Anal. Chem. 1982, 54, 101-105
be of immense value. ]Further comparisons of this technique with other mass spectrometric techniques (especially the triple quadrupole method) will provide a great deal of insight into the fundamental processes involved.
101
(16) Burnier, R. 6.;Byrd, G. D.; Freiser, B. S. Anal. Cbem. 1980, 52, 1641. (17) Burnier, R. C.; Byrd, G. D.; Freiser, 8. S. J . Am. Chem. SUC. 1981, 103, 4360. (18) Fedor, D. M.; Cody, R. B.; Burinsky, D. J.; Frelser, B. S.; Cooks, R. G. Int. J . Mass Spectrom. Ion Phys. 1981, 39, 55. (19) Franchetti, '4.; Freiser, B. S.; Cooks, R. 0. Org. Mass Spectrom. 1978, 73, 106. (20) Sigsby, M. L.; Day, R. J.; Cooks, R. G. Org. Mass Spectrom. 1979, 14, 273. (21) Sigsby, M. L.; Day, R. J.; Cooks, R. G. Org. Mass Spectrom. 1878, 14, 556. (22) Freas, R. B.; Ridge, D. P. J. Am. Chem. SOC. 1980, 102, 7129. (23) McLuckey, S. A.; Cameron, D.; Cooks, R. G. J. Am. Chem. SOC. 1981, 103, 1313. (24) Beauchamp. J. L.; Caserio, M. C.; McMahon, T. B. J . Am. Cbem. SOC. 1974, 9 6 , 6243. (25) Beauchamp, J. L.; Caserio, M. C. J . Am. Chem. SOC. 1972, 94, 2638. (26) Bomse, D. S.;Beauchamp, J. L. J. Am. Chem. SOC. 1981, 703, 3292. (27) Burnier, R. C.; Cody, R. B.; Freiser, B. S., unpublished results. (28) McIver, R. T. Workshop on Newer Aspects of Ion Cyclotron Resonance (Fourier Transform Mass Spectrometry), 29th Annual Conference of Mass Spectrometry and Allied Topics, Minneapolis, MN, 1981; p 791. (29) Cody, R. B.; Freiser, B. S. Anal. Chem. 1979, 5 7 , 547. (30) McLafferty, H. F.; Sakai, I. Org. Mass Spectrom. 1973, 7 , 971.
ACKNOWLEDGMENT Acknowledgment is made to Nicolet 1:nstrument Corp. for their technical assistance. LITERATURE CITIED (1) Kondrat, R. W.; Cooks, R. G. Anal. Chem. 1978, 50, 81 A. (2) McLafferty, F. W.; Bockhoff, F. M. Anal. C,hem. 1978, 50, 69. (3) Bozorgzodeh, M. H.; Morgan, R. P.; Beynon, J. H. Analyst (London) in711 II~ .-.-, in.? ."-, R-.-. (4) (a) Yost, R. A.; Enke, C . G. J. Am. Chem. SOC. 1978, 100, 2274. (b) Yost, R. A.; Enke, C. G. Anal. Chem. 1979, 51, 1251 A. (5) Yost, R . A.; Enke, C. G.;McGilvery, D. C.;'Smith, D.; Morrison, J. D. Int. J . Mass Soectronl. Ion Phvs. 1979. 90. 127. (6) Kaplan, F. J . i m . Chem. Soc,'1968, 90, 4483 (7) VanderHart, W. J.; Van Sprang, H. A. J . Am. Chem. SOC. 1967, 89, 32. (8) Henis, J. M. S. J . Am Chem. SOC.1968, 9 0 , 845. (9) Cody, R. B.; Freiser, El. S. Int. J . Mass Spectrom. Ion Phys., in press. (10) Comisarow, M. B.; Marshall, A. G. Chem. F'hys. Left. 1974, 2 5 , 282. (11) Parisod, G.; Comisarow, M. B. Adv. Mass Spectrom. 1980, 8 , 216. (12) Ghaderi, S.; Kulkarni, P. S.; Ledford, E. B.; Wilkins, C. L.; Gross, M. L. Anal. Chem. 1981, 59, 428. (13) Lehman, T. A,; Bursey, M. M. "Ion Cyclotron Resonance Spectrometry"; Wley-lnterscience, New York, 1976. (14) Beauchamp, J. L. Annu. Rev. Phys. Chem. 1971, 22, 527. (15) Cody, R. 8.; Burnier, R . C.; Reents, W. D., Jr.; Carlin, T. J.; McCrery, D. A.; Lengel, R. K.; heiser, B. S . Int. J . Mass Spectrom. Ion Phys. 1980, 33, 37.
RECEIVED for review August 11, 1981. Accepted October 19, 1981. Acknowledgment is made to the Department of Energy (DE-AC02-80ER10689) for supporting this research and the National Science Foundation (CHE-8002685) for providing funds to purchase the FTMS.
Fourier Transform Infrared Spectrometry, Carbon- 13 Nuclear Magnetic Resonance Spectrometry, and Photoacoustic Spect roscollpy of a SiIica- I mmobiIized Ligand D. E. Leyden," D. S. Kendall, L. W. Burggraf, F. J. IPern, and M. DeBello Department of Chemistry, University of Denim-, Denver, Colorado 80208
Infrared spectrometry, ultraviolet absorbance spectrometry, photoacoustlc spectralrcopy, and solld-sate 13C NMR spectrometry have in combination produceid useful information about an immobllirecl acetoacetamlde. The lmmobllired acetoacetamide formed by reacting dikotene wlth ellher amlnopropylsilane or (annlnoethyl)amlnopiropylsllane bound to silica gel is strongly hydrogen bonded and largely in the keto form. The keto form c,an be converted to the enolate Ion only In very alkaline conditions. In weakly acidic solutions the keto form complexes vvlth several metal Ions.
The modification of oxide surfaces such as silica gel by silylation ( I ) has had a large impact on such diverse and important areas of cheinistry as chromab3graphy (2), chemical photoconversion (3),immobilized enzymes ( 4 ) ,and catalysis (5). A wide variety of silanes can react with surface silanol groups. These silanes1 can themselves be further modified, providing the ability to prepare surfaces for specific purposes. For example, silanes r3uch as [N-(Z-aminoethy1)-3-aminopropyl] trimethoxysilane, essentially an ethylenediamine organofunctional silane, have been used on siliceous surfaces to preconcentrate metal ions for multielement determination by X-ray spectrometry. Modification to the dithiocarbamate
derivative also proved useful (6). In order to understand more completely the nature of the interactions between metal ions and immobilized ligands, between immobilized ligands and the surface, and between the surface and bound complexes, we have undertaken to investigate in detail a particular ligand and several of its metal complexes. An immobilized acetoacetamide, shown in Figure 2, was chosen because of its known synthesis (7), ease of preparation, and spectroscopic characteristics. The chemistry of immobilized ligands and metal complexes will not necessarily be that of analogous moieties in solution. A variety of spectroscopic and chemical techniques have been applied to elucidate similarities and differences. Comparisons of the spectroscopy and solution chemistry of model compounds to the immobilized group were valuable. The immobilized acetacetamide was also investigated in order to understand its selectivity in binding metal ions. It was reported to show significant affinity only for uranyl, ferric, and cupric ions (7). In contrast, most metals can form 0diketone complexes. Over 100 P-diketones have been successfully used to synthesize metal complexes (8). EXPERIMENTAL SECTION Silylation of' Silica Surfaces. Baker chromatographic grade silica gel, 60-200 mesh, 300 m2 8-l surface area, was used as received. (3-Aminopropyl)trimethoxysilane(APT) and [N-(2-
0003-2700/82/0354-0101$01.25/0 0 1981 American Chemical Society
102
ANALYTICAL CHEMISTRY, VOL. 54,NO. 1, JANUARY 1982
Table I. Acetoacetamide Infrared Spectra (Reported in Wavenumbers)
*
model compound dilute solutionC model compound neat immobilized APT-acetoacetamide immobilized AEAPT-acetoace tamide a See text for assignment. (2000-1400).
NH stretch
amide I1 overtone
Iret o ne carbonyl
amide I (carbonyl)
3439 3359 3289 3305
3077 3089
1714 17 20 1720
1681 1645 1647
3335
3095
1722
1635
* N-(1,4-Dimethylpentyl)acetoacetamide.
aminoethyl)-3-aminopropyl]trimethoxysilane(AEAPT), both obtained from Dow Corning Corp., were applied to the silica gel from a 5% (v/v) solution in dry toluene; for each 5 g of silica gel 25 mL of solution was used. The contact time was 2 h with occasional stirring of the mixture. After the reaction, the silica gel was separated by filtration,washed with toluene and methanol, and then dried at 80 "C in a vacuum oven for at least 3 h. Samples of fumed silica (Cab-o-sil, Cabot Corp.) were silylated in a similar manner. Preparation of Immobilized Acetoacetamides. APT or AEAPT was first immobilized on silica surfaces, as described above. Diketene in chloroform was reacted with the primary amino groups in order to form the acetoacetamides (7). Bis[ N-(1,4-dimethylpentyl)acetoacetamido]copper(II). N-(1,4-Dimethylpentyl)acetoacetamide(Aldrich) was used as received. Two grams of CuS04-5Hz0was dissolved in 20 mL of water. Sufficient aqueous ammonia was added to dissolve the cupric hydroxide after which 1g of the ligand was added. The green oil which separated was extracted with benzene. After the mixture was dried over NazS04,the benzene was evaporated, leaving a green oil which was dissolved in benzene and petroleum ether (1:9, respectively). When placed in a freezer, both a green oil and green crystals separated. The crystals were recrystallized from CHCl, and petroleum ether (1:l). The melting point was 181-182 "C. A copper determination by atomic absorption spectrometry gave 13.5% (13.8% calculated). Infrared Spectra. Infrared spectra were obtained with a Nicolet MX-1 Fourier transform infrared spectrometer. Samples were prepared as Nujol or halocarbon oil mulls between KBr plates, or as solids in KBr pellets. The software supplied by the vendor permitted spectral substraction and other manipulations. Ultraviolet and Visible Spectra. Solution spectra were run on a Cary Varian 219 spectrophotometer. Two methods were used for solid-statespectra. In the first, a slurry of the ground modified silica gel was prepared with cyclohexane. This slurry was placed in a 1-mm cell, and spectra were acquired by using a similar cell containing only silica gel and cyclohexane as the reference (9, IO). Unmodified silica gel was used to dilute the sample. The second method was photoacoustic spectroscopy. Photoacoustic spectra were measured by using instrumentation and techniques described previously (11). NMR Spectra. Cross-polarization magic angle spinning I3C NMR spectra were obtained at the Colorado State University Regional NMR Center using a Nicolet NT-200 wide bore Fourier transform spectrometer. The cross polarization contact time was usually 3 ms and approximately 5000 scans were accumulated. Tetramethylsilane was used as the chemical shift reference.
RESULTS AND DISCUSSION Immobilized acetoacetamides were prepared by reacting diketene with both bound [N-(2-aminoethyl)-3-aminopropyllsilane (AEAPT) and (3-aminopropy1)silane (APT). N-(1,4-Dimethylpentyl)acetoacetamidewas used as a model compound for solution phase chemical and spectra studies. Figure 1shows portions of the infrared spectra of the bound acetoacetamide prepared from APT (A), the neat liquid model compound (B), and the model compound in dilute solution (C). Table I shows the band positions and assignments. In dilute solution little intermolecular interaction occurs and the bands can be assigned as those expected for a nonhydrogen
amide I1
1648
1527 1552 1554 1545
0.01 M in CCl, (3800-2600)and C,Cl,
I 13800
enola
n
1
'
3200
'
$600
'
ZOO0
WAVENUMBERS
'
(700
'
1'400
Figure 1. Infrared spectra of (A) immobilized acetoacetamide prepareci from APT, (6)neat N-(l,4dimethylpentyl)acetoacetamide, and (C) 0.01 M N-(1,4-dimethyipentyI)acetoacetamide in carbon tetrachloride (3800-2600cm-') and tetrachloroethylene (2000-1400cm-').
bonded secondary amide and a ketone carbonyl (12). The band at 1648 cm-l is the enol CO absorption as substantiated by the observation that the relative intensity and position of the band are unaffected by dilution from 0.01 to 0.001 M. The infrared spectrum of the model compound as a neat liquid is considerably different, as expected for concentrated amides which may form hydrogen bonded networks (13). The amide I and I1 bands have shifted toward each other, indicative of strong hydrogen bonding of both the amide NH and the amide carbonyl and also of a trans configuration. The amide I1 overtone and the NH stretching frequency are also characteristic of a strongly hydrogen bonded, trans, secondary amide. The spectra of the immobilized acetoacetamide are very similar to that of the neat model compound. All the observed bands are attributable to the keto form. These results suggest that on the surface both the bound amide NH and CO are strongly hydrogen bonded and are in the trans configuration. Because the reaction conditions were chosen in an attempt to achieve monolayer coverage (14),many of these hydrogen bonds are probably formed with unreacted surface silanols. Numerous studies have shown that silica gel will have on the order of 7.5 pmol m-2 of silanol groups (15). It is the reaction of the aminosilanes with the silica surface which determines the surface loading of ligand. The aminosilane coverage, as found by carbon determination, is never, in our experience, more than 2.5 pmol m-2 for the method of preparation used here. Although some silanes will react with two surface silanol sites, there will be residual silanol on some of the silanes after reaction. Thus, there should be on the order of 3.5 pmol m-2 of unreacted silanol present after conversion to the acetoacetamide which is available to participate in hydrogen bonding. The I3C solid-state NMR spectrum of the immobilized acetoacetamide confirms the structure suggested by the in-
ANALYTICAL CHEMISTRY, VOL. 54, NO. 1, JANUARY 1982 0 - S I CH2Ct12CH2NI-I CH2CH2NHCOCH2COCH, f e c c c b c o d C 1
d e
PPM 0
205
b 170
f
250
200
is0
100
so
o
-so
c
508
d
30 I
-100 ppri
cross polarization with magic angle spinning
Figure 2. Solid-state NMR spectra of AEAPT-acetoacetamide. Strlucture and chemical shift
assignments are shown. frared data and places semiquantitative limits on the amount of enol tautomer present. The I3C spectrum of the acetoacetamide prepared from immobilized AEAPT is shown in Figure 2, with the structure and resonance assignments in parts per million. The assignments are based on chemical shifts from published spectra (16) and calculated from empirical additivity rules, (17). The four methylene groups a t 51 ppm are not resolved, but this peak is by far the most intense. Additivity constants for the --Si(OX)3group (X is H or Si) were based on the solid-state spactrum of immobilized CH3NHCH2CH2CH2Si(OX)3(18). Observed shifts were 10.0, 23.5, and 49.9 ppm for the methylene carbons a, 0,and y to the silicon, respectively, and 34.4 ppm for the methyl carbon. If empirical additivity constants are assumed to be valid, then the only other parameters needed to calculate chemical shifts are those for the Si(OX)3group. Therie were calculated by subtracting from the observed shifts ithe known additivity constants (17) and were found to be -2.4, -0.6, and -3.5 ppm for the effect of the Sli(OX)3group on the carbons CY, 0,and y to the silicon, respectively. The 13C NMR spectrum of the acetoacetamide is interpretable by considering only the keto florm. The most easily observed enol resonance would be the enolic methine carbon. In the enol form of acetylacetone it occurs at 100.3ppm (16). On the basis of the srnlallest observed peak of the keto form relative to the background, there is probably less than 15% enol present. This if, in qualitative agreement with the infrared results discussled above. However, the ultraviolet absorption spectra of the immobilized acetoacetamildes indicate the presence of a small fraction of both enol and enolate. The immobilized acetoacetamide prepared from APT on silica gel has transmission absorption maxima a,t, 254 and 291 nnn as slurries in cyclohexane, while the same ligand immobilized on fumed silica has maxima at 242 and 285 nm in its photoacoustic spectrum. The acetoacetamide prepared from AElAPT on silica gel has broad and poorly resollved bands centered at 259 and 294 nm when spectra are taken from cyclohexane slurries in transmission cells and a t 245 and 288 nm in the photoacoustic spectrum of the ligand on fumed silica. Ik is uncertain whether the photoacoustic absorption maxima are at slightly lower wavelengths because of substrate differences, the presence of solvent, or technique dependent factors. N-(1,4-Dimethylpenty1)acetoacetamide in cyclohexane solution shows a single absorption maximum in the ultraviolet region at 245 nm with a molar absorptivity of 4200 cm-l mol-’ L. This transition
103
is a a-a* transition of the enol tautomer. The bands a t shortest wavelength in the immobilized acetoacetamides are undoubtedly from a-a* transitions of the enol form. The immobilized acetoacetamides on silica gel are red shifted compared to the model compound. This is to be expected for a a-a* transition in a more polar environment. The excited state is likely t u be more polar than the ground state and thus stabilized to a greater extent on a polar surface (19). The amount of enol present under these conditions has not been determined for the model compound. N,N-Diethylacetoacetamide is reported to be 67.2% keto at 25 “C as the pure liquid (20). In chloroform,,,A is 256 nm and the molar absorptivity is 3200 (21). Often a larger fraction of the enol tautomer is piresent in nonpolar solvents than in the neat liquid. For example, ethyl acetoacetate is 7.9% enol in the neat liquid and 25% enol in diethyl ether (22). From these results it is reasonable to propose for the immobilized and model acetoacetamides under consideration that the absorption band near 250 nm is an enol TIT* transition with a molar absorptivity of about lo4 cm-l m o r L for the pure enol. Along with this assumption, the density of the modified silica gel and the measured absorbance can be used to calculate that the “observed” molar absorptivity is about 800. From this one can proporie that there is roughly 8% enol on the surface. This is consistent with the infrared and 13C NMR results which suggest little enol. A more accurate determination of the enol percentage would require measurements to substantiate the many assumptions used, such as the transferability of absorptivities. When N-(1,4-dimethylpentyl)acetoacetamideis adsorbed on silica gel, no ultraviolet absorbance bands are observed. The absorption bands of the keto form would be too weak to observe by the slurry technique if they are similar in intensity to those of normal amides and ketones. Thus there is less enol present in tho adsorbed model compound than in the immobilized acetoacetamide. The silica surface is polar and can function as a donor in hydrogen bonding. It is reasonable that the keto tautomer of the adsorbed model compound, which can form intermolecular hydrogen bonds, is preferentially stabilized on the surface. This is the case with acetylacetone (23). That the immobilized acetoacetamide, though predominantly in the keto form, has a larger percentage of the enol tautomer than the adsorbed acetoacetamide must mean that at least some of the immobilized moieties are not able to fully participate in hydrogen bonding. The 290-nm band observed in the spectrum of the immobilized acetoacetamide is too intense to be completely attributable to the keto form, although the wavelength maximum is at the expected position. Treatment of the modified silica gel with aqueous sodium hydroxide increases the intensity of this band, establishing it, at least in part, as an enolate a-a* absorption. The immobilized acetoacetamide is largely in the keto form. There is, however, a small percentage of enol and enolate present on the surface, accounting for the ultraviolet absorbance spectrum. Many 0-diketones can form metal complexes. The majority of metal complexes formed by 0-diketones contain the ligands as enolate ions (24). The enolic proton is replaced by a metal ion. When metal complexes are formed, both the vibrational and electronic spectra’are changed, often substantially. The infrared spectra of metal acetylacetonates are quite characteristic (25). The carbonyl stretching bands characteristic of the keto and enol forms of the uncomplexed ligand disappear. New bands appear in the 1500-1600 cm-l range which are quite intense. Usually there are two, the higher frequency band is assigned as an asymmetric C-0 stretch and the lower frequency band as a C-C stretching mode. For instance, many metal acetylacetonate complexes have intense bands near 1580
104
ANALYTICAL CHEMISTRY, VOL. 54, NO. 1, JANUARY 1982
Table 11. Properties of Metal Ion Complexes Fe( 111) APTacetoFe( 111) AEAPTacetamide acetoacetamide preparation pH I R S abs in 1500-1 800 cm-' range, cm-' UV abs, nm
2 1719 1650 1565 26 1 298
2 1720 1634 1561 272 295 (sh)
visible abs, nm
metal capacity, mmol/g
0.10
0.20
Fe( 11) AEAPTacetoacetamide
Cu(11) APTacetoacetamide
Cu(11) AEAPTacetoacetamide
11 1696 ( w ) 1571 (s, sh)
11 1595' 1547
11 1565 1520
263 215 (sh) 355
238d 28Bd
237d 290d
693d
690d
0.43
0.52
modela compound basic 1600 (m) 1561 (s) 1507 isj 232 272 374
550 (sh)e 630e 654e 674f 695d
Bis[N-(1,4-dimethylpentyl)acetoacetamido]copper(11). Abbreviations: s, strong; sh, shoulder; m, medium; w, weak. Photoacoustic spectroscopy. e Carbon tetrachloride as solvent. f Methanol as solvent . a
' Uncomplexed ligand subtracted.
and 1520 cm-l. In acetylacetonate complexes variation of the metal ion does not appreciably affect the C-0 stretching frequency (26). The a--n* enol absorption in the ultraviolet is replaced by a a-a* enolate absorption upon metal complex formation. In addition, charge transfer bands are often observed. The enolate a-a* transition is often observed a t a slightly lower energy than that of the enol, but normally is equally intense. For example, the enol form of acetylacetone absorbs at about 273 nm in a variety of solvents. The enolate ion, formed in basic solution, absorbs at 294 nm. Both transitions have a molar absorptivity of about lo4. Ferric acetylacetonate absorbs a t 235, 272, 351, and 431 nm in ethanol (27). The 272-nm absorption is the a-a* enolate absorption, while the 351 and 431 nm bands are charge transfer bands. The latter is responsible for the characteristic red color of a-diketone complexes with ferric ion. For this complex the r-a* enolate absorption happens to occur near the acetylacetone R-T* transition, but this is not usual. In a series of y-substituted acetylacetone complexes of copper(I1) the a-a* transitions were in the 290-315 nm range (28). For acetoacetamides similar results are observed. Bis[diethylacetoacetamido]copper(II) absorbs a t 1565, 1525, and 1500 cm-' in the 1500-1800 cm-l region of the infrared. It is not necessary to make exact assignments in order to attribute these bands to the enolate form of the acetoacetamide. The same compound absorbs at 260 nm with a molar absorptivity of about 1.6 X lo4 in chloroform (21). Thus the infrared spectrum and usually the ultraviolet spectrum are a strong indication of the formation of a metal enolate complex. The model complex which was prepared, bis[N-(1,4-dimethylpentyl)acetoacetamido]copper(II),absorbs strongly at 232 and 272 nm in cyclohexane. Bands which were observed in the 1500-1800 cm-l region in the infrared region are also consistent with an enolate complex. Spectra data of the acetoacetamide-metal ion complexes are given in Table 11. Upon treatment of the immobilized acetoacetamide prepared from AEAPT on silica gel with a solution of Cu(NH3)Z' in aqueous ammonia, a dark green immobilized complex was formed; 0.52 mmol g-l of copper was bound. The material had infrared bands at 1565 (5) and 1520 (m) cm-'. Clearly, a copper(I1)-enolate complex had been formed. The strongly basic conditions promoted proton removal and enolate formation. Only a small fraction of ligand in the keto form was observed. When the acetoacetamide prepared from APT on silica gel was similarly treated, comparable results were obtained and 0.43 mmol Cu g-' was bound. IR bands at 1595
and 1547 cm-l were observed after subtraction of the spectrum of the free ligand. The product of the APT reaction had a substantial amount of acetoacetamide still in the keto form. The difference in percentage of enolate formed and the differences in IR frequencies reflect variations in interactions with the surface and in the structure of the complex. However, it is clear that copper(II)-enolate complexes are formed under basic conditions. The immobilized acetoacetamide copper(I1) complex prepared from AEAPT shows UV absorption bands a t 237 nm and 290 nm. The latter band is a t the same wavelength as the immobilized enolate ion prepared in aqueous sodium hydroxide. As shown in Table 11, the ultraviolet spectra of the immobilized copper complexes prepared under basic conditions as measured by both photoacoustic and transmission measurements compare very well to the spectra of the complex formed with the model compound. The table also presents results for the visible spectrum of the model compound. It is clear that the envelope of d-d transitions shows strong solvent effects. An immobilized iron(II1) enolate complex was also prepared with the acetoacetamide prepared from AEAPT. The modified silica gel was first treated with aqueous ammonia. After the excess ammonia was removed by filtration and the silica gel washed with water, ferric chloride in ethanol was added to the silica gel, forming an intensely colored complex. After the appropriately weighted spectrum of the unbound ligand was substracted to remove a contribution from a small amount of unchanged ligand, absorption bands were observed at 1696 (w) and 1571 (strong, broad) cm-l. Clearly the enolate ion has been formed and must be involved in iron bonding. The band a t 1696 cm-l, probably from a shifted ketone carbonyl, may be due to unbound ligand. Ultraviolet transitions were observed at 263,275 (shoulder), and about 355 nm. One of the first two transitions is a TIT* enolate band. The 355-nm band is not observed in the uncomplexed ligand and is undoubtedly a charge transfer band. This broad band extends into the visible region to about 420 nm, accounting for the intense color, similar to that of iron(II1) acetylocetonate. Thus, in sufficiently basic solution the immobilized acetoacetamide can form metal-enolate complexes in high yield. The competition between metal ions and protons for the enolate determines the degree of complex formation. This is reflected in the synthesis of many p-diketones complexes. A base is used to remove the enolic proton. Rather stringent conditions are necessary to form the enolate ion or complexes from the immobilized acetoacetamide.
ANALYTICAL CHEMISTRY, VOL. 54, NO. 1, JANUARY 1982
It has been reported that in neutral or acidic solutions only Fe(III), Cu(II), and UCh?+ ions would bind to an immobilized acetoacetamide (7). For example, Fe(1II) binds to the AEAPT acetoacetamide on silic,agel at pH 2 to the extent of 0.20 mmol g-' for one preparation. At this loading, a substantial portion of the acetoacetamide ligands must be involved in the binding, as blank silica gel and ,4EAPT treated silica gels will not bind iron at this pH. Yet the infrared spectrum of this material reveals the ligand to be in the keto form. The spectrum looks very much like that of the free ligand. The acetoacetamide/AEAPT from which the compleir was made absorbed a t 1721,1634, and 1544 cm-'. The immobilized iron complex absorbed at 1720, 1634, and 1561 cm-l. Clearly, the immobilized keto form is ablle to bind iron at pH 2. The ultraviolet spectra are also consieitent with keto binding. The iron(II1) complexes observed at pH 2 with the APT and AEAPT acetoacetamides have ultraviolet absorption bands very similar to the uncomplexed ligand, which has bands due to a small proportion of the enol form. The intensities are similar and the maxima are shifted only a few nanometers. Thus, there is a low percentage of immobilized enol which does not form complexes and continues to absorb in the ultraviolet region. The charge transfer band(s) observed with the iron(II1) enolate complex prepared unlder basic conditions are not observed with the keto complexes formed at pH 2. On the silica surface the enolate ion or enlolate complexes cannot form to any appreciable extent in (contact with neutral or acidic solution. The only reason that the immobilized acetoacetamide can bind ferric and uranyl ions a t lower pH values is that keto complexes are formed. It is not unexpected that the keto form of the acetoacetamide can form complexes with some metals. Complexes containing P-diketonen in the keto form have been isolated and characterized. Octahedral complexes were prepared in inert solvents at low tlemperatures from1 group 4 tetrahalides with either 3,3-dimethylacetylacetone or 3-methylacetylacetone as bidentate keto ligands (29). The carbonyl groups adsorbed near 1690 cni-2, Another group prepared transition metal complexes containing acetylacetoiie as a neutral ligand. The infrared absorption near 1700 cm-l established that the ligand was in the keto form (30). The crystal structure of one of this series of compounds showed the acetylacetone ligand as bidentate and nonplmar. In this compound, NiBr2(acacH)2, the keto carbonyl absiorption was at 1693 cm-' (31). The studies we have reported are inlsensitive to the stoichiometry of the complexes. The immobilized acetoacetamide ligands are to a good approximation independent of other ligands bound to the same metal ion insofar as the vibrational and electronic spectra of their complexes are concerned (25). Thus it was not possibHe to decide whether 1:l or 1 2 complexes are formed on the surface. In favorable cases, it is possible to determine the stoicliiometry of the immobilized complexes, Photoacoustic spectroscopy and metal binding studies were applied to investigate Cu(I1) binding to an immobilized ethylenediamine analogue (32). The results of this study indicate that some of the bound ligands form 1:2 complexes and the remainder form only 1:l complexes. The conclusions of this study are of two types. First, we have determined specific structural features of an immobilized ligand and its Complexes. The acetoacetamides are strongly hydrogen bonded with the amide group in the normal trans
105
configuration. The ligand is largely in the keto form on the surface but a small amount of enol tautomer is also present. The keto tautomer forms metal complexes with Fe(II1) and UO+: (7). Enolate complexes are formed at higher pH values. Second, the benefits of an approach in which several spectroscopic techniques and chemical studies are used in combination to study modified surfaces is demonstrated. The various techniques used are complementary and suitable for a wide range of applications.
ACKNOWLEDGMENT NMR spectra were obtained with the aid of the Colorado State University Regional NMR Center, funded by National Science Foundlation Grant CHE 78-18581.
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RECEIVED for review June 22,1981. Accepted September 22, 1981. This research was supported in part by Research Grant CHE-78-23123 from the National Science Foundation and by the AMAX Foundation.
