Analysis of spin states, spin barriers, and trans-effects involved in the coordination and stabilization of dinitrogen by biomimetic iron complexes |
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Authors: | Gerrit Moritz Markus Reiher Bernd Artur Hess |
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Affiliation: | 1. Institut für Physikalische Chemie, Universit?t Jena, Helmholtzweg 4, 07743, Jena, Germany
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Abstract: | Coordination of dinitrogen to Sellmann-type iron (II) complexes in a sulfur-dominated coordination sphere, which emulates
the environment of iron centers in the FeMo-cofactor of nitrogenase, is analyzed with respect to spin states, spin barriers,
and the effect of trans-ligands. Such detailed investigations became only recently feasible when the reliability of density functional methods, which
are the only quantum chemical methods capable of describing large transition metal complexes, could significantly be improved
for the calculation of energies for states of different spin. It is found that the actual binding energy of dinitrogen is
of sufficient magnitude for a reasonably strong fixation of N2 by Sellmann-type coordination compounds. However, potential fixation is determined by additional factors which reduce the
binding energy. One factor is the change in spin state of the N2-free metal fragment, which lowers the total energy and quenches the thermodynamic stabilization effect of the binding energy.
In addition, the metal fragment rearranges and gains even more stabilization energy for the un-coordinated state. Apart from
these thermodynamical effects, the existence of spin barriers, which must be overcome upon binding of dinitrogen, leads to
kinetical effects, which cannot be neglected. |
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