Redox-active Ligand Research Articles

Tailoring the redox capabilities of organic ligands for metal-ligand coordination with vanadium single-sites

The creation of single-site metal centers (SSMCs) through the formation of metal-organic coordination networks is an area of interest due to the proven ability of SSMCs to improve selectivity for heterogeneous catalysts. In order to better understand the reactivity potential for the SSMCs it is necessary to study the ligand-metal interaction in the metal-organic coordination networks. In the work reported here, we demonstrate the ability to tune the oxidation state of vanadium from II to IV through the tailoring of redox-active ligands. Using the N-heterocyclic ligands of bipyrimidine (BP), bispyrimidinyltetrazine (BMTZ), and biimidazole (BIM) complexed with metallic V, we have shown that the oxidation state of the V metal centers can be tuned to V(II) for BP, V(III) for BMTZ, and V(IV) for BIM. These redox-active ligands provide similar coordination environments when complexed into one dimensional chains but result in different oxidation states for the single-site metal center.

Periphery Modification of Tetrathiafulvalenes: Recent Development and Applications.

Tetrathiafulvalene (TTF) and its analogs are fascinating molecules in materials science based on their excellent electron-donating abilities. This personal account describes recent advances in the synthesis of TTF analogs for functional materials via the palladium-catalyzed modification of peripheries of TTF analogs. We first consider three types of molecules: fluorophore-TTF hybrid molecules, multi-redox systems, and an organic ligand for metal-organic frameworks. These molecules were successfully synthesized via Stille coupling or palladium-catalyzed direct C-H arylation and their structural, electrochemical, and optical properties were clarified. Subsequently, phosphorus-substituted TTF analogs were successfully synthesized for future applications of redox-active phosphine ligands for metal catalysts. The development of these molecules can significantly affect the advancement of chemical science.

Iron-Based, Symmetric, Non-Aqueous Redox Flow Battery

The redox flow battery (RFB) is a promising technology for the long-duration storage of energy from domestic- to utility-scale. A RFB stores charge in a liquid electrolyte that is charged and discharged in an electrochemical cell reactor. Since storage and reaction site are separated, energy and power of the system can be scaled independently via electrolyte tank and cell stack dimensions. Iron as one of earth’s most abundant elements is an attractive active material, especially in symmetric RFBs using the same electrolyte on both negative and positive side of the battery. One way to realize such a system is by combining iron with redox-active ligands in non-aqueous solvents.[1],[2] We study the iron tris(2,2’-bipyridine) complex which shows a metal-centred reversible oxidation and three ligand-based reductions that are solvent-dependent.[3] The influence of electrolyte composition on charge and discharge reactions as well as on viscosity and conductivity is further investigated. Charge-discharge cycling experiments are carried out in 3-electrode H-cell and flow cell setups. We determine failure modes for the positive side due to electrode kinetics and for the negative side due to solubility changes. We propose improvements to this non-aqueous RFB system based on the selection of electrode materials and cycling conditions.[1] J. Mun et al., J. Electrochem. Soc. 2018, 165, 215-219.[2] C. X. Cammack et al., DaltonTrans . 2021, 50, 858.[3] D. M. Cabral, P. C. Howlett, D. R. MacFarlane, Electrochim. Acta 2016, 220, 347-353.

Electrochemistry of Innocent Cyanocobalt Corroles

The electrochemical and spectroscopic properties of numerous transition metal corroles have been examined under a wide variety of solution conditions with particular interest being placed on the innocent or non-innocent nature of the corrole macrocycle and oxidation state of the central metal ion. The innocence of the redox active corrole ligand, as well as the half-wave potentials and sites of electron transfer are known to be strongly influenced by the type of central metal ion, degree of axial coordination (i.e. four-, five- or six-coordinate) and type of peripheral substituents at the meso or β-pyrrole positions of the corrole ligand. Our laboratories have previously investigated five- and six-coordinate cobalt corroles with P, N or O donor ligands and a wide variety of peripheral corrole substituents. In this work, we describe the intriguing electrochemical and spectroscopic properties of several innocent cobalt(III) corroles which are axially ligated by two pyridine molecules, one or two cyanide ions or one pyridine and one cyanide ion. Examples of the investigated corroles are shown in Chart 1 and represented as [(R)3CorCo(Py)2], [(R)3CorCo(Py)(CN)]-, [(R)3CorCo(CN)]- and [(R)3CorCo(CN)2]2- where (R)3Cor is the corrole macrocycle and R is one of several meso-substituents. Figure 1

2Fe-2S] Cluster Supported by Redox-Active o-Phenylenediamide Ligands and Its Application toward Dinitrogen Reduction.

As prevalent cofactors in living organisms, iron-sulfur clusters participate in not only the electron-transfer processes but also the biosynthesis of other cofactors. Many synthetic iron-sulfur clusters have been used in model studies, aiming to mimic their biological functions and to gain mechanistic insight into the related biological systems. The smallest [2Fe-2S] clusters are typically used for one-electron processes because of their limited capacity. Our group is interested in functionalizing small iron-sulfur clusters with redox-active ligands to enhance their electron storage capacity, because such functionalized clusters can potentially mediate multielectron chemical transformations. Herein we report the synthesis, structural characterization, and catalytic activity of a diferric [2Fe-2S] cluster functionalized with two o-phenylenediamide ligands. The electrochemical and chemical reductions of such a cluster revealed rich redox chemistry. The functionalized diferric cluster can store up to four electrons reversibly, where the first two reduction events are ligand-based and the remainder metal-based. The diferric [2Fe-2S] cluster displays catalytic activity toward silylation of dinitrogen, affording up to 88 equiv of the amine product per iron center.

Charge frustration in ligand design and functional group transfer.

Molecules with different resonance structures of similar importance, such as heterocumulenes and mesoionics, are prominent in many applications of chemistry, including 'click chemistry', photochemistry, switching and sensing. In coordination chemistry, similar chameleonic/schizophrenic entities are referred to as ambidentate/ambiphilic or cooperative ligands. Examples of these had remained, for a long time, limited to a handful of archetypal compounds that were mere curiosities. In this Review, we describe ambiphilicity - or, rather, 'charge frustration' - as a general guiding principle for ligand design and functional group transfer. We first give a historical account of organic zwitterions and discuss their electronic structures and applications. Our discussion then focuses on zwitterionic ligands and their metal complexes, such as those of ylidic and redox-active ligands. Finally, we present new approaches to single-atom transfer using cumulated small molecules and outline emerging areas, such as bond activation and stable donor-acceptor ligand systems for reversible 1e- chemistry or switching.

Accurate fragment-based 51-V chemical shift predictions in molecular crystals

Nuclear magnetic resonance (NMR) spectroscopy plays a crucial role in determining molecular structure for complex biological and pharmaceutical compounds. NMR investigations are increasingly reliant on computation for mapping spectral features to chemical structures. Here we benchmark the accuracy of fragment-based 51V chemical shielding tensor calculations using a training set comprised of 10 biologically and pharmaceutically relevant oxovanadium complexes. Using our self-consistent reproduction of the Madelung potential (SCRMP) electrostatic embedding model, we demonstrate comparable performance between fragment methods and computationally demanding cluster-based techniques. Specifically, fragment methods employing hybrid density functionals are capable of reproducing the experimental 51V isotropic chemical shifts with a training set rms error of ~9 ​ppm, representing a 20% improvement over traditional plane wave techniques. We provide training set-derived linear regression models for mapping the absolute shieldings obtained from computation to the experimentally determined chemical shifts using four common density functionals; PBE0, B3LYP, PBE, and BLYP. Finally, we establish the utility of fragment methods and the reported regression parameters examining four oxovanadium structures excluded from the training set including the tetracoordinate oxovanadium silicate (Ph3SiO)3VO, VO15NGlySalbz which contains redox-active ligands, and the solid-state form of the common 51V NMR reference compound VOCl3.

Acute Toxicity Evaluation of Non-Innocent Oxidovanadium(V) Schiff Base Complex

The vanadium(V) complexes have been investigated as potential anticancer agents which makes it essential to evaluate their toxicity for safe use in the clinic. The large-scale synthesis and the acute oral toxicity in mice of the oxidovanadium(V) Schiff base catecholate complex, abbreviated as [VO(HSHED)dtb] containing a redox-active ligand with tridentate Schiff base (HSHED = N-(salicylideneaminato)-N’-(2-hydroxyethyl)-1,2-ethylenediamine) and dtb = 3,5-di-(t-butyl)catechol ligands were carried out. The body weight, food consumption, water intake as well biomarkers of liver and kidney toxicity of the [VO(HSHED)dtb] were compared to the precursors, sodium orthovanadate, and free ligand. The 10-fold scale-up synthesis of the oxidovanadium(V) complex resulting in the preparation of material in improved yield leading to 2–3 g (79%) material suitable for investigating the toxicity of vanadium complex. No evidence of toxicity was observed in animals when acutely exposed to a single dose of 300 mg/kg for 14 days. The toxicological results obtained with biochemical and hematological analyses did not show significant changes in kidney and liver parameters when compared with reference values. The low oral acute toxicity of the [VO(HSHED)dtb] is attributed to redox chemistry taking place under biological conditions combined with the hydrolytic stability of the oxidovanadium(V) complex. These results document the design of oxidovanadium(V) complexes that have low toxicity but still are antioxidant and anticancer agents.

Mononuclear and Dinuclear Copper Complexes of Tridentate Redox-active Ligands with Tunable H-bonding Donors: Structure, Spectroscopy and H+ /e- Reactivity.

In this research article, we describe the synthesis and characterization of mononuclear and dinuclear Cu complexes bound by a family of tridentate redox-active ligands with tunable H-bonding donors. The mononuclear Cu-anion complexes were oxidized to the corresponding "high-valent" intermediates by oxidation of the redox-active ligand. These species were capable of oxidizing phenols with weak O-H bonds via H-atom abstraction. Thermodynamic analysis of the H-atom abstractions, which included reduction potential measurements, pKa determination and kinetic studies, revealed that modification of the anion coordinated to the Cu and changes in the H-bonding donor did not lead to major differences in the reactivity of the "high-valent" CuY complexes (Y: hydroxide, phenolate and acetate), which indicated that the tridentate ligand scaffold acts as the H+ and e- acceptor.

Valence tautomeric transition of bis(o-dioxolene) cobalt complex in solid state and solution

Valence tautomer transition occurs mainly in 3d metalorganic complexes with redox-active ligands and makes them potential candidates for single-molecular switches. The transition occurs under temperature, pressure, or light-induced stimuli and is strongly affected by the intermolecular interactions. However single-crystal x-ray diffraction is not always applicable to such systems when crystal structure is destroyed upon transition or system is studied in the solution. Such an example is bis(o-semiquinonato) cobalt complex with TEMPO-functionalized iminopyridine ancillary ligand. In this work we apply two complementary techniques—ligand-sensitive Fourier transform infrared spectroscopy (FTIR) and metal sensitive Co K-edge x-ray absorption spectroscopy (XAS). In a solid state, a temperature hysteresis of magnetization larger than 40 K was observed upon cyclic cooling-heating. So, the temperature of phase transition upon cooling is about 40 K lower than that upon heating. In solution, the x-ray absorption spectra for high-temperature and low-temperature states were similar to that in the solid form, but the hysteresis was absent. Two methods are can probe valence tautomer transition, but XAS has an advantage for the liquid phase analysis and FTIR has larger sensitivity to the ligand related interactions in solid.

Graphite Conjugation of a Macrocyclic Cobalt Complex Enhances Nitrite Electroreduction to Ammonia

This work reports on the generation of a graphite-conjugated diimine macrocyclic Co catalyst (GCC-CoDIM) that is assembled at o-quinone edge defects on graphitic carbon electrodes. X-ray photoelectron spectroscopy and X-ray absorption spectroscopy confirm the existence of a new Co surface species with a coordination environment that is the same as that of the molecular analogue, [Co(DIM)Br2]+. GCC-CoDIM selectively reduces nitrite to ammonium with quantitative Faradaic efficiency and at a rate that approaches enzymatic catalysis. Preliminary mechanistic investigations suggest that the increased rate is accompanied by a change in mechanism from the molecular analogue. These results provide a template for creating macrocycle-based electrocatalysts based on first-row transition metals conjugated to an extreme redox-active ligand.

Controlled Single-Electron Transfer via Metal-Ligand Cooperativity Drives Divergent Nickel-Electrocatalyzed Radical Pathways.

Electrocatalysis enables the construction of C-C bonds under mild conditions via controlled formation of carbon-centered radicals. For sequences initiated by alkyl halide reduction, coordinatively unsaturated Ni complexes commonly serve as single-electron transfer agents, giving rise to the foundational question of whether outer- or inner-sphere electron transfer oxidative addition prevails in redox mediation. Indeed, rational design of electrochemical processes requires the discrimination of these two electron transfer pathways, as they can have outsized effects on the rate of substrate bond activation and thus impact radical generation rates and downstream product selectivities. We present results from combined synthetic, electroanalytical, and computational studies that examine the mechanistic differences of single electron transfer to alkyl halides imparted by Ni metal-ligand cooperativity. Electrogenerated reduced Ni species, stabilized by delocalized spin density onto a redox-active tpyPY2Me polypyridyl ligand, activates alkyl iodides via outer-sphere electron transfer, allowing for the selective activation of alkyl iodide substrates over halogen atom donors and the controlled generation and sequestration of electrogenerated radicals. In contrast, the Ni complex possessing a redox-innocent pentapyridine congener activates the substrates in an inner-sphere fashion owning to a purely metal-localized spin, thereby activating both substrates and halogen atom donors in an indiscriminate fashion, generating a high concentration of radicals and leading to unproductive dimerization. Our data establish that controlled electron transfer via Ni-ligand cooperativity can be used to limit undesired radical recombination products and promote selective radical processes in electrochemical environments, providing a generalizable framework for designing redox mediators with distinct rate and potential requirements.

Redox-active ligands: Recent advances towards their incorporation into coordination polymers and metal-organic frameworks

• Developments in coordination polymers (CPs) and metal–organic frameworks (MOFs). • Electroactive CPs and MOFs possess unique redox-state dependent properties. • An exploration of key redox-active ligands investigated in the past eight years. • Emerging redox-active ligands inspired by organic photovoltaic devices (OPVs). • CP and MOFs with applications in solar cells, photo switches and photoelectrocatalysis. Over the past three decades, the field of coordination polymers (CPs) and metal–organic frameworks (MOFs) has developed rapidly due to the potential applications of these materials in gas storage, separations, catalysis and switching. Introducing redox properties into CPs and MOFs to electrochemically modulate their properties for the development of solid state electronic devices has been an interesting strategy applied in recent years. Notably, a challenge within this area is the engineering of framework materials with desired redox properties targeted at a specific function. This review explores some of the key redox-active ligands that have been investigated within the previous eight years as redox-active components of solid state materials and examines a number of key strategies for their integration into CPs and MOFs. This review further highlights emergent redox-active ligands in the literature, which take inspiration from the rich field of organic electronics. While these ligands remain largely unexplored in the field of MOFs and CPs, they offer new opportunities for the improvement of solar cells, light induced photo switches, efficient photoelectrocatalysts and long range, rapid charge transfer in MOF materials.

Buffer anion effects on water oxidation catalysis: The case of Cu(III) complex

Water oxidation is the bottleneck of artificial photosynthesis. Since the first ruthenium-based molecular water oxidation catalyst, the blue dimer, was reported by Meyer's group in 1982, catalysts based on transition metals have been widely employed to explore the mechanism of water oxidation. Because the oxidation of water requires harsh oxidative conditions, the stability of transition complexes under the relevant catalytic conditions has always been a challenge. In this work, we report the redox properties of a CuIII complex (TAML-CuIII) with a redox-active macrocyclic ligand (TAML) and its reactivity toward catalytic water oxidation. TAML-CuIII displayed a completely different electrochemical behavior from that of the TAML-CoIII complex previously reported by our group. TAML-CuIII can only be oxidized by one-electron oxidation of the ligand to form TAML•+-CuIII and cannot achieve water activation through the ligand-centered proton-coupled electron transfer that takes place in the case of TAML-CoIII. The generated TAML•+-CuIII intermediate can undergo further oxidation and ligand hydrolysis with the assistance of borate anions, triggering the formation of a heterogeneous B/CuOx nanocatalyst. Therefore, the choice of the buffer solution has a significant influence on the electrochemical behavior and stability of molecular water oxidation catalysts.

Exploring Ligand-Centered Hydride and H-Atom Transfer.

The nickel(II) complex [ON(H)O]Ni(PPh3) ([ON(H)O]2- = bis(3,5-di-tert-butyl-2-phenoxy)amine), bearing a protonated redox-active ligand, was examined for its ability to serve as a hydrogen atom (H•) and hydride (H-) donor. Deprotonation of [ON(H)O]Ni(PPh3) afforded the square-planar anion {[ONOcat]Ni(PPh3)}1-, whereas hydrogen atom transfer from [ON(H)O]Ni(PPh3) to TEMPO• in the presence of added PPh3 afforded five-coordinate [ONO]Ni(PPh3)2 that has been structurally characterized. In solution, this five-coordinate complex exists in equilibrium with four-coordinate [ONO]Ni(PPh3), and this ligand exchange equilibrium correlates with a valence tautomerization between the redox-active ligand and the nickel center. Abstraction of a hydride from [ON(H)O]Ni(PPh3) in the presence of PPh3 afforded the octahedral complex, [ONOq]Ni(OTf)(PPh3)2, which was characterized as an S = 1, nickel(II) complex. Bond dissociation free energy (BDFE) and hydricity (ΔG°H-) measurements benchmark the thermodynamic propensity of this complex to participate in ligand-centered H• and H- transfer reactions.

Stimulation of Redox-Induced Electron Transfer by Interligand Hydrogen Bonding in a Cobalt Complex with Redox-Active Guanidine Ligand.

Octahedrally coordinated cobalt(II) complexes with a redox‐active bisguanidine ligand and acac co‐ligands were synthesized and their redox chemistry analysed in detail. The N−H functions in a bisguanidine ligand with partially alkylated guanidino groups form N−H⋅⋅⋅O hydrogen bonds with the acac co‐ligands, thereby massively influencing the redox chemistry. For all complexes, the first one‐electron oxidation is metal‐centred, leading to CoIII complexes with neutral bisguanidine ligand units. Further one‐electron oxidation is ligand‐centred in the case of Co–bisguanidine complexes with fully alkylated guanidino groups, giving CoIII complexes with radical monocationic bisguanidine ligands. On the other hand, the hydrogen‐bond strengthening upon oxidation of the Co–bisguanidine complex with partially alkylated guanidino groups initiates metal reduction (CoIII→CoII) and two‐electron oxidation of the guanidine ligand, providing the first example for the stimulation of redox‐induced electron transfer by interligand hydrogen bonding.

Redox-Active Ligands in Electroassisted Catalytic H+ and CO2 Reductions: Benefits and Risks

In the past decade, the use of redox-active ligands has emerged as a promising strategy to improve catalyst selectivity, efficiency, and stability in electroassisted H+ and CO2 reductions. Partial ...

Iron(II) spin crossover complexes with a sulfur rich ligand backbone

One goal that many scientists pursue is the unification of several interesting chemical or physical properties in one system, as only multifunctional materials will meet the challenges of today's technologies. With this background, three novel iron(II) coordination compounds with a Schiff base-like N2O2 coordinating ligand L bearing a sulfur-rich backbone are investigated in this work. Two of the complexes, the mononuclear [FeL(py)2] (py = pyridine) and the coordination polymer {[FeL(bpee)]}n [bpee = trans-1,2-bis(4-pyridyl)ethene], show spin crossover behavior followed using magnetic susceptibility measurements and Mössbauer spectroscopy. To get a closer insight into different states of the complex spin crossover behavior of {[FeL(bpee)]}n, XPS measurements were conducted at different temperatures. Furthermore, impedance spectroscopic measurements at variable temperatures were performed to get insight into the electrical conductivity of this system. All iron(II) complexes were electrochemically characterized using cyclovoltammetric measurements, supplemented by DFT computation. Apparently, the extension by a sulfur backbone leads to a stabilization of the HOMO. Due to this, the complexes are more difficult to oxidize than comparable systems. With {[FeL(azpy)]}n, another coordination polymer, this time axially decorated by the redox-active ligand azpy (azpy = 4,4′-azopyridine), is investigated, which is, however, a pure high spin complex.

Iron(II), Cobalt(II), and Nickel(II) Complexes of Bis(sulfonamido)benzenes: Redox Properties, Large Zero-Field Splittings, and Single-Ion Magnets.

Metal complexes of 1,2-diamidobenzenes have been long studied because of their intriguing redox properties and electronic structures. We present here a series of such complexes with 1,2-bis(sulfonamido)benzene ligands to probe the utility of these ligands for generating a large zero-field splitting (ZFS, D) in metal complexes that possibly act as single-ion magnets. To this end, we have synthesized a series of homoleptic ate complexes of the form (X)n[M{bis(sulfonamido)benzene}2] (n equals 4 minus the oxidation state of the metal), where M (Fe/Co/Ni), X [K+/(K-18-c-6)+/(HNEt3)+, with 18-c-6 = 18-crown ether 6], and the substituents (methyl and tolyl) on the ligand [bmsab = 1,2-bis(methanesulfonamido)benzene; btsab = 1,2-bis(toluenesulfonamido)benzene] were varied to analyze their effect on the ZFS, possible single-ion-magnet properties, and redox behavior of these metal complexes. A combination of X-ray crystallography, (spectro)electrochemistry, superconducting quantum interference device magnetometry, high-frequency electron paramagnetic resonance spectroscopy, and Mössbauer spectroscopy was used to investigate the electronic/geometric structures of these complexes and the aforementioned properties. These investigations show that the cobalt(II) complexes display very high negative D values in the range of -100 to -130 cm-1, and the nickel(II) complexes display very high positive D values of 76 and 58 cm-1. In addition, the cobalt(II) complexes shows barriers of 200-260 cm-1 and slow relaxation of the magnetization in the absence of an external magnetic field, underscoring the robustness of this class of complexes. The iron(II) complex exhibits a D value of -3.29 cm-1 and can be chemically oxidized to an iron(III) complex that has D = -1.96 cm-1. These findings clearly show that bis(sulfonamido)benzenes are ideally suited to stabilize ate complexes, to generate very high ZFSs at the metal centers with single-ion-magnet properties, and to induce exclusive oxidation at the metal center (for iron) despite the presence of ligands that are potentially noninnocent. Our results therefore substantially enhance the scope for this class of redox-active ligands.

Metal-Organic Frameworks Derived from Calcium and Strontium Complexes of a Redox-Active Ligand.

The reactions of monomeric [(dpp-Bian)M(thf)4] (M = Ca (1a), Sr (1b); dpp-Bian = 1,2-bis[(2,6-diisopropylphenyl)imino]acenaphthene) with 4,4'-bipyridyl (4,4'-bipy) proceed with electron transfer from dpp-Bian2- to 4,4'-bipy0 to afford calcium and strontium complexes containing simultaneously radical-anionic dpp-Bian- and 4,4'-bipy- ligands. In tetrahydrofuran (thf) the reactions result in 1D coordination polymers [{(dpp-Bian)M(4,4'-bipy)(thf)2}·4thf]n (M = Ca (2a), Sr (2b)), while in a thf/benzene mixture the reaction between 1a and 4,4'-bipy affords the 2D metal-organic framework [{(dpp-Bian)Ca(4,4'-bipy)2}·2thf·2C6H6]n (3). The structures of compounds 2a,b and 3 have been determined by single-crystal X-ray analyses. The presence of the ligand-localized unpaired electrons allows the use of ESR spectroscopy for characterization of the compounds 2a,b and 3. DFT calculations of model calcium complexes with the dpp-Bian, 4,4'-bipy, and thf ligands confirm the energetically favorable open-shell configurations of the molecules bearing the 4,4'-bipy fragments. The magnetic susceptibility measurements confirm the presence of two unpaired electrons per monomeric unit in 2a,b and 3. The thermal stability of compounds 2a,b and 3 was studied by thermogravimetric analysis (TGA). To the best of our knowledge, 3 is the first MOF simultaneously containing two different paramagnetic bridging ligands inside the framework.