Ferrocene has been adsorbed on the surface of silica and activated carbon within the pores by dry grinding in the absence of a solvent at room temperature. While the dry adsorption and translational mobility of ferrocene within the pores are already established on the centimeter scale, there is little systematic understanding of the surface site-to-site motions of the ferrocene molecules and their orientation with respect to the surface. In this paper, silica and activated carbon, both widely applied in academia and industry as adsorbents, are used as support materials. Using variable-temperature 13C and 2H solid-state NMR and T 1 relaxation time measurements, the dynamics of ferrocene on the surfaces of silica and activated carbon within the pores has been quantitatively characterized on the molecular scale. The obtained data indicate that ferrocene molecules show a liquid-like behavior on the surface. Fast exchange between isotropically moving molecules and surface-attached molecular states of ferrocene has been found in samples with submonolayer surface coverages. The surface-attached molecular states have been characterized by the free energies ΔG ⧧ of 6.1 kcal/mol for silica and ΔG ⧧ of 6.2 kcal/mol for activated carbon at 223 and 263 K, respectively. The horizontally oriented ferrocene molecules are the most thermodynamically stable states on the surfaces of both materials. These molecules exhibit fast C5 rotation of the Cp rings, as established by low-temperature 13C and 2H NMR. The interactions of ferrocene with the pore surfaces have been characterized by adsorption enthalpies measured as −8.4 to −7.0 kcal/mol and −6.7 kcal/mol for activated carbon and silica, respectively. It has been suggested that the ferrocene–surface interactions for both support materials have a polar character.

Triphenylphosphine oxide (TPPO, 1) has been adsorbed on neutral alumina by dry grinding of the components in the absence of a solvent. The adsorption proves translational mobility of 1 on the surface of alumina. Different surface coverages from a densely packed monolayer (99% coverage) to a dilute sub-monolayer (25%) have been produced. The samples have been studied by diverse multinuclear 1H, 13C, and 31P variable temperature solid-state nuclear magnetic resonance (NMR) techniques. The interactions of 1 with the surface are determined by hydrogen bonding of the P=O group to OH groups on the surface. The 31P solid-state NMR spectra prove that even at low temperatures, the molecules of 1 are highly mobile on the surface. Using T1 and T2 relaxation time analyses of the 31P resonance in the solid state at variable temperatures allowed the identification and quantification of two different modes of mobility. Besides the translational mobility that consists of jumps from one hydrogen-bonding OH site on the surface to an adjacent one, a rotational movement around the axis defined by the P=O group of 1 occurs.

When grinding nickelocene with silica in the absence of a solvent at room temperature, it adsorbs on the surface within the pores. This has also been demonstrated visually by adsorbing green nickelocene in the pores of a large colorless silica gel specimen. While this dry adsorption and translational mobility of nickelocene within the pores is proven visually, the site-to-site mobility of the nickelocene molecules and their orientation toward the surface are not yet understood. In this contribution, mesoporous silica is used as the support material for a systematic solid-state NMR study of these issues. Paramagnetic 1 H VT solid-state NMR and T 1 relaxation times have been powerful tools for studying the dynamics of nickelocene on the silica surface. Herewith, the mobility of the surface-adsorbed nickelocene molecules in the pores could be quantified on the molecular scale. According to the obtained data, the nickelocene molecules move like a liquid on the surface. Isotropically moving molecules exchange places rapidly with surface-attached molecular states of nickelocene in a sample with submonolayer surface coverage. This finding is corroborated by a macroscopic visualization experiment. The states of the surface-attached horizontally oriented nickelocene molecules that are prevalent at temperatures below 200 K have been quantified. The temperature dependencies of the rate k in coordinates of ln(k) versus 1/T and ln(k/T) versus 1/T form ideal straight lines that allow the determination of the kinetic parameters E act = 5.5 kcal/mol, A = 1.1 × 10 10 , ΔH ‡ = 5.0 kcal/mol, and ΔS ‡ = −15 eu. Investigating a sample with equal amounts of nickelocene and ferrocene in a submonolayer amount of 80% overall surface coverage shows that the different metallocenes mix on the molecular level on the silica surface.

Benzene-d 6 and cyclohexane-d 12 have been adsorbed on the surface within the pores of high-surface-area activated carbon (AC). Their molecular motions have been characterized by variable-temperature 2H and 13C solid-state NMR spectroscopy. Three different states of benzene molecules on the AC surface have been found: isotropically moving molecules, bound molecules, and intermediates between these states. In contrast to cyclohexane, benzene assumes stationary states that are stabilized by π–p interactions with the AC surface. Hereby, fast in-plane C6 rotations take place. The adsorption enthalpy −ΔH 0 for benzene on the surface of AC within the pores was determined as 4.6 ± 0.3 kcal/mol. The strongly adsorbed molecules undergo slow exchange with isotropically moving, liquid-like molecules. In contrast to this, exchange between the molecules in a liquid-like state with benzene in semibound states (T-complexes) is very fast, requiring only low activation energies E act and ΔH ‡ of 3.1 and 2.7 kcal/mol, respectively.