of 38
Exploring the Limits of Dative Boratrane Bonding: Iron as a
Strong Lewis Base in Low-Valent Non-Heme Iron-Nitrosyl
Complexes
Hai T. Dong
1
,
Matthew J. Chalkley
2
,
Paul H. Oyala
2
,
Jiyong Zhao
3
,
E. Ercan Alp
3
,
Michael Y.
Hu
3
,
Jonas C. Peters
2,*
,
Nicolai Lehnert
1,*
1
Department of Chemistry and Department of Biophysics, University of Michigan, Ann Arbor,
Michigan 48109-1055, United States
2
Department of Chemistry and Chemical Engineering, California Institute of Technology,
Pasadena, California 91125, United States
3
Advanced Photon Source (APS), Argonne National Laboratory (ANL), Argonne, Illinois 60439,
United States
Abstract
We previously reported the synthesis and preliminary characterization of a unique series of low-
spin (ls) {FeNO}
8−10
complexes supported by an ambiphilic trisphosphineborane ligand,
[Fe(TPB)(NO)]
+/0/−
. Herein, we use advanced spectroscopic techniques and density functional
theory (DFT) calculations to extract detailed information as to how the bonding changes across the
redox series. We find that, despite the highly reduced nature of these complexes, they feature an
NO
+
ligand throughout with strong Fe-NO
π
-backbonding and essentially closed-shell electronic
structures of their FeNO units. This is enabled by an Fe-B interaction that is present throughout
the series. In particular, the most reduced [Fe(TPB)(NO)]
complex, an example of a ls-{FeNO}
10
species, features a true reverse dative Fe
B bond where the Fe center acts as a strong Lewis-base.
Hence, this complex is in fact electronically similar to the ls-{FeNO}
8
system, with two additional
electrons “stored” on site in an Fe-B single bond. The outlier in this series is the ls-{FeNO}
9
complex, due to spin polarization (quantified by pulse EPR spectroscopy), which weakens the Fe-
NO bond. These data are further contextualized by comparison with a related N
2
complex,
[Fe(TPB)(N
2
)]
, which is a key intermediate in Fe(TPB)-catalyzed N
2
fixation. Our present study
finds that the Fe
B interaction is key for storing the electrons needed to achieve a highly reduced
state in these systems, and highlights the pitfalls associated with using geometric parameters to try
to evaluate reverse dative interactions, a finding with broader implications to the study of transition
metal complexes with boratrane and related ligands.
Graphical Abstract
*
Corresponding Authors.
lehnertn@umich.edu, jpeters@caltech.edu.
Supporting Information.
Details of the pulse EPR simulations, coordinates of optimized molecules, and Table S1 can be found in the SI.
This material is available free of charge via the Internet at
http://pubs.acs.org
.
The authors declare no competing financial interest.
HHS Public Access
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Inorg Chem
. Author manuscript; available in PMC 2021 October 19.
Published in final edited form as:
Inorg Chem
. 2020 October 19; 59(20): 14967–14982. doi:10.1021/acs.inorgchem.0c01686.
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We use advanced spectroscopic methods and DFT calculations to interrogate the electronic
structure of our unique redox series of [Fe(TPB)(NO)]
+/0/−
complexes. We find that the Fe
B
interaction is key for storing the electrons needed to achieve a highly reduced state in these
systems. Comparison is further made to the related N
2
complex, [Fe(TPB)(N
2
)]
, which is a key
intermediate in Fe(TPB)-catalyzed N
2
fixation.
1. Introduction
Heme and non-heme iron-nitrosyl units are highly prevalent in biology, and (bio)inorganic
chemists have pondered their electronic structures and reactivity patterns for decades to
better understand these systems. In particular, heme-nitrosyls are relevant in NO sensing,
transport and as intermediates in nitrogen-cycle enzymes,
1
12
whereas non-heme iron
centers are particularly relevant in bacterial NO reductases (NORs).
13
16
Transition metal
nitrosyl (M-NO) complexes represent some of the earliest recognized examples of redox
non-innocence, leading to the development of the Enemark-Feltham notation, {MNO}
n
,
which classifies a M-NO complex by its total number of valence electrons
n
(= metal(d) +
NO(
π
*) electrons).
17
In this regard, NO can coordinate to metals in three different oxidation
states (NO
+/0/−
). In the case of non-heme iron enzymes, for example, binding of •NO to the
Fe(II) form generates high-spin (hs) {FeNO}
7
adducts, which, in general, have Fe
III
-NO
type electronic structures.
18
20
Due to their highly covalent Fe-NO bonds, these complexes
are usually stable and unreactive (with NORs being potential exceptions), but capable of
undergoing reduction at mild potentials to form very reactive hs-{FeNO}
8
complexes.
21
24
The latter species have been shown to undergo a number of different reactions, including N-
N coupling to form N
2
O,
25
27
disproportionation to form dinitrosyl iron complexes
(DNICs),
28
and protonation to generate HNO.
29
Recent findings by Balkus and coworkers
show that non-heme iron enzymes are also involved in biosynthetic pathways of natural
products containing the N-nitroso group, with a hs-{FeNO}
6
intermediate potentially
involved in this reaction.
30
Understanding how the electron distribution effects the reactivity
and stability of Fe-NO complexes is of critical importance to develop a better understanding
of their many roles both in signaling and energy-transducing reactions in biology.
Given the non-innocent nature of the •NO ligand, redox series of non-heme Fe-NO
complexes with the same ligand scaffold are of particular value for developing a better
understanding of the electronic structure of the M-NO unit. This is highlighted by two well-
studied examples of {FeNO}
6
8
complexes, a cyclam-supported low-spin (ls) system from
Wieghardt and coworkers
31
and a TMG
3
tren-supported hs system from Lehnert and
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coworkers,
24
which revealed significantly different electronic structures. The former is best-
described as Fe
II
coordinated to NO
+/0/−
in turn; whereas the latter is best described as
Fe
IV/III/II
antiferromagnetically coupled to
3
NO
. However, going beyond the {FeNO}
8
oxidation state has been a challenge in both hs and ls Fe-NO complexes, because it means
that either Fe(I) or NO
2−
species must be stabilized. More recently, two redox series that
expand the accessible Enemark-Feltham states for Fe have been reported. The first, from
Peters and coworkers, consisted of a ls-{FeNO}
8−10
redox series supported by a
trisphosphineborane ligand (TPB = tris[2-(di-iso-propylphosphino)phenyl]borane); see
Scheme 1).
32
These compounds, denoted [Fe(TPB)(NO)]
+/0/−
, are surprisingly stable and
could be characterized by X-ray crystallography. The only other series of mononitrosyl
complexes that reaches beyond the {FeNO}
8
state, [Fe(TIMEN
Mes
)(NO)]
2/+/+/0/−
, was
recently reported by Meyer and coworkers.
23
Therein, the hs-{FeNO}
7−9
redox states were
isolable while the putative {FeNO}
10
immediately undergoes NO insertion into the tris-
carbene supporting ligand.
In this study, we present a full spectroscopic and electronic structure analysis of the
[Fe(TPB)(NO)]
+/0/−
series. In these complexes, a second redox-active unit, namely a borane,
is positioned in close proximity to the metal by the ligand architecture. Ambiphilic ligands
that utilize Lewis base donors both to coordinate a metal center and position a Lewis acid
(LA) in its proximity have become increasingly popular in the past two decades.
33
35
However, given the constraints imposed by the ligand scaffolds used, evaluating the degree
M-LA bonding is often challenging.
Herein, we demonstrate the utility of force constants derived from quantum-chemistry
centered normal coordinate analysis (QCC-NCA) of nuclear resonance vibrational
spectroscopy (NRVS) data in deconvoluting the electronic structure and bonding at Fe in a
highly covalent ligand sphere comprised of nitrosyl, boratrane, and phosphine ligands. We
find that, despite their low formal Fe redox states, an NO
+
redox state with strong Fe-NO
π
-
bonds is maintained throughout the redox series. This is made possible because of the high
degree of structural and electronic flexibility in the TPB ligand, demonstrated via the
breaking of an
η
4
-BCCP donor interaction present in the most oxidized complex, and
formation of a reverse dative Fe
B bond in the most reduced complex. Similarly, a reverse
dative Fe
B bond has also been identified in the structurally related [Fe(TPB)(N
2
)]
complex by NRVS, underscoring the relevance of this interaction in promoting small
molecule functionalization (
i.e
., N
2
fixation).
36
These conclusions are corroborated by
continuous wave and pulse electron paramagnetic resonance spectroscopy (EPR) and density
functional theory (DFT) calculations.
2 Experimental Section
All complexes including
57
Fe complexes were prepared as previously reported and obtained
as pure compounds, as determined by Mössbauer and IR spectroscopy.
32
Efforts to label the
complexes with
15
NO were largely unsuccessful. However, trace amounts of the ls-{FeNO}
9
complex [Fe(TPB)(
15
NO)], sufficient for pulse EPR measurements, could be obtained via
reaction of [Fe(TPB)(N
2
)] with [TBA][
15
NO
2
] followed by extraction by pentane and
filtration through celite.
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NRVS measurements.
Nuclear resonance vibrational spectroscopy (NRVS) data were obtained as described
previously
3
at beamline 3-ID at the Advanced Photon Source (APS) at Argonne National
Laboratory. Samples were loaded in copper sample holders with lucite lids. During data
collection, samples were maintained at cryogenic temperatures using a liquid helium-cooled
cryostat. Spectra of solid samples were recorded from 0 to +90 meV in 0.25 meV steps.
Multiple scans were taken, normalized to the intensity of the incident beam, and added
together to achieve adequate signal to noise ratios; the final spectra represent averages
between 6 and 10 scans. The program Phoenix
4
was used to convert the raw NRVS data to
the vibrational density of states (VDOS).
Pulse EPR measurements for the ls-{FeNO}
9
complex.
All pulse X-band (
ν
9.7 GHz) EPR and electron nuclear double resonance (ENDOR)
experiments were performed using a Bruker (Billerica, MA) ELEXSYS E580 pulse EPR
spectrometer equipped with a Bruker MD-4 resonator. Temperature control for experiments
at 7 K was achieved using an ER 4118HV-CF5-L Flexline Cryogen-Free VT cryostat
manufactured by ColdEdge (Allentown, PA), while ENDOR experiments at 5 K were
performed using an Oxford Instruments CF935 helium flow cryostat. An Oxford Instruments
Mercury ITC was used for temperature regulation with both cryostats.
X-band Electron spin-echo detected field swept spectra (ESE-EPR) were acquired using the
2-pulse Hahn echo sequence
π
2
τ
π
τ
ec
o
, while the magnetic field was varied. The
“CW-EPR like” 1
st
derivative spectrum was generated by use of the pseudomodulation
function in EasySpin, an EPR simulation toolbox for use with Matlab.
37
,
38
Pulse X-band ENDOR was acquired using the Davies pulse sequence (
π
T
RF
π
RF
T
RF
π
/2 –
τ
π
− echo), where
T
RF
is the delay between mw pulses and RF pulses,
π
RF
is the
length of the RF pulse and the RF frequency is randomly sampled during each pulse
sequence.
X-band Hyperfine sublevel correlation (HYSCORE) spectra were acquired using the 4-pulse
sequence (
π
/2 −
τ
π
/2 −
t
1
π
t
2
π
/2 − echo), where
τ
is a fixed delay, while
t
1
and
t
2
are independently incremented by Δ
t
1
and Δ
t
2
, respectively. The time domain data was
baseline-corrected (third-order polynomial) to eliminate the exponential decay in the echo
intensity, apodized with a Hamming window function, zero-filled to eight-fold points, and
fast Fourier-transformed to yield the 2-dimensional frequency domain. The intensity of this
FT data was plotted as a series of contours on a logarithmic scale, in colors ranging from
blue to red in increasing intensity.
EPR Simulations.—
Simulations of all EPR data were achieved using the EasySpin
simulation toolbox (release 5.2.28) with Matlab 2019a.
37
For more details of these
simulations, we refer readers to the SI.
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DFT Calculations using Gaussian 09 and Normal Coordinate Analysis.
Geometry optimization of the ls-{FeNO}
8−10
complexes was carried out using the BP86 and
B3LYP functionals with the TZVP basis set, using both closed shell and broken symmetry
wavefunctions (see text). All calculations were performed using the program Gaussian 09.
39
Subsequent frequency calculations on the optimized structures show no imaginary
frequencies, indicating that true energy minima were obtained. The DFT-calculated force
constants in Cartesian coordinates were extracted from the Gaussian output files and
transformed into internal coordinates using a modified version of the program Redong.
Modified normal coordinate analysis (NCA) programs based on QCPE 576 were used for
the subsequent fitting of the experimental NRVS data. The fitting was performed by
adjusting a minimal set of force constants (in the spirit of the QCC-NCA approach)
40
to
reproduce the vibrations of the Fe-N-O units in the ls-{FeNO}
8−10
series of complexes (see
text).
DFT Calculations using ORCA 4.0.
The Gaussian-optimized structures of the ls-{FeNO}
8−10
complexes were used for following
single-point calculations (BP86/TZVP) with ORCA 4.0 to predict Mössbauer and EPR
parameters, and to further analyze the electronic structures of the complexes. This includes
the use of unrestricted corresponding orbitals (UCOs) for the ls-{FeNO}
9
complex.
41
3. Results and Analysis
3.1 Nuclear Resonance Vibrational Spectroscopy (NRVS) for the ls-{FeNO}8–10 Series
The Fe-NO bonding in the ls-{FeNO}
8−10
series is evaluated and analyzed herein based on
NRVS measurements (see Figure 1). NRVS is a vibrational technique that selectively detects
vibrations that involve the
57
Fe center, making it well-suited for the identification of Fe-
ligand stretching and bending modes. The experimental NRVS data of the ls-{FeNO}
8
complex reveal an intense band at 610 cm
−1
and weaker signals at 537 and 540 cm
−1
. The
feature at 610 cm
−1
is assigned to the FeNO stretch (see below), whereas those at 537 and
540 cm
−1
are in the correct range for Fe-N-O bending modes. With an Fe-NO stretch of 610
cm
−1
, this complex has one of the strongest transition metal-NO bonds observed to this date
and the strongest for an iron compound,
42
surpassing even ls-{FeNO}
6
complexes in hemes
(with typical Fe-NO stretching frequencies around 590 cm
−1
).
43
,
44
In IR spectroscopy, the
N-O stretch of this complex is observed at 1745 cm
−1
. The NRVS data of the ls-{FeNO}
10
complex are remarkably similar to those of the ls-{FeNO}
8
species described above. In
particular, its Fe-NO stretch is observed as the most intense signal at 602 cm
−1
, with the
weaker features at 525 and 543 cm
−1
again associated with Fe-N-O bending modes (see
Figure 1). The N-O bond of this complex is the weakest (and most activated) in the series,
with an N-O stretching frequency of 1568 cm
−1
as determined by IR spectroscopy.
The intense, high-energy NRVS feature of the ls-{FeNO}
9
species, observed at 583 cm
−1
, is
again assigned to the Fe-NO stretch. This mode is significantly shifted compared to 610 cm
−1
ν
= −27 cm
−1
) and 602 cm
−1
ν
= −19 cm
−1
) in the other two complexes,
respectively, which, as we will show below, is due to spin polarization. The Fe-N-O bending
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modes are similarly shifted as well (506 and 522 cm
−1
, see Figure 1). The N-O stretch of
this complex is located at 1667 cm
−1
.
In summary, comparison of the Fe-NO and N-O stretching frequencies along the ls-
{FeNO}
8−10
series does not reveal a consistent trend. In a simple
π
-backbonding model
(between the Fe-d and NO(
π
*) orbitals), we would anticipate that concomitant with the
observed stepwise weakening of the N-O bond along the ls-{FeNO}
8−10
series there would
be a stepwise strengthening of the FeNO bond. Instead, for the ls-{FeNO}
8/9
pair, both the
Fe-NO and N-O stretching frequencies (and bond strengths) decrease in the ls-{FeNO}
9
compound. This trend is then reversed in the ls-{FeNO}
9/10
pair (now showing a pattern that
would be in line with an increase in
π
-backbonding upon reduction), creating a discontinuity
in the observed behavior. Thus, it is clear that a more detailed analysis, one that considers all
available experimental data supported by electronic structure calculations, is necessary.
DFT Calibration for the ls-{FeNO}
8−10
Series.—
In our previous report, the ls-
{FeNO}
8
and ls-{FeNO}
10
complexes were described as closed shell systems, on the basis
of their diamagnetic ground states (from multinuclear nuclear magnetic resonance (NMR)
spectroscopy). Alternatively, diamagnetic ground states could also arise from strong
antiferromagnetic coupling between a hs iron center and a triplet NO
ligand, which is often
observed for non-heme Fe-NO complexes.
24
Furthermore, a recent interrogation of a related
redox series, [Fe(TPB)(NNMe
2
)]
+/0/−
, by experiment and theory revealed antiferromagnetic
coupling between the Fe center and a hydrazyl radical anion, [NNMe
2
]
•−
in some redox
states.
45
Therefore, we decided to re-evaluate whether the ground states of the [Fe(TPB)
(NO)]
+/0/−
complexes are best described by closed shell (CS) or broken-symmetry (BS)
wave functions. As in previous work, we applied both the gradient-corrected functional
BP86 and the hybrid functional B3LYP in combination now with the TZVP basis set for
these calculations.
46
50
While BP86 has previously been shown to be a reliable functional in
predicting geometric structures and spectroscopic properties of iron-nitrosyl complexes,
B3LYP tends to underestimate the covalency of the Fe-N-O moiety.
51
,
52
However, hybrid
functionals like B3LYP with a higher percentage of Hartree-Fock exchange often allow for
the geometry optimization of BS states in strongly spin-coupled systems, which is difficult
with gradient-corrected functionals like BP86.
To our surprise, the structural features derived from X-ray crystallography were well-
reproduced by
both the CS and BS state in B3LYP calculations on the ls-{FeNO}
8
complex
(see Table S1). For example, the N-O bond length only deviates by 0.01 Å for both states
(1.16/1.17/1.17 Å for exp/CS/BS). Similarly, the Fe-NO bond distance shows very good
agreement with the experimental data, with just 0.01–0.02 Å deviation for both states
(1.66/1.65/1.68 Å for exp/CS/BS). Both calculations show moderate agreement with the
experimental Fe-B bond distance (2.31/2.37/2.37 Å for exp/CS/BS). Finally, the BS state
shows better agreement with the experimental data for the Fe-N-O angle (176/172/175° for
exp/CS/BS). Thus, although purely structural comparisons do not distinguish between a CS
or BS electronic structure for the ls-{FeNO}
8
complex, the accuracy of the predicted NRVS
spectra is dramatically different, as shown in Figure 2. Whereas the predicted spectrum for
CS shows very good agreement with experiment, the BS calculation shows large deviations
from the experimental data (Fe-NO stretch: 610/654/490 cm
−1
for exp/CS/BS). Interestingly,
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the BS-predicted Fe-NO stretch at 490 cm
−1
is in line with experimentally determined Fe-
NO stretching frequencies in complexes featuring
3
NO
ligands,
24
,
53
,
54
suggesting that the
disagreement is not an artifact of the calculation. In summary, this result shows that the CS
wavefunction provides a better representation of the ground state electronic structure of the
ls-{FeNO}
8
complex, which differs from most other (trigonal-bipyramidal) non-heme iron-
NO complexes.
21
24
Comparing CS solutions calculated with both B3LYP and BP86, we find that the BP86
functional not only better reproduces the vibrational and structural data for the ls-
{FeNO}
8−10
series, but is also able to accurately predict the isomer shift (
δ
) and quadrupole
splitting (|Δ
eq
|) derived from Mössbauer spectroscopy and the hyperfine parameters derived
from pulse EPR spectroscopy (Table 1). Thus, we confirm that a CS, highly covalent
description of the ground state in the [Fe(TPB)(NO)]
+/0/−
complexes is most appropriate.
The BP86-optimized structures show very good agreement with the crystal structures of all
three compounds, as further demonstrated by the structural overlays in Figure 3. The ls-
{FeNO}
8
complex has a distinct distorted trigonal-bipyramidal geometry, where one of the
P-Fe-P angles in the trigonal plane is expanded to 154° allowing for an unusual
intramolecular
η
4
-BCCP interaction. Both of these features are well reproduced in the DFT
optimized structure. As the compound is reduced to the ls-{FeNO}
9
state, the complex
becomes more symmetric (closer to an actual trigonal-bipyramidal geometry), and the
unusually large P-Fe-P angle decreases from 154° to 126°. The ls-{FeNO}
10
complex is the
most symmetric with only about 1° difference between the three P-Fe-P angles.
The BP86 calculations reproduce the vibrational properties of the ls-{FeNO}
8−10
complexes, especially the Fe-NO and N-O stretching frequencies, quite well with respect to
experimental data (Figures S1). Importantly, the calculations capture the lack of a correlation
between the change in Fe-NO and N-O stretching frequencies along the ls-{FeNO}
8−10
series (see Table 1). Thus, we use these calculations as the basis to further analyze the NRVS
data and refine the force constants of the Fe-N-O units in the three complexes. In this way,
we further address the question of whether the reduction along the ls-{FeNO}
8−10
series is
metal- or NO-based.
QCC-NCA for the ls-{FeNO}
8−10
Series.—
To obtain simulations of the NRVS data of
the ls-{FeNO}
8−10
complexes and determine high-quality (experimental) force constants for
their Fe-N-O units, a quantum-chemistry centered normal coordinate analysis (QCC-NCA)
was performed.
45
,
55
This process allows us to correct the DFT-calculated force constants,
vibrational frequencies and NRVS intensities by fitting the experimental NRVS data, starting
from the DFT-predicted force field. In this way, we obtain high-quality force constants for
the modes of interest that afford detailed insight into the changes in Fe-NO and N-O
bonding along the ls-{FeNO}
8−10
series, independent of potential vibrational (mode)
mixing. In the spirit of the QCC-NCA approach,
55
only the small number of force constants
relevant to the Fe-N-O unit are varied, while the DFT-predicted force constants of the
[Fe(TPB)] frame are kept unchanged.
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For the ls-{FeNO}
8
complex, the Fe-NO force constant was corrected from the calculated
value of 4.95 to 4.53 mdyn/Å to fit the Fe-NO stretch at 610 cm
−1
(DFT-calculated value:
638 cm
−1
). Since the Fe-N-O unit is close to linear, the Fe-N-O unit has two linear bending
vibrations, which are assigned to the modes at 537 and 540 cm
−1
in the NRVS data, with
force constants of 0.41 and 0.57 mdyn•Å. The relatively high anisotropy of the two linear
bends is consistent with the strong deviation from trigonal symmetry in the FeP
3
plane. The
experimental N-O force constant of 12.5 mdyn/Å is close to the initial, DFT-calculated
value. Vibrational assignments are listed in Table 2, and the experimental and QCC-NCA
simulated NRVS data are compared in Figure 1. All force constants that were fit are listed in
Table 3.
The same process was applied to the ls-{FeNO}
9
and ls-{FeNO}
10
compounds, and the
resulting QCC-NCA simulated NRVS data are compared to experiment in Figure 1.
Vibrational assignments are provided in Table 2, and key force constants of the ls-
{FeNO}
8−10
series are listed in Tables 1 and 3. Reduction of the ls-{FeNO}
8
to the ls-
{FeNO}
9
complex causes both the FeNO and N-O bonds to become weaker (with force
constants decreasing from 4.53/12.5 to 4.15/11.3 mdyn/Å, respectively), confirming that this
unusual drop in both the Fe-NO and N-O stretching frequencies is not caused by unforeseen
mode mixing.
Whereas this trend is not in agreement with a simple change in
π
-backbonding, as discussed
above, this type of behavior actually resembles that observed for the hs-{FeNO}
7/8
complexes, [Fe(TMG
3
tren)(NO)]
2+/+
, where reduction leads to a decrease in
π
-donation
from the
3
NO
ligand to the hs-Fe center.
24
Reduction from the ls-{FeNO}
9
to the ls-
{FeNO}
10
state causes a further weakening of the N-O bond (N-O force constant: 11.3 vs
9.79 mdyn/Å), but at the same time, the Fe-NO bond now becomes stronger (Fe-NO force
constant: 4.15 to 4.45 mdyn/Å). This is opposite to the trend observed for the ls-{FeNO}
8/9
pair, but in agreement with the trends derived from the vibrational frequencies (see above).
A distinct Fe-B stretching mode is not observed in the experimental NRVS data. Because of
this, we were unable to optimize the corresponding Fe-B force constants via the QCC-NCA
process, and Table 3 lists the DFT-calculated Fe-B force constants. Nonetheless, the close
agreement between the DFT-predicted and the experimental force constants gives us
confidence that the Fe-B force constants are accurate (±10%).
In the ls-{FeNO}
8
and ls-{FeNO}
9
complexes, the Fe-B interaction is relatively weak, with
a calculated force constant of ~0.5 mdyn/Å. Reduction to the ls-{FeNO}
10
state then causes
a remarkable increase in the Fe-B bond strength, with the Fe-B force constant increasing to
1.56 mdyn/Å. The data thus suggest that an Fe-B single bond forms in the ls-{FeNO}
10
state
via a reverse dative bond with the Fe center serving as a Lewis base, donating a pair of
electrons to the borane Lewis acid. This clearly shows that
d
z
2
is doubly occupied in the ls-
{FeNO}
10
state. Relatedly, a dative B
Cu bond has previously been identified
computationally and spectroscopically in [Cu(TPB)]
,
56
and Fe-B flexibility has been
implicated as a key feature in stabilizing Fe across formal redox states.
57
,
58
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3.2 Pulse EPR Measurements of the ls-{FeNO}
9
Complex
The ls-{FeNO}
9
complex [Fe(TPB)(NO)] has an
S
t
= 1/2 ground state and is therefore EPR
active. As previously reported, this complex displays an axial EPR signal (
g
= 1.99, 1.99,
2.45; see Figure S4) with a large
g
z
value (2.45). This is consistent with the approximate
trigonal-bipyramidal geometry of the complex and an electronic structure in which the
electron hole is mostly located in the
xy
-plane (with the Fe-NO vector corresponding to the
z
-axis) and on the metal center (directly indicated by the large
g
shift). This leads to strong
2
nd
order spin-orbit coupling in the
z
direction. Indeed, similar axial EPR spectra with large
g
z
shifts have been measured for a number of TPB and P
3
Si
(features Si in place of B)
complexes with similar electronic structures (
i.e.
, e
g
3
ground states).
59
As these complexes
vary primarily in the identity of their axial ligand, information about that Fe-L interaction
can be extracted from the
g
-anisotropy. This is further analyzed in the Discussion section, in
direct comparison to the isoelectronic N
2
-adduct [Fe(TPB)(N
2
)]
.
Interestingly, if we include all (P
3
E
)Fe-L complexes (E = B in TPB, Si) with an e
g
3
ground
state for which an X-ray structure and EPR spectrum has been measured, we find a strong
linear correlation between Δ
g
z
and the Fe–P distance (R
2
= 0.92). This suggests that the
covalency of the Fe-P bond and/or the out-of-plane displacement of the Fe center might play
a key role in determining Δ
g
z
. Furthermore, we find that the Fe center in [Fe(TPB)(NO)] has
a Δ
g
z
that lies between those found for formally Fe
I
and Fe
III
ions in (P
3
E
)Fe-L complexes.
Given the vibrational and computational data are consistent with an NO
+
ligand state and
thus the Fe is formally Fe
−I
, this demonstrates the tremendous ability of a covalently bonded
NO
+
ligand to accept electron density.
Analysis of X-band hyperfine sublevel correlation (HYSCORE) spectroscopy acquired on
samples prepared with natural abundance (
14
N) and
15
N labeled NO bound (see Figure 4)
allowed us to accurately determine relatively weak hyperfine coupling constants to the
coordinated
14/15
N(O) and
11
B centers providing further insight into the electron spin
distribution in the complex. The observed coupling to the
14
N nucleus is largely axial
consistent with the axial g-tensor observed in the CW EPR measurements. Simulation of the
15
N HYSCORE data allowed for determination of the nitrogen hyperfine coupling tensor as
A(
15
N) = [8.4, 11.6, −5.4] MHz, independent of any influence from the nuclear quadrupole
interaction present in the natural abundance data due to the presence of the
I
= 1
14
N
nucleus. Accounting for the relative gyromagnetic ratios of
14/15
N (
γ
14
N/
γ
15
N = −0.7129)
the
14
N hyperfine coupling tensor is A(
14
N) = [−6.0, −8.3, 3.8] MHz, which can be
decomposed into an isotropic component
a
iso
(
14
N) = −3.5 MHz and an anisotropic
component of
T
(
14
N) = [−2.5, −4.8, 7.3] MHz. The small
a
iso
value indicates that minimal
spin (estimated: 0.002 e
) is an a
1
-type orbital (s or p
z
) with most spin (estimated: 0.065 e
)
in the e-symmetric p
x
and p
y
set. These results would be consistent with a spin polarization
mechanism that transfers electron density from the
d
xy
/d
x
2
−y
2
orbitals into the p
x
/p
y
orbitals
of the NO ligand. The total spin density of −0.07 e
on the N atom is consistent with the
DFT predictions for a CS state. Comparison of these hyperfine parameters with those
similarly extracted for [Fe(TPB)(NNMe
2
)]
+/−
further supports the CS rather than a BS
electronic ground state for the ls-{FeNO}
9
complex.
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Comparison of the HYSCORE data of the
14
N and
15
N isotopologues allows for accurate
determination of not only the hyperfine coupling constants, but also the
electric
interaction
of the
I
= 1
14
N nuclear quadrupole with the inhomogeneous electric field induced by
electron density in p-orbitals at the nucleus. This interaction is parameterized by the nuclear
quadrupole coupling constant (
e
2
qQ/h
= 0.8) and the electric field gradient (EFG)
asymmetry (
η
= 0). The low magnitude of
e
2
qQ/h
and negligible EFG rhombicity indicates
nearly spherical charge density about the nitrogen nucleus in this complex, in agreement
with the linear Fe-N-O unit and equal spin distribution in the p
x
and p
y
orbitals.
The hyperfine coupling to boron with A(
11
B) = [14.7, 14.7, 18.0] MHz can be decomposed
into
a
iso
(
11
B) = 15. 8 MHz and a small anisotropic contribution of
T
(
11
B) = [−1.1, −1.1, 2.2]
MHz. These data indicate that significantly less electron density is on that ligand (0.006 e
in a
1
type orbitals and 0.017 e
in e-type orbitals) and are consistent with the DFT results.
We interpret these results as being consistent with the lack of available orbitals of
appropriate symmetry to accept electron density from the xy-plane via spin polarization. X-
band ENDOR experiments to determine the hyperfine coupling to
31
P of the phosphine
ligands are best modeled with a single class of fairly isotropic coupling constants, A(
31
P) =
[82, 70, 70] MHz, which corresponds to
a
iso
(
31
P) = 74 MHz and an anisotropic component
of
T
(
31
P) = [8, −4, −4] MHz. The large hyperfine coupling to the
31
P centers again supports
the idea that the electron hole is mostly located in the
xy
-plane.
3.3 Electronic Structure Analysis
The ls-{FeNO}
8
Complex
has eight valence electrons, as indicated by the Enemark-Feltham
index, and, as discussed above, the complex has a closed-shell singlet ground state, which
means that of the total seven valence MOs (5 Fe(d) + 2 NO(
π
*) orbitals), four valence MOs
are doubly occupied, and three are empty. The MOs themselves are strongly mixed, and
Scheme 2 represents a simplified version of the bonding scheme. Here, the Fe-N-O unit
corresponds to the molecular
z
-axis. The strong distortion away from C
3
symmetry towards
a T-shaped geometry in the FeP
3
plane, characterized by a large P-Fe-P angle (154°), causes
a large energy splitting between the d
xy
and
d
x
2
−y
2
orbitals of 1.97 eV, as indicated in
Scheme 3. In this geometry, the lower energy orbital, d
xy
(HOMO-1), is essentially
σ
-
nonbonding with respect to the phosphine ligands (80% Fe character). Whereas, the lowest
unoccupied molecular orbital (LUMO), the empty
d
x
2
−y
2
orbital, shows strong antibonding
(
σ
*) interactions with the in-plane phosphine donors (see Scheme 3). Unexpectedly, the
d
x
2
−y
2
orbital also has a strong admixture of one of the NO(
π
*) orbitals (38% Fe, 14%
NO), but because the MO is unoccupied, it does not play a role for bonding in the ls-
{FeNO}
8
complex. This type of admixture, however, becomes relevant in the more reduced
species.
The highest occupied molecular orbital (HOMO) of the ls-{FeNO}
8
complex is the doubly-
occupied d
z
2 orbital, which has a notable contribution from the unoccupied boron(p)-orbital
(43% Fe, 12% B). This leads to a stabilization of the
d
z
2
orbital, which normally is the
highest energy orbital in a trigonal-bipyramidal coordination geometry. This weak Lewis
base (Fe) – Lewis acid (B) interaction (Fe-B force constant: 0.51 mdyn/Å) is indicative of a
fractional Fe-B bond order. Hence, despite the relatively short Fe-B distance (2.31 Å), the
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bonding between the doubly-occupied
d
z
2
orbital and the unoccupied boron(p)-orbital is
reduced by poor orbital overlap resulting from the tilt of the BC(Ph)
3
plane away from the
Fe-B axis.
The lowest-lying valence orbitals are the doubly-occupied, Fe-NO
π
-bonding combinations
of the d
xz
_
π
*
x
and d
yz
_
π
*
y
orbitals (HOMO-2 and HOMO-3). These bonds are very
covalent, with about 60% Fe(d) and 30% NO(
π
*) contribution.
Based on this analysis, and assigning MOs to the atom or group with the dominant charge
contribution, the ls-{FeNO}
8
complex can formally be assigned an Fe(0)-NO
+
type
electronic structure with all 8 valence electrons originating primarily from the Fe center, and
two very strong
π
-backbonds with the NO
+
ligand (consistent with the large Fe-NO force
constant of 4.53 mdyn/Å). The presence of an NO
+
ligand explains the absence of spin
polarization in this system. This is similar to six-coordinate ls-{FeNO}
6
complexes in
hemes, which have been shown to have a closed-shell Fe(II)-NO
+
type ground state with no
spin polarization.
48
,
60
In this sense, the FeNO unit in the ls-{FeNO}
8
complex could be
considered an electronic analog to that of heme ls-{FeNO}
6
complexes, where two
additional electrons of the Fe center are stabilized by the
d
z
2
−B(p)
interaction. This
becomes more evident in the ls-{FeNO}
10
system (see below).
Finally, the crystal structure of the ls-{FeNO}
8
complex reveals a unique
π
-bond between
the iron center and the C=C bond of one of the aromatic benzene rings. This interaction is
unique in the ls-{FeNO}
8
complex and explains the observed, significant contributions of
phenyl orbitals to the valence MOs in this complex, which complicates the analysis.
However, this interaction does not affect the FeNO moiety significantly.
The ls-{FeNO}
9
Complex
has an EPR-active
S
t
= 1/2 ground state, which provides
additional spectroscopic handles to further interrogate its ground state electronic structure.
Due to spin-polarization effects, the
α
- and
β
-spin covalencies differ in the ls-{FeNO}
9
complex, which complicates the analysis of its electronic structure. As we might expect
based on its more C
3
-symmetric structure, reduction of the ls-{FeNO}
8
complex results in
an orbital ordering more similar to that of a canonical trigonal bipyramid (see Scheme 3).
The SOMO of the ls-{FeNO}
9
complex is the
d
x
2
−y
2
orbital, as indicated in Scheme 4,
pointing towards an iron-based reduction (in agreement with the EPR results). Because of
this, the Fe-P covalency in the xy-plane is reduced, and the energy splitting between the
d
x
2
−y
2
and d
xy
orbitals decreases to 1.02 eV. Accordingly, the d
xy
orbital is now higher in
energy than the
d
z
2
orbital, and becomes the SOMO-1. The two lowest energy valence
orbitals remain the Fe-NO
π
-bonding interactions, which again correspond to the bonding
combinations of the d
xz
and d
yz
orbitals and the NO(
π
*
x/y
) orbitals. Finally, the d
z2
orbital is
again lowered in energy by the Fe-B interaction. Scheme 4 shows the resulting bonding
scheme of the ls-{FeNO}
9
complex, which points towards an unusual Fe(−I)-NO
+
type
ground state.
The experimental data show that the Fe-NO bond becomes weaker upon reduction of the ls-
{FeNO}
8
to the ls-{FeNO}
9
state, as reflected by a drop of the corresponding Fe-NO force
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constant from 4.53 to 4.15 mdyn/Å and of the Fe-NO stretch from 610 to 583 cm
−1
. This
indicates a reduction in the covalency of the two Fe-NO
π
-bonds in the ls-{FeNO}
9
state.
The DFT calculations underestimate the weakening of the Fe-NO stretch (Δ = −27 cm
−1
experimentally versus −17 cm
−1
by DFT) and the weakening of the N-O stretch (Δ = −78 cm
−1
experimentally versus −59 cm
−1
by DFT). Nonetheless, DFT captures the seemingly
counterintuitive trend that the Fe-NO and N-O bond both weaken upon reduction.
Due to spin polarization, both Fe-NO
π
-bonds are stronger and more covalent in
β
-spin
compared to
α
-spin, which manifests itself in the appearance of about −0.1 negative spin
density on the NO ligand, in the
π
*
x/y
orbitals. This finding is supported by the pulse EPR
measurements, showing weak, mostly anisotropic hyperfine coupling with the
14
N atom of
the coordinated NO ligand. Based on this finding alone, one would predict that the N-O
stretch should increase in energy in the reduced complex, but this is not the case
experimentally. The reason for the sharp drop in the N-O stretch from 1745 to 1667 cm
−1
upon reduction requires an increase in the occupation of the NO(
π
*
x/y
) orbitals in the
reduced complex, without increasing the Fe-NO bond strength. This in fact is the case. As
shown in Scheme 4, both the
d
x
2
−y
2
SOMO (41% Fe(d) and 4% NO character) and the
doubly-occupied d
xy
orbital (63% Fe(d) and 5% NO character) of the ls-{FeNO}
9
complex
show a distinct admixture of the NO(
π
*
x/y
) orbitals. Occupation of these MOs effectively
transfers electron density into the NO(
π
*
x/y
) orbitals, weakening the N-O bond, but
without
significantly affecting the Fe-NO bond strength. Although one might initially dismiss this
orbital interaction as an artefact of DFT, the available data show that this is a real effect.
Indeed, it is significant and likely underestimated in the DFT calculations, considering the
larger experimental shift in the N-O stretch (Δ = −78 cm
−1
) compared to Δ = −59 cm
−1
predicted by DFT. Using a linear scaling approach, we can roughly estimate from the N-O
stretches of free NO
+
(2387 cm
−1
) and NO (1876 cm
−1
; Δ
500 cm
−1
) that a shift in the N-
O stretch of ~80 cm
−1
requires an increase in the occupation of the NO(
π
*
x/y
) orbitals by
0.16 electrons (assuming similar electronic structures), which is slightly underestimated in
the calculations (Loewdin charges for NO: ls-{FeNO}
8
: +0.02; ls-{FeNO}
9
: −0.11, Δ(e
) =
0.13).
Further support for the importance of spin polarization effects to the bonding in the ls-
{FeNO}
9
complex is that the Fe-B interaction is predicted to be similarly polarized. Except
in this case the relevant ligand orbital is B(p
z
) with asymmetry in the
d
z
2
−B
p
z
interaction.
This bond is distinctively more covalent in
β
-spin (22% B(p
z
) admixture into
d
z
2
) compared
to
α
-spin (10% B(p
z
) contribution), again resulting in about −0.1 negative spin density on
the boron atom. This is supported by the pulse EPR measurements, showing weak hyperfine
coupling to the
11
B nucleus with a relatively larger component of its unpaired spin in an a
1
-
type (s or p
z
) orbital. The DFT calculations predict that the Fe-B bond interaction becomes
slightly weaker in the ls-{FeNO}
9
compared to the ls-{FeNO}
8
complex (due to the spin
polarization), although in the absence of any vibrational information, it is difficult to confirm
this. Therefore, we consider the Fe-B bond to be largely unchanged in the ls-{FeNO}
9
complex.
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Based on these observations, it is puzzling that despite the iron-based reduction in the ls-
{FeNO}
9
relative to the ls-{FeNO}
8
complex, both the experimental and DFT-calculated
Mössbauer isomer shifts only show a very small change (see Table 1). The main reason for
this finding is the fact that the occupation of the
d
x
2
−y
2
orbital in the ls-{FeNO}
9
complex
leads to the weakening of the Fe-P interactions, since the
d
x
2
−y
2
orbital is Fe-P antibonding.
This is reflected in the corresponding Fe-P force constants, which drop from an average
value of ~1.6 mdyn/Å to ~1.1 mdyn/Å upon reduction. This decrease in the Fe-P bonding
partially compensates for the electron that is added to the
d
x
2
−y
2
orbital, as does the transfer
of electron density from the
xy
-plane to the NO(
π
*) orbitals (see above). This “redox
buffering” causes a negligible change in the effective nuclear charge of the iron center upon
reduction, and minimizes the change in the Mössbauer isomer shift.
The ls-{FeNO}
10
Complex
is completely diamagnetic with a CS ground state, as shown in
Scheme 5. Compared to the ls-{FeNO}
9
complex, the extra electron is located in the
d
x
2
−y
2
orbital, completing the d
10
shell of the iron center. Therefore, once again, the reduction is
iron-centered. As a consequence of the now
d
xy
,d
x
2
−y
2
4
electron configuration, the ls-
{FeNO}
10
complex adapts an almost perfect trigonal symmetry of the FeP
3
unit, causing the
d
xy
and
d
x
2
−y
2
orbitals to form a degenerate set (Scheme 3).
In agreement with this analysis, both orbitals show identical charge contributions, with 62%
Fe(d) character and a 5% contribution from the NO(
π
*) orbitals. Likewise, the lowest lying
valence orbitals are also a now degenerate d
xz
/d
yz
pair. This pair shows 53% Fe(d) and 38%
NO(
π
*) contributions, indicating the presence of a very covalent Fe-NO bond, similar to
that in the ls-{FeNO}
8
complex (60% Fe and 30% NO). Indeed, the similar orbital
contributions of the corresponding d
xz
_
π
*
x
and d
yz
_
π
*
y
bonding pairs and the similar Fe-
NO force constants of 4.53 and 4.45 mdyn/Å are strongly suggestive of similar Fe-NO
bonding interactions in the ls-{FeNO}
8
and ls-{FeNO}
10
complexes. Nonetheless, the N-O
stretching frequency in the ls-{FeNO}
10
complex is 177 cm
−1
lower than in the ls-{FeNO}
8
complex, and the N-O force constant is reduced by about 2.7 mdyn/Å. As discussed for the
ls-{FeNO}
9
compound, this is best explained not by increased Fe-NO
π
-backbonding but
rather by the transfer of electron density from the xy-plane into the NO(
π
*) orbitals. Indeed,
in the ls-{FeNO}
10
complex, the
d
xy
/d
x
2
−y
2
pair contains 5% NO(
π
*) character each, as
indicated in Scheme 5. Once again, this likely represents a lower bound on the magnitude of
this effect, given the reduction in the N-O stretching frequency (Δ
exp
= −99 cm
−1
vs Δ
DFT
=
−85 cm
−1
compared to ls-{FeNO}
9
) is underestimated in the calculations.
Due to the formal d
10
configuration, the Fe center becomes unusually low-valent (Fe(−II)) in
the ls-{FeNO}
10
complex. However, this charge accumulation on the Fe center is largely
compensated by a dramatic strengthening of the Fe-B interaction, indicated by the increase
in the Fe-B force constant to 1.56 mdyn/Å, which corresponds to the formation of an Fe-B
σ
single bond. Here, the iron center becomes a Lewis base and donates one electron pair,
located in the doubly-occupied
d
z
2
orbital, to the boron center, which therefore functions as
a Lewis acid in the ls-{FeNO}
10
complex. This mechanism is key to the stabilization of the
ls-{FeNO}
10
system. Because of the formation of a full Fe-B single bond, the
d
z
2
orbital
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drops in energy after reduction and is now located significantly below the
d
xy
/d
x
2
−y
2
degenerate pair. Orbital analysis further reveals that the corresponding (bonding) MO has
35% Fe(d) and 23% B(p
z
) charge contributions (the rest is ligand contribution), in agreement
with a very covalent Fe-B interaction. Thus, the ls-{FeNO}
10
complex has an Fe(-II)-NO
+
type electronic structure, but with the electron pair in the
d
z
2
orbital being strongly stabilized
by donation to the boron Lewis acid. In this sense, the ls-{FeNO}
10
complex contains two
non-innocent ligands and could be designated as ls-{BFeNO}
10
. The NO ligand is more
activated due to the aforementioned admixture of NO(
π
*) character into the
d
xy
/d
x
2
−y
2
pair.
Curiously, the ls-{FeNO}
10
complex has the Fe center with the most positive effective
nuclear charge, based on the Mössbauer isomer shift. We attribute the positive isomer shift
of the complex relative to the ls-{FeNO}
9
system to (a) the newly formed Fe-B single bond,
which reduces the electron density on the Fe center, and (b) the onset of Fe-P backbonding.
Our observations emphasize the uniqueness of the TPB coligand scaffold and its ability to
stabilize extremely low-valent metal centers through an adjustable interaction between the
metal center and the empty p
z
orbital of boron. Surprisingly, the effect on the N-O bond
strength observed for the ls-{FeNO}
8
and ls-{FeNO}
10
pair is not so much due to changes
in the Fe-NO
π
-bond itself, but due to a secondary effect, i.e. the admixture of NO(
π
*)
character into the
d
x
2
−y
2
/d
xy
orbital pair as discussed above.
4. Discussion
In previously characterized redox series of Fe-NO complexes, Mössbauer spectroscopy has
been a key tool for understanding the redox state of the Fe center. In the cyclam-ac
supported Fe-NO series from Wieghardt and coworkers, the change in isomer shift (
δ
) across
redox states is linear (Δ
δ
~ 0.2 mm/s per redox state), which has been interpreted in terms of
NO-centered redox changes, dramatically affecting the Fe-NO bond and, in turn, the isomer
shift.
11
,
31
In the TMG
3
tren supported Fe-NO series from Lehnert and coworkers even larger
changes in the isomer shift are observed (Δ
δ
~ 0.4 mm/s per redox state), which, in
combination with other findings, was taken as evidence of Fe-centered redox changes.
24
,
61
More recently, Meyer’s hs-{FeNO}
7−9
series with the TIMEN
Mes
coligand has also been
shown to follow metal-centered reductions, with changes in isomer shift of Δ
δ
~ 0.2 mm/s.
23
A direct comparison between the [Fe(TPB)(NO)]
+/0/−
and the [Fe(TIMEN
Mes
)(NO)]
+/0/−
complexes in Figure 6 highlights the stark contrast in stability and reactivity of these low-
valent FeNO systems.
62
In addition, the FeNO redox series studied here presents a notable
difference to the previously reported examples in that the Mössbauer isomer shift does not
trend linearly with the redox state of the complex, and the complete range spans less than
0.1 mm/s (0.17–0.26).
32
This small overall range speaks to a consistent effective nuclear
charge at Fe across our redox series. Similar observations have been reported in a recent
study by Moore et al. on a bimetallic Fe-Ti system. In this case, redox-induced changes of
the effective neutral charge at Fe are buffered by the Lewis acidic Ti center. Thus, changes in
the covalency of the Fe-Ti interaction minimize changes in the isomer shift across the redox
series.
63
The main objective of this study was therefore to interrogate the electronic
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