Anion-directed synthesis of metal−organic frameworks based on 2-picolinate Cu(II) complexes: a ferromagnetic alternating chain and two unprecedented ferromagnetic fish backbone chains

Abstract

Three new polynuclear copper (II) complexes of 2-picolinic acid (Hpic), {[Cu₂(pic)₃(H₂O)]ClO₄}_n (1), {[Cu₂(pic) (H₂O)]BF₄}_n (2) and [Cu₂(pic)₃(H₂O) ₂(NO₃)]_n (3), have been synthesized by reaction of the “metalloligand” [Cu(pic) ₂] with the corresponding copper(II) salts. The compounds are characterized by single-crystal X-ray diffraction analyses and variable-temperature magnetic measurements. Compounds 1 and 2 are isomorphous and crystallize in the triclinic system with space group P1̄, while 3 crystallizes in the monoclinic system with space group P2₁/n. The structural analyses reveal that complexes 1 and 2 are constructed by “fish backbone” chains through syn−anti (equatorial−equatorial) carboxylate bridges, which are linked to one another by syn−anti (equatorial−axial) carboxylate bridges, giving rise to a rectangular grid-like two-dimensional net. Complex 3 is formed by alternating chains of syn−anti carboxylate-bridged copper (II) atoms, which are linked together by strong H bonds involving coordinated nitrate ions and water molecules and uncoordinated oxygen atoms from carboxylate groups. The different coordination ability of the anions along with their involvement in the H-bonding network seems to be responsible for the difference in the final polymeric structures. Variable-temperature (2−300 K) magnetic susceptibility measurement shows the presence of weak ferromagnetic coupling for all three complexes that have been fitted with a fish backbone model developed for 1 and 2 (J = 1.74 and 0.99 cm⁻¹; J‘ = 0.19 and 0.25 cm⁻¹, respectively) and an alternating chain model for 3 (J = 1.19 cm⁻¹ and J‘ = 1.19 cm⁻¹).