A theoretical comparative study of complexes of porphyrin (P), porphyrazine (Pz), and phthalocyanine (Pc) with metal (M) = Fe, Co, Ni, Cu, and Zn has been carried out using a DFT method. The calculations provide a clear elucidation of the ground states for the MP/Pz/Pc molecules and for a series of [MP/Pz/Pc](x-) and [MP/Pz/Pc](y+) ions (x = 1, 2, 3, 4; y = 1, 2). There are significant differences among MP, MPz, and MPc in the electronic structure and other calculated properties. For FeP/Pz and CoP/Pz, the first oxidation occurs at the central metal, while it is the macroring of FePc and CoPc that is the site of oxidation. The smaller coordination cavity results in a stronger ligand field in Pz than in P. However, the benzo annulation produces a surprisingly strong destabilizing effect on the metal-macrocycle bonding. The effects of Cl axial bonding upon the electronic structures of the iron(III) complexes of P, Pz, and Pc were examined, as was the bonding of pyridine (py) to NiP, NiPz, and NiPc. The porphinato core size plays a crucial role in controlling the spin state of Fe(III) in these complexes. FePc(Cl) is predicted to be a pure intermediate-spin system, whereas NiPz(py)(2) and NiPc(py)(2) are metastable in high-spin (S = 1) states. The NiPz/Pc-(py)(2) binding energy curve has only a shallow well that facilitates decomposition of the complex. The NiP-(py)(2) bond energy is small, but the relatively deep well in the binding energy curve ought to make this system stable to decomposition.

Comparative study of metal‐porphyrins, ‐porphyrazines, and ‐phthalocyanines

The electronic structures, spin-states, and geometrical parameters of tetra-, penta-, and hexa-coordinated iron-porphyrins are investigated applying density functional theory (DFT) based calculations, utilizing the plane-wave pseudopotential as well as localized basis set approaches. The splitting of the spin multiplet energies are investigated applying various functionals including recently developed hybrid meta-GGA (M06 family) functionals. Almost all of the hybrid functionals accurately reproduce the experimental ground state spins of the investigated Fe-porphyrins. However, the energetic ordering of the spin-states and the energies between them are still an issue. The widely used B3LYP provides consistent results for all chosen systems. The GGA+U functionals are found to be equally competent. After assessing the performance of various functionals in spin-state calculations, the potential energy surfaces of the oxygen binding process by heme is investigated. This reveals a "double spin-crossover" feature for the lowest energy reaction path that is consistent with previous CASPT2 calculations but predicting a lowest energy singlet state. The calculations have hence captured the spin-crossover as well as spin-flip processes. These are driven by the intra-atomic orbital polarization on the central metal atom due to the atomic and orbitals rearrangements. The nature of the chemical bonding and a molecular orbital analysis are also performed for the geometrically simple but electronic structurally complicated system tetra-coordinated planar Fe porphyrin in comparison to the penta-coordinated systems. This analysis explains the observed paradoxical appearance of certain peaks in the local density of states (DOS).

Electronic structure, spin-states, and spin-crossover reaction of heme-related Fe-porphyrins: a theoretical perspective.

Structure and magnetic properties of dinuclear [Mn(III)(salen)(H2O)]2(ClO4)2 and polynuclear [Mn(III)(salen)(NO3)]n

The behaviors of a large number of GGA, meta-GGA, and hybrid-GGA density functionals in describing the spin-state energetics of iron porphyrins and related compounds have been investigated. There is a large variation in performance between the various functionals for the calculations of the high-spin state relative energies. Most GGA and meta-GGA functionals are biased toward lower-spin states and so fail to give the correct ground state for the high-spin systems, for which the meta-GGA functionals show more or less improvement over the GGA ones. The GGA functionals that use the OPTX correction for exchange show remarkably high performance for calculating the high-spin state energetics, but their results for the intermediate-spin states are somewhat questionable. A heavily parameterized GGA functional, HCTH/407, provides results which are in qualitative agreement with the experimental findings for the iron porphyrins [FeP, FeP(Cl), FeP(THF)2], but its relative energies for the high-spin states are probably somewhat too low. The high-spin state relative energies are then even more underestimated by the corresponding meta-GGA functional tau-HCTH. For the hybrid-GGA functionals, the Hartree-Fock (HF)-type (or exact) exchange contribution strongly stabilizes the high-spin states, and so the performance of such functionals is largely dependent upon the amount of the HF exchange admixture in them. The B3LYP, B97, B97-1, and tau-HCTH-hyb functionals are able to provide a satisfactory description of the energetics of all the systems considered.

Assessment of the performance of density-functional methods for calculations on iron porphyrins and related compounds.

Two new iron(III) complexes with salicylaldimine ligands, [Fe(L1)Cl]·CH3CN (1) and [Fe(L2)Cl(H2O)]·1/3CH3Cl (2) [H2L1 = N,N′-(1,1-dimethylethylene)bis(salicylaldimine), H2L2 = N,N′-(1,1-dimethylethylene)bis(3-methoxysalicylaldimine)], have been prepared and characterized. The X-ray crystal structure analysis shows that the iron(III) ion in complex 1 is in a distorted square pyramidal environment, while in 2, the iron(III) center is six-coordinate with distorted octahedral geometry. The Schiff base ligands L1 and L2 occupied the basal sites, with chloride and water occupying the apical sites. Magnetic susceptibility and Mossbauer effect measurements showed a high-spin configuration (S = 5/2) for Fe(III) in 1 and 2. The temperature dependence of the magnetic data for complexes 1 and 2 is analyzed in terms of the spin-Hamiltonian formalism, which gives an accurate estimate of the zero-field splitting of the ground state of the iron(III) ions as D = 10.2 cm−1 (E = 3.4 cm−1) and D = 7.2 cm−1 (E = 2.4 cm−1, zJ′ = −0.5 cm−1), respectively, in the above two complexes. It has been found for the first time that complexes 1 and 2 activate the epoxidation of cis-stilbene by NaOCl. With 1, the epoxides were produced in 40% yield with cis∶trans = 20∶80, and with 2, the epoxides were produced in 90% yield with cis∶trans = 40∶60.

Structure, magnetic properties and epoxidation activity of iron(III) salicylaldimine complexes

Accurate and detailed measurements of average magnetic susceptibility (4–100 K) and magnetization (2–20 K and 10–50 kOe) are reported on a number of high spin iron (III) prophyrins, namely protoporphyrin‐, octaethylporphyrin‐, and deuteroporphyrin iron (III) chlorides. Adequate precautions were taken to ensure that the crystallites did not orient during the magnetization measurements, in high magnetic fields at low temperatures. The experimental magnetization data show complete saturation below 4 K at magnetic fields above 40 kOe and the saturation moment 〈μ〉 lies between 3.0–3.4 in these compounds, indicating large deviation from the expected value of 5.0, due to sizeable zero‐field splitting. The magnetization results at low temperatures show varying degrees of exchange interaction, which was considered within molecular field framework to quantitatively account for the data. A fit to the data gave reasonable values for the zero‐field splitting and exchange‐interaction parameters.

V. R. Marathe

Papers

Low temperature magnetization study on high spin iron (III) porphyrins