Powerful Bis-facially Pyrazolate-Bridged Dinuclear Ruthenium Epoxidation Catalyst

Abstract

ABSTRACT:A new bis-facial dinuclear ruthenium complex, {[Ru II (bpy)] 2 (μ-bimp)(μ-Cl)} 2+ , 2 2+ , containing a hexadentate pyrazolate-bridging ligand (Hbimp) and bpy as auxiliary ligands has been synthesized and fully characterized in solution by spectrometric, spectroscopic and electrochemical techniques. The new compound has been tested with regard to its capacity to oxidize water and alkenes. The in situ generated bis-aqua complex, {[Ru II (bpy)(H 2 O)] 2 (μ-bimp)} 3+ , 3 3+ , results in low efficiencies and selectivities when oxidizing water, but is an excellent catalyst for the epoxidation of a wide range of alkenes. High turnover numbers (TN), up to 1900, and turnover frequencies (TOF), up to 73 min -1 , are achieved using PhIO as oxidant. Moreover, 3 3+ presents an outstanding stereospecificity for both cis and trans olefins towards the formation of their corresponding epoxides due to specific interactions transmitted by its ligand scaffold. A mechanistic analysis of the epoxidation process has been performed based on DFT calculations in order to better understand the putative cooperative effects within this dinuclear catalyst.The epoxidation of olefins, a process of great industrial and economical importance, has historically constituted a great challenge for the organic synthetic chemists. 1,2 Epoxides constitute a family of essential chemicals, particularly for the synthesis of various polymers (polyglycols, polyamides, polyurethanes, etc.), 3,4 and fine chemicals such as pharmaceuticals, food additives or flavor and fragrance compounds. 5 For instance, propylene oxide monopolizes the epoxide chemical business with a yearly 8 million Ton production and an expected annual increase of 5%. 6 Ru complexes are excellent catalysts for redox transformations such as alcohol oxidation, 7,8,9,10,11,12,13,14 sulfoxidation, 15,16,17,18 water oxidation 19,20,21,22,23,24,25,26,27,28 and epoxidation. 14,29,30,31,32,33,34,35,36,37 In all these cases, a Ru IV =O or Ru V =O group has been shown to be the active catalytic unit. Most of the literature related to redox catalysis using Ru complexes is based on mononuclear complexes, since they are generally easily accessible from a synthetic point of view. In sharp contrast, two powerful diruthenium epoxidation catalysts in terms of epoxide selectivity and substrate conversion have been recently reported by our research group. 38,39 In addition, these new catalysts display distinctive reactivity with regards to cis and trans alkenes. Both features are proposed to be caused by a hydrogen bonding interaction between the second Ru IV =O site and the substrate employed, together with steric effects.Our group has an extensive experience on the synthesis, characterization and evaluation of the oxidative catalytic performance of dinuclear Ru complexes, most of them inspired by the well-known {[Ru II (trpy)] 2 (μ-bpp)(μ-Cl)} 2+ water oxidation catalyst. 40 Modifications around this paradigmatic compound, like the replacement of the trpy auxiliary ligands by facially coordinating ...

Strengths, Weaknesses, Opportunities and Threats: Computational Studies of Mn- and Fe-Catalyzed Epoxidations

“…Being in the second row of the transition metal series, and being located bellow iron, ruthenium systems often share some similarities to their iron counterparts, having attained the attention of several researchers [119,120]. Unlike iron, di-nuclear ruthenium complexes are common and have been extensively studied using DFT methods [121,122].…”

Ditopic Hexadentate Ligands with a Central Dihydrobenzo‐diimidazole Unit Forming a [2x2] Zn4 Grid Complex

“…[6,7] The resulting complexes feature different catalytic, magnetic, and photophysical properties and may enable new cooperative functionality. [8][9][10][11][12] In this context, a number of bimetallic complex catalysts have been reported, e. g., in asymmetric allylic alkylation, [13] water oxidation, [14][15][16][17][18] epoxidation, [19] and transfer hydrogenation of ketones. [20] The probably most adaptable and established ligands for this purpose are derivatives of 2,2':6',2"-terpyridine (terpy) type containing, inter alia, pyrazole, tetrazole, and imidazole subunits.…”

Ruthenium Catalyst for Epoxidation Reaction

“…One study undertook the synthesis of bis-facial dinuclear ruthenium complex that included a hexadentate pyrazolate-bridging ligand (Hbimp) and bpy as auxiliary ligands [34]. Additionally, the ability of water and alkene oxidation of this complex was assessed.…”