CO-Induced Dinitrogen Fixation and Cleavage Mediated by Boron

Screening seven-electron boron-centered radicals for dinitrogen activation

The activation of dinitrogen is significant as nitrogen-containing compounds play an important role in industries. However, the inert NN triple bond caused by its large HOMO-LUMO gap (10.8 eV) and high bond dissociation energy (945 kJ mol-1 ) renders its activation under mild conditions particularly challenging. Recent progress shows that a few main group species can mimic transition metal complexes to activate dinitrogen. Here, we demonstrate that a series of seven-electron (7e) boron-centered radical can be used to activate N2 via density functional theory calculations. It is found that boron-centered radicals containing amine ligand perform best on the thermodynamics of dinitrogen activation. In addition, when electron-donating groups are introduced at the boron atom, these radicals can be used to activate N2 with low reaction barriers. Further analysis suggests that the electron transfer from the boron atom to the π* orbitals of dinitrogen is essential for its activation. Our findings suggest great potential of 7e boron radicals in the field of dinitrogen activation.

Binding Properties of Small Electrophilic Anions [B6 X5 ]- and [B10 X9 ]- (X=Cl, Br, I): Activation of Small Molecules Based on π-Backbonding

Superelectrophilic anions constitute a special class of molecular anions that show strong binding of weak nucleophiles despite their negative charge. In this study, the binding characteristics of smaller gaseous electrophilic anions of the types [B6 X5 ]- and [B10 X9 ]- (with X=Cl, Br, I) were computationally and experimentally investigated and compared to those of the larger analogues [B12 X11 ]- . The positive charge of vacant boron increases from [B6 X5 ]- via [B10 X9 ]- to [B12 X11 ]- , as evidenced by increasing attachment enthalpies towards typical σ-donor molecules (noble gases, H2 O). However, this behavior is reversed for σ-donor-π-acceptor molecules. [B6 Cl5 ]- binds most strongly to N2 and CO, even more strongly than to H2 O. Energy decomposition analysis confirms that the orbital interaction is responsible for this opposite trend. The extended transition state natural orbitals for chemical valence method shows that the π-backdonation order is [B6 X5 ]- >[B10 X9 ]- >[B12 X11 ]- . This predicted order explains the experimentally observed red shifts of the CO and N2 stretching fundamentals compared to those of the unbound molecules, as measured by infrared photodissociation spectroscopy. The strongest red shift is observed for [B6 Cl5 N2 ]- : 222 cm-1 . Therefore, strong activation of unreactive σ-donor-π-acceptor molecules (commonly observed for cationic transition metal complexes) is achieved with metal-free molecular anions.

Achieving Control over the Reduction/Coupling Dichotomy of N2 by Boron Metallomimetics

We report a detailed computational and experimental study of the fixation and reductive coupling of dinitrogen with low-valent boron compounds. Consistent with our mechanistic findings, the selectivity toward nitrogen fixation or coupling can be controlled through either steric bulk or the reaction conditions, allowing for the on-demand synthesis of nitrogen chains. The electronic structure and intriguing magnetic properties of intermediates and products of the reaction of dinitrogen with borylenes are also elucidated using high-level computational approaches.

Predicting dinitrogen activation by borenium and borinium cations

The activation of thermodynamically stable and kinetically inert dinitrogen (N₂) has been a great challenge due to the significant strength of the triple bond. Recently, in an experimental study on N₂ activation by boron species, a highly reactive two-coordinated borylene broke through the limitations of traditional strategies of N₂ activation by metal species. Still, studies on metal-free N₂ activation remain underdeveloped. Here, we systematically investigate a frustrated Lewis pair (FLP) combining carbene and borenium (or borinium) cations to screen potential candidates for N₂ activation via density functional theory calculations. As a result, we found that two FLPs (closed form FLP, borenium and open form FLP, borinium) are able to activate N₂ in a thermodynamically and kinetically favorable manner, with a low energy barrier of 9.6 and 7.3 kcal mol^-1, respectively. Furthermore, aromaticity was found to play an important role in the stabilization of the products, supported by nucleus-independent chemical shift (NICS), anisotropy of the current-induced density (ACID) and electron density of delocalized bonds (EDDB) analysis. Our findings provide an alternative approach for metal-free N₂ activation, highlighting the importance of FLP chemistry and aromaticity in N₂ activation.

Recent Progress in Dinitrogen Activation by Gas-Phase Metal Species

Understanding the mechanisms to activate and functionalize dinitrogen (N₂) is of great importance for the rational design of nitrogen-fixation catalysts. Reactions of gas-phase species with N₂ are being actively studied to understand the bond activation and formation processes at a strictly molecular level. This Perspective provides an overview of the recent progress in combined experimental and theoretical studies on the activation and functionalization of N₂ by gas-phase metal species. New mechanistic insights into N₂ molecular adsorption, N≡N cleavage, and N-X (X = C, B, and H) formation have been introduced, in which the new reaction channels of ejecting neutral metal fragments and the coupling reactions of N₂ with other molecules are highlighted. Finally, the current challenges and outlooks of N₂ activation in the gas phase are discussed as well.

Nitrogen fixation and transformation with main group elements

Nitrogen fixation is essential for the maintenance of life and development of society, however, the large bond dissociation energy and nonpolarity of the triple bond constitute a considerable challenge. The transition metals, by virtue of their combination of empty and occupied d orbitals, are prevalent in the nitrogen fixation studies and are continuing to receive a significant focus. The main group metals have always been considered incapable in dinitrogen activation owing to the absence of energetically and symmetrically accessible orbitals. The past decades have witnessed significant breakthroughs in the dinitrogen activation with the main group elements and compounds via either matrix isolation, theoretical calculations or synthetic chemistry. The successful reactions of the low-valent species of the main group elements with inert dinitrogen have been reported via the π back-donation from either the d orbitals (Ca, Sr, Ba) or p orbitals (Be, B, C…). Herein, the significant achievements have been briefly summarized, along with predicting the future developments.

Spectroscopic Identification of the Heterocumulenic Isocyanatoborane Radical HBNCO

The highly elusive isocyanatoborane radical HBNCO has been generated by the reaction of laser-ablated boron atoms with HNCO and also by the light-induced chemical transformation of the hydrogen-bonded molecule-radical complex BNH···CO in solid neon matrix. IR spectroscopic and theoretical studies indicate that the HBNCO radical possesses a quasilinear B═N═C═O heterocumulenic structure with the unpaired electron mainly located at the boron atom. This is in sharp contrast to the bonding properties of the isoelectronic analogues HCCCO and NCCO, in which the unpaired electron is located at the terminal CO moiety.

Predicting Dinitrogen Activation by Five-Electron Boron-Centered Radicals

Due to the high bond dissociation energy (945 kJ mol^-1) and the large highest occupied molecular orbital-lowest unoccupied molecular orbital (HOMO-LUMO) gap (10.8 eV), dinitrogen activation under mild conditions is extremely challenging. On the other hand, the conventional Haber-Bosch ammonia synthesis under harsh conditions consumes more than 1% of the world's annual energy supply. Thus, it is important and urgent to develop an alternative approach for dinitrogen activation under mild conditions. In comparison with transition metals, main group compounds are less explored for nitrogen activation. Here, we carry out density functional theory calculation to screen boron radicals for dinitrogen activation. As a result, the experimentally available seven-electron boron-centered radicals are found to be inactive to N₂ activation, whereas some five-electron boron-centered radicals become favorable for dinitrogen activation, inviting experimental chemists' examination. The principal interacting spin-orbital analyses suggest that a five-electron boron-centered radical can mimic a transition metal on a synergic interaction with dinitrogen in the transition states.

Infrared Spectroscopy and Bonding of the B(NN)3+ and B2(NN)3,4+ Cation Complexes

The boron-dinitrogen cation complexes B(NN)₃⁺ and B₂(NN)_3,4⁺ are produced in the gas phase and are studied by infrared photodissociation spectroscopy in the N-N stretching vibrational frequency region. The geometric and electronic structures are determined by comparison of the experimental spectra with density functional theory calculations. The B(NN)₃⁺ cation is characterized to have a closed-shell singlet ground state with planar D_3h symmetry. The B₂(NN)₃⁺ cation is determined to have a B═B bonded (NN)₂BBNN structure with C_2v symmetry. Two isomers of the B₂(NN)₄⁺ cation contribute to the experimental spectrum. One is a N₂-tagged complex involving a B₂(NN)₃⁺ core ion. Another one is a B-B bonded B₂(NN)₄⁺ complex with a planar D_2h structure. Bonding analyses reveal that the B-NN interactions in these complexes come mainly from covalent orbital interactions, with the NN → B σ donation being stronger than the B → NN π back-donation.

Cleavage of the N≡N Triple Bond and Unpredicted Formation of the Cyclic 1,3-Diaza-2,4-Diborete (FB)2 N2 from N2 and Fluoroborylene BF

A complete cleavage of the triple bond of N₂ by fluoroborylene (:BF) was achieved in a low-temperature N₂ matrix by the formation of the four-membered heterocycle FB(μ-N)₂ BF, which lacks a trans-annular N-N bond. Additionally, the linear complex FB=N-N=BF and cyclic FB(η² -N₂ ) were formed. These novel species were characterized by their matrix infrared spectra and quantum-chemical calculations. The puckered four-membered-ring B₂ N₂ complex shows a delocalized aromatic two-electron π-system in conjugation with the exo-cyclic fluorine π lone pairs. This work may contribute to a rational design of catalysts based on borylene for artificial dinitrogen activation.

CO-Induced Dinitrogen Fixation and Cleavage Mediated by Boron

The boron atoms react with carbon monoxide and dinitrogen forming the end-on bonded NNBCO complex in solid neon or in nitrogen matrices. The NNBCO complex rearranges to the (η2 -N2 )BCO isomer with a more activated side-on bonded dinitrogen ligand upon visible light excitation. (η2 -N2 )BCO and its weakly CO-coordinated complexes further isomerize to the NBNCO and B(NCO)2 molecules with N-N bond being completely cleaved under UV light irradiation. The geometries, energies and vibrational spectra of the molecules are calculated with quantum chemical methods and the electronic structures are analyzed with charge- and energy-partitioning methods.

Intriguing structural, bonding and reactivity features in some beryllium containing complexes

We highlighted our contributions to Be chemistry which include bond-stretch isomerism in Be₃²⁻ species, Be complexes bound with noble gas, CO, and N₂, Be based nanorotors, and intriguing bonding situations in some Be complexes.