DFT Study of Imine-Exchange Reactions in Iron(II)-Coordinated Pincers

Abstract

The imine bond is among the most applied motifs in dynamic covalent chemistry. Although its uses are varied and often involve coordination to a transition metal for stability, mechanistic studies on imine exchange reactions so far have not included metal coordination. Herein, we investigated the condensation and transimination reactions of an Fe²⁺ -coordinated diimine pyridine pincer, employing wB97XD/6-311G(2d,2p) DFT calculations in acetonitrile. We first experimentally confirmed that Fe²⁺ is strongly coordinated by these pincers, and is thus a justified model ion. When considering a four-membered ring-shaped transition state for proton transfers, the required activation energies for condensation and transimination reaction exceeded the values expected for reactions known to be spontaneous at room temperature. The nature of the incoming and exiting amines and the substituents on the para-position of the pincer had no effect on this. Replacing Fe²⁺ with Zn²⁺ or removing it altogether did not reduce it either. However, the addition of two ethylamine molecules lowered the energy barriers to be compatible with experiment (19.4 and 23.2 kcal/mol for condensation and transimination, respectively). Lastly, the energy barrier of condensation of a non-coordinated pincer was significantly higher than found for Fe²⁺ -coordinating pincers, underlining the catalyzing effect of metal coordination on imine exchange reactions.

Keywords: DFT calculations; imines; pincer compounds; proton shuttle.