Novel B(Ar′)₂(Ar′′) hetero-tri(aryl)boranes: a systematic study of Lewis acidity

Abstract

A series of homo- and hetero-tri(aryl)boranes incorporating pentafluorophenyl, 3,5-bis(trifluoromethyl)phenyl, and pentachlorophenyl groups, four of which are novel species, have been studied as the acidic component of frustrated Lewis pairs for the heterolytic cleavage of H2. Under mild conditions eight of these will cleave H2; the rate of cleavage depending on both the electrophilicity of the borane and the steric bulk around the boron atom. Electrochemical studies allow comparisons of the electrophilicity with spectroscopic measurements of Lewis acidity for different series of boranes. Discrepancies in the correlation between these two types of measurements, combined with structural characterisation of each borane, reveal that the twist of the aryl rings with respect to the boron-centred trigonal plane is significant from both a steric and electronic perspective, and is an important consideration in the design of tri(aryl)boranes as Lewis acids.

How Boron Doping Shapes the Optoelectronic Properties of Canonical and Phenylene‐Containing Oligoacenes: A Combined Experimental and Theoretical Investigation

“…This ordering agrees with the energy of the respective, boron‐centered LUMOs of the parent species [0.30 eV ( DBI ), 0.72 eV ( DBA ), and 0.87 eV ( DBP ) at the SCS‐MP2/def2‐TZVP level of theory], which may apparently serve as a qualitative measure of the Lewis acidity of the compounds (see experimental part above and reference ).…”

“…The existence of diadducts was ruled out in all three cases by experimental and theoretical means. Our results thus confirm that electrochemical measurements may indeed serve as valuable diagnostic tools for assessing the Lewis acidity of a given redox‐active borane . Here, different spectroscopical methods do not necessarily give consistent results (cf.…”

“…Detailed investigations by Wildgoose et al. have led to the conclusion that “ electrochemical studies allow comparisons of the electrophilicity with spectroscopic measurements of Lewis acidity for different series of boranes ” . It is therefore understandable, why DBI , which is the best electron acceptor among the aryl boranes under investigation here, also turned out to be the strongest Lewis acid.…”

Lewis Acid‐Mediated One‐Electron Reduction of Nitrous Oxide

“…Apart from electronic reasons, one can invoke steric hindrance around the boron center in the conceivable [B(C 6 Cl 5 ) 3 ] .− radical anion as a possible reason for the attenuated reactivity. The steric congestion was revealed by analyzing crystals of (Cp* 2 Co) + [(B(C 6 Cl 5 ) 3 ] .− (Figure S24, SI), which were obtained despite the limited lifetime of [B(C 6 Cl 5 ) 3 ] .− at room temperature ,. Note that the geometry of B(C 6 Cl 5 ) 3 is largely retained in the reduced radical anion, which is similarly observed for trimesitylborane…”

“…Subsequently, we investigated the possibility of reducing N 2 O with the less powerful reductant Cp* 2 Fe ( E 1/2 =−0.59 vs. Fc/Fc + in CH 2 Cl 2 ) in the presence of B(C 6 F 5 ) 3 . In contrast to Cp* 2 Co, Cp* 2 Fe is not able to convert B(C 6 F 5 ) 3 to its radical anion state due to thermodynamic mismatch (B(C 6 F 5 ) 3 was reported to have an E 1/2 of at least −1.52 vs. Fc/Fc + in CH 2 Cl 2 ) –. Still, this system is able to reduce O 2 to give a borane‐capped peroxide, and to reduce a range of chalcogen‐based small molecules …”