Optical recognition of alkyl nitrile by a homochiral iron(II) spin crossover host

A homochiral complex 1.MeCN was synthesized by the multicomponent self-assembly of (R)-phenylethylamine, 1-methyl-2-imidazolecarboxaldehyde and iron(II) ions in acetonitrile solution. X-ray crystallography analysis revealed that complex 1.MeCN crystallized in the chiral space group P2(1). The octahedral coordination mononuclear [FeL3](2+) cations are stacked into a left-handed double helix supramolecular structure along the a axis with uncoordinated acetonitrile molecules filling the helical channel. Interestingly, when 1.MeCN redissolved in racemic lactonitrile (LN) or methylglutaronitrile (MGN), the [FeL3](2+) cations can recognize one enantiomeric alkyl nitrile by forming 1.1/3(R)-LN or 1.1/3(S)-MGN crystals. 1.1/3(R)-LN and 1.1/3(S)-MGN crystallized in the P2(1)2(1)2(1) space group, and the [FeL3](2+) cations are stacked in a triple helix mode with the enantiomeric alkyl nitrile captured in the helical channel. Magnetic measurement indicated that 1.MeCN displayed spin-crossover at 355 K, while the transition temperature became 220 K after desolvation. However, 1.1/3(R)-LN and 1.1/3(S)-MGN exhibited incomplete and reversible spin-crossover behaviors at about 363 K. The results demonstrated that the mononuclear iron(II) complex could be used as a host for racemic alkyl nitrile separation, and the spin-crossover property was influenced by the process of chiral recognition.