Catalyst-Controlled Intermolecular Homobenzylic C(sp3)-H Amination for the Synthesis of ß-Arylethylamines.

The combination of a tailored sulfamate with a C4-symmetrical rhodium(II) tetracarboxylate allows to uncover a selective intermolecular amination of unactivated homobenzylic C(sp3)-H bonds. The reaction has a broad scope (>30 examples) and proceeds with a high level of regioselectivity with homobenzylic/benzylic ratio of up to 35:1, thereby providing a direct access to ß-arylethylamines that are of utmost interest in medicinal chemistry. Computational investigations evidenced a concerted mechanism, involving an asynchronous transition state. Based on a combined activation strain model and energy decomposition analysis, the regioselectivity of the reaction was found to rely mainly on the degree of orbital interaction between the [Rh2]-nitrene and the C-H bond. The latter is facilitated at the homobenzylic position due to the establishment of specific noncovalent interactions within the catalytic pocket.

Second Coordination Sphere Effect Shifts CO2 to CO Reduction by Iron Porphyrin from Fe0 to FeI.

Iron porphyrins are among the most studied molecular catalysts for carbon dioxide (CO2 ) reduction and their reactivity is constantly being enhanced through the implementation of chemical functionalities in the second coordination sphere inspired by the active sites of enzymes. In this study, we were intrigued to observe that a multipoint hydrogen bonding scheme provided by embarked urea groups could also shift the redox activation step of CO2 from the well-admitted Fe(0) to the Fe(I) state. Using EPR, resonance Raman, IR and UV-Visible spectroscopies, we underpinned a two-electron activation step of CO2 starting from the Fe(I) oxidation state to form, after protonation, an Fe(III)-COOH species. The addition of another electron and a proton to the latter species converged to the cleavage of a C-O bond with the loss of water molecule resulting in an Fe(II)-CO species. DFT analyses of these postulated intermediates are in good agreement with our collected spectroscopic data, allowing us to propose an alternative pathway in the catalytic CO2 reduction with iron porphyrin catalyst. Such a remarkable shift opens new lines of research in the design of molecular catalysts to reach low overpotentials in performing multi-electronic CO2 reduction catalysis.

Photocatalytic generation of a non-heme Fe(iii)-hydroperoxo species with O2 in water for the oxygen atom transfer reaction.

Coupling a photoredox module and a bio-inspired non-heme model to activate O2 for the oxygen atom transfer (OAT) reaction requires a vigorous investigation to shed light on the multiple competing electron transfer steps, charge accumulation and annihilation processes, and the activation of O2 at the catalytic unit. We found that the efficient oxidative quenching mechanism between a [Ru(bpy)3]2+ chromophore and a reversible electron mediator, methyl viologen (MV2+), to form the reducing species methyl viologen radical (MV˙+) can convey an electron to O2 to form the superoxide radical and reset an Fe(iii) species in a catalytic cycle to the Fe(ii) state in an aqueous solution. The formation of the Fe(iii)-hydroperoxo (FeIII-OOH) intermediate can evolve to a highly oxidized iron-oxo species to perform the OAT reaction to an alkene substrate. Such a strategy allows us to bypass the challenging task of charge accumulation at the molecular catalytic unit for the two-electron activation of O2. The FeIII-OOH catalytic precursor was trapped and characterized by EPR spectroscopy pertaining to a metal assisted catalysis. Importantly, we found that the substrate itself can act as an electron donor to reset the photooxidized chromophore in the initial state closing the photocatalytic loop and hence excluding the use of a sacrificial electron donor. Laser Flash Photolysis (LFP) studies and spectroscopic monitoring during photocatalysis lend credence to the proposed catalytic cycle.

Rhodium(II)-Catalyzed Enantioselective Intermolecular Aziridination of Alkenes.

C4-Symmetrical dirhodium(II) tetracarboxylates are highly efficient catalysts for the asymmetric intermolecular aziridination of substituted alkenes with sulfamates. The reaction proceeds with high levels of efficiency and chemoselectivity to afford aziridines with excellent yields of up to 95% and enantiomeric excesses of up to 99%. The scope of the alkene aziridination includes mono-, di-, and trisubstituted olefins as well as the late-stage functionalization of complex substrates. The reaction can be performed on a gram-scale with a catalyst loading of 0.1 mol %. Our DFT study led us to propose a two-spin-state mechanism, involving a triplet Rh-nitrene species as key intermediate to drive the stereocontrolled approach and activation of the substrate.

Proton-controlled Action of an Imidazole as Electron Relay in a Photoredox Triad.

Electron relays play a crucial role for efficient light-induced activation by a photo-redox moiety of catalysts for multi-electronic transformations. Their insertion between the two units reduces detrimental energy transfer quenching while establishing at the same time unidirectional electron flow. This rectifying function allows charge accumulation necessary for catalysis. Mapping these events in photophysical studies is an important step towards the development of efficient molecular photocatalysts. Three modular complexes comprised of a Ru-chromophore, an imidazole electron relay function, and a terpyridine unit as coordination site for a metal ion were synthesized and the light-induced electron transfer events studied by laser flash photolysis. In all cases, formation of an imidazole radical by internal electron transfer to the oxidized chromophore was observed. The effect of added base evidenced that the reaction sequence depends strongly on the possibility for deprotonation of the imidazole function in a proton-coupled electron transfer process. In the complex with MnII present as a proxy for a catalytic site, a strongly accelerated decay of the imidazole radical together with a decreased rate of back electron transfer from the external electron acceptor to the oxidized complex was observed. This transient formation of an imidazolyl radical is clear evidence for the function of the imidazole group as an electron relay. The implication of the imidazole proton and the external base for the kinetics and energetics of the electron trafficking is discussed.

Dissection of Light-Induced Charge Accumulation at a Highly Active Iron Porphyrin: Insights in the Photocatalytic CO2 Reduction.

Iron porphyrins are among the best molecular catalysts for the electrocatalytic CO2 reduction reaction. Powering these catalysts with the help of photosensitizers comes along with a couple of unsolved challenges that need to be addressed with much vigor. We have designed an iron porphyrin catalyst decorated with urea functions (UrFe) acting as a multipoint hydrogen bonding scaffold towards the CO2 substrate. We found a spectacular photocatalytic activity reaching unreported TONs and TOFs as high as 7270 and 3720 h-1 , respectively. While the Fe0 redox state has been widely accepted as the catalytically active species, we show here that the FeI species is already involved in the CO2 activation, which represents the rate-determining step in the photocatalytic cycle. The urea functions help to dock the CO2 upon photocatalysis. DFT calculations bring support to our experimental findings that constitute a new paradigm in the catalytic reduction of CO2 .

Intercepting a transient non-hemic pyridine N-oxide Fe(III) species involved in OAT reactions.

In the context of bioinspired OAT catalysis, we developed a tetradentate dipyrrinpyridine ligand, a hybrid of hemic and non-hemic models. The catalytic activity of the iron(III) derivative was investigated in the presence of iodosylbenzene. Unexpectedly, MS, EPR, Mössbauer, UV-visible and FTIR spectroscopic signatures supported by DFT calculations provide convincing evidence for the involvement of a relevant FeIII-O-NPy active intermediate.

Tracking light-induced electron transfer toward O2 in a hybrid photoredox-laccase system.

Photobiocatalysis uses light to perform specific chemical transformations in a selective and efficient way. The intention is to couple a photoredox cycle with an enzyme performing multielectronic catalytic activities. Laccase, a robust multicopper oxidase, can be envisioned to use dioxygen as a clean electron sink when coupled to an oxidation photocatalyst. Here, we provide a detailed study of the coupling of a [Ru(bpy)3]2+ photosensitizer to laccase. We demonstrate that efficient laccase reduction requires an electron relay like methyl viologen. In the presence of dioxygen, electrons transiently stored in superoxide ions are scavenged by laccase to form water instead of H2O2. The net result is the photo accumulation of highly oxidizing [Ru(bpy)3]3+. This study provides ground for the use of laccase in tandem with a light-driven oxidative process and O2 as one-electron transfer relay and as four-electron substrate to be a sustainable final electron acceptor in a photocatalytic process.

Atropisomeric Hydrogen Bonding Control for CO2 Binding and Enhancement of Electrocatalytic Reduction at Iron Porphyrins.

The manipulation of the second coordination sphere for improving the electrocatalytic CO2 reduction has led to breakthroughs with hydrogen bonding, local proton source, or electrostatic effects. We have developed two atropisomers of an iron porphyrin complex with two urea functions acting as multiple hydrogen-bonding tweezers to lock the metal-bound CO2 in a similar fashion found in the carbon monoxide dehydrogenase (CODH) enzyme. The αα topological isomer with the two urea groups on the same side of the porphyrin provides a stronger binding affinity to tether the incoming CO2 in comparison to the αß disposition. However, the electrocatalytic activity of the αß atropisomer outperforms its congener with one of the highest reported turnover frequencies at low overpotential. The strong H/D kinetic isotope effect (KIE) observed for the αα system indicates the existence of a tight water hydrogen-bonding network for proton delivery which is disrupted by addition of an acid source. The small H/D KIE for the αß isomer and the enhanced electrocatalytic performance on addition of stronger acid indicate the free access of protons to the bound CO2 on the opposite side of the urea arm.

A Reversible Electron Relay to Exclude Sacrificial Electron Donors in the Photocatalytic Oxygen Atom Transfer Reaction with O2 in Water.

Using light energy and O2 for the direct chemical oxidation of organic substrates is a major challenge. A limitation is the use of sacrificial electron donors to activate O2 by reductive quenching of the photosensitizer, generating undesirable side products. A reversible electron acceptor, methyl viologen, can act as electron shuttle to oxidatively quench the photosensitizer, [Ru(bpy)3 ]2+ , generating the highly oxidized chromophore and the powerful reductant methyl-viologen radical MV+. . MV+. can then reduce an iron(III) catalyst to the iron(II) form and concomitantly O2 to O2.- in an aqueous medium to generate an active iron(III)-(hydro)peroxo species. The oxidized photosensitizer is reset to its ground state by oxidizing an alkene substrate to an alkenyl radical cation. Closing the loop, the reaction of the iron reactive intermediate with the substrate or its radical cation leads to the formation of two oxygenated compounds, the diol and the aldehyde following two different pathways.

Second-Sphere Biomimetic Multipoint Hydrogen-Bonding Patterns to Boost CO2 Reduction of Iron Porphyrins.

Inspired by nature's orchestra of chemical subtleties to activate and reduce CO2 , we have developed a family of iron porphyrin derivatives in to which we have introduced urea groups functioning as multipoint hydrogen-bonding pillars on the periphery of the porphyrinic ring. This structure closely resembles the hydrogen-bond stabilization scheme of the carbon dioxide (CO2 ) adduct in the carbon monoxide dehydrogenase (CODH). We found that such changes to the second coordination sphere significantly lowered the overpotential for CO2 reduction in this family of molecular catalysts and importantly increased the CO2 binding rate while maintaining high turnover frequency (TOF) and selectivity. Entrapped water molecules within the molecular clefts were found to be the source of protons for the CO2 reduction.

Water Molecules Gating a Photoinduced One-Electron Two-Protons Transfer in a Tyrosine/Histidine (Tyr/His) Model of Photosystem†II.

We investigate a biomimetic model of a TyrZ /His190 pair, a hydrogen-bonded phenol/imidazole covalently attached to a porphyrin sensitizer. Laser flash photolysis in the presence of an external electron acceptor reveals the need for water molecules to unlock the light-induced oxidation of the phenol through an intramolecular pathway. Kinetics monitoring encompasses two fast phases with distinct spectral properties. The first phase is related to a one-electron transfer from the phenol to the porphyrin radical cation coupled with a domino two-proton transfer leading to the ejection of a proton from the imidazole-phenol pair. The second phase concerns conveying the released proton to the porphyrin N4 coordinating cavity. Our study provides an unprecedented example of a light-induced electron-transfer process in a TyrZ /His190 model of photosystem II, evidencing the movement of both the phenol and imidazole protons along an isoenergetic pathway.

Successive light-induced two electron transfers in a Ru-Fe supramolecular assembly: from Ru-Fe(ii)-OH2 to Ru-Fe(iv)-oxo.

In the present work we describe the synthesis and study of a RuII-FeII chromophore-catalyst assembly designed to perform the light-induced activation of an iron bound water molecule and subsequent photo-driven oxidation of a substrate. Using a series of spectroscopic techniques, we demonstrate that excitation of the chromophore unit with 450 nm light, in the presence of a sacrificial electron acceptor, triggers a cascade of electron transfers leading to the formation of a high valent iron(iv)-oxo center from an iron(ii)-bound water molecule. The activity of this catalytic center is illustrated by the oxidation of triphenyl phosphine.

A nickel dimethyl glyoximato complex to form nickel based nanoparticles for electrocatalytic H2 production.

We report on the electrochemical alteration of a nickel(II) bis-glyoximato complex into nickel-based nanoparticles at a glassy carbon electrode under acid reducing conditions. These particles show electrocatalytic activity towards hydrogen production at +410 mV compared to the bare glassy carbon electrode. Mechanistic insights are discussed based on DFT calculations.

Gold(I) complexes of phosphanyl gallanes: from interconverting to separable coordination isomers.

Gallant exchange: Upon coordination of phosphanyl gallane ligands to AuCl, both neutral and zwitterionic complexes coexist. NMR spectroscopy provides direct evidence for the transfer of the chloride between gold and gallium in diphosphanyl gallane. The introduction of a third phosphanyl buttress allows the separation and structural characterization of the two coordination isomers (see picture; Au yellow, P red, Cl green, Ga blue).

Ambiphilic diphosphine-borane ligands: metal-->borane interactions within isoelectronic complexes of rhodium, platinum and palladium.

Coordination of an ambiphilic diphosphine-borane (DPB) ligand to the RhCl(CO) fragment affords two isomeric complexes. According to X-ray diffraction analysis, each complex adopts a square-pyramidal geometry with trans coordination of the two phosphine buttresses and axial RhB contacts, but the two differ in the relative orientations around the rhodium and boron centres. DFT calculations on the actual complexes provide insight into the influence of the pi-accepting CO co-ligand, compared with previously reported complexes [Rh(mu-Cl)(dpb)]2 and [RhCl(dmap)(dpb)]. In addition, comparison of the nu(CO) frequency of [RhCl(CO)(dpb)] with that of the related borane-free complex [RhCl(CO)(iPr2PPh)2] substantiates the significant electron-withdrawing effect that the sigma-accepting borane moiety exerts on the metal. Valence isoelectronic [PtCl2(dpb)] and [PdCl2(dpb)] complexes have also been prepared and characterized spectroscopically and structurally. The pronounced influence of the transition metal on the magnitude of the M-->B interaction is highlighted by geometric considerations and NBO analyses.

Group 10 and 11 metal boratranes (Ni, Pd, Pt, CuCl, AgCl, AuCl, and Au+) derived from a triphosphine-borane.

The ambiphilic triphosphine-borane ligand 1 {TPB = [o-iPr2P-(C6H4)3B} readily coordinates to all group 10 and 11 metals to afford a complete series of metal boratranes (TPB)[M] 2-8 (2: M = Ni, 3: M = Pd, 4: M = Pt, 5: M = CuCl, 6: M = AgCl, 7: M = AuCl, 8: M = Au+). Spectroscopic and structural characterization unambiguously establishes the presence of M-B interactions in all of these complexes. The first evidence for borane coordination to copper and silver is provided, and the Au-->B interaction is shown to persist upon chloride abstraction. Experimental and theoretical considerations indicate that the M-->B interaction is strongest in the Pt and Au complexes. The influence of the oxidation state and charge of the metal is substantiated, and the consequences of relativistic effects are discussed. The coordination of the sigma-acceptor borane ligand is found to induce a significant bathochromic shift of the UV-vis spectra, the Ni, Pd, and Pt complex presenting strong absorptions in the visible range. In addition, all of the group 10 and 11 metal boratranes adopt C3 symmetry both in the solid state and in solution. The central M-->B interaction is found to moderately influence the degree of helicity and configurational stability of these three-bladed propellers, and DFT calculations support a dissociative pathway for the inversion process.