Isolation of Iron(II) aqua and hydroxyl complexes featuring a tripodal H-bond donor and acceptor ligand

Article abstract of DOI:10.1021/ic500102c

A tripodal ligand platform, tris(5-cycloiminopyrrol-2-ylmethyl)amine (H₃[N(pi^Cy)₃]), that features a hydrogen bond-accepting secondary coordination sphere when bound anionically to an iron center is reported. Neutral coordination to iron affords ligand tautomerization, resulting in a hydrogen bond-donating secondary coordination sphere, and formation of the tris(5-cyclohexyl-amineazafulvene-2-methyl)amine, H ₃[N(afa^Cy)₃], scaffold. Both binding motifs result in formation of stable, high-spin iron(II) complexes featuring ancillary water, triflate, or hydroxo ligands. Structural analysis reveals that these complexes exhibit distorted trigonal-bipyramidal geometries with coordination of the apical nitrogen to iron as well as three equatorial amine or imine nitrogens, depending on the axial ancillary ligand. Formation of the aqua complex K[(N(pi^Cy)₃)Fe(OH₂)] (3) illustrated the propensity of the ligand to be hydrogen bond-accepting, whereas the iron triflate species [N(afa^Cy)₃Fe](OTf)₂ (4) features a hydrogen bond-donating secondary coordination sphere. The ability of each of the three arms of the ligand to tautomerize independently was observed during the formation of the iron-hydroxyl species [N(afa^Cy) ₂(pi^Cy)]FeOH (5) and characterized by X-ray crystallography and IR spectroscopy. The combined data for the iron complexes established that each arm of the tripodal ligand can tautomerize independently and is likely dependent on the electronic needs of the iron center when binding various substrates.

Full text of DOI:10.1021/ic500102c

Article
pubs.acs.org/IC
Isolation of Iron(II) Aqua and Hydroxyl Complexes Featuring a
Tripodal H-bond Donor and Acceptor Ligand
Ellen M. Matson, Jeffrey A. Bertke, and Alison R. Fout*
School of Chemical Sciences, University of Illinois at Urbana−Champaign, 600 S. Mathews Avenue Urbana, Illinois 61801, United
States
S
* Supporting Information
ABSTRACT: A tripodal ligand platform, tris(5-cycloimino-
pyrrol-2-ylmethyl)amine (H3[N(piCy)3]), that features a
hydrogen bond-accepting secondary coordination sphere
when bound anionically to an iron center is reported. Neutral
coordination to iron affords ligand tautomerization, resulting
in a hydrogen bond-donating secondary coordination sphere,
and formation of the tris(5-cyclohexyl-amineazafulvene-2-
methyl)amine, H3[N(afaCy)3], scaffold. Both binding motifs
result in formation of stable, high-spin iron(II) complexes
featuring ancillary water, triflate, or hydroxo ligands. Structural analysis reveals that these complexes exhibit distorted trigonal-
bipyramidal geometries with coordination of the apical nitrogen to iron as well as three equatorial amine or imine nitrogens,
depending on the axial ancillary ligand. Formation of the aqua complex K[(N(piCy)3)Fe(OH2)] (3) illustrated the propensity of
the ligand to be hydrogen bond-accepting, whereas the iron triflate species [N(afaCy)3Fe](OTf)2 (4) features a hydrogen bond-
donating secondary coordination sphere. The ability of each of the three arms of the ligand to tautomerize independently was
observed during the formation of the iron−hydroxyl species [N(afaCy)2(piCy)]FeOH (5) and characterized by X-ray
crystallography and IR spectroscopy. The combined data for the iron complexes established that each arm of the tripodal ligand
can tautomerize independently and is likely dependent on the electronic needs of the iron center when binding various
substrates.
earliest examples of a nonheme, ancillary ligand that includes a
secondary coordination sphere of a metal ion, reported by
Kitajima and co-workers45 involves the formation of Mn(O2)-
(3,5-iPr2pzH)(HB(3,5-iPr2pz)3), in which a single hydrogen
bond is shown to stabilize the Mn(III)−peroxo compound.45
Borovik and co-workers have established that appropriately
placed hydrogen bonds are capable of regulating the micro-
environment at the metal center and that these site-specific
modulations in structure can be correlated to reactiv-
ity.12,14,18−20,22,24,46 For example, varying the number of
hydrogen bonds (from three to zero) within a tripodal cobalt
system, through the development of hybrid urea-carboxamide
ligands, resulted in a variance of stability of the resultant Co−
OH complexes.46a Inspired by this work, we sought to design a
single ligand scaffold that, depending on substrate ligation or
chemical environment, was flexible in the hydrogen bonding
environment. Our goal was twofold: (1) to incorporate a
secondary coordination sphere in which the hydrogen bonding
to substrates was flexible in that as many as three hydrogen
bonds or as few as none could be accessed for binding to the
substrate, and (2) the coordination of the ligand to a metal
could adapt electronically to allow the metal center to traverse
various oxidation states.
INTRODUCTION
■
Analogous to the enhanced reactivity observed in metal-
loenzymes by appropriately positioned amino acid hydrogen
bonds for the binding, orientation, and activation of
substrates,1,2 ligand design may be used to influence the
reactivity at a metal center through secondary interactions.
These interactions add structural stability to the active site of
metalloenzymes and play a key role in regulating activity.2
Heme and nonheme enzymes that bind and activate dioxygen
for a variety of functions combine primary iron ligand fields
with defined networks of hydrogen bonding interactions.3−17
Although the secondary coordination sphere hydrogen bonding
interactions of bioinspired metal complexes are crucial to the
stabilization of coordinated oxo,18,19 hydroxo,16,20−27 and
hydroperoxo28−32 intermediates, as well as the activation of
small molecules,33,34 developing synthetic systems that mimic
not only the structural but also the functional properties of
metalloenzymes has been challenging. Synthetic difficulties
arise from the necessity to involve multiple proton−electron
transfers in the sustainable transformation of small molecules
such as O2, H2, CO2, and N2.35−39
Synthetic systems capable of mediating multiple proton−
electron transformations have involved highly specialized and
synthetically challenging ligands such as picket-fence,40,41
crown,42 Pacman,43 and Hangman44 porphyrins, all of which
feature flanking hydrogen bonding networks. One of the
Received: January 16, 2014
Published: April 23, 2014
© 2014 American Chemical Society
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The formation of iron−hydroxyl species derived from either
O2 or water has been observed; however, nonheme
mononuclear Fe−OH (FeII/FeIII) species are limited16,20−27
given their propensity to form multinuclear hydroxo- or oxo-
bridged complexes. With the addition of very bulky ligands or
secondary hydrogen bonding networks the formation of
mononuclear Fe−OH species has been developed. The work
by Borovik12,18,19 is an elegant display of the importance of
hydrogen bonding motifs in the preparation and character-
ization of iron complexes supported by the urea-based, tripodal
ligand framework tris{(N′-tert-butylureayl)-N-ethylene]-
amine.23 Upon exposure to oxygen, formation of the iron-
(II)/iron(III)−hydroxo species is observed, stabilized by the
intramolecular hydrogen bonding framework. The work by
Borovik has focused on intramolecular hydrogen bonds and
intermolecular hydrogen bonds;24 very bulky ligands21,26b have
also been used to stabilize monomeric Fe−OH complexes.
Although numerous ligands have been constructed that
feature hydrogen bonding with substrates on the metal center,
fewer ligand constructs feature hydrogen bond acceptors47,48 in
the secondary coordination sphere. Ligands that incorporate
both hydrogen bond donors and acceptors are exceedingly
rare.47,49 Therefore, we sought to design a ligand scaffold that
contains a well-defined primary coordination sphere for binding
of a metal center and that features a secondary coordination
sphere that, through ligand tautomerization, may be hydrogen
bond donating and/or hydrogen bond accepting. To satisfy the
aforementioned design criteria and electronic structure of the
metal center we sought a pyrryl-based tripodal ligand
framework, analogous to the tris(5-arylpyrrol-2-ylmethyl)amine
(H3tpaAr), for the primary coordination sphere at the metal
center.27,50 This tripodal framework recently reported by
Chang and co-workers has been shown to support a family of
high-spin iron(II) complexes.51 Upon oxidation of these
compounds with Me3NO, a transient Fe(IV)−oxo is formed,
which is capable of functionalizing the proximal aryl-CH bonds
of the ligand or abstracting H· from ethereal solvents.51 This
result implied that appending a secondary coordination sphere
to the ligand framework may allow for the desired secondary
coordination sphere interactions with metal-ligand multiple
bonds.
ligand tris(pyrryl)amine (H3tpa) by addition of a variety of
functional groups. Chiefly, our interests focused on the
selection of a pendant functional group that could serve as
both a hydrogen bond donor and acceptor, depending on
chemical environment and exposure to substrate. Literature
precedent displays the ability of a pyrryl-2-imine to engage in
tautomerization (Scheme 1).55 Formation of a ligand frame-
Scheme 1
work where the metal center could bind in the pyrryl pocket
gives the opportunity for a hydrogen bond-accepting secondary
coordination sphere in the case of anionic coordination
(abbreviated (piCy)), while a neutral coordination environment,
in the case of the amine−azafulvene tautomer (abbreviated
(afaCy)) gives rise to a hydrogen bond donating periphery. The
multidentate ligand architecture allows for flexibility in the
secondary coordination sphere in that the various arms of the
ligand may independently tautomerize to be either hydrogen
bond donating or accepting, depending on chemical environ-
ment and electronic needs of the metal center.
Previous work with the development of functionalized
polypyrryl compounds has illustrated a preparatory pathway
for the generation of these ancillary ligands. In the case of
Chang and co-workers, assembly of a tripodal pyrrole ligand
framework is accomplished by triple Mannich condensations of
2-R-pyrrole (R = mesityl, 2,4,6-triisopropylphenyl, 2,6-difluor-
ophenyl, tert-butyl, phenyl).50 Following failed synthetic
attempts at the generation of the desired ligand framework
H3(N(piCy)3 by traditional routes, an alternative preparatory
pathway was explored. Recent work by the Beer54 and Love55
research groups has shown the utility of the installation of 5′-
imine substituents following the assembly of the polypyrrole
framework for the functionalized tris(5-R-imino-pyrryl)ethane
ligand H3tpeNR (R = cyclohexyl, benzyl) in good yields. While
these ligands were suggested to afford secondary coordination
spheres to metal centers possessing a pseudotetrahedral ligand
field, the expected tripodal coordination of the ligand
framework was sparsely observed.55 We postulated installation
of the functional group off the 5′ carbons of the tris(pyrryl-2-
methyl)amine ligand scaffold would afford the desired
coordination geometry for the metal complexes.
An adapted procedure from that developed for the
installation of the aldehyde functional group in the synthesis
of tris(5-formylpyrrol-2-ylmethyl)ethane (H3tpeCO) via a
Vilsmeier−Haack formylation was explored for the tris(pyrryl)-
amine. In a similar fashion, the formyl derivative of tris-
(pyrryl)amine, H3tpaCO, was synthesized by the dropwise
addition of POCl3 to a solution of H3tpa in DMF at −10 °C
(Scheme 2). Following workup, the proligand was isolated as a
brown solid in low yields (41%). While the formation of
H3tpaCO was confirmed by 1H (Supporting Information, Figure
S1) and 13C NMR and IR spectroscopies, the lack of purity of
the sample due to contamination by NMR silent species and
the inability to separate the compound from the residual
reaction solvent, DMF, prohibited confirmation of formation by
elemental analysis. Stirring H3tpaCO with an excess of
Our research program focuses on the utilization of earth-
abundant metals, such as iron, to activate small molecules
through the influence of ligand design. In this Report, we
present the new ligand platform tris(5-cycloiminopyrrol-2-
ylmethyl)amine (H3[N(piCy)3]), in which the secondary
coordination sphere can accommodate noncovalent interac-
tions (hydrogen bonds) with exogenous ligands bound to the
metal center. The synthesis, spectroscopic characterization, and
reactivity of a new series of tripodal iron(II) complexes is
discussed. This system is distinguished from other hydrogen
bond cavities by its flexibility to traverse trianionic coordination
to neutral coordination at the metal center while promoting a
secondary coordination sphere that can be either hydrogen
bond donating or accepting. This flexibility of the ligand
scaffold through tautomerization allows for the isolation of
stable iron(II) aqua, triflate, and hydroxyl complexes.
RESULTS AND DISCUSSION
■
Synthesis and Characterization of H3(N(piCy)3. To
generate a series of pyrrole-based tripodal ligands with a
secondary coordination sphere for the stabilization of high-
valent MO complexes, we invoked modification of the parent
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Scheme 2
cyclohexylamine in a dry acetonitrile solution resulted in the
quantitative formation of the desired imine ligand architecture
via condensation (Scheme 2). The ligand could also be
synthesized in situ by the addition of cyclohexylamine to the
H3tpaCO prior to isolation. H3[N(piCy)3]·H2O (1) is isolated as
a beige powder in good yields from the H3tpa starting material
(53%). Characterization of the ligand by elemental analysis
revealed encapsulation of one water molecule, indicating a high
affinity for substrate binding to the ligand prior to metalation.
magnetic moment of complex 2 of 5.73 μB was determined
by Evans’ method, consistent with the formation of a high spin,
trivalent iron species. Characterization by IR spectroscopy
revealed an intense feature located at 1581 cm−1, assigned to
the CN stretch of the ligand. This IR stretch is consistent
with a previously characterized Zn complex (CN at 1586
cm−1) featuring a similar bidentate coordination of the tpe
ligand.55
To definitively identify the molecular structure of complex 2
and better understand the coordination geometry of the
tris(pyrryl)amine ligand framework with hydrogen bond-
accepting secondary coordination sphere, crystals suitable for
X-ray diffraction were grown from a concentrated solution of
THF and benzene (1:1), layered with hexanes. Refinement
revealed an iron(III) center in a pseudo-octahedral coordina-
tion environment. One face of the octahedron is derived from
anionically coordinated pyrrole nitrogens of the ligand
framework, while the other face is composed of imine nitrogens
datively bound to the metal center. Complex 2 crystallizes in a
hexagonal unit cell, with the C3 axis of the molecule sitting
along a symmetry operation (see Figure 1). Thus, only a third
Similar behavior has been observed for the trispyrrylethane
55
ligand H3tpeNCy
.
In the case of H3tpeNCy, crystallographic
characterization also revealed binding of a water molecule
within the pocket of the ligand, stabilized by hydrogen bonding.
Tautomerization of the proposed ligand system has been
observed in a similar framework in the case of H3tpeNCy
.
Addition of H3tpeNCy to CoCl2 under aerobic conditions
resulted in the oxidation of the metal center to cobalt(III), with
two neutrally coordinated ligands bound to the metal center
and three outer sphere chlorine atoms, [Co(H3tpeNCy)2]-
[Cl]3.55 Structural analysis of the product revealed tautomeriza-
tion of the ligand to afford the amine−azafulvene species
(Scheme 1), where the amine proton is proposed to derive
from a proton migration from the pyrrole nitrogen to the imine
moiety.55 The isomerization of this ligand indicates potential
for ligand-mediated modulation of the oxidation state of a
transition metal center in coordination complexes of this type
and enables this ligand species to serve as both a hydrogen
bond donor and acceptor.
Synthesis of [(N(piCy)3)Fe]. Following synthesis and
characterization of the desired ligand 1, metalation was
attempted. Addition of 3.1 equiv of KH to a THF solution of
complex 1 resulted in a vigorous effervescence of hydrogen and
the in situ generation of K3[(N(piCy)3]. Addition of the
potassium salt of complex 1 to a THF solution of FeCl3
resulted in an immediate color change to dark brown (Scheme
3). Following workup, the product, (N(piCy)3)Fe (2), was
isolated as a brown powder in high yields (68%). Analysis of 2
Figure 1. Molecular structure of 2 shown with 50% probability
ellipsoids. Hydrogen atoms were removed for clarity. Left view of
complex shown perpendicular to the Fe···N1 axis similar to subsequent
complexes; right view displays pseudo-octahedral coordination
geometry of 2. Selected bond lengths (Å) and angles (deg): Fe1···
N1, 2.949 58(10); Fe1−N2, 2.029 88(5); Fe1−N5, 2.263 96(6); N2−
Fe1−N5, 74.976(3); N2−Fe1−N2a, 100.626(3), N5−Fe1−N2a,
158.4037(8); N5−Fe1−N5a, 85.0094(18).
1
by H NMR spectroscopy revealed a paramagnetic product
composed of ill-defined, broad resonances ranging from −15.7
to 33.0 ppm (Supporting Information, Figure S3). The
Scheme 3
of the molecule is chemically unique, with the remaining three
arms generated by a symmetry operation. The iron center is
removed from the traditional tris(pyrryl)amine pocket, with an
Fe1−N1 interaction of 2.949 58(10) Å, indicative of dative
binding of iron to the apical nitrogen. The Fe1−N2 distance of
2.029 88(5) Å is consistent with anionic Fe−N distances
reported previously for pyrryl-based ligand frameworks.50 The
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Scheme 4
Fe1−N5 distance of 2.263 96(6) Å is consistent with a dative
interaction.
the iron(II) species is in a pseudo-trigonal bipyramidal
coordination geometry with a water molecule occupying the
position trans to the apical nitrogen and bound to the metal
center (Figure 2, Table 1). The Fe−O1 distance of 2.080(2) Å
compares favorably to that of Fe(II) aqua complexes reported
in the literature, indicating a dative interaction between the two
atoms.57 The equatorial plane is composed of the anionically
bound pyrryl nitrogen (Npyr) atoms.
Synthesis and Characterization of Iron(II) Complexes.
Since using an Fe(III) starting material afforded the
coordinatively saturated and undesired octahedral iron
complex, metalation of H3[(N(piCy)3] was then attempted in
analogy to the generation of [Fe(tpaR)]−, reported by Chang
and co-workers to synthesize Fe(II) derivatives.50 Deprotona-
tion of H3[(N(piCy)3] was accomplished once again by addition
of 3.1 equiv of KH to a THF solution of 1. After it was stirred
for 3 h at room temperature, the mixture was filtered over
Celite to remove excess KH. Addition of the freshly generated
K3[(N(piCy)3] in situ to a slurry of FeCl2 in THF resulted in an
instantaneous color change to yellow-orange, with concurrent
dissolution of the starting material (Scheme 4). Following
workup, the product K[(N(piCy)3Fe(II)H2O] (complex 3) was
isolated as a yellow powder in high yields (73%).
Although the protons of the water molecule within the
coordination sphere of the metal center were not observed on
1
the H NMR time scale, potentially due to proximity to the
paramagnetic iron center, calculated positions indicate the
protons of the water molecule are engaging in hydrogen
bonding with two of the pendant imine nitrogen atoms,
supported by N6···H6 and N7···H7 distances of 1.90(2) and
1.81(2) Å, respectively. The N6−O1 and N7−O1 distances of
2.739(3) and 2.650(4) Å are well within the range of donor−
acceptor atoms participating in hydrogen bonding interactions.
The resulting hydrogen bonding is further manifested in the
elongation of two of the Fe−Npyr bond distances (2.108(2) and
2.118(2) Å), as compared to the third arm, which is not
engaging in secondary interactions (Fe−Npyr bond distance of
2.044(2) Å) and more closely resembles the previously
reported Fe−Npyr distances of [Fe(tpaR)]− (R = Ph, Mes,
tBu) (2.008(3)−2.041(2) Å).50 As a result, no distinct υOH
stretches were observed for the aqua complex presumably
masked by the strong intramolecular hydrogen bonding within
the cavity. Furthermore, the Fe−N1 distance of 2.262(3) Å is
significantly elongated from that of Chang’s [Fe(tpaR)]−
complexes (2.144(1)−2.172(2) Å),50 while the N1−Fe−Npyr
bond angles of complex 3, ranging from 78.70(10)−
80.21(10)°, further indicate a deviation from the traditional
geometry, as previously characterized M[Fe(tpaR)] (M = Na,
K) complexes have N1−Fe−Npyr bond angles ranging from
82.00(9)−83.84(6)°.50 This interaction of the Fe(II) metal
center with a water molecule engaged in hydrogen bonding
with the ancillary coordination environment is proposed to be
responsible for perturbing the metal center from the plane of
the pyrrole rings, thus giving rise to the observed distorted
trigonal bipyramidal geometry.
1
Analysis of complex 3 by H NMR spectroscopy revealed
four distinct paramagnetically shifted and broadened reso-
nances, ranging from 0.2 to 95.6 ppm (Supporting Information,
Figure S4). Signals integrating to three hydrogens located at
28.6, 32.3, and 95.6 ppm are assigned to the aryl C−H protons
of the pyrrole moieties and the imine C−H protons of the
ligand bound to the iron center. The six methylene hydrogens
of the ligand framework are located at 12.3 ppm. Additionally, a
cluster of signals ranging from 3.7 to 7.3 ppm were assigned to
the cyclohexyl protons of the ligand framework. The number of
1H NMR resonances is indicative of a C3 symmetric molecule.
IR spectroscopy further corroborated the C3 symmetric ligand
scaffold with a single, broad and intense υCN signal at 1602
cm−1, indicative of a single ligand tautomer present in all three
arms of the complex. The solution magnetic moment for
complex 3 as determined by the Evans’ method (μeff = 5.39 μB)
was consistent with a high-spin, Fe(II) metal center. This
electronic state is similar to that observed by Chang and co-
workers for their M[Fe(tpaR)] (M = Na, K; R = mesityl, 2,4,6-
triisopropylphenyl, 2,6-difluorophenyl, tert-butyl, phenyl) com-
plexes with μeff values ranging from 5.15 to 5.42 μB.50
Orange crystals of complex 3 suitable for X-ray analysis were
grown by vapor diffusion of hexanes into a concentrated THF
solution of the complex. Refinement of the data revealed that
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Because of our interest in probing the ability of each arm of
the ligand to tautomerize independently, metalation was
explored by addition of complex 1 to 1 equiv of Fe(OTf)2-
(MeCN)2 (OTf = trifluoromethanesulfonate) without the
addition of base. Upon addition of a THF solution of the
ligand to a THF slurry of Fe(OTf)2, an instantaneous color
change to yellow was noted (Scheme 4). To ensure
consumption of starting materials, the reaction was stirred for
30 min before the solvents were removed under reduced
pressure. Following workup, the product [N(afaCy)3Fe](OTf)2
(4) was isolated as a yellow powder in quantitative yields.
Chart 1. Bond Length Comparisons between Complexes 4
(left) and 3 (right)
a
1
Analysis of complex 4 by H NMR spectroscopy revealed a
complicated spectrum composed of paramagnetic resonances
ranging from −5.19 to 39.24 ppm (Supporting Information,
Figure S5). While the number of peaks is consistent with the
desired complex [N(afaCy)3Fe](OTf)2 adopting an unsymmet-
ric conformation in solution, definitive peak assignment was
impossible due to overlapping resonances and dynamic
broadening. Characterization by 19F NMR revealed a signal at
−79.59 ppm, corresponding to the fluorine atoms of the
trifluoromethylsulfonate counteranions. The single resonance
indicates uniformity in the chemical environment of fluorine,
suggesting dissociation of both (OTf)− moieties in solution on
the NMR time scale. Characterization by IR spectroscopy
revealed an intense feature centered at υCN of 1637 cm−1, a
value blue-shifted (by ∼35 cm−1) from complex 3, consistent
with previously reported azafulvene derivatives of transition
metal complexes.56 The magnetic data from a series of
independently synthesized samples of complex 4, revealed an
average μeff of 5.37 μB, consistent with a high-spin, S = 2, Fe(II)
metal center.
Structural characterization of 4 was obtained by X-ray
diffraction studies from a concentrated THF solution of the
compound layered with diethyl ether. Refinement revealed a
five-coordinate Fe(II) center in a pseudo-trigonal bipyramidal
geometry, composed of the tripodal ligand framework with a
triflate anion bound trans to the apical nitrogen (Figure 2,
Table 1). The equatorial plane of complex 4 is composed of the
pyrryl nitrogen atoms, with Fe−Npyr bond distances ranging
from 2.060(2) to 2.081(2) Å. The Fe−N1 distance of 2.268(2)
Å is akin to complex 3, indicating a similar displacement of the
iron center from the equatorial plane.
a
Bond distances of the azafulvene substituent (4) listed in red denote
contraction, while those in blue signify elongation from anionically
coordinated pyrrole−imine (3) arm.
hydrogen bonding to the inner-sphere triflate is proposed to be
a result of the steric bulk of the counterion.
To evaluate the ability of the ligand framework to engage in
asymmetric tautomerization another synthetic route was
sought. Exposure of Fe[N(SiMe3)2]2(py)2 to 1 equiv of
complex 1 resulted in an instantaneous color change to a
pale orange-brown, with concurrent formation of 2 equiv of
HN(SiMe3)2 as confirmed by H NMR spectroscopy (Scheme
4). Characterization of the paramagnetic, iron-containing
product by H NMR spectroscopy revealed 10 resonances
paramagnetically shifted and broadened ranging from −3.86 to
28.82 ppm, consistent with C3 symmetry in solution
(Supporting Information, Figure S6). Assignment of the signals
was made possible by integration, with the three resonances
located at −3.86, 27.71, and 28.82 ppm integrating to 3H
corresponding to the imine C−H and aryl protons of the
pyrrolidine moiety, respectively, while the signal integrating to
6H located at 16.20 ppm is assigned to the methylene protons.
Resonances ranging from 5.49 to 8.23 ppm collectively
integrate to 33H, which corresponds to the flanking cyclohexyl
substituents adjacent to the imine nitrogens. An identical
species with similar spectral features is noted upon deproto-
nation of H3[N(afaCy)3] with LiN(SiMe3)2 and addition of the
solution to a THF slurry of FeCl2.
1
1
Yellow crystals of 5 suitable for analysis by X-ray
crystallography were grown from a concentrated THF solution
layered with hexanes. Structural refinement revealed a pseudo-
trigonal bipyramidal Fe(II) center with a hydroxyl moiety
occupying the coordination site trans to the apical nitrogen
(Figure 2, Table 1). The Fe1−O1 bond distance of 2.0339(12)
Å is akin to that of [FeIIH3L(OH)]2− (L = N[CH2CH2N−C-
(O)NHC(CH3)3]3), previously reported by Borovik and co-
workers, with Fe−O bond distances of 2.051(3) and 2.044(3)
Å;23 however, it is significantly longer than sterically
unencumbered monometallic FeII(OH) complexes with Fe−
O bond distances ranging from 1.830(8) to 1.938(3)
Å.13,16,21,24 The equatorial plane is composed of the pyrrole
nitrogens, with Fe−N bond distances ranging from 2.1020(14)
to 2.1264(14) Å, which are significantly longer than those
reported by Chang and co-workers for their family of
Although the gross structural features of 3 and 4 are similar, a
comparison of intraligand bond distances revealed bond
contractions and elongations consistent with the formation of
the amine−azafulvene tautomer in the latter complex upon
coordination to the metal center (Chart 1). The bond distances
compare favorably to Fe(II) complexes reported by Love and
co-workers, in which tautomerization of the N-donor-extended
dipyrromethane ligand framework upon coordination to FeBr2
affords the amine−azafulvene product FeBr2[H2L] (L = [2-
(tBu-NHCH)-C4H4N]2CMe2).57 This type of amino/imino
ligand tautomerization was also observed in tetradentate ligand
tris(2-amino-oxazoline)amine, which upon metalation tauto-
merizes to the 2-imino-oxazolidine.47 The hydrogen atoms
adjacent to the nitrogen atoms of the pendant imine moieties of
4 were located and refined, further supporting ligand
tautomerization during the formation of complex 4. The N−
H functionalities are rotated away from the triflate molecule
bound to the iron center and possess hydrogen bonding
interactions to the outer-sphere triflate counterions, with N−O
bond distances ranging from 2.910(3)−2.980(3) Å. A lack of
t
[Fe(tpaR)]− (R = Bu, Ph, Mes) complexes (2.008(3)−
2.031(1) Å).50 This elongation is once again attributed to the
structural constraints placed on complex 5 by the interactions
of the secondary coordination sphere with the hydroxyl
substituent.
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Figure 2. Molecular structures of 3, 4, and 5 shown with 50% probability ellipsoids. Solvent molecules, selected hydrogen atoms, potassium
countercations (3), and outer sphere triflate anions (4) were removed for clarity.
Table 1. Spectroscopic Data and Structural Parameters of 1, H3(tpeNcy), 2, 3, 4, and 5
a
bond
1
H3(tpeNCy)·H2O
2
3
4
5
Fe1−N1
2.94958(10)
2.02988(5)
2.26396(6)
2.262(3)
2.044(2)
2.118(2)
2.108(2)
2.080(2)
2.268(2)
2.076(2)
2.060(2)
2.081(2)
2.1603(19)
2.2675(14)
2.1020(14)
2.1091(14)
2.1264(14)
2.0339(12)
0.757(19)
1.54(2)
Fe1−N2
Fe1−N3
Fe1−N4
Fe1−O1
O1−H5
O1−H6
O1−H7
C25−N5
C35−N6
C45−N7
O1···N5
O1···N6
O1···N7
N1−Fe1−N2
N1−Fe1−N3
N1−Fe1−N4
0.843(19)
0.863(19)
1.277(4)
1.290(4)
1.281(4)
1.75(2)
1.24(2)
1.24(2)
1.24(2)
3.12(2)
2.89(2)
2.87(2)
1.350(3)
1.298(4)
1.299(4)
1.315(4)
1.281(2)
1.297(2)
1.300(2)
2.7939(18)
2.5653(18)
2.650(2)
2.739(3)
2.650(4)
78.70(10)
80.21(9)
79.68(10)
1603 cm−1
5.39 μB
b
79.83(9)
77.08(8)
77.66(9)
1637 cm−1
5.37 μB
78.04(5)
78.16(5)
79.09(5)
1624,1655 cm−1
CN stretch
1638 cm−1
1639 cm−1
1581 cm−1
magnetic moment
5.73 μB
5.31 μB
a
b
Bond lengths are listed in Å. Bond angles are given in degrees.
Binding of the oxygen atom to the iron center was made
exposure of Fe[N(SiMe3)2]2(py)2 to 1 equiv of complex 1,
deprotonation of water by an amide gives rise to the transient
Fe(OH)[N(SiMe3)2](py)2 species. Subsequent deprotonation
of a pyrrole of the ligand then coordinates the ligand to the
Fe(II) center. Hydrogen bonding directs the formation of the
trigonal bipyramidal structure observed for 5. Alternatively, one
can invoke the deprotonation of both pyrrole N−H bonds by
the amines of the Fe[N(SiMe3)2]2(py) starting material,
releasing 2 equiv of amine, HN(SiMe3)2. Water then
coordinates to the metal center, and the third arm binds
datively, engaging in hydrogen bonding between the amine N−
H and the oxygen atom of water. Finally, deprotonation of the
water complex by a pendant imine gives rise to the formation of
the hydroxyl species 5.
However, we were unable to determine if the hydrogen
bonding network of the ligand scaffold prevents the ligand-
bound water molecule from interacting with the metal center
prior to coordination of the pyrroles during formation of
complexes 3 and 5. Two different synthetic pathways were
targeted to gain understanding of the mechanism for formation
of 5. When complex 4 is exposed to Li2O, a salt metathesis
occurs, resulting in the release of 2 equiv of LiOTf, which can
possible by refinement of H5, H6, and H7 in the structure of 5.
The O1−H5 bond distance of 0.757(19) Å is consistent with a
single covalent bonding interaction between these two atoms,
while the O1−H6 and O1−H7 distances of 1.54(2) and
1.75(2) Å, respectively, are reminiscent of hydrogen bonding
interactions. Thus, the secondary coordination sphere of
complex 5 is best defined as an Fe−OH moiety with hydrogen
bonding interactions to the imine nitrogen of a pyrrolidine−
imine arm of the ligand. The oxygen atom is also engaged in
hydrogen bonding from the adjacent, pendant N−H sub-
stituents (N6−H6 = 1.028(19) Å; N7−H7 = 0.90(2) Å)
formed as a result of tautomerization to the datively
coordinated amine−azafulvene. All three O1−NX (X = 5, 6,
7) distances fall within the appropriate range for hydrogen
bonding interactions (2.5653(18)−2.7939(18) Å). These two
distinct coordination modes of the ligand arms as pyrryl−imine
and azafulvene−amine are observed in the solid-state IR
spectroscopy, as denoted by the two distinct CN stretches
located at 1624 and 1655 cm−1, respectively.
Mechanistic Considerations. Mechanistically, two pro-
cesses can be invoked for the formation of 5. In the first, upon
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Glass Contour System (SG Water USA, Nashua, NH) and stored over
4 Å molecular sieves purchased from Strem following literature
procedure prior to use. Chloroform-d, tetrahydrofuran-d8, and
acetontrile-d3 were purchased from Cambridge Isotope Laboratories
and were degassed and stored over 4 Å molecular sieves prior to use.
Lithium hexamethyldisilazane was purchased from Sigma-Aldrich and
recrystallized from toluene under an inert atmosphere prior to use.
Potassium hydride was purchased from Sigma-Aldrich, washed with
hexanes to remove mineral oil, and dried under reduced vacuum prior
to use. Ferric chloride and ferrous chloride were purchased from Strem
and used as received. Cyclohexylamine was purchased from Acros and
used as received. Dimethylformamide (DMF) and sodium hydroxide
were purchased from Fisher and used as received. POCl3 and
hydrochloride etherate (2.0 M) were purchased from Sigma-Aldrich
and used as received. Tris(pyrryl-2-methyl)amine52 and ferrous
triflate53 were prepared according to literature procedures. Celite
545 (J. T. Baker) was dried in a Schlenk flask for 24 h under dynamic
vacuum while heating to at least 150 °C prior to use in a drybox. NMR
spectra were recorded at room temperature on a Varian spectrometer
operating at 500 MHz (1H NMR) and 126 MHz (13C NMR) and
referenced to the residual solvent resonance (δ in parts per million and
J in Hz). For paramagnetic molecules, the 1H NMR data are reported
with the chemical shift, followed by the peak width at half height in
Hertz, the integration value, and where possible, the peak assignment.
Elemental analysis was performed by Complete Analysis Laboratories,
Inc. in Parsippany, NJ and by the University of Illinois at Urbana−
Champaign School of Chemical Sciences Microanalysis Laboratory in
Urbana, IL.
be washed away with hexanes, and the subsequent formation of
complex 5 via the deprotonation of a pendant amine by [O2−].
Alternatively, the addition of HCl·Et2O to 3 cleanly resulted in
1
formation of 5, as assayed by H NMR spectroscopy. This
process is chemically reversible, as addition of 1 equiv of base
KCH2Ph to complex 5 results in the quantitative conversion to
the iron(II) aqua species 3.
The formation of complex 5 illustrates the propensity of
H3[N(piCy)3] to behave as both a hydrogen bond acceptor and
donor. The anionically coordinated arm, featuring an imine−
pyrrolidene extended network, makes available an imine
nitrogen for hydrogen bonding with a H atom donor, such as
the hydroxyl moiety in complex 5. Ligand tautomerization to
form the amine−azafulvene places hydrogen atoms on the
pendant nitrogens available to engage in hydrogen bonding
with the O atom acceptor. While solid-state characterization
through IR and X-ray crystallography defines complex 5 as the
hydroxyl species, the equivalency of the three arms of the tripod
on the NMR time scale may more aptly define this compound
as the hydronium species. The flexible network of hydrogen
bonding results in the generation of stable Fe(II) species.
CONCLUSION
■
We investigated the formation of high-spin iron(II) complexes
featuring the tris(5-cyclohexyliminopyrryl)amine H3[N(piCy)3],
ligand (1). The secondary coordination sphere of 1 is capable
of tautomerizing from the imine−pyrrole to the amine−
azafulvene both in a symmetric and unsymmetric fashion,
depending on additional axial coordination of a substrate as
well as the oxidation state of iron. Trianionic coordination of
the ligand to iron was achieved by prior in situ deprotonation of
the ligand with potassium hydride and subsequent addition of
FeCl2 to obtain K[(N(piCy)3Fe(H2O)] (3) in good yields. The
propensity of the ligand scaffold to form hydrogen bonds was
first exemplified in formation of the free ligand by inclusion of a
single water molecule and again during formation of 3.
Formation of [N(afaCy)3Fe](OTf)2, compound 4, exposed
the ligand’s electronic flexibility as it tautomerized from the
trianionic hydrogen bond-accepting pyrrole−imine platform to
the neutral azafulvene-amine hydrogen bond-donating archi-
tecture. IR spectroscopy as well as crystal structure determi-
nation was consistent with the ligand tautomerization.
Excitingly the various arms of the ligand can tautomerize
independently, depending on substrate as well as electronic
need of the iron center, which was evident in formation of the
iron−hydroxyl complex [N(piCy)(afaCy)2]FeOH (5). One arm
of the ligand scaffold is the anionic pyrrole, while the other two
arms of the tripodal system are coordinated to iron as the
neutral azafulvene. Both complexes 3 and 4 could be converted
to 5, as well as 5 to 3, clearly illustrating the flexibility of each
arm of the ligand scaffold to engage in independent
tautomerization. This insight into the dynamic nature of the
ligand framework allows for the future exploration of
multielectron transformations featuring the tripodal hydrogen
bond donor or acceptor ligand H3[N(piCy)3.
Preparation of H3(tpaCO). A 100 mL three-neck round-bottom
flask was charged with tris(pyrryl-2-methyl)amine (H3tpa) (2.000 g,
0.0079 mol) and approximately 40 mL of DMF. The tan solution was
cooled to −10 °C. POCl3 (4.4 mL, 0.0175 mol) was added dropwise
over 10 min with vigorous stirring. Following addition of the POCl3,
the solution was dark red. The mixture was removed from the ice bath
and heated to 60 °C for 2 h. The contents of the round-bottom flask
were poured into an aqueous solution of NaOH (6.00 g, 0.15 mol),
resulting in formation of a dark brown mixture. The contents were
heated to 80 °C for 1 h. The product was subsequently extracted with
dichloromethane and chloroform (1:1, 3 × 300 mL). Volatiles were
then removed under reduced pressure. The product was precipitated
from the crude reaction mixture as a brown powder (1.471 g, 0.0043
1
mol, 41%) by stirring in an excess of acetonitrile for 16 h. H NMR
(CDCl3, 25 °C): δ = 3.70 (s, 6H, −CH2), 6.18 (d, J = 5, 3H, Ar−CH),
6.92 (d, J = 5, 3H, Ar−CH), 9.40 (s, 3H, aldehyde−CH), 11.29 (br, s,
3H, pyr−NH). 13C NMR (CDCl3, 25 °C): δ = 51.80 (methylene-C),
111.07 (pyr-C), 123.55 (pyr-C), 133.40 (pyr-C), 139.49 (pyr-C),
179.91 (aldehyde-C). IR = 1651 cm−1 (CO), 3304 cm−1 (N−H).
Preparation of H3[N(piCy)3]·H2O (1). The proligand H3tpaCO was
synthesized in situ. The red oil is diluted with dry acetonitrile.
Cyclohexylamine (H2NCy) (2 mL) is added, and the mixture is stirred
for 16 h. The product H3[N(piCy)3]·H2O precipitates from solution as
a tan powder, which is isolated by filtration (2.534 g, 4.17 mmol, 53%).
Analysis for C36H51N7·H2O: Calcd C, 72.08; H, 8.91; N, 16.34. Found
1
C, 72.09; H, 8.94; N, 15.83%. H NMR (CDCl3, 25 °C): δ = 1.19−
1.85 (m, 30 H, Cy−CH), 3.09 (m, 3H, Cy−CH), 3.59 (s, 6H, −CH2),
6.02 (d, J = 4, 3H, Ar−CH), 6.89 (d, J = 4, 3H, Ar−CH), 8.02 (s, 3H,
imine−CH). 13C NMR (CDCl3, 25 °C): δ = 25.30 (Cy-C3,C5), 26.12
(Cy-C4), 35.33 (Cy-C2,C6), 50.82 (methylene-C), 69.12 (Cy-C1),
108.77 (pyr-C), 115.09 (pyr-C), 130.49 (pyr-C), 134.61 (pyr-C),
150.12 (imine-C). IR = 1638 cm−1 (CN), 3266 cm−1 (N−H).
Preparation of [N(piCy)3Fe] (2). A 20 mL scintillation vial was
charged with H3[N(piCy)3]·H2O (0.050 g, 0.083 mmol) and
approximately 7 mL of tetrahydrofuran (THF). KH (3.1 equiv,
0.011 g, 0.268 mmol) was weighed by difference and added as a solid
to the solution of ligand, resulting in vigorous effervescence. After it
was stirred for 2 h, the solution was filtered over Celite to remove
excess KH and was added to a 20 mL scintillation vial charged with 1
equiv of FeCl3 (0.014 g, 0.086 mmol) and approximately 3 mL of
THF. An immediate color change to dark brown was observed. The
mixture was stirred for 3 h, after which time solvents were removed
EXPERIMENTAL SECTION
■
General Considerations. All manipulations were carried out in
the absence of water and dioxygen using standard Schlenk techniques
or in an MBraun inert atmosphere drybox under a dinitrogen
atmosphere except where specified otherwise. All glassware was oven-
dried for a minimum of 8 h and cooled in an evacuated antechamber
prior to use in the drybox. Solvents were dried and deoxygenated on a
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under reduced pressure. The product was dissolved in benzene and
filtered over Celite to remove KCl. Volatiles were removed in vacuo to
afford the product as a dark brown powder (0.037 g, 0.059 mmol,
68%). Crystals suitable for X-ray analysis were grown from a
concentrated solution of THF and benzene (1:1) layered with
hexanes. IR: 1581 cm−1 (CN). Analysis for C36H51N7FeKCl: Calcd
C, 64.18; H, 7.18; N, 14.55. Found C, 64.61; H, 7.48; N, 14.21. Due to
similar solubilities of KCl byproduct, complete separation was not
pressure. The product was isolated as brown-yellow powder with LiCl
as a contaminant.
Synthesis of 5 via Addition of Acid to 3. A 20 mL scintillation
vial was charged with complex 3 (0.020 g, 0.029 mmol) and
approximately 3 mL of THF. HCl (1 equiv, 2 M) in diethyl ether
(0.015 mL, 0.030 mmol) was added via microsyinge with vigorous
stirring. An immediate color change to yellow-brown was noted. After
this solution was stirred for 30 min at room temperature, the solvents
were removed under reduced pressure. The product, complex 5, was
1
possible. H NMR (THF-d8, 25 °C) Broad resonances ranging from
1
characterized by H NMR spectroscopy. Because of the inability to
−15.7 to 33.0 ppm could not be reliably integrated or assigned
separate complex 5 from KCl, yields could not be determined via mass.
The reaction was repeated in a J. Young tube in the presence of an
internal standard of mesitylene. Conversion to compound 5 was found
to be quantitative (>99%).
Synthesis of 3 via Deprotonation of 5. A 20 mL scintillation
vial was charged with complex 5 (0.020 g, 0.031 mmol) and
approximately 4 mL of THF and cooled to −35 °C. KCH2Ph (1.5
equiv, 0.006 g, 0.044 mmol) was weighed by difference and added as
an orange solid to the iron species with vigorous stirring. A color
change to brown was noted. After this solution was stirred for 4 h at
room temperature, the solvents were removed under reduced pressure.
The residual oil was washed copiously with hexanes to remove residual
salts and the byproduct, toluene. The product was isolated as a brown-
orange powder in good yields (0.018 g, 0.026 mmol, 84%).
(Supporting Information, Figure S3). μeff = 5.73 μB.
Preparation of K[N(piCy)3Fe(OH2)] (3). A 20 mL scintillation vial
was charged with H3[N(piCy)3]·H2O (0.050 g, 0.083 mmol) and
approximately 5 mL of THF. With vigorous stirring, KH (3.1 equiv,
0.011 g, 0.268 mmol) was weighed by difference and added as a solid.
Evolution of hydrogen was noted. After it was stirred for 2 h, the
mixture was filtered over Celite and added to a slurry of FeCl2 (0.011
g, 0.087 mmol) in THF with two drops of pyridine (to aid in
solubility). An instantaneous color change to orange was observed.
Following 3 h of stirring at room temperature, solvents were removed
under reduced pressure. The product was washed with hexanes to
remove excess pyridine and was extracted with diethyl ether. Following
the removal of volatiles, the product was isolated as an orange powder
(0.042 g, 0.061 mmol, 73%). Crystals suitable for X-ray diffraction
were grown from a concentrated solution of complex 3 in THF that
was layered with hexanes at room temperature. Analysis for
FeKC36H50N7O·C4H8O: Calcd C, 62.89; H, 7.65; N, 12.84. Found
Synthesis of 5 via Addition of Base to 4. A 20 mL scintillation
vial was charged with complex 3 (0.050 g, 0.053 mmol) and
approximately 7 mL of THF. Li2O (1 equiv, 0.002 g, 0.066 mmol) was
weighed by difference and added as solid to the aforementioned
solution. The mixture was stirred overnight at room temperature. After
this time, the solution was filtered over Celite to remove excess Li2O,
the solvents were removed under reduced pressure, and the residual
brown oil was washed with diethyl ether to remove LiOTf formed over
the course of the reaction. The product was dried under vacuum,
resulting in the isolation of a yellow brown powder (0.033 g, 0.051
1
C, 62.45; H, 7.26; N, 13.17%. H NMR (THF-d8, 25 °C): δ = 95.60
(859, 3H, CH), 32.39 (60, 3H, CH), 28.56 (70, 3H, CH), 12.34 (88,
6H, CH2), 3.73−7.34 (33H, Cy−CH2), 0.17 (141, 3H, CH). IR: 1603
cm−1 (CN).
Preparation of [N(afaCy)3Fe]OTf)2 (4). A 20 mL scintillation vial
was charged with Fe(OTf)2(MeCN)2 (0.043 g, 0.100 mmol) and
approximately 10 mL of THF. H3[N(piCy)3]·H2O (0.060 g, 0.100
mmol) was weighed by difference and added as a solid to the cloudy
white solution at room temperature, resulting in an instantaneous
color change to yellow. After this solution was stirred for 30 min, the
solvents were removed under reduced pressure, leaving a viscous
yellow oil. Trituration with diethyl ether to remove excess free ligand
left the product [N(afaCy)3Fe]OTf)2 as a yellow powder (0.092 g,
0.098 mmol, 98%). Crystals suitable for X-ray analysis were grown
from a concentrated solution of 4 in THF layered with diethyl ether.
Analysis for FeC36H51N7O6F6S2: Calcd C, 48.77; H, 5.49; N, 10.48.
Found C, 48.82; H, 5.39; N, 10.37%. 19F NMR (CD3CN, 25 °C): δ =
−79.59 (SO3CF3) IR: 1637 cm−1 (CN), 3218 cm−1 (NH) 1739
cm−1 (SO).
1
mmol, 95%) identified as compound 5 via H NMR spectroscopy.
ASSOCIATED CONTENT
* Supporting Information
■
S
Spectral data for complexes 1−5; selected crystallographic data
and .cif files for 2−5. This material is available free of charge via
the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: fout@illinois.edu.
■
Preparation of [N(piCy)(afaCy)2]FeOH (5). A 20 mL scintillation
vial was charged with Fe[N(SiMe3)2]2(py)2 (0.041 g, 0.085 mmol)
and approximately 3 mL of THF. In a separate vial, H3[N(piCy)3·H2O
(0.050 g, 0.086 mmol) was dissolved in 5 mL of THF. The solution of
ligand was added dropwise to the metal complex, giving rise to an
instantaneous color change to yellow-brown. After the solution was
stirred for 30 min, the solvents were removed under reduced pressure,
and the resulting brown residue was washed with diethyl ether. The
product was isolated as a brown powder in high yields (0.055 g, 0.085
mmol, 99%). Analysis for FeC36H51N7O·C4H8O: Calcd C, 66.19; H,
8.19; N, 13.51. Found C, 66.60; H, 8.47; N, 13.45%. 1H NMR (THF-
d8, 25 °C): δ = 28.82 (27, 3H, CH), 27.71 (20, 3H, CH), 16.20 (57,
6H, CH2), 5.49−8.23 (33H, Cy−CH2), −3.86 (51, 3H, CH). IR:
1624, 1655 cm−1 (CN).
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors thank the University of Illinois at Urbana−
Champaign for financial support and Dr. Danielle Gray for
crystallographic assistance.
REFERENCES
■
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Alternative Synthesis of 5. A 20 mL scintillation vial was charged
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