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International audience; Six- and seven-membered cyclic hydroxamic acids are found as terminal binding units in different families of siderophores, including exochelins and mycobactins. The simplest models of these preorganized chelating ligands were known, but their coordination chemistry with Fe3+, the target metal ion of siderophores, had never been reported. Four complexes were synthesized and studied: two Fe3+ complexes, one with the six-membered ring hydroxamate PIPO− and one with the seven-membered ring hydroxamate AZEPO−, and the two corresponding Ga3+ complexes. X-ray diffraction studies showed that the interligand repulsion energies were better minimized in the case of the AZEPO− complexes whatever the metal cation considered, and that the Fe−O bond distances were shorter in [Fe(AZEPO)3] by comparison with [Fe(PIPO)3].

The syntheses and properties of expanded 4-tert-butyl-mercaptocalix[4]arenes, in which the methylene linkers are replaced by -CH

International audience; Chiral gold(I) acetylide trinuclear complexes 1 - 3 based on the cyclotribenzylene platform and terminal PR3 ligands (R = Ph, Et, and Cy, respectively), were characterized and their light emission studied. They exhibited long-lived blue phosphorescence in CHCl3 and a weak fluorescence in the UV. In MeOH/CHCl3 mixtures of > 1:1 volume ratio, 1 and 2 exhibited a new emission band at ca 540 nm that developed at the expense of the UV emission. DLS studies demonstrated the presence of molecular aggregates of Ø 30 - 80 nm. The green emission observed in MeOH-rich solvent mixtures was therefore induced by aggregation, and could originate from Au•••Au interactions. The AIE spectrum of 3 was observed only in solutions containing 99% of MeOH, and correlated with its solid state emission. The AIE profiles of the enantiomers of 1 differed from that of rac-1, suggesting that the latter is a true racemate.

Chiral gold(I) acetylide trinuclear complexes 1-3 based on the cyclotribenzylene platform and terminal PR

International audience; Actinide-based mineral phases occurring in contaminated soils can be solubilized by organic chelators excreted by plants, such as citrate. Herein, the efficiency of citrate towards U and Pu extraction is compared to that of siderophores, whose primary function is the acquisition of iron(III) as an essential nutrient and growth factor for many soil microorganisms. To that end, we selected desferrioxamine B (DFB) as an emblematic bacterial trishydroxamic siderophore and a synthetic analog, abbreviated (LCy,Pr)H2, of the tetradentate rhodotorulic acid (RA) produced by yeasts. Firstly, the uranyl speciation with both ligands was assessed in the pH range 2–11 by potentiometry and visible absorption spectrophotometry. Equilibrium constants and absorption spectra for three [UO2(DFB)Hh](h–1)+ (h = 1–3) and five [UO2(LCy,Pr)lHh](2+h–2l)+ (–1  h  1 for l = 1 and h = 0–1 for l = 2) solution complexes were determined at 25.0 °C and I = 0.1 M KNO3. Similar studies for the Fe3+/(LCy,Pr)2– system revealed the formation of five species having [Fe(LCy,Pr)]+, [Fe(LCy,Pr)OH], [Fe(LCy,Pr)(OH)2]–, [Fe(LCy,Pr)2H], and [Fe2(LCy,Pr)3] compositions. Then, the ability of DFB, (LCy,Pr)H2, and citrate to solubilize either U or Pu from pitchblende-rich soils (soils 1 and 2) or freshly plutonium-contaminated soils (LBS and PG) was evaluated by performing batch extraction tests. U was extracted significantly only by citrate after a day. After one week, the amount of U complexed by citrate only slightly exceeded that measured for the siderochelates, following the order citrate > (LCy,Pr)H2  DFB  H2O, and were comparatively very low. Pu was also more efficiently extracted by citrate than by DFB after a day, but only by a factor of ~2–3 for the PG soil, while the Pu concentration in the supernatant after one week was approximately the same for both natural chelators. It remained nearly constant for DFB between the 1st and 7th day, but drastically decreased in the case of citrate, suggesting chemical decomposition in the latter case. For the Fe-rich soils 1 and 2, the efficiencies of the three chelators to solubilize Fe after a day were of the same order of magnitude, decreasing in the order DFB > citrate > (LCy,Pr)H2. However, after a week DFB had extracted ~1.5 times more Fe, whereas the amount extracted by the other chelators stayed constant. For the less Fe-rich LBS and PG soils contaminated by Pu, the amounts of extracted Fe were higher, especially after 7 days, and the DFB outperformed citrate by a factor of nearly 3. The higher capacity of the hexadentate DFB to extract Pu in the presence of Fe and its lower ability to mobilize U qualitatively agree with the respective complexation constant ratios, keeping in mind that both Pu-containing soils had a lower iron loading. Noticeably, (LCy,Pr)H2 has roughly the same capacity as DFB to solubilize U, but it mobilizes less Fe than the hexadentate siderophore. Similarly, citrate has the highest capacity to extract Pu, but the lowest to extract Fe. Therefore, compared to DFB, (LCy,Pr)H2 shows a better U/Fe extraction selectivity and citrate shows a better Pu/Fe selectivity.

International audience

Cryptophanes are receptors consisting of two bridging concave cyclotribenzylene (CTB) subunits. The formation of cryptophanes in basic aqueous solutions (0.1 m MOD; M = Li, Na, K, and Cs) by disulfide bridge formation has been investigated. Two original CTBs, 1H6 and 2H6, derived from cyclotriphenolene through the introduction of mercaptomethyl and mercapto substituents, respectively, were synthesized. Cryptophane formation by slow oxygenation of 16– was observed only in the presence of Me4N+ as template, which yielded the diastereomeric forms in a ratio of around 82:18. By contrast, the rigid CTB 2H6 yielded a cryptophane stereoselectively without a template. Interestingly, air oxidation of a 1:1 mixture of 16– and 26– in the presence of Me4N+ (0.5–1 equiv./16–) in 0.1 m LiOD led to the exclusive formation of the cryptophane complex [(1·1)6–⊃+NMe4], (2·2)6– being formed only in very minor amounts. No mixed species were detected in the reaction mixtures. The stabilities of the diastereomers of the protonated cryptophanes were calculated in the gas phase by DFT at the ωB97XD 6-311G(d,p) level of theory.

Purpose Currently, the most commonly used chelator for labelling antibodies with 89Zr for immunoPET is desferrioxamine B (DFO). However, preclinical studies have shown that the limited in vivo stability of the 89Zr-DFO complex results in release of 89Zr, which accumulates in mineral bone. Here we report a novel chelator DFOcyclo*, a preorganized extended DFO derivative that enables octacoordination of the 89Zr radiometal. The aim was to compare the in vitro and in vivo stability of [89Zr]Zr-DFOcyclo*, [89Zr]Zr-DFO* and [89Zr]Zr-DFO. Methods The stability of 89Zr-labelled chelators alone and after conjugation to trastuzumab was evaluated in human plasma and PBS, and in the presence of excess EDTA or DFO. The immunoreactive fraction, IC50, and internalization rate of the conjugates were evaluated using HER2-expressing SKOV-3 cells. The in vivo distribution was investigated in mice with subcutaneous HER2+ SKOV-3 or HER2− MDA-MB-231 xenografts by PET/CT imaging and quantitative ex vivo tissue analyses 7 days after injection. Results 89Zr-labelled DFO, DFO* and DFOcyclo* were stable in human plasma for up to 7 days. In competition with EDTA, DFO* and DFOcyclo* showed higher stability than DFO. In competition with excess DFO, DFOcyclo*-trastuzumab was significantly more stable than the corresponding DFO and DFO* conjugates (p

International audience

Currently, the most commonly used chelator for labelling antibodies with 89Zr for immunoPET is desferrioxamine B (DFO). However, preclinical studies have shown that the limited in vivo stability of the 89Zr-DFO complex results in release of 89Zr, which accumulates in mineral bone. Here we report a novel chelator DFOcyclo*, a preorganized extended DFO derivative that enables octacoordination of the 89Zr radiometal. The aim was to compare the in vitro and in vivo stability of [89Zr]Zr-DFOcyclo*, [89Zr]Zr-DFO* and [89Zr]Zr-DFO. The stability of 89Zr-labelled chelators alone and after conjugation to trastuzumab was evaluated in human plasma and PBS, and in the presence of excess EDTA or DFO. The immunoreactive fraction, IC50, and internalization rate of the conjugates were evaluated using HER2-expressing SKOV-3 cells. The in vivo distribution was investigated in mice with subcutaneous HER2+ SKOV-3 or HER2− MDA-MB-231 xenografts by PET/CT imaging and quantitative ex vivo tissue analyses 7 days after injection. 89Zr-labelled DFO, DFO* and DFOcyclo* were stable in human plasma for up to 7 days. In competition with EDTA, DFO* and DFOcyclo* showed higher stability than DFO. In competition with excess DFO, DFOcyclo*-trastuzumab was significantly more stable than the corresponding DFO and DFO* conjugates (p

International audience; Many siderophores incorporate as bidentate chelating subunits linear and more seldomly cyclic hydroxamate groups. In this work, a comparative study of the uranyl binding properties in aqueous solution of two monohydroxamic acids, the prototypical linear N-methylacetohydroxamic acid (NMAH) and the cyclic analog 1-hydroxypiperidine-2-one (PIPOH), has been carried out. The complex [UO2(PIPO)(2)(H2O)] crystallized from slightly acidic water solutions (pH < 5), and its molecular structure was determined by X-ray diffraction. The uranyl speciation in the presence of both ligands has been thoroughly investigated in a 0.1 M KNO3 medium at 298.2 K by the combined use of four complementary techniques, i.e., potentiometry, spectrophotometry, Raman spectroscopy, and affinity capillary electrophoresis. Preorganization of the hydroxamate ligand for chelation by incorporation into a cyclic structure, as in PIPO-, results in an increase of nearly one order of magnitude in the formation constants of the uranyl complexes of 1 : 1 and 1 : 2 metal/ligand stoichiometries.

This study compares the binding properties towards the uranyl cation of three hydroxamates derived from benzohydroxamic acid (BHAH), N-methylacetohydroxamic acid (NMAH) and 1-hydroxypiperidine-2-one (PIPOH), as models of the binding sites found in natural chelators that could be involved in the mobilization of uranyl in the vicinity of abandoned mines. Solution speciation studies of the UO 2 2+ /BHAH system by affinity capillary electrophoresis allowed to estimate stability constants for both ML [log K110 = 7.4(1)] and ML2 [log K120 = 7.0(1)] species in aqueous media [0.1 M (H,Na)ClO4, 25 °C], which lie in-between those reported by us elsewhere for NMA− and PIPO−. By contrast, gas phase studies demonstrate that one of the U=O bonds can be activated in the NMA− and PIPO− complexes, whereas this is not the case for the BHA− complex.

Six- and seven-membered cyclic hydroxamic acids, such as 1-hydroxypiperidine-2-one (1H, 1,2-PIPOH) and 1-hydroxyazepan-2-one (2H), have recently been identified in some mixed siderophores as one of their three chelating subunits. Compared to their ubiquitous noncyclic counterparts, cyclic hydroxamates are preorganized for metal binding. Surprisingly, the coordination chemistry of these bidentate, monoanionic ligands remains virtually unknown, even in the case of iron(III). We report herein the first structural study of the complexes of 1– and of 6–, an unsaturated seven-membered ring analog of 2–, with tetravalent cations of transition metals (zirconium and hafnium), lanthanide (cerium), and actinides (thorium and uranium). Structural characterization by means of X-ray crystallography of the corresponding ML4 complexes evidenced distorted square antiprismatic coordination geometries with the exception of U4+, which favors a dodecahedral arrangement.

An octadentate tetrahydroxamic calix[4]arene podand incorporating 1-hydroxypiperidine-2-one (1,2-PIPOH) binding units has been designed as a specific chelator for tetravalent metal cations like Zr4+ or Hf4+. This receptor, which can be considered as the first ever abiotic ligand possessing only cyclic six-membered hydroxamate groups, has been synthesized and characterized in its tetraprotonated form (1H4). Contrary to expectation, however, this new chelator did not form a 1 : 1 complex upon reaction with M(acac)4 (M = Zr and Hf; acac = acetylacetonate), but rather self-assembled into a dimeric species of 2 : 2 stoichiometry. The latter could be characterized in solution by mass spectrometry and NMR spectroscopy as its monopotassium adduct ([M2K(1)2]+), pointing to the ionophoric character of the M2(1)2 complex.

Disulfide-bond formation is an expeditious coupling reaction to make macropolycyclic symmetrical receptors. It is used here for the preparation of unsymmetrical cavitands based on α-cyclodextrin (α-CDX) and cyclotriveratrylene (CTV) analogues. Accordingly, diastereomeric hemicryptophanes 2a and 2b were obtained in 11 % isolated yield by the iodine oxidation of a 1:1 mixture of racemic cyclotrithiophenolene (3) and the C3-symmetric trithiol derivative of permethylated α-cyclodextrin (PM α-CDX) 4. Remarkably, the target hemicryptophanes were obtained in a 5:3 diastereomeric ratio. The reaction produced mainly (34 %) a singly disulfide-bridged PM α-CDX dimer 7, however no traces of triply disulfide-bridged head-to-head PM α-CDX dimer were detected. In addition, the formation of the known S-cryptophane-0.0.0 was observed.

Topological constructions made from closed curves range from simple links to intricate knots and started to capture the chemists' attention in the early sixties. These mathematical objects result from particular embeddings of a single or a set of closed curves in the three-dimensional space that show an infinite variety of presentations. Simple catenanes, higher order interlocked macrocycles, and molecular knots can be synthesized via the metal template approach, just as simple macrocycles. However, this requires that rigid presentations with appropriate geometrical characteristics be identified prior to molecular design, and those selected for the metal-templated synthesis of some of these fascinating molecules are reviewed here.

[Extract] Chirality is probably one of the most significant topics in chemistry. The strong connection between chirality and symmetry has made it appealing from the mathematical and aesthetic viewpoints, and the recent interest in topologically chiral interlocked and knotted molecules has increased its intellectual attraction, raising the concept of a hierarchy in chirality [1]. The most fascinating aspect of chirality stems from the dynamic properties of molecules and supramolecular assemblies, rather than their static properties, because they are the cause of many intriguing and sometimes paradoxical issues. At the same time, dynamic chirality is also the most useful topic because of the numerous applications it underpins, from chiral recognition to molecular motors.

International audience; An optically pure hemicryptophane cavitand, in which the primary side of permethylated a-cyclodextrin (alpha-CD) is capped with a C-3-chiral cyclotribenzylene (CTB) derivative, responds to its solution environment, either by electronic circular dichroism or 1HNMR spectroscopy. In particular, the primary methoxy (6'OMe) protons of the alpha-CD component behave as solvent sensitive NMR probes. For example, they are more shielded in C2D2 Cl-4 than in CDCl3, and their chemical shifts show opposite temperature dependence. A similar contrasted behavior was observed in benzene vs. toluene, suggesting that the observed effects are related to the size of the solvent molecule. Molecular Dynamics simulations indicated that C2H2Cl4 and CHCl3 were included in the alpha-CD component of the hemicryptophane, but not in the CTB concavity, with binding energies of - 21.1 kJ x mol(-1) and - 32.7 kJ x mol(-1), respectively.

The oxidation of racemic methylthio-substituted C3-symmetric cyclotriveratrylene (CTV) 2 produces the corresponding trisulfoxides 3 as follows. Oxidation with m-chloroperoxybenzoic acid (m-CPBA) in dichloromethane leads to the formation of a distribution of the four possible diastereomers that is relatively close to the statistical outcome, in which the C3-symmetric stereoisomers are the minor species. In contrast, the optically active Davis reagent [oxaziridine (+)-4] in carbon tetrachloride at room temperature leads to a biased (73 %) distribution in favor of the C3-symmetric MRRR/PSSS diastereomer of 3 with 56 % ee (PSSS enantiomer according to the model of Davis). Interestingly, when the oxidation is carried out in toluene at reflux, the distribution is switched to approximately equal amounts of the C3-symmetric MSSS/PRRR and asymmetric MRSS/PSRR diastereomers of 3. Overall, however, these results point to the negligible role played by the chirality of the CTV platform on the induction of chirality at the sulfur prochiral centers.

A hybrid C 3 -symmetric cavitand 1, in which permethylated α-cyclodextrin (PM α-CDX) is capped with cyclotriveratrylene (CTV), has been prepared in 8 % yield by intramolecular cyclization of a vanillyl alcohol derivative attached to the primary rim of the CDX platform. The reaction proceeds diastereoselectively (dr ≈ 6:1), the chirality of the α-glucopyranosyl units controlling the chirality of the CTV component. Interestingly, in polar solvents, 1 shows self-complexation properties as the primary methoxy groups of the CDX component are directed towards the CTV cavity.

Catenanes are molecules made of two or more interlocked rings, and rotaxanes are composed of rings threaded onto axles that look like dumbbells because they are terminated by stoppers large enough to prevent unthreading (Figure 1) [1]. [ n ]Catenanes are constituted by n interlocked rings, whereas [ n ]rotaxanes are made of n-m rings and m dumbbell components. Both constitute the prototypical molecular objects featuring the so-called “mechanical bond”. Whereas the first syntheses of catenanes and rotaxanes date back to the late sixties [2], the real burst of these exotic-at-that-time classes of compounds really started a decade later, after the very first template methods (metal coordination [3], and π-π donor-acceptor interactions [4]) were discovered and developed [5, 6]. Since then catenanes and rotaxanes, from the status of molecular curiosities (in terms of topology and synthetic access), attained the rank of useful synthetic molecules that could be endowed with functions. Indeed, the last two decades have seen the development of molecular machines that rely heavily on catenanes and rotaxanes, taking advantage of the mechanical bonds for the generation and observation of net molecular motions [7, 8]. This contribution will account for the metal template technology, as among all the methods developed for making interlocked molecules, it is certainly the one that proved itself to be the most versatile and productive.

The past ten years have seen a spectacular development of chemical sensors based on the monomer-excimer dual luminescence of aromatic systems, such as pyrene. Either in the form of integrated or multicomponent molecular devices these chemosensors have been attracting a high interest above all because of their unique ratiometric properties. This review will focus on the latter systems, which can be classified into two classes: Firstly, the assembly of receptor-effector conjugates is triggerred by the analyte of interest. As a result, the sensor shows monomer to excimer fluorescence switching upon substrate binding. Secondly, the supramolecular assembly that constitutes the sensor is perturbed by interaction with the analyte. This induces a conformational change or the exchange of a component of the system, which is the cause of the luminescence switch effect.

Molecular recognition between receptor and substrate is optimized when these compounds show complementary shapes, sizes, and interacting moieties. A family of C3v-symmetric macrotricycles 1–4 is presented that incorporate resorcinol- and mesitylene-derived “walls” and “cap”, respectively. These compounds feature, in principle, a tetrahedral π-basic cavity. This paper reports the effect of substituents in the “walls” and the “cap” on the shapes of the macrotricycles in solution (1H NMR), the solid state (X-ray diffraction), and gas phase (calculations). Substitution of the lower position of the “walls” by Br (in 3) or MeS (in 4) has the same effect as ethyl substitution of the “cap” (in 2), that is, imparting high rigidity to the molecules and deforming their expected spherical shape to a cylindrical one.

A sterically encumbered [N(2)S(6)] macropentacycle (5) related to diazamacrobicycles and cryptands has been synthesized in 53% yield by the [1+1] condensation reaction between functionalized macrocyclic and macrotricyclic precursors. A macrononacycle (18) resulting from the corresponding [2+2] condensation was isolated in 7% yield from the reaction mixture. Both compounds showed broad features in their room-temperature (1)H NMR spectra, but their maximal average symmetry (D(3h) and D(2h), respectively) was achieved at high temperature (380 K). At low temperature (200 K, CD(2)Cl(2) solution), the macropentacycle is "frozen" to a single asymmetric (C1) conformation on the (1)H NMR time scale, which has also the molecular structure observed in the solid state by X-ray crystallography: pseudo-triple helical ( not equalC(3)) shape, io (in, out) form resulting from the endo/exo configuration at the nitrogen bridgehead atoms, and similar orientations of the tosyl substituents. The solution dynamics of the molecule can be described by coupled bridgehead nitrogen inversion, triple helix symmetrization, and reversal of triple helix handedness, with DeltaGc = 54.2 kJ mol(-1) in CD(2)Cl(2) at 300 K. Adoption of the io form by macropentacycle 5 in the crystal and in solution at low-temperature most probably results from the steric crowding and strain introduced by the [15]ane-N(2)S(2) macrocyclic bridging subunits.

Photoinduced processes have been determined in a [2]catenane containing a zinc(II) porphyrin, a gold(III) porphyrin, and two free phenanthroline binding sites, Zn-Au(+), and in the corresponding copper(I) phenanthroline complex, Zn-Cu(+)-Au(+). In acetonitrile solution Zn-Au(+) is present in two different conformations: an extended one, L, which accounts for 40 % of the total, and a compact one, S. In the L conformation, the electron transfer from the excited state of the Zn porphyrin to the gold-porphyrin unit (k = 1.3x10(9) s(-1)) is followed by a slow recombination (k = 8.3x10(7) s(-1)) to the ground state. The processes in the S conformation cannot be clearly resolved but a charge-separated (CS) state is rapidly formed and decays with a lifetime on the order of fifty picoseconds. In the catenate Zn-Cu(+)-Au(+), the zinc-porphyrin excited state initially transfers energy to the Cu(I)-phenantholine unit, producing a metal-to-ligand charge-transfer (MLCT) excited state localized on the copper complex with a rate k = 1.4x10(9) s(-1). From this excited state the transfer of an electron to the gold-porphyrin unit takes place, producing the CS state Zn-Cu(2+)-Au(.), which decays with a lifetime of 10 ns. The results are discussed in comparison with the closely related [2]rotaxane, in which a further charge shift from the copper center to the zinc-porphyrin unit leads to the fully CS state. Even in the absence of such full charge separation, it is shown that the lifetimes of the CS states are increased by a factor of about 2-2.5 over those of the corresponding rotaxanes.

STM investigations on the adsorption of submonolayers of large organic molecules smaller than 6 nm were carried out on copper surfaces to compare two different deposition methods: the thermal sublimation and the pulse-injection. The blue lander, a member of the lander family, could be deposited intact with the thermal procedure, therefore it was used to test the performances of the pulse-injection. On the contrary, the bis-porphyrin, which was decomposed by the thermal procedure, was deposited intact with the pulse injection.

The bis-porphyrin system ZnP 2 , in which two zinc porphyrins are connected by a phenanthroline linker in an oblique fashion, acts as a bifunctional receptor towards the complexation of free-base meso-5,10-bis(4'-pyridyl)-15,20-diphenylporphyrin (4'-cisDPyP). In solution, NMR spectroscopy evidenced quantitative formation of the tris-porphyrin macrocyclic assembly ZnP 2 (4'-cisDPyP), in which the two fragments are held together by two axial 4'-N(pyridyl)-Zn interactions. The remarkable stability of the edifice (an association constant of about 6 x 10 8 M - 1 was determined by UV/Vis absorption and emission titration experiments in toluene) is due to the almost perfect geometrical match between the two interacting units. The macrocycle was crystallized and studied by X-ray diffraction, which confirmed the excellent complementarity of the two components. Photoinduced energy transfer from the singlet excited state of the zinc porphyrin chromophores to the free-base porphyrin occurs with an efficiency of 98% (k e n = 2 x 10 1 0 s - 1 in toluene, ambient temperature) with a mechanism consistent with a dipole - dipole process with a low orientation factor.

The synthesis of a new RuII complex [Ru(L1)(4,7-dpphen)]2+, bearing a tetradentate bis-1,10-phenanthroline ligand L1, is described. X-ray crystal structure analysis shows that the tetradentate ligand L1 coils around the metal and occupies both axial and two of the equatorial positions of the complex. 1H NMR studies clearly indicate that the complex has C2 symmetry in solution. The room temperature luminescence properties of [Ru(L1)(4,7-dpphen)]2+ have been studied, as have those of the related complex [Ru(L2)(4,4′-dmbp)]2+. For both complexes, the luminescence quantum yields and lifetimes are three to ten times higher than those of [Ru(bpy)3]2+. These unusual results are explained by the geometrical constraints imposed by the tetradentate ligands L1 and L2 in their respective complexes, resulting in an effective decoupling between the 3MLCT and 3MC excited states. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003)

A ruthenium(II) precursor complex containing a bis-phenanthroline ligand, leading to a controlled helical structure around the metal centre, and a third phenanthroline-type ligand has been prepared. The spatial arrangement of the system is such that two chemical groups are disposed trans to one another on an axis running through the Ru centre. By appropriate functional group transformation, these two groups can be attached to monosubstituted zinc(II) porphyrins (PZn) to afford a linear array consisting of a central ruthenium(II) complex and two peripheral PZn motifs. (© Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002)

A linear bis-porphyrin bridged by a 5,5′-diphenyl-2,2′-bipyridine rod-like spacer complexing a [Ru(phen)2]2+ fragment has been synthesized in 7.4% yield by one-pot condensation of 3,5-di-tert-butylbenzaldehyde, 4,4′-dimethyl-3,3′-dihexyl-2,2′-methylenedipyrrole and the [Ru(phen)2]2+ complex of 5,5′-bis(p-formylphenyl)-2,2′-bipyridine, followed by chloranil oxidation. The protected dialdehyde (5,5′-bis[(5,5-dimethyl-1,3-dioxan-2-yl)phenyl]-2,2′-bipyridine) was obtained in 80% yield by Suzuki coupling of 2-[4-(5,5-dimethyl-1,3-dioxan-2-yl)phenyl]-4,4,5,5-tetramethyl-1,3-dioxaborolane and 5,5′-dibromo-2,2′-bipyridine, using [Pd(PPh3)4] as catalyst. A new procedure is reported for the preparation of 5,5′-dibromo-2,2′-bipyridine, which is obtained in 80% yield by Stille homocoupling of 2,5-dibromopyridine in the presence of hexamethylditin.

A series of [2]-rotaxanes has been synthesized in which two Zn(II)-porphyrins (ZnP) electron donors were attached as stoppers on the rod. A macrocycle attached to a Au(III)-porphyrin (AuP+) acceptor was threaded on the rod. By selective excitation of either porphyrin, we could induce an electron transfer from the ZnP to the AuP+ unit that generated the same ZnP*+-AuP* charge-transfer state irrespective of which porphyrin was excited. Although the reactants were linked only by mechanical or coordination bonds, electron-transfer rate constants up to 1.2x10(10) x s(-1) were obtained over a 15-17 A edge-to-edge distance between the porphyrins. The resulting charge-transfer state had a relatively long lifetime of 10-40 ns and was formed in high yield (80%) in most cases. By a simple variation of the link between the reactants, viz. a coordination of the phenanthroline units on the rotaxane rod and ring by either Ag+ or Cu+, we could enhance the electron-transfer rate from the ZnP to the excited 3AuP+. We interpret our data in terms of an enhanced superexchange mechanism with Ag+ and a change to a stepwise hopping mechanism with Cu+, involving the oxidized Cu(phen)22+ unit as a real intermediate. When the ZnP unit was excited instead, electron transfer from the excited 1ZnP to AuP+ was not affected, or even slowed, by Ag+ or Cu+. We discuss this asymmetry in terms of the different orbitals involved in mediating the reaction in an electron- and a hole-transfer mechanism. Our results show the possibility to tune the rates of electron transfer between noncovalently linked reactants by a convenient modification of the link. The different effect of Ag+ and Cu+ on the rate with ZnP and AuP+ excitation shows an additional possibility to control the electron-transfer reactions by selective excitation. We also found that coordination of the Cu+ introduced an energy-transfer reaction from 1ZnP to Cu(phen)2+ (k = 5.1x10(9) x s(-1)) that proceeded in competition with electron transfer to AuP+ and was followed by a quantitative energy transfer to give the 3ZnP state (k = 1.5x10(9) x s(-1)).

A bis-chelating ligand (L1), made of two 7-(p-anisyl)-1,10-phenanthroline (phen) subunits connected with a p-(CH(2))(2)C(6)H(4)(CH(2))(2) spacer through their 4 positions, has been prepared, using Skraup syntheses and reaction of the anion of 4-methyl-7-anisyl-1,10-phenanthroline with alpha,alpha'-dibromo-p-xylene. Its Fe(II) complex, [FeL1(dmbp)](PF(6))(2), was prepared in one step by reaction of L1 with [Fe(dmbp)(3)](PF(6))(2) (dmbp = 4,4'-dimethyl-2,2'-bipyridine). On the other hand, its Ru(II) complex, [RuL1(dmbp)](PF(6))(2), was prepared in two steps from Ru(CH(3)CN)(4)Cl(2) and L1, followed by reaction with dmbp. X-ray crystal structure analyses show that in the two octahedral complexes, ligand L1 coils around the metal by coordination of the axial and two equatorial positions. It defines a 21 A long axis (O.O distance) running through the central metal and the terminal anisyl substituents. The complexes were also characterized by (1)H NMR, mass spectrometry, cyclic voltammetry, electronic absorption, and, in the case of Ru(II), fluorescence spectroscopy.

A [2]-catenane made with Zn(II) and Au(III) porphyrin-incorporating marcrocycles has been synthesized using the transition metal templated technique, starting from either preformed Au or Zn porphyrin-containing macrocycles. The highest yield (11.5%) was obtained in the latter case. Removal of the copper(I) template metal afforded the free [2]-catenane, whose 1H-NMR properties differ dramatically from those of the parent Cu(I) complex, suggesting a changeover in the molecular structure upon demetallation.

A [2]-rotaxane Zn2−Au+ made from a dumbbell component ended by Zn(II) porphyrin stoppers and a ring component incorporating a Au(III) porphyrin has been assembled in 13% yield using the transition metal templating route. 1H NMR studies show that its conformation in solution is very different from those of its complexes with Cu+, Ag+, and Li+. In particular, removal of the templating metal resulted in a changeover of the molecule, the threaded macrocycle undergoing a pirouetting motion placing the Au(III) porphyrin in the cleft formed by the two Zn(II) porphyrin stoppers. At room temperature, the changeover could be either complete or partial, depending on the solvent used. Photoinduced electron transfer from one of the Zn(II) porphyrins to the Au(III) porphyrin of the macrocycle was evidenced in the case of the free rotaxane and its Cu(I) complex, Zn2Cu+Au+. In the former case, the photoinduced electron transfer process could be clearly resolved for an extended conformation that is characterized by the Zn...

A dicopper(I)-complexed [3]-rotaxane was prepared in 34% yield by a transition-metal-templated strategy involving a bis-chelating molecular thread, macrocycles incorporating the complementary chelating unit, and porphyrin stoppers as key components. A tetracopper(I)-complexed [5]-rotaxane was obtained as a byproduct in 8% yield. Coordination chemistry was performed either at the 2,9-diphenyl-1,10-phenanthroline (dpp) chelates or at the porphyrin stoppers. The latter were metalated by either Zn(II), Au(III), or Zn(II) and Au(III). Both Cu(I) template cations could be removed by competitive complexation with CN- when at least one porphyrin stopper was complexed with Au(III), the other one being metalated with either Au(III) or Zn(II). This provided two strictly defined [3]-rotaxanes, in which the macrocycle and the molecular thread are not held together by metal coordination. Rather, dethreading is prevented by the bulky metalloporphyrin stoppers. Unexpectedly, when both stoppers were complexed with Zn(II),...

Energy and electron transfer processes taking place in [3]-rotaxanes containing various photo- and electro-active metal-complex fragments, such as zinc(II)- and gold(III)-porphyrin stoppers, copper(I)- and silver(I)-bis-phenanthroline complex moieties, were studied by spectroscopic steady state and time resolved techniques. Energy transfer from the zinc porphyrin excited singlet state to the MLCT excited state of the Cu(I) complex fragments was observed in the [3]-rotaxane [PZnCuCuPZn]2+, with a lifetime of 180 ps. In the rotaxane [PZnCu-PZn]+energy transfer occurred only from the porphyrin (τ=490 ps) close to the Cu(I) complex, while the lifetime of the distant zinc porphyrin was unaffected. Quenching of the distant porphyrin was partly restored in [PZnCuAgPZn]2+, while the one close to the Cu(I) complex displayed a luminescence lifetime of 300 ps. In the case of the free [3]-rotaxane [PZn--PAu]+, the data suggest that a very slow electron transfer process takes place between the porphyrin stoppers probably through the extended “necklace’' spacer.

Rotaxanes are molecular architectures ideally suited for building integrated, multicomponent modular systems, displaying novel chemical and physical properties. In this new field of functional rotaxanes, those incorporating transition metals, which are incidentally used as synthetic templates, are particularly attractive for their photophysical and electronic properties as well as their dynamic behaviour. It is believed that they will provide, in the future, the basic elements for constructing nanoscale machines and motors.

Bis-porphyrin conjugates have been synthesized for performing long-range photoinduced charge separation. The electron donor is a zinc(II) porphyrin in the excited state and the electron acceptor is a gold(III) porphyrin. These elements or their free-base analogs have been assembled following two different strategies. In a first approach, the porphyrinic subunits, separated by one or more 2,9-diphenyl-1,10-phenanthroline spacers, have been incorporated into rotaxane structures containing up to four threaded macrocycles by copper(I) templated synthesis. In a second approach, a triad having strictly controlled geometry has been prepared following a new gathering strategy. This triad, consisting of two porphyrinic moieties assembled via a central bis(phenyl-terpyridine)ruthenium(II) complex, uses coordination rather than covalent chemistry to interconnect the components.

Considered only laboratory curiosities when first synthesized in the late sixties, rotaxanes (a cyclic component threaded onto a dumbbell-shaped linear component) are now relatively easily available through template synthesis. This has led to the development of functional rotaxanes, for example rotaxanes displaying electrochemically or photochemically triggered intramolecular motions like translation of the ring along the dumbbell, or electron transfer between their mechanically linked components (figure).

The enantiomers of topologically chiral molecular objects cannot be interconverted by a continuous deformation. It must be noted that this behavior is opposite to that of molecules showing classical or Euclidian chirality. Interlocked oriented rings and the trefoil knot are prototypical topologically chiral objects. We have been designing a transition-metal templated route to the corresponding real molecules (i.e., [2]-catenanes and molecular knots) using copper(I) as the template and 2,9-diphenyl-1,10-phenanthroline (dpp) based ligands. The precursor to the catenane species was a tetrahedral Cu(dpp)2+ complex in which the two ligands fit in around Cu(I), thereby allowing the interlocking process. In the case of the knot, the precursor was a double-stranded helical complex in which two bis-phenanthroline ligands wrap around two Cu(I) ions. The compounds were obtained as racemates and were resolved as their Cu(I) complexes either by HPLC on chiral stationary phases or by diastereoselective crystallization. For the trefoil knot (K · 2Cu+), the latter method proved to be the most efficient. It took advantage of the dicationic nature of the species to be resolved. The original triflate anion was exchanged by an optically pure anion (S)-(+)-1,1′-binaphthyl-2,2′-diyl phosphate ((+)-BNP-) and the diastereomer (+)-K · 2Cu+ · 2(+)BNP- was selectively crystallized in a mixture of nitromethane and benzene. Removal of the Cu(I) ions by treatment with cyanide afforded an enantiomerically pure molecular knot (K), showing an optical rotatory power close to +2,000° · mol-1 · L · dm-1. Chirality 10:125–133, 1998. © 1998 Wiley-Liss,Inc.

Topological constructions made from closed curves range from simple links to intricate knots and started to capture the chemists' attention in the early sixties. These mathematical objects result from particular embeddings of a single or a set of closed curves in the three-dimensional space that show an infinite variety of presentations. Simple catenanes, higher order interlocked macrocycles, and molecular knots can be synthesized via the metal template approach, just as simple macrocycles. However, this requires that rigid presentations with appropriate geometrical characteristics be identified prior to molecular design, and those selected for the metal-templated synthesis of some of these fascinating molecules are reviewed here.