Electronic configuration and structure of paramagnetic iron dinitrosyl complexes

Abstract

The electronic and geometric structures of paramagnetic iron dinitrosyl complexes have been investigated using electron spin resonance, infrared spectroscopy, and X-ray crystallography. It is concluded that these compounds are best described as 17 electron complexes with a d 9 configuration rather than the d 7 configuration assumed by most previous investigators. The anisotropy of the g values, determined from the electron spin resonance spectra of frozen solutions, varies considerably from complex to complex. The results are consistent with the supposition that all of the complexes have a distorted tetrahedral geometry, but the nature of the distortion changes as the ligands are varied. As a result of this variation there are changes in the nature of the spin-containing d orbital. Ligands containing hard, nonpolarizable donor atoms such as oxygen or fluorine produce a distortion towards a planar geometry, placing the odd electron in a predominantly d, 2-, 2 orbital, while those containing softer donor atoms such as phosphorus or sulfur give complexes with a different type of distortion, leading to placement of the odd electron in a predominantly dz2 orbital. Nitrogen and halide donor ligands produce smaller distortions, leading to spin-containing molecular orbitals with contributions from a mixture of d orbitals. In accordance with this model, the crystal structure of [Fe(NO),I,]-has been found to be only slightly distorted from regular tetrahedral coordination about the iron atom. TRACI R. BRYAR et DONALD R. EATON. Can. J. Chem. 70, 1917Chem. 70, (1992. Faisant appel 2 la resonance paramagnetique Clectronique (rpe), a la spectroscopie infrarouge et i la diffraction des rayons X, on a CtudiC les structures Clectroniques et geometriques des complexes paramagnetiques du dinitrosyl fer. On en conclut que la meilleure f a~o n de dCcrire ces composCs est de les considerer comme des complexes 2 17 klectrons possidant une configuration d 9 plut6t que la configuration d7 utilisee par la plupart des auteurs qui en ont trait6 anterieurement. L'anisotropie des valeurs g, determinee par les spectres rpe de solutions congelees, varie beaucoup d'un complexe 2 l'autre. Les resultats sont en accord avec l'hypothkse selon laquelle tous les complexes possedent une gkomktrie tCtraCdrique dCformCe, mais que la nature de la distorsion varie avec la nature des ligands. Un rksultat de cette variation amkne des changements dans la nature de l'orbitale contenant le spin. Les ligands contenant des atomes donneurs durs et non-polarisables, comme l'oxygkne ou le fluor, provoquent une distorsion vers une gComCtrie plane forqant 1'Clectron celibataire 2 1 occuper une orbitale principalement dX2-y2 alors que ceux qui contient des atomes donneurs plus mous, comme le phosphore ou le soufre, donnent des complexes~avec un type de distorsion different, conduisant . [Traduit par la rkdaction]

Mechanism of [4Fe-4S](Cys)4 Cluster Nitrosylation Is Conserved among NO-responsive Regulators

“…The sample buffer was 25 mM HEPES, 2.5 mM CaCl 2 , 100 mM NaCl, 100 mM NaNO 3 , pH 7.5. B, simulated EPR spectra of persulfide-ligated DNIC (g Ќ ϭ 2.044, g ϭ ϭ 2.032) (31,46) and two thiol-coordinated DNICs at g(ϳ) 2.04 (g Ќ ϭ 2.045, g ϭ ϭ 2.023) (29,44) and g(ϳ) 2.02 (g Ќ ϭ 2.023, g ϭ ϭ 2.016) (11,47) that were used to deconvolute the recorded (74 K) EPR spectra in A. C, EPR spectra for nitrosylated FNR (black lines), together with simulations of the experimental data (R 2 (ϳ) 0.90) (gray lines) obtained by the combination of varying amounts of the spectra shown in B.…”

Functional models of [Fe—S] nitrosyl proteins

“…32, 33 Investigations into the structures and physicochemi cal properties of synthetic models of active sites of non heme [Fe-S] nitrosyl proteins are related to application of knowledge in practice for the selective delivery of nitro gen monoxide in vivo. [34][35][36][37][38][39][40][41][42][43][44][45] In recent years, a search for, and studies of, new nitrogen monoxide donors have at tracted considerable attention of experts in practical medi cine. There are several classes of compounds that gener ate nitrogen monoxide during metabolic processes.…”

13. Reactivity of iron-sulfur clusters with nitric oxide

“…DNICs are paramagnetic (S=1/2) with a characteristic g-value of ≈ 2.03 that is consistent across all ligands. They exhibit distorted tetrahedral geometry and roughly linear Fe-NO bonds, and EPR, IR, and crystallographic structural data are available for a very large range of complexes of this type [84,85]. As described above, DFT calculations combined with spectroscopic observations revealed that that the S=1/2 DNIC, which is represented as {Fe(NO)2} 9 in the Enemark-Feltham notation, is best described by two resonance structures consisting of a high spin Fe 3+ (S= 5/2) antiferromagnetically coupled to two NO -ligands (each S=1), and a high spin Fe 2+ (S=2) antiferromagnetically coupled to an overall quartet 4 (NO)2 -ligand (S=3/2) [76].…”