Pierre Mobian

Transition metal complexes as molecular machine prototypes.

The field of molecular machines, i.e. multicomponent systems able to undergo large amplitude motions under the action of an external signal, has experienced a spectacular development since the beginning of the 1990s. Transition metal complexes have played an important role in this context, often as components of catenanes and rotaxanes. The present tutorial review will discuss a few systems of this type, taken from the contributions of our group or from others. The stimulus responsible for the controlled motion of the machine can be chemical, electrochemical, or photochemical. Examples of these three categories will be considered.

Towards the stereoselective synthesis of inherently chiral pseudorotaxanes.

Herein is reported an investigation towards the stereoselective synthesis of inherently chiral pseudorotaxanes. Chiral ammonium threads were readily prepared in five steps from racemic or enantiopure (M or P) salts of di-n-propyl-1,13-dimethoxyquinacridinium cation. Their self-assembly with DB24C8 or disymmetrically oriented DB24C8F6 rings formed pseudorotaxanes as shown by 1H and 19F NMR spectroscopy as well as MS measurements. A determination of the association constants (Ka) was afforded. The crucial role played by the ammonium counter-ion in the threading process was further demonstrated as salts of TRISPHAT (tris(tetrachlorobenzenediolato)phosphate(V)) anion were quite more effective than their PF6- analogues (x 7.3). A general lack of diastereoselectivity (de <or= 8%) was unfortunately observed.

Synthesis, resolution, and VCD analysis of an enantiopure diazaoxatricornan derivative.

Using simple organic synthetic transformations, a novel diazaoxatricornan derivative, the 12 c-methyl-12-phenyl-8-propyl-12,12 c-dihydro-8 H-4-oxa-8,12-diazadibenzo[ cd, mn]pyrene ( 6a), was prepared. This novel chiral cup-shaped molecule was isolated in racemic form and in excellent yield after the addition of methyl lithium to the BF 4 salt of a novel unsymmetrical diazaoxatriangulenium cation. Compound 6a was found to be stable under classical laboratory conditions-something not obvious considering the extreme stability of the carbenium ion precursor, the electron-rich nature of the core, and the strain induced by the pyramidalization of the central carbon. The enantiomers were readily separated by chiral stationary phase chromatography, and the absolute configuration of (-)-( S)- 6a was determined by a comparison of the experimental and theoretical vibrational circular dichroism (VCD) spectra. This isolation of (-)-( S)- 6a and (+)-( R)- 6a constitutes thus the first report of a nonracemic closed-capped chiral bowl molecule for which the chirality is due to the intrinsic dissymmetry of the central core of the structure only.

Stereoselective Synthesis of Biphenolate/Binaphtolate Titanate and Zirconate Alkoxide Species: Structural Characterization and Use in the Controlled ROP of Lactide

Well-defined biphenol/binaphtolate group 4 alkoxide salt species [(Ph-Biphen-O)(2)M(O(i)Pr)]Li(THF) (2a, M = Ti; 4a, M = Zr) and [(Ph-binapht-O)(2)M(O(i)Pr)]Li(THF) (2b, M = Ti; 4b, M = Zr) were found to be readily accessible in good yields via alcohol elimination routes and/or substitution reactions from the corresponding pro-ligands Ph-Biphen-OH (1a) and rac-Ph-Binapht-OH (1b). As established via X-ray crystallographic analysis, the molecular structures of the Ti derivatives 2a and 2b consist of Li(+) salts of anionic Ti-O(i)Pr moieties in which the Ti center adopts a distorted tbp geometry and is effectively chelated by two biphenolate/binaphtolate units. Remarkably, the solution and solid state data for salt species 2a,b agree with the sole presence of one diastereomer (with a (Δ, aS, aS)/(Λ, aR, aR) configuration), thus indicating that formation of the Ti and Zr alkoxide complexes 2a,b/4a,b proceeds stereoselectively. In contrast, the neutral biphenolate/binaphtolate Zr complexes (Ph-biphen-O)(2)Zr(THF)(2) (3a) and (Ph-binapht-O)(2)Zr(THF)(2) (3b) were both isolated and X-ray characterized as stereomers in a heterochiral configuration (Δ, aR, aS)/(Λ, aS, aR). The Ti and Zr alkoxide anionic chelates were found to initiate the ROP of rac-lactide in a controlled manner for production of narrowly disperse and ester-end group PLA, as deduced from SEC, kinetic, and MALDI-TOF data. The Zr-O(i)Pr derivatives 4a,b exhibit superior performance to their Ti counterparts (whether regarding activity, polymerization control, or stereoselectivity) to produce narrowly disperse and heterotactically enriched PLA (P(r) = 0.67, PDI < 1.15). The significantly decreased Lewis acidity of the Zr metal center in anions 4a,b (versus neutral analogues) due to the anionic charge and a likely substantial electronic π donation of the four Zr-O(ArO) oxygens to the Zr metal center may rationalize the moderate polymerization activity. Control experiments suggest that the nature of the countercation has little influence on lactide ROP activity and stereocontrol.

From Photoinduced Charge Separation to Light-driven Molecular Machines

The photochemical properties of transition metal complexes, such as those of iridium(III) or ruthenium(II),can be exploited in various ways to generate charge-separated (CS) states, in relation to the mimicry ofthe natural photosynthetic reaction centres, or to set multicomponent compounds or assemblies in motion.The first part of the present chapter summarizes the work carried out in our groups (Bologna and Strasbourg)in recent years with iridium(III)-terpy complexes (terpy: 2,2′,6′,6′′-terpyridine).The synthesis of multicomponent iridium(III) complexes in reasonable yields has been achieved and theirphotochemical properties have been investigated. Unexpectedly, the excited state lifetimes of some of thesecompounds are very long at room temperature (several microseconds) in fluid solution, making the Ir(terpy)2 3+fragment an interesting chromophore. Once attached to electron donor (D) groups, dyads of the Ir(terpy)2 3+-Dtype undergo fast photoinduced electron transfer. In addition Ir(terpy)2 3+in the ground state is a relatively good electron acceptor, displaying interesting properties as electronrelay in porphyrinic triads. A triad, consisting of an Ir(terpy)2 3+central core, a Zn porphyrin as the primary donor on one side and a gold(III) porphyrin as theterminal acceptor on the other side, leads to a relatively long-lived CS state (close to the microsecond).The other section of the present chapter deals with light-driven molecular machines built around Ru(bpy)3 2+derivatives, including catenanes and rotaxanes. In order to set the system in motion, a dissociativeligand field (LF) state is generated from the light-absorbing metal-to-ligand charge transfer (MLCT) state,originating in the expulsion of a given ligand in a perfectly controlled fashion. This step israpidly followed by coordination of another ligand to afford a kinetically stable new complex. Theprocess can be inverted by thermal energy, so as to regenerate the starting state of the system.

A remarkable solvent effect on the nuclearity of neutral titanium(IV)-based helicate assemblies.

The spontaneous self-assembly of a neutral circular trinuclear Ti(IV) -based helicate is described through the reaction of titanium(IV) isopropoxide with a rationally designed tetraphenolic ligand. The trimeric ring helicate was obtained after diffusion of n-pentane into a solution with dichloromethane. The circular helicate has been characterized by using single-crystal X-ray diffraction study, (13) C CP-MAS NMR and (1) H NMR DOSY solution spectroscopic, and positive electrospray ionization mass-spectrometric analysis. These analytical data were compared with those obtained from a previously reported double-stranded helicate that crystallizes in toluene. The trimeric ring was unstable in a pure solution with dichloromethane and transformed into the double-stranded helicate. Thermodynamic analysis by means of the PACHA software revealed that formation of the double-stranded helicates was characterized by ΔH(toluene)=-30 kJ mol(-1) and ΔS(toluene)=+357 J K(-1)  mol(-1) , whereas these values were ΔH(CH2 Cl2 )=-75 kJ mol(-1) and ΔS(CH2 Cl2 )=-37 J K(-1)  mol(-1) for the ring helicate. The transformation of the ring helicate into the double-stranded helicate was a strongly endothermic process characterized by ΔH(CH2 Cl2 )=+127 kJ mol(-1) and ΔH(n-pentane)=+644 kJ mol(-1) associated with a large positive entropy change ΔS=+1115 J K(-1) ⋅mol(-1) . Consequently, the instability of the ring helicate in pure dichloromethane was attributed to the rather high dielectric constant and dipole moment of dichloromethane relative to n-pentane. Suggestions for increasing the stability of the ring helicate are given.

Spontaneous symmetry breaking during self-assembly of a double stranded biphenolate-based Ti(IV)-helicate.

A binuclear Ti(IV)-based helicate synthesized from a symmetric tetrahydroxyheptaphenylene strand was self-assembled in solution and shown to undergo a spontaneous head-to-tail differentiation according to single-crystal X-ray diffraction.

Copper-Complexed Pirouetting (2)pseudorotaxanes with Sulfur-Containing End-Groups Attached to the Thread: Synthesis, Electrochemical Studies, and Deposition on Gold Electrodes

Two copper [2]pseudorotaxanes incorporating a macrocycle with two chelating sites (2,9-diphenyl-1,10-phenanthroline and 2,2′,6′,2″-terpyridine) and a thread based on crescent-shaped 2,2′-bipyridine derivatives have been synthesized and characterized. One of the threads was a hindering 2,2′-bipyridine functionalized by two thioctic-ended arms. The second thread was an 8,8′-diphenyl-3,3′-biisoquinoline functionalized by two thioether-ended arms. The electrochemical studies of the two copper [2]pseudorotaxanes, both in solution and anchored on a gold surface, showed only fast-moving systems in solution. The reasons of the inertness of the deposited complexes on gold electrode have been explored.

Synthesis, topology and energy analysis of crystalline resorcinol-based oligophenylene molecules with various symmetries

We describe the development of a set of highly symmetric multitopic oligophenylene molecules decorated with hydroxyl functions that are all derived from the resorcinol moiety. All these resorcinol-based oligophenylene self-assembled structures and the corresponding methoxy protected precursors were found to be highly crystalline materials, with two compounds giving rise to porous 3D-networks. The solid-state organisation of these molecules has been studied from three complementary viewpoints using metrical, topological and energy analyses. A quantitative interaction energy has been associated to each supramolecular interaction responsible for a given topology, even for the quite new and highly complex topologies evidenced. Thermal and luminescent properties of these materials have also been examined.