Benzie, Jordon William (2020-12). NICKELOCENE, FERROCENE, AND BENZENE ADSORBED ON SILICA AND ACTIVATED CARBON: A SOLID-STATE NMR STUDY. Doctoral Dissertation.
The ultimate goal of this thesis was to explore a new strategy for creating a nickel single atom catalyst (SAC) on a silica or activated carbon surface. It has been demonstrated in a preliminary experiment earlier that SAC formation might be possible by reducing nickelocene that had been dispersed on silica in a monolayer, with hydrogen. A Ni(0) catalyst had been obtained that was active for the cyclotrimerization of acetylenes. In order to explore this venue in detail, the objective of this thesis was to thoroughly investigate dynamic processes on silica surfaces that involve the supported molecular nickel complex nickelocene. As models for the paramagnetic and moderately sensitive nickelocene, benzene and the diamagnetic ferrocene have been employed. The studies relied mostly on solid-state NMR analyses because a plethora of different insights can be gained by this technique, especially with respect to molecular motions on surfaces. Dynamic processes do not only occur when catalysts immobilized covalently via linkers are suspended in a solvent. It has been discovered previously that metal complexes adsorbed via van der Waals interactions can also display different modes of mobility on diverse surfaces within pores, even in the absence of a solvent. Benzene and ferrocene can easily be obtained as deuterated species, which allow deuterium solid-state NMR analyses of their orientation on surfaces and their dynamics. Ferrocene can additionally be investigated as representative for all metallocenes and in particular for nickelocene that is slightly more reactive. For benzene, slow exchange between semi-bound and stationary, firmly adsorbed states has been found. In contrast to intuition, ferrocene is mostly adsorbed lying sideways on the surface. Nickelocene in comparison shows similar behavior, but a more robust interaction with the surface. When ferrocene and nickelocene are adsorbed in a sub-monolayer on the same surface they mix on the molecular level. This could be determined by analyzing the proton wideline signal halfwidths and relaxation times of both components at variable temperatures. The strong interactions between nickelocene and ferrocene on the support surface might even allow one to create well-defined dual Ni/Fe atom catalysts on the surface. The results described in this thesis enhanced the fundamental knowledge about how to create and characterize precursors for well-defined, atom-efficient, recyclable, supported catalysts which can lead to more sustainable processes in the future.