Ru-bis(pyridine)pyrazolate (bpp)-Based Water-Oxidation Catalysts Anchored on TiO2: The Importance of the Nature and Position of the Anchoring Group.

Three distinct functionalisation strategies have been applied to the in,in-[{Ru(II)(trpy)}2(µ-bpp)(H2O)2](3+) (trpy=2,2':6',2''-terpyridine, bpp=bis(pyridine)pyrazolate) water-oxidation catalyst framework to form new derivatives that can adsorb onto titania substrates. Modifications included the addition of sulfonate, carboxylate, and phosphonate anchoring groups to the terpyridine and bis(pyridyl)pyrazolate ligands. The complexes were characterised in solution by using 1D NMR, 2D NMR, and UV/Vis spectroscopic analysis and electrochemical techniques. The complexes were then anchored on TiO2-coated fluorinated tin oxide (FTO) films, and the reactivity of these new materials as water-oxidation catalysts was tested electrochemically through controlled-potential electrolysis (CPE) with oxygen evolution detected by headspace analysis with a Clark electrode. The results obtained highlight the importance of the catalyst orientation with respect to the titania surface in regard to its capacity to catalytically oxidize water to dioxygen.

Synthesis and Isomeric Analysis of RuII Complexes Bearing Pentadentate Scaffolds.

A RuII-pentadentate polypyridyl complex [RuII(κ-N5-bpy2PYMe)Cl]+ (1+, bpy2PYMe = 1-(2-pyridyl)-1,1-bis(6-2,2'-bipyridyl)ethane) and its aqua derivative [RuII(κ-N5-bpy2PYMe)(H2O)]2+ (22+) were synthesized and characterized by experimental and computational methods. In MeOH, 1+ exists as two isomers in different proportions, cis (70%) and trans (30%), which are interconverted under thermal and photochemical conditions by a sequence of processes: chlorido decoordination, decoordination/recoordination of a pyridyl group, and chlorido recoordination. Under oxidative conditions in dichloromethane, trans-12+ generates a [RuIII(κ-N4-bpy2PYMe)Cl2]+ intermediate after the exchange of a pyridyl ligand by a Cl- counterion, which explains the trans/cis isomerization observed when the system is taken back to Ru(II). On the contrary, cis-12+ is in direct equilibrium with trans-12+, with absence of the κ-N4-bis-chlorido RuIII-intermediate. All these equilibria were modeled by density functional theory calculations. Interestingly, the aqua derivative is obtained as a pure trans-[RuII(κ-N5-bpy2PYMe)(H2O)]2+ isomer (trans-22+), while the addition of a methyl substituent to a single bpy of the pentadentate ligand leads to the formation of a single cis isomer for both chlorido and aqua derivatives [RuII(κ-N5-bpy(bpyMe)PYMe)Cl]+ (3+) and [RuII(κ-N5-bpy(bpyMe)PYMe)(H2O)]2+ (42+) due to the steric constraints imposed by the modified ligand. This system was also structurally and electrochemically compared to the previously reported [RuII(PY5Me2)X]n+ system (X = Cl, n = 1 (5+); X = H2O, n = 2 (62+)), which also contains a κ-N5-RuII coordination environment, and to the newly synthesized [RuII(PY4Im)X]n+ complexes (X = Cl, n = 1 (7+); X = H2O, n = 2 (82+)), which possess an electron-rich κ-N4C-RuII site due to the replacement of a pyridyl group by an imidazolic carbene.

Molecular water oxidation mechanisms followed by transition metals: state of the art.

One clean alternative to fossil fuels would be to split water using sunlight. However, to achieve this goal, researchers still need to fully understand and control several key chemical reactions. One of them is the catalytic oxidation of water to molecular oxygen, which also occurs at the oxygen evolving center of photosystem II in green plants and algae. Despite its importance for biology and renewable energy, the mechanism of this reaction is not fully understood. Transition metal water oxidation catalysts in homogeneous media offer a superb platform for researchers to investigate and extract the crucial information to describe the different steps involved in this complex reaction accurately. The mechanistic information extracted at a molecular level allows researchers to understand both the factors that govern this reaction and the ones that derail the system to cause decomposition. As a result, rugged and efficient water oxidation catalysts with potential technological applications can be developed. In this Account, we discuss the current mechanistic understanding of the water oxidation reaction catalyzed by transition metals in the homogeneous phase, based on work developed in our laboratories and complemented by research from other groups. Rather than reviewing all of the catalysts described to date, we focus systematically on the several key elements and their rationale from molecules studied in homogeneous media. We organize these catalysts based on how the crucial oxygen-oxygen bond step takes place, whether via a water nucleophilic attack or via the interaction of two M-O units, rather than based on the nuclearity of the water oxidation catalysts. Furthermore we have used DFT methodology to characterize key intermediates and transition states. The combination of both theory and experiments has allowed us to get a complete view of the water oxidation cycle for the different catalysts studied. Finally, we also describe the various deactivation pathways for these catalysts.

Powerful Bis-facially Pyrazolate-Bridged Dinuclear Ruthenium Epoxidation Catalyst.

A new bis-facial dinuclear ruthenium complex, {[Ru(II)(bpy)]2(µ-bimp)(µ-Cl)}(2+), 2(2+), containing a hexadentate pyrazolate-bridging ligand (Hbimp) and bpy as auxiliary ligands has been synthesized and fully characterized in solution by spectrometric, spectroscopic, and electrochemical techniques. The new compound has been tested with regard to its capacity to oxidize water and alkenes. The in situ generated bis-aqua complex, {[Ru(II)(bpy)(H2O)]2(µ-bimp)}(3+), 3(3+), is an excellent catalyst for the epoxidation of a wide range of alkenes. High turnover numbers (TN), up to 1900, and turnover frequencies (TOF), up to 73 min(-1), are achieved using PhIO as oxidant. Moreover, 3(3+) presents an outstanding stereospecificity for both cis and trans olefins toward the formation of their corresponding epoxides due to specific interactions transmitted by its ligand scaffold. A mechanistic analysis of the epoxidation process has been performed based on density functional theory (DFT) calculations in order to better understand the putative cooperative effects within this dinuclear catalyst.

Dinuclear ruthenium complexes containing the Hpbl ligand: synthesis, characterization, linkage isomerism, and epoxidation catalysis.

Three dinucleating Ru-Cl complexes containing the hexadentate dinucleating ligand [1,1'-(4-methyl-1H-pyrazole-3,5-diyl)bis(1-(pyridin-2-yl)ethanol)] (Hpbl) and the meridional 2,2':6',2″-terpyridine ligand (trpy) have been prepared and isolated. These complexes include {[RuCl(trpy)]2(µ-pbl-κ-N(3)O)}(+) (1a(+)), {[RuCl(trpy)]2(µ-Hpbl-κ-N(3)O)}(2+) (1b(2+)), and {[RuCl(trpy)]2(µ-Hpbl-κ-N(2)O(2))}(2+) (1c(2+)) and were characterized by analytic and spectroscopic techniques. In addition, complexes 1b(2+) and 1c(2+) were characterized in the solid state by monocrystal X-ray diffraction analysis. The coordination versatility of the Hpbl ligand allows the presence of multiple isomers that can be obtained depending on the Ru oxidation state and were thoroughly characterized by electrochemical techniques, namely, cyclic voltammetry and coulometry. Finally, 1a(+) and its recently reported mononuclear analogue, in-[RuCl(Hpbl)(trpy)](+), have been tested as catalysts for epoxidation of cis-ß-methylstyrene.

Synthesis, characterization, and linkage isomerism in mononuclear ruthenium complexes containing the new pyrazolate-based ligand Hpbl.

A new tetradentate dinucleating ligand [1,1'-(4-methyl-1H-pyrazole-3,5-diyl)bis(1-(pyridin-2-yl)ethanol)] (Hpbl) containing an O/N mixed donor set of atoms has been synthesized and characterized by analytical and spectroscopic techniques. The Ru-Cl and Ru-aqua complexes containing this ligand of general formula [Ru(II)X(Hpbl)(trpy)](y+) (trpy = 2,2':6',2″-terpyridine; X = Cl, y = 1; X = H2O, y = 2) have been prepared and thoroughly characterized by spectroscopic and electrochemical techniques. The Ru-aqua complex 2 undergoes N → O linkage isomerization as observed electrochemically, and the related thermodynamic and kinetic parameters are extracted from cyclic voltammetry experiments together with DIGISIM, a CV simulation package. Under basic conditions an additional isomer is observed where the pyrazolyl group in the Hpbl ligand is replaced by the geminal pyridyl group. Further structural and electronic characterization of all the isomers has been carried out by means of DFT calculations.

Insights into the light-driven hydrogen evolution reaction of mesoporous graphitic carbon nitride decorated with Pt or Ru nanoparticles.

Ru or Pt nanoparticles have been prepared following the organometallic approach and deposited onto the surface of mesoporous graphitic carbon nitride (mpg-CN). Three different Ru-based samples have also been compared to investigate the effect of 4-phenylpyridine as a stabilizing agent. The photocatalytic performance towards the hydrogen evolution reaction (HER) has been tested showing that all hybrid systems clearly outperform the photocatalytic activity of bare mpg-CN. In particular, Pt-decorated mpg-CN yields the largest H2 production upon visible-light irradiation (870 µmol h-1 g-1, TOF = 14.1 h-1, TON = 339 after 24 h) when compared with the Ru-based samples (137-155 µmol h-1 g-1, TOFs between 2.3-2.7 h-1, TONs between 54-57 after 24 h). Long-term photochemical tests (up to 65 h irradiation) show also an improved stability of the Pt-based samples over the Ru counterpart. Photophysical experiments aimed at rationalizing the photocatalytic performance of the different hybrid systems elucidate that the enhanced activity of the Pt-decorated mpg-CN over the Ru-based analogues arises from improved electron transfer kinetics from mpg-CN to the metal nanoparticles.

Synthesis, structure, and reactivity of new tetranuclear Ru-Hbpp-based water-oxidation catalysts.

The preparation of three new octadentate tetranucleating ligands made out of two Ru-Hbpp-based units [where Hbpp is 3,5(bispyridyl)pyrazole], linked by a xylyl group attached at the pyrazolate moiety, of general formula (Hbpp)(2)-u-xyl (u = p, m, or o) is reported, together with its dinucleating counterpart substituted at the same position with a benzyl group, Hbpp-bz. All of these ligands have been characterized with the usual analytical and spectroscopic techniques. The corresponding tetranuclear ruthenium complexes of general formula {[Ru(2)(trpy)(2)(L)](2)(µ-(bpp)(2)-u-xyl)}(n+) [L = Cl or OAc, n = 4; L = (H(2)O)(2), n = 6] and their dinuclear homologues {[Ru(2)(trpy)(2)(L)](µ-bpp-bz)}(n+) [L = Cl or OAc, n = 2; L = (H(2)O)(2), n = 3] have also been prepared and thoroughly characterized both in solution and in the solid state. In solution, all of the complexes have been characterized spectroscopically by UV-vis and NMR and their redox properties investigated by means of cyclic voltammetry techniques. In the solid state, monocrystal X-ray diffraction analysis has been carried out for two dinuclear complexes {[Ru(2)(trpy)(2)(L)](µ-bpp-bz)}(2+) (L = Cl and OAc) and for the tetranuclear complex {[Ru(2)(trpy)(2)(µ-OAc)](2)(µ-(bpp)(2)-m-xyl)}(4+). The capacity of the tetranuclear aqua complexes {[Ru(2)(trpy)(2)(H(2)O)(2)](2)(µ-(bpp)(2)-u-xyl)}(6+) and the dinuclear homologue {[Ru(2)(trpy)(2)(H(2)O)(2)](µ-bpp-bz)}(3+) to act as water-oxidation catalysts has been evaluated using cerium(IV) as the chemical oxidant in pH = 1.0 triflic acid solutions. It is found that these complexes, besides generating significant amounts of dioxygen, also generate carbon dioxide. The relative ratio of [O(2)]/[CO(2)] is dependent not only on para, meta, or ortho substitution of the xylylic group but also on the concentration of the starting materials. With regard to the tetranuclear complexes, the one that contains the more sterically constrained ortho-substituted ligand generates the highest [O(2)]/[CO(2)] ratio.

Molecular catalysts that oxidize water to dioxygen.

During the past four years we have witnessed a revolution in the field of water-oxidation catalysis, in which well-defined molecules are opening up entirely new possibilities for the design of more rugged and efficient catalysts. This revolution has been stimulated by two factors: the urgent need for clean and renewable fuel and the intrinsic human desire to mimic nature's reactions, in this case the oxygen-evolving complex (OEC) of the photosystem II (PSII). Herein we give a short general overview of the established basis for the oxidation of water to dioxygen as well as presenting the new developments in the field. Furthermore, we describe the new avenues these developments are opening up with regard to catalyst design and performance, together with the new questions they pose, especially from a mechanistic perspective. Finally the challenges the field is facing are also discussed.

Dissimilar catalytic behavior of molecular or colloidal palladium systems with a new NHC ligand.

In this work, we describe the synthesis of a new N-heterocyclic carbene (NHC) ligand, derived from a hybrid pyrazole-imidazolium scaffold, namely 1-[2-(3,5-dimethylpyrazol-1-yl)ethyl]-3-((S)-1-phenylethyl)-3H-imidazol-2-ylidene (L). This ligand has been used as a stabilizer for the organometallic synthesis of palladium(0) nanoparticles (Pd NPs). L presents a better stabilizing effect than its pre-carbenic HLCl counterpart, allowing the formation of isolated Pd NPs while HLCl yields aggregated ones. Additionally, molecular Pd(ii) coordination compounds of L and HLCl were synthesized and characterized to better understand the coordination modes of these ligands. Both molecular and colloidal Pd systems have been further tested in catalytic C-C coupling processes. Three different types of reactions have been observed depending on the catalytic system: (i) the Suzuki-Miyaura reaction takes place with Pd molecular complexes; (ii) a secondary reaction, the dehalogenation of the substrate, is always detected and (iii) the C-C homocoupling between two molecules of bromoarenes is observed with colloidal catalysts.

Mononuclear ruthenium compounds bearing N-donor and N-heterocyclic carbene ligands: structure and oxidative catalysis.

A new CNNC carbene-phthalazine tetradentate ligand has been synthesised, which in the reaction with [Ru(T)Cl3] (T = trpy, tpm, bpea; trpy = 2,2';6',2''-terpyridine; tpm = tris(pyrazol-1-yl)methane; bpea = N,N-bis(pyridin-2-ylmethyl)ethanamine) in MeOH or iPrOH undergoes a C-N bond scission due to the nucleophilic attack of a solvent molecule, with the subsequent formation of the mononuclear complexes cis-[Ru(PhthaPz-OR)(trpy)X]n+, [Ru(PhthaPz-OMe)(tpm)X]n+ and trans,fac-[Ru(PhthaPz-OMe)(bpea)X]n+ (X = Cl, n = 1; X = H2O, n = 2; PhthaPz-OR = 1-(4-alkoxyphthalazin-1-yl)-3-methyl-1H-imidazol-3-ium), named 1a+/2a2+ (R = Me), 1b+/2b2+ (R = iPr), 3+/42+ and 5+/62+, respectively. Interestingly, regulation of the stability regions of different Ru oxidation states is obtained by different ligand combinations, going from 62+, where Ru(iii) is clearly stable and mono-electronic transfers are favoured, to 2a2+/2b2+, where Ru(iii) is almost unstable with regard to its disproportionation. The catalytic performance of the Ru-OH2 complexes in chemical water oxidation at pH 1.0 points to poor stability (ligand oxidation), with subsequent evolution of CO2 together with O2, especially for 42+ and 62+. In electrochemically driven water oxidation, the highest TOF values are obtained for 2a2+ at pH 1.0. In alkene epoxidation, complexes favouring bi-electronic transfer processes show better performances and selectivities than those favouring mono-electronic transfers, while alkenes containing electron-donor groups show better performances than those bearing electron-withdrawing groups. Finally, when cis-ß-methylstyrene is employed as the substrate, no cis/trans isomerization takes place, thus indicating the existence of a stereospecific process.

Chemical, electrochemical and photochemical molecular water oxidation catalysts.

Hydrogen release from the splitting of water by simply using sunlight as the only energy source is an old human dream that could finally become a reality. This process involves both the reduction and oxidation of water into hydrogen and oxygen, respectively. While the first process has been fairly overcome, the conversion of water into oxygen has been traditionally the bottleneck process hampering the development of a sustainable hydrogen production based on water splitting. Fortunately, a revolution in this field has occurred during the past decade, since many research groups have been conducting an intense research in this area. Thus, while molecular, well-characterized catalysts able to oxidize water were scarce just five years ago, now a wide range of transition metal based compounds has been reported as active catalysts for this transformation. This review reports the most prominent key advances in the field, covering either examples where the catalysis is triggered chemically, electrochemically or photochemically.

Synthesis and Reactivity of Nickel(II) Complexes of the Pyridine-Based Phosphorus-Containing Macrocycle 6-Phenyl-15-aza-6-phospha-3,9- dithiabicyclo[9,3,1]pentadeca-1(15),11,13-triene.

The reactivity of the phosphomacrocycle 6-phenyl-15-aza-6-phospha-3,9-dithiabicyclo[9,3,1]pentadeca-1(15), 11,13-triene (L) toward different Ni(II) salts has been studied. The reaction of L with 1 molar equiv of Ni(II) perchlorate in acetonitrile solution gives the complex [Ni(L)(CH(3)CN)(2)](ClO(4))(2) (1), which crystallizes in the monoclinic space group P2(1)/c (No. 14) with a = 12.704(2) Å, b = 13.538(2) Å, c = 17.377(2) Å, beta = 107.554(8) degrees, and Z = 4. The Ni(II) atom in 1 shows an octahedral environment defined by the four donor atoms of L and two acetonitrile molecules in a cis disposition. The reaction of L with Ni(II) perchlorate in 2:1 molar ratio yields the green complex [Ni(Lox)(2)](ClO(4))(2) (2) in which the phosphine groups of the ligand L are in their phosphine-oxide form (Lox). The same complex is also obtained by adding more ligand L to an acetonitrile solution of 1. Complex 2 crystallizes in the triclinic space group P&onemacr; (No. 2) with a = 10.564(1) Å, b = 11.975(2) Å, c = 9.124(2) Å, alpha = 109.79(1) degrees, beta = 95.67(2) degrees, gamma = 109.19(1) degrees, and Z = 1. The octahedral coordinated Ni(II) atom in 2 lies on a crystallographic center of symmetry, and it is surrounded by two pyridine nitrogen atoms, two thioether sulfur atoms, and two phosphine-oxide oxygen atoms provided by two facially coordinated Lox ligands. The reaction of L with Ni(II) chloride and Ni(II) thiocyanate yields the complexes [NiCl(2)(L)] (3) and [Ni(NCS)(2)(L)] (4), respectively, which are also obtained by adding Me(4)NCl or KSCN to an acetonitrile solution of 1. Complex 4 crystallizes in the monoclinic space group P2(1)/n (No. 14), with a = 11.139(2) Å, b = 13.270(2) Å, c = 14.885(1) Å, beta = 92.34(1) degrees, and Z = 4, and its structure is similar to that of complex 1, simply replacing the two acetonitrile molecules by two SCN(-) anions. The reactions of the complex 1 with different chelating (en, bipy, o-phen) and nonchelating (py) N-donor ligands have also been carried out. In all cases the two acetonitrile molecules of 1 are easly replaced by the N-donor ligands, but the macrocycle L molecule remains coordinated to the Ni(II) atom. The crystal structure of the complex [Ni(L)(py)(2)](ClO(4))(2) (5) is also elucidated. Crystal data: monoclinic space group P2(1)/c (No. 14) a = 13.063(2) Å, b = 16.615(3) Å, c = 14.973(3) Å, beta = 91.76(2) degrees, Z = 4.

A Ru-Hbpp-based water-oxidation catalyst anchored on rutile TiO2.

Two organic ligands based on bis-(2-pyridyl)pyrazole (Hbpp) functionalized with a para-methylenebenzoic acid (Hbpp-R(a)) or its ester derivative (Hbpp-R(e)) were prepared and characterized. The ester-functionalized ligand was then used to prepare a series of related dinuclear ruthenium complexes of general formula [Ru(II)2(L-L)(bpp-R(n))(trpy)2](m+) (L-L=mu-Cl, mu-acetato, or (H2O)2; n=e or a; trpy=2,2':6',2''-terpyridine; m=2 or 3). The complexes were characterized in solution by 1D and 2D NMR spectroscopy, UV/Vis spectroscopy, and electrochemical techniques. The [Ru(II)2(mu-Cl)(bpp-R(e))(trpy)2](PF6)(2) complex was further characterized in the solid state by X-ray diffraction. The complexes containing the free carboxylic acid ligand were anchored onto rutile TiO2 and treated with 0.1 M triflic acid solution to generate the homologous water-oxidation catalysts TiO(2)-[Ru(II) (2)(H2O)(2)(bpp-R(a))(trpy)2]2+. This new hybrid material catalytically oxidizes water to molecular oxygen in a heterogeneous manner using Ce(IV) as chemical oxidant. The generation of molecular oxygen is accompanied by the formation of carbon dioxide as well as some leaching of the Ru catalyst.

Ru complexes that can catalytically oxidize water to molecular dioxygen.

The main objective of this review is to give a general overview of the structure, electrochemistry (when available), and catalytic performance of the Ru complexes, which are capable of oxidizing water to molecular dioxygen, and to highlight their more relevant features. The description of the Ru catalysts is mainly divided into complexes that contain a Ru-O-Ru bridging group and those that do not. Finally a few conclusions are drawn from the global description of all of the catalysts presented here, and some guidelines for future catalyst design are given.

Exploring the interaction of mercury(II) by N(2)S(2) and NS(3) anthracene-containing macrocyclic ligands: photophysical, analytical, and structural studies.

The complexation properties toward Hg(II) of six macrocyclic ligands, 3,11-dithia-7,17-diazabicyclo[11.3.1]heptadeca-1(17),13,15-triene (L1), 7-(9-anthracenylmethyl)-3,11-dithia-7,17-diazabicyclo[11.3.1]heptadeca-1(17),13,15-triene (L2), 7-(10-methyl-9-anthracenylmethyl)-3,11-dithia-7,17-diazabicyclo[11.3.1]heptadeca-1(17),13,15-triene (L3), 7,7'-[9,10-anthracenediylbis(methylene)]bis-3,11-dithia-7,17-diazabicyclo[11.3.1]heptadeca-1(17),13,15-triene (L4), 1,4,7-trithia-11-azacyclotetradecane (L5), and 11,-(anthracen-9-ylmethyl)-1,4,7-trithia-11-azacyclotetradecane (L6), were studied. The stoichiometries of the formed species were determined from absorption and fluorescence titrations. In these anthracene-containing macrocycles, a fluorescent quenching of the emission was found upon Hg(II) addition. The X-ray crystal structure of [HgCl2(L2)] x 1/2CH2Cl2 was determined. The asymmetric unit contains two independent [HgCl2(L2)] molecules and one dichloromethane molecule. Each Hg(II) ion is coordinated by the pyridine nitrogen, the two sulfur atoms of one L2 molecule, and two chloride ions. Analytical studies using solvent extraction separation of Hg(II) from aqueous solutions were performed to determine the Hg(II) extraction capability of ligands L1, L2, and L5.

Structural and EPR studies on single-crystal and polycrystalline samples of copper(II) and cobalt(II) complexes with N2S2-based macrocyclic ligands.

The properties of Cu(II) and Co(II) complexes with oxygen- or nitrogen-containing macrocycles have been extensively studied; however, less attention has been paid to the study of complexes containing sulfur atoms in the first coordination sphere. Herein we present the interaction between these two metal ions and two macrocyclic ligands with N2S2 donor sets. Cu(II) and Co(II) complexes with the pyridine-containing 14-membered macrocycles 3,11-dithia-7,17-diazabicyclo[11.3.1]heptadeca-1(17),13,15-triene (L) and 7-(9-anthracenylmethyl)-3,11-dithia-7,17-diazabicyclo[11.3.1]heptadeca-1(17),13,15-triene (L1) have been synthesized. The X-ray structural analysis of {[Co(ClO4)(H2O)(L)][Co(H2O)2(L)]}(ClO4)3 shows two different metal sites in octahedral coordination. The EPR spectra of powdered samples of this compound are typical of distorted six-coordinated Co(II) ions in a high-spin (S=3/2) configuration, with the ground state being S=1/2 (g1=5.20, g2=3.20, g3=1.95). The EPR spectrum of [Cu(ClO4)(L)](ClO4) was simulated assuming an axial g tensor (g1=g2=2.043, g3=2.145), while that of [Cu(ClO4)(L1)](ClO4) slightly differs from an axial symmetry (g1=2.025, g2=2.060, g3=2.155). These results are compatible with a Cu(II) ion in square-pyramidal coordination with N2S2 as basal ligands. Single-crystal EPR experiment performed on [Cu(ClO4)(L1)](ClO4) allowed determining the eigenvalues of the molecular g tensor associated with the copper site, as well as the two possible orientations for the tensor. On the basis of symmetry arguments, an assignment in which the eigenvectors are nearly along the Cu(II)-ligand bonds is chosen.

Structural and magnetic properties of a complete halide series of Ni(II) complexes with a pyridine-containing 14-membered macrocycle.

The complete halide series of Ni(II) complexes containing the tetradentate macrocyclic ligand 3,11-dithia-7,17-diazabicyclo[11.3.1]heptadeca-1(17),13,15-triene (L), was fully characterized by X-ray diffraction. The fluoro, chloro, and bromo complexes are dinuclear species with formula [{Ni(L)}2(mu-X)2]2+ (X = halide), whereas only mononuclear species with formula [Ni(Y)(solv)(L)]n+ (Y = halide or solvent molecule), were obtained with I. To date, the fluoro derivative is the first nonorganometallic coordination compound containing the Ni(mu-F)2Ni core. The magnetic properties of these halo complexes have been studied. Intramolecular interactions were observed in the three dinuclear complexes, being antiferromagnetic in the fluoro derivative and ferromagnetic in both the chloro and bromo ones. The two iodo derivatives are paramagnetic species, as would be expected for mononuclear octahedral Ni(II) complexes. Density functional theory calculations led us to relate the magnetic behaviors of these compounds to the values of the corresponding Ni-X-Ni(i) angle. The analysis of the singly occupied molecular orbitals gave a clear comprehension of the different magnetic exchange interactions found in these Ni(II) complexes.

Color tuning of a nickel complex with a novel N2S2 pyridine-containing macrocyclic ligand.

The novel pyridine-containing 14-membered macrocycle 3,11-dithia-7,17-diazabicyclo[11.3.1]heptadeca-1(17),13,15-triene (L), which contains an N2S2 donor set, was synthesized, and its protonation behavior was studied by absorption titration with CH3SO3H. The reaction of L with Pd(II) was studied spectroscopically, and the square-planar complex [Pd(L)](BF4) was isolated and characterized. The reactions between L and NiX2 x 6 H2O (X = BF4, ClO4) in ethanol or acetonitrile afforded the octahedral complexes [Ni(CH3CN)(H2O)(L)](X)2 and [Ni(H2O)2(L)](X)2, respectively. The square-planar complexes [Ni(L)](X)2 were obtained by heating these octahedral complexes. Spectrophotometric titrations of [Ni(L)](BF4)2 were performed with neutral and negatively charged ligands. The color of nitromethane solutions of this square-planar complex turns from red to cyan, purple, blue, yellow-green, and pink following addition of halides, acetonitrile, water, pyridine, and 2,2'-bipyridine, respectively. X-ray structural analyses were carried out on the {[Ni(ClO4)(H2O)(L)][Ni(H2O)2(L)]}(ClO4)3, [Ni(CH3CN)(H2O)(L)](ClO4)2, [{Ni(L)}2(mu-Cl)2](ClO4)2, and [{Ni(L)}2(mu-Br)2]Br2 x 2 CH3NO2 complexes.

New fluorescence PET systems based on N2S2 pyridine-anthracene-containing macrocyclic ligands. spectrophotometric, spectrofluorimetric, and metal ion binding studies.

Three new fluorescent devices for protons and metal ions have been synthesized and characterized, and their photophysical properties have been explored; these are the macrocycles 7-(9-anthracenylmethyl)-3,11-dithia-7,17-diazabicyclo[11.3.1]heptadeca-1(17),13,15-triene (L1) and 7-(10-methyl-9-anthracenylmethyl)-3,11-dithia-7,17-diazabicyclo[11.3.1]heptadeca-1(17),13,15-triene (L2) and the bis macrocycle 7,7'-[9,10-anthracenediylbis(methylene)]bis-3,11-dithia-7,17-diazabicyclo[11.3.1]heptadeca-1(17),13,15-triene (L3). All these systems have a pyridil-thioether-containing macrocycles as a binding site and an anthracene moiety as a signaling agent. The coordination properties of these ligands toward Cu(II), Co(II), Ni(II), Zn(II), and Pd(II) have been studied in solution and in the solid state. The addition of these metal ions to dichloromethane solutions of L1, L2, and L3 produce strong changes in the absorption and emission spectra of these ligands. The stoichiometry of the species, formed at 298 K, have been determined from absorption and fluorescence titrations. The Co(II) and Cu(II) complexes of L1 have been studied by EPR spectroscopy. This last complex and its free ligand have also been characterized by X-ray crystallography.