Valence Electron Cloud Asymmetry from Two Points of View:  A Correlation between Mössbauer Quadrupole Splittings and <sup>57</sup>Fe NMR Chemical Shifts of Diamagnetic Iron(II) Porphyrinates

Polam, a Jayapal Reddy; Wright, a Joshua L.; Christensen, Alex; Flint, b Holger; Winkler, Bjoern; Grodzicki, b and Michael; Trautwein, b Alfred X.

doi:10.1021/ja954096m

Cited by 39 publications

(33 citation statements)

References 29 publications

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“…At 250 K, the 57 Fe Mössbauer spectrum of solid 2 shows a high-spin Fe(II) species comprising 46(5)% of the sample with an isomer shift (δ) of 0.880(9) mm/s and quadrupole splitting (Δ E Q ) of 3.22(2) mm/s (Figure A, green). ,− The remaining sample has a broad, poorly resolved signal (blue) potentially composed of multiple sites with an δ of 0.23(2) mm/s and Δ E Q of 0.81(2) mm/s which we assign as low-spin Fe(II) sites. These parameters are outside of the typical ranges for both low-spin Fe(II) (δ: 0.36–0.52 mm/s; Δ E Q : 0.23–0.52 mm/s) and low-spin Fe(III) (δ: 0.20–0.28 mm/s; Δ E Q : 1.5–1.7 mm/s) reported for TPA ligated 6-coordinate complexes but are similar to other low-spin Fe(II) species …”

Section: Results and Discussionsupporting

confidence: 43%

Reversible Switching of Organic Diradical Character via Iron-Based Spin-Crossover

et al. 2020

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Organic diradicals are uncommon species that have been intensely studied for their unique properties and potential applicability in a diverse range of innovative fields. While there is a growing class of stable and well characterized organic diradicals, there has been recent focus on how diradical character can be controlled or modulated with external stimuli. Here we demonstrate that a diiron complex bridged by the doubly oxidized ligand tetrathiafulvalene-2,3,6,7tetrathiolate (TTFtt 2−) undergoes a thermally induced Fe-centered spin-crossover which yields significant diradical character on TTFtt 2−. UV-vis-Near-IR, Mössbauer, NMR, and EPR spectroscopies with magnetometry, crystallography, and advanced theoretical treatments suggest that this diradical character arises from a shrinking TTFtt 2− π-manifold from the Fe(II)-centered spin-crossover. The TTFtt 2− centered diradical is predicted to have a singlet ground state by theory and variable temperature EPR. This unusual phenomenon demonstrates that inorganic spin transitions can be used to modulate organic diradical character. Results and Discussion Synthesis and Structural Parameters Complex 1 was synthesized via reaction with the deprotected proligand 2,3,6,7-tetrakis(2cyanoethylthio)tetrathiafulvalene (TTFtt(C2H4CN)4) in good yield. Complex 1 was insoluble in all solvents we investigated which precluded detailed characterization but is pure as indicated by combustion analysis and behaves as a suitable synthon for subsequent chemistry. Complex 1 can be doubly oxidized with [Cp2Fe][BAr F 4] to form 2 which is more soluble, enabling common solution characterization including 1 H NMR and cyclic voltammetry measurements (Figure S1-S2). Oxidation from 1 to 2 could be ligandcentered (TTFtt 4− →TTFtt 2−), metal-centered (2 Fe(II)→2 Fe(III)), or some intermediate case, but the data acquired for 2 supports a TTFtt 2− structure arising from ligandcentered oxidation (Chart 1B, see below). Compound 2 was structurally characterized via singlecrystal X-ray diffraction (SXRD) at 293 K (2-HT; Figure S3) and 100 K (2-LT; Figure 1). In both structures TTFtt 2− is bridged between two TPA-capped Fe centers with two outer-sphere BAr F 4 − counter anions. The most striking difference between these temperatures is markedly longer Fe bond lengths in 2-HT. The Fe-Npyridine and Fe-Namine bond lengths in 2-LT are 1.958(6)-1.979(6) and 2.017(6) Å (Figure 1), respectively. These values are consistent with Fe-N bonds in other low-spin complexes with a Fe-TPA moiety. 16,17 In 2-HT, these bonds are 0.18-0.19 and 0.244(11) Å longer than their counterparts at 100 K, respectively, and are consistent with high-spin Fe-TPA complexes. The shorter Fe bonds at lower temperature indicate that 2 exhibits a temperature dependent spincrossover as observed in related compounds. 16,21

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Section: Results and Discussionsupporting

confidence: 43%

Reversible Switching of Organic Diradical Character via Iron-Based Spin-Crossover

et al. 2020

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“…The 57 Fe chemical shift tensor orientations were generally close to molecular symmetry axes, although reversed from previous work, but with the skew of the shielding tensor again reversing sign on transition from strong to weak ligand fields. Also, the results showed that the paramagnetic contribution overwhelmingly dominated the total absolute shielding, as noted in previous work [70]. Interestingly, when MbCO models having distorted Fe-C-O geometries (as seen in protein X-ray crystal structures) were employed, the experimental 57 Fe chemical shifts were in poor accord with the theoretical results.…”

Section: Dft Investigation Of Metal Ion Nmr and 57 Fe Mö Ssbauer Quadsupporting

confidence: 48%

NMR, IR, Mössbauer and quantum chemical investigations of metalloporphyrins and metalloproteins

Sanders

Arnold

Oldfield

2001

J. Porphyrins Phthalocyanines

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ABSTRACT:We review contributions made towards the elucidation of CO and O 2 binding geometries in respiratory proteins. Nuclear magnetic resonance, infrared spectroscopy, Mössbauer spectroscopy, X-ray crystallography and quantum chemistry have all been used to investigate the Fe-ligand interactions. A principal result is that CO in most heme proteins has an essentially linear and untilted geometry (t = 4°, b = 7°) which is in extremely good agreement with a recently published X-ray synchrotron structure. CO/O 2 discrimination is thus attributable to polar interactions with the distal histidine residue, rather than major Fe-C-O geometric distortions.

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“…For example, six-coordinate Fe(II/III) heme complexes with imidazole (Im) and pyridine have been extensively characterized by X-ray crystallography,14 computational methods15 and spectroscopy16. In contrast, only limited data have been reported for alkylamine-ligated iron(III) porphyrins.…”

Section: Introductionmentioning

confidence: 99%

Alkylamine-Ligated H93G Myoglobin Cavity Mutant: A Model System for Endogenous Lysine and Terminal Amine Ligation in Heme Proteins such as Nitrite Reductase and Cytochrome f

Perera

Dawson

2011

Inorg. Chem.

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His93Gly sperm whale myoglobin (H93G Mb) has the proximal histidine ligand removed to create a cavity for exogenous ligand binding, providing a remarkably versatile template for the preparation of model heme complexes. The investigation of model heme adducts is an important way to probe the relationship between coordination structure and catalytic function in heme enzymes. In this study, we have successfully generated and spectroscopically characterized the H93G Mb cavity mutant ligated with less common alkylamine ligands (models for Lys or the amine group of N-terminal amino acids) in numerous heme iron states. All complexes have been characterized by electronic absorption and magnetic circular dichroism spectroscopy in comparison with data for parallel imidazole-ligated H93G heme iron moieties. This is the first systematic spectral study of models for alkylamine- or terminal amine-ligated heme centers in proteins. High-spin mono- and low-spin bis-amine-ligated ferrous and ferric H93G Mb adducts have been prepared together with mixed-ligand ferric heme complexes with alkylamine trans to nitrite or imidazole as heme coordination models for cytochrome c nitrite reductase or cytochrome f, respectively. Six-coordinate ferrous H93G Mb derivatives with CO, NO and O2 trans to the alkylamine have also been successfully formed, the latter for the first time. Finally, a novel high-valent ferryl species has been generated. The data in this study represent the first thorough investigation of the spectroscopic properties of alkylamine-ligated heme iron systems as models for naturally occurring heme proteins ligated by Lys or terminal amines.

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Valence Electron Cloud Asymmetry from Two Points of View: A Correlation between Mössbauer Quadrupole Splittings and ⁵⁷Fe NMR Chemical Shifts of Diamagnetic Iron(II) Porphyrinates

Cited by 39 publications

References 29 publications

Reversible Switching of Organic Diradical Character via Iron-Based Spin-Crossover

Reversible Switching of Organic Diradical Character via Iron-Based Spin-Crossover

NMR, IR, Mössbauer and quantum chemical investigations of metalloporphyrins and metalloproteins

Alkylamine-Ligated H93G Myoglobin Cavity Mutant: A Model System for Endogenous Lysine and Terminal Amine Ligation in Heme Proteins such as Nitrite Reductase and Cytochrome f

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Valence Electron Cloud Asymmetry from Two Points of View: A Correlation between Mössbauer Quadrupole Splittings and 57Fe NMR Chemical Shifts of Diamagnetic Iron(II) Porphyrinates

Cited by 39 publications

References 29 publications

Reversible Switching of Organic Diradical Character via Iron-Based Spin-Crossover

Reversible Switching of Organic Diradical Character via Iron-Based Spin-Crossover

NMR, IR, Mössbauer and quantum chemical investigations of metalloporphyrins and metalloproteins

Alkylamine-Ligated H93G Myoglobin Cavity Mutant: A Model System for Endogenous Lysine and Terminal Amine Ligation in Heme Proteins such as Nitrite Reductase and Cytochrome f

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Valence Electron Cloud Asymmetry from Two Points of View: A Correlation between Mössbauer Quadrupole Splittings and ⁵⁷Fe NMR Chemical Shifts of Diamagnetic Iron(II) Porphyrinates