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 H 2 bpda and H 2 ppda were synthesized and characterized. The two Cu(II) dimers [Cu II 2 (Hbpda) 2 (ClO 4 ) 2 ] ( 1 ) and [Cu II 2 (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 Et 3 N, the tetranuclear cluster (HNEt 3 )[Cu II 4 (bpda) 2 (μ 3 -OH) 2 (ClO 4 )(DMF) 3 ](ClO 4 ) 2 ( 3 ) and hexanuclear cluster (HNEt 3 ) 2 [Cu II 6 (ppda) 6 (H 2 O) 2 (CH 3 OH) 2 ](ClO 4 ) 2 ( 4 ) were prepared under similar reaction conditions. The symmetrical and unsymmetrical arrangement of the ligand donors in ligands H 2 bpda and H 2 ppda 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 II 3 Cu I 2 (bpda) 3 (μ 3 -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 μ 3 -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 .