Les calmars géants

8 páginas, 6 figurasPeer reviewe

Nonlinear-Optical Properties of α‑Diiminedithiolatonickel(II) Complexes Enhanced by Electron-Withdrawing Carboxyl Groups

We report the synthesis, characterization, nonlinear-optical (NLO) properties, and density functional theory (DFT) calculations for three nickel diiminedithiolate complexes [Ni­(4,4′-R₂carboxy-bpy)­(L)] [R = methyl, L = 1,2-benzenedithiolate (bdt), <b>1</b>; R = ethyl, L = 5,6-dihydro-1,4-dithine-2,3-dithiolate (dddt), <b>2</b>; R = ethyl, L = 1-(<i>N</i>-methylindol-5-yl)­ethene-1,2-dithiolate (mi-5edt), <b>3</b>]. The crystal structure of <b>1</b> shows a square-planar coordination for the nickel ion and bond distances consistent with a diiminedithiolate description for the complex. For all complexes, the cyclic voltammetry measurements show two reversible reduction processes (−1.353/–1.380 V and −0798/–0.830 V, respectively) and an anodic wave (+0.372/+0.601 V). The UV–vis spectra present a band around 600–700 nm (ε = 4880–6000 dm³ mol^–1 cm^–1) mainly attributed to a charge-transfer highest occupied molecular orbital (HOMO)–lowest unoccupied molecular orbital (LUMO) transition, which shows a large negative solvatochromic shift, characteristic of push–pull complexes, and is responsible for the NLO properties of these molecules. The charge-transfer character of this electronic transition is confirmed by DFT calculations, with the HOMO mainly centered on the dithiolate moiety and the LUMO on the bpy ligand, with important contribution given by the carboxyl groups (≈13%). Small contributions from the nickel­(II) ion are present in both of the frontier orbitals. The carboxyl groups enhance the optical properties of this class of complexes, confirmed by comparison with the corresponding unsubstituted compounds. The second-order NLO properties have been measured by an electric-field-induced second-harmonic-generation technique using a 10^–3 M solution in <i>N</i>,<i>N</i>-dimethylformamide and working with a 1.907 μm incident wavelength, giving for μβ_1.907 (μβ₀) values of −1095 (−581), −2760 (−954), and −1650 (−618) × 10^–48 esu for <b>1</b>–<b>3</b>, respectively. These values are among the highest in the class of square-planar push–pull complexes, similar to those found for dithionedithiolate compounds. Moreover, spectroelectrochemical experiments demonstrate the possibility of using these complexes as redox-switchable NLO chromophores

Nonlinear-Optical Properties of α‑Diiminedithiolatonickel(II) Complexes Enhanced by Electron-Withdrawing Carboxyl Groups

We report the synthesis, characterization, nonlinear-optical (NLO) properties, and density functional theory (DFT) calculations for three nickel diiminedithiolate complexes [Ni­(4,4′-R₂carboxy-bpy)­(L)] [R = methyl, L = 1,2-benzenedithiolate (bdt), <b>1</b>; R = ethyl, L = 5,6-dihydro-1,4-dithine-2,3-dithiolate (dddt), <b>2</b>; R = ethyl, L = 1-(<i>N</i>-methylindol-5-yl)­ethene-1,2-dithiolate (mi-5edt), <b>3</b>]. The crystal structure of <b>1</b> shows a square-planar coordination for the nickel ion and bond distances consistent with a diiminedithiolate description for the complex. For all complexes, the cyclic voltammetry measurements show two reversible reduction processes (−1.353/–1.380 V and −0798/–0.830 V, respectively) and an anodic wave (+0.372/+0.601 V). The UV–vis spectra present a band around 600–700 nm (ε = 4880–6000 dm³ mol^–1 cm^–1) mainly attributed to a charge-transfer highest occupied molecular orbital (HOMO)–lowest unoccupied molecular orbital (LUMO) transition, which shows a large negative solvatochromic shift, characteristic of push–pull complexes, and is responsible for the NLO properties of these molecules. The charge-transfer character of this electronic transition is confirmed by DFT calculations, with the HOMO mainly centered on the dithiolate moiety and the LUMO on the bpy ligand, with important contribution given by the carboxyl groups (≈13%). Small contributions from the nickel­(II) ion are present in both of the frontier orbitals. The carboxyl groups enhance the optical properties of this class of complexes, confirmed by comparison with the corresponding unsubstituted compounds. The second-order NLO properties have been measured by an electric-field-induced second-harmonic-generation technique using a 10^–3 M solution in <i>N</i>,<i>N</i>-dimethylformamide and working with a 1.907 μm incident wavelength, giving for μβ_1.907 (μβ₀) values of −1095 (−581), −2760 (−954), and −1650 (−618) × 10^–48 esu for <b>1</b>–<b>3</b>, respectively. These values are among the highest in the class of square-planar push–pull complexes, similar to those found for dithionedithiolate compounds. Moreover, spectroelectrochemical experiments demonstrate the possibility of using these complexes as redox-switchable NLO chromophores

Heteroleptic Co(III) bisdithiocarbamato-dithione complexes: Synthesis, structure and bonding of [Co(Et₂dtc)₂(R₂pipdt)]BF₄ (R = Me, 1; Ph, 2; pipdt = piperazin-2,3-dithione) complexes

The reaction between the binuclear cobalt complex, [Co2(Et2dtc)5]+, and Me2pipdt and Ph2pipdt ligands has provided almost quantitatively the cobalt tris-chelate heteroleptic complexes [Co(Et2dtc)2(R2pipdt)]BF4 (1 and 2). The molecular structure of 2 shows the metal in a distorted octahedral geometry. The nature of the bonding in these complexes has been elucidated with the support of DFT TD-DFT calculations. Both chelating S,S donors work as weak-field ligands. The comparison of the chemical reactivity for the homoleptic dithiocarbamate complex [Co(Et2dtc)3] and the heteroleptic [Co(Et2dtc)2(Ph2pipdt)]+ derivative shows that the global softness σ is significantly higher in [Co(Et2dtc)2(Ph2pipdt)]+ than in the homoleptic dithiocarbamate complex, due to a reduction of nephelauxetic effect induced by the dithioxamide ligand. The kinetics for the reaction between the reagents in CH2Cl2 has been followed spectrophotometrically as a function of temperature in pseudo-first order conditions with respect to R2pipdt ligands. Kinetic results further support a reaction mechanism involving a one-end reversible dissociation of the [Co2(Et2dtc)5]+ dimer forming a reactive cobalt(III)dithiocarbamato center susceptible to attack by nucleophiles. The effectiveness and versatility of the above reaction is an easy and clean method to provide heteroleptic-dithiocarbamates with a variety of suitable ligands of interest for applicative purposes.</p

Spectral Tuning and Emission Enhancement through Lanthanide Coordination in a Dual Visible–Near-Infrared Emissive Cyanide-Bridged Heterometallic Ru(II)–Er(III) Complex

Owing to their unique luminescent properties and photosensitizing capability, cyanoruthenium(II) complexes with diimine ligands are the subject of intense research striving for routes for tuning their electronic properties and improving their emission quantum yield. In this work, we describe a heterometallic d-f cyanide-bridged Ru(II)–Er(III) assembly obtained by the direct reaction of trivalent erbium salt with the neutral [Ru(bipy)2(CN)2] (bipy = 2,2′-bipyridine) metalloligand. This strategy allows for accommodating inorganic negatively charged anions such as nitrate and oxalate in the coordination sphere of the lanthanide ion. As a result, a dimeric tetranuclear discrete molecular architecture is obtained, where the two constituting monomeric Ru(II)-CN-Er(III) units are bridged by an oxalate anion coordinating two Er(III) ions in a bis-bidentate fashion. Strikingly, this heterometallic compound shows intense dual emission in the visible and near-infrared spectral ranges under single-wavelength excitation in both solution and the crystalline state. The effect of Er(III) coordination through a cyanide bridge is thoroughly discussed, also with the support of DFT calculations, to highlight the factors that induce the observed spectral hypsochromism and, more importantly, the remarkable 10-fold-increased emission quantum yield of the [Ru(bipy)2(CN)2] moiety in the visible range. We show that the described coordination mode induces an energy raise of the emissive triplet metal-to-ligand charge transfer (3MLCT) state and even a more pronounced lifting of the nonemissive Ru(II) triplet metal-centered (3MC) states, suppressing thermal deactivation channels. Furthermore, owing to the reduced number of water molecules and quenching groups surrounding the lanthanide ion in the molecular architecture, relatively intense erbium emission at 1.5 μm telecom wavelength is detected through sensitization from the Ru(II) metalloligand. We suggest that this compound can find applications as an efficient solid-state dual emitter

Synthesis and Physical Properties of K₄[Fe(C₅O₅)₂(H₂O)₂](HC₅O₅)₂·4H₂O (C₅O₅^2– = Croconate): A Rare Example of Ferromagnetic Coupling via H-bonds

The reaction of the croconate dianion (C₅O₅)^2– with a Fe­(III) salt has led, unexpectedly, to the formation of the first example of a discrete Fe­(II)–croconate complex without additional coligands, K₄[Fe­(C₅O₅)₂(H₂O)₂]­(HC₅O₅)₂·4H₂O (<b>1</b>). <b>1</b> crystallizes in the monoclinic <i>P</i>2₁/<i>c</i> space group and presents discrete octahedral Fe­(II) complexes coordinated by two chelating C₅O₅^2– anions in the equatorial plane and two trans axial water molecules. The structure can be viewed as formed by alternating layers of <i>trans</i>-diaquabis­(croconato)­ferrate­(II) complexes and layers containing the monoprotonated croconate anions, HC₅O₅^–, and noncoordinated water molecules. Both kinds of layers are directly connected through a hydrogen bond between an oxygen atom of the coordinated dianion and the protonated oxygen atom of the noncoordinated croconate monoanion. A H-bond network is also formed between the coordinated water molecule and one oxygen atom of the coordinated croconate. This H-bond can be classified as strong–moderate being the O···O bond distance (2.771(2) Å) typical of moderate H-bonds and the O–H···O bond angle (174(3)°) typical of strong ones. This H-bond interaction leads to a quadratic regular layer where each [Fe­(C₅O₅)₂(H₂O)₂]^2– anion is connected to its four neighbors in the plane through four equivalent H-bonds. From the magnetic point of view, these connections lead to an <i>S</i> = 2 quadratic layer. The magnetic properties of <b>1</b> have been reproduced with a 2D square lattice model for <i>S</i> = 2 ions with <i>g</i> = 2.027(2) and <i>J</i> = 4.59(3) cm^–1. This model reproduces quite satisfactorily its magnetic properties but only above the maximum. A better fit is obtained by considering an additional antiferromagnetic weak interlayer coupling constant (<i>j</i>) through a molecular field approximation with <i>g</i> = 2.071(7), <i>J</i> = 2.94(7) cm^–1, and <i>j</i> = −0.045(2) cm^–1 (the Hamiltonian is written as <i>H</i> = –<i>JS_iS_j</i>). Although this second model might still be improved since there is also an extra contribution due to the presence of ZFS in the Fe­(II) ions, it confirms the presence of weak ferromagnetic Fe–Fe interactions through H-bonds in compound <b>1</b> which represents one of the rare examples of ferromagnetic coupling via H-bonds

Role of the Acceptor in Tuning the Properties of Metal [M(II) = Ni, Pd, Pt] Dithiolato/Dithione (Donor/Acceptor) Second-Order Nonlinear Chromophores: Combined Experimental and Theoretical Studies

The mixed-ligand complexes [M­(II)­(Et₂dazdt)­(mnt)] (M = Ni, <b>1</b>; Pd, <b>2</b>; Pt, <b>3</b>) [Et₂dazdt = <i>N</i>,<i>N</i>′-diethyl-perhydrodiazepine-2,3-dithione; mnt = maleonitrile-2,3-dithiolate] have been prepared and fully characterized. X-ray diffractometric studies on <b>1</b>–<b>3</b> (the structure of <b>1</b> was already known) show that the crystals are isostructural (triclinic, <i>P</i>–1), and two independent molecular entities are present in the unit cell. These entities differ in the orientation of the ethyl substituents with respect to the epta-atomic ring. In the C₂S₂MS₂C₂ dithiolene core the four sulfur atoms define a square-planar coordination environment of the metal where the M–S bond distances involving the two ligands are similar, while the C–S bond distances in the C₂S₂ units exhibit a significant difference in Et₂dazdt (dithione) and mnt (dithiolato) ligands. <b>1</b>–<b>3</b> show in the visible region one or two moderately strong absorption peaks, having ligand-to-ligand charge-transfer (CT) character with some contribution of the metal, and show negative solvatochromism and molecular quadratic optical nonlinearity, which was determined by the EFISH (electric-field-induced second-harmonic generation) technique. These complexes are redox active and show two reversible reduction waves and one irreversible oxidation wave. Theoretical calculations based on DFT and TD-DFT calculations on complexes <b>1</b>–<b>3</b> as well as on [Pt­(Bz₂pipdt)­(mnt)] (<b>4</b>) and [Pt­(Bz₂pipdt)­(dmit)] (<b>5</b>) highlight the factors which affect the optical properties of these second-order redox-active NLO chromophores. Actually, the torsion angle of the dithione system (δ₂) inversely correlates either with the oscillator strengths of the main transition of the complexes or with their beta values. The high beta value of <b>5</b> can be attributed both to its lowest torsion angles and to the extent of the π system of its dithiolate ligand, dmit

New BDH-TTP/[M^III(C₅O₅)₃]^3– (M = Fe, Ga) Isostructural Molecular Metals

Two new isostructural molecular metals(BDH-TTP)₆[M^III(C₅O₅)₃]·CH₂Cl₂ (BDH-TTP = 2,5-bis­(1,3-dithiolan-2-ylidene)-1,3,4,6-tetrathiapentalene, where M = Fe (<b>1</b>) and Ga (<b>2</b>))have been prepared and fully characterized. Compound <b>1</b> is a molecular conductor showing paramagnetic behavior, which is due to the presence of isolated [Fe­(C₅O₅)₃]^3– complexes with high-spin <i>S</i> = ⁵/₂ Fe­(III) metal ions. The conductivity originates from the BDH-TTP organic donors arranged in a κ-type molecular packing. At 4 kbar, compound <b>1</b> behaves as a metal down to ∼100 K, showing high conductivity (∼10 S cm^–1) at room temperature. When applying a pressure higher than 7 kbar, the metal–insulator (M-I) transition is suppressed and the compound retains the metallic state down to low temperatures (2 K). For <b>1</b>, ESR signals have been interpreted as being caused by the fine structure splitting of the high-spin (<i>S</i> = ⁵/₂) state of Fe­(III) in the distorted octahedral crystal field from the ligands. At 4 kbar, the isostructural compound <b>2</b> behaves as a metal down to ∼100 K, although it is noteworthy that the M-I transition is not suppressed, even at pressures of 15 kbar. For <b>2</b>, only the signal assigned to delocalized π-electrons has been observed in the ESR measurements

New BDH-TTP/[M^III(C₅O₅)₃]^3– (M = Fe, Ga) Isostructural Molecular Metals

Two new isostructural molecular metals(BDH-TTP)₆[M^III(C₅O₅)₃]·CH₂Cl₂ (BDH-TTP = 2,5-bis­(1,3-dithiolan-2-ylidene)-1,3,4,6-tetrathiapentalene, where M = Fe (<b>1</b>) and Ga (<b>2</b>))have been prepared and fully characterized. Compound <b>1</b> is a molecular conductor showing paramagnetic behavior, which is due to the presence of isolated [Fe­(C₅O₅)₃]^3– complexes with high-spin <i>S</i> = ⁵/₂ Fe­(III) metal ions. The conductivity originates from the BDH-TTP organic donors arranged in a κ-type molecular packing. At 4 kbar, compound <b>1</b> behaves as a metal down to ∼100 K, showing high conductivity (∼10 S cm^–1) at room temperature. When applying a pressure higher than 7 kbar, the metal–insulator (M-I) transition is suppressed and the compound retains the metallic state down to low temperatures (2 K). For <b>1</b>, ESR signals have been interpreted as being caused by the fine structure splitting of the high-spin (<i>S</i> = ⁵/₂) state of Fe­(III) in the distorted octahedral crystal field from the ligands. At 4 kbar, the isostructural compound <b>2</b> behaves as a metal down to ∼100 K, although it is noteworthy that the M-I transition is not suppressed, even at pressures of 15 kbar. For <b>2</b>, only the signal assigned to delocalized π-electrons has been observed in the ESR measurements