Molecular tectonics. Porous hydrogen-bonded networks built from derivatives of 9,9'-spirobifluorene

Molecules with multiple sites that induce strong directional association tend to form open networks with significant volumes available for the inclusion of guests. Such molecules can be conveniently synthesized by grafting diverse sticky sites onto geometrically suitable cores. The characteristic inability of 9,9'-spirobifluorene to form close-packed crystals suggests that it should serve as a particularly effective core for the elaboration of molecules designed to form highly porous networks. To test this hypothesis, various new tetrasubstituted 9,9'-spirobifluorenes with hydrogen-bonding sites at the 3,3',6,6'-positions or 2,2',7,7'-positions were synthesized by multistep routes. Four of these compounds were crystallized, and their structures were determined by X-ray crystallography. In all cases, the compounds form extensively hydrogen-bonded networks with high porosity. In particular, 43% of the volume of crystals of 3,3',6,6'-tetrahydroxy-9,9'-spirobifluorene (28) is available for the inclusion of guests, whereas the porosity is only 28% in crystals of tetrakis(4-hydroxyphenyl)methane, a close model that lacks the spirobifluorene core. Similarly, the porosities found in crystals of 2,2',7,7'-tetra(acetamido)-9,9'-spirobifluorene (33) and 2,2',7,7'-tetrasubstituted tetrakis(diaminotriazine) 39 are 33% and 60%, respectively. Moreover, the porosity of crystals of 2,2',7,7'-tetrasubstituted tetrakis(triaminotriazine) 40 is 75%, the highest value yet observed in crystals built from small molecules. These observations demonstrate that a particularly effective strategy for engineering molecules able to form highly porous networks is to graft multiple sticky sites onto spirobifluorenes or other cores intrinsically resistant to close packing.     

Molecular tectonics. Porous hydrogen-bonded networks built from derivatives of pentaerythrityl tetraphenyl ether

The symmetric four-armed geometry of pentaerythrityl tetraphenyl ether (5) makes it a valuable starting point for building complex molecular and supramolecular structures. In particular, it provides a core to which multiple sites of attractive intermolecular interaction can be attached, thereby creating compounds predisposed to form complex networks by association. To facilitate exploitation of the pentaerythrityl tetraphenyl ether core in such ways, we have prepared more than 20 new derivatives by efficient methods. Of special interest are compounds 3 and 4, which incorporate four diaminotriazine groups attached to the meta and para positions of the pentaerythrityl tetraphenyl ether core. Crystallization of compounds 3 and 4 from DMSO/dioxane is directed by hydrogen bonding of the diaminotriazine groups according to well-established motifs, thereby producing three-dimensional networks. In forming these networks, each molecule of compound 3 forms a total of 12 hydrogen bonds with six others, whereas each molecule of compound 4 forms a total of 16 hydrogen bonds with four others. Both networks are highly porous and define significant interconnected channels for the inclusion of guests. In crystals of compounds 3 and 4, the fraction of the volume accessible to guests is 66% and 57%, respectively. In both cases, the pentaerythrityl tetraphenyl ether cores adopt conformations that deviate substantially from tetrahedral geometry. It is noteworthy that the inherent flexibility of the core does not favor the formation of close-packed guest-free structures.     

Molecular tectonics. Dendritic construction of porous hydrogen-bonded networks

Molecules that associate to form porous networks can be made by attaching multiple hydrogen-bonding sites to suitable cores. Pentaerythrityl tetraphenyl ether, a four-armed core, is the progenitor of dendritic derivatives with more arms, including dipentaerythrityl hexaphenyl ether 7. An advantage of such dendritic derivatives is that the resulting networks are held together by larger numbers of intermolecular hydrogen bonds. [structure: see text]     

Molecular tectonics. Construction of porous hydrogen-bonded networks from bisketals of pentaerythritol

2,4,8,10-Tetraoxaspiro[5,5]undecanes tetrasubstituted at the 3 and 9 positions with groups incorporating diaminotriazines can be used for the construction of extensively hydrogen-bonded networks by the strategy of molecular tectonics. Four such compounds, tectons 1-4, were made by short and efficient syntheses involving bisketalization of pentaerythritol and subsequent reactions. Unlike tectons typically used in previous studies, compounds 1-4 are flexible and chiral, and they orient four sticky diaminotriazine groups in a distorted tetrahedral geometry. Tecton 1 crystallizes from DMF/toluene as an inclusion compound of approximate composition 1.8DMF.xH2O. In the resulting structure, each tecton participates in a total of 16 hydrogen bonds. Eight of these bonds involve four principal neighbors, and the tectons linked in this way define a distorted diamondoid network. Despite 8-fold interpenetration, 60% of the volume of the network is available for including guests. The guests are disordered and occupy parallel helical channels that have cross sections of approximately 11 x 12 A2 at the narrowest points. These channels provide access to the interior of the crystals and permit guests to be exchanged quantitatively without loss of crystallinity. It is noteworthy that tecton 1, despite its flexibility, small size, and structural simplicity, is apparently unable to find a periodic three-dimensional structure in which the dictates of hydrogen bonding and close packing are satisfied simultaneously.     

Engineering hydrogen-bonded molecular crystals built from derivatives of hexaphenylbenzene and related compounds

Hexakis[4-(2,4-diamino-1,3,5-triazin-6-yl)phenyl]benzene (4) incorporates a disc-shaped hexaphenylbenzene core and six peripheral diaminotriazine groups that can engage in hydrogen bonding according to established motifs. Under all conditions examined, compound 4 crystallizes as planned to give closely related noninterpenetrated three-dimensional networks built from sheets in which each molecule has six hydrogen-bonded neighbors. In the structure of compound 4, the number of hydrogen bonds per molecule and the percentage of volume accessible to guests approach the highest values so far observed in molecular networks. Analogue 5 (which has the same hexaphenylbenzene core but only four diaminotriazine groups at the 1,2,4,5-positions) and analogue 7 (in which the two unsubstituted phenyl groups of compound 5 are replaced by methyl groups) crystallize according to a closely similar pattern. Analogues with flatter pentaphenylbenzene or tetraphenylbenzene cores crystallize differently, underscoring the importance of maintaining a consistent molecular shape in attempts to engineer crystals with predetermined properties.     

Molecular tectonics: from molecular recognition of anions to molecular networks

One may apply concepts developed in the context of molecular recognition of anions by synthetic receptors in solution to the design of molecular tectons capable of generating molecular networks with anionic species in the crystalline phase. With respect to that, bis-cyclic amidinium dications are interesting tectons because they offer two positive charges allowing strong electrostatic charge–charge interactions with anions and four acidic protons divergently oriented and capable of forming two sets of two H-bond chelates. The latter characteristic is of interest for the generation of supramolecular chirality taking place within the second coordination sphere around anionic metal complexes adopting an octahedral coordination geometry.     

Molecular tectonics: design of luminescent H-bonded molecular networks

Using bis-amidinium dications as tetra H-bond donor tectons and Au(CN)(2)(-) anion, neutral 1-D networks based on a bis monohapto mode of H-bonding are obtained. Owing to the short metal-metal distance within the network, luminescent crystals are obtained. The emission phenomena may be tuned by the nature of the spacer connecting the two cyclic amidinium groups.     

Molecular tectonics: design and generation of charge-assisted, H-bonded, hybrid molecular networks based on amidinium cations and thio- or isothio-cyanatometallates

Upon combining the bis-amidinium dication 1-2H(+) with thiocyanatometallate M(SCN)(4)(2-) (M = Pd, Hg) or isothiocyanatometallate Cu(NCS)(4)(2-) anions (behaving as H-bond donor and acceptors, respectively) three new hybrid molecular networks have been obtained in the crystalline phase and structurally characterized by X-ray diffraction on single crystals. Whereas for the combination of tecton 1-2H(+) and both Pd(SCN)(4)(2-) and Hg(SCN)(4)(2-) anions analogous 1-D H-bonded networks were observed, for the Cu(NCS)(4)(2-) anion a 2-D network was obtained. Based on structural features of both components, the formation of the two types of networks is discussed.     

Reversible anion exchange and sensing in large porous materials built from 4,4'-bipyridine via pi...pi and H-bonding interactions

Two unusual d10 compounds, [Zn2(bipy)3(H2O)8(ClO4)2(paba)2].2(bipy).4H2O (1) and [Cd2(bipy)4(H2O)6(ClO4)2(paba)2].(bipy).5H2O (2) (bipy = 4,4'-bipyridine, paba = p-aminobenzoate), were obtained from reaction of the metal salts, bipy and paba in an EtOH/H2O mixture. The bipy ligands in the two compounds exhibit two new modes of coordinating to transition metal ions, resulting in the formation of large porous frameworks. Immersion of single crystals of 1 in an aqueous solution of NH4PF6 results in the formation of its hexafluorophosphate derivative 3 as shown by single crystal diffraction. Immersion of crystals of 3 in NaClO4 regenerates 1. Furthermore, compound 1 also shows interesting anion sensing properties in an EtOH/H2O mixture.     

Molecular recognition of adeninium cations on anionic metal-oxalato frameworks: an experimental and theoretical analysis

Reactions of adenine with water-soluble oxalato complexes at acidic pH give the compounds (1H,9H-ade)2[Cu(ox)2(H2O)] (1) [H2ade=adeninium cation (1+), ox=oxalato ligand (2-)] and (3H,7H-ade)2[M(ox)2(H2O)2].2H2O [M(II)=Co (2), Zn (3)]. The X-ray single crystal analyses show that the supramolecular architecture of all compounds is built up of anionic sheets of metal-oxalato-water complexes and ribbons of cationic nucleobases among them to afford lamellar inorganic-organic hybrid materials. The molecular recognition process between the organic and the inorganic frameworks determines the isolated tautomeric form of the adeninium cation found in the crystal structures: the canonical 1H,9H for compound 1, and the first solid-state characterized 3H,7H-adeninium tautomer for compounds 2 and 3. Density functional theory calculations have been performed to study the stability of the protonated nucleobase forms and their hydrogen-bonded associations by comparing experimental and theoretical results.     

A theoretical study of cohesion, structural deformation, inclusion, and dynamics in porous hydrogen-bonded molecular networks

Molecules with multiple sites of hydrogen bonding attached to suitable cores tend to crystallize as open networks. The resulting crystals can have the following unusual properties: They can include significant amounts of guest molecules; the guests are typically located in channels and can be exchanged without loss of crystallinity; and the geometry of the networks can change in response to new guests. We have found that DFT calculations can provide accurate simulations of the unusual structure and properties of such materials, represented by crystals of prototypic tetrapyridinone 1. These calculations have yielded three key insights that cannot be obtained directly from experiments. (1) The hypothetical porous network obtained by removing guests from crystals of compound 1 is highly flexible, and its deformations are inherently anisotropic, leading to lengthening or shortening of the channels along the c axis and no significant changes along the a and b axes. (2) Quantitative analysis of the total cohesive energy has revealed that hydrogen bonding within the network makes a dominant contribution, along with interactions of guests with the network. (3) Differences in the overall stability of crystals of compound 1 as the guests are varied do not arise primarily from significant changes in the cohesive energy of the network itself; instead, differences in guest-guest interactions play a key role, resulting from the nature of the guests and constraints imposed by the surrounding network. These insights, together with the results of ab initio molecular dynamics, help explain how hydrogen-bonded networks can be robust yet permit molecular movement that underlies the exchange of guests and adaptive porosity. These insights promise to be of general value to scientists studying ordered molecular materials in which strong directional interactions are prominent.     

Deformation of porous molecular networks induced by the exchange of guests in single crystals

A strategy for making molecular networks that are porous and deformable is revealed by the behavior of compound 1, in which groups that form hydrogen bonds are attached to a nominally tetrahedral Si core. Compound 1 crystallizes from various carboxylic acids to produce a porous hydrogen-bonded diamondoid network, with up to 65% of the volume available for including guests. Changing the guests expands or contracts the network up to 30% in one direction, and single crystals can accommodate these exchange-induced deformations without loss of crystallinity. This resilience appears to result in part from the incorporation of flexible Si nodes in an otherwise robust network maintained by multiple hydrogen bonds. In certain cases, exchange is faster than deformation of the network, allowing the isolation of metastable structures with a new guest included in an otherwise unchanged network. Such processes can provide new materials that would be difficult or impossible to obtain in other ways.     

Functional self-assembly of hydrogen-bonded 3D coordination networks constructed from 1,2,4,5-benzenetetracarboxylic acid

Hydrothermal synthesis, characterization (IR, TG/DTA, element analysis, inductively coupled plasma (ICP)) and single-crystal X-ray structures of H₄Btec hydrate and its two cobalt complexes, colorless [H₄Btec · 2H₂O] _n (I), pink [Co(H₂O)₆(H₂Btec)] _n (II), and nacarat {[Co(H₂O)₃(H₂Btec)(Phen)] · H₂O} _n (III) (H₄Btec = 1,2,4,5-benzenetetracarboxylic acid, Phen = 1,10-phenanthroline) have been solved. The results showed that I forms a 3D O-H⋯O hydrogen-bonded network generated from H₄Btec and water molecules, II presents a 3D network constructed by mononuclear [Co(H₂O)₆]²⁺ cations and H₂Btec²⁻ dianions through extensive hydrogen-bonding interactions, and III gives rise to a pseudo-octahedral coordination geometry. Extensive hydrogen-bonding interactions have significant effects in configuring a 3D network constructed by mononuclear [Co(H₂Btec)(Phen)(H₂O)₃] neutral molecules and a water molecules. The article was submitted by the authors in English.     

Many faces of dipyrrins: from hydrogen-bonded networks to homo- and heteronuclear metallamacrocycles

The amphoteric 5-(4-cyanophenyl)dipyrrin ligand, offering three distinct states, i.e., cationic, neutral, and anionic, has been exploited for the formation of a 1-D hydrogen-bonded network in its protonated form and both homo- and heterobinuclear metallamacrocycles, in its neutral and deprotonated states, respectively, with a variety of coordination modes.     

Two-dimensional networks of ethylenedithiotetrathiafulvalene derivatives with the hydrogen-bonded functionality of uracil, and channel structure of its tetracyanoquinodimethane complex

Ethylenedithiotetrathiafulvalene (EDT-TTF) derivatives with N1-butyluracil or N1-phenyluracil moiety were designed and synthesized as new hydrogen-bonded electron-donor molecules with the aim of introducing multiple S...S interactions into the hydrogen-bonded structures composed of the TTF-nucleobase systems. In the crystals of the EDT-TTF derivatives, two-dimensional sheet and layer structures were formed through pi...pi, multiple S...S interactions, and complementary double hydrogen bonds. In the tetracyanoquinodimethane (TCNQ) charge-transfer complex of the EDT-TTF-N1-butyluracil dyad with a segregated column, a layer structure of the electron-donor molecules was constructed through the noncovalent interactions. The n-butyl group of the uracil moiety served to separate the space between the donor layers, resulting in construction of a channel structure. Disordered TCNQ molecules were located in the microporous space of the channel. The TCNQ complex exhibited high electric conductivity (sigmart= 2.1 S cm(-1)) in a single crystal.     

Molecular tectonics: on the formation of 1-D silver coordination networks by thiacalixarenes bearing nitrile groups

Two new p-tert-butylthiacalix[4]arene derivatives 2 and 3 decorated at the lower rim with four nitrile groups have been prepared and structurally characterised in the crystalline phase. The two ligands, differing by the length of the spacer between the calix moiety and the nitrile group, adopt the 1,3-alternate conformation in the solid state. The ligand 3 bearing four (CH(2))(3)CN fragments behaves as a tecton in the presence of silver salts (AgX, X = BF(4), PF(6) or SbF(6)) and leads to the formation of analogous 1-D linear coordination networks. The tecton 3 acts as a bischelate unit and bridges consecutive silver cations adopting a tetrahedral coordination geometry. Anions and solvent molecules occupy the free space between networks and exhibit no specific interactions with the cationic architecture.