Quantitative Account of the Bonding Properties of a Rubredoxin Model Complex [Fe(SCH₃)₄]^q, q = −2, −1, +2, +3

Abstract

Iron–sulfur clusters play important roles in biology as parts of electron-transfer chains and catalytic cofactors. Here, we report a detailed computational analysis of a structural model of the simplest natural iron–sulfur cluster of rubredoxin and its cationic counterparts. Specifically, we investigated adiabatic reduction energies, dissociation energies, and bonding properties of the low-lying electronic states of the complexes [Fe­(SCH3)4]2–/1–/2+/3+ using multireference (CASSCF, MRCISD), and coupled cluster [CCSD­(T)] methodologies. We show that the nature of the Fe–S chemical bond and the magnitude of the ionization potentials in the anionic and cationic [Fe­(SCH3)4] complexes offer a physical rationale for the relative stabilization, structure, and speciation of these complexes. Anionic and cationic complexes present different types of chemical bonds: prevalently ionic in [Fe­(SCH3)4]2–/1– complexes and covalent in [Fe­(SCH3)4]2+/3+ complexes. The ionic bonds result in an energy gain for the transition [Fe­(SCH3)4]2– → [Fe­(SCH3)4]− (i.e., FeII → FeIII) of 1.5 eV, while the covalent bonds result in an energy loss for the transition [Fe­(SCH3)4]2+ → [Fe­(SCH3)4]3+ of 16.6 eV, almost half of the ionization potential of Fe2+. The ionic versus covalent bond character influences the Fe–S bond strength and length, that is, ionic Fe–S bonds are longer than covalent ones by about 0.2 Å (for FeII) and 0.04 Å (for FeII). Finally, the average Fe–S heterolytic bond strength is 6.7 eV (FeII) and 14.6 eV (FeIII) at the RCCSD­(T) level of theory.

3-Input AND Molecular Logic Gate with Enhanced Fluorescence Output: The Key Atom for the Accurate Prediction of the Spectra

“…The Fe 2+ complexes are more stable than the Fe 3+ by about 5.1 eV in all complexes (Table ), either including explicitly a water molecule or not. It should be noted that our methodology predicts very well the atomic Fe 2+ → Fe 3+ oxidation in excellent agreement with the RCCSD­(T)/AVTZ value of 30.10 eV and the experimental value of 30.651 eV . It has been reported that the complexation stabilizes the Fe 3+ complexes significantly.…”

“…or not. It should be noted that our methodology predicts very well the atomic Fe 2+ → Fe 3+ oxidation in excellent agreement with the RCCSD(T)/AVTZ value of 30.10 eV31 and the experimental value of 30.651 eV 32. It has been reported that the complexation stabilizes the Fe 3+ complexes significantly.…”

“…Oxidation of Iron Cation: Fe 2+ → Fe 3+ . The Fe 2+ complexes are more stable than the Fe 3+ by about 5.1 eV in all complexes (Table 6), either including explicitly a water 31 Here, the oxidation energy of 5.1 eV (= 243 nm) corresponds to the excitations at the UV area of the absorption spectra with high oscillator strength, showing that the energy excitation at about the 35th singlet excited state and the oxidation of Fe 2+ are two contradictory processes. The main peaks in this case are found in 4.86 eV (1), 5.05 eV (1-H + ), 5.14 eV (1-Na + ), and 4.79 eV (1-H + -Na + ), see Table 6.…”

“…Moreover, the ionic bonds between iron and ligands result in larger stabilization than the covalent bonds. Furthermore, in some cases, for instance, in [Fe­(SCH 3 ) 4 ] 2– / – complexes, Fe 3+ can form more stable complexes than Fe 2+ . Here, the oxidation energy of 5.1 eV (= 243 nm) corresponds to the excitations at the UV area of the absorption spectra with high oscillator strength, showing that the energy excitation at about the 35th singlet excited state and the oxidation of Fe 2+ are two contradictory processes.…”

Molybdenum–Sulfur Bond: Electronic Structure of Low-Lying States of MoS

“…70 Thus, while the Mo + ( 6 S) + S − ( 2 P) asymptote can yield a 5 Π state, that is, the same as the ground state of MoS, these asymptotic products are lying 5.015 eV higher than Mo( 7 S) + S( 3 P). Of course, in complexes included Mo−S bonds, as in the case of the Fe−S bonds, 71 this energy difference between ionic and covalent bonds can be provided by the interactions with solvents, other ions, ligands, and so forth. Thus, the data of the MoS diatomic molecule could be useful in understanding solid MoS 2 and molybdenum complexes.…”