Effect of Ligand Field Strength on the Spin Crossover Behaviour in 5‐X‐SalEen (X=Me, Br and OMe) Based Fe(III) Complexes

Abstract

The reaction of Fe(NCS) 3 prepared in situ in MeOH with 5-X-SalEen ligands (5-X-SalEen = condensation product of 5-substituted salicylaldehyde and N-ethylethylenediamine) provided three Fe(III) complexes, [Fe(5-X-SalEen) 2 ]NCS; X=Me (1), X=Br (2), X=OMe (3). All the complexes reveal similar structural features but a very different magnetic profile. Complex 1 shows a gradual spin crossover while complexes 2 and 3 show a sharp spin transition. The T 1/2 for complex 2 is 237 K while for complex 3 it is much higher with a value of 361 K. The spin transition temperature is shifted towards higher temperature with increasing electron-donation ability of the ligand substituents. This experimental observation has been rationalized with DFT calculations. UV-Vis and cyclic voltammetry studies support the fact that the electron density on the ligand increases from Me to Br to OMe substituents. To understand the change in spin states, temperature-dependent EPR spectra have been recorded. The spin state equilibrium in the liquid state has been probed with Evans NMR spectroscopic method, and thermodynamic parameters have been evaluated for all complexes.

Study of Spin Crossover Property of a Series of X‐OMe‐SalEen (X=6, 5 and 4) Based Fe(III) Complexes

“…The energy gap between high spin and low spin state (ΔE HS‐LS ) are 5.99 kJ mol −1 and 4.02 kJ mol −1 for 1 and 3 respectively, and they lie within the reported range of values for spin crossover [23–24] . Intermediate spin states are higher in energy than the high and low spin states [16] . The optimized structures at high and low spin have been provided in figure S22 and S23, Supporting Information.…”

“…The effect of solvent on SCO behavior is absent for complexes 1 and 3, but visible for complex 2. The magnetic data of complex 3 (having NCSe À anion) and that of a similar complex (having NCS À anion) are identical, [16] so it can be ascertained that anions have no role to play in the SCO behavior. Taking into account the weak interactions, the magneto-structural correlation has been done for the complexes using the Hirshfeld surface analysis.…”

“…[23][24] Intermediate spin states are higher in energy than the high and low spin states. [16] The optimized structures at high and low spin have been provided in figure S22 and S23, Supporting Information. To understand the spin transition behavior, d-orbital splitting energies have been studied (Figure 14).…”

“…Previous studies reveal the use of various ligand substituents that alter the electron density on the metal center, thereby changing the nature of the spin transition [15] . Recently, we reported a paper where the effect of the different substituents on SCO property has been studied for [Fe(5‐X‐SalEen) 2 ]NCS complexes [16] . Now, we attempted to study the π‐donation ability of the methoxy group at various isomeric positions of the phenoxide ring (ortho, meta, and para) coordinated directly to Fe(III), and thereby investigate the SCO behavior and the spin transition temperature for the three complexes formed.…”

Magnetic properties of the mononuclear iron (III) complexes with biphenyl‐disubstituted Schiff base ligand: EPR and SQUID study

“…Interesting to create new multifunctional fluorescent magnetic materials based on iron metal complexes which can exist in two different spin states and can be converted from low‐spin (LS) to high‐spin (HS) states upon external action like heat, light irradiation, pressure, and magnetic fields or by chemical alteration (ligand substitution, present of anion, and solvates). [ 13 ] Among the most extensively studied spin‐crossover (SCO) materials are complexes of Fe 2+ and Fe 3+ , which have caused the interest of many researchers as potential multifunctional materials in fields such as molecular electronics, memory storage, sensors, and photoelectric devices. Most previous publications on this topic are devoted to Fe 2+ SCO, while Fe 3+ complexes have a number of advantages (e.g., structural, magnetic, photomagnetic, and relaxation).…”