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.