A remarkably stable hydrogen-bonded porphyrin·iron(terpyridine) ion pair

Article abstract of DOI:10.1039/b106430b

Even in highly competitive solvents such as DMSO, strong bimolecular association and subsequent fluorescence quenching result from the combination of hydrogen bonding and ion pairing between a porphyrinic bis(carboxylate) dianion and an iron(terpyridine)

Full text of DOI:10.1039/b106430b

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A remarkably stable hydrogen-bonded porphyrin·iron(terpyridine) ion
pair
Tyler B. Norsten, Kelly Chichak and Neil R. Branda*†
Department of Chemistry, University of Alberta, Edmonton, AB, Canada T6G 2G2
Received (in Columbia, MO, USA) 10th May 2001, Accepted 18th July 2001
First published as an Advance Article on the web 3rd September 2001
Even in highly competitive solvents such as DMSO, strong
bimolecular association and subsequent fluorescence
quenching result from the combination of hydrogen bonding
and ion pairing between a porphyrinic bis(carboxylate)
dianion and an iron(terpyridine) bis(urea).
Electrospray mass spectrometry confirmed this claim as peaks
at m/z 1738.7 and 880.3 corresponding to [M + Na]+ and [M +
2Na]2+, respectively, were observed.
Assembly 1 is freely soluble in highly polar solvents such as
DMSO and DMF, but is only sparingly soluble in all other
common organic solvents. GCOSY and T-ROESY experiments
aided in assigning the protons of complex 1. The complex
In naturally occurring light harvesting systems, the relative
orientation and proximity of the photoactive components play
fundamental roles in the effective operation of the systems.
These complex molecular devices rely primarily on non-
covalent interactions to place the chromophores in optimal
locations to facilitate beneficial electronic communication.
Accordingly, significant efforts have been devoted to develop
artificial multi-component photoactive arrays, with a particular
emphasis on those involving porphyrins.1–7 Because the
photochemical characteristics are ultimately governed by the
assemblies’ topologies, their fabrication hinges on the tailoring
of productive molecular recognition interactions such as
coordination bonding,2,3 ion pairing,4 p–p stacking5,6 and
hydrogen bonding.3,7 The hydrogen bond is a particularly
serviceable driving force to align building blocks due to its
directionality and its ease of tailoring. This is especially true
when several hydrogen bonds are united into a multipoint
recognition site. The utility of the hydrogen bond is diminished,
however, in polar competitive solvents that can better solvate
the hydrogen bonding surface. The implementation of cooper-
ative or ionic hydrogen bonding motifs, where the hydrogen
bond partners are of opposite charge, can often overcome these
destructive solvation effects.8 Strong complexation is especially
critical in order to study energy/electron transfer processes at
the low concentrations required by luminescence spectroscopy
without having to add excessive quantities of quencher.
We describe here the synthesis and characterization of the
novel non-covalently bound porphyrinic assembly 1. The
building blocks were designed to harness the beneficial
recognition attributes of both ion pairing and hydrogen bonding
interactions. The first interaction is provided by the attraction
between the Fe2+ dication complex 3a and the porphyrinic
bis(carboxylate) dianion 4. The carboxylate groups on the
porphyrin serve dual roles in that they also act as charged
hydrogen bond acceptors for the neutral bidentate urea
hydrogen bond donors on the iron(terpyridine) fragment. The
result is the self-assembly of neutral complex 1, which retains
both its structural integrity and topology even at low concentra-
tions in polar solvents such as DMSO.
Assembly 1 was prepared by mixing an equimolar mixture of
building blocks 3a and 4 in methanol (Scheme 1). The neutral
complex precipitated from the solution and was easily isolated
in high purity and in nearly quantitative yield. Owing to the
charges balancing in the final complex, 2 equivalents of
[Bu4N][BF4] were produced in the reaction and were washed
1
away during the filtration. The integration of signals in the H
NMR spectrum of the isolated solid supports the claim that the
1:1 complex is the product of the self-assembly process.
Scheme 1 Reagents and conditions: (a) SnCl2·2H2O, EtOH; (b) tri-
phosgene, CH2Cl2, then 3,5-di-tert-butylaniline, 78% for three steps; (c)
NaH, CH3I, DMF, 50 °C, 99%; (d) Fe(H2O)6(BF4)2, acetone, quantitative;
(e) 4, MeOH, 97%. The double bonds have been removed from structure 1
for clarity. Observed intermolecular nOe’s (represented as arrows) and
complex induced chemical shifts in the 1H NMR spectrum (shown in
parentheses) are highlighted.
† Current address: Department of Chemistry, Simon Fraser University,
8888 University Drive, Burnaby, B.C., Canada V5A 1S6. E-mail:
nbranda@sfu.ca
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Chem. Commun., 2001, 1794–1795
This journal is © The Royal Society of Chemistry 2001
DOI: 10.1039/b106430b

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induced shift (CIS) values for complex 1 in the 1H NMR
spectrum in DMSO-d6 and the observed intermolecular nuclear
Overhauser enhancements (nOe's) are highlighted in Scheme 1.
Both experiments support the proposed structure of 1, where 3a
is straddling across the porphyrin macrocycle and is not lying to
its side. The significant downfield shift (Dd > 3 in DMSO-d6)
observed for the urea N–H protons in assembly 1 is indicative of
effective hydrogen bonding even in such a polar and com-
petitive solvent. The role of the hydrogen bonds is also to steer
the iron(terpyridine) fragment into a position where it lies
directly over the porphyrin plane. This guidance is successful as
diagnosed by the upfield shifts observed for the hydrogen atoms
of 3a lying directly over the porphyrin plane and within the
shielding region of the macrocycle (Dd are as large as 23.52).
The signals for the four hydrogen atoms on the terminal
pyridine ring of 3a are unique in that they appear as broad peaks
in the spectrum. We attribute this to the fact that 3a can be
thought of as a ‘spit on a barbecue’ in which the terpyridine can
slowly rotate above the porphyrin ring. The terpyridine protons
can, therefore, range in distance from 3.5 to 14.5 Ň from the
plane of the macrocycle at any given moment affording a
variety of possible conformers that can exist within the NMR
time-scale. Variable temperature NMR experiments failed to
alter the shape of these broadened signals.
The strength of the binding between 3a and 4 was too large to
be accurately measured by 1H NMR spectroscopy even in
DMSO-d6. Isothermal titration calorimetry (ITC) experiments
in DMSO, however, indicated a binding stoichiometry of 1+1
for 3a and 4 and an impressive value for the association constant
(Ka) of (2.47 0.44) 3 106 M21. When the ITC experiments
were repeated replacing 3a with the N,NA-dimethylated analog
3b, that can only associate through ion pairing, the heat released
upon binding was so small that the association constant was
impossible to estimate. The titration of 5 with 3a also revealed
a similar trend, despite the fact that the 3a·5 complex can be
isolated as a solid in a similar fashion as for 1. In this case, the
hydrogen bonds are not suitably positioned to operate in unison
and direct the formation of a strapped 1+1 complex. Although
the 1H NMR spectrum in DMSO-d6 does reveal a 1+1
stoichiometry between 3a and 5, the signals for the urea N–H
protons shift only 1 ppm downfield, and there is no observable
shift of the signals corresponding to the C–H protons on the
iron(terpyridine) fragment. This indicates that 3a does not
reside over the plane of 5 and the 1+1 complex should really be
thought of as an aggregate (3a·5)n. These experiments clearly
highlight that ion pairing contributes to the association of 1;
however, the cooperative hydrogen bonds aid in aligning the
building blocks into close proximity so that these ion pairing
attractive forces can be maximized.
The relative positioning of 3a and 4 within 1 has a significant
impact on the photophysical behavior of the final assembly.
Studies using steady-state fluorescence spectroscopy to monitor
the changes in the emission intensities of DMSO solutions of 4
and 5 as the porphyrins were treated with aliquots of 3a are
shown in Fig. 1. The immediate quenching of the fluorescence
of 4 is most likely a direct result of the straddling nature of the
iron(terpyridine) fragment which positions the two chromo-
phores into the most intimate arrangement possible and ensures
maximum through-space communication. The fluorescence
quenching of porphyrin 4 by 3a is clearly a result of both strong
bimolecular association and optimal spatial positioning of the
two chromophores. The N,NA-dimethylated analog 3b, on the
other hand, only slightly quenched the fluorescence of 4
presumably in a dynamic, collision-based process. A similar
low level of quenching was obtained when porphyrin 5 was
titrated with 3a. Despite the fact that both hydrogen bonding
and ion pairing are present in the (3a·5)n polymolecular
assembly, the terpyridine fragment cannot form a strapped
arrangement, and any through-space communication between
Fig. 1 Stern–Volmer quenching when a DMSO solution of 4 is titrated with
3a (2), with 3b (8), and when 5 is titrated with 3a (5) (lex = 415 nm, lem
= 633 nm). Concentrations: [4] and [5] = 1.0 3 1026 M, [3a] and [3b] =
2 3 1025 M.
the chromophores is significantly reduced. Impressively, sim-
ilar photophysical behavior of assembly 1 was observed in a
10% H2O/MeCN solution attesting to the strength of the
association between building blocks 3a and 4 even in an
aqueous environment.
This work was supported by the Natural Sciences and
Engineering Research Council of Canada and the University of
Alberta. We are grateful to Pablo Ballester (Universitat de les
Illes Balears) for his helpful suggestions.
Notes and references
‡ These distances were estimated from the lowest energy structure of
assembly 1 using computer-assisted molecular modeling.
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