A Ru(terpy)(phen)-incorporating ring and its light-induced geometrical changes

Article abstract of DOI:10.1039/b503411f

A Ru(terpy)(phen) motif has been inscribed in a 39-membered ring by functionalizing the ligands and subsequently performing a cyclization reaction on the complex; by visible light irradiation, a dramatic geometrical changeover of the cyclic complex takes

Full text of DOI:10.1039/b503411f

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A Ru(terpy)(phen)-incorporating ring and its light-induced geometrical
changes{
Sylvestre Bonnet, Jean-Paul Collin* and Jean-Pierre Sauvage
Received (in Cambridge, UK) 9th March 2005, Accepted 27th April 2005
First published as an Advance Article on the web 24th May 2005
DOI: 10.1039/b503411f
A Ru(terpy)(phen) motif has been inscribed in a 39-membered
ring by functionalizing the ligands and subsequently performing
a cyclization reaction on the complex; by visible light
irradiation, a dramatic geometrical changeover of the cyclic
complex takes place which can be reversed thermally.
Including and controlling molecular motions via various types of
signals is a key feature for the development of mechanical devices
at the nanoscopic level.1 Photonic signals are particularly
Scheme 1
promising and several molecular machines have recently been
reported which are set in motion by light irradiation.2 Our group
formation of two isomeric complexes containing L1, L2 and the
ancillary sixth ligand L seemed to be possible.
in particular has described various ruthenium(II)-complexed
rotaxanes and catenanes whose movements are induced by
populating a dissociative excited state from a metal-to-ligand
charge transfer (MLCT) state, this latter state being simply formed
by visible light irradiation.3
The only complex formed by reacting RuL1Cl3 and L2 was the
one for which the sterically hindering mesityl group was in close
contact with the terpy nucleus, the sixth ligand Cl2 being located
inside the ring cavity.9 For the moment, the reason for this
selectivity is unclear, as well as the reaction mechanism leading to
Rings are essential components of molecular machine proto-
types, especially within the catenane family.4 A limited number of
ruthenium(II)-incorporating macrorings have been made.5 Since
Ru(II) is substitutionallly inert, the so-called self-assembly
approach cannot be utilized.6 In the present report, we would
like to describe the synthesis of a novel Ru(II)-containing cycle,
+
this isomer. [RuL1L2(Cl)]th (thermal isomer) was subjected to
ligand exchange under light irradiation, in CH3CN–H2O (5 : 1).
Clean substitution of the chloride ligand for CH3CN was
observed, as well as quantitative isomerisation to the other isomer,
considered as the photoisomer. The substitution–isomerisation
reaction 1+ A2+ is represented in Fig. 1. The isomerisation reaction
is remarkably clean, although its mechanism is still unclear. Ligand
the ligand set consisting of
a terpy derivative (terpy 5
2,29,69,20-terpyridine) and a phen chelate (phen 5 1,10-phenan-
throline). The sixth ligand can thermally or photochemically be
substituted by another monodentate ligand.7 It consists of pyridine
(py), acetonitrile or S-linked DMSO. In addition, a new
photoisomerisation reaction of the ruthenium(II) complex leads
to a dramatic change of the ring shape under the action of visible
light, the reverse process regenerating the initial state by a thermal
reaction.
The embedding of a Ru(terpy)(phen)2+ moiety in a ring is
certainly not trivial since the terpy fragment occupies three
meridional sites of the metal octahedron and the phen chelate is
almost opposite to the terpy in the metal coordination sphere.
We thought that a convenient way to connect the phen and the
terpy fragments was to interlink a lateral position of the phen
(3 position) and the para position (49) of the central pyridinic
nitrogen atom of the terpy. This strategy should allow formation
of a large ruthenium(II)-containing ring (Scheme 1).
The starting terpy and phen ligands used are represented in
Fig. 1.8 Since the phen chelate L2 is asymmetrically substituted, the
{ Electronic supplementary information (ESI) available: 1H NMR spectra
of the complexes 1th to 5th2+. See http://www.rsc.org/suppdata/cc/b5/
b503411f/
*jpcollin@chimie.u-strasbg.fr
+
2+
Fig. 1 Synthesis of the photoisomer Ru(L1)(L2)(py)]photo (3photo2+).
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Chem. Commun., 2005, 3195–3197 | 3195

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exchange was again carried out but, this time, thermally.
2+
[RuL1L2(CH3CN)]photo was heated in refluxing pyridine and,
after work-up, a quantitative yield of the pyridine complex
2+
[RuL1L2(py)]photo was obtained. It is noteworthy that the py
complex obtained belongs to the photoisomer family and, thus, no
isomerisation takes place during ligand exchange under the
2+
relatively harsh conditions used. [RuL1L2(py)]photo was the
starting complex to prepare the ruthenium(II)-containing ring.
All the organic chemistry transformations involving the ligands
were carried out on the complex. Due to its chemical stability, no
decomplexation problems were encountered.
2+
Fig. 3 Thermal isomerisation of the photoisomer 5photo and the
photochemical back reaction. The pyridine ligand moves from an external
position to an intra-ring location while the –(CH2)18– fragment undergoes
a folding–unfolding process.
The sequence of reactions leading to the 39-membered ring
2+
5photo (still belonging to the photoisomer family) is depicted in
Fig. 2. The synthetic procedure is particularly efficient since the
overall yield is y70% from [Ru L1L2(py)]photo2+. The yield of the
ring closing metathesis (RCM) reaction leading to the cyclic
compound is similar to those corresponding to the preparation of
other macrocyclic transition metal complexes of the catenane or
knot families.10 It is very likely that the geometry of the
photoisomer 4photo2+, precursor of the ring, is favourable to the
formation of a cycle. Indeed, the two –(CH2)8–CHLCH2 fragments
can easily be oriented parallel to one another, with close proximity
of the terminal olefins.
whole isomerisation process can thus be summarized by the
following equation:
1Þ DMSO, D
2Þ Pyridine, D
2z
photo
2z
?
5
5
th
/
hu
This photochemical–thermal isomerisation of the ruthenium-
containing ring is accompanied by a dramatic geometrical change
of the compound. Molecular modelling studies11 suggest that the
distance between the two oxygen atoms borne by the phenyl rings
Although the photoisomers are very inert towards isomerisation
for all members of the family, a procedure was discovered which
˚
of the terpy and phen ligands varies from 9.7 A for the
2+
2+
allowed conversion of the photoisomer 5photo to the thermal
photochemical isomer 5photo to 17.7 A for 5th2+. The –(CH2)–18
˚
isomer 5th2+. This procedure is quite general and can also be
applied to acyclic complexes. The reaction is represented in a very
schematic fashion in Fig. 3. It has been carried out with either
pyridine or CH3CN as the entering ligand from the intermediate
DMSO complex.
linker which connects these two oxygen atoms undergoes a
folding–stretching process, as depicted in Fig. 3. In the past, the
photochemical cis–trans isomerisation of an azo (–NLN–) bond
has been used to modify the shape of cyclic compounds.12 Related
geometrical changes have also been triggered by a chemical means
in dinuclear copper(II) complexes.13
The conversion of 5photo2+ to 5th2+ is performed in two steps: (1)
substitution of py by DMSO and isomerisation, (2) substitution of
DMSO by py. Its overall yield is above 80%. In order to perform a
In conclusion, a Ru(terpy)(phen) subunit could be inscribed
in a ring by connecting the terpy unit and the phen motif by
a –(CH2)–18 linker via two appropriate positions. A novel
photochemical–thermal isomerisation interconverts two very
distinct situations: by heating the macrocyclic complex
(photoisomer) in DMSO and, subsequently, in pyridine, the folded
–(CH2–)18 fragment is stretched. In parallel, the sixth ligand
(pyridine), originally located outside the ring, moves to an
intracavity position. The process is reversible, since the starting
form is regenerated by visible light irradiation.
2+
complete cycle, the photochemical reaction leading back to 5photo
was carried out in the usual way (irradiation performed at room
temperature with a 1000 W xenon arc-lamp filtered by a water
filter) in pyridine. It turned out to be virtually quantitative. The
We would like to thank the CNRS and the European
Commission for financial support. The Re´gion Alsace is also
gratefully acknowledged for a fellowship to S.B. We also thank
Johnson Matthey Inc. for a loan of RuCl3.
Sylvestre Bonnet, Jean-Paul Collin* and Jean-Pierre Sauvage
Laboratoire de Chimie Organo-Mine´rale, UMR 7513 du CNRS, Institut
Le Bel, Universite´ Louis Pasteur, 4 rue Blaise Pascal, 67000 Strasbourg
Cedex, France. E-mail: jpcollin@chimie.u-strasbg.fr
Notes and references
1 V. Balzani, M. Venturi and A. Credi, Molecular Devices and Machines,
Wiley-VCH, Weinheim, 2003; J.-P. Sauvage, Acc. Chem. Res., 1998, 31,
611; V. Balzani, A. Credi, F. M. Raymo and F. J. Stoddart, Angew.
Chem., Int. Ed., 2000, 39, 3348; Molecular Machines and Motors,
Structure & Bonding, ed. J.-P. Sauvage, Springer, Berlin, Heidelberg,
2001, vol. 99.
Fig. 2 Preparation of the ruthenium(II)-incorporating 39-membered ring
5photo2+; [Ru] 5 RuCl2[P(C6H11)3]2(LCHPh).
3196 | Chem. Commun., 2005, 3195–3197
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2 R. Ballardini, V. Balzani, M. T. Gandolfi, L. Prodi, M. Venturi,
D. Philp, H. G. Ricketts and J. F. Stoddart, Angew. Chem., Int. Ed.
Engl., 1993, 32, 1301; A. Livoreil, J.-P. Sauvage, N. Armaroli,
V. Balzani, L. Flamigni and B. Ventura, J. Am. Chem. Soc., 1997,
119, 12114; N. Armaroli, V. Balzani, J.-P. Collin, P. Gavin˜a,
J.-P. Sauvage and B. Ventura, J. Am. Chem. Soc., 1999, 121, 4397;
A. M. Brouwer, C. Frochot, F. G. Gatti, D. A. Leigh, L. Mottier,
F. Paolucci, S. Roffia and G. W. H. Wurpel, Science, 2001, 291, 2124;
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J. F. Stoddart, M. Venturi and S. Wenger, Chem. Eur. J., 2000, 6, 3558;
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3 J.-P. Collin, A.-C. Laemmel and J.-P. Sauvage, New J. Chem., 2001, 25,
22; D. Pomeranc, D. Jouvenot, J.-C. Chambron, J.-P. Colllin, V. Heitz
and J.-P. Sauvage, Chem. Eur. J., 2003, 9, 4247; P. Mobian, J. M. Kern
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sodium hydroxide in methanol to yield quantitatively the corresponding
chalcone, which was reacted with acetylpyridinium iodide and an excess
of ammonium acetate in methanol to give 49-(3-bromophe-
nyl)-2,29;69,20-terpyridine (yield 5 63%). To this bromoterpyridine was
coupled p-anisylboronic acid in the presence of sodium carbonate and
Pd(PPh3)4 in a toluene–water mixture to give L1 (yield 5 70%). L2: 1,10-
phenanthroline hydrochloride was brominated with Br2 in nitrobenzene
to yield 3-bromo-1,10-phenanthroline (26%). The p-anisylboronic
acid was coupled to this bromophenanthroline using the standard
Suzuki cross-coupling conditions (yield 5 90%). To the resulting
3-anisyl-1,10-phenanthroline in dried toluene or ether was added an
ether solution of mesityllithium monoanion, to afford the two
corresponding regioisomers 3-anisyl-2-mesityl-1,10-phenanthroline
(yield 5 20%) and 8-anisyl-2-mesityl-1,10-phenanthroline (L2,
yield 5 26%).
9 Complex 1+?PF6 was prepared in the following way: 1.0 equiv. of L2
2
and 1.5 equiv. of Ru(L1)Cl3 were mixed together with an excess of LiCl
and NEt3 in a 1 : 4 water–ethanol mixture and heated to reflux for 4 h.
A saturated aqueous solution of KPF6 was added to precipitate the
crude mixture, which was filtered, washed with water and recovered
with acetone. The ruthenium complex was purified by (1) precipitation
of an acetone solution into toluene and filtration and (2) silica gel
4 Molecular Catenanes, Rotaxanes and Knots, ed. J.-P. Sauvage and
C. Dietrich-Buchecker, Wiley-VCH, Weinheim, 1999.
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C. A. Bessel, J. Chem. Soc., Dalton Trans., 1999, 2281; D. J. Cardenas,
P. Gavina and J.-P. Sauvage, J. Am. Chem. Soc., 1997, 119, 2656.
6 M. Fujita, Acc. Chem. Res., 1999, 32, 53.
chromatography of the solid filtrate, to yield 1+?PF6 as a violet solid
2
(yield 5 30%). All the ruthenium(II) complexes , 1+ to 62+, were
1
1
characterized by H 1D, 2D COSY and ROESY, H–13C HSQC and
HMBC NMR spectroscopy, UV-vis spectroscopy and high resolution
electrospray mass spectrometry.
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11 MM2 force field calculations performed with Chem3D, see: M. J. Dudek
and J. W. Ponder, J. Comput. Chem., 1995, 16, 791.
7 For examples of Ru(terpy)(phen)L2+ complexes and their light-induced
substitution reactions, see: S. Bonnet, J.-P. Collin, J.-P. Sauvage and
E. R. Schofield, Inorg. Chem., 2004, 43, 8346; S. Bonnet, J.-P. Collin,
N. Gruber, J.-P. Sauvage and E. R. Schofield, Dalton Trans., 2003,
4654; C. R. Hecker, P. E. Fanwick and D. R. McMillin, Inorg. Chem.,
1991, 30, 659.
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Chem. Soc., 1980, 102, 5860.
13 L. Fabbrizzi, F. Foti, S. Patroni, P. Pallavicini and A. Taglietti, Angew.
Chem., Int. Ed., 2004, 43, 5073.
8 The detailed synthesis of L1 and L2 will be reported elsewhere.
L1 : 3-bromobenzaldehyde and 2-acetylpyridine were coupled with
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Chem. Commun., 2005, 3195–3197 | 3197