TY - JOUR
T1 - O2 Activation and Catalytic Alcohol Oxidation by Re Complexes with Redox-Active Ligands
T2 - A DFT Study of Mechanism
AU - Dinda, Shrabani
AU - Genest, Alexander
AU - Rösch, Notker
N1 - Publisher Copyright:
© 2015 American Chemical Society.
PY - 2015/8/7
Y1 - 2015/8/7
N2 - As a contribution to understanding catalysis by transition metal complexes with redox-active ligands (here: catecholate - cat), we report a computational study on the mechanism of a catalytic cycle where (i) O2 is activated at the metal center of the catecholate complex [ReV(O)(cat)2]- to yield [ReVII(O)2(cat)2]-, which (ii) subsequently is applied to oxidize alcohols. We were able to identify the steps where the redox-active ligands played a crucial role as e- buffer. For O2 homolysis, a series of sequential 1e- steps leads to superoxo and bimetallic intermediates, followed by facile cleavage of the bimetallic peroxo O-O linkage. The trans-cis isomerization of trans-[ReV(O)(cat)2]- is the crucial step of O2 activation, with an absolute free energy barrier of 16.8 kcal mol-1 in methanol. Due to the ionic character of intermediates, all reaction barriers of O2 activation are significantly lowered in a polar solvent, thus rendering O2 homolysis kinetically accessible. With computational results for the activation barriers of all elementary steps as well as the calculated solvent effects, we are able to rationalize all pertinent experimental findings. For catalytic alcohol oxidation, we propose a novel cooperative mechanism that involves two units of the metal complexes, ruling out the reaction via a seven-coordinated active oxidant, as previously hypothesized. We present in detail calculated energies and barriers for the reaction steps of the oxidation of methanol as model alcohol as well as the energetics of crucial steps of the experimentally studied oxidation of benzyl alcohol, both transformations for methanol as solvent.
AB - As a contribution to understanding catalysis by transition metal complexes with redox-active ligands (here: catecholate - cat), we report a computational study on the mechanism of a catalytic cycle where (i) O2 is activated at the metal center of the catecholate complex [ReV(O)(cat)2]- to yield [ReVII(O)2(cat)2]-, which (ii) subsequently is applied to oxidize alcohols. We were able to identify the steps where the redox-active ligands played a crucial role as e- buffer. For O2 homolysis, a series of sequential 1e- steps leads to superoxo and bimetallic intermediates, followed by facile cleavage of the bimetallic peroxo O-O linkage. The trans-cis isomerization of trans-[ReV(O)(cat)2]- is the crucial step of O2 activation, with an absolute free energy barrier of 16.8 kcal mol-1 in methanol. Due to the ionic character of intermediates, all reaction barriers of O2 activation are significantly lowered in a polar solvent, thus rendering O2 homolysis kinetically accessible. With computational results for the activation barriers of all elementary steps as well as the calculated solvent effects, we are able to rationalize all pertinent experimental findings. For catalytic alcohol oxidation, we propose a novel cooperative mechanism that involves two units of the metal complexes, ruling out the reaction via a seven-coordinated active oxidant, as previously hypothesized. We present in detail calculated energies and barriers for the reaction steps of the oxidation of methanol as model alcohol as well as the energetics of crucial steps of the experimentally studied oxidation of benzyl alcohol, both transformations for methanol as solvent.
KW - O homolysis
KW - catalytic alcohol oxidation
KW - cooperativity within dinuclear complexes
KW - oxorhenium complex
KW - redox-active ligand
UR - http://www.scopus.com/inward/record.url?scp=84938723311&partnerID=8YFLogxK
U2 - 10.1021/acscatal.5b00509
DO - 10.1021/acscatal.5b00509
M3 - Article
AN - SCOPUS:84938723311
SN - 2155-5435
VL - 5
SP - 4869
EP - 4880
JO - ACS Catalysis
JF - ACS Catalysis
IS - 8
ER -