Oxygen activation and intramolecular C-H bond activation by an amidate-bridged diiron(II) complex

Article abstract of DOI:10.1021/ic2007183

A diiron(II) complex containing two μ-1,3-(κN:κO)-amidate linkages has been synthesized using the 2,2′,2′′- tris(isobutyrylamido)triphenylamine (H₃L^iPr) ligand. The resulting diiron complex, 1, reacts with dioxygen (or iodosylbenzene

Full text of DOI:10.1021/ic2007183

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pubs.acs.org/IC
Oxygen Activation and Intramolecular CꢀH Bond Activation by an
Amidate-Bridged Diiron(II) Complex
Matthew B. Jones, Kenneth I. Hardcastle, Karl S. Hagen, and Cora E. MacBeth*
Department of Chemistry, Emory University, Atlanta, Georgia 30322, United States
S Supporting Information
b
Scheme 1
ABSTRACT: A diiron(II) complex containing two μ-1,3-
(kN:kO)-amidate linkages has been synthesized using
the 2,20,200-tris(isobutyrylamido)triphenylamine (H3LiPr)
ligand. The resulting diiron complex, 1, reacts with dioxygen
(or iodosylbenzene) to effect intramolecular CꢀH bond
activation at the methine position of the ligand isopropyl
group. The ligand-activated product, 2, has been isolated
and characterized by a variety of methods including X-ray
crystallography. Electrospray ionization mass spectroscopy
of 2 prepared from18O2 was used to confirm that the oxygen
atom incorporated into the ligand framework is derived
from molecular oxygen.
bromide yields PPh4[KFe2(LiPr)2] (1) as a bright-yellow solid
in 78% yield. This route is similar to the route reported by
Stavropoulos and co-workers16 for the preparation of a related
multinuclear iron(II) complex from the H3LiPr ligand with one
acterial multicomponent monooxygenases (BMMs) react
B
with dioxygen to form intermediates capable of cleaving
important difference: the route presented here incorporates a
noncoordinating countercation (PPh4þ) that leads to the for-
mation of a single asymmetric diiron(II) core structure upon
crystallization that is soluble in a variety of organic solvents. The
alternative route, which incorporates only coordinating potas-
sium ions, results in the formation of a dimer of diiron(II) cores
(i.e., a dimer of dimers) upon crystallization.16 Crystals of 1
suitable for single-crystal X-ray diffraction studies were grown by
vapor diffusion of diethyl ether into a DMF solution of the
complex.
CꢀH bonds in hydrocarbon substrates.1,2 Within the BMM family,
the hydroxylase components of soluble methane mononoxygenase3
and toluene/o-xylene monooxygenase4 both contain structurally
similar nonheme, diiron active sites that incorporate bridg-
ing carboxylate ligands.5 Consequently, synthetic carboxylate-
bridged diiron(II) complexes have been extensively studied as
both structural and functional models of these active sites.5ꢀ13
These synthetic studies have illustrated the importance of li-
gand design in the assembly of diiron complexes that display bio-
mimetic reactivity.5 Recently, a diiron(IV) complex, [(FeIV)2-
(μ-O)(L)2]2þ [where L = N,N-bis(30,50-dimethyl-40-methoxy-
pyridyl-20-methyl)-N0-acetyl-1,2-diaminoethane], containing a
(μ-oxo)bis(μ-1,3-(kN:kO)-amidate)diiron(IV) core was pre-
pared by bulk electrolysis.14 This complex was shown to be a
very reactive dehydrogenating agent, attacking strong CꢀH and
OꢀH bonds. These results suggested to us that diiron(II)
complexes incorporating bridging amidate ligands could be
promising synthetic candidates for creating functional models
of diiron(II) hydroxylases. With this goal in mind, we set out to
explore the reactivity of a diiron(II) complex that incorporates
bridging μ-1,3-(kN:kO)-amidate ligands. Herein, we describe
the reactivity of one such complex and demonstrate that these
species can react directly with dioxygen or oxygen-atom-transfer
reagents to afford intramolecular aliphatic CꢀH oxygenation.
The target diiron(II) complex was prepared using the tris-
(isobutyrylamido)triphenylamine (H3LiPr) ligand15 by the route
shown in Scheme 1. Treatment of an N,N-dimethylformamide
(DMF) solution of H3LiPr with 3 equiv of potassium hydride, 1
equiv of iron(II) acetate, and 0.5 equiv of tetraphenylphosphonium
The solid-state structure of 1 is shown in Figure 1 and reveals
an asymmetric diiron(II) core supported by two bridging
μ-1,3-(kN:kO)-amidate ligands. One amidate arm from each
ligand adopts the bridging coordination mode. The remaining
amidate ligands bind to the iron centers in a monodentate,
N-amidate coordination mode17 with average FeꢀNamidate bond
lengths of 2.108 Å for Fe1 and 2.102 Å for Fe2. Both Fe1 and Fe2
exhibit slightly distorted tetrahedral coordination geometries,
with τ4 values of 0.94 and 0.99, respectively.18 The tertiary nitrogen
atoms (N1 and N5) of the ligand backbone are effectively non-
coordinating because each is positioned ca. 2.5 Å from their
respective iron center. The iron(II) centers in 1 are separated by
4.138 Å. A potassium ion is also situated near the diiron(II) core
and is coordinated by three carbonyl groups [two from the
diiron(II) core and one from a neighboring molecule in the unit
cell (Figure S1 in the Supporting Information)]. This potassium
ion is positioned equidistant from the iron(II) centers (ca. 4.13 Å).
Received: April 7, 2011
Published: June 13, 2011
r
2011 American Chemical Society
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dx.doi.org/10.1021/ic2007183 Inorg. Chem. 2011, 50, 6402–6404
|

Inorganic Chemistry
COMMUNICATION
Scheme 2
Figure 1. Molecular structure of 1 illustrated by thermal ellipsoids
þ
drawn at 50% probability. Hydrogen atoms, PPh4 counterion and
solvents of crystallization have been removed for clarity. Selected bond
lengths (Å) and angles (deg): Fe1ꢀN2 2.116(3); Fe1ꢀN3 2.110(4);
Fe1ꢀN4 2.099(4); Fe1ꢀO4 1.985(3); Fe2ꢀN6 2.120(3); Fe2ꢀN7
2.084(4); Fe2ꢀN8 2.102(4); Fe2ꢀO1 1.999(3); K1ꢀO2 2.604(3);
K1ꢀO5 2.568(3); N2ꢀFe1ꢀN4 111.92(15); N2ꢀFe1ꢀN3 104.46(13);
N2ꢀFe1ꢀO4 110.40(13); N3ꢀFe1ꢀN4 115.21(15); N3ꢀFe1ꢀO4
109.80(14); N4ꢀFe1ꢀO4 105.13(14); N6ꢀFe2ꢀN7 108.65(15);
N6ꢀFe2ꢀN8 109.60(14); N6ꢀFe2ꢀO1 109.39(13); N7ꢀFe2ꢀN8
109.09(15); N7ꢀFe2ꢀO1 110.36(14); N8ꢀFe2ꢀO1 109.74(14);
O5ꢀK1ꢀO1 141.02(11).
In coordinating solvents (e.g., DMSO-d6 and CD3CN), the
1H NMR spectrum of 1 exhibits a complex pattern of para-
magnetically broadened peaks consistent with a structure of low
symmetry. The electrospray ionization mass spectroscopy (ESI-
MS) spectrum of a tetrahydrofuran solution of 1 displays peaks
and an isotope pattern consistent with the [K(FeII)2(LiPr)2]ꢀ
formulation (m/z 1145.34). The magnetic moment of 1 was
determined to be 7.3(2) μB (Evans’ method, DMSO-d6, 298 K).
This value is comparable to the μeff values measured for
carboxylate-bridged diiron(II) complexes that exhibit similar
FeꢀFe distances.19,20
Figure 2. Molecular structure of 2 depicted with thermal ellipsoids
drawn at 50% probability. Hydrogen atoms and the counterion are
removed for clarity. Selected bond lengths (Å) and angles (deg):
Fe1ꢀN1 2.274(3); Fe1ꢀN2 1.994(3); Fe1ꢀN3 1.992(3); Fe1ꢀN4
1.998(3); Fe1ꢀO4 1.882(3); O4ꢀFe1ꢀN1 155.13(11); O4ꢀFe1ꢀN2
81.86(11); O4ꢀFe1ꢀN3 104.86(12); O4ꢀFe1ꢀN4 122.02(12);
N2ꢀFe1ꢀN3 122.28(11); N3ꢀFe1ꢀN4 111.00(12); N1ꢀFe1ꢀN2
76.42(10); N1ꢀFe1ꢀN3 77.42(10); N1ꢀFe1ꢀN4 78.18(11).
To explore the reactivity of 1 with oxygen-atom-transfer
reagents, a DMF solution of the complex was treated with 2
equiv of iodosylbenzene (PhIO; Scheme 2). The reaction color
changed from bright yellow to dark red within 5 min. The
crude product of this reaction was investigated by ESI-MS, and
a strong molecular ion peak was observed at a mass-to-charge
ratio (m/z = 568.18) consistent with a mononuclear iron complex
containing a modified ligand. To unambiguously identify the
reaction product, the crude reaction mixture was crystallized by
vapor diffusion of diethyl ether into a concentrated DMF
solution of the product. This procedure affords analytically pure
2 in 43% yield as a tetraphenylphosphonium salt. The molecular
structure of 2 was determined by single-crystal X-ray crystal-
lography and is shown in Figure 2. The structure confirms that
the product of this reaction is a mononuclear iron(III) complex
containing a modified ligand in which an oxygen atom has
replaced a methine proton on one of the isopropyl groups. This
type of intramolecular isopropyl group activation has been
observed before in synthetic cobalt21 and copper complexes.22,23
The five-coordinate iron(III) center in 2 displays a coordina-
tion geometry intermediate between trigonal-bipyramidal and
square-pyramidal (τ5 = 0.55).24 Its coordination sphere is made
up of three N-amidate donors, an alkoxide from the ligand
modification site, and the tertiary nitrogen of the ligand back-
bone. Complex 2 displays an average FeꢀNamidate bond length of
1.995(3) Å, which is ∼0.1 Å shorter than the average FeꢀNamidate
bond lengths in 1. Complex 2 also displays a Fe1ꢀO4 (iron
alkoxide) bond distance of 1.882(3) Å, which is comparable to
other iron(III) terminal alkoxide species25,26 and shorter than
typical FeII/IIIꢀO(H)R (alcohol) bond lengths (ca. 2.1 Å).27 In
solution, 2 is dark red and exhibits a charge-transfer band at
450 nm (ε = 3000 Mꢀ1 cmꢀ1, DMF) in its electronic absorbance
spectrum. The solution-state magnetic moment of 2 was mea-
sured to be 5.9(2) μB (DMF-d7, 298 K), consistent with a high-
5
spin iron(III) center (S = /2). Analysis of 2 by cyclic voltam-
metry (25 °C, DMF, 0.2 M TBAPF6) revealed a single, reversible
event at E1/2 = ꢀ1.53 (ΔEp = 0.080 V; ipc/ipa = 0.99) versus the
ferrocene/ferrocenium (Figure S6 in the Supporting Information)
that is tentatively assigned to the FeII/FeIII couple.
In order to probe the reactivity of 1 with dioxygen, an
anaerobic DMF solution of 1 was reacted with an excess of dry
dioxygen gas. After 15 min, the solution had changed from yellow
to dark red. The reaction mixture was stirred for a additional 2 h
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Inorganic Chemistry
COMMUNICATION
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: cora.macbeth@emory.edu.
’ ACKNOWLEDGMENT
The authors thank the donors of the American Chemical
Society Petroleum Research Fund and the Emory University
Research Committee for financial support and Dr. Fred Strobel
for assistance with MS experiments.
Figure 3. (A) HR-ESI mass spectrum of [2] prepared from 16O2 (m/z
568.1778 obsd (568.1773 calcd)); and (B) HR-ESI mass spectrum of
[2] prepared from 18O2 (m/z = 570.1823 obsd (570.1815 calcd)).
’ REFERENCES
and then concentrated to dryness. The crude product was then
recrystallized under an inert atmosphere. The product of this
reaction displayed spectroscopic (1H NMR, IR, ESI-MS, and
UVꢀvis) characteristics identical with those observed for 2. To
verify that dioxygen was the source of the oxygen atom incorpo-
rated in 2 prepared by this route, the reaction was repeated using
18O2 (98%) as the oxidant. High-resolution ESI-MS of the
product isolated from this reaction confirms that the oxygen
atom inserted into the ligand backbone is derived from dioxygen
because the mass-to-charge ratio for 2 prepared from 18O2 (m/z
570.1815) is two units greater than the mass-to-charge ratio
observed for 2 prepared from 16O2 (Figure 3). The reaction of 1
with dioxygen at room temperature was followed using electronic
absorption spectroscopy to estimate the yield of 2 formed
during this reaction (Figure S5 in the Supporting Information).
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this reaction. The maximum yield of 2 obtained by either route
does not appear to exceed 50%. Attempts to completely character-
ize the other byproduct of these reactions have not been
successful, but mass spectroscopy of the reaction mixtures
suggests that at least one of the byproducts is the protonated
(uncoordinated) H3LiPr ligand. Studies addressing the mecha-
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intermediates involved in the formation of 2 from the diiron(II)
complex 1 are unknown. We speculate, however, that this
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contain μ-oxo, μ-hydroxo, or, in the case of the dioxygen
reaction, μ-peroxo bridging ligands because both PhIO (an
oxygen-atom-transfer reagent) and dioxygen can be used as
oxidants in this CꢀH activation reaction.
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In summary, we have shown that a diiron(II) complex containing
bridging amidate ligands can promote intramolecular aliphatic
CꢀH activation of an isopropyl group. We have demonstrated
that both an oxygen-atom-transfer reagent and dioxygen can be
used as oxidants in this reaction. This study supports the idea that
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promising candidates for the development of functional models
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identifying the intermediates involved in these reactions and
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’ ASSOCIATED CONTENT
S
Supporting Information. Complete experimental details
b
and CIF files for all compounds. This material is available free of
charge via the Internet at http://pubs.acs.org.
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dx.doi.org/10.1021/ic2007183 |Inorg. Chem. 2011, 50, 6402–6404