Approaching a ‘naked’ boryl anion: amide metathesis as a route to calcium, strontium and potassium boryl complexes

Amide metathesis has been used to generate the first structurally characterized boryl complexes of calcium and strontium, {(Me 3 Si) 2 N}M{B(NDippCH) 2 }(thf) n (M = Ca, n = 2; M = Sr, n = 3), through the reactions of the corresponding bis(amides), M{N(SiMe 3 ) 2 } 2 (thf) 2 , with (thf) 2 Li{B(NDippCH) 2 }. Most notably, this approach can also be applied to the analogous potassium amide K{N(SiMe 3 )}, 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 contacts of > 3.1 A. 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.