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.

Synthesis and Derivatives of Beryllium Triflate.

Various pathways for the synthesis of beryllium triflate were investigated. The reaction of triflic acid or trimethylsilyl triflate with beryllium metal in liquid ammonia led to the formation of mono-, di-, and tetra-nuclear beryllium ammine complexes. Utilization of SMe2 as a solvent gave homoleptic Be(OTf)2. Various beryllium triflate complexes with N- and O-donor ligands as well as the complex anions [Be(OTf)4]2- and [Be2(OTf)6]2- were synthesized to evaluate the reactivity and solution properties of beryllium triflate. This showed that OTf- is not a weakly coordinating anion for Be2+ cations and that it exhibits good bridging properties.

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.

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.

Bonding situations in tricoordinated beryllium phenyl complexes.

The bonding situation in the tricoordinated beryllium phenyl complexes [BePh3 ]- , [(pyridine)BePh2 ] and [(trimethylsilyl-N-heterocyclic imine)BePh2 ] is investigated experimentally and computationally. Comparison of the NMR spectroscopic properties of these complexes and of their structural parameters, which were determined by single crystal X-ray diffraction experiments, indicates the presence of π-interactions. Topology analysis of the electron density reveals elliptical electron density distributions at the bond critical points and the double bond character of the beryllium-element bonds is verified by energy decomposition analysis with the combination of natural orbital for chemical valence. The present beryllium-element bonds are highly polarized and the ligands around the central atom have a strong influence on the degree of π-delocalization. These results are compared to related triarylboranes.

Beryllium-Mediated Halide and Aryl Transfer onto Silicon.

The reactivity of hexamethylcyclotrisiloxane (D3 ) towards BeCl2 , BeBr2 , BeI2 and [Be3 Ph6 ]3 was investigated. While BeCl2 only showed unselective reactivity, BeBr2 , BeI2 and [Be3 Ph6 ] cleanly react to the trinuclear complexes [Be3 Br2 (OSiMe2 Br)4 ], [Be3 I2 (OSiMe2 I)4 ] and [Be3 Ph2 (OSiMe2 Ph)4 ]. These unprecedented bromide, iodide and phenyl transfer reactions from a group II metal onto silicon offer a versatile access to previously unknown diorgano bromo and iodo silanolates.

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.

Plasmachemical Syntheses of RuF6, RhF6, and PtF6.

Starting from the respective metal M, we have synthesized the hexafluorides MF6 of M = Ru, Rh, and Pt by the use of a laser-based heating system and a remote fluorine plasma source using a mixture of Ar and NF3 as the feed gas. The formation of the hexafluorides was confirmed by several different spectroscopic methods, including IR, Raman, UV/vis, and NMR spectroscopy. In addition, we present first experimental hints that RuF6 is more reactive than PtF6, because RuF6 is able to oxidize lower fluorides of platinum to PtF6.

Bismuth in Dynamic Covalent Chemistry: Access to a Bowl-Type Macrocycle and a Barrel-Type Heptanuclear Complex Cation.

Dynamic covalent chemistry (DCvC) is a powerful and widely applied tool in modern synthetic chemistry, which is based on the reversible cleavage and formation of covalent bonds. One of the inherent strengths of this approach is the perspective to reversibly generate in an operationally simple approach novel structural motifs that are difficult or impossible to access with more traditional methods and require multiple bond cleaving and bond forming steps. To date, these fundamentally important synthetic and conceptual challenges in the context of DCvC have predominantly been tackled by exploiting compounds of lighter p-block elements, even though heavier p-block elements show low bond dissociation energies and appear to be ideally suited for this approach. Here we show that a dinuclear organometallic bismuth compound, containing BiMe2 groups that are connected by a thioxanthene linker, readily undergoes selective and reversible cleavage of its Bi-C bonds upon exposure to external stimuli. The exploitation of DCvC in the field of organometallic heavy p-block chemistry grants access to unprecedented macrocyclic and barrel-type oligonuclear compounds.

Reactivity of Diphenylberyllium as a Brønsted Base and Its Synthetic Application.

Diphenylberyllium [Be3 Ph6 ] 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.

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.

Ligand Influence on Structural and Spectroscopic Properties of Beryllium Oxocarboxylates.

Aluminum-based adjuvants for vaccines and beryllium ions interact with the same immune receptor. The Be4O core, which is also found in beryllium oxocarboxylates, has been proposed to be the binding species in the latter case. However, this is not proven due to a lack of suitable probes for the Be4O moiety. Therefore, a versatile synthetic route to beryllium oxocarboxylates has been developed to investigate the steric and electronic influence of the ligands onto their molecular and spectroscopic properties. The oxocarboxylates exhibit extremely narrow line widths in 9Be NMR spectroscopy, and the chemical shift is only influenced by the sterics of the ligands. The mean variation of the atomic distances in the central Be4O building block is extremely small over all investigated compounds, and even the C-C distances are only little perturbed by the properties of the ligands. Vibrational spectroscopy showed Be-O bands; however, further distinctions could not be drawn.

Diphenylberyllium Reinvestigated: Structure, Properties, and Reactivity of BePh2 , [(12-crown-4)BePh]+ , and [BePh3 ].

The first synthesis of BePh2 was accomplished almost a century ago. However, its structure has remained unknown so far, while the corresponding aryls of the elements adjacent to beryllium in the periodic table are well investigated. Herein, we present an improved synthesis for diphenylberyllium and show by X-ray diffraction that it forms a trinuclear complex in the solid state. NMR spectroscopy revealed that this structure is also retained in solution but exhibits dynamic behavior. Its stability against heat and coordinating solvents is discussed and the possible obstacles to the synthesis of BePh2 from BeCl2 are examined. In the process of this study two ether adducts, BePh2 ⋅Et2 O and Be2 Ph4 ⋅Et2 O, have been characterized as well as the previously unknown triphenylberyllate anion. From the latter several single-crystal structures were obtained under various conditions, in which [BePh3 ]- is either isolated or acts as a ligand for Li+ . Furthermore, the crown ether induced selfionization of BePh2 is described and the resulting [(12-crown-4)BePh]+ cation was isolated, which shows an unusual 4+1 coordination around the Be atom.

A Second Modification of Beryllium Bromide: ß-BeBr2.

The synthesis of a second beryllium bromide modification, ß-BeBr2, was accomplished through recrystallization of α-BeBr2 from benzene in the presence of cyclo-decamethylpentasiloxane. This phase was analyzed via single-crystal X-ray diffraction and IR and Raman spectroscopy as well as density functional theory calculations. This enabled a comparison to α-BeBr2 and the α and ß phases of beryllium chloride and iodide.

Recent Contributions to the Coordination Chemistry of Beryllium.

Given the alleged extreme toxicity of beryllium and its compounds, it is the least investigated nonradioactive element. However, beryllium exhibits unique properties among the elements and therefore the related coordination chemistry is unprecedented. With the emergence of s-block catalysis and the introduction of low-valent magnesium complexes as versatile reducing agents, the interest in related beryllium compounds also has increased. Therefore, some quite remarkable progress on the coordination and organometallic chemistry of beryllium has been made in the last two decades. This Minireview article gives an oversight over the relatively recent developments in this field of beryllium research. An emphasis is placed on the correlation between the structure and reactivity of the described compounds.

Understanding the Localization of Berylliosis: Interaction of Be2+ with Carbohydrates and Related Biomimetic Ligands.

The interplay of metal ions with polysaccharides is important for the immune recognition in the lung. Due to the localization of beryllium associated diseases to the lung, it is likely that beryllium carbohydrate complexes play a vital role for the development of berylliosis. Herein, we present a detailed study on the interaction of Be2+ ions with fructose and glucose as well as simpler biomimetic ligands, which emulate binding motives of saccharides. Through NMR and IR spectroscopy as well as single-crystal X-ray diffraction, complemented by competition reactions we were able to determine a distinctive trend in the binding affinity of these ligands. This suggests that under physiological conditions beryllium ions are only bound irreversibly in glycoproteins or polysaccharides if a quasi ideal tetrahedral environment and κ4 -coordination is provided by the respective biomolecule. Furthermore, Lewis acid induced conversions of the ligands and an extreme increase in the Brønstedt acidity of the present OH-groups imply that upon enclosure of Be2+ , alterations may be induced by the metal ion in glycoproteins or polysaccharides. In addition the frequent formation of Be-O-heterocycles indicates that multinuclear beryllium compounds might be the actual trigger of berylliosis. This investigation on beryllium coordination chemistry was supplemented by binding studies of selected biomimetic ligands with Al3+ , Zn2+ , Mg2+ , and Li+ , which revealed that none of these beryllium related ions was tetrahedrally coordinated under the give conditions. Therefore, studies on the metabolization of beryllium compounds cannot be performed with other hard cations as a substitute for the hazardous Be2+ .

Understanding the Localization of Berylliosis: Interaction of Be2+ with Carbohydrates and Related Biomimetic Ligands.

Invited for the cover of this issue is the group of Magnus R. Buchner at Philipps-Universität Marburg. The image depicts a piece of beryllium decomposing sugar cubes. Read the full text of the article at 10.1002/chem.201903439.

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 ß-H can form stable beryllium complexes, whereas other aldehydes are oligo- and polymerized or decomposed by beryllium halides.

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.

Electrospray Ionization Mass Spectrometric Study of the Gas-Phase Coordination Chemistry of Be2+ Ions with 1,2- and 1,3-Diketone Ligands.

Electrospray ionization mass spectrometry (ESI MS) is a powerful technique for the study of coordination complexes because of its ability to analyze solution systems involving very low concentrations of metal complexes. In this work, the coordination chemistry of Be ions with a selection of well-known 1,3-diketone and related 1,2-diketone ligands has been investigated using ESI MS. With acetylacetone (Hacac), a range of acac-containing ions is observed, including [Be(acac)2H]+, [Be(acac)(MeOH) n]+ ( n = 1, 2), and polynuclear species such as the dinuclear [Be2(acac)3]+ and trinuclear [Be3O(acac)3]+. Density functional theory calculations indicate that the latter species has a central Be3(µ3-O) core, with each Be chelated (as opposed to being bridged) by an acac ligand. The effect of changing the substituents on 1,3-diketone was explored by an investigation of mixtures of Be2+ with other 1,3-diketones such as dibenzoylmethane (Hdbm), where the [Be(dbm)2H]+ ion showed a lesser tendency to undergo fragmentation and aggregation processes. Comparisons with the corresponding aluminum acetylacetone system were also made. In contrast, mixtures of Be2+ and the 1,2-diketones diacetyl and phenanthrenequinone showed poor metal-ligand interactions. Be2+ interacted with the 1,2-diketone benzil [PhC(O)C(O)Ph], forming the [Be(benzil) n]2+ ( n = 2-4) ions. The synthesis (from BeCl2) and X-ray structures of the dibenzoylmethanato (dbm) complex Be(dbm)2 and the benzil complex [BeCl2(benzil)] are also reported.