Recent advances in stable main group element radicals: preparation and characterization

Radical species are significant in modern chemistry. Their unique chemical bonding and novel physicochemical properties play significant roles not only in fundamental chemistry, but also in materials science. Main group element radicals are usually transient due to their high reactivity. Highly stable radicals are often stabilized by π-delocalization, sterically demanding ligands, carbenes and weakly coordinating anions in recent years. This review presents the recent advances in the synthesis, characterization, reactivity and physical properties of isolable main group element radicals.

Diradical Anion of Potassium Aggregate: Reduction of Dimer Boroxide Complex

Crystalline aggregates containing metal cation clusters "wrapped" by reduced hydrocarbon anions have been presented. Initially, a dimeric complex (2) possessing a bimetallic K₂O₂ core was synthesized from the reaction between an anthracene substituted boronic acid (2-(anthracen-9-yl)phenyl)(hydroxy)(mesityl)borane (1-H) and KN(SiMe₃)₂ in THF solution. The B-O bond length (1.281(4) Å) in complex 2 is comparable to those observed in oxoboranes, indicating this may be a double bond, which is supported by its Wiberg bond order (1.9) predicted by density functional theory calculations. Subsequently, potassium aggregate complexes, diradical 3 and dihydro anion 4, were obtained through the reduction reactions of dimeric complex 2 and 1-H, respectively. They exhibit similar K₃O₂B₂ aggregate structures, but differ significantly in the geometry of the anthracene units. Complex 3 features a triplet diradical character with planar anthracene units that carry the unpaired electrons, while the anthracene groups in complex 4 display a puckered structure due to the addition of hydrogen atoms.

Aryl Insertion vs Aryl-Aryl Coupling in C,C-Chelated Organoborates: The "Missing Link" of Tetraarylborate Photochemistry

The photoreactivity of 9-borafluorene-based, C,C-chelated organoborates was investigated. Unlike the related tetraarylborates, the charge-transfer transitions imparted by the biphenyl chelate lead to selective insertion of one aryl substituent into the endocyclic B-C bond of the 9-borafluorene moiety, resulting in the formation of boratanorcaradienes. This photoreaction likely proceeds according to a Zimmerman rearrangement, which is analogous to one of the initially proposed mechanisms for tetraarylborates and provides additional insight into these long-debated photochemical reactions.