Long but Strong C-C Single Bonds: Challenges for Theory.

Theoretical challenges in describing molecules with anomalously long single C-C bonds are analyzed in terms of the relative contributions of stabilizing and destabilizing intramolecular interactions. Diamondoid dimers that are stable despite the presence of C-C bonds up to 1.7 Šlong, as well as other bulky molecules stabilized due to intramolecular noncovalent interactions (London dispersions) are discussed. The unexpected stability of highly crowded molecules, such as diamondoid dimers and tert-butyl-substituted hexaphenylethanes, calls for reconsideration of the "steric effect" traditionally thought to destabilize the molecule. Alternatively, "steric attraction" helps to understand bonding in sterically overloaded molecules, whose structural and energetic analysis requires a proper theoretical description of noncovalent interactions.

Sterically controlled mechanochemistry under hydrostatic pressure.

Mechanical stimuli can modify the energy landscape of chemical reactions and enable reaction pathways, offering a synthetic strategy that complements conventional chemistry. These mechanochemical mechanisms have been studied extensively in one-dimensional polymers under tensile stress using ring-opening and reorganization, polymer unzipping and disulfide reduction as model reactions. In these systems, the pulling force stretches chemical bonds, initiating the reaction. Additionally, it has been shown that forces orthogonal to the chemical bonds can alter the rate of bond dissociation. However, these bond activation mechanisms have not been possible under isotropic, compressive stress (that is, hydrostatic pressure). Here we show that mechanochemistry through isotropic compression is possible by molecularly engineering structures that can translate macroscopic isotropic stress into molecular-level anisotropic strain. We engineer molecules with mechanically heterogeneous components-a compressible ('soft') mechanophore and incompressible ('hard') ligands. In these 'molecular anvils', isotropic stress leads to relative motions of the rigid ligands, anisotropically deforming the compressible mechanophore and activating bonds. Conversely, rigid ligands in steric contact impede relative motion, blocking reactivity. We combine experiments and computations to demonstrate hydrostatic-pressure-driven redox reactions in metal-organic chalcogenides that incorporate molecular elements that have heterogeneous compressibility, in which bending of bond angles or shearing of adjacent chains activates the metal-chalcogen bonds, leading to the formation of the elemental metal. These results reveal an unexplored reaction mechanism and suggest possible strategies for high-specificity mechanosynthesis.

Multigram Synthesis of Dimethyl Stellane-1,5-Dicarboxylate as a Key Precursor for ortho-Benzene Mimics.

Herein, we present previously unavailable C(sp3 )-rich polycyclic hydrocarbon scaffolds that have the potential to become valuable tools in medicinal chemistry and crop science as saturated bioisosteres of benzenoids. We have developed a scalable protocol (up to 50 g from a single synthetic run) for the synthesis of tricyclo[3.3.0.03,7 ]octane (bisnoradamantane or stellane) 1,5-dicarboxylic acid derivatives. X-ray crystallographic analysis of the stellane 1,5-dicarboxylic acid dimethyl ester has revealed that this scaffold is an optimal saturated isostere for ortho-disubstituted benzene where substituents exhibit in-plane topology. The synthetic protocol is based on the oxidative cyclization of dimethyl octahydropentalene-2,5-dicarboxylate (DMOD) through lithiation followed by I2 oxidation. The reaction outcome is determined by the stereochemistry of the substrate. While the endo,endo cis-DMOD, exclusively gives the "unwanted" Claisen cyclization product, the exo,endo cis- and exo,exo cis- stereoisomers afford the desired stellane 1,5-dicarboxylic acid dimethyl ester quantitatively. DFT computations have revealed that the reaction proceeds via the dianion of dimethyl octahydropentalene-2,5-dicarboxylate, which undergoes SET oxidation by I2 to form a radical anion. The subsequent cyclization followed by a second SET oxidation gives the desired stellane derivative.

Synthesis and Functionalization of Isomeric Sesquihomodiamantenes.

anti- and syn-sesquihomodiamantenes (SDs) were prepared and structurally characterized. anti-SD and parent sesquihomoadamantene were CH-bond functionalized by utilizing a phase-transfer protocol. The density functional theory-computed ionization potentials of unsaturated diamondoid dimers correlate well with the experimental oxidation potentials obtained from cyclic voltammetry. Similar geometries ensue for both the reduced and ionized SD states, whose persistence is supported by the ß-hydrogen's spatial sheltering. This makes SDs promising building blocks for the construction of diamond materials with high stability and carrier mobility.

Experimental measurement of the diamond nucleation landscape reveals classical and nonclassical features.

Nucleation is a core scientific concept that describes the formation of new phases and materials. While classical nucleation theory is applied across wide-ranging fields, nucleation energy landscapes have never been directly measured at the atomic level, and experiments suggest that nucleation rates often greatly exceed the predictions of classical nucleation theory. Multistep nucleation via metastable states could explain unexpectedly rapid nucleation in many contexts, yet experimental energy landscapes supporting such mechanisms are scarce, particularly at nanoscale dimensions. In this work, we measured the nucleation energy landscape of diamond during chemical vapor deposition, using a series of diamondoid molecules as atomically defined protonuclei. We find that 26-carbon atom clusters, which do not contain a single bulk atom, are postcritical nuclei and measure the nucleation barrier to be more than four orders of magnitude smaller than prior bulk estimations. These data support both classical and nonclassical concepts for multistep nucleation and growth during the gas-phase synthesis of diamond and other semiconductors. More broadly, these measurements provide experimental evidence that agrees with recent conceptual proposals of multistep nucleation pathways with metastable molecular precursors in diverse processes, ranging from cloud formation to protein crystallization, and nanoparticle synthesis.

Monochromatic Photocathodes from Graphene-Stabilized Diamondoids.

The monochromatic photoemission from diamondoid monolayers provides a new strategy to create electron sources with low energy dispersion and enables compact electron guns with high brightness and low beam emittance for aberration-free imaging, lithography, and accelerators. However, these potential applications are hindered by degradation of diamondoid monolayers under photon irradiation and electron bombardment. Here, we report a graphene-protected diamondoid monolayer photocathode with 4-fold enhancement of stability compared to the bare diamondoid counterpart. The single-layer graphene overcoating preserves the monochromaticity of the photoelectrons, showing 12.5 meV ful width at half-maximum distribution of kinetic energy. Importantly, the graphene coating effectively suppresses desorption of the diamondoid monolayer, enhancing its thermal stability by at least 100 K. Furthermore, by comparing the decay rate at different photon energies, we identify electron bombardment as the principle decay pathway for diamondoids under graphene protection. This provides a generic approach for stabilizing volatile species on photocathode surfaces, which could greatly improve performance of electron emitters.

Diamondoid Nanostructures as sp3 -Carbon-Based Gas Sensors.

Diamondoids, sp3 -hybridized nanometer-sized diamond-like hydrocarbons (nanodiamonds), difunctionalized with hydroxy and primary phosphine oxide groups, enable the assembly of the first sp3 -C-based chemical sensors by vapor deposition. Both pristine nanodiamonds and palladium nanolayered composites can be used to detect toxic NO2 and NH3 gases. This carbon-based gas sensor technology allows reversible NO2 detection down to 50 ppb and NH3 detection at 25-100 ppm concentration with fast response and recovery processes at 100 °C. Reversible gas adsorption and detection is compatible with 50 % humidity conditions. Semiconducting p-type sensing properties are achieved from devices based on primary phosphine-diamantanol, in which high specific area (ca. 140 m2 g-1 ) and channel nanoporosity derive from H-bonding.

Hybrid metal-organic chalcogenide nanowires with electrically conductive inorganic core through diamondoid-directed assembly.

Controlling inorganic structure and dimensionality through structure-directing agents is a versatile approach for new materials synthesis that has been used extensively for metal-organic frameworks and coordination polymers. However, the lack of 'solid' inorganic cores requires charge transport through single-atom chains and/or organic groups, limiting their electronic properties. Here, we report that strongly interacting diamondoid structure-directing agents guide the growth of hybrid metal-organic chalcogenide nanowires with solid inorganic cores having three-atom cross-sections, representing the smallest possible nanowires. The strong van der Waals attraction between diamondoids overcomes steric repulsion leading to a cis configuration at the active growth front, enabling face-on addition of precursors for nanowire elongation. These nanowires have band-like electronic properties, low effective carrier masses and three orders-of-magnitude conductivity modulation by hole doping. This discovery highlights a previously unexplored regime of structure-directing agents compared with traditional surfactant, block copolymer or metal-organic framework linkers.

Aerobic Aliphatic Hydroxylation Reactions by Copper Complexes: A Simple Clip-and-Cleave Concept.

A simple imine clip-and-cleave concept has been developed for the selective hydroxylation of non-activated CH bonds in aliphatic aldehydes with dioxygen through a copper complex. The synthetic protocol involves reaction of a substrate aldehyde with N,N-diethyl-ethylendiamine to give the corresponding imine, which is used as a bidentate donor ligand forming a copper(I) complex with [Cu(CH3 CN)4 ][CF3 SO3 ]. After exposure of the reaction mixture to dioxygen acidic cleavage and aqueous workup liberates the corresponding ß-hydroxylated aldehyde. The concept for the hydroxylation of trimethylacetaldehyde as well as adamantane and diamantane 1-carbaldehydes was investigated and the corresponding ß-hydroxy aldehydes were obtained with high selectivities. The results of low temperature stopped-flow measurements indicate the formation of a bis(µ-oxido)dicopper complex as reactive intermediate. According to density functional theory (DFT, RI-BLYP-D3/def2-TZVP(SDD)/ COSMO(CH2 Cl2 )//RI-PBE-D3/def2-TZVP(SDD)) computations CH bonds suitably predisposed to the [Cu2 O2 ]2+ core undergo hydroxylation in a concerted step with particularly low activation barriers, which explains the experimentally observed regioselectivities.

Intramolecular London Dispersion Interaction Effects on Gas-Phase and Solid-State Structures of Diamondoid Dimers.

The covalent diamantyl (C28H38) and oxadiamantyl (C26H34O2) dimers are stabilized by London dispersion attractions between the dimer moieties. Their solid-state and gas-phase structures were studied using a multitechnique approach, including single-crystal X-ray diffraction (XRD), gas-phase electron diffraction (GED), a combined GED/microwave (MW) spectroscopy study, and quantum chemical calculations. The inclusion of medium-range electron correlation as well as the London dispersion energy in density functional theory is essential to reproduce the experimental geometries. The conformational dynamics computed for C26H34O2 agree well with solution NMR data and help in the assignment of the gas-phase MW data to individual diastereomers. Both in the solid state and the gas phase the central C-C bond is of similar length for the diamantyl [XRD, 1.642(2) Å; GED, 1.630(5) Å] and the oxadiamantyl dimers [XRD, 1.643(1) Å; GED, 1.632(9) Å; GED+MW, 1.632(5) Å], despite the presence of two oxygen atoms. Out of a larger series of quantum chemical computations, the best match with the experimental reference data is achieved with the PBEh-3c, PBE0-D3, PBE0, B3PW91-D3, and M06-2X approaches. This is the first gas-phase confirmation that the markedly elongated C-C bond is an intrinsic feature of the molecule and that crystal packing effects have only a minor influence.

Overcoming lability of extremely long alkane carbon-carbon bonds through dispersion forces.

Steric effects in chemistry are a consequence of the space required to accommodate the atoms and groups within a molecule, and are often thought to be dominated by repulsive forces arising from overlapping electron densities (Pauli repulsion). An appreciation of attractive interactions such as van der Waals forces (which include London dispersion forces) is necessary to understand chemical bonding and reactivity fully. This is evident from, for example, the strongly debated origin of the higher stability of branched alkanes relative to linear alkanes and the possibility of constructing hydrocarbons with extraordinarily long C-C single bonds through steric crowding. Although empirical bond distance/bond strength relationships have been established for C-C bonds (longer C-C bonds have smaller bond dissociation energies), these have no present theoretical basis. Nevertheless, these empirical considerations are fundamental to structural and energetic evaluations in chemistry, as summarized by Pauling as early as 1960 and confirmed more recently. Here we report the preparation of hydrocarbons with extremely long C-C bonds (up to 1.704 Å), the longest such bonds observed so far in alkanes. The prepared compounds are unexpectedly stable--noticeable decomposition occurs only above 200 °C. We prepared the alkanes by coupling nanometre-sized, diamond-like, highly rigid structures known as diamondoids. The extraordinary stability of the coupling products is due to overall attractive dispersion interactions between the intramolecular H•••H contact surfaces, as is evident from density functional theory computations with and without inclusion of dispersion corrections.

From isolated diamondoids to a van-der-Waals crystal: A theoretical and experimental analysis of a trishomocubane and a diamantane dimer in the gas and solid phase.

The electronic properties of sp2/sp3 diamondoids in the crystalline state and in the gas phase are presented. Apparent differences in electronic properties experimentally observed by resonance Raman spectroscopy in the crystalline/gas phase and absorption measurements in the gas phase were investigated by density functional theory computations. Due to a reorganization of the molecular orbitals in the crystalline phase, the HOMO (highest occupied molecular orbital) and LUMO (lowest unoccupied molecular orbital) energy gaps are lowered significantly by 0.5 eV-1 eV. The π → π* transition is responsible for large absorption in both gas and crystalline phases. It further causes a large increase in the Raman intensity of the C=C stretch vibration when excited resonantly. By resonance Raman spectroscopy we were able to determine the C=C bond length of the trishomocubane dimer to exhibit 1.33 Å in the ground and 1.41 Å in the excited state.

Hybrid Group IV Nanophotonic Structures Incorporating Diamond Silicon-Vacancy Color Centers.

We demonstrate a new approach for engineering group IV semiconductor-based quantum photonic structures containing negatively charged silicon-vacancy (SiV(-)) color centers in diamond as quantum emitters. Hybrid diamond-SiC structures are realized by combining the growth of nano- and microdiamonds on silicon carbide (3C or 4H polytype) substrates, with the subsequent use of these diamond crystals as a hard mask for pattern transfer. SiV(-) color centers are incorporated in diamond during its synthesis from molecular diamond seeds (diamondoids), with no need for ion-implantation or annealing. We show that the same growth technique can be used to grow a diamond layer controllably doped with SiV(-) on top of a high purity bulk diamond, in which we subsequently fabricate nanopillar arrays containing high quality SiV(-) centers. Scanning confocal photoluminescence measurements reveal optically active SiV(-) lines both at room temperature and low temperature (5 K) from all fabricated structures, and, in particular, very narrow line widths and small inhomogeneous broadening of SiV(-) lines from all-diamond nanopillar arrays, which is a critical requirement for quantum computation. At low temperatures (5 K) we observe in these structures the signature typical of SiV(-) centers in bulk diamond, consistent with a double lambda. These results indicate that high quality color centers can be incorporated into nanophotonic structures synthetically with properties equivalent to those in bulk diamond, thereby opening opportunities for applications in classical and quantum information processing.

SF5-Enolates in Ti(IV)-Mediated Aldol Reactions.

The F···Ti bonding in the transition structures determines high trans- and syn-diastereoselectivities for aldol reactions of SF5-acetates with aldehydes in the presence of TiCl4 in the non-nucleophilic solvent CH2Cl2. Such bonding is canceled in nucleophilic solvents where opposite cis-stereochemistry is observed. The potential of thus obtained stereoisomeric SF5-aryl acrylates as dipolarophiles in the preparation of SF5-containing heterocycles is demonstrated.

Defying Stereotypes with Nanodiamonds: Stable Primary Diamondoid Phosphines.

Direct unequal C-H bond difunctionalization of phosphorylated diamantane was achieved in high yield from the corresponding phosphonates. Reduction of the functionalized phosphonates provides access to novel primary and secondary alkyl/aryl diamantane phosphines. The prepared primary diamantyl phosphines are quite air stable compared to their adamantyl and especially alkyl or aryl analogues. This finding is corroborated by comparing the singly occupied molecular orbital energy levels of the corresponding phosphine radical cations obtained by density functional theory computations.

Toward an Understanding of Diamond sp(2)-Defects with Unsaturated Diamondoid Oligomer Models.

Nanometer-sized doubly bonded diamondoid dimers and trimers, which may be viewed as models of diamond with surface sp(2)-defects, were prepared from corresponding ketones via a McMurry coupling and were characterized by spectroscopic and crystallographic methods. The neutral hydrocarbons and their radical cations were studied utilizing density functional theory (DFT) and ab initio (MP2) methods, which reproduce the experimental geometries and ionization potentials well. The van der Waals complexes of the oligomers with their radical cations that are models for the self-assembly of diamondoids, form highly delocalized and symmetric electron-deficient structures. This implies a rather high degree of σ-delocalization within the hydrocarbons, not too dissimilar to delocalized π-systems. As a consequence, sp(2)-defects are thus also expected to be nonlocal, thereby leading to the observed high surface charge mobilities of diamond-like materials. In order to be able to use the diamondoid oligomers for subsequent surface attachment and modification, their C-H-bond functionalizations were studied, and these provided halogen and hydroxy derivatives with conservation of unsaturation.

Inverted Carbon Geometries: Challenges to Experiment and Theory.

Disproving a long C-C-bond textbook example: The reported 1.643 Å C-C bond in 5-cyano-1,3-dehydroadamantane was redetermined and "only" amounts to 1.584 Å. While this value is well reproduced with ab initio methods, some common DFT approaches perform poorly and are only consistent with CCSD(T)/cc-pVTZ optimizations for noninverted carbons. Large deviations from experiment were also found for other molecules with atypical electron density distributions, e.g., cubane, bicyclo[2.2.0]hexane, and bicyclo[2.1.0]- and bicyclo[1.1.1]pentane, thereby presenting challenging structures for some DFT implementations.

Elemental F2 with transannular dienes: regioselectivities and mechanisms.

Three reaction paths, namely, molecule-induced homolytic, free radical, and electrophilic, were modeled computationally at the MP2 level of ab initio theory and studied experimentally for the reaction of F2 with the terminal dienes of bicyclo[3.3.1]nonane series. The addition of fluorine is accompanied by transannular cyclization to the adamantane derivatives in which strong evidence for the electrophilic mechanism both in nucleophilic (acetonitrile) and non-nucleophilic (CFCl3 , CHCl3 ) solvents were found. The presence of KF in CFCl3 and CHCl3 facilitates the addition and substantially reduces the formation of tar products.

Beyond the corey reaction II: dimethylenation of sterically congested ketones.

Bulky methyl ketones show significantly decreased reactivities toward the Corey-Chaykovsky methylenation reagent dimethylsulfoxonium methylide (DMSM). The excess of base and temperature increase opens an alternative reaction channel that instead leads to the corresponding cyclopropyl ketones. Computations suggest that the initial reaction step involves the methylene group transfer from DMSM on the ketone enolate followed by the intramolecular cyclization. The key step is associated with a barrier of 22 ± 3 kcal mol(-1) and is driven by exothermic elimination of DMSO.

Functionalization of homodiamantane: oxygen insertion reactions without rearrangement with dimethyldioxirane.

Homodiamantane bromination and nitroxylation are accompanied by contraction of the seven-membered ring to give the corresponding substituted 1-diamantylmethyl derivatives. In contrast, CH-bond hydroxylations with dimethyldioxirane retain the cage and give both apically and medially substituted homodiamantanes. The product ratios are in accord with the barriers for the oxygen insertion computed with density functional theory methods only if solvation is included through a polarizable continuum model. B3LYP-D3 and M06-2X computations with a 6-31G(d,p) basis set on the oligomeric van der Waals complexes predict the potential of homodiamantane derivatives for surface modifications with conformationally slightly flexible diamondoid homologues.