Toward chiral iron(II) spin-crossover grafted resins with imidazole Schiff-base ligands

• Spin-crossover iron(II) enantiomeric complexes 1-4 were synthesized.
• Chiral polymers 5-8 were synthesized by grafting 1-4 to the merrifield’s peptide resin.
• The spin-crossover behavior presented in monomer complexes 1-4 was vanished in polymers 5-8.

Using chiral imidazole Schiff-base as ligands, two couples of mononuclear iron(II) enantiomeric complexes fac-Λ-[Fe(R-L)3](ClO4)2·H2O (1), fac-Δ-[Fe(S-L)3](ClO4)2·H2O (2), fac-Λ-[Fe(R-L)3](BF4)2·H2O (3), fac-Δ-[Fe(S-L)3](BF4)2·H2O (4) (L = 1-(2-naphthyl)-N-((1-methyl-imidazol-2-yl)methylene)ethanamine) have been successfully synthesized and characterized. The CD spectra of 1 and 2, 3 and 4, were basically mirror images confirming their enantiomers. X-ray crystallography revealed that complexes 1–4 all crystallized in the chiral space group P213 with the iron(II) center surrounded by three bidentate ligands. Magnetic measurements revealed incomplete spin transition for 1, and gradual spin crossover (T1/2 = 150 K) for 3. However, the spin-crossover behaviors were vanished after grafting 1–4 to the Merrifield's peptide resin, and the iron(II) centers in the resin remained in a high-spin state in the temperature range of 2–300 K.

Graphic abstractTwo couples of mononuclear spin-crossover iron(II) enantiomeric complexes 1–4 and their Merrifield's peptide resin-supported chiral compounds 5–8 have been successfully synthesized. Magnetic measurements revealed that monomer complexes 1–4 displayed obvious spin-crossover properties, while the spin-crossover behavior presented in monomer complexes 1–4 was vanished after it was grafted to the resin.Figure optionsDownload as PowerPoint slide