The rigidity and chelation effect of ligands on the hydrogen evolution reaction catalyzed by Ni(II) complexes

With increasing interest in nickel-based electrocatalysts, three heteroleptic Ni(II) dithiolate complexes with the general formula [Ni(II)L(L')2] (1-3), L = 2-(methylene-1,1'-dithiolato)-5,5'-dimethylcyclohexane-1,3-dione and L' = triphenylphosphine (1), 1,1'-bis(diphenylphosphino)ferrocene (DPPF) (2), and 1,2-bis(diphenylphosphino)ethane (DPPE) (3), have been synthesized and characterized by various spectroscopic techniques (UV-vis, IR, 1H, and 31P{1H} NMR) as well as the electrochemical method. The molecular structure of complex 2 has also been determined by single-crystal X-ray crystallography. The crystal structure of complex 2 reveals a distorted square planar geometry around the nickel metal ion with a NiP2S2 core. The cyclic voltammograms reveal a small difference in the redox properties of complexes (ΔE° = 130 mV) while the difference in the catalytic half-wave potential becomes substantial (ΔEcat/2 = 670 mV) in the presence of 15 mM CF3COOH. The common S^S-dithiolate ligand provides stability, while the rigidity effect of other ligands (DPPE (3) > DPPF (2) > PPh3 (1)) regulates the formation of the transition state, resulting in the NiIII-H intermediate in the order of 1 > 2 > 3. The foot-of-the-wave analysis supports the widely accepted ECEC mechanism for Ni-based complexes with the first protonation step as a rate-determining step. The electrocatalytic proton reduction activity follows in the order of complex 1 > 2 > 3. The comparatively lower overpotential and higher turnover frequency of complex 1 are attributed to the flexibility of the PPh3 ligand, which favours the easy formation of a transition state.

Crystal structure, Hirshfeld surface analysis and spectroscopic characterization of the di-enol tautomeric form of the compound 3,3'-[(2-sulf-an-yl-idene-1,3-di-thiole-4,5-di-yl)bis-(sulfane-di-yl)]bis-(pentane-2,4-dione)

The reaction between [TBA]2[Zn(dmit)2] and 3-chloro-2,4-penta-nedione yielded single crystals of the title compound, (3E,3'E)-3,3'-[(2-sulfanylidene-1,3-dithiole-4,5-diyl)bis(sulfanediyl)]bis(4-hydroxypent-3-en-2-one), C13H14O4S5, after solvent evaporation. The title compound crystallizes in the triclinic space group P with two mol-ecules related by an inversion center present in the unit cell. The central thione ring moiety contains a carbon-carbon double bond covalently linked to two sulfoxide substituents located outside of the plane of the ring. The S-C-C-S torsion angles are -176.18 (8) and -0.54 (18)°. Intra-molecular hydrogen bonds occur within the two dione substituents (1.67-1.69 Å). Adjacent asymmetric units are linked by C-H⋯S (2.89-2.90 Å), S⋯S [3.569 (1) Å] and O⋯H [2.56-2.66 Å between non-stacked thione rings] short contacts.

Redox, transmetalation, and stacking properties of tetrathiafulvalene-2,3,6,7-tetrathiolate bridged tin, nickel, and palladium compounds

Here we report that capping the molecule TTFtt (TTFtt = tetrathiafulvalene-2,3,6,7-tetrathiolate) with dialkyl tin groups enables the isolation of a stable series of redox congeners and facile transmetalation to Ni and Pd. TTFtt has been proposed as an attractive building block for molecular materials for two decades as it combines the redox chemistry of TTF and dithiolene units. TTFttH₄, however, is inherently unstable and the incorporation of TTFtt units into complexes or materials typically proceeds through the in situ generation of the tetraanion TTFtt^4-. Capping of TTFtt^4- with Bu₂Sn²⁺ units dramatically improves the stability of the TTFtt moiety and furthermore enables the isolation of a redox series where the TTF core carries the formal charges of 0, +1, and +2. All of these redox congeners show efficient and clean transmetalation to Ni and Pd resulting in an analogous series of bimetallic complexes capped by 1,2-bis(diphenylphosphino)ethane (dppe) ligands. Furthermore, by using the same transmetalation method, we synthesized analogous palladium complexes capped by 1,1'-bis(diphenylphosphino)ferrocene (dppf) which had been previously reported. All of these species have been thoroughly characterized through a systematic survey of chemical and electronic properties by techniques including cyclic voltammetry (CV), ultraviolet-visible-near infrared spectroscopy (UV-vis-NIR), electron paramagnetic resonance spectroscopy (EPR), nuclear magnetic resonance spectroscopy (NMR) and X-ray diffraction (XRD). These detailed synthetic and spectroscopic studies highlight important differences between the transmetalation strategy presented here and previously reported synthetic methods for the installation of TTFtt. In addition, the utility of this stabilization strategy can be illustrated by the observation of unusual TTF radical-radical packing in the solid state and dimerization in the solution state. Theoretical calculations based on variational 2-electron reduced density matrix methods have been used to investigate these unusual interactions and illustrate fundamentally different levels of covalency and overlap depending on the orientations of the TTF cores. Taken together, this work demonstrates that tin-capped TTFtt units are ideal reagents for the installation of redox-tunable TTFtt ligands enabling the generation of entirely new geometric and electronic structures.

Crystal structures of 4,4'-(disulfane-1,2-diyl)bis(5-methyl-2H-1,3-dithiol-2-one) and 4,4'-(diselanane-1,2-diyl)bis(5-methyl-2H-1,3-dithiol-2-one)

The two title compounds, C8H6O2S6 and C8H6O2S4Se2, are isotypic with very similar cell parameters. The complete mol-ecules constitute the asymmetric units, despite being chemically perfectly symmetric. The most prominant differences in the metrical parameters arise from the distinct sizes of sulfur and selenium in the dichalcogenide bridges, with C-S-S-C and C-Se-Se-C torsion angles of 70.70 (5) and 68.88 (3)°, respectively. The crystal packing is determined by weak non-classical hydrogen-bonding inter-actions. One carbonyl oxygen but not the other participates in C-H⋯O inter-actions zigzagging along the b axis, forming infinite chains. This is complemented by an intra-molecular C-H⋯S inter-action and further inter-molecular C-H⋯S (C-H⋯Se) inter-actions, resulting in a three-dimensional network. The inter-actions involving the bridging chalcogenides form chains protruding along the c axis.

Gold(I) complexes with alkylated PTA (1,3,5-triaza-7-phosphaadamantane) phosphanes as anticancer metallodrugs

New stable thiolate gold(I) derivatives containing the alkylated phosphanes [PTA-CH2Ph]Br and [PTA-CH2COOMe]Br derived from 1,3,5-triaza-7-phosphaadamantane (PTA) have been prepared by different routes of synthesis. By the use of basic media to deprotonate the corresponding thiol in the former and by transmetallation reactions from tin (IV) complexes, in the later, thus avoiding side reactions on the phosphane. Strong antiproliferative effects are observed for most of the compounds, including the chloro- and bromo precursors with the series of phosphanes derived from PTA, in human colon cancer cell lines (Caco-2, PD7 and TC7 clones). Apoptosis-induced cell death is found for all compounds, being the thiolate derivatives with [PTA-CH2Ph]Br the most effective, as shown by an annexin-V/propidium iodide double-staining assay.

Self-association and columnar liquid crystalline phase of cationic alkyl-substituted-bipyridine benzenedithiolato gold(III) complexes

The introduction of a Au(III) ion into a mesogenic core, [M(Bdt)(Cnbpy)](+) (Bdt = 1,2-benzenedithiolato and Cnbpy = 4,4'-di-alkyl-2,2'-bipyridine (n = 13 (4,4'-di-tridecyl-2,2'-bipyridine (C13bpy)) and 8,10 (4,4'-di-(3-octyltridecyl)-2,2'-bipyridine (C8,10bpy)))), leads to the formation of ionic molecular assemblies in crystalline and mesophases. Successive syntheses of precursor complexes, [AuCl2(Cnbpy)]PF6 (n = 13 (1) and 8,10 (2)), followed by the target complexes, [Au(Bdt)(Cnbpy)]PF6 (n = 13 (3) and 8,10 (4)), were achieved. The crystallographic analysis of 3 revealed that the central cationic cores form a characteristic one-dimensional columnar structure, which is an essential property for the formation of columnar structures in the liquid crystalline phase. The central cores of the [Au(Bdt)(C13bpy)](+) cations in 3 stack alternatively so as to cancel out their dipole moments and the counteranions lie between the cationic columns. Furthermore, the introduction of the branched alkyl tails in 4 induces the formation of a rectangular columnar liquid crystalline phase (Col(r)) with C2/m symmetry, and it melts to an isotropic liquid at 69 °C. The Col(r) phase of 4 is partially similar in its liquid crystalline structure to the neutral [Pt(Bdt)(C8,10bpy)] liquid crystal in a hexagonal columnar phase reported previously, while complex 4 shows a lower clearing point than that of the Pt analogue. Based on these results, the present study demonstrates the designability of the liquid crystalline structures of the [M(Bdt)(C8,10bpy)] skeleton together with their assembled structures and physico-chemical properties.