Main Group Redox Catalysis: New Frontiers with Germanium and Tin

ConspectusThe quest for sustainable alternatives to precious transition metals has driven a paradigm shift toward main group redox catalysis. Among p-block elements, low-valent group 14 compounds (such as tetrylenes and tetryliumylidenes) are particularly attractive because their ambiphilic nature, featuring a lone pair of electrons and vacant p-orbital, closely mimics the electronic configuration of transition metals. Despite their well-established ability to activate inert bonds in stoichiometric reactions, translating this reactivity into genuine catalytic redox cycles has remained a formidable challenge. The primary obstacle lies in the often prohibitive reductive elimination from the high-valent state, which precludes catalyst turnover.Over the past three years, we have addressed this long-standing limitation by employing rigid, tridentate carbodiphosphorane (CDP) and acridine-based pincer ligands. These ligand frameworks not only stabilize low-valent Ge(II) and Sn(II) centers but also promote the crucial reductive elimination step, thereby enabling complete E(II)/E(IV) or E(II)/E(III)/E(IV) (E = Ge or Sn) catalytic cycles. In this Account, we summarize our systematic progress in unlocking the redox potential of germanium and tin. We first describe the synthesis and electronic properties of CDP-ligated Sn(II) and Ge(II) complexes, which exhibit strong nucleophilicity and versatile reactivity toward electrophiles. Building on this foundation, we demonstrate that these complexes serve as efficient redox catalysts for the catalytic activation of both C(sp²)-F and C(sp³)-F bonds. Notably, the Sn(II) platform enables selective defluorination of trifluoromethyl alkenes to either gem-difluoroalkenes or monofluoroalkenes by simply tuning the solvent, temperature and silane loading. Mechanistic studies into the elementary steps validate a Sn(II)/Sn(IV) redox cycle that closely mirrors classical transition metal catalysis, comprising C-F oxidative addition, F/H ligand metathesis and C-H reductive elimination.We further extended this catalytic platform to nitrous oxide (N₂O) activation and the chemodivergent reduction of nitroarenes. With PhSiH₃ as the reductant, nitroarenes can be selectively converted to either anilines or arylhydroxylamines simply by controlling the reaction temperature. In contrast, switching to the less reactive Et₂SiH₂ delivers azoxybenzenes or hydrazines, with selectivity again dictated by temperature. These results underline the exceptional tunability of our Sn(II)/Sn(IV) catalytic system. In parallel, a T-shaped stannyliumylidene ion stabilized by an acridine-based PNP pincer ligand exhibits pronounced ambiphilicity and serves as an efficient catalyst for the transfer hydrogenation of azoarenes and imines, operating through an FLP-type concerted mechanism. Finally, we disclosed a distinct mechanistic landscape for the germanium congener. Unlike the Sn(II) system, the Ge(II) catalyst promotes the transfer hydrogenation of azoarenes via a single-electron transfer pathway, involving a Ge(II)/Ge(III)/Ge(IV) redox cycle. The formation of azo radical anions and a germanium radical cation have been unequivocally confirmed by electron paramagnetic resonance spectroscopy and radical trapping experiments. Moreover, the Ge(II) catalyst is also effective in the transfer hydrogenation of imines and N-heteroarenes, proceeding through a Ge(II)/Ge(IV) redox cycle.Collectively, this work establishes divalent group 14 compounds as a new class of main group redox catalysts, offering sustainable and conceptually novel alternatives to transition metals. The mechanistic diversity, which spans two-electron and single-electron transfer pathways, highlights how rational ligand design and element selection can unlock diverse chemical transformations.