Optical reading of the open and closed states of a molecular turnstile

Article abstract of DOI:10.1039/c4cc01071j

A molecular turnstile composed of a hydroquinone based rotor and a stator bearing a tridentate coordinating site can be reversibly switched between open and closed states. The locking and unlocking processes may be read optically.

Full text of DOI:10.1039/c4cc01071j

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Optical reading of the open and closed states
of a molecular turnstile†‡
Cite this: Chem. Commun., 2014,
50, 5040
Nicolas Zigon, Patrick Larpent, Abdelaziz Jouaiti, Nathalie Kyritsakas and
Mir Wais Hosseini*
Received 10th February 2014,
Accepted 31st March 2014
DOI: 10.1039/c4cc01071j
www.rsc.org/chemcomm
A molecular turnstile composed of a hydroquinone based rotor and
a stator bearing a tridentate coordinating site can be reversibly
switched between open and closed states. The locking and unlocking
processes may be read optically.
The control of intramolecular movements has attracted con-
siderable efforts over the last two decades and is still of current
interest.1–3 Among many dynamic systems such as motors,
rotors and machines, molecular turnstiles form an interesting
ensemble.4 Over the last few years, we have investigated a series
of porphyrins5 or Pt(II) organometallic6 based turnstiles.
Herein, we report on a new system for which the open and
closed states display different luminescence properties.
The turnstile 1 (Scheme 1) is composed of a rotor based on a
hydroquinone moiety equipped with two divergently oriented
benzonitrile units as monodentate coordinating sites and a
stator bearing a tridentate chelating unit. The terphenyl group
was chosen as the hinge due to its luminescence properties. As
the external effector leading to the closed state of the turnstile
1-Pd, Pd(II) was used.
The rational behind the choice of the latter was its square
planar coordination geometry allowing thus its simultaneous
binding by the rotor and stator and its propensity to behave as a
heavy atom and thus to quench the emission of the terphenyl
moiety.
For the open state of the turnstile 1 (Fig. 1O1), the rotor
should freely rotate around the stator, whereas in the presence
of Pd(II), the turnstile would be locked in its closed state
(Fig. 1C).
Scheme 1 Structures of compounds 1–3 (L = DMAP or CNÀ).
The synthesis of 1 (see ESI‡) was achieved in 8 steps. The
precursor macrocyclic compound 3 was obtained in 90% yield
upon condensation of the handle 2, prepared in 6 steps,5g with
2,5-dibromohydroquinone in DMF in the presence of Cs2CO3.
The turnstile 1 was obtained in 68% yield by coupling
4-benzonitrile boronic acid with the dibromo compound 3 in
the presence of Pd(PPh3)4. The closed state of the turnstile
(1-Pd) was generated quantitatively upon metallation of compound 1
by Pd(OAc)2.
In addition to solution characterization, the solid-state
structure of 1-Pd was studied by X-ray diffraction (Fig. 2, see
ESI‡). Single crystals of 1-Pd were obtained at 25 1C upon slow
diffusion of Et2O into a CHCl3 solution of 1-Pd. The latter
crystallizes (monoclinic P21/n) without any solvent. The Pd(II)
cation, adopting a strongly distorted square planar geometry
(NPdNcis angle in the 80.71–99.81 range and NPdNtrans angles of
161.61 and 175.91), is coordinated to the tridentate chelating
moiety of the stator (Pd–N distance in the 1.92–2.02 Å range) and
to the N atom of the benzonitrile unit of the rotor (dPdN = 2.01 Å).
Molecular Tectonic Laboratory, UMR UDS-CNRS 7140, icFRC, University of
Strasbourg, Institut Le Bel, 4, rue Blaise Pascal, F-67000 Strasbourg, France.
E-mail: hosseini@unistra.fr
† This contribution is dedicated to J.-P. Sauvage on the occasion of his 70th
birthday.
‡ Electronic supplementary information (ESI) available: Experimental details,
characterisation of new compounds, additional 2D-NMR and UV/luminescence
spectra. CCDC 985992. For ESI and crystallographic data in CIF or other electro-
nic format see DOI: 10.1039/c4cc01071j
Fig. 1 Schematic representation of the turnstile in its open (O1 and O2)
and closed (C) states.
5040 | Chem. Commun., 2014, 50, 5040--5042
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These observations have been further substantiated by
1H–1H NOESY experiments (Fig. 4). Indeed, in addition to the
expected Hn/Hl and HN/HL correlations, only through space
correlations between the HQ and HR atoms of the benzonitrile
moiety bound to the Pd(II) centre and hydrogen atoms of the
oligoethylenglycol chains (OCH2) are observed.
Fig. 2 X-Ray structure of 1-Pd. H atoms are omitted for clarity. For bond
angles and distances see the text.
The return to the open state was achieved using a CNÀ anion
as an external ligand competing with the benzonitrile moiety.
The addition of 1 eq. of CNÀ (Fig. 3c) leads to the opening of the
turnstile (Fig. 1O2) leading to 1–Pd–CNÀ (Scheme 1). This was
1
evidenced by H-NMR, which revealed a unique singlet for Hn
and two doublets for Hq and Hr indicating the equivalence of
the two benzonitrile moieties and thus the unlocking of the
turnstile. The fact that Ha and Hb are only slightly shifted and
no NH signal is observed indicate that the Pd(II) remains bound
to the handle and its coordination sphere is completed by a
CNÀ ligand. Further addition of 3 eq. of CNÀ (Fig. 3d) leads to
the coexistence of the two open states O1 (1) and O2 (1-Pd-CNÀ)
resulting from the strong affinity of the cyanide anion for Pd(II).
Fig. 3 Portions of 1H-NMR (CD2Cl2, X00 MHz, X = 5 for (a) and (b) and
X = 3 for (c–e), 298 K): (a) turnstile 1, (b) 1-Pd, (c) 1-Pd + 1 eq. of TBACN, Finally, the complete return to the turnstile 1 is achieved by
addition of 8 eq. of CNÀ anion (Fig. 3e).
(d) 1-Pd + 3 eq. of TBACN, (e) 1-Pd + excess of TBACN. For signal
assignment see Scheme 1.
Interestingly, owing to the rather weak binding propensity of
benzonitrile towards transition metals, the open state O2 was
reached in the presence of p-dimethylaminopyridine (DMAP), a
The solution dynamic behaviour of 1 was studied at 25 1C by
stronger ligand replacing the bound benzonitrile within the
1- and 2-D NMR. In the aromatic region of the 1H-NMR
coordination sphere of Pd(II). The process was again monitored
spectrum (Fig. 3a), the signal at 7.15 ppm corresponding to
Hn appears as a unique singlet whereas signals corresponding
by 1H-NMR at 25 1C in CD2Cl2 (Fig. 5). Upon addition of 1 eq. of
DMAP, the two singlets corresponding to Hn and HN (Fig. 5a)
to Hq and Hr at 7.84 and 7.77 ppm, respectively, appear as two
were transformed into a unique singlet (Fig. 5b). The same
doublets (see Scheme 1 for assignment). These observations
behaviour was observed for the four doublets corresponding to
imply a free rotation of the rotor around the stator defining
Hq, Hr, HQ and HR, which appeared as two doublets (Fig. 5b).
thus the open state of the turnstile (Fig. 1O1).
The closed state of the turnstile (1-Pd) was obtained upon
binding of Pd(II) behaving as the external effector. Indeed the
These observations imply the equivalence of the two benzonitrile
units resulting from the free rotation of the handle (Fig. 1O2).
Taking advantage of the strong basicity of DMAP, the return to
latter, in a square planar geometry, is bound simultaneously to the
the closed state of the turnstile was achieved upon addition of
tridentate moiety of the handle and one of the two monodentate
methanesulfonic acid (MsOH) causing the protonation of DMAP
sites of the rotor freezing thus the rotational movement (Fig. 1C).
and its replacement by one of the two benzonitrile groups
This is evidenced by the disappearance of the NH signal and by the
(Fig. 5c). This, as expected, leads to the splitting of signals
shift of signals corresponding to Ha and Hb hydrogen atoms of the
corresponding to Hn, Hr, and Hq. Finally, the open state O2
central pyridyl moiety of the handle by 0.04 and À0.60 ppm
was regenerated upon deprotonation of DMAP-H+ by addition of
respectively (Fig. 3b). Furthermore, the insertion of Pd(II) caused
1.5 eq. of NEt3 (Fig. 5d).
the splitting of the Hn signal into two singlets Hn at 6.93 and HN at
7.84 ppm corresponding to the unbound and bound benzonitrile
moieties (Fig. 3b). As expected, the two doublets corresponding to
Hq and Hr were also split into four doublets assigned (Scheme 1) to
Hq (7.73 ppm), Hr (7.69 ppm), HN (9.22 ppm) and HR (8.08 ppm).
Fig. 5 Portions of 1H-NMR (CD2Cl2, X00 MHz, X = 5 for (a) and (b) and
X = 3 for (c) and (d), 298 K): (a) 1-Pd, (b) 1-Pd + 1 eq. of DMAP, (c) addition
Fig. 4 A portion of the NOESY spectrum of 1-Pd (CD2Cl2, 298 K, 500 MHz).
of 1 eq. MsOH to 1-Pd-DMAP, (d) addition of 1.5 eq. Et3N. For signal
For signal assignment see Scheme 1.
assignment see Scheme 1. * correspond to signals of DMAP.
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In conclusion, the purely organic turnstile 1, composed of a
rotor based on a hydroquinone derivative bearing two peripheral
coordinating sites of the benzonitrile type connected to a stator
equipped with a chelating moiety, undergoes a reversible locking–
unlocking process between an open and a closed state. The closed
state is generated upon binding of Pd(II) as an effector. The switch
between the locked and unlocked states is achieved either by a
competitive external ligand such as CNÀ or DMAP or in the latter
case by an acid (MsOH) and a base (Et3N). Interestingly, owing to
the emissive nature of the turnstile, whereas the open state O1 is
strongly luminescent, for the closed state, the emission is
quenched by the presence of Pd(II) leading thus to an optical
reading of the two states.
Fig. 6 UV-visible spectra (CH2Cl2, 298 K) of 1 (black) and 1-Pd (red).
Financial support by the University of Strasbourg, the
International Centre for Frontier Research in Chemistry (PhD
fellowship to P.L.), Strasbourg, the Institut Universitaire de
France, the CNRS and the Ministry of Education and Research
(PhD fellowship to N.Z.) is acknowledged.
Fig. 7 Picture of 1 (left) and 1-Pd (right) in CH2Cl2 (298 K, aerated, lexc
365 nm) and emission spectra for iso-absorbing solutions of the open 1
=
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5042 | Chem. Commun., 2014, 50, 5040--5042
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