A Ru-pentadentate polypyridyl complex [Ru(κ-N-bpy2PYMe)Cl] (1, bpy2PYMe = 1-(2-pyridyl)-1,1-bis(6-2,2'-bipyridyl)ethane) and its aqua derivative [Ru(κ-N-bpy2PYMe)(HO)] (2) were synthesized and characterized by experimental and computational methods. In MeOH, 1 exists as two isomers in different proportions, cis (70%) and trans (30%), which are interconverted under thermal and photochemical conditions by a sequence of processes: chlorido decoordination, decoordination/recoordination of a pyridyl group, and chlorido recoordination. Under oxidative conditions in dichloromethane, trans-1 generates a [Ru(κ-N-bpy2PYMe)Cl] intermediate after the exchange of a pyridyl ligand by a Cl counterion, which explains the trans/cis isomerization observed when the system is taken back to Ru(II). On the contrary, cis-1 is in direct equilibrium with trans-1, with absence of the κ-N-bis-chlorido Ru-intermediate. All these equilibria were modeled by density functional theory calculations. Interestingly, the aqua derivative is obtained as a pure trans-[Ru(κ-N-bpy2PYMe)(HO)] isomer (trans-2), while the addition of a methyl substituent to a single bpy of the pentadentate ligand leads to the formation of a single cis isomer for both chlorido and aqua derivatives [Ru(κ-N-bpy(bpyMe)PYMe)Cl] (3) and [Ru(κ-N-bpy(bpyMe)PYMe)(HO)] (4) due to the steric constraints imposed by the modified ligand. This system was also structurally and electrochemically compared to the previously reported [Ru(PY5Me)X] system (X = Cl, n = 1 (5); X = HO, n = 2 (6)), which also contains a κ-N-Ru coordination environment, and to the newly synthesized [Ru(PY4Im)X] complexes (X = Cl, n = 1 (7); X = HO, n = 2 (8)), which possess an electron-rich κ-NC-Ru site due to the replacement of a pyridyl group by an imidazolic carbene.
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 ...
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.