Ligand, oxygen, and carbon monoxide affinities of iron(II) modified "capped" porphyrins

Hashimoto, Toshiaki; Baldwin, Jack E.; Basolo, Fred; Dyer, Robert; Crossley, Maxwell J.

doi:10.1021/ja00372a003

Cited by 98 publications

(90 citation statements)

References 6 publications

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“…These can either be in the form of nanophases or specific types of Fe–N x sites with a higher affinity to oxygen as compared with nitrite (Supplementary Note 2). The steric effects on the relative affinity of different substrates on iron centres is documented for enzymes3031. In the presence of nitrite in solution, the catalytic activity is almost completely decreased to metal-free level suggesting that nitrite also interacts with these other active sites, be it not as strong (Supplementary Fig.…”

Section: Resultsmentioning

confidence: 96%

In situ electrochemical quantification of active sites in Fe–N/C non-precious metal catalysts

2016

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The economic viability of low temperature fuel cells as clean energy devices is enhanced by the development of inexpensive oxygen reduction reaction catalysts. Heat treated iron and nitrogen containing carbon based materials (Fe–N/C) have shown potential to replace expensive precious metals. Although significant improvements have recently been made, their activity and durability is still unsatisfactory. The further development and a rational design of these materials has stalled due to the lack of an in situ methodology to easily probe and quantify the active site. Here we demonstrate a protocol that allows the quantification of active centres, which operate under acidic conditions, by means of nitrite adsorption followed by reductive stripping, and show direct correlation to the catalytic activity. The method is demonstrated for two differently prepared materials. This approach may allow researchers to easily assess the active site density and turnover frequency of Fe–N/C catalysts.

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Section: Resultsmentioning

confidence: 96%

In situ electrochemical quantification of active sites in Fe–N/C non-precious metal catalysts

2016

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“…However, the CO affinity was 2 ± 3 orders of magnitue higher than those of Hb and Mb. With the aim of controlling the CO affinity, other models including capped, [12] cyclophane, [13] pocket, [14] hybrid, [9a] and bis-handle porphyrins [15] were synthesized. Systematically decreasing the size of the available distal cavity decreased CO affinity, while leaving O 2 affinity roughly comparable to those of unmodified porphyrins.…”

Section: Introductionmentioning

confidence: 99%

Iron Twin‐Coronet Porphyrins as Models of Myoglobin and Hemoglobin: Amphibious Electrostatic Effects of Overhanging Hydroxyl Groups for Successful CO/O₂ Discrimination

Tani¹,

Matsu-ura²,

Ariyama³

et al. 2003

Chemistry A European J

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Inspired by the observation of polar interactions between CO and O(2) ligands and the peptide residues at the active site of hemoglobin and myoglobin, we synthesized two kinds of superstructured porphyrins: TCP-IM, which contains a linked imidazole ligand, and TCP-PY, which contains a linked pyridine ligand, and examined the thermodynamic, kinetic, and spectroscopic (UV/Vis, IR, NMR, and resonance Raman) properties of their CO and O(2) complexes. On both sides of each porphyrin plane, bulky binaphthyl bridges form hydrophobic cavities that are suitable for the binding of small molecules. In the proximal site, an imidazole or pyridine residue is covalently fixed and coordinates axially to the central iron atom. In the distal site, two naphtholic hydroxyl groups overhang toward the center above the heme. The CO affinities of TCPs are significantly lower than those of other heme models. In contrast, TCPs have moderate O(2) binding ability. Compared with reported model hemes, the binding selectivity of O(2) over CO in TCP-IM and TCP-PY complexes is greatly improved. The high O(2) selectivity of the TCPs is mainly attributable to a low CO affinity. The comparison of k(on)(CO) values of TCPs with those of unhindered hemes indicates the absence of steric hindrance to the intrinsically linear CO coordination to Fe(II) in TCP-IM and TCP-PY. The abnormally large k(off)(CO) values are responsible for the low CO affinities. In contrast, k(off)(O(2)) of TCP-PY is smaller than those of other pyridine-coordinated model hemes. For the CO adducts of TCPs, unusually low nu(Fe-CO) and unusually high nu(C-O) frequencies are observed. These results can be ascribed to decreased back-bonding from the iron atom to the bound CO. The lone pairs of the oxygen atoms of the hydroxyl groups prevent back-bonding by exertion of a strong negative electrostatic interaction. On the other hand, high nu(Fe-O(2)) frequencies are observed for the O(2) adducts of TCPs. In the resonance Raman (RR) spectrum of oxy-TCP-IM, we observed simultaneous enhancement of the Fe-O(2) and O-O stretching modes. Furthermore, direct evidence for hydrogen bonding between the hydroxyl groups and bound dioxygen was obtained by RR and IR spectroscopy. These spectroscopic data strongly suggest that O(2) and CO binding to TCPs is controlled mainly by the two different electrostatic effects exerted by the overhanging OH groups: destabilization of CO binding by decreasing back-bonding and stabilization of O(2) binding by hydrogen bonding.

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“…14 Neither of the two reported [Fe(OEP)(CO)] crystal structures show nearest-neighbor interactions that could compress the FeCO bond. 15 The high δ Fe–C frequency in 6-c heme carbonyls has been attributed to diminished pseudo-Jahn–Teller instability along the e symmetry displacement because of an increased energy gap between a 1 and e orbitals caused by σ* interaction between the iron d z 2 orbital and the σ orbital from the trans axial ligand.…”

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confidence: 99%

The Diagnostic Vibrational Signature of Pentacoordination in Heme Carbonyls

et al. 2014

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Heme-carbonyl complexes are widely exploited for the insight they provide into the structural basis of function in heme-based proteins, by revealing the nature of their bonded and nonbonded interactions with the protein. This report presents two novel results which clearly establish a FeCO vibrational signature for crystallographically verified pentacoordination. First, anisotropy in the NRVS density of states for νFe–C and δFeCO in oriented single crystals of [Fe(OEP)(CO)] clearly reveals that the Fe–C stretch occurs at higher frequency than the FeCO bend and considerably higher than any previously reported heme carbonyl. Second, DFT calculations on a series of heme carbonyls reveal that the frequency crossover occurs near the weak trans O atom donor, furan. As νFe–C occurs at lower frequencies than δFeCO in all heme protein carbonyls reported to date, the results reported herein suggest that they are all hexacoordinate.

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Ligand, oxygen, and carbon monoxide affinities of iron(II) modified "capped" porphyrins

Cited by 98 publications

References 6 publications

In situ electrochemical quantification of active sites in Fe–N/C non-precious metal catalysts

In situ electrochemical quantification of active sites in Fe–N/C non-precious metal catalysts

Iron Twin‐Coronet Porphyrins as Models of Myoglobin and Hemoglobin: Amphibious Electrostatic Effects of Overhanging Hydroxyl Groups for Successful CO/O₂ Discrimination

The Diagnostic Vibrational Signature of Pentacoordination in Heme Carbonyls

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Ligand, oxygen, and carbon monoxide affinities of iron(II) modified "capped" porphyrins

Cited by 98 publications

References 6 publications

In situ electrochemical quantification of active sites in Fe–N/C non-precious metal catalysts

In situ electrochemical quantification of active sites in Fe–N/C non-precious metal catalysts

Iron Twin‐Coronet Porphyrins as Models of Myoglobin and Hemoglobin: Amphibious Electrostatic Effects of Overhanging Hydroxyl Groups for Successful CO/O2 Discrimination

The Diagnostic Vibrational Signature of Pentacoordination in Heme Carbonyls

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Iron Twin‐Coronet Porphyrins as Models of Myoglobin and Hemoglobin: Amphibious Electrostatic Effects of Overhanging Hydroxyl Groups for Successful CO/O₂ Discrimination