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

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

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).

ASSESSMENT OF SOME RECENTLY DEVELOPED DENSITY FUNCTIONALS FOR CALCULATIONS ON IRON PORPHYRINS

The behaviors of several recently developed density functionals (M05, M05-2x, M06, M06-2x, M06-L, KT1, KT2) in describing the spin-state energetics of iron porphyrins [ FeP , FeP ( Cl ), FeP ( THF )2, FeTpivPP ] have been investigated, where KT1 and KT2 are pure GGA (generalized gradient approximation) functionals; M06-L is a meta-GGA functional that contains the kinetic-energy density τ[=∑(∇ϕi)2]; M05, M05-2x, M06, and M06-2x are hybrid meta-GGA functionals that include both the kinetic-energy density and Hartree–Fock exchange. The results reveal that KT1 and KT2 are biased toward lower-spin states and so fail to give the correct ground state for the high-spin systems, but KT2 shows a significant improvement over KT1. M05, M05-2x, M06, and M06-2x yield results which are in qualitative agreement with the experimental findings for FeP ( Cl ) and FeP ( THF )2, but their relative energies for the high-spin states are actually underestimated greatly. Among the various functionals tested here, only M06-L is able to provide a satisfactory description of the energetics of FeP , FeP ( Cl ), and FeP ( THF )2, for which the ground-state multiplicities are surely known. FeTpivPP is predicted to be high spin by the M06-L functional, in agreement with the early experimental magnetic measurement, but different from the previous results (and conclusion) obtained with other various functionals.

Spectroscopic identification of reactive porphyrin motions

Nuclear resonance vibrational spectroscopy (NRVS) reveals the vibrational dynamics of a Mössbauer probe nucleus. Here, (57)Fe NRVS measurements yield the complete spectrum of Fe vibrations in halide complexes of iron porphyrins. Iron porphine serves as a useful symmetric model for the more complex spectrum of asymmetric heme molecules that contribute to numerous essential biological processes. Quantitative comparison with the vibrational density of states (VDOS) predicted for the Fe atom by density functional theory calculations unambiguously identifies the correct sextet ground state in each case. These experimentally authenticated calculations then provide detailed normal mode descriptions for each observed vibration. All Fe-ligand vibrations are clearly identified despite the high symmetry of the Fe environment. Low frequency molecular distortions and acoustic lattice modes also contribute to the experimental signal. Correlation matrices compare vibrations between different molecules and yield a detailed picture of how heme vibrations evolve in response to (a) halide binding and (b) asymmetric placement of porphyrin side chains. The side chains strongly influence the energetics of heme doming motions that control Fe reactivity, which are easily observed in the experimental signal.

Inter-Ring Interactions in [Fe(TalkylP)(Cl)] (alkyl = ethyl, n-propyl, n-hexyl) Complexes: Control by meso-Substituted Groups

Syntheses, molecular structures and magnetic susceptibilities of three meso-substituted high-spin iron(III) porphyrinate complexes ([Fe(TEtP)(Cl)], [Fe(TPrP)(Cl)], and [Fe(THexP)(Cl)]) are described. It was determined that the inter-ring interactions within each dimeric unit change upon alteration of the alkyl groups at the meso-positions. Magnetic exchange couplings between iron centers of the dimers are in accord with the trends in structural inter-ring geometries. Crystal data for [Fe(TEtP)(Cl)]: a = 10.1710(5) Å, b = 11.309(3) Å, c = 12.170(3) Å, α = 91.774(9) °, β = 113.170(14) °, γ = 112.149(9) °, V = 1165.2(4) Å(3), triclinic, P1̄, Z = 2, R(1) = 0.0844 and ωR(2) = 0.2073 for observed data. Crystal data for [Fe([Fe(TPrP)(Cl)])(Cl)]: a = 13.040(2) Å, b = 15.221(2) Å, c = 14.6681(9) Å, β = 109.997(11) °, V = 2735.9(7) Å(3), monoclinic, P2(1)/n, Z = 4, R(1) = 0.0477 and ωR(2) = 0.1176 for observed data. Crystal data for [Fe(THexP)(Cl)]: a = 10.246(7) Å, b = 12.834(4) Å, c = 17.420(15) Å, α = 69.74(3) °, β = 87.52(4) °, γ, = 84.89(3) °, V = 2140(2) Å(3), triclinic, P1̄, Z = 2, R(1) = 0.1024 and ωR(2) = 0.2659 for observed data.

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

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.

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

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.