Modular Synthesis of Bidentate Triazolyl-Functionalized N-Heterocyclic Carbenes and Their Palladium Complexes

Abstract

New heterobidentate N-heterocyclic carbene-triazolyl ligands and several of their palladium(II) complexes have been synthesized in a modular fashion using click chemistry. These complexes are the first examples where triazolyl-substituted NHCs exhibit bidentate behavior, which was confirmed by NMR and X-ray diffraction studies. The synthesis of the complexes could be achieved in relatively few steps by introducing the diversity at a late stage in the synthesis. The complexes are active precatalysts in the transfer semihydrogenation of alkynes to Z-alkenes, with activity and selectivity depending on the triazolyl substituent and the NHC nitrogen substituent. Selectivities as high as 99% were observed.

Bulky Picolyl Substituted NHC Ligands and Their Pd0 Complexes

“…[5] A significant influence of the nitrogen donor on the activity and selectivity were found. [7] The results of our screening with differently substituted NHC-picolyl ligands are summarized in Table 1.…”

“…In previous publications, we have shown the effect of various nitrogen donors on the coordinative properties of the ligands and the catalytic activity and selectivity of their zero-and divalent palladium complexes. [5][6][7]26] The use of picoline as donor in combination with NHC ligands is also known, and the coordination chemistry of these flexible aromatic have been studied by NMR and X-ray diffraction, confirming their bidentate nature. The complexes are active precatalysts in the transfer semihydrogenation of alkynes to Z-alkenes, with activity and selectivity depending on the picolyl substituent to a high degree.…”

Cationic Rh and Ir complexes containing bidentate imidazolylidene–1,2,3-triazole donor ligands: synthesis and preliminary catalytic studies

“…A saturated aqueous sodium EDTA solution (preferred) or saturated aqueous ammonium chloride solution was used to remove the copper catalyst. During the course of our studies, Elsevier et al 5 and Chen et al 6 reported the synthesis of a number of analogous imidazolium-1,2,3-triazole ligands with either Br or PF 6 − as the counter anion and their complexes with palladium. Ligand 1d with an aniline functional group (n = 1; R 2 = NH 2 , Scheme 1) was synthesised by the reduction of the corresponding nitro compound 1c with hydrazine hydrate in the presence of palladium on activated carbon.…”

“…There have been only two reports of late transition metal complexes containing mixed NHC-1,2,3-triazolyl bidentate ligands, and each of these reported the synthesis of palladium complexes. 5,6 More recently, Cowie and co-workers reported the synthesis of a number of imidazolium-1,2,3-triazolyl salts but then alkylated the 1,2,3-triazolyl moiety to triazolium. They † Electronic supplementary information (ESI) available: Syntheses and characterization of ligands 1b-c and 1e-g; Rh(I) complexes 2b-d and 3b-d; Rh(III) complexes 4b-d; Ir(III) complexes (5b-c).…”

Crystal structure of di-μ-chlorido-bis[1-(2-(4-nitrophenyl)amidoethyl)-3- (2,4,6-trimethylphenyl)imidazol-2-ylidene-κC]di(dichlorido)dipalladium( II)) (Pd–Pd) dichloromethane solvate, C43H46Cl6N8O6Pd2

“…It was shown thatakinetically stable trans-isomer was obtained when two equivalents of NHC were coordinated to palladium [3].When only one equivalent is used, the dinuclear species is formed preferentially over thee xpectedm ononuclear species in which the ligand coordinates bidentate [4][5][6]. However, theamide does play arole, in contrast to earlier reports [3].…”