Coordination chemistry of Co complexes containing tridentate SNS ligands and their application as catalysts for the oxidation of n-octane

Abstract

The selective oxidation of saturated hydrocarbons to terminal oxygenates under mild catalytic conditions has remained a centuries long challenge in chemical catalysis. In an attempt to address this challenge, two series of tridentate donor ligands {2,6-bis(RSCH2)pyridine and bis(RSCH2CH2)amine [R = alkyl, aryl]} and their respective cobalt complexes {Co[2,6-bis(RSCH2)pyridine]Cl2 and Co[bis(RSCH2CH2)amine]Cl2} were synthesized and characterized. Crystal structures of Co[2,6-bis(RSCH2)pyridine]Cl2 [R = -CH3 (), -CH2CH3 (), -CH2CH2CH2CH3 () and -C6H5 ()] are reported in which crystallized as a homo-bimetallic dimer that incorporated two bridging chloride atoms in an octahedral geometry around each cobalt center, while , and crystallized as mono-metallic species characterized by trigonal bipyramidal arrangement of ligands around each cobalt center. As catalysts for the homogeneous selective oxidation of n-octane, the catalysts yielded ketones as the dominant products with a selectivity of ca. 90% for the most active catalyst Co[bis(CH2CH2SCH2CH2)amine]Cl2 () at a total n-octane conversion of 23%. Using tert-butyl hydroperoxide (TBHP) as an oxidant, optimization of reaction conditions is also reported.

trans-Bis[8-(benzylsulfanyl)quinoline-κ2 N,S]dichloridocobalt(II)

“…The central Co II atom, located on a crystallographic center of inversion, is coordinated by two N and two S atoms from two symmetry-equivalent ligands 1 situated in the equatorial plane and two Cl atoms in the axial positions. The Co-N [2.1543 (17) A ˚] and Co-S [2.4856 (5) A ˚] bond lengths are within the range of those found in dichloridocobalt(II) complexes with a nitrogen-and sulfur-containing multidentate ligand (Soobramoney et al, 2014;Lei et al, 2018). In addition, weak intermolecular C-HÁ Á Á interactions between the 8-(benzylsulfanyl)quinoline ligands are data reports observed in the crystal packing of 2 (Karle et al, 2007), forming a chain along the a-axis direction (Fig.…”

“…Dichloridocobalt(II) complexes with hetero donor ligands (e.g., nitrogen-and sulfur-containing multidentate ligands) also exhibit interesting catalytic activities, e.g. in the oxidation reaction of n-octane (Soobramoney et al, 2014) and in the photochemical-driven hydrogen evolution from water (Lei et al, 2018); however, they are still limited in number. Herein, we report the structure determination of a new dichloridocobalt(II) complex 2 with 8-(benzylsulfanyl)quinoline (1) as an N,S-ligand (Kita et al, 2002) by single-crystal X-ray analysis.…”

Water-soluble SNS cationic palladium(II) complexes and their Suzuki–Miyaura cross-coupling reactions in aqueous medium

“…Our study commenced with the preparation of the SNS pincer ligand 19a using literature reported protocols [ 36 ] as shown in Scheme 1 . Treatment of in situ-generated thiophenolate with 2,6-bis(chloromethyl)pyridine ( 18 ) afforded pincer ligand 19a in 73% yield.…”

Cobalt aminodiphosphine complexes as catalysts in the oxidation of n-octane

“…29,30 In this work, a new approach has been undertaken in using cobalt aminodiphosphine complexes in the C-H activation of n-octane. 40,45 Phosphine based ligands have not been widely explored due to ligand degradation or loss of ligand from the metal complex. [31][32][33][34] The development of synthetic models has been biologically inspired by a number of enzymes such as methane monooxygenase and cytochrome P450, which make use of a reactive iron-oxo species in the oxidation of a number of alkanes.…”

“…3,17,[42][43][44] More recently Tordin and coworkers have used cobalt complexes with tripodal 4N ligands in the oxidation of alkanes, while SNS cobalt complexes have also been studied in the oxidation of noctane. 40,45 Phosphine based ligands have not been widely explored due to ligand degradation or loss of ligand from the metal complex. 46 However, Wong and co-workers have used ruthenium based phosphine complexes in the oxidation of noctane and reported low conversions.…”