Energy & Fuels 1994,8, 172-178
172
New lr9SnNMR Tagging Reagents for Labile Hydrogen Functional Group Analysis Mao-Chun Ye and John G. Verkade* Department of Chemistry and Ames Laboratory, Iowa State University, Ames, Iowa 50011-3111 Received March 12, 1993. Revised Manuscript Received October 20, 1999
MeaSnNHPh and (n-Bu3Sn)zNPh are shown to be excellent stannylating reagents for -OH and -SH functional groups present in coal materials. The latter reagent is not as prone to exchange phenomena as the former, however, thus facilitating resolution of 119Sn NMR peaks at 240 K for model mixtures derivatized with (n-Bu3Sn)zNPh. Because of self-associationof derivatized compounds in noncoordinating solvents such as hexane, l19Sn NMR spectra are best measured in pyridine, wherein this phenomenon is quenched by solvent coordination. A model mixture of four phenols and a similar mixture of five thiophenols in pyridine derivatized with (n-Bu3Sn)zNPh showed wellresolved 1Wn NMR peaks whose calculated integrations reflected the original amounts of phenols added within f1.7 % . l19Sn NMR spectra at 240 K of 29 phenols, thiophenols, carboxylic acids, and alcohols derivatized with (n-BusSn)nNPh in pyridine reveal that the l19Sn chemical shift ranges of these compound classes do not overlap. Preliminary results on a pyridine extract and alow-temperature pyrolysate of an Illinois No. 6 coal sample derivatized with (n-Bu&)zNPh indicate detectable amounts of phenols, of which four were tentatively identified in the condensate.
Introduction The identification and quantitation of heteroatom functionalities possessing labile hydrogens in complex mixtures and coal-derived liquids continues to be a challenge. As alternative and complementary approaches to frequently employed GC/MS techniques, IR and NMR spectroscopic procedures have been under development in recent years. NMR experiments have generally employed the use of derivatization reagents which replace labile hydrogens with an NMR-active nucleus (such as 29Si, l19Sn,and 3lP) or with functionalities bearing a 19F nucleus removed from the heteroatom by three bonds. A few years ago, we summarized the history of such efforts in our report on the use of phospholane 31PNMR tagging reagents for identifying organic compounds bearing a variety of labile hydrogen functional groups known to occur in coal.' Since that time, we have described superior phospholane 3lP NMR tagging reagents for identifying labile hydrogen functional groups,2 and the use of such reagents in speciating and quantifying phenols in coal condensate^,^ in quantifying phenols in coal resids4 and in quantifying moisture in solid coals.5 Reports have also recently appeared on the l3C and 29SiNMR spectroscopic identification of acetylated6 a n d trimethylsilylated species? respectively, and also on the use of Ph2P(0)Cl as a reagent for the quantitation of total phenolic Abstract published in Advance ACS Abstracts, December 1,1993. (1)Wroblewski, A. E.;Lensink, C.; Markuszewski, R.; Verkade, J. G. Energy Fuels 1988,2,765. (2)Lensink, C.; Verkade, J. G. Energy Fuels 1990,4 , 197. (3)Wroblewski, A. E.;Lensink, C.; Verkade, J. G. Energy Fuels 1991, 5,491. (4)Mohan, T.; Verkade, J. G. Energy Fuels 1993,7 , 222. (5)Reinartz, K.; Wroblewski,A. E.; Verkade, J. G. Energy Fuels, 1991, 5,786. (6)Yu, S. K. T.; Green, J. B. Anal. C h e n . 1989,61,1260. (7)(a) Tuel, A.; Hommel, H.; Legrand, A. P.; Kovata, E. J. Langmuir 1990,6,770.(b) Howarth, 0.W.; Ratcliffe, G. S.; Burchill, P. Fuel 1990, 69,297.
0887-0624/94/2508-0172$04.50/0
-OHcontent in a coal liquids and for the characterization of moisture types in The methylation of reduced heteroatom functionalities in coal liquids with l3C and 2H-enriched CH31has also been described.1° To our knowledge only two reports on the application of stannylating reagents for l W n NMR spectroscopic identification and quantitation of labile hydrogen functional groups in coal-derived materials have appeared.11J2 In both reports (n-Bu3Sn)20 was used as the reagent, and water from the reaction was azeotropically distilled from the refluxing toluene solution: Z-H
+ (n-Bu3Sn),0
A
Z-SnBu,
+ H20
(1)
Although the total range of l19Sn chemical shifts for phenols, alcohols, and thiols (ca. 60 ppm) makes the Sn( n - B u ) ~group an attractive derivatizing functionality, there are three disadvantages that detract from its current usefulness: (1)its method of introduction via reaction 1 involves heating to 111"C to azeotropicallyremove water, which in some complex mixtures may cause side reactions, (2) the water produced in the reaction may lead to undesirable hydrolyses, (3) the l19Snchemical shifts of its derivatives are bothersomely concentration and solvent dependent,"J2 often rendering assignments in unknown mixtures highly uncertain. Although MeaSnCl was found to react under much milder conditions with thiols and thiophenols, it was discarded as a candidate reagent in favor of (n-Bu3Sn)zO owing to fewer manipulations and to a lower mammalian toxicity of the latter compound.12 The abundance and NMR receptivity of l19Sn (8.58% and 4.4 X 103,respectively) lie between those of 29Si(4.7 74, (8)Dadey, E.J.; Smith, S. L.; Davis, B. H. Energy Fuels 1988,2,326. (9)Wroblewski, A. E.and Verkade, J. G. Energy Fuekr 1992,4,331. (10)Ebert, L. B.; Rose, K. D.; Scanlon, J. C. Fuel 1989,68,935. (11)Raffi, E.;Faure, R.; Louis, L.; Vincent, E. J.; Metzger, J. Anal. Chem. 1985.57. - - - , -, 28.54. ----
(12)Raffi, E.; Ngassoum, M.; Faure, R.; Foon, R.; Lena, L.; Metzger,
J. Fuel 1991,70, 132.
0 1994 American Chemical Society
New "9Sn NMR Tagging Reagents 3.7 X 10") and 31P(100.0%,0.066) which would seem to
make the l19Sn nucleus less desirable as a probe than 31P, though more attractive than 29Si. However, the -2000 ppm range of "9Sn NMR chemical shifts is considerably larger than that of 31P(- 700 ppm) and %Si(-400 ppm),13 which would render the I19Sn nucleus the most attractive NMR probe in terms of potential dispersion of chemical shifts of derivatized phenols, for example. In the present paper we explore the relative reactivities of stannylating reagents of the types R3SnNHPh, R3SnNMez, R3SnOzCMe, and R3SnOPh (wherein R = n-Bu or Me) with a variety of labile hydrogen functional groups relevant to coal-derived liquids, including carboxylicacids, phenols, alcohols,and amines. We show here, among other things, that (1) the concentration dependence of P 9 S n is virtually eliminated in pyridine as a solvent, (2) the range of P 9 S n values in pyridine for phenols, for example, derivatized with the (n-Bu)aSn group is an unprecedented 55 ppm, while for trimethylstannyl phenolates the chemical shift range is 33 ppm, (3) R3Sn group exchange in pyridine decreases in ease from Me3Sn to (n-Bu)sSn and with decreasing temperature, (4) (n-Bu)3SnNHPh is currently the optimum l W n NMR tagging reagent for identifying and quantitating labile hydrogen functional groups in model mixtures, and (5) several phenols can be identified in an Illinois No. 6 low-temperature condensate. Experimental Section Pyridine was distilled from CaHz and stored under nitrogen. Other solventa were dried by standard methods. Samples of the model compounds were purchased from vatious sources and used as received. Trimethylstannyldimethylamide was purchased from Aldrich Co. Trimethylstannyl phenoxide," trimethylstannyl2-methylphenoxide,14trimethylstannyl ethoxide,16trimethylstannyl acetate,la trimethylstannyl propanoate,16tributylstannyl phenoxide," tributylstannyl 2-methylphenoxide,11tributylstannyl acetate," and tributylstannyl propanoate17were prepared by literature methods. Because experimental details for the preparations of N-trimethylstannylanilinel* and N,N-bis(tri-nbutylstannyl)aniline19 were lacking in previous reports, our preparations are herein described. N-Trimethylstannylaniline.To a stirred solution of MeaSnNMez (4.50 g, 0.0217 mol) in 15 mL of anhydrous ether was added dropwise aniline (2.01 g, 0.0216 mol) in 25 mL of ether a t room temperature. After 2 h of stirring, the solvent was removed under vacuum. The residue was distilled under reduced pressure to give 4.72 g (85.6%) of the product. Bp, 61-62 "C/0.02 Torr (lit.'* 77 "C/0.05 Torr; 1H NMR (CDCls); 7.06 (t, 2 H, m-Ph protons), 6.55 (t, 1H, p-Ph proton), 6.45 (d, 2 H, o-Ph protons), 2.99 (e, 1 H, NH), 0.41 (s, 9 H, MesSn). NJV-Bis(tri-n-butylstanny1)aniline.To 13.0g (39.0mmol) of n-BusSnNMe2'8 was added aniline 1.85 g (20 mmol) at -78 "C. The mixture was then allowed to warm to room temperature after which it was heated in an oil bath a t 110 "C for 1.5 h. The gas that escaped from the reaction mixture was trapped at -78 "C to give 1.5 mL of a liquid. The residue was distilled under reduced pressure to give 11.0 g (81.9%)of the product. Bp, 184185 W 0 . 5 Torr (lit.lB160-180 "C/O.l mm); lH NMR (CDCl3): (13) Harris and Mann. NMR and the Periodic Table;Academic Press: New York, 1978. (14) Kozuka, S.; Kikuchi, A.; Yamaguchi, S. Bull. Chem. SOC.Jpn. 1981,54, 307. (15) Baldwin, J. C.; Lappert, M. F.; Pedley, J. B.; Poland, J. S. J. Chem. Soc., Dalton Trans. 1972, 1943. (16) Okawara, R.; Webster, D. E.; Rochow, E. G. J. Am. Chem. SOC. 1960,82, 3287. (17) Vilarem, M.; Maire, J. C. Compt. Rend. 1966, C262, 480. (18)Jones, K.; Lappert, M. F. J. Chem. SOC. 1965,1944. (19) Davies, A. G.; Kennedy, J. 0. J. Chem. SOC.(C)1971, 68.
Energy & Fuels, Vol. 8, No.1, 1994 173 7.01 (t,2 H, m-Ph protons), 6.62 (m, 3 H, 0,p-Ph protons), 1.46 (m, 12 H, Sn(CH2)2CH2),1.29 (m, 12 H, SnCHzCHz), 0.95 (t, 12 H, SnCHz), 0.87 (t, 18 H, Sn(CH2)3CHs). The Reaction between MetSnNMez and MeCOJL Although the first step of this reaction to produce MeCOzSnMes was reported earlier,20 we report here conditions permitting further reaction giving MeC(0)NMez and (Me3Sn)zO. To a solution of acetic acid (1.00 g, 16.6 mmol) in 10 mL of anhydrous THF wasaddeddropwise MeaSnNMez (6.90g, 33.2 mmol). During the addition, a white solid precipitated which disappeared upon heating to 50 "C. After stirring overnight at that temperature, the solvent was removed and the residue was fractionally distilled under reduced pressure to give a 1:l mixture of MeC(0)NMez and (Me3Sn)zO. Bp, 29-30 "C/0.7 Torr; lH NMR (CDC13) of MeC(0)NMeZ: 2.94 (s, 3 H,NCH3),2.87 (s, 3 H,NCH3), 2.01(s, 3 H, CH&(O)); lH NMR of (Me3Sn)zO: 0.255 (s,18 H, CH3);13C NMR (CDCl3) of MeC(0)NMez: 168.99 (C(O)), 36.97 (NCH,), 34.56 (NCHa),21.32 (CHsC(0));13CNMR of (Me3Sn)zO: -2.34; llBSnNMR of (Me3Sn)zO (CDC13): 106; IR of MeC(0)NMez: 1651 cm-l (C=O). Coal and Coal Products. Coal extracts and condensates were prepared from Illinois No. 6 coal, by literature methods.' NMR Spectroscopy. lH NMR and 13C NMR spectra were obtained on a Nicolet-300 spectrometer. "9Sn NMR spectra were recorded on a Bruker WM-200 spectrometer at 74.63 MHz. For trimethylstannyl derivatives, benzene-& was placed in a sealed 5-mm NMR tube as a field lock. Tri-n-butylstannyl derivatives had to be measured a t low temperature (240K)and so acetoned6 was used as field-lock solvent. Tetramethylstannane (Aldrich) was used as an external standard and all chemical shifts are reported relative to MehSn = 0. An inverse-gated decoupling technique was used, and 5-5 delay times between pulses were applied. A typical analysis was carried out as follows. To a 10-mm NMR tube was added a model compound under Nz after which the weight added was determined. Anhydrous pyridine (about 2.0 mL) was added using a syringe. A stoichiometric amount of either MeaSnNHPh or (n-BusSn)zNPhwas then added by syringe and the tube shaken to mix the solution. The standard solutions used to measure the concentration effect were prepared in 5-mL volumetric flasks a t room temperature, after which they were transferred to 10-mm NMR tubes. Solutions were ca. 0.4 M in tin, thus allowing adequate spectra to be obtained with a 20-min acquisition time in 200 scans. Chemical shifts are precise to h0.1 ppm as determined by repeated measurements.
Results and Discussion Effect of Concentration on "9Sn NMR Chemical Shifts. Several factors (e.g., concentration, solvent effects,
temperature) affect the I19Sn NMR chemical shifts of organotin compounds. The concentration effect is due to intermolecular association via donor-acceptor interactions between a donor atom such as a carbonyl oxygen and a Lewis acidic trialkylstannyl tin.21 A typical example of the variation of P 9 S n with concentration is that of trimethylstannyl formate whose I19Sn chemical shift moved from 152 to 2.5 ppm when the concentration increased from 0.05 to 3.0 Mez2 Compared with trimethylstannyl carboxylates, trimethylstannyl phenoxides and alkoxides displayed a weaker sensitivity of their 6119Snvalues t o concentration changes.23 For the purpose of speciating compounds with functionalities bearing labile hydrogens, however, this sensitivity must be minimized. (20) (a) George, T. A.; Lappert, M. F. J. Chem. SOC. ( A ) 1969,992. (b) Chandra, G.; George, T. A.; Lappert, M. F. J.Chem. SOC.(C)1969,2565. (21) Ye, M.-C.; Verkade, J. G., submitted for publication. (22) Smith, P. J.; White, R. F. M.; Smith, L. J . Orgunometal. Chem. 1972, 40, 341. (23) Annual Reports on NMR spectroscopy; Webb, G. A., Ed., Academic Press: New York, 1978; Vol. 8, p 291.
Ye and Verkade
174 Energy & Fuels, Vol. 8, No. I , 1994
I
1
0
2
3
Molarity
f 132;
I
I
I
1
2
3
Molarity
Figure 1. l19Sn NMR chemical shifts of MesSnOPhas a function of concentration in n-hexane (top). 119Sn NMR chemical shifts of MesSnOEt as a functionof concentration in n-hexane (bottom).
Curiously, the chemical shift behavior with increasing concentration is opposite for these compounds. The chemical shift of MesSnOPh and MesSnOzCMe in pyridine shows no trend with concentration change from 0.1 to 1.5 and 0.02 to 0.3 M, respectively. The constancy of P S n with concentration for these compounds can be ascribed to the strong coordinating ability of pyridine. Thus, the fully characterized adduct Me3SnCbNC&I6, for example, contains a pyridine whose nitrogen is coordinated to the tin atom.24 Also noteworthy is the strong upfield movement of l19Sn chemical shifts of MesSnOPh and MesSnOzCMe from noncoordinating solvents to pyridine. Thus W S n for MesSnOPh changed from 136.3ppm in a 1.5 n-hexane solution to 1.25 ppm in pyridine at the same concentration, while P S n for MesSnOzCMe changed from 136.9 ppm in n-hexane to -16.6 ppm in pyridine at the same concentration. This clearly indicates that these compounds are strongly coordinated in pyridine (by at least one and possibly two solvent molecules),since it is common for the chemical shifts of heavier nuclei to move upfield upon increasing their coordination number. In the solvent pyridine, this coordination effect is expected to essentially prohibit self-association of trialkylstannyl derivatives. Evaluation of Stannylating Reagents. Trimethylstannyl dimethylamide is a very active stannylating agentsz5 In a preliminary experiment, we reacted trimethylstannyl dimethylamide with phenol, thiophenol, and acetic acid in pyridine a t room temperature. After about 15 min their l19Sn NMR spectra showed that the reagent peak had disappeared and a new peak formed for each product (i.e., MesSnOPh, -10.3 ppm; MeaSnSPh, 53.0 ppm; MeaSnOC(O)Me,-30.0ppm). Curiously,these shifta differed from those obtained for the isolated compounds in separate pyridine solutions (1.25,58.2, and -16.6 ppm, respectively). That Me2NH released in reaction 2 was Z-H
-Ba!ahc!a I
I
I
120
100
I
80
-
12 pharms 6 thkb 6 mbphml8
-4add8 I
80
I
I
40
20
I
0
I
-20
I
I
P
-40
I"@Sn (ppm)
Figure 2. LleSn NMR chemical shift ranges for labile hydrogen functional groups of comparable compounds in each class derivatized by (n-BusSn)zOand recorded at room temperature in CHCla or as neat liquidsl1J2(a),derivatized by (n-Bu&M"h and recorded at 297K in pyridine (b), derivatized by (n-BusSn)zNPh and recorded at 240K in pyridine (c), and derivatized by MesSnNHPh and recorded at room temperature in pyridine (d).
In our experiments, n-hexane and pyridine were chosen as solvents in order to test the effect of a non-coordinating and a coordinating medium, respectively,on the variability of "9Sn chemical shifts of trimethylstannyl phenoxide, ethoxide, and acetate. Figures 1 and 2 illustrate the chemical shift change with concentration of the first two compounds in n-hexane. Apparently both compounds have a nonlinear concentration dependency, with a 3.8 and an 11.6 ppm change for MesSnOPh and MesSnOEt, respectively, over the concentration range 0.2 to 3.0 M.
-
+ Me3SnNMe2
PY
+
Me,SnZ HNMe, (Z = PhO, PhS, MeC(0)O) (2)
competing with pyridine in the initial experiment to drive the chemical shift of the coordinated tin further upfield was shown by adding one drop of dimethylamine to the trimethylstannyl phenoxide in pyridine solution. Its chemical shift moved from +1.25 to -42.8 ppm. It is not clear whether the dimethylamine replaced pyridine in the complex or that it coordinated to a five-coordinatepyridine complex to give a six-coordinate compound. It was observed that excess trimethylstannyl dimethylamide reacts further with trimethylstannyl acetate. In the earlier literature," only MeaSnOC(0)Me was obtained in this reaction, but in reaction 3 are shown the products 2Me,SnNMe,
+ HOC(0)Me
-
(Me,Sn),O
+ Me,NC(O)Me
(3)
obtained by using a 2:l molar ratio of reactants. The same products were also obtained by treating MeaSnNMez with MesSnOC(0)Me in a 1:lmolar ratio in THF at 45 "C. The pathway for the second step may resemble that described (24) Hulme, R. J. Chem. SOC.1963,1524. (25) Comprehensiue Organometallic Chemistry;Wilkinson, G.,Stone, F. G. A., Abel, E., Eda.; Pergamon Press: New York, 1982; Vol. 2, p 601.
New "9Sn NMR Tagging Reagents Table 1.
Energy &Fuels, Vol. 8, No. 1, 1994 175
NMR Chemical Shifts (ppm) of Phenols, Carboxylic Acids, Alcohols, and Thiophenole Derivatized with MesSnNHPh in Pyridine at Room Temperature compound PSn compound WSn 30.0 acetic acid 2-methoxyphenol -16.9 propionic acid -15.2 23.6 2,3,64rimethylphenol 11.3 menthol 91.4 2,3,5-trimethylphenol 7.9 82.6 1-butanol 5-indanol isopentanol 6.4 81.9 3,5-dimethylphenol ethanol 5.9 79.6 o-cresol 2-hydroxyethanethiol 69.1," 67.gb 5.5 p-cresol benzyl alcohol 73.0 3.2 m-cresol m-thiocresol 59.9 3.8 3-ethylphenol 58.2 thiophenol 0.8 phenol -1.5 67.6 2,4-dimethylbenzenethiol catechol o-thiocresol -3.7 64.2 2-naphthol 2,5-dimethylbenzenthiol 66.6 4-amino-2-chlorobenzoicacid -18.0 -17.5 a-methylcinnamic acid
a
11%
Tentatively assigned to Sn-0 tin. b Tentatively assigned to Sn-S tin. Table 3. l1$SnNMR Chemical Shifts of Trimethylstannyl Coml)oundsa 61Wn (ppm) compound CHClS (0.1 M) pyridine Abl%n MesSnNHC(0)CFs 77.0 -26.89 103.9 54.9 32.38 22.6 MesSnNHPh 73.3b 74.6 -1.3 MesSnNMez 136.9 -16.6 153.5 MeaSnOC(0)Me 1.3 149.7 151.0 MesSnOPh 136.8 80.0 56.8 MesSnOEt
Table 2. 11% NMR Chemical Shifts of Phenols and Carboxylic Acids Derivatized with (n-BusSn)zNPh in Pyridine a t Room Temperature 6119Sn (ppm) compound 72.1 2,3,6-trimethylphenol 65.6 2,6-dimethylphenol 39.6 5-indanol 35.3 3,5-dimethylphenol 32.0 o-cresol 28.8 phenol 18.8 2-naphthol 11.3 propionic acid 9.2 acetic acid 7.6 4-amino-2-clorobenzoicacid 2.5 a-methylcinnamic acid
a
earlier20for the reaction of MeaSnNMea and carboxylic acid esters in reaction 4. Because of these side reactions, Me,SnNMe,
+ ROC(0)Me
-
Me,SnOR
+ Me,NC(O)Me
(4)
MesSnNMe2 was removed from our consideration as a suitable reagent. The potential reagents Me3SnNHC(O)CF3 and MeaSnNHC(0)Me were easily prepared by condensation of trimethyltin hydroxide with the corresponding amide.21 However, these reagents were unreactive toward alcohols under mild conditions, and so these reagents were discarded. The reagents Me3SnNHPh18and (n-Bu3Sn)2NPhlgwere prepared via reaction 5. Preliminary experiments in which xR,SnNMe,
+ H,NPh
-
Me,SnNHPh, (n-Bu,Sn),NPh (5) x = 1,R = Me; x = 2, R = n-Bu
these reagents were reacted with model compounds in pyridine at room temperature indicated quantitative reactions in all cases (Tables 1 and 2). Even with the sterically hindered 2,6-dimethylphenol, the reaction with MesSnNHPh is complete in 15 min. The P 9 S n ranges for the derivatives summarized in Figure 2 show that good separation of the various classes of compounds derivatized is achieved for MesSnNHPh at room temperature and for (n-Bu,Sn),NPh at 240 K. From this figure it is also seen that the 6119Sn value shifts upfield for a given derivative as the temperature is lowered. This is expected, since higher-coordination tin complexes involving the solvent are expected to be less dissociated at lower temperature.
At room temperature. Measured as a 0.1 M solution in hexane.
Solvent Dependency of 6Wn. According to the literature,22the upfield progression for the W9Sn ranges for trimethyltin derivatives of alcohols,thiophenols, thiols, phenols, and carboxylic acids in nonpolar solvents is roughly carboxylic acids = phenols < thiophenols r thiols < alcohols. Thus, the greater the electron-withdrawing power of the group attached to the MesSn group, the lower the field of the P 9 S n value that might be expected. The opposite trend is seen in pyridine wherein the upfield progression of the P S n ranges of trimethyltin derivatives is alcohols < thiophenols and thiols < phenols < carboxylic acids (Figure 2). This order is reasonable if it is considered that increasing electron-withdrawing power of the substituents is expected to increase the tendency of the tin substituent to expand its coordination number by binding to pyridine, thus leading to increasingly upfield shifts. I t is odd, however, that RS and ArS substituents appear to be more electron withdrawing than RO for MeaSn derivatives. Notwithstanding, the electron induction rationale is also consistent with the very small AG1lgSnvalue of -1.3 ppm for MesSnNMee and the somewhat larger value of 22.55 ppm for MesSnNHPh from an essentially noncoordinating solvent to pyridine (Table 3). For n-BusSn derivatives, the upfield progression for the 6119Sn ranges in noncoordinating solvents is phenols < alcohols < thiols and thiophenols. Here the last twoclasses are reversed from their positions for MesSn derivatives. In pyridine, the upfield progression for the P 9 S n ranges of the n-BusSn derivatives is thiols and thiophenols < alcohols < phenols. Thus, for n-BusSn derivatives, alcohols appear to behave in a more electron-withdrawing manner than thiols and thiophenols, which seems quite reasonable. Reagent Reactivities. It can be supposed that the greater the contrast in the acidity (or basicity) of the
Ye and Verkade
176 Energy &Fuels, Vol. 8,No. 1, 1994
Table 4. Quantitative Determination of Mixtures of Model Compounds with MeaSnNHPh and (n-BuaSn)rNPh in
Pyridine
compound
menthol butanol ethanol benzyl alcohol 95.0
90.0
85.0
80.0
75.0
70.0
2,6-dimethylphenol
Figure 3. W n NMR spectrum of alcohols derivatized with MesSnNHPh with peaks from left to right representing L-menthol, butanol, ethanol, benzyl alcohol, and 19-thioethanol.
-
reagent and the substrate in reaction 6, the greater the R,SnE
+ Z-H
Z-SnR,
+ HE
(6)
reactivity of the reagent in terms of driving the reaction to completion. Thus the stronger the affinity of the E moiety in R3SnE for aproton (i.e., the greater its Bronsted basicity) and the greater the acidity of Z-H, the greater the degree of completion of reaction 6. Thus, the acidity order of the substrates HOC(0)Me > HOPh > H,NC(O)CF,
> HOEt > H,NPh > HNMe,
is consistent with our observation that MqSnNMez readily reacts with the first five members of this series, Me,SnNHPh reacts with the first four, MesSnOEt reacts with the first three, and MesSnOPh reacts with HOC(0)Me. Moreover, none of these reagents react appreciably with the remaining substrates in each case, and MeaSnOC(0)Me was found to be stable in solutions of all of the substrates. The reagent reactivity order Me,SnNMe, > Me,SnNHPh > Me,SnOEt > (M~,S~INHC(O)CF,)~' > Me,SnOPh > Me,SnOC(O)Me allows the selection of a reagent based on the functional groups to be derivatized in a mixture for l19Sn NMR spectral analysis. This reagent reactivity order and its rationale (which is based essentially on electron induction arguments - see above), are also in accord with the AP9Sn values in Table 3 which rise in the same order. Application of MeaSnNHPh and (n-BuaSn)rNPh in Analysis. The ll@SnNMR spectrum of a mixture of four alcohols derivatized with MesSnNMez is shown in Figure 3. The chemical shift range for the alcohols is 18 ppm, which is quite sufficient to fully resolve the peaks. A comparison (Table 4) of the calculated amounts of the alcohols from peak integration and the added amounts is quite satisfying (averageerror *0.6%). The chemical shift range for five thiophenols and pentane thiol was 17 ppm. Although these peaks are broader than for the alcohols, the peaks are quite well resolved. A t room temperature, only a single broad 119Sn NMR peak was observed for a mixture of phenols and carboxylic acids derivatized with MeaSnNHPh, presumably owing to exchange of the Me& moiety. A VT 119Sn NMR experiment was then carried out on a mixture of tri(26) Although we did not test this reagent for its range of reactivity, ita placement in thispositionof the order is inferredfrom the data obtained for all the other reagents discussed in this section.
0.157 0.193 0.169 0.155
10.0 12.3 10.8 9.9
91.4 82.6 80.3 73.4
10.0 12.4 10.8 9.7
-4.4 -16.8 -16.5 -23.4
10.0 10.1 9.54 11.2
70.1 66.5 61.8 64.0 63.2
10.0 10.2 12.4 11.8 12.7
(n-BusSn)2NPhb
65.0
ppm
relative relative amount amountc 6 W n amountc (mmol) (calcd) (ppm) (found) MesSnNHPha
2,3,5-trimethylphenol
3,4-dimethylphenol m-methylphenol
0.369 0.373 0.360 0.406
10.0 10.1 9.76 11.8
(n-Bu3Sn)2NPhb 2,4-dmethylthiophenol 2,5-dimethylthiophenol o-methylthiophenol m-methylthiophenol thiophenol
0.333 0.340 0.405 0.386 0.410
10.0 10.2 12.2 11.6 12.3
At room temperature. At 240 K. e These amounts are relative to the integral (taken to be 10.0 arbitraryunits) of the first compound in each series.
methylstannyl phenoxide and o-methylphenoxide, and on a mixture of trimethylstannyl acetate and propionate. In the former case, a relatively broad peak at 4.20 ppm was observed at 298 K which split into two peaks (-7.36 and -10.97 ppm) at 273 K. Lowering the temperature to 253 K further sharpened the peaks which now appeared at -16.63 and -20.00 ppm. No significant increase in resolution was observed upon further lowering the temperature. For the Me3SnOC(O)Me/Me3SnOC(O)Etmixture, a sharp peak was observed at -15.07 ppm at 298 K which became broad as the temperature was lowered. However, no peak separation was observed down to 238 K. Thus, trimethylstannanecarboxylic acids are more labile to exchangethan trimethylstannylaryl oxides. Since one of our purposes was to obtain stannylating reagents for the labile functional groups in coal, in which phenols and carboxylicacids dominate, a trimethylstannyl reagent appears to be inadequate owing to exchange phenomena. The tributylstannyl group is more bulky than trimethylstannyl, and therefore was believed to be less prone to exchange. Indeed, the NMR spectrum of a mixture of tributylstannyl phenoxide and o-methylphenoxide in pyridine displayed no evidence of exchange at room temperature. However, a mixture of tributylstannyl acetate and propionate did show exchange at room temperature. A VT NMR experiment revealed that this exchange can be essentially halted at lower temperatures and a stable, high resolution spectrum was obtained at 240K. In Figure 4 is a room temperature l19Sn NMR spectrum of a mixture of phenols and carboxylic acids derivatized with (n-Bu3Sn)zNPh wherein all the phenol peaks are sharp but the two carboxylic acids are broad. Figure 5 shows the same sample measured at 240 K, from which we see that the two carboxylic acid peaks are now sharp. The 119Sn NMR spectrum of a mixture of four different phenols and a similar mixture of five different thiophenols derivatized with (n-Bu3Sn)aNPh in pyridine at 240 K was examined. The integrations of the peaks gave quantitations of the components of the mixtures that matched reasonably well (avg error *1.7% in each case) with the original amounts added (Table 4).
New "9Sn NMR Tagging Reagents
Energy & Fuels, Vol. 8, No. 1, 1994 177
70.0
50.0 40.0 30.0 20.0 10.0 PPm Figure 4. Room temperature llQSnNMR spectrum of hydroxyl compounds derivatized with (n-BusSn)zNPh with peaks from left to right representing n-BusSnOH, 1,3,64rimethylphenol,2,6-dimethylphenol, m-cresol, phenol, 8-quinolinol, 2-naphthol, propanoic acid, and acetic acid.
90.0
80.0
30.0
20.0
60.0
10.0
0.0
-10.0
-20.0
-30.0
-40.0
PPm Figure 5. The 240 K 1Wn NMR spectrum of hydroxyl compounds derivatized with (n-Bu$%)zNPh with peaks from left to right representing n-BusSnOH, 8-quinolinol, 2,3,64rimethylphenol, 2,6-dimethylphenol, m-cresol, phenol, 2-naphthol, propanoic acid, and acetic acid.
Table 5. lisSa NMR Chemical Shifts of Phenols, Thiols, Thiophenols, Carboxylic Acids, and Alcohols Derivatized with Reagent (n-BuaSn)aNPhat 240 K compound 2-naphthol 3-ethylphenol o-cresol o-phenylphenol phenol p-cresol 3,5-dimethylphenol m-cresol 5-indanol 2,6-dimethylphenol 2-methoxylphenol 2,3,6-trimethylphenol 4-methylcatechol 8-quinolinol 2,4-dimethylthiophenol
P S n (ppm) -30.7 -28.7 -28.4 -27.5 -26.0 -25.7 -24.4 -24.4
compound thiophenol o-cresylthiol m-cresylthiol butanethiol isopropanethiol 2-hydroxyethanethiol acetic acid m-hydroxybenzoicacid propionic acid benzoic acid 4-methoxybenzoicacid ethanol 1-butanol benzyl alcohol
-22.4
-8.5 -2.8 0.1
4.9," 9.5" 9.7
6119Sn (ppm) 60.1 64.6 60.6 72.9 65.1 73.1: 46.8c -37.7 -40.6,' -29.1e -35.7 -41.7 -40.6 58.1 58.9 45.2
67.5
0 Insufficient data for assignment. * Tentatively assigned to Sn-S tin. Tentatively assigned to Sn-0 tin. d Tentativelyassigned to SnOzC tin. e Tentatively assigned to SnOC tin.
100.0
80.0
60.0
40.0 PPm
20.0
0.0
-30.0
-50.0
Figure 6. The 240 K l%n NMR spectrum a 2-h pyrolysis (420 "C)condensate of Illinois No. 6 derivatized with (n-Bu3Sn)zNPhwith the assignments of the major peaks from left to right representing n-BusNHPh, 2,4-dimethylphenol, m-cresol, phenol, and o-cresol.
Table 5 lists the 119Sn chemical shifts of a variety of phenols, thiophenols, carboxylic acids and alcohols measured at 240 K. As depicted in Figure 2, t h e 6119Snranges for tributylstannylated derivatives of these compound
classes do not overlap. This is clearly another advantage of the n-BuaSn moiety as a derivatizing group.
Analysis of Coal Materials. A t 240 K the l19SnNMR spectrum of a pyridine extract of Illinois No. 6 coal
178 Energy & Fuels, Vol. 8, No. 1, 1994
derivatized with (n-BusSn)20 revealed only a broad peak ranging from-15 to-36 ppm. Despite the lackof resolution of this spectrum (part of which may be due to the presence of paramagnetic species) it is possible to conclude from Figure 2 that the broad peak arises only from phenols. A preliminary result obtained on a 20-h pyrolysis (420 "C) condensate of Illinois No. 6 coal is shown in Figure 6. Four main peaks were tentatively identified as 2,4dimethylphenol, m-cresol and 3,5-dimethylphenol,phenol, and o-cresol. No significant amount of carboxylic acids were found to be present. Conclusions. The results of the present study demonstrate that trialkylstannylaniline derivatives such as MesSnNHPh and (n-Bu3Sn)zNPhreact cleanly under mild conditions to give derivatized phenols, carboxylic acids, alcohols, and thiols that can be characterized by l19Sn NMR spectroscopy. Derivative formation is quantitative in every case, even for hindered phenols such as 2,6- and 2,3,6-trimethylphenol. The chemical shift ranges for derivatized phenols, carboxylic acids, alcohols, and thiols
Ye and Verkade are quite separate (especially in the case of n-Bu3Sn derivatives), making it easy to identify the presence of these four classes of compounds. Even though the trimethylstannyl derivatives of phenols and carboxylic acids undergo exchange, the Me3Sn derivatives of a mixture of alcohols, thiophenols, and thiols showed good resolution. The reagent (n-Bu3Sn)nNPh seems to be more useful in the analysis of coal material. The application of this reagent in the analysis of amixture of phenols and carboxylic acids, and also of a coal condensate, gave promising preliminary results.
Acknowledgment. Ames Laboratory is operated for the U. S. Department of Energy by Iowa State University under Grant No. 2-7405-ENG-82. This work was supported, in part, by the Assistant Secretary for Fossil Energy through the Pittsburgh Energy Technology Center. Partial support through DOE Grant No. DE-FG 2288P C88923 is also acknowledged.
