An Unusual Example of Hypervalent Silicon: A Five‐Coordinate Silyl Group Bridging Two Palladium or Nickel Centers through a Nonsymmetrical Four‐Center Two‐Electron Bond

Abstract

Pd and Ni dimers supported by PSiP ligands in which two hypervalent five-coordinate Si atoms bridge the two metal centers are reported. Crystallographic characterization revealed a rare square-pyramidal geometry at Si and an unusual asymmetric M 2 Si 2 core (M = Pd or Ni). DFT calculations showed that the unusual structure of the core is also found in a model in which the phosphine and Si centers are not part of a pincer group, thus indicating that the observed geometry is not imposed by the PSiP ligand. NBO analysis showed that an asymmetric four-center two-electron (4c-2e) bond stabilizes the hypervalent Si atoms in the M 2 Si 2 core.Since the discovery of the first hypervalent Si compounds, [SiF 6 ] 2À and trans-SiF 4 (NH 3 ) 2 , in the 19th century, [1] there has been considerable research into the structure, bonding, and properties of these species.[2] These five-and six-coordinate Si compounds cannot be described in terms of two-center twoelectron (2c-2e) bonds. Instead, three-center four-electron (3c-4e) [3] or three-center two-electron (3c-2e) [4] bonds are used, depending on the environment of Si.[5] As a result of their ability to stabilize electron-rich groups through hypervalency, hypervalent Si species are often postulated as crucial intermediates in organic reactions, [6] and there is continued interest in understanding the structure of this important class of compounds.Over the last decade, pincer ligands with two phosphine donors and a central anionic Si donor (PSiP ligands) have attracted attention owing to the high trans influence of the Si atom and the rigidity of the pincer framework.[7] Turculet and co-workers have demonstrated that complexes supported by this ligand can undergo reversible SiÀC bond cleavage, [7f] whereas Takaya and Iwasawa utilized catalysts with PSiP ligands for the carboxylation of allenes under mild conditions.[7d] Complexes supported by the PSiP ligand have not only given rise to interesting reactivity but have also led to the formation of structurally remarkable species. For example, while studying the mechanism of the carboxylation reaction, Takaya and Iwasawa were able to isolate a number of complexes with h 2 -Si À H bonds; such complexes mimic the transition state for oxidative addition of a Si À H bond. [7h,j] Herein, we report the preparation of a Pd dimer supported by two PSiP ligands in which an unusual five-coordinate hypervalent silyl group bridges two Pd centers. Computational analysis revealed that the key interaction is a rare example of a four-center two-electron (4c-2e) bond. In this study, this type of bond was shown to stabilize hypervalent Si for the first time. A closely related Ni analogue of the Pd dimer, with similar bonding features, has also been synthesized and characterized by X-ray crystallography.Recently, both our research group [7q] and Mitton and Turculet

Synthesis, Structure, and Reactivity of a Mononuclear η2‐(Ge–H)palladium(0) Complex Bearing a PGeP‐Pincer‐Type Germyl Ligand: Reactivity Differences between Silicon and Germanium

“…4 was converted into the unknown complex completely after 20 h. The structure of the unknown product was determined to be a germyl‐bridged Pd dimer 5 by X‐ray crystallographic analysis (Figure ). Nova and Hazari reported preparation of a Si analogue of 5 by dehydrogenative dimerization of the palladium hydride having a PSiP‐pincer type ligand, which was regarded as an unusual example of hypervalent five‐coordinate silicon‐bridged Pd dimer complex . The Pd–Pd distance of 5 is 2.582(1) Å, which is comparable to that of Nova and Hazari's Si‐analogue [2.5860(5) Å] in spite of larger covalent radius of the bridging germyl ligand.…”

Chemical Research Society of India Awards: R. Nolte, L. F. Tietze, A. B. Holmes, W. B. Tolman / SCF Industrial Chemistry Division Prize: M. Maudit and F. Caijo / Hans Fischer Memorial Prize: S. Huber / Elected to the Académie des Sciences: A. Bousseksou and O. Eisenstein

“…Eisenstein is interested in the use of theoretical methods for studying the structures, properties, and reactivity of transition‐metal complexes. She has recently reported in Angewandte Chemie on hypervalent silicon compounds 8…”

Preise der indischen Chemical Research Society: R. Nolte, L. F. Tietze, A. B. Holmes, W. B. Tolman / Preis der Division de Chimie Industrielle der SCF: M. Maudit und F. Caijo / Hans‐Fischer‐Preis: S. Huber / In die Académie des Sciences gewählt: A. Bousseksou und O. Eisenstein

“…Eisenstein interessiert sich für den Einsatz theoretischer Methoden zur Untersuchung der Strukturen, Eigenschaften und Reaktivitäten von Übergangsmetallkomplexen. In der Angewandten Chemie hat sie kürzlich über hypervalente Siliciumverbindungen berichtet 8…”