Thermally sensitive polymeric zinc dihydride ZnH2 n can conveniently be prepared by the reaction of ZnEt2 with AlH3(NEt3). When reacted with CO2 (1 bar) in the presence of chelating N-donor ligands L ...n = N,N,N′,N′-tetramethylethylenediamine (TMEDA), N,N,N′,N′-tetramethyl-1,3-propanediamine (TMPDA), N,N,N′,N″,N′′-pentamethyldiethylenetriamine (PMDTA), and 1,4,7,10-tetramethyl-1,4,7,10-tetraazacyclododecane (Me4TACD), insertion into the Zn–H bond readily occurred. Depending on the denticity n, formates (L n )Zn(OCHO)2 were isolated and structurally characterized, either as a molecule (L n = TMEDA, TMPDA, PMDTA) or a charge-separated ion pair (L n )Zn(OCHO)OCHO (L n = Me4TACD). The reaction of ZnH2 n with the mild Lewis acid BPh3 in the presence of chelating N-donor ligands L n gave a series of hydridotriphenylborates, either as a contact ion pair (L2)Zn(H)(HBPh3) (L2 = TMEDA, TMPDA) or a separated ion pair (L n )Zn(H)HBPh3 (L n = PMDTA, Me4TACD). In the crystal, the contact ion pair (TMEDA)Zn(H)(HBPh3) showed a bent Zn–H–B bridge indicative of a delocalized Zn–H–B interaction. In contrast, a linear Zn–H–B bridge for (TMPDA)Zn(H)(HBPh3) was observed, suggesting a contact ion pair. In THF solution, both complexes show an exchange with free BPh3 as well as HBPh3−. DFT calculations suggest the presence of HBPh3− anion with a highly polarized B–H bond that interacts with the Lewis acidic zinc hydride cation (L2)Zn(H)+. The hydridotriphenylborates (L n )Zn(H)(HBPh3) underwent CO2 insertion to give (formato)zinc (formoxy)triphenylborate complexes (L n )Zn(OCHO)(OCHO)BPh3 (L n = TMPDA, PMDTA, Me4TACD). For L n = TMEDA, a dinuclear complex (L n )2Zn2(μ-OCHO)3(OCHO)BPh3 was isolated. Hydridotriphenylborates (L n )Zn(H)(HBPh3) catalyzed the hydrosilylation of CO2 (1 bar) by n BuMe2SiH in THF at 70 °C to give formoxysilane and (methoxy)silane.
Octahedral complexes of hexavalent molybdenum containing an (OSSO)-type bis(phenolate) ligand were prepared and structurally characterized. These complexes were screened as catalysts for the ...deoxydehydration of anhydroerythritol using 3-octanol as reducing agent. Display omitted
Bio-based polyols can be converted to olefins and furan derivatives in one step by combined reduction and dehydration (deoxydehydration, DODH). A series of octahedral complexes of hexavalent molybdenum containing an (OSSO)-type bis(phenolate) ligand were prepared and structurally characterized. These complexes were screened as catalyst precursors for the deoxydehydration of anhydroerythritol using 3-octanol as reducing agent. Microwave heating allows a lower reaction temperature.
Zinc has been an element of choice for carbon dioxide reduction in recent years. Zinc compounds have been showcased as catalysts for carbon dioxide hydrosilylation and hydroboration. The extent of ...carbon dioxide reduction can depend on various factors, including electrophilicity at the zinc center and the denticity of the ancillary ligands. In a few cases, the addition of Lewis acids to zinc hydride catalysts markedly influences carbon dioxide reduction. These factors have been investigated by exploring elementary reactions of carbon dioxide hydrosilylation and hydroboration by using cationic zinc hydrides bearing tetradentate tris2‐(dimethylamino)ethylamine and tridentate N,N,N′,N′′,N′′‐pentamethyldiethylenetriamine in the presence of triphenylborane and tris(pentafluorophenyl)borane.
Exploring elementary reactions: Cationic zinc hydrides are successfully probed for CO2 reduction. The mechanistic pathways operative in cationic zinc hydride catalyzed hydrosilylation and hydroboration are established through experiments and computations. The coordination number at zinc and secondary Lewis acid dictate the efficacy.
Cationic calcium hydride complexes with a Ca3(μ3‐H)2 core were prepared from different organocalcium precursors and Ph2SiH2. The hydride complexes were found to be active in the catalytic ...hydrosilylation and hydrogenation of 1,1‐diphenylethene.
Cationic alkyl complexes of the rare‐earth metals LnRm(L)n(3–m)+ (R=alkyl; m=1, 2; L=Lewis base) were virtually unknown species until recently. Because of their increased Lewis ...acidity/electrophilicity they should have considerable potential as homogeneous catalysts in olefin polymerization and in organic transformations. They can be generated by treating the neutral rare‐earth metal precursors containing at least two alkyl groups R with suitable Lewis or Brønsted acids in the presence of weakly coordinating anions. Not only monocationic but also dicationic alkyl derivatives have been shown to be accessible. In the context of modeling homogeneous ethylene polymerization using a mixture consisting of LnR3/AlR3/NMe2HPhB(C6F5)4, such dications were discovered. Some thermally robust examples of mono‐ and dicationic alkyl complexes have been structurally characterized as solvent‐separated ion pairs. Neutral and monoanionic macrocycles such as crown ethers or aza‐crown ethers as well as amidinato, β‐diketiminato, and substituted cyclopentadienyl ligands are suitable ancillary ligands to stabilize the cationic alkyl fragments.
Group 4 metal initiators with a tetradentate bis(phenolato) ligand polymerized meso-lactide efficiently under ring-opening to give syndiotactic polylactide. L-Lactide was converted faster than ...rac-lactide and meso-lactide.
•This review summarizes and classifies cationic rare-earth metal hydrides, a relatively recent addition to the growing body of molecular rare-earth metal hydrides.•The latter has been reviewed ...several times in the literature.•The introduction of cationic charges can be expected to reduce the nuclearity of rare-earth metal hydride fragments, at the same time to result in increase of Lewis acidity and electrophilicity.
A survey of the literature on cationic rare-earth metal hydride complexes is presented. This overview follows systematics based on the nature of supporting ligands and provides comparisons between neutral and cationic hydrides with respect to their structure and reactivity.
When light alkali metal amides M(Me3TACD)n (M=Li, Na, K; (Me3TACD)H=1,4,7‐trimethyl‐1,4,7,10‐tetraazacyclododecane) were treated with H2SiPh2 in THF, M{(H2SiPh2)Me3TACD}(thf)x containing a pendant ...hypervalent dihydridosilicate were formed and characterized by elemental analysis, NMR/IR spectroscopy, and single‐crystal X‐ray crystallography. The lithium complex catalyzed the hydrosilylation of styrene derivatives under mild conditions with anti‐Markovnikov regioselectivity.
Silicon time! Light alkali metal amides M{(H2SiPh2)Me3TACD}(thf)x (M=Li, Na, K) containing a pendant hypervalent dihydridosilicate were characterized. The lithium complex catalyzed the regioselective hydrosilylation of styrene derivatives (see figure).
