Nanostructured Silicas, a Platform for the ... - ACS Publications

Article Cite This: J. Phys. Chem. C XXXX, XXX, XXX−XXX

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Nanostructured Silicas, a Platform for the Observation of Transient Radicals: Application to Sulfinyl Radicals François Vibert,† Emily Bloch,‡ Michèle P. Bertrand,† Stéphane Gastaldi,*,† and Eric Besson*,† †

Aix Marseille Univ, CNRS, ICR, Marseille, France Aix Marseille Univ, CNRS, MADIREL, Marseille, France

‡

S Supporting Information *

ABSTRACT: Diazenes, precursors of sulfinyl radicals, were used to functionalize nanostructured SBA-15 silicas either in the framework or on the pore by formation of covalent links, or by simple adsorption on the surface. Depending on their design and on the experimental conditions, these materials proved to be effective tools to study the behavior and the reactivity of arylsulfinyl radicals made persistent because of confinement effects. When the covalently linked precursors were irradiated at room temperature, a spectacular increase of the lifetime of the expected sulfinyl radicals was registered (up to 17 h). At higher temperature, upon thermal initiation at 473 K, the decomposition of the adsorbed precursor enabled visualization of the rearrangement of the corresponding arylsulfinyl radicals into sulfonyl radicals via O−S coupling.

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INTRODUCTION

being readily decomposed upon irradiation or thermolysis without the need of any coreagent. In this article, we report the contrasting behavior of arylsulfinyl radicals generated in different types of nanostructured silicas: first, when the radical precursor is covalently linked to the silica but localized either in its framework or on its surface and, second, when the precursor is simply adsorbed on the surface of silica.

Sulfinyl radicals are mainly known as intermediates in oxidation of sulfur-containing compounds particularly in the atmosphere1,2 or in biological systems.3 In solution, their oxidation leads to thiosulfonates.4,5 In contrast to sulfanyl and sulfonyl radicals, sulfinyl radicals have not been used to prepare sulfurcontaining targets through their addition onto π systems. This can be explained by the fast fragmentation of β-alkylsulfinyl radicals6,7 which is the only synthetic process reported in the literature.8−11 Most alkyl- and arylsulfinyl radicals are transient at room temperature as illustrated by their extinction rate constants close to the diffusion-controlled limit.12−15 In spite of the lack of applications in organic synthesis, a broad range of reactions enables the formation of phenylsulfinyl radicals. They can be generated by photolysis or thermolysis of a wide range of precursors such as thiosulfonates,16 thiosulfinates, 17 sulfonamides,18 arylsulfenyl nitrates,19 diarylsulfoxides,16,20 or aryl benzyl sulfoxides.20,21 The formation of sulfinyl radicals was also evidenced when sulfinyl chlorides were irradiated22 or reduced by zinc(0).23 In a previous report, we have shown that the design of nanostructured silicas functionalized with radical precursors enabled us to dramatically improve the lifetime of transient radicals such as arylsulfanyl radicals.24,25 This increase in the lifetime was observed regardless of whether the precursors were positioned in the framework25 or on the pores24 of the silica. For the traceless generation of sulfanyl radicals, a diazene-based precursor was designed. Diazene precursors present the double advantage of being stable all along the silica synthesis and of © XXXX American Chemical Society

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EXPERIMENTAL SECTION Experimental procedures and characterizations for the synthesis of organic compounds and the derived materials (NMR, SAXS, nitrogen adsorption/desorption analysis, ATG) are fully described in the Supporting Information. Experimental Procedures for EPR Analysis. EPR experiments were performed on an ELEXSYS Bruker instrument, and the Bruker BVT 3000 setup was used to control the temperature. The photolysis instrument (ORIEL version 66901 with an energy supplier version 68911) is equipped with a 300X UXL306 arc Xe lamp (200−800 nm) with an optical fiber (1 m, version 77620). In a 4 mm quartz glass tube, 20 mg of functionalized silica was degassed with a 10−5 mbar vacuum pump. EPR spectra for the direct observation of sulfur-centered radicals were recorded with the following parameters: modulation amplitude = 1 G, receiver gain = 90 dB, modulation frequency = 100 kHz, power Received: November 3, 2017 Revised: December 6, 2017 Published: December 7, 2017 A

DOI: 10.1021/acs.jpcc.7b10879 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C = 2 mW, sweep width = 200 G, conversion time = 24 ms, sweep time = 25 s, number of scans = 2.

Scheme 3. Arylsulfinyl Radical Generation through Diazene Decomposition: (A) In the Framework (SBAn-1) and (B) in the Pore (SBAn-2) of the Silica

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RESULTS AND DISCUSSION The synthesis of mesoporous silicas via the sol−gel process in the presence of structure-directing agents enables organic− inorganic hybrid nanostructured materials to be obtained in which the organic moiety can be selectively located either on the pore surface or inside the wall of the silica pores.26,27 Indeed, the number of trialkoxysilane groups in the organic precursor determines its location in the silica. The presence of only one silylated group enables the location of the organic moiety on the pore surface whereas an additional trialkoxysilane function positions the organic linkage in the framework, thanks to the hydrophilicity of the two trihydroxysilyl groups generated at each end under the acidic conditions of the sol− gel process. As depicted in Schemes 1 and 2, the silylated Scheme 1. Synthesis of Wall-Functionalized Silicas from Symmetrical Diazene Precursor

distribution of 1 and 2 in the framework and all along the pore, respectively.26,27,29 The resulting materials were named SBAn-1 in which 1 specifies the nature of the organic precursor and n indicates the TEOS/radical precursor molar ratio determined after the characterization of the hybrid silicas. All previous reports on the observation by EPR of sulfinyl and sulfonyl radicals are related to the decomposition of various precursors most often by photolysis at low temperature in solution12,13,16,22 either directly, or with the relay of spin traps.21,30 In the following, the detection of these radicals was achieved in the solid state, at room temperature or at 473 K, depending on whether the cleavage of the diazene precursors was accomplished via photolysis or thermolysis. The behavior of the arylsulfinyl radicals generated from the photolysis of precursors SBAn-1 and SBAn-2 was investigated by EPR. In a typical procedure, the functionalized material was degassed (10−5 mbar) in a 4 mm quartz glass tube, and arylsulfinyl radicals were generated via the light-induced decomposition of the diazene inside the spectrometer cavity (xenon lamp (200−800 nm)). An anisotropic signal, resulting from the immobilization of the radicals, was recorded at room temperature after irradiation whether the radical precursor was located in the framework (SBAn-1) or on the pore (SBAn-2) (Figure 1). The giso Landé factors of these signals were in agreement with the formation of arylsulfinyl radicals.21,22,31,32 It must be underlined that the g-tensor was axisymmetrical when the radicals were localized in the framework. It was rhombic when the radicals were formed on the pore surface. Representative spectra are shown in Figure 1. For both SBAn-1 and SBAn-2, giso values ranging from 2.008 to 2.010 were calculated from graphically determined gx, gy, and gz. Both types of spectra are consistent with the formation of arylsulfinyl radicals. The radical half-lifetimes recorded for the arylsulfinyl radicals generated from the various hybrid materials are collected in Table 1. In the case of SBAn-2, a t-butyl radical was generated concomitantly with the arylsulfinyl radical. It must be underlined that no t-butyl radical was ever detected in

Scheme 2. Synthesis of Pore-Functionalized Silicas from Dissymmetrical Diazene Precursor

radical precursors 1 and 2 were straightforwardly prepared from the corresponding thioethers 425 and 628 through a two-step sequence, i.e., a mono-oxidation of the sulfur atom with mCPBA followed by hydrosilylation in the presence of Karstedt’s catalyst. Similarly, to the previously reported generation of arylsulfanyl radical,25,28 the photoinduced fragmentation of the diazene moiety should lead to a β-sulfur carbon-centered radical which would undergo a fast β-fragmentation releasing the arylsulfinyl radical (Scheme 3).6,7 2D Hexagonal SBA type silicas were prepared by cocondensation of bridged organotriethoxysilane 1 or organotriethoxysilane 2 with tetraethyl orthosilicate (TEOS) in the presence of P123 (PEO20PPO70PEO20) as structure-directing agent. The P123 role was crucial for (i) the nanostructuration of the functionalized silicas and (ii) the homogeneous B


Article

The Journal of Physical Chemistry C

(see SI), which did not allow the mechanism involved to be determined. Several competitive processes are likely to contribute to the disappearance of the sulfur-centered radicals. The half-lifetimes were graphically estimated from the fitted decay curves. When the radical was located in the framework, the values of half-lifetimes for SBA64-1 and SBA112-1 reached 6.1 and 10.9 h, respectively (Table 1). Their magnitude is in the same order as that registered for analogous face-to-face arylsulfanyl radicals.25 As regards the arylsulfinyl radicals located in the pore (SBAn-2), their half-lifetime ranged from 17.0 to 0.4 h. These latter values are higher than those observed for the arylsulfanyl radicals generated from analogous diazenebased precursors.28 As previously demonstrated, this discrepancy between the half-lifetimes can be related to the compactness of the organic monolayer which disfavors the coupling reactions in the case of the higher loadings.24 The SBAn-1 and SBAn-2 thermogravimetric analyses (TGAs) indicated that the thermal decomposition of the diazene moieties occurred at 473 K. Like the irradiation, the thermolysis led to the formation of the arylsulfinyl radical according to the same fragmentation process (Scheme 3A). The EPR study, under thermal fragmentation conditions, showed the presence of an isotropic signal at 2.009 g-factor, i.e., a value close to the giso values recorded at 293 K (Figure 2). The EPR observation

Figure 1. EPR spectra of arylsulfinyl radical at room temperature (a) in the framework (SBA64-1) and (b) on the pore (SBA23-2).

Table 1. Half-Lifetime of the Arylsulfinyl Radicals Generated at 293 K by Photolysis of SBAn-1 and SBAn-2 a b c d e

SBA64-1 SBA112-1 SBA23-2 SBA42-2 SBA62-2

gx

gy

gz

gisoa

2.015 2.015 2.015 2.015 2.015

2.007 2.007 2.008 2.007 2.007

2.007 2.007 2.003 2.003 2.002

2.010 2.010 2.009 2.008 2.008

t1/2b 6.1 10.9 17.0 9.9 0.4

Figure 2. EPR spectra for the thermolysis of SBA112-1 at 473 K (g = 2.009).

h h h h h

of sulfinyl radicals generated from the thermal decomposition of sulfoxides and thiosulfonates has been reported but, no information about their persistence was given.16,21 With both SBA64-1 and SBA112-1 materials, the temperature was maintained at 473 K, and the half-lifetimes of the anchored arylsulfinyl radicals were measured by monitoring the decay of the signal just after it reached its highest intensity. In this case, there was no stationary state. In spite of the high temperature, half-lifetimes as long as 3 and 7 min were measured (Table 2). It must be underlined that no other signal was detected. Sulfinyl radicals exhibiting half-lifetimes of the same order of magnitude were generated from silicas samples with the precursors located on the pores (SBAn-2). However, in this case, once the sulfinyl radical signal had disappeared a low intensity signal remained (g = 2.004). This g-factor fits with the typical Landé factor reported for an arylsulfonyl radical.13 The above results show that functionalized mesoporous silicas are powerful tools to observe a transient radical as elusive

a

giso = (gx + gy+ gz)/3. bDetermined from the double integrated EPR signal at 293 K.

any of the reported EPR experiments. This observation argues in favor of a fast evolution of the t-butyl radicals, probably via dimerization or disproportionation, and these processes would occur before the decay of arylsulfinyl radical starts being monitored. In all experiments, the irradiation was maintained until the signal reached a quasistationary state, at which point the UV lamp was switched off. The half-lifetimes of the anchored arylsulfinyl radicals were measured by monitoring the decay of the signal intensity after turning off the irradiation. They were determined from the double integrated EPR signal. The decay curves were fitted with exponential or multiexponential models C


Article

The Journal of Physical Chemistry C Table 2. Half-Lifetime of the Aryl Sulfinyl Radicals Generated at 473 K by Thermolysis SBAn-1 and SBAn-2 Precursors entry a b c d e

SBA64-1 SBA112-1 SBA23-2 SBA42-2 SBA63-2

g

t1/2 (min)

2.007 2.009 2.009 2.008 2.009

3 7 2 2 2

as the sulfinyl radical when it is covalently linked to the silica. However, the fate of the sulfinyl radical was not strictly identical depending on its close environment. The formation of sulfonyl radical was evidenced under thermolytic conditions when the precursor was linked to the pore of the hybrid material. In order to evaluate the incidence of the anchoring on both the arylsulfinyl radical persistence and its evolution, we decided to use a nanostructured silica as a mere support on which the radical precursor would be adsorbed (Scheme 4). In such a Scheme 4. Radical Precursor Adsorbed on SBA-15 Silica

system, the absence of covalent linkage between the precursor and the silica should increase the mobility of the organic molecules. To this end, diazene 7 was adsorbed on a nonfunctionalized SBA-15 silica (7*). The irradiation of this system (7*) at 360 nm did not enable the observation of sulfinyl radical at room temperature. However, at 173 K a signal similar to the one registered for SBA64-1 was observed (Figure 1). These results are in agreement with the studies reported by Gilbert.22 Much to our surprise, when the adsorbed precursor (7*) was submitted to the conditions for a thermal diazene homolysis, 11 min after the beginning of the heating of the probe, a clean EPR signal was recorded at 435 K (Figure 3a). Its g-factor of 2.009 fitted with the formation of an arylsulfinyl radical. Once the temperature was stabilized at 475 K, a second signal appeared for which a 2.004 g-factor was measured (Figure 3b). After 1.9 h at the same temperature, the arylsulfinyl signal had totally disappeared in favor of the second signal which matched with an arylsulfonyl radical (g = 2.004) (Figure 3c).13 Amazingly, when the EPR tube was cooled at 298 K, that signal was still observed without loss of intensity over 16 h showing an impressive persistence of the corresponding arylsulfonyl radicals at room temperature. The adsorption of 7 on silica enabled visualization of the rearrangement of arylsulfinyl radicals into arylsulfonyl radicals. Evidence of this disproportionation has already been reported in the literature.21,33 It proceeds through dimerization via the formation of a sulfur−oxygen bond, followed by subsequent homolysis of the resulting sulfenylsulfinate intermediate leading to phenylsulfonyl and phenylsulfanyl radicals (Scheme 5). Evidence for the concomitant formation of phenylsulfanyl radical could not be identified. Two main explanations can be put forward. First, the g-factor of an aryl sulfanyl radical24,25 is on the same order of magnitude as that of an arylsulfinyl radical; thus, the arylsulfanyl radical signal might be hidden by the signal of the arylsulfinyl radical. Second, our previous

Figure 3. Evolution of the EPR spectra of 7 adsorbed on SBA-15: (a) arylsulfinyl radical at 435 K, (b) mixture of arylsulfinyl and arylsulfonyl radicals at 475 K, and (c) arylsulfonyl radicals at 475 K.

Scheme 5. Formation of Phenylsulfonyl Radical from Phenylsulfinyl Radical

attempts to observe the arylsulfanyl radical expected from a precursor adsorbed on a nonfunctionalized SBA-15 silica at 473 K failed.25 The important point is that this last experiment D


Article

The Journal of Physical Chemistry C

(4) Kice, J. L. In Free Radical; Kochi, J. K., Ed.; Wiley: New York, 1973; Vol. 2, Chapter 2, pp 711−740. (5) Chatgilialoglu, C. In The Chemistry of Sulfones and Sulfoxides; Patai, S., Rappoport, Z., Stirling, C., Eds.; John Wiley & Sons Ltd.: New York, 1988; pp 1081−1087. (6) Wagner, P. J.; Sedon, J. H.; Lindstrom, M. J. Rates of radical β cleavage in photogenerated diradicals. J. Am. Chem. Soc. 1978, 100, 2579−2580. (7) Iino, M.; Matsuda, M. Studies of sulfinyl radicals. 2. Reversibility of addition to styrene. J. Org. Chem. 1983, 48, 3108−3109. (8) For selected examples see references 8−11: Boothe, T. E.; Greene, J. L., Jr.; Shevlin, P. B.; Willcott, M. R., III; Inners, R. R.; Cornelis, A. The stereochemistry of free-radical eliminations on βphenylsulfinyl radicals. J. Am. Chem. Soc. 1978, 100, 3874−3879. (9) Russell, G. A.; Tashtoush, H.; Ngoviwatchai, P. Alkylation of.beta.-substituted styrenes by a free radical addition-elimination sequence. J. Am. Chem. Soc. 1984, 106, 4622−4623. (10) Caddick, S.; Aboutayab, K.; West, R. I. Novel intramolecular radical displacement reactions of 2-indolyl aryl sulfides and sulfoxides. J. Chem. Soc., Chem. Commun. 1995, 1353−1354. (11) Mouriès, V.; Delouvrié, B.; Lacôte, E.; Fensterbank, L.; Malacria, M. Radical β-elimination of a sulfinyl group to afford allenes. Eur. J. Org. Chem. 2002, 2002, 1776−1787. (12) Howard, J. A.; Furimsky, E. An electron spin resonance study of the tert-butylsulfinyl radical. Can. J. Chem. 1974, 52, 555−556. (13) Bennett, J. E.; Brunton, G.; Gilbert, B. C.; Whittall, P. E. A kinetic−electron spin resonance study of the self-reactions of some aromatic and aliphatic sulphonyl radicals and aromatic sulphinyl radicals. J. Chem. Soc., Perkin Trans. 2 1988, 1359−1364. (14) Mizuno, H.; Matsuda, M.; Iino, M. Studies of sulfinyl radicals. I. Thermal decompositions of benzhydryl p-tolyl sulfoxide and benzhydryl methyl sulfoxide. J. Org. Chem. 1981, 46, 520−525. (15) Darmanyan, A. P.; Gregory, D. D.; Guo, Y.; Jenks, W. S. generation and decay of aryl sulfinyl and sulfenyl radicals: a transient absorption and computational study. J. Phys. Chem. A 1997, 101, 6855−6863. (16) Gilbert, B. C.; Gill, B.; Sexton, M. D. Detection of aromatic sulphinyl (ArSO·) radicals during the photolysis and thermolysis of Saryl arenethiosulphonates and diaryl sulphoxides. J. Chem. Soc., Chem. Commun. 1978, 78−79. (17) Koch, P.; Ciuffarin, E.; Fava, A. Thermal disproportionation of aryl arenethiolsulfinates. Kinetics and mechanism. J. Am. Chem. Soc. 1970, 92, 5971−5977. (18) Booms, R. E.; Cram, D. J. Stereochemistry of sulfur compounds. III. Radical-chain mechanism for racemization of sulfinamides. J. Am. Chem. Soc. 1972, 94, 5438−5446. (19) Topping, R. M.; Kharasch, N. Sulfenic acids and their derivatives. XLI. Sulfenyl nitrates and sulfinyl radicals. J. Org. Chem. 1962, 27, 4353−4356. (20) Miller, E. G.; Rayner, D. R.; Thomas, H. T.; Mislow, K. Thermal racemization of diaryl, alkyl aryl, and dialkyl sulfoxides by pyramidal inversion. J. Am. Chem. Soc. 1968, 90, 4861−4868. (21) Chatgilialoglu, C.; Gilbert, B. C.; Gill, B.; Sexton, M. D. Electron spin resonance studies of radicals formed during the thermolysis and photolysis of sulphoxides and thiolsulphonates. J. Chem. Soc., Perkin Trans. 2 1980, 1141−1150. (22) Gilbert, B. C.; Kirk, C. M.; Norman, R. O. C; Laue, H. A. H. Electron spin resonance studies. Part 51. Aliphatic and aromatic sulphinyl radicals. J. Chem. Soc., Perkin Trans. 2 1977, 497−501. (23) Barnard, D. The reaction of sulphinyl chlorides with zinc. J. Chem. Soc. 1957, 4673−4676. (24) Vibert, F.; Marque, S. R. A.; Bloch, E.; Queyroy, S.; Bertrand, M. P.; Gastaldi, S.; Besson, E. Arylsulfanyl radical lifetime in nanostructured silica: dramatic effect of the organic monolayer structure. Chem. Sci. 2014, 5, 4716−4723. (25) Vibert, F.; Marque, S. R. A.; Bloch, E.; Queyroy, S.; Bertrand, M. P.; Gastaldi, S.; Besson, E. Design of wall-functionalized hybrid silicas containing diazene radical precursors. epr investigation of their photolysis and thermolysis. J. Phys. Chem. C 2015, 119, 5434−5439.

highlighted the amazing persistence of the arylsulfonyl radical at 475 K. To conclude, SBA-15 silicas are versatile nanostructured tools to observe and to study the behavior of transient arylsulfinyl radical. At room temperature, only the signal of sulfinyl radical was detected by EPR whatever the model (SBAn-1, SBAn-2, or 7*), while at high temperature it was possible to find evidence for either the persistence of the isolated radical or its evolution depending on the design of the functionalized nanostructured silica. In the case of SBAn-1, in which the precursor was localized in the framework, the absence of byproduct resulting from the formation of an arylsulfenylsulfinate intermediate was expected since the recombination of two sulfinyl radicals is prevented because of (i) the distance separating them and (ii) their anchoring. Similarly, when the radical precursor was covalently linked to the silica surface (SBAn-2), the main species observed was still the arylsulfinyl radical; however, a weak signal fitting to the corresponding aryl sulfonyl radical was detected. The low intensity of the signal can be explained by the low probability to have two radicals close enough to recombine. In other words, these two systems enabled the observation of the arylsulfinyl radical without or with little interference from other radicals thanks to the absence of diffusion and to the low concentration in radicals. In contrast, in the noncovalent system, the evolution of the transient species was merely slowed down which enabled the observation of their transformation into new transient radicals, themselves stabilized by adsorption onto silica.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b10879. Characterizations for the organic compounds and the derived materials (NMR, SAXS, nitrogen adsorption/ desorption analysis, ATG), and EPR decay curves (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Stéphane Gastaldi: 0000-0003-4363-8322 Author Contributions

All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the Agence Nationale de la Recherche for funding (ANR-12-JS07-005). REFERENCES

(1) Urbanski, S. P.; Wine, P. H. In S-Centered Radicals; Alfassi, Z. B., Ed.; Wiley: Chichester, 1999; pp 97−140. (2) Reisenauer, H. P.; Romański, J.; Mlostoń, G.; Schreiner, P. R. Reactions of the methylsulfinyl radical [CH3(O)Ṡ] with oxygen (3O2) in solid argon. Chem. Commun. 2015, 51, 10022−10025. (3) Razskazovskii, Y. V.; Becker, D.; Sevilla, M. D. In S-Centered Radicals; Alfassi, Z. B., Ed.; Wiley: Chichester, 1999; pp 245−276. E


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