J. Phys. Chem. 1994, 98, 4919-4922
4919
NMR Study of the Solution-State Dynamics and Solid-state Structure of Tri-n-butyltin Fluoride Y. W. Kim, A. Labouriau, C. M. Taylor, William L. Earl,' and L. C. Werbelow' CST-1. IUS G740, Las Alamos National Laboratory, Los Alamos, New Mexico 87545 Received: September 13, 1993; I n Final Form: February 8, 1994'
Dynamics and structure of tri-n-butyltin fluoride in n-hexane solutions were probed using (tin-1 19) nuclear magnetic resonance spin relaxation methodologies. Significant relaxation-induced polarization transfer effects were observed and exploited. The experimental observations indicate that the tri-n-butyl fluoride exists in a polymeric form in solution. For a 0.10% (w/w) solution at 25 OC,NMR reveals significant orientational/ exchange relaxation on both the microsecond and nanosecond time scales. Solution-state and solid-state parameters are compared and contrasted.
Introduction Trialkyltin fluorides, characterized by several unusual physicochemical properties,l-3 provide interesting systems for spectroscopic investigation. Solid-state NMR,4-6 and X-ray9.10 data all indicate that trialkyltin fluorides exist as polymeric species in the solid state where the tin atoms are pentacoordinate with each tin bonded to two divalent fluorines. Whether or not the two fluorine nuclei are equidistant from the tin atom is somewhatunclear sincetheoreticalcalculations indicate that there are two different tin-fluorine positions that are energy minima7J whereas solid-state NMR studies suggest equivalent Sn-F bonds.4 The first evidence of bridging fluorine atoms in solutions of trialkyltin fluorides with nonpolar solvents was demonstrated by Dunn and 0ldfield.Il Subsequently, van der Kelen and coworkers12 presented evidence that the solution process is characterized by a slow permeation of solvent into the solid polymeric matrix. To date, lI9Sn NMR has been used extensively for the study of trialkyltin compounds and their complexes in coordinating and noncoordinating solvents by only one group of ~ 0 r k e r s . l ~ These previous studies dealt with trialkyltin chloridesand bromides and did not include any data on the more strongly associating trialkyltin fluorides. Furthermore, although it is well documented that nuclear spin relaxation is a powerful probe of site-specific microdynamical behavior, the NMR relaxation characteristics of these novel systems have never been explored. In this work, the application of relaxation-induced polarization and coherence transfer,14 a robust technique which has been applied successfully in the study of other macromolecular system~,Is-~~ is exploited to quantify the microdynamics of this associative polymeric system.
The tri-n-butyltin fluoride, obtained from Pfaltz and Bauer (Waterbury, CT), was used without further purification. A viscous, homogeneous solution was prepared by mixing tri-nbutyltin fluoride in n-hexane followed by subsequent heating at 40 OC for 48 h. A 0.10% (w/w) solution of tri-n-butyltin fluoride yielded a solution which was viscous, indicating formation of the associative polymer, yet resulted in NMR lines which were sufficiently narrow to observe multiplet structure. Higher concentrations of the trialkyltin fluoride resulted in NMR signals which were too broad to be useful. Over the course of this investigation, it was discovered that the dynamics of tri-n-butyltin fluoride in n-hexane changed radically over a narrow temperature range. At high temperatures (above 70 "C), autoassociation was minimal, with small oligomers being the most probable species. At low temperatures (below 0 "C), the solubility was insufficient to maintain the polymer in solution and a suspension rather than solution resulted. In this instance, broad, solid-like powder patterns were observed. Theory Transverse Relaxation. In this study, it is noted that the effective spin-spin relaxation rate is large compared with the effectivespin-lattice relaxation rate. This simplifiesthe treatment of the decay of the transverse magnetization. If only C S h n(tin chemical shielding anisotropy) and Dsn-p (tin-fluorine dipolar) adiabatic (zero-frequency spectral density) contributions are retained, a completedescriptionof the relaxation features of I-spin single quantum coherence in the AXX' (AX2) spin system is given by
where
Experimental Section All Il9Sn NMR spectra were recorded on a Varian Unity 400 spectrometer operating at 149.16 MHz at ambient temperature. Spin-lattice relaxation times were measured by the inversionrecovery method using a relaxation delay of 1.O s and six variable delays. For each delay, 122 260 scans were accumulated. Effective TIvalues were calculated using a three-parameter fitting procedure for each peak. The solid-state, magic angle spinning (MAS) lI9Sn spectrum was recorded at ambient temperature using a 90° pulse of 6 ps with a spinning rate of 7.63 kHz. The MAS spectrum was simulated using ANTIOPE,20 a computer program which runs on IBM-PC compatible computers. Solution and solid spectra were referenced with respect to external tetramethyltin. Abstract published in Advance ACS Abstracts. April 1, 1994.
0022-3654/94/2098-4919$04.50/0
Spin I refers to 1%n, and spins S and S' tag the two 19F nuclei. The various spectral density terms appearing in eq 1 aredefined by the relationships 0 1994 American Chemical Society
4920 The Journal of Physical Chemistry, Vol. 98, No. 18, 1994
-
PXCSA - P X C S A (Sn-F)Sn(O)= ( l / (Sn-F)Sn
lo) [YSnYFh/rSn-,3 -
uSn-F/3
1 (%nAUSn)
r2(DSn-FCSASn)
No attempt is made to isolate or otherwise distinguish the direct (YSnYFh /rsnq3) and indirect ( & , - F / 3 ) dipolar contributions. In the context of a simple t w o - s t a t e m ~ d e l ,theautocorrelation ~~-~~ and cross-correlation terms, ~ ( 7 and ) ~ ( q q ' )can be defined as
+
I,(?)) = s2,,I, (1 - s2,,)7,
-
s',,I,
where the correlation time, I,, is associated with slow, relatively isotropic motions. The term I, identifies with the correlation time, characterizing other motions that effect a relatively rapid partial averaging of the appropriate interaction, S,, = /2. The angle defines the angle between the principal axes of the (cross-) correlating or interfering interactions, 7 and q'. At this point, only adiabatic terms are considered, and the contribution of I, can be ignored. Although it is questionable whether the two Sn-F dipolar couplings are identical on the time scale of I,, the functional form of eq l b is preserved if it is understood that all spectral densities are the average over pairs of terms such as
The two potentially inequivalent fluorines associated with any given tin atom are denoted F1 and FZ. In these expressions, it is assumed that the Sn-F internuclear vector and the principal axis of the anisotropic indirect dipolar coupling (AJ) are collinear. Furthermore, if the assumed local symmetry about the "9Sn nucleus has a three-fold rotational axis, all T ~ appearing S in these definitions can be assumed equal. This greatly simplifies subsequent analysis. Although the image provided by eq 1 is highly illustrative, in the limit of resolved multiplet structure, the alternative operator description,
+ 2m(S, + Sj)+ ( 2 d - l)SS,']
-
(
- -
DXD / T2)m = (4/3) [J&-F(O)- (-l) mKFSn-F(o)I +
(8/3) [$:*(o)
40
30
20
i0
0
prrrpsrrprrrinii--'?---T -20 -30 -40 -50
-10
-80
p.p.m. Figure 1. 1~9SnNMRspectrumofa0.1% (w/w) solutionof tri-n-butyltin fluoride in n-hexane. This spectrum was obtained at 25 O C in an applied field of 9.3 T. Beneath the experimental spectrum, the three individual members of the triplet, obtained by a least-squares fit, are indicated.
In the absence of additional broadening mechanisms, ( 1 / ~ 7 ' 2 ) - 1 , (l/rTz)+l, and (l/?rT~)ocan be measured from the half-width at half-height of the three components of the l19Sn triplet. (It is important to recognize that 119Sn is characterized by a negative gyromagnetic ratio. Hence, for a positively signed isotropic J coupling, the low field component of the Il9Sn triplet is identified with the transition, I@)lacu) la)laa).) From eq 4, it follows that
-
DXCSA
where
and
(3)
proves more useful. In eq 3, single quantum coherence observed within theZ-manifold between states 1p)laa) 1a)lacu)is denoted as the coherence between states la)lj3@) lB)Ipj3) is denoted as (Z+)m=-l, and the summed coherence between states IMlaB) + I ~ a ) I / d 2 I ~ ) @ B ) + I B C ~ ) I INIaB) / ~ ~ , -Pa)}/ 4 2 Ia){la/3)- 10cu)]/d2 is denoted as (1,)m-O. The time evolution of each of the operators, (I+)m, is characterized by a unique relaxation constant given as follows:
-
-r?---n-rr--r-r-
Longitudinal Relaxation. In the limit where spin Z is relaxed dominantly by shielding anisotropy, the relaxation features can be described by the following set of equations:
+
KDXCSA (Sn-F)Sn
= (1/4)Z+[l
Kim et al.
- 2mK(Sn-F)Sn(o)l (4)
The most interesting feature of these expressions is the prediction that each of the three components decays as a single exponential: (1 / Tl)m = 4FSA - 8mKDXCSA.
Results and Discussion A typical I19Sn spectrum is shown in Figure 1. Superimposed upon this spectrum are the three, best-fit Lorentzian composites. Although thesplittings and widths of thesecompositesaresimilar, little error is introduced by the Lorentzian appro~imation.2~ The integrated intensities of these three, computer-fit peaks deviate by less than 2% from a 1:2:1 ratio. Furthermore, tin-1 19 clearly exhibits equal scalar couplings to each neighboring fluorine-19
The Journal of Physical Chemistry, Vol. 98, No. 18, 1994 4921
Tri-n-Butyltin Fluoride in n-Hexane
TABLE 1: ll%n Nuclear Spin Relaxation Parameters of a 0.10% (w/w)Solution of Tri-rrbutyltin Fluoride in rrHexane at 25 OC and B,, = 9.3 T ( 1 / Tdm-o
(~/Tz)~-+I -(16/3)qi2&(0)
p.p.m. Figure 2. lL9SnMAS NMR spectrum (spinning rate 7.63 kHz)of trin-butyltin fluoride obtained at 149.16 MHz and ambient temperature.
* 0.41 ms-l
(4/3)@2EF(0) -0.02 0.20 ms-1 4.52 0.18 ms-I (l/Tl),,,=-I 12 2 S-I 3.11 f 0.06 ms-I (l/Tl),,,=o 7 & 1 s-' 4.47 f 0.20 ms-l. (l/TI),,,=+l 3 1 s-l 1.30 0.25 mS-' -8q2?&,(~~~) 4.5 0.5 8-l 5.72
(1/Tdm=-1
*
*
are identical, each member of the triplet splits into a 1:6:15:20: 156:1septet. It is anticipated that each of these components will be differentially broadened. However, because nooperators with proton character are observable, H-Sn dipolar, Sn shielding anisotropy interference effects are not exposed. Most importantly, H-Sn dipolar interactions contribute very little to the overall line widths. Utilization of eq 5 leads directly to the relationship [(I/TJ+ - (l/T*)-l/[(l/T2)+ + 2(1/7,2)0 + (l/T*)-l =
35/(1 + 9t2) (7) where 5 =: ( w s ~ A u s ~ ) / [ Y s ~ Y1/&n-F3) F ~ ( - h J s p F / 3 ] . In Table 1, it is shown that this ratio equal 0.14 f 0.03. Hence, f = 0.047 f 0.01. As noted earlier, the solid-state study yielded Au = -327 f 15 ppm or ~WS,,AQ~ = 48.5 f 2.0 kHz. Ifthe solid-state and solution-state values for Au are equal, then [YsnYFh ( l/rsWF3) - & , - F / 3 ] -2.3 f 0.5 kHz. This value must be considered as a lower limit because complete correlation and lack of homogeneous broadening are assumed. From the solid-state studies, the value of this dipolar coupling is -3.2 kHz. There is good evidence435 that the sign of the one bond scalar coupling constant, 'Js,,F, is positive. Since the high-field component is narrower than the low-field component, the cross-correlation term is negative. The cross term scales as (~s,,)~Aa, which suggests that Au is negative. Based on experiment, the effective longitudinal relaxation rates, measured in s-l, can be fit to the expression
-
-200
-100
0
100
200
p.p.m. Figure3. Computer simulation(representedas dashed linesslightlyoffset) of the high-frequency spinningsideband manifold of the MAS spectrum shown in Figure 2. The center band is indicated by the arrow.
nucleus. Hence, as an initial approximation, the relaxation characteristics of this system will be modeled as an AX2 spin system. Two features of the spectrum shown in Figure 1 attract immediate attention. First, the isotropic Sn-F scalar coupling, 1350 f 20 Hz, is large. Second, dramatic differential line broadening indicates significant temporal interference between chemical shielding and dipolar interactions.14 For the fluoro-tin spin systems considered in this work, the chemical shielding anisotropy (CSA) dominates the nuclear spin relaxation behavior of Il9Sn and, toa lesser extent, 19F. However, Sn-Fdirect (Dsn-p) and anisotropic, indirect (AJsn-~) dipolar interactions play subtle yet important roles in the nuclear spin relaxation process. Figure 2 presents the 119Sn MAS spectrum of tri-n-butyltin fluoride. An iterative fit20 of the high-frequency, spinningsideband manifold is shown in Figure 3. From this fit, using arguments analogous to those made by other workers,4v5 it is possible to determine values for the shielding anisotropy, Au = -327 f 15 ppm and [YSnYFh( l/rsn-:)
( s / T ~ == ) ~(7.5 f 1.2)
+ m(4.5 f 0.5) +
(3m2- 2)(0.3 f 0.7) (8) The differentials, linear in m, are remarkably large and imply that f = 0.10 f 0.02. For a shielding anisotropy equal to -327 ppm, [ Y S ~ Y (Fl/rsn-F3) ~ - A&,-F/3]- 4 . 8 kHz. It is important to recognize that the foregoing argument presumes single exponential relaxation of each spectral component. However, in contrast to the influence on line width differentials, the effect of proton-tin dipolar tin shielding anisotropy interference cannot be dismissed so simply. The most important influence of neighboring methylene proton-tin dipolar interactions is summarized by the following couplings:
where
- AJsnq/3] = -3.2 f 0.2 kHz
For the solution studies, the measured TI and T2 values and the derived spectral densities are entered in Table 1 . Although proton-tin dipolar couplings are similar in magnitude to the tin-fluoride direct dipolar couplings, Table 1 shows that this interaction assumes a relatively insignificant role in the analysis of line width or "T2"data. If the three methylene groups
D
= J:n-F(WSn) + JSn-F(WH) and the number of nearby protons (characterized by momentum operators, M ) is n = 6. These additional couplings will render the decay of each m-component multiexponential, which will result in exaggerated ASnH
4922
Kim et al.
The Journal of Physical Chemistry, Vol. 98, No. 18, 1994
relaxation rate differentials. Hence, the experimental, singleexponential TI values may be expected to overestimate (. However, themean value (-3.5 kHz) deduced from Tzdata (-2.3 kHz) and TI data (-4.8 kHz) is in excellent agreement with the value deduced from solid-state measurements (-3.2 kHz). This finding suggests that the electronic environments seen by the tin atom in both the solid-state and (dilute) solution-state structures of tri-n-butyltin fluoride are similar. The quadratic correction (eq 8), which can be explained by incorporating dipole-dipole interference effects, appears negligible. Another notable feature of this study is the efficient decay of athermal longitudinal magnetization. Since ( l / r l ) = ( 2 / 1 ~ ) ( ~ ~ ~ / -wS z~) /)( l~ 4-, ~7,2002) ~ ( ~ if A m c is on the order of 50 kHz, then TNO( 1 - S z ) / (1 + T ? W O ~ ) = 1/2. Thus, in this study, it appears that S2 is relatively small and T,UO is on the order of unity. Additional dynamic information from the relaxation data can be obtained from the TI/ T2 ratio. Assuming that the dissipation of athermal one-spin order is dominated by tin shielding anisotropy, a two-state model (eq 2) predicts that the ratio T I / T ~ can be written as
+ (1 + w2rm2)-') + (1 - S2)7,( 1 + w027,2)-']/ [2S*7,( 1 + w;7m2)-1 + 2( 1 + S2)7,( 1 + w&,2)-1] = (2/3)S2(7,J7,)(l + wt7,2)(1 - S2)-'
T1/T2= [S27,(4/3
Extending the argument of the preceding paragraph, it follows that T1/T2 = S ~ T ~ =W650 O f 100 or S ~ T ,= 700 f 100 ns. Hence, it follows that T , > 1 ps.
Conclusions In all likelihood, the molecular dynamics of this associative system are complicated and characterized by motions on many different time scales.25 Without performing tedious relaxation dispersion studies,26 this assertion is somewhat speculative. However, the high degree of temporal correlation between competing relaxation pathways observed in this study is unambiguous. Any dynamical model must predict the presence of correlated modulations of the shielding and dipolar couplings. This realization strongly suggests that these polymeric species exist with a well-defined solution-state structure Nuclear spin relaxation is induced via thermal reorientation of this structure.
Of course, this structure is constructed as an average over time scales short compared to the nanosecond scale. In addition, the similar magnitudes of the dipolar and anisotropic shielding interactions in both the solution-state and solid-state studies indicate similar structures of the associated tri-n-butyltin fluorides in both phases.
Acknowledgment. The authors thank Prof. John Waugh for providing a copy of thecomputer program, ANTIOPE, and Prof. Rod Wasylishen for his independent analysis of the solid-state measurements. References and Notes (1) Davies, R., Smith, P. J. In ComprehensiueOrganometallic Chemistry; Wilkinson, G., Stone, F. G. A., Abel, E. W., Eds.; Pergamon: Oxford, 1982; p 519ff. (2) Dandge, D. K.; Taylor, C.; Heller, J. P.; Wilson, K. V.; Brumley, N. J. Macromol. Sci. Chem. 1989, A26, 1451. (3) Dandge, D. K.; Taylor, C.; Heller, J. P. J . Polymer Sci. A 1989, 27, 3. (4) Bai, H.; Harris, R. K.; Reuter, H. J . Organomet. Chem. 1991, 408, 167. ( 5 ) Bai, H.; Harris, R. K. J . Magn. Reson. 1992, 96, 24. (6) Harris, R. K.; Reams, P.; Packer, K. J. J. Mol. Struct. 1986, 141, 13. (7) Yasuda, K.; Okawara, R. J . Organomet. Chem. 1965, 3, 76. (8) Okawara, R.; Wada, M. Adu. Organomef. Chem. 1967, 5, 137. (9) Murdock, A.; Platt, R. H. J . Chem. SOC.(A). 1971, 1191. (IO) Clark, H. C.; OBrian, R. J.; Trotter, J. J . Chem. SOC.1964, 2332. (11) Dunn, P.; Oldfield, D. J . Macromol. Sci. Chem. 1970, A4, 1157. (12) van der Kelen, G. P.; van den Berghe, E. V.; Verdonck, L. In Organometallic Compounds Vol. 1 ; Sawyer, A. K., Ed.; Dekker: New York, 1971; p 12lff. (13) Nadvornik, M.; Holecek, J.; Handlir, K.; Lycka, A. J . Organomet. Chem. 1984, 275, 43. Holecek, J.; Cerny, V.; Handlir, K.; Lycka, A.; Nadvornik, M.Polyhedron 1987,6, 1037. (14) Werbelow,L. G. In NuclearMagneticProbesofMolecularDynamics; Tycko, R., Ed.; Plenum; New York, in press. (15) Fuson, M.M.; Miller, J. B. Macromolecules 1993, 26, 3218. (16) Chung, J.; Oldfield, E.; Thevand, A.; Werbelow, L. J . Magn. Reson. 1992, 100,69. (17) Oldfield, E.;Chung, J.; Le, Bowers, T.; Patterson, J. Macromolecules 1992, 25, 3027. (18) Daragan, V. A,; Mayo, K. H. J . Am. Chem. SOC.1992,114,4326. (19) Fuson, M. M.; Anderson, D. J.; Liu, F.; Grant, D. M. Macromolecules 1991, 24, 2594. (20) de Bouregas, F. S.; Waugh, J. S . J . Magn. Reson. 1992, 96, 280. Drakenberg, T.; (21) WennerstrBm, H.; Lindman, B.; SBderman, 0.; Rosenholm, J. B. J . Am. Chem. SOC.1979, 101, 6860. (22) Lipari, G.; Szabo, A. J. Am. Chem. SOC.1982, 104,4546. (23) Elbayed, K.; Canet, D. Mol. Phys. 1989, 68, 1033. (24) Geschke, D. Zeit. Phys. 1968, 212, 169. (25) Cohen-Addad, J. P. Prog. NMR Spectrosc. 1993, 25, 1. (26) Koenig, S. H.; Brown, R. D. Prog. NMR Spectrosc. 1990, 22,487.
