A photochromic system based on photochemical or thermal chelate exchange on Ru(phen)2L2+ (L = diimine or dinitrile ligand)

Article abstract of DOI:10.1039/b004982o

In the presence of a bidentate chelate containing two convergent nitrile groups, Ru(phen)₂L²⁺ (L = sterically hindering aromatic diimine ligand; phen = 1,10-phenanthroline) undergoes a photochemical reaction leading to quantitative replacement of the diimine by the bis-nitrile ligand; the reverse reaction takes place upon heating, also quantitatively.

Full text of DOI:10.1039/b004982o

A photochromic system based on photochemical or thermal chelate exchange
on Ru(phen)2L2+ (L = diimine or dinitrile ligand)
Etienne Baranoff, Jean-Paul Collin,* Yoshio Furusho, Anne-Chantal Laemmel and Jean-Pierre Sauvage*
Laboratoire de Chimie Organo-Minérale, UMR 7513 du CNRS, Université Louis Pasteur, Faculté de Chimie, 4, rue
Blaise Pascal, 67070 Strasbourg Cedex, France. E-mail: sauvage@chimie.u-strasbg.fr
Received (in Cambridge, UK) 21st June 2000, Accepted 25th August 2000
First published as an Advance Article on the web
photoproduct, Ru(phen)2(dCN)2+, was isolated quantitatively
and authenticated by comparison with a separately prepared
sample.10 To our knowledge, it is the first example of a
photochemical chelate-to-chelate exchange reaction. Crystals
were grown from MeOH–p-xylene and an X-ray structure of the
compound was obtained.11 The ORTEP diagram is shown in
Fig. 2. The ruthenium–nitrogen bond lengths range from
2.04(1) to 2.085(8) Å and are similar to those observed in the
Ru(phen)32+ and Ru(phen)2(dpq)2+ complexes12 (dpq = dipyr-
ido[3,2-d:2A,3A-f]quinoxaline). As also indicated by the N–Ru–
N angles the geometry about ruthenium atom is distorted from
octahedral. The most noticeable feature is the constraint
imposed on the dCN ligand as reflected by the Ru–N(6)–C(40)
and Ru–N(5)–C(2) angles (167 and 168° respectively) as well
as the large dihedral angle between the two phenyl groups.
In order to demonstrate that dCN and 6,6A-dmbp or biq can be
interchanged, it was essential to show that the starting complex
can be regenerated. This is indeed the case: reaction (2) takes
place quantitatively.
In the presence of a bidentate chelate containing two
convergent nitrile groups, Ru(phen)2L2+ (L = sterically
hindering aromatic diimine ligand; phen = 1,10-phenan-
throline) undergoes a photochemical reaction leading to
quantitative replacement of the diimine by the bis-nitrile
ligand; the reverse reaction takes place upon heating, also
quantitatively.
Multicomponent molecular systems for which certain parts can
be set into motion while others can be considered as motionless
are promising models of biological molecular machines and
motors as well as potential switches.1,2 The signals sent to the
molecules in order to trigger the movement can be of an
electrochemical3 or chemical4 nature, photoinduced redox
processes being also used in a few cases.5 In the search for non-
sacrificial photochemical reactions usable for moving molec-
ular fragments,6 it occurred to us that ligand-field (LF) excited
states of d6 transition metal complexes are strongly antibonding
and can lead, in some cases, to effective ligand exclusion and
substitution by solvent molecules.7 In the present work, we
would like to describe a system in which two different chelates,
an aromatic diimine and a dinitrile, can be efficiently and
cleanly interchanged under the action of light or heat. In
addition, significant photochromism is observed. The ligands
used are represented in Fig. 1.
(2)
For example, a yellow solution of Ru(phen)2(dCN)2+ and biq
in ethyleneglycol was heated at 190 °C for 4 h in the dark. The
colour of the solution changed to deep red. After removal of the
solvent followed by chromatography, Ru(phen)2(biq)2+ was
obtained in 91% yield. Moreover, the photochemical reaction
(6,6A-dmbp or biq release) and the thermal back reaction (dCN
The aromatic diimines 6,6A-dmbp and biq are sterically
hindering so that their removal from a ruthenium(II) trischelate
complex will be favoured over that of phen, also considering
that rotation about the C–C bond between both halves of the
chelate facilitates stepwise decoordination. The bis-nitrile dCN
has been previously described and used by Angelici et al. to
make a few complexes (Mn(
), Fe(II) and Pt(II)).8 Although no
I
structural data are available, dCN is expected to behave as a
bidentate chelate.
The following photochemical reactions have been shown to
take places quantitatively under light irradiation (300 nm @ l @
800 nm):9
(1)
The efficiency of reaction (1) is solvent dependent, the best
solvents being acetone or 1,2-dichloroethane (1,2-DCE). The
Fig. 2 ORTEP view of the cationic part of [Ru(phen)2(dCN)](PF6)2(p-
xylene)2(MeOH) with partial labelling scheme. Ellipsoids are scaled to
enclose 30% of the electronic density. Hydrogen atoms are omitted for
clarity. Selected distances (Å) and angles (°): Ru–N(1) 2.059(9), Ru–N(2)
2.054(9), Ru–N(3) 2.061(9), Ru–N(4) 2.085(8), Ru–N(5) 2.05(1), Ru–N(6)
2.04(1), N(5)–Ru–N(6) 85.1(3), Ru–N(5)–C(25) 168(1), Ru–N(6)–C(40)
167(1), C(26)–C(25)–N(5) 177(1), C(39)–C(40)–N(6) 172(1).
Fig. 1 The various chelates used in photochemically active ruthenium
complexes.
DOI: 10.1039/b004982o
Chem. Commun., 2000, 1935–1936
This journal is © The Royal Society of Chemistry 2000
1935

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Fig. 3 Electronic spectra of a 1,2-DCE solution of Ru(phen)2(biq)2+ in the
presence of dCN (10 eq) after visible light irradiation: 1 (0 s); 2 (20 s); 3 (40
s); 4 (60 s); 5 (90 s); 6 (120 s).
release with recoordination of 6,6A-dmbp or biq) could be
carried out on the same reaction mixture containing an excess of
both ligands, dCN and the aromatic diimine, in addition to the
starting ruthenium(II
)
complex. The solvent, EtOCH2
CH2OCH2CH2OH, in which all the components (ligands and
complexes) are soluble, turned out to be compatible with both
photochemical and thermal processes.
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a NMR tube (f = 5.0 mm) by the use of a Hanimex side projector (150
W halogen lamp).
Interestingly, the absorption spectra of Ru(phen)2(dCN)2+
and Ru(phen)2(biq)2+, for instance, are significantly different.
dCN is a poor s-donor and good p-acceptor, stabilizing the dp
orbitals of Ru(t2g) and thus leading to a relatively high MLCT
(Ru ? phen) excited state. In Ru(phen)2(biq)2+, the nature of
the MLCT bond is different: biq is a better p-acceptor than phen
so that the charge transfer is directed from the metal centre to
biq and no more to the phen ligands. The photochromism of the
system is illustrated in Fig. 3.
10 A mixture of dCN (79 mg) and cis-Ru(phen)2Cl2 (0.272 mmol), EtOH
(10 mL) and H2O (10 mL) was refluxed for 2 h under an argon
atmosphere. To the mixture was added a KPF6 aqueous solution to form
a yellow-orange solid, which was filtered off and washed with water.
Chromatography (SiO2, acetone/H2O/KNO3(aq.) = 100/10/1) followed
by anion exchange gave [Ru(phen)2(dCN)](PF6)2 as a yellow solid in
53% yield. 1H NMR (400 MHz, CD2Cl2) d 9.93 (dd, J = 5.21 Hz, 1.26
Hz, 2H, phen-2H), 8.82 (dd, J = 8.12 Hz, 1.22 Hz, 2H, phen-4H), 8.46
(dd, J = 8.26 Hz, 1.22 Hz, 2H, phen-7H), 8.33 (dd, J = 8.29 Hz, 5.22
Hz, 2H, phen-3H), 8.30 (d, J = 8.88 Hz, 2H, phen-5H), 8.17 (d, J =
8.88 Hz, 2H, phen-6H), 7.89 (dd, J = 5.30 Hz, 1.24 Hz, 2H, phen-9H),
7.65 (ddd, J = 8.92 Hz, 7.60 Hz, 1.72 Hz, 2H, dCN-5H), 7.59 (dd, J =
8.24 Hz, 5.30 Hz, 2H, phen-8H), 7.46 (dd, J = 7.79 Hz, 1.69 Hz, 2H,
dCN-3H), 7.18 (d, J = 8.61 Hz, 2H, dCN-6H), 7.06 (dt, J = 7.64 Hz,
0.52 Hz, 2H, dCN-4H), 4.61 (s, 4H, dCN-CH2); UV/Vis (1,2-DCE):
lmax (e) = 382 nm (14 100 L mol21 cm21).
Without displaying colour changes as large as those obtained
with organic photochromic compounds,13,14 Ru(phen)2(biq)2+
(deep red; lmax = 525 nm in 1,2-DCE) changes colour under
irradiation to afford an orange solution of Ru(phen)2(dCN)2+
(lmax = 382 nm in 1,2-DCE) within a few tens of seconds.
In conclusion, the present system is particularly promising
for the construction of molecular photomechanical devices,
combining motion and photochromism.
This work was performed with financial support from CNRS
and the European Community. We thank André De Cian and
Jean Fischer for the X-ray structure. A. C. L. acknowledges
support from the French Ministry of Education and Y. F. thanks
Osaka Prefecture (Japan) for a fellowship.
11 Crystal data for C57H52F12N6O3P2Ru+[Ru(phen)2(dCN)(PF6)2](p-xy-
lene)2(MeOH), yellow crystal: M
= 1257.06, orthorhombic, a =
17.9740(6), b = 21.1890(9), c = 29.793(1) Å, U = 11 346(1) Å3, T =
294 K, space group Pbca, Z = 8, Dc = 1.47 g cm23, m = 0.422 mm21
.
Crystal dimensions: 0.20 3 0.18 3 0.14 mm, F000: 5112, wavelength:
0.71073 Å, radiation: Mo-Ka graphite monochromated, diffractometer:
KappaCCD, f and w scans, hkl limits: 0.22/ 2 27,27/ 2 38,38, q limits:
2.5/27.49°, number of data measured: 22153, number of data with I >
3s(I): 4186, weighting scheme: 4Fo2/(s2(Fo2) + 0.0064 Fo4), number of
variables: 727, R(F): 0.066, R(F)w: 0.095, GOF: 1.553, largest peak in
final difference: 0.940 e Å23. Package used: OpenMolen, Interactive
Structure Solution, Nonius B.V., Delft, The Netherlands, 1997. CCDC
182/1761. See http://www.rsc.org/suppdata/cc/b0/b004982o/ for crys-
tallograhic files in .cif format.
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