High-resolution Tin-119 NMR of some solid organotin compounds

JOURNAL

OF MAGNETIC

High-Resolution

RESONANCE

73,389-398

(1987)

Tin-119 NMR of Some Solid Organotin Compounds

RICHARD A. KOMOROSKI,* RICHARD G. PARKER, AND ANTHONY M. MAZANY BF Goodrich Research and Development Center, Brecksville, Ohio 44141 AND

THOMAS A. EARLY General Electric Company, NMR Instruments, Fremont, California 94539 Received May 15, 1986; revised January 26, 1987 High-resolution “?Sn NMR spectra were obtained for a number of solid organotin compounds using cross polarization, magic-angle spinning, and dipolar decoupling. Chemical-shift anisotropies range from about 40 ppm or less for tetrahedral tetraphenyltin to about 1100 ppm for polymeric di(n-octyl)tin maleate. For chlorine-containing compounds, a centerband pattern of two, relatively broad peaks is observed at low field (33.56 MHz). This pattern is attributed in part to the effect of the quadrupole moment of the directly bonded chlorine nuclei on the Sn-CI dipolar coupling. Spectra at high field (111.9 MHz) support this interpretation. Information on the number of crystallographically nonequivalent tin atoms is obtained in several cases and is correlated with available X-my crystallographic and other spectroscopic data. The spectrum of triphenyltin chloride at 111.9 MHz is considerably more complex than at 33.56 MHz and is not consistent with the published X-ray structure. o 1987 Academic mess, hc. INTRODUCTION

High-resolution, solid-state NMR employing the combined techniques of cross polarization, dipolar decoupling, and magic-angle spinning (CPMAS) is being applied widely to the characterization of many types of compounds and polymers (I). Although the overwhelming majority of studies involves 13C, other nuclei such as “N, 3’P, 29Si, and “0 are being used. One nucleus that has been studied little is ‘19Sn, which has a spin of f and a natural abundance of 8.58%. Since the early report of Lippmaa and co-workers (2), three short reports have appeared very recently on the application of ’ 19Sn CPMAS NMR to organotin compounds (3-5). These studies found that ’ 19Sn CPMAS spectra could be obtained readily and could yield information on the solidstate structure. We report here some ‘19Sn results for a number of solid organotin compounds, focusing on the salient spectral characteristics and on the correlation of the ‘19Sn data with known solid-state structures. * Current address: Departments of Radiology and Pathology, University of Arkansas for Medical Sciences, 430 1 West Markham, Little Rock, Arkansas 72205. 389

0022-2364187 $3.00 CopyrightQ 1987 by Academic

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Tetraphenyltin (Aldrich Chemical Co.) tetracyclohexyltin (Fluka AG), tributyltin acetate (Pfaltz and Bauer), triphenyltin chloride (Morton Thiokol), diethyltin dichloride (Alfa Inorganics), and di(n-octyl)tin maleate polymer (Thermolite 8 13) (M & T Chemicals) were obtained commercially and used without further purification. High-resolution, 1’9Sn NMR spectra were obtained at 2.1 T (33.56 MHz) using cross polarization, dipolar decoupling, and magic-angle spinning. The instrument was a Bruker SXP spectrometer retrofitted with a B-DR90C unit. The Bruker 13C CPMAS probe was modified for operation in the range of 32-37 MHz. Rotors for the powdered samples were made of Dehin and were of the Andrew geometry with an o.d. of 9 mm. The magic angle was adjusted by minimization of the linewidth for a separate sample of dibutyltin oxide. Typical conditions were: 90 RF pulse, 4.5 ps; contact time, 2 ms; pulse repetition time, 4- 15 s; MAS rate, 2.3-4.7 kHz; spectral width, 50 kHz in 4096 points, data acquisition, 30-50 ms. Chemical shifts were referenced to the instrument frequency which was set relative to a solution of tetravinyltin (20 ~01% in CCL,), taken as - 157.4 ppm from tetramethyltin (6). Tin-l 19 spectra at 7.05 T (111.9 MHz) were acquired for diethyltin dichloride and triphenyltin chloride on a General Electric GN-300WB spectrometer with Chemagnetics CPMAS accessory, Rotors were of the double-bearing design. Conditions were comparable to those for the low-field spectra. RESULTS

AND

DISCUSSION

In Table 1 are the r19Sn isotropic chemical shifts and shift anisotropies of the solid compounds examined. Where available, previous solution results are also listed. The results in Table 1 represent a broad range of environments experienced by the tin nucleus. The spectrum of tetracyclohexyltin is a single, narrow line with a width of about 10 Hz (spectrum not shown). Because it gives a strong signal, it proved to be good for obtaining the Hartmann-Hahn match (I). The lack of MAS sidebands indicates a relatively small chemical-shift anisotropy, as expected for this symmetric environment. We obtained an isotropic chemical shift of 93 ppm, in good agreement with that obtained by Lippmaa and co-workers (2). The spin relaxation behavior of tin in tetracyclohexyl tin is favorable for CPMAS studies. Approximate measurements indicate a cross polarization time of several hundred microseconds; the maximum intensity is obtained at a contact time of about 1.6 ms. Also, the proton T1 is not long since maximum intensity at constant contact time is obtained for pulse delays of 2 s or greater. The above conditions appear to be applicable to all the compounds reported here. However, we cannot generalize to the majority of tin compounds. Limited experiments on organotin compounds not reported here suggest the possibility of long proton Ti values in some cases. Tetraphenyltin also yielded a single, narrow line by CPMAS. The spectrum without MAS was a relatively featureless line about 40 ppm in width. The isotropic chemical shift of - 117 ppm is significantly different than the - 137 ppm reported for a 30% solution in C2H3C13 at 100°C (6). We do not know the origin of this difference. The crystal structure of tetraphenyltin shows that the molecule is tetrahedral, and there is

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TABLE 1 “?Sn Chemical Shifts and Chemical-Shift Anisotropies of Some Solid Organotin Compounds’ Compound

6(solution)

@~clo-C6HMn

(C6Hs)&’ (n-Bu)rSnO

(n-Bu),SnOAc (C&),SnCl Et2SnC12

-137*

-55.5-106.9g -48d 126’

[-Sn(n-CsH17)r-OzCCH=CHC0r-]l,

Go -93 (-92’) -117 -173

-48, -54 (I:1 “) -19, -38 (1:3.6h) 72, 95 (l:l.5h) -295

Uh (talc,’

-173

68

Aa or oqqb
a In parts per million from MeJGn, at 33.56 MHz. bAa = IJ,, - q3, estimated from slow-spinning experiments. ’ From Ref. (2). *From Ref. (6). ‘From “Comprehensive Organometallic Chemistry” (G. Wilkinson, Ed.), p. 528, Pergamon Press, Oxford, 1982. ‘Calculated Q,, from f (ui, + g2r + urr). s From Ref. (13). The chemical shift of this compound varies considerably depending on solvent and concentration. h Relative peak intensities.

only one environment for tin in the crystal (7). Perhaps phenyl-ring rotation in solution yields a significantly different average electronic environment for tin relative to the rigid case. Figure 1 shows the ‘19Sn CPMAS spectrum of dibutyltin oxide at two MAS speeds. This result is undoubtedly more typical for tin compounds than those for the symmetric molecules just discussed. This contention is borne out by previously published work (3-5). The CPMAS spectrum is a pattern of sidebands covering about 800 ppm, with the isotropic shift occurring at - 173 ppm. Figure 2 shows the full CSA powder pattern for the spectrum taken without MAS and with principal values indicated. One-third the sum of the principal values also yields a gim of - 173 ppm. Dibutyltin oxide was the compound used for adjustment of the magic angle at low field because it displayed narrow MAS sidebands with linewidths very sensitive to the angle setting. The minimum linewidth obtained for this compound was about 60 Hz. We did not have the preferable alternative method of using the ‘Li linewidth (of a lithium salt added to the analytical sample) readily available to us (8). Hence the acquisition of spectra with the narrowest possible lines was difficult at low field. The single, narrow line observed for the isotropic chemical shift of dibutyltin oxide suggests a single tin environment for this solid, which is consistent with what is known about the solid-state structure (9). Miissbauer spectra (10) suggest that for the dialkyltin oxides all the tin atoms have coordination number five with the presence of fourmembered distannoxane rings. However, infrared spectra have been interpreted in

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FIG. 1. Tin- I 19 CPMAS spectra of dibutyltin oxide at MAS speeds of (A) 4.7 kHz and (B) 2.3 kHz. The centerband artifact is marked by “X”.

FIG. 2. Solid-state powder pattern of dibutyltin million from tetramethyltin.

oxide. The principal values are indicated in parts per

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terms of a linear polymer of alternating tin and oxygen atoms, with weak interactions between the chains (II). Only one environment for tin is present in either case. An alternate structure (12) containing both five- and four-coordinate tin atoms can be eliminated on the basis of the NMR results. Figure 3 shows the ‘19Sn CPMAS spectrum of tri(n-butyl)tin acetate. In addition to the spectrum of MAS sidebands, it is noted that the isotropic pattern consists of two lines of equal intensity, with chemical shifts of -48 and -54 ppm. This indicates that the tin atoms in the crystal occupy at least two nonequivalent environments. The r19Sn chemical shift of this compound in solution occurs over a wide range (Table l), depending on concentration and solvent (13). The unassociated, four-coordinate species resonates at the low-field end of this range. The resonance moves upfield in solvents of increasing coordinating capability, consistent with a change from four-coordinate to five-coordinate tin (13). Our CPMAS result suggests a highly coordinated environment for tin in the solid. Infrared spectroscopic results for tri(n-butyl)tin acetate have been interpreted in terms of a low-molecular-weight, associated species, possibly a cyclic trimer (14). To our knowledge, the crystal structure of tri(n-butyl)tin acetate has not been reported. The crystal structures of some organotin acetates have been reported (15-18). They vary considerably depending on the organic functionally, but all have a single environment for tin in the crystal. Another compound examined was polymeric di(n-octyl)tin maleate. This compound had an isotropic chemical shift of -295 ppm, and the relatively large shift anisotropy of about 1100 ppm. Only one environment was seen for tin in this compound. The solid-state ‘19Sn shielding tensor of diethyltin dichloride without MAS is approximately axially symmetric with principal values given in Table 1. The value of gim calculated from these principal values is about 68 ppm. This (along with the MAS result below) is significantly different from the value of 126 ppm reported in solution (Table 1). We attribute this to the effects of molecular association which are maximized Oiso t

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Tin- 119 CPMAS spectrum of tri(n-butyl)tin

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in the crystal. The ’ i9Sn chemical shift of organotin halides are highly concentrationdependent in solution (6), a fact that has been ascribed to the formation of associated structures at higher concentrations (6). Figure 4 shows the CPMAS spectrum of diethyltin dichloride at 33.56 MHz. The isotropic pattern for this compound consists of two, relatively broad resonances (at 72 and 95 ppm) of intensity ratio 1: 1.5. The residual linewidths of 300-400 Hz could not be reduced by careful adjustment of the magic angle on the analytical sample. A similar pattern, but with a different intensity ratio, was observed for triphenyltin chloride (but with no MAS sidebands) (Table l), and for dimethyltin dichloride and dibenzyltin dichloride. The solid-state, isotropic chemical shifts for the latter two compounds are not reported here. These compounds gave very weak spectra under the conditions used here, with the typical MAS sideband pattern barely visible. We attribute this to long ‘H T, values. These compounds were not pursued further. The crystal structure of diethyltin dichloride has been determined (19). Only one environment exists for tin in this structure. In addition, the crystal structure of dimethyltin dichloride (20, 21) shows only a single environment for tin. X-ray crystallographic results (22) reported for triphenyltin chloride are consistent with two equally populated environments for tin (see below). These results, taken in conjunction with the NMR results reported here, lead us to conclude that the two-peak pattern in the ’ 19Snspectra of the chlorotin compounds arises from a source other than the crystalline environment. We consider the most likely source to be the effect of the quadrupolar “T~‘C~ nuclei on the Sn-Cl dipolar coupling (23,24). When a spin-$ nucleus is dipolarcoupled to a quadrupolar nucleus, MAS may not average the resonance for the spin1 nucleus into a single, narrow line (23, 24). Instead, complicated patterns are seen which arise from residual dipolar coupling between the spin-f and quadrupolar nuclei. The magnitude of the effect depends on the ratio of the quadrupole coupling constant to the Zeeman frequency of the quadrupolar nucleus and is larger at lower magnetic fields. Lippmaa and co-workers (2) examined di(n-butyl)tin dichloride by i19Sn CPMAS NMR. Unlike the results for the chlorotin compounds we examined, a very broad line (200 ppm total width) was observed, presumably without MAS sidebands. They

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4. Tin-l 19 CPMAS spectrum of diethyltin dichloride. The centerband artifact is marked by “x”.

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cite several possible causes for the broadening, one of which is the effect of the quadrupolar chlorine nuclei mentioned above. This effect could be the cause of the line broadening they observe given the very low field (0.94 T) used in their experiments. Other possible explanations for the two-peak behavior do not fit the experimental data. A residual homonuclear “9Sn dipolar coupling is unlikely, based on the internuclear distances involved and the low natural abundance of ii9Sn. The same is also true of a 19Sn-“’ Sn heteronuclear dipolar coupling. MAS at several kilohertz should easily remove such interactions. The observed peak separation is much too large for an isotope effect of 35,37C1on the i19Sn chemical shift. Moreover, the observed peak intensities do not agree with the known chlorine isotope ratio. If the two-peak behavior arose from a Sn-Cl scalar coupling, four equally spaced resonances should be seen. Simulated spectra have been obtained previously for the case where the quadrupolar nucleus has a spin of g as for 35,37Cl (23, 24). The detailed shape of the spectrum depends on several factors, including the magnitudes of the quadrupole coupling and Zeeman frequencies, the symmetry of the quadrupole coupling, the number of bonded quadrupolar isotopes, and their spin. The peak separations and linewidths in the experimental spectra of the chlorotin compounds at 33.56 MHz are of the correct magnitude, being one to two times the dipolar coupling of 320 Hz (100 ppm at 33.56 MHz) for a 119Sn-35C1pair separated by 2.40 A. To confirm the previously mentioned source of the two-peak behavior, we obtained i19Sn MAS spectra at high field (7.05 T). Figure 5 shows the 111.9 MHz spectrum of

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FIG. 5. The “‘Sn CPMAS spectrum of diethyltin dichloride at 1 I 1.9 MHz. The zero position of the scale is arbitrary.

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diethyltin dichloride. Unlike at 33.56 MHz where the two-peak behavior is clearly resolved, the pattern has collapsed at 111.9 MHz, yielding single, broad peaks for most MAS sidebands, as expected. Several sidebands show a barely resolved, twopeak structure. We conclude that the low-field spectrum is dominated by the quadrupole effect. The full ‘19Sn CPMAS spectrum of triphenyltin chloride at 111.9 MHz consists of a center pattern and first- and second-order spinning sidebands spanning about 100 ppm. Surprisingly, the isotropic pattern has not collapsed as for diethyltin dichloride, but consists of seven relatively narrow lines spanning about 10 ppm. Figure 6 shows the spectral pattern of triphenyltin chloride. It was generated by adding, in the computer, the sideband patterns to that of the centerband, which was identified using Dixon’s TOSS sequence for sideband suppression (25). Hence all of the signal intensity is accounted for in Fig. 6. It appears that the minimization of the effect of the quadrupolar chlorine nucleus as well as the chemical shift spread with increasing field has allowed this underlying structure to be observed. Deconvolution of the experimental pattern of Fig. 6 into Gaussian lines shows that each of the three upper and three lower peaks in the spectrum is of approximately unit intensity and that the center peak is of double intensity. The two humps appearing to the high-field sides of the downfield peaks were included with their respective primary peaks. The intensities of these humps suggest they arise from a 35C1-37C1isotope effect. We are not able to explain the pattern shown in Fig. 6. The X-ray results reported for this compound show monoclinic crystals with a space group PZ,/a with eight molecules in the unit cell (22). The Sn coordination is tetrahedral. There are two molecules in the asymmetric unit, and hence we expect two peaks of equal intensity in the ‘19Sn spectrum. The spectrum of Fig. 6 suggests eight unique environments,

FIG. 6. The ‘19Sn CPMAS spectrum of triphenyltin chloride at 111.9 MHz. The spectrum is the computergenerated sum of the centerband and the major sideband patterns. The zero position of the scale is arbitrary.

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with two coincidentally appearing at the same chemical shift. One possible explanation is that we are observing the effect of “9Sn-35 Cl scalar coupling; however, the spacings between the various lines are not equal, leading us to reject any interpretation based on scalar coupling. The sample gave a single ’ 19Sn NMR peak in CDC13, indicating that impurities in the compound are not giving rise to the complicated pattern. Because we did not perform any X-ray studies on the crystals used for the NMR work, we cannot be sure that the commercially available crystals we examined have the same crystal structure as that used in the previous X-ray study. Careful *19SnCPMAS NMR, at fields higher than 7.05 T, on Ph3SnC1 crystals of known structure is necessary to shed further light on this problem. CONCLUSIONS

Tin-l 19 CPMAS NMR holds promise as a sensitive technique for studying the structure of solid organotin compounds. It should be particularly valuable for molecular association effects, since discrete, known structures in the solid state can be characterized by high-resolution ‘19Sn NMR and the results compared to those in the solution state. The variety of structures available to solid-state tin compounds, including monomeric, cyclic, and polymeric species, as well as noncrystalline and mixed-coordination species, should provide a fertile area for ‘19Sn CPMAS NMR. In addition, the chemical-shift anisotropy should shed additional light on the sometimes complicated and confusing behavior of isotropic lL9Sn chemical shifts (6). The limitations we have experienced at low field can be alleviated on more modem solid-state NMR equipment. Higher fields lessen the effect of the quadrupolar chlorine nuclei, although the chemical-shift anisotropy pattern is then spread over a wider frequency range, yielding more MAS sidebands. Such sidebands can be eliminated using techniques developed for this purpose (25). Also, because of the large shift anisotropies seen for the tin nucleus in many environments, the angle stability and adjustability of double-bearing rotor geometries are necessary to make ’ 19Sn CPMAS NMR more straightforward. REFERENCES 1. 2. 3. 4. 5. 6.

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A. FIFE, “Solid State NMR for Chemists,” C.F.C. Press, Guelph, 1983. T. LIPPMAA, M. A. ALLA, T. J. PEHK, AND G. ENGELHARDT, J. Am. Chem. Sot. 100, 1929 (1978). K. HARRIS, K. J. PACKER, AND P. REAMS, J. Mu@. Reson. 61,564 (1985). K. HARRIS, K. J. PACKER, AND P. REAMS, Chem. Phys. Lett. 115, 16 (1985). K. HARRIS, T. N. MITCHELL, AND G. J. NESBI~, Magn. Reson. Chem. 23, 1080 (1985). K. HARRIS AND B. E. MANN, “NMR and the Periodic Table,” p. 35 1, Academic Press, London, 1978. K. BEL’SKII, A. A. SIMONENKO, V. 0. REIKHFEL’D, AND I. E. SARATOV, J. Organomet. Chem. 244, 125 (1983). S. FRYE AND G. E. MACIEL, J. Magn. Reson. 48, 125 (1982). C. POLLER, “The Chemistry of Organotin Compounds,” p. 2 13, Academic Press, New York, 1970. I. GOL’DANSKII, E. F. MAKAROV, R. A. STWAN, V. A. TRUKHTANOV, AND V. V. KHRAPOV, Dokl.

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