Iron(II) complexes supported by sulfonamido tripodal ligands: Endogenous versus exogenous substrate oxidation

Article abstract of DOI:10.1021/ic501531g

High-valent iron species are known to act as powerful oxidants in both natural and synthetic systems. While biological enzymes have evolved to prevent self-oxidation by these highly reactive species, development of organic ligand frameworks that are capable of supporting a high-valent iron center remains a challenge in synthetic chemistry. We describe here the reactivity of an Fe(II) complex that is supported by a tripodal sulfonamide ligand with both dioxygen and an oxygen-atom transfer reagent, 4-methylmorpholine-N-oxide (NMO). An Fe(III)-hydroxide complex is obtained from reaction with dioxygen, while NMO gives an Fe(III)-alkoxide product resulting from activation of a C-H bond of the ligand. Inclusion of Ca²⁺ ions in the reaction with NMO prevented this ligand activation and resulted in isolation of an Fe(III)-hydroxide complex in which the Ca²⁺ ion is coordinated to the tripodal sulfonamide ligand and the hydroxo ligand. Modification of the ligand allowed the Fe(III)-hydroxide complex to be isolated from NMO in the absence of Ca²⁺ ions, and a C-H bond of an external substrate could be activated during the reaction. This study highlights the importance of robust ligand design in the development of synthetic catalysts that utilize a high-valent iron center. (Chemical Equation Presented).

Full text of DOI:10.1021/ic501531g

Article
pubs.acs.org/IC
Iron(II) Complexes Supported by Sulfonamido Tripodal Ligands:
Endogenous versus Exogenous Substrate Oxidation
Sarah A. Cook, Joseph W. Ziller, and A. S. Borovik*
Department of Chemistry, University of CaliforniaIrvine, 1102 Natural Sciences II, Irvine, California 92697, United States
S
* Supporting Information
ABSTRACT: High-valent iron species are known to act as powerful oxidants in
both natural and synthetic systems. While biological enzymes have evolved to
prevent self-oxidation by these highly reactive species, development of organic
ligand frameworks that are capable of supporting a high-valent iron center remains
a challenge in synthetic chemistry. We describe here the reactivity of an Fe(II)
complex that is supported by a tripodal sulfonamide ligand with both dioxygen and
an oxygen-atom transfer reagent, 4-methylmorpholine-N-oxide (NMO). An
Fe(III)−hydroxide complex is obtained from reaction with dioxygen, while
NMO gives an Fe(III)−alkoxide product resulting from activation of a C−H bond
of the ligand. Inclusion of Ca2+ ions in the reaction with NMO prevented this
ligand activation and resulted in isolation of an Fe(III)−hydroxide complex in
which the Ca2+ ion is coordinated to the tripodal sulfonamide ligand and the
hydroxo ligand. Modification of the ligand allowed the Fe(III)−hydroxide complex
to be isolated from NMO in the absence of Ca2+ ions, and a C−H bond of an external substrate could be activated during the
reaction. This study highlights the importance of robust ligand design in the development of synthetic catalysts that utilize a high-
valent iron center.
Chart 1. Ligand Derivatives Described in This Report
INTRODUCTION
■
High-valent iron species are often reactive intermediates and
are understood to be involved in C−H bond functionalization
of a variety of substrates. For example, nonheme iron-
containing monooxygenases utilize a mononuclear iron(IV)−
oxo unit as the active species to perform a diverse set of
reactions, including hydroxylation, halogenation, desaturation,
and epoxidation.1 These diverse and important reactions have
made nonheme high-valent iron complexes a target for
synthetic chemists, both for understanding the functional
aspects of active sites in enzymes and for developing new
synthetic oxidants for chemical transformations such as C−H
activation.2,3
One challenge associated with preparing complexes that can
support oxidized Fe centers and harnessing their reactivity for
substrate activation is designing supporting ligands that can
withstand the highly reactive nature of these species. Indeed,
several well-characterized Fe(IV)−oxo4 and Fe(IV)−imido5
species have been shown to undergo self-decay via reactivity
with the supporting ligand. In this report, we describe C−H
activation of the tripodal ligand N,N′,N″-[2,2′,2″-nitrilotris-
(ethane-2,1-diyl)]tris(2,4,6-trimethylbenzenesulfonamido)
([MST]3− = mesityl sulfonamide tripod, Chart 1) upon
oxidation of its Fe(II) complex with an oxygen-atom (O-
atom) transfer reagent to generate an Fe(III)−alkoxide species.
The ligand [MST]3− contains mesityl groups whose methyl
positions are susceptible to oxidation. Redesign of the ligand to
remove the methyl groups that are positioned closest to the
metal center (Chart 1, [TST]3− = tolyl sulfonamide tripod)
allowed us to observe an intermediate species, possibly an
Fe(IV)−oxo complex, which was capable of activating C−H
bonds on external substrates to give an Fe(III)−hydroxide
species.
EXPERIMENTAL SECTION
■
General Methods. Syntheses of metal complexes were completed
under a nitrogen atmosphere in a VAC drybox. Solvents were sparged
with argon and dried over columns containing Q-5 and molecular
sieves. All reagents were purchased from commercial suppliers and
used as received unless otherwise noted. Sodium hydride as a 30%
suspension in mineral oil was filtered and washed five times each with
Et2O and pentane and dried under vacuum. H3MST,6a
Received: June 27, 2014
© XXXX American Chemical Society
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NMe4[FeIIMST],6b and H3TST7 were prepared according to literature
procedures.
(s), 816 (s), 666 (s), 553 (s). λmax, nm (DCM, ε, M−1 cm−1): 355
(5500). EPR (1:1 DCM:THF, 77 K): g = 9.7, 4.3.
Complex Syntheses and Reactivity Studies. NMe4[FeIIMST-
(OH2)]. A suspension of NMe4[FeIIMST] (0.100 g, 0.12 mmol) in 4
mL of THF at room temperature was treated with 3 μL (0.17 mmol)
of H2O via syringe, causing the solution to become homogeneous. The
reaction was stirred vigorously for 5 min, after which the solvent was
removed under vacuum. The resulting residue was redissolved in
dichloromethane (DCM) and filtered through Celite to remove fine
particulate solid. The product was crystallized from the DCM filtrate
via diffusion of pentane to give 95 mg (93%) of NMe4[FeIIMST-
(OH2)] as colorless needle crystals. Anal. Calcd for NMe4[FeIIMST-
(OH2)], C37H59N5O7S3Fe: C, 53.03; H, 7.10; N, 8.36. Found: C,
53.00; H, 7.03; N, 8.38. FTIR (KBr disc, cm−1, selected bands, strong
(s), medium (m), weak (w)): 3255 (m), 2968 (m), 2852 (m), 1604
(w), 1491 (m), 1255 (s), 1127 (s), 974 (s), 812 (s), 654 (s).
NMe4[FeIII−O−MST]. A solution of NMe4[FeIIMST] (0.250 g, 0.30
mmol) in 15 mL of DCM at room temperature was treated with a
solution of NMO (72 mg, 6.1 mmol) in 5 mL of DCM, resulting in an
immediate color change to red. The reaction was stirred for 3 h and
then filtered through Celite. The product was recrystallized twice from
DCM via pentane diffusion to give 128 mg (50%) of orange crystals.
Anal. Calcd for NMe 4 [FeI I I −O−MST]·0. 5CH2 Cl2 ,
C37.5H57N5O7S3Cl2Fe: C, 51.34; H, 6.55; N, 7.98. Found: C, 51.69;
H, 6.51; N, 7.77. FTIR (KBr disc, cm−1, selected bands, strong (s),
medium (m), weak (w)): 3029 (w), 2932 (m), 2855 (m), 1604 (m),
1488 (m), 1292 (s), 1136 (s), 958 (s), 797 (s), 654 (s). λmax, nm
(DCM, ε, M−1 cm−1): 351 (7500). EPR (1:1 DCM:THF, 77 K): g =
9.0, 4.2.
Ligand Isolation Studies. In a typical experiment, a DCM
solution of the reaction mixture was brought out of the dry box and
extracted with 1 M HCl. The organic layer was washed with brine,
dried over MgSO4, and filtered. The DCM solution was then passed
through a plug of silica, which was flushed with additional DCM. The
ligand was eluted from the silica with 5% MeOH in DCM and the
solvent removed under vacuum. The products were analyzed by
electrospray ionization mass spectrometry (ESI-MS) and NMR
spectroscopy.
Electronic Absorption Studies. In a typical experiment, a 0.2
mM stock solution of the metal complex was prepared in the glove
box, and 3 mL of the solution was transferred to a quartz cuvette,
which was sealed with a rubber septum. The cuvette was brought out
of the glove box and allowed to equilibrate in the sample holder at 25
°C for 10 min before NMO was added as a 30 mM solution via
syringe.
Substrate Oxidation Studies. In a typical experiment, a solution
of NMO was added in one portion to a solution containing the FeII
complex and DHA. After 3 h, the solvent was evaporated to dryness
and the resulting yellow residue stirred in Et2O. Et2O was then filtered
through Celite, passed through a silica plug, and evaporated to give the
DHA products as an off-white residue. The residue was redissolved in
CDCl3, and the ratio of products was determined by integration of
1
their signals in the H NMR spectrum. The Fe-containing products
were redissolved in DCM and recrystallized by Et2O layering.
Physical Methods. Electronic absorption spectra were recorded in
a 1.0 or 0.1 cm quartz cuvette on a Cary 50 spectrophotometer or an
8453 Agilent UV−vis spectrometer equipped with an Unisoku
Unispeks cryostat. Negative mode electrospray ionization mass spectra
were collected using a Micromass MS Technologies LCT Premier
Mass Spectrometer. X-band (9.28 GHz) EPR spectra were collected as
frozen solutions using a Bruker EMX spectrometer equipped with an
ER041XG microwave bridge. IR spectra were recorded on a Varian
800 Scimitar Series FTIR spectrometer as KBr disks or as a solution
using a Beckman liquid IR cell.
X-ray Crystallographic Methods. A Bruker SMART APEX II
diffractometer was used to collect all data. The APEX28 program
package was used to determine the unit-cell parameters and for data
collections. The raw frame data was processed using SAINT9 and
SADABS10 to yield the reflection data file. Subsequent calculations
were carried out using the SHELXTL11 program. Structures were
solved by direct methods and refined on F2 by full-matrix least-squares
techniques. Analytical scattering factors12 for neutral atoms were used
throughout the analysis. Hydrogen atoms were included using a riding
model. Hydrogen atoms H(1) of NMe4[FeIIITST(OH)] and H(1)
and H(2) NMe4[FeIIMST(OH2)] were located from a difference-
Fourier map and refined (x, y, z, and Uiso). Data sets of both
NMe4[FeIIITST(OH)] and NMe4[FeIII−O−MST] contained several
high residuals in the final difference-Fourier map. It was not possible to
determine the nature of the residuals, although it is probable that a
pentane or DCM solvent molecule was present. The SQUEEZE
routine in the PLATON13 program package was used to account for
the electrons in the solvent-accessible voids. In the NMe4[FeIIMST-
(OH2)] structure, the (NMe4)+ counterion was disordered. Carbon
atoms C(35)−C(40) were included using multiple components with
partial site-occupancy factors.
15-crown-5⊃Ca−(μ-OH)−FeIIIMST]OTf.
A solution of
NMe4[FeIIMST] (50 mg, 0.061 mmol) and Ca⊃15-crown-5(OTf)2
(37 mg, 0.067 mmol) in 3 mL of DCM at room temperature was
treated with a solution of NMO (14 mg, 0.12 mmol) in 2 mL of DCM,
resulting in an immediate color change to orange. After 5 h, the
reaction mixture was filtered through Celite and the product was
recrystallized twice via pentane diffusion to give 60 mg (85%) of
yellow-orange crystals. Anal. Calcd for [15-crown-5⊃CaII−(μ-OH)−
FeIIIMST]OTf·0.5CH2Cl2, C44.5H67CaClF3N4O15S4Fe: C, 44.00; H,
5.56; N, 4.61. Found: C, 44.13; H, 5.31; N, 4.50. FTIR (KBr disc,
cm−1, selected bands, strong (s), medium (m), weak (w)): 3379 (m),
2937 (m), 2868 (m), 1604 (w), 1266 (s), 1144 (s), 1090 (s), 1031 (s),
955 (s), 811 (s), 659 (s), 638 (s). λmax, nm (DCM, ε, M−1 cm−1): 383
(6000). EPR (1:1 DCM:THF, 77 K): g = 9.4, 4.7, 4.2
NMe4[FeIITST(OH2)]. A solution of H3TST (0.200 g, 0.33 mmol) in
4 mL of anhydrous dimethylacetimide (DMA) at room temperature
was treated with 3 equiv of solid NaH (24 mg, 1.0 mmol), causing H2
evolution and precipitation of the deprotonated ligand. After the
evolution of H2 gas ceased, Fe(OAc)2 (57 mg, 0.33 mmol) and
NMe4OAc (44 mg, 0.33 mmol) were added to the heterogeneous
mixture, which was then stirred for 3 h. One equivalent (6 μL) of H2O
was then added via syringe and the reaction mixture filtered through a
medium-porosity frit to remove 3 equiv of insoluble NaOAc (79 mg,
0.96 mmol). Vapor diffusion of Et2O into the pale yellow filtrate gave
the product as pale blue crystals in 90% yield. Anal. Calcd for
NMe4[FeIITST(OH2)], C31H47N5O7S3Fe: C, 49.40; H, 6.28; N, 9.29.
Found: C, 49.13; H, 6.23; N, 9.14. FTIR (KBr disc, cm−1, selected
bands, strong (s), medium (m), weak (w)): 3257 (m), 3037 (w), 2896
(w), 2845 (m), 1599 (w), 1494 (m), 1246 (s), 1129 (s), 973 (s), 815
(s), 663 (s), 597 (m), 555 (s).
NMe4[FeIIITST(OH)]. A solution of NMe4[FeIITST(H2O)] (0.100 g,
0.13 mmol) in 6 mL of DCM at room temperature was treated with a
solution of NMO (15 mg, 0.13 mmol) in 2 mL of DCM, causing an
immediate color change to red. The reaction was stirred for 4 h, during
which time the color faded to orange. After filtering through Celite, the
product was recrystallized twice by layering the DCM solution under
Et2O to give 60 mg (60%) of yellow-orange crystals. Analy. Calcd for
NMe4[FeIIITST(OH)], C31H46N5O7S3Fe: C, 49.46; H, 6.16; N, 9.30.
Found: C, 49.55; H, 6.00; N, 8.98. FTIR (KBr disc, cm−1, selected
bands, strong (s), medium (m), weak (w)): 3450 (m), 3036 (w), 2962
(w), 2859 (m), 1599 (w), 1490 (m), 1270 (s), 1138 (s), 1091 (s), 962
RESULTS AND DISCUSSION
■
Reactivity of the [FeIIMST]− Complex with Dioxygen.
We previously reported the preparation of bimetallic complexes
that are supported by [MST]3−.6,14 We discovered that treating
NMe4[FeIIMST] or NMe4[MnIIMST] with dioxygen in the
presence of a second metal ion resulted in formation of
Fe(III)− or Mn(III)−hydroxide complexes with the second
metal ion coordinated through the hydroxide ligand and two of
the sulfonamido ligand arms of [MST]3− (Scheme 1).6 In the
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Scheme 1. Preparative Route to Heterobimetallic Complexes6
absence of a secondary metal ion, this reaction is sluggish, with
the Mn(II) complex reacting so slowly that completion of the
reaction could not be observed. The Fe(II) complex reacts
faster than the Mn(II) complex, and analysis of the reaction
mixture after 5 h suggested formation of the analogous
monometallic NMe4[FeIIIMST(OH)] salt. The electrospray
ionization mass spectrum (ESI-MS) contained a negative ion
peak whose mass-to-charge ratio (m/z) of 762 corresponds to
the formulation [FeMST(OH)]− (Figure S1, Supporting
Information), and based on charge balance, the Fe center
must be in the 3+ oxidation state. This oxidation state is
supported by EPR spectroscopy, which exhibits a rhombic
signal with g values of 4.2 and 8.6 that are consistent with a
high-spin Fe(III) center (Figure S2, Supporting Information).
Furthermore, a broad band was observed at a frequency of 3463
cm−1 in the FTIR spectrum of the reaction mixture, which is
consistent with a vibration from a hydroxo ligand (Figure 1,
solid black trace, and Figure S3, Supporting Information).15
corresponding to the hydroxo ligand and the N−H vibrations
of DPH were replaced by the O−H band of the Fe(II)−aquo
complex within 2 h.16 This increase in the rate of [FeIIMST-
(OH2)]− formation suggests that the observed instability of
NMe4[FeIIIMST(OH)] results from H-atom abstraction from
the solvent or from an external substrate to give
NMe4[FeIIMST(OH2)].
Reactivity of the [FeIIMST]− Complex with Oxygen-
Atom Transfer Reagents. The bimetallic Fe(III)−hydroxide
complexes could also be generated using O-atom transfer
reagents such as 4-methylmorpholine-N-oxide (NMO) in place
of dioxygen. For instance, reaction of NMe4[FeIIMST] and
CaII⊃15-crown-5(OTf)2 with 2 equiv17 of NMO gave [(15-
crown-5)CaII−(μ-OH)−FeIIIMST]OTf in 85% crystalline
yield, which is similar to the yield obtained from dioxygen
(60−70%, eq 1).6b In addition, the [FeIIMST]− complex reacts
(1)1.1Ca(15‐crown‐5)(OTf)2
(2)2NMO
NMe4[FeIIMST] ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ [(15‐crown‐5)
DCM,N2,rt
−NMe4OTf
CaII−(μ‐OH)−FeIIIMST]OTf
(1)
with 2 equiv of NMO in the absence of a second metal ion, as
evidenced by a rapid color change to orange. On the basis of
the match in reactivity between dioxygen and NMO in the
bimetallic systems, the analogous [FeIIIMST(OH)]− complex
was predicted to be the major product formed from this
reaction (eq 2). Evidence for this product was again observed
2NMO
NMe4[FeIIMST] ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ NMe4[FeIIIMST(OH)]
DCM,N2,rt
+ NMe4[FeIII − O − MST]
Figure 1. FTIR spectra showing the change in the product from
reaction of NMe4[FeIIMST] with O2. Spectra were collected on a 24
mM DCM solution over the course of 7 days (dashed black) with the
first spectrum collected after 5 h (solid black).
(2)
50%
by FTIR spectroscopy, in which an identical O−H vibration at
3460 cm−1 was replaced by a second O−H vibration at 3300
cm−1 after 9 days (Figures S7−S9, Supporting Information).
However, the ESI mass spectrum contained only a minor peak
corresponding to the [FeIIIMST(OH)]− ion, while a dominant
ion peak was observed two mass units lower at a m/z of 760
(Figure S10, Supporting Information). Structural determination
of the crystallized product via X-ray diffraction methods
revealed the source of the dominant molecular ion: an ortho
methyl group on an arm of the ligand had been hydroxylated to
form an Fe(III) product with a coordinated alkoxide ([FeIII−
O−MST]−, Figure 2). This Fe−alkoxide species was the major
species formed and was isolated in 50% crystalline yield.
The Fe(III) center in [FeIII−O−MST]− has an N4O primary
coordination environment in which all five donors are provided
by the [MST]3− ligand. The anionic nitrogen atoms and neutral
apical nitrogen donor provide the base of a distorted trigonal
Attempts to crystallize the proposed Fe(III)−hydroxide
product instead resulted in isolation of the Fe(II)−aquo salt,
NMe4[FeIIMST(OH2)], whose molecular structure was
determined by XRD methods (Figure S4, Supporting
Information). The isolated product presumably results from
instability of the initial Fe(III)−hydroxide species in solution,
which is supported by an FTIR study on the reaction mixture
after removal of the excess dioxygen. Over the course of 7 days,
the O−H vibration from the putative [FeIIIMST(OH)]−
complex decreased in intensity with a concomitant increase in
a new O−H vibration at 3300 cm−1 that matches the vibration
observed for the aquo ligand of independently prepared
[FeIIMST(OH2)]− (Figures 1, S5, and S6, Supporting
Information). When 0.5 equiv of diphenylhydrazine (DPH)
were added to the initial reaction mixture, the O−H band
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Chart 2. Ligand Products Isolated from the Reaction Shown
in Eq 3
Figure 2. Thermal ellipsoid diagram depicting [FeIII−O−MST]−.
Ellipsoids are drawn at the 50% probability level, and all hydrogen
atoms are omitted for clarity. Selected bond lengths (Angstroms) and
angles (degrees): Fe1−N1, 2.358(2); Fe1−N2, 2.034(2); Fe1−N3,
2.030(2); Fe1−N4, 1.999(2); Fe1−O1, 1.805(1); O1−Fe1−N1,
171.09(6); N2−Fe1−N3, 122.11(7); N2−Fe1−N4, 115.18(7); N3−
Fe1−N4, 108.08(7).
observed from reaction of pure [FeIII−O−MST]− with NMO;
only the singly hydroxylated ligand product 2 was observed
after isolation of the ligand products from the reaction (eq 4).
Moreover, free H3MST and 2 show no reactivity with NMO,
which suggests that the ligand must be coordinated to the metal
center in order to become activated.
bipyramid (τ = 0.82)18 with an average Fe−Neq distance of
2.021(2) Å and an Fe−N1 distance of 2.358(2) Å. The
deprotonated hydroxyl group that resulted from functionaliza-
tion of a mesityl group completes the coordination sphere of
the Fe center. In order to accommodate the binding of the
hydroxyl group, the functionalized mesityl group twists above
the Fe−Neq plane, whereas those on the two unfunctionalized
ligand arms point outward from the complex. The oxygen atom
of the deprotonated hydroxyl group tilts away from the Fe−N1
vector toward N4 with an O1−Fe−N1 bond angle of
171.09(6)° and an Fe−O1 distance of 1.805(1) Å (see Table
S2, Supporting Information, for additional metrical parame-
ters).
2NMO
aq.acid extraction
NMe4[FeIII − O − MST] ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ 2
DCM,N2,rt
100%
(4)
Ligand products from reaction of NMe4[FeIIMST] with
NMO in the presence of Ca2+ ions (eq 1) were also analyzed
after extraction of the ligand from the metal complex. None of
the oxidized ligand products shown in Chart 2 were observed.
Other than a small amount of an unidentified ligand product
(<5% overall), only unfunctionalized H3MST was isolated. One
possibility for this lack of ligand oxidation could be
preassociation of a Ca2+ ion within the secondary coordination
sphere of the [FeIIMST]− complex, which positions the mesityl
groups far enough away from the metal center to prevent
hydroxylation upon addition of NMO.
The reaction of NMe4[FeIIMST] with NMO was further
probed by determining the extent of oxidation of the [MST]3−
ligand after isolation of the ligand from the complex. The metal
ion was removed from the ligand in an aqueous acid workup to
give a mixture of H3MST and oxidized ligand species, which
were recovered in a combined yield of 82% (eq 3). Analysis of
the ligand products by NMR spectroscopy provided an
estimate of the yields of the Fe(III)− hydroxide and
Fe(III)−alkoxide products. The hydroxylated ligand 1, which is
Reactivity of the [FeIITST]− Complex. Oxidation of the
ligand observed from reaction of NMe4[FeIIMST] with NMO
highlights a limitation of complexes of [MST]3− in oxidation
reactions, and modification of the ligand is required to prevent
this undesirable reactivity. We therefore replaced the
susceptible mesityl groups of the ligand with tolyl groups
([TST]3−) in order to eliminate this pathway and redirect the
reactivity toward external substrates with C−H bonds.19
Unlike [FeIIMST]−, the four-coordinate [FeIITST]− complex
could not be cleanly isolated due to partial coordination of
adventitious water to form [FeIITST(OH2)]−, which could not
be removed during purification. Therefore, [FeIITST(OH2)]−
was independently prepared and used as the starting complexes
for all oxidation reactions. In order to verify that the presence
of an aquo ligand would not influence the reactivity relative to
[FeIIMST]−, the analogous [FeIIMST(OH2)]− complex was
also prepared and its oxidation with O2 and NMO were
investigated (eqs 5a and 5b). The [FeIIMST(OH2)]− complex
2NMO
aq.acid extraction
NMe4[FeIIMST] ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ ligand products
DCM,N2,rt
(3)
isolated from [FeIII−O−MST]−, makes up 56% of the ligand
products, while unfunctionalized H3MST, which we propose is
isolated from [FeIIIMST(OH)]− or [FeIIMST(OH2)]−, makes
up 25% (Chart 2). The remaining 19% of the products from
[MST]3− consists of three species that have been further
oxidized beyond hydroxylation of the ortho metal group. Two
of the species are assigned to cyclization of one arm through
the sulfonamide nitrogen atom and the hydroxylated ortho
carbon atom of the activated ligand arm. Of these two products,
one retained the hydroxyl functionality (12%, 2) and the other
was further oxidized to the carbonyl (5%, 3). In the final ligand
species, two of the ligand arms contained the cyclized carbonyl
product (2%, 4). Note that no further ligand oxidation is
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NMe4[FeIIMST(OH2)]
NMO
⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ NMe4[FeIII − O − MST]
DCM,N2,rt
+ NMe4[FeIIIMST(OH)]
(5a)
excess O2
NMe4[FeIIMST(OH2)] ⎯⎯⎯⎯⎯⎯⎯⎯→ NMe4[FeIIIMST(OH)]
DCM,rt
(5b)
reacted with O2 and NMO in a manner similar to that of
[FeIIMST]−, and a similar ratio of ligand oxidation products
was observed from reaction with excess NMO.20
Oxidation of the [FeIITST(OH2)]− complex with NMO
resulted in initial formation of a red species that faded to orange
over the course of several minutes (eq 6). When this reaction
NMO
NMe4[FeIITST(OH2)] ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ NMe4[FeIIITST(OH)]
DCM,N2,rt
Figure 4. Thermal ellipsoid diagram depicting [FeIIITST(OH)]−
(bond lengths in Angstroms and angles in degrees). Ellipsoids are
drawn at the 50% probability level, and only the hydroxo hydrogen
atom is shown. The NMe4+ counterion is omitted for clarity. Fe1−N1,
2.329(2); Fe1−N2, 2.053(2); Fe1−N3, 2.031(2); Fe1−N4, 2.022(2);
Fe1−O1, 1.831(1); O1···O2, 2.743; O1−Fe1−N1, 173.56(6); N2−
Fe1−N3, 124.46(7); N2−Fe1−N4, 110.42(6); N3−Fe1−N4,
111.26(6).
(6)
was monitored optically, an intermediate with peaks at 380 and
895 nm appeared and then converted to a new species
containing a single optical feature at 352 nm (Figure 3).
(OH)]− points nearly parallel to the Fe1−O1 vector, forming a
negatively polarized fence around the hydroxo ligand. A short
distance (2.743 Å) between the oxygen atom of the hydroxo
ligand and one of these sulfonamide oxygen atoms (O2) is
suggestive of an intramolecular hydrogen-bonding interaction
between these two groups (see Table S3, Supporting
Information, for additional metrical parameters).
The mechanism for oxidation of [FeIITST(OH2)]− with
NMO is still under investigation, but observation of an
absorbance band at λmax = 895 nm indicates that the reaction
may have involved an Fe(IV)−oxo intermediate. There is a
growing body of data to suggest that synthetic nonheme
Fe(IV)−oxo complexes have optical features between 800 and
900 nm that arise from d−d transitions, and these features
appear to be independent of spin state and molecular
structure.2c,d,f For example, we previously characterized the
related trigonal bipyramidal Fe(IV)−oxo complex [FeIVH3buea-
(O)]− ([H3buea, tris[(N′-tert-butylureaylato)-N-ethylene]-
aminato), which exhibits a band at 808 nm.22 Similarly, the
Fe(IV)−oxo species supported by the macrocyclic ligand
1,4,8,11-tetramethyl-1,4,8,11-tetraaza-cyclotetradecane (TMC)
exhibits a band at 820 nm in the optical spectrum despite
having a different coordination geometry (tetragonal) and spin
state (S = 1).23 Reactivity of this putative Fe(IV)−oxo
intermediate with a C−H bond in the solvent could possibly
be the source of the characterized [FeIIITST(OH)]− product.
Observation of a putative Fe(IV)−oxo species that did not
react with the ancillary tripodal ligand suggested that we might
be able to intercept this reactive intermediate to activate a C−H
bond on an external substrate. Indeed, reaction of [FeIITST-
(OH2)]− with NMO in the presence of 1 equiv of
dihydroanthracene (DHA) in dichloromethane resulted in
20% conversion to the oxidized products anthracene (A),
9,9′,10,10′-tetrahydro-9,9′-bianthracene (B), and anthraqui-
none (C, Figures 5 and S18, Supporting Information).24 For
comparison, no conversion of DHA was observed for identical
reactions with NMe4[FeIIMST] or NMe4[FeIIMST(OH2)].
When reaction of the TST complex was conducted in
acetonitrile instead of dichloromethane, 50% of the DHA was
Figure 3. Electronic absorption spectra for oxidation of a 0.2 mM
DCM solution of NMe4[FeIITST(H2O)] by NMO at 25 °C showing
(A) conversion of the Fe(II) complex (dashed black) to the
intermediate species (solid black) and (B) further reaction of the
intermediate to the final NMe4[FeIIITST(OH)] product (dotted
black). (Inset of B) Decay of the low-energy band in a 5 mM DCM
solution.
Spectroscopic and analytical data are consistent with this final
species being [FeIIITST(OH)]−, which was isolated in 60%
crystalline yield.21 In addition, no evidence for formation of the
hydroxylated ligand product was observed in the analysis of the
isolated ligand after the oxidation reaction of [FeIITST]−.
The molecular structure of NMe4[FeIIITST(OH)], deter-
mined using X-ray diffraction methods, revealed similar
coordination properties of the Fe center as the Fe(III)−
alkoxide species of [MST]3− (Figure 4). The [FeIIITST(OH)]−
ion consists of a five-coordinate Fe(III) center in trigonal
bipyramidal geometry (τ = 0.82) that is established by the four
nitrogen donors of [TST]3− and a terminal hydroxo ligand. The
average Fe−Neq distance of 2.035(2) Å and the Fe−N1
distance of 2.329(2) Å are similar to the distances in [FeIII−O−
MST]− (2.021 and 2.358 Å). The Fe−O1 distance is slightly
longer at 1.831(1) Å compared to 1.803 Å, and the oxygen
atom of the hydroxo ligand is also tilted out of the Fe−N1
vector with an O1−Fe−N1 angle of 173.56(6)°. In contrast to
the Fe(III)−alkoxide complex of MST3−, one sulfonamido
oxygen atom on each of the three ligand arms of [FeIIITST-
E
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Article
Figure 5. Oxidation products of dihydroathracene: (A) anthracene, (B) 9,9′,10,10′-tetrahydro-9,9′-bianthracene, and (C) anthraquinone and
percent conversion obtained from reaction in DCM and acetonitrile.
P. A.; Saleh, L.; Bollinger, J. M.; Krebs, C.; Fitzpatrick, P. F. J. Am.
Chem. Soc. 2007, 129, 11334−11335. (e) Tinberg, C. E.; Lippard, S. J.
Acc. Chem. Res. 2011, 44, 280−288. (f) Panay, A. J.; Lee, M.; Krebs, C.;
Bollinger, J. M.; Fitzpatrick, P. F. Biochemistry 2011, 50, 1928−1933.
(2) For recent reviews on the syntheses, characterizations, and
reactivities of synthetic Fe(IV)−oxo complexes, see: (a) Shan, X.; Que,
L., Jr. J. Inorg. Biochem. 2006, 100, 421−433. (b) Groves, J. T. J. Inorg.
Biochem. 2006, 100, 434−447. (c) Que, L. Acc. Chem. Res. 2007, 40,
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converted to oxidized products (Figure 5). The
NMe4[FeIIITST(OH)] product was crystallized from this
reaction in 90% yield.
CONCLUSIONS
■
In this article, we showed that reaction of NMe4[FeIIMST] with
dioxygen produced an Fe(III)−hydroxide complex that is
analogous to the bimetallic complexes formed in the presence
of a secondary metal ion. However, unlike the bimetallic
Fe(III)−hydroxide products, the NMe4[FeIIIMST(OH)] spe-
cies is not stable in solution and converts to NMe4[FeIIMST-
(OH2)]. The reactivity of [FeIIMST]− alone further deviates
from the reactivity in the presence of Ca2+ ions when the
oxygen-atom transfer reagent NMO is used as the oxidant.
While the same bimetallic Fe(III)−hydroxide complex was
isolated from NMO and Ca2+ as from dioxygen, a new product
was isolated when NMO was reacted with NMe4[FeIIMST]
alone. This product was determined to be an Fe(III)−alkoxide
species that resulted from activation of a C−H bond in a
mesityl group of the ligand (BDEC−H ≈ 88 kcal mol−1). This
result highlights the oxidizing power of this system as well as a
limitation of the [MST]3− ligand system in generating high-
valent iron species. Oxidation of the ligand was prevented via
substitution of the mesityl groups with the tolyl derivative, and
the C−H bonds of an external substrate could then be
activated. An intermediate species was observed in the reaction
between NMe4[FeIITST(OH2)] and NMO, which was
postulated to be an Fe(IV)−oxo species. The Fe(III)−
hydroxide complex was identified as the metal-containing
product of this reaction.
(3) For examples of well-characterized synthetic Fe(IV)−oxo
complexes that react with C−H bonds, see: (a) Kaizer, J.; Klinker,
E. J.; Oh, N. Y.; Rohde, J.-U.; Song, W. J.; Stubna, A.; Kim, J.; Munck,
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E.; Nam, W.; Que, L. J. Am. Chem. Soc. 2004, 126, 472−473.
(b) Martinho, M.; Banse, F.; Bartoli, J.-F.; Mattioli, T. A.; Battioni, P.;
Horner, O.; Bourcier, S.; Girerd, J.-J. Inorg. Chem. 2005, 44, 9592−
9596. (c) Sastri, C. V.; Lee, J.; Oh, K.; Lee, Y. J.; Lee, J.; Jackson, T. A.;
Ray, K.; Hirao, H.; Shin, W.; Halfen, J. A.; Kim, J.; Que, L.; Shaik, S.;
Nam, W. Proc. Natl. Acad. Sci. U.S.A. 2007, 104, 19181−19186.
(d) England, J.; Martinho, M.; Farquhar, E. R.; Frisch, J. R.; Bominaar,
E. L.; Munck, E.; Que, L., Jr. Angew. Chem., Int. Ed. 2009, 48, 3622−
̈
3626. (e) Yoon, J.; Wilson, S. A.; Jang, Y. K.; Seo, M. S.; Nehru, K.;
Hedman, B.; Hodgson, K. O.; Bill, E.; Solomon, E. I.; Nam, W. Angew.
Chem., Int. Ed. 2009, 48, 1257−1260. (f) Thibon, A.; Bartoli, J.-F.;
Bourcier, S.; Banse, F. Dalton Trans. 2009, 9587−9594. (g) Wang, D.;
Farquhar, E. R.; Stubna, A.; Munck, E.; Que, L. Nat. Chem. 2009, 1,
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145−150. (h) Comba, P.; Maurer, M.; Vadivelu, P. Inorg. Chem. 2009,
48, 10389−10396. (i) Xue, G.; De Hont, R.; Munck, E.; Que, L. Nat.
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Chem. 2010, 2, 400−405. (j) Seo, M. S.; Kim, N. H.; Cho, K.-B.; So, J.
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E.; Park, S. K.; Clemancey, M.; Garcia-Serres, R.; Latour, J.-M.; Shaik,
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ASSOCIATED CONTENT
* Supporting Information
■
́
́
̈
S
Munck, D. E.; Luis, J. M.; Que, L., Jr.; Costas, M. Chem.Eur. J. 2011,
̈
Crystallographic data (CIF), metrical parameters, and support-
ing figures and tables. This material is available free of charge
via the Internet at http://pubs.acs.org.
17, 1622−1634.
(4) (a) Lange, S. J.; Miyake, H.; Que, L. J. Am. Chem. Soc. 1999, 121,
6330−6331. (b) Jensen, M. P.; Lange, S. J.; Mehn, M. P.; Que, E. L.;
Que, L. J. Am. Chem. Soc. 2003, 125, 2113−2128. (c) Harman, W. H.;
Chang, C. J. J. Am. Chem. Soc. 2007, 129, 15128−15129. (d) Bigi, J. P.;
Harman, W. H.; Lassalle-Kaiser, B.; Robles, D. M.; Stich, T. A.; Yano,
J.; Britt, R. D.; Chang, C. J. J. Am. Chem. Soc. 2012, 134, 1536−1542.
AUTHOR INFORMATION
Corresponding Author
*E-mail: aborovik@uci.edu.
■
(e) England, J.; Guo, Y.; Farquhar, E. R.; Young, V. G., Jr; Munck, E.;
̈
Notes
Que, L., Jr. J. Am. Chem. Soc. 2010, 132, 8635−8644.
(5) (a) Jensen, M. P.; Mehn, M. P.; Que, L., Jr. Angew. Chem., Int. Ed.
2003, 42, 4357−4360. (b) King, E. R.; Betley, T. A. Inorg. Chem. 2009,
48, 2361−2363.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
We are thankful for the NSF Center for C−H Functionalization
(CHE-1205646) and the NIH (GM05081).
■
(6) (a) Park, Y. J.; Ziller, J. W.; Borovik, A. S. J. Am. Chem. Soc. 2011,
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(15) Attempts to establish dioxygen as the source of the hydroxo
ligand via reactivity with 18O2 were inconclusive, presumably due to
the fast exchange of the hydroxo ligand with adventitious water. In
order to test this premise, NMe4[FeIIIMST(OH)] was treated with 1
equiv of H218O and the mixture was analyzed by ESI-MS, which
indicated 18O incorporation (Figure S11, Supporting Information).
Furthermore, the O−H vibration of the hydroxo ligand was broad
(fwhm ≈ 110 cm−1), which hindered determination of the relative
energies of the ν(16OH) and ν(18OH). Note that a shift of 10 cm−1 is
predicted based on a harmonic O−H oscillator model.
(16) The [FeIIMST(OH2)]− product was crystallized from this
reaction in 85% yield.
(17) Two equivalents of NMO were used in reactions with
NMe4[FeIIMST] because it gave the highest yield of crystalline
products.
(18) Addison, A. W.; Rao, T. N.; Reedijk, J.; van Rijn, J.; Verschoor,
G. C. J. Chem. Soc., Dalton Trans. 1984, 1349−1356.
(19) The H3TST ligand has been previously utilized by Mountford to
support early transition metal centers as polymerization catalysts.
Schwarz, A. D.; Herbert, K. R.; Paniagua, C.; Mountford, P.
Organometallics 2010, 29, 4171−4188.
(20) For [FeIIMST(OH2)]− we observed 10% more unfunctionalized
H3MST compared to the products found for [FeIIMST]−.
(21) Similar to NMe4[FeIIIMST(OH)], the NMe4[FeIIITST(OH)]
product is unstable in DCM solution and converts to
NMe4[FeIITST(OH2)] but on a longer time scale than the [MST]3−
analog. After 14 days, the band associated with the ν(OH) of
NMe4[FeIIITST(OH)] was still observable in the FTIR spectrum of
the original reaction mixture. In comparison, complete loss of the
ν(OH) band from NMe4[FeIIIMST(OH)] was observed after only 7
days. The cause for this difference in stability between the Fe(III)−
hydroxide complexes of [TST]3− and [MST]3− is still under
investigation.
(22) Lacy, D. C.; Gupta, R.; Stone, K. L.; Greaves, J.; Ziller, J. W.;
Hendrich, M. P.; Borovik, A. S. J. Am. Chem. Soc. 2010, 132, 12188−
12190.
(23) Lim, M. H. Proc. Natl. Acad. Sci. U.S.A. 2003, 100, 3665−3670.
(24) The assignment of 9,9′-10,10′-tetrahydro-9,9′-bianthracene as a
product was based on a match of NMR spectra that we obtained from
our reactions with that reported for this compound: J. Org. Chem.
2002, 67, 8002−8009. We previously reported formation of this
product with a different tripodal Fe(II) system: J. Am. Chem. Soc.
2005, 127, 11596−11597.
G
dx.doi.org/10.1021/ic501531g | Inorg. Chem. XXXX, XXX, XXX−XXX