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

Abstract

We report here on the synthesis and characterisation of a first iron(II) spin-crossover coordination polymer with the dca spacer ligand, having the formula [Fe(aqin)2(dca)]ClO4.MeOH (aqin=8-aminoquinoline, dca=dicyanamide), which displays a two-step complete spin transition. Variable-temperature magnetic susceptibility measurements and Mössbauer spectroscopy have revealed that the two relatively gradual steps are centred at 215 and 186 K and are separated by an inflection point at about 201 K, at which 50 % of the complex molecules undergo a spin transition. The two steps are related to the existence of two crystallographically inequivalent metal sites, as confirmed by the structural and Mössbauer studies. The crystal structure was resolved at 293 K (HS form) and 130 K (LS form). Both spin-state structures belong to the triclinic P1 space group (Z=2). The complex assumes a linear chain structure, in which the active iron(II) sites are linked to each other by anionic dicyanamide ligands acting as chemical bridges. The Fe-Fe distances through the dca ligand are 8.119(1) and 7.835(1) A in the high-spin and low-spin structures, respectively. The polymeric chains extend along a (1, 0, -1) axis and are packed in sheets, between which the perchlorate anions and methanol molecules are inserted. The complex molecules are linked together by pi-stacking interactions and H-bonding between the H-donor aqin ligands and the perchlorate ions. These structural features provide a basis for cooperative interactions in the crystal lattice. Analysis of the two-step spin-crossover character in this compound suggests that covalent interactions through the spacer ligand do not provide the main mechanism of cooperativity.

On the Role of Intermolecular Interactions on Structural and Spin‐Crossover Properties of 2D Coordination Networks [Fe(bbtr)3]A2 (bbtr=1,4‐bis(1,2,3‐triazol‐1‐yl)butane; A=ClO4−, BF4−)

“…[13] In order to enhance the perturbation transmission produced by the change of spin state, the idea of direct connection of the SCO centres through the covalent bonds of bridging ligand molecules was applied. [14,15] Materials fulfilling these requirements are coordination networks based on 1H,2,4-triazole or 4-substituted 1,2,4-triazole rings, [16] cyanometallate [17] or dicyanamide [18] anions and bisA C H T U N G T R E N N U N G (pyridylo)- [12,13,15,19,20] and bis- [21][22][23][24][25] or trisA C H T U N G T R E N N U N G (azolylo)-type [26] ligands. Many of them show cooperative spin transition, but coordination networks exhibiting gradual SCO are also known.…”

A Spin‐Crossover Cluster of Iron(II) Exhibiting a Mixed‐Spin Structure and Synergy between Spin Transition and Magnetic Interaction

“…[21] To date, this singular switchable behavior and these phenomena have not been extended to clusters with higher nuclearity.…”

Spiral Assembly of the 1D Chain Sheet of Fe(NCBH3)2(bpa)2·(biphenyl) (bpa = 1,2-bis(4-pyridyl)ethane) and its Stepwise Spin-Crossover Phenomenon

“…It is understood from the results of X-ray structure that the spin-state change intermittently occurs in all 1D sheets. It is well established that the two-step spin crossover can be observed in two cases: (a) coexistence of two crystallographically distinct metal sites at all temperatures [24][25][26][27]; (b) appearance of two crystallographically independent metal sites as a result of a crystallographic phase transition [23,28,29]. Recently, a two-step SCO was observed for several systems involving crystallographically unique metal center whatever the temperature.…”