Crystal Structures of Spiroborates Derived from 2,2 - ACS Publications

In crystals of certain salts (NH4+, NH2Me2+, NMe4+, PPh4+, and MBA+), the packing of spiroborate 4 can be considered to define homochiral layers; in o...
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CRYSTAL GROWTH & DESIGN

Crystal Structures of Spiroborates Derived from 2,2′-Dihydroxybiphenyl

2008 VOL. 8, NO. 1 308–318

Emilie Voisin, Thierry Maris, and James D. Wuest* Département de Chimie, UniVersité de Montréal, Montréal, Québec H3C 3J7 Canada ReceiVed August 24, 2007

ABSTRACT: Salts of bis[[1,1′-biphenyl]-2,2′-diolato-O,O′] borate (4) were prepared by the reaction of boric acid with 2,2′-dihydroxybiphenyl in the presence of various cations. Ten salts were crystallized, and their structures were solved by X-ray diffraction. In all cases examined, spiroborate 4 favors enantiomeric (R,R) and (S,S) structures with a flattened D2 geometry. Moreover, the crystals analyzed all incorporate the two enantiomers in a 1:1 ratio, even when the cations themselves are chiral and enantiomerically pure. In crystals of certain salts (NH4+, NH2Me2+, NMe4+, PPh4+, and MBA+), the packing of spiroborate 4 can be considered to define homochiral layers; in other salts, however, the organization of the anions is helical (NMe4+), columnar (NPr4+), or can be described as either layered or columnar (NBu4+ and NPent4+). These variations presumably arise because neighboring anions are not positioned reliably by strong directional interactions between them. Derivatives of spiroborate 4 that can engage in such interactions are expected to have well-defined molecular geometries and to serve as effective subunits for engineering crystals and other ordered materials with predetermined structures and properties. Introduction Despite decades of research in crystal engineering, the structures of molecular crystals cannot in general be predicted reliably.1 This is in part because (1) complex molecules can often adopt multiple conformations that have similar energies; and (2) many intermolecular interactions are weak and nondirectional, making them incapable of forcing neighboring molecules to assume particular orientations. However, when molecules have well-defined geometries and engage in multiple interactions that are strong and directional, it is frequently possible to foresee how they will crystallize. Such molecules, which have been called tectons,2 have proven to be effective for the purposeful construction of crystals and other ordered materials with particular architectures and properties.3,4 An attractive strategy for creating tectons is composed of two steps: (1) Selecting sticky sites that regulate intermolecular association according to reliable patterns, which have been called supramolecular synthons;5 and (2) grafting these sites to core structures that orient them properly and simultaneously introduce other desirable properties. Cores that can be varied methodically without changing certain common features are particularly appealing, because they allow the potential of tecton-based crystal engineering to be explored systematically. For example, structures 1a-d6 and rigidified analogues 2a-d7 show how sticky sites (represented by •) can be oriented by cores that share a nominally tetrahedral geometry, yet have different sizes and charges. Identifying promising new families of cores is therefore an important component of the strategy of building ordered materials from tectons. Here we report an initial exploration of the potential of tectons 3a-d with cores derived from 2,2′-dihydroxybiphenyl.8–15 Because derivatives of 2,2′-dihydroxybiphenyl favor nonplanar conformations, such tectons can in principle adopt structures that are either achiral ((R,S)-3) or chiral ((S,S)-3 or its enantiomer). Surprisingly, no structural studies of the unsubstituted cores of tectons 3a-d have ever been reported.14–16 In this paper, we describe the structures of * Author to whom correspondence [email protected].

may

be

addressed.

E-mail:

a series of salts of simple spiroborate 4, thereby creating a foundation for more ambitious studies of tectons 3a-d and related compounds.17

Results and Discussion Synthesis and Crystallization of Salts of Spiroborate 4. As outlined in Table 1, salts of spiroborate 4 could be prepared from boric acid and 2,2′-dihydroxybiphenyl in good yield by very simple one-step procedures based on methods previously reported.8 All 10 salts in Table 1 were crystallized, and their structures were determined by X-ray diffraction. Structure of Spiroborate Salt 4 · NH4+. Exposure of a solution of salt 4 · NH4+ in dimethylformamide (DMF) to vapors of CHCl3 induced the formation of crystals of composition 4 · NH4+ · 0.5 DMF · 0.5 CHCl3.18 These crystals proved to belong to the monoclinic space group P2/n. In this structure, spiroborate

10.1021/cg700803k CCC: $40.75  2008 American Chemical Society Published on Web 12/07/2007

Crystal Structures of Spiroborates

Crystal Growth & Design, Vol. 8, No. 1, 2008 309 Table 1. Synthesis of Salts of Spiroborate 4

entry 1 2 3 4 5 6 7 8 9 10

conditions B(OH)3, B(OH)3, B(OH)3, B(OH)3, B(OH)3, B(OH)3, B(OH)3, B(OH)3, B(OH)3, 18 h B(OH)3,

product

yield (%)

NH4OH, methanol/water, 1 h DMF, reflux 24 h NMe4OH, DMF, reflux 24 h NEt4OH, DMF, reflux 24 h NPr4OH, DMF, reflux 24 h NBu4OH, DMF, reflux 24 h NPent4OH, DMF, reflux 24 h PPh4Br, DMF, reflux 24 h (S)-(-)-R-methylbenzylamine, methanol, room temperature,

4 · NH4+ 4 · NH2Me2+ 4 · NMe4+ 4 · NEt4+ 4 · NPr4+ 4 · NBu4+ 4 · NPent4+ 4 · PPh4+ 4 · (S)-(-)-MBA+

70 79 80 87 61 60 74 64 74

(()-R-methylbenzylamine, methanol, room temperature, 18 h

4 · (()-MBA+

93

4 exists exclusively as its chiral atropisomer, and a 1:1 mixture of enantiomers (S,S)-4 and (R,R)-4 is present. To distinguish these enantiomers in views of the structures of salts of spiroborate 4, they are shown in red and blue, respectively. Our observations are consistent with calculations at the B3LYP/631G* level, which predict that the chiral isomer of spiroborate 4 is more stable than the achiral isomer by 0.45 kcal/mol.10,11 The results are also in agreement with what is known as Wallach’s rule, which notes the tendency of racemic mixtures to crystallize as racemic crystals instead of as conglomerates consisting of crystals composed of a single enantiomer.19 Detailed studies of compounds related to spiroborate 4 suggest that interconversion of the various chiral and achiral forms should be fast under the conditions of synthesis and crystallization.20

Views of the structure of salt 4 · NH4+ appear in Figures 1–4, and crystallographic details are provided in Tables 2 and 3. The unit cell contains spiroborate 4 in two slightly different conformations, only one of which will be described in detail.

Figure 1 shows one of the conformers of the (S,S)-enantiomer of spiroborate 4 as viewed along each of three axes of C2 symmetry, thereby revealing that a flattened geometry of D2 symmetry is adopted. In the preferred geometry, the two aryl-aryl bonds are approximately parallel, as predicted by previous calculations.11 The interaryl angle between the average planes of the phenyl groups is approximately 40° (Table 3), which is a normal value for derivatives of biphenyl. As a consequence of the characteristic geometry of the spiroborate core, the CPh · · · Bcore · · · CPh angles defined by the central boron atom of the core (Bcore) and the 5, 5′, 5″, and 5′″ positions of the phenyl groups (CPh) range from 68.55(15)° to 171.35(15)°. If the core of hypothetical tecton 3c were to adopt a similar structure, its sticky sites would therefore be expected to deviate very substantially from a tetrahedral orientation, whereas the corresponding angles in tectons 1a-d and 2a-d should be close to the ideal tetrahedral value. Close examination of the central B(OC)4 unit in spiroborate salt 4 · NH4+ reveals that the average B-O bond length (1.467(3) Å) has a value similar to those found in salts of tetramethoxyborate21 and tetraphenoxyborate.22 In these acyclic models, the B(OC)4 unit is elongated tetragonally, and the unit taken in isolation has approximate D2d symmetry. The B(OC)4 unit of spiroborate 4 is similarly distorted, with two O-B-O angles that are much smaller (102.01(10)°) than the average of the other four (113.32(8)°). Again, the unit has approximate D2d symmetry, and four O-B-O-C torsion angles have values in the range 164.1(2)-166.0(2)°. Theoretical and experimental analy-

Figure 1. Representations of the structure of crystals of spiroborate 4 · NH4+ grown from CHCl3/DMF, showing one of the conformers of the (S,S)-enantiomer of spiroborate 4 as viewed along each of three axes of C2 symmetry. For simplification, all atoms are drawn in red, and included cations and neutral guests are omitted for clarity.

310 Crystal Growth & Design, Vol. 8, No. 1, 2008

Voisin et al.

Figure 2. Representations of the structure of crystals of spiroborate 4 · NH4+ grown from CHCl3/DMF. (a) View along the b-axis showing alternating layers composed of a single enantiomer. To simplify interpretation, (S,S)-enantiomers are drawn with all atoms in red and (R,R)-enantiomers with all atoms in blue. Between anionic layers composed of a single enantiomer of spiroborate 4 are spaces for incorporating the counterions. Molecules of solvent are omitted for clarity, and the cations are shown with hydrogen atoms in white and nitrogen atoms in blue. (b) Similar view with the atoms of spiroborate 4 shown as spheres of van der Waals radii to reveal the cross sections of the channels. Cations and neutral guests are omitted for clarity.

Figure 3. Views of specific interactions between anions in crystals of spiroborate 4 · NH4+ grown from CHCl3/DMF. The interactions are represented by broken lines. (a) Edge-to-face aromatic interactions and C-H · · · O interactions define chains composed of a single enantiomer of spiroborate 4 (shown here as the (S,S)-enantiomer in red). (b) Edge-to-face aromatic interactions between adjacent layers of the (S,S)-enantiomer (red) and the (R,R)-enantiomer (blue).

ses of related compounds that incorporate the units M(OH)4 or M(OC)4 (M ) C, Si, or B-) have revealed similar features,23 suggesting that hypothetical tectons 3a-d can be expected to have related molecular structures. The crystal structure of salt 4 · NH4+ can be described in various ways. To facilitate comparison with other salts, we view the structure of salt 4 · NH4+ as consisting of corrugated layers of anion 4 that lie parallel to the bc-plane (Figure 2). Each layer is composed of a single enantiomer, and the adjacent layers contain the other enantiomer. The homochiral layers can be considered to be built from homochiral chains of anions linked by edge-to-face aromatic interactions (3.09(1) and 3.10(1) Å) and C-H · · · O interactions (2.58(1) and 2.86(1) Å) (Figure 3a).24 Chains composed of a single enantiomer form the layers by packing closely according to van der Waals interactions, whereas neighboring chains of opposite configuration in adjacent layers participate in edge-to-face aromatic interactions (2.94(1) and

2.95(1) Å) (Figure 3b). The B · · · B distance between the closest anions in a single homochiral chain is 12.030(4) Å, whereas the shortest distance between anions in the same homochiral layer (but not in the same chain) is 7.745(4) Å and that between two anions of opposite configuration in adjacent layers is 6.652(4) Å. The resulting anionic network is open and defines parallel channels aligned with the b-axis. The channels have a cross section 3.9 × 6.4 Å2 and include the NH4+ counterions and neutral guests (Figure 2b).25 Approximately 27% of the volume of the crystals is accessible to the counterions (10%) and neutral guests (17%), as estimated by standard methods.26,27 The neutral guests are disordered, but the cations are held in position by three N-H · · · O hydrogen bonds with nearby spiroborate anions, one (R,R) (H · · · O distance of 2.02(1) Å) and the other (S,S) (H · · · O distances of 2.20(1) and 2.15(2) Å) (Figure 4). These hydrogen bonds link alternating (R,R)- and (S,S)-enantiomers

Crystal Structures of Spiroborates

Crystal Growth & Design, Vol. 8, No. 1, 2008 311

Figure 4. Views of the structure of crystals of spiroborate 4 · NH4+ grown from CHCl3/DMF, with hydrogen bonds between the anions and cations represented by broken lines. The (S,S)-enantiomer of spiroborate 4 is drawn in red and the (R,R)-enantiomer in blue. Only one part of the disordered cations is shown. The cations are drawn with hydrogen atoms in white and nitrogen atoms in blue.

of spiroborate 4 into zigzag chains aligned with the a-axis and thereby ensure that each homochiral layer adheres strongly to the two adjacent layers of opposite configuration. Structure of Spiroborate Salt 4 · NH2Me2+. Slow cooling of a hot solution of salt 4 · NH2Me2+ in DMF produced crystals of composition 4 · NH2Me2+ · 1 DMF (Table 2).18 The crystals proved to belong to the monoclinic space group P21/c and to have a structure closely similar to that of the NH4+ salt. Again, spiroborate 4 exists exclusively as its chiral atropisomer, and the crystals are racemic, with alternating layers composed of the (S,S)-enantiomer and the (R,R)-enantiomer (Figure 5). The geometry of the spiroborate anion itself is closely similar to those observed in salt 4 · NH4+. In particular, the CPh · · · Bcore · · · CPh angles have nontetrahedral values, the B(OC)4 unit is elongated tetragonally, and its local symmetry is approximately D2d (Table 3). Each alternating layer is made up of homochiral chains in which the anions are linked by edgeto-face aromatic interactions similar to those observed in the

salt 4 · NH4+ (2.73(1) and 2.61(1) Å).28 The homochiral chains pack closely to form layers held together by van der Waals interactions.28 The B · · · B distance between adjacent anions in a single homochiral chain is 12.052(2) Å, the shortest B · · · B distance between other anions in the same homochiral layer is 7.035(2) Å, and the shortest B · · · B distance between anions of opposite configuration in adjacent layers is 9.404(2) Å. The resulting anionic network is open and defines channels of cross section 2.7 × 8.5 Å2 (Figure 5b).25 The channels are parallel to the a-axis and contain the cations and included DMF. Approximately 36% of the volume of the crystals is accessible to the counterions (15%) and neutral guests (21%), as estimated by standard methods.26,27 The neutral guests are disordered, but the cations are ordered, in part because they form a bifurcated hydrogen bond with oxygen atoms of spiroborate 4 (Figure 6). No hydrogen-bonded chains of alternating (S,S)-enantiomers and (R,R)-enantiomers exist in the structure of salt 4 · NH2Me2+, whereas they are present in salt 4 · NH4+. Nevertheless, the structures of the two salts are otherwise closely similar. This similarity suggests that hydrogen bonding does not play a dominant role in determining how the two salts crystallize, and it supports our decision to base analysis and description of all structures on how spiroborate 4 packs. Structure of Spiroborate Salt 4 · NMe4+. The NH4+ and NH2Me2+ salts of spiroborate 4 crystallize to give similar structures with room for including neutral guests, and hydrogen bonding does not appear to play a critical role in determining the overall architecture. For these reasons, we expected somewhat larger cations, even those unable to engage in strong hydrogen bonding, to be incorporated without major structural changes. Slow cooling of a hot solution of salt 4 · NMe4+ in DMF yielded crystals of composition 4 · NMe4+ · 1 DMF,18 which were found to belong to the tetragonal space group I41/a (Table 2). As expected, key structural features proved to be similar to those observed in salts 4 · NH4+ and 4 · NH2Me2+. In particular, the crystals are racemic, and the (S,S)-enantiomer and (R,R)-enantiomer of spiroborate 4 have their standard geometry, with nontetrahedral CPh · · · Bcore · · · CPh angles and a tetragonally elongated B(OC)4 unit that has approximate D2d local symmetry (Table 3). However, spiroborate anions in the structure of salt 4 · NMe4+ pack to form homochiral helices aligned with the c-axis, rather than homochiral layers as observed in the corresponding salts 4 · NH4+ and 4 · NH2Me2+ (Figures 7, 8). The principal forces

Table 2. Crystallographic Data for Spiroborate Salts 4 · NH4+, 4 · NH2Me2+, 4 · NMe4+, 4 · NEt4+, and 4 · NPr4+ compound

4 · NH4+

4 · NH2Me2+

4 · NMe4+

4 · NEt4+

4 · NPr4+

formula mw crystal system space group a (Å) b (Å) c (Å) R (°) β (°) γ (°) V (Å3) Z T (K) Fcalc (g cm-3) λ (Å) µ (mm-1) R1, I > 2σ(I) (all) WR2, I > 2σ(I) (all) measured reflections independent reflections

C52H48B2Cl3N3O9 986.90 monoclinic P2/n 12.8717(4) 12.0303(4) 14.9133(5) 90 90.344(2) 90 2309.29(13) 2 100(2) 1.419 1.54178 2.318 0.0768 (0.0861) 0.2383 (0.2474) 24719 4526

C26H24BNO4 425.28 monoclinic P21/c 12.0516(2) 12.5212(2) 17.4375(3) 90 100.040(1) 90 2591.03(7) 4 291(2) 1.278 1.54178 0.700 0.0585 (0.0663) 0.1729 (0.1829) 26189 4695

C28H28BNO4 453.21 tetragonal I41/a 30.5053(9) 30.5053(9) 12.1560(4) 90 90 90 11312.1(6) 16 220(2) 1.236 1.54178 0.667 0.0715 (0.1109) 0.1983 (0.2183) 34747 5558

C32H36BNO4 509.43 monoclinic P21/n 18.0047(8) 17.8601(8) 18.6114(9) 90 115.519(2) 90 5400.9(4) 8 100(2) 1.253 1.54178 0.642 0.0421 (0.0557) 0.1038 (0.1087) 48563 10524

C36H44BNO4 565.53 monoclinic P21/c 23.105(4) 17.302(4) 16.392(3) 90 102.49(3) 90 6398(2) 8 100(2) 1.167 1.54178 0.584 0.0623 (0.1169) 0.1539 (0.1812) 21203 11552

NPr4+

A B

NBu4+ NPent4+ PPh4+ (S)-(-)-MBA+ e,f (()-MBA+ e

A B A B NH2Me2 NMe4+ NEt4+

a Additional entries correspond to nonequivalent spiroborates in unit cell. b Measure of deviation from tetrahedral geometry in spiroborate 4 defined by angles between the central boron atom of the core (Bcore) and the 5, 5′, 5″, and 5′″ positions of the phenyl groups (CPh). c Measure of tetragonal distortion in spiroborate 4. d Measure of closeness to ideal D2d symmetry of the B(OC)4 unit of spiroborate 4. e MBA ) R-methylbenzylammonium. f For simplification, parameters are given for only one of the four similar but nonequivalent spiroborates in the unit cell.

42.1(4) 42.6(4) 41.8(2) 41.2(6) 39.0 (2) 39.1(2) 42.3(4) 40.4(4) 43.7(2) 40.9(4) 43.0(6) 43.5(3) 42.7(3) 43.8(3) 164.1(2)-166.0(2) 165.4(2)-166.1(2) 163.97(10)-166.53(12) 161.6(3)-167.6(3) 159.82(11)-169.57(11) 159.74(11)-168.32(11) 159.5(2)-171.9(2) 160.8(2)-168.6(2) 164.16(12)-167.42(11) 155.64(18)-170.76(18) 156.3(3)-169.7(4) 164.74(14)-165.62(15) 164.96(15)-165.84(15) 164.43(15)-164.70(16) 113.32(8) 112.80(8) 112.84(12) 112.8(3) 112.46(12) 112.91(12) 112.5(3) 112.6(3) 111.81(13) 112.45(19) 112.1(4) 112.77(7) 112.95(7) 113.22(8) 102.01(10), 102.01(10) 102.98(10), 102.98(10) 101.67(11), 104.14(11) 102.8(3), 102.9(3) 103.04(11), 104.22(12) 102.29(11), 103.27(11) 103.6(2), 103.7(2) 102.9(2), 103.7(2) 104.63(12), 105.14(12) 102.89(18), 104.36(18) 100.0(4), 108.0(4) 103.07(6), 103.07(6) 102.72(7), 102.72(7) 102.23(8), 102.23(8) 1.467(3) 1.469(3) 1.465(2) 1.464(4) 1.467(18) 1.467(18) 1.464(4) 1.465(4) 1.471(2) 1.470(3) 1.482(6) 1.466(2) 1.461(2) 1.468(2) 68.55(15) 68.85(15) 69.20(11) 68.50(3) 68.81(11) 68.20(11) 69.5(2) 69.0(2) 69.46(11) 67.88(18) 69.0(4) 68.29(13) 68.64(14) 69.01(15) 171.35(15) 169.02(15) 172.28(11) 174.62(3) 171.83(11) 173.29(11) 166.8(2) 171.5(2) 165.43(11) 172.03(18) 174.4(4) 169.83(13) 170.59(14) 170.91(15) A B +

NH4+

range of four largest O-B-O-C torsional angles (°)d average of other O-B-O angles (°)c two smallest O-B-O angles (°)c average B-O bond length (Å) smallest CPh · · · Bcore · · · CPh angle (°)b cation

aniona

largest CPh · · · Bcore · · · CPh angle (°)b

Table 3. Selected Bond Lengths, Bond Angles, and Torsional Angles for Spiroborate 4 in Salts with Various Cations

average interary torsional angle (°)e

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Voisin et al.

of cohesion within the homochiral helices are van der Waals interactions, whereas adjacent helices of opposite configuration participate in edge-to-face aromatic interactions (3.18(1) and 3.19(1) Å).28 The shortest B · · · B distance within a single homochiral helix is 8.929(7) Å, whereas the shortest B · · · B distance between anions of opposite configuration in adjacent helices is 7.653(7) Å. Parallel channels of cross section 3.7 × 8.8 Å2 lie between the anionic helices, run along the c-axis, and accommodate the cations and included DMF.25 Approximately 40% of the volume of the crystals is accessible to the counterions (19%) and neutral guests (21%).26,27 The neutral guests are disordered, but the cations are ordered, in part because various weak C-H · · · O interactions and C-H · · · π interactions link each NMe4+ cation to nearby spiroborate anions.28 Structure of Spiroborate Salt 4 · NEt4+. Inclusion of DMF in crystals of salt 4 · NMe4+ suggested that even larger cations could be accommodated without forcing the structure to change substantially. Crystals of composition 4 · NEt4+ were grown by placing a solution of salt 4 · NEt4+ in DMF in contact with vapors of diethyl ether (Table 2). The crystals proved to belong to the monoclinic space group P21/n, to have a structure closely related to those of other salts of spiroborate 4, and to have a unit cell containing spiroborate 4 in two closely similar conformations, only one of which will be analyzed in detail. Again, the crystals were found to be racemic, and spiroborate 4 has the characteristic geometry observed in other salts, with nontetrahedral CPh · · · Bcore · · · CPh angles and a tetragonally elongated B(OC)4 unit that has approximate D2d local symmetry (Table 3). Crystals of salt 4 · NEt4+ consist of alternating homochiral layers composed of the (S,S)-enantiomer and the (R,R)-enantiomer, as in the case of salts 4 · NH4+ and 4 · NH2Me2+ (Figure 9).28 This similarity confirms that N-H · · · O hydrogen bonds between spiroborate anion 4 and the counterion are not a prerequisite for the formation of layered structures. Each layer in crystals of salt 4 · NEt4+ is parallel to the bcplane, and the spiroborate anions are linked in both the b and c directions by edge-to-face aromatic interactions (2.77(1) Å).28 Longer edge-to-face aromatic interactions (2.91(1) Å) join spiroborate anions of opposite configuration in adjacent layers.28 The B · · · B distances between the closest neighbors within a single homochiral layer are 8.743(8) and 9.317(8) Å, whereas the shortest B · · · B distance between anions of opposite configuration is 8.709(8) Å. Parallel channels of cross section 2.5 × 9.3 Å2 run along the c-axis,25 and approximately 38% of the volume of the crystals is accessible to the counterions, which leave no space for including neutral guests (Figure 9b).26,27 Various weak C-H · · · O interactions and C-H · · · π interactions link each NEt4+ cation to nearby spiroborate anions.28 Structure of Spiroborate Salt 4 · NPr4+. The observation that crystals of salt 4 · NEt4+ included no neutral guests suggested that larger tetraalkylammonium cations would be difficult or impossible to accommodate without significant structural alteration. Diffusion of vapors of diethyl ether into a solution of salt 4 · NPr4+ in DMF produced crystals of composition 4 · NPr4+ belonging to the monoclinic space group P21/c (Table 2). The crystals proved again to be racemic, and Table 3 confirms that the molecular geometry of spiroborate 4 remains essentially unchanged. However, the overall structure is significantly different (Figures 10, 11a), and the (S,S)- and (R,R)-enantiomers of spiroborate 4 are no longer segregated in homochiral layers or helices. Instead, the structure can be described as being built from columns of alternating (S,S)- and (R,R)-enantiomers aligned

Crystal Structures of Spiroborates

Crystal Growth & Design, Vol. 8, No. 1, 2008 313

Figure 5. Representations of the structure of crystals of spiroborate 4 · NH2Me2+ grown from DMF. (a) View along the a-axis showing adjacent layers, each composed of a single enantiomer, with (S,S)-enantiomers in red and (R,R)-enantiomers in blue. Spaces between the homochiral anionic layers define channels that include NH2Me2+ and molecules of DMF. The guest molecules are omitted for clarity, and the cations are shown with carbon atoms in gray, hydrogen atoms in white, and nitrogen atoms in blue. (b) Similar view with the atoms of spiroborate 4 shown as spheres of van der Waals radii to reveal the cross sections of the channels. Cations and neutral guests are omitted for clarity.

Figure 6. View of the structure of crystals of spiroborate 4 · NH2Me2+ grown from DMF, with hydrogen bonds between the anion, the cation, and DMF represented by broken lines. Only one part of the disordered DMF molecule is shown. Carbon atoms are drawn in gray, hydrogen in white, boron in pink, nitrogen in blue, and oxygen in red.

with the c-axis (Figure 11a). For the two closely similar conformations of spiroborate 4 found in the unit cell, the B · · · B distances between adjacent anions within a single heterochiral column are 8.314(4) and 8.478(4) Å, and neighboring anions engage in edge-to-face aromatic interactions (2.83(1) and 3.01(1) Å).28 Spaces between the columns define parallel channels of cross section 7.0 × 7.7 Å2 that run along the a-axis and include the NPr4+ cations,25 which form various weak interactions with nearby spiroborate anions.28 Approximately 49% of the volume of the crystals is accessible to the counterions.26,27 Comparison of the structures of salts of spiroborate 4 with NH4+, NH2Me2+, NMe4+, NEt4+, and NPr4+ leads to the following initial conclusions: The geometry of anion 4 and the formation of racemic crystals are deep-seated structural preferences that are not easily altered, whereas the exact packing of the anions varies widely according to the identity of the cations. Structure of Spiroborate Salts 4 · NBu4+ and 4 · NPent4+. Further support for these conclusions was obtained by analyzing crystals of salts 4 · NBu4+ and 4 · NPent4+, which incorporate even larger tetraalkylammonium cations. Crystals of composition 4 · NBu4+ and 4 · NPent4+ were obtained by allowing vapors of diethyl ether to diffuse into solutions of the salts in DMF. Crystals of both salts proved to belong to the

Figure 7. View along the c-axis of the structure of crystals of spiroborate 4 · NMe4+ grown from DMF. The (S,S)-enantiomer of spiroborate anion 4 (red) and the (R,R)-enantiomer (blue) pack to define homochiral helices that run along the c-axis. Channels between the helices accommodate the NMe4+ ions and included DMF. Guest molecules are omitted for clarity, and the cations are shown with carbon atoms in gray, hydrogen atoms in white, and nitrogen atoms in blue.

monoclinic space group P21/c and to have closely similar structural parameters (Table 4). Despite the increased size of the cations, the principal structural features remain unchanged; in particular, crystals were again found to be racemic, and Table 3 confirms that the characteristic geometry of spiroborate 4 is retained. Like the structure of the salt 4 · NPr4+, those of the NBu4+ and NPent4+ salts can be described as consisting of columns of alternating (S,S)- and (R,R)-enantiomers aligned with the c-axis (Figure 11b). Alternatively, the NBu4+ and NPent4+ salts can also be considered to form layered structures similar to those obtained from the salts of NH4+, NH2Me2+, and NMe4+. The B · · · B distance between adjacent anions within a single heterochiral column is 9.605(4) Å in the NBu4+ salt, the anions are separated by intervening cations so that no direct interanionic

314 Crystal Growth & Design, Vol. 8, No. 1, 2008

Figure 8. Representations of the structure of crystals of spiroborate 4 · NMe4+ grown from DMF. (a) Single homochiral helix built from the (R,R)-enantiomer of spiroborate 4 with all atoms shown in blue. (b) Alternative view of the helix, with each central boron atom represented by a small pink sphere and connected by lines to the boron atoms of neighboring anions in van der Waals contact.

interactions are present, and approximately 55% of the volume of the crystals is accessible to the counterions.26,27 Comparison of these values with those observed in the analogous NPr4+ salt reveals that space for accommodating the larger NBu4+ and NPent4+ cations is created in part by increasing the interanionic separation. Structure of Spiroborate Salt 4 · PPh4+. A further attempt to alter the basic structural preferences of spiroborate 4 was made by replacing ammonium cations with PPh4+. Crystals of composition 4 · PPh4+ were grown by allowing vapors of diethyl ether to diffuse into a solution of the salt in DMF. The resulting crystals were found to belong to the monoclinic space group

Voisin et al.

P21/n (Table 4), and their structure proved to be analogous to those of other salts. Again, the crystals are racemic, spiroborate 4 adopts its favored geometry (Table 3), and the anions are arranged in homochiral layers similar to those observed in salts 4 · NH4+, 4 · NH2Me2+, and 4 · NEt4+ (Figure 12). Neighboring anions in the same layer engage in edge-to-face aromatic interactions (3.17(1) and 3.22(1) Å).28 Spaces between adjacent homochiral layers define channels of cross section 8.1 × 8.1 Å2 that are parallel to the a-axis and include the PPh4+ cations,25 which form various weak interactions with nearby spiroborate anions.28 Approximately 58% of the volume of the crystals is accessible to the counterions.26,27 Structures of Spiroborate Salts 4 · (S)-r-Methylbenzylammonium (MBA) and 4 · (()-MBA. Our analysis of the structures of salts of spiroborate 4 with a wide range of achiral cations reveals a consistent preference for the formation of racemic crystals. In these crystals, multiple anion-cation contacts are present, and the identity of the cations clearly plays an important role in determining how the anions are positioned. We reasoned that the use of a chiral cation might therefore favor structures no longer built from equal amounts of the (S,S)-enantiomer and the (R,R)-enantiomer of spiroborate 4. Crystals of composition 4 · (S)-MBA · 2 MeOH were grown by cooling a hot solution of the salt in MeOH.18 The crystals proved to belong to the monoclinic space group P2 (Table 4) and to have a structure closely analogous to those of other salts. Despite the presence of a chiral cation in enantiomerically pure form, the crystals are again composed of a 1:1 mixture of the (S,S)-enantiomer and the (R,R)-enantiomer of spiroborate 4. Anion 4 adopts its normal geometry (Table 3) and yields homochiral layers similar to those observed in other salts.28 Approximately 45% of the volume of the crystals is accessible to the counterions (31%) and neutral guests (14%), as estimated by standard methods.26,27 A network of N-H · · · O and O-H · · · O hydrogen bonds links the spiroborate anions to intervening cations and included MeOH, both within homochiral layers and between adjacent layers of opposite configuration.28 In a similar experiment, crystals of composition 4 · (()MBA · 2 MeOH were grown by cooling a hot solution of the salt in MeOH.18 The crystals proved to belong to the monoclinic space group P2/n (Table 4) and to have a structure closely

Figure 9. Representations of the structure of crystals of spiroborate 4 · NEt4+ grown from diethyl ether/DMF. (a) View along the c-axis showing alternating homochiral layers of (S,S)-enantiomers (red) and (R,R)-enantiomers (blue). Cations are shown with all atoms in green. (b) Similar view with the cations omitted and the atoms of spiroborate 4 shown as spheres of van der Waals radii to reveal the cross sections of the channels.

Crystal Structures of Spiroborates

Crystal Growth & Design, Vol. 8, No. 1, 2008 315

Figure 10. Representations of the structure of crystals of spiroborate 4 · NPr4+ grown from diethyl ether/DMF. (a) View along the ac-diagonal, with (S,S)-enantiomers in red and (R,R)-enantiomers in blue. Cations are shown in green. (b) Similar view with the cations omitted and the atoms of spiroborate 4 shown as spheres of van der Waals radii to reveal the cross sections of the channels.

Figure 11. (a) Representation of two heterochiral columns in the structure of crystals of spiroborate 4 · NPr4+ grown from diethyl ether/DMF. (b) Representation of two heterochiral columns in the structure of crystals of spiroborate 4 · NBu4+ grown from diethyl ether/DMF. In both images, the cations are omitted for clarity, and the atoms of spiroborate 4 are shown as spheres of van der Waals radii, with (S,S)-enantiomers in red and (R,R)-enantiomers in blue.

analogous to those of other salts, including the enantiomerically pure (S)-MBA salt. Again, the crystals are composed of a 1:1 mixture of the (S,S)-enantiomer and the (R,R)-enantiomer of spiroborate 4. Anion 4 adopts its normal geometry (Table 3) and yields homochiral layers similar to those observed in other cases.28 Approximately 45% of the volume of the crystals is accessible to the counterions (32%) and neutral guests (13%), as estimated by standard methods.26,27 It is noteworthy that the racemic salt crystallizes in essentially the same way as the enantiomerically pure form. Conclusions Our observations suggest that tectons derived from anionic spiroborate 4 can be used confidently to engineer crystals and

other ordered molecular materials. In salts incorporating a wide range of cations, spiroborate 4 exhibits a consistent preference for enantiomeric (R,R) and (S,S) structures with a flattened D2 geometry. Moreover, crystals of salts of spiroborate 4 reliably incorporate the two enantiomers in a 1:1 ratio, even when the cations themselves are chiral and enantiomerically pure. In crystals of certain salts (NH4+, NH2Me2+, NEt4+, PPh4+, and MBA+), the packing of spiroborate 4 defines homochiral layers; in other salts, however, the organization of the anions is helical (NMe4+) or columnar (NPr4+, NBu4+, and NPent4+). These variations presumably arise because the positions of neighboring anions are not controlled by strong directional interactions between them, but rather by a complex interplay of electrostatic effects, van der Waals contacts, and aromatic interactions. In

316 Crystal Growth & Design, Vol. 8, No. 1, 2008

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Table 4. Crystallographic Data for Spiroborate Salts 4 · NBu4+, 4 · NPent4+, 4 · PPh4+, 4 · (S)-(-)-MBA+, and 4 · (()-MBA+ compound

4 · NBu4+

4 · NPent4+

4 · PPh4+

formula mw crystal system space group a (Å) b (Å) c (Å) R (°) β (°) γ (°) V (Å3) Z T (K) Fcalc (g cm-3) λ (Å) µ (mm-1) R1, I > 2σ(I) (all) WR2, I > 2σ(I) (all) measured reflections independent reflections

C40H52BNO4 621.64 monoclinic P21/c 11.1413(3) 16.3143(4) 19.2076(4) 90 90.428(1) 90 3491.12(15) 4 100(2) 1.183 1.54178 0.580 0.0452 (0.0601) 0.1093 (0.1146) 20941 6715

C44H60BNO4 677.74 monoclinic P21/c 12.041(4) 17.599(5) 19.680(6) 90 94.518(9) 90 4157 4 280(2) 1.083 1.54178 0.522 0.0600 (0.0717) 0.1341 (0.1354) 50429 7528

C48H36BO4P 718.55 monoclinic P21/n 11.866(2) 14.925(3) 21.304(4) 90 95.84(3) 90 3753.4(13) 4 291(2) 1.272 1.54178 1.01 0.0569 (0.0765) 0.1154 (0.1166) 32131 7121

principle, conversion of spiroborate 4 into tectons by the attachment of suitable sticky sites can remedy this problem of inconsistent packing and lead to the creation of networks with a narrow range of architectures predetermined by the intrinsic chiral D2 geometry of the spiroborate core and the particular orientation of the sticky sites. In addition to providing a core with a well-defined geometry, spiroborate 4 has many other appealing characteristics: (1) It can be synthesized in one step from boric acid and 2,2′dihydroxybiphenyl; (2) derivatives can be readily obtained by electrophilic substitution of 2,2′-dihydroxybiphenyl;29 and (3) the core is charged and must cocrystallize with counterions, which can be varied to introduce additional diversity. Together, these features make spiroborate 4 and its derivatives attractive candidates for engineering crystals and other ordered materials with novel structures and properties. Experimental Section General Procedure for Synthesizing Ammonium Bis[[1,1′-biphenyl]-2,2′-diolato-O,O′] Borates. Unless noted otherwise, all ammonium salts of spiroborate 4 were synthesized by the following procedure. A mixture of boric acid (0.33 g, 5.3 mmol), 2,2′-dihydroxybiphenyl (2.0 g, 11 mmol), and an ammonium hydroxide (5.9 mmol) in DMF (25 mL) was heated at reflux for 24 h. The mixture was then cooled to 25 °C, and diethyl ether was added. The resulting precipitate was separated

4 · (S)-(-)-MBA+ C34H36BNO6 565.45 monoclinic P2 14.5833(3) 12.0045(2) 17.6323(3) 90 104.137(1) 90 2993.32(10) 4 200(2) 1.255 1.54178 0.684 0.0443 (0.0471) 0.1251 (0.1297) 40206 10427

4 · (()-MBA+ C34H36BNO6 565.45 monoclinic P2/n 14.449(3) 12.011(2) 17.798(4) 90 104.99(3) 90 2983.32(10) 4 150(2) 1.259 0.71073 0.085 0.0542 (0.1267) 0.1199 (0.1502) 21756 6810

by filtration to give the desired ammonium spiroborate as a colorless solid. In the case of the NH2Me2+ salt, the cation was derived from decomposition of DMF, and no ammonium hydroxide was added to the reaction mixture. Crystallization of the salts was achieved by slowly cooling hot solutions in DMF or by exposing solutions in DMF at 25 °C to vapors of CHCl3 or diethyl ether. The purified products were characterized by standard methods as summarized below. Dimethylammonium Bis[[1,1′-biphenyl]-2,2′-diolato-O,O′] Borate (4). Yield 79%: mp 252 °C; 1H NMR (400 MHz, DMSO-d6) δ 2.46 (s, 6H), 6.90 (dd, 4H, 3J ) 7.5 Hz, 4J ) 1.2 Hz), 6.98 (td, 4H, 3J ) 7.5 Hz, 4J ) 1.2 Hz), 7.25 (td, 4H, 3J ) 7.5 Hz, 4J ) 1.7 Hz), 7.34 (dd, 4H, 3J ) 7.5 Hz, 4J ) 1.7 Hz), 8.16 (s, 2H); 13C NMR (100 MHz, DMSO-d6) δ 35.2, 120.8, 122.9, 128.6, 129.1, 132.6, 157.9. Anal. Calcd for C26H24BNO4 + 1 DMF: C, 69.89%; H, 6.27%; N, 5.62%. Found: C, 69.72%; H, 6.06%; N, 5.13%. Tetramethylammonium Bis[[1,1′-biphenyl]-2,2′-diolato-O,O′] Borate (4). Yield 80%: mp >280 °C; 1H NMR (400 MHz, DMSO-d6) δ 3.03 (s, 12H), 6.89 (dd, 4H, 3J ) 7.5 Hz, 4J ) 1.2 Hz), 6.97 (td, 4H, 3 J ) 7.5 Hz, 4J ) 1.2 Hz), 7.24 (td, 4H, 3J ) 7.5 Hz, 4J ) 1.7 Hz), 7.34 (dd, 4H, 3J ) 7.5 Hz, 4J ) 1.7 Hz); 13C NMR (100 MHz, DMSOd6) δ 55.2, 120.8, 122.9, 128.6, 129.1, 132.6, 157.9. Anal. Calcd for C28H28BNO4 + 1 DMF + 0.5 H2O: C, 69.54%; H, 6.78%; N, 5.23%. Found: C, 69.17%; H, 6.48%; N, 5.04%. Tetraethylammonium Bis[[1,1′-biphenyl]-2,2′-diolato-O,O′] Borate (4). Yield 87%; mp 236 °C; 1H NMR (400 MHz, DMSO-d6) δ 1.11 (m, 12H), 3.14 (q, 8H, 3J ) 7.3 Hz), 6.89 (dd, 4H, 3J ) 7.5 Hz, 4 J ) 1.1 Hz), 6.97 (td, 4H, 3J ) 7.5 Hz, 4J ) 1.1 Hz), 7.24 (td, 4H, 3 J ) 7.5 Hz, 4J ) 1.7 Hz), 7.35 (dd, 4H, 3J ) 7.5 Hz, 4J ) 1.7 Hz);

Figure 12. Representations of the structure of crystals of spiroborate 4 · PPh4+ grown from diethyl ether/DMF. (a) View along the a-axis showing alternating homochiral layers of (S,S)-enantiomers (red) and (R,R)-enantiomers (blue). Cations are shown in green. (b) Similar view with the cations omitted and the atoms of spiroborate 4 shown as spheres of van der Waals radii to reveal the cross sections of the channels.

Crystal Structures of Spiroborates 13

C NMR (100 MHz, DMSO-d6) δ 7.9, 52.2, 120.8, 122.9, 128.6, 129.0, 132.7, 158.0. Anal. Calcd for C32H36BNO4: C, 75.44%; H, 7.12%; N, 2.75%. Found: C, 75.46%; H, 7.36%; N, 2.69%. Tetrapropylammonium Bis[[1,1′-biphenyl]-2,2′-diolato-O,O′] Borate (4). Yield 61%: mp 244 °C; 1H NMR (400 MHz, DMSO-d6) δ 0.86 (t, 12H, 3J ) 7.2 Hz), 1.57 (m, 8 H), 3.09 (t, 8H, 3J ) 7.4 Hz), 6.88 (dd, 4H, 3J ) 7.5 Hz, 4J ) 1.0 Hz), 6.96 (td, 4H, 3J ) 7.5 Hz, 4J ) 1.0 Hz), 7.23 (td, 4H, 3J ) 7.5 Hz, 4J ) 1.7 Hz), 7.33 (dd, 4H, 3J ) 7.5 Hz, 4J ) 1.7 Hz); 13C NMR (100 MHz, DMSO-d6) δ 11.4, 15.6, 60.1, 120.8, 122.9, 128.6, 129.1, 132.7, 158.0. Anal. Calcd for C36H44BNO4: C, 76.46%; H, 7.84%; N, 2.48%. Found: C, 76.04%; H, 7.92%; N, 2.56%. Tetrabutylammonium Bis[[1,1′-biphenyl]-2,2′-diolato-O,O′] Borate (4). Yield 60%: mp 229 °C; 1H NMR (400 MHz, DMSO-d6) δ 0.91 (t, 12H, 3J ) 7.3 Hz), 1.26 (m, 8 H), 1.50 (m, 8H), 3.11 (m, 8H), 6.89 (dd, 4H, 3J ) 7.5 Hz, 4J ) 1.0 Hz), 6.97 (td, 4H, 3J ) 7.5 Hz, 4J ) 1.0 Hz), 7.24 (td, 4H, 3J ) 7.5 Hz, 4J ) 1.6 Hz), 7.34 (dd, 4H, 3J ) 7.5 Hz, 4J ) 1.6 Hz); 13C NMR (100 MHz, DMSO-d6) δ 14.3, 20.0, 23.9, 58.4, 120.7, 122.9, 128.5, 129.0, 132.6, 158.0. Anal. Calcd for C40H52BNO4: C, 77.28%; H, 8.43%; N, 2.25%. Found: C, 77.27%; H, 8.21%; N, 2.32%. Tetrapentylammonium Bis[[1,1′-biphenyl]-2,2′-diolato-O,O′] Borate (4). Yield 74%: mp 244 °C; 1H NMR (400 MHz, DMSO-d6) δ 0.88 (t, 12H, 3J ) 7.2 Hz), 1.20 (m, 8 H), 1.30 (m, 8H), 1.51 (m, 8H), 3.11 (m, 8H), 6.89 (dd, 4H, 3J ) 7.5 Hz, 4J ) 1.0 Hz), 6.96 (td, 4H, 3J ) 7.5 Hz, 4J ) 1.0 Hz), 7.23 (td, 4H, 3J ) 7.5 Hz, 4J ) 1.6 Hz), 7.33 (dd, 4H, 3J ) 7.5 Hz, 4J ) 1.6 Hz); 13C NMR (100 MHz, DMSO-d6) δ 14.6, 21.6, 22.4, 28.7, 58.5, 120.7, 122.9, 128.5, 129.0, 132.7, 158.0. Anal. Calcd for C44H60BNO4: C, 77.97%; H, 8.92%; N, 2.07%. Found: C, 77.87%; H, 9.22%; N, 2.12%. Ammonium Bis[[1,1′-biphenyl]-2,2′-diolato-O,O′] Borate (4). A solution of boric acid (0.33 g, 5.3 mmol) in methanol (10 mL) was added to a mixture of 2,2′-dihydroxybiphenyl (2.0 g, 11 mmol) and 28% aqueous ammonium hydroxide (1 mL) in hot water (10 mL). Volatiles were then removed by evaporation under reduced pressure, and the residual pink solid was dissolved in DMF. Addition of CHCl3 induced the precipitation of ammonium bis[[1,1′-biphenyl]-2,2′-diolatoO,O′] borate (4), which was separated by filtration and dried in vacuo. Yield 70%: mp >280 °C; 1H NMR (400 MHz, DMSO-d6) δ 6.88 (dd, 4H, 3J ) 7.5 Hz, 4J ) 1.2 Hz), 6.97 (td, 4H, 3J ) 7.5 Hz, 4J ) 1.2 Hz), 7.07 (s, 4H), 7.24 (td, 4H, 3J ) 7.5 Hz, 4J ) 1.7 Hz), 7.34 (dd, 4H, 3J ) 7.5 Hz, 4J ) 1.7 Hz); 13C NMR (100 MHz, DMSO-d6) δ 120.8, 122.9, 128.6, 129.1, 132.6, 157.9. Anal. Calcd for C24H20BNO4 + 0.5 DMF + 0.5 CHCl3: C, 63.28%; H, 4.90%; N, 4.26%. Found: C, 63.38%; H, 4.92%; N, 4.56%. Tetraphenylphosphonium Bis[[1,1′-biphenyl]-2,2′-diolato-O,O′] Borate (4). A mixture of boric acid (0.33, 5.3 mmol), 2,2′-dihydroxybiphenyl (2.0 g, 11 mmol), and tetraphenylphosphonium bromide (2.7 g, 6.4 mmol) in DMF (25 mL) was heated at reflux for 24 h. The solution was cooled to 25 °C, and diethyl ether was added. The resulting precipitate was separated by filtration and dried in vacuo to give tetraphenylphosphonium bis[[1,1′-biphenyl]-2,2′-diolato-O,O′] borate (4) as a colorless solid. Yield 64%: mp >250 °C; 1H NMR (400 MHz, DMSO-d6) δ 6.89 (dd, 4H, 3J ) 7.5 Hz, 4J ) 1.1 Hz), 6.96 (td, 4H, 3J ) 7.5 Hz, 4J ) 1.1 Hz), 7.23 (td, 4H, 3J ) 7.5 Hz, 4J ) 1.7 Hz), 7.32 (dd, 4H, 3J ) 7.5 Hz, 4J ) 1.7 Hz), 7.75 (m, 8H), 7.80 (m, 8H), 7.94 (m, 4H); 13C NMR (100 MHz, DMSO-d6) δ 117.6 (JPC ) 89 Hz), 119.9, 122.0, 127.7, 128.2, 130.4 (JPC ) 13 Hz), 131.8, 134.5 (JPC ) 10 Hz), 135.3, 157.0. Anal. Calcd for C44H60BNO4 + 0.75 DMF: C, 78.04%; H, 5.38%; N, 1.36%. Found: C, 77.43%; H, 5.08%; N, 0.90%. (S)-r-Methylbenzylammonium Bis[[1,1′-biphenyl]-2,2′-diolatoO,O′] Borate (4). A mixture of boric acid (0.17 g, 2.7 mmol) and 2,2′dihydroxybiphenyl (1.0 g, 5.4 mmol) in methanol (25 mL) was stirred at 25 °C. (S)-R-Methylbenzylamine (0.39 g, 3.2 mmol) was slowly added. The solution was stirred overnight, and half of the solvent was subsequently removed by partial evaporation under reduced pressure. A colorless solid was then separated by filtration and washed with small amounts of methanol to give a pure sample of (S)-R-methylbenzylammonium bis[[1,1′-biphenyl]-2,2′-diolato-O,O′] borate (1.0 g, 2.0 mmol, 74%): mp >250 °C; 1H NMR (400 MHz, DMSO-d6) δ 1.46 (d, 3H, 3J ) 6.8 Hz), 4.37 (q, 1H, 3J ) 6.8 Hz), 6.89 (dd, 4H, 3J ) 7.5 Hz, 4J ) 1.0 Hz), 6.97 (td, 4H, 3J ) 7.5 Hz, 4J ) 1.0 Hz), 7.24 (td, 4H, 3J ) 7.5 Hz, 4J ) 1.6 Hz), 7.33 (dd, 4H, 3J ) 7.5 Hz, 4J ) 1.6 Hz), 7.41 (m, 5H), 8.16 (s, 3H); 13C NMR (100 MHz, DMSO-d6) δ 21.5, 50.8, 120.8, 122.9, 127.5, 128.6, 129.1, 129.4, 129.6, 132.6, 140.0, 157.9.

Crystal Growth & Design, Vol. 8, No. 1, 2008 317 Anal. Calcd for C32H28BNO4: C, 76.66%; H, 5.63%; N, 2.79%. Found: C, 76.00%; H, 5.62%; N, 2.78%. (()-r-Methylbenzylammonium Bis[[1,1′-biphenyl]-2,2′-diolatoO,O′] Borate (4). An analogous procedure was used to convert boric acid (0.33 g, 5.3 mmol), 2,2′-dihydroxybiphenyl (2.0 g, 11 mmol), and (()-R-methylbenzylamine (0.78 g, 6.4 mmol) into a sample of the corresponding spiroborate, which was isolated as a colorless solid (2.5 g, 5.0 mmol, 93%): mp >250 °C; 1H NMR (400 MHz, DMSO-d6) δ 1.46 (d, 3H, 3J ) 6.8 Hz), 4.36 (q, 1H, 3J ) 6.8 Hz), 6.89 (dd, 4H, 3J ) 7.5 Hz, 4J ) 0.9 Hz), 6.97 (td, 4H, 3J ) 7.5 Hz, 4J ) 0.9 Hz), 7.24 (td, 4H, 3J ) 7.5 Hz, 4J ) 1.6 Hz), 7.34 (dd, 4H, 3J ) 7.5 Hz, 4J ) 1.6 Hz), 7.41 (m, 5H), 8.09 (s, 3H); 13C NMR (100 MHz, DMSO-d6) δ 20.8, 50.0, 119.9, 122.0, 126.6, 127.7, 128.2, 128.5, 128.7, 131.8, 139.3, 157.0. Anal. Calcd for C32H28BNO4 + 1 CH3OH: C, 74.44%; H, 6.06%; N, 2.63%. Found: C, 74.59%; H, 5.52%; N, 2.79%. X-Ray Crystallographic Studies. Data were collected using (1) a Bruker AXS SMART 4K/Platform diffractometer, (2) a Bruker Microstar diffractometer with Cu KR radiation, or (3) a Nonius Kappa CCD diffractometer with Mo KR radiation. Structures were solved by direct methods using SHELXS-97 and refined using SHELXL-97.30 All nonhydrogen atoms were refined anisotropically, whereas hydrogen atoms were placed in ideal positions and defined as riding atoms.

Acknowledgment. We are grateful to the Natural Sciences and Engineering Research Council of Canada, the Ministère de l′Éducation du Québec, the Canada Foundation for Innovation, the Canada Research Chairs Program, and Université de Montréal for financial support. We thank Prof. Jurgen Sygusch for providing access to a diffractometer equipped with a rotating anode, and we are grateful to Dr. Alan J. Lough for collecting diffraction data for salt 4 · (()-MBA. Supporting Information Available: Additional crystallographic details, including ORTEP drawings and tables of structural data. This material is available free of charge via the Internet at http://pubs.acs.org.

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CG700803K