Reversible Redox Events Research Articles

Indenoannulated Tridecacyclene: An All-Carbon Seven-Stage Redox-Amphoter.

We disclose an indenoannulated tridecacyclene comprising a central cyclooctatetraene moiety with multiple adjacent pentagonal rings which is accessible in a concise synthetic sequence. The saddle-shaped geometry of the non-benzenoid polycyclic scaffold and its unique packing behavior in the solid state were characterized by X-ray crystallography. In electrochemical studies, the compound undergoes seven reversible redox events comprising five reductions and two oxidations. The dicationic and dianionic species obtained by chemical oxidation and reduction, respectively, were characterized spectroscopically in solution. Density functional theory calculations were applied to provide insights into aromaticity evolution in the respective charged species, highlighting the beneficial effect of the non-benzenoid moieties on charge stabilization.

Diffusional Electron Transport Coupled to Thermodynamically Driven Electron Transfers in Redox-Conductive Multivariate Metal-Organic Frameworks.

The development of redox-conductive metal-organic frameworks (MOFs) and the fundamental understanding of charge propagation through these materials are central to their applications in energy storage, electronics, and catalysis. To answer some unresolved questions about diffusional electron hopping transport and redox conductivity, mixed-linker MOFs were constructed from two statistically distributed redox-active linkers, pyromellitic diimide bis-pyrazolate (PMDI) and naphthalene diimide bis-pyrazolate (NDI), and grown as crystalline thin films on conductive fluorine-doped tin oxide (FTO). Owing to the distinct redox properties of the linkers, four well-separated and reversible redox events are resolved by cyclic voltammetry, and the mixed-linker MOFs can exist in five discrete redox states. Each state is characterized by a unique spectroscopic signature, and the interconversions between the states can be followed spectroscopically under operando conditions. With the help of pulsed step-potential spectrochronoamperometry, two modes of electron propagation through the mixed-linker MOF are identified: diffusional electron hopping transport between linkers of the same type and a second channel that arises from thermodynamically driven electron transfers between linkers of different types. Corresponding to the four redox events of the mixed-linker MOFs, four distinct bell-shaped redox conductivity profiles are observed at a steady state. The magnitude of the maximum redox conductivity is evidenced to be dependent on the distance between redox hopping sites, analogous to the situation for apparent electron diffusion coefficients, Deapp, that are obtained in transient experiments. The design of mixed-linker redox-conductive MOFs and detailed studies of their charge transport properties present new opportunities for future applications of MOFs, in particular, within electrocatalysis.

Homotrinuclear ruthenium(ii) and rhodium(i) complexes of redox-active tris(ferrocenyl)arene-based tris-phosphanes.

Homotrinuclear complexes of the C 3-symmetric tris(ferrocenyl)arene-based tris-phosphanes 1a-d with ruthenium(ii) ([1a-d(Ru)3]) and rhodium(i) ([1a-d(Rh)3]) were prepared and fully characterised. Complexes [1a-d(Ru)3] and [1a-d(Rh)3] are electrochemically active. The nature of the arene core in 1a-d ranging from benzene, 1,3,5-trifluorobenzene and mesitylene to s-triazine allows to fine-tune the exact oxidation potentials for tailoring the electrochemical response. With a BArF 4 --based supporting electrolyte, a distinct separation of the three iron-centred oxidations of the ligand backbone is observable. Under these conditions, these oxidations are mostly reversible but, especially for the third oxidation, already show signs of irreversibility. In general, while the coordinated metal complex fragment does not strongly alter the electrochemical response of the arene-trisferrocenyl core 1a-d, there are observable differences. Rhodium(i) complexes are oxidised at slightly higher potentials than ruthenium(ii) complexes. In both cases, individual oxidation states for the C6H3(CH2)3-based ligand (1d) are difficult to address and the C3N3-based ligand (1c) shows the most complicated and least reversible electrochemistry with severely broadened third oxidations and reduced reversibility in cyclic voltammetry. The most well-suited system for potential applications in redox-switchable catalysis, in all cases, is the C6H3-based ligand (1a), showing entirely reversible and well-separated redox events.

A Conformationally Stable π-Expanded X-Type Double Helicene Comprising Dihydropyracylene Units with Multistage Redox Behavior.

A π-expanded X-type double [5]helicene comprising dihydropyracylene moieties was synthesized from commercially available acenaphthene. X-ray crystallographic analysis revealed the unique highly twisted structure of the compound resulting in the occurrence of two enantiomers which were separated by chiral HPLC, owing to their high conformational stability. The compound shows strongly bathochromically shifted UV/vis absorption and emission bands with small Stokes shift and considerable photoluminescence quantum yield and circular polarized luminescence response. The electrochemical studies revealed five facilitated reversible redox events, including three reductions and two oxidations, thus qualifying the compound as chiral multistage redox amphoter. The experimental findings are in line with the computational studies based on density functional theory pointing towards increased spatial extension of the frontier molecular orbitals over the polycyclic framework and a considerably narrowed HOMO-LUMO gap.

Synthesis and Properties of Fluorenone-Containing Cycloparaphenylenes and Their Late-Stage Transformation.

Cycloparaphenylenes (CPPs) are the smallest possible armchair carbon nanotubes, the properties of which strongly depend on their ring size. They can be further tuned by either peripheral functionalization or by replacing phenylene rings for other aromatic units. Here we show how four novel donor-acceptor chromophores were obtained by incorporating fluorenone or 2-(9H-fluoren-9-ylidene)malononitrile into the loops of two differently sized CPPs. Synthetically, we managed to perform late-stage functionalization of the fluorenone-based rings by high-yielding Knoevenagel condensations. The structures were confirmed by X-ray crystallographic analyses, which revealed that replacing a phenylene for a fused-ring-system acceptor introduces additional strain. The donor-acceptor characters of the CPPs were supported by absorption and fluorescence spectroscopic studies, electrochemical studies (displaying the CPPs as multi-redox systems undergoing reversible or quasi-reversible redox events), as well as by computations. The oligophenylene parts were found to comprise the electron donor units of the macrocycles and the fluorenone parts the acceptor units.

Lewis Acid Supported Nickel Nitrenoids.

Metalation of the polynucleating ligand F,tbs LH6 (1,3,5-C6 H9 (NC6 H3 -4-F-2-NSiMe2 t Bu)3 ) with two equivalents of Zn(N(SiMe3 )2 )2 affords the dinuclear product (F,tbs LH2 )Zn2 (1), which can be further deprotonated to yield (F,tbs L)Zn2 Li2 (OEt2 )4 (2). Transmetalation of 2 with NiCl2 (py)2 yields the heterometallic, trinuclear cluster (F,tbs L)Zn2 Ni(py) (3). Reduction of 3 with KC8 affords [KC222 ][(F,tbs L)Zn2 Ni] (4) which features a monovalent Ni centre. Addition of 1-adamantyl azide to 4 generates the bridging μ3 -nitrenoid adduct [K(THF)3 ][(F,tbs L)Zn2 Ni(μ3 -NAd)] (5). EPR spectroscopy reveals that the anionic cluster possesses a doublet ground state (S = ). Cyclic voltammetry of 5 reveals two fully reversible redox events. The dianionic nitrenoid [K2 (THF)9 ][(F,tbs L)Zn2 Ni(μ3 -NAd)] (6) was isolated and characterized while the neutral redox isomer was observed to undergo both intra- and intermolecular H-atom abstraction processes. Ni K-edge XAS studies suggest a divalent oxidation state for the Ni centres in both the monoanionic and dianionic [Zn2 Ni] nitrenoid complexes. However, DFT analysis suggests Ni-borne oxidation for 5.

Lewis Acid Supported Nickel Nitrenoids

AbstractMetalation of the polynucleating ligand F,tbsLH6 (1,3,5‐C6H9(NC6H3−4‐F−2‐NSiMe2tBu)3) with two equivalents of Zn(N(SiMe3)2)2 affords the dinuclear product (F,tbsLH2)Zn2 (1), which can be further deprotonated to yield (F,tbsL)Zn2Li2(OEt2)4 (2). Transmetalation of 2 with NiCl2(py)2 yields the heterometallic, trinuclear cluster (F,tbsL)Zn2Ni(py) (3). Reduction of 3 with KC8 affords [KC222][(F,tbsL)Zn2Ni] (4) which features a monovalent Ni centre. Addition of 1‐adamantyl azide to 4 generates the bridging μ3‐nitrenoid adduct [K(THF)3][(F,tbsL)Zn2Ni(μ3‐NAd)] (5). EPR spectroscopy reveals that the anionic cluster possesses a doublet ground state (S = ). Cyclic voltammetry of 5 reveals two fully reversible redox events. The dianionic nitrenoid [K2(THF)9][(F,tbsL)Zn2Ni(μ3‐NAd)] (6) was isolated and characterized while the neutral redox isomer was observed to undergo both intra‐ and intermolecular H‐atom abstraction processes. Ni K‐edge XAS studies suggest a divalent oxidation state for the Ni centres in both the monoanionic and dianionic [Zn2Ni] nitrenoid complexes. However, DFT analysis suggests Ni‐borne oxidation for 5.

Electrochromism in Isoreticular Metal-Organic Framework Thin Films with Record High Coloration Efficiency.

The power of isoreticular chemistry has been widely exploited to engineer metal-organic frameworks (MOFs) with fascinating molecular sieving and storage properties but is underexplored for designing MOFs with tunable optoelectronic properties. Herein, three dipyrazole-terminated XDIs (X = PM (pyromellitic), N (naphthalene), or P (perylene); DI = diimide) with different lengths and electronic properties are prepared and employed as linkers for the construction of an isoreticular series of Zn-XDI MOFs with distinct electrochromism. The MOFs are grown on fluorine-doped tin oxide (FTO) as high-quality crystalline thin films and characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). Due to the constituting electronically isolated XDI linkers, each member of the isoreticular thin film series exhibits two reversible one-electron redox events, each at a distinct electrochemical potential. The orientation of the MOFs as thin films as well as their isoreticular nature results in identical cation-coupled electron hopping transport rates in all three materials, as demonstrated by comparable apparent electron diffusion coefficients, Deapp. Upon electrochemical reduction to either the [XDI]•- or [XDI]2- state, each MOF undergoes characteristic changes in its optical properties as a function of linker length and redox state of the linker. Operando spectroelectrochemistry measurements reveal that Zn-PDI@FTO (PDI = perylene diimide) thin films exhibit a record high coloration efficiency of 941 cm2 C-1 at 746 nm, which is attributed to the maximized Faradaic transformations at each electronically isolated PDI unit. The electrochromic response of the thin film is retained to more than 99% over 100 reduction-oxidation cycles, demonstrating the applicability of the presented materials.

Tetra(peri-naphthylene)anthracene: A Near-IR Fluorophore with Four-Stage Amphoteric Redox Properties.

A novel, benign synthetic strategy towards soluble tetra(peri-naphthylene)anthracene (TPNA) decorated with triisopropylsilylethynyl substituents has been established. The compound is perfectly stable under ambient conditions in air and features intense and strongly bathochromically shifted UV/vis absorption and emission bands reaching to near-IR region beyond 900 nm. Cyclic voltammetry measurements revealed four facilitated reversible redox events comprising two oxidations and two reductions. These remarkable experimental findings were corroborated by theoretical studies to identify the TPNA platform a particularly useful candidate for the development of functional near-IR fluorophores upon appropriate functionalization.

Tromancenium complexes with steric and electronic modulation

A series of new heteroleptic cationic (η7-cycloheptatrienyl)(η5-cyclopentadienyl)manganese(I) (“tromancenium”) sandwich complexes containing the cycloheptatrienyl ligand and various sterically bulky cyclopentadienyl ligands was synthesized in a one-pot two-step procedure via their cymantrene precursors by high-power LED photochemistry followed by oxidation. In comparison to their less-substituted congeners which display only partially reversible redox events, these new air-stable tromancenium salts show reversible Mn(I)/Mn(II) and quasi-reversible Mn(I)/Mn(0) redox couples, as intended by the conceptual design of this work. Full spectroscopic and structural characterization of by 1H, 13C, 55Mn NMR, IR, UV–vis spectroscopy, high-resolution mass spectrometry and single crystal structure analysis is reported.

Dual Emission, Aggregation, and Redox Properties of Boron Difluoride Hydrazones Functionalized with Triphenylamines

Abstractπ‐Conjugated molecular materials often exhibit interesting photoluminescence, redox, and optoelectronic properties that grant them utility as light‐harvesting materials in solar cells, as the emissive component of organic light‐emitting diodes, and as fluorescence sensors. Preparing dye conjugates by appending multiple dyes together to generate π‐conjugated molecules is a common strategy to enhance the properties of these materials as it often imparts traits that cannot be achieved by the building blocks independently. Herein, we report molecular materials that incorporate boron difluoride hydrazone (BODIHY) and triphenylamine (TPA) units that exhibit charge‐transfer character, dual‐emission, and aggregation‐induced emission. The title compounds have low‐energy absorption bands (λabs=455–493 nm) that are red‐shifted relative to BODIHYs with smaller π‐systems and exhibit dual‐emission in the solution, solid, and aggregate states. TPA–BODIHY (donor‐acceptor) conjugates show multiple reversible redox events and, in combination, the data presented herein indicate that there is electronic communication throughout the donor‐acceptor and acceptor‐donor‐acceptor π‐systems. These results are rationalized with the use of computational chemistry and solid‐state structural analysis. This study provides new opportunities for the design of molecular materials that are comprised of redox‐active photoluminescent dyes, including BODIHY, and their potential applications.

Organic Four-Electron Redox Systems Based on Bipyridine and Phenanthroline Carbene Architectures.

Novel organic redox systems that display multistage redox behaviour are highly sought‐after for a series of applications such as organic batteries or electrochromic materials. Here we describe a simple strategy to transfer well‐known two‐electron redox active bipyridine and phenanthroline architectures into novel strongly reducing four‐electron redox systems featuring fully reversible redox events with up to five stable oxidation states. We give spectroscopic and structural insight into the changes involved in the redox‐events and present characterization data on all isolated oxidation states. The redox‐systems feature strong UV/Vis/NIR polyelectrochromic properties such as distinct strong NIR absorptions in the mixed valence states. Two‐electron charge–discharge cycling studies indicate high electrochemical stability at strongly negative potentials, rendering the new redox architectures promising lead structures for multi‐electron anolyte materials.

Organische Vier‐Elektronen Redox‐Systeme basierend auf Bipyridin‐ und Phenanthrolin‐Carben‐Architekturen

AbstractNeuartige organische Redoxsysteme, die ein mehrstufiges Redoxverhalten aufweisen, sind für eine Reihe von Anwendungen von besonderem Interesse, wie z. B. organische Batterien oder elektrochrome Materialien. Hier beschreiben wir eine einfache Synthesestrategie, um bekannte zwei‐Elektronen redoxaktive Bipyridine in stark reduzierende vier‐Elektronen Redox‐Systeme zu überführen. Diese zeichnen sich durch eine vollständig reversible Redoxchemie sowie bis zu fünf isolierbare Oxidationsstufen aus. Wir geben strukturelle und spektroskopische Einblicke in die Redox‐Ereignisse begleitenden Änderungen und präsentieren Charakterisierungsdaten zu allen isolierten Oxidationszuständen. Die Redoxsysteme weisen starke UV/Vis/NIR polyelektrochrome Eigenschaften auf, wie z. B. ausgeprägte, intensive NIR‐Absorptionen in den gemischtvalenten Zuständen. Zwei‐Elektronen Ladungs/Entladungs‐Studien weisen auf eine hohe elektrochemische Stabilität bei stark negativen Potentialen hin, was die neuen Redox‐Architekturen zu vielversprechenden Leitstrukturen für Multi‐Elektronen Anolyt‐Materialien macht.

Heterodi-, -tri- and -tetrametallic Transition-Metal-Complexes

Complexes trans-[Pt(CCMc)2(PBu3)2)] (Mc = Fc = Fe(ƞ5-C5H4)(ƞ5-C5H5) (3), [1,2] Mc = Rc = Ru(ƞ5-C5H4)(ƞ5-C5H5) (4)) were synthesized by treatment of cis-[PtCl2(PBu3)2] with McCCH (Mc = Fc (1), Rc (2)). The synthesis of trans-[Pt(PR3)2(CCFc)(CCRʹ)] (R = Bu: Rʹ = H (7a), Rʹ = PPh2 (7b), Rʹ = Rc (7c); R = Ph: Rʹ = 4-C5H4N (8a), Rʹ = C6H4-4-CN (8b), Rʹ = 2,2ʹ-bipyridin-5-yl (8c), Rʹ = C6H4-4-CCH (8d)) was realized by the reaction of trans-[PtCl(PR3)2(CCFc)] (R = Bu (5), R = Ph (6)) with alkynes HCCRʹ. Treatment of 8a with [RuCl2(cymene)]2 (cymene = η6-1-Me-4-iPr-C6H4) or [Rh(η5-C5Me5)Cl2]2 and of 8c with [Mo(CO)4(2,5-norbornadiene)] or [Mn(CO)5Br] led to heterotrimetallic trans-[Pt(PPh3)2(CCFc)(CCRʹ-{M})] (Rʹ = 4-C5H4N: {M} = Ru(η6-cymene)Cl2 (10a), = Rh(η5-C5Me5)Cl2 (10b); Rʹ = 2,2ʹ-bipyridin-5-yl: {M} = Mo(CO)4 (11a), = Mn(CO)3Br (11b)). When 8b,c were reacted with [{[Ti](µ-σ,π-CCSiMe3)2}Cu(MeCN)][PF6] ([Ti] = Ti(η5-C5H4SiMe3)2) then heterotetrametallic trans-[Pt(PPh3)2(CCFc)(CCRʹ-{M})][PF6] ({M} = [Ti](µ-σ,π-CCSiMe3)2Cu: R’ = 2,2ʹ-bipyridin-5-yl (11c), Rʹ = -C6H4-4-CN (11d) were produced. Compounds 3, 4 and 7c were characterized by single-crystal X-ray diffraction studies establishing the trans arrangement at Pt. The electrochemical behaviour of all compounds was investigated by CV showing reversible redox events for Fc in 3, 7a,c, 8a−d, 10a,b, 11b,c in the range of E°1′ = -120 − -85 mV, while M feature irreversible redox processes between 745 and 1230 mV.

Synthesis and Electrochemical Behavior of Metal Carbonyl Isocyanoferrocene Compounds [M(CO)6‐n(C≡NFc)n] (M=Cr, Mo, W; n=1, 2, 3)

AbstractThe synthesis of [M(CO)6‐n(C≡NFc)n] (n=1: M=Cr, 3 a; M=Mo, 3 b; M=W, 3 c. n=2: M=Mo, 5. n=3: M=Mo, 6. Fc=Fe(η5−C5H4)(η5−C5H5)) by the reaction of [M(CO)5(thf)] (M=Cr, 1 a; M=Mo, 1 b; M=W 1 c) (synthesis of 3 a–c), [Mo(CO)4(nbd)] (4) (synthesis of 5) or [Mo(CO)6] (synthesis of 6) with FcN≡C (2) is discussed. IR studies confirm the formation of cis‐[Mo(CO)4(C≡NFc)2] (5) and mer‐[Mo(CO)3(C≡NFc)3] (6). The electrochemical behavior of 2, 3 a–c, 5 and 6 is reported. Compound 2 shows a reversible Fc redox event at E°1′=355 mV (ΔEp=60 mV). Upon complexation of 2 to M(CO)5 this wave is slightly shifted to E°′=335 (3 a), 345 (3 b) and 340 mV (3 c) showing that in 3 a–c the FcN≡C ligand is more easy to oxidize. The substitution of further CO ligands in 3 b by FcN≡C as characteristic for 5 and 6 influences the respective redox events insignificantly. Reversible oxidation of Cr occurs at 1045 mV (3 a) (ΔEp=60 mV), while for 3 b,c (3 b: 1170 mV, 3 c: 1290 mV) irreversible oxidations were observed. In addition, in situ spectroelectrochemical IR studies were carried out on 3 a–c. The results were further confirmed by DFT calculations.

Modulating the frontier orbitals of L(X)Ga-substituted diphosphenes [L(X)GaP]2 (X = Cl, Br) and their facile oxidation to radical cations.

Modulating the electronic structures of main group element compounds is crucial to control their chemical reactivity. Herein we report on the synthesis, frontier orbital modulation, and one-electron oxidation of two L(X)Ga-substituted diphosphenes [L(X)GaP]2 (X = Cl 2a, Br 2b; L = HC[C(Me)N(Ar)]2, Ar = 2,6-i-Pr2C6H3). Photolysis of L(Cl)GaPCO 1 gave [L(Cl)GaP]22a, which reacted with Me3SiBr with halide exchange to [L(Br)GaP]22b. Reactions with MeNHC (MeNHC = 1,3,4,5-tetramethylimidazol-2-ylidene) gave the corresponding carbene-coordinated complexes L(X)GaPP(MeNHC)Ga(X)L (X = Cl 3a, Br 3b). DFT calculations revealed that the carbene coordination modulates the frontier orbitals (i.e. HOMO/LUMO) of diphosphenes 2a and 2b, thereby affecting the reactivity of 3a and 3b. In marked contrast to diphosphenes 2a and 2b, the cyclic voltammograms (CVs) of the carbene-coordinated complexes each show one reversible redox event at E 1/2 = -0.65 V (3a) and -0.36 V (3b), indicating their one-electron oxidation to the corresponding radical cations as was confirmed by reactions of 3a and 3b with the [FeCp2][B(C6F5)4], yielding the radical cations [L(X)GaPP(MeNHC)Ga(X)L]B(C6F5)4 (X = Cl 4a, Br 4b). The unpaired spin in 4a (79%) and 4b (80%) is mainly located at the carbene-uncoordinated phosphorus atoms as was revealed by DFT calculations and furthermore experimentally proven in reactions with n Bu3SnH, yielding the diphosphane cations [L(X)GaPHP(MeNHC)Ga(X)L]B(C6F5)4 (X = Cl 5a, Br 5b). Compounds 2-5 were fully characterized by NMR and IR spectroscopy as well as by single crystal X-ray diffraction (sc-XRD), and compounds 4a and 4b were further studied by EPR spectroscopy, while their bonding nature was investigated by DFT calculations.

Beyond the Textbook: Introducing Undergraduates to Practical Electrochemistry

Electrochemistry is broad and interdisciplinary by nature and as such has become a powerful tool in science, technology, engineering, math, and medical (STEMM) fields ranging from energy storage to biotechnology. However, traditional undergraduate chemistry education does not offer students the opportunity to create direct connections between topics learned in a lecture course and applications beyond the examples depicted in textbooks. Here, we offer a framework that introduces learners to hands-on cyclic voltammetry in a general chemistry laboratory course setting or in outreach activities. In 2–4 h, learners practice electrode care, data acquisition, and the pattern recognition needed for discerning reversible, quasireversible, and irreversible solution-phase redox events.

Electronic Structure of Superoxidized Radical Cationic Dodecaborate-Based Clusters.

The expanding field of boron clusters has attracted continuous theoretical efforts to understand their diverse structures and unique bonding. We recently discovered a new reversible redox event of B12(O-3-methylbutyl)12 in which the superoxidized radical cationic form [B12(O-3-methylbutyl)12]•+ was identified and isolated for the first time. Herein, comprehensive (TD-)DFT studies in tandem with electrochemical experiments were employed to demonstrate the generality of the reported behavior across perfunctionalized B12(OR)12 clusters (R = aryl or alkyl). While the spin density of radical cationic clusters is delocalized in the core region, the oxidation brings about notable gains of positive partial charges on the supporting groups whose electronics can readily tune the redox potential of the 0/•+ couple. The underlying changes of frontier orbitals were elucidated, and the resulting [B12(OR)12]•+ species manifest a general diagnostic absorption as a consequence of mixed local/charge-transfer excitations.

Axial Redox Tuning at a Tetragonal Cobalt Center.

Square pyramidal cobalt complexes were prepared to study their multielectron redox properties. To build a stable redox-active cobalt complex, the combination of a tridentate acriPNP (acriPNP- = 4,5-bis(diisopropylphosphino)-2,7,9,9-tetramethyl-9H-acridin-10-ide) ligand with a bidentate ligand, such as 2,2'-bipyridine, 2-(o-phenyl)pyridine, biphenylene, and their analogues, was employed. In a cobalt complex having a tetragonal structure, the dx2-y2 orbital possesses an antibonding character and must remain empty for its structural integrity, while the dz2 orbital acts as a redox-active frontier molecular orbital (FMO). Tuning the redox potential of the Co(II/I) couple was successfully achieved by introducing a different axial donor. The reduction of Co(II) to Co(I) occurs at -2.6 V for a neutral donor but shifts to -3.4 V for an anionic donor. Since the redox-active dz2 orbital is close in energy to other ligand-based orbitals, multielectron redox activity is also observed. Electrochemical measurements indicate three reversible redox events within a window of -3.0-0.0 V vs Fc/Fc+ in tetrahydrofuran (THF). These redox processes are fully reversible for over 100 cycles, reflecting the electrochemical stability of these cobalt complexes. Surprisingly, the oxidation potential of the acriPNP ligand varies dramatically from +0.15 to -2.4 V, which is probably due to the cobalt contribution on the amido-based molecular orbital. The electronic structure of the cobalt complexes was examined structurally, spectroscopically, and theoretically.

Iron-iminopyridine complexes as charge carriers for non-aqueous redox flow battery applications

Non-aqueous redox flow batteries (RFBs) have been gaining increased attention in the energy storage arena. Some of their attractive features include the promise for high energy densities, wider voltage windows compared to aqueous systems, as well as wider operating temperature ranges. One of the major challenges in the development of these systems is the lack of electroactive materials that can undergo reversible redox events within the larger potential window of organic solvents. Herein, we present the design, synthesis, and measurement of electrochemical properties of some iron-based imine- and iminopyridine complexes as promising candidates for RFB applications. Synthesized complexes afforded three reversible redox couples over a ~3 V range. The redox events were also computationally explored and are in good agreement with experimental data. Theoretical calculations show that the redox event at the positive potential can be characterized as metal-centered event corresponding to Fe3+→ Fe2+ reduction, while the two redox events at negative potentials are associated with the consecutive reductions of iminopyridine or diimine moieties. This work has led to the identification of a promising anolyte material, [tris(imino)pyridine)Fe][OTf]2 (1), which is soluble in acetonitrile and is synthesized in two simple steps. This species shows outstanding performance when cycled between 1.0 V and 2.35 V, with 93% coulombic efficiency, 84% energy efficiency, and a capacity decay rate of 0.61% per cycle at 5 mA cm−2. Further modifications to this kind of charge carrier may lead to the development of high energy density materials for grid scale energy storage applications.