Non-heme High-Spin {FeNO}^6–8 Complexes: One Ligand Platform Can Do It All

Abstract

Heme and non-heme iron-nitrosyl complexes are important intermediates in biology. While there are numerous examples of low-spin heme iron-nitrosyl complexes in different oxidation states, much less is known about high-spin (hs) non-heme iron-nitrosyls in oxidation states other than the formally ferrous NO adducts ({FeNO} in the Enemark-Feltham notation). In this study, we present a complete series of hs-{FeNO} complexes using the TMGtren coligand. Redox transformations from the hs-{FeNO} complex [Fe(TMGtren)(NO)] to its {FeNO} and {FeNO} analogs do not alter the coordination environment of the iron center, allowing for detailed comparisons between these species. Here, we present new MCD, NRVS, XANES/EXAFS, and Mössbauer data, demonstrating that these redox transformations are metal based, which allows us to access hs-Fe(II)-NO, Fe(III)-NO, and Fe(IV)-NO complexes. Vibrational data, analyzed by NCA, directly quantify changes in Fe-NO bonding along this series. Optical data allow for the identification of a "spectator" charge-transfer transition that, together with Mössbauer and XAS data, directly monitors the electronic changes of the Fe center. Using EXAFS, we are also able to provide structural data for all complexes. The magnetic properties of the complexes are further analyzed (from magnetic Mössbauer). The properties of our hs-{FeNO} complexes are then contrasted to corresponding, low-spin iron-nitrosyl complexes where redox transformations are generally NO centered. The hs-{FeNO} complex can further be protonated by weak acids, and the product of this reaction is characterized. Taken together, these results provide unprecedented insight into the properties of biologically relevant non-heme iron-nitrosyl complexes in three relevant oxidation states.

The Fe2(NO)2 Diamond Core: A Unique Structural Motif In Non‐Heme Iron–NO Chemistry

“…[14] Similar reactivity is observed for our new hs-{FeNO} 7 complex [Fe(BMPA-tBu 2 PhO)(NO)(OTf)] (3), which contains the weak field ligand [N-(3,5-di-tert-butyl-2-hydroxybenzyl)-N,N-di-(2-pyridylmethyl)]amine (BMPA-tBu 2 PhOH). Previous studies have shown fast DNIC formation following reduction of our hs-{FeNO} 8 model complex, [Fe(TMG 2 dien)(NO)] + .…”

“…Clearly,more work is necessary to elucidate the biologicallyrelevant reactivity of non-heme hs-{FeNO} 8 complexes. [14] Herein, we further investigated the reactivity of hs-{FeNO} 8 complexes with TPA( tris(2-pyridylmethyl)amine) and related coligands.I np articular, we report the serendipitous discovery of aF e 2 (NO) 2 diamond core structure,w hich is unprecedented in non-heme iron-NO chemistry.T his core structure is stabilized by ac hange in spin state of the iron centers to low-spin (ls) Fe II .Incontrast, aTPA derivative with aw eaker ligand field that cannot support the spin-state change to ls proceeds to DNIC formation. [10] In addition, DNICs with histidine ligation have been proposed to form at the non-heme diiron core of the ferric uptake regulation protein (Fur), [11] in serum albumin, [12] and in ferritin.…”

“…[14] Similar reactivity is observed for our new hs-{FeNO} 7 complex [Fe(BMPA-tBu 2 PhO)(NO)(OTf)] (3), which contains the weak field ligand [N-(3,5-di-tert-butyl-2-hydroxybenzyl)-N,N-di-(2-pyridylmethyl)]amine (BMPA-tBu 2 PhOH). [14] However,j ust as in the previous studies,n o intermediate of the process [following Eq. Nitrosylation of this yellow solid in THF using dried NO gas resulted in the formation of pure complex 3 (see Supporting Information).…”

An efficient, step-economical strategy for the design of functional metalloproteins

“…67, 69-71 We cannot rule out the possibility of X-ray induced reduction of the Fe center to generate an {FeNO} 8 species that would be anticipated to display such an acute Fe-N-O angle. 70,71 Alternatively, it is noteworthy that the NO ligand occupies a rather confined pocket defined by the Tyr70 side chains which may impose steric constraints on the ligand geometry.…”

Stepwise nitrosylation of the nonheme iron site in an engineered azurin and a molecular basis for nitric oxide signaling mediated by nonheme iron proteins

“…[24][25][26][27] Lehnert group reported the first series of high-spin {FeNO} [6][7][8] complexes and demonstrated that the redox transformation of high-spin {FeNO} [6][7][8] were all ironbased contrasted to corresponding low-spin complexes where redox transformation was generally NO centered. 28 Most recently, Meyer and coworkers synthesized an complete series of five {FeNO} [6][7][8][9][10] complexes with one ligand scaffold. 29 While significant progress has been made in the chemistry of nonheme iron and NO reaction, the transformation from {FeNO} 7 to {Fe(NO) 2 } 9 has been rarely reported.…”