Since the discovery of the first stable N-heterocyclic carbene (NHC) in the beginning of the 1990s, these divalent carbon species have become a common and available class of compounds, which have found numerous applications in academic and industrial research. Their important role as two-electron donor ligands, especially in transition metal chemistry and catalysis, is difficult to overestimate. In the past decade, there has been tremendous research attention given to the chemistry of low-coordinate main group element compounds. Significant progress has been achieved in stabilization and isolation of such species as Lewis acid/base adducts with highly tunable NHC ligands. This has allowed investigation of numerous novel types of compounds with unique electronic structures and opened new opportunities in the rational design of novel organic catalysts and materials. This Review gives a general overview of this research, basic synthetic approaches, key features of NHC–main group element adducts, and might be...

NHCs in Main Group Chemistry.

Charge carrier trapping, recombination and transfer during TiO2 photocatalysis: An overview

The emergence of N-heterocyclic carbenes as ligands across the Periodic Table had an impact on various aspects of the coordination, organometallic, and catalytic chemistry of the 3d metals, including Cu, Ni, and Co, both from the fundamental viewpoint but also in applications, including catalysis, photophysics, bioorganometallic chemistry, materials, etc. In this review, the emergence, development, and state of the art in these three areas are described in detail.

N-Heterocyclic Carbene Complexes of Copper, Nickel, and Cobalt

Metal-halide perovskites have been widely investigated in the photovoltaic sector due to their promising optoelectronic properties and inexpensive fabrication techniques based on solution processing. Here we report the development of inorganic CsPbBr3-based photoanodes for direct photoelectrochemical oxygen evolution from aqueous electrolytes. We use a commercial thermal graphite sheet and a mesoporous carbon scaffold to encapsulate CsPbBr3 as an inexpensive and efficient protection strategy. We achieve a record stability of 30 h in aqueous electrolyte under constant simulated solar illumination, with currents above 2 mA cm−2 at 1.23 VRHE. We further demonstrate the versatility of our approach by grafting a molecular Ir-based water oxidation catalyst on the electrolyte-facing surface of the sealing graphite sheet, which cathodically shifts the onset potential of the composite photoanode due to accelerated charge transfer. These results suggest an efficient route to develop stable halide perovskite based electrodes for photoelectrochemical solar fuel generation. While photoelectrochemical cells may offer access to solar fuels from a single integrated device, halide perovskite photoelectrodes are difficult to use due to their inherent moisture sensitivity. Here, the authors protect perovskite photoanodes with graphite sheets to boost their stability in water.

/pdf/graphite-protected-cspbbr3-perovskite-photoanodes-484zz124gt.pdf

Graphite-protected CsPbBr3 perovskite photoanodes functionalised with water oxidation catalyst for oxygen evolution in water

This review serves to document advances in the synthesis, versatile bonding, and reactivity of molecular main group metal hydrides within Groups 1, 2, and 12-16. Particular attention will be given to the emerging use of said hydrides in the rapidly expanding field of Main Group element-mediated catalysis. While this review is comprehensive in nature, focus will be given to research appearing in the open literature since 2001.

Molecular Main Group Metal Hydrides

The trapping of a silicon(I) radical with N-heterocyclic carbenes is described The reaction of the cyclic (alkyl)(amino) carbene [cAACMe] (cAACMe=:C(CMe2)2(CH2)NAr, Ar=2,6-iPr2C6H3) with H2SiI2 in a 3:1 molar ratio in DME afforded a mixture of the separated ion pair [(cAACMe)2Si:]+I− (1), which features a cationic cAAC–silicon(I) radical, and [cAACMe−H]+I− In addition, the reaction of the NHC–iodosilicon(I) dimer [IAr(I)Si:]2 (IAr=:C{N(Ar)CH}2) with 4 equiv of IMe (:C{N(Me)CMe}2), which proceeded through the formation of a silicon(I) radical intermediate, afforded [(IMe)2SiH]+I− (2) comprising the first NHC–parent-silyliumylidene cation Its further reaction with fluorobenzene afforded the CAr−H bond activation product [1-F-2-IMe-C6H4]+I− (3) The isolation of 2 and 3 confirmed the reaction mechanism for the formation of 1 Compounds 1–3 were analyzed by EPR and NMR spectroscopy, DFT calculations, and X-ray crystallography

/pdf/trapping-a-silicon-i-radical-with-carbenes-a-cationic-caac-2jkyxiqz31.pdf

Trapping a Silicon(I) Radical with Carbenes: A Cationic cAAC–Silicon(I) Radical and an NHC–Parent-Silyliumylidene Cation

Progress in frustrated Lewis pair (FLP) chemistry has revealed the importance of the main group elements in catalysis, opening new avenues in synthetic chemistry. Recently, new reactivities of frustrated Lewis pairs have been uncovered that disclose that certain combinations of Lewis acids and bases undergo single-electron transfer (SET) processes. Here an electron can be transferred from the Lewis basic donor to a Lewis acidic acceptor to generate a reactive frustrated radical pair (FRP). This minireview aims to showcase the recent advancements in this emerging field covering the synthesis and reactivities of frustrated radical pairs, with extensive highlights of the results from Electron Paramagnetic Resonance (EPR) spectroscopy to explain the nature and stability of the different radical species observed.

https://www.onlinelibrary.wiley.com/doi/pdf/10.1002/ange.202010633

Frustrated Radical Pairs: Insights from EPR Spectroscopy.

Photocatalytic removal of single volatile organic compounds (VOCs) has been widely investigated; however, photodecomposition of VOC mixtures has been rarely addressed, which may bring safety doubts in indoor air purification due to possible formation of harmful compounds. Here we show that in photocatalytic oxidation of formaldehyde–toluene and formaldehyde–xylene mixtures, the introduction of Pd cocatalyst on TiO2 photocatalyst successfully inhibits the formation of toxic methylphenols, thus promoting the complete mineralization of VOC mixtures into CO2 via the harmless benzaldehyde intermediates. Mechanistic analysis reveals that the loading of Pd cocatalyst effectively removes the inherent surface −OH groups of TiO2, which significantly promotes the activation of O2 into radical dotOH radicals. The Pd cocatalyst also directs the radical dotOH radicals to attack the methyl group instead of the aromatic ring for the formation of benzaldehyde and its further oxidation to CO2, thus yielding a better overall photocatalytic performance.

Inhibit the formation of toxic methylphenolic by-products in photo-decomposition of formaldehyde-toluene/xylene mixtures by Pd cocatalyst on TiO2

Advances in the chemistry of metal-free systems known as frustrated Lewis pairs (FLPs) has exposed new reactivity of the p-block elements, particularly in small-molecule activation and catalysis. Typically, the mode of activation by FLPs has been predicated on a heterolytic two-electron process, although recently, select FLPs have been shown to participate in single-electron processes. Here, we report the reaction of diaryl substituted esters with FLPs. This results in divergent pathways, one whereby the diaryl moiety is stabilized by the Lewis basic phosphine, and the alternative pathway, wherein a single-electron transfer process occurs, generating the [Mes3P]+⋅/[C(H)Ar2]⋅ radical ion pair. The latter species undergoes a homocoupling reaction to yield tetraphenylethane derivatives. In the presence of olefins, this reactivity can be harnessed through an sp2-sp3 C–C heterocoupling reaction to generate α,β-substituted olefins. Notably, this work showcases an FLP approach to metal-free radical C–H bond activation with subsequent C–C bond formation, which also displays complementary reactivity to other approaches.

Emma Richards

Papers

Trapping a Silicon(I) Radical with Carbenes: A Cationic cAAC–Silicon(I) Radical and an NHC–Parent-Silyliumylidene Cation

Frustrated Radical Pairs: Insights from EPR Spectroscopy.

Inhibit the formation of toxic methylphenolic by-products in photo-decomposition of formaldehyde-toluene/xylene mixtures by Pd cocatalyst on TiO2

Radical Reactivity of Frustrated Lewis Pairs with Diaryl Esters

Investigations into the Photophysical and Electronic Properties of Pnictoles and Their Pnictenium Counterparts