Synthetic Mechanism of a Fe(II) N‑Heterocyclic Carbene Bidentate Complex Revealed by Electronic Structure Methods
Abstract
Octahedral Fe(II) complexes with bidentate N-heterocyclic (NHC) ligands are solid candidates for photoactive materials based on first-row transition metals. Despite the remarkable advances in ligand design and excited-state control, the theoretical basis for describing the complexation mechanism from a molecular and electronic point of view is lacking. This work reveals the molecular motions that drive the formation of bidentate [Fe(C^N)3]2+ complexes and how they couple with the electronic structure. Quantum chemistry methods are used to describe the chemical reactions that lead to the [Fe(pyIm)3]2+ (pyIm = pyridine-imidazol-2-ylidene) complex as a model case. The molecular model employed is based on the canonical synthesis using FeCl2 in an organic solvent and a strong Brønsted base to generate the pyIm ligand in situ. The energy profiles indicate that almost all reactivity takes place in the quintet state, whereas the singlet ground state is only populated in the last coordination step. Both d-activated dissociative interchange (Id) and purely dissociative (D) mechanisms compete, although the former is expected to be slightly more favorable. A global description of the coordination mechanism, consistent with the available experimental data, is provided through an analysis of the kinetic competition between the pathways and the thermodynamic stability of the intermediates.
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