Sb(V) dihalide corroles: efficient singlet oxygen photosensitisers.

Sb(V) dihalide corrole complexes, in particular difluoro-5,15-di(4-cyanophenyl)-10-(2,4,5-trimethoxyphenyl)corrolatoantimony(V) (complex 1), show distinct emission properties and efficient intersystem crossing rates. Furthermore, complex 1 is characterised by its extended triplet excited state lifetime and an impressive singlet oxygen quantum yield exceeding 95%. This emphasises its potential for effective photooxidation reactions, positioning it as a leader in Sb(V) complex applications.

Do Corroles Stabilize Tetravalent Antimony?

Antimony corrole cations have been prepared by one-electron oxidation of antimony(III) congeners with silver(I) and copper(II) salts. Isolation and crystallization was successful for the first time, and the X-ray crystallographic investigation unraveled structural similarities with antimony(III)corroles. EPR experiments showed strong hyperfine interactions of the unpaired electron with the 121 Sb (I=5/2) and 123 Sb nuclei (I=7/2). A DFT analysis supports the description of the oxidized form as a SbIII corrole radical with less than 2 % SbIV character. In the presence of water or a fluoride source like PF6 - , the compounds undergo a redox disproportionation to yield known antimony(III)corroles and either difluorido-antimony(V)corroles, or bis-µ-oxido-di[antimony(V)corroles] via novel cationic hydroxo-antimony(V) derivatives.

Flagellin lysine methyltransferase FliB catalyzes a [4Fe-4S] mediated methyl transfer reaction.

The methyltransferase FliB posttranslationally modifies surface-exposed ɛ-N-lysine residues of flagellin, the protomer of the flagellar filament in Salmonella enterica (S. enterica). Flagellin methylation, reported originally in 1959, was recently shown to enhance host cell adhesion and invasion by increasing the flagellar hydrophobicity. The role of FliB in this process, however, remained enigmatic. In this study, we investigated the properties and mechanisms of FliB from S. enterica in vivo and in vitro. We show that FliB is an S-adenosylmethionine (SAM) dependent methyltransferase, forming a membrane associated oligomer that modifies flagellin in the bacterial cytosol. Using X-band electron paramagnetic resonance (EPR) spectroscopy, zero-field 57Fe Mössbauer spectroscopy, methylation assays and chromatography coupled mass spectrometry (MS) analysis, we further found that FliB contains an oxygen sensitive [4Fe-4S] cluster that is essential for the methyl transfer reaction and might mediate a radical mechanism. Our data indicate that the [4Fe-4S] cluster is coordinated by a cysteine rich motif in FliB that is highly conserved among multiple genera of the Enterobacteriaceae family.

PorphyStruct - A Digital Tool for the Quantitative Assignment of Non-Planar Distortion Modes in Four-Membered Porphyrinoids.

Invited for the cover of this issue are Jens Krumsieck and Martin Bröring from the TU Braunschweig, Germany. The cover image allows a glance into a futuristic lab with an innovative imaging device analyzing structural details of a metal corrole. Read the full text of the article at 10.1002/chem.202101243.

PorphyStruct: A Digital Tool for the Quantitative Assignment of Non-Planar Distortion Modes in Four-Membered Porphyrinoids.

PorphyStruct, a new digital tool for the analysis of non-planar distortion modes of different porphyrinoids, and its application to corrole structures is reported. The program makes use of the normal-coordinate structure decomposition technique (NSD) and employs sets of normal modes equivalent to those established for porphyrins in order to describe the out-of-plane dislocation pattern of perimeter atoms from corroles, norcorroles, porphycenes and other porphyrinoids quantitatively and in analogy to the established terminology. A comparative study of 17 porphyrin structures shows very similar results to the original NSD analysis and no systematic error. Application to corroles is successful and reveals the necessity to implement an extended basis of normal modes for a large share of experimental structures. The results frequently show the concomitant occurence of several modes but remain interpretable. For group XI metal corroles the phenomenon of supersaddling was unravelled, allowing for more in-depths discussions of structure-function correlations.

Zinc- and Cadmium meso-Aryl-Isoporphyrins: Non-Aromatic NIR Dyes with Distinct Conformational Features.

A series of pyrrolyl and dipyrrinyl isoporphyrins carrying different phenyl and thienyl groups is reported. The compounds are obtained by a one-pot approach in the presence of a template reagent. Thienyl derivatives gave better yields, and were the only subclass to form with steric hindrance. The structural analyses carried out on compounds 1 and 14 revealed distinct conformational differences which are likely to result from an intramolecular NH… Cl hydrogen bridge of the pyrrolyl subclass. In addition, this hydrogen bridge strongly favors one of the two possible atropisomers. Hindered rotation of the meso-aryl groups is observed only in the cases of methylbenzothienyl derivatives 10 and 15 and leads to the observation of several diastereomers. NIR absorptions up to 923 nm are found throughout. Electrochemical investigations into the 1e- and 2e- reduced species unravel axial ligand exchange dynamics for the zinc isoporphyrin radical, and the probable formation of a zinc phlorinate.

Iron(II) Spin Crossover Complexes Based on a Redox Active Equatorial Schiff-Base-Like Ligand.

In this work, two iron(II) coordination compounds with a N2O2 coordinating Schiff base-like ligand bearing a redox active tetrathiafulvalene (TTF) unit and pyridine or trans-1,2-bis(4-pyridylethylene) as an axial ligand are synthesized. Crystals suitable for single X-ray structure analysis were obtained for the new ligand. The complexes were characterized by magnetic susceptibility measurements, T-dependent UV-vis spectroscopy, and cyclic voltammetry. Both complexes display spin transition behavior below room temperature with T1/2 values of 146 and 156 K. The mononuclear iron(II) complex [FeTTFL(py)2] is relatively stable up to 400 K compared to similar complexes, showing no loss of axial ligands upon heating. Temperature dependent Mössbauer spectroscopy was conducted for the coordination polymer {[FeTTFL(bpee)]}n to get more information regarding the origin of the stepwise spin crossover (SCO) behavior observed in the magnetic measurements. The change of the spin state is accompanied by a change of the optical properties, which can be monitored by VT-UV-vis spectroscopy for the mononuclear complex and has been analyzed in theoretical studies. The redox behavior of the iron(II) complexes reveals three reversible redox steps which are located at the iron center and at the TTF unit of the ligand. Oxidation of the TTF unit induces characteristic changes in the UV-vis spectrum that can be followed by spectroelectrochemical UV-vis spectroscopy. Addressing the potential of the iron-centered redox process results in similar changes in the UV-vis spectrum, which indicates an electronic coupling of the redox active unit with the metal center under certain circumstances.

RirA of Dinoroseobacter shibae senses iron via a [3Fe-4S]1+ cluster co-ordinated by three cysteine residues.

In the marine bacterium, Dinoroseobacter shibae the transcription factor rhizobial iron regulator A (RirA) is involved in the adaptation to iron-limited growth conditions. In vitro iron and sulfide content determinations in combination with UV/Vis and electron paramagnetic resonance (EPR) spectroscopic analyses using anaerobically purified, recombinant RirA protein suggested a [3Fe-4S]1+ cluster as a cofactor. In vivo Mössbauer spectroscopy also corroborated the presence of a [3Fe-4S]1+ cluster in RirA. Moreover, the cluster was found to be redox stable. Three out of four highly conserved cysteine residues of RirA (Cys 91, Cys 99, Cys 105) were found essential for the [3Fe-4S]1+ cluster coordination. The dimeric structure of the RirA protein was independent of the presence of the [3Fe-4S]1+ cluster. Electro mobility shift assays demonstrated the essential role of an intact [3Fe-4S]1+ cluster for promoter binding by RirA. The DNA binding site was identified by DNase I footprinting. Mutagenesis studies in combination with DNA binding assays confirmed the promoter binding site as 3'-TTAAN10AATT-5'. This work describes a novel mechanism for the direct sensing of cellular iron levels in bacteria by an iron-responsive transcriptional regulator using the integrity of a redox-inactive [3Fe-4S]1+ cluster, and further contributes to the general understanding of iron regulation in marine bacteria.

The radical SAM protein HemW is a heme chaperone.

Radical S-adenosylmethionine (SAM) enzymes exist in organisms from all kingdoms of life, and all of these proteins generate an adenosyl radical via the homolytic cleavage of the S-C(5') bond of SAM. Of particular interest are radical SAM enzymes, such as heme chaperones, that insert heme into respiratory enzymes. For example, heme chaperones insert heme into target proteins but have been studied only for the formation of cytochrome c-type hemoproteins. Here, we report that a radical SAM protein, the heme chaperone HemW from bacteria, is required for the insertion of heme b into respiratory chain enzymes. As other radical SAM proteins, HemW contains three cysteines and one SAM coordinating an [4Fe-4S] cluster, and we observed one heme per subunit of HemW. We found that an intact iron-sulfur cluster was required for HemW dimerization and HemW-catalyzed heme transfer but not for stable heme binding. A bacterial two-hybrid system screen identified bacterioferritins and the heme-containing subunit NarI of the respiratory nitrate reductase NarGHI as proteins that interact with HemW. We also noted that the bacterioferritins potentially serve as heme donors for HemW. Of note, heme that was covalently bound to HemW was actively transferred to a heme-depleted, catalytically inactive nitrate reductase, restoring its nitrate-reducing enzyme activity. Finally, the human HemW orthologue radical SAM domain-containing 1 (RSAD1) stably bound heme. In conclusion, our findings indicate that the radical SAM protein family HemW/RSAD1 is a heme chaperone catalyzing the insertion of heme into hemoproteins.

Heme and nitric oxide binding by the transcriptional regulator DnrF from the marine bacterium Dinoroseobacter shibae increases napD promoter affinity.

Under oxygen-limiting conditions, the marine bacterium Dinoroseobacter shibae DFL12T generates energy via denitrification, a respiratory process in which nitric oxide (NO) is an intermediate. Accumulation of NO may cause cytotoxic effects. The response to this nitrosative (NO-triggered) stress is controlled by the Crp/Fnr-type transcriptional regulator DnrF. We analyzed the response to NO and the mechanism of NO sensing by the DnrF regulator. Using reporter gene fusions and transcriptomics, here we report that DnrF selectively repressed nitrate reductase (nap) genes, preventing further NO formation. In addition, DnrF induced the expression of the NO reductase genes (norCB), which promote NO consumption. We used UV-visible and EPR spectroscopy to characterize heme binding to DnrF and subsequent NO coordination. DnrF detects NO via its bound heme cofactor. We found that the dimeric DnrF bound one molecule of heme per subunit. Purified recombinant apo-DnrF bound its target promoter sequences (napD, nosR2, norC, hemA, and dnrE) in electromobility shift assays, and we identified a specific palindromic DNA-binding site 5'-TTGATN4ATCAA-3' in these target sequences via mutagenesis studies. Most importantly, successive addition of heme as well as heme and NO to purified recombinant apo-DnrF protein increased affinity of the holo-DnrF for its specific binding motif in the napD promoter. On the basis of these results, we propose a model for the DnrF-mediated NO stress response of this marine bacterium.

Viking Helmet Corroles: Activating Inert Oxidometal Corroles.

Chemically inert oxidometal(V) corrols of molybdenum and rhenium undergo clean ligand-exchange reactions upon the action of SiCl4 . The resulting dichlorido complexes show trigonal prismatic coordination of the metal ion with the chlorine atoms residing in a cis configuration, and were studied by optical and resonance spectroscopy as well as DFT calculations. In situ reactivity studies with carbon nucleophiles indicate high reactivity for chlorine replacement. Treatment with sodium cyclopentadienide paves the way to robust molybdenum corrolocene half-sandwich complexes. These organometallic compounds are the first corrole species that stabilize an air-stable and diamagnetic low spin d2 -MoIV center. Structural, spectroelectrochemical, and chemical investigations prove a reversible MoIV /MoV redox couple close to the Fc/Fc+ potential for these systems. The high stability of the compounds in both redox states calls for future applications in catalysis and as redox switch.

Design and Evaluation of a Boron Dipyrrin Electrophore for Redox Flow Batteries.

A boron dipyrrin (BODIPY) dye was designed as a molecular single-component electrophore for redox flow batteries. All positions of the BODIPY core were assessed on the basis of literature data, in particular cyclic voltammetry and density functional calculations, and a minimum required substitution pattern was designed to provide solubility, aggregation, radical cation and anion stabilities, a large potential window, and synthetic accessibility. In-depth electrochemical and physical studies of this electrophore revealed suitable cathodic behavior and stability of the radical anion but rapid anodic decomposition of the radical cation. The three products that formed under the conditions of controlled oxidative electrolysis were isolated, and their structures were determined by spectroscopy and comparison with a synthetic model compound. From these structures, a benzylic radical reactivity, initiated by one-electron oxidation, was concluded to play the major role in this unexpected decomposition.

Heterocorrole Conformations: Little Saddling, Much Ruffling.

10-Heterocorrole complexes with oxygen, sulfur, and selenium at position 10 of the macrocycle and with the divalent ions of nickel, copper, and palladium were prepared and investigated. The focus was set on the size adaptation and matching mechanisms of cavity size versus ionic radius in corrole-type macrocycles. A full set of single-crystal X-ray analytical data revealed that in all but one case the N4 binding site of the ring-contracted tetrapyrrole was larger than necessary to bind the metal ion without deformation. In-plane size adaptation through M-N bond-length elongation by 2.5-3.2 % was effective, as well as pronounced out-of-plane ruffling of the macrocycle for those compounds with a more severe size mismatch. Such ruffling had been excluded for corroles previously, but is apparently the most efficient mechanism to adapt to small central ions.

Circularly Polarized Luminescence from Axially Chiral BODIPY DYEmers: An Experimental and Computational Study.

With our new home-built circularly polarized luminescence (CPL) instrument, we measured fluorescence and CPL spectra of the enantiomeric pairs of two quasi-isomeric BODIPY DYEmers 1 and 2, endowed with axial chirality. The electronic circular dichroism (ECD) and CPL spectra of these atropisomeric dimers are dominated by the exciton coupling between the main π-π* transitions (550-560 nm) of the two BODIPY rings. Compound 1 has strong ECD and CPL spectra (glum =4×10-3 ) well reproduced by TD-DFT and SCS-CC2 (spin-component scaled second-order approximate coupled-cluster) calculations using DFT-optimized ground- and excited-state structures. Compound 2 has weaker ECD and CPL spectra (glum =4×10-4 ), partly due to the mutual cancellation of electric-electric and electric-magnetic exciton couplings, and partly to its conformational freedom. This compound is computationally very challenging. Starting from the optimized excited-state geometries, we predicted the wrong sign for the CPL band of 2 using TD-DFT with the most recommended hybrid and range-separated functionals, whereas SCS-CC2 or a DFT functional with full exact exchange provided the correct sign.

Metal-Assisted One-Pot Synthesis of Isoporphyrin Complexes.

Zinc and cadmium complexes of meso-arylisoporphyrins carrying a pyrrolyl or dipyrrinyl substituent at the sp(3) carbon atom were obtained through a simple one-pot variation of the Alder-Longo porphyrin synthesis. Key to the formation and stabilization of isoporphyrins is the presence of metal acetates during the oxidative macrocyclization step. The characteristic Q-bands of isoporphyrins are found in the NIR region between 750 nm and 880 nm. All of the isolated pyrrolyl- and dipyrrinyl-appended isoporphyrins are stable under typical laboratory conditions and allow chemical transformations like BF2 coordination, transmetalation, and ligand exchange.

Acidic Condensation of BODIPYs with Aldehydes: A Quick and Versatile Route to Alkenyl-BODIPYs and C(sp(3) )-Connected DYEmers.

The condensation of aldehydes with BODIPY (boron dipyrrin) luminophores was investigated. Formaldehyde can be used to connect two BODIPYs at each of the three pyrrolic C positions (α-, ß-, and ß'-positions) in a quick and highly selective manner, yielding new DYEmers (di- and oligomeric BODIPY derivatives) with varied photophysical properties. Benzaldehydes form DYEmers only at the ß- and the ß'-positions. For aliphatic aldehydes the DYEmer formation competes with the elimination of water from a proposed alcohol intermediate, leading to the formation of α- and ß-alkenyl-BODIPYs. 2-Phenylacetaldehyde and similar precursors exclusively yield elimination products. These acid-mediated transformations are valuable alternatives to the well-established, base-promoted Knoevenagel condensation protocol that is typically employed in the preparation of BODIPYs with near infrared (NIR)-shifted absorptions.

The auxiliary [4Fe-4S] cluster of the Radical SAM heme synthase from Methanosarcina barkeri is involved in electron transfer.

The heme synthase AhbD catalyzes the oxidative decarboxylation of two propionate side chains of iron-coproporphyrin III to the corresponding vinyl groups of heme during the alternative heme biosynthesis pathway occurring in sulfate-reducing bacteria and archaea. AhbD belongs to the family of Radical SAM enzymes and contains two [4Fe-4S] clusters. Whereas one of these clusters is required for substrate radical formation, the role of the second iron-sulfur cluster is not known. In this study, the function of the auxiliary cluster during AhbD catalysis was investigated. Two single cluster variants of AhbD from M. barkeri carrying either one of the two clusters were created. Using these enzyme variants it was shown that the auxiliary cluster is not required for substrate binding and formation of the substrate radical. Instead, the auxiliary cluster is involved in a late step of AhbD catalysis most likely in electron transfer from the reaction intermediate to a final electron acceptor. Moreover, by using alternative substrates such as coproporphyrin III, Cu-coproporphyrin III and Zn-coproporphyrin III for the AhbD activity assay it was observed that the central iron ion of the porphyrin substrate also participates in the electron transfer from the reaction intermediate to the auxiliary [4Fe-4S] cluster. Altogether, new insights concerning the completely uncharacterized late steps of AhbD catalysis were obtained.

Cationic nickel porphyrinoids with unexpected reactivity.

Cationic nickel(ii) complexes of two ring-contracted porphyrinoid ligands distantly related to the corrins were prepared by metal templated macrocyclisation. The compounds show reversible electron transfer processes and were found to be the first porphyrinoid-based catalysts for C-C cross-coupling.

Spin Crossover and Valence Tautomerism in Neutral Homoleptic Iron Complexes of Bis(pyridylimino)isoindolines.

Homoleptic iron complexes of six bis(pyridylimino)isoindoline (bpi) ligands with different substituents (H, Me, Et, tBu, OMe, NMe2) at the 4-positions of the pyridine moieties have been prepared and studied with regard to temperature-dependent spin and redox states by a combination of (57)Fe Mössbauer spectroscopy, SQUID magnetometry, single-crystal X-ray diffraction analysis, X-band EPR, and (1)H NMR spectroscopy. While the H-, methyl-, and ethyl-substituted complexes remain in a pure high-spin state irrespective of the temperature, the 4-tert-butyl-substituted derivative shows spin-crossover behavior. The methoxy- and dimethylamino-substituted compounds were found to easily undergo oxidation. In the crystalline state, valence tautomeric behavior was observed for the methoxy derivative as a thermally activated charge-transfer transition, accompanied by a spin crossover above 200 K. The valence tautomerism leads to a chelate with one of the bpi ligands as a dianion radical L(2-·) and with an effective spin of S=2.

Design of BODIPY Dyes as Photosensitisers in Multicomponent Catalyst Systems for Light-Driven Hydrogen Production.

A study of visible-light-driven hydrogen production using a multicomponent system consisting of different boron dipyrromethene (BODIPY) dyes, triethylamine and [{Pd(PPh3)Cl2}2] from THF/water mixtures is presented. A trio of meso-mesityl BODIPY dyes display the best activities and long-term stabilities of more than ten days with the 2,6-diiodo derivative showing the best performance.