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.

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.

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.

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.

A Three-Membered Cyclic Phosphasilene.

Small unsaturated phosphacycles are versatile reagents owing to their strain and the added functionality of the double bond and the phosphorus lone pair. Herein we report the synthesis and isolation of the smallest possible cyclic phosphasilene as a stable adduct with an N-heterocyclic carbene (NHC). First reactivity studies show a) that the PSi2 ring is a competent ligand to the Fe(CO)4 fragment via the phosphorus lone pair and b) that the abstraction of the NHC by BPh3 results in the rapid head-to-head or head-to-tail dimerization of the PSi2 unit. The relatively facile NHC cleavage indicates that the P=Si double bond is available for further manipulation.

Stable unsaturated silicon clusters (siliconoids).

Unsaturated silicon clusters are key intermediates of silicon deposition processes from the gas phase, which is reflected in the recently introduced term "siliconoids" for silicon rich clusters with at least one hemispheroidally coordinated vertex. Unlike the case of metalloid clusters of heavier Group 14 elements, stable homonuclear derivatives containing silicon have become available just recently. This review summarizes the developments since the first report on a stable unsaturated silicon cluster in 2005. The synthesis, structure and reactivity under retention of the unsaturated vertices (i.e. the functionalization) of stable siliconoids is discussed within the broader context of soluble Zintl anions of silicon and metalloid clusters. A structural parameter is introduced as a quantitative measure to characterize the "hemispheroidality" of an unsubstituted vertex of a siliconoid.

Isolation and Versatile Derivatization of an Unsaturated Anionic Silicon Cluster (Siliconoid).

The characteristic features of bulk silicon surfaces are echoed in the related partially substituted-and thus unsaturated-neutral silicon clusters (siliconoids). The incorporation of siliconoids into more-extended frameworks is promising owing to their unique electronic features, but further developments in this regard are limited by the notable absence of functionalized siliconoid derivatives until now. Herein we report the isolation and full characterization of the lithium salt of an anionic R5 Si6 -siliconoid, thus providing the missing link between silicon-based Zintl anions and siliconoid clusters. Proof-of-principle for the high potential of this species for the efficient transfer of the intact unsaturated R5 Si6 moiety is demonstrated by clean reactions with representative electrophiles of Groups 13, 14, and 15.