Peter Schweyen

The Corrole Radical

The reaction of 5,10,15-trimesitylcorrole (H3 cor) with tungsten hexachloride and tungsten hexacarbonyl resulted in the unexpected formation of the 3,17-dichloro-5,10,15-trimesitylcorrole radical (H2 cor*) as an air-stable product. X-ray crystallography proved the planarization of the corrole radical structure, which was rationalized by the reduced steric hindrance of two versus three hydrogen atoms inside the N4 cavity. Although the aromaticity was lost, no specific changes in C-C or C-N bond distances could be observed. The regioselectivity of the two-fold chlorination is the result of the nucleophilic attack of chloride ions to an oxidized corrole macrocycle, and is supported by DFT results. The corrole radical acts as a dianionic ligand and allows the insertion of the divalent zinc(II) cation, which usually does not form neutral corrole complexes.

N-Heterocyclic Carbene Stabilized Boryl Radicals

The reaction of the 2-(trimethylsilyl)imidazolium triflate 9 with diarylboron halides (4-R-C6 H4 )2 BX (R=H, X=Br; R=CH3 , X=Cl; R=CF3 , X=Cl) afforded the NHC-stabilized borenium cations 10 a-c. Cyclic voltammetry revealed a linear correlation between the Hammett parameter σp of the para substituent and the half-wave potential. Chemical reduction with decamethylcobaltocene, [(C5 Me5 )2 Co], furnished the corresponding radicals 11 a-c; their characterization by EPR spectroscopy confirmed the paramagnetic character of 11 a-c, with large hyperfine coupling constants to the boron isotopes 11 B and 10 B, while delocalization of the unpaired electron into the NHC is negligible. DFT calculations of the percentage of spin density distribution between the carbene (NHC) and the boryl fragments (BR2 ) revealed for 11 a-c a spin density ratio (BR2 /NHC) of ca. 9:1, which underlines their distinct boryl radical character. The molecular structure of the most stable species 11 c was established by X-ray diffraction analysis.

Synthesis and Electronic Ground-State Properties of Pyrrolyl-Based Iron Pincer Complexes: Revisited

The pyrrolyl-based iron pincer compounds [(tBuPNP)FeCl] (1), [(tBuPNP)FeN2] (2), and [(tBuPNP)Fe(CO)2] (3) were prepared and structurally characterized. In addition, their electronic ground states were probed by various techniques including solid-state magnetic susceptibility and zero-field 57Fe Mössbauer and X-band electron paramagnetic resonance spectroscopy. While the iron(II) starting material 1 adopts an intermediate-spin (S = 1) state, the iron(I) reduction products 2 and 3 exhibit a low-spin (S = 1/2) ground state. Consistent with an intermediate-spin configuration for 1, the zero-field 57Fe Mössbauer spectrum shows a characteristically large quadrupole splitting (ΔEQ ≈ 3.7 mm s-1), and the solid-state magnetic susceptibility data show pronounced zero-field splitting (|D| ≈ 37 cm-1). The effective magnetic moments observed for the iron(I) species 2 and 3 are larger than expected from the spin-only value and indicate an incompletely quenched orbital angular momentum and the presence of spin-orbit coupling in the ground state. The experimental findings are complemented by density functional theory computations, which are in good agreement with the experimental data. Most notably, these calculations reveal a low-lying (S = 2) excited state for complex 1; furthermore, the computed Mössbauer parameters for all complexes studied herein are in excellent agreement with the experimental findings.

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.

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.

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.

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.