Structural Insights into the Two-Step Spin-Crossover Compound Fe(3,4-dimethyl-pyridine)2[Ag(CN)2]2

Abstract

The crystal structure of the polymeric spin crossover compound Fe(3,4-dimethyl-pyridine)2[Ag(CN)2]2 has been solved and its temperature dependence followed by means of single-crystal and powder X-ray diffraction. This compound presents a two-step spin transition with relatively abrupt steps centred at ca. 170 K and 145 K and a plateau at around 155 K. The origin of the two-step transition is discussed in light of these structural studies. The observations are compatible with a mostly disordered state between the two steps, consisting of mixing of high-spin and low-spin species, while weak substructure reflections in the mixed phase could indicate some degree of long-range order of the high-spin and low-spin sites.

Two-Step Spin Crossover in Hofmann-Type Coordination Polymers [Fe(2-phenylpyrazine)2{M(CN)2}2] (M = Ag, Au)

“…The transition temperatures are 175 K ( T c1 ) and 125 K ( T c2 ), with a narrow pseudoplateau appearing between 145 and 149 K. In the subsequent heating mode, these parameters are completely retained, and the transition curve reproduces the curve in the cooling regime. It was found that AgPhpz is characterized by transition temperatures similar to those of Ag-containing SCO complexes based on pyridine-type ligands. ,,, …”

“…Complex AgPhpz undergoes a gradual two-step transition over a temperature range of ∼100 K (Figure 4a) like for similar Ag-containing SCO systems. 32,36,44 The χ M T values are 3.53 cm 3 K mol −1 at 273 K, 1.86 cm 3 K mol −1 at 147 K, and 0.23 cm 3 K mol −1 at 50 K, which allow one to roughly estimate the completeness of the transition at 0%, 47%, and 94%, respectively. This corroborates with the complete stepped transformation of a Fe(II) ion from the HS state (S = 2, paramagnetic) to the LS state (S = 0, diamagnetic).…”

“…It was found that AgPhpz is characterized by transition temperatures similar to those of Ag-containing SCO complexes based on pyridine-type ligands. 33,36,44,66 The χ M T value of AuPhpz is 3.06 cm 3 K mol −1 at 273 K, indicating the HS state of a Fe(II) ion. Upon further cooling, χ M T gradually decreases to 0.43 cm 3 K mol −1 at 100 K, corresponding to the practically complete LS fraction.…”

“…To date, quite a few examples of materials showing multistep SCO behavior are known. − ,− There are several examples of three-, − four-, ,,,− six/five-step spin transtions and even an example of a seven/eight-step spin transition. It should be noted that a significant number of such systems are Hofmann clathrate analogues, especially with dicyanometallic [M­(CN) 2 ] − (M = Ag, Au) fragments and pyridine derivatives (ranging from simple monosubstituted to more complicated ligand systems). , In these complexes, Fe­(II) ions have a pseudooctahedral coordination environment [FeN 6 ] with two axial azine ligands and four dicyanometallic [M­(CN) 2 ] − fragments that occupy equatorial positions.…”

Synthesis and Applications of New Spin Crossover Compounds

“…The optical microscopy technique to investigate the thermal and the spatio-temporal properties of the Hofmann-related SCO single crystal [Fe(2-pytrz) 2 {Pt(CN) 4 }]_3H 2 O was described to show a first-order SCO behavior from a full high-spin (HS) state at high temperatures to intermediate, high-spin low-spin (HS-LS) states [14]. The precise crystallographic investigation on the polymeric SCO Hofmann-like compound Fe(3,4-dimethyl-pyridine) 2 [Ag(CN) 2 ] 2 was reported, and its temperature dependence was followed by the means of a single-crystal and powder X-ray diffraction [15]. These very important article reported in the special issue demonstrate a soft X-ray-induced excited spin state trapping (SOXEISST) effect in Hofmann-like SCO coordination polymers of Fe II (4-methylpyrimidine) 2 [Au(CN) 2 ] 2 and Fe II (pyridine) 2 [Ni(CN) 4 ] [16].…”