Self-assembly directed by NH⋅⋅⋅O hydrogen bonding: New layered molecular arrays derived from 4-tert- butylbenzoic acid and aliphatic diamines

Abstract

¹H and ¹³C NMR titrations in both CDCl₃ and CD₃OD demonstrate that 4-tert-butylbenzoic acid interacts with both propane-1,2-diamine and propane-1,3-diamine to yield 1:2 host–guest complexes in these solvents. Based on this observation, the isolation of new three-dimensional molecular arrays through cocrystallization of the above diamines and 4-tert-butylbenzoic acid (in a 1:2 molar ratio) has been achieved. X-ray studies of these self-assembled structures show that they incorporate [propane-1,2-diamine⋅(4-tert-butylbenzoic acid)₂] or [propane-1,3-diamine⋅(4-tert-butylbenzoic acid)₂] hydrogen-bonded motifs. Three structural derivatives of the latter type (two monohydrate forms and one anhydrous form) have been characterized. The structures are compared with a previously described three-dimensional array based on the “parent” [ethane-1,2-diamine⋅(benzoic acid)₂] motif. Similarities occur between each of the structures. In each, a two-dimensional “ionic” layer consisting of an extensive network of hydrogen bonds is sandwiched between two “less polar” aromatic ring-containing layers. In the respective ionic layers, the carboxylic acid protons have been transferred onto the amines to yield diammonium cations, with all ammonium protons being involved in hydrogen bonding. In part, the adoption of these unusual layered structures seems to reflect a tendency toward maximization of both the number and strengths of the hydrogen bond interactions in the respective ionic layers.

A major thrust in recent chemical research has been the development of supramolecular chemistry, broadly the chemistry of large multicomponent molecular assemblies in which the component structural units are held together by a variety of weaker (noncovalent) interactions. The latter include hydrogen bonding, dipole stacking, π-stacking, van der Waals forces, and favorable hydrophobic interactions (1). The spontaneous self-assembly of synthetic supramolecular structures, stabilized by interactions of the above type, now has resulted in a wide range of new architectures. One category of this type involves the formation of (stacked) networks between organic components held together in regular repeating arrays (2). The development of controlled strategies for the noncovalent synthesis of such supramolecular frameworks from suitable host and guest components remains a major goal of much of this research. A further motivation for the strong interest in such systems has been the prospect of developing new materials exhibiting potentially exploitable properties (and especially novel opto-electronic properties; refs. 3 and 4).

We have initiated a program involving the self-assembly of complementary organic components to document structure-function relationships within well defined three-dimensional networks. Our approach has been to study the effect of incremental host/guest structural variation on the solid-state arrangements adopted by the resulting networks.

We present here the results of an investigation leading to four new (trilayered) host–guest networks based on structural motifs of the form [propane-1,2-diamine⋅(4-tert-butylbenzoic acid)₂] and [propane-1,3-diamine⋅(4-tert-butylbenzoic acid)₂⋅xH₂O] (x = 0 or 1). This study extends the results reported in our prior communication in which the structure of the corresponding parent supramolecular array, based on a [ethane-1,2-diamine⋅(benzoic acid)₂] motif, was investigated (5, 6).

Materials and Methods

General.

All reagents were analytical reagent grade where available. The ¹H and ¹³C NMR titrations were carried out on a Bruker AMX 300 spectrometer at 25°C. The diamines were obtained commercially and distilled before use. The calculated amount of the solid benzoic acid derivative, previously dried over P₄O₁₀, was added to a solution of the required diamine dissolved in CD₃OD (0.5 ml) to produce a solution of concentration 0.075 mol⋅dm⁻³. Solid carboxylic acid (≈4 mg) then was added incrementally to the NMR tube (the precise amount obtained by the weight difference before and after the addition). Chemical-shift data for the corresponding ¹H and ¹³C spectra were recorded after each addition. The chemical shifts were plotted against the ratio of acid to diamine at each point; a typical titration curve is given in Fig. 1. Electrospray ionization mass spectra were obtained on Bruker BioApex 47e Fourier transform MS instrument. Samples of the 1:2 host–guest complexes were prepared by mixing stoichiometric amounts of the required reactants in methanol and then allowing the solvents to evaporate. The resulting solids were isolated, and an ≈1-mg sample was diluted 1:1,000 in methanol in each case. Raman spectra were collected by using a Renishaw Raman System 3000 microprobe equipped with a charge-coupled device camera. An argon ion laser emitting at 514.5 nm was used for sample excitation. All experiments were performed by using the ×50 objective. The laser power at the sample was ≈1 mW. Spectra were collected over a range from 3,800 to 200 cm⁻¹ using an exposure time of 10 s; the overall collection time was ≈3 min.

Figure 1.

¹H NMR titration plot for the addition of 4-tert-butylbenzoic acid to propane-1,2-diamine in CD₃OD at 25°C.

Crystal Isolation.

Transparent crystals of each of the host–guest species of stoichiometry [propane-1,2-diamine⋅(4-tert-butylbenzoic acid)₂] and [propane-1,3-diamine⋅(4-tert-butylbenzoic acid)₂⋅xH₂O] (x = 0 or 1) were isolated. In all cases except one, crystals were obtained on mixing stoichiometric amounts of the required reactants in methanol and then allowing the solvents to evaporate slowly at room temperature over 1–3 days; two forms of the latter host–guest species (with x = 1) were obtained from separate crystallizations. All crystals were collected and air-dried before being subjected to x-ray structure determinations. For anhydrous [propane-1,3-diamine⋅(4-tert-butylbenzoic acid)₂], crystals formed rapidly from absolute ethanol; these were collected after 18 h. A sample of each host–guest type was subjected to microanalysis. [ethane-1,2-Diamine⋅(benzoic acid)₂]: Found: C, 63.2; H, 6.8; N, 9.2. C₁₆H₂₀N₂O₄ requires C, 63.1; H, 6.6; N, 9.2%; [propane-1,2-diamine⋅(4-tert-butylbenzoic acid)₂]: Found: C, 70.0; H, 9.4; N, 6.5. C₂₅H₃₈N₂O₄ requires C, 69.7; H, 8.9; N, 6.5%; [propane-1,3-diamine⋅(4-tert-butylbenzoic acid)₂]: Found: C, 70.0; H, 9.2; N, 6.0. C₂₅H₃₈N₂O₄ requires C, 69.7; H, 8.9; N, 6.5%.

Computational Methods.

All density functional theory calculations were run using GAUSSIAN 98 (version A.9; ref. 7) and either SGI Origin 3000 or Compaq Alpha Server SC computer systems.

Structure Determinations.

For the triclinic structure a full sphere of four-circle diffractometer data were measured (monochromatic Mo Kα radiation, λ = 0.7107₃ Å), N_(total) reflections merging to N unique (R_int quoted), N_o with I > 3σ(I) considered “observed” and used in the full matrix least-squares refinement, anisotropic thermal parameter forms being refined for C, N, O (x, y, z, U_iso)_H treatment as indicated. For the others full spheres of charge-coupled device data (Bruker AXS instrument) were measured and reduced similarly (“empirical”/multiscan absorption correction), the observed criterion being F > 4σ(F). Conventional residuals R and R_w {weights: [σ²(F) + 0.0004 F²]⁻¹} are quoted at convergence. Neutral atom (complex)-scattering factors were used, and computations were made by using the XTAL 3.7 program system (8). Pertinent results are given below and in Figs. 2–5, the latter showing 20% (room temperature)/50% (low temperature) displacement ellipsoids for the nonhydrogen atoms, with hydrogen atoms having arbitrary radii of 0.1 Å.

Figure 2.

[H₃NCH₂CH(CH₃)NH₃⋅(4-tert-butylbenzoic acid-C₆H₄CO₂)₂]. (Upper) Unit cell contents projected down b. (Lower) View of the cation/carboxylate hydrogen-bonded web about z = 0.5. Protonated base cations and carboxylate anions are denoted as b and a, respectively.

Figure 5.

[H₃N(CH₂)₃NH₃⋅(4-tert-butylbenzoic acid-C₆H₄CO₂)₂]. H₂O [β (triclinic) form]. (Upper) Unit cell contents projected down c. (Lower) A section of the cell about x = 0.5, showing the hydrogen bonding.

Crystal/Refinement Data.

Achievement of well formed crystals of optimal size presented difficulties, the crystals not being amenable to cutting, presumably because of crazing in the crystal along planes evident in the structures as subsequently determined (see Figs. 2–5).

[H₃NCH₂CH(CH₃)NH₃⋅(4-tert-butylbenzoate)₂].

C₂₅H₃₈N₂O₄,M = 430.6. Monoclinic, space group C2 (C, No. 5), a = 19.242(8), b = 6.145(2), c = 21.133(8) Å, β = 102.149(6)°, V = 2,443 ų. D_c (Z = 4) = 1.17₁ g cm⁻³. μ_Mo = 0.8 cm⁻¹; specimen: 0.55 × 0.45 × 0.21 mm. T_min,max = 0.79, 0.96. 2θ_max = 58°; N_t = 11,168, n = 3,158 (R_int = 0.015), N_o = 2,972; R = 0.032, R_w = 0.040. |Δρ_max| = 0.25(1) e Å⁻³. T ≈ 153 K.

Variata.

CH₃ and NH₃ were distinguished on the basis of refinement behavior and lattice interactions. Friedel data were merged, and no attempt was made to establish the absolute chirality of the specimen crystallographically. (x, y, z, U_iso)_H were refined throughout.

[H₃N(CH₂)₃NH₃⋅(4-tert-butylbenzoate)₂].

M = 430.6. Orthorhombic, space group Fdd2 (C, No. 43), a = 19.211(5), b = 78.89(2), c = 6.486(17) Å, V = 9,830 ų. D_c (Z = 16) = 1.16₄ g cm⁻³. μ_Mo = 0.8 cm⁻¹; specimen: 0.39 × 0.35 × 0.03 mm (no correction). 2θ_max = 57°; N_t = 23,964, n = 3,259 (R_int = 0.096), N_o = 1,302; R = 0.062, R_w = 0.14. |Δρ_max| = 0.54 e Å⁻³. T ≈ 294 K.

Variata.

Acquisition of a single crystal specimen was not possible for this material; accordingly a single reciprocal lattice component dissected from the overall diffraction pattern was used. The hydrogen-bonded component of the structure is poorly defined, with carboxylate and cation groups being modeled as disordered over two sets of sites and occupancies set at 0.5 after trial refinement. (x, y, z, U_iso)_H were constrained in refinement.

[H₃N(CH₂)₃NH₃⋅(4-tert-butylbenzoate)₂·H₂O].

This monohydrate, M = 448.6, was obtained in two phases, “α” (orthorhombic) and “β” (triclinic).

The α Form.

Orthorhombic, space group Pbca (D, No. 62), a = 16.384(1), b = 8.305(2), c = 38.222(5) Å, V = 5,201 ų. D_c (Z = 8) = 1.14₆ g cm⁻³. μ_Mo = 0.8 cm⁻¹; specimen: 0.45 × 0.35 × 0.11 mm; T_min,max = 0.88, 0.96. 2θ_max = 50°; N_t = 51,388, n = 4,515 (R_int = 0.044), N_o = 3,781; R = 0.057, R_w = 0.088. |Δρ_max| = 0.70(2) e Å⁻³. T ≈ 153 K.

Variata.

(x, y, z, U_iso)_H were constrained in refinement.

The β Form.

Triclinic, space group P (C, No. 2), a = 19.344(5), b = 13.768(4), c = 9.813(7) Å, α = 87.99(5), β = 89.07(4), γ = 87.91(2)°, V = 2,610 ų. D_c (Z = 4) = 1.14₂ g cm⁻³. μ_Mo = 0.8 cm⁻¹; specimen: 0.35 × 0.25 × 0.12 mm (no correction). 2θ_max = 50°; N_t = 14,461, n = 7,247 (R_int = 0.045), N_o = 3,134; R = 0.051, R_w = 0.060. |Δρ_max| = 0.51(5) e Å⁻³. T ≈ 295 K.

Variata.

(x, y, z, U_iso)_H were refinable for those hydrogen atoms involved in hydrogen bonding, those for the remainder being constrained.

Results and Discussion

NMR Studies.

In previous studies both ¹H and ¹³C NMR titrations have proven useful for detecting host–guest complex formation in solution and yielding the stoichiometry of the host–guest product(s) so formed (9). The technique also has proved generally of value for indicating the host–guest interaction sites. In the present study the titration procedure involved the monitoring of the induced chemical shift of a proton or carbon signal from a ⋅⋅⋅CH₂⋅⋅⋅ group adjacent to the amine as carboxylic acid was added incrementally to a solution of the diamine in CDCl₃ or CD₃OD. As also reported for ethane-1,2-diamine (10), both the ¹H and ¹³C NMR titrations in each case demonstrated that 4-tert-butylbenzoic acid yields a 2:1 (acid/diamine) species in both chloroform and methanol with both propane-1,2-diamine and propane-1,3-diamine. In particular cases a clear 1:1 end point was also apparent, indicating the stepwise formation of the 2:1 species (for example, see Fig. 1). Parallel titrations in which benzoic acid was substituted for 4-tert-butylbenzoic acid also indicated similar 2:1 (and, in particular instances, also 1:1) host–guest behavior.

Description of the Structures.

In our previous study (10) cocrystallization of a 2:1 mixture of benzoic acid and ethane-1,2-diamine from methanol yielded the corresponding 2:1 supramolecular array incorporating a hydrogen-bonded network in which the acid protons have been transferred to the amine nitrogens, with the formation of charge-separated hydrogen bonds. In this structure, which crystallizes in the space group Pca2₁, the doubly protonated ethane-1,2-diammonium cations straddle planes at ≈z = 3/8, 7/8, hydrogen bonding to deprotonated carboxylate “heads” of the benzoate moieties that approach from either side with planes parallel making up “sandwiches” with aromatic hydrocarbon peripheries enclosing the hydrogen-bonded ionic component of the structure. Hydrogen bonds occur between adjacent benzoate anions and ethane-1,2-diammonium cations from two different motif units, with each benzoate participating in three strong hydrogen bonds. A pair of benzoate sheets thus effectively sandwich an ionic sheet of protonated ethane-1,2-diamine ions, with the charge separation present resulting in the amine layer being formally positively charged, and the adjacent benzoate layer bears a formal negative charge. Overall, the layers yield a ...bab′bab′... pattern in which b and b′ represent nonidentical benzoate layers, and a is the hydrogen-bonded amine layer. The stacking of these planar layers leads to the adjacent layers being bound together by edge-to-face (T-oriented) aromatic interactions that result in a herringbone arrangement. As well as this x-ray study, a neutron diffraction investigation and a density functional theory computational study were used to investigate the nature of the intermolecular interactions in this unusually layered molecular system. With respect to the above, related proton-transfer behavior in a series of supramolecular structures derived from 3,5-dinitrobenzoic acid with diamines (including ethane-1,2,-diamine) has been reported recently (11).

The present arrays all show similarities to the structure of our previously reported array discussed above. In each case the 1:2 (diamine/4-tert-butylbenzoate) stoichiometries of the new systems (two are hydrated) are confirmed. Also in each, sandwich structures occur wherein tert-butyl peripheries enclose diamine/carboxylate/(water) cores. Three products incorporating propane-1,3-diamine and one with propane-1,2-diamine were investigated and yielded structures of diverse complexity, albeit in each case the interface between the hydrocarbon layers presumably comprises the prominent cleavage plane of the crystalline material.

The simplest of these arrays are the two anhydrous salts. The [H₃NCH₂CH(CH₃)NH₃]²⁺ derivative crystallizes with one molecule devoid of crystallographic symmetry as the asymmetric unit of a chiral structure (Fig. 2 Upper), albeit derivative of a racemic amine, in which the planes of the sandwiches lie normal to c*. The detail of the hydrogen-bonded network is shown in Fig. 2 (Lower), wherein one of the carboxylate oxygens, O(21), is “chelated” by the two charged NH₃ groups of the base [O(21)⋅⋅⋅H(01a, 04c) (x, 1 + y, z) 1.95(2), 1.96(2) Å], and the others interact predominantly on a 1:1 basis with NH₃ hydrogens to make up the web [O(11)⋅⋅⋅H(01c) (1 − x, y, 1 − z) 1.85(3), O(12)⋅⋅⋅H(01b), H(04b) (½ − x, y − ½, 1 − z) 1.90(3), 1.83(3), O(22)⋅⋅⋅H(04a) 1.81(3) Å] such that all oxygen and NH₃ hydrogen atoms are involved in mutual interactions with H⋅⋅⋅O ranged between 1.81(3) and 1.96(3) Å and N⋅⋅⋅H between 0.87(3) and 0.95(3) Å. The C, N—C—C—N torsion angles are 51.3(7), −72.0(2)°, whereas the dihedral angles between carboxylate and aromatic C₆ planes for the pair of anions are 7.2(1), 6.1(1)°.

The structure of the other anhydrous salt, namely that incorporating the [H₃N(CH₂)₃NH₃]²⁺ cation, also has one formula unit devoid of crystallographic symmetry comprising the asymmetric unit and, regrettably, is confused by disorder about the central plane of the sandwich. The latter involves the cation and the carboxylate component of the anion (Fig. 3), with a complexity that does not justify discussion within the present space constraints. This complex, however, also has been crystallized as a monohydrate, which, remarkably, has been defined in two polymorphs, similar to the above in respect of their sandwich aspect but with the acid/base contributors to the hydrogen bonding about the central plane augmented by the addition of the water molecule. In the orthorhombic α form, one formula unit devoid of crystallographic symmetry comprises the asymmetric unit of the structure; in the triclinic β form, there are two such units.

Figure 3.

[H₃N(CH₂)₃NH₃⋅(4-tert-butylbenzoic acid-C₆H₄CO₂)₂]. Unit cell contents projected down c.

In the orthorhombic α form (Fig. 4), the polar components of the various moieties are brought to a focus about (¼, y, ¼, etc.) as hydrogen-bonded columns with more tenuous links across x = ½. Within the column, O(11) also hydrogen-bonds to cationic hydrogens [O(11)⋅⋅⋅H(01c) (1 − x, ½ + y, 1½ − z) 1.8 Å], as do O(12) [O(12)⋅⋅⋅H(05b) (x, 1 + y, z) 1.9 Å] and O(21) [O(21)⋅⋅⋅H(05a) (x, 1 + y, z), H(05c) (½ − x, ½ + y, z) 1.9(x2) Å]. O(12) also hydrogen-bonds to the water molecule [O(12)⋅⋅⋅H(1b) (½ − x, ½ + y, z) 1.9 Å] as does O(22) [O(22)⋅⋅⋅H(01a) 1.8 Å]; the water molecule oxygen also is an acceptor to cationic hydrogen: O(1)⋅⋅⋅H(01b) 1.9 Å. All cationic hydrogens thus interact with water or anionic oxygen atoms without chelation (if that were a possibility in the present system); torsion angles about successive C—C bonds are 178.8(2), 60.7(3)°, whereas aromatic C₆/carboxylate interplanar dihedral angles are 15.71(9), 2.84(9)°.

Figure 4.

[H₃N(CH₂)₃NH₃⋅(4-tert-butylbenzoic acid-C₆H₄CO₂)₂]. H₂O [α (orthorhombic) form]: unit cell contents projected down b.

The β phase is similar, the structure being bounded by two faces of the cell (x = 0, 1), with the hydrogen-bonding entities disposed about x = 0.5 (Fig. 5). In this case, however, these interactions are dispersed through that sheet, with the cations lying within the sheet here rather than along a column as in the α phase. Cation torsions are −168.9(5), −179.5(5)° (cation 1), 84.0(6), 172.8(5)° (cation 2), with the C₆/CO₂ dihedrals being 0.9(2), 6.0(2), 11.9(2), and 6.2(2)°.

The ionic layer in each of the present structures contains a hydrogen-bonded network that thus resembles the pattern found in the crystal of the parent [propane-1,2-diamine⋅(4-tert-butylbenzoic acid)₂] structure (10). However the “herringbone” stacking of the benzoic acid groups observed in the parent structure does not occur in the present series, which presumably reflects steric perturbation preventing adoption of a similar arrangement by the bulky tert-butyl groups in the para positions of the carboxylic acid moieties. Nevertheless, in each of the present structures the presence of an ionic/polar layer sandwiched between two “aromatic” layers is preserved.

Mass Spectroscopic Studies.

Peaks for both the propane-1,2-diamine and propane-1,3-diamine “adducts” with 4-tert-butylbenzoic acid were observed in the electrospray ionization mass spectra as ions corresponding to [diamine/carboxylic acid⋅H]⁺ and [diamine/(carboxylic acid)₂⋅H]⁺. As well as these parent peaks, a large number of cluster progressions, built on the addition of diamine/carboxylic acid fragments, also were observed. The respective spectra were complex and very dependent on the electrospray and source chamber conditions, and the nature of these latter clusters was not pursued further. However, the experiments demonstrate that gas-phase clusters occur that are related closely to the respective solid-state structures and solution species described herein, adding further evidence for the strength of the intermolecular interactions in such systems.

IR and Raman Studies.

The IR and Raman spectra of the representative arrays, [ethane-1,2-diamine⋅(benzoic acid)₂] and [propane-1,3-diamine⋅(4-tert-butylbenzoic acid)₂⋅H₂O] (α form), have been investigated. The spectra of solid samples of both products are in general accord with the observed proton transfers discussed above having occurred. By way of example, the Raman spectrum of the first of these products is shown in Fig. 6 together with the spectra of its separate components. Prominent bands assigned to ν(C⩵O) and ν(C⋅⋅⋅O⋅⋅⋅H) in the IR and Raman spectra of the free acid dimer (12) are not observed in the spectra of the solid host–guest product. Rather, bands assigned to ν_S (COO⁻) and γ_S (COO⁻), characteristic of the −CO moiety (12, 13), are observed (e.g., at 1,391 and 840 cm⁻¹, respectively, in the Raman spectrum given in Fig. 6); these and others correspond closely with Raman bands observed for the free benzoate ion. Some broadening, even splitting, of the band assigned to ν_S (COO⁻), is observed and may arise from the inequivalence of hydrogen bonding as shown in the corresponding crystal structures. Similar spectral behavior was observed for the solid [propane-1,3-diamine⋅(4-tert-butylbenzoic acid)₂] array.

Figure 6.

Raman spectra of (a) solid [ethane-1,2-diamine⋅(benzoic acid)₂], (b) ethane-1,2-diamine, and (c) benzoic acid.

The Raman spectra in the high wavenumber region is also in accordance with proton transfer having occurred in each case. For example, in free ethane-1,2-diamine, strong, broad bands at 3,303 and 3,362 cm⁻¹ may be assigned to ν(NH₂). These bands disappear on host–guest complex formation, which is consistent with the formation of ⋅⋅⋅NH, which is a poor Raman scatterer.

A benzene ring mode lying near 800 cm⁻¹ in the spectra of each of the free acids moves to a higher wavenumber and increases in Raman-band intensity on substitution of tert-butyl- for H⋅⋅⋅, which is consistent with the former enhancing the electron density of the delocalized π-cloud. The π-stacking in the host–guest array, as shown in the crystal structure, would be expected to reduce the π-electron delocalization in the ring. Such diminished delocalization is indicated by a shift to lower wavenumber and a reduction in the intensity of this Raman band. In the case of the benzoic acid-containing array, the band disappears completely, which may reflect the close π-stacking that occurs in this product (relative to the situation in the 4-tert-butylbenzoic acid-containing array).

Computational Studies.

Calculations of sufficient accuracy to take account of long-range hydrogen-bonding forces were found to be impractical for the present systems. In our previous studies of the 1:2 ethane-1,2-diamine/benzoic acid assembly we selected an isolated, discrete 1:2 benzoic acid/diamine unit as a key repeating element present in the structure. However, this approach of course does not account for the full array of hydrogen bonding and/or π-stacking interactions involved in the assembly of a structure of this type. In fact, calculations of this nature proved unsuccessful (even with the incorporation of an Onsager solvation model; ref. 14) in modeling the proton transfer observed in the solid state (10). In the present study we repeated these calculations with a larger basis set incorporating diffuse functions and a density functional theory function that is better suited for studying hydrogen bonds (pw91pw91/6311+G*; refs. 15 and 16). However, as before, proton transfer was not predicted; because of this, an alternative strategy was used.

The proton transfer in systems of this type may be considered to parallel the formation of zwitterions within amino acids. Within an appropriate pH range, amino acids have been well documented to exist as zwitterions in solution. However, in the gas phase the situation is less straightforward. For example, gas-phase calculations predict that glycine will exist in its nonzwitterionic form (17), although subsequently it has been suggested that arginine exists as a zwitterion in this phase (18). Certainly, the conventional wisdom is that zwitterion formation is promoted by the presence of solvent molecules (17). To probe the issue of proton transfer (or pseudo-zwitterion formation) in the present systems, a series of model calculations was run on 1:1 adducts of aminoethane and benzoic acid. Optimizations at the pw91pw91/6–311+G* level in the absence of a solvent (or dielectric) field, commencing from either proton-transferred or neutral hydrogen-bonded forms, yielded identical minima that corresponded to conventional “neutral” hydrogen bonds. In contrast, the same calculations repeated in the presence of a polarizable conductor calculation model (COSMO; ref. 19) using the dialectic of water yielded the proton-transferred hydrogen-bonded form of this adduct.

Although it is clear that further work is required to understand the nature of the interactions in the present systems more fully, it seems evident that the formation of the observed proton-transferred hydrogen bonds is promoted in some way by the presence of the extended hydrogen-bonded networks and/or by the substantial dielectric that will exist between the charged layers of the present arrays.

Concluding Remarks.

In summary, it is demonstrated that benzoic acid and/or its 4-tert-butyl derivative self-assembles with ethane-1,2-diamine, propane-1,2-diamine, or propane-1,3-diamine to yield related 2:1 aggregates held together by extensive hydrogen-bonding and π-stacking interactions. The resulting structures consist of layered three-dimensional arrays in which hydrogen-bonded layers are sandwiched between layers incorporating the tert-butyl-substituted aryl rings. The former layers incorporate charge-separated hydrogen bonds. Evidence suggests that the repeating trilayered arrangement generated in the present structures is to a large degree a reflection of the tendency for formation of the extensive hydrogen networks (incorporating the strong charge-separated bonds) in these systems.