CRYSTAL GROWTH & DESIGN 2007 VOL. 7, NO. 5 989-1000
ReView Supramolecular and Host-Guest Chemistry of Bis-phenol and Analogues Rupam J. Sarma† and Jubaraj B. Baruah* Department of Chemistry, Indian Institute of Technology, Guwahati, 781 039, India ReceiVed December 10, 2006
ABSTRACT: Recent developments on structural studies of bis-phenols and related compounds are systematically discussed. The effects of substituents on bis-phenols to control the size and shape of cavities in assemblies of bis-phenols and the weak interactions in anion-assisted assemblies of bis-phenols are narrated to illustrate pseudo-polymorphism. The possible impact of the occurrence of a high Z′ value in different polymorphs of bis-phenol and related compounds is presented. 1. Introduction Self-assembly and molecular recognition are two interrelated phenomena having important implications in chemistry and biology. Design and synthesis of host molecules capable of binding various guests including cations, anions, and neutral molecules have been studied from supramolecular perspectives. In this regard, the synthetic and structural aspects of cavitands capable of binding small guest molecules have been investigated by different groups.1-5 Among the other cyclic phenolic derivatives, calixarenes are extensively studied and reviewed.6-10 Both calix-arenes and their derivatives, cavitands, are capable of adopting a bowl-shaped conformation that enables accommodation of guest molecules. On the other hand, bis-phenols are generally considered useful building blocks for polyethers and polycarbonates, and this aspect has already been reviewed.11 However, the self-assembly of discrete bis-phenol units leading to supramolecular structures has immense scope in guest binding and vis-a`-vis molecular recognition. Bis-phenols leading to ladder-like structures are structurally comparable to metalloorganic ladder frames, and they have been discussed recently.12 Bis-phenol compounds can have more than one independent molecule or formula unit in a unit cell giving rise to a higher Z′ value,13 and the polymorphism occurring due to a different Z′ value has been of great interest in recent years.14-16 The aim of this review is to present the structural aspects of bis-phenols and their analogous compounds and hence identify the possibility of using them for constructing host-guest assemblies. Selective examples have been chosen to illustrate both abovementioned aspects in bis-phenol chemistry published in the last 15 years. In this review, we emphasize the fact that understanding of different bis-phenol synthons in a systematic manner is of importance as far their self-assemblies are concerned. * To whom correspondence should be addressed. E-mail: juba@ iitg.ernet.in. † Present address: Department of Organic Chemistry, University of Geneva, Sciences II 30, quai Ernest Ansermet CH-1211 Geneva 4, Switzerland.
2. General Description of Bis-phenols Bis-phenols, in a broad sense, can be defined as those organic compounds in which appropriately designed spacers connect two phenol units to each other. As the definition suggests, there are numerous ways by which the phenol units can be connected to construct bis-phenol motifs. However, the terminology has been restricted to those bis-phenols that have a methylene bridge separating the two phenol units, such as described by structure 1. Within this structural framework, some examples are bisphenol A (2), bis-phenol C (3), and bis-phenol F (4), tris-phenol, tetrakis-phenol, etc. The interesting feature of these compounds is their easy synthetic procedures. Direct condensation of phenols with carbonyl compounds is one of the most general yet effective ways to prepare bis-phenols. The condensation of phenols with carbonyl compounds can be performed in mineral acids,17-24 heteropolyacids as catalysts (e.g., phosphotungstic acid, H3PW12O40‚25H2O), cation exchange resin as catalyst25 (such as DOWEX 50WX426), with base,27-28 or under photochemical conditions.29 3. Structural Features of Bis-phenols The ability to predict and control intermolecular interactions in the solid state may have ramifications in the design and synthesis of solids and crystalline materials that have desired composition, topology, and reactivity.30 In this regard, the hydrogen-bonded assemblies of bis-phenols that have trigonal or tripodal geometries are interesting because they lead to the formation of network structures. Thus, the study of such systems would help in understanding different types of interactions between bis-phenol molecules in the solid state. Generally, O-H‚‚‚O hydrogen-bonding interactions between hydroxy groups are strong and highly directional. The hydroxy groups of phenolic derivatives are more acidic than that of alcohols, and hence they can form stronger hydrogen bonds as donors with suitable hydrogen-bonding acceptors. In certain cases, the
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oxygen atom of the hydroxy group of the phenol can also behave as an efficient hydrogen-bond acceptor. It is reasonable to expect that the presence of two such O-H groups in a bis-phenol would impart intrinsic molecular and supramolecular dimensions to a bis-phenol. Apart from O-H‚‚‚O interactions, a few other intermolecular interactions31 such as C-H‚‚‚O, O-H‚‚‚π, O-H‚‚‚N are also important in determining the solid-state structures of substituted bis-phenols. However, these C-H‚‚‚O and O-H‚‚‚π interactions are weak in nature compared to conventional hydrogen bonds. The bis-phenolic units are constructed around a sp3 carbon, and thus it should provide geometrical features while forming self-assembled structures. As far as the synthesis of porous materials is concerned, it is desirable to identify molecules that can generate open framework structures by self-assembly. In this regard, the presence of strong hydrogen-bonding groups and structural rigidity is underlined. The bis-phenol, bis(4-hydroxyphenyl)(phenyl)-methane (7) crystallizes in the R3h space group (rhomohedral) as observed in β-hydroquinone lattice,32-35 Dianins’ compound36 and in some of the analogues, and more recently in 2,2,6,6-tetramethyl4,4-terphenyldiol.37 Molecule 7 has “T” geometry (Figure 1), and the hydroxy groups form strong O-H‚‚‚O hydrogen bonds compared to the methyl-substituted analogue. The crystal structure shows that one of the hydroxy groups (O2-H) of six bis-phenol molecules are hydrogen bonded via a molecule of water resulting in the formation of a cyclic hexameric network “A” (that leads to the formation of a water channel having diameter of 5.6 Å) along the c-direction.38 The hexameric units are connected through helical chains of hydrogen bonds involving the hydroxy groups. The situation is in sharp contrast to the hydrogen-bonding pattern observed in bis(4-hydroxy-3,5dimethylphenyl)-phenyl methane (Figure 1), indicating that the presence of methyl substituents ortho- to the hydroxy group restricts the orientation of the O-H‚‚‚O bonds along with a concomitant reduction of the bond strengths. The O1H1o‚‚‚O1′ (dH‚‚‚O 1.860 Å; < O-H‚‚‚O 171.95°) hydrogenbonding interactions between the hydroxy groups of the bisphenol molecules generate helical chains (B) that run down the c-axis. These types of hexameric water channels were reported recently in calixarene derivatives.39 The dihydroxy aromatic compounds have been very useful in guest-host binding ability, although they are comparable to bis-phenolic compounds in terms of the number of phenolic groups; the structural aspects are not dealt in this review but are discussed in terms of structural rationalization. The crystal structure of dihydroxy aromatics such as 1,4-dihydroxybenzene (hydroquinone) and its polymorphs including the β-hydro-
Review
quinone lattice is well established.32-35 The β-hydroquinone lattice belongs to the space group R3h and has the ability to form inclusion compounds with different guest molecules,39-40 including Ne, HF, H2S, MeOH, and even C60. This structure of hydroquinone is also important because of its close resemblance to the rhombohedral lattice of β-polonium, which is also known to form inclusion compounds. The crystal structure of the bisphenol 4,4′-terphenyldiol (8) as a 1:1 inclusion complex with dimethylsulfoxide was recently reported.41 However, the homologous bis-phenol 2,2′,6,6′-tetramethyl-4,4′-terphenyldiol (9) does exhibit polymorphism (Figure 2), and two polymorphs are reported.41 This compound 9 can crystallize in both rhombohedral and monoclinic space groups as polymorphs, thereby revealing striking similarities. Compounds such as 2,5-diphenylhydroquinones (10)42a and 1,1-bis(4-hydroxyphenyl)-cyclohexane (11) are also known to form inclusion compounds with suitable guests molecules. Thus, compound 10, which is axially chiral, have been used for studying molecular inclusion where it behaves as a chiral host. It is found that N-alkylchinchonidinium halides42b are effective substrates for the resolution of these chiral bis-phenols. Similarly, the inclusion complex of 11, water, and acetylacetone (1:1:1) is also characterized43 that reveals selective inclusion of the enol form of acetylacetone as in Figure 3.
With this objective, the hydrogen-bonded assembly of the bis-phenols such as 4,4′-bis(4-hydroxyphenyl)cyclohexanone and 4,4′-bis(3-methyl-4-hydroxyphenyl)cyclohexanone (12a,b) have been studied. While the former gives rise to self-inclusion host-guest structures, the latter exhibits interpenetration of nonidentical networks.44
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Figure 1. (a) Structure of 7; (b) formation of hexameric network (A) of hydrogen bonds involving the phenol (O2-H) and a molecule of water (O3) and the helical chains (B) involve hydrogen bonding between the O1-H groups; (c) hydrogen bond in the region B.
The inclusion of three picoline isomers with 2,2′-binaphthol occurs due to the formation of strong intermolecular O-H‚‚‚N hydrogen-bonding interactions.45 It is observed that 2- and 4-picoline can selectively be enclathrated, which has been confirmed by thermal stability measurements and competition experiments. The structure of the 2-picoline inclusion compound is stabilized by strong intermolecular host‚‚‚guest interactions that involve Ohost-H‚‚‚Nguest interactions, with a hydrogenbonding distance of 2.697(2) Å. Inclusion complexes of bisphenols with amines have been studied44 from the viewpoint of supramolecular chemistry, and it is observed that like neutral phenols, phenolates are also important supramolecular motifs. It is also reported that 2,2′-bis(4-hydroxyphenyl)propane forms 1:1 complexes between and several alcohols, amines, hydrazine.46 In case of the complex of 2,2′-bis(4-hydroxyphenyl)propane with methyl hydrazine (13), it is observed that the two components are connected by O-H‚‚‚N hydrogen bonds;47 one of the hydroxy groups of the phenol is linked to the amino group, while the other is linked to the methylamine nitrogen resulting in a centrosymmetric dimeric structure.
Figure 2. Structures of 4,4′-terphenyldiol, 8 and 2,2′,6,6′-tetramethyl4,4′-terphenyldiol, 9; (a) polymorph of 9 that contains O-H‚‚‚O hexamers, in the R3h space group and (b) the other polymorph of containing infinite O-H‚‚‚O bonded chains in the P21/c space group.
The substituent effect and molecular recognition in bis-phenol can be best understood from the crystal structures of some bisphenols, bis(4-hydroxy-3,5-dimethyl phenyl)(aryl)methanes (15).38,49-52
Hydrogen-bonding interactions between the bis-phenol 14 and bis (2-aminoethyl)-amine in the 1:3 inclusion complex lead to the formation of novel three-dimensional (3D) frameworks.48 The molecular structure shows that in this inclusion complex one of the bis-phenol molecules occupies a general position while the other is across a two-fold axis, together with the amine molecule, each with an occupancy of 0.5. The bis-phenol molecule that occurs in the general position is neutral, while the one lying on the 2-fold axis is anionic, because both of the protons are transferred to the amine.
The bis(4-hydroxy-3,5-dimethylphenyl) (4-nitrophenyl)methane (16) forms inclusion complexes with benzene (16) (Figure 4). In this case, hydrogen-bonding interactions between the hydroxy groups of the bis-phenol leads to the formation of infinite O2-H‚‚‚O1 (dH‚‚‚O1 2.569 Å,
