Surface-mediated synthesis and spectroscopic characterization of tantalum clusters on silica

Tantalum clusters were synthesized on the surface of porous silica by treatment of adsorbed Ta(CH(2)Ph)(5) in H(2) at temperatures in the range of 523-723 K. The surface species were characterized by UV-vis, far-infrared, and extended X-ray absorption fine-structure (EXAFS) spectroscopies, each of which provided evidence of Ta-Ta bonds similar to those in well-characterized molecular tantalum clusters. The Ta-Ta distance determined by EXAFS spectroscopy was 2.93 A. The chemistry of the cluster synthesis is similar to that of syntheses of similar tantalum clusters in solution. The supported clusters formed at 523 K are characterized by an EXAFS first-shell Ta-Ta coordination number of nearly 2, indicative of tri-tantalum clusters, although it is expected that a mixture of clusters was present, and reduction in H(2) at higher temperatures led to larger tantalum clusters. This is the first example of the surface-mediated synthesis of an early transition metal cluster, and the supported clusters reported here are the first to have been characterized by all three of the spectroscopic methods mentioned above. The similarity of the surface synthesis to that in solution points to opportunities to extend this new class of material to other early transition metal clusters on various supports.