No abstract

Efficient polyoxometalate (POM)-based Lewis acid-base catalysts of the rare-earth-metal-containing POMs (TBA(6)RE-POM, RE = Y(3+), Nd(3+), Eu(3+), Gd(3+), Tb(3+), or Dy(3+)) were designed and synthesized by reactions of TBA(4)H(4)[γ-SiW(10)O(36)] (TBA = tetra-n-butylammonium) with RE(acac)(3) (acac = acetylacetonato). TBA(6)RE-POM consisted of two silicotungstate units pillared by two rare-earth-metal cations. Nucleophilic oxygen-enriched surfaces of negatively charged POMs and the incorporated rare-earth-metal cations could work as Lewis bases and Lewis acids, respectively. Consequently, cyanosilylation of carbonyl compounds with trimethylsilyl cyanide ((TMS)CN) was efficiently promoted in the presence of the rare-earth-metal-containing POMs via the simultaneous activation of coupling partners on the same POM molecules. POMs with larger metal cations showed higher catalytic activities for cyanosilylation because of the higher activation ability of C═O bonds (higher Lewis acidities) and sterically less hindered Lewis acid sites. Among the POM catalysts examined, the neodymium-containing POM showed remarkable catalytic performance for cyanosilylation of various kinds of structurally diverse ketones and aldehydes, giving the corresponding cyanohydrin trimethylsilyl ethers in high yields (13 substrates, 94-99%). In particular, the turnover frequency (714,000 h(-1)) and the turnover number (23,800) for the cyanosilylation of n-hexanal were of the highest level among those of previously reported catalysts.

Polyoxometalates (POMs) with heterodinuclear lanthanoid cores, TBA8H4[{Ln(μ2-OH)2Ln'}(γ-SiW10O36)2] (LnLn'; Ln = Gd, Dy; Ln' = Eu, Yb, Lu; TBA = tetra-n-butylammonium), were successfully synthesized through the stepwise incorporation of two types of lanthanoid cations into the vacant sites of lacunary [γ-SiW10O36](8-) units without the use of templating cations. The incorporation of a Ln(3+) ion into the vacant site between two [γ-SiW10O36](8-) units afforded mononuclear Ln(3+)-containing sandwich-type POMs with vacant sites (Ln1; TBA8H5[{Ln(H2O)4}(γ-SiW10O36)2]; Ln = Dy, Gd, La). The vacant sites in Ln1 were surrounded by coordinating W-O and Ln-O oxygen atoms. On the addition of one equivalent of [Ln'(acac)3] to solutions of Dy1 or Gd1 in 1,2-dichloroethane (DCE), heterodinuclear lanthanoid cores with bis(μ2-OH) bridging ligands, [Dy(μ2-OH)2Ln'](4+), were selectively synthesized (LnLn'; Ln = Dy, Gd; Ln' = Eu, Yb, Lu). On the other hand, La1, which contained the largest lanthanoid cation, could not accommodate a second Ln'(3+) ion. DyLn' showed single-molecule magnet behavior and their energy barriers for magnetization reversal (ΔE/kB) could be manipulated by adjusting the coordination geometry and anisotropy of the Dy(3+) ion by tuning the adjacent Ln'(3+) ion in the heterodinuclear [Dy(μ2-OH)2Ln'](4+) cores. The energy barriers increased in the order: DyLu (ΔE/kB = 48 K) < DyYb (53 K) < DyDy (66 K) < DyEu (73 K), with an increase in the ionic radii of Ln'(3+); DyEu showed the highest energy barrier.

Ein Yttrium‐verbrücktes Silicowolframat‐Dimer (siehe Bild) katalysiert die Cyanosilylierung von Ketonen und Aldehyden mit Trimethylsilylcyanid (TMSCN). Die Reaktionen verlaufen selektiv und geben die entsprechenden Cyanhydrintrimethylsilylether. Die katalytische Aktivität des Dimers ist vor allem für Aldehyde beeindruckend, mit einer Umsatzzahl von 18 000 und einer Umsatzfrequenz von 540 000 h−1 für n‐Hexanal.

Heterodinuclear lanthanoid cores are synthesized through stepwise incorporation of two types of lanthanoid cations into the vacant sites of lacunary [γ-SiW10O36] 8units without the use of templating cations. The compounds (III) crystallize in the monoclinic space group P21/c with Z = 4. (V) and (VII) crystallize in the monoclinic space group P2 1 /n with Z = 2. Compounds (Va-c) display single-molecule magnet behavior and their energy barriers for magnetization reversal can be manipulated by adjusting the coordination geometry of the Dy 3+ cation by varying the adjacent Ln 3+ cation in the [Dy(μ-OH)2Ln] 4+ core (Ln: Eu, Yb, Lu). -(SATO, R.; SUZUKI, K.; SUGAWA, M.; MIZUNO*, N.; Chem. -Eur.