Tris(pentafluoroethyl)silanol Derivatives and the Lewis Amphoteric Tris(pentafluoroethyl)silanolate Anion, [Si(C₂F₅)₃O]⁻

Abstract

While alkyl-substituted siloxanes are widely known, virtually nothing is known about perfluoroalkyl siloxanes and their congener species, the silanols and silanolates. We recently reported on the tris-(pentafluoroethyl)silanide ion, [Si(C 2 F 5 ) 3 ] À , which features Lewis amphoteric character deriving from the pentafluoroethyl substituents and their strong electron-withdrawing properties. Transferring this knowledge, we investigated the Lewis amphoteric behavior of the tris(pentafluoroethyl) silanolate, [Si(C 2 F 5 ) 3 O] À . In order to examine such Lewis amphoteric behavior, we first developed a strategy for the synthesis of the corresponding silanol Si(C 2 F 5 ) 3 OH, which readily condenses at room temperature to the hexakis (pentafluoroethyl)disiloxane, (C 2 F 5 ) 3 SiOSi(C 2 F 5 ) 3 . Deprotonation of Si(C 2 F 5 ) 3 OH employing a sterically demanding phosphazene base allows the characterization of the first example of a dimeric triorganosilanolate: the dianionic hexakis(pentafluoroethyl)disilanolate, [{Si(C 2 F 5 ) 3 O} 2 ] 2À , implies Lewis amphoteric character of the monomeric [Si-(C 2 F 5 ) 3 O] À anion.

The Field of Main Group Lewis Acids and Lewis Superacids: Important Basics and Recent Developments

“…The first one is the elusive tris(pentafluoroethyl)silanolate anion [Si(C 2 F 5 ) 3 O] − ( 1 ) (Scheme 2,a) which is isoelectronic to O=P(C 2 F 5 ) 3 . The oxidation of (C 2 F 5 ) 3 SiH by NO 2 at −50 °C in the presence of molecular sieves results in (C 2 F 5 ) 3 SiOH formation, which after addition of two equivalents of tmg 3 P=N t Bu (tmg – tetramethylguanidyl) and warming up to room temperature forms [tmg 3 P=N(H) t Bu] 2 [(C 2 F 5 ) 3 SiO) 2 ] featuring dianion 2 (Scheme 2,b) [47a] . Formally 2 can be considered as a dimer of 1 , where the oxygen atom acts as LB, and the silicon atom acts as LA, increasing its coordination number to five.…”

“… Schematic representation of the structures of (a) elusive monomeric [Si(C 2 F 5 ) 3 O] − ( 1 ); (b) experimentally observed dimeric [(Si(C 2 F 5 ) 3 O) 2 ] 2− ( 2 ) [47a] . (c) experimentally observed structure of [Si(C 2 F 5 ) 3 ] − ( 3 ) [47b] .…”