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 <i>N</i>-methylimidazole leads to the boronium cation [(<i>N</i>-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