TY - JOUR
T1 - Heterometallic Co III Zn II schiff base catalyst for mild hydroxylation of C(sp 3 )–H bonds of unactivated alkanes
T2 - Evidence for dual mechanism controlled by the promoter
AU - Nesterova, Oksana V.
AU - Kasyanova, Katerina V.
AU - Buvaylo, Elena A.
AU - Vassilyeva, Olga Yu
AU - Skelton, Brian W.
AU - Nesterov, Dmytro S.
AU - Pombeiro, Armando J.L.
PY - 2019/3/1
Y1 - 2019/3/1
N2 - The novel Schiff base complex [Co III Zn II L 3 Cl 2 ]·CH 3 OH (1) was synthesized by interaction of zinc powder, cobalt(II) chloride and methanol solution of the pre-formed HL in air (HL is the product of condensation of o-vanillin and methylamine) and characterized by IR, UV-Vis and NMR spectroscopy, ESI-MS and single crystal X-ray diffraction analysis. In the heterometallic core of 1 the two metal centers are bridged by deprotonated phenoxy groups of the L − ligands with the cobalt-zinc separation of 3.123 Å. Catalytic investigations demonstrated a pronounced activity of 1 towards mild alkane oxidation with m-chloroperbenzoic acid (m-CPBA) as an oxidant and cis-1,2-dimethylcyclohexane (cis-1,2-DMCH) as the model substrate. The influence of the nature of different promoting agents of various acidities (from HOTf to pyridine) on the catalytic process was studied in detail and a pronounced activity of 1 in the presence of nitric acid promoter was found, also showing a high retention of stereoconfiguration of the substrate (>99% for cis-1,2-DMCH). The best achieved yield of tertiary cis-alcohol based on the oxidant was 61%, with a turnover number (TON) of 198 for nitric acid as promoter. The 18 O-incorporations into the alcohols when the reactions were performed under 18 O 2 atmosphere using acetic and nitric acid promoters, suggest that the cis-1,2-DMCH hydroxylation proceeds by two distinct pathways, a non-stereoselective and a stereoselective one (with and without involvement of a long-lived free carbon radical, respectively). The former dominates in the case of acetic acid promoter and the latter is realized in the case of HNO 3 promoter.
AB - The novel Schiff base complex [Co III Zn II L 3 Cl 2 ]·CH 3 OH (1) was synthesized by interaction of zinc powder, cobalt(II) chloride and methanol solution of the pre-formed HL in air (HL is the product of condensation of o-vanillin and methylamine) and characterized by IR, UV-Vis and NMR spectroscopy, ESI-MS and single crystal X-ray diffraction analysis. In the heterometallic core of 1 the two metal centers are bridged by deprotonated phenoxy groups of the L − ligands with the cobalt-zinc separation of 3.123 Å. Catalytic investigations demonstrated a pronounced activity of 1 towards mild alkane oxidation with m-chloroperbenzoic acid (m-CPBA) as an oxidant and cis-1,2-dimethylcyclohexane (cis-1,2-DMCH) as the model substrate. The influence of the nature of different promoting agents of various acidities (from HOTf to pyridine) on the catalytic process was studied in detail and a pronounced activity of 1 in the presence of nitric acid promoter was found, also showing a high retention of stereoconfiguration of the substrate (>99% for cis-1,2-DMCH). The best achieved yield of tertiary cis-alcohol based on the oxidant was 61%, with a turnover number (TON) of 198 for nitric acid as promoter. The 18 O-incorporations into the alcohols when the reactions were performed under 18 O 2 atmosphere using acetic and nitric acid promoters, suggest that the cis-1,2-DMCH hydroxylation proceeds by two distinct pathways, a non-stereoselective and a stereoselective one (with and without involvement of a long-lived free carbon radical, respectively). The former dominates in the case of acetic acid promoter and the latter is realized in the case of HNO 3 promoter.
KW - O isotopic labelling
KW - Metal complex catalysis
KW - Stereoselective C-H oxidation
UR - http://www.scopus.com/inward/record.url?scp=85063493031&partnerID=8YFLogxK
U2 - 10.3390/catal9030209
DO - 10.3390/catal9030209
M3 - Article
AN - SCOPUS:85063493031
SN - 2073-4344
VL - 9
JO - Catalysts
JF - Catalysts
IS - 3
M1 - 209
ER -