Boron has one valence electron less than ac arbon atom and an availablev acant p z orbital. The incorporation of sp 2 -hybridized boron atoms into the host lattice of ap olycyclic aromatic hydrocarbon (PAH) is formally relatedt oo xidative doping. Ab oron-containing B-PAH has an energetically low-lying LUMO and an arrow HOMO-LUMOg ap, which renders it as trong Lewis acid/electron acceptora nd promotes fluorescencei nt he visible range of the electromagnetic spectrum. Many methods have been developed to access B-PAHs that are deliberately designed for specific tasks. Herein, we highlight recentb reakthroughs in the field of B-PAH synthesis and the scope of their applications, which range from Lewis acid and redox catalysis to device fabrication.W ew illa lso report on the dynamic covalent chemistry of neutral and anionic B-PAHs, as it is ap otential limitationi nt he design of catalysts ystems but can also provide ap owerful synthetic tool for the preparation of otherwise inaccessible B-PAHs.

Dimeric aryl(hydro)boranes can provide suitable platforms for the synthesis of boron-containing graphene flakes through reductive B-B coupling. Two-electron reduction of 1,2:1,2-bis(4,4'-di-tert-butyl-2,2'-biphenylylene)diborane(6) (4) with LiNaph/THF establishes a B-B σ bond but can be accompanied by substituent redistribution. In the singly rearranged product, Li2[6], only one 1,2-phenyl shift has occurred. The doubly ring-contracted product, Li2[7], consists of two 9H-9-borafluorenyl moieties that are linked via their boron atoms. When the amount of LiNaph/THF is increased to 4 equiv, Li2[6] is subsequently observed as the dominant species. Addition of 11 equiv of LiNaph/THF results in over-reduction with hydride elimination to afford the doubly boron-doped dibenzo[g,p]chrysene Li2[1]. In contrast, excess KC8 reduces 4 to the corresponding dihydro-dibenzo[g,p]chrysene, K2[5], with a trans-HB-BH core. Hydride abstraction from K2[5] with 1 equiv of 4 leads to K[8], in which the central B-B bond is bridged by a single hydrogen atom. K[8] is also obtained upon treatment of 4 with 1 equiv of KC8. All products have been characterized by multinuclear NMR spectroscopy and X-ray crystallography.

Reduction of the bis(9-borafluorenyl)methane 1 with excess lithium furnishes the red dianion salt Li2[1]. The corresponding dark green monoanion radical Li[1] is accessible through the comproportionation reaction between 1 and Li2[1]. EPR spectroscopy on Li[1] reveals hyperfine coupling of the unpaired electron to two magnetically equivalent boron nuclei (a((11)B) = 5.1 ± 0.1 G, a((10)B) = 1.7 ± 0.2 G). Further coupling is observed to the unique B-CH-B bridgehead proton (a((1)H) = 7.2 ± 0.2 G) and to eight aromatic protons (a((1)H) = 1.4 ± 0.1 G). According to X-ray crystallography, the B···B distances continuously decrease along the sequence 1 → [1](•-) → [1](2-) with values of 2.534(2), 2.166(4), and 1.906(3) Å, respectively. Protonation of Li2[1] leads to the cyclic borohydride species Li[1H] featuring a B-H-B two-electron-three-center bond. This result strongly indicates a nucleophilic character of the boron atoms; the reaction can also be viewed as rare example of the protonation of an element-element σ bond. According to NMR spectroscopy, EPR spectroscopy, and quantum-chemical calculations, [1](2-) represents a closed-shell singlet without any spin contamination. Detailed wave function analyses of [1](•-) and [1](2-) reveal strongly localized interactions of the two boron pz-type orbitals, with small delocalized contributions of the 9-borafluorenyl π systems. Overall, our results provide evidence for a direct B-B one-electron and two-electron bonding interaction in [1](•-) and [1](2-), respectively.

Deprotonation of the doubly arylene-bridged diborane(6) derivative 1H with (Me Si) CLi or (Me Si) NK gives the B-B σ-bonded species M[1H] in essentially quantitative yields (THF, room temperature; M=Li, K, arylene=4,4'-di-tert-butyl-2,2'-biphenylylene). With nBuLi as the base, the yield of Li[1H] drops to 20 % and the 1,1-bis(9-borafluorenyl)butane Li[2H] is formed as a side product (30 %). In addition to the 1,1-butanediyl fragment, the two boron atoms of Li[2H] are linked by a μ-H bridge. In the closely related molecule Li[3H], the corresponding μ-H atom can be abstracted with (Me Si) CLi to afford the B-B-bonded conjugated base Li [3] (THF, 150 °C; 15 %). Li[1H] and Li[2H] were characterized by NMR spectroscopy and X-ray crystallography.

Targeted C(sp3)–H activation or nucleophilic substitution reactions have been achieved through the interaction of a diborane dianion with haloalkanes.