Adsorption of the metallocenes ferrocene, chromocene, cymantrene and nickelocene on favorable solid supports proceeds in the absence of a solvent.1,2 Dry grinding of the metallocenes with amorphous mesoporous silica or activated carbon leads to adsorption in monolayers within minutes. The translational mobility of the metallocene molecules on the surfaces in the pores has been proven by placing large clear single pieces of silica gel in contact with the solid colorful metallocenes and visualizing their migration into the support on the macroscopic scale.1 The large silica gel specimens also allow one to determine the loadings gravimetrically. 1H, 2H, and 13C solid-state NMR confirms the fast isotropic reorientation of the adsorbed metallocene molecules within the pores of the silica. All anisotropic solid-state interactions are averaged out already at room temperature. The diamagnetic and paramagnetic (solid!) materials are even amenable to measurements with a solution NMR instrument. The relative adsorption strengths of ferrocene on different silica supports, nanotubes, and activated carbon have been determined by a novel straightforward method recording the desorption temperatures. The reversibility of the adsorption, and therewith the exclusion of chemical reactions with the support surface, has also been demonstrated by competition experiments using ferrocene, doubly deuterated ferrocene, and cymantrene. Nickelocene, adsorbed in a sub-monolayer, in the absence of a solvent, can be reduced and small Ni(0) aggregates, or even single atoms on the surface are formed. This is achieved by reaction with hydrogen under mild conditions, again in the absence of a solvent. The resulting material has been tested regarding its catalytic activity and selectivity for the cyclotrimerization of phenylacetylene1 and the hydrophosphonation of acetylene. The catalytic characteristics of the Ni(0) material are amazing and unprecedented. They show that dry adsorption of metallocenes, followed by reduction of the metal centers under mild conditions, represents a new exciting pathway to inexpensive, efficient surface-supported heterogeneous catalysts with unprecedented activities, selectivities, and lifetimes.

Metallocenes adsorbed on silica and activated carbon: Solid-State NMR and catalysis

253rd National Meeting (2017)

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