New avenues for ligand-mediated processes--expanding metal reactivity by the use of redox-active catechol, o-aminophenol and o-phenylenediamine ligands

Reversible multi-electron storage in dual-site redox-active supramolecular cages

Redox-active M₆L₄ cages display multiple reversible redox-events, enabling switching from overall +12 to −4 charged species with reversible storage of 16 electrons.

A coordinatively unsaturated iridium complex with an unsymmetrical redox-active ligand: (spectro)electrochemical and reactivity studies

Metal–ligand cooperativity can be used in iridium complexes with an unsymmetrically substituted redox-active diamidobenzene ligand for bond activation reactions.

Rigidification of a macrocyclic tris-catecholate scaffold leads to electronic localisation of its mixed valent redox product

One-electron oxidation of the compound shown shows no evidence for intervalence charge transfer in the macrocylic ligand radical product. In contrast, related [{Pt(L)}₃(μ₃-ctc˙)]⁺ (H₆ctc = cyclotricatechylene), exhibits class II mixed valency.

Structure and catalytic properties of novel copper isatin Schiff base complexes

Isatin Schiff bases in combination with copper ions act as non-innocent ligands amplifying the alcohol oxidation process.

Recent progress in ligand-centered homogeneous electrocatalysts for hydrogen evolution reaction

Recent advances in metal and metal-free ligand-centred electrocatalytic H₂ evolution have been reviewed.

Di-tert-butylcatecholate derivatives of titanocene

Titanocene dichloride reacts with sodium 3,5-di-tert-butyl-catecholate to give the mixed-valence binuclear Ti(iii,iv) complex Cp₂Ti(μ-Cat³⁵)₂TiCp.

First-Row Transition Metal (De)Hydrogenation Catalysis Based On Functional Pincer Ligands

The use of 3d metals in de/hydrogenation catalysis has emerged as a competitive field with respect to "traditional" precious metal catalyzed transformations. The introduction of functional pincer ligands that can store protons and/or electrons as expressed by metal-ligand cooperativity and ligand redox-activity strongly stimulated this development as a conceptual starting point for rational catalyst design. This review aims at providing a comprehensive picture of the utilization of functional pincer ligands in first-row transition metal hydrogenation and dehydrogenation catalysis and related synthetic concepts relying on these such as the hydrogen borrowing methodology. Particular emphasis is put on the implementation and relevance of cooperating and redox-active pincer ligands within the mechanistic scenarios.

Six-coordinate [CoIII(L)2]z (z = 1-, 0, 1+) complexes of an azo-appended o-aminophenolate in amidate(2-) and iminosemiquinonate π-radical (1-) redox-levels: the existence of valence-tautomerism

Aerobic reaction of the ligand H2L1, 2-(2-phenylazo)-anilino-4,6-di-tert-butylphenol, CoCl2·6H2O and Et3N in MeOH under refluxing conditions produces, after work-up and recrystallization, black crystals of [Co(L1)2] (1). When examined by cyclic voltammetry, 1 displays in CH2Cl2 three one-electron redox responses: two oxidative, E11/2 = 0.30 V (peak-to-peak separation, ΔEp = 100 mV) and E21/2 = 1.04 V (ΔEp = 120 mV), and one reductive E1/2 = -0.27 V (ΔEp = 120 mV) vs. SCE. Consequently, 1 is chemically oxidized by 1 equiv. of [FeIII(η5-C5H5)2][PF6], affording the isolation of deep purple crystals of [Co(L1)2][PF6]·2CH2Cl2 (2), and one-electron reduction with [CoII(η5-C5H5)2] yielded bluish-black crystals of [CoIII(η5-C5H5)2][Co(L1)2]·MeCN (3). A solid sample of 1 exhibits temperature-independent (50-300 K) magnetism, revealing the presence of a free radical (S = 1/2), which exhibits an isotropic EPR signal (g = 2.003) at 298 K and at 77 K an eight-line feature characteristic of hyperfine-interaction of the radical with the Co (I = 7/2) nucleus. Based on X-ray structural parameters of 1-3 at 100 K, magnetic and EPR spectral behaviour of 1, and variable-temperature (233-313 K) 1H NMR spectral features of 1-3 and 13C NMR spectra at 298 K of 2 and 3 in CDCl3 point to the electronic structure of the complexes as either [CoIII{(LAP)2-}{(LISQ)}˙-] or [CoIII{(L1)2}˙3-] (delocalized nature favours the latter description) (1), [CoIII{(LISQ)˙-}2][PF6]·2CH2Cl2 (2) and [CoIII(η5-C5H5)2][CoIII{(LAP)2-}2]·MeCN (3) [(LAP)2- and (LISQ)˙- represent the redox-level of coordinated ligands o-amidophenolate(2-) ion and o-iminobenzosemiquinonate(1-) π-radical ion, respectively]. Notably, all the observed redox processes are ligand-centred. To the best of our knowledge, this is the first time that six-coordinate complexes of a common tridentate o-aminophenolate-based ligand have been structurally characterized for the parent 1, its monocation 2 and the monoanion 3 counterparts. Temperature-dependent 1H NMR spectra reveal the existence of valence-tautomeric equilibria in 1-3. Density Functional Theory (DFT) calculations at the B3LYP-level of theory corroborate the electronic structural assignment of 1-3 from experimental data. The origins of the observed UV-VIS-NIR absorptions for 1-3 have been assigned, based on time-dependent (TD)-DFT calculations.

Catalytic Aerobic Oxidation of Alcohols by Copper Complexes Bearing Redox-Active Ligands with Tunable H-Bonding Groups

In this research article, we describe the structure, spectroscopy, and reactivity of a family of copper complexes bearing bidentate redox-active ligands that contain H-bonding donor groups. Single-crystal X-ray crystallography shows that these tetracoordinate complexes are stabilized by intramolecular H-bonding interactions between the two ligand scaffolds. Interestingly, the Cu complexes undergo multiple reversible oxidation-reduction processes associated with the metal ion (Cu^I, Cu^II, Cu^III) and/or the o-phenyldiamido ligand (L^2-, L^•-, L). Moreover, some of the Cu^II complexes catalyze the aerobic oxidation of alcohols to aldehydes (or ketones) at room temperature. Our extensive mechanistic analysis suggests that the dehydrogenation of alcohols occurs via an unusual reaction pathway for galactose oxidase model systems, in which O₂ reduction occurs concurrently with substrate oxidation.

Cobalt complexes with hemilabile o-iminobenzoquinonate ligands: a novel example of redox-induced electron transfer

The tetracoordinated square-planar CoIII complex (imSQC(O)Ph)CoIII(APC(O)Ph) (1) bearing a radical anion and the closed-shell o-amidophenolate forms of the functionalized o-aminophenol H2LC(O)Ph were synthesized. The intermediate spin state (SCo = 1) CoIII center was found for compound 1. The cyclic voltammogram of derivative 1 contains two oxidative processes and one reductive redox process as well as an additional multi-electron wave at high negative potentials above -2 V, which can involve both the ligand and metal center. One-electron oxidation of 1 by silver triflate produces the [(imSQC(O)Ph)CoII(imQC(O)Ph)]OTf·2toluene (2) derivative with the trigonal prismatic coordination environment of the metal arising from the additional coordination of -C(O)Ph hemilabile groups. This is a first example of a trigonal prismatic coordination polyhedron in cobalt-based complexes featuring o-iminobenzoquinone ligands. The trigonal prismatic geometry achieved by the unique flexibility of the ligand allows metal-to-ligand redox-induced electron transfer (RIET). Chemical oxidation of complex 1 promotes the reduction of CoIII to CoII in compound 2 due to the redox-active nature of o-iminobenzoquinonate ligands. Remarkably, this is the first example of RIET in cobalt-based derivatives with this type of ligand. The oxidative states of the ligands and cobalt ion in both complexes were unequivocally established according to the X-ray data collection by using the utility of "metric oxidation state" (MOS). The spin states of the metal centers were unambiguously determined by density functional theory. The strong antiferromagnetic exchange via metal-ligand interactions is dominant in compounds 1 and 2, giving the doublet (S = 1/2) and triplet (S = 1) ground spin state, respectively.

Cycloaddition versus Cleavage of the C=S Bond of Isothiocyanates Promoted by Digallane Compounds with Noninnocent α-Diimine Ligands

Whereas the chemistry of single-bond activation by compounds of the main group elements has undergone some development in recent years, the cleavage of multiple bonds remains underexplored. Herein, the reactions of two digallanes bearing α-diimine ligands, namely, [L¹ Ga-GaL¹ ] (1, L¹ =dpp-dad=[(2,6-iPr₂ C₆ H₃ )NC(CH₃ )]₂ ) and [L² Ga-GaL² ] (2, L² =dpp-bian=1,2-[(2,6-iPr₂ C₆ H₃ )NC]₂ C₁₀ H₆ ), with isothiocyanates are reported. Reactions of 1 or 2 with isothiocyanates in 1:2 molar ratio proceeded with [2+4] cycloaddition of the C=S bond across the C₂ N₂ Ga metallacycle with formation of C-C and S-Ga single bonds to afford [L¹ (RN=C-S)Ga-Ga(S-C=NR)L¹ ] (3, R=Me; 4, R=Ph) and [L² (RN=C-S)Ga-Ga(S-C=NR)L² ] (8, R=allyl; 9, R=Ph). In the cases of 8 and 9, this cycloaddition is reversible. The digallanes reacted with 2 equiv of PhNCS in the presence of Na metal or at high temperatures through a unique reductive cleavage of the C=S bond to yield the disulfide-bridged digallium species [Na(THF)₃ ]₂ [L¹ Ga(μ-S)₂ GaL¹ ] (5), [L² Ga(μ-S)₂ GaL² ] (10), and [Na(DME)₃ ][L² Ga(μ-S)₂ GaL² ] (11). Moreover, products 4 and 5 can further react with an excess of isothiocyanate, through cleavage of the C=S bond or cycloaddition, to give the bis- or mono-S-bridged complexes [Na(THF)₂ ]₂ [L¹ (PhN=C-S)Ga(μ-S)₂ Ga(S-C=NPh)L¹ ] (6) and [L¹ (PhN=C-S)Ga(μ-S)Ga(S-C=NPh)L¹ ] (7). All the newly prepared compounds were characterized by elemental analysis, single-crystal X-ray diffraction, IR spectroscopy, NMR (3-9) or ESR spectroscopy (11), and DFT calculations.

Catalytic Applications of Vanadium: A Mechanistic Perspective

The chemistry of vanadium has seen remarkable activity in the past 50 years. In the present review, reactions catalyzed by homogeneous and supported vanadium complexes from 2008 to 2018 are summarized and discussed. Particular attention is given to mechanistic and kinetics studies of vanadium-catalyzed reactions including oxidations of alkanes, alkenes, arenes, alcohols, aldehydes, ketones, and sulfur species, as well as oxidative C-C and C-O bond cleavage, carbon-carbon bond formation, deoxydehydration, haloperoxidase, cyanation, hydrogenation, dehydrogenation, ring-opening metathesis polymerization, and oxo/imido heterometathesis. Additionally, insights into heterogeneous vanadium catalysis are provided when parallels can be drawn from the homogeneous literature.

Transition metal-catalyzed dehydrogenation of amines

This review focuses on the use of homogeneous transition metal complexes for the catalytic dehydrogenation of amines for synthetic purposes, and for hydrogen storage applications. The catalytic dehydrogenation of primary, secondary and cyclic amines is reviewed looking at reaction conditions, different catalysts and common side reactions. Recent developments in this active field of research showcase how cooperative ligands and photocatalysts can overcome the need for noble metals or harsh reaction conditions.

Ligand-Induced Reductive Elimination of Ethane from Azopyridine Palladium Dimethyl Complexes

Reductive elimination (RE) is a critical step in many catalytic processes. The reductive elimination of unsaturated groups (aryl, vinyl and ethynyl) from Pd(II) species is considerably faster than RE of saturated alkyl groups. Pd(II) dimethyl complexes ligated by chelating diimine ligands are stable toward RE unless subjected to a thermal or redox stimulus. Herein, we report the spontaneous RE of ethane from (azpy)PdMe₂ complexes and the unique role of the redox-active azopyridine (azpy) ligands in facilitating this reaction. The (azpy)PdMe₂ complexes are air- and moisture-stable in the solid form, but they readily produce ethane upon dissolution in polar solvents at temperatures from 10 °C to room temperature without the need for an external oxidant or elevated temperatures. Experimental and computational studies indicate that a bimolecular methyl transfer precedes the reductive elimination step, where both steps are facilitated by the redox-active azopyridine ligand.

Structural snapshots of the rearrangement of the bis(di-tert-butyl-aminophenyl)amine pincer ligand in the presence of transition metal ions

The bis(di-tert-butyl-aminophenyl)amine ligand H3LN,N,N was reacted with Mn(ii), Co(ii), Fe(iii) and Cu(ii) salts in air. The ligand undergoes oxidative transformations, which involve intra and intermolecular C-N and N-N bond formations. A rare aromatic C-N bond cleavage leading to a C-O bond has also been observed.

Tuning of Thiyl/Thiolate Complex Near-Infrared Chromophores of Platinum through Geometrical Constraints

The chemistry of radical-ligand complexes of the transition metals has developed into a vibrant field of research that spans from fundamental studies on the relationship between the chemical and electronic structures to applications in catalysis and functional materials chemistry. In general, fine-tuning of the relevant properties relies on an increasingly diversifying pool of radical-proligand structures. Surprisingly, the variability of the conformational freedom and the number of distinct bonding modes supported by many radical proligands is limited. This work reports on the angular constraints and relative geometric alignment of metal and ligand orbitals as key parameters that render a series of chemically similar thiyl/thiolate complexes of platinum(II) electronically and spectroscopically distinct. The use of conformational flexible thiophenols as primary ligand scaffolds is essential to establishing a defined radical-ligand [(^areneS)₂Pt^II]^•+ core whose electronic structure is modulated by a series of auxiliary coligands at platinum.

Reactivity of Two-Electron-Reduced Boron Formazanate Compounds with Electrophiles: Facile N-H/N-C Bond Homolysis Due to the Formation of Stable Ligand Radicals

The reactivity of a boron complex with a redox-active formazanate ligand, LBPh₂ [L = PhNNC(p-tol)NNPh], was studied. Two-electron reduction of this main-group complex generates the stable, nucleophilic dianion [LBPh₂]^2–, which reacts with the electrophiles BnBr and H₂O to form products that derive from ligand benzylation and protonation, respectively. The resulting complexes are anionic boron analogues of leucoverdazyls. N–C and N–H bond homolysis of these compounds was studied by exchange NMR spectroscopy and kinetic experiments. The weak N–C and N–H bonds in these systems derive from the stability of the resulting borataverdazyl radical, in which the unpaired electron is delocalized over the four N atoms in the ligand backbone. We thus demonstrate the ability of this system to take up two electrons and an electrophile (E⁺ = Bn⁺, H⁺) in a process that takes place on the organic ligand. In addition, we show that the [2e^–/E⁺] stored on the ligand can be converted to E^• radicals, reactivity that has implications in energy storage applications such as hydrogen evolution.

A boron complex with a redox-active formazanate ligand in its two-electron-reduced state is shown to react with electrophiles (BnBr and H⁺). The resulting “borataleucoverdazyl” products have weak N−C and N−H bonds; homolytic cleavage reactions lead to stable ligand-based radicals. Thus, the accumulation of [2e⁻/E⁺] on the formazanate ligand and conversion to E^• radicals are demonstrated, and their potential relevance in energy-related electrocatalysis (e.g., proton reduction) is discussed.

When Do Strongly Coupled Diradicals Show Strongly Coupled Reactivity? Thermodynamics and Kinetics of Hydrogen Atom Transfer Reactions of Palladium and Platinum Bis(iminosemiquinone) Complexes

The 2,2'-biphenylene-bridged bis(iminosemiquinone) complexes ( ^tBuClip)M [ ^tBuClipH₄ = 4,4'-di- tert-butyl- N, N'-bis(3,5-di- tert-butyl-2-hydroxyphenyl)-2,2'-diaminobiphenyl; M = Pd, Pt] can be reduced to the bis(aminophenoxide) complexes ( ^tBuClipH₂)M by reaction with hydrazobenzene (M = Pd) or by catalytic hydrogenation (M = Pt). The palladium complex with one aminophenoxide ligand and one iminosemiquinone ligand, ( ^tBuClipH)Pd, is generated by comproportionation of ( ^tBuClip)Pd with ( ^tBuClipH₂)Pd in a process that is both slow (0.06 M^-1 s^-1 in toluene at 23 °C) and only modestly favorable ( K_com = 1.9 in CDCl₃), indicating that both N-H bonds have essentially the same bond strength. The mono(iminoquinone) complex ( ^tBuClipH)Pt has not been observed, indicating that the platinum analogue shows no tendency to comproportionate ( K_com < 0.1). The average bond dissociation free energies (BDFE) of the complexes have been established by equilibration with suitably substituted hydrazobenzenes, and the palladium bis(iminosemiquinone) is markedly more oxidizing than the platinum compound, with hydrogen transfer from ( ^tBuClipH₂)Pt to ( ^tBuClip)Pd occurring with Δ G° = -8.9 kcal mol^-1. The palladium complex ( ^tBuClipH₂)Pd reacts with nitroxyl radicals in two observable steps, with the first hydrogen transfer taking place slightly faster than the second. In the platinum analogue, the first hydrogen transfer is much slower than the second, presumably because the N-H bond in the monoradical complex ( ^tBuClipH)Pt is unusually weak. Using driving force-rate correlations, it is estimated that this bond has a BDFE of 55.1 kcal mol^-1, which is 7.1 kcal mol^-1 weaker than that of the first N-H bond in ( ^tBuClipH₂)Pt. The two radical centers in the platinum, but not the palladium, complex thus act in concert with each other and display a strong thermodynamic bias toward two-electron reactivity. The greater thermodynamic and kinetic coupling in the platinum complex is attributed to the stronger metal-ligand π interactions in this compound.

Uncoupled Redox-Inactive Lewis Acids in the Secondary Coordination Sphere Entice Ligand-Based Nitrite Reduction

Metal complexes composed of redox-active pyridinediimine (PDI) ligands are capable of forming ligand-centered radicals. In this Forum article, we demonstrate that integration of these types of redox-active sites with bioinspired secondary coordination sphere motifs produce direduced complexes, where the reduction potential of the ligand-based redox sites is uncoupled from the secondary coordination sphere. The utility of such ligand design was explored by encapsulating redox-inactive Lewis acidic cations via installation of a pendant benzo-15-crown-5 in the secondary coordination sphere of a series of Fe(PDI) complexes. Fe(15bz5PDI)(CO)2 was shown to encapsulate the redox-inactive alkali ion, Na+, causing only modest (31 mV) anodic shifts in the ligand-based redox-active sites. By uncoupling the Lewis acidic sites from the ligand-based redox sites, the pendant redox-inactive ion, Na+, can entice the corresponding counterion, NO2-, for reduction to NO. The subsequent initial rate analysis reveals an acceleration in anion reduction, confirming this hypothesis.

Radical Relatives: Facile Oxidation of Hetero-Diarylmethene Anions to Neutral Radicals

Furan and thiophene diarylmethenes are potential redox-active ligands for metal centers that could be exploited in the development of nontraditional, stoichiometric, and catalytic redox reactions. As such, we describe here the selective meso-deprotonations of dithiophene, difuran, and diimine-difuran diarylmethanes to form the π-conjugated anions, for which only the diimino-difuryl anion is truly isolable and studied by X-ray crystallography. In all cases, facile one-electron oxidation of these anions occurs, which allows the isolation of the neutral dithienyl and diimino-difuryl radicals. UV-Visible and time-dependent density functional theory studies reveal that the oxidation of the dithienyl anion to its radical is associated with an increase in the highest (singly) occupied molecular orbital-lowest unoccupied molecular orbital gap, evident through a hypsochromic shift of the main absorption band in the electronic spectrum, whereas oxidation of the diimino-difuryl anion causes only minor spectroscopic changes. Electrochemical studies support the stability of the radicals with respect to the anion, showing strongly negative oxidation potentials. The control of the redox activity of these diarylmethene carbanions through variation of the nature of the substituents, donor-atom, and the conjugated π-system and their potential as ligands for redox-inert metal centers makes them intriguing candidates as noninnocent partners for redox reactions.

Alkali Cation Effects on Redox-Active Formazanate Ligands in Iron Chemistry

Noncovalent interactions of organic moieties with Lewis acidic alkali cations can greatly affect structure and reactivity. Herein, we describe the effects of interactions with alkali-metal cations within a series of reduced iron complexes bearing a redox-active formazanate ligand, in terms of structures, magnetism, spectroscopy, and reaction rates. In the absence of a crown ether to sequester the alkali cation, dimeric complexes are isolated wherein the formazanate has rearranged to form a five-membered metallacycle. The dissociation of these dimers is dependent on the binding mode and size of the alkali cation. In the dimers, the formazanate ligands are radical dianions, as shown by X-ray absorption spectroscopy, Mössbauer spectroscopy, and analysis of metrical parameters. These experimental measures are complemented by density functional theory calculations that show the spin density on the bridging ligands.

Backbone Dehydrogenation in Pyrrole-Based Pincer Ligands

Treatment of both [CoCl( ^tBuPNP)] and [NiCl( ^tBuPNP)] ( ^tBuPNP = anion of 2,5-bis((di- tert-butylphosphino)methyl)pyrrole) with one equivalent of benzoquinone affords the corresponding chloride complexes containing a dehydrogenated PNP ligand, ^tBudPNP ( ^tBudPNP = anion of 2,5-bis((di- tert-butylphosphino)methylene)-2,5-dihydropyrrole). Dehydrogenation of PNP to dPNP results in minimal change to steric profile of the ligand but has important consequences for the resulting redox potentials of the metal complexes, resulting in the ability to isolate both [CoH( ^tBudPNP)] and [CoEt( ^tBudPNP)], which are more challenging (hydride) or not possible (ethyl) to prepare with the parent PNP ligand. Electrochemical measurements with both the Co and Ni dPNP species demonstrate a substantial shift in redox potentials for both the M(II/III) and M(II/I) couples. In the case of the former, oxidation to trivalent Co was found to be reversible, and subsequent reaction with AgSbF₆ afforded a rare example of a square-planar Co(III) species. Corresponding reduction of [CoCl( ^tBudPNP)] with KC₈ produced the diamagnetic Co(I) species, [Co(N₂)( ^tBudPNP)]. Further reduction of the Co(I) complex was found to generate a pincer-based π-radical anion that demonstrated well-resolved EPR features to the four hydrogen atoms and lone nitrogen atom of the ligand with minor contributions from cobalt and coordinated N₂. Changes in the electronic character of the PNP ligand upon dehydrogenation are proposed to result from loss of aromaticity in the pyrrole ligand, resulting in a more reducing central amido donor. DFT calculations on the Co(II) complexes were performed to shed further insight into the electronic structure of the pincer complexes.

A highly active diradical cobalt(iii) catalyst for the cycloisomerization of alkynoic acids

The first cobalt-catalysed cycloisomerisation of alkynoic acids is reported, thanks to the design of a well-defined diradical cobalt(iii) catalyst, in the absence of any additives. The high efficiency, regioselectivity and chemoselectivity are comparable to those of noble metal-based systems. The unique reactivity might be attributed to second coordination sphere effects.

Reversible Ligand-Centered Reduction in Low-Coordinate Iron Formazanate Complexes

Coordination of redox-active ligands to metals is a compelling strategy for making reduced complexes more accessible. In this work, we explore the use of redox-active formazanate ligands in low-coordinate iron chemistry. Reduction of an iron(II) precursor occurs at milder potentials than analogous non-redox-active β-diketiminate complexes, and the reduced three-coordinate formazanate-iron compound is characterized in detail. Structural, spectroscopic, and computational analysis show that the formazanate ligand undergoes reversible ligand-centered reduction to form a formazanate radical dianion in the reduced species. The less negative reduction potential of the reduced low-coordinate iron formazanate complex leads to distinctive reactivity with formation of a new N-I bond that is not seen with the β-diketiminate analogue. Thus, the storage of an electron on the supporting ligand changes the redox potential and enhances certain reactivity.

Nickel-Alkyl Complexes with a Reactive PNC-Pincer Ligand

Based on previous work related to the design and application of rigid tridentate phosphine-pyridine-phenyl coordination offered by a PNC-pincer ligand upon cyclometalation to nickel, the synthesis, spectroscopic and solid state characterization and redox-reactivity of two NiII(PNC) complexes featuring either a methyl (2CH3 ) or CF3 co-ligand (2CF3 ) are described. One-electron oxidation is proposed to furnish C-C reductive elimination, as deduced from a combined chemical, electrochemical, spectroscopic and computational study. One-electron reduction results in a ligand-centered radical anion, as supported by electrochemistry, UV spectroelectrochemistry, EPR spectroscopy, and DFT calculations. This further attenuates the breadth of chemical reactivity offered by such PNC-pincer ligands.

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 [Ni^II (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 ][Ni^II (Bdt)(BdtSQ)] [R=nC₁₆ H₃₃ (NC16₄ Ni) and C8,10 (NC8,10₄ Ni); C8,10=6-octylhexadecyl] or [NMe₂ R₂ ][Ni^II (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.

Photolytic Reactivity of Organometallic Chromium Bipyridine Complexes

Known stable [Cr(bpy)₂(Ph)₂](BPh₄) complexes undergo reductive elimination of biphenyl with visible-light photolysis using household incandescent or compact fluorescent light bulbs. A series of [Cr(R-bpy)₂(Ar)₂](X) complexes (R = H or CMe₃; Ar = Ph, C₆H₄-CMe₃, or C₆H₄-OMe; X = I, BPh₄, or PF₆) were prepared, and the effect of varying the bipyridine and aryl ligands on the UV-visible spectra and electrochemistry of the chromium(III) complexes was investigated. Photolysis of a mixture of two different bis(aryl) complexes gave only the homocoupled biaryl products by ¹H NMR and gas chromatography/mass spectrometry analysis. The initial product of photoinduced reductive elimination of [Cr(bpy)₂(Ar)₂](PF₆) was trapped with bipyridine to generate [Cr(bpy)₃](PF₆) and with benzoyl peroxide to form [Cr(bpy)₂(O₂CPh)₂](PF₆). The latter chromium(III) bis(benzoate) complex was also synthesized by the addition of bipyridine and PhCO₂H to Cp₂Cr, followed by air oxidation. The neutral Cr(bpy)(S₂CNMe₂)Ph₂ complex also generated biphenyl upon visible-light photolysis. While the treatment of Cr(^tBu-bpy)(dpm)Cl₂ [dpm = (OC^tBu)₂CH] with AgO₂CPh gave trans-Cr(^tBu-bpy)(dpm)(O₂CPh)₂, reaction of the dichloro precursor with PhMgCl produced anionic [Cr(^tBu-bpy)Ph₃]^- with [Mg(dpm)(THF)₄]⁺ as the countercation, with both complexes characterized by single-crystal X-ray diffraction. Protonolysis of Cr(bpy)Ph₃(THF) with 8-hydroxyquinoline produced Cr(bpy)(quin)Ph₂, which generated biphenyl under visible-light photolysis, and the initial product of reductive elimination was trapped by bipyridine or benzoyl peroxide. A related Cr(bpy)(quin)₂ complex was synthesized by protonolysis of Cr(bpy)[N(SiMe₃)₂]₂ and characterized by single-crystal X-ray diffraction.

Copper-Catalyzed Aziridination with Redox-Active Ligands: Molecular Spin Catalysis

Small-molecule catalysts as mimics of biological systems illustrate the chemists' attempts at emulating the tantalizing abilities displayed by nature's metalloenzymes. Among these innate behaviors, spin multistate reactivity is used by biological systems as it offers thermodynamic leverage towards challenging chemical reactivity but this concept is difficult to translate into the realm of synthetic organometallic catalysis. Here, we report a rare example of molecular spin catalysis involving multistate reactivity in a small-molecule biomimetic copper catalyst applied to aziridination. This behavior is supported by spin state flexibility enabled by the redox-active ligand.

Unveiling the redox-active character of imidazolin-2-thiones derived from amino-substituted N-heterocyclic carbenes

Spectroscopic, structural and computational studies on the amino-substituted imidazolin-2-thiones reveal the imidazolyl ring to be redox active.

Aminotroponiminates: ligand-centred, reversible redox events under oxidative conditions in sodium and bismuth complexes

Redox-active bismuth complexes based on a new aminotroponiminate ligand with ferrocenyl substituents have been synthesised and characterised.

Redox-active diaminoazobenzene complexes of rhodium(iii): synthesis, structure and spectroscopic characterization

Coordination diversity of an aromatic diamine with Rh(iii) is presented together with the elucidation of the molecular and electronic structures, electron transfer, and electronic transitions.