Synthesis and Characterization of Cu(II) and Mixed-Valence Cu(I)Cu(II) Clusters Supported by Pyridylamide Ligands.

A group of copper complexes supported by polydentate pyridylamide ligands Hbpda and Hppda were synthesized and characterized. The two Cu(II) dimers [Cu(Hbpda)(ClO)] () and [Cu(ppda)(DMF)] () were constructed by using neutral ligands to react with Cu(II) salts. Although the dimers showed similar structural features, the second-sphere interactions affect the structures differently. With the application of EtN, the tetranuclear cluster (HNEt)[Cu(bpda)(μ-OH)(ClO)(DMF)](ClO) () and hexanuclear cluster (HNEt)[Cu(ppda)(HO)(CHOH)](ClO) () were prepared under similar reaction conditions. The symmetrical and unsymmetrical arrangement of the ligand donors in ligands Hbpda and Hppda led to the dramatic conformation difference of the two Cu(II) complexes. As part of our effort to explore mixed-valence copper chemistry, the triple-decker pentanuclear cluster [CuCu(bpda)(μ-O)] () was prepared. XPS examination demonstrated the localized mixed-valence properties of complex . Magnetic studies of the clusters with EPR evidence showed either weak ferromagnetic or antiferromagnetic interactions among copper centers. Due to the trigonal-planar conformation of the trinuclear Cu(II) motif with the μ-O center, complex exhibits geometric spin frustration and engages in antisymmetric exchange interactions. DFT calculations were also performed to better interpret spectroscopic evidence and understand the electronic structures, especially the mixed-valence nature of complex .