Isolation of a potassium bis(1,2,3-triazol-5-ylidene)carbazolide: a stabilizing pincer ligand for reactive late transition metal complexes

Abstract

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...

Isolation and Structures of 1,2,3‐Triazole‐Derived Mesoionic Biscarb­enes with Bulky Aromatic Groups

“…The UV absorption band at around λ = 400 nm and a near-IR band at λ = 750 nm is consistent with the simulated electronic transition spectrum of an anionic isomer of 3a (see the Supporting Information). An alkali-metal-coordinated anion radical complex [9] is also possible, because the stabilization energy by coordination with metal cations was comparable to that of the free anion radical, according to the calculations. Other possibilities such as the generation of triazolylidene-to-pyridine charge transfer induced by the positive metal center can be ruled out on the basis of the calculations.…”

“…[9] In this paper, we report the first isolation of free tridentate carbenes and an iron complex. Schubert's sequential method was used to synthesize the carbenes, as shown in Scheme 1.…”

Copper(I) Complexes of Anionic Tridentate CNC Pincer Ligands

“…The copper center adopts a pseudo T‐shaped arrangement, with a large C–Cu–C angle (151.41(12)°] due to the steric constraints of the CNC pincer. Copper complexes that display even more closer T‐shape have also been observed but involve much bulkier CNC ligands …”

Comparing Isomeric Tridentate Carbazole‐Based Click Ligands: Metal Complexes and Redox Chemistry

“…[9] Very recently corresponding Co complexes have been investigated as well. [10] Here, we presentacomparative study of the two Hbtc ligands (Scheme 1) based on first investigations on the coordination behavior of the btc2 anion and an extension of the scarce knowledgeo nb tc1 complexes.B oth ligands proved redox active, and through detailede xamination of corresponding Ru, Zn, andN ic omplexes we were able to reveal common properties and differences of the two isomeric ligand systems. [10] Here, we presentacomparative study of the two Hbtc ligands (Scheme 1) based on first investigations on the coordination behavior of the btc2 anion and an extension of the scarce knowledgeo nb tc1 complexes.B oth ligands proved redox active, and through detailede xamination of corresponding Ru, Zn, andN ic omplexes we were able to reveal common properties and differences of the two isomeric ligand systems.…”