Chemistry and Relevant Biomimetic Applications of Group 6 Metals Systems Supported by Scorpionates

The structural and functional properties of group 6 metal complexes with scorpionate ligands, used as synthetic analogues for the binding sites of the molybdenum and tungsten enzymes, are the subject of the present review. The group 6 elements molybdenum and tungsten are the only second and third row transition metals essential to all forms of life on Earth. Molybdenum is found at the active sites of nitrogenase and all of the more than 50 known Mo-molybdopterin (Mo- MPT) enzymes that play vital roles in plant, animal, and human health, the carbon, sulfur, and nitrogen cycles, biofeedback systems, and the control of global climate; tungsten is also associated with MPTbased ligands in all its known biological manifestations. Chemical approaches to molybdenum enzyme sites have been directed toward mimicking a portion of the structural center in order to ascertain the role of that particular feature of the center on the chemical reactivity and the spectroscopic properties of the center. Here we attempt to analyze the overall progress on synthetic analogues of these enzyme centers and dissect the contributions of systems in which coordination spheres contain poly(pyrazolyl)borate ligands, focusing primarily on research that has appeared since the structures of the active sites of representative enzymes have become known.