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

The combination of iron-(II) perchloratewith the octacyanidorhenate-(V)ions and two isomers of benzylpyridine results in heterometallic cyanido-bridgedframeworks whose structure and physical properties are governed bythe position of a benzyl substituent of the pyridine ligand. The 3-benzylpyridinegenerates the coordination layers crystallizing in the centrosymmetricspace group which does not show significant thermal Fe-(II) spin crossover,whereas the 4-benzylpyridine induces the chiral three-dimensionalcoordination network exhibiting thermal spin transition of Fe-(II)sites. We present two novel coordination polymers based on octacyanidorhenate-(V)metalloligands and Fe-(II) complexes with two different positionalisomers of a benzylpyridine ligand, namely 3-benzylpyridine (3-benzpy)that leads to a two-dimensional {[Fe-II(3-benzpy)(4)](3)[Re-V(CN)(8)](2)}center dot 2H(2)O (1) coordination network and 4-benzylpyridine(4-benzpy) giving a three-dimensional {[Fe-II(4-benzpy)(4)](5)[Re-V(CN)(8)](3)}-(ClO4)center dot 2-(4-benzpy)center dot 6H(2)O center dot MeOH (2) coordination framework. 1 is a layered coordinationpolymer of a honeycomb topology, crystallizing in the centrosymmetricspace group, which exhibits only residual thermal spin-crossover (SCO)effect on Fe-(II) complexes at low temperatures. The lack of a significantSCO effect is probably caused by strong supramolecular interactionswhich do not allow the coordination framework to undergo the structuralchange required by the spin transition. On the other hand, 2 crystallizes as the chiral, cationic three-dimensional pillaredcyanido-bridged framework, consisting of coordination layers of adeformed square grid topology, further bonded together by additionalFe-(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 confirmedby the single-crystal X-ray diffractions studies and the second-harmonicgeneration (SHG) effect detected at room temperature. 2 exhibits a pronounced two-step, and incomplete thermal SCO effectof embedded Fe-(II) complexes. The intricate course of the spin transitionin 2 is related to the presence of five crystallographicallydistinguishable Fe-(II) centers with different supramolecular environments.The first noticeable SCO step is realized by the two Fe-(II) complexesundergoing the spin transition between 250 and 150 K, while the secondis related to the incomplete spin transition of one type of remainingFe-(II) complexes. Therefore, 2 is a rare example of achiral SCO material showing also the nontrivial course of the spintransition. These properties were achieved by the subtle modificationof the pyridine-based ligand indicating the advantage of iron-(II)-octacyanidorhenate-(V)systems in the formation of functional spin transition materials.

Automated summary

The combination of iron-(II) perchlorate with the octacyanidorhenate-(V) ions and two isomers of benzylpyridine results in heterometallic cyanido-bridged frameworks. The structure and physical properties of these frameworks are determined by the position of the benzyl substituent of the pyridine ligand. The coordination layers formed by 3-benzylpyridine do not exhibit significant thermal spin crossover, while the chiral three-dimensional coordination network formed by 4-benzylpyridine displays thermal spin transition of Fe-(II) sites.