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.

Behavior of beryllium halides and triflate in acetonitrile solutions

The solution behavior of beryllium halides and triflate in acetonitrile was studied by NMR, IR and Raman spectroscopy. Thereby mononuclear units [(MeCN)2BeX 2] (X = Cl, Br, I, OTf) were identified as dominant species in these solutions. The solid state structure of [(MeCN)2Be(OTf)2] has been determined by X-ray diffraction. If only one equivalent of MeCN is used the dinuclear compounds [(MeCN)BeX 2]2 (X = Cl, Br, I) are formed. Partial halide and triflate dissociation into the monomeric complexes as well as the formation of hetero-halide complexes [(MeCN)2BeClBr], [(MeCN)2BeClI] and [(MeCN)2BeBrI] was observed.

Speciation of Be2+ in acidic liquid ammonia and formation of tetra- and octanuclear beryllium amido clusters

The hexa-μ₂-amido-tetraammine-tetraberyllium compounds [Be₄(NH₂)₆(NH₃)₄]X₂ (X = Cl, Br, I, CN, SCN, N₃) have been prepared from beryllium metal and NH₄X or [Be(NH₃)₄]X₂ in liquid ammonia at ambient temperature. The obtained compounds feature an adamantyl shaped complex cation, which was examined via X-ray diffraction, IR, Raman and NMR spectroscopy. The speciation in solution was studied via¹⁵N labeling experiments supplemented with quantum chemistry. Hereby, the intermediates [Be₂(NH₂)(NH₃)₆]³⁺ and [Be₃(NH₂)₃(NH₃)₆]³⁺ were identified. Reactivity studies provided bis(N-acetimidoylacetamidinato-N,N′)-beryllium(ii), when [Be₄(NH₂)₆(NH₃)₄]²⁺ was treated with acetonitrile. While the unprecedented octa-nuclear complex cation [Be₈O(NH₂)₁₂(C₅H₅N)₄]²⁺ was received from pyridine. This cluster proves that the [Be₄O]⁶⁺ core can be stabilized without bidentate O-donor ligands.

The boundaries of beryllium metal oxidation in acidic ammonia have been explored. This enabled the isolation of the tetra- and octa-nuclear beryllium amide complexes. The latter exhibits a completely new structural motive in coordination chemistry.

Probing the electronic boundaries between trigonal and tetrahedral coordination at beryllium

The influence of the beryllium atom's partial charge on its coordination sphere was investigated experimentally and computationally on NEt₃ adducts to BeCl₂, BeBr₂ and BrI₂.

Beryllium coordination chemistry and its implications on the understanding of metal induced immune responses

The coordination chemistry of beryllium with ligands containing biologically relevant functional groups is discussed. The geometry, speciation and reactivity of these compounds, aids a better understanding of metal ion induced immune reactions.

Solution Behavior of Beryllium Halides in Dimethylformamide

The reactivity of beryllium compounds in N,N-dimethylformamide (DMF) is fairly uncharted. However, as a versatile O-donor solvent, DMF enables reaction conditions that are inaccessible in solvents more commonly applied in beryllium chemistry. Here we present a comprehensive study on the interaction of beryllium halides BeX₂ (X = F, Cl, Br, I) with DMF. Through NMR and IR spectroscopy as well as single-crystal X-ray diffraction, we were able to characterize several Be(DMF)_1-4X₂ species and distinguish the competitiveness of the investigated halides against DMF. On the basis of titration experiments, [BeCl(DMF)₃]⁺ was identified as the dominant compound when BeCl₂ was dissolved in DMF. The unpredicted high beryllium-halide bond strength in chloro complexes as well as the instability of iodo compounds is discussed. The latter showed a distinctively different coordination chemistry compared to the other beryllium halides. In addition to that, the first example of a compound with the hexaiodidodiberyllate anion ([Be₂I₆]^2-) was characterized. On the basis of our results, BeBr₂ is a superior starting material compared to the other beryllium halides for the synthesis of coordination chemistry compounds.

Beryllium-Induced Conversion of Aldehydes

Aldehydes play a key role in the human metabolism. Therefore, it is essential to know their reactivity with beryllium compounds in order to assess its effects in the body. The reactivity of simple aldehydes towards beryllium halides (F, Cl, Br, I) was studied through solution and solid‐state techniques and revealed distinctively different reactivities of the beryllium halides, with BeF₂ being the least and BeI₂ the most reactive. Rearrangement and aldol condensation reactions were observed and monitored by in situ NMR spectroscopy. Crystal structures of various compounds obtained by Be²⁺‐catalyzed cyclization, rearrangement, and aldol addition reactions or ligation of beryllium halides have been determined, including unprecedented one‐dimensional BeCl₂ chains and the first structurally characterized example of an 1‐iodo‐alkoxide. Long‐term studies showed that only aldehydes without a β‐H can form stable beryllium complexes, whereas other aldehydes are oligo‐ and polymerized or decomposed by beryllium halides.