Ligand-based Reduction Research Articles

Heteroleptic palladium(II) complexes of dipyrrin-1,9-dione supported by intramolecular hydrogen bonding

The dipyrrin-1,9-dione framework, which is characteristic of the propentdyopent pigments deriving from heme metabolism, coordinates metal ions as monoanionic bidentate donors. The resulting analogs of dipyrrinato complexes undergo reversible ligand-based reductions, thus showcasing the ability of the dipyrrindione scaffold to act as an electron reservoir. Herein we report the synthesis and characterization of three heteroleptic palladium complexes of the redox-active dipyrrindione ligand. Primary amines were chosen as additional ligands so as to assemble complexes of planar geometries with complementary interligand hydrogen-bonding. Full chemical characterization confirms the hydrogen bonding interactions between the primary amine ligands and the acceptor carbonyl groups on the dipyrrolic ligand. The resulting heteroleptic compounds display reversible one-electron reduction events that are centered on the dipyrrindione ligand as revealed by voltammetry and spectroelectrochemistry data. Within these planar Pd(II) complexes, the propentdyopent motif therefore combines reversible ligand-based redox chemistry with interligand hydrogen bonding in the primary coordination sphere of the metal center.

Divergent uranium- versus phosphorus-based reduction of Me3SiN3 with steric modification of phosphido ligands.

We describe an example of a two-electron metal- and ligand-based reduction of Me3SiN3 using uranium(iv) complexes with varying steric properties. Reaction of (C5Me5)2U(CH3)[P(SiMe3)(Ph)] with Me3SiN3 produces the imidophosphorane complex, (C5Me5)2U(CH3)[N Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> P(SiMe3)2(Ph)] through oxidation of phosphorus. However, a similar reaction with a more sterically encumbering phosphido ligand, (C5Me5)2U(CH3)[P(SiMe3)(Mes)] forms the U(iv) complex, (C5Me5)2U[κ2-(N,N)–N(SiMe3)P(Mes)N(SiMe3)]. In probing the mechanism of this reaction, a U(vi) bis(imido) complex, (C5Me5)2U(NSiMe3){N[P(SiMe3)(Mes)]} was isolated. DFT calculations show an intramolecular reductive cycloaddition reaction leads to the formation of the U(iv) bis(amido)phosphane from the U(vi) bis(imido) complex. This is a rare example of the isolation of a reaction intermediate in f element chemistry.

Electrochemical studies of cobalt(II) diphenylazodioxide complexes

The electrochemical behavior of the unusual cobalt(II) diphenylazodioxide complex salts [Co(az)4](PF6)21 and [Co(bpy)(az)2](PF6)22 has been studied by cyclic voltammetry. Each complex displays two quasireversible redox couples, which are proposed to correspond to a reduction of Co(II) to Co(I), followed by a ligand-based reduction. Irreversible reductions of 1 are observed at more negative potentials, and are proposed to arise from deposition of elemental Co and the decomposition of transiently formed Co(-I) species. Spectroelectrochemical experiments on both 1 and 2, involving electrolytic reduction followed by reoxidation, are consistent with the quasireversibility observed in the CV measurements.

Spectroelectrochemical, photochemical and theoretical study of octaazamacrocyclic nickel(II) complexes exhibiting unusual solvent-dependent deprotonation of methylene group

The reduction potentials and the reversibility of corresponding redox processes in a series of nickel(II) complexes, featuring 15-membered (1–5) and 14-membered (6) octaazamacrocyclic ligands derived from 1,2- and 1,3-diketones and S-methylisothiocarbohydrazide, strongly depend on the number of phenyl and methyl substituents on the macrocycle. The reaction mechanism for the ligand-based reduction of 1–5 is proposed. One-electron reduction generates unstable ligand-centred radical anions that lose a hydrogen atom from the macrocycle, and transform to stable monoanionic EPR silent species. After the re-oxidation at the strongly shifted anodic potential, a tautomerised neutral Ni(II) complex can be reduced again to the nickel(II) ligand-based radical. This was confirmed by NMR, UV–vis–NIR and EPR spectroelectrochemical measurements in different media. Density functional theory (DFT) calculations were performed to support the assignment of species generated upon reduction. Additionally, we showed that photochemical stability and photochemically induced reactions of 1–5 strongly depend on the form of the complex in the solution, where the deprotonated form is more reactive.

Double bridged μ2-Halide Cu (II) complexes for the electrocatalytic reduction of CO2

Two dicopper (II) complexes of general formula [CuLX2]2 {L = 2, 5-diphenyl-3, 4(2-pyridyl) cyclopenta-2, 4-dien-1-one, X = Cl (1) or Br (2)} have been synthesized and characterized by elemental analysis, IR, UV–Vis spectroscopic methods. The crystal structures of both complexes have been determined by single-crystal X-ray diffraction; 1 and 2 have binuclear structures and Cu (II) centres in both complexes adopt a distorted square pyramidal geometry. Two [CuLX] (X = Cl or Br) units in both complexes are linked via μ-X coordination bridge modes with the Cu–Cu distances of 3.534 and 3.716 Å for 1 and 2 respectively. The electrochemical behaviour of the free L ligand and the corresponding Cu (II) complexes was studied in acetonitrile. The cyclic voltammetry of the complexes 1 and 2 show three ligand based reduction processes and two metal-centred reductions that are assigned to copper (II) to copper (I) and copper (I) to copper (0). The electrocatalytic activity for the reduction of CO2 of the two complexes was investigated; both complexes are active for the CO2 reduction and result in the formation of CO.

Tris(β-ketoiminato)ruthenium(III) complexes: Electrochemical and computational chemistry study

The electronic and electrochemical properties are reported here for the first time for a series of five tris(β-ketoiminato)ruthenium(III) complexes. Since the β-ketoiminato ligand is unsymmetrical, both fac and mer isomers are theoretically possible for these octahedral complexes. Density functional theory calculations show that for complexes containing an H on the imino position, both the fac and mer are energetically possible, while for complexes with a Ph on the imino position, the mer isomer is energetically favoured, due to the steric hindrance caused by the Ph group in the fac isomer. Electrochemistry, utilizing cyclic voltammetry, showed RuIII/IV oxidation, RuIII/II reduction, as well as ligand based reduction of the RuII complex. Different RuIII/IV and RuIII/II redox couples were observed for the different fac and mer isomers of the tris(β-ketoiminato)ruthenium(III) complexes.

Neocuproine as a Redox-Active Ligand Platform on Iron and Cobalt.

A family of bis(neocuproine) complexes of Fe2+ and Co2+ have been investigated for neocuproine redox noninnocence. A series of redox isomers of M(neocuproine)2n+ (where n = 2, 1, 0 for Co and n = 2, 0 for Fe) have been synthesized and thoroughly characterized. The electronic structure of these complexes has been rigorously investigated using a variety of techniques, including X-ray absorption spectroscopy, Mössbauer spectroscopy, X-ray diffraction, electron paramagnetic resonance spectroscopy, and magnetic measurements. All of these techniques are consistent with ligand-based reduction events to generate radical neocuproine complexes. Thus, neocuproine adds to a growing family of chelating N-donor type ligands that participate in redox noninnocence and may be useful for future catalyst and reaction design.

Single-walled carbon nanotube/sulfanyl porphyrazine hybrids deposited on glassy carbon electrode for sensitive determination of nitrites

The metallation reaction of 2-[2-(3,5-dibutoxycarbonylphenoxy)ethoxy]ethylsulfanyl-substituted porphyrazine with manganese and nickel salts led to novel porhyrazine derivatives with d-block metal ions in the core. Voltammetric studies of nickel(II) porphyrazine in organic electrolyte revealed the existence of well-defined one-electron ligand-based reductions and ligand-based one-electron oxidation redox pairs. Manganese porphyrazine however, presented both metal-based and ligand-based reductions and oxidation processes. The glassy carbon electrode with deposited single-walled carbon nanotube/sulfanyl porphyrazine hybrids was examined for the electrocatalytic oxidation of nitrites. The results indicated that when nickel(II) porphyrazine is present, it significantly contributes to the promotion of the electrocatalytic current for nitrites oxidation. The response of the proposed modified electrode was linear considering the nitrites concentration in the range of 5–200 μM and with a detection limit (LOD) and quantitation limit (LOQ) equal to 1.08 and 3.60 μM, respectively. The proposed voltammetric sensor was also applied for the determination of nitrites in tap water and mineral water samples.

Electronic Communication between Dithiolato-Bridged Diiron Carbonyl and S-Bridged Redox-Active Centres

The catalytic potential of linked redox centres is exemplified by the catalytic site of [FeFe]-hydrogenases, which feature a diiron subsite linked by a cysteinyl S atom to a 4Fe4S cube. The investigation of systems possessing similarly-linked redox sites is important because it provides a context for understanding the biological system and the rational design of abiological catalysts. The structural, electrochemical and spectroscopic properties of Fe2(CO)5(CH3C(CH2S)2CH2SPhNO2, I-bzNO2 and the aniline analogue, I-bzNH2, are described and IR spectroelectrochemical studies have allowed investigation of the reduction products and their reactions with CO and protons. These measurements have allowed identification of the nitrobenzenyl radical anion, quantification of the shifts of the (CO) bands on ligand-based reduction compared with NO2/NH2 exchange and protonation of the pendent ligand. The strength of thioether coordination is related to the electronic effects, where competitive binding studies with CO show that CO/thioether exchange can be initiated by redox processes of the pendent ligand. Stoichiometric multi electron/proton transfer reactions of I-bzNO2 localised on nitrobenzene reductions occur at mild potentials and a metal-centred reduction in the presence of protons does not lead to significant electrocatalytic proton reduction.

Impact of Ligand Substitutions on Multielectron Redox Properties of Fe Complexes Supported by Nitrogenous Chelates.

Redox flow batteries (RFBs) have recently been recognized as a potentially viable technology for scalable energy storage. To take full advantage of RFBs, one possible approach for achieving high energy densities is to maximize a number of redox events by utilizing charge carriers capable of multiple one-electron transfers within the electrochemical window of solvent. However, past efforts to develop more efficient electrolytes for nonaqueous RFBs have mostly been empirical. In this manuscript, we shed light on design principles by theoretically investigating the effects of systematically substituting pyridyl moieties with imine ligands within a series of Fe complexes with some experimental validation. We found that such replacement is an effective strategy for reducing the molecular weight-to-charge ratios of these complexes. Simultaneously, calculations suggest that the reduction potentials and ligand-based redox activity of such substituted N-heterocyclic Fe compounds might be maintained within their +4 → −1 charge states. Additionally, by theoretically examining the role of coordination geometry, vis-à-vis reducing the number of redox noninnocent ligands within the first coordination sphere, we have demonstrated that Fe complexes with one such ligand were also capable of supporting multielectron reduction events and exhibited reduction potentials similar to their parent analogs supported by two or three of the same multidentate ligands. However, some differences in redox nature within the lower (+2 → −1) charge states were also noticed. Specifically, complexes containing two bidentate ligands, or one tridentate ligand, exhibited ligand-based reductions, whereas compounds with one bidentate ligand exhibited metal-centered reductions. The current results pave the way toward the design of the next-generation of Fe complexes with lower molecular weights and greater stored energy for redox flow batteries.

Electrochemical and electronic properties of a series of substituted polypyridine ligands and their Co(II) complexes

DFT calculations show that, due to Jahn-Teller distortion, the d7 [Co(N,N)3]2+ complexes, with S = ½ (N,N = bipyridine or substituted bipyridine ligand) have two longer axial and four shorter equatorial Co-N bonds (elongation Jahn-Teller), while [Co(terpyridine)2]2+ with S = ½, instead has two shorter central (axial) Co-N bonds and four longer distal Co-N bonds (compression Jahn-Teller), since in the latter, the distal Co-N bonds are more flexible than the Co-N axial bonds in the rigid structure of the tridentate terpyridine ligand. The same trend is observed for the related high spin S = 3/2 Co(II) complexes, though less pronounced. The cyclic voltammograms of [Co(terpyridine)2]2+ and a series of the [Co(N,N)3]2+ complexes show at least three chemically as well as electrochemically reversible redox couples, namely CoIII/II, CoII/I and a ligand based reduction of the polypyridine-Co(I) complex. The reduction of the uncoordinated free polypyridine ligand is more than 0.5 V more negative than the reduction of the coordinated ligand in the polypyridine-Co(I) complex.

Electrochemical properties of a series of Co(II) complexes, containing substituted phenanthrolines

Electrochemical studies of a series of substituted phenanthroline-Co(II) complexes all show generally similar behaviour, namely a chemically and electrochemically reversible CoIII/II redox couple, as well as a chemically and electrochemically reversible CoII/I redox couple, followed by a ligand-based reduction. Electron donating- or -withdrawing substituents on the phenanthroline ligands which are coordinated to the Co metal, directly influence the electron density on the Co metal, due to good communication between these substituents and the Co metal via the aromatic rings of the heterocyclic substituted phenanthroline-Co(II) complexes, leading to either more negative (for electron donating groups) or more positive (for electron withdrawing groups) redox potentials respectively. Linear relationships relating E°′(CoIII/II) oxidation and E°′(CoII/I) reduction to various experimental and empirical values, as well as to theoretically calculated energies, show that the electron density on Co is linearly influenced by the electronic properties of the ligands attached to the Co metal. All these established relationships can be used in the design of new substituted phenanthroline-Co(II) complexes with specific customized redox properties as required, for example, for the application of such Co(II) complexes as redox mediator for dye-sensitized solar cells.

Hydrogen Evolution Reactions Catalyzed by a Bis(thiosemicarbazone) Cobalt Complex: An Experimental and Theoretical Study

The synthesis and characterization of a dinuclear bis(thiosemicarbazone) cobalt complex [Co2 L2 (NCS)2 ] is reported. This complex exhibits significant catalytic activity for hydrogen production in DMF by using triethylammonium (Et3 NHBF4 ) as the proton source. Cyclic voltammetry data allowed a maximum turnover frequency of 130 s-1 for 1 m proton concentration to be determined. The catalytic nature of the process and the production of dihydrogen were confirmed by gas analysis during controlled potential electrolysis experiments. Quantum chemical calculations show that the complex displays a ligand-assisted metal-centered reactivity and supports a catalytic mechanism involving ligand-based reduction and protonation steps followed by metal-centered processes.

Tuning the Mesomorphism and Redox Response of Anionic-Ligand-Based Mixed-Valent Nickel(II) Complexes by Alkyl-Substituted Quaternary Ammonium Cations.

The combination of the redox-active mesogenic anion [NiII (Bdt)(BdtSQ)]- (Bdt=1,2-benzenedithiolato; BdtSQ=1,2-dithia-semi-benzoquinonato) with alkyl-substituted ammonium cations afforded a series of redox-active ionic complexes of the type [NR4 ][NiII (Bdt)(BdtSQ)] [R=nC16 H33 (NC164 Ni) and C8,10 (NC8,104 Ni); C8,10=6-octylhexadecyl] or [NMe2 R2 ][NiII (Bdt)(BdtSQ)] [R=nC16 H33 (NMe2 C162 Ni) and C8,10 (NMe2 C8,102 Ni)]. X-ray crystallographic analyses of NMe2 C162 Ni and NC164 Ni revealed the formation of cation-dependent integrated ionic layers separated by interdigittated alkyl chains. Complexes NMe2 C162 Ni and NC164 Ni commonly form crystalline phases at room temperature, whereas complexes NMe2 C8,102 Ni and NC8,104 Ni, which contain branched alkyl chains, form a metastable mesophase and an amorphous phase at the same temperature, respectively. Furthermore, complexes NMe2 C162 Ni, NMe2 C8,102 Ni, and NC164 Ni commonly form a smectic A phase (SmA) at 375, 317, and 342 K, respectively. For the four complexes, well-defined cyclic voltammetry responses, derived from ligand-based oxidation and reduction, were observed in solution and the condensed phases, that is, upon casting these complexes on an indium-doped tin oxide working electrode. The present study demonstrates the tunability of the mesomorphism of ionic molecular assemblies composed of alkyl-substituted quaternary ammonium cations, while maintaining the well-defined redox responses of the anions even in the condensed phases.

Plutonium coordination and redox chemistry with the CyMe4-BTPhen polydentate N-donor extractant ligand.

Complexation of Pu(iv) with the actinide extractant CyMe4-BTPhen (2,9-bis(5,5,8,8-tetramethyl-5,6,7,8-tetrahydro-1,2,4-benzotriazin-3-yl)-1,10-phenanthroline) was followed by vis-NIR spectroscopy in acetonitrile solution. The solid-state structure of the crystallized product suggests that Pu(iv) is reduced to Pu(iii) upon complexation. Analysis by DFT modeling is consistent with metal-based rather than ligand-based reduction.

Tetrametallic Thorium Compounds with Th4E4 (E = S, Se) Cubane Cores.

Tetrametallic thorium compounds with a Th4E4 core (E = S, Se) having a distorted cubane structure can be prepared by ligand-based reductions of elemental E with thorium chalcogenolates, prepared by in situ oxidation of Th metal with a 3:1 mixture of PhEEPh and F5C6EEC6F5. Four compounds, (py)8Th4S4(μ2-SPh)4(SC6F5)4, (py)8Th4S4(μ2-SPh)4(SeC6F5)4, (py)8Th4Se4(μ2-SePh)4(SeC6F5)4, and (py)8Th4Se4(μ2-SePh)4(SC6F5)4, were isolated and characterized by NMR spectroscopy and X-ray diffraction. These compounds clearly demonstrate the chemical impact of ring fluorination, with the less-nucleophilic EC6F5 ligands occupying the terminal binding sites and the EPh ligands bridging two metal centers. For this series of compounds, crystal packing and intermolecular π···π and H-bonding interactions result in a consistent motif and crystallization in a body-centered tetragonal unit cell. Solution-state 77Se NMR spectroscopy reveals that the solid-state structures are maintained in pyridine.

Electrocatalytic reduction of CO2 using rhenium complexes with dipyrido[3,2-a:2′,3′-c]phenazine ligands

A series of rhenium(I) complexes (1–4) with substituted dipyrido[3,2-a:2′,3′-c]phenazine (dppz) ligands were synthesized and analyzed as electrocatalysts for CO2 reduction. Cyclic voltammetry study shows that complexes 1–4 exhibit dppz ligand-based and metal center-based quasi-reversible reductions under N2. Under a CO2 atmosphere, complexes 1–4 exhibit electrocatalytic response consistent with CO2→CO reduction with the presence of the gaseous phase product CO confirmed by gas chromatography (GC). Re(2,2′-bpy)(CO)3Br (Lehn catalyst), a benchmark for electrocatalytic reduction of CO2 to CO, was also prepared and tested for electrocatalytic properties for comparison. The results suggest that chemical modification on the dppz ligand leads to interesting electrocatalytic properties and the large conjugation of the dppz ligand can help store multiple electrons and lower the energy barrier for CO2 reduction.

Chromium(III) Bis-Arylterpyridyl Complexes with Enhanced Visible Absorption via Incorporation of Intraligand Charge-Transfer Transitions.

A series of chromium(III) bis-arylterpyridyl complexes containing intraligand charge-transfer (ILCT) excited states were prepared and characterized. These complexes show significant absorption in the visible region due to the ILCT bands. The ILCT bands are tunable across the UV and visible spectrum via incorporation of electron-withdrawing and electron-donating groups on the aryl ring. The absorption of Cr(4'-(4-methoxyphenyl)-2,2':6',2″-terpyridine)23+ (4) in particular is much stronger in the visible region (ε = 11 900 M-1 cm-1 at 450 nm and ε = 5090 M-1 cm-1 at 500 nm) than that of the parent complex Cr(tpy)23+ (tpy = 2,2':6',2″-terpyridine; ε = 2160 M-1 cm-1 at 450 nm, and ε = 170 M-1 cm-1 at 500 nm). Emission experiments on this series reveal Cr(III)-based phosphorescence with lifetimes from 140 to 600 ns upon excitation into the ILCT bands, which indicates funneling of the excitation energy from ligand-localized excited states to Cr(III)-based excited states. Cyclic voltammograms exhibit at least three reversible ligand-based reductions. The first reduction shows shifts of up to -160 mV compared to Cr(tpy)23+. The excited-state reduction potential of these complexes ranges from +0.95 to +1.04 V vs the ferrocene/ferrocenium couple, making them potent photooxidants.

Osmium-hydride-carbonyl complex with thioether containing Schiff base ligand: Synthesis, crystal structure, electrochemistry and catalytic transfer hydrogenation

A new osmium(II) carbonyl complex, [Os(H)(CO)(PPh3)2(L)] (1) with thioether containg NNS donor ligand (where L = 2-(ethylthio)-N-((pyridine-2-yl)methylene))benzenamine) is synthesized and characterized by various spectroscopic techniques. The distorted octahedral environment around osmium in the complex is confirmed by single crystal X-ray structure. Cyclic voltammogram in acetonitrile exhibits Os(II)/Os(III) oxidation and ligand based reduction. The complex effectively catalyzed the transfer hydrogenation reaction of ketones in high yields in i-PrOH. The electronic structure and redox properties are interpreted by DFT studies.

Tetra(3-(1,5-diphenyl-4,5-dihydro-1H-pyrazol-3-yl)phenoxy) substituted cobalt, iron and manganese phthalocyanines: Synthesis and electrochemical analysis

This work reports on the synthesis, characterization of new symmetrical substituted cobalt (II) (4), iron (II) (5) and manganese (III) (6) phthalocyanines bearing tetra 3-(1,5-diphenyl-4,5-dihydro-1H-pyrazol-3-yl)phenoxy groups and electrochemical properties of 4–6. The new compounds (3–6) have been characterized by IR, NMR spectroscopy, electronic spectroscopy and mass spectroscopic techniques. Electrochemical studies of cobalt (II) (4), iron (II) (5) and manganese (III) (6) phthalocyanines were examined by using cyclic voltammetry (CV) and square wave voltammetry (SWV) techniques. Voltammetric analyses of phthalocyanines supported the proposed structure of the complexes. Electrochemical studies reveal that all phthalocyanines gave metal and ligand based reduction process during the cathodic potential scans, but they were electropolymerized on the working electrode during the anodic potential scan.