Non-Innocent Ligands: New Opportunities in Iron Catalysis Blanchard, Sébastien; Derat, Etienne; Desage-El Murr, Marine ...
European Journal of Inorganic Chemistry,
January 2012, Letnik:
2012, Številka:
3
Book Review, Journal Article
Recenzirano
Introduced in the late sixties, non‐innocent (or redox) ligands have been extensively studied for their unusual and intriguing chemical behavior. Their ability to delocalize and/or provide electrons ...to the metal center of organometallic complexes confers them undisputable chemical interest and has proved valuable in the development of novel synthetic methodologies. This review will focus on the chemistry and applications of low‐valent iron complexes bearing potentially non‐innocent ligands. Because of the elusive nature of these ligands, and whenever possible, theoretical calculations and analysis of spectroscopic data will be presented in an effort to provide insights into the catalytic activity of the complexes.
The ability of non‐innocent ligands (NILs) to act as a storage/supply unit of electrons promotes one‐ or two‐electron transfers, allowing iron catalysts to perform reactions that were previously restricted to catalysts based on noble metals. Progress in this promising area of research is reviewed with special emphasis on DFT calculations and analysis of spectroscopic data.
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.
ElectroNic: Electronic transfer from a nickel complex bearing redox‐active ligands to form radical species is studied. Combined UV/Vis and DFT experiments suggest that the redox‐active ligands in the complex sustain this reactivity, and have a greater influence than the metal‐based reactivity. This reactivity can be extended to a complex bearing a single “redox‐equivalent”.
A well-defined copper complex bearing iminosemiquinone ligands performs single electron reduction of an electrophilic CF3+ source into CF˙3 radicals. This redox behavior is enabled by the ligand ...which shuttles through two different redox states (iminosemiquinone and iminobenzoquinone) while the copper center is preserved as a Cu(ii). This system was used in the trifluoromethylation of silyl enol ethers, heteroaromatics and in the hydrotrifluoromethylation of alkynes. This is the first example of cooperative redox catalysis for the controlled generation of CF˙3 radicals.
This account describes some advances we have made in the field of iron catalysis. Two types of reactivity have been uncovered. Based on the use of an iron(II) precatalyst in the presence of NaBH4, ...the first one consists in a SET which can be useful for the reductive dehalogenation of iodide and bromide derivatives. Switching to the non‐innocent bis‐iminopyridine ligands promotes a previously undescribed Csp2−H activation reaction leading to biaryl derivatives. First clues into the intricate nature of the mechanism were obtained and suggested that the redox‐active bis‐iminopyridine ligand acts as an electron reservoir. The resulting buildup of electron density triggers the C−H bond breaking. All these findings are discussed in light of the existing literature and perspectives are given.
Complexes of triaryl- and trialkylboranes with N-heterocyclic carbenes (NHCs) participate in Suzuki−Miyaura cross-coupling reactions and provide coupled products in good yields under base-free ...conditions. The reaction can be applied to Csp2−Csp2 and Csp2−Csp3 carbon−carbon bond formation with triflates, iodides, bromides, and chlorides. These results enrich the utility of NHC−borane complexes, which can be added to the toolkit of Suzuki−Miyaura cross-couplings, along with boronic acids and organotrifluoroborates.
Pyrane to oxepane in one step: Highly regio‐ and stereoselective homologations dependent on the nature of carbonyl function and catalysts have been observed during the formation of enoxysilanes (see ...scheme). Trimethylsilyl trifluoromethanesulfonate (TMSOTf) and AlCl3 were identified as suitable catalysts to promote the homologation and ring‐enlargement.
Flavins and their alloxazine isomers are key chemical scaffolds for bioinspired electron transfer strategies. Their properties can be fine‐tuned by functional groups, which must be introduced at an ...early stage of the synthesis as their aromatic ring is inert towards post‐functionalization. We show that the introduction of a remote metal‐binding redox site on alloxazine and flavin activates their aromatic ring towards direct C−H functionalization. Mechanistic studies are consistent with a synthetic sequence involving ground‐state single electron transfer (SET) with an electrophilic source followed by radical‐radical coupling. This unprecedented reactivity opens new opportunities in molecular editing of flavins by direct aromatic post‐functionalization and the utility of the method is demonstrated with the site‐selective C6 functionalization of alloxazine and flavin with a CF3 group, Br or Cl, that can be further elaborated into OH and aryl for chemical diversification.
Selective functionalization of flavin analogues is desirable for property tuning but requires de novo synthesis. A synthetic strategy involving the introduction of a remote redox site followed by copper complexation turns an alloxazine and a flavin into redox‐active structures and activates their aromatic ring for site‐selective aromatic C−H functionalization. This unprecedented mechanism opens new vistas in the chemistry of flavins and analogues.
We report the design of a unique bioinspired ligand merging redox-active catechol and flavin-related alloxazine substructures. Upon coordination with a Ni(
ii
) salt, this hybrid ligand forms a ...trinuclear complex containing three Ni
II
centers and three redox-active ligands. This air-stable metallic triangle stabilizing protons and electrons was studied by X-ray crystallography,
infra
-red (IR) and UV-vis spectroscopy, SQUID magnetometry (Superconducting QUantum Interference Device), MAS-NMR (Magic Angle Spinning-Nuclear Magnetic Resonance), EPR (Electron Paramagnetic Resonance), electrochemistry and DFT (Density Functional Theory). This multidisciplinary approach supports the presence of protons located on the organic ligands, and an electronic structure involving three high spin Ni(
ii
) ions strongly ferromagnetically coupled with radicals. Both ligand and complex provide a new design for molecular bricks and bioinspired energy storage devices.
A new class of redox-active ligands merging catechol and alloxazine structures is reported. A trimetallic triangular complex is formed upon complexation to nickel.
Potential inversion refers to the situation where a protein cofactor or a synthetic molecule can be oxidized or reduced twice in a cooperative manner; that is, the second electron transfer is easier ...than the first. This property is very important regarding the catalytic mechanism of enzymes that bifurcate electrons and the properties of bidirectional redox molecular catalysts that function in either direction of the reaction with no overpotential. Cyclic voltammetry is the most common technique for characterizing the thermodynamics and kinetics of electron transfer to or from these molecules. However, a gap in the literature is the absence of analytical predictions to help interpret the values of the voltammetric peak potentials when potential inversion occurs; the cyclic voltammograms are therefore often analyzed by simulating the data, with no discussion of the possibility of overfitting and often no estimation of the error on the determined parameters. Here we formulate the theory for the voltammetry of freely diffusing or surface-confined two-electron redox species in the experimentally relevant irreversible limit where the peak separation depends on the scan rate. We explain why the model is intrinsically underdetermined, and we illustrate this conclusion by analysis of the voltammetry of a nickel complex with redox-active iminosemiquinone ligands. Being able to characterize the thermodynamics of two-electron electron-transfer reactions will be crucial for designing more efficient catalysts.
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