Structural flexibility within a sterically hindered ligand platform: Mononuclear iron(II) carboxylate complexes as subsite models for diiron(II) centers

Article abstract of DOI:10.1016/S0020-1693(02)01200-8

The synthesis and characterization of a series of mononuclear iron(II) carboxylate complexes are described. By using sterically hindered carboxylate ligands, 2,6-di(p-tolyl)benzoate (Ar^TolCO₂^-) and 2,6-di(4-tert-butylphenyl)benzoate (Ar^4-tBuPhCO₂^-), a series of four-, five-, and six-coordinate iron(II) complexes were synthesized. The compounds are [Fe(O₂CAr^Tol)₂(1-BnIm)₂] (3), [Fe(O₂CAr^Tol)₂(1-MeBzIm)₂] (4), [Fe(O₂CAr^4-tBuPh)₂(2,2′-bipy) ₂] (5), [Fe(O₂CAr^Tol)₂(TMEDA)] (6), and [Fe(O₂CAr^Tol)₂(BPTA)] (7). Structural analyses of 3-7 revealed that the overall stereochemistry of the [Fe(O₂CAr′)₂L_n] units is dictated by electronic and steric factors of the N-donor ligands (L), as well as by the flexible coordination of the carboxylate ligands. Distinctive M?ssbauer parameters obtained for these and related compounds facilitated the spectral assignment of a diiron(II) complex having asymmetric metal sites, [Fe₂(μ-O₂CAr^Tol)₃(O ₂CAr^Tol)(2,6-lutidine)] (2). Well-defined mononuclear iron carboxylate complexes thus may serve as subsite models for higher nuclearity species in both synthetic and biological systems.

Full text of DOI:10.1016/S0020-1693(02)01200-8

Inorganica Chimica Acta 341 (2002) 1ꢁ11
/
www.elsevier.com/locate/ica
Structural flexibility within a sterically hindered ligand platform:
mononuclear iron(II) carboxylate complexes as subsite models
for diiron(II) centers
Dongwhan Lee, Stephen J. Lippard *
Department of Chemistry, Massachusetts Institute of Technology, 77 Massachusetts Avenue Room 18-498, Cambridge, MA 02139, USA
Received 28 August 2001; accepted 10 December 2001
Dedicated to Professor Kenneth N. Raymond on the occasion of his 60th birthday
Abstract
The synthesis and characterization of a series of mononuclear iron(II) carboxylate complexes are described. By using sterically
hindered carboxylate ligands, 2,6-di(p-tolyl)benzoate (ArTolCO2ꢀ) and 2,6-di(4-tert-butylphenyl)benzoate (Ar4-tBuPhCO2ꢀ), a series
of four-, five-, and six-coordinate iron(II) complexes were synthesized. The compounds are [Fe(O2CArTol)2(1-BnIm)2] (3),
[Fe(O2CArTol)2(1-MeBzIm)2] (4), [Fe(O2CAr4-tBuPh)2(2,2?-bipy)2] (5), [Fe(O2CArTol)2(TMEDA)] (6), and [Fe(O2CArTol)2(BPTA)]
(7). Structural analyses of 3ꢁ7 revealed that the overall stereochemistry of the [Fe(O2CAr?)2Ln] units is dictated by electronic and
/
steric factors of the N-donor ligands (L), as well as by the flexible coordination of the carboxylate ligands. Distinctive Mossbauer
¨
parameters obtained for these and related compounds facilitated the spectral assignment of a diiron(II) complex having asymmetric
metal sites, [Fe2(m-O2CArTol)3(O2CArTol)(2,6-lutidine)] (2). Well-defined mononuclear iron carboxylate complexes thus may serve as
subsite models for higher nuclearity species in both synthetic and biological systems.
# 2002 Elsevier Science B.V. All rights reserved.
Keywords: Carboxylate shifts; Mononuclear iron complexes; Mossbauer spectroscopy; m-Terphenyl
¨
1. Introduction
fragment, which can support unusual coordination
geometries in mononuclear complexes. The two-coordi-
nate iron(II) thiolate complexes [Fe(SArMes)-
{N(SiMe3)2}] and [Fe(SArMes)2] [10,11] illustrate this
application. Two mesityl groups flanking the metal
binding sites apparently shield them against unwanted
oligomerization reactions. Control over coordination
number and geometry can thus be achieved by inter-
ligand steric interactions in the second coordination
sphere.
Coordinatively unsaturated redox-active transition
metal ions bind and activate small molecule substrates
in living systems [1]. Sulfido-bridged three-coordinate
iron centers occur in the nitrogen-fixing FeMo cofactors
[2]. One carboxylate and two histidine ligands bind non-
heme iron in selected dioxygen-activating enzymes [3ꢁ
/
5]. Efforts to investigate structureꢁfunction relation-
/
ships for such catalytic sites have been greatly assisted
by low molecular weight analogs having similar chemi-
cal properties [6,7].
Sterically hindered ligands are commonly exploited to
stabilize low-coordinate metal centers [8,9]. Among the
tactics used to afford steric bulk is the m-terphenyl
Recently, we [12ꢁ15] and others [16,17] have em-
/
ployed the m-terphenyl fragment to access coordina-
tively unsaturated diiron carboxylate complexes.
Following a divergent synthetic route, a variety of
tetracarboxylate diiron(II) compounds were prepared
from a convenient synthetic precursor, [Fe2(m-O2-
CArTol)2(O2CArTol)2(THF)2] (1), where ArTolCO2ꢀ ꢂ
/
2,6-di(p-tolyl)benzoate [12]. Weakly coordinating THF
molecules in 1 are readily displaced by N-donor ligands
under mild conditions to afford diiron(II) complexes
* Corresponding author. Tel.: ꢃ
8150
E-mail address: lippard@lippard.mit.edu (S.J. Lippard).
/
1-617-253 1892; fax: ꢃ1-617-258
/
0020-1693/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved.
PII: S 0 0 2 0 - 1 6 9 3 ( 0 2 ) 0 1 2 0 0 - 8

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D. Lee, S.J. Lippard / Inorganica Chimica Acta 341 (2002) 1ꢁ11
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bearing a close structural resemblance to the active sites
of selected non-heme diiron enzymes [3,18,19]. These
ligand substitution reactions, together with the flexible
coordination of the terphenyl carboxylates, afforded
doubly-, triply-, and quadruply-bridged diiron com-
plexes sharing the common {Fe2(O2CArTol)4} compo-
nent [12,13,20]. In an extension of this approach we
explored the chemistry of sterically more demanding
monodentate and polydentate N-donor ligands. Our
goal was to evaluate how variation in steric bulk and
ligand donor strength would affect the nuclearity and
coordination geometry of the resulting complexes.
In this report, we describe the synthesis and char-
CArTol)2(THF)2] (1) [12] were prepared according to
literature procedures. The syntheses of 2,6-di(4-tert-
butylphenyl)benzoic acid (Ar4-tBuPhCO2H) and [Fe2(m-
O2CArTol)3(O2CArTol)(2,6-lutidine)] (2) are reported
elsewhere [20]. Air-sensitive manipulations were carried
out under nitrogen in a Vacuum Atmospheres drybox or
by standard Schlenk line techniques.
2.2. Physical measurements
FT-IR spectra were recorded on a Bio Rad FTS-135
instrument with Win-IR software. UVꢁ/Vis spectra were
measured on a Hewlett-Packard 8453 diode array
spectrophotometer. Mossbauer spectra were obtained
¨
on an MS1 spectrometer (WEB Research Co.) with a
57Co source in a Rh matrix maintained at room
temperature (r.t.) in the MIT Department of Chemistry
Instrumentation Facility. Solid samples were prepared
acterization of neutral mononuclear iron(II) complexes
of the carboxylate ligands Ar?CO2 (Ar?ꢂ
ArTol or
/
ꢀ
Ar4-tBuPh). Four-, five-, and six-coordinate compounds
were readily obtained from a common synthetic pre-
cursor with the proper choice of N-donor ligands.
Unique structural and physical properties of the com-
by suspending ꢀ0.05 mmol of the powdered material in
/
pounds [Fe(O2CAr?)2Ln] (Lꢂ/1-BnIm (3); 1-MeBzIm
Apeizon N grease and packing the mixture into a nylon
sample holder. All data were collected at 4.2 K and the
isomer shift (d) values are reported with respect to
natural iron foil that was used for velocity calibration at
r.t. The spectra were fit to Lorentzian lines by using the
WMOSS plot and fit program [27].
(4); 2,2?-bipy (5); TMEDA (6); BPTA (7)) are described.
The Mossbauer parameters of these compounds were
¨
used to facilitate spectral assignment of [Fe2(m-O2C-
ArTol)3(O2CArTol)(2,6-lutidine)] (2), which has asym-
metric iron centers. Carboxylate shifts upon changes in
the N-donor ligands and their structural implications
are discussed. A portion of this work has been pre-
viously communicated [15].
2.3. Preparation of compounds
2.3.1. [Fe(O2CArTol)2(1-BnIm)2] (3)
To a rapidly stirred pale yellow CH2Cl2 (2 ml)
solution of 1 (98 mg, 67 mmol) was added dropwise 1-
benzylimidazole (1-BnIm) (43 mg, 0.27 mmol) dissolved
in CH2Cl2 (1 ml). The resulting colorless solution was
stirred for 3 h and filtered. Vapor diffusion of pentanes
into the filtrate afforded colorless blocks of 3 (102 mg,
0.105 mmol, 78%), which were suitable for X-ray
2. Experimental
2.1. Materials and methods
All reagents were obtained from commercial suppliers
and used as received unless otherwise noted. Dichlor-
omethane was distilled over CaH2 under nitrogen.
Diethyl ether, pentanes, and THF were saturated with
nitrogen and purified by passage through activated
Al2O3 columns under nitrogen [21]. The compounds
crystallography. FT-IR (KBr, cmꢀ1
)
3141(w),
3121(m), 2920(w), 1608(vs), 1585(vs), 1515(vs),
1497(m), 1454(vs), 1438(m), 1400(m), 1346(vs),
1305(m), 1283(m), 1239(m), 1144(w), 1106(s), 1088(s),
1031(m), 1020(m), 946(m), 940(w), 838(s), 820(s), 800(s),
783(s), 765(s), 752(s), 736(s), 725(s), 708(s), 693(s),
655(s), 544(m), 458(w). Anal. Calc. for C62H54FeN4O4:
Fe(OTf)2×
(ArTolCO2H) [23ꢁ
butylamine (BPTA) [26], and [Fe2(m-O2CArTol)2(O2-
/
2MeCN [22], 2,6-di(p-tolyl)benzoic acid
/
25], N,N?-bis(2-pyridylmethyl)-tert-

D. Lee, S.J. Lippard / Inorganica Chimica Acta 341 (2002) 1ꢁ
/
11
3
C, 76.38; H, 5.58; N, 5.75. Found: C, 75.83; H, 5.92; N,
5.75%.
blocks of 6 (308 mg, 80%) were obtained and analyzed
by X-ray crystallography.
2.3.2. [Fe(O2CArTol)2(1-MeBzIm)2] (4)
2.3.4.2. Method B. To a rapidly stirred CH2Cl2 (3 ml)
solution of 1 (153 mg, 0.105 mmol) was added dropwise
TMEDA (32 ml, 0.21 mmol). The reaction mixture was
stirred for 0.5 h and filtered. Vapor diffusion of
To a rapidly stirred pale yellow CH2Cl2 (3 ml)
solution of 1 (101 mg, 69.1 mmol) was added dropwise
1-methylbenzimidazole (1-MeBzIm) (39 mg, 0.30 mmol)
in CH2Cl2 (3 ml). The reaction immediately turned into
a heterogeneous mixture. Volatile fractions were re-
moved under reduced pressure and the residual off-
pentanesꢁ
/
hexanes (1:1) into the filtrate afforded color-
less blocks of 6 (148 mg, 91%). FT-IR (KBr, cmꢀ1
)
2916(m), 2865(m), 1545(vs), 1514(vs), 1459(vs), 1411(s),
1382(vs), 1291(w), 1125(w), 1112(w), 1029(m), 954(w),
855(m), 835(m), 818(m), 800(vs), 766(s), 738(m), 711(s),
586(m), 546(m), 528(s). Anal. Calc. for C48H50FeN2O4:
C, 74.41; H, 6.50; N, 3.62. Found: C, 74.56; H, 6.88; N,
3.63%.
white solid was extracted into MeOHꢁMeCN (1:1, 3 ml)
/
and filtered. Vapor diffusion of Et2O into the filtrate
afforded colorless blocks of 4 (103 mg, 0.112 mmol,
81%), which were suitable for X-ray crystallography.
FT-IR (KBr, cmꢀ1
) 3111(w), 3056(m), 2918(m),
1565(vs), 1513(vs), 1453(vs), 1361(vs), 1297(m),
1255(m), 12 039(s), 1186(m), 1149(m), 1130(w),
1111(w), 1071(w), 1020(w), 1006(w), 916(w), 882(m),
855(m), 835(m), 821(m), 802(m), 784(m), 763(s), 586(w),
541(w), 525(w), 454(vw). Anal. Calc. for C58H50FeN4O4:
C, 75.48; H, 5.46; N, 6.07. Found: C, 75.50; H, 5.42; N,
6.12%.
2.3.5. [Fe(O2CArTol)2(BPTA)] (7)
To a rapidly stirred CH2Cl2 (2 ml) solution of 1 (102
mg, 69.8 mmol) was added dropwise BPTA (36 mg, 0.14
mmol) dissolved in CH2Cl2 (1 ml). The intense yellow
solution was stirred for 15 min and volatile fractions
were removed under reduced pressure. The residual
orange solid was extracted into PhCl (2 ml) and
insoluble material was filtered off. Vapor diffusion of
pentanes into the yellow filtrate afforded yellow needles
of 7 (102 mg, 0.111 mmol, 80%), which were suitable for
X-ray crystallography. FT-IR (KBr, cmꢀ1) 3053(m),
3024(m), 2974(m), 2919(m), 1623(vs), 1606(vs), 1576(s),
1515(vs), 1477(s), 1442(vs), 1404(s), 1372(s), 1354(vs),
1189(w), 1149(w), 1021(m), 833(m), 799(s), 767(vs),
2.3.3. [Fe(O2CAr4-tBuPh)2(2,2?-bipy)] (5)
To a rapidly stirred MeCN (5 ml) solution of
Fe(OTf)2×
/
2MeCN (112 mg, 0.257 mmol) was added
NaO2CAr4-tBuPh (208 mg, 0.509 mmol) to afford a pale
yellow solution. A portion of 2,2?-bipyridine (2,2?-bipy)
(40 mg, 0.256 mmol) in MeCN (3 ml) was added
dropwise. The color of the solution turned dark
burgundy red and crystalline material began to deposit
within minutes. Dark red blocks of 5 (150 mg, 56%)
were isolated and analyzed by X-ray crystallography.
FT-IR (KBr, cmꢀ1) 3057(m), 2962(s), 2903(s), 2866(s),
1598(vs), 1579(vs), 1547(vs), 1513(vs), 1473(s), 1460(vs),
1442(vs), 1406(s), 1381(vs), 1363(vs), 1268(m), 1020(m),
856(m), 842(m), 827(m), 806(m), 781(s), 763(s), 752(m),
701(m), 532(m); UVꢁ
/
Vis (CH2Cl2) (lmax, nm (o, Mꢀ1
cmꢀ1)) 433 (1300). Anal. Calc. for C58H55FeN3O4×
/
C6H5Cl: C, 74.89; H, 5.89; N, 4.09. Found: C, 75.00;
H, 6.20; N, 4.14%.
2.4. X-ray crystallography
Intensity data were collected on a Bruker (formerly
736(m), 695(w), 686(w), 578(s); UVꢁVis (CH2Cl2) (lmax,
/
Siemens) CCD diffractometer with graphite-monochro-
˚
0.71073 A), controlled by a
nm (o, Mꢀ1 cmꢀ1)) 350 (sh, 290), 537 (200). Anal. Calc.
for C64H66FeN2O4: C, 78.19; H, 6.77; N, 2.85. Found:
C, 78.10; H, 6.83; N, 2.84%.
mated Mo Ka radiation (lꢂ
/
Pentium-based PC running the SMART software package
[28]. Single crystals were mounted at r.t. on the tips of
quartz fibers, coated with Paratone-N oil, and cooled to
188 K under a stream of cold nitrogen maintained by a
Bruker LT-2A nitrogen cryostat. Data collection and
reduction protocols are described elsewhere [29]. The
structures were solved by direct methods and refined on
F2 by using the SHELXTL software package [30].
Empirical absorption corrections were applied with
SADABS [31], part of the SHELXTL program package,
and the structures were checked for higher symmetry by
the program PLATON [32]. All non-hydrogen atoms were
refined anisotropically unless otherwise noted. Hydro-
gen atoms were assigned idealized positions and given
thermal parameters equivalent to either 1.5 (methyl
hydrogen atoms) or 1.2 (all other hydrogen atoms)
2.3.4. [Fe(O2CArTol)2(TMEDA)] (6)
2.3.4.1. Method A. A portion of N,N,N?,N?-tetra-
methylethylenediamine (TMEDA) (75 ml, 0.497 mmol)
was added dropwise to a rapidly stirred MeCN (10 ml)
solution of Fe(OTf)2×
/
2MeCN (219 mg, 0.502 mmol).
The colorless solution was treated with NaO2CArTol
(325 mg, 1.00 mmol) and the heterogeneous mixture was
stirred overnight. Volatile fractions were removed under
reduced pressure and the residual off-white solid was
extracted into CH2Cl2 (5 ml). Insoluble material was
filtered off and pentanesꢁ/hexanes (1:1) were allowed to
diffuse into the pale yellow filtrate at r.t. Colorless

4
D. Lee, S.J. Lippard / Inorganica Chimica Acta 341 (2002) 1ꢁ11
/
Table 1
Summary of X-ray crystallographic data
3
4
5×2MeCN
6
7×PhCl
Empirical formula
Formula weight
Temperature (8C)
Space group
C
974.9
62H54FeN4O4
C58H50FeN4O4
922.9
C68H72FeN4O4
1065.2
C48H50FeN2O4
774.8
C64H59ClFeN3O4
1025.4
ꢀ85
ꢀ85
C2/c
ꢀ85
ꢀ85
P21/n
ꢀ85
¯
¯
¯
/
/
P1/
/
P1/
/
P1
˚
a (A)
12.7648(4)
14.5563(5)
15.4066(6)
73.8100(10)
67.795(2)
84.0580(10)
2545.26(15)
2
14.453(17)
23.21(2)
15.667(10)
14.1779(12)
14.4349(13)
14.7856(13)
78.700(3)
84.547(2)
83.7120(10)
2941.0(4)
2
12.4478(4)
19.9561(6)
17.2865(5)
13.3303(4)
16.6989(4)
26.8209(6)
99.7700(10)
102.9470(10)
104.31
˚
b (A)
˚
c (A)
a (8)
b (8)
114.03(5)
110.7520(10)
g (8)
3
V (A )
˚
4800(8)
4
4015.5(2)
4
5475.4(2)
4
1.244
Z
rcalc (g cmꢀ3
)
1.272
0.349
1.277
0.366
1.203
0.308
1.282
0.423
m(Mo Ka) (mmꢀ1
u Limits (8)
Total no. of data
)
0.375
1.81ꢁ
/
28.30
1.75ꢁ
/
28.29
1.94ꢁ
/
28.34
1.62ꢁ/28.31
1.61ꢁ28.30
/
16198
11287
640
4892
3479
310
18565
12990
690
24856
9231
522
34765
24237
1333
No. of unique data
No. of parameters
a
R (%)
wR2 (%)
˚
Max, min peaks (e A
6.80
11.10
3.80
9.40
6.21
14.23
4.06
9.27
7.03
12.10
b
ꢀ3
)
0.310, ꢀ0.341
0.197, ꢀ0.225
0.445, ꢀ0.339
0.355, ꢀ0.413
0.291, ꢀ0.328
a
Rꢂa jjFojꢀjFcjj/a jFoj.
b
wR2ꢂ{a [w(Fo2ꢀFc2)2]/a [w(Fo2)2]}1/2
.
times the thermal parameter of the carbon atom to
which they were attached. The hydrogen atoms asso-
ciated with the disordered solvent molecules were not
included in the refinement. The butyl groups on one of
the Ar4-tBuPhCO2ꢀ ligands in 5 were disordered over two
positions. The atoms were equally distributed and
refined isotropically. The disordered ethylene carbon
atoms of the TMEDA ligand in 6 were equally
distributed over two positions and refined isotropically.
Each of the chlorine atoms associated with the two PhCl
solvent molecules in the structure of 7 were disordered
over two positions. One was refined at 0.7 and 0.3
occupancy; the other refined to 0.6 and 0.4. Crystal-
lographic information is provided in Table 1.
3 is shown in Fig. 1; selected bond lengths and angles are
listed in Table 2. The pseudo-tetrahedral iron center in 3
is coordinated by two monodentate carboxylate and two
1-BnIm ligands. The metrical parameters are normal for
high-spin iron(II) centers and comparable to those of
related compounds [Fe(O2CArMes)2L2] (Lꢂ
1-MeIm or
/
pyridine) built on sterically more demanding carbox-
ylate ligands, 2,6-di(mesityl)benzoate (ArMesCO2ꢀ) [33].
When a similar procedure was used for 1 and 1-
MeBzIm, a six-coordinate mononuclear complex 4 was
isolated following recrystallization from MeOHꢁ
/
MeCNꢁEt2O. The crystal structure of 4 is shown in
/
Fig. 1; selected bond lengths and angles are listed in
Table 2. A crystallographic C2-symmetry bisects the
molecule and two symmetry-related bidentate carbox-
ylate and 1-MeBzIm ligands support a highly distorted
octahedral iron center. The binding of the carboxylate
3. Results
ligands is asymmetric, as reflected in the two distinct
˚
0.31 A (Table 2). The
3.1. Synthesis, and structural characterization of
[Fe(O2CArTol)2(1-BnIm)2] (3), [Fe(O2CArTol)2(1-
MeBzIm)2] (4), [Fe(O2CAr4-tBuPh)2(2,2?-bipy)] (5),
[Fe(O2CArTol)2(TMEDA)] (6), and
FeÃ
/
O distances with DFeÃOꢀ
/
longer FeÃ
atoms and the average FeÃ
significantly longer than that (ꢀ
increase in coordination number, however, the NÃ
/O bond is positioned trans to the N-donor
˚
/
O distance of ꢀ
/
2.23 A is
1.99 A) in 3. Despite an
FeÃ
˚
[Fe(O2CArTol)2(BPTA)] (7)
/
/
/
N angle of 114.99(12)8 in 4 is significantly larger than
that in 3, presumably due to the steric interaction
between cis-coordinated bulky N-donor ligands. Much
Compound 3 was prepared from a reaction between 1
and 1-BnIm in CH2Cl2. Even when a 2:1 ratio of 1-BnIm
to 1 was used, only a mononuclear complex was
isolated, which incorporated two N-donor ligands per
iron(II). The preparation was thus optimized by using
smaller NÃ
/
FeÃ
/
N angles of 90.3(1)ꢁ97.6(2)8 were ob-
/
tained for related six-coordinate iron(II) complexes
[Fe(O2CArMes)2L2] having sterically less demanding
ligands such as 2-picoline or 2,5-lutidine [33].
ꢁ/4 equiv of 1-BnIm per 1, which afforded colorless
blocks of 3 in good yield (78%). The crystal structure of

D. Lee, S.J. Lippard / Inorganica Chimica Acta 341 (2002) 1ꢁ
/
11
5
Fig. 1. ORTEP diagrams of [Fe(O2CArTol)2(1-BnIm)2] (3) (left) and [Fe(O2CArTol)2(1-MeBzIm)2] (4) (right) with thermal ellipsoids at 50%
probability.
Ligand substitution reactions of 1 with bidentate
imine or amine donors afforded five-coordinate iron(II)
complexes 5 and 6 having N2O3 donor atom sets. When
a CH2Cl2 solution of 1 was treated with 2 equiv of 2,2?-
bipy, the color of the solution turned bluish purple and a
highly intractable solid material deposited immediately.
In order to enhance the solubility of this 2,2?-bipy
however, binding of h2-carboxylate in 6 is highly
˚
asymmetric (DFeÃO
O(2A) bond positioned trans to the amine donors.
$
/
0.289 A), with a longer Fe(1)Ã
/
Reaction of 1 with 2 equiv of BPTA in CH2Cl2
afforded yellow needles of 7 in good yield (80%). The
crystal structure of 7 is shown in Fig. 3; selected bond
lengths and angles are available in Table 2. X-ray
structure analysis revealed two chemically identical but
crystallographically inequivalent molecules in the unit
cell. The tridentate BPTA ligand binds in a meridional
configuration, positioning the two pyridine nitrogen
atoms at the pseudoaxial sites of a distorted trigonal
adduct(s), the less polar Ar4-tBuPhCO2 ligand was used
ꢀ
to replace ArTolCO2ꢀ. Compound 5 was isolated in
modest yield (ꢀ
/
56%) from a reaction between
2MeCN, NaO2CAr4-tBuPh, and 2,2?-bipy in a
Fe(OTf)2×
/
1:2:1 ratio in MeCN. The use of the polar solvent
MeCN apparently facilitates isolation of the neutral
product 5, which precipitates directly from the reaction
mixture in analytically pure, crystalline form. The
crystal structure of 5 is shown in Fig. 2; selected bond
lengths and angles are available in Table 2. Compound 5
has one bidentate and one monodentate carboxylate
bipyramidal. The N(1P)Ã
/
Fe(1)Ã
/
N(1Q) angles are
152.28(18) and 153.09(18)8. A pair of monodentate
carboxylates and a tertiary amine complete the N3O2
donor atom set. The FeÃ
/
Npyridine distances of 2.100(4)
to 2.137(4) A in 7 are comparable to the FeÃ
/
˚
/
˚
Nimidazole/pyridine distances of 2.074(4)ꢁ
/
2.141(3) A in 3ꢁ
Namine distances
2.353(4) A. A similar trend was observed for
5, but significantly shorter than the FeÃ
/
ligands. The symmetric bonding a bidentate carboxylate
˚
O distances of 2.194(2) and
˚
of 2.336(4)ꢁ
/
(DFeÃO
$
/
0.023 A) has FeÃ
/
dimanganese(III) and dimanganese(IV) complexes sup-
ported by BPTA [26]. Compound 7 displays an intense
˚
2.171(2) A, which are significantly longer than that of
˚
the monodentate carboxylate, 2.015(2) A.
(oꢂ
assigned as a MLCT transition. Similar transitions
occur at ꢀ360 nm in related complexes having compar-
able o values per iron(II)-pyridine unit (500ꢁ
1000 Mꢀ1
cmꢀ1) [34].
/
1300 Mꢀ1 cmꢀ1) visible band at lmax
ꢂ/433 nm,
Compound 6 can be prepared either by ligand
substitution of 1 with 2 equiv of TMEDA or by direct
/
self-assembly from Fe(OTf)2×
and TMEDA in a 1:2:1 ratio in THF. Colorless blocks
were obtained in excellent yield (80ꢁ91%) following
recrystallization from CH2Cl2ꢁpentanesꢁhexanes. The
/
2MeCN, NaO2CArTol
,
/
/
/
/
crystal structure of 6 is shown in Fig. 2; selected bond
lengths and angles are available in Table 2. The highly
3.2. Mossbauer spectroscopy
¨
distorted trigonal bipyramidal iron atom (O(1A)Ã
/
Zero-field Mossbauer spectra (Figs. 4 and 5) of solid
¨
Fe(1)Ã
/
O(1B)ꢂ157.80(6)8) is coordinated by one biden-
/
samples of 2ꢁ5 and 7 were collected at 4.2 K. The
/
tate and one monodentate carboxylate, as in 5. The
flexible ethylene linker between the two N-donor atoms
corresponding parameters derived from fits of the
spectra are provided in Table 3 along with those of
related diiron(II) complexes. Consistent with the pre-
sence of two distinct iron(II) centers in the dinuclear
affords a slightly expanded NÃ
/
FeÃN angle of 82.66(6)8
/
compared with the 75.57(11)8 value in 5. Unlike 5,

6
D. Lee, S.J. Lippard / Inorganica Chimica Acta 341 (2002) 1ꢁ11
/
Table 2
unit, 2 exhibits two overlapping doublets with equal
intensities. Compounds 3ꢁ5 and 7 display single sharp
(Gꢂ0.24ꢁ
0.31 mm sꢀ1) quadrupole doublets. The
˚
Selected bond lengths (A) and angles (8) for 3, 4, 5×2MeCN, 6, and
7×PhCl
/
a
/
/
isomer shifts and quadrupole splittings of these com-
pounds are typical of high-spin iron(II) sites in a N/O
Bond lengths
Bond angles
3
coordination environment [35ꢁ37] and comparable to
/
Fe(1)ÃO(1)
Fe(1)ÃO(3)
Fe(1)ÃN(1)
Fe(1)ÃN(3)
1.989(3)
1.996(3)
2.074(4)
2.121(4)
O(1)ÃFe(1)ÃO(3)
O(1)ÃFe(1)ÃN(1)
O(3)ÃFe(1)ÃN(1)
O(1)ÃFe(1)ÃN(3)
O(3)ÃFe(1)ÃN(3)
N(1)ÃFe(1)ÃN(3)
147.58(13)
107.68(14)
95.25(14)
90.13(14)
109.02(14)
98.89(15)
those obtained for related diiron(II) complexes (Table 3)
[20].
4
4. Discussion
Fe(1)ÃO(1)
Fe(1)ÃO(2)
Fe(1)ÃN(1)
2.076(3)
2.384(2)
2.128(2)
O(1)ÃFe(1)ÃO(1A)
O(1)ÃFe(1)ÃN(1)
O(1)ÃFe(1)ÃN(1A)
O(1)ÃFe(1)ÃO(2)
O(1)ÃFe(1)ÃO(2A)
N(1)ÃFe(1)ÃO(2)
N(1)ÃFe(1)ÃO(2A)
O(2)ÃFe(1)ÃO(2A)
N(1)ÃFe(1)ÃN(1A)
156.77(1)
107.93(8)
84.75(8)
58.58(7)
102.22(8)
93.72(9)
139.58(7)
79.51(11)
114.99(12)
4.1. Synthesis of iron(II) carboxylate complexes
Synthetic routes to the ArTolCO2ꢀ-supported iron(II)
complexes developed in this and previous investigations
are summarized in Scheme 1. The iron(II) centers in
these compounds are nestled within the hydrophobic
5×2MeCN
Fe(1)ÃO(1A)
Fe(1)ÃO(2A)
Fe(1)ÃO(1B)
Fe(1)ÃN(1)
Fe(1)ÃN(2)
2.194(2)
2.171(2)
2.015(2)
2.131(3)
2.141(3)
O(1A)ÃFe(1)ÃO(2A)
O(1A)ÃFe(1)ÃO(1B)
O(1B)ÃFe(1)ÃN(1)
O(1B)ÃFe(1)ÃN(2)
O(1B)ÃFe(1)ÃO(2A)
N(1)ÃFe(1)ÃO(2A)
N(2)ÃFe(1)ÃO(2A)
N(1)ÃFe(1)ÃO(1A)
N(2)ÃFe(1)ÃO(1A)
N(1)ÃFe(1)ÃN(2)
60.08(9)
100.92(9)
141.94(11)
112.81(10)
101.80(10)
112.95(10)
101.85(10)
84.19(10)
144.82(10)
75.57(11)
cavity afforded by the p-substituted aryl groups flanking
ꢀ
the carboxylate unit on Ar?CO2
(Ar?ꢂ
ArTol or
/
Ar4-tBuPh). This steric shield attenuates the tendenacy
of kinetically labile iron-carboxylate units to assemble
into higher nuclearity species [38]. The nuclearity and
coordination geometry of these compounds are appar-
ently dictated by the electronic and steric properties of
the N-donor ligands. Structural diversity within the
[Fe(O2CArTol)2L2] modules is further extended by the
flexible coordination of the carboxylate ligands, which
6
Fe(1)ÃO(1A)
Fe(1)ÃO(2A)
Fe(1)ÃO(1B)
Fe(1)ÃN(1)
Fe(1)ÃN(2)
2.0641(14) O(1A)ÃFe(1)ÃO(1B)
2.3527(14) O(1A)ÃFe(1)ÃO(2A)
1.9590(13) O(1B)ÃFe(1)ÃO(2A)
2.1868(16) O(1A)ÃFe(1)ÃN(1)
2.2119(16) O(1A)ÃFe(1)ÃN(2)
O(1B)ÃFe(1)ÃN(1)
157.80(6)
59.07(5)
104.11(5)
103.28(6)
91.06(6)
96.89(6)
100.73(6)
114.16(6)
147.65(6)
82.66(6)
translates into distinctive Mossbauer parameters ob-
¨
tained for these compounds (vide infra).
O(1B)ÃFe(1)ÃN(2)
4.1.1. Dimetallic core disassembly upon ligand
substitution with monodentate N-donors
N(1)ÃFe(1)ÃO(2A)
N(2)ÃFe(1)ÃO(2A)
N(1)ÃFe(1)ÃN(2)
Minimal structural perturbation is introduced into the
carboxylate-bridged diiron(II) core of 1 upon displace-
ment of the weakly bound THF ligands by MeCN,
pyridine, or 1-methylimidazole [12,20]. Binding of
sterically more demanding monodentate N-donor li-
gands such as 1-BnIm or 1-MeBzIm, however, dissoci-
ates the dinuclear core into mononuclear iron(II)
dicarboxylate complexes 3 or 4. Members of this rare
class of iron(II) compounds having two carboxylate and
two N-donor ligands were previously prepared by using
7×PhCl
Fe(1)ÃO(1A)
b
b
c
2.116(3)
2.150(3)
2.007(3)
1.995(3)
2.121(4)
2.100(4)
2.137(4)
2.122(4)
2.353(4)
2.336(4)
O1AÃFe1ÃO1B
O1AÃFe1ÃN1P
O1AÃFe1ÃN1Q
O1AÃFe1ÃN01
O1BÃFe1ÃN1P
O1BÃFe1ÃN1Q
O1BÃFe1ÃN01
N1PÃFe1ÃN1Q
N1PÃFe1ÃN01
N1QÃFe1ÃN01
87.80(14)
91.68(13)
101.40(14)
98.98(15)
96.72(14)
96.74(15)
142.17(14)
142.12(13)
91.92(15)
93.50(14)
109.69(14)
107.76(15)
129.76(13)
125.96(13)
152.28(18)
153.09(18)
76.28(15)
77.25(17)
76.53(15)
77.25(17)
c
b
c
b
c
Fe(1)ÃO(1B)
Fe(1)ÃN(1Q)
Fe(1)ÃN(1P)
Fe(1)ÃN(01)
b
c
b
c
b
c
b
c
b
c
b
c
b
c
a dinucleating dicarboxylate XDK [10] or a m-terphe-
ꢀ
b
c
nyl-derived
ArMesCO2
ligand.
In
[Fe(m-
b
c
XDK)(C5H5N)2] (A), the iron atom is coordinated by
three carboxylate oxygen and two pyridine nitrogen
atoms [34]. Four- or six-coordinate iron(II) centers can
be stabilized within the [Fe(O2CArMes)2L2] platform
[33]. Related mononuclear iron(II) complexes
[Fe(OAc)2(C5H5N)4] (B) and [Fe(OAc)3(C5H5N)]ꢀ (C)
can be prepared by chemical reduction of a basic iron
acetate cluster [Fe3O(OAc)6(C5H5N)3] in a mixture of
pyridine and AcOH [39,40]. Without an effective steric
shield or conformational rigidity, however, these com-
b
c
b
c
a
Numbers in parentheses are estimated standard deviations of the
3.
last significant figures. Atoms are labeled as indicated in Figs. 1ꢁ
/
b
c
Molecule 1.
Molecule 2.

D. Lee, S.J. Lippard / Inorganica Chimica Acta 341 (2002) 1ꢁ
/
11
7
Fig. 2. ORTEP diagrams of [Fe(O2CAr4-tBuPh)2(2,2?-bipy)] (5) (left) and [Fe(O2CArTol)2(TMEDA)] (6) (right) with thermal ellipsoids at 50%
probability.
change in the carboxylate denticity and correlates with
the electron density at the metal center. Coordination of
the less electron-donating ligand 1-MeBzIm (pKaꢂ
/5.7)
compared to 1-BnIm (pKaꢂ6.7) [41,42] enhances the
/
Lewis acidity at the metal center, inducing a carboxylate
shift from monodentate to bidentate chelating. A recent
structural analysis of iron(II) complexes [Fe(O2-
CArMes)2L2] (Lꢂ
notion.
/N-donor ligands) [33] supports this
4.1.2. Effects of conformationally restricted bidentate N-
donors
The five-coordinate complexes 5 and 6 are favored
when 2,2?-bipy and TMEDA, respectively, are em-
ployed. Although an identical N2O3 donor atom set
occurs in 5 and 6, the bidentate ligands in these
complexes have significantly different donor abilities,
as indicated by the pKa values of 4.3 for 2,2?-bipy [43]
and 9.0 for TMEDA [44]. Both the electronic and steric
properties of the ligands in 6 suggest that it, like 3, might
adopt a four-coordinate geometry due to the enhanced
donor ability of the amine groups as well as the steric
crowding induced by four methyl groups on TMEDA.
Compound 5, on the other hand, might favor six-
coordination, since 2,2?-bipy has a pKa value even lower
Fig. 3. ORTEP diagram of [Fe(O2CArTol)2(BPTA)] (7) with thermal
ellipsoids at 50% probability.
pounds exist in equilibrium with higher nuclearity
species in solution.
The shift in the coordination number from four-
coordinate in 3 to six-coordinate in 4 results from a

8
D. Lee, S.J. Lippard / Inorganica Chimica Acta 341 (2002) 1ꢁ11
/
Fig. 4. Zero-field Mossbauer spectra (experimental data (j), calculated
¨
fit(ꢁ
/
)) recorded at 4.2 K for a solid sample of [Fe2(m-O2CArTol)3(O2-
CArTol)(2,6-lutidine)] (2). The upper curves show two subsets for the
calculated spectra. See Table 3 for derived Mossbauer parameters.
¨
than that (5.7) of 1-MeBzIm. The solid state structures
of 5 and 6, however, indicate the failure of these simple
arguments to predict the correct geometry, since both
are five-coordinate. The bidentate ligands 2,2?-bipy and
TMEDA afford smaller NÃ
/
FeÃ
/
N bite angles (75.6ꢁ
/
82.78) than that (98.98) in 3, promoting coordination
numbers higher than four. They are conformationally
restrictive, however, and may not allow the structural
rearrangement required to alleviate steric crowding in
six-coordination.
Recent DFT calculations of [Zn(OAc)x(Im)yLz]nꢃ
models revealed an energetically very flat potential
surface of carboxylate coordination, which allows
barrierless interconversion between different carboxy-
late binding modes [45]. If similar isoenergetic processes
are operative for the [Fe(O2CArTol)2Ln] module, the
correlation between electronic and stereochemical prop-
erties becomes less than straightforward.
Fig. 5. Zero-field Mossbauer spectra (experimental data (j), calculated
¨
fit(ꢁ
)) recorded at 4.2 K for solid samples of [Fe(O2CArTol)2(1-BnIm)2]
/
(3) (A); [Fe(O2CArTol)2(1-MeBzIm)2] (4) (B); [Fe(O2CAr4-tBuPh)2(2,2?-
bipy)] (5); (C) [Fe(O2CArTol)2(BPTA)] (7). See Table 3 for derived
Mossbauer parameters.
¨
4.1.3. Flexible tridentate N-donor ligand
ters [26,46ꢁ48]. Unlike TACN- or [HBpz]3-derived
/
Binding of the three nitrogen atoms of BPTA
afforded the five-coordinate iron(II) complex 7, which
has two monodentate carboxylate ligands. Amine/imine
mixed-donor ligands derived from the N,N-bis(2-pyr-
idylmethyl)amine (BPA) module have been extensively
employed to stabilize biomimetic polymanganese clus-
tridentate ligands [10], conformationally flexible BPA
analogs allow both facial and meridional coordination
[47,49]. This feature prompted us to investigate the
structural properties of its iron(II) complexes. In order
to prevent the formation of unwanted bis(chelate)

D. Lee, S.J. Lippard / Inorganica Chimica Acta 341 (2002) 1ꢁ
/
11
9
Table 3
4.2 K zero-field Mossbauer parameters for [Fe (m-O CArTol)3(O2CArTol)(2,6-lutidine)] (2), [Fe(m-O2CArTol)2(1-BnIm)2] (3), [Fe(O2CArTol)2(1-
¨
2
2
MeBzIm)2] (4), [Fe(O2CAr4-tBuPh)2(2,2?-bipy)] (5), [Fe(O2CArTol)2(BPTA)] (7), and related diiron(II) complexes
Compound
d (mm sꢀ1
)
DEQ (mm sꢀ1
)
GL (mm sꢀ1
)
GR (mm sꢀ1
)
Coordination number Reference
2
1.05(2)
1.23(2)
1.08(2)
1.16(2)
1.13(2)
1.10(2)
1.26(2)
2.18(2)
2.80(2)
2.46(2)
2.82(2)
2.91(2)
3.52(2)
2.90(2)
3.02(2)
0.31
0.28
0.26
0.24
0.29
0.26
0.27
0.25
0.31
0.28
0.31
0.26
0.28
0.26
0.28
0.25
site 1
site 2
4
5
4
6
5
5
5
5
This work
3
This work
This work
This work
This work
[20]
4
5
7
[Fe2(m-O2CArTol)2(O2CArTol)2(THF)2]
a
a
[Fe2(m-O2CArTol)2(O2CArTol)2(C5H5N)2] 1.19(2)
[20]
a
Two iron atoms are equivalent.
compounds [Fe(BPA)2]X2 [50], a tert-butyl group was
installed on the amine nitrogen atom. As shown in Fig.
3, meridional coordination of BPTA provides a pair of
cis oriented coordination sites for carboxylate binding.
Monodentate coordination of the carboxylate ligands
[35]. Among biologically relevant oxidation and spin
states, only high-spin (Sꢂ2) iron(II) has a unique range
of isomer shift (d) values [36,37]. The coordination
environment and degree of metalÃligand covalency can
be deduced from the quadrupole splitting parameter,
DEQ, which reflects the electric field gradient (EFG)
generated by the charges surrounding the iron site.
/
/
alleviates steric crowding between the ArTolCO2
ꢀ
ligands and tert-butyl group on BPTA. Carboxylate
shifts [51] thus allow both N2O3 and N3O2 donor
combinations without a change in overall coordination
Mossbauer parameters obtained for well-defined iron
¨
carboxylate complexes thus can be used to aid the
assignment of spectra for structurally related biological
iron clusters. Even for structurally characterized iron
model compounds, however, an assignment of quad-
ruple doublets to individual subsites can be ambiguous
when multiple asymmetric iron(II) centers occur within
a given molecule [34,52,53]. We therefore investigated
number for 5ꢁ
/7.
4.2. Mossbauer spectroscopy
¨
Mossbauer spectroscopy is an excellent method for
¨
establishing the oxidation and spin states of iron atoms
Scheme 1.

10
D. Lee, S.J. Lippard / Inorganica Chimica Acta 341 (2002) 1ꢁ11
/
the Mossbauer parameters of the mononuclear iron(II)
¨
ligands stabilize four-, five-, and six-coordinate com-
plexes having rare coordination geometries. Structural
variations within the [Fe(O2CAr?)2Ln] module are
dictated by the steric and electronic properties of the
N-donor ligands and accommodated by flexible coordi-
complexes prepared here as subsite models that can be
used to deconvolute more complicated spectra obtained
for structurally related polyiron clusters.
Compound 2 is one such species, in which there are
two distinct iron sites, Fe(1) (four-coordinate) and Fe(2)
(five-coordinate), within the dinuclear unit. The asym-
metric coordination environment of the two iron atoms
in 2 results in two overlapping quadrupole doublets
nation of the carboxylate ligands. Mossbauer para-
¨
meters obtained for these compounds were used to
assist spectral assignment of a dinuclear complex.
Structurally related mononuclear complexes thus may
serve as useful subsite models for higher nuclearity
species in both synthetic and biological systems.
displayed in the 4.2 K zero-field Mossbauer spectra (Fig.
¨
4). Valence bond theory dictates that the lower-coordi-
nate Fe(1) site should have greater s-character relative to
Fe(2) and a negatively shifted d value. The quadrupole
doublet having the small isomer shift of 1.05 mm sꢀ1 is
therefore assigned to Fe(1) and the one with the larger
isomer shift of 1.23 mm sꢀ1 to Fe(2). This assignment
6. Supplementary material
Full tables of crystallographic parameters and struc-
ture refinement, positional and isotropic thermal para-
meters, bond distances, bond angles, and components of
the anisotropic thermal parameters have been deposited
requires that the smaller DEQ value (2.18 mm sꢀ1
)
belongs to Fe(1), which indeed displays a more sym-
metric tetrahedral coordination sphere. Consistent with
this analysis is the observation that, in the four-
coordinate complex 3, the d and DEQ values lie at the
/
with the Cambridge Data Centre, CCDC Nos. 192206ꢁ
192209 (for 3ꢁ5 and 7). Copies of this data may be
obtained free of charge from The Director, CCDC, 12
Union Road, Cambridge CB2 1EZ, UK (fax: ꢃ44-
/
/
low end of the range (dꢂ
/
1.08ꢁ
/
1.16 mm sꢀ1; DEQꢂ
2.46ꢁ
/
3.52 mm sꢀ1) of parameters for [Fe(O2CAr?)2Ln]
/
compounds (Table 3). The subspectrum of 2 displaying
the larger DEQ value (2.80 mm sꢀ1) is assigned to the
more asymmetric, highly distorted five-coordinate atom
1223-336-033; e-mail: deposit@ccdc.cam.ac.uk or www:
http://www.ccdc.cam.ac.uk).
Fe(2). Comparable quadrupole splittings (2.90ꢁ3.02 mm
/
sꢀ1) were obtained for 1 and [Fe2(m-O2CArTol)2-
(O2CArTol)2(C5H5N)2] [12], in which analogous five-
coordinate iron(II) centers are coordinated by O5 and
NO4 donor sets, respectively. Deviations from a per-
fectly symmetric distribution of donor atoms are
Acknowledgements
This work was supported by a grant from the
National Science Foundation. D.L. was the recipient
of a Corning Foundation Science Fellowship in chem-
istry. We thank Ms. J. Kuzelka for help in acquiring the
reflected in similarly large DEQ values of 2.82ꢁ
/
3.52
mm sꢀ1 exhibited by 4, 5, and 7.
Mossbauer spectra.
¨
As summarized in Table 3, a distribution of DEQ
values occurs for the [Fe(O2CAr?)2L2] series of com-
pounds. Asymmetry in coordination geometry as well as
the positioning of different donor atoms apparently
reinforce the intrinsic EFG of high-spin iron(II). A
References
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¨
and overall stereochemistry of the iron(II) complexes
cannot be made, however, in part owing to the limited
number of well-defined basis sets available. Analysis of
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