2,2′-Bipyridine-6,6′-diyl bisnitroxide as a paramagnetic host: Encapsulation of a zinc(II) ion

A chelate complex of zinc(II) and 2,2′-bipyridine-6,6′-diyl bis(tert-butyl nitroxide) (bpybNO) with a metal/ligand ratio of 1/2 was structurally characterized to be [Zn(bpybNO)2][Zn(hfac)3]2, where Hhfac stands for 1,1,1,5,5,5-hexafluoropentane-2,4-dione. The magnetic susceptibility measurement indicates the presence of considerable antiferromagnetic interaction among the four S = 1/2 spins. The exchange parameter J was estimated as 2J/kB = −103(1) K, on the basis of a tetrahedral coupling model. The antiferromagnetic coupling is stronger after complexation than before. The density-functional theory calculation on related model compounds supports the present analysis and clarified the role of the zinc ion as a superexchange coupler.

Graphical abstractA chelate complex [Zn(bpybNO)2][Zn(hfac)3]2 was structurally and magnetically characterized. The superexchange coupling via diamagnetic zinc(II) ion was suggested, and the exchange parameter J was estimated as 2J/kB = −103(1) K on the average among the four radical S = 1/2 spins.Figure optionsDownload full-size imageDownload as PowerPoint slideHighlights► A biradical bipyridine derivative was utilized in supramolecular chemistry. ► Full structural and magnetic characterization of the zinc complex. ► Superexchange coupling across the zinc ion is discussed.