Control of Spin State by Ring Conformation of Iron(III) Porphyrins. A Novel Model for the Quantum-Mixed Intermediate Spin State of Ferric Cytochromec‘ from Photosynthetic Bacteria

Abstract

(2,3,7,8,12,13,17,18-Octaethyl-5,10,15,20-tetraphenylporphinato)iron(III) chloride FeIII(OETPP)Cl and (2,3,7,8,12,13,17,18-octamethyl-5,10,15,20-tetraphenylporphinato)iron(III) chloride FeIII(OMTPP)Cl complexes have been synthesized and characterized by 1H NMR and X-ray crystallography. Both molecules are severely nonplanar and assume saddle shapes in solid state. Variable-temperature 1H NMR studies confirm that the conformational distortions are maintained in solution with ΔG ⧧ = 15.8 and 10.1 kcal/mol for ring inversion for Fe(OETPP)Cl and Fe(OMTPP)Cl, respectively. EPR (g ⊥ = 5.2−5.3 at 77 K), magnetic moments (μeff = 4.7−5.2 μB at 300 K), and structural data (Fe−Np = 2.03 Å, Fe−Cl = 2.24−2.25 Å) all indicate that unlike high-spin FeIII(TPP)Cl and FeIII(OEP)Cl (S = 5/2), FeIII(OETPP)Cl and FeIII(OMTPP)Cl these complexes are of the uncommon quantum-mixed S = 5/2, 3/2 intermediate-spin state. Saddle-shaped ring deformations lower the symmetries of the complexes into C 2v . Other than the nonaxial symmetric EPR spectra of both complexes, 1H NMR spectrum of FeIII(OETPP)Cl shows large asymmetry to the methylene proton shifts. Certain cytochromes c‘ from photosynthetic bacteria reported to be of similar quantum-mixed intermediate-spin and showed EPR signals of rhombic symmetry have been noted to be with saddle-shaped deformations. These anomalous spin states and electronic structure asymmetry are ascribed to the ring deformation of the porphyrin macrocycle.

Ab initio multiconfiguration reference perturbation theory calculations on the energetics of low-energy spin states of iron(III) porphyrins

“…In view of the tendency of DFT to unduly favor spinpairing in transition metal ions as in the case of Fe III (P)Cl, we have attempted to theoretically model an [(N-carboxamido) 4 FeCl] -complex, a unique high-spin Fe(IV) complex that has been studied and structurally characterized by Collins, M and co-workers [28]. According to our recent work (Ghosh A, Taylor PR, unpublished results), PW91 predicts that the S=1 and S=2 states are approximately of the same energy, but B3LYP favors the S=2 state by 0.3 eV, comparable TZP-quality basis sets being used in both series of calculations.…”

“…Neglecting S=3/2, 5/2 spin admixture due to spinorbit coupling {this effect is important for Fe(OETPP)Cl (OETPP=b-octaethyl-meso-tetraphenylporphyrinato) [4]} [5,6], spectroscopic, magnetic susceptibility, and structural studies suggest that the iron(III) chloride derivatives of common porphyrins (e.g. octaethylporphyrin) exhibit a sextet ground state (see e.g.…”

Paramagnetic NMR Shifts for Saddle‐Shaped Five‐Coordinate Iron(III) Porphyrin Complexes with Intermediate‐Spin Structure

“…[1] It has also been conjectured that a saddle deformation of the macrocycle results in a symmetry reduction, that is, a C 2v local symmetry, to raise the d x 2 Ày 2 orbital energy by spatial mixing of the d x 2 Ày 2 and d z 2 orbitals as shown by early spin-restricted ZINDO calculations. [2] Nevertheless, the definite composition, which is a mixture of the S = 3/2 and S = 5/2 spin states, still remains debatable.…”

“…[2] Nevertheless, the definite composition, which is a mixture of the S = 3/2 and S = 5/2 spin states, still remains debatable. [1,[3][4][5] When the axial ligand is changed from the Cl À ion to a weaker ligand ClO 4 À , Fe(OETPP)ClO 4 recently has been characterized by a pure intermediate-spin state (S = 3/2) electronic structure. [4,6] Generally, the properties determined by Mçssbauer spectroscopy, EPR and magnetic susceptibility measurements in the series of Fe(OETPP)X (X = Cl, Br, I, ClO 4 ) complexes are similar to planar five-coordinate iron-(III) porphyrin complexes.…”

Models for Cytochromes c′: Observation of an Extremely Labile Spin State in Monoimidazole Complexes of Saddle‐Shaped Iron(III) Porphyrinates

“…Chemical shifts of the major signals in these complexes are listed in Table 2. Curie plots of the signals for 1 – 3 exhibited a good linearity, as shown in the Supporting Information, which suggests that these complexes maintain a single spin state in the temperature range 298–173 K. The negative δ ( meta )− δ ( para ) values, together with the negative Curie slopes of the meta signals, indicate that the meso carbon atoms have negative spin,7, 8 which in turn indicates that 1 – 3 adopt the S =3/2 state; the S =5/2 Fe III complexes have positive spin at the meso carbon atoms and thus give positive δ ( meta )− δ ( para ) values, as in the case of [Fe(oetpp)]Cl 6. 9 The solution magnetic moment of 2 determined by the Evans method was 4.2 μ B at 298 K and was invariant down to 173 K. This value is close to the spin‐only value expected for the S =3/2 state.…”