Multinuclear beryllium amide and imide complexes: structure, properties and bonding

The beryllium amide and imide complexes [Be(HNMes)2]3, [(py)2Be(HNMes)2], [Be(HNDipp)2]2, [Be(NPh2)(μ2-HNDipp)]2 and [Be(NCPh2)2]3 have been prepared and characterised with NMR and IR spectroscopy as well as single crystal X-ray diffraction. Analysis of the localised molecular orbitals (LMOs) and intrinsic atomic orbital (IAO) atomic charges in the framework of the intrinsic bond orbital (IBO) localization method revealed a covalent bonding network consisting of 2-electron-2-centre and 2-electron-3-centre σ bonds, in which one electron pair of the anionic N-donor ligands is involved. The electron deficiency at the beryllium atoms is partially compensated through additional electron donation from the lone pair at the nitrogen atoms.

Multinuclear Beryllium Chloro Carboxylates

Reaction of 1 equiv of BeCl2 with mesityl (Mes) or o-tolyl (o-Tol) carboxylic acid in benzene gives hexanuclear heterocyles [BeCl(MesCO2)]6 and [BeCl(o-TolCO2)]6, respectively. Small amounts of the oxocarboxylates [Be4O(MesCO2)6] and [Be4O(o-TolCO2)6] are also formed. If chloroform is used as the solvent, a mixture of these complexes together with the unprecedented tertranuclear cage compounds [Be4Cl2(MesCO2)6] and [Be4Cl2(o-TolCO2)6] is obtained.

Formation, Structure and Reactivity of a Beryllium(0) Complex with Mgδ+-Beδ- Bond Polarization

Attempts to create a novel Mg-Be bond by reaction of [(DIPePBDI*)MgNa]2 with Be[N(SiMe3)2]2 failed; DIPePBDI*=HC[(tBu)C=N(DIPeP)]2, DIPeP=2,6-Et2C-phenyl. Even at elevated temperatures, no conversion was observed. This is likely caused by strong steric shielding of the Be center. A similar reaction with the more open Cp*BeCl gave in quantitative yield (DIPePBDI*)MgBeCp* (1). The crystal structure shows a Mg-Be bond of 2.469(4) Å. Homolytic cleavage of the Mg-Be bond requires ΔH=69.6 kcal mol-1 (cf. CpBe-BeCp 69.0 kcal mol-1 and (DIPPBDI)Mg-Mg(DIPPBDI) 55.8 kcal mol-1). Natural-Population-Analysis (NPA) shows fragment charges: (DIPePBDI*)Mg +0.27/BeCp* -0.27. The very low NPA charge on Be (+0.62) compared to Mg (+1.21) and the strongly upfield 9Be NMR signal at -23.7 ppm are in line with considerable electron density on Be and the formal oxidation state assignment of MgII-Be0. Despite this Mgδ+-Beδ- polarity, 1 is extremely thermally stable and unreactive towards H2, CO, N2, cyclohexene and carbodiimide. It reacted with benzophenone, azobenzene, phenyl acetylene, CO2 and CS2. Reaction with 1-adamantyl azide led to reductive coupling and formation of an N6-chain. The azide reagent also inserted in the Cp*-Be bond. The inertness of 1 is likely due to bulky ligands protecting the Mg-Be unit.

Identification of Global Minimum of HNBeCO Complex

Multiple bonding has always excited chemists. Recently, triple bonding between beryllium and N atoms in the HNBeCO complex has been reported based on experimental infrared spectroscopy and theoretical calculations. However, the present work reports a different structure based on a detailed potential energy surface scan. The global minimum geometry features only a weak partial Be-N double bond. The global minimum geometry lies very deep in the potential energy surface with respect to the reported one. Isomerization kinetics reveals that the reported structure has to overcome a very small barrier (5.4 kcal/mol) to isomerize to the global one. Although the previously reported structure is a real minimum, the present study identifies a much lower energy structure. A re-examination of the experimental spectra might show that the global minimum has also been observed.

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.

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.

Triple bonding between beryllium and nitrogen in HNBeCO

The HNBeCO complex is generated via the reaction of a beryllium atom with a HNCO molecule in a solid neon matrix, which is identified via infrared absorption spectroscopy with isotopic substitutions. The complex is characterized to have a linear structure with a very short Be-N bond distance. Bonding analyses indicate that the complex involves an unprecedented HNBeCO triple bond consisting of two degenerate electron-sharing π bonds and a dative σ bond with the π bonds being much stronger than the σ bond.

Base-stabilized formally zero-valent mono and diatomic molecular main-group compounds

Various compounds are known for transition metals in their formal zero-oxidation state, while similar compounds of main-group elements are recently realized and limited to only a few examples. Lewis-base-stabilized mono and diatomic molecular species (B₂, C, C₂, Si, Si₂, Ge, Ge₂, Sn, P₂, As₂, Sb₂) represent groundbreaking examples of main-group compounds with formally zero-oxidation state. In recent years, the isolation of low-valent main-group compounds has attracted increasing attention of both experimental and theoretical chemists. This is not only due to their fascinating electronic structures and exceptional reactivities, but also their use as valuable precursors for the synthesis of exotic yet important chemical species. This has led to a better understanding of the intricate balance of the donor-acceptor properties of the ligand(s) used to stabilize elements in a formally zero-oxidation state. Owing to the unusual oxidation state of the central element, many compounds containing formally zero-valent elements can efficiently activate otherwise inert small molecules. This review describes the synthesis, characterization, and reactivity of reported mono and diatomic formal zero-oxidation state main-group compounds. This review also emphasizes the comparative description of systems where different ligands are used to stabilize an element in its formal zero-oxidation state.

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.

Be-Be π bonding and predicted superconductivity in MBe2 (M=Zr, Hf)

Beryllium, an s-block element, forms an aromatic network of delocalized Be-Be π bonds in alloys ZrBe2 and HfBe2. This gives rise to a structure that fits description as stacked [Be2]4- layers with tetravalent cations in between. The [Be2]4- sublattice is isoelectronic and isostructural to graphite, as well as the [B]-2 sublattice in MgB2, and it bears identical manifestations of π bonding in its electronic band structure. These come in the form of degeneracies at K and H in the Brillouin zone, separated in energy as the result of interlayer orbital interactions. Zr and Hf use their valence d orbitals to form bonds with the layers, leading to nearly identical band structures. Like MgB2, ZrBe2 and HfBe2 are computed to be phonon-mediated superconductors at ambient pressures, with respective critical temperatures of 11.4 K and 8.8 K. The coupling strength between phonons and free electrons is very similar, so that the difference in critical temperatures is controlled by the mass of constituent interlayer ions.

s-Block Multiple Bonds: Isolation of a Beryllium Imido Complex

A common feature of d- and p-block elements is that they participate in multiple bonding. In contrast, the synthesis of compounds containing homo- or hetero-nuclear multiple bonds involving s-block elements is extremely rare. Herein, we report the synthesis, molecular structure, and computational analysis of a beryllium imido (Be=N) complex (2), which was prepared via oxidation of a molecular Be⁰ precursor (1) with trimethylsilyl azide Me₃ SiN₃ (TMS-N₃ ). Notably, compound 2 features the shortest known Be=N bond (1.464 Å) to date. This represents the first compound with an s-block metal-nitrogen multiple bond. All compounds were characterized experimentally with multi-nuclear NMR spectroscopy (¹ H, ¹³ C, ⁹ Be) and single-crystal X-ray diffraction studies. The bonding situation was analyzed with density functional theory (DFT) calculations, which supports the existence of π-bonding between beryllium and nitrogen.

Hungry for charge – how a beryllium scorpionate complex “eats” a weakly coordinating anion

We present the reaction of a tris(pyrazolyl) beryllium scorpionate (TpBe) complex with a weakly coordinating anion (WCA), which yields the heteroleptic complex TpBeOC(CF3)3 1 (TpBeOR F). The product 1 has been characterized by multinuclear NMR spectroscopy (1H, 9Be, 13C) and single-crystal X-ray diffraction (scXRD). Quantum chemical calculations (DFT, NPA, LOL) were performed to study the bonding nature in 1.

9Be nuclear magnetic resonance spectroscopy trends in discrete complexes: an update

9Be solution NMR spectroscopy is a useful tool for the characterisation of beryllium complexes. An updated comprehensive table of the 9Be NMR chemical shifts of beryllium complexes in solution is presented. The recent additions span a greater range of chemical shifts than those previously reported, and more overlap is observed between the chemical shift regions of four-coordinate complexes and those with lower coordination numbers. Four-coordinate beryllium species have smaller ω 1/2 values than the two- and three-coordinate species due to their higher order symmetry. In contrast to previous studies, no clear relationship is observed between chemical shift and the size and number of chelate rings.

Quantum information-based analysis of electron-deficient bonds

Recently, the correlation theory of the chemical bond was developed, which applies concepts of quantum information theory for the characterization of chemical bonds, based on the multiorbital correlations within the molecule. Here, for the first time, we extend the use of this mathematical toolbox for the description of electron-deficient bonds. We start by verifying the theory on the textbook example of a molecule with three-center two-electron bonds, namely, diborane(6). We then show that the correlation theory of the chemical bond is able to properly describe the bonding situation in more exotic molecules which have been synthesized and characterized only recently, in particular, the diborane molecule with four hydrogen atoms [diborane(4)] and a neutral zerovalent s-block beryllium complex, whose surprising stability was attributed to a strong three-center two-electron π bond stretching across the C-Be-C core. Our approach is of high importance especially in the light of a constant chase after novel compounds with extraordinary properties where the bonding is expected to be unusual.

RJ. s-Block carbodicarbene chemistry: C(sp3)–H activation and cyclization mediated by a beryllium center

The first examples of carbodicarbene (CDC)-s-block complexes have been synthesized. Unusual C–H bond activation and cyclization discovered.

Bis(trimethylsilyl)amide complexes of s-block metals with bidentate ether and amine ligands

The synthesis of the bis(trimethylsilyl)amide complexes of alkali and alkaline-earth metals with bidentate ether and amine bases 1,2-bis(dimethylamino)ethane (tmeda), dimethyl-methoxyethylamine (dmmea), and 1,2-dimethoxyethane (dme) succeedsviaaddition of these bases to coligand-free complexes orvialigand exchange of thf adducts.

Salt metathesis routes to homoleptic near-linear Mg(ii) and Ca(ii) bulky bis(silyl)amide complexes

A series of bulky bis(silyl)amide ligands have been utilised to stabilise rare examples of formally 2-coordinate linear or near-linear Mg and Ca complexes.

Off the Beaten Track-A Hitchhiker's Guide to Beryllium Chemistry

This Minireview aims to give an introduction to beryllium chemistry for all less-experienced scientists in this field of research. Up to date information on the toxicity of beryllium and its compounds are reviewed and several basic and necessary guidelines for a safe and proper handling in modern chemical research laboratories are presented. Interesting phenomenological observations are described that are related directly to the uniqueness of this element, which are also put into historical context. Herein we combine the contributions and experiences of many scientist that work passionately in this field. We want to encourage fellow scientists to reconcile the long-standing reservations about beryllium and its compounds and motivate intense research on this spurned element. Who on earth should be able to deal with beryllium and its compounds if not chemists?

Homopolar dihydrogen bonding in ligand stabilized diberyllium hydride complexes, Be2(CH3)2H2L2 (L = H(-), CO, N-heterocyclic carbene and CN(-))

The structure and bonding of diberyllium hydride complexes Be2(CH3)2H2L2, where L = H(-) (), CO (), NHC () and CN(-) () were studied in detail and compared with isostructural diborane (). The bonding in Be2(CH3)2H2L2 was analyzed considering the interaction of two Be(CH3)L fragments with the bridging (μ-H)2 fragment. The geometrical and molecular orbital (MO) analyses indicate that the extent of back donation from Be→L (L = CO, NHC and CN(-)) is less in Be2(CH3)2H2L2 as compared to that in Be(CH3)L fragments. The NBO, MO and QTAIM analyses indicate a significant charge concentration at the bridging (μ-H)2 in Be2(CH3)2H2L2, whereas charge depletion is observed at the (μ-H)2 fragment in diborane. The accumulation of electron density at the bridging (μ-H)2 is duly reflected in the detection of a (μ-H)-(μ-H) bond path in Be2(CH3)2H2L2, in spite of having longer (μ-H)-(μ-H) distances. The interaction between the bridging H-atoms in diberyllium hydride complexes can be considered as homopolar hydride-hydride dihydrogen bonding. The direction of the charge flow is from (μ-H)2 to BH2 in diborane, whereas from Be(CH3)L to (μ-H)2 in Be2(CH3)2H2L2. Hence, the bridging (μ-H)2 fragments in Be2(CH3)2H2L2 are electron rich as compared to the (μ-H)2 fragment in diborane. The experimental report of the ring opening of the (Ar)NHC (Ar = 2,4,6-trimethylphenyl) ligand through hydride transfer from [Be(CH3)H((Ar)NHC)]2 corroborates well with the presence of electron rich bridging (μ-H)2 in diberyllium hydride complexes.

Computational Predictions of the Beryllium Analogue of Borole, Cp(+), and the Fluorenyl Cation: Highly Stabilized, non-Lewis Acidic Antiaromatic Ring Systems

A computational study of a set of synthetically unknown beryllium-containing rings, anionic analogues of antiaromatic boroles, has been carried out to investigate their structure, stability, and potential reactivity. The results indicate that these compounds should be electronically viable (as assessed from HOMO-LUMO and singlet-triplet gaps) and therefore potential targets for synthesis. In strong contrast with boroles, these beryllium species are predicted to be not Lewis acidic but rather Lewis basic, with reactivity centered on the endocyclic Be-C bond.