Attempted synthesis of axial–equatorial pyrazine-bridged copper(II) complexes toward homometallic ferromagnetic compounds

The introduction of a relatively bulky alkyl group at 2-position of pyrazine afforded unique μ-pyz complexes, namely, dinuclear [{Cu(hfac)2}2(μ-2-isobutylpyrazine)], trinuclear [{Cu(hfac)2}3(μ-2-isopropylpyrazine)2], and polymeric [Cu(hfac)2(μ-ibupyz)]n (Hhfac stands for 1,1,1,5,5,5-hexafluoropentane-2,4-dione). These compounds have an axial–equatorial dinuclear moiety, which is expected to show ferromagnetic interaction. However, they exhibited practically negligible or very weak antiferromagnetic interaction. This may be due to violation of orthogonality because of the steric hindrance of the isobutyl or isopropyl groups. We analyzed the relationship between the structural deformation parameter defined as the N(eq)⋯N(ax)–Cu angle and the exchange coupling parameter. A critical angle, at which the sign of the magnetic coupling changes from positive to negative, is estimated to be 165(2)°. The present results are reasonably explained in this relation.

Graphical abstractWe synthesized unique di-, tri-, and polynuclear compounds having an axial–equatorial-type pyrazine bridge. In contrast to the expectation of ferromagnetic interaction, they exhibited practically negligible antiferromagnetic interaction. The steric hindrance of the isobutyl group has been successfully utilized to afford the axial coordination structure, but at the same time it failed the orthogonality between the magnetic orbitals of the two Cu spins.Figure optionsDownload full-size imageDownload as PowerPoint slideHighlights► Axial–equatorial pyrazine-bridged Cu2+ compounds were synthesized and characterized. ► Sterically bulky 2-isobutyl- and 2-isopropylpyrazines were employed. ► The magneto-structure relationship is discussed based on the orbital orthogonality. ► A critical geometry, at which the sign of the magnetic coupling changes, is proposed.