Muonium adduct of benzaldehyde : A novel probe of cation-molecule interactions in zeolite catalysts and of solvation and electronic substituent effects?

Christopher J. Rhodes,"** Chantal S. Hinds" and Ivan D. Reidb a School of Pharmacy and Chemistry, Liverpool John Moores University, Byrom St., Liverpool, U K L3 3AF

Paul Scherrer Institute, CH-3057 Villigen, Switzerland

The adduct radicals (ArCHOMu') have been formed by muonium addition to the carbonyl group of benzaldehyde and its derivatives. The muon coupling is found to be highly sensitive to the nature of substituents in the benzene ring, being increased by electron releasing groups. This bears relevance to their properties in Group 1 and Group 2 exchanged zeolite-X, in which the coupling is decreased by interactions between the aromatic n-electrons and the cation: the effect increasing in the order of decreasing ionic radius, as the electron withdrawing power of the ring is increased by polarization of the n-electrons towards the cation. For the radical derived from benzaldehyde itself, a strong dependence is also found on the solvent in which it is dissolved. It is suggested that this may be used in the study of solvent effects, particularly H-bonding, pertinent to organic and bio-organic media. Electronic substituent effects are also studied in substituted PhCHOMu' radicals, and in cyclohexadienyl radicals formed by muonium addition to the aromatic ring, the latter showing evidence of captodative stabilization, when an electron donor substituent is present with the electron accepting CHO group.

Stable free radicals, nearly always nitroxides, have been used as probes for environmental and electronic effects since pion- eering studies in the 1 9 6 0 ~ . ~ 9 ~ The basis of this is that the 14N coupling in a radical R,NO', measured by EPR spectroscopy, is very sensitive both in the electron demand of the groups, R, (with good correlations being made with Hammett 0-

parameters) and to the local environment of the radical. In one s t ~ d y , ~ in which were correlated the I4N couplings for three different nitroxides with solvent polarity parameters obtained by various means, success was met only for the so- named 'cybotactic' scales;8 the correlations being very poor with 'dipole moments ' and 'relative permittivities', which are properties of the bulk medium. Nitroxides, especially di-tert- butyl nitroxide, have also been used as probes for acid sites in zeolites, for instance, in cation-exchanged zeolite-)<, where linear correlations were found for the 14N parallel and iso- tropic couplings with the electronegativity of the metal, and was rationalized by the association between the N-0 unit and Lewis-sites present of differing strength, depending on the nature of the ~ a t i o n . ~ Thus they do not probe the electrostatic field from the cation directly, but only the way that cations modify the overall zeolite framework acidity, and hence the Lewis-acid strength. Using muon spin rotation spectroscopy, we have studied the radical Me,C-OMu' formed from acetone adsorbed into NaX'' and the coupling is increased by ca. 20% from that measured in liquid acetone;''-'8 this we attributed to electrostatic interactions directly with Na' cations. We note that an overview of EPR studies applied to zeolites up to the mid-1970s is available."

Prompted by results from previous work on radicals (R,COMu') formed from muonium addition to non-aromatic carbonyl (C=O) compounds,1 '-I8 which show a sensitivity of the muon coupling both to the nature of the R group and to the solvent, we have undertaken the present study of benz- aldehyde adducts (PhCHOMu') since both solvent and sub- stituent effects can be investigated for this readily functionalized system. This approach has the potential advan-

t Presented at the 29th International Conference of the ESR Group of the Royal Society of Chemistry, University of Edinburgh, 1996.

tage that radicals are produced by muon implantation in sample media, including aqueous systems, which are a problem for EPR methods. Moreover, there is no limitation that only stable radicals can be utilized as probes; certainly transient radicals have been thoroughly studied by EPR, but it is not possible to generate them under steady-state condi- tions in all media, particularly solids such as zeolites or in aqueous (membrane) media by conventional methods involv- ing the photolysis of complex mixtures or flow systems.

The very great sensitivity of pSR" (because of single- particle counting) makes radical detection intrinsically much easier than by EPR, and only one reagent is required as the 'muonium-trap'. We now present results for three different applications in which the PhCHOMu' probe shows promise: electrostatic effects of cations on molecules adsorbed in zeo- lites, solvent interactions and electronic substituent effects.

Experimental The TF-pSR measurements were made using the pE4 beam- line at the Paul Scherrer Institute, Villigen, Switzerland. Samples of the benzaldehydes (Aldrich) either neat or in solu- tion were sealed into 35 mm od thin-walled Pyrex ampoules, following deoxygenation by four freeze-pumpthaw cycles. Following ion-exchange (vide infra) and drying under vacuum overnight at 400 "C, benzaldehyde was adsorbed from the gas- phase onto the zeolite cooled to 77 K in a Pyrex ampoule which was then sealed. Full details of the TF-pSR technique have been given previously,' but specifically, for each experi- ment, the sample was exposed to the beam of spin-polarized positive muons while an external magnetic field of 3 kG (0.3 T) was applied transverse to the beam direction; typically, 4 x lo7 good decay events were accumulated in 4 data histo- grams. The data were analysed by fitting the usual theoretical function in Fourier space, which allowed the determination of the muon precession frequencies. Since the hyperfine coupling is relatively small for these radicals, the signals were partly overlapped by the intense signal from muons in diamagnetic environments; therefore, the diamagnetic component was fil- tered out, prior to analysis, as shown in the spectrum of Fig. l.

J . Chem. SOC., Faraday Trans., 1996,92(21), 4265-4269 4265

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c Table 1 Muon hyperfine couplings for XC,H,CHOMu' radicals"

substituent coupling/MHz

0 20 . 40 60 80 frequency/M Hz

Fig. 1 TF-pSR spectrum for m-ClC,H,CHOMu' radicals in the absence of the 27 MHz diamagnetic frequency; a is the cyclotron fre- quency

Zeolite ion-exchange procedure

The zeolite ion-exchange procedure used is as described else- where.,~~ The zeolite NaX (obtained as Molecular Sieve Type 13X from UOP) was slurried with 0.1 M NaCl (10% weight slurry) three times for approximately 1 h each. The zeolite was suction filtered and washed with deionized water, to ensure that no other exchangeable cations were present. The sample was dried and then rehydrated in a desiccator over a brine solution. The NaX sample was weighed and the wt.% of Na+

for which the value of x has been determined as 251 for full hydra t i~n .~ Once the Na+ content is known, the appropriate amount of the exchange cation is added as a 0.1 M solution and slurried at a set temperature for up to 3 days. The per- centage of ion-exchange was determined initially by titration for alkane cations and additionally by atomic absorption spectrometry.

calculated, given its formula Na,,[(AlO,),,(Sio,),o,] xH20,

Results and Discussion Electronic substituent effects in benzaldehydemuonium adducts [ XC,H,CHOMu' ]

Table 1 shows the muon couplings for the radicals XC,H,CHOMu', and it is clear that electron releasing substit- uents increase the coupling from that in the unsubstituted C,H,CHOMu', while electron withdrawing groups decrease it; indeed, a good correlation is obtained (r = 0.97) with Hammett cr, and cP constants, which shows that the coupling is sensitive to the electronic demand of the aromatic group (the p-C1 value is not included in the correlation since it was measured in ethanol solution, and from our later discussion of

ma 13-

10-

.C

.0 ?

f.

Fig. 2 Plot of A(0Mu) for PhCHOMu' radicals in different solvents against ET(30), solvent polarity parameters : a, cyclohexane; by diethyl ether; c, benzaldehyde; d, methanol; e, formamide and f, ethanol

m-C1 p-c1 P-H m-Me p-Me m-Me0 p-Me0

o-Me0 0-CHO

10.64 9.04*

12.61 13.16 14.25 13.10 15.21 13.30 11.10

In each case measured in a sample of the pure substituted benz- aldehyde. Measured in ethanol solution.

solvent effects will be artifically low). This may be explained by the relative contribution of the canonical structures.

ArCH'OMu(1) + ArCH-O+ 'Mu(I1)

which is one description of the C=O n-bonding in the C-O(Mu) unit. Electron withdrawing groups will tend to weight 11, which has two consequences: first, the spin density on the 0 is increased, increasing the negative contribution to the muon coupling from spin-polarization of the 0-Mu bonding electrons ; secondly, since structure I1 represents stronger C=O z-overlap and hence bonding, less vibrational excursion of the muon occurs from the radical plane and into the region of positive ~oup l ing : '~ ,~* the (positive) coupling is therefore reduced. (The coupling in PhCHOMu' is positive because the hyperconjugative transfer of positive spin to the muon from the carbon atom, by its excursion from the radical nodal region, dominates over 0-Mu spin-polarization.) This argument runs counter for electron releasing substituents which favour I, so reducing the 0 spin density but increasing the out-of-plane amplitude of the muon by the weaker C=O(Mu) z-bond.

Ca tion-exc hanged zeoli te-X

This section is chosen to follow immediately on from the last, since both sets of results relate to polarization of the n-system, either by substituents in the aromatic ring or by the local elec- trostatic environment of a zeolite. We mentioned already that nitroxide radicals have been used as probes of Lewis acidity in ze~l i tes ,~ in which the I4N coupling is increased by adsorption at sites of increasing acid strength, because the N-+O dipole moment is raised, so weighting the canonical structure N+'- 0-. As part of our ongoing effort to determine the properties of organic radicals in zeolite catalyst^,^^-^^ we formed Me,COMu' in NaX," since it is related to the 'H,COMe radical, which is believed by some2, to be an inter- mediate in the methanol-to-gasoline (MTG) process. We dis- covered that its motion was slow on the p timescale of pSR, since the spectrum revealed A, , A,, , A, anisotropic muon- hyperfine components, rather than sharp 'liquid-like' lines, which we have seen for more weakly polar radicals in zeolites, and that the isotropic coupling obtained as their average was significantly increased from that measured directly in liquid acetone (by ca. 20%) or in n-hexane solution (by ca. 7%). Our explanation is that association with Na+ cations increases the C-+O(Mu) dipole so increasing the contribution of the C'-O(Mu) canonical structure over that C--O+'(Mu), having the effect of increasing the muon coupling as the 0 spin density falls and the out-of-plane 0-Mu vibration increases in amplitude because the C=O n-bond is now weaker.

To try and develop a probe of wider potential application than Me,COMu', which can be more readily functionalized, we repeated this experiment with PhCHOMu' adsorbed into NaX and other X-zeolites with different Group 1 and Group 2

4266 J . Chem. SOC., Faraday Trans., 1996, Vol. 92

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Table 2 Muon hyperfine couplings for PhCHOMu' radicals adsorbed in zeolite-X

cation coupling/MHz

Li +

Na+ K + Mg2 ' Ca2 +

Sr2 +

Ba2 +

12.61 12.68 13.06 11.94 12.54 12.96 14.12

cations (Table 2). If we compare the muon couplings with those measured in solution (Table 3), it is apparent that the solution values are generally smaller, but their variations are so large as to be best explained by specific solvation effects, which reduce the coupling from that in the least polar medium, cyclohexane, of 13.46 MHz (next section). Clearly, the effect found for Me,COMu' would be expected to increase with the polarizing power of the medium, i.e., as their ionic radius decreases; however, the contrary is evident. This argues against there being a similar predominant electrostatic inter- action with the C-OMu unit, and it is more likely that the cation is associated with the wider and more polarizable n- system, particularly at the benzene ring: we note that NMR measurements on aromatic compounds in zeolites demon- strate a strong association of the cation with the aromatic ring, and for benzene there is a preferential reorientation of the molecule about the cation, via its six-fold axis.27

We can then account for the phenomenon by an argument similar to that earlier to explain the influence of ring substit- uents on the muon coupling: namely, that as the ring n-system is associated with more polarizing M" + cations, its electron demand will increase (as it does when an electron withdrawing substituent is introduced) and so the canonical structure C--O+'Mu will be favoured so decreasing the coupling con- stant, for the reasons discussed in the earlier section.

Influence of solvent on the muon coupling in PhCHOMu'

EPR s t u d i e ~ ~ ~ , ~ ~ of radicals R,COH' show that the O H proton coupling is highly dependent both on temperature and on solvent; similar effects have also been demonstrated for the muon coupling in the Me,COMu' r a d i ~ a 1 . l ~ ~ ~ ~ The main dif- ference between Me,COH' and Me,COMu' is that the greater librational amplitude of the lighter muon causes its coupling to be positive, even at temperatures where a sign change is observed for Me2COH' (the latter occurs when the proton's vibrational amplitude from the radical plane is insufficient to transfer more positive spin from the carbon atom than the negative spin density arising from spin-polarization of the 0-H bond. Indeed, a negative muon coupling has yet to be observed for a R,COMu' radical.

The evidence is that solvent variations on proton/muon couplings in radicals of this kind are caused by H-bonding, and so there is the potential that they might be used deliber- ately as probes of H-bonding effects in various media. This would not be possible in many systems using those flow or

Table 3 Muon h y p e h e couplings for PhCHOMu' radicals in dif- ferent solvents

solvent coupling/MHz

ethanol formamide methanol diethyl ether benzaldeh yde c yclohexane

7.78 8.18 8.46

11.08 12.61 13.48

photolytic methods normally associated with generating R,COH' radicals from alcohols and a peroxide for EPR detec- ti~n,~'*'' but is readily achieved through the implantation of positive muons into materials containing a particular carb- onyl compound to trap them (as muonium).11-'8 We have chosen benzaldehyde because the essential molecule may be functionalized appropriately for particular applications.

In Table 3 is shown the muon coupling in PhCHOMu' measured in different solvents. We have noted previously" that the muon coupling in the Me2COMu' radical located in NaX zeolite is enhanced from that in acetone or n-hexane solution, and is thought to be due to the strong electrostatic field from Na' cations. This follows the general principle that an overall increase in environmental polarity will serve to further 'separate' any charges present, and so will act in the direction of the molecular ((240) dipole, so disfavouring the canonical structure Me,C-O+'(Mu), while weighting that Me,C'OMu- hence, in accord with our earlier discussion of electronic substituent effects in XC,H,CHOMu' radicals, enhancing the muon coupling. In contrast, the data in Table 3 show that the more polar solvents (as deduced from their dielectric constants) actually decrease the coupling, and so we sought instead an explanation in terms of specific solvation, making recourse to published solvent 'cybotactic' scales;30 that derived from the ionization of tert-butyl chloride does not cover the range of solvents used here, so we chose the E,(30) scale which does, and is derived from shifts in the energy of a charge-transfer band in an aromatic ylide.

The plot of A(p) vs. E,(30) (Fig. 2) is not entirely convincing (r = 0.90) and shows that there are significant differences in the influence of solvent on the charge-transfer process and on the dynamics of the muon in the PhCHOMu' radical. We note that the three polar protic media exert the greatest effect and must H-bond to the 0-atom in PhCHOMu', which will itself H-bond (Mu-bond) to a second solvent molecule (III). This is supported by the contrast between diethyl ether and the two alcohols: given the combined inductive effect of the two ethyl groups, ether must have the greater donor power, but the alcohols exert the greater effect since an interaction between the 0-Mu group and two alcohol molecules will more greatly impede the out-of-plane amplitude of the muon and so reduce its coupling. The situation is analogous to the reduction in the coupling in the Me,COMu' radical in acetone solution by the addition of H,0.15

Ring adducts : cyclohexadienyl radicals

In addition to the adducts of the carbonyl group, observed at low frequencies, much higher frequencies were detected, which we ascribe to the formation of cyclohexadienyl radicals by muonium addition to the aromatic ring (Table 4). Cyclo- hexadienyl radicals have previously been studied in detail using TF-pSR, initially in connection with the differential spin delocalizing power of substituents in ortho-, meta- and para- positions,31 an ipso-isomer being detected only in PhCF,. In an extension of this work, we used the cyclohexadienyl radical as a model system to probe the effect of dual substituents, to decide on the reality or otherwise of 'captodative stabilization' in organic radicals,,' in which the presence simultaneously of electron withdrawing ('capto') and electron donor ('dative') groups is proposed to confer a stability on a free radical which exceeds that expected from their cumulative effect. Taking the premise that radical stabilization and spin delocalization are related, we were able to use the muon coup- ling (as a measure of spin delocalization within the system) to determine the nature of the substituent interactions.

H,. ,Mu---S

Ph H-Z ,c-o.

J. Chem. SOC., Faraday Trans., 1996, Vol. 92 4267

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Table 4 Muon hyperfine couplings for cyclohexadienyl radicals formed by muonium addition to substituted benzaldehydes"

Table 6 Muon hyperline couplings measured for cyclohexadienyl radicals formed by muonium addition to PhCHO in different solvents

compound coupling/MHz solvent coupling/MHz

C,H,CHO m-Me0 p-Me0 o-Me0 p-Me p-c1 m-C1 0-CHO

439, 463,494 359,382 409,474 436 459 467 432 495

PhCHO HCONH, Et,O EtOH MeOH cyclo hexane

439 (o), 463 (p), 494 (m) 435,458 439, 462 435,459 435, 458 439, 463

" Converted to x y values, which are shown for particular ring posi- tions in Table 5.

Assuming only a cumulative effect, we would expect the relation in eqn. (1) to hold:

A, = A&Il(l - A,)

where A, is the muon coupling for the substituted radical, Ah is the coupling for the unsubstituted case (benzene itself, 514.6 M H z ) , ~ ~ and A , is a delocalization parameter similar to those previously derived for benzyl radicals, S33 and C T ' . ~ ~ We found, as we do now, that much better accommodation of the experi- mental A , values is possible by the introduction of a substit- uent interaction parameter A, , , as in eqn. (2):

A, = A&lI(l - d,)rr(l - A,,)

Clearly, if A,, is positive, there is a synergetic interaction between the substituents (so the spin delocalization and stabil- ity, is enhanced), and if it is negative, the interaction is antago- nistic and presumably destabilizing. The detection, therefore, of cyclohexadienyl radicals in substituted benzaldehydes, allows, in addition to the environmental probe provided by the carbonyl adducts (vide supra), the opportunity to examine further the (captodative) matter with the strongly electron withdrawing CHO (capto) group present with a number of potential dative substituents; A,, values thus determined are listed in Table 5, along with those previously determined from related systems.

We see that certain common features emerge. When there is a direct through-conjugation between the substituents and the delocalized radical system (Le., 1,3- and 1,5-substitution) the captodative proposal is borne out since the pairs of substit- uents MeO/CN, MeO/CHO, Cl/CHO show all positive A,, values, while those for like-pairs, MeO/MeO, Me/Me, F/F, are negative, the exception being the 1,3-C1/Cl pair in which A,, is very weakly positive. For all other arrangements of sub- stituents, while there are appreciable substituent interactions, the signs of the Ax, parameters scatter.

This may relate to the fact that meta-positions are substi- tuted, since it has been shown that meta-substituents in benzyl radicals affect the spin density distribution by a polar rather than a spin delocalizing effect.35 We propose that the stabili- zation of a radical may be complicated by polar influences of substituents that change the overall electronic distribution of the cyclohexadienyl moiety, and that, in such cases, the spin

densities alone do not always account for the total substituent effect on the stability of the radical. A theoretical treatment indicates that there is a significant inductive destabilizing effect in methyl radicals substituted with electron withdrawing groups,36 but which falls in delocalized systems, allowing the spin delocalizing influence to dominate. We note that there is a very pronounced antagonistic effect when pairs of donor, and in the one case of acceptor (CHO/CHO), substituents occupy the adjacent 1,2-positions, that for the CHO/CHO pair being the strongest of all the substituent interactions that we have measured.

Following another theoretical which concluded that polar solvents would greatly enhance the captodative effect, we measured the muon couplings in MeO/CN- substituted cyclohexadienyl radicals dissolved in different media.38 Indeed, we did find small effects of this kind in a (90 : 10) formamide : methanol solvent, yet measurements of dissociation equilibria for captodatively substituted radical dimers found n~ th ing .~ ' To the best of our knowledge, there has been no systematic investigation reported of solvent effects on radicals with a single substituent; however, we were able to detect at least the ortho- and para-muonium adducts of PhCHO in different solvents (along with the PhCHOMu' adduct), and there are clear reductions in the couplings for both, when measured in the polar protic media, methanol, ethanol and formamide (Table 6). To compare with the varia- tion in coupling constants already discussed for pairwise sub- stituent interactions, we may express these reductions by dsolv values, defined in eqn. (3):

(3) Expressed in units of 1004,01v, these amount to +0.91(EtOH), + 0.9 l(MeOH), + 0.91(HCONH2) for para-muonium adducts, and +0.86(EtOH), + 1.08(MeOH), + 1.08(HCONH2) for ortho-muonium adducts, and so the solvent effect appears comparable in magnitude with that of a second substituent.

Conclusions We have found that radicals of the type PhCHOMu' are readily detected by TF-pSR when benzaldehyde and deriv- atives are irradiated with polarized, positive muons. The muon coupling is extremely sensitive to the environment and we have found three distinct areas where these could find application. First, they are an effective probe of molecular interactions with cations in zeolite catalysts, since the muon

Table 5 A,, parameters determined for the interaction of two substituents in the cyclohexadienyl radical (in units of 1004,,)

position of X, Y" (defined in) 1,5 192 174 1,3 294 273

X = M e Y = M e

-0.13 - 1.81 + 0.36 - 0.45 + 0.01 - 0.99

F F

- 1.93 - 1.62 + 1.12 -0.71 - 0.90 + 1.53

c1 OMe c1 OMe

- 2.62 -0.46 -7.07 - + 4.44

+0.04 -3.37 - 1.45

+0.08 -2.53

-

-

OMe CN CHO OMe CHO CHO CHO CN OMe OMe CHO Me C1 CHO

+2.62 +2.62 + 6.59 +6.59 - - - +0.72 -2.60 - - - - 11.24 -1.20 -2.96 +6.13 -4.15 -0.15 -1.85 - +5.17 +6.98 - +7.26 - +1.08 - -1.18 -1.18 -2.59 -1.37 - - - -0.96 -

- - - - -

" Data for CHO substituents are from this work, all other values are from ref. 28.

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coupling varies by ca. 4% in the Li+ - K + series, by ca. 9% in the similar Mg2 + - Sr2 + group, but by 18% in the full Mg2 +

- Ba2 + alkaline-earth cation series. Compared with nitrox- ides, which are used with EPR spectroscopy as environmental probes, the sensitivity of PhCHOMu' to solvation effects is enormous: an increase of 73% in the muon coupling in cyclo- hexane compared with ethanol; the I4N coupling in di-tert- butyl nitroxide shows only 6% variation in n-hexanelethanol media ! The related cyclohexadienyl adducts, formed by muonium addition to the benzene ring in substituted benz- aldehydes, has also provided further information for our work on captodative stabilization in radicals, and we have found a definite enhancement of the degree of spin delocalization onto the CHO group in polar protic media in both ortho- and para-muonium adducts of benzaldehyde itself.

We envisage that these radicals could be useful in studies of electrostatic effects in surface catalysts, particularly cation- molecule interactions in zeolites, and provide cybotactic probes of solvation and other medium effects. They are also extremely sensitive to substituent effects and could find appli- cation in this area also. A further potential advantage is that they may be suitably functionalized, for instance to change the propensity for localization in more or less hydrophobic/ hydrophilic regions, and are likely to find particular applica- tion in studies of H(Mu)-bonding interactions in a range of systems such as membrane models.

We thank the SERC/EPSRC, the Leverhulme Trust and the Paul Scherrer Institute for grants in support of this work.

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