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PMO-Immobilized AuI–NHC Complexes: HeterogeneousCatalysts for Sustainable ProcessesEls De Canck+,[a] Fady Nahra+,[b] Kevin Bevernaege,[a, b] Sofie Vanden Broeck,[b]

Judith Ouwehand,[a] Diederick Maes,[a, b] Steven P. Nolan,*[b] and Pascal Van Der Voort*[a]

1. Introduction

The field of gold chemistry and catalysis continues to flourishafter a decade of important advances.[1] The exploration of

gold(I)-NHC chemistry (NHC=N-heterocyclic carbene) hasgained increased attention mainly due to the ever-growing de-

velopment of NHC ligand design and the tenability of the ar-chitecture.[2,3] In recent years, transition metal hydroxide com-

plexes have emerged as simple and versatile reagents that

forgo the need for external bases.[4] The strong stabilizingpower of N-heterocyclic carbene (NHC) ligands has thus been

leveraged to isolate several of these reactive metal-hydroxidespecies.[5] Among these species, gold(I)-NHC hydroxides have

proven highly efficient in several catalytic transformations aswell as being extremely useful synthons to access a variety offunctionalized gold(I)-NHC complexes.[5a–d,6]

The high stability of these NHC-based complexes combinedwith their equally efficient reactivity make them excellent can-

didates for immobilisation onto solid support. The heterogeni-zation of these homogeneous catalysts would allow easy re-

covery, recycling and reuse, therefore reducing costs andwaste production significantly.[7]

The immobilization of homogeneous metal-catalysts ontovarious solid supports by means of covalently attaching the

ligand to that support has been widely explored in a recent

search for more sustainable and greener methodologies forcatalysis.[8] Advantages, such as easy recovery and reuse of the

catalyst and/or lowering metal contamination of the final prod-ucts are mainly sought after.[7] Contrary to other metal systems

such as palladium and rhodium, gold(I) is thought to be abetter candidate for these supported-homogeneous systemssince the oxidation state of gold(I) does not change through-

out the processes ; a phenomenon that is typically linked withleaching.[9] In that context, we sought the development of im-mobilized catalysts that combine the performance of homoge-neous AuI-NHC catalysts with the ease of recovery and recy-

cling of heterogeneous systems.A promising candidate for the immobilization of gold(I)-NHC

catalysts are the Periodic Mesoporous Organosilicas (PMOs).[10]

This novel class of hybrid nanoporous materials are preparedvia the hydrolysis and condensation of a bis-silane (most com-

monly denoted as (R’O)3Si-R-Si(OR’)3) around a structure-direct-ing agent (SDA) in acidic or alkaline environment. The addition

of the SDA leads to the formation of porous organosilica mate-rials with mesopores generally between 2 to 30 nm. The pres-

ence of the organic bridge R, completely embedded between

the two silicon atoms, leads to highly tunable hybrid materials.The nature of the R group significantly influences the proper-

ties of the resulting solid material (e.g. , hydrophilicity/hydro-phobicity, rigidity/flexibility) ; different types of bridges are al-

ready employed for various high-end applications.[10,11] Thesematerials possess several advantages over their silica counter-

A stable periodic mesoporous organosilica (PMO) with accessi-ble sulfonic acid functionalities is prepared via a one-pot-syn-

thesis and is used as solid support for highly active catalysts,consisting of gold(I)-N-heterocyclic carbene (NHC) complexes.The gold complexes are successfully immobilized on the nano-porous hybrid material via a straightforward acid–base reactionwith the corresponding [Au(OH)(NHC)] synthon. This catalystdesign strategy results in a boomerang-type catalyst, allowing

the active species to detach from the surface to perform thecatalysis and then to recombine with the solid after all the

starting material is consumed. This boomerang behavior is as-sessed in the hydration of alkynes. The tested catalysts werefound to be active in the latter reaction, and after an acidicwork-up, the IPr*-based gold catalyst can be recovered andthen reused several times without any loss in efficiency.

[a] Dr. E. De Canck,+ K. Bevernaege, J. Ouwehand, D. Maes,Prof. Dr. P. Van Der VoortCenter for Ordered MaterialsOrganometallics and Catalysis (COMOC)Department of ChemistryGhent University, Campus SterreKrijgslaan 281—Building S39000 Ghent (Belgium)E-mail : Pascal.VanDerVoort@UGent.be

[b] Dr. F. Nahra,+ K. Bevernaege, S. Vanden Broeck, D. Maes, Prof. Dr. S. P. NolanDepartment of Chemistry and Center for Sustainable ChemistryGhent University, Campus SterreKrijgslaan 281—Building S3, 9000 Ghent (Belgium)E-mail : Steven.Nolan@UGent.be

[++] These authors contributed equally to this publication.

Supporting Information and the ORCID identification number(s) for theauthor(s) of this article can be found under :https://doi.org/10.1002/cphc.201701102.

An invited contribution to a Special Issue on Reactions in ConfinedSpaces

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ArticlesDOI: 10.1002/cphc.201701102

parts, such as their higher hydrothermal stability and the pres-ence of functionalized and stable C@C bonds instead of labile

siloxane bridge, which are more useful in grafting proce-dures.[10,12]

Metal-immobilization on PMOs has already been reportedusing various metals (e.g. , Cu, Pd, Ru and Rh).[10] In that con-

text, immobilization either as metal-complexes or as nanoparti-cles has been performed. Several reports also described PMOsas support for gold nanoparticles.[13] In that manner, phenyl-ene-bridged materials loaded with gold nanoparticles wereevaluated for the Ullmann coupling of iodobenzene and phe-nol.[13a] Also, similar materials were used in the aerobic oxida-tion of alcohols and were recycled for 7 runs.[13b] Bifunctional

PMO materials with thiol/sulfonic acid functions were also ex-amined as support for gold nanoparticles and used in the hy-

dration of alkynes.[13c]

However, examples of anchored gold(I) complexes for cata-lytic applications are rather scarce and are limited to the im-

mobilization of phosphine-based gold(I) catalysts, with whichthe hydration of alkenes was briefly investigated.[14] Neverthe-

less, after an example on immobilized N-acyclic carbene com-plexes of gold,[7d] a recent example of silica-immobilized NHC-

gold(I) complexes was reported with promising results in catal-

ysis, recyclability and flow applications.[7c]

In this work, the hydrothermal stability and tunability of

PMOs is fully exploited for the immobilization of highly-activegold(I)-NHC catalysts.[3b–f] In this study, a stable sulfonic-acid-

based PMO material is selected as this allows for easy anchor-ing of a gold(I)-NHC complex, via an acid-base reaction, to

form an anchored gold(I) sulfonate (Figure 1). The catalytic per-

formance of the resulting heterogeneous catalyst is evaluated

in the hydration of alkynes and its recyclability is subsequentlyinvestigated.

2. Results and Discussion

The sulfonic-acid-functionalized PMO material is selected dueto its ease of synthesis via a one-pot procedure and the pres-

ence of an -SO3H function as an anchoring point for gold.[15] Inaddition, recent reports have shown that sulfonic acid-based

gold(I) complexes are excellent precursors for cationic gold(I),

exhibiting high catalytic activity in several reactions.[16,17] Thepreparation of PMO-SO3H is based upon a procedure recently

developed by one of our groups (Figure 1).[18] It uses a mixtureof two bis-silanes, 1,2-bis(triethoxysilyl) ethane and 1-thiol-1,2-

bis(triethoxysilyl) ethane in a 1:1 mixture, which are co-con-densed around the commonly used structure-directing agent

(SDA) P123. The latter thiol containing precursor is synthesized

using the thiol acid-ene ”click“ chemistry.[18a] Hydrogen perox-ide is added during the synthesis to in situ oxidize all thiol

groups and convert them to sulfonic acid functions. After-wards, the SDA is removed via an extraction procedure with

acidified ethanol and the mesoporous solid support PMO-SO3His obtained as a white powder. By using this one-pot synthesismethod, an additional post-treatment for the transformation of

-SH into -SO3H with an oxidant and strong acid is avoided.Next to -SO3H functions, the solid also contains acidic sila-

nols. These functionalities are preferentially deactivated by acapping agent, hexamethyldisilazane (HMDS), in a straightfor-ward procedure (Figure 1). The reaction between the Si-OH onthe surface and the trimethyl silyl functions completely deacti-

vates the silanols and subsequently forms Si-O(SiMe3) groups.

Figure 1. Synthesis of the PMO-based gold(I)-NHC catalysts.

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In this study, two [Au(OH)(NHC)] complexes are anchoredonto the PMO material as it was previously shown that these

complexes are highly active; [Au(OH)(IPr)] and [Au(OH)(IPr*)](IPr=N,N’-bis[2,6-(di-iso-propyl)phenyl]imidazol-2-ylidene;

IPr*=N,N’-1,3-bis[2,6-bis(diphenylmethyl)-4-methylphenyl]imi-dazol-2-ylidene).[16,19] The IPr*-based complex is bulkier than itsIPr counterpart and thus offers significantly higher steric pro-tection of the gold moiety; this could allow for a better protec-tion of the active catalyst through the recycling process. The

complexes are prepared according to a previously describedsilver-free procedure starting from the [AuCl(NHC)] relatives

using sodium hydroxide and tert-amyl alcohol.[5d]

The NHC-based gold hydroxides were readily reacted with

the capped and non-capped PMO-SO3H materials in stoichio-metric amounts based on the pre-determined acidity of the

latter (Figure 1). The resulting support is characterized via vari-

ous analysis techniques and the results of nitrogen sorption,elemental analysis (CHNS) and acidity are shown in Table 1.

The nitrogen sorption results (Figure 2) clearly show iso-therms of type IV with H1 hysteresis, indicating the formation

of mesoporous materials with cylindrical and uniform pores inthe range of 4–5.5 nm as revealed by the narrow pore size dis-

tribution. The specific surface area, pore volume and size all

decrease after the two functionalization steps, due to the addi-tion of the trimethyl silyl group and the large gold(I)-NHC com-

plex. Both functionalities result in a significant mass increaseand decoration of the pore walls. The X-ray diffraction pattern

of PMO-SO3H (Supporting Information; Figure S1) shows an in-tense (100) diffraction and two usually weaker second-order

signals which can be contributed to the (110) and (200) reflec-tions. This evidently indicates that a PMO with a 2D hexagonal

pore (p6mm) structure is obtained. Both techniques show thatthe obtained PMO materials are well-ordered and structured.

Elemental analysis (CHNS) provides the amount of sulfur

present in the entire material (PMO-SO3H), i.e. , 0.78 mmolg@1

which is in agreement with previously reported values.[18b] Itmust be noted that this is the total amount of sulfur, includingalso functionalities that are present inside the pore walls and

not reachable for any chemical modification. This immediatelyexplains the discrepancy between the S content and the acidi-

ty obtained by acid base titration (0.62 mmolg@1). The acid-

base titration can only reach the sulfonic acid functions thatare on the surface of the material and therefore are available

for modification reactions. The acidity value gives an estimateof the maximum possible loading of gold(I)-NHC complexes

onto the PMO material as it also includes a contribution of thesilanols still present on the PMO-SO3H. After capping and im-

mobilization of the gold(I)-NHC complexes, only a slight de-

crease in sulfur content is observed and in combination withthe nitrogen sorption and XRD results, this also points towards

Table 1. Structural and chemical characterization of the prepared PMO materials.

SBET[a]

[m2 g@1]Vp

[b]

[mLg@1]dp

[c]

[nm]S content[e]

[mmolg@1]N content[e]

[mmolg@1][H+][d]

[mmolg@1]Au content[f]

[mmolg@1]

PMO-SO3H 590 0.95 5.5 0.78 - 0.62 -cPMO-SO3H 361 0.50 5.0 0.77 - n.d. -PMO-SO3-Au(IPr) 250 0.35 4.3 0.59 0.88 - 0.44cPMO-SO3-Au(IPr) 225 0.31 4.1 0.77 0.52 - 0.26cPMO-SO3-Au(IPr*) 240 0.31 4.0 0.75 0.22 - 0.11

[a] Calculated via BET equation. [b] Determined at P/P0=0.95. [c] Determined via the BJH method on the desorption branch. [d] Determined via acid-basetitration. [e] Determined via elemental analysis (CHNS). [f] Determined based on the nitrogen content and the reacted amounts of [Au(OH)(NHC)] after stoi-chiometric reaction with the PMO (stoichiometry is based on acidity). n.d.=not determined.

Figure 2. Nitrogen sorption isotherm of PMO-SO3H (A) and cPMO-SO3-Au(IPr*) (B). Pore size distribution of PMO-SO3H (C) and cPMO-SO3H- Au(IPr*) (D). cPMO-SO3H has a similar spectrum as PMO-SO3H and therefore is omitted for clarity. Isotherm B is vertically offset by 400 mL (STP) g@1.

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a retention of the PMO structure after functionalization. Moreimportantly, the addition of nitrogen on the structure (&0.52

and 0.22 mmolg@1 for IPr- and IPr*-based complexes, respec-tively) highlights the successful immobilization of the com-

plexes as these ligands contain two nitrogen atoms. The result-ing cPMO-SO3-Au(NHC) materials hold a gold(I) complex load-ing of 0.26 and 0.11 mmolg@1 (for IPr and IPr*, respectively).This loading is based on half the nitrogen content as well asthe amount of [Au(OH)(NHC)] that reacted during the anchor-

ing reaction; both values matched perfectly.Spectroscopic analysis provides more information on the

chemical structure of the solids. Infrared spectroscopy firstshows the successful synthesis of PMO-SO3H, in agreement

with previously reported data,[18b] and moreover demonstratesthe effectiveness of the HMDS treatment (Figures 3 and 4). The

band at &3750 cm@1, originating from the SiO-H stretch of the

isolated silanols, disappears completely (Figure 3). Furthermore,in the spectrum of cPMO-SO3-Au(IPr*) one can observe new

signals appearing between 3070 and 3030 cm@1 due to the ar-omatic rings of the NHC units. The presence of the NHC ligand

is also evident in the region between 1600 and 1390 cm@1

(Figure 4). This is also observed for the other two gold-based

materials (see supporting information). Additionally, the in situ

hydrogen peroxide treatment oxidized the thiol functions com-pletely towards -SO3H moieties, since the typical S-H stretch

around 2550 cm@1 is absent. This is also confirmed by theRaman spectroscopy measurements where no S-H stretch is

visible (Figure 5). The Raman spectrum also shows that no di-sulfide (S-S) bridges are present since the typical strong peak

in the 450–600 cm@1 range is absent as well (Figure 5). The for-

mation of disulfides can occur during PMO synthesis, as previ-ously reported.[18a] Moreover, typical bands around 1410 and

1040 cm@1 for sulfonic acids appear in the Raman spectrumconfirming the oxidation of the sulfur functions (Figure 5).

With the immobilization process now confirmed, the catalyt-

ic activity was investigated in the hydration of alkynes. The hy-dration of alkynes is an atom-economical and straightforward

reaction that allows rapid access to ketones.[16,19] Gold com-plexes are well-known catalysts for this transformation; electro-

philic gold(I) is highly efficient in activating the C/C bond to-wards the nucleophilic attack of water.[20] Although recent re-

Figure 3. Infrared spectrum of A. PMO-SO3H; B. cPMO-SO3H and C. cPMO-SO3-Au(IPr*). Only the relevant region between 4000 and 2700 cm@1 isshown.

Figure 4. Infrared spectrum of A. PMO-SO3H; B. cPMO-SO3H and C. cPMO-SO3-Au(IPr*). Only the relevant region between 1800 and 750 cm@1 is shown.

Figure 5. Raman spectrum of cPMO-SO3H. Only the relevant regions be-tween 3300-2400 and 1800–750 cm@1 are shown.

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ports have described highly efficient methods to performalkyne hydration, these procedures still make use of silver (or

other halide abstractors) or/and acid to afford high conversionrates at very low catalyst loadings.[21] Silver- and acid-free pro-

tocols have emerged as a suitable alternative; among theseprotocols sulfate- or sulfonate-based gold(I)-NHC catalysts

have shown some promising activity.[6e,16,17b,d, 19,22] In that re-spect, the PMO-based gold(I) materials seem to be well-suitedto be used as catalysts in the hydration of alkynes. Also, as

noted previously, the gold hydroxides that where anchored inthis study have been prepared using a silver-free protocol, in

order to prevent any silver contamination. At first, the PMO-SO3-Au(IPr) catalyst (Cat. 1) was tested at different loadings

(Table 2, entries 1–3). This resulted in yields of 62% and 91%after one hour using 5 mol% and 10 mol% of Cat. 1, respec-tively. After the reaction time was increased to 5 h (Table 2,

entry 5), an increase in yield from 62% to 76% was observedusing only 5 mol% of the catalyst (Table 2, entries 2 vs. 5). Fur-

thermore, a blank reaction was performed confirming that thePMO material on its own did not show any catalytic activity

(Table 2, entry 4). These reactions all made use of a catalystwhere the PMO material was not capped. The improvement

made by capping can clearly be seen when the reaction with a

capped material is considered (Cat. 2, Table 2, entry 6). In thiscase, a yield of 91% is reached after 5 h, whereas the uncap-

ped material only reached 76% after 5 h at the same catalystloading (Table 2, entries 5 vs. 6). It should be noted that when

the capped cPMO-SO3-Au(IPr*) catalyst (Cat. 3) was used, a62% yield was obtained (Table 2, entry 7), which is relatively

high since IPr* catalysts are reported to have a much lower ac-

tivity than the IPr congeners in gold catalysis. Finally, when thecatalyst loadings of Cat. 1 and Cat. 3 were increased up to

7.5 mol%, a quantitative yield was obtained in both cases asindicated by 1H NMR spectroscopy, using 1,3,5-trimethoxyben-

zene as internal standard (Table 2, entries 8–9).Following the above optimization, the recycling process was

next investigated using both capped catalysts, Cat. 2 and Cat.

3 (Table 3). Reactions were conducted as previously described,

followed by simple filtration and washing to recover the cata-lysts. To the recovered catalysts, were then added the sub-

strate and the solvent mixture to start the second cycle. Un-fortunately, both catalysts showed poor activity in the second

run with 63% and 4% yield obtained for Cat. 2 and Cat. 3, re-spectively (Table 3, entries 1–4). Although Cat. 2 gave a better

yield, decomposition of the IPr-based catalyst was observed

after the 1st run and therefore it was disregarded. Cat.3showed no decomposition, however, deactivation of the cata-

lyst was still significant. At this point, it was decided to add areactivation step between the cycles. An acid treatment[23] was

applied after each cycle, using 2 equiv of H2SO4 (relative to thegold content) in diethyl ether followed by filtration, the recov-ered catalyst gave 98% and 99% yield in the second and third

runs, respectively (Table 3, entries 5–7). Note that similar treat-ment of non-metallated support led to no catalytic activity inthe hydration of aklynes.

3. Conclusions

In conclusion, we have developed a straightforward methodol-ogy to access PMO-supported gold(I)-NHC catalysts. The an-choring of the gold complexes was shown to be successful.

These materials were excellent catalysts for the hydration of di-phenylacetylene. The capped PMO material showed superior

reactivity in catalysis to the non-capped analogiues, highlight-ing a trend for future studies. The bulkier IPr*-based catalyst

was found to be better suited for the recycling process, proba-

bly due to the better shielding of the gold moiety. Finally, anacid work-up was necessary after each cycle in order to reacti-

vate the catalyst and obtain high yields in subsequent cycles.Further investigation of the deactivation process as well as the

synthesis of new types of NHC-based materials is still ongoingin our groups. We are convinced that this methodology will

Table 2. Hydration reactions using the PMO-based gold(I)-NHC catalysts:Optimisation[a]

Entry Catalyst [mol%] Time [h] Yield [%][b]

1 Cat. 1 [2.5] 1 592 Cat. 1 [5] 1 623 Cat. 1 [10] 1 914 PMO-SO3H [5] 5 05 Cat. 1 [5] 5 766 Cat. 2 [5] 5 917 Cat. 3 [5] 5 628 Cat. 3 [7.5] 3 >999 Cat. 2 [7.5] 3 >99

[a] Reaction conditions: 0.05 mmol of diphenylacetylene in a mixture ofdioxane and H2O (0.2 mL, 2:1), 80 8C, unless otherwise stated. [b] Yield de-termined by 1H NMR, using 1,3,5-trimethoxybenzene as internal standardand are average of two runs. Cat. 1=PMO-SO3-Au-IPr (not capped); Cat.2=cPMO-SO3-Au(IPr) (capped); Cat. 3=cPMO-SO3-Au(IPr*) (capped).

Table 3. Hydration reactions using the PMO-based gold(I)-NHC catalysts:Recycling[a]

Without an activation step

Entry Cycle Catalyst [mol%] Yield [%][b]

1 1 Cat. 2 [7.5] >992 1 Cat. 3 [7.5] >993 2 Cat. 2 [7.5] 634 2 Cat. 3 [7.5] 4

With an activation step

5 1 Cat. 3 [7.5] >996 2 Cat. 3 [7.5] 987 3 Cat. 3 [7.5] 99

[a] Reaction conditions: diphenylacetylene (1 equiv), 7.5 mol% of catalyst(50 mg), dioxane/H2O (0.25m, 2:1), 80 8C, 3 h, unless stated otherwise.[b] Yield determined by 1H NMR, using 1,3,5-trimethoxybenzene as inter-nal standard. Cat. 1=PMO-SO3-Au-IPr (not capped) ; Cat. 2=cPMO-SO3-Au(IPr) (capped); Cat. 3=cPMO-SO3-Au(IPr*) (capped)

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move forward the discussion[24] in this research area and leadto significant advance in the ultimate quest for more sustain-

able chemistry and catalysis.

Experimental Section

Materials

EO20PO70EO20 (P123), reagent grade acetone, toluene, diethyl etherand pentane are purchased from Sigma Aldrich. Potassium chlo-ride, sodium chloride, concentrated hydrochloric acid (37%), con-centrated sulfuric acid, sodium hydroxide (98%) and hydrogen per-oxide (H2O2, 30 wt%) are obtained from Carl Roth. Ethanol (96%) isobtained from Fiers. The silanes 1,2-bis(triethoxysilyl)ethane (97%)and hexamethyldisilazane (HMDS, 98.5%) are acquired from ABCR.All reagents are used as received without any further purification.The precursor 1-thiol-1,2-bis(triethoxysilyl)ethane is preparedfollowing the recipe of Esquivel et al.[18a] [Au(OH)(IPr)] and[Au(OH)(IPr*)] were synthesised according to our previously report-ed procedure.[5d] 1H Nuclear Magnetic Resonance (NMR) spectrawere recorded on Bruker Advance 300 and 400 MHz Ultrashieldspectrometers at 298 K.

One-Pot Synthesis of the Sulfonic-Acid-Functionalized PMO

The structure-directing agent P123 (0.84 g) and KCl (5.36 g) areadded to water (29.52 mL) and HCl (4.2 mL). The mixture is vigo-rously stirred at 45 8C until P123 is completely dissolved. After-wards, a mixture of 50% of 1-thiol-1,2-bis(triethoxysilyl) ethane(1,00 mL) and 50% of 1,2-bis(triethoxysilyl) ethane (0.96 mL) isadded dropwise to the solution at 45 8C. After stirring for threehours, a total amount of 3 mL of H2O2 is added. This solution is fur-ther stirred for another 21 h, maintaining 45 8C and subsequentlyaged for 24 h at 95 8C under static conditions. After cooling downof the mixture and subsequent filtration, the white powder is thor-oughly washed with water (3V15 mL) and ethanol (2V15 mL). TheSDA is removed via an extraction procedure. The white powder issuspended in a solution of 300 mL ethanol and 8 mL of HCl for12 h at 80 8C. This process is repeated twice. After each filtration,the solid is washed with ethanol and after the final extraction pro-cedure, it is dried under vacuum at 120 8C. The resulting material isdenoted as PMO-SO3H.

HMDS Treatment of PMO-SO3H

The PMO material is treated with HMDS to deactivate any silanolspresent on the surface of the solid. PMO-SO3H is place in a flack,completely covered with pure HMDS and stirred for 5 h at roomtemperature while allowing any NH3 gases to escape. The solidsare filtered and washed with pentane. The white powder is subse-quently transferred to a flask and stirred in pentane for 1.5 h at35 8C to remove any remaining HMDS from the pores. Afterwards,the mixture is filtered and washed extensively with pentane. Final-ly, the product is dried overnight under vacuum at 100 8C. The re-sulting material is denoted as cPMO-SO3H.

Immobilization of AuI-NHC

PMO-SO3H (capped or non-capped, 1 equiv) and [Au(OH)(NHC)](1 equiv based on acidity of the initial PMO) were added into a vialfollowed by 3 mL of toluene. The mixture was stirred at 35 8C for4 h. The solvent was then removed under vacuum using a rotary

evaporator. THF was added and the solid was collected via filtra-tion, washed with THF and pentane and then dried under vacuumovernight.

Analysis and Characterization Methods

X-ray diffraction measurements are performed on an ARL X’TRADiffractometer from Thermo using radiation Cu Ka (45 kV and44 mA) and a Peltier cooled lithium drifted silicon solid stage de-tector. Nitrogen sorption experiments are performed on a Tristar3000 (Micromeritics) at @196 8C. Samples are pre-treated at 120 8Cfor 16 hours while degassing. The specific surface area (SBET) is de-termined using the Brunauer-Emmett-Teller (BET) equation, thepore diameter (dp) is derived from the Barrett-Joyner-Halenda (BJH)theory using the desorption branch and the pore volume (Vp) isdetermined at P/P0=0.95. Diffuse Reflectance Infrared FourierTransform (DRIFT) and Raman spectroscopy are recorded on ahybrid apparatus of Thermo Scientific containing a Nicolet 6700 IRand NXR FT-Raman spectrometer with a nitrogen cooled MCT-Adetector and InGaAs detector, respectively. The DRIFT spectra areobtained using a specially made Graseby Specac diffuse reactantcell, operating under vacuum and at elevated temperature (120 8C).Elemental analysis (CHNS) is performed on a Thermo Flash 2000 el-emental analyser using V2O5 as catalyst.

1H Nuclear Magnetic Reso-nance (NMR) spectra are recorded on a Bruker Advance (300 and400 MHz) Ultrashield spectrometer at 298 K CDCl3 as NMR solvent.

The acidity (H+) of the solids, expressed in mmolg@1, is determined

by performing an acid-base titration. First, a certain amount of ma-terial is stirred in 20 mL of 2m of NaCl for 24 h. Next, the solutionis titrated with standardized NaOH and phenolphthalein as indica-tor.

Catalytic Tests

0.05 mmol of diphenylacetylene (1 equiv) and the indicatedamount of the corresponding catalyst were stirred at 80 8C in a 1,4-dioxane/H2O mixture (0.2 mL, 2:1). After the indicated time,0.05 mmol (1 equiv) of the internal standard, 1,3,5-trimethoxyben-zene, in 0.4 mL of THF, was added, followed by 4 mL of diethylether. The mixture was then filtered through a microfilter andwashed with ether. The filtrate was then evaporated, and 0.5 mL ofdeuterated chloroform was added. A sample was taken to performthe 1H NMR spectroscopy.

Recycling Tests

The amounts of diphenylacetylene were calculated based on the7.5 mol% of catalyst used in the reaction (see Table 3). After the re-action was completed, an equimolar amount of internal standard(1,3,5-trimethoxybenzene) in 0.4 mL of THF was added. Now thereaction mixture was filtered and washed with diethyl ether andpentane. The resulting filtrate was evaporated at room tempera-ture at the rotavapor and the residue was diluted with CDCl3 toperform the 1H NMR. The remaining catalyst was dried undervacuum. The catalyst is reused as is when no activation step is de-scribed.

In the case of an activation step: Acid work-up: After each cycle,the recovered solid is suspended in ether and 2 equiv of sulfuricacid is added (based on initial gold content). The mixture is stirredfor 10 min, filtered, washed with ether and dried under highvacuum. The resulting solid is then reused in the next cycle.

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Acknowledgements

The authors gratefully thank Ghent University for financial sup-

port during this research. Funda AliÅ from our department is sin-cerely thanked for performing the elemental analysis and XRD

measurements. Additionally, Bram Vanderhaeghe is acknowl-

edged for practical assistance during the experiments. SPNthanks King Abdullah University of Science and Technology

(Award No. OSR-2015-CCF-1974-03.) for support. This work waspartially possible by financial support from the Research Founda-

tion—Flanders (FWO), through Grant Number 3G006813. Umi-core AG & Co. KG is gratefully acknowledged for the gift of gold

salts.

Keywords: alkyne hydration · gold · heterogeneous catalysis ·N-heterocyclic carbene · periodic mesoporus organosilica

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Manuscript received: October 11, 2017Accepted manuscript online: November 9, 2017Version of record online: January 5, 2018

ChemPhysChem 2018, 19, 430 – 436 www.chemphyschem.org T 2018 Wiley-VCH Verlag GmbH&Co. KGaA, Weinheim436

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