Boronyl chemistry: the BO group as a new ligand in gas-phase clusters and synthetic compounds.

Boronyl (BO) is a monovalent σ radical with a robust B≡O triple bond. Although BO/BO(-) are isovalent to CN/CN(-) and CO, the chemistry of boronyl has remained relatively unknown until recently, whereas CN/CN(-) and CO are well-known inorganic ligands. Further analogy may be established for BO versus H or Au ligands, which are all monovalent σ radicals. This Account intends to provide an overview of research activities over the past few years that are relevant to the development of boronyl chemistry, in particular, in size-selected gaseous clusters containing BO. The systems covered herein include transition metal boronyl clusters, carbon boronyl clusters, boron oxide clusters and boron boronyl complexes, the boronyl boroxine, and the first synthetic Pt-BO bulk compound. In these boronyl clusters and compounds, the BO groups show remarkable structural and chemical integrity as a ligand. Among transition metal boronyls, gold monoboronyl clusters Aun(BO)(-) and Aun(BO) (n = 1-3) have been characterized, and they are shown to possess electronic and structural properties similar to the corresponding Au(n+1)(-) and Au(n+1) bare clusters, demonstrating the BO/Au analogy. The Au-B bonding in the Au-BO clusters is highly covalent. A recent advance in boronyl chemistry is the successful synthesis and isolation of the first boronyl compound, trans-[(Cy3P)2BrPt(BO)]. This unique Pt-BO compound and other potential transition metal boronyl compounds may find applications in catalysis and as chemical building blocks. Carbon boronyl clusters versus boron carbonyl clusters is a topic of interest in designing new aromatic complexes. Experimental and theoretical data obtained to date show that carbon boronyl clusters are generally far more stable than their boron carbonyl counterparts, highlighting the potency of boronyl as a ligand in aromatic compounds. Notably, in light of the BO/H analogy, the perfectly hexagonal (CBO)6 cluster is a carbon boronyl analogue of benzene. The BO groups also dominate the structures and bonding of boron oxide clusters and boron boronyl complexes, in which BO groups occupy terminal, bridging, or face-capping positions. The bridging η(2)-BO groups feature three-center two-electron bonds, akin to the BHB τ bonds in boranes. A close isolobal analogy is thus established between boron oxide clusters and boranes, offering vast opportunities for the rational design of novel boron oxide clusters and compounds. Boron boronyl clusters may also serve as molecular models for mechanistic understanding of the combustion of boron and boranes. An effort to tune the B versus O composition in boron oxide clusters leads to the discovery of boronyl boroxine, D3h B3O3(BO)3, an analogue of boroxine and borazine and a new member of the "inorganic benzene" family. Furthermore, a unique concept of π and σ double conjugation is proposed for the first time to elucidate the structures and bonding in the double-chain nanoribbon boron diboronyl clusters, which appear to be inorganic analogues of polyenes, cumulenes, and polyynes. This Account concludes with a brief outlook for the future directions in this emerging and expanding research field.