Role of Steric Hindrance in the Crystal Packing of Z ... - ACS Publications

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Cite This: Cryst. Growth Des. XXXX, XXX, XXX−XXX

Role of Steric Hindrance in the Crystal Packing of Z′ = 4 Superstructure of Trimethyltin Hydroxide S. I. Dey,† A. Schönleber,* S. Mondal,‡ S. I. Ali,§ and S. van Smaalen* Laboratory of Crystallography, University of Bayreuth, 95440 Bayreuth, Germany S Supporting Information *

ABSTRACT: The room-temperature crystal structure of trimethyltin hydroxide, (CH3)3SnOH, has been described by Anderson et al. [Cryst. Growth Des. 2011, 11, 820−826] as a 2a × 2b × 8c, 32-fold superstructure. We report a a × b × 8c, eight-fold superstructure with orthorhombic P21cn symmetry and Z′ = 4. Structured diffuse scattering observed at the positions of presumed superlattice reflections along a* and b* might have appeared as Bragg reflections in the experiment by Anderson et al. Alternatively, Anderson et al. and the present work might have studied different polymorphs of (CH3)3SnOH. Crystalline (CH3)3SnOH constitutes polymeric chains arranged parallel to c. In the eight-fold superstructure at 220 K, the polymeric chains possess a distorted zigzag arrangement of linked linear O−Sn−O units with bent angle at oxygen of ∼139.2°. This structure is essentially different from the 83-helical arrangement in the published 32-fold superstructure model. The origin of the distorted zigzag structure is explained by steric hindrance between hydrogen atoms of adjacent hydroxy groups and (CH3)3Sn groups. Frustration in the packing of the chains is determined by steric hindrance between methyl groups of neighboring chains, which prevents the formation of interchain C−H···O hydrogen bonds.

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INTRODUCTION The origin of multiple copies of a molecule in the asymmetric unit of a molecular crystal (Z′ > 1 structures) is often argued to be frustrated intermolecular interactions, also known as “synthon frustrations”.1,2 Such frustrations occur when intermolecular interactions favor certain orientations and conformations of the molecules while intramolecular interactions favor another conformation.3−11 A complete crystalchemical analysis is an important tool for developing and understanding the origin of low and high Z′ structures arising from variations of reaction conditions and/or phase transitions as a function of thermodynamic conditions.5,6,12−14 Different conformations and different environments of the independent molecules in high Z′ structures can be described in an elegant way as a commensurate modulation of a basic structure with a small unit cell, employing the higher-dimensional superspace approach.15−19 With one molecule in the basic structure unit cell, the modulation describes different modifications to the molecular structure for the independent copies of this molecule in the corresponding supercell. The superspace approach has the advantage that it establishes unique relations between different phases of a compound and also between different compounds, which are isomorphous,18,20−22 and that it removes correlations between structural parameters.18,23 However, not all high Z′ structures can be described as modulated structures.10,14 Trimethyltin hydroxide, (CH3)3SnOH (Figure 1), exists as a dimer in solution, and it is a polymeric compound in the solid © XXXX American Chemical Society

Figure 1. Asymmetric unit of the eight-fold superstructure of (CH3)3SnOH, containing four formula units arranged as part of a polymeric chain. Viewing direction along [21̅0]. Hydrogen atoms are omitted. The molecular graphic has been prepared with Diamond.24

state.25 A phase transition has been reported to occur at Tc ≈ 176 K, from a 32-fold superstructure toward a two-fold superstructure at low temperatures.26 It attracted our attention as the room-temperature phase (2a × 2b × 8c supercell) with monoclinic space group Pn (b-unique) consists of 32 formula units in the asymmetric unit (Z′ = 32).26 The proposed superstructure model26 consists of four crystallographically independent chains, each containing eight independent molecules (CH3)3SnOH, resulting in Z′ = 32. The chains are slightly distorted with respect to each other. Individual chains are formed by connecting (CH3)3Sn groups through oxygen resulting in (Sn−O−)n infinite chains with nearly equal Sn−O bond lengths. Received: September 13, 2017 Revised: January 4, 2018 Published: January 22, 2018 A

DOI: 10.1021/acs.cgd.7b01295 Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

Article

An earlier study27 has used a different setting of the unit cell. Transformed to the setting of Anderson et al.,26 Kasai et al.27 proposed the space group P21mn for the a × b × c basicstructure unit cell, and they proposed Pn (c unique) for an intermediate a × b × 8c supercell. They mention to have observed the 2a × 2b × 8c supercell as a “true” unit cell but do not provide the space group. It is noticed that P21mn is identical to the present space group for the basic structure, while Pn (c unique) is a monoclinic subgroup of presently P21cn for the a × b × 8c supercell. The space group Pn (b unique)26 for the 2a × 2b × 8c supercell is a subgroup of P21cn for the a × b × 8c supercell, which itself is a subgroup of P21mn for the a × b × c basic-structure unit cell. Since Kasai et al.27 do not report a structure model, we restrict the discussion in the present article to a comparison with the model by Anderson et al.26 We have performed single-crystal X-ray diffraction experiments in the room-temperature phase at T = 220 K. Inspection of the diffraction images revealed the absence of superlattice reflections along a* and b*. Instead, pronounced diffuse scattering has been observed in planes perpendicular to c containing main reflections. Strong superlattice reflections are observed along c*; hence, our data support an eight-fold, a × b × 8c superlattice. The superstructure is described as a commensurately modulated structure within the (3 + 1)dimensional [(3 + 1)D] superspace approach. The symmetry is given by the orthorhombic superspace group P21mn(00σ3)0s0, No. 31.1.9.7,28,29 resulting in the orthorhombic space group P21cn for the corresponding eight-fold superstructure. The four-fold smaller unit cell together with the higher symmetry results in a reduction of Z′ from published26 32 toward presently 4 (Figure 1). A complete crystal-chemical analysis is done to understand the origin of modulation (periodic distortion) in the chains and the interactions between the chains.

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Table 1. Experimental and Crystallographic Data Crystal Data chemical formula Mr crystal dimensions (mm3) crystal system superspace group T (K) a (Å) b (Å) c (Å) V (Å3) Z wavevector q

(CH3)3SnOH 180.82 0.07 × 0.07 × 0.30 orthorhombic P21mn(00σ3)0s0 220.0 (3) 6.61986 (19) 11.06277 (31) 4.13537 (15) 302.85 (2) 2

(0, 0, 83 ) t0 = 0 a × b × 8c P21cn synchrotron 0.56000 2.128

commensurate section supercell supercell space group radiation type wavelength (Å) μ (mm−1) Diffraction Data [sin(θ)/λ]max (Å−1) Δφ,Δω (deg) exposure time (s) crystal-to-detector (mm) absorption correction criterion of observability Unique Reflections all (obs/all) m = 0 (obs/all) m = ± 1 (obs/all) m = ± 2 (obs/all) m = ± 3 (obs/all) m = −4 (obs/all) Rint (obs/all) Refinement GoF (obs/all) Robs/wRall all (obs/all) m = 0 (obs/all) m = ±1 (obs/all) m = ±2 (obs/all) m = ±3 (obs/all) m = −4 (obs/all) No. of parameters H atom treatment absolute structure parameter30 Δρmin/Δρmax (e/Å3)

EXPERIMENTAL SECTION

Diffraction Experiment and Data Integration. Single crystals of (CH3)3SnOH have been obtained from Alfa Aesar (98% purity). They were used as purchased. The crystals were stored in argon atmosphere at T = 250 K because they were found to decompose in air at ambient conditions. Single-crystal X-ray diffraction experiments were performed at beamline F1, Hasylab, Desy, Hamburg, employing a fourcircle kappa diffractometer with a MAR-CCD detector and radiation of wavelength λ = 0.56000 Å (Table 1). The temperature of the crystal was maintained at T = 220.0 (3) K by an open-flow nitrogen-gas cryostat by Oxford Cryosystems. This temperature has been chosen to prevent decomposition of the crystal while still being well above the transition temperature (Tc ≈ 176 K).26 A complete data collection of diffracted intensities was done by φ and ω scans at different detector positions (Table 1). After repeated temperature cycling in a DSC measurement, Anderson et al.26 have observed features that hint toward a possible further phase transition at around T ≈ 250 K. Therefore, an additional data collection was performed at a temperature of 270 K, before cooling the crystal to T = 220 K. The analysis of those data confirmed that the crystal still is in the room temperature phase at T = 220 K. No indication concerning an additional phase transition was observed. Data processing has been performed with the software Eval15.31 In a first step the diffraction peaks were indexed in the published 2a × 2b × 8c superstructure setting,26 resulting a primitive monoclinic superlattice with refined lattice parameters 2a = 13.2347 (9) Å, 2b = 22.1139 (8) Å, and 8c = 33.0642 (15) Å and β = 89.978(4)°. Only 30 observed reflections (with intensities I > 3σ(I)) were found among 19009 integrated superlattice positions (hkl) with h = 2n + 1, and only 75 observed reflections existed among 39896 superlattice positions

0.81311 1 2, 16 150 empirical, multiscan I > 3σ(I) 5119/10600 1180/1377 1629/2542 1158/2757 743/2545 409/1379 0.0222/0.0280 2.16/1.67 0.0437/0.0573 0.0270/0.0347 0.0484/0.0458 0.0878/0.0938 0.1144/0.1348 0.0712/0.0788 193 mixed 0.5 −2.27/1.53

(hkl) with k = 2n + 1. All these 105 observed reflections have l = 8n. Visual inspection of the frames showed diffuse planes perpendicular to c containing the main reflections (Figure 2 and Supporting Information), which suggest that these 105 reflections actually are part of structured diffuse scattering, and they were not used in the further analysis. Superlattice reflections have been found at positions (hkl) with h = 2n, k = 2n, and l ≠ 8n (l = 8n are main reflections; Figure 2 and Supporting Information). In order to describe the eight-fold superstructure as a commensurately modulated structure within the (3 + 1)D superspace approach, diffraction peaks were alternatively indexed by four integers (hklm) (Figure 2). This indexing is based on the basic unit cell defined by a, b,

(

and c, and the modulation wave vector q = 0, 0,

3 8

) (Table 1). Free

refinement of the lattice parameters led to all angles equal to 90° B



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Lattice parameters, modulation wave vector, and reflection conditions (h, 0, l, m), m = 2n; (h, k, 0, 0), h + k = 2n; and (h, 0, 0, 0), h = 2n point toward an orthorhombic noncentrosymmetric superspace group P21mn(00σ3)0s0, No. 31.1.9.728,29 (see Supporting Information for details). Structure Solution and Refinement. The structure was solved in (3 + 1)D superspace by the charge flipping algorithm using the program Superflip.33,34 The resulting higher-dimensional electron density map revealed the position of the atom Sn1 along with initial values for its atomic modulation functions (AMFs) being centered on the mirror plane perpendicular to b. Structure refinement was done with the software package Jana2006.35 Detailed information on the structure refinement strategy is given in the Supporting Information. Positions of the oxygen and carbon atoms were determined along with initial values of their AMFs from difference Fourier maps. The oxygen atom O1 and the carbon atom C1 are centered on the mirror plane perpendicular to b, and the second carbon atom C2 is in a general position. The asymmetric unit possesses half of a formula unit (Z′ = 0.5). Up to four harmonic waves for displacive modulation for all atoms were introduced into the model. Refinement was found to diverge due to large correlations between parameters. Anisotropic ADPs of light atoms were found to be nonpositive definite. Analysis of the (3 + 1)-dimensional Fourier maps revealed that the amplitude of the displacive AMFs of the oxygen and carbon atoms are significantly larger than that of the tin atom. Therefore, in an alternate model, crenel functions36 were introduced for all atoms by splitting the continuous displacive AMFs along the internal dimension xs4 (Figure 3 and Supporting Information). Atoms Sn1, O1, and C1 are located on 1 the mirror plane (my, s): x1, − x 2 , x3 , x4 + 2 . They are split along xs4 into four positions each, Sn1 into Sn1a, Sn1b, Sn1c, and Sn1d, and similar for O1 and C1. Their AMFs are described by crenel functions of width Δs4 = 0.125 (Figure 3 and Supporting Information). The C2 atom, which is in a general position, is split into formally eight atoms, C2a, C2b, C2c, C2d, C3a, C3b, C3c, and C3d (see Supporting Information for Fourier maps of all atoms). Hydrogen atoms were added to the carbon atoms by a riding model. Hydrogen atoms attached to the oxygen atoms were added and refined applying distance and angle restraints d(O−H) = 0.82 ± 0.01 Å and ∠(Sn−O− H) = 105 ± 1°, using the ADP constraints Uiso(H) = 1.5Ueq(O). In the final model all non-hydrogen atoms were described with anisotropic ADPs and the refinement converged without any nonpositive definite ADPs at Robs F = 0.0437 (Table 1). The two superspace models (crenel functions vs continuous harmonic waves) provide equivalent descriptions of the crystal structure. The superstructure depends on the location of the origin along the internal dimension, that is, on the value t0 of t. Structure refinements were performed for t0 = 0 (resulting in supercell space group P21cn), 1 1 t0 = 32 (supercell space group Pc), and t0 = 16 (supercell space group P21cn). Refinements showed that the model at t0 = 0 fits best to the diffraction data (see Supporting Information). This model was therefore used for further analysis. The commensurately modulated structure model and the corresponding eight-fold superstructure are equivalent. Since the supercell space group is noncentrosymmetric, inversion twinning was tested. Refinement of the twin volumes resulted in a value of 0.5 within its standard uncertainty (absolute structure parameter30 = 0.42 (12)) and hence was fixed to 0.5 (Table 1). Isotropic extinction was introduced and refined with marginal improvement to the statistical parameters.

Figure 2. (a) Schematic representation of the diffraction image indexed in an 2a × 2b × 8c supercell. Black circles represent observed reflections, while open circles and ellipses indicate unobserved reflections. The horizontal gray lines represent diffuse scattering. (b) Small part of a measured frame with strong main reflections (hkl0) and weaker superstructure reflections (hklm with m = ±1, ±2, ±3, −4) 3 indexed on an a × b × c unit cell with q = 8 c*. The diffraction image shows pronounced diffuse scattering (indicated by arrows) in the layers defined by main reflections. The complete measured frame is given in the Supporting Information.

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within three times their standard uncertainties, while the nonzero component of q refined toward 0.374971(10), which is equal to 3 8 within three standard uncertainties. Integrated intensities of diffraction peaks were obtained by Eval15. Absorption correction was performed by Sadabs,32 employing Laue symmetry mmm. The ratio of the average intensities of main and superlattice reflections is ⟨I⟩|m|=0/⟨I⟩|m|=1/⟨I⟩|m|=2/⟨I⟩|m|=3/⟨I⟩|m|=4 ≈ 160:16:4:3:8 and of the average significance is ⟨I/σ(I)⟩|m|=0/⟨I/ σ(I)⟩|m|=1/⟨I/σ(I)⟩|m|=2/⟨I/σ(I)⟩|m|=3/⟨I/σ(I)⟩|m|=4 ≈ 25:20:9:8:15, which indicates pronounced modulation.

DISCUSSION Symmetry of the Low- and High-Temperature Superstructures. The room-temperature phase of (CH3)3SnOH is described as a commensurately modulated crystal structure with symmetry according to the noncentrosymmetric superspace 3 group P21mn(00σ3)0s0 with σ3 = 8 , and alternatively as an eight-fold superstructure with space group P21cn. The lowtemperature phase has been described as modulated structure C



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point of 388 K26 and might become the stable phase at high pressures. Phase Transitions and Frustrated Interactions. The crystal structures of (CH3)3SnOH are based on slightly distorted C3SnO2 trigonal bipyramids, with oxygen as the apexes and three carbon atoms defining the equatorial plane. Polymeric chains are formed by sharing oxygen atoms between neighboring bipyramids (Figure 4). Two symmetry-related polymeric chains are contained in the orthorhombic unit cells (Figure 5b and ref 37).

Figure 4. Single chain of the eight-fold superstructure viewed along (a) −a and (b) b. The plane through O1a and (0, 0, z) is highlighted in blue. Rotations ϕ1 through ϕ8 are indicated. The rotations are more visible along −a than in projection along b (compare to Figure 3). Molecular graphics has been prepared with Diamond.24

Distortions of the bipyramids are small in both the two- and eight-fold superstructures. Angles O−Sn−O are within the range of 175.5−178.3° and angles C−Sn−O are close to 90°,

Figure 3. (xsi, xs4)-Sections (i = 1, 2) of the Fourier map centered at the atoms (a) Sn1 (light blue) and Sn1iv (dark blue), and (b) O1 (orange) and O1 iv (dark brown). Symmetry code is (iv) 1 x1, − x 2 , x3 , x4 + 2 . Open circles indicate the xs4 values of the tsections relevant to the commensurate superstructure. Contour lines of equal density are drawn at intervals of 5 e/Å3 in (a) and 0.25 e/Å3 in (b). The widths of the maps are 4 Å.

with noncentrosymmetric superspace group P21212(00σ3)00s 1 and σ3 = 2 . The equivalent two-fold superstructure has space group P212121.37 A group/subgroup relation does not exist between these superspace groups. Therefore, these different symmetries are in agreement with a first-order phase transition at Tc = 176 K.26 Both noncentrosymmetric superspace groups are subgroups of a single centrosymmetric superspace group, Pmmn(00σ3)0s0 (No. 59.1.9.2 in ref 28). Structure refinements according to this centrosymmetric superspace symmetry did not lead to a good fit to the diffraction data (see Supporting Information). However, the common supergroup suggests the existence of a virtual high-temperature phase with space group Pmmn, a unit cell approximately equal to the basic-structure unit cell at 220 K, and chains (CH3)3SnOH with disordered conformations, which would be thermodynamically stable beyond the melting

Figure 5. One unit cell of the superstructure projected along (a) −a and (b) −c. Dashed gray lines indicate short interchain H···H contacts (see Table 3 and Supporting Information). Molecular graphics has been prepared with Diamond.24 D



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Contacts between neighboring chains are realized through shortest interatomic distances between hydrogen atoms. In the low-temperature phase at T = 100 K, these distances are dCH···HC = 2.53 Å between methyl groups.37 Within a single chain, the shortest nonbonded contact is dOH···HC = 2.63 Å between a hydroxy group and a methyl group. For the eightfold superstructure at 220 K, we have presently found shortest contact distances of dCH···HC = 2.19 Å and dOH···HC = 2.26 Å (Figure 6 and Table 3).

while distances are nearly equal among the Sn−O bonds as well as for the Sn−C bonds (see Supporting Information and ref 37). The bipyramids are arranged along the polymeric chains in a zigzag way, with angles Sn−O−Sn within the range of 139.2− 143.1° (Figure 4). The bond bending at oxygen is enforced by the presence of lone pairs on oxygen. It is in agreement with similar bond bending in related compounds, like trimethyltin fluoride,38 trimethyltin cyanide,39 trimethyltin methoxide,40 trimethyltin methylsulfinate,41 hydroxotriphenyltin, hydroxotriphenyllead,42 and triethyltin hydroxide43 (see Supporting Information for distances and angles). The short c axis of 4.1 Å of the basic structure precludes an ordered crystal structure with bond bending at oxygen. A linear arrangement (Sn−O)∞ is highly unstable, and bond bending at oxygen thus is a strong driving force for superstructure formation. The two-fold superstructure at low temperatures is the minimum solution for the superstructure. Through the two-fold screw axis, its symmetry determines that the zigzag configuration of the (Sn−O)∞ polymeric chain exists in a single plane, which can be defined as the plane containing the O atom and the c axis.37 Despite very similar values for all Sn− O−Sn angles, the oxygen atoms within the eight-fold superstructure strongly deviate from a single plane (Figure 5b). The eight-fold superstructure is composed of four crystallographically independent monomers, arranged consecutively on a single chain (Figure 1). The other four monomers are related to the independent atoms through the c glide, while the second chain follows the 21 and n glide operations. For each oxygen atom, we can now define the plane containing this atom and the c axis. Employing the plane containing O1a and the c axis as reference, nonzero interplanar angles ϕn (n = 1, ..., 8) define deviations from the planar zigzag arrangement (Figure 4a). Values for these parameters are collected in Table 2. They confirm the strong deviations from a single plane for the oxygen atoms.

Figure 6. Nonbonded interactions within a single chain: (a) O1a− H1o1a, (b) O1b−H1o1b, (c) O1c−H1o1c, and (d) O 1d−H1o1d with methyl groups. O−H···H−C (dashed blue) and C−H···H−C (dashed gray) shorter than 2.45 Å are shown. Viewing direction along [1̅10]. a1 = H1o1a−H3c2bviii, a2 = H1o1a−H3c1a; b = H1o1b− H2c2c; c1 = H1o1c−H2c3c, c2 = H3c2c−H2c1d; d = H1o1d−H3c1d (see Table 3 and Supporting Information). Symmetry code: (viii) 1 x , − y , z − 2 . Molecular graphics has been prepared with Diamond.24

Table 2. Rotations [deg] of Oxygen Atoms about the c Axis with Respect to Plane through O1a and the c Axisa angle

observed value [deg]

83 screw [deg]

ϕ1 ϕ2 ϕ3 ϕ4 ϕ5 ϕ6 ϕ7 ϕ8

0 147.28 −36.71 162.56 14.79 −132.49 51.50 −147.77

0 135 −90 45 180 −45 90 −135

Table 3. Distances between Hydrogen Atoms Shorter than 2.45 Å for Interchain (Figure 5) and Intrachain Contacts (Figure 6)a reference Figure 5

Figure 6a Figure 6b Figure 6c

See Figure 4 for the definition of angles ϕn (n = 1, ..., 8). The third column gives rotation angles for the case of an 83 screw axis as symmetry of the chain.

a

Figure 6d

e f g h a1 a2 b c1 c2 d

atoms

distance (Å)

C3a−H3c3a···H2c3bv−C3bv C3b−H3c3b···H2c3dv−C3dv C2c−H2c2c···H3c3dv−C3dv C3d−H3c3d···H2c2cii−C2cii O1a−H1o1a···H3c2bviii−C2bviii O1a−H1o1a···H3c1a−C1a O1b−H1o1b···H2c2c−C2c O1c−H1o1c···H2c3c−C3c C2c−H3c2c···H2c1d−C1d O1d−H1o1d···H3c1d−C1d

2.347 2.359 2.374 2.374 2.395 2.440 2.244 2.357 2.183 2.272

a

t-Plots of the distances are given in the Supporting Information. Symmetry code: (ii) x + 1/2, −y + 1/2, −z + 1/2; (v) x − 1/2, −y + 1/2, −z + 1/2; (viii) x, −y, z − 1/2

Anderson et al.26 have proposed that the structure of each chain in their 32-fold superstructure model possesses approximate symmetry according to an 83 screw axis. Symmetry of the present structure model, in particular the c-glide symmetry of single chains, implies that 83 symmetry cannot apply to the structure of the chain in the present model since (83)4 = 21 provides an essentially different relation between oxygen atoms than the c-glide does. For the crystallographically independent atoms, an approximate 83 symmetry is not found either, as is demonstrated by the values in Table 2.

They indicate steric hindrance within the chains (twice the van der Waals radii for hydrogen atom is 2.2−2.4 Å).44 In order to avoid short contacts between hydrogen atoms, the hydroxy groups must displace away from the (CH3)3Sn groups. This displacement is hampered by the bonds between each oxygen atom and its two neighboring tin atoms, whose bonds can only twist and turn. This explains the shift of the oxygens out of the E



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zigzag planes. The angles ∠(Sn−O−Sn) are as large as 143.1° (at T = 100 K, ∠(Sn−O−Sn) = 139.4°), which means decreased bending of these bridges to compensate the frustration arising due to steric hindrance. Distances between oxygen atoms and hydrogen atoms of methyl groups are longer than 3.0 Å (dH···O = 3.15−3.94 Å). Such large distances do not support the presence of interchain C−H···O weak hydrogen bonds (standard values dH···O = 2.0− 3.0 Å).45 The present structural model does not include disorder. Nevertheless, the diffraction patterns at T = 220 K (present work) and at T = 100 K37 do show diffuse scattering in addition to Bragg reflections. Structured diffuse scattering is concentrated in planes (hkl) perpendicular to c* (l = integer). This implies partial pseudo-one-dimensional order, i.e., a decoupling of the relative positions or relative conformations of different chains.

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Phone: +49 921 55 3879. Fax: +49 921 55 3770. ORCID

A. Schönleber: 0000-0003-2516-2332 S. Mondal: 0000-0003-2633-3842 S. van Smaalen: 0000-0001-9645-8240 Present Addresses †

King Abdullah University of Science and Technology (KAUST), Core Labs, Thuwal, 23955-6900, Saudi Arabia. ‡ CSIR-Central Glass and Ceramic Research Institute, Kolkata 700032, India. § Department of Chemistry, Moyna College, Vidyasagar University, Anandapur-721629, India.

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CONCLUSIONS The eight-fold superstructure of trimethyltin hydroxide has been successfully described as a commensurately modulated structure within the (3 + 1)D superspace approach. The description employing a basic-structure unit cell (Z′ = 0.5) and modulation functions defined by crenel functions enabled a stable refinement of the crystal structure. A complete structure model is established with physically meaningful anisotropic ADPs of all the atoms. The corresponding eight-fold superstructure has four independent formula units (Z′ = 4). The (CH3)3Sn groups are bridged by oxygen atoms and form a zigzag arrangement along the chains. The oxygen atoms of adjacent formula units are shifted out of the zigzag planes as compared to the low-temperature phase.37 This shift within the chains is caused by nonbonded interactions between the hydroxy groups and the methyl groups (dO−H···H−C < 2.4 Å). The oxygen atoms are, however, bonded to two tin atoms, which allows only twist and turn of the hydroxy groups. Hence, the eight-fold superstructure is the result of a competition between dense packing within chains and optimal conformation of the hydroxy groups. Interchain steric hindrance (dC−H···H−C < 2.4 Å) prevents formation of interchain C−H···O weak hydrogen bonds.

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Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Carsten Paulmann for assistance in the diffraction experiments at beamline F1 of Hasylab at DESY, Hamburg. The authors thank Karen Friese and Vaclav Petricek for useful discussions. Financial support has been obtained from the German Science Foundation (DFG) under grant number SCHO 830/3-1.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.cgd.7b01295. Choice of superspace group, refinement details, a complete measured frame corresponding to the section given in Figure 2, I/σ(I) versus l-indices in 32-fold superstructure, (3 + 1)-dimensional Fourier maps of the Sn1, O1, C1, and C2,C3 atoms, eight-fold supercell with ellipsoids for ADPs, t-plots of interatomic distance and bond angles and torsion angles, table of comparison of statistical parameters for refinement, and tables of interatomic distances and angles (PDF) Accession Codes

CCDC 1574281 and 1574285 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033. F



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