The Influence of Weakly Coordinating Cations on the O−H⋅⋅⋅O− Hydrogen Bond of Silanol-Silanolate Anions

The reaction of a saline phosphazenium hydroxide hydrate with siloxanes led to a novel kind of silanol-silanolate anions. The weakly coordinating behavior of the cation renders the formation of silanol-silanolate hydrogen bonds possible, which otherwise suffer from detrimental silanolate–oxygen cation interactions. We investigated the influence of various weakly coordinating cations on silanol-silanolate motifs, particularly with regard to different cation sizes. While large cations favor the formation of intramolecular hydrogen bonds resulting in cyclic structures, the less bulky tetramethyl ammonium cation encourages the formation of polyanionic silanol-silanolate chains in the solid state.     

Boron-Made N2: Realization of a B≡B Triple Bond in the B2Al3− Cluster

Until now, all B≡B triple bonds have been achieved by adopting two ligands in the L→B≡B←L manner. Herein, we report an alternative route of designing the B≡B bonds based on the assumption that by acquiring two extra electrons, an element with the atomic number Z can have properties similar to those of the element with the atomic number Z+2. Specifically, we show that due to the electron donation from Al to B, the negatively charged B≡B kernel in the B₂Al₃⁻ cluster mimics a triple N≡N bond. Comprehensive computational searches reveal that the global minimum structure of B₂Al₃⁻ exhibits a direct B–B distance of 1.553 Å, and its calculated electron vertical detachment energies are in excellent agreement with the corresponding values of the experimental photoelectron spectrum. Chemical bonding analysis revealed one σ and two π bonds between the two B atoms, thus confirming a classical textbook B≡B triple bond, similar to that of N₂.     

α-Boryl Carbanions: The Influence of Geminal Heteroatoms in C−C Bond Formation

The wide applications of alpha-boryl carbanions in selective coupling with organohalides, imines/carbonyls and conjugated unsaturated substrates has become an interesting tool for organic synthesis. Strategically, the inclusion of heteroatoms, such as Si, S, N, F, Cl, Br and I in the alpha position opens a new venue towards multifunctionalities in molecular design. Here, a conceptual and practical view on powerful carbanions, containing α-silicoboron, α-thioboron, α-haloboron and α-aminoboron is given, as well as a prespective on their efficient application for selective electrophilic trapping.     

Electrooxidative Activation of B−B Bond in B2cat2: Access to gem-Diborylalkanes via Paired Electrolysis

This report describes the unprecedented electrooxidation of a solvent (e.g., DMF)-ligated B₂cat₂ complex, whereby a solvent-stabilized boryl radical is formed via quasi-homolytic cleavage of the B−B bond in a DMF-ligated B₂cat₂ radical cation. Cyclic voltammetry and density functional theory provide evidence to support this novel B−B bond activation strategy. Furthermore, a strategy for the electrochemical gem-diborylation of gem-bromides via paired electrolysis is developed for the first time, affording a range of versatile gem-diborylalkanes, which are widely used in synthetic society. Notably, this reaction approach is scalable, transition-metal-free, and requires no external activator.     

N−X⋅⋅⋅O−N Halogen Bonds in Complexes of N-Haloimides and Pyridine-N-oxides: A Large Data Set Study

N−X⋅⋅⋅⁻O−N⁺ halogen-bonded systems formed by 27 pyridine N-oxides (PyNOs) as halogen-bond (XB) acceptors and two N-halosuccinimides, two N-halophthalimides, and two N-halosaccharins as XB donors are studied in silico, in solution, and in the solid state. This large set of data (132 DFT optimized structures, 75 crystal structures, and 168 ¹H NMR titrations) provides a unique view to structural and bonding properties. In the computational part, a simple electrostatic model (SiElMo) for predicting XB energies using only the properties of halogen donors and oxygen acceptors is developed. The SiElMo energies are in perfect accord with energies calculated from XB complexes optimized with two high-level DFT approaches. Data from in silico bond energies and single-crystal X-ray structures correlate; however, data from solution do not. The polydentate bonding characteristic of the PyNOs’ oxygen atom in solution, as revealed by solid-state structures, is attributed to the lack of correlation between DFT/solid-state and solution data. XB strength is only slightly affected by the PyNO oxygen properties [(atomic charge (Q), ionization energy (I_s,min) and local negative minima (V_s,min)], as the σ-hole (V_s,max) of the donor halogen is the key determinant leading to the sequence N-halosaccharin>N-halosuccinimide>N-halophthalimide on the XB strength.     

The nido-Cage⋅⋅⋅π Bond: A Non-covalent Interaction between Boron Clusters and Aromatic Rings and Its Applications

Non-covalent interactions involving multicenter multielectron skeletons such as boron clusters are rare. Now, a non-covalent interaction, the nido-cage⋅⋅⋅π bond, is discovered based on the boron cluster C₂B₉H₁₂⁻ and an aromatic π system. The X-ray diffraction studies indicate that the nido-cage⋅⋅⋅π bonding presents parallel-displaced or T-shaped geometries. The contacting distance between cage and π ring varies with the type and the substituent of the aromatic ring. Theoretical calculations reveal that this nido-cage⋅⋅⋅π bond shares a similar nature to the conventional anion⋅⋅⋅π or π⋅⋅⋅π bonds found in classical aromatic ring systems. This nido-cage⋅⋅⋅π interaction induces variable photophysical properties such as aggregation-induced emission and aggregation-caused quenching in one molecule. This work offers an overall understanding towards the boron cluster-based non-covalent bond and opens a door to investigate its properties.     

The ionization of sulfurous acid in Na−Mg−Cl solutions at 25°C

The stoichiometric pK ₁ ^* and pK ₂ ^* for the ionization of sulfurous acid has been determined from emf measurements in NaCl solutions with varying concentrations of added MgCl₂ (m=0.1, 0.2 and 0.3) from I=0.5 to 6.0 molal at 25°C. These experimental results have been treated using both the ion pairing and Pitzer's specific ion-interaction models. The Pitzer parameters for the interaction of Mg²⁺ with SO₂ and HSO ₃ ^– yielded =0.085±0.004, ⁽⁰⁾ = 0.35±0.02, ⁽¹⁾ = 1.2±0.04, and C = –0.072±0.007. The Pitzer parameters ⁽⁰) = –2.8±0.4, ⁽¹⁾ = 12.9±2.9 and ⁽²⁾ = –2071±57 have been determined for the interactions of Mg²⁺ with SO ₃ ^2– . The calculated values of pK ₁ ^* and pK ₂ ^* using Pitzer's equations reproduce the measured values to within ±0.04 pK units. The ion pairing model with log K_MgSO3=2.36±0.02 and log_MgSO3 = 0.1021, reproduces the experimental values of pK ₂ ^* to ±0.01. These results demonstrate that treating the data by considering the formation of MgSO₃ yields a better fit of the experimental measurements with fewer adjustable parameters. With these derived coefficients obtained from the Pitzer equations and the ion pairing model, it is possible to make reliable estimates of the activity coefficients of HSO ₃ ^– and SO ₃ ^2– in seawater, brines and marine aerosols containing Mg²⁺ ions.     

Primary Sulfonamide Functionalization via Sulfonyl Pyrroles: Seeing the N−Ts Bond in a Different Light

Despite common occurrence in molecules of value, methods for transforming sulfonamides are distinctly lacking. Here we introduce easy-to-access sulfonyl pyrroles as synthetic linchpins for sulfonamide functionalization. The versatility of the sulfonyl pyrrole unit is shown by generating a variety of products through chemical, electrochemical and photochemical pathways. Preliminary results on the direct functionalization of primary sulfonamides are also provided, which may lead to new modes of activation.     

Solid State Chemistry of A4B3−Molecules

Abstract

The system P₄S₃?P₄Se₃?As₄S₃?As₄Se₃ was investigated by thermal and X-ray methods. Five regions of solid solubility with different crystal structures were found. All transform at higher temperatures into the plastically-crystalline state with β-P₄S₃?structure.

The substituted species P_4-nAs_nS_mSe_3-m (n = 0–4, m = 0–3) are formed in molten mixtures of A₄B₃?molecules (FIGURE 1). They were identified by HPLC and mass-spectrometric measurements.

After long equilibration times P₄Se₃, As₄S₃ and As₄Se₃ decompose peritectoidally into the resp. A₄B₄?species and an amorphous product.     

Proving and Probing the Presence of the Elusive C−H⋅⋅⋅O Hydrogen Bond in Liquid Solutions at Room Temperature

Hydrogen bonds (H bonds) play a major role in defining the structure and properties of many substances, as well as phenomena and processes. Traditional H bonds are ubiquitous in nature, yet the demonstration of weak H bonds that occur between a highly polarized C−H group and an electron-rich oxygen atom, has proven elusive. Detailed here are linear and nonlinear IR spectroscopy experiments that reveal the presence of H bonds between the chloroform C−H group and an amide carbonyl oxygen atom in solution at room temperature. Evidence is provided for an amide solvation shell featuring two clearly distinguishable chloroform arrangements that undergo chemical exchange with a time scale of about 2 ps. Furthermore, the enthalpy of breaking the hydrogen bond is found to be 6–20 kJ mol⁻¹. Ab-initio computations support the findings of two distinct solvation shells formed by three chloroform molecules, where one thermally undergoes hydrogen-bond making and breaking.     

Comment on “Realization of Lewis Basic Sodium Anion in the NaBH3− Cluster”

We challenge the interpretation of the chemical bond in NaBH₃⁻ proposed by Liu et al. We argue that NaBH₃⁻ has an electron-sharing Na−BH₃⁻ covalent bond rather than a dative bond Na⁻→BH₃.     

Incommensurate Phase in Λ-cobalt (III) Sepulchrate Trinitrate Governed by Highly Competitive N−H⋅⋅⋅O and C−H⋅⋅⋅O Hydrogen Bond Networks**

Phase transitions in molecular crystals are often determined by intermolecular interactions. The cage complex of [Co(C₁₂H₃₀N₈)]³⁺ ⋅ 3 NO₃⁻ is reported to undergo a disorder-order phase transition at T_c1 ≈133 K upon cooling. Temperature-dependent neutron and synchrotron diffraction experiments revealed satellite reflections in addition to main reflections in the diffraction patterns below T_c1. The modulation wave vector varies as function of temperature and locks in at T_c3≈98 K. Here, we demonstrate that the crystal symmetry lowers from hexagonal to monoclinic in the incommensurately modulated phases in T_c1<T<T_c3. Distinctive levels of competitions: trade-off between longer N−H⋅⋅⋅O and shorter C−H⋅⋅⋅O hydrogen bonds; steric constraints to dense C−H⋅⋅⋅O bonds give rise to pronounced modulation of the basic structure. Severely frustrated crystal packing in the incommensurate phase is precursor to optimal balance of intermolecular interactions in the lock-in phase.     

Reassessment of the Mechanisms of Thermal C−H Bond Activation of Methane by Cationic Magnesium Oxides: A Critical Evaluation of the Suitability of Different Density Functionals

The mechanisms of the thermal reactions of the two iconic magnesium oxide cations MgO^.+ and Mg₂O₂^.+ with methane have been re-evaluated at the CCSD(T)/CBS//CCSD/def2-TZVP level of theory. For the reaction of MgO^.+ with CH₄, only the classical hydrogen-atom transfer (HAT) was found; in contrast, for the Mg₂O₂^.+/CH₄ couple, both HAT and proton-coupled electron-transfer (PCET) exist as mechanistic variants. In order to evaluate the suitability of density functional theory (DFT) methods, the reactions were computed by using 27 density functionals. The results obtained demonstrate that the various DFT methods often deliver rather different results for both geometric and energetic features. As to the prediction of the apparent barriers, pure functionals give the largest mean absolute errors. BMK, ωB97XD, and the double-hybrid functional mPW2PLYP were confirmed to come closest to the results provided by CCSD(T)/CBS. Thus, mechanistic conclusions based on a single DFT method should be viewed with great caution. In summary, this study may assist in the selection of a suitable quantum chemical method to unravel the mechanistic details of C−H bond activation by charged metal oxides.     

Benzyl Borane NHC Adducts: Beyond B−C Bond Scission

Benzyl-substituted boronates and borates are widely employed as mild sources in radical or anionic transfer reactions of benzyl entities. In this process the B−C bond to the benzyl moiety is essentially ruptured. In contrast, reactions with retention of the B−C bond are poorly investigated although several other reactive sites in benzyl–boron systems are clearly inherent. In this respect, the novel reactivity of the representative borane adduct IiPr−BH₂Bn [IiPr=:C{N(iPr)CH}₂, Bn=CH₂C₆H₅] is demonstrated. Dihalogenation of the BH₂ entity is observed with BCl₃ and BBr₃, whereas BI₃ either affords IiPr−BHI₂ or proceeds with borylation of the aromatic phenyl ring to give a hydride-bridged bisborylated species. The photochemical mono- and dihalogenation of the benzylic CH₂ group was demonstrated with elemental bromine Br₂. The brominated product IiPr−BBr₂−CHBr−C₆H₅ was borylated at the benzylic carbon atom in an umpolung event with BI₃ to afford the zwitterion IiPr−BI−CH(BI₃)−C₆H₅.     

B−B Bond Nucleophilicity in a Tetraaryl μ-Hydridodiborane(4) Anion

The tetraaryl μ-hydridodiborane(4) anion [ 2 H]⁻ possesses nucleophilic B−B and B−H bonds. Treatment of K[ 2 H] with the electrophilic 9-H-9-borafluorene (HBFlu) furnishes the B₃ cluster K[ 3 ], with a triangular boron core linked through two BHB two-electron, three-center bonds and one electron-precise B−B bond, reminiscent of the prominent [B₃H₈]⁻ anion. Upon heating or prolonged stirring at room temperature, K[ 3 ] rearranges to a slightly more stable isomer K[ 3 a ]. The reaction of M[ 2 H] (M⁺=Li⁺, K⁺) with MeI or Me₃SiCl leads to equimolar amounts of 9-R-9-borafluorene and HBFlu (R=Me or Me₃Si). Thus, [ 2 H]⁻ behaves as a masked [:BFlu]⁻ nucleophile. The HBFlu by-product was used in situ to establish a tandem substitution-hydroboration reaction: a 1:1 mixture of M[ 2 H] and allyl bromide gave the 1,3-propylene-linked ditopic 9-borafluorene 5 as sole product. M[ 2 H] also participates in unprecedented [4+1] cycloadditions with dienes to furnish dialkyl diaryl spiroborates, M[R₂BFlu].     

Retracted: The Effect of Air on the Oxidation Decomposition of Cationic Exchange Resins in a Ternary Li2CO3−Na2CO3−K2CO3 Molten-Salt System

The molten-salt oxidation method (MSO) can be applied for disposal of spent cationic exchange resins (CERs) after the treatment of nuclear industry wastewater. In this work, the oxidation decomposition of resins in carbonate molten salt in N₂ and air atmospheres was investigated. The SEM morphology and FTIR spectrograms indicated that the addition of air obviously prompted the oxidation decomposition of the benzene ring, S−O bond and S−C bond in residues and the decomposition efficiency of resins reached 98.69 % at 800 °C. The XPS analysis showed the conversion of sulfur species in residues. The peroxide and superoxide ions in carbonate molten salt prompted the decomposition of thiophene sulfur and resulted in the formation of sulfate. The retention rate of sulfur in spent salt was 84.36 % at 800 °C. This work provided more theoretical guidance for the treatment of resins and technical support for the sustainable development of nuclear industry.     

An Experimental Exploration of C−H⋅⋅⋅X Hydrogen Bond in [CHCl3−X(CH3)2] Complexes (X=O,S, and Se)

Among the conglomeration of hydrogen bond donors, the C−H group is prevalent in chemistry and biology. In the present work, CHCl₃ has been selected as the hydrogen bond donor and are X(CH₃)₂ are the hydrogen bond acceptors. Formation of C−H⋅⋅⋅X hydrogen bond under the matrix isolation condition is confirmed by the observation of red-shift in the C−H stretching frequency of CHCl₃ and comparison with the simulated spectra. Stabilisation energy of all the three complexes is almost equal although the observed red-shift for the C−H⋅⋅⋅O complex is less compared to the C−H⋅⋅⋅S/Se complexes. The nature and origin of the hydrogen bond have been delineated using Natural Bond Orbital, Atoms in Molecules, Non-Covalent Interaction analyses, and Energy Decomposition Analysis. Charge transfer is found to be proportional to the observed red-shift. This work provides the first impression of C−H⋅⋅⋅Se hydrogen bond and its comparison with C−H⋅⋅⋅O/S hydrogen bond interaction under experimental condition.     

The Influence of Secondary Interactions on the [N−I−N]+ Halogen Bond

[Bis(pyridine)iodine(I)]⁺ complexes offer controlled access to halonium ions under mild conditions. The reactivity of such stabilized halonium ions is primarily determined by their three-center, four-electron [N−I−N]⁺ halogen bond. We studied the importance of chelation, strain, steric hindrance and electrostatic interaction for the structure and reactivity of halogen bonded halonium ions by acquiring their ¹⁵N NMR coordination shifts and measuring their iodenium release rates, and interpreted the data with the support of DFT computations. A bidentate ligand stabilizes the [N−I−N]⁺ halogen bond, decreasing the halenium transfer rate. Strain weakens the bond and accordingly increases the release rate. Remote modifications in the backbone do not influence the stability as long as the effect is entirely steric. Incorporating an electron-rich moiety close by the [N−I−N]⁺ motif increases the iodenium release rate. The analysis of the iodine(I) transfer mechanism highlights the impact of secondary interactions, and may provide a handle on the induction of stereoselectivity in electrophilic halogenations.     

The Importance of 1,5-Oxygen⋅⋅⋅Chalcogen Interactions in Enantioselective Isochalcogenourea Catalysis

The importance of 1,5-O⋅⋅⋅chalcogen (Ch) interactions in isochalcogenourea catalysis (Ch=O, S, Se) is investigated. Conformational analyses of N-acyl isochalcogenouronium species and comparison with kinetic data demonstrate the significance of 1,5-O⋅⋅⋅Ch interactions in enantioselective catalysis. Importantly, the selenium analogue demonstrates enhanced rate and selectivity profiles across a range of reaction processes including nitronate conjugate addition and formal [4+2] cycloadditions. A gram-scale synthesis of the most active selenium analogue was developed using a previously unreported seleno-Hugerschoff reaction, allowing the challenging kinetic resolutions of tertiary alcohols to be performed at 500 ppm catalyst loading. Density functional theory (DFT) and natural bond orbital (NBO) calculations support the role of orbital delocalization (occurring by intramolecular chalcogen bonding) in determining the conformation, equilibrium population, and reactivity of N-acylated intermediates.     

Anion⋅⋅⋅Anion Attraction in Complexes of MCl3− (M=Zn,Cd, Hg) with CN−

High-level ab initio calculations show that the MCl₃⁻ anions comprising Group 2B M atoms Zn, Cd, and Hg form a stable complex with the CN⁻ anion, despite the like charge of the two ions. The complexation occurs despite a negative π-hole region above the M atom of MCl₃⁻. The dimerization distorts the planar geometry of MCl₃⁻ into a pyramidal shape which reduces the negative potential above the M atom, facilitating a close approach of the two anions, with R(M⋅⋅⋅C)∼2 Å, and an overall attractive electrostatic attraction within the dimer. In the gas phase, this dimer is less stable than the pair of separated ions by some 30 kcal/mol. However, the dissociation must surmount an energy barrier of roughly 25 kcal/mol which occurs at an intermolecular distance of 4 Å. In aqueous solution, the dimerization process is exothermic and barrier-free, with a binding energy in the 11–18 kcal/mol range.