Synthesis and electronic structure determination of N-alkyl-substituted bis(imino)pyridine iron imides exhibiting spin crossover behavior

Three new N-alkyl substituted bis(imino)pyridine iron imide complexes, ( ^iPrPDI)FeNR ( ^iPrPDI = 2,6-(2,6- ⁱPr ₂-C ₆H ₃-N=CMe) ₂C ₅H ₃N; R = 1-adamantyl ( ¹Ad), cyclooctyl ( ^CyOct), and 2-adamantyl ( ²Ad)) were synthesized by addition of the appropriate alkyl azide to the iron bis(dinitrogen) complex, ( ^iPrPDI)Fe(N ₂) ₂. SQUID magnetic measurements on the isomeric iron imides, ( ^iPrPDI)FeN ¹Ad and ( ^iPrPDI)FeN ²Ad, established spin crossover behavior with the latter example having a more complete spin transition in the experimentally accessible temperature range. X-ray diffraction on all three alkyl-substituted bis(imino)pyridine iron imides established essentially planar compounds with relatively short Fe-N _imide bond lengths and two-electron reduction of the redox-active bis(imino)pyridine chelate. Zero- and applied-field Mössbauer spectroscopic measurements indicate diamagnetic ground states at cryogenic temperatures and established low isomer shifts consistent with highly covalent molecules. For ( ^iPrPDI)FeN ²Ad, Mössbauer spectroscopy also supports spin crossover behavior and allowed extraction of thermodynamic parameters for the S = 0 to S = 1 transition. X-ray absorption spectroscopy and computational studies were also performed to explore the electronic structure of the bis(imino)pyridine alkyl-substituted imides. An electronic structure description with a low spin ferric center (S = 1/2) antiferromagnetically coupled to an imidyl radical (S _imide = 1/2) and a closed-shell, dianionic bis(imino)pyridine chelate (S _PDI = 0) is favored for the S = 0 state. An iron-centered spin transition to an intermediate spin ferric ion (S _Fe = 3/2) accounts for the S = 1 state observed at higher temperatures. Other possibilities based on the computational and experimental data are also evaluated and compared to the electronic structure of the bis(imino)pyridine iron N-aryl imide counterparts.