Molecular Magnetism and Iron(II) Spin‐State Equilibrium as Structural Probes in Heterodinuclear d–f Complexes

Abstract

. Highly preorganized cryptands [5] and podands [6] have been used to control the coordination sphere around Ln"', but it was recently realized that less constrained ligands may also lead to well-defined lanthanide complexes. E.g., Lnr" ions react with linear heterocyclic oligotridentate ligands such as L3

Deciphering the Influence of Meridional versus Facial Isomers in Spin Crossover Complexes

“…(18), see below) . Let's stress here that changing the size or the charge of the innocent (i.e., non‐paramagnetic) capping cations (Ca 2+ , Y 3+ , La 3+ ) in the helicates [CaFe( L5 ) 3 ] 4+ , [YFe( L5 ) 3 ] 5+ and [LaFe( L5 ) 3 ] 5+ has very limited impact on the Fe II ‐centred spin‐crossover processes, a consequence of the large flexibility of the capping unit and its minor influence on the Fe II coordination sphere, except for the preorganization of a single facial isomer (Figure S25) . Altogether, the opposite trends displayed by Δ H SCO (i.e., increase) and Δ S SCO (i.e., decrease) in going from the mononuclear [Fe( L k ) 3 ] 2+ complexes (i.e., mixture of mer / fac isomers) to the dinuclear helicates [LnM( L k ) 3 ] 5+ (i.e., pure facial isomer) produces a significant shift of T 1/2 =Δ H SCO /Δ S SCO toward higher temperature (Δ T 1/2 =30–60 K), which eventually delays the SCO process in the pure facial isomers found in the helicates.…”

Structural, Magnetic, and Electrochemical Properties of Dinuclear Triple Helices: Comparison with Their Mononuclear Analogues

“…[25,44] Molar susceptibilities were converted to effective magnetic moments m eff by Equation (10). [19] Elemental analyses were performed at the Microanalytical Laboratory of the University of Geneva. Metal contents were determined by atomic absorption spectroscopy after acidic mineralization.…”

“…[28] The transition is described by the equilibrium constant K sc , which is the ratio between the molar fractions of the high-and lowspin configurations and may be obtained from Equation (6). [19] Here x HS and x LS are the molar fractions of highand low-spin Fe II , c obs is the observed molar magnetic susceptibility, c HS and c LS are the susceptibilities for completely high-spin and low-spin complexes, m eff is the effective magnetic moment and m HS and m LS are the effective magnetic moments for pure high-spin and low-spin complexes. , typical for this type of complex.…”

“…, typical for this type of complex. [19,28] The transition temperature T 1/2 , for which x HS x LS 0.5, was 299 K. The plot of ln K sc against 1/T was slightly curved; possible reasons for this deviation are: a) a nonzero value of m LS , as observed for similar complexes [29] (however, variation of the value in the range 0 ± 0.7 BM did not improve the fit); b) the presence of Fe III impurities, which we discount since this would require more than 10 % Fe III ; and c) a change in the mer/fac isomer ratio with temperature. This has already been observed for tris(2-methylaminopyridine)-iron(ii) iodide.…”

Monitoring the Spin States of Ferrous Ions by Fluorescence Spectroscopy in Spin‐Crossover–Fluorescent Hybrid Materials

“…This may be realized by tailoring the functionality of organic ligands coordinated to SCO centers or by introducing other functions into SCO systems that couple with the SCO. To this end, fluorescent groups have recently been added to SCO systems using both methods, and the spin‐state switches of metallic ions have been demonstrated to modulate the fluorescence signals . However, metallic ions such as Co II and Fe II can easily quench fluorescence and are often viewed as “killers” of fluorophores, so it is counterintuitive to combine fluorescence and SCO within a system.…”