Chapter 2. A Triad of Highly-Reduced, Linear Iron Nitrosyls: {Fe(NO)} 8-10
2.2 Results
2.2.2 Characterization and Electronic Structure of {Fe(NO)} 8-10
The infrared spectra of {(P3B)Fe(NO)}8-10 demonstrate an approximately 100 cm-1 decrease in the stretching frequency of the NO bond upon each successive reduction (Figure 2.3). This behaviour is reminiscent of that seen in transition metal complexes of π- accepting ligands such as N2 and CO.16,18 However, this behaviour 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 of Enemark-Feltham number.6,11 This observation suggested to us that the Fe–NO
linkage remains linear throughout the redox series described here, behaviour that is rare in redox series of metal nitrosyls.4,5 The crystal structures of {{(P3B)Fe(NO)}8-10 confirm that the Fe–N–O angle is highly linear in each complex: 175.8(3)° in {(P3B)Fe(NO)}8, 176.18(6)° in {(P3B)Fe(NO)}9, and 179.05(12)° in {(P3B)Fe(NO)}10.
Figure 2.3: IR spectra for {(P3B)Fe(NO)}8-10 highlighting the NO stretch.
The crystal structures19 (Figure 2.4) of {(P3B)Fe(NO)}8-10 further reveal that the only significant ligand rearrangement across the series is the presence of an intramolecular η4-BCCP interaction in {(P3B)Fe(NO)}8. Variable temperature 1H and 31P NMR experiments indicate that this interaction is maintained in solution. In contrast, both {(P3B)Fe(NO)}9 and {(P3B)Fe(NO)}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 (Figure 2.5), the Wiberg bond indices (Figure 2.5) find that the Fe–N bond order increases slightly from {(P3B)Fe(NO)}8 to {(P3B)Fe(NO)}10 with a concomitant decrease in the N–O bond order.20,21 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.20 Although the Fe–B bond
Figure 2.4: X-ray crystal structures of {(P3B)Fe(NO)}8-10 with hydrogens and counteranions omitted for clarity. Fe is orange, phosphorus is purple, nitrogen is pink, oxygen is red, boron is brown, and carbon is blue.
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, and suggests increased Fe–B bonding upon reduction. Finally, the Fe–P distances are significantly shorter in {(P3B)Fe(NO)}10 than in both {(P3B)Fe(NO)}8, and {(P3B)Fe(NO)}9, potentially suggestive of increased Fe–P backbonding in the most reduced species.
Figure 2.5: Experimental bond lengths from X-ray crystallography and Wiberg bond indices from DFT, along with a comparison of experimental and computational NO stretching frequencies.
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.6,11,13,15,17,22,23 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, as the decreasing ability of the NO ligand to accept electron density through backbonding leads the Fe-center to become more electron rich.6 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.22
The zero-field, 80 K Mössbauer spectra of {(P3B)Fe(NO)}8-10 (Figure 2.6) show only very minimal changes in the isomer shift as a function of the overall redox state. The linear and therefore strongly π-bonded NO ligand enforces low-spin configurations for all three redox states, leading to short metal–ligand bonds and correspondingly low isomer shifts.24 These low isomer shifts are consistent with the behavior we have observed in other highly-reduced (P3B)Fe complexes, related tris(phosphine)silyl supported Fe complexes, and a very recently reported series of linear {FeNO}6-8 with four N-heterocyclic carbene ligands.15,18,25,26 Even within this context, the {(P3B)Fe(NO)} 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. The nitrosyl ligand enhances the already strong covalency between the Fe and the P3B ligand, which we have previously noted leads to a
non-classical relationship between isomer shift and redox state in these species. Low- spin complexes of these types feature low isomer shifts regardless of formal oxidation state.26
Figure 2.6: Zero-field Mӧssbauer spectra of {(P3B)Fe(NO)}8-10 obtained as microcrystalline material suspended in a boron nitride matrix at 80 K. Data is shown as black dots and the simulation with the isomer shift (δ) and quadrupole splitting (Δeq) given below shown as a red line.
The electronic structures of metal nitrosyls are still debated,27 but linear NO complexes are most commonly described by a π-accepting NO+ resonance form. In Fe–
(NO+) complexes, the (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 NO−ligand.[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 (P3B)Fe scaffold.15,18,25,26 Relative to the (P3B)Fe(NO) complexes, the (P3B)Fe(N2) 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. For comparison, the Wiberg bond index calculation for [(P3B)Fe(N2)]− (isoelectronic to {(P3B)Fe(NO)}9) provides an Fe–B bond order of 0.5526 compared to 0.4402 and an Fe–N bond order of 0.9796 compared to 1.5958.
In contrast, the (P3B)Fe(NR) (R = adamantyl or 4-methoxyphenyl) 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. For comparison, the Wiberg bond index calculation for (P3B)Fe(NAd) (Ad = adamantyl) (isoelectronic to {(P3B)Fe(NO)}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. It therefore seems apparent that neither limiting scenario (Fe–(NO+) vs Fe–(NO−)) reliably describes the bonding situation observed in the {(P3B)Fe(NO)}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 {(P3B)Fe(NO)}9 (Figure 2.7). 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 in the cryogenic electronic spectrum of [Cr(CN)5(NO)]3−, characterized by Gray and coworkers in 1966. 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 this vibronic progression 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.28 Likewise, cooling of {(P3B)Fe(NO)}8 and {(P3B)Fe(NO)}10, which exhibit absorption features at a similar wavelength, does not lead to the emergence of any vibronic coupling. Due to the similarities in M–N–O angle, N–O distance, and ν(NO) of [Cr(CN)5(NO)]3− and {(P3B)Fe(NO)}9, and the observation that the calculated dxy-Fe–N π* gap (SOMO-LUMO gap) in {(P3B)Fe(NO)}9 is 520 nm, we also assign this absorption feature to a dxy-M–N π* transition.29 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 2.7: Density-corrected room temperature (red) and cryogenic (black) UV-Vis spectra of {(P3B)Fe(NO)}9 in 2-MeTHF highlighting the region that demonstrates vibronic coupling.
The solution magnetic susceptibility of {(P3B)Fe(NO)}9 is 1.7μB (C6D6, RT), consistent with an S = ½ species. The X-band EPR spectrum of {(P3B)Fe(NO)}9 (Figure 2.8) 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.30 Unrestricted DFT
Figure 2.8: CW X-band EPR spectrum of {(P3B)Fe(NO)}9 at 77 K in a 2-MeTHF glass (black) and its simulation (red). Simulation parameters: g = [2.50048, 1.99439, 1.96918]
and HStrain = [450.420, 159.384, 205.277].
calculations using BP86/def2-TZVPP (Fe, B, P, N, O) and 631-G(d) (C, H)31–35 reproduce the structure of {(P3B)Fe(NO)}9 well and support this description (Figure 2.9).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.27 Attempts to optimize such wavefunctions with BP86 led to their collapse back to the low-spin wavefunction.
Calculations using B3LYP were observed to converge broken symmetry wavefunctions, but
this functional predicted the ground states to be intermediate spin, rather than the observed low-spin.