Synthesis, characterization and reactivity of transition metal clusters and their role towards organic transformation

Abstract

Transition metal cluster containing main group atoms as bridging ligands have drawn increased attention in recent years, mainly because of their unusual structures and novel chemical reactivity, as well as for their potential in the field of material science and catalysis. In the last three decades, varieties of synthetic methodologies have been developed for the synthesis of metal clusters containing chalcogens with unique structural features and properties. The chalcogen ligands have been known to act as bridging units and support the metal fragments in various cluster growth reactions. Designing of systematic synthetic routes to clusters containing metal-chalcogen bonds with new geometries and coordination modes led to the development of models and precursors for the synthesis of new materials. Some of these metal-chalcogen containing building blocks have been of great interest due to their unusual structural features and tunable opto-eletronic properties. Moreover, mixed-metal clusters have also been of tremendous importance due to their use as valuable precursors for the preparation of supported bimetallic and multimetallic heterogenous catalysts. In view of the enormous potential of transition metal clusters, we started our investigation to synthesise novel chalcogenide transition metal clusters containing ligands like phosphines, carbonyls, acetylides, alkynes etc. and understand their role in supporting cluster molecules and to explore the reactivity of metal clusters towards cluster growth reactions. Structural diversity of transition metal clusters can be achieved by using different types of ligands that play an important role to support the cluster framework and assist in the tuning of the cluster behaviour. This has prompted us to design transition metal cluster containing diphosphine groups of varied chain length and understand their potential in linking cluster cluster molecule. Furthermore, phosphines are one of the most widely utilized ligands in transition metal complex chemistry due to their extreme versatility in bonding and reactivity.
Most of these diphosphine ligands have been found to adopt a variety of bonding modes on the cluster framework, including monodentate with a pendant phosphine center, chelating a single metal atom in the multimetallic cluster, bridging across a metal-metal bond and forming an intermolecular link across two clusters. The bonding modes adopted by these diphosphine ligands are influenced by the flexibility and length of the organic or organometallic backbone. In an effort to prepare novel clusters with structural identity, we sought to explore the possibility of incorporating both diphosphine ligands and chalcogen atoms in the cluster framework and study
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their combined effect. We have been able to synthesize several homo- and hetero-metallic transition metal clusters containing chalcogens and diphosphines as supporting ligands. To understand the influence of different diphosphine ligands towards metal chalcogenide clusters we studied the reaction of triiron ditelluride carbonyl cluster and triironditelluride phosphine cluster with two different diphosphine ligands, bis(diphenylphosphino)methane and bis(diphenylphosphino)ethane. Synthesis and characterization of four new iron-palladium mixed metal clusters containing diphosphine ligand have been carried out and shows interesting bonding features and coordination modes. The contrasting results show the difference in reactivity between the cluster species and the influence of phosphines in controlling the cluster synthesis. Our aim has also been to synthesize complexes containing several metal binding sites for the synthesis of multimetallic system. In an effort to synthesize such molecules we have focussed our study on the preparation of dithiocarboxylate-alkyne metal complexes by sunlight mediated reaction process and use them to obtain mutimetallic complexes. A variety of organic transformations are supported and catalysed by metal complexes, wherein the necessary steric and electronic requirements for such transformations are offered by the metal centres and ligands. To understand the exact behaviour of the synthesized molecule on this front some investigation on the metal mediated transformation of different alkynes was undertaken.