Adsorption of Ruthenium and Iron Metallocenes on Silica: A Solid-State NMR Study

Ruthenocene,
bis­(indenyl)­ruthenium, bis­(tetrahydroindenyl)­ruthenium,
cyclododecane, ferrocene, and ferrocene-d2 have been adsorbed on silica surfaces by grinding the polycrystalline
materials with silica. The adsorption process proceeds without solvent
and is practically complete within 2 h. Its progress is monitored
by 1H, 13C, and 2H solid-state NMR
spectroscopy. The transition from the crystal lattice to the surface
species that are highly mobile is proven by strongly reduced chemical
shift anisotropies and diminished dipolar interactions. Furthermore,
the residual line widths are reduced. All solid-state NMR spectra
indicate that the transition from a monolayer to the crystalline state
is abrupt, and no multiple layers form on the surfaces. A correlation
between surface coverage and 2H residual line widths has
been established. Besides a hydrophobic dry silica surface, wet and
TMS-capped silica have been used as supports. The adsorption leads
to the highest surface coverages and most mobile species for the surface
of rigorously dried silica. The 2H MAS spectra of surface-adsorbed
ferrocene-d2 prove that the motion of
the metallocenes on the surfaces is fast and nearly isotropic, as
in solution. Consequently, it is demonstrated that 1H and 13C NMR spectra of adsorbed ferrocene can be recorded using
a conventional liquids NMR instrument.