From Achiral Ligands to Chiral Coordination Polymers: Spontaneous Resolution, Weak Ferromagnetism, and Topological Ferrimagnetism

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Abstract
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 L1 (1,4-bis(2-pyridyl)-1-amino-2,3-diaza-1,3-butadiene) and L2 (1,4-bis(2-pyridyl)-1,4-diamino-2,3-diaza-1,3-butadiene) induces spontaneous resolution, yielding conglomerates of chiral compounds [Mn3(L1)2(N3)6]n (1) and [Mn2(L2)2(N3)3]n(ClO4)n·nH2O (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 L3 (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 fish-scale-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, [Mn2(L3)(N3)4]n (3a), and a conglomerate, [Mn2(L3)(N3)4]n·nMeOH (3b). On the other hand, the ligand without amino and methyl substituents (L4, 1,4-bis(2-pyridyl)-2,3-diaza-1,3-butadiene) does not induce spontaneous resolution. The resulting compound, [Mn2(L4)(N3)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 1D 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.

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