A Carborane-Derived\nProton-Coupled Electron Transfer\nReagent

Reagents capable of concerted proton–electron\ntransfer (CPET)\nreactions can access reaction pathways with lower reaction barriers\ncompared to stepwise pathways involving electron transfer (ET) and\nproton transfer (PT). To realize reductive multielectron/proton transformations\ninvolving CPET, one approach that has shown recent promise involves\ncoupling a cobaltocene ET site with a protonated arylamine Brønsted\nacid PT site. This strategy colocalizes the electron/proton in a matter\ncompatible with a CPET step and net reductive electrocatalysis. To\nprobe the generality of such an approach a class of C,C′-diaryl-<i>o</i>-carboranes is herein explored as a conceptual substitute\nfor the cobaltocene subunit, with an arylamine linkage still serving\nas a colocalized Brønsted base suitable for protonation. The\nfeatured <i>o</i>-carborane (Ph<b>Cb</b>Ph<sup>N</sup>) can be reduced and protonated to generate an N–H bond with\na weak effective bond dissociation free energy (BDFE<sub>eff</sub>) of 31 kcal/mol, estimated with measured thermodynamic data. This\nN–H bond is among the lowest measured element–H bonds\nfor analyzed nonmetal compounds. Distinct solid-state crystal structures\nof the one- and two-electron reduced forms of diaryl-<i>o</i>-carboranes are disclosed to gain insight into their well-behaved\nredox characteristics. The singly reduced, protonated form of the\ndiaryl-<i>o</i>-carborane can mediate multi-ET/PT reductions\nof azoarenes, diphenylfumarate, and nitrotoluene. In contrast to the\naforementioned cobaltocene system, available mechanistic data disclosed\nherein support these reactions occurring by a rate-limiting ET step\nand not a CPET step. A relevant hydrogen evolution reaction (HER)\nreaction was also studied, with data pointing to a PT/ET/PT mechanism,\nwhere the reduced carborane core is itself highly stable to protonation.