Organometallic chemistry of group VI metals in the void space of zeolites

Abstract

Many salient features about the structures, dynamics and transport of organometallic complexes of group VI metals (M(CO) 6, M=Cr, Mo, W) and ligands (C 6H 6, C 5H 5N, P(CH 3) 3) occluded within the void space of faujasitic zeolites have been deduced from X-ray and neutron diffraction, EXAFS, inelastic neutron scattering, RMN, IR absorption, Raman scattering investigations as well as from theoretical methods. The faujasitic zeolites are porous crystalline aluminosilicates with M ′ n [(SiO 2) 192− n (AlO 2) n ] (M ′ n FAU) formulae per unit cell, the M(CO) 6 and ligand molecules can gain free access to the inner void space (supercages) through the smallest section called the windows. The silicon–aluminum ratios (Si/Al) of the framework are found to be in the 1–100 range whereas M′ + cations compensate the AlO − 2 charges of the framework (M +′=H +, Li +, Na +, K +, Rb +, Cs +). From the in situ spectroscopic investigations and molecular modeling a coherent picture of the reactions between M(CO) 6 and ligands under thermal activation has been drawn as a function of the aluminum content of the zeolite. Among the available in situ spectroscopic investigations (far-IR, Raman, UV–visible, NMR) diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) in the mid-IR region provides an efficient tool for dehydration, sorption and cosorption as well as subsequent reactions under thermal activation even at low coverage. In siliceous faujasite (Si/Al=100) the M(CO) 6 and ligand molecules are randomly distributed within the void space and the molecular motions approach the rapid isotropic limits of liquids. The chemical behavior upon thermal activation is found to be analogous to that observed in solution. In Na 56FAU (Si/Al=2.5) the reagents are trapped in well-defined sorption sites in close proximity. Upon gentle thermal activation a fast reaction occurs to form M(CO) 3(η 6-C 6H 6) or Mo(CO) 3(C 5H 5N) 3, or Mo(CO) 4(P(CH 3) 3) 2 as major species inside the supercage through a concerted mechanism including the electrostatic field and the basicity of the framework oxygens. In Na 85FAU (Si/Al=1.25) the M(CO) 6 and ligand molecules are not encapsulated in close proximity. M(CO) 6 reacts thermally in the void space in the absence of ligand to lose sequentially three CO ligands and form predominantly anchored M(CO) 3(O z ) 3 species in which the three vacant coordination sites are occupied by three O z framework oxygens. The substitution kinetics have been interpreted on the basis of structure and dynamics of reactant and product guests in the zeolites. The kinetics data reveal that the available porous volume of zeolite can provide a precisely defined activating environment to obtain faster reactions than in homogeneous media. The dynamics of the supramolecular assembly of M(CO) 6, ligands and extraframework cations M′ + in the vicinity of the oxygen atoms of the inner surface can undergo rapid and specific organometallic reactions under geometry, size, shape and electrostatic constraints.