Electron Paramagnetic Resonance Signature of Tetragonal Low Spin Iron(V)-Nitrido and-Oxo Complexes Derived from the Electronic Structure Analysis of Heme and Non-Heme Archetypes

Article abstract of DOI:10.1021/jacs.8b11429

Iron(V)-nitrido and-oxo complexes have been proposed as key intermediates in a diverse array of chemical transformations. Herein we present a detailed electronic-structure analysis of [Fe^V(N)(TPP)] (1, TPP^2- = tetraphenylporphyrinato), and [Fe^V(N)(cyclam-ac)]⁺ (2, cyclam-ac = 1,4,8,11-tetraazacyclotetradecane-1-acetato) using electron paramagnetic resonance (EPR) and ⁵⁷Fe M?ssbauer spectroscopy coupled with wave function based complete active-space self-consistent field (CASSCF) calculations. The findings were compared with all other well-characterized genuine iron(V)-nitrido and-oxo complexes, [Fe^V(N)(MePy₂tacn)](PF₆)₂ (3, MePy₂tacn = methyl-N′,N″-bis(2-picolyl)-1,4,7-triazacyclononane), [Fe^V(N){PhB(t-BuIm)₃}]⁺ (4, PhB(^tBuIm)₃^- = phenyltris(3-tert-butylimidazol-2-ylidene)borate), and [Fe^V(O)(TAML)]^a' (5, TAML^4- = tetraamido macrocyclic ligand). Our results revealed that complex 1 is an authenticated iron(V)-nitrido species and contrasts with its oxo congener, compound I, which contains a ferryl unit interacting with a porphyrin radical. More importantly, tetragonal iron(V)-nitrido and-oxo complexes 1-3 and 5 all possess an orbitally nearly doubly degenerate S = 1/2 ground state. Consequently, analogous near-axial EPR spectra with g_|| < g_a?1 ≤ 2 were measured for them, and their g_|| and g_a?1 values were found to obey a simple relation of ga?1 2 + (2-g_a1)² = 4. However, the bonding situation for trigonal iron(V)-nitrido complex 4 is completely different as evidenced by its distinct EPR spectrum with g_|| < 2 < g_a?1. Further in-depth analyses suggested that tetragonal low spin iron(V)-nitrido and-oxo complexes feature electronic structures akin to those found for complexes 1-3 and 5. Therefore, the characteristic EPR signals determined for 1-3 and 5 can be used as a spectroscopic marker to identify such highly reactive intermediates in catalytic processes.

Full text of DOI:10.1021/jacs.8b11429

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Article
EPR Signature of Tetragonal Low Spin Iron(V)-Nitrido
and -Oxo Complexes Derived from the Electronic
Structure Analysis of Heme and Non-Heme Archetypes
Hao-Ching Chang, Bhaskar Mondal, Huayi Fang, Frank Neese, Eckhard Bill, and Shengfa Ye
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b11429 • Publication Date (Web): 08 Jan 2019
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EPR Signature of Tetragonal Low Spin Iron(V)-Nitrido and -Oxo
Complexes Derived from the Electronic Structure Analysis of Heme and
Non-Heme Archetypes
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Hao-Ching Chang,†‡ Bhaskar Mondal,§‡ Huayi Fang,§& Frank Neese,† Eckhard Bill*§ and Shengfa Ye*†
†Max-Planck-Institut für Kohlenforschung, Kaiser-Wilhelm-Platz 1, D-45470 Mülheim an der Ruhr, Germany
§Max-Planck-Institut für Chemische Energiekonversion, Stiftstr. 34-36, D-45470 Mülheim an der Ruhr, Germany
ABSTRACT: Iron(V)-nitrido and -oxo complexes have been proposed as key intermediates in a diverse array of chemical
transformations. Herein we present a detailed electronic-structure analysis of [FeV(N)(TPP)] (1, TPP2– = tetraphenylporphyrinato),
and [FeV(N)(cyclam-ac)]+ (2, cyclam-ac = 1,4,8,11-tetraazacyclotetradecane-1-acetato) using electron paramagnetic resonance
(EPR) and 57Fe Mössbauer spectroscopy coupled with wavefunction based complete active-space self-consistentꢀfield (CASSCF)
calculations. The findings were compared with all other well-characterized genuine iron(V)-nitrido and -oxo complexes,
[FeV(N)(MePy2tacn)](PF6)2 (3, MePy2tacn = methyl-N',N''-bis(2-picolyl)-1,4,7-triazacyclononane), [FeV(N){PhB(t-BuIm)3}]+ (4,
−
PhB(tBuIm)3 = phenyltris(3-tert-butylimidazol-2-ylidene)borate), and [FeV(O)(TAML)]− (5, TAML4− = tetraamido macrocyclic
ligand). Our results revealed that complex 1 is an authenticated iron(V)-nitrido species and contrasts with its oxo congener,
compound I, which contains a ferryl unit interacting with a porphyrin radical. More importantly, tetragonal iron(V)-nitrido and -oxo
complexes 1–3 and 5 all possess an orbitally nearly doubly degenerate S = 1/2 ground state. Consequently, analogous near-axial
EPR spectra with g|| < g⊥ < 2 were measured for them, and their g|| and g⊥ values were found to obey a simple relation of
. However, the bonding situation for trigonal iron(V)-nitrido complex 4 is completely different as evidenced by
its distinct EPR spectrum with g|| < 2 < g⊥. Further in-depth analyses suggested that tetragonal low spin iron(V)-nitrido and -oxo
complexes feature electronic structures akin to those found for complexes 1–3 and 5. Therefore, the characteristic EPR signals
determined for 1–3 and 5 can be used as a spectroscopic marker to identify such highly reactive intermediates in catalytic processes.
coupled, thus yielding an overall doublet ground state (Stot
=
INTRODUCTION
1/2).7 However, model complexes of compound I all feature an
Stot = 3/2 ground state due to moderately strong ferromagnetic
coupling.8
High-valent iron complexes featuring oxo (O2–) or nitrido
(N3–) coordination are invoked as key intermediates in O2 and
N2 activation processes. In biology, several nonheme
iron(IV)-oxo intermediates have been trapped in the reactions
of a series of O2-activating iron enzymes, and were thoroughly
characterized by absorption, resonance Raman (rR), and 57Fe
Mössbauer spectroscopy.2 In parallel, synthetic chemists have
1
Chart 1. Compound I and Compound II in Heme
Containing Enzymes
O–
O–
O
O
O
O
prepared dozens of non-heme iron(IV)-oxo models in order to
N
N
N
N
N
N
N
N
FeIV
FeIV
3
understand their structure-function relation.
Perferryl
complexes have also been proposed in the chemistry of non-
heme iron enzymes.4 Compound I, formally an iron(V)-oxo
heme species, is pivotal intermediate of many heme-
containing oxygenases and peroxidases (e.g. chloroperoxidase,
horseradish peroxidase, and cytochrome P450 family),5 which
play crucial roles in a range of biological processes including
mitochondrial respiration, steroid regulation and degradation
of xenobiotics.5b–e However, 57Fe Mössbauer measurements
revealed that one of the oxidizing equivalents of compound I,
in fact, is allocated to the porphyrin ligand, because its
Mössbauer spectroscopic features are essentially identical to
those of its one-electron reduced species, compound II
O
O
O–
O–
S
S
Cys
Cys
Compound I
Compound II
In contrast to a large number of iron-oxo compounds, only a
few iron-nitrido complexes have been investigated to date,9
despite the strong motivation to develop new nitrogen fixation
protocols that may compete with the industrial Haber–Bosch
process.10 In 1988, Nakamoto and Wagner reported in situ
generation and detection of the first iron(V)-nitrido species,
[FeV(N)(TPP)] (1, TPP2– = tetraphenylporphyrinate dianion), a
nitrido congener of compound I, using rR spectroscopy. 11
6
consisting of a triplet Fe(IV)=O unit (Chart 1). EPR
investigations showed that the ferryl moiety and the porphyrin
π radical of compound I are weakly antiferromagnetically
Complex
1 was generated by photo-oxidation of the
corresponding iron(III)-azido precursor in the Raman laser
beam. The rR spectra of 1 revealed the Fe–N stretching
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Page 2 of 16
vibration at 876 cm–1 as well as several marker bands of the
nitrido complexes discussed above. Furthermore, iron(V)-oxo
species have been advocated as actual oxidants for a range of
1
2
3
4
5
6
7
8
porphyrin ligand. Complex 1 was proposed to be a high spin
(S = 3/2) iron(V)-nitrido compound without a ligand radical,11b
by referring to its isoelectronic [MnIV(O)(TPP)] complex.12
This electronic structure assignment is qualitatively distinct
not only from that determined for compound I, but also from
those published later on for other well-characterized
authenticated iron(V)-nitrido and -oxo complexes supported
by innocent non-heme ligands, in as much as all possess low
spin (S = 1/2) ground states (vide infra). Thus, the electronic
structure of complex 1 needs to be further scrutinized by
thorough spectroscopic investigations.
nonheme iron complexes which catalyze regio- and stereo-
21
selective C–H and C=C bond functionalization.
Such
iron(V)-oxo intermediates were generated by O–O bond
cleavage of the corresponding metastable iron(III)-
acetylperoxo precursors, [FeIII(OOAc)–]2+. We recently carried
out
a
detailed electronic structure analysis on
3,6,9,15-
[FeV(O)(OAc)(PyNMe3)]2+ (6, PyNMe3
=
tetraazabicyclo[9.3.1]pentadeca-1(15),11,13-triene-3,6,9-
9
trimethyl), an prototypical example of this type of catalysts.
22
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However, our results
showed that complex 6 is best
formulated as an intermediate spin iron(IV) center
antiferromagnetically coupled to an O–O σ* radical, viz.
[FeIV(O∙∙∙OAc)2–•]2+. As a result, 6 can be viewed as a three-
electron reduced form of O2 in which the O–O bond was not
completely broken,21b as evidenced by a non-negligible
negative spin population (–0.14) observed for the O atom in
the acetate moiety, while the adjacent FeIVO unit featuring a
large positive spin population (+1.17). A similar bonding
Chart 2. Iron(V) Complexes Discussed in the Current
Work
situation
was
also
encountered
for
[FeV(O)(TMC)(NC(O)CH3)]+ (7, TMC = 1,4,8,11-tetramethyl-
1,4,8,11-tetraazacyclotetradecane)
and
[FeV(O)(TMC)(NC(OH)CH3)]2+ (7-H+). 23 Sizeable negative
spin populations found on the N atoms of the trans ligands
support the notion that complexes 7 and 7-H+ contain a triplet
•
•
ferryl unit that interacts with N=C(O–)CH3 or N=C(OH)CH3
ligands in an antiferromagnetic fashion.22
Complex 1 cannot be generated by photolysis in fluid
solution, because
a mixed-valent iron(III/IV) -nitrido
porphyrin dimer [(TPP)Fe]2N with an S = 1/2 ground state
forms instead.24 Similarly, complexes 2 and 3 also undergo
facile decay by dimerization of the FeV=N groups, eventually
yielding monomeric ferrous complexes and releasing N2.16,25
Since complexes 1–3 cannot persist in fluid solutions, their
reactivity studies relied on in situ spectroscopic methods. For
instance, complexes [FeV(N)L]2+, 2 and 3 were shown to be
capable of activating C–H or C=C bonds of organic substrates
on the basis of the mass fragmentation analysis.16,17,26 Nitride
addition to CO and tri-n-butylphosphine for complexes 2 and
Recently, a handful of non-heme iron(V)-nitrido complexes
(Chart 2), namely, [FeV(N)(cyclam-ac)]+ (2, cyclam-ac =
13
cyclam-1-acetato),
cyclam-ac
[FeV(N)(Me3-cyclam-ac)]+ (2, Me3-
14
=
4,8,11-trimethylcyclam-1-acetato),
[FeV(N)(N3)(cyclam)]+ (2),15 and [FeV(N)(MePy2tacn)](PF6)2
(3,
MePy2tacn
=
methyl-N',N''-bis(2-picolyl)-1,4,7-
2, respectively, was monitored by time-resolved Fourier-
triazacyclononane),16 were synthesized by bulk photolysis of
their ferric-azido precursors in frozen solutions. Furthermore,
27
transform infrared spectroscopy.
Furthermore, it was
reported that treating complex 4, the most stable iron(V)-
nitrido compound, with cobaltocene and water leads to
formation of ammonia.18 Interestingly, in addition to initiating
H- and O-atom transfer processes,28 complexes 5 and 5 could
act as co-catalysts for photochemical water oxidation.29
complex
[FeV(N)L]2+
(L
=
2,6-bis(1,1-
di(aminomethyl)ethyl)pyridine) was produced in gas phase
and detected by collision-induced dissociation of electrospray
ionization mass spectrometry.17 Complexes 2 and 3 have been
characterized by 57Fe Mössbauer and X-ray absorption
spectroscopy coupled with DFT calculations.13,14,16 It has been
concluded that both complexes are best described as genuine
low spin iron(V)-nitrido compounds. In addition to these
tetragonal species, Smith, Meyer, and coworkers reported
synthesis and spectroscopic and structural characterization of a
trigonal iron(V)-nitrido compound, [FeV(N)(PhB(tBuIm)3)]+
Although complexes 1–5 exhibit diverse chemical activity,
detection of similar iron(V)-nitrido and -oxo intermediates in
catalytic processes is rather challenging. Typically, 57Fe
Mössbauer and X-ray absorption spectroscopy are employed
to identify such species. However, for both types of
measurements there are some requirements for the sample
preparation and/or the availability of sophisticated facilities.
More importantly, to reach unequivocal assignments of
electronic structures, reference compounds, which are often
homologous iron complexes with different oxidation states,
are usually needed. Due to these limitations, alternative
spectroscopic technique that allows to detect transient iron(V)-
nitrido and -oxo complexes with higher efficiency and higher
sensitivity is highly desirable.
−
(4, PhB(tBuIm)3 = phenyltris(3-tert-butylimidazol-2-ylidene
borate). This complex was also found to possess an S = 1/2
ground state despite featuring
a different coordination
geometry.18
Two bona fide iron(V)-oxo species have been reported thus
far, [FeV(O)(TAML)]− (5, TAML4− = tetraamido macrocyclic
ligand),19 and its biuretamide derivative (5).20 EPR and 57Fe
Mössbauer measurements suggested that both complexes have
a low spin ground state, in analogy to the non-heme iron(V)-
2
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Journal of the American Chemical Society
The present work serves as a dual purpose. We first present
an Oxford Instruments Variox cryostat. Isomer shifts are
quoted relative to iron metal at 300 K.
1
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4
5
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a combined spectroscopic and computational study of the
electronic structure of complex 1 in comparison with well-
characterized iron(V) complexes 2–5. This enables us to
identify the unique bonding feature of tetragonal low spin
iron(V)-nitrido and -oxo complexes. On the basis of that, we
propose characteristic EPR signatures for such species. Note
that correlation of the electronic structure of trigonal iron(V)-
nitrido complex 4 with its g factors was published earlier by
Smith, Kirk and Hoffman and coworkers. 30
Computational Setup. All calculations were performed by
using the ORCA quantum chemical program.33 For geometry
optimizations, the BP8634 functional was used in combination
with the resolution of the identity (RI)35 approximation. All
atoms were described by the triple-ζ quality def2-TZVP basis
set in conjunction with the def2-TZV/J auxiliary basis set
required for the RI approximation.36 Solvation effects were
taken into account by employing the conductor like
polarizable continuum model (CPCM), 37 for which, to be
consistent with the experiment, acetonitrile ( = 36.6) was
chosen as the solvent. Numerical frequency calculations
verified the optimized structures to be local minima on the
potential energy surface.
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MATERIALS AND METHODS
Sample preparation and photolysis. The ferric azido
31
complexes, [FeIII(N3)(TPP)] (1pro
)
and [FeIII(N3)(cyclam-
ac)](PF6) (2pro),13a were synthesized by following the
published procedures. Dry and degassed solvents were used to
prepare the samples. The ferric azido precursors were
dissolved in a 1:9 dichloromethane:toluene mixture for 1pro or
1:9 methanol:n-butyronitrile for 2pro to give 2 mM stock
solutions. Aliquots of the azide solutions were loaded into
standard 4 mm quartz EPR tubes before freezing in liquid
nitrogen. Then, the tubes were placed in a finger Dewar filled
with liquid nitrogen and photolyzed by an LED LUXEON III
Star LED lamp (dominant wavelength of 470 nm). The entire
photolysis to generate complex 2 in the EPR tubes was
completed within 30 minutes, whereas for complex 1, the
irradiation had to last for ca. 20 hours. To prepare Mössbauer
samples, droplets of the frozen solution of fully 57Fe-enriched
1pro (1.7 mM in the solvent mixture) were collected in liquid
nitrogen and crushed into fine powder, which was then
photolyzed for 18 hours accompanied by periodic manual
stirring. The powder was subsequently recovered from liquid
nitrogen slurry and transferred to Mössbauer sample cups (ca.
0.7 mL). The photolyzed samples were always stored in liquid
nitrogen to avoid decomposition of the desired iron(V)-nitrido
species. An EPR sample of photolyzed 1pro was subjected to
rR measurements to validate the formation of 1.
The complete active space self-consistent field (CASSCF)
calculations38 were performed with the def2-TZVPP basis set
along with the def2-TZVPP/C auxiliary basis set for the RI
approximation. In the case of complexes 1–3 and 5, we first
tested CASSCF(11,9) calculations, for which the active space
consists of five d-orbitals, three nitrido- or oxo-2p based
orbitals, respectively, and the bonding combination (σeq) with
respect to the interaction between the Fe dx²-y² orbital and the
equatorial ligands. It turned out that the CASSCF(11,9)
computations predicted an erroneous ground state with an
electron configuration of (nb)2( *eq)1 instead of (nb)2(*Fe–N)1.
As a consequence, the computed g-values deviate from the
experiment values significantly. We then enlarged the active
space by adding three t2g-derived 4d orbitals (4dxy, 4dxz, and
4dyz), and the resulting CASSCF(11,12) computations
provided a correct ground state as evidenced by the calculated
g-values closely matching the experiment. For complex 1, in
order to allow development of radical character in the
porphyrin ligand, we also added four porphyrin π-orbitals,
namely, a1u, a2u, and two eg orbitals on top of CASSCF(11,12).
The resulting CASSCF(15,16) calculations with an active
space containing more than 14 orbitals were treated by
iterative-configuration expansion configuration interaction
(ICE-CI), an approximated version of the full configuration
interaction recently developed by our group. For complex 4,
we employed an active space distributed 13 electrons into 14
orbitals CASSCF(13,14), including five Fe d-orbitals, three
nitrido 2p based orbitals, two bonding partners of the dxy and
dx²-y² orbitals, and four 4d orbitals (4dxy, 4dx²-y², 4dxz and 4dyz).
To capture dynamic correlation effects, N-electron valence
perturbation theory of second order (NEVPT2)39 calculations
were performed on top of the CASSCF wavefunctions.
EPR Measurements. Continuous-wave (cw) X-band EPR
measurements were performed on a Bruker E500 ELEXSYS
spectrometer equipped with the Bruker dual-mode cavity
(ER4116DM) or a standard cavity (ER4102ST) and an Oxford
Instruments helium flow cryostat (ESR 900). The microwave
bridge was a high-sensitivity Super-X bridge (Bruker ER-
049X) with integrated microwave frequency counter. The
magnetic field controller (ER032T) was calibrated with a
Bruker NMR field probe (ER035M). EPR simulations have
been done with our own routines, esim_gfit and esim_sx. For
spin quantitation, the experimental derivative spectra were
numerically integrated by using the routine eview, and the
results were corrected for their g value dependence for field-
swept spectra by using Aasa and Vänngård approximation,32
i.e. dividing the integrals by the factor,
For g-value calculations using the multi-reference
CASSCF/NEVPT2 method,40 we first diagonalized the spin-
orbit coupling (SOC) matrix constructed by the five roots from
the state-average CASSCF calculation, for which the diagonal
elements were replaced by the NEVPT2 excitation energies.
The g-values were then computed by using Gerloch-
McMeeking equation in the basis of the relativistic
wavefunctions, the eigenvectors of the SOC matrix.41
gi2
gi
2
3
gPav
=
+
i
i
.
3
9
57Fe Mössbauer Measurements. 57Fe Mössbauer spectra
were recorded on a conventional spectrometer with alternating
constant acceleration of the γ-source (57Co/Rh, 1.8 GBq),
which was kept at room temperature. The minimum
experimental line width was 0.24 mm/s (full width at half-
height). The sample temperature was maintained constant in
RESULTS AND DISCUSSION
Spectroscopic Characterizations of Iron(V)-Nitrido
Species. In the earlier work, the electronic structure of
complex 1 was deduced only from its vibrational frequencies
determined by the rR measurements.11 In order to gain more
insights into its nature, we carried out more thorough
3
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spectroscopic characterizations. In the present work, complex
Page 4 of 16
i.e. the shorter the iron-ligand distance, the more negative the
1 was prepared by irradiating frozen solutions of 1pro in quartz
EPR tubes for 20 hours. The samples thus obtained are closer
to the usual conditions of chemical reactions in comparison
with the previous work, where complex 1 was generated by
photolysis of a solid thin film of 1pro deposited on a cold tip in
the incident Raman beam at 30 K.11 Despite the different
preparation protocol employed, the rR spectra measured for
our photolyzed samples revealed signals at 883, 1371 and
1569 cm−1 (Figure S5), which reasonably match the Fe–N
stretching vibration and the marker bands of the porphyrin
ligand reported before for 1 (876, 1373, 1576 cm−1,
respectively).11b The difference can be attributed to the solvent
effect. Thus, the rR investigations confirmed the successful
generation of complex 1.
isomer shift.42 Consequently, to reach more reliable conclusion
about the iron oxidation state, it is necessary to compare the
isomer shifts of related complexes with similar chemical
bonding.
1
2
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The zero-field Mössbauer spectrum (Figure 1) of the
photolyzed sample, which is prepared in a similar way by
starting from 57Fe-enriched 1pro, exhibits two quadrupole
doublets. The minor component can be attributed to the
unreacted precursor as compared to the Mössbauer spectrum
independently measured for 1pro (Figure S6). The newly
formed major component that is assigned to 1 accounting for
67% of the total iron content in the sample has an isomer shift
of 0.02 mm/s and a quadruple splitting of 2.49 mm/s. Notably,
the isomer shift of 1 is comparable to those found for
complexes 2, 2′, 2″, and 3 (Table 1), indicating that the iron
oxidation states of 1 is also +V. The more negative isomer
shifts observed for complexes 4, 5, and 5′ mainly originate
from the more contracted Fe–N/O bonds. Typically, the iron-
ligand distance is a more critical factor than the dN
configuration of the iron center to determine the isomer shift,
Figure 1. Mössbauer spectrum of 18h-photolyzed 1pro measured
at 80 K. The simulation (red line) is composed of two
components. Parameters: δ = 0.02, |ΔEQ| = 2.49, Γ = 0.40 mm/s,
w2/1 = 1.32 (67%, green line), and δ = 0.40, |ΔEQ| = 0.59, Γ = 0.30
mm/s, w2/1 = 1.10 (33%, blue line). Γ is the full-width at half
maximum of the Lorentzian lines and w2/1 is the asymmetric
broadening factor for the high-energy line of the doublets. The
asymmetric broadening is introduced to mimics the effects of not
perfectly fast spin relaxation for a half-integer spin species.
Table 1. Spectroscopic Parameters of Iron(V) Complexes
FeV complex Fe–N/O distance δ / mms−1
|ΔEQ| / mms−1 g-valuesa
ref.
1
2
0.02
2.49
1.67
1.83, 1.70, 1.0 (1.766, 1.718, 0.931)
1.75, 1.64, 1.0b (1.542, 1.510, 0.512)
this work
13a
1.61 Å
−0.04
2′
2″
3
1.68, 1.55, 0.92
14
−0.04
−0.01
−0.45c
−0.42
−0.44
1.90
1.02
4.78c
4.25
4.27
1.75, 1.63, 0.99
14,15
16
1.64 Å
1.59, 1.33, 0.9 (0.974, 0.962, 0.041)
4
1.506(2) Å
1.58 Å
2.299, 1.971, 1.971 (2.275, 1.990, 1.981) 18
5
1.99, 1.97, 1.74 (2.033, 1.947, 1.803)
1.983, 1.935, 1.726
19
20
5′
aIn parentheses present the g-values calculated at the CASSCF/NEVPT2 level. bEPR data obtained in this work. cValues recorded at 78 K.
Complex 1pro produces a nearly axial EPR spectrum with
effective g factors of 6.02, 5.89 and 2.01 (Figure 2, traces a),
typical for high spin iron(III) porphyrin complexes (S = 5/2)
with a positive axial zero-field splitting. After 20-hour
photolysis, the signal of 1pro is attenuated, and a weak yet
perceptible asymmetric zero-crossing signal around 400 mT
appears with a very shallow trough extending to the high field
(Figure 2, traces b), rendering an almost axial spectrum with g||
< g⊥ < 2. The resonances are attributed to complex 1. A
reasonable fit gave g factors of 1.83, 1.70 and 1.0 for 1,
wherein gmin was estimated on the basis of the integrated
absorption spectrum and fixed in the simulation. Double
integration of the spectra, for which the g-dependence of the
field-swept spectra was adjusted by Aasa-Vænngård factors,32
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revealed that the yield of the 1pro-to-1 conversion is 71%
(Figures S3 and S4), comparable to that determined by the
Mössbauer measurements. This observation hence confirms
our assignment of the emerged EPR signal to 1.
Remarkably, such unconventional EPR spectra with three g
factors all significantly lower than 2 were also observed for
complexes 2, 214 and 316a (Table 1).
1
2
3
Taken together, complex 1 must feature qualitatively the
same electronic structure as those determined for 2 and 3. This
notion is consistent with the observation that the Fe–N
stretching frequency measured for 1 (883 cm–1) is comparable
to those for 2 (864 cm–1)13c and 3 (866 cm–1).16b Therefore,
complex 1 is a genuine iron(V)-nitrido species and possesses a
low spin rather than high spin ground state. Different from
complexes 2 and 3 whose precursors are both low spin
complexes, 1 is evolved from a high spin complex. Thus, the
formation of 1 must involve a change in the spin state. Our
B3LYP calculations predicted the quartet state to be ~15
kcal/mol higher in energy than the doublet ground state. One
can anticipate an even large gap for the sextet state in which
all iron-nitrido antibonding orbitals are singly occupied. As
such, the large driving force and the efficient spin orbital
coupling (SOC) of the iron center may render the required spin
transition easily occur. Clearly, our findings show that low
spin state of the ferric azido precursors is not the prerequisite
for the photochemical generation of iron(V)-nitrido species.43
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The EPR spectrum of 1 differs markedly from those of the
various forms of compound I, whose Stot = 1/2 ground state
results from (weak) antiferromagnetic coupling between a
triplet ferryl core and a porphyrin π-radical.7 Because the
isotropic exchange coupling (J) competes with the axial zero-
field splitting of the ferryl moiety (DFe=O), the nature of the
ground state depends on the degree of the resulting mixing of
Stot = 1/2 and 3/2. As a consequence, the EPR spectra of the
variants of compound I in different enzymes vary depending
on the relative magnitudes of J and DFe=O. For instance, g
factors below 2 have been observed for compound I in
chloroperoxidase6c (g|| = 2 and broad g
≈ 1.73, J/D ≈ 1), and
⊥
the EPR spectrum of horseradish peroxidase shows an
exceedingly broad feature at g ≈ 1.99 due to a much smaller
J/DFe=O value and conformational strains.7a In general, the spin
Hamiltonian analyses7a,c render the sharp g|| feature close to 2
nearly independent of the J/DFe=O value, whereas g⊥ can be
much smaller. Interestingly, the synthetic porphyrin model
complexes of compound I show distinct Stot = 3/2 ground
eff
eff
states with effective g values of g
⊥
≈ 4 and g
= 2,
||
independent of various porphyrin substitutions.8 The situation
for compound I and its models is thus distinct from that
observed for 1, which features g|| < g⊥ < 2. This finding further
corroborates that complex 1 possesses a different electronic
structure from compound I.
Figure 2. X-Band EPR spectra of 1 and 2 in situ prepared from
the azide precursors. Spectra a and c are 1pro and 2pro; spectra b
and d were obtained after photolysis of 1pro (a) and 2pro (c) (black
traces). The insets show amplified signals (green traces) and their
integrated absorption spectra (blue lines) at higher field region.
Simulations are shown in red dashed lines. Conditions: 10 K with
0.2 µW microwave power and 0.75 mT modulation amplitude.
The EPR spectrum of complex 5 displays a near-axial
pattern of g|| < g⊥ < 2,19 similar to that found for complexes 1–
3, but has much smaller g shifts, the deviation of the measured
g value from spin-only g value, 2. In contrast, a distinct EPR
spectrum with g|| < 2 < g⊥ is observed for complex 4.18 These
observations hence give rise to a question about how to
correlate the different g factors determined for complexes 1–5
with their electronic structures.
For comparison, photolysis of 2pro was carried out at the
same conditions. Unlike that of 1pro, the photoreaction of 2pro
in the EPR tubes completed within half an hour. Low spin
ferric azido complex 2pro elicits a rhombic spectrum with large
g-anisotropy13a (Figure 2, traces c, gmax = 2.60, gmid = 2.29, and
gmin = 1.82). After photolysis, it completely changed into a
wide-split spectrum at low g values that we attributed to the
photolysis product, 2 (Figure 2, traces d). The simulations
yield g factors of 1.75, 1.64 and 1.0 for 2, similar to those
detected for 1. Double integration of the spectra demonstrated
nearly full recovery of the spin in the conversion of 2pro to 2.
Ligand Field Analysis of Electronic Structures of
Iron(V)-Nitrido/-Oxo Complexes and Their g Values. In
this section, we first present a ligand-field bonding analysis of
iron(V)-nitrido and -oxo complexes in tetragonal and trigonal
coordination environments. On the basis of that, a quantitative
model to rationalize the g values of tetragonal low spin
iron(V)-nitrido and –oxo complexes (1–3 and 5) is developed.
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In the next two sections, the approximation used to derive this
In the case of trigonal coordination geometry, a 2+1+2
ligand field splitting with the energetic ordering of 2σ*eq
σ*Fe-E ≤ 2π*Fe-E (Scheme 1b) is often proposed, where σ*eq is
1
2
3
4
5
6
7
8
model will be verified by more rigorous multireference
electronic-structure calculations using the CASSCF/NEVPT2
approach and finally the validity of the model will be carefully
evaluated.
As elaborated elsewhere44 the interaction of the iron center
with oxo and nitrido ligands (E) is rather covalent and entails
two π-bonds between the Fe-dxz/yz and E-px/y orbitals, and one
<
2
2
the equatorial σ*-combination between the dxy and dx –y
orbitals and the equatorial donors of tripodal ligands. Note that
the equatorial σ-antibonding interaction in pseudo-tetrahedral
geometry is much weaker than the corresponding one in
distorted octahedral or square pyramidal coordination
2
2
2
arrangements. In the latter cases, the four lobes of the dx –y
orbital all directly point to the donor atoms. Therefore, in a
trigonal coordination environment the σ*eq orbitals usually
σ-bond involving the Fe-dz and E-pz orbitals. The resulting
antibonding molecular orbitals are labeled as π*Fe-E with a
two-fold degeneracy and σ*eq, respectively. For tetragonal
coordination geometry, the remaining dxy orbital is essentially
9
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2
have the lowest energy. Due to the 3dz -4s-4pz mixing, σ*Fe-E
2
2
is typically situated at lower energy than π*Fe-E
.
9b Note that for
a non-bonding (nb) orbital, whereas dx –y interacts strongly
with the equatorial donors of the supporting ligand, yielding
the σ*eq molecular orbital. Thus, one envisions a 1+2+1+1
ligand field splitting pattern (Scheme 1a) with the energetic
ordering of nb < π*Fe-E < σ*eq (the σ*-orbital in the equatorial
plane) < σ*Fe-E, as proposed for complex [VIV(O)(H2O)5]2+ by
Ballhausen and Gray. 45 For low spin d3 centers, the only
unpaired electron must occupy one of the doubly degenerate
π* orbitals, and the resulting electron configuration of
a trigonal iron(IV)-nitrido complex supported by a bulky
guanidinate ligand, DFT calculations suggest that the π*Fe-N
46
orbitals lie above σ*Fe-N
.
Despite this complexity, for low
spin iron(V) complexes, the singly occupied molecular orbital
(SOMO) must be one of the two σ*eq-orbitals. Consequently,
3
2
2
the ground state is predicted to feature a (σ*xy, x –y ) electron
configuration, and to be of 2E symmetry in the C3v point
group. Similar to the tetragonal situation discussed above, the
double degeneracy of 2E cannot remain.
2
(nb)2(π*)1 leads to a ground state of E symmetry in the C4v
point group. However, even in ideal cases where the
supporting ligands possess four-fold rotation axes, such as
TPP, Jahn-Teller distortions should lower the symmetry of the
entire complex and lift the double degeneracy of the 2E state.
To gain further insight into the correlation between the
electronic structure and the EPR g values of low spin iron(V)
complexes, one needs to consider SOC between the ground
state and low lying excited states with the same spin as the
ground state. G anisotropy and g shifts are predominantly
originated from the mixing of excited states into the ground
state under the influence of SOC and the resulting partial
restoration of the orbital angular moment.47 The sign of the g
shifts can be predicted by using the following rule.47
A
DOMO-to-SOMO (DOMO = doubly occupied molecular
orbital transition) causes a positive g shift, whereas a SOMO-
to-VMO (VMO = virtual molecular orbital) transition gives a
negative g shift. The magnitude of the g shift is inversely
proportional to the excitation energy.
As will be verified below, due to the overwhelming iron-
nitrido and –oxo interaction, complexes 1–3 and 5 feature an
orbitally near doubly degenerate ground state. More
importantly, the energy separation between the ground state
with an electron configuration of (nb)2(π*y)1 and the first
excited state (nb)2(π*x)1 is comparable to the effective SOC
constant of iron(V) (~578 cm–1).48 Thus, we assume that the
Scheme 1. Qualitative orbital splitting pattern for iron(V)
complexes.
2
SOC within the effective E ground state essentially dictates
the g values, and the contributions from the higher lying
excited states are negligible. According to the above rule, for
1–3, 5, the lowest-energy SOMO-to-VMO excitation
(nb)2(π*y)1 → (nb)2(π*x)1 should give a dominant down-shift
of one g value (g||), as experimentally measured, whereas for
trigonal complex 4, the lowest-energy DOMO-to-SOMO
excitation of σ*xy → σ*x²-y² should introduce a positive g shift
in the z direction along the Fe–N bond. The (smaller) negative
shift found for g⊥ is in accord with the two higher lying
SOMO-to-VMO excitations of σ*x²-y² → π*x/y.
In order to rationalize more quantitatively the g values of 1–
3 and 5, which largely determined by the intra-2E excitations
((nb)2(π*y)1 →(nb)2(π*x)1), we first consider an ideal situation
2
where complexes have an exact doubly degenerate E ground
ˆ
ˆ
lz sz
state. In this case, one can show that only the
term
contributes non-vanishing matrix elements to the SOC
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Hamiltonian (For details, see the Supporting Information), compute the g values as defined for Kramers doublets in a
1
which, hence, can be written as
weak-field approximation by using
2
3
4
5
6
7
8
9
ˆ
ˆ
DE
g =
ˆ
HSOC =zlz sz
.
mB B
Here to a good approximation the SOC operator is treated as a
single-electron operator. 47a
To this end, the Zeeman matrix for the magnetic field along
the Z direction can be computed as follows,
Furthermore, to simplify the calculation, one can use
+2sin2 j
0
æ
ç
ö
÷
ˆ . These are
complex d-orbitals, which are eigenfunctions of lz
m B
B
related to the usual real d-orbitals by a unitary transformation.
Specifically, the two degenerate real dxz and dyz orbitals in C4v
symmetry correspond to the complex d+1 and d–1 orbitals.
Thus, the four basis functions of the 2E state can be
characterized by the orbital and spin magnetic quantum
ç
è
÷
ø
0
-2sin2 j
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Therefore,
DE = 4mB Bsin2 j
numbers, LM and SM, viz.
.
LM SM
(1)
Specifically,
Similarly, for the magnetic field in the equatorial plane, the
Zeeman matrix is
1
1
E » + aF2ezFe
+1+ = aFed+a1 +aN p+a1
,
,
,
,
2
2
æ
ç
ç
è
ö
÷
÷
ø
0
sin2j
m B
B
1
1
E » + aF2ezFe
-1- = aFed-b1 +aN p-b1
sin2j
0
2
2
and
1
1
E » - aF2ezFe
g^ = 2 sin2j
(2)
+1- = aFed+b1 +aN p+b1
2
2
Note that the final g matrix computed by this approach is
only determined by the mixing angle  and is independent of
the metal–ligand covalency parameterized by α values (for
details, see the Supporting Information).
1
1
E » - aF2ezFe
-1+ = aFed-a1 +aN p-a1
2
2
One can eliminate  in eqs 1 and 2, and obtain a direct
relation between the two g factors.
Here the coefficients Fe and N denote the contributions from
iron 3d- and nitrido or oxo p-orbitals, and the indices α and β
at the d and p functions denote the spin part. Apparently,
is the eigenfunction of the SOC operator, and its
LM SM
(3)
energy E is obtained by acting the SOC Hamiltonian on itself.
Furthermore, Fe is the effective SOC constant of Fe(V),
whereas SOC of the ligand-atoms is neglected. In summary, as
expected, the E ground state in perfect C4v symmetry is split
by the first order SOC into two Kramers doublets.
This equation represents the lower quadrant of a full cycle
with a radius of 2 and the origin at (0,2) (Figure 3). In the
present case, if the energy gap between the two components of
2E is zero, then g|| = g⊥ = 0. In the present situation, the two
components of the lowest energy Kramers doublet have orbital
angular momenta of ±ħ and spin angular momenta of ∓ħ/2,
respectively; therefore, the magnetic moment arising from the
orbital angular momentum exactly cancel out that from the
spin angular momentum. However, if the energy separation is
close to infinite, then g|| = g⊥ = 2, because the orbital angular
momentum is completely quenched and the system has an
2
Lowering the symmetry from C4v to the actual symmetry C1
of the complexes under investigation leads to mixing of
and
, because eventually only SM is a
+1SM
-1SM
good quantum number. Such mixing can be parameterized in
terms of a mixing angle  ( [0, π/4]), which yields the wave
functions of the lowest-energy Kramers doublet as
orbitally non-degenerate ground state. The
determined experimentally for complexes 1–3 and 5 all obey
g
values
eq. 3 nicely.
1
1
2
a = sinj +1+ + cosj -1+
2
1
1
2
b = sinj -1- + cosj +1-
2
Furthermore, the Zeeman splitting is described by
Here µB is the Bohr magneton, ge ≈ 2 is the spin-only g value,
and B is the magnetic field. For a given doublet, one can
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that the two Fe–N bonds (1.981, 1.974 Å) along the x-
1
2
3
4
direction are considerably shorter than those (2.000, 1.996 Å)
along the y-direction (Figure S19). These geometric
distortions raise the π*x orbital and simultaneously lower the
π*y orbital.
5
6
7
8
Unexpectedly, the computed spin population of the iron
center in complex 1 is less than that of the nitrido ligand. In
line with this observation, the π*x,y orbitals contains more N-
px,y contribution than that from the Fe-dxz,yz atomic orbitals.
Thus, the iron-nitrido interaction features so-called “inverted”
bonding,44c,51 in contrast to usual situations where the metal d
character prevails in metal-ligand antibonding orbitals. Thus,
there is substantial radical character in the nitrido ligand of
complex 1, and its electronic structure is best described as a
resonance hybrid between two limiting bonding situations,
FeV(SFe = 1/2)N3- ↔ FeII(SFe = 0)N•(SN = 1/2), in the latter case
the iron center featuring an electron configuration of
(dxy)2(dxz)2(dyz)2. This bonding description is consistent with
that deduced from the earlier ground-state DFT calculations.13b
However, for iron(V)-oxo complex 5, the spin population of
the iron center is higher than that of the oxo group (Figure
S17). The difference clearly originates from considerably
higher energy of the nitrido p-orbitals than the oxo p-orbitals.
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Figure 3. Schematic relationship of g factors of tetragonal FeV
complexes. The g⊥ is the average of the two slightly different g
values of each compound.
Furthermore, as analyzed in our earlier work on related
Ab Initio Calculations of Electronic Structures of
Iron(V)-Nitrido/-Oxo Complexes. As analyzed above, to
rationalize g values of transition complexes, one need to
consider the SOC between the ground state and low lying
excited states, especially for complexes 1–5 which likely
feature orbital near degeneracy. In this regard, DFT is not a
method of choice, because it cannot treat the ground and
excited states on an equal footing.49 Therefore, it is necessary
to employ wavefunction based highly correlated
CASSCF/NEVPT2 approach. In our earlier work on the
spectroscopy and reactivity of high valent iron-oxo
complexes,22,50 this method has been shown to deliver reliable
results not only for the ground state but also for the excited
states. The balanced active space consists of the Fe-centered
3d orbitals and their ligand centered bonding partners. For
complexes 1–3, the active space has to include three t2g
derived 4d orbitals (4dxy, 4dxz, and 4dyz); otherwise the
CASSCF calculations predicted erroneous ground states (for
details, see the Supporting Information). To examine the
electronic structure of complex 1 in an unbiased manner, we
further added four porphyrin π-orbitals, namely, a1u, a2u, and
two eg, into the active space, which should allow the system to
develop a porphyrin radical in the calculations. Hereafter, we
first discuss the ground state of complexes 1–5, and then
discuss their excited states.
22,50
iron(IV)-oxo complexes,
the unpaired electron in the
SOMO (π*y) is expected to contribute positive spin density in
the Fe-dyz and N-py atomic orbitals, while negative spin
density on the iron center, which reduces the total spin
population, mainly stems from the spin polarization. Because
in the present case, the nitrido ligand has a larger spin
population than the iron center, spin polarization induces some
marginal negative spin density in the Fe-dxz and -dz2 atomic
orbitals as suggested by the occupation numbers of the
DOMOs (πx and σz) substantially deviating from their
anticipated value (2), and those of the corresponding VMOs
(π*x and σ*z) considerably differing from 0. Consequently, the
spin density does not exactly resemble the shape of the SOMO
and shows a negative fraction in the xz plane (Figure 4b). The
situation found for complex 1 is exclusively different from
those for 6, 7 and 7-H+ where the peripheral groups of the
central FeIVO unit possess sizeable negative spin density.22
As displayed in Figure 4, our CASSCF(15,16) calculations
on complex 1 revealed that its ground state features a principal
electron
configuration
(74%)
x
of
(nb
dxy)2(σeq)2(σz)2(πx/y)4(a1u)2(a2u)2(π* )1(π* )0(eg-x/y)0(σ*eq)0(σ* )0.
y
z
Thus, our theoretical results reinforced that complex 1 cannot
be formulated as an iron(IV)-nitrido species interacting with a
porphyrin radical. The same bonding picture was delivered by
the CASSCF(11,12) calculations (Figure S13); therefore, in
the following we employed the smaller active space to
compute its low lying ligand-field excite states. The predicted
ground-state electron configuration of complex 1 corresponds
2
to one component of the E state. To accommodate such a
ground state for complex 1, the optimized geometry shows
that the Fe center is situated above the porphyrin plane, and
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(Figure S19), an analogous situation found for complex 1.
1
2
Such geometric distortions stabilize one of the two
components of the 2E ground states, and destabilize the other.
3
4
5
6
7
8
9
Table 2 summarizes the calculated energies of important
excited states for all complexes under investigation.
Complexes 1–3 feature a very low-lying excited state with an
electron configuration of (nb)2(π*x)1, which lies above the
ground state by only several hundred wavenumbers. Thus,
complexes 1–3 possess an orbitally near degenerate ground
2
state of effectively E symmetry, consistent with the ligand
field analysis. It should be noted that adjusting the Fe–N
distance in the (FeN)2+ core, the only geometric freedom of
this moiety, cannot lift the double degeneracy of the two Fe–N
π-bonds. Therefore, the small energy separation must arise
from much weaker interactions between the iron center and
the supporting ligand as found for 1–3. The excitation energy
of π*y → σ*z computed for 1 is much lower than those for 2
and 3, mainly because the lack of a trans ligand in 1 stabilizes
the σz* orbital. In line with this reasoning, the Fe–nitrido bond
length (1.56 Å) estimated for complex 1 is slightly shorter
than those (~ 1.60 Å) for complexes 2 and 3.
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Complex 5 features a similar electronic structure as 1–3,
except for the considerably larger energy separation between
2
the two components of E. Furthermore, for complex 5, the
excitation energy of π*y → σ*eq was predicted to be much
higher than that of π*y → σ*z. This is due to the strong σ-
donating capability of TAML, which raises the σ*eq orbital
above σ*z.52 The excitation from the nb dxy orbital to the Fe–E
π* orbital can be used to gauge the differential bonding
strength between the iron-nitrido and -oxo π-interactions.53
These excited states of complexes 1–3 were found to lie much
higher in energy than that of 5, thereby suggesting that the π-
bond of iron(V)-nitrido complexes is substantially stronger
than that of iron(V)-oxo species. For complexes 1 and 5, both
species featuring the same coordination geometry, our
calculations showed that the excitation from π*y to the vacant
σ*z orbital requires similar energy, although the π*y orbital of
1 is by far more destabilized than that of 5. These findings
show that iron(V)-nitrido complexes have stronger σ-bonds
than iron(V)-oxo compounds. As a consequence, the iron-oxo
interaction is more vulnerable to subtle perturbations. To test
the ligand effect, we calculated the gap between the two
components of the effective 2E ground state of the hypothetical
nitrido congener of 5, [FeV(N)(TAML)]2− (5-N). The obtained
value of 1000 cm-1 is higher than those found for complexes
1–3 but lower than that for 5. Therefore, not only the distinct
iron(V)-nitrido and -oxo bonding strengths but also the strong
donating capability of the TAML ligand lead to the larger
energy separation for complex 5 compared to 1–3. Because
there are four negatively charged donors in TAML, the gap
estimated for 5 is probably close to the maximum value that
can be reached in the iron(V)-nitrido and -oxo chemistry.
Figure 4. Electronic structure of complex 1. (a) Natural orbitals
obtained from the ground-state CASSCF(15,16) calculation. The
occupation number of each orbital is shown below the orbital
label (nb = non-bonding) and atomic contributions to the
molecular orbitals are shown for the important orbitals. The
double d-shell is omitted for clarity. (b) Spin density and
population obtained at the CASSCF(15,16) level.
Relative to 1, similar leading electron configurations were
found for the ground states of complexes 2, 3, and 5 (Figures
S14, S15, and S17). For complex 2, the double degeneracy of
2
the effective E ground state is lifted by the interaction of the
iron center with the trans π-donating acetate ligand. The
optimized geometries of complexes 3 and 5 reveals that the
iron centers move out of the equatorial plane and that the
computed equatorial metal-ligand bond distances along the x-
direction substantially differ from those along the y-direction
Table 2. CASSCF/NEVPT2 excitation energy (cm–1) for complexes 1–5.
Excitation
Excited state
1
π*y → π*x
(nb)2(π*x)1
630
π*y → σ*eq
(nb)2(σ*eq)1
3870
nb → π*y
(nb)1(π*y)2
22950
π*y → σ*z
(nb)2(σ*z)1
13260
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2
400
4890
2790
5710
5020
29370
28910
20770
20180
20160
20880
13630
14440
20480
20580
20010
20580
14610
14800
1
2
3
4
5
6
7
8
2′
2′′
3
500
450
130
5
2470
2380
5′
Excitation
Excited state
4
σ*xy → σ*x²-y²
σ*x²-y² → π*x
σ*x²-y² → π*y
σ*x²-y² → σz*
(σ*xy)1(σ*x²-y²
)
(σ*xy)2(π*x)1
(σ*xy)2(π*y)1
(σ*xy)2(σ*z)1
2
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4220
20020
22500
22280
The differential bonding strength between the iron-nitrido
and -oxo interactions explain why complex 1 features a
distinctly different electronic structure compared to compound
I. Our calculations show that, due to the much stronger iron-
nitrido π-interactions, the two π*Fe–N orbitals (–3.8 eV) of the
hypothetical one-electron reduced form of complex 1 are
situated at higher energy than the porphyrin a1u (–5.2 eV) and
a2u (–5.0 eV) orbitals (Figure S18), in analogy to 1.
Consequently, the electron residing in the π*Fe–N orbital is
more likely to be removed in the one-electron oxidation
components. Nevertheless, the estimated
g values of
complexes 1–3 and 5 reproduced the near-axial pattern with g||
< g⊥ < 2 and the lowest g factors (g||) were found to align
along the Fe–E bonds (Figure S22). For complex 4, the largest
g value was predicted along the Fe–N bond, and the other two
are situated in the equatorial plane (Figure S22). In contrast to
the ab initio results, the DFT computed g factors of complexes
1–3 and 5 are all very close to 2 (Table S1), further
corroborating the notion that often DFT methods cannot be
applied to orbitally near degenerate systems.
process. In other words, if
a species formulated as
As discussed above, complexes 1–3 and 5 all possess a low-
lying excited state. Thus, the large deviations of the estimated
g values are likely result from the error in the computed
excitation energy of this state. Taking complex 1 as an
example, we examined its influence on the g values. In a series
of five-root CASSCF/NEVPT2 calculations, we systematically
varied the transition energy of π*y → π*x from 0–6000 cm-1
and kept the energy of other excited states fixed at the initially
calculated values (Figure 5b). In parallel, we also carried out
similar two-root CASSCF/NEVPT2 computations, where only
the SOC of the effective 2E ground state was taken into
account (Figure 5a). The results obtained from both
calculations are essentially identical. A similar behavior was
also found for complex 2 (Figure S21). These findings suggest
that the g values of complexes 1 and 2 are almost completely
[FeIV(N)(TPP•+)]0 were to be generated in the photolysis, the
electron transfer from the singly occupied π*Fe–N orbitals to the
vacant porphyrin π*-orbital would have a tremendous driving
force and would happen spontaneously. Further experimental
investigations are required to verify this interpretation.
In agreement with an earlier study reported by Cutsail III et
al.,30 complex 4 has essentially an orbitally non-degenerate
ground state with a leading electron configuration (78%) of
(σ*xy)2(σ*x²-y²)1 (Figure S16). The considerably large energy
2
gap (4220 cm–1) of the two components of E mainly results
from the strong Jahn-Teller distortion in the equatorial plane,
as evidenced by three distinct Fe–C bond lengths (1.932,
1.947, and 1.969 Å) shown in the crystal structure of 4. As
depicted in Figure S16, both σ*xy and σ*x²-y² orbitals are
essentially non-bonding in nature, because they contain
predominant iron 3d character (94% and 84%, respectively)
and rather limited C lone-pair character (< 5 %). The
excitations of σ*x²-y² → π*x/y for complex 4 are, in fact,
equivalent to those of nb → π*y for complexes 1–3, because in
both transitions one electron is promoted from the nb orbital to
the Fe–N π* orbital. These excitations of 1–4 were estimated
to have comparable energy despite their different iron-nitrido
bond orders. This observation is consistent with the notion that
the SOMO (σ*x²-y²) of complex 4 is raised to higher energy due
to the significant Jahn-Teller distortion. For complex 4, the
excitation energy of σ*x²-y² → π*x/y is comparable to that of
σ*x²-y² → σ*z, thereby suggesting that the π-bond in 4 is as
2
determined by the SOC between the two components of E,
which verifies the assumption of the ligand field model.
Specifically, as the excitation energy changes from 0 to 3000
cm-1, the g|| and g⊥ values rockets from 0 to 1.8 and 2.0,
respectively. As the excitation energy further increases, the g||
component slowly approaches to 2, while g⊥ levels off at 2.
Thus, the g|| value is more sensitive to the variation of the
excitation energy, because it gets saturated at higher excitation
energy than g⊥. To achieve better agreement with the
experimental g values of complex 1 indicated by grey dashed
lines in Figure 5, the excitation energy should be in the range
of 600–800 cm–1, at most 200 cm–1 above the calculated
excitation energy (Table 2). This error is definitely beyond the
accuracy of any quantum chemical calculations. Thus, our
theoretical results clearly demonstrated that a minor change in
the excitation energy of π*y → π*x has drastic influence on the
g values, in particular g||. This explains the large error in the
calculated g values of complexes 2 and 3, because their first
excited states are below 2500 cm-1.
2
strong as its σ-bond. This finding is due to the 3dz -4s-4pz
mixing,30 which significantly drops the energy of the σz*
orbital.
Ab Initio Calculations of the g Values of Iron(V)-Nitrido
and -Oxo Complexes The computed g values of complexes
1–5 by using CASSCF/NEVPT2 approach are summarized in
Table 1. The theoretical results of complexes 1, 4 and 5 are in
reasonable agreement with the experiment. However, for
complexes 2 and 3, our computations do not achieve
quantitative agreement, especially for the lowest
g
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tetragonal low spin iron(V)-nitrido and -oxo complexes. As a
1
2
3
4
consequence, their EPR spectra would show a near-axial
pattern with g|| considerably less than 2, and, more critically,
the g|| and g⊥ values fit eq. 3, in analogy to those measured for
complexes 1–3 and 5.
5
6
7
8
Eq. 3 has been shown to succeed in correlating the g|| and g⊥
values of complexes 1–3 and 5, because our numerical
calculations revealed that the contributions from the higher
lying excited states, other than the first excited state, to the g
shifts are negligible (Figure 5). On the basis of the electronic
structures found for complexes 1–3 and 5, tetragonal low spin
iron(V)-nitrido and -oxo complexes may be classified into two
classes according to their equatorial coordination strength. On
one hand, if complexes feature weak equatorial coordination,
as exemplified by complexes 1–3, they typically have a small
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energy gap of ~1000 cm–1 for the effective E ground state.
Their closely lying excited states likely arise from promoting
the α-electron residing in the SOMO (π*Fe-E) to the equatorial
σ-antibonding orbital (σ*eq). These excited states were
computed to be situated at ~5000 cm-1 above the ground state
for complexes 1–3. Because the π-bonds of iron(V)-oxo
complexes are much weaker than those of iron(V)-nitrido
compounds, the corresponding excitations (π*Fe-O → σ*eq) for
iron(V)-oxo complexes should have much higher energy. On
the other hand, if tetragonal low spin iron(V)-nitrido and -oxo
complexes, such as 5, are supported by very strong equatorial
ligands, such systems often possess an energy separation of at
2
most 2500 cm–1 for the effective E ground state. However,
different from the situation discussed above, the closely lying
excited states probably originate from exciting the β-electron
in the doubly occupied nb orbitals to the SOMO (π*Fe-E). Our
calculations on complexes 1–3 and 5 show the lower bound of
the energy of these excited states is ~14000 cm-1. Taken
together, for both classes the energy of other excited states is
at least four times higher than the energy separation of the
effective 2E ground state. Therefore, the in-state SOC
essentially determines the g values of tetragonal iron(V)-
nitrido and -oxo complexes, which provides a rationale for the
general applicability of eq. 3.
As elaborated in our earlier work,22 complexes 6, 7 and 7-
H+ feature different bonding situations from those found for
1–3 and 5. As a consequence of their distinct electronic
structures, the g values of 6, 7 and 7-H+ are all close to 2.
More importantly, our reactivity studies revealed that
complexes 5 and 5 only can initiate one-electron chemistry, in
agreement with experimental findings,28 whereas 6 can
function as a two-electron oxidant.22 The reactions of C–H and
C=C bond oxidation with complex 6 proceed without an
intervening intermediate, which nicely explain the stereo-
specificity observed experimentally.
Figure 5. The g values of complex 1 as a function of the
excitation energy of π*y π*x calculated by using
CASSCF(11,12)/NEVPT2 calculations averaging two doublets
(a) and five doublets (b). The experimental g values are denoted
by dashed lines at g = 1.00, 1.70, and 1.83.
→
Given the electronic-structures of complexes 1–3 and 5, we
surmise that probably all tetragonal low spin iron(V)-nitrido
and -oxo complexes feature effective 2E ground states.
Because of the exceedingly strong - and π- donating
capability of the nitrido and oxo ligands, the overwhelming
iron-nitrido and -oxo bonding overrides any other metal-ligand
interactions, which in turn slightly lift the double degeneracy
of 2E. Bendix et al. proposed that the π* orbitals in
[CrV(N)Cl4]2– can be significantly destabilized and hence lie
higher in energy than the σ*eq orbital.54 Consequently, the
classical 1-2-1-1 orbital splitting (Scheme 1a) does not hold
true for [CrV(N)Cl4]2–. Thus, one can envisage a ground-state
electron configuration of (nb)2(σ*eq)1 for a low-spin iron(V)-
nitrido and -oxo complex with a very weak equatorial
coordination. To test this hypothesis, we computationally
examined the corresponding hypothetical iron(V) complexes,
[FeV(N)Cl4]2– and [FeV(O)Cl4]– (Figure S20). It turns out that
both complexes feature qualitatively the same electronic
structure as those found for complexes 1–3 and 5. This finding
further corroborates our proposed general bonding feature for
Chart 3. Open-shell Square Planar Nitrido Complexes
Recently, de Bruin and Schneider and coworkers reported
synthesis and characterization of two pincer rhodium(IV)- and
iridium(IV)-nitrido complexes, [RhN{N(CHCHPtBu2)2)}] (8)
55
and [IrN{N(CHCHPtBu2)2)}] (9)
(Chart 3). Despite
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possessing a square planar coordination geometry, both
Author Contributions
1
2
3
4
5
6
complexes feature a similar electronic structure to those of 1–3
and 5 with a single electron occupied in the nearly degenerate
π*Rh/Ir=N orbitals. Unsurprisingly, their measured g values (for
8, g = 2.04, 1.93, 1.70, and for 9, g = 1.885, 1.631 and 1.320)
also reasonably obey eq. 3 (Figure 3), which provides an
independent support for our ligand field model.
‡H.-C. C. and B. M. contributed equally to the present work.
Present Address
&H. F. Department of Chemistry, Fudan University, Shanghai
200433, China
7
8
CONCLUSION
Our experimental and theoretical investigations evidenced
that complex 1, a nitrido congener of compound I, is a bona
fide low spin (S = 1/2) iron(V)-nitrido complex. The
multireference CASSCF/NEVPT2 calculations revealed that
tetragonal iron(V)-nitrido complexes 1–3 all feature a unique
electronic structure having an orbitally near degenerate ground
state with an electron configuration of (nb)2(π*Fe–N)1. A similar
bonding situation was also found for tetragonal iron(V)-oxo
complex 5, but the gap between the two components of the
Notes
9
The authors declare no competing financial interest.
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ACKNOWLEDGMENT
We are indebted to Dr. Maurice van Gastel and Mr. Dennis Skerra
for resonance Raman measurements and gratefully acknowledge
the financial supports from the Max-Planck Society.
ABBREVIATIONS
Cyclam: 1,4,8,11-tetraazacyclotetradecane.
2
effective E ground state is larger. As a manifestation of their
analogous electronic structures, their EPR spectra exhibit a
near-axial pattern with g|| < g⊥ < 2, and the lowest g
component is considerably lower than 2. On the basis of their
unique bonding features, a simple equation to correlate their g||
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Further in-depth electronic-structure analysis suggested that
tetragonal low spin iron(V)-nitrido and -oxo complexes
possess electronic structures akin to those found for complexes
1–3 and 5. Thus, the EPR signatures determined for
complexes 1–3 and 5 can be used as a spectroscopic marker to
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This work provides deep insight into the electronic
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ASSOCIATED CONTENT
Supporting Information
This material is available free of charge via the Internet at
http://pubs.acs.org
EPR spectra of azide precursors (1pro and 2pro) and the samples
after photolysis, Raman spectrum of photolyzed 1pro, Mössbauer
spectrum of 1pro, the effect of the double d-shell on the ground
state of complex 2, CASSCF electronic structures of complexes
1–5, key geometrical parameters of complexes 1–5 and the
hypothetical complexes, dependence of g-tensors on the *y →
*x excitation energy for complex 2, orientation of the g matrices
for complexes 1–5.
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AUTHOR INFORMATION
Corresponding Author
eckhard.bill@cec.mpg.de;
shengfa.ye@kofo.mpg.de
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