Steric Mapping, Ligand Dynamics, and Cycloisomerization Catalysis with Redox Robust Mn^I/0/‑I Dicarbenes

Manganese is perhaps the most electronically versatile element, yet the redox properties, reactivity, and catalytic applications of low-valent Mn⁰/Mn^-I complexes remain underexplored due to the propensity for Mn⁰ to dimerize, quenching high-energy metalloradicals. We report a series of redox-active monometallic Mn^I, Mn⁰ and Mn^-I complexes containing a BH₂-bridged dicarbene, 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 experimental values. Despite the lack of steric bulk at the BH₂-bridged Mn⁰, the ^tBu groups at the dicarbene provide adequate steric protection to prevent dimerization, with percent buried volume (%V _bur) serving as a valuable steric ranking tool. We also show that a %V _bur > 83% prevents dimerization for a diverse array of Mn⁰ complexes from the literature. Ligand sterics of BPh₂-and BH₂-bridged complexes dictate reaction outcomes when Mn^I and Mn^-I are exposed to nucleophiles and electrophiles, respectively, while Mn⁰ facilitates the 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 provides valuable insights for designing chelating ligands that can selectively leverage Mn^I/0/‑I states in redox-mediated catalytic reactions.