A significant number of isolable silylenes are currently known. They have quickly developed from laboratory curiosities to useful ligands in metal‐mediated homogeneous catalysis. This includes their ...utilization in various catalytic transformations, such as C−C cross‐coupling, cyclotrimerization, hydroformylation, borylation, deuteration, hydrosilylation, amination, hydrogenation, and transfer semi‐hydrogenation reactions. Recent studies suggest that the silylene ligands surpass the steering properties of their phosphine and N‐heterocyclic carbene (NHC) analogues and provide excellent chemo‐, regio‐, and stereoselectivites. Mechanistic studies suggest that their promoted performance of metal‐mediated catalytic transformations results from a strong σ‐donor character along with cooperative effects of their SiII centers. This Minireview covers the most recent advances in the field.
No longer laboratory curiosities: Stable silylenes are a versatile family of powerful steering ligands that increase catalytic activity and selectivity. This Minireview covers the recent progress of silylene ligands in homogeneous catalysis.
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Gottfried Huttner, formerly Professor of Inorganic Chemistry at Heidelberg University, passed away on July 21, 2021. With his pioneering contributions, he sustainably enriched the fields of X‐ray ...crystallography, organometallic chemistry and main group chemistry. He will be remembered as an outstanding researcher and charismatic teacher.
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Electrocatalytic water splitting is the key process for the formation of green fuels for energy transport and storage in a sustainable energy economy. Besides electricity, it requires water, an ...aspect that seldomly has been considered until recently. As freshwater is a limited resource (<1% of earth's water), lately, plentiful reports were published on direct seawater (around 96.5% of earth's water) splitting without or with additives (buffers or bases). Alternatively, the seawater can be split in two steps, where it is first purified by reverse osmosis and then split in a conventional water electrolyser. This quantitative analysis discusses the challenges of the direct usage of non-purified seawater. Further, herein, we compare the energy requirements and costs of seawater purification with those of conventional water splitting. We find that direct seawater splitting has substantial drawbacks compared to conventional water splitting and bears almost no advantage. In short, it is less promising than the two-step scenario, as the capital and operating costs of water purification are insignificant compared to those of electrolysis of pure water.
In this analysis, we show that direct seawater splitting with or without additives faces significant challenges and bears almost no advantage with respect to the costs and energy demands to purify water prior to water electrolysis.
Transition metal hydroxides (M‐OH) and their heterostructures (X|M‐OH, where X can be a metal, metal oxide, metal chalcogenide, metal phosphide, etc.) have recently emerged as highly active ...electrocatalysts for hydrogen evolution reaction (HER) of alkaline water electrolysis. Lattice hydroxide anions in metal hydroxides are primarily responsible for observing such an enhanced HER activity in alkali that facilitate water dissociation and assist the first step, the hydrogen adsorption. Unfortunately, their poor electronic conductivity had been an issue of concern that significantly lowered its activity. Interesting advancements were made when heterostructured hydroxide materials with a metallic and or a semiconducting phase were found to overcome this pitfall. However, in the midst of recently evolving metal chalcogenide and phosphide based HER catalysts, significant developments made in the field of metal hydroxides and their heterostructures catalysed alkaline HER and their superiority have unfortunately been given negligible attention. This review, unlike others, begins with the question of why alkaline HER is difficult and will take the reader through evaluation perspectives, trends in metals hydroxides and their heterostructures catalysed HER, an understanding of how alkaline HER works on different interfaces, what must be the research directions of this field in near future, and eventually summarizes why metal hydroxides and their heterostructures are inevitable for energy‐efficient alkaline HER.
This review brings out the key advancements made in the field of alkaline HER with metal hydroxides and their heterostructures and also provides a detailed and critical analysis of strategies and perspectives used with highlights on future prospects at the end.
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The first chelating bis(N-heterocyclic silylene)xanthene ligand SiII(Xant)SiII as well as its Ni complexes SiII(Xant)SiIINi(η2-1,3-cod) and SiII(Xant)SiIINi(PMe3)2 were synthesized and fully ...characterized. Exposing SiII(Xant)SiIINi(η2-1,3-cod) to 1 bar H2 at room temperature quantitatively generated an unexpected dinuclear hydrido Ni complex with a four-membered planar Ni2Si2 core. Exchange of the 1,3-COD ligand by PMe3 led to SiII(Xant)SiIINi(PMe3)2, which could activate H2 reversibly to afford the first SiII-stabilized mononuclear dihydrido Ni complex characterized by multinuclear NMR and single-crystal X-ray diffraction analysis. SiII(Xant)SiIINi(η2-1,3-cod) is a strikingly efficient precatalyst for homogeneous hydrogenation of olefins with a wide substrate scope under 1 bar H2 pressure at room temperature. DFT calculations reveal a novel mode of H2 activation, in which the SiII atoms of the SiII(Xant)SiII ligand are involved in the key step of H2 cleavage and hydrogen transfer to the olefin.
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This account is a review on the synthesis and transition‐metal coordination chemistry of N‐heterocyclic silylenes (NHSi’s) over the last 20 years till the present time (2012). Recently, fascinating ...and novel synthetic methods have been developed to access transition‐metal–NHSi complexes as an emerging class of compounds with a wealth of intriguing reactivity patterns. The striking influence of coordinating NHSi’s to transition‐metal complex fragments affording different reactivities to the “free” NHSi is a connecting theme (“leitmotif”) throughout the review, and highlights the potential of these compounds which lie at the interface of contemporary main‐group and classical organometallic chemistry towards new molecular catalysts for small‐molecule activation.
Simply Si: A comprehensive review on the synthesis, structural elucidation, and spectroscopic properties of N‐heterocyclic silylene (NHSi) transition‐metal complexes (Groups 4 to 12) is presented. A particular focus on the emerging reactivity of these complexes, particularly with respect to small‐molecule activation and catalysis, is a connecting theme throughout the review.
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Over the years, cobalt phosphates (amorphous or crystalline) have been projected as one of the most significant and promising classes of nonprecious catalysts and studied exclusively for the ...electrocatalytic and photocatalytic oxygen evolution reaction (OER). However, their successful utilization of hydrogen evolution reaction (HER) and the reaction of overall water‐splitting is rather unexplored. Herein, presented is a crystalline cobalt phosphate, Co3(OH)2(HPO4)2, structurally related to the mineral lazulite, as an efficient precatalyst for OER, HER, and water electrolysis in alkaline media. During both electrochemical OER and HER, the Co3(OH)2(HPO4)2 nanostructure was completely transformed in situ into porous amorphous CoOx
(OH) films that mediate efficient OER and HER with extremely low overpotentials of only 182 and 87 mV, respectively, at a current density of 10 mA cm−2. When assemble as anode and cathode in a two‐electrode alkaline electrolyzer, unceasing durability over 10 days is achieved with a final cell voltage of 1.54 V, which is superior to the recently reported effective bifunctional electrocatalysts. The strategy to achieve more active sites for oxygen and hydrogen generation via in situ oxidation or reduction from a well‐defined inorganic material provides an opportunity to develop cost‐effective and efficient electrocatalysts for renewable energy technologies.
A crystalline lazulite cobalt phosphate is identified as a low‐cost preelectrocatalyst for generating remarkably active and durable electrocatalysts for unifying the hydrogen evolution reaction, oxygen evolution reaction, and overall water‐splitting in alkaline media. Under oxidizing and reducing electrochemical environments, the restructuring (corrosion) of highly crystalline particles results in two different in situ‐generated amorphously active phases, yielding low overpotentials and cell potential.
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A facile synthesis and isolation of pristine silicon tetrakis(trifluoromethanesulfonate), Si(OTf)4, is reported, acting as the first neutral silicon‐based Lewis superacid suitable towards soft and ...hard Lewis bases. Its OTf groups have a dual function: they are excellent leaving groups and modulate the degree of reactivity towards soft and hard Lewis bases. Exposed to soft Lewis donors, Si(OTf)4 leads to L2Si(OTf)4 complexes (L=isocyanide, thioether and carbonyl compounds) with retention of all Si−OTf bonds. In contrast, it can cleave C−X bonds (X=F, Cl) of hard organic Lewis bases with a high tendency to form SiX4 (X=F, Cl) after halide/triflate exchange. Most notable, Si(OTf)4 allows a gentle oxydefluorination of mono‐ and bis(trifluoromethyl)benzenes, resulting in the formation of the corresponding benzoylium species, which are stabilized by the weakly coordinating Si(OTf)6 dianion.
The title compound is the first neutral silane acting as a hard and soft Lewis superacid as exemplified by extensive scaling. Its distinct reactivity towards soft Lewis bases L leads to unprecedented L2Si(OTf)4 complexes, while the reactions with halocarbons afford electrophilic cations, including a straight access to benzoylium cations from mono‐ and bis(trifluoromethyl)benzenes, stabilized by the Si(OTf)62− weakly coordinating dianion.
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The first neutral bis(metallylene)pyridine pincer-type ENE ligands (E = SiII, GeII) were synthesized, and their coordination chemistry and reactivity toward iron was studied. First, the ...unprecedented four-coordinate complexes κ 2 E,E ′-ENEFeCl 2 were isolated. Unexpectedly and in contrast to other related pyridine-based pincer-type Fe(II) complexes, the N atom of pyridine is reluctant to coordinate to the Fe(II) site due to the enhanced σ-donor strength of the E atoms, which disfavors this coordination mode. Subsequent reduction of κ 2 Si,Si ′-SiNSiFeCl 2 with KC8 in the presence of PMe3 or direct reaction of the ENE ligands using Fe(PMe3)4 produced the highly electron-rich iron(0) complexes ENEFe(PMe 3 ) 2 . The reduction of the iron center substantially changes its coordination features, as shown by the results of a single-crystal X-ray diffraction analysis of SiNSiFe(PMe 3 ) 2 . The iron center, in the latter, exhibits a pseudosquare pyramidal (PSQP) coordination environment, with a coordinative (pyridine)N→Fe bond, and a trimethylphosphine ligand occupying the apical position. This geometry is very unusual for Fe(0) low-spin complexes, and variable-temperature 1H and 31P NMR spectra of the ENEFe(PMe 3 ) 2 complexes revealed that they represent the first examples of configurationally stable PSQP-coordinated Fe(0) complexes: even after heating at 70 °C for >7 days, no changes are observed. The substitution reaction of ENEFe(PMe 3 ) 2 with CO resulted in the isolation of ENEFe(CO) 2 and the hitherto unknown κ 2 E,E ′-ENEFe(CO) 2 L (L = CO, PMe3) complexes. All complexes were fully characterized (NMR, MS, XRD, IR, and 57Fe Mössbauer spectroscopy), showing the highest electron density on the iron center for pincer-type complexes reported to date. DFT calculations and 57Fe Mössbauer spectroscopy confirmed the innocent behavior of these ligands. Moreover, preliminary results showed that these complexes can serve as active precatalysts for the hydrosilylation of ketones.
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This account describes recent progress (>2006) in the synthesis and structural characterization of isolable N-heterocyclic silylenes (NHSi's) and their fascinating reactivities with respect to an ...emergent topic in main-group chemistry: metal-free small-molecule activation. Since the seminal discovery of stable N-heterocyclic silicon analogues of nucleophilic Wanzlick−Arduengo-type carbenes in 1994, new types of NHSi's have emerged with unique electronic features and strikingly different reactivities. Among them, the first zwitterionic (ylide-like) silylene LSi: (L = CH(CCH2)CMeNAr2; Ar = 2,6-Pr i 2C6H3) and unprecedented N-heterocyclic bis(silylenes) with amidinate ligands and Si(I)−Si(I) bonds were synthesized. Their striking electronic structures open new doorways to metal-free activation of C−H, C−X, Si−X, E−H (E = group 15, group 16 elements), P−P, E−O (E = C, N), and E−E bonds (E = O, S, Se, Te).
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