Two mononuclear iron(ii) complexes of 4-phenylpyrazole-5-carbaldehyde derived ligands are stabilised in different spin states

Abstract

The polydentate acyclic ligand 4-phenylpyrazole-5-carbaldehyde azine, H₂L¹, has been prepared and characterised. A second 4-phenylpyrazole-5-carbaldehyde derived ligand, H₂L², was prepared in situ by condensation with o-phenylendiamine. Two mononuclear iron(II) complexes, [Fe^II(HL²)(MeOH)(NCSe)]·H₂O and [Fe^II(H₂L¹)₂](BF₄)₂·solvents, have been synthesised from 1 : 1 and 2 : 3 M:L reactions, respectively. No dinuclear, helical, species were obtained from 2 : 3 M:L reactions regardless of the solvent employed (MeOH, MeCN or NO₂Me). An X-ray structure determination on the latter complex reveals that it is low spin at 90 K. Mössbauer spectra confirm this, and show that the former complex is, in contrast, high spin, even at 4.6 K.