Redox Robust Mn(I/0/-I) Dicarbenes: Electrochemistry, Steric Mapping, and Borate-Directed C-C Bond Formation

Manganese is perhaps the most electronically versatile element, accessing eleven different formal oxidation states (-3 to +7). However, the redox properties and reactivities of low valent monometallic Mn(0) and Mn(-I) systems remain severely underexplored due to the propensity of Mn0 to dimerize and quench high-energy metalloradicals. We report a new series of redox active monometallic Mn(I), Mn(0) and Mn(-I) complexes containing a BH2-bridged dicarbene ligand coordinated to Mn, all which have been characterized using a suite of experimental and cutting-edge computational (DFT) methods. Slow electron transfer kinetics at Mn(I/0) are observed, with computations and electrochemical simulations in excellent agreement with experiment. Despite the lack of steric bulk at the BH2-bridged Mn(0), the tBu groups at the dicarbene provide sufficient steric protection to prevent dimerization, with percent buried volume (%V_bur) serving as a steric ranking tool for to predict whether Mn(0) complexes will remain monometallic in solution. We show that when %V_bur > 83% prevents dimerization for a diverse array of of Mn(0) complexes from the literature. Ligand sterics of the BPh2-bridged and BH2 bridged complexes dictate reaction outcomes when Mn(I) and Mn(-I) are exposed to nucleophiles and electrophiles, respectively, while the Mn(0) metalloradicals facilitate radical cycloisomerization catalysis of 6-iodo-1-hexene at room temperature. This work underscores the importance of ligand sterics in rationalizing reactivity patterns at Mn and offers valuable insights for designing chelating ligands that can selectively leverage Mn(I/0/-I) states in redox-mediated reactions.