Synthesis and Reactivity of Heteroditopic Dicarbene Rhodium(I) and Iridium(I) Complexes Bearing Chelating 1,2,3-Triazolylidene–Imidazolylidene Ligands

Abstract

1,2,3-Triazol-5-ylidenes (tzNHC) have become a popular class of NHC ligands in homogeneous catalysis. Herein, we introduce chelate monovalent Rh- and Ir(cod) complexes bearing bidentate ligands that combine this tzNHC and an Arduengo-type NHC motif. The reactivity of these complexes with H2 and CO gas has been investigated, leading to an interesting octahedral [Ir(tzNHC-CH2-NHC)(CO)2(H)2]OTf complex and [M(tzNHC-CH2-NHC)(CO)2]OTf complexes. The carbonyl stretching frequencies of the latter indicate that the ligand has stronger electron-donating properties than classic di-NHC ligands. The square planar rhodium and iridium NHC-tzNHC complexes have been applied in transfer hydrogenation employing isopropyl alcohol as the hydrogen donor, in which they show moderate activity (Ir > Rh) toward a range of ketones as well as for an aldehyde, an imine, and a diene. The new dicarbene complexes proved to be more active for this reaction than the analogues in which the triazolyl moiety coordinates through a nitrogen donor.

Synthesis, Structure and Antitumoural Activity of Triazole‐Functionalised NHC–Metal Complexes

“…In agreement with the literature, [13,14] the methylation is occurring only at the imidazole moiety since the functionalisation of the 3-N triazole ring requires the use of methylating agents stronger than methyl iodide. In fact, by using methyl triflate or Meerwein's salt, trimethyloxonium tetrafluoroborate, in chlorinated solvents proligands 2 and 3 were obtained in moderate yields.…”

“…In the 1 H NMR spectra, the signal relative to the imidazole C2-H atom is absent, confirming the deprotonation of the imidazolium salt. The formation of the silver(I) complex was also assessed through 13 C{ 1 H} NMR spectra that show a signal at δ = 182.0 ppm associable to the C2 carbon atom, in the typical range for carbene carbon atoms coordinated to an Ag I centre. [15][16][17] Scheme 2.…”

“…C 41.17, H 4.06, N 16.85; found C 41.52, H 4.31, N 16.28. 1 H NMR (CDCl 3 ): δ = 4.08 (s, 3 H, CH 3 ), 4.16 (s, 3 H, CH 3 ), 5.63 (s, 2 H, CH 2 ), 7.39 (s, 5 H, CH Ar), 7.63 (s, 1 H, CH), 8.27 (s, 1 H, CH tz), 9.72 (s, 1 H, CH im) ppm 13. C{ 1 H} NMR (CDCl 3 ): δ = 36.7 (CH 3 ), 37.3 (CH 3 ), 54.7 (CH 2 ), 121.3 (CH), 124.4 (C), 126.9 (C), 128.5 (CH Ar), 129.2 (CH Ar), 129.3 (CH Ar), 133.7 (CH tz), 137.8 (CH im) ppm; C Ar signal not detected.…”

Hydrogen Transfer Reactions of Carbonyls, Alkynes, and Alkenes with Noble Metals in the Presence of Alcohols/Ethers and Amines as Hydrogen Donors

“…Continuing the success of the NHC-type ligand family, a series of cationic chelate rhodium complexes (iridium as well) have been synthesized, bearing a mixed 1,2,3-triazol-5-ylidene (tzNHC)-NHC moiety (Scheme 6, equation B) [45]. The dicarbene tzNHC-NHC derivatives showed significantly higher catalytic activities in the comparison with the ones with one NHC and a secondary nitrogen donor.…”

Imino Transfer Hydrogenation Reductions

“…[175][176][177][178] A carbene derived from triazole was used in IPA with K2CO3 as base, for direct reductive amination of aldehydes with primary amines to form secondary amine products. [179] Other recent examples include; i) a base-free catalyzed reduction of a series of C═O bonds and of the C═N bond of benzylideneaniline (>99% conversion was achieved in 48h, with 0.1 mol% catalyst), [180] ii) heteroditopic dicarbene Rh(I) and Ir (I) complexes containing 1,2,3-triazolylidene-Imidazolylidene ligands, mostly tested on acetophenone but with one imine example, [181] iii) Ir complexes of N-benzyl substituted N-heterocyclic carbenes where 0.5 mol% catalyst is used with 5% KOH in IPA in reductions to give products in >99% yields [182] and iv) catalyst system, was prepared and used to reduce C=O and C=N bonds. [191] Some cobalt examples have recently been published; cobalt on a heterogeneous support was used in TH of C=O, C=N and C=C bonds using IPA as the reducing agent.…”