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

Abstract

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 P2₁. 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 P2₁2₁2₁ 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.