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

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 (1a) 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]} (2) is a three-dimensional porous coordination polymer formed by flat {Fe­[Pd­(SCN)4]}∞ layers pillared by the pz ligand. Thermal analysis of 1a shows a clear desorption of the two coordinated CH3OH molecules giving a rather stable phase (1b), which presumably is a polymorphic form of 2. The magnetic properties of the three derivatives are typical of the high-spin Fe­(II) compounds. However, compounds 1b and 2, with coordination sphere [FeN6], show thermal spin crossover behavior at pressures higher than ambient pressure (105 MPa).