Influence of PNIPAm on log Kf of a copolymerized 2,2′-bipyridine: Revised bifunctional ligand design for ratiometric metal-ion sensing

Article abstract of DOI:10.1039/c5dt01688f

Here we describe the synthesis of a model compound (1) based upon a previously reported bifunctional 2,2′-bipyridine (2). Ligand pK_a and thermodynamic stability constants were investigated by potentiometric titrations for 1 in order to assess the metal-binding capabilities of 2 following subsequent incorporation within a temperature-responsive polymer that functions as a fluorescent metal-ion indicator. While the logK_Cu1 measured here was found to be 8.86 ± 0.05 at 25 °C, this value was previously seen to fall 2.8 orders of magnitude following copolymerization of 2 with poly(N-isopropylacrylamide) (PNIPAm). This drop in affinity was attributed to stabilization of the neutral ligand by the polymer environment and elevated temperatures at which metal-binding experiments were performed. ΔH (-54.4 kJ mol^-1) and ΔS (-12.8 J K^-1 mol^-1) were therefore determined through variable temperature titrations in order to establish the temperature dependence of logK_Cu1. Doing so enabled elucidation of the overall effect that the polymer environment exerts on thermodynamic stability of copolymerized 2. Specifically, the polymer indicator was found to decrease the thermodynamic stability by 2.2 orders of magnitude, whereas elevated temperatures account for the additional 0.6 order of magnitude drop observed. This finding has implications regarding the design of future bifunctional ligands for ratiometric sensing within our temperature-responsive polymer indicator.

Full text of DOI:10.1039/c5dt01688f

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Influence of PNIPAm on log Kf of a copolymerized
2,2’-bipyridine: revised bifunctional ligand design
for ratiometric metal-ion sensing†
Cite this: Dalton Trans., 2015, 44,
11887
Justin O. Massing and Roy P. Planalp*
Here we describe the synthesis of a model compound (1) based upon a previously reported bifunctional
2,2’-bipyridine (2). Ligand pKa and thermodynamic stability constants were investigated by potentiometric
titrations for 1 in order to assess the metal-binding capabilities of 2 following subsequent incorporation
within a temperature-responsive polymer that functions as a fluorescent metal-ion indicator. While the
log KCu1 measured here was found to be 8.86 0.05 at 25 °C, this value was previously seen to fall 2.8
orders of magnitude following copolymerization of 2 with poly(N-isopropylacrylamide) (PNIPAm). This
drop in affinity was attributed to stabilization of the neutral ligand by the polymer environment and elev-
ated temperatures at which metal-binding experiments were performed. ΔH (−54.4 kJ mol−1) and ΔS
(−12.8 J K−1 mol−1) were therefore determined through variable temperature titrations in order to establish
the temperature dependence of log KCu1. Doing so enabled elucidation of the overall effect that the
polymer environment exerts on thermodynamic stability of copolymerized 2. Specifically, the polymer
indicator was found to decrease the thermodynamic stability by 2.2 orders of magnitude, whereas elev-
ated temperatures account for the additional 0.6 order of magnitude drop observed. This finding has
implications regarding the design of future bifunctional ligands for ratiometric sensing within our temp-
erature-responsive polymer indicator.
Received 4th May 2015,
Accepted 30th May 2015
DOI: 10.1039/c5dt01688f
www.rsc.org/dalton
rendering this metal less bioavailable and substantially less
toxic.7,8 Therefore, a means of accurately evaluating bioavail-
Introduction
Copper’s ability to access several oxidation states renders it able copper levels in water samples is highly desirable, in that
extremely useful for small molecule activation in living it avoids costly and unnecessary treatment of nontoxic bodies
systems.1,2 However, copper mismanagement is known to of water. Quantifying bioavailable Cu(II), however, remains a
promote oxidative stress, a hallmark of cancer3 and neuro- challenging task due to its low concentrations. While fluo-
degeneration.4 Thus, uptake, trafficking, and storage of this rescence spectroscopy is well suited to address this issue, most
metal within cellular compartments are strictly regulated by fluorescent indicators are restricted to a less desirable turn-off
membrane transporters, metallochaperones, and metalloregu- response owing to this metal’s paramagnetic nature.9 And
latory proteins, respectively.5 Environmental contamination, while turn-on and ratiometric indicators exist for Cu(II),
however, provides an alternate means of disrupting homeo- these sensors too are limited by poor water solubility10 and
static control, leading to systemic copper overload and con- irreversibility.11
comitant oxidative stress.6 While analytical methods (e.g., atomic
To address these issues, we recently reported a water-
absorption spectroscopy) are in place to ensure appropriate soluble polymer indicator capable of sensing paramagnetic
copper concentrations are maintained, these techniques take metals ratiometrically.12–14 We achieved this by physically
into account total concentrations only. The issue here lies in separating metal recognition from the fluorescence response
the fact that much of the aqueous copper present is bound by along a temperature-responsive polymer. Poly(N-isopropyl-
naturally occurring ligands, such as humic and fulvic acids, acrylamide) (PNIPAm) was chosen due to its well-defined struc-
tural transition from an expanded coil to a collapsed globule
at 32 °C. This lower critical solution temperature (LCST) may
Department of Chemistry, University of New Hampshire, Durham, NH 03824, USA.
be systematically raised and lowered by increasing hydrophilic
E-mail: roy.planalp@unh.edu
and hydrophobic interactions, respectively.15 For instance,
†Electronic supplementary information (ESI) available. See DOI: 10.1039/
c5dt01688f
incorporating a charge-neutral ligand (e.g., 2,2′-bipyridine)
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would serve to lower the LCST. Binding of cationic species,
however, elevates the LCST by introducing charge along the
Experimental methods
polymer backbone, thereby causing the polymer to expand All reactions were performed under nitrogen atmosphere in
when temperature is held constant. Incorporation of a donor flame-dried glassware equipped with a magnetic stir bar
and acceptor fluorophore pair within this scaffold permits unless otherwise stated. Formation of 4′-methyl-[2,2′-bipyri-
interrogation of the polymer structure, which may then be cor- dine]-4-carbaldehyde (4) from 4,4′-dimethyl-2,2′-bipyridine (3)
related with bioavailable metal concentrations (Fig. 2). We was achieved as previously reported.16 Anhydrous solvents
have therefore initiated a research program dedicated to the were obtained from an Innovative Technology Inc. Solvent
rational design and development of bifunctional ligands Delivery System. 1H NMR spectra were recorded on a Varian
capable of modulating these macromolecular transformations Mercury 400 MHz NMR spectrometer and are reported in ppm
through metal coordination.
relative to the deuterated solvent used (CDCl3 at 7.26 ppm).
Our group has designed and synthesized a variety of bifunc- Spectral data are reported as s = singlet, d = doublet, t = triplet,
tional ligands bearing both a polymerizable acrylamide and a q = quartet, p = pentet, hept = heptet, m = multiplet; coupling
chelating functionality. In order to characterize the metal constant(s) in Hz; integration. Proton-decoupled 13C NMR
binding capabilities of these compounds, we also synthesize spectra were recorded on a Varian Mercury 400 (101 MHz)
the isobutyramide analogues that more closely resemble these NMR spectrometer and are reported in ppm relative to the
agents following copolymerization with NIPAm (Fig. 1). Here deuterated solvent used (CDCl3 at 77.16 ppm). Mass spectra
we report the synthesis of an isobutyramide analogue (1) of a data were obtained on either a JEOL accuTOF DART mass
previously reported bifunctional 2,2′-bipyridine (2). We then spectrometer or Bruker amaZon SL ion trap LC/MS. Potentio-
evaluated the pKa and thermodynamic stability constants of 1 metric titrations were performed using a 785 DMP Titrino
by potentiometric titrations. Moreover, we determined thermo- equipped with an Accumet double junction pH electrode and
dynamic parameters (ΔH and ΔS) relevant to formation of the thermally-regulated titration vessel maintained at either 278,
Cu12+ complex to better understand the effect temperature 298, or 318 K. Samples were prepared using freshly degassed
and the polymer environment exert on 2 following incorpor- water and the ionic strength adjusted to 0.1 M with NaNO3.
ation within our indicator system.
Samples were purged with N2 prior to measurement, while an
inert atmosphere previously saturated with 0.1 M NaNO3 was
maintained throughout the course of the experiment. Data
were analyzed using Hyperquad2008,17 while the autoproto-
lysis constant of H2O was defined as 14.43, 13.77, and 13.34 at
278, 298, and 318 K, respectively. Distribution diagrams were
generated using HySS 2008.
Synthesis
N-((4′-Methyl-[2,2′-bipyridin]-4-yl)methylene)propan-1-amine
(5). To a solution of 4 (2.832 g, 14.29 mmol) in DCM (60 mL)
were added finely powdered 3 Å molecular sieves (2.4 g), fol-
lowed by n-propylamine (1.75 mL, 21.4 mmol) dropwise. The
reaction mixture was allowed to stir at room temperature
under N2 for 30 h before filtering through Celite. The filtrate
was concentrated under reduced pressure to afford 5 as a
pale yellow oil (3.364 g, 98%). 1H NMR (400 MHz, CDCl3)
δ 8.74–8.70 (m, 1H), 8.60 (dd, J = 1.7, 0.8 Hz, 1H), 8.56–8.53
(m, 1H), 8.33 (d, J = 1.4 Hz, 1H), 8.24 (dt, J = 1.7, 0.8 Hz, 1H),
7.70 (dd, J = 5.0, 1.6 Hz, 1H), 7.13 (ddd, J = 5.1, 1.7, 0.8 Hz,
1H), 3.63 (td, J = 6.9, 1.4 Hz, 2H), 2.42 (d, J = 0.7 Hz, 3H), 1.75
(h, J = 7.2 Hz, 2H), 0.96 (t, J = 7.4 Hz, 3H). 13C NMR (101 MHz,
CDCl3) δ 159.28, 157.27, 155.62, 149.72, 149.20, 148.30, 144.37,
125.09, 122.13, 121.08, 120.68, 63.86, 24.01, 21.38, 12.05;
HRMS (ESI): Exact mass cald for C15H17N3 [M + H]+, 240.1501.
Found 240.1482.
Fig. 1 Model bifunctional ligand (1) based upon
thesized copolymerizable 2,2’-bipyridine (2).
a previously syn-
Fig. 2 Incorporation of a charge-neutral ligand within temperature-
responsive PNIPAm depresses its LCST, yielding a collapsed globule
above this temperature. This conformation facilitates efficient energy
transfer between a donor and acceptor fluorophore pair. When temp-
erature is held constant, metal coordination affords polymer expansion
and a concomitant decrease in energy transfer. This design provides a
modular platform for ratiometric sensing of any metal through rational
design of the corresponding bifunctional ligand.
N-((4′-Methyl-[2,2′-bipyridin]-4-yl)methyl)propan-1-amine
(6). 5 (3.283 g, 13.72 mmol) was dissolved in MeOH (60 mL).
NaBH4 (1.342 g 35.48 mmol) was added to the stirred solution,
affording vigorous effervescence. Stirring was continued under
N2 for 18 h followed by concentration under reduced pressure.
The resulting pale yellow residue was then dissolved in H2O
11888 | Dalton Trans., 2015, 44, 11887–11892
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(100 mL), made basic by addition of 1.0 M NaOH(aq) (pH = 11 125.21, 124.98, 122.33, 122.24, 120.89, 120.21, 119.09, 50.47,
as evidenced by paper), and extracted with DCM (5 × 10 mL). 49.60, 48.48, 48.22, 30.85, 30.36, 22.70, 21.40, 21.06, 20.04,
The combined organic fractions were dried over Na2SO4 and 19.93, 11.50, 11.37; LC/MS (ESI): Exact mass cald for
concentrated under reduced pressure to
a
colorless oil C19H25N3O [M + H]+, 312.2076. Found 312.2461.
1
(3.136 g, 95%). H NMR (400 MHz, CDCl3) δ 8.62 (dd, J = 5.0,
0.7 Hz 1H), 8.54 (dd, J = 5.0, 0.7 Hz, 1H), 8.32 (dd, J = 1.7,
0.8 Hz, 1H), 8.23 (dt, J = 1.6, 0.8 Hz, 1H), 7.33 (ddt, J = 5.1, 1.6,
0.7 Hz, 1H), 7.13 (ddd, J = 5.0, 1.7, 0.8 Hz, 1H), 3.89 (s, 2H),
2.67–2.55 (m, 2H), 2.44 (s, 3H), 1.54 (h, J = 7.4 Hz, 2H), 0.93
(t, J = 7.4 Hz, 3H). 13C NMR (101 MHz, CDCl3) δ 156.49, 156.16,
150.99, 149.41, 149.15, 148.33, 124.89, 123.19, 122.22, 120.67,
53.15, 51.62, 23.43, 21.39, 11.97; HRMS (ESI): Exact mass cald
for C15H19N3 [M + H]+, 242.1657. Found 242.1650.
Results and discussion
Initial oxidation of 4,4′-dimethyl-2,2′-bipyridine (3) with SeO2
yielded the desired aldehyde (4) in addition to the undesired
carboxylic acid (Scheme 1). Pure 4 was obtained in 36% yield
through formation of a water-soluble sulfonate adduct with
NaHSO3(aq) that was then liberated at elevated pH values and
subsequently extracted into DCM. Condensation between 4
and propylamine cleanly afforded the desired imine product 5
in 98% yield, followed by reduction with NaBH4 to generate
the penultimate amine 6 in 95% yield.13 Amide formation at
0 °C with either isobutyryl or acryloyl chloride in the presence
of Et3N provided the corresponding isobutyramide (1) and
acrylamide (2) derivatives in 95% and 51% yield, respectively.
Further, both 1 and 2 exist as a 2 : 1 mixture of rotamers as
evidenced by the 1H NMR spectra. pKa and thermodynamic
stability constants for 1 were then investigated by potentio-
metric titrations, whereas 2 was copolymerized and its ability
to quantify metal-ion concentrations in wastewater samples
evaluated.13
We initially interrogatted the pKa value of 1 (Fig. 3), which
we found to be 4.81. This value is somewhat elevated relative
to that seen for 2,2′-bipyridine (4.41),18 and may be reasoned
through inductive contributions from the 4 and 4′ alkyl substi-
tuents. Owing to the weakly basic nature of 1, there is poor
competition between protons and metal for this ligand. Thus,
at acidic pH, 1 would largely exist as metallated species,
thereby complicating thermodynamic stability determination.
It was therefore necessary to perform potentiometric titrations
in the presence of tris(2-aminoethyl)amine (TREN) (eqn (1)).19,20
N-((4′-Methyl-[2,2′-bipyridin]-4-yl)methyl)-N-propylacrylamide
(2). Et3N (0.50 mL, 3.6 mmol) and 6 (0.289 g, 1.20 mmol) were
dissolved in DCM (15 mL) and allowed to stir for 10 min at
0 °C before adding acryloyl chloride (0.11 mL, 1.4 mmol). The
reaction mixture was allowed to slowly warm to room tempera-
ture with continued stirring for 12 h under N2. The reaction
mixture was filtered through Celite and washed with 0.1 M
HCl(aq) (2 × 50 mL). The aqueous layer was extracted with DCM
(3 × 25 mL), and the organic fractions combined, dried over
Na2SO4, and concentrated under reduced pressure to yield 2 as
1
a yellow oil (0.182 g, 51%). H NMR (400 MHz, CDCl3) δ 8.65
(d, J = 5.1 Hz, 1H), 8.61 (d, J = 4.9 Hz, 2H), 8.54 (dd, J = 5.0,
0.8 Hz, 3H), 8.25 (d, J = 14.0 Hz, 6H), 7.25–7.19 (m, 2H),
7.19–7.11 (m, 4H), 6.67 (dd, J = 16.7, 10.3 Hz, 2H), 6.50 (d, J =
2.1 Hz, 1H), 6.44 (dt, J = 10.6, 2.0 Hz, 3H), 5.79 (dd, J = 10.3,
2.1 Hz, 2H), 5.67 (dd, J = 8.6, 3.7 Hz, 1H), 4.76 (s, 4H), 4.69
(s, 2H), 3.49–3.42 (m, 2H), 3.37–3.30 (m, 4H), 2.45 (s, 9H),
1.71–1.57 (m, 6H), 0.92 (td, J = 7.6, 3.5 Hz, 9H). 13C NMR
(101 MHz, CDCl3) δ 167.01, 166.91, 157.04, 156.57, 155.83,
155.53, 149.98, 149.68, 149.23, 149.08, 148.55, 148.20, 147.90,
129.18, 127.73, 127.32, 125.24, 125.06, 122.74, 122.35, 121.16,
120.39, 119.19, 50.84, 49.91, 48.99, 22.71, 21.44, 21.09, 11.62,
11.37; HRMS (ESI): Exact mass cald for C18H21N3O [M + H]+,
296.1763. Found 296.1714.
N-((4′-Methyl-[2,2′-bipyridin]-4-yl)methyl)-N-propylisobutyr-
amide (1). 6 (1.434 g, 5.942 mmol) and Et3N (1.793 g,
17.72 mmol) were dissolved in DCM (40 mL) and allowed to
stir for 10 min at 0 °C before adding isobutyryl chloride
(0.67 mL, 6.4 mmol). The reaction was allowed to warm to
room temperature and continue stirring under N2. After 24 h
the reaction mixture was filtered through Celite, washed with
0.1 M HCl(aq) (3 × 50 mL), and combined organic fractions
dried over Na2SO4. The pale yellow solution was then concen-
trated under reduced pressure to a peach colored residue
(1.756 g, 95%). 1H NMR (400 MHz, CDCl3) δ 8.68–8.64 (m, 1H),
8.62–8.58 (m, 2H), 8.56–8.51 (m, 3H), 8.27 (dd, J = 1.8, 0.9 Hz,
1H), 8.23 (dd, J = 2.0, 1.0 Hz, 3H), 8.23–8.21 (m, 2H), 7.19–7.07
(m, 6H), 4.69 (s, 4H), 4.65 (s, 2H), 3.43–3.36 (m, 2H), 3.31–3.23
(m, 4H), 2.90 (hept, J = 6.7 Hz, 2H), 2.65 (hept, J = 6.5 Hz, 1H),
2.45 (s, 3H), 2.44 (s, 6H), 1.68–1.52 (m, 6H), 1.23 (d, J = 6.7 Hz,
12H), 1.12 (d, J = 6.7 Hz, 6H), 0.90 (q, J = 7.3 Hz, 9H). 13C NMR
(101 MHz, CDCl3) δ 177.88, 177.81, 157.06, 156.62, 155.95,
Mð1Þx2þ þ TRENH3
Ð MTREN2þ þ 1Hþ þ 2Hþ ð1Þ
3þ
We independently characterized this competing ligand’s
pKa and thermodynamic stability constants with Cu(II), Ni(II),
155.58, 149.92, 149.63, 149.27, 149.17, 148.67, 148.46, 148.36, Scheme 1 Synthesis of copolymerizable 2 and model compound 1.
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Fig. 5 Distribution diagram for 1 : 1 : 1 Zn(II) : 1 : TREN·3HCl mixture ([Zn]
= [1] = [TREN·3HCl] = 1.8 mM, 25 °C, I = 0.1 M NaNO3).
Fig. 3 pH profile of 1 + 3 mol equiv. HNO3 ([1] = 1.9 mM, 25 °C, I =
0.1 M NaNO3).
and Zn(II). Ni(II) and Zn(II) were included owing to their inter-
ferring nature in wastewater samples. Values obtained were
found to be in good agreement with those reported in the lit-
erature.18 Again, as previously stated, the weak basicity of 1
necessitated the use of TREN as a competing ligand during
Table 1 Protonation and formation constants for 1 and 2,2’-bipyridine
with Ni(II), Cu(II), and Zn(II) (I = 0.1 M, 25 °C)
1
2,2′-Bipyridine18
log KLH
4.86 0.03
7.47 0.08
6.52 0.08
4.40 0.06
8.86 0.05
5.67 0.02
3.71 0.01
5.8 0.1
4.41 0.04
7.04 0.03
6.82 0.09
6.30 0.03
8.12 0.03
5.51 0.03
3.4 0.1
5.12 0.09
4.51 0.04
3.7 0.1
subsequent titrations conducted in the presence of metal ions. log KNiL
Below a pH of 4, Cu(II) is largely sequestered as Cu12+ with a
log KNiL2
log KNiL3
log KCuL
log Kf of 8.86 (Fig. 4). This value, as expected, is elevated with
respect to the analagous complex between Cu(II) and 2,2′-bipyr- log KCuL2
idine (8.12)18 given the heightend basicity of 1. Raising the pH
log KCuL3
log KZnL
log KZnL2
serves to deprotonate TREN, thereby allowing this tetradentate
4.5 0. 1
4.11 0.08
ligand to efficiently bind Cu(II). The amount of CuTREN2+ stea- log KZnL3
dily increases between a pH of 4 and 8 and quickly drops after
this due to formation of the CuOHTREN+ species. Moreover,
formation of CuTREN2+ between this pH range is accompanied
good agreement with those reported for the analagous 2,2′-
bipyridine species (log KCuL2 = 5.51; log KCuL3 = 3.4).18 Zn(II)
was found to exhibit a similar speciation profile (Fig. 5); for-
mation constants between our model ligand and this metal
are presented in Table 1.
2+
2+
by appearance of Cu12 and Cu13 with log Kf values of 5.66
and 3.71, respectively. These values too were found to be in
Though Ni(II) is similar to Cu(II) and Zn(II), we were unable
to detect a NiOHTREN+ species during potentiometric titra-
tions between Ni(II), TREN, and 1 (Fig. 6). Below a pH of 4,
60% and 20% of Ni(II) is bound as the mono (log KNi1 = 7.47)
and bis species (log KNi12 = 6.52), respectively, while the
remaining 20% is free Ni(II) in solution. At neutral pH,
2+
however, Ni(II) exists as equal amounts of Ni12 and NiTREN
(c. 43% each) while the remaining metal is complexed as
either Ni13 (log KNi13 = 4.40) or NiTRENH3+. Increasing the
2+
2+
pH beyond this point yields a decrease in Ni12
and
NiTRENH3+, whereas Ni13 and NiTREN2+ steadily increase
and eventually plateau at pH 11.
2+
Thermodynamic stability constants reported in Table 1 for
Ni(II), Cu(II), and Zn(II) were found to be in agreement with the
Irving-Williams series (Ni(II) < Cu(II) ≫ Zn(II)) as was expected.
Furthermore, these numbers compare well with those pre-
viously reported for 2,2′-bipyridine.18
Fig. 4 Distribution diagram for 1 : 1 : 1 Cu(II) : 1 : TREN·3HCl mixture
([Cu] = [1] = [TREN·3HCl] = 1.8 mM, 25 °C, I = 0.1 M NaNO3).
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the gas constant (R = 8.314 J K−1 mol−1) gave ΔH and ΔS,
respectively (Table 2). Obtained values were found to be in
agreement with those reported in the literature. Log Kf values
between Cu(II) and TREN were similarly established at these
three separate temperatures. The CuOHTREN+ species was not
included in this analysis as it does not exist at pH values rele-
vant to the formation of Cu12+ seen here. Potentiometric titra-
tions were then performed to elucidate the pKa value of 1 at
278, 298, and 318 K, followed by determination of its thermo-
dynamic stability constant with an equivalent of Cu(II) at these
temperatures. Plotting the ln Keq of the latter against 1/T
afforded the corresponding Van‘t Hoff plot illustrating the
temperature dependence of log KCu1 (Fig. 7). Data refinement
at each temperature was carried out using the necessary con-
stants obtained at the corresponding values. As expected, pro-
tonation and complexation are exothermic processess and are
Fig. 6 Distribution diagram for 1 : 1 : 1 Ni(II) : 1 : TREN·3HCl mixture
([Ni] = [1] = [TREN·3HCl] = 1.8 mM, 25 °C, I = 0.1 M NaNO3).
therefore characterized by
a negative ΔH. Furthermore,
increasing the temperature from 298 to 318 K was found to
lower the log KCu1 more than half an order of magnitude. This
observation, in combination with the hydrophobic PNIPAm
environment further explains the marked decrease in log KCu2
recognized following copolymerization. Thus, in the case of
copolymerized 2, the PNIPAm environment was found to effec-
tively lower the log Kf by 2.2 orders of magnitude as opposed
to 2.8. Future ligand designs will therefore account for this
observation.
Although we have synthesized and characterized the thermo-
dynamic stability between Cu(II) and 1, the values obtained
for this model compound do not accurately translate to those
observed with the polymer indicator incorporating 2. Previous
copolymerization of 2 and PNIPAm was found to depress the
log Kf by nearly three orders of magnitude.13 Moreover, for-
mation of the bis and tris chelates seen here were unobserved
in the polymer indicator given the low percentage (2%) of 2
incorporated within PNIPAm. While the apparent drop in
thermodynamic stability was attributed to globular PNIPAm
(hydrophobic) stabilizing the neutral ligand relative to the
charged complex, the effect of performing experiments at
45 °C on the log Kf was not considered. Metal coordination is
an exothermic process, and should therefore result in
decreased affinity at elevated temperatures. We therefore evalu-
ated the temperature dependence of log KCu1 through a series
of variable temperature titrations and Van‘t Hoff plots. Doing
so enabled the effect of the polymer environment on thermo-
dynamic stability to be evaluated.
The first three pKa values for TREN were determined at 278,
298, and 318 K, and are presented in Table 2. Plotting the
ln Keq against 1/T yielded the corresponding linear Van‘t Hoff
plots (ESI†). Multiplying the negative slope and intercept by
Fig. 7 Cu12+ Van‘t Hoff plot.
Table 2 Protonation and formation constants for TREN and 1 with Cu(II) at 278, 298, and 318 K (I = 0.1 M) and the corresponding ΔH and ΔS values
ΔH
ΔS
278 K
298 K
318 K
(kJ mol−1
)
(J K−1 mol−1
)
TREN
log KLH
10.65 0.02
10.23 0.01
9.17 0.01
20.17 0.02
3.77 0.03
5.013 0.009
9.56 0.03
10.12 0.04
9.51 0.02
8.6 0.2
18.75 0.04
3.59 0.01
4.86 0.03
8.86 0.05
9.62 0.01
8.96 0.01
7.88 0.01
17.84 0.01
3.42 0.03
4.724 0.004
8.28 0.03
−43.5 (−47.2)a
−52.6 (−53.6)a
−59.3 (−54.5)a
−99.0 (−99.0)a
−14.7
47.4 (35)a
log KLH2
log KLH3
log KCuL
log KCuLH
log KLH
5.9 (−0.4)a
−35.8 (−15)a
28.8 (73.6)a
19.4
1
−12.2 (−15)b
−54.4 (−46.0)b
52.0 (32)b
log KCuL
−12.8 (−13)b
a Literature values for TREN. b Literature values for 2,2′-bipyridine.
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4 K. J. Barnham, C. L. Masters and A. I. Bush, Nat. Rev. Drug
Discovery, 2004, 3, 205.
Conclusions
In summary, we have synthesized a bifunctional ligand (2) and
its isobutyramide analogue (1), and evaluated the latter’s
metal binding abilities. The former compound was previously
incorporated within a temperature-responsive polymer able to
ratiometrically sense aqueous Cu(II).13 The log Kf of 2 copoly-
merized with PNIPAm was seen to fall nearly three orders of
magnitude relative to that value measured for 1 here at 45 °C.
We therefore established the temperature dependence of
log KCu1 to better understand the degree to which PNIPAm
influences the thermodynamic stability of copolymerized
ligands with metal-ion contaminants. In the case of 2, it was
5 A. K. Boal and A. C. Rosenzweig, Chem. Rev., 2009, 109,
4760.
6 C. A. Flemming and J. T. Trevors, Water, Air, Soil Pollut.,
1989, 44, 143.
7 M. S. Shuman and J. L. Cromer, Environ. Sci. Technol.,
1979, 13, 543.
8 D. M. Di Toro, H. E. Allen, H. L. Bergman, J. S. Meyer,
P. R. Paquin and R. C. Santore, Environ. Toxicol. Chem.,
2001, 20, 2383.
9 R. Bergonzi, L. Fabbrizzi, M. Licchelli and C. Mangano,
Coord. Chem. Rev., 1998, 170, 31.
found that PNIPAm depresses log KCu2 by 2.2 orders of magni- 10 S. Goswami, S. Maity, A. C. Maity, A. K. Maity, A. K. Das
tude relative to the model 1 evaluated here. This finding has and P. Saha, RSC Adv., 2014, 4, 6300.
implications regarding the design of future bifunctional 11 Z. Shi, X. Tang, X. Zhou, J. Cheng, Q. Han, J.-a. Zhou,
ligands to be incorporated within our polymer-based indicator.
Furthermore, this modular sensing platform enables any
B. Wang, Y. Yang, W. Liu and D. Bai, Inorg. Chem., 2013,
52, 12668.
metal ion of any oxidation state to be accurately detected 12 J. Du, S. Yao, W. R. Seitz, N. E. Bencivenga, J. O. Massing,
through rational design of the appropriate bifunctional
ligand.21
R. P. Planalp, R. K. Jackson, D. P. Kennedy and
S. C. Burdette, Analyst, 2011, 136, 5006.
13 S. Yao, A. M. Jones, J. Du, R. K. Jackson, J. O. Massing,
D. P. Kennedy, N. E. Bencivenga, R. P. Planalp,
S. C. Burdette and W. R. Seitz, Analyst, 2012, 137, 4734.
14 J. Osambo, W. Seitz, D. Kennedy, R. Planalp, A. Jones,
R. Jackson and S. Burdette, Sensors, 2013, 13, 1341.
15 H. Feil, Y. H. Bae, J. Feijen and S. W. Kim, Macromolecules,
1993, 26, 2496.
16 G. F. Strouse, J. R. Schoonover, R. Duesing, S. Boyde,
W. E. Jones Jr. and T. J. Meyer, Inorg. Chem., 1995, 34, 473.
17 P. Gans, A. Sabatini and A. Vacca, Talanta, 1996, 43, 1739.
18 A. E. Martell and R. M. Smith, NIST Critically Selected Stabi-
lity Constants of Metal Complexes, Version 8.0, National
Institute of Science and Technology (NIST), Gaithersburg,
MD, 2004.
19 H. Ackermann and G. Schwarzenbach, Helv. Chim. Acta,
1949, 32, 1543.
20 R. Nakon, P. R. Rechani and R. J. Angelici, J. Am. Chem.
Soc., 1974, 96, 2117.
21 J. O. Massing and R. P. Planalp, Tetrahedron Lett., 2015,
DOI: 10.1016/j.tetlet.2015.04.079.
Acknowledgements
We gratefully acknowledge the National Science Foundation
(CHE-1012897) and the University of New Hampshire Disser-
tation Year Fellowship for financial support.
Notes and references
1 I. Bertini, H. B. Gray, E. I. Stiefel and J. S. Valentine, Biologi-
cal Inorganic Chemistry: Structure and Reactivity, University
Science Books, Sausalito, CA, 2007.
2 M. R. Halvagar, P. V. Solntsev, H. Lim, B. Hedman,
K. O. Hodgson, E. I. Solomon, C. J. Cramer and
W. B. Tolman, J. Am. Chem. Soc., 2014, 136, 7269.
3 T. Finkel, M. Serrano and M. A. Blasco, Nature, 2007, 448,
767.
11892 | Dalton Trans., 2015, 44, 11887–11892
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