MECHANISMS OF CHOSEN ENZYME REACTIONS - New Trends in Enzyme Catalysis and Biomimetic Chemical Reactions - page 93

into steps that represent the process taking place under certain idealized conditions (e.g.
in the gas phase or a neutral solution) and the interaction of the reactants (A) and
products (B) with the metal complex (M): It is obvious that
where One of the fundamental principles of catalysis under
mild conditions is that the catalyst should provide for a chemical process with smooth
thermodynamic relief on each step along the reaction path without deep energy holes or
high hills. It means that the Gibbs energy of the substrate A adsorption and product B
desorption can not be very high by the absolute value, i.d.
Because and have different signs and, therefore, the difference between
these values should be even less, we can take We conclude
that the difference in the estimated energy of plausible intermediates of the dinitrogen
reduction, which takes place in the coordinate sphere of transition metal, cannot be
essentially higher than that in water solution. Therefore, the values of the reducing
agents, redox potential which are required to reduce dinitrogen through the intermediates
or can not be markedly different from those presented in Fig.3.5, and the
conclusion of thermodynamic preference for the four-electron mechanism is still rational
energy of plausible intermediates of the dinitrogen reduction, which takes place in the
coordinate sphere of transition metal, cannot be essentially higher than that in water
solution. Therefore, the values of the reducing agents, redox potential which are required
to reduce dinitrogen through the intermediates or can not be markedly
different from those presented in Fig.3.5, and the conclusion of thermodynamic
preference for the four-electron mechanism is still rational.
4. In the four-electron process, in order to avoid a drastic change in the electrostatic
status of the system and to minimize reorganization energy due to electron transfer, the
following requirements should be fulfilled: a) multi-orbital binding of dinitrogen in a
metalcluster when a transfer of electrons from the filled d-orbitals to the anti-binding of
the ligand is accompanied by the reverse transfer of electrons from the binding orbitals
of the ligand to empty d-orbitals of the metal, b) essential delocalization of the
electrostatic charge among the electron donor atoms with only a slight change of local4.
In the four-electron process, in order to avoid a drastic change in the electrostatic status