Tailoring the Use of 8-Hydroxyquinolines for the Facile Separation of Iron, Dysprosium and Neodymium.

Permanent magnets (PMs) containing rare earth elements (REEs) can generate energy in a sustainable manner. With an anticipated tenfold increase in REEs demand by 2050, one of the crucial strategies to meet the demand is developing of efficient recycling methods. NdFeB PMs are the most widely employed, however, the similar chemical properties of Nd (20-30 % wt.) and Dy (0-10 % wt.) make their recycling challenging, but possible using appropriate ligands. In this work, we investigated commercially available 8-hydroxyquinolines (HQs) as potential Fe/Nd/Dy complexing agents enabling metal separation by selective precipitation playing on specific structure/property (solubility) relationship. Specifically, test ethanolic solutions of nitrate salts, prepared to mimic the main components of a PM leachate, were treated with functionalized HQs. We demonstrated that Fe3+ can be separated as insoluble [Fe(QCl,I)3] from soluble [REE(QCl,I)4]- complexes (QCl,I -: 5-Cl-7-I-8-hydoxyquinolinate). Following that, QCl - (5-Cl-8-hydroxyquinolinate) formed insoluble [Nd3(QCl)9] and soluble (Bu4N)[Dy(QCl)4]. The process ultimately gave a solution phase containing Dy with only traces of Nd. In a preliminary attempt to assess the potentiality of a low environmental impact process, REEs were recovered as oxalates, while the ligands as well as Bu4N+ ions, were regenerated and internally reused, thus contributing to the sustainability of a possible metal recovery process.

Glassy Powder Derived from Waste Printed Circuit Boards for Methylene Blue Adsorption.

Electronic waste (e-waste) is one of the fastest-growing waste streams in the world and Europe is classified as the first producer in terms of per capita amount. To reduce the environmental impact of e-waste, it is important to recycle it. This work shows the possibility of reusing glassy substrates, derived from the MW-assisted acidic leaching of Waste Printed Circuit Boards (WPCBs), as an adsorbent material. The results revealed an excellent adsorption capability against methylene blue (MB; aqueous solutions in the concentration range 10-5 M-2 × 10-5 M, at pH = 7.5). Comparisons were performed with reference samples such as activated carbons (ACs), the adsorbent mostly used at the industrial level; untreated PCB samples; and ground glass slides. The obtained results show that MW-treated WPCB powder outperformed both ground glass and ground untreated PCBs in MB adsorption, almost matching AC adsorption. The use of this new adsorbent obtained through the valorization of e-waste offers advantages not only in terms of cost but also in terms of environmental sustainability.

Near-IR Absorbers Based on Pt(II)-Dithiolene Donor-Acceptor Charge-Transfer (CT) Systems: A Structural Analysis to Highlight DA Interactions.

The packing interactions of a series of electron donor (D) and electron acceptor (A) charge transfer (CT) near-IR absorbers based on platinum-dithiolene complexes are reinvestigated here as a case study also by using the Hirshfeld surface analysis. This analysis on systems, which exhibit the 1:1, 2:1 and 2:2 columnar stacking patterns between D and A, allows us to point out that several interactions of atoms and fragments are involved in the stacking interactions but also that only a limited fraction of these interactions, limited to the 1:1 D/A columnar stacking case, can be relatable to the absorption features of this class of compounds.

Anion-Induced Structural Diversity and Optical Chromism in a Series of Cyano-Bridged Heterometallic 3d-4f Coordination Polymers.

The self-assembly reaction of the neutral dicyano-bis(1,10-phenanthroline) iron(II) complex with lanthanide ions (Ln = Eu(III), Gd(III), Er(III)) provided two different classes of heterometallic cyano-bridged 3d-4f architectures depending on the nature of the counteranion, irrespective of the size of the 4f metal ion. Tetranuclear oligomers with a squared Fe2Ln2 core were isolated when using nitrate salts, whereas unusual 1D polymeric chains were obtained when resorting to triflate salts under the same synthetic conditions. It is shown that the different structural motifs have a remarkable impact on the thermal stability and the optical properties of the compounds, which display a notable optical ipsochromism of the parent Fe(II) complex upon coordination with the Ln ion. This effect is significantly more pronounced in the polymeric chain than in the Fe2Ln2 oligomer both in solution and in the solid state. Structural evidence suggests that this behavior is likely related to the geometry of the CN-Ln bridge. On the other hand, more extended π-stacking interactions in the oligomer give rise to a broad charge-transfer absorption (600-1500 nm), making this compound promising as NIR absorber. Density Functional Theory calculations and electrochemical studies demonstrate that the observed negative chromism originates from the stabilization of a mixed metal/cyanide character HOMO with respect to a phenanthroline-centered LUMO.

Halogen-Bonding-Mediated Radical Reactions: The Unexpected Behavior of Piperazine-Based Dithiooxamide Ligands in the Presence of Diiodine.

N,N'-Dialkylpiperazine-2,3-dithiones (R2pipdt) were recognized as a class of hexa-atomic cyclic dithiooxamide ligands with peculiar charge-transfer donor properties toward soft electron-acceptors such as noble metal cations and diiodine. The latter interaction is nowadays better described as halogen bonding. In the reaction with diiodine, R2pipdt unexpectedly provides the corresponding triiodide salts, differently from the other dithiooxamides, which instead typically achieve ligand·nI2 halogen-bonded adducts. In this paper, we report a combined experimental and theoretical study that allows elucidation of the nature of the cited products and the reasons behind the unpredictable behavior of these ligands. Specifically, low-temperature single-crystal X-ray diffraction measurements on a series of synthetically obtained R2pipdt (R = Me, iPr, Bz)/I3 salts, complemented by neutron diffraction experiments, were able to experimentally highlight the formation of [R2pipdtH]+ cations with a -S-H bond on the dithionic moiety. Differently, with R = Ph, a benzothiazolylium cation, resulting from an intramolecular condensation reaction of the ligand, is obtained. Based on density functional theory (DFT) calculations, a reasonable reaction mechanism where diiodine plays the fundamental role of promoting a halogen-bonding-mediated radical reaction has been proposed. In addition, the comparison of combined experimental and computational results with the corresponding reactions of N,N'-dialkylperhydrodiazepine-2,3-dithione (R2dazdt, a hepta-atomic cyclic dithiooxamide), which provide neutral halogen-bonded adducts, pointed out that the difference in the torsion angle of the free ligands represents the structural key factor in determining the different reactivities of the two systems.

Homogeneous Gold Catalysis Using Complexes Recovered from Waste Electronic Equipment.

Despite the greater awareness of elemental sustainability and the benefits of the circular economy concept, much waste electrical and electronic equipment (WEEE) is still destined for landfill. Effective methods for valorizing this waste within our society are therefore imperative. In this contribution, two gold(III) complexes obtained as recovery products from WEEE and their anion metathesis products were investigated as homogenous catalysts. These four recovery products were successfully applied as catalysts for the cyclization of propargylic amides and the condensation of acetylacetone with o-iodoaniline. Impressive activity was also observed in the gold-catalyzed reaction between electron-rich arenes (2-methylfuran, 1,3-dimethoxybenzene, and azulene) and α,ß-unsaturated carbonyl compounds (methyl vinyl ketone and cyclohexenone). These recovered compounds were also shown to be effective catalysts for the oxidative cross-coupling reaction of aryl silanes and arenes. When employed as Lewis acid catalysts for carbonyl-containing substrates, the WEEE-derived gold complexes could also be recovered at the end of the reaction and reused without loss in catalytic activity, enhancing still further the sustainability of the process. This is the first direct application in homogeneous catalysis of gold recovery products sourced from e-waste.

Semirigid Ligands Enhance Different Coordination Behavior of Nd and Dy Relevant to Their Separation and Recovery in a Non-aqueous Environment.

Rare-earth elements are widely used in high-end technologies, the production of permanent magnets (PMs) being one of the sectors with the greatest current demand and likely greater future demand. The combination of Nd and Dy in NdFeB PMs enhances their magnetic properties but makes their recycling more challenging. Due to the similar chemical properties of Nd and Dy, their separation is expensive and currently limited to the small scale. It is therefore crucially important to devise efficient and selective methods that can recover and then reuse those critical metals. To address these issues, a series of heptadentate Trensal-based ligands were used for the complexation of Dy3+ and Nd3+ ions, with the goal of indicating the role of coordination and solubility equilibria in the selective precipitation of Ln3+-metal complexes from multimetal non-water solutions. Specifically, for a 1:1 Nd/Dy mixture, a selective and fast precipitation of the Dy complex occurred in acetone with the Trensalp-OMe ligand at room temperature, with a concomitant enrichment of Nd in the solution phase. In acetone, complexes of Nd and Dy with Trensalp-OMe were characterized by very similar formation constants of 7.0(2) and 7.3(2), respectively. From the structural analysis of an array of Dy and Nd complexes with TrensalR ligands, we showed that Dy invariably provided complexes with coordination number (cn) of 7, whereas the larger Nd experienced an expansion of the coordination sphere by recruiting additional solvent molecules and giving a cn of >7. The significant structural differences have been identified as the main premises upon which a suitable separation strategy can be devised with these kind of ligands, as well as other preorganized polydentate ligands that can exploit the small differences in Ln3+ coordination requirements.

A cascade biorefinery for grape marc: Recovery of materials and energy through thermochemical and biochemical processes.

The agro-industrial sector makes a high contribution to greenhouse gas emissions; therefore, proper waste management is crucial to reduce the carbon footprint of the food chain. Hydrothermal carbonization (HTC) is a promising and flexible thermochemical process for converting organic materials into energy and added-value products that can be used in different applications. In this work, grape marc residues before and after an extraction process for recovering polyphenols were hydrothermally treated at 220 °C for 1 h. The resulting hydrochar and process water were investigated to test an innovative cascade approach aimed at a multiple product and energy recovery based on the integration of HTC with anaerobic digestion. The results show that this biorefinery approach applied to grape marc could allow to diversify and integrate its potential valorisation options. The produced hydrochars possess an increased fixed carbon content compared to the feedstock (up to +70 %) and, therefore, can be used in soil, immobilizing carbon in a stable form and partially replacing peat in growing media (up to 5 % in case of hydrochar from grape marc after extraction), saving the consumption of this natural substrate. In addition, energy can be recovered from both hydrochar by combustion and from process water through anaerobic digestion to produce biogas. Hydrochars show good properties as solid fuel similar to lignite, with an energy content of around 27 MJ kg-1 (+30 % compared to the feedstock). The anaerobic digestion of the process water allowed obtaining up to 137 mL of biomethane per gram of fed COD. Finally, while HTC process waters are suitable for biological treatment, attention must be paid to the presence of inhibiting compounds that induce acute toxic effects in aerobic conditions. The proposed approach is consistent with the principles of circular economy and could increase the overall sustainability and resilience of the agro-industrial sector.

From Waste to Green Applications: The Use of Recovered Gold and Palladium in Catalysis.

The direct use in catalysis of precious metal recovery products from industrial and consumer waste is a very promising recent area of investigation. It represents a more sustainable, environmentally benign, and profitable way of managing the low abundance of precious metals, as well as encouraging new ways of exploiting their catalytic properties. This review demonstrates the feasibility and sustainability of this innovative approach, inspired by circular economy models, and aims to stimulate further research and industrial processes based on the valorisation of secondary resources of these raw materials. The overview of the use of recovered gold and palladium in catalytic processes will be complemented by critical appraisal of the recovery and reuse approaches that have been proposed.

Characterization and Structural Insights of the Reaction Products by Direct Leaching of the Noble Metals Au, Pd and Cu with N,N'-Dimethyl-piperazine-2,3-dithione/I2 Mixtures.

In the context of new efficient and safe leaching agents for noble metals, this paper describes the capability of the Me2pipdt/I2 mixture (where Me2pipdt = N,N'-dimethyl-piperazine-2,3-dithione) in organic solutions to quantitatively dissolve Au, Pd, and Cu metal powders in mild conditions (room temperature and pressure) and short times (within 1 h in the reported conditions). A focus on the structural insights of the obtained coordination compounds is shown, namely [AuI2(Me2pipdt)]I3 (1), [Pd(Me2pipdt)2]I2 (2a) and [Cu(Me2pipdt)2]I3 (3), where the metals are found, respectively, in 3+, 2+ and 1+ oxidation states, and of [Cu(Me2pipdt)2]BF4 (4) and [Cu(Me2dazdt)2]I3 (5) (Me2dazdt = N,N'-dimethyl-perhydrodizepine-2,3-dithione) compared with 3. Au(III) and Pd(II) (d8 configuration) form square-planar complexes, whereas Cu(I) (d10) forms tetrahedral complexes. Density functional theory calculations performed on the cationic species of 1-5 help to highlight the nature of the bonding in the different complexes. Finally, the valorization of the noble metals-rich leachates is assessed. Specifically, gold metal is quantitatively recovered from the solution besides the ligands, showing the potential of these systems to promote metal recycling processes.

Insight into the Properties of Heteroleptic Metal Dithiolenes: Multistimuli Responsive Luminescence, Chromism, and Nonlinear Optics.

A comprehensive investigation of the functional properties of heteroleptic donor-M-acceptor dithiolene complexes Bu4N[MII(L1)(L2)] is presented (M = Pd, Pt). The acceptor L1 consists of the chiral (R)-(+)α-methylbenzyldithiooxamidate ((R)-α-MBAdto), the donor L2 is 2-thioxo-1,3-dithiole-4,5-dithiolato (dmit) in 1 (Pd) and 2 (Pt), 1,2-dicarbomethoxyethylenedithiolate (ddmet) in 3 (Pd) and 4 (Pt), or [4',5':5,6][1,4]dithiino[2,3-b]quinoxaline-1',3'-dithiolato (quinoxdt) in 5 (Pd) and 6 (Pt). L1 is capable of undergoing proton exchange and promoting crystal formation in noncentrosymmetric space groups. L2 has different molecular structures while it maintains similar electron-donating capabilities. Thanks to the synergy of the ligands, 1-6 behave as H+ and Ag+ switchable linear chromophores. Moreover, the compounds exhibit a H+-switchable second-order NLO response in solution, which is maintained in the bulk for 1, 3, and 4 when they are embedded into a PMMA poled matrix. 5 and 6 show unique anti-Kasha H+ and Ag+ tunable colored emission originating from the quinoxdt ligand. A correlation between the electronic structure and properties is shown through density functional theory (DFT) and time-dependent DFT calculations.

Uncommon Optical Properties and Silver-Responsive Turn-Off/On Luminescence in a PtII Heteroleptic Dithiolene Complex.

Complex [Pt(iPr2 pipdt)(Quinoxdt)] (iPr2 pipdt=1,4-diisopropyl-piperazine-2,3-dithione; Quinoxdt=[1,4]dithiino[2,3-b]quinoxaline-2,3-dithiolate) exhibits a remarkable green emission at 570 nm (room temperature), which is above the lowest excited state. The complex is characterized by negative solvatochromism as well as a high second-order polarizability. Addition of AgI ions induces 1) hypsochromic shift of the lowest frequencies and 2) reversible quenching of luminescence. The corresponding Ni and Pd complexes have also been prepared and investigated to assist interpretation of optical properties within the triad. Computational studies based on DFT and time-dependent DFT highlight the electronic properties of [Pt(iPr2 pipdt)(Quinoxdt)]. The preferential site of interaction between the Pt complex and incoming AgI is evidenced by the shape of the Fukui functions, pointing to the thiolic sulfur and platinum atoms as the most reactive sites towards a soft cation. Calculated optical properties are in agreement with experimental findings. This study sheds light on the structure-property relationship for this class of compounds.

Optically Multiresponsive Heteroleptic Platinum Dithiolene Complex with Proton-Switchable Properties.

Both linear- and nonlinear-optical properties of Bu4N[Pt(L1)(L2)] (1; L1 = [4',5':5,6][1,4]dithiino[2,3-b]quinoxaline-1',3'dithiolato; L2 = (R)-α-MBAdto dithiooxamidate, where (R)-α-MBA = (R)-(+)-α-methylbenzyl) upon HCl addition at room temperature change dramatically: the color turns from deep blue to green; the luminescence switches from deep red to green; the nonlinear-optical response (first hyperpolarizability) increases by a factor of 12. Thus, 1 behaves as a unique multiresponsive optical switch whose properties can be followed by the naked eye.

A Platinum-Dithiolene Monoanionic Salt Exhibiting Multiproperties, Including Room-Temperature Proton-Dependent Solution Luminescence.

The platinum salt C[PtL2], where C = [(R)-Ph(Me)HC*-NMe3](+) and [PtL2](-) = radical monoanion based on [4', 5': 5, 6][1, 4]dithiino[2,3-b]quinoxaline-1',3'dithiolato, shows a variety of properties both in solution and in the solid state thanks to the electronic and/or structural features of the ligand. The complex crystallizes in the chiral space group P1 due to the presence of the enantiopure cation (R)-Ph(Me)HC*-NMe3(+), and it shows paramagnetic behavior relatable to the [PtL2](-) radical monoanion. This anionic complex is redox active and shows a strong near-infrared absorbance peak at 1085 nm tunable with the oxidation state of the complex. This complex exhibits a proton-dependent emission at 572 nm in solution at room temperature. The excitation band corresponds to the HOMO-1 (π-orbitals of the S2C2S2 system) → LUMO (π-orbitals of the quinoxaline and benzene-like moieties) transition suggesting that emission is mainly ligand centered in character. The luminescent properties are highly unusual, since the emission falls well above the energy of the lowest energy absorption (anti-Kasha behavior). Joint experimental and density functional theory (DFT) and time-dependent DFT studies are discussed to provide a satisfactory structure/property relationship.

Switching-on luminescence in anilate-based molecular materials.

A simple change of one chloro substituent on the chloranilate ligand with a cyano group dramatically affects the electronic properties of the anilate moiety inducing unprecedented luminescence properties in the class of anilate-based ligands and their metal complexes. Here we report on the optimized synthesis and full characterization, including photoluminescence, of the chlorocyananilate ligand (ClCNAn(2-)) (dianion of the 3-chloro-6-cyano-2,5-dihydroxybenzoquinone, H2ClCNC6O4), a unique example of a heterosubstituted anilate ligand whose electronic properties, optical properties and coordination chemistry have never been investigated to date, even though it has been known since 1966. The synthesis and full characterization of its tris-chelated metal complexes with Cr(iii), Fe(iii), and Al(iii) metal ions are also described herein. These complexes, formulated as [A]3[M(III)(ClCNAn)3] (A = (n-Bu)4N(+) or Ph4P(+); M(III) = Cr (), Fe (), Al ()), are isostructural. While and are potential molecular building blocks for the preparation of molecule-based magnets or paramagnetic conducting organic-inorganic hybrid systems, , instead, where the coordinated Al(iii) metal ions retain the luminescence of the ligand, represents a unique building block to achieve heterobimetallic assemblies showing emissive properties under visible light irradiation.

Ln3Q9 as a molecular framework for ion-size-driven assembly of heterolanthanide (Nd, Er, Yb) multiple near-infrared emitters.

A unique example of discrete molecular entity Nd(y)Er(x)Yb(3-(x+y))Q9 (1) (Q = quinolinolato) containing three different lanthanides simultaneously emitting in three different spectral regions in the NIR, ranging from 900 to 1600 nm, has been synthesized and fully chararacterized. A simple molecular strategy based on tuning metal composition in the Ln3Q9 framework, which contains inequivalent central and terminal coordination sites, has allowed a satisfactory ion-size-driven control of molecular speciation close to 90%. In 1 the central position of the larger Nd ion is well distinguished from the terminal ones of the smaller Yb(3+) and Er(3+), which are almost "vicariants" as found in the heterobimetallic Er(x)Yb(3-x)Q9 (2). The Ln3Q9 molecular architecture, which allows communication between the ions, has proved to afford multiple NIR emission in 1 and 2, and is promising to develop a variety of multifunctional materials through the variation of the Ln composition.

Ultrafast electronic and vibrational relaxations in mixed-ligand dithione-dithiolato Ni, Pd, and Pt complexes.

Ultrafast excited-state dynamics of planar Pt, Pd, and Ni dithione-dithiolato complexes were investigated by transient absorption spectroscopy on the femtosecond-picosecond timescale. All studied complexes show a common photobehaviour, although individual kinetics parameters and quantum yields vary with the metal, the dithione ligand and, namely the solvent (DMF, MeCN). Laser pulse irradiation at 800 nm populates the lowest singlet excited state of a dithiolato → dithione charge transfer character, (1)LL'CT. The optically excited state undergoes a solvation-driven sub-picosecond electronic relaxation that enhances the dithione/dithiolato charge separation. The (1)LL'CT state decays with a 1.9-4.5 ps lifetime by two simultaneous pathways: intersystem crossing (ISC) to the lowest triplet state (3)LL'CT and non-radiative decay to the ground state. ISC occurs on a ∼6 ps timescale, virtually independent of the metal, whereas the rate of the non-radiative decay to the ground state decreases on going from Ni (2 ps) to Pd (3 ps) and Pt (∼10 ps). (3)LL'CT is initially formed as a vibrationally excited state. Its equilibration (cooling) takes place on a picosecond timescale and is accompanied by a competitive decay to the ground state. Equilibrated (3)LL'CT is populated with a quantum yield of less than 50%, depending on the metal: Pt > Pd > Ni. (3)LL'CT is long-lived for Pt and Pd (≫500 ps) and short-lived for Ni (∼15 ps). Some of the investigated complexes also exhibit spectral changes due to vibrational cooling of the singlet (2-3 ps, depending on the solvent). Rotational diffusion occurs with lifetimes in the 120-200 ps range. Changing the dithione (Bz2pipdt/(i)Pr2pipdt) as well as dithiolate/diselenolate (dmit/dsit) ligands has only small effects on the photobehavior. It is proposed that the investigated dithione-dithiolato complexes could act as photooxidants (*E(o) ≈ +1.2 V vs. NHE) utilizing their lowest excited singlet ((1)LL'CT), provided that the excited-state electron transfer is ultrafast, competitive with the picosecond decay. On the other hand, the efficiency of any triplet-based processes would be severely limited by the low quantum yield of the triplet population.

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.

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, 1; Pd, 2; Pt, 3) [Et2dazdt = N,N'-diethyl-perhydrodiazepine-2,3-dithione; mnt = maleonitrile-2,3-dithiolate] have been prepared and fully characterized. X-ray diffractometric studies on 1-3 (the structure of 1 was already known) show that the crystals are isostructural (triclinic, P-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. 1-3 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 1-3 as well as on [Pt(Bz2pipdt)(mnt)] (4) and [Pt(Bz2pipdt)(dmit)] (5) 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 5 can be attributed both to its lowest torsion angles and to the extent of the π system of its dithiolate ligand, dmit.