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 H₂bpda and H₂ppda were synthesized and characterized. The two Cu(II) dimers [Cu^II₂(Hbpda)₂(ClO₄)₂] (1) and [Cu^II₂(ppda)₂(DMF)₂] (2) 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 Et₃N, the tetranuclear cluster (HNEt₃)[Cu^II₄(bpda)₂(μ₃-OH)₂(ClO₄)(DMF)₃](ClO₄)₂ (3) and hexanuclear cluster (HNEt₃)₂[Cu^II₆(ppda)₆(H₂O)₂(CH₃OH)₂](ClO₄)₂ (4) were prepared under similar reaction conditions. The symmetrical and unsymmetrical arrangement of the ligand donors in ligands H₂bpda and H₂ppda 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 [Cu^II₃Cu^I₂(bpda)₃(μ₃-O)] (5) was prepared. XPS examination demonstrated the localized mixed-valence properties of complex 5. 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 μ₃-O center, complex 5 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 5.