RESUMEN
Homoatomic polyanions of post-transition main-group metals, namely, Zintl anions, are precast in analogous Zintl phases and can react in solution to form new materials. Despite comprehensible reaction approaches, the formed products cannot be planned in advance, as hitherto undetected and therefore disregarded side reactions take place. The outcomes and interpretations of the reactions of Zintl anions are so far based mainly on crystal structures, which only allow characterization of the product that has the lowest solubility. Here we present the results of our investigation of the stability of highly charged tin Zintl anions in liquid ammonia, which is not exclusively based on solution effects but also on the oxidative influence of the solvent. This allows for a deeper understanding of the ongoing processes in solution and opens doors to the directed synthesis of transition metal complexes of Sn4 (4-) , here shown by its reactivity towards MesCu.
RESUMEN
Copper-catalyzed asymmetric conjugate addition reactions are a very powerful and widely applied method for enantioselective carbon-carbon bond formation. However, structural and mechanistic insight into these famous reactions has been very limited so far. In this article, the first direct experimental detection of transmetalation intermediates in copper-catalyzed reactions is presented. Special combinations of (1)H,(31)P HMBC spectra allow for the identification of complexes with chemical bonds between the alkyl groups and the copper complexes. For the structural characterization of these transmetalation intermediates, a special approach is applied, in which samples using enantiopure ligands are compared with samples using enantiomeric mixtures of ligands. It is experimentally proven, for the first time, that the dimeric copper complex structure is retained upon transmetalation, providing an intermediate with mixed trigonal/tetrahedral coordination on the copper atoms. In addition, monomeric intermediates with one ligand, but no intermediates with two ligands, are detected. These experimental results, in combination with the well-known optimal ligand-to-copper ratio of 2:1 in synthetic applications, allow us to propose that a binuclear transmetalation intermediate is the reactive species in copper-catalyzed asymmetric conjugate addition reactions. This first direct experimental insight into the structure of the transmetalation intermediate is expected to support the mechanistic and theoretical understanding of this important class of reactions and to enable their further synthetic development. In addition, the special NMR approach presented here for the identification and characterization of intermediates below the detection limit of (1)H NMR spectra can be applied also to other classes of catalyses.
RESUMEN
The reaction of Rb4Sn4 with ZnPh2 in liquid ammonia in the presence of [2.2.2]-cryptand yielded crystals of Rb6[(η(2)-Sn4)Zn(η(3)-Sn4)]·5NH3, which could be characterized by single crystal X-ray diffraction. This is the first example of a successful solution reaction of the highly charged tetrahedral Sn4(4-) anions. The homoleptic [E4ZnE4](6-) complex (E = tetrel element) was previously known only for E=Ge and Si/Ge.
RESUMEN
The new Zintl anion [Sn(4)Bi(4)](4-), synthesized by dissolving CsSnBi in liquid ammonia, forms a monocapped nortricyclane-like cage. Its electron localization function (ELF) analysis shows evidence for 3-centre bonding. The analogous reaction with KSnBi results in a crystallographically fully ordered tetrahedral [Sn(2)Bi(2)](2-) ion.