A "molecular pivot-hinge" based on the pH-regulated intramolecular switching of Pt-Pt and π-π interactions

Article abstract of DOI:10.1021/ja066914o

The binuclear cycloplatinated complex {[Pt(L)]₂(μ-dppm)}²⁺ (1), where HL is a new cyclometalating ligand 2-phenyl-6-(1H-pyrazol-3-yl)pyridine, is found to behave as a molecular pivot-hinge in which the closing and opening of the hing

Full text of DOI:10.1021/ja066914o

Published on Web 12/05/2006
A “Molecular Pivot-Hinge” Based on the pH-Regulated Intramolecular
Switching of Pt-Pt and π-π Interactions
Chi-Kin Koo,† Benny Lam,† Sze-Kar Leung,† Michael Hon-Wah Lam,*,† and Wai-Yeung Wong‡
Department of Biology & Chemistry, City UniVersity of Hong Kong, Tat Chee AVenue, Hong Kong SAR, China, and
Department of Chemistry, Hong Kong Baptist UniVersity, Waterloo Road, Kowloon Tong, Hong Kong SAR, China
Received September 26, 2006; E-mail: bhmhwlam@cityu.edu.hk
The use of explicit conformational features and intra- and
intermolecular interactions in specifically designed organic, coor-
dination/organometallic, and supramolecular systems to mimic
mechanical motions in the macroscopic world is of great research
and applied interest.1 Elegant molecular devices such as molecular
clips, gears, hinges, ratchets, rotors, scissors, shuttles, switches, and
tweezers with different supramolecular architectures have been
actively explored in recent years.1,2 Many of these devices make
use of steric, electrostatic, and π-π interactions as well as stimuli-
induced conformational isomerization to bring about their mimetic
motions. On the other hand, the metal-metal interaction has
Figure 1. ORTEP plots of 1 before (a) and after (b) deprotonation of the
scarcely been utilized in the design of molecular devices.3
1-pyrazolyl-NH on the C,N,Npyrazolyl ligands. Hydrogen atoms, except the
1-pyrazolyl-NH, are omitted for clarity.
The dimeric d8 cyclometalated systems are well-known for their
intramolecular d8-d8 interactions.4 Among the various cyclometa-
lating ligands involved, the tridentate 6-phenyl-2,2′-bipyridine
(HC∧N∧N) ligand (Scheme 1) is probably the most widely studied
because of the special photophysical properties of its cyclometalated
complexes.4,5 We have designed and synthesized an analogous
tridentate π-conjugated C,N,Npyrazolyl cyclometalating ligand, 2-phenyl-
6-(1H-pyrazol-3-yl)pyridine (HL) (Scheme 1), that contains a
Cphenyl, a Npyridyl, and a Npyrazolyl donor moiety as well as a
1-pyrazolyl-NH that can still be available for further chemical
interactions, such as protonation/deprotonation, after cyclometala-
tion. Reaction of HL with K2PtCl4 affords the cycloplatinated
complex [Pt(L)Cl] which further reacts with bis(diphenylphosphi-
no)methane (dppm) to give the binuclear {[Pt(L)]2(µ-dppm)}(ClO4)2
(1‚2ClO4) (Figure 1).6 Complex 1 represents a molecular “pivot-
hinge” where the two [Pt(L)] moieties are held by the diphen-
ylphosphino functionalities of the µ-dppm with the bridging
methylene as the pivot. Opening and closing of the pivot-hinge is
regulated by the deprotonation/protonation of the 1-pyrazolyl-NH
of the C,N,Npyrazolyl ligands.
Complex 1 is luminescent both in solid state and in solution at
room temperature. Emission λmax of 1 in acetonitrile at 606 nm is
typical of analogous binuclear cycloplatinated systems and can be
attributed to the triplet metal-metal-to-ligand charge transfer
(3MMLCT) (dσ*(Pt)fπ*(L)) transition.7,8 Addition of triethylamine
leads to a dramatic change in the emission color from orange to
green.6 In aqueous DMF solutions buffered at increasing pH, the
same phenomenon is also observed with a gradual shift of the
emission maxima from 605 to 507 nm with a structured shoulder
at 545 nm (Figure 2). Compared to the photophysical characteristics
of the monomeric [Pt(L)Cl]6 and other similar cycloplatinated
systems, the new emission peaks at high pH can be assigned the
triplet metal-to-ligand charge transfer (3MLCT) (dπ(Pt)fπ*(L))
transition. Spectrofluorometric titration in acetonitrile showed that
the spectral response reached a plateau after 2 equiv of base was
Figure 2. (Top) Photograph showing the dramatic change in the lumines-
cent responses of 1 (concentration ) 2.5 × 10-5 M) in 2:1 (v/v) DMF:
aqueous buffer of different pH (left to right): 2.23, 2.57, 2.87, 3.47, 3.62,
3.80, 4.35, 4.65, and 4.85. (Bottom) Spectral traces of the spectrofluoro-
metric titration of 1. The inset shows the plot of relative emission intensity
at 507 (9) and 605 (2) nm against pH. Excitation λ ) 355 nm.
added.6 This is consistent with the deprotonation of the 1-pyrazolyl-
NH functionality on both cyclometalating ligands of 1.
The origin of such a spectrofluorometric response of 1 is revealed
by X-ray crystallography. Figure 1a shows the crystal structure of
1 which is comparable to its analogous {[Pt(C∧N∧N)]2(µ-dppm)}2+
,
where the two cycloplatinated [Pt(L)] moieties are parallel to each
other with Pt-Pt separation of 3.193(2) Å, indicating the presence
of d8-d8 interaction.7
Figure 1b shows the crystal structure of deprotonated 1 obtained
by slow evaporation of its acetone solution in the presence of excess
† City University of Hong Kong.
‡ Hong Kong Baptist University.
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J. AM. CHEM. SOC. 2006, 128, 16434-16435
10.1021/ja066914o CCC: $33.50 © 2006 American Chemical Society

C O M M U N I C A T I O N S
Scheme 1
triethylamine. The two [Pt(L-)] moieties are not parallel to each
other anymore, and the Pt-Pt separation has increased to 5.555(2)
Å. Instead of intramolecular d8-d8 interaction, the deprotonated 1
possesses a pair of intramolecular π-π interactions between the
two deprotonated pyrazolyl rings on the cyclometalating ligands
and two phenyl rings of the µ-dppm. Interplanar distances between
the interacting aromatic rings are 3.54-3.75 Å. It seems that, upon
deprotonation of the 1-pyrazolyl-NH on C,N,Npyrazolyl, Coulombic
repulsion between the negatively charged cyclometalating ligands,
together with the compensation from the establishment of the two
intramolecular π-π interactions between the pyrazolyl rings and
the phosphino phenyl rings, cleaves the Pt-Pt interaction between
the two [Pt(L-)] moieties. Such a cleavage diminishes the 3MMLCT
emission and enhances the 3MLCT emissions from the two
noninteracting [Pt(L-)] units.
Figure 4. Crystal structures showing the opening and closing of the
molecular pivot-hinge by the protonation and deprotonation of the 1-pyra-
zolyl-NH of the C,N,Npyrazolyl ligands of the binuclear cycloplatinated
complex 1. One phosphino phenyl ring on each side of the µ-dppm bridge
is omitted for clarity.
π-π interactions in both organic and aqueous organic media using
the binuclear cycloplatinated complex 1 with the new C,N,Npyrazolyl
ligand. The dynamic actions involved resemble mechanical motions
of a pivot-hinge and are unambiguously captured by X-ray
crystallography.
The switching from the lower energy 3MMLCT emission to the
higher energy 3MLCT emission via the cleavage of the Pt-Pt
interaction can be reversed by the addition of acid (Figure 3). Such
Acknowledgment. The work described in this paper was funded
by the Research Grants Council of Hong Kong SAR, China (CityU
1108/02P).
Supporting Information Available: Experimental details and
characterizations of 1‚(ClO4)2, X-ray crystallographic data of 1 (pro-
tonated and deprotonated forms), and spectroscopic/spectrofluorometric
titrations of 1. This material is available free of charge via Internet at
http://pubs.acs.org.
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a switching process can be repeated without observable loss in
fidelity. In essence, the pH-regulated formation and cleavage of
the Pt-Pt d8-d8 interaction in 1 mimics the mechanical motions
of a pivot-hinge (Figure 4). The methylene bridge of the µ-dppm
acts as the pivot upon which the two cyclometalated [Pt(L)] units
can be swung toward opposing directions. The pivot-hinge is closed
under the Pt-Pt d8-d8 interaction, holding the two [Pt(L)] units
together when the 1-pyrazolyl-NH functionality of the C,N,Npyrazolyl
ligands is protonated. Upon deprotonation of the 1-pyrazolyl-NH,
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