pubs.acs.org/crystal Published on Web 01/21/2011 r 2011 American Chemical Society

DOI: 10.1021/cg101464j

2011, Vol. 11820–826

Trimethyltin Hydroxide: A Crystallographic and High Z0 Curiosity

Kirsty M. Anderson, Sarah E. Tallentire, Michael R. Probert, Andr�es E. Goeta,Budhika G. Mendis, and Jonathan W. Steed*

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 crystallographi-cally independent SnMe3OHunits arranged in four independent coordination polymer strands.We suggest that aZ0=4value ismore appropriate than Z0 = 32, reflecting the polymeric structure of the compound. DSC, single crystal and XRPD studiesshow that on cooling below ca.160K the structure undergoes a first order phase change to a symmetricZ0=1 structure with justone crystallographically unique SnMe3OHunit. The phase change is reversible, and onwarming past 176K the highZ0 structureis regenerated, in an endothermic transition. The Z0 =1 and 4 structures are an enantiotropic pair, and trimethyltin hydroxiderepresents a case where the higherZ0 structure is themost stable form at high temperature with the highZ0 value possibly arisingfrom a consideration of the dynamics of the crystal as a whole.

Introduction

Recent studies on structureswhich crystallizewithmore thanone molecule in the asymmetric unit, i.e. have Z0>1,1 haveshown that the origins of this phenomenonhave implications ina number of fundamental fields including crystal structureprediction and polymorphism studies,2-24 and a web resourcebringing together a database of high Z0 crystal information isnow available.25 The simple parameter Z0 is the tip of ametaphorical iceberg of complex phenomena that arise from,or are implicated in, effects such as frustration between com-peting packing motifs, the size and shape of a molecule,26-28

intermolecular interactions,29,30 formation of false conglom-erates,31 crystal nucleation32 and growth and many otherinherent properties of molecular-scale behavior. It is fair tosay that perhaps there is not a “one rule fits all” explanation forthe formation of crystal structures withZ0>1but that there area number of factorswhich can contribute to crystallizationwithZ0>1. The inability of the parameterZ0 to completely describecertain aspects of packing complexity has led to the use ofseveral other parameters such as Z00 (the total number ofmolecules in the asymmetric unit23) and Zr (the number oftypes of chemical residue in the asymmetric unit33) alongwith amore comprehensive nomenclature system capable of describ-ing, for example, cases where a Z0=1 value arises from thepresence of two independent half molecules.34

The number of molecules known to crystallize with Z0>1is increasing as faster and more powerful diffractometers anddata processing techniques become available, allowing manystructures with large numbers of atoms in the asymmetric unitto be solved and refined with little or no difficulty.35-37

Despite these improvements the number of known structureswith larger Z0 values is still relatively low. A search of theNovember 2009 version of the Cambridge Structural Data-base (CSD)35 shows only 50 structures with Z0 g 9, only 34 ofwhichhave full 3Dcoordinates andare not recorded ashavingany “errors”. This number canbe reduced even further if someof the purported high Z0 structures are examined in detail.25

Webelieve that this small subset of structureswhich crystallizewith very largeZ0 values represent the most extreme examplesof the Z0 > 1 phenomenon, and therefore detailed study ofthese structures in particular could lead to new insights intocrystal packing and growth phenomena.36

The largest value of Z0 in the CSD is a value of 32 for theroom temperature structure of trimethyltin hydroxidemakingthis compound the currentZ0 “world record holder” for smallmolecules. This fascinating structure was first published in1965,37 and although there are no coordinates available in theCSD from this determination, the paper contains a carefulstudy of the intensity data and related structural conclusions.The structure was redetermined in 2004 at 150 K and foundto be in the Sohnke space group P212121 with Z0 =1 anddeposited as a private communication in the CSD.38 In 2003we also published a preliminary account of our Z0 = 1determination at 120 K in P21/c.

1 These more recent determi-nations show the compound to exist as a 1D coordinationpolymer comprising a trigonal bipyramidal tin(IV) center,with the methyl groups equatorial and a bridging hydroxylligand, consistentwith its tendency topolymerize in solution.39

The Sn-O-Sn angle is bent at ca. 140� (Figure 1), as found inwell-characterized analogues such as triethyltin hydroxide.40

In view of the lack of 3D structural data available for theoriginalZ0 =32 report and the advances in variable tempera-ture diffraction data collection, since its publication, we haveundertaken a series of experiments to aid understanding ofthis interesting compound.

Results and Discussion

DSC Analysis. Trimethyltin hydroxide is commerciallyavailable in the form of large, needle-shaped, crystals witha significant propensity for twinning along the needle axis.The compound has a melting point of 388 K but sublimes attemperatures above ca. 353 K (80 �C).39 Samples of “asreceived” and vacuum-sublimed (313 K) SnMe3OH wereanalyzed by differential scanning calorimetry, Figure 2 andSupporting Information Figure S1. On warming from 113 to373 K the “as received” material undergoes an endothermic

*Corresponding author. Fax: þ44 (0)191 384 4737. Tel: þ44 (0)191 3342085. E-mail: jon.steed@durham.ac.uk.

Article Crystal Growth & Design, Vol. 11, No. 3, 2011 821

phase transition with onset temperature 176 K (-97 �C,ΔH = 1.57 J g-1). The only other notable feature is thesublimation endotherm with onset ca. 330 K. On repeatedtemperature cycling, however, a new endotherm with onset245 K gradually appears. This event is followed by anapparent recrystallization exotherm and lower temperaturesublimation endotherm. Repeating the DSC measurementson the vacuum-sublimed sample gave similar results, sug-gesting that vacuum sublimation may result in a phasechange to a metastable solid form that converts back to the“as synthesized” polymorph on storage. Hence experimentsconcentrated on the “as synthesized” formwhich also provedto exhibit higher quality crystals.

The clear phase transition in this material at 176 Kimmediately suggests an explanation for the different Z0

values observed for the 1965 determination (which was

carried out at room temperature) and the more recent lowtemperature determinations at 150 and 120 K. As a resultfurther structural studies were undertaken at a variety oftemperatures in order to redetermine, and fully resolve, thehighZ0 form and relate it to the lower temperature structure.

Single Crystal X-ray Crystallography. In the 1965 report,analysis of the datawas initially carried out on a small subcellwith Z0=2 (subcell 1, Table 1); however when the structurewas solved the Sn-OH-Sn bridges appeared to be approxi-mately linear. This was correctly considered to be chemicallyunlikely given that other similar compounds exhibit markedbending at the bridging oxygen atom. Careful scrutiny of theintensity data revealed the presence of a noncrystallographic83 helical twist to the Sn-O-Sn-O- strand which could bedescribed using subcell 2 which has Z0=8. When indexingthe data using subcell 2 the authors noted that some reflec-tions were not included and postulated that the “true cell”must be four times larger than subcell 2 (Table 1); theyfurther suggested that this large cell arose due to an inter-chain disorder.

The twinning of the trimethyltin hydroxide needle shapedcrystalsmakes finding a single crystal for data collection verydifficult. Sublimed crystals also proved to be twinned as wellas being smaller and were unsuitable for single crystal datacollection. Around 50 “as received” samples were screened,but the majority gave diffraction which was clearly frommore than one crystal. Reflections arising from multiplecrystals are readily confused with reflections indicating alarger unit cell than subcell 1, and hence higher Z0; however,collecting data on a carefully selected sample using a stan-dard laboratoryMoKR source at room temperaturewewereable to reproduce the room temperature diffraction patternincluding the weaker spots that lead to the large (Z0=32)room temperature cell. We were subsequently able to carryout a full data collection at this temperature and obtain asolution in a similar cell setting to the “true cell” observed in1965 (structure 1), although it should be noted that thesample quality is not ideal, which, along with crystal degra-dation toward the end of the data collection, resulted inresiduals that are slightly higher than desirable, and only thetin atoms could be refined anisotropically.

The room temperature structure 1 is shown inFigure 3 andconsists of four independent polymeric chains (A-D)aligned along the c-axis, each containing eight SnMe3OHunits.

It is clear from Figure 3 that the formula unit of thisparticular structure is much more realistically represented as{(SnMe3(μ-OH))8}¥ and hence Z0 is more formally 4(corresponding to the number of crystallographically inde-pendent polymeric chains) rather than the value of 32 (thenumber of independent “SnMe3OH” monomer units).Figure 4 shows the packing of the structure along the c-axis

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

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

Table 1

1965 report 2004 CSD entry this work

subcell 1 subcell 2 “true cell” TMESNH01 1 2

T/K RT RT RT 150 K RT 120 Ka/A 6.67 6.67 13.34 6.664(1) 13.358(4) 10.793(2)b/A 4.15 33.20 33.20 8.317(2) 22.436(7) 6.6883(13)c/A 11.21 11.21 22.42 10.818(1) 33.343(10) 8.3828(17)R/deg 90 90 90 90 90 90β/deg 90 90 90 90 90.006(6) 90.130(3)γ/deg 90 90 90 90 90 90space group P21nm Pn Pn? P212121 Pn P21/cZ0 2 8 32 1 32/8 = 4 1

822 Crystal Growth & Design, Vol. 11, No. 3, 2011 Anderson et al.

with independent polymer chains highlighted. Sn-O-Snangles are all bent as expected, with an average value of ca.140�. The 83 helical nature of the polymer strands is apparentfrom Figure 3, where in chains A, B and C the first four tinatoms from the left look approximately coplanar in theperspective presented followed by an undulation which isreversed in D. Figure 4 also demonstrates that the methylgroups in all the chains are not in register, although it isremarkable that over the eight formula units shown the

cross-sections of the four independent polymer strands areremarkably similar to one another. Hence the lower sym-metry results from the way in which each strand is packedwith respect to those around it, with the helical strands fitt-ing together to allow close packing, rather than remain-ing exactly parallel, Figure 5. The situation thus closelyresembles the Z0 = 16 hydrogen bonded polymer[UO2Cl2(H2O)3] 3 15-crown-5.

41,42 Comparison of the struc-ture of 1 with the 150 K structure in P212121 (Figure 4b)

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 SnMe3OHviewed along the c-axis (A, green; B, orange; C, blue;D, red), (b) theZ0 =1 structure TMESNH01 along b (150 K, P212121)

38 and (c) the Z0 = 1 disordered structure 2 along c (120 K, P21/c).

Article Crystal Growth & Design, Vol. 11, No. 3, 2011 823

shows that the differences are in the positions of the methylgroups, 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 DSCresults, the crystal used in the structure of 1 was cooledslowly to 120 K. Figure 6 shows rotation photographs takenat two different temperatures, 233 and 120 K, clearly show-ing additional peaks in the higher temperature diffractionpattern at 233K.Unit cell determination at 120K gave goodagreement with the P21/c unit cell published in preliminaryform in 20031 for a crystal of the same substance that wasflash frozen to 120 K. We now report full crystallographicdetails of this 120 K determination (structure 2).

Structure 2 has Z0 = 1 and refines to a satisfactoryconventionalR factor of 0.0384.Unlike structure 1, structure2 exhibits 2-fold disorder of the oxygen atoms and methylgroupswith Sn-O-Sn angles of 139.9(10)� and 140.7(9)� forthe two components (Figure 7). While the structure isbroadly similar to 1 in terms of the overall geometry, thehelical twist of the individual polymer chains is absent.

Structure 2 was obtained using a sealed tube Mo KRsource and a KappaCCD detector, and, while plausible, itis not possible to completely rule outmissedweak reflections,particularly since disorder is present in the model. We there-fore attempted to study the disappearance of the weakerreflections upon cooling by selecting a very small crystaland collecting data using an intense Cu KR source on a

Bruker MicroStar H rotating anode equipped with a Proteum135CCDdetector. Differences in observed average structureusing different radiation sources have been noted in othersystems.43 On this instrument we noticed crystal qualitydegrading very quickly over time even at lower temperatures.To test whether this was due to reaction in the air or someform of radiation damage a long needle crystal was selectedand mounted so that only the top third of the crystal was inthe beam. The crystal was exposed to intense copper radia-tion for ca. 30 min, then removed from the machine andexamined again under themicroscope. It is clear fromFigure8, which shows the crystal after irradiation, that the part ofthe crystal which was in the beam has been greatly damagedwhereas the remaining two-thirds of the crystal remain un-changed, confirming that the crystal undergoes radiationdamage. We postulate that the radicals generated by theradiation damage cause chemical degradation which even-tually causes catastrophic decomposition of the polymerchains and hence loss of long-range order. This effect isnoticeably less using Mo KR radiation compared to Cu KRand suggests that in order to study small crystals of thissystem fully the tunablewavelength of synchrotron radiationcould be required.

Despite the problems with radiation damage we were ableto undertake unit cell determinations during cooling over arelatively short time period while cooling several smallcrystals of trimethyltin hydroxide from room temperatureat varying rates. The behavior of the samples proved to bedependent on the cooling rate with lower intensity satellitereflections observed at slower cooling rates. We again foundthat the structure undergoes a phase change from the largecell found for structure 1 to the small cell found for structure2. However, using this intense source even with slow coolingthe satellite reflections indicative of the larger cell did notcompletely disappear at 120 K. We suggest that with slowcooling (3 K per hour) the majority of the polymer chainswithin the structure undergo the phase transition but defectsof lower symmetry remain giving rise to weak satellitereflections.

Powder X-ray Diffraction. To eliminate the difficulties infinding untwinned single crystals of trimethyltin hydroxidewe 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 andconsequent increasing symmetry.

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

824 Crystal Growth & Design, Vol. 11, No. 3, 2011 Anderson et al.

variable temperature X-ray powder diffraction (XRPD)using CuKR radiation. The “as received” sample was cooledfrom 300 to 100 K on a Bruker D8 diffractometer cooling at15 K h-1, and XRPD patterns were collected approximatelyevery 5 K, Figure 9. These patterns reveal a distinct firstorder phase change beginning at 180 K, in broad agreementwith the complex corresponding cooling features in the DSCscan. Refinement of the unit cell parameters gave a sharpdiscontinuity at 146-151 K consistent with the change fromstructure 1 to structure 2 (Supporting Information FiguresS2 and S3). We note the presence of these locked-in reflec-tions cause the lattice parameters from Rietveld refinementto appear as if the transition occurs at a lower temperaturethan actually seen in the XRD data, Figure 9. Thus theXRPD data confirms that the low temperature structuralphase transition is in no way a twinning artifact. In the first

sample “lingering” peaks attributable to structure 1 areobservable well below the phase change and appear to be“locked in”. Repetition of the experiment on a differentsample from 300 to 12 K resulted in the complete disappear-ance of these peaks on cooling. This observation is consistentwith the single crystal work and confirms that careful/slowcooling is required to give complete interchain ordering.

The VT XRPD data also exhibits a slight discontinuity inthe refined lattice parameters at 224 K, consistent with thepeaks that grow into the DSC scan with onset 245 K in theheating cycle that was attributed tomaterial sublimed duringthe DSC scan. As a result the XRPD experiments were alsorepeated on the sublimed sample. The powder diffractometeris able to access very much lower temperatures than thecalorimeter, and as a result the XRPD pattern was probedfrom 300 K down to 12 K; raw data and refined latticeparameters are given in Supporting Information FiguresS4-S6. The data suggest a total of three phase changes oncooling through this temperature range beginning at ca. 245,177, and 78 K. The first two features are also evident in theDSC trace of sublimed trimethyltin hydroxide. The thirdtransition is the most significant and appears related to thefirst order phase change to structure 2 observed for the “asreceived” sample but occurs some 60 K lower. The precisenature of these phase changes is unclear, but it is possible thatsublimation results in a disordered sample that requiresslower ormore extensive cooling in order to adopt the highersymmetry 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 damagemadeTEMmeasurement extremely challengingwith diffrac-tion patterns observable for only a fraction of a secondbefore crystallinity was lost as a result of beam-inducedsample degradation. Rapid measurement of the diffractionpatterns did give some results, and representative TEMimages and diffraction patterns are shown in the SupportingInformation, however the low signal-to-noise ratio in thediffraction patterns meant that it was not possible to observethe change in size and symmetry of the unit cell using thismethod.

Figure 7. X-ray crystal structure 2 ofMe3SnOHat 120K exhibiting2-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.

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

Article Crystal Growth & Design, Vol. 11, No. 3, 2011 825

Alternative Polymorph and Crystal Packing Considera-

tions. During our work we have not explicitly observed thealternativeP212121 structure reported in 2004 (structure 3).

38

This third form appears to represent an alternative poly-morph to structures 1 and 2. Like structure 2 the individualpolymer chains are aligned giving rise to Z0 =1; howeverwhile 2 is disordered, structure 3 is fully ordered and thecross-section of the chains suggests that the oxygen atompositions from one chain to the next are correlated in 3,whereas they are randomly distributed in 2. Communicationwith the Edinburgh group indicates that the crystals used inthe determination of structure 3 were obtained by slow, lowtemperature hydrolysis ofMe3SnHwith the crystals formingduring the reaction. The crystal decomposes if data collec-tion is carried out at 220 K, hence the choice of 150 K. It istherefore possible that structure 3 represents a metastablepolymorph and is perhaps related to the material generatedby sublimation during theDSC experiment, although there isno evidence to confirm this speculation at present.

Interchain interactions were studied by Hirshfeld surfacefingerprint analysis with the aid of the program Crystal-Explorer.44 A fingerprint plot for the simplest structure, 3, isshown in Figure 10. Trimethyltin hydroxide is unusual inthat the OH group does not undergo any hydrogen bondinginteractions because of the lack of suitable acceptor atoms,and the packing is completely dominated byH 3 3 3Hcontactswhich account for some 95.7% of the surface of a section ofpolymer. The remainder of the surface comprises the cova-lent bonds that propagate the polymer chain. Thus thepacking in trimethyltin hydroxide is determined purely bythe shape of the polymer chains rather than any specificdirectional interactions.

In addition to diffractionmethods, trimethyltin hydroxidehas previously been studied by 119Sn and 13C solid-stateNMR spectroscopy.45-47 The CP-MAS 119Sn NMR spec-trum shows only a single resonance, inconsistent with the 32independent tin atoms found in the room temperaturestructure 1. Similarly the CP-MAS 13C NMR spectrumshows just two peaks at 6.2 and 3.5 ppm in a 1:2 ratio with119Sn satellites. These peaks can be assigned to the methylcarbon atoms that lie in and out of the plane of the Sn-Ozigzag chain, respectively. The fact that both 119Sn and 13C

solid state NMR spectroscopy suggest a high symmetrystructure suggests that solid state NMR spectroscopy inthe presence of an anisotropic atom such as tin is notsensitive to the small differences in solid state environmentfound in structure 1. Because of the low temperature phasechange revealed in the present work the CP-MAS 13C NMRspectrum of the “as received” trimethyltin hydroxide wasexamined from -115 to þ50 �C. The spectrum remainslargely unchanged over the entire temperature range,although we see a modest chemical shift change from 3.08to 3.41 ppm over this range. However, on warming from 25to 50 �C there is a substantial diminution in the intensity ofthe peak at 6.14 ppm relative to the peak at 3.41 ppm. Weascribe this behavior to increasing chain flexibility as thematerial nears the sublimation point, and it does not appearrelated to the crystallographic symmetry.

Conclusions

Overall we conclude that the phase transition observed at176KbyDSCcanbe attributed toa transitionbetweenplanarand 83 helical chains, with interchain interactions betweenthe helical chains at high temperature being responsible for thelower symmetry. The helical twist may arise as a consequenceof the alleviation of interstrand steric interactions in thecrystal arising from increased thermal twisting motion withinthe chain. Below the phase transition, depending on samplesize and cooling rate, the majority of the sample adopts aZ0=12-fold disordered structure in P21/c. The ordered Z0 =1polymorph in space groupP212121 observedby theEdinburghgroup is apparently metastable and may be related to theunidentified phase obtained by sublimation during the DSCexperiment. The crystal packing in all forms of trimethyltinhydroxide is dominated byH 3 3 3Hvan derWaals interactionsandhence the shape of thepolymer strandswithnodirectionalinterstrand interactions. The room temperature structure, asdescribed in the seminal 1965 determination, comprises a totalof 32 crystallographically independent SnMe3OH units ar-ranged in four independent coordination polymer strandswith an Sn-O-Sn angle of around 140�, consistent with arange of related compounds. Because the bridging oxygenatom is shared equally between the 5-coordinate tin(IV)centers, we suggest that a lower Z0=4 value is more appro-priate than Z0=32 reflecting the polymeric structure of thecompound: a structure that is to at least some extent retainedin solution.39 However, this assignment is certainly subjectivein labile, coordination interactions of this type.48 The lowersymmetry compared to the low temperatureZ0=1formsmayarise from the accommodation of interstrand steric inter-actions as off-axis thermal motion of the individual chainsincreases. The reversibility of the transition with onset 176 Kbetween structure 2 and structure 1 suggests that the tworepresent an enantiotropic polymorphic pair, and the positiveenthalpy change observed from the low temperature to thehigh temperature form is consistent with the Burger andRamberger heat of transition rule for enantiotropic pairs.Thus the classic trimethyltin hydroxide represents a casewhere the higher Z0 structure is the most stable form at hightemperature with the high Z0 value possibly arising from aconsideration of the dynamics of the crystal as a whole as thetemperature increases, rather than static packing conceptssuch as synthon frustration29 or nucleation and growthprocesses occurring during the crystal formation.49,50

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

826 Crystal Growth & Design, Vol. 11, No. 3, 2011 Anderson et al.

Experimental Section

Crystals of trimethyltin hydroxide were obtained from Alfa andwere used without further purification for the “as received” sample.The sublimed sample was purified by vacuum sublimation in aSchlenk tubeheated to40 �Cwithanoil bath. Trimethyltin hydroxidecan become contaminated with dimethyltin oxide, however solidstate 13CMASNMR results on both the “as received” and sublimedsamples are essentially identical.

Crystal Data for Structure 1. C24H72O8Sn8:M=1438.34, color-less 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) A3, 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 reflectionscollected, 49093 unique (Rint= 0.0589). FinalGooF=0.910,R1=0.0681, wR2 = 0.2221, R indices based on 7214 reflections withI >2σ(I) (refinement on F2), 897 parameters, 2 restraints. Lp andabsorption corrections applied, μ = 3.960 mm-1. Absolute struc-ture parameter = 0.44(9).51

Crystal Data for Structure 2. C3H10OSn: M = 180.80, colorlessneedle, 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) A3,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 reflectionscollected, 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 andabsorption corrections applied, μ = 4.087 mm-1. The ratio ofdisordered components was modeled as a fixed proportion of 0.5based on the best fit to the data.

For both structures CH hydrogen atoms were placed in calcu-lated positions and allowed to ride on the parent atom. OHhydrogen atoms were not included in the model for 1 while theywere located in 2 by difference Fourier synthesis, and again a ridingmodel was adopted with a fixed, isotropic atomic displacementparameter.

Powder diffraction datawere recorded on samples of trimethyltinhydroxide with copper KR1/KR2 radiation on a Bruker D8 diffrac-tometer equipped with a Lynxeye psd and an Oxford CryosystemspHeniX cryostat. Data sets were collected from 10 to 80� 2θ in20 min time slices as the sample was cooled at 15 K h-1 from 300 to15 K. Unit cell parameters were extracted by Rietveld refinement.All Rietveld refinements were performed using the Topas Academicsoftware suite controlled by local routines.52 The supercell modelwas used to fit all experimental data to allow volume evolution to befollowed over the whole temperature range of cells.

A total of 50 parameters were refined in each sequential refine-ment, these included: 15 background parameters, a sample heightcorrection, 1 parameter to describe axial divergence, 4 latticeparameters, 4 peak shape parameters, a scale factor and 24 param-eters to describe an eighth order spherical harmonic function usedto model the significant preferred orientation present due to theneedle-like shape of the crystals. The isotropic thermal displacementparameters were kept fixed at the values obtained from the singlecrystal study.

Lattice parameters, R factors and the first Rietveld plot for eachsample range are given in Supporting Information Figures S2-S4.

Acknowledgment. We would like to thank the EPSRC forfunding, Dr. Ehmke Pohl for the use of the BrukerMicroStarH rotating anode equippedwith a Proteum135CCDdetectorand Prof. SimonParsons (Edinburgh) for additional informa-tion on the structure of TMESNH01.

Supporting Information Available: X-ray crystallographic files inCIF format for 1 and 2. DSC scans (multiple temperature cycles) ofthe vacuum-sublimed SnMe3OH. Rietveld plots for the initial roomtemperature data collection for each of the “as received” and thesublimed samples. Refined lattice parameters, volume andRwp valuesfor eachof the sequentialRietveld refinementson thedataobtainedbycooling eachof the“as received”and sublimed samples. Solid state 13CMASNMRspectra at various temperatures.Thismaterial is availablefree of charge via the Internet at http://pubs.acs.org.

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Software for Powder Diffraction Data; Bruker AXS: Karlsruhe, 2004.