A template-directed synthetic approach to halogen-bridged mixed-valence platinum complexes on artificial peptides in solution

Article abstract of DOI:10.1039/b711804j

A template-directed synthetic approach to halogen-bridged mixed-valence platinum complexes has been performed in organic media using, for instance, a synthetic peptide bearing two bis(ethylenediamine)-based Pt(iv) complexes with two axial bromide anionic ligands, [1a(Pt(iv)Br₂(en)) ₂](RSO₃)₄, and a [Pt(ii)(en) ₂](RSO₃)₂ complex (R = (C₁₂H ₂₅OCH₂)₂CHO(CH₂)₃-). The Royal Society of Chemistry.

Full text of DOI:10.1039/b711804j

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An international journal of inorganic chemistry
www.rsc.org/dalton
Number 46 | 14 December 2007 | Pages 5349–5472
ISSN 1477-9226
COMMUNICATION
Mitsuhiko Shionoya et al.
A template-directed synthetic
PERSPECTIVE
Paul S. Wheatley et al.
approach to halogen-bridged mixed- Synthesis of microporous materials
valence platinum complexes on
artiꢀcial peptides in solution
using macrocycles as structure
directing agents
1477-9226(2007)46;1-Z

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COMMUNICATION
www.rsc.org/dalton | Dalton Transactions
A template-directed synthetic approach to halogen-bridged mixed-valence
platinum complexes on artificial peptides in solution†
Kentaro Tanaka,a,b Kenji Kaneko,a Yusuke Watanabea and Mitsuhiko Shionoya*a
Received 1st August 2007, Accepted 6th September 2007
First published as an Advance Article on the web 19th September 2007
DOI: 10.1039/b711804j
A
template-directed synthetic approach to halogen-
bridged mixed-valence metal complexes reported so far have
attractive physicochemical properties such as intense intervalence
charge-transfer absorption,7,8 semiconductivity9 and large third-
order nonlinear optical susceptibilities10 arising from strong
electron-lattice interactions and tunable electronic states.11 Most
of them are generated in the solid state and only a few examples
in solution or in liquid crystal,12,13 and thus, to the best of
our knowledge, there is no report on discrete halogen-bridged
mixed-valence metal complexes that exist in solution. Herein,
we report a template-directed synthetic approach in solution to
halogen-bridged mixed-valence metal complexes using synthetic
peptides bearing a discrete number of Pt(II or IV)(en)2 units
(en = ethylenediamine) on the side chains and Pt(IV or II)(en)2
counterparts (Fig. 1). The interactions between these Pt(II) and
Pt(IV) complexes were examined by photometric titration, 1H
NMR spectroscopy and mass spectrometry.
The basic design concept for the synthetic peptides is as
follows. Each building block of peptide templates has a side-chain
ethylenediamine unit as an optically-active metal binding site on
an oligo-ether-containing main chain. The number of building
blocks on one peptide template can be predetermined in the
synthetic procedure. The length of the main chain was determined
by molecular modelling so as to adjust the distance around 11–
bridged mixed-valence platinum complexes has been per-
formed in organic media using, for instance, a synthetic
peptide bearing two bis(ethylenediamine)-based Pt(IV)
complexes with two axial bromide anionic ligands,
[1a(Pt(IV)Br2(en))2](RSO3)4, and a [Pt(II)(en)2](RSO3)2 com-
plex (R = (C12H25OCH2)2CHO(CH2)3–).
Recently, a number of excellent examples of synthetic ap-
proaches to molecular wires consisting of a discrete number of
metals in the solid state1 and in solution2 have been reported. In
particular, a template-directed strategy is a key to the alignment
of functional subunits in a way that their number, composition,
sequence and direction are programmable.2,3 Biomacromolecules
such as nucleic acids and peptides have a structural basis as a
molecular template for aligning functional building blocks in a way
that their number and sequence are predetermined. For instance,
artificial DNAs are known to act as templates for homogeneous
and heterogeneous metal arrays.2,4
In this study, we used synthetic peptide templates for an
approach to quasi-1D halogen-bridged mixed-valence metal com-
plexes with a discrete number of alternating transition metal (M)
and halide (X) ions.5,6 A series of polymeric quasi-1D halogen-
˚
12 A between the adjacent two metal centers on the template. This
distance was expected to be equivalent to that between two metal
centers M in a linearly arranged -(M-X-Mꢀ-X-)n- unit where X
and Mꢀ denote a halogen bridging ligand and a counterpart of M
in the mixed-valence complex, respectively.
Synthetic procedures for the peptide templates and their Pt(II)
and Pt(IV) complexes are briefly summarized in Scheme 1. For
instance, a dipeptide (1a(CF3COOH)4) was prepared according
aDepartment of Chemistry, Graduate School of Science, The University of
Tokyo, Hongo, Bunkyo-ku, Tokyo, 113-0033, Japan. E-mail: shionoya@
chem.s.u-tokyo.ac.jp; Fax: +81 3 5841 8061; Tel: +81 3 5841 8061
bDepartment of Chemistry, Graduate School of Science, Nagoya Univer-
sity, Furo-cho, Chikusa-ku, Nagoya, 464-8602, Japan. E-mail: kentaro@
chem.nagoya-u.ac.jp; Fax: +81 52 789 2940; Tel: +81 52 789 2940
† Electronic supplementary information (ESI) available: Experimental
details. See DOI: 10.1039/b711804j
Fig. 1 Schematic representation of a discrete association of Pt(II) and Pt(IV)Br2 species with the aid of synthetic peptide templates used in this study.
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Scheme 1 Synthetic routes to (a) peptide templates and (b) their platinum complexes.
to a solid-phase synthetic procedure in Fmoc chemistry using
building block 1 (Scheme 1a) prepared from Na-t-Boc-b-amino-
L-alanine as the starting material (see ESI†). The resulting
peptides were purified by reversed-phase HPLC with CF3COOH-
containing eluents and isolated as CF3COOH salts. The
Pt(II) complex-pendant water-soluble dipeptide, [1a(Pt(II)(en))2]-
(CF3COO)4, was synthesized by the reaction of acid-free 1a
with [Pt(II)Br2(en)] in 75% yield (2 steps). The dinuclear Pt(II)
complex was then oxidized by bromination, followed by anion
−
exchange with RSO3 (R = (C12H25OCH2)2CHO(CH2)3–) to
afford lipophilic [1a(Pt(IV)Br2(en))2](RSO3)4 in 49% yield (2 steps)
(Scheme 1b). We thereby expected that a halogen-bridged trinu-
clear platinum complex would be formed as a thermodynamically
stable clip-shaped structure in a self-assembled manner from
[1a(Pt(IV)Br2(en))2](RSO3)4 and [Pt(II)(en)2](RSO3)2 in organic
solvents. Other peptides, [1b–d(Pt(IV)Br2(en))n](RSO3)2n: (n = 3,
9, 10) and [1a–d(Pt(II)(en))n](RSO3)2n: (n = 2, 3, 9, 10), were also
prepared according to the same synthetic procedure (see ESI†).
Photometric titration experiments were firstly performed to see
interactions between [1a(Pt(IV)Br2(en))2](RSO3)4 and [Pt(II)(en)2]-
(RSO3)2. When [1a(Pt(IV)Br2(en))2](RSO3)4 (25 lM) was titrated
with [Pt(II)(en)2](RSO3)2 in CH2Cl2 at 293 K, new peaks gradually
appeared at 360 and 390 nm (Fig. 2). This result indicates a
significant interaction between [1a(Pt(IV)Br2(en))2](RSO3)4 and
[Pt(II)(en)2](RSO3)2 even under dilute conditions.14 The molar
extinction coefficient at 360 nm as a 1 : 1 complex was estimated
to be more than 8 × 103 M−1 cm−1.
Fig. 2 Photometric titration spectra of dinuclear [1a(Pt(IV)Br2(en))2]-
(RSO3)4 with [Pt(II)(en)2](RSO3)2 in CH2Cl2 at 293 K (l = 1). [[1a(Pt(IV)Br2-
(en))2](RSO3)4] = 25 lM. [[Pt(II)(en)2](RSO3)2] = 0, 5, 10, 15, 20, 25, 38, 50,
75 and 100 lM. Inset; The reaction scheme of [1a(Pt(IV)Br2(en))2](RSO3)4
and [Pt(II)(en)2](RSO3)2.
of [1a(Pt(IV)Br2(en))2](RSO3)4 in CD2Cl2 ([[1a(Pt(IV)Br2(en))2]-
(RSO3)4] = 5 mM, [[Pt(II)(en)2](RSO3)2] = 2.5 mM) at 293 K,
the signals assigned to the methylene protons of ethylenediamine
moieties in [1a(Pt(IV)Br2(en))2](RSO3)4 shifted to upfield with
broadening (Fig. S3†). The spectra continuously changed even
when the ratio of Pt(II) to Pt(IV) reached higher than 1 : 1,
indicating that these Pt(II) and Pt(IV) complexes significantly
interact with each other in a faster equilibrium than the NMR
timescale but not quantitatively under this condition. It is a
noteworthy observation that the CH2 proton signals of ethylenedi-
amine moieties for [Pt(II)(en)2](RSO3)2 showed downfield shift in
contrast with [1a(Pt(IV)Br2(en))2](RSO3)4, suggesting a significant
interaction between Pt(IV) and Pt(II)species.
Photometric titration experiments of [1b(Pt(IV)Br2(en))3]-
(RSO3)6 with [1a(Pt(II)(en))2](RSO3)4 and [1d(Pt(IV)Br2(en))10]-
(RSO3)20 with [1c(Pt(II)(en))9](RSO3)18 showed a new absorption
peak at 360 and 357 nm, respectively, similar to the above-
mentioned result (Fig. S1 and S2, ESI†), suggesting a quite similar
interaction between the Pt(II) and Pt(IV) species.
The interactions of [1a(Pt(IV)Br2(en))2](RSO3)4 with [Pt(II)-
(en)2](RSO3)2 were then examined by 1H NMR titration ex-
periment. Upon addition of [Pt(II)(en)2](RSO3)2 to a solution
The magnitude of association seemed to vary with the
constituent concentrations, as compared with the NMR and
photometric titration studies. This is probably because the
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concentration-dependent self-association of counter anions with
long-alkyl-chains affects the stability of complexes formed be-
tween the Pt(II) and Pt(IV) species in a rather complicated
manner.
Notes and references
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The electrospray ionization-time-of-flight (ESI-TOF) mass
spectrum of
a mixture of [1a(Pt(IV)Br2(en))2](RSO3)4 and
[Pt(II)(en)2](RSO3)2 in a 1 : 1 ratio showed a series of peaks around
2064.48 corresponding to a doubly-charged cationic species,
[1a(Pt(IV)Br2(en))2·Pt(II)(en)2·(RSO3)4]2+, as shown in Fig. 3. This
result indicates the formation of a mixed-valence trinuclear com-
plex in solution containing a 1 : 1 mixture of [1a(Pt(IV)Br2(en))2]4+
and [Pt(II)(en)2]2+ species.
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14 It is well known that a series of analogous quasi-1D halogen-bridged
platinum complexes show an intensive CT band around 360 nm
which can shift widely depending on the counter anions.6c,8a,b,11a,12 The
assignment of the absorption band around 360 nm observed in this
work will be reported elsewhere after further investigation.
In summary, we have designed and synthesized novel synthetic
peptides and their complexes bearing a discrete number of
Pt(II) or Pt(IV) ions for a template-directed synthetic approach
to discrete halogen-bridged mixed-valence platinum complexes
in solution. In the 1H NMR, UV-vis absorption and ESI-
TOF mass spectral studies using [1a(Pt(IV)Br2(en))2](RSO3)4 and
[Pt(II)(en)2](RSO3)2, a significant interaction was observed be-
tween [1a(Pt(IV)Br2(en))2]4+ and [Pt(II)(en)2]2+ and the formation
of a mixed-valence trinuclear complex in solution containing a
1 : 1 mixture of the Pt(IV) and Pt(II) species was confirmed by
mass spectrometry. The present study would provide a new clue to
a template-directed synthesis of discrete halogen-bridged mixed-
valence platinum complexes with novel functions depending
on their number and sequence in relation to nanomagnetism,
conductivity or catalysis.
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