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

Three new polynuclear copper(II) complexes of 2-picolinic acid (Hpic), {[Cu2(pic)3(H2O)]ClO4} n (1), {[Cu2(pic)3(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).