Room-Temperature Spin Crossover in a Solution of Iron(II) Complexes with <i>N</i>,<i>N</i>′‑Disubstituted Bis(pyrazol-3-yl)pyridines

Here, we report a
combined study of the effects of two chemical
modifications to an N,N′-disubstituted
bis­(pyrazol-3-yl)­pyridine (3-bpp) and of different solvents on the
spin-crossover (SCO) behavior in otherwise high-spin iron­(II) complexes
by solution NMR spectroscopy. The observed stabilization of the low-spin
state by electron-withdrawing substituents in the two positions of
the ligand that induce opposite electronic effects in SCO–active
iron­(II) complexes of isomeric bis­(pyrazol-1-yl)­pyridines (1-bpp)
was previously hidden by NH functionalities in 3-bpp precluding the
molecular design of SCO compounds with this family of ligands. With
the recent SCO-assisting substituent design, the uncovered trends
converged toward the first iron­(II) complex of N,N′-disubstituted 3-bpp to undergo an almost complete
SCO centered at room temperature in a less polar solvent of a high
hydrogen-bond acceptor ability.