Synthesis and Guest Recognition of Switchable Pt-Salphen Based Molecular Tweezers

Molecular tweezers are artificial receptors that have an open cavity generated by two recognition units pre-organized by a spacer. Switchable molecular tweezers, using a stimuli-responsive spacer, are particularly appealing as prototypes of the molecular machines that combine mechanical motion and allosteric recognition properties. In this present study, the synthesis of switchable molecular tweezers composed of a central terpyridine unit substituted in 4,4″ positions by two Pt(II)-salphen complexes is reported. The terpyridine ligand can be reversibly converted upon Zn(II) coordination from a free ‘U’-shaped closed form to a coordinated ‘W’ open form. This new substitution pattern enables a reverse control of the mechanical motion compared to the previously reported 6,6″ substituted terpyridine-based tweezers. Guest binding studies with aromatic guests showed an intercalation of coronene in the cavity created by the Pt-salphen moieties in the closed conformation. The formation of 1:1 host-guest complex was investigated by a combination of NMR studies and DFT calculations.


General procedures
Reagent grade tetrahydrofuran was distilled from sodium and benzophenone. Tetrahydrofuran and triethylamine were degassed by three freeze-pump-thaw cycles before being used in the Sonogashira coupling reactions. All others chemicals were purchased from commercial suppliers and used without further purification. Complex 5 was synthetized according to the literature. [1] Flash column chromatography was performed using silica gel from Merck (40-63 µm) or GraceResolv High Resolution Flash Cartridges (particle size 40 µm). Thin layer chromatography was performed using aluminium plates pre-coated with silica gel or neutral aluminum oxide 60 F254 0.20 mm layer thickness purchased from VWR. Absorption spectra were recorded on a JASCO V-670 spectrophotometer. Infrared spectra were recorded on a Bruker tensor 27 ATR spectrometer. Electrospray ionisation (ESI) mass spectrometry was performed on a Bruker microTOF spectrometer.

Titration procedures
1 H NMR titrations were performed using CDCl 3 dried over molecular sieves (4 Å) and passed through dried neutral aluminum oxide. Metal salts and tris(2-aminoethyl)amine were used without any purification. All solutions of tweezers, metal salts and ligands, used for titrations were prepared in volumetric flasks, and additions were made with Hamilton syringes. Tweezers 1 opening: To 0.5 mL of closed tweezers 1 (1.0 × 10 -3 M) dissolved in DMSO-d 6 in an NMR tube (5 mm), were added 0.2 eq of ZnCl 2 (4 µL of a 2.5 × 10 -2 M solution in D 3 CCN). After each metal addition, the tube was heated at reflux during 5 seconds, then cooled at room temperature, and the 1 H NMR spectrum was recorded.
Guest binding: To 0.5 mL of closed tweezers 1 (2.0 × 10 -3 M) dissolved in CDCl 3 in an NMR tube (5 mm), were added coronene as a solid. After each metal addition, the tube was heated at reflux during 5 seconds, then cooled at room temperature, and the 1 H NMR spectrum was recorded.
UV-visible absorption spectra were recorded on a JASCO V-670 spectrophotometer at 25°C. CHCl 3 was dried over molecular sieves 4 Å and neutralized on neutral Al 2 O 3 . Metal salts were used without any purification. Solutions of tweezers, metals, used for titrations were prepared in volumetric flasks, and additions were made with Hamilton syringes. The metal salt concentrations in the stock solutions were checked by titration with a terpyridine solution. Curve fitting were performed by a nonlinear least-squares fit of the absorbance versus the concentration of guest added using the Matlab program developed by P. Thordarson. [4] The titrations monitored by UV-Visible spectroscopy have been performed according to the following general procedure:

Computational details
Calculations were performed with the Gaussian 09 software. [5] Complete geometry optimizations were carried out using the density functional theory method with the conventional Becke-3-Lee-Yang-Parr (B3LYP) exchange-correlation functional and 6-31G**/LanL2DZ. The platinum atoms were modeled using the effective core potential and the corresponding valence orbitals LanL2DZ in order to decrease the number of basis functions. The other atoms were described by the double zeta 6-31G** base which takes into account the polarization orbitals of all atoms, including hydrogen atoms. Vibrational analysis was performed at the same level in order to check the obtaining of a minimum on the potential energy surface.