Pierre Roussel

Iulian Stefan

Is the Interpretation of Delayed-Choice Experiments Misleading?

The interpretation of an experimental realization of Wheeler's delayed-choice gedanken experiment is discussed and called into question.

Once more, we find that nature behaves in agreement with the prediction of quantum mechanics even in surprising situations where a tension with relativity seems to appear.

It is this quotation from J.S. Bell [START_REF] Bell | Speakable and Unspeakable in Quantum Mechanics[END_REF] which brings Vincent Jacques's report [START_REF] Jacques | [END_REF] to an end. What is it about? In this "Experimental realization of Wheeler's delayed-choice gedanken experiment " a Mach-Zehnder interferometer is used either in a "closed" configuration, the usual one, with a phase adjustment leading to the extinction of one photo-multiplicator (PM2), or in an "open" configuration where the second beam-splitter BS2 has been removed leading to an equal probability for the triggering of PM1 or PM2, as illustrated in the next drawing. "Closed" configuration (with a particular phase adjustment). It is argued by the author following Wheeler [START_REF] Wheeler | Quantum theory and measurement[END_REF] that in the closed configuration each photon detected at PM1"has arrived by both routes" whereas in the open configuration "the photon has travelled only one route".

Because of the short pulse duration in the presented experiment and since BS2 can be set or removed while the photon is travelling in the interferometer arm(s), hence after it passed BS1, it results what one could call a "delayed choice effect" : "Thus one decides the photon shall have come by one route or by both routes after it has already done its travel" [START_REF] Wheeler | Quantum theory and measurement[END_REF] or else again "We, now, by moving the mirror in or out have an unavoidable effect on what we have the right to say about the already past history of that photon."

In this interpretation, a change is then produced at the location of BS1 and up to BS2 by the arbitrary choice of setting in or not BS2. Here is the delayed-choice effect, the specific "tension with relativity". In Wheeler's article consequences could be found back at the time of Big Bang (see footnote further). But is this effect or this interpretation true? Does QM require or predict such a specific effect? We will argue that no, and in order to emphasize that nothing new is invented by the present authors, quotations from the founding fathers of quantum mechanics will be called as often as possible.

A tension with relativity is indeed at work each time a "wave packet reduction" occurs within the QM description of a micro-physics experiment and we will examine whether or not it applies here and if something more has to be added.

To begin, one has to go back to a long lasting debate between A. Einstein and N. Bohr. The former, as reported by the latter in 1955[4], had argued that in a double-slit diffraction experiment :

If in the experiment the electron is recorded at one point A of the plate then it is out of the question of ever observing an effect of this electron at another point (B), although the laws of ordinary wave propagation offer no room for a correlation between two such events. and indeed one could find the same type of remark much earlier (in 1927) at the fifth Conseil de Physique Solvay where Einstein commenting this time a single hole diffraction experiment declared:

If one uses Schrödinger's waves, the interpretation of |Ψ 2 | within a theory of individual events claiming to be complete, implies in my opinion a contradiction with the relativity postulate. Does the reported experiment deserve more than Einstein's remarks? It seems that no.

In the "open" configuration (no second beam-splitter BS2) of Wheeler's proposal, nothing more and nothing less is at work than in the Einstein-Bohr gedanken experiment. The space open to the detection of a "diffracted" electron becomes here the two tubes of space following the first beam splitter BS1 in the two arms of the interferometer which are together open to the detection of the photon. The two tubes of space happen to go across one each other without any particular consequences in this open configuration. Here again " If in the experiment the photon is detected at one point A (wherever is this point) it is out of the question...etc..." However after detection, the statement that the photon was (or travelled) in the arm corresponding to the triggered PM is as illegitimate as would be to draw any trajectory ending at the electron impact in the Einstein-Bohr experiment, or more generally to draw a trajectory between two subsequent position measurements, for, as stated by Schrödinger:

In general ... a variable has no definite value before I measure it ; then measuring it does not mean ascertaining the value that it has and still less the value that it had.

As regards these questions, a central issue is the status of the wave function. It is not the particle which is diffracted or split on the beam splitter mirror, or which travel "both routes" (or "one route"). The wave is not the object as ascertained by Bohr [START_REF] Bohr | Can quantum-mechanical description of physical reality be considered complete?[END_REF] :

The diffraction by the slit of the plane wave giving the symbolic representation of its state will imply etc... If a strange situation appears since a "symbolic representation" is diffracted by a real slit, nevertheless no doubt for Bohr that the particle is not the wave nor a model of it. That Ψ is not a model is stressed at length and with more clarity by Schrödinger [START_REF] Schrödinger | The present situation in quantum mechanics : a translation of Shrödinger's "cat paradox[END_REF] in its article written (in 1935) in reply to that of EPR (which contains the cat paradox chapter) where chapter 7 is untitled " The Ψ-function as Expectation catalog" where he explains -the well known statistical interpretation of Max Born!-that Ψ is "the means for predicting probability of measurement results". Not only the wave is not the particle, the object, but the wave has not to be considered as real when the "reduction" occurs : as written by Bell, a tension with relativity (a contradiction for Einstein), the same observation as that concerning the EPR, Einstein Podolsky Rosen, correlations.

Up to the location of the second beam splitter BS2, the probability of detecting a photon is one half all along each arm for each triggered event. If time is considered in the reported particular experimental situation, this probability has a time evolution to match the wave packet propagation.This does not depend on the presence or not of BS2. Of course it is no more true after the place of BS2 for if absent it leaves the sharing of the probability between the two arms unchanged whereas if present it changes drastically the subsequent wave function (or may do so if the phases are so adjusted) and consequently the detection probabilities.

It must be clearly settled that no change in the wave function is introduced by BS2 between BS1 and BS2. Assuming on the contrary that the photon was in one or the other arm in the open configuration would simply mean that after BS1 the photon is now in a mixture of state, instead of a pure state ("a complete expectation-catalog"," a maximum knowledge state", a Ψ-function in Schrödinger wording) as precisely required and demonstrated by the observed results in the closed configuration. Only a measurement, a wave packet reduction, is able to produce a change from a pure state to a mixture of states.

The use of Bohr's complementary principle is called in Wheeler's demonstration as well as in Jacques's article: Such an experiment supports Bohr's statement that the behavior of a quantum system is determined by the type of measurements performed on it. Moreover, it is clear that for the two complementary measurements considered here, the corresponding experimental settings are mutually exclusive ; that is, BS2 cannot be simultaneously present and absent.

Is this call to complementarity appropriate? This principle (actually more a warning than a principle) stresses that the use of experimental settings corresponding to conjugate variables are exclusive. After a common preparation leading to an initial state and its corresponding (pure state) wave function, the changes brought by the further instrument will be different depending on the variable to be measured and the actual subsequent measurement results will also be different. This principle does not claim that the initial state is changed as finally suggested by the "delayed choice" experiment or rather its interpretation. No doubt that after the location of BS2 results will depend in an exclusive way on the presence or not of BS2 but nothing has to be changed before.

If one is now convinced that this "delayed choice effect" is not really required by QM, why did J.A. Wheeler invent it? And finally, did really J.A. Wheeler believe himself in this effect he contributed so much to propagate?

This may be questioned when in the same article one finds:

"No elementary phenomenon is a phenomenon until it is a registered (observed) phenomenon". It is wrong to speak of the "route " of the photon in the experiment of the beam splitter. It is wrong to attribute a tangibility to the photon in all its travel from the point of entry to its last instant of flight.

This quotation 1 begins with the so often reported statement which during decades suggested a role for the observer's conscience ; but the rest of the quotation seems much closer to standard QM. Why has this part of Wheeler's article ignored for so long? If the beautiful experiment reported by Vincent Jacques and performing Wheeler's gedanken experiment does not produce any "delayed choice effect", it does exemplify the observation of a wave packet reduction in a very special situation : because of the short duration of the light pulse the wave packet is split into two parts with no connexity, which "travel" separately along the spectrometer arms.

1

 1


Dr Aymen R Skhiri

Ben Ridha

Jean-François Salem
email: jean-francois.soule@univ-rennes1.fr

Henri Soulé
email: henri.doucet@univ-rennes1.fr

Doucet

Dr J.-F Soulé

H Doucet

Salem Ben

Unprecedented Access to β-Arylated Selenophenes through Palladium-Catalysed Direct Arylation

ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Unprecedented access to -arylated selenophenes via palladium-catalysed direct arylation

Aymen Skhiri, [a,b] Ridha Ben Salem,* [b] Jean-François Soulé,* [a] Henri Doucet* [a] Abstract: Several methods allow the access to -arylated selenophenes; whereas the synthesis of -arylated selenophenes remains very challenging. We demonstrated that the Pd-catalysed coupling of benzenesulfonyl chlorides with selenophenes affords regiospecifically the -arylated selenophenes. The reaction proceeds with easily accessible catalyst, base and substrates, and tolerates a variety of substituents both on the benzene and selenophene moieties. This transformation allows the programmed synthesis of polyarylated selenophenes with potential applications in pharmaceutical and material chemistry, as the installation of aryl groups at the desired positions can be achieved.

(Hetero)arylated selenophenes are important structures with several applications such as in the fields of pharmaceutical chemistry and organo-electronics. [1] If the biological or physical properties of several 2-(hetero)arylselenophenes have been studied in detail, on the other hand, 3-arylselenophenes have attracted less attention. This is certainly due to the lack of convenient synthetic methods for the preparation of such structures. [2] Currently, -arylated selenophenes can be prepared via Suzuki coupling using 3-bromoselenophene derivatives [START_REF] Barancelli | C3-arylation of selenophenes via Suzuki coupling: a) D[END_REF] (Scheme 1) or via Negishi coupling. [START_REF] Schumacher | C3-arylation of selenophenes via Negishi coupling[END_REF] Scheme 1. Reported pathways for the synthesis of -arylated selenophenes from selenophenes.

Since one decade, the metal-catalysed functionalization of C-H bonds has emerged as a very powerful method for the simpler access to molecules useful to materials or biological applications. If specific C-H bonds of (hetero)arenes can be coupled with arenes, it provides one of the simplest way for access to bi(hetero)aryls. [5,6] From thiophene derivatives, by tuning the reaction conditions, both the and -arylated thiophenes can be easily obtained using Pdcatalysed direct arylation. [7,[START_REF] Okazawa | For Pd-catalysed direct -arylations of thiophenes: a)[END_REF] In contrast, relatively little effort has been expended toward developing such metal-catalysed direct arylations for the synthesis of arylated selenophenes. In 2008, Mori, Koumura et al. reported the first Pd-catalysed direct arylation of a selenophene derivative (Scheme 2, top). Using aryl iodides as aryl source and PdCl2(PPh3)2 catalyst, they obtained regioselectively the C5-arylated 2-formylselenophenes. [9a] Then, in 2014, Schneider et al. reported condition allowing the access to -arylselenophenes from selenophene and aryl halides (Scheme 2, middle). To our knowledge, the metal-catalysed direct arylation at -position of selenophenes has not been reported so far (Scheme 2, bottom). [9b] Therefore, it was of particular interest to us to achieve an arylation at the 3-or 4positions of selenophenes. Dong et al. reported in 2009 an example of Pd-catalysed direct arylation, via a desulfitative coupling, of a quinoline using p-TolSO2Cl as aryl source. [10,11] Following this seminal result, the use of ArSO2Cl as aryl source for the Pd-catalysed direct arylations has been extended to a variety of heteroarenes by several groups. [12,[START_REF] Zhang | For Pd-catalysed desulfitative couplings with heteroarenes: a)[END_REF] In 2014, we described a novel access to -arylated thiophenes from thiophenes and ArSO2Cl, using desulfitative Pd-catalysed direct arylation. [14] However, to our knowledge, the desulfitative direct arylation of selenophenes with ArSO2Cl has not been reported.

Herein, we describe an environmentally benign general protocol allowing the completely regioselective access to -arylated selenophenes, using Pd-catalysed desulfitative direct arylation, from selenophenes and ArSO2Cl (Scheme 2, bottom). The influence of the ArSO2Cl and selenophenes substituents is reported. The high yielding programmed synthesis of a tetraarylselenophene is also described.

Based on our previous results as a starting point, [14] we first examined the influence of several reaction conditions on the product formation for the reaction of 1.5 equiv. of 4-nitrobenzenesulfonyl chloride with selenophene at 140 °C during 4 h (Scheme 3 and Table 1 in SI). Using 10 mol% Pd(OAc)2 catalyst and 3 equiv. of Li2CO3 as the base in 1,4-dioxane, the C3-arylated selenophene 1a was regiospecifically obtained in 56% yield. No formation of the C2-arylated selenophene derivative 1b was observed by GC/MS and 1 H NMR analysis of the crude mixture. The use of PdCl(C3H5)(dppb), PdCl2, PdCl2(MeCN)2 or Pd(PPh3)4 catalysts also led regiospecifically to product 1a, but in lower yields. On the other hand, in the presence of a larger excess (6 equiv.) of Li2CO3 base, product 1a was obtained 79% yield. Then, we examined the influence of other bases such as Na2CO3, K2CO3, Cs2CO3, K3PO4 or KOAc; however, lower yields in 1a were obtained. Then, the influence of the para-substituents on ArSO2Cl for reaction with selenophene was examined using 10 mol% Pd(OAc)2 catalyst and 6 equiv. of Li2CO3 in dioxane (Scheme 4). First we employed electron-deficient ArSO2Cl. Cyano-and trifluoromethylsubstituents on ArSO2Cl gave regiospecifically the C3-arylated selenophenes 2 and 3 in 88% and 83% yields, respectively. Relatively good yields were also obtained for the coupling of 4chloro and 4-fluoro-benzenesulfonyl chlorides with selenophene, as the desired products 4 and 5 were isolated in 57% and 72% yields, respectively. It should be noted that no cleavage of the C-Cl bond was observed allowing further transformations. Good yields of 71% and 73% in 6 and 7 were also obtained from PhSO2Cl and p-TolSO2Cl; whereas, lower yields of 54% and 31% in 8 and 9 were produced from the electron-rich 4-tert-butyl-and 4-methoxybenzenesulfonyl chlorides. The influence of meta-substituents on the ArSO2Cl partner was also evaluated. ArSO2Cl bearing CF3, Cl or F meta-substituents were successfully coupled with selenophene, affording regiospecifically the desired C3-arylated selenophenes 10-12 in 74-82% yields. ArSO2Cl containing two CF3, Cl or F substituents also gave the expected products 13-15 in good to high yields; whereas, the electron-rich 3,4-dimethoxybenzenesulfonyl chloride afforded 16 in only 24% yield. ArSO2Cl containing cyano or chloro ortho-substituents were also tolerated affording 18 and 19 in 88% and 76% yields, respectively; whereas a nitro orthosubstituent gave 17 in only 31% yield. Polyfluorinated molecules are ubiquitous in medicinal chemistry, owing to fluorine atom properties which induce a dramatic change in the molecules behaviour. The use of ArSO2Cl containing fluorine atoms should offer a straightforward route to (poly)fluorinated 3-arylselenophenes. Indeed, from ArSO2Cl containing one to three fluoro substituents, the 3-arylselenophenes 21-23 were produced in 86-90% yields. As the use of ArSO2Cl as coupling partners in Pd-catalysed direct arylations tolerates bromo and iodo substituents, [14b] the behaviour of several (poly)halobenzenesulfonyl chlorides for coupling with selenophene was investigated (Scheme 5). Satisfactory yields in 24 and 25 were obtained in the presence of ArSO2Cl containing 2-or 4-bromo-substituents. An additional ortho-ethyl-substituent exhibits a minor influence, as with 2-ethyl-4bromobenzenesulfonyl chloride, a high yield of 74% in the target product 26 was obtained. 2-Bromobenzenesulfonyl chlorides bearing CF3 or F substituents also led to the expected 3-arylated selenophenes 27-29 in 75-83% yield. The reactivity of two dibromobenzenesulfonyl chlorides, which can be easily obtained by reaction of dibromobenzenes with chlorosulfonic acid, is also described in the scheme 5. In both cases, the target products 30 and 31 were obtained in good yields, without cleavage of both C-Br bonds. Then, although C-I bonds on benzene rings are known to be very reactive in the presence of palladium catalysts, we examined the reactivity of 4-iodobenzenesulfonyl chloride in the presence of selenophene. The reaction affords 32 in only 33% yield, but without cleavage of the C-I bond. The regioselectivity of the arylation of 2-or 3-substituted selenophenes was also determined (Scheme 6). We first examined the reactivity of 2-arylselenophenes, which could be easily obtained via direct arylation of selenophene by aryl bromides using reported conditions. [9b] High yields in 33-36 were obtained from four 2arylselenophenes using CF3-or ArSO2Cl as aryl source. In cases, a complete regioselectivity in favour of the arylation at C4-position of the 2-arylselenophene was observed. A bromo-substituent at C2-position on selenophene was also tolerated, affording the 4-aryl-2-bromoselenophenes 37-39 in 61-70% yields, without cleavage of the selenophenyl C-Br bond. Moreover, in the presence of 3,4-dibromobenzenesulfonyl chloride and 4iodobenzenesulfonyl chloride, the 4-arylated 2-bromoselenophene derivatives 40 and 41 were obtained in moderate yields, but without cleavage of the C-halo bonds on both coupling partners. The reaction of 3-(4-methoxyphenyl)selenophene 9 with 4nitrobenzenesulfonyl chloride regioselectively affords the 3,4diarylselenophene 42, which contains two different aryl groups, in 62% yield (Scheme 6, bottom). It was also possible to directly prepare the 3,4-diarylselenophene 43, bearing two identical aryl groups, by reaction of selenophene with 3 equiv. of benzenesulfonyl chloride derivative. Scheme 6. Influence of substituents on selenophene.

Then, from the previously obtained 2,4-diarylselenophene 34 and 4bromochlorobenzene in the presence of 2 mol% Pd(OAc)2 catalyst, the 2,3,5-triarylselenophene 44 containing three different aryl groups was obtained in 82% yield (Scheme 7, top). Finally, the tetraarylated selenophene 45 could be obtained by reaction of the 3,4-diarylselenophene 42 with 3 equiv. of 4-bromobenzonitrile in the presence of Pd(OAc)2 catalyst (Scheme 7, bottom). Although the mechanism cannot yet be elucidated, a catalytic cycle shown on scheme 8 can be proposed. The first step is probably the oxidative addition of the ArSO2Cl to Pd(II) to afford the Pd(IV) intermediate A as for Dong reaction. [10] Then, after elimination of SO2, the coordination of selenophene gives B. which affords C after migration of the aryl group to the -carbon atom of selenophene. [START_REF] Steinmetz | For mechanistic studies on the regioselectivity of the carbopalladation of thiophenes and relative deprotonation rates[END_REF] Finally, base-assisted proton abstraction gives the -arylated selenophene with regeneration of the Pd(II) species. However, an Heck-type Pd(0)/Pd(II) mechanism, with carbo-palladation followed by an anti-β-hydride elimination or a base-assisted E2 elimination is also possible. [16]

Scheme 8. Proposed catalytic cycle

In summary, we report here the first procedure promoting the hard-to-achieve arylation at -position of selenophene derivatives. reaction proceeds with easily accessible phosphine-free Pd(OAc)2 catalyst and Li2CO3 as base and tolerates a wide variety of substituents both on the ArSO2Cl and selenophene coupling partners. Moreover, this procedure allows the programmed synthesis of polyarylated selenophenes as the installation of aryl groups at the desired positions can be achieved. Due to the wide availability of diversely functionalised ArSO2Cl, this strategy (no expensive base and ligand) should be very attractive to synthetic or material chemists for access to -arylated and polyarylated selenophenes. The palladium-catalysed coupling of benzenesulfonyl chlorides with selenophene derivatives allows the access to -arylated selenophenes with complete regioselectivity. The reaction proceeds with easily accessible catalyst, base and substrates, without oxidant and tolerates a variety of substituents both on the benzene and selenophene moieties.

Scheme 2 .

 2 Scheme 2. Pd-catalysed direct arylation of selenophenes.

Scheme 3 .

 3 Scheme 3. Influence of the reaction conditions.

Scheme 4 .

 4 Scheme 4.Influence of the substituents on ArSO2Cl.

Scheme 5 .

 5 Scheme 5. Influence of bromo-or iodo-substituents on ArSO2Cl.

Scheme 7 .

 7 Scheme 7. Programmed polyarylations of selenophene.

Keywords:

  Palladium • catalysis • C-H functionalization • selenophenes • benzenesulfonyl chlorides Entry for the Table of Contents -regioselectivity Aymen Skhiri, Ridha Ben Salem,* Jean-François Soulé,* Henri Doucet * Page -Page Unprecedented access to -arylated selenophenes via palladium-catalysed direct arylation


Jacques Simonet
email: jacques.simonet@univ-rennes1.fr

Large-scale cathodic carboxylation of copper surfaces

Keywords: Cathodic doping of copper, Carboxylation of copper, Carbon dioxide electroreduction, Electrochemically modified surfaces, Metal-carbonaceous composites

Large scale carboxylation of copper can easily be achieved by reduction of CO 2 solubilised in aprotic polar solvents in the presence of tetramethylammonium salts (TMeA + X - ). Carbon dioxide could be inserted into the metal matrix (presumably in the form of the carbon dioxide anion radical) at high surface concentrations (up to 10 -7 mole cm -2 ), most probably organized in multi-layers. With significant amounts of electricity (>0.1 x 10 -2 C cm - 2 ), this cathodic procedure leads to an impressive corrosion phenomenon. Importantly, the interfacial carboxylation of copper may efficiently protect the interface against anodic corrosion (gain up to ΔE ≈ + 1.3 V). Moreover, the insertion process is reversible and anodic oxidation leads to release of gaseous CO 2 . Lastly, the external poly-carboxylated layers can easily be tailored to produce a variety of chemically modified interfaces.

Introduction

Carbon dioxide is known to react under a wide range of cathodic processes and many papers have recently been published on its catalytic reduction at different metallic surfaces [START_REF] Pletcher | The cathodic reduction of carbon dioxide-What can it realistically achieve? A mini review[END_REF][START_REF] Ganesh | Electrochemical conversion of carbon dioxide into renewable fuel chemicals -The role of nanomaterials and the commercialization[END_REF][START_REF] Lee | The electrochemical reduction of carbon dioxide (CO 2 ) to methanol in the presence of pyridoxine (vitamin B 6 )[END_REF][START_REF] Lv | Studies on the faradaic efficiency for electrochemical reduction of carbon dioxide to formate on tin electrodes[END_REF][START_REF] Scialdone | Electrochemical reduction of carbon dioxide to formic acid at a tin cathode in divided and undivided cells: effect of carbon dioxide pressure and other operating parameters[END_REF][START_REF] Wang | Nanostructured non precious metal catalysts for electrochemical reduction of carbon dioxide[END_REF][START_REF] Jianping | Preparation of a silver electrode with a three-dimensional surface and its performance in the electrochemical reduction of carbon dioxide[END_REF][START_REF] Khoshro | A study of the catalytic activity of symmetric and unsymmetric macrocyclic [N 4 2-] coordinated nickel complexes for electrochemical reduction of carbon dioxide[END_REF][START_REF] Bashir | Electrocatalytic reduction of carbon dioxide on SnO 2 /MWCNT in aqueous electrolyte solution[END_REF][START_REF] Fenwick | Electrocatalytic reduction of nitrogen and carbon dioxide to chemical fuels: challenges and opportunities for a solar fuel device[END_REF][START_REF] Grace | Electrochemical reduction of carbon dioxide at low overpotential on a polyaniline/Cu 2 O nanocomposite based electrode[END_REF][START_REF] García | Electro and photoelectrochemical reduction of carbon dioxide on multimetallic porphyrins/polyoxotungstate modified electrodes[END_REF][START_REF] Qiao | Highly-active copper oxide/copper electrocatalysts induced from hierarchical copper oxide nanospheres for carbon dioxide reduction reaction[END_REF][START_REF] Kumar | New trends in the development of heterogeneous catalysts for electrochemical CO 2 reduction, C1 Cat[END_REF]. Contributions based on the reduction of CO 2 to methanol, carbon monoxide, methane or salts such as oxalates might appear as a partial solution to decreasing the environmental nuisance of this effluent. The electrochemical reactivity of this molecule might possibly be employed for valorization of this pollutant via reduction processes or other modes of sequestration in conducting solids. Moreover, the electrophilicity of CO 2 has also been used to promote efficient reactions with different allotropic forms of carbon under cathodic activation in aprotic media. In this way, cathodically doped graphite, glassy carbon and graphene can be powerfully transformed into carboxylated materials [START_REF] Simonet | Conditions for a large sequestration of carbon dioxide into glassy carbon. generation of carbon carboxylate versatile material[END_REF][START_REF] Jouikov | Efficient cathodic carboxylation of graphene[END_REF].

Additionally, a recent development might permit the reduction and/or reversible sequestration of CO 2 into transition metals, as recently demonstrated with gold and silver [17,

A C C E P T E D M A N U S C R I P T 2 18]
, with the concomitant creation of carboxylated layers at the surface of these metals, allowing facile surface functionalization. Although this work is still in progress, many metals might exhibit similar carboxylation and reversible CO 2 sequestration behaviours. The present report on the reactivity of copper should be considered essentially as a preview of the ability of copper to be carboxylated to a large extent. Additionally, this metal, which is of considerable interest in organic chemistry as a catalyst and as a reducing species toward carbon-bromide bonds [START_REF] Jitaru | Solubility of carbon dioxide into usual organic solvents[END_REF], could be used as an electrode up to +1.2 V vs. an Ag/AgCl electrode when the surface has been carboxylated and then easily transformed into chemically modified layers. Other materials, including platinum, palladium, rhodium, iron, titanium, cobalt and nickel, appear to show promising results, but are not discussed in this paper.

Importantly, in some ways, the cathodic CO 2 insertion in copper discussed here may be viewed as a chemically reversible procedure that could possibly be put into practice to selective extract CO 2 from certain conducting fluids.

Strategy and experimental conditions

In the classical chemical literature, the catalytic conversion of carbon dioxide into renewable fuel chemicals (CO 2 saturated at room temperature in aqueous solutions or in organic polar solvents mixed with proton donors) was generally achieved using smooth transition metal electrodes as catalysts, but rarely with good selectivity [START_REF] Pletcher | The cathodic reduction of carbon dioxide-What can it realistically achieve? A mini review[END_REF]. In contrast, a totally different approach is presented here which concerns experiments conducted in dry polar aprotic solvents and containing exclusively tetra-methylammonium salts (TMeA + X -, essentially associated with tetrafluoroborate, sulphate and perchlorate). Iodides, bromides, and chlorides could be used as well, but part of the present study also concerns the behavior of the electrode within the anodic range, and it appears judicious to leave halides aside for the time being. Moreover, tetramethylammonium TMeA + salts were used in the present paper because no β-protons are available in the TMeA+ cation (ie there is no possibility of a Hofmann elimination) to protonate the CO 2 anion radical regarded as the main transient in the electrochemical process. Na+ could also be used, but is not described in this communication.

The choice of tetramethylammonium TMeA + salts is very possibly the origin of the successful process described hereafter with Cu. Additionally, the smallsize of the TMeA + ion may increase the rate of insertion inside the solid bulk.

In the present report, 0.1 M solutions of tetramethylammonium tetrafluoroborate in dimethylformamide (DMF) were exclusively employed. Chemicals were purchased from

A C C E P T E D M A N U S C R I P T 3
Aldrich (synthesis quality). Therefore, DMF was used as received (water content <1000 ppm). All electrochemical experiments were performed under an inert atmosphere by bubbling argon through the solution. The electrolytic solution was saturated with CO 2 (purchased from Air Liquide) by bubbling for 5 min at room temperature. Carbon dioxide was used as a saturated solution in DMF (0.199 mol L -1 at room temperature [START_REF] Jitaru | Solubility of carbon dioxide into usual organic solvents[END_REF]).

Potentials are all referred to an aqueous Ag/AgCl/KCl sat electrode. The electrochemical instrumentation has been described previously [START_REF] Simonet | Conditions for a large sequestration of carbon dioxide into glassy carbon. generation of carbon carboxylate versatile material[END_REF][START_REF] Jouikov | Efficient cathodic carboxylation of graphene[END_REF].

The copper electrodes (disk diameter: 1.5 and 2 mm) used in the voltammetry have apparent surface areas of 1.8 and 3.2 mm 2 , respectively. Before use, the electrode surfaces were carefully polished with silicon carbide paper (Struers 500 and 1200) or with Norton polishing paper (type 02 and 03) and rinsed with water, then with alcohol and lastly with acetone. After repeated scans or fixed potential electrolyses, the electrodes and metallic plates (exclusive use of pristine copper of purity 99.98%) were sonicated for two minutes in water.

Finally, before analysis, the electrodes were dried under a hot air flow for about 30s. The coulometry measurements and electrolyses reported in this work were carried out using a three-electrode, two-compartment cell with a total catholyte volume of about 5 ml.

Carboxylation of copper

The copper plates and polished Cu micro-electrodes were first rinsed with aqueous 0.1 M sulfuric acid, washed with water, and finally dried at 60°C. They may then be easily carboxylated. When large amounts of CO 2 are inserted into the metal, the facile anodic oxidation of Cu at E ≥ 0.2 V should prevent a priori any reliable anodic quantifications that concern superficial concentrations of trapped CO 2 , as previously noted with gold [START_REF] Jouikov | Cathodic carboxylation of gold in thick Au-CO 2 layers. A model for a reversible electrochemical sequestration of CO 2[END_REF].

Figure 1

However, under the conditions specified in Figure 1, copper surface carboxylation was successfully achieved in the course of repeated scans. The cathodic reduction (first scan) exhibits two broad steps, the first being assigned to the reduction of the adsorbed CO 2 . After several scans between 0V and -2.0V, the electrode surface was totally deactivated, presumably owing to the reaction of the CO 2 anion radicals with the copper surface. Fixed

A C C E P T E D M A N U S C R I P T

4 potential electrolyses carried out with Cu sheets showed progressive corrosion of the surface (Figure 2A and2B). The surface as modified after only one cathodic scan could be indirectly functionalized with amino-acceptors (such as 1-aminoanthraquinone) or with 2bromomethylanthraquinone, making it possible to indirectly estimate, via grafting of πacceptors, the apparent level of surface carboxylation (Γ CO2 ≈ 3 ± 2 x 10 -9 mol cm -2 ). FTIR analysis (absoption: 1780 cm -1 , stretching v(C=O); 1150 cm -1 , v(C-O); 950 cm -1 -H) confirmed the reactivity of Cu surfaces towards CO 2 . Lastly, the carboxylation procedure led to strongly hydrophilic interfaces (Figure 2C) that remained quite stable over time.

Anodic Cu passivation via carboxylation

The cathodic carboxylation process produces an efficient passivation of the Cu surface even when small amounts of electricity are applied, and reveals the high efficiency of the method. This is depicted in Figure 3. Therefore, after a single cathodic sweep down to -2.0 V (part A)reduction of CO 2 at Cu according to step a, the subsequent anodic exploration can reach quite a positive potential (+1.2V) without observing any corrosion of copper (part B). By contrast, a sharp step b is observed (first anodic scan only) which is assigned to the specific oxidation of the carboxylate interface, similar to a process observed [START_REF] Jouikov | Cathodic carboxylation of gold in thick Au-CO 2 layers. A model for a reversible electrochemical sequestration of CO 2[END_REF] with gold, which is known not to corrode within this potential range. The integration of the peak allows the thickness of the carboxylate layer to be estimated (based on a one-electron process possibly assigned to the cleavage of the Cu-CO 2 bond). Coulometric values obtained over the course of several experiments then gave Γ CO2 = 1.5 ± 2 x 10 -7 mole cm -2 , which can be interpreted as the total disappearance of a thick carboxylated multilayer. This value can be compared with the amount of electricity injected in the course of the reduction (obtained according to A) which gives Γ CO2 = 4 ± 1 x 10 -7 mole cm -2 . The values are quite comparable and any discrepancy could be explained by the fast concomitant cathodic corrosion depicted in Figure 2B.

The anodic pulse allows the total decarboxylation of the metal and permits the regeneration of a clean Cu interface. Under these conditions, one might obtain strongly reactive copper interfaces (similarly to the spirit of reference [START_REF] Rieke | Preparation and chemistry of the active copper species derived from CuI-PBu 3 , CuI-PPh 3 , and CuCN-nLiX complexes[END_REF] under a quite heavy process, generating Cu* strongly active toward organic electrophiles such as R-Brs) in polar solvents after getting rid of any residual CO 2 by simple bubbling of argon. This could be applied to the surface modification of other transition metals in which CO 2 insertion is observed.

A C C E P T E D M A N U S C R I P T

Conclusion

The cathodic carboxylation of copper under the experimental conditions given above allow the reversible formation of thick {Cu-CO 2 ‾ -TMeA + } multi-layers which may be removed by a simple and efficient anodic process. The method could be considered as a facile process to temporarily store CO 2 in a transition metal. Moreover, the efficient insertion of carbon dioxide in its reduced form (associated to a bulky ammonium cation or an alkaline metal cation like Na + or K + ) during extensive electrolyses would provoke a deep burst and corrosion of the pristine Cu surface. The granular aspect of the produced surface can be seen as a conglomerate of micro-particles carboxylated (size < 100 nm) at the surface which can be easily scratched off the transitory interface. The bursting of the interface by means of internal carboxylation concomitantly with the insertion of the salt makes it possible to extract the Cu nanoparticles, as depicted in Scheme1. This assumes a continuous degradation of the surface and depends on the applied potential. The generated copper boundaries showed a strong wetting ability. A pristine, clean and very active Cu surface, free of oxides, can easily be generated by anodic oxidation at E < +1.3 V. The present electrochemical method could be compared with the purely organic approach proposed by Rieke [START_REF] Rieke | Preparation and chemistry of the active copper species derived from CuI-PBu 3 , CuI-PPh 3 , and CuCN-nLiX complexes[END_REF] (reduction of CuI-PBu 3 using preformed lithium naphtalenide), allowing the formation of a copper interface reactive toward organic electrophiles.

Figure 3 Scheme 1 Figure captions Figure 1

 31captions1 Figure 3

Figure 2 :

 2 Figure 2: Reduction of CO 2 saturated at room temperature in DMF containing TMeABF 4 (0.1 M) at -1.8 V. Amount of electricity: 10 -2 C cm -2 .

Figure 3 :Scheme 1 : 9 Figure 1 ACCEPTED 11 Figure 3

 3191113 Figure 3: Carboxylation of smooth copper micro-electrode. Area: 3.2 mm 2 . Electrolyte: DMF + TMeABF 4 (0.1 M). Scan rate: 50 mV s -1 . (A) First cathodic scan immediately followed by (B) showing two successive recurrent oxidation sweeps up to + 1.5 V within the interval 0V to +1.5 V (beware that cathodic and anodic potential scales are different).

 Experimental conditions permit the insertion of the CO 2 anion radical.  A "reversible" carboxylation/de-carboxylation process is revealed.  Coverage of the Cu surface by carboxylates results in a strongly hydrophilic material.  Chemical insertion of the reduced form of CO 2 causes bursting of the Cu metal.


Guirec Le Bozec
email: guirec.lebozec@ensc-rennes.fr

Sylvain Giraudet

Laurent Le Polles

Pierre Le Cloirec

H NMR investigations of activated carbon loaded with volatile organic compounds: quantification, mechanisms and diffusivity determination

Keywords: Solid NMR, Activated carbon, Volatile Organic Compound, Adsorption mechanisms, Diffusion coefficients

Three volatile organic compounds (benzene, cyclohexane, dichloromethane) were adsorbed onto activated carbon fiber cloth. 1 H (MAS and PFG) NMR techniques were carried out and the signals were analyzed in terms of peak surface areas and shifts. These techniques were shown to be very useful for determining i) the intrinsic quantification of adsorbed molecules (VOC and/or water) in the porosity of the materials; the adsorption capacities ranged from 0.2 to 4 mol.kg -1 , ii) the mechanisms of interactions between adsorbed organic molecules and the carbon walls; illustrations of positions of the molecule inside the pore volume are proposed; the proton-wall distance was less than 0.15 nm, iii) the diffusivities; surface diffusion coefficients (D S ) were estimated at ≈ 4.10 -12 m 2 .s -1 for cyclohexane, ≈ 1.10 -11 m 2 .s -1 for benzene and ≈ 4.10 -11 m 2 .s -1 for dichloromethane.

INTRODUCTION

In both industrialized and developing countries, volatile organic compound (VOC) emissions are among the causes of atmospheric pollution. They act as chemical precursors of pollutants like tropospheric ozone and can also directly impact human health. One of the main VOC treatment processes is adsorption onto active carbon (grains or fiber cloths). The high porosity of these materials enables high retention rates and a very wide range of gas flow rates (100 to 10,000 m 3 /h) for a large diversity of organic molecules. In this context, knowledge of adsorbent supports and analyses of physical and chemical phenomena involved in the adsorption process are of utmost importance. These studies use several characterization methods and techniques. In general, porous volumes, specific surface areas, and pore size distributions can be deduced from the adsorption isotherm curves of nitrogen, as well as gases such as helium or CO 2 [START_REF] Sweatman | Characterization of Porous Materials by Gas Adsorption: Comparison of Nitrogen at 77 K and Carbon Dioxide at 298 K for Activated Carbon[END_REF] . These isotherms are described by means of the BET model 2 or, more recently, a model based on density functional theory [START_REF] Ravikovich | Density functional theory model of adsorption on amorphous and microporous solids[END_REF][START_REF] Neimark | Quenched solid density functional theory end pore size analysis of micro-mesoporous carbons[END_REF] calculations. From a chemical point of view, Boehm's method leads to the characterization and quantification of surface functional groups [START_REF] Boehm | Some aspects of the surface chemistry of carbon blacks and other carbons[END_REF] . Calorimetric measurements (microcalorimetry [START_REF] Toeckli | The enthalpies of immersion of active carbons in relation to the Dubinin theory for the volume filling of micropores[END_REF][START_REF] Toeckli | The external surface of microporous carbons derived from adsorption and immersion studies[END_REF][START_REF] Gregg | Adsorption surface and porosity[END_REF][START_REF] Groszek | Study of the Active Carbon-Water Interaction by Flow Adsorption Microcalorimetry[END_REF] , differential scanning calorimetry 10,[START_REF] Mcnaughton | Differential scanning calorimetry[END_REF][START_REF] Baudu | First approach of desorption energies of water and organic molecules onto active carbon by differential scanning calorimetry studies[END_REF] ) provide energetic values of adsorption interactions, in particular enthalpies and Gibbs energies. Isotherm, kinetic and breakthrough curves result from experimental data obtained through indirect measurements.

Adsorbed quantities are deduced from the difference between non-adsorbed compound concentrations (remaining in the reactor atmosphere) and initial concentrations. Quantitative analyses of organics in the gas phase are performed by gas chromatography separation (the case of multicomponent adsorption) [START_REF] Sudibandriyo | Adsorption of Methane, Nitrogen, Carbon Dioxide, and Their Binary Mixtures on Dry Activated Carbon at 318.2 K and Pressures up to 13.6 MPa[END_REF] . The detection and quantification of organics use spectroscopic or flame ionization detection (FID) methods [START_REF] Cloirec | Les composés organiques volatils dans l'environnement[END_REF][START_REF] Farmer | Continuous hourly analysis of C2-C10 non-methane hydrocarbon compounds in urban air by GC-FID[END_REF] . From these experimental data, diffusion coefficient values are obtained by indirect means such as parametric correlations and adjustments in adsorption kinetic models [START_REF] Giraudet | Adsorption and Electrothermal Desorption of Volatile Organic Compounds and Siloxanes onto an Activated Carbon Fiber Cloth for Biogas Purification[END_REF] .

Earlier nuclear magnetic resonance (NMR) investigations of organic compounds adsorbed onto active carbons and non-carbon lattices were conducted in the late 1980s [START_REF] Bronnimann | C NMR Study of methanol in HY zeolite[END_REF]18 . Due to significant technical progress, NMR now appears as an additional analytical tool to enhance studies of adsorbed compounds onto host porous active carbons [START_REF] Gun'ko | Characteristics of adsorption phase with water/organic mixtures at a surface of activated carbons possessing intraparticle and textural porosities[END_REF] . Most notably, NMR methods provide a dual benefit: non-destructivity and direct measurements of the adsorbed phase inside porous materials. Several studies have been carried out on non-carbon materials (MCM-41, zeolites, silicates). Pore size distributions have been characterized [START_REF] Schmidt | Pore size determination of MCM-41 mesoporous materials of 1 H NMR Spectroscopy, N 2 adsorption, and HREM. A preliminary study[END_REF] . Furthermore, using pulsed field gradient (PFG) NMR methods, information concerning the diffusion and adsorbed phase behavior inside the porosity has been obtained onto mesoporous [START_REF] Stallmach | Pulsed field gradient NMR studies of diffusion in MCM-41 mesoporous solids[END_REF][START_REF] Dvoyaskin | Pulsed field gradient NMR study of surface in mesoporous adsorbents[END_REF] and nanoporous adsorbents [START_REF] Valiulllin | Concentration-dependent selfdiffusion of liquids in nanopores: A nuclear magnetic resonance study[END_REF] . NMR techniques applied to carbon materials revealed two specific features. . Nevertheless, fruitful studies using 1 H, [START_REF] Sudibandriyo | Adsorption of Methane, Nitrogen, Carbon Dioxide, and Their Binary Mixtures on Dry Activated Carbon at 318.2 K and Pressures up to 13.6 MPa[END_REF] C, [START_REF] Gun'ko | Characteristics of adsorption phase with water/organic mixtures at a surface of activated carbons possessing intraparticle and textural porosities[END_REF] F, and 11 B NMR probes have been published [START_REF] Forse | Nuclear magnetic resonance study of ion adsorption on microporous carbide-derived carbon[END_REF][START_REF] Deschamps | Exploring electrolyte organization in supercapacitor electrodes with solid-state NMR[END_REF][START_REF] Mueller | Diffusion of Methane and Carbon Dioxide in Carbon Molecular Sieve Membranes by Multinuclear Pulsed Field Gradient NMR[END_REF] . Assuming a relationship between the peak shifts and the adsorption cavity porous diameter, resulting from the shielding effect at the carbon surface, authors have principally investigated the adsorbate behavior in the porosity. Furthermore, in order to improve the spectrum deconvolution, 2 H and 31 P magic-angle spinning (MAS) NMR techniques have been used to describe multicomponent adsorption phenomena onto activated carbon [START_REF] Shaw | Magic-Angle Spinning NMR Study of the Competitive Adsorption of an Organophosphate and an Organophosphonate on Activated Carbon[END_REF][START_REF] Harris | Phosphorus-31 NMR studies of adsorption onto activated carbon[END_REF] and to evaluate the effects of carbon substrates activation 30 . The combination of PFG and MAS condition was also carried out, providing diffusion results for liquid crystal confined into microporous glasses [START_REF] Romanova | Diffusion Studies in confined liquid crystal by MAS PFG NMR[END_REF] .

Studies using 1 H MAS NMR in porous carbon have been conducted [START_REF] Xu | Water and small organic molecules as probes for geometric confinement in well-ordered mesoporous carbon materials[END_REF] . Nevertheless, conclusive results remain scarce in the literature for the characterization of the adsorption of organic molecules using NMR methods (quantification of the adsorbed compounds, determination of the diffusion coefficients, characterization of the adsorbed phase for multicomponent adsorption, etc.).

The present work focused on the use of [START_REF] Sweatman | Characterization of Porous Materials by Gas Adsorption: Comparison of Nitrogen at 77 K and Carbon Dioxide at 298 K for Activated Carbon[END_REF] H NMR to study the mechanisms of volatile organic compounds adsorbed onto activated carbons. Both 1 H MAS NMR and PFG NMR were used to observe and discuss spectral features according to the adsorbate/adsorbent characteristics. In addition, [START_REF] Sweatman | Characterization of Porous Materials by Gas Adsorption: Comparison of Nitrogen at 77 K and Carbon Dioxide at 298 K for Activated Carbon[END_REF] H PFG NMR gave selective access to the diffusion coefficients of adsorbed molecules at the micrometer scale [START_REF] Levitt | Spin Dynamics. Basic of Nuclear Magnetic Resonance[END_REF][START_REF] Duer | Solid-state NMR spectroscopy: principles and applications[END_REF] .

MATERIALS AND METHODS

NMR experiments

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Langmuir [START_REF] Sweatman | Characterization of Porous Materials by Gas Adsorption: Comparison of Nitrogen at 77 K and Carbon Dioxide at 298 K for Activated Carbon[END_REF] H MAS NMR was employed to average out chemical shift anisotropies, homonuclear protonproton dipolar coupling and reduce the effect of magnetic susceptibility inhomogeneities. [START_REF] Sweatman | Characterization of Porous Materials by Gas Adsorption: Comparison of Nitrogen at 77 K and Carbon Dioxide at 298 K for Activated Carbon[END_REF] H MAS NMR enables the isotropic chemical shifts of adsorbed compounds to be extracted and provides information about their interaction with the carbon surface. Quantitative measurements were carried out using the areas of each peak, taking into account the number of hydrogen atoms, and using a calibration with known quantities of adamantane (C 10 H 16 ). [START_REF] Sweatman | Characterization of Porous Materials by Gas Adsorption: Comparison of Nitrogen at 77 K and Carbon Dioxide at 298 K for Activated Carbon[END_REF] H NMR spectra were obtained using a BRUKER AV300 (7T) spectrometer with a 4 mm MAS probe. Complementary experiments were performed using a 900 MHz BRUKER spectrometer equipped with a 1.3 mm MAS probe in order to evaluate the influence of H-H dipolar coupling on the proton linewidth.

On the other hand, 1 H PFG NMR was used to estimate the self-diffusion coefficients (D), also called the diffusivity, of adsorbed molecules. This technique is based on an echo NMR experiment carried out with a spatial encoding of nuclei. After a diffusion time (∆), the magnitude of the sample's signal is measured. Diffusivities are deduced from the correlation between this magnitude and the gradient value [START_REF] Callaghan | Principles of Nuclear Magnetic Resonance Microscopy[END_REF][START_REF] Callaghan | Diffusion in porous systems and the influence of pore morphology in pulsed field gradient spin-echo nuclear magnetic resonance studies[END_REF] . Correlatively, the mean squared molecular displacement (r) during the diffusion time (∆) is given by the Einstein equation [START_REF] Ruthven | Adsorption and Diffusion[END_REF] .

ݎ = √2. Δ. ܦ (1)
Then, then a molecular velocity v m can be deduced:

ݒ = ݎ Δ (2)
Measurements were performed using a PFGSTE (Pulsed Field Gradient Stimulated Echo) experiment on a BRUKER AV300 spectrometer equipped with a PFG BRUKER probe (30 T/m).

Thermogravimetric analyses (TGA)

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Thermogravimetric measurements were carried out by means of a TA Instrument SDT Q600 analyzer. A loaded sample of THC515 was analyzed at atmospheric pressure under flow of nitrogen. Mass variation was followed versus temperature until 250°C. At this temperature, for all samples, the mass reached a minimum, which indicated a total desorption of the VOC.

Actived carbon

A commercial microporous active carbon, reference THC515, produced by Dacarb (Asnièressur-Seine, France) was used. This sorbent is an activated carbon fiber cloth (ACFC). It is particularly suitable for polluted gas treatments. Compared to granular activated carbons, they have larger external surface areas directly linked to nanopores. This specificity leads to a decrease of the intra-particular mass transfer resistance and then to a 2 to 20 times faster adsorption kinetics. Moreover, fast adsorption/desorption cycles using the Joule effect could be easily implemented [START_REF] Giraudet | Adsorption and Electrothermal Desorption of Volatile Organic Compounds and Siloxanes onto an Activated Carbon Fiber Cloth for Biogas Purification[END_REF] . BET analysis gave a specific surface area of 1768 m 2 .g -1 for THC515.

According to the Horvath-Kawazoe (HK) model or the non-linear Density Functional Theory (DFT), the pore width distribution was centered on sizes of 0.46 nm and 0.49 nm, respectively (Figure 1). Determination of surface functional groups were performed by Boehm'method previously showing weak concentrations (about 0.154 meq/g for basic group and 0.231 for acid groups) [START_REF] Boulinguiez | Chemical transformations of sulfur compounds adsorbed onto activated carbon materials during thermal desorption[END_REF] .

Sample preparation

ACFCs with adsorbed organic compounds were prepared in 2.0 L glass sealed reactors. Three

VOCs were chosen to have a wide range of molecular structures and physical and chemical properties: benzene, cyclohexane and dichloromethane (DCM). Briefly, 100 mg of THC515 was placed in the batch reactor in a central position and a known volume (a few µL) of liquid VOC was injected using a syringe. The adsorption lasted for 48 hours, previously shown to be sufficient to reach equilibrium. The carbon samples were not dried, i.e. directly put into the vessels from the atmosphere, without any drying operation.

RESULTS AND DISCUSSION

MAS effects

Figure 2 presents the 1 H NMR spectra of benzene, DCM and cyclohexane. A comparison of static and MAS spectra shows the effect of magic-angle spinning on the proton NMR signal. In these three cases, the static peaks exhibit a large magnitude with a correlated much lower full width at half maximum (FWHM) between -4 and -8 ppm. These values are consistent with those of -10 to -6 previously obtained for H 2 in a microporous carbon [START_REF] Anderson | NMR Methods for Characterizing the Pore Structures and Hydrogen Storage Properties of Microporous Carbons[END_REF] .

In most cases, static NMR linewidths hinder a quantitative analysis of NMR spectra but, as

Anisotropy effects

FWHMs decrease from about 8 ppm of chemical shift in the static case to approximately 1 ppm under MAS conditions (preserving the integrated intensities), confirming the results for comparable spinning rates in a mesoporous carbon [START_REF] Xu | Water and small organic molecules as probes for geometric confinement in well-ordered mesoporous carbon materials[END_REF] . A spinning rate of 10 kHz is high enough Page 8 of 41 ACS Paragon Plus Environment Langmuir to average out the spectra anisotropies. A higher spinning rate (12 kHz, 15 kHz) has no impact on the spectra (no evidence for spinning sidebands or modification of lineshapes).

In addition, a hypothetical broad signal was carefully searched for. Such a signal can be generated by a part of the adsorbed molecules exhibiting large dipolar coupling [START_REF] Romanova | Diffusion Studies in confined liquid crystal by MAS PFG NMR[END_REF][START_REF] Xu | Water and small organic molecules as probes for geometric confinement in well-ordered mesoporous carbon materials[END_REF] . For this specific purpose, high field 1 H NMR recordings correlated with very high MAS frequencies were carried out with the aim of detecting such interaction effects. 15 kHz, 30 kHz and 45 kHz MAS signals were recorded on a cyclohexane/THC515 sample using a 900 MHz spectrometer confirming the absence of remaining homonuclear dipolar broadening. The three spectra gave the same integration values.

Therefore, it can be considered that the entire signal was contained in the main NMR peak of the spectra and that MAS frequencies of 10 to 12 kHz were sufficient to average out all the anisotropies fully and reveal completely the signal of all the adsorbed molecules.

Two remarks can be made: i) classic peak integration methods can lead to quantitative measurements (this point was confirmed by a set of thermogravimetric analyses (TGA) whose results differ by less than 10% relative error from NMR integrations), ii) in terms of diffusometry, classic static PFG measurements can be envisaged.

The relevance of MAS NMR spectroscopy for the quantitative and qualitative analysis of the adsorbed organic compounds in either mono-or multicomponent adsorption was thus confirmed.

Magnetic inhomogeneities (distribution effects)

The consistent MAS width revealed that the average value of anisotropic bands was not unique, but rather varied from one adsorption site to another. The origin of such fluctuations was explained by the magnetic inhomogeneities in the lattice and a given distribution model was

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proposed. In order to confirm this point, transversal relaxation times (T 2 ) were measured. The NMR signal relaxation time is a direct consequence of the defocalization of the set of resonant nuclei. This defocalization is due to two main factors: i) molecular interactions and ii) inhomogeneities of the NMR fields. The latter directly results from the microscopic irregularity of the host lattice and lead to the distribution effect observed on the spectra. The relaxation time of the signal without those inhomogeneities is commonly called the "true T 2 " while the real relaxation time measured is called "T 2 *". The presence of inhomogeneities could be observed through a significant difference between T 2 * and T 2 . The classic Spin-Echo measurement method of T 2 was used [START_REF] Hahn | Spin Echo[END_REF] . This pulse sequence leads to the spins refocusing, and does not impact the precession speed, allowing the impact of the magnetic inhomogeneities to be artificially compensated. T 2 measurements were carried out on three samples. In all cases, a significant difference (of the order of 1. ppm for DCM, and 6 ppm for water. This shift was 5.5 ppm for benzene (Figure 2). Secondly, these shifts were dependent on the adsorbed amounts. For the three VOCs, higher amounts were associated with an enlargement towards high ppm values, which induced a shift of approximately 1 ppm of the MAS averaged signal. This phenomenon was already obtained on mesoporous and microporous carbons [START_REF] Xu | Water and small organic molecules as probes for geometric confinement in well-ordered mesoporous carbon materials[END_REF][START_REF] Anderson | NMR Methods for Characterizing the Pore Structures and Hydrogen Storage Properties of Microporous Carbons[END_REF] . Thirdly, for increasing loadings of VOCs, the initial adsorbed water signal vanished while a new peak located a few ppm away appeared. The origin of lineshape modifications can be related to the population of various sites presenting unequal interactions with the molecules. The magnetic field experienced by the nucleus corresponded to the applied NMR, slightly impacted by the shielding effect produced by the magnetic susceptibility at the carbon surface [START_REF] Ganguli | The Magnetic and Other Properties of the Free Electrons in Graphite[END_REF][START_REF] Heremans | Magnetic Susceptibility of Carbon Structures[END_REF][START_REF] Heremans | Magnetic susceptibility of carbon structures[END_REF] . A classic interpretation of this phenomenon can be provided. The induced field is caused by the ring currents generated by the π electrons of the graphite layers. Since it goes against the excitation (Lenz's law), the actual field experienced by the absorbed nucleus above the graphitic plane is lower than the original NMR field [START_REF] Pople | Proton Magnetic Resonance of Hydrocarbons[END_REF] . For the storage of hydrogen in porous carbons, a phenomenological model to interpret chemical shift corrections (∆δ) in graphitic cavities has been proposed [START_REF] Anderson | NMR Methods for Characterizing the Pore Structures and Hydrogen Storage Properties of Microporous Carbons[END_REF] . According to this approach, the impact of a graphitic plane on the chemical shift at the vicinity of the surface is described by Equation (3): Its correction to the applied field is given by Equation ( 4) proposed by Carrington [START_REF] Carrington | Introduction to Magnetic Resonance with Applications to Chemistry and Chemical Physics[END_REF] .

∆ߜ = ܽ ݀ ଷ (3)
∆ߪ = - ߤ 4ߨ . ݁ ଶ . ܴ ଶ √2. ݉ . 1 ݀ ଷ (4)
where e is the elementary charge, m e the electron mass, R the ring current radius and ߤ the magnetic constant. Finally, taking into account two parallel walls in a slit pore of width w gives:

∆ߜ = ܽ ൬ 1 ݀ ଷ + 1 ݓ( -݀) ଷ ൰ (5)
Considering a proton located in the middle of the cavity:

∆ߜ = 16 ܽ ݓ ଷ (6)
At a microscopic level, theoretical calculations show the most energetically favorable position for benzene adsorbed on a graphene sheet. These studies depict a molecular plane strictly parallel to the graphene layer and give an equilibrium separation of 0.36 nm between the benzene and graphitic planes [START_REF] Tabony | Nuclear magnetic resonance studies of the melting and orientation of benzene adsorbed upon graphite[END_REF]47,[START_REF] Chakarova-Käck | Application of van der Waals Density Functional to an Extended System: Adsorption of Benzene and Naphthalene on Graphite[END_REF] . Therefore, in a pore thinner than 0.72 nm in the case of monolayer adsorption, the molecule is considered to be at the center of the cavity.

According to the textural characterization, a narrow pore distribution was observed for THC515, with micropore sizes centered at 0.45 nm. On the basis of this value, using Equation ( 4), the a value was calculated at -0.031 ppm.nm [START_REF] Ravikovich | Density functional theory model of adsorption on amorphous and microporous solids[END_REF] . This approach, despite its phenomenological character, allows the shift dependence of the molecules with respect to the pore diameter to be calculated.

However, several observations can be made from Figures 5 and6. Figure 5 shows that slitshaped pores with widths between 0.43 and 0.49 nm are involved in benzene adsorption (shown in Figure 1 using the Horvath-Kawazoe model). The observed experimental shifts, between -4 and -7 ppm, can be converted into a pore size by applying Equation (5). Figure 5 shows that the 1 H spectra are related to pore sizes between 0.43 and 0.49 nm.

Furthermore, the NMR peak enlargement towards high values of ppm proves that small cavities are favored during the adsorption process. However, a limitation occurred for ultramicropores, with a diameter less than 0.4 nm. The minimum shift values observed for C 6 H 6 were of the order of -7 ppm. Therefore, there is no experimental evidence for the occupation of pores smaller than 0.4 nm. The 10 ppm case of DCM results from geometrical factors. For a given pore size, a proton from DCM will be located closer than C 6 H 6 , leading to a strong 1 H shift. Figure 7 shows the shift of non-centered protons. Independently of slit size, it appears that for a wall/proton distance less than 0.15 nm, the shift was clearly higher than 10 ppm. Consequently, it may also be anticipated that for micropores thinner than 0.5 to 0.6 nm, multilayer adsorption was unlikely. The conformations of cyclohexane suggested higher fluctuations of chemical shifts. The spectra gave a value of 7 ppm, which was less than the shift measured for dichloromethane. The steric congestion of cyclohexane may explain its adsorption in larger pores than benzene or dichloromethane molecules (Figure 7).

VOC adsorption and water desorption mechanisms

The adsorption competition of water/VOC was well described by MAS NMR. Figure 3(a) shows the water peak of spectra recorded with increasing amounts of adsorbed dichloromethane.

Water is gradually desorbed from the micropores revealing a preferential adsorption of VOC in micropores. Such a phenomenon was also observed onto micro/macroporous carbon by Gun'ko et al. (2008) for a water/benzene mixture [START_REF] Gun'ko | Characteristics of adsorption phase with water/organic mixtures at a surface of activated carbons possessing intraparticle and textural porosities[END_REF] . Interestingly, the initial water peak magnitude continuously decreased and was gradually replaced by another one with a chemical shift of -0.5 ppm and increasing intensity. The -0.5 ppm chemical shift of the new peak remained much lower than that of liquid water, revealing a weaker but still significant influence of the graphitic network. Thus, the presence of adsorbed water inside the microporosity was still assumed, with a decrease in the wall/H 2 O molecule interaction. In other terms, migration of H 2 O towards larger micropores and/or mesopores was revealed. In addition, the very flat component appearing in

Measurement of self-diffusion constants

PFG recordings were conducted on wet THC515 with three VOCs for various diffusion time ∆ values (from 40 ms to 180 ms). Measurements were limited to 180 ms due to the T 2 relaxation effect. Over this period, the signal/noise ratio was too weak. Figure 8 shows the cylcohexane Ψ/Ψ° ratio evolution for increasing gradient intensities for ∆ = 80 ms. Experimental and fitted model curves are presented. The commonly used model equation is:

Ψ Ψ =  . exp -γ ଶ . G ଶ . δ ଶ . ܦ ௦, . (Δ - δ 3 )൨ (6)
Equation ( 6) involves many groups of molecules with self-diffusion D s,i . p i is the fraction of molecules in group i, γ the proton gyromagnetic ratio, G the applied magnetic field gradient and δ the pulse duration.

In porous systems, the diffusivity is generally given by [START_REF] Zeigermann | Diffusion in Hierarchical Mesoporous Materials: Applicability and Generalization of the Fast-Exchange Diffusion Model[END_REF] :

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ܦ ௦ =  ௗ௦ . ܦ ௦,ௗ௦ +  ௩ . ܦ ௦,௩
Where p ads and D s,ads are respectively the fraction and the self-diffusion constant of adsorbed molecules, and p v and D s,v respectively the fraction and the self-diffusion of molecules in the vapor phase in equilibrium with the adsorbed one.

The agreement between the model and measurements was obtained using a single diffusivity value. Consequently, a single dominant diffusion phenomenon was implied. According to the wall/nucleus distance previously evaluated, and the unique peak obtained in MAS spectra, the dominant diffusion mechanism was expected to be surface diffusion. Consequently, p v can be considered negligible.

Three diffusion mechanisms are involved in porous structures and their respective predominance depends on both pore size and the molecule free mean path (FMP) [START_REF] Ruthven | Principles of Adsorption and Adsorption Processes[END_REF] . But, it should be noted that FMP must be distinguished from the molecular displacements below deduced from PFG experiments. FMP corresponds to the mean distance covered by a molecule between two collisions with another molecule. If the pore diameter is larger than the FMP, collisions with walls are not influential and a molecular diffusion process is predominant. If the FMP increases or the pore size is smaller, the host frame presence appears in the diffusion process. The contribution of wall collisions becomes significant and Knudsen's diffusion mechanism is then effective. Lastly, if the FMP is of the same order as the pore diameter, the wall/molecule interaction is dominant and the surface diffusion mechanism is expected [START_REF] Pople | Proton Magnetic Resonance of Hydrocarbons[END_REF] .

Modeling has provided orders of magnitude of D s for these diffusion processes [START_REF] Do | Dynamics of a semi-batch adsorber with constant molecular supply rate: a method for studying adsorption rate of pure gases[END_REF][START_REF] Prasetyo | Surface diffusion of strong adsorbing vapours on porous carbon[END_REF][START_REF] Chiang | Diffusivity of Microporous Carbon for Benzene and Methyl-Ethyl Ketone adsorption[END_REF] . In nanopores (< 2 nm), surface diffusion is largely dominant with D s values lower than 10 -8 m 2 .s -1 .

For cyclohexane, a D s of the order of 4.10 -12 m 2 .s -1 was measured. D s ≈ 1.10 -11 m 2 .s -1 and D s ≈ 4.10 -11 m 2 .s -1 were obtained for benzene and DCM, respectively. Those values were in agreement Page 15 of 41

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Langmuir with those of 10 -10 m 2 .s -1 to 5.10 -9 m 2 .s -1 in studies carried out for cyclohexane onto actived carbon micro-meso hierarchical pore systems [START_REF] Furtado | Guest diffusion in interpenetrating networks of microand mesopores[END_REF] and hexane in MCM-41 mesoporous silicas 55 . In addition, they confirmed the cyclohexane MAS results. In fact, the lower diffusivity of cyclohexane corroborated the previous assumption of its reduced mobility in porosity as this molecule has a more complex and voluminous structure.

Diffusivities Ds and corresponding molecular velocities are reported in Figure 8. A slight decrease in these parameters is observed with the observation time. This feature is explained by a short ∆ time with respect to the time interval between two redirections of diffusing molecules in the host frame. The phenomenon is illustrated considering the molecule displacement in Figure 9. This schematic presentation demonstrates that with a short ∆, the measured molecular displacement doesn't take account of the redirection phenomenon and, as a result, provides v m and D s values greater than the macroscopic one. Using a sufficiently long ∆, the mean number of redirections per time unit becomes constant, and the limit values of D s and v m are reached.

Interestingly, in the latter conditions, the NMR diffusion coefficient value leads to the diffusivity that could be observed at a macroscopic scale.

Confirmation of water behavior

Two sets of recording were carried out on wet THC515. The first sample contained only adsorbed water (4 mol.kg -1 ) while the second was loaded with a water/cyclohexane mixture.

Adsorbed water concentration was also of the order of 4 mol.kg -1 and cyclohexane was doubly The D s values from PFG measurements were of the order of 5.10 -13 m 2 .s -1 for the first sample (water only) and 10 -11 m 2 .s -1 for the second (water/deuterated cyclohexane). The ratio of 50 between the two D s values remained constant despites ∆ variations. Figure 10

List of figures

  First, a shift of a few ppm towards strong fields was observed for all NMR signals compared to the signal for pure organic compounds. Second, large peak broadness of the order of 10 ppm was noted. Complex signals are produced in the case of multicomponent adsorption or for carbons dependence of the chemical shift vs. adsorption sites. In such a case, line broadening and overlapping signals hide the spectrum deconvolution,24

illustrated in Figure 2 ,

 2 magic-angle spinning conditions enhance the detection and enable a clear distinction of VOC signals in the case of multicomponent adsorption. For instance, Figures 2(b) and 2(c) illustrate the value of MAS for spectral analysis. Figure 2(b) represents the NMR spectra of the adsorption of a mixture of cyclohexane/benzene. The MAS signal clearly reveals two separate peaks corresponding to each of the adsorbed molecules. The spectrum in Figure 2(c) was obtained from DCM adsorbed onto a wet host carbon. Water molecules, present in most experiments, and VOC are easily distinguishable and can be quantified. It should also be noted that the spectrum reveals two adsorbed populations of water molecules. This dual peak will be explained below. signals of VOCs adsorbed onto carbon exhibit some characteristic features of solid state NMR spectra: relatively short attenuation times and a static linewidth of a few kHz (2-4 kHz). The 1 H static NMR linewidth can be explained by different mechanisms: i) homogenous broadening arising from 1 H-1 H dipolar interactions, chemical shift anisotropy and a contribution of the T 2 relaxation time, ii) inhomogeneous broadening corresponding to a distribution of chemical shifts. However, the observed 1 H NMR linewidth remains relatively small for a real solid state NMR case dominated by homonuclear proton-proton dipolar interactions. One must keep in mind that a proton-proton dipolar interaction for two nuclei separated by one Angstrom gives rise to a dipolar coupling constant of 120 kHz. The linewidth measured (a few kHz) can probably be explained by two factors: i) a relatively fast and isotropic reorientation of the adsorbed molecules, giving rise to a motional averaging of the dipolar interactions, and ii) the dilution of the molecules on the surface, limiting the possibilities of intermolecular dipolar interactions. Furthermore, the values of T 2 obtained under MAS conditions (about 2 ms in all cases) suggest that the 1 H homogeneous linewidth is only 35 Hz, confirming that the majority of the additional broadening observed in Figures 2(a), 2(b) and 2(c) is a result of a distribution of chemical shifts and, therefore, of chemical environments.

2 *.Figures 3

 23 Figures3 and 4illustrate the 1 H NMR spectra of cyclohexane and DCM adsorbed on THC515, for adsorbed loadings between 0.2 and 4 mol.kg -1 . Three features can be noted. Firstly, a

Page 11 of 41 ACS

 41 Paragon Plus EnvironmentLangmuirwhere (d) is the distance to the plane and (a) is a constant depending on the magnetic moment of the adsorbent layer. Such a surface is highly anisotropic and gives rise to a strong modification of the adsorbed molecule chemical shift tensor.

Page 12 of 41 ACS

 41 Paragon Plus Environment Langmuir phenomena. This interval perfectly matches the BET pore size distribution peak of THC515

Figure 3 (

 3 Figure 3(d)/curve (d) could be attributed to residual extra-porous adsorbed water. PFG

deuterated C 6 D 12 .

 612 Consequently, only adsorbed water molecules were analyzed by NMR. The 1 H MAS spectra of these samples are shown in the inset of Figure 10. The change in water molecule behavior was thus highlighted.

  represents the molecular velocities values and their constant ratio. It reveals a six times higher water mobility in the presence of VOC. The results obtained using MAS NMR were thus confirmed. A lower interaction with the host frame induced both a lower peak shift towards strong fields and a greater mobility. Nevertheless, these D s values verified that surface diffusion was occurring, excluding the assumption of extra-porous water and confirming the dual presence of water and VOC in micropores.CONCLUSIONThe possibility of using 1 H NMR as a tool for the detection, identification and quantification of VOCs adsorbed onto wet ultra-microporous activated carbon has been assessed. Under static conditions, water, cyclohexane, benzene and DCM gave rise to very wide signals (several tens of ppm) significantly shifted towards low ppm values (5 to 10 ppm) compared to the chemical shifts of the liquid solutions of these compounds. The origin of the spectral width of the recorded peaks could be attributed to the combination of very low transversal relaxation time values, strong chemical shift anisotropy effects and homonuclear dipolar interactions. The latter averaged under MAS conditions gave spectral bandwidths as small as 1 ppm, thus allowing the separation and identification of each VOC signal. Furthermore, resorting to a semi-empirical model enabled the averaged chemical shifts to be linked to the magnetic effects of the carbon walls and therefore to the resonating proton/pore wall distance. Thus, despite the short signal attenuation time, PFG studies could be carried out for diffusion times reaching 40 ms. This study

Figure 1 .

 1 Figure 1.THC515 pore width distribution (HK method). Inset: scanning electronic

Figure 2 .

 2 Figure 2. Static and MAS (9 kHz) spectra on THC515: (a) benzene (2.0 mol.kg -1 ); (b) mixture of benzene (2.0 mol.kg -1 )/cyclohexane (1.2 mol.kg -1 ); (c) dichloromethane (DCM) (2.5 mol.kg -1 )/water (2.3 mol.kg -1 ).

Figure 3 .

 3 Figure 3. (3a) 1 H MAS spectra (spinning rate 9 kHz). Spectra of cyclohexane adsorbed onto non-dried THC515 for various loadings; (a) 0.2 mol.kg -1 , (b) 0.7 mol.kg -1 , (c) 2.5 mol.kg -1 , (d) 4.0 mol.kg -1 . Fig. (3b) represents high ppm values of the spectra, containing the water molecule

Figure 4 .

 4 Figure 4. MAS 9 kHz spectra of DCM adsorbed onto non-dried THC515 for various loadings; (a) 0.1 mol.kg -1 , (b) 0.4 mol.kg -1 , (c) 1.7 mol.kg -1 , (d) 3.3 mol.kg -1 . Figure (4b)

Figure 5 .

 5 Figure 5.Correlation between the pore width and the NMR shift according to Equation (3).

Figure 6 .

 6 Figure 6. Correlation between the wall/nucleus distance in different porous cavities according to

Figure 7 .

 7 Figure 7.Illustrations of H/wall distance for benzene, DCM and cyclohexane molecules,

Figure 8 .Figure 9 .

 89 Figure 8.NMR echo intensity with respect to the gradient for ∆ = 80 ms. Inset: diffusivity

Figure 10. Measured H 2 OFigure 1 .Figure 2 .Figure 3 .Figure 4 .Figure 5 .Figure 6 .

 2123456 Figure 10. Measured H 2 O Molecular velocities v m1 (only H 2 0 loaded) and v m2 (mixture

Figure 7 .Figure 8 .Figure 9 .

 789 Figure 7. Illustrations of H/wall distance for benzene, DCM and cyclohexane molecules,

Figure 10 .

 10 Figure 10. Measured H 2 O Molecular velocities v m1 (only H 2 0 loaded) and v m2 (mixture

Page 17 of 41 ACS Paragon Plus Environment Langmuir showed

  diffusion coefficients tending towards a constant value, revealing the macroscopic aspect of the diffusion phenomenon. These results suggest the possibility of obtaining exploitable values in the current kinetic models of mass transfer in porous materials. The use of various techniques of solid state NMR, including Magic Angle Spinning, Pulsed Field Gradient and even Magnetic Resonance Imaging, applied to the study of small molecule interactions with carbon surfaces constitutes a rapidly growing field of research. So far, NMR studies focusing on the application for supercapacitors are the most advanced 56 , 57 , 58 ,[START_REF] Forse | NMR Study of ion Dynamics and Charge Storage in ionic Liquid Supercapacitors[END_REF] (ref, x, y, z, t) but, as shown in this paper, we believe this combination of NMR techniques constitutes also a very promising tool for studying the VOC adsorption onto activated carbons.

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ACS

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Abir Sarbajna

Pragati Pandey

S M Wahidur Rahaman

Kuldeep Singh

Akshi Tyagi

Pierre H Dixneuf

Jitendra K Bera

A Triflamide-tethered NHC-Rh(I) Catalyst for Hydroalkoxylation Reactions: Ligand Promoted Nucleophilic Activation of Alcohols

Keywords: alkyne hydroalkoxylation, nucleophilic addition, ligand design, triflamide, cyclization

A triflamide-tethered NHC-bound Rh(I) dicarbonyl catalyst is found highly effective for both intermolecular hydroalkoxylation and intramolecular heteroannulation reactions. The involvement of both amido nitrogen and triflato oxygen of the triflamide functionality for alcohol activation and 1,2-hydrogen shift respectively is proposed.

Scheme 1. Nucleophilic addition across a terminal alkyne.

Enol ethers are an important functionality in organic synthesis being used widely in cross coupling and ring-closing metathesis, cycloaddition reactions and for the synthesis of pharmaceuticals. 4 While metal catalyzed intramolecular hydroalkoxylations are well reported, 5 intermolecular reaction between alcohol and alkyne is difficult 6 and often demands strong base or harsh conditions. 7 Kirchner reported the addition of allylic alcohols to phenylacetylene by [RuCl(tris(pyrazolyl)borate)(pyridine)2] catalyst. 8 A [PdMo3S4 (1,4,7-triazacyclononane)3Cl] 3+ cubane cluster with CF3SO3Ag was reported to catalyze the AM hydroalkoxylation of alkynic acid esters. 9 Kakiuchi first reported intermolecular AM hydroalkoxylation of terminal acetylenes by a 8-quinolinolato ligated rhodium(I) dicarbonyl complex. 10 In most cases, products were obtained with high Z selectivity. [START_REF] Lam | Messerle and co-workers employed a dicarbonyl(5phenyldipyrrinato)rhodium(I) catalyst and obtained E selective enol ethers[END_REF] Computational studies by Wang and co-workers revealed that the 8-quinolinolato oxygen atom facilitates alcohol deprotonation to a more potent Onucleophile which readily adds to Rh-vinylidene followed by proton transfers. 12 N-heterocyclic carbene (NHC) derived ligands are widely used in organometallic catalysis owing to their ability to form robust compounds with a variety of metal ions and for their relatively easy steric and electronic tunability. 13 We postulated that a NHC with pendant basic group would be ideally suited for alcohol activation and promote its nucleophilic attack to the metal-vinylidene. [START_REF] Au | s were reported as catalysts for hydroalkoxylation of internal alkynes[END_REF] Herein we report the design, synthesis and catalytic activity of a Rh(I)-NHC dicarbonyl complex featuring a tethered triflamide group for intermolecular addition of alcohol to terminal alkynes and intramolecular cycloisomerization of alkynols (Scheme 2). The involvement of both the amido nitrogen and the triflato oxygen in alcohol activation and proton transfer steps is highlighted.

Scheme 2. Catalyst design.

Reaction of 2-isopropyl-1-(trifluoromethylsulfonyl)-aziridine and 1-mesitylimidazole afforded a zwitterionic imidazolium salt (LH) in high yield (90%). Subsequent reaction of LH with AgOAc in presence of K2CO3 in refluxing THF gave a neutral disilver complex 1 in 82% yield (Scheme 3a, Figure S2). Transmetallation of 1 with [Rh(µ-Cl)(COD)]2 (COD = 1,5cyclooctadiene) dimer afforded 2 in 90% yield. The molecular structure revealed a neutral square planar Rh complex chelate bound to L via carbene carbon and triflamido nitrogen. The metal coordination sphere is completed by a COD (Scheme 3b). The Rh1-C2 and Rh1-N3 bond lengths are 2.012(2) and 2.227(2) Å respectively. When CO was bubbled into a dichloromethane solution of 2 for 30 minutes, a yellow amorphous solid 3 was isolated in quantitative yield. Although the crystal structure could not be obtained, 3 was identified as [Rh(CO)2L]. The carbene carbon was observed at δ 172.1 ppm as a doublet (J = 48 Hz) in the 13 C NMR spectrum. Two carbonyls resonate at 171 and 165 ppm. Sharp absorptions were observed at 2079 and 2005 cm -1 for carbonyls in 3, which The catalytic utility of several Rh compounds was tested towards intermolecular hydroalkoxylation to understand the role of participating and ancillary ligands. Both [Rh(µ-Cl)(COD)]2 and [Rh(µ-Cl)(CO)2]2 were found ineffective at 2 mol% catalyst loading using 1 mmol phenylacetylene and methanol (1 mL) in toluene co-solvent (1 mL) (Scheme 4). This suggests that a ligand framework is probably required for catalyst activity. However, both [RhLʹ(COD)Cl] and [RhLʹ(CO)2Cl] (Lʹ=N,N'dimethylimidazoline-2-ylidene) were found ineffective. Even 2 with an appended triflamide did not show any catalytic activity. However, when the COD ligand is replaced by two carbonyls in 3, modest yield of (2-methoxyvinyl)benzene (41%, Table S2) was obtained after 24 h at 70°C. A host of other solvents such as THF, acetonitrile, 1,4-dioxane and DMSO were screened but did not increase the yield significantly (41-51%, Table S3). However, the use of amide based dimethylformamide and N,N-dimethylacetamide (DMAc) led to a considerable increase in the product formation. [START_REF] Rh | I) catalysts depicted in Scheme 4 were ineffective even in methanol-DMAc solvent combination[END_REF] Quantitative conversion was achieved after 48 h by using a methanol-DMAc solvent combination (Table S3). The C-O bond formation took place regioselectively at terminal carbon of the phenylacetylene and showed E isomer as the major product. The progress of the reaction was monitored by 1 H NMR analysis. Initially the Z isomer was predominant (kinetic product), but the thermodynamic E product gradually increased over time (See, Figure S4). Use of stronger bases such as Et3N and pyridine led to decreased yields. [START_REF]conditions suggesting that the co-solvent plays an important role in the reaction[END_REF] Scheme 4. Evaluation of Rh(I) catalysts for intermolecular hydroalkoxylation.

The substrate scope for 3 was tested under optimized reaction conditions and the results are collected in Table 1. For electron rich aromatic alkynes 1-ethynyl-4-methoxybenzene, 1-ethynyl-4-methylbenzene and 1-ethynyl-2,4,5trimethylbenzene, high yields were obtained (entries 2-4, Table 1).

3-ethynylthiophene was converted to 3-(2methoxyvinyl)thiophene in quantitative yield (entry 5). The best result was obtained for 2-ethynyl-6-methoxynaphthalene with 97% E-selectivity (entry 6, Table 1). The reaction was extended to ethanol and quantitative conversion to (2ethoxyvinyl)benzene was obtained with 3:2 E/Z selectivity (entry 7, Table 1). The reaction also showed poor activity for aliphatic alkyne 1-octyne (entry 8, Table 1). All catalysis reactions were limited to terminal alkynes. For an internal alkyne diphenylacetylene, no addition product was detected implying the intermediacy of a metal-vinylidene for AM hydroalkoxylation. 10 The prerequisite of π-acidic carbonyl ligands for catalyst activity could be rationalized owing to their ability to favor the metal-vinylidene formation and increase electrophilicity of the vinylidene carbon. We further carried out reactions with a series of alcohols under optimized conditions. Methanol and ethanol afforded excellent yields (vide supra), but sterically demanding aliphatic alcohols n-butanol, i butanol, n-hexanol and n-octanol were found unreactive. When i PrOH was used, dehydrogenation took place and acetone was obtained. For benzyl alcohol, the reaction showed no product formation. Interestingly, moderate yield of 35% was obtained for phenol in presence of a catalytic amount of 2,6-lutidine. The inability of bulkier alcohols to give hydroalkoxylated product suggests that the ligand-appended triflamide unit participates in the alcohol activation through hydrogen bonding interactions. Surprisingly, CF3CH2OH was also found unreactive under the reaction conditions. Although the acidity of CF3CH2OH is higher than ethanol, the conjugate base CF3CH2O -is not sufficiently nucleophilic to attack the Rhvinylidene. High solvation of fluorinated alcohols may also contribute to lack of activity. [START_REF] Takamuku | [END_REF] The hydroalkoxylation proceeds in a slightly alkaline DMAc solvent. We pondered whether 3 could activate terminal alkyne with acidic nucleophile. Remarkably, the selectivity in the addition of benzoic acid across phenylacetylene was completely reversed as M product was obtained in near quantitative yield (Scheme 5). In a separate experiment, when phenylacetylene, 3 and methanol were refluxed along with a few drops of ethereal hydrochloric acid, quantitative formation of the M hydration product acetophenone was observed similar to the acidic nucleophile. The contrast in product regioselectivity for acidic/basic nucleophiles demonstrates that a switch between metal-π-alkyne and metal-vinylidene intermediates could be achieved by changing the nucleophile and/or by controlling the acidity of the reaction medium. Mechanistic investigation by deuterium labeling experiments was difficult due to rapid exchange of protons between methanol and phenylacetylene. DFT calculations at BPW91 level of theory were carried out using Gaussian 09 to rationalize a probable mechanism depicted in Scheme 6. [START_REF]Phenylacetylene was chosen as the substrate and DMAc as solvent. The catalyst was simplified to reduce the computational cost[END_REF] At first, alkyne substitution to one of the carbonyls in 3 takes place (A) directed by the strong trans effect of NHC. Rearrangement of terminal alkyne to vinylidene on a metal site has been studied extensively by computational and experimental techniques. 19 For Kakiuchi's catalyst, indirect 1,2-hydrogen shift promoted by 8-quinolinolato oxygen atom was proposed to be the most favorable pathway. 12 Nevertheless, a Rh-vinylidene intermediate B was computed to be endergonic by 8.1 kcal/mol. Next, we optimized a low lying intermediate C where the methanol proton migrates to the triflamide nitrogen (N-H = 1.06, O…H = 1.73 Å). The Cα-O bond length is 1.43 Å. A relevant transition state could not be located suggesting a facile proton movement without much of a kinetic barrier. NBO analyses showed negative charges on both Rh (-0.449) and Cβ (-0.446) whereas Cα (+0.221) was electrophilic. Hence, a proton transfer from protonated ligand to Cα was not considered. Due to a large Cβ-H separation, a rotamer intermediate along Rh-Cα was optimized. D facilitates a hydrogen migration from amido N to Cβ via TSDE (Ea = 8.3 kcal/mol). Intermediate E is a Fischer type carbene containing two diastereomeric β-hydrogen atoms which undergoes 1,2-βhydrogen shift to form a thermodynamically more stable metalalkene complex. 20 A relevant transition state TSEF was located (rate limiting step) which revealed the involvement of a triflato oxygen for hydrogen shift (O … H = 1.85, Cα-H= 1.25, Cβ-H = 1.56 Å), resulting in an enol ether coordinated Rh complex F. Overall, the addition reaction of phenylacetylene to methanol is exothermically driven as a weak π bond is broken and a strong σ bond is formed. We further evaluated the catalytic potential of 3 for intramolecular reactions. 2-(phenylethynyl)phenol was used as a model substrate and the reaction was performed in chloroform at 70°C. The 2-phenylbenzofuran was obtained in moderate yield (56%), but could be improved significantly on adding 5 mol% of Cs2CO3 as an additive (95%, entry 1, Table 2). Other aromatic alkynol derivatives also showed excellent yields although 2-(t-butylethynyl)phenol was less reactive (entries 2-4, Table 2). The reaction was extended to (2-(phenylethynyl)phenyl)methanol where two products are possible. NMR analysis showed that the 5-exo-dig cyclized product formed exclusively (entry 5, Table 2). Intramolecular cycloisomerization is limited to internal alkynols, since a terminal alkynol (entry 6) rearranges to the corresponding metalvinylidene that is rapidly trapped by an alcohol to afford a stable Fischer carbene complex. 21 This suggests that π-activation mechanism at the Rh center is the preferred pathway for intramolecular reactions that is in contrast to the intermediacy of metal-vinylidene for intermolecular reactions. Nonetheless, catalyst 3 is active for both inter and intramolecular hydroalkoxylations.

In conclusion, we have synthesized a triflamide-tethered NHC-Rh(I)(CO)2 complex 3 which is an effective and versatile catalyst for both inter-and intramolecular hydroalkoxylation of terminal alkynes and alkynols respectively. The AM selectivity of the addition product was reversed for acidic nucleophiles. The potential of both amido nitrogen and triflato oxygen of the appended triflamide functionality for alcohol activation and hydrogen shift is realized. Further studies for water addition by a triflamide appendage are being actively pursued in our laboratory. [a] Alkynol (1 mmol) and Cs2CO3 (0.05 mmol) were sequentially added to a 3 mL CHCl3 solution of the catalyst 3 (0.02 mmol) and the closed reaction vessel was heated at 70°C for 24h. [b] Isolated yields are reported. n.r. = no reaction.

Experimental Section

General procedure for intermolecular hydroalkoxylation: Alkyne (1 mmol), mesitylene (internal standard, 1 mmol) and MeOH (1 mL) were sequentially added to a 1 mL DMAc solution of the catalyst 3 (0.02 mmol) and the closed reaction vessel was heated at 70ºC for 24h. After completion of the reaction, the solutions were diluted to 10 mL with Et2O and washed with LiCl solution (0.1 M, 2×10 mL). The organic phase was dried with saturated brine solution (20 mL) and Na2SO4, and the solvent was removed in vacuo. Products were analyzed using GC-MS and 1 H NMR spectroscopy.

Intramolecular hydroalkoxylation: Alkynol (1 mmol), mesitylene (internal standard, 1 mmol) and Cs2CO3 (0.05 mmol) were sequentially added to a 2 mL chloroform solution of the catalyst 3 (0.02 mmol) and the closed reaction vessel was heated at 70ºC for 24h. After completion of reaction, the solvent was removed in vacuo. The residue was purified by column chromatography on silica gel (eluent: hexane: ethyl acetate = 10:1) to give desired product.

Scheme 3 .

 3 Scheme 3. (a) Synthesis of complexes 1, 2 and 3. (b) Molecular structure of 2 with the anisotropic displacement parameters depicted at the 50% probability level. Hydrogen atoms are omitted for clarity.

  [a] Alkyne (1 mmol) and alcohol (1 mL) were sequentially added to a 1 mL DMAc solution of 3 (0.02 mmol) and the closed reaction vessel was heated at 70°C for 24h. Isolated yields are reported. E/Z ratio determined by NMR and GC analyses in presence of internal standard mesitylene (1 mmol). n.d. = not determined.

Scheme 5 .

 5 Scheme 5. Selectivity of addition products.

Scheme 6

 6 Scheme 6Mechanism for intermolecular hydroalkoxylation catalyzed by 3. Energies are shown in parentheses in kcal/mol relative to A.

Table 1 .

 1 Substrate scope of intermolecular hydroalkoxylation reaction.

	[a]

Table 2 .

 2 Substrate scope for intramolecular hydroalkoxylation reaction.

	[a]

Acknowledgements

This work is financially supported by the DST, CSIR and DAE, India. A.S., K.S., A.T. thank UGC, P.P. thanks CSIR, India for fellowships.

Alcohol Activation