Molecular Magnesium Hydrides Mukherjee, Debabrata; Okuda, Jun
Angewandte Chemie International Edition,
February 5, 2018, Volume:
57, Issue:
6
Journal Article
Peer reviewed
Solid magnesium hydride MgH2∞ has been pursued as a potential hydrogen‐storage material. Organic chemists were rather interested in soluble magnesium hydride reagents from mid‐20th century. It was ...only in the last two decades that molecular magnesium hydride chemistry received a major boost from organometallic chemists with a series of structurally well‐characterized examples that continues to build a whole new class of compounds. More than 40 such species have been isolated, ranging from mononuclear terminal hydrides to large hydride clusters with more than 10 magnesium atoms. They provide not only insights into the structure and bonding of Mg−H motifs, but also serve as models for hydrogen‐storage materials. Some of them are also recognized to participate in catalytic transformations, such as hydroelementation. Herein, an overview of these molecular magnesium hydrides is given, focusing on their synthesis and structural characterization.
Searching for the right size: Molecular magnesium hydrides, including larger clusters have been studied as potential hydrogen‐storage materials. An overview of structurally well‐defined molecular magnesium hydrides including larger clusters is compiled with emphasis on their synthesis, characterization, and properties.
The application of solid calcium hydride CaH2 has been mostly confined to its use as a desiccant, although its catalytic activity has long been known. Since the first isolation of a well‐defined ...molecular calcium hydride in 2006, the past decade has witnessed a gradual emergence of this new family of compounds. Although the detrimental Schlenk equilibrium has kept the number of examples low, the novelty of their reactivity, especially in small‐molecule activation, holds great promise. This Minireview gives an overview of the molecular calcium hydrides reported to date, highlighting their synthesis, structure, and reactivity.
More than just a drying agent: Structurally well‐defined molecular calcium hydrides are summarized in this Minireview, with a focus on their synthesis, structure, and reactivity, including catalysis.
Objective: To study the oldest Yakushi Buddha bronze statues holding a medicinal container made in the Asuka (Hakuhō) and Nara periods.Methods: The author investigated the Yakushi Buddha bronze ...statues holding a medicinal container by referring to the 12 volumes of Encyclopedia of National Treasures, Important Cultural Properties published by the Mainichi Press from 1997-2000. The author visited the Shin Yakushi Temple (Nara) and the Shojuraiko Temple (Shiga) to study the Yakushi Buddha bronze statues.Results and Discussion: The author confirmed that the oldest Yakushi Buddha bronze statue (73.0 cm in height) has been kept in the Shin Yakushi Temple (Nara), as Mrs. Masako Kida suggested. However, the original statue had been stolen, and a replica was produced with complete fidelity from a model. This statue holding a medicinal container is called the Ko Yakushi Buddha and was made during the Hakuho and Nara periods (A.D. 645-794). The second oldest Yakushi Buddha bronze statue holding a medicinal container is that of the Shojuraiko Temple (Shiga). It is 42.5 cm in height and was made during the Nara period (A.D. 710-794).
Triphenylborane (BPh3) was found to catalyze the reduction of tertiary amides with hydrosilanes to give amines under mild condition with high chemoselectivity in the presence of ketones, esters, and ...imines. N,N‐Dimethylacrylamide was reduced to provide the α‐silyl amide. Preliminary studies indicate that the hydrosilylation catalyzed by BPh3 may be mechanistically different from that catalyzed by the more electrophilic B(C6F5)3.
The simple borane triphenylborane (BPh3) catalyzes the chemoselective reduction of tertiary amides by hydrosilanes to give amines in the presence of halogen, ester, nitro, cyano, ketone, and imine groups. Conjugated amides are reduced to α‐silyl amides.
Reaction of dibenzyl calcium complex Ca(Me4TACD)(CH2Ph)2, containing the neutral NNNN‐type macrocyclic ligand Me4TACD (Me4TACD=1,4,7,10‐tetramethyl‐1,4,7,10‐tetraazacyclododecane), with ...triphenylsilane gave the cationic dinuclear calcium hydride Ca2H2(Me4TACD)2(PhCHSiPh3)2 which was characterized by NMR spectroscopy and single‐crystal X‐ray diffraction. The cation can be regarded as the ligand‐stabilized dimeric form of hypothetical CaH+. Hydrogenolysis of benzyl calcium cation Ca(Me4TACD)(CH2Ph)(thf)+ gave dicationic calcium hydrides Ca2H2(Me4TACD)2BAr42 (Ar=C6H4‐4‐tBu; C6H3‐3,5‐Me2) containing weakly coordinating anions. In THF, they catalyzed the isotope exchange of H2 and D2 to give HD and the hydrogenation of unactivated 1‐alkenes.
The solution is calcium hydride: The cationic dinuclear calcium hydride Ca2H2(Me4TACD)22+ was characterized by NMR spectroscopy and single‐crystal X‐ray diffraction. The complex catalyzes the HD exchange between H2 and D2 as well as the hydrogenation of unactivated 1‐alkenes.
A Hexagonal Planar Metal Complex Tauchert, Michael E.; Okuda, Jun
Angewandte Chemie International Edition,
March 9, 2020, Volume:
59, Issue:
11
Journal Article
Peer reviewed
A six‐coordinate ML3Z3‐type transition‐metal complex with a hexagonal planar geometry has been isolated and characterized, extending the scope of six‐coordinate metal coordination compounds to those ...with a geometry beyond octahedral and trigonal prismatic.
Flattened: A six‐coordinate ML3Z3‐type transition‐metal complex with a hexagonal planar geometry has been isolated and characterized, extending the scope of six‐coordinate metal coordination compounds to those with a geometry beyond octahedral and trigonal prismatic.
Chemistry freshmen learn to distinguish between nitrate NO3− and nitrite NO2− or sulfate SO42− and sulfite SO32−, but how many would know about carbonite CO22− in contrast to carbonate CO32−? This is ...not surprising for an anion that has never been observed in solution. However, it might be a crucial intermediate in the reductive activation of CO2. This review will track back carbonite and reflect on its relevance to carbon dioxide binding in metal complexes.
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•This review describes carbonite, which is the dianion of carbon dioxide as well as the conjugated base of dihydroxycarbene.•Carbonite is considered an important intermediate in the reductive activation of CO2.•We discuss electronic und structural features of CO22− and its metal derivatives.
Protonolysis of ZnH2n with the conjugated Brønsted acid of the bidentate diamine TMEDA (N,N,N′,N′‐tetramethylethane‐1,2‐diamine) and TEEDA (N,N,N′,N′‐tetraethylethane‐1,2‐diamine) gave the zinc ...hydride cation (L2)ZnH+, isolable either as the mononuclear THF adduct (L2)ZnH(thf)+BArF4− (L2=TMEDA; BArF4−=B(3,5‐(CF3)2‐C6H3)4−) or as the dimer {(L2)Zn)}2(μ‐H)22+BArF4−2 (L2=TEEDA). In contrast to ZnH2n, the cationic zinc hydrides are thermally stable and soluble in THF. (L2)ZnH+ was also shown to form di‐ and trinuclear adducts of the elusive neutral (L2)ZnH2. All hydride‐containing cations readily inserted CO2 to give the corresponding formate complexes. (TMEDA)ZnH+BArF4− catalyzed the hydrosilylation of CO2 with tertiary hydrosilanes to give stepwise formoxy silane, methyl formate, and methoxy silane. The unexpected formation of methyl formate was shown to result from the zinc‐catalyzed transesterification of methoxy silane with formoxy silane, which was eventually converted into methoxy silane as well.
Insoluble, thermally unstable zinc dihydride ZnH2n can be converted into robust zinc hydride cations (L2)ZnH+ which catalyze the homogeneous hydrosilylation of CO2 to give methoxy silane. Ligand L2 can be as simple as chelating TMEDA.
Incorporating artificial metal‐cofactors into protein scaffolds results in a new class of catalysts, termed biohybrid catalysts or artificial metalloenzymes. Biohybrid catalysts can be modified ...chemically at the first coordination sphere of the metal complex, as well as at the second coordination sphere provided by the protein scaffold. Protein‐scaffold reengineering by directed evolution exploits the full power of nature's diversity, but requires validated screening and sophisticated metal cofactor conjugation to evolve biohybrid catalysts. In this Minireview, we summarize the recent efforts in this field to establish high‐throughput screening methods for biohybrid catalysts and we show how non‐chiral catalysts catalyze reactions enantioselectively by highlighting the first successes in this emerging field. Furthermore, we shed light on the potential of this field and challenges that need to be overcome to advance from biohybrid catalysts to true artificial metalloenzymes.
Evolution revolution: In the past, incorporating artificial cofactors into proteins hampered high‐throughput approaches for the directed evolution of biohybrid catalysts. This Minireview provides an overview of the challenges, approaches, and success stories in the directed evolution of biohybrid catalysts and their transformation into highly specialized and enantioselective artificial metalloenzymes.
Chemo‐ and regioselectivity are often difficult to control during olefin hydrosilylation catalyzed by d‐ and f‐block metal complexes. The cationic hydride of calcium CaH+ stabilized by an NNNN ...macrocycle was found to catalyze the regioselective hydrosilylation of aliphatic olefins to give anti‐Markovnikov products, while aryl‐substituted olefins were hydrosilyated with Markovnikov regioselectivity. Ethylene was efficiently hydrosilylated by primary and secondary hydrosilanes to give di‐ and monoethylated silanes. Aliphatic hydrosilanes were preferred over other commonly employed hydrosilanes: Arylsilanes such as PhSiH3 underwent scrambling reactions promoted by the nucleophilic hydride, while alkoxy‐ and siloxy‐substituted hydrosilanes gave isolable alkoxy and siloxy calcium derivatives.
The CaH effect: The calcium hydride cation CaH+ catalyzes the regioselective hydrosilylation of ethylene, α‐olefins, and styrene derivatives. Aliphatic hydrosilanes are preferred over commonly employed silanes such as PhSiH3 as the latter undergoes scrambling reactions.
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