The Zintl Phase Cs7NaSi8 – From NMR Signal Line Shape Analysis and Quantum Mechanical Calculations to Chemical Bonding

Powder samples as well as red and transparent single crystals of the Zintl phase Cs₇NaSi₈ were synthesized and characterized by means of X‐ray diffraction and differential thermal analysis. Cs₇NaSi₈ was found to be isotypic to the recently reported phase Rb₇NaSi₈. It crystallizes in the Rb₇NaGe₈ structure type forming trigonal pyramidal Si₄^4– anions. Two unique environments of the cations are observed, a linear arrangement [Na(Si₄)₂]^7– with short Na–Si distances of 3.0 Å and a Cs2 atom coordinated by six Si₄^4– anions with long Cs–Si distances of 4.2 Å. The bonding situation was investigated by a combined application of ²⁹Si, ²³Na, and ¹³³Cs solid‐state NMR spectroscopy and quantum mechanical calculations of the NMR coupling parameters. In addition the electronic density of states (DOS), the electron localizability indicator (ELI) and the atomic charges using the QTAIM approach were studied. Good agreement of the calculated and experimental values of the NMR coupling parameters was obtained. An anisotropic bonding situation of the silicon atoms is indicated by the chemical shift anisotropy being similar to Rb₇NaSi₈. Confirmation is given by the observation of one lone‐pair‐like feature for each silicon atom and two types of two‐center Si–Si bonds using the ELI. Calculation and NMR spectroscopic determination of the ²³Na and ¹³³Cs electric field gradients prove anisotropies of the charge distribution around the cations. Due to the similar values for the Na atoms in M₇NaSi₈ (M = Rb, Cs) equal bonding situations can be concluded. The much larger anisotropy of the charge distribution of the Cs atoms can be addressed as the main difference to Rb₇NaSi₈.     

Unravelling Local Atomic Order of the Anionic Sublattice in M(Al1−xGax)4 with M=Sr and Ba by Using NMR Spectroscopy and Quantum Mechanical Modelling

The quasibinary section of the intermetallic phases MAl₄ and MGa₄ with M=Sr and Ba have been characterised by means of X‐ray diffraction (XRD) studies and differential thermal analysis. The binary phases show complete miscibility and form solid solutions M(Al_1?xGa_x)₄ with M=Sr and Ba. These structures crystallise in the BaAl₄ structure type with four‐ and five‐bonded Al and/or Ga atoms (denoted as Al(4b), Al(5b), Ga(4b), and Ga(5b), respectively) that form a polyanionic Al/Ga sublattice. Solid state ²⁷Al NMR spectroscopic analysis and quantum mechanical (QM) calculations were applied to study the bonding of the Al centres and the influence of Al/Ga substitution, especially in the regimes with low degrees of substitution. M(Al_1?xGa_x)₄ with M=Sr and Ba and 0.925≤x≤0.975 can be described as a matrix of the binary majority compound in which a low amount of the Ga atoms has been substituted by Al atoms. In good agreement with the QM calculations, ²⁷Al NMR investigations and single crystal XRD studies prove a preferred occupancy of Al(4b) for these substitution regimes. Furthermore, two different local Al environments were found, namely isolated Al(4b1) atoms and Al(4b2), due to the formation of Al(4b)–Al(4b) pairs besides isolated Al(4b) atoms within the polyanionic sublattice. QM calculations of the electric field gradient (EFG) using superlattice structures under periodic boundary conditions are in good agreement with the NMR spectroscopic results.     

Electronic Structure,Chemical Bonding,and Solid‐State NMR Spectroscopy of the Digallides of Ca,Sr, and Ba

Delving into digallides : The characteristics of the chemical bonding of the digallides of the alkaline‐earth metals (see figure) have been studied by application of experimental methods, such as single‐crystal X‐ray diffraction and solid‐state NMR spectroscopy, in combination with quantum mechanical calculations.

     

The Solid Solution Sr1−xBaxGa2: Substitutional Disorder and Chemical Bonding Visited by NMR Spectroscopy and Quantum Mechanical Calculations

Complete miscibility of the intermetallic phases (IPs) SrGa₂ and BaGa₂ forming the solid solution Sr_1?xBa_xGa₂ is shown by means of X‐ray diffraction, thermoanalytical and metallographic studies. Regarding the distances of Sr/Ba sites versus substitution degree, a model of isolated substitution centres (ISC) for up to 10 % cation substitution is explored to study the influence on the Ga bonding situation. A combined application of NMR spectroscopy and quantum mechanical (QM) calculations proves the electric field gradient (EFG) to be a sensitive measure of different bonding situations. The experimental resolution is boosted by orientation‐dependent NMR on magnetically aligned powder samples, revealing in first approximation two different Ga species in the ISC regimes. EFG calculations using superlattice structures within periodic boundary conditions are in fair agreement with the NMR spectroscopy data and are discussed in detail regarding their application on disordered IPs.     

Metathetical Reactions in the System Na(K)PF6 – LiBF4 – Aprotic Media

¹⁹F NMR spectroscopy, X‐ray powder diffraction, elemental analysis, and ab initio quantum chemical calculations were used to study metathetical reactions between potassium or sodium hexafluorophosphate and lithium tetrafluoroborate in a mixture of propylene carbonate (PC) – dimethyl carbonate (DMC). It was shown that the increase in size of the cations in the second coordination sphere from Na⁺ to K⁺ results in an increase of the equilibrium conversion. This is in agreement with the influence of the cation size on the solubility of tetrafluoroborates in the media investigated.     

Vibrational Spectra of Cluster Anions. 2. Vibrational Spectra of Compounds with the Cluster Anions [E4]4−: M4E4 (M = K,Rb, Cs; E = Ge,Sn) and β‐Na4Sn4

Vibrational spectra of the compounds M₄E₄ (M = K, Rb, Cs; E = Ge, Sn) and of β‐Na₄Sn₄ with the cluster anions [E₄]^4? were analysed based on the point group of isolated tetrahedranide units. The lower individual symmetry of the anions in the real structure being more patterned and complex primarily affects the spectra of the tetrahedro‐tetragermanides. ν₃(F₂) clearly splits both in Raman and IR and in the case of K₄Sn₄ only in IR. Rb₄Sn₄ and Cs₄Sn₄ exhibit very simple spectra with three bands in Raman and one band in IR. The breathing mode ν₁(A₁) for the quasi isolated [E₄]^4? cluster appears only in the Raman spectrum and is hardly influenced by the structural environment and by the nature of the alkali metal cations: ν₁(A₁) = 274 cm^?1 ([Ge₄]^4?) and 183‐187 cm^?1 ([Sn₄]^4?), respectively. The calculated valence force constants f_d(E–E) are: [Ge₄]^4? : f_d = 0.89 Ncm^?1 ( K ), 0.87 Ncm^?1 ( Rb ), 0.86 Ncm^?1 ( Cs ) and [Sn₄]^4? : 0.67 Ncm^?1 ( Na ), 0.66 Ncm^?1 ( K ), 0.67 Ncm^?1 ( Rb ), 0.68 Ncm^?1 ( Cs ). Both, the frequencies and the force constants fit well into the range previously reported.     

Si44– in Solution – First Solvate Crystal Structure of the Ligand‐free Tetrasilicide Tetraanion in Rb1.2K2.8Si4·7NH3

We report the first solvate structure of the silicide anion Si₄^4–, which provides circumstantial evidence of the stability of the highly charged anion in liquid ammonia solutions. The solvate Rb_1.2K_2.8Si₄ · 7NH₃ crystallized from a mixture of the ternary compound K₆Rb₆Si₁₇ with the transition metal complex [(C₆H₅)₃P]₂Ni(CO)₂ [bis(triphenylphosphine)dicarbonylnickel] in the presence of the chelating agents 18‐crown‐6 (1,4,7,10,13,16‐hexaoxacyclooctadecane) and [2.2.2]cryptand (4,7,13,16,21,24‐hexaoxa‐1,10‐diazabicyclo[8.8.8]hexacosane) in liquid ammonia. Single X‐ray diffraction analysis confirms the presence of the Si₄^4– anion in the crystal structure of Rb_1.2K_2.8Si₄ · 7NH₃, which represents the first solvate compound of the naked tetrasilicide tetraanion. All five crystallographically independent cation positions show mixed occupancy by Rb⁺ and K⁺.     

Alkalimetallbismutide ABi und ABi2 (A = K,Rb, Cs) — Synthesen,Kristallstrukturen, Eigenschaften

Alkali Metal Bismuthides ABi and ABi₂ — Synthesis, Crystal Structure, Properties The Zintl phases ABi (A = K/Rb/Cs; monoclinic, space group, P2₁/c, a = 1422.3(2)/1474.2(2)/1523.7(3), b = 724.8(1)/750.2(1)/773.7(1), c = 1342.0(2)/1392.1(2)/1439.9(2) pm and β = 113.030(3)/113.033(2)/112.722(3)°, Z = 16) crystallize with the β‐CsSb structure type containing chains of two‐connected Bi atoms. Hence, and according to calculated electronic structures, they are semiconductors with small band gaps of approx. 0.5 eV. In contrast, the compounds ABi₂ (A = K/Rb/Cs; cubic, space group Fd3¯m, a = 952.1(2)/962.4(8)/972.0(3) pm, Z = 8) belong to the Laves phases, showing a typical metallic electrical conductivity and no band gaps.     

Reactions of Beryllium Halides in Liquid Ammonia: The Tetraammineberyllium Cation [Be(NH3)4]2+, its Hydrolysis Products,and the Action of Be2+ as a Fluoride‐Ion Acceptor

The first structural characterization of the text‐book tetraammineberyllium(II) cation [Be(NH₃)₄]²⁺, obtained in the compounds [Be(NH₃)₄]₂Cl₄ ? 17NH₃ and [Be(NH₃)₄]Cl₂, is reported. Through NMR spectroscopic and quantum chemical studies, its hydrolysis products in liquid ammonia were identified. These are the dinuclear [Be₂(μ‐OH)(NH₃)₆]³⁺ and the cyclic [Be₂(μ‐OH)₂(NH₃)₄]²⁺ and [Be₃(μ‐OH)₃(NH₃)₆]³⁺ cations. The latter species was isolated as the compound [Be₃(μ‐OH)₃(NH₃)₆]Cl₃ ? 7NH₃. NMR analysis of solutions of BeF₂ in liquid ammonia showed that the [BeF₂(NH₃)₂] molecule was the only dissolved species. It acts as a strong fluoride‐ion acceptor and forms the [BeF₃(NH₃)]^? anion in the compound [N₂H₇][BeF₃(NH₃)]. The compounds presented herein were characterized by single‐crystal X‐ray structure analysis, ⁹Be, ¹⁷O, and ¹⁹F NMR, IR, and Raman spectroscopy, deuteration studies, and quantum chemical calculations. The extension of beryllium chemistry to the ammine system shows similarities but also decisive differences to the aquo system.     

The Metallic Zintl Phase Ba3Si4 – Synthesis,Crystal Structure,Chemical Bonding,and Physical Properties

The Zintl phase Ba₃Si₄ has been synthesized from the elements at 1273 K as a single phase. No homogeneity range has been found. The compound decomposes peritectically at 1307(5) K to BaSi₂ and melt. The butterfly‐shaped Si₄⁶⁻ Zintl anion in the crystal structure of Ba₃Si₄ (Pearson symbol tP28, space group P4₂/mnm, a = 8.5233(3) Å, c = 11.8322(6) Å) shows only slightly different Si‐Si bond lengths of d(Si–Si) = 2.4183(6) Å (1×) and 2.4254(3) Å (4×). The compound is diamagnetic with χ ≈ −50 × 10⁻⁶ cm³ mol⁻¹. DC resistivity measurements show a high electrical resistivity (ρ(300 K) ≈ 1.2 × 10⁻³ Ω m) with positive temperature gradient dρ/dT. The temperature dependence of the isotropic signal shift and the spin‐lattice relaxation times in ²⁹Si NMR spectroscopy confirms the metallic behavior. The experimental results are in accordance with the calculated electronic band structure, which indicates a metal with a low density of states at the Fermi level. The electron localization function (ELF) is used for analysis of chemical bonding. The reaction of solid Ba₃Si₄ with gaseous HCl leads to the oxidation of the Si₄⁶⁻ Zintl anion and yields nanoporous silicon.     

Synthesis,Crystal, and Electronic Structure of the New Ternary Zintl Phase Eu2–xMg2–yGe3 (x = 0.1; y = 0.5)

The new ternary phase Eu_2–xMg_2–yGe₃ (x = 0.1, y = 0.5) was obtained by solid‐state synthesis and the structure determined by means of Single Crystal X‐ray Diffraction. The compound crystallizes with the orthorhombic space group Cmcm (no. 63) having structural features of the low‐temperature modification of LaSi. The crystal structure contains two different types of germanium anions: isolated Ge^4– and $\rm^{2}_{\infty}$ [Ge^2–x–y] chains. The cation substructure is partially disordered and is best represented assuming a split position. The chemical bonding is well represented by the Zintl‐Klemm concept. Resistivity measurements reveal that the compound is metallic. DFT band structure calculations were carried out on the ideal stoichiometric compound Eu₂Mg₂Ge₃, showing that this model (x = 0; y = 0) would be also metallic as a consequence of the ecliptic stacking of anions. Susceptibility and specific heat measurements evidence the presence of weak, and probably frustrated, antiferromagnetic interactions between disordered europium atoms.     

From a Si3‐Cyclopropene to a Si3S‐Bicyclo[1.1.0]butane to a Si3S‐Cyclopropene to a Si3S2‐Bicyclo[1.1.0]butane: Back‐and‐Forth,and In‐Between

Compact and highly reactive bicyclo[1.1.0]butanes constitute one of the most fascinating classes of organic compounds. Furthermore, interplay of bicyclo[1.1.0]butanes with their valence isomers, such as buta‐1,3‐dienes and cyclobutenes, is among the fundamental pericyclic transformations in organic chemistry. Herein we report the back‐and‐forth interconversion between the cyclotrisilenes and thiatrisilabicyclo[1.1.0]butanes, allowing for the synthesis of novel representatives of such classes of highly reactive organometallics. The peculiar structural and bonding features of the newly synthesized compounds, as well as the mechanism of their isomerization, were verified both experimentally and computationally.     

Schrittweise Darstellung von verzweigten tripodalen Chlor‐Methyl‐Siloxanen der allgemeinen Formel tBuSi[{OSiMe2}yOSiMe3–xClx]3 [x = 0–3; y = 0–2] sowie Synthese der Silanole tBuSi[(OSiMe2)xOH]3 [x = 1, 2] und des bicyclischen Siloxans tBuSi(OSiMe2O)3SitBu

The branched tripodal chloro‐methyl‐siloxanes of the general formula tBuSi[{OSiMe₂}_yOSiMe_3–xCl_x]₃ [x = 0–3; y = 0–2] were synthesized, starting with tert‐Butyl‐trisilanol ( 1 ). The treatment of 1 with the chloro‐methyl‐silanes (Me_3–xSiCl_x+1) (x = 0–3) in the presence of triethylamine leads to the compounds tBuSi(OSiMe₂Cl)₃ ( 2 ), tBuSi(OSiMeCl₂)₃ ( 3 ) and tBuSi(OSiCl₃)₃ ( 4 ). The siloxanes 2 – 4 are colourless oily liquids, which can be purified by distillation. Their yields decrease with the number of chloro substituents. In the reaction of compound 2 with three equivalents of water the silantriol tBuSi(OSiMe₂OH)₃ ( 5 ) is generated which is used to create the branched tripodal chloro‐methyl‐siloxanes tBuSi(OSiMe₂OSiMe₃)₃ ( 6 ), tBuSi(OSiMe₂OSiMe₂Cl)₃ ( 7 ), tBuSi(OSiMe₂OSiMeCl₂)₃ ( 9 ) and tBuSi(OSiMe₂OSiCl₃)₃ ( 10 ). Compound ( 7 ) is only a side product with a yield of 25 %., The cyclic tBuSi[{(OSiMe₂)₂Cl}(OSiMe₂)₃O] ( 8 ) can be isolated and characterised. The transformation of the compound tBuSi(OSiMe₂OSiMe₂Cl)₃ ( 7 ) into the trisilanol tBuSi(OSiMe₂OSiMe₂OH)₃ ( 11 ) allows to prepare the tripodale siloxane tBuSi(OSiMe₂OSiMe₂OSiMe₃)₃ ( 12 ) in good yields., The reaction of tBuSi(OSiMe₂Cl)₃ ( 2 ) with tert‐butyl trisilanol 1 leads to the formation of bicyclic tBuSi(OSiMe₂O)₃SitBu ( 13 ). An X‐ray structure determination on 13 reveals a [3.3.3]‐bicycle with a C₃ axis, which crystallizes in the cubic crystal system in the space group Pa . The reported compounds 2 – 13 were characterised by NMR‐ and IR spectroscopy ( 5 , 11 ) and show correct elemental analyses. The ²⁹Si‐NMR‐data of the compounds show interesting trends with respect to the Si–O chain length and the chloro substistuents.     

Structure and dynamics of homoleptic beryllocenes: a solid-state 9Be and 13C NMR study

The correlation between anisotropic 9Be NMR (quadrupolar and chemical shielding) interactions and the structure and dynamics in [Cp2Be], [Cp2*Be], and [(C5Me4H)2Be] is examined by solid-state 9Be NMR spectroscopy, as well as by ab initio and hybrid density functional theory calculations. The 9Be quadrupole coupling constants in the three compounds correspond well to the relative degrees of spherical ground-state electronic symmetry of the environment about beryllium. Theoretical computations of NMR interaction tensors are in excellent agreement with experimental values and aid in understanding the origins of NMR interaction tensors and their correlation to molecular symmetry. Variable-temperature (VT) 9Be and 13C NMR experiments reveal a highly fluxional structure in the condensed phase of [Cp2Be]. In particular, the pathway by which the Cp rings of [Cp2Be] 'invert' coordination modes is examined in detail using hybrid density functional theory in order to inspect variations of the 9Be NMR interaction tensors. The activation energy for the 'inversion' process is found to be 36.9 kJ mol(-1) from chemical exchange analysis of 13C VT CP/MAS NMR spectra. The low-temperature (ca. -100 degrees C) X-ray crystal structures of all three compounds have been collected and refined, and are in agreement with previously reported structures. In addition, the structure of the same Cp2Be crystal was determined at 20 degrees C and displays features consistent with increased intramolecular motion, supporting observations by 9Be VT NMR spectroscopy.     

Two Hexagonal Series of Lanthanoid(III) Oxide Fluoride Selenides: M6O2F8Se3 (M = La – Nd) and M2OF2Se (M = Nd,Sm, Gd – Ho)

Two hexagonal series of lanthanoid(III) oxide fluoride selenides with similar structure types can be obtained by the reaction of the components MF₃, M₂O₃, M, and Se in sealed niobium tubes at 850 °C using CsI as fluxing agent. The compounds with the lighter and larger representatives (M = La – Nd) occur with the formula M₆O₂F₈Se₃, whereas with the heavier and smaller ones (M = Nd, Sm, Gd – Ho) their composition is M₂OF₂Se. For both systems single‐crystal determinations were used in all cases. The compounds crystallize in the hexagonal crystal system (space group: P6₃/m) with lattice parameters of a = 1394–1331 pm and c = 403–372 pm (Z = 2 for M₆O₂F₈Se₃ and Z = 6 for M₂OF₂Se). The (M1)³⁺ cations show different square antiprismatic coordination spheres with or without an extra capping fluoride anion. All (M2)³⁺ cations exhibit a ninefold coordination environment shaped as tricapped trigonal prism. In both structure types the Se^2– anions are sixfold coordinated as trigonal prisms of M³⁺ cations, being first condensed by edges to generate trimeric units and then via faces to form strands running along [001]. The light anions reside either in threefold triangular or in fourfold tetrahedral cationic coordination. For charge compensation, both structures have to contain a certain amount of oxide besides fluoride anions. Since F^– and O^2– can not be distinguished by X‐ray diffraction, bond‐valence calculations were used to address the problem of their adjunction to the available crystallographic sites.     

Polymorphism of N,N'-diacetylbiuret studied by solid-state 13C and 15N NMR spectroscopy, DFT calculations, and X-ray diffraction

The molecular configuration and crystal structure of solid polycrystalline N,N′′‐diacetylbiuret (DAB), a potential nitrogen‐rich fertilizer, have been analyzed by a combination of solid‐ and liquid‐state NMR spectroscopy, X‐ray diffraction, and DFT calculations. Initially a pure NMR study (“NMR crystallography”) was performed as available single crystals of DAB were not suitable for X‐ray diffraction. Solid‐state ¹³C NMR spectra revealed the unexpected existence of two polymorphic modifications (α‐ and β‐DAB) obtained from different chemical procedures. Several NMR techniques were applied for a thorough characterization of the molecular system, revealing chemical shift anisotropy (CSA) tensors of selected nuclei in the solid state, chemical shifts in the liquid state, and molecular dynamics in the solid state. Dynamic NMR spectroscopy of DAB in solution revealed exchange between two different configurations, which raised the question, is there a correlation between the two different configurations found in solution and the two polymorphic modifications found in the solid state? By using this knowledge, a new crystallization protocol was devised which led to the growth of single crystals suitable for X‐ray diffraction. The X‐ray data showed that the same symmetric configuration is present in both polymorphic modifications, but the packing patterns in the crystals are different. In both cases hydrogen bonds lead to the formation of planes of DAB molecules. Additional symmetry elements, a two‐fold screw in the case of α‐DAB and a c‐glide plane in the case of β‐DAB, lead to a more symmetric (α‐DAB) or asymmetric (β‐DAB) intermolecular hydrogen‐bonding pattern for each molecule.     

Li4Ca3Si2N6 and Li4Sr3Si2N6 – Quaternary Lithium Nitridosilicates with Isolated [Si2N6]10– Ions

The isotypic nitridosilicates Li₄Ca₃Si₂N₆ and Li₄Sr₃Si₂N₆ were synthesized by reaction of strontium or calcium with Si(NH)₂ and additional excess of Li₃N in weld shut tantalum ampoules. The crystal structure, which has been solved by single‐crystal X‐ray diffraction (Li₄Sr₃Si₂N₆: C2/m, Z = 2, a = 6.1268(12), b = 9.6866(19), c = 6.2200(12) Å, β = 90.24(3)°, wR₂ = 0.0903) is made up from isolated [Si₂N₆]^10– ions and is isotypic to Li₄Sr₃Ge₂N₆. The bonding angels and distances within the edge‐sharing [Si₂N₆]^10– double‐tetrahedra are strongly dependent on the lewis acidity of the counterions. This finding is discussed in relation to the compounds Ca₅Si₂N₆ and Ba₅Si₂N₆, which also exhibit isolated [Si₂N₆]^10– ions.     

Investigation of (1R,2S)‐(−)‐Ephedrine by Cryogenic Terahertz Spectroscopy and Solid‐State Density Functional Theory

The terahertz (THz) spectrum of the pharmaceutical (1R,2S)‐(?)‐ephedrine from 8.0 to 100.0 cm^?1 is investigated at liquid‐nitrogen (78.4 K) temperature. The spectrum exhibits several distinct features in this range that are characteristic of the crystal form of the compound. A complete structural analysis and vibrational assignment of the experimental spectrum is performed using solid‐state density functional theory (DFT) and cryogenic single‐crystal X‐ray diffraction. Theoretical modeling of the compound includes an array of density functionals and basis sets with the final assignment of the THz spectrum performed at a PW91/6‐311G(d,p) level of theory, which provides excellent solid‐state simulation agreement with experiment. The solid‐state analysis indicates that the seven experimental spectral features observed at low temperature consist of 13 IR‐active vibrational modes. Of these modes, nine are external crystal vibrations and provide approximately 57 % of the predicted spectral intensity. This study demonstrates that the THz spectra of complex pharmaceuticals may be well reproduced by solid‐state DFT calculations and that inclusion of the crystalline environment is necessary for realistic and accurate simulations.