Phenanthrene is a polycyclic aromatic hydrocarbon (PAH) composed of three fused benzene rings. It can be used, e.g., to create materials with conjugated π-systems. Replacing one or more carbon atoms in phenanthrene with boron could be useful for tuning the compound’s electronic and photophysical properties. However, boron-doped phenanthrene analogues are not well-studied. While the 9-borataphenanthrene anion has been prepared before, neither neutral 9-boraphenanthrene nor 9-boraphenanthrene radicals had been known so far.

David J. D. Wilson, La Trobe University, Melbourne, Australia, Robert J. Gilliard, Jr., Massachusetts Institute of Technology (MIT), Cambridge, USA, and colleagues have investigated bis(boraphenanthrenes) and isolated the first structurally characterized examples of neutral bis(9-boraphenanthrene) and its corresponding biradical (pictured, NHC = N-heterocyclic carbene, CAAC = cyclic (alkyl)(amino)carbene). The team first prepared a neutral bis(chloroboracycle) as a precursor. This precursor was obtained from 9-chloro-9-borafluorene and 1,4-bis((trimethylsilyl)ethynyl)benzene.

When the precursor was reduced with potassium graphite (KC₈) in the presence of the NHC 1,3-diisopropylimidazol-2-ylidene (IⁱPr), the NHC 1,3,4,5-tetramethylimidazol-2-ylidene (IMe₄), or the CAAC (2,6-diisopropylphenyl)-4,4-diethyl-2,2-dimethyl-pyrrolidin-5-ylidene (^EtCAAC), the team obtained neutral, NHC-substituted bis(9-boraphenanthrene)s or a CAAC-substituted biradical, respectively. The products were obtained in yields of 65 % (for IⁱPr), 68 % (for IMe₄), and 61 % (for ^EtCAAC), respectively.

The researchers found that the neutral bis(9-boraphenanthrene)s shows an interesting dual emission in solution due to so-called excited state conjugation enhancement (ESCE). According to the team, the synthesized biradical species shows the highest reported degree of biradical character among known boron-based PAHs.

• Bis(9-Boraphenanthrene) and Its Stable Biradical,
  Samir Kumar Sarkar, Kimberly K. Hollister, Andrew Molino, Akachukwu D. Obi, Chun-Lin Deng, Bi Youan E. Tra, Brennan M. Stewart, Diane A. Dickie, David J. D. Wilson, Robert J. Gilliard,
  J. Am. Chem. Soc. 2023.
  https://doi.org/10.1021/jacs.3c07236