Optimizing Group Transfer Catalysis by Copper Complex with Redox-Active Ligand in an Entatic State.

Metalloenzymes use earth-abundant non-noble metals to perform high-fidelity transformations in the biological world. To ensure chemical efficiency, metalloenzymes have acquired evolutionary reactivity-enhancing tools. Among these, the entatic state model states that a strongly distorted geometry induced by ligands around a metal center gives rise to an energized structure called entatic state, strongly improving the reactivity. However, the original definition refers both to the transfer of electrons or chemical groups, whereas the chemical application of this concept in synthetic systems has mostly focused on electron transfer, therefore eluding chemical transformations. Here we report that a highly strained redox-active ligand enables a copper complex to perform catalytic nitrogen- and carbon-group transfer in as fast as 2 min, thus exhibiting a strong increase in reactivity compared with its unstrained analogue. This report combines two reactivity-enhancing features from metalloenzymes, entasis and redox cofactors, applied to group-transfer catalysis.

Time-resolved X-ray absorption spectroelectrochemistry of redox active species in solution.

Electrochemistry and electrocatalysis have been receiving increased attention recently due to their crucial contribution to electrical-to-chemical conversion systems. We describe here the development and operation of a new spectroelectrochemical transmission cell for time-resolved X-ray absorption spectroscopy of solutions. X-ray absorption spectra were recorded on the ROCK beamline of SOLEIL under constant and scanning potentials. Spectra were recorded at a frequency of 2 Hz during a cyclic voltammetry experiment performed on a 20 mM solution of FeIIICl3·6H2O at 20 mV s-1 scanning speed. Spectra with good signal-to-noise ratios were obtained when averaging ten spectra over 5 s, corresponding to a 100 mV potential range. A 90% conversion rate from Fe(III) to Fe(II) was spectroscopically demonstrated in cyclic voltammetry mode.

Base-controlled mechanistic divergence between iron(iv)-oxo and iron(iii)-hydroperoxo in the H2O2 activation by a nonheme iron(ii) complex.

Activation of hydrogen peroxide by FeII salts (Fenton systems) leads to a myriad of oxidizing agents whose nature, FeIVO, or hydroxyl radicals and FeIII species, is dictated by the reaction conditions, in particular the pH value. Using the non heme FeII complex [FeII(L52)(CH3CN)]2+ (1) (where L52 is the pentadentate ligand N-methyl-N,N',N'-tris(2-pyridylmethyl)ethane-1,2-diamine) we have observed the simultaneous formation of two reaction intermediates, [FeIV(O)(L52)]2+ and [FeIII(OOH)(L52)]2+, in its reaction with excess hydrogen peroxide in the presence of sub-stoichiometric amounts of triethylamine. Kinetic and spectroscopic monitoring of the reaction mixture and of independently prepared [FeIV(O)(L52)]2+ in the presence of the different constituents of the reaction mixture allows drawing a mechanistic scheme. These two reactive species are formed simultaneously following two independent and competitive pathways. [FeIV(O)(L52)]2+ is obtained via heterolytic O-O cleavage of the oxidant assisted by the base in a peroxidase-like mechanism whereas [FeIII(OOH)(L52)]2+ is generated upon homolytic O-O cleavage of hydrogen peroxide. The relative contribution of these two pathways can be tuned by adjusting the amount of base used.

Selective Formation of an FeIV O or an FeIII OOH Intermediate From Iron(II) and H2 O2 : Controlled Heterolytic versus Homolytic Oxygen-Oxygen Bond Cleavage by the Second Coordination Sphere.

We demonstrate that the devised incorporation of an alkylamine group into the second coordination sphere of an FeII complex allows to switch its reactivity with H2 O2 from the usual formation of FeIII species towards the selective generation of an FeIV -oxo intermediate. The FeIV -oxo species was characterized by UV/Vis absorption and Mössbauer spectroscopy. Variable-temperature kinetic analyses point towards a mechanism in which the heterolytic cleavage of the O-O bond is triggered by a proton transfer from the proximal to the distal oxygen atom in the FeII -H2 O2 complex with the assistance of the pendant amine. DFT studies reveal that this heterolytic cleavage is actually initiated by an homolytic O-O cleavage immediately followed by a proton-coupled electron transfer (PCET) that leads to the formation of the FeIV -oxo and release of water through a concerted mechanism.

Copper-Catalyzed Aziridination with Redox-Active Ligands: Molecular Spin Catalysis.

Small-molecule catalysts as mimics of biological systems illustrate the chemists' attempts at emulating the tantalizing abilities displayed by nature's metalloenzymes. Among these innate behaviors, spin multistate reactivity is used by biological systems as it offers thermodynamic leverage towards challenging chemical reactivity but this concept is difficult to translate into the realm of synthetic organometallic catalysis. Here, we report a rare example of molecular spin catalysis involving multistate reactivity in a small-molecule biomimetic copper catalyst applied to aziridination. This behavior is supported by spin state flexibility enabled by the redox-active ligand.

Circumventing Intrinsic Metal Reactivity: Radical Generation with Redox-Active Ligands.

Nickel complexes have gained sustained attention as efficient catalysts in cross-coupling reactions and co-catalysts in dual systems due to their ability to react with radical species. Central to this reactivity is nickel's propensity to shuttle through several accessible redox states from Ni0 to NiIV . Here, we report the catalytic generation of trifluoromethyl radicals from a nickel complex bearing redox-active iminosemiquinone ligands. This unprecedented reactivity is enabled through ligand-based oxidation performing electron transfer to an electrophilic CF3+ source while the nickel oxidation state is preserved. Additionally, extension of this reactivity to a copper complex bearing a single redox equivalent is reported, thus providing a unified reactivity scheme. These results open new pathways in radical chemistry with redox-active ligands.

Structural, magnetic and optical properties of an Fe(III) dimer bridged by the meridional planar divergent N,N'-bis(salicyl)hydrazide and its photo- and electro-chemistry in solution.

{Fe(III)Cl(DMF)(2)}(2)(L) where L is N,N'-bis(salicyl)hydrazide has been synthesized as red crystals and characterized using single-crystal diffraction, infrared and UV-vis spectroscopies, and its magnetic properties studied. The dimeric unit in the structure is formed through the two meridional sets of divergent O, N, O coordinating atoms of the hexacoordinated and quadruply charged ligand. With the presence of the inversion symmetry the Fe atoms are strictly planar with the ligand. The magnetic exchange interaction is found to be antiferromagnetic with a J = -5.98(3) cm(-1) through the rare Fe-N-N-Fe pathway. Irradiation of the FeCl(3)/H(4)L red DMF solution in the visible region of the spectrum resulted in its complete discoloration and from which the unknown colorless salt [Fe(II)(DMF)(6)][Fe(II)Cl(4)] and the neutral ligand have been identified by single crystal diffraction. The UV-visible spectra of FeCl(3), H(4)L and their mixture in DMF solution indicate that the iron complex is the absorbing species and the presence of the free ligand in the irradiated solution suggests that the ligand is potentially acting as a catalyst to the photoreduction of Fe(III) to Fe(II), while electrochemistry points to a mixed-valent (Fe(II)-Fe(III)) intermediate in the process.