Selective Pathway toward Heteroleptic Spin-Crossover Iron(II) Complexes with Pyridine-Based N-Donor Ligands.

A new synthetic pathway is devised to selectively produce previously elusive heteroleptic iron(II) complexes of terpyridine and N,N'-disubstituted bis(pyrazol-3-yl)pyridines that stabilize the opposite spin states of the metal ion. Such a combination of the ligands in a series of the heteroleptic complexes induces the spin-crossover (SCO) not experienced by the homoleptic complexes of these ligands or shifts it to lower/higher temperatures respective to the SCO-active homoleptic complex. The midpoint temperatures of the resulting SCO span from ca. 200 K to the ambient temperature and beyond the highest temperature accessible by NMR spectroscopy and SQUID magnetometry. The proposed "one-pot" approach is applicable to other N-donor ligands to selectively produce heteroleptic complexes─including those inaccessible by alternative synthetic pathways─with highly tunable SCO behaviors for practical applications in sensing, switching, and multifunctional devices.

Unravelling of a [High Spin-Low Spin] ↔ [Low Spin-High Spin] Equilibrium in Spin-Crossover Iron(II) Dinuclear Helicates Using Paramagnetic NMR Spectroscopy.

Spin-crossover between high-spin (HS) and low-spin (LS) states of selected transition metal ions in polynuclear and polymeric compounds is behind their use as multistep switchable materials in breakthrough electronic and spintronic devices. We report the first successful attempt to observe the dynamics of a rarely found broken-symmetry spin state in binuclear complexes, which mixes the states [HS-LS] and [LS-HS] on a millisecond timescale. The slow exchange between these two states, which was identified by paramagnetic NMR spectroscopy in solutions of two spin-crossover iron(II) binuclear helicates that are amenable to molecular design, opens a path to double quantum dot cellular automata for information storage and processing.

Room-Temperature Spin Crossover in a Solution of Iron(II) Complexes with N,N'-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.