Catalytic properties of biomimetic metallomacrocycles intercalated in layered double hydroxides and smectite clay: the importance of edge-site access

Abstract

Anionic and cationic forms of two biomimetic macrocyclic metal complexes, namely, Co(II)-tetrasulfophthalocyanine anions, [CoPcTs] 4−, and Co(II)-tetra( N-methyl-4-pyridiniumyl)porphyrin cations, [CoTMPyP] 4+, have been intercalated in Mg Al layered double hydroxides (LDH) and a fluorohectorite smectite clay, respectively. The catalytic activities of the intercalated complexes for the dioxygen oxidation of 2,6-di-tert-butylphenol have been compared with those for the corresponding complexes under homogeneous reaction conditions. [CoPcTs] 4− intercalated in Mg 2Al- and Mg 4Al-layered double hydroxides exhibit dramatically enhanced catalytic activity and longevity relative the homogenous metal complex. In contrast, [CoTMPyP] 4+ intercalated in F-hectorite reduces the activity of the metallomacrocycle. The activities are correlated with the orientation of the macrocyclic complex in the galleries of the layered host. In the LDH intercalates, the plane of the complex is perpendicular to the host layers, whereas in F-hectorite the plane is inclined at an angle of 27°. The vertical orientation of the macrocycle in LDHs optimizes the accessibility of the metal complex centers near the external edge sites of the crystallites, which are the most important sites for reaction with the substrate. Metallomacrocycles bound at intracrystal gallery sites or at the external basal planes of the host are not readily accessible for reaction and do not contribute greatly to the overall catalytic activity.