Catalyst Activation with Cp*Rh^III/Ir^III–1,2,3-Triazole-Based Organochalcogen Ligand Complexes: Transfer Hydrogenation via Loss of Cp* andN-MethylmorpholineN-Oxide Based vs Oppenauer-Type Oxidation

Abstract

The reactions of 1-benzyl-4-((phenylthio/phenylseleno)methyl)-1H-1,2,3-triazole (L1/L2) and 4-phenyl-1-((phenylthio/phenylseleno)methyl)-1H-1,2,3-triazole (L3/L4), synthesized using the click reaction, with [(η5-Cp*)RhCl(μ-Cl)]2 and [(η5-Cp*)IrCl(μ-Cl)]2 at room temperature followed by treatment with NH4PF6 result in complexes of the type [[(η5-Cp*)M(L)Cl] (1–8). Their HR-MS and 1H, 13C{1H}, and 77Se{1H} NMR spectra have been found characteristic. The single-crystal structures of 2, 3, and 6 have been established by X-ray crystallography. There is a pseudo-octahedral “piano-stool” disposition of donor atoms around Rh/Ir. In 1, 2, 5, and 6 N(3) of the triazole skeleton coordinates with Rh/Ir, whereas in the other four complexes the nitrogen involved is N(2). These complexes have been explored as catalysts for N-methylmorpholine N-oxide (NMO) based and Oppenauer-type oxidation of alcohols and transfer hydrogenation (TH) of carbonyl compounds with 2-propanol. Oppenauer type oxidation is somewhat slower than that based on NMO. The homogeneous nature of TH is supported by a poisoning test. The catalytic processes are more efficient with Rh complexes than the corresponding Ir complexes. The complexes having N(2) coordinated with the metal have shown better activity than those in which N(3) is involved in ligation. The reactivity with respect to ligands is in the order Se > S. In TH the species formed with loss of Cp* appears to be involved in catalysis with Rh as well as Ir complexes. Such a loss is noticed in the case of Rh for the first time. Generally results of DFT calculations are consistent with the experimental results.

The Golden Age of Transfer Hydrogenation

“…418 The complexes 127 having the N(2) atom of 1,2,3-triazole coordinated to the rhodium center have shown better activity in the TH of ketones than 126 in which N(3) is involved in ligation. Moreover, the reactivity with respect to the ligands is in the order Se > S. 479 The mechanistic studies indicated that the Rh species that had lost the Cp* ligand was an intermediate in the catalytic cycle. The iridium analogues of complexes 123, 124, 126, and 127 were also synthesized and exhibited even better catalytic performances in TH reactions than these rhodium counterparts (Scheme 46).…”

Palladium(ii) complexes bearing the 1,2,3-triazole based organosulfur/ selenium ligand: synthesis, structure and applications in Heck and Suzuki–Miyaura coupling as a catalyst via palladium nanoparticles

“…Thus argon or nitrogen atmosphere 22,23 is required during catalysis to protect catalyst from deactivation. In view of moisture and air sensitivity of these complexes, a number of Pd(II) species bearing chalcogen (S, Se or Te) ligands, [26][27][28] have been explored as possible alternatives to catalyse various chemical transformations 29,30 including C-C coupling reactions. [31][32][33][34][35][36][37][38][39][40][41][42][43][44][45][46][47][48] The catalytic activity of some of these complexes (thermally stable and resistant to aerial oxidation) not only rivals but vastly outperforms that of the corresponding phosphorus analogues.…”

“…These ligands are 1,2,3-triazole based and in the recent past, applications of such organochalcogen ligands in designing catalysts for transfer hydrogenation and oxidation have been reported by our group. 29,30 The synthesis of palladium complexes of a (P, C, S) pincer ligands having 1,2,3-triazole skeleton and their applications in Heck coupling are known. 49 The presence of 1,2,3-triazole unit in a ligand offers an interesting alternative to the traditional phenyl based framework and has been found to render interesting variation in catalytic activities of metal complexes of such ligands, due to potential of this unit to act as a N donor through two types of nitrogen atoms: viz., N(2) and N(3).…”

Synthesis and Characterization of Benzimidazolium Salts Bearing Triazole Groups

“…This method has been widely used for synthesizing several ligands comprising 1,4-disubstituted 1,2,3-triazolyl groups [9][10][11][12][13][14]. The metal complexes of these ligands have drawn attention because of their catalytic activities in many organic transformations [15][16][17][18].…”