  We used density functional theory to computationally design a Zr
organometallic catalyst for selectively oxidizing substrates using molecular
oxygen as oxidant without coreductant. Each selective oxidation cycle involves
four general steps: (a) a peroxo or weakly adsorbed O2 group releases an O atom
to substrate to form substrate oxide and an oxo group, (b) an oxygen molecule
adds to the oxo group to generate an eta2-ozone group, (c) the eta2-ozone group
rearranges to form an eta3-ozone group, and (d) the eta3-ozone group releases
an O atom to substrate to form substrate oxide and regenerate the peroxo or
weakly adsorbed O2 group. This catalyst could potentially be synthesized via
the condensation reaction Zr(N(R)R')4 + 2
C6H4-1,6-(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] + 4
N(R)(R')H where R and R' are CH3, CH2CH3, or other alkyl groups. For direct
ethylene epoxidation, the computed enthalpic energetic span (i.e., effective
activation energy for the entire catalytic cycle) is 27.1 kcal/mol, which is
one of the lowest values for catalysts studied to date. We study reaction
mechanisms and the stability of different catalyst forms as a function of the
oxygen atom chemical potential. Notably, an aromatic linkage in each ligand
prevents this catalyst from deactivating to form an inactive octahedral-like
structure 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 to
catalyst, this catalyst is useful for directly epoxidizing alkenes such as
ethylene that do not contain allylic H atoms. To better understand the reaction
chemistry, we computed net atomic charges and bond orders for the two
catalytically relevant reaction cycles. These results quantify electron
transfer and bond forming and breaking during the catalytic process.
