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′)2 n+ (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′)2 is 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′)2 demonstrate the endothermic LS to HS conversion and entropic preference of the HS state. Entropy changes for both Fe(Tp′)2 isomers are significantly larger than for the majority of iron scorpionate SCO systems. Solid-state magnetic and thermochemical measurements show cis-Fe(Tp′)2 to 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′)2 shows T c ≈ 270 K (solution) and T c ≈ 385 K (solid), with the remarkably wide ΔT c ≈ 115 K; trans-Fe(Tp′)2 shows T c ≈ 278 K (solution) and T c ≈ 372 K (solid). Solid-state T c values are among the highest seen for iron(II) molecular systems. The large solution/solid ΔT c difference 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′)2 crystallography 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′)2 is 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.

Utility of all-pyrazole heteroscorpionates in f-element chemistry

“…21–23 To overcome this, Vahrenkamp and coworkers, reacted potassium trispyrazolylborate (KTp) with thioimidazoles in a melt, to generate phase pure thiol-functionalized Tp − ligands. 19 Desrochers and coworkers expanded this work with benzotriazoles and Tp Me2− , 11,16 and Warren et al , contributed with the substitution of 3′-pyridyl,5-methyl-pyrazole and Tp i Pr2− . 20 Our group is also interested in improving upon the desirable properties of homologous Tp − ligands such as Tp − , Tp Me2 , Tp i Pr2− etc.…”

“…[8][9][10] Tp − , the first example of a scorpionate, is a homoscorpionate: all of the donor sites are equivalent ( pyrazole N); on the other hand, heteroscorpionates are an evolution of scorpionates, defined as anionic, 6e − donor, face-capping ligand where not all three donor sites are identical. 11 Most research efforts involving scorpionates focus on generating different types of homologous scorpionate ligands (and derivative metal-scorpionate complexes) including: trisimidazolylborate, 12 trisbenzotriazolylborate, 13 trismercaptoarylborates and phenyltris(2-pyridyl) borates, 14 and tris(2-pyridyl)amines/phosphines. 15 We recommend the following reviews by Marques and coworkers for f-element scorpionate materials 3,4 and Smith, 2 and Pawar et al, 14 for the versatility of scorpionate ligands in coordination chemistry.…”

“…There have been several groups interested in developing chemistry to generate heteroscorpionates via selective functionalization of Tp − . 11,[16][17][18][19][20][21][22][23] The most effective method (Fig. 1) involves the substitution of a normal pyrazole for another N-or S-donor aromatic ring.…”

“…1) involves the substitution of a normal pyrazole for another N-or S-donor aromatic ring. 11 Initial attempts at generating functionalized Tp − involved reacting different types of aromatic amines with pyrazoles and MBH 4 (M = alkali metal) in a melt, however, this led to uncontrolled substitutions and mixed phases. [21][22][23] To overcome this, Vahrenkamp and coworkers, reacted potassium trispyrazolylborate (KTp) with thioimidazoles in a melt, to generate phase pure thiol-functionalized Tp − ligands.…”

“…[21][22][23] To overcome this, Vahrenkamp and coworkers, reacted potassium trispyrazolylborate (KTp) with thioimidazoles in a melt, to generate phase pure thiol-functionalized Tp − ligands. 19 Desrochers and coworkers expanded this work with benzotriazoles and Tp Me2− , 11,16 and Warren et al, contributed with the substitution of 3′-pyridyl,5-methylpyrazole and Tp iPr2− . 20 Our group is also interested in improving upon the desirable properties of homologous Tp − ligands such as Tp − , Tp Me2 , Tp iPr2− etc., by generating all-pyrazole heteroscorpionates, with 2 identical binding sites via pyrazole nitrogens (claws) and 1 unique binding site via an n-nitropyrazole (stinger, n = 3 or 4), corresponding to a selective single substitution of one of nine sites available for the nitro group (Fig.…”