A Carborane-Derived Proton-Coupled Electron Transfer Reagent

Reagents capable of concerted proton–electron
transfer (CPET)
reactions can access reaction pathways with lower reaction barriers
compared to stepwise pathways involving electron transfer (ET) and
proton transfer (PT). To realize reductive multielectron/proton transformations
involving CPET, one approach that has shown recent promise involves
coupling a cobaltocene ET site with a protonated arylamine Brønsted
acid PT site. This strategy colocalizes the electron/proton in a matter
compatible with a CPET step and net reductive electrocatalysis. To
probe the generality of such an approach a class of C,C′-diaryl-o-carboranes is herein explored as a conceptual substitute
for the cobaltocene subunit, with an arylamine linkage still serving
as a colocalized Brønsted base suitable for protonation. The
featured o-carborane (PhCbPhN) can be reduced and protonated to generate an N–H bond with
a weak effective bond dissociation free energy (BDFEeff) of 31 kcal/mol, estimated with measured thermodynamic data. This
N–H bond is among the lowest measured element–H bonds
for analyzed nonmetal compounds. Distinct solid-state crystal structures
of the one- and two-electron reduced forms of diaryl-o-carboranes are disclosed to gain insight into their well-behaved
redox characteristics. The singly reduced, protonated form of the
diaryl-o-carborane can mediate multi-ET/PT reductions
of azoarenes, diphenylfumarate, and nitrotoluene. In contrast to the
aforementioned cobaltocene system, available mechanistic data disclosed
herein support these reactions occurring by a rate-limiting ET step
and not a CPET step. A relevant hydrogen evolution reaction (HER)
reaction was also studied, with data pointing to a PT/ET/PT mechanism,
where the reduced carborane core is itself highly stable to protonation.