Synthesis, Structure, and Reactivity of Zwitterionic Divalent Rare-Earth Metal Silanides

Abstract

ABSTRACT:The synthesis and structures of the first zwitterionic divalent rare-earth metal silanides of the formula [Si(SiMe 2 OMe) 3 - 3 ] 2 M, (M-3), where M = Eu, Yb, Sm, is reported. M-3 compounds feature spirocyclic bicyclooctane structures in which the central rare-earth metal ions are being octahedrally coordinated by six methoxy groups. Zwitterionic silanides are an emerging class of donorsubstituted silyl anions 1, 2 that are potentially useful as ambidentate ligands to support main group and transition metals.3-10 Interest in these types of highly reactive zwitterions primarily arises from the "naked" silyl anion that is rigidly locked and insulated from the metal cation by internal donor bridges. Unlike tetracoordinate borate-based zwitterions 11 , zwitterionic silanides are tri-coordinated and consequently contain a stereochemically active electron pair localized at the silyl anion, which behaves as a Lewis base that can bind to electrophilic transition and main group metal centers (Scheme 1). Scheme 1. Design of hetero-bimetallic zwitterions.In this regard, we recently reported the synthesis and discrete structures of zwitterionic alkali metal silanides of the formula [Si(SiMe 2 OCH 2 CH 2 OMe) 3 - 6 ]M (M = Li, Na, K) and demonstrated that the charge separated, "naked" silyl anion is available for additional metal binding, allowing for the synthesis of novel zwitterionic hetero-bimetallic silanides. 6,8 In addition, we succeeded in synthesizing and structurally characterizing novel zwitterionic alkaline earth metal silanides of formula [Si(SiMe 2 OMe) 3 - 3 ] 2 M (M = Mg, Ca, Sr, Ba). These discrete homoleptic complexes represent the first examples of metalcontaining zwitterions that are composed of two "naked" silyl anions.7 On the other hand, zwitterionic silanides of the rare-earth metals regardless of their oxidation state have not been prepared so far. Herein, we report on the synthesis and structures of the first zwitterionic divalent europium, ytterbium and samarium silanides and the reaction behavior of the divalent ytterbium silanide toward BPh 3 and W(CO) 6 (1), with commercially or synthetically readily available metal alkoxides, M(OR) 2 (M = Ca, Sr, Ba) in tetrahydrofuran (THF) as the solvent (Scheme 2).7 Owing to their zwitterionic nature, M-3 compounds proved to be poorly soluble, even in THF, and precipitated from the reaction mixture as crystalline materials, which greatly facilitated their isolation and purification. For the synthesis of the zwitterionic divalent rare earth silanides M-3 (M = Eu, Yb, Sm), assumed to be of low solubility similar to that of their alkaline earth counterparts, salt metathesis as an alternative route was envisioned because of the lack of commercially available divalent rare-earth alkoxides. , generated upon standing at room temperature for several hours crystalline precipitates of the europium, ytterbium and samarium silanides, M-3, in acceptable isolated yields (Scheme 2). The use of lithium silanide, Li-2, as the nucleophile was crucial to the isola...

Using Functionalized Silyl Ligands To Suppress Solvent Coordination to Silyl Lanthanide(II) Complexes

“…A signal with a similar chemical shift at −148.6 ppm was reported by Niemeyer for the ate complex K[(Me 3 Si) 3 SiYb{N(SiMe 3 ) 2 } 2 ]. 17 Most recently Krempner and co-workers 18 described the ytterbium silanide [Si(SiMe 2 OMe) 3 -κ 3 ] 2 Yb, where the interaction of the ligand with the Yb atom is only occurring via the OMe groups and no interaction between the silanide atom and the lanthanide is observed. The respective 29 Si NMR signal for this compound was found at −202.9 ppm.…”

The Tris(pentafluoroethyl)silanide Anion

“…[8] Although functional silyl anions (SiCl 3 À , [9] Si(OR) n Me 3Àn À ) [10] are known, they have been postulated as reactive intermediates generated in situ in the presence of quenching agents. [8] In contrast to vivid activity in the field of alkyl- [2][3][4][5][6][7] and silyl-substituted [11][12][13][14] silanides and their coordination compounds,virtually nothing is known about perfluoroalkyl derivatives. [8] In contrast to vivid activity in the field of alkyl- [2][3][4][5][6][7] and silyl-substituted [11][12][13][14] silanides and their coordination compounds,virtually nothing is known about perfluoroalkyl derivatives.…”

“…[8] Mixed alkyl amino silyl anions (Li[Si(NEt 2 )Ph 2 ], Li[Si-(NEt 2 ) 2 Ph],L i[Si(NEt 2 )PhMe]) represent the first examples of stable functionalized silyl anions. [8] In contrast to vivid activity in the field of alkyl- [2][3][4][5][6][7] and silyl-substituted [11][12][13][14] silanides and their coordination compounds,virtually nothing is known about perfluoroalkyl derivatives.…”

Das Tris(pentafluorethyl)silanid‐Anion

“…[8] Funktionelle Silylanionen (SiCl 3 À , [9] Si(OR) n Me 3Àn À [10] )s ind prinzipiell bekannt, werden jedoch als reaktive Intermediate beschrieben, die in situ generiert und direkt mit geeigneten Reagentien abgefangen werden müssen. [8] Im Unterschied zu den weitgehend untersuchten Alkyl- [2][3][4][5][6][7] und Silyl-substituierten [11][12][13][14] Silaniden und deren Koordinationsverbindungen ist praktisch nichts über die entsprechenden Perfluoralkylderivate bekannt. [8] Im Unterschied zu den weitgehend untersuchten Alkyl- [2][3][4][5][6][7] und Silyl-substituierten [11][12][13][14] Silaniden und deren Koordinationsverbindungen ist praktisch nichts über die entsprechenden Perfluoralkylderivate bekannt.…”

“…[8] Gemischte Alkylaminosilylanionen (Li[Si-(NEt 2 )Ph 2 ], Li[Si(NEt 2 ) 2 Ph],L i[Si(NEt 2 )PhMe]) repräsentieren die ersten Beispiele fürs tabile funktionalisierte Silylanionen. [8] Im Unterschied zu den weitgehend untersuchten Alkyl- [2][3][4][5][6][7] und Silyl-substituierten [11][12][13][14] Silaniden und deren Koordinationsverbindungen ist praktisch nichts über die entsprechenden Perfluoralkylderivate bekannt.…”