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 MnI, 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 MnI/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 (%Vbur) serving as a valuable steric ranking tool. We also show that a %Vbur > 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 MnI 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 MnI/0/-I states in redox-mediated catalytic reactions.