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

We report an incredibly reducing and redox-active Mn -I dianion, [Mn(CO) 3 (Ph 2 B( t Bu NHC) 2 )] 2- (NHC = N-heterocyclic carbene), furnished via 2e - reduction of the parent 16e - Mn I complex with Na 0 or K 0 . Cyclic voltammograms show a Mn 0/-I redox couple at -3.13 V vs Fc +/0 in tetrahydrofuran (THF), -3.06 V in 1,2-dimethoxyethane, and -2.85 V in acetonitrile. The diamagnetic Mn -I dianion 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)] + , [ n Bu 4 N] + ; M = Na, K). Countercation identity does not significantly alter 13 C NMR spectral signatures with [ n Bu 4 N] + and Na + , suggesting minimal ion pairing in solution. IR spectroscopy reveals a significant decrease in CO stretching frequencies from Mn I to 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 -I dianion behaves as a chemical reductant, smoothly releasing 1e - or 2e - to regenerate the oxidized Mn 0 or Mn I species in solution. The reducing potential of [Mn(CO) 3 (Ph 2 B( t Bu NHC) 2 )] 2- surpasses the naphthalenide anion in THF (-3.09 V) and represents one of the strongest isolable chemical redox agents.