The second example of spin conversion governed by molecular vibrations: a novel ferrous complex resulting from hexacoordination of a Schiff base with an N4O2 donor set

The crystal structure of [FeII(3-MeO,5-NO2-sal-N(1,10)-NMe(4,7))] (2) is identical to that of [FeII(5-NO2-sal-N(1,4,7,10))] (1), except for substitution of two strong N–H⋯O(nitro) bonds (1) by one weak N–C–H⋯O(nitro) contact (2) between adjacent molecules along the 1D chains. Unlike 1 (two-step spin-state transition), 2 experiences an incomplete (17%) and progressive (70–240 K) spin-state conversion (Mössbauer and magnetic studies). A theoretical approach based on the Ising-like model, completed with harmonic oscillators associated with the 15 vibrational modes of the coordination octahedron, fits the thermal variation of the high-spin (HS) fraction with a 2303 K energy gap between the lowest low spin (LS) and HS electrovibrational states, an average vibrational frequency ωLS=725 K and an average ωLS/ωHS=1.724 ratio. These results illustrate the effect of intramolecular vibrations spin-state conversions.