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Nickel superoxide dismutase (NiSOD) is an enzyme that protects cells against O2·-. While the structure of its active site is known, the mechanism of the catalytic cycle is still not elucidated. Its active site displays a square planar NiII center with two thiolates, the terminal amine and an amidate. We report here a bioinspired NiII complex built on an ATCUN-like binding motif modulated with one cysteine, which demonstrates catalytic SOD activity in water (kcat = 8.4(2) × 105 M-1 s-1 at pH = 8.1). Its reactivity with O2·- was also studied in acetonitrile allowing trapping two different short-lived species that were characterized by electron paramagnetic resonance or spectroelectrochemistry and a combination of density functional theory (DFT) and time-dependent DFT calculations. Based on these observations, we propose that O2·- interacts first with the complex outer sphere through a H-bond with the peptide scaffold in a [NiIIO2·-] species. This first species could then evolve into a NiIII hydroperoxo inner sphere species through a reaction driven by protonation that is thermodynamically highly favored according to DFT calculations.

International audience; We report on the isolation of a new family of μ-carboxylato-bridged metallocrown (MC) compounds by self-assembly of the recently isolated hexadentate tris(2-pyridylmethyl)amine ligand tpada2- incorporating two carboxylate units with metal cations. Twelve-membered MCs of manganese of the type 12-MC-3, namely [{MnII(tpada)}3(M)(H2O)n]2+ (Mn3M) (M = Mn2+ (n = 0), Ca2+ (n = 1) or Sr2+ (n = 2)), were structurally characterized. The metallamacrocycles connectivity consisting in three -[Mn-O-C-O]- repeating units is provided by one carboxylate unit of the three tpada2- ligands, while the second carboxylate coordinated a fourth cation in the central cavity of the MC, Mn2+ or an alkaline-earth metal, Ca2+ or Sr2+. The Mn3Ca and {Mn3Sr]2 join the small family of heterometallic manganese-calcium complexes and even rarer manganese-strontium complexes as models of the OEC of photosystem II (PSII). A 8-MC-4 of strontium of the molecular wheel type with four –[Sr-O] repeating unit was also isolated by self-assembly of the tpada2- ligand with Sr2+. This complex, namely [Sr(tpada)(OH2)]4 (Sr4), does not incorporate any cation in the central cavity but instead four water molecules coordinated to each Sr2+. Electrochemical investigations coupled to UV-visible absorption and EPR spectroscopies as well as electrospray mass spectrometry reveal the stability of the 12-MC-3 tetranuclear structures in solution, both in the initial oxidation state, MnII3M, as well as in the three-electrons oxidized state, MnIII3M. Indeed, the cyclic voltammogram of all these complexes exhibits three-successive reversible oxidation waves between +0.5 and +0.9 V corresponding to the successive one-electron oxidation of the Mn(II) ion into Mn(III) of the three {Mn(tpada)} units constituting the ring, which are fully maintained after bulk electrolysis.

The superoxide dismutase (SOD) activity of mononuclear NiII complexes, whose structures are inspired by the NiSOD, has been investigated. They have been designed with a sulfur-rich pseudopeptide li...

We report on the synthesis and structural characterization of the cobalt pentanuclear helicate complex from the rigid tetradentate bis(2-pyridyl)-3,5-pyrazolate ligand bpp

International audience; With the intention to investigate the redox properties of polynuclear complexes as previously reported for the pentamanganese complex [{Mn(II)(μ-bpp)3}2Mn(III)Mn(II)2(μ3-O)](3+) (2(3+)), we focused on the analogous pentairon complex that was previously isolated as all-ferrous. As Masaoka and co-workers recently published, aerobic synthesis leads to the [{Fe(II)(μ-bpp)3}2Fe(III)Fe(II)2(μ3-O)](3+) complex (1(3+)). This species exhibits in acetonitrile solution four reversible one-electron oxidation waves. Accordingly, the three oxidized species 1(4+), 1(5+), and 1(6+) with a 3Fe(II)2Fe(III), 2Fe(II)3Fe(III), and 1Fe(II)4Fe(III) composition, respectively, were generated by bulk electrolysis and isolated. Mössbauer spectroscopy allowed us to determine the spin states of all the iron ions and to unambiguously locate the sites of the successive oxidations. They all occur in the μ3-oxo core except for the 1(4+) to 1(5+) process that presents a striking electronic rearrangement, with both metals in axial position being oxidized while the core is reduced to the [Fe(III)Fe(II)2(μ3-O)](5+) oxidation level. This strongly differs from the redox behavior of the Mn5 system. The origin of this electronic switch is discussed.

International audience; The heterodinuclear complexes [Fe(III)Mn(II)(L-Bn)(μ-OAc)(2)](ClO(4))(2) (1) and [Fe(II)Mn(II)(L-Bn)(μ-OAc)(2)](ClO(4)) (2) with the unsymmetrical dinucleating ligand HL-Bn {[2-bis[(2-pyridylmethyl)aminomethyl]]-6-[benzyl-2-(pyridylmethyl)aminomethyl]-4-methylphenol} were synthesized and characterized as biologically relevant models of the new Fe/Mn class of nonheme enzymes. Crystallographic studies have been completed on compound 1 and reveal an Fe(III)Mn(II)μ-phenoxobis(μ-carboxylato) core. A single location of the Fe(III) ion in 1 and of the Fe(II) ion in 2 was demonstrated by Mössbauer and (1)H NMR spectroscopies, respectively. An investigation of the temperature dependence of the magnetic susceptibility of 1 revealed a moderate antiferromagnetic interaction (J = 20 cm(-1)) between the high-spin Fe(III) and Mn(II) ions in 1, which was confirmed by Mössbauer and electron paramagnetic resonance (EPR) studies. The electrochemical properties of complex 1 are described. A quasireversible electron transfer at -40 mV versus Ag/AgCl corresponding to the Fe(III)Mn(II)/Fe(II)Mn(II) couple appears in the cyclic voltammogram. Thorough investigations of the Mössbauer and EPR signatures of complex 2 were performed. The analysis allowed evidencing of a weak antiferromagnetic interaction (J = 5.72 cm(-1)) within the Fe(II)Mn(II) pair consistent with that deduced from magnetic susceptibility measurements (J = 6.8 cm(-1)). Owing to the similar value of the Fe(II) zero-field splitting (D(Fe) = 3.55 cm(-1)), the usual treatment within the strong exchange limit was precluded and a full analysis of the electronic structure of the ground state of complex 2 was developed. This situation is reminiscent of that found in many diiron and iron-manganese enzyme active sites.

The tripodal H(3)thqtcn ligand allows the synthesis of well-defined neutral monomeric syn-tris(hydroxyquinolinate) complexes of lanthanides. Pure [Ln(thqtcn)] complexes (Ln = Nd, 1; Er, 2; Yb, 3) of the triply deprotonated ligand thqtcn(3-) were prepared. Crystallographic characterization was carried out for complexes 1 and 3, showing that the ligand is flexible enough to wrap around Ln(III) of different size with a tricapped trigonal-prism coordination geometry. The partially protonated H(1.5)thqtcn(1.5-) ligand also binds strongly to Ln(III) ions in methanol and water (at pH approximately 5). The X-ray diffraction study shows that protonated complexes crystallize as chiral dimers of formula [Ln(H(1.5)thqtcn)](2)(OTf)(3) x 3 MeOH (Ln = Nd, 4; Yb, 5) in which two equivalent monomeric complexes of the partially protonated H(1.5)thqtcn(1.5-) are bridged by very strong hydrogen bonds between the phenol oxygen atoms. The ligand thqtcn(3-) sensitizes efficiently the near-infrared emission of Er, Nd (0.10% Qy), and Yb (0.60% Qy). For the first time, the effect of ligand protonation on the efficiency of the solid-state luminescence emission of lanthanides complexes is demonstrated by the decrease of the luminescence quantum yield observed for [Yb(H(1.5)thqtcn)](2)(OTf)(3) (0.26%) with respect to [Yb(thqtcn)] (0.60%). The water-soluble H(3)thqtcn-SO(3) analogue of H(3)thqtcn and its lanthanide complexes has been prepared. The solution quantum yields of the thqtcn-SO(3)(3-) complexes were measured in water at pH 7.4 (0.016% for Nd(III) and 0.14% for Yb(III)) and in deuterated water (Nd, 0.047%; Yb, 0.55%), and they are among the highest reported in the literature for Yb(III) in aqueous solutions. The high thermodynamic and kinetic stability in water at physiological pH of the gadolinium complex of thqtcn-SO(3)(3-) indicate that the lanthanide complexes of thqtcn(3-) and thqtcn-SO(3)(3-) are highly resistant to hydrolysis and therefore are well suited for the development of luminescent devices and for application as probes in biomedical imaging.

Novel copper(II)-nitroxide complexes exhibiting a spin-transition-like behavior have been prepared and characterized. They include meso, chiral, and racemic 2-(3-pyridyl)-nitronyl nitroxides differently substituted in positions 4 and/or 5 by ethyl groups and pyrimidyl nitroxides. Depending on the stoichiometry of the reaction, tetranuclear and binuclear complexes were obtained whose structures are cyclic. The tetranuclear species, which include two intracyclic and two exocyclic metal sites, are similar to the previously reported complex of the tetramethylated analogue, while the binuclear complexes involve only endocyclic metal ions and have uncoordinated N-oxyl groups. The tetranuclear complexes exist as two isomers depending on the temperature of crystallization: at room temperature, N-oxyl ligand coordination is axial-axial, while it is axial-equatorial at low temperature. Unexpectedly, this isomerism concerns N-oxyl bonding to the exocyclic metal centers for the derivatives of 4,5-diethyl-substituted ligands while it involves the endocyclic metal site in the complex of the monoethylated ligand, which converts reversibly from a high-spin state to a low-spin state, as observed for the complex of the tetramethylated ligand. Binuclear complexes are diamagnetic at room temperature but convert to a paramagnetic state on warming (90-110 degrees C); the transition is irreversible and sharp.

The synthesis, structural characterization, and electronic properties of a new series of high-spin six-coordinate dihalide mononuclear MnII complexes [Mn(tpa)X2] (tpa=tris-2-picolylamine; X=I (1), Br (2), and Cl (3)) are reported. The analysis of the crystallographic data shows that in all investigated complexes the manganese ion lies in the center of a distorted octahedron with a cis configuration of the halides imposed by the tpa ligand. By a multifrequency high-field electron paramagnetic resonance investigation (95-285 GHz), the electronic properties of 1-3 were determined (DI=-0.600, DBr=-0.360, DCl=+0.115 cm-1), revealing the important effect of (i) the nature of the halide and (ii) the configuration (cis/trans) of the two halides on the magnitude of D. The spin Hamiltonian parameters obtained by density functional theory calculations initiated from the crystal structure of 1-3 are in reasonable agreement with the experimental values. The absolute value of D is consistently overestimated, but the sign and the trend over the chemical series is well reproduced. Theoretical models (cis- and trans-[Mn(NH3)4X2], X=I, Br, Cl and F) have been used to investigate the different contributions to D and also to understand the origin of the experimentally observed changes in D within the series reported here. This study reveals that the spin-spin coupling contributions to the D tensor are non-negligible for the lighter halides (F, Cl) but become insignificant for the heavier halides (I, Br). The four different types of excitations involved in the spin-orbit coupling (SOC) part of the D tensor contribute with comparable magnitudes and opposing signs. The general trend observed for halide MnII complexes (DIDBrDCl) can be explained by the fact that the halide SOC dominates the D value in these systems with a major contribution arising from interference between metal- and halide-SOC contributions, which are proportional to the product of the SOC constants of Mn and X.

The crystal and molecular structure and the stability of lead and calcium complexes of two chelates containing picolinate chelating groups in different geometries have been investigated in order to relate the ligand affinity and selectivity for lead over calcium with the ability of the ligand to accommodate a stereochemically active lone pair. The crystal structures of the lead complexes of the diprotonated and monoprotonated tripodal ligand tpaa2- show that the three picolinate arms of the tripodal ligand coordinate the lead in an asymmetric way leaving a gap in the coordination sphere to accommodate the lead lone pair. As a consequence of this binding mode, one picolinate arm is very weakly bound and therefore can be expected to contribute very little to the complex stability. Conversely, the geometry of the dipodal ligand H2dpaea is designed to accommodate the lead lone pair; in the structure of the [Pb(dpaea)] complex the donor atoms of the ligand occupy only a quarter of the coordination sphere, reducing the sterical interaction between the lead lone pair with respect to the H3tpaa complexes. As a result, in the lead structures of H2dpaea all the ligand donor atoms are strongly bound to the metal ion leading to increased stability. The high value of the formation constant measured for the lead complex of the dipodal dpaea2- (log beta11(Pb)=12.1(3)) compared to the lower value found for the one of the tripodal tpaa3- (log beta11(Pb)=10.0(1)) provides direct evidence of the influence of the stereochemically active lead lone pair on complex stability. As a result, since the ligand geometry has little effect on the stability of the calcium complex, a remarkable increase in the Pb/Ca selectivity is observed for dpaea-(10(6.6)) compared to tpaa3- (10(1.5)), making the dipodal ligand a good candidate for application as extracting agent for the lead removal from contaminated water.

International audience; We have previously investigated cis/trans isomerization processes in phenoxido-bridged mixed-valent Fe$^{II}$Fe$^{III}$ complexes that contain either one aniline or one anilide ligand. In this work, we compare the properties of similar complexes bearing one terminal protic ligand, either aniline or 1H-benzimidazole. Whatever the ligand, $^1$H NMR spectroscopy clearly evidences that the complexes are present in CH$_3$CN as a mixture of cis- and trans-isomers in a close to 1:1 ratio. We show here that addition of NEt$_3$ indeed allows the deprotonation of these ligands, the resulting complexes bearing either anilide or benzimidazolide that are coordinated to the ferric site. The latter are singular examples of a high-spin ferric ion coordinated to a benzimidazolide ligand. Whereas the trans-isomer of the anilide complex is the overwhelming species, benzimidazolide species are mixtures of cis- and trans-isomers in equal proportions. Moreover, cyclic voltammametry studies show that Fe$^{II}$Fe$^{III}$ complexes with 1H-benzimidazole are more stable than their aniline counterparts, whereas the reverse is observed for the deprotonated species.

International audience; Uranyl(VI), uranyl(V), and uranium(IV) complexes supported by ferrocene-based tetradentate Schiff-base ligands were synthesized, and their solid-state and solution structures were determined. The redox properties of all complexes were investigated by cyclic voltammetry. The bulky salfen-Bu-t(2) allows the preparation of a stable uranyl(V) complex, while a stable U(IV) bis-ligand complex is obtained from the salt metathesis reaction between [UI4(OEt2)(2)] and K(2)salfen. The reduction of the [U(salfen)(2)] complex leads to an unprecedented intramolecular reductive coupling of the Schiff-base ligand resulting in a C-C bond between the two ferrocene-bound imino groups.

The reaction of UH3 or U metal with triflic acid results in the formation of a mixture of species including U(OTf)4 and leads to the reproducible isolation of the mononuclear U(IV) hydroxo complex [U(OTf)3(OH)(py)4] (1) and the U(IV) dinuclear mu-oxo-complex [{U(OTf)2(py)3}2{mu-O}{mu-OTf}2] (2). The X-ray crystal structures of these complexes have been determined. Analytically pure complex 1 can be prepared in a 17-27% yield providing a good precursor for the synthesis and study of the reactivity of the hydroxo complexes with different coordination environments. Two practical synthetic methods for the preparation of Lewis base adducts of U(OTf)3 are described. Analytically pure [U(OTf)3(py)4] (4) was easily and reproducibly prepared (50-60% yield) by protonolysis of the amide U{N(SiMe3)2}3 with pyridinium triflate in pyridine. Salt metathesis of UI3(thf)4 with potassium triflate in acetonitrile resulted in the complete substitution of the iodide counterions by triflate producing the acetonitrile solvate [U(OTf)3(MeCN)3]n (3). The solid-state structure of 3 shows the formation of a unique U(III) coordination polymer in which the metal ions are connected by three triflates acting as bidentate bridging ligands to form a 1D chain.

The isolation, structural characterization, and electronic properties of a series of high-spin mononuclear five-coordinated Mn(II) complexes, [Mn(terpy)(X)(2)] (terpy = 2, 2':6',2' '-terpyridine; X = I(-) (1), Br(-) (2), Cl(-) (3), or SCN(-) (4)), are reported. The X-ray structures of the complexes reveal that the manganese ion lies in the center of a distorted trigonal bipyramid for complexes 1, 2, and 4, while complex 3 is better described as a distorted square pyramid. The electronic properties of 1-4 were investigated by high-field and high-frequency EPR spectroscopy (HF-EPR) performed between 5 and 30 K. The powder HF-EPR spectra have been recorded in high-field-limit conditions (95-285 GHz) (DgbetaB). The spectra are thus simplified, allowing an easy interpretation of the experimental data and an accurate determination of the spin Hamiltonian parameters. The magnitude of D varies between 0.26 and 1.00 cm(-)(1) with the nature of the anionic ligand. Thanks to low-temperature EPR experiments, the sign of D was unambiguously determined. D is positive for the iodo and bromo complexes and negative for the chloro and thiocyano ones. A structural correlation is proposed. Each complex is characterized by a significant rhombicity with E/D values between 0.17 and 0.29, reflecting the distorted geometry observed around the manganese. Finally, we compared the spin Hamiltonian parameters of our five-coordinated complexes and those previously reported for other analogous series of dihalo four- and six-coordinated complexes. The effect of the coordination number and of the geometry of the Mn(II) complexes on the spin Hamiltonian parameters is discussed.

Magnetization measurements and variable temperature optical spectroscopy have been used to investigate, within the 4-300 K temperature range, the electronic structure of the reduced high-potential iron protein (HiPIP) from Chromatium vinosum and the model compounds (Cat)(2)[Fe(4)S(4)(SR)(4)], where RS(-) = 2,4,6-triisopropylphenylthiolate (1), 2,6-diphenylphenylthiolate (2), diphenylmethylthiolate (3), 2,4,6-triisopropylbenzylthiolate (4, 4'), 2,4,6-triphenylbenzylthiolate (5, 5'), 2,4,6-tri-tert-butylbenzylthiolate (6), and Cat(+) = (+)NEt(4) (1, 2, 3, 4', 5', 6), (+)PPh(4) (4, 5). The newly synthesized 2(2)(-), 3(2)(-), 5(2)(-), and 6(2)(-) complexes are, as 1(2)(-) and 4(2)(-), excellent models of the reduced HiPIPs: they exhibit the [Fe(4)S(4)](3+/2+) redox couple, because of the presence of bulky ligands which stabilize the [Fe(4)S(4)](3+) oxidized core. Moreover, the presence of SCH(2) groups in 4(2)(-), 5(2)(-), and 6(2)(-), as in the [Fe(4)S(4)] protein cores, makes them good biomimetic models of the HiPIPs. The X-ray structure of 2 is reported: it crystallizes in the orthorhombic space group Pcca with no imposed symmetry and a D(2)(d)()-distorted geometry of the [Fe(4)S(4)](2+) core. Fit of the magnetization data of the reduced HiPIP and of the 1, 2, 3, 4, 5, and 6 compounds within the exchange and double exchange theoretical framework leads to exchange coupling parameters J = 261-397 cm(-)(1). A firm determination of the double exchange parameters B or, equivalently, the transfer integrals beta = 5B could not be achieved that way. The obtained |B| values remain however high, attesting thus to the strength of the spin-dependent electronic delocalization which is responsible for lowest lying electronic states being characterized by delocalized mixed-valence pairs of maximum spin (9)/(2). Electronic properties of these systems are then accounted for by the population of a diamagnetic ground level and excited paramagnetic triplet and quintet levels, which are respectively J and 3J above the ground level. Optical studies of 1, 2, 4', 5', and 6 but also of (NEt(4))(2)[Fe(4)S(4)(SCH(2)C(6)H(5))(4)] and the isomorph (NEt(4))(2)[Fe(4)S(4)(S-t-Bu)(4)] and (NEt(4))(2)[Fe(4)Se(4)(S-t-Bu)(4)] compounds reveal two absorption bands in the near infrared region, at 705-760 nm and 1270-1430 nm, which appear to be characteristic of valence-delocalized and ferromagnetically coupled [Fe(2)X(2)](+) (X = S, Se) units. The |B| and |beta| values can be directly determined from the location at 10|B| of the low-energy band, and are respectively of 699-787 and 3497-3937 cm(-)(1). Both absorption bands are also present in the 77 K spectrum of the reduced HiPIP, at 700 and 1040 nm (Cerdonio, M.; Wang, R.-H.; Rawlings, J.; Gray, H. B. J. Am. Chem. Soc. 1974, 96, 6534-6535). The blue shift of the low-energy band is attributed to the inequivalent environments of the Fe sites in the protein, rather than to an increase of |beta| when going from the models to the HiPIP. The small differences observed in known geometries of [Fe(4)S(4)](2+) clusters, especially in the Fe-Fe distances, cannot probably lead to drastic changes in the direct Fe-Fe interactions (parameter beta) responsible for the delocalization phenomenon. These differences are however magnetostructurally significant as shown by the 261-397 cm(-)(1) range spanned by J. The cluster's geometry, hence the efficiency of the Femicro(3)-S-Fe superexchange pathways, is proposed to be controlled by the more or less tight fit of the cluster within the cavity provided by its environment.

Two new terpyridine dimanganese oxo complexes [Mn(2)(III,IV)(mu-O)(2)(terpy)(2)(CF(3)CO(2))(2)](+) (3) and [Mn(2)(III,III)(mu-O)(terpy)(2)(CF(3)CO(2))(4)] (4) (terpy = 2,2':6,2' '-terpyridine) have been synthesized and their X-ray structures determined. In contrast to the corresponding mixed-valent aqua complex [Mn(2)(III,IV)(mu-O)(2)(terpy)(2)(H(2)O)(2)](3+) (1), the two Mn atoms in 3 are not crystallographically equivalent. The neutral binuclear monooxo manganese(III,III) complex 4 exhibits two crystallographic forms having cis and trans configurations. In the cis complex, the two CF(3)CO(2)(-) ligands on each manganese adopt a cis geometry to each other; one CF(3)CO(2)(-) is trans to the oxygen of the oxo bridge while the second is cis. In the trans complex, the two coordinated CF(3)CO(2)(-) have a trans geometry to each other and are cis to the oxo bridge. The electrochemical behavior of 3 in organic medium (CH(3)CN) shows that this complex could be oxidized into its corresponding stable manganese(IV,IV) species while its reduced form manganese(III,III) is very unstable and leads by a disproportionation process to Mn(II) and Mn(IV) complexes. Complex 4 is only stable in the solid state, and it disproportionates spontaneously in CH(3)CN solution into the mixed-valent complex 3 and the mononuclear complex [Mn(II)(terpy)(2)](2+) (2), thereby preventing the observation of its electrochemical behavior.

Reference EPFL-ARTICLE-203059doi:10.1021/ic990122wView record in Web of Science Record created on 2014-11-07, modified on 2017-11-27

A new type of octacoordinated ternary β-diketonates complexes of terbium and europium has been prepared using the anionic tetradentate terpyridine-carboxylate ligand (L) as a sensitizer of lanthanide luminescence in combination with two β-diketonates ligands 2-thenoyltrifluoroacetyl-acetonate (tta(-)) for Eu(3+) and trifluoroacetylacetonate (tfac(-)) for Tb(3+). The solid state structures of the two complexes [Tb(L)(tfac)2] (1) and [Eu(L)(tta)2] (2) have been determined by X-ray crystallography. Photophysical and (1)H NMR indicate a high stability of these complexes with respect to ligand dissociation in solution. The use of the anionic tetradentate ligand in combination with two β-diketonates ligands leads to the extension of the absorption window toward the visible region (390 nm) and to high luminescence quantum yield for the europium complex in the solid state (Φ = 66(6)%). Furthermore, these complexes have been incorporated in polymer matrixes leading to highly luminescent flexible layers.

The Cu(I) and Cu(II) complexes of the new 1,8-diferro-cenylmethyl-4,11-dimethyl-1,4,8,11-tetraazacyclotetra-decane ligand (denoted L) have been isolated and characterized by X-ray structure determination and electrochemical studies. The Cu(I) complex presents an unprecedented stability toward dioxygen. The two complexes adopt two energetically distinct and stable geometries, which differ mainly by the relative positioning of the substituents above or below the cyclam plane. Triggered by a copper-centered electron transfer, a fast and reversible motion of the noncoordinating subunits is obtained in homogeneous solution and in the solid state.

A rational approach to the formation of pure heteropolymetallic lanthanide complexes that uses a two-step assembly strategy and exploits the different size requirements of the two metals included in the final structure is described. The investigation of the assembly of [LnL2](Otf) (L = 2,2':6',2' '-terpyridine-6-carboxylate) complexes into hexametallic rings hosting an additional hexacoordinated lanthanide cation was crucial for the development of this strategy. The formation and size of the cyclic assembly are controlled by the ionic radius and by the coordination number of the lanthanides. The rather high luminescence quantum yield of the heptaeuropium complex (25%) indicates that the ring structure is well adapted to include highly luminescent lanthanide complexes in nanosized architecture. The use of a stepwise synthetic strategy leads to the selective assembly of large heteropolymetallic rings. The addition of a smaller lanthanide ion to the [EuL2](Otf) complex in anhydrous acetonitrile leads selectively to heterometallic species with the Eu ions located on the peripheral sites and the smaller ion occupying only the central site. The high selectivity is the result of the different size requirements of the two metal sites present in the cyclic structure. The heterometallic structure of the isolated [Lu subset (EuL2)6](Otf)9 complex was confirmed by X-ray diffraction and by high resolution solid-state photophysical studies. The described synthetic approach allowed us to obtain the first example of selective assembly of two different lanthanide ions in a large polymetallic structure characterized in solution and in the solid state and will make the isolation of planned dimetallic combinations presenting different lanthanide emitters in the peripheral sites possible.

As a model of the active site of [NiFe] hydrogenases, a dinuclear nickel-ruthenium complex [Ni(xbsms)Ru(CO)2Cl2] was synthesized and fully characterized. The three-dimensional structure reveals a nickel center in a square-planar dithioether-dithiolate environment connected to a ruthenium moiety via a Ni(mu-SR)2Ru bridge. This complex catalyzes hydrogen evolution by electroreduction of the weakly acidic Et3NH+ ions in N,N-dimethylformamide and is therefore the first functional bioinspired model of [NiFe] hydrogenases.

The reaction of the lanthanide salts LnI3(thf)4 and Ln(OTf)3 with tris(2-pyridylmethyl)amine (tpa) was studied in rigorously anhydrous conditions and in the presence of water. Under rigorously anhydrous conditions the successive formation of mono- and bis(tpa) complexes was observed on addition of 1 and 2 equiv of ligand, respectively. Addition of a third ligand equivalent did not yield additional complexes. The mono(tpa) complex [Ce(tpa)I3] (1) and the bis(tpa) complexes [Ln(tpa)2]X3 (X = I, Ln = La(III) (2), Ln = Ce(III) (3), Ln = Nd(III) (4), Ln = Lu(III) (5); X = OTf, Ln = Eu(III) (6)) were isolated under rigorously anhydrous conditions and their solid-state and solution structures determined. In the presence of water, 1H NMR spectroscopy and ES-MS show that the successive addition of 1-3 equiv of tpa to triflate or iodide salts of the lanthanides results in the formation of mono(tpa) aqua complexes followed by formation of protonated tpa and hydroxo complexes. The solid-state structures of the complexes [Eu(tpa)(H2O)2(OTf)3] (7), [Eu(tpa)(mu-OH)(OTf)2]2 (8), and [Ce(tpa)(mu-OH)(MeCN)(H2O)]2I4 (9) have been determined. The reaction of the bis(tpa) lanthanide complexes with stoichiometric amounts of water yields a facile synthetic route to a family of discrete dimeric hydroxide-bridged lanthanide complexes prepared in a controlled manner. The suggested mechanism for this reaction involves the displacement of one tpa ligand by two water molecules to form the mono(tpa) complex, which subsequently reacts with the noncoordinated tpa to form the dimeric hydroxo species.

Reaction of the unsymmetrical phenol ligand 2-((bis(2-pyridylmethyl)amino)methyl)-6-(((2-pyridylmethyl)benzylamino)methyl)-4-methylphenol (HL-Bn) or its 2,6-dichlorobenzyl analogue (HL-BnCl(2)) with Fe(H(2)O)(6)(ClO(4))(2) in the presence of disodium m-phenylenedipropionate (Na(2)(mpdp)) followed by exposure to atmosphere affords the diiron(II,III) complexes [Fe(2)(L-Bn)(mpdp)(H(2)O)](ClO(4))(2) and [Fe(2)(L-BnCl(2))(mpdp)(CH(3)OH)](ClO(4))(2), respectively. The latter complex has been characterized by X-ray crystallography. It crystallizes in the monoclinic system, space group P2(1)/n, with a = 13.3095(14) A, b = 20.1073(19) A, c = 19.4997(19) A, alpha = 90 degrees, beta = 94.471(2) degrees, gamma = 90 degrees, V = 5202.6(9) A(3), and Z = 4. The structure of the compound is very similar to that of [Fe(2)(L-Bn)(mpdp)(H(2)O)](BPh(4))(2) determined earlier, except for the replacement of a water by a methanol on the ferrous site. Magnetic measurements of [Fe(2)(L-Bn)(mpdp)(H(2)O)](BPh(4))(2) reveal that the two high-spin Fe ions are moderately antiferromagnetically coupled (J = -3.2(2) cm(-)(1)). Upon dissolution in acetonitrile the terminal ligand on the ferrous site is replaced by a solvent molecule. The acetonitrile-water exchange has been investigated by various spectroscopic techniques (UV-visible, NMR, Mössbauer) and electrochemistry. The substitution of acetonitrile by water is clearly evidenced by Mössbauer spectroscopy by a reduction of the quadrupole splitting value from 3.14 to 2.41 mm/s. In addition, it causes a 210 mV downshift of the oxidation potential of the ferrous site and a similar reduction of the stability domain of the mixed-valence state. Exhaustive electrolysis of a solution of [Fe(2)(L-Bn)(mpdp)(H(2)O)](2+) shows that the aqua diferric species is not stable and undergoes a chemical reaction which can be partly reversed by reduction to the mixed-valent state. This and other electrochemical observations suggest that upon oxidation of the diiron center to the diferric state the aqua ligand is deprotonated to a hydroxo. This hypothesis is supported by Mössbauer spectroscopy. Indeed, this species possesses a large quadrupole splitting value (DeltaE(Q)or= 1.0 mm.s(-)(1)) similar to that of analogous complexes with a terminal phenolate ligand. This study illustrates the drastic effects of aqua ligand exchange and deprotonation on the electronic structure and redox potentials of diiron centers.

The synthesis of the potentially heptadentate ligand tris[6-((2-N,N-diethylcarbamoyl)pyridyl)methyl]amine, tpaam, containing three pyridinecarboxamide arms connected to a central nitrogen is described. Lanthanide complexes of this ligand are prepared and characterized. The crystallographic structure of the complexes of three lanthanide ions (La, Nd, Lu) is determined. The lanthanide(III) complexes of tpaam crystallize as monomeric species (in the presence of chloride or iodide counterions) in which the ligand tpaam acts as a N4O3 donor. The crystal structures presented here show that the Ln[bond]O and Ln[bond]N(pyridyl) distances in the complexes of tpaam are similar to those found for the tpaa complexes (H(3)tpaa = alpha,alpha',alpha' '-nitrilotri(6-methyl-2-pyridinecarboxylic acid) despite the difference in charge. A lengthening of the Ln[bond]N(apical) distance is observed in the tpaam complexes compared to the tpa (tris[(2-pyridyl)methyl]amine) complexes which is more marked for larger lanthanides than for smaller ones. The solution structures of the tpaam complexes were analyzed across the 4f series and compared to the solution structures of the lanthanide complexes of the tetradentate ligand tpa. Proton NMR studies are in agreement with the presence of C(3)(v) symmetric solution species for both ligands. For the larger lanthanides, the cation moves away from the apical nitrogen compared to the position occupied in tpa complexes, whereas for the smaller lanthanides, the metal ion is located in a similar position for the two ligands. Quite surprisingly, the formation constant of the Eu(tpaam)Cl(3) complex in D(2)O at 298 K (log beta(110) = 2.34(4)) is very similar to the one reported for Eu(tpa)Cl(3) (log beta(110) = 2.49(4) at 298 K in D(2)O) indicating that the addition of three amide groups to the ligand tpa does not lead to any increase in stability of the lanthanide complexes of tpaam compared to those of tpa.

Three manganese(II) complexes, namely [Mn(1)(ClO(4))(2)] (3), [Mn(1)(acac)(2)] (4), and [Mn(2)(1)(acac)(4)] (5), were isolated from solutions of Mn(ClO(4))(2) or Mn(acac)(2), and an easily accessible diimine ligand (1S,2S)-N,N'-bis-pyridin-2-ylmethylene-cyclohexane-1,2-diamine (1). Their structure was determined by X-ray crystallography, and these complexes proved to be catalysts for asymmetric sulfide oxidation by H(2)O(2). Enantiomeric excesses ranging from 5% to 62% were obtained with a variety of aryl alkyl sulfides. We also observed an interesting "chirality switch" effect by the achiral acac anion reversing the enantioselectivity of the complex [Mn(1)(ClO(4))(2)] from the S to the R sulfoxide enantiomer.

The crystal structure of the complex [U(tpa)(2)]I(3), 1 (tpa = tris[(2-pyridyl)methyl]amine), has been elucidated. The complex exists as only one enantiomer in the crystal leading to the chiral space group P2(1)2(1)2(1). The coordination geometry of the metal can be described as a distorted cube. Accidental oxidation of [U(tpa)(2)]I(3) led to the isolation of the unusual mononuclear bishydroxo complex of uranium(IV) [U(tpa)(2)(OH)(2)]I(2).3CH(3)CN, 2, which was structurally characterized. The controlled reaction of [U(tpa)(2)]I(3) with water resulted in the oxidation of the metal center and led to the formation of protonated tpa and of the trinuclear U(IV) oxo complex ([U(tpa)(mu-O)I](3)(mu(3)-I))I(2), 3. The solid state and solution structures of this trimer are reported. The pathway suggested for the formation of this complex is the oxidation of the [U(tpa)(2)]I(3) complex by H(2)O to form a U(IV) hydroxo complex which then decomposes, eliminating mono-protonated tpa. The comparison with the reported reaction with water of cyclopentadienyl derivatives points to a higher reactivity toward water reduction of the bis(tpa) complex with respect to the cyclopentadienyl derivatives. The reaction of U(III) with methanol in the presence of the supporting ligand tpa leads to formation of alkoxo complexes similarly to what is found for amide or cyclopentadienyl derivatives. The monomethoxide complex [U(tpa)I(3)(OMe)], 4, has been prepared in good yield by alcoholysis of the U(III) mono(tpa) complex. The crystal structure of this complex has been determined. The reaction of [U(tpa)(2)]I(3) with 2 equiv of methanol in acetonitrile allows the isolation of the bismethoxo complex of U(IV) [U(tpa)I(2)(OMe)(2)], 5, in 35-47% yield, which has been fully characterized. To account for the oxidation of U(III) to U(IV) the suggested mechanism assumes that hydrogen is evolved in both reactions.

Reactions of the unsymmetrical phenol ligand 2-(bis(2-pyridylmethyl)aminomethyl)-6-((2-pyridylmethyl)(benzyl)aminomethyl)-4-methylphenol with Mn(OAc)(2).4H(2)O or Mn(H(2)O)(6)(ClO(4))(2) in the presence of NaOBz affords the dimanganese(II) complexes 1(CH(3)OH), [Mn(2)(L)(OAc)(2)(CH(3)OH)](ClO(4)), and 2(H(2)O), [Mn(2)(L)(OBz)(2)(H(2)O)](ClO(4)), respectively. On the other hand, reaction of the ligand with hydrated manganese(III) acetate furnishes the mixed-valent derivative 3(H(2)O), [Mn(2)(L)(OAc)(2)(H(2)O)](ClO(4))( 2). The three complexes have been characterized by X-ray crystallography. 1(CH(3)OH) crystallizes in the monoclinic system, space group P2(1)/c, with a = 10.9215(6) A, b = 20.2318(12) A, c = 19.1354(12) A, alpha = 90 degrees, beta = 97.5310(10) degrees, gamma = 90 degrees, V = 4191.7 A(3), and Z = 4. 2(H(2)O) crystallizes in the monoclinic system, space group P2(1)/n, with a = 10.9215(6) A, b = 20.2318(12) A, c = 19.1354(12) A, alpha = 90 degrees, beta = 97.5310(10) degrees, gamma = 90 degrees, V = 4191.7 A(3), and Z = 4. 3(H(2)O) crystallizes in the monoclinic system, space group P2(1)/c, with a = 11.144(6) A, b = 18.737(10) A, c = 23.949(13) A, alpha = 90 degrees, beta = 95.910(10) degrees, gamma = 90 degrees, V = 4974(5) A(3), and Z = 4. Magnetic measurements revealed that the three compounds exhibit very similar magnetic exchange interactions -J = 4.3(3) cm(-)(1). They were used to establish tentative magneto-structural correlations which show that for the dimanganese(II) complexes -J decreases when the Mn-O(phenoxo) distance increases as expected from orbital overlap considerations. For the dimanganese(II,III) complexes, crystallographic results show that the Mn(II)-O(phenoxo) and Mn(III)-O(phenoxo) bond lengths are inversely correlated. An interesting magneto-structural correlation is found between -J and the difference between these bond lengths, delta(Mn)(-)(O) = d(Mn)()II(-)(O) - d(Mn)()III(-)(O): the smaller this difference, the larger -J. Electrochemical studies show that the mixed-valence state is favored in 1-3 by ca. 100 mV with respect to analogous complexes of symmetrical ligands, owing to the asymmetry of the electron density as found in the analogous diiron complexes.

Reactions of Mn(II)(PF(6))(2) and Mn(II)(O(2)CCH(3))(2).4H(2)O with the tridentate facially capping ligand N,N-bis(2-pyridylmethyl)ethylamine (bpea) in ethanol solutions afforded the mononuclear [Mn(II)(bpea)](PF(6))(2) (1) and the new binuclear [Mn(2)(II,II)(mu-O(2)CCH(3))(3)(bpea)(2)](PF(6)) (2) manganese(II) compounds, respectively. Both 1 and 2 were characterized by X-ray crystallographic studies. Complex 1 crystallizes in the monoclinic system, space group P2(1)/n, with a = 11.9288(7) A, b = 22.5424(13) A, c =13.0773(7) A, alpha = 90 degrees, beta = 100.5780(10 degrees ), gamma = 90 degrees, and Z = 4. Crystals of complex 2 are orthorhombic, space group C222(1), with a = 12.5686(16) A, b = 14.4059(16) A, c = 22.515(3) A, alpha = 90 degrees, beta = 90 degrees, gamma = 90 degrees, and Z = 4. The three acetates bridge the two Mn(II) centers in a mu(1,3) syn-syn mode, with a Mn-Mn separation of 3.915 A. A detailed study of the electrochemical behavior of 1 and 2 in CH(3)CN medium has been made. Successive controlled potential oxidations at 0.6 and 0.9 V vs Ag/Ag(+) for a 10 mM solution of 2 allowed the selective and nearly quantitative formation of [Mn(III)(2)(mu-O)(mu-O(2)CCH(3))(2)(bpea)(2)](2+) (3) and [Mn(IV)(2)(mu-O)(2)(mu-O(2)CCH(3))(bpea)(2)](3+) (4), respectively. These results have shown that each substitution of an acetate group by an oxo group is induced by a two-electron oxidation of the corresponding dimanganese complexes. Similar transformations have been obtained if 2 is formed in situ either by direct mixing of Mn(2+) cations, bpea ligand, and CH(3)COO(-) anions with a 1:1:3 stoichiometry or by mixing of 1 and CH(3)COO(-) with a 1:1.5 stoichiometry. Associated electrochemical back-transformations were investigated. 2, 3, and the dimanganese [Mn(III)Mn(IV)(mu-O)(2)(mu-O(2)CCH(3))(bpea)(2)](2+) analogue (5) were also studied for their ability to disproportionate hydrogen peroxide. 2 is far more active compared to 3 and 5. The EPR monitoring of the catalase-like activity has shown that the same species are present in the reaction mixture albeit in slightly different proportions. 2 operates probably along a mechanism different from that of 3 and 5, and the formation of 3 competes with the disproportionation reaction catalyzed by 2. Indeed a solution of 2 exhibits the same activity as 3 for the disproportionation reaction of a second batch of H(2)O(2) indicating that 3 is formed in the course of the reaction.

The tripodal ligand (alpha,alpha',alpha' 'nitrilotri(6-methyl-2-pyridinecarboxylic acid)) (H(3)tpaa) forms a Gd(III) complex which has a relaxivity (r(1p) = 13.3 mM(-1) s(-1) at 25 degrees C and at 60 MHz) remarkably higher than those of the currently clinically used contrast agents based on octacoordinate polyaminocarboxylate complexes (3.5-4.7 mM(-1) s(-1)) and a reasonably good thermodynamic stability. The crystal structure of the ligand and of its La, Nd, Eu, Gd, Tb, Ho, Tm, Yb, and Lu complexes have been determined by X-ray crystallography. The neutral H(3)tpaa molecule adopts, in the solid state, a preorganized tripodal conformation in which the three H(3)tpaa arms are located on the same side of the molecule, ready to bind a metal ion in a heptadentate coordination mode. The structures of the Ln(III) complexes vary along the series for their nuclearity and number of water molecules coordinated to the metal, and a tetrameric structure is observed for the La(3+) ion (9- and 10-coordinate metal centers), dimeric structures are formed from the Nd(3+) ion through the Yb(3+) ion (9-coordinate), and a monomeric structure results for Lu(3+) (8-coordinate). The relaxivity studies presented here suggest that the high relaxivity of the Gd(tpaa) complex is mainly the consequence of a shorter bound water proton-Gd(III) distance associated with a probable water coordination equilibrium between tris(aqua) and bis(aqua) complexes, giving raise to a mean number of coordinated water molecules q2. Both effects are strongly related to the ligand flexibility, which allows for a large volume available for water binding. The observed rapid water exchange rate is probably due to the presence of a low-energy barrier between 10-, 9-, and 8- coordinate geometries. Although the low solubility of the Gd complex of tpaa prevents its practical application as an MRI contrast agent, the straightforward introduction of substituents on the pyridine rings allows us to envisage ligands with a higher water solubility, containing functional groups leading to macromolecular systems with very high relaxivity.

The coordination of Fe2+ with mono-bipy, bis(bipy), and bis(phen) carboxamide or carboxyester-bridged derivatives of ferrocene (bipy = 2,2‘-bipyridine; phen = 1,10-phenanthroline) resulted in the unprecedented formation of stable Fe-bis(bipy)- and bis(phen)-like complexes, where the iron(II) environment is provided by the four nitrogen atoms of the bipy or phen subunits and by the two oxygen atoms of carbonyl groups belonging to the amide or ester linkages.

This report covers studies in trivalent lanthanide complexation by two simple cyclohexanetriols that are models of the two coordination sites found in sugars and derivatives. Several complexes of trivalent lanthanide ions with cis,cis-1,3,5-trihydroxycyclohexane (L(1)()) and cis,cis-1,2,3-trihydroxycyclohexane (L(2)()) have been characterized in the solid state, and some of them have been studied in organic solutions. With L(1)(), Ln(L)(2) complexes are obtained when crystallization is performed from acetonitrile solutions whatever the nature of the salt (nitrate or triflate) [Ln(L(1)())(2)(NO(3))(2)](NO(3)) (Ln = Pr, Nd); [Ln(L(1)())(2)(NO(3))H(2)O](NO(3))(2) (Ln = Eu, Ho, Yb); [Ln(L(1)())(2)(OTf)(2)(H(2)O)](OTf) (Ln = Nd, Eu). Lanthanum nitrate itself gives a mixed complex [La(L(1)())(2)(NO(3))(2)][LaL(1)()(NO(3))(4)] from acetonitrile solution while [La(L(1)())(2)(NO(3))(2)](NO(3)) is obtained using dimethoxyethane as reaction solvent and crystallization medium. With L(2)(), Ln(L)(2) complexes have also been crystallized from methanol solution [Ln(L(2)())(2)(NO(3))(2)]NO(3), (Ln = Pr, Nd, Eu). Single-crystal X-ray diffraction analyses are reported for these complexes. Complex formation in solution has been studied for several triflate salts (La, Pr, Nd, Eu, and Yb) with L(1 )()and L(2)(), respectively in acetonitrile and in methanol. In contrast to the solid state, both structures Ln(L) and Ln(L)(2) equilibrate in solution, as was demonstrated by low-temperature (1)H NMR and electrospray ionization mass spectrometry experiments. Competing experiments in complexing abilities of L(1)() and L(2)() with trivalent lanthanide cations have shown that only L(2)() exhibits a small selectivity (NdPrYbLaEu) in methanol.

High levels of stereoinduction are obtained in the epoxidation of 1,2-dihydronaphthalene catalyzed by sterically crowded manganese chiroporphyrin catalysts, and asymmetric induction is correlated to the degree of nonplanar distortion of the porphyrin free base in solution.