Homoatomic triple bonds between main-group elements have been restricted to alkynes, dinitrogen, and a handful of reactive compounds featuring trans-bent heavier elements of groups 13 and 14. Previous attempts to prepare a compound with a boron-boron triple bond that is stable at ambient temperature have been unsuccessful, despite numerous computational studies predicting their viability. We found that reduction of a bis(N-heterocyclic carbene)-stabilized tetrabromodiborane with either two or four equivalents of sodium naphthalenide, a one-electron reducing agent, yields isolable diborene and diboryne compounds. Crystallographic and spectroscopic characterization confirm that the latter is a halide-free linear system containing a boron-boron triple bond.

The metals of the s block of the periodic table are well known to be exceptional electron donors, and the vast majority of their molecular complexes therefore contain these metals in their fully oxidized form. Low-valent main-group compounds have recently become desirable synthetic targets owing to their interesting reactivities, sometimes on a par with those of transition-metal complexes. In this work, we used stabilizing cyclic (alkyl)(amino)carbene ligands to isolate and characterize the first neutral compounds that contain a zero-valent s-block metal, beryllium. These brightly coloured complexes display very short beryllium-carbon bond lengths and linear beryllium coordination geometries, indicative of strong multiple Be-C bonding. Structural, spectroscopic and theoretical results show that the complexes adopt a closed-shell singlet configuration with a Be(0) metal centre. The surprising stability of the molecule can be ascribed to an unusually strong three-centre two-electron π bond across the C-Be-C unit.

The complexation of two equivalents of a cyclic (alkyl)(amino)carbene (CAAC) to tetrabromodiborane, followed by reduction with four equivalents of sodium naphthalide, led to the formation of the CAAC-stabilized linear diboracumulene (CAAC)2B2. The capacity of the CAAC ligand to facilitate B2 →CAAC donation of π-electron density resulted in important differences between this species and a previously reported complex featuring a B≡B triple bond stabilized by cyclic di(amino)carbenes, including a longer B-B bond and shorter B-C bonds. Frontier orbital analysis indicated sharing of valence electrons across the entire linear C-B-B-C unit in (CAAC)2B2, which is supported by natural population analysis and cyclic voltammetry.

Many metal-containing compounds, and some metal-free compounds, will bind carbon monoxide. However, only a handful of metal-containing compounds have been shown to induce the coupling of two or more CO molecules, potentially a method for the use of CO as a one-carbon-atom building block for the synthesis of organic molecules. In this work, CO was added to a boron-boron triple bond at room temperature and atmospheric pressure, resulting in a compound into which four equivalents of CO are incorporated: a flat, bicyclic, bis(boralactone). By the controlled addition of one CO to the diboryne compound, an intermediate in the CO coupling reaction was isolated and structurally characterized. Electrochemical measurements confirm the strongly reducing nature of the diboryne compound.

Human dihydroorotate dehydrogenase (DHODH) represents an important target for the treatment of hyperproliferative and inflammatory diseases. In the cell DHODH catalyzes the rate-limiting step of the de novo pyrimidine biosynthesis. DHODH inhibition results in beneficial immunosuppressant and antiproliferative effects in diseases such as rheumatoid arthritis. Here, we present high-resolution X-ray structures of human DHODH in complex with a novel class of low molecular weight compounds that inhibit the enzyme in the nanomolar range. Some compounds showed an interesting dual binding mode within the same cocrystal strongly depending on the nature of chemical substitution. Measured in vitro activity data correlated with the prevailing mode of binding and explained the observed structure-activity relationship. Additionally, the X-ray data confirmed the competitive nature of the inhibitors toward the putative ubiquinone binding site and will guide structure-based design and synthesis of molecules with higher activity.