Porphyrin dyads linked by a rotatable 3,3′-biphenyl scaffold: A new binding motif for small ditopic molecules

Article abstract of DOI:10.1039/c2ob25147g

Synthetic access to a set of metallo- and free base bis-porphyrins has been provided by a stepwise approach involving sequential peptide and Suzuki couplings. Linking these porphyrins through a 3,3′-biphenyl bridge enables cooperative binding to ditopic ligands such as the bipyridyls. Association constants and binding stoichiometry has been determined by spectroscopic/ spectrophotometric means and the differences in the binding affinities of a small series of diaza ligands is discussed in the context of structural fit and microscopic association constants.

Full text of DOI:10.1039/c2ob25147g

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PAPER
Porphyrin dyads linked by a rotatable 3,3′-biphenyl scaffold: a new binding
motif for small ditopic molecules†‡
Amy R. Mulholland,a Pall Thordarson,b Emily J. Mensfortha and Steven J. Langford*a
Received 19th January 2012, Accepted 17th April 2012
DOI: 10.1039/c2ob25147g
Synthetic access to a set of metallo- and free base bis-porphyrins has been provided by a stepwise
approach involving sequential peptide and Suzuki couplings. Linking these porphyrins through a
3,3′-biphenyl bridge enables cooperative binding to ditopic ligands such as the bipyridyls. Association
constants and binding stoichiometry has been determined by spectroscopic/spectrophotometric means
and the differences in the binding affinities of a small series of diaza ligands is discussed in the context
of structural fit and microscopic association constants.
event. Since the Lewis acidity of a metalloporphyrin is linked to
1
Introduction
substrate affinity, it should be possible to tune the system to
match certain ligands with certain metal centres. Here, we
present a new method for the preparation of prototypical metallo-
porphyrin and free base dimers and discuss the preliminary
binding studies on the Zn(II)/Zn(II) dimer with nitrogenous bases
such as 4,4′-dipyridyl.
Research into the chemistry and biological role of porphyrins
has led to important advances in understanding the structure and
function of enzymes,1 the photophysical phenomena behind
light harvesting,2 oxygen transport in living organisms3 and the
driving factors in porphyrin biosynthesis.4 Porphyrins feature
unique redox, optical and association properties,5 which make
them ideal candidates for use in the construction of functional
molecular devices. Notable examples include a processive oxi-
dation catalyst for the epoxidation of polymers,6 a light-powered
molecular pedal7 and a dithienylethene-porphyrin based photo-
switch with supramolecular memory,8 all of which demonstrate
how the prudent installation of a porphyrin unit can result in a
device which offers great improvement to nanoscale process and
function.
Recognition processes are fundamental to the operation of
many molecular devices, particularly chemoswitchable elements.
We considered a bis-porphyrin unit featuring a biphenyl linkage
as a suitable host with which to study binding processes in semi-
flexible divalent receptors. Intended as a model system for more
elaborate receptors based on the same core binding motif, the
coordination of a ditopic ligand to one of two trans-oriented
metal centres, followed by rotation about the biphenyl C–C axis
to form a 1 : 1 complex could be interpreted as a switching
2
Results and discussion
2.1 Synthesis
A stepwise approach to the construction of porphyrin dimers
bridged by biphenyl (Scheme 1) enabled us to access both sym-
metric and asymmetric dimers in reasonable yield, without the
need for extensive purification required through the alternative
statistical approach of post metallation. Initially, mono-carboxy-
tetraphenylporphyrin (TPP-COOH) was coupled to 3-iodoaniline
using standard amide coupling conditions in 74% yield.§ The
product 1 was obtained in good purity through simple recrystalli-
sation from DCM/Et2O. Suzuki coupling of 3-aminophenylboro-
nic acid and 1 was used to obtain 2a in 85% yield. The use of
KOH in THF–H2O (20 : 1) over K2CO3 in DMF gave rise to a
significant improvement in both yield and reaction time (1.5 h).¶
The free-base precursor 2a was then coupled with TPP-COOH
to produce a 2H/2H dimer (3a) in high yield, with subsequent
metallation yielding metallodimers such as Zn(II)/Zn(II) 3d in
aSchool of Chemistry, Monash University, Clayton, Victoria 3800,
Australia. E-mail: steven.langford@monash.edu; Fax: +61 3 9905
4597; Tel: +61 3 9905 4569
bSchool of Chemistry, The University of New South Wales, Sydney, NSW
2052, Australia
§The 3-bromoaniline analogue can also be made using this procedure
however the lower reactivity of the bromide, in our hands, limits the
yield to <20% in the subsequent coupling step, even with high catalyst
loading and an extended reaction time.
†This article is part of the Organic & Biomolecular Chemistry 10th
Anniversary issue.
‡Electronic supplementary information (ESI) available: NMR, UV-vis
and Mass spectra, X-ray structures for 3d and details of the binding
studies. CCDC 844463. For ESI and crystallographic data in CIF or
other electronic format see DOI: 10.1039/c2ob25147g
¶When a catalytic system comprised of K2CO3/DMF/Pd(PPh3)4 was
used, a competing reductive dehalogenation compromised the yield of
the target compound 2a to 54% and required an extended reaction dur-
ation of 2 days.
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Scheme 1 Synthetic pathway to the porphyrin dyads.
near quantitative yield.k Alternatively, metallation of the precur-
sor 2a with Zn(II), followed by coupling with a suitably metal-
lated TPP-COOH, was used to generate mixed metalloporphyrin
dimers such as 3b and 3c. By introducing the metal centres at
the last step, and linking the two fragments using very mild con-
ditions, we have also avoided problems associated with transme-
tallation under conditions involving metal-based catalysts, such
as those which can occur in Suzuki couplings.9 We attribute the
lower yields of the asymmetric dimers to a competing interaction
of the Zn(II) porphyrin and HATU which decreases the efficiency
of the coupling process. These reactions failed to reach com-
pletion even after an extended reaction time. As well as the spec-
troscopic and crystallographic information, the intermediates and
porphyrin dimers were characterised by a combination of
1H-NMR, 2D-COSY, 13C-NMR, ESI or MALDI-TOF analysis
(see ESI‡).
width at half maximum (BWHM) is greater by 2 nm (14 nm).
There are only two main Q-bands, a consequence of the higher
symmetry of metallated porphyrins. The asymmetric dimers
display greater increases in the BWHM (16 and 19 nm for the
Zn(II)/2H and Ni(II)/Zn(II) complexes, respectively), reflecting an
overall broadening caused by overlapping transitions which arise
from the two different porphyrin units.10 The symmetrical
dimers 3a and 3d display no evidence of exciton coupling,
having very similar peak maxima and band widths to TPP and
Zn(II)TPP, respectively. This is given as supporting evidence that
the two porphyrins exist predominantly in a trans conformation,
a finding which complements earlier studies on pyridyl and phe-
nanthroline linked bis-porphyrin systems.11 Molecular modelling
indicates that the maximum cavity size of the porphyrin dyads
when cofacially aligned is 10.6 Å, a suitable distance for
inclusion of small molecules such as 4,4′-dipyridyl. Rotation
about the amide groups is possible, reducing the cavity size to
8.5 or 6.5 Å, depending on whether one or both amides rotate
inwards.
2.2 Spectroscopy
The combination of zinc(II) and free base porphyrins held in a
cofacial orientation by rigid or semi-flexible linkers5b,11,12 has
formed the basis of many studies in energy transfer.12a,13 The
emission spectra of dimer 3c and the Zn(II)/Zn(II) analogue 3d
are shown in Fig. 1. Whilst the preferred geometry is expected to
be trans with respect to the two porphyrin units, it was possible
to observe a photoinduced process from the zinc(II) to the free
base porphyrin consistent with excitation energy transfer (EET)
as evinced by the enhanced fluorescence associated with the
free-base component of the dimer.
The free base dimer 3a displays an aetio-type spectrum with a
Soret band centred at 419 nm and four Q-bands of decreasing
intensity (ESI‡). Relative to this dimer, the Soret band of the
Zn(II)/Zn(II) dimer is slightly red shifted (423 nm) and the band
kWhilst we were able to obtain a high-resolution ESI mass spectrum of
the 2H/2H dimer, the other complexes required MALDI-TOF analysis.
In all cases, the sinapinic acid matrix caused partial demetallation of the
zinc compounds and peaks were observed for [M + H]+, [M–Zn + 3H]+
and [M–2Zn + 5H]+.
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(Ar–Ar planar angles = 37° and 35.8°). Molecules of 3b inter-
link through an intermolecular hydrogen bonding array (Fig. 2)
through the amide groups (N–H⋯O = 3.03–3.36 Å) leading to
an infinite array with porphyrins splaying out from the central
hydrogen bonded core.
2.4 Binding studies
The coordination chemistry of zinc(II) porphyrins has been
studied in detail using a variety of monomeric and multimeric
precursors.5a,14,15 When considering the binding of bidentate
N-donating ligands such as 4,4′-dipyridyl to our Zn(II)/Zn(II)
dimer 3d, excluding polymerisation, two discrete ensemble scen-
arios can be envisioned as shown in Scheme 2;16 one involving
a 1 : 1 inclusion complex c-ZnZn·NN (Path A)17 and one the
formation of a 2 : 2 “sandwich” complex c-(ZnZn)2·(NN)2 (Path
B).18 In both cases, addition of excess ligand would eventually
lead to the formation of a 1 : 2 complex ZnZn·(NN)2. In our
case, the formation of a 1 : 1 or 2 : 2 Zn(II)/Zn(II)-dipyridyl
inclusion complex is formally expected to take place over two
steps. Initially an “open” 1 : 1 complex o-ZnZn·NN forms
which could then either go on to form the “closed” 1 : 1 (c-
ZnZn·NN) or 2 : 2 [c-(ZnZn)2·(NN)2]. The formation of these
cyclic structures would be driven by entropic factors which can
be quantified by the dimensionless intramolecular binding con-
stant Kintra. A related system where either a 1 : 1 or 2 : 2 complex
forms upon addition of a bidentate ligand to a ditopic host
similar to our Zn(II)/Zn(II) dimer has been described.17,19 Here, it
was shown that the different equilibria can all be linked via stat-
istical factors to two simple thermodynamic identities: the effec-
tive molarity (EM) and the microscopic binding constant (Km)
for each N-ligand to Zn(II) porphyrin interaction.†† Here EM =
Fig. 1 Emission spectra of the Zn(II)/Zn(II) and the Zn(II)/2H dimers
3d and 3c, respectively. Spectra recorded in CHCl3 at a concentration of
2.2 × 10−5 M in porphyrin, excitation wavelength as indicated.
K
intra/Km which is a measure of the chelate effect for these inter-
actions.16b,20 The relationship between EM and Km and the step-
wise constants for Paths A and B is also shown in Scheme 2.
The terminology used here for the 2 : 2 complexation equilibria
is based on literature with KF = formation constant and KB
=
Fig. 2 Two dimers of 3d.2py shown as a H-bonded pair. Selected
hydrogen atoms and lattice solvent have been omitted for clarity.
breaking constant.15c,17,19 Neglecting all steric considerations
and assuming that the Kintra is the same for Path A and B (which
is very unlikely), the only factor that would affect the ratio of
1 : 1 complex (Path A) and 2 : 2 complex (Path B) would be the
concentration of host ZnZn and guest NN with the formation of
the 2 : 2 less favourable at low concentrations. In most cases
however, the formation of 1 : 1 vs. 2 : 2 complex is dictated by
steric grounds.
If a trans orientation of the two porphyrin rings in 3d is true,
then for the formation of a 1 : 1 complex there is an energetic
barrier associated with the rotation of the 3,3′-biphenyl unit such
that the two zinc porphyrins adopt coplanar geometry. Whilst
4,4′-dipyridyl may be of a suitable size to fit within the cavity,
there are two aspects which may inhibit closed 1 : 1 complex
2.3 Crystallography
Crystals of 3d suitable for X-ray crystallography were grown
by vapour diffusion of diethyl ether into CH2Cl2 solutions of
3b containing an excess of pyridine (Fig. 2). The compound
crystallised with a triclinic space group with two unique struc-
tures identified to be mainly due to rotation about the axis of
chirality.** Clearly evident in both structures is the axial coordi-
nation of pyridine to each metal centre (Zn–N = 2.152 and
2.218 Å) on the same face. The amide bonds adopt a trans
configuration with the biphenyl adopting a non-planar orientation
**Crystal data for 3d: C120H94N12O4Zn2 M = 1898.81, red plate, 0.02 ×
0.01 × 0.01 mm, triclinic, space group P1, a = 10.090 (2), b = 30.792 (6),
c = 31.472 (6) Å, α = 103.63 (3), β = 91.68 (3), γ = 93.67 (3)°, V =
9473(3) Å3, Z = 4, Dc = 1.331 mg m−3, F000 = 3960, T = 123 (2) K,
166 140 reflections collected, 45 862 unique (Rint = 0.0653). Final GoF
= 1.016, R1 = 0.0992, wR2 = 0.2726. CCDC 844463 contains the sup-
plementary crystallographic data for this paper.
††The calculated EM values are of the same order of magnitude but
slightly higher than the concentration at which the NMR binding studies
were undertaken. At the host concentration used for the UV-vis spectro-
photometric titrations (1 × 10−6 M) an all-or-none process involving the
formation of a ‘cyclic’ 1 : 1 complex is most likely to be the dominant
interaction, however there may be some competition from oligomeric
species at the host concentration of the 1H-NMR titration (10−3 M).
ˉ
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Scheme 2 Summary of the key binding equilibria for the coordination of a ditopic ligand (NN) to the Zn(II)/Zn(II) dimer (ZnZn). o-ZnZn·NN
stands for open 1 : 1 complex, c-ZnZn·NN for the closed 1 : 1 complex and c-(ZnZn)2·(NN)2 for the closed 2 : 2 sandwich complex. In all cases the
final product in the presence of excess NN is the 1 : 2 complex ZnZn·(NN)2 with corresponding equilibrium constant β12 shown at the bottom of this
scheme. Path A indicates the formation of ZnZn·(NN)2 via the 1 : 1 intramolecular complex c-ZnZn·NN while Path B goes through the 2 : 2 sandwich
complex c-(ZnZn)2·(NN)2. The various microscopic equilibria are also shown with the corresponding equilibrium constants based on the microscopic
binding constant (Km) based on the equilibria shown at the top for a monodenate ligand binding N to ZnZn and the effective molarity (EM) as Kintra
/
Km (see box bottom left) with Kintra the intramolecular binding constant. The macroscopic (measurable) stepwise binding constants for Paths A and B
are indicated by dotted boxes surrounding their definition. Cooperativity, higher order and degenerate equilibria for geometric isomers have been
omitted for clarity.
formation (other than the energy cost of a global conformational
change in the structure of the dimer); the first being a steric hin-
derance provided by the 2,2′ and 6,6′ protons of the biphenyl
fragment, and the second a lowering of the basicity of the
second binding site on coordination of the first (i.e. negative
cooperativity).15a
For this reason, 1,2-bis(4-pyridyl)ethane was also included in
our investigations. The binding of these two ligands was ana-
1
lysed by a combination of UV-Vis spectrophotometric and H
NMR titration experiments. As detailed below, fitting of the
UV-Vis binding data to both the 1 : 1 (Path A) and 2 : 2 (Path B)
complex model in Scheme 2 indicated that the data gave a better
fit to the former. The binding constants obtained from UV-Vis
titrations were then used to fit the observed H NMR data to
both binding models, the 1 : 1 binding was again in much better
agreement with the experimental data.
Fig. 3 UV-vis spectra (CHCl3, 25 °C) of the Zn(II)/Zn(II) dimer (1 μM)
on titration with 4-methylpyridine.
1
25 °C in CHCl3. Similarly, titration of the Zn(II)/Zn(II) dimer with
4-methylpyridine gave Ka 1.94 0.01 × 103 M−1
Titration of 4-methylpyridine or pyridine into a solution of the
Zn(II)/Zn(II) dimer results in a bathochromic shift in the Soret
band from 423 to 429 nm and an increase in absorption intensity
concomitant with band sharpening (Fig. 3). Non-linear global
regression analysis21 of the titration data, assuming a statistical
binding model in which the coordination of the first pyridine
ligand is unrelated to the second (that is, the two subunits of the
Zn(II)/Zn(II) dimer are effectively disconnected), gave an association
constant for the binding of pyridine Ka 1.27 0.03 × 103 M−1 at
.
The 1,2-bis(4-pyridyl)ethane UV-Vis titration was more
complex, showing evidence of sequential equilibria analogous to
the association behaviour of calix-bisporphyrins.17 At a [G]/[H]
ratio of 0–1760 a simple two-state process with an isosbestic
point at 425 nm and peak maximum (of the bound 1 : 1
complex) at 428 nm predominates. With increasing ligand con-
centration, the emergence of a second equilibrium causes a
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Fig. 4 UV-vis spectra (CHCl3, 25 °C) of the Zn(II)/Zn(II) dimer (1 μM) on titration with (a) 1,2-bis(4-pyridyl)ethane; (b) expansion of a; (c) 4,4′-
dipyridyl; (d) expansion of c.
Table 1 Binding constants and calculated microscopic binding
constants (Km) and effective molarities (EM) for complexes formed
between the Zn(II)/Zn(II) dimer host and the 1,2-bis(4-pyridyl)ethane
and 4,4′-dipyridyl guests in CHCl3, at 25 °C. The results are based on
fitting of UV-Vis titration data to both the cyclic 1 : 1 (Path A in
Scheme 2) and 2 : 2 sandwich (Path B in Scheme 2) stoichiometries
gradual shift in the peak maximum (to 429 nm) and is
accompanied by a second isosbestic point at 428 nm (Fig. 4a,b).
Similar behaviour was seen in the 4,4′-dipyridyl titration (Fig. 4
c,d). The UV-Vis titration data for the binding of 1,2-bis(4-
pyridyl)ethane and 4,4′-dipyridyl to the Zn(II)/Zn(II) dimer was
fitted to both the 1 : 1 closed-complex (Path A, Scheme 2) and
2 : 2 (sandwich) closed-complex (Path B, Scheme 2). The fitting
Zn(II)/Zn(II) dimer +
Zn(II)/Zn(II) dimer +
1,2-bis(4-pyridyl)ethanea
4,4′-dipyridyla
was performed using
a Matlab-based non-linear global
regression program21 on titration data obtained from three or
four independent sets of measurements. The results for both
binding models, including the corresponding calculated micro-
scopic binding constants (Km) and effective molarities (EM) are
summarised in Table 1.
Formation of a c-ZnZn·NN 1 : 1 complex (Path A)
K1 [M−1
K2 [M−1
Km
]
]
4.69 0.21 × 104
3.08 0.22 × 102
1.9 0.2 × 103
1.3 0.3 × 104
5.1 1.3 × 102
1.3 0.3 × 103
[M−1 b
]
EM [M]c
6.5 0.6 × 10−3
3.9 1.4 × 10−3
The UV-vis titration data fitting results are strongly in favour
of the 1 : 1 over the 2 : 2 stoichiometry model for the binding of
4,4′-dipyridyl and 1,2-bis(4-pyridyl)ethane to the Zn(II)/Zn(II)
dimer. In the latter case, the data could only be fitted to the
1 : 1 model whereas for 4,4′-dipyridyl the quality of the
1 : 1 model was significantly better than the 2 : 2 as evident by:
(i) the larger estimated uncertainties for the 2 : 2 model (Table 1)
and (ii) that the covariance of the fit21 was 3–5 fold higher for
the 2 : 2 than the 1 : 1 model (both models have the same
number of unknown parameters in the fitting process).
Formation of a c-(ZnZn)2·(NN)2 2 : 2 sandwich complex (Path B)
d
KF [M−1
]
]
—
—
—
3.4 1.3 × 1014
0.53 0.14
d
d
KB [M−1
Km
1.8 0.8 × 103
[M−1 e
]
EM [M]f
—
15
7
d
a Averages of three or four independent determination are shown
(estimated uncertainties are at the 95% confidence limit). In each case
the binding constants were obtained by a global non-linear regression
approach using a Matlab-based program.21 b Calculated from Km
=
√(K1K2/4). c Calculated from EM = 2/K2. d Attempts to fit the data to
this model gave poor fit with very inconsistent results between
independent titration data sets (many orders of magnitude difference).
For the 4,4′-dipyridyl guest, the calculated microscopic Km
=
1.3 × 103 M−1 from the 1 : 1 intramolecular binding model is in
excellent agreement with the measured binding constant for pyri-
dine to the Zn(II)/Zn(II) dimer discussed above. The correspond-
ing Km value for the 1,2-bis(4-pyridyl)ethane guest is ca. 50%
higher, as is the calculated Ka for the binding of 4-methylpyri-
dine to this host. This difference is to be expected for alkyl-sub-
stituted pyridine guests; with reports that alkylpyridines have Ka
values greater than their unsubstituted counterparts (Ka = 1600
M−1 for 4-methylpyridine vs. Ka = 920 M−1 for pyridine upon
binding to Zn(II)TPP in CDCl3 at 20 °C).22 The calculated effec-
tive molarities (EM) for both guests are in the mM region which
is similar to the EM’s reported for the 1 : 1 intramolecular com-
plexes formed between DABCO and an bis-porphyrin
e Calculated from Km = [(KFKB)/16)]1. f Calculated from EM = 8/KB.
4
isophthalic acid derivative.18 In contrast, the 2 : 2 sandwich
model for the 4,4′-dipyridyl guest gives a suspiciously large cal-
culated EM value of 15 M (Table 1).
1
A H-NMR titration (in conjunction with 2D-COSY analysis)
was used to gain further insight into the structural changes that
occur on binding and hence support the findings from the
UV-Vis titrations. The most obvious change after addition of
0.25–0.5 molar equivalents of 4,4′-dipyridyl (relative to the
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dimer) is the appearance of two additional peaks at 2.72 and
5.00 ppm, attributed to the dipyridyl protons of the closed 1 : 1
complex (see ESI‡). These protons show quite a significant shift
upfield (cf. the free ligand) due to the combined anisotropy of
the porphyrin ring current. A similar effect was observed in the
titration of 1,2-bis(4-pyridyl)ethane and the zinc dimer, with
peaks observed at 4.90, 2.28 and 1.04 ppm, corresponding to the
2,6/2′,6′, 3,5/3′,5′ and ethane protons of the ligand, respectively.
Both systems are in fast exchange and as the concentration of the
free ligand rises (after addition of >0.5 equivalents) the average
signal attributed to the protons of the ligand shifts downfield.
Changes in the chemical shift and multiplicity of the porphyri-
nic and biphenyl protons gave greatest information on the mode
of binding (see ESI‡). The β-pyrrolic protons shift downfield
and initially display greater complexity in multiplicity as
inherent symmetry is diminished through complexation. The
biphenyl 2,2′ protons (which resonate at 8.16 ppm in the
1H-NMR spectrum of the unbound zinc dimer) gradually shift
upfield as the ligand equivalence approaches 1, and then begin
shifting downfield at higher ligand concentrations. The 6,6′
protons of the biphenyl fragment display similar but opposite be-
haviour, first shifting downfield as the closed 1 : 1 complex
forms, and then returning to a position only slightly downfield of
its original position in the spectrum of the unbound complex.
Overall, the cyclical behaviour of the proton shifting pattern
seems indicative of a reversible process, with the formation of a
closed 1 : 1 (c-ZnZn·NN) system being followed by re-opening
of the complex and formation of a 1 : 2 supramolecule
Fig. 5 1H-NMR (CDCl3, 400 MHz, 300 K) binding isotherms of the
2/2′ proton resonances in the Zn(II)/Zn(II) dimer as a function of added
guest. (a) Experimental data for 4,4′-dipyridyl (■) and the corresponding
calculated binding isotherm for the 1 : 1 intramolecular (–) and 2 : 2
sandwich ( ) binding models. (b) Experimental data for the 1,2-bis(4-
pyridyl)ethane (■) and the corresponding calculated binding isotherm
for the 1 : 1 intramolecular (–) and 2 : 2 sandwich ( ) binding models.
The results for the 6/6′ protons (not shown) are similar.
1
(ZnZn·(NN)2, Scheme 2). Comparing the H-NMR titrations of
the two ligands, the 2/2′ and 6/6′ protons of the biphenyl frag-
ment display their greatest displacement in the 1,2-bis(4-pyridyl)-
ethane titration; after adding one equivalent of the ligand the 2/2′
protons reach a total upfield shift of 0.38 ppm, by comparison,
the total shift is 0.31 ppm when one equivalent of 4,4′-dipyridyl
is added. The moderate difference in the K1 values calculated
from the UV-vis titration data (Table 1) is reflected in the result
obtained here.
values calculated from the Km for this guest obtained from the
1 : 1 model (Table 1) and various EM values from mM to 1000
1
M. The best (albeit unsatisfactory) fit to the H-NMR data with
the 2 : 2 model was obtained with an EM of 100 M, however
this was still inferior to the fit obtained for the 1 : 1 intramolecu-
lar binding model as shown in Fig. 5b.
Additionally, 2D-NOESY analysis23 suggests close contacts
(≈3.5–4 Å) between the 6/6′ protons and the four pyridyl
protons on the 1,2-bis(4-pyridyl)ethane ligand, in agreement
with the calculated distances (4–4.5 Å) for these interactions
from simple force-field based molecular modelling24 of the
closed (c-ZnZn·NN) complex. In contrast, the calculated dis-
tances for the same 6/6′ to ligand contacts exceed 6 Å for several
different possible conformers of the 2 : 2 sandwich [c-
(ZnZn)2·(NN)2], as determined by molecular modelling. This
further supports the view that a closed 1 : 1 (c-ZnZn·NN)
complex was formed.
We also considered the formation of a 2 : 2 sandwich [c-
(ZnZn)2·(NN)2, Scheme 2) complex when analysing the
1H-NMR titration data. There are a number of possible geometric
configurations which could possibly give rise to a stable 2 : 2
complex, some of which may be associated with less strain than
a simple 1 : 1 complex. We used the binding constants obtained
from the UV-vis data (Table 1) to fit the observed chemical shifts
in the 2/2′ and 6/6′ biphenyl protons to both the 1 : 1 intramole-
cular and 2 : 2 sandwich model for the 4′4-dipyridyl guest as
shown in Fig. 5a. In the case of 1,2-bis(4-pyridyl)ethane the
2 : 2 fitting process was performed using a series of KF and KB
Conclusions
In conclusion, a new method for the construction of biphenyl-
linked porphyrin dimers has been developed and the host–guest
chemistry of the Zn(II)/Zn(II) complex has been explored. The
synthetic strategy is mild and should be conducive to the syn-
thesis of more sophisticated symmetric and asymmetric systems
with different metal centres and aryl substitution patterns. We
have shown that the prototypical systems in this work are suit-
able hosts for small ditopic molecules, and that there is a unique
chemical shifting pattern of the 2/2′ and 6/6′ biphenyl protons
1
that can be followed by 2D-COSY and H-NMR. This enables
the complex to be distinguished between ‘open’ and ‘closed’
forms, and is useful when considering these hosts for molecular
recognition processes. The quantitative data obtained from UV-
vis titration indicated a moderate difference in the association be-
haviour of the two ditopic ligands with 1,2-bis(4-pyridyl)ethane
having a higher K1 value than 4,4′-dipyridyl. This is attributed to
the combined effect of a higher Km and a better structural ‘fit’ of
this ligand to the host. Preliminary work has indicated that other
ditopic ligands such as DABCO show complex association be-
haviour and this will be reported in due course.
6050 | Org. Biomol. Chem., 2012, 10, 6045–6053
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Multichromophoric systems bearing redox-active groups capable
of electron transfer processes as a result of switching are also
under development.
and removing the solvent in vacuo. The crude material was
purified by column chromatography (40–63 μm SiO2, DCM :
acetone 10 : 1) to yield 36.3 mg of the target compound (85%).
1H-NMR (400 MHz, CDCl3): δ 8.90–8.88 (apparent-d, 6H);
8.83–8.82 (d, 2H, 3J 4.8 Hz); 8.35–8.33 (d, 2H, 3J 8.0 Hz);
4
Experimental
8.29–8.27 (m, 3H); 8.25–8.22 (m, 6H); 8.06–8.05 (t, 1H, J 1.6
Hz); 7.80–7.73 (m, 10H); 7.52–7.48 (t, 1H, 3J 8.0 Hz);
7.45–7.42 (m, 1H); 7.24–7.22 (m, 1H); 7.09–7.07 (m, 1H);
General remarks
4
6.98–6.97 (t, 1H, J 2.0 Hz); 6.68–6.65 (m, 1H); 3.68 (bs, 2H);
Column chromatography was performed using Davisil LC60A
40–63 μm mesh silica and the specified eluent mixture,
expressed as volume to volume (v/v) ratios. High resolution ESI
spectra were recorded on an Agilent Technologies 6220 Accu-
rate-Mass TOF LC/MS spectrometer. MALDI spectra were
recorded on an Applied Biosystems 4700 Proteomics Analyser
MALDI-TOF/TOF in reflectron mode with a mass range of
800 to 3500 Da, focus mass of 1400 Da at 1500 shots per
−2.71 (bs, 2H). 13C-NMR (100 MHz, CDCl3): δ 165.9; 146.8;
146.1; 142.6; 142.0; 142.0; 141.8; 138.3; 134.9; 134.5; 134.2;
129.7; 129.5; 127.8; 126.7; 125.4; 123.5; 120.6; 120.4; 119.2;
119.2; 118.3; 117.7; 114.4; 113.9. HR-MS (ESI, +ve):
C57H41N6O requires m/z 825.3342, found m/z 825.3330
[M + H]+.
1
spectra. H and 13C nuclear magnetic resonance (NMR) spectra
Zinc metallation of 2a
were recorded using a Bruker DRX 400 MHz spectrometer
1
(400 MHz H, 100 MHz 13C) or Bruker DPX 300 MHz spec-
Compound 2a (31.4 mg, 38.1 μmol) was dissolved in a
2 : 1 mixture of CHCl3: saturated zinc acetate in methanol
(10 ml) and the reaction heated to reflux for 2 h, in the dark.
Once complete the reaction mixture was cooled to room
temperature, transferred to a separating funnel and washed with
water (3×) before drying (Na2SO4) and removing the solvent
in vacuo to yield 29.9 mg (88%) of a pink-purple microcrystal-
trometer (300 MHz 1H, 75 MHz 13C), as solutions in the deuter-
ated solvents specified. Chemical shifts (δ) were calibrated
against the residual solvent peak. All reagents and solvents used
were commercial grade. Mono-carboxytetraphenylporphyrin was
synthesised using a standard Adler procedure25 and was sub-
sequently metallated with nickel to generate Ni(II)TPP-COOH.
1
line compound (2b). H-NMR (400 MHz, CDCl3): δ 8.95–8.68
(m, 8H); 8.47 (bs, 1H); 8.28–8.21 (m, 8H); 8.07–8.05 (d, 2H,
Synthesis of 5-carboxamide-N-(3-iodophenyl)-10,15,20-
triphenylporphyin (1)
3
3J 8.0 Hz); 7.82–7.72 (m, 10H); 7.65–7.63 (bd, 1H, J 7.6 Hz);
3
3
7.36–7.32 (t, 1H, J 7.6 Hz); 6.69–6.63 (2 × d, 2H, J 7.6 Hz);
3
6.35–6.31 (t, 1H, J 7.6 Hz); 3.67–3.67 (m, 1H); 3.58–3.56 (bd,
To a solution of TPP-COOH (95.8 mg, 0.145 mmol), EDCI·HCl
(139 mg, 0.727 mmol), HOBt·H2O (22.3 mg, 0.146 mmol) and
pyridine (182 μl) in anhydrous DMF (5.90 ml) was added 3-
iodoaniline (20.0 μl, 0.166 mmol). The reaction was stirred at
RT for 2 d before removing the solvent under high vacuum,
passing the crude material through a silica plug and then recrys-
tallising from DCM/diethyl ether to yield 92.7 mg (74%) of a
powdery purple compound. 1H-NMR (400 MHz, CDCl3): δ
1H, J 7.2 Hz); −1.31 (bs, 2H). 13C-NMR (75 MHz, CDCl3): δ
3
166.2; 150.1; 150.0; 149.9; 149.6; 147.1; 143.2; 141.6; 138.3;
135.0; 134.6; 132.0; 131.8; 131.8; 131.3; 129.2; 127.3; 126.4;
125.1; 123.3; 121.0; 120.9; 120.8; 120.8; 119.7; 119.7; 119.7;
119.6; 119.6; 119.5; 119.0; 115.2; 114.0. HR-MS (ESI, +ve):
C57H39N6OZn requires m/z 887.2477, found m/z 887.2475
[M + H]+. λmax (CHCl3): 424 (log ε, 5.64); 554 (4.18); 595 nm
(3.68).
3
8.89–8.86 (m, 6H); 8.80–8.79 (d, 2H, J 4.8 Hz); 8.38–8.36 (d,
2H, 3J 8.0 Hz); 8.26–8.21 (m, 9H); 8.08 (bs, 1H); 7.81–7.73 (m,
3
10H); 7.59–7.56 (m, 1H); 7.21–7.17 (t, 1H, J 8.0 Hz); −2.76
(bs, 2H). 13C-NMR (100 MHz, CDCl3): δ 165.7; 146.3; 142.0;
142.0; 139.2; 134.9; 134.5; 133.8; 133.7; 130.7; 129.1; 127.8;
126.7; 126.7; 125.5; 120.7; 120.4; 119.6; 118.2; 94.3. HR-MS
(ESI, +ve): C51H35IN5O requires m/z 860.1886, found m/z
860.1880 [M + H]+.
Synthesis of the 2H/2H porphyrin dimer (3a)
The free base biphenyl amine porphyrin (10.9 mg, 13.2 μmol),
EDCI·HCl (12.6 mg, 65.8 μmol), HOBt·H2O (3.02 mg,
19.7 μmol), pyridine (10.0 μl) and TPP-COOH (13.0 mg,
19.7 μmol) were combined under an argon atmosphere and
anhydrous DMF (2.00 ml) was added. The reaction was stirred
at RT, protected from light for 3 d after which time water was
added and the product extracted into DCM (3×), washed with
water (4×), dried (Na2SO4) and the solvent removed under
reduced pressure. The crude material was then purified by
column chromatography (40–63 μm SiO2, DCM : ethyl acetate
20 : 1) to yield 17.5 mg of the target compound (91%). 1H-NMR
(400 MHz, CDCl3): δ 8.89–8.82 (m, 16H); 8.39–8.37 (d, 4H,
3J 8.0 Hz); 8.33–8.31 (m, 6H); 8.22–8.18 (m, 14H); 7.87–7.86
(m, 2H); 7.81–7.72 (m, 18H); 7.59–7.58 (m, 4H); −2.75 (bs,
4H). HR-MS (ESI, +ve): C102H69N10O2 requires m/z 1465.5605,
found m/z 1465.5585 [M + H]+. λmax (CHCl3): 419 (log ε,
5.94); 516 (4.48); 551 (4.12); 590 (3.95); 645 nm (3.80).
Synthesis of 5-(3′-amino[1,1′-biphenyl]-3-yl)benzamide)-
10,15,20-triphenylporphyrin (2a)
The porphyrin iodide 1 (44.6 mg, 51.9 μmol), KOH (14.6 mg,
0.259 mmol), 3-aminophenylboronic acid hydrochloride
(13.5 mg, 77.8 μmol) and THF : H2O (20 : 1, 7 ml) were com-
bined in a 2-neck flask and the solution was degassed by three
freeze–thaw cycles. Pd(PPh3)4 (5.99 mg, 5.18 μmol) was added
and the reaction heated to reflux, with stirring, under nitrogen
and protected from light. At completion (1.5 h) the reaction was
cooled to RT, water was added and the product extracted into
DCM (3×) and washed with water (3×) before drying (Na2SO4)
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Synthesis of the Ni(II)/Zn(II) porphyrin dimer (3b)
12H); 8.16 (bs, 2H); 7.85–7.84 (m, 2H); 7.78–7.72 (m, 18H);
7.59–7.57 (m, 4H). MS (MALDI, sinapinic acid):
C102H65N10O2Zn2 requires m/z 1593, found m/z 1593 [M + H]+;
2b (14.3 mg, 16.1 μmol) and HATU (6.73 mg, 17.7 μmol) were
combined under an argon atmosphere and anhydrous DMF
(2.00 ml) was added. Ni(II)TPP-COOH (12.7 mg, 17.7 μmol) in
DMF (1.50 ml) and DIEA (15.0 μl) was then added via syringe
and the reaction stirred at RT for 3 d in the dark, after which
time water was added and the product extracted into DCM (3×),
washed with water (4×), dried (Na2SO4) and the solvent
removed under reduced pressure. The crude material was then
purified by column chromatography (40–63 μm SiO2) applying
gradient elution (100% DCM → 3% EtOAc in DCM → 30%
acetone in DCM) to yield 11.0 mg of the target compound
(43%). 1H-NMR (400 MHz, CDCl3): δ 8.98–8.92 (m, 8H);
8.77–8.70 (m, 8H); 8.38–8.36 (d, 2H, 3J 8.0 Hz); 8.30–8.28
C
102H69N10O2 requires m/z 1466, found m/z 1467 [M − 2Zn +
5H]+. λmax (CHCl3): 423 (log ε, 5.97); 553 (4.60); 595 nm
(4.06).
Acknowledgements
Part of this research was undertaken on the MX2 beamline at
the Australian Synchrotron, Victoria, Australia. We are grateful
to Dr Toby Bell for helpful discussions, Dr Craig Forsyth and
Dr David Turner for X-ray crystallography support, Ms Shane
Reeve for her assistance with MALDI-TOF analysis and the
Sir James McNeill Foundation for a postgraduate scholarship (to
AM). This work is supported by the ARC Discovery grant
program (DP1093337).
3
(apparent-d, 3H); 8.23–8.21 (m, 9H); 8.18–8.16 (d, 2H, J 8.0
Hz); 8.13–8.11 (apparent-d, 2H); 8.02–7.99 (m, 6H); 7.84–7.83
(m, 1H); 7.78–7.64 (m, 19H); 7.56–7.55 (m, 4H). MS (MAL-
DI-TOF, sinapinic acid): C102H65N10NiO2Zn requires m/z 1585,
found m/z 1587 [M + H]+; C102H67N10NiO2 requires m/z 1522,
found m/z 1523 [M − Zn + 3H]+. λmax (CHCl3): 423 (log ε,
5.84); 531 (4.32); 550 (4.37); 594 nm (3.77).
Notes and references
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Synthesis of the Zn(II)/2H porphyrin dimer (3c)
TPP-COOH (7.43 mg, 11.3 μmol) in anhydrous DMF (1.50 ml)
and DIEA (11.0 μl) was added to the 2b (9.10 mg, 10.2 μmol)
and HATU (7.79 mg, 20.5 μmol) under an argon atmosphere.
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time water was added and the product extracted into DCM (3×),
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removed under reduced pressure. The crude material was then
purified by column chromatography (40–63 μm SiO2, 5%
EtOAc in DCM) to yield 5.65 mg of the target compound
(36%). 1H-NMR (400 MHz, CDCl3): δ 8.98–8.95 (m, 6H);
3
8.92–8.91 (d, 2H, J 4.8 Hz); 8.87–8.85 (m, 6H); 8.81–8.80 (d,
3
3
2H, J 4.8 Hz); 8.35–8.33 (d, 4H, J 8.0 Hz); 8.28–8.19 (m,
18H); 8.10 (bs, 2H); 7.78–7.70 (m, 20H); 7.53–7.52 (m, 4H);
−2.75 (bs, 2H). 13C-NMR (100 MHz, CDCl3): δ 166.0; 165.9;
150.4; 150.3; 150.2; 149.7; 146.9; 146.2; 142.8; 142.7; 142.1;
142.0; 141.7; 141.7; 138.6; 138.5; 134.9; 134.8; 134.5; 134.4;
134.1; 133.9; 132.3; 132.2; 132.1; 131.5; 129.7; 127.8; 127.5;
126.7; 126.6; 125.5; 125.3; 123.6; 123.6; 121.5; 121.4; 120.6;
120.4; 119.6; 119.5; 119.3; 119.2; 119.1; 118.3. MS (MALDI,
sinapinic acid): C102H67N10O2Zn requires m/z 1529, found m/z
1529 [M + H]+; C102H69N10O2 requires m/z 1466, found m/z
1467 [M − Zn + 3H]+. λmax (CHCl3): 421 (log ε, 5.91); 515
(4.32); 552 (4.39); 593 (4.02); 647 nm (3.67).
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and stirred at RT for 1 h. After transferring the reaction to a sep-
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3
(d, 4H, J 8.0 Hz); 8.31–8.30 (apparent-d, 6H); 8.23–8.21 (m,
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This journal is © The Royal Society of Chemistry 2012
Org. Biomol. Chem., 2012, 10, 6045–6053 | 6053