Nucleophilic and Electrophilic Molybdenum Terminal Oxo Complexes By Coordination-Induced Bond Weakening of Hydroxo O-H Bonds.

The extent of coordination-induced bond weakening in aquo and hydroxo ligands bonded to a molybdenum(III) center complexed by a dianionic, pentadentate ligand system was probed by reacting the known complex (B2Pz4Py)Mo(III)-NTf2, I, with degassed water or dry lithium hydroxide. The aquo adduct was not observed, but two LiNTf2 stabilized hydroxo complexes were fully characterized. Computational and experimental work showed that the O-H bond in these complexes were significantly weakened (to ≈ 57 kcal mol-), such that these compounds could be used to form the diamagnetic, d2 neutral molybdenum oxo complex (B2Pz4Py)Mo(IV)O, 2, by hydrogen atom abstraction using the aryl oxyl reagent ArO• (Ar = 2,4,6-tri-tert-butylphenyl). Oxidation of the neutral hydroxo derivative further facilitated production of 2 by significantly enhancing the acidity of the hydroxyl proton. Speciation in these processes was probed by electrochemical and chemical experiments. The terminal oxo complex 2 was smoothly oxidized by one electron to the cationic [(B2Pz4Py)Mo(V)O]+[A]- derivatives [2]+[A]- (A = NTf2 or Al[(OC(CF3)3]4 depending on the oxidizing agent used). Both Mo(IV) and Mo(V)+ oxo complexes were fully characterized and their nucleophilic and electrophilic reaction behavior probed by conducting reactions with the Lewis acid B(C6F5)3 and the Lewis base PPh3. Neutral oxo complex 2 reacts only with the Lewis acid, while the cationic [2]+[A]- reacts only with PPh3, the former by adduct formation, the latter via phosphine oxidation.