N-Heterocyclic carbene, carbodiphosphorane and diphosphine adducts of beryllium dihalides: synthesis, characterisation and reduction studies

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.

Enforcing Metal-Arene Interactions in Bulky p-Terphenyl Bis(anilide) Complexes of Group 2 Metals (Be-Ba): Potential Precursors for Low-Oxidation-State Alkaline Earth Metal Systems

An extremely bulky p-terphenyl bis(aniline), p-C6H4{C6H4[N(H)TCHP]-2}2 (TCHP = 2,4,6-tricyclohexylphenyl) TCHPTerphH2, has been developed. Deprotonation of a less bulky analogue, DipTerphH2 (Dip = 2,6-diisopropylphenyl), with BePh2 affords the bimetallic system, [(BePh)2(μ-DipTerph)] 1. Treating either TCHPTerphH2 or DipTerphH2 with Mg{CH2(SiMe3)}2 gives the monomeric bis(anilide) complexes [Mg(ArTerph)] (Ar = Dip 2, TCHP 3) which display rare examples of η6-arene coordination to the metal center. Treating 2 with THF leads to partial dissociation of the Mg···arene interaction and formation of [Mg(DipTerph)(THF)] 4. Reactions of the bis(aniline)s with the group 2 metal amides [M{N(SiMe3)2}2] afford dimeric, structurally analogous compounds [{M(ArTerph)}2] (Ar = Dip, M = Ca 5, Sr 6, Ba 7; Ar = TCHP, M = Ca 8, Sr 9, Ba 10) which display intermolecular M···arene interactions in the solid state. Computational studies have shown that the intramolecular M···η6-arene interactions in models of the ether-free metal bis(anilide) compounds are largely electrostatic in nature. Reductions of these compounds with alkali metals led to mixtures of unidentified products.

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.

Reactivity of free N-heterocyclic carbenes in dichloromethane

The reactivity of free 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene (MeIPr) and 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene (IDipp) in dichloromethane was investigated. MeIPr reacts relatively slowly and selectively with the solvent under the formation of imidazolium salt [MeIPrH]Cl, which was characterised through NMR and IR spectroscopy as well as single crystal X-ray diffraction. Through deuterium labelling experiments the reaction rate was determined. In contrast IDipp reacts unselectively to various imidazolium salts. Due to the slow decomposition rates of MeIPr and IDipp in CH2Cl2, reactions of the free carbenes with BeBr2 to [(MeIPr)BeBr2], [(MeIPr)2BeBr2] and [(IDipp)BeBr2] can be performed.

Low-Coordinate Magnesium Sulfide and Selenide Complexes

The reactions of [{(iPrDipNacNac)Mg}2] 1 (iPrDipnacnac = HC(iPrCNDip)2) with Ph3P═O at 100 °C afforded the phosphinate complex [(iPrDipNacNac)Mg(OPPh3)(OPPh2)] 3. Reactions of 1 with Ph3P═E (E = S, Se) proceeded rapidly at room temperature to low-coordinate chalcogenide complexes [{(iPrDipNacNac)Mg}2(μ-S)] 4 and [{(iPrDipNacNac)Mg}2(μ-Se)] 5, respectively. Similarly, reactions of RNHC═S ((MeCNR)2C═S with R = Me, Et, or iPr) with 1 afforded NHC adducts of magnesium sulfide complexes, [{(iPrDipNacNac)Mg(RNHC)}(μ-S){Mg(iPrDipNacNac)}] 6, that could alternatively be obtained by adding the appropriate RNHC to sulfide complex 4. Complex 4 reacted with 1-adamantylazide (AdN3) to give [{(iPrDipNacNac)Mg}2(μ-SN3Ad)] 7 and can form various simple donor adducts in solution, of which [(iPrDipNacNac)Mg(OAd)}2(μ-S)] 8a (OAd = 2-adamantanone) was structurally characterized. The nature of the ionic Mg-E-Mg unit is described by solution and solid-state studies of the complexes and by DFT computational investigations.

Beryllium compounds for the carbon-halogen bond activation of phenyl halides: the role of non-innocent ligands

Beryllium is a metallomimetic main-group element, i.e., it behaves similarly to transition metals (TMs) in some bond activation processes. To investigate the ability of Be compounds to activate C-X bonds (X = F-I), we have computationally investigated, using DFT methods, the reaction of (CAAC)2Be (CAAC = 1-(2,6-diisopropylphenyl)-3,3,5,5-tetramethylpyrrolidin-2-ylidene) and a series of five-membered heterocyclic beryllium bidentate ligands with phenyl halides. We have analysed all plausible reaction mechanisms and our results show that, after the initial C-X oxidative addition, migration of the phenyl group occurs towards the less electronegative heteroatom. Our theoretical study highlights the important role of bidentate non-innocent ligands in providing the required electrons for the initial Ph-X oxidative addition. In contrast, the monodentate ligand, CAAC, does not favour this oxidative addition.

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.

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.

Reactivity of Diphenylberyllium as a Brønsted Base and Its Synthetic Application

Diphenylberyllium [Be₃Ph₆] is shown here to react cleanly as a Brønsted base with a vast variety of protic compounds. Through the addition of the simple molecules tBuOH, HNPh₂ and HPPh₂, as well as the more complex 1,3‐bis‐(2,6‐diisopropylphenyl)imidazolinium chloride, one or two phenyl groups in diphenylberyllium were protonated. As a result, the long‐postulated structures of [Be₃(OtBu)₆] and [Be(μ‐NPh₂)Ph]₂ have finally been verified and shown to be static in solution. Additionally [Be(μ‐PPh₂)(HPPh₂)Ph]₂ was generated, which is only the second beryllium‐phospanide to be prepared; the stark differences between its behaviour and that of the analogous amide were also examined. The first crystalline example of a beryllium Grignard reagent with a non‐bulky aryl group has also been prepared; it is stabilised with an N‐heterocyclic carbene.

Hydrolysis and oxidation products of phosphine adducts to beryllium chloride

The synthesis of bis(diphenylphosphino)ethane (dppe) and PMe3 mono-adducts [(dppe)BeCl2]n and [(PMe3)BeCl2]2 is described and their spectroscopic properties discussed. Hydrolysis of these two compounds and of the bis(diphenylphosphino)propane (dppp) adduct to BeCl2 gave [dppeH2][BeCl4], [Me3PH]n[Be4Cl9]n and [dpppH2][Be2Cl6], which have been isolated and structurally characterized by single crystal X-ray diffraction. The reactions of [(PMe3)BeCl2]2 with p-cresole gave [Me3PH]2[Be2Cl4(OC7H7)2]. This phenoxide together with [(Me3PO)2Be2Cl4], the oxidation product of [(PMe3)BeCl2]2, have also been structurally characterized.

Accessing the main-group metal formyl scaffold through CO-activation in beryllium hydride complexes

Carbon monoxide (CO) is an indispensable C1 building block. For decades this abundant gas has been employed in hydroformylation and Pausen-Khand catalysis, amongst many related chemistries, where a single, non-coupled CO fragment is delivered to an organic molecule. Despite this, organometallic species which react with CO to yield C1 products remain rare, and are elusive for main group metal complexes. Here, we describe a range of amido-beryllium hydride complexes, and demonstrate their reactivity towards CO, in its mono-insertion into the Be-H bonds of these species. The small radius of the Be²⁺ ion in conjunction with the non-innocent pendant phosphine moiety of the developed ligands leads to a unique beryllium formyl complex with an ylidic P-^COC fragment, whereby the carbon centre, remarkably, datively binds Be. This, alongside reactivity toward carbon dioxide, sheds light on the insertion chemistry of the Be-H bond, complimenting the long-known chemistry of the heavier Alkaline Earth hydrides.

Stoichiometric carbon monoxide insertion processes leading to metal-formyl complexes are scarce, even for transition metals. Here, light is shed on the underexplored chemistry of beryllium hydrides leading to a stable example of a main group metal-formyl complex.

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.

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.