Flavia Artizzu

Near infrared light emission quenching in organolanthanide complexes

We investigate the quenching of the near infrared light emission in Er3+ complexes induced by the resonant dipolar interaction between the rare-earth ion and high frequency vibrations of the organic ligand. The nonradiative decay rate of the lanthanide ion is discussed in terms of a continuous medium approximation, which depends only on a few, easily accessible spectroscopic and structural data. The model accounts well for the available experimental results in Er3+ complexes, and predicts an ∼100% light emission quantum yield in fully halogenated systems.

A Family of Layered Chiral Porous Magnets Exhibiting Tunable Ordering Temperatures

A simple change of the substituents in the bridging ligand allows tuning of the ordering temperatures, Tc, in the new family of layered chiral magnets A[M(II)M(III)(X2An)3]·G (A = [(H3O)(phz)3](+) (phz = phenazine) or NBu4(+); X2An(2-) = C6O4X2(2-) = 2,5-dihydroxy-1,4-benzoquinone derivative dianion, with M(III) = Cr, Fe; M(II) = Mn, Fe, Co, etc.; X = Cl, Br, I, H; G = water or acetone). Depending on the nature of X, an increase in Tc from ca. 5.5 to 6.3, 8.2, and 11.0 K (for X = Cl, Br, I, and H, respectively) is observed in the MnCr derivative. Furthermore, the presence of the chiral cation [(H3O)(phz)3](+), formed by the association of a hydronium ion with three phenazine molecules, leads to a chiral structure where the Δ-[(H3O)(phz)3](+) cations are always located below the Δ-[Cr(Cl2An)3](3-) centers, leading to a very unusual localization of both kinds of metals (Cr and Mn) and to an eclipsed disposition of the layers. This eclipsed disposition generates hexagonal channels with a void volume of ca. 20% where guest molecules (acetone and water) can be reversibly absorbed. Here we present the structural and magnetic characterization of this new family of anilato-based molecular magnets.

Mixed-ligand Pt(II) dithione-dithiolato complexes: influence of the dicyanobenzodithiolato ligand on the second-order NLO properties

The mixed-ligand dithiolene complex [Pt(Bz(2)pipdt)(dcbdt)] (1) bearing the two ligands Bz(2)pipdt = 1,4-dibenzyl-piperazine-3,2-dithione and dcbdt = dicyanobenzodithiolato, has been synthesized, characterized and studied to evaluate its second-order optical nonlinearity. The dithione/dithiolato character of the two ligands gives rise to an asymmetric distribution of the charge in the molecule. This is reflected by structural data showing that in the C(2)S(2)PtS(2)C(2) dithiolene core the four sulfur atoms define a square-planar coordination environment of the metal where the Pt-S bond distances involving the two ligands are similar, while the C-S bond distances in the C(2)S(2) units exhibit a significant difference in Bz(2)pipdt (dithione) and dcbdt (dithiolato). 1 shows a moderately strong absorption peak in the visible region, which can be related to a HOMO-LUMO transition, where the dcbdt ligand (dithiolato) contributes mostly to the HOMO, and the Bz(2)pipdt one (dithione) mostly to the LUMO. Thus this transition has ligand-to-ligand charge transfer (CT) character with some contribution of the metal and undergoes negative solvatochromism and molecular quadratic optical nonlinearity (μβ(0) = -1296 × 10(-48) esu), which was determined by the EFISH (electric-field-induced second-harmonic generation) technique and compared with the values of similar complexes on varying the dithiolato ligand (mnt = maleonitriledithiolato, dmit = 2-thioxo-1,3-dithiole-4,5-dithiolato). Theoretical calculations help to elucidate the role of the dithiolato ligands in affecting the molecular quadratic optical nonlinearity of these complexes.

Innocence and noninnocence of the ligands in bis(pyrazine-2,3-dithiolate and -diselonate) d8-metal complexes. A theoretical and experimental study for the Cu(III), Au(III) and Ni(II) cases

In this paper we present an experimental and theoretical study to investigate the electronic structures of [ML₂]⁻ (M(III) = Cu, L = pdt and pds, pyrazine-2,3-dithiolate and -diselonate; M(III) = Au, L = pds) with the aim of elucidating the nature of the bonding and to establish the innocent-noninnocent character of the ligand in these complexes. Calculations based on DFT methods have been performed to obtain geometry optimizations, harmonic frequencies, IR intensities and Raman scattering activities. The experimental vibrational spectra are accurately reproduced by the calculations, which show that CC, CN, and CX (X = S, Se) vibrations are extensively mixed with other modes, and thus unsuitable to work as vibrational markers. Geometry optimization performed at the DFT level provides geometrical parameters in good agreement with the available structural data. The energetic sequence and nature of the redox-active molecular orbitals help to elucidate the observed electrochemical behaviour. Accordingly, the quasi-reversible redox couple for the reduction processes exhibited by [ML₂]²⁻ (L = pdt and pds), that appears at negative values and depends both on the ligand and on the metal, is related to the LUMO which is a σ antibonding combination of the ligand orbitals (sulfur or selenium atoms) and the 3d(xy) (Cu) and 5d(xy) (Au) metal orbitals. The HOMO is a π-orbital with a b(2g) symmetry, predominantly ligand in character with a small contribution of the nd(xz) atomic orbitals in antibonding combination with chalcogen atom orbitals. The low energy of the metal d-orbitals compared to the ligand orbitals, due to the high effective nuclear charge of the metals, explains their small participation to this orbital. Thus in [ML₂]⁻ the metals approach the oxidation state 3+ and the ligand a dichalcogenolate description and thus a prevalent innocent character. However the same ligand shows a noninnocent character in complexes with a different d⁸ metal such as Ni(II) whose d-orbitals lie at higher energies and mix at a higher extent with the ligand orbitals in the HOMO.

Structural Diversity and Physical Properties of Paramagnetic Molecular Conductors Based on Bis(ethylenedithio)tetrathiafulvalene (BEDT-TTF) and the Tris(chloranilato)ferrate(III) Complex

Electrocrystallization of bis(ethylenedithio)tetrathiafulvalene (BEDT-TTF) in the presence of the tris(chloranilato)ferrate(III) [Fe(Cl2An)3](3-) paramagnetic chiral anion in different stoichiometric ratios and solvent mixtures afforded three different hybrid systems formulated as [BEDT-TTF]3[Fe(Cl2An)3]·3CH2Cl2·H2O (1), δ-[BEDT-TTF]5[Fe(Cl2An)3]·4H2O (2), and α‴-[BEDT-TTF]18[Fe(Cl2An)3]3·3CH2Cl2·6H2O (3). Compound 1 presents an unusual structure without the typical alternating organic and inorganic layers, whereas compounds 2 and 3 show a segregated organic-inorganic crystal structure where layers formed by Λ and Δ enantiomers of the paramagnetic complex, together with dicationic BEDT-TTF dimers, alternate with layers where the donor molecules are arranged in the δ (2) and α‴ (3) packing motifs. Compound 1 behaves as a semiconductor with a much lower conductivity due to the not-layered structure and strong dimerization between the fully oxidized donors, whereas 2 and 3 show semiconducting behaviors with high room-temperature conductivities of ca. 2 S cm(-1) and 8 S cm(-1), respectively. The magnetic properties are dominated by the paramagnetic S = 5/2 [Fe(Cl2An)3](3-) anions whose high-spin character is confirmed by electron paramagnetic resonance and magnetic susceptibility measurements. The correlation between crystal structure and conductivity behavior was studied by means of tight-binding band structure calculations, which support the observed conducting properties.

Halogen-bonding in a new family of tris(haloanilato)metallate(III) magnetic molecular building blocks

Here we report on new tris(haloanilato)metallate(III) complexes with general formula [A]3[M(X2An)3] (A = (n-Bu)4N(+), (Ph)4P(+); M = Cr(III), Fe(III); X2An = 3,6-dihalo derivatives of 2,5-dihydroxybenzoquinone (H4C6O4), chloranilate (Cl2An(2-)), bromanilate (Br2An(2-)) and iodanilate (I2An(2-))), obtained by a general synthetic strategy, and their full characterization. The crystal structures of these Fe(III) and Cr(III) haloanilate complexes consist of anions formed by homoleptic complexes formulated as [M(X2An)3](3-) and (Et)3NH(+), (n-Bu)4N(+), or (Ph4)P(+) cations. All complexes exhibit octahedral coordination geometry with metal ions surrounded by six oxygen atoms from three chelate ligands. These complexes are chiral according to the metal coordination of three bidentate ligands, and both Λ and Δ enantiomers are present in their crystal lattice. The packing of [(n-Bu)4N]3[Cr(I2An)3] (5a) shows that the complexes form supramolecular dimers that are held together by two symmetry related I···O interactions (3.092(8) Å), considerably shorter than the sum of iodine and oxygen van der Waals radii (3.50 Å). The I···O interaction can be regarded as a halogen bond (XB), where the iodine behaves as the XB donor and the oxygen atom as the XB acceptor. This is in agreement with the properties of the electrostatic potential for [Cr(I2An)3](3-) that predicts a negative charge accumulation on the peripheral oxygen atoms and a positive charge accumulation on the iodine. The magnetic behaviour of all complexes, except 5a, may be explained by considering a set of paramagnetic non-interacting Fe(III) or Cr(III) ions, taking into account the zero-field splitting effect. The presence of strong XB interactions in 5a are able, instead, to promote antiferromagnetic interactions among paramagnetic centers at low temperature, as shown by the fit with the Curie-Weiss law, in agreement with the formation of halogen-bonded supramolecular dimers.

Synthesis and physical properties of K 4[Fe(C5O5)2(H 2O)2](HC5O5) 24H2O (C5O5)2- = croconate): A rare example of ferromagnetic coupling via H-bonds

The reaction of the croconate dianion (C(5)O(5))(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, K(4)[Fe(C(5)O(5))(2)(H(2)O)(2)](HC(5)O(5))(2)·4H(2)O (1). 1 crystallizes in the monoclinic P2(1)/c space group and presents discrete octahedral Fe(II) complexes coordinated by two chelating C(5)O(5)(2-) anions in the equatorial plane and two trans axial water molecules. The structure can be viewed as formed by alternating layers of trans-diaquabis(croconato)ferrate(II) complexes and layers containing the monoprotonated croconate anions, HC(5)O(5)(-), 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(C(5)O(5))(2)(H(2)O)(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 S = 2 quadratic layer. The magnetic properties of 1 have been reproduced with a 2D square lattice model for S = 2 ions with g = 2.027(2) and J = 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 (j) through a molecular field approximation with g = 2.071(7), J = 2.94(7) cm(-1), and j = -0.045(2) cm(-1) (the Hamiltonian is written as H = -JS(i)S(j)). 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 1 which represents one of the rare examples of ferromagnetic coupling via H-bonds.

Chameleon behaviour of iodine in recovering noble-metals from WEEE: towards sustainability and “zero” waste

An effective and sustainable method for the selective leaching of metals from the non-ferrous metal fraction of Waste Electric and Electronic Equipment (WEEE) is described here. This method consists of a sequence of steps which involve the selective leaching of the different metals from the shredded sample by using environmentally friendly lixiviants in water. In particular: (1) a refluxing citric acid (3 M) solution which dissolves Sn, Zn, Pb, Ni, and other base metals; (2) NH3 in combination with an IO3−/I− mixture which allows one to oxidize Cu and Ag, and to separate them by selective AgI precipitation; (3) an I−/I2 mixture (5.3 : 1 molar ratio) which is capable of leaching quantitatively Au metal from the solid residue. Each step is followed by a further treatment for: (i) high-rate metal and reagent recovery, in the case of NMs; (ii) inertization, in the case of heavy metals. The “chameleon” behaviour of iodine, which shows versatile redox/complexing/precipitating capabilities, allows one to achieve, on the one hand, a selective noble-metal (NM) leaching and, on the other hand, a simple and effective reagent and metal recovery. The comparison of the above described method with a similarly effective one previously patented by some of the same authors, allows one to point out that a significant improvement in sustainability is achieved in terms of lixiviant employment, which is of low cost, easily available and recyclable, and able to work in water solutions, while maintaining or improving its environmentally friendly character.

Controlling Nd-to-Yb energy transfer through a molecular approach

Nd-to-Yb energy transfer (ET) at the molecular level has been investigated in the novel heterobimetallic NdYb2Q9 (Q = 8-quinolinolate) complex where the Q ligand works as a high cross-sectional optical antenna. The remarkable ion size-driven templating effect of Nd3+ allows for good control of molecular speciation through a simple one-pot synthetic procedure. Short intermetallic distances and enhanced donor (Nd)/acceptor (Yb) spectral overlap in the molecular assembly strongly favor Nd-to-Yb energy transfer, which reaches nearly unitary efficiency, while detrimental processes such as concentration quenching and energy back transfer are ruled out. The devised approach allows us to obtain a controlled assembly of heterolanthanide compounds whose features hold potential for the development of highly performing Yb3+-based optical materials.

Tailoring functionality through synthetic strategy in heterolanthanide assemblies

An overview of the different strategies proposed for the preparation of heterolanthanide assemblies suitable to work as (multi-) functional materials, highlighting their structure/property relationship, is provided. Three classes of compounds are selected: multi-dimensional coordination frameworks, polynuclear discrete molecules and flexible large molecules formed by two or more coordinating units connected by a linker. Synthetic approaches and potential functionalities are discussed on the basis of lanthanide ion discrimination and the structural arrangement of the heterometallic assembly.