Heterobimetallic MOFs Containing Tetrathiocyanometallate Building Blocks: Pressure-Induced Spin Crossover in the Porous {Fe^II(pz)[Pd^II(SCN)₄]} 3D Coordination Polymer

Abstract

Here we describe the synthesis, structure, and magnetic properties of two related coordination polymers made up of self-assembling Fe­(II) ions, pyrazine (pz), and the tetrathiocyanopalladate anion. Compound {Fe­(MeOH)₂[Pd­(SCN)₄]}·pz (<b>1a</b>) is a two-dimensional coordination polymer where the Fe­(II) ions are equatorially coordinated by the nitrogen atoms of four [Pd­(SCN)₄]^2– anions, each of which connects four Fe­(II) ions, forming corrugated layers {Fe­[Pd­(SCN)₄]}_∞. The coordination sphere of Fe­(II) is completed by the oxygen atoms of two CH₃OH molecules. The layers stack one on top of each other in such a way that the included pz molecule establishes strong hydrogen bonds with the coordinated methanol molecules of adjacent layers. Compound {Fe­(pz)­[Pd­(SCN)₄]} (<b>2</b>) is a three-dimensional porous coordination polymer formed by flat {Fe­[Pd­(SCN)₄]}_∞ layers pillared by the pz ligand. Thermal analysis of <b>1a</b> shows a clear desorption of the two coordinated CH₃OH molecules giving a rather stable phase (<b>1b</b>), which presumably is a polymorphic form of <b>2</b>. The magnetic properties of the three derivatives are typical of the high-spin Fe­(II) compounds. However, compounds <b>1b</b> and <b>2</b>, with coordination sphere [FeN₆], show thermal spin crossover behavior at pressures higher than ambient pressure (10⁵ MPa)