Isocyanide or nitrosyl complexation to hemes with varying tethered axial base ligand donors: synthesis and characterization

Abstract

A series of ferrous-heme 2,6-dimethylphenyl isocyanide (DIMPI) and ferrous-heme mononitrosyl complexes have been synthesized and characterized. The heme portion of the complexes studied is varied with respect to the nature of the axial ligand, including complexes, where it is covalently tethered to the porphyrinate periphery. Reduced heme complexes, [(F8)FeII], [(PPy) FeII], [(PIm)FeII], and [(PImH)FeII], where F8 = tetrakis(2,6-difluorophenyl)-porphyrinate and PPy, PIm, and PImH are partially fluorinated tetraaryl porphyrinates with covalently appended axial base pyridyl/imidazolyl or histamine moieties, were employed; PImH is a new construct. Room temperature addition of DIMPI to these iron(II) complexes affords the bis-isocyanide species [(F8)FeII-(DIMPI)2] in the case of [(F8)FeII], while for the other hemes, mono-DIMPI compounds are obtained, [(PPy)FeII-(DIMPI)] [(2)-DIMPI], [(PIm)FeII-(DIMPI)] [(3)-DIMPI], and [(PImH) FeII-(DIMPI)] [(4)-DIMPI]. The structures of complexes (3)-DIMPI and (4)-DIMPI have been determined by single crystal X-ray crystallography, where interesting H…F(porphryinate aryl group) interactions are observed. 19F-NMR spectra determined for these complexes suggest that H…F(porphyrinate aryl groups) attractions also occur in solution, the H atom coming either from the DIMPI methyl groups or from a porphyinate axial base imidazole or porphyrinate pyrrole. Similarly, we have used nitrogen monoxide to generate ferrous-nitrosyl complexes, a five-coordinate species for F8, [(F8)FeII-(NO)], or low-spin six-coordinate compounds [(PPy)FeII-(NO)], [(PIm)FeII-(NO)], and [(PImH)FeII-(NO)]. The DIMPI and mononitrosyl complexes have also been characterized using UV–Vis, IR, 1H-NMR, and EPR spectroscopies.

A Six-Coordinate Peroxynitrite Low-Spin Iron(III) Porphyrinate Complex—The Product of the Reaction of Nitrogen Monoxide (·NO(g)) with a Ferric-Superoxide Species

“…Under these conditions, we also do not observe formation of any ferrous/ferric nitrosyl complex. As we have shown earlier, the ferrous nitrosyl 31 complex has characteristic UV–vis and EPR spectroscopic features, shown in Figure S9.…”

“…For peroxynitrite binding to a metal ion, several possible binding modes and conformations must be considered, including N atom 11 vs O atom ligation, cis vs trans isomers (Figure 6a and b, respectively), and even O,O′-chelation. 31 Peroxynitrite N-binding to a heme-Fe(III) has been proposed based on theoretical calculations, 13 but peroxo anion O atom binding is observed experimentally (and supported by DFT calculations) for several porphyrinate cobalt(III)-peroxynitrite complexes. 19a …”

Tetranuclear Fe Clusters with a Varied Interstitial Ligand: Effects on the Structure, Redox Properties, and Nitric Oxide Activation

“…The nature and type of interaction with axial ligand has been previously demonstrated to effect the level of NO activation in mononuclear Fe complexes. 16 Analogous shifts in the distance between Fe and axial ligands trans to coordinated N 2 have been reported for monoiron models of nitrogenase. 17 …”

Modeling Tryptophan/Indoleamine 2,3-Dioxygenase with Heme Superoxide Mimics: Is Ferryl the Key Intermediate?

“…The ferrous heme complex, [(THF) 2 (TPP)­Fe II ], exhibited significant electronic absorption perturbations from 426 (Soret; ε = 16 × 10 5 M –1 cm –1 ), 536 (ε = 6.2 × 10 4 M –1 cm –1 ) and 558 (ε = 6.5 × 10 4 M –1 cm –1 ) nm to 416 (Soret; ε = 10 × 10 5 M –1 cm –1 ), and 540 (ε = 9.0 × 10 4 M –1 cm –1 ) nm in 9:1 DCM:THF solvent mixture at −80 °C upon the bubbling of dry O 2(g) , indicating the formation of the EPR-silent, end-on ferric superoxo species, [(THF)­(TPP)­Fe III (O 2 –• )] (Figure A,B). , Isotope-sensitive ν­(Fe–O) and ν­(O–O) resonance Raman frequencies for this species were centered at 579 (Δ 18 O 2 = −26) and 1170 (Δ 18 O 2 = −60) cm –1 , respectively (Figure C). Subsequent addition of 2 equiv of 4 methylimidazole (Im) led to the formation of the Im-coordinated superoxo adduct, [(Im)­(TPP)­Fe III (O 2 –• )] (418 (Soret; ε = 7.5 × 10 5 M –1 cm –1 ) and 543 (ε = 6.5 × 10 4 M –1 cm –1 ) nm); − ν­(Fe–O): 577 (Δ 18 O 2 = −23) and ν­(O–O): 1170 (Δ 18 O 2 = −56) cm –1 (Figure D). − Similarly, the rest of the ferric superoxo compound series was prepared and characterized: [(THF)­(F 20 TPP)­Fe III (O 2 –• )], [(Im)­(F 20 TPP)­Fe III (O 2 –• )], [(THF)­(TMPP)­Fe III (O 2 –• )] and [(Im)­(TMPP)­Fe III (O 2 –• )] (Figures S1 and S2). …”