Abstract
Amide metathesis has been used to generate the first structurally characterized boryl complexes of calcium and strontium, {(Me3Si)2N}M{B(NDippCH)2}(thf)n (M=Ca, n=2; M=Sr, n=3), through the reactions of the corresponding bis(amides), M{N(SiMe3)2}2(thf)2, with (thf)2Li‐ {B(NDippCH)2}. Most notably, this approach can also be applied to the analogous potassium amide K{N(SiMe3)2}, leading to the formation of the solvent‐free borylpotassium dimer [K{B(NDippCH)2}]2, which is stable in the solid state at room temperature for extended periods (48 h). A dimeric structure has been determined crystallographically in which the K+ cations interact weakly with both the ipso‐carbons of the flanking Dipp groups and the boron centres of the diazaborolyl heterocycles, with K⋅⋅⋅B distances of >3.1 Å. These structural features, together with atoms in molecules (QTAIM) calculations imply that the boron‐containing fragment closely approaches a limiting description as a “free” boryl anion in the condensed phase.
Highlights
Boryl anions, [BX2]À, featuring an electronic sextet at boron,[1,2] were long targeted as synthetic reagents offering the possibility for “umpolung” nucleophilic reactivity at boron
Given the ready availability of the thf-free potassium amide K{N(SiMe3)2}, we hypothesized that analogous chemistry might permit access to a donor-free borylpotassium species. We envisaged that such a compound would be of interest from a structural perspective (given the dimeric, predominantly ionic structures determined recently for related potassium aluminyl systems of the type [K(AlX2)]2),[10] and as an alternative lithium-free source of the boryl anion of use, for example, in the formation of f-element or early d-block complexes.[13,14]
Combination of2Li{B(NDippCH)2}, K{N(SiMe3)2} and hexane at 77 K, followed by warming to room temperature, and crystallization from the same solvent at 243 K leads to the formation of deep yellow single crystals in ca. 50 % yield. This product can be shown by X-ray crystallography to be the solvent-free centrosymmetric borylpotassium dimer, 3 (Scheme 2), which is stable in the solid state at room temperature for 2 days under an inert atmosphere
Summary
Boryl (or “boranyl”) anions, [BX2]À, featuring an electronic sextet at boron,[1,2] were long targeted as synthetic reagents offering the possibility for “umpolung” nucleophilic reactivity at boron.
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