Elusive Silane–Alane Complex [SiH⋅⋅⋅Al]: Isolation, Characterization, and Multifaceted Frustrated Lewis Pair Type Catalysis

Abstract

The super acidity of the unsolvated Al(C6F5)3 enabled isolation of the elusive silane-alane complex [Si-H⋅⋅⋅Al], which was structurally characterized by spectroscopic and X-ray diffraction methods. The Janus-like nature of this adduct, coupled with strong silane activation, effects multifaceted frustrated-Lewis-pair-type catalysis. When compared with the silane-borane system, the silane-alane system offers unique features or clear advantages in the four types of catalytic transformations examined in this study, including: ligand redistribution of tertiary silanes into secondary and quaternary silanes, polymerization of conjugated polar alkenes, hydrosilylation of unactivated alkenes, and hydrodefluorination of fluoroalkanes.

Electrophilic Activation of Silicon–Hydrogen Bonds in Catalytic Hydrosilations

“…100 Hz JSiH values reported for η 1 -H-SiR3 complexes with boranes, alanes, and iridium. [27,28] The greater degree of Si-H activation in 34 is consistent with a structure that had previously been reported for a closely related ruthenium complex [Cp(Me3P)2Ru(η 2 -HSiCl3)] + (Scheme 40). [77] This structure's short Ru-H (dRu-H = 1.60(5) Å) and Ru-Si (dRu-Si = 2.329(1) Å) distances and elongated Si-H bond (dSi-H = 1.77(5) Å) point to significant activation of the Si-H bond.…”

“…Notably however, two examples of adducts of main group Lewis acids (boranes and alanes) with silanes were recently isolated and structurally characterized. [27,28] These structures mark an exciting advance in the study of main group-catalyzed hydrosilations by providing the first direct evidence for η 1 -silane intermediates in main group chemistry. These advances, described below, offer the opportunity for direct structural comparison of these electrophilic silicon species with those involving transition metals.…”

“…Mechanistic comparisons of electrophilic silane activations are enabled by major recent advances that elucidate the structures of key silicon intermediates. For example, borane-silane [27] and alane-silane [28] adducts have only recently been isolated and characterized, and these studies provide definitive support for intermediates that had long been inferred based on indirect experimental evidence. Similarly, structural data was recently obtained for a cationic ruthenium silylene complex that catalyzes olefin hydrosilation reactions.…”

“…An isolable adduct between triethylsilane and Al(C6F5)3 (5), the second-row analogue of the of the widely-used B(C6F5)3, has recently been reported. [28] The X-ray crystallographic data for Et3Si-H···Al(C6F5)3 (5•HSiEt3) suggest a weak silane-alane interaction, with a Si-H distance of 1.475(16) Å, and an Al-H distance of 1.865(16) Å (Scheme 9). The 1 JSi-H coupling constant of the silane-alane adduct in cyclohexane-d12 indicates that the Si-H bond is partially activated by interaction with Al(C6F5)3 (JSi-H = 180 Hz for free Et3SiH versus 104 Hz for Et3Si-H···Al(C6F5)3).…”

Solid frustrated-Lewis-pair catalysts constructed by regulations on surface defects of porous nanorods of CeO2

“…Hydrogen activation by either transition metal complexes or metal-based heterogeneous catalysts have been well developed and successfully applied for various reactions4. Recently, frustrated Lewis pairs (FLPs), Lewis acids and bases that are sterically prevented from interaction to form Lewis acid–base adjuncts, can efficiently and cooperatively activate many small molecules (for example, H 2 , CO 2 , and NO) and even strong C–H bond for important catalytic reactions including hydrogenation, hydroamination and carbon dioxide reduction5678910111213141516171819202122232425.…”