Herein we report the syntheses of highly rigid silicon‐ and tin‐containing cyclic and bicyclic compounds with unsaturated organic backbones. The syntheses proceed via di‐lithiated diethynylbenzene ...derivatives. The protocols for the known precursor compounds were significantly improved in a two‐step one‐pot reaction with yields up to 45 % by alternating addition of the lithiation reagent and starting material. A total of ten compounds were synthesized and the molecular structure in the solid state was elucidated for six novel compounds. Host‐guest‐experiments with selected molecules were conducted to investigate on the potential of the complexation of small anions. All compounds were characterized by multinuclear NMR spectroscopy and elemental analyses or mass spectrometry and in most parts by SC‐XRD analyses.
Highly rigid silicon‐ and tin‐containing macrocyclic and bicyclic cage compounds with diethynylbenzene backbones were synthesized and characterized and their potential as Lewis acids was explored.
Connecting multiple organic spacer functions via silicon core units leads to various organosilanes that are excellently suited as backbones for poly‐Lewis acids (PLAs). Using ethynyl spacer groups, ...rigid bis‐ and tris‐dioxabenzoborole (BCat)‐substituted PLAs were prepared. The fixed orientation of the Lewis acidic functions of the PLAs is reflected in their solid‐state structures. Further flexible PLAs were obtained by hydroboration of vinylsilanes using 9‐borabicyclo3.3.1nonane (9‐BBN), which shows the flexibility of the backbone motif. Host‐guest experiments of the bidentate representatives with pyridine demonstrate the ability of both PLA‐types (BCat or 9‐BBN) for complexing neutral guest molecules. The rigid host system shows additionally a 1 : 1 adduct formation using a bridged diamine as guest compound.
Bis‐ and tris‐Lewis acids, based on alkynylsilanes with terminal boron functions, were prepared and tested for their ability to accept neutral Lewis base molecules.
The structures of the three para‐substituted halotetrafluoropyridines with chlorine, bromine, and iodine have been determined in the solid state (X‐ray diffraction). The structures of these compounds ...and that of pentafluoropyridine were also determined in the gas phase (electron diffraction). Structures in the solid state of the bromine and iodine derivatives exhibit halogen bonding as a structure‐determining motif. On the way to an investigation of halogen bond formation of halotetrafluoropyridines in the solid state with the stronger Lewis base pyridine, co‐crystals of benzene adducts were investigated to gain an understanding of the influence of aryl–aryl interactions. These co‐crystals showed halogen bonding only for the two heavier halotetrafluoropyridines. In the pyridine co‐crystals halogen bonding was observed for all three para‐halotetrafluoropyridines. The formation of homodimers and heterodimers with pyridine is also supported by quantum‐chemical calculations of electron density topologies and natural bond orbitals.
Halogen‐bond formation in co‐crystals containing halogentetrafluoropyridines is markedly dependent on the type of halogen atom, the electronegativity of its substituent, and the halogen bond acceptor and it is less pronouncedly dependent on additional π–π stacking interactions.
The formation of host–guest (H‐G) complexes between 1,8‐bis(diethylgallanyl)ethynylanthracene (H) and the N‐heterocycles pyridine and pyrimidine (G) was studied in solution using a combination of NMR ...titration and diffusion NMR experiments. For the latter, diffusion coefficients of potential host–guest structures in solution were compared with those of tailor‐made reference compounds of similar shape (synthesized and characterized by NMR, HRMS, and in part XRD). Highly dynamic behavior was observed in both cases, but with different host–guest species and equilibria. With increasing concentrations of the pyridine guest, the equilibrium H2⇄H2κ1‐G1⇄HG2 is observed (in the second step a host dimer coordinates one guest molecule); for pyrimidine the equilibrium H2→H1κ2‐G1⇄HG2 is observed (the formation of a 1:1 aggregate is the second step).
Dynamic host–guest complexation was studied using a new combination of NMR titration and diffusion NMR experiments. The host is the bidentate 1,8‐bis(diethylgallanyl)ethynylanthracene, the guests are pyrimidine and pyridine. The analysis of the diffusion NMR experiments requires tailor‐made reference compounds of the same shape for comparison.
Starting from 1,2‐diethynylbenzene, a series of bidentate Lewis acids was prepared by means of hydrometalations, in particular hydrosilylation, hydroboration, hydroalumination and terminal metalation ...based on group 13 and 14 elements. In the case of terminal alkyne metalation, the Lewis‐acidic gallium function was introduced using triethylgallium under alkane elimination. A total of six different Lewis acids based on a semiflexible organic scaffold were prepared, bearing −SiClMe2, −SiCl2Me, −SiCl3, −B(C6F5)2, −AlBis2 (Bis=bis(trimethylsilyl)methyl) and −GaEt2 as the corresponding functional units. In all cases, the Lewis acid functionalisation was carried out twice and the products were obtained in good to excellent yields. In the case of the twofold gallium Lewis acid, a different structural motif in the form of a polymer‐like chain was observed in the solid state. All new bidentate Lewis acids were characterised by multinuclear NMR spectroscopy, CHN analysis and X‐ray diffraction experiments.
Six bidentate Lewis acids, based on elements of groups 13 and 14, were obtained by hydrometalation and terminal metalation reactions of o‐diethynylbenzene, a well accessible bidentate scaffold.
1,5‐Dialkynylanthracenes and 1,8‐dialkynylanthracenes have been functionalised by a series of hydrometallation reactions, namely, hydrosilylation, hydroboration and hydrogallation. Nine ...anthracene‐based Lewis acidic compounds with a semiflexible organic framework bearing SiCl3, SiCl2Me, SiClMe2, B(C6F5)2 and GaCl2 substituents were obtained. In all cases, the substrate could be functionalised twice, and bidentate Lewis acids were obtained. By using Piers' borane HB(C6F5)2, a fourfold‐substituted anthracene species functionalised with four extremely electronegatively substituted boron atoms could be generated in quantitative yield. All of the poly‐Lewis acids were characterised by multinuclear NMR spectroscopy and, in part, by mass spectrometry and X‐ray diffraction experiments.
A series of B‐, Si‐ and Ga‐containing poly‐Lewis acids were synthesised by converting 1,5‐ and 1,8‐dialkynylanthracene derivatives in hydrometallation reactions. Some of the compounds reacted with diethyl ether, which demonstrates the variable behaviour of the Lewis acidic compounds towards Lewis basic molecules.
ConspectusThe structures of molecules can be different in different phases. Intermolecular forces, even those of weak noncovalent interactions (WNCIs), can lead to a preference for quite different ...conformations in the solid, the gas, and the liquid phases. WNCIs can cause variations in bond lengths, angles, and torsional angles. Since structure is a fundamental concept in chemistry, the knowledge of structural changes with phase is important to understand the source and effects of distorting contributions from WNCIs but also as a predictive tool for the design and stabilization of new bonding situations.X-ray crystallography is ubiquitous and now mostly straightforward to perform, but facilities for the determination of accurate gas-phase structure determination are rare, and gas-phase work is laborious and time-consuming. There are currently about 1.25 million crystal structures and more than 12 500 experimental gas-phase structures, but the intersection of the two data sets that can tell us about the structural differences of the same molecule in different phases is surprisingly small.In this Account, we describe several cases of WNCI-dominated systems for which accurate experimental structure determinations exist for both the gas phase and the solid state and, in one case, also for solution. The examples include aryl-aryl, aryl-alkyl, and alkyl-alkyl interactions; systems with chalcogen and halogen bonding; and fluorine-based interactions in arylboranes. We work out the role of WNCIs in stabilizing large, strained, or sterically overloaded molecules. We will show how flexible molecules will fold under the action of WNCIs when isolated in the gas and how they fold or unfold when they are embedded in an environment of neighbors in crystals. We will show how they can vary in strength when the substitution patterns in aryl groups are changed by different halogens and how intramolecular WNCIs, such as those forming rings, change when such systems experience additional intermolecular WNCIs.Overall, we hope that this Account will give the reader an idea of the type and magnitude of structural changes that can be expected from a free molecule in the gas phase or a single molecule calculated by quantum chemistry compared with one embedded in a crystal. This should define the limits of comparability and provide some predictive concepts of the distortions and variations to be expected.
α,ω‐Bis(1,8‐dichloroanthracen‐10‐yl)alkanes with (CH2)n‐linker units (n=1–4) were synthesized starting from 1,8‐dichloroanthracen‐10(9H)‐one. This was transformed into anthracenes with allyl, ...bromomethyl and propargyl substituents in position 10; these were converted in various C−C‐bond formation reactions (plus hydrogenation), leading to two anthracene units flexibly linked by α,ω‐alkandiyl groups. 1,2‐Ethandiyl‐ and 1,3‐propandiyl‐linked derivatives were functionalized with ethynyl groups in positions 1, 8, 1’ and 8’, and these terminally functionalized by Me3Sn groups using Me2NSnMe3. All linked bisanthracenes were subjected to UV light induced cyclomerization and a series of 9,10 : 9’,10’‐photo‐cyclomers were obtained. Their thermal cycloreversion and (repeated) switchability was demonstrated. 1,3‐Bis{1,8‐bis(trimethylstannyl)ethynylanthracen‐10‐yl}propane served as model compound for photo‐switchable acceptor molecules and its open and closed forms were characterized by NMR and DOSY experiments.
Light on the switch: Linked bisanthracenes bearing four (partly metal‐containing) substituents undergo photo‐cyclomerization that enables the parallelization of all four functions and can be thermally reopened by cycloreversion which allows repeated switchability.
1,8‐Dichloroanthracenes bearing EMe3 substituents at the 10‐position (E = Si, Ge, Sn) have been synthesised by salt elimination reactions. The key compound, 10‐bromo‐1,8‐dichloroanthracene (2), was ...quantitatively obtained by conversion of 1,8‐dichloroanthracene with elemental bromine in dichloromethane. The EMe3‐substituted anthracene compounds 1,8‐dichloro‐10‐(trimethylsilyl)‐ (3), 1,8‐dichloro‐10‐(trimethylgermyl)‐ (4) and 1,8‐dichloro‐10‐(trimethylstannyl)anthracene (5) were completely characterised by multinuclear NMR spectroscopy and mass spectrometry. Their molecular structures in the crystalline state were analysed by X‐ray diffraction experiments and compared with the crystal structure of 10‐tert‐butyl‐1,8‐dichloroanthracene (1). It was found that the level of deformation of the anthracene backbone continuously increases along the series of anthracene substituents SnMe3 < GeMe3 < SiMe3 < CMe3. Owing to the good agreement of experimental structural parameters with the results of quantum chemical calculations, the molecular deformations can be regarded as inherent molecular properties.
10‐Bromo‐1,8‐dichloroanthracene was quantitatively obtained by bromination of 1,8‐dichloroanthracene and converted into 1,8‐dichloro‐10‐(trimethylelement)anthracenes (with E = Si, Ge, Sn). Their crystal structures show a butterfly‐like deformation of the molecules, which is also observed in case of the tert‐butyl compound (E = C).
Reactions between 1,8‐dichloroanthracenes with substituents in position 10 and ortho‐chloroaryne afford mixtures of 1,8,13‐ (syn) and 1,8,16‐trichlorotriptycenes (anti). The syn/anti ratio is ...dependent on these substituents. Electropositive substituents like SiMe3 and GeMe3 lead to preferred formation of the syn‐isomer, whereas CMe3 groups exclusively afford the anti‐isomer. Different quantum chemical calculations including location of transition states give conflicting results, but indicate the importance of dispersion forces for an at least qualitative prediction of results. The syn‐trichlorotriptycenes with SiMe3 and GeMe3 substituents were characterized by using NMR spectroscopy, mass spectrometry, and X‐ray diffraction experiments.
Hocus pocus syn‐salabim: The orientation of a 4+2 cycloaddition between aryne and anthracene molecules can be controlled by substituents at position 10 of the anthracene. Electropositive SiMe3 or GeMe3 substituents in 1,8‐dichloroanthracenes steer regiochemistry towards the desired syn‐addition product, whereas CMe3 gives 100 % of the anti‐isomer.
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