Unravelling the underlying mechanism of the reduction of aldehydes/ketones with metal borohydride in an aprotic solvent.

The reduction mechanism of aldehyde/ketones with M(BH4)n is not fully understood, even though the hydroboration mechanism of weak Lewis base borane complexes is known to involve a four-membered ring transition state. Herein, the reduction mechanism of M(BH4)n in aprotic solvents has been elucidated for a six-membered ring, in which hydride transfer to the C atom from the B atom, formation of an L·BH3 adduct, and disproportionation of (BH3(OR)-) borane are involved. The metal cations and solvents participate in and significantly influence the reaction procedure. We believe that this mechanistic study would provide a further reference for the application of M(BH4)n in organic reactions.

Potassium Decahydrido-closo-Decaborane Urea Complex as a Potential Solid-State Electrolyte for Potassium Metal Batteries.

All-solid-state potassium metal batteries have been considered promising candidates for large-scale energy storage because of abundance and wide availability of K resources, elimination of flammable liquid organic electrolytes, and incorporation of high-capacity K metal anode. However, unideal K-ion conductivities of most reported K-ion solid electrolytes have restricted the development of these batteries. Herein, a novel K2B10H10·CO(NH2)2 complex is reported, forming by incorporating urea into K2B10H10, to achieve an enhanced K-ion conductivity. The crystal structure of K2B10H10·CO(NH2)2 was determined as a monoclinic lattice with the space group of C2/c (No. 15). K2B10H10·CO(NH2)2 delivers an ionic conductivity of 2.7 × 10-8 S cm-1 at 25 °C, and reaching 1.3 × 10-4 S cm-1 at 80 °C, which is about 4 orders of magnitude higher than that of K2B10H10. One possible reason is the anion expansion in size due to the presence of dihydrogen bonds in K2B10H10·CO(NH2)2, resulting in an increase in the K-H bond distance and the electrostatic potential, thereby enhancing the mobility of K+. The K-ion conductivity is also higher than those of most hydridoborate-based K-ion conductors reported. Besides, K2B10H10·CO(NH2)2 reveals a wide electrochemical stability window and remarkable interface compatibility with K metal electrodes, suggesting a promising electrolyte for all-solid-state K metal batteries.

Silylium ion-mediated cage-opening functionalization of closo-B10H102- salts.

The cage-opening functionalization of stable closo-B10H102- salts is a great way to get various boron clusters. However, the known methods to mediate cage-opening functionalization rely on the use of strong acids, which suffer from low efficiency and narrow substrate scope. Herein, an efficient method to synthesize 6-substituted decaboranyl ethers and sulfides has been developed. The reaction was mediated by trimethylsilyl trifluoromethanesulfonate (TMSOTf) and occurred at room temperature. Six 6-substituted ethers were obtained in 65-92% yields and five 6-substituted sulfides were prepared in 38-58% yields. The reaction had excellent regioselectivity, affording the single B(6) regioisomer in all cases. The interaction between the B-H bonds of the boron cage and the silylium ion was believed to be the key factor in the reaction.

Synthesis, Formation Mechanism, and Structure of K[BH3S(CH3)BH3] and Its Application in Preparation of KB3H8.

The design, synthesis, and applications of new boranes are eternal topics in boron chemistry. A new bis(borane)alkanethiolate salt, K[BH3S(CH3)BH3], was synthesized in high yield by the reaction of K with (CH3)2S·BH3 at room temperature. The formation mechanism was elucidated based on experimental and theoretical studies. The single-crystal structure of the K[BH3S(CH3)BH3]·18-crown-6 adduct was determined in which the B-S-B bonding information of K[BH3S(CH3)BH3] was illustrated for the first time. Using K[BH3S(CH3)BH3] as a starting material, KB3H8 was successfully synthesized.

Coordination mode and stability of the tetrahydroborate ligand in group 10 metal pincer complexes.

The coordination mode of the BH4- ligand in transition metal tetrahydroborate complexes is mainly dominated by the nature of the metal centres. However, other factors can also play important roles sometimes. In order to rationalize the coordination modes and the stability of the BH4- ligand in group 10 metal tetrahydroborate pincer complexes, [2,6-(tBu2PO)2C6H3]Pt(η1-HBH3) and [C6H4-o-(NCH2PtBu2)2B]M(η2-H2BH2) (M = Ni, Pt) were prepared and characterized. A structural comparison of [2,6-(tBu2PCH2)2C6H3]Ni(BH4), [2,6-(tBu2PO)2C6H3]M(BH4) and [C6H4-o-(NCH2PtBu2)2B]M(BH4) (M = Ni, Pd, and Pt) indicates that the M-P bond length, the P-M-P bite angle and the trans-influence of the central atom in the pincer platform also affect the coordination mode of the BH4- ligand. The nickel complexes tend to adopt a monodentate coordination mode while the palladium and platinum complexes can adopt either the monodentate or the bidentate mode depending on the structural features of the pincer platforms. Longer M-P bonds and smaller P-M-P bite angles favour the bidentate mode. The stability of the BH4- ligand is influenced by both the coordination mode and the nature of the metal centre. The BH3 species is released more easily from complexes with less electron rich metal centres. Following the series of Ni, Pd, and Pt, complexes with the same pincer ligand more easily lose a BH3 moiety.

Synthesis of K[B3H7NH2BH2NH2B3H7] for a K-ion solid-state electrolyte.

All-solid-state K batteries are ideal energy storage devices for grid-scale applications of renewable energies. A novel electrolyte K[B3H7NH2BH2NH2B3H7] with weakly coordinating anions was synthesized. It has a high K+ conductivity of 1.01 × 10-4 S cm-1 at 75 °C, which is probably due to the increased electrostatic potential and size of the anions.

Which Type of Pincer Complex Is Thermodynamically More Stable? Understanding the Structures and Relative Bond Strengths of Group 10 Metal Complexes Supported by Benzene-Based PYCYP Pincer Ligands.

The influence of the pincer platform composition and substitution on the reactivity and physical properties of pincer complexes can be easily explored through different experimental techniques. However, the influence of these factors on the molecular structures and thermodynamic stability of pincer complexes is usually very subtle and cannot always be unambiguously established. To rationalize this subtle influence, a survey of crystallographic data from 130 group 10 metal pincer complexes supported by benzene-based PYCYP pincer ligands, [2,6-(R2PY)2C6H3-nR'n]MX (Y = CH2, NH, O, S; M = Ni, Pd, Pt; R = tBu, iPr, Ph, Cy, Me; R' = CO2Me, tBu, CF3, Ac; n = 0-2; X = F, Cl, Br, I, H, SH, SPh, SBn, Ph, Me, N3, NCS), was carried out. Theoretical calculations for some selected complexes were performed to evaluate the relative bond strength. It was found that the M-Cipso bond length decreases following the linker series of CH2 > NH > O and that the relative M-Cipso bond strength increases following the linker series of CH2 < NH < O. In most cases, the M-P bond length decreases following the linker series of NH > CH2 > O. The relative M-P bond strength increases following the linker series of CH2 < NH < O. A comparison of the thermochemical balance for the isodesmic displacement of the side-arm interactions with PH3 as a probe ligand indicated that the Ni-P bond in a PCCCP-type pincer complex is far less difficult to break compared with that in a POCOP-type complex. As a result, with the same donor substituents and the same auxiliary ligand, the POCOP-type pincer complexes are thermodynamically more stable than the PCCCP complexes. The influence of other backbone and donor substitutions as well as the pincer platform composition on the M-Cipso, M-P, and M-X bond lengths, relative bond strengths, and P-M-P bite angles was also discussed.

Improved Methods for the Synthesis of KB3H8, NH3B3H7, and N-Alkyl Analogues of NH3B3H7.

Improved methods for the synthesis of KB3H8, NH3B3H7, and N-alkyl analogues of NH3B3H7 have been developed based on previous works. KB3H8 was synthesized by the reaction of metallic potassium (K) with borane dimethyl sulfide ((CH3)2S·BH3) with high yield and atom-economy. In the preparation of NH3B3H7 and its N-alkyl analogues, KB3H8 served as a starting material and was converted to THF·B3H7 first through reactions with HCl diethyl ether solution or oxidation agent CoCl2. Then, the formed THF·B3H7 in situ reacted with the corresponding ammonia or amines to form the amine borane final products. This work paves an alternative way for preparing organic-inorganic hybrid materials containing B, N, and C atoms.

Efficient synthesis of primary and secondary amides via reacting esters with alkali metal amidoboranes.

Amides are one of the most important organic compounds that are widely applied in medicine, biochemistry, and materials science. To find an efficient synthetic method of amides is a challenge for organic chemistry. We report here a facile synthesis method of primary and secondary amides through a direct amidation of esters with sodium amidoboranes (NaNHRBH3, R = H, Me), at room temperature without using catalysts and other reagents. This process is rapid and chemoselective, and features quantitative conversion and wide applicability for esters tolerating different functional groups. The experimental and theoretical studies reveal a reaction mechanism with nucleophilic addition followed by a swift proton transfer-induced elimination reaction.

Aggregation-Induced Fluorescence of Carbazole and o-Carborane Based Organic Fluorophore.

Carbazole based fluorophores 9-butyl-3,6-bis-(phenylethynyl)-9H-carbazole (1) and 9-butyl-3,6-bis-(2-phenyl-o-carborane)-9H-carbazole (2) were synthesized via Sonogashira type cross-coupling reaction and followed by insertion with decaborane. Compound 1 exhibited far more intense fluorescence than 2 in THF solution, while in solid state 2 exerted stronger fluorescence than 1. The fluorescence quenching behavior of 2 in THF solution could be attributed to the intramolecular charge transfer of donor-acceptor system in 2, which was confirmed by electrochemical experiments and DFT calculations. The fluorescence enhancement of 2 in solid state can be ascribed to aggregational induced packing which was evidenced by crystallographic study. In addition, compound 2 showed typical aggregation induced emission (AIE) behavior.