Counterion effect on the spin-transition properties of the second generation iron(III) dendrimeric complexes

Abstract

The magnetic properties and the influence of counterions on the spin crossover properties of two novel Fe(III) dendrimeric complexes of the second generation, namely [Fe(L)2]+X−, where L=3,5-di(3,4,5-tris(tetradecyloxy)benzoyloxy)benzoyl-4-oxy-salicylidene-N’-ethyl-N-ethylenediamine X=Cl− (1), ClO4− (2), have been studied for the first time by magnetic susceptibility measurements and electron paramagnetic resonance (EPR) method in a wide (4.2–300K) temperature range. EPR results showed that compound 1 contains about 98% of high-spin (HS, S=5/2) and ∼2% of low-spin (LS, S=1/2) Fe(III) centers, and undergoes an antiferromagnetic ordering below 7K. The EPR integrated intensity of a broad line (g≈2), corresponding to the HS iron(III) centers, passes through a broad maximum at Tmax≈100K, which is indicative of short-range correlation effects. The anomalous broadening of this EPR line at low temperatures with the critical exponent β=1.5 upon approaching the long-range ordering transition (TNEPR=7K) from above indicates the quasi-two-dimensional antiferromagnetic nature of magnetism in complex 1. The spin-crossover effect is completely suppressed in compound 1. The complex with ClO4− counterion demonstrates a different magnetic behavior. EPR data showed that compound 2 contains about 77% of LS and ∼23% of HS Fe(III) centers at TNEPR=10.2K. It displays a partial spin crossover (S=5/2↔1/2) above 150K and undergoes the antiferromagnetic ordering below 10.2K. The obtained results and the results of DFT calculations allowed us to conclude that a bilayered packing with a chain structure of Fe(III) centers in ionic bilayers is formed in compound 1, whereas a dimeric structure of Fe(III) centers is formed in compound 2. Thus, the ability of the counterion to form an effective network of hydrogen bonds and its size define the packing motif of the [Fe(L)2]+ complexes. Therefore, the replacing of the counterion has a significant impact on the magnetic properties of the compound.