Infrared and Photoacoust ic Spectroscopic Studies of a Sil ica-immobilized fl-Diketone

D. S. KENDALL, D. E. LEYDEN,* L. W. BURGGRAF, and F. J . PERN Department of Chemistry, University of Denver, Denver, Colorado 80208

Infrared and photoacoustic spectroscopies have in combination produced useful information about an immobil ized fl-diketone, formed by the reaction of acctylacetone with p-chloromethyl- phenyltrimethoxysilane previously immobil ized on silica. Evi- dence confirming the synthesis of immobil ized 3-benzyl-2,4-pen- tanedione is presented. Comparisons with 3-benzyl-2,4-penta- nedione, a model for the surface bonded ligand, were valuable. The bound fl-diketone is largely in the keto tautomer on the surface. The photoacoustic spectrum shows that the remainder is in the form of an intermolccular hydrogen-bonded enol. In basic solution the enolate ion and metal-enolate complexes can be formed. Infrared spectra show that the keto form can bind metals in acidic solutions. Index Headings: Infrared spectrometry; Photoacoustic spectrom- etry; Surface studies.

INTRODUCTION

There are many branches of chemistry in which surface properties are important. In order to characterize a sur- face completely, especially a modified surface, it will often be necessary to combine a variety of spectroscopic techniques with chemical studies. Infrared spectrometry using Fourier transform methods and ultraviolet and visible spectra obtained by photoacoustic techniques is valuable for the study of complex organic functional groups on surfaces.

An important type of surface modification is silylation of oxide surfaces such as silica gel. 1 Silicas silylated with organofunctional sflanes can be further modified to im- mobilize metal-chelating ligands. Such materials are use- ful for the preconcentration of metal ions before deter- ruination 2 and for metal ion chromatography, a

In order to understand the nature of surface immobi- lized ligands and their metal ion complexes more com- pletely, spectroscopic studies of an immobilized t-dike- tone were undertaken. Comparison of chemical and spec- tral properties to those of a model compound and a model complex was valuable. However, it must be ex- pected that differences between immobilized species and their solution analogues will be important. Interactions between bound species and the surface, especially surface sflanol groups, may affect the properties of the immobi- lized moieties. A detailed knowledge of these interactions should lead to a more rational approach to syntheses and to applications.

I. EXPERIMENTAL

A. A p p a r a t u s and Procedures . Infrared spectra

Received 12 November 1981; revision received 11 January 1982. * Author to whom correspondence should be addressed. Present ad-

dress: Department of Chemistry, Colorado State University, Fort Collins, CO 80523.

436 Volume 36, Number 4, 1982

were obtained using a Nicolet MX-1 Fourier transform infrared spectrometer. Samples were prepared as halo- carbon oil mulls between KBr plates. Most spectra were the average of 320 scans with a resolution of 2 cm -1. The software supplied by the vendor permitted spectral sub- traction and other manipulations.

Ultraviolet spectra were measured by photoacoustic spectroscopy. The instrumentation and techniques have been described previously. 4

B. Syntheses . Spectroscopic studies were performed on an immobilized fl-diketone, a model compound, and the Cu(II) complex of the model compound.

1. Immobil ized 3-Benzyl-2,4-pentanedione. p-Chloro- methylphenyltrimethoxysflane (Dow Corning) was first immobilized on silica gel or fumed silica. The silica gel was Baker chromatographic grade, 60 to 200 mesh, 300 m2g -1. The silica was refluxed in a 4% (v/v) solution of the sflane in dry toluene for 2 h. The solid product was washed sequentially in toluene and methanol and then dried overnight at 80°C in a vacuum oven.

The immobilized benzylchloride was reacted with 2,4- pentanedione as described by Waddell et al. 5 Essentially, a mixture of the reactants is refluxed in acetone in the presence of K2CO3. After 23 h of refluxing, the modified silica was washed and dried. Qualitative solid state car- bon-13 NMR spectra showed that the p-choloromethyl- phenyltrimethoxysflane had not exclusively given p-cho- loromethylphenyl groups on the surface, although the nature of the other products has not yet been established. Energy dispersive x-ray fluorescence spectroscopy re- vealed that 75% of the chlorine content of the immobi- lized benzyl chloride sflane was lost during reaction with 2,4-pentanedione. Thus it is clear that the yield of the desired product was less than 100%. However, the results that follow show that the functional properties of the modified silica are derived from immobilized 3-benzyl-2, 4-pentanedione, the structure of which is shown in Fig. 1.

2. 3-(Benzyl)-2,4.pentanedione. This model ligand was prepared by the method of Morgan and Taylor. 6 Purifi- cation was by vacuum distillation and was checked by gas chromatography. The structure was confirmed by 1H-NMR.

3. Bis[3-benzylacetylacetonato]copper(II). This com- plex has been reported by both Morgan and Taylor 6 and by Hauser and Harris. 7 The melting point reported by the latter authors, 205 to 207°C, is the correct one. The synthesis used for this work consisted of combining a methanol solution of 3-benzyl-2,4-pentanedione with aqueous ammoniacal copper(II). Recrystallization was from chloroform-petroleum ether.

H. RESULTS AND DISCUSSION

A. Immobi l ized 3-Benzyl-2,4-pentanedione. For

APPLIED SPECTROSCOPY

both infrared and ultraviolet studies, the model com- pound, 3-benzyl-2,4-pentanedione, proved extremely val- uable. This compound is 46% enol at 38°C (neat) as determined by NMR. s In dilute solution in nonpolar solvents an even higher percentage of enol is present. 9

The carbonyl region of the infrared spectra of the model fl-diketone and the model compound adsorbed on silica gel is shown in Fig. 2. The spectrum of the adsorbed compound has had the spectrum of silica gel (with ad- sorbed water) subtracted. The ketone carbonyl bands are at 1725 and 1700 cm -1 in the neat model compound and at 1721 and 1693 cm -~ for the adsorbed compound. The presence of a ketone carbonyl doublet is not unusual for a fl-diketone. The shift to lower frequency with adsorp- tion is an indication of hydrogen bond formation. The enol adsorption band is at 1604 cm -~ in the neat model compound and at 1606 cm -~ in the adsorbed species. Fig. 3 shows spectra of the adsorbed compound with the spectrum of silica subtracted using several difference factors. An enol band is present which is obscured by the presence of a 1630 cm ~ band from the silica gel and its adsorbed water. As this latter absorption is subtracted, the enol band is definitely revealed, although its exact size and shape is somewhat dependent on the difference factor used in the subtraction procedure.

There is relatively less enol present when the model

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14flVENUHBEB8 FIG. 2. Infrared spectra of (A) neat 3-benzyl-2,4-pentanedione, (B) 3- benzyl-2,4-pentanedione adsorbed on silical gel after subtraction of blank silica gel, and (C) immobilized 3-benzyl-2,4-pentanedione analog after subtraction of blank silica gel.

compound, 3-benzyl-2,4-pentanedione, is adsorbed on sil- ica gel than when it is neat. The exact enol-to-keto ratio appears to be dependent on the fraction of the surface covered. The enol-to-keto ratio is lower for those mole- cules adsorbed initially than for those molecules ad- sorbed later. This is not unexpected since the keto form can form strong hydrogen bounds with the surface. Sim- ilar results have been observed with acetylacetone. 1° If it is assumed that the surface species have the same struc- ture and molar absorptivities as the tautomers in the neat liquid, it is straightforward to calculate for any specific sample the percentage of enol on the surface from the relative intensities of the infrared bands and the NMR results on the neat compound. From the spectra displayed in Fig. 2 this calculation indicates approxi- mately 15% enol on the surface. However, as shown below, it is probable that the enol species present on the surface is different from the solution species.

The carbonyl region of the infrared spectrum of the immobilized 3-benzyl-2,4-pentanedione is shown in Fig. 2C, again after subtraction of the spectrum of silica gel and its associated water. There is a ketone carbonyl band at 1701 cm -1 and in the 1600 cm -1 region there are two smaller bands at 1606 and 1595 cm-L The similarity of the spectrum to the spectra in Fig. 1A of the model fl- diketone strongly suggests the presence of an immobi- lized 3-benzyl-2,4-pentanedione on the surface. The small differences between the spectra and the apparent pres- ence of two enol-like bands near 1600 cm -1 may well be

APPLIED SPECTROSCOPY 43'7

due to heterogeneity of the surface, to differences in interactions with the surface, or to a phenyl ring vibra- tion. As with the adsorbed species, the immobilized moi- ety is largely in the keto form. This is consistent with the likely probability that the immobilized fl-diketone forms hydrogen bonds with residual silanol groups. The ketone carbonyl vibration of the immobilized species is not split into a doublet as are those of the adsorbed and neat model compounds. The doublet is due to coupling of the two carbonyls and in-phase and out-of-phase carbonyl stretching vibrations are possible. ~1 When the two car- bonyls are trans to each other, only the out-of-phase vibration is infrared active. Thus both the adsorbed and neat 3-benzyl-2,4-pentanedione have the carbonyls cis to each other. The immobilized analogue may well have the carbonyls trans to each other.

Although the infrared spectrum contains information only about the ground electronic state, the ultraviolet spectrum contains information about both the ground and excited electronic states. In addition, since absorp- tivities for electronic transitions span a greater range than those of vibrational transitions, ultraviolet spectra can in favorable situations selectively probe moieties with high absorptivities. Although for 3-benzyl-2,4-pen- tanedione, the enol infrared absorption is well separated from the ketone carbonyl band, the ultraviolet spectrum nevertheless provides additional information.

In cyclohexane, 3-benzyl-2,4-pentanedione has an ab- sorption maximum at 287 nm (E = 2100 L mo1-1 cm -~) which shows a small red shift in ethanol (hmax = 290 nm). This is characteristic of the ~r -* ~r* enol adsorption. The keto band, which is not observed, is expected to occur at approximately the same wavelength with an absorptivity at least an order of magnitude less. ~2

The photoacoustic spectra of the immobilized fl-dike-

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438 Volume 36, Number 4, 1982

tone and the adsorbed model compound are shown in Fig. 4. The adsorption maximum for immobilized 3-ben- zyl-2,4-pentanedione and adsorbed 3-benzyl-2,4-pen- tanedione are at 271 and 265 nm, respectively. The similarity of the two spectra supports the proposed struc- ture of the immobilized ligand.

The observed ultraviolet spectra of both the adsorbed and immobilized fl-diketones are different from the so- lution spectra of the model compound. Compared with the solution spectrum in cyclohexane, the spectrum of the adsorbed fl-diketone is blue shifted 22 nm {2890 cm-1), whereas the immobilized moiety is blue shifted 16 nm (2060 cm-1). In nonpolar solvents the s-cis enol form is the dominant enol form due to intramolecular hydro- gen bonding. 13 The observed hypsochromic shifts are too large to be a result of changing the environment of this enol form. With ethanol rather than cyclohexane as the solvent, the band maximum undergoes a bathochromic shift of only 3 nm (360 cm-~). This red shift is expected for a ~r --* qr* transition when observed in a more polar solvent. ~4 Thus both the magnitude and the direction of the observed shifts argue against the presence of the same s-cis enol conformer being present on the silica surface as is present in solution.

It is necessary to consider the possibility that the observed surface spectra arise from the keto tautomer. Leermakers e t a l . 15 studied the spectra of ketones ad- sorbed on silica gel. They found blue shifts on adsorption almost as large as those observed here. However, the molar absorptivities of the adsorbed ketones were of the same low magnitude as the solution species. It seems improbable that the observed photoacoustic spectra could be that of the keto form since the observed inten- sity is high. Also, if the observed spectra were attributed to the keto tautomer, it would be necessary to explain the absence of ultraviolet absorption by the enols ob- served in the infrared. Another alternative is that the enolate ion gives rise to the observed surface spectra. If an ethanol solution of 3-benzyl-2,4-pentanedione is treated with potassium hydroxide, a solution of the eno- late ion is formed. The lowest energy ~r ~ 7r* transition has a maximum at 307 nm. It is most improbable that adsorption on silica gel would cause a hypsochromic shift of 42 nm. Since the infrared spectra are also not consist- ent with the presence of an enolate, this explanation is discounted.

A specific interaction between the adsorbed or immo- bilized ligand and the silica surface is most likely respon- sible for the observed ultraviolet spectra. Fig. 1 shows the intermolecularly hydrogen-bonded enol structure which is believed to give rise to the observed spectra. As discussed below, a s-trans rather than the s-cis conformer shown may be present. The adsorbed and immobilized fl-diketones are largely keto on the surface, since the keto tautomer can form hydrogen bonds with the surface as shown in Fig. 1, but there is a small fraction of enol that must be favored at some sites. The change from intra- molecular to intermolecular hydrogen bonding of the enol would be expected to affect the electronic spectra. Eliminating the possibility of an intramolecular hydrogen bond in 2,4-pentanedione by forming the mono-methyl ether (4-methoxy-3-penten-2-one) changes the ultravi- olet maximum from 273 to 254 nm. TM The enol form of 1,3-cyclohexanedione is locked in the s-trans configura-

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tion and is unable to form an intramolecular hydrogen bond. The absorption is at 253 nm in ethanol, 251 nm in relatively concentrated cyclohexane solutions, and 236 nm in dilute cyclohexane solutions. 17 The wavelength of maximum absorption depends on the extent of interrmo- lecular hydrogen bonding. In diluted cyclohexane solu- tions enol monomers are present, whereas in more con- centrated cyclohexane solutions intermolecularly hydro- gen-bonded dimers and polymers are predominant. If 2,4- pentanedione and 1,3-cyclohexanedione are considered to have essentially the same chromophore, it is observed that the change from intramolecular to intermolecular to no hydrogen bonding results in a regular decrease in the wavelength maximum. Intramolecular hydrogen bonding requires, of course, the s-cis conformer. If intramolecular hydrogen bonding is disrupted or replaced by intermo- lecular hydrogen bonding the s-trans conformation as well as nonplanar configurations also become possibili- ties. For some enols in some solvents the s-trans confor- mation is found, is Steric factors also affect the s-cis-to-s- trans ratio. ~a For some 1-acetylcyclohexane derivatives with enol conformations similar to those of the fl-dike- tone under discussion, a form similar to the s-trans enol of the fl-diketone is more stable. 19 In summary, it is reasonable to assume that the large blue shifts observed on adsorption or immobilization are due to changes in the hydrogen bonding of the enol tautomer.

The presence of an intermolecularly hydrogen-bonded enol rather than an intramolecularly hydrogen-bonded enol is not inconsistent with the infrared results. Consider dimedone (5,5-dimethyl-l,3-cyclohexanedione) whose enol cannot form an intramolecular hydrogen bond. 2° Yet in the solid state dimedone is entirely enolic. In solution

the keto form is in equilibrium with the enolic conformer. In chloroform the keto form is in equilibrium with the enol dimer for the concentration range reported, 0.25 to 2.0% (w/v). The enol dimer, which is intermolecularly hydrogen bonded, has a carbonyl adsorption at 1607 cm -1. Infrared studies were not done on solutions with concentrations low enough to observe enol monomers. 11 Thus the infrared spectra of the immobilized and ad- sorbed species is consistent with the presence of an intermolecularly hydrogen-bonded enol.

B. Metal Ion Complexat ion. The immobilized 3- benzyl-2,4-pentanedione will bind copper(II), iron(III), and nickel(II) ions, the only ones tested. Metal ion ca- pacity and infrared studies of the immobilized complexes were compared with bis[3-benzylacetylacetonato]-cop- per(II) as a model compound.

The infrared spectra of fl-diketone complexes are char- acteristic and different from those of the uncomplexed • ligand. This is because the majority of metal complexes formed by fl-diketones contain the ligands as enolate ions. 21 The carbonyl stretching bands characteristic of the keto and enol tautomers of the uncomplexed ligand disappear and new bands appear in the 1500 to 1600 cm -1 range which are quite intense. 22 Usually there are two, the higher frequency band is assigned as an asymmetric CO stretch and the lower frequency band as a CC stretch- ing mode. For instance, many metal acetylacetonate com- plexes have intense bands near 1580 and 1520 cm -1. In acetylacetonate complexes, variation of the metal ion does not appreciably affect the CO stretching fre- quency. 23

The model complex, bis[3-benzylacetylacetonato]cop- per(II), does not exhibit the keto and enol absorptions of

APPLIED SPECTROSCOPY 439

the ligands, but has an intense enolate absorption at 1567 cm -t. An impure, noncrystalline ferric complex of the ligand yielded intense absorptions at 1570 and 1522 cm -1. Copper chelates of r-substituted acetylacetones often do not exhibit the 1520 cm -~ band. 7

The immobilized fl-diketone was equilibrated with cop- per(II) solutions at various pH values. The samples were then dried and infrared spectra obtained. Fig. 5 shows the spectra in the 1400 to 1800 cm -1 region along with the spectra of the model complex. The enolate absorption is at 1565 cm - ' in the model complex and at 1572 cm-' in the immobilized complex formed at a pH of 8.5. As the pH is increased, proton removal is facilitated and the relative amount of complex increases. In sufficiently basic solution, the immobilized fl-diketone can form metal- enolate complexes in high yield, a conclusion supported by copper binding determinations.

At pH values of 2, 3, and 4 the immobilized 3-benzyl- 2,4-pentanedione will bind Fe(III) ions to an extent of 0.06, 0.14, and 0.15 mmol g-~, respectively. In contrast, silica gel with attached benzylchloride groups will only bind 0.01 mmol g-X Fe(III) ions at a pH of 2. Thus the immobilized fl-diketone can bind ferric ions in mildly acidic solutions. Yet, the infrared spectrum of this ma- terial reveals the ligand to be in the keto form. It seems most probable that the immobilized keto form is able to bind iron. On the silica surface the enolate ion or enolate complexes cannot form to any appreciable extent in contact with neutral or acidic solution. The only reason the immobilized fl-diketone can bind ferric or other ions at lower pH values is that keto complexes are formed.

It is not unexpected that the keto tautomer can form complexes with some metals. Such complexes have been

z

NAVENUHBER8 Fro. 5. Infrared spect ra of (A) bis(3-benzylacetylacetonato) copper(II) , and the immobil ized fl-diketone on silica gel after equil ibration wi th Cu(II) at pH 8.5 (B), pH 6.0 (C), and pH 4.0 (D).

440 Volume 36, Number 4, 1982

isolated and characterized. Octahedral complexes were prepared from group IV tetrahalides with either 3,3-di- methylacetylacetone or 3-methylacetylacetone as biken- tate keto ligands. 24 The carbonyl groups absorbed near 1690 cm -1. Another group prepared transition metal com- plexes containing acetylacetone as a neutral ligand. The infrared adsorption near 1700 cm -1 established that the ligand was in the keto form. 25 The crystal structure of one of this series of compounds showed the acetylacetone ligand as bidentate and nonplanar. In this compound, NiBr2(acacH)z, the keto carbonyl adsorption was at 1693 e r a - 1 . 26

III. CONCLUSIONS

The utility of infrared and photoacoustic spectroscopy in studying surface-bound ligands and metal ion com- plexes has been illustrated. Elsewhere the benefits of coupling photoacoustic spectroscopy with chemical bind- ing studies has been demonstratedY Solid state NMR studies of silylated surfaces 2s is also an important spec- troscopy that can complement the other techniques. A combination of spectroscopic and chemical studies is synergistic and is the best approach to the study of functionalized surfaces.

ACKNOWLEDGMENTS

This work was supported in part by research grant CHE-78-23123 from the National Science Foundation. The use of the Colorado State University Regional NMR Center funded by National Science Foundation Grant CHE-78-18581 is acknowledged.

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