Design of porphyrin-based ligands for the assembly of [d-block metal?:?calcium] bimetallic centers

Article abstract of DOI:10.1039/c6dt04647a

The association of different metals in stable, well-defined molecular assemblies remains a great challenge of supramolecular chemistry. In such constructs, the emergence of synergism, or cooperative effects between the different metal centers is particularly intriguing. These effects can lead to uncommon reactivity or remarkable physico-chemical properties that are not otherwise achievable. For example, the association of alkaline or alkaline-earth cations and transition metals is pivotal for the activity of several biomolecules and human-made catalysts that carry out fundamental redox transformations (water oxidation, nitrogen reduction, water-gas shift reaction, etc.). In many cases the precise nature of the interactions between the alkaline-earth cations and the redox-active transition metals remains elusive due to the difficulty of building stable molecular heterometallic assemblies that associate transition metals and alkaline or alkaline-earth cations in a controlled way. In this work we present the rational design of porphyrin-based ligands possessing a second binding site for alkaline-earth cations above the porphyrin macrocycle primary complexation site. We demonstrate that by using a combination of crown ether and carboxylic acid substituents suitably positioned on the periphery of the porphyrin, bitopic ligands can be obtained. The binding of calcium, a typical alkaline-earth cation, by the newly prepared ligands has been studied in detail and we show that a moderately large binding constant can be achieved in protic media using ligands that possess some degree of structural flexibility. The formation of Zn-Ca assemblies discussed in this work is viewed as a stepping stone towards the assembly of well defined molecular transition metal-alkaline earth bimetallic centers using a versatile organic scaffold.

Full text of DOI:10.1039/c6dt04647a

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Efficient extraction of sulfate from water using
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Design of porphyrin-based ligands for the assembly of
Received 00th January 20xx,
Accepted 00th January 20xx
[d-block metal:calcium] bimetallic centers
Matthieu Koepf,a Jesse J. Bergkamp,a‡ Anne-Lucie Teillout,a• Manuel J. Llansola-Portoles,a±‖
†
DOI: 10.1039/x0xx00000x
Gerdenis Kodis,a Ana L. Moore,a* Devens Gust,a* and Thomas A. Moorea*
www.rsc.org/
The association of different metals in stable, well-defined molecular assemblies remains
a great challenge of
supramolecular chemistry. In such constructs, the emergence of synergism, or cooperative effects between the different
metal centers is particularly intriguing. These effects can lead to uncommon reactivity or remarkable physico-chemical
properties that are not otherwise achieveable. For example, the association of alkaline or alkaline-earth cations and
transition metals is pivotal for the activity of several biomolecules and human-made catalysts that carry out fundamental
redox transformations (water oxidation, nitrogen reduction, water-gas shift reaction, etc.). In many cases the precise
nature of the interactions between the alkaline-earth cations and the redox-active transition metals remains elusive due to
the difficulty of building stable molecular heterometallic assemblies that associate transition metals and alkaline or
alkaline-earth cations in a controlled way. In this work we present the rational design of porphyrin-based ligands
possessing a second binding site for alkaline-earth cations above the porphyrin macrocycle primary complexation site. We
demonstrate that by using a combination of crown ether and carboxylic acid substituents suitably positioned on the
periphery of the porphyrin, bitopic ligands can be obtained. The binding of calcium, a typical alkaline-earth cation, by the
newly prepared ligands has been studied in detail and we show that a moderately large binding constant can be achieved
in protic media using ligands that possess some degree of structural flexibility. The formation of Zn-Ca assemblies
discussed in this work is viewed as a stepping stone towards the assembly of well defined molecular transition metal-
alkaline earth bimetallic centers using a versatile organic scaffold.
observed in both natural and synthetic systems, is the
modulation of transition-metal based redox processes induced
by redox-inactive alkaline or alkaline-earth cations.11-49 In
Introduction
Synthetic ligands designed to assemble hetero-metallic
complexes are key to various fields of research, including
molecular recognition, molecular devices, and catalysis.1, 2 In
the latter case, the association of different metals within a
well-defined organic framework can lead to the appearance of
cooperative behavior, or synergism, between the metallic
centers, sometimes giving rise to unique reactivity patterns
and unprecedented catalytic activities.3-10
nature, the oxygen evolving complex (OEC) found in
Photosystem II of photosynthetic organisms offers
a
remarkable example of such synergism; the association of a
calcium ion with four redox-active manganese centers is
critical for the enzyme's activity for water-oxidation.17, 19, 31, 41,
50-53
In the area of synthetic chemistry, some notable examples
of such synergism have been described as well. Since 1910,
iron doped with potassium has been known as a superior
catalyst for the Haber-Bosch process, providing higher
selectivity and activity than pure iron in the reduction of
molecular nitrogen to ammonia.11 Following this early
example, the alteration of the performance of heterogeneous
catalysts upon doping with alkaline or alkaline-earth cations
has been explored for platinum-catalyzed water-gas shift
reactions,20, 23 manganese-catalyzed water oxidation,22, 32, 34, 54
and other major industrially relevant processes.12, 13, 18
One important aspect of hetero-metallic synergism,
a School of Molecular Sciences, Arizona State University, Tempe, AZ 85287, USA.
† Current address: Laboratoire de Chimie et Biologie des Métaux, CEA–CNRS–UGA
UMR 5249, 17 rue des Martyrs, Bât. K’, 38054 Grenoble Cedex 9, France
‡ Current address: Department of Chemistry, California State University, 9001
Stockdale Highway, Bakersfield, CA, 93311, USA.
• Current address: Laboratoire de Chimie Physique d'Orsay, Université Paris-Sud
Bât. 349 - Campus d'Orsay, 15, avenue Jean Perrin, 91405 Orsay Cedex, France.
±
Current address: Institute for Integrative Biology of the Cell (I2BC), CEA, CNRS,
Université Paris-Sud, Université Paris-Saclay, F-91198, Gif-sur-Yvette cedex, France.
Electronic Supplementary Information (ESI) available: [detailed synthetic
procedure and characterization of the ligands, additional 1H NMR information,
details of the fluorescence anisotropy measurements, and UV vis titrations]. See
DOI: 10.1039/x0xx00000x
The study of synergistic effects between transition-metal
molecular complexes and alkaline, alkaline-earth, or other
redox-inactive cations in homogeneous conditions has
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attracted attention as well. Early studies highlighted the highlighted.27 Lability is partly due to the use of manganese-
effects of magnesium salts on the product distribution and oxo bridges as a major structural element of the heterometalic
kinetics of iron-porphyrin mediated electro-catalytic carbon assemblies. The stability of such bridges is indeed closely
dioxide reduction14 as well as the enhancement of the oxygen related to the redox state of the manganese ions and the pH of
transfer rate between chemically generated tetraamido the solution.72 Thus the development of robust, well-defined
manganese-oxo complexes and unsaturated substrates in the hetero-metallic complexes that associate manganese (or other
presence of alkali and alkaline-earth cations.15 Following these transition metals) and alkaline or alkaline-earth cations
examples, systematic studies were undertaken to investigate remains challenging in aqueous and more generally in protic
the control of the rates and stoichiometry of electron transfer media. The limited availability of appropriate ligand hampers
between organic or organometallic substrates and iron-, the extensive investigation of alkaline-earth / transition metal
manganese- and cobalt-oxo and peroxo complexes by the synergism in catalytic processes.
addition of scandium, calcium and other redox-inactive
In this manuscript, we present the design of novel organic
cations.21, 26, 28, 30, 42, 44, 45, 55 In the case of catalytically active scaffolds able to promote the association of first-row
iron and manganese complexes, the alteration of the high- transition metals and alkali-earth metals within compact, well-
valent metal-oxo derivatives’ properties by the addition of defined structures in protic media. Moreover, in order to
redox inactive cations was found to greatly affect the overcome some of the stability issues discussed previously, we
mechanisms of oxidative C-H bond cleavage, sulfoxidation, and have focused on the development of assemblies that do not
epoxidation reactions of benchmark substrates.33, 35, 36, 42, 43, 46, feature structural metal-oxo bridges. The synthesis and
56
Following these lines, the influence of calcium, barium, and characterization of various ligands will be presented as well as
1
strontium cations on the kinetics of direct oxygen activation by the results of H-NMR spectroscopy, UV-Vis spectroscopy, and
various iron(II), and manganese(II) complexes was investigated time resolved fluorescence anisotropy studies that investigate
38, 45, 57
as well.29,
Collectively, these examples highlight the the metal binding properties of the ligands in solution. We will
potential of redox-inactive cations to control specific catalytic focus on the complexation of Zn2+ and Ca2+ as model
processes in homogeneous conditions. However, in these transition-metal and alkaline-earth cations, respectively.
examples, the association of the different metals does not Because these metal ions lack unpaired electrons, NMR
result from the specific coordination of the cations within a spectroscopy can be used for characterization and binding
well-defined ligand scaffold. Thus, the resulting heterometallic studies.
assemblies are essentially transient species, and the control
and fine-tuning of their reactivity remains challenging.
Results and discussion
This limitation has recently been circumvented by the
introduction of ingenious ligands and assembly procedures,
developed mainly in the context of manganese chemistry.
These procedures permitted the preparation of well-defined
heterometallic Mn3O4An clusters (with A = Na, Ca, Sr, Sc, Zn, Y,
La, Ce, Nd, Eu, Gd, Tb, Dy, Yb, or Lu, and n = 1, 2).25, 39, 40, 48, 58, 59
Design of porphyrin-based N,O-bitopic ligands
Porphyrins and related tetrapyrrolic macrocycles are
known to strongly coordinate most of the transition-metals
within an N4-chelating motif.73,
74
Stable manganese, iron,
cobalt, and nickel porphyrins are commonly prepared and
used as active catalysts for various reactions such as carbon
dioxide reduction, oxygen reduction, water oxidation, and
oxygen transfer reactions.75-81 The beta- and meso-positions of
These constructs provided
a direct way to probe the
modulation of the redox properties of the manganese centers
upon doping the clusters with redox-inactive cations.25, 39, 40, 48
A linear correlation between the reduction potential of the
clusters and the Lewis acidity of the redox-inactive cations was
demonstrated.25, 39, 40, 48 However such a simple correlation is
not universal for assemblies of transition metals and redox-
inactive cations within a well-defined scaffold.60 In addition,
the reactivity of calcium- and scandium-doped Mn3O4A
clusters towards oxo transfer reactions on phosphines was
tested as a first step towards the investigation of the catalytic
activity of such species.37
Some of the aforementioned studies25,
attracted
58, 59
particular attention in the context of water oxidation, through
introduction of classes of synthetic models precisely mimicking
the core of the OEC.61 Other than these remarkable examples,
only
a limited number of studies have reported the
preparation of discrete alkaline-earth/manganese hetero-
62-71
and like the examples discussed
Figure 1: Bitopic N,O-ligands studied in this work. Class I ligands (M = H2) are
based on a strapped architecture imposing strong geometrical constraints
on the oxygen-rich binding site. Class II ligands (M = Zn) offer a more flexible
architecture for the oxygen-rich binding site. Reference ligands (M = Zn)
feature the main binding elements of class-II ligands grafted onto the N4-
binding site in a non-interacting fashion.
metallic assemblies,27,
above, most of them rely on the incorporation of oxo-bridges
within their structures. The labile nature of such assemblies in
aqueous conditions when submitted to a redox cycle has been
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the porphyrin macrocycle are readily available for synthetic functionalities distributed on the periphery of the porphyrin
modifications, making the latter a useful scaffold for building macrocycle as promising N,O-bitopic ligands (Figure 1). The
multi-topic ligands around a primary N4-chelating motif. It is architectures were designed to possess either a completely
noteworthy that alkaline-earth metals do not form stable preorganized (rigid) or a rather flexible oxygen-rich binding
porphyrin complexes in protic media, with the exception of site.
magnesium.82 In fact, alkaline-earth cations, and particularly
Class I ligands feature a fairly rigid (highly preorganized)
calcium, exhibit a strong preference for oxygen-rich ligands.83- structure in which the crown ether is covalently attached to
85
Therefore we investigated different strategies for building a the porphyrin macrocycle via two opposite meso-positions to
second coordination site for alkaline-earth cations using form a strap above the tetrapyrrolic N4-binding site (Figure 1).
oxygen donors positioned directly above the porphyrin The introduction of a 2-carboxyphenyl substituent on a third
macrocycle. Since crown ethers and carboxylic acid groups are meso-position of the porphyrin can lead to two thermally
among the most widely used ligands for the complexation of stable atropisomers (1a and 1b) due to the high energy barrier
alkaline-earth metals in protic media,85-89 we have investigated for rotation about the bonds joining the ortho-substituted aryl
combinations of these binding motifs around the porphyrin rings to the porphyrin macrocycle. Compound 1a is the
scaffold in order to prepare ligands exhibiting two highly primary target because for this atropisomer, the carboxylic
differentiated binding sites, i.e. bitopic N,O-ligands.
acid is orientated towards the crown ether and is expected to
A variety of crown ether appended porphyrins have favor the endo coordination of alkaline-earth cations within
already been reported. Two main designs can be distinguished, the cavity formed by the crown ether strap and the porphyrin.
wherein the crown ether moiety is: i) grafted on the periphery
Class II ligands were designed to offer a much more flexible
of the tetrapyrrolic macrocycle forming a simple dangling (adaptable) oxygen-rich binding site. For this purpose, a crown
secondary binding site or, ii) anchored at two points on the ether and a dicarboxylic acid moiety were grafted on two
porphyrin, forming a strap above the N4 chelating site.90 The opposite meso-positions of the porphyrin macrocycle. As in the
functionalization of the porphyrin macrocycle with carboxylic case of the class I ligands two thermally stable atropisomers
acid groups has been reported as well,81, 91-95 and was found to can be expected (2a and 2b) depending on the relative
be
a
viable strategy for preparing heterobimetallic orientation of the substituents with respect to the plane of the
complexes.96 To the best of our knowledge, only one type of porphyrin. Compound 2a is expected to allow the crown ether
carboxylate functionalized porphyrin has been tested for and the carboxylic acids to act in a cooperative way to
calcium binding and it led to the aggregation of the coordinate alkaline-earth cations, and position them above the
macrocycles in solution.93 It is worth noting that the porphyrin plane.
functionalization of a porphyrin with a crown ether bearing a
hanging carboxylic acid group was described as well, without Synthesis of the ligands
any indication of the ability of this specific ligand to form
alkaline-earth (heterometallic) complexes.97
The synthetic route to class I ligands was derived from a
strategy reported by Conte et al.,98 and is discussed in the
Inspired by these precedents, we defined two classes of
supporting information (cf. SI, section 1.2).
ligands combining carboxylic acids and polyether
Scheme 1: Synthetic scheme for ligand 2a. The same synthetic strategy was followed to prepare ligand 2b, starting from the atropisomer 11b (for which the
nitro group and diester substituents are facing towards the two opposite sides of the porphyrin macrocycle). a) BF3.Et2O, CH3Cl, Ar, RT; b) DDQ (dichloro-dicyano
quinone), CH2Cl2 RT; c) Zn(OAc)2, CHCl3, MeOH, RT; d) Zn, AcOH (acetic acid), CH2Cl2, Ar, RT; e) ClCH2COCl, K2CO3, CH2Cl2, Ar, O°C ─ RT; f) EtOH, Ar, 70°C; g) KOH,
EtOH, H2O, Ar, 50°C.
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Class II ligands were obtained following the general
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synthetic strategy depicted in Scheme 1. The precursor
porphyrin (11) was obtained as a mixture of two atropisomers
from the statistical acid-catalyzed condensation of
dipyrrylmethane with 2-nitrobenzaldeyhde and the acetal
derivative of dimethyl 2,2'-(2-formylbenzamido)diacetate.
Each atropisomer of the desired mixed-porphyrin, 11a and
11b
, could be isolated in its pure form after column
chromatography. Importantly, only relatively slow rates of
interconversion between the two atropisomers were observed
above 60°C. The latter could thus be engaged in further
reactions in their pure forms and were not subjected to
significant re-equilibration during the subsequent synthetic
steps. Reduction of the nitro functionality was conducted using
zinc(0) in the presence of acetic acid at room temperature.
Next, the amino-porphyrin 10a( or 10b) was acylated with
chloroacetyl chloride at low temperature, followed by
treatment of the resulting derivative 9a( or 9b) with 4-aza-18-
crown-6 in gently refluxing methanol. Finally, saponification of
the diester led to the desired ligands 2a(or 2b). In sharp
contrast to ligands 1a and 1b, 2a and 2b were poorly soluble in
most non-protic solvents in their free-base forms. Their
solubility increased, however, after metalation with zinc.
Therefore, only their zinc complexes will be considered in the
following studies.
Detailed synthetic protocols for the preparation of ligands
1a
R2 are described in the supporting information (cf. SI section
1).The conformations of ligands 1a 1b 2a and 2b were
, 1b, 2a, 2b, as well as for the reference compounds R1, and
,
,
1
assigned using H-NMR spectroscopy (cf. SI section 2.3 and 2.5
Figure 2: Effect of the addition of Ca2+ on the 1H-NMR spectra of 2a (A) and 2b (B).
for the detailed analysis).
The spectra were recorded for
2 mM solutions of the ligands in MeOD-5%
diisopropylethylamine (DIPEA) at 298 K. The aliphatic region is not shown for
clarity. Top spectra: free ligand (2 mM), and subsequent spectra recorded after the
addition of 1 and 5 equivalents of CaCl2. Bottom: spectra obtained after the
addition of excess of ethylenediaminetetraacetic acid (EDTA). Note the growth of a
sharp set of signals in the spectra of 2a as compared to the rather broad
massifs observed in the case of 2b upon addition of Ca2+. Triangles of identical
Calcium binding
1H-NMR spectroscopy
The interaction of Ca2+ with the newly prepared N,O-class I
and II ligands was probed using 1H-NMR spectroscopy in MeOD
containing 5% diisopropylethylamine (DIPEA). The organic base
was introduced to ionize the carboxylic acid substituents, thus,
permitting an optimal interaction of the ligand with Ca2+. The
choice of a bulky amine such as DIPEA was critical to avoid the
formation of competitive inclusion complexes of the
ammonium ions within the crown ether cavity and direct
coordination to the zinc(II) center.
color indicate spin systems exhibiting
J coupling interactions in COSY
experiments (cf. SI section 2.14); the vertical lines are added as a reference for
the initial chemical shifts of the protons of the free ligands.
interactions with Ca2+ ions (see Figure 2A and cf. SI section
2.8). Therefore, extensive studies were conducted on class II
N,O-ligands (2a and 2b).
We initially focused on the atropisomers 1a and 2a only. For
the first studies, aliquots of calcium chloride were added to 2
mM solutions of the porphyrins. In the case of ligand 1a no (or
only minor) evolution of the spectrum was observed at low
Ca2+ concentration (< 10 mM), but gradual shifts of the peak
positions were observed at higher Ca2+ concentrations (>20
mM), principally in the aromatic region (cf. SI section 2.7).
These observations show that 1a do not strongly interact with
the Ca2+ ions. On the contrary, ligand 2a exhibited dramatic
perturbations of its spectrum in the presence of low
concentrations of Ca2+ (< 1 mM) in both the aromatic and the
aliphatic regions, suggesting the development of strong
As shown in Figure 2, 2a and 2b are greatly affected by the
addition of Ca2+ in solution (cf. SI sections 2.8 and 2.9 for
spectra observed at intermediate Ca2+ concentrations). For
both atropisomers the appearance of new sets of signals is
observed, as well as a general broadening of the spectra which
is indicative of the existence of dynamic exchange among the
different species present in solution. The dynamic nature, and
thus fully reversible interaction, between the Ca2+ and the
porphyrin ligands was confirmed by the addition of
a
competing chelating agent, ethylenediaminetetraacetic acid
(EDTA), which led to the recovery of the initial spectra for both
ligands. Interestingly, while the introduction of Ca2+ results in a
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Table 1: Characteristic rotational correlation times determined from time-resolved fluorescence anisotropy measurements of 2 mM solutions of ligands 2a and 2b in MeOH-
5% DIPEA in presence of increasing amounts of calcium chloride. The samples were excited at 590 nm and the time-resolved fluorescence decays were recorded at 680 nm.
The measurements were performed at 298 K (cf. SI section 3 for the decay traces and a detailed description of the measurements). θ1, θ2 and θ3 are the rotational
correlation times that fit best the experimental data, r1, r2 and r3 are the pre-exponential factors (amplitudes) of each component used for the fitting (see SI section 3.1
Equation S2).
2a
2b
equiv. Ca2+
0
θ1 (ps)
370
r1(0)
0.12
θ2 (ns)
r2(0)
-
θ1 (ps)
480
r1(0)
0.17
θ2 (ns)
r2(0)
-
θ3 (ps)
r3(0)
-
-
-
-
± 10
370
± 0.02
0.13
± 10
± 0.03
0.1
0.2
0.4
1
-
-
-
-
-
-
N/A
N/A
N/A
480a
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
N/A
± 10
360
± 0.02
0.13
± 10
390
± 0.02
0.15
± 10
± 0.03
0.10
1.1
± 0.1
1.4
0.04
± 0.01
0.06
0.08
3.6
0.01
120
0.03
370a
± 0.02
0.11
± 0.01
± 0.7
± 0.01
± 20
± 0.01
5
370a
N/A
N/A
N/A
N/A
N/A
± 0.02
± 0.1
± 0.01
a fixed values; refer to the text for explanations.
general broadening and increase in complexity of the spectra affected by dilution, confirming that free ligand does not
of both atropisomers, the spectra of 2a consistently exhibit aggregate in solution (cf. SI section 3.2).
sharper and simpler signal patterns. This is particularly
noticeable for the addition of 1 to 5 equivalents of Ca2+. In this Evolution of the fluorescence anisotropy decay of ligand 2a in
concentration range, 2b features broad and poorly resolved the presence of Ca2+
signals (see Figure 2B), whereas in the case of 2a a distinct set
Next, calcium chloride aliquots were added to the ligand
of peaks emerges after the addition of 5 equivalents of Ca2+ solutions while keeping the concentration of porphyrin
(arrows Figure 2A). The assignment of this new set of peaks to constant. In the case of the ligand 2a, no major evolution of
a single, well defined species is further supported by the the fluorescence anisotropy decay is detected up to the
occurrence of a coherent spin-spin coupling pattern associated addition of 0.4 equivalents of Ca2+ (cf. SI section 3.4). The
with these signals (cf. SI section 2.10). Collectively, these system remains best described by a single-exponential decay
observations suggest that upon addition of 5 equivalents of in each case, and the rotational correlation times determined
Ca2+ the cis-atropisomer 2a forms a dominant, discrete stay close to the value observed for the ligand alone (θ1(2a) ~
[porphyrin
⊂
heterogeneous mixture of species in equilibrium is obtained.
Ca2+], complex whereas for the trans-isomer 2b
a
370 ps). The presence of complexed species under these
1
conditions is, however, indicated by H-NMR spectroscopy (cf.
In order to gain more insights into the nature of the species SI section 2.8). Therefore, minimal differences in the
observed in the NMR experiments, time-resolved fluorescence hydrodynamic properties of the complexed [2a Ca] and the
anisotropy was used to investigate their hydrodynamic free ligand 2a are inferred; the species present in solution
properties. The experimental conditions (solvent, remain undistinguishable by fluorescence anisotropy
concentration, temperature) were kept identical for both measurements, and θ1(2a) ~ θ1([2a
Ca]) ~ 370 ps.§ Notably,
⊂
⊂
techniques to permit a fair comparison.
this observation points towards the formation of discrete
mononuclear complexes between the calcium ions and the
porphyrin ligand 2a at low Ca2+ concentration. Upon increasing
the Ca2+ concentration (≥ 1 equivalent added) a bi-exponential
decay is required to describe the system (cf. SI section 3.4). In
order to minimize the variables and the error in the
determination of the rotational correlation times of the new
species, the first component was fixed to the rotation time
determined for the free/complexed monomeric ligands
Time-resolved fluorescence anisotropy
Free ligands (2a and 2b
)
As shown in Table 1
,
in the absence of Ca2+, each
atropisomer exhibits a single-component anisotropy decay,
with characteristic rotational correlation times of θ1(2a) = (370
± 10) ps and θ1(2b) = (480 ± 10) ps (cf. SI sections 3.2 and 3.3).
The difference observed between the rotation correlation
times of 2a and 2b points towards a marked difference in their
hydrodynamic properties (i.e. size, shape and solvatation cf. SI
section 3.1, Equation S3), and is compatible with the
observation of the two atropisomers.99 Importantly, in the
absence of Ca2+ the rotational correlation time of 2a is not
2a/[2a⊂Ca] (θ1 ~ 370 ps). The second component of the
fluorescence anisotropy decay is found to be significantly
longer, with a rotational correlation time of θ2 = (1.1 ± 0.1) ns.
This observation indicates the formation of multimeric
assemblies of the ligand 2a at higher Ca2+ concentration. The
appearance of an equilibrium between monomeric and
multimeric porphyrin species in the presence of excess Ca2+ is
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further supported by the observation of an even slower equivalent of Ca2+ for 2b and 2a, respectively) pointing
component in the fluorescence anisotropy decay after the towards a higher propensity of the trans-atropisomer 2b to
addition of 5 equivalents of Ca2+, with θ2 = (1.4 ± 0.1) ns, which form extended assemblies in the presence of divalent cations.
is indicative of the aggregation of 2a into larger assemblies. This behavior can be explained by the antipodal orientation of
This behavior reflects the multivalent nature of the crown the crown ether and diacid moieties in the latter atropisomer,
ether and diacid moieties, both of which can lead to the and parallels the 1H-NMR titration experiments discussed
development of coordination bonds with multiple cations and above. The observation of a shorter rotational correlation time
in different geometries. In the following the multimeric (θ3) is surprising, since an increase in the rotation speed of the
assemblies discussed above will be noted as [2ax
⊂
Can]
.
ligand upon addition of Ca2+ would suggest an apparent
decrease of its hydrodynamic radius. The occurrence of a rapid
Evolution of the fluorescence anisotropy decay of ligand 2b in depolarization induced by Förster resonance energy transfer
the presence of Ca2+
(FRET) within the extended porphyrin aggregates [2by Cam]
⊂
Upon the addition of 1 equivalent of Ca2+, the fluorescence provides a possible explanation for the observation of this fast
anisotropy decay of 2b is characterized by a triple exponential component in the fitting of the fluorescence anisotropy
decay (Table 1; cf. SI section 3.5). Assuming the co-existence of decay.100 The occurrence of FRET in 2b assemblies would
different species in equilibrium, including monomeric 2b suggest a mode of association which favors a close proximity
species, one component was fixed to the rotational correlation of the chromophores within the assemblies, and clearly differs
time determined for the free ligand θ1 ~ 480 ps. The two from the mode of association of the cis-atropisomer 2a
additional components are then characterized with rotation observed upon the addition of 1 equivalent of Ca2+.
correlation times of θ2= (3.6 ± 0.7) ns and θ3 = (120 ± 20) ps
respectively.§§ The longest rotation correlation-time (θ2) can discrete monomeric porphyrin complexes between the ligand
be attributed to the presence of multimeric [2by
Cam] species 2a and Ca2+ ions at low Ca2+ concentration, we turned to UV-
in solution, resulting from calcium-induced aggregation of Visible absorption spectroscopy to get an estimation of the
ligand 2b Interestingly, under identical conditions, association constant of the complexes in methanol.
θ2([2by
θ2([2ax
After having confirmed the preferential formation of
⊂
.
⊂
Cam]) was found to be significantly longer than
UV-Visible absorption spectroscopy
⊂
Can]) (3.6 ns and 1.1 ns upon the addition of 1
Figure 3: Evolution of the UV-Visible absorption spectra of ligands 2a, ;
R2 and 8a upon addition of Ca2+ in MeOH-5% DIPEA, at 298 K. (A) ~5 μM solution of 2a
free ligand (solid black line); after addition of 10 equivalents of CaCl2 (solid red line); after the addition of an excess of ethylenediaminetetraacetic acid (EDTA)
(dashed blue line); inset: details of the Soret band. (B) ~ 1 μM solution of 2a with the addition of 0 (black curve) to 4 equivalents of CaCl2 (blue curve); inset:
absorbance at 412.2 nm (black squares) and 412.8 nm (red circles) and corresponding fitted curves assuming the formation of a 1:1 [2a
μM solution of R2 with the addition of 0 (black curve) to 180 (blue curve) equivalents of CaCl2; inset: absorbance at 411.0 nm (black squares) and 411.3 nm (red
squares) and corresponding fitted curves assuming the formation of a 1:1 [R2 Ca] complex. (D) ~2.5 μM solution of 8a (see scheme 2) with the addition of 0
(black curve) to 250 (blue curve) equivalents of CaCl2; inset: absorbance at 411.9 nm (black squares) and 412.8 nm (red squares) and corresponding fitted curves
⊂Ca] complex. (C) ~2.5
⊂
assuming the formation of a 1:1 [2a
⊂Ca] complex.
6 | J. Name., 2012, 00, 1-3
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carboxylic acids.93 In sharp contrast, ligand R2 bearing only a
Ligand 2a
crown ether substituent was found competent for Ca2+
As shown in Figure 3A and in the Supporting Information complexation. As was observed for 2a, the addition of calcium
(cf. SI section 4.2), the addition of calcium chloride aliquots to chloride aliquots to a micromolar solution of R2 induces a
a micromolar solution of 2a induces a gradual shift of the gradual shift of the spectrum of the ligand, which results in a
ligand’s spectrum, resulting in a slight red shift (0.6 nm) and a minor red shift (0.3 nm) and slight decrease of the absorption
decrease in the absorption amplitude of the Soret band of the intensity of the Soret band of the porphyrin (Figure 3C and cf.
porphyrin. Addition of a competitor for binding the Ca2+ SI section 4.4).
cations, namely EDTA, results in the recovery of the initial
The titration of a solution of R2 at 2.5 μM in MeOH-5%
spectrum of 2a, and confirms that the spectral evolution DIPEA, with up to 180 equivalents of Ca2+, leads to the gradual
followed in these experiments is due to the specific interaction shifting of the maximum of the Soret band absorption from
between 2a and Ca2+ ions.
411.0 nm to 411.3 nm. In that concentration range a well-
To limit the aggregation of the species observed by 1H- defined isosbestic point centered at 411.9 nm is maintained
NMR spectroscopy and fluorescence
anisotropy (Figure 3C and cf. SI section 4.4). Increasing the ratio of Ca2+ to
measurements (experiments carried out in the millimolar R2 above 200 leads to the loss of the isosbestic point and
range), the UV-Visible titration experiments were executed suggests the occurrence of additional or competing
under high-dilution conditions (1 μM). At this concentration coordination events at high Ca2+ concentration. Considering
and with the addition of amounts up to 4.5 equivalents of Ca2+, the large body of studies reporting the coordination of divalent
the maximum of the Soret band absorption shifts from 412.2 cations by crown ether derivatives,86-88 it is reasonable to
nm to 412.8 nm. Importantly, a well-defined isosbestic point assume that the first binding event (< 180 equivalents of Ca2+
centered at 414.1 nm is observed (cf. SI section 4.2), and added) corresponds to the formation of a simple 1:1 [R2⊂Ca]
implies the establishment of a simple equilibrium between the molecular complex. Thus, using the same model as above we
free ligand 2a and a well-defined complex.§§§ This observation obtain an estimated association constant of Ka(R2) ~ (8.1 ±0.4)
suggests an initial bimolecular equilibrium, with the formation x 103 M-1).
of simple 1:1 [2a
⊂
Ca] molecular complexes (cf. SI section 4.1,
Ligand 8a behaved very similarly to R2. Upon addition of 0
Equation S4). The evolution of the absorption of 2a at 412.2 to 250 equivalents of Ca2+ to a solution 8a at 2.5 μM in MeOH-
and 412.8 nm is well described using this simple model and 5% DIPEA, a modest red shift (0.9 nm) and decrease of the
leads to the estimation of an association constant of Ka (2a) ~ absorption intensity of the Soret-band is observed (Figure 3D).
(2.4 ±0.2) x 106 M-1 for this first binding event (Figure 3B insert; During the titration the maximum of the Soret-band
cf. SI section 4.1).
absorption shifts from 411.9 nm to 412.8 nm. Furthermore, a
In order to probe the degree of cooperativity between the well-defined isosbestic point appears at 413.0 nm, and
crown ether and the diacid moieties for the complexation of suggests the occurrence of a simple equilibrium under the
Ca2+ by 2a, the binding abilities of reference compounds R1 conditions tested (cf. SI section 4.5). For the same reason as
and R2 (Figure 1) and of the diester precursor 8a (Scheme 2) before, we attributed this first event to the formation of a
were investigated as well.
simple 1:1 [8a⊂Ca] molecular complex, and could estimate an
association constant of Ka(8a) ~ (2.9 ±0.1) x 104 M-1.
Reference ligands R1
,
R2 and 8a
As summarized in Table
2
,
the UV-Visible titrations
R2, and 8a) and target (2a
In the case of R1 which bears the diacid moiety only, slow conducted on the reference (R1
)
precipitation of the ligand occurred upon addition of Ca2+ (cf. ligands, point towards a cooperative binding of Ca2+ by the
SI section 4.3), and therefore, no further analysis was diacid groups and the crown ether moiety in 2a; the latter
conducted on this compound. It is worth noting that this exhibiting the largest association constant of all the tested
behavior is reminiscent of the previously reported alkaline- ligands. Importantly, for the structures and conditions
earth induced aggregation of porphyrins bearing multiple investigated in the present work, the presence of the crown
ether moiety is critical to avoid calcium-induced precipitation
of the acid-functionalized porphyrins. The ester precursor of
2a (i.e. 8a) shows a surprisingly high affinity for Ca2+, with an
Table 2: Association constants for the first elemental Ca2+ complexation event
observed for micromolar solutions of R1, R2, 8a, and 2a, in MeOH containing 5%
association constant Ka(8a) about one order of magnitude
of DIPEA at 298 K.
higher than Ka(R2) and two orders of magnitude lower than
Ligand
Ka (M1)
R1
R2
8a
2a
Ka(2a). Just as in the case of calcium-binding proteins, in which
carbonyls groups from the peptidic backbone are frequently
found in the coordination sphere of Ca2+,84 it is plausible that
for 8a, the carbonyl oxygen atoms of the ester groups are
involved in the cooperative binding of Ca2+ with the crown
ether moiety and lead to the increase of the association
constant as compared to R2. Furthermore, it is interesting to
note that 8a shows very little tendency to aggregate or diverge
from a simple binding isotherm during the titration with Ca2+.
8.1 ±0.4
x 103
2.9 ±0.1
x 104
2.4 ±0.2
x 106
n/aa
a No association constant estimated due to precipitation of the ligand upon
exposure to Ca2+
.
Determined from 2.5 μM ligand solution in MeOH-5%
b
DIPEA, upon addition of 0 to 180 equivalents of Ca2+
.
c Determined from 2.5
to 250
Determined from a 1 μM ligand solution in MeOH-5%
DIPEA upon the addition of 0 to 4 equivalents of Ca2+
μM ligand solution in MeOH-5% DIPEA upon addition of
0
equivalents of Ca2+
.
d
.
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A robust isosbestic point is observed, even at high Ca2+ to 8a alkaline-earth metals in discrete heterometalic assemblies.
ratios (up to 250) and supports the establishment of a simple Operating in protic media, these constructs can provide
equilibrium between two well-defined species (presumably frameworks for developing bio-inspired catalytic systems such
[8a] and [8a⊂Ca]). This contrasts with the more complex as the water oxidizing complex in photosynthesis. Indeed, after
behavior observed for 2a, which rapidly diverges from a single suitable functionalization porphyrins complexes are amenable
equilibrium at high concentration of Ca2+ and/or of the ligand. to electrocatalysis at significantly positive potentials.
However, despite this slightly more complex behavior, the
introduction of ionizable groups within the ligand scaffold (i.e.,
Acknowledgements
the carboxylic acids groups) increases the association constant
with Ca2+ by two orders of magnitude, which is expected for
The work at ASU was supported by the Center for Bio-Inspired
Solar Fuel Production, an Energy Frontier Research Center
funded by the U.S. Department of Energy, Office of Science,
Office of Basic Energy Sciences under Award Number
DESC0001016, and the Office of Basic Energy Sciences, Division
of Chemical Sciences, Geosciences, and Energy Biosciences,
Department of Energy under contract DE-FG02-03ER15393.
ligands that are able to provide charge compensation upon
complexation.87,
88
The enhanced association constant
observed for 2a is of particular relevance for using this class of
ligands in aqueous environment. Indeed, the association
constants between macrocyclic ligands and alkaline and
alkaline-earth cations are three to four orders of magnitude
larger in pure methanol than in water, due to the decrease in
the desolvation enthalpy of the cations in solvents of lower
dielectric constant.87,
88
Thus 2a represents a promising
Notes and references
§
scaffold for building ligands for the assembly of discrete
transition metal/alkaline-earth hetero-metallic complexes in
aqueous conditions.
The presence of free and complexed species of quasi identical
hydrodynamic radius is further supported by experiments
conducted at low concentration. For a 0.1 mM solution of 2a in
MeOH-5% DIPEA, at 298 K, and in the presence of 0.1 mM of
CaCl2, the fluorescence anisotropy decay of the porphyrin
remains best fitted with a single exponential decay with a life
time of 370 ps. In similar conditions the 1H NMR spectrum
exhibits a well-defined set of peaks that can be attributed to
monomeric complexed species, cf. SI section 3.2 (anisotropy)
Conclusions
The introduction of crown ether and carboxylic acid
substituents on porphyrin macrocycles is a fruitful strategy for
building a secondary binding site for oxophilic cations in the
apical position of the N4-chelating motif of porphyrins. In this
work we used this strategy to design a versatile class of bitopic
N,O-ligands, able to drive the formation of stable transition
metal/alkaline-earth hetero-metallic complexes in solution. As
a proof of principle, we characterized the association of Zn2+
and Ca2+ cations in protic media.
and section 2.11 (1H NMR).
§§
The fitting of the anisotropy decay of 2b in the presence of
Ca2+ with three floating variables led to values of the three
components reasonably close to those determined upon fixing
one component to the rotation correlation time of the
monomeric ligand (cf. SI section 3.5).
§§§
Upon addition of more Ca2+, or in presence of a higher
concentration of the ligand, the isosbestic point was rapidly lost
and no meaningful fitting could be undertaken.
We demonstrated that the use of semi-flexible oxygen rich
substituents is critical for the design of a competent binding
site for labile alkaline-earth cations at the periphery of the
rigid porphyrin macrocycle. It is likely that the introduction of
flexible, partly pre-organized, binding motif is advantageous
because the binding pocket remains free to reorganize in order
to optimize its interaction with the desired cations. This
approach led us to the synthesis of a ligand exhibiting a
moderately large association constant with the labile Ca2+
(Ka(2a) ~ 2.4 (±0.2) x 106 M-1) in methanol. This result is
encouraging for the development of fully water soluble ligands
based on the same binding motifs and the association of
alkaline-earth cations and transition metals within a compact
organic scaffold in aqueous environment. Importantly, the
cooperative binding of Ca2+ by the crown ether and the diacid
moiety grafted on two opposite meso-positions of the
porphyrin macrocycle in 2a implies the binding of the Ca2+ in
the apical position of the N4-chelating motif of the porphyrin
and in close proximity to a metal atom chelated by the
porphyrin macrocycle.
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