Odd–Even Effect on the Spin-Crossover Temperature in Iron(II) Complex Series Involving an Alkylated or Acyloxylated Tripodal Ligand

In the context of magneto-structural study, a relatively short alkyl group was introduced to anionic spin-crossover (SCO) building blocks based on [Fe­(py₃CR)­(NCS)₃]⁻, where py₃CR stands for tris­(2-pyridyl)­methyl derivatives. The linear alkyl and acyloxyl derivatives of Me₄N­[Fe­(py₃CR)­(NCS)₃] with R = C_nH_2n+1 (n = 1–7) and C_nH_2n+1CO₂ (n = 1–6) were synthesized, and the magnetic study revealed that all the compounds investigated here exhibited SCO. The SCO temperature (T_1/2) varied in 289–338 K for the alkylated compounds, and those of the acyloxylated ones were lower with a narrower variation width (T_1/2 = 216–226 K). The crystal structures of the former with n = 3, 4, and 5 and the latter with n = 1, 4, 5, and 6 were determined, and various molecular arrangements were characterized. There is no structural evidence for a molecular fastener effect. The plots on T_1/2 against n displayed a pronounced odd–even effect; the SCO temperatures of the homologues with even n are relatively higher than those of the homologues with odd n. The odd–even effect on T_1/2 may be related with the entropy difference across the SCO, rather than crystal field modification or intermolecular interaction. The present work will help molecular design to fine-tune T_1/2 by means of simple chemical modification like alkylation and acyloxylation.