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Halogen Bonding Motifs in Polyhedral Phenylsilsesquioxanes: Effects of Systematic Variations in Geometry or Substitution Published as part of the Crystal Growth & Design virtual special issue on Halogen Bonding in Crystal Engineering: Fundamentals and Applications. Mark F. Roll,*,† Jeffrey W. Kampf,‡ and Richard M. Laine†,§ †
Macromolecular Science and Engineering, ‡Department of Chemistry, and §Department of Materials Science and Engineering, University of Michigan, Ann Arbor, Michigan 48109-2136, United States
bS Supporting Information ABSTRACT: Herein we describe the halogen bonding coordination found in the crystal structures of two homologous series of hybrid silsesquioxane (Polyhedral Oligomeric SilsesquioxaneS, or POSS) molecules, with systematic variations in halogen substitution and cluster geometry. Each of these compounds presents highly coordinated three-dimensional motifs. The first examples are a series of three para-iodinated octa-, deca-, and dodecameric phenylsilsesquioxanes, which show a range of symmetry dependent coordination motifs. The second set of examples deal with the structures of the octa-, hexadeca-, and tetraicosa-brominated octaphenylsilsesquioxane, where again significant short contacts are found, which drive the formation different cluster packing motifs. These structures provide unique examples of the complexities of three-dimensional packing motifs and their relationship to the assembly of tunable materials from nanobuilding blocks.
’ INTRODUCTION Nano-building blocks, as tools for the production of materials with precisely tunable properties, have been proposed and investigated.18 Such a macromolecule would possess threedimensional symmetry and easily modifiable peripheral functionality to allow the assembly process to be subtly altered as needed.13 We have previously reported the merits of octaphenylsilsesquioxane (OPS)4 for such a role, describing the bromination5,6 and iodination7,8 of OPS in order to provide access to peripheral functionality. Silsesquioxanes are thermally and chemically robust hybrid compounds consisting of an organic component, or radical, covalently bonded to an inorganic silicate backbone. Through carefully developed synthetic routes, closed, polyhedral oligomer (POSS) species may be isolated. In this paper we will consider T8, T10, and T12 species (Figure 1), where “T” refers to the monomeric PhenylSiO3/2 unit. Examples of crystalline bromo-6 and iodo-7,8 derivatives have been presented to demonstrate the ability to purify these compounds by recrystallization to provide well-ordered precursors for successive synthetic modifications.9 Now we turn our attention to an investigation of the crystalline ordering of these molecules in the solid state. The intermolecular coordination of molecular crystals is a topic of great current interest.10,11 Desiraju’s review10 of supermolecular coordination motifs, or synthons, now has >2300 citations. Some synthon examples include hydrogen bonding, r 2011 American Chemical Society
halogen 3 3 3 halogen and iodine 3 3 3 nitro short contacts, and π 3 3 3 π stacking.1014 Recently, the topic of “halogen bonding” was the focus of a symposium at the Fall 2009 National ACS Meeting in Washington DC. and this provides further impetus for studying the supermolecular coordination in halogenated polyhedral phenylsilsesquioxanes.15 Desiraju defines crystal engineering as “the understanding of intermolecular interactions in the context of crystal packing and the utilization of such understanding in the design of new solids with desired physical and chemical properties.”11 This is the same fundamental goal for nano-building blocks, so it is natural to explore them simultaneously. The study of the halogen 3 3 3 halogen intermolecular interactions has been well outlined and studied by several groups,1013,1624 so our work begins with Desiraju’s suggested “study of the packing modes, in the context of these interactions...”11 We intend these studies to set the stage for the design of tailored materials. With these structureproperty relationships, open structures for infiltration, sequestration, or “guesthost” materials can be targeted. In addition, tighter intermolecular packing of electron rich molecules might be useful for the production of high dielectric materials. Received: March 13, 2011 Revised: August 8, 2011 Published: August 10, 2011 4360
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Figure 1. Closed polyhedral silsesquioxane oligomer (POSS) units and the PhSiO3/2 monomer.
Figure 2. Type I (A), Type II (B), and X3 (C) halogen 3 3 3 halogen short contacts.
We have two distinct sets of halogenated polyhedral phenylsilsesquioxanes to examine: Three iodinated macromolecules with identical functionality and variations in core geometry and three brominated macromolecules with identical core geometry and variations in functionality. We initially hypothesize that the combination of halogen bonding forces and the unique 3-D structure of polyhedral silsesquioxanes will present complex coordination patterns, with a large number of short contacts per molecule. For the iodinated series, we hypothesize that the cubic system will pack in a straightforward manner with halogen bonding interactions on the corners of the molecule to give an open coordination structure. In contrast, the decahedral and dodecahedral systems will have more complicated organization, a larger number of short contacts per molecule, and a more tightly packed coordination structure. For the brominated series, we hypothesize that, given the same core geometry, as the degree of subsitution increases, the number of intermolecular short contacts will increase, and the coordination structures will increase in density. In particular, the synthons found in halogen substituted aromatic molecules have been a subject of much investigation. There are three 2-D halogen 3 3 3 halogen synthons: Type I, Type II, and X3, as seen in Figure 2.10,11,13,16,17 These different modes are characterized by geometries derived from crystal structure data. Type I contacts typically have a “slipped-stack” orientation; Type II contacts have nearly perpendicular orientation, and X3 contacts offer a trimeric, triangular motif.10,11,13,16,17 Recent theoretical analysis was done with respect to the underlying physical phenomena.16,19 Calculations by Bosch, shown in Figure 3, describe the permanent polarization of iodobenzene.19 Thus, the motifs seen in Figure 2 can be described as intermolecular, quasi-electrostatic interactions. Each synthon has a characteristic set of short contacts, often defined as an interatomic
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Figure 3. (A) Calculated electrostatic potentials for iodobenzene. (B) General schematic model of halobenzene molecule.19
distance less than the sum of the van der Waals (vdW) radii. In general terms, these halogen bonding forces are considered to be 2σ(I)]
7.4% (9.7%)
4.6% (6.6%)
9.2%
solvent occupied volume % (#)27
39% (1)
24% (4)
30% (2)
Before use of SQUEEZE27 to account for disordered solvent.
Table 2. Crystal Structure Data for Octa-, Hexadeca-, and Tetraicosa-Brominated OPS [o-Br-C6H4SiO1.5]86
a
[2,6-Br2-C6H3SiO1.5]86
[Br3C6H2SiO1.5]86
included solvent
m-xylene
carbon disulfide
o-dichlorobenzene
space group unit cell dimensions
I4(1)/a, tetragonal a = 20.5738(10) Å
P1, triclinic a = 12.7172(9) Å
P1, triclinic a = 15.5726(10) Å
b = 20.5738(10) Å
b = 12.7392(9) Å
b = 20.4685(13) Å
c = 36.261(4) Å
c = 13.0890(9) Å
c = 20.6449(13) Å
α = 90°
α = 97.592(1)°
α = 65.3990(10)°
β = 90°
β = 109.075(1)°
β = 74.6160(10)°
γ = 90°
γ = 116.231(1)°
γ = 82.1720(10)°
unit cell volume
15348.5(19) Å3
1699.3(2) Å3
5766.3(6)Å3
Z, density R-factor, [I > 2σ(I)]
8, 1.671 mg/m3 5.3% (5.3%)a
1, 2.318 mg/m3 2.7%
2, 2.007 mg/m3 6.3% (12.7%)a
solvent occupied volume % (#)27
31% (1)
7% (1)
41% (1)
Before use of SQUEEZE27 to account for disordered solvent.
Figure 4. Four coplanar I8OPS molecules coordinated via I 3 3 3 I Type II short contacts (distances in Å).
The crystal structures of the iodinated octaphenylsilsesquioxane show examples of I 3 3 3 I motifs, as well as C 3 3 3 I short contacts. Recently, Desiraju, et al. described I 3 3 3 I short contacts in the I3 motif of 3.753.79 Å (∼8% below van der Waals sum),24 and a tetrahedral I4 synthon in tetra(p-iodophenyl)methane with I 3 3 3 I short contacts of 3.949 and 4.161 Å.17 Figure 4 shows two
pairs of Type II contacts (3.80 Å) found in the I8OPS structure. Though there are three coordinating iodines, the coordination angles C16I3I1 = 90.6° and C4I1I3 = 163.5° are best described by two Type II contacts, not an I3 motif, as in the phosphazene structure of Jung et al. (Figure S.1 of the Supporting Information).28 In Figure 5, the combination of four one-dimensional ArC 3 3 3 I short contacts in I8OPS forms a square pore between adjacent molecules. A similar square motif is found in the crystal structure of diiodoacetylene, with slightly larger coordination distances (Figure S.2 of the Supporting Information).29 Desiraju’s analysis of the (4iodophenyl)triphenylmethane structure noted a ArC 3 3 3 I distance of 3.75 Å (Figure S.3 of the Supporting Information).17 In contrast, in I8OPS the analogous ArC 3 3 3 I distance is 3.395 Å, almost 10% shorter. The short ArC 3 3 3 I contact in I8OPS is a result of three simultaneous short contacts of I3 with C15 and C16. The combination of 3-D symmetry and multifunctionality in I8OPS multiplies the degree of supermolecular coordination, allowing for possible “guest/host” materials. The calculated density of the tetra(p-iodophenyl)methane is 2.2 g/cm3, while the density of I8OPS is 1.64 g/cm3. In fact, the density of the (4-iodophenyl)triphenylmethane is 1.55 g/cm3, even through it is primarily hydrocarbon. The PLATON software suite was used 4362
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Figure 6. C 3 3 3 I and C 3 3 3 C short contacts found in I10dPS (Å). Figure 5. Square ArC 3 3 3 I coordination motif as seen in I8OPS.
Table 3. Partial Listing of Short Contacts Found in p-Iodinated Polyhedral Phenylsilsesquioxanes atom 1
atom 2
distance (Å)
% under
no. per
vdW sum
molecule
3.99%
8
I8OPS I3
I1
3.802
Type II: C16I3I1 = 90.6°; C4I1I3 = 163.5° I3 I3
C15 C16
3.55 3.349
3.53% 8.99%
8 4
I10dPS C15
C21
3.296
3.06%
4
C29
I3
3.408
7.27%
4
C28
I3
3.607
1.95%
4
C9
I4
3.578
2.73%
4
C10
I4
3.54
3.74%
4
O3
I1
3.474
0.73%
4
H3A H2A
C1 C4
2.755 2.791
5.00% 3.76%
2 2
I1
I4
3.682
7.0%
4
I12DPS Type II: C22I4I1 = 99.5°; C4I1I4 = 172.8° I4
Si2
4.063
0.4%
4
to calculate the solvent accessible voids in the unit cell, giving one void comprising 39% of the unit cell volume. This interconnected void structure is visualized in Figure S.4 of the Supporting Information and indeed appears to be a possible host for guest/host materials. This initial discussion highlights one difference seen in the silsesquioxane structures, the presence of multiple, unique motifs along different crystallographic directions. Thus, I8OPS exhibits supermolecular synthons analogous to published examples, with more significant short contacts, indicating stronger intermolecular interaction. At the end of this section, Table 3 lists numerical details for short contacts discussed in this section, which range from 4 to 9% less than the sum of the van der Waals radii of the two atoms. A complete listing of short contacts in the I8OPS structure is given in Table S.1 in the Supporting Information.
Figure 7. I 3 3 3 O and C 3 3 3 H interactions found in I10dPS (Å).
Deca(p-iodophenyl)silsesquioxane: [p-I-C6H4SiO1.5]10 (I10dPS). In [p-I-C6H4SiO1.5]10 (I10dPS) there are four voids
comprising 24% of the unit cell, as calculated by PLATON,27 occupied by disordered ethyl acetate solvents. In this case, it appears the voids are held open by C 3 3 3 I (coordination of aromatic iodides and aromatic carbons), I 3 3 3 O (coordination of aromatic iodides and cage oxygens), H 3 3 3 H, and C 3 3 3 H (edgeface interactions between aromatic moieties) short contacts. Figure 6 shows that C 3 3 3 C and C 3 3 3 I short contacts including four distinct C 3 3 3 I short contacts (C10 3 3 3 I4, C9 3 3 3 I4, C28 3 3 3 I3, C29 3 3 3 I3) are found, ranging from 3.4 to 3.6 Å. The ArC 3 3 3 I distances for I3 and I4 are 3.80 and 3.58 Å, respectively (Figure S.5); again similar to those seen in the (4-iodophenyl)triphenylmethane system. Figure 7 shows the coordination of aromatic iodines to oxygens in the silsesquioxane cage. Short contacts have been reported in halogen bonding thyroid protein studies and are consistent with the halogen bonding concept developed by Metrangolo et al.23 An additional coordination feature seen in I10dPS is C 3 3 3 H, edge-face short contacts between adjacent aromatic rings, which partially occupy two symmetric orientations (Figure 7). As recently discussed by Desiraju, an aromatic C 3 3 3 H short contact (as seen in phase I of benzene in Figure S.6 of the Supporting Information)17 is shorter than 2.9 Å. The crystal structure of I10dPS demonstrates that the rigid silsesquioxane core appears to dictate which short contacts actively drive the intermolecular coordination in the crystal structure. As described in Table 1 and visulaized in Figure S.7 of the Supporting Information, there are a total of four solvent accessible and occupied void channels in the I10dPS structure, which comprise 24% of the total unit cell volume. This lower 4363
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Table 4. Partial Listing of Short Contacts in Brominated OPS Derivatives atom 1
atom 2
length (Å)
Br3 Br3
Br3 Br30
3.544 3.758
% under
no. per
VdW sum
molecule
4.2% 1.57%
1 4
o-Br8OPS
Br4 synthon, tetrahedral coordination Br16OPS Br5
Br8
3.455
6.6%
4
Type II: Br8Br5C14 = 108.9°; C23Br8Br5 = 158.5° Br4
Figure 8. I12DPS molecules coordinated via I 3 3 3 I and Si 3 3 3 I short contacts.
volume unit cell implies that this compound would be less suitable for guest/host materials than I8OPS. At the end of this section, Table 3 lists the numerical details for the discussed short contacts found in the I10dPS structure. A complete listing of the short contacts is given in Table S.2 of the Supporting Information. While I8OPS exhibits significant I 3 3 3 I coordination, I10dPS shows none. Both structures exhibit C 3 3 3 I halogen bonding, with ArC 3 3 3 I distances similar to or below those reported for I8OPS. Finally, C 3 3 3 C, C 3 3 3 H, and H 3 3 3 H short contacts are seen in the I10dPS though I8OPS shows none. By changing the geometry of the core, we see activation and deactivation of different coordination modes or synthons. Dodeca(p-iodophenyl)silsesquioxane: [p-I-C6H4SiO1.5]12 (I12DPS). We expect and find a combination of new coordination motifs in addition to the previously described examples in [p-IC6H4SiO1.5]12 (I12DPS) based on the discussion of the I8OPS and I10dPS structures. In the crystal structure of I12DPS, m-xylene solvent molecules occupy two voids comprising 30% of the unit cell, as calculated by PLATON.27 Figure 8 shows a Type II I 3 3 3 I short contact (C22I4I1 = 99.5°; C4I1I4 = 172.8°; 7% less than the van der Waals sum), as found in the I8OPS structure. However, a new type of short contact is also seen, a minimal (
