Rational Design of Iron Spin-Crossover Complexes Using Heteroscorpionate Chelates

Abstract

The optical, structural, and magnetic properties of iron(II,III) sandwich complexes, Fe(Tp′)2n+(Tp′ = bis(3,5-dimethylpyrazolyl)benzotriazolylborate), are described. The intensely colored FeII(Tp′)2(orange) and FeIII(Tp′)2+(purple) show strong MLCT bands. Geometric isomerism for M(Tp′)2is established crystallographically in the racemate of chiral cis-Fe(Tp′)2. For the first time, paramagnetic 11B NMR describes solution-phase low-spin (LS, S = 0) to high-spin (HS, S = 2) crossover behavior in Fe(Tp′)2. Thermochemical parameters for solution-phase SCO of Fe(Tp′)2demonstrate the endothermic LS to HS conversion and entropic preference of the HS state. Entropy changes for both Fe(Tp′)2isomers are significantly larger than for the majority of iron scorpionate SCO systems. Solid-state magnetic and thermochemical measurements show cis-Fe(Tp′)2to be thermally stable up to 520 K, allowing experimental investigation of a solid-state SCO magnetic hysteresis of over 45 K. A large solution vs solid-state SCO difference was observed: cis-Fe(Tp′)2shows Tc≈ 270 K (solution) and Tc≈ 385 K (solid), with the remarkably wide ΔTc≈ 115 K; trans-Fe(Tp′)2shows Tc≈ 278 K (solution) and Tc≈ 372 K (solid). Solid-state Tcvalues are among the highest seen for iron(II) molecular systems. The large solution/solid ΔTcdifference is explained by "anchoring" intermolecular interactions in the solid state that prevent thermal expansion of the LS iron(II) coordination sphere in its transition to the HS state. DFT calculations, validated against LS cis-Fe(Tp′)2crystallography and LS to HS SCO thermochemical parameters, demonstrate the role the benzotriazole rings play in its structural and optical properties. The Lewis basicity of M(Tp′)2is shown with the structural characterization of the air-stable tin(II) adduct [cis-Fe(Tp′)2-SnCl2]; tin(II) coordination does not alter the iron(II) spin state. The Tp′ chelate adds functionality (asymmetry, chirality, chemical reactivity) to the array of iron SCO materials for potential incorporation into nanoscale magnetic switches and spintronic devices.