Transition metal catalysed photo-induced oxidative C-H bond functionalization and water oxidation

Abstract

A series of cyclometalated gold(III) complexes with N-heterocyclic carbine (NHC) or alkynyl as auxiliary ligand were synthesized and characterized. Complexes [AuIII(R1

–C^N^C)(NHC)](OTf) and [AuIII(C^N^C)(C≡CR2)] (HC^N^CH = 2,6-di(naphthalene-2-yl)pyridine; R1 = H or 4-methoxyphenyl; R2 =aryl) are emissive in solution at room temperature with quantum yields in the range of 0.65–11.4% and lifetimes ranging from 98 to 506 s. [AuIII(4-(4-OMePh)–C^N^C)(NHC)](OTf) showed prominent photochemical properties. This complex effectively catalyses photo-induced oxidation of secondary amines (to the corresponding imines) and -functionalization of tertiary amines in good to excellent yields; it also acts as a photosensitizer for hydrogen generation in a water/acetonitrile mixture, producing more than 350 turnovers of hydrogen after 4 hours of irradiation.

Palladium(II) meso-tetrakis(pentafluorophenyl)porphyrin was found to be an efficient and robust catalyst for the photo-induced oxidative C–H bond functionalization reactions. Several kinds of -functionalized tertiary amines were obtained in good to excellent yields by irradiating a mixture of palladium(II) catalyst, corresponding tertiary amine and nucleophile under aerobic conditions. The nucleophiles for these reactions include cyanide, nitromethane, dimethyl malonate, diethyl phosphite and acetone. Two examples of novel intramolecularly cyclized amines were also described. Comparison of the UV-vis absorption spectra before and after reaction indicated that the palladium catalyst was highly robust. The practical potential of this catalyst was shown by the success in reactions at a low catalyst loading and on a large scale. The palladium(II) catalyst could also sensitize photo-induced oxidation of sulfide to sulfoxide and photo-induced hydrogen production in a water/acetonitrile mixture with up to 240 turnovers.

[FeIII(L-N4Me2)Cl2][FeCl4] (L-N4Me2 = N,N’-dimethyl-2,11-diaza[3,3] (2,6)pyridinophane) was demonstrated to be an active catalyst for water oxidation. When cerium ammonium nitrate (CAN) was used as the oxidant, the iron(III) catalyst oxidized water to oxygen with up to 93 turnovers after 30 minutes in 0.1 M nitric acid, whereas changing the oxidant to sodium periodate (NaIO4) resulted in only 44 turnovers of oxygen after 30 minutes. The mechanism of the reaction was explored by high resolution electrospray ionization mass spectrometry (ESI-MS), 18O labeling, UV-vis absorption spectroscopy, kinetic plots and DFT calculations. In the case of using CAN, an FeIV-oxo species was detected by ESI-MS and UV-vis absorption spectroscopy. The rate of oxygen evolution was found to be linearly dependent on both concentrations of catalyst and oxidant. 18O labeling studies confirmed that the origin of oxo ligands was from water and was irrespective of the choice of oxidant. This reaction was proposed to involve a coupling between an FeIV-oxo species and a hydroxocerium(IV) radical. In the case of using NaIO4, an FeV-dioxo species was detected by ESI-MS as a major species, and a small amount of FeIV-oxo species was detected by UV-vis absorption spectroscopy. As the rate of oxygen evolution was found to be linearly dependent on the concentration of catalyst only, the reaction was proposed to involve a cis-FeV-dioxo species. DFT calculations showed that the cis-FeV-dioxo species was capable of oxidizing water to oxygen through the formation of an [FeIII(L-N4Me2)(OO?)(OH)]+ intermediate.