Solid-state NMR spectroscopy is a versatile tool for gaining insights into structures and dynamic processes in materials and at their surfaces. For example, a molecular catalyst tethered to a solid support by a bifunctional phosphine linker can entail issues that need to be addressed before the catalyst is used. Typical linkers are phosphines with ethoxysilane groups, in combination with silica as the support. The proper covalent binding of the linker to the support can be checked by 29Si CP/MAS. 31P and 13C CP/MAS confirm that the linker remains structurally intact after immobilization. These techniques also confirm the coordination of the metal complex to the phosphine group before (and after!) catalysis. 1- and 2D HRMAS techniques that allow high-resolution MAS measurements in the presence of solvents, give unique insights regarding structure and dynamics. E.g., the migration of metal complexes from one surface-bound linker to the next can be monitored. This allows the creation of tailored heterobimetallic catalysts, e.g., for Pd(0)/Cu(I) Sonogashira systems. It will also be demonstrated that solid-state NMR techniques allow the comprehensive investigation of adsorption processes. The adsorption of metallocenes with high melting points like ferrocene, chromocene, and nickelocene on solid supports proceeds in the absence of a solvent. Dry grinding of the metallocenes with amorphous mesoporous silica or activated carbon leads to adsorption in monolayers within minutes. Dynamic 1H and 13C solid-state NMR allows the quantification of the fast isotropic reorientation of the surface-adsorbed molecules within the pores. All prevalent anisotropic interactions are completely averaged out at RT, even quadrupolar interactions in the 2H MAS spectra of deuterated metallocenes and the CSA of the paramagnetic solid-state NMR signals of adsorbed nickelocene and chromocene. The solids are even amenable to 1H, 13C, and 2H measurements with a standard solution NMR instrument. Nickelocene, adsorbed in a submonolayer, can be reduced to Ni(0) aggregates, or even single atoms on the surface. The resulting material is catalytically active regarding the cyclotrimerization of phenylacetylene. The results show that dry adsorption of metallocenes in monolayers, followed by reduction of the metal centers under mild conditions, represents a new exciting pathway to inexpensive, efficient surface-supported heterogeneous catalysts.

Solid-state NMR spectroscopy for the comprehensive structural and dynamic characterization of covalently bound and surface-adsorbed catalysts

255th National Meeting (2018)

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