Ge(9) {Si(SiMe(3) )(3) }(3) {SnPh(3) }]: A Tetrasubstituted and Neutral Deltahedral Nine-Atom Cluster

Reaching neutral territory: The title compound, the first tetrasubstituted deltahedral Zintl cluster, is no longer an ion (see picture; Ge?green, Si?purple, Sn?blue). It is a neutral molecule formed by a reaction of the trisilylated anion with Ph(3) SnCl.     

[Ge2]4− Dumbbells with Very Short Ge−Ge Distances in the Zintl Phase Li3NaGe2: A Solid‐State Equivalent to Molecular O2

The novel ternary Zintl phase Li₃NaGe₂ comprises alkali‐metal cations and [Ge₂]^4? dumbbells. The diatomic [Ge₂]^4? unit is characterized by the shortest Ge?Ge distance (2.390(1) Å) ever observed in a Zintl phase and thus represents the first Ge=Ge double bond under such conditions, as also suggested by the (8?N) rule. Raman measurements support these findings. The multiple‐bond character is confirmed by electronic‐structure calculations, and an upfield ⁶Li NMR shift of ?10.0 ppm, which was assigned to the Li cations surrounded by the π systems of three Ge dumbbells, further underlines this interpretation. For the unperturbed, ligand‐free dumbbell in Li₃NaGe₂, the π‐ bonding p_y and p_z orbitals are degenerate as in molecular oxygen, which has singly occupied orbitals. The partially filled π‐type bands of the neat solid Li₃NaGe₂ cross the Fermi level, resulting in metallic properties. Li₃NaGe₂ was synthesized from the elements as well as from binary reactants and subsequently characterized crystallographically.     

A10LaCdSb9 (A=Ca,Yb): A Highly Complex Zintl System and the Thermoelectric Properties

Two new Zintl compounds A₁₀LaCdSb₉ (A=Ca, Yb), namely, Ca_9.81(1)La_0.97(1)Cd_1.23(1)Sb₉ and Yb_9.78(1)La_0.97(1)Cd_1.24(1)Sb₉, have been designed and synthesized by applying the Zintl concept. Although both compounds are isoelectronic with their Ca₁₁InSb₉ and Yb₁₁InSb₉ analogues, they crystallize in a new structure type with the orthorhombic space group Ibam (No.72) and feature very complex anion structures, which are composed of unique [Cd₂Sb₆]^12? clusters, dumbbell‐shaped [Sb₂]^4? dimers, and isolated [Sb]^3? anions. For Yb_9.78(1)La_0.97(1)Cd_1.24(1)Sb₉, an extremely low lattice thermal conductivity of 0.29 W m^?1 K^?1 was observed at 875 K, which almost approaches the lowest reported limit of nonglassy or nonionically conducting bulk materials. According to thermogravimetric (TG) and differential scanning calorimetry (DSC) analyses, both compounds show very good thermal stability and no melting or phase transition processes were found below 1173 K. Although related thermoelectric property studies on Yb_9.78(1)La_0.97(1)Cd_1.24(1)Sb₉ only present a maximum ZT of 0.11 at 920 K, owing to its low Seebeck coefficients, these materials are still very promising for their high temperature stability and low thermal conductivity. Furthermore, as mixed cations exist with different charges, it makes this system very flexible in tuning the related electrical properties.     

Cationic Channels with Partial Anion Occupation in the Zintl Phases Ba2Mg12Ge7.33 and Ba6Mg17.4Li2.6Ge12O0.64

In the system Ba/(Mg, Li)/Ge, two new Zintl phases with the composition Ba₂Mg₁₂Ge_7.33 (P6₃/m, Z = 1, a = 1121.7(5) pm, c = 440.2(2) pm) and Ba₆Mg_17.4Li_2.6Ge₁₂O_0.64 (P6₃/m, Z = 1, a = 1537.8(8) pm, c = 454.6(2) pm) are found and structurally characterized. Their structures are described with respect to the Zintl‐Klemm concept, structure directing rules, and chemical twinning. These new compounds contain as a specific structural feature cationic channels with partial anion occupation which allows to adjust the electron count. In Ba₂Mg₁₂Ge_7.33, the channels are formed by Mg²⁺ cations and are partially filled with germanium dumb‐bells, while the channels in Ba₆Mg_17.4Li_2.6Ge₁₂O_0.64 are formed by Li⁺ and Mg²⁺ cations and host O^2— anions. The electronic structure of both compounds has been investigated using Extended‐Hückel calculations with special emphasis on the states of the cationic channels and their interstitial heteroatoms. The potentiality of using the electron localization function (ELF) to find missing atoms in structures has been tested and verified for both compounds.     

K6Pb8Cd: A Zintl Phase with Oligomers of Pb4 Tetrahedra Interconnected by Cd Atoms

Unprecedented (Pb ₄ )Cd(Pb ₄ )Cd(Pb ₄ )Cd(Pb ₄ ) tetramers (see picture) are present in the structure of the title compound, in which they coexist with isolated Pb₄ tetrahedra. Since Cs₆Ge₈Zn has the same stoichiometry as K₆Pb₈Cd but contains exclusively (Ge₄)₂Zn dimers, this can be viewed as a disproportionation reaction of the type (a).     

Mercury and cadmium pnictidehalides: the inverted Zintl phases

The structures of the mercury and cadmium pnictidehalides M_aZ_bX_c (M = Cd, Hg; Z = P, As, Sb; X = Cl, Br, I) are discussed on the basis of the Zintl--Klemm concept. Primary attention is paid to the relationship between the crystal and electronic structures of the compounds in question.     

Zintl Defects in Intermetallic Clathrates

In many cases, idealized crystal structure models cannot rationalize the actual properties of intermetallic compounds. For a realistic approach in materials research, microstructures and defects need to be taken into account. In case of clathrate compounds, particularly the intrinsic framework vacancies (denoted as Zintl defects) demand consideration. Consequently, clathrate research produces evidence that modern-day structure chemistry involves the utilization of advanced X-ray diffraction methods combined with elaborated bulk phase analyses, the investigation of phase relations, and the study of mutual interrelations in the triangle chemical bonding–structure–properties. Herein, we review some fundamental contributions to the specific defect chemistry of intermetallic clathrates.     

Zintl Phases: Transitions between Metallic and Ionic Bonding

Experimental studies on compounds of alkali and alkaline earth metals with semi- and metametals have considerably broadened the basis for a discussion of the transition from metallic to ionic bonding. Current interest is focused mainly upon the elucidation of the principles governing the structure of such compounds which are subject to a wide range of variation within this class of materials. A new definition of the term Zintl phase is proposed after consideration of available findings.     

Intermetallics as zintl phases: Yb2Ga4Ge6 and RE3Ga4Ge6 (RE=Yb, Eu): structural response of a [Ga4Ge6]4- framework to reduction by two electrons

Two new intermetallic compounds, Yb(2)Ga(4)Ge(6) and Yb(3)Ga(4)Ge(6), were obtained from reactions in molten Ga. A third compound, Eu(3)Ga(4)Ge(6), was produced by direct combination of the elements. The crystal structures of these compounds were studied by single-crystal X-ray diffraction. Yb(2)Ga(4)Ge(6) crystallizes in an orthorhombic cell with a=4.1698(7), b=23.254(4), c=10.7299(18) A in the polar space group Cmc2(1). The structure of RE(3)Ga(4)Ge(6) is monoclinic, space group C2/m, with cell parameters a=23.941(6), b=4.1928(11), c=10.918(3) A, beta=91.426(4) degrees for RE=Yb, and a=24.136(2), b=4.3118(4), c=11.017(1) A, beta=91.683(2) degrees for RE=Eu. The refinement [I>2 sigma(I)] converged to the final residuals R(1)/wR(2)=0.0229/0.0589, 0.0411/0.1114, and 0.0342/0.0786 for Yb(2)Ga(4)Ge(6), Yb(3)Ga(4)Ge(6), and Eu(3)Ga(4)Ge(6), respectively. The structures of these two families of compounds can be described by a Zintl concept of bonding, in which the three-dimensional [Ga(4)Ge(6)](n-) framework serves as a host and electron sink for the electropositive RE atoms. The structural relation of RE(3)Ga(4)Ge(6) to of Yb(2)Ga(4)Ge(6) lies in a monoclinic distortion of the orthorhombic cell of Yb(2)Ga(4)Ge(6) and reduction of the [Ga(4)Ge(6)] network by two electrons per formula unit. The results of theoretical calculations of the electronic structure, electrical transport data, and thermochemical and magnetic measurements are also reported.     

On the Reactivity of Silylated Ge9 Clusters: Synthesis and Characterization of [ZnCp*(Ge9{Si(SiMe3)3}3)], [CuPiPr3(Ge9{Si(SiMe3)3}3)], and [(CuPiPr3)4{Ge9(SiPh3)2}2]

We report on the synthesis of new derivatives of silylated clusters of the type [Ge₉(SiR₃)₃]^? (R = SiMe₃, Me = CH₃; R = Ph, Ph = C₆H₅) as well as on their reactivity towards copper and zinc compounds. The silylated cluster compounds were synthesized by heterogeneous reactions starting from the Zintl phase K₄Ge₉. Reaction of K[Ge₉{Si(SiMe₃)₃}₃] with ZnCl₂ leads to the already known dimeric compound [Zn(Ge₉{Si(SiMe₃)₃}₃)₂] ( 1 ), whereas upon the reaction with [ZnCp*₂] the coordination of [ZnCp*]⁺ to the cluster takes place (Cp*=1,2,3,4,5‐pentamethylcyclopentadienyl) under the formation of [ZnCp*(Ge₉{Si(SiMe₃)₃}₃)] ( 2 ). A similar reaction leads to [CuPiPr₃(Ge₉{Si(SiMe₃)₃}₃)] ( 3 ) from [CuPiPr₃Cl] (iPr=isopropyl). Further we investigated the novel silylated cluster units [Ge₉(SiPh₃)₃]^? ( 4 ) and [Ge₉(SiPh₃)₂]^? ( 5 ), which could be identified by mass spectroscopy. Bis‐ and tris‐silylated species can be synthesized by the respective stoichiometric reactions, and the products were characterized by ESI‐MS and NMR experiments. These clusters show rather different reactivity. The reaction of the tris‐silylated anion 4 with [CuPiPr₃Cl] leads to [(CuPiPr₃)₃Ge₉(SiPh₃)₂]⁺ as shown from NMR experiments and to [(CuPiPr₃)₄{Ge₉(SiPh₃)₂}₂] ( 6 ), which was characterized by single‐crystal X‐ray diffraction. Compound 6 shows a new type of coordination of the Cu atoms to the silylated Zintl clusters.     

The Largest Metalloid Group 14 Cluster,Ge18[Si(SiMe3)3]6: An Intermediate on the Way to Elemental Germanium

The oxidation of [Ge₉(Hyp)₃]^? (Hyp=Si(SiMe₃)₃) with an Fe^II salt leads to Ge₁₈(Hyp)₆ ( 1 ), the largest Group 14 metalloid cluster that has been structurally characterized to date. The arrangement of the 18 germanium atoms in 1 shows similarities to that found in the solid‐state structure Ge(cF136). Furthermore, 1 can be described as a macropolyhedral cluster of two Ge₉ units. Quantum‐chemical calculations further hint at a strained arrangement so that 1 can be considered as a first trapped intermediate on the way from Ge₉ units to elemental germanium with the clathrate‐II structure (Ge(cF136)).     

[(Me3Si)Si]3EtGe9Pd(PPh3), a Pentafunctionalized Deltahedral Zintl Cluster: Synthesis,Structure, and Solution Dynamics

The title compound, which has a ten‐atom deltahedral cluster core of Ge₉Pd, was synthesized through insertion of Pd(PPh₃) into the tetrasubstituted nona‐germanium cluster [(Me₃Si)Si]₃EtGe₉ through a reaction of the latter with Pd(PPh₃)₄. This first reaction of neutral tetrasubstituted nine‐atom clusters shows that they retain reactivity despite their neutral charge. The Ge₉Pd core is the first that incorporates a 5‐connected transition metal other than from Group VI, a noble metal in this case. Single‐crystal X‐ray diffraction shows that the ten‐atom core is a closo‐cluster with the expected shape of a bicapped square antiprism. ¹H and ¹³C NMR spectroscopy show that, in contrast to the parent tetra‐substituted [(Me₃Si)Si]₃EtGe₉, the new compound does not exhibit dynamics. Relativistic DFT calculations are used to explain the differences.     

Linking Deltahedral Zintl Clusters with Conjugated Organic Building Blocks: Synthesis and Characterization of the Zintl Triad [R‐Ge9‐CHCHCHCH‐Ge9‐R]4−

The accessibility of triads with deltahedral Zintl clusters in analogy to fullerene–linker–fullerene triads is another example for the close relationship between fullerenes and Zintl clusters. The compound {[K(2.2.2‐crypt)]₄[RGe₉‐CH?CH? CH?CH‐Ge₉R]}(toluene)₂ (R=(2Z,4E)‐7‐amino‐5‐aza‐hepta‐2,4‐dien‐2‐yl), containing two deltahedral [Ge₉] clusters linked by a conjugated (1Z,3Z)‐buta‐1,3‐dien‐1,4‐diyl bridge, was synthesized through the reaction of 1,4‐bis(trimethylsilyl)butadiyne with K₄Ge₉ in ethylenediamine and crystallized after the addition of 2.2.2‐cryptand and toluene. The compound was characterized by single‐crystal structure analysis as well asNMR and IR spectroscopy.     

(Ge4Bi14)4−: A Case of “Element Segregation” on the Molecular Level

Extraction of a solid with the nominal composition “K₂GeBi” with 1,2‐diaminoethane (en) in the presence of 4,7,13,16,21,24‐hexaoxa‐1,10‐diazabicyclo[8.8.8]hexacosane (crypt‐222) afforded the salt [K(crypt‐222)]₄(Ge₄Bi₁₄). The 18‐atom Zintl anion (Ge₄Bi₁₄)⁴⁻ has a heretofore unknown molecular topology, which can be thought of as the formal condensation product of two E₁₁³⁻ cages along a shared Ge₄ waist. In this way, (Ge₄Bi₁₄)⁴⁻ represents the largest and most structurally complex Bi‐containing polyanion. We describe its stepwise formation, its geometric and electronic structure, and comment on relative stabilities of isomers with different distributions of the four Ge atoms on the 18 positions that were investigated using DFT calculations.