N-donor effects on carboxylate binding in mononuclear iron(II) complexes of a sterically hindered benzoate ligand

Article abstract of DOI:10.1021/ic000531o

Using the sterically hindered 2,6-dimesitylbenzoate ligand Mes₂ArCO₂^-, a series of mononuclear Fe(II) carboxylate complexes has been obtained with the general formula (Mes₂ArCO₂)₂Fe(base)₂ (base = 1-methylimidazole (MeIm), pyridine (Py), 2-picoline (2-Pic), 2,5-lutidine (2,5-Lut), 2,6-lutidine (2,6-Lut), (base)₂ = N,N,N′,N′-tetramethylethylenediamine (TMEDA)). For the monodentate base adducts, single-crystal X-ray diffraction studies revealed several different structural types ranging from distorted tetrahedral to distorted octahedral that correlate with the degree of α-substitution of the N-donors. Increasing α-substitution leads to the lengthening of the Fe-N bond, which in turn results in a change in carboxylate binding mode from η¹ to η². We surmise that this change is due to an electrostatic effect and is driven by increasing the Lewis acidity of the Fe center. Such a simple process for inducing carboxylate shifts could play a critical role in biological systems.

Full text of DOI:10.1021/ic000531o

6086
Inorg. Chem. 2000, 39, 6086-6090
N-Donor Effects on Carboxylate Binding in Mononuclear Iron(II) Complexes of a Sterically
Hindered Benzoate Ligand
John R. Hagadorn, Lawrence Que, Jr.,* and William B. Tolman*
207 Pleasant St S.E., Department of Chemistry and Center for Metals in Biocatalysis,
University of Minnesota, Minneapolis, Minnesota 55455
ReceiVed May 19, 2000
Using the sterically hindered 2,6-dimesitylbenzoate ligand Mes2ArCO2-, a series of mononuclear Fe(II) carboxylate
complexes has been obtained with the general formula (Mes2ArCO2)2Fe(base)2 (base ) 1-methylimidazole (MeIm),
pyridine (Py), 2-picoline (2-Pic), 2,5-lutidine (2,5-Lut), 2,6-lutidine (2,6-Lut), (base)2 ) N,N,N′,N′-tetramethyl-
ethylenediamine (TMEDA)). For the monodentate base adducts, single-crystal X-ray diffraction studies revealed
several different structural types ranging from distorted tetrahedral to distorted octahedral that correlate with the
degree of R-substitution of the N-donors. Increasing R-substitution leads to the lengthening of the Fe-N bond,
which in turn results in a change in carboxylate binding mode from η1 to η2. We surmise that this change is due
to an electrostatic effect and is driven by increasing the Lewis acidity of the Fe center. Such a simple process for
inducing carboxylate shifts could play a critical role in biological systems.
Introduction
essential for the formation of reactive intermediates for these
diiron enzymes, it is less clear whether similar processes play
integral roles in the chemistry of mononuclear Fe active sites
supported by carboxylates. It is noteworthy that both mono-
and bidentate carboxylates have been identified in X-ray
structures of mononuclear Fe centers in proteins.6
To better understand the roles that supporting ligands play
in the aforementioned systems and ultimately improve our
understanding of metalloenzyme mechanism, we7 and others8
have been interested in studying Fe carboxylate complexes that
Carboxylate ligation plays a variety of critical roles in mono-
and dinuclear Fe-containing metalloproteins.1 In addition to
providing anionic donors to the metal center, carboxylate
residues may adopt a variety of binding modes, thus providing
a flexible binding environment that yields multiple coordination
motifs. This phenomenon is well documented for the O2-
activating non-heme diiron enzymes such as methane monooxy-
genase (MMO), ribonucleotide reductase (RNR), and stearoyl-
ACP ∆9-desaturase (∆9D). For MMO and RNR, crystallographic2
and EXAFS3 studies have revealed dramatic changes in car-
boxylate binding geometries4 and Fe-Fe distances (∆ ≈ 1.7
Å) for species that are involved in their catalytic cycles.
Similarly, the addition of substrate to ∆9D has been shown to
induce changes in coordination number and stereochemistry that
likely include a change in carboxylate binding mode from
bidentate to monodentate.5 While core flexibility is undoubtedly
(6) (a) Kauppi, B.; Lee, K.; Carredano, E.; Parales, R. E.; Gibson, D. T.;
Eklund, H.; Ramaswamy, S. Structure 1998, 6, 571-586. (b) Han,
S.; Eltis, L. D.; Timmis, K. N.; Muchmore, S. W.; Bolin, J. T. Science
1995, 270, 976-980. (c) Senda, T.; Sugiyama, K.; Narita, H.;
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Y. J. Mol. Biol. 1996, 255, 735-752. (d) Boyington, J. C.; Gaffney,
B. J.; Amzel, M. Science 1993, 260, 1482-1486. (e) Minor, W.;
Steczko, J.; Bolin, J. T.; Otwinowski, Z.; Axelrod, B. Biochemistry
1993, 32, 6320-6323. (f) Roach, P. L.; Clifton, I. J.; Fu¨lo¨p, V.; Harlos,
K.; Barton, G. J.; Hajdu, J.; Andersson, I.; Schofield, C. J.; Baldwin,
J. E. Nature 1995, 375, 700-704.
(1) For recent review articles, see: (a) Lange, S. J.; Que, L., Jr. Curr.
Opin. Chem. Biol. 1998, 2, 159-172. (b) Que, L., Jr.; Ho, R. Y. N.
Chem. ReV. 1996, 96, 2607-2624. (c) Wallar, B. J.; Lipscomb, J. D.
Chem. ReV. 1996, 96, 2625-2658. (d) Feig, A. L.; Lippard, S. J. Chem.
ReV. 1994, 94, 759-805.
(2) (a) Eklund, H.; Eriksson, M.; Uhlin, U.; Nordlund, P.; Logan, D. J.
Biol. Chem. 1997, 378 (8), 821-825 and references contained within.
(b) Andersson, M. E.; Ho¨gbom, M.; Rinaldo-Matthis, A.; Andersson,
K. K.; Sjo¨berg, B.-M.; Nordlund, P. J. Am. Chem. Soc. 1999, 121,
2346-2352. MMO structures: (c) Rosenzweig, A. C.; Nordlund, P.;
Takahara, P. M.; Frederick, C. A.; Lippard, S. J. Chem. Biol. 1995, 2,
409-418. (d) Rosenzweig, A. C.; Frederick, C. A.; Lippard, S. J.;
Nordlund, P. Nature 1993, 366, 537-543. (e) Elango, N.; Radhakrish-
nan, R.; Froland, W. A.; Wallar, B. J.; Earhart, C. A.; Lipscomb, J.
D.; Ohlendorf, D. H. Protein Sci. 1997, 6, 556-568.
(3) (a) Riggs-Gelasco, P. J.; Shu, L.; Chen, S.; Burdi, D.; Huynk, B. H.;
Que, L., Jr.; Stubbe, J. J. Am. Chem. Soc. 1998, 120, 849-860. (b)
Shu, L.; Nesheim, J. C.; Kauffmann, K.; Mu¨nck, E.; Lipscomb, J. D.;
Que, L., Jr. Science 1997, 275, 515-518.
(4) Rardin, R. L.; Tolman, W. B.; Lippard, S. J. New J. Chem. 1991, 15,
417-430.
(7) (a) Hagadorn, J. R.; Que, L., Jr.; Tolman, W. B. J. Am. Chem. Soc.
1998, 120, 13531-13532. (b) Hagadorn, J. R.; Que, L., Jr.; Tolman,
W. B. J. Am. Chem. Soc. 1999, 121, 9760-9761.
(8) Selected examples of carboxylate-rich Fe complexes: (a) Du Bois,
J.; Mizoguchi, T. J.; Lippard, S. J. Coord. Chem. ReV. 2000, 200-
202, 443-485 and references contained within. (b) Hagen, K. S.;
Lachicotte, R. J. Am. Chem. Soc. 1992, 114, 8741-8742. (c) Randall,
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10.1021/ic000531o CCC: $19.00 © 2000 American Chemical Society
Published on Web 11/30/2000

Carboxylate Binding in Mononuclear Iron(II) Complexes
Inorganic Chemistry, Vol. 39, No. 26, 2000 6087
0.686 mmol) and (Mes2ArCO2)Li(Et2O)0.25(CH3CN)0.30 (0.542 g, 1.37
mmol) to form a yellow suspension. After stirring overnight, the volatile
components were removed under reduced pressure. The resulting yellow
solid was extracted with CH2Cl2 (25 mL). The cloudy solution was
filtered, the filtrate was concentrated to 10 mL, and HMDSO (20 mL)
was layered on the solution. After 12 h, yellow crystals had formed.
Cooling to -30 °C afforded additional product (0.39 g, 57% yield).
1H NMR (CDCl3): δ (ω1/2, Hz) 142 (870) (Py, R-CH), 36 (320) (Py,
â-CH), 12.8 (90) (Ar, m-H), 8.5 (80) (Pyr, γ-CH), 5.2 (70) (Mes, m-H),
3.0 (80) (Mes, o-CH3), 1.6 (70) (Ar, p-H), 0.7 (60) (Mes, p-CH3). Anal.
Calcd (Found) for C60H60N2O4Fe: C, 72.14 (72.26); H, 6.17 (6.16);
N, 2.70 (2.76).
(Mes2ArCO2)2Fe(2-Pic)2. This complex was prepared analogously
to (Mes2ArCO2)2Fe(Py)2. The product was isolated as yellow crystals
(40% yield). 1H NMR (CDCl3): δ (ω1/2, Hz) 120.0 (1300) (Pic, R-CH),
49.5 (240) (Pic, â-CH), 33.0 (260) (Pic, â-CH), 14.8 (190) (Ar, m-H),
5.3 (190) (Mes, o-CH3 and m-H), 2.6 (150) (Pic, γ-H), 0.3 (100) (Mes,
p-CH3), -1.6 (240) (Ar, p-H), -19.6 (690) (Pic, R-CH3). Anal. Calcd
(Found) for C64H64N2O4Fe: C, 77.81 (76.82); H, 6.74 (6.74); N, 2.93
(2.96).
model features of various Fe-containing metalloenzymes. To
this end, we are using sterically hindered benzoate ligands to
control coordination number and nuclearity in mono- and
dinuclear Fe(II) carboxylates. Our previously reported results
have focused on diferrous derivatives and novel mixed-valence
diiron complexes prepared with the 2,6-dimesitylbenzoate ligand
(Mes2ArCO2-).7 Here we present a unique series of structurally
characterized mononuclear Fe(II) carboxylates prepared with
simple mono- and bidentate N-donors. These complexes greatly
expand the list of known structure types for Fe(II) carboxylates,
and include coordinatively unsaturated species made accessible
by the extremely bulky nature of the supporting ligands. We
have also identified a correlation between the structure type and
the degree of R-substitution in a series of related N-donors, thus
providing insights into the manner by which N-donors may
affect carboxylate binding modes and geometry at mononuclear
Fe centers. A small portion of this work was reported in a
communication.7a
(Mes2ArCO2)2Fe(2,5-Lut)2. This complex was prepared analogously
to (Mes2ArCO2)2Fe(Py)2. The product was isolated as yellow crystals
(61% yield). 1H NMR (CDCl3): δ (ω1/2, Hz) 125.6 (1600) (Lut, R-CH),
47.9 (260) (Lut, â-CH), 15.7 (260), 6.3 (270), 5.0 (190), 3.2 (200), 1.8
(250) (Lut, â-CH3), -0.2 (160), -2.0 (250), -36.8 (930) (Lut, R-CH3).
Anal. Calcd (Found) for C64H68N2O4Fe: C, 78.03 (77.48); H, 6.96
(6.95); N, 2.84 (2.91).
Experimental Section
General Considerations. All manipulations were carried out under
nitrogen using standard Schlenk-line and glovebox techniques. Tetra-
hydrofuran (THF), toluene, diethyl ether, and pentane were distilled
from Na/benzophenone. Hexamethyldisiloxane (HMDSO) and N,N,N′,N′-
tetramethylethylenediamine (TMEDA) were distilled from Na. Di-
chloromethane, acetonitrile, and 1-methylimidazole (MeIm) were
distilled from CaH2. Pyridine (Py), 2-picoline (2-Pic), 2,5-lutidine (2,5-
Lut), and 2,6-lutidine (2,6-Lut) were vacuum distilled from CaH2.
CDCl3 was vacuum transferred from CaH2. Mes2ArCO2Li(solvent)n was
prepared as described,7a but better yields (>95%) of crystalline product
were obtained by the addition of CH3CN to concentrated Et2O solutions
of the lithium carboxylate followed by cooling to -40 °C. For individual
batches, solvent content and composition were determined by 1H NMR
spectroscopy. Commercial FeCl2 was dried by treatment with Me3SiCl.9
Fe(OTf)2‚2CH3CN (OTf ) O3SCF3) was prepared by the addition of
2.2 equiv of Me3SiOTf to FeCl2 in 2:1 CH2Cl2/CH3CN, followed by
(Mes2ArCO2)2Fe(2,6-Lut)2. This complex was prepared analogously
to (Mes2ArCO2)2Fe(Py)2. The product was isolated as colorless crystals
(25% yield). 1H NMR (CDCl3): δ (ω1/2, Hz) 40.7 (2000), 32.4 (860),
29.1 (1000), 24.7 (2200), 16.2 (960), 12.0 (1200), 7.1 (400), 2.2 (600),
1.2 (140), -5.8 (510), -25.0 (680), -74.5 (1440). Anal. Calcd (Found)
for C64H68N2O4Fe: C, 78.03 (78.19); H, 6.96 (6.98); N, 2.84 (2.85).
(Mes2ArCO2)2Fe(TMEDA). Dichloromethane (5 mL) and toluene
(3 mL) were added to (Mes2ArCO2)Li(Et2O) (0.519 g, 1.29 mmol) to
form a cloudy solution. To this solution was added a CH2Cl2 solution
(5 mL) of [(TMEDA)FeCl2]2 (0.160 g, 0.319 mmol) to yield a cloudy,
pale yellow solution. After stirring overnight, the volatile materials were
removed under reduced pressure. The solid was extracted with toluene
(15 mL) and filtered through a glass frit. Addition of pentane (10 mL)
to the filtrate and cooling to -30 °C gave the product as colorless
crystals (total yield from two crops: 0.36 g, 64%). 1H NMR (CDCl3):
δ (ω1/2, Hz) 108 (720) (TMEDA, NCH3), 86 (780) (TMEDA, NCH2),
13.2 (60) (Ar, m-H), 5.4 (100) (Mes, o-CH3), 2.5 (70) (Mes, m-H), 1.8
(50) (Ar, p-H), -2.2 (60) (Mes, p-CH3). Anal. Calcd (Found) for
C56H66N2O4Fe: C, 75.83 (75.71); H, 7.50 (7.56); N, 3.16 (3.17).
10
crystallization from CH3CN/Et2O. [(TMEDA)FeCl2]2 was prepared
by the addition of 1.1 equiv of TMEDA to a CH2Cl2 suspension of
anhydrous FeCl2 followed by crystallization at -40 °C. 1H NMR spectra
of Fe complexes were acquired on ca. 4 mM CDCl3 solutions using a
500 MHz Varian spectrometer. Selected acquisition parameters for
paramagnetic complexes: relaxation delay ) 0.03 s, acquisition time
) 0.064 s, line broadening ) 30 Hz, sweep width ) 100-250 ppm.
Chemical shifts (δ) are given relative to residual protium in the
deuterated solvent at δ 7.24 ppm for CDCl3. Line widths at half peak
height (ω1/2) are reported in hertz. Elemental analyses were determined
by Atlantic Microlab, Inc. Details of X-ray structure determinations
and crystallographic information files are presented as Supporting
Information.
Results
As we reported earlier,7a the salt-metathesis reaction of 2
equiv of (Mes2ArCO2)Li(solvent)n with Fe(OTf)2‚2CH3CN (OTf
) O3SCF3) in toluene/CH3CN yields the diferrous complex
(Mes2ArCO2)4Fe2(CH3CN)2 (Scheme 1). When bulkier, more
basic N-donors (e.g., MeIm, pyridines, or TMEDA) are added
to solutions of the diiron complex, new species form im-
(Mes2ArCO2)2Fe(MeIm)2. Dichloromethane (10 mL) was added to
FeCl2 (52.0 mg, 0.410 mmol) and (Mes2ArCO2)Li(Et2O) (0.334 g, 0.821
mmol) to form a tan suspension, to which MeIm (62.0 mg, 0.76 mmol)
was added. After stirring overnight, the volatile materials were removed
under reduced pressure. The white solid was extracted with toluene
(15 mL) and filtered through a frit. Concentration of the filtrate to 7
mL, addition of pentane (8 mL), and cooling to -30 °C gave the product
as colorless microcrystals (70 mg, 20%). H NMR (CDCl3): δ (ω1/2
Hz) 58 (380) (MeIm, R-CH), 36 (50) (MeIm, â-CH), 29 (350) (MeIm,
R-CH), 15.1 (50) (MeIm NCH3), 11.8 (50) (Ar, m-H), 5.7 (60) (Mes,
m-H), 2.8 (80) (Mes, o-CH3), 1.4 (60) (Ar, p-H), 1.2 (50) (Mes, p-CH3).
Anal. Calcd (Found) for C58H62N4O4Fe: C, 74.51 (74.61); H, 6.68
(6.91); N, 5.99 (5.79).
1
mediately (by H NMR, CDCl3) and the diiron complex is no
longer observed. These same mononuclear derivatives can be
synthesized conveniently in good yields by salt-metathesis
reactions in the presence of 2-3 equiv of the bases. The
preparation of (Mes2ArCO2)2Fe(TMEDA), which features a
bidentate N-donor ligand, was conveniently carried out by
reaction of the lithium benzoate with [(TMEDA)FeCl2]2 in a
CH2Cl2-toluene mixture. The mesityl groups of the ligands
impart the products with good solubility in aromatic hydrocarbon
solvents and CH2Cl2. All compounds were isolated as either
colorless (MeIm, 2,6-Lut, TMEDA) or yellow (Py, 2-Pic, 2,5-
Lut) crystalline solids. Combustion analysis revealed that the
complexes all share the empirical formula (Mes2ArCO2)2Fe-
(base)2.
1
,
(Mes2ArCO2)2Fe(Py)2‚CH2Cl2. A mixture of toluene (20 mL) and
Py (0.162 g, 2.05 mmol) was added to Fe(OTf)2‚2CH3CN (0.299 g,
(10) (a) Davies, S. C.; Hughes, D. L.; Leigh, G. J.; Sanders, J. R.; de Souza,
J. S. J. Chem. Soc., Dalton Trans. 1997, 1981-1988. (b) Hagadorn,
J. R.; Arnold, J.; Unpublished results.

6088 Inorganic Chemistry, Vol. 39, No. 26, 2000
Hagadorn et al.
Figure 2. Drawing of the X-ray structure of (Mes2ArCO2)2Fe(Py)2
showing 50% thermal ellipsoids. Hydrogen atoms are omitted for clarity.
complex, the Fe-N bonds are short (2.103(2) and 2.106(2) Å)
and the N-Fe-N angle is large at 116.07(9)°.
The compounds (Mes2ArCO2)2Fe(2,5-Lut)2 (Figure 3) and
(Mes2ArCO2)2Fe(2-Pic)2 (Figures S2 and S3) are 6-coordinate
complexes with cis-coordinated N-donors and two bidentate
carboxylates. Because these complexes adopt similar structures,
only that of the 2,5-Lut derivative will be discussed in detail.
The Fe-N bond distances (2.181(2) Å) are longer than those
in the unsubstituted Py derivative (average 2.105 Å) due to the
R-substitution of the N-donor and a higher overall coordination
number. Due to differing trans ligands, the bidentate carboxy-
lates are bound asymmetrically (∆FeO ) 0.12 Å), with the longer
Fe-O bonds being trans to the N-donors (Table 1). Their
average value of 2.18 Å is considerably longer than those for
the aforementioned 4-coordinate complexes (average 1.99 Å).
The addition of a second R-methyl group to the N-donor
results in another change of coordination environment. Similar
to the aforementioned mono R-substituted Py derivatives, (Mes2-
ArCO2)2Fe(2,6-Lut)2 forms a 6-coordinate complex, but the
N-donors are coordinated trans to each other (Figure 4). The
sterically hindered 2,6-Lut has a fairly long Fe-N bond of
2.241(3) Å that is ca. 0.15 Å longer than the Fe-N bonds of
(Mes2ArCO2)2Fe(Py)2. Unlike the 2-Pic and 2,5-Lut derivatives,
the carboxylates are bound symmetrically with Fe-O distances
of 2.189(2) and 2.190(2) Å that are equal within experimental
error.
Figure 1. 1H-NMR spectra of (Mes2ArCO2)2Fe(base)2 (a) base ) Py,
(b) base ) MeIm, and (c) (base)2 ) TMEDA at 298 K in CDCl3.
Scheme 1
The use of the bidentate N-donor TMEDA yielded yet another
coordination geometry in the solid state. As shown in Figure 5,
(Mes2ArCO2)2Fe(TMEDA) has a distorted trigonal bipyramidal
Fe center (τ ) 0.26)11 with O2 and O3 occupying the
pseudoaxial sites (O2-Fe-O3 154.74(8)°). The 5-coordinate
complex features one η2-chelated and one monodentate car-
boxylate. The bidentate benzoate features Fe-O distances
(average 2.149 Å) that are comparable to those found in the
aforementioned 6-coordinate derivatives. The monodentate
Fe1-O3 bond of 1.924(2) Å, however, is even shorter than those
observed for the MeIm and Py adducts. The chelating TMEDA
is bound in typical fashion with an N-Fe-N angle of 82.65-
(8)°, which is smaller than the corresponding values in the other
compounds with monodentate N-donors.
The solid-state structures of all six mononuclear derivatives
were determined by single-crystal X-ray diffraction. Although
they share the composition of two carboxylates and two
N-donors per Fe atom, several distinct structural types were
observed. These approximate tetrahedral, trigonal bipyramidal,
and octahedral ligand environments. The complexes (Mes2-
7a
ArCO2)2Fe(MeIm)2 (Figure S7) and (Mes2ArCO2)2Fe(Py)2
(Figure 2) feature similar pseudotetrahedral Fe centers. For the
core of the Py adduct, a pair of monodentate carboxylates are
bound to Fe with relatively short Fe-O distances of 1.980(2)
and 1.961(2) Å for O1 and O3, respectively (Table 1). The other
oxygen atoms are much farther removed with Fe-O distances
of 2.666(2) and 3.329(2) Å. As expected for the four-coordinate
1H NMR spectroscopic data in CDCl3 have been obtained
for the various complexes to assess the extent to which the solid-
state structures are retained in solution. As shown in Figure 1a,
b, and c, respectively, (Mes2ArCO2)2Fe(Py)2, (Mes2ArCO2)2-
Fe(MeIm)2, and (Mes2ArCO2)2Fe(TMEDA) exhibit relatively
sharp resonances that span 140 ppm, consistent with the presence
of high-spin Fe(II) centers. Tentative assignments for many
signals can be made based on integration and by comparison
(11) Addison, A. W.; Rao, T. N.; Reedjik, J.; van Rijn, J.; Verschoor, G.
C. J. Chem. Soc., Dalton Trans. 1984, 1349-1356.

Carboxylate Binding in Mononuclear Iron(II) Complexes
Inorganic Chemistry, Vol. 39, No. 26, 2000 6089
Table 1. Line Drawings of Core Structures and Selected Bonds and Angles
Figure 4. Drawing of the X-ray structure of (Mes2ArCO2)2Fe(2,5-
Lut)2 showing 50% thermal ellipsoids. Hydrogen atoms are omitted
for clarity.
Figure 3. Drawing of the X-ray structure of (Mes2ArCO2)2Fe(2,5-
Lut)2 showing 50% thermal ellipsoids. Hydrogen atoms are omitted
for clarity.
protons of the bulky benzoate ligands. The observation that there
are only five benzoate resonances in the pyridine and MeIm
complexes is consistent with their crystal structures showing
that the benzoates are structurally equivalent. For the TMEDA
complex, however, the observation of only five benzoate
resonances indicates that the monodentate and bidentate ben-
zoates in this complex must interconvert rapidly on the NMR
time scale. The NMR spectrum of the 2-picoline complex can
be assigned by analogy to the other complexes above, but those
of the two lutidine complexes are more difficult to interpret.
The bulky benzoate ligand in the latter complexes gives rise to
more signals than the five observed in the previous complexes,
suggesting that the symmetry of the benzoate has been disrupted.
While specific assignments of the NMR signals for these
complexes will require further experiments (e.g., atom substitu-
tion), the adoption of structures different from those character-
ized in the solid state and/or a fluxional process(es) is indicated.
to related compounds.12 Thus, the pyridine complex has
resonances at δ 142, 36, and 8.5 ppm (Figure 1a) that can be
assigned to the pyridine R-, â-, and γ-hydrogens, respectively.
Similarly, the MeIm complex (Figure 1b) features resonances
at δ 58.1 (R-CH), 35.9 (â-CH), 29.3 (R-CH), and 15.1 (N-
CH3) ppm from the coordinated MeIm ligands, while the
TMEDA complex (Figure 1c) exhibits peaks at δ 108 and 86
ppm arising from the diamine methyls and methylenes, respec-
tively. For each of the aforementioned complexes, there are only
five resonances in the diamagnetic region; they have the
appropriate relative intensities to be assigned to the five unique
(12) (a) Chiou, Y.-M.; Que, L., Jr. Inorg. Chem. 1995, 34, 3270-3278.
(b) Chiou, Y.-M.; Que, L., Jr. J. Am. Chem. Soc. 1995, 117, 3999-
4013. (c) Borovik, A. S.; Hendrich, M. P.; Holman, T. R.; Mu¨nck,
E.; Papaefthymiou, V.; Que, L., Jr. J. Am. Chem. Soc. 1990, 112,
6031-6038. (d) Zang, Y.; Kim, J.; Dong, Y.; Wilkinson, E. C.;
Appelman, E. H.; Que, L., Jr. J. Am. Chem. Soc. 1997, 119, 4197-
4205.
Discussion
We have found for the (Mes2ArCO2)2Fe(base)2 series that
simple variation of the base results in several different structural

6090 Inorganic Chemistry, Vol. 39, No. 26, 2000
Hagadorn et al.
A key point to be taken from the solid state structural analysis
of the monodentate base series is that the carboxylate binding
mode and overall stereochemistry appear to be controlled by
varying the Lewis acidity of the Fe center. This variation in
Lewis acidity was accomplished indirectly by changing the
degree of R-substitution in a set of monodentate bases. We
acknowledge that fluxional behavior and/or adoption of alterna-
tive structures in solution in some cases, as well as the operation
of unspecified crystal packing forces in the solids, are important
caveats. Related studies of diiron complexes have revealed
similar effects wherein carboxylate binding mode differences
are observed in complexes with divergent steric repulsions or
N-donors.8e,f These findings are especially relevant to our results
and suggest a generality to the notion of controlling carboxylate
binding through subtle modification of Lewis acidity at the metal
center.
In addition to the monodentate bases, we have also studied a
derivative of the bidentate TMEDA. The use of this base
introduces changes in steric bulk, donor ability, and geometry
that complicate straightforward comparisons to the previous
results. The ethylenediamine backbone results in a relatively
fixed N-Fe-N angle (82.65(8)°) that is considerably smaller
than the related values for the other complexes (Table 1). This
“tying back” of the N-donors is expected to favor higher
coordination number species. Moreover, the Fe-N bond lengths
of (Mes2ArCO2)2Fe(TMEDA) (average 2.191 Å) are comparable
to those of the 2-Pic and 2,5-Lut derivatives. From these data,
we may simplistically expect bidentate carboxylates to be
favored. What is observed, however, is a 5-coordinate species
with one monodentate and one bidentate carboxylate. Most
likely, the considerable steric bulk of the tetra(methyl)-
substituted base prevents the bidentate coordination of both
carboxylates.
Figure 5. Drawing of the X-ray structure of (Mes2ArCO2)2Fe(TMEDA)
showing 50% thermal ellipsoids. Hydrogen atoms are omitted for clarity.
types in the solid crystalline state. Shown in Table 1 are four
representatives of those types drawn above the corresponding
bases used to prepare them. Notwithstanding the ubiquitous
influence of crystal packing forces on molecular structures,
trends among the structures that may be rationalized by more
specific metal-ligand interactions are apparent. The most
obvious trend is observed for the monodentate bases, where the
degree of steric bulk in the R-position correlates with the
observed complex geometry. For the least hindered N-donors,
MeIm and Py, the Fe center is close to tetrahedral with the
carboxylates adopting monodentate binding modes. This low
coordinate geometry for carboxylate-rich Fe(II) complexes is
rare; to our knowledge the only closely related complex is the
5-coordinate (XDK)Fe(Py)2,8e,13 which features one monodentate
carboxylate with the other bound in an asymmetric fashion with
a long Fe-O interaction of 2.326(5) Å.
Increasing the steric bulk of the base by the addition of a
single R-Me group results in a counterintuitive structural change.
While increasing steric bulk around a metal center typically
results in the lowering of the coordination number, just the
opposite was observed. A comparison of bond lengths for (Mes2-
ArCO2)2Fe(2-Pic)2 and (Mes2ArCO2)2Fe(2,5-Lut)2 with the
aforementioned tetrahedral derivatives makes the effect of the
R-Me group apparent. It creates a steric interaction with the
Fe, forcing the Fe-N bond to lengthen by about 0.1 Å.12d,14
We hypothesize that this weakened interaction increases the
Lewis acidity of the Fe, such that binding of additional ligand
donors (here, bidentate coordination of the carboxylates)
becomes favored. The addition of a second R-Me group (i.e.,
2,6-Lut) further weakens the Fe-N interaction. However, the
2,6-Lut groups adopt trans positions, possibly to minimize steric
interactions between the hindered 2,6-Lut ligands, and the
carboxylates become symmetrically chelated.
In summary, the sterically hindered benzoate ligand Mes2-
ArCO2- has allowed us to access a range of new mononuclear
Fe(II) carboxylates. Structural analysis of the monodentate base
series has revealed the effect of N-donors on carboxylate binding
mode. The simple 0.1 Å change in Fe-N distance from ca. 2.1
to 2.2 Å upon R-methyl substitution of the N-donors correlates
with a shift of the carboxylates from a monodentate to a
bidentate binding mode. We surmise that this change is due to
an electrostatic effect, and is driven by increasing the Lewis
acidity of the Fe center. Such a simple process for inducing
carboxylate shifts4 could play a critical role in biological systems
that require control of coordination sites. Thus, our results in
conjunction with those of others8 suggest that a subtle confor-
mational change in a protein could result in the necessary
lengthening or shortening of a metal-nitrogen bond, thereby
altering the preferred carboxylate binding mode during catalysis.
Acknowledgment. We gratefully acknowledge the National
Institutes of Health for funding (GM38767 to L.Q.) and a
postdoctoral fellowship (F32-GM19374 to J.R.H.).
Supporting Information Available: X-ray crystallographic files
in CIF format for all new complexes. This material is available free of
charge via the Internet at http://pubs.acs.org.
(13) XDK is the dicarboxylate ligand based on m-xylylenediamine-bis-
(Kemp’s triacid)imide.
(14) Constable, E. C.; Baum, G.; Bill, E.; Dyson, R.; van Eldik, R.; Fenske,
D.; Kaderli, S.; Morris, D.; Neubrand, A.; Neuburger, M.; Smith, D.
R.; Wieghardt, K. Chem. Eur. J. 1999, 5, 498-508.
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