A Neutral Beryllium(I) Radical

Mono-Ortho-Beryllated Carbodiphosphoranes: Synthesis, Structure, Bonding and Reactivity

The reaction of organoberyllium compounds with hexaphenylcarbodiphosphorane yields mono-ortho-beryllated complexes, which feature a double dative Be=C bond. The bonding situation in these compounds together with a simple carbodiphosphorane and an N-heterocyclic carbene adduct was analysed with energy decomposition analysis in combination with natural orbital for chemical valence as well as with quantum theory of atoms-in-molecules. Furthermore, the driving forces accountable for mono-ortho-beryllation were elucidated along with the reactivity of the Be=C bond.

Low oxidation state and hydrido group 2 complexes: synthesis and applications in the activation of gaseous substrates

Numerous industrial processes utilise gaseous chemical feedstocks to produce useful chemical products. Atmospheric and other small molecule gases, including anthropogenic waste products (e.g. carbon dioxide), can be viewed as sustainable building blocks to access value-added chemical commodities and materials. While transition metal complexes have been well documented in the reduction and transformation of these substrates, molecular complexes of the terrestrially abundant alkaline earth metals have also demonstrated promise with remarkable reactivity reported towards an array of industrially relevant gases over the past two decades. This review covers low oxidation state and hydrido group 2 complexes and their role in the reduction and transformation of a selection of important gaseous substrates towards value-added chemical products.

A nucleophilic beryllyl complex via metathesis at [Be-Be]2

Owing to its high toxicity, the chemistry of element number four, beryllium, is poorly understood. However, as the lightest elements provide the basis for fundamental models of chemical bonding, there is a need for greater insight into the properties of beryllium. In this context, the chemistry of the homo-elemental Be-Be bond is of fundamental interest. Here the ligand metathesis chemistry of diberyllocene (1; CpBeBeCp)-a stable complex with a Be-Be bond-has been investigated. These studies yield two complexes with Be-Be bonds: Cp*BeBeCp (2) and [K{(HCDippN)2BO}2]BeBeCp (3; Dipp = 2,6-diisopropylphenyl). Quantum chemical calculations indicate that the Be-Be bond in 3 is polarized to such an extent that the complex could be formulated as a mixed-oxidation state Be0/BeII complex. Correspondingly, it is demonstrated that 3 can transfer the 'beryllyl' anion, [BeCp]-, to an organic substrate, by analogy with the reactivity of sp2-sp3 diboranes. Indeed, this work reveals striking similarities between the homo-elemental bonding linkages of beryllium and boron, despite the respective metallic and non-metallic natures of these elements.

Synthesis and Reactivity of Tricoordinate Organoberyllium Azides

A series of terminal mono- and disubstituted beryllium azides of the form [(CAAC)Be(N3)R] (R=CAACH, Dur; CAACH/CAAC=1-(2,6-diisopropylphenyl)-3,3,5,5-tetramethylpyrrolidin-2-yl/idene, Dur=2,3,5,6-tetramethylphenyl) and [L2Be(N3)2] (L=CAACNH=1-(2,6-diisopropylphenyl)-3,3,5,5-tetramethylpyrrolidin-2-imine, IiPrMe=1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene), respectively, were synthesized and characterized by NMR spectroscopy and X-ray crystallography. Thermolysis and photolysis products of these first examples of tricoordinate azidoberyllium complexes evidence extensive ligand scrambling and the formal insertion of nitrenes into the CAAC-Be bond, generating cyclic alkyl(amino)imine (CAAI) ligands. Furthermore, the reaction with a small N-heterocyclic carbene (NHC) leads to unexpected CAAC-NHC ligand exchange, while the reaction with pentaphenylborole yields the first γ-azide adduct of a borole, long postulated to be the first step in the synthesis of 1,2-azaborinines from boroles and azides.

Cesium Reduction of a Lithium Diamidochloroberyllate

Room temperature reaction of elemental cesium with the dimeric lithium chloroberyllate [{SiNDipp}BeClLi]2 [{SiNDipp} = {CH2SiMe2N(Dipp)}2, where Dipp = 2,6-di-isopropylphenyl, in C6D6 results in activation of the arene solvent. Although, in contrast to earlier observations of lithium and sodium metal reduction, the generation of a mooted cesium phenylberyllate could not be confirmed, this process corroborates a previous hypothesis that such beryllium-centered solvent activation also necessitates the formation of hydridoberyllium species. These observations are further borne out by the study of an analogous reaction performed in toluene, in which case the proposed generation of formally low oxidation state beryllium radical anion intermediates induces activation of a toluene sp3 C-H bond and the isolation of the polymeric cesium benzylberyllate, [Cs({SiNDipp}BeCH2C6H5)]∞.

Recent progress in beryllium organometallic chemistry

Beryllium possesses a unique amalgamation of characteristics, its electronegativity included, that not only make it a vital component in a wide range of technical sectors and consumer industries, but also make it an interesting candidate for forming covalently bonded compounds. However, the extremely toxic nature of beryllium, which can cause chronic beryllium disease, has limited the exploration of its chemistry, making beryllium one of the least studied (non-radioactive) elements. The development of selective chelating ligands, sterically encumbered substituents and, moreover, the boom of N-heterocyclic carbenes in organometallic chemistry and main group chemistry has revived the interest in beryllium chemistry. Therefore, some quite remarkable progress in the coordination and organometallic chemistry of beryllium has been made in the last two decades. For example, low oxidation state beryllium compounds, antiaromatic/aromatic beryllium compounds, where beryllium is involved in π-electron delocalization, and the isolation of beryllium-beryllium bonded species have all been achieved. This article provides an oversight over the recent developments in the organometallic chemistry of beryllium.

Tricoordinate Beryllium Radicals and Their Reactivity

The one-electron reduction of [(CAAC)Be(Dur)Br] (CAAC = cyclic alkyl(amino)carbene, Dur = 2,3,5,6-tetramethylphenyl = duryl) with lithium sand in diethyl ether yields the first neutral, tricoordinate, and moderately stable beryllium radical, [(CAAC)(Et2O)BeDur]• (2-Et2O), which undergoes a facile second one-electron reduction concomitant with the insertion of the beryllium center into the endocyclic C-NCAAC bond and a cyclopropane-forming C-H bond activation of an adjacent methyl group. In situ generation of 2-Et2O and addition of PMe3 yield the stable analogue, [(CAAC)(Me3P)BeDur]• (2-PMe3), which serves as a platform for PMe3-ligand exchange with stronger donors, generating the radicals [(CAAC)LBeDur]• (2-L, L = isocyanides, pyridines, and N-heterocyclic carbenes). X-ray structural analyses show trigonal-planar beryllium centers and strong π backbonding from the metal to the CAAC ligand. The EPR signals of all six isolated [(CAAC)LBeDur]• radicals display significant, albeit small, hyperfine coupling to the 9Be nucleus. DFT calculations show that the spin density is mostly delocalized over the CAAC π framework and, where present, the isocyanide CN moiety, with only a small proportion (3-6%) on the beryllium center. 2-PMe3 proved thermally unstable at 80 °C, first undergoing radical hydrogen abstraction with the solvent, followed by insertion of beryllium into the endocyclic C-NCAAC bond and PMe3 transfer to the former carbene carbon atom. The reactions with diphenyl disulfide and phenyl azide occur at the beryllium center and yield the corresponding Be(II) phenyl sulfide and amino complexes, respectively, the latter concomitant with radical transfer and hydrogen abstraction by the beryllium-bound nitrogen center.

A Preference for Heterolepticity - Schlenk Type Equilibria in Organometallic Beryllium Systems

The reaction of homoleptic beryllium halide with diphenyl beryllium complexes leads to the clean formation of heteroleptic beryllium Grignard compounds [(L)1-2 BePhX]1-2 (X=Cl, Br, I; L=C-, N-, O-donor ligand). The influence of ligands and solvent on these compounds, their formation and exchange equilibria in solution were investigated, together with the factors determining the complex constitution.

Diberyllocene, a stable compound of Be(I) with a Be-Be bond

The complex diberyllocene, CpBeBeCp (Cp, cyclopentadienyl anion), has been the subject of numerous chemical investigations over the past five decades yet has eluded experimental characterization. We report the preparation and isolation of the compound by the reduction of beryllocene (BeCp₂) with a dimeric magnesium(I) complex and determination of its structure in the solid state by means of x-ray crystallography. Diberyllocene acts as a reductant in reactions that form beryllium-aluminum and beryllium-zinc bonds. Quantum chemical calculations indicate parallels between the electronic structure of diberyllocene and the simple homodiatomic species diberyllium (Be₂).

Ligand exchange at tetra-coordinated beryllium centres

Mono and dinuclear phosphine complexes of beryllium halides [(PMe₃)₂BeX₂], [(PMe₃)BeX₂]₂ and [(PCy₃)BeX₂]₂ (X = Cl, Br, I) were synthesised and characterised via NMR and IR spectroscopy as well as single crystal X-ray diffraction experiments. Dissociation and ligand exchange processes at these complexes were investigated through variable temperature NMR experiments in combination with line shape analysis and complemented by quantum chemical calculations. The PMe₃ dissociation energy is smallest in [(PMe₃)₂BeCl₂], while PMe₃ exchange is similar in energy in all mononuclear [(PMe₃)₂BeX₂] complexes and follows an interchange mechanism. While [(PMe₃)BeX₂]₂ dissociates homolytically, [(PCy₃)BeX₂]₂ cleaves one phosphine ligand. These distinctive dissociation processes account for the different chemical behaviour of these complexes.

Five-membered N-heterocyclic beryllium(I) compounds: fluctuating electronic structures with ambiphilic reactivity

The structure, bonding, and reactivity of the five-membered N-heterocyclic beryllium compounds (NHBe), BeN₂C₂H₄ (1) and BeN₂(CH₃)₂C₂H₂ (2) were studied at the M06/def2-TZVPP//BP86/def2-TZVPP level of theory. The molecular orbital analysis indicates that NHBe is an aromatic 6π-electron system with an unoccupied σ-type spⁿ-hybrid orbital on Be. Energy decomposition analysis combined with natural orbitals for chemical valence has been carried out with Be and L (L = N₂C₂H₄ (1), N₂(CH₃)₂C₂H₂ (2)) in their different electronic states as fragments at the BP86/TZ2P level of theory. The results indicate that the best bonding representation can be considered as an interaction between Be⁺ having the 2s⁰2p_x¹2p_y⁰2p_z⁰ electronic configuration and L^-. Accordingly, L^- forms two donor-acceptor σ-bonds and one electron sharing π-bond with Be⁺. Compounds 1 and 2 show high proton and hydride affinity at beryllium, indicating its ambiphilic reactivity. The protonated structure results from adding a proton on the lone pair of electrons in the doubly excited state. On the other hand, the hydride adduct is formed by donating electrons from the hydride to an unoccupied σ-type spⁿ-hybrid orbital on Be. These compounds show very high exothermic reaction energy for adduct formation with two electron donor ligands such as cAAC, CO, NHC, and PMe₃.

Inducing Nucleophilic Reactivity at Beryllium with an Aluminyl Ligand

The reactions of anionic aluminium or gallium nucleophiles {K[E(NON)]}₂ (E = Al, 1; Ga, 2; NON = 4,5-bis(2,6-diisopropylanilido)-2,7-ditert-butyl-9,9-dimethylxanthene) with beryllocene (BeCp₂) led to the displacement of one cyclopentadienyl ligand at beryllium and the formation of compounds containing Be-Al or Be-Ga bonds (NON)EBeCp (E = Al, 3; Ga, 4). The Be-Al bond in the beryllium-aluminyl complex [2.310(4) Å] is much shorter than that found in the small number of previous examples [2.368(2) to 2.432(6) Å], and quantum chemical calculations suggest the existence of a non-nuclear attractor (NNA) for the Be-Al interaction. This represents the first example of a NNA for a heteroatomic interaction in an isolated molecular complex. As a result of this unusual electronic structure and the similarity in the Pauling electronegativities of beryllium and aluminium, the charge at the beryllium center (+1.39) in 3 is calculated to be less positive than that of the aluminium center (+1.88). This calculated charge distribution suggests the possibility for nucleophilic behavior at beryllium and correlates with the observed reactivity of the beryllium-aluminyl complex with N,N'-diisopropylcarbodiimide─the electrophilic carbon center of the carbodiimide undergoes nucleophilic attack by beryllium, thereby yielding a beryllium-diaminocarbene complex.

Beryllium-centred C-H activation of benzene

Reaction of BeCl₂ with the dilithium diamide, [{SiN^Dipp}Li₂] ({SiN^Dipp} = {CH₂SiMe₂NDipp}₂), provides the dimeric chloroberyllate, [{SiN^DippBeCl}Li]₂, en route to the 2-coordinate beryllium amide, [SiN^DippBe]. Lithium or sodium reduction of [SiN^DippBe] in benzene, provides the relevant organoberyllate products, [{SiN^DippBePh}M] (M = Li or Na), via the presumed intermediacy of transient Be(I) radicals.

A linear Di-coordinate boron radical cation

The pursuit of di-coordinate boron radical has been continued for more than a half century, and their stabilization and structural characterization remains a challenge. Here we report the isolation and structural characterization of a linear di-coordinate boron radical cation, achieved by stabilizing the two reactive atomic orbitals of the central boron atom by two orthogonal π-donating and π-accepting functionalities. The electron deficient radical cation undergoes facile one-electron reduction to borylene and binds Lewis base to give heteroleptic tri-coordinate boron radical cation. The co-existence of half-filled and empty p orbitals at boron also allows the CO-regulated electron transfer to be explored. As the introduction of CO promotes the electron transfer from a tri-coordinate neutral boron radical to a boron radical cation, the removal of CO under vacuum furnishes the reverse electron transfer from borylene to yield a solution consisting of two boron radicals.

Recent advances in stable main group element radicals: preparation and characterization

Radical species are significant in modern chemistry. Their unique chemical bonding and novel physicochemical properties play significant roles not only in fundamental chemistry, but also in materials science. Main group element radicals are usually transient due to their high reactivity. Highly stable radicals are often stabilized by π-delocalization, sterically demanding ligands, carbenes and weakly coordinating anions in recent years. This review presents the recent advances in the synthesis, characterization, reactivity and physical properties of isolable main group element radicals.

The oxidation state in low-valent beryllium and magnesium compounds

Low-valent group 2 (E = Be and Mg) stabilized compounds have been long synthetically pursued. Here we discuss the electronic structure of a series of Lewis base-stabilized Be and Mg compounds. Despite the accepted zero(0) oxidation state nature of the group 2 elements of some recent experimentally accomplished species, the analysis of multireference wavefunctions provides compelling evidence for a strong diradical character with an oxidation state of +2. Thus, we elaborate on the distinction between a description as a donor-acceptor interaction L(0) ⇆ E(0) ⇄ L(0) and the internally oxidized situation, better interpreted as a diradical L(-1) → E(+2) ← L(-1) species. The experimentally accomplished examples rely on the strengthened bonds by increasing the π-acidity of the ligand; avoiding this interaction could lead to an unprecedented low-oxidation state.

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.

Access to a Labile Monomeric Magnesium Radical by Ball-Milling

In order to isolate a monometallic Mg radical, the precursor (Am)MgI⋅(CAAC) (1) was prepared (Am=tBuC(N‐DIPP)₂, DIPP=2,6‐diisopropylphenyl, CAAC=cyclic (alkyl)(amino)carbene). Reduction of a solution of 1 in toluene with the reducing agent K/KI led to formation of a deep purple complex that rapidly decomposed. Ball‐milling of 1 with K/KI gave the low‐valent Mg^I complex (Am)Mg⋅(CAAC) (2) which after rapid extraction with pentane and crystallization was isolated in 15 % yield. Although a benzene solution of 2 decomposes rapidly to give Mg(Am)₂ (3) and unidentified products, the radical is stable in the solid state. Its crystal structure shows planar trigonal coordination at Mg. The extremely short Mg−C distance of 2.056(2) Å indicates strong Mg−CAAC bonding. Calculations and EPR measurements show that most of the spin density is in a π* orbital located at the C−N bond in CAAC, leading to significant C−N bond elongation. This is supported by calculated NPA charges in 2: Mg +1.73, CAAC −0.82. Similar metal‐to‐CAAC charge transfer was calculated for M⁰(CAAC)₂ and [M^I(CAAC)₂⁺] (M=Be, Mg, Ca) complexes in which the metal charges range from +1.50 to +1.70. Although the spin density of the radical is mainly located at the CAAC ligand, complex 2 reacts as a low‐valent Mg^I complex: reaction with a I₂ solution in toluene gave (Am)MgI⋅(CAAC) (1) as the major product.

Organic radical materials in biomedical applications: State of the art and perspectives

Owing to their unique chemical reactivities and paramagnetism, organic radicals with unpaired electrons have found widespread exploration in physical, chemical, and biological fields. However, most radicals are too short-lived to be separated and only a few of them can maintain stable radical forms via stereochemical strategies. How to utilize these raw radicals for developing stable radical-containing materials have long been a research hotspot for many years. This perspective introduces fundamental characteristics of organic radical materials and highlights their applications in biomedical fields, particularly for bioimaging, biosensing, and photo-triggered therapies. Molecular design of these radical materials is considered with reference to their outstanding imaging and therapeutic performances. Various challenges currently limiting the wide applications of these organic radical materials and their future development are also discussed.

Recent advances in the domain of Cyclic (alkyl)(amino) carbenes

Isolation of cyclic (alkyl) amino carbenes (cAACs) in 2005 has been a major achievement in the field of stable carbenes due to their better electronic properties. cAACs and bicyclic(alkyl)(amino)carbene (BicAAC) in essence are the most electrophilic as well as nucleophilic carbenes are known till date. Due to their excellent electronic properties in terms of nucleophilic and electrophilic character, cAACs have been utilized in different areas of chemistry, including stabilization of low valent main group and transition metal species, activation of small molecules, and catalysis. The applications of cAACs in catalysis have opened up new avenues of research in the field of cAAC chemistry. This review summarizes the major results of cAAC chemistry published until August 2021.

π Back-Donation from a Beryllium Dibromide Fragment at the Expense of Its σ Strength

It is common knowledge that metal-to-ligand π back-donation requires filled atomic orbitals at the metal center. However, we show through a combined experimental and theoretical approach that Be(II)→N-heterocyclic carbene (NHC) π back-donation is present in the two carbene adducts [(iPr)BeBr₂] (1) and [(iPr)₂BeBr₂] (2) (iPr = 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene). These complexes were characterized with NMR, IR, and Raman spectroscopy as well as with single-crystal X-ray diffractometry. The unusual bonding situation is understood from the results of energy decomposition analysis in combination with natural orbital for chemical valence and quantum theory of atoms-in-molecules analysis. The obtained findings shed light on the unusually high Be-C bond strength in carbene adducts to beryllium compounds and rationalize their geometry and reactivity.

s-Block chemistry in weakly coordinating solvents

Alkaline earth metal catalysis has been a growing field in recent years. To enhance reactivity and to understand the metal-substrate interactions in more detail, reactions are increasingly carried out in weakly coordinating solvents. This article gives an overview over the two main approaches to facilitate this, which are either through the employment of highly dipolar haloaryls as solvents, or by increasing the solubility of the ligand systems. The resulting coordination modes and reactivities are presented together with the synthetic strategies. Additionally, the latest results of group 1 complex chemistry in aliphatic solvents are illustrated and future challenges are highlighted.

New horizons in low oxidation state group 2 metal chemistry

Since the seminal report on Mg in the +I oxidation state in 2007, low-valent complexes featuring a Mg^I-Mg^I bond developed from trophy molecules to state-of-the-art reducing agents. Despite increasing interest in low-valency of the other group 2 metals, this area was restricted for a long time to a rare example of a Ca^I(arene)Ca^I inverse sandwich. This feature article focuses on the most recent developments in the field, highlighting recent breakthroughs for Be, Mg and Ca. The more exotic metal Be was the first to be isolated as a zero-valent complex which could be oxidized to a Be^I species. There also has been interest in breaking the Mg^I-Mg^I bond with superbulky β-diketiminate ligands (BDI) that suppress (BDI)Mg-Mg(BDI) bond formation. This led to Mg-Mg bond elongation or Mg-N bond cleavage. Several reports on attempts to isolate (BDI)Mg˙ radicals by combinations of ligand bulk, addition of neutral ligands or UV(vis) irradiation led to reduction of the aromatic solvents, underscoring the high reactivity of these open shell species. Only recently, zero-valent complexes of Mg were introduced. Double reduction of a (BDI)MgI complex with Na gave [(BDI)Mg^-]Na⁺. This Mg⁰ complex crystallized as a dimer in which the Na⁺ cations bridge the two (BDI)Mg^- anions which react as Mg nucleophiles. Thermal decomposition led to spontaneous formation of Na⁰ and a trinuclear (BDI)MgMgMg(BDI) complex. This mixed-valence Mg₃-complex is a prime example of the fleeting multinuclear Mg_n intermediates discussed on the way from Mg metal to Grignard reagent. Attempts to prepare low-valent Ca^I compounds by reduction of (BDI)CaI led to dearomatization of the arene solvents: (BDI)Ca(arene)Ca(BDI). Reduction in alkanes prevented this decomposition pathway but led to N₂ reduction and isolation of (BDI)Ca(N₂)Ca(BDI), representing the first example of molecular nitrogen fixation with an early main group metal. As the N₂^2- anion reacts in most cases as a very strong two-electron reductant, LCa(N₂)CaL could be seen as a synthon for hitherto elusive Ca^I-Ca^I complexes. Theoretical calculations suggest that participation of Ca d-orbitals is relevant for N₂ activation. These most recent developments in low-valent group 2 metal chemistry will revive this area and undoubtly lead to new reactivities and applications.

A Neutral Beryllium(I) Radical

The reduction of a cyclic alkyl(amino)carbene (CAAC)-stabilized organoberyllium chloride yields the first neutral beryllium radical, which was characterized by EPR, IR, and UV/Vis spectroscopy, X-ray crystallography, and DFT calculations.