Revisiting [Fe(bipy)2Et2]: electronic structure and new considerations on the thermal decomposition mechanism

Abstract

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 κ² to κ¹. 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 [(κ²-bipy)(κ¹-bipy)FeEt₂], 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) Fe^III. 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.