Non-Heme Iron-Nitrosyl Complexes: Intermediates in Bacterial NO reductases

Two major types of NO Reductases have been identified in bacteria: NorBC which contains a unique heme/non-heme diiron active site, and flavodiiron proteins (FNOR), which utilize a more common non-heme/non-heme diiron core for catalysis. In general, two different approaches for NOR catalysis are possible: the enzymes could either bind just one molecule of NO and activate it, for example by reduction, for reaction with the second molecule of NO (mononuclear mechanism). Alternatively, the active site could bind two equivalents of NO, leading to a central dinitrosyl intermediate (dinuclear mechanism). Model complex studies are ideal to shed light on these issues by defining the fundamental chemistry that these different types of nitrosyl complexes could mediate. Model compounds can be designed to specifically mimic parts of an enzyme's active site, and to study the electronic structures and reactivities of otherwise labile intermediates, since reactions can be run at very low temperatures, and the conditions (presence of reductants and protons) can be ideally controlled.

Ferrous non-heme iron-nitrosyls in non-heme iron enzymes have previously mostly been studied as models for corresponding O2 complexes [1-3]. However, in order to further develop the mechanism of NorBC and to evaluate the different mechanistic proposals, deeper insight into the properties of non-heme iron-nitrosyls and corresponding, reduced species is now of key significance. Although a number of such non-heme {FeNO}7 model complexes (in the Enemark-Feltham notation) have been reported, the large majority of these compounds are low-spin (ls, S = 1/2) and hence, better model ferrous heme-NO adducts. In contrast, the FeB center of NorBC forms a high-spin (hs) {FeNO}7 complex (S = 3/2), as evident from EPR spectroscopy. We recently reported the first series of biomimetic model systems for the FeB(II)-NO adduct, [Fe(BMPA-Pr)(NO)]X (1-X; BMPA-Pr- = N-propanoate-N,N-bis-(2-pyridylmethyl)amine; X = Cl-, ClO4-, I-, OTf-) and studied the properties of these compounds using spectroscopy and DFT calculations. The TPA complex [Fe(TPA)(CH3CN)(NO)](ClO4)2 (2) was also synthesized for comparison. Elucidation of the exact coordination environment of the 1-X complexes was carried out by X-ray crystallography. Excitingly, the structure of 1-Cl reveals a geometry which is very similar to that of the non-heme FeB center of NorBC as shown in Figure 1, left.

Here, pyridine and amine groups replace the His ligands found in the protein. 1-Cl shows a bent Fe-N-O unit with an angle of 152o, and Fe-NO and N-O distances of 1.783 and 1.154 /Å, respectively. In comparison, crystals of both 1-ClO4 and 1-OTf show a surprising propensity of the complexes for the formation of unique metallacrown hexamers (Figure 1, right). Initial reactivity studies with five-coordinate (5C) [Fe(Porph)(NO)] (Porph2- = TPP2-, To-F2PP2-) and six-coordinate (6C) [Fe(To-F4PP-BzIM)(NO)] complexes showed no reactivity upon simple mixing of the heme- and non-heme iron-nitrosyls. Considering that these complexes are also unreactive towards additional NO gas, the frequently cited radical type N-N coupling mechanism for NorBC is in fact unlikely. Furthermore, the electronic structure of non-heme ferrous-nitrosyls, best described as hs Fe(III)-NO- species [1], does not support the proposed N-N radical type coupling mechanism. However, cyclic voltammetry (CV) studies show a surprisingly low reduction potential for complexes 1-X of only about -290 mV (vs. SHE) as shown in Figure 2, emphasizing the possibility for other, redox-mediated N-N coupling mechanisms.

In order to further investigate the properties of non-heme iron-NO complexes in different oxidation states, we then turned to the TMG3tren ligand platform. 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 (or {FeNO}7 in the Enemark-Feltham notation). We were able to prepare the complete series of hs-{Fe-NO}6-8 complexes using the TMG3tren coligand. Redox transformations from the hs-{FeNO}7 complex [Fe(TMG3tren)(NO)]2+ to its {FeNO}6 and {FeNO}8 analogs do not alter the coordination environment of the iron center, allowing for detailed comparisons between these species. We used X-ray crystallography and MCD, NRVS, XANES/EXAFS and Mössbauer spectroscopy, coupled to DFT calculations, to demonstrating that these redox transformations are all metal based, which allows us to access hs-Fe(II)-NO-, Fe(III)-NO- and Fe(IV)-NO- complexes. In contrast, in low-spin iron-nitrosyl complexes, all redox transformations are generally NO-centered. Figure 3 shows a comparison between our hs-{FeNO}6-8 series with the ls-{FeNO}6-8 complexes from Wieghardt's group, based on the cyclam-acetate ligand [4].

In the hs-{FeNO}6-8 species, the NO- ligand acts predominantly as a strong π-donor ligand. Here, the covalency of the Fe-NO bond increases as the iron-center is oxidized, as evident from an increase in the Fe-NO bond strength along the hs-{FeNO}8, hs-{FeNO}7 and hs-{FeNO}6 series. This is demonstrated by a stepwise increase in the Fe-NO stretching frequency from 435 to 484 to 594 cm-1 along this series. This means that upon reduction of the hs-{FeNO}7 to the hs-{FeNO}8 complex, the Fe-NO bond becomes weaker (and less covalent), concomitant with an increase of the radical character on the NO- ligand. In this way, the hs-{FeNO}8 complex is activated for N-N bond formation in the semireduced and superreduced mechanisms of flavodiiron NO reductases (FNORs). These results are therefore relevant for the ongoing effort to understand the mechanism of FNORs.

T. C. Berto, M. B. Hoffman, Y. Murata, K. B. Landenberger, E. E. Alp, J. Zhao, N. Lehnert
"Structural and Electronic Characterization of Non-Heme Fe(II)-Nitrosyls as Biomimetic Models of the FeB Center of Bacterial Nitric Oxide Reductase (NorBC)"
J. Am. Chem. Soc. 2011, 133, 16714-16717
(selected for Journal cover: Issue 42, October 26, 2011)

T. C. Berto, A. Speelman, S. Zheng, N. Lehnert
"Mono- and Dinuclear Non-Heme Iron-Nitrosyl Complexes: Models for Key Intermediates in Bacterial Nitric Oxide Reductases"
Coord. Chem. Rev. 2013, 257, 244-259

A. L. Speelman, N. Lehnert
"Characterization of a High-Spin Non-Heme {FeNO}8 Complex: Implications for the Reactivity of Iron Nitroxyl Species in Biology"
Angew. Chem. Int. Ed. 2013, 52, 12283-12287

A. L. Speelman, N. Lehnert
"Heme versus Non-Heme Iron-Nitroxyl {FeN(H)O}8 Complexes: Electronic Structure and Biologically Relevant Reactivity"
Acc. Chem. Res. 2014, 47, 1106-1116

A. L. Speelman, B. Zhang, A. Silakov, K. M. Skodje, E. E. Alp, J. Zhao, M. Y. Hu, E. Kim, C. Krebs, N. Lehnert
"Unusual Synthetic Pathway for an {Fe(NO)2}9 Dinitrosyl Iron Complex (DNIC) and Insight into DNIC Electronic Structure via Nuclear Resonance Vibrational Spectroscopy"
Inorg. Chem. 2016, 55, 5485-5501

A. L. Speelman, B. Zhang, C. Krebs, N. Lehnert
"Structural and Spectroscopic Characterization of a High-Spin {FeNO}6 Complex with an Iron(IV)-NO- Electronic Structure"
Angew. Chem. Int. Ed. 2016, 55, 6685-6688

A. L. Speelman, C. J. White, B. Zhang, E. E. Alp, J. Zhao, M. Hu, C. Krebs, J. Penner-Hahn, N. Lehnert
"Non-Heme High-Spin {FeNO}6-8 Complexes: One Ligand Platform Can Do It All"
J. Am. Chem. Soc. 2018, 140, 11341-11359

A. Banerjee, J. Li, A. L. Speelman, C. J. White, P. L. Pawlak, W. W. Brennessel, N. Lehnert, F. A. Chavez
"A Structural Model for the Iron-Nitrosyl Adduct of Gentisate Dioxygenase"
Eur. J. Inorg. Chem. 2018, 4797-4804

K. Fujisawa, S. Soma, H. Kurihara, A. Ohta, Y. Miyakawa, H. T. Dong, N. Lehnert
"Stable Ferrous Mononitroxyl {FeNO}8 Complex with a Hindered Hydrotris(pyrazolyl)borate Coligand: Structure, Spectroscopic Characterization, and Reactivity towards NO and O2"
Inorg. Chem. 2019, 58, 4059-4062

H. T. Dong, A. L. Speelman, C. E. Kozemchak, D. Sil, C. Krebs, N. Lehnert
"The Fe2(NO)2 Diamond Core: A Unique Structural Motif in Non-Heme Iron-NO Chemistry"
Angew. Chem. Int. Ed. 2019, 58, 17695-17699

N. Lehnert, K. Fujisawa, S. Camarena, H. T. Dong, C. J. White
"Activation of Non-Heme Iron-Nitrosyl Complexes: Turning up the Heat"
ACS Catal. 2019, 9, 10499-10518

H. T. Dong, M. J. Chalkley, P. H. Oyala, J. Zhao, E. E. Alp, M. Y. Hu, J. C. Peters, N. Lehnert
"Exploring the Limits of Dative Boratrane Bonding: Iron as a Strong Lewis Base in Low-Valent Non-Heme Iron-Nitrosyl Complexes"
Inorg. Chem. 2020, 59, 14967-14982

[1] Brown, C. A.; Pavlosky, M. A.; Westre, T. E.; Zhang, Y.; Hedman, B.; Hodgson, K. O.; Solomon, E. I. J. Am. Chem. Soc. 1995, 117, 715-732.
[2] Jackson, T. A.; Yikilmaz, E.; Miller, A.-F.; Brunold, T. C. J. Am. Chem. Soc. 2003, 125, 8348-8363.
[3] Diebold, A. R.; Brown-Marshall, C. D.; Neidig, M. L.; Brownlee, J. M.; Moran, G. R.; Solomon, E. I. J. Am. Chem. Soc. 2011, 133, 18148-18160.
[4] Serres, R. G.; Grapperhaus, C. A.; Bothe, E.; Bill, E.; Weyhermüller, T.; Neese, F.; Wieghardt, K. J. Am. Chem. Soc. 2004, 126, 5138-5153.