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Logo of nihpaAbout Author manuscriptsSubmit a manuscriptHHS Public Access; Author Manuscript; Accepted for publication in peer reviewed journal;
 
Angew Chem Int Ed Engl. Author manuscript; available in PMC 2017 September 19.
Published in final edited form as:
PMCID: PMC5079689
NIHMSID: NIHMS824008

A Triad of Highly-Reduced, Linear Iron Nitrosyls: {FeNO}8–10

Abstract

Given the importance of Fe–NO complexes in both human biology and the global nitrogen cycle, there has been interest in understanding their diverse electronic structures. Herein we describe an interesting redox series of isolable iron nitrosyl complexes stabilized by a tris(phosphine)borane (TPB) ligand. These structurally characterized iron nitrosyl complexes reside in the following highly reduced Enemark-Feltham numbers: {FeNO}8, {FeNO}9, and {FeNO}10. These {FeNO}8–10 compounds are each low-spin, and feature linear yet strongly activated nitric oxide ligands. Use of Mössbauer, EPR, NMR, UV/Vis, and IR spectroscopies, in conjunction with DFT calculations, provide insight into the electronic structures of this uncommon redox series of iron nitrosyls. In particular, the data collectively suggest that {TPBFeNO}8–10 are all remarkably covalent. This covalency is likely responsible for the stability of this system across three highly reduced redox states that correlate with unusually high Enemark-Feltham numbers.

Keywords: Iron, Nitric Oxide, Nitrosyl, Boratrane, Metal-Ligand Covalency

Graphical Abstract

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Model iron-nitrosyl complexes have been well-studied due to their biological significance and fascinating underlying electronic structures. However, species with high Enemark-Feltham numbers (8–10) remain rare. Herein we report an unusual redox series of {FeNO}8–10 complexes supported by a tris(phosphine)borane. These species maintain a linear Fe–N–O angle throughout the series; the underlying reasons for this atypical behavior have been investigated spectroscopically.

Due to the prevalence of both heme and non-heme iron nitrosyls in biology,[1] iron model complexes bearing nitrosyl ligands have been the subject of study for decades.[2] Many examples of {FeNO}6 and {FeNO}7 complexes are known[3] and, more recently, several {FeNO}8 complexes have been thoroughly characterized.[45] However, {FeNO}9 complexes are unknown, and the unique properties of the only known example of an {FeNO}10 complex, [Fe(CO)3NO], prompted its reinvestigation in 2014 by Plietker and coworkers.[6] The activated NO stretching frequency (1647 cm−1) and long N–O bond (1.212 Å) but linear Fe–N–O angle (180°) observed in this latter complex stand in contrast to the iron nitrosyl literature, where strong activation of the coordinated NO unit leads to a bent geometry.[4] Therefore, we sought to investigate the Fe–NO unit under local three-fold symmetry on a ligand platform that would engender strong Fe–N multiple bonding while also providing redox flexibility. With this in mind, we pursued the synthesis of a series of Fe–NO complexes supported by a tris(phosphine)borane (TPB) ligand (Scheme 1).[7] Herein we report the synthesis of a monoiron-mononitrosyl complex that has been crystallographically characterized across three oxidation states, including highly unusual examples of {FeNO}9 and {FeNO}10 complexes. These nitrosyl complexes are distinct in their retention of a strictly linear Fe–N–O unit across the series; a high degree of covalency factilitates this atypical structural behavior.[8]

Scheme 1
Synthesis of {TPBFeNO}8–10.

In our exploration of Fe–N2 chemistry, the TPB ligand has demonstrated the ability to support strong Fe–N π-bonding, while geometric (and electronic) flexibility of the Fe–B interaction allows for stabilization of highly reduced Fe species. Given that [TPBFeN2] has an irreversible oxidation event that causes dissociation of the N2 ligand to generate [TPBFe]+, we hypothesized that nitrosonium hexafluorophosphate ([NO][PF6]) could serve both to oxidize [TPBFeN2] and to act as a source of in situ nitric oxide to bind Fe.[9] Successful isolation of the {TPBFeNO}8 cation as its BArF4 salt (after salt metathesis with NaBArF4; BArF4 = B(3,5-(CF3)2-C6H3)4) proved viable (Scheme 1). The cyclic voltammetry of {TPBFeNO}8 (Figure 1) demonstrated both a quasi-reversible reduction at −0.56 V vs. Fc/Fc+ and a second, fully reversible feature at −1.97 V vs. Fc/Fc+. We attribute these redox features to the {FeNO}8/9 and {FeNO}9/10 couples, respectively. Synthetic generation of neutral {TPBFeNO}9 via cobaltocene reduction of {TPBFeNO}8, and anionic {TPBFeNO}10 by Na/Hg reduction of {TPBFeNO}9, provided the desired {TPBFeNO}8–10 series (Scheme 1). In contrast to most other reports of highly reduced Fe–NO complexes,[4,13b] these three species are stable both in solution and in the solid state at room temperature under an inert atmosphere.

Figure 1
(top) Cyclic voltammetry of {TPBFeNO}8 in a THF solution of 0.1 M [TBA][PF6] under argon at 100 mV sec−1. (bottom) Thin-film IR spectra of {TPBFeNO}8–10 highlighting their ν(NO) stretching frequencies.

The infrared spectra of {TPBFeNO}8–10 demonstrate an approximately 100 cm−1 decrease in the stretching frequency of the NO bond upon each successive reduction (Figure 1). This behavior is reminiscent of that seen in transition metal complexes of π-accepting ligands such as N2 and CO.[910] However, this behavior stands in contrast to that of most previously characterized iron nitrosyls, which more typically show much larger changes (between 200–350 cm−1) in the NO stretching frequency per unit change in their Enemark-Feltham number.[4a,4f] This observation suggested to us that the Fe–NO linkage remains linear throughout the redox series described here, behavior that is rare in redox series of metal nitrosyls.[3] The crystal structures of {TPBFeNO}8–10 confirm that the Fe–N–O angle is highly linear in each complex: 175.8(3)° in {TPBFeNO}8, 176.18(6)° in {TPBFeNO}9, and 179.05(12)° in {TPBFeNO}10.

The crystal structures[11] (Figure 2) of {TPBFeNO}8–10 further reveal that the only significant ligand rearrangement across the series is the presence of an intramolecular η4-BCCP interaction in {TPBFeNO}8. Variable temperature 1H and 31P NMR experiments indicate that this interaction is maintained in solution (Figure S1–S4). In contrast, both {TPBFeNO}9 and {TPBFeNO}10 demonstrate approximate three-fold symmetry both in solution and in the solid state. Although Fe–N–O linearity is maintained, the N–O bond does lengthen about 0.03 Å upon each reduction, in agreement with the activation observed by IR spectroscopy. Although the Fe–N bond distance remains fairly constant (Table S3), the Wiberg bond indices (Table S27) find that the Fe–N bond order increases slightly from {TPBFeNO}8 to {TPBFeNO}10 with a concomitant decrease in the N–O bond order.[12] In addition to Fe–N bonding, the Wiberg Bond Index finds significant Fe–O bonding (bond order of ~0.5). This through bonding interaction has been previously interpreted as indicative of a highly covalent interaction.[12a] Although the Fe–B bond distance increases with reduction, suggestive of a weakening Fe–B interaction, the boron does become more pyramidalized, which agrees with both the DFT calculations (vide infra) and the 11B NMR spectra (Figure S5, S12), and suggests increased Fe–B bonding upon reduction. Finally, the Fe–P distances are significantly shorter in {TPBFeNO}10 than in both {TPBFeNO}8, and {TPBFeNO}9, potentially suggestive of increased Fe–P backbonding in the most reduced species.

Figure 2
(top) X-ray crystal structures of {TPBFeNO}8 (left), {TPBFeNO}9 (middle), and {TPBFeNO}10 (right). Thermal ellipsoids are set at 50% probability. Hydrogen atoms, solvent molecules, and counterions are omitted for clarity. The η4-BCCP interaction ...

Mössbauer spectroscopy has been frequently employed in the study of iron nitrosyl complexes as an experimental probe of the relative state of oxidation of the iron center.[4a,4f,6b,8,13] Typically octahedral {FeNO}6 complexes have isomer shifts between 0.0 and 0.05 mm s−1, {FeNO}7 complexes have isomer shifts between 0.25 and 0.33 mm s−1 and {FeNO}8 complexes have isomer shifts between 0.4 and 0.5 mm s−1. These significant changes in the isomer shift occur despite NO-centered reduction due to the decreasing ability of the NO ligand to accept electron density through backbonding.[4a] For the high-spin, pseudo-C3 {FeNO}6–8 recently reported by Lehnert and coworkers, even larger changes in isomer shift (~0.4 mm s−1) per unit reduction are observed. These authors suggest that such a large shift is indicative of metal-centered reduction.[13a]

The zero-field, 80 K Mössbauer spectra of {TPBFeNO}8–10 (Figure 2) show only very minimal changes in the isomer shift as a function of the overall redox state. The linear and therefore strongly π-bound NO ligand enforces low-spin configurations for all three redox states, leading to short metal–ligand bonds and correspondingly low isomer shifts.[15] These low isomer shifts are consistent with the behavior we have observed in other highly-reduced TPBFe complexes, related tris(phosphine)silyl supported Fe complexes, and a very recently reported series of linear {FeNO}6–8 in tetragonal symmetry.[8,16] Even within this context, the {TPBFeNO} system is remarkable for the minimal change in isomer shift observed across the redox series; this suggests a high degree of metal–ligand covalency that buffers against any buildup of electron density on the iron center upon successive reductions.[17]

The electronic structures of metal nitrosyls are still debated,[18] but linear NO complexes are most commonly described by a π-accepting NO+ resonance form. In Fe–(NO+) complexes, the ν(NO) stretching frequency is typically found between 1900 and 2000 cm−1.[19] There are also cases where a linear nitrosyl is considered to be a π-donating NOligand.[4e,6b] These two limiting resonance forms indicate formal charge transfer either from the NO to the metal or from the metal to the NO. To help determine which, if either, of these limiting cases more accurately describes the complexes featured herein, we draw comparisons to the known dinitrogen (N2 is isolobal to NO+) and imido (NR is isolobal to NO) complexes of the TPBFe scaffold.[8,16b] Relative to the {TPBFeNO} complexes, the TPBFeN2 complexes have longer Fe–N (~1.78 Å) distances and shorter Fe–B distances (~1.3 Å), suggesting a weaker π-interaction between the Fe and the N2 ligand and more σ-backdonation into the borane.[20] In contrast the TPBFe(NR) complexes have similarly short Fe–N (1.66 Å) distances but much longer Fe–B (2.6–2.8 Å) distances, arising from a distortion to a more tetrahedral symmetry at iron that further enhances Fe–N π-bonding, and a more electron poor Fe center with minimal backdonation into the borane.[21] It therefore seems apparent that neither limiting scenario (Fe–(NO+) vs Fe–(NO)) reliably describes the bonding situation observed in the {TPBFeNO}8–10 series. A description of the entire ligand sphere about the iron center as covalent seems more appropriate than descriptions that imply significant charge transfer.

This covalent description is further supported by the cryogenic-temperature (−180 °C) UV-vis spectrum of {TPBFeNO}9 (Figure 3). Upon cooling, a vibronic progression with spacing of 452, 457, 476, 509, 499 cm−1 emerges on the electronic transition centered at 521 nm. To our knowledge, a related vibronic progression has been observed in only one other M–NO system, characterized by Gray and coworkers in 1966 in the cryogenic electronic spectrum of [Cr(CN)5(NO)]3−. In that case, the electronic transition featuring the vibronic progression was centered at 470 nm and was attributed to a transition from a metal-based dxy or dx2-y2 orbital into a Cr–N π* orbital. Although this previous study investigated a series of isoelectronic, pentacyano metal (M = V, Cr, Mn, Fe) nitrosyl complexes, only [Cr(CN)5(NO)]3− revealed vibronic coupling upon cooling. The orbital contribution from Cr and NO π* to the Cr–N π* orbital was deduced to be nearly equal, leading the authors to posit that this might be requisite for the observation of vibronic coupling.[23] Likewise, cooling of {TPBFeNO}8 and {TPBFeNO}10, which exhibit absorption features at a similar wavelength, does not lead to the emergence of any vibronic coupling (Figure S17–18). Due to the similarities in M–N–O angle, N–O distance, and ν(NO) (Table S1) of [Cr(CN)5(NO)]3− and {TPBFeNO}9, and the observation that the calculated dxy-Fe–N π* gap (SOMO-LUMO gap) in {TPBFeNO}9 is 520 nm, we also assign this absorption feature to a dxy-M–N π* transition.[24] Based on our own observations, those of Gray and coworkers, and the paucity of metal–NO complexes demonstrating such vibronic coupling, it appears that vibronic coupling of this type is only (albeit not necessarily) observed in highly covalent, linear M–NO units.

Figure 3
Density-corrected[22] room temperature (solution) and cryogenic (frozen glass) UV/Vis spectra of {TPBFeNO}9 in 2-MeTHF highlighting the region that demonstrates vibronic coupling.

The solution magnetic susceptibility of {TPBFeNO}9 is 1.7μB (C6D6, RT), consistent with an S = ½ species. The X-band EPR spectrum of {TPBFeNO}9 (Figure S33) shows a nearly axial signal with significant g anisotropy and broad features. We favor an electronic structure consistent with a description in which the SOMO consists of an iron d-orbital that is primarily of dxy or dx2-y2 parentage, akin to ferrocenium.[25] Unrestricted DFT calculations using BP86/def2-TZVVP (Fe, B, P, N, O); and 631-G(d) (C, H)[26] reproduce the structure of {TPBFeNO}9 well and support this description.[27]

The collective data presented here lend support to an electronic description of {TPBFeNO}8–10 in which reduction of the system across the Enemark-Feltham numbers from 8 to 9 and 9 to 10 occurs in primarily Fe-based orbitals that are orthogonal to both the Fe–NO π-bonds and the Fe–B σ-bond. However, as the Fe becomes more electron rich, backbonding into the NO, σ - backbonding into the borane, and presumably π-backbonding into the phosphines all become stronger leading to very little overall change in the electron density at the Fe-center (i.e. its relative state of oxidation.

This high degree of covalency, particularly in the Fe–NO bond but also between the Fe and the TPB ligand, leads to the atypical maintenance of a linear Fe–N–O geometry upon successive reductions, and appears to be key to the ability of the TPB ligand to support an Fe–NO unit across three redox states. Finally, trigonal symmetry, rather than tetragonal symmetry, gives rise to an additional non-bonding orbital, providing access to unusually high Enemark-Feltham numbers (8, 9, and 10).

Figure 4
A molecular orbital diagram for {TPBFeNO}9. The energies given are relative to the SOMO which was set to be 0 eV. The molecular orbitals depicted (Isovalue = 0.05) are from the α-spin manifold as are the energies. The energy and appearance of ...

Supplementary Material

Supplemental Information

Acknowledgments

This research was supported by the NIH (GM-070757) and an NSF Graduate Research Fellowship to MJC. We thank Larry Henling and Dr. Michael K. Takase for crystallographic assistance. We acknowledge Dr. Gaël Ung for preliminary data on {TPBFeNO}8 complex and helpful discussions.

Footnotes

Supporting information for this article is given via a link at the end of the document.

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21. For comparison, the Wiberg bond index calculation for [TPBFeNAd]+ (isoelectronic to {TPBFeNO}9) provides an Fe–B bond order of 0.2718 compared to 0.4402 and an Fe–N bond order of 1.7980 compared to 1.5958.
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27. Broken symmetry calculations, in which an S = 1 NO is antiferromagnetically coupled to a metal center are often used in the case of linear M–NO complexes. Attempts to optimize such wavefunctions with BP86 led to their collapse back to the low-spin wavefunction. Further DFT discussion including low-spin and broken symmetry calculations with B3LYP are discussed in the supporting information.