Formation, Stability, and Structures of Borenium and Boronium Cations Derived from Pentamethylazaferrocene–Boranes by Hydride or Chloride Abstraction Reactions

Hydride
abstraction reactions from 1′,2′,3′,4′,5′-pentamethylazaferrocene–boranes
(PMAF-boranes, where borane = BH3, 9-bora[3.3.1]­bicyclononane)
using trityl cation in CH2Cl2 yielded the corresponding
borenium cations, whose stability depends on the counteranion of the
trityl salt. If the [BF4]− salt was employed,
then fluoride abstraction from the anion occurred immediately, yielding
PMAF-BF3 and other identifiable byproducts. If the [B­(C6F5)4]− salt was employed,
then the borenium ions were stable in solution at room temperature
and could be characterized by 1H, 13C, and 11B NMR spectroscopy. Double hydroboration of [PMAF-BH2]+ with 1,5-cycloooctadiene yields the borenium
ion of PMAF-9-bora[3.3.1]-bicyclononane, [PMAF-9BBN]+.
Reaction of [PMAF-BH2]+ with excess PMAF yields
the boronium cation [(PMAF)2BH2]+, which was isolated and crystallographically characterized as its
[B­(C6F5)4]− salt.
Reaction of [PMAF-9BBN]+ with 2 equiv of N-methylimidazole leads to the boronium cation [(N-methylimidazole)2(BBN)]+, which was also isolated
and crystallographically characterized as its [B­(C6F5)4]− salt. Chloride abstraction
from PMAF-BCl3 with AlCl3 in CH2Cl2 yielded the borenium cation [PMAF-BCl2]+, which was characterized in solution by 1H, 13C, and 11B NMR spectroscopy. A single-crystal X-ray structure
was also obtained of [PMAF-BCl2]+[AlCl4]−, revealing structural changes upon borenium
ion formation, including rehybridization of the B center and a bending
of the BCl2 moiety toward the Fe center. The Lewis acidity
of all three borenium cations was assessed by the Gutmann–Beckett
method, and [PMAF-BH2]+ was found to be more
acidic than B­(C6F5)3. DFT calculations
were performed on [PMAF-BH2]+ using the BP86
method with the LANL2DZ basis set for Fe and the TZVP basis set for
C, H, N, and B. The optimized structure revealed substantial bending
of the BH2 moiety toward Fe with a dip angle of −23.7°.
Inspection of the frontier MOs reveal that the electrophilic B center
is stabilized through delocalized interactions that involve orbitals
on Fe, the pyrrolyl ring, and the pentamethylcyclopentadienyl ring.