A theoretical study of the primary oxo transfer reaction of a dioxo molybdenum(VI) compound with imine thiolate chelating ligands: A molybdenum oxotransferase analogue
Lm. Thomson et Mb. Hall, A theoretical study of the primary oxo transfer reaction of a dioxo molybdenum(VI) compound with imine thiolate chelating ligands: A molybdenum oxotransferase analogue, J AM CHEM S, 123(17), 2001, pp. 3995-4002
The reaction mechanism of an analogue system of the molybdenum oxotransfera
ses was investigated at the density functional (B3P86) level of theory. Kin
etic measurements by Schultz and Helm suggest that the reaction MoO2(t-BuL-
NS)(2) + X --> MoO(t-BuL-NS)(2) + OX (t-BuL-NS = bis(4-tert-butylphenyl)-2
pyridylmethanethiolate( 1-)) occurs through an associative transition state
. Our results on the model reaction, MoO2(SCH2CHNH)(2) + P(CH3)(3) --> MoO(
SCH2CHNH)(2) + OP(CH3)(3), support this hypothesis, and indicate that this
reaction proceeds through a two-step rilechanism via an associative interme
diate. The DeltaH(double dagger) for the first, and rate-determining, step
was predicted to be 9.4 kcal/mol, and DeltaH(double dagger) for the second
step (release of the OP(CH3)(3) product) was predicted to be 3.3 kcal/mol.
These results are in good agreement with the experimental system, for which
the rate determining DeltaH(double dagger) = 9.6(6) kcal/mol. Shultz and H
elm's experimental model undergoes a significant Ligand rearrangement in th
e oxo transfer reaction: the reactant, MoO2(t-BuL-NS)(2), has a trans-S arr
angement of the ligands, while the product, MoO(t-BuL-NS)(2), has a trans-N
arrangement. To investigate the driving force behind the ligand rearrangem
ent, four model compounds, that systematically removed the unsaturation at
the N and the chelate character of the ligands, were modeled at the B3P86 l
evel of theory. For all models of the dioxo species, the trans-N isomer was
higher in energy than the trans-S isomer. The analysis of these results in
dicated that a trans influence accounts for approximately 16% of the energy
difference, the unsaturation at the nitrogens accounts for approximate to
26%, and the ring strain from the chelator accounts for approximate to 58%
of the energy difference between the two isomers (trans-N and trans-S). For
all models of the monooxo species, only the trans-N species was a stable g
eometry. Therefore, for the reverse oxo transfer reaction, ligand rearrange
ment must occur after or during the attack of the OX substrate.