Highly Strained Arene-Fused 1,2-Diborete Biradicaloid

Area of Science:

• Organoboron Chemistry
• Radical Chemistry
• Aromaticity and Electronic Structure

Background:

• Cyclic alkyl(amino)carbenes (CAACs) are versatile ligands in stabilizing reactive main group species.
• Naphthalene derivatives offer a platform for exploring novel electronic structures through functionalization.
• Understanding the reduction pathways of polyborylated aromatic compounds is crucial for developing new materials.

Purpose of the Study:

• To investigate the stepwise electrochemical reduction of a CAAC-stabilized 2,3-bis(dibromoboryl)naphthalene.
• To characterize the resulting radical, diborete, and dianion species using spectroscopic and crystallographic methods.
• To elucidate the electronic structure and bonding characteristics of the novel 1,2-diborete intermediate.

Main Methods:

• Stepwise electrochemical reduction of the precursor molecule.
• Isolation and characterization of reduction products, including radicals and cyclic boron compounds.
• X-ray crystallography for structural determination of the 1,2-diborete.
• Density Functional Theory (DFT) and multireference calculations for electronic structure analysis.
• Electron Paramagnetic Resonance (EPR) spectroscopy to probe biradical character.

Main Results:

• Successful isolation of mono- and bis(boryl) radicals, a 1,2-diborete, and a diboron dianion through controlled reduction.
• X-ray analysis revealed a strained, twisted four-membered ring in the diborete with an elongated B-B bond.
• Computational studies indicated an open-shell singlet biradicaloid ground state for the diborete, slightly favored over its triplet state.
• Reactions of the diborete with CO and phenyl azide yielded bis(borylene) and a diazadiboretidine, respectively.

Conclusions:

• The stepwise reduction of the naphthalene precursor provides access to a diverse range of boron species with unique bonding and electronic properties.
• The 1,2-diborete exhibits significant biradical character, challenging traditional aromaticity concepts.
• This work expands the known chemistry of low-valent boron compounds and highlights the utility of CAAC stabilization.