Kinetics and Thermodynamics of the Monomer− Dimer Equilibria of

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Organometallics 2010, 29, 6384–6392 DOI: 10.1021/om100752j

Kinetics and Thermodynamics of the Monomer-Dimer Equilibria of Dialkoxydibutylstannanes Sarah R. Whittleton, Alfred J. Rolle, Russell J. Boyd,* and T. Bruce Grindley* Department of Chemistry, Dalhousie University, Halifax, N.S., B3H 4J3 Canada Received July 31, 2010

Enthalpies and entropies of dimerization have been determined as functions of concentration for dibutyltin dipropoxide, dibutyltin dibutoxide, and dibutyltin diisopropoxide in toluene-d8 and cyclohexane-d12 solutions from the variation in 119Sn NMR chemical shifts with temperature. The values of ΔH and ΔS obtained in toluene-d8 for dibutyltin dibutoxide were -69.5 ( 3.0 kJ mol-1 and -197 ( 10 J mol-1 K-1, respectively, and for dibutyltin diisopropoxide -67.1 ( 1.4 kJ mol-1 and -244 ( 9 J mol-1 K-1, respectively. Enthalpies and entropies of activation for this process for dibutyltin diisopropoxide have been derived by simulation of the temperature-dependent broadening of the 119Sn NMR spectra. The free energy of activation for dimerization was 33 kJ mol-1. The same thermodynamic parameters for a greater range of dibutyltin dialkoxides were derived theoretically by using MP2 single-point calculations on B3LYP-optimized geometries using the LANL2DZdp basis set with diffuse and polarization functions and its effective core potential for tin and 6-311G(2d,p)// 6-31G(d,p) for other atoms. Quite good agreement with the experimental results was achieved with this level of theory. The present study confirms that steric effects dictate the degree of dimerization for dibutyltin dialkoxides, with monomers becoming favored at room temperature when the alkoxide groups are changed from primary to secondary.

Introduction Dialkyltin dialkoxides are key intermediates or catalysts in a number of processes. They have been used extensively in carbohydrate chemistry because they facilitate regioselective monofunctionalization of diols and polyols.1-3 Recently, they have become of interest as catalysts for the capture of carbon dioxide in the synthesis of alkyl carbonates, starting *Corresponding authors. (R.J.B.) E-mail [email protected]. Tel: 902-494 8883. Fax: 902-494-1310. (T.B.G.) E-mail: Bruce.Grindley@ Dal.Ca. Tel: 902-494-2041. Fax: 902-494-1310. (1) Grindley, T. B. Adv. Carbohydr. Chem. Biochem. 1998, 53, 17–142. (2) Grindley, T. B. Applications of Organotin Derivatives for Carbohydrate Synthesis. In Tin, Fundamentals and Applications, Gielen, M., Davies, A. G., Tiekink, E. R. T., Pannell, K. H., Eds.; John Wiley: Chichester, UK, 2008; pp 491-508. (3) David, S.; Hanessian, S. Tetrahedron 1985, 41, 643–663. (4) Sakakura, T.; Choi, J.-C.; Saito, P.; Masuda, T.; Sako, T.; Oriyama, T. J. Org. Chem. 1999, 64, 4506–4508. (5) Choi, J.-C.; Kohno, K.; Ohshima, Y.; Yasuda, H.; Sakakura, T. Catal. Commun. 2008, 9, 1630–1633. (6) Kohno, K.; Choi, J.-C.; Ohshima, Y.; Yili, A.; Yasuda, H.; Sakakura, T. J. Organomet. Chem. 2008, 693, 1389–1392. (7) Choi, J.-C.; Sakakura, T.; Sako, T. J. Am. Chem. Soc. 1999, 121, 3793–3794. (8) Ballivet-Tkatchenko, D.; Chermette, H.; Plasserauda, L.; Walter, O. Dalton Trans. 2006, 5167–5175. (9) Ballivet-Tkatchenko, D.; Douteau, O.; Stutzmann, S. Organometallics 2000, 19, 4563–4567. (10) Ballivet-Tkatchenko, D.; Chambrey, S.; Keiski, R.; Ligabue, R.; Plasseraud, L.; Richard, P.; Turunen, H. Catal. Today 2006, 115, 80–87. (11) Ballivet-Tkatchenko, D.; Burgat, R.; Chambrey, S.; Plasseraud, L.; Richard, P. J. Organomet. Chem. 2006, 691, 1498–1504. (12) Beckmann, J.; Dakternieks, D.; Duthie, A.; Lewcenko, N. A.; Mitchell, C. Angew. Chem., Int. Ed. 2004, 43, 6683–6685. pubs.acs.org/Organometallics

Published on Web 11/11/2010

materials for the industrial production of isocyanates and polycarbonates.4-13 They have also attracted attention as transesterification catalysts.14-18 In all of these processes, reversible dimerization or oligomerization of the dialkyltin dialkoxide is critical (Figure 1). For dibutyltin dialkoxides, early studies by IR spectroscopy,19 by molecular weight determination, and by indirectly detected 119Sn NMR spectroscopy20 established that the position of the equilibrium is highly dependent on the size of the alkoxyl alkyl group; to the limit of observation, the dimethoxide exists entirely as a dimer, while the di-tertbutoxide is present as a monomer.20 The structures of dialkyltin dialkoxides with less bulky alkoxyl groups were confirmed to be dimers by the X-ray structure of dimethyltin dimethoxide.7 The diisopropoxide was shown to be present as a mixture of the dimer and the monomer at room temperature, shifting toward the monomer when the temperature (13) Du, Y.; Kong, D. L.; Wang, H. Y.; Cai, F.; Tian, H. S.; Wang, J. Q.; He, L. N. J. Mol. Catal. A 2005, 241, 233–237. (14) Otera, J. Acc. Chem. Res. 2004, 37, 288–296. (15) Peng, Z. H.; Orita, A.; An, D.; Otera, J. Tetrahedron Lett. 2005, 46, 3187–3189. (16) Xiang, J. N.; Orita, A.; Otera, J. Angew. Chem., Int. Ed. 2002, 41, 4117–4119. (17) Angiolini, L.; Caretti, D.; Mazzocchetti, L.; Salatelli, E.; Willem, R.; Biesemans, M. J. Organomet. Chem. 2006, 691, 3043–3052. (18) Poelmans, K.; Pinoie, V.; Verbruggen, I.; Biesemans, M.; Van Assche, G.; Deshayes, G.; Degee, P.; Dubois, P.; Willem, R. Appl. Organomet. Chem. 2007, 21, 504–513. (19) Mendelsohn, J.; Pommier, J. C.; Valade, J. C. R. Hebd. Seances Acad. Sci., Ser. C 1966, 263, 921–924. (20) Smith, P. J.; White, R. F. M.; Smith, L. J. Organomet. Chem. 1972, 40, 341–353. r 2010 American Chemical Society

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approaches gave surprisingly close agreement, as will be described herein.

Relative Energy of Dimerization ¼ EDimer - 2ðEMonomer Þ

Figure 1. Dimerization of dialkyltin dialkoxides.

was raised or when the neat liquid was diluted with carbon tetrachloride.20 These general trends were confirmed recently.8 In the original study, the variation in 119Sn NMR chemical shifts with temperature for the diisopropoxide was used to derive an approximate value for ΔH for the equilibrium, þ100 ( 12 kJ mol-1.20 However, the accuracy of this ΔH value is uncertain because the average chemical shift was measured only over a small temperature range, because the limiting value of the chemical shift used for the monomer appears to be too large by >20 ppm, as determined by comparison with the value measured herein and because spectra were not recorded at temperatures below room temperature to provide an accurate limiting value of the chemical shift for the dimer.20 Nevertheless, this ΔH value was recently used as the experimental target for a theoretical study of the dimerization of dimethyltin dimethoxide.21 Accurate data are needed to validate the results of any theoretical evaluation of the relative stabilities of organotin species. Recent improvements in computer technology and in theoretical methods have advanced the study of heavy-element systems using computational chemistry. Computational chemistry has been used to study organotin systems,22 with effective core potentials being employed to increase computational efficiency and to account for the relativistic effects associated with the presence of the heavy element, tin. Wakamatsu et al recently reported theoretical results for the dimerization of dimethyltin dimethoxide,21 using MP2/LANL2DZ for tin and MP2/6-311þG* for the other elements; it was necessary to include electron correlation in order to obtain reasonable estimates of the energies associated with dimerization. We have found that, although geometries are predicted well for alkyltin halides with a variety of basis sets and effective core potentials,23 addition of diffuse and polarization functions to the above basis sets for tin and the other heavy atoms gave improved measures of the energies of complexation of some tin complexes.24 In this publication, we report the careful measurement of thermodynamic parameters for the dimerization of three dibutyldialkoxytin derivatives whose 119Sn NMR spectra showed observable changes with temperature (Figure 1 and eqs 1-3) and have also calculated the same parameters for a wider range of compounds using the method described earlier.24 These two (21) Wakamatsu, K.; Orita, A.; Otera, J. Organometallics 2008, 27, 1092–1097. (22) Whittleton, S. R.; Boyd, R. J.; Grindley, T. B. Computational Methods for Organotin Compounds, in Tin Chemistry: Fundamentals, Frontiers, and Applications. In Tin Chemistry: Fundamentals, Frontiers, and Applications; Gielen, M., Davies, A. G., Tiekink, E. R. T., Pannell, K. H., Eds.; John Wiley: Chichester, UK, 2008; pp 267-281. (23) Whittleton, S. R.; Boyd, R. J.; Grindley, T. B. J. Phys. Chem. A 2006, 110, 5893–5896. (24) Whittleton, S. R.; Boyd, R. J.; Grindley, T. B. Can. J. Chem. 2009, 87, 974–983.

ð1Þ

ΔHReaction ¼ ΔHDimer - 2ðΔHMonomer Þ

ð2Þ

ΔGReaction ¼ ΔGDimer - 2ðΔGMonomer Þ

ð3Þ

Results and Discussion Thermodynamics of Equilibria. Tin-119 NMR chemical shifts were determined for compounds 3, 4, 5, and 6 as functions of solvent, concentration, and temperature. The full set of data is provided in tables in the file of Supporting Information. The 119Sn NMR chemical shift for dibutyltin di-tert-butoxide (6) was relatively constant under all conditions, -30 ppm in toluene-d8, similar to the value previously reported,20 consistent with a monomer being present under all conditions. For the diisopropoxide 5, the 119Sn NMR chemical shifts changed by about 160 ppm over a range of temperature of about 60 K; the precise values depended on the concentration and the solvent, consistent with what is expected for a dimerization equilibrium (see Figures 1 and 2). In addition, line broadening was observed that reached a maximum as the temperature was lowered to about 240 K, then decreased as the temperature was lowered further, consistent with slowing of a dynamic process on the NMR time scale. The kinetics of this process will be described and analyzed in the next section. It should be noted that these low-temperature spectra contained no observable signals in the -200 to -300 ppm range, as would be expected if hexacoordinate tin in trimers was present (see Figure 3). Signals in this range were observed for dibutyltin dialkoxides where both alkoxides are derived from a single 1,2- or 1,3-diol (dibutylstannylene acetals) and were ascribed to hexacoordinate tin atoms in trimers and higher oligomers.20,25,26 This lack of observation of a signal from a trimer is consistent with the results of the calculations of Wakamatsu et al.21 for dimethyltin dimethoxide, which is expected to have a greater tendency to form a trimer than the more hindered dibutyl derivatives studied here. In the analysis of the monomer-dimer equilibria to follow, the following abbreviations were used in the equations that can be defined to describe the equilibrium (Figure 1): CSM = chemical shift monomer, CSD = chemical shift dimer, c = initial concentration assuming that all material is present in the monomeric form, CSO = chemical shift observed, x = concentration of the dimer at equilibrium, K = equilibrium constant. For compound 5, the equilibria were completely shifted to dimer at the low-temperature end of the range and to monomer at the high-temperature end, and the CSM and CSD values were taken from the limiting values of chemical shifts at the temperature extremes. The variations in these limiting chemical shifts with temperature appeared to be quite small; for 5 the chemical shift of the broad signal of the dimer below coalescence varied less than (25) Grindley, T. B.; Thangarasa, R. J. Am. Chem. Soc. 1990, 112, 1364–1373. (26) Grindley, T. B.; Thangarasa, R.; Bakshi, P. K.; Cameron, T. S. Can. J. Chem. 1992, 70, 197–204.

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Figure 2. 186.5 MHz temperature.

119

Whittleton et al.

Sn NMR spectra of a 2.0 M solution of dibutyltin diisopropoxide (5) in toluene-d8 as a function of

Figure 3. Potential equilibrium showing trimer formation.

the uncertainty in the measurements of the chemical shifts ((1 ppm) over the range from 180 to 220 K for the 0.12 M sample in toluene-d8. For 3 and 4, where the limiting chemical shifts for the monomers were not available, initial values were estimated and then adjusted to yield the largest correlation coefficients for the linear plots of ln K versus 1/T.

CSO ¼ CSMðc - 2xÞ=c þ 2xCSD=c

ð4Þ

K ¼ x=ðc - 2xÞ2

ð5Þ

These plots gave excellent straight lines (i.e., see Figure 4 and Supporting Information, all correlation coefficients >0.994) with slopes and intercepts that were used to obtain ΔH and ΔS, respectively, reported in Table 1. The trends in limiting chemical shift values are rather surprising, with the values for the primary alkyl groups being intermediate between those of the isopropyl and tert-butyl groups for monomers and perhaps for dimers; the 119Sn chemical shifts for tetracoordinate tin in monomeric Bu2Sn(OR)2 are -19, -10.5, and -30 ppm for R = primary alkyl groups (propyl and butyl), isopropyl and tert-butyl, respectively, and for pentacoordinate tin in dimers the corresponding values are -160 ppm, -149 ppm, and unevaluated experimentally. The ΔH and ΔS values for the dibutoxy derivative 4 and the diisopropoxy derivative 5 in toluene-d8 solutions vary somewhat with concentration, as shown in Table S17.

Average values are listed in Table 1. The uncertainties are the standard deviations of the measurements, and the average ΔG’s were derived from the ΔG values at particular concentrations. Values were also measured at one concentration in cyclohexane-d12 to simulate gas phase conditions, and the results were similar to the average values in toluene-d8. The values previously obtained for ΔH for 4 and 5 were -59 ( 12 and -100 ( 16 kJ mol-1, respectively,20 if the equilibria are written in the association direction, compared to -69.5 ( 3.0 and -67.1 ( 1.4 kJ mol-1 obtained here. The previous values were obtained on neat samples with measurement of the 119Sn NMR data via the heteronuclear double resonance technique using 1H NMR spectra on a 60 MHz spectrometer over a limited temperature range using values of limiting chemical shifts that were probably substantially in error. Here, the similarly sized ΔH values obtained for the two compounds, with that for the diisopropoxy derivative being smaller, which fits what is expected on the basis of the steric effects of the substituents; the bulkier isopropoxy group experiences a greater increase in steric repulsion in the more sterically restricted pentacoordinate environment of the dimer. Kinetics of Equilibration. Activation energies for the dimerization process of dialkyltin dialkoxides have never been measured before. As mentioned above, the line widths of the average signal of compound 5 in toluene-d8 broadened as the

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Figure 4. Plot of ln K derived from average chemical shifts versus 1/T for a 1.0 M solution of 4 in toluene-d8, r = 0.999. Table 1. Data for Dimerization of Dibutyltin Dialkoxides compd

solvent

CS monomer (ppm)

CS dimer (ppm)

ΔH (kJ mol-1)

ΔS (J mol-1 K-1)

ΔG298 (kJ mol-1)

3 4 4 5 5

tol-d8 tol-d8 cyclohex-d12 tol-d8 cyclohex-d12

-19 -19 -19 -8.5 -10.5

-160 -160 -160 -149 -149

-66 ( 5 -69.5 ( 3.0 -73 ( 5 -67.1 ( 1.4 -75 ( 5

-187 ( 10 -197 ( 9.6 -192 ( 10 -244 ( 8.5 -261 ( 10

-10.4 ( 1.0 -10.5 ( 0.3 -15.7 ( 1.0 þ6.69 ( 0.50 þ2.8 ( 1.0

Figure 5. Experimental (left) and simulated (right) 186.5 MHz 119Sn NMR spectra of a 0.20 M solution of 5 in toluene-d8 as a function of temperature. The temperatures used are shown next to the experimental spectra, while the rate constants used in the simulations in the direction dimer to monomer are shown beside the simulated spectra.

temperature was lowered from room temperature, reaching a maximum near 240 K, then sharpened as the temperature was lowered further. Because the positions of the equilibria change rapidly to favor the dimer as the temperature is lowered, monomer signals were never observed in the spectra below the coalescence temperature. This line broadening was simulated for exchange going from monomer to dimer and from dimer to monomer for 0.12 and 0.20 M solutions in toluene-d8 using the density matrix program embedded in the Bruker software, Topspin version 2.1, which is based on a program written by Rohonczy.27 Populations of the individual species at the simulated temperatures were obtained by using the ΔH and ΔS values determined above to calculate populations. Figure 5 shows experimental and simulated spectra for the 0.20 M solution. Rate constants obtained for exchange in the two directions were used to calculate (27) Rohonczy, J. Kem. Kozl. 1992, 74, 161–200. (28) Sandstr€ om, J. Dynamic NMR Spectroscopy; Academic Press: London, 1982.

ΔGq values using the Eyring equation.28 Plots of ΔGq versus T were then used to determine ΔHq and ΔSq. The results are summarized in Table 2, and the complete set of data is given in the Supporting Information. These values have large but undetermined uncertainties because it was difficult to define line widths in the absence of exchange and because the populations used result from extrapolations based on chemical shifts above coalescence. Because the dimer is more stable in enthalpy terms but is disfavored by entropy, the ΔHq value should be larger in the D f M direction and the ΔSq values should be positive in the D f M direction but negative in the M f D direction. The observations fit these expectations. However, by the theory of microscopic reversibility, the difference between the ΔHq values for exchange in the two directions (M f D and D f M) should equal the much more accurately determined ΔH o value for the equilibrium between the dimer and the monomer, and the same should be true for differences in the ΔS values. The differences in ΔHq values are 41 and 43 kJ mol-1

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Table 2. Kinetic Data for the Dimerization of Dibutyldiisiopropoxystannane (5) in Toluene-d8 dimer f monomer

monomer f dimer

conc (M)

ΔHq (kJ mol-1)

ΔSq (J mol-1 K-1)

ΔGq240 (kJ mol-1)

ΔHq (kJ mol-1)

ΔSq (J mol-1 K-1)

ΔGq240 (kJ mol-1)

0.12 0.20

56.9 77.8

92 172

35 ( 4 37 ( 4

16 31

-71 -11

33 ( 4 33 ( 4

Figure 6. Structures of the Bu2Sn(OR)2 monomers.

for the 0.12 and 0.20 M solutions, respectively, compared to 67.1 ( 1.4 kJ mol-1 from the equilibration data in Table 1, whereas the differences in ΔSq values are 163 and 184 J mol-1 K-1, compared to 244 ( 8.5 J mol -1 K-1 from above (Table 1). These differences of differences probably reflect the inaccuracies in the enthalpies and entropies of activation. Nevertheless, the ΔGq values agree quite well: the differences in ΔGq298 values are 2 and 4 ( 8 kJ mol-1 for the 0.12 and 0.20 M solutions, respectively, while the average equilibriumderived ΔGo value was 6.7 ( 0.5 kJ mol-1.

Computational Results Methods. All geometry optimizations and frequency calculations were performed in the gas phase using the B3LYP29-31 hybrid functional. Tin atoms were described using the Los Alamos National Laboratory double-ζ basis set (LANL2DZdp) with diffuse and polarization functions32 and its effective core (29) Becke, A. D. J. Chem. Phys. 1993, 98, 5648–5652. (30) Becke, A. D. J. Chem. Phys. 1993, 98, 1372–1377. (31) Lee, C.-H.; Yang, W.; Parr, R. G. Phys. Rev. B 1988, 37, 785– 789. (32) Check, C. E.; Faust, T. O.; Bailey, J. M.; Wright, B. J.; Gilbert, T. M.; Sunderlin, L. S. J. Phys. Chem. A 2001, 105, 8111–8116.

potential developed by Hay and Wadt.33 All non-tin atoms in this study (H, C, and O) were treated at the MP2/6-311G(2d,p)// B3LYP/6-31G(d,p) level. Single-point energies were calculated with the MP2 level of theory on the B3LYP-optimized geometries. These basis sets and effective core potentials have been shown to accurately predict the geometries23 and energies24 of organotin species. Previous examination21 of the monomerdimer equilibrium of dimethyltin dimethoxide showed that although optimization at either the B3LYP and MP2 level resulted in geometries that were close to X-ray geometries, it was necessary to include electron correlation at the MP2 level in order to obtain energies that were close to experimental thermodynamics of dimerization. Geometries of Monomers. Minimization of the geometries of the Bu2Sn(OR)2 monomers results in the same general shape, shown in Figure 6, with a tetrahedral tin center, two dicoordinate oxygen atoms, and butyl and alkoxy groups in all anti conformations. The relevant geometric parameters are reported in Table 3, focusing on the parameters involved in dimerization and affected by choice of R. In the monomers, tin-carbon bonds are longer than tin-oxygen bonds by 0.201-0.207 A˚, while — (C,Sn,C) are greater than — (O,Sn,O) by 4.2-10.6. Table 3 and Figure 6 show little deviation between the (33) Wadt, W. R.; Hay, P. J. J. Chem. Phys. 1985, 82, 284–298.

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Table 3. Geometric Parameters of Monomeric and Dimeric Bu2Sn(OR)2 (r in A˚, — in deg)a R exptlb Me Et Pr Bu i-Pr t-Bu

dimer monomer dimer monomer dimer monomer dimer monomer dimer monomer dimer monomer dimer

r(Sn-O)

r(Sn-O0 )

r(Sn-C)

r(Sn 3 3 3 O0 )

— (O,Sn,O)

— (C,Sn,C)

— (O0 ,Sn,O0 )

— (Sn,O0 ,Sn)

2.038 1.944 1.985 1.945 1.990 1.945 1.990 1.945 1.991 1.944 1.980 1.943 1.958

2.038

2.126c 2.147 2.152 2.147 2.153 2.146 2.153 2.147 2.154 2.148 2.159 2.150 2.151

2.324

93.2 108.7 92.6 108.9 92.9 108.5 92.9 108.9 92.9 107.0 97.1 109.8 103.8

135.1 118.7 122.3 118.7 123.7 118.4 124.2 118.4 123.8 117.6 127.8 114.0 114.9

68.9

111.1

68.3

110.8

68.9

110.4

69.0

110.4

68.8

110.8

70.7

108.9

80.7

99.1

2.046 2.044 2.045 2.047 2.022 1.950

2.296 2.328 2.325 2.318 2.516 4.131

In dimeric stannylene acetals, r(Sn-O) and r(Sn-O0 ), respectively, refer to the intramolecular bonds between tin and dicoordinate (O) and tricoordinate oxygen (O0 ), while r(Sn 3 3 3 O0 ) refers to the intermolecular bond between tin and tricoordinate oxygen. Angles containing a tricoordinate oxygen are — (O0 ,Sn,O0 ) and — (Sn,O0 ,Sn). Because monomers contain no tricoordinate oxygens, these parameters are not included in this table. b X-ray diffraction results for (MeO)2SnMe2.7 c Average value. a

geometries of the six Bu2Sn(OR)2 monomers, with all bond lengths and angles ranging within 0.004 A˚ and 4.7. Apparently, the size of the R groups does not drastically affect the geometry of the monomeric dialkoxydibutyltin deriveratives. Geometries of Dimers. All the dimeric dialkoxydibutyltin derivatives have the same general shape (see Figure 7), with tin adopting a distorted trigonal-bipyramid geometry, all butyl groups in anti,anti conformations, and all primary alkoxy groups in all anti conformations where applicable. All tin-oxygen bonds in dimers are in the same molecular plane, where one oxygen from each monomer is involved in an intermolecular bond (Sn 3 3 3 O) and becomes tricoordinate (O0 ), while the other remains dicoordinate. This creates a four-membered Sn2O2 ring involving r(Sn 3 3 3 O0 ), — (O0 ,Sn,O0 ), and — (Sn,O0 ,Sn). The geometric parameters are reported in Table 3. It should be noted that the computational study of Me2Sn(OMe)2 examined five different energetically stable dimer conformers, but the one present in the crystal structure7 was calculated to be significantly more stable than the others.21 On the basis of those results, the current study used the geometry of the crystal structure of dimethyltin dimethoxide7 as the starting point for optimization of all dimers. As shown in Table 3, most structural features are very similar to those present in the crystal structure of Me2Sn(OMe)2 and will not be discussed further. It should be emphasized that Sn-O bonds between monomers within the dimers are the longest bonds, ranging from 2.296 to 2.516 A˚. In the dimers, intramonomer Sn-O bonds were calculated to have unequal lengths, with the bond to the tricoordinate oxygen atom being 0.042 to 0.061 A˚ longer than the bond to the dicoordinate oxgen atom. In the X-ray crystal structure of (MeO)2SnMe2, these bonds had the same lengths,7 although they were different in crystal structures of dimers of dibutyltin derivatives of 1,2-diols.34,35 Another instance where there was a difference between the crystal structure7 and the calculated values was in the size of — (C,Sn,C); the crystal structure value was 135.1(3)o, whereas the calculated value for (MeO)2SnBu2 was 122.3. It has been known for some time that the sizes of these — (C,Sn,C) influence the magnitudes of 1JSn,C, and relationships between these values have been developed for both dimethyl36 (34) Bates, P. A.; Hursthouse, M. B.; Davies, A. G.; Slater, S. D. J. Organomet. Chem. 1989, 363, 45–60. (35) Cameron, T. S.; Bakshi, P. K.; Thangarasa, R.; Grindley, T. B. Can. J. Chem. 1992, 70, 1623–1630. (36) Lockhart, T. P.; Manders, W. F. J. Am. Chem. Soc. 1987, 109, 7015–7020. (37) Holecek, J.; Lycka, A. Inorg. Chim. Acta 1986, 118, L15–L16. (38) Holecek, J.; N avornı´ k, K.; Handlı´ r, K.; Lycka, A. J. Organomet. Chem. 1986, 315, 299–308. (39) Casella, G.; Ferrante, F.; Saielli, G. Inorg. Chem. 2008, 47, 4796– 4807.

and dibutyl37,38 tin derivatives. These relationships have also been evaluated theoretically.39 For the dibutyltin derivatives, the following relationship applies for 119Sn:37 1

J Sn, C ¼ 9:99θ - 746

For Bu2Sn(OMe)2 (1), this relationship gives 138 for — (C,Sn,C) from the 1JSn,C value in CDCl3,38 slightly larger than the value in Me2Sn(OMe)2 obtained from the X-ray study.7 The values obtained for 5 and 6, where the tin atom is tetracoordinate, were 126 and 124 from 1JSn,C values in CCl4,38 again larger than calculated. Here, the 1JSn,C values for neat samples of 3 and 4, which had chemical shifts that are consistent with dimer only, were 624 and 631 Hz, yielding — (C,Sn,C) values of 137 and 138, respectively, 13 and 11 larger than calculated, but similar to the value for 1. It is probable that the potential energy surface with respect to changing the size of this angle is fairly flat over the range of angles calculated and observed because opening this angle does not result in significant increase in steric effects. It should be noted that while the general structures are similar for all Bu2Sn(OR)2 dimers, when R= t-Bu, the intermolecular distance in the dimer is 4.131 A˚, almost 2 A˚ longer than in the other dimers. Unlike dimers with the isopropyl or butyl alkoxides, the tert-butyl groups cannot rotate to allow closer approach, resulting in much longer r(Sn 3 3 3 O0 ). This unlikely geometry may be a consequence of the well-known inability of density functional theory to correctly handle van der Waals interactions40-42 and that the B3LYP functional underestimates the dispersion force between the tert-butyl groups; therefore the monomers do not bond but instead are separated by 4.131 A˚. Because of this decreased intermolecular bonding, the differences between bond angles and bond lengths are not as pronounced as those mentioned above when R is methyl to isopropyl. Although the large steric bulk of t-Bu prevents shorter intermolecular bonds, it should be noted that the relative placement of the Sn, O, O0 , and tin butyl groups is consistent with the five other Bu2Sn(OR)2 dimers. Unlike the monomeric species, the size of R influences the geometric parameters of the Bu2Sn(OR)2 dimers. As the steric bulk increases from Me to t-Bu, r(Sn-C) increases by up to (40) Sun, Y. Y.; Kim, Y. H.; Lee, K.; Zhang, S. B. J. Chem. Phys. 2008, 129. (41) Jurecka, P.; Cerny, J.; Hobza, P.; Salahub, D. R. J. Comput. Chem. 2007, 28, 555–569. (42) Wodrich, M. D.; Jana, D. F.; Schleyer, P. V.; Corminboeuf, C. J. Phys. Chem. A 2008, 112, 11495–11500.

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Figure 7. Structures of the Bu2Sn(OR)2 dimers. 0.008 A˚, while the r(Sn-O0 ) decrease by as much as 0.097 A˚. Large alkoxyl groups also decrease intermolecular bonding, with r(Sn 3 3 3 O0 ) increasing by almost 2 A˚ as the steric bulk of R increases. To accommodate the growing steric bulk, angles — (C,Sn,C) and — (O,Sn,O) increase by as much as 4.5 as R increases from methyl to isopropyl. Intermolecular angles are less influenced by R, as the — (O0 ,Sn,O0 ) and — (Sn,O0 ,Sn) in these species differ by less than 2.4. While the Bu2Sn(O-t-Bu)2 dimer has a geometry distinct from other acyclic dimers, these trends are more pronounced, with angles differing by as much as 13 when compared to the methoxyl dimer. Effects of Dimerization. Dimerization changes the monomer geometry substantially. The creation of five-coordinate tin atoms causes the tin butyl groups to shift with — (C,Sn,C) increasing by as much as 10.2 and — (O,Sn,O) decreasing by

as much as 16.1. This forces the intramolecular tin-oxygen bonds to increase by as much as 0.046 and 0.102 A˚ for r(Sn-O) and r(Sn-O0 ), respectively. These geometric changes are smaller for the t-Bu dimer, with the two Bu2Sn(O-t-Bu)2 molecules within the dimer retaining their monomeric geometries. Upon dimerization, angles change by less than 6.0 and 0.9, respectively, for — (C,Sn,C) and — (O,Sn,O), while bond lengths increase by only 0.015 and 0.007 A˚ for r(Sn-O) and r(Sn-O0 ), respectively. Excluding the Bu2Sn(O-t-Bu)2 species, the extent of the aforementioned geometric trends depends on the size of the alkoxy group. Although the steric bulk of R does not affect monomer geometry, it dictates dimer geometries, and therefore the effects of dimerization effects change from Bu2Sn(OMe)2 to Bu2Sn(O-i-Pr)2. To accommodate the increasing steric bulk of R, the — (C,Sn,C) opens, while there is a smaller decrease in — (O,Sn,O). In addition, Table 3 shows that the larger the size of

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Organometallics, Vol. 29, No. 23, 2010

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Table 4. Energetics and Thermodynamics of the Dimerization of Dibutyltin Dialkoxides, Bu2Sn(OR)2

1 2 3 4 5 6

rel energies (kJ mol-1)

ΔH (kJ mol-1)

-86.6 -89.9 -84.3 -85.9 -54.9 -48.7

-87.6 -91.5 -85.6 -87.5 -54.4 -47.6

ΔHexp (kJ mol-1)

-73.6 -68.6

R, the larger the increase in r(Sn-O) and r(Sn-C), while there is a smaller increase in r(Sn-O0 ). Thermodynamics of Dimerization. The size of the alkoxy group (R) has been shown to influence the geometries of the dimers, as well as to dictate the degree of geometric change upon dimerization. All of the aforementioned trends in geometry are reflected in the energetics and thermodynamics of the dibutyltin dialkoxides (Table 4). The relative energies of two monomeric dibutyltin dialkoxides are compared with those of their corresponding dimer in Table 4. For all Bu2Sn(OR)2, the relative energies of the dimers are lower than those of their respective monomers. The enthalpies of dimerization (ΔH) show similar trends, with all values being substantially negative. The extent to which dimerization is favored decreases as the steric bulk of R increases, with relative energies and ΔH increasing by 37.9 and 40.0 kJ mol-1, respectively over the full series. However, for the primary dibutyltin dialkoxides the relative energies and enthalpies are similar, increasing by less than 6 kJ mol-1 as size increases from the dimethoxide to the dibutoxide. Branching is calculated to have a larger effect, with a 33 kJ mol-1 increase on changing from the dibutoxide to the diisopropoxide. The calculated entropy changes for all Bu2Sn(OR)2 are strongly negative, consistent with the loss of translational and rotational entropy on dimerization. Furthermore, the Gibbs energies of the reaction show that dimerization becomes disfavored at room temperature when steric interactions between the monomeric components are increased by changing the alkoxy groups from primary to secondary or tertiary. Comparison with Experimental Results and Previous Theoretical Calculations. The difference between the calculated and experimental enthalpies of dimerization for dibutyltin dibutoxide (4) is quite small, 13.9 kJ mol-1 (see Table 4), particularly since solvation is not included in the calculation. Both the calculated enthalpy and entropy differences are more negative than the experimental values. The calculation of a more negative value for entropy than observed is not unexpected because B3LYP density functional calculations are known to give infrared frequencies that are slightly too large.43 The difference between experimental and calculated enthalpy differences for 5, the diisopropoxide, is also good, 14.2. kJ mol-1, but now the dimer is calculated to be less stable than the experimental measurement indicates. Changing to the branched diisopropoxide must be the point at which the known40,41 inability of B3LYP to correctly account for dispersion forces comes into play. The experimental ΔG298 for the tert-butyl derivative must be at least þ12 kJ mol-1 because there were no marked changes in its 119Sn NMR chemical shifts or line widths for solutions in toluene-d8 from -80 to þ60 C. The one previous theoretical study of dimerization of dialkyltin dialkoxides by Wakamatsu et al.21 used MP2 optimization with LANL2DZdp basis sets for tin but without additional diffuse and polarization functions. Their work largely focused on dimethyltin dimethoxide, but a ΔE value for dimerization of dibutyltin dimethoxide (1) of -102.1 kJ mol-1 was calculated, compared with -86.6 kJ mol-1 obtained here. Dibutyltin dibutoxide (5) is calculated to have a similar ΔE value for dimerization (-85.9 kJ mol-1). The experimental ΔH value (43) Scott, A. P.; Radom, L. J. Phys. Chem. 1996, 100, 16502–16513.

ΔS (J mol-1 K-1) -221 -241 -240 -247 -222 -173

ΔSexp (J mol-1 K-1)

-213 -255

ΔG298 (kJ mol-1) -21.6 -19.6 -14.1 -13.7 11.7 4.1

ΔG298exp (kJ mol-1)

-10.5 -10.9 7.1

for 5 was -73.6 kJ mol-1 (see Table 4), suggesting that inclusion of diffuse and polarization functions on tin improves the accuracy of the theoretical calculations.

Conclusions The very large changes in 119Sn NMR chemical shifts (>130 ppm) with temperature over a fairly limited temperature range (