Self-Assembly of Dinitrosyl Iron Units into Imidazolate-Edge-Bridged Molecular Squares: Characterization Including Mössbauer Spectroscopy

Abstract

Imidazolate-containing {Fe(NO)2}9 molecular squares have been synthesized by oxidative CO displacement from the reduced Fe(CO)2(NO)2 precursor. The structures of complex 1 [(Imidazole)Fe(NO)2]4, (Ford, Li, et al. Chem. Comm. 2005, 477–479), 2 [(2-Isopropylimidazole)Fe(NO)2]4, and 3 [(Benzimidazole)Fe(NO)2]4, as determined by X-ray diffraction analysis, find precise square planes of irons with imidazolates bridging the edges and nitrosyl ligands capping the irons at the corners. The orientation of the imidazolate ligands in each of the complexes results in variations of the overall structures, and molecular recognition features in the available cavities of 1 and 3. Computational studies show multiple low energy structural isomers and confirm that the isomers found in the crystallographic structures arise from intermolecular interactions. EPR and IR spectroscopic studies, and electrochemical results suggest that the tetramers remain intact in solution in the presence of weakly-coordinating (THF) and non-coordinating (CH2Cl2) solvents. Mössbauer spectroscopic data for a set of reference dinitrosyl iron complexes, reduced {Fe(NO)2}10 compounds A ((NHC-iPr)2Fe(NO)2), and C ((NHC-iPr)(CO)Fe(NO)2), and oxidized {Fe(NO)2}9 compounds B ([(NHC-iPr)2Fe(NO)2][BF4]), and D ((NHC-iPr)(SPh)Fe(NO)2) (NHC-iPr = 1,3-diisopropylimidazol-2-ylidene) demonstrate distinct differences of the isomer shifts and quadrupole splittings between the oxidized and reduced forms. The reduced compounds have smaller positive isomer shifts as compared to the oxidized compounds ascribed to the greater π-backbonding to the NO ligands. Mössbauer data for the tetrameric complexes 1, 2, and 3 demonstrate larger isomer shifts, most comparable to compound D; all four complexes contain cationic {Fe(NO)2}9 units bound to one anionic ligand and one neutral ligand. At RT the paramagnetic, S = ½ per iron, centers are not coupled.

Synthetic Advances Inspired by the Bioactive Dinitrosyl Iron Unit

“…9 In contrast to Lippard's (Ar-nacnac)-DNICs, the Mossbauer spectroscopic parameters of the prototypic NHC-DNICs in Figure 9 show that the oxidized {Fe(NO) 2 } 9 complexes have larger positive isomer shifts than the reduced {Fe(NO) 2 } 10 analogues (Table 2). 40 40 These studies involving structural and spectroscopic characterization of the series of NHC-DNICs confirmed the suitability of NHC ligands as mimics of imidazole/histidine in stable model DNICs of different redox levels. A fundamental expectation, experimentally observed, is that the Fe−NO bond should be stronger in the reduced complexes and NO release is favored in the oxidized {Fe(NO) 2 } 9 form.…”

New members of a class of dinitrosyliron complexes (DNICs): The characteristic EPR signal of the six-coordinate and five-coordinate {Fe(NO)2}9 DNICs

“…In addition, DNICs with EPR signal deviating from g = 2.03 were also observed upon addition of nitrite and ascorbate to Clostridium botulinum or stimulation of cytotoxic activated macrophage [3,4,18]. In inorganic chemistry, although the four-coordinate {Fe(NO) 2 } 9 DNICs containing the varieties of coordinating ligands were well-known [12][13][14][15][16][17][19][20][21][22][23][24], few five-coordinate {Fe(NO) 2 } 9 DNICs were reported [25,26]. Nitrosylation of complex [(6-Me 3 -TPA)Fe II (CatH)] + (6-Me 3 -TPA = tri(6-methyl-2-pyridylmethyl)amine, CatH = 1,2-catecholate monoanion) resulted in the formation of [(6-Me 3 -TPA)Fe III (Cat)] + and [(6-Me 3 -TPA)Fe (NO) 2 ] + via the disproportionation of [(6-Me 3 -TPA)Fe(CatH) (NO)] + [25].…”

“…The neutral square-pyramidal {Fe(NO) 2 } 9 DNIC 4-PyImiS displayed a well-resolved 13-line EPR signal with g = 2.018 (a N(NO) = 4.0 G, a N(imine) = 9.6 G and a N(pyridine) = 3.2 G) at 280 K ( Fig. 3), deviating from the tetrahedral {Fe(NO) 2 } 9 DNICs displaying EPR signal at g = 2.03 [12][13][14][15][16][17][19][20][21][22][23][24]. In spite of the trigonal bipyramidal {Fe(NO) 2 } 9 DNIC [(TMEDA)Fe(NO) 2 (I)] characterized with EPR signal g = 2.03 (Table 1) [26], presumably, the EPR spectra (the g value and the pattern of the hyperfine splitting) may serve as an efficient tool for the discrimination of the four-coordinate tetrahedral {Fe(NO) 2 } 9 DNICs [L 2 Fe(NO) 2 ] − (L= thiolate, imidazolate) (g = 2.03) and the five-/ six-coordinate {Fe(NO) 2 } 9 DNICs (e.g.…”

“…In the EPR experiments, complex 1-TPA displays an isotropic EPR signal g = 2.018 (298 K) in CH 2 Cl 2 (Fig. 1), deviating from the characteristic g value 2.03 of the four-coordinate, tetrahedral {Fe(NO) 2 } 9 DNICs[2,[12][13][14][15][16][17][19][20][21][22][23][24]27,28].Ina similar fashion, treatment of THF solution of [Fe(CO) 2 (NO) 2 ] + with 2,6-bis[1-(2,6-diisopropylphenylimino)ethyl]pyridine ( iPr PDI) in a 1:1 molar ratio led to the formation of the cationic fivecoordinate {Fe(NO) 2 } 9 [(NO) 2 Fe( iPr PDI)][BF 4 ] (2-iPr PDI) (Schemes 1 and 2b), characterized by IR, UV-vis, EPR spectroscopy, SQUID, XAS and single-crystal X-ray diffraction. The IR v NO stretching frequencies (v NO 1794 s, 1721 s cm − 1 (THF)) imply the formation of the fiveScheme 1.…”

Nitrosylation of Nitric‐Oxide‐Sensing Regulatory Proteins Containing [4Fe‐4S] Clusters Gives Rise to Multiple Iron–Nitrosyl Complexes

“…S1 and Table S1) indicate a significant change in the iron environment, consistent with the formation of iron-nitrosyl species. [12, 23] NRVS spectra are shown in Fig. S2, where they are compared to the previously published NRVS spectrum of D14C variant of Pyrococcus furiosus [4Fe-4S] ferrodoxin.…”