Crystal Structures and Magnetic Properties of Coordination Polymer Complexes of Metal Ions With Kojic Acid Ligands

The crystal structures and magnetic properties of coordination complexes [Cu(C 6 H 5 O 4 ) 2 ] ( 1 ) and [Co(C 6 H 5 O 4 ) 2 ] ( 2 ) constructed from kojic acid were investigated. Single‐crystal X‐ray analyses revealed that both complexes formed one‐dimensional coordination polymers; however, their chain‐connectivity patterns differed owing to the distinct coordination environments of Cu(II) and Co(II). Complex 1 adopted a square‐pyramidal geometry arising from Jahn–Teller elongation, whereas complex 2 featured an octahedral Co(II) center, generating a structurally different polymeric network. Magnetic susceptibility measurements showed paramagnetic behaviors for both complexes without any magnetic phase transition above 2 K. Molecular‐orbital calculations demonstrated that the unpaired electron in 1 was strongly localized in the equatorial CuO plane, resulting in negligible spin density along the axial direction. Although a relatively short intermolecular Cu•••O contact (2.82 Å) with a sizable McConnell spin‐density product (∼0.13) was present, the poor orbital overlap along the axial direction suppressed superexchange, yielding essentially no magnetic coupling. In contrast, 2 contained Co–O•••H–O–Co hydrogen‐bond pathways, but the corresponding McConnell products (∼4 × 10 −3 ) are far below the threshold required for appreciable magnetic exchange. Therefore, the magnetic behavior of 2 is dominated by single‐ion effects, including multiplet splitting and zero‐field splitting, rather than by intermolecular superexchange.