Taming the Cationic Beast: Novel Developments in the Synthesis and Application of Weakly Coordinating Anions

Abstract

This Review gives a comprehensive overview of the most topical weakly coordinating anions (WCAs) and contains information on WCA design, stability, and applications. As an update to the 2004 review, developments in common classes of WCA are included. Methods for the incorporation of WCAs into a given system are discussed and advice given on how to best choose a method for the introduction of a particular WCA. A series of starting materials for a large number of WCA precursors and references are tabulated as a useful resource when looking for procedures to prepare WCAs. Furthermore, a collection of scales that allow the performance of a WCA, or its underlying Lewis acid, to be judged is collated with some advice on how to use them. The examples chosen to illustrate WCA developments are taken from a broad selection of topics where WCAs play a role. In addition a section focusing on transition metal and catalysis applications as well as supporting electrolytes is also included.

Stable Salts of Heteroleptic Iron Carbonyl/Nitrosyl Cations

“…1876 cm −1 ) and similar to [Mn(NO) 4 ] + (1978/1866/1855 cm −1 ). Furthermore, the close agreement between the experimental vibrations in the solid state and the ideal, non‐distorted calculated frequencies of [Fe(CO) 4 (NO)] + ( C 2 v ) and [Fe(CO)(NO) 3 ] + ( C 3 v ), indicate that the [F‐{Al(OR F ) 3 } 2 ] − anion truly induces pseudo‐gas‐phase conditions in condensed phases …”

“…With that in mind, we made use of the versatility of carbon monoxide (CO) as a ligand by starting from Fe(CO) 5 as a Fe 0 source in combination with an oxidative approach. In order to compensate for the lability of the Fe−CO bond in the resulting iron cations, we utilized the weakly coordinating anion (WCA) [F‐{Al(OR F ) 3 } 2 ] − (R F =C(CF 3 ) 3 ) in combination with [NO] + as oxidant. The inevitable and (in this case) desirable coordination of the resulting NO (g) to the iron center led to the formation of the novel heteroleptic 18 valence electron (VE) iron(I) carbonyl/nitrosyl cations [Fe(CO) 4 (NO)] + and [Fe(CO)(NO) 3 ] + with the respective Enemark–Feltham numbers {FeNO} 8 and {FeNO} 10 .…”

Stabile Salze heteroleptischer Eisen‐Carbonyl/Nitrosylkationen

“…1876 cm −1 ) und ähnlich zu denen des [Mn(NO) 4 ] + (1978/1866/1855 cm −1 ). Zudem zeigt die gute Übereinstimmung der Experimentaldaten aus den Festkörpern 1 und 2 mit denen der idealen, unverzerrten, berechneten Gasphasenkationen [Fe(CO) 4 (NO)] + ( C 2 v ) und [Fe(CO)(NO) 3 ] + ( C 3 v ), dass das [F‐{Al(OR F ) 3 } 2 ] − Anion echte pseudo‐Gasphasenbedingungen in kondensierten Phasen ermöglicht.…”

A Dicationic Bismuth(III) Lewis Acid: Catalytic Hydrosilylation of Olefins

“…Despite the demonstration of activation of sp 2[37–39] and sp 3 C–H bonds, the potential of this non‐toxic element as a reactive cation is not well explored. The isolation of such a Lewis acidic dication requires the employment of a WCA and weakly nucleophilic solvents , , . Herein, we present the synthesis and electronic structure of the reactive dication [Tp Me2 Bi] 2+ isolated with the WCA [B(C 6 F 5 ) 4 ] – anion in chlorobenzene and ortho ‐dichlorobenzene.…”