Manganese(ii)-carboxylate-pseudohalide systems derived from 1,4-bis(4-carboxylatopyridinium-1-methylene)benzene: structures and magnetism

Abstract

The reactions of manganese(II) acetate or perchlorate, sodium azide or sodium cyanate, and the zwitterionic dicarboxylate ligand 1,4-bis(4-carboxylatopyridinium-1-methylene)benzene (L) under different conditions yielded three different Mn(II) coordination polymers with mixed carboxylate and azide (or cyanate) bridges: {[Mn (L¹)_0.5(N₃)(OAc)]·3H₂O}_n (1), {[Mn₄(L¹)(N₃)₈(H₂O)₄(CH₃OH)₂]·[L¹]}_n (2), and {[Mn₃(L¹)(NCO)₆(H₂O)₄]·[L¹]·[H₂O]₂}_n (3). The compounds exhibit diverse structures and magnetic properties. In 1, the 1D uniform anionic [Mn(N₃)(COO)₂]_n chains with the (μ-EO-N₃)(μ-COO)₂ triple bridges (EO = end-on) are interlinked by the dipyridinium L ligands into highly undulated 2D layers. Magnetic studies on 1 reveal that the mixed triple bridges induce antiferromagnetic coupling between Mn(II) ions. Compounds 2 and 3 consist of 1D neutral polymeric chains and co-crystallized zwitterions, and the chains are formed by the L ligands interlinking linear polynuclear units. The polynuclear unit in 2 is tetranuclear with (μ-EO-N₃)₂ as central bridges and (μ-EO-N₃)₂(μ-COO) as peripheral bridges, while that in 3 is trinuclear with (μ-NCO)₂(μ-COO) bridges. Magnetic studies demonstrate that the magnetic coupling through the mixed azide/isocyanate and carboxylate bridges in 2 and 3 is antiferromagnetic. An expression of magnetic susceptibility based on a 2-J model for linear tetranuclear systems of classical spins has been deduced and applied to 2.