Polyhedral Oligomeric Silsesquioxane D3h-(RSiO1.5)14.

While smaller polyhedral oligomeric silsesquioxanes TnRn (POSS) are readily accessible or even commercially available, unambiguously authenticated larger systems (n>12) have barely been reported. Synthesis and isolation procedures are lengthy, and yields are often very low. Herein, we present the surprisingly straightforward and high-yielding access to the phenyl-substituted derivative of a so far only postulated second D3h-symmetric T14 isomer and with that the largest crystallographically characterized POSS cage with organic substituents. Treatment of the commercially available incompletely condensed T7Ph7(OH)3 silsesquioxane with catalytic amounts of trifluoromethanesulfonic acid results in high yields of the T14Ph14 framework, which is isolated in crystalline form by a simple work-up. D3h-T14Ph14 was analyzed by single crystal X-ray diffraction, multinuclear NMR spectroscopy and thermal analysis. The relative energies of all four theoretically possible T14Ph14 isomers were determined by optimization of the corresponding structure using DFT methods.

Tetrylene-Functionalized Si7-Siliconoids.

The core expansion of metallic or metalloid clusters by the addition of further homo- or heteronuclear vertices is pivotal to the nucleation and growth of particles. The exohedral grafting of a low-valent functionality followed by endohedral incorporation have been identified as key steps. Following previous work on the Si6 series, we now report the synthesis and full characterization of the amidinatotetrylene-functionalized seven-vertex siliconoids Si7R5[E(NtBu)2CPh] (E = Si, Ge, Sn). In the case of the silylene derivative, the solid-state structure was determined by single crystal X-ray diffraction.

Experimental molecular structures in the gas phase at the upper size limit: The case of Si6Tip6.

Currently, the largest (ramax = 19.9 Å) and by far the most complicated (234 atoms, C1 symmetry, 696 independent geometrical parameters, and 27 261 interatomic terms) experimental molecular structure of a cage-type Si6Tip6 (Tip = 2,4,6-iPr3C6H2) isomer has been investigated in the gas phase by the electron diffraction method (Tav = 645 K) supplemented with theoretical simulations. A detailed analysis of the current possibilities for experimentally investigating large molecular structures is performed. A series of density functional theory approximations and the role of dispersion interactions have been benchmarked using the obtained data. Based on the refined geometry of Si6Tip6, various quantum-chemical methods have been applied for the investigation of the electronic structure of its Si6 core. In particular, natural bond orbital, quantum theory of atoms in molecules, interacting quantum atoms, fractional occupation number weighted density, and complete active space self-consistent field (CASSCF) methods were utilized. The diradical character of the molecule has been assessed by the UHF and CASSCF approximations. The problem of bonding between the hemispheroidal silicon atoms has been investigated.

Stable Silapyramidanes.

Starting from tetrakis(trimethylsilyl)cyclobutadiene and an amidinate-supported silylene of the Roesky-type, a sequence of addition and reduction cleanly gives the elusive silapyramidane via an isolable cyclobutene intermediate with an exocyclic Si═C bond. The silapyramidane features an unusually shielded 29Si NMR resonance at -448.3 ppm for the apex silicon atom. Treatment with Fe2(CO)9 results in the formation of the corresponding silapyramidane-iron complex. Silapyramidane also reacts with the cyclobutadiene starting material to cleanly afford a fluorescent spirobis(silole).

σ,π-Conjugated Bis(germylene) Adducts with NHC and CAACs.

Heavier tetrylenes attract attention for their potential in synthesis, catalysis and small molecule activation. The coordination by N-heterocyclic carbenes (NHCs) and cyclic (alkyl)(amino)carbenes (CAACs) results in substantial structural and electronic differences although typically only one of these yields stable derivatives for one and the same tetrylene. We now report both NHC- and CAAC-coordination to a bridged bis(germylene) motif. The NHC-coordinated bis(germylene) exhibits pyramidal germanium centers with lone pairs of electrons, while with CAAC an unprecedented stable bis(germene) with two Ge=C bonds is isolated. Spectroscopic and crystallographic evidence as well as DFT calculations confirm the effects of σ,π-conjugation between the two germanium centers in both cases. The coordination of NHC is reversible as the reaction with BPh3 liberates the transient bis(germylene) and thus provides an alternative low-temperature route towards polymers with Ge=Ge bonds.

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.

Low-Valent Mx Al3 Cluster Salts with Tetrahedral [SiAl3 ]+ and Trigonal-Bipyramidal [M2 Al3 ]2+ Cores (M=Si/Ge).

Schnöckel's [(AlCp*)4 ] and Jutzi's [SiCp*][B(C6 F5 )4 ] (Cp*=C5 Me5 ) are landmarks in modern main-group chemistry with diverse applications in synthesis and catalysis. Despite the isoelectronic relationship between the AlCp* and the [SiCp*]+ fragments, their mutual reactivity is hitherto unknown. Here, we report on their reaction giving the complex salts [Cp*Si(AlCp*)3 ][WCA] ([WCA]- =[Al(ORF )4 ]- and [F{Al(ORF )3 }2 ]- ; RF =C(CF3 )3 ). The tetrahedral [SiAl3 ]+ core not only represents a rare example of a low-valent silicon-doped aluminium-cluster, but also-due to its facile accessibility and high stability-provides a convenient preparative entry towards low-valent Si-Al clusters in general. For example, an elusive binuclear [Si2 (AlCp*)5 ]2+ with extremely short Al-Si bonds and a high negative partial charge at the Si atoms was structurally characterised and its bonding situation analysed by DFT. Crystals of the isostructural [Ge2 (AlCp*)5 ]2+ dication were also obtained and represent the first mixed Al-Ge cluster.

Diboriranide σ-Complexes of d- and p-Block Metals.

Diboriranides are the smallest conceivable monoanionic aromatic cycles, yet only limited examples have been reported and their reactivity and complexation behavior remain completely unexplored. We report a straightforward synthesis of the first peraryl diboriranide c-(DurB)2 CPh- as its lithium salt in three steps via the corresponding non-classical diborirane from a readily available 1,2-dichlorodiborane(4) (Dur=2,3,5,6-tetramethylphenyl). With the preparation and complete characterization of representative complexes with tin, copper, gold and zinc, we demonstrate the strong preference of the diboriranide for σ-type coordination modes towards main group and transition metal centers under unperturbed retention of the three-membered B2 C-ring's 2e- π-system.

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.

Transition-Metal Complexes of Heavier Cyclopropenes: Non-Dewar-Chatt-Duncanson Coordination and Facile Si═Ge Functionalization.

Transition-metal complexes of cyclopropenes occur as fleeting intermediates of numerous metal-catalyzed organic transformations. A heavier analogue has now been obtained from the reaction of an NHC-stabilized silagermenylidene, bis(1,5-cyclooctadiene)nickel(0), and 1 equiv of N-heterocyclic carbene (NHC). The residual chloro functionality at the germanium end of the coordinated Ge═Si moiety of the thus formed 1H-disilagermirene is easily exchanged by treatment with anionic nucleophiles, which provides access to a series of differently substituted Si2Ge-cyclopropenes as nickel complexes in excellent yields. NMR spectroscopic data, X-ray crystallographic analysis, and DFT calculations indicate a coordination mode different from the metallacyclopropane and π-complex extremes of the Dewar-Chatt-Duncanson model: the σ-component of the Ge═Si double bond acts as donor and acceptor, leaving behind a nearly unsupported Si-Ge π-bond.

Transition Metal Complexes of Heavier Vinylidenes: Allylic Coordination vs Vinylidene-Alkyne Rearrangement at Nickel.

Transition metal π-allyl complexes are key reagents/intermediates of various catalytic and stoichiometric allylation reactions. We now report the first transition metal complex of a heavier allylic π-system. The η3-Si2Ge allyl nickel complex is formally obtained by the oxidative addition of the Si-Cl bond of the heavier vinylidene [R2(Cl)Si-(R)Si═(NHC)Ge:] to [Ni(COD)2] (R = 2,4,6-triisopropylphenyl; NHC = 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene; COD = 1,5-cyclooctadiene). Due to geometric constraints, the coordination to the Ni(II) center occurs through the formal Si═Ge double bond instead of the residual lone pair of electrons at germanium. In contrast, the Si-N bond of the analogous vinylidene [R2(Me2N)Si-(R)Si═(NHC)Ge:] (obtained by nucleophilic substitution of Cl by NMe2) does not oxidatively add to Ni(0), and a hydridosilagermene-η2-nickel complex is obtained instead. The formation of this complex necessarily implies the isomerization of the heavier vinylidene to the corresponding heteroalkyne with the Si≡Ge triple bond in the coordination sphere of nickel followed by the activation of a C-H bond of one of the isopropyl groups of an N-heterocyclic carbene (NHC) ligand.

A Mixed Heavier Si=Ge Analogue of a Vinyl Anion.

The versatile reactivities of disilenides and digermenide, heavier analogues of vinyl anions, have significantly expanded the pool of silicon and germanium compounds with various unexpected structural motifs in the past two decades. We now report the synthesis and isolation of a cyclic heteronuclear vinyl anion analogue with a Si=Ge bond, potassium silagermenide as stable thf-solvate and 18-c-6 solvate by the KC8 reduction of germylene or digermene precursors. Its suitability as synthon for the synthesis of functional silagermenes is proven by the reactions with chlorosilane and chlorophospane to yield the corresponding silyl- and phosphanyl-silagermenes. X-ray crystallographic analysis, UV/Vis spectroscopy and DFT calculations revealed a significant degree of π-conjugation between N=C and Si=Ge double bonds in the title compound.

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 Cp2 MCl2 (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.

Free Radical Chemistry of Phosphasilenes.

Understanding the characteristics of radicals formed from silicon-containing heavy analogues of alkenes is of great importance for their application in radical polymerization. Steric and electronic substituent effects in compounds such as phosphasilenes not only stabilize the Si=P double bond, but also influence the structure and species of the formed radicals. Herein we report our first investigations of radicals derived from phosphasilenes with Mes, Tip, Dur, and NMe2 substituents on the P atom, using muon spin spectroscopy and DFT calculations. Adding muonium (a light isotope of hydrogen) to phosphasilenes reveals that: a) the electron-donor NMe2 and the bulkiest Tip-substituted phosphasilenes form several muoniated radicals with different rotamer conformations; b) bulky Dur-substituted phosphasilene forms two radicals (Si- and P-centred); and c) Mes-substituted phosphasilene mainly forms one species of radical, at the P centre. These significant differences result from intramolecular substituent effects.

Equilibrium Formation of Stable All-Silicon Versions of 1,3-Cyclobutanediyl.

Main group analogues of cyclobutane-1,3-diyls are fascinating due to their unique reactivity and electronic properties. So far only heteronuclear examples have been isolated. Here we report the isolation and characterization of all-silicon 1,3-cyclobutanediyls as stable closed-shell singlet species from the reversible reactions of cyclotrisilene c-Si3 Tip4 (Tip=2,4,6-triisopropylphenyl) with the N-heterocyclic silylenes c-[(CR2 CH2 )(NtBu)2 ]Si: (R=H or methyl) with saturated backbones. At elevated temperatures, tetrasilacyclobutenes are obtained from these equilibrium mixtures. The corresponding reaction with the unsaturated N-heterocyclic silylene c-(CH)2 (NtBu)2 Si: proceeds directly to the corresponding tetrasilacyclobutene without detection of the assumed 1,3-cyclobutanediyl intermediate.

Boron and Phosphorus Containing Heterosiliconoids: Stable p- and n-Doped Unsaturated Silicon Clusters.

Unsaturated silicon clusters (siliconoids) are short-lived intermediates during the transition from molecules to the elemental bulk; stable representatives reiterate surface features of silicon materials. The incorporation of suitable heteroatoms into the cluster scaffold of stable siliconoids extends this analogy to the technological process of silicon doping. Here, we report boron- and phosphorus-containing heterosiliconoids with BSi5 and PSi5 core based on the global minimum Si6R6 platform (dubbed benzpolarene for its relationship to benzene). The reductive cleavage of an SiR2 moiety (R = 2,4,6-iPr3C6H2) from Si6R6 selectively yields a dianionic Si5R42- cluster as its lithium salt. Treatment with Me3SiCl affords the corresponding trimethylsilyl-substituted (Me3Si)2Si5R4. Reaction of Si5R42- with iPr2NECl2 (E = B, P) yields the unprecedented p- and n-doped heterosiliconoids iPr2NESi5R4. Their peculiar electronic features are compared to those of the hexasilabenzpolarene starting material on grounds of NMR spectroscopy, X-ray diffraction, and DFT calculations.

Persistent Digermenes with Acyl and α-Chlorosilyl Functionalities.

We report the preparation of α-chlorosilyl- and acyl-substituted digermenes. Unlike the corresponding transient disilenes, these species with a Ge=Ge double bond show an unexpectedly low tendency for cyclization, but in turn are prone to thermal Ge=Ge bond cleavage. Triphenylsilyldigermene has been isolated as a crystalline model compound, and is the first fully characterized example of a neutral digermene with an A2 GeGeAB substitution pattern. Spectroscopic and computational evidence prove the constitution of 1-adamantoyldigermene as a first persistent species with a heavy double bond conjugated with a carbonyl moiety.

Permethylated Disila[2]metallocenophanes of Group 14 and 15 Elements.

Permethylated disila[2]metallocenophanes of silicon, germanium, tin, lead, 2 a-d, (tetrelocenophanes) and antimony, 3 a,b, (pnictogenocenophanes) are described. In the case of antimony, a chloro-substituted stibonocenophane, 3 a, as well as cationic stibonocenophanium tetrachloroaluminate and tetraphenylborate salts, 3 b[X] (X=[AlCl4 ], [BPh4 ]), were synthesized. These represent the first examples of metallocenophanes of any Group 15 element. All compounds were studied in solution and in the solid state. Without exception the ansa-bridge exerts a strong influence on the bending angle of the two Cp-ligands. For disila[2]plumbocenophane, 2 d, its reactivity towards Group 15 halides was probed. Treatment of disila[2]plumbocenophane, 2 d, with two equivalents of phosphorus(III) chloride or arsenic(III) chloride, results in a ring-opening reaction and gives the bis(dihalopnictogenyl)-substituted products, 4 a,b.

Multiple Ether-Functionalized Phosphonium Ionic Liquids as Highly Fluid Electrolytes.

Ionic liquids (ILs) are promising electrolytes, although their often high viscosity remains a serious drawback. The latter can be addressed by the introduction of multiple ether functionalization. Based on the highly atom efficient synthesis of tris(2-ethoxyethyl) phosphine, several new phosphonium ionic liquids were prepared, which allows studying the influence of the ether side chains. Their most important physicochemical properties have been determined and will be interpreted using established approaches like ionicity, hole theory, and the Walden plot. There is striking evidence that the properties of phosphonium ionic liquids with the methanesulfonate anion are dominated by aggregation, whereas the two triple ether functionalized ILs with the highest fluidity show almost ideal behavior with other factors being dominant. It is furthermore found that the deviation from ideality is not significantly changed upon introduction of the ether side chains, although a very beneficial impact on the fluidity of ILs is observed. Multiple ether functionalization therefore proves as a powerful tool to overcome the disadvantages of phosphonium ionic liquids with large cations.