Stimuli-responsive reversible assembly of 2D and 3D metallosupramolecular architectures

Article abstract of DOI:10.1021/ja907297z

(Figure Presented) The discovery of interconvertible platinum coordination modes, which reveals and masks cis coordinating groups upon addition of acid and base, respectively, has been exploited to facilitate stimuli-responsive assembly and disassembly of both two- and three-dimensional metallosupramolecular architectures. Treatment of a binclear platinum complex with acid along with ditopic and tritopic donor ligands generated a molecular square and a trigonal prism, respectively, in good to high yield. These complexes were unambiguously identified using electrospray mass spectrometry, ¹H NMR spectroscopy, and X-ray crystallography. Both assemblies can be disassembled into their constituent parts simply by treatment with base, and the prism can be cycled between the assembled and disassembled states by the alternate addition of acid and base.

Full text of DOI:10.1021/ja907297z

Published on Web 10/26/2009
Stimuli-Responsive Reversible Assembly of 2D and 3D Metallosupramolecular
Architectures
Paul J. Lusby,*,† Peter Mu¨ller,† Sarah J. Pike,† and Alexandra M. Z. Slawin‡
School of Chemistry, UniVersity of Edinburgh, Joseph Black Building, West Mains Road, Edinburgh EH9 3JJ, U.K.,
and School of Chemistry, UniVersity of St. Andrews, Purdie Building, St. Andrews, Fife KY16 9ST, U.K.
Received August 28, 2009; E-mail: Paul.Lusby@ed.ac.uk
Scheme 1. Stimuli-Responsive Assembly and Disassembly of 2D
and 3D Metallosupramolecular Architectures
In the last 20 years, supramolecular chemistry has witnessed an
explosion in discrete nanoscale self-assembled cages and capsules.1-3
Nonetheless, many of these artificial systems lack the dynamic
function that often defines their naturally occurring biological
counterparts, such as the ability to spontaneously assemble (and
then disassemble) in response to a specific stimulus or a subtle
change in the local environment (e.g., a pH change).4 The
demonstration of stimuli-responsive dynamic function in metallo-
supramolecular systems, such as reversible switching between
different assemblies in solution, remains rare.5 In this communica-
tion, we report the discovery of a reversible, acid-base-switchable
platinum cyclometalation reaction, which has been exploited to
create stimuli-responsive metallosupramolecular architectures that
disassemble upon reaction with base and reassemble through a
proton-activated process.
This research was initiated by a study of the neutral pseudo-
square-planar carboplatinum complex [LPt(DMAP)] (H2L ) 2,6-
diphenylpyridine, DMAP ) 4-dimethylaminopyridine). Rather
unexpectedly, when this complex was treated with 1 equiv or an
excess of p-toluenesulfonic acid (TsOH), clean and immediate
formation of [HLPt(DMAP)OTs] was observed, where TsOH had
effectively added across one of the Pt-C bonds. The structure of
[HLPt(DMAP)OTs] was confirmed by X-ray crystallography (see
the Supporting Information) using crystals grown from diisopropyl
ether and chloroform. It appears that this process is driven by a
slight shortening of the remaining Pt-C bond, from 2.06 Å in
[LPt(DMAP)] to 1.95 Å in [HLPt(DMAP)OTs] (see the Supporting
Information), at the expense of the added chelate. Nonetheless, the
energetics of this process appear to be finely balanced, as simply
treating [HLPt(DMAP)OTs] with base, either heterogeneously using
K2CO3 or in solution with the phosphazene base P1-tBu,6 resulted in
rapid conversion back to the starting material, [LPt(DMAP)]. This
process could be monitored using 1H NMR spectroscopy, and studies
in different deuterium-labeled solvents indicated that the TsO- anion
of [HLPt(DMAP)OTs] is readily displaced by better ligands such as
pyridine and even acetonitrile. In effect, this protonation/deprotonation
event reveals/masks cis coordinating groups at a square-planar Pt(II)
center, a process we therefore reasoned could be used to reversibly
assemble metallosupramolecular architectures, similar to those previ-
ously reported by Stang2 and Fujita.3
yellow solution, after which time a second equivalent of 4,4′-bipy
To explore this concept, [(LPt)2(4,4′-bipy)] was prepared by
stirring 2 equiv of the known complex7 [LPt(DMSO)] with 4,4′-
bipyridine in dichloromethane. The product possessed relatively
low solubility, which facilitated the isolation of an orange solid
through filtration. Despite the low solubility, treatment of [(LPt)2(4,4′-
bipy)] with a dichloromethane solution of (+)-camphor-10-sulfonic
acid (CSA) resulted in dissolution within minutes to give a pale-
1
was added (Scheme 1). The H NMR spectrum of the resulting
solution indicated the rapid formation of predominantly a single
species that did not appear to change over time. As an alternative
to sequential addition, treatment of [(LPt)2(4,4′-bipy)] directly with
the CSA salt of 4,4′-bipyridine (i.e., 4,4′-bipy·2CSA) gave the same
1H NMR spectrum. To aid in the isolation, NH4PF6 was added to
the solution, and following recrystallization from nitromethane and
diethyl ether, the product was obtained as a yellow solid. The
electrospray mass spectrum of this sample (Figure 1a) showed
† University of Edinburgh.
‡ University of St. Andrews.
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10.1021/ja907297z CCC: $40.75  2009 American Chemical Society

C O M M U N I C A T I O N S
Figure 3. X-ray crystal structure of the molecular square [(HLPt)4(4,4′-
bipy)4](PF6)4. The carbon atoms of 4,4′-bipy are shown in red, carbon atoms
of HL in blue, platinum atoms in magenta, and nitrogen atoms in pale-
blue. The PF6 counteranions and five nitromethane solvent molecules have
been removed for clarity.
ether. The solid-state structure (Figure 3) confirmed a tetrameric
molecular-square arrangement and also the connectivity of the
kinetic D2h product. However, the view along the plane of the square
clearly shows that the product adopts a lower-symmetry arrange-
ment in the solid state due to quite different 4,4′-bipy conformations.
The 4,4′-bipy units that lie trans to the nitrogen donor of HL adopt an
essentially planar conformation (torsion angle ) 9°) and lie perpen-
dicular to the plane of the four platinum ions. In contrast, the other
bipy units adopt a nonplanar orientation (torsion angle ) 46°) in which
two hydrogen atoms from each constituent pyridine moiety point
slightly toward the center of the square. The adoption of this
conformation appears to be caused by π-π interactions between these
constituent pyridine groups and the noncoordinating phenyl moiety
of HL (centroid-centroid distances of between 3.41 and 3.52 Å).11
In addition, the noncoordinating phenyl groups of HL are oriented
either above or below the plane of the four platinum ions, presumably
to avoid unfavorable steric interactions between adjacent sites.
To explore the stimuli-responsive disassembly process, a dichlo-
romethane solution of P1-tBu was added to [(LPt)4(4,4′-bipy)4](PF6)4
(Scheme 1). Almost immediately, a darkening of the solution was
observed, and after the solution was stirred overnight at room
temperature, an orange solid was filtered off and showed spectro-
scopic properties identical to those of the material already assigned
as [(LPt)2(4,4′-bipy)]. Unsurprisingly, this base-induced disassembly
of the tetramer is slow in comparison to the reaction of the
mononuclear complex [HLPt(DMAP)OTs] to give [LPt(DMAP)],
which under similar conditions is complete by the time an NMR
spectrum can be recorded (less than 5 min).
To take this chemistry from two to three dimensions, the
switchable self-assembly with the triazine ligand tpt12 was also
investigated, both by sequential addition of CSA and tpt to
[(LPt)2(4,4′-bipy)] (Scheme 1) and by direct addition of the salt.
Again, formation was rapid at room temperature, and after exchange
of the counteranions by treatment with NH4PF6, a yellow solid was
isolated in 97% yield. The electrospray mass spectrum of this
product showed several peaks, many of which could be assigned
to fragments of the cage, but those at m/z 1359 and 2112 compared
well to the predicted isotopic distribution for the intact 3+ and 2+
species, respectively (see the Supporting Information), thus sup-
porting the formation of the molecular trigonal prism [(HLPt)6(4,4′-
bipy)3(tpt)2](PF6)6. In addition, the 1H NMR spectrum of this product
(Figure 4a) showed the correct ratio of triazine signals (HC and HD),
bipy signals (HA and HB), and signals from HL (e.g., HE), and the
NOE spectrum showed a cross-peak between HB and HC. The large
upfield shifts of the triazine pyridyl signals (HC and HD) relative to
those for the free triazine ligand are likely caused by π-π interactions
with the noncoordinating phenyl group of HL, which further supports
Figure 1. (a) Electrospray mass spectrum of [(HLPt)4(4,4′-bipy)4](PF6)4.
The insets show (b) the experimental isotope pattern and (c) the predicted
isotopic distribution for the tricationic molecular square [[(HLPt)4(4,4′-
bipy)4](PF6)]3+
.
several peaks between m/z 500 and 1500, but a closer inspection
of the peak at m/z 823.5 (Figure 1b) revealed a third of a unit peak
separation, and the peak pattern compared well to the predicted
isotopic distribution (Figure 1c) for the tricationic molecular square8
[[(HLPt)4(4,4′-bipy)4](PF6)]3+. The peak at m/z 1307 showed a half-
unit separation, and although this was consistent with the 2+
molecular square [[(HLPt)4(4,4′-bipy)4](PF6)2]2+, the peak over-
lapped with that for the singly charged half-molecular-square
fragment [(HLPt)2(4,4′-bipy)2](PF6)]+, prohibiting further predicted
isotope comparison. Virtually all of the peaks within the m/z
500-1500 region could be readily assigned to fragments of
[(HLPt)4(4,4′-bipy)4](PF6)4.
[(HLPt)4(4,4′-bipy)4](PF6)4 could potentially adopt several dif-
ferent isomeric forms. However, the 1H NMR spectrum of [(HLPt)4-
(4,4′-bipy)4](PF6)4 in CD3NO2 (Figure 2) indicated a single product
with 14 different proton environments (for the full assignment, see
the Supporting Information), which eliminated lower-symmetry
isomers and suggested that the product was one of two possibilities:
either the D2h-symmetric isomer shown in Scheme 1 or the C4h
isomer in which the Pt corner pieces all point in a clockwise
direction. Further NMR analysis revealed an nuclear Overhauser
effect (NOE) cross-signal between the different ortho bipy protons
(HB and HB′) but not between the meta bipy protons (HA and HA′),
which eliminated the C4h isomer.9 In effect, the formation of the
D2h metallocycle in methylene chloride at room temperature is a
kinetically controlled four-component self-assembly process involv-
ing two acceptor and two donor components, which explains the
absence of any entropically favored triangular species.10
Single crystals of [(HLPt)4(4,4′-bipy)4](PF6)4 suitable for analysis
by X-ray crystallography were grown from nitromethane and diethyl
Figure 2. 1H NMR spectrum (CD3NO2, 400 MHz, 300 K) of the molecular
square [(HLPt)4(4,4′-bipy)4](PF6)4. The assignments correspond to the
lettering shown in Scheme 1.
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C O M M U N I C A T I O N S
In conclusion, the discovery of a pH-switchable platinum
coordination mode has been exploited to switch on the self-assembly
of two- and three-dimensional metallosupramolecular architectures.
The efficacy of this self-asssembly process was demonstrated
through the formation of single species in good to excellent yields,
and in both cases, the self-assembly process could be simply
reversed in full by treatment with a slight excess of base. We are
currently looking at ways to link this responsive behavior to other
functions such as transport, catalysis, and sensing.
Acknowledgment. We thank Dr. Perdita E. Barran and Mr.
Martin De-Cecco for their assistance with the mass spectrometry.
This work was supported by the EPSRC and the Royal Society.
P.J.L is a Royal Society University Research Fellow.
Supporting Information Available: Experimental procedures and
spectroscopic data for all new compounds; full crystallographic details
for [LPt(DMAP)], [HLPt(DMAP)OTs], and [(HLPt)4(4,4′-bipy)4](PF6)4
(CIF). This material is available free of charge via the Internet at http://
pubs.acs.org.
References
Figure 4. 1H NMR spectra (CD2Cl2, 400 MHz, 300 K) showing solution
switching between assembled and disassembled states of the trigonal prism
[(HLPt)6(4,4′-bipy)3(tpt)2]6+: (a) spectrum of [(HLPt)6(4,4′-bipy)3(tpt)2]-
(PF6)6; spectra recorded (b) 2 h after the addition of 12 equiv of P1-tBu to
[(HLPt)6(4,4′-bipy)3(tpt)2](PF6)6; (c) 30 min after subsequent addition of
12 equiv of CSA; (d) 2 h after subsequent addition of 12 equiv of P1-tBu;
and (e) 30 min after subsequent addition of 12 equiv of CSA. The
assignments correspond to the lettering shown in Scheme 1.
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kinetically controlled, five-component self-assembly process involving
three ditopic acceptors and two tritopic donor units.
The increased solubility of the prism [(HLPt)6(4,4′-bipy)3(tpt)2]-
(PF6)6 in dichloromethane, relative to that of the square, allowed
the stimuli-responsive switching to be monitored in solution using
1H NMR spectroscopy (Figure 4). This experiment was initiated
by the addition of 12 equiv of P1-tBu (2 equiv per Pt ion) to a
CD2Cl2 solution of [(HLPt)6(4,4′-bipy)3(tpt)2](PF6)6. After 2 h,
complete disappearance of the signals assigned to [(HLPt)6(4,4′-
bipy)3(tpt)2](PF6)6 (HA-HE) and the emergence of new resonances
corresponding to the disassembled components tpt (Hc and Hd) and
[(LPt)2(4,4′-bipy)] (Ha, Hb, and He) was observed. Addition of 12
equiv of CSA to the same sample resulted in the disappearance of
the signals due to free tpt and [(LPt)2(4,4′-bipy)] and the appearance
of a new set of signals (Figure 4c). This spectrum showed a striking
similarity to the spectrum of [(HLPt)6(4,4′-bipy)3(tpt)2](PF6)6 (Figure
4a), indicating that the addition of CSA effects the reassembly of
the trigonal prism in solution. The subtle differences and slight
broadening in the spectrum of the CSA-reassembled species are
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solution to give [(HLPt)6(4,4′-bipy)3(tpt)2](PF6)m(CSA)6-m. A sec-
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