Computationally Designed Zirconium Organometallic Catalyst for Direct Epoxidation of Alkenes without Allylic H Atoms: Aromatic Linkage Eliminates Formation of Inert Octahedral Complexes

摘要

We used density functional theory to computationally design a Zrorganometallic catalyst for selectively oxidizing substrates using molecularoxygen as oxidant without coreductant. Each selective oxidation cycle involvesfour general steps: (a) a peroxo or weakly adsorbed O2 group releases an O atomto substrate to form substrate oxide and an oxo group, (b) an oxygen moleculeadds to the oxo group to generate an eta2-ozone group, (c) the eta2-ozone grouprearranges to form an eta3-ozone group, and (d) the eta3-ozone group releasesan O atom to substrate to form substrate oxide and regenerate the peroxo orweakly adsorbed O2 group. This catalyst could potentially be synthesized viathe condensation reaction Zr(N(R)R')4 + 2C6H4-1,6-(N(C6H3-2',6'-(CH(CH3)2)2)OH)2 -->Zr(C6H4-1,6-(N(C6H3-2',6'-(CH(CH3)2)2)O)2)2 [aka Zr_Benzol catalyst] + 4N(R)(R')H where R and R' are CH3, CH2CH3, or other alkyl groups. For directethylene epoxidation, the computed enthalpic energetic span (i.e., effectiveactivation energy for the entire catalytic cycle) is 27.1 kcal/mol, which isone of the lowest values for catalysts studied to date. We study reactionmechanisms and the stability of different catalyst forms as a function of theoxygen atom chemical potential. Notably, an aromatic linkage in each ligandprevents this catalyst from deactivating to form an inactive octahedral-likestructure that contains the same atoms as the dioxo complex, Zr(Ligand)2(O)2.Due to a side reaction that can transfer an allylic H atom from alkene tocatalyst, this catalyst is useful for directly epoxidizing alkenes such asethylene that do not contain allylic H atoms. To better understand the reactionchemistry, we computed net atomic charges and bond orders for the twocatalytically relevant reaction cycles. These results quantify electrontransfer and bond forming and breaking during the catalytic process.