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.

The element boron is known to have a variety of ways to relieve its inherent electron deficiency. The acceptance of an electron pair (Lewis acidity) has applications in catalysis [1] and activation of element-element bonds (frustrated Lewis pairs). [2] The combination of boron with p-donating substituents (e.g. BF 3 ) and its incorporation into organic p-conjugated systems allows the empty p z orbital of boron to participate in p bonding and p conjugation, respectively, and the latter enables the use of boron in optoelectronic materials with unique properties. [3] The absence of p-donating substituents at the boron center may result in multiple-center bonding to form nonclassical frameworks (e.g. B 2 H 6 or clusters). In addition, organoboranes and -diboranes(4) are prone to accept a single electron by chemical reduction. [4] Likewise, hydrogen atom abstraction from N-heterocyclic carbene (NHC)-stabilized boranes (NHC-BH 3 ) can lead to neutral, persistent [4f] boryl radicals of the type NHC-BH 2 C, [5] which have been studied by means of cyclic voltammetry, EPR, and UV/Vis spectroscopy as well as trapping reactions. [4][5][6] However, examples of isolated boron radicals are rare owing to the reactive nature of the species, and only little is known about their structural properties. Steric protection of the boron center combined with spin delocalization over the organic substituents, both achieved by substitution with mesityl groups (Mes = 2,4,6-trimethylphenyl), has occasionally enabled isolation and structural characterization of radical anions such as [Li ([12]crown-4) 2 ][BMes 3 ] (1) or [K([18]crown-6)-(thf) 2 ][Mes 2 BB(Ph)Mes] (2). [7]Our group has recently studied a persistent radical anion as an intermediate in the stepwise reduction of 1-ferrocenyl-2,3,4,5-tetraphenylborole (3). [8] Boroles are a class of antiaromatic compounds with interesting chemical and photophysical properties [9,10] that are well-known for their ability to accept two electrons with formation of an aromatic borole dianion. [11,12] Encouraged by these recent results on the radical anion [3]C À , which indicated the presence of a highly unusual C 4 B p system bearing five electrons, [8] we set out to isolate and characterize a stable borol radical anion. As we report here, this was possible by choice of steric protection and an appropriate reducing agent.The synthesis of MesBC 4 Ph 4 (1-mesityl-2,3,4,5-tetraphenylborole, 4) by means of the commonly employed tin-boron exchange reaction [11][12][13][14] was unsuccessful because of the low reactivity of dihalo(mesityl)boranes (MesBX 2 ; X = Cl, Br). However, 4 was obtained in 41 % yield by functionalization of the boron center in 1-chloro-2,3,4,5-tetraphenylborole (5) through nucleophilic displacement of the chlorine ligand with LiMes (Scheme 1). [14b] A more efficient alternative was found to be the salt-elimination reaction of MesBCl 2 with 1,4dilithio-2,3,4,5-tetraphenylbuta-1,3-diene (6) which provided 4 in 66 % yield. Formation of a Lewis acid-base adduct with Et 2 O, as previou...

A new compound with the formula L-B2-L wherein the stabilizing ligand (L) is 1,3-bis[diisopropylphenyl]-4,5-dihydroimidazol-2-ylidene (SIDip) has been synthesized, isolated, and characterized. The π-acidity of the SIDip ligand, intermediate between the relatively non-acidic IDip (1,3-bis[diisopropylphenyl]imidazol-2-ylidene) ligand and the much more highly acidic CAAC (1-[2,6-diisopropylphenyl]-3,3,5,5-tetramethylpyrrolidin-2-ylidene) ligand, gives rise to a compound with spectroscopic, electrochemical, and structural properties between those of L-B2-L compounds stabilized by CAAC and IDip. Reactions of all three L-B2-L compounds with CO demonstrate the differences caused by their respective ligands, as the π-acidities of the CAAC and SIDip carbenes enabled the isolation of bis(boraketene) compounds (L(OC)B-B(CO)L), which could not be isolated from reactions with B2(IDip)2. However, only B2(IDip)2 and B2(SIDip)2 could be converted into bicyclic bis(boralactone) compounds.

Completing the set: Although 1,2‐azaborine is known, little is known about the 1,3‐ and 1,4‐analogues. Now a simple, controlled synthesis of 1,2‐di‐tert‐butyl‐1,2‐azaborine from acetylene and di‐tert‐butyliminoborane has been achieved by a metal‐mediated formal [2+2+2] cycloaddition reaction. A 1,2‐azaborete pianostool complex was identified as an reaction intermediate.