Immobilizing molecularly-precise metal complexes onto relatively inert substrates, such as silica, provides an attractive platform for the purposeful design of active sites in heterogeneous catalysts. Specific steric and electronic variations of the ligand environment enable the development of structure-activity relationships and the knowledge-driven design of catalysts. At present, however, the three-dimensional environment of the catalytic active site is generally not known for heterogeneous single-site catalysts. We explored the degree to which, dynamic nuclear polarization (DNP)-enabled, NMR-based distance measurements could be used for the determination of such 3D structures. Furthermore, when NMR is used to measure the magnitudes of some anisotropic interactions, such as chemical shift anisotropy and dipolar couplings, we can start to move beyond the static picture of the active site and further elucidate the dynamics of the active site and how these can impact catalysis. <br/>            For the structure determination we have opted for the measurement of surface-to-atom distances between specific sites on the catalytic site and the support. These are enabled by accurate multi-spin simulations of the experiments and facilitate the 3D structure determination of naturally-abundant surface species. We first demonstrate the approach on an alumina-supported organoiridium(III) pincer complex for which the entire three dimensional structure can be condensed into three parameters. We then use plane-wave DFT to model potential configurations of the complex and isolate the one that agrees with the NMR measurements. We then expand this approach to metal complexes featuring a larger number of free degrees of freedom and show that easily-measured vertical distances are sufficient to fully constrain the three-dimensional conformation of the complex. Examples include a surface-tethered Zn(II) phenanthroline complex, a grafted Sc(III) amidinate complex and a Zr(IV) alkoxide. Lastly we will touch upon how these structures evolve dynamically at room temperature and some new NMR methods that enable their measurement in a far shorter timeframe.

Probing the structure and dynamics of catalytic sites in three dimensions

ACS Spring 2022

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