Theoretical Study of Spin Crossover in 30 Iron Complexes

Area of Science:

• Inorganic Chemistry
• Computational Chemistry
• Materials Science

Background:

• Iron complexes are vital for oxidative metabolism and have potential in technological applications.
• Spin crossover (SCO) behavior in these complexes is governed by the balance between high- and low-spin states.
• This balance is influenced by physical factors including dispersion, relativistic effects, and vibrational entropy.

Purpose of the Study:

• To investigate the entropy-enthalpy compensation in 30 iron SCO systems.
• To evaluate the performance of 12 density functionals in modeling SCO phenomena.
• To assess the impact of vibrational entropy, zero-point energies, dispersion, and relativistic effects on SCO.

Main Methods:

• Computational study of 30 iron SCO systems with known thermochemical data.
• Application of 12 different density functionals.
• Calculation of first-coordination sphere entropy from numerical frequencies.
• Inclusion of zero-point energies, dispersion, and relativistic effects in calculations.

Main Results:

• A general entropy-enthalpy compensation was identified across SCO systems (R = 0.82, p = 0.002).
• Computed first-coordination sphere entropy values closely reproduced experimental data.
• Dispersion and relativistic effects significantly favor the low-spin state, with magnitudes up to 33 kJ/mol and 24 kJ/mol, respectively.
• B3LYP* functional demonstrated accuracy when dispersion and relativistic effects were included.

Conclusions:

• Entropy-enthalpy compensation is a key factor in rational SCO design.
• Vibrational entropy from the first coordination sphere should be incorporated into SCO modeling.
• Accurate SCO modeling requires the inclusion of dispersion and relativistic effects, with B3LYP* being a suitable functional.