Michael.cowley82

Alkali metals in water are always at the brink of explosion. Herein, we show that this vigorous reaction can be kept in a non-exploding regime, revealing a fascinating richness of hitherto unexplored chemical processes. A combination of high-speed camera imaging and visible/near-infrared/infrared spectroscopy allowed us to catch and characterize the system at each stage of the reaction. After gently placing a drop of a sodium/potassium alloy on water under an inert atmosphere, the production of solvated electrons became so strong that their characteristic blue color could be observed with the naked eye. The exoergic reaction leading to the formation of hydrogen and hydroxide eventually heated the alkali metal drop such that it became glowing red, and part of the metal evaporated. As a result of the reaction, a perfectly transparent drop consisting of molten hydroxide was temporarily stabilized on water through the Leidenfrost effect, bursting spectacularly after it had cooled sufficiently.

The vigorous reaction of alkali metals in water was kept in a non-exploding regime through gently placing a drop of Na/K alloy on water. Initially, solvated electrons are produced (blue color), before the exoergic reaction leading to H₂ and OH⁻ heats the drop such that it becomes red, and part of the metal evaporates. A transparent drop of molten hydroxide is then temporarily stabilized on the water through the Leidenfrost effect before bursting spectacularly.

As a result of the increased polarity of the metal–carbon bond when going down the group of the periodic table, the heavier alkali metal organyl compounds are generally more reactive and less stable than their lithium congeners. We now report a reverse trend for alkali metal carbenoids. Simple substitution of lithium by the heavier metals (Na, K) results in a significant stabilization of these usually highly reactive compounds. This allows their isolation and handling at room temperature and the first structure elucidation of sodium and potassium carbenoids. The control of stability was used to control reactivity and selectivity. Hence, the Na and K carbenoids act as selective carbene-transfer reagents, whereas the more labile lithium systems give rise to product mixtures. Additional fine tuning of the M−C interaction by means of crown ether addition further allows for control of the stability and reactivity.

Heavy but stable: The reactivity of s-block metal organyl compounds generally increases when going down the group of the periodic table, often limiting applications of the heavier congeners. However, the reverse of this trend has now been demonstrated for alkali metal carbenoids by replacement of Li by Na or K. The resulting compounds exhibit an increased thermal stability, which allows their isolation and selective application.

We demonstrate that polymer electron acceptors with excellent all-polymer solar-cell (all-PSC) device performance can be developed from polymer electron donors by using BN units. By alleviating the steric hindrance effect of the bulky pendant moieties on the conjugated polymers that contain BN units, the π–π stacking distance of polymer backbones is decreased and the electron mobility is consequently enhanced by nearly two orders of magnitude. As a result, the power conversion efficiency of all-PSCs with the polymer acting as the electron acceptor is greatly improved from 0.12 % to 5.04 %. This PCE value is comparable to that of the best all-PSCs with state-of-the-art polymer acceptors.

From giver to taker: Incorporation of BN units into polymer electron donors has resulted in a series of polymer electron acceptors. Extending the length of the repeating units of the conjugated polymers alleviates the effect of steric hindrance from the pendant groups and promotes the π–π stacking of the polymer backbones. The all-polymer solar-cell device shows a power conversion efficiency (PCE) exceeding 5.0 %.

The first frustrated Lewis pair-catalyzed cycloisomerization of a series of 1,5-enynes was developed. The reaction proceeds via the π-activation of the alkyne and subsequent 5-endo-dig cyclization with the adjacent alkene. The presence of PPh₃ was of utmost importance on the one hand to prevent side reactions (for example, 1,1-carboboration) and on the other hand for the efficient protodeborylation to achieve the catalytic turnover. The mechanism is explained on the basis of quantum-chemical calculations, which are in full agreement with the experimental observations.

Cycloisomerization without a metal: The first FLP-catalyzed C−C bond forming reaction is described as proceeding through a cycloisomerization/protodeborylation sequence. The reaction mechanism is supported by X-ray crystal structure analysis of intermediates, kinetic experiments, and by quantum-mechanical calculations.

At ultrahigh pressure (>110 GPa), H₂S is converted into a metallic phase that becomes superconducting with a record T_c of approximately 200 K. It has been proposed that the superconducting phase is body-centered cubic H₃S (Im m, a=3.089 Å) resulting from the decomposition reaction 3 H₂S2 H₃S+S. The analogy between H₂S and H₂O led us to a very different conclusion. The well-known dissociation of water into H₃O⁺ and OH⁻ increases by orders of magnitude under pressure. H₂S is anticipated to behave similarly under pressure, with the dissociation process 2 H₂SH₃S⁺+SH⁻ leading to the perovskite structure (SH⁻)(H₃S⁺). This phase consists of corner-sharing SH₆ octahedra with SH⁻ ions at each A site (the centers of the S₈ cubes). DFT calculations show that the perovskite (SH⁻)(H₃S⁺) is thermodynamically more stable than the Im m structure of H₃S, and suggest that the A site hydrogen atoms are most likely fluxional even at T_c .

Under ultrahigh pressure (>110 GPa), H₂S is converted into a metallic phase that becomes superconducting with a record T_c of approximately 200 K. It is proposed that in this phase a dissociation of 2 H₂S into H₃S⁺ and SH⁻ is present, leading to the perovskite structure (SH⁻)(H₃S⁺). This phase consists of corner-sharing SH₆ octahedra with SH⁻ ions at the center of each S₈ cube.

Nanostructured Group 14 semiconductors attract significant attention because of a broad range of potential applications. In their Communication on page 2441 ff., T. Fässler, D. Fattakhova-Rohlfing et al. describe a general and controllable fabrication method for Ge nanomorphologies with tunable composition using the controlled reaction of [Ge₉]⁴⁻ Zintl clusters to a solid germanium phase. The image was designed by Christoph Hohmann, Nanosystems Initiative Munich.

We report the spectroscopic identification of the [B₃(NN)₃]⁺ and [B₃(CO)₃]⁺ complexes, which feature the smallest π-aromatic system B₃⁺. A quantum chemical bonding analysis shows that the adducts are mainly stabilized by L[B₃L₂]⁺ σ-donation.

Minimal π systems: The [B₃(NN)₃]⁺ and [B₃(CO)₃]⁺ complexes are identified through infrared photodissociation spectroscopy. The complexes feature the smallest π-aromatic system B₃⁺. Quantum chemical bonding analysis reveals that the adducts are stabilized by L[B₃L₂]⁺ σ-donation.

High-molar-mass poly(alkylphosphinoboranes) are currently not accessible by conventional metal-catalyzed dehydropolymerization. However, the mild thermolysis of a Lewis base stabilized phosphinoborane is an alternative metal-free approach. In their Communication on page 13782 ff., M. Scheer, I. Manners, and co-workers describe their successful synthesis of poly(tert-butylphosphinoborane) by this promising strategy.

Cation–π interactions are one of the most important classes of noncovalent bonding, and are seen throughout biology, chemistry, and materials science. However, in almost every documented case, these interactions play only a supporting role to much stronger covalent or dative bonds, thus making examples of exclusive cation–π bonding exceedingly rare. In this study, a neutral diboryne molecule is found to encapsulate the light alkali metal cations Li⁺ and Na⁺ in the absence of a net charge, covalent bonds, or lone-pair donor groups. The resulting encapsulation complexes are, to our knowledge, the first structurally authenticated species in which a neutral molecule binds the light alkali metals exclusively through cation–π interactions.

No help required: Cation–π interactions are one of the most important classes of noncovalent bonding; however, examples of exclusive cation–π bonding are exceedingly rare. A neutral diboryne molecule has been found to encapsulate Li⁺ and Na⁺ in the absence of a net charge, covalent bonds, or lone-pair donor groups. In the resulting encapsulation complexes, a neutral molecule binds the light alkali metals exclusively through cation–π interactions.

Photolysis of the cyclic phosphine oligomer [PPh]₅ in the presence of pentaarylboroles leads to the formation of 1,2-phosphaborines by the formal insertion of a phenylphosphinidene fragment into the endocyclic CB bond. The solid-state structure features a virtually planar central ring with bond lengths indicating significant delocalization. Appreciable ring current in the 1,2-phosphaborine core, detected in nuclear independent chemical shift (NICS) calculations, are consistent with aromatic character. These products are the first reported 1,2-BPC₄ conjugated heterocycles and open a new avenue for BP as a valence isoelectronic substitute for CC in arene systems.

Jamming PB into benzene: 1,2-Phosphaborines were synthesized by the ring expansion reaction of boroles with the cyclic phosphine [PPh]₅ under UV irradiation. The products were structurally characterized revealing a planar central ring. The nature of the bonding was analyzed computationally and indicated that the heterocycle had appreciable aromatic character.

We report the evaporation of a stable cyclic silylene and its oxidation (with ozone or N₂O) through oxygen atom transfer to form the corresponding silanone under matrix isolation conditions. As uncomplexed silanones are rare owing to their very high reactivity, this method provides an alternative route to these sought-after molecules. The silanone, as well as a novel bicyclic silane with a bridgehead silicon atom derived from an intramolecular silylene CH bond insertion, were characterized by comparison of high-resolution infrared spectra with density functional theory (DFT) computations at the M06-2X/cc-pVDZ level of theory.

One atom at a time: Oxygen atom transfer from ozone to a stable silylene provides access to a new cyclic silanone. This bimolecular atom transfer reaction was achieved under matrix isolation conditions through co-deposition of the silylene and ozone. Conclusive evidence for the silanone is provided by comparison of experimental and computed IR spectra, including isotopological ¹⁶O/¹⁸O replacements. Atom colors: Si=purple, C=black, O=red, H=blue.

We report the first direct spectroscopic observation by electron paramagnetic resonance (EPR) spectroscopy of a triplet diradical that is formed in a thermally induced rotation around a main-group π bond, that is, the SiSi double bond of tetrakis(di-tert-butylmethylsilyl)disilene (1). The highly twisted ground-state geometry of singlet 1 allows access to the perpendicular triplet diradical 2 at moderate temperatures of 350–410 K. DFT-calculated zero-field splitting (ZFS) parameters of 2 accurately reproduce the experimentally observed half-field transition. Experiment and theory suggest a thermal equilibrium between 1 and 2 with a very low singlet–triplet energy gap of only 7.3 kcal mol⁻¹.

A triplet diradical that is formed in a thermally induced rotation around a main-group π bond, that is the SiSi double bond of 1, was directly observed by EPR spectroscopy. Both experiment and theory support a thermal equilibrium between singlet 1 and the perpendicular triplet diradical 2.

The regioregular synthesis of the first azaborine oligomers and a corresponding conjugated polymer was accomplished by Suzuki–Miyaura coupling methods. An almost perfectly coplanar syn arrangement of the heterocycles was deduced from an X-ray crystal structure of the dimer, which also suggested that NH⋅⋅⋅π interactions play an important role. Computational studies further supported these experimental observations and indicated that the electronic structure of the longer azaborine oligomers and polymer resembles that of poly(cyclohexadiene) more than poly(p-phenylene). A comparison of the absorption and emission properties of the polymer with those of the oligomers revealed dramatic bathochromic shifts upon chain elongation, thus suggesting highly effective extension of conjugation.

A BN serial: A regioregular conjugated polymer and short model oligomers constructed solely from 1,2-azaborine units by Suzuki–Miyaura cross-coupling have been synthesized and found to adopt an unusual syn conformation stabilized by NH⋅⋅⋅π interactions. The optoelectronic properties of the polymer more closely resemble the computationally predicted properties of poly(cyclohexadiene) rather than those of poly(p-phenylene). pin=pinacol.