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Here, we present the development and characterization of the novel PhenTAA macrocycle as well as a series of [Ni(R2PhenTAA)]ncomplexes featuring two sites for ligand-centered redox-activity. These differ in the substituent R (R = H, Me, or Ph) and overall charge of the complex n (n = −2, −1, 0, +1, or +2). Electrochemical and spectroscopic techniques (CV, UV/vis–SEC, X-band EPR) reveal that all redox events of the [Ni(R2PhenTAA)] complexes are ligand-based, with accessible ligand charges of −2, −1, 0, +1, and +2. The o-phenylenediamide (OPD) group functions as the electron donor, while the imine moieties act as electron acceptors. The flanking o-aminobenzaldimine groups delocalize spin density in both the oxidized and reduced ligand states. The reduced complexes have different stabilities depending on the substituent R. For R = H, dimerization occurs upon reduction, whereas for R = Me/Ph, the reduced imine groups are stabilized. This also gives electrochemical access to a [Ni(R2PhenTAA)]2–species. DFT and TD–DFT calculations corroborate these findings and further illustrate the unique donor–acceptor properties of the respective OPD and imine moieties. The novel [Ni(R2PhenTAA)] complexes exhibit up to five different ligand-based oxidation states and are electrochemically stable in a range from −2.4 to +1.8 V for the Me/Ph complexes (vs Fc/Fc+).

Single-atom catalysts often show exceptionally high performance per metal loading. However, the isolated atom sites tend to agglomerate during preparation and/or high-temperature reaction. Here we show that in the case of Rh/Al2O3this deactivation can be prevented by dissolution/exsolution of metal atoms into/from the support. We design and synthesise a series of single-atom catalysts, characterise them and study the impact of exsolution in the dry reforming of methane at 700–900 °C. The catalysts' performance increases with increasing reaction time, as the rhodium atoms migrate from the subsurface to the surface. Although the oxidation state of rhodium changes from Rh(iii) to Rh(ii) or Rh(0) during catalysis, atom migration is the main factor affecting catalyst performance. The implications of these results for preparing real-life catalysts are discussed.

The redox noninnocence of the TAML scaffold in cobalt-TAML (tetra-amido macrocyclic ligand) complexes has been under debate since 2006. In this work, we demonstrate with a variety of spectroscopic measurements that the TAML backbone in the anionic complex [CoIII(TAMLred)]–is truly redox noninnocent and that one-electron oxidation affords [CoIII(TAMLsq)]. Multireference (CASSCF) calculations show that the electronic structure of [CoIII(TAMLsq)]is best described as an intermediate spin (S= 1) cobalt(III) center that is antiferromagnetically coupled to a ligand-centered radical, affording an overall doublet (S = 1/2) ground-state. Reaction of the cobalt(III)-TAML complexes with PhINNs as a nitrene precursor leads to TAML-centered oxidation and produces nitrene radical complexes without oxidation of the metal ion. The ligand redox state (TAMLredor TAMLsq) determines whether mono- or bis-nitrene radical complexes are formed. Reaction of [CoIII(TAMLsq)]or [CoIII(TAMLred)]–with PhINNs results in the formation of [CoIII(TAMLq)(N•Ns)]and [CoIII(TAMLq)(N•Ns)2]–, respectively. Herein, ligand-to-substrate single-electron transfer results in one-electron-reduced Fischer-type nitrene radicals (N•Ns–) that are intermediates in catalytic nitrene transfer to styrene. These nitrene radical species were characterized by EPR, XANES, and UV–vis spectroscopy, high-resolution mass spectrometry, magnetic moment measurements, and supporting CASSCF calculations.

A new family of low-coordinate Co complexes supported by three redox-noninnocent tridentate [OCO] pincer-type bis(phenolate) N-heterocyclic carbene (NHC) ligands are described. Combined experimental and computational data suggest that the charge-neutral four-coordinate complexes are best formulated as Co(II) centers bound to closed-shell [OCO]2–dianions, of the general formula [(OCO)CoIIL] (where L is a solvent-derived MeCN or THF). Cyclic voltammograms of the [(OCO)CoIIL] complexes reveal three oxidations accessible at potentials below 1.2 V vs Fc+/Fc, corresponding to generation of formally Co(V) species, but the true physical/spectroscopic oxidation states are much lower. Chemical oxidations afford the mono- and dications of the imidazoline NHC-derived complex, which were examined by computational and magnetic and spectroscopic methods, including single-crystal X-ray diffraction. The metal and ligand oxidation states of the monocationic complex are ambiguous; data are consistent with formulation as either [(SOCO)CoIII(THF)2]+containing a closed-shell [SOCO]2–diphenolate ligand bound to a S= 1 Co(III) center, or [(SOCO•)CoII(THF)2]+with a low-spin Co(II) ion ferromagnetically coupled to monoanionic [SOCO•]−containing a single unpaired electron distributed across the [OCO] framework. The dication is best described as [(SOCO0)CoII(THF)3]2+, with a single unpaired electron localized on the d7Co(II) center and a doubly oxidized, charge-neutral, closed-shell SOCO0ligand. The combined data provide for the first time unequivocal and structural evidence for [OCO] ligand redox activity. Notably, varying the degree of unsaturation in the NHC backbone shifts the ligand-based oxidation potentials by up to 400 mV. The possible chemical origins of this unexpected shift, along with the potential utility of the [OCO] pincer ligands for base-metal-mediated organometallic coupling catalysis, are discussed.

Coordination of FeCl3to the redox-active pyridine–aminophenol ligand NNOH2in the presence of base and under aerobic conditions generates FeCl2(NNOISQ) (1), featuring high-spin FeIIIand an NNOISQradical ligand. The complex has an overall S= 2 spin state, as deduced from experimental and computational data. The ligand-centered radical couples antiferromagnetically with the Fe center. Readily available, well-defined, and air-stable 1catalyzes the challenging intramolecular direct C(sp3)–H amination of unactivated organic azides to generate a range of saturated N-heterocycles with the highest turnover number (TON) (1 mol% of 1, 12 h, TON = 62; 0.1 mol% of 1, 7 days, TON = 620) reported to date. The catalyst is easily recycled without noticeable loss of catalytic activity. A detailed kinetic study for C(sp3)–H amination of 1-azido-4-phenylbutane (S1) revealed zero order in the azide substrate and first order in both the catalyst and Boc2O. A cationic iron complex, generated from the neutral precatalyst upon reaction with Boc2O, is proposed as the catalytically active species.