Molecule of the Week (05)
This week’s molecule comes out of recent research at Iowa State University focused on the catalytic activity of zirconium (IV) complexes involving a new class of scorpionate ligands (Dunne, J.F.; Su, J.; Ellern, A.; Sadow, A.D., Organometallics, 2008, 27, 2399-2401). I found this to be of particular interest since the Zubris lab has also been investigating the catalytic activity of zirconium complexes with new and interesting ligands. First developed by DuPont chemist Swiatoslav Trofimenko in the 1960s, the tridentate tris(pyrazolyl)borate (“Tp,” “scorpionate”) ligands coordinate to a metal in a facial manner creating steric bulk on one side, partially shielding the metal center and preventing unwanted side reactions. The chemists at ISU have been exploring the reactivity of a similar class of complexes in olefin catalysis. What makes these ligands significant is their ability to withstand the high energy of a reactive, cationic zirconium during polymerization. The carbon-boron bonds, the researchers believe, allow the ligand to withstand decomposition and isomerization tendencies, thereby maintaining its structural integrity in the presence of cationic zirconium. Previous research has shown that the nitrogen-boron bonds found in the traditional Tp ligands, are subject to isomerization and cleavage in many transition metal complexes. The ISU chemists anticipate that the bulk of these ligands will protect and stabilize the zirconium complex, allowing the polymerization of olefins and possible stereoselective polymerizations with analogous chiral zirconium complexes they hope to synthesize.