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
C
4
v
-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 F
2,
R
1(I>2σ(I))
= 0.078, wR
2(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.
