The Perfluoro-o-phenylene-mercury Trimer [Hg(o-C6F4)]3 - a Textbook Example of Phase-Dependent Structural Differences.

The geometric and electronic structure of [Hg(o-C6F4)]3 (1) in the gas phase, i. e. free of intermolecular interactions, was determined by a synchronous gas-phase electron diffraction/mass spectrometry experiment (GED/MS), complemented by quantum chemical calculations. 1 is stable up to 498 K and the gas phase contains a single molecular form: the trimer [Hg(o-C6F4)]3. It has a planar structure of D3h symmetry with a Hg-C distance of 2.075(5) Šand a Hg-Hg distance of 3.614(7) Š(both rh1). Structural differences between the crystalline and gaseous state have been analyzed. Different DFT functional-basis combinations were tested, demonstrating the importance to consider the relativistic effects of the mercury atoms. The combination PBE0/MWB(Hg),cc-pVTZ(C,F) turned out to be the most appropriate for the geometry optimization of such organomercurials. The electronic structure of 1, the nature of the chemical bonding in C-Hg-C fragments and the nature of the Hg⋅⋅⋅Hg interactions have been analyzed in terms of the Natural Bond Orbital (NBO) and Quantum Theory of Atoms in Molecules (QTAIM) approaches. The influence of the nature of halogen substitution on the structure of the molecules in the series [Hg(o-C6H4)]3, [Hg(o-C6F4)]3, [Hg(o-C6Cl4)]3, [Hg(o-C6Br4)]3 was also analyzed.

Tridentate Lewis Acids Based on Tribenzotriquinacene Chalices.

Chalice-shaped tridentate poly-Lewis acids (PLA) based on the tribenzotriquinacene (TBTQ) scaffold have been synthesised. Stannylation of the alkyne units, attached via phenyl-spacers to the benzhydrylic positions to the TBTQ scaffold, with Me2NSnMe3 afforded the trimethyltin substituted TBTQ derivative. Replacement of these tin functions with other elements resulted in rigid boron- and aluminium-functionalised PLAs. More flexible PLAs were obtained by hydrometallation reactions of the terminal alkyne groups of 4b,8b,12b-tris((ethynyl)phenyl)tribenzotriquinacene. The resulting poly-Lewis acids were tested for their acceptor abilities in host-guest experiments with suitable bases. Preliminary tests with pyridine led to the synthesis of a large tridentate base with three pyridyl groups attached to a TBTQ backbone. Complexation of this Lewis base with the PLAs resulted in the formation of aggregates, which were studied in solution in more detail by 1H DOSY NMR experiments regarding their size. Further experiments were performed with the tridentate bases 1,4,7-trimethyl-1,4,7-triazacyclononane and tris((dimethylphosphino)methyl)phenylsilane.

Bidentate boron Lewis acids: synthesis by tin boron exchange reaction and host-guest complex formation.

Four bidentate boron Lewis acids based on the 1,8-diethynylanthracene backbone have been synthesized by a tin-boron exchange reaction with various chloroboranes, yielding the products in good to excellent yields. Complexation experiments of the host compounds with pyridine, pyrimidine and TMEDA demonstrated striking differences in terms of formation and solubility of the supramolecular adducts. The host-guest complexes were investigated by multinuclear NMR spectroscopy and structurally characterized by X-ray diffraction experiments, illustrating the adaptation of the host system upon adduct formation with different neutral guest molecules.

Hexadentate poly-Lewis acids based on the bowl-shaped tribenzotriquinacene.

Hexadentate poly-Lewis acids (PLA) based on the bowl-shaped tribenzotriquinacene (TBTQ) have been synthesised. The introduction of three n-propyl groups into the benzhydrylic positions of the TBTQ backbone has significantly increased the solubility of the subsequently derived compounds. Semi-flexible PLAs containing boron and aluminium were obtained by hydrometallation of the corresponding 2,3,6,7,10,11-hexaalkynyl-TBTQ. Other rigid hexadentate PLAs were synthesised by stannylation of the corresponding alkyne units with Me3SnNMe2 followed by tin-element exchange reactions. The Lewis acidity of these PLAs was investigated in host-guest experiments with pyridine. Further experiments with bidentate bases showed correlations between their flexibility, their Lewis basicity and the complexation behaviour towards the synthesised PLAs. Addition of bis((dimethylphosphino)methyl)dimethylsilane (BisPhos) to solutions of the rigid alkynyl PLAs led to the formation of 3 : 1 adducts. Single crystal X-ray diffraction was used to further elucidate the host-guest connectivtiy. In addition, a sixfold pnictogen-bonding donor was synthesised by tin-antimony exchange.

Phase-Dependence of Molecular Structures Arising from Weak London Dispersion Interactions.

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.

Accurate single crystal and gas-phase molecular structures of acenaphthene: a starting point in the search for the longest C-C bond.

The molecular structure of acenaphthene has been determined experimentally in the gas phase using gas electron diffraction intensities and literature-available rotational constants. Supplementary high-level quantum-chemical calculations were utilized in refinements of the semi-empirical equilibrium structure. In this work we investigate on how different schemes of GED data averaging and weighting can be used for obtaining the most accurate and precise structural parameters. Single-crystal X-ray diffraction experiments at different temperatures have been performed and the solid-state structure of acenaphthene has been determined. Both gas and solid-state acenaphthene molecules are planar and possess a non-twisted ethylene bridge. The aliphatic C-C bond in the ethylene fragment is elongated to 1.560(4) Å in the gas phase and 1.5640(4) Å in the solid phase. Based on the experimental data several theoretical approximations have been calibrated and predictions for other molecules were made, taking into account dispersion and electrostatic interactions. Particular derivatives of acenaphthene may potentially have significantly elongated C-C bonds up to 1.725 Å. However, among the experimental gas-phase structures available to date probably the longest C-C bond (re,(av) = 1.750(28) Å at w = 0.93) was determined in a carbaborane derivative 1,2-(SeH)2-closo-1,2-C2B10H10.

Gas-phase equilibrium molecular structures and ab initio thermochemistry of anthracene and rubrene.

Semi-experimental gas-phase structures of anthracene and rubrene (5,6,11,12-tetraphenyltetracene) were determined by means of gas electron diffraction (GED). The use of the flexible restraints in the refinement of the GED data successfully resolves non-equivalent C-C bond lengths. The tetracene core of an isolated rubrene molecule was found to exhibit a twist distortion of about 18°; this is less than DFT calculations predict (30-40°). The modified Feller-Peterson-Dixon method in conjunction with high-level DLPNO-CCSD(T) calculations was employed to resolve the discrepancy between the available experimental gas-phase enthalpies of formation for rubrene. The theoretical value of meets its recent experimental counterpart (765.6 ± 8.4 kJ mol-1) and is in strong disagreement with the previous estimation (882 kJ mol-1).

Synthesis of bi- and tetradentate poly-Lewis acids by hydroalumination of poly-alkynyl-anthracene derivatives.

Photo-dimers of 1,5-diethynylanthracene and 1,5-bis[(trimethylsilyl)ethynyl]anthracene, obtained by UV radiation of the monomers, were treated with the bulky aluminium hydride [(SiMe3)2HC]2AlH in hydroalumination reactions. Hydroalumination of the tetraethynyl-substituted anthracene photo-dimers (syn and anti) led to fourfold aluminium-functionalized Janus-like products with two aluminium functions on each side oriented towards the same direction. The reactions of the monomeric anthracene derivatives with [(SiMe3)2HC]2AlH afforded semi-flexible bidentate Lewis acids, which are interesting building blocks for molecular chains bearing multiple Lewis-acidic functions. The aluminium-functionalized anti-photo-dimer was treated with various donor molecules to gain insight into its host-guest chemistry. All poly-Lewis acids and their adducts were characterized by X-ray diffraction experiments, multinuclear NMR spectroscopy and by elemental analyses.

Host-guest chemistry of a bidentate silyl-triflate bis-Lewis acid - complex complexation behaviour unravelled by diffusion NMR spectroscopy.

The bidentate silicon-based Lewis acid, bis(dimethyl-(trifluoromethylsulfonyl)silylethyl)dimethylsilane, Me2Si[(CH2)2SiMe2OTf]2, was prepared in a two-step synthesis starting from dimethyldivinylsilane by hydrosilylation with dimethylchlorosilane and subsequent Lewis acidity enhancement of the terminal silicon atoms by substituting the chlorine with triflate groups using silver triflate. The potential of the resulting Me2Si[(CH2)2SiMe2OTf]2 for binding of Lewis basic guests was explored in reactions with mono- and bifunctional aromatic nitrogen bases. A 1 : 2-adduct with pyridine and a 2 : 2-adduct with 4,4'-bipyridine was structurally characterised in the solid state. In solution, diffusion NMR spectroscopy revealed the existence of complex dynamic equilibria of oligomers which are formed by the host with bidentate guests. The size of the oligomers is significantly determined by the spatial arrangement of the docking sites within the guests and depends on the host-guest ratio.

Monitoring dynamic pre-crystallization aggregation processes in solution by VT-DOSY-NMR spectroscopy.

Equimolar mixtures of pyridine (Py) with para-halotetrafluoropyridine (BrTFP and ITFP) were investigated by VT-diffusion NMR experiments. The formation of a halogen-bond-stabilized ITFP·Py complex was detected upon cooling a solution in methylcyclohexane-d14 to 260 K; this allows monitoring a halogen-bond-driven aggregation process preceding crystallization in solution.

Bidentate Lewis Acids Derived from o-Diethynylbenzene with Group 13 and 14 Functions.

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 , -SiCl2 Me, -SiCl3 , -B(C6 F5 )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.

The Structure of Bis(catecholato)silanes: Phase Adaptation by Dynamic Covalent Chemistry of the Si-O Bond.

Catechols occupy a unique role in the structural, bio-, and geochemistry of silicon. Although a wealth of knowledge exists on their hypercoordinate complexes, the structure of tetracoordinate bis(catecholato)silane, Si(catH)2 1, has been enigmatic since its first report in 1951. Indeed, the claim of a planar-tetracoordinated silicon in 1 triggered a prominent debate, which is unsettled to this day. Herewith, we present a comprehensive structural study on 1 and derivatives in the gas phase by electron diffraction, in a neon matrix by IR spectroscopy, in solution by diffusion NMR spectroscopy, and in the solid-state by X-ray diffraction and MAS NMR spectroscopy, complemented by high-level quantum-chemical computations. The compound exhibits unprecedented phase adaptation. In the gas phase, the monomeric bis(catecholato)silane is tetrahedral, but in the condensed phase, it is metastable toward oligomerization up to a degree controllable by the type of catechol, temperature, and concentration. For the first time, spectroscopic evidence is obtained for a rapid Si-O σ-bond metathesis reaction. Hence, this study sorts out a long-lasting debate and confirms dynamic covalent features for our Earth's crust's most abundant chemical bond.

Hydrogen-bond-induced selectivity of a head-to-head photo-dimerisation of dialkynylanthracene - access to tetradentate Lewis acids.

Using 2-hydroxypropyl-protecting groups, 1,8-dialkynylanthracene photo-dimers were prepared in head-to-head-configuration by UV irradiation on a multi-gram scale. In non-polar solvents, the combination of non-covalent hydrogen bonds and π-π-interactions induces the formation of the syn-isomer in up to 85% yield. Instead, more polar solvents or irradiation of unprotected 1,8-diethynylanthracene led to formation of the corresponding anti-isomer in large excess. Cleavage of the protecting groups under basic conditions affords a rigid hydrocarbon skeleton with four directional functions. This was used as a building block for a tetradentate boron Lewis acid. Its applicability as a host for Lewis-base substrates was demonstrated by the formation of adducts with various nitrogen bases. Adduct formation with hydrazine leads to impressive networks between the tetraboron host and the substrate molecules.

A Rational Approach to Tetra-Functional Photo-Switches.

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)ethynyl]anthracen-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.

Halogen Bonds of Halotetrafluoropyridines in Crystals and Co-crystals with Benzene and Pyridine.

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.

Regiochemical Control in Triptycene Formation-An Exercise in Subtle Balancing Multiple Factors.

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.

Facile routes to abnormal-NHC-cobalt(ii) complexes.

Deprotonation of [IPrPh]I (1) with Co{N(SiMe3)2}2 readily affords the abnormal N-heterocyclic carbene (aNHC) complex (aIPrPh)2CoI2 (2) (aIPrPh = 1,3-bis(2,6-iPr2C6H3)-2-phenyl-imidazol-4-ylidene). Treatment of 1 with NaHBEt3 yields (aIPrPh)BEt3 (3) that serves as an aNHC-transfer agent and yields (aIPrPh)Co{N(SiMe3)2}2 (4) on reaction with Co{N(SiMe3)2}2.

Gas-phase structure of 1,8-bis[(trimethylsilyl)ethynyl]anthracene: cog-wheel-type vs. independent internal rotation and influence of dispersion interactions.

The gas-phase structure of 1,8-bis[(trimethylsilyl)ethynyl]anthracene (1,8-BTMSA) was determined by a combined gas electron diffraction (GED)/mass spectrometry (MS) experiment as well as by quantum-chemical calculations (QC). DFT and dispersion corrected DFT calculations (DFT-D3) predicted two slightly different structures for 1,8-BTMSA concerning the mutual orientation of the two -C-C[triple bond, length as m-dash]C-SiMe3 units: away from one another or both bent to the same side. An attempt was made to distinguish these structures by GED structural analysis. To probe the structural rigidity, a set of Born-Oppenheimer molecular dynamics (BOMD) calculations has been performed at the DFT-D level. Vibrational corrections Δr = ra - re were calculated by two BOMD approaches: a microcanonically (NVE) sampled ensemble of 20 trajectories (BOMD(NVE)) and a canonical (NVT) trajectory thermostated by the Noose-Hoover algorithm (BOMD(NVT)). In addition, the conventional approach with both, rectilinear and curvilinear approximations (SHRINK program), was also applied. Radial distribution curves obtained with models using both MD approaches provide a better description of the experimental data than those obtained using the rectilinear (SHRINK) approximation, while the curvilinear approach turned out to lead to physically inacceptable results. The electronic structure of 1,8-BTMSA was investigated in terms of an NBO analysis and was compared with that of the earlier studied 1,8-bis(phenylethynyl)anthracene. Theoretical and experimental results lead to the conclusion that the (trimethylsilyl)ethynyl (TMSE) groups in 1,8-BTMSA are neither restricted in rotation nor in bending at the temperature of the GED experiment.

Methylene-bridged, intramolecular donor-acceptor systems based on rare-earth metals and phosphinomethanides.

New geminally bonded intramolecular donor-acceptor systems, Cp2LnCH2PR2 (Ln = Y, Ho, Er, R = CHMe2, CMe3), were prepared by salt elimination reactions between the dicyclopentadienyl-element chlorides (Cp2LnCl, Ln = Y, Ho, Er) and diorganylphosphinomethanides (R2PCH2Li; R = CHMe2, CMe3). These compounds, 1-6, were characterized by elemental analyses, mass spectrometry and X-ray diffraction experiments and the yttrium species additionally by NMR spectroscopy. In the solid state the molecular structures differ from each other, depending on the steric demand of the phosphorus substituents. For all iso-Pr-substituted compounds, dimers [Cp2LnCH2P(CHMe2)2]2 with six-membered Ln-C-P-Ln-C-P rings in a chair-like conformation were observed. The sterically more demanding tert-butyl groups prevent dimerization but instead lead to species that undergo complexation of LiCl units by two monomers: [Cp2LnCH2P(CMe3)2·LiCl]. The solution NMR data for the yttrium compounds are consistent with the solid-state structures. Conversion with phenylacetylene afforded heteroleptic cyclobutane-like alkynyl-rare-earth metal complexes [Cp2Ln(µ-C[triple bond, length as m-dash]CPh)]2 [Ln = Y (7), Ho (8), Er (9)]. Treatment of compounds 1-6 with 1,8-diethynylanthracene led to single metalation and dimerization and products with similar structural motifs as observed for complexes 7-9. Reactions with dihydrogen and carbon dioxide resulted in Y-C bond breaking, yielding Cp2YH/R2PCH3 and CO2 insertion products, respectively.

1,8-Bis(phenylethynyl)anthracene - gas and solid phase structures.

1,8-Bis(phenylethynyl)anthracene (1,8-BPEA) was synthesized by a twofold Kumada cross-coupling reaction. The molecular structure of 1,8-BPEA was determined using a combination of gas-phase electron diffraction (GED), mass spectrometry (MS), quantum chemical calculations (QC) and single-crystal X-ray diffraction (XRD). Five rotamers of the molecule with different orientations of phenylethynyl groups were investigated by DFT calculations. According to these, molecules of C2 symmetry with co-directional rotation of the phenylethynyl groups are predicted to exist in the gas phase at 498 K. This was confirmed by a GED/MS experiment at this temperature. The bonding of this conformer was studied and described in terms of an NBO-analysis. Dispersion interactions in the solid state structure and in the free molecule are discussed. In the solid this symmetry is broken; the asymmetric unit of the single crystal contains 3.5 molecules and a herringbone packing motif of π-stacked dimers and trimers. The π-stacking in the dimers is between the anthracene units, and the trimers are linked by π-stacking between phenyl and anthracene units. The interaction between these stacks can be described in terms of σ(C-H)π interactions.