Pushing the Limits of Organometallic Redox Chemistry with an Isolable Mn(−I) Dianion

We report an incredibly reducing and redox-active Mn–Idianion, [Mn(CO)3(Ph2B(tBuNHC)2)]2–(NHC = N-heterocyclic carbene), furnished via 2e–reduction of the parent 16e–MnIcomplex with Na0or K0. Cyclic voltammograms show a Mn0/–Iredox couple at −3.13 V vs Fc+/0in tetrahydrofuran (THF), −3.06 V in 1,2-dimethoxyethane, and −2.85 V in acetonitrile. The diamagnetic Mn–Idianion is stable in solution and solid-state at room temperature, tolerating a wide range of countercations ([M(2.2.2)crypt]+, [M(18-crown-6)]+, [nBu4N]+; M = Na, K). Countercation identity does not significantly alter 13C NMR spectral signatures with [nBu4N]+and Na+, suggesting minimal ion pairing in solution. IR spectroscopy reveals a significant decrease in CO stretching frequencies from MnIto Mn–I(ca. 240 cm–1), consistent with a drastic increase in electron density at Mn. State-of-the-art DFT calculations are in excellent agreement with the observed IR spectral data. Moreover, the Mn–Idianion behaves as a chemical reductant, smoothly releasing 1e–or 2e–to regenerate the oxidized Mn0or MnIspecies in solution. The reducing potential of [Mn(CO)3(Ph2B(tBuNHC)2)]2–surpasses the naphthalenide anion in THF (−3.09 V) and represents one of the strongest isolable chemical redox agents.