Unsupported μ-oxo- and μ-hydroxo-iron(III) dimers and mononuclear iron(III) complexes with pyridylbis(aminophenol) ligands

Article abstract of DOI:10.1002/ejic.200900794

Unsupported hydroxo- and oxo-bridged diiron(III) and mononuclear iron(III) complexes with pyridylbis(aminophenol) ligands (L^amine)^2- and (L^tBu-amine)^2- were synthesized and characterized [H₂L^amin

Full text of DOI:10.1002/ejic.200900794

FULL PAPER
DOI: 10.1002/ejic.200900794
Unsupported µ-Oxo- and µ-Hydroxo-Iron(III) Dimers and Mononuclear
Iron(III) Complexes with Pyridylbis(aminophenol) Ligands
Rajendra Shakya,[a] Douglas R. Powell,[a] and Robert P. Houser*[a]
Keywords: Iron / N,O ligands / Bridging ligands / Bioinorganic chemistry
Unsupported hydroxo- and oxo-bridged diiron(III) and mono- groups of (LtBu-amine 2–
nuclear iron(III) complexes with pyridylbis(aminophenol) li- Monomeric 4 and 5 are isostructural with pentadentate li-
gands (Lamine 2– tBu-amine 2– were synthesized and char- gands (Lamine 2– tBu-amine 2–, respectively, and a chloro li-
and (L or (L
acterized [H2Lamine = 2,2Ј-(2-methyl-2-(pyridin-2-yl)propane-
1,3-diyl)bis(azanediyl)bis(methylene)diphenol; H2LtBu-amine
6,6Ј-(2-methyl-2-(pyridin-2-yl)propane-1,3-diyl)bis(azane-
diyl)bis(methylene)bis(2,4-di-tert-butylphenol)]. Dimeric
)
prevent intramolecular H-bonding.
)
)
)
)
gand in the sixth position of the six-coordinate iron(III) moie-
ties. Conversion of 2 to its protonated hydroxo-bridged ana-
log [(FeLtBu-amine)2(µ-OH)]+ by treatment with mild acid was
monitored spectroscopically and electrochemically, but the
reaction was not reversible by treatment with base. Similarly,
hydroxo-bridged 1 could not be deprotonated to generate its
oxo-bridged analog, [(FeLamine)2(µ-O)], suggesting that the
pKa values of the hydroxo groups in 1 and [(FeLtBu-amine)2(µ-
OH)]+ are quite large. The CV of 2 revealed well-defined
ligand-based redox couples, but no metal-based redox cou-
ples. When 2 was treated with mild acid, metal-based redox
couples for [(FeLtBu-amine)2(µ-OH)]+ grew in.
=
[(FeLamine)2(µ-OH)]BPh4 (1) and [(FeLtBu-amine)2(µ-O)] (2), and
monomeric [FeLtBu-amine(OCH3)] (3) were synthesized from
ferric perchlorate. Monomeric [FeLamineCl] (4) and
[FeLtBu-amineCl] (5) were synthesized from ferric chloride.
Complex 1 is comprised of two [FeLamine +
(Lamine 2–
is pentadentate, bridged by a single hydroxo li-
gand. Intramolecular H-bonding between the NH groups on
]
units, where
)
one [FeLamine +
] unit and the phenolato O atoms of the other
[FeLamine]+ unit stabilizes the structure. Complex 2 is an oxo-
bridged dimer, but the steric constraints from the tert-butyl
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2009)
and one is a non-heme diiron(II) complex with a tetrapodal
polyamine ligand, pyN4.[4] Only one of these is a non-heme
diiron(III) complex, [(Fe(salten))2(µ-OH)]BPh4, where
Introduction
Oxo- and hydroxo-bridged binuclear iron complexes have
been studied extensively due to their relevance to important
metalloproteins and metalloenzymes including hemer-
ythrin, methane monooxygenase (MMO), ribonucleotide
reductase, and purple acid phosphatases, where the ubiqui-
tous diiron motif in these enzymes is essential to their func-
tion.[1] Although the diiron moieties in these metalloprote-
ins and metalloenzymes are often bridged by other bridging
units, such as carboxylate groups from aspartate or gluta-
mate amino acid side chains, unsupported hydroxo- or oxo-
bridged species have been reported as possible intermedi-
ates in the hydroxylation mechanism of MMO based on
related cytochrome P450 chemistry.[2] While there are a
large number of structurally characterized unsupported
oxo-bridged diiron complexes,[1a] to the best of our knowl-
edge, only six unsupported hydroxo-bridged diiron com-
plexes are known. Four of these are porphyrin species,[3]
H2salten
=
4-azaheptane-1,7-bis(salicylideneiminate).[5]
Here we report a new example of an unsupported hydroxo-
bridged diiron(III) complex, where the ligand plays an im-
portant role in stabilizing this unusual structure, along with
related oxo-bridged diiron(III) and mononuclear iron(III)
complexes.
In our laboratory we have generated a series of pyr-
idylamide ligands and studied the coordination chemistry
of transition-metal complexes of various oxidation states
and nuclearities.[6] We recently extended the pyridylamide
family of ligands to a new family of ligands, including the
pyridylbis(acetamide) ligand H2pp(ac)2 (Scheme 1), that
was found to support several novel mixed-valence copper
complexes.[7] Most recently, we developed and synthesized
, ,
three new ligands H2Limine H2Lamine and H4Lamide
(Scheme 1) that share a common backbone with H2pp(ac)2
but contain phenol groups in place of the methyl in
H2pp(ac)2, and were found to support copper(II) complexes
of varying nuclearity.[8] Here we present two new iron(III)
complexes with the previously reported H2Lamine ligand,
and three new iron(III) complexes with a new ligand,
[a] Department of Chemistry and Biochemistry,
University of Oklahoma,
Norman, Oklahoma, 73019, USA
Fax: +1-405-325-6111
E-mail: houser@ou.edu
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.200900794.
H2LtBu-amine
.
Eur. J. Inorg. Chem. 2009, 5319–5327
© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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R. Shakya, D. R. Powell, R. P. Houser
FULL PAPER
Using H2Lamine and H2LtBu-amine we synthesized five new
iron(III) complexes, including hydroxo- and oxo-bridged bi-
nuclear iron(III) complexes, and mononuclear iron(III)
complexes. Ligand H2Lamine, upon reacting with Fe(ClO4)3·
6H2O in methanol in the presence of Et3N to deprotonate
the phenol group, produced a dark red solution from which
a red powder was isolated by solvent evaporation. After
counterion metathesis by treatment with NaBPh4, X-ray
quality crystals of the complex were isolated by solvent
evaporation. The complex was formulated as the hydroxo-
bridged species [(FeLamine)2(µ-OH)]BPh4 (1) by X-ray crys-
tallography and elemental analysis. The ESI-MS peak at
m/z = 879 corresponds to the hydroxo-bridged diiron(III)
cation, and a broad band at 3420 cm–1 in the FTIR con-
firms the hydroxo group as the bridging unit.
Combining the tert-butyl-substituted ligand H2LtBu-amine
with Fe(ClO4)3·6H2O and excess Et3N in methanol at
–20 °C for ca. 24 h yielded a purple solution, which upon
removal of solvent produced a dark purple powder.
Crystallization by slow evaporation from a solution of 1:1
acetonitrile/hexanes yielded red crystals of [(FeLtBu-amine)2-
(µ-O)] (2) in poor yield. The absence of a strong band for
the perchlorate anion at ca. 1100 cm–1 in the FTIR spec-
trum of 2 combined with the ESI-MS peak envelope at m/z
= 1328 of a methanol solution of 2 indicated a diiron(III)
species with a bridging oxo group. This synthetic procedure
produced 2 as a minor product (30% yield), while the major
product obtained was a dark blue powder which, upon
characterization by FTIR spectroscopy, ESI-MS, and ele-
mental analysis, was characterized as the mononuclear
iron(III) methoxide species [FeLtBu-amine(OCH3)] (3).
The non-coordinating perchlorate anion and solvent
choice are key to the syntheses of 1 and 2, since using ferric
chloride in dichloromethane instead of ferric perchlorate in
methanol leads to simple iron(III) chloride monomers. The
reaction of either H2Lamine or H2LtBu-amine with anhydrous
FeCl3 in dichloromethane using Et3N to deprotonate the
ligands yielded mononuclear complexes [FeLamineCl] (4)
and [FeLtBu-amineCl]·H2O (5), respectively. Interestingly,
FeCl3 with H2LtBu-amine and Et3N in methanol produced 2.
Scheme 1. Pyridylbis(amine), pyridylbis(imine), and pyridylbis-
(amide) ligands used in this study and related previous work.
Results and Discussion
Syntheses
The differences between H2Lamine and H2LtBu-amine cen-
ter on the substituents on the phenol rings (Scheme 1 and
2). H2Lamine contains no substituents on the phenol rings,
while H2LtBu-amine contains tert-butyl substituents in the 3-
and 5-positions. Both H2Lamine and H2LtBu-amine contain
the same potential N3O2 donor atom set, with pyridyl and
amine N-donors, and when deprotonated, phenolate O-do-
nors. While the unsubstituted ligand H2Lamine was synthe-
sized as reported previously from our laboratory,[8] the new
ligand H2LtBu-amine was synthesized following a similar pro-
cedure by Schiff base condensation of 2-methyl-2-pyridine-
2-ylpropane-1,3-diamine (ppda, Scheme 1)[9] with 3,5-di-
tert-butylsalicylaldehyde, followed by reduction with
NaBH4.
X-ray Structures
Unlike the coordination behavior of (Lamine 2–
lated ligand (Limine 2–
with copper(II) where the pyridyl
)
and re-
)
group does not coordinate,[8] the pyridyl group coordinates
to the iron centers in complexes 1–5, resulting six-coordi-
nate complexes after the bridging O(H) group or the coor-
dinating anion (Cl– or CH3O–) coordination is accounted
for. The X-ray structure of 1 reveals that it consists of two
FeIII ions linked together by a single bridging hydroxo li-
gand and intramolecular H-bonding (Figure 1). Each FeIII
ion is coordinated in a distorted octahedral geometry, with
the N3O2 ligand donor atom set from the deprotonated
pentadentate ligand (Lamine)2–, and a bridging hydroxo li-
gand rounding out the coordination environment. The over-
Scheme 2. Structures of ligands H2Lamine and H2LtBu-amine and the
syntheses of 1–5. (i) Fe(ClO4)3·6H2O, Et3N, CH3OH. (ii) NaBPh4,
CH3OH. (iii) FeCl3, Et3N, CH2Cl2.
–
all +1 charge of the complex is balanced by one BPh4
counteranion.
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Eur. J. Inorg. Chem. 2009, 5319–5327

Iron(III) Complexes with Pyridylbis(aminophenol) Ligands
Table 1. Selected bond lengths [Å] and angles [°] for 1.
Complex 1
Fe1A–O1
Fe1A–O1A
Fe1A–O28A
Fe1A–N9A
Fe1A–N13A
Fe1A–N20A
Fe1A···Fe1B
O1–Fe1A–O1A
O1–Fe1A–O28A
O1–Fe1A–N9A
O1–Fe1A–N20A
O1A–Fe1A–O28A
O1A–Fe1A–N9A
O1A–Fe1A–N13A
O28A–Fe1A–N13A
O28A–Fe1A–N20A
N9A–Fe1A–N13A
N9A–Fe1A–N20A
N20A–Fe1A–N13A
O1–Fe1A–N13A
O1A–Fe1A–N20A
O28A–Fe1A–N9A
Fe1B–O1–Fe1A
2.0174(16)
1.9171(16)
1.9113(16)
2.160(2)
2.2119(19)
2.180(2)
3.7616(7)
105.60(7)
93.61(7)
88.57(7)
87.11(7)
90.40(7)
90.62(7)
88.46(7)
92.82(7)
91.80(7)
84.66(7)
86.64(8)
78.57(7)
164.47(7)
166.93(7)
177.25(7)
138.64(9)
Fe1B–O1
2.0033(17)
1.9327(16)
1.9108(17)
2.151(2)
2.177(2)
2.169(2)
Fe1B–O1B
Fe1B–O28B
Fe1B–N9B
Fe1B–N13B
Fe1B–N20B
O1–Fe1B–O1B
O1–Fe1B–O28B
O1–Fe1B–N9B
O1–Fe1B–N13B
O1B–Fe1B–O28B
O1B–Fe1B–N9B
O1B–Fe1B–N20B
O28B–Fe1B–N13B 90.69(7)
O28B–Fe1B–N20B 90.58(7)
N9B–Fe1B–N13B
N9B–Fe1B–N20B
92.78(7)
94.78(7)
91.41(7)
94.30(7)
97.98(7)
91.99(7)
85.08(7)
Figure 1. Representation of the X-ray structure of 1 with all H
atoms except the N9A and N20A amine and µ-hydroxo H atoms
removed for clarity. Heteroatoms are shown with 50% probability
thermal ellipsoids. H-bonding interactions are represented by
dashed lines.
78.55(8)
83.56(8)
N20B–Fe1B–N13B 87.01(7)
The coordination environments around Fe1A and Fe1B
in 1, while similar, are not the same. One iron center (Fe1A)
has the pyridyl N atom (N13A) oriented trans to the bridg-
ing hydroxo ligand (O1), while the other iron atom (Fe1B)
has one of the amine N atoms (N20B) trans to the bridging
hydroxo ligand and the pyridyl N atom (N13B) is cis. The
O1–Fe1B–N20B
O1B–Fe1B–N13B
O28B–Fe1B–N9B
174.46(7)
168.31(7)
167.97(7)
Fe–N bond length of the pyridyl N atom trans to the hy- [FeLamine +
] units together, a property not present in either
droxo group is slightly longer than the distance of the cis of the reported unsupported hydroxo-bridged non-heme di-
pyidyl N atom, 2.2119(19) Å vs. 2.151(2) Å, respectively. iron complexes discussed above. In fact, the Fe–O(H)–Fe
The bond lengths and angles involving the iron atoms bond angle in 1 is closer to the Fe–O(H)–Fe bond angle of
(Table 1) are otherwise typical of six-coordinate FeIII spe- 123.1(2)° in the bis(µ-acetato)diiron(III) species [(FeTp)2(µ-
cies with similar ligands. While the coordination environ- OH)(µ-AcO)2], where Tp– is tris(pyrazolyl)hydroborate and
ment around the two iron centers in 1 closely resembles AcO– is acetate.[10]
the only reported unsupported hydroxo-bridged non-heme
The structure of 2 is similar to 1 in that it consists of two
diiron(III) complex, [(Fe(salten))2(µ-OH)]BPh4,[5] the key [FeLtBu-amine]+ units bridged, in this case, by an oxo ligand.
difference between these two compounds lies in their coor- Each FeIII is coordinated in a distorted octahedral geome-
dination modes. In the salten complex, the two [Fe- try comprised of a N3O2 ligand atom donor set from the
(salten)]+ units of the dimetallic cations are crystallographi- deprotonated (LtBu-amine 2–
ligands, and a single bridging
)
cally equivalent and no H-bonding interactions exist be- oxo ligand in place of the hydroxo ligand in 1 (Figure 2).
tween the two units, whereas in 1 the distance of one of The Fe–N(py) bond lengths in 2 are slightly longer than the
the iron atoms from the bridging hydroxo oxygen is slightly corresponding distances in 1, possibly due to the stronger
longer than the other [Fe1A–O1, 2.0174(16) Å vs. Fe1B– donating ability of the bridging oxo group compared to the
O1, 2.0033(17) Å], and important H-bonding between each hydroxo group (Table 2). The average Fe–O(phenolate)
[FeLamine]+ unit defines the structure.
bond length of 1.962 Å in 2 is also slightly larger than the
The differences in coordination modes on each iron in 1 corresponding average bond length for 1, which is 1.912 Å.
are a consequence of H-bonding interactions between the Other bond lengths and angles involving the iron atoms in 2
amine NH groups of one [FeLamine]+ unit (N9A and N20A) are generally typical of FeIII complexes with similar ligands.
with the phenolate O atoms of the other [FeLamine +
]
unit
(O1B and O28B). The amine NH groups from the other no H-bonding interactions between the (LtBu-amine 2–
ligand (N9B and N20B) are not involved in intra- or inter- ligands in 2. The pyridyl groups in both of the
molecular H-bonding. The Fe–O(H)–Fe bond angle in 1 is [FeLtBu-amine +
units of 2 are oriented trans to the bridging
An important difference between 1 and 2 is that there is
)
]
138.64(9)°, which is significantly smaller than the corre- oxo group, which orients the phenolato O atoms such that
sponding angle of 159.45(14)° in [(Fe(salten))2(µ-OH)]+. they are not available for H-bonding. Stated another way,
The Fe···Fe distance of 3.7616(7) Å in 1 is also shorter than in order for the phenolato O atoms to be able to form H-
the Fe···Fe distance of 3.928 Å in [(Fe(salten))2(µ-OH)]+.[5] bonds with the amine NH groups from the adjacent ligand,
The Fe–O(H)–Fe bond angle in 1 is also smaller than the the ligand must be coordinated with an amine group trans
Fe–O(H)–Fe bond angle of 146.7(2)° in the unsupported to the bridging ligand, a condition that is not met in 2.
hydroxo-bridged non-heme diiron(II) complex with tetra-
podal polyamine ligand pyN4.[4] These differences are likely [FeLtBu-amine +
a consequence of the H-bonding interactions pulling the 169.4(6)° is much larger than the corresponding Fe–O(H)–
As a result of the absence of any H-bonding between
]
units in 2, the Fe–O–Fe bond angle of
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R. Shakya, D. R. Powell, R. P. Houser
FULL PAPER
tances in [(Fe(cbpN))2(µ-O)].[11] The Fe···Fe distance of
3.621(4) Å in 2 is shorter than the corresponding distance
in 1, due to the shorter Fe–O(oxo) distances in 2.
The structures of 4 and 5 are similar, consisting of two
monomers connected by H-bonding. Each monomer in 4
and 5 is comprised of an FeIII ion in a distorted octahedral
geometry from the N3O2 ligand donor atom set, and a Cl–
ligand. The crystal structures and selected bond lengths and
angles for complexes 4 and 5 are shown in Figures 3 and 4,
and Tables 3 and 4, respectively. The bond lengths angles
around the iron atoms in both 4 and 5 are typical of six-
coordinate, high spin FeIII complexes.[12] The X-ray struc-
ture of 4 consists of two symmetrically unrelated units con-
nected by H-bonding between the amine NH groups from
one monomer and the phenolate O atoms from the other
(Figure 3). The H-bonding in 4 is reminiscent of the H-
bonding observed in 1, not surprising in light of the fact
that they both contain ligand (Lamine)2–.
Figure 2. Representation of the X-ray structure of 2 with all H
atoms removed for clarity. Heteroatoms are shown with 50% prob-
ability thermal ellipsoids.
Table 2. Selected bond lengths [Å] and angles [°] for 2.
Complex 2
Fe1–O1
Fe1–O1A
Fe1–O36A
Fe1–N17A
Fe1–N22A
Fe1–N28A
Fe1···Fe2
1.844(10)
1.963(9)
Fe2–O1
1.793(10)
1.971(9)
1.954(10)
2.163(12)
2.261(12)
2.251(12)
Fe2–O1B
Fe2–O36B
Fe2–N17B
Fe2–N22B
Fe2–N28B
1.962(10)
2.155(12)
2.334(12)
2.243(12)
3.621(4)
O1–Fe1–O1A
O1–Fe1–O36A
O1–Fe1–N17A
O1–Fe1–N28A
O1A–Fe1–N17A
O1A–Fe1–N22A
O36A–Fe1–O1A
O36A–Fe1–N22A
O36A–Fe1–N28A
N17A–Fe1–N22A
N17A–Fe1–N28A
N28A–Fe1–N22A
O1–Fe1–N22A
O1A–Fe1–N28A
O36A–Fe1–N17A
Fe1–O1–Fe2
103.4(4)
O1–Fe2–O1B
O1–Fe2–O36B
104.5(4)
103.3(4)
88.4(5)
85.5(4)
90.2(4)
92.4(4)
96.9(4)
85.1(4)
90.0(4)
80.6(4)
80.4(4)
76.2(4)
159.9(4)
166.2(4)
164.3(4)
102.5(4)
88.3(4)
85.6(4)
89.4(4)
92.2(4)
98.7(4)
82.8(4)
90.2(4)
83.8(4)
79.4(4)
77.6(4)
162.4(4)
165.5(4)
164.5(4)
169.4(6)
O1–Fe2–N17B
O1–Fe2–N28B
O1B–Fe2–N17B
O1B–Fe2–N22B
O36B–Fe2–O1B
O36B–Fe2–N22B
O36B–Fe2–N28B
N17B–Fe2–N22B
N17B–Fe2–N28B
N28B–Fe2–N22B
O1–Fe2–N22B
O1B–Fe2–N28B
O36B–Fe2–N17B
Figure 3. Representation of the X-ray structure of 4 with all H
atoms except amine H atoms removed for clarity. Heteroatoms are
shown with 50% probability thermal ellipsoids. H-bonding interac-
tions are represented by dashed lines.
Fe angle of 138.64(9)° in 1. However, this bond angle is
smaller than the linear bond angle of 180°, which is seen in
many unsupported oxo-bridged diiron(III) complexes,[1a]
but significantly larger than the Fe–O–Fe bond angle of
144.5(2)° observed in [(Fe(cbpN))2(µ-O)] where cbpN is a
hydroxy-benzophenone-substituted triazacyclononane ligand
that shares a N3O2 donor set similar to (LtBu-amine)2–.[11] The
near-linear Fe–O–Fe bond angle in 2 is likely due to the steric
constraints imposed by the bulky tert-butyl groups on (LtBu-
amine 2–
)
combined with the absence of any H-bonding that
could pull the [FeLtBu-amine]+ units together. The Fe–O(oxo)
distances in 2 of 1.844(10) Å and 1.793(10) Å are shorter
than the Fe–O(H) distances in 1, but longer than the Fe–
O(oxo) distances found in unsupported µ-oxo-bridged di-
iron complexes reported in the literature [average Fe–
O(oxo) 1.774 Å].[1a] However, one of the Fe–O(oxo) dis-
Figure 4. Representation of the X-ray structure of 5 with all H
atoms except amine protons removed for clarity. Heteroatoms are
shown with 50% probability thermal ellipsoids. H-bonding interac-
tances in 2 is even longer than the 1.807 Å Fe–O(oxo) dis- tions are represented by dashed lines.
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Iron(III) Complexes with Pyridylbis(aminophenol) Ligands
Table 3. Selected bond lengths [Å] and angles [°] for 4.
Complex 4
ure S1, Supporting Information). All the bands of binuclear
complexes 1 and 2 are more intense than those of the mo-
nonuclear complexes 3–5. The absorption bands observed
in the near-UV region (below 300 nm) are assigned as π–π*
transitions involving the aromatic units.[12] There is also an
intense high energy band in the 300–400 nm region assigned
to pπ–dσ* charge-transfer transitions.[14] However, these
charge-transfer bands in binuclear complexes 1 and 2 ap-
pear only as shoulders. Based on an explanation proposed
by Reem et al.,[15] the absorption bands in the oxo dimer
region (300–400 nm) for the binuclear complexes are related
to the bridging angles. For the bent Fe–O–Fe geometries in
1 and 2, the peaks below 300 nm are assigned as the highest
energy π-derived transitions, and the other lower energy
transitions are assigned to the oxo-pz–Fe-dxz CT transi-
tions. Complexes 1–5 also possess spectral features in the
lower energy region (400–600 nm). For 1 and 2, the low
energy band is assigned as a hydroxo/oxo-to-iron(III)
LMCT band, and the high intensity of this band overlaps
with a band due to a phenolate-to-iron(III) LMCT. For the
mononuclear complexes 3–5, this low energy band is solely
attributed to a pπ–dπ* phenolate-to-iron(III) LMCT band.
In 1 and 2, the bridging oxo and hydroxo groups signifi-
cantly influence the position of the low-energy LMCT tran-
sitions.[16] While the position of this LMCT band in 1 is at
520 nm (ε = 6,600 –1 cm–1), in 2 it is blue-shifted to 446 nm
(ε = 6,700 –1 cm–1), which may be partly due to the
stronger π-donating ability of the oxo bridge in 2 compared
to the hydroxo bridge in 1.[17]
Fe1–O1A
Fe1–O28A
Fe1–N9A
Fe1–N14A
Fe1–N20A
Fe1–Cl1
1.925(2)
1.926(2)
2.176(2)
2.175(2)
2.187(3)
2.3598(9)
97.40(9)
87.67(9)
92.31(9)
95.79(7)
86.90(9)
90.38(9)
100.14(6)
84.49(9)
78.96(9)
87.78(7)
85.69(10)
89.75(7)
169.49(9)
166.75(9)
171.65(7)
Fe2–O1B
Fe2–O28B
Fe2–N9B
Fe2–N14B
Fe2–N20B
Fe2–Cl2
1.943(2)
1.922(2)
2.202(3)
2.203(3)
2.171(3)
2.3386(9)
O1A–Fe1–O28A
O1A–Fe1–N14A
O1A–Fe1–N9A
O1A–Fe1–Cl1
O28A–Fe1–N14A
O28A–Fe1–N20A
O28A–Fe1–Cl1
N9A–Fe1–N14A
N9A–Fe1–N20A
N9A–Fe1–Cl1
N14A–Fe1–N20A
N20A–Fe1–Cl1
O1A–Fe1–N20A
O28A–Fe1–N9A
N14A–Fe1–Cl1
O1B–Fe2–O28B
O1B–Fe2–N14B
O1B–Fe2–N9B
O1B–Fe2–Cl2
O28B–Fe2–N14B
O28B–Fe2–N20B
O28B–Fe2–Cl2
N9B–Fe2–N14B
N9B–Fe2–N20B
N9B–Fe2–Cl2
N20B–Fe2–N14B
N20B–Fe2–Cl2
O1B–Fe2–N20B
O28B–Fe2–N9B
N14B–Fe2–Cl2
100.48(9)
85.31(9)
89.64(9)
98.78(7)
86.90(10)
91.43(10)
97.92(7)
87.56(10)
77.40(10)
86.73(7)
83.63(10)
91.09(7)
163.26(9)
168.02(9)
172.95(7)
Table 4. Selected bond lengths [Å] and angles [°] for 5.
Complex 5
Fe1–O1A
Fe1–O36A
Fe1–N17A
Fe1–N22A
Fe1–N28A
Fe1–Cl1
O1A–Fe1–O36A
O1A–Fe1–N17A
O1A–Fe1–N22A
O1A–Fe1–Cl1
O36A–Fe1–N22A
O36A–Fe1–N28A
O36A–Fe1–Cl1
N17A–Fe1–N22A
N28A–Fe1–N17A
N17A–Fe1–Cl1
N28A–Fe1–N22A
N28A–Fe1–Cl1
O1A–Fe1–N28A
1.8982(15) Fe2–O1B
1.9081(15) Fe2–O36B
1.8986(16)
1.8936(16)
2.171(2)
2.171(2)
2.240(2)
2.3568(9)
99.67(7)
89.84(7)
93.65(7)
97.42(5)
90.41(7)
89.89(7)
93.98(5)
87.47(8)
80.49(7)
86.22(6)
78.85(8)
89.16(6)
167.95(7)
170.38(7)
167.23(5)
2.231(2)
2.189(2)
2.172(2)
2.3766(8)
Fe2–N17B
Fe2–N22B
Fe2–N28B
Fe2–Cl2
100.42(6)
O1B–Fe2–O36B
O1B–Fe2–N17B
O1B–Fe2–N22B
O1B–Fe2–Cl2
O36B–Fe2–N22B
O36B–Fe2–N28B
O36B–Fe2–Cl2
N17B–Fe2–N22B
N17B–Fe2–N28B
N17B–Fe2–Cl2
N22B–Fe2–N28B
N28B–Fe2–Cl2
O1B–Fe2–N28B
O36B–Fe2–N17B
N22B–Fe2–Cl2
90.37(7)
91.49(7)
93.16(5)
91.47(7)
90.30(7)
98.18(5)
78.80(7)
79.00(7)
90.48(6)
87.87(7)
85.58(6)
169.28(7)
The transformation of an oxo- to a hydroxo-bridged sys-
tem is considered obligatory in heme-copper oxidases,[18]
and such transformations have also been considered as a
part of the oxygen binding process in hemerythrin.[19] The
protonation of the oxo group upon treatment with HClO4
has been observed in various heme[3a,3c] and non-heme[10]
oxo-bridged diiron model compounds. Structurally, the pro-
tonation of the oxo group rehybridizes the bridging oxygen
atom and the Fe–O–Fe unit becomes more bent along with
significantly longer Fe–O bond lengths.
O36A–Fe1–N17A 165.71(7)
N22A–Fe1–Cl1 168.35(5)
To explore the possible transformation of
2 to
[(FeLtBu-amine)2(µ-OH)]+ by protonating the oxo bridging
group, we examined the UV/Vis spectra of 2 after titration
with acid. Dichloromethane solutions of 2 were titrated
with 8ϫ10–5  HClO4, also in dichloromethane, and fol-
lowed by UV/Vis spectroscopy (Figure 5). After one equiva-
lent of acid had been added, the low-energy LMCT transi-
tion is red-shifted from 446 nm to 520 nm, suggesting the
protonation of the oxo group into a hydroxo group to form
[(FeLtBu-amine)2(µ-OH)]+. Back-titrating with base caused
the transition at 520 nm to shift toward higher energy, but it
was not possible to regenerate the spectrum of 2. Hydroxo-
bridged dimer 1 was treated with base to attempt to gener-
ate the oxo-bridged species [(FeLamine)2(µ-O)]. The transi-
tion associated with the hydroxo group at 516 nm was blue-
shifted upon addition of base (Et3N or proton sponge), but
like the back titration of [(FeLtBu-amine)2(µ-OH)]+, the
The structure of 5 consists of two six-coordinate mono-
mers connected by weak H-bonding between the Cl– ligand
and an NH group of one monomer with the corresponding
groups from a second monomer (Figure 4). The N···Cl dis-
tances, which range from 3.214(2) Å to 3.368(2) Å, are typi-
cal of N–H···Cl hydrogen bonds.[13] The phenolate oxygen
of one monomer is pointed away from the amine nitrogen
of the other, probably due to the combined effects of steric
hindrance of the tert-butyl groups and the symmetry of the
pseudo inversion-related units. However, the coordination
environment of the iron center in 5 is essentially the same
as that seen in 4.
Spectroscopic Properties
Complexes 1–5 all exhibit strong bands in the near UV- amount of the blue-shift was less than expected. It is likely
region and less intense bands in the visible region (see Fig- that the pKa of the hydroxo groups in 1 and [(FeLtBu-amine)2-
Eur. J. Inorg. Chem. 2009, 5319–5327
© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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5323

R. Shakya, D. R. Powell, R. P. Houser
FULL PAPER
(µ-OH)]+ are fairly high, thus requiring a much stronger
base to deprotonate all of the hydroxo species.
Figure 6. Cyclic voltammograms of complexes 1 (dashed line) and
2 (solid line) vs. Ag/AgCl, sample concentration ca. 1.0 m in
CH2Cl2, scan rate 150 mVs–1; 0.1  TBAH supporting electrolyte.
Figure 5. Titration of 0.10 m CH2Cl2 solution of 2 with a 0.08 m
CH2Cl2 solution of HClO4 added via syringe in the volumes indi-
cated.
scan rates. This suggests a very stable FeIII oxidation state
in 2. However, in the positive potential range, 2 exhibits two
quasi-reversible one-electron redox couples at 550 mV (∆E
= 130 mV) and 825 mV (∆E = 156 mV), and an irreversible
feature, possibly involving a two-electron oxidation, cen-
tered at 1086 mV (∆E = 270 mV) vs. Ag/AgCl. These fea-
tures are attributed to the oxidation of the phenolate groups
leading to formation of phenoxyl radical species based on
similarities to phenoxyl radicals and transition-metal com-
plexes of phenoxyl radicals reported in the literature.[21]
However, metal-centered redox features do appear in the
CVs of related LtBu-amine monomers 3 and 5 (Figures S4
and S5). 3 and 5 both exhibit one quasi-reversible
feature corresponding to the FeIII/FeII redox couple at
–830 mV and –670 mV, respectively, and two quasi-revers-
ible couples in the +800 to +1200 mV range corresponding
to ligand oxidation. These observations suggest that fac-
tor(s) that lead to the absence of metal-centered redox fea-
tures in the dimeric oxo-bridged 2 are not present in mono-
meric species 3 and 5, despite containing the same ligand.
In order to further explore the protonation of the bridg-
The conversion of mononuclear complex 4 to the corre-
sponding hydroxo-bridged diiron(III) species 1 was also
probed by titration experiments followed by UV/Vis spec-
troscopy. Dichloromethane solutions of 4 were titrated with
Et3N, and dichloromethane solutions of 1 were titrated with
acid (HCl). The titration experiment carried out on 1 and
4 revealed that under the appropriate pH conditions, mono-
nuclear complex 4 can be reversibly converted into the cor-
responding binuclear 1. When 4 was titrated with Et3N, the
position of the low energy LMCT band was shifted from
520 nm to 486 nm, which is the same energy as that of the
hydroxide-bridged diiron complex 1, suggesting the conver-
sion of 4 to 1. Likewise, when 1 was titrated with HCl, the
position of the low energy LMCT band was red-shifted to
520 nm, indicating the conversion of 1 to 4 (Figure S2).
Electrochemistry
The redox behavior of complexes 1–5 were studied by ing oxo group, dichloromethane solutions of 2 were treated
cyclic voltammetry in dichloromethane. The cyclic voltam- with varying amounts of 8ϫ10–5  HClO4, and their CVs
mogram (CV) of 1 (Figure 6, dashed trace) exhibits two were recorded. The CVs recorded after treatment of 2 with
quasi-reversible redox waves centered at E1/2 = –630 mV 25 and 50 µL of the HClO4 solution revealed the appear-
(∆E = 120 mV) and –1190 mV (∆E = 165 mV) vs. Ag/AgCl. ance of two irreversible redox waves centered at –640 mV
These peaks are believed to correspond to the FeIIIFeIII
/
and –1300 mV (Figure 7), which are similar to the redox
FeIIIFeII and FeIIIFeII/FeIIFeII couples, respectively. In the waves observed for the metal-centered processes in hydroxo-
positive potential range, there is an irreversible electrochem- bridged binuclear complex 1. The ligand-centered redox
ical wave at +830 mV vs. Ag/AgCl assigned to ligand oxi- processes observed in 2 were unaffected by the addition of
dation. The CV of related Lamine monomer 4 (Figure S3) HClO4, suggesting the bridging oxo group in 2 is being pro-
has a single quasi-reversible feature centered at –450 mV tonated to a hydroxo-bridged species. The higher redox po-
corresponding to the FeIII/FeII redox couple, and an tentials of these newly appeared redox features compared
irreversible wave at +1200 mV corresponding to ligand oxi- to the redox potentials of the metal-centered processes in 1
dation.
are likely due to the tert-butyl substituents in 2. Upon in-
Interestingly, in contrast to similar oxo-bridged com- creasing the amount of HClO4, the intensity of the metal-
plexes with the TPA series of ligands [TPA = tris(2-pyr- centered redox processes increases, but some of the ligand-
idylmethyl)amine],[20] no metal-centered redox waves were centered processes disappear, probably due to decomposi-
observed for 2 from 0 to –1.8 V (Figure 6, solid trace) at all tion of the binuclear species upon protonation of the phe-
5324
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© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2009, 5319–5327

Iron(III) Complexes with Pyridylbis(aminophenol) Ligands
nolate O atoms. These electrochemical observations corro- treated with mild acid were measured, metal-based redox
borate the protonation of the bridging oxo group observed couples grew in close to the potentials of the metal-based
in the UV/Vis experiment.
FeIIIFeIII/FeIIIFeII and FeIIIFeII/FeIIFeII couples observed
for 1. These observations suggest that the metal-based re-
dox couples in 2 are outside of the potential range mea-
sured, making the iron(III) oxidation state in 2 extremely
stable.
Experimental Section
General Procedures: Unless otherwise stated, all reagents were used
as received from commercial sources. The starting material for both
of the ligands, 2-methyl-2-pyridin-2-ylpropane-1,3-diamine (ppda),
was synthesized according to the literature procedure.[9] 2,2Ј-(2-
Methyl-2-(pyridin-2-yl)propane-1,3-diyl)bis(azanediyl)bis(methyl-
ene)diphenol (H2Lamine) was synthesized according to our pub-
lished procedure.[8] Solvents were doubly purified using alumina
columns in a MBraun solvent purification system (MB-SPS). Infra-
red spectra were measured from 4000 to 400 cm–1 as KBr pellets
on a NEXUS 470 FTIR spectrometer. 1H NMR spectra were mea-
sured using a Varian 300 MHz instrument. ESI-MS (positive) spec-
tra were measured in Q-TOF quadrupole time-of-flight mass spec-
trometer (Micromass, Manchester, U. K.) containing a Z-spray
electrospray ionization source (ESI). Elemental analyses were per-
formed by Atlantic Microlabs, Norcross, GA. UV/Vis spectra were
measured using a Shimadzu UV2401PC spectrophotometer in the
range of 250 to 1000 nm. Cyclic voltammetry experiments were per-
formed using a BAS 50W potentiometer. A standard three-elec-
trode cell was employed with a glassy-carbon working electrode, a
Pt-wire auxiliary electrode, and a Ag/AgCl reference electrode un-
der an inert atmosphere at room temperature. X-Band EPR spectra
were recorded for frozen CH2Cl2 solutions of the complexes using
a Bruker EMX spectrometer at 77 K. Solid state magnetic suscep-
tibility was measured using a Johnson Matthey Magnetic Suscep-
tibility Balance (MSB – AUTO) with a magnetic field strength of
Figure 7. Cyclic voltammograms of complex 2 vs. Ag/AgCl with
varying amounts of 0.08 m HClO4 in methanol added, sample
concentration ca. 1 m in CH2Cl2, scan rate 150 mVs–1; 0.1 
TBAH supporting electrolyte.
Conclusions
In summary, iron(III) complexes of pyridylbis(amino-
phenol) ligands (Lamine 2– and (LtBu-amine 2–
) were synthe-
)
sized and characterized. Iron dimers [(FeLamine)2(µ-OH)]-
BPh4 (1) and [(FeLtBu-amine)2(µ-O)] (2), and monomer
[FeLtBu-amine(OCH3)] (3) were synthesized from ferric per-
chlorate, while monomers [FeLamineCl] (4) and [FeLtBu-amine
-
Cl] (5) were synthesized from ferric chloride. Iron(III) di-
mers 1 and 2 are examples of unsupported O(H)-bridged
diiron species, where no other bridging atoms support the
structures. While this is not unusual for oxo-bridged di-
iron(III) complexes,[1a] 1 is a rare example of unsupported
hydroxo-bridged diiron species,[3–5] being only the second
example of a non-heme diiron(III) complex with an unsup-
ported µ-OH ligand. The unique intramolecular H-bonding
in 1 between the amine NH groups of one [FeLamine]+ unit
4.5
kGauss
and
measurement
rangeϮ1.999ϫ10–4
toϮ5ϫ10–10 cgs at 22 °C. A narrow bore sample tube with
0.400 cm outer diameter and 0.200 cm inner diameter was packed
with sample to a minimum height of 15 mm. Solution magnetic
susceptibilities of compounds 1, 4, and 5 in CD3CN and 2 in
CDCl3 were measured by Evans/NMR method.[22]
Caution! Perchlorate salts of metal complexes with organic ligands
are potentially explosive. Although no difficulty was encountered
during the syntheses, they should be prepared in small amounts
and handled with caution.
with the phenolato O atoms on the other [FeLamine +
] sup-
ports the µ-OH group and stabilizes the structure of 1. In
contrast, steric constraints introduced by the tert-butyl
6,6Ј-[2-Methyl-2-(pyridin-2-yl)propane-1,3-diyl]bis(azanediyl)bis(meth-
ylene)bis(2,4-di-tert-butylphenol) (H2LtBu-amine): H2LtBu-amine was
prepared by the condensation of 2-methyl-2-pyridin-2-ylpropane-
1,3-diamine (0.82 g, 5.00 mmol) with 3,5-di-tert-butylsalicylalde-
groups in the [FeLtBu-amine +
] units of 2 orient the ligands
such that intramolecular H-bonding is not possible.
The conversion of oxo-bridged 2 to its protonated hy-
droxo-bridged analog [(FeLtBu-amine)2(µ-OH)]+ by treatment hyde (2.34 g, 10.0 mmol) in 50 mL of CH3OH. The solution was
stirred at 50 °C for 2 h, yielding a pale yellow solution. NaBH4
(0.57 g, 15.0 mmol) was then added at 0 °C in small portions and
the solution was stirred at room temperature for 2 h. The solvent
was evaporated; the product was extracted with CH2Cl2, then dried
with MgSO4 and isolated after solvent evaporation (2.28 g, 76%
yield). C39H59N3O2·CH3OH (633.49): calcd. C 75.78, H 10.02, N
6.63; found C 75.83, H 9.72, N 5.94. H NMR (300 MHz, CDCl3,
293 K): δ = 1.28–1.48 (m, 39 H), 2.85–3.13 (m, 4 H), 3.89–3.91 (m,
4 H), 6.85 –7.21 (m, 4 H), 7.30–7.70 (m, 3 H), 8.58 (d, J = 0.9 Hz,
with mild acid was monitored spectroscopically and electro-
chemically. However, the reaction was not reversible under
the conditions tested, where solutions of [(FeLtBu-amine)2(µ-
OH)]+ were treated with base to try to regenerate 2. Simi-
larly, hydroxo-bridged 1 could not be deprotonated to gen-
erate its oxo-bridged analog, [(FeLamine)2(µ-O)], suggesting
that the pKa values of the hydroxo groups in 1 and
[(FeLtBu-amine)2(µ-OH)]+ are quite large. Finally, the CV of
2 revealed well-defined ligand-based redox couples, but no
1
1 H) ppm. FTIR (KBr): ν = 3301 (O–H), 2956 (s, tBu–CH), 2905
˜
metal-based redox couples. When the CVs of solutions of 2 (s, tBu–CH), 2867 (s, tBu–CH) 1591 (s), 1480 (s), (C=Npy,
Eur. J. Inorg. Chem. 2009, 5319–5327
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5325

R. Shakya, D. R. Powell, R. P. Houser
FULL PAPER
C=Caromatic), 1390, 1361, 1301, 1236, 1203, 1165, 1124, 1092, 1026,
995, 925, 876, 821, 788, 748, 723, 672, 648, 542 cm–1. ESI-MS
(CH3OH): m/z = 602 [H2LtBu-amine + H]+.
was stirred at room temperature for 2 h and then filtered to remove
any unreacted solids. X-ray quality crystals were obtained from the
solution after crystallization by slow evaporation of the solvent
(0.065 g, 70% yield). C23H25ClFeN3O2 (466.10): calcd. C 59.18, H
5.40, N 9.00; found C 59.34, H 5.79, N 8.56. UV/Vis (CH2Cl2):
λmax (ε, –1 cm–1) = 277 (9,250), 318 (5,010), 519 (2,590) nm. FTIR
[(FeLamine)2(µ-OH)]BPh4 (1): A 1 mL CH3OH solution of Fe(ClO4)3·
6H2O (0.115 g, 0.250 mmol) was added dropwise to a 10 mL
CH3OH solution of H2Lamine (0.095 g, 0.25 mmol) and Et3N
(0.07 mL, 0.5 mmol). The resulting dark red solution was stirred
for 2 h at room temperature and filtered to discard any unreacted
solids. A powder was isolated after removal of the solvent, was
redissolved in CH3OH, and to this was added NaBPh4 (0.13 g,
0.40 mmol) for counterion metathesis. The resulting solution was
warmed to 50 °C for 2 h and then cooled down to room tempera-
ture. X-ray quality crystals were obtained by slow evaporation of
the solvent (0.11 g, 70% yield). C72H83BFe2N6O9 (1298.50): calcd.
C 66.57, H 6.44, N 6.47; found C 66.45, H 6.26, N 6.57. UV/Vis
(CH2Cl2): λmax, (ε, –1 cm–1) = 275 (23,100), 320 (10,200), 516
(KBr): ν = 3248, 1594 (s), 1569, 1478 (s, C=N , C=Caromatic), 1453,
˜
py
1398, 1384, 1273, 1172, 1062, 1037, 909, 820, 758, 609, 510, 474
cm–1. ESI-MS (CH3OH): m/z
=
431 [Fe (Lamine)]+. EPR
(9.457 GHz, mod. amp. 10.0 G, CH2Cl2, 77 K): g = 9.24 (weak)
and g = 4.18 (strong). Solution magnetic moment (Evans method,
19.8 °C, 1.68ϫ10–2 , [D3]acetonitrile): 2.64 µB/Fe. Solid-state
magnetic moment (MSB-Auto, 4.5 kG, 22.0 °C): 2.82 µB/Fe.
[FeLtBu-amineCl] (5): Anhydrous FeCl3 (0.0162 g, 0.100 mmol) was
added to
a
3 mL CH2Cl2 solution of HLtBu-amine (0.060 g,
0.100 mmol) and Et3N (0.04 mL, 0.3 mmol). The resulting dark
blue solution was stirred at room temperature for 2 h and then
filtered to remove any unreacted solids. The blue powder was iso-
lated from the reaction solution. X-ray quality crystals were ob-
tained after recrystallization by slow evaporation of ethyl ether
(0.052 g, 75% yield). Complex 5 is hygroscopic, and despite our
efforts to dry it, water was always present in the elemental analysis.
C39H57ClFeN3O2·H2O (708.36): calcd. C 66.05, H 8.39, N 5.92;
found C 66.02, H 8.37, N 5.64. UV/Vis (CH2Cl2): λmax (ε, –1 cm–1)
(6,580) nm. FTIR (KBr): ν = 3420 (µ-OH), 3133, 3054, 2906, 1595
˜
(s), 1570, 1479 (s, C=Npy, C=Caromatic), 1455, 1427, 1389, 1268,
1150, 1112, 1064, 1031, 909, 881, 757, 732, 705, 613, 604, 510, 477
cm–1. ESI-MS (CH3OH): m/z = 431 [FeLamine]+, 879 [Fe(Lamine)2(µ-
OH)]+. EPR (9.457 GHz, mod. amp. 10.0 G, CH2Cl2, 77 K): g =
4.20 (strong) and g = 2.03 (weak). Solution magnetic moment (Ev-
ans method, 19.8 °C, 6.40ϫ10–3 , [D3]acetonitrile): 2.47 µB/Fe.
Solid-state magnetic moment (MSB-Auto, 4.5 kG, 20.0 °C):
2.38 µB/Fe.
= 285 (12,700), 333 (6,630), 584 (3,100) nm. FTIR (KBr): ν = 3217,
˜
3175, 2951 (s, tBu-CH), 2902 (s, tBu-CH), 2864 (s, tBu-CH), 1601
(s), 1562, 1466 (s, C=Npy, C=Caromatic), 1438, 1412, 1390, 1360,
1299, 1266, 1240, 1203, 1168, 1075, 834, 544, 477 cm–1. ESI-MS
(CH3OH): m/z = 655 [Fe(LtBu-amine)]+. EPR (9.457 GHz, mod.
amp. 10.0 G, CH2Cl2, 77 K): g = 9.2 (weak) and g = 4.23(strong).
Solution magnetic moment (Evans method, 19.8 °C, 1.2ϫ10–2 ,
[D3]acetonitrile): 3.068 µB/Fe. Solid-state magnetic moment (MSB-
Auto, 4.5 kG, 22.0 °C): 2.940 µB/Fe.
[(FeLtBu-amine)2(µ-O)] (2): A 1 mL CH3OH solution of Fe(ClO4)3·
6H2O (0.046 g, 0.10 mmol) was added dropwise to a 2 mL CH3OH
solution of H2LtBu-amine (0.060 g, 0.10 mmol) and Et3N (0.06 mL,
0.45 mmol) at –20 °C. The resulting purple solution was stirred for
3 h at –20 °C and then stirred for another 16 h at room tempera-
ture. The solvent was removed under reduced pressure. X-ray qual-
ity crystals of 2 were obtained by slow evaporation of a 1:1 acetoni-
trile/hexane solution of the product mixture (0.020 g, 30% yield).
C78H114Fe2N6O5 (1326.75): calcd. C 70.57, H 8.66, N 6.33; found
C 70.70, H 8.68, N 6.34. UV/Vis (CH2Cl2): λmax (ε, –1 cm–1) = 291
X-ray Crystal Structure Determination: X-ray quality crystals of 1
were obtained by slow evaporation of a methanol solution of 1.
Single crystals of 2 were obtained by slow evaporation of a 1:1
acetonitrile/hexane solution of 2. Single crystals of 4 and 5 were
obtained by slow evaporation of methanol or ethyl ether solutions,
respectively. Intensity data for all the compounds were collected
using a diffractometer with a Bruker APEX ccd area detector[23]
and graphite-monochromated Mo-Kα radiation (λ = 0.71073 Å).
The samples were cooled to 100(2) K. Cell parameters were deter-
mined from a non-linear least-squares fit of the data. The data were
corrected for absorption by the semi-empirical method.[24] The
structure was solved by direct methods and refined by full-matrix
least-squares methods on F2.[25] Hydrogen atom positions of hydro-
gen atoms bonded to carbon atoms were initially determined by
geometry and refined by a riding model. Hydrogen atoms bonded
to nitrogen or oxygen atoms were located on a difference map, and
their positions were refined independently. Non-hydrogen atoms
were refined with anisotropic displacement parameters. Hydrogen
atom displacement parameters were set to 1.2 (1.5 for methyl) times
the displacement parameters of the bonded atoms. Crystal data for
1·(CH4O)2·(H2O) and 2·(C2H3N)5 are summarized in Table 5, and
crystal data for 4 and 5·(C4H10O) are summarized in Table 6. Se-
lected bond lengths and angles for 1–2 and 4–5 are summarized in
Tables 1, 2, 3, and 4.
(28,500), 446 (6,720) nm. FTIR (KBr): ν = 2953 (s, tBu–CH), 2901
˜
(s, tBu–CH), 2866 (s, tBu-CH), 1597 (s), 1573, 1467 (s, C=Npy,
C=Caromatic), 1439, 1412, 1391, 1361, 1300, 1268, 1239, 1203, 1167,
1090, 834, 745, 544, 477 cm–1. ESI-MS (CH3OH): m/z = 655
[FeLtBu-amine]+, 1328 ([Fe(LtBu-amine)2(µ-O)]
+
H)+. EPR
(9.457 GHz, mod. amp. 10.0 G, CH2Cl2, 77 K): g = 4.21 (strong)
and g = 2.02 (weak). Solution magnetic moment (Evans method,
19.8 °C, 9.30ϫ10–3 , [D1]chloroform): 2.016 µB/Fe. Solid-state
magnetic moment (MSB-Auto, 4.5 kG, 22.0 °C): 1.919 µB/Fe.
[FeLtBu-amine(OCH3)] (3): This complex is the major product in the
synthesis of 2 (see above). A dark blue powder was isolated from
the recrystallization mother liquor (after crystals of 2 were removed
by decanting the solvent) by removal of solvent under vacuum. The
powder was redissolved in dichloromethane, washed with water,
and the organic layer was dried with anhydrous MgSO4 and then
filtered. A blue powder was isolated by removal of the solvent un-
der vacuum (0.046 g, 60% yield). C40H60FeN3O3·CH2Cl2 (770.35):
calcd. C 63.81, H 8.10, N 5.45; found C 64.52, H 8.01, N 5.35.
UV/Vis (CH2Cl2): λmax (ε, –1 cm–1) = 283 (9,850), 335 (5,610), 600
(3,100) nm. FTIR (KBr): ν = 3257, 2953 (s, tBu-CH), 2901 (s, tBu-
˜
CH), 2866 (s, tBu-CH), 1660, 1603 (s), 1562, 1466 (s, C=Npy, C=Ca-
romatic), 1439, 1412, 1361, 1300, 1267, 1240, 1203, 1168, 1107, 1018,
870, 834, 623, 546, 485 cm–1. ESI-MS (CH3OH): m/z = 655
[Fe(LtBu-amine)]+.
CCDC-736595 (for 1·2CH4O·H2O), -736596 (for 2·5C2H3N),
-736597 (for 4), -736598 (for 5·C4H10O) contain the supplementary
crystallographic data for this paper. These data can be obtained
free of charge from The Cambridge Crystallographic Data Centre
via www.ccdc.cam.ac.uk/data_request/cif.
[FeLamineCl] (4): Anhydrous FeCl3 (0.0324 g, 0.200 mmol) was
added to a 5 mL CH2Cl2 solution of HLamine (0.076 g, 0.20 mmol)
and Et3N (0.08 mL, 0.60 mmol). The resulting dark red solution
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Eur. J. Inorg. Chem. 2009, 5319–5327

Iron(III) Complexes with Pyridylbis(aminophenol) Ligands
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Table 5. Crystallographic data for complexes 1 and 2.
1·2CH4O·H2O
2·5C2H3N
Formula
Fw
Space group
a [Å]
b [Å]
c [Å]
C72H81BFe2N6O8
1280.94
P1
C88H129Fe2N11O5
1532.72
P21/n
13.661(3)
31.106(7)
20.710(5)
90
94.797(8)
90
8770(3)
4
1.161
0.385
2.08 to 18.00
0.1483
0.3354
¯
11.448(2)
15.643(4)
19.253(4)
71.920(8)
88.393(7)
84.240(7)
3261.0(12)
2
1.305
0.506
1.38 to 26.00
0.0453
0.1292
α [°]
β [°]
γ [°]
V [Å3]
Z
ρcalcd. [mg/m–3]
µ [mm–1]
θ [°]
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wR2[b]
GOF on F2
1.051
1.174
[a] R1 = Σ||Fo| – |Fc||/Σ|Fo|. [b] wR2 = {Σ[w(Fo – Fc2)2]/Σ[w(Fo ) ]}1/2
.
2
2 2
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Table 6. Crystallographic data for complexes 4 and 5.
4
5·C4H10O
Formula
Fw
Space group
a [Å]
b [Å]
c [Å]
C23H25ClFeN3O2
466.76
Cc
14.304(3)
16.283(4)
18.928(4)
90
101.987(8)
90
2807.8(16)
8
1.438
0.848
1.92 to 26.00
0.0342
C43H67ClFeN3O3
765.30
P1
¯
13.737(3)
14.409(3)
23.904(5)
89.985(5)
81.727(5)
67.005(6)
4302.0(16)
4
1.182
0.452
1.63 to 26.00
0.0456
0.1155
α [°]
β [°]
γ [°]
V [Å3]
Z
ρcalcd. [mg/m–3]
µ [mm–1]
θ [°]
R1[a]
[b]
wR2
0.0815
1.001
GOF on F2
1.007
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2 2
[a] R1 = Σ||Fo| – |Fc||/Σ|Fo|. [b] wR2 = {Σ[w(Fo – Fc2)2]/Σ[w(Fo ) ]}1/2
.
Supporting Information (see also the footnote on the first page of
this article): Crystallographic data in CIF format, UV/Vis spectra
of complexes 1–5 (Figure S1), reversible conversion of 1 and 4 fol-
lowed by UV/Vis spectroscopy (Figure S2), and cyclic voltammog-
rams for 3–5 (Figures S3–S5).
Acknowledgments
This work was supported by the National Science Foundation
(NSF CHE-0616941). Additionally, we thank the NSF for the pur-
chase of a CCD-equipped X-ray diffractometer (CHE-0130835)
and Prof. Dr. A. Grohmann (Technische Universität Berlin) for
fruitful discussions and the gift of several grams of 2-methyl-2-(2-
pyridinyl)-1,3-propanediol.
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Received: August 14, 2009
Published Online: October 28, 2009
Eur. J. Inorg. Chem. 2009, 5319–5327
© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
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