Reactions of CB₉H₁₀^- with Electrophiles, Including the Regioselective Mono- and Dihalogenation of the Lower Belt

Abstract

The 11B NMR spectrum of CpFe(CO)2(CB9H10) dissolved in dichloromethane shows the presence of two isomers in a 75:25 ratio. In the predominant isomer, the C4v-symmetry CB9H10- anion is coordinated to the iron atom via the B10−H10 bond (the antipodal B−H bond). In the less abundant isomer, the CB9H10- anion is coordinated to the iron atom via the B6−H6 bond, one of four equivalent B−H bonds (the lower-belt B−H bonds) adjacent to B10−H10. In spite of the preference of the electrophilic cation [CpFe(CO)2]+ for the antipodal B−H bond, mono- and dihalogenation with the electrophilic reagents (i) anhydrous hydrogen fluoride, (ii) N-chlorosuccinimide, (iii) N-bromosuccinimide, and (iv) elemental iodine occurred almost exclusively at lower-belt boron atoms (>95% regioselectivity). The new anions 6-CB9H9X- (X = F, Cl, Br, I) were isolated as cesium salts. Little or no isomeric 10-CB9H9X- was observed. Dihalo derivatives were predominantly 6,8-CB9H8X2-, with varying amounts of 6,7-CB9H8X2- present. Small amounts of 6,10-CB9H8X2- were present in some reaction mixtures. In contrast to the nearly exclusive lower-belt halogenations, electrophilic H/D exchange of CB9H10- with 20% aqueous DCl occurred equally rapidly at the antipodal and lower-belt boron atoms, forming 6,7,8,9,10-CB9H5D5-. The order of deuteration rates for 6-CB9H9F- under the same conditions was B8 > B10 > {B7,B9}. No deuterium exchange was observed even after 1 month for 6-CB9H9I- under the same conditions. All of the carborane cluster anions were characterized by a combination of 1D and 2D NMR and mass spectroscopy. The anions 6-CB9H9F-, 6,7-CB9H8F2-, and 6,8-CB9H8F2- are the first examples of 10-vertex closo-borane or -heteroborane clusters containing B−F bonds. The structure of Ag(C6H6)2(6,8-CB9H8F2) was determined by X-ray crystallography: orthorhombic, space group P212121, a = 7.945(2) Å, b = 11.274(2) Å, c = 20.756(4) Å, V = 1859.2(7) Å3, Z = 4, T = −100 °C, least-squares refinement on F2, R1(I>2σ(I)) = 0.078, wR2(all data) = 0.197. The difluorinated carborane cluster has the same overall structure as the parent ion CB9H10-, but B6 and B8 each bear a single, terminal fluorine atom. The silver(I) ion is coordinated to two benzene molecules in distorted η2 fashion and to hydrogen atoms H9 and H10 from the same 6,8-CB9H8F2- anion.