Boronyl Chemistry: The BO Group as a New Ligand in Gas-Phase Clusters and Synthetic Compounds

CONSPECTUS: Boronyl (BO) is a monovalent sigma 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 sigma 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 dusters, carbon boronyl clusters, boron oxide clusters and boron boronyl complexes, the boronyl boroxine, and the first synthetic Pt-BO bulk compound. In these boronyl dusters and compounds, the BO groups show remarkable structural and chemical integrity as a ligand. Among transition metal boronyls, gold monoboronyl clusters Au-n(BO)(-) and Au-n(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 Aun+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)(2)BrPt(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 eta(2)-BO groups feature three-center two-electron bonds, akin to the BHB tau bonds in boranes. A dose isolobal analogy is thus established between boron oxide clusters and boranes, offering vast opportunities for the rational design of novel boron oxide dusters 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 dusters leads to the discovery of boronyl boroxine, D-3h B3O3(BO)(3), an analogue of boroxine and borazine and a new member of the inorganic benzene family. Furthermore, a unique concept of pi and sigma 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.