Synergistic Effect of Sn and Fe in Fe–N_<i>x</i> Site Formation and Activity in Fe–N–C Catalyst for ORR

Iron–nitrogen–carbon (Fe–N–C) materials emerged as one of the best non-platinum group material (non-PGM) alternatives to Pt/C catalysts for the electrochemical reduction of O2 in fuel cells. Co-doping with a secondary metal center is a possible choice to further enhance the activity toward oxygen reduction reaction (ORR). Here, classical Fe–N–C materials were co-doped with Sn as a secondary metal center. Sn–N–C according to the literature shows excellent activity, in particular in the fuel cell setup; here, the same catalyst shows a non-negligible activity in 0.5 M H2SO4 electrolyte but not as high as expected, meaning the different and uncertain nature of active sites. On the other hand, in mixed Fe, Sn–N–C catalysts, the presence of Sn improves the catalytic activity that is linked to a higher Fe–N4 site density, whereas the possible synergistic interaction of Fe–N4 and Sn–Nx found no confirmation. The presence of Fe–N4 and Sn–Nx was thoroughly determined by extended X-ray absorption fine structure and NO stripping technique; furthermore, besides the typical voltammetric technique, the catalytic activity of Fe–N–C catalyst was determined and also compared with that of the gas diffusion electrode (GDE), which allows a fast and reliable screening for possible implementation in a full cell. This paper therefore explores the effect of Sn on the formation, activity, and selectivity of Fe–N–C catalysts in both acid and alkaline media by tuning the Sn/Fe ratio in the synthetic procedure, with the ratio 1/2 showing the best activity, even higher than that of the iron-only containing sample (jk = 2.11 vs 1.83 A g–1). Pt-free materials are also tested for ORR in GDE setup in both performance and durability tests

Platinum(II) Complexes with Novel Diisocyanide Ligands: Catalysts in Alkyne Hydroarylation

A series of novel diisocyanide ligands (<i>o</i>-CNC₆H₄O)₂Y (diNC-1: Y = P­(O)­Ph; diNC-2: Y = <i>o</i>-C­(O)­C₆H₄C­(O); diNC-3: Y = <i>m</i>-C­(O)­C₆H₄C­(O); diNC-4: Y = C­(O)­C₂H₄C­(O); diNC-5: Y = <i>trans</i>-C­(O)­C₂H₂C­(O)) was successfully synthesized by reaction of lithium 2-isocyanophenate (generated in situ from benzoxazole and <i>n</i>-BuLi) and a diacylic or phosphonic dichloride. The corresponding platinum­(II) complexes of general formula [PtX₂(diNC)]_1,2 (X = Cl, Me; diNC = diisocyanide ligand) were isolated by simple substitution of 1,5-cyclooctadiene in the starting [PtX₂(COD)] complexes. The structure of the complexes, mononuclear or dinuclear, was confirmed by single-crystal X-ray analysis. A dinuclear complex of formula {(μ-diNC)­[<i>cis</i>-PtCl₂(PPh₃)]₂} could also be obtained with the diisocyanide ligand having a rigid fumaryl bridge and consequently the isocyanide moieties pointing in opposite directions. All the complexes were employed as catalysts in the hydroarylation of alkynes, showing generally good activity and selectivity toward the <i>trans</i>-hydroarylation product. With <i>N</i>-methylindole as aromatic substrate the major product was instead a heterocycle:alkyne 2:1 adduct

Platinum(II) Complexes with Novel Diisocyanide Ligands: Catalysts in Alkyne Hydroarylation

A series of novel diisocyanide ligands (<i>o</i>-CNC₆H₄O)₂Y (diNC-1: Y = P­(O)­Ph; diNC-2: Y = <i>o</i>-C­(O)­C₆H₄C­(O); diNC-3: Y = <i>m</i>-C­(O)­C₆H₄C­(O); diNC-4: Y = C­(O)­C₂H₄C­(O); diNC-5: Y = <i>trans</i>-C­(O)­C₂H₂C­(O)) was successfully synthesized by reaction of lithium 2-isocyanophenate (generated in situ from benzoxazole and <i>n</i>-BuLi) and a diacylic or phosphonic dichloride. The corresponding platinum­(II) complexes of general formula [PtX₂(diNC)]_1,2 (X = Cl, Me; diNC = diisocyanide ligand) were isolated by simple substitution of 1,5-cyclooctadiene in the starting [PtX₂(COD)] complexes. The structure of the complexes, mononuclear or dinuclear, was confirmed by single-crystal X-ray analysis. A dinuclear complex of formula {(μ-diNC)­[<i>cis</i>-PtCl₂(PPh₃)]₂} could also be obtained with the diisocyanide ligand having a rigid fumaryl bridge and consequently the isocyanide moieties pointing in opposite directions. All the complexes were employed as catalysts in the hydroarylation of alkynes, showing generally good activity and selectivity toward the <i>trans</i>-hydroarylation product. With <i>N</i>-methylindole as aromatic substrate the major product was instead a heterocycle:alkyne 2:1 adduct

Tuning the Framework of Thioether-Functionalized Polyazamacrocycles: Searching for a Chelator for Theranostic Silver Radioisotopes

Silver-111 is an attractive unconventional candidate for targeted cancer therapy as well as for single photon emission computed tomography and can be complemented by silver-103 for positron emission tomography noninvasive diagnostic procedures. However, the shortage of chelating agents capable of forming stable complexes tethered to tumor-seeking vectors has hindered their in vivo application so far. In this study, a comparative investigation of a series of sulfur-containing structural homologues, namely, 1,4,7-tris[2-(methylsulfanyl)ethyl)]-1,4,7-triazacyclononane (NO3S), 1,5,9-tris[2-(methylsulfanyl)ethyl]-1,5,9-triazacyclododecane (TACD3S), 1,4,7,10-tetrakis[2-(methylsulfanyl)ethyl]-1,4,7,10-tetraazacyclotridecane (TRI4S), and 1,4,8,11-tetrakis[2-(methylsulfanyl)ethyl]-1,4,8,11-tetraazacyclotetradecane (TE4S) was conducted to appraise the influence of different polyazamacrocyclic backbones on Ag+ complexation. The performances of these macrocycles were also compared with those of the previously reported Ag+/[111Ag]Ag+-chelator 1,4,7,10-tetrakis[2-(methylsulfanyl)ethyl]-1,4,7,10-tetraazacyclododecane (DO4S). Nuclear magnetic resonance data supported by density functional theory calculations and X-ray crystallographic results gave insights into the coordination environment of these complexes, suggesting that all of the donor atoms are generally involved in the metal coordination. However, the modifications of the macrocycle topology alter the dynamic binding of the pendant arms or the conformation of the ring around the metal center. Combined pH/pAg-potentiometric and spectroscopic experiments revealed that the 12-member N4 backbone of DO4S forms the most stable Ag+ complex while both the enlargement and the shrinkage of the macrocyclic frame dwindle the stability of the complexes. Radiolabeling experiments, conducted with reactor-produced [111Ag]Ag+, evidenced that the thermodynamic stability trend is reflected in the ligand’s ability to incorporate the radioactive ion at high molar activity, even in the presence of a competing cation (Pd2+), as well as in the integrity of the corresponding complexes in human serum. As a consequence, DO4S proved to be the most favorable candidate for future in vivo applications