Axial phenoxide coordination on di-iron(III) bisporphyrin: Insights from experimental and DFT studies

Bhowmik, Susovan; Sil, Debangsu; Patra, Ranjan; Rath, Sankar Prasad

doi:10.1007/s12039-011-0156-6

Cited by 38 publications

(7 citation statements)

References 33 publications

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“…Thus, the spin‐density calculations can qualitatively explain the nature and positioning of the proton signals. Similar calculations have also been performed to explain the 1 H NMR shift patterns of the molecules with paramagnetic metal ions 3g. 9a, 20…”

Section: Resultsmentioning

confidence: 97%

“…However, the phenolate resonances of complex 2 a are shifted both upfield and downfield in the 1 H NMR spectrum, as shown in Figure 11 and Figure 12, which indicates π‐spin delocalization from the Fe III center onto the phenolate ligand 3g. 9 The ortho protons of the axial phenolate ligand are closest to the paramagnetic Fe center and have extremely small T 1 (spin‐lattice relaxation) values, thereby resulting in a very broad signal at δ =−90.8 ppm.…”

Section: Resultsmentioning

confidence: 99%

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Unusual Stabilization of an Intermediate Spin State of Iron upon the Axial Phenoxide Coordination of a Diiron(III)–Bisporphyrin: Effect of Heme–Heme Interactions

Bhowmik

Dey

Sahoo

et al. 2013

Chemistry A European J

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The binding of a series of substituted phenols as axial ligands onto a diiron(III)bisporphyrin framework have been investigated. Spectroscopic characterization revealed high-spin states of the iron centers in all of the phenolate complexes, with one exception in the 2,4,6-trinitrophenolate complex of diiron(III)bisporphyrin, which only stabilized the pure intermediate-spin (S=3/2) state of the iron centers. The average FeN (porphyrin) and FeO (phenol) distances that were observed with the 2,4,6-trinitrophenolate complex were 1.972(3) Å and 2.000(2) Å, respectively, which are the shortest and longest distances reported so far for any Fe(III) porphyrin with phenoxide coordination. The alternating shift pattern, which shows opposite signs of the chemical shifts for the meta versus ortho/para protons, is attributed to negative and positive spin densities on the phenolate carbon atoms, respectively, and is indicative of π-spin delocalization onto the bound phenolate. Electrochemical data reveals that the E1/2 value for the Fe(III) /Fe(II) couple is positively shifted with increasing acidity of the phenol. However, a plot of the E1/2 values for the Fe(III) /Fe(II) couple versus the pKa values of the phenols shows a linear relationship for all of the complexes, except for the 2,4,6-trinitrophenolate complex. The large deviation from linearity is probably due to the change of spin for the complex. Although 2,4,6-trinitrophenol is the weakest axial ligand in the series, its similar binding with the corresponding Fe(III) monoporphyrin only results in stabilization of the high-spin state. The porphyrin macrocycle in the 2,4,6-trinitrophenolate complex of diiron(III)bisporphyrin is the most distorted, whilst the "ruffling" deformation affects the energy levels of the iron d orbitals. The larger size and weaker binding of 2,4,6-trinitrophenol, along with hemeheme interactions in the diiron(III)bisporphyrin, are responsible for the larger ring deformations and eventual stabilization of the pure intermediate-spin states of the iron centers in the complex.

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Section: Resultsmentioning

confidence: 97%

Section: Resultsmentioning

confidence: 99%

Unusual Stabilization of an Intermediate Spin State of Iron upon the Axial Phenoxide Coordination of a Diiron(III)–Bisporphyrin: Effect of Heme–Heme Interactions

Bhowmik

Dey

Sahoo

et al. 2013

Chemistry A European J

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“…Such axial ligation by imidazoles also brings prominent changes in the redox potential of the Fe(III)/Fe(II) couple. It was demonstrated earlier that a change of the iron(III) spin state significantly affects the Fe(III)/Fe(II) redox potential. , The high-spin complex 2 ·(TP) 2 shows the redox couple at −0.76 V, which is positively shifted by 380–500 mV, in complexes 3 ·(L 1 ) 2 (L 2 ) 2 , upon the addition of imidazoles (L 2 ). Similarly, a shift of 180–290 mV is observed for 2 ·(DCTP) 2 upon sixth-axial ligation by L 2 .…”

Section: Results and Discussionmentioning

confidence: 95%

“…Our group has been actively working on porphyrin dimers to unmask the role of interheme interactions and the effect of axial ligation over various structural and physiochemical properties of the complex. ,− Along with interheme interactions, the nature of the axial ligands also controls the redox potentials in multiheme cytochromes . Previous investigations were done on the synthetic analogues of DHC2 where the iron centers contain either bis(imidazole) or bis(pyridine) ligation .…”

Section: Introductionmentioning

confidence: 99%

Heme–Heme Interactions in Diheme Cytochromes: Effect of Mixed-Axial Ligation on the Electronic Structure and Electrochemical Properties

et al. 2021

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Diheme cytochromes, the simplest members in the multiheme family, play substantial biochemical roles in enzymatic catalysis as well as in electron transfer. A series of diiron(III) porphyrin dimers have been synthesized as active site analogues of diheme cytochromes. The complexes contain six-coordinated iron(III) having thiophenol and imidazole at the fifth and sixth coordination sites, respectively. The iron centers in the complexes have been found to be in a low-spin state, as confirmed through solid-state Mössbauer and electron paramagnetic resonance (EPR) spectroscopic investigations. Mössbauer quadrupole splitting of complexes having mixed ligands is substantially larger than that observed when both axial ligands are the same. Rhombic types of EPR spectra with narrow separation between g x , g y , and g z clearly distinguish heme thiolate coordination compared to bis(imidazole)-ligated low-spin heme centers. The redox potential in diheme cytochromes has been found to be tuned by interheme interactions along with the nature of axial ligands. The effect of mixed-axial ligation within the diiron(III) porphyrin dimers is demonstrated by a positive shift in the Fe(III)/Fe(II) redox couple upon thiophenolate coordination compared to their bis(imidazole) analogues. The pK a of the imidazole also decides the extent of the shift for the Fe(III)/Fe(II) couple, while the potential of the mixed-ligated diiron(III) porphyrin dimer is more positive compared to their monomeric analogue. A variation of around 1.1 V for the Fe(III)/Fe(II) redox potential in the diiron(III) porphyrin dimer can be achieved with the combined effect of axial ligation and a metal spin state, while such a large variation in the redox potential, compared to their monomeric analogues, is attributed to the heme–heme interactions observed in dihemes. Moreover, theoretical calculations also support the experimental shifts in the redox potential values.

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“…We have been engaged in exploiting the significance and outcome of the heme–heme interactions using metalloporphyrin dimers. − In the present work, we explore the structure–function relationship of the diheme cytochrome c using a series of synthetic diheme analogs. For that, covalently linked porphyrin dimer with highly flexible ethane bridge has been employed as a diheme analog in which two-heme units orient in an anti -conformation.…”

Section: Introductionmentioning

confidence: 99%

Diheme Cytochrome c: Structure–Function Correlation and Effect of Heme−Heme Interactions

Khan¹,

Banerjee²,

Kumar

et al. 2018

Inorg. Chem.

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We explore here the structure-function relationship of the diheme cytochrome c using synthetic diheme analogs which serve as a convenient tool to investigate various aspects of Nature's sophisticated design in vitro. A large series of diiron ethane-bridged porphyrin dimers, both in the oxidized and the reduced states, are synthesized and their structural, chemical, and electrochemical properties have been scrutinized. Interestingly, the iron-to-iron nonbonding separation observed in such dihemes ranges from 9.49 to 10.06 Å which is very similar to the separation of 9.4 and 9.9 Å observed in the crystal structures of diheme cytochromes c isolated from Geobacter sulfurreducens and Haemophilus influenza, respectively. The Fe/Fe redox couple in the diheme complex is shifted toward more positive than their monomeric analog. Present study unmasks the electronic structure and properties of diheme centers and also highlights the significance of their structural arrangement and axial ligand orientation, and heme-to-heme separation. The Atoms in Molecules (AIM) analysis suggests long-range attractive dispersion forces between the heme units for the observed structure and properties in dihemes.

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Axial phenoxide coordination on di-iron(III) bisporphyrin: Insights from experimental and DFT studies

Cited by 38 publications

References 33 publications

Unusual Stabilization of an Intermediate Spin State of Iron upon the Axial Phenoxide Coordination of a Diiron(III)–Bisporphyrin: Effect of Heme–Heme Interactions

Unusual Stabilization of an Intermediate Spin State of Iron upon the Axial Phenoxide Coordination of a Diiron(III)–Bisporphyrin: Effect of Heme–Heme Interactions

Heme–Heme Interactions in Diheme Cytochromes: Effect of Mixed-Axial Ligation on the Electronic Structure and Electrochemical Properties

Diheme Cytochrome c: Structure–Function Correlation and Effect of Heme−Heme Interactions

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