Silicon Containing Nine Atom Clusters from Liquid Ammonia Solution: Crystal Structures of the First Protonated Clusters [HSi9]3-and [H2{Si/Ge}9]2-

Selective synthesis of germasila-adamantanes through germanium-silicon shift processes

The regioselective synthesis of germasila-adamantanes with the germanium atoms in the bridgehead positions is described starting from cyclic precursors by a cationic sila-Wagner-Meerwein (SWM) rearrangement reaction. The SWM rearrangement allows also a deliberate shift of germanium atoms from the periphery and within the cage structures into the bridgehead positions. This opens the possibility for a synthesis of germasila-adamantanes of defined germanium content and controlled regiochemistry. In the same way that sila-adamantane can be regarded as a molecular building block of elemental silicon, the germasila-adamantane molecules represent cutouts of silicon/germanium alloys.

Functionalized nona-silicide [Si9R3] Zintl clusters: a new class of superhalogens

Superatoms, due to their various applications in redox and materials chemistry, have been a major topic of study in cluster science. Superhalogens constitute a special class of superatoms that mimic the chemistry of halogens and serve as building blocks of novel materials such as super and hyper salts, perovskite-based solar cells, solid-state electrolytes, and ferroelectric materials. These applications have led to a constant search for new class of superhalogens. In this study, using density functional theory, we show that recently synthesized [Si₉{Si (^tBu)₂H}₃] and [Si₉{Si (TMS)₃}₃] Zintl clusters not only behave like halogens but also when functionalized with suitable ligands exhibit superhalogen characteristics. Frontier molecular orbital (FMO) analyses give insights into the electron-accepting nature of the Zintl clusters. Additional bonding techniques such as energy density at the bond critical point (BCP) and adaptive natural density partitioning (AdNDP) gives complementary information about the nature of bonding in Si₉-based Zintl clusters. The potential of these Zintl clusters in the synthesis of new electrolytes in Li-ion batteries is also investigated.

Siliconoid Expansion by a Single Germanium Atom through Isolated Intermediates

The growth of (semi-)metal clusters is pivotal for nucleation processes in gaseous and condensed phases. We now report the isolation of intermediates during the expansion of a stable unsaturated silicon cluster (siliconoid) by a single germanium atom through a sequence of substitution, rearrangement and reduction. The reaction of ligato-lithiated hexasilabenzpolarene LiSi6 Tip5 (1Li⋅(thf)2 , Tip=2,4,6-triisopropylphenyl) with GeCl2 ⋅NHC (NHC=1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene) initially yields the product with exohedral germanium(II) functionality, which then inserts into an Si-Si bond of the Si6 scaffold. The concomitant transfer of the chloro functionality from germanium to an adjacent silicon preserves the electron-precise nature of the formed endohedral germylene. Full incorporation of the germanium heteroatom to the Si6 Ge cluster core is finally achieved either by reduction under loss of the coordinating NHC or directly by reaction of 1Li⋅(thf)2 with GeCl2 ⋅1,4-dioxane.

Chemi-Inspired Silicon Allotropes-Experimentally Accessible Si9 Cages as Proposed Building Block for 1D Polymers, 2D Sheets, Single-Walled Nanotubes, and Nanoparticles

Numerous studies on silicon allotropes with three-dimensional networks or as materials of lower dimensionality have been carried out in the past. Herein, allotropes of silicon, which are based on structures of experimentally accessible [Si9]4- clusters known as stable anionic molecular species in neat solids and in solution, are predicted. Hypothetical oxidative coupling under the formation of covalent Si-Si bonds between the clusters leads to uncharged two-, one- and zero-dimensional silicon nanomaterials not suffering from dangling bonds. A large variety of structures are derived and investigated by quantum chemical calculations. Their relative energies are in the same range as experimentally known silicene, and some structures are even energetically more favorable than silicene. Significantly smaller relative energies are reached by the insertion of linkers in form of tetrahedrally connected Si atoms. A chessboard pattern built of Si9 clusters bridged by tetrahedrally connected Si atoms represents a two-dimensional silicon species with remarkably lower relative energy in comparison with silicene. We discuss the structural and electronic properties of the predicted silicon materials and their building block nido-[Si9]4- based on density functional calculations. All considered structures are semiconductors. The band structures exclusively show bands of low dispersion, as is typical for covalent polymers.

Unsaturated Amido-Substituted Six-Vertex Mixed Silicon Germanium Clusters

The synthesis of mixed silicon and germanium clusters SixGe6-x{N(SiMe3)Dipp}4 1-3 (x = 3.7, 3.1, 2.1) with amido-substituents and two unsubstituted germanium atoms was achieved in co-reductions using the tribromosilane {N(SiMe3)Dipp}SiBr3...

Crystal structure of (4,7,13,16,21,24-hexaoxa-1,10-diazabicyclo[8.8.8]hexacosane-κ 8 N 2, O 6) potassium cyclopentadienide, [K([2.2.2]crypt)]Cp, C23H41KN2O6

C23H41KN2O6, monoclinic, P21/c (no. 14), a = 10.9817(6) Å, b = 23.8330(15) Å, c = 9.7379(7) Å, β = 94.705(5)°, V = 2540.1(3) Å3, Z = 4, R gt (F) = 0.0327, wR ref(F 2) = 0.0682, T = 120 K.

Molecular Silicon Clusters

The continuously decreasing size of device features in microelectronics draws growing attention to the structuring of silicon at the molecular level with powerful tools provided by synthetic chemistry. Silicon clusters are of particular importance in this regard not only as potential precursors for silicon deposition but also as well-defined model systems for bulk and surfaces of silicon at the nanoscale as well as possible starting points for future construction of molecularly precise device structures. This review aims to give a comprehensive overview about the state of the art in the synthesis of molecular silicon clusters, which are grouped into (1) electron-precise saturated clusters, (2) soluble polyhedral Zintl anions, and (3) unsaturated silicon clusters, the so-called siliconoids. Particular attention is paid to functionalization as it is generally considered a necessary prerequisite for the design and construction of more extended systems. The interrelations between the three different classes of molecular silicon clusters, e.g., arising from the introduction of negatively charged functional groups, are highlighted on grounds of NMR properties and computed electronic structures.

FLP-type nitrile activation and cyclic ether ring-opening by halo-borane nonagermanide-cluster Lewis acid-base pairs

Even though homoatomic nine-atom germanium clusters are known for two decades, their chemical properties are still rarely investigated. We now discovered that Zintl ion main group-element clusters possess a reactive lone pair of electrons, and we show a new pathway to bind ligands with functional groups to the [Ge₉] cluster core through Ge-C bond formation. We report on the reactivity of [Ge₉{Si(TMS)₃}₂]^2- (TMS = trimethylsilyl) towards a series of Lewis acidic bromo-boranes. The reaction of [Ge₉{Si(TMS)₃}₂]^2- and DAB ^o-tol-Br (DAB = 1,3,2-diazaborolidine; o-tol = 2-methylphenyl) resulted, depending on the reaction protocol, either in the formation of [Ge₉{Si(TMS)₃}₂DAB ^o-tol]^- (1a) with direct Ge-B interactions, or in [Ge₉{Si(TMS)₃}₂(CH₂)₄O-DAB ^o-tol]^- (2a) featuring a ring-opened thf moiety. Ring opening reactions occur for all bulkier DAB^R-Br [R: o-xyl (2,6-dimethylphenyl), Mes (2,4,6-trimethylphenyl), Dipp (2,6-diisopropylphenyl)], DAB(ii)^Dipp-Br and acyclic ( ⁱ Pr₂N)₂BBr without Ge-B bond formation as shown for the structural characterization of the ring-opened products of thf (3, 4) and trimethylene oxide (5). In contrast to thf, the activation of CH₃CN requires the simultaneous presence of Lewis-acid and Lewis-basic reactants allowing the formation of [Ge₉{Si(TMS)₃}₂CH₃C[double bond, length as m-dash]N-DAB^Mes]^- (6a). Within the presented compounds, 3 and 4 show an unusual substitution pattern of the three ligands at the [Ge₉] core in the solid state. The [Ge₉] cluster/borane systems correspond to intermolecular frustrated Lewis pairs (FLPs), in which the [Ge₉] cluster with several lone pairs represents the Lewis base, and the borane is the Lewis acid.

Chalcogen-Expanded Unsaturated Silicon Clusters: Thia-, Selena-, and Tellurasiliconoids

Reactions of silylenes with heavier chalcogens (E) typically result in Si=E double bonds or their π-addition products. In contrast, the oxidation of a silylene-functionalized unsaturated silicon cluster (siliconoid) with Group 16 elements selectively yields cluster expanded siliconoids Si7 E (E=S, Se, Te) fully preserving the unsaturated nature of the cluster scaffold as evident from the NMR signatures of the products. Mechanistic considerations by DFT calculations suggest the intermediacy of a Si6 siliconoid with exohedral Si=E functionality. The reaction thus may serve as model system for the oxidation of surface-bonded silylenes at Si(100) by chalcogens and their diffusion into the silicon bulk.

Indirect and Direct Grafting of Transition Metals to Siliconoids

Unsaturated charge‐neutral silicon clusters (siliconoids) are important as gas‐phase intermediates between molecules and the elemental bulk. With stable zirconocene‐ and hafnocene‐substituted derivatives, we here report the first examples containing directly bonded transition‐metal fragments that are readily accessible from the ligato‐lithiated Si₆ siliconoid (1Li) and Cp₂MCl₂ (M=Zr, Hf). Charge‐neutral siliconoid ligands with pending tetrylene functionality were prepared by the reaction of amidinato chloro tetrylenes [PhC(NtBu)₂]ECl (E=Si, Ge, Sn) with 1Li, thus confirming the principal compatibility of such low‐valent functionalities with the unsaturated Si₆ cluster scaffold. The pronounced donor properties of the tetrylene/siliconoid hybrids allow for their coordination to the Fe(CO)₄ fragment.

Crystal structure of (1,4,7,10,13,16-hexaoxacyclooctadecane-κ6O6) 1,2,3,4,5-pentamethyl-cyclopenta-2,4-dien-1-yl(potassium, rubidium) — ammonia (1/2), [K0.3Rb0.7(18-crown-6)]Cp*⋅2 NH3, C22H45K0.3N2O6Rb0.7

C22H45K0.26N2O6Rb0.74, orthorhombic, Pbca (no. 61), a = 10.1587(8) Å, b = 16.2842(15) Å, c = 32.862(3) Å, V = 5436.2(8) Å3, Z = 8, Rgt(F) = 0.0386, wRref(F2) = 0.0616, T = 120(2) K.

Silicon clusters with six and seven unsubstituted vertices via a two-step reaction from elemental silicon

Unsaturated silicon clusters with only partial substitution, and thus, "naked" Si atoms are well studied species as they are proposed intermediates in gas-phase deposition processes. Although a remarkable number of stable molecular clusters has been reported, they are typically still obtained by multi-step syntheses. Herein we introduce a newly developed synthetic approach which led to the formation of the anionic species {Si(TMS)3}3Si9 - (1a) and {Si(TMS)3}2Si9 2- (1b), and an extension of this synthetic protocol resulted in the first covalent attachment of ligands through metal atoms to these clusters, (SnCy3)3Si9 - (2a) and (SnCy3)2Si9 2- (2b). The influence of the substituents on the electron localization in the central Si9 unit is analyzed by means of intrinsic bond orbital (IBO) analysis and partial atomic charge distribution. The IBO analyses reveal a new type of delocalization including 5-center-6-electron besides 3-center-2-electron bonds. The Raman spectra of 1b and 2b allow an assignment of the Si-Si intra-cluster vibrations by comparison to calculated (DFT-PBE0) spectra. The anions are formed in a one-step synthesis from binary K12Si17 which can easily be obtained by fusing the elements K and Si. The anions are characterized by ESI mass spectrometry and comprehensive NMR studies (1H, 13C, 29Si, 119Sn). Attempts to crystallize 1a and 2a as their (K-222crypt)+ salts yielded after the loss of one of the substituents single crystals containing 1b and 2b. The single crystal X-ray structure analyses reveal the presence of anionic siliconoids with surfaces of seven unsubstituted silicon atoms.

Enhancing the Variability of [Ge9 ] Cluster Chemistry through Phosphine Functionalization

The synthetic approach towards molecules that contain Ge atoms with oxidation state 0, and which are exclusively connected to other Ge atoms, is explored by using anionic clusters extracted from binary solids. Besides providing a novel variable method for the introduction of alkenyl moieties to [Ge₉ ] cluster compounds, this work expands the spectrum of mixed-functionalized [Ge₉ ] cluster anions, which are suitable for the straightforward synthesis of zwitterionic compounds upon coordination to metal cations. In detail, the synthesis of a series of mixed-functionalized [Ge₉ ] clusters is reported, including [Ge₉ {Si(TMS)₃ }₃ PRR^I ] (R=tBu, R^I =(CH₂ )₃ CH=CH₂ ; 2) and [Ge₉ {Si(TMS)₃ }₂ PRR^I ]^- (R and R^I : alkyl, alkenyl, aryl, aminoalkyl; 3 a to 11 a, TMS: (trimethyl)silyl). In 2 and 3 a, pentenyl functionalization of the [Ge₉ ] clusters was achieved by reaction of the novel chlorophosphine tBu{(CH₂ )₃ CH=CH₂ }PCl (1) with silylated [Ge₉ ] clusters. Furthermore, the reactivity of the cluster anions 3 a to 11 a towards NHC^Dipp MCl (NHC^Dipp =1,3-di(2,6-diisopropylphenyl)imidazolylidine; M=Cu, Ag) showed a dependency on the steric demand of the phosphine either zwitterions (3-MNHC^Dipp to 7-MNHC^Dipp ) featuring P-M interactions are formed, or Ge-M coordination (8-MNHC^Dipp to 11-MNHC^Dipp ) occurs. For M=Ag, the formation of zwitterionic complexes was unequivocally proven by NMR investigations showing ¹ J(³¹ P-¹⁰⁷ Ag/¹⁰⁹ Ag) spin-spin coupling.

Site-selective functionalization of Si6R6 siliconoids

The recent progress in the synthesis of partially substituted neutral silicon clusters (siliconoids) revealed unique structures and electronic anisotropies that are reminiscent of bulk and nano surfaces of silicon. Here, we report the selective 2-lithiation of the global minimum Si6R6 siliconoid at a different vertex than in the previously reported isomeric 4-lithiated derivative (R = 2,4,6- i Pr3C6H2). In order to enable an intuitive distinction of the vertices of the global minimum Si6R6 scaffold (which can be considered the silicon analogue of benzene in terms of thermodynamic stability), we introduce a novel nomenclature in analogy to the ortho-meta-para nomenclature of disubstituted benzenes. By treatment of the 2-lithiated Si6 cluster with Me3SiCl, SiCl4 H3B·SMe2, (Me2N)2PCl as well as with carboxylic acid chlorides RCOCl (R = t Bu, Ph) various 2-functionalized Si6 clusters were obtained and characterized in solution and - in most cases - the solid state. The structural and spectroscopic effect of the position of the newly introduced functional group is discussed by comparison to the corresponding 4-functionalized derivatives.

Atomically Precise Expansion of Unsaturated Silicon Clusters

Small- to medium-sized clusters occur in various areas of chemistry, for example, as active species of heterogeneous catalysis or as transient intermediates during chemical vapor deposition. The manipulation of stable representatives is mostly limited to the stabilizing ligand periphery, virtually excluding the systematic variation of the property-determining cluster scaffold. We now report the deliberate expansion of a stable unsaturated silicon cluster from six to seven and finally eight vertices. The consecutive application of lithium/naphthalene as the reducing agent and decamethylsilicocene as the electrophilic source of silicon results in the expansion of the core by precisely one atom with the potential of infinite repetition.

Elusive Zintl Ions [μ-HSi4 ]3- and [Si5 ]2- in Liquid Ammonia: Protonation States, Sites, and Bonding Situation Evaluated by NMR and Theory

The existence of [μ-HSi₄ ]^3- in liquid ammonia solutions is confirmed by ¹ H and ²⁹ Si NMR experiments. Both NMR and quantum chemical calculations reveal that the H atom bridges two Si atoms of the [Si₄ ]^4- cluster, contrary to the expectation that it is located at one vertex Si of the tetrahedron. The calculations also indicate that in the formation of [μ-HSi₄ ]^3- , protonation is driven by a high charge density and an increase of electron delocalization compared to [Si₄ ]^4- . Additionally, [Si₅ ]^2- was detected for the first time and characterized by NMR. Calculations show that it is resistant to protonation, owing to a strong charge delocalization, which is significantly reduced upon protonation. Thus, our methods reveal three silicides in liquid ammonia: unprotonated [Si₅ ]^2- , terminally protonated [HSi₉ ]^3- , and bridge-protonated [μ-HSi₄ ]^3- . The protonation trend can be roughly predicted by the difference in charge delocalization between the parent compound and the product, which can be finely tuned by the presence of counter ions in solution.

An anionic heterosiliconoid with two germanium vertices

Reduction of dismutational 1,4-digermatetrasilabenzene isomer gives lithium digermatetrasila-benzpolarenide (R = Tip = 2,4,6-iPr₃C₆H₂). The positions of its Ge vertices yield mechanistic insights; its reaction with SiCl₄ proves its nucleophilicity.

Anionic Siliconoids from Zintl Phases: R3 Si9 - with Six and R2 Si9 2- with Seven Unsubstituted Exposed Silicon Cluster Atoms (R=Si(tBu)2 H)

Neutral and anionic silicon clusters (siliconoids) are regarded as important model systems for bulk silicon surfaces. For 25 years their formation from binary alkali metal silicide phases has been proposed, but experimentally never realized. Herein the silylation of a silicide, leading to the anionic siliconoids (Si(tBu)₂ H)₃ Si₉ ^- (1 a) and (Si(tBu)₂ H)₂ Si₉ ^2- (2 a) with the highest known number of ligand-free silicon atoms is reported for the first time. The new anions are obtained in a one-step reaction of K₁₂ Si₁₇ /NH₃ (liq.) and Si(tBu)₂ HCl/THF. Electrospray ionization spectrometry and ¹ H, ¹³ C, ²⁹ Si, as well as ²⁹ Si-HMBC (heteronuclear multiple bond correlation) NMR spectroscopy, confirm the attachment of three silyl groups at a [Si₉ ]^4- cluster under formation of 1 a, in accordance with calculated NMR shifts. During crystal growth the siliconoid di-anion 2 a is formed. The single-crystal X-ray structure determination reveals that two silyl groups are connected to the deltahedral Si₉ cluster core, revealing seven unsubstituted exposed silicon cluster atoms with a hemispheroidal coordination. The negative charges -1 and -2 are delocalized over the six and seven siliconoid Si atoms in 1 a and 2 a, respectively.

Crystal structure of [(1,2-η)-1,2,3,4,5-pentamethyl-cyclopenta-2,4-dien-1-yl] (1,4,10,13-tetraoxa-7,16-diazacyclooctadecane-κ6N2,O4) rubidium (I), [Rb(diaza-18-crown-6)]Cp*, C22H41N2O4Rb

C22H41N2O4Rb, monoclinic, P21/n (no. 14), a = 10.8160(9) Å, b = 17.7253(16) Å, c = 13.2179(12) Å, β = 93.961(8)°, V = 2528.0(4) Å3, Z = 4, Rgt(F) = 0.0376, wRref(F2) = 0.0694, T = 120(2) K.

Crystal Structure of the Protonated Germanide Cluster [HGe9]3−

A single crystal X-ray diffraction study of the new compound [Rb([2.2.2]crypt)]2[Rb([18]crown−6)][HGe9]·4NH3 revealed the presence of the first protonated nine-atom germanide cluster [HGe9]3−. It forms from Rb4Ge9 in liquid ammonia, so that [Ge9]4− can be considered as the base and [HGe9]3− its formally conjugated acid. The H atom is attached to a germanium vertex atom of the basal square plane, as it is known for [RGe9]3− (R = C5H9, Mes, etc.) or [HE9]3− (E = Si, Sn). In addition, the proton could be located unambiguously in the Fourier difference map. [HGe9]3− also represents a nido cluster species with 22 cluster-bonding electrons, which can be considered the most stable structure for nine-atom cluster species for all group 14 elements.

[(μ2-H)(η2-Ge4)ZnPh2]3−, an edge-on protonated E4 cluster establishing the first three-center two-electron Ge–H–Ge bond

The first example of a protonated and a rare example of a metal complex of the tetrahedral tetrel cluster anion [Ge₄]⁴⁻ was obtained from a solution of K₆Rb₆Ge₁₇ in liquid ammonia in the presence of ZnPh₂ and [18]crown-6.