Tetranuclear CoII, MnII, and CuII complexes of a novel binucleating pyrazolate ligand preorganized for the self-assembly of compact [2 x 2]-grid structures

A straightforward synthesis toward the preparation of the rigid pyrazolate ligand L-H featuring bipyridyl side-arms is described, starting from 2,2'-bipyridyl-N-oxide as the sole organic building block. In this context, optimized procedures for the synthesis of the organic intermediates 6-acetyl-2,2'-bipyridine 1 and 6-methylcarboxy-2,2'-bipyridine 2 are reported. The new ligand comprises two proximate terpyridine-like binding sites and is shown to form discrete [2 x 2]-grid complexes with Co-II, Mn-II, and Cu-II in a highly selective self-assembly process, even in the presence of excess metal precursor. The thus obtained complexes [Co4L4][Na(NO3)(4)](NO3) (3), [Mn4L4](PF6)(4) (4), and [CU4L4](ClO4)(4) (5) are fully characterized, including X-ray crystallographic analyses, and their magnetic properties are discussed. All three complexes show weak to moderate antiferromagnetic coupling between the four nuclei. The stability of the grid structures proved very high, as dissociation or exchange between metal ions in solution was not observed in a set of competition experiments.