A boron-centered radical: a potassium-crown ether stabilized boryl radical anion.

A boron radical contact ion-pair Mes2B{4-(3,5-dimethylpyridinyl)}K(18-crown-6)(THF) (1K) has been isolated and characterized by electron paramagnetic resonance (EPR) spectroscopy, UV-vis absorption spectroscopy and single crystal X-ray diffraction. The geometry, bonding and spin density distribution are shown to be affected by the NK interaction. The unpaired electron resides mainly on the boron atom and falls between those of triarylboron radical anions and neutral boron radicals. The work provides a novel boron-centered radical intermediate, connecting anionic and neutral boryl radicals.