Recent interest in
Cu-exchanged zeolite catalysts for methane hydroxylation
has been broadened to small-pore Cu-zeolites such as Cu-SSZ-13 (Cu-CHA),
Cu-SSZ-16 (Cu-AFX), and Cu-SSZ-39 (Cu-AEI), which were reported to
produce more methanol per copper atom than the medium-pore Cu-ZSM-5
(Cu-MFI) and large-pore Cu-mordenite (Cu-MOR) zeolites do. To elucidate
the nature of such fascinating catalytic activities, theoretical investigations
based on density functional theory (DFT) were performed on the direct
conversion of methane to methanol by [Cu2(μ-O)]2+-exchanged AEI, CHA, AFX, and MFI zeolites in periodic systems.
DFT computational results show that the important activation energies
for C–H bond dissociation by [Cu2(μ-O)]2+-AEI, -CHA, and -AFX zeolites are lower than those for [Cu2(μ-O)]2+-MFI zeolite. Moreover, the rate-determining
methanol desorption and N2O decomposition by [2Cu]2+-AEI zeolite are also found to require low barriers, which
renders [Cu2(μ-O)]2+-AEI zeolite highly
active for the direct conversion of methane to methanol. Molecular
orbital analyses show that AEI, CHA, AFX, and MFI zeolites exert similar
confinement effects that stabilize the transition state for C–H
bond cleavage. In addition, a decrease in the Cu–O–Cu
angle, due to a change in the zeolite ring structure, lowers the acceptor
orbital energy of [Cu2(μ-O)]2+-zeolite,
which further stabilizes the transition state. We conclude that these
two factors play important roles in the activation of methane.
A RuII complex with a π-expanded diimine ligand performs photocatalytic H2 evolution and hydrogenation of organic substrates under visible-light irradiation, involving an intermediate with the dihydrogenated ligand; a mechanistic insight into the H2 evolution is also gained.
A tin(II) complex coordinated by a sterically demanding o-phenylenediamido ligand is synthesized. The ligand is redox-active to reach a tin(II) complex with the diiminobenzosemiquinone radial anion in the oxidation by AgPF 6 . The tin(II) complex reacts with a series of nosylazides (x-NO 2 C 6 H 4 −SO 2 −N 3 ; x = o, m, or p) at −30 °C to yield the corresponding nitrene radical bound tin(II) complexes. The nitrene radical complexes exhibit C(sp 3 )−H activation and amination reactivity.
Formation of an active
oxygen species at the dicopper site of pMMO
is studied by using density functional theory (DFT) calculations.
The role of the amino acid residues of tyrosine (Tyr374) and glutamate
(Glu35) located in the second coordination sphere of the dicopper
site is discussed in detail. The phenolic proton of the tyrosine residue
is transferred to the Cu2O2 core in a two-step
manner via the glutamate residue, and an electron is directly transferred
to the Cu2O2 core. These proton- and electron-transfer
processes induce the O–O bond cleavage of the μ–η2:η2-peroxodicopper(II) species to form the
(μ-oxo)(μ-hydroxo)CuIICuIII species,
which is able to play a key role of methane hydroxylation at the dicopper
site of pMMO (Inorg. Chem.2013527907). This proton-coupled electron-transfer mechanism is a little different
from that in tyrosinase in that the proton of substrate tyrosine is
directly transferred to the dicopper site (J. Am. Chem. Soc.20061289873) because there
is no proton acceptor in the vicinity of the dicopper site of tyrosinase.
The rate-determining step for the formation of the (μ-oxo)(μ-hydroxo)CuIICuIII species is determined to be the O–O
bond cleavage. These results shed new light on the interpretation
of the role of the tyrosine and glutamate residues located in the
second coordination sphere of the dicopper site of pMMO.
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