Dihydrogen Activation by Frustrated Carbene-Borane Lewis Pairs: An Experimental and Theoretical Study of Carbene Variation

Abstract

A variety of Lewis acid-base pairs consisting of tris(pentafluorophenyl)borane, B(C(6)F(5))(3), in combination with sterically demanding five- and six-membered N-heterocyclic carbenes (NHCs) of the imidazolin-2-ylidene, imidazolidin-2-ylidene, and tetrahydropyrimidin-2-ylidene types were investigated with respect to their potential to act as frustrated Lewis pairs (FLP) by reaction with dihydrogen (H(2)) and tetrahydrofuran (THF). A sufficient degree of "frustration" was usually established by introduction of a 1,3-di-tert-butyl or 1,3-diadamantyl carbene substitution pattern, which allows an unquenched acid-base reactivity and thus leads to heterolytic dihydrogen activation and ring-opening of THF. In contrast, 1,3-bis(2,6-diisopropylphenyl)-substituted carbenes showed ambiguous behavior, and the corresponding five-membered imidazolin-2-ylidene formed a stable carbene-B(C(6)F(5))(3) adduct, whereas fast C-F activation and formation of a zwitterionic pyrimidinium-fluoroborate was observed for the six-membered tetrahydropyrimidin-2-ylidene. A stable adduct was also isolated for the combination of the acyclic carbene bis(diisopropylamino)methylene with B(C(6)F(5))(3), and consequently no reactivity toward H(2) and THF was observed. To rationalize the reactivity of the carbene-borane Lewis pairs, the thermodynamics of adduct formation with B(C(6)F(5))(3) were calculated for 10 different carbenes; the stability (or instability) of these adducts can be used as a good measure of the degree of "frustration".

Reactivity of a Frustrated Lewis Pair and Small‐Molecule Activation by an Isolable Arduengo CarbeneB{3,5‐(CF3)2C6H3}3 Complex

“…These distances are similar to the corresponding BC bond lengths that were established for compound 2 b in two different crystal modifications (1.663(5) and 1.684(2) Å) 9. 15 However, these bond lengths are longer than that observed in the B(C 6 F 5 ) 3 adduct of the sterically less encumbered NHC 1,3,4,5‐tetramethylimidazolin‐2‐ylidene (1.641(2) Å) 27…”

London Dispersion Interactions in Pnictogen Cations [ECl2]+ and [E=E]2+ (E=P, As, Sb) Supported by Anionic N‐Heterocyclic Carbenes

“…In recent years, anionic N ‐heterocyclic carbenes have emerged as an important class of novel NHC ligands . Contributions of our group have been based on the consideration that frustrated Lewis pairs, composed of bulky imidazolin‐2‐ylidenes and boranes such as B(C 6 F 5 ) 3 , would form abnormal carbene adducts, in which the weakly coordinating anionic (WCA) borate unit is attached to the 4‐position of the N ‐heterocycle . The corresponding anionic carbenes were termed WCA‐NHCs and introduced as ancillary ligands in transition metal complexes and homogeneous catalysts .…”

Trans-Metal-Trapping Meets Frustrated-Lewis-Pair Chemistry: Ga(CH2SiMe3)3-Induced C–H Functionalizations

“…While sharing many coordination features with phosphines, NHCs offer greater potential for subtle variations of their steric/electronic properties. 18d,25,26 In addition to their tuneability, the N-substituents are responsible for inducing “steric pressure” toward the LA component by being directed toward the carbene lone pair. 18d Although most systems use a boron complex as the LA component, 18 the use of other group 13 elements, in particular aluminum, 27 is steadily growing in popularity with some other recent examples reported using gallium 28 and indium.…”