Double End-on Bridges Research Articles

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)(ClO(4))(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 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, [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(4))(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 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.

Structural and magnetic properties of three one-dimensional azido-bridged copper(II) and manganese(II) coordination polymers.

Three one-dimensional (1D) azido-bridged coordination polymers of formula [Cu(L)(N3)2]n (1), [Cu2(Me-L)(N3)4]n (2), and [Mn(L)(N3)2]n (3) have been synthesized and structurally characterized, and their magnetic properties studied, where L and Me-L are 2-(pyrazol-1-ylmethyl)pyridine and 2-(3-methylpyrazol-1-ylmethyl)pyridine, respectively. Compound 1 consists of 1D chains in which the Cu(II) ions with a square pyramidal geometry are alternately bridged by an end-to-end (EE) and an end-on (EO) azido ligands, both adopting a basal-apical disposition. Compound 2 exhibits an unprecedented chain topology built via three different kinds of EO azido bridges. Four Cu(II) ions in the square pyramidal environment are alternately bridged by single and double EO bridges to form a tetranuclear cyclic ring, and neighboring rings are interlinked by double EO bridges to generate a "chain of rings". The intraannular double azido ions are disposed between metal ions in a basal-basal fashion, and the other two kinds of azido ions adopt the basal-apical disposition. Compound 3 consists of 1D concave-convex chains in which cis-octahedrally coordinated Mn(II) ions are alternately bridged by double EE and double EO bridges. There exist pi-pi interactions between the ligands bound to the neighboring Mn(II) ions bridged by the EO bridges. Temperature- and field-dependent magnetic analyses reveal alternate ferromagnetic interactions for 1, dominating ferromagnetic interactions for 2, and alternating ferro- and antiferromagnetic interactions through the EO and EE azido bridges for 3, respectively.

New one-dimensional azido-bridged manganese(II) coordination polymers exhibiting alternating ferromagnetic-antiferromagnetic interactions: structural and magnetic studies.

Five Mn(II)[bond]azido coordination polymers of formula [Mn(L)(N(3))(2)](n) have been synthesized and crystallographically characterized, and their magnetic properties studied, where L's are the bidentate Schiff bases obtained from the condensation of pyridine-2-carbaldehyde with aniline (1) and its derivatives p-toluidine (2), m-toluidine (3), p-chloroaniline (4), and m-chloroaniline (5). All the complexes consist of the zigzag Mn(II)[bond]azido chains in which the Mn(II) ions are alternately bridged by two end-to-end (EE) and two end-on (EO) azido ligands, the cis-octahedral coordination being completed by the two nitrogen atoms of the Schiff base ligands. Compound 2 is unique in that the Mn[bond](EE-N(3))(2)[bond]Mn ring adopts an unusual twist conformation with the two linear azido bridges crossing each other. By contrast, the rings in the other compounds take the usual chair conformation with the two azido bridges parallel. The double EO bridging fragments in the complexes are similar with the bridging angles (Mn[bond]N[bond]Mn) ranging from 99.6 degrees to 104.0 degrees. Magnetic analyses reveal that alternating ferro- and antiferromagnetic interactions are mediated through the alternating EO and EE azido bridges with the J(F) and J(AF) parameters in the ranges of 4.1-8.0 and -11.8 to -15.4 cm(-1), respectively. Finally, the magnetostructural correlations are investigated. The present complexes follow the general trend that the ferromagnetic interaction through the double EO bridge increases with the Mn[bond]N[bond]Mn bridging angle, while the antiferromagnetic interaction through the double EE bridge is dependent on the distortion of the Mn[bond](N(3))(2)[bond]Mn ring from planarity toward the chair conformation and the Mn[bond]N[bond]N angle.