Preparation of Heterobimetallic Catalysts

In an attempt to synthesise mesoporous silicas doped with truly heterobimetallic nanoparticles, the use of single-source precursors has been proposed. The method of preparation of the silicas has employed true liquid crystal templating (TLCT), a methodology used to synthesise mesoporous silicas, previously employed in one-pot syntheses to prepare silicas containing heterobimetallic nanoparticles from separate metal precursors. Using single source precursors was expected to lead to bimetallic nanoparticles of uniform composition dispersed throughout the silica framework.
In this work several attempts were made to prepare heterobimetallic complexes that possessed an equivalent metallic ratio (1 : 1), which were to be used as precursors in the one-pot synthesis mentioned above. However, none of them produced complexes of the desired stoichiometry. In these attempts, two different approaches were used namely a polydentate ligand (TTHA, Triethylene tetraaminehexaacetic acid) and usage of halogeno bridges. From these methods of preparation only one stoichiometrically precise TTHA complex was isolated, that of Rh-Zn (1 : 1) obtained as a single crystal. Apart from that two more heterobimetallic complexes e.g. Cr-Rh (3 : 1) TTHA complex and one halobridged Pd-Pt (3 : 1) complex, were isolated.
Despite the imperfect stoichiometry of the TTHA complexes, some (Rh-Ni, Cr-Pd, Rh-Pd) were nonetheless used in the preparation of mesoporous silicas. The combinations Cr/Pd and Rh/Pd evidently led to deposition of nanoparticles of the different metals, although there was some evidence for the formation of heterobimetallic nanoparticles with Rh/Ni.
In investigating the possible preparation of heterobimetallic complexes between gold and platinum or palladium, the oxidation of the Group 10 metal by gold was observed and studied. One unexpected outcome of the reaction between PtII and AuIII as their NBu4+ salts was the observation of the bimetallic complex [NBu4]2[Cl3Pt{-(2:2–trans-butadiene)}PtCl3] and possible mechanisms of formation are discussed.