Trapping of a Terminal Intermediate in the Boron-Mediated Dinitrogen Reduction: Mono-, Tri-, and Tetrafunctionalized Hydrazines in Two Steps from N2.

The addition of chlorotrimethylsilane to a boron-mediated, transition-metal-free N2 activation reaction leads to the isolation of multiple potassium boryl(silyl)hydrazido species, likely trapping products of a terminal dinitrogen complex of boron. One of these silylated N2 species can be protonated or methylated, providing access to mono- to tetrafunctionalized hydrazines in two steps from N2 and in the absence of transition metals.

All-Inorganic sp-Chain Ligands: Isoelectronic B/N Analogues of E. O. Fischer's Alkynylcarbynes.

Borylation of a tungsten-bound N2 ligand and halide abstraction provides access to a cationic complex with an unprecedented linear NNBR ligand. This complex undergoes [3+2] cycloaddition with azides, and an unexpected chain-extension reaction with an iminoborane, leading to a complex with a five-atom B/N chain. These two [NNBR]-containing complexes, inorganic analogues of E. O. Fischer's alkynylcarbynes, are very rare examples of molecules containing all-inorganic chains of sp-hybridized atoms.

Synthesis and Reactivity of a Dialane-Bridged Diradical.

Radicals of the lightest group 13 element, boron, are well established and observed in numerous forms. In contrast to boron, radical chemistry involving the heavier group 13 elements (aluminum, gallium, indium, and thallium) remains largely underexplored, primarily attributed to the formidable synthetic challenges associated with these elements. Herein, we report the synthesis and isolation of planar and twisted conformers of a doubly CAAC (cyclic alkyl(amino)carbene)-radical-substituted dialane. Extensive characterization through spectroscopic analyses and X-ray crystallography confirms their identity, while quantum chemical calculations support their open-shell nature and provide further insights into their electronic structures. The dialane-connected diradicals exhibit high susceptibility to oxidation, as evidenced by electrochemical measurements and reactions with o-chloranil and a variety of organic azides. This study opens a previously uncharted class of dialuminum systems to study, broadening the scope of diradical chemistry and its potential applications.

Bypassing Ammonia: From N2 to Nitrogen Heterocycles without N1 Intermediates or Transition Metals.

Diboradiazene compounds, derived in one step from the boron-mediated reduction of dinitrogen (N2), were treated separately with sulfur and acetic anhydride, providing heterocyclic compounds that are BN isosteres of thiophene and 1,3-oxazole, respectively. These simple reactions represent the final steps in two-step routes to complex heterocycles from N2 that both circumvent the need for transition metal reagents and completely bypass the traditional intermediate ammonia.

Structure and bonding of proximity-enforced main-group dimers stabilized by a rigid naphthyridine diimine ligand.

The development of ligands capable of effectively stabilizing highly reactive main-group species has led to the experimental realization of a variety of systems with fascinating properties. In this work, we computationally investigate the electronic, structural, energetic, and bonding features of proximity-enforced group 13-15 homodimers stabilized by a rigid expanded pincer ligand based on the 1,8-naphthyridine (napy) core. We show that the redox-active naphthyridine diimine (NDI) ligand enables a wide variety of structural motifs and element-element interaction modes, the latter ranging from isolated, element-centered lone pairs (e.g., E = Si, Ge) to cases where through-space π bonds (E = Pb), element-element multiple bonds (E = P, As) and biradical ground states (E = N) are observed. Our results hint at the feasibility of NDI-E2 species as viable synthetic targets, highlighting the versatility and potential applications of napy-based ligands in main-group chemistry.

Thienyl-Substituted Diboranes(4): Electronic Stabilization of Radicals Versus Increased Reactivity towards Bond Activation.

Low-valent main group chemistry involves a balancing act between steric and electronic stabilization of the electron-rich low oxidation state main group centers and their desired reactivity. Herein we show that the combination of sterically shielding mesityl and rotationally flexible 2-thienyl groups, the latter having the potential to be either electronically stabilizing or activating, at a diborane(4) provides a platform for both radical anion stabilization and unusual bond activation and rearrangement reactions. The addition of a Lewis base to a 1,2-dimesityl-1,2-dithienyldiborane(4) (1) results in direct and unprecedented C-H borylation of one thienyl substituent with cleavage of the B-B bond. The facile one-electron reduction of 1 yields a stable diboron radical anion through delocalization of its unpaired electron over the entire planar 1,2-dithienyldiboron framework, as evidenced by EPR spectroscopy and DFT calculations. The two-electron reduction of 1 with magnesium anthracene under more forcing conditions results in B-B bond cleavage and replacement of one thienyl sulfur atom by a mesitylboron moiety, leading to the formation of a magnesium complex of an η5 -diborafulvene dianion. Salt metathesis of the latter with [(η6 -p-cymene)RuCl2 ] affords a mixed ruthenium sandwich complex of an η5 -borylborole dianion. Calculations highlight both the structural and electronic changes in the boron-substituted heterocyclic C4 B dianion upon switching coordination from magnesium (diborafulvene dianion) to ruthenium (borylborole dianion).

Electron-Precise Dicationic Tetraboranes: Syntheses, Structures and Rearrangement to an Alkylidene Borate-Borenium Zwitterion and a 1,3-Azaborinine.

Carbene-stabilized symmetrical and unsymmetrical dicationic tetraboranes, featuring an electron-precise tetraborane chain, were synthesized and fully characterized. Reactions of these tetraboranes with reductants/bases give rise to different outcomes according to the conditions employed, including: 1) reduction and rearrangement of the tetraborane chain to give a zwitterionic alkylidene borate-borenium species; 2) cleavage of the tetraborane chain to afford a 1,3-azaborinine; and 3) reduction of the supporting ligands to provide a diamino dipotassium salt. The zwitterionic alkylidene borate-borenium species can be viewed as an analogue of the base-stabilized diborenes. NMR spectroscopy and DFT calculations reveal a highly polarized B-B bond in the zwitterionic alkylidene borate-borenium, in which the formal oxidation states of the boron atoms can be considered as -1 and +2. These results suggest the considerable potential of tetraboranes as synthons for low-valent boron species.

Metal-Free Intermolecular C-H Borylation of N-Heterocycles at B-B Multiple Bonds.

Carbene-stabilized diborynes of the form LBBL (L=N-heterocyclic carbene (NHC) or cyclic alkyl(amino)carbene (CAAC)) induce rapid, high yielding, intermolecular ortho-C-H borylation at N-heterocycles at room temperature. A simple pyridyldiborene is formed when an NHC-stabilized diboryne is combined with pyridine, while a CAAC-stabilized diboryne leads to activation of two pyridine molecules to give a tricyclic alkylideneborane, which can be forced to undergo a further H-shift resulting in a zwitterionic, doubly benzo-fused 1,3,2,5-diazadiborinine by heating. Use of the extended N-heteroaromatic quinoline leads to a borylmethyleneborane under mild conditions via an unprecedented boron-carbon exchange process.

Evidence for Borylene Carbonyl (LHB═C═O) and Base-Stabilized (LHB═O) and Base-Free Oxoborane (RB≡O) Intermediates in the Reactions of Diborenes with CO2.

Doubly N-heterocyclic-carbene-stabilized diborenes undergo facile reactions with CO2, initially providing dibora-ß-lactones. These lactones convert over time to their 2,4-diboraoxetan-3-one isomers through a presumed dissociative pathway and hypovalent boron species borylene carbonyls (LHB═C═O) and base-stabilized oxoboranes (LHB═O). Repeating these reactions with doubly cyclic(alkyl)(amino)carbene-stabilized diborenes allowed the isolation of a borylene carbonyl intermediate, whereas a base-stabilized oxoborane could be inferred by the isolation of a boroxine from the reaction mixture. These results, supported by calculations, confirm the presumed mechanism of the diboralactone-to-diboraoxetanone isomerization while also establishing a surprising level of stability for three unknown or very rare hypovalent boron species: base-stabilized derivatives of the parent borylene carbonyl (LHB═C═O) and parent oxoborane (LHB═O) as well as base-free oxoboranes (RB≡O).

Recent Advances in the Synthesis and Reactivity of Transition Metal σ-Borane/Borate Complexes.

The coordination of an element-element σ bond to a transition metal (TM) is both a fundamentally intriguing binding mode and of critical importance to metal-mediated bond activation mechanisms and catalysis, particularly the hotly contested field of C-H activation. TM σ complexes of dihydrogen (i.e., H-H) and silanes (H-SiR3) have been extensively studied, the latter being of interest as models for the (generally unstable and unisolable) σ complexes of alkanes (i.e., H-CR3). TM σ complexes of hydroboranes and hydroborates (i.e., H-BR2, H-BR3, (H-)2BR2) are somewhat less well studied but similarly have relevance to catalytic borylation reactions that are of high current interest to organic synthesis. Our two research groups have made significant contributions to elaborating the family of σ-borane/-borate complexes using two distinct approaches: while the Ghosh group generally starts from hydrogen-rich tetracoordinate boron species such as borates, the Braunschweig group starts from hypovalent and/or hypocoordinate boron building blocks. Through these two approaches, a wide range of species containing one or two σ-bound B-H ligands have been prepared, some with additional chelating donor sites. Over the past 2 years, the body of work on σ-borane/-borate complexes from our two research groups has significantly expanded, with a combined nine published articles in 2019-2020 alone. Very recent work from the Braunschweig group has led to the synthesis of the first bis(σ)-borane complexes of group 6 metals, as well as the synthesis of a series of novel bis(σ)-borane and bis(σ)-borate complexes of ruthenium and iridium, the former being useful precursors for pentacoordinate borylene complexes of Ru. Recent work from the Ghosh group has uncovered a remarkable diversity of structures with σ(B-H)-bound ligands from the combination of borohydrides and nitrogen/chalcogen-containing groups and heterocycles. These reactions, while in some cases producing conventional scorpionate-type chelating products, more frequently undergo fascinating rearrangements with unpredictable outcomes. This Account aims to highlight this recent acceleration of research progress in this area, particularly the distinct but related approaches of-and complexes produced by-our two research groups, in addition to relevant works from other groups where appropriate.

Probing the Potential of Hitherto Unexplored Base-Stabilized Borylenes in Dinitrogen Binding.

Computational investigations were carried out to probe the potential of several dicoordinate, singly base-stabilized borylenes of the form [L→BR] (L=neutral Lewis base) in dinitrogen binding. The calculated reaction free energies and activation barriers associated with the formation of mono- and diborylene-N2 adducts suggest the presence of thermally surmountable kinetic barriers towards their possible isolation. Our results show that the exergonicity of dinitrogen activation and fixation is linearly dependent on the natural charge at the boron center, which can be tuned to design novel boron-based compounds with potential applications to small-molecule activation. EDA-NOCV analysis reveals strong binding of dinitrogen to these base-stabilized borylenes.

Probing the Potential of Hitherto Unexplored Base-Stabilized Borylenes in Dinitrogen Binding.

Invited for the cover of this issue are Felipe Fantuzzi, Holger Braunschweig, Ashwini K. Phukan and co-workers at the University of Kent, Julius-Maximilians-Universität Würzburg and Tezpur University. The image depicts a borylene "fishing" for a molecule of nitrogen. Read the full text of the article at 10.1002/chem.20210123.

An Unsymmetrical, Cyclic Diborene Based on a Chelating CAAC Ligand and its Small-Molecule Activation and Rearrangement Chemistry.

A one-pot synthesis of a CAAC-stabilized, unsymmetrical, cyclic diborene was achieved via consecutive two-electron reduction steps from an adduct of CAAC and B2 Br4 (SMe2 )2 . Theoretical studies revealed that this diborene has a considerably smaller HOMO-LUMO gap than those of reported NHC- and phosphine-supported diborenes. Complexation of the diborene with [AuCl(PCy3 )] afforded two diborene-AuI π complexes, while reaction with DurBH2 , P4 and a terminal acetylene led to the cleavage of B-H, P-P, and C-C π bonds, respectively. Thermal rearrangement of the diborene gave an electron-rich cyclic alkylideneborane, which readily coordinated to AgI via its B=C double bond.

Diphosphino-Functionalized 1,8-Naphthyridines: a Multifaceted Ligand Platform for Boranes and Diboranes.

A 1,8-naphthyridine diphosphine (NDP) reacts with boron-containing Lewis acids to generate complexes featuring a number of different naphthyridine bonding modes. When exposed to diborane B2 Br4 , NDP underwent self-deprotonation to afford [NDP-B2 Br3 ]Br, an unsymmetrical diborane comprised of four fused rings. The reaction of two equivalents of monoborane BBr3 and NDP in a non-polar solvent provided the simple phosphine-borane adduct [NDP(BBr3 )2 ], which then underwent intramolecular halide abstraction to furnish the salt [NDP-BBr2 ][BBr4 ], featuring a different coordination mode from that of [NDP-B2 Br3 ]Br. Direct deprotonation of NDP by KHMDS or PhCH2 K generates mono- and dipotassium reagents, respectively. The monopotassium reagent reacts with one or half an equivalent of B2 (NMe2 )2 Cl2 to afford NDP-based diboranes with three or four amino substituents.

Twisting versus Delocalization in CAAC- and NHC-Stabilized Boron-Based Biradicals: The Roles of Sterics and Electronics.

Invited for the cover of this issue is Bernd Engels, Holger Braunschweig, Volker Engel and their coworkers at University of Würzburg. The image depicts bridged boron compounds which possess fascinating relationships between their composition and their geometrical and electronic structures, the latter ranging from closed-shell to biradical triplet or singlet ground state. Read the full text of the article at 10.1002/chem.202004619.

Boron- versus Nitrogen-Centered Nucleophilic Reactivity of (Cyano)hydroboryl Anions: Synthesis of Cyano(hydro)organoboranes and 2-Aza-1,4-diborabutatrienes.

Cyclic alkyl(amino)carbene-stabilized (cyano)hydroboryl anions were synthesized by deprotonation of (cyano)dihydroborane precursors. While they display boron-centered nucleophilic reactivity towards organohalides, generating fully unsymmetrically substituted cyano(hydro)organoboranes, they show cyano-nitrogen-centered nucleophilic reactivity towards haloboranes, resulting in the formation of hitherto unknown linear 2-aza-1,4-diborabutatrienes.

Twisting versus Delocalization in CAAC- and NHC-Stabilized Boron-Based Biradicals: The Roles of Sterics and Electronics.

Twisted boron-based biradicals featuring unsaturated C2 R2 (R=Et, Me) bridges and stabilization by cyclic (alkyl)(amino)carbenes (CAACs) were recently prepared. These species show remarkable geometrical and electronic differences with respect to their unbridged counterparts. Herein, a thorough computational investigation on the origin of their distinct electrostructural properties is performed. It is shown that steric effects are mostly responsible for the preference for twisted over planar structures. The ground-state multiplicity of the twisted structure is modulated by the σ framework of the bridge, and different R groups lead to distinct multiplicities. In line with the experimental data, a planar structure driven by delocalization effects is observed as global minimum for R=H. The synthetic elusiveness of C2 R2 -bridged systems featuring N-heterocyclic carbenes (NHCs) was also investigated. These results could contribute to the engineering of novel main group biradicals.

Controlled Synthesis of Oligomers Containing Main-Chain B(sp2 )-B(sp2 ) Bonds.

A number of novel alkynyl-functionalized diarylbis(dimethylamino)diboranes(4) are prepared by salt metathesis, and the appended alkynyl groups are subjected to hydroboration. Their reactions with monohydroboranes lead to discrete boryl-appended diborane(4) species, while dihydroboranes induce their catenation to oligomeric species, the first known examples of well-characterized macromolecular species with B-B bonds. The oligomeric species were found to comprise up to ten repeat units and are soluble in common organic solvents. Some of the oligomeric species have good air stability and all were characterized by NMR and vibrational spectroscopy and size-exclusion chromatography techniques.

Multiple complexation of CO and related ligands to a main-group element.

The ability of an atom or molecular fragment to bind multiple carbon monoxide (CO) molecules to form multicarbonyl adducts is a fundamental trait of transition metals. Transition-metal carbonyl complexes are vital to industry, appear naturally in the active sites of a number of enzymes (such as hydrogenases), are promising therapeutic agents, and have even been observed in interstellar dust clouds. Despite the wealth of established transition-metal multicarbonyl complexes, no elements outside groups 4 to 12 of the periodic table have yet been shown to react directly with two or more CO units to form stable multicarbonyl adducts. Here we present the synthesis of a borylene dicarbonyl complex, the first multicarbonyl complex of a main-group element prepared using CO. The compound is additionally stable towards ambient air and moisture. The synthetic strategy used--liberation of a borylene ligand from a transition metal using donor ligands--is broadly applicable, leading to a number of unprecedented monovalent boron species with different Lewis basic groups. The similarity of these compounds to conventional transition-metal carbonyl complexes is demonstrated by photolytic liberation of CO and subsequent intramolecular carbon-carbon bond activation.

Adducts of the Parent Boraphosphaketene H2 BPCO and their Decarbonylative Insertion Chemistry.

The first examples of Lewis base adducts of the parent boraphosphaketene (H2 B-PCO) and their cyclodimers are prepared. One of these adducts is shown to undergo mild decarbonylation and phosphinidene insertion into a B-C bond of a borole, forming very rare examples of 1,2-phosphaborinines, B/P isosteres of benzene. The strong donor properties of these 1,2-phosphaborinines are confirmed by the synthesis of their π complexes with the Group 6 metals.