Approaching the Silylium (R₃Si⁺) Ion:  Trends with Hexahalo (Cl, Br, I) Carboranes as Counterions

Abstract

A systematic investigation has been made into the degree of silylium ion character in species of the type i-Pr 3 SiY where Yis a weakly coordinating hexahalo-carborane anion, X 6 -CB 11 H 6 -, with X ) Cl, Br, I. By X-ray crystal structural criteria such as the average C-Si-C angle (117.3(4)°, 117.0(13)°, and 115.6(9)°for X ) Cl, Br, I, respectively) and the downfield 29 Si chemical shift (115, 110, and 97 ppm for X ) Cl, Br, I, respectively) the hexachloro-carborane anion is the least coordinating. Thus, the "ion-like" species i-Pr 3 Si δ+ (Cl 6 -CB 11 H 6 ) δbecomes the closest structural approach to date to a condensed phase silylium ion. With the aid of ab initio molecular orbital theory and silicon chemical shift calculations we address the question of how much silylium ion character this species represents. From structural, energetic, and NMR criteria (where expectation values for the condensed phase 29 Si chemical shifts are revised downward) we conclude that i-Pr 3 Si δ+ (Cl 6 -CB 11 H 6 ) δmay have >50% silylium ion character.

Silver and Copper Complexes with closo-Polyhedral Borane, Carborane and Metallacarborane Anions: Synthesis and X-ray Structure

“…Two [1-CB11H11-12-Br] − anions are coordinated to the silver atom via BH groups (Ag…H(B) distances of 2.20 and 2.27 Å) and the third anion is coordinated by the bromine atom (Ag…Br(B) distance of 2.642 Å), the benzene molecule is coordinated to the silver atom in a η 1 -fashion with Ag-C distance of 2.485 Å (Figure 65) [92]. The silver salt of 12-bromo-1-carba-closo-dodecaborate anion, Ag[1-CB 11 H 11 -12-Br], was prepared using the standard metathesis approach [92] The silver salts of 6,7,8,9,10,11,12-hexahalogeno-1-carba-closo-dodecaborate anions, Ag[1-CB11H6-6,7,8,9,10,11,12-X6] (X = Cl [94], Br [86], I [94]), were prepared from the treatment of the corresponding cesium salts with AgNO3 in aqueous solution. The crystals of {Ag(η 2 -para-xylene)[1-CB11H6-6,7,8,9,10,11,12-Cl6]}n were obtained by crystallization from 1,4-xylene.…”

“…The silver atom is in distorted octahedral arrangement The silver salts of 6,7,8,9,10,11,12-hexahalogeno-1-carba-closo-dodecaborate anions, Ag[1-CB11H6-6,7,8,9,10,11,12-X6] (X = Cl [94], Br [86], I [94]), were prepared from the treatment of the corresponding cesium salts with AgNO3 in aqueous solution. The crystals of {Ag(η 2 -para-xylene)[1-CB11H6-6,7,8,9,10,11,12-Cl6]}n were obtained by crystallization from 1,4-xylene.…”

“…Cl distances range from 2.640(1) to 2.926(1) Å. The p-xylene is coordinated in asymmetric η 2 fashion with Ag-C bonds of 2.506(3) and 2.481(4) Å (Figure 67) [94].…”

Optimizing the Least Nucleophilic Anion. A New, Strong Methyl+ Reagent

“…Without methylation, lower solubility, quadrupole-broadened B-H resonances, and a single indistinct C-H resonance render the hexahalo carborane salts less amenable to NMR characterization. Illustrative of these points is the characterization of the silylium species, i-Pr 3 Si(HCB 11 Me 5 Br 6 ), 20 whose solubility in toluene (>20 mM) is more than an order of magnitude higher than the most soluble of the unmethylated analogues, i-Pr 3 Si(HCB 11 H 5 Cl 6 ), 22 enabling collection of the 29 Si NMR spectrum in minutes.…”

Silicenium Ions: Quantum Chemical Computations

“…Reed's group reported the X-ray structures of trialkyl-silicenium ions R 3 Si C complexed to low-nucleophilic hexahalo carborane anions 90,91 . The authors varied systematically both the alkyl substituents R (R D Me, Et, i-Pr and t-Bu) and the halogen substitution of the X 6 CB 11 H 6 (X D Cl, Br, I) counterions.…”