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

Abstract

Hydride abstraction reactions from 1′,2′,3′,4′,5′-pentamethylazaferrocene−boranes (PMAF-boranes, where borane = BH 3 , 9-bora[3.3.1]bicyclononane) using trityl cation in CH 2 Cl 2 yielded the corresponding borenium cations, whose stability depends on the counteranion of the trityl salt. If the [BF 4 ] − salt was employed, then fluoride abstraction from the anion occurred immediately, yielding PMAF-BF 3 and other identifiable byproducts. If the [B(C 6 F 5 ) 4 ] − salt was employed, then the borenium ions were stable in solution at room temperature and could be characterized by 1 H, 13 C, and 11 B NMR spectroscopy. Double hydroboration of [PMAF-BH 2 ] + with 1,5-cycloooctadiene yields the borenium ion of PMAF-9-bora[3.3.1]-bicyclononane, [PMAF-9BBN] + . Reaction of [PMAF-BH 2 ] + with excess PMAF yields the boronium cation [(PMAF) 2 BH 2 ] + , which was isolated and crystallographically characterized as its [B(C 6 F 5 ) 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(C 6 F 5 ) 4 ] − salt. Chloride abstraction from PMAF-BCl 3 with AlCl 3 in CH 2 Cl 2 yielded the borenium cation [PMAF-BCl 2 ] + , which was characterized in solution by 1 H, 13 C, and 11 B NMR spectroscopy. A single-crystal X-ray structure was also obtained of [PMAF-BCl 2 ] + [AlCl 4 ] − , revealing structural changes upon borenium ion formation, including rehybridization of the B center and a bending of the BCl 2 moiety toward the Fe center. The Lewis acidity of all three borenium cations was assessed by the Gutmann−Beckett method, and [PMAF-BH 2 ] + was found to be more acidic than B(C 6 F 5 ) 3 . DFT calculations were performed on [PMAF-BH 2 ] + 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 BH 2 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.

Fundamental and Applied Properties of Borocations

“…6 compared to that of À12. 3 for Cp*Fe(C 5 H 4 BBr 2 ), indicating a greater interaction between Fe and boron for the neutral borane [14]. These results again suggest an enhanced hard Lewis acidity for borocations due to the unit positive charge.…”

“…This result can again be attributed to the large degree of positive charge localised at boron in 1, which is generated by the bonding of boron to three highly electronegative atoms. Finally, Brunker and co-workers used the Gutmann-Beckett method to probe the Lewis acidity of a number of pentamethylazaferrocene ligated borenium cations including 4 and 5 [14]. Cation 4 was found to be more Lewis acidic towards Et 3 PO than 5, consistent with the reduced electronic stabilisation of boron bonded to two hydride substituents relative to two chloride π donor substituents.…”

“…Mesoionic carbene boranes were also effective for the hydroboration of allylbenzene on activation with [Ph 3 C][B(C 6 F 5 ) 4 ], presumably proceeding via borenium cations or their functional equivalents [82]. Brunker and co-workers also utilised a borenium cation produced by hydride abstraction, in this case from pentamethylazaferrocene-BH 3 , to hydroborate 1,5-cyclo-octadiene to generate [pentamethylazaferrocene-BBN] + [14]. In contrast to the reactive borenium hydroborating agents discussed above, the B-H containing borenium cation 2 did not hydroborate phenylacetylene, an indication of considerable π delocalisation reducing Lewis acidity towards soft nucleophiles [31].…”

Straightforward and Controlled Shape Access to Efficient Macrocyclic Imidazolylboronium Anion Receptors

“…Boronium macrocycle 4a was unambiguously characterized by using spectroscopicm ethods.I n particular, 1 HNMR spectroscopy revealed the highly symmetrical structure, andt he chemical shift of the boron atom in the 11 BNMR spectrum is consistent with ab oronium speciesi n [D 6 ]DMSO. [10,12,13] High-resolutionm asss pectrometry (HRMS) confirmed the dimeric macrocyclic structure with only two species detected after positive-mode electrosprayi onization (ESI): the bis-cation without bromide counterions ([4aÀ2Br] 2 + : m/z exptl 330.7102; m/z calcd 330.7109) and the monocation-bromide complex ([4aÀBr] + : m/z exptl 740.3409; m/z calcd 740.3406). In our hands, no suitable crystalsw ere obtained to determine the spatial arrangement of the imidazolyl units.…”

“…Interestingly,d ue to the bulky 9-BBN group,t he two imidazolyl moieties of boronium 2b adoptapincer-like shape in which the bromide counterion displays an umber of close contacts with the cation.I nt his case, the bromine counterionf orms noncovalent hydrogen-bonding interactions with the electropositive (C2ÀH) + of the imidazolyl moieties (Figure 1, left). [12] In contrast with other known imidazoliumr eceptors, both imidazolyl cores in 2b are facing each other and conjointly bind the same bromide anion.I np articular,t he cationic charge is delocalized between the boron-centered cation [13] and the imidazolyl moieties, which confers am ore electropositive( C ÀH) + interaction.…”