CRYSTAL GROWTH & DESIGN
Crystal Structures of Spiroborates Derived from [1,1′-Binaphthalene]-2,2′-diol (BINOL) Tao Tu, Thierry Maris, and James D. Wuest* Département de Chimie, UniVersité de Montréal, Montréal, Québec H3C 3J7 Canada
2008 VOL. 8, NO. 5 1541–1546
ReceiVed January 23, 2008; ReVised Manuscript ReceiVed January 29, 2008
ABSTRACT: Ammonium salts of bis[[1,1′-binaphthalene]-2,2′-diolato-O,O’] borate (4) were prepared by the reaction of boric acid with both racemic and enantiomerically pure [1,1′-binaphthalene]-2,2′-diol (BINOL). Four salts were crystallized, and their structures were solved by X-ray diffraction. In all cases examined, spiroborate 4 was found to adopt a flattened D2 conformation similar to the one favored by analogous spiroborates derived from 2,2′-dihydroxybiphenyl. Spiroborate 4 has an essentially invariant molecular geometry, can be synthesized and derivatized by straightforward methods, and is readily available in both racemic and enantiomerically pure forms of high configurational stability. For these reasons, it is an attractive starting point for creating crystals and other ordered materials with predetermined structures and properties. Introduction An effective strategy for engineering ordered materials with predictable structures and properties is to build them from molecules that have two characteristic features: (1) well-defined geometries and (2) an ability to hold neighboring molecules in predetermined positions by engaging in strong directional interactions.1,2 Such molecules, which have been called tectons, are useful subunits for purposeful nanoscale construction.3 They can be made by the simple expedient of selecting functional groups that participate in reliable patterns of molecular association, which have been called supramolecular synthons,4 and then by grafting them to cores that ensure proper orientation and simultaneously introduce other desirable properties. In hypothetical tecton 1, for example, sites that control association (represented by b) are attached to a spiroborate core 2 derived from 2,2′-dihydroxybiphenyl.
In an initial exploration of the potential of spiroborates in crystal engineering and other areas of materials science,5,6 we noted that core 2 has many appealing features: (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;7 (3) the core is charged and must cocrystallize with counterions, which can be exchanged to create libraries of salts with diverse properties; (4) core 2 reliably favors enantiomeric structures (S,S)-2 and (R,R)* Author to whom correspondence may be addressed. E-mail: james.d.wuest@ umontreal.ca.
Figure 1. Representative views of the structure of (S,S)-spiroborate 2 as found in crystals of salt 2 · NH4+grown from CHCl3/DMF. The same structure is viewed along each of three axes of C2 symmetry. For simplification, all atoms are drawn in red.
2, and the alternative meso isomer (R,S)-2 is not observed; and (5) (S,S)-2 and (R,R)-2 dependably adopt structures with welldefined flattened D2 geometries (Figure 1). Together, these properties make hypothetical tecton 1 and related spiroborates attractive candidates for engineering crystals and other ordered materials with novel structures and properties.
Interconversion of the various chiral and achiral forms of spiroborate 2 is fast under the conditions of synthesis and crystallization, and salts tend to form racemic crystals composed of enantiomers (S,S)-2 and (R,R)-2 in a 1:1 ratio, even when the counterion itself is chiral and enantiomerically pure.5 In contrast, hypothetical tecton 3 incorporates spiroborate core 4, which can be prepared by the reaction of boric acid with [1,1′binaphthalene]-2,2′-diol (BINOL) under conditions permitting the retention of configuration.8 Related complexes of BINOL
10.1021/cg7008013 CCC: $40.75 2008 American Chemical Society Published on Web 04/16/2008
1542 Crystal Growth & Design, Vol. 8, No. 5, 2008
Tu et al.
Figure 2. Views of the structure of (S,S)-spiroborate 4 as found in crystals of salt (()-4 · NH2Et2+grown from CH3CN/DMF. The structure is viewed along each of three axes of C2 symmetry. For simplification, all atoms are drawn in red. Figure 4. Representation of the structure of crystals of spiroborate salt (()-4 · NH2Et2+ grown from CH3CN/DMF, showing a central homochiral chain of (S,S)-enantiomers of spiroborate 4 and the four adjacent homochiral chains of (R,R)-enantiomers as viewed along the c-axis. (S,S)-enantiomers are drawn with all atoms in red, (R,R)-enantiomers are shown in blue, and all intervening cations are omitted for clarity.
Figure 3. View of the structure of crystals of spiroborate salt (()4 · NH2Et2+ grown from CH3CN/DMF. To simplify interpretation, (S,S)enantiomers of spiroborate 4 are drawn with all atoms in red and (R,R)enantiomers with all atoms in blue. The cations are shown with carbon atoms in gray, hydrogen atoms in white, and nitrogen atoms in blue. Broken lines with dashes are used to represent hydrogen bonds, which link spiroborate 4 and NH2Et2+ into homochiral chains. Broken lines with dots highlight face-to-face aromatic interactions between chains.
have been used extensively in asymmetric catalysis,9,10 yet structural studies of salts of simple spiroborate 4 are rare.10 These studies have yielded a set of data that is too small to allow definitive analysis, but it is noteworthy that the known structures all share the following common features: (1) Only the enantiomeric (S,S)-4 and (R,R)-4 stereoisomers have been observed (shown in red and blue, respectively, in subsequent figures to distinguish them clearly), and the meso (R,S) isomer appears to be disfavored; and (2) spiroborate 4 adopts a flattened D2 conformation closely analogous to the one preferred by spiroborate 2. Because these two features are shared by salts with a variety of different cations, they appear to reflect an inherent preference of spiroborate 4. To further assess the suitability of derivatives of BINOL for engineering crystals and other molecular materials, we have crystallized additional salts of spiroborate 4 in both racemic and enantiomerically pure forms, and we have determined their structures by X-ray diffraction. The results of this study provide a foundation for more ambitious investigations of tecton 3 and related derivatives of BINOL designed to form strongly associated molecular networks. Results and Discussion Synthesis of Salts of Spiroborate 4. Ammonium salts of spiroborate 4 were prepared in excellent yield by treating boric acid and a suitable amine with either (()- or (S)-[1,1′binaphthalene]-2,2′-diol according to a previously reported method.8
Structure of Spiroborate Salt (()-4 · NH2Et2+. Exposure of a solution of salt (()-4 · NH2Et2+ in DMF to vapors of CH3CN induced the formation of crystals of composition 4 · NH2Et2+. No molecules of solvent were included. The crystals proved to belong to the monoclinic space group Cc. Views of the structure appear in Figures 2–4, and crystallographic details are provided in Tables 1 and 2. Spiroborate 4 exists exclusively as its chiral atropisomer, and the crystals consist of a 1:1 mixture of enantiomers (S,S)-4 and (R,R)-4. Figure 2 shows the (S,S)enantiomer of spiroborate 4 as viewed along each of three axes of C2 symmetry, thereby revealing a flattened geometry of D2 symmetry similar to the one adopted by spiroborate 2 (Figure 1). In the preferred geometry of both spiroborates, the two aryl-aryl bonds are approximately parallel. In these ways, the behavior of spiroborate 4 closely resembles that of analogue 2. The results are also consistent with Wallach’s rule, which states that racemic mixtures tend to crystallize as racemic crystals containing equal amounts of both enantiomers, rather than as a conglomerate of crystals each composed of a single enantiomer.11 The central B(OC)4 unit in spiroborate salt (()-4 · NH2Et2+ has an average B-O bond length (1.471(4) Å) similar to those found in salts of spiroborate 2,5 tetramethoxyborate,12 and tetraphenoxyborate.13 In all these models, the B(OC)4 unit is elongated tetragonally, and the unit taken in isolation has approximate D2d symmetry. Analogously, the B(OC)4 unit of spiroborate 4 has two O-B-O angles that are much smaller (101.1(2)° and 103.2(2)°) than the average of the other four (113.3(2)°). Again, the unit has approximate D2d symmetry, and four O-B-O-C torsional angles have values in the range 160.6(2)°-169.6(2)°. Theoretical and experimental studies suggest that these tetragonal distortions are a structural feature shared by all subunits M(OH)4 or M(OC)4, where M ) C, Si, or B-.14 In assessing the shapes of hypothetical tectons 1, 3, and related compounds, it is useful to evaluate how the characteristic geometry of the central B(OC)4 unit will control the orientation of peripheral sites of association (represented by b). Of particular practical interest are sites of association introduced by simple methods of synthesis, such as direct electrophilic substitution of the precursors, 2,2′-dihydroxybiphenyl and BINOL. Electrophilic substitution favors derivatives of 2,2′dihydroxybiphenyl substituted in the 5,5′-positions7 and deriva-
Crystal Structures of Spiroborates
Crystal Growth & Design, Vol. 8, No. 5, 2008 1543
Table 1. Crystallographic Data for Spiroborate Salts (()-4 · NH2Et2+, (S,S)-4 · NH2Et2+, (S,S)-4 · NH2Bn2+, and (S,S)-4 · NH2Bn2+ · 0.5 CH3CN · 1.5 H2O compound
(()-4 · NH2Et2+
(S,S)-4 · NH2Et2+
(S,S)-4 · NH2Bn2+
(S,S)-4 · NH2Bn2+ · 0.5 CH3CN · 1.5 H2O
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 (Cu KR Å) µ (Cu KR mm-1) R1, I > 2σ(I) (all) WR2, I > 2σ(I) (all) measured reflections independent reflections
C44H36BNO4 653.55 monoclinic Cc 28.665(6) 15.857(3) 7.3927(15) 90 91.81(3) 90 3358.6(12) 4 100(2) 1.292 1.54178 0.644 0.0480 (0.0489) 0.1147 (0.1150) 16991 5028
C44H36BNO4 653.55 orthorhombic P21212 18.6055(5) 21.7959(6) 13.1561(4) 90 90 90 5335.1(3) 6 100(2) 1.220 1.54178 0.608 0.0684 (0.0889) 0.1498 (0.1586) 31973 10391
C54H40BNO4 777.68 hexagonal P65 24.969(1) 24.969(1) 16.924(1) 90 90 120 9137.7(7) 9 100(2) 1.272 1.54178 0.622 0.1041 (0.1052) 0.2708 (0.2716) 122474 10581
C110H89B2N3O11 1650.46 monoclinic P21 15.2276(3) 15.6443(3) 19.1539(3) 90 108.837(1) 90 4318.55(14) 2 100(2) 1.269 1.54178 0.643 0.0437 (0.0445) 0.1195 (0.1213) 59036 13980
Table 2. Selected Bond Lengths, Bond Angles, and Torsional Angles for Spiroborate 4 in Salts with Various Cations
aniona
salt +
(()-4 · NH2Et2 (S,S)-4 · NH2Et2+ (S,S)-4 · NH2Bn2+ (S,S)-4 · NH2Bn2+ · 0.5 CH3CN · 1.5 H2O
largest smallest average CN · · · Bcore · · · CN CN · · · Bcore · · · CN B-O bond angle (°)b angle (°)b length (Å)
two smallest O-B-O angles (°)c
average of other range of four largest average O-B-O O-B-O-C inter-aryl angles (°)c torsional angles (°)d torsional angle (°)e
A B A Bf A
152.1(2) 143.1(4) 148.8(4) 144.5(5) 139.9(2) 150.2(2)
60.0(2) 61.4(4) 60.0(4) 63.4(5) 63.9(2) 59.6(2)
1.471(4) 1.468(5) 1.466(5) 1.467(8) 1.477(9) 1.469(3)
101.1(2), 103.2(2) 103.1(2), 103.1(2) 102.0(3), 102.6(3) 102.2(5), 102.6(5) 101.5(2), 103.6(2) 101.8(2), 102.7(2)
113.3(2) 112.8(2) 113.2(3) 113.1(5) 113.0(2) 113.2(2)
160.6(2)-169.6(2) 162.4(3)-168.7(3) 165.2(3)-166.1(3) 161.5(5)-172.0(4) 166.0(8), 167.2(9) 165.6(2)-166.1(2)
53.3(3) 55.9(6) 54.4(6) 58.0(8) 57.5(8) 52.4(3)
B
154.8(2)
58.1(2)
1.469(3)
101.8(2), 102.4(2)
113.3(2)
158.8(2)-173.3(2)
51.6(3)
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 6, 6′, 6′′, and 6′′′ positions of the naphthyl groups (CN). 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 Measured between the average aryl planes. f Measured on one disordered part of anion B.
tives of BINOL substituted in the 6,6′-positions.15 In the structure of the salt (()-4 · NH2Et2+, the CN · · · Bcore · · · CN angles defined by the central boron atom of the spiroborate core (Bcore) and the 6, 6′, 6′′, and 6′′′ positions of the naphthyl groups (CN) range from 60.0(2)° to 152.1(2)° (Table 2). If the core of hypothetical tecton 3 were to adopt a similar structure, sites of intermolecular association at the 6, 6′, 6′′, and 6′′′ positions would therefore deviate significantly from a tetrahedral orientation. In contrast, the CPh · · · Bcore · · · CPh angles of spiroborate 2, which are defined by the central boron atom and the 5, 5′, 5′′, and 5′′′ positions of the phenyl groups (CPh), range from an average low value of 69° to an average high value of 171°.5 In part, the different degrees of deviation from an ideal tetrahedral geometry in spiroborates 2 and 4 are determined by the interaryl torsional angles. In various salts of spiroborate 2, the average value of this angle (42°) is typical of other derivatives of biphenyl;5 in spiroborate salt (()-4 · NH2Et2+, however, the average angle is markedly larger (53°), as is normally observed in derivatives of BINOL. As a result, hypothetical tectons 1 and 3 are expected to have well-defined geometries that orient the peripheral sticky sites in distinctly different ways, and association is unlikely to produce networks with the same connectivity and architecture. The crystal structure of spiroborate salt (()-4 · NH2Et2+ can be described as consisting of parallel homochiral chains of either the (S,S)- or the (R,R)-enantiomer of anion 4, linked by intervening counterions (Figures 3 and 4). The integrity of
individual chains is maintained by ionic interactions and by the formation of a total of three significant N-H · · · O hydrogen bonds per anion, involving both N-H bonds of the cation as donors and three oxygen atoms of each spiroborate as acceptors (Figure 3). One of the two N-H bonds of each cation engages in a single short hydrogen bond (1.95(1) Å), and the other forms longer bifurcated hydrogen bonds with two different oxygen atoms (2.36(1) Å and 2.42(1) Å). Two important C-H · · · O interactions between each anion and cation (2.55(1) Å and 2.59(1) Å) make an additional contribution to the cohesion of the chains.16 Each homochiral hydrogen-bonded chain is aligned with the c-axis and is surrounded by four chains of opposite configuration (Figure 4). Interactions between the chains include π-stacking (closest centroid-centroid distance ) 3.850(2) Å) and edgeto-face aromatic interactions. The B · · · B distance between the closest anions in a single homochiral chain is 7.393(4) Å, and the shortest B · · · B distance between chains of opposite configuration is 8.404(4) Å. Structure of Spiroborate Salt (S,S)-4 · NH2Et2+. The high configurational stability of BINOL gave us an attractive opportunity to compare the structure of crystals of salts of spiroborate 4 in racemic and enantiomerically pure form. Similar experiments cannot be carried out with analogous spiroborate 2, which is configurationally unstable under the normal conditions of synthesis and crystallization. There are few previous
1544 Crystal Growth & Design, Vol. 8, No. 5, 2008
Figure 5. View of the structure of crystals of spiroborate salt (S,S)4 · NH2Et2+ grown from CH3CN/DMF, showing part of a central hydrogen-bonded helix and two neighboring helices. In the central helix, spiroborate 4 is drawn with all atoms in red; in the neighbors, anion 4 is shown in contrasting pink. In all three helices, the intervening cations are drawn with carbon atoms in gray, hydrogen atoms in white, and nitrogen atoms in blue. Broken lines of two types are used to represent hydrogen bonds between spiroborate 4 and NH2Et2+, as well as faceto-face aromatic interactions between helices.
Figure 6. Representation of the helical hydrogen-bonded chain observed in crystals of spiroborate salt (S,S)-4 · NH2Et2+ grown from CH3CN/ DMF. In the image, the central boron atom of each (S,S)-spiroborate 4 is shown by a red sphere, and each cation is represented by a blue sphere. Broken lines connecting the spheres represent connectivity controlled by hydrogen bonding. Hydrogen atoms are omitted from (S,S)-spiroborate 4 for clarity.
structural studies of salts of spiroborate 4,10 and none of these studies allows comparison of both enantiomerically pure and racemic forms. Exposure of a solution of salt (S,S)-4 · NH2Et2+in DMF to vapors of CH3CN yielded crystals of composition 4 · NH2Et2+, with no guests included. The crystals were found to belong to the orthorhombic space group P21212. Views of the structure appear in Figures 5 and 6, and crystallographic details are presented in Tables 1 and 2. The unit cell contains (S,S)-spiroborate 4 in two slightly different conformations. Both assume a flattened geometry of D2 symmetry, as in the corresponding racemic crystal, and the two aryl-aryl bonds are approximately parallel. In both conformers, the central B(OC)4 unit has a normal average B-O bond length (approximately 1.47 Å), is elongated tetragonally, and has approximate D2d symmetry. Two O-B-O angles are much smaller than the average of the other four, and the four largest O-B-O-C torsional angles have values in the normal range (Table 2). The CN · · · Bcore · · · CN angles that measure distortion from a tetrahedral geometry range from approximately 60° to 150°, as in the case of the racemic salt, and the interaryl torsional angles have an average value of approximately 55°. Together, these data establish that spiroborate 4 adopts essentially the same molecular structure in both racemic and enantiomerically pure salts.
Tu et al.
The crystal structure of spiroborate salt (S,S)-4 · NH2Et2+, like that of the racemate, consists of homochiral hydrogen-bonded chains in which spiroborate 4 is linked to intervening counterions (Figure 5). Four N-H · · · O hydrogen bonds are formed per anion, and each N-H bond of the cation engages in two asymmetric bifurcated hydrogen bonds with oxygen atoms of spiroborate 4 (H · · · O distances ranging from 1.90(1) Å to 2.35(1) Å). Additional contributions to the stability of the chains are made by ionic interactions, C-H · · · O interactions between the anions and cations (H · · · O distances ranging from 2.71(1) Å and 2.89(1) Å), and π-stacking interactions of neighboring anions within the same chain (closest centroid-centroid distance ) 4.152(3) Å). In racemic crystals of spiroborate salt (()-4 · NH2Et2+, the homochiral hydrogen-bonded chains can pack efficiently by surrounding themselves with chains of opposite configuration, thereby creating multiple face-to-face and edge-to-face aromatic interactions. The same effective packing of chains cannot be achieved in enantiomerically pure crystals of spiroborate salt (S,S)-4 · NH2Et2+. Instead, the hydrogen-bonded chains adopt a helical structure (Figure 6), with three spiroborates per repeat. The helices are aligned with the a axis, are surrounded by six other helices, and interact with their neighbors by engaging in various face-to-face and edge-to-face aromatic interactions. The average B · · · B distance between the closest anions in a single homochiral helix is 6.64(5) Å, and the shortest B · · · B distance between adjacent chains is approximately 12 Å. The racemic and enantiomerically pure salts have similar structures that differ primarily in the detailed architecture of the hydrogen-bonded chains. This difference appears to arise because chains of opposite configuration in a racemic mixture of (S,S)- and (R,R)spiroborate 4 can pack more efficiently than chains of the same configuration. Indeed, the calculated density of the racemic crystals (1.292 g cm-3) is significantly higher than that of the enantiomerically pure form (1.220 g cm-3). Structures of Spiroborate Salt (S,S)-4 · NH2Bn2+. To further probe the packing of (S,S)-spiroborate 4, we prepared its dibenzylammonium salt and crystallized it by cooling a hot solution in CH3CN. This procedure produced two types of crystals. One type proved to have the composition (S,S)4 · NH2Bn2+ and to belong to the hexagonal space group P65. Crystallographic details are presented in Tables 1 and 2.17 The unit cell contains (S,S)-spiroborate 4 in two slightly different conformations, one of which is partly disordered and has not been analyzed in detail. As noted in all other structures of spiroborate 4, both conformers adopt a flattened D2 geometry with parallel inter-aryl bonds. The central B(OC)4 unit has a normal average B-O bond length (1.467(8) Å in the ordered conformer), is elongated tetragonally, and has approximate D2d symmetry. As usual, two O-B-O angles are much smaller than the others, and the O-B-O-C torsional angles are normal (Table 2). The CN · · · Bcore · · · CN angles range from approximately 60° to 145°, and the inter-aryl torsional angles have an average value of approximately 58°. Together, these data support our conclusion that the geometry of spiroborate 4 is essentially invariant in all its salts. The crystal structure of spiroborate salt (S,S)-4 · NH2Bn2+ is very similar to those of salts (()-4 · NH2Et2+ and (S,S)-4 · NH2Et2+, and it consists of homochiral hydrogen-bonded helices in which (S,S)-spiroborate 4 is linked to intervening counterions. One of the two N-H bonds of each cation forms a single short N-H · · · O hydrogen bond (1.96(1) Å for the helix composed of ordered (S,S)-spiroborate 4), and the other forms longer bifurcated hydrogen bonds with two different oxygen atoms
Crystal Structures of Spiroborates
Crystal Growth & Design, Vol. 8, No. 5, 2008 1545
hydrogen bonds with two different oxygen atoms of spiroborate 4 (1.93(2) Å and 2.56(2) Å). The other N-H bond does not interact directly with spiroborate 4 but instead donates an N-H · · · O hydrogen bond (1.94(2) Å) to an intervening molecule of H2O, which in turn forms an O-H · · · O hydrogen bond with a third oxygen atom of spiroborate 4 (1.89(2) Å). Additional interactions within the chains include C-H · · · O hydrogen bonds between the anions and cations (H · · · O distances ranging from 2.38(1) Å to 2.58(1) Å) and various aromatic interactions between neighboring anions and cations. Each chain is surrounded by six others and engages in aromatic interactions with them. The B · · · B distances between the closest anions within the two types of homochiral chains are 7.994(3) Å and 8.259(3) Å. Conclusions
Figure 7. View of the structure of crystals of spiroborate salt (S,S)4 · NH2Bn2+ · 0.5 CH3CN · 1.5 H2O grown from CH3CN, showing two different hydrogen-bonded chains. In one chain, spiroborate 4 is drawn with all atoms in red; in the neighboring chain, anion 4 is shown in contrasting pink. Included cations and H2O are drawn with carbon atoms in gray, hydrogen atoms in white, nitrogen atoms in blue, and oxygen atoms in red. Broken lines are used to represent hydrogen bonds. In the cations, hydrogen atoms are omitted for clarity, except those involved in hydrogen bonding.
(2.09(1) Å and 2.36(1) Å). Cohesion of the helices is also ensured by ionic interactions, C-H · · · O interactions between the anions and cations (2.71(1) Å), π-stacking interactions between neighboring anions (closest centroid-centroid distance ) 3.88(1) Å), and π-stacking interactions between adjacent anions and cations (closest centroid-centroid distance ) 4.43(1) Å). The helices incorporate three spiroborates per repeat, are surrounded by six other helices, and interact with neighboring helices by engaging in various edge-to-face aromatic interactions. The B · · · B distance between the closest anions in a single homochiral helix is 6.820(1) Å, and the shortest B · · · B distance between adjacent chains is approximately 13 Å. The second type of crystals of salt (S,S)-4 · NH2Bn2+ grown from CH3CN was found to have the composition (S,S)4 · NH2Bn2+ · 0.5 CH3CN · 1.5 H2O and to belong to the monoclinic space group P21. A view of the structure appears in Figure 7, and crystallographic details are provided in Tables 1 and 2. The unit cell contains (S,S)-spiroborate 4 in two slightly different conformations. As observed in all other structures of spiroborate 4, both conformers adopt a flattened D2 geometry with nearly parallel inter-aryl bonds. As usual, the B(OC)4 core has B-O bonds approximately 1.47 Å in length, is elongated tetragonally, and has approximate D2d symmetry. Two O-B-O angles are much smaller than the others, and the O-B-O-C torsional angles are normal (Table 2). The CN · · · Bcore · · · CN angles range from approximately 60° to 155°, and the inter-aryl torsional angles have an average value of approximately 52°. Together, these data provide further evidence that spiroborate 4 has an essentially invariant geometry. The crystal structure of the CH3CN/H2O solvate of spiroborate salt (S,S)-4 · NH2Bn2+ closely resembles those of salts (()4 · NH2Et2+, (S,S)-4 · NH2Et2+, and unsolvated (S,S)-4 · NH2Bn2+. The new structure incorporates two different kinds of linear hydrogen-bonded chains in which (S,S)-spiroborate 4 is linked to adjacent counterions (Figure 7). In one kind of chain, one of the two N-H bonds of each cation forms asymmetric bifurcated
Our observations suggest that tectons derived from anionic spiroborates 2 and 4 are promising subunits for constructing crystals and other ordered molecular materials by design. Like spiroborate 2, spiroborate 4 reliably adopts a flattened D2 geometry. Our work, in combination with earlier structural studies of other salts,10 shows that the preferred geometry is largely independent of the identity of the cation, even when it can interact with spiroborate 4 by forming hydrogen bonds. This geometric preference ensures that peripheral sites of intermolecular association added to spiroborate 4 will be oriented in a predictable way. Moreover, spiroborate 4 has many other appealing characteristics: (1) It can be synthesized in one step from boric acid and BINOL; (2) it is readily available in either racemic or enantiomerically pure forms of high configurational stability, unlike analogue 2; (3) derivatives can be readily obtained by electrophilic substitution of BINOL;15 and (4) spiroborate 4 is charged and must cocrystallize with counterions, which can be varied to introduce additional structural diversity. For these reasons, spiroborate 4 and its derivatives are attractive candidates for engineering crystals and other ordered materials with novel structures and properties. Experimental Section Racemic Diethylammonium Bis[[1,1′-binaphthalene]-2,2′-diolato-O,O′] Borate (4). (()-[1,1′-Binaphthalene]-2,2′-diol (2.50 g, 8.73 mmol) was added to a stirred solution of boric acid (0.270 g, 4.37 mmol) and diethylamine (1.35 mL, 13.0 mmol) in acetonitrile (18 mL). The mixture was heated at reflux for 16 h and then cooled to 25 °C. The crystalline precipitate was separated by filtration and dried in vacuo to give racemic diethylammonium bis[[1,1′-binaphthalene]-2,2′-diolatoO,O’] borate (4) (5.01 g, 7.67 mmol, 88%) as analytically pure colorless plates: mp > 300 °C; IR (KBr) 3059, 2372, 1593, 1468, 1334, 1248, 1008, 810, 742 cm-1; 1H NMR (400 MHz, DMSO-d6) δ 7.95 (m, 8H), 7.26–7.34 (m, 8H), 7.15 (m, 8H), 2.86 (q, 3J ) 7.3 Hz, 4H), 1.10 (t, 3 J ) 7.3 Hz, 6H); 13C NMR (100 MHz, DMSO-d6) δ 156.4, 133.1, 129.3, 128.5, 128.4, 126.1, 125.2, 124.9, 122.8, 122.2, 41.8, 11.5; MS (MALDI) m/e 727.3, 580.2, 226.8, 73.7. Anal. Calcd for C44H36BNO4: C, 80.86; H, 5.55; N, 2.14. Found: C, 80.53; H, 5.63; N, 2.25. Diethylammonium Bis[[(S)-1,1′-binaphthalene]-2,2′-diolato-O,O′] Borate ((S,S)-4). The preceding method was used to convert (S)-[1,1′binaphthalene]-2,2′-diol and diethylamine into diethylammonium bis[[(S)1,1′-binaphthalene]-2,2′-diolato-O,O’] borate ((S,S)-4) in 95% yield as colorless needles: mp > 300 °C; [R]20D +277.2 (c1.25, DMSO); IR (KBr) 3481, 3062, 2364, 1587, 1463, 1310, 1248, 1006, 806, 742 cm-1; 1 H NMR (400 MHz, DMSO-d6) δ 7.95 (m, 8H), 7.26–7.34 (m, 8H), 7.15 (m, 8H), 2.86 (q, 3J ) 7.3 Hz, 4H), 1.10 (t, 3J ) 7.3 Hz, 6H); MS (MALDI) m/e 580.2, 226.8, 73.7. DibenzylammoniumBis[[(S)-1,1′-binaphthalene]-2,2′-diolato-O,O’] Borate ((S,S)-4). The same method was used to convert (S)-[1,1′binaphthalene]-2,2′-diol and dibenzylamine into diethylammonium bis[[(S)-1,1′-binaphthalene]-2,2′-diolato-O,O’] borate ((S,S)-4) in 98%
1546 Crystal Growth & Design, Vol. 8, No. 5, 2008 yield as colorless needles: mp > 300 °C; [R]20D +182.0 (c1.01, DMSO); IR (KBr) 3390, 3062, 2364, 1591, 1463, 1332, 1244, 1070, 1008, 813, 748 cm-1; 1H NMR (400 MHz, CDCl3) δ 7.90 (m, 8H), 7.15–7.49 (m, 18H), 6.83 (m, 4H), 6.30 (m, 4H), 3.26 (m, 4H); MS (MALDI) m/e 580.2, 197.9. Anal. Calcd for C54H40BNO4 · H2O: C, 81.51; H, 5.32; N, 1.76. Found: C, 81.44; H, 5.16; N, 1.91. X-Ray Crystallographic Studies. Data were collected using a Bruker AXS SMART 4K/Platform diffractometer with Cu KR radiation. Structures were solved by direct methods using SHELXS-97 and refined using SHELXL-97.18 All non-hydrogen 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.
Tu et al.
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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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