Trimethyltin Hydroxide: A Crystallographic and High Z′ Curiosity

Jan 21, 2011 - The Z. 0. = 1 and 4 structures are an enantiotropic pair, and trimethyltin hydroxide represents a case where the higher Z. 0 structure ...
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DOI: 10.1021/cg101464j

Trimethyltin Hydroxide: A Crystallographic and High Z0 Curiosity Kirsty M. Anderson, Sarah E. Tallentire, Michael R. Probert, Andres E. Goeta, Budhika G. Mendis, and Jonathan W. Steed*

2011, Vol. 11 820–826

Department of Chemistry, Durham University, South Road, Durham, U.K., DH1 3LE Received November 5, 2010; Revised Manuscript Received January 6, 2011

ABSTRACT: The remarkable room temperature structure of trimethyltin hydroxide comprises a total of 32 crystallographically independent SnMe3OH units arranged in four independent coordination polymer strands. We suggest that a Z0 = 4 value is more appropriate than Z0 = 32, reflecting the polymeric structure of the compound. DSC, single crystal and XRPD studies show that on cooling below ca.160 K the structure undergoes a first order phase change to a symmetric Z0 =1 structure with just one crystallographically unique SnMe3OH unit. The phase change is reversible, and on warming past 176 K the high Z0 structure is regenerated, in an endothermic transition. The Z0 = 1 and 4 structures are an enantiotropic pair, and trimethyltin hydroxide represents a case where the higher Z0 structure is the most stable form at high temperature with the high Z0 value possibly arising from a consideration of the dynamics of the crystal as a whole.

Introduction Recent studies on structures which crystallize with more than one molecule in the asymmetric unit, i.e. have Z0 > 1,1 have shown that the origins of this phenomenon have implications in a number of fundamental fields including crystal structure prediction and polymorphism studies,2-24 and a web resource bringing together a database of high Z0 crystal information is now available.25 The simple parameter Z0 is the tip of a metaphorical iceberg of complex phenomena that arise from, or are implicated in, effects such as frustration between competing packing motifs, the size and shape of a molecule,26-28 intermolecular interactions,29,30 formation of false conglomerates,31 crystal nucleation32 and growth and many other inherent properties of molecular-scale behavior. It is fair to say that perhaps there is not a “one rule fits all” explanation for the formation of crystal structures with Z0 >1 but that there are a number of factors which can contribute to crystallization with Z0 >1. The inability of the parameter Z0 to completely describe certain aspects of packing complexity has led to the use of several other parameters such as Z00 (the total number of molecules in the asymmetric unit23) and Zr (the number of types of chemical residue in the asymmetric unit33) along with a more comprehensive nomenclature system capable of describing, for example, cases where a Z0 = 1 value arises from the presence of two independent half molecules.34 The number of molecules known to crystallize with Z0 >1 is increasing as faster and more powerful diffractometers and data processing techniques become available, allowing many structures with large numbers of atoms in the asymmetric unit to be solved and refined with little or no difficulty.35-37 Despite these improvements the number of known structures with larger Z0 values is still relatively low. A search of the November 2009 version of the Cambridge Structural Database (CSD)35 shows only 50 structures with Z0 g 9, only 34 of which have full 3D coordinates and are not recorded as having any “errors”. This number can be reduced even further if some of the purported high Z0 structures are examined in detail.25 *Corresponding author. Fax: þ44 (0)191 384 4737. Tel: þ44 (0)191 334 2085. E-mail: [email protected]. pubs.acs.org/crystal

Published on Web 01/21/2011

We believe that this small subset of structures which crystallize with very large Z0 values represent the most extreme examples of the Z0 > 1 phenomenon, and therefore detailed study of these structures in particular could lead to new insights into crystal packing and growth phenomena.36 The largest value of Z0 in the CSD is a value of 32 for the room temperature structure of trimethyltin hydroxide making this compound the current Z0 “world record holder” for small molecules. This fascinating structure was first published in 1965,37 and although there are no coordinates available in the CSD from this determination, the paper contains a careful study of the intensity data and related structural conclusions. The structure was redetermined in 2004 at 150 K and found to be in the Sohnke space group P212121 with Z0 = 1 and deposited as a private communication in the CSD.38 In 2003 we also published a preliminary account of our Z0 = 1 determination at 120 K in P21/c.1 These more recent determinations show the compound to exist as a 1D coordination polymer comprising a trigonal bipyramidal tin(IV) center, with the methyl groups equatorial and a bridging hydroxyl ligand, consistent with its tendency to polymerize in solution.39 The Sn-O-Sn angle is bent at ca. 140 (Figure 1), as found in well-characterized analogues such as triethyltin hydroxide.40 In view of the lack of 3D structural data available for the original Z0 = 32 report and the advances in variable temperature diffraction data collection, since its publication, we have undertaken a series of experiments to aid understanding of this interesting compound. Results and Discussion DSC Analysis. Trimethyltin hydroxide is commercially available in the form of large, needle-shaped, crystals with a significant propensity for twinning along the needle axis. The compound has a melting point of 388 K but sublimes at temperatures above ca. 353 K (80 C).39 Samples of “as received” and vacuum-sublimed (313 K) SnMe3OH were analyzed by differential scanning calorimetry, Figure 2 and Supporting Information Figure S1. On warming from 113 to 373 K the “as received” material undergoes an endothermic r 2011 American Chemical Society

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phase transition with onset temperature 176 K (-97 C, ΔH = 1.57 J g-1). The only other notable feature is the sublimation endotherm with onset ca. 330 K. On repeated temperature cycling, however, a new endotherm with onset 245 K gradually appears. This event is followed by an apparent recrystallization exotherm and lower temperature sublimation endotherm. Repeating the DSC measurements on the vacuum-sublimed sample gave similar results, suggesting that vacuum sublimation may result in a phase change to a metastable solid form that converts back to the “as synthesized” polymorph on storage. Hence experiments concentrated on the “as synthesized” form which also proved to exhibit higher quality crystals. The clear phase transition in this material at 176 K immediately suggests an explanation for the different Z0 values observed for the 1965 determination (which was

Figure 1. Solid-state chemical structure of “SnMe3OH”.

Figure 2. DSC scans (multiple temperature cycles) of “as received” SnMe3OH.

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carried out at room temperature) and the more recent low temperature determinations at 150 and 120 K. As a result further structural studies were undertaken at a variety of temperatures in order to redetermine, and fully resolve, the high Z0 form and relate it to the lower temperature structure. Single Crystal X-ray Crystallography. In the 1965 report, analysis of the data was initially carried out on a small subcell with Z0 =2 (subcell 1, Table 1); however when the structure was solved the Sn-OH-Sn bridges appeared to be approximately linear. This was correctly considered to be chemically unlikely given that other similar compounds exhibit marked bending at the bridging oxygen atom. Careful scrutiny of the intensity data revealed the presence of a noncrystallographic 83 helical twist to the Sn-O-Sn-O- strand which could be described using subcell 2 which has Z0 = 8. When indexing the data using subcell 2 the authors noted that some reflections were not included and postulated that the “true cell” must be four times larger than subcell 2 (Table 1); they further suggested that this large cell arose due to an interchain disorder. The twinning of the trimethyltin hydroxide needle shaped crystals makes finding a single crystal for data collection very difficult. Sublimed crystals also proved to be twinned as well as being smaller and were unsuitable for single crystal data collection. Around 50 “as received” samples were screened, but the majority gave diffraction which was clearly from more than one crystal. Reflections arising from multiple crystals are readily confused with reflections indicating a larger unit cell than subcell 1, and hence higher Z0 ; however, collecting data on a carefully selected sample using a standard laboratory Mo KR source at room temperature we were able to reproduce the room temperature diffraction pattern including the weaker spots that lead to the large (Z0 = 32) room temperature cell. We were subsequently able to carry out a full data collection at this temperature and obtain a solution in a similar cell setting to the “true cell” observed in 1965 (structure 1), although it should be noted that the sample quality is not ideal, which, along with crystal degradation toward the end of the data collection, resulted in residuals that are slightly higher than desirable, and only the tin atoms could be refined anisotropically. The room temperature structure 1 is shown in Figure 3 and consists of four independent polymeric chains (A-D) aligned along the c-axis, each containing eight SnMe3OH units. It is clear from Figure 3 that the formula unit of this particular structure is much more realistically represented as {(SnMe3(μ-OH))8}¥ and hence Z0 is more formally 4 (corresponding to the number of crystallographically independent polymeric chains) rather than the value of 32 (the number of independent “SnMe3OH” monomer units). Figure 4 shows the packing of the structure along the c-axis

Table 1 1965 report

T/K a/A˚ b/A˚ c/A˚ R/deg β/deg γ/deg space group Z0

2004 CSD entry

this work

subcell 1

subcell 2

“true cell”

TMESNH01

1

2

RT 6.67 4.15 11.21 90 90 90 P21nm 2

RT 6.67 33.20 11.21 90 90 90 Pn 8

RT 13.34 33.20 22.42 90 90 90 Pn? 32

150 K 6.664(1) 8.317(2) 10.818(1) 90 90 90 P212121 1

RT 13.358(4) 22.436(7) 33.343(10) 90 90.006(6) 90 Pn 32/8 = 4

120 K 10.793(2) 6.6883(13) 8.3828(17) 90 90.130(3) 90 P21/c 1

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Figure 3. Room temperature structure of SnMe3OH (1) showing the four independent polymeric chains (A, B, C, D).

Figure 4. (a) Independent polymer chains in structure 1 of SnMe3OH viewed along the c-axis (A, green; B, orange; C, blue; D, red), (b) the Z0 = 1 structure TMESNH01 along b (150 K, P212121)38 and (c) the Z0 = 1 disordered structure 2 along c (120 K, P21/c).

with independent polymer chains highlighted. Sn-O-Sn angles are all bent as expected, with an average value of ca. 140. The 83 helical nature of the polymer strands is apparent from Figure 3, where in chains A, B and C the first four tin atoms from the left look approximately coplanar in the perspective presented followed by an undulation which is reversed in D. Figure 4 also demonstrates that the methyl groups in all the chains are not in register, although it is remarkable that over the eight formula units shown the

cross-sections of the four independent polymer strands are remarkably similar to one another. Hence the lower symmetry results from the way in which each strand is packed with respect to those around it, with the helical strands fitting together to allow close packing, rather than remaining exactly parallel, Figure 5. The situation thus closely resembles the Z 0 = 16 hydrogen bonded polymer [UO2Cl2(H2O)3] 3 15-crown-5.41,42 Comparison of the structure of 1 with the 150 K structure in P212121 (Figure 4b)

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shows that the differences are in the positions of the methyl groups, which are exactly aligned in the latter structure and, more significantly, in the positions of the oxygen atoms. In order to study the phase change suggested by the DSC results, the crystal used in the structure of 1 was cooled slowly to 120 K. Figure 6 shows rotation photographs taken at two different temperatures, 233 and 120 K, clearly showing additional peaks in the higher temperature diffraction pattern at 233 K. Unit cell determination at 120 K gave good agreement with the P21/c unit cell published in preliminary form in 20031 for a crystal of the same substance that was flash frozen to 120 K. We now report full crystallographic details of this 120 K determination (structure 2). Structure 2 has Z0 = 1 and refines to a satisfactory conventional R factor of 0.0384. Unlike structure 1, structure 2 exhibits 2-fold disorder of the oxygen atoms and methyl groups with Sn-O-Sn angles of 139.9(10) and 140.7(9) for the two components (Figure 7). While the structure is broadly similar to 1 in terms of the overall geometry, the helical twist of the individual polymer chains is absent. Structure 2 was obtained using a sealed tube Mo KR source and a KappaCCD detector, and, while plausible, it is not possible to completely rule out missed weak reflections, particularly since disorder is present in the model. We therefore attempted to study the disappearance of the weaker reflections upon cooling by selecting a very small crystal and collecting data using an intense Cu KR source on a

Figure 5. Ribbon cartoon generated from the oxygen atom positions in structure 1 showing the packing of the independent helices. The figures define the sheet of the twisting surface containing the tin atoms and OH groups.

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Bruker MicroStar H rotating anode equipped with a Proteum 135 CCD detector. Differences in observed average structure using different radiation sources have been noted in other systems.43 On this instrument we noticed crystal quality degrading very quickly over time even at lower temperatures. To test whether this was due to reaction in the air or some form of radiation damage a long needle crystal was selected and mounted so that only the top third of the crystal was in the beam. The crystal was exposed to intense copper radiation for ca. 30 min, then removed from the machine and examined again under the microscope. It is clear from Figure 8, which shows the crystal after irradiation, that the part of the crystal which was in the beam has been greatly damaged whereas the remaining two-thirds of the crystal remain unchanged, confirming that the crystal undergoes radiation damage. We postulate that the radicals generated by the radiation damage cause chemical degradation which eventually causes catastrophic decomposition of the polymer chains and hence loss of long-range order. This effect is noticeably less using Mo KR radiation compared to Cu KR and suggests that in order to study small crystals of this system fully the tunable wavelength of synchrotron radiation could be required. Despite the problems with radiation damage we were able to undertake unit cell determinations during cooling over a relatively short time period while cooling several small crystals of trimethyltin hydroxide from room temperature at varying rates. The behavior of the samples proved to be dependent on the cooling rate with lower intensity satellite reflections observed at slower cooling rates. We again found that the structure undergoes a phase change from the large cell found for structure 1 to the small cell found for structure 2. However, using this intense source even with slow cooling the satellite reflections indicative of the larger cell did not completely disappear at 120 K. We suggest that with slow cooling (3 K per hour) the majority of the polymer chains within the structure undergo the phase transition but defects of lower symmetry remain giving rise to weak satellite reflections. Powder X-ray Diffraction. To eliminate the difficulties in finding untwinned single crystals of trimethyltin hydroxide we also studied both “as received” and sublimed samples by

Figure 6. Rotation photographs of the single crystal used in structure 1 at 233 K and 120 K showing disappearance of satellite peaks and consequent increasing symmetry.

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Figure 9. Two dimensional film representation of powder diffraction data recorded while cooling a sample of the “as received” trimethyltin hydroxide from 300 to 100 K in 5 K steps. The sharp first order phase transition beginning at ca. 180 K corresponding to the change from structure 1 to structure 2 is clearly evident.

Figure 7. X-ray crystal structure 2 of Me3SnOH at 120 K exhibiting 2-fold disorder.

Figure 8. Radiation damage in a crystal of trimethyltin hydroxide; the top third of the crystal was exposed to intense Cu KR radiation.

variable temperature X-ray powder diffraction (XRPD) using Cu KR radiation. The “as received” sample was cooled from 300 to 100 K on a Bruker D8 diffractometer cooling at 15 K h-1, and XRPD patterns were collected approximately every 5 K, Figure 9. These patterns reveal a distinct first order phase change beginning at 180 K, in broad agreement with the complex corresponding cooling features in the DSC scan. Refinement of the unit cell parameters gave a sharp discontinuity at 146-151 K consistent with the change from structure 1 to structure 2 (Supporting Information Figures S2 and S3). We note the presence of these locked-in reflections cause the lattice parameters from Rietveld refinement to appear as if the transition occurs at a lower temperature than actually seen in the XRD data, Figure 9. Thus the XRPD data confirms that the low temperature structural phase transition is in no way a twinning artifact. In the first

sample “lingering” peaks attributable to structure 1 are observable well below the phase change and appear to be “locked in”. Repetition of the experiment on a different sample from 300 to 12 K resulted in the complete disappearance of these peaks on cooling. This observation is consistent with the single crystal work and confirms that careful/slow cooling is required to give complete interchain ordering. The VT XRPD data also exhibits a slight discontinuity in the refined lattice parameters at 224 K, consistent with the peaks that grow into the DSC scan with onset 245 K in the heating cycle that was attributed to material sublimed during the DSC scan. As a result the XRPD experiments were also repeated on the sublimed sample. The powder diffractometer is able to access very much lower temperatures than the calorimeter, and as a result the XRPD pattern was probed from 300 K down to 12 K; raw data and refined lattice parameters are given in Supporting Information Figures S4-S6. The data suggest a total of three phase changes on cooling through this temperature range beginning at ca. 245, 177, and 78 K. The first two features are also evident in the DSC trace of sublimed trimethyltin hydroxide. The third transition is the most significant and appears related to the first order phase change to structure 2 observed for the “as received” sample but occurs some 60 K lower. The precise nature of these phase changes is unclear, but it is possible that sublimation results in a disordered sample that requires slower or more extensive cooling in order to adopt the higher symmetry phase. The P212121 structure reported in 200438 may also correspond to one of these phases. We also attempted to study the phase behavior of the “as received” sample using TEM in diffraction mode at -70 and -150 C. The sensitivity of the sample to radiation damage made TEM measurement extremely challenging with diffraction patterns observable for only a fraction of a second before crystallinity was lost as a result of beam-induced sample degradation. Rapid measurement of the diffraction patterns did give some results, and representative TEM images and diffraction patterns are shown in the Supporting Information, however the low signal-to-noise ratio in the diffraction patterns meant that it was not possible to observe the change in size and symmetry of the unit cell using this method.

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solid state NMR spectroscopy suggest a high symmetry structure suggests that solid state NMR spectroscopy in the presence of an anisotropic atom such as tin is not sensitive to the small differences in solid state environment found in structure 1. Because of the low temperature phase change revealed in the present work the CP-MAS 13C NMR spectrum of the “as received” trimethyltin hydroxide was examined from -115 to þ50 C. The spectrum remains largely unchanged over the entire temperature range, although we see a modest chemical shift change from 3.08 to 3.41 ppm over this range. However, on warming from 25 to 50 C there is a substantial diminution in the intensity of the peak at 6.14 ppm relative to the peak at 3.41 ppm. We ascribe this behavior to increasing chain flexibility as the material nears the sublimation point, and it does not appear related to the crystallographic symmetry.

Figure 10. Hirshfeld surface fingerprint plot for structure 3 with H 3 3 3 H interactions colored. The gray regions represent covalent bonds that propagate the chain. There are no OH hydrogen bonds.

Alternative Polymorph and Crystal Packing Considerations. During our work we have not explicitly observed the alternative P212121 structure reported in 2004 (structure 3).38 This third form appears to represent an alternative polymorph to structures 1 and 2. Like structure 2 the individual polymer chains are aligned giving rise to Z0 = 1; however while 2 is disordered, structure 3 is fully ordered and the cross-section of the chains suggests that the oxygen atom positions from one chain to the next are correlated in 3, whereas they are randomly distributed in 2. Communication with the Edinburgh group indicates that the crystals used in the determination of structure 3 were obtained by slow, low temperature hydrolysis of Me3SnH with the crystals forming during the reaction. The crystal decomposes if data collection is carried out at 220 K, hence the choice of 150 K. It is therefore possible that structure 3 represents a metastable polymorph and is perhaps related to the material generated by sublimation during the DSC experiment, although there is no evidence to confirm this speculation at present. Interchain interactions were studied by Hirshfeld surface fingerprint analysis with the aid of the program CrystalExplorer.44 A fingerprint plot for the simplest structure, 3, is shown in Figure 10. Trimethyltin hydroxide is unusual in that the OH group does not undergo any hydrogen bonding interactions because of the lack of suitable acceptor atoms, and the packing is completely dominated by H 3 3 3 H contacts which account for some 95.7% of the surface of a section of polymer. The remainder of the surface comprises the covalent bonds that propagate the polymer chain. Thus the packing in trimethyltin hydroxide is determined purely by the shape of the polymer chains rather than any specific directional interactions. In addition to diffraction methods, trimethyltin hydroxide has previously been studied by 119Sn and 13C solid-state NMR spectroscopy.45-47 The CP-MAS 119Sn NMR spectrum shows only a single resonance, inconsistent with the 32 independent tin atoms found in the room temperature structure 1. Similarly the CP-MAS 13C NMR spectrum shows just two peaks at 6.2 and 3.5 ppm in a 1:2 ratio with 119 Sn satellites. These peaks can be assigned to the methyl carbon atoms that lie in and out of the plane of the Sn-O zigzag chain, respectively. The fact that both 119Sn and 13C

Conclusions Overall we conclude that the phase transition observed at 176 K by DSC can be attributed to a transition between planar and 83 helical chains, with interchain interactions between the helical chains at high temperature being responsible for the lower symmetry. The helical twist may arise as a consequence of the alleviation of interstrand steric interactions in the crystal arising from increased thermal twisting motion within the chain. Below the phase transition, depending on sample size and cooling rate, the majority of the sample adopts a Z0 =1 2-fold disordered structure in P21/c. The ordered Z0 = 1 polymorph in space group P212121 observed by the Edinburgh group is apparently metastable and may be related to the unidentified phase obtained by sublimation during the DSC experiment. The crystal packing in all forms of trimethyltin hydroxide is dominated by H 3 3 3 H van der Waals interactions and hence the shape of the polymer strands with no directional interstrand interactions. The room temperature structure, as described in the seminal 1965 determination, comprises a total of 32 crystallographically independent SnMe3OH units arranged in four independent coordination polymer strands with an Sn-O-Sn angle of around 140, consistent with a range of related compounds. Because the bridging oxygen atom is shared equally between the 5-coordinate tin(IV) centers, we suggest that a lower Z0 =4 value is more appropriate than Z0 = 32 reflecting the polymeric structure of the compound: a structure that is to at least some extent retained in solution.39 However, this assignment is certainly subjective in labile, coordination interactions of this type.48 The lower symmetry compared to the low temperature Z0 = 1 forms may arise from the accommodation of interstrand steric interactions as off-axis thermal motion of the individual chains increases. The reversibility of the transition with onset 176 K between structure 2 and structure 1 suggests that the two represent an enantiotropic polymorphic pair, and the positive enthalpy change observed from the low temperature to the high temperature form is consistent with the Burger and Ramberger heat of transition rule for enantiotropic pairs. Thus the classic trimethyltin hydroxide represents a case where the higher Z0 structure is the most stable form at high temperature with the high Z0 value possibly arising from a consideration of the dynamics of the crystal as a whole as the temperature increases, rather than static packing concepts such as synthon frustration29 or nucleation and growth processes occurring during the crystal formation.49,50

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References

Experimental Section Crystals of trimethyltin hydroxide were obtained from Alfa and were used without further purification for the “as received” sample. The sublimed sample was purified by vacuum sublimation in a Schlenk tube heated to 40 C with an oil bath. Trimethyltin hydroxide can become contaminated with dimethyltin oxide, however solid state 13C MAS NMR results on both the “as received” and sublimed samples are essentially identical. Crystal Data for Structure 1. C24H72O8Sn8: M = 1438.34, colorless needle, monoclinic, space group Pn (No. 7), a = 13.358(4), b = 22.436(7), c = 33.343(10) A˚, β = 90.006(6), V = 9993(5) A˚3, Z = 8, Dc = 1.912 g/cm3, F000 = 5440, SMART 6k, Mo KR radiation, λ = 0.71073 A˚, T = 293(2) K, 2θmax = 58.5, 108639 reflections collected, 49093 unique (Rint = 0.0589). Final GooF = 0.910, R1 = 0.0681, wR2 = 0.2221, R indices based on 7214 reflections with I >2σ(I) (refinement on F2), 897 parameters, 2 restraints. Lp and absorption corrections applied, μ = 3.960 mm-1. Absolute structure parameter = 0.44(9).51 Crystal Data for Structure 2. C3H10OSn: M = 180.80, colorless needle, monoclinic, space group P21/c (No. 14), a = 10.793(2), b = 6.6883(13), c = 8.3828(17) A˚, β = 90.130(3), V = 605.1(2) A˚3, Z = 4, Dc = 1.985 g/cm3, F000 = 344, KappaCCD, Mo KR radiation, λ = 0.71073 A˚, T = 120(2) K, 2θmax = 49.8, 1798 reflections collected, 757 unique (Rint = 0.0638). Final GooF = 1.174, R1 = 0.0387, wR2 = 0.0718, R indices based on 495 reflections with I > 2σ(I) (refinement on F2), 80 parameters, 36 restraints. Lp and absorption corrections applied, μ = 4.087 mm-1. The ratio of disordered components was modeled as a fixed proportion of 0.5 based on the best fit to the data. For both structures CH hydrogen atoms were placed in calculated positions and allowed to ride on the parent atom. OH hydrogen atoms were not included in the model for 1 while they were located in 2 by difference Fourier synthesis, and again a riding model was adopted with a fixed, isotropic atomic displacement parameter. Powder diffraction data were recorded on samples of trimethyltin hydroxide with copper KR1/KR2 radiation on a Bruker D8 diffractometer equipped with a Lynxeye psd and an Oxford Cryosystems pHeniX cryostat. Data sets were collected from 10 to 80 2θ in 20 min time slices as the sample was cooled at 15 K h-1 from 300 to 15 K. Unit cell parameters were extracted by Rietveld refinement. All Rietveld refinements were performed using the Topas Academic software suite controlled by local routines.52 The supercell model was used to fit all experimental data to allow volume evolution to be followed over the whole temperature range of cells. A total of 50 parameters were refined in each sequential refinement, these included: 15 background parameters, a sample height correction, 1 parameter to describe axial divergence, 4 lattice parameters, 4 peak shape parameters, a scale factor and 24 parameters to describe an eighth order spherical harmonic function used to model the significant preferred orientation present due to the needle-like shape of the crystals. The isotropic thermal displacement parameters were kept fixed at the values obtained from the single crystal study. Lattice parameters, R factors and the first Rietveld plot for each sample range are given in Supporting Information Figures S2-S4.

Acknowledgment. We would like to thank the EPSRC for funding, Dr. Ehmke Pohl for the use of the Bruker MicroStar H rotating anode equipped with a Proteum 135 CCD detector and Prof. Simon Parsons (Edinburgh) for additional information on the structure of TMESNH01. Supporting Information Available: X-ray crystallographic files in CIF format for 1 and 2. DSC scans (multiple temperature cycles) of the vacuum-sublimed SnMe3OH. Rietveld plots for the initial room temperature data collection for each of the “as received” and the sublimed samples. Refined lattice parameters, volume and Rwp values for each of the sequential Rietveld refinements on the data obtained by cooling each of the “as received” and sublimed samples. Solid state 13C MAS NMR spectra at various temperatures. This material is available free of charge via the Internet at http://pubs.acs.org.

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