New directions of cluster chemistry - The story of the [Re ₆(μ₃-Se)₈]²⁺ clusters

Transition metal clusters are a unique class of chemical substances. Not only do they have well-defined molecular structures, they also exhibit interesting and potentially useful properties that are inherent to metal-metal bonded species. In this Comment our recent results in developing synthetic methodologies necessary to bring a cluster system out of the limited sphere of fundamental cluster chemistry and into general synthetic applicability are summarized. Specifically, we have explored the uses of site-differentiated cluster complexes, featuring the [Re₆(μ₃-Se) ₈]²⁺ core, as stereospecific building blocks for supramolecular construction. A great variety of cluster-supported molecular and supramolecular architectures have been realized, some of which display rather interesting electronic and spectroscopic properties. Following a distinctly different line of research, we have also pursued the activation of nitriles toward nucleophilic attack, as a result of their prior coordination to the Lewis acidic rhenium sites of the cluster. The isolation and characterization of imino ester complexes proved that the cluster core acts as a powerful Lewis acid catalyst. Subsequent photocleavage in the presence of nitriles, regenerating the procatalytic solvate, marks the completion of one cycle of a potentially photocatalytic process. It is not unreasonable to imagine applying cluster-based Lewis acid activation to a range of substrates beyond nitriles and alcohols. The work therefore marks the beginning of what promises to be an exciting new chapter in cluster chemistry.