Synthesis and characterization of new trinuclear copper complexes

This report describes our approach towards modeling the copper cluster active sites of nitrous oxide reductase and the multicopper oxidases/oxygenases. We have synthesized two mesitylene-based trinucleating ligands, MesPY1 and MesPY2, which employ bis(2-picolyl)amine (PY1) and bis(2-pyridylethyl)amine (PY2) tridentate copper chelates, respectively. Addition of cuprous salts to these ligands leads to the isolation of tricopper(I) complexes [(Mes-PY1)CuI3(CH3CN)3](ClO4)3·0.25Et2O (1) and [(Mes-PY2)CuI3](PF6)3 (3). Each of the three copper centers in 1 is most likely four-coordinate, with ligated acetonitrile as the fourth ligand; by contrast, the copper centers in 3 are three-coordinate, as determined by X-ray crystallography. The synthesis of [(Mes-PY1)CuII3(CH3CN)2(CH3OH)2](ClO4)6·(CH3OH) (2) was accomplished by addition of three equivalents of the copper(II) salt, Cu(ClO4)2·6H2O, to the ligand. The structure of 2 shows that two of the copper centers are tetracoordinate (with MeCN solvent ligation), but have additional weak axial (fifth ligand) interactions with the perchlorate anions; the third copper is unique in that it is coordinated by two MeOH solvent molecules, making it overall five-coordinate. For complexes 2 and 3, one copper ion center is located on the opposite side of the mesitylene plane as the other two. These observations, although in the solid state, must be taken into account for future studies where intramolecular tricopper(I)/O2 (or other small molecules of interest) interactions in solution are desirable.