Tuning ligand field strength with pendent Lewis acids: access to high spin iron hydrides

Geometrically flexible 9-borabicyclo[3.3.1]nonyl units within the secondary coordination sphere enable isolation of high-spin Fe( ii )-dihydrides stabilized by boron-hydride interactions and a rare example of an isolable S = 3/2 reduction product. The borane-capped Fe( ii )-dihydride: (1) rapidly deprotonates E-H (E = N, O, P, S) bonds to afford borane-stabilized Fe adducts and (2) releases H 2 upon exposure to π-acids. The Lewis acids provide an avenue for redox-leveling in analogy to the near constant operating potential for N 2 reduction in nitrogenase. Pendent borane Lewis acids provide an avenue for changing a ligand's field strength through acid/base interactions; this strategy was highlighted within a series of biologically-relevant high spin iron hydrides.