Spin coupling in Roussin's red and black salts

Area of Science:

• Inorganic Chemistry
• Computational Chemistry
• Materials Science

Background:

• Density functional theory (DFT) has described the electronic structure of Roussin's red and black salts.
• Detailed studies on spin coupling in these nitrosylated iron-sulfur clusters were previously lacking.
• Understanding spin coupling is crucial for predicting magnetic properties and designing novel materials.

Purpose of the Study:

• To perform a detailed analysis of spin coupling in Roussin's red and black salts for the first time.
• To evaluate Heisenberg coupling constants (J) using broken-symmetry DFT calculations.
• To elucidate the factors contributing to the observed spin coupling behavior in nitrosylated iron-sulfur clusters.

Main Methods:

• Broken-symmetry density functional theory (DFT) calculations, primarily using the OLYP/STO-TZP level of theory.
• Application of both the Noodleman and Yamaguchi formulas for the evaluation of Heisenberg coupling constants (J).
• Investigation of three specific nitrosylated binuclear clusters: [Fe(2)(NO)(2)(Et-HPTB)(O(2)CPh)](2+), [Fe(NO)(2){Fe(NO)(NS(3))}-S,S'], and Roussin's red salt anion [Fe(2)(NO)(4)(micro-S)(2)](2-).
• Analysis of the spin coupling in Roussin's black salt anion, [Fe(4)(NO)(7)(micro(3)-S)(3)](-).

Main Results:

• Heisenberg coupling constants (J) for clusters 2 and 3 were found to be significantly high (≈10^3 cm(-1)), an order of magnitude greater than for cluster 1 (≈10^2 cm(-1)).
• In Roussin's black salt anion (4), the apical-basal coupling constant (J(12)) was approximately 40 times larger (≈4000 cm(-1)) than the basal-basal coupling (J(22) ≈10^2 cm(-1)).
• These J values are exceptionally high compared to non-nitrosylated iron-sulfur clusters.
• Analysis of spin-dependent bonding energies indicated that lower individual Fe(NO)(x) site spins in nitrosylated systems lead to smaller denominators in J calculation formulas, resulting in high J values.

Conclusions:

• Nitrosylated iron-sulfur clusters, including Roussin's salts, exhibit exceptionally high spin coupling constants (J).
• The high J values are attributed to reduced individual iron site spins within the Fe(NO)(x) units.
• This detailed spin coupling analysis provides fundamental insights into the magnetic properties of these important inorganic compounds.