Adsorption of Metallocenes on Silica

Abstract

The adsorption of the metallocenes ferrocene, doubly deuterated ferrocene, cymantrene and nickelocene, as well as molybdenum hexacarbonyl, proceeds in the absence of a solvent. Large single pieces of silica gel were placed in contact with the solid metallocenes and the adsorption process was visualized on a macroscopic scale and the maximal loadings were determined. H, H, and C solid-state NMR studies confirmed fast isotropic reorientation of the surface-adsorbed metallocene molecules within the pores of the silica. All prevalent anisotropic solid-state interactions were averaged out. The solid diamagnetic and paramagnetic materials were amenable to measurements with a standard solution NMR instrument. All metallocenes adsorbed in a monolayer. In the case of ferrocene and cymantrene, different C MAS signals were obtained for the cyclopentadienyl ring carbon nuclei and assigned to one ring interacting with the surface and one ring pointing away from it. The relative adsorption strengths of ferrocene on different silica supports, nanotubes, and activated carbon were determined by a novel straightforward method recording the desorption temperature. The reversibility of adsorption has been demonstrated by competition experiments using ferrocene, doubly deuterated ferrocene, and cymantrene. Adsorbed nickelocene could be reduced to small Ni aggregates on the surface and the catalytic activity of the resulting material for the cyclotrimerization of phenylacetylene was proven.

Gold‐Catalyzed Intermolecular Formal Insertion of Aryldiazo Esters into Cp−H Bonds of Iron and Ruthenium Metallocenes

“…Although the chemistry of metallocenes adsorbed on silica gel has not been well investigated, very recently Cluff and Blümer studied the adsorption of ferrocene on silica gel by solid‐state NMR spectroscopic methods, and discovered that in the presence of air, adsorbed ferrocene is oxidized to ferrocenium salts at acidic surface silanol sites . Based on this observation, we hypothesize that our aerobic oxidation could proceed through initial electron‐transfer from iron to a dioxygen molecule, thereby forming ferrocenium ion I and superoxide radical anion intermediate, O 2 .…”

Structures and Dynamics of Secondary and Tertiary Alkylphosphine Oxides Adsorbed on Silica

“…Indeed,t he 31 Pw ideline and MAS spectra ( Figure 9) of 4a show that the CSA is only reduced, but not averaged out entirely.T he size of the molecule andt he long alkyl chains are not expected to slow the motion of an adsorbed species because it has been demonstrated earlier that paraffin, cyclododecane, or metallocenes with cycloalkyl substituents are highly mobile on silica and other surfaces. [13][14][15][16] Upon adsorption of 4 to form 4a all CSA parameters and the span of the signal are changed significantly,t he latter by nearly 100 ppm (Table2). Therefore, it can be concluded that both P=Og roups in the molecule are interacting equallyw ith the surfaceo fs ilica, but they actually do impedee ach other's mobility to some extent.…”

“…Encouraged by the meaningful results obtained with 1 Hw ideline NMR of adsorbed metallocenes [12][13][14][15][16] andm obile species in polymers [50] we probed 4 and 4a with this technique. The proton wideline spectrum of 4 shows as ignal that is broadened by large homonuclear dipolar interactions [17][18][19] and spans from about À110ppm to + 120 ppm, with ah alfwidth of ca.…”

“…[8] Early research in our group resulted in a study of paramagnetic chromocenes in polycrystalline and adsorbed form, [9,10] while Oprunenkoa nd Günther investigated biphenylene chromium tricarbonyl complexes. [11] Later,w e studied diverse diamagnetic and paramagnetic metallocenes and demonstrated that they can be adsorbed as solids on amorphous high surface area silica gel [12][13][14][15] and on various carbon supports. [16] In all the cases of successful solid/solid adsorption described in the literature, [6,7,9,[11][12][13][14][15][16] unique solid-state NMR [17][18][19][20] features were found due to the high translational mobilities of the adsorbate molecules on the surfaces.…”

“…Therefore, the anisotropic interactions dominating the signal shapes of crystalline solids, [17][18][19][20] dipolar and quadrupolar interactions, as well as the Chemical Shift Anisotropy (CSA) [17,18] of the signals of the adsorbeds pecies, are averaged out. [6,7,9,[11][12][13][14][15][16] Signals with larger residual linewidths, but devoid of rotational sidebands, result. The broader isotropic lines are ac onsequence of the less uniform surroundings of the nuclei on the support surfaces, as compared to their environment in asingle crystal.…”