S. Ernst and G. Haberhauer
Conclusion
All in all, we were able to show that it is possible to control
the unidirectional open–close mechanism of metal-ion-
driven molecular hinges. Whether or not and to what extent
that mechanism works can be determined by the choice of
the solvent, the choice of the metal ion, and the choice of
the combination of bridges and clamps. By variation of the
metal ions and the clamp-bridge combinations, the height of
the amplitude of motion can be influenced. Quantum me-
chanical calculations show that it is possible to reach ampli-
tudes from 45 up to 1908. This combination of initializing or
preventing a movement by choosing the adequate chemical
conditions and the control of the amplitude of motion can
be used for more complex switching or motion processes of
molecular machines.
Figure 8. Relative shift of the absorptions maximum (Dlmax) of the bipyri-
dine band of the complex [4·Zn
(MeCN)2]2+ in relation to the dihedral
AHCTUNGTRENNUNG
angle N1-C2-C2’-N1’ calculated by TD-DFT/6-31G* (lmax =326 nm for
N1-C2-C2’-N1’=08).
Experimental Section
of 1 in methanol, the amplitude of motion in the closing
process amounts to 1508 and is much lower than with Zn2+
in dichloromethane/acetonitrile.
General remarks: All chemicals were reagent grade and used as pur-
chased. Reactions were monitored by TLC analysis using silica gel 60
F254 thin-layer plates. Flash chromatography was carried out on silica gel
60 (230–400 mesh). 1H and 13C NMR spectra were measured using
Bruker Avance DMX 300 and Avance DRX 500 spectrometers. All
chemical shifts (d) are given in ppm relative to TMS at 258C. The spectra
were referenced to deuterated solvents indicated in brackets in the ana-
lytical data. HRMS spectra were recorded using a Bruker BioTOF III In-
strument. IR spectra were measured using a Varian 3100 FTIR Excalibur
Series spectrometer. UV and CD absorption spectra were taken using a
Jasco J-815 spectrophotometer.
The fact that the formation of the complex is successful in
methanol with Hg2+ whereas no complex is found in di-
chloromethane/acetonitrile can probably be ascribed to the
metal ions that are differently solvated in distinct solvents:
in dichloromethane/acetonitrile the large Hg2+ ion is com-
plexed by acetonitrile and therefore it is too large to get
into the cavity between the bipyridine unit and the clamp.
As mentioned before, a complex in which the metal ion is
placed outside the cavity cannot be formed. In methanol/
acetonitrile the Hg2+ ion is complexed by the smaller meth-
anol molecules so that it can get into the cavity. Neverthe-
less, no ideal complex is formed and the resulting enlarge-
ment of the cavity by Hg2+ leads to a dihedral angle that
differs from 08 and amounts to approximately 308.
The hinges 2 and 3 show a similar behavior to (pS)-1. The
same solvent and metal ion effects can be observed, but the
changes in the CD and UV spectra are less distinctive.
Hinge 3 shows, for example, a bathochromic shift of the bi-
pyridine band from 306 to 331 nm upon addition of Zn2+ in
dichloromethane, which is almost identical to the values
found for 1. This is consistent with the above-described cal-
culations that predict a motion amplitude of 1908.
Hinge 1: Caesium carbonate (130 mg, 0.400 mmol) was added to a solu-
tion of scaffold 12 (27 mg, 0.048 mmol) and bipyridine
8 (25 mg,
0.048 mmol) in anhydrous acetonitrile (35 mL) and the mixture was
heated to reflux at 908C for 5 h. After cooling to room temperature,
ethyl acetate (60 mL) and water (15 mL) were added. The organic layer
was washed with water (3ꢃ10 mL), dried (MgSO4), and concentrated in
vacuo. Column chromatography of the residue on silica gel (CH2Cl2/
EtOAc/MeOH, 75:25:3) afforded hinge 1 (11 mg, 25%) as a white solid.
M.p. >2508C; 1H NMR (500 MHz, CD3OD/CDCl3): d=7.62 (dd,
3J
3J
3J
N
E
ACHTUNGTREN(NUNG H,H)=7.9 Hz,
ACHTUNGTRENNUNG
E
ACHTUNGTRENNUNG
(H,H)=8.1 Hz, 4J
R
2
3
2H; Har), 5.27 (d, J
N
ACHTUNGTREN(NUNG H,H)=6.5 Hz,
2H; NHCHCH), 4.87 (d, 2J
ACHTUNGTRENNUNG
3J
A
E
ACHTUNGTRENNUNG
3
3
0.92 (d, J
CH
G
G
ACHTUNGTRENNUNG
N
N
ACHTUNGTRENNUNG
There is also an experimental affirmation of the calcula-
tions made for 2. According to the calculations, the dihedral
angles of (pS)-2 and (aS)-2 differ significantly from 1808.
This has to result in a shift of the maximum of the absorp-
tion band of the bipyridine. In the UV spectrum there is
indeed an absorption maximum located at 295 nm that is
shifted hypsochromically (12 nm) compared to the absorp-
tion maximum of 1. A specific value for the amplitude of
motion of 2 cannot be deduced from the spectra, but in ac-
cordance with the calculation, it should be much lower than
that for hinge 1.
(125 MHz, CD3OD/CDCl3): d=170.2, 162.0, 161.5, 155.8, 152.4, 146.1,
141.3, 135.0, 132.5, 130.0, 129.6, 121.7, 120.3, 117.7, 114.7, 112.0, 58.1,
50.4, 46.8, 34.0, 32.3, 19.1, 18.5, 18.3, 17.6, 9.6 ppm; IR (KBr): n˜ =3327,
2920, 2851, 1724, 1660 cmÀ1; UV/Vis (MeOH): l (loge)=307 nm (3.92) ;
HRMS (ESI): m/z calcd for C52H61N10O6 [M+H]+: 921.4776; found:
921.4796.
Hinge 2: Caesium carbonate (98 mg, 0.300 mmol) was added to a solu-
tion of scaffold 12 (19 mg, 0.034 mmol) and bridge 11 (18 mg,
0.034 mmol) in anhydrous acetonitrile (25 mL) and the mixture was
heated to reflux at 908C for 5 h. After cooling to room temperature,
ethyl acetate (30 mL) and water (15 mL) were added. The organic layer
was washed with water (3ꢃ10 mL), dried (MgSO4), and concentrated in
vacuo. Column chromatography of the residue on silica gel (CH2Cl2/
EtOAc/MeOH, 75:25:2) provided hinge 2 (5.0 mg, 16%) as a white solid.
13414
ꢂ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2009, 15, 13406 – 13416