Homogeneous and Immobilized Nickel Catalysts and Mobility Studies of Metallocenes Adsorbed on High Surface Area Materials

Abstract

This thesis will explore two different facets of the quest for superior immobilized catalysts: (1) cyclotrimerization of acetylenes with nickel catalysts and (2) solid-state NMR studies of the adsorption and subsequent mobility of organometallic solids on silica gel and other relevant surfaces.

The cyclotrimerization of acetylenes has a wide range of applications and can be catalyzed by inexpensive immobilized nickel complexes. However, the selectivity of these catalysts changes over time, and the activity declines. In this study, tridentate ligands are employed to reduce leaching and decomposition.

The chelate nickel complexes incorporating tridentate phosphine ligands (CO)Ni[(PPh2CH2)3CMe] (2), (CO)Ni[(PPh2CH2CH2)3SiMe] (6), and Ph3PNi[(PPh2CH2CH2)3SiMe] (7), as well as the bidentate complex (CO)2Ni[(PPh2CH2)2CMeCH2PPh2] (3) and the heterobimetallic complex (CO)2Ni[(PPh2CH2)2CMeCH2Ph2PAuCl] (4) have been synthesized and characterized in solution and in the solid state. While complexes 2-4 displayed low catalytic activity, complex 7 led to 100% conversion of the substrate within four hours and was highly selective throughout the catalytic reaction.

Various metallocenes have been adsorbed on high surface area materials including mesoporous silica, carbon nanotubes and activated carbon. The adsorption progresses quickly onto a favorable support, even when the solids contact each other without a solvent, and is practically complete within two hours. 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. 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 linewidths has been established. Besides a hydrophobic dry silica surface, wet and TMScapped silica have been used as supports. 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. Additionally, it is possible to visually monitor the adsorption and translational mobility of ferrocene and other metallocenes in large pieces of silica gel.