Structural Dynamics of Spin Crossover in Iron(II) Complexes with Extended-Tripod Ligands

Abstract

Selective manipulation of spin states in iron(II) complexes by thermal or photonic energy is a desirable goal in the context of developing molecular functional materials. As dynamic spin-state equilibration in isolated iron(II) complexes typically limits the lifetime of a given spin state to nanoseconds, synthetic strategies need to be developed that aim at inhibited relaxation. Herein we show that modulation of the reaction coordinate through careful selection of the ligand can indeed massively slow down dynamic exchange. Detailed structural analysis of [FeL] 2+ and [ZnL] 2+ (L: tris(1-methyl-2-{[pyridin-2-yl]-methylene}hydrazinyl)phosphane sulfide) with crystallographic and computational methods clearly reveals a unique trigonal-directing effect of the extended-tripod ligand L during spin crossover, which superimposes the ubiquitous [FeN 6 ] breathing with trigonal torsion, akin to the archetypal Bailar twist. As a consequence of the diverging reaction coordinates in [FeL] 2+ and in the tren-derived complex [Fe(tren)py 3 ] 2+ , their thermal barriers differ massively, although the spin crossover energies are close to identical. As is shown by time-resolved transient spectroscopy and dynamic 1 H-NMR line broadening, reference systems deriving from tren (tris-(2-aminoethyl)amine), which greatly lack such trigonal torsion, harbor very rapid spin-state exchange.

Exploring the light-induced dynamics in solvated metallogrid complexes with femtosecond pulses across the electromagnetic spectrum

“…For this complex, the reaction coordinate cannot be approximated by a simple average Fe-N bond elongation, e.g., it is anisotropic (Nance et al, 2015;Canton et al, 2014;2015;. As revealed by powerful studies combining ligand design, crystallography, and modeling tools, such anisotropy is the key to stabilizing the HS state (Steinert et al, 2016;Stock et al, 2016;2017a;2017b;Phan et al, 2017;and Kroll et al, 2019). A significant increase in the HS lifetime of mononuclear complexes occurs when radial and angular motions are coupled during the rearrangement of the ligand scaffold (Marchivie et al, 2005;Buhks et al, 1980;Hauser et al, 1991;1995;2004;and Gütlich et al, 1994), e.g., through a trigonal prismatic deformation known as the Bailar twist (Bailar, 1958), which was already proposed in the 1980s (Rodger and Johnson, 1988;Vanquickenborne and Pierloot, 1981;McCusker et al, 1992;and McCusker et al, 1993).…”

Revealing Hot and Long-Lived Metastable Spin States in the Photoinduced Switching of Solvated Metallogrid Complexes with Femtosecond Optical and X-ray Spectroscopies

“…Extended HS lifetimes have been realized for mononuclear complexes by designing sophisticated ligands that strongly couple radial and angular motions across the LS → HS transition, − e.g. through the Bailar twist …”

Spin-Crossover Complexes

“…This Special Issue also contains 11 original research articles that are devoted to the synthesis and characterization of various molecular systems of SCO complexes. For the investigation of isolated mononuclear SCO complexes, Stock and co-workers [11] report structural dynamics in isolated LS Fe(II) complexes from extended-tripod ligands by means of laser flash photolysis and find that the slowing-down of dynamic exchange from the metastable HS state may arise from the trigonal torsion of a coordination octahedron. With respect to mononuclear SCO complex solids, Kimura and Ishida [12] describe the substitution effect on the pybox ligand in an Fe(II) SCO complex and reveal that the decrease in LFS energy originates from the electron-donating ability of substituents; moreover, the comparison between the solid and solution states may shed light on the possibility of separating the inner and outer effects from a substitution effect.…”