In this study, in order to explain the solvent and spin state effects on the molecular structure of catechol-Fe complex [Fe(cat) 3] n﹣ where n = 2 and 3, Hartree Fock (HF)-Density Functional Theory (DFT) hybrid calculations are performed at the B3LYP/6-311g(d,p) level of theory. The binding energies of Fe 2+ and Fe 3+ in high-spin state are higher than intermediate and low-spin states which show that the complex formation in a high spin state is more favorable. The calculated binding energies at different solvents indicate that the binding energies in polar solvents are lower than non-polar solvents. Furthermore, spectroscopic studies including FTIR and Raman spectrum in various solvents reveal that the formation of intermolecular bonds between the oxygen atom of carbonyl group and the hydrogen atom of solvent causes a spectral red shift. The calculated FTIR and geometry parameters are in good agreement with previous experimental data. Donor-acceptor interaction energies are evaluated due to the importance of the charge transfer in the complex formation. It is observed that the free electrons of oxygen atom interact with the antibonding orbitals of the iron. Finally, some correlations between the quantum chemical reactivity indices of the complexes and solvent polarity are considered. The study indicates a linear correlation between chemical hardness and binding energies of [Fe(cat) 3] 3﹣ complex.
Iron is a pivotal nutrient for life which is one of the most abundant elements on the earth. It plays an important role in biological processes such as oxygen transport, energy generation, electron transfer and DNA synthesis [
Catechol, which is a phenolic compound known as pyrocatechol or 1,2-dihy- droxybenzene with the molecular formula C6H4(OH)2, acts as iron chelating agent [
Recent theoretical study based on HF/DFT hybrid functional has considered the ferric complexes of catecholic ligands which are close to the complexes of the present study [
In the present study, we have performed a theoretical investigation on the structure, binding energies, stability and spectroscopic properties of the [Fe(cat)3]n− complexes (for Fe(II) and Fe(III) oxidation states) in the gas phase and different solvents using the first principles HF/DFT hybrid approach. The thermodynamic stability of all the possible spin states for both oxidation states is verified. The natural bond orbital (NBO) analysis [
The model compounds were subjected to geometry optimizations followed by computations of time dependent density functional theory (TD-DFT) electronic absorption spectra, IR spectra, and pre-resonance Raman spectra. The geome- tries of the complexes were optimized at the HF/DFT hybrid B3LYP [
In order to estimate the zero-point vibrational energies (ZPVEs), frequency calculations were performed for all structures in the different spin states. All minimum structures were verified with the real frequencies. Geometry optimization was taken to be converged if the maximum atomic force was smaller than 0.00045 Hartree/Bohr. No symmetry was imposed in all the calculations. The raw vibrational frequencies were scaled by a factor of 0.9668, which produced good agreement with the experiment for a wide range of systems [
Solvent effects were taken into account by the conductor like polarizable continuum model (CPCM) [
After the full optimization, the total energy of the considered spin states for both Fe2+ and Fe3+ oxidation states for [Fe(cat)3]n− complexes are calculated and the relative energy values for all the states are compared in
The metal-ligand binding energies were computed according to Equation (1) [
Here Ecomplex, Emetal and Eligand is the energy of [Fe(cat)3]n− complex, metal and ligand respectively. The calculated binding energy values are compared in
Phase | Spin State | Relative Energy | Binding Energy | Enthalpy |
---|---|---|---|---|
Fe2+ | LS | 12.15 | 203.53 | 206.73 |
IS | 9.29 | 206.40 | 210.50 | |
HS | 0 | 215.69 | 219.74 | |
Fe3+ | LS | 7.40 | 511.75 | 515.24 |
IS | 6.57 | 512.59 | 516.94 | |
HS | 0 | 519.15 | 523.47 |
is observed that Fe3+ complexes are more stable than Fe2+ complexes. In both cases, the values of the binding energy of the high-spin and intermediate-spin state complexes are more negative than the low-spin state complex. In case of Fe2+, the low-spin and intermediate spin complexes are less stable than the high-spin state by approximately 12.15 and 9.29 kcal∙mol−1 respectively. In case of Fe3+, the low-spin and intermediate spin complexes are less stable than the high-spin state by approximately 7.4 and 6.57 kcal∙mol−1 respectively. Compar- ing all the calculated binding energy values, it is concluded that Fe3+ at high-spin state is the most preferable to form the iron-catecolate complex. According to Boys-Bernardi counterpoise (CP) correction method [
state only. The BSSE-corrected energy
The effects of temperature and contributions from zero point energy on the calculated formation energies of the [Fe(cat)3]n− are explicitly taken into account by additional frequency calculations (at 298.15 K and 1atm pressure). The enthalpy of formation is calculated according to the Equation (1). Based on the calculated enthalpy, it is observed that the correction terms range upto 4.5 kcal∙mol−1 which is in the acceptable range as observed in previous theoretical studies [
The geometrical parameters at different spin states for Fe3+ complexes in gas phase are given in
Spin state | |||||
---|---|---|---|---|---|
LS | 1.96 (±0.01) | 1.31 (±0.00) | 90.08 (±3.91) | 125.05 (±0.09) | 111.64 (±0.42) |
IS | 2.02 (±0.09) | 1.30 (±0.01) | 90.17 (±5.72) | 125.04 (±0.65) | 113.07 (±2.37) |
HS | a,b2.06 (±0.00) | 1.30 (±0.00) | a,b90.34 (±7.22) | 125.34 (±0.00) | 114.44 (±0.00) |
aX-ray analysis on a tris-catecholato Fe(III) crystal gave
their average values of approximately 90˚. The present
In order to study the solvent effects on the iron binding with catechol, we have studied [Fe(cat)3]3− complex at the HS state in various solvents with the increasing dielectric constant including chloroform, acetone, ethanol, methanol, acetonitrile, DMSO and water (as shown in
Media | ε (dielectric constant) | Ebind (kcal∙mol−1) | Carbonyl stretching frequency (cm−1) | Carbon-Carbon stretching frequency (cm−1) |
---|---|---|---|---|
Gas | - | −519.15 | 1470.72 | 1260.25 |
Chloroform | 4.81 | −200.50 | 1454.45 | 1235.16 |
Acetone | 20.7 | −134.17 | 1450.11 | 1228.03 |
Ethanol | 24.5 | −130.70 | 1449.93 | 1227.67 |
Methanol | 32.5 | −126.82 | 1449.76 | 1227.24 |
Acetonitrile | 37.5 | −125.75 | 1449.55 | 1226.93 |
DMSO | 46.8 | −123.06 | 1449.40 | 1226.08 |
Water | 80 | −119.58 | 1449.27 | 1225.99 |
agreement with a recent theoretical investigation [
The IR spectra of [Fe(cat)3]3− complex in gas phase as well as solvents are calculated. The C-O and C-C vibrational modes are given in
and 1449 cm−1 at those solvents. Each of these two major peaks have originated from the in-plane deformation mode involving the C-C and C-O stretching and the C-H bending of catecholate. These two major IR peaks are close in frequency to those previously observed for mefp-1 adsorbed on an iron substrate at 1258 and 1485 cm−1 [
In order to study the effect of solvents on the preresonance Raman intensities, a wide variety of solvents with increasing dielectric constants were used. Using the optimized geometry for every solvent, the excitation wavelength was calculated with TD-DFT method. Using those wavelengths, we calculated preresonance raman with Raman Optical Activity (ROA).
solvent | Preresonance raman (theoretical) | Exp. Raman |
---|---|---|
Gas | 503, 600, 608, 759, 1098, 1278, 1312, 1485, 1519 | |
Chloroform | 511, 618, 1118, 1232, 1315, 1472, 1537 | |
Acetone | 511, 619, 769, 1073, 1120, 1222, 1312, 1465, 1543 | |
Ethanol | 511, 620, 769, 1073, 1120, 1222, 1311, 1465, 1542 | |
Methanol | 511, 620, 769, 1073, 1120, 1221, 1311, 1465, 1543 | |
Acetonitrile | 511, 620, 769, 1073, 1120, 1221, 1311, 1465, 1543 | |
Dmso | 511, 620, 769, 1120, 1220, 1310, 1464, 1543 | |
Water | 511, 620, 769, 1073, 1120, 1220, 1311, 1464, 1543 | a529, 584, 634, 1270, 1324, 1422, 1484 b533, 621, 800, 1154, 1258, 1320, 1487, 1572 c531, 591, 638, 815, 1152, 1274, 1326, 1426, 1491, 1571 d550, 596, 637, 1270, 1322, 1423, 1476 |
aSynthetic tris(DOPA-PEG) Fe(III) complex formed at pH ≈ 12 [
polarity has increased. The present calculation agrees reasonably with the previous Raman experiments on the synthetic and natural cross-linked MAPs, where the low-frequency (512 - 614 cm−1) modes arose from the chelation of Fe by catecholate [
was located at 521 nm, agreeing with the experimental wavelength (492 nm) of the maximum peak found for the tris(DOPA-PEG) Fe(III) [
To characterize the electronic transitions, five frontier MOs were pictorized ranging from the highest occupied MO-2(HOMO-2) to the lowest unoccupied MO + 1(LUMO + 1) (
In the natural bond orbital (NBO) analysis, the electronic wave function is elucidated in terms of occupied Lewis and unoccupied Lewis localized orbitals. The strength of donor-acceptor interactions, E(2), are evaluated by second-order perturbation theory [
Donor-acceptor interaction | Gas phase | Chloroform | Acetone | Ethanol | Methanol | Acetonitrile | DMSO | Water |
---|---|---|---|---|---|---|---|---|
LPO8→LP*Fe | 12.21 | 12.85 | 13.01 | 13.02 | 13.03 | 13.04 | 13.06 | 13.10 |
LPO9→LP*Fe | 12.21 | 12.84 | 12.96 | 12.99 | 13.01 | 13.01 | 13.05 | 13.06 |
LPO19→LP*Fe | 12.21 | 12.84 | 13.01 | 13.02 | 13.03 | 13.03 | 13.02 | 13.06 |
LPO20→LP*Fe | 12.21 | 12.85 | 12.96 | 12.99 | 13.02 | 13.03 | 13.05 | 13.06 |
LPO30→LP*Fe | 12.21 | 12.86 | 12.95 | 12.99 | 13.01 | 13.02 | 13.03 | 13.03 |
LPO31→LP*Fe | 12.21 | 12.84 | 12.96 | 12.99 | 12.99 | 13.01 | 13.02 | 13.08 |
The HOMO-LUMO energy gap (Eg) is an important factor for the evaluation of polarizability of a molecule [
Quantum reactivity indices such as the HOMO-LUMO gap (Eg), chemical hardness (η) and electronic chemical potential (μ) are reported in
Media | Catecholate | [Fe(Cat)3]3− | ||
---|---|---|---|---|
Eg (eV) | Eg(eV) | η(eV) | −μ(eV) | |
Gas | 3.167 | 2.762 | 1.381 | 6.336 |
Chloroform | 4.370 | 2.816 | 1.408 | 0.589 |
Acetone | 4.669 | 2.832 | 1.416 | 2.113 |
Ethanol | 4.686 | 2.835 | 1.418 | 2.180 |
Methanol | 4.702 | 2.837 | 1.419 | 2.271 |
Acetonitrile | 4.710 | 2.843 | 1.422 | 2.293 |
DMSO | 4.713 | 2.846 | 1.423 | 2.354 |
Water | 4.710 | 2.851 | 1.426 | 2.433 |
The effect of oxidation states and spin states of iron for the binding in [Fe(cat)3]n− complexes are studied theoretically using quantum chemical approach based on the B3LYP/6-311G (d,p) level of the theory. Our study shows that Fe3+ in high spin state is the most preferable oxidation state to form the iron-catechol complex. The effects of various solvents with increasing dielectric constants on the structure, binding energy, FT-IR, preresonance Raman and UV-vis spectra of the [Fe(cat)3]3− complex were investigated. The binding energies in polar solvents are lower than those in non-polar ones.
The simulated FT-IR showed that both the C-O and C-C vibrational modes have shifted to a lower frequency (red shift) when the solvent polarity increases. The preresonance Raman spectra of [Fe(cat)3]3− clearly indicated that, in presence of solvents, the Raman peaks shifted (blue shifts). The simulated UV-vis spectra of the [Fe(cat)3]3− complex were also consistent with the previous measurements and calculations. Furthermore, through the NBO analysis, the metal-ligand interactions were studied and verified that charge transfer occurs from the oxygen atoms of the ligand to the Fe (III). Also, based on this analysis it is concluded that the charge transfer in the gas phase is higher than in solution.
Finally, according to quantum chemical reactivity indices, the correlations between the electronic potential and binding energy of the [Fe(cat)3]3− complex as well as the solvent dielectric constant were obtained, respectively. The present metal−ligand binding energies, structures, and atomic charges of the metal-ca- techolate complexes will serve as a keystone for such modeling using molecular dynamics or Monte Carlo simulations.
The Regional Scientific Computing Center for Lower Saxony (RRZN) of the University of Hannover is acknowledged for high performance computing facilities to perform this research activity.
Matin, M.A., Islam, M.M., Bredow, T. and Aziz, M.A. (2017) The Effects of Oxidation States, Spin States and Solvents on Molecular Structure, Stability and Spectroscopic Properties of Fe-Catechol Complexes: A Theoretical Stu- dy. Advances in Chemical Engineering and Science, 7, 137-153. https://doi.org/10.4236/aces.2017.72011