Revisiting [Fe(bipy)2Et2]: electronic structure and new considerations on the thermal decomposition mechanism
Résumé
The present computational investigation discredits a previously proposed mechanism for the thermal decomposition of the title compound in THF, according to which β-H elimination from one Et ligand would occur after an initial rearrangement of one bipy ligand from κ$^2$ to κ$^1$. It is alternatively proposed that the initial step is full replacement of one bipy ligand with one solvent molecule, to yield 5-coordinate [(bipy)(THF)FeEt2]. The title compound, the 5-coordinate [(κ$^2$-bipy)(κ$^1$-bipy)FeEt 2 ], the 5-coordinate [(bipy)(THF)FeEt2] and the 6-coordinate [(bipy)(THF)2FeEt2] complexes have been analyzed in all possible spin states. While [(bipy)2FeEt2] is diamagnetic, but suggested by the calculation to rather adopt an open-shell (broken-symmetry) singlet with a low spin (S = ½) Fe III and non-innocent bipy ligands, [(bipy)2]•-, the other three systems have a broken-symmetry triplet ground state (S = 1 overall) with an intermediate spin (S = 3/2) FeIII. The low computed Gibbs energy cost for the bipy/THF substitution (5.6 kcal mol-1) makes [(bipy)(THF)FeEt2] a kinetically competent intermediate. Full bipy dissociation prior to the β-H elimination step is also consistent with the previously observed dependence of the decomposition rate on the solvent donor properties.
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