A tosylimido analogue of a nonheme oxoiron(IV) complex

Article abstract of DOI:10.1002/anie.200602799

(Figure Presented) High-valent versatility: N4Py is a versatile pentadentate ligand demonstrated to stabilize both high-valent oxo and imido compounds. The [Fe^IV(NTs)(N4Py)]²⁺ complex (see DFT-optimized structure) has been characterized spectroscopically and formulated as having an S=1 iron(IV) center with an Fe=N bond (1.73 A) that is longer than the Fe=O bond of its oxo analogue (1.65 A).

Full text of DOI:10.1002/anie.200602799

Communications
the generation and characterization of the tosylimidoiron(IV)
analogue of the well-characterized oxoiron(IV) complex
[FeIV(O)(N4Py)]2+ (N4Py = N,N-bis(2-pyridylmethyl)bis(2-
pyridyl)methylamine).[13] This first example of a pair of
complexes with a ligand that can support both an oxo- and
imidoiron(IV) unit provides a unique opportunity for a
comparative study.
The oxoiron(IV) complex [FeIV(O)(N4Py)]2+ (1) can be
readily generated in high yield by oxygen-atom transfer from
solid iodosylbenzene (PhIO) to [FeII(NCMe)(N4Py)]2+ in
MeCN.[8] In an analogous manner, the tosylimidoiron(IV)
complex [FeIV(NTs)(N4Py)]2+ (2) was prepared by treating
the iron(II) precursor in MeCN with solid mesityl-N-tosyl-
imidoiodinane (MsINTs; Ms = 2,4,6-Me3C6H2, Ts = tosyl) at
room temperature. Within minutes of the addition, the orange
solution of the iron(II) starting material faded to a gold color.
Complex 2 exhibits a visible spectrum (Figure 1) with an
Iron Imido Complexes
DOI: 10.1002/anie.200602799
A Tosylimido Analogue of a Nonheme
Oxoiron(IV) Complex**
Eric J. Klinker, Timothy A. Jackson, Michael P. Jensen,
Audria Stubna, Gergely Juhµsz, Emile L. Bominaar,
Eckard Münck,* and Lawrence Que, Jr.*
High-valent nonheme iron complexes play key roles as
intermediates in biology and catalysis.[1–3] Examples of
oxoiron(IV) centers in nonheme ligand environments have
been characterized in both enzymes[4,5] and model systems[6]
and have been shown to participate in the hydroxylation of
[7,8]
ꢀ
C H bonds.
The analogous imidoiron(IV) unit should be
Figure 1. Visible spectra of [FeIV(X)(N4Py)]2+ in MeCN(X =O, 1;
X=NTs, 2). Inset: Vertically expanded lower-energy region.
capable of isolobal amination reactions and has been
implicated in such iron-catalyzed reactions;[9] it is also
proposed to be an intermediate in the formation of an
isolated amidoiron(III) complex.[10] The only two structurally
characterized examples of an imidoiron(IV) unit show this
group in a tetrahedral geometry supported by a tridentate
pyrazolyl/bis(phosphino)borate ligand[11] or as part of a novel
{FeIII3FeIV(NR)4} cubane cluster.[12] In this paper, we report
intense band at l = 445 nm (e = 2700mꢀ1 cmꢀ1) and a much
weaker and broader feature at l = 660 nm (e = 250mꢀ1 cmꢀ1,
Figure 1 inset). The latter may correspond to the band at l =
695 nm (e = 400mꢀ1 cmꢀ1, Figure 1 inset) observed for 1 that
arises from ligand-field transitions characteristic of an S = 1
oxoiron(IV) center.[14] Complex 2 has a Mössbauer spectrum
at 4.2 K that consists of a doublet with a quadrupole splitting
of 0.93 mmsꢀ1 and an isomer shift of 0.02 mmsꢀ1, parameters
that fall within the range of those found for nonheme
oxoiron(IV) complexes.[2] It gives rise to a high-resolution
electrospray mass spectrum with a major ion species at
m/z 741.0860 that corresponds to [{FeIV(NTs)(N4Py)}-
(SO3CF3)]+ (m/z calculated 741.0859).
[*] Dr. A. Stubna, Dr. G. Juhµsz, Prof. Dr. E. L. Bominaar,
Prof. Dr. E. Münck
Department of Chemistry
Carnegie Mellon University
Pittsburgh, PA 15213 (USA)
Fax: (+1)412-268-1061
E-mail: emunck@cmu.edu
E. J. Klinker, Dr. T. A. Jackson, Dr. M. P. Jensen, Prof. Dr. L. Que, Jr.
Department of Chemistry and
1
The H NMR spectrum of 2 (Figure 2) is similar to, but
distinct from, that of 1. Like 1, complex 2 exhibits two equally
intense sets of paramagnetically shifted pyridine ring protons,
indicating the presence of an effective mirror plane of
symmetry on the NMR time scale. The pyridine protons
display a unique shift pattern arising from the S = 1 iron(IV)
center[13] in which one pyridine b-proton signal is downfield-
shifted to about d = 45 ppm, the other b-proton signal is
upfield-shifted to about d = ꢀ15 ppm, and the g-proton peak
is downfield-shifted to about d = 18 ppm. The 1H NMR
spectrum of 2 also shows the presence of a small amount of
1; the latter arises from trace water in the solvent that
Center for Metals in Biocatalysis
University of Minnesota
Minneapolis, MN55455 (USA)
Fax: (+1)612-624-7029
E-mail: que@chem.umn.edu
[**] Supported by NIH grants GM-33162 (L.Q.) and GM-22701 (E.M.),
an NIH postdoctoral fellowship to T.A.J., and an NSF graduate
research fellowship to A.S. XAS data were collected at the Stanford
Synchrotron Radiation Laboratory (SSRL), which is supported by the
US Department of Energy and the National Institutes of Health.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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Angewandte
Chemie
Figure 3. Fourier transform of the Fe K-edge EXAFS data and EXAFS
spectrum (inset) of [FeIV(NTs)(N4Py)]2+. Experimental data are shown
in dashed lines and fits in solid lines. See the Supporting Information
for EXAFS and Fourier transform range.
Figure 2. 1H NMR spectrum of 2 in CD3CN. Peaks assigned to a minor
impurity of 1 are marked by ( ). Insets show the d=6–10 ppm region
*
for 2 under conditions to enhance paramagnetically shifted protons
(b) and under normal 1H NMR conditions (c).
ray crystal structure of 1. The single scatterer at 1.73 Š is thus
hydrolyzes part of the MsINTs oxidant to MsIO (2 is not
converted into 1 by the addition of trace water). The tosyl
aromatic protons are observed at d = 8.50 and 7.71 ppm
(Figure 2b), with integrations that match the values found for
attributed to the N atom of the NTs ligand.
ꢀ
Notably, the Fe N bond length found for 2 (1.73 Š) is
significantly longer than the bond lengths observed crystallo-
[13]
ꢀ
ꢀ
graphically for the Fe O bond in 1 and for the Fe N bonds
of two other iron(IV) imido complexes (ca. 1.65 Š).[11,12]
Relative to the latter two complexes, the longer bond in 2
can be rationalized by the difference in the coordination
numbers of the iron(IV) centers. The previously characterized
complexes have a tetrahedral geometry about the iron center
with a strong trigonal distortion for which the p-antibonding
molecular orbitals (MOs) associated with the imido ligand are
1
the pyridine proton peaks of 2. With H NMR parameters
used for diamagnetic compounds, these tosyl resonances
become obscured by peaks at d = 7.75 and 7.37 ppm (Fig-
ure 2c) corresponding to the protons of free tosylamide. The
integrations of these peaks relative to that of the aromatic
protons of the iodomesitylene by-product (d = 6.95 ppm)
reveal an amount of free tosylamide (20%) that corresponds
well to the amount of 1 present in the sample (15%). These
data further support the formulation of 2 as [FeIV(NTs)-
(N4Py)]2+.
unoccupied.[3] These complexes thus have a formal Fe NR
ꢁ
triple bond and a collinear Fe-N-R unit.[3,11,12] In contrast, the
corresponding MOs for octahedral 2 are singly occupied, thus
=
Fe K-edge X-ray absorption spectroscopy (XAS) provides
additional insight into the geometric and electronic structure
of 2. Complexes 1 and 2 exhibit 1s!3d transitions at about
7114 eV with areas of 25 and 18 units, respectively (Support-
ing Information).[15] As discussed previously, the relatively
giving rise to an Fe N double bond instead. This experimen-
tal result is supported by a DFT geometry-optimized struc-
ture for 2 (Figure 4a and the Supporting Information), which
is constrained to have Cs symmetry to be consistent with the
ꢀ
NMR results and displays a Fe NTs bond length of 1.75 Š.
ꢀ
=
large value observed for 1 is a result of the short Fe O bond
The effects of the different Fe X units in 1 and 2 on their
length of 1.64 Š;[15] consequently, the pre-edge area of 18
ground-state electronic structures were revealed by high-field
Mössbauer analysis; the experimental results were comple-
mented by DFT calculations that were based on the
geometry-optimized structures for 1 and 2 (see the Supporting
Information). Variable-temperature Mössbauer spectra for 1
and 2 in acetonitrile were obtained in applied magnetic fields
of up to 8.0 T; spectra of each compound at 4.2 K are shown in
Figure 5. The spectra were simulated by using the S = 1 spin
Hamiltonian of Equation (1), in which all symbols have their
conventional meanings.[17]
suggests a smaller degree of distortion for the iron center of 2,
ꢀ
namely, a relatively long Fe NTs bond. Indeed, a similar
value was recently observed for the thiolate-ligated oxo-
iron(IV) complex [FeIV(O)(tmcs)]+ (tmcs = monoanion of 1-
mercaptoethyl-4,8,11-trimethyl-1,4,8,11-tetraazacyclotetrade-
cane), which, on the basis of an EXAFS analysis, exhibits a
[16]
ꢀ
somewhat longer Fe O bond (1.70 Š).
An analysis of the EXAFS data obtained for 2 provides
metric details of the iron coordination environment. In
Figure 3 are displayed the R’-space and k-space EXAFS
spectra of 2 along with the best fit to these data (see the
Supporting Information for fitting details). The EXAFS data
for 2 are well fit with three shells consisting of one N scatterer
at 1.73 Š, five N scatterers at 1.97 Š, and six C scatterers at
2.87 Š. The latter two shells can be associated respectively
with N and C atoms of the N4Py ligand, as observed in the X-
H ¼ DðS2ꢀ = Þ þ EðS2ꢀS2Þ þ bB g S þ S A Iꢀgnbn I B þ HQ
2
3
z
x
y
ꢀ
ꢁ
ð1Þ
e Q Vzz
12
15
HQ
¼
3 I2zꢀ þ hðI2xꢀI2yÞ
4
Analysis of the spectra of 1 shows that the magnetic
hyperfine tensor, A, and the zero-field splitting (ZFS) tensor
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7395

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are both axial (that is, Ax = Ay and E/D = 0) and that Az lies
along the symmetry axis of the ZFS tensor (Table 1). DFT
computations for 1 also yield an axial A tensor, with Az
ꢀ
aligned within 108 of the Fe O bond. Our combined DFT/
crystal-field studies of previous complexes[14,16,18] and 1
suggest that the z axis of the ZFS tensor and therefore of
ꢀ
Equation (1) is along the Fe O vector.
Table 1: Properties of [FeIV(NTs)(N4Py)]2+ (2) and [FeIV(O)(N4Py)]2+ (1)
complexes.[a]
[FeIV(NTs)(N4Py)]2+
[FeIV(O)(N4Py)]2+
t1/2 at 258C [h]
3 h
60 h
r(Fe N/O) [Š]
1.73 (1.75)
1.97 (2.01)
445 {2700}
660 {250}
0.02 (0.13[c])
+0.98 (1.13)
ꢀ0.7 (ꢀ1.2)
29
1.64[b] (1.65)
1.96 (2.01)
ꢀ
lmax [nm] {e [mꢀ1 cmꢀ1]}
695 {400}
ꢀ0.04 (0.03[c])
+0.93 (1.03)
0<h<0.5 (0.01)
22
d [mmsꢀ1
]
DEQ [mmsꢀ1
]
h
D [cmꢀ1
E/D
]
0.23
0
Ax/gnb [T][d,e]
Ay/gnbn [T][d,e]
Az/gnbn [T][d,e]
Eo [eV]
ꢀ20 (ꢀ20.5)
ꢀ21 (ꢀ18.3)
ꢀ3 (ꢀ5)
7123.8
7113.9
18
ꢀ21 (ꢀ20.3)
ꢀ21 (ꢀ20.8)
ꢀ5 (ꢀ4.1)
7123.7[f]
Figure 4. Molecular structure of [FeIV(NTs)(N4Py)]2+ based on DFT
geometry optimization (a), and contour plots of the Fe 3dz2-based
MOs of [FeIV(O)(N4Py)]2+ (b) and [FeIV(NTs)(N4Py)]2+ (c) based on
DFT computations. Fe purple, C black, Nblue, O red, S yellow. The
contributions to the MOs from the Fe 3dz2 and oxo O and imido N
2p orbitals are displayed. For clarity in (c), the tolyl ring is not shown.
Epre-edge [eV]
pre-edge area
7114.3[f]
25[f]
[a] Values in parentheses derived from DFT calculations (see the
Supporting Information for details). [b] Obtained by X-ray crystallogra-
phy; see reference [13]. [c] For the N4Py ligand, for reasons not yet
understood, our computations yield d values noticeably larger than
those observed experimentally. Rotation of the tosyl group around the
ꢀ
Fe Nbond, followed by geometry optimization, changed the d value by
less than 0.02 mmsꢀ1. [d] Final A values were obtained by adding the
calculated spin–dipolar contribution to the experimental value Aiso
=
(Ax +Ay +Az)/3. We used this procedure because DFT Aiso values
generally are in poor agreement with experimental data. The z axes of
the electric-field gradient tensor and the spin–dipolar part of A are
ꢀ
coincident within 58 with the Fe NTs bond axis. [e] At 4.2 K the internal
field Bint ꢂ{g?A?S(S+1)/D}bB/gnbn and thus the product g?A?, rather
than A?, is determined, thereby illustrating the point for axial symmetry.
In our simulations we have fixed g? =2. The real A? value may be slightly
smaller but probably not by more than 10%. As an example, the
correction is about 1% for [FeIV(O)(tmcs)]+ as calculated by time-
dependent DFT.[16] [f] From reference [15].
The applied-field Mössbauer spectra of 2 are quite distinct
from those of 1 (Figure 5b,d). These differences arise from a
substantial anisotropy of the magnetic hyperfine field (Bint
=
ꢀhSiA/gn bn) in the xy plane of 2 (for example, 3.3 T in the
spectrum of Figure 5d). This anisotropy can be caused by a
rhombic ZFS tensor (E/D ¼ 0) and/or by a rhombic A tensor
(Ax ¼ Ay). Although the Mössbauer data do not allow us to
determine these three parameters independently, the expres-
sion A? = (Ax + Ay)/2 is nearly independent of the E/D value
used to fit the spectra. For Cs symmetry, implied by the
1H NMR spectrum of 2 (Figure 2), the anisotropy of A is
determined by the spin–dipolar contribution, AS-D. As DFT
calculations on 2 yield a nearly axial AS-D, (Ajj/gnbn = ꢀ3.5, ꢀ5,
+ 8.5 T for j = x, y, and z, respectively), we conclude that the
magnetic anisotropy of 2 is a result of a finite E/D value of
Figure 5. Mössbauer spectra of 1 (a, b) and 2 (c, d) in CH3CNat
4.2 K. Spectra in (a) and (c) were obtained in zero field while those in
(b) and (d) were obtained in a magnetic field of 8.0 T applied parallel
to the observed g rays. The solid lines are spectral simulations based
on the S=1 Hamiltonian of Equation (1) using the parameters in
Table 1 and in Table S2 of the Supporting Information. Both 1 and 2
contained minor contaminants; for clarity, their contributions were
removed from the spectra. A complete set of data, spectral simula-
tions, and information about the contaminants are given in the
Supporting Information.
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Angewandte
Chemie
0.23, determined by fitting the Mössbauer spectra of 2 with a
nearly axial A tensor. For 2 we found that A?/gnbn is about
ꢀ20.5 T if S = 1 is assumed, a value which fits well in the range
ꢀ20 T to ꢀ23 T observed for S = 1 complexes.[6,16,19] In
contrast, fitting the spectra of 2 under assumption of an S =
2 ground state yielded A?/gnbn ꢂ ꢀ6.5 T, which, in magnitude,
is substantially below ꢀA?/gnbn = 16–20 T observed for S = 2
FeIV species.[4,20]
Nonheme oxoiron(IV) complexes exhibit large D values,
which arise from spin–orbit coupling of the S = 1 ground state
with low-lying S = 2 excited states.[16,18] Importantly, this
coupling modifies the D value but affects the g and A tensors
of Equation (1) in a minor way at most. Spin–orbit coupling
with low-lying S = 1 excited states would give rise to
substantially smaller A?/gnbn values owing to partial
unquenching of the orbital angular momentum and give a
positive contribution to A.[21] Because this is not the case for 1
and 2, we conclude that spin–orbit coupling of the ground
state with low-lying S = 2 and/or S = 0 excited states is
responsible for the large D value, as shown in the Supporting
Information.
Keywords: bioinorganic chemistry · density functional
calculations · imido ligands · iron · Mössbauer spectroscopy
.
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The DFT calculations also shed light on the dominant
=
bonding interactions that determine the differing Fe X bond
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electronic configuration is observed, thus consistent with
=
=
formal Fe O and Fe NR bonds. As evidenced by the almost
equal mixture of Fe 3d and oxo O 2p or imido N 2p character
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are apparent in the Fe 3dz2-based MOs. For 1, a strong s-
antibonding interaction between the 3dz2 orbital and the
O pz orbital (Figure 4a) splits the spin-up Fe 3dx2ꢀy2- and 3dz2
-
based MOs by roughly 0.6 eV (Supporting Information). In
contrast, the corresponding MOs of 2 are nearly degenerate
(split by only about 0.05 eV). This is a result of a less favorable
interaction between the Fe 3dz2 and the N pz-like orbital of the
tosylimido ligand (Figure 4c), the latter of which is stabilized
relative to the corresponding N 2py and 2px orbitals owing to
ꢀ
the N S bonding interaction. Thus, these DFT studies reveal
=
that, despite the fact that both 2 and 1 contain formal Fe N
=
ꢀ
and Fe O bonds, respectively, the Fe O s interaction of 1 is
ꢀ
significantly stronger than the corresponding Fe NTs inter-
action in 2, which accounts in part for the relatively long Fe
NTs bond length of 1.73 Š.
ꢀ
ꢀ
The longer Fe N bond of 2 suggests that the tosylimido
group may not be able to stabilize the iron(IV) center as well
as the oxo ligand in 1. Indeed, 2 has a half-life of three hours
for self-decay at room temperature, which is a factor of 20
shorter than that of 1. This difference is also consistent with
the faster transfer of NTs from PhINTs (versus PhIO) to the
pendant phenyl group of [FeII(6-Ph-tpa)]2+ (tpa = tris(2-
pyridylmethyl)amine).[9a] Further studies are clearly war-
ranted to understand the relative oxidative abilities of FeIV
=
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O and FeIV NR units and to place them within the greater
=
context of recently reported high-valent nitridoiron com-
plexes.[22]
Received: July 13, 2006
Published online: October 13, 2006
Angew. Chem. Int. Ed. 2006, 45, 7394 –7397
ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
7397