Syntheses, crystal structures, and magnetic properties of cyano-bridged Mn(II)–Nb(IV) bimetal assemblies

In this paper, we report the syntheses, crystal structures, and magnetic properties of two manganese(II)–octacyanoniobate(IV)-based magnets: MnII2[NbIV(CN)8](3-pyridinemethanol)8·2H2O (1) and MnII2[NbIV(CN)8](3-aminopyridine)8·2H2O (2). 1 and 2 possess 3D network structures where a [Nb(CN)8]4− ion is connected to four Mn ions, and a Mn ion is connected to two [Nb(CN)8]4− ions via bridged cyanides. The coordination geometry of [Nb(CN)8] in 1 is square antiprism, while that in 2 is dodecahedron. Magnetic measurements show that 1 and 2 are ferrimagnets with Curie temperatures (TC) of 24 and 43 K, respectively, although the coordination numbers of the bridged CN groups around Nb and Mn ions are the same. The difference in TC can be understood by considering the difference in the coordination geometries of [Nb(CN)8]. Molecular orbital calculations of [NbIV(CN)8] using the discrete variational (DV)-Xα method indicate that the population of the 2p orbital on the bridged N with a dodecahedron geometry is larger than that with a square antiprism geometry. The calculation results suggest that the charge population of the 2p orbital on the cyano-nitrogen affects the superexchange interaction for the magnetic orbital of Nb C N Mn, which determines the TC value.