Macrocyclic Rhodium(III) Hydrides and a Monomeric Rhodium(II) Complex

A rhodium(II)/rhodium(III) redox couple for C-H bond amination with alkylazides: a rhodium(III)-nitrenoid intermediate with a tetradentate [14]-macrocyclic ligand

The catalytic activity of a rhodium(II) dimer complex, [RhII(TMAA)]2 (TMAA = tetramethyltetraaza[14]annulene), in C-H amination reactions with organic azides is explored. Organic azides (N3-R) with an electron-withdrawing group such as a sulfonyl group (trisylazide; R = S(O)2iPr3C6H2 (Trs)) and a simple alkyl group (R = (CH2)4Ph, (CH2)2OCH2Ph, CH2Ph, or C6H4NO2) are employed in intra- and intermolecular C-H bond amination reactions. The spectroscopic analysis using ESI-mass and EPR spectroscopy techniques on the reaction intermediate generated from [RhII(TMAA)]2 and N3-R reveals that a rhodium(III)-nitrenoid species is an active oxidant in the C-H bond amination reaction. DFT calculations suggest that the species can feature a radical localised nitrogen atom. The DFT calculation studies also indicate that the amination reaction involves hydrogen atom abstraction from the organic substrate R'-H by the NR moiety of 2N˙R and successive rebound of the generated organic radical intermediate R'˙ to [RhIII(NH-R)(TMAA)], giving [RhII(TMAA)] and R'-NH-R (amination product).

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.

Photochemical H2 Evolution from Bis(diphosphine)nickel Hydrides Enables Low-Overpotential Electrocatalysis

Molecules capable of both harvesting light and forming new chemical bonds hold promise for applications in the generation of solar fuels, but such first-row transition metal photoelectrocatalysts are lacking. Here we report nickel photoelectrocatalysts for H₂ evolution, leveraging visible-light-driven photochemical H₂ evolution from bis(diphosphine)nickel hydride complexes. A suite of experimental and theoretical analyses, including time-resolved spectroscopy and continuous irradiation quantum yield measurements, led to a proposed mechanism of H₂ evolution involving a short-lived singlet excited state that undergoes homolysis of the Ni-H bond. Thermodynamic analyses provide a basis for understanding and predicting the observed photoelectrocatalytic H₂ evolution by a 3d transition metal based catalyst. Of particular note is the dramatic change in the electrochemical overpotential: in the dark, the nickel complexes require strong acids and therefore high overpotentials for electrocatalysis; but under illumination, the use of weaker acids at the same applied potential results in a more than 500 mV improvement in electrochemical overpotential. New insight into first-row transition metal hydride photochemistry thus enables photoelectrocatalytic H₂ evolution without electrochemical overpotential (at the thermodynamic potential or 0 mV overpotential). This catalyst system does not require sacrificial chemical reductants or light-harvesting semiconductor materials and produces H₂ at rates similar to molecular catalysts attached to silicon.

UV-visible spectroscopy of macrocyclic alkyl, nitrosyl and halide complexes of cobalt and rhodium. Experiment and calculation

Visible light photolysis of Co(NH₃)₅CH₃²⁺ generates alkyl radicals, unlike the halide complexes Co(NH₃)₅X²⁺ which produce halogen anions, X⁻.

Kinetics and mechanism of the reduction of a macrocyclic Rh(III) complex by chromium(II) ions: pH-controlled selectivity to rhodium(II) vs. rhodium(III) hydride

Aqueous chromium(II) ions reduce a macrocyclic Rh(III) complex L(1)(H(2)O)(2)Rh(3+) (L(1) = 1,4,8,11-tetraazacyclotetradecane) to the hydride L(1)(H(2)O)RhH(2+) in two discrete, one-electron steps. The first step generates L(1)(H(2)O)Rh(2+) with kinetics that are first order in each rhodium(III) complex and Cr(H(2)O)(6)(2+), and inverse in [H(+)], k/M(-1) s(-1) = 0.065/(0.0031 + [H(+)]). Further reduction of L(1)(H(2)O)Rh(2+) to L(1)(H(2)O)RhH(2+) is kinetically independent of [H(+)], k/M(-1) s(-1) = 0.30. The difference in [H(+)] dependence allows relative rates of the two steps to be manipulated to generate either L(1)(H(2)O)Rh(2+) or L(1)(H(2)O)RhH(2+) as the final product.

Hydrogen atom donors: recent developments

This review highlights recent developments in the field of hydrogen atom transfer (HAT) reagents that circumvent the disadvantages of classical group 14 reagents, such as Bu₃SnH. Special emphasis is laid on the lowering of bond dissociation energies (BDEs) of molecules that could, as yet, not be used as HAT reagents and on the use of organometallic HAT reagents.

Aqueous rhodium(iii) hydrides and mononuclear rhodium(ii) complexes

In aqueous solutions, as in organic solvents, rhodium hydrides display the chemistry of one of the three limiting forms, i.e. {Rh(I)+ H+}, {Rh(II)+ H.}, and {Rh(III)+ H-}. A number of intermediates and oxidation states have been generated and explored in kinetic and mechanistic studies. Monomeric macrocyclic rhodium(II) complexes, such as L(H2O)Rh2+ (L = L1 = [14]aneN4, or L2 = meso-Me6[14]aneN4) can be generated from the hydride precursors by photochemical means or in reactions with hydrogen atom abstracting agents. These rhodium(II) complexes are oxidized rapidly with alkyl hydroperoxides to give alkylrhodium(III) complexes. Reactions of Rh(II) with organic and inorganic radicals and with molecular oxygen are fast and produce long-lived intermediates, such as alkyl, superoxo and hydroperoxo complexes, all of which display rich and complex chemistry of their own. In alkaline solutions of rhodium hydrides, the existence of Rh(I) complexes is implied by rapid hydrogen exchange between the hydride and solvent water. The acidity of the hydrides is too low, however, to allow the build-up of observable quantities of Rh(I). Deuterium kinetic isotope effects for hydride transfer to a macrocyclic Cr(v) complex are comparable to those for hydrogen atom transfer to various substrates.

Transition Metal Ion Initiated Chain Reactions between tert-Butyl Hydroperoxide and Rhodium Hydrides. Rapid Hydrogen Atom Abstraction from Rhodium Hydrides by Methyl Radicals

In the absence of catalysts, there is no reaction between t-BuOOH and macrocyclic rhodium hydrides LRhH(2+) (L(1) = [14]aneN(4), L(2) = Me(6)-[14]aneN(4)). The addition of Fe(aq)(2+) to such solutions results in rapid and catalytic oxidation of rhodium hydrides (LRhH(2+) + (CH(3))(3)COOH + H(+) --> LRh(3+) + CH(4) + (CH(3))(2)CO). This reaction occurs by a chain mechanism featuring methyl radicals and LRh(2+) as reactive intermediates. The rates are independent of the absolute concentrations of iron but vary with the ratio of the two oxidation states, -d[LRhH(2+)]/dt = k(obs)[t-BuOOH](2)[Fe(2+)]/[Fe(3+)]. The rate constants k(obs) are directly proportional to [H(+)]. In 0.10 M aqueous HClO(4) at 25 degrees C, the values are 80.8 M(-)(1) s(-)(1) (L(1)RhH(2+)) and 7.0 M(-)(1) s(-)(1) (L(2)RhH(2+)). A macrocyclic cobalt(II) complex, L(2)Co(2+), is also an effective chain initiator. The reaction occurs according to the rate law -d[L(1)RhH(2+)]/dt = 254[t-BuOOH][L(1)RhH(2+)], independent of [H(+)]. This reaction is not catalytic in L(2)Co(2+), which is slowly consumed in the initiation and termination steps. Methyl radicals react rapidly with L(1)RhH(2+) (k = 1.0 x 10(9) M(-)(1) s(-)(1)), L(2)RhH(2+) (1.4 x 10(8)), and L(1)Rh(2+) (8.6 x 10(8)).