Structure and Dynamics of Paramagnetic Iron(II) Sites from the DFT-Assisted Solid-State NMR of Molecular and Surface Species

The rational development of molecular catalysts, supported on an inorganic material, requires advanced analytic methodologies to probe the structure, dynamics, and electronic properties of the catalytic center(s). While spectroscopic tools, in particular solid-state MAS NMR, have greatly advanced our understanding of metal sites in molecules and materials, it remains challenging to address the detailed structure and dynamics of paramagnetic centers. Fe(II) sites are ubiquitous across catalytic systems from enzymes to molecular and heterogeneous catalysts. Across supported systems, silica-supported Fe(II) species have recently been shown to enable the nonoxidative coupling of methane, and the surface sites have been investigated via Surface Organometallic Chemistry. Here, we show how state-of-the-art MAS NMR experiments combined with quantum computations enable us to resolve the structure and the dynamics of a dimeric Fe(II) complex, used for the production of isolated iron sites on the surface of amorphous silica. We also show through analysis of the 1H and 13C NMR signatures of the silica-supported species that a large structural difference exists between molecular and supported species; the latter displaying a remarkably homogeneous, monomeric, and highly dynamic single-atom sites.