Synthesis and Electronic Structure Determination of N-Alkyl-Substituted Bis(imino)pyridine Iron Imides Exhibiting Spin Crossover Behavior

摘要

Three new N-alkyl substituted bis(imino)-pyridine iron imide complexes, (~(iPr)PDI)FeNR (~(iPr)PDI = 2, 6-(2,6-~iPr_2-C_6H_3-N=CMe)_2C_5H_3N; R = 1-adamantyl (~1Ad), cyclooctyl (~(Cy)Oct), and 2-adamantyl (~2Ad)) were synthesized by addition of the appropriate alkyl azide to the iron bis(dinitrogen) complex, (~(iPr)PDI)Fe(N_2)_2· SQUID magnetic measurements on the isomeric iron imides, (~(iPr)PDI)FeN~1 Ad and (~(iPr)PDI)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 Mossbauer spectroscopic measurements indicate diamagnetic ground states at cryogenic temperatures and established low isomer shifts consistent with highly covalent molecules. For (~(iPr)PDI)FeN~2Ad, Moessbauer 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.