Allosterically Driven Assembly of a Multisite Cage-Based [2]Semirotaxane.

The assembly of a [2]semirotaxane from a half-dumbbell endowed with a pyrazine coordination site and a bis-Zn(II) porphyrin cage as a multisite ring is reported. The threading is allosterically driven by the coordination of silver(I) ions to the multiple binding sites of the cage linkers, as shown by NMR studies. Addition of chloride ions destabilizes [2]semirotaxane, leading to its disassembly into its cage and half-dumbbell components.

Photophysical and Computational Insights into Ag(I) Complexation of Porphyrinic Covalent Cages Equipped with Triazoles-Incorporating Linkers.

The photophysical characterization of four supramolecular complexes based on covalent cages 2H-S-2H, 2H-L-2H, Zn-S-2H, and Zn-L-2H, consisting in either two free-base porphyrins or one Zn(II) porphyrin and one free-base porphyrin connected by four flexible linkers of different lengths incorporating triazole binding sites, and their Ag(I) complexation are reported. The complexation processes have been followed by means of absorption and emission spectroscopies, and a comprehensive computational study explains the behavior of the free-base porphyrin-containing cages. Absorption and emission features have been interpreted on the bases of conformational changes, metalation processes, and modification of energy transfer efficiencies occurring in the different cases. In all cages, except 2H-L-2H, the coordination of four Ag(I) ions to the lateral triazole groups of the linkers leads to the enlargement of their cavity. Only for 2H-L-2H is a different behavior observed, where the process of silver metalation of the porphyrins' core prevails.

Allosteric Control of Naphthalene Diimide Encapsulation and Electron Transfer in Porphyrin Containers: Photophysical Studies and Molecular Dynamics Simulation.

The complexation processes of N,N'-dibutyl-1,4,5,8-naphthalene diimide (NDI) into two types of π-electron-rich molecular containers consisting of two Zn(II)-porphyrins connected by four flexible linkers of two different lengths, were characterized by means of absorption and emission spectroscopies and molecular dynamics simulation. Notably, the addition of NDI leads to a strong quenching of the fluorescence of both cages only when they are in an open conformation suitable for guest encapsulation, a situation triggered by silver(I) ions binding to the lateral triazoles. Molecular dynamics simulations confirm the fast binding of NDI, likely assisted by NDI-silver(I) interactions. Upon NDI complexation, the two porphyrin macrocycles get closer, with an optimized face to face orientation, suggesting an induced-fit mechanism through π-π interactions with the NDI aromatic cycle. Ultrafast transient absorption experiments allowed to identify the process of quenching of the Zn-porphyrin fluorescence as an efficient photoinduced electron transfer reaction between the cage porphyrin and the included NDI guest. The process occurs on fast and ultrafast time scales in the two complexes (1.5 ps and ≤300 fs) leading to a short-lived charge separated state (charge recombination lifetimes in the order of 30-40 ps). The combined computational and experimental approach used here is able to furnish a reliable model of the NDI-cage complexation mechanism and of the corresponding electron transfer reaction, attesting the allosteric control of both processes by the silver(I) ions.

Positive Allosteric Control of Guests Encapsulation by Metal Binding to Covalent Porphyrin Cages.

The allosteric control of the receptor properties of two flexible covalent cages is reported. These receptors consist of two zinc(II) porphyrins connected by four linkers of two different sizes, each incorporating two 1,2,3-triazolyl ligands. Silver(I) ions act as effectors, responsible for an on/off encapsulation mechanism of neutral guest molecules. Binding silver(I) ions to the triazoles opens the cages and triggers the coordination of pyrazine or the encapsulation of N,N'-dibutyl-1,4,5,8-naphthalene diimide. The X-ray structure of the silver(I)-complexed receptor with short connectors is reported, revealing the hollow structure with a cavity well-defined by two eclipsed porphyrins. Rather unexpectedly, the crystallographic structure of this receptor with pyrazine as a guest molecule showed that the cavity is occupied by two pyrazines, each binding to the zinc(II) porphyrin in a monotopic fashion.

Chemically Induced Breathing of Flexible Porphyrinic Covalent Cages.

The synthesis of two flexible bis-porphyrin cages 3 and 4, incorporating respectively Zn(II) porphyrins and free-base porphyrins, is reported. In both cages, the four covalent linkers that bind the two porphyrins are functionalized with two 1,2,3-triazolyl ligands. These cages were characterized by NMR and HRMS, and for cage 3 incorporating 1,4-diazabicyclo[2.2.2]octane (DABCO), an X-ray crystallographic structure was obtained. Chemically induced conformational changes are studied and compared to those of two related cages with longer flexible linkers. Binding of four silver(I) ions to the peripheral ligands opens the flattened structures in solution and locks the two porphyrins in a face-to-face disposition. Addition of an excess of acid fully expands the cages due to electrostatic repulsion between the positively charged sites. These two reversible processes allow for a chemically induced breathing of the flexible structures.

A Porphyrin Coordination Cage Assembled from Four Silver(I) Triazolyl-Pyridine Complexes.

The synthesis of a zinc(II) porphyrin 1 with four appended triazolyl-pyridine chelates is reported. Complexation of the porphyrin peripheral ligands with Ag(I) ions in a 1:2 binding stoichiometry afforded quantitatively the coordination cage [Ag4 (1)2 ](4+) . The assembly and disassembly processes of the cage were investigated in solution using UV/Vis spectroscopy. The mathematical analysis of the data obtained in the UV/Vis titration of 1 with Ag(I) confirmed the assembly in CH2 Cl2 /MeOH (90:10) solution of a species having a 1:2 porphyrin/silver stoichiometry and assigned to it an overall stability constant of 5.0×10(26) M(-5) . The use of a model system allowed an independent assessment of a microscopic binding constant value (Km ) for the interaction between the triazolyl-pyridine ligand and Ag(I) . The coincidence that existed between the Km values extracted from the model system and the titration of 1 provided an indication of the quality and fit of the data analysis. It also allowed the calculation of the average effective molarity (EM) value for the three intramolecular processes that led to the cage assembly as 2.6 mM. Simulated speciation profiles supported the conclusion that at millimolar concentration and working under strict stoichiometric control of the silver/porphyrin ratio, the cage [Ag4 (1)2 ](4+) was the species exclusively assembled in solution. On the other hand, when the concentration of added Ag(I) was approximately 2.6 mM, 50 % of the coordination cage disassembled into open aggregates.

Control of the cavity size of flexible covalent cages by silver coordination to the peripheral binding sites.

The synthesis of covalent cages consisting of two porphyrins connected by four flexible spacers each incorporating two 1,2,3-triazolyl ligands is reported. Binding of four silver(I) to the peripheral ligands induces conformational changes in solution and locks the porphyrins in a face-to-face disposition.

Synthesis and solution studies of silver(I)-assembled porphyrin coordination cages.

The synthesis and the characterization of two porphyrin coordination cages are reported. The design of the cage formation is based on the coordination of silver(I) ions to the pyridyl units of 3-pyridyl appended porphyrins. (1)H/(109)Ag NMR spectroscopy, and diffusion-ordered spectroscopy (DOSY) experiments demonstrate that both the free base porphyrin 2H-TPyP and the Zn-porphyrin Zn-TPyP form the closed cages, [Ag4(2H-TPyP)2](4+) and [Ag4(Zn-TPyP)2](4+), respectively, upon addition of two equivalents of Ag(+). The complexation processes are characterized in details by means of absorption and emission spectroscopy in diluted CH2Cl2 solutions. The data are discussed in the frame of the point-dipole exciton coupling theory; the two porphyrin monomers, in fact, experience a rigid face-to-face geometry in the cages and a weak inter-porphyrin exciton coupling. An intermediate species is observed, for Zn-TPyP, in a porphyrin/Ag(+) stoichiometric ratio of about 1:0.5 and is tentatively ascribed to an oblique open form. The occurrence of a photoinduced electron-transfer reaction within the cages is excluded on the basis of the experimental outcomes and thermodynamic evaluations. Photophysical experiments evidence different reactivities of singlet and triplet excited states in the assemblies. A lower fluorescence quantum yield and triplet formation is discussed in relation to the constrained geometry of the complexes. Unusually long triplet excited state lifetimes are measured for the assemblies.

Transition-metal-complexed catenanes and rotaxanes: from dynamic systems to functional molecular machines.

Transition metal-based catenanes and rotaxanes constitute a specific class of mechanically interlocked molecules whose metal centers are essential both as templates in the construction of the compounds and for their ability to induce large-amplitude motions. In the present chapter we will first present a historical perspective of the field of interlocking compounds in general, in relation to molecular machines, starting with old work dating back to the 1980s and 1990s. Copper was shown many years ago to be the metal of choice for synthesizing the compounds via a template approach and for setting the molecules in motion using a redox signal (Cu(II)/Cu(I)). In a second paragraph, we will discuss various rotaxanes able to undergo a pirouetting motion of the axis within the threaded ring. Two families of such molecules will be mentioned: (1) a porphyrin-containing [2]rotaxane whose pirouetting motion is induced by a chemical reaction and (2) electrochemically driven systems. In this second category of [2]rotaxanes, the rate of motion could be dramatically increased by gradually modifying structural parameters and, in particular, by making the metal center less and less hindered by its surrounding ligands. The third section will be devoted to molecular shuttles and muscles, both families of compounds being reminiscent of linear machines such as biological muscles. By replacing the classical 2,9-diaryl-1,10-phenanthroline chelate (highly shielding and hindering) used by our group since the 1980s by an endocyclic but non-sterically hindering 3,3'-biisoquinoline derivative, the shuttling rate was increased in spectacular fashion, demonstrating the importance of steric factors in transition metal-based molecular machines. The same 3,3'-biisoquinoline motif was also used in the elaboration of a three-station shuttle, leading to long-distance (>20 Å) transport of a ring along the axis on which it is threaded. Finally, porphyrin-containing [3]rotaxanes and [4]rotaxanes, the latter displaying an overall cyclic structure, will be discussed and shown to behave as adjustable and switchable receptors. The synthesis of such compounds is a particularly challenging task in itself. In addition, the new receptors display fascinating properties such as, in particular, their ability to compress various guests and to expel them from their binding site using a chemical signal.

Synthesis of [5]rotaxanes containing bi- and tridentate coordination sites in the axis.

A new example of a linear [5]rotaxane has been synthesized by using the traditional "gathering-and-threading" approach but based on an unusual axle incorporating a symmetrical bis(bidentate) chelating fragment built on a 4,7-phenanthroline core. The stoppering reaction is particularly noteworthy since, instead of using a trivial bulky stopper as precursor to the blocking group, two semistoppered copper-complexed [2]pseudorotaxanes (namely [2]semirotaxanes) are used, which leads to the desired [5]rotaxane in good yield. The efficiency of the method relies on the use of "click" chemistry, with its very mild conditions, and on the protection by a transition-metal (copper(I)) of the various coordinating groups present in the fragments to be interconnected (terpy and bidentate chelating groups), thus inhibiting potential detrimental side reactions during the copper-catalyzed stoppering reaction. Since the external fragments and the central core of the system contain tri- and bidentate chelating units, respectively, the axle of the final [5]rotaxane incorporates two types of coordinating units: two external terpy groups (terpy: 2,2':6',2''-terpyridine) and two central bidentate ligands. Such a situation enables the system to tidy two different metals centers, and to localize them in a priori well-defined positions. This is what was observed when mixing the free ligand with a mixture of Zn(2+) and Li(+) : the zinc(II) ions were unambiguously shown to occupy the external sites, whereas the Li(+) cations were found in the central part of the [5]rotaxane. An X-ray diffraction study carried out on a [3]pseudorotaxane, the axis of which is similar to the central part of the [5]rotaxane axle, demonstrates that Zn(2+) is clearly five-coordinate, the fifth ligand being a counterion, even when the coordination site of the pseudorotaxane is designed for four-coordinate metals, which is in marked contrast with copper(I) or Li(+) .

Copper-complexed catenanes and rotaxanes in motion: 15 years of molecular machines.

In this review, we try to summarise the work performed in our laboratories in the course of the last 15 years, in the field of catenane- and rotaxane-based molecular machines containing copper(I), copper(II) or zinc(II) atoms. We put our work into perspective, showing how the properties of the compounds made have been gradually improved, mostly in terms of motion rate. In parallel, the function of the molecular machines elaborated have been made more and more complex. Instead of discussing all the systems elaborated and studied in our team, we preferred to select a few representative examples and show what were the principles which guided us for improving their performances and how the compounds were experimentally modified to afford new functions and faster-responding molecular machines. Starting from an electrochemically-driven "swinging" [2]catenane, reported in 1994, whose rearrangement was disappointingly slow, we could recently elaborate fast moving pirouetting copper-complexed [2]rotaxanes or molecular shuttles. The rearrangement mechanism of the pirouetting systems, as studied by experimental and computational methods, led us to synthesise interlocking compounds with a very open structure around the copper centre, allowing facile ligand exchange. With such compounds, whose copper centres are highly accessible, the minutes or hours required for the first generation of molecular machines to rearrange were converted to milliseconds or seconds, demonstrating that the rate limiting step in the electrochemically-steered copper catenanes and rotaxanes is probably decoordination of ligands to be replaced by entering ligands leading to the new form of the species. In addition to the electrochemically-piloted systems, we discuss a few compounds, which were not set in motion using an electrochemical signal but rather a chemical stimulus, including porphyrin-containing [2]rotaxanes or a "molecular muscle", based on a [2]rotaxane dimer of the hermaphroditic type. In these two machines, the stimulus is based on metal complexation, decomplexation or exchange (copper(I) being replaced by zinc(II) and vice versa for the "muscle"). Using non-sterically hindering ligands of the 8,8'-diaryl-3,3'-biisoquinoline, it is likely that the chemically driven motions used for contracting or stretching the "muscle" could be replaced by electrochemical signals, which are certainly more promising in terms of future devices.

Separation of geometric isomers of a dicopper complex by using a (19)F-labeled ligand: dynamics, structures, and DFT calculations.

Introducing a fluorine group on two pyridines of the HL(CH(3)) ligand (2,6-bis[(bis(2-pyridylmethyl)amino)methyl]-4-methylphenol) allows the separation of two geometric isomers after complexation by two copper(II) ions. Methods for isolating the isomers (1(meso) and 1(rac)) as a mu-phenoxo,mu-hydroxo dicopper(II) complex as a crystalline product have been developed. Both isomers (1(meso) and 1(rac)) have been characterized by X-ray crystallography and (19)F NMR. The isomerism is determined by the disposition of the fluorine atoms with respect to the plane containing the Cu(2)O(2) core. Density functional theory calculations using different functionals were performed to provide additional support for the existence of these two forms. Dissolution of 1(meso) in acetone or acetonitrile causes its spontaneous isomerization into the 1(rac) form at room temperature. Combined experimental studies (UV-vis, (19)F NMR) and theoretical calculations support this process. Paramagnetic (19)F NMR appears as a unique and powerful probe for distinguishing the two isomers and supplying direct evidence of this isomerization process in solution.

Copper(II) and cobalt(III) pyridoxal thiosemicarbazone complexes with nitroprusside as counterion: syntheses, electronic properties, and antileukemic activity.

In this paper the syntheses of new pyridoxal thiosemicarbazone copper(II) and cobalt(III) complexes with nitroprusside as a counterion and tests on the antileukemic activity of three of these complexes toward human cell lines U937 and CEM are reported. Nitroprusside was chosen in order to test if its ability to release NO can increase the biological activity already shown by these complexes. The compounds were characterized by spectroscopic and magnetic methods and by single-crystal X-ray diffraction.

Imidazole and imidazolate iron complexes: on the way for tuning 3D-structural characteristics and reactivity. Redox interconversions controlled by protonation state.

X-ray structures for six Fe(II) and Fe(III) complexes from two closely heptadentate N-tripodal ligands, L1H(3) = tris[(imidazol-4-yl)-3-aza-3-butenyl]amine and L2H(3) = tris[(imidazol-2-yl)-3-aza-3-butenyl]amine, are described: three complexes in the L1 series (namely, [Fe(II)(L1H(3))](2+) and [Fe(III)(L1H(3))](3+) at low pH and [Fe(III)(L1)](0) at high pH) and three complexes in the L2 series (namely, [Fe(II)(L2H(3))](2+) at low pH and [Fe(II)(L2H)](0) and [Fe(III)(L2)](0) at high pH). Most of these complexes are stable in both Fe(II) and Fe(III) redox states and with the ligand in various protonation states. In the solid state, hydrogen bonds networks were obtained. Structural differences induced by 2- or 4-imidazole substitution are described and discussed. In solution, interconversions between different forms, with regard to oxidation and protonation states, were investigated by UV-visible spectroscopy, cyclic voltammetry, and potentiometry. The deprotonation pattern of these polyimidazole iron(II) and iron(III) complexes is described in detail. pK(a)s of the imidazolate/imidazole moieties in MeOH/H(2)O are reported. Two new species, namely, [Fe(II)(L1)](-) and [Fe(II)(L2)](-), were shown to be obtained in DMSO upon strong base addition and characterized by UV-vis spectroscopy and cyclic voltammetry. Half-wave potentials of Fe(III)/Fe(II) complexes with ligand moieties in several protonation states are reported, both in DMSO and in MeOH/H(2)O. Because of the presence of free imidazole groups coordinated to the iron, the potential of the iron(III)/iron(II) couples can be tuned by pH. A shift of DeltaE = E(deprot) - E(prot) ranging from -270 to -320 mV per exchanged proton in DMSO was measured. This study shows moreover that interconversions (with regard to both redox and protonation states) can be reversed several times. As the complexes have been isolated in order to be tested as superoxide dismutase mimics, preliminary reactions with dioxygen and with superoxide, considered as oxidant and reducer of biological importance, are reported. In these two series, O(2)(-) behaves either as a base or as a reducer and no adducts have been observed.

Structural and magnetic properties of carboxylato-bridged manganese(II) complexes involving tetradentate ligands: discrete complex and 1D polymers. Dependence of J on the nature of the carboxylato bridge.

The crystal structure of an inorganic linear polymer consisting of Mn(II) and an N-centered tripodal ligand N,N-bis(2-(6-methyl)pyridylmethyl)glycinate is presented (1, C(16)H(20)N(3)O(3)F(6)P(1)Mn(1), a = 9.993(2) A, b = 13.285(3) A, c = 16.040(3) A, orthorhombic, Pnam, Z = 4). The polymeric structure is ensured by carboxylato ligands connecting two Mn(II) in a rather rare syn-anti geometry. The magnetic properties of this infinite chain have been investigated, together with the magnetic properties of a dimeric Mn(II) compound (3) from a closely related ligand [N,N-bis[(1-methylimidazol-2-yl)-methyl)glycinate] involving an unusual bis(monatomic-carboxylato) bridge. The inorganic polymer 1 shows a pseudo-2D magnetic structure, with a major interaction pathway along the chain (J/k = -0.172 +/- 0.005 K) and an interchain minor one (zJ'/k = -0.006 +/- 0.004 K). These properties are reminiscent of those from a closely related previously reported inorganic Mn(II) polymer (2 obtained from manganese(II) and N,N-(2-pyridylmethyl)((1-methylimidazol-2-yl)methyl)glycinate). The dimer 3 shows a small antiferromagnetic coupling of J/k = -0.693 +/- 0.016 K. To address the influence of the carboxylato bridging mode on the magnetic properties, these complexes are compared to a series of compounds involving carboxylato bridges of several geometries between Mn(II) ions. Carboxylato bridges induce usually antiferromagnetic coupling, with the magnitude of the interaction (/J/) increasing with the number of bridges. The J value is dependent on the bridging mode. The syn-syn bridge is an efficient pathway, even by comparison with the monatomic [(mu-eta(1)-carboxylato)] bridge.