Homomeric chains of intermolecular bonds scaffold octahedral germanium perovskites.

Perovskites with low ionic radii metal centres (for example, Ge perovskites) experience both geometrical constraints and a gain in electronic energy through distortion; for these reasons, synthetic attempts do not lead to octahedral [GeI6] perovskites, but rather, these crystallize into polar non-perovskite structures1-6. Here, inspired by the principles of supramolecular synthons7,8, we report the assembly of an organic scaffold within perovskite structures with the goal of influencing the geometric arrangement and electronic configuration of the crystal, resulting in the suppression of the lone pair expression of Ge and templating the symmetric octahedra. We find that, to produce extended homomeric non-covalent bonding, the organic motif needs to possess self-complementary properties implemented using distinct donor and acceptor sites. Compared with the non-perovskite structure, the resulting [GeI6]4- octahedra exhibit a direct bandgap with significant redshift (more than 0.5 eV, measured experimentally), 10 times lower octahedral distortion (inferred from measured single-crystal X-ray diffraction data) and 10 times higher electron and hole mobility (estimated by density functional theory). We show that the principle of this design is not limited to two-dimensional Ge perovskites; we implement it in the case of copper perovskite (also a low-radius metal centre), and we extend it to quasi-two-dimensional systems. We report photodiodes with Ge perovskites that outperform their non-octahedral and lead analogues. The construction of secondary sublattices that interlock with an inorganic framework within a crystal offers a new synthetic tool for templating hybrid lattices with controlled distortion and orbital arrangement, overcoming limitations in conventional perovskites.

Pseudodiproline (Pro-Cyp) Oligomers Fold into Helical Polyproline Type secondary structures.

The synthesis and conformational properties of oligo-proline mimetics composed of dimeric and tetrameric Pro-Cyp constructs linked by a hydroxymethylene unit are reported. Oligomers were studied both in the solid state and in solution, unveiling right-handed helical conformation depending on the configuration of the vicinally substituted trans-cyclopentane carboxylic acid unit (Cyp). Unlike polyproline oligomers, the alternating synthetic Pro-Cyp counterparts are not stabilized by n-π* interactions but rely instead on the steric demands of the extended backbone conformation within the hydroxymethylene-linked Pro-Cyp repeating units.

Refreshing the Legacy of Rudolf Nietzki: Benzene-1,2,4,5-tetramine and Related Compounds.

Like hydroquinones and quinones, aromatic compounds with multiple NH2 groups and the corresponding quinonediimines have the potential to serve as components of useful redox-active organic materials. Benzene-1,2,4,5-tetramine (BTA) and its oxidized form BTA-H2 offer a promising redox pair of this type, and the compounds have proven to be useful in many areas of chemistry. However, key aspects of their behavior have remained poorly studied, such as the nature of their protonated forms, their preferred molecular structures, their reactivity, and their organization in condensed phases. In the present work, we have used a combination of improved methods of synthesis, computation, spectroscopic studies, and structural analyses to develop a deeper understanding of BTA, BTA-H2, their salts, and related compounds. The new knowledge is expected to accelerate exploitation of the compounds in areas of materials science where desirable properties can only be attained by properly controlling the organization of molecular components.

Designing Tetraoxa[8]circulenes To Serve as Hosts and Sensors.

In D4-symmetric tetraoxa[8]circulenes, alternating fused benzene and furan rings form an octagonal array. These compounds are little known despite their novel properties, which include extended planar π-conjugation and a formally antiaromatic cyclooctatetraene core. Tetraoxa[8]circulenes can be formed by acid-induced cyclocondensations of suitable quinones, but existing methods often give very low yields. In addition, π-stacking of simple tetraoxa[8]circulenes reduces solubility and limits opportunities to form homogeneous mixtures or cocrystals with other compounds. To help make tetraoxa[8]circulenes more useful, we have developed better ways to synthesize them, and we have used these methods to produce awkwardly shaped derivatives with large concave electron-rich aromatic surfaces. These compounds crystallize to form open structures that can accommodate various guests, including C60. Analysis of the structures shows that the cyclooctatetraene core of the hosts exhibits surprising variations in C-C bond lengths and conjugation, which appear to be related to the gain or loss of aromaticity. This allows tetraoxa[8]circulenes to serve as sensitive probes of local molecular environment and to be used as sensors of electron-deficient species such as nitroaromatic compounds.

Diphenoquinhydrones and Related Hydrogen-Bonded Charge-Transfer Complexes.

Benzoquinone and hydroquinone cocrystallize to form quinhydrone, a 1:1 complex with a characteristic structure in which the components are positioned by hydrogen bonds and charge-transfer interactions. We have found that analogous diphenoquinhydrones can be made by combining 4,4'-diphenoquinones with the corresponding 4,4'-dihydroxybiphenyls. In addition, mixed diphenoquinhydrones can be assembled from components with different substituents, and mismatched quinhydrones can be made from benzoquinones and dihydroxybiphenyls. In all cases, the components of the resulting structures are linked in alternation by O-H···O hydrogen bonds to form essentially planar chains, which stack to produce layers in which π-donors and π-acceptors are aligned by charge-transfer interactions. Geometric parameters, computational studies, and spectroscopic properties of diphenoquinhydrones show that the key intermolecular interactions are stronger than those in simple quinhydrone analogues. These findings demonstrate that the principles of modular construction underlying the formation of classical quinhydrones can be generalized to produce a broad range of hydrogen-bonded charge-transfer materials in which the components are positioned by design.

Diphenoquinones Redux.

Benzoquinones can undergo reversible reductions and are attractive candidates for use as active materials in green carbon-based batteries. Related compounds of potential utility include 4,4'-diphenoquinones, which have extended quinonoid structures with two carbonyl groups in different rings. Diphenoquinones are a poorly explored class of compounds, but a wide variety can be synthesized, isolated, crystallized, and fully characterized. Experimental and computational approaches have established that typical 4,4'-diphenoquinones have nearly planar cores in which two cyclohexadienone rings are joined by an unusually long interannular C═C bond. Derivatives unsubstituted at the 3,3',5,5'-positions react readily by hydration, dimerization, and other processes. Association of diphenoquinones in the solid state normally produces chains or sheets held together by multiple C-H···O interactions, giving structures that differ markedly from those of the corresponding 4,4'-dihydroxybiphenyls. Electrochemical studies in solution and in the solid state show that diphenoquinones are reduced rapidly and reversibly at potentials higher than those of analogous benzoquinones. Together, these results help bring diphenoquinones into the mainstream of modern chemistry and provide a foundation for developing redox-active derivatives for use in carbon-based electrochemical devices.

Surprising Chemistry of 6-Azidotetrazolo[5,1-a]phthalazine: What a Purported Natural Product Reveals about the Polymorphism of Explosives.

6-Azidotetrazolo[5,1-a]phthalazine (ATPH) is a nitrogen-rich compound of surprisingly broad interest. It is purported to be a natural product, yet it is closely related to substances developed as explosives and is highly polymorphic despite having a nearly planar structure with little flexibility. Seven solid forms of ATPH have been characterized by single-crystal X-ray diffraction. The structures show diverse patterns of molecular organization, including both stacked sheets and herringbone packing. In all cases, N···N and C-H···N interactions play key roles in ensuring molecular cohesion. The high polymorphism of ATPH appears to arise in part from the ability of virtually every atom of nitrogen and hydrogen in the molecule to take part in close N···N and C-H···N contacts. As a result, adjacent molecules can adopt many different relative orientations that are energetically similar, thereby generating a polymorphic landscape with an unusually high density of potential structures. This landscape has been explored in detail by the computational prediction of crystal structures. Studying ATPH has provided insights into the field of energetic materials, where access to multiple polymorphs can be used to improve performance and clarify how it depends on molecular packing. In addition, our work with ATPH shows how valuable insights into molecular crystallization, often gleaned from statistical analyses of structural databases, can also come from in-depth empirical and theoretical studies of single compounds that show distinctive behavior.

The Role of Hydrogen Bonds in Interactions between [PdCl4]2- Dianions in Crystal.

[PdCl4]2- dianions are oriented within a crystal in such a way that a Cl of one unit approaches the Pd of another from directly above. Quantum calculations find this interaction to be highly repulsive with a large positive interaction energy. The placement of neutral ligands in their vicinity reduces the repulsion, but the interaction remains highly endothermic. When the ligands acquire a unit positive charge, the electrostatic component and the full interaction energy become quite negative, signalling an exothermic association. Raising the charge on these counterions to +2 has little further stabilizing effect, and in fact reduces the electrostatic attraction. The ability of the counterions to promote the interaction is attributed in part to the H-bonds which they form with both dianions, acting as a sort of glue.

Phosphangulene: A Molecule for All Chemists.

Phosphangulene (1) is a hexacyclic triarylphosphine with a distinctive conical shape and other features that allow the compound to be viewed from diverse perspectives and to be embraced by chemists from different parts of the field as a molecule worthy of special attention. In recent work, phosphangulene and its derivatives have proven to be effective tools for probing general principles that govern molecular organization in solids. The phosphangulene family is particularly well-suited for these studies because systematic structural changes in the compounds are easy to introduce. In crystals of phosphangulene itself, molecules are stacked efficiently like hats, giving rise to an R3m structure that is polar and pyroelectric. Simple conversion of the compound into phosphangulene oxide (7a) or other chalcogenides blocks effective stacking and forces crystallization to produce alternative structures that have many suboptimal intermolecular interactions and vary little in energy as their geometries are altered. This leads to high levels of polymorphism, and phosphangulene oxide (7a) belongs to the elite set of compounds known to exist in five or more forms characterized by single-crystal X-ray diffraction. For similar reasons, phosphangulene chalcogenides form crystals with complex unit cells in which multiple inequivalent molecules are needed to optimize packing, and the compounds are also predisposed to form solvates and mixed crystals containing other molecules. For example, crystallization of a 1:1 mixture of phosphangulene and oxide 7a yielded needles composed of pure phosphangulene along with crystals of the oxide containing substantial amounts of phosphangulene. Phosphangulene has one known polymorph, and its crystallization rejects the oxide. In contrast, the oxide is highly polymorphic, and its crystallization is prone to errors in which molecules in the lattice are replaced by other compounds. Packing in crystals of the oxide appears to be so ineffective that the orientation and even the identity of the molecular components can be varied without imposing severe energetic penalties.Because substituted members of the phosphangulene family have awkward curved shapes that cannot be packed efficiently, they have emerged as highly effective partners for cocrystallizing fullerenes and for using concave-convex interactions to control how fullerenes can be organized in materials. This can be achieved without eliminating fullerene-fullerene contacts of the type needed to ensure conductivity. In addition, phosphangulene has created unlimited opportunities for making complex structures with large curved aromatic surfaces based on a new strategy in which the central atom of phosphorus is used to form covalent bonds with other elements or to introduce coordinative interactions with metals. In these ways, recent work has put phosphangulene in the spotlight as a compound of unusually broad interest and shown that it can appropriately be called a molecule for all chemists.

ROY Reclaims Its Crown: New Ways To Increase Polymorphic Diversity.

Chemical compounds that exist in multiple crystalline forms are said to exhibit polymorphism. Polymorphs have the same composition, but their structures and properties can vary markedly. In many fields, conditions for crystallizing compounds of interest are screened exhaustively to generate as many polymorphs as possible, from which the most advantageous form can be selected. We report new ways to search for polymorphs and increase polymorphic diversity, based on crystallization induced by suitably designed mixed-crystal seeds. The potential of the strategy has been demonstrated by using it to produce new polymorphs of the benchmark compound ROY as single crystals structurally characterized by X-ray diffraction. This allows ROY to reclaim its crown as the most polymorphic compound in the Cambridge Structural Database. More generally, the methods promise to become valuable tools for polymorphic screening in all fields where crystalline solids are used.

Modular Construction of Porous Hydrogen-Bonded Molecular Materials from Melams.

Ordered materials with predictable structures and properties can be made by a modular approach, using molecules designed to interact with neighbors and hold them in predetermined positions. Incorporating 4,6-diamino-1,3,5-triazin-2-yl (DAT) groups in modules is an effective way to direct assembly because each DAT group can form multiple N-H⋅⋅⋅N hydrogen bonds according to established patterns. We have found that modules with high densities of N(DAT)2 groups can be made by base-induced double triazinylations of readily available amines. The resulting modules can form structures held together by remarkably large numbers of hydrogen bonds per molecule. Even simple modules with only 1-3 N(DAT)2 groups and fewer than 70 non-hydrogen atoms can crystallize to form highly open networks in which each molecule engages in over 20 N-H⋅⋅⋅N hydrogen bonds, and more than 70 % of the volume is available for accommodating guests. In favorable cases, guests can be removed to create rigorously porous crystalline solids analogous to zeolites and metal-organic frameworks.

Controlling Molecular Organization by Using Phenyl Embraces of Multiple Trityl Groups.

Sixfold phenyl embraces are well-established aromatic interactions that are strong and directional. In addition, functional groups that are able to participate, such as triphenylmethyl (trityl), are easily incorporated in molecular structures. As a result, embraces offer a possible way to control molecular organization in materials. To test this notion, we used a hybrid organic-inorganic strategy to make compounds with multiple trityl groups. Trityl-substituted alkynylpyridines 3-5 react with Pd(II) to form square-planar 4:1 complexes with multiple divergent trityl groups poised to engage in embraces. The complexes were crystallized, and their structures were determined by X-ray diffraction. Surprisingly, few structures in this set of compounds were found to incorporate sixfold embraces. Our observations suggest that predictable molecular organization cannot normally be achieved using these embraces, which must compete with alternative aromatic interactions of similar energy.

Amidine/Amidinate Cobalt Complexes: One-Pot Synthesis, Mechanism, and Photocatalytic Application for Hydrogen Production.

A new synthetic route was carried out via a one-pot reaction to prepare a novel series of amidine/amidinate cobalt complexes 8-10 by mixing ligand 2 (6-pyridin-2-yl-[1,3,5]-triazine-2,4-diamine) with Co(II) in acetonitrile or benzonitrile. We observed that a change of solvent from methanol (used in complex 7, previously reported) to nitrile solvents (MeCN and PhCN) led to the in situ incorporation of the amidine group, ultimately forming 8-10. So far, this is a unique method reported to introduce amidine/amidinate groups into a pyridinyl-substituted diaminotriazine complex. Remarkably, the single crystal X-ray diffraction study (SCXRD) of these new compounds reveals associations involving Janus DATamidine and Janus DATamidinate. A mechanism is proposed to explain the formation of amidine/amidinate groups by investigating the single crystal structures of the possible intermediates 11 and 12 where the cobalt ion acts as a template. These amidine/amidinate cobalt complexes were used as a model to assess the photocatalytic activity for the hydrogen evolution reaction (HER). Complexes 9 and 10 show a 74% and 86% enhancement, respectively, of the catalytic activity towards the HER compared to complex 7. This highlights the structure-property relationship. By examining the novel cobalt complexes described here, we discovered the following: (i) a method to introduce an amidine group into a pyridine DAT-based complex, (ii) the efficiency of amidine complexes to form multiple hydrogen bonds to direct the molecular organization, (iii) the plausible mechanism of formation of amidines based on the SCXRD study, (iv) the modification of the final structure and hence the final properties by varying the reaction conditions, and (v) the utility of amidine complexes towards photocatalytic HER activity.

Building Large Structures with Curved Aromatic Surfaces by Complexing Metals with Phosphangulene.

Phosphangulene (1) is a hexacyclic triarylphosphine with a distinctive conical shape and an electron-rich aromatic surface that is geometrically and electronically complementary to fullerenes such as C60 and C70. As a result, suitable derivatives of phosphangulene can cocrystallize with fullerenes or even bind them in solution. Surprisingly, previous work has largely overlooked the potential of phosphangulene to form complexes with metals, which offers a simple way to create large molecular structures with curved aromatic surfaces. To explore this approach, we have prepared and characterized a series of complexes of phosphangulene with Ag+ and Cu+. Our results show that Phang ligands are exceptional for many reasons. In particular, they can yield metal complexes with unique coordination, and the metal centers hold the concave aromatic surfaces of multiple ligands in various divergent arrays. Moreover, the rigid conical structure of phosphangulene gives the complexes an awkward shape that cannot be packed efficiently without complementary partners. As a result, metal complexes of phosphangulene are predisposed to cocrystallize with fullerenes, thereby yielding materials in which metals and fullerenes are brought together in ordered arrangements.

Triptycene 1,2-Quinones and Quinols: Permeable Crystalline Redox-Active Molecular Solids.

Suitably designed quinones and quinols are promising modules for the programmed construction of ordered redox-active molecular solids. To explore this potential, we have synthesized compounds 1-4, in which multiple 1,2-benzoquinone and 1,2-quinol units are attached to a triptycene core. The resulting molecules have topologies that disfavor efficient packing, and structural studies show that they crystallize to form open networks held together by characteristic attractive intermolecular forces, including O-H···O hydrogen bonds, C-H···O interactions, π-stacking, and dipolar interactions. Remarkably, the resulting solids are permeable and can undergo reversible redox reactions without loss of crystallinity. Our work may thereby help lead to the design of robust carbon-based batteries with electrodes derived from quinones, quinols, and other redox-active molecules abundantly produced by nature.

Molecular Organization of 2,1,3-Benzothiadiazoles in the Solid State.

Derivatives of 2,1,3-benzothiadiazole (1) are widely used in many areas of science and are particularly valuable as components of active layers in various thin-film optoelectronic devices. Even more effective benzothiadiazoles are likely to result if a deeper understanding of their preferred patterns of molecular association can be acquired. To provide new insight, we have analyzed the structures of compounds in which multiple benzothiadiazole units are attached to well-defined planar and nonplanar molecular cores. Our results show that molecular organization can be controlled in complex structures by using directional S···N bonding of benzothiadiazole units and other characteristic interactions. Moreover, the observed structures are distinctly different from those of analogous arenes. Replacing benzene rings in arenes by thiadiazoles thereby provides a strategy for making new compounds with extended systems of π-conjugation and unique patterns of molecular organization, including the ability to co-crystallize with the fullerenes C60 and C70.

Predictably Ordered Open Hydrogen-Bonded Networks Built from Indeno[1,2-b]fluorenes.

Predictably ordered materials can be constructed by a modular strategy in which properly designed molecular components are positioned in space by reliable interactions. In principle, this approach can be used to control the arrangement of adjacent systems of π-conjugation, thereby creating molecular materials with valuable optoelectronic properties. To explore this possibility, we have synthesized compounds in which 2,4-diamino-1,3,5-triazinyl groups are attached to derivatives of 6,12-dihydroindeno[1,2-b]fluorene to produce molecules with well-defined cruciform topologies, extended π-conjugated aromatic cores, and an ability to form multiple hydrogen bonds. These compounds crystallize to form robust open hydrogen-bonded networks with parallel indenofluorenyl cores, significant volume (64-70%) available for accommodating guests, and characteristic luminescence. Our results will help permit the rational design of complex new molecular materials in which multiple optoelectronically active components are arranged in productive ways.

Engineering Hydrogen-Bonded Hexagonal Networks Built from Flexible 1,3,5-Trisubstituted Derivatives of Benzene.

2,4-Diamino-1,3,5-triazinyl (DAT) groups are known to form N-H···N hydrogen bonds according to reliable patterns of self-association. In compounds 3a-c, three DAT groups are attached to trigonally substituted phenyl cores via identical flexible arms. Crystallization of compounds 3a-c produces robust networks in which each molecule is linked to its immediate neighbors by a total of 10-12 hydrogen bonds. In compound 3a, the DAT groups are designed to lie close to the plane of the phenyl core, thereby giving hydrogen-bonded sheets built from hexameric rosettes. In contrast, the more highly substituted phenyl cores of analogues 3b and 3c favor conformations in which the DAT groups are no longer coplanar, leading predictably to the formation of three-dimensional networks. In general, the nominally trigonal topologies of compounds 3a-c favor structures in which hexagonal networks are prominent, so they behave like trimesic acid despite their greater complexity and flexibility. The structures of all crystals incorporate open networks with significant fractions of volume accessible to guests (32-60%). Despite their flexibility, compounds 3a-c appear to be unable to assume conformations that pack efficiently and simultaneously allow the DAT groups to engage in normal hydrogen bonding.

Building Giant Carbocycles by Reversible C-C Bond Formation.

We describe a simple way to build giant macrocyclic hydrocarbons by the reversible formation of carbon-carbon bonds. Specifically, extended spirobifluorene-substituted derivatives of Wittig's hydrocarbon were synthesized and found to undergo oligomerization, giving the largest hydrocarbon that has been crystallized and characterized by X-ray diffraction to date.

N,N,N-Tri-butyl-butan-1-aminium (T-4)-(cyano-κC)tri-hydro-borate.

In the crystal structure of the title salt, C16H36N(+)·CH3BN(-), the tetra-n-butyl-ammonium cations and [BH3(CN)](-) anions are connected via weak C-H⋯N inter-actions, forming chains along the b-axis direction. The anion is almost linear with an N-C-B angle of 178.7 (2)°. The C-N-C angle values at the core of the tetra-n-butyl-ammonium cation range from 105.74 (11) to 111.35 (11)° with an average of 109.49 (11)°, close to the ideal tetra-hedral value.