Structure and Dynamics of Homoleptic Beryllocenes: A Solid-State9Be and13C NMR Study

Revisiting the origin of the bending in group 2 metallocenes AeCp2 (Ae = Be-Ba)

Metallocenes are well-established compounds in organometallic chemistry, and can exhibit either a coplanar structure or a bent structure according to the nature of the metal center (E) and the cyclopentadienyl ligands (Cp). Herein, we re-examine the chemical bonding to underline the origins of the geometry and stability observed experimentally. To this end, we have analysed a series of group 2 metallocenes [Ae(C₅R₅)₂] (Ae = Be-Ba and R = H, Me, F, Cl, Br, and I) with a combination of computational methods, namely energy decomposition analysis (EDA), polarizability model (PM), and dispersion interaction densities (DIDs). Although the metal-ligand bonding nature is mainly an electrostatic interaction (65-78%), the covalent character is not negligible (33-22%). Notably, the heavier the metal center, the stronger the d-orbital interaction with a 50% contribution to the total covalent interaction. The dispersion interaction between the Cp ligands counts only for 1% of the interaction. Despite that orbital contributions become stronger for heavier metals, they never represent the energy main term. Instead, given the electrostatic nature of the metallocene bonds, we propose a model based on polarizability, which faithfully predicts the bending angle. Although dispersion interactions have a fair contribution to strengthen the bending angle, the polarizability plays a major role.

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.

Stack by Stack: From the Free Cyclopentadienylgermanium Cation Via Heterobimetallic Main-Group Sandwiches to Main-Group Sandwich Coordination Polymers

Heterobimetallic cationic sandwich complexes [M(μ-Cp)M'Cp]+ of group 13 (M=Ga, In) and group 14 (M'=Ge, Sn) elements have been prepared as [WCA]- salts (WCA=Al(ORF )4 ; ORF =OC(CF3 )3 ). Their molecular structures include free apical gallium or indium atoms. The sandwich complexes were formed in the reactions of [M(HMB)]+ [WCA]- (HMB=C6 Me6 ) with the free metallocenes [M'Cp2 ]. Their structures are related to known stannocene and stannocenium salts; the unprecedented germanium analogues, namely the free germanocenium cation [GeCp]+ and the corresponding triple-decker complex cation [CpGe(μ-Cp)GeCp]+ , are described herein. By variation of the reaction conditions, these sandwich complexes can be transformed into the group 13/14 mixed cationic coordination polymer [{In(HMB)(μ-SnCp2 )}n ][WCA]n . This polymeric chain motif was also successfully replicated by the synthesis of complexes [{Ga/In(HMB)(μ-FeCp2 )}n ][WCA]n containing FeCp2 as a bridging ligand.

Diphenylberyllium Reinvestigated: Structure, Properties, and Reactivity of BePh2 , [(12-crown-4)BePh]+ , and [BePh3 ]. Chemistry 2020;26:9915-9922. [PMID: 31957173 PMCID: PMC7496417 DOI: 10.1002/chem.202000259]

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.

Solid-state Be-9 NMR of beryllium compounds

Despite the favorable NMR-spectroscopic features of the 9Be isotope, the exploration of solid Beryllium compounds by this method has been very limited, owing to safety concerns regarding their preparation and handling. The present review aims to be the first comprehensive one on this topic, summarizing the classical and modern methodologies available for determining the relevant 9Be NMR observables in the solid state. Results on molecular crystals, oxidic materials, and intermetallic systems will be discussed in terms of their informational content in relation to atomic and electronic structure and dynamics, leading to suggestions for future paths of investigation.

Recent advances in solid-state nuclear magnetic resonance spectroscopy of exotic nuclei

We present a review of recent advances in solid-state nuclear magnetic resonance (SSNMR) studies of exotic nuclei. Exotic nuclei may be spin-1/2 or quadrupolar, and typically have low gyromagnetic ratios, low natural abundances, large quadrupole moments (when I > 1/2), or some combination of these properties, generally resulting in low receptivities and/or prohibitively broad line widths. Some nuclides are little studied for other reasons, also rendering them somewhat exotic. We first discuss some of the recent progress in pulse sequences and hardware development which continues to enable researchers to study new kinds of materials as well as previously unfeasible nuclei. This is followed by a survey of applications to a wide range of exotic nuclei (including e.g., ⁹Be, ²⁵Mg, ³³S, ³⁹K, ⁴³Ca, ^47/49Ti, ⁵³Cr, ⁵⁹Co, ⁶¹Ni, ⁶⁷Zn, ⁷³Ge, ⁷⁵As, ⁸⁷Sr, ¹¹⁵In, ¹¹⁹Sn, ^121/123Sb, ^135/137Ba, ^185/187Re, ²⁰⁹Bi), most of them quadrupolar. The scope of the review is the past ten years, i.e., 2007-2017.

Linear dicoordinate beryllium: a 9Be solid-state NMR study of a discrete zero-valent s-block beryllium complex

Beryllium-9 (9Be) quadrupolar coupling and chemical shift tensor data are reported for bis(1-(2,6-diisopropylphenyl)-3,3,5,5-tetramethylpyrrolidine-2-ylidene)beryllium (Be(CAAC)2). These are the first such data for beryllium in a linear dicoordinate environment. The 9Be quadrupolar coupling constant, 2.36(0.02) MHz, is the largest recorded in the solid state to date for this isotope. The span of the beryllium chemical shift tensor, 22(2) ppm, covers about half of the known 9Be chemical shift range, and the isotropic 9Be chemical shift, 32.0(0.3) ppm, is the largest reported in the solid state to our knowledge. DFT calculations reproduce the experimental data well. A natural localized molecular orbital approach has been used to explain the origins and orientation of the beryllium electric field gradient tensor. The single-crystal X-ray structure of a second polymorph of Be(CAAC)2 is also reported. Inspection of the powder X-ray diffraction data shows that the new crystal structure is part of the bulk product next to another crystalline phase. Therefore, experimental X-ray powder data for the microcrystalline powder sample and the SSNMR data do not fully match either the originally reported crystal structure (Arrowsmith et al. Nat. Chem. 2016, 8, 890–894) or the new polymorph. The ability of solid-state NMR and powder X-ray diffraction to characterize powdered samples was thus particularly useful in this work.

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?

Low-temperature NMR studies of Zn tautomerism and hindered rotations in solid zincocene derivatives

Using a combination of NMR methods we have detected and studied fluxional motions in the slip-sandwich structure of solid decamethylzincocene (I, [(eta5-C5Me5)Zn(eta1-C5Me5)]). For comparison, we have also studied the solid iminoacyl derivative [(eta5-C5Me5)Zn(eta1-C(NXyl)C5Me5)] (II). The variable temperature 13C CPMAS NMR spectra of I indicate fast rotations of both Cp* rings in the molecule down to 156 K as well as the presence of an order-disorder phase transition around 210 K. The disorder is shown to be dynamic arising from a fast combined Zn tautomerism and eta1/eta5 reorganization of the Cp* rings between two degenerate states A and B related by a molecular inversion. In the ordered phase, the degeneracy of A and B is lifted; that is, the two rings X and Y are inequivalent, where X exhibits a larger fraction of time in the eta5 state than Y. However, the interconversion is still fast and characterized by a reaction enthalpy of DeltaH = 2.4 kJ mol-1 and a reaction entropy of DeltaS = 4.9 J K-1 mol-1. In order to obtain quantitative kinetic information, variable temperature 2H NMR experiments were performed on static samples of I-d6 and II-d6 between 300 and 100 K, where in each ring one CH3 is replaced by one CD3 group. For II-d6, the 2H NMR line shapes indicate fast CD3 group rotations and a fast "eta5 rotation", corresponding to 72 degrees rotational jumps of the eta5 coordinated Cp* ring. The latter motion becomes slow around 130 K. By line shape analysis, an activation energy of the eta5 rotation of about 21 kJ mol-1 was obtained. 2H NMR line shapes analysis of I-d6 indicates fast CD3 group rotations at all temperatures. Moreover, between 100 and 150 K, a transition from the slow to the fast exchange regime is observed for the 5-fold rotational jumps of both Cp* rings, exhibiting an activation energy of 18 kJ mol-1. This value was corroborated by 2H NMR relaxometry from which additionally the activation energies 6.3 kJ mol-1 and 11.2 kJ mol-1 for the CD3 rotation and the molecular inversion process were determined.

Solid-State 93Nb and 13C NMR Investigations of Half-Sandwich Niobium(I) and Niobium(V) Cyclopentadienyl Complexes

Solid-state 93Nb and 13C NMR experiments, in combination with theoretical calculations of NMR tensors, and single-crystal and powder X-ray diffraction experiments, are applied for the comprehensive characterization of structure and dynamics in a series of organometallic niobium complexes. Half-sandwich niobium metallocenes of the forms Cp'Nb(I)(CO)4 and CpNb(V)Cl4 are investigated, where Cp = C5H5- and Cp' = C5H4R- with R = COMe, CO2Me, CO2Et, and COCH2Ph. Anisotropic quadrupolar and chemical shielding (CS) parameters are extracted from 93Nb MAS and static NMR spectra for seven different complexes. It is demonstrated that 93Nb NMR parameters are sensitive to changes in temperature and Cp' ring substitution in the Cp'Nb(I)(CO)4 complexes. There are dramatic differences in the 93Nb quadrupolar coupling constants (C(Q)) between the Nb(I) and Nb(V) complexes, with C(Q) between 1.0 and 12.0 MHz for Cp'Nb(CO)4 and C(Q) = 54.5 MHz for CpNbCl4. The quadrupolar Carr-Purcell Meiboom-Gill (QCPMG) pulse sequence is applied to rapidly acquire, in a piecewise fashion, a high signal-to-noise ultra-wide-line 93Nb NMR spectrum of CpNbCl4, which has a breadth of ca. 400 kHz. Solid-state 93Nb and 13C NMR spectra and powder XRD data are used to identify a new metallocene adduct coordinated at the axial position of the metal site by a THF molecule: CpNb(V)Cl4.THF. 13C MAS and CP/MAS NMR experiments are used to assess the purity of samples, as well as for measuring carbon CS tensors and the rare instance of one-bond 93Nb, 13C J-coupling, 1J(93Nb,13C). Theoretically calculated CS and electric field gradient (EFG) tensors are utilized to determine relationships between tensor orientations, the principal components, and molecular structures.