Spin-Crossover Iron(II) Coordination Polymer with Zigzag Chain Structure

Abstract

We report on the synthesis and characterization of a first iron(II) spin-crossover coordination polymer of formula [Fe(pyim) 2 (bpy)](ClO 4 ) 2 ‚2C 2 H 5 OH, where pyim ) 2-(2-pyridyl)imidazole and bpy ) 4,4′-bipyridine, with zigzag chain structure. The chains are linked by π stacking into a two-dimensional network. Variable-temperature magnetic susceptibility measurements have revealed the occurrence of the HS T LS transition without hysteresis. The spin conversion temperature T 1/2 (temperature for which the HS fraction is equal to 0.5) is found equal to 205 K. The crystal structure was resolved at 293 K (high-spin form) and at 173 K (low-spin form). Both spin-state structures belong to the monoclinic space group C2/c (Z ) 4). The complex assumes one-dimensional zigzag chain structure, in which the iron(II) active sites are linked to each other by a chemical bridge as the rigid rodlike 4,4′-bipyridine molecule. The Fe-Fe separation via the bpy ligand equals 11.523(2) and 11.201(2) Å in the high-spin and low-spin structures, respectively. The polymeric chains extended along the c axis are stacked in the b direction forming two-dimensional sheets of bound molecules. The sheets are linked together by several intermolecular H-bonds in the a direction. The ensemble of structural features of the complex, potentially leading to the cooperativity, provides a basis for strong interactions. However, they are not strong enough in comparison with the HS-LS energy gap and result in a relatively gradual spin transition of the complex. The structural features and magnetic properties of the complex have been discussed and compared with those for earlier reported iron(II) spin-crossover coordination polymers.

Spin-Crossover Behavior of Hofmann-Type-Like Complex Fe(4,4’-bipyridine)Ni(CN)4·nH2O Depending on Guest Species

“…11.3 Å was observed ( Figure S2). This distance agrees with that of 11.5 Å observed in a known spin cross over (SCO) complex [Fe(bpy)(biim) 2 ](ClO 4 ) 2¨2 C 2 H 5 OH (biim = 2-(2-pyridyl)imidazole)), which contains bpy working as a bridge between two Fe(II) atoms [34]. Based on the above information, the basic structure of 1 is the layered structure similar to that of the Hofmann-type host complex and the layers are linked by bridging bpy ligands.…”

Crystal structure of 2-(2′-pyridyl)imidazole, C8H7N3

“…It is also known to yield to iron(II) complexes exhibiting spin crossover behaviour in solution [16,17] as well as in the solid state [18][19][20][21][22]. Yet its crystal structure had not been determined.…”

First Dicyanamide‐Bridged Spin‐Crossover Coordination Polymer: Synthesis, Structural, Magnetic, and Spectroscopic Studies

“…Other polymers have since been synthesised, with bridging units such as 4,4'-dipyridyl, [22][23][24][25] 1,2-di(4-pyridyl)ethene, [26] and 1,4-bis(4-pyridyl)butadiyne. [27] However, many of these new polymers, [22,23,28] as well as covalently linked spin-crossover dimers, [29] have a lower cooperativity than might be expected on the basis of the chosen strategy, irrespective of the Fe-Fe separation. This highlights the fact that the mere presence of a bridging unit capable of linking metal sites together with short Fe-Fe distances is not sufficient to build a highly cooperative spincrossover system.…”

“…We introduced a way of overcoming this problem through the use of the self-assembly strategy with mixed ligands, [22] that is, with different molecules acting as spacer and coligand. To date, this approach has led to the synthesis of the aforementioned family of 2-(2-pyridyl)imidazole polymers [22,28] and of the linear chain polymer [FeA C H T U N G T R E N N U N G (aqin) 2 A C H T U N G T R E N N U N G (4,4'-bpy)]A C H T U N G T R E N N U N G (ClO 4 ) 2 ·2 EtOH (4,4'-bpy = 4,4'-bipyridyl) (1), which exhibits a spin-crossover phenomenon.…”

“…To date, this approach has led to the synthesis of the aforementioned family of 2-(2-pyridyl)imidazole polymers [22,28] and of the linear chain polymer [FeA C H T U N G T R E N N U N G (aqin) 2 A C H T U N G T R E N N U N G (4,4'-bpy)]A C H T U N G T R E N N U N G (ClO 4 ) 2 ·2 EtOH (4,4'-bpy = 4,4'-bipyridyl) (1), which exhibits a spin-crossover phenomenon. [23] We present here a novel polymer, (2), obtained by way of the same synthetic strategy.…”