“Piano‐Stool” Complexes of Ruthenium(II) Designed with Arenes and N‐[2‐(Arylchalcogeno)ethyl]morpholines: Highly Active Catalysts for the Oxidation of Alcohols with N‐Methylmorpholine N‐Oxide, tert‐Butyl Hydroperoxide and Sodium Periodate and Oxychloride

Abstract

The reactions of [{(η6‐C6H6)RuCl(μ‐Cl)}2] and [{(η6‐p‐cymene)RuCl(μ‐Cl)}2] with N‐[2‐(arylchalcogeno)ethyl]morpholines (L) (aryl = Ph/2‐pyridyl for S, Ph for Se, 4‐MeOC6H4 for Te) and NH4PF6 result in “piano‐stool” complexes of RuII of composition [RuCl(η6‐C6H6)(L)][PF6]/[RuCl(η6‐p‐cymene)(L)][PF6], which give characteristic 1H, 13C{1H}, 77Se{1H}, and 125Te{1H} NMR spectra. Some of them have also been characterized by X‐ray crystallography [Ru–S, Ru–Se, and Ru–Te bond lengths: 2.3815(12)/2.3742(14), 2.4837(14), and 2.6143(7) Å, respectively]. The cyclic voltammograms show that all the complexes undergo irreversible oxidation (E${1 \over 2}$ = 0.290–0.586 V). All the ruthenium complexes have been explored for their catalytic activity in the oxidation of primary and secondary alcohols with N‐methylmorpholine N‐oxide (NMO), tBuOOH, NaOCl, and NaIO4 (TON values upto 9.8 × 104). The efficiency of the catalytic oxidation reaction decreases in the order Te > Se > S. The intermediate species involved in the oxidation reactions appear to incorporate the RuIV=O group.

Synthesis and molecular studies of half-sandwich arene ruthenium and Cp*Rh/Cp*Ir complexes containing (3-picolyl)thiourea derivatives as N,S-bidentate donor ligands

“…Furthermore the distances between metal and centroid of arene/Cp* ring are in range 1.678-1.797 Å , while the M-Cl bond lengths are in the range of 2.3932-2.4212 Å . Both sets of values are close to those found in related complexes reported in the literature [28][29][30][31][32].…”

Reactions of benzene based half sandwich ruthenium(II) complex with 2,6-bis((phenylseleno)methyl)pyridine: Preferential substitution of ring resulting in a catalyst of high activity for oxidation of alcohols

“…The advantages of 1 in comparisons to recently reported Ru based good catalytic species [30,33,36,[41][42][43][44][45] for oxidation of alcohols are: (i) high efficiency/yield as they are needed in less quantity and (ii) short reaction time. The half sandwich Ru(II) complexes containing chalcogenated pyrrolidine, benzotriazole, Schiff base and morpholine known to catalyze the oxidation of alcohols efficiently with NMO [23,25,46,47] (TON up to 9.8 × 10 4 ) are comparable with the present complexes (TON up to 9.7 × 10 4 ). The cyclic voltammetric experiment performed at 298 K in CH 3 CN (0.01 M NBu 4 ClO 4 as supporting electrolyte) for 1 at scan rate 100 mVs − 1 (anodic sweep) reveals an irreversible oxidation by one electron transfer with E 1/2 value 0.587 V (vs. Ag/AgCl) (see Table S3 and Fig.…”

“…S2 in online Supplementary material for details) which are not extreme, implying that both species, Ru(II) and Ru(III), are equally stabilized by the same set of ligand. Such E 1/2 value has been reported earlier [47][48][49] favourable for catalytic oxidation process, of course no one to one relationship unequivocally has been established. The investigations to understand further the properties of 1 and 2 are in progress.…”

“…The percentage yields and conversions do not differ much. The earlier work carried out on the mechanism of such catalytic oxidation suggests that Ru(IV) = O species [23,[25][26][27][28][29][30][31][32][33][34][35]46,47] are involved. On adding NMO to the solution of complex 1 the signal in its 77 Se{ 1 H} NMR spectrum goes to higher frequency ≥353 ppm.…”

“…In UV-visible spectrum, on addition of NMO to a dichloromethane solution of 1, a new shoulder at 392 nm appears which is believed [23,25,[32][33][34][35][36][37]46,47] to be due to Ru(IV) = O species, reported to be responsible for transfer of the oxygen to alcohol substrates resulting in their catalytic oxidation. IR spectrum of the residue left after evaporating off solvent from the mixture of NMO with 1, exhibits a very strong band at 845 cm − 1 (ν P-F at 839 cm − 1 is of medium intensity only), which further supports the formation of Ru (IV) = O species [23,25,28,33,[36][37][38][39][40]46,47] which is probably responsible for catalytic oxidation of alcohols. The 1 H NMR spectrum of 1 recorded after adding NMO becomes broad, indicating the formation of paramagnetic species as intermediate.…”