Tuning Magnetic Dimensionality via Metal-Ion Dilution in a Polyoxometalate-Templated 2D Framework.

The rational design of superparamagnetic molecular magnets remains a central challenge in molecular materials chemistry. Herein, we report a silicotungstate-templated, 2D cyanoferrate coordination polymer and its four isostructural Zn2+-diluted analogs, {(SiW12O40)@[TpFe(CN)3]2Fe3- xZnx(MeOH)6∙6MeOH}n (Tp = Tri(pyrazolyl)borate; 1-5, x = 0-3). The neutral hybrid layers adopt a cyanide-bridged honeycomb topology, with (SiW12O40)4- anions located in the cavities and interlayer metal-to-metal separations exceeding 10 Å. Magnetic study revealed that 1 (x = 0) behaves as a metamagnet with an ordering temperature of 15.5 K, featuring intralayer ferromagnetic and interlayer antiferromagnetic coupling. Remarkably, Zn2+ doping disrupts the 2D magnetic layer, triggering a dimensional crossover in magnetism. As a result, compounds 2 (x = 0.41) and 3 (x = 0.93) exhibit slow relaxation of the magnetization, suggesting a superparamagnetic behavior, with effective energy barriers of 40.9 and 46.3 K, respectively. This transition from 2D to lower-dimensional magnetism arises from a fragmentation of the honeycomb network into isolated spin chains (clusters) via metal-ion dilution. Such a metal-doping-induced "top-down" transformation of magnetic dimensionality provides a novel alternative to conventional bottom-up synthetic strategies for engineering low-dimensional magnetic materials.