Chirality and Spin Crossover in Iron(II)–Octacyanidorhenate(V) Coordination Polymers Induced by the Pyridine-Based Ligand’s Positional Isomer

We present two novel coordination polymers based on octacyanidorhenate(V) metalloligands and Fe(II) complexes with two different positional isomers of a benzylpyridine ligand, namely 3-benzylpyridine (3-benzpy) that leads to a two-dimensional {[FeII(3-benzpy)4]3[ReV(CN)8]2}·2H2O (1) coordination network and 4-benzylpyridine (4-benzpy) giving a three-dimensional {[FeII(4-benzpy)4]5[ReV(CN)8]3}(ClO4)·2(4-benzpy)·6H2O·MeOH (2) coordination framework. 1 is a layered coordination polymer of a honeycomb topology, crystallizing in the centrosymmetric space group, which exhibits only residual thermal spin-crossover (SCO) effect on Fe(II) complexes at low temperatures. The lack of a significant SCO effect is probably caused by strong supramolecular interactions which do not allow the coordination framework to undergo the structural change required by the spin transition. On the other hand, 2 crystallizes as the chiral, cationic three-dimensional pillared cyanido-bridged framework, consisting of coordination layers of a deformed square grid topology, further bonded together by additional Fe(II) complexes. The structure is completed by noncoordinated 4-benzpy, water, and methanol molecules, as well as perchlorate counterions. The chiral character of the structure of 2 was confirmed by the single-crystal X-ray diffractions studies and the second-harmonic generation (SHG) effect detected at room temperature. 2 exhibits a pronounced two-step, and incomplete thermal SCO effect of embedded Fe(II) complexes. The intricate course of the spin transition in 2 is related to the presence of five crystallographically distinguishable Fe(II) centers with different supramolecular environments. The first noticeable SCO step is realized by the two Fe(II) complexes undergoing the spin transition between 250 and 150 K, while the second is related to the incomplete spin transition of one type of remaining Fe(II) complexes. Therefore, 2 is a rare example of a chiral SCO material showing also the nontrivial course of the spin transition. These properties were achieved by the subtle modification of the pyridine-based ligand indicating the advantage of iron(II)–octacyanidorhenate(V) systems in the formation of functional spin transition materials.