Both Spacer Length and Parity Influence the Thermal and Light‐Induced Properties of Iron(II) α,ω‐Bis(tetrazole‐1‐yl)alkane Coordination Polymers

Abstract

A new series of [mu-tris-{1,n-bis(tetrazol-1-yl)alkane-N4,N4'}iron(II)] bis(perchlorate) spin-crossover coordination polymers ([Fe(nditz)3](ClO4)2]; n = 4-9) has been synthesised and characterised. The ditetrazole bridging ligands provide octahedral symmetry at the iron(II) centres while allowing the distance between iron(II) centres to be varied. These polymers have therefore been investigated to determine the effects of spacer length on their thermal and light-induced spin-transition behaviour. An increase in the number of carbon atoms in the spacer (n) raises the thermal spin-crossover temperature, while decreasing the stability of the light-induced metastable state generated through the light-induced excited spin state trapping (LIESST) effect by irradiating the sample at 530 nm. Remarkably, however, the parity of the spacer also has an effect, enabling the series of complexes to be divided into two sub-series depending on whether the bridging ligand possesses an even or an odd number of carbon atoms. An explanation at the molecular level using the single configurational coordinate (SCC) model is presented.

Counterion Effect on the Spin‐Transition Properties of the Cation [Fe(btzx)3]2+ (btzx=m‐Xylylenebis(tetrazole))

“…[11] Weinberger and co-workers recently published a study on the relationship between the size of the alkyl spacer and the T 1/2 (temperature at which half of the metal centres are in LS states, the other half being in the HS state) and the optical properties of the complexes. [12] An effect of the length of the spacer on the thermomagnetic and magneto-optical properties is expected, but a correlation with the parity (number of carbons) of the spacer has also been established. Further systematic studies are necessary to better understand the role played by each chemical piece constituting the metal-organic network.…”

“…Most importantly, the size of the counterion has an effect on the quality of the crystallisation. [12] In addition, the exchange of counterions very often results in an annihilation of the SCO properties, resulting from slight structural variations able to modify the ligand-field strength or form a complete different structure. Consequently, its influence on the spin transition is very difficult to investigate.…”

“…So far, the only report mentioning the preparation of Fe II spin-transition compounds with two different counterions involves the ligand btzb (btzb = 1,4-bis-A C H T U N G T R E N N U N G (tetrazole-1-yl)butane). [8,13] [FeA C H T U N G T R E N N U N G (btzb) 3 6 ] 2 shows a twostep steep transition, while the perchlorate derivative shows a steep, incomplete transition, [13] or a gradual one-step transition, [12] suggesting two different phases.…”

“…It is known that the crystallinity of bis(tetrazole)-based compounds is dependent on the anion used. [12] The role of the anion in this type of materials is to act as a template for the generation of the cationic polymeric network. Thus, the size of the anion is crucial to obtain suitable single crystals of the material, as observed for…”

Control of Spin-Crossover Phenomena in One-Dimensional Triazole-Coordinated Iron(II) Complexes by Means of Functional Counter Ions

“…The hysteresis width of T 1/2 decreases with increasing n, showing the even-odd (parity) effect, which is known for 1D iron(II) SCO coordination polymers based on bridging di-tetrazoles [28]. Moreover, we developed [Fe(II)(R-trz) 3 ]@Nafion films exhibiting spin transition around room temperature, in which the Nafion membrane behaves as a counter anion as well as a transparent substrate [21], and investigated the condensed photogenerated HS state below 35 K [29].…”