Cooperative Spin Transitions in Iron(II) Derivatives of 1,2,4-Triazole

Abstract

Complexes of stoichiometry Fe( trzH )2( trz )X, where trzH is 1,2,4-triazole and X = BF4, ClO4, PF6, have been prepared. All three salts display temperature-induced singlet (1A1) ↔ quintet (5T2) transitions which are generally discontinuous and associated with a broad hysteresis loop. The salts are strongly thermochromic, being white above and pink-lilac below the transition region. Several samples of each salt were obtained and the detailed behaviour varied somewhat from one sample to another. For the fluoroborate and perchlorate salts the transition is centred above room temperature while for the hexafluorophosphate salt it occurs below room temperature. The perchlorate salt was also isolated in a form which showed a continuous transition but the nature of its electronic properties changed as the sample aged. In one form of the hexafluorophosphate salt the hysteresis loop spans room temperature and it has been possible to characterize both the singlet and the quintet state species at that temperature. For all three salts the transition has been characterized by measurement of the temperature dependence of the magnetism and Mossbauer and electronic spectra. These measurements establish that the complexes all contain an FeN6 coordination core and this must be achieved through bridging of the triazole units.

Triazole‐Based One‐Dimensional Spin‐Crossover Coordination Polymers

“…[8a] In many instances, especially with bulky R groups or long alkyl substituents, the formed coordination polymeric cationic chains tend to be insoluble and a precipitate rapidly forms. [12,17] This is also the case in the Htrz/trz system, [10] most likely due to the extended network of strong interchain interactions that forms (see the section on Structural Aspects). A direct consequence of this is the difficulty to isolate crystals of a significant size in these systems.…”

“…44 This in general allows supporting observations from magnetic measurements, while in certain cases gives additional insight into local modifications due or related to the SCO. In 1D triazole‐based materials, Mössbauer spectroscopy has been used mostly to detect the SCO and quantify the residual HS fractions at low temperature 10a,b. 11a,b,c,g,h, 13, 16, 17b,c, 18, 25b,c, 45 This is likely due to the presence of only one type of environment for the Fe II as detected by Mössbauer spectroscopy, which initially was put forward to support the proposed 1D nature of these materials 10a.…”

“…[8] After early studies in the 1990s concerned with the processing of selected SCO compounds into bulk sensors and opto-electronic displays, more recently a surge of research activity in this domain is related with both the nanostructuration of SCO materials and the advent of new detection techniques or new associated physical properties. [9] Polymeric one-dimensional (1D) triazole-based Fe II materials [10][11][12][13][14][15][16][17][18] of general formula [FeA C H T U N G T R E N N U N G (Rtrz) 3 ][A] 2 ·x H 2 O (Rtrz = 4substituted-1,2,4-triazole, and A = monovalent anion) have been at the centre of both early studies and the more recent developments. They indeed have allowed the design of early optical devices prototypes, [19] the elaboration of SCO nanoparticles, [20,21] supramolecular gels, [22] thin films, [23,24] dendrimers, [25] contrast agents for Magnetic Resonance Imaging [26] and liquid crystals, [12c, 27] as well as the electrical addressing of SCO [28] or the use of plasmons to detect SCO.…”

Water effect on the spin-transition behavior of Fe(ii) 1,2,4-triazole 1D chains embedded in pores of MCM-41

“…[8][9][10] Several SCO nanoparticles based on Fe(II) complexes were prepared and embedded in matrices, such as SiO 2 , 11 Ni, 12 and CsFe[Cr(CN) 6 ], to name a few. [16][17][18] Both are known to exhibit a hysteretic SCO behavior with sharp spin transition (ST) above room temperature. 14,15 In this work, two iron(II) SCO compounds were embedded in mesostructured silica MCM-41.…”

Spin-crossover Compounds with Wide Thermal Hysteresis

“…However, two examples exhibit more cooperative spin transitions (Scheme 4): 22 (T 1/2 = 329 K and ¦T = 37 K) 70,71 and the "A" modification of 23 (T 1/2 = 365 K and ¦T = 40 K). 23,72 Although these compounds have been intensely studied for 20 years, the first X-ray structures have only recently become available. 73,74 The high-spin and low-spin structures of 23 are most informative.…”