Anna Pintus

Gold(III) Complexes of Asymmetrically Aryl‐Substituted 1,2‐Dithiolene Ligands Featuring Potential‐Controlled Spectroscopic Properties: An Insight into the Electronic Properties of bis(Pyren‐1‐yl‐ethylene‐1,2‐dithiolato)Gold(III)

The electrochemical, UV/Vis-NIR absorption, and emission-spectroscopic features of (TBA(+))(1(-)) and the corresponding neutral complex 1 were investigated (TBA(+)=tetrabutylammonium; 1(-)=[Au(III)(Pyr,H-edt)(2)](-); Pyr,H-edt(2-)=pyren-1-yl-ethylene-1,2-dithiolato). The intense electrochromic NIR absorption (λ(max)=1432 nm; ε=13000 M(-1) cm(-1) in CH(2)Cl(2)) and the potential-controlled visible emission in the range 400-500 nm, the energy of which depends on the charge of the complex, were interpreted on the grounds of time-dependent DFT calculations carried out on the cis and trans isomers of 1, 1(-), and 1(2-). In addition, to evaluate the nonlinear optical properties of 1(x-) (x=0, 1), first static hyperpolarizability values β(tot) were calculated (β(tot)=78×10(-30) and 212×10(-30) esu for the cis isomer of 1(-) and 1, respectively) and compared to those of differently substituted [Au(Ar,H-edt)(2)](x-) gold dithiolenes [Ar=naphth-2-yl (2), phenyl (3); x=0, 1].

Synthesis and Characterization of Novel Gold(III) Complexes of Asymmetrically Aryl-Substituted 1,2-Dithiolene Ligands Featuring Potential-Controlled Spectroscopic Properties

The tetrabutylammonium (TBA(+)) salts of square-planar monoanionic gold complexes of the unsymmetrically substituted Ar,H-edt(2-) 1,2-dithiolene ligands (Ar,H-edt(2-)=arylethylene-1,2-dithiolato; Ar=phenyl (1(-)), 2-naphthyl (2(-)), and 1-pyrenyl (3(-))) were synthesized and characterized by spectroscopic and electrochemical methods and the corresponding neutral species (1, 2, and 3, respectively) were obtained in CH(2)Cl(2) solution at room temperature by diiodine oxidation. The single-crystal X-ray diffraction structural data collected for (TBA(+))(2(-)), supported by DFT theoretical calculations, are consistent with the ene-1,2-dithiolate form of the ligand and the Au(III) oxidation state. All complexes feature intense near-IR absorptions (at about 1.5 microm) in their neutral states and Vis-emitting properties in the 400-550 nm range, the energy of which is controlled by the charge of the complex in the case of the 3(-)/3 couple. The spectroscopic and electrochemical features of 1(x-) and 2(x-) (x=0, 1), both in their cis and trans conformations, were investigated by means of DFT and time-dependent (TD) DFT calculations.

Cationic and anionic 1D chains based on NH+⋯N charge-assisted hydrogen bonds in bipyridyl derivatives and polyiodides

The possibility of constructing extended networks based on NH+⋯N charge-assisted hydrogen bonding and N⋯I interactions was explored. The organic modules 3,5-di-(3-pyridyl)-1,2,4-thiadiazole (L1) and 3,5-di-(4-pyridyl)-1,2,4-thiadiazole (L2) possess two pyridyl groups, allowing them to act both as hydrogen-bond acceptors and hydrogen-bond donors with H+ acting as the main linker between the molecular units. The crystal structure of the ionic compounds (HL1)I3, (HL1)I5, (HL1)IBr2, (HL2)I3, and (HL2)IBr2 are described; the position of the nitrogen atoms in the outwards pyridyl rings in L1 and L2 leads to the formation of 1D helices of interacting cations (HL1)+ and zig-zag chains of interacting cations (HL2)+. In the case of (HL1)I5, cationic helices of (HL1)+ and helices of I5− take place in a highly shape-complementary arrangement. The crystal structure of the bis-adduct L1·2I2 features the presence of intermolecular iodine–iodine long contacts to form infinite I2 chains. A comparative structural analysis carried out using the XPac procedure identifies three common molecular arrangements and confirms the importance of directional interactions and molecular shape of the target molecules in directing the packing preferences of this family of structures.

Allosteric modulation of Euphorbia peroxidase by nickel ions

A class III peroxidase, isolated and characterized from the latex of the perennial Mediterranean shrub Euphorbia characias, contains one ferric iron–protoporphyrin IX pentacoordinated with a histidine ‘proximal’ ligand as heme prosthetic group. In addition, the purified peroxidase contained 1 mole of endogenous Ca2+ per mole of enzyme, and in the presence of excess Ca2+, the catalytic efficiency was enhanced by three orders of magnitude. The incubation of the native enzyme with Ni2+ causes reversible inhibition, whereas, in the presence of excess Ca2+, Ni2+ leads to an increase of the catalytic activity of Euphorbia peroxidase. UV/visible absorption spectra show that the heme iron remains in a quantum mechanically mixed‐spin state as in the native enzyme after addition of Ni2+, and only minor changes in the secondary or tertiary structure of the protein could be detected by fluorescence or CD measurements in the presence of Ni2+. In the presence of H2O2 and in the absence of a reducing agent, Ni2+ decreases the catalase‐like activity of Euphorbia peroxidase and accelerates another pathway in which the inactive stable species accumulates with a shoulder at 619 nm. Analysis of the kinetic measurements suggests that Ni2+ affects the H2O2‐binding site and inhibits the formation of compound I. In the presence of excess Ca2+, Ni2+ accelerates the reduction of compound I to the native enzyme. The reported results are compatible with the hypothesis that ELP has two Ni2+‐binding sites with opposite functional effects.

[Au(pyb-H)(mnt)]: A novel gold(III) 1,2-dithiolene cyclometalated complex with antimicrobial activity (pyb-H = C-deprotonated 2-benzylpyridine; mnt = 1,2-dicyanoethene-1,2-dithiolate)

The novel heteroleptic cyclometalated complex [AuIII(pyb-H)(mnt)] (1; pyb-H=C-deprotonated 2-benzylpyridine; mnt =1,2-dicyanoethene-1,2-dithiolate) was tested against a panel of ten Gram positive (belonging to the Staphylococcus, Streptococcus spp. and Bacillus clausii), Gram negative (E. coli, K. pneumoniae, P. aeruginosa) bacteria and three yeasts belonging to the Candida spp. Complex 1 showed a remarkable bacteriostatic antimicrobial activity against staphylococci, with Minimum Inhibitory Concentration (MIC) values of 1.56 and 3.13μg/mL for S. haemoliticus and S. aureus, respectively. Spectroscopic and electrochemical measurements, supported by Density Functional Theory (DFT) calculations, were exploited to fully investigate the electronic structure of complex 1 and its relationship with the antimicrobial activity.

Structural tailoring of the NIR-absorption of bis(1,2-dichalcogenolene) Ni/Pt electrochromophores deriving from 1,3-dimethyl-2-chalcogenoxo-imidazoline-4,5-dichalcogenolates

The choice of the metal ion M and the terminal Y and donor X chalcogen species (M = Ni, Pt; and X, Y = S, Se) in square-planar complexes deriving from 1,3-dimethyl-2-chalcogenoxo-imidazoline-4,5-dichalcogenolates allows fine-tuning of both the redox stability and the energy of the particularly intense NIR electrochromic absorption, thanks to the subtle contribution of M, X, and Y to the relevant frontier molecular orbitals, investigated at the IEF-PCM DFT and TD-DFT level.

Structural diversity in the products formed by the reactions of 2-arylselanyl pyridine derivatives and dihalogens

The reactivity of the 2-arylselanyl pyridine derivatives L1–L4 towards dihalogens X2 (X = I, Br) and interhalogens IX (X = Cl, Br) was studied in CHCl3 or CH3CN. The solid products obtained were structurally characterized and their nature points out the preference for CT spoke adducts and for seesaw insertion adducts to be formed at the N-donor and Se-donor site, respectively. DFT calculations were performed to provide a rationale for the structural diversity observed in the products obtained.

CdII/ZnII discrimination using 2,5-diphenyl[1,3,4]oxadiazole based fluorescent chemosensors

The coordination properties and photochemical response of two fluorescent ligands 2,5-bis{2-[bis(2-pyridylmethyl)aminomethyl]phenyl}[1,3,4]oxadiazole (L1) and 2,5-bis{2-[(1,4,7-triazacyclononane-1-yl)methyl]phenyl}[1,3,4]oxadiazole (L2) towards ZnII and CdII are reported. The ligands contain the 2,5-diphenyl[1,3,4]oxadiazole (PPD) fluorescent probe linked to two coordinating units, namely dipicolylamine (DPA) for L1 and 1,4,7-triazacyclononane (TACN) for L2, through a methylene spacer. Both ligands form mononuclear and dinuclear complexes with these metal ions in aqueous or hydro-alcoholic media. With regard to L1, only the mononuclear CdII complex is highly fluorescent at 368 nm, while the mononuclear ZnII and dinuclear CdII and ZnII complexes are not emissive. Ligand L2 forms an excimer emitting at 474 nm only in the presence of one equivalent of CdII, and potentiometric, spectrophotometric, spectrofluorimetric, and NMR studies and DFT calculations were performed to explain this peculiarity. The discrimination between CdII and ZnII is mainly due to the different coordination environments of the metal ions in the mononuclear complexes; namely CdII is coordinated by both the side arms of each ligand, while ZnII is bound only to one coordinating unit.

Density functional theory modelling of protective agents for carbonate stones: a case study of oxalate and oxamate inorganic salts

Sulphur and nitrogen oxide pollutants cause acid rain that can eventually lead to the dissolution of calcite in marble and limestones. Calcium oxalate is an inorganic protective agent, which is obtained by treatment with ammonium oxalate. The functionalization of oxalic acid to give monoesters and monoamides (oxamates) allows tailoring the solubility of the relevant ammonium and calcium salts. In this context, theoretical calculations carried out at the Density Functional Theory (DFT) level were exploited to investigate the capability of oxalate, methyloxalate, phenyloxalate, oxamate, methyloxamate, and phenyloxamate to interact with the calcium carbonate lattice. An in-depth validation based on the structural data showed that DFT calculations with the PBE0 functional along with a single or triple-zeta def2 basis set allow understanding the different reactivity of the oxalate and oxamate derivatives and their efficiency in interacting with stones containing calcium carbonate, such as Carrara marble and biomicritic limestones.

CCDC 924216: Experimental Crystal Structure Determination

Related Article: M. Carla Aragoni, Massimiliano Arca, Andrea Bencini, Claudia Caltagirone, Alessandra Garau, Francesco Isaia, Mark E. Light, Vito Lippolis, Carlos Lodeiro, Marta Mameli, Riccardo Montis, M. Cristina Mostallino, Anna Pintus, Stefano Puccioni|2013|Dalton Trans.|42|14516|doi:10.1039/C3DT51292D