Linear or Cyclic Clusters of Cu(II) with a Hierarchical Relationship

The polydentate ligand 2,6-bis­(5-(2-hydroxyphenyl)-pyrazol-3-yl)-pyridine, H₄L, exhibits a series of coordination pockets favoring the establishment of metal sequences with predetermined motifs, together with a degree of flexibility for the formation of clusters with various overall topologies. With Cu­(II) under strong basic conditions it has a marked tendency to stabilize a cyclic [Cu₁₆L₈] cluster. The sequential formation of this compound via [Cu₇L₈]^2– intermediates, recognized in its structure, is suggested by crystallographic evidence, which shows the persistent formation of the complex salt (NBu₄)₂[Cu₇L₈] in the presence of the organic cation. Also, the crystallographic identification of the related cluster [Cu₁₁L₅(OH)₂(py)₁₂] from similar reaction conditions underscores the rich multiplicity of species attainable from this simple reaction system.