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

The 11 B NMR spectrum of CpFe(CO) 2 (CB 9 H 10 ) dissolved in dichloromethane shows the presence of two isomers in a 75:25 ratio. In the predominant isomer, the C 4 v -symmetry CB 9 H 10 - anion is coordinated to the iron atom via the B10−H10 bond (the antipodal B−H bond). In the less abundant isomer, the CB 9 H 10 - 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-CB 9 H 9 X - (X = F, Cl, Br, I) were isolated as cesium salts. Little or no isomeric 10-CB 9 H 9 X - was observed. Dihalo derivatives were predominantly 6,8-CB 9 H 8 X 2 -, with varying amounts of 6,7-CB 9 H 8 X 2 - present. Small amounts of 6,10-CB 9 H 8 X 2 - were present in some reaction mixtures. In contrast to the nearly exclusive lower-belt halogenations, electrophilic H/D exchange of CB 9 H 10 - with 20% aqueous DCl occurred equally rapidly at the antipodal and lower-belt boron atoms, forming 6,7,8,9,10-CB 9 H 5 D 5 - . The order of deuteration rates for 6-CB 9 H 9 F - under the same conditions was B8 > B10 > {B7,B9}. No deuterium exchange was observed even after 1 month for 6-CB 9 H 9 I - 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-CB 9 H 9 F -, 6,7-CB 9 H 8 F 2 -, and 6,8-CB 9 H 8 F 2 - are the first examples of 10-vertex closo -borane or -heteroborane clusters containing B−F bonds. The structure of Ag(C 6 H 6 ) 2 (6,8-CB 9 H 8 F 2 ) was determined by X-ray crystallography: orthorhombic, space group P 2 1 2 1 2 1, a = 7.945(2) Å, b = 11.274(2) Å, c = 20.756(4) Å, V = 1859.2(7) Å 3, Z = 4, T = −100 °C, least-squares refinement on F 2, R 1 ( I >2σ( I )) = 0.078, w R 2 (all data) = 0.197. The difluorinated carborane cluster has the same overall structure as the parent ion CB 9 H 10 -, but B6 and B8 each bear a single, terminal fluorine atom. The silver(I) ion is coordinated to two benzene molecules in distorted η 2 fashion and to hydrogen atoms H9 and H10 from the same 6,8-CB 9 H 8 F 2 - anion.