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Borane coordination, B-H borane bond activation, and borane catenation via metal-mediated dehydrocoupling to form electron-precise B-B bonds are reported. The reaction of trans -[M(IMes) 2 Cl 4 ] (M = W, Mo) (IMes = 1,3-bis(2,4,6-trimethylphenyl)imidazolin-2-ylidene) with borates Li[BH 3 R] (R = Mes, Dur; Mes = 2,4,6-Me 3 C 6 H 2 and Dur = 2,3,5,6-Me 4 C 6 H) afforded the complexes [M(IMes)(η 2 -H 2 BR) 2 (η 1 -H 2 BR)] (M = W: R = Mes 1 , R = Dur 3 ; M = Mo: R = Mes 2 , R = Dur 4 ). Three borane ligands are coordinated in 1 - 4 to the group 6 metal atom via five (σ-B-H) bonds. Reaction of 1 with the phosphines PMe 3 and PEt 3 , respectively, led to the elimination of one of the borane ligands and afforded the hydrido (σ-B-H)-boryl bis(σ-B-H)-borane complexes trans -[W(IMes)(PR 3 )(η 1 -HBMes)(η 2 -H 2 BMes)(H)] (R = Me 5 , R = Et 6 ), in which the metal atom inserted into one of the remaining σ-B-H bonds of the borane ligands. Reaction of 1 with an additional equivalent borane BH 2 Mes resulted in borane dehydrocoupling and formation of complex [W(IMes)(η 4 -BH 2 Mes-BMes-BMes-BH 2 Mes)] 7 , featuring a unique B 4 chain as a ligand. Reaction of trans -[W(IMes) 2 Cl 4 ] with NaBH 4 also led to B-B coupling, and the metallaborane cluster [{W(IMes)(BH 4 )} 2 (B 5 H 9 )] 9 was formed, in which two tungsten atoms bridge a B 5 chain.

The photolytic or oxidative liberation of a cyclic (amino)(alkyl)carbene (CAAC)-stabilized arylborylene in the presence of organoazides yielded borylene-organoazide complexes (4a,b) has been achieved in a manner akin to the first step of the Staudinger reaction. Similarly, a CAAC-stabilized aminoborylene also afforded borylene-organoazide complexes (6a-c), which further undergo rearrangement to produce aminoborane triazene species (7a,b).

Borylation of a tungsten-bound N 2 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., (© 2024 Wiley-VCH GmbH.)

Diboradiazene compounds, derived in one step from the boron-mediated reduction of dinitrogen (N 2 ), 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 N 2 that both circumvent the need for transition metal reagents and completely bypass the traditional intermediate ammonia., (© 2024 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH.)

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., (© 2024 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH.)

The addition of chlorotrimethylsilane to a boron-mediated, transition-metal-free N 2 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 N 2 species can be protonated or methylated, providing access to mono- to tetrafunctionalized hydrazines in two steps from N 2 and in the absence of transition metals.

Hydrosilanes undergo mild, uncatalyzed single and double 1,2-addition across the B-B triple bonds of diborynes, leading to an unsymmetrical silyldiborene and compounds with novel non-cluster three-membered B 2 Si rings. The reactions are a new addition to the very few catalyst- and alkali-metal-free methods available for the construction of B-Si bonds.

The reaction of a pyridyl-substituted, doubly Lewis base-stabilised diborene with different amounts of copper(I) precursors led to the formation of the first chelating π-diborene complexes, the first π-diborene complexes in which metals are bound to both faces of the BB bond, and the first mixed-metal π-diborene species.

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., (© 2023 The Authors. Chemistry - A European Journal published by Wiley-VCH GmbH.)

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 η 5 -diborafulvene dianion. Salt metathesis of the latter with [(η 6 -p-cymene)RuCl 2 ] affords a mixed ruthenium sandwich complex of an η 5 -borylborole dianion. Calculations highlight both the structural and electronic changes in the boron-substituted heterocyclic C 4 B dianion upon switching coordination from magnesium (diborafulvene dianion) to ruthenium (borylborole dianion)., (© 2023 The Authors. Chemistry - A European Journal published by Wiley-VCH GmbH.)

The Fischer carbene synthesis, involving the conversion of a transition metal (TM)-bound CO ligand to a carbene ligand of the form [=C(OR')R] (R, R' = organyl groups), is one of the seminal reactions in the history of organometallic chemistry. Carbonyl complexes of p-block elements, of the form [E(CO) n ] (E = main-group fragment), are much less abundant than their TM cousins; this scarcity and the general instability of low-valent p-block species means that replicating the historical reactions of TM carbonyls is often very difficult. Here we present a step-for-step replica of the Fischer carbene synthesis at a borylene carbonyl involving nucleophilic attack at the carbonyl carbon followed by electrophilic quenching at the resultant acylate oxygen atom. These reactions provide borylene acylates and alkoxy-/silyloxy-substituted alkylideneboranes, main-group analogues of the archetypal transition metal acylate and Fischer carbene families, respectively. When either the incoming electrophile or the boron center has a modest steric profile, the electrophile instead attacks at the boron atom, leading to carbene-stabilized acylboranes - boron analogues of the well-known transition metal acyl complexes. These results constitute faithful main-group replicas of a number of historical organometallic processes and pave the way to further advances in the field of main-group metallomimetics., (© 2023. The Author(s).)

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-E 2 species as viable synthetic targets, highlighting the versatility and potential applications of napy-based ligands in main-group chemistry., (© 2022 The Authors. Journal of Computational Chemistry published by Wiley Periodicals LLC.)

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., (© 2022 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH.)

A naphthyridine diimine (NDI) supported bis(germylene) NDI-Ge 2 containing two dicoordinate, coplanar Ge(II) atoms has been synthesised. Computational investigations on NDI-Ge 2 indicated the two Ge(II) atoms are nearly independent. The EDA-NOCV analysis of the [NDI-Ge 2 ][Fe 2 (CO) 6 ] complex revealed the six-electron donor behavior of NDI-Ge 2 , the first example for group-14-element-based bidentate ligands.

The halocarbyne complexes [M(CX)(CO) 2 (Tp*)] (M = Mo, W; X = Cl, Br; Tp* = hydrotris(dimethylpyrazolyl)borate) react with [AuCl(SMe 2 )], [Pt(η 2 -H 2 CCH 2 )(PPh 3 ) 2 ] or [Pt(η 2 -nbe) 3 ] (nbe = norbornene) to furnish rare examples of μ 2 -halocarbyne complexes [MAu(μ 2 -CX)Cl(CO) 2 (Tp*)], [MPt(μ 2 -CCl)(CO) 2 (PPh 3 ) 2 (Tp*)] and [W 2 Pt(μ 2 -CCl) 2 (CO) 4 (Tp*) 2 ]. The complex [WPt(μ 2 -CCl)(CO) 2 (PPh 3 ) 2 (Tp*)] spontaneously rearranges to the μ 2 -carbido complex [WPt(μ 2 -C)Cl(CO) 2 (PPh 3 ) 2 (Tp*)] during silica-gel chromatography. One phosphine ligand of [WPt(μ 2 -CCl)(CO) 2 (PPh 3 ) 2 (Tp*)] is readily substituted by CO to afford [WPt(μ 2 -CCl)(CO) 3 (PPh 3 )(Tp*)]. These μ 2 -halocarbyne complexes have been interrogated by spectroscopic, crystallographic and computational methods, the latter by reference to data for terminal halocarbyne precursors [M(CX)(CO) 2 (Tp*)].

We report the reduction of bulky ferrocenyl-based NHC-stabilised aluminium(iii) diiodide [Fc*(NHC)AlI 2 ] (Fc* = 2,5-bis(3,5-di- tert -butylphenyl)-1-ferrocenyl) in different hydrocarbon solvents (hexane, benzene, toluene, and p -xylene), which results in different outcomes. Reduction in hexane with an equivalent amount of KC 8 generates the diiododialane [(Fc*(NHC)AlI) 2 ], whereas complete reduction in hexane leads to an unusual C-H activation at an N-Me group of one NHC unit. In contrast, reaction in aromatic solvents result in hitherto unknown Birch-type reductions of the corresponding solvent molecules by transient aluminium radicals of the type [LAlR 2 ]˙, which is ultimately bound to two aluminium centers., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

While a stable base-free arylalumylene bearing a sterically encumbered terphenyl substituent has been reported previously, we herein report that our attempts to form a base-stabilised arylalumylene bearing a relatively small terphenyl substituent and an N -heterocyclic carbene base led instead to a "masked" dialumene (LRAl[double bond, length as m-dash]AlRL), self-stabilised by one peripheral aromatic group. Intriguingly, examining the behavior of this species or its transient dialumene formed from reducing the diiodoarylalane in aromatic solvents under different conditions reveals that they both decouple into the desired base-stabilised arylalumylene. This transient acyclic, dicoordinate alumylene is highly reactive, deconstructing benzene and toluene to furnish dialuminium derivatives of pentalene, providing the first example of a neutral Al I compound able to deconstruct these less reactive arenes. Computational insights were also gained on the dialumene dissociation and on the mechanism of arene deconstruction by alumylene., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

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., (© 2022 Wiley-VCH GmbH.)

Boron chemistry has experienced tremendous progress in the last few decades, resulting in the isolation of a variety of compounds with remarkable electronic structures and properties. Some examples are the singly Lewis-base-stabilised borylenes, wherein boron has a formal oxidation state of +I, and their dimers featuring a boron-boron double bond, namely diborenes. However, no evidence of a Wanzlick-type equilibrium between borylenes and diborenes, which would open a valuable route to the latter compounds, has been found. In this work, we combine DFT, coupled-cluster, multireference methods, and natural bond orbital/natural resonance theory analyses to investigate the electronic, structural, and kinetic factors controlling the reactivity of the transient CAAC-stabilised cyanoborylene, which spontaneously cyclotetramerises into a butterfly-type, twelve-membered (BCN) 4 ring, and the reasons why its dimerisation through the boron atoms is hampered. The computations are also extended to the NHC-stabilised borylene counterparts. We reveal that the borylene ground state multiplicity dictates the preference for self-stabilising cyclooligomerisation over boron-boron dimerisation. Our comparison between NHC- vs. CAAC-stabilised borylenes provides a convincing rationale for why the reduction of the former always gives diborenes while a range of other products is found for the latter. Our findings provide a theoretical background for the rational design of base-stabilised borylenes, which could pave the way for novel synthetic routes to diborenes or alternatively non-dimerising systems for small-molecule activation., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

Doubly N-heterocyclic-carbene-stabilized diborenes undergo facile reactions with CO 2 , initially providing dibora-β-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).

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 B 2 Br 4 (SMe 2 ) 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(PCy 3 )] afforded two diborene-Au I π complexes, while reaction with DurBH 2 , P 4 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 Ag I via its B=C double bond., (© 2021 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH.)

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., (© 2021 The Authors. Chemistry - A European Journal published by Wiley-VCH GmbH.)

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 B 2 Br 4 , NDP underwent self-deprotonation to afford [NDP-B 2 Br 3 ]Br, an unsymmetrical diborane comprised of four fused rings. The reaction of two equivalents of monoborane BBr 3 and NDP in a non-polar solvent provided the simple phosphine-borane adduct [NDP(BBr 3 ) 2 ], which then underwent intramolecular halide abstraction to furnish the salt [NDP-BBr 2 ][BBr 4 ], featuring a different coordination mode from that of [NDP-B 2 Br 3 ]Br. Direct deprotonation of NDP by KHMDS or PhCH 2 K generates mono- and dipotassium reagents, respectively. The monopotassium reagent reacts with one or half an equivalent of B 2 (NMe 2 ) 2 Cl 2 to afford NDP-based diboranes with three or four amino substituents., (© 2021 The Authors. Chemistry - A European Journal published by Wiley-VCH GmbH.)

The chemistry of aluminacyclopentadienes, also known as alumoles, has experienced a resurgence in recent years, resulting in the preparation of a number of new entries in this class of compound. In contrast, their reactivity remains relatively unexplored. Thus far, their Lewis acid-base chemistry has been studied, as well as various ring-insertion reactions with unsaturated species such as alkynes and azides. Herein we present a range of new ring expansion reactions of an alumole with heteroatom-containing unsaturated species, providing novel six-, seven- and eight-membered cyclic products with high heteroatom content. Additionally, an unusual reaction was observed when the alumole was treated with morpholine-N-oxide, involving a stepwise insertion of two oxygen atoms, first into the exocyclic Al-C bond and then the endocyclic Al-C bond.

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., (© 2021 The Authors. Chemistry - A European Journal published by Wiley-VCH GmbH.)

The NHC-stabilised diboryne (B 2 (SIDep) 2 ; SIDep=1,3-bis(2,6-diethylphenyl)imidazolin-2-ylidene) undergoes a high-yielding P-P bond activation with tetraethyldiphosphine at room temperature to form a B 2 P 2 heterocycle via a diphosphoryldiborene by 1,2-diphosphination. The heterocycle can be oxidised to a radical cation and a dication, respectively, depending on the oxidant used and its counterion. Starting from the planar, neutral 1,3-bis(alkylidene)-1,3-diborata-2,4-diphosphoniocyclobutane, each oxidation step leads to decreased B-B distances and loss of planarity by cationisation. X-ray analyses in conjunction with DFT and CASSCF/NEVPT2 calculations reveal closed-shell singlet, butterfly-shaped structures for the NHC-stabilised dicationic B 2 P 2 rings, with their diradicaloid, planar-ring isomers lying close in energy., (© 2021 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH.)

The first examples of Lewis base adducts of the parent boraphosphaketene (H 2 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., (© 2021 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH.)

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., (© 2021 Wiley-VCH GmbH.)

The synthesis and reactivity of dialkynyldiboranes(4), a little-studied family of diboranes, are presented herein. Three dialkynyldiboranes(4) were prepared via two different salt metathesis pathways. The three reactive sites of these dialkynyldiboranes(4) are then selectively addressed by judicious application of reagents: addition of an amine N-oxide leads to oxygen insertion into the B-B bond, dicobaltoctacarbonyl binds to the alkynyl C[triple bond, length as m-dash]C bonds, while Sonogashira-Hagihara cross-coupling conditions lead to double C-C bond formation at the alkynyl C-H groups.

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-SiR 3 ) have been extensively studied, the latter being of interest as models for the (generally unstable and unisolable) σ complexes of alkanes (i.e., H-CR 3 ). TM σ complexes of hydroboranes and hydroborates (i.e., H-BR 2 , H-BR 3 , (H-) 2 BR 2 ) 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.

The concepts of aromaticity and antiaromaticity have a long history, and countless demonstrations of these phenomena have been made with molecules based on elements from the p, d, and f blocks of the periodic table. In contrast, the limited oxidation-state flexibility of the s-block metals has long stood in the way of their participation in sophisticated π-bonding arrangements, and truly antiaromatic systems containing s-block metals are altogether absent or remain poorly defined. Using spectroscopic, structural, and computational techniques, we present herein the synthesis and authentication of a heterocyclic compound containing the alkaline earth metal beryllium that exhibits significant antiaromaticity, and detail its chemical reduction and Lewis-base-coordination chemistry., (© 2020 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH.)

Transition metal borylene complexes of the type [(OC)5M[double bond, length as m-dash]BN(SiMe3)(tBu)] (M = Cr, Mo, W) have been synthesised by salt elimination of the corresponding dibromoborane and the dianionic metallates Na2[M(CO)5]. The borylene complexes have been characterised by multinuclear solution-state NMR spectroscopy and solid-state molecular structure determination. The group 6 borylene complexes can be used to effectively transfer the borylene ligand to other transition metal complexes by replacing one or two carbonyl ligands upon irradiation of the reaction mixture with UV light. This borylene transfer reaction led to the formation of new terminal and bridging borylene complexes which cannot be formed by the corresponding salt elimination reactions, including a rare example of a bis(terminal borylene) complex and only the second reported terminal borylene complex of an early transition metal (vanadium).

The industrial reduction of dinitrogen (N 2 ) to ammonia is an energy-intensive process that consumes a considerable proportion of the global energy supply. As a consequence, species that can bind N 2 and cleave its strong N-N bond under mild conditions have been sought for decades. Until recently, the only species known to support N 2 fixation and functionalization were based on a handful of metals of the s and d blocks of the periodic table. Here we present one-pot binding, cleavage and reduction of N 2 to ammonium by a main-group species. The reaction-a complex multiple reduction-protonation sequence-proceeds at room temperature in a single synthetic step through the use of solid-phase reductant and acid reagents. A simple acid quench of the mixture then provides ammonium, the protonated form of ammonia present in fertilizer. The elementary reaction steps in the process are elucidated, including the crucial N-N bond cleavage process, and all of the intermediates of the reaction are isolated.

Boron's unique position in the Periodic Table, that is, at the apex of the line separating metals and nonmetals, makes it highly versatile in chemical reactions and applications. Contemporary demand for renewable and clean energy as well as energy-efficient products has seen boron playing key roles in energy-related research, such as 1) activating and synthesizing energy-rich small molecules, 2) storing chemical and electrical energy, and 3) converting electrical energy into light. These applications are fundamentally associated with boron's unique characteristics, such as its electron-deficiency and the availability of an unoccupied p orbital, which allow the formation of a myriad of compounds with a wide range of chemical and physical properties. For example, boron's ability to achieve a full octet of electrons with four covalent bonds and a negative charge has led to the synthesis of a wide variety of borate anions of high chemical and electrochemical stability-in particular, weakly coordinating anions. This Review summarizes recent advances in the study of boron compounds for energy-related processes and applications., (© 2019 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.)

We present herein an in-depth study of complexes in which a molecule containing a boron-boron triple bond is bound to tellurate cations. The analysis allows the description of these salts as true π complexes between the B-B triple bond and the tellurium center. These complexes thus extend the well-known Dewar-Chatt-Duncanson model of bonding to compounds made up solely of p block elements. Structural, spectroscopic and computational evidence is offered to argue that a set of recently reported heterocycles consisting of phenyltellurium cations complexed to diborynes bear all the hallmarks of π-complexes in the π-complex/metallacycle continuum envisioned by Joseph Chatt. Described as such, these compounds are unique in representing the extreme of a metal-free continuum with conventional unsaturated three-membered rings (cyclopropenes, azirenes, borirenes) occupying the opposite end., (© 2020 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.)

Herein we present the first solid-state structures of tetraiododiborane(4) (B 2 I 4 ), which was long believed to exist in all phases as discrete molecules with planar, tricoordinate boron atoms, like the lighter tetrahalodiboranes(4) B 2 F 4 , B 2 Cl 4 , and B 2 Br 4 . Single-crystal X-ray diffraction, solid-state NMR, and IR measurements indicate that B 2 I 4 in fact exists as two different polymeric forms in the solid state, both of which feature boron atoms in tetrahedral environments. DFT calculations are used to simulate the IR spectra of the solution and solid-state structures, and these are compared with the experimental spectra., (© 2019 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.)

The reactivity of a diruthenium tetrahydride complex towards three selected dihydroboranes was investigated. The use of [DurBH 2 ] (Dur=2,3,5,6-Me 4 C 6 H) and [(Me 3 Si) 2 NBH 2 ] led to the formation of bridging borylene complexes of the form [(Cp*RuH) 2 BR] (Cp*=C 5 Me 5 ; 1 a: R=Dur; 1 b: R=N(SiMe 3 ) 2 ) through oxidative addition of the B-H bonds with concomitant hydrogen liberation. Employing the more electron-deficient dihydroborane [3,5-(CF 3 ) 2 -C 6 H 3 BH 2 ] led to the formation of an anionic complex bearing a tetraarylated chain of four boron atoms, namely Li(THF) 4 [(Cp*Ru) 2 B 4 H 5 (3,5-(CF 3 ) 2 C 6 H 3 ) 4 ] (4), through an unusual, incomplete threefold dehydrocoupling process. A comparative theoretical investigation of the bonding in a simplified model of 4 and the analogous complex nido-[1,2(Cp*Ru) 2 (μ-H)B 4 H 9 ] (I) indicates that there appear to be no classical σ-bonds between the boron atoms in complex I, whereas in the case of 4 the B 4 chain better resembles a network of three B-B σ bonds, the central bond being significantly weaker than the other two., (© 2019 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.)

The steric and electronic properties of aryl substituents in monoaryl borohydrides (Li[ArBH 3 ]) and dihydroboranes were systematically varied and their reactions with [Ru(PCy 3 ) 2 HCl(H 2 )] (Cy: cyclohexyl) were studied, resulting in bis(σ)-borane or terminal borylene complexes of ruthenium. These variations allowed for the investigation of the factors involved in the activation of dihydroboranes in the synthesis of terminal borylene complexes. The complexes were studied by multinuclear NMR spectroscopy, mass spectrometry, X-ray diffraction analysis, and density functional theory (DFT) calculations. The experimental and computational results suggest that the ortho-substitution of the aryl groups is necessary for the formation of terminal borylene complexes., (© 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

The scandium Cp/COT hybrid sandwich compound [(η 5 -C 5 H 5 )Sc(η 8 -C 8 H 8 )] is resistant to metallation via conventional alkyllithium and lithium amide bases. In this work, clean, selective, stoichiometric and high-yielding mono- and dimetallation is accomplished using tandem trans-metal-trapping (TMT) involving LiTMP and iBu 2 AlTMP with deprotonation occurring selectively at the Cp and Cp/COT rings respectively, providing the first examples of selective metallation of a sandwich complex featuring a group 3 element.

A set of diboryldiborenes are prepared by the mild, catalyst-free, room-temperature diboration of the B-B triple bonds of doubly base-stabilized diborynes. Two of the product diboryldiborenes are found to be air- and water-stable in the solid state, an effect that is attributed to their high crystallinity and extreme insolubility in a wide range of solvents.

The lability of B=B, B-P, and B-halide bonds is combined in the syntheses of the first diiododiborenes. In a series of reactivity tests, these diiododiborenes undergo cleavage of all six of their central bonds in different ways, leading to products of B=B hydrogenation and dihalogenation as well as halide exchange., (© 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Reaction of AsCl3 with Pt(0) complexes [Pt(PCy3)2], [Pt(PCy3)(IMes)] and [Pt(IMes)2] (IMes = 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene) resulted in oxidative addition of As-Cl bonds at the Pt centres to form complexes of the form trans-[PtCl(AsCl2)L2]. Two of these compounds were characterised by X-ray diffraction, making them the first structurally characterised examples of AsX2 ligands (X = halogen). AsBr3 also underwent oxidative addition to [Pt(PCy3)2], forming trans-[PtBr(AsBr2)(PCy3)2] in situ, as judged by 31P NMR spectroscopy. This reaction was unselective, yielding several products, of which a Pt3As2 cluster could be identified by single-crystal X-ray diffraction.

A number of recently discovered nucleophilic boron compounds, such as boryl anions and borylenes, are breaking the rules regarding boron and boron-containing compounds and their reputation as Lewis acids/electrophiles. In a similar fashion, the B-H bonding pair electrons in boranes also show nucleophilicity which is ascribed to the lower electronegativity of boron relative to that of hydrogen. However, this nucleophilicity of the B-H bond has received far less attention. Explorations of the nucleophilicity of the B-H bonding pair electrons have led to the formation of B-H-B bonded units and B-H⋅⋅⋅H-Y dihydrogen bonds, based on which new chemistry has been uncovered, including the elucidation of the mechanism of formation of aminodiborane (ADB), the diammoniate of diborane (DADB), and lithium or sodium salts of octahydrotriborates (B 3 H 8 - ), as well as the development of more convenient and straightforward synthetic routes to these reagents. Moreover, the recognition of the nucleophilic properties of the B-H bonding pair electrons will also help to more deeply understand the different mechanisms operating in hydroboration reactions., (© 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

A new structural class of boron-based diradicals is prepared by Lewis base addition to and reduction of singly cyclic (alkyl)(amino)carbene (CAAC) stabilised diborylalkenes. The diradicals feature a perpendicular olefinic bridge preventing delocalization between the B(CAAC) π systems, making the coupling between the spin centres very weak. DFT calculations indicate that the compounds are ground-state (open-shell) singlets, with triplet states negligibly higher in energy, thus making the triplet states easily populated thermally., (© 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

The reaction of the tetrahalodiboranes(4) B 2 F 4 , B 2 Cl 4 , and B 2 Br 4 with a Lewis basic platinum(0) complex led to the isolation of the cis-bis(difluoroboryl) complex cis-[(Cy 3 P) 2 Pt(BF 2 ) 2 ] (1) and the novel borylborato complexes trans-[(Cy 3 P) 2 Pt{B(X)-BX 3 }] (2, X = Cl; 3, X = Br), respectively. The trans influence of the borylborato group was found to be one of the strongest ever observed experimentally. Furthermore, the reactivity of little-explored diaryldifluorodiboranes(4) F 2 B-BMes 2 and the new derivative F 2 B-BAn 2 (An = 9-anthryl) toward a range of platinum(0) complexes was investigated. Reactions with relatively nonbulky platinum(0) complexes led to the formation of unsymmetrical cis-bis(boryl) complexes cis-[(R 3 P) 2 Pt(BF 2 )(BMes 2 )] (6, R = Me; 7, R = Et) as well as the first example of a fourfold-unsymmetrical bis(boryl) complex, [(Me 3 P)(Cy 3 P)Pt(BF 2 )(BMes 2 )] (12). The use of a more bulky Pt complex provided access to the unprecedented dinuclear bis(boryl) complexes [{( iPr 3 P)Pt} 2 (μ-BF 2 )(μ-BAr 2 )] (8, Ar = Mes; 9, Ar = An), which feature two different μ 2 -bridging boryl ligands.

Cyclic (alkyl)(amino)carbenes (CAACs) and N-heterocyclic carbenes (NHCs) are widely used as stabilizing ligands in transition metal and main group element chemistry. Variations in their stabilizing properties have been cursorily explained in the literature by the greater π-donating and σ-accepting properties of CAACs relative to NHCs and their differing steric demands; however, a more precise understanding, in particular a disentanglement of steric and electronic effects, is lacking. The recently reported compounds (E)(L)BB(L)(E) (L = NHC (I)/CAAC (II) and E = SPh) present a unique opportunity to investigate the differences between NHC and CAAC donors, as both forms are stable but differ considerably in their geometrical and electronic properties. The NHC systems possess a singlet ground state with a planar central SBBS unit, while their CAAC counterparts show a triplet ground state with a twisted SBBS unit. Steric effects were found to be important in this case; however, it remained unclear how the different forms of twisting in I and II depend on the interplay of steric and electronic effects. In the present work we disentangle both effects. Our investigations explain all of these effects by MO considerations and show that for this kind of system the size of the singlet-triplet gaps are the key determinants of the differences. The different sizes of the S-T gaps result from variations in the antibonding effects within the highest occupied (HOMOs) and lowest unoccupied molecular orbitals (LUMOs). Our explanation seems to contradict the general scientific consensus about variations in the HOMO and LUMO of these two classes of cyclic carbenes; however, comparisons to the Kekulé biradicaloids recently presented by Bertrand and co-workers indicate the generality of our approach.

Diaryldihalodiboranes(4) were reacted with bis(amidinato)- and bis(guanidinato)silylenes to generate the first neutral diborane-centered radicals. These formally non-aromatic 5π electron systems are stable in the solid state as well as in solution and were characterized by solid-state structure determination, high-resolution mass spectrometry, and EPR spectroscopy. The reactivity of one of these radicals with the oxidant 1,4-benzoquinone led to ring-opening and B-O bond formation., (© 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)