Kinetic studies of the coordination of mono-And ditopic ligands with first row transition metal ions

Article abstract of DOI:10.1021/acs.inorgchem.5b02931

The reactions of the ditopic ligand 1,4-bis-(2,2′:6′,2 -Terpyridin-4′-yl)benzene (1) as well as the monotopic ligands 4′-phenyl-2,2′:6′,2 -Terpyridine (2) and 2,2′:6′,2 -Terpyridine (3) with Fe²⁺, Co²⁺, and Ni²⁺ in solution are studied. While the reaction of 1 with Fe²⁺, Co²⁺, and Ni²⁺ results in metallo-supramolecular coordination polyelectrolytes (MEPEs), ligands 2 and 3 give mononuclear complexes. All compounds are analyzed by UV/vis and fluorescence spectroscopy. Fluorescence spectroscopy indicates that protonation as well as coordination to Zn²⁺ leads to an enhanced fluorescence of the terpyridine ligands. In contrast, Fe²⁺, Co²⁺, or Ni²⁺ quench the fluorescence of the ligands. The kinetics of the reactions are studied by stopped-flow fluorescence spectroscopy. Analysis of the measured data is presented and the full kinetic rate laws for the coordination of the terpyridine ligands 1, 2, and 3 to Fe²⁺, Co²⁺, and Ni²⁺ are presented. The coordination occurs within a few seconds, and the rate constant increases in the order Ni²⁺ < Co²⁺ < Fe²⁺. With the rate constants at hand, the polymer growth of Ni-MEPE is computed.

Full text of DOI:10.1021/acs.inorgchem.5b02931

Article
pubs.acs.org/IC
Kinetic Studies of the Coordination of Mono- and Ditopic Ligands
with First Row Transition Metal Ions
Stefanie Martina Munzert, Guntram Schwarz, and Dirk G. Kurth*
Chemische Technologie der Materialsynthese, Julius-Maximilians-Universitat Wurzburg, Rontgenring 11, D-97070 Wurzburg,
̈
̈
̈
̈
Germany
S
* Supporting Information
ABSTRACT: The reactions of the ditopic ligand 1,4-bis-
(2,2′:6′,2″-terpyridin-4′-yl)benzene (1) as well as the mono-
topic ligands 4′-phenyl-2,2′:6′,2″-terpyridine (2) and
2,2′:6′,2″-terpyridine (3) with Fe2+, Co2+, and Ni2+ in solution
are studied. While the reaction of 1 with Fe2+, Co2+, and Ni2+
results in metallo-supramolecular coordination polyelectrolytes
(MEPEs), ligands 2 and 3 give mononuclear complexes. All
compounds are analyzed by UV/vis and fluorescence spec-
troscopy. Fluorescence spectroscopy indicates that protonation
as well as coordination to Zn2+ leads to an enhanced
fluorescence of the terpyridine ligands. In contrast, Fe2+,
Co2+, or Ni2+ quench the fluorescence of the ligands. The
kinetics of the reactions are studied by stopped-flow
fluorescence spectroscopy. Analysis of the measured data is presented and the full kinetic rate laws for the coordination of
the terpyridine ligands 1, 2, and 3 to Fe2+, Co2+, and Ni2+ are presented. The coordination occurs within a few seconds, and the
rate constant increases in the order Ni2+ < Co2+ < Fe2+. With the rate constants at hand, the polymer growth of Ni-MEPE is
computed.
interactions. For instance, we could make predictions of the size
of metallo-supramolecular assemblies as well as their dynamic
response to an external stimuli if the binding constants and
ligand substitution rates were available.
The reaction of metal ions and ligands in solutions generally
involves successive replacement of coordinated solvent
molecules by the appropriate donor atoms of the incoming
coordinating ligands. According to Eigen and Wilkins9 the
central metal ion (M2+) and the ligand (L) first form an outer-
sphere complex followed by the rate-determining step, the
replacement of a solvent molecule (S) by the ligand in the inner
sphere complex:10−12
INTRODUCTION
■
Metal ion coordination of organic ligands and metal ions results
in a plethora of structures ranging from mononuclear
compounds, to discrete nanosized assemblies, soluble coordi-
nation polymers, gels, and solid state networks.1−4 The metal
ions bring structural and functional properties to these systems
that are relevant for functional materials. The ligand operates as
organizing element in positioning and ordering the metal ions
in the final assembly. The interplay of coordination geometry,
thermodynamic, and kinetic properties of the metal ions and
the structural features of the ligands results in one or a set of
well-defined metallo-supramolecular architectures. If under the
particular conditions the metal ion ligand interaction is
reversible, the resulting macromolecular assemblies are
equilibrium structures that can respond to external stimuli.
Reversible metal ion coordination offers an attractive alternative
to reversible covalent bonds as pH, counterions, and solvent are
additional variables in the equilibrium. Their uses are as rich as
their structures ranging from catalysis,5 molecular magnetism,6
electrochromic devices,7 and gas storage1,2 to self-healing
materials.8
K0
kex
[M(S)m]2+ + L ⇌ [M(S)m]2+, L → [M(S)m−1L]2+, S
(1)
with K0 being the equilibrium constant for the formation of the
outer-sphere complex, and kex being the rate constant for
exchange of a solvent molecule between the inner sphere and
the bulk solvent. Reactions following this rate law are
controlled by the rate of water ligand dissociation, are largely
insensitive to the nature of the incoming ligands and are
governed by ligand field effects.13 The reaction scheme predicts
the rate law to be first-order in M2+ and in L. With M2+ in
Despite the widespread use of metal ion coordination the
mechanisms of formation in MOFs, coordination networks and
soluble polymers are poorly understood. Tailoring the structure
and properties of ever increasing complex metallo-supra-
molecular architectures requires a better understanding of the
kinetic and thermodynamic properties of metal ion ligand
Received: December 18, 2015
© XXXX American Chemical Society
A
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excess, k is the second-order rate constant for the overall
forward reaction:
Scheme 1. Metal Ion Induced Self-Assembly of the Metal
Ions Fe2+, Co2+, Ni2+, and Zn2+ with 1,4-Bis(2,2′:6′,2″-
terpyridine-4′-yl)benzene (1) Resulting in Metallo-
a
k = K0kex
(2)
Supramolecular Coordination Polyelectrolytes (MEPEs)
If a negative volume of activation is measured, the mechanism
is associative (A). In this case, the intermediate shows an
increased coordination number. In contrast, in dissociative
substitution mechanism (D), the intermediate shows a reduced
coordination number. Due to the mainly bond-breaking
character of dissociative substitution mechanisms the volume
of activation is positive. When the reaction is concerted and
there is partial, and equal, association and dissociation of the
entering and leaving groups, it is an interchange mechanism.
Moreover, we can distinguish the interchange mechanism (I)
with an associative (Ia) or a dissociative activation mode (Id), in
which the transition states are characterized mainly by bond-
formation or bond-breaking, respectively.12−15 The water
exchange reaction of first row transition metal ions, i.e. Mn2+,
Fe2+, Co2+, Ni2+ as well as the coordination of these metal ions
to bi- and terpyridine ligands were studied by Caldin,10
Ellgen,11 Hayward,16 Hubbard,17 Merbach,18 Moore,19 van
Eldik,20 and Wilkins.21,22 In water exchange studies, Co2+ and
Ni2+ show positive volume of activation indicating a dissociative
interchange (Id) mechanism. However, Fe2+ shows an almost
zero value and the value for Mn2+ is negative. These results are
rationalized in terms of a mechanistic changeover along the
series with Mn2+ showing associative and Ni2+ dissociative
behavior, respectively.17,18 Measurements of volumes of
activation indicate that coordination reactions of the metal
ions Fe2+, Co2+, and Ni2+ with bi- and terpyridines take place as
an Id mechanism. Thus, the changeover in the solvent-exchange
mechanism for the first row transition-metal elements also
applies to complex formation reactions involving these metal
ions and neutral ligands.20 Finally, there is evidence that the
chemical nature of the entering ligand also affects the rate, e.g.
in chelation-controlled substitutions.23
Coordination polymers based on polytopic pyridine ligands
and their interactions with metal salts have been investigated by
Constable,24,25 Kurth,7,26,27 Newkome,28 Stuart,4 Rowan,8,29
and Schubert.30,31 The terpyridine receptor, connected back to
back, is ideal if linear assemblies are of concern. The binding
strength is sufficient to support macromolecular assemblies
even with kinetically labile metal ions. With many metal ions
coordination results in a pseudooctahedral geometry. In case of
rigid bis-terpyridines, such as 1, rigid-rod like metallo-
supramolecular coordination polyelectrolytes (MEPEs) form
in solution (Scheme 1).
The assembly of metallo-polymers from ditopic bisterpyr-
idines is assumed to proceed in two steps. Wilkins et al.22 have
shown that the addition of a terpyridine ligand to the metal ion
is controlled by the first attachment, followed by rapid
completion of the chelate.35 Each step is associated with a
binding constant, K1 and K2, respectively (Scheme 1).22
Generally, K2 ≫ K1 for the coordination of terpyridines with
the metal ions Fe2+, Co2+, and Ni2+. Hence, the bis-tpy-
complex, that is the species of metal ion complexed to two
terpyridine ligands (see Scheme 1) is the preferred species even
if an excess of metal ions is present in solution.36−38 If K1 is
larger than K2, species with one metal ion coordinated to one
ligand-molecule would be favored. Therefore, coordination of
ligand 1 with an excess of Zn2+ leads to [Zn2(1)]4+
complexes.4,22
a
4′-Phenyl-2,2′:6′,2′′-terpyridine (2) and 2,2′:6′,2″-terpyridine (3)
form mononuclear complexes with Fe2+, Co2+, Ni2+, and Zn2+. The
compounds are shown in part a. The ligands are trans−trans
configured in ethanol and cis−cis in HOAc due to hydrogen bonding
between the protons and the free electron pair of the central nitrogen
atom.32−34 As shown in part b, K1 and K2 are the binding constants for
the stepwise coordination of 1 to the metal ion, whereas part c
illustrates the overall reaction of the components. The acetate
counterions are omitted for clarity.
Previously, we showed that ditopic bis-terpyridines form
extended metallo-supramolecular coordination polyelectrolytes
(MEPEs) in solution with transition metal ions such as Fe2+,
Co2+, or Ni2+.39 The MEPEs can be incorporated in various
architectures, including thin films,39 liquid crystals,40 or
nanostructures41 and exhibit low- or high-spin states, and it is
possible to switch between the two states at moderate
temperatures. Changing the spin state modifies magnetic and
optical properties of the complex. Thus, MEPEs are suitable
systems for spin-crossover or thermochromic materials.42
Finally, MEPEs exhibit excellent electrochromic properties.7,43
In order to understand the growth and complexation of MEPE
polymers based on Fe2+, Co2+, Ni2+, and ligand 1, we present a
comprehensive analysis of the coordination kinetics using
stopped-flow measurements in solution. The monotopic ligands
4′-phenyl-2,2′:6′,2″-terpyridine (2) and 2,2′:6′,2″-terpyridine
(3) (Scheme 1) are used for control experiments. Based on the
kinetic data presented by Wilkins et al.22 it is expected that
coordination in MEPEs occurs on a time scale, which is readily
available by conventional stopped-flow methods.
B
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acid, but less soluble in polar solvents such as water, EtOH, or
MeOH. At a concentration of at least 10−3 M, which is needed
for measuring meaningful absorption spectra, ligand 1 forms a
precipitate in EtOH or MeOH giving rise to scattering. For this
reason, the absorption spectrum of ligand 1 is recorded in
acetic acid solution (75% vol) (Figure 1a). The UV/vis
absorption spectra of MEPEs recorded in acetic acid solution
(75 vol %), EtOH, or H2O do not show any significant
difference in the spectral envelope.
Protonated ligand 1 displays two distinct absorption bands
associated with π−π* transitions at 292 and 321 nm. Due to
the low solubility in EtOH, ligand 1 shows a broad and
featureless absorption band at around 290 nm in ethanol (see
Figure S1 in the Supporting Information); at a concentration
sufficient for measuring absorption spectra the solution turns
turbid due to agglomeration of ligand 1.32−34 Similar to the
protonated ligand 1, the spectra of MEPEs show two π−π*
transitions with band maxima occurring at around 287 and 321
nm, respectively. Coordinating to the metal ion forces the
terpyridine receptor to the cis−cis configuration.34,44 The blue
color of Fe-MEPE originates from the additional absorption
band at approximately 580 nm, a metal-to-ligand-charge-
transfer (MLCT) band, typical for Fe-terpyridine complexes.45
Co-MEPE has a red color originating from the d−d transitions
in the range from 400 to 550 nm.46 The orange color of Ni-
MEPE is associated with a d−d transition at 341 nm, the π−π*
transitions are slightly shifted to 294 and 332 nm. [Zn(1)]2+ is
colorless due to its d10 electron configuration.31
RESULTS AND DISCUSSION
■
Here, we are interested in the overall rate that is the formation
of the bis-terpyridine complexes. The rate is determined from
the fluorescence quenching of the ligands 1, 2, and 3 upon
metal ion binding with Fe2+, Co2+, and Ni2+. We assume that
the emission signal is proportional to the concentration of
uncoordinated terpyridine receptors. Furthermore, we expect
that the selected metal ions have a strong preference for
binding two terpyridine receptors because K2 ≫ K1. As a result,
the emission decay that is the consumption of terpyridine
receptors is directly related to the overall rate of two terpyridine
groups coordinating to the metal ion. The emission as a
function of time is monitored with the stopped-flow technique.
The rate, v, of coordination is obtained from the slope of the
emission decay. First, we present the absorption and
fluorescence properties of the ligands and the corresponding
complexes, followed by a discussion of stopped-flow analysis.
Here, we focus on the coordination of the ligands to Fe2+, Co2+,
and Ni2+ given that the coordination of Zn2+ to ligand 1 does
not lead to polymers as mentioned above.
Figure 1 shows the UV/vis absorption spectra of ligand 1 in
acetic acid solution (75 vol %), as well as of Fe-, Co, Ni-, and
Next, we present the fluorescence data of the complexes.
While aqueous acetic acid would be the solvent of choice, we
decided to measure the fluorescence spectra in EtOH. First,
acetic acid can cause corrosion of the stopped-flow apparatus.
Second, fluorescence spectra are recorded at concentrations of
10−4 M and below. At these concentrations, ligand 1 and all
other species are soluble in EtOH. Finally, the protons of acetic
acid would compete with the metal ions in coordination, adding
a further variable to the equation. Figure 2a shows the
fluorescence spectra of ligand 1 and [Zn(1)]2+ recorded in
ethanol.
The concentration of 1 is 10−5 M in both samples. At this
concentration we obtain a clear solution, yet enough signal to
record fluorescence spectra. The low intensity of the Rayleigh
scattering signal suggests that ligand 1 is dissolved and that
aggregates, which form at higher concentrations, are absent.
Formation of aggregates at higher concentrations is readily
observed by an increasing Rayleigh scattering signal.47
Dissolved in EtOH ligand 1 shows a broad emission band
with a maximum at 360 nm (Figure 2a). We note that in acetic
acid solution (75 vol %) 1 reveals an enhanced emission band
at 438 nm. Similarly, [Zn(1)]2+ shows an increased emission,
with a maximum at 389 nm. Protonation of the terpyridine
receptors as well as coordination of Zn2+ enhances the
fluorescence intensity due to the chelation-enhanced-fluores-
cence (CHEF) effect (see Figure S2a in the Supporting
Information).48
Figure 1. Absorption spectra of ligand 1 in acetic acid solution (75 vol
%), as well as of [Zn(1)]2+, Fe-, Co-, and Ni-MEPE in (a) acetic acid
solution (75 vol %), (b) EtOH, and (c) H2O recorded at 20 °C. The
concentration of the metal ions and of ligand 1 is 10−3 M in all
samples.
Ligands 2 and 3 show fluorescence bands at 338, 344, and
352 nm, respectively (see Figure 2b). The complexes
[Zn(2)2]2+ and [Zn(3)2]2+ also show an increased emission
intensity compared to the uncoordinated ligands, due to the
CHEF effect.
[Zn(1)]2+ in (a) acetic acid solution (75 vol %), (b) EtOH, and
(c) H2O.
With simple counterions like acetate, MEPEs are soluble in
water, aqueous acetic acid, and polar solvents like EtOH or
MeOH. On the other hand, ligand 1 is soluble in aqueous acetic
The fluorescence intensities of the monotopic ligands 2 and
3 in EtOH (intensity maxima 229 and 174 au) in Figure 2b are
higher compared with the intensity of the ditopic ligand 1 in
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Figure 3. Fluorescence spectra (a) of ditopic ligand 1 on addition of
Ni(OAc)2·(H2O)4 (excitation wavelength λe = 292 nm) and (b)
fluorescence intensity at λmax = 360 nm as a function of stoichiometry,
Figure 2. Fluorescence spectra of (a) ligand 1 (dissolved in acetic acid
solution (75 vol %) and EtOH) and [Zn(1)]2+ (excitation wavelength
λe = 292 nm, dissolved in EtOH), (b) 2 and [Zn(2)2]2+ (λe = 289 nm,
EtOH), 3 and [Zn(3)2]2+ (λe = 281 nm, EtOH). Spectra are recorded
at 20 °C with 10−5 M concentration of 1, 2, and 3, respectively.
y = [Ni2+]/[1]. Spectra are recorded in EtOH at 20 °C with 10−5
concentration of 1.
M
as a function of increasing Ni2+ concentration and thus as a
function of decreasing concentration of uncoordinated ligand 1.
The dependence of fluorescence intensity IF on the
concentration, c, is given by a form of the Lambert−Beer law:
EtOH (Figure 2a, intensity maximum 69 au). It appears, that a
photoinduced-electron-transfer (PET)48 effect may be respon-
sible for the decreased fluorescence of the free and
unprotonated ditopic ligand 1 in EtOH (see Figure S2b in
the Supporting Information). We assume, that the PET effect
in EtOH is more effective for the ditopic ligand 1, consisting of
two terpyridine receptors, as for the monotopic ligands 2 and 3.
In contrast, the complexes containing Fe2+, Co2+, and Ni2+ do
not show fluorescence (see Figure S3 in the Supporting
Information). The fluorescence quenching is associated with
the coordination of the metal ion to the terpyridine receptor.
As reported in the literature, the presence of transition-metal
ions with empty or half-filled d-orbitals like Cr3+, Mn2+, Fe2+,
Fe3+, Co2+, Ni2+, and Cu2+ quenches the fluorescence
presumably through a ligand-to-metal charge-transfer mecha-
nism giving rise to a nonradiative deactivation of the excited
singlet state of the ligand.49−52 In the nonfluorescent
complexes, the excited singlet state of the ligands 1, 2, and 3
are likely to act as energy donors. The transition metal ions
Fe2+, Co2+ and Ni2+ possess empty or half-filled orbitals (d6, d7,
and d8 configuration, respectively), which can be involved in an
energy-transfer mechanism where they act as energy-transfer
acceptors in a ligand-to-metal charge-transfer. This leads to a
nonradiative deactivation of the photoexcited fluorophores 1, 2,
and 3.49−52 Due to the d10 configuration, Zn2+ does not
participate in such an energy-transfer process.50 Degassing the
solutions has no significant effect on the fluorescence properties
of all samples.
IF = I0Φ(1 − e−εcd
)
(3)
with I0 being the intensity of incident radiation, Φ being the
fluorescence quantum yield, ε being the absorptivity, and d
being the path length. For low concentrations (εcd ≪ 1), the
fluorescence intensity is proportional to the concentration:47
IF = I0Φεcd
(4)
This is consistent with the observed linearity in Figure 3b,
where the intensity is proportional to the concentration of Ni2+
and uncoordinated terpyridine receptor, respectively.
To quench the emission of the ditopic ligand completely by
coordination with Ni2+, the metal ion to ligand ratio has to be y
= 1. In this case, both terpyridine receptors of the ligand are
occupied, except one receptor at the very end of the Ni-MEPE
chains. If y = 0.5, two ligands will associate to [Ni(1)2]2+
complexes, because K2 ≫ K1. As shown in Figure 3,
fluorescence intensity for Ni-MEPE at y = 0.5 is quenched to
half of the ligand’s initial intensity (y = 0). The metal ion
quenches only the coordinating terpyridine receptor, not the
entire ditopic ligand. The same results are obtained for Fe2+ and
Co2+ (see Figure S4 in the Supporting Information). This result
proves that in the investigated concentration range, fluores-
cence quenching is linearly related to the coordination of
bisterpyridine metal ion complexes.
We use fluorescence spectroscopy to monitor the emission
intensity decay of the fluorescing ligands 1, 2, and 3 as a
function of coordination to the metal ions Fe2+, Co2+, and Ni2+.
Controlled mixing of the reagents is achieved with a stopped-
flow method. In this technique, the ligand solutions 1, 2, or 3
and the solutions containing the metal ions Fe2+, Co2+, or Ni2+
are forced with sample syringes into the observation cell, where
Figure 3 shows (a) the fluorescence spectra and (b) the
fluorescence intensity of ligand 1 as a function of Ni2+
concentration. The stoichiometry, y, of metal ion to ligand
concentration is varied from y = 0 (no Ni2+ metal ions present)
to y = 1.0. As the stoichiometry approaches one, the
fluorescence is fully quenched as discussed above. Figure 3b
shows the fluorescence intensity at λmax = 360 nm as a function
of y. We observe a linear decrease of the fluorescence intensity
D
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the reaction mixture is studied by fluorescence spectroscopy.
For the reasons mentioned above the reactions are carried out
in ethanol at concentrations of 10−4 M and below. For the
concentration range of interest a calibration curve is recorded
that relates ligand concentration to emission signal as described
in the Experimental Section. In the concentration range a linear
relationship between concentration of ligand and emission
signal is observed.
1.0, the emission of uncoordinated terpyridine receptors is
detected (Figure 4, thick solid line). These results are in
agreement with the steady-state measurements discussed above.
The reaction rate is determined from the slopes of the
concentration/time traces at t = 0 (see the Experimental
Section). The data is extrapolated to t = 0 because the dead
time of the stopped-flow measurement prevents recording data
at t = 0. The slope is determined from the curve fit of the
experimental data. The resulting rates, v, for the reaction of 1
and Ni2+ are summarized in Table 1.
The data is analyzed in the following way. The relation
between the rate of a reaction and the concentration of a
chemical species is given by
Table 1. Rates v at t = 0 for the Reaction of 1 with Ni2+ at
Different Concentrations in EtOH at 20 °C
v = k[M2+]a0[L]b0
(5)
with v being the rate of the reaction, k being the rate constant,
[L]0 being the initial concentration of the respective ligand 1, 2,
or 3, and [M2+]0 the initial concentration of the respective
transition metal ion. The exponents, a and b, are defined as the
order of the reaction.15 Wilkins et al.22 proved that the
coordination reactions are first-order in metal ions and first-
order in monotopic terpyridine ligands. However, the order of
reaction in ditopic terpyridine ligand 1 is investigated. For this
purpose, the order is determined by keeping the concentration
[L]0 constant, while [M2+]0 is varied and vice versa. With a and
b at hand, we are able to calculate the rate constant k and
therefore the full rate law for the coordination reaction.
[1]0 (mol L−1
)
[Ni2+]0 (mol L−1
)
v (mol L−1 s−1
)
1.0 × 10−4
1.0 × 10−4
1.0 × 10−4
1.0 × 10−4
1.0 × 10−4
1.0 × 10−4
2.5 × 10−4
5.0 × 10−4
7.5 × 10−4
1.0 × 10−3
1.0 × 10−4
2.7 × 10−4
5.5 × 10−4
7.8 × 10−4
1.1 × 10−3
Higher initial concentrations of the metal ion Ni2+ lead to
higher rates, as Figure 4 already indicated. The reaction order a
is calculated by plotting ln[v] and ln[Ni2+]0 (see Figure S6 in
the Supporting Information) according to
We use the method of initial rates15 to estimate the order of
the reaction related to each reactant. The initial rate varies with
the concentration of the respective reactant. Higher initial
concentrations of the metal ion result in a faster quenching of
the terpyridine’s emission. Thus, the rate of coordination
increases with increasing initial concentration of the metal ion
(see Figure 4 and Figure S5 in the Supporting Information).
v = k[Ni2+]a0[1]b0
⇔ln[v] = aln[Ni2+]0 + ln[k] + b ln[1]0
The data follows a straight line with a slope of approximately
one indicating that the reaction is first order in Ni2+. Likewise,
the order in ditopic ligand 1 is determined by using a constant
initial concentration of the metal ion [Ni2+]0 and varied initial
concentrations of ditopic ligand [1]0. Applying the same
procedure as for the metal ion the order in 1 is found to be one.
The overall order of the coordination reaction of 1 and Ni2+ is
therefore second-order. With a and b equal to one, the rate law
reduces to
(6)
(7)
v = k[Ni2+]0[1]0
(8)
The reaction order can be confirmed for the coordination of
Fe2+ and Co2+ to the ditopic terpyridine ligand 1. The rate
constant k is now determined by plotting v, that is the slope at t
= 0, against different initial concentrations of [Ni2+]0 and [1]0.
Figure 5 shows the obtained data, for k we obtain a value of k =
(8.0 0.4) × 103 M−1 s−1.
Figure 4. Concentration of uncoordinated ditopic ligand [1] as a
function of time for different stoichiometries, y = [Ni2+]0/[1]0 in
EtOH at 20 °C. The initial concentration of [1]0 is 10−4 M, while that
of Ni2+ is increased from 5 × 10−5 to 10−3 M. The concentration of
uncoordinated 1 is determined from the emission intensity via a
calibration curve (excitation wavelength λe = 292 nm).
As the reaction proceeds, the concentration of uncoordinated
terpyridine receptors decreases, which is monitored as decaying
emission signal. Plotting the emission intensity against time
results in decreasing exponential curves, as shown exemplary in
Figure 4 for the reaction of 1 and Ni2+. Under these
experimental conditions, the coordination of 1 to Ni2+ takes
place within a few seconds. Figure 4 shows that y = [Ni2+]0/
[1]0 = 1.0 is sufficient to reduce the concentration of
uncoordinated ditopic ligand 1 to zero. Due to the ditopic
structure of 1 and the strong preference for forming the bis-
terpyridine complex, y = 1.0 is sufficient to occupy all
terpyridine receptors except at the end of the chain. If y <
Figure 5. Plot of v versus [Ni2+]0[1]0 for coordination of 1 and
Ni(OAc)2·(H2O)4 in EtOH at 20 °C. The slope is the rate constant k,
which is calculated to be (8.0 0.4) × 103 M−1 s−1.
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Table 2. Rate Constants k (M−1 s−1) for the Reaction of Ligands 1, 2, and 3 with Co2+, Ni2+, and Fe2+ Determined by Stopped-
a
Flow Experiments in EtOH at 20 °C and for Comparison Literature Values for the Coordination of the Metal Ions to Ligand 3
1
2
3
3 (ref 22)
Ni2+
Co2+
Fe2+
(8.0 0.4) × 103
(1.8 0.1) × 105
(3.9 0.8) × 105
(5.5 0.3) × 103
(1.3 0.1) × 105
(1.9 0.1) × 105
(4.3 0.2) × 103
(1.4 0.1) × 105
(2.0 0.2) × 105
1.4 × 103
2.4 × 104
5.6 × 104
a
The literature values are measured in water at 25 °C and bromide as counter ion by usage of UV−vis spectroscopy22.
Therefore, the full rate law for the coordination of Ni2+ to 1
results in
the fact that the reactivity of the terpyridine receptors toward
metal ions is simply additive.
Kinetics of Polymer Growth. Next, we want to verify if
the kinetic data determined for the formation of mononuclear
complexes is also valid for MEPE growth. In a first
approximation we assume an isodesmic step-growth polymer-
ization for the formation of MEPE chains, which means that the
reactivity of binding is independent of chain-length. Thus, not
only monomers can add to chains, but chains of different length
can react with each other.54−58 We assume that there is no
energy difference between adding a monomer to an oligomer or
to another monomer. The monomer is defined as [M(1)]2+.
Thus, the reactivity of polymerizable groups should be
independent of chain-length. MEPEs based on ligand 1 are
linear and incapable of forming angled structures. In Scheme 2
the formation of MEPE polymers is shown, with k being the
rate constant of the polymerization process.
v = (8.0 0.4) × 103 M−1 s−1 [Ni2+]0[1]0
(9)
The same procedure is repeated for the reactions of 2 and 3
with Fe2+, Co2+, and Ni2+, resulting in [Co(2)2]2+, [Ni(2)2]2+,
[Fe(2)2]2+, [Co(3)2]2+, [Ni(3)2]2+, and [Fe(3)2] 2+ (see Figure
S7 in the Supporting Information). Analogously to the reaction
of 1 and Ni2+ the reaction orders a and b are one, and therefore
all evaluated reactions of the metal ions Fe2+, Co2+, and Ni2+
with 1, 2, and 3 are first order in the reactants and second order
in total, which is in agreement with work of Wilkins22 and
Ellgen et al.11 If we compare the results, as summarized in
Table 2, we see that there is not a simple relation between the
metal ion, the ligand, and the resulting rates for coordination.
The obtained rate constants are of the same order of
magnitude as presented in a publication by Wilkins et al. for the
coordination of 3 to the same metal ions in water at 25 °C,
bromide as counterion, and usage of the time-dependent
absorption spectra at wavelength of 320 to 335 nm (see Table
Scheme 2. Self-Assembly of MEPEs, with k Being the Rate
Constant of the Polymerization Process
2).22 The reaction rates decrease in the order Fe2+ > Co2+
>
Ni2+. The rate constants are sensitive to the ionic radius ri and
the electronic configuration of the metal(II) ion. The reaction
rates decrease in the order Fe2+ > Co2+ > Ni2+. The rate
constants are sensitive to the ionic radius ri and the electronic
configuration of the metal(II) ion. The following values apply
to the used acetate tetrahydrate complexes Fe(OAc)2·4H2O (ri
= 78 pm, d6 configuration), Co(OAc)2·4H2O (ri = 74 pm, d7
configuration) and Ni(OAc)2·4H2O (ri = 69 pm, d8
configuration).53 The coordination involves the replacement
of a ligand coordinated to the metal ion, e.g. H2O or OAc−, by
the entering ligand 1, 2, or 3 in solution. As discussed in the
Introduction, coordination reactions of the metal ions Fe2+,
Co2+, and Ni2+ and terpyridine take place via an Id mechanism.
The ionic radius decreases from Mn2+ to Ni2+ and leaves less
and less space for the entering ligand. The t2g orbitals are
nonbonding whereas eg* orbitals are antibonding. For σ-
bonded octahedral complexes the t2g orbitals are spread out
between the ligands and therefore their gradual filling will
electrostatically disfavor the approach of a terpyridine molecule
toward the octahedron. Thus, the possibility of bond-making is
decreased for increased number of electrons filled in t2g orbitals.
The eg* orbitals are directed toward the ligands and their
increased occupancy will increase the tendency of bond-
breaking, which leads to a weaker coordination. Both effects,
combined with steric effects due to a decrease in the ionic
radius lead to an increasing dissociative character of the
exchange mechanism and thus, a decreasing rate constant from
Fe2+ to Ni2+ for the coordination to the terpyridine ligands 1, 2,
and 3.18,20,53 Not surprisingly, ligands 2 and 3 exhibit similar
rates. However, coordination reactions of the ditopic ligand 1
proceed approximately twice as fast than the monotopic
ligands. While the observed reactivity may be an intrinsic
function of ligand 1, e.g. a result of its structure, it may hint to
We assume that k equals the values found for reaction of 1
with the corresponding metal ion (see also Table 2). We
assume a first-order kinetic with respect to the monomer,
[M(1)]2+ (see Scheme 2). Generally, polymer growth of a
linear step-growth polymerization can be described as
1
X = [M(1)]2+kt + 1 =
̅
n
0
1 − p
(10)
with X being the number-average degree of polymerization,
̅
n
[M(1)]20+ the concentration of monomers at the beginning of
the polymerization, k the rate constant, t the time, and p the
monomer conversion.59 Carothers60 established a connection
of the weight-average degree of polymerization, X , to the
̅
w
monomer conversion, p:
1 + p
X
̅
=
w
1 − p
with X being defined as
(11)
̅
w
M
Mm
̅
w
X
̅
=
w
(12)
with M being the weight-average molar mass of the polymer,
̅
w
that is the weight-average molar mass of the MEPEs, and Mm
being the molar mass of the monomer, [M(1)]2+ (see also
Scheme 2). With eqs 10 and 11 the weight-average degree of
polymerization of the MEPE polymer chains, X , can be
̅
w
described as a function of time:
F
DOI: 10.1021/acs.inorgchem.5b02931
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Inorganic Chemistry
= 2[M(1)]20+kt + 1
Article
down as the chains grow due to dissociation and exchange
processes of the highly anisometric assemblies. This may
further reduce the final conversion, p. Also, it is well-known that
polymerization critically depends on the experimental con-
ditions. The rate constant, k, shown in Table 2 is determined in
ethanolic solutions, whereas the polymer growth, shown in
Figure 6, is measured in 0.1 M KOAc acetic acid solution (75
vol %). Further studies shall elucidate the details of the growth
mechanism of MEPE.
X
̅
(13)
w
With X at hand, the monomer conversion, p, can be calculated
̅
w
by rearranging eq 11:
X
X
̅
− 1
+ 1
̅
w
w
p =
(14)
By usage of eqs 13 and 14 the monomer conversion, p, at a
time, t, can be calculated if the rate constant k is known.59 In
Figure 6 the monomer conversion, p, of Ni-MEPE is computed
by using eqs 13 and 14 with [M(1)]20+ = 6.7 mmol L−1 and k =
8.0 × 103 M−1 s−1 (see Table 2).
CONCLUSIONS
■
In this work we present data on the overall rates of the
coordination reaction of terpyridine ligands and Fe2+, Co2+, and
Ni2+. We make use of the fact that metal ion coordination
quenches the fluorescence of the ligands. The rates are such
that the reactions can be followed by stopped-flow measure-
ments. The coordination reactions occur within a few seconds,
but there are remarkable differences between the metal ions,
due to the electron configuration and the ionic radii. The
reaction rates decrease in the order Fe2+ > Co2+ > Ni2+. On the
other side, monotopic ligands 2 or 3 show similar reaction
rates, however, ligand 1 reveals a higer rate constant. Since
ditopic terpyridine ligands and metal ions form polymers,
future work will focus on the coordination kinetics and the
details of polymer growth of MEPEs.
EXPERIMENTAL SECTION
■
Synthesis. The ligands 1,4-bis(2,2′:6′,2″-terpyridine-4′-yl)benzene
(1)61 and 4′-phenyl-2,2′:6′,2″-terpyridine (2)62 were synthesized and
characterized according to literature procedures. 2,2′:6′,2″-Terpyridine
(3) and all other chemicals were purchased from Aldrich and used
without further purification. Fe(OAc)2 was synthesized from iron
powder under reflux in acetic acid solution (75 vol %) according to a
literature procedure.63 The MEPEs investigated were prepared by
metal ion induced self-assembly of the ditopic ligand 1,4-bis(2,2′:6′,2″-
terpyridine-4′-yl)benzene (1) in acetic acid solution (75 vol %) with
Fe2+, Co2+, or Ni2+, respectively. The synthesis of MEPE was done
under conductometric control because the chain-length of MEPE
depends critically on the stoichiometry, y = [M2+]/[L], that is the ratio
of the concentrations of metal ion, [M2+], and ligand, [L].64 The
ligands 4′-phenyl-2,2′:6′,2″-terpyridine (2) and 2,2′:6′,2″-terpyridine
(3) form mononuclear complexes, which are used for control
experiments. A typical titration procedure for Fe-MEPE was adapted
to a previous publication:64 26.1 mg (468 μmol) iron powder was
dispersed in 26 mL of degassed acetic acid solution (75 vol %). The
dispersion was heated under reflux and under argon until the iron
powder had completely disappeared and reacted to 81.4 mg (468
μmol) Fe(OAc)2, which took ∼2 h. In a separate 100 mL two-necked
flask equipped with a conductivity sensor, ligand 1 (243 mg, 450
μmol) was dissolved in 50 mL of acetic acid solution (75 vol %). The
solution containing the freshly synthesized Fe(OAc)2 was cooled to
room temperature and titrated under argon to the ligand solution
while the conductivity was metered. Each addition is accompanied by a
corresponding decrease in the conductivity. Because of the small
amount that was added (100 μL), the end point or 1:1 stoichiometry
was recognized by the observation that the conductivity did not
change for two consecutive additions. The solvent was removed under
reduced pressure, and the MEPE was isolated as solid powder. A
typical titration procedure for [Fe(2)2]2+ and [Fe(3)2]2+, respectively,
is as follows: 58.8 mg (338 μmol) Fe(OAc)2 was prepared as described
above. In a separate 100 mL two-necked flask equipped with a
conductivity sensor, ligand 2 (201 mg, 650 μmol) and ligand 3 (282
mg, 650 μmol), respectively, was dissolved in 50 mL of acetic acid
solution (75 vol %). The solution containing the synthesized
Fe(OAc)2 was titrated to the ligand solution while the conductivity
Figure 6. Time dependence of the monomer conversion, p, for
polymer chain-growth of Ni-MEPE, calculated using eqs 13 and 14.
(a) Computed conversion as a function of time ([M(1)]20+ = 6.7
mmol/L) with (b) region as p approaches 1. For comparison, polymer
growth is computed with the found rate constant k = 8.0 × 103 M−1
s−1 from Table 2 (solid line) and also with rate constants ranging from
k = 8.0 × 100 M−1 s−1 to k = 8.0 × 10−3 M−1 s−1. Finally, this figure
shows the MEPE growth in 0.1 M KOAc acetic acid solution (75 vol
%) at a temperature of 20 °C ([M(1)]20+ = 6.7 mmol/L), measured
experimentally by static light scattering (for details concerning the
theory of static light scattering, see Chapter 4 in the Supporting
Information).
For verification of the computed data, static light scattering
(SLS) is used to investigate the growth of Ni-MEPE polymers
(for details concerning the theory of static light scattering, see
Chapter 4 in the Supporting Information). A solution of ligand
1 in 0.1 M KOAc acetic acid solution (75 vol %) and a solution
of Ni2+ in 0.1 M KOAc acetic acid solution (75 vol %) are
mixed and the weight-average molar mass, M of the resulting
̅
w
Ni-MEPE is obtained by using the SLS technique. With M at
̅
w
hand, the monomer conversion, p, is calculated according to
eqs 13 and 14. The results are shown in Figure 6. We note that
in a first approximation MEPE growth follows the predicted
curve based on the rate of complex formation determined
above. However, a closer look at the data (Figure 6b) shows
that the MEPE does not quite reach the computed conversion,
p. The deviation toward full conversion is not surprising,
because we assume isodesmic step-growth polymerization in a
first approximation. In addition, it is not clear, if the system
reaches equilibrium, in particular the reaction rate may slow
G
DOI: 10.1021/acs.inorgchem.5b02931
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
was metered as described above. The solvent was removed under
reduced pressure, and the complexes were isolated as solid powder.
Absorption and Fluorescence. Absorbance spectra were
recorded on a JASCO V-650 spectrophotometer, emission spectra
were measured on a JASCO FP-8300 spectrofluorometer. The spectra
were recorded at 20 °C with solutions of the different MEPEs and
monotopic complexes all containing a constant ligand concentration of
10−3 M in UV/vis measurements and 10−5 M in fluorescence
spectroscopy measurements.
ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.5b02931.
Details on UV−vis absorption spectroscopy, fluorescence
spectroscopy, stopped-flow, and SLS (PDF)
Stopped-Flow. The stopped-flow measurements were performed
using the fluorescent substances (1, 2, and 3) at different
concentrations between 10−5 and 10−4 M ultrasonicated in ethanol
for at least 30 min. The measurements were carried out in ethanol
instead of acetic acid solution (75 vol %) to avoid possible corrosion to
the stopped-flow apparatus. To make sure, that the ligands 1, 2, and 3
are dissolved in EtOH, emission spectra were monitored and the
intensity of the Rayleigh scattering band was checked. A low intensity
of the band indicated, that the ligands were dissolved and that there
were no scattering aggregates present in solution. These dissolved
substances were reacted with different concentrations in the range of 5
× 10−5 to 10−3 M of the respective quenching substances Fe(OAc)2·
4H2O, Co(OAc)2·4H2O, and Ni(OAc)2·4H2O in ethanol. The
coordination kinetics of MEPE and metal complexes were monitored
by fluorescence detection using a BioLogic SFM-300 stopped-flow
module attached to a JASCO J-815 spectropolarimeter with an
addition photomultiplier tube for fluorescence detection. For the
concentration range between 10−5 and 10−4 M a calibration curve was
recorded that relates ligand concentration of 1, 2, and 3 to the
emission signal, which follows a linear relationship. The slope of the
calibration curves correlates the emission signal to the concentration of
the ligand. Ligand 1 was excited at λe = 292 nm, ligand 2 at λe = 289
nm and ligand 3 at λe = 281 nm. Emission intensity between 387 and
447 nm was detected using a cutoff filter supplied by Bio-Logic. The
dead time of these experiments was 3.8 ms. Every stopped-flow mixing
experiment was performed with 151 μL of ligand, dissolved in EtOH
and 151 μL of metal acetate, dissolved in EtOH, each with a flow rate
of 4.00 mL s−1. The temperature was set to 20 °C with a FL 300
Julabo temperature controller. Fluorescence-time traces were moni-
tored using Biokine32 (BioLogic). The concentration of ligand is
related to the emission intensity via a calibration curve. In the
concentration range of 10−5 to 10−4 M we observe a linear relationship
between concentration of ligand and emission signal. Every
concentration/time plot was fitted to the following equation:
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: dirk.kurth@matsyn.uni-wuerzburg.de.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors would like to thank Prof. Dr. Markus Sauer and
Dr. Soren Doose of the Theodor-Boveri Institute (University
̈
Wurzburg, Germany), where the stopped-flow measurements
̈
were performed, and Christian Bauer for his help preparing the
solutions and performing the SLS measurements. Light
scattering experiments were possible through support of the
Deutsche Forschungsgemeinschaft (INST 93/774-1 FUGG).
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