Intramolecular Halo Stabilization of Silyl Cations-Silylated Halonium- and Bis-Halo-Substituted Siliconium Borates.

The stabilizing neighboring effect of halo substituents on silyl cations was tested for a series of peri-halo substituted acenaphthyl-based silyl cations 3. The chloro- (3 b), bromo- (3 c), and iodo- (3 d) stabilized cations were synthesized by the Corey protocol. Structural and NMR spectroscopic investigations for cations 3 b-d supported by the results of density functional calculations, which indicate their halonium ion nature. According to the fluorobenzonitrile (FBN) method, the silyl Lewis acidity decreases along the series of halonium ions 3, the fluoronium ion 3 a being a very strong and the iodonium ion 3 d a moderate Lewis acid. Halonium ions 3 b and 3 c react with starting silanes in a substituent redistribution reaction and form siliconium ions 4 b and 4 c. The structure of siliconium borate 4 c2 [B12 Br12 ] reveals the trigonal bipyramidal coordination environment of the silicon atom with the two bromo substituents in the apical positions.

An Experimental Acidity Scale for Intramolecularly Stabilized Silyl Lewis Acids.

A new NMR-based Lewis acidity scale is suggested and its application is demonstrated for a family of silyl Lewis acids. The reaction of p-fluorobenzonitrile (FBN) with silyl cations that are internally stabilized by interaction with a remote chalcogenyl or halogen donor yields silylated nitrilium ions with the silicon atom in a trigonal bipyramidal coordination environment. The 19 F NMR chemical shifts and the 1 J(CF) coupling constants of these nitrilium ions vary in a predictable manner with the donor capability of the stabilizing group. The spectroscopic parameters are suitable probes for scaling the acidity of Lewis acids. These new probes allow for the discrimination between very similar Lewis acids, which is not possible with conventional NMR tests, such as the well-established Gutmann-Beckett method.

Single-Electron Transfer Reactions in Frustrated and Conventional Silylium Ion/Phosphane Lewis Pairs.

Silylium ions undergo a single-electron reduction with phosphanes, leading to transient silyl radicals and the corresponding stable phosphoniumyl radical cations. As supported by DFT calculations, phosphanes with electron-rich 2,6-disubstituted aryl groups are sufficiently strong reductants to facilitate this single-electron transfer (SET). Frustration as found in kinetically stabilized triarylsilylium ion/phosphane Lewis pairs is not essential, and silylphosphonium ions, which are generated by conventional Lewis adduct formation of solvent-stabilized trialkylsilylium ions and phosphanes, engage in the same radical mechanism. The trityl cation, a Lewis acid with a higher electron affinity, even oxidizes trialkylphosphanes, such as tBu3 P, which does not react with either B(C6 F5 )3 or silylium ions.

The Silicon Version of Phosphine Chalcogenides: Synthesis and Bonding Analysis of Stabilized Heavy Silaaldehydes.

The synthesis of chalcogena-silaaldehydes (Ch = S, Se, Te), 15, stabilized by interaction with N-heterocylic carbenes (NHCs) is reported. Compounds 15 are formed by reaction of the NHC-stabilized hydridosilylene 13a with elemental chalcogens in moderate yields. X-ray diffraction analysis of all three variants of the chalcogena-silaaldehydes, 15, revealed short Si/Ch distances which are close to expected values for Si═Ch double bonds. The results of natural bond orbital and natural resonance theory analyses indicate strongly polarized Si-Ch bonds and suggest the occurrence of negative hyperconjugation, which is responsible for the short Si/Ch distances. These results indicate the isolobal relation between NHC-stabilized heavy silaaldehydes 15 and the well-known phosphine chalcogenides, R3PCh.