Künstliche molekulare Maschinen

Stereocontrol in dinuclear triple lithium-bridged titanium(IV) complexes: solving some stereochemical mysteries

Compounds 1 a-f-H2 form "monomeric" triscatecholate titanium(IV) complexes [Ti(1 a-f)3](2-), which in the presence of Li cations are in equilibrium with the triple lithium-bridged "dimers" [Li3(Ti(1 a-f)3)2](-). The equilibrium strongly depends on the donor ability of the solvent. Usually, in solvents with high donor ability, the stereochemically labile monomer is preferred, whereas in nondonor solvents, the dimer is the major species. In the latter, the stereochemistry at the complex units is "locked". The configuration at the titanium(IV) triscatecholates is influenced by addition of chiral ammonium countercations. In this case, the induced stereochemical information at the monomer is transferred to the dimer. Alternatively, the configuration at the metal complexes can be controlled by enantiomerically pure ester side chains. Due to the different orientation of the ester groups in the monomer or dimer, opposite configurations of the triscatecholates were observed by circular dichroism (CD) spectroscopy for [Ti(1 c-e)3](2-) or [Li3(Ti(1 c-e)3)2](-). A surprising exception was found for the dimer [Li3(Ti(1 f)3)2](-). Herein, the dimer is the dominating species in weak donor (methanol), as well as strong donor (DMSO), solvents. This is due to the bulkiness of the ester substituent destabilizing the monomer. Due to the size of the substituent in [Li3(Ti(1 f)3)2](-) the esters have to adopt an unusual conformation in the dimer resulting in a stereocontrol of the small methyl group. Following this, opposite stereocontrol mechanisms were observed for the central metal-complex units of [Li3(Ti(1 c-e)3)2](-) or [Li3(Ti(1 f)3)2](-).

Intramolecular interactions of trityl groups

Trityl group, Tr, is a molecular dynamic rotor of which the conformation and helicity depend on other groups in the close vicinity. Interactions with another covalently linked Tr group and with other substituents are analyzed in terms of transfer of chirality to the trityl group. Two trityl groups in a molecule can mutually interact at a distance of two, three, or five bonds. Despite its size, a Tr group attached to a cyclohexane or cyclopentane ring through an oxygen or nitrogen atom adopts either an axial or equatorial position, depending on additional stabilizing interactions, such as hydrogen bonding.

Evaluation of weak interactions in [2]pseudorotaxanes

Viologens readily thread bis-p-phenylene crown ethers to form [2]pseudorotaxanes. However, the binding of sterically hindered 3,3'-dimethylviologens is very weak. Density functional theory (DFT) calculations indicated that the additional energy cost of "flattening" is substantial, 55 kJ mol(-1), and prevents the formation of a stable host-guest complex. The structures of [2]pseudorotaxanes determined by X-ray crystallography are in good agreement with the NMR characterisation and DFT results. Their association constants and thermodynamic parameters in solution were measured by using a dilution method and, for the first time, by host-guest nuclear Overhauser effect (NOE) correlations. The NOE approach was subsequently applied to study the sterically hindered analogues and it was shown that the binding in 3,3'-dimethyl-N,N-dibenzyl [2]pseudorotaxane is by 8.5 kJ mol(-1) weaker than in its regular analogue. The proposed technique helps to quantify weak interactions in [2]pseudorotaxanes and can be applied to other host-guest complexes.

Fe-terpyridyl complex based multiple switches for application in molecular logic gates and circuits

The Fe(ii)-terpyridyl binary complex is used as a molecular switch upon external chemical stimuli. The stability of different switching states, their reversibility upon further chemical stimulus, prompted construction of solution based molecular logic gates and circuits.

Anion sensing by solution- and surface-assembled osmium(II) bipyridyl rotaxanes

We report the preparation of [2]rotaxanes containing an electrochemically and optically active osmium(II) bipyridyl macrocyclic component mechanically bonded with cationic pyridinium axles. Such interlocked host systems are demonstrated to recognise and sense anionic guest species as shown by (1)H NMR, luminescence and electrochemical studies. The rotaxanes can be surface assembled on to gold electrodes through anion templation under click copper(I)-catalysed Huisgen cycloaddition conditions to form rotaxane molecular films, which, after template removal, respond electrochemically and selectively to chloride.

Complete photochromic structural changes in ruthenium(II)-diimine complexes, based on control of the excited states by metalation

The thermal and photochemical reactions of a newly synthesized complex, [Ru(II)(TPA)(tpphz)](2+) (1; TPA=tris(2-pyridylmethyl)amine, tpphz=tetrapyrido[3,2-a:2',3'-c:3'',2''-h: 2''',3'''-j]phenazine), and its derivatives have been investigated. Heating a solution of complex 1 (closed form) and its derivatives in MeCN caused the partial dissociation of one pyridylmethyl moiety of the TPA ligand and the resulting vacant site on the Ru(II) center was occupied by a molecule of MeCN from the solvent to give a dissociated complex, [Ru(II)(η(3)-TPA)(tpphz)(MeCN)](2+) (1', open form), and its derivatives, respectively, in quantitative yields. The thermal dissociation reactions were investigated on the basis of kinetics analysis, which indicated that the reactions proceeded through a seven-coordinate transition state. Although the backwards reaction was induced by photoirradiation of the MLCT absorption bands, the photoreaction of complex 1' reached a photostationary state between complexes 1 and 1' and, hence, the recovery of complex 1 from complex 1' was 67%. Upon protonation of complex 1 at the vacant site of the tpphz ligand, the efficiency of the photoinduced recovery of complex 1+H(+) from complex 1'+H(+) improved to 83%. In contrast, dinuclear μ-tpphz complexes 2 and 3, which contained the Ru(II)(TPA)(tpphz) unit and either a Ru(II)(bpy)2 or Pd(II)Cl2 moiety on the other coordination edge of the tpphz ligand, exhibited 100% photoconversion from their open forms into their closed forms (2'→2 and 3'→3). These results are the first examples of the complete photochromic structural change of a transition-metal complex, as represented by complete interconversion between its open and closed forms. Scrutinization by performing optical and electrochemical measurements allowed us to propose a rationale for how metal coordination at the vacant site of the tpphz ligand improves the efficiency of photoconversion from the open form into the closed form. It is essential to lower the energy level of the triplet metal-to-ligand charge-transfer excited state ((3)MLCT*) of the closed form relative to that of the triplet metal-centered excited state ((3)MC*) by metal coordination. This energy-level manipulation hinders the transition from the (3)MLCT* state into the (3)MC* state in the closed form to block the partial photodissociation of the TPA ligand.

Stimuli-responsive folding and unfolding of a polymer bearing multiple cerium(IV) bis(porphyrinate) joints: mechano-imitation of the action of a folding ruler

A pivotal guest role: a new porphyrin polymer, poly(PorZn⋅DD) (pink/purple), composed of a porphyrinatozinc and a porphyrin double-decker complex as a repeating unit was synthesized. In poly(PorZn⋅DD), porphyrinatozinc complexes recognize a divalent amine (tan/red) to induce an intramolecular pivoting motion through the rotation of porphyrin double-decker complexes and the polymer undergoes shortening and compaction.

Covalent cargo loading to molecular shuttles via copper-free "click chemistry"

An important prerequisite for molecular shuttle-based functional devices is the development of adequate linker chemistries to load and transport versatile cargoes. Copper-free "click chemistry" has not been applied before to covalently load cargo onto molecular shuttles propelled by biological motors such as kinesin. Due to the high biocompatibility and bioorthogonality of the strain-promoted azide-alkyne cycloaddition, this approach has pronounced advantages compared to previous methods.

Oligopyrenotides: chiral nanoscale templates for chromophore assembly

Getting organized: DNA-like supramolecular polymers formed of short oligopyrenotides serve as a helical scaffold for the molecular assembly of ligands. The cationic porphyrin meso-tetrakis(1-methylpyridin-4-yl)porphyrin interacts with the helical polymers in a similar way as with poly(dA:dT).

Synthesis of multivalent host and guest molecules for the construction of multithreaded diamide pseudorotaxanes

A series of di-, tri- and tetravalent axles and wheels for the synthesis of pseudorotaxanes bearing the tetralactam macrocycle/diamide axle binding motif was prepared. Starting from iodinated monovalent precursors, Sonogashira cross-coupling reactions were utilized to couple the binding sites to appropriate spacer groups. Through this "Lego" or "toolbox" approach, the convergent synthesis of host and guests with a well-defined number of the binding sites is possible. In addition, the spatial arrangement of the binding sites can be controlled through the quite rigid connections between linker and binding sites. Although a quantitative assessment of binding strengths was not possible by NMR titration experiments, typical and significant shifts of the signals of the diamide moiety indicate qualitatively the formation of pseudorotaxanes from the axle and wheel precursors.

A unique non-catenane interlocked self-assembled supramolecular architecture and its photophysical properties

A novel, interlocked, self-assembled (M(2)L(2))(2) molecular architecture was constructed from an arene-Ru acceptor and a 1,4-di(pyridin-4-yl)buta-1,3-diyne donor. Two M(2)L(2) units, with cavities of ~7.21 Å, spontaneously interlock, with one unit encapsulating a twin in a non-catenane fashion. The dimeric host-guest complex thus formed is unique among two-dimensional self-assemblies and is stabilized by π-π interactions between the M(2)L(2) units.

Control of rotor function in light-driven molecular motors

A study is presented on the control of rotary motion of an appending rotor unit in a light-driven molecular motor. Two new light driven molecular motors were synthesized that contain aryl groups connected to the stereogenic centers. The aryl groups behave as bidirectional free rotors in three of the four isomers of the 360° rotation cycle, but rotation of the rotors is hindered in the fourth isomer. Kinetic studies of both motor and rotor functions of the two new compounds are given, using (1)H NMR, 2D-EXSY NMR, and UV-vis spectroscopy. In addition, we present the development of a new method for introducing a range of aryl substituents at the α-carbon of precursors for molecular motors. The present study shows how the molecular system can be photochemically switched between a state of free rotor rotation and a state of hindered rotation and reveals the dynamics of coupled rotary systems.