From achiral ligands to chiral coordination polymers: Spontaneous resolution, weak ferromagnetism, and topological ferrimagnetism

Using the achiral diazine ligands bearing two bidentate pyridylimino groups as sources of conformational chirality, five azido-bridged coordination polymers are prepared and characterized crystallographically and magnetically. The chirality of the molecular units is induced by the coordination of the diazine ligands in a twisted chiral conformation. The use of L-1 (1,4-bis(2-pyridyl)-1-amino-2,3-diaza-1,3-butadiene) and L-2 (1,4-bis(2-pyridyl)-1,4-diamino-2,3-diaza-1,3-butadiene) induces spontaneous resolution, yielding conglomerates of chiral compounds [Mn-3(L-1)(2)(N-3)(6)](n) (1) and [Mn-2(L-2)(2)(N-3)(3)](n)(ClO4)(n)(.)nH(2)O (2), respectively, where triangular (1) or double helical (2) chiral units are connected into homochiral one-dimensional (1D) chains via single end-to-end (EE) azido bridges. The chains are stacked via hydrogen bonds in a homochiral fashion to yield chiral crystals. When L-3 (2,5-bis(2-pyridyl)-3,4-diaza-2,4-hexadiene) is employed, a partial spontaneous resolution occurs, where binuclear chiral units are interlinked into fishscale-like homochiral two-dimensional (2D) layers via single EE azido bridges. The layers are stacked in a heterochiral or homochiral fashion to yield simultaneously a racemic compound, [Mn-2(L-3)(N-3)(4)](n) (3a), and a conglomerate, [Mn-2(L-3)(N-3)(4)](n)(.)nMeOH (3b). On the other hand, the ligand without amino and methyl substituents (L-4, 1, 4-bis(2-pyridyl)-2,3-diaza-1,3-butadiene) does not induce spontaneous resolution. The resulting compound, [Mn-2(L)(N-3)(4)](n) (4), consists of centrosymmetric 2D layers with alternating single diazine, single EE azido, and double end-on (EO) azido bridges, where the chirality is destroyed by the centrosymmetric double EO bridges. These compounds exhibit very different magnetic behaviors. In particular, 1 behaves as a metamagnet built of homometallic ferrimagnetic chains with a unique "fused-triangles" topology, 2 behaves as a 1 D antiferromagnet with alternating antiferromagnetic interactions, 3a and 3b behave as spin-canted weak ferromagnets with different critical temperatures, and 4 also behaves as a spin-canted weak ferromagnet but exhibits two-step magnetic transitions.