Borole systems tend to undergo various reactions driven by the disruption of its destabilizing antiaromatic character. As a consequence, the isolation and characterization of free boroles is challenging, especially when the substituents around the C 4 B framework are sterically less demanding. In the present paper we report the synthesis of 1-bromo-2,3,4,5tetraphenylborole. The title compound readily undergoes a dimerization/rearrangement reaction in analogy to the previously reported 1-chloro-2,3,4,5-tetraphenyborole to form an isostructural product identified by X-ray crystallography. Additionally we present the formation of Lewis acidÀbase adducts of the title compound with 3,5-lutidine, PCy 3 , N-hetrocyclic carbene, cyclic (amino)(alkyl)carbene, and THF. The latter compounds were analyzed by single-crystal X-ray diffraction and compared.

The reactions of isonitriles with a variety of metalloboryl and metalloborylene species are shown to yield a range of products, none of which display the typical Lewis acid-base reactivity previously observed between such metal complexes and other Lewis bases. Insertions of one, or several, isonitriles into the metal-boron bond give cyclic and spiro compounds with bonding controlled by the electron count at the metal and in the ring.

Boryl, borylene, and base-stabilized borylene complexes of manganese and iron undergo a range of different reactions when treated with isonitriles including single, double, and partial isonitrile insertions into metal-boron bonds, ring formation, isonitrile coupling, and the liberation of new monovalent boron species. Two of the resulting cyclic species have also been found to react selectively with anhydrous HCl to form ring-opened products. The diverse isonitrile-promoted reactivity of transition-metal-boron compounds has been explored computationally.

Reaction of N-heterocyclic carbenes with ferroborirene complex [{(η(5)-C5Me5)Fe(CO)2}{BC2(SiMe3)2}] results in heterolytic Fe-B bond cleavage, yielding borironium ions, a new class of boron-containing heterocycles. The reaction rests on the surprising ability of the reactive [(η(5)-C5Me5)Fe(CO)2](-) anion to act as a leaving group. The properties of these species were investigated by multinuclear NMR spectroscopy, as well as single-crystal X-ray diffraction.