Complete rearrangement of a multi-porphyrinic rotaxane by metallation-demetallation of the central coordination site

Article abstract of DOI:10.1039/a805746j

A new multiporphyrinic [2]rotaxane has been made in which a gold(III) porphyrin is part of the ring; rotation of the string-like fragment within the ring between two diametrically opposed positions is triggered by metallation-demetallation of the central

Full text of DOI:10.1039/a805746j

Complete rearrangement of a multi-porphyrinic rotaxane by
metallation–demetallation of the central coordination site
Myriam Linke, Jean-Claude Chambron, Vale´rie Heitz, Jean-Pierre Sauvage* and Vincent Semetey
Laboratoire de Chimie Organo-Mine´rale, UMR 7513 du CNRS, Universite´ Louis Pasteur, Institut Le Bel, 4, rue
Blaise Pascal, 67000 Strasbourg, France. E-mail: sauvage@chimie.u-strasbg.fr
Received (in Basel, Switzerland) 22nd July 1998, Accepted 25th September 1998
A new multiporphyrinic [2]rotaxane has been made in which
a gold(III) porphyrin is part of the ring; rotation of the string-
like fragment within the ring between two diametrically
opposed positions is triggered by metallation–demetallation
of the central coordination site.
to 6 (tosyl chloride, NEt3, CH2Cl2) and subsequently to 7 (NaI,
acetone; 30% yield from 4). Macrocycle 11 was prepared from
3 and 7 (Cs2CO3 in DMF, 55 °C; 31% yield). Rotaxane 14 was
synthesized following a strategy previously developed in our
group for making porphyrin-stoppered rotaxanes.9 An equimo-
+
lar mixture of 8, 11 and Cu(MeCN)4 led quantitatively to
In relation to molecular switches,1 machines and motors,2,3 it is
of special interest to be able to control at will, amongst other
properties, the shape of multicomponent molecular systems.
The triggering signal, responsible for the rearrangement of the
compound can be photochemical,4 electrochemical5 or chem-
ical.6 Catenanes and rotaxanes are ideally suited for undergoing
large amplitude motions under the action of an external
stimulus.7,8
In the present work, we show that complexing or decom-
plexing an appropriate metal in a coordination site can bring to
close proximity, or spread a long distance apart, given
porphyrinic components of the system. The principle is depicted
in Fig. 1.
prerotaxane 12 (not drawn) in which the open chain fragment 8
has been threaded through the ring 11 thanks to the gathering
effect of copper(i). The terminal porphyrinic blocking groups of
13 were built from 9, 10 and 12 (CF3CO2H in CH2Cl2;
chloranil). 14† was obtained after metallation [Zn(OAc)2] in
13% yield from 8.
The conformation of 14 is indeed similar to what the drawing
of Fig. 3 suggests. In particular, NOE effects measured on H5,6
and Hpy demonstrate unambiguously that a close proximity
The compound made and studied is a [2]rotaxane in which
the string-like fragment bears two zinc(ii) porphyrins as
blocking groups. The ring through which the string is threaded
incorporates a gold(iii) porphyrin. It should be noted that these
metalloporphyrins are key components of multichromophoric
systems undergoing photoinduced electron transfer and pro-
posed as models of given fragments of the photosynthetic
reaction centre.9
The organic compounds used as intermediates in the
synthesis of the [2]rotaxane are represented in Fig. 2. The
tetraaryl porphyrin 1 was prepared in 6% yield using pyrrole
and a 1:1 mixture of the appropriate aldehydes (CF3CO2H, in
CH2Cl2 followed by chloranil treatment10). After metallation
(KAuCl4) to afford 2 (76%), demethylation (BBr3) furnished 3
in almost quantitative yield. 5 was obtained by reacting 411 with
2-bromoethanol (K2CO3, refluxing DMF) and it was converted
Fig. 1 Control of the mutual arrangement between the gold porphyrin (PAu+
incorporated in the ring, black diamond) and the zinc porphyrins (PZn end-
function of the dumbell, white diamond) by complexation/decomplexation
of a metal centre (black circle) within/from the central coordination site. (a)
The chemical structure of the two organic constitutive fragments of the
rotaxane (ring and thread) is such that, in the complex, the gold porphyrin
is remote from the two zinc porphyrins. (b) After removal of the central
metal, weak forces may favour an attractive interaction between PAu+ and
the PZn nuclei, leading to a situation in which PAu+ is pinched between the
two PZn units. The interconversion between the situations [(a) and (b)]
implies a half-turn rotation of the threaded fragment (axle) within the ring
(wheel), the latter being artificially considered as fixed. This motion is
reminiscent of the process taking place in the rotary motor of ATP-
synthase.3b
Fig. 2 Intermediates used in the synthesis of the [2]rotaxane.
Chem. Commun., 1998, 2469–2470
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Interestingly, the interconversion between 14 and 15, although
leading to dramatic geometrical changes, is quantitative and
reversible. This changeover process can be triggered by other
metals such as Ag+ and Li+.
We thank the Ministry of Education for a fellowship (to
M. L.).
Notes and references
† Selected data: 14: dH(CD2Cl2, 400 MHz) 10.13 (s, 4H), 9.60 (d, 2H), 9.47
(d, 2H), 9.43 (s, 2H), 9.30 (s, 2H), 8.90 (d, 2H), 8.52 (d, 2H), 8.42 (d, 4H),
8.31 (d, 2H), 8.17 (d, 4H), 8.13 (d, 4H), 8.04 (t, 2H), 7.98 (s, 2H), 7.97 (d,
2H), 7.89 (d, 4H), 7.86 (t, 2H), 7.78 (d, 4H), 7.75 (s, 2H), 7.62 (d, 4H), 7.54
(d, 4H), 6.63 (d, 4H), 4.95 (m, 4H), 4.41 (m, 4H), 3.95 (t, 8H), 3.81 (t, 8H),
2.44 (s, 12H), 2.17 (t, 8H), 2.06 (t, 8H), 1.84 (s, 12H), ≈ 1.68 (m, 16H), 1.60
(s, 36H), 1.51 (s, 36H), ≈ 1.50 (m, 16H), ≈ 1.40 (m, 16H), 0.87 (t, 12H),
0.83 (t, 12H); m/z (FAB) 3783.6 (M+); l(CH2Cl2)/nm 415, 538, 574.
15: dH(CD2Cl2, 400 MHz) 10.07 (s, 4H), 9.56 (d, 2H), 9.39 (d, 2H), 9.36
(s, 2H), 9.15 (d, 4H), 8.80 (s, 2H), 8.57 (d, 2H), 8.46 (d, 2H), 8.26 (d, 4H),
8.21 (d, 4H), 8.18 (d, 4H), 8.11 (d, 2H), 8.09 (d, 4H), 8.00 (t, 2H), 7.99 (d,
2H), 7.91 (d, 4H), 7.90 (s, 2H), 7.85 (d, 4H), 7.82 (t, 2H), 7.46 (s, 2H), 7.39
(d, 4H), 4.82 (m, 4H), 4.43 (m, 4H), 3.91 (t, 8H), 3.70 (t, 8H), 2.40 (s, 12H),
2.27 (s, 12H), 2.12 (t, 8H), 1.93 (t, 8H), 1.68 (t, 8H), 1.55 (s, 36H), 1.47 (s,
36H), ≈ 1.40 (m, 16H), ≈ 1.35 (m, 8H), ≈ 1.25 (m, 8H), ≈ 1.20 (m, 8H),
0.85 (t, 12H), 0.72 (t, 12H); m/z (FAB) 3720.6 (M+); l(CH2Cl2)/nm 413,
538, 574.
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Chem Commun., 1998, 2469–2470