Triptycene-based, carboxylate-bridged biomimetic diiron(II) complexes

Article abstract of DOI:10.1002/ejic.201201387

A triptycene-based bis(benzimidazole) ester ligand, L3, was designed to enhance the electron-donating ability of the heterocyclic nitrogen atoms relative to those of the first-generation bis(benzoxazole) analogs, L1 and L2. A convergent synthesis of L3 was designed and executed. Three-component titration experiments using UV/Vis spectroscopy revealed that the desired diiron(II) complex could be obtained with a 1:2:1 ratio of L3/Fe(OTf)₂(MeCN) ₂/external carboxylate reactants. X-ray crystallographic studies of two diiron complexes derived in this manner from L3 revealed their formulas to be [Fe₂L3(μ-OH)(μ-O₂CR)(OTf)₂], where R = 2,6-bis(p-tolyl)phenyl (7) or triphenylmethyl (8). The structures are similar to that of a diiron complex derived from L1, [Fe₂L1(μ-OH)(μ- O₂CAr^Tol)(OTf)₂] (9), a notable difference being that, in 7 and 8, the geometry at iron more closely resembles square-pyramidal than trigonal-bipyramidal. Moessbauer spectroscopic analyses of 7 and 8 indicate the presence of high-spin diiron(II) cores. These results demonstrate the importance of substituting benzimidazole for benzoxazole for assembling biomimetic diiron complexes with syn disposition of two N-donor ligands, as found in O₂-activating carboxylate-bridged diiron centers in biological systems. Copyright

Full text of DOI:10.1002/ejic.201201387

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DOI:10.1002/ejic.201201387
Triptycene-Based, Carboxylate-Bridged Biomimetic
Diiron(II) Complexes
Yang Li,[a] Chan Myae Myae Soe,[a] Justin J. Wilson,[a]
Suan Lian Tuang,[a] Ulf-Peter Apfel,[a] and Stephen J. Lippard*[a]
Keywords: Enzyme models / Iron / Oxygenases / Ligand design / Triptycene
A triptycene-based bis(benzimidazole) ester ligand, L3, was
designed to enhance the electron-donating ability of the het-
erocyclic nitrogen atoms relative to those of the first-genera-
tion bis(benzoxazole) analogs, L1 and L2. A convergent syn-
thesis of L3 was designed and executed. Three-component
titration experiments using UV/Vis spectroscopy revealed
that the desired diiron(II) complex could be obtained with a
1:2:1 ratio of L3/Fe(OTf)2(MeCN)2/external carboxylate reac-
tants. X-ray crystallographic studies of two diiron complexes
derived in this manner from L3 revealed their formulas to be
[Fe2L3(μ-OH)(μ-O2CR)(OTf)2], where R = 2,6-bis(p-tolyl)-
phenyl (7) or triphenylmethyl (8). The structures are similar
to that of a diiron complex derived from L1, [Fe2L1(μ-OH)(μ-
O2CArTol)(OTf)2] (9), a notable difference being that, in 7 and
8, the geometry at iron more closely resembles square-pyr-
amidal than trigonal-bipyramidal. Mössbauer spectroscopic
analyses of 7 and 8 indicate the presence of high-spin di-
iron(II) cores. These results demonstrate the importance of
substituting benzimidazole for benzoxazole for assembling
biomimetic diiron complexes with syn disposition of two N-
donor ligands, as found in O2-activating carboxylate-bridged
diiron centers in biological systems.
to serving as a structural scaffold, the active site cavity also
provides a hydrophobic environment where catalytic inter-
mediates can avoid being prematurely deactivated by adven-
titious reactants.
Introduction
Bacterial multicomponent monooxygenases (BMMs), in-
cluding soluble methane monooxygenase (sMMO),[1] tolu-
ene monooxygenase (ToMO),[2,3] and phenol hydroxylase
(PH),[4] belong to a unique family of carboxylate-bridged
diiron enzymes.[5–8] The diiron active sites of BMMs have
an oxygen-rich coordination environment composed of ter-
minal and bridging carboxylates, water and/or hydroxides,
and two imidazole donors that bind in a syn fashion with
respect to the diiron vector. Even though the active sites of
members of the BMM family are very similar in structure,
each enzyme catalyzes the oxidation or epoxidation of a
specific set of hydrocarbon substrates.
The coordination environment of the diiron core of solu-
ble methane monooxygenase hydroxylase (sMMOH) is pro-
vided by a highly preorganized set of amino acid residues.
The proper orientation of these coordinating residues is
maintained by the predefined conformation of the folded
protein. The empty active site of the apo-form of sMMOH,
for example, is structurally analogous to that containing the
coordinated diiron unit.[9] This high degree of preorganiza-
tion is a feature common to many enzymes.[10] In addition
The synthesis of faithful small-molecule models of the
active sites of diiron enzymes is a significant challenge in
bioinorganic chemistry.[11–17] In addition to providing a
platform for studying the mechanisms of natural systems,
biomimetic models may also function as catalysts to per-
form similar chemical transformations. Over the past three
decades, many different ligands have been employed to
mimic the diiron cores of BMMs. Among them, ligands
based on tris(2-pyridylmethyl)amine (TPA) and sterically
encumbering 2,6-diarylbenzoate ligands have been exten-
sively investigated (Figure 1).[13,14,18,19] Although much in-
sight has been gained from studying these systems, the
model compounds generally fall short of accurately repli-
cating both the structural and electronic properties of the
enzyme active site, not to mention their functions. TPA-
based diiron complexes can generate intermediates close to
those observed in BMMs upon reaction with dioxygen.
These intermediates usually exhibit poor reactivity with hy-
drocarbons, which may be due to the fact that they have a
low-spin rather than a high-spin electronic configura-
tion[20–22] as a result of their nitrogen- rather than oxygen-
rich coordination environments.[23–25] High-spin diiron
complexes of this ligand class are more reactive.[26–30] Steri-
cally encumbering 2,6-diarylbenzoate ligands form diiron
complexes that closely mimic the oxygen-rich coordination
sphere found in the active sites of BMMs.[31–34] The use of
[a] Department of Chemistry, Massachusetts Institute of
Technology,
Cambridge, MA 02139, USA
Fax: +1-617-258-8150
E-mail: lippard@mit.edu
Homepage: http://web.mit.edu/lippardlab/
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201201387.
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dendrimer-appended carboxylates allowed preparation of
Here, we describe an advanced design of highly preorga-
complexes having a hydrophobic sheath, similar to the envi- nized ligands for the synthesis of biomimetic models of the
ronment surrounding the diiron active sites of the diiron active site of sMMOH (Figure 2). Continuing along
BMMs.[35] Attempts to model the syn disposition of the two lines reported previously,[36] we present the synthesis of a
histidine ligands in this system, however, led only to the new ligand with a triptycene backbone, L3. This molecule
formation of complexes with nitrogen donors in the anti differs from previously reported triptycene L1[36] by replace-
disposition. Thus, although certain structures and reactivit- ment of the benzoxazole arms with more nucleophilic benz-
ies of the BMM active site can be modeled, no small mole- imidazole donors. The synthesis and structures of two carb-
cule yet exists that can successfully mimic both features.
oxylate-bridged diiron(II) complexes of L3 are reported,
and the results are compared to those for the L1 analogue.
Results and Discussion
Ligand Design and Synthesis
We previously reported a unique dinucleating ligand hav-
ing two benzoxazole N,O-donor arms linked through a rigid
triptycene backbone (Figure 2, L1).[36] With this ligand, we
were able to crystallize and obtain structural data for the
diiron complex, [Fe2L1(μ-OH)(μ-O2CArTol)(OTf)2], where
each iron atom is coordinated by the nitrogen atom of the
benzoxazole group and an oxygen atom from the ester. Be-
cause the active site of sMMOH utilizes carboxylate rather
than ester groups, the analogous ligand, H2L2Ph4, which be-
ars sterically protected bis(benzoxazole) diacid ligands, was
synthesized.[44] The coordination of H2L2Ph4 to iron in the
presence of an external carboxylate afforded the unantici-
pated triiron complex, [NaFe3(L2Ph4)2(μ3-O)(μ-O2CCPh3)2-
Figure 1. Ligands employed in diiron modeling.
Our group has recently focused on constructing faithful
structural models by using tailor-made dinucleating ligands.
A particularly challenging aspect is to model the syn dispo-
sition of the two histidine ligands. This orientation of nitro-
gen donors may be crucial for the oxidation of methane
by intermediate Q of sMMOH.[22] We have prepared and
reported several ligands with the goal of modeling this
structural feature.[14,16,17,36] 1,2-Diethynylbenzene-based
scaffolds were effectively employed for this purpose.[37–41]
The low-energy barrier to rotation about the alkynyl C–C
bond, however, led to the formation of undesired metallo-
polymers and bis(ligand)iron complexes.[38,42] To circum-
vent this problem, we recently introduced a syn N-donor
macrocyclic ligand, H2PIM,[43] which afforded a diiron
complex with remarkable structural similarity to the active
site of sMMOH.
(H2O)3](OTf)2, having a “basic iron acetate” core.[44]
A
noteworthy feature of the [NaFe3(L2Ph4)2(μ3-O)(μ-
O2CCPh3)2(H2O)3](OTf)2 structure is the lack of iron coor-
dination at the nitrogen atoms of the benzoxazole arms. We
hypothesized that these nitrogen atoms do not bind to the
iron centers because of the low basicity of the benzoxazole
moiety (pKb = 13.2). Much more basic nitrogen donors are
provided by the bis(benzimidazole) ligand, L3 (Figure 2).
The pKb of benzimidazole is 8.2, which is five orders of
magnitude more basic than that of benzoxazole; this in-
crease in basicity was expected to enhance the binding affin-
ity of L3 for iron. The pKb of benzimidazole is also a better
match than that of benzoxazole to the pKb of histidine (pKb
≈ 8.0). From a biomimetic point of view, L3 more accu-
rately matches the electronic properties of the coordinating
residues present in the primary coordination sphere in
BMM diiron active sites.
The synthetic route to benzimidazole ligand L3 requires
preparation of methyl 2-amino-3-(methylamino)benzoate
(5), which was obtained by the route depicted in Scheme 1.
Although 5 has an apparently simple structure, differentia-
tion of its two nitrogen atoms by selective protection of ni-
trogen at the 3-position with a methyl group is a synthetic
challenge. For instance, the direct functionalization of 2,3-
diaminobenzoate by methylation or reductive amination
would not be selective, giving low yields of the desired prod-
ucts and requiring difficult purification protocols. Although
a procedure to replace the chlorine atom in 3-chloro-2-ni-
trobenzoic acid with methylamine under high pressure has
been reported,[45] we obtained a mixture of compounds by
Figure 2. More basic benzimidazole ligand.
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this method. After performing a tedious recrystallization however, is expected to yield the same product regardless of
step, the product was isolated in very low yield. Therefore, which amide bonds were present. Hence, cyclization of the
this reaction is not practical for large-scale synthesis. To crude mixture in glacial acetic acid afforded the desired
prepare large quantities of the desired diamine compound ligand, L3, as the major product in 48% yield from diacid
5, several synthetic pathways were explored, ultimately lead- 6.
ing to the discovery of an efficient route. The optimized
synthesis of diamine 5 is delineated in Scheme 1. Methyl 3-
chloro-2-nitrobenzoate (1) can be readily prepared from 3-
chloro-2-nitrobenzoic acid in 99% yield, using diazometh-
ane generated from Diazald and 10% aq. NaOH in ethanol.
A C–N bond coupling reaction of 1 with N-methylacet-
amide was achieved under Lindenschmidt’s conditions,
Pd(TFA)2, BINAP, and Cs2CO3.[46] Methyl 3-(N-methyl-
acetamido)-2-nitrobenzoate (2) was obtained in high yield
(82%) on a large scale. Next, hydrolysis of both the ester
and amide groups with excess 10% aq. NaOH in ethanol at
80 °C gave 3-(methylamino)-2-nitrobenzoic acid (3). Crude
acid 3 was then treated with diazomethane to yield methyl
3-(methylamino)-2-nitrobenzoate (4) in 95% yield over two
steps from 2. Lastly, hydrogenation of 4 afforded methyl 2-
amino-3-(methylamino)benzoate (5) in 95% yield. This five-
step synthesis of diamine 5 is efficient, high-yielding (73%
overall yield), and can be readily scaled up. Moreover, this
approach can be utilized for the synthesis of diamine 5 ana-
logs, facilitating the preparation of other L3 ligand variants.
Three-Component Titration Experiments
With diester ligand L3 in hand, we next explored its iron
coordination chemistry. Because the active site of sMMOH
contains two iron atoms surrounded by two nitrogen and
four carboxylate donors, we wished to reproduce this stoi-
chiometry in our model compounds. Although ligand L3 is
designed to bind to two iron atoms, external carboxylate
ligands are needed to complete the coordination sphere of
the diiron core. To determine the ratio of iron(II) and exter-
nal carboxylates that the L3 platform can accommodate, we
carried out a series of UV/Vis spectrophotometric ti-
trations.[42,47,48]
We first examined the coordination behavior of ligand
L3 with an iron(II) salt in the absence of an external carb-
oxylate ligand. As shown in Figure 3, when various aliquots
of Fe(OTf)2 were added to a solution of L3 in dichloro-
methane, the absorption band at 345 nm decreased con-
comitantly with increases at 365 and 386 nm, features that
are characteristic of an Fe–O(H)–Fe species. The appear-
ance of an isosbestic point at 308 nm indicates a single-step
conversion between two different species, suggesting that
the apo L3 ligand binds to iron(II). These spectroscopic
features did not change significantly when greater than
2.0 equiv. of Fe(OTf)2 were added (Figure 3, inset). Al-
though the exact iron-to-ligand ratio cannot be determined
with certainty, these titration results reveal that ligand L3
can bind to more than one iron atom. This result is encour-
aging given that some other ligand systems developed in
our laboratory could only produce complexes with a 1:1
iron-to-ligand ratio.[38,42]
Scheme 1. Diaminobenzoate synthesis.
With diamine 5 in hand, the amide coupling reaction be-
tween 5 and 6 was carried out (Scheme 2).[36] In contrast to
the synthesis of benzoxazole ligand L1, the coupling reac-
tion between diacid 6 and diamine 5 generated an insepa-
rable mixture of compounds. Because compound 5 contains
two different amine functionalities, several different prod-
ucts can be obtained depending on which amine group re-
acts to form the amide bond. The final cyclization step,
Figure 3. UV/Vis absorption spectra obtained from addition of
Fe(OTf)2 (0.5 equiv.) to L3 (10 μm in dichloromethane). The inset
shows the absorbance changes at 386 nm, where the error bars for
each data point were calculated from triplicate experiments.
We next sought to determine the stoichiometry of carb-
Scheme 2. Synthesis of ligand L3.
oxylate binding to the diiron complex of L3. To a premixed
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solution containing L3 and Fe(OTf)2 in a 1:2 ratio, the Iron Complexes of L3
carboxylate Et3NH(O2CArTol
) [triethylammonium 2,6-
By combining L3, carboxylate, and iron(II) in a 1:1:2 ra-
tio, as determined to be optimal for diiron complex forma-
tion according to the titration experiments described above,
crystals of [Fe2L3(μ-OH)(μ-O2CR)(OTf)2] [where R = ArTol
(7) or Ph3C (8)] were obtained in moderate yield. Figures 5
and 6 depict the structures for 7 and 8, respectively, along
with relevant bond metrics. Compounds 7 and 8 are struc-
tural analogs, differing only in the identity of their bridging
carboxylate ligand. Each iron center is five-coordinate,
comprising O-ester and N-benzimidazole groups from L3,
a terminal triflate (OTf–), a bridging hydroxide, and a
bridging carboxylate. The Fe–O distances of the triflate li-
gands vary between 2.184 and 2.284 Å and are longer than
other Fe–O distances in these structures (Figures 5 and 6).
Additionally, one of the terminal triflate ligands forms a
hydrogen-bonding interaction with the bridging hydroxide.
bis(p-tolyl)benzoate] was added in small portions and bind-
ing was monitored by UV/Vis absorption spectroscopy. The
addition of up to 1.0 equiv. of Et3NH(O2CArTol) resulted
in an increase in absorption bands at 365 and 386 nm. Fig-
ure 4a shows a plot of the absorption changes at 386 nm
upon addition of Et3NH(O2CArTol) to L3. When greater
than 1.0 equiv. of the carboxylate were introduced, the band
at 386 nm decreased while another appeared at 345 nm. The
band at 345 nm arises from the free ligand, L3. We could
reason that the addition of a large excess of carboxylate
leads to extrusion of iron from L3, forming iron carboxylate
complexes and free L3.[18,19] These studies suggested that
only 1.0 equiv. of carboxylate should be utilized to prepare
diiron complexes supported by L3.
Figure 4. UV/Vis titration experiments: (a) addition of Et3NH-
(O2CArTol) to a premixed solution of L3 (10 μm in dichlorometh-
ane) and Fe(OTf)2(MeCN)2 (1:2); (b) addition of Fe(OTf)2(MeCN)2
to a premixed solution of L3 (10 μm in dichloromethane) and
Et3NH(O2CArTol) (1:1); (c) addition of Na(O2CCPh3) to a pre-
mixed solution of L3 (10 μm in dichloromethane) and Fe(OTf)2-
(MeCN)2 (1:2); (d) addition of Fe(OTf)2(MeCN)2 to a premixed
solution of L3 (10 μm in dichloromethane) and Na(O2CCPh3) (1:1).
Each plot shows the absorbance changes at 386 nm. The error bars
for each data point were calculated from triplicate experiments.
Figure 5. Molecular structure of [Fe2L3(μ-OH)(μ-O2CArTol)-
(OTf)2] (7) with 50% probability thermal ellipsoids and a partial
numbering scheme (hydrogen atoms and solvent molecules are
omitted for clarity). Iron, green; carbon, gray; oxygen, red; nitro-
gen, blue; sulfur, yellow; fluorine, yellow-green. Selected bond
lengths (Å) and angles (°): Fe1–N1, 2.084(6); Fe2–N2, 2.100(6);
Fe1–O5, 2.091(6); Fe2–O7, 2.122(5); Fe1–O10, 2.074(5); Fe2–O11,
2.068(5); Fe1–O9, 1.943(5); Fe2–O9, 1.962(5); Fe1–O12, 2.284(3);
Fe2–O15, 2.212(6); Fe1–O9–Fe2, 126.5(3); O9–Fe1–O10, 97.9(2);
O9–Fe2–O11, 95.0(2); O9–Fe1–N1, 167.3(3); O9–Fe2–N2,
168.1(2); O10–Fe1–O12, 170.50(16); O11–Fe2–O15, 163.6(2).
To investigate whether the ratio of iron, external carb-
oxylates, and L3 depends on the order of addition, a final
set of titrations was carried out by adding Fe(OTf)2 to a
premixed solution of L3 and Et3NH(O2CArTol) (1:1). Fig-
ure 4b shows a plot of the UV/Vis absorbance changes at
386 nm following addition of Fe(OTf)2 to L3/Et3NH-
(O2CArTol). The absorbance increased linearly until
2.0 equiv. of Fe(OTf)2 were added. Treatment with ad-
Compounds 7 and 8 are also structurally similar to the
ditional equivalents of Fe(OTf)2 led to a slight decrease in diiron complex derived from the bis(benzoxazole) ligand
the absorption at 386 nm, which is most likely a result of L1. Despite having an identical set of ligands, the diiron
competitive binding of excess FeII to the external carboxyl- complex of L1, [Fe2L1(μ-OH)(μ-O2CArTol)(OTf)2] (9),[36]
ate groups.
exhibits some notable differences from benzimidazole ana-
Similar titration experiments were carried out with so- logs 7 and 8. A comparison of relevant structural param-
dium 2,2,2-triphenylacetate instead of 2,6-bis(tolyl)benzo- eters of these three complexes is provided in Table 1. Even
ate and yielded comparable results (Figure 4c and 4d). though a more strongly donating benzimidazole ligand is
Taken together, these studies suggest that at least one exter- present in 7 and 8, the Fe–N distances of approximately
nal carboxylate can bind to a diiron–L3 complex.
2.1 Å are similar to those in the benzoxazole structure. The
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(Table 1). For 7 and 8 these distances contract significantly.
The Fe–O distances in these complexes range from 2.609 to
2.859 Å. These shorter distances could in part be due to
more rigorous square-pyramidal geometry enforced by the
benzimidazole ligands.
Table 1. Selected distances (Å), angles (°), and values of the geo-
metric parameter τ in complexes 7–9. Numbers in parentheses are
standard deviations in the last digit(s).
7
8
9
Fe1···Fe2
N1···N2
Fe1···O1
Fe2···O2
O1···O2
Fe–OH–Fe
τ (Fe1)
3.4867(14)
7.336(8)
2.690(6)
2.696(5)
5.134(7)
126.5(3)
0.058(6)
0.075(5)
3.4445(10)
7.232(6)
2.859(3)
2.609(4)
5.168(4)
123.3(2)
0.205(4)
0.303(4)
3.4436(13)
7.321(8)
2.987(5)
3.060(5)
5.075(6)
124.4(3)
0.303(5)
0.408(5)
τ (Fe2)
The dinucleating ligand conformations are similar for
structures 7–9 (Table 1). The distance between nitrogen do-
nor atoms (N1···N2) varies from 7.232 to 7.336 Å, whereas
the distances between furan oxygen atoms (O1···O2) fall be-
tween 5.075 and 5.168 Å. These small structural variations
reflect the preorganization of the triptycene backbone. Ro-
tation about the C–C bond between the benzofuran back-
bone and the benzoxazole or benzimidazole arms of the
ligands will allow for some structural flexibility, however,
as may be required for efficient metal coordination. These
dihedral angles in 9 are 2.69 and 3.76° for the two arms. In
the benzimidazole structures the analogous dihedral angles
range from 4.47 to 7.70°, which indicates somewhat greater
distortion from coplanarity of the two π systems. This sub-
tle change in dihedral angle may be a consequence of the
preferred coordination geometry of the benzimidazole com-
pared to that of the benzoxazole structures.
Figure 6. Molecular structure of [Fe2L3(μ-OH)(μ-O2CCPh3)-
(OTf)2] (8) with 50% probability thermal ellipsoid and a partial
numbering scheme (hydrogen atoms and solvent molecules are
omitted for clarity). Iron, green; carbon, gray; oxygen, red; nitro-
gen, blue; sulfur, yellow; fluorine, yellow-green. Selected bond
lengths (Å) and angles (°): Fe1–N1, 2.106(4); Fe2–N2, 2.113(4);
Fe1–O5, 2.133(4); Fe2–O7, 2.128(4); Fe1–O10, 2.073(4); Fe2–O11,
2.062(4); Fe1–O9, 1.961(4); Fe2–O9, 1.953(4); Fe1–O12, 2.230(4);
Fe2–O15, 2.184(4); Fe1–O9–Fe2, 123.3(2); O9–Fe1–O10, 97.04(15);
O9–Fe2–O11, 93.23(15); O9–Fe1–N1, 158.25(17); O9–Fe2–N2,
177.68(17); O10–Fe1–O12, 170.52(15); O11–Fe2–O15, 159.51(16).
other Fe–L distances are similar as well. The parameter τ
[τ = (β – α)/60, β Ͼ α], provides an assessment of a five-
coordinate structure having a geometry between that of an
idealized trigonal bipyramid (τ = 1) and a square pyramid
(τ = 0).[49–51] The τ values for the iron atoms in 7–9 are
summarized in Table 1. The two iron atoms of 7 attain a
nearly perfect square-pyramidal coordination geometry as
evidenced by τ values close to 0 (0.05 and 0.08). The iron
atoms of 8 and 9 exhibit a higher degree of distortion to-
ward trigonal-bipyramidal stereochemistry. The τ values for
8 are 0.20 and 0.30, whereas those for 9 are 0.30 and 0.41.
The greater distortion of 9 toward trigonal-pyramidal ge-
ometry may be a consequence of the weaker donor ability
of the benzoxazole nitrogen atoms. Although compounds
7 and 8 have a hydroxido-bridged diiron(II) core, similar
electronic effects may be at play in determining the coordi-
nation geometry. The Fe···Fe separations of 8 and 9 are
3.44 Å, whereas in 7 the distance is slightly elongated,
3.49 Å. Although minimal, this structural difference may
be associated with the greater degree of square-pyramidal
character of 7 relative to those of 8 and 9. Another signifi-
cant difference between benzimidazole structures 7 and 8
and benzoxazole structure 9 is the presence of weak interac-
tions at the available remaining coordination sites at the
FeII centers and the oxygen atoms of the furan ring (O1
and O2). In 9, these Fe–O distances are 2.987 and 3.060 Å
Mössbauer Spectra of Complexes 7 and 8
Zero-field 57Fe Mössbauer spectra of polycrystalline
samples of complexes 7 and 8 were recorded at 77 K. Both
spectra show a single quadrupole doublet, as shown in Fig-
ure 7 for 7. Table 2 lists the parameters of complexes 7 and
8. The isomer shift (δ) of 7 is 1.218 mms–1 and the quadru-
pole splitting (ΔEQ) is 1.960 mms–1. These parameters are
characteristic of high-spin diiron(II) complexes. The values
compare favorably to those for the benzoxazole analog 9,
which has an isomer shift of 1.214 mms–1 and a quadrupole
splitting of 1.925 mms–1. In addition, the isomer shift (δ)
of 8 is 1.224 mms–1 and the quadrupole splitting (ΔEQ) is
2.064 mms–1. Although the solid-state structures indicate
slightly different coordination geometries at the two iron
centers, these differences have a minimal effect on electronic
properties, as reflected by a single isomer shift.
Table 2. Mössbauer fits for complexes 7 and 8.
7
8
Isomer shift δ (mms–1)
1.218
1.960
1.224
2.064
Quadrupole splitting ΔEQ (mms–1)
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X-ray Diffraction Studies
Single crystals suitable for X-ray analysis were coated with Para-
tone-N oil, suspended in a small fiber loop, and placed in a cooled
dry N2 gas stream on a Bruker APEX CCD X-ray diffractometer.
Data were acquired by using graphite monochromated Mo-Kα radi-
ation (λ = 0.71073 Å) at 100(2) K and a combination of φ and ω
scans traversing their respective scanning angles at 0.5° increments
for ω scans and 0.45° increments for φ scans. Data collection, in-
dexing, reduction, and final unit cell refinement procedures were
carried out by using APEX2; absorption corrections were applied
by using the program SADABS. All structures were solved with
direct methods with SHELXS[52] and refined against F2 on all data
by full-matrix least-squares with SHELXL,[53] following established
refinement strategies.[54] Data collection and refinement parameters
are summarized in Table 3, and specific refinement details are given
below.
For compound 7, the crystal diffracted only to a maximum θ value
of 23.4°. A molecule of dichloromethane and diethyl ether are pres-
ent in the asymmetric unit, along with the diiron complex. The
diethyl ether molecule was refined with similarity restraints on the
thermal ellipsoids and carbon–carbon bond lengths. The triflate
ligand coordinated to Fe1 is disordered, adopting two orientations.
The disorder was modeled by restraining both components to have
similar bond lengths and thermal ellipsoid parameters. The occu-
pancy factors of the disordered components were refined, con-
straining the sum of the occupancy factors to unity. The major
component refined to an occupancy value of 0.68. The hydrogen
atom of the bridging hydroxido ligand was located on a difference
Fourier map, and the O–H distance was constrained to 0.84 Å. The
thermal ellipsoid of the hydrogen atom was constrained to be 1.5
times that of the oxygen atom to which it is attached. At the final
stages of refinement, a large residual electron density peak of
4.38 eÅ–3 remained. This peak is located 0.85 Å from H10, an atom
on the triptycene backbone. The origin of this large residual elec-
tron density remains uncertain, but may reflect series termination
errors owing to the low resolution and intensity of the data set.
Figure 7. Zero-field 57Fe Mössbauer spectrum of complex 7 re-
corded at 77 K. The red line is the best fit to the data.
Conclusions
With the goal of increasing the electron-donating proper-
ties of our first-generation triptycene-based ligand, L1, we
designed and synthesized an analogous ligand, L3, which
introduces a more basic benzimidazole nitrogen donor in
place of the benzoxazole nitrogen donor of L1. Ligand L3
was obtained in a 15-step convergent synthesis. Titration
experiments were conducted to determine the appropriate
combination of iron(II), carboxylate, and L3 required to
assemble a diiron complex. The X-ray crystal structures of
two such carboxylate-bridged diiron complexes reveal that
L3 binds to a diiron unit in a fashion similar to that of
L1. Complexes of L3 exhibit some structural differences in
comparison to those of L1. Most notably, diiron complexes
7 and 8 feature coordination geometries closer to square-
pyramidal and a close Fe–O interaction with benzofuran
backbone. The iron oxidation states of the new complexes
were confirmed by utilizing Mössbauer spectroscopy. On-
going work is focused on the hydrolysis of the ester moieties
of L3 to form a more biomimetic carboxylate ligand.
Crystals of 8 diffracted well. The asymmetric unit consists of the
diiron complex and a molecule of dichloromethane. The latter is
disordered, having two orientations that were refined with similar-
ity restraints on both the bond lengths and thermal ellipsoids. The
major component of this disorder refined to an occupancy factor
of 0.53. The hydrogen atom of the bridging hydroxido ligand was
located on a difference Fourier map and refined as described for 7.
The absolute structure (Flack) parameter, refined by using the race-
mic twin law, converged to a value of 0.395 with a standard uncer-
tainty of 0.019, indicating that the crystal is most likely a twin
with a minor domain of the opposite absolute structure comprising
approximately 40% of the crystal.
Experimental Section
General Methods and Materials: Reagents purchased from TCI, Al-
drich Chemical Co., and Alfa Aesar were used as received, unless
otherwise noted. The synthesis of compound 6 was described pre-
viously.[36] Column chromatography was carried out with Silicycle
CCDC-909410 (for 7) and -909411 (for 8) contain the supplemen-
tary crystallographic data for this paper. These data can be ob-
tained free of charge from The Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif.
1
60 Å, ultrapure silica gel. H and 13C NMR spectra were recorded
with a 300 MHz or 500 MHz Varian Mercury spectrometer. IR
spectra were measured with a ThermoNicolet Avatar 360 spectro-
photometer controlled by the OMNIC software. Low-resolution
electrospray ionization mass spectra were acquired with an Agilent
57Fe Mössbauer Spectroscopy: Mössbauer spectra were recorded
with a MSI spectrometer (WEB Research Co.) with a 57Co source
in a Rh matrix maintained at room temperature. Solid samples
were prepared by suspension in Apiezon M grease and placed in a
Technologies 1100 series LC-MSD trap. High-resolution mass spec- nylon sample holder. Data were acquired at 77 K, and the isomer
tra were obtained with a Bruker Daltonics APEXIV 4.7 Tesla Fou-
rier Transform ion cyclotron resonance mass spectrometer (FT-
ICRNS) at the MIT Department of Chemistry Instrument Facility
(DCIF).
shift (δ) values are reported with respect to metallic iron that was
used for velocity calibration at room temperature. The spectra were
fit to Lorentzian lines by using the WMOSS plot and fit program
(WEB Research Co.).
Eur. J. Inorg. Chem. 2013, 2011–2019
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Table 3. Crystal data and structure refinement for complexes 7 and 8.
7·(CH2Cl2)·(Et2O)
8·(CH2Cl2)
Empirical formula
Formula weight
Temperature (K)
Wavelength (Å)
Crystal system
Space group
Unit cell dimensions
a (Å)
C72H60Cl2F6Fe2N4O16S2
1597.96
100(2)
0.71073
monoclinic
P21/c
C67H48Cl2F6Fe2N4O15S2
1509.81
100(2)
0.71073
orthorhombic
P212121
19.9244(14)
20.6794(15)
19.7602(14)
16.0657(9)
19.7408(11)
19.9989(11)
b (Å)
c (Å)
β (°)
118.8530(10)
7131.0(9)
4
1.488
0.627
Volume (Å3)
Z
6342.6(6)
4
1.581
0.698
Dcalcd. (gcm–3)
Absorption coefficient (mm–1)
F(000)
3280
3080
Crystal size (mm3)
Theta range (°)
Index ranges
0.40ϫ0.10ϫ0.10
1.55 to 23.38
–22 Յ h Յ 22
–23 Յ k Յ 23
–22 Յ l Յ 22
97291
0.20ϫ0.14ϫ0.06
1.63 to 25.14
–18 Յ h Յ 19
–23 Յ k Յ 23
–23 Յ l Յ 23
101906
Reflections collected
Independent reflections
Maximum θ (°)
Completeness to 2θ (%)
Absorption correction
Max. and min. transmission
Refinement method
10347 [R(int) = 0.0640]
23.38
99.6
11325 [R(int) = 0.1103]
25.14
99.8
semiempirical from equivalents
0.9400 and 0.7876
full-matrix least-squares on F2
10347/244/946
1.054
R1 = 0.0917, wR2 = 0.2417
R1 = 0.1204, wR2 = 0.2682
4.375 and –0.698
semiempirical from equivalents
0.9593 and 0.8729
full-matrix least-squares on F2
11325/56/913
1.047
R1 = 0.0569, wR2 = 0.1190
R1 = 0.0860, wR2 = 0.1314
1.029 and –0.435
Data/restraints/parameters
Goodness-of-fit on F2
Final R indices [IϾ2σ(I)]
R indices (all data)
Largest diff. peak and hole (eÅ–3)
Methyl 3-Chloro-2-nitrobenzoate (1): Diazomethane, CH2N2, gen-
erated from Diazald and 10% NaOH in EtOH, was bubbled into
a suspension of 3-chloro-2-nitrobenzoic acid (10.0 g, 50 mmol) in
Et2O (250 mL) under a steam of nitrogen. The reaction was moni-
tored by TLC until completion. The resulting solution was passed
through a short pad of silica gel. The product, which was retained
on the column, was eluted with acetone. After rotary evaporation
of the acetone eluate, methyl 3-chloro-2-nitrobenzoate (1) was ob-
tained as a white solid. Yield 10.6 g (99%). Melting point: 105–
were washed with 10% aq. HCl (50 mL) and brine (50 mL), dried
with Na2SO4, filtered, and concentrated to dryness. Flash column
chromatography of the resulting residue (dichloromethane/acetone
= 1:0–4:1) gave methyl 3-(N-methylacetamido)-2-nitrobenzoate (2)
as a yellow solid. Yield 8.2 g (82%). Melting point: 120–122 °C. 1H
NMR (300 MHz in CDCl3): δ = 1.81 (s, 3 H), 3.14 (s, 3 H), 3.91
(s, 3 H), 7.55 (dd, J1 = 1.5, J2 = 7.8 Hz, 1 H), 7.68 (t, J = 7.8 Hz,
1 H), 8.10 (dd, J1 = 1.5, J2 = 7.8 Hz, 1 H) ppm. 13C NMR (75 MHz
in CDCl3): δ = 22.23, 37.10, 53.51, 124.36, 130.13, 131.61, 131.82,
1
106 °C. H NMR (300 MHz in CDCl3): δ = 3.90 (s, 3 H), 7.53 (t,
134.48, 136.67, 162.79, 170.38 ppm. IR (KBr): ν = 3068, 3034,
˜
J = 8.1 Hz, 1 H), 7.70 (dd, J1 = 1.5, J2 = 8.1 Hz, 1 H), 7.97 (dd,
2958, 2851, 1738, 1671, 1546, 1442, 1384, 1355, 1290, 1201, 1164,
1001, 859, 834, 769, 701, 637, 606, 586, 573 cm–1. HRMS (ESI+)
m/z = 253.0809 [M + H]+ (calcd. for C11H13N2O5: 253.0824).
J1 = 1.5, J2 = 7.8 Hz, 1 H) ppm. 13C NMR (75 MHz in CDCl3): δ
=
53.49, 124.43, 126.38, 129.83, 130.15, 130.92, 134.91,
162.68 ppm. IR (KBr): ν = 3107, 3066, 2957, 2906, 2850, 1732,
˜
3-(Methylamino)-2-nitrobenzoic Acid (3): A solution of methyl 3-
(N-methylacetamido)-2-nitrobenzoate (2) (8.0 g, 31.7 mmol) in a
mixture of 10% aq. NaOH and EtOH (520 mL:200 mL) was heated
at 80–85 °C for 10 h. The resulting orange-red solution was cooled
to 0 °C. Concentrated HCl was added slowly to adjust the pH to
1.5–2.0. The resulting mixture was extracted with EtOAc (4ϫ
1552, 1444, 1384, 1288, 1216, 1172, 978, 853, 765, 718 cm–1. LRMS
(ESI+) m/z: 237.9, 239.9 [M + Na]+ (calcd. for C8H6ClNNaO4:
238.0, 240.0).
Methyl 3-(N-Methylacetamido)-2-nitrobenzoate (2): A flask con-
taining methyl 3-chloro-2-nitrobenzoate (1) (8.6 g, 40 mmol),
Cs2CO3 (15.6 g, 48 mmol, 1.2 equiv.), Pd(TFA)2 (TFA = tri- 150 mL). The combined organic layers were washed with brine
fluoroacetate) (664 mg, 5 mol-%), and BINAP (1.25 g, 5 mol-%) (30 mL), dried with Na2SO4, filtered, and concentrated to dryness.
was evacuated and back-filled with nitrogen three times. N-Methyl-
acetamide (4.1 g, 56 mmol, 1.4 equiv.) was added with a syringe,
followed by addition of anhydrous toluene (240 mL). The resulting
mixture was heated at 80–85 °C for three days under a nitrogen
atmosphere. The resulting black solution was cooled to room tem-
perature. Water (300 mL) was added, and the resulting mixture was
extracted with EtOAc (4ϫ 100 mL). The combined organic layers
The desired product, 3-(methylamino)-2-nitrobenzoic acid (3)
(7.2 g), was obtained as a red solid. The crude 3 was used in the
next step without further purification. 1H NMR (500 MHz in
CD3OD): δ = 2.96 (s, 3 H), 6.80 (dd, J1 = 1.5, J2 = 7.5 Hz, 1 H),
7.04 (dd, J1 = 1.5, J2 = 8.5 Hz, 1 H), 7.46 (dd, J1 = 7.5, J2 = 8.5 Hz,
1 H) ppm. 13C NMR (125 MHz in CD3OD): δ = 30.30, 116.78,
116.82, 132.60, 133.43, 135.29, 146.18, 170.80 ppm. IR (KBr): ν =
˜
Eur. J. Inorg. Chem. 2013, 2011–2019
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3392, 2911, 1700, 1615, 1575, 1512, 1448, 1253, 1180, 1078, 1053,
= 32.52, 41.89, 52.61, 54.60, 111.30, 114.40, 118.79, 120.35, 121.30,
799, 765 cm–1. HRMS (ESI+) m/z = 197.0566 [M + H]+ (calcd. for 122.96, 124.12, 124.51, 125.47, 125.74, 126.10, 126.46, 128.30,
C8H9N2O4: 197.0562).
137.52, 141.59, 143.84, 145.94, 146.10, 146.37, 146.62, 150.57,
166.70 ppm. IR (KBr): ν = 3127, 3065, 2947, 2839, 1711, 1636,
˜
Methyl 3-(Methylamino)-2-nitrobenzoate (4): Crude 3-(meth-
ylamino)-2-nitrobenzoic acid (3) (1 g) was suspended in Et2O
(150 mL). In a well-ventilated hood, diazomethane, generated by
slow addition of 10% aq. NaOH to a solution of Diazald (2 equiv.)
in EtOH, was bubbled into the ethereal solution under a stream of
nitrogen until the reaction was complete, as evaluated by TLC. This
reaction was repeated seven times on similar 1 g scales until all of
the above crude material was converted. The combined red oil was
purified by column chromatography (hexanes/EtOAc = 1:0, 10:1–
4:1) to obtain methyl 3-(methylamino)-2-nitrobenzoate (4) as a red
oil. Yield: 6.0 g (90% in two steps). 1H NMR (300 MHz in CDCl3):
δ = 3.00 (d, J = 4.8 Hz, 3 H), 3.88 (s, 3 H), 6.75 (dd, J1 = 1.2, J2
= 7.2 Hz, 1 H), 6.91 (d, J = 8.7 Hz, 1 H), 7.42 (dd, J1 = 7.2, J2 =
8.7 Hz, 1 H), 7.54 (br. s, 1 H) ppm. 13C NMR (75 MHz in CDCl3):
δ = 30.20, 53.28, 115.59, 116.11, 130.16, 132.27, 134.77, 145.60,
1606, 1449, 1384, 1294, 1254, 1207, 1118, 1061, 836, 752, 713,
680 cm–1. HRMS (ESI+) m/z = 711.2244 [M + H]+ (calcd. for
C44H31N4O6: 711.2244).
[Fe2L3(μ-OH)(μ-O2CArTol)(OTf)2] (7): In
a
nitrogen-filled
glovebox, a solution of 2,6-bis(p-tolyl)benzoic acid (HO2CArTol
)
(12.8 mg, 0.042 mmol, 1.0 equiv.) in THF (2 mL) was added to a
solution of Fe(OTf)2(MeCN)2 (36.9 mg, 0.085 mmol, 2.0 equiv.) in
MeCN (2 mL), followed by addition of wet Et3N (11.7 μL,
0.085 mmol, 2.0 equiv.). The mixture was stirred at room tempera-
ture for 5 min. Ligand L3 (30 mg, 0.042 mmol) dissolved in di-
chloromethane (2 mL) was added, and the resulting red solution
was stirred for 30 min. All the volatiles were removed under vac-
uum. The resulting yellow solid was dissolved in dichloromethane/
MeOH (2 mL/0.5 mL) to form a dark-red solution. Vapor diffusion
of Et2O into the solution at room temperature over 3 d gave ruby-
colored crystals. X-ray structural analysis of a crystal revealed its
composition to be [Fe2L3(μ-OH)(μ-O2CArTol)(OTf)2]·CH2Cl2·
Et2O. After washing with Et2O and drying under vacuum, orange-
168.08 ppm. IR (KBr): ν = 3398, 3002, 2952, 2839, 1732, 1613,
˜
1573, 1511, 1446, 1358, 1280, 1208, 1119, 1079, 1054, 983, 853,
792, 754, 704, 547, 482 cm–1. HRMS (ESI+) m/z = 211.0714 [M +
H]+ (calcd. for C9H11N2O4: 211.0719).
red crystals were obtained. Yield 22.4 mg (34.8%). IR (KBr): ν =
˜
Methyl 2-Amino-3-(methylamino)benzoate (5): A mixture of methyl
3-(methylamino)-2-nitrobenzoate (4) (2.0 g, 9.5 mmol) and 10%
Pd/C (100 mg, 5% w/w) in EtOAc/MeOH (50 mL/25 mL) was hy-
drogenated with hydrogen from a balloon for 60 h. The catalyst
was removed by filtration through filter paper, and the resulting
solution was concentrated to dryness. Flash column chromatog-
raphy of the residue (hexanes/EtOAc = 10:1–2:1) gave methyl 2-
amino-3-(methylamino)benzoate (5) as a brown solid. Yield 1.43 g
(83%). Melting point: 53–55 °C. 1H NMR (500 MHz in CDCl3): δ
= 2.87 (s, 3 H), 3.87 (s, 3 H), 4.50–5.10 (br. s, 3 H), 6.73 (dd, J1 =
7.5, J2 = 8.5 Hz, 1 H), 6.80 (dd, J1 = 1.0, J2 = 7.5 Hz, 1 H), 7.45
(dd, J1 = 1.0, J2 = 8.0 Hz, 1 H) ppm. 13C NMR (125 MHz in
CDCl3): δ = 31.56, 51.78, 111.69, 115.29, 117.40, 121.17, 138.04,
3491, 3132, 3023, 2961, 2920, 1672, 1620, 1582, 1449, 1409, 1327,
1298, 1237, 1158, 1027, 756, 637 cm–1. [Fe2L3(μ-OH)(μ-O2CArTol)-
(OTf)2]·CH2Cl2 or C68H50Cl2F6Fe2N4O15S2 (1523.87): calcd. C
53.60, H 3.31, N 3.68; found C 53.51, H 3.15, N 3.75.
[Fe2L3(μ-OH)(μ-O2CCPh3)(OTf)2] (8): In
a
nitrogen-filled
glovebox, solution of NaO2CCPh3 (13.1 mg, 0.042 mmol,
a
1.0 equiv.) in THF (2 mL) was added to a solution of Fe(OTf)2-
(MeCN)2 (36.9 mg, 0.085 mmol, 2.0 equiv.) in MeCN (2 mL). The
mixture was stirred at room temperature for 5 min. A solution of
ligand L3 (30 mg, 0.042 mmol) in dichloromethane (2 mL) was
added, and the resulting red solution was stirred for 30 min. All
the volatiles were removed under vacuum. The resulting yellow so-
lid was dissolved in dichloromethane/MeOH (2 mL/0.5 mL) to
yield a dark-red solution. Vapor diffusion of Et2O at room tem-
perature over 5 d gave ruby-colored crystals. X-ray structural analy-
sis of this crystal revealed the formula to be [Fe2L3(μ-OH)(μ-
O2CCPh3)(OTf)2]·CH2Cl2. After washing with Et2O and drying un-
der vacuum, orange-red crystals were obtained. Yield 28 mg (43%).
141.13, 169.35 ppm. IR (KBr): ν = 3541, 3482, 3404, 3273, 2948,
˜
2858, 2805, 1677, 1622, 1570, 1493, 1469, 1455, 1438, 1384, 1287,
1239, 1190, 1087, 811, 745 cm–1. HRMS (ESI+) m/z = 181.0974 [M
+ H]+ (calcd. for C9H13N2O2: 181.0977).
Ligand L3: Pyridine (646 μL, 8 mmol, 40 equiv.) was added to a
suspension of diacid 6 (84.4 mg, 0.2 mmol) in anhydrous dichloro-
methane (80 mL) under a nitrogen atmosphere. Thionyl chloride
(584 μL, 8 mmol, 40 equiv.) was added, and the resulting solution
was stirred at room temperature for 60 h. The volatiles were re-
moved by rotary evaporation, and the residue was dried further
under high vacuum for 8 h. To the resulting solid were added anhy-
drous dichloromethane (40 mL) and pyridine (646 μL, 8 mmol,
40 equiv.), followed by methyl 2-amino-3-(methylamino)benzoate
(5) (72 mg, 0.4 mmol, 2 equiv.). The resulting light-brown solution
was stirred at room temperature for 30 h, after which all the vola-
tiles were removed under vacuum. This crude material was used in
the next step without further purification. A solution of the crude
solid in HOAc (20 mL) was heated at reflux for 1 h. The acetic acid
was removed under reduced pressure. The residue, purified by flash
column chromatography with silica gel neutralized by Et3N
(dichloromethane/EtOAc = 20:1–6:1), gave ligand L3 as a pale yel-
low solid. Yield 68 mg (48% in 3 steps). Melting point: 333–335 °C.
1H NMR (500 MHz in CDCl3): δ = 4.08 (s, 3 H), 4.33 (s, 3 H),
IR (KBr): ν = 3514, 3134, 3055, 2963, 2860, 1664, 1597, 1446,
˜
1347, 1305, 1238, 1161, 1030, 877, 754, 637 cm–1. [Fe2L3(μ-OH)(μ-
O2CCPh3)(OTf)2]·(CH2Cl2)1.25 or C67.25H48.5Cl2.5F6Fe2N4O15S2
(1531.08): calcd. C 52.76, H 3.19, N 3.66; found C 52.64, H 2.92,
N 3.79.
UV/Vis Spectrophotometric Studies: Stock solutions of L3 (10 μm
in dichloromethane), Fe(OTf)2(MeCN)2 (4.0 mm in MeCN),
Et3NHO2CArTol (4.0 mm, prepared by mixing equimolar amounts
of Et3N and HO2CArTol in THF), and carboxylate NaO2CCPh3
(4.0 mm in MeOH) were prepared in a glovebox.
Reaction of L3 with Fe(OTf)2(MeCN)2: A portion of the L3 stock
solution (4.0 mL) was added to a UV/Vis quartz cuvette, sealed
tightly with a septum cap, and brought outside the glovebox. Ali-
quots (5 μL, 0.25 equiv. relative to L3) of the Fe(OTf)2(MeCN)2
stock solution were added with a 50 μL syringe through the septum
seal into the cuvette. The cell was shaken gently before the elec-
tronic absorption spectra were recorded.
5.75 (s, 1 H), 6.87 (s, 1 H), 7.03–7.08 (m, 2 H), 7.28 (d, J = 7.5 Hz, Reaction of the Carboxylate Ligand with a Mixture of 2.0 equiv.
2 H), 7.37 (dd, J1 = 7.5, J2 = 8.0 Hz, 2 H), 7.44 (d, J = 8.0 Hz, 2
Fe(OTf)2(MeCN)2 and L3: Inside a nitrogen-filled glovebox, a mix-
ture containing L3 stock solution (4 mL) and Fe(OTf)2(MeCN)2
H), 7.48–7.51 (m, 1 H), 7.60–7.64 (m, 4 H), 7.66–7.69 (m, 1 H),
8.02 (d, J = 7.5 Hz, 2 H) ppm. 13C NMR (125 MHz in CDCl3): δ stock solution (2.0 equiv.) was added to a UV/Vis quartz cuvette,
Eur. J. Inorg. Chem. 2013, 2011–2019
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FULL PAPER
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sealed tightly with a septum cap, and brought outside the glovebox.
Aliquots (5 μL, 1.0 equiv. relative to L3) of carboxylate in THF
stock solution were added with a 50 μL syringe through the septum
seal to the cuvette. The cell was shaken gently before the electronic
absorption spectra were recorded.
Reaction of Fe(OTf)2(MeCN)2 with a Mixture of 1.0 equiv. Carb-
oxylate and L3: Inside a nitrogen-filled glovebox, a mixture con-
taining L3 stock solution (4 mL) and carboxylate stock solution
(1.0 equiv.) was added to a UV/Vis quartz cuvette, sealed tightly
with a septum cap, and brought outside the glovebox. Aliquots
(5 μL, 0.5 equiv. relative to L3) of Fe(OTf)2(MeCN)2 in MeCN
stock solution were added with a 50 μL syringe through the septum
seal to the cuvette. The cell was shaken gently before the electronic
absorption spectra were recorded.
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Acknowledgments
[31] D. Lee, S. J. Lippard, J. Am. Chem. Soc. 1998, 120, 12153–
The work was supported by the National Institute of General Med-
ical Sciences (grant GM032134). Ulf-Peter Apfel holds a Feodor
Lynen research fellowship from the Alexander von Humboldt
Foundation. Justin J. Wilson is the grateful recipient of a David H.
Koch Graduate Fellowship. Instrumentation at the MIT Depart-
ment of Chemistry Instrumentation Facility is maintained with
National Institutes of Health (NIH) Grant 1S10RR13886-01.
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