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...
