Coordination-number dependence of reactivity in an imidoiron(III) complex

Article abstract of DOI:10.1002/anie.200601927

(Chemical Equation Presented) Reaction of the {L^MeFe} fragment with adamantyl azide (AdN₃) in the presence of 4-tert-butylpyridine (tBuPy) gives an S = 3/2 species that has been examined by ¹H NMR, IR, EPR, and Moessbauer

Full text of DOI:10.1002/anie.200601927

Communications
Coordination Chemistry
DOI: 10.1002/anie.200601927
Coordination-Number Dependence of Reactivity
in an Imidoiron(III) Complex**
Nathan A. Eckert, Sridhar Vaddadi, Sebastian Stoian,
Rene J. Lachicotte, Thomas R. Cundari,* and
Patrick L. Holland*
Mononuclear oxoiron species are postulated as intermediates
in enzymatic oxidations, and have been characterized in heme
systems and in the nonheme monoiron enzyme TauD.[1] Many
of the differences in reactivity between the enzymatic
intermediates are attributed to the spectator ligands.[2]
Recently, the observation and isolation of synthetic iron oxo
(LnFeO)[3] and iron imido (LnFeNR)[4] complexes has permit-
ted the systematic variation of the ligand environment in
species with Fe–O and Fe–N multiple bonds. The six-
coordinate [(tmc)(L)Fe(O)]2+ system (tmc = 1,4,8,11-tetra-
methyl-1,4,8,11-tetraazacyclotetradecane) in particular has
been amenable to variation of the ligand L.[3,5] In this context,
it is important to understand how the number and identity of
the spectator ligands modulate the reactivity of compounds
with iron–ligand multiple bonds. As a part of this effort, this
communication describes data that suggest the formation of a
metastable three-coordinate iron(III) imido complex. Unex-
pectedly, coordination of a fourth ligand induces hydrogen-
atom abstraction reactivity. These results contrast with the
[*] S. Vaddadi, Prof. T. R. Cundari
Department of Chemistry
University of North Texas
Denton, TX 76203 (USA)
Fax: (+1)940-565-4318
E-mail: tomc@unt.edu
Homepage: http://www.chem.unt.edu/faculty/cundari.htm
Dr. N. A. Eckert, Dr. R. J. Lachicotte, Prof. P. L. Holland
Department of Chemistry
University of Rochester
Rochester, NY 14627 (USA)
Fax: (+1)585-276-0205
E-mail: holland@chem.rochester.edu
Homepage: http://hollandimac.chem.rochester.edu
Dr. S. Stoian
Department of Chemistry
Carnegie Mellon University
Pittsburgh, PA 15213 (USA)
[**] The authors acknowledge funding from the National Science
Foundation (CHE-0134658 to P.L.H. and MCB-0424494 to Eckard
Münck), the A. P. Sloan Foundation (P.L.H.), the University of
Rochester (Weissberger Fellowship; N.A.E.), and the U.S. Depart-
ments of Education and Energy (DEFG02-03ER15387; T.R.C.).
Calculations were performed on the UNT computational chemistry
resource, for which S.V. and T.R.C. acknowledge the NSF for support
through grant CHE-0342824. We thank Eckard Münck for support in
recording and interpreting the EPR and Mössbauer spectra, and for
valuable discussions.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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typical situation in which more unsaturated metals give higher
reactivity. We also report electronic structure calculations that
indicate possible reasons for this interesting phenomenon.
Addition of organic azides to low-valent metal precursors
is a well-precedented tactic for introducing the imido ligand
into late transition metal complexes.[4,6] We recently demon-
strated that the diketiminate-supported dinitrogen complex
[LMeFeNNFeLMe] is a source of the iron(I) fragment {LMeFe}
doublet (j D j > 30 cmꢀ1). Quantitation (by simulation or by
double integration) of the observed signal yields a spin
concentration of up to 73% of the expected Fe concentration.
Mössbauer spectra of samples at 150 K showed two quadru-
pole doublets in a 3:1 ratio with DEQ = 0.98(4) mmsꢀ1 and d =
0.47(2) mmsꢀ1 for the majority species (see Supporting
Information). Thus, one major product is formed in roughly
70% yield, and we label this majority species A.
[7]
(LMe = 2,4-bis(2,6-diisopropylphenylimino)pent-3-yl).
We consider three assignments for A that are consistent
with these combined data: the four-coordinate iron(I) azide
complex [LMeFe(N3Ad)(tBuPy)] (1), the three-coordinate
iron(III) imido complex [LMeFeNAd] (2), and the four-
coordinate iron(III) imido complex [LMeFe(NAd)(tBuPy)]
(3). A dinuclear structure is disfavored for steric reasons, and
is difficult to reconcile with an S = 3/2spin state. Known
transition-metal azide complexes present an intense band
around 2100 cmꢀ1, but solution FT-IR spectra of A showed no
Addition of adamantyl azide (AdN3) to a blue solution of
[LMeFeNNFeLMe] and 4-tert-butylpyridine (tBuPy, 2equiv)
gave effervescence and immediate formation of an orange-
red solution.[8] The 1H NMR and X-band EPR spectra of
samples generated in this way are shown in Figure 1.
bands between 1600 and 3000 cmꢀ1 [9]
. As shown below, kinetic
evidence is also inconsistent with this assignment. Therefore
possibility 1 is unlikely. We infer that A is an iron(III) imido
complex in an unusual intermediate-spin electronic config-
uration.
To help distinguish between assignments 2 and 3, hybrid
QM/MM computations were carried out on full models with
density-functional treatment of the metal core and molecular
mechanics treatment of the aryl and adamantyl (excepting the
carbon bonded to the imido nitrogen) groups.[10] In model 2,
the S = 3/2electronic state is lowest in energy by 10 kcal
molꢀ1. Model 3 has a calculated S = 5/2ground state with a
ꢀ1
low-lying S = 3/2excited state 4 kcalmol
higher in energy.
The experimental observation of a single ground spin state up
to 40 K by EPR, and no sign of an S = 5/2signal, implicate
[LMeFeNAd] (2) as the most reasonable assignment for A.
Consistent with this idea, the experimental Mössbauer isomer
shift (d = 0.47(4) mmsꢀ1) is closer to that calculated for
quartet 2 (d = 0.40 mmsꢀ1) than that calculated for quartet 3
(d = 0.62mms ꢀ1).[10]
In the S = 3/2ground state of computational model 2, the
= ꢀ
ꢀ
Fe N C unit is nearly linear, with an Fe N distance of 1.68 Š
and Fe-N-C angle of 174.18. The ordering of the d orbitals is
similar to that for other three-coordinate iron-diketiminate
complexes (Figure 2).[11] In the plane of the diketiminate, the
Figure 1. a) 1H NMR spectrum of A in C6D6, generated from mixing
[LMeFeNNFeLMe], 2 equiv AdN3, and 2 equiv tBuPy. The solvent signal
is indicated with S, and the tBuPy signal with P. b) Solid line: X-band
EPR spectrum of A diluted to toluene/C6D6 (9:1). Conditions:
9.65 GHz, 0.75 mT modulation amplitude, 0.2 mW microwave power,
T=7.5 K. The dotted line shows the simulation that gave the values in
the text.
d
yz orbital is high in energy from s antibonding interactions
with the diketiminate nitrogen atoms and p antibonding
interactions with the imido group. The inability to occupy this
orbital explains the stability of the quartet spin state. There is
near degeneracy of the doubly occupied dxy orbital and the
singly occupied dx2ꢀy2 orbital,[11] and spin–orbit coupling
between these orbitals rationalizes the large zero-field split-
ting.
1
The H NMR spectra of the solution display resonances
for a paramagnetic complex containing only LMe, an ada-
mantyl group, and perhaps tBuPy (Figure 1a). The NMR-
active species is formed in about 70% yield. The X-band EPR
spectrum (Figure 1b) corresponds to an isolated Kramers
doublet with geff = 6.12, 1.94, and 1.42. The EPR spectrum is
reminiscent of a spin quartet and can be simulated as an S =
3/2species with E/D = 0.33, gx = 1.94, gy = 2.20, and gz = 1.94.
The temperature dependence of the signal shows that the
ground Kramers doublet is well separated from the excited
We have not been able to isolate A because it converts to a
purple high-spin iron(III) complex (B) over a few hours at
room temperature. X-ray crystallography identifies B as a
neutral, high-spin iron(III) adamantylamido complex with
ꢀ
coordinated tBuPy (Figure 3a). There is a C C bond between
the erstwhile isopropyl group of the diketiminate and a
carbon atom of the diketiminate backbone, rendering it a
ꢀ
dianionic ligand. The Fe N distances of 1.905(3) Š and
ꢀ
1.925(3) Š in the new ligand are similar to the Fe N(ada-
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Scheme 1. Proposed assignments for the discussed compounds. tBuPy=4-tert-butyl-
pyridine.
lecular HAA reaction is much faster than the intramolecular
HAA reaction to form B. The observed pseudo-first-order
rate constant has a first-order dependence on [CHD] at
ꢀ51(1)8C in [D8]toluene, showing that the tBuPy adduct of A
reacts directly with CHD. This observation is inconsistent
with identifying A as the potential azide complex 1 or another
compound that irreversibly forms the imido complex. The
combined spectroscopic and kinetic results are most readily
explained using the reaction manifold depicted in Scheme 1.
In the presence of tBuPy, A also reacts rapidly with indene
at low temperature, but does not react with 9,10-dihydroan-
thracene (DHA) even at room temperature before intra-
molecular conversion to B. We attribute this large difference
Figure 2. Qualitative ligand-field orbital-splitting diagram for
[LMeFeNAd] (2) obtained from density functional calculations.
ꢀ
to a steric effect, because the three substrates have similar C
H bond energies (CHD 76(1) kcalmolꢀ1, indene 79(1) kcal
molꢀ1, DHA 78(1) kcalmolꢀ1).[15] The lack of reactivity with
DHA argues against HAA by free radicals.
Figure 3. Plots of the X-ray crystal structures of a) B and b) [LMeFe-
(NHAd)(tBuPy)] showing 50% probability ellipsoids. In B, the new
ꢀ
C C bond is indicated with an arrow. Hydrogen atoms are omitted for
clarity.
The QM/MM calculations indicate some differences
between three-coordinate (2) and four-coordinate (3) iron-
(III) imido complexes that might explain the higher reactivity
of the latter. In 3, the geometry at iron is pseudotetrahedral,
which results in a smaller splitting of the ligand-field orbitals
(despite the increased coordination number) and hence an
S = 5/2ground state. (Because no such species is evident in
EPR spectra, we suspect that the active species is present in
extremely low concentration and is not observed.) As a result
of populating the second p* orbital in the high-spin state of
ꢀ
mantylamido) distance of 1.882(3) Š, and to the Fe N(aryl-
amido) distance of 1.880(2) Š in [LMeFe(NHdipp)(OTf)]
(dipp = 2,6-diisopropylphenyl,
fonyl).[12] Hydrogen-atom abstraction (HAA) by an iron-
imido intermediate accounts for the formation of
Tf = trifluoromethanesul-
B
(Scheme 1). The key hydrogen-atom abstraction step of this
reaction is similar to those attributed to inferred iron(IV)
imido species, and isolated nickel(III) and cobalt(III) imido
complexes.[6,13]
The HAA reaction was examined through kinetic studies
in which disappearance of A was monitored by 1H NMR
spectroscopy at 31(1)8C.[14] The observed rate constants show
a first-order dependence on [tBuPy] with an intercept near
zero, suggesting that a pyridine adduct of A is the only species
undergoing intramolecular HAA. Addition of 10 equiv
(based on [LMeFeNNFeLMe]) of 1,4-cyclohexadiene (CHD)
to A in C6D6 leads to rapid formation of [LMeFe(NHAd)-
(tBuPy)], isolated in 91% yield (Figure 3b). This intermo-
ꢀ
four-coordinate 3, the Fe N bond is lengthened to 1.74 Š
(compared to 1.68 Š for the quartet state of three-coordinate
2). Binding of pyridine also causes substantial bending of the
imido ligand (Fe-N-C 155.18). Finally, the nitrogen atom has
much more spin density in four-coordinate 3 (0.82eꢀ) than in
three-coordinate 2 (0.23eꢀ).
In conclusion, we have presented spectroscopic studies on
an observable species A with an S = 3/2ground state.
Considering that A is the majority species, that its ground
state is consistent with that calculated for [LMeFeNAd], that it
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10782; d) S. D. Brown, J. C. Peters, J. Am. Chem. Soc. 2004, 126,
reacts directly with hydrogen-atom donors when pyridine is
added, and can undergo an intramolecular reaction with the
ligand isopropyl groups in the presence of pyridine, it is most
reasonable to assign A as a three-coordinate iron(III) imido
complex with an intermediate-spin electronic configuration.
While other formulations of A that can rapidly interconvert
with an imido complex are also compatible with the data, they
are unnecessarily complicated.
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Given this interpretation, it appears that the three-
coordinate imido complex becomes active for HAA only
upon addition of a fourth ligand. Calculations suggest that the
ligand-induced change in reactivity is accompanied by 1) a
weaker ligand field at iron, which has been observed to
increase hydrogen-atom abstraction reactivity in octahedral
iron(IV) oxo complexes;[5] 2) a weaker Fe N bond and a
ꢀ
more bent Fe-N-C angle, which decreases the reorganization
energy for proton-coupled electron transfer;[16] and 3) a
possible spin-state change that leads to enhanced radical
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The ability to modulate the reactivity of the imido complex
through addition of a fourth ligand suggests that this
diketiminate system will be useful for elucidating the effects
of ligand field strength and geometry on the reactions of
metal–nitrogen multiple bonds.
[8] If no pyridine donor is present, one obtains a different product
without an imido group. This reaction will be reported sepa-
rately.
[9] H. V. R. Dias, S. A. Polach, S.-K. Goh, E. F. Archibong, D. S.
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[10] The Gaussian 98 package was used.[10a] Hybrid quantum
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employed the Universal Force Field[10b] to describe the methyl
and 2,6-diisopropylphenyl groups of the b-diketiminate, the tBu
group of tBuPy (if present), and the entire adamantyl group
except the alpha carbon. The remaining atoms were modeled at
the B3LYP/6-31G(d) level of theory.[10c] Hybrid QM/MM
calculations were performed with the ONIOM[10d] formalism.
Mössbauer isomer shift calculations used the Scheme in
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Received: May 16, 2006
Revised: July 12, 2006
Published online: September 22, 2006
Keywords: coordination number · hydrogen-atom abstraction ·
.
imido ligands · iron
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