II. EXPERIMENTAL

B. X-ray powder diffraction

PXRD data of compounds 1 and 2 were recorded at ambient temperature [293(2) K] using a Bruker D8 Advance diffractometer operating in the Bragg–Brentano geometry with CuK _α radiation (λ = 1.5418 Å). Indexing of PXRD patterns using EXPO 2014 (Altomare et al., Reference Altomare, Cuocci, Giacovazzo, Moliterni, Rizzi, Corriero and Falcicchio2013) showed an orthorhombic (in 1) and a monoclinic (in 2) unit cells. Given the volume of unit cells and a consideration of expected density of these compounds, the number of formula units in the unit cell of 1 and 2 turned out as 8 and 4, respectively. Although the correct assignment of space group from the systematic absences of reflections in the PXRD patterns is not often obvious, a statistical analysis of PXRD data using the FINDSPACE module of EXPO 2014 (Altomare et al., Reference Altomare, Cuocci, Giacovazzo, Moliterni, Rizzi, Corriero and Falcicchio2013) indicated Pbca and P2₁/c as the most likely space groups for 1 and 2, respectively, which were chosen for structure solution. The structures of 1 and 2 were solved by global optimization of structural models in direct space based on a Monte Carlo search using the simulated annealing technique (in parallel tempering mode) as implemented in the program FOX (Favre-Nicolin and Cerny, Reference Favre-Nicolin and Cerny2002). The optimization of isolated molecules was performed using the energy gradient method as incorporated in MOPAC 9.0 (Stewart, Reference Stewart2007).

The best solution (i.e., structure with the lowest R _wp) was used as the initial structural model of 1 and 2 for Rietveld refinement (Rietveld, Reference Rietveld1967), which was carried out using the GSAS program (Larson and Von Dreele, Reference Larson and Von Dreele2000). A pseudo-Voigt peak profile function was used during refinement and the background of the PXRD patterns was modeled by a Shifted Chebyshev function of first kind with 16 points in 1 and 2, regularly distributed over the entire 2θ range. Standard restraints were applied to bond lengths and bond angles, and planar restraint was used for the phenyl ring in 2. The standard restraints on the bond distances and bond angles of 1 and 2 are listed in Table SI. Common isotropic displacement parameters were refined for carbon, oxygen, sulfur atoms (in 1), and silicon, oxygen, carbon atoms of methyl and phenyl groups, and spiro-fused rings (in 2). Hydrogen atoms in the molecules of 1 and 2 were placed in calculated positions with a fixed U _iso (0.07 Å² in 1 and 0.1 Å² in 2) value. In the final stage of refinement, a preferred orientation correction using the generalized spherical harmonic model was applied. The final Rietveld plots of 1 and 2 (Figure 1) showed good agreement between the observed PXRD profiles and calculated powder patterns. A summary of crystal data and relevant refinement parameters for 1 and 2 is listed in Table I.

Figure 1.

Final Rietveld plot for (a) C₁₅H₂₄O₆S₂ (1), (b) C₁₈H₂₂O₂Si (2). The intensity in the high-angle region has been multiplied by a factor of 5.

Table I.

Crystal data and structure refinement parameters of C₁₅H₂₄O₆S₂ (1) and C₁₈H₂₂O₂Si (2).

C. Hirshfeld surface analysis

The Hirshfeld surfaces (Hirshfeld, Reference Hirshfeld1977; Spackman and Jayatilaka, Reference Spackman and Jayatilaka2009) and associated two-dimensional (2D) fingerprint plots (Spackman and Mckinnon, Reference Spackman and McKinnon2002; Parkin et al., Reference Parkin, Barr, Dong, Gilmore, Jayatilaka, McKinnon, Spackman and Wilson2007) were generated using the Crystal Explorer 3.1 program (Wolff et al., Reference Wolff, Grimwood, McKinnon, Turner, Jayatilaka and Spackman2012). For each point on the Hirshfeld isosurface, two distances d _e, the distance from the point to the nearest nucleus external to the surface, and d _i, the distance to the nearest nucleus internal to the surface, are defined. The normalized contact distance (d _norm) based on d _e and d _i is given by

(1) $$d_{{\rm norm}} = \displaystyle{{d_i - r_i^{{\rm vdw}}} \over {r_i^{{\rm vdw}}}} + \displaystyle{{d_{\rm e} - r_{\rm e}^{{\rm vdw}}} \over {r_{\rm e}^{{\rm vdw}}}}, $$

where r _i ^vdW and r _e ^vdW are the van der Waals radii of the particular atoms involved in the close contact to the surface. The value of d _norm is negative or positive depending on the intermolecular contacts being shorter or longer than the van der Waals separations. The parameter d _norm displays a surface with a red–white–blue color scheme, where the bright red spots highlight shorter contacts, the white areas represent contacts around the van der Waals separation, and the blue regions are devoid of close contacts.

III. RESULT AND DISCUSSION

The molecular views of 1 and 2 with atom labeling scheme are shown in Figure 2. The molecule 1 consists of a cis-fused 6, 5 bicyclic ring (C1–C9 atoms) with the C12–C1–C2–H2 torsion angle of 41°, while that in 2 comprises a 5, 6, 5 tricyclic ring skeleton with the norbornene fragment (C1–C7 atoms) spiro-fused with the tetrahydrofuran (THF) moiety (C7–C10, O1 atoms). The ring puckering parameters (Cremer and Pople, Reference Cremer and Pople1975) for the five-membered ring containing atom C9 in 1 (Table II) indicate a half-chair conformation. The six-membered cyclohexene ring (C1–C6 atoms) in 1 adopts a half-chair conformation with the methylmethanesulfonate group at C6 in an equatorial position. The torsion angle C12–C1–C6–C14 of –58.6(6)° in 1 indicates a syn-clinal orientation of two methanesulfonate groups about the C1–C6 bond. Similarly, the ring puckering parameters (Table II) of the 6,5 bridged norbornene system (C1–C7 atoms) in 2 show conformation close to a boat for the cyclohexene moiety (C1–C4, C6 and C7 atoms) and an envelope for the THF ring (C7–C10 and O1 atoms). An extended molecular conformation in 2 is established by torsion angles C7–C4–C5–Si of 176.6(2)° and C4–C5–Si–C13 of −175.9(2)°.

Figure 2.

Molecular views of (a) C₁₅H₂₄O₆S₂ (1), (b) C₁₈H₂₂O₂Si (2) with the atom-labeling scheme.

Table II.

Ring-puckering parameters (Å, °) of C₁₅H₂₄O₆S₂ (1) and C₁₈H₂₂O₂Si (2).

The geometrical parameters (Table III) of molecules with S = O bond distances of 1.414(13)–1.432(12) Å in 1 and Si–C bond lengths of 1.863(3)–1.885(4) Å in 2 agree well with the corresponding mean values (S = O: 1.426 Å for 791 structures and Si–C: 1.864 Å for 277 structures) obtained with MOGUL (Bruno et al., Reference Bruno, Cole, Kessler, Luo, Motherwell, Purkis, Smith, Taylor, Cooper, Harris and Orpen2004) from the CSD search (version 5.37, November 2015 release). An overlay of molecular conformations as determined by the X-ray analysis and solid state DFT calculations of 1 and 2 is shown in Figure 3. The rms deviations between the geometrically optimized bond lengths, bond angles (Table III), and the corresponding crystallographically determined values are 0.04 Å, 2.9° in 1 and 0.03 Å, 2.6° in 2; the corresponding rms Cartesian displacements for 1 and 2 are 0.33 and 0.20 Å, respectively. Close agreement between the X-ray analyzed structure and that obtained via quantum mechanical calculations probably indicates that the compounds 1 and 2 are stable conformers.

Figure 3.

Superposition of molecular conformations as obtained from the X-ray structure analysis (green) and solid state DFT calculation (yellow) for (a) C₁₅H₂₄O₆S₂ (1) and (b) C₁₈H₂₂O₂Si (2).

Table III.

Selected bond lengths (Å) and bond angles (°) of C₁₅H₂₄O₆S₂ (1) and C₁₈H₂₂O₂Si (2) as determined from X-ray structure analysis and DFT calculations.

In the absence of any strong hydrogen bond donating group such as –OH or –NH, weak intermolecular C–H ⋯ O hydrogen bonds influence the packing of molecules of 1 and 2 in the crystalline state. Intermolecular C15–H15C ⋯ O6 [C ⋯ O 3.122(12) Å, H ⋯ O 2.17 Å, C–H ⋯ O 169°] hydrogen bonds connect molecules in 1 forming a polymeric C² ₂(8) chain propagating along the [100]-direction. Further linking of molecules via C15–H15A ⋯ O3 [C ⋯ O 3.363(14) Å, H ⋯ O 2.50 Å, C–H ⋯ O 150°] hydrogen bonds generate a 1D strip built with edge fused R ² ₂(15) rings (Figure 4). The parallel strips in 1 are joined by C13–H13B ⋯ O4 [C ⋯ O 3.518(13) Å, H ⋯ O 2.56 Å, C–H ⋯ O 169°] hydrogen bonds to produce a 2D framework in the (010) plane (Figure 4). In 2, pairs of C8–H8B ⋯ O2 [C ⋯ O 3.578(11) Å, H ⋯ O 2.63 Å, C–H ⋯ O 169°] hydrogen bonds join centrosymmetric molecules into dimers with an R ² ₂(8) synthon. The propagation of R ² ₂(8) rings via C17–H17 ⋯ π (C13–C18) interactions results in a 2D architecture parallel to (102) plane (Figure 5).

Figure 4.

Two-dimensional framework in the (010) plane formed by the C–H ⋯ O hydrogen bonds in C₁₅H₂₄O₆S₂ (1).

Figure 5.

Two-dimensional architecture in the (102) plane formed by the C–H ⋯ O hydrogen bonds and C–H ⋯ π interactions in C₁₈H₂₂O₂Si (2).

The Hirshfeld surfaces of 1 and 2 mapped over a d _norm range of −0.5 to 1.5 Å are illustrated in Figure 6(a) and (c). The red spots labeled as ‘a/a′’, ‘b’ and ‘c′’ in Figure 6(a) and (c) are because of the C–H ⋯ O interactions in 1 and 2. Other red patches in Figure 6(a) correspond to the H ⋯ H interactions in 1. In the 2D fingerprint plot, two sharp spikes labeled as a/a′ in Figure 6(b) in the region of 2.1 < d _e + d _i < 2.4 Å are characteristics of C15–H15C ⋯ O6 bonded C² ₂(8) chain in 1. Spikes corresponding to relatively weaker hydrogen bond (C15–H15A ⋯ O3) are masked within (a/a′). Spikes labeled as a/a′ in Figure 6(d) in the region of 2.5 < d _e + d _i < 2.85 Å are characteristics of C8–H8B ⋯ O2 bonded R ² ₂(8) ring in 2. The wings marked with black circles in Figure 6(d) are because of C–H ⋯ π interactions in 2. The central spikes [Figure 6(b) and (d)] extending upto (d _i, d _e) region of (0.9 Å, 0.9 Å) in 1 and (1.2 Å, 1.2 Å) in 2 reveal relatively high percentage of H ⋯ H contacts in both cases. The relative contribution of different interactions to the Hirshfeld surfaces of 1, 2 and a few fused 6, 5 cyclic systems retrieved from the CSD are shown in Figure 7. It is evident from Figure 7 that H ⋯ H and O ⋯ H interactions can account for about 85% of the Hirshfeld surface area, 85.7% in 1 and 83.7% in 2, whereas the corresponding values for 3′-(2-hydroxypropyl)-5-methyldihydro-4′H-spiro[furan-3,2′-[5]oxatricyclo[4.2.1.0^3,7]nonane]-2,4′-dione (OTAKOK) (Kotha et al., Reference Kotha, Dipak and Mobin2011) and 3′-isopropyl-7a′-methylhexahydrospiro[[1,3]dioxolane-2,5′-inden]-1′(6′H)-one (XIMJEK) (Hog et al., Reference Hog, Mayer and Trauner2012) are 98.4 and 99.7%, respectively. An approximately twofold decrease in the contribution of O ⋯ H contacts from 33.3% in 1 to 18.7% in 2 is a consequence of reduced percentage of oxygen atoms in the molecular formula of 2 (4.4%) in comparison with that in 1 (12.8%).

Figure 6.

Hirshfeld surface and 2D fingerprint plot of (a) C₁₅H₂₄O₆S₂ (1), (b) C₁₈H₂₂O₂Si (2).

Figure 7.

Relative contribution of different interactions to the Hirshfeld surfaces of C₁₅H₂₄O₆S₂ (1), C₁₈H₂₂O₂Si (2) and a few related structures from the CSD.