On-Surface Synthesis and Real-Space Visualization of Aromatic P3 N3.

On-surface synthesis is at the verge of emerging as the method of choice for the generation and visualization of unstable or unconventional molecules, which could not be obtained via traditional synthetic methods. A case in point is the on-surface synthesis of the structurally elusive cyclotriphosphazene (P3 N3 ), an inorganic aromatic analogue of benzene. Here, we report the preparation of this fleetingly existing species on Cu(111) and Au(111) surfaces at 5.2 K through molecular manipulation with unprecedented precision, i.e., voltage pulse-induced sextuple dechlorination of an ultra-small (about 6 Å) hexachlorophosphazene P3 N3 Cl6 precursor by the tip of a scanning probe microscope. Real-space atomic-level imaging of cyclotriphosphazene reveals its planar D3h -symmetric ring structure. Furthermore, this demasking strategy has been expanded to generate cyclotriphosphazene from a hexaazide precursor P3 N21 via a different stimulation method (photolysis) for complementary measurements by matrix isolation infrared and ultraviolet spectroscopy.

Synthesis, electronic nature, and reactivity of selected silylene carbonyl complexes.

Room-temperature stable main group element carbonyl complexes are rare. Here we report on the synthesis of two such complexes, namely gallium-substituted silylene-carbonyl complexes [L(X)Ga]2SiCO (X = I 2, Me 3; L = HC[C(Me)NDipp]2, Dipp = 2,6-iPr2C6H3) by reaction of three equivalents of LGa with IDippSiI4 (IDipp = 1,3-bis(2,6-iPr2C6H3)-imidazol-2-ylidene) or by salt elimination from [L(Br)Ga]2SiCO with MeLi. Both silylene carbonyl complexes were spectroscopically characterized as well as with single crystal X-ray diffraction (sc-XRD), while their electronic nature and the specific influence of the Ga-substituents X was evaluated by quantum chemical computations. In addition, we report the oxidative addition reaction of [L(Br)Ga]2SiCO with NH3, yielding [L(Br)Ga]2Si(H)NH24, demonstrating the promising potential of such complexes for small molecule activation.

Equilibrating parent aminomercaptocarbene and CO2 with 2-amino-2-thioxoacetic acid via heavy-atom quantum tunneling.

The search for methods to bind CO2 and use it synthetically as a C1-building block under mild conditions is an ongoing endeavor of great urgency. The formation of heterocyclic carbene-carbon dioxide adducts occurs rapidly when the carbene is generated in solution in the presence of CO2. Here we demonstrate the reversible formation of a complex of the hitherto unreported aminomercaptocarbene (H2N-C̈-SH) with CO2 isolated in solid argon by photolysis of 2-amino-2-thioxoacetic acid. Remarkably, the complex disappears in the dark as deduced by time-dependent matrix infrared measurements, and equilibrates back to the covalently bound starting material. This kinetically excluded process below ca. 8 K is made possible through heavy-atom quantum mechanical tunneling, as also evident from density functional theory and ab initio computations at the CCSD(T)/cc-pVTZ level of theory. Our results provide insight into CO2 activation using a carbene and emphasize the role of quantum mechanical tunneling in organic processes, even involving heavy atoms.

London Dispersion Helps Refine Steric A-Values: Dispersion Energy Donor Scales.

We suggest a scale of dispersion energy donors (DEDs) that allows for direct comparisons with steric effects. This scale is based on the classic A-values and allows groups to reorient to minimize strain, thereby providing an advantage over raw group polarizabilities. The A-value can no longer be considered purely a steric factor. Even for groups that do not participate in charge transfer or electrostatic interactions, the A-value includes Pauli repulsion (steric hindrance) and attractive London dispersion (LD) interactions. Although the common assumption is that, at the distances found in monosubstituted cyclohexanes, steric demands are the key factors influencing conformer preferences, we show in this computational study that there is a non-negligible LD part. We use this system to build a DED scale and a complementary steric scale. These scales are quantitatively comparable, as they are based on the same system, and allow for comparison of the two competing interactions in experimentally relevant settings. In addition, we show that LD interactions can be used to explain puzzling data regarding relative group sizes.

London Dispersion Helps Refine Steric A-Values: The Halogens.

Halogens are rarely considered as dispersion energy donors for organic reaction design. Here, we re-examine one of the textbook examples for assessing steric hindrance, the A-value, and demonstrate that even in this system, halogens cannot be treated solely as classic repulsive hard spheres. A significant part of the steric demand of the halogens is compensated by attractive London dispersion (LD) interactions, explaining the experimental lack of a clear trend when going down the halogens' row. Beyond monohalogenated cyclohexanes, dihalo- and perhalocyclohexanes also show significant LD interactions. We also explored several other small organic systems containing halogens. Our findings show that organic chemists should treat halogens as possible sources of LD interactions in reaction design, as these atoms can change the landscape of the potential energy surface and reverse trends of conformer stabilities and reaction selectivities.

Diamantanethiols on Metal Surfaces: Spatial Configurations, Bond Dissociations, and Polymerization.

We report the on-surface chemistry of diamantanethiols on metal surfaces by combining low-temperature STM studies with quantum mechanical density functional theory computations. First, we examined the spatial configurations of diamantanethiols on metal surfaces, in which the thiol-substrate confinement plays a key role. We then thermally desorbed the diamantanethiols from the substrate surfaces to determine whether the C-S or S-metal bonds preferentially break. Finally, we explored diamantane-4,9-dithiol and its polymerization on metal surfaces, forming linear nanodiamond disulfur chains. This work broadens the fundamental knowledge of functionalized diamondoid behavior on surfaces and provides a novel approach to link diamantane as necklace-chain nanodiamond hybrid materials.

Heavy-Atom Tunneling Processes during Denitrogenation of 2,3-Diazabicyclo[2.2.1]hept-2-ene and Ring Closure of Cyclopentane-1,3-diyl Diradical. Stereoselectivity in Tunneling and Matrix Effect.

Triplet cyclopentane-1,3-diyl diradical (T-DR) was generated via photolysis of 2,3-diazabicyclo[2.2.1]hept-2-ene (AZ) under low-temperature matrix conditions. Temperature independency of T-DR decay and the kinetic isotope effect of T-DR-d6 provided experimental evidence in favor of heavy-atom (carbon) tunneling process during the decay of T-DR to bicyclo[2.1.0]pentane (CP) via singlet S-DR. For the first time, the formation of CP was confirmed using low-temperature infrared spectra. Computations of the heavy-atom tunneling process using the small-curvature tunneling method demonstrated a fast reaction from S-DR to CP. Moreover, we observed heavy-atom tunneling during denitrogenation of AZ. Stereoselectivity in the tunneling process of T-DR-d6 was observed at 7 K to form retention-CP-d6 in higher amounts compared to inversion-CP-d6. Photolysis of AZ-d6 yielded inv-CP-d6 and ret-CP-d6 in environment- and temperature-dependent ratios. Moreover, because of the prominent matrix effect, T-DR decayed more rapidly in Ar than in glassy organic matrices.

Flat corannulene: when a transition state becomes a stable molecule.

Flat corannulene has been considered so far only as a transition state of the bowl-to-bowl inversion process. This study was driven by the prediction that substituents with strong steric repulsion could destabilize the bowl-shaped conformation of this molecule to such an extent that the highly unstable planar geometry would become an isolable molecule. To examine the substituents' effect on the corannulene bowl depth, optimized structures for the highly-congested decakis(t-butylsulfido)corannulene were calculated. The computations, performed with both the M06-2X/def2-TZVP and the B3LYP/def2-TZVP methods (the latter with and without Grimme's D3 dispersion correction), predict that this molecule can achieve two minimum structures: a flat carbon framework and a bowl-shaped structure, which are very close in energy. This rather unusual compound was easily synthesized from decachlorocorannulene under mild reaction conditions, and X-ray crystallographic studies gave similar results to the theoretical predictions. This compound crystallized in two different polymorphs, one exhibiting a completely flat corannulene core and the other having a bowl-shaped conformation.

Equatorial Sulfur Atoms in Bambusurils Spawn Cavity Collapse.

This study confirms the hypothesis that bambusurils (BUs) with equatorial sulfur atoms cannot assume an anion-binding jigger conformation due to strong intramolecular van der Waals attractive interactions. NMR, X-ray crystallography, and computation with newly synthesized eq-semithio-BU[4] and ax-semiaza-eq-semithio-BU[4]s indicate that they all assume compact conformations. Intramolecular distances and a torsional angle serve as reliable indicators of the conformation of any BU. Chemoselective alkylation at the peripheral (equatorial) thiourea functions provides a convenient entry to novel hetero-BUs.

Carbon Tunneling in the Automerization of Cyclo[18]carbon.

Cyclo[18]carbon (C18 ), a recently synthesized carbon allotrope, was found to have a polyynic ground-state structure with D9h symmetry and formally alternating single and triple bonds. Yet, under less influencing experimental conditions this molecule might undergo an automerization reaction between its two degenerate geometries through a cumulenic (non-alternating, adjacent double bonds) D18h transition state. Herein, we discuss the role of quantum mechanical tunneling (QMT) in this degenerate reaction. Our computations predict that at the experimental temperature (5 K) the reaction in the gas phase is completely driven by an extremely rapid heavy atom tunneling (k=2.1×108  s-1 ). Even when approaching room temperature, the QMT rate is still an order of magnitude faster than the semi-classical one. We propose an experimental test to support our prediction, by measuring a characteristic tunneling energy splitting within the radio wave region. Additionally, we examine the role of QMT in other hypothetical C4n+2 carbon clusters.

Chemical mimicry of viral capsid self-assembly via corannulene-based pentatopic tectons.

Self-assembly of twelve pentatopic tectons, which have complementary edges or can be linked using either digonal or trigonal connectors, represents the optimal synthetic strategy to achieve spherical objects, such as chemical capsids. This process requires conditions that secure uninterrupted equilibria of binding and self-correction en route to the global energy minimum. Here we report the synthesis of a highly soluble, deca-heterosubstituted corannulene that bears five terpyridine ligands. Spontaneous self-assembly of twelve such tectons with 30 cadmium(II) cations produces a giant icosahedral capsid as a thermodynamically stable single product in high yield. Nuclear magnetic resonance (NMR) methods, mass spectrometry analyses, small-angle X-ray scattering, transmission electron microscopy, and atomic force microscopy indicate that this spherical capsid has an external diameter of nearly 6 nm and shell thickness of 1 nm, in agreement with molecular modeling. NMR and liquid chromatography evidences imply that chiral self-sorting complexation generates a racemic mixture of homochiral capsids.

Intramolecular van der Waals Interactions Challenge Anion Binding in perthio-Bambusurils.

Bambusurils (BUs) are known to be rigid cavitands that feature an extended, jigger-like conformation, and the BU[6]s strongly bind anions within their hydrophobic cavity. These features are not necessarily shared by the family of perthio-BUs. This study reveals that perthio-BUs assume a compact conformation and perthio-BU[6]s are poor anion binders, crystallizing as anion-free species from solutions containing halide salts. Computational studies show that the equatorial sulfur atoms compete against guest anions for binding with the glycoluril methine groups via strong van der Waals (vdW) attractive interactions. These competitive contacts not only account for the diminished anion-binding of perthio-BUs, but also explain their compact conformation. The semithio- and perthio-BU[4]s form linear coordination polymers with HgII in the solid-state regardless of their intrinsic molecular conformation. The strong involvement of sulfur atoms in intramolecular interactions differentiates the equatorial from the axial (peripheral) heteroatoms, thus offering chemoselective and regioselective transformations.

Catalysis: energy is the measure of all things.

Is there any place in the extremely well-established field of catalytic kinetics for new interpretations or novel models that can change the basic doctrines and viewpoints of catalytic cycles? Since the first introduction of the energy span model, we started to believe that indeed there is. Herein we will evaluate from mathematical, physical and applied perspectives the advantages in using the energy representation for the depiction of the kinetics of catalysis, compared to the usual rate constant representation. We will discuss many concepts such as the energy span, the determining states, the determining zone, graph theory, affinity, degrees of rate control, robustness and volcano plots. Special emphasis will be put on the effect of reactants' and products' concentrations, and how to easily and accurately understand their influence on the kinetics. Ultimately, we will try to explore if energy truly is the measure of all things.

On the Power of Geometry over Tetrel Bonds.

Tetrel bonds are noncovalent interactions formed by tetrel atoms (as σ-hole carriers) with a Lewis base. Here, we present a computational and molecular orbital study on the effect of the geometry of the substituents around the tetrel atom on the σ-hole and on the binding strengths. We show that changing the angles between substituents can dramatically increase bond strength. In addition, our findings suggest that the established Sn > Ge > Si order of binding strength can be changed in sufficiently distorted molecules due to the enhancement of the charge transfer component, making silicon the strongest tetrel donor.

Tuning the Spin, Aromaticity, and Quantum Tunneling in Computationally Designed Fulvalenes.

Pentafulvalene is a symmetrical unsaturated hydrocarbon built from two five-membered rings connected by an exocyclic double bond, where each ring is one electron short of being a 6π-electron aromatic system. Here, we show computationally that by selectively introducing electron withdrawing and donating substituents, we can design pentafulvalene derivatives that exhibit tunable aromaticity properties. Pentafulvalene can be shaped into a species with connected aromatic-antiaromatic rings, which can also achieve π-bond shifting by carbon tunneling. We propose an NMR technique that can experimentally prove such tunneling mechanism. In addition, we devised a doubly aromatic fulvalene involving both Hückel and Baird aromaticities. These results can open possibilities to create novel molecules in terms of spin state, aromaticity, and reactivity by quantum tunneling.

Synthetic Evolution of the Multifarene Cavity from Planar Predecessors.

The stepwise evolution of curved multifarene structures from planar precursors is demonstrated, highlighting three architectural design elements: 1) employment of various aromatic units, 2) changing the hybridization of the linking atoms from sp2 to sp3 , and 3) rigidification of the system by the introduction of five-membered rings. Similar design elements have been employed to transform graphene sheets into curved carbon structures. Specifically, the stepwise synthetic evolution of multifarene[2+2], which has a curved, quite rigid structure, begins with a planar, tetraimine precursor, conversion to pairs of vicinal diamines, and the transformation of these pairs to cyclic thiourea groups. This process was probed by NMR spectroscopy and X-ray crystallography. Since varying the carbonylation conditions resulted in carbamates or thiocarbamates rather than the urea or thiourea isomers, the isomeric interconversion was studied both experimentally and by DFT computations. The carbamate versus urea preference was found to reflect either kinetic or thermodynamic control, respectively.

Aza-Bambusurils En Route to Anion Transporters.

Previous calculations of anion binding with various bambusuril analogs predicted that the replacement of oxygen by nitrogen atoms to produce semiaza-bambus[6]urils would award these new cavitands with multiple anion binding properties. This study validates the hypothesis by efficient synthesis, crystallography, thermogravimetric analysis and calorimetry. These unique host molecules are easily accessible from the corresponding semithio-bambusurils in a one-pot reaction, which converts a single anion receptor into a potential anion channel. Solid-state structures exhibit simultaneous accommodation of three anions, linearly positioned within the cavity along the main symmetry axis. The ability to hold anions at a short distance of about 4 Šis reminiscent of natural chloride channels in E. coli, which exhibit similar distances between their adjacent anion binding sites. The calculated transition-state energy for double-anion movement through the channel suggests that although these host-guest complexes are thermodynamically stable they enjoy high kinetic flexibility to render them efficient anion channels.

Enhanced anion binding by heteroatom replacement in bambusurils.

This study was driven by the hypothesis that heteroatom replacement in bambusurils could significantly modify their anion binding properties. Indeed, calculations with various glycoluril and bambusuril analogs predict that such replacements significantly alter their molecular electrostatic potential and binding properties. Both polarization and electrostatic interactions contribute to anion binding, leading to a general trend of affinity among the neutral molecules: X = S > O > NH. In bambusurils the heteroatom replacement at the portal carbonyls affect the induced dipole more significantly than replacements at the equatorial carbonyls. The stronger polarization and stronger anion binding manifest the increased aptitude of the portal heteroatoms as electron sinks. Notably, this study predicts that protonated aza-bambusurils would not only bind multiple anions along their main axis, but could also function as synthetic anion channels.

Dual-functional semithiobambusurils.

Semithiobambusurils, which represent a new family of macrocyclic host molecules, have been prepared by a convenient, scalable synthesis. These new cavitands are double functional: they strongly bind a broad variety of anions in their interiors and metal ions at their sulfur-edged portals. The solid-state structure of semithiobambus[4]uril with HgCl2 demonstrates the ability of these compounds to form linear chains of coordination polymers with thiophillic metal ions. The crystal structure of semithiobambus[6]uril with tetraphenylphosphonium bromide exhibits the unique anion-binding properties of the host cavity and the characteristics of the binding site.

Chemisorbed monolayers of corannulene penta-thioethers on gold.

Penta(tert-butylthio)corannulene and penta(4-dimethylaminophenylthio)corannulene form highly stable monolayers on gold surfaces, as indicated by X-ray photoelectron spectroscopy (XPS). Formation of these homogeneous monolayers involves multivalent coordination of the five sulfur atoms to gold with the peripheral alkyl or aryl substituents pointing away from the surface. No dissociation of C-S bonds upon binding could be observed at room temperature. Yet, the XPS experiments reveal strong chemical bonding between the thioether groups and gold. Temperature-dependent XPS study shows that the thermal stability of the monolayers is higher than the typical stability of self-assembled monolayers (SAMs) of thiolates on gold.