Synthesis and characterization of a tris(2-hydroxyphenyl)methane-based cryptand and its triiron(iii) complex

Article abstract of DOI:10.1039/c2dt30312d

Reaction of tris(5-amino-2-ethoxy-3-isopropylphenyl)methane and pyridine-2,6-dicarbonyl-dichloride affords a multi-dentate cryptand in 48% yield. Metallation with iron(iii) chloride results in a substantial conformational change of this ligand to give a trianionic triiron(iii) complex. Ferric cations line the periphery of the internal cavity with each adopting a square pyramidal N₃Cl₂ coordination environment.

Full text of DOI:10.1039/c2dt30312d

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COMMUNICATION
Synthesis and characterization of a tris(2-hydroxyphenyl)methane-based
cryptand and its triiron(III) complex†
Gary L. Guillet,a Forrest T. Sloane,a Matthieu F. Dumont,b Khalil A. Abbouda and Leslie J. Murray*a
Received 11th February 2012, Accepted 16th April 2012
DOI: 10.1039/c2dt30312d
Previously, multimetallic complexes of cofacial diporphyrins,
pacman porphyrins, and bis(TREN) cryptands, among others,
have been carefully designed to preorganize metal centers
around an active site, in which the ligand orients accessible
metal coordination sites to react with substrates cooperatively.7
Rational catalyst design, however, requires synthetic control of
the electronic and structural properties of the metal centres,
which cannot be achieved without sacrificing synthetic ease or
yield for porphyrin- and TREN-based ligands. Despite these
drawbacks, macrobicycles remain attractive candidates because
libraries of multimetallic complexes can be readily synthesized
using a modular approach. We sought to develop macrobicyclic
ligands, in which synthetic precursors with versatile functional
groups could be easily accessed. In this regard, tris(2-hydroxy-
phenyl)methanes are an ideal candidate because the synthesis of
these triarylmethanes tolerates various functionalities on the
reactant phenol and salicylaldehyde.8 Here we report the syn-
thesis and metallation of the first cryptand to be synthesized
from tris(5-amino-2-ethoxy-3-isopropylphenyl)methane and the
corresponding triiron(III) complex. This result demonstrates a
route to potentially create tunable ligands for metal clusters by
varying the functional groups on the phenols and
salicylaldehyde.
Reaction
of
tris(5-amino-2-ethoxy-3-isopropylphenyl)
methane and pyridine-2,6-dicarbonyl-dichloride affords a
multi-dentate cryptand in 48% yield. Metallation with iron(III)
chloride results in a substantial conformational change of
this ligand to give a trianionic triiron(III) complex. Ferric
cations line the periphery of the internal cavity with each
adopting
environment.
a
square pyramidal N3Cl2 coordination
Proteins that catalyze multielectron redox reactions combine a
tailored active site pocket with multimetallic assemblies, where
the metal clusters provide the redox equivalents to drive the reac-
tion, and the active site enforces substrate selectivity.1 The
design of synthetic catalysts to replicate the reactivity of metallo-
proteins is a long standing research goal with substantial focus
on the synthesis and properties of metal complexes that are struc-
tural mimics of enzymatic metal clusters. However, these struc-
tural analogues2 have yet to operate as effective catalysts for
multielectron redox reactions.3
The design of molecular receptors that selectively recognize
specific analytes is well-developed in organic synthesis.4,5 These
types of molecules are constructed with a carefully tailored
binding site that recognizes the target analyte selectively based
on size, as dictated by the shape and volume of the binding site
relative to the target molecule, and electrostatic or hydrogen
bonding interactions between functional groups incorporated on
the interior surface of the binding cavity and complementary
groups on the analyte. Metal ions, such as copper(II), have been
incorporated into organic receptors, where the metal ion serves
as a Lewis acid for analyte recognition rather than as a redox-
active catalytic centre.6 Realizing the interplay between the
active site and metal cofactors in enzyme systems, our approach
focuses on incorporating multiple metal coordination sites into
molecular receptors as a means to combine metal cooperativity
with substrate selectivity.
Reaction of phenol 1 and salicylaldehyde 28 with thionyl
chloride in methanol affords the substituted triphenoxymethane
3 (Scheme 1).9 From prior reports, we anticipated that the alkoxy
groups would preferentially align to one molecular face with the
nitro-substituents in 4 oriented to the opposite face.9,10 Alky-
lation followed by nitration afforded 4, and subsequent reduction
yielded our desired building block 5 in multigram quantities.9,11
Ease of purification for each synthetic step compensated for the
low overall yield of 5. Condensation of two equivalents of 5
with three of pyridine-2,6-dicarbonyl dichloride afforded the
desired cryptand H6L in 48% yield (Scheme 1). We attribute the
unusually high yield for the final step in our synthetic route to
the alignment of the amines to one molecular face, which has
been proposed for cyclophanes assembled from 1,3,5-
triethylbenzene.12
From the single crystal X-ray structure, the pyridine dicarbox-
amide arms of H6L adopt a propeller arrangement and preassem-
ble around a 126 Å3 central cavity (Fig. 1).13 The distances
between the amide nitrogen atoms range from 6.577(4) Å for
N6–N3 to 7.130(3) Å for N9–N3.‡ Expectedly, these metrics are
similar to the distances between similar substituents in com-
pounds analogous to 5, suggesting that distances between metal
aDepartment of Chemistry, Center for Catalysis, University of Florida,
Gainesville, FL 32611-7200, USA. E-mail: murray@chem.ufl.edu;
Tel: +001 352-392-0564
bDepartment of Physics and National High Magnetic Field Laboratory,
University of Florida, Gainesville, FL 32611-7200, USA
†Electronic supplementary information (ESI) available: Synthesis, spec-
troscopic data, crystallographic data and cyclic voltammagrams. CCDC
866977–866978. For ESI and crystallographic data in CIF or other elec-
tronic format see DOI: 10.1039/c2dt30312d
7866 | Dalton Trans., 2012, 41, 7866–7869
This journal is © The Royal Society of Chemistry 2012

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Fig. 2 A portion of the crystal structure of [K(MeCN)2]3[(FeCl2)3L]
depicting the triiron(III) anionic complex. The iron centers all adopt a
square pyramidal coordination environment and surround the central
cavity. The two tris(alkoxyphenyl)methane caps adopt opposite propeller
conformations in contrast to the free ligand. Fe, Cl, C, N, and O atoms
are represented by orange, green, grey, red and blue spheres, respectively.
Isopropyl chains, ethoxy groups, K+ ions, H-atoms and solvent mol-
ecules are omitted for clarity.
Scheme 1 Synthesis of the multi-dentate cryptand, H6L. (i) SOCl2,
MeOH (ii) EtI, NaH, DMF, (iii) HNO3, TFA, DCM (iv) RANEY Ni, H2(g)
,
THF/EtOH.
(III) complexes are observed, including the potassium adduct of
the parent complex, K[(FeCl2)3L]2−, as well as species in which
one or more chloride ligands are substituted for hydroxide. In
the UV-visible absorption spectrum of the product complex, we
observe a strong absorption band centred at 294 nm (ε = 43 900
M−1 cm−1) and a broad feature in the visible region (524 nm, ε
= 3980 M−1 cm−1). These absorption maxima are comparable to
those reported for other ferric complexes of substituted pyridine-
2,6-dicarboxamides.16,17 Attempts to deprotonate the H6L with
stronger bases, such as methyllithium, were unsuccessful and
afforded only a mixture of products. Although related triaryl-
methanes undergo oxidation via formal hydride loss at the
1
methine carbon atom,10b the H-NMR spectra of reaction mix-
tures suggest that this proton can be abstracted by stronger bases,
subsequently leading to ligand decomposition.
Fig. 1 Crystal structure of H6L·(THF)3(H2O)2. The ligand adopts a
propeller-like structure in which the three pyridine dicarboxamide arms
define an internal cavity. A guest THF molecule is held within the cavity
by two hydrogen bonds. C, N, O and H atoms are represented by grey,
blue, red and light gray spheres, respectively. Isopropyl chains, ethoxy
groups, hydrogen atoms (except the two that are depicted), and non-
coordinated solvent molecules were omitted for clarity.
Diffusion of diethyl ether into an acetonitrile solution of the product
afforded dark red–brown crystals of [K(MeCN)2]3[(FeCl2)3L]
(Fig. 2).§ In the molecular structure of the complex, the iron–
iron distance is 6.723(2) Å and the distance between the chloride
donors housed within the interior of the complex is 3.413(3) Å.
Each potassium cation is coordinated by two acetonitrile mol-
ecules from the crystallization solvent and two oxygen donors –
one from an amide and the other from the ethoxy group on an
adjacent ligand arm (Fig. 3, O1 and O4A). From the large
metal–metal distances and absence of bridging ligands between
the iron centres, we expected minimal electronic communication
between the metal centres in the complex.
binding sites can be varied by rational choice of the tripodal tri-
amine. Guest solvent molecules occupy the central cavity and
grooves between the arms of H6L. The THF guest within the
cavity forms hydrogen bonds with two amides on one pyridine
dicarboxamide arm with N–H⋯O bond distances of 2.35(3) and
2.14(3) Å and angles of 146(3)° and 158(3)°, which are compar-
able to those for interactions between THF and other amides.14
The four remaining amides from the other pyridine dicarboxa-
mide arms form hydrogen bonds with THF and water molecules,
all of which occupy the grooves between ligand arms (Fig. S3†).
Treatment of a THF solution of H6L with potassium hexa-
methyldisilazide followed by ferric chloride at −78 °C afforded
the dark red–brown trianionic [(FeCl2)3L]3− complex. In ESI
(−)/MS of the product solution,15 ions corresponding to triiron
The steric demands of H6L enforce a 1 : 1 stoichiometry for
each pyridine-2,6-dicarboxamide donor and iron(III) cation,
which has not been realized for monometallic ferric complexes
employing
phenyl-substituted
pyridine-2,6-dicarboxamide
ligands.16–18 In [(FeCl2)3L]3−, each arm coordinates one iron(III)
with the metal adopting a slightly distorted square pyramidal
coordination geometry (τ = 0.05 with a 0.9% tetragonal distor-
tion19) (Fig. 3). The basal plane is comprised of the pyridyl
N-atom, two deprotonated carboxamides and one chloride anion,
with an axial chloride donor completing the primary
This journal is © The Royal Society of Chemistry 2012
Dalton Trans., 2012, 41, 7866–7869 | 7867

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Fig. 3 Portions of the crystal structures of [K(MeCN)2]3[(FeCl2)3L]
(left) and H6L (right) depicting the conformational changes upon metal-
lation. The pyridyl ring in the metal complex is almost perpendicular to
the neighbouring phenyl rings (left) whereas the angles between the
planes of two phenyl rings and the pyridyl are 36° and 50° in H6L
(right). Each K+ cation is coordinated to two acetonitrile molecules from
the crystallization solvent, and two O-atoms from an amide (O1) and an
ethoxy group (O4A). Fe, Cl, K, C, N and O atoms are represented as
orange, green, sky blue, gray, blue and red, respectively. H-atoms and
solvent are omitted for clarity.
Fig. 4 Cyclic voltammogram of [K(MeCN)2]3[(FeIIICl2)3L]. Scan rate
of 100 mV s−1 using 2 mm Pt working, Pt wire counter, and Ag/Ag+
reference electrode in 0.1 M Bu4NPF6 DMF solution.
(Fig. 4).16 The large ΔEp of 256 mV is consistent with a struc-
tural change upon reduction,24 although we cannot discount
weak electrostatic coupling between the metal centres, which
may give rise to three overlapping waves. No change was
observed in the amplitude of either the cathodic or anodic waves
after repeated scanning, suggesting that there is no ligand or
complex decomposition during the reaction. Electrochemical
data collected on H6L and the potassium salt of L6−, which was
generated in situ with KHMDS, support our assignment of this
process as metal-based (Fig. S7†). From magnetic susceptibility
measurements on [K(MeCN)2]3[(FeCl2)3L], the saturation value
of χMT at room temperature of 7.4 emu K mol−1 is comparable
to the expected theoretical value for three isolated S = 3/2 iron
(III) centres (Fig. S10†).
In conclusion, we have reported the synthesis and characteriz-
ation of a tris(hydroxyphenyl)methane-based cryptand and its
corresponding trianionic triiron(III) complex. For the former, the
ligand arms preassemble a central void space, capable of accom-
modating guest species. The triiron(III) complex serves as a syn-
thetic proof of principle as the metal cations decorate the internal
cavity, and are primed to react cooperatively with a bound sub-
strate. Although the metal centers appear to be site-isolated in
the iron(III) complex, we are exploring routes to displace the
equatorial chloride ligands to allow substrates to bind and bridge
the metal centres and turn on cooperativity. Preliminary work
suggests that the chloride–chloride distance depends on the
metal oxidation state, opening the possibility of designing com-
plexes as redox-dependent host–guest systems.
coordination sphere. The metal ion sits 0.570(2) Å above the
N3Cl plane, comparable to other square pyramidal iron(III) com-
plexes with N3-donor sets.20 The Namide–Fe bond distances of
2.064(5) and 2.104(5) Å and the Namide–Fe–Namide bond angle
of 145.1(2)° are similar to those reported for the high-spin [Fe-
(PyPSMe)2]− complex.17 Consistent with these structural simi-
larities to the high-spin complex, a room temperature magnetic
moment of 10.08 μB was determined for [(FeCl2)3L]3− by
Evan’s method using the parallel field formula,21 which agrees
with three S = 3/2 ferric ions. Coordination of the K+ cation to
O1 likely weakens the donor strength of N1, and results in the
longer Namide–Fe bond distance as compared to N3–Fe bond.
Comparison of the structures of the [(FeCl2)3L]3− and the as-
synthesized ligand reveals a significant structural change upon
metal binding. In H6L, both triphenoxymethane caps have the
same chirality and the amide groups in each arm are not coplanar
with either the pyridyl or phenyl rings (Fig. 3, right). Conse-
quently, the angles between the planes of neighbouring pyridyl
and phenyl aromatic rings are 36° and 50°, with almost an up-
down conformation for the two phenyl rings relative to the
pyridyl one. After deprotonation and metallation, however, the
angles between these aromatic ring planes increase to 75° and
83° with concomitant reorganization from the helical arrange-
ment in H6L to one in which the two triphenoxymethane caps
adopts opposite propeller conformations (Fig. 2 and 3). The
almost perpendicular disposition of the pyridyl ring versus the
two neighbouring phenyl rings in [(FeCl2)3L]3− is consistent
with that observed for monometallic complexes of substituted N,
N′-diphenylpyridine-2,6-dicarboxamide.16,17 This structural
change consequently rotates the equatorial chloride away from
the interior cavity, and could be an important feature for substrate
access and/or product egress in future efforts. The significant
ligand reorganization after metallation is reminiscent of the con-
formational changes observed during catalytic turnover in many
enzyme systems22 as well as the structural changes reported for
molecular devices.23 Methods to exploit this rearrangement to
enhance reactivity or to respond to external stimuli are currently
being explored.
This work was funded by the University of Florida and the
National Science Foundation (CHE-0821346). KAA acknowl-
edges the National Science Foundation (CHE-1048604) and the
University of Florida for funding the purchase of the X-ray
equipment. The authors thank Noelle Elliot for help with mass
spectrometry and Prof. Mark Meisel for discussions on molecu-
lar magnetism.
Notes and references
The cyclic voltammogram of [K(MeCN)2]3[(FeCl2)3L] in
‡Crystal data for H6L·(THF)3·(H2O)2: C101H129N9O17, M = 1714.13,
triclinic, space group P1, a = 17.9708(2), b = 19.0228(3), c = 19.2837
(3) Å, α = 73.327(1)°, β = 67.465(1)°, γ = 66.260(1)°, V = 5504.7(1)
Å3, Z = 2, T = 100 K. 67 204 reflections collected, 18 677 independent
reflections, 9470 reflections with (I > 2σ(I)), R1 (I > 2σ(I)) = 0.1096,
wR2 (all data) = 0.1609.
ˉ
1
anhydrous DMF shows a quasi-reversible response with E =
2
−403 mV (vs. Fc–Fc+) assigned to the FeIII–FeII couple, which
is comparable to the value reported by Mukherjee and co-
workers for a bis(pyridine-2,6-dicarboxamido)iron(III) complex
7868 | Dalton Trans., 2012, 41, 7866–7869
This journal is © The Royal Society of Chemistry 2012

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§Crystal data for [K(MeCN)2]3[(FeCl2)3L]: C101.5H107Cl6Fe3-
K3N15.5O12, M = 2233.58, cubic, space group P213, a = b = c = 22.853
(2) Å, α = β = γ = 90°, V = 11934(1) Å3, Z = 4, T = 100 K, 5832 reflec-
tions collected, 5832 independent reflections, 4639 reflections with (I >
2σ(I)), R1 (I > 2σ(I)) = 0.0794, wR2 (all data) = 0.1684.
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This journal is © The Royal Society of Chemistry 2012
Dalton Trans., 2012, 41, 7866–7869 | 7869