Debangsu Sil

Effect of Heme–Heme Interactions and Modulation of Metal Spins by Counter Anions in a Series of Diiron(III)-μ-hydroxo Bisporphyrins: Unusual Stabilization of Two Different Spins in a Single Molecular Framework

A new family of five ethene-bridged diiron(III)-μ-hydroxo bisporphyrins with the same core structure but different counter anions, represented by the general formula [Fe2 (bisporphyrin)]OH·X (X=counter anion), is reported herein. In these complexes, two different spin states of Fe are stabilized in a single molecular framework. Protonation of the oxo-bridged dimer 1 by strong Brønsted acids such as HI, HBF4, HPF6, HSbF6 , and HClO4 produces the μ-hydroxo complexes with I5(-)(2), BF4(-)(3), PF6(-)(4), SbF6(-)(5), and ClO4(-)(6) as counter anions, respectively. The X-ray structures of 2 and 6 have been determined, which provide a rare opportunity to investigate structural changes upon protonation. Spectroscopic characterization has revealed that the two iron(III) centers in 2 are nonequivalent with nearly high and admixed-intermediate spins in both the solid state and solution. Moreover, the two different Fe(III) centers of 3-5 are best described as having admixed-high and admixed-intermediate spins with variable contributions of S=5/2 and 3/2 for each state in the solid, but two different admixed-intermediate spins in solution. In contrast, the two Fe(III) centers in 6 are equivalent and are assigned as having high and intermediate spin states in the solid and solution, respectively. The X-ray structures reveal that the Fe-O bond length increases on going from the μ-oxo to the μ-hydroxo complexes, and the Fe-O(H)-Fe unit becomes more bent, with the dihedral angle decreasing from 150.9(2)° in 1 to 142.3(3)° and 143.85(2)° in 2 and 6, respectively. Variable-temperature magnetic data have been subjected to a least-squares fitting using the expressions derived from the spin Hamiltonians H=-2JS1·S2 -μ·B+D[S(2)(z) - 1/3S(S + 1)] (for 2, 3, 4, and 5) and H=-2JS1·S2 (for 6). The results show that strong antiferromagnetic coupling between the two Fe(III) centers in 1 is attenuated to nearly zero (-2.4 cm(-1)) in 2, whereas the values are -46, -32.6, -33.5, and -34 cm(-1) for 3, 4, 5, and 6, respectively.

Spin-State Ordering in Hydroxo-Bridged Diiron(III)bisporphyrin Complexes

We report the synthesis, structure, and spectroscopic characterization of 1,2-bis[μ-hydroxo iron(III) 5-(2,3,7,8,12,13,17,18-octaethylporphyrinyl)]ethane with PF6(–) and SbF6(–) counteranions. The two iron centers are nonequivalent with admixed intermediate spin state (S = 3/2 with a minor contribution of S = 5/2) on each metal both in the solid and in solution. The molecules are compared with previously known μ-hydroxo complexes with other counterions, such as I3(–), BF4(–), and ClO4(–), which demonstrates that the nature of the counterion can affect the spin-state ordering dramatically. To understand how the spin-state ordering is affected by external perturbations, we also have done a comprehensive computational study. The calculations show that subtle environmental perturbations affect the spin-state ordering and relative energies and are likely to be the root cause of the variation in spin-state ordering observed experimentally.

Axial Thiophenolate Coordination on Diiron(III)bisporphyrin: Influence of Heme–Heme Interactions on Structure, Function and Electrochemical Properties of the Individual Heme Center

The binding of a series of substituted thiophenols as axial ligands on a highly flexible ethane-bridged diiron(III)bisporphyrin framework has been investigated as a model of diheme proteins. Spectroscopic characterization reveals a high-spin (S = 5/2) state of iron for all of the pentacoordinate thiophenolato complexes. In the UV-visible spectra of the complexes, the positions of the Soret and band I have been found to be dependent on the pKa of thiophenols. The alternating shift pattern, which has opposite sign of the chemical shifts for meta- vs. ortho- and para-protons in the (1)H NMR spectra, is attributed to negative and positive spin densities, respectively, on thiophenolate carbon atoms and is indicative of π-spin delocalization to the bound thiophenolate ligand. The Fe(III)/Fe(II) redox couple of the complexes bears a linear relationship with the pKa of thiophenol and is found to be positively shifted with decreasing pKa. The effect of the electronic nature of the substituent on the thiophenolate ring has also been demonstrated in which a large potential range of 540 mV was observed (in contrast to the value of only 270 mV in case of monoheme analogues) for the Fe(III)/Fe(II) redox couple on going from monoheme to diheme and is attributed to the interheme interaction. Also, the Fe(III)/Fe(II) redox potential of the thiophenolato complexes has been found to be more positively shifted compared to their phenolato analogues, which was further supported by DFT calculation. The addition of another thiophenol at the sixth axial position of the five-coordinate thiophenolato complex causes a change in iron spin from high (S = 5/2) to low (S = 1/2) along with a large positive shift of 490 mV for the Fe(III)/Fe(II) redox couple.

Switching orientation of two axial imidazole ligands between parallel and perpendicular in low-spin Fe(III) and Fe(II) nonplanar porphyrinates.

We have reported here the synthesis, structure, and properties of low-spin bis-imidazole-coordinated Fe(III) and Fe(II) complexes of 5,10,15,20-tetrakis(pentafluorophenyl)-2,3,7,8,12,13,17,18-octachloroporphyrin, [Fe(III)(TFPPCl(8))(L)(2)]ClO(4) and Fe(II)(TFPPCl(8))(L)(2) (L = 1-methylimidazole, 4-methylimidazole, imidazole). The X-ray structure of Fe(II)(TFPPCl(8))(1-MeIm)(2) is reported here, which demonstrated the near-perpendicular axial ligand orientation (dihedral angle between two 1-methylimidazoles is 80.7°) for Fe(II) porphyrins in a highly saddle-distorted macrocyclic environment. Oxidation of Fe(II)(TFPPCl(8))(L)(2) using thianthrenium perchlorate produces [Fe(III)(TFPPCl(8))(L)(2)]ClO(4), which was also isolated in the solid state and characterized spectroscopically. The complex gives rhombic EPR spectra in both solid and solution phases at 77 K and thus represents a rare example of nearly parallel axial ligand orientations for the unhindered imidazoles in a saddle-distorted porphyrin macrocycle. Geometry optimization using DFT also converged to the parallel axial alignment when 1-methylimidazole was used as the axial ligand (the dihedral angle between two axial ligands is 8.6°). The potential energy surface (PES) scan results also show that the relatively parallel axial orientations are energetically preferred for Fe(III), while perpendicular orientations are preferred for the Fe(II) complexes reported here. Bulk oxidation of Fe(II)(TFPPCl(8))(L)(2) in dichloromethane at a constant potential under nitrogen converts it to [Fe(III)(TFPPCl(8))(L)(2)]ClO(4), which gives identical EPR spectra at 77 K and which upon reduction regenerates Fe(II)(TFPPCl(8))(L)(2) again. Thus, we have demonstrated here very rare examples of Fe porphyrins in which the relative axial imidazole orientations switch between parallel and perpendicular just upon changing the oxidation states of iron from +3 to +2, respectively, in a nonplanar porphyrinic environment. These observations could be immensely important for understanding the possible effects of axial histidine orientations on similar macrocyclic deformations observed in various heme proteins.

A Highly Oxidized Cobalt Porphyrin Dimer: Spin Coupling and Stabilization of the Four‐Electron Oxidation Product

A highly oxidized cobalt porphyrin dimer is reported. Each cobalt(II) ion and porphyrin ring underwent 1e oxidation with iodine as the oxidant to give a 4e-oxidized cobalt(III) porphyrin π-cation radical dimer. The bridging ethylene group allows for substantial conjugation of the porphyrin macrocycles, thus leading to a strong antiferromagnetic coupling between the π-cation radicals and to stabilization of the singlet state. X-ray crystallography clearly showed that the complex may be considered as a real supramolecule rather than two cobalt(III) porphyrin π-cation radicals that interact through space.

Diiron(III)-μ-Fluoro Bisporphyrins: Effect of Bridging Ligand on the Metal Spin State.

A hitherto unknown family of diiron(III)-μ-fluoro bisporphyrins has been synthesized and structurally characterized. Fluoride abstraction from SbF6 (-) and BF4 (-) resulted in the synthesis of the μ-fluoro complexes of ethane- and ethene-bridged diiron(III) bisporphyrins. Two such complexes were structurally characterized, which revealed a single fluoro bridge between two iron centers with a remarkably bent Fe-F-Fe unit. Although isoelectronic with the μ-hydroxo complexes, the μ-fluoro species are quite divergent in terms of the electronic structure and properties. UV/Vis spectroscopy of the μ-fluoro complex exhibits a large redshift (ca. 18 nm) of the Soret band in comparison to their μ-hydroxo analog. Combined analysis by single crystal X-ray structure determination and Mössbauer and (1) H NMR spectroscopy revealed the presence of two equivalent iron(III) centers in the μ-fluoro complexes in both solid and solution phases. In contrast, the iron(III) centers of the μ-hydroxo complexes are known to be inequivalent. Variable-temperature magnetic studies show a weak antiferromagnetic interaction between the iron(III) centers of the μ-fluoro complexes with coupling constants (J) ranging from -33 to -40 cm(-1) . The experimental results were further supported by DFT calculations.

Experimental and Theoretical Investigation of a Series of Novel Dimanganese(III) μ-Hydroxo Bisporphyrins: Magneto–Structural Correlation and Effect of Metal Spin on Porphyrin Core Deformation

The synthesis, structure, and properties of a new family of five ethane-bridged dimanganese(III) μ-hydroxo bisporphyrins with the same core structure but different counteranions are reported here. Additions of 10% Brønsted acids such as HI, HBF4, HSbF6, HPF6, and HClO4 to a dichloromethane solution of the dichloro dimanganese(III) bisporphyrin produces complexes having a remarkably bent μ-hydroxo group with I3(-), BF4(-), SbF6(-), PF6(-), and ClO4(-) as counteranions, respectively. The X-ray structures of all complexes have been determined, which have revealed the presence of two equivalent high-spin manganese(III) centers with equally distorted porphyrin rings in the complexes, in sharp contrast with the case for the diiron(III) μ-hydroxo bisporphyrin analogues. (1)H NMR spectra have shown highly deshielded meso resonances, unlike the case for the diiron(III) analogues, where the meso resonances are highly shielded. The variable-temperature magnetic data have been subjected to a least-squares fit which provides a moderate antiferromagnetic coupling through the hydroxo bridge between two zero-field split Mn(III) centers with coupling constant (J) values ranging from -29.5 to -38.6 cm(-1). Fairly good correlations are observed for J with Mn-O(H) distances and Mn-O(H)-Mn angles for all the complexes except for that having an I3(-) counteranion. DFT calculations support the stabilization of two equivalent high-spin Mn(III) porphyrin cores in the complexes and have also explored the role of metal spin in controlling porphyrin ring deformation. Unlike diiron(III) μ-hydroxo bisporphyrin complexes, the dimanganese(III) analogues do not have easily accessible spin states of the metal attainable by subtle environmental perturbations and, therefore, can only stabilize the high-spin state with a variety of counteranions.

Effect of Inter-Porphyrin Distance on Spin-State in Diiron(III) μ-Hydroxo Bisporphyrins.

The synthesis, structure, and properties of bischloro, μ-oxo, and a family of μ-hydroxo complexes (with BF4 (-) , SbF6 (-) , and PF6 (-) counteranions) of diethylpyrrole-bridged diiron(III) bisporphyrins are reported. Spectroscopic characterization has revealed that the iron centers of the bischloro and μ-oxo complexes are in the high-spin state (S=(5) /2 ). However, the two iron centers in the diiron(III) μ-hydroxo complexes are equivalent with high spin (S=(5) /2 ) in the solid state and an intermediate-spin state (S=(3) /2 ) in solution. The molecules have been compared with previously known diiron(III) μ-hydroxo complexes of ethane-bridged bisporphyrin, in which two different spin states of iron were stabilized under the influence of counteranions. The dimanganese(III) analogues were also synthesized and spectroscopically characterized. A comparison of the X-ray structural parameters between diethylpyrrole and ethane-bridged μ-hydroxo bisporphyrins suggest an increased separation, and hence, less interactions between the two heme units of the former. As a result, unlike the ethane-bridged μ-hydroxo complex, both iron centers become equivalent in the diethylpyrrole-bridged complex and their spin state remains unresponsive to the change in counteranion. The iron(III) centers of the diethylpyrrole-bridged diiron(III) μ-oxo bisporphyrin undergo very strong antiferromagnetic interactions (J=-137.7 cm(-1) ), although the coupling constant is reduced to only a weak value in the μ-hydroxo complexes (J=-42.2, -44.1, and -42.4 cm(-1) for the BF4 , SbF6 , and PF6 complexes, respectively).