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).

A Computational Study on the Mechanism of Catalytic Cyclopropanation Reaction with Cobalt N-Confused Porphyrin: The Effects of Inner Carbon and Intramolecular Axial Ligand.

The factors that affect acceleration and high trans/cis selectivity in the catalytic cyclopropanation reaction of styrene with ethyl diazoacetate by cobalt N-confused porphyrin (NCP) complexes were investigated using density functional theory calculations. The reaction rate was primarily related to the energy gap between the cobalt-carbene adduct intermediates, A and B, which was affected by the NCP skeletons and axial pyridine ligands more than the corresponding porphyrin complex. In addition, high trans/cis stereoselectivity was determined at the TS1 and, in part, in the isomerization process at the carbon-centered radical intermediates, Ctrans and Ccis.

C(sp3)-H bond activation by the carboxylate-adduct of osmium tetroxide (OsO4).

The reaction of osmium tetroxide (OsO4) and carboxylate anions (acetate: X- = AcO- and benzoate: X- = BzO-) gave 1 : 1 adducts, [OsO4(X)]- (1X), the structures of which were determined by X-ray crystallographic analysis. In both cases, the carboxylate anion X coordinates to the osmium centre to generate a distorted trigonal bipyramidal osmium(VIII) complex. The carboxylate adducts show a negative shift of the redox potentials (E1/2) and a red shift of the νOsO stretches as compared to those of tetrahedral OsO4 itself. Despite the negative shift of E1/2, the reactivity of these adduct complexes 1X was enhanced compared to that of OsO4 in benzylic C(sp3)-H bond oxidation. The reaction obeyed the first-order kinetics on both 1X and the substrates, giving the second-order rate constant (k2), which exhibits a linear correlation with the C-H bond dissociation energy (BDEC-H) of the substrates (xanthene, 9,10-dihydroanthracene, fluorene and 1,2,3,4-tetrahydronaphthalene) and a kinetic deuterium isotope effect (KIE) of 9.7 (k2(xanthene-h2)/k2(xanthene-d2)). On the basis of these kinetic data together with the DFT calculation results, we propose a stepwise reaction mechanism involving rate-limiting benzylic hydrogen atom abstraction and subsequent rebound of the generated organic radical intermediate to a remaining oxido group on the osmium centre.

Tin(II)-Nitrene Radical Complexes Formed by Electron Transfer from Redox-Active Ligand to Organic Azides and Their Reactivity in C(sp3)-H Activation.

A tin(II) complex coordinated by a sterically demanding o-phenylenediamido ligand is synthesized. The ligand is redox-active to reach a tin(II) complex with the diiminobenzosemiquinone radial anion in the oxidation by AgPF6. The tin(II) complex reacts with a series of nosylazides (x-NO2C6H4-SO2-N3; x = o, m, or p) at -30 °C to yield the corresponding nitrene radical bound tin(II) complexes. The nitrene radical complexes exhibit C(sp3)-H activation and amination reactivity.

Photocatalytic hydrogen evolution using a Ru(ii)-bound heteroaromatic ligand as a reactive site.

A RuII complex, [RuII(tpphz)(bpy)2]2+ (1) (tpphz = tetrapyridophenazine, bpy = 2,2'-bipyridine), whose tpphz ligand has a pyrazine moiety, is converted efficiently to [RuII(tpphz-HH)(bpy)2]2+ (2) having a dihydropyrazine moiety upon photoirradiation of a water-methanol mixed solvent solution of 1 in the presence of an electron donor. In this reaction, the triplet metal-to-ligand charge-transfer excited state (3MLCT*) of 1 is firstly formed upon photoirradiation and the 3MLCT* state is reductively quenched with an electron donor to afford [RuII(tpphz˙-)(bpy)2]+, which is converted to 2 without the observation of detectable reduced intermediates by nano-second laser flash photolysis. The inverse kinetic isotope effect (KIE) was observed to be 0.63 in the N-H bond formation of 2 at the dihydropyrazine moiety. White-light (380-670 nm) irradiation of a solution of 1 in a protic solvent, in the presence of an electron donor under an inert atmosphere, led to photocatalytic H2 evolution and the hydrogenation of organic substrates. In the reactions, complex 2 is required to be excited to form its 3MLCT* state to react with a proton and aldehydes. In photocatalytic H2 evolution, the H-H bond formation between photoexcited 2 and a proton is involved in the rate-determining step with normal KIE being 5.2 on H2 evolving rates. Density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations on the reaction mechanism of H2 evolution from the ground and photo-excited states of 2 were performed to have a better understanding of the photocatalytic processes.

Role of Amino Acid Residues for Dioxygen Activation in the Second Coordination Sphere of the Dicopper Site of pMMO.

Formation of an active oxygen species at the dicopper site of pMMO is studied by using density functional theory (DFT) calculations. The role of the amino acid residues of tyrosine (Tyr374) and glutamate (Glu35) located in the second coordination sphere of the dicopper site is discussed in detail. The phenolic proton of the tyrosine residue is transferred to the Cu2O2 core in a two-step manner via the glutamate residue, and an electron is directly transferred to the Cu2O2 core. These proton- and electron-transfer processes induce the O-O bond cleavage of the µ-η2:η2-peroxodicopper(II) species to form the (µ-oxo)(µ-hydroxo)CuIICuIII species, which is able to play a key role of methane hydroxylation at the dicopper site of pMMO ( Inorg. Chem. 2013 , 52 , 7907 ). This proton-coupled electron-transfer mechanism is a little different from that in tyrosinase in that the proton of substrate tyrosine is directly transferred to the dicopper site ( J. Am. Chem. Soc. 2006 , 128 , 9873 ) because there is no proton acceptor in the vicinity of the dicopper site of tyrosinase. The rate-determining step for the formation of the (µ-oxo)(µ-hydroxo)CuIICuIII species is determined to be the O-O bond cleavage. These results shed new light on the interpretation of the role of the tyrosine and glutamate residues located in the second coordination sphere of the dicopper site of pMMO.