Evidence for the Iron(III) Oxidation State in Bis(imino)pyridine Catalysts. A Density Functional Theory Study

Abstract

We report a theoretical analysis based on density functional theory devoted to the study of the activated iron bis(imino)pyridine catalysts {[2,6-(2-C6H4(CH3))2-C5H3N]FenRx]n−x} (n = 2, 3; x = 1, 2; R = Me, Cl). The aim of this work is to obtain detailed information on the nature (oxidation state and spin multiplicity) of the iron active species, by studying the coordination and insertion of the first ethylene molecule with the 10 most reasonable activated species that can be formed out of {[2,6-(2-C6H4(CH3))2-C5H3N]FeCl2} after reaction with MAO. The relatively small exothermicity of the coordination reaction of ethylene, calculated for disubstituted species, allowed us to exclude them from further examination. The coordination and insertion reaction pathway for the two most reactive activated catalysts, i.e., monomethylated Fe(III) and Fe(II) species, were then evaluated on the B3LYP/Lacvp** potential energy surface (PES), taking into account all possible spin states and several coordination modes of the ethylene molecule. These reactions take place at the quintet PES for the Fe(II) and quartet PES for the Fe(III) species. For the latter, more favorable reaction and activation enthalpies for the insertion reaction were calculated: Fe(II), ΔH(298 K) = –14.1 kcal/mol and ΔH⧧(298 K) = +21.6 kcal/mol; Fe(III), ΔH(298 K) = −22.8 kcal/mol and ΔH⧧(298 K) = +10.0 kcal/mol. Assuming similar insertion barriers for the second insertion reaction, the β-hydrogen transfer termination reaction (ΔH⧧ = +11.9 kcal/mol) is favored over further chain growth for [FeIIMe]+, whereas for [FeIIIMe]2+ catalyst chain growth and termination reaction are clearly in competition (ΔH⧧ = +10.0 and +12.1 kcal/mol, respectively). On the basis of these results, we conclude that the most activated species has oxidation state III and is likely expected to produce oligomers in agreement with results from experimental studies.