Iron-borane catalyzed carbonyl hydroboration and isolation of an iron(I)-ketyl radical

Hydroboration of carbonyl compounds has proven a pivotal route to access alcohols and other C1 moieties in recent years. Despite this, iron-based catalyst systems are somewhat limited with very little mechanistic understanding of these systems developed. Here we show that an iron metalloborane complex [{(^iPrDPB^Ph)Fe}₂(μ-1,2-N₂)] (A) is an efficient pre-catalyst for hydroboration of ketones, cyclic esters and CO₂ with mild conditions. Mechanistic insights reveal a previously unreported direct iron(0)-mediated ligand-to-ligand hydride transfer (LLHT) process is in operation with B-H bond breaking being rate determining, indicating the importance of mechanistic studies on well-known transformations. An iron(I)-benzophenone ketyl radical with a unique S = 1 antiferromagnetic ground state has been isolated and fully characterized.