Oxoiron(IV) Complex of the Ethylene-Bridged Dialkylcyclam Ligand Me2EBC

Article abstract of DOI:10.1021/acs.inorgchem.5b00861

We report herein the first example of an oxoiron(IV) complex of an ethylene-bridged dialkylcyclam ligand, [Fe^IV(O)(Me₂EBC)(NCMe)]²⁺ (2; Me₂EBC = 4,11-dimethyl-1,4,8,11-tetraazabicyclo[6.6.2]hexadecane). Complex 2 has been characterized by UV-vis, ¹H NMR, resonance Raman, M?ssbauer, and X-ray absorption spectroscopy as well as electrospray ionization mass spectrometry, and its properties have been compared with those of the closely related [Fe^IV(O)(TMC)(NCMe)]²⁺ (3; TMC = 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane), the intensively studied prototypical oxoiron(IV) complex of the macrocyclic tetramethylcyclam ligand. Me₂EBC has an N4 donor set nearly identical with that of TMC but possesses an ethylene bridge in place of the 1- and 8-methyl groups of TMC. As a consequence, Me₂EBC is forced to deviate from the trans-I configuration typically found for Fe^IV(O)(TMC) complexes and instead adopts a folded cis-V stereochemistry that requires the MeCN ligand to coordinate cis to the Fe^IV = O unit in 2 rather than in the trans arrangement found in 3. However, switching from the trans geometry of 3 to the cis geometry of 2 did not significantly affect their ground-state electronic structures, although a decrease in ν(Fe = O) was observed for 2. Remarkably, despite having comparable Fe^IV/III reduction potentials, 2 was found to be significantly more reactive than 3 in both oxygen-atom-transfer (OAT) and hydrogen-atom-transfer (HAT) reactions. A careful analysis of density functional theory calculations on the HAT reactivity of 2 and 3 revealed the root cause to be the higher oxyl character of 2, leading to a stronger O-H bond specifically in the quintet transition state. (Figure Presented).

Full text of DOI:10.1021/acs.inorgchem.5b00861

Article
pubs.acs.org/IC
Oxoiron(IV) Complex of the Ethylene-Bridged Dialkylcyclam Ligand
Me2EBC
Jason England,†,⊥ Jai Prakash,† Matthew A. Cranswick,†,∥ Debasish Mandal,§ Yisong Guo,*,‡
Eckard Munck,*,‡ Sason Shaik,*,§ and Lawrence Que, Jr.*,†
̈
†Department of Chemistry and Center for Metals in Biocatalysis, University of Minnesota, Minneapolis, Minnesota 55455, United
States
‡Department of Chemistry, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213, United States
§Institute of Chemistry and the Lise Meitner-Minerva Center for Computational Quantum Chemistry, The Hebrew University of
Jerusalem, 91904 Jerusalem, Israel
S
* Supporting Information
ABSTRACT: We report herein the first example of an
oxoiron(IV) complex of an ethylene-bridged dialkylcyclam
ligand, [FeIV(O)(Me2EBC)(NCMe)]2+ (2; Me2EBC = 4,11-
dimethyl-1,4,8,11-tetraazabicyclo[6.6.2]hexadecane). Complex
1
2 has been characterized by UV−vis, H NMR, resonance
Raman, Mossbauer, and X-ray absorption spectroscopy as well
̈
as electrospray ionization mass spectrometry, and its proper-
ties have been compared with those of the closely related
[FeIV(O)(TMC)(NCMe)]2+ (3; TMC = 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane), the intensively studied
prototypical oxoiron(IV) complex of the macrocyclic tetramethylcyclam ligand. Me2EBC has an N4 donor set nearly identical
with that of TMC but possesses an ethylene bridge in place of the 1- and 8-methyl groups of TMC. As a consequence, Me2EBC
is forced to deviate from the trans-I configuration typically found for FeIV(O)(TMC) complexes and instead adopts a folded cis-V
stereochemistry that requires the MeCN ligand to coordinate cis to the FeIVO unit in 2 rather than in the trans arrangement
found in 3. However, switching from the trans geometry of 3 to the cis geometry of 2 did not significantly affect their ground-
state electronic structures, although a decrease in ν(FeO) was observed for 2. Remarkably, despite having comparable FeIV/III
reduction potentials, 2 was found to be significantly more reactive than 3 in both oxygen-atom-transfer (OAT) and hydrogen-
atom-transfer (HAT) reactions. A careful analysis of density functional theory calculations on the HAT reactivity of 2 and 3
revealed the root cause to be the higher oxyl character of 2, leading to a stronger O---H bond specifically in the quintet transition
state.
Since the turn of the century, a sizable family of synthetic
oxoiron(IV) complexes has been reported.11−13 More than 90%
of these complexes exhibit an intermediate spin (S = 1) ground
state rather than the S = 2 spin state established for the
enzymatic intermediates. Much of our understanding of the
spectroscopic and reactivity properties of these complexes has
derived from the study of complexes containing solvent-
occupied labile coordination sites, cis and/or trans to the
FeIVO moiety. The most extensively studied is the complex
of 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane
(TMC; Chart 1), which coordinates in the equatorial plane
with a trans-I stereochemistry,14 resulting in a labile
coordination site trans to the FeIVO unit.15 The array of
complexes derived therefrom has been pivotal in building our
current understanding of the reactivity properties of the
oxoiron(IV) unit.16−18 A further variation of the TMC ligand
is the ethylene-bridged dialkylcyclam ligand 4,11-dimethyl-
INTRODUCTION
■
Mononuclear nonheme iron enzymes are responsible for a
remarkable number of biologically important transformations,
including DNA and RNA repair, post-translational hydrox-
ylation of amino acids, oxygen sensing, the biosynthesis of
antibiotics, and histone demethylation in transcriptional
regulation.1,2 In the majority of cases, the active oxidant
responsible for effecting these chemical transformations is an
oxoiron(IV) species, which has been found to possess an S = 2
ground state in all examples spectroscopically characterized
thus far.3−8 Although it is not entirely clear whether these
enzymatic intermediates possess an octahedral or a trigonal-
bipyramidal geometry,9,10 three of their ligands typically derive
from the protein backbone, which leaves up to three
coordination sites available for occupation by substrate, oxidant,
or cofactor. Most importantly, these coordination sites are
oriented cis with respect to one another and, by extension, the
incipient FeIVO moiety, a feature that is thought to be key to
their function.
Received: April 16, 2015
© XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.5b00861
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Article
CH2), −29 (1H, CH2), −31 (3H, NMe), −45 (3H, NMe), −67 (1H,
CH2), −81 (1H, CH2), −83 (1H, CH2), −107 (1H, CH2), −124 (1H,
CH2), −169 (1H, CH2), −211 (1H, CH2). MS (ESI+): m/z 475.4
([FeIV(O)(Me2EBC)(OTf)]+), 163.2 ([FeIV(O)(Me2EBC)]2+).
Physical Methods. NMR spectra were recorded in deuterated
solvents using either Varian Inova 500 or 300 MHz spectrometers, and
chemical shifts (ppm) were referenced to residual protic solvent peaks.
Elemental analyses were carried out by Atlantic Microlab (Norcross,
GA). UV−vis spectroscopic measurements were performed using a
HP8453A diode-array spectrometer equipped with a cryostat from
Unisoku Scientific Instruments (Osaka, Japan). Electrospray ionization
mass spectrometry (ESI-MS) experiments were carried out on a
Bruker BioTOF II mass spectrometer using a spray chamber voltage of
4000 V and a carrier gas temperature tailored according to the stability
of the complex (200 and 70 °C for complexes 1 and 2, respectively).
Chart 1
1,4,8,11-tetraazabicyclo[6.6.2]hexadecane (Me2EBC; Chart 1),
which is effectively identical with TMC in terms of the
coordination environment but enforces a cis-coordination
geometry.14 Although the Me2EBC ligand has been shown by
Busch and co-workers to be able to stabilize the MnIV oxidation
state19,20 and an oxoiron(V) oxidant has been invoked in the
catalytic epoxidation and cis-dihydroxylation of olefins with
H2O2,21 the corresponding iron(IV) chemistry has yet to be
described. Herein, we report the synthesis, spectroscopic
characterization, and reactivity of the oxoiron(IV) complex
[FeIV(O)(Me2EBC)(NCMe)]2+ (2) and a comparison of its
properties with those of [FeIV(O)(TMC)(NCMe)]2+ (3).
Mossbauer spectra were recorded with home-built spectrometers using
̈
Janis Research Super-Varitemp dewars, which allowed studies in the
temperature range of 1.5−200 K and applied magnetic fields of up to
8.0 T. Mossbauer spectral simulations were performed using the
̈
WMOSS software package (SEE Co., Edina, MN), and isomer shifts
(δ) are quoted relative to iron metal at 298 K.
X-ray Crystallography. A selected single crystal of 1(ClO4)2 was
placed on the top of a 0.1-mm-diameter glass capillary and mounted
on a Bruker SMART V5.054 CCD area detector diffractometer for
data collection at 123(2) K. A preliminary set of cell constants was
collected from reflections harvested from three sets of 20 frames.
These initial sets of frames were oriented such that orthogonal wedges
of reciprocal space were surveyed. Using 54 reflections (14 were not
indexed), initial orientation matrixes were produced. These reflections
were indexed as a two-component twin related by a 180° rotation
about the a axis (see the CIF file for additional details) using
CELL_NOW.24 Data collection was carried out using Mo Kα radiation
(graphite monochromator) with a frame time of 60 s and a detector
distance of 4.8 cm. A randomly oriented region of the reciprocal space
was surveyed to the extent of one sphere and to a resolution of 0.84 Å,
with four major sections of frames being collected using 0.30° steps in
ω at four different ϕ settings and a detector position of −28° in 2θ.
The intensity data were corrected for absorption and decay
(TWINABS).25 Final cell constants were calculated from 2947 strong
reflections taken from the actual data collection after integration
(SAINT).26 Please refer to Table S1 for a summary of the
crystallographic and structure refinement data.
EXPERIMENTAL SECTION
■
General Considerations. All reagents were purchased from
commercial sources and used as received, unless otherwise noted.
The complexes [FeII(Me2EBC)(Cl)2] (4)22 and [FeII(Me2EBC)-
(OTf)2] (5)21 and oxidant 2-(tert-butylsulfonyl)iodosylbenzene [2-
(tBuSO2)C6H4IO]23 were synthesized according to previously
published procedures. All moisture- and oxygen-sensitive compounds
were prepared using standard Schlenk-line techniques, and a nitrogen-
filled glovebox was used for any subsequent manipulation and storage
of these compounds.
Preparation of Complexes. [FeII(Me2EBC)(NCMe)2](ClO4)2
[1(ClO4)2]. A total of 2 equiv of AgClO4 (0.24 g, 1.15 mmol) was
added to a solution of 4 in MeCN (8 mL) and the resultant mixture
stirred for 3 h. Subsequently, it was filtered to remove AgCl
precipitate, which was itself washed with 3 mL of MeCN. The two
batches of filtrate were combined and reduced in volume to
approximately 3 mL. Diethyl ether (15 mL) was added, thereby
causing precipitation of a solid, which was isolated by filtration, washed
with diethyl ether, and dried under vacuum to give the product as a
pale-green-brown powder (0.32 g, 94% yield). X-ray diffraction-quality
crystals of 1(ClO4)2 were obtained by vapor diffusion of Et2O into a
concentrated MeCN solution of the complex. 1H NMR (CD3CN, 293
K, all peaks appear as broad singlets): δ 283 (2H, CH2), 246 (2H,
CH2), 128 (2H, CH2), 107 (2H, CH2), 84 (2H, CH2), 72 (6H, NMe),
60 (2H, CH2), 40 (2H, CH2), 23 (2H, CH2), 11 (2H, CH2), −8.1
(2H, CH2), −55 (2H, CH2). MS (ESI+): m/z 409.2 ([FeII(Me2EBC)-
(ClO4)]+). Anal. Calcd (found) for C18H36Cl2FeN6O8: C, 36.55
(36.29); H, 6.14 (6.30); N, 14.22 (13.79).
The structures were solved and refined using Bruker SHELXTL.27
The space group P21/c was determined based on systematic absences
and intensity statistics. A direct methods solution was calculated that
provided most non-hydrogen atoms from the E-map. The remaining
non-hydrogen atoms were located by full-matrix least-squares/
difference Fourier cycles. All non-hydrogen atoms were refined with
anisotropic displacement parameters. All hydrogen atoms were placed
in ideal positions and refined as riding atoms with relative isotropic
displacement parameters. The pattern of difference Fourier peaks
suggests a minor occupancy of the enantiomorph at 5−10%.
Crystallographic data for 1(ClO4)2 have been deposited with the
Cambridge Crystallographic Data Centre with a deposition number of
CCDC 1029038.
Resonance Raman Spectroscopy. Resonance Raman spectra of
2 were collected in a frozen solution at 77 K using a Spectra-Physics
model 2060 Kr+ laser and an ACTON AM-506 monochromator
equipped with a Princeton LN/CCD data collection system. Samples
in a 20:1 CH3CN/CH2Cl2 or a 20:1 CD3CN/CD2Cl2 solution were
frozen onto a gold-plated copper coldfinger in thermal contact with a
Dewar flask containing liquid nitrogen. Throughout, the mono-
chromator slit width was set for a band pass of 4 cm−1, and a 135°
[FeII(Me2EBC)(OTf)2] (5). Compound 5 was synthesized following an
earlier published literature procedure.21 1H NMR (CD2Cl2, 293 K, all
peaks appear as broad singlets): δ 309.7 (2H, CH2), 246.5 (2H, CH2),
126.1 (2H, CH2), 88.6 (6H, NMe), 88.2 (2H, CH2), 77.6 (2H, CH2),
72.1 (2H, CH2), 63.7 (2H, CH2), 23.3 (2H, CH2), 18.4 (2H, CH2),
1.7 (2H, CH2), −6.1 (2H, CH2), −109.2 (2H, CH2). MS (ESI+): m/z
459.3 ([FeII(Me2EBC)(OTf)]+).
[FeIV(O)(Me2EBC)(NCMe)]2+ (2). To a stirring MeCN solution of 5
at 0 °C was added 1.5 equiv of 2-(tBuSO2)C6H4IO dissolved in
CH2Cl2. The progress of the reaction was monitored spectrophoto-
metrically and, after 3−5 min, resulted in the maximal formation of 2.
1H NMR (CD3CN, 293 K, all peaks appear as broad singlets): δ 38
(1H, CH2), 29 (1H, CH2), 21 (1H, CH2), 20 (1H, CH2), 15 (1H,
CH2), 9.4 (1H, CH2), 4.6 (1H, CH2), 4.1 (1H, CH2), −5.5 (2H,
CH2), −9.4 (1H, CH2), −9.8 (1H, CH2), −12 (1H, CH2), −19 (1H,
backscattering geometry was used. Spectra were collected using λex
=
406.7 nm at 20 mW, and plotted spectra were composed from an
average of 32 scans with collection times of 30 s. Raman frequencies
were calibrated to indene prior to data collection. All spectra were
intensity-corrected to the 710 or 773 cm−1 solvent peak of CD3CN or
CH3CN, respectively.
B
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X-ray Absorption Spectroscopy (XAS). XAS data were collected
on beamline 7-3 of the Stanford Synchrotron Radiation Lightsource
(SSRL) of the SLAC National Accelerator Laboratory. Fe K-edge XAS
data were collected for frozen samples of 2 prepared in tandem XAS/
Reactivity Studies. All of the computational details are given in the
Supporting Information. A few details follow here. The hydrogen-
atom-transfer (HAT) process for 2 and 3 reacting with 9,10-
dihydroanthracene (DHA) was studied by means of DFT calculations.
Geometry optimization was conducted with UB3LYP. For all of the
species, we used the LACVP*(Fe)/6-31G*(rest) basis set, labeled B1.
To avoid the self-interaction errors, we neutralized the charge of the
oxoiron(IV) reagents with ClO4 for 2 and triflate (CF3SO3 )
counterions for 3 and verified that there are no consequences to the
usage of different counterions. Geometry optimization at the B1 level
of theory was carried out with the Jaguar 8.0 program.35 A subsequent
frequency calculation were also done at the same level to confirm the
nature of the optimized structures as local minima (no imaginary
frequency) or transition states (one imaginary frequency) and to
evaluate the zero-point vibrational energy and thermal corrections to
the Gibbs free energy at T = 273 K, which is the experimental
temperature.
The energies were further corrected with two larger basis sets,
LACV3P+*(Fe)/6-311+G*, labeled B2, and the all-electron Def2-
TZVPP basis set, labeled B3. Solvent effect corrections on B1
geometries were carried out at the B2 and B3 levels, at the latter using
the PCM model with the default UFF radii in Gaussian09. Tunneling
and kinetic isotope (KIE) effects were calculated to validate the
mechanism.36
Reaction Kinetics. All kinetic studies were performed under a
nitrogen atmosphere. MeCN solutions of 2 (0.5 mM) were prepared
by treating 1 with 1.5 equiv of 2-(tBuSO2)C6H4IO dissolved in
CH2Cl2 or trifluoroethanol at 0 °C. Organic substrates dissolved in
either CH2Cl2 [PPh3, DHA, 1,4-cyclohexadiene (CHD)] or MeCN
(thioanisole and benzyl alcohol) were added in >10-fold excess to the
aforementioned solutions of 2 at either −40 or 0 °C. The resulting
reactions were monitored by decay of the near-IR feature (λmax = 800
nm) associated with the oxoiron(IV) complex and found to display
Mossbauer cups with [Fe]total ∼ 4−5 mM. The SPEAR storage ring of
̈
the SSRL was operated at 3.0 GeV and ∼350 mA, and energy
resolution of the focused incoming X-rays was achieved using a
Si(220) double-crystal monochromator, which was detuned to 50% of
maximal flux to attenuate second-harmonic X-rays. The sample
temperature was controlled utilizing an Oxford Instruments CF1208
continuous-flow liquid-helium cryostat. Data were obtained as
fluorescence excitation spectra with a 31-element solid-state
germanium detector array (Canberra). An iron foil spectrum was
recorded concomitantly for internal energy calibration, and the first
inflection point of the K-edge was assigned to 7112.0 eV. The edge
energy was monitored during data collection for red shifts indicative of
sample photoreduction, but none were observed in the present study.
A 3 μm manganese foil and a Soller slit were utilized to attenuate the
signal-to-noise of the incoming data counts. The data set obtained by
averaging 15 scans was used in spectral analysis.
−
−
Data reduction, averaging, and normalization were performed using
the program EXAFSPAK.28 Following calibration and averaging of the
data, background absorption was removed by fitting a Gaussian
function to the preedge region and then subtracting this function from
the entire spectrum. A three-segment spline with fourth-order
components was then fit to the extended X-ray absorption fine
structure (EXAFS) region of the spectrum in order to extract χ(k).
Theoretical phase and amplitude parameters for a given absorber−
scatterer pair were calculated using FEFF 8.4029 and utilized by the
“opt” program of the EXAFSPAK package during curve fitting.28
Parameters for 2 were calculated using the crystal structure
coordinates of 1, modified to possess bond distances and ligands
similar to those of the crystallographically determined structure of 3.
In all analyses, the coordination number of a given shell was a fixed
parameter and was varied iteratively, while bond lengths (r) and mean-
square deviations (σ2) were allowed to freely float. The amplitude
reduction factor S0 was fixed at 0.9, while the edge shift parameter E0
was allowed to float as a single value for all shells [thus, in any given fit,
the number of floating parameters was typically equal to (2 × number
pseudo-first-order kinetics. Plots of the observed rate constants (kobs
)
versus substrate concentration were found to be linear in all cases
(Figures S1−S6), thereby yielding second-order rate constants (k2).
RESULTS AND DISCUSSION
■
Synthesis and Characterization of [FeII(Me2EBC)-
(NCMe)2](X)2 [1(X)2; X = OTf, ClO4]. Although the iron(II)
starting complex 5 could be generated by the combination of
equimolar quantities of Me2EBC and [FeII(OTf)2(NCMe)2] in
CH2Cl2 solution, it was more reliably and cleanly prepared by
metathesis of the chloride ligands in 4 using AgOTf. It was
previously demonstrated, using 19F NMR spectroscopy, that in
MeCN solution 5 actually exists as the solvento complex
1(OTf)2.21 Hence, efforts were made to grow crystals of the
latter suitable for X-ray crystallographic analysis. This was
unsuccessful, so other salts of 1 were prepared in MeCN
solution via silver salt metathesis, and the resulting materials
were recrystallized.
of shells) + 1]. The goodness-of-fit F was defined simply as ∑(χexptl
−
χcalc).2 For fits to unfiltered data, a second goodness-of-fit parameter,
2
F-factor, was defined as [∑k6(χexptl − χcalc)2/∑k6χexptl
]
1/2. In order to
account for the effect that additional shells have on improving the fit
quality, a third goodness-of-fit metric F′ = F2/(NIDP − NVAR) was
employed, where NVAR is the number of floated variables in the fit and
NIDP is the number of independent data points, which is defined as
NIDP = 2ΔkΔr/π. In the latter equation, Δk is the k range over which
the data are fit, while Δr is the back-transformation range employed in
fitting Fourier-filtered data. F′ is thus of principal utility in fitting
Fourier-filtered data but can also be employed for unfiltered data by
assuming a large value of Δr.
Preedge analysis was carried out by normalization of the data in the
Ifeffit program Athena,30 and fitting the restricted preedge region
(7105−7120 eV), as suggested by Westre et al.,31 using the program
Fityk.32 The preedge transition was fit using a Gaussian function, and a
pseudo-Voigt baseline function composed of 50:50 Gaussian/
Lorentzian functions.
Density Functional Theory (DFT) Calculations. Structural and
Spectroscopic Studies. DFT calculations were performed using
Becke’s three-parameter hybrid (B3LYP) functional and basis set 6-
311G provided by Gaussian09 software package.33 The initial
structural model for 2 was constructed using the crystal structure of
1 by replacing one of the CH3CN ligands with an oxygen atom. The
self-consistent-field procedure and geometry optimization were
terminated upon reaching the default convergence criteria. The
quadrupole splitting ΔEQ was calculated using the property keyword
of the Gaussian code and Q(57Fe) = 0.17 barn. The 57Fe isomer shift δ
was evaluated from the DFT charge density at the iron nucleus using
the calibration given by Vrajmasu et al.34
It was found that recrystallization of 1(ClO4)2 yielded pale-
green-brown crystals suitable for X-ray analysis, and the
structure of the [FeII(Me2EBC)(NCMe)2]2+ ion (1) is
presented in Figure 1. As anticipated, the dication is six-
coordinate and pseudo-C2-symmetric, and the Me2EBC ligand
adopts the cis-V stereochemistry typically seen in its complexes,
with the two MeCN ligands occupying cis-coordination
sites.14,19−22 This is in sharp contrast to the five-coordinate
trans-I geometry displayed in all X-ray structures of nonbridged
tetralkylcyclam iron(II) complexes reported thus far,14,18,37,38 in
which a nominally vacant site is trans to the axial noncyclam
ligand and all four N-alkyl substituents are oriented syn with
respect to one another. Interestingly, the average Fe−N3°‑amine
bond length in 1 is 2.06 Å, considerably shorter than those
found in high-spin FeII(Me2EBC) complexes 4 (2.27 Å) and 5
C
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Figure 2. 1H NMR spectra of 5 recorded in CD3CN (top) and
CD2Cl2 (bottom) at room temperature. The NMe resonances are
■
marked with “ ”.
methyl substituents. This is consistent with 1 possessing C2
symmetry in solution, but in such a scenario, one would expect
the spectrum to contain 12 peaks that integrate to two protons
each. Hence, one peak (two protons) is missing, presumably
due to either severe line broadening resulting from very close
proximity to the metal center, which can lead to inaccurate peak
integration and even complete loss of a signal, or overlap with
the intense solvent signals in the diamagnetic region.
Figure 1. Structure of the dicationic component of 1(ClO4)2, depicted
using 50% probability ellipsoids. Hydrogen atoms and ClO4
counterions have been omitted for clarity. Atom color scheme: C,
gray; N, blue; Fe, magenta.
−
(2.20 Å) but comparable to those in low-spin FeII(R3TACN)
complexes (Table 1).21,22 In addition, the Fe−NMeCN bond
lengths of 1 (1.94 Å) compare well to those found in the low-
spin complexes [FeII(TPA)(NCMe)2]2+ (1.93 Å) and
[FeII(Me3TACN)(NCMe)3]2+ (1.98 Å) but are about 0.2 Å
shorter than those seen in the high-spin complexes
[FeII(Me3TACN)(NCMe)2(OTf)]+ (2.14 Å) and [FeII(6-
Me3TPA)(NCMe)2]2+ (2.17 Å).39,40 These bond-length differ-
ences reflect the decrease in the ionic radius upon a change in
the spin state and are thus diagnostic of the spin state of the
iron(II) center. Thus, the iron(II) center in 1 is likely to be
Interestingly, contrary to expectations based upon Curie’s
law by which the chemical shifts would be expected to increase
linearly with 1/T, the chemical shift range decreased to +190 to
−50 ppm at −40 °C (Figure S8). These results suggest the
existence of a rapid equilibrium between high- and low-spin
forms of 1, i.e., spin-crossover behavior, which has been noted
for a wide array of FeII(N6) complexes.45,46 Similar behavior
was also reported for the related [FeII(BPMEN)(NCMe)2]-
(ClO4)2 [BPMEN = N,N′-dimethyl-N,N′-bis(2-pyridylmethyl)-
1,2-diaminoethane] and [FeII(BQMEN)(NCMe)2](ClO4)2
[BQMEN = N,N′-dimethyl-N,N′-bis(8-quinolylmethyl)-1,2-
low-spin, a property that is confirmed by Mossbauer studies
̈
(Figure S7). As such, 1(ClO4)2 represents the first instance of a
low-spin iron(II) tetraalkylcyclam complex.
diaminoethane] complexes.47−49 The H NMR spectrum of 5
1
On the basis of the crystal structure of 1(ClO4)2, one might
expect the peaks in its 1H NMR spectrum in a CD3CN solution
to be restricted to the diamagnetic region (0−10 ppm), but this
is not the case. In fact, its room temperature 1H NMR spectrum
shows strongly paramagnetically shifted resonances spread out
over the chemical shift range of +280 to −60 ppm (Figure 2,
top), suggesting that 1 is a high-spin iron(II) complex in
solution at 25 °C. More specifically, the 1H NMR spectrum of 1
contains 11 peaks that integrate to two protons each, plus one
peak that integrates to six protons, which can be assigned to the
obtained in CD2Cl2 at 25 °C exhibits an even larger spread of
chemical shifts (Figure 2, bottom) that presumably arises only
from the high-spin complex. Titration of this solution with
CD3CN resulted first in the appearance of a spectrum with
twice the number of signals, consistent with the loss of 2-fold
symmetry, and upon addition of more CD3CN, a spectrum like
that of 1 in pure CD3CN was obtained (Figure S9). These
changes reflect the stepwise replacement of the triflate ligands
of 5 with CD3CN via the C1-symmetric monotriflate complex,
which was also corroborated by 19F NMR (Figure S10). They
a
Table 1. Bond Lengths (Å) of [FeII(Me2EBC)(X)2] and Related Complexes
complex
S
Fe−NMe
Fe−Nbridge
Fe−X
ref
[FeII(Me2EBC)(NCMe)2](ClO4)2 [1(ClO)4]
[FeII(Me2EBC)Cl2] (4)
0
2
2
2
2
0
2
0
2
0
2
2.073(4), 2.094(4)
2.027(4), 2.037(4)
1.942(4), 1.946(4)
2.4260(5), 2.4273(4)
2.1243(11), 2.1244(11)
2.018(4)
this work
22
2.2748(13), 2.2866(14)
2.2574(13), 2.2634(13)
2.1973(13), 2.1974(13)
[FeII(Me2EBC)(OTf)2] (5)
2.1992(13), 2.1993(13)
21
[FeII(TMC)(NCS)](OTf)
[FeII(TMC)(O2SPh)](OTf)
[FeII(Me,HPyTACN)(NCMe)2](PF6)2
[FeII(Me,HPyTACN)(OTf)2]
[FeII(Me,6MePyTACN)(NCMe)2](SbF6)2
[FeII(Me,6MePyTACN)(OTf)2]
2.20
2.22
2.00
2.21
2.03
2.23
2.05
2.20
41
1.996(2)
37
42
43
44
43
[FeII(Me3TACN)(NCMe)3](BPh4)2
[FeII(Me3TACN)(NCMe)2(OTf)](OTf)
1.98
2.14
40
40
a
Abbreviations used: Me,HPyTACN = 1-(pyridyl-2′-methyl)-4,7-dimethyl-1,4,7-triazacyclononane; Me,6MePyTACN = 1-(6′-methylpyridyl-2′-methyl)-
4,7-dimethyl-1,4,7-triazacyclononane; Me3TACN = 1,4,7-trimethyl-1,4,7-triazacyclononane.
D
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Inorganic Chemistry
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also show that the triflate and MeCN ligands in the
monotriflate complex are in slow exchange with respect to
the possible positional isomers.
associated with 1, it exhibits two prominent ion fragments
(Figures S11 and S12), with m/z 475 and 163 possessing
isotope distribution patterns consistent with respective
formulations as [FeIV(O)(Me2EBC)(OTf)]+ and [FeIV(O)-
(Me2EBC)]2+.
Preparation and Characterization of 2. The reaction of
1 with 1.5 equiv of 2-(tBuSO2)C6H4IO at 0 °C results in the
rapid formation of yellow species 2, possessing a half-life (t1/2
)
The 1H NMR spectrum of 2 exhibits a set of 24
paramagnetically shifted resonances in the chemical shift
range of +40 to −210 ppm (Figure 4) and is free of starting
of 5 h and characterized by an intense UV band at 282 nm (εmax
∼ 13000 M−1 cm−1) and weak near-IR features centered at 800
(εmax ∼ 270 M−1 cm−1) and 945 nm (Figure 3 and Table 2).
Figure 4. 1H NMR spectrum of 2 recorded in CD3CN at room
■
temperature. The NMe resonances are marked with “ ”.
Figure 3. Electronic spectra of 2 (solid) and 3 (dashed) recorded in a
MeCN solution at 0 °C.
material 1 (Figure 2). Many upfield-shifted resonances are
observed, which is typical of S = 1 iron(IV) centers, while the
reverse is normally found for S = 2 iron(II) complexes.18,54,55
Of the 24 peaks, there are two peaks that integrate to three
protons each, corresponding to the two methyl groups and 22
signals with one-proton integration assigned to the diaster-
eotopic methylene protons. (As was the case in 1, two protons
are missing from the spectrum of 2.) Complex 2 has twice the
number of peaks as that seen for 1, which is indicative of
formation of the C1-symmetric complex 2, where a MeCN
ligand is coordinated cis to the oxo-atom donor and in the
slow-exchange regime relative to the two possible positional
isomers.
Resonance Raman spectroscopy of a 10 mM solution of 2 in
20:1 MeCN/CH2Cl2 using λex = 406.7 nm yields a resonance-
enhanced band at 824 cm−1, which is in the energy range
typically associated with ν(FeO) of oxoiron(IV) complexes
(Figure 5). Generation of 2 using an 18O-labeled oxidant leads
to loss of the 824 cm−1 feature and the appearance of two new
resonance-enhanced bands at 766 and 791 cm−1. Assuming that
Table 2. Spectroscopic Properties of Oxoiron(IV)
Tetraalkyltetraazamacrocyclic Complexes
a
b
2
3
λmax (nm) [εmax
282, 540, 800, 945 [13000, 80, 282, 824 [10000,
(M−1 cm−1)]
270, 190]
400]
ν(FeO) (cm−1
)
824 [−45]
839 [−35]
[Δ18O]
Mossbauer Analysis
̈
δ (mm s−1
)
0.13(1) (0.15)
0.60(2) (0.34)
0.4 (0.9)
0.17
1.24
0.5
ΔEQ (mm s−1
)
η
c
D (cm−1
)
26(2)
26.95
Ax,y,z/gnβn(T)
−22(1), −21(1), −3(2) (−21, −21.3, −20.3,
d
−20, −5)
XAS Analysis
−2.9
E0 (eV)
7124.5
7124.5
7114.1
33
Epreedge (eV)
7114.0
33
e
preedge area
r(FeO) (Å)
1.64 (1.65)
1.64
r(Fe−N)ave (Å)
2.04 (2.08)
2.08
a
Italicized numbers within parentheses represent values obtained from
DFT calculations. The calculated A tensor was obtained by taking the
experimentally determined Aiso and adding the spin-dipolar term
b
c
d
obtained from DFT. Data from ref 52. Reference 53. Reference 54.
e
Weighted preedge areas scaled to values expected for the pure
oxoiron(IV) complex, assuming that other impurities present do not
contribute significantly to the preedge area.
[Titration of 2-(tBuSO2)C6H4IO into solutions of 1 provided
maximal yields of 2 when 1.5 equiv of the oxidant was used.]
The latter is emblematic of S = 1 oxoiron(IV) complex
formation and is attributed to symmetry-forbidden ligand-field
(d → d) transitions.50,51 A further noteworthy point is that the
near-IR feature of 2 shows rich vibronic structure, which was
also seen for 315 but is not apparent in the UV−vis spectra of
the majority of S = 1 oxoiron(IV) complexes. The successful
formation of an oxoiron(IV) complex is further evidenced by
the ESI-MS spectrum of 2. In addition to the features
Figure 5. Resonance Raman spectrum of 2 in 20:1 MeCN/CH2Cl2
(top) and its 18O-labeled isotopomer in 20:1 CD3CN/CD2Cl2
(bottom). λex = 406.7 nm; power = 20 mW; [Fe] = 10 mM.
E
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the 766 and 791 cm−1 bands are due to a ν(FeO) Fermi
doublet (ν0 = 779 cm−1), this corresponds to a 45 cm−1 shift
upon 18O labeling (equal weighting was given to the two
peaks), which is significantly larger than that predicted by
Hooke’s law for a diatomic harmonic oscillator (−37 cm−1).
For most nonheme FeIVO complexes thus far characterized
by resonance Raman spectroscopy, the 18O shifts observed are
either as-predicted for a diatomic vibration by Hooke’s law or
slightly lower.11 There are, however, the three following
exceptions: ν(FeO) for [(6-Me3TPA)FeIIIOFeIV(O)(6-
Me3TPA)]3+ observed at 840 cm−1 with a 43 cm−1 downshift
upon 18O labeling of the oxo group,56 that for [(cis-β-
BPMCN)FeIV(O)]2+ at 822 cm−1 with a 40 cm−1 downshift,57
and that for [(Me,HPyTACN)FeIV(O)]2+ at 831 cm−1 with a 43
cm−1 downshift.58 Although the reasons for such discrepancies
are presently unclear, the assignment of these vibrations as
ν(FeO) is not in doubt. ν(FeO) of 2 is approximately 15
cm−1 lower in energy than that of 3, which implies that the
FeO bond in the former is weaker than that in the latter. On
the basis of previously established trends,52,59 this might be
expected to translate into 2 displaying greater reactivity than 3.
⎛
⎞
⎟
2
2
3
2
2
̂
̂ ̂
+ E(Sx − Sy ) + 2βB·S + S·A·I
⎜
H = D Sz
−
⎝
⎠
+ g β B·I + HQ
(1a)
(1b)
n
n
where
⎤
eQVzz ⎡
2
15
4
2
2
̂
̂ ̂
+ η(Ix − I )
HQ
=
3I
−
⎢
⎥
⎦
z
y
⎣
12
D and E are the axial and rhombic zero-field-splitting (ZFS)
parameters, respectively, and A is the magnetic hyperfine
coupling tensor. The quadrupole interactions (HQ) depend on
the principal components of the electric field gradient (EFG)
tensor V and are described by Vzz and the asymmetric
parameter η = (Vxx − Vyy)/Vzz. For all simulations, we assumed
that the ZFS, EFG, and A tensors are collinear. In addition, E/
D is fixed at zero and g values are assumed to be isotropic (g =
2.0). From the variable-field measurements, we determine that
2 has a large and positive D value (26 cm−1) that arises from
spin−orbit coupling of the S = 1 ground state with S = 2, 1, and
0 excited states. The D value of 2 is within experimental error
the same as that for 3, indicating that the S = 2 excited states of
the two complexes are at comparable energies. This congruence
of D values has important consequences for understanding the
hydrogen-atom-transfer (HAT) reactivity of these two
complexes (see Reactivity section below).
The 4.2 K zero-field Mossbauer spectrum of a frozen
̈
solution of 2 contains a quadrupole doublet representing 75%
of the total iron, with ΔEQ = 0.60 mm s−1 and δ = 0.13 mm s−1
(Figure 6, top). This δ value is close to that obtained for 3
In addition to the major S = 1 oxoiron(IV) species, 2
contains 20% mononuclear high-spin iron(III) species (Figure
S13) and an additional diamagnetic species representing 5% of
the total iron (the gray solid curve above the top spectrum in
Figure 6). The latter has an isomer shift δ of 0.44 mm s−1 and a
quadrupole splitting ΔEQ of 1.13 mm s−1 and likely
corresponds to a minor diferric decay product.
The X-ray absorption near-edge structure region of the Fe K-
edge XAS spectrum of 2 exhibits features at 7114.01 eV
(preedge) and 7118.34 eV (rising edge) and a K-edge energy of
7124.54 eV. The first of these features derives from 1s → 3d
transitions at the iron center, which increase in probability as
the degree of 4p and 3d orbital mixing increases, and the
second at approximately 7118 eV most likely arises from edge
transitions (e.g., 1s → 4p). The energies of the aforementioned
preedge and K-edge are effectively identical with those of the
corresponding features in 3,52 which indicates that any
differences between the electronic environments of these two
complexes are not sufficient to impact the energy of these
transitions. The preedge feature was found to have an area of
33 units, after scaling according to the amount of oxoiron(IV)
present in the sample (it is assumed other components do not
contribute significantly to the preedge area), which is identical
with that reported for 3.
Figure 6. Mossbauer spectra of 2 recorded at the various temperatures
̈
and parallel applied fields indicated. The sample contained 20% high-
spin ferric impurity, and its 4.2 K spectral features were simulated
(Figure S13) and removed from the data. Solid red lines are spectral
simulations using the parameters listed in Table 2. The solid gray line
is a spectral simulation of a diamagnetic impurity representing 5% of
the total iron.
The k3-weighted EXAFS and Fourier-transformed data of 2
are displayed in Figure 7, and the results of EXAFS analysis
(Table S2) are listed in Table 2. The Fourier-transformed data
exhibit at least two main features at r′ ∼ 1.5 and 2.5 Å. The
peak at r′ ∼ 1.5 Å is best fit with four Fe−N/O scatterers at
2.04 Å, which can be attributed to the nitrogen donors of the
Me2EBC ligand, plus a short Fe−O scatterer at 1.64 Å, a
distance that is characteristic of the FeIVO bond. The r′ ∼ 2.5
Å feature can be fit by six Fe−C scatterers at a distance of 2.94
Å, which is typically seen for macrocyclic polyamine ligands and
corresponds to the carbon framework of the Me2EBC ligand.
These values are broadly similar to those found for 3,52 with the
main distinction being that the average Fe−N distance in 2 is
(Table 2)15 and typical of S = 1 oxoiron(IV) complexes.11 The
magnetic hyperfine interactions and parameters obtained from
the applied field measurements shown in Figure 6 confirm this
assumption. Interestingly, the ΔEQ value obtained for 2 is
approximately half the size of that for 3, which may reflect a
decrease in the axial symmetry accompanying the fact that the
MeCN ligand is cis, and not trans, to the oxo ligand.
The Mossbauer spectra of 2, shown in Figure 6, have been fit
̈
using the S = 1 spin Hamiltonian
F
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Inorganic Chemistry
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being further out of the oxo pocket (Figure S15). Thus, the
hydrogen atoms (Hcavity) in 3 are closer to the oxo moiety and
may shield it further from incoming substrates. Also, the
coordination of CH3CN cis to the FeIVO unit in 2 provides a
“gap” (with an HMe−O−HMe angle of 111°) in the steric barrier
surrounding the oxo ligand (the hydrogen atoms on CH3CN
are at least 4.74 Å away with an FeIVO···H angle of 88.8°;
Figure S16). The aforementioned differences in substrate access
to the FeIVO unit may be a factor in modulating its reactivity.
Reactivity of 2 (vs 3). The intermolecular oxygen-atom-
transfer (OAT) and HAT reactivities of 2 were probed with
various substrates and found to exhibit second-order kinetics in
all cases. The observed rate constants (k2) are listed in Table 3.
Table 3. Oxidation Rate Constants (k2, M−1 s−1) for
a
[FeIV(O)(L)]2+ Complexes 2 and 3
Figure 7. Fourier-transformed Fe K-edge EXAFS data (dotted black
line) and corresponding best fit (solid red line) for 2. The inset shows
unfiltered EXAFS data k3[χ(k)].
T (°C)
L = Me2EBC (2)
L = TMC (3)
0.39
xanthene
DHA
0
12.8
0.04 Å shorter, presumably because of the smaller/tighter
binding cavity provided by Me2EBC relative to TMC. The
distances found experimentally are in reasonable agreement
with values predicted by DFT calculations (Table 2).
DFT calculations at the B3LYP/6-311G level of theory were
carried out for 2 (Table S3), and relevant comparisons with
experimental data are presented in Table 2. The calculated
FeO distance is in excellent agreement with the EXAFS-
derived value, while the calculated average Fe−N distance is
0.04 Å longer than the value obtained from EXAFS. An
overestimation of the Fe−N bond lengths of this magnitude is
typically seen when using the B3LYP functional and was
observed as well in the case of 3.52 The reliability of these
optimized geometries was supported by good agreement
0 25
7.4
0.14
b
27 (KIE = 25)
0.5 (KIE = 10)
CHD
0
0
7.4
0.12
fluorene
PPh3
0.09
9.0
9.2 × 10−4
4.2 × 10−2
5.9
−40
0
c
−
d
PhSMe
Ered vs Fc+/0
0
0.41
9 × 10−4
0.02
e
25
0.08
a
b
c
For product yields, see Table S5. Data from ref 16. Too fast to
d
determine by conventional kinetics. We were not able to reproduce
the k2 values reported for the reaction of 3 with PhSMe in ref 60,
which were about 1 order of magnitude higher (Figure S17 and Table
S4). Obtained by Fukuzumi’s method61 (Figure S18).
e
between the calculated Mossbauer parameters and the
̈
Complex 2 oxidized PPh3 and PhSMe to corresponding oxides
at rates consistent with the relative oxidizabilities of the two
substrates. HAT reactions of 2 with substrates like xanthene,
DHA, CHD, and fluorine were also studied. As found for other
oxoiron(IV) complexes, a linear correlation was obtained when
log k2′ values of HAT reactions (k2′ = k2 divided by the number
of equivalent C−H bonds on the substrate) were plotted versus
the strength of the substrate C−H bonds being cleaved,
indicating that the rate-determining step involves C−H bond
cleavage (Figure 9). Interestingly, a large nonclassical H/D KIE
of 25 was measured for the oxidation of dihydroanthracene by
2 at 25 °C.
corresponding experimental values. Space-filling models for
the two geometry-optimized structures are compared in Figure
8. Initial inspection of these models suggested that the FeIVO
When the rate constants associated with the oxidation of
substrates by 2 are compared with the corresponding values for
Figure 8. van der Waals radii space-filling models derived from
geometry-optimized structures of 2 (left) and 3 (right). Atom color
scheme: C, gray; H, white; N, blue; methyl C of the Me2EBC
supporting ligand for emphasis, green.
unit of 2 is not as shielded as that of 3 by the
tetraazamacrocyclic ligand framework. This impression was
validated by a comparison of the distances between the oxo
ligand and the hydrogen atoms forming the oxo cavity (Hcavity),
along with the corresponding FeIVO···Hcavity angles in 2 and
3. In the former complex, the O···Hcavity distances range
between 2.40 and 2.80 Å with an average value of 2.61 Å,
whereas they vary between 2.25 and 2.51 Å with an average
value of 2.38 Å in the latter (Figure S14). Additionally, the
smaller average FeIVO···Hcavity angle of 91.3° (ranging
between 84.0 and 98.3°) in 2 compared to 98.6° (ranging
between 99.3 and 101.1°) in 3 is consistent with the oxo ligand
Figure 9. Plots of log k2′ values obtained at 0 °C versus the bond
dissociation energies of substrate C−H bonds oxidized by 2 (blue
squares) and 3 (red circles). Solid lines (red and blue) represent the
best fits to the data.
G
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3 in Table 3, it is evident that 2 is more reactive than 3 in both
HAT (30−100-fold) and OAT (200−600-fold) reactions.
There is also a large difference between the H/D KIE values
for the oxidation of DHA, 25 for 2 versus 10 for 3.16 These
reactivity differences are quite intriguing, given the fact that the
macrocyclic ligands of 2 and 3 provide essentially identical
donors but adopt different topologies. (Although it is not
strictly the case, 2 and 3 may be construed as approximate
geometrical isomers of each other.) Furthermore, the fact that
the D values determined for 2 and 3 are the same within
experimental error suggests that their electronic structures are
comparable. Thus, the typical argument used in the two-state-
reactivity (TSR) model of related complexes having different S
= 2 excited-state energy levels62 cannot be applied to the 2/3
pair.
reduction potentials between the two members of each pair,
0.11 V for the BQCN pair63 and 0.25 V for the BP pair.59 In
both cases, the isomer with the more positive reduction
potential is the more reactive complex. These pairwise trends
are in line with that found for a series of five [FeIV(O)(N5)]
complexes, where N5 is a pentadentate ligand with a
combination of pyridine and tertiary amine donors.59 [FeIV(O)-
(BP1)] and [FeIV(O)(BP2)] are in fact members of this series.
Observed for the series is a linear correlation for the logarithms
of the second-order OAT rate constants for PhSMe versus their
FeIV/III reduction potentials, and the HAT values exhibit
correlations with similar slopes. However, the even smaller
difference (0.06 V) between the FeIV/III potentials of found for
2 and 3 (Table 3) makes it difficult to apply the reduction
potential argument to rationalize the observed reactivity
differences. Clearly, other factors besides the redox potential
must modulate the reactivity.
Another obvious factor to consider is the accessibility of the
FeO unit for reaction with substrates. On the basis of DFT
calculations of 2 and 3 (vide supra, Figure 8), the FeO unit
of 2 appears to be more accessible than that of 3. The oxo
ligand of 3 is well shielded by the hydrogen atoms of the TMC
macrocycle, particularly for a substrate attack trajectory
perpendicular to the FeO bond that is required for
interaction with the singly occupied π* orbitals of the S = 1
FeO unit.16,38 On the other hand, the more open topology of
the Me2EBC macrocycle should allow easier access to the Fe
O unit via this trajectory. This possibility of a π* attack
trajectory was tested by comparing the oxidation rates of DHA
and CHD, substrates with comparable C−H bond strength but
different steric bulk. However, as can be seen in Table 3, the
rates of DHA and CHD oxidation for 2 are not much different
from each other, and the corresponding rates for 3 are also
similar to each other but much slower than those for 2. From
these comparisons, we surmise that there is no significant steric
barrier for the HAT reactions of these two complexes. It is
likely then that the HAT reactions of 2 and 3 do not occur via
the π* trajectory but instead take place by the much less
hindered σ* trajectory, which is available only on the S = 2
excited-state surface.38,51,65,66 In support of the generality of
this σ* mechanism for HAT, we note that comparable DHA
and CHD oxidation rates have, in fact, been observed for a
number of S = 1 oxoiron(IV) complexes.67 Comba and co-
workers also reached a similar conclusion in their efforts to
understand the relative HAT reactivity of the BP isomers
discussed earlier. Their DFT and force-field calculations carried
out on this pair of complexes afford computed barriers for the
rate-determining hydrogen-atom-abstraction step on the S = 2
excited-state surface via the σ* trajectory that differ by 0.6 kcal
mol−1, in accordance with the experimental data.64,68
A role for ligand topology in modulating the reactivity has
been demonstrated for two pairs of isomers among the
oxoiron(IV) complexes characterized to date. These are the
only two examples for which the reactivity data has been
reported (Figure 10). One pair consists of the cis-α and cis-β
Figure 10. Two pairs of oxoiron(IV) complexes that differ with
respect to the ligand topology.
isomers of [FeIV(O)(BQCN)]2+ described by Nam et al., where
the quinoline rings are oriented respectively trans and cis to
each other.63 The cis-α isomer is more reactive than the cis-β
isomer by 10−20-fold with respect to HAT and 100-fold with
respect to OAT. The other pair consists of the [FeIV(O)-
(BP1)]2+ and [FeIV(O)(BP2)]2+ complexes of Comba et al.,64
which differ in the orientations of two pyridine rings relative to
the FeO bond; the BP2 isomer with two pyridine rings
perpendicular to the FeO bond is more reactive by 20−40-
fold with respect to HAT and 100-fold with respect to OAT.59
Thus, the effects of a change in the ligand topology are
comparable for the BQCN and BP pairs of complexes.
Additional DFT calculations were thus carried out to
investigate the reactivity differences between 2 and 3 with
respect to DHA oxidation via HAT. The UB3LYP/B1-
optimized geometries for 2 and 3 together with relevant
iron−ligand distances in angstroms are presented in Figure S19.
The main conclusions derived from these calculations are
presented here in the main text; all of the corresponding
reactivity data are reported in Table S6. Both reactions exhibit a
typical TSR scenario,62,65,66,69 in which the S = 2 transition
state, 5TS, lies well below that of the S = 1 species, 3TS. At all of
the levels of theory employed, 2 was found to be more reactive
than 3 (Table S6), in line with experimental observations.
Table 4 shows the free-energy barriers at the highest level
The reactivity difference within each pair of complexes can
be qualitatively rationalized by differences in the FeIV/III
H
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Table 4. Calculated Imaginary Frequencies in the TSs, Free-
Energy Barriers, and KIEs for the S = 1 Process and for a
TSR Scenario via the S = 2 TS
KIE
S = 1
νim
→
a
a
oxidant species (cm−1
)
ΔG⧧
ΔG⧧
S = 1 S = 2 KIEexp
273 K
exp
2
3TS
5TS
3TS
5TS
i1986
32.2
284
20
20
15
15
25
25
10
10
b
b
i1464
i2088
i1346
17.5
14.8
17.0
3
34.1
20.3
54
a
In kilocalories per mole using B3 for the computational values and
b
the Eyring equation for the experimental values. These are tunneling-
corrected barriers (the correction is 1.3 kcal mol−1 for 2 and 3). The
barrier datum for 3 in ref 65 was obtained with a different solvent
correction protocol than that in the present study (using PCM in
Gaussian09). Note that the higher reactivity of 2 versus 3 is
reproduced here at all of the computational levels.
(ΔG‡), imaginary frequencies of the TSs (νim), and the
corresponding KIE values calculated for the reactions
5
commencing on the S = 1 reactant state and passing via TS,
along with experimental values. It is seen that the calculations
reproduce the trends in the free-energy barriers for the TSR
scenario, although the absolute values of the barriers after
tunneling corrections are a bit larger than the experimental
values by about 3 kcal mol−1. The calculations also reproduce
the observation that both HAT processes involve significant
tunneling, which is larger for 2. The computed KIE values are
accordingly 20 for 2 and 15 for 3, in reasonable agreement with
the experimentally observed KIEs of 25 and 10, respectively.
Table 4 further lists the calculated KIEs for the two reactions
proceeding on the S = 1 surface. These huge KIEs are clearly
much larger than the ones involving passage on the S = 2 surface,
thus supporting the previous predictions that KIE can serve as a
probe of the reacting spin state.65
Figure 11. UB3LYP/B1-optimized geometries with important geo-
metrical parameters (distances in angstroms and angles in degrees) for
the TSs. Only selected hydrogen atoms are shown for clarity.
identical for 2 and 3 (see Table S8), as also deduced
experimentally from their essentially identical D values (see
Table 1).
Having ascertained that the S = 1/S = 2 energy gap does not
contribute to the terms in eqs 2a and 2b, we can inspect the
distortion and interaction energies in Table 5 (see the details in
5
As clearly illustrated in Figure 11, TS(2) has the scissile
hydrogen atom equidistant from the carbon and oxygen atoms
Table 5. Distortion and Interaction Energies (kcal mol−1)
Calculated at the B3 Level of Theory
(rCH ∼ rOH), and the C−H bond is slightly more elongated
5
than that in TS(3). This may account for the finding that the
5
imaginary frequency of TS(2) is a little bit higher (∼120
reaction
ΔE⧧
ΔE⧧
5
cm−1) than that of TS(3), thereby leading to a higher KIE.
dist
int
2 + DHA
3 + DHA
21.4
18.1
−7.3
−2.4
What then is the factor that lowers the HAT barrier for 2
relative to 3? To answer this question, one can generally
express the barrier as a sum of the following two terms:70−74
ΔE⧧ = ΔE⧧ + ΔE⧧
Tables S10, parts 1 and 2, and S11 presented in the Supporting
Information). It is seen that the reaction of 2 with DHA
possesses a higher distortion energy (ΔE⧧dist = 21.4 kcal mol−1)
dist
int
ΔE⧧dist is the distortion energy of the two reactants in their 5TS
geometries relative to their relaxed geometries at the S = 1
than the corresponding reaction of 3 (ΔE⧧ = 18.1 kcal
dist
reactant state. ΔE⧧ is the interaction energy of the two
mol−1). This energy difference of 3.3 kcal mol−1 is mainly due
to the higher distortion of the DHA substrate (2.5 kcal mol−1)
in the 5TS for 2. However, Table 5 also shows that the
int
5
fragments in TS. ΔE⧧ and ΔE⧧ were calculated using the
dist
int
following equations:
interaction energy ΔE⧧ is significantly more favorable in
ΔE⧧ = E⧧(S) + E⧧(5O) − [E(S) + E(3O)]
int
(2a)
(2b)
5TS(2+DHA) than in 5TS(3+DHA), indicating that 5TS-
(2+DHA) is stabilized relatively more by electrostatic
dist
ΔE⧧ = E(5TS) − [E⧧(S) + E⧧(5O)]
int
5
interaction and orbital mixing than TS(3+DHA) and suffers
where E⧧(S) and E⧧(5O) are single-point energies of the
separated substrate and quintet-state oxidant fragments in the
TS. Because the reactants start at the S = 1 ground state and
cross over to the S = 2 surface during TSR, we must first
ascertain that the initial S = 1/S = 2 energy gap does not affect
the relative distortion energies. Indeed, these energy gaps are
less Pauli repulsion. Our calculations show that the electrostatic
interaction contributes only ca. 1 kcal mol−1, and the Pauli
5
repulsions are also slightly less for TS(2+DHA) (see the
average distances in Figure S20. Hence, TS stabilization is
dominated mainly by orbital mixing. Orbital mixing is, in turn,
controlled by the ability of the oxidant to make an O---H bond
I
DOI: 10.1021/acs.inorgchem.5b00861
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
in the corresponding TS. This capability of the oxidant can be
predicted from the spin density on the oxygen atom of the
FeO oxidant.75 Even at the stage of the relaxed quintet-state
oxidants, the spin-density values on the oxygen atoms of 2 and
3 are different, i.e., 0.74 and 0.64, respectively. En route to 5TS,
the difference between these values grows, becoming 0.71 and
0.54 for 2 and 3, respectively (see also Tables S12 and S13). So,
we can safely argue that the oxyl character (O•) of the FeO
moiety of 2 is significantly larger than that of 3. Consequently,
the O---H bond in the TS is stronger for 2 than for 3, and this
is the main reason why 5TS(2+DHA) is stabilized relative to its
fragments. It follows therefore that the reactants 2 and DHA
pay slightly more in terms of distortion energies in order to
gain a more favorable bond-making interaction in the TS. So,
considering all of the aforementioned factors, we can
understand why 2 is more reactive than 3.
AUTHOR INFORMATION
■
Corresponding Authors
*E-mail: ysguo@andrew.cmu.edu.
*E-mail: emunck@cmu.edu.
*E-mail: sason@yfaat.ch.huji.ac.il.
*E-mail: larryque@umn.edu.
Present Addresses
⊥J.E.: Division of Chemistry and Biological Chemistry, School
of Physical and Mathematical Sciences, Nanyang Technological
University, 21 Nanyang Link, Singapore 637371, Singapore.
∥M.A.C.: Colorado State University - Pueblo, Pueblo, CO
81001.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
CONCLUSIONS
This work was supported by grants from the U.S. National
Science Foundation (Grants CHE-1058248 and CHE-1361773
to L.Q. and Grant CHE-1305111 to E.M.) and a postdoctoral
fellowship from the National Institutes of Health (NIH;
ES017390 to M.A.C.). S.S. acknowledges support from the
Israel Science Foundation (Grant 1183/13). XAS data were
collected on beamline 7-3 at the SSRL. Use of the SSRL of the
SLAC National Accelerator Laboratory is supported by the U.S.
Department of Energy, Office of Science, Office of Basic Energy
Sciences, under Contract DE-AC02-76SF00515. The SSRL
Structural Molecular Biology Program is supported by the DOE
Office of Biological and Environmental Research and by the
NIH, National Institute of General Medical Sciences (NIGMS;
including P41GM103393). The contents of this publication are
solely the responsibility of the authors and do not necessarily
represent the official views of NIGMS or NIH. We thank Drs.
Matthew Latimer, Allyson Aranda, and Erik Nelson for their
excellent technical support of our XAS experiments. X-ray
crystallographic data collection and structure solutions were
conducted by Dr. Victor G. Young, Jr. at the X-ray
Crystallographic Laboratory, Department of Chemistry, Uni-
versity of Minnesota.
■
Herein, we have detailed the synthesis, spectroscopic proper-
ties, and reactivity of the oxoiron(IV) complex 2 and compared
it with the extensively studied complex 3. Both Me2EBC and
TMC are tetradentate N-tetraalkylcyclam ligands that possess
essentially identical donor atoms. Whereas the former enforces
a geometry that allows the coordination of two additional cis
ligands, the latter forms complexes with these sites disposed
trans to one another. As a consequence, 2 and 3 represent a
unique opportunity to compare two systems possessing
effectively identical coordination spheres, differing only in the
position of the solvent-labile site cis/trans to FeIVO, in terms
of their spectroscopic and reactivity properties. Indeed, the
differing coordination geometries of 2 and 3 result in distinct
UV−vis, Raman, and Mossbauer features. However, both
̈
complexes have comparable FeIV/III potentials and exhibit
essentially identical ZFS values, which implies that their
respective S = 2 excited states are at comparable energies.
Despite these similarities, 2 is 30−600-fold more reactive than
3 with respect to HAT and OAT reactions. DFT calculations
reproduce the higher HAT reactivity of 2, which is attributed to
5
the greater oxyl character acquired by the oxo atom of the 2
fragment in 5TS. The reactivity data presented herein
comparing 2 and 3, together with the recent results of Mandal
et al.65 and Nam et al. on Fe(13-TMC) complexes,60,76 serve to
emphasize what a gold mine of information is provided by even
the subset of oxoiron(IV) complexes supported by cyclam
ligands and demonstrate that our understanding of the
reactivity of nonheme oxoiron(IV) complexes remains
incomplete and justifies continued investigation.
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