A Novel Carbene Ruthenium Complex as Reusable and Selective Two‐Electron Catalyst for Alkene Epoxidation

Abstract

A new ruthenium aquo catalyst with the formula trans-[Ru(II)A C H T U N G T R E N N U N G (CN-Me)A C H T U N G T R E N N U N G (trpy)OH 2 ]A C H T U N G T R E N N U N G (PF 6 ) 2 [where trpy = 2,2':6',2''-terpyridine and CN-Me = 3-methyl-1-(pyridin-2-yl)-imidazolylidene] has been prepared and thoroughly characterized by spectroscopic and electrochemical techniques. The complex has been tested in epoxidation catalysis both in dichloromethane and dichloromethane:ionic liquid media, displaying excellent performances and selectivities. Reuse of the catalyst in ionic liquid:solvent media has been explored for the first time in ruthenium-mediated epoxidation catalysis and its performance is fully maintained for up to ten runs.Keywords: epoxidation; ionic liquids; reutilization; ruthenium-carbene Polypyridyl ruthenium complexes have been widely developed during the past decades given their attractive chemical, photochemical and electrochemical properties [1] that permit their application in a broad scope of fields such as photochemistry/photophysics [2] or bioinorganics [3] as well as in a large diversity of catalytic processes.[4] In the specific field of redox catalysis, the rationalization of catalyst performance on the basis of electronic and structural parameters is particularly complicated due to the involvement of distinct redox state species and so the development of new metal complexes with novel properties is essential to understand and optimize the mechanisms followed by such catalytic species.In this sense, the use of N-heterocyclic carbene (NHC) species as ligands for transition metal complexes has experienced a growing interest throughout the last years [5,6] given their particular electronic properties. A wide scope of carbenic d block metal complexes have been tested as catalysts for a diversity of redox chemical transformations [7] and, in the specific case of ruthenium-NHC complexes, applications such as olefin metathesis, [5c,8] hydrogenation/transfer hydrogenation [9] or alkylation [10] have been extensively developed. However, the use of Ru-NHC species in oxidative processes is still quite limited. [9d-e,11] Olefin epoxidation is a key chemical transformation for the synthesis of highly functionalized organic species and the use of several transition metal complexes for this type of process has been well documented. [12] Among these, the use of NHC as ligands for epoxidation catalysts has been investigated mainly with Mo species [13] and also with a few examples of Mn [14] or W [13a] complexes but, although Ru complexes are powerful catalysts broadly studied for epoxidation processes in homogeneous catalysis, [15] a unique example of epoxidation mediated by a Ru-NHC catalyst has been reported to date.[11a]On the other hand, heterogenization and reuse of catalysts are fields of unquestionable importance especially towards their application in large-scale processes.[16] Room-temperature ionic liquids (RTIL) have been broadly studied as alternative solvent or co-solvent media in catal...

A Ruthenium(II) Aqua Complex as Efficient Chemical and Photochemical Catalyst for Alkene and Alcohol Oxidation

“…This fact could be explained on the basis of electronic effects. In the trans Ru5b isomer, the more sigma‐donor pyrazolyl ring of the bidentate pypz‐H ligand is coordinated trans to the monodentate Cl ligand (see Scheme ), allowing a better distribution of the electronic density through elongation of the Ru‐Cl bond length, as we have observed in another family of complexes containing the highly sigma‐donor carbene CN‐Me ligand . In the corresponding cis isomer, the chelate character of the trpy ligand prevents such structural rearrangement.…”

“…All bond lengths and angles are within the expected values for this type of complexes. [14,15] Regarding the two dimeric compounds (see SI), their structures show again distorted octahedral geometries around the metal centers, with the deprotonated pyrazole rings of the corresponding pypz-H ligands bridging the two ruthenium atoms in both cases. The trpy and pypz-arrangement around each Ru metal center corresponds to that of a cis isomer and this leads to an additional hypothesis to explain the lower occurrence of Ru5a in the synthesis of the chloro complex, as the easy deprotonation of the pyrazole proton in Ru5a could result in its conversion into the dinuclear compounds described.…”

“…In the trans Ru5b isomer, the more sigma-donor pyrazolyl ring of the bidentate pypz-H ligand is coordinated trans to the monodentate Cl ligand (see Scheme 2), allowing a better distribution of the electronic density through elongation of the Ru-Cl bond length, as we have observed in another family of complexes containing the highly sigma-donor carbene CN-Me ligand. [15] In the corresponding cis isomer, the chelate character of the trpy ligand prevents such structural rearrangement. On the other hand, the trans isomer probably allows a stronger hydrogen bond interaction between the chloride ligand and the H proton in cis, be- longing to a pyridyl ring spatially closer to what the smaller pyrazolyl ring would be (H(1)-Cl distance in the trans complex Ru5b is 2.768 Å as inferred from X-ray diffraction analysis, see below).…”

Tuning two-electron transfer in terpyridine-based platinum(ii) pincer complexes

“…For example, Wilkinson's catalyst (RhCl(PPh 3 ) 3 ) for olen hydrogenation, 1 Shilov's catalyst (Pt(OH 2 ) 2 Cl 2 ) for alkane oxidation, 2 and Heyduk's and Nocera's photocatalyst 3 for the reduction of hydrohalic acids to hydrogen. The relative instability of the middle electron conguration, d 7 in the above examples, compared to the initial and the two-electron-oxidized nal congurations, d 8 and d 6 respectively, has been shown to be effective in generating cooperative two-electron transfer. 4 Similar relative instability of the middle electron conguration leading to two-electron transfer has been reported for a few ruthenium complexes such as reversible oxidation of 4+ .…”

Synergistic oxygen atom transfer by ruthenium complexes with non-redox metal ions

“…The absorption spectrum of compound 1 is highly consistent with previous results for the (bpy) 2 Ru(IV)vO species, [76][77][78][79] which gives the first evidence of our proposed reaction pathway: Ru(II) is oxidized to the Ru(IV)vO species using PhI(OAc) 2 as a stoichiometric oxidant. 11,25,80 Upon addition of 20 equiv. of cyclooctene as the substrate, this yellow solution of high valent species, (bpy) 2 Ru(IV)vO, gradually changes back to deep purple.…”