There are only a few examples of small boron (0) compounds (six or fewer B atoms). For example, no boron(0) species with four boron atoms had been isolated so far.

Rei Kinjo and colleagues, Nanyang Technological University, Singapore, have synthesized a neutral tetraboron species coordinated by cyclic (alkyl)(amino)carbene (cAAC) ligands (pictured, substituents truncated). The team used a cAAC- and trimethylphosphine-coordinated 1,2-diboraallene as a starting point and reacted this compound with a cAAC·B₂Br₄ adduct to give the desired B₄ species.

The product is stable enough for isolation and characterization using NMR spectroscopy and X-ray crystallography. The team found that the four central boron atoms and the neighboring carbon atoms form a planar structure. Density functional theory calculations confirmed a delocalization of electrons along three of the boron atoms and the adjacent carbons. According to the researchers, an investigation of the reactivity of the tetraboron species is underway.

• Crystalline Tetraatomic Boron(0) Species,
  Wei Lu, Yongxin Li, Rei Kinjo,
  J. Am. Chem. Soc. 2019.
  https://doi.org/10.1021/jacs.9b02173