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A combined synthetic and theoretical investigation of N‐heterocyclic carbene (NHC) adducts of magnesium amidoboranes is presented, which involves a rare example of reversible migratory insertion within a normal valents‐block element. The reaction of (NHC)Mg(N(SiMe₃)₂)₂(1) and dimethylamine borane yields the tris(amide) adduct (NHC−BN)Mg(NMe₂BH₃)(N(SiMe₃)₂) (2; NHC−BN = NHC−BH₂NMe₂). In addition to Me₂N=BH₂capture at the^NHCC−Mg bond, mechanistic investigations suggest the likelihood of aminoborane migratory insertion from an RMg(NMe₂BH₂NMe₂BH₃) intermediate. To elucidate these processes, the carbene complexes (NHC)Mg(NMe₂BH₃)₂(8) and (NHC)Mg(NMe₂BH₂NMe₂BH₃)₂(9) were synthesized, and a dynamic migration of Me₂N=BH₂between Mg−N and^NHCC−Mg bonds was observed in9. This unusual reversible migratory insertion is presumably induced by dissimilar charge localization in the⁻{NMe₂BH₂NMe₂BH₃} anion, as well as the capacity of NHCs to reversibly capture Me₂N=BH₂in the presence of Lewis acidic magnesium species.

A combined synthetic and theoretical investigation of N‐heterocyclic carbene (NHC) adducts of magnesium amidoboranes is presented, which involves a rare example of reversible migratory insertion within a normal valents‐block element. The reaction of (NHC)Mg(N(SiMe₃)₂)₂(1) and dimethylamine borane yields the tris(amide) adduct (NHC−BN)Mg(NMe₂BH₃)(N(SiMe₃)₂) (2; NHC−BN = NHC−BH₂NMe₂). In addition to Me₂N=BH₂capture at the^NHCC−Mg bond, mechanistic investigations suggest the likelihood of aminoborane migratory insertion from an RMg(NMe₂BH₂NMe₂BH₃) intermediate. To elucidate these processes, the carbene complexes (NHC)Mg(NMe₂BH₃)₂(8) and (NHC)Mg(NMe₂BH₂NMe₂BH₃)₂(9) were synthesized, and a dynamic migration of Me₂N=BH₂between Mg−N and^NHCC−Mg bonds was observed in9. This unusual reversible migratory insertion is presumably induced by dissimilar charge localization in the⁻{NMe₂BH₂NMe₂BH₃} anion, as well as the capacity of NHCs to reversibly capture Me₂N=BH₂in the presence of Lewis acidic magnesium species.

The addition of non‐benzenoid quinones, acenapthenequinone or aceanthrenequinone, to the 9‐carbene‐9‐borafluorene monoanion (1) affords the first examples of dianionic 10‐membered bora‐crown ethers (2–5), which are characterized by multi‐nuclear NMR spectroscopy (¹H,¹³C,¹¹B), X‐ray crystallography, elemental analysis, and UV/Vis spectroscopy. These tetraoxadiborecines have distinct absorption profiles based on the positioning of the alkali metal cations. When compound4, which has a vacant C₄B₂O₄cavity, is reacted with sodium tetrakis[3,5‐bis(trifluoromethyl)phenyl]borate, a color change from purple to orange serves as a visual indicator of metal binding to the central ring, whereby the Na⁺ion coordinates to four oxygen atoms. A detailed theoretical analysis of the calculated reaction energetics is provided to gain insight into the reaction mechanism for the formation of2–5. These data, and the electronic structures of proposed intermediates, indicate that the reaction proceeds via a boron enolate intermediate.

The addition of non‐benzenoid quinones, acenapthenequinone or aceanthrenequinone, to the 9‐carbene‐9‐borafluorene monoanion (1) affords the first examples of dianionic 10‐membered bora‐crown ethers (2–5), which are characterized by multi‐nuclear NMR spectroscopy (¹H,¹³C,¹¹B), X‐ray crystallography, elemental analysis, and UV/Vis spectroscopy. These tetraoxadiborecines have distinct absorption profiles based on the positioning of the alkali metal cations. When compound4, which has a vacant C₄B₂O₄cavity, is reacted with sodium tetrakis[3,5‐bis(trifluoromethyl)phenyl]borate, a color change from purple to orange serves as a visual indicator of metal binding to the central ring, whereby the Na⁺ion coordinates to four oxygen atoms. A detailed theoretical analysis of the calculated reaction energetics is provided to gain insight into the reaction mechanism for the formation of2–5. These data, and the electronic structures of proposed intermediates, indicate that the reaction proceeds via a boron enolate intermediate.

Borepin, a 7‐membered boron‐containing heterocycle, has become an emerging molecular platform for the development of new materials and optoelectronics. While electron‐deficient borepins are well‐established, reduced electron‐rich species have remained elusive. Herein we report the first isolable, crystalline borepin radical (2 a,2 b) and anion (3 a,3 b) complexes, which have been synthesized by potassium graphite (KC₈) reduction of cyclic(alkyl)(amino) carbene‐dibenzo[b,d]borepin precursors. Borepin radicals and anions have been characterized by EPR or NMR, elemental analysis, X‐ray crystallography, and cyclic voltammetry. In addition, the bonding features have been investigated computationally using density functional theory.

Borepin, a 7‐membered boron‐containing heterocycle, has become an emerging molecular platform for the development of new materials and optoelectronics. While electron‐deficient borepins are well‐established, reduced electron‐rich species have remained elusive. Herein we report the first isolable, crystalline borepin radical (2 a,2 b) and anion (3 a,3 b) complexes, which have been synthesized by potassium graphite (KC₈) reduction of cyclic(alkyl)(amino) carbene‐dibenzo[b,d]borepin precursors. Borepin radicals and anions have been characterized by EPR or NMR, elemental analysis, X‐ray crystallography, and cyclic voltammetry. In addition, the bonding features have been investigated computationally using density functional theory.