Two Cyanide-Bridged Mn^II–Nb^IV Coordination Chain Ferrimagnets Promoted by Interchain Ferromagnetic Interactions

Two new coordination chain compounds based on octacyanoniobate­(IV) are reported: {[Mn^II(bpy)­(H₂O)₂]₂[Nb^IV(CN)₈]·5H₂O}_<i>n</i> (<b>1</b>) and {[Mn^II(bpy)­(H₂O)₂]­[Mn^II(bpy)₂]­[Nb^IV(CN)₈]·5H₂O}_<i>n</i> (<b>2</b>) (bpy = 2,2′-bipyridine). Both have the topology of vertex-sharing CN-bridged squares. The Mn₂Nb₂ units are mutually parallel in <b>1</b> and perpendicular in <b>2</b>. The antiferromagnetic intrachain interactions between the Mn^II and Nb^IV centers in <b>1</b> and <b>2</b>, typical for Mn^II-[Nb^IV(CN)₈] family of compounds, lead to ferrimagnetic ground state of each single chain. Both compounds show bulk ferrimagnetic ordering below their critical temperatures (<i>T</i>_c) of 6.4 K in <b>1</b> and 4.7 K in <b>2</b>. Such a behavior is due to the ferromagnetic interchain interactions, assured by the π–π contacts between the 2,2′-bipyridine rings and by the hydrogen bonds between the terminal cyanide ligands of the [Nb^IV(CN)₈]-building blocks and coordination/crystallization water molecules. <b>1</b> and <b>2</b> are rare examples of ferrimagnets promoted by interchain ferromagnetic exchange, and the <i>T</i>_c of <b>1</b> is one of the highest among similar 1-D coordination polymers