Redox Activity and Bond Activation in Iridium–Diamidobenzene Complexes: A Combined Structural, (Spectro)electrochemical, and DFT Investigation

Switching the Photoreactions of Ir(III) Diamine Complexes between C-N Coupling and Dehydrogenation under Visible Light Irradiation

The selective photoreactions under mild conditions play an important role in synthetic chemistry. Herein, efficient and mild protocols for switching the photoreactions of Ir(III)-diamine complexes between the interligand C-N coupling and dehydrogenation are developed in the presence of O₂ in EtOH solution. The photoreactions of achiral diamine complexes rac-[Ir(L)₂(dm)](PF₆) (L is 2-phenylquinoline or 2-(2,4-difluorophenyl)quinoline, dm is 1,2-ethylenediamine, 1,2-diaminopropane, 2-methyl-1,2-diamino-propane, or N,N'-dimethyl-1,2-ethylenediamine) are competitive in the oxidative C-N coupling and dehydrogenation at room temperature, which can be switched into the interligand C-N coupling reaction at 60 °C, affording hexadentate complexes in good to excellent yields, or the dehydrogenative reaction in the presence of a catalytic amount of TEMPO as an additive, affording imine complexes. Mechanism studies reveal that ¹O₂ is the major reactive oxygen species, and metal aminyl is the key intermediate in the formation of the oxidative C-N coupling and imine products in the photoreaction processes. These will provide a new and practical protocol for the synthesis of multidentate and imine ligands in situ via the postcoordinated strategy under mild conditions.

Rhodium Diamidobenzene Complexes: A Tale of Different Substituents on the Diamidobenzene Ligand

Diamidobenzene ligands are a prominent class of redox-active ligands owing to their electron reservoir behaviour, as well as the possibility of tuning the steric and the electronic properties of such ligands through the substituents on the N-atoms of the ligands. In this contribution, we present Rh(iii) complexes with four differently substituted diamidobenzene ligands. By using a combination of crystallography, NMR spectroscopy, electrochemistry, UV-vis-NIR/EPR spectroelectrochemistry, and quantum chemical calculations we show that the substituents on the ligands have a profound influence on the bonding, donor, electrochemical and spectroscopic properties of the Rh complexes. We present, for the first time, design strategies for the isolation of mononuclear Rh(ii) metallates whose redox potentials span across more than 850 mV. These Rh(ii) metallates undergo typical metalloradical reactivity such as activation of O₂ and C–Cl bond activations. Additionally, we also show that the substituents on the ligands dictate the one versus two electron nature of the oxidation steps of the Rh complexes. Furthermore, the oxidative reactivity of the metal complexes with a [CH₃]⁺ source leads to the isolation of a unprecedented, homobimetallic, heterovalent complex featuring a novel π-bonded rhodio-o-diiminoquionone. Our results thus reveal several new potentials of the diamidobenzene ligand class in organometallic reactivity and small molecule activation with potential relevance for catalysis.

Diamidobenzene ligands are versatile platforms in organometallic Rh-chemistry. They allow the isolation of tunable mononuclear ate-complexes, and the formation of a unprecedented homobimetallic, heterovalent 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.

Radical-Type Reactivity and Catalysis by Single-Electron Transfer to or from Redox-Active Ligands

Controlled ligand-based redox-activity and chemical non-innocence are rapidly gaining importance for selective (catalytic) processes. This Concept aims to provide an overview of the progress regarding ligand-to-substrate single-electron transfer as a relatively new mode of operation to exploit ligand-centered reactivity and catalysis based thereon.

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.

Redox-Active Ligand Assisted Multielectron Catalysis: A Case of CoIII Complex as Water Oxidation Catalyst

Water oxidation is the key step in both natural and artificial photosynthesis to capture solar energy for fuel production. The design of highly efficient and stable molecular catalysts for water oxidation based on nonprecious metals is still a great challenge. In this article, the electrocatalytic oxidation of water by Na[(L^4-)Co^III], where L is a substituted tetraamido macrocyclic ligand, was investigated in aqueous solution (pH 7.0). We found that Na[(L^4-)Co^III] is a stable and efficient homogeneous catalyst for electrocatalytic water oxidation with 380 mV onset overpotential in 0.1 M phosphate buffer (pH 7.0). Both ligand- and metal-centered redox features are involved in the catalytic cycle. In this cycle, Na[(L^4-)Co^III] was first oxidized to [(L^2-)Co^IIIOH] via a ligand-centered proton-coupled electron transfer process in the presence of water. After further losing an electron and a proton, the resting state, [(L^2-)Co^IIIOH], was converted to [(L^2-)Co^IV═O]. Density functional theory (DFT) calculations at the B3LYP-D3(BJ)/6-311++G(2df,2p)//B3LYP/6-31+G(d,p) level of theory confirmed the proposed catalytic cycle. According to both experimental and DFT results, phosphate-assisted water nucleophilic attack to [(L^2-)Co^IV═O] played a key role in O-O bond formation.