

XPS

analysis in order to study the oxidation state of the metal. Figure 4 presents the Cu 2p spectra for Cu/NH2/MCM-41 and CuI and two important pieces of information can be obtained from them. First, the absence of satellite peaks around 945 eV means that there are


16

Page 17 of 45

no Cu(II) species in the materials,70 so the metal was not oxidized in the immobilization process. This is a desirable feature since Cu(I)

species

are

recognized

to

be

essential

for

catalytic

activity.4 Choi & Jang71 carried out comparative studies with both Cu(I) and Cu(II) salts and found the former to be more active in A3-coupling. Second, the binding energies slightly shifted to higher values when the Cu species were grafted on functionalized silica, as compared to CuI. Such a shift is consistent with Cu atoms coordination with a more electronegative atom, such as nitrogen in the amine functionality, and confirms the expected bonding scheme.

932.9 eV

Cu 2p

Intensity Intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


932.0 eV

951.7 eV 952.9 eV

Cu/NH Cu-NH-MCM-41 2/MCM-41 970 970

960 960

CuI 950 950

940 940

930 930

920 920

Binding Binding Energy energy (eV) (eV)

Figure 4. Cu 2p XPS spectra of CuI (green) and Cu/NH2/MCM-41 (black).


17


Page 18 of 45

The functionalized material was tested for the A3-coupling of the model reagents: cyclohexanecarboxyaldehyde, piperidine and phenylacetylene (Scheme 1). The reaction conditions were initially chosen according to previous experience from our group. Initially, the reaction was carried out in a solvent-free fashion at 100oC for 5 minutes in a microwave reactor and a catalyst load of 0.4 mol% (Table 2, entries 1). This provided moderate yields of 42%. Interestingly, upon reducing the load incrementally all the way to 0.02 mol%, this yield level remained remarkably stable. In order to push the reaction yield, the reaction time was varied.

the At

the low charge of 0.02 mol%, excellent yield of 95% in 2h were measured. 0.02 mol% was the minimum feasible charge at our reaction scale (0.5 mL). We thus increased the scale of the reaction to 1 mL, which enabled the use of 0.01 mol% of catalyst. This lower catalyst load provided a good yield of 85% after 2h of reaction (Table 2, entry 11). We set the optimized reaction conditions at 0.02 mol% of catalyst for 2h.

Cu-NH-MCM-41 0.02 mol% Cu o 100 C (MW), 2h, neat

N H H

O

N

- H 2O

Scheme 1. A3-coupling reaction scheme between cyclohexanecarboxyaldehyde, piperidine and phenylacetylene.


18

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Table 2. Catalyst load and reaction time optimization. All the reactions were carried out with the selected model substrates, under solvent-free conditions and 100oC microwave heating. Entry

Catalyst load (mol%)

1 2 3 4 6 7 8 9 10 11

0.4 0.2 0.1 0.05 0.02 0.02 0.02 0.02 0.02 0.01

Reaction time (min) 5 5 5 5 5 15 30 60 120 120

NMR yield (%) 42 41 42 48 43 79 85 92 95 85

A comparative kinetic study was carried out with 0.02 mol% of catalyst for the reaction catalyzed with either immobilized copper on Cu/NH2/MCM-41 or CuI alone as catalyst, using the exact same amount

of

copper

in both

cases.

The

recovered products were

systematically digested and submitted to ICP-OES Cu analysis, leading to a measure of the quantity of released Cu into the reaction mixture over time. Figure 5 summarizes the kinetic data obtained

for

product

formation

(solid

lines)

and

copper

concentration in solution (dotted lines).


19


Page 20 of 45

Figure 5. Kinetic study of the A3-coupling reaction of cyclohexanecarboxaldehyde, piperidine and phenylacetylene catalyzed by CuI or Cu/NH2/MCM-41 (solid lines – left ordinate) and Cu leaching into products (dotted lines – right ordinate). Reactions carried out with 0.02 mol% catalyst and 100°C under microwave heating.

With

the

supported

Cu/NH2/MCM-41

catalyst,

the

reaction

proceeded very fast, reaching a yield of 79% after 15 min and 95% after 2h, under microwave heating at 100°C with a catalyst load of 0.02 mol%. Using ICP-OES, we noticed that about 55 to 60% of the Cu species were released during the reaction. This value remained fairly stable over the reaction period. Homogeneous CuI however


20

Page 21 of 45 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


had a slower start, with a conversion of 47% at 15 min of reaction and 74% yield at 2 h, all other experimental conditions being identical. Intuitively, 100% of the Cu from CuI present was in solution. These measurements show two aspects. First, this proves that our system could keep at least a part of the Cu species immobilized, thus limiting contamination of the product. Secondly, the reaction was accelerated and overall more efficient across the reaction period for the heterogeneous system compared to the homogenous one. This is counter intuitive since in most systems, mass

transfer

causes

homogeneous

systems

to

outperform

heterogeneous ones.72 In order to rationalize this reactivity, we considered

the

possibility

of

having

a

confinement

effect,

affording accelerated rates in the pores of our catalyst.73 The

reactants

were

also

submitted

to

the

same

reaction

conditions, but this time replacing Cu/NH2/MCM-41 for a combination of homogeneous CuI plus NH2/MCM-41 or homogeneous CuI plus MCM-41. Both blanks gave the same results as the homogeneous catalyst itself (Supporting Information, Figure S6), indicating that the support does not play a role in the system unless copper species are effectively immobilized on the surface and allowed to permeate the pores of the material. Therefore, the immobilization of metal within the porous structure was demonstrated to be the key step for the observed catalytic activity increase.


21


Page 22 of 45

Indeed, porous materials are known for having a special sorption equilibrium caused by the curvature of the pores, which leads to an increased concentration of specific reagents inside the nanostructures. Such confinement effect has long been known on zeolites, the microporous relatives of mesoporous silicas, having Derouane and coworkers investigated the subject decades ago.74–76 In fact, the confinement effect on zeolites is considered one of the most interesting features of such materials.77 Although a solid literature exists on the confinement effect for zeolites, the knowledge of such phenomenon in materials with larger pores is still being built.78 The Kim group, for example, has shown that dyes

confined

properties

in

that

the

mesopores

differ

of

MOFs

significantly

from

have the

photophysical non-confined

molecules.79 Another interesting work was published by the Goswami group, in which the authors were able to conduct polyol metal nanoparticles synthesis inside the pores of polystyrene and silica gel

particles.80

conditions.

In

Such

methods

addition,

are

Herrmann

not &

possible Iglesias

under

ambient

studied

the

condensation of acetone in aluminosilicates with different types of pores using DFT calculations.81 By studying the transition states, the authors demonstrated that the rate constants greatly depend on the size and shape of the confining voids. There is also a study by Bereckzi et al.72 that found similar results to ours. The authors immobilized a Cu (II) complex on mesoporous silica


22

Page 23 of 45 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


aerogel and used it to catalyze the oxidation of phenol. They found that the immobilized catalyst had up to 15 times higher TOFs than the purely homogeneous complex and attributed this fact to the confinement of species inside the mesopores. Porous structures can, thus, concentrate catalytic species in their interiors, which allows the reagents to experience a local higher catalyst concentration. This enabled us to use of such a low catalyst amount, which is, the lowest ever reported in the literature for A3-coupling. The small scale of the reaction made the effective reutilization of the material impossible, which is an open avenue for further improving this system in the future. Nevertheless, the low catalyst load still gives our system maximum TON

and

TOF

of

4750

and

2375

h-1,

respectively,

which

are

competitive with the best values encountered in the literature for this reaction. Such systems often require multiple recycling steps to

achieve

high

TONs.

Montmorillonite-stabilized

copper

nanoparticles, for example, afforded a TON of 5640 over three runs24 and a catalyst based on copper supported on organosilica and ionic liquid framework gave a TON of

4526 over 7 runs.32 Most

works

the

found

in

the

literature

for

same

process

have

significantly lower TONs (i.e. below 1000), even after multiple cycles of reuse.25,27,28,30,36–38,82,83 The low catalyst load limits the copper contamination in the product to a maximum of about 50 ppm (in the extreme case of total


23


Page 24 of 45

leaching), which is already below the limits for oral exposure stablished by both the U.S. Food and Drugs Administration42 and the European Medicines Agency41 (300 and 250 ppm respectively), being good for most applications. ICP-OES measurements on the products showed that the system was able to scavenge about 40% of the total copper species used (Figure 5). Given the known ability of amine groups of coordinating to copper, it is reasonable to assume that the functionalized materials would be able to retrieve the metals from the reaction mixture, but it must be also taken into account the presence of other amines in the reaction mixture (reagents and products) that could compete with the support for the copper ions, being a possible cause of leaching. Despite such leaching, our system

remains

competitive

with

other

known

catalysts,

which

typically lead to more leaching. The combination of low metal load with the retention of part of it on the support led to final contamination in the product as low as only 30 ppm. In order to prove the versatility of the system, different aldehydes, amines and alkynes were used to build a substrate scope (Table 3, expanded on Table S2). A3-coupling is a well-known reaction and a range of different propargylamines were obtained in good to excellent yields. Aromatic aldehydes (entries 2-3) were less

active

than

the

aliphatic

model

substrate

and

required

slightly higher reaction temperatures. Product was observed with both cyclic and non-cyclic secondary amines (entries 4-6) and


24

Page 25 of 45 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


morpholine (entry 6) was slightly less reactive than the other ones due to the presence of the oxygen atom in the ring, which is electron-withdrawing

and

reduces

the

basicity

of

the

amine.

Aromatic and non-aromatic alkynes were also used (entries 7-8) and the former provided slightly higher yield since the resonance structures contribute to make the terminal hydrogen more acidic and the C-H bond more easily activated. The system proved to be selective

to

the

propargylamine,

having

all

the

additional

functional groups used in the scope remained untouched in the final product. Table 3. A3-coupling substrate scope using Cu/NH2/MCM-41.a Entr y

Aldehyde O

1

O

2

O

3

Amine

Alkyne

Product

H N

N

H N

N

H N

H3CO

Yield (%) 95

68b

N

79b

H3CO O

O

4

H N

N

87

O


25


O

5

Page 26 of 45

N

H N

91

N

O H N

6

O

7

59

H N

N

91 Br Br

O

8

H N

N

79

aReaction

conditions as described in the typical procedure.

bAromatic

aldehydes were reacted at 120oC.

Experimental All the reagents used were purchased from Sigma-Aldrich and used as provided, unless specified. Synthesis of catalysts: MCM-41 was synthesized according to a procedure described by us (Moura) in a previous report.60 The functionalization was carried out by modifying a method used by Zapata et al.64 Initially, a suspension containing 0.5 g of parent MCM-41 in 10 mL of dry toluene was prepared in a polypropylene tube

and

sonicated

for

1h.

Then,

25

mL

of

a

(3-

aminopropyl)trimethoxysilane (APTMS) solution in dry toluene was added to the suspension, being the amount of APTMS calculated according to the desired degree of functionalization. In this work,


26

Page 27 of 45 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


the loadings were 1.5, 3.0, 4.5 and 6.0 mmol g-1. The suspensions containing both MCM-41 and APTMS were magnetically stirred at 1600 rpm and 30°C for 24h. Each of the functionalized materials was, then, vacuum filtered and washed with 300 mL of ethanol P.A., ovendried for 18h at 115°C and washed with dry toluene in a soxhlet extractor for 3h, followed by extra rinsing with ethanol P.A. and oven-dried for extra 18h at 100°C. The four functionalized fine white powders obtained were named after their nitrogen loading: N1.5, N3.0, N4.5 and N6.0. Once the materials were obtained, N1.5 was used to immobilize copper (I) species by suspending 200 mg of N1.5 in 200 mL of a 0.4 mmol L-1 CuI solution in acetonitrile. The suspension of stirred at 800 rpm for 24h at room temperature. The greenish powder obtained was vigorously washed with approximately 300 mL of acetonitrile, then dried in vacuum oven at 50°C for 18h, maintaining its greenish color. Characterization of the catalysts: Parent MCM-41 and the four functionalized materials were characterized by N2 sorption, solid state

29Si

analysis

and (TGA),

13C

CPMAS X-ray

NMR

spectroscopy,

photoelectron

thermogravimetric

spectroscopy

(XPS),

inductively coupled plasma atomic emission spectroscopy (ICP-OES), and transmission electron microscopy. N2 sorption isotherms were obtained on a Micrometric TriStar 3000, 55 points scan at 77.35 K, having the samples being previously degassed at 100°C for 10 hours; the isotherms were analyzed by Brunauer, Emmett and Teller (BET)


27


Page 28 of 45

and Barrett, Joyner and Halenda (BJH) methods. Solid state NMR was performed on a 400 MHz Varian VNMRS equipment using 7.5 mm zirconia rotors spinning at 5 kHz with cross polarization and magical angle spinning (CPMAS), 20 ms acquisition time, 4 s recycle delay on 1024 scans, operating at Larmor frequencies of 100.53 MHz for and 79.42 MHz for in

concentrated

concentration

29Si.

trace

measured

13C

The copper-loaded materials were digested metal by

grade

HNO3

inductively

and

had

coupled

their

copper

plasma

atomic

emission spectroscopy (ICP-OES) on a Thermo iCap 6500 Duo Series spectrometer. Catalytic tests: The amine was chosen as the limiting reagent and the A3-coupling reaction reagents were added in a molar ration 1.6:1.3:1.6 of aldehyde:amine:alkyne and a catalyst load of 0.02 mol%.

A

typical

procedure

cylohexanecarboxaldehyde,

128

would L

of

take

piperidine,

200 180

L L

of of

phenylacetylene and 2.7 mg of Cu/NH2/MCM-41 (0.0954 mmol g-1 of Cu). The reagents and catalyst were added to a V-shaped microwave tube (0.5-2 mL) with a magnetic stir bar, sealed and set in a Biotage microwave reactor for 2h at 100oC and low frequency, 5s of pre-stirring. Following, 100 L of diphenylmethane 99% were added to the crude product along with around 5 mL of ethyl acetate. The mixture was homogenized and filtered on celite, then the solvent was removed and the absolute yield was calculated by proton NMR using diphenylmethane as internal standard. Products


28

Page 29 of 45 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


were

also

qualitatively

analyzed

by

GC-MS.

The

proton

NMR

experiments were performed with 16 scans on a 500 MHz AVIIIHD Bruker

spectrometer. Additionally, the products

were properly

digested and the leached copper amount was also measured by ICPOES. The digestion was carried out in a multi-step procedure consisted by wet ashing with concentrated sulfuric acid, followed by an open digestion step with concentrated nitric acid. Conclusion Our

results

position

amine-functionalized

MCM-41

in

a

strategic position as a non-innocent support for copper (I) species in the A3-coupling reaction. The support did not only deliver the catalytic species, but also enabled their confinement inside the pores, which made possible the use of the lowest catalyst loads ever reported for this reaction, working better than the purely homogeneous catalyst itself. Furthermore, even though leaching of the metal species was observed at the end of the reaction, the support is still able to scavenge part of those leached species, leading to a product contamination as low as 30 ppm, which is one order of magnitude lower than the concentration permitted by regulatory agencies. The products were, therefore, obtained from minimum metal usage and dispense costly and energy-consuming metal purification steps. This work showcases the power of mesoporous silica as a catalytic support by challenging current boundary


29


conditions

exploits

one

of

the

multiple

Page 30 of 45

possibilities

for

improving supported metal catalysis.

Acknowledgements We thank the Natural Science and Engineering Research Council of Canada

(NSERC)

Discovery

Grant

and

accelerator

programs,

the

Canada Foundation for Innovation (CFI), the Canada Research Chairs (CRC), the Centre for Green Chemistry and Catalysis (CGCC) and McGill University for their financial support. We also thank Dr. Robin Stein for the precious help with Solid State NMR, Andrew Golsztajn for the dedication on ICP Analysis and Alain Y. Li for his priceless help and advice. Supporting information TGA

profiles

description

and of

XPS the

atomic

percentage

calculation

of

tables,

effective

as

well

degrees

as of

functionalization by both methods. Full N2 sorption isotherms for all materials and TEM image depicting the porous structure of the synthesized MCM-41. Information on the identification of the main product, including detailed

1H

NMR and mass spectra. Also results

of catalytic control experiments and an expanded scope table.

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FOR TABLE OF CONTENTS USE ONLY

A3-coupling reaction happens with low catalyst load and yields lowcontaminated product, limiting the use of metal and eliminating the need for product purification.


45

Amine-functionalized mesoporous silica as a support for on-demand

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