Well-defined molecular catalysts for the reduction of N2 to NH3 with protons and electrons remain very rare despite decades of interest, and are currently limited to systems featuring Mo or Fe. This report details the synthesis of a molecular Co complex that generates superstoichiometric yields of NH3 (>200% NH3 per Co-N2 precursor) via the direct reduction of N2 with protons and electrons. While the NH3 yields reported herein are modest by comparison to previously described Fe and Mo systems, they intimate that other metals are likely to be viable as molecular N2 reduction catalysts. Additionally, comparison of the featured tris(phosphine)borane Co-N2 complex with structurally related Co-N2 and Fe-N2 species shows how remarkably sensitive the N2 reduction performance of potential pre-catalysts are. These studies enable consideration of structural and electronic effects that are likely relevant to N2 conversion activity, including π-basicity, charge state, and geometric flexibility.

An acidic hydride! Thanks to the presence of a π-acceptor cyclic (alkyl)(amino)carbene (CAAC) and of two electron-withdrawing nitrile groups, a borohydride reacts with a base to give a carbenestabilized boryl anion, which reacts with carbon and metal electrophiles at the boron center.

The CAAC [CAAC=cyclic (alkyl)(amino)carbene] family of carbene ligands have shown promise in stabilizing unusually low-coordination number transition-metal complexes in low formal oxidation states. Here we extend this narrative by demonstrating their utility in affording access to the first examples of two-coordinate formal Fe(0) and Co(0) [(CAAC)2M] complexes, prepared by reduction of their corresponding two-coordinate cationic Fe(I) and Co(I) precursors. The stability of these species arises from the strong σ-donating and π-accepting properties of the supporting CAAC ligands, in addition to steric protection.

The synthesis and X-ray crystal structure of a potassium adduct of a monoanionic CNC-pincer ligand featuring two mesoionic carbenes is reported. Owing to the peculiar electronic and steric properties of this ligand, the first neutral stable Ni(II)-hydride, and an unusual Cu(II) complex displaying a seesaw geometry, have been isolated.It is well known that tridentate pincer ligands not only give rise to robust catalysts, but also allow for isolating extremely reactive metal centers. 1 A large number of both neutral and monoanionic pincer ligands featuring N-heterocyclic carbenes (NHCs) have been prepared, and the corresponding complexes used as catalysts for various chemical transformations. 2 However, only four complexes, featuring a strongly donating amido-moiety as the central coordinating atom, flanked by two NHC wing-tip groups in a CNC-fashion, have been reported (A-C) (Chart 1). 3 Moreover, pincer ligands based on the novel generation of carbenes, namely mesoionic carbenes (MICs), 4 which are even stronger donors than NHCs, have been even less explored. For the CCC-tridentate binding mode, a handful of examples are known with imidazol-4-ylidenes, 4,5 whereas with 1,2,3-triazol-5-ylidenes, 6 binuclear bridged complexes 7 or mononuclear complexes with bidentate ligands where the central C-atom does not ligate, 8 are exclusively found. A neutral CNC-analogue of tridentate terpyridine, [2,6-bis(1,2,3-triazol-5-ylidene)-pyridine], is the only example of a bisMIC pincer acting as a tridentate ligand. 9 Here we report the synthesis of the first stable anionic CNC-tridentate ligand featuring terminal 1,2,3-triazol-5-ylidenes and a central amido functionality, its mononuclear tridentate Ni(II)-hydride and Cu(II) complexes.The planar carbazole backbone with its rigid geometry seemed an attractive choice for the design of a bis(mesoionic carbene)amido pincer-type ligand. The synthesis of the dicationic salt precursor, namely the bis(1,2,3-triazolium)carbazole 1, was achieved in 43% yield by an adapted version of the formal 1,3-dipolar cycloaddition between a 1,3-diaza-2-azoniaallene salt and a 1,8-diethynylcarbazole (Scheme 1). 6a Addition of 3 equivalents of potassium hexamethyldisilazide (KHMDS) to a THF solution of 1 at À78 1C resulted only in the deprotonation of the carbazole, keeping unchanged the two pendant 1,2,3-triazolium moieties. The cationic salt 2 was isolated in 93% yield as an air-and moisture-stable red solid. The monodeprotonation of 1 is indicated by the absence of the N-H resonance in the 1 H NMR spectrum, and by the presence of a triazolium C-H signal (2H) at 10.03 ppm (see ESI †). The structure of 1 was confirmed by an X-ray diffraction study (Fig. 1). When a large excess of KHMDS (5 equivalents) was added to a diethylether suspension of 1 at À78 1C, the potassium salt 3 could be isolated in good yield (72%) after extraction with hexanes. In the 1 H NMR spectrum, the disappearance of the acidic N-H and triazolium C-H signals confirmed the formation of the triply deprotonated compound 3. In th...