Pyrazole's substituents effect on the spin state of [Fe(bpp)2]2+complexes

2,6-dipyrazolyl pyridine (bpp) skeleton is a good building unit for iron(II) spin crossover research. Various functional groups can be appended on pyrazolyl and pyridine rings thereby modifying potentially the spin state properties of [Fe(bpp)2]2+ derivatives. In this work, we have worked on substituted pyrazole by grafting an ester function as an electro-attractor group and a methyl substituent as an electro-donating group in 3 and 5 positions, respectively. Mossbauer analyses of the mononuclear complex 1 [FeL2](ClO4)2 (L= diethyl-1,1'-(pyridine-2,6-diyl)bis(5-methyl-1H-pyrazole-3-carboxylate) revealed high-spin iron(II) ions at room temperature. On cooling down to liquid helium temperature, magnetic susceptibility measurements confirmed a paramagnetic state, suggesting that the steric influence of the pyrazole substituents was to great to allow a thermally induced spin crossover to occur.