Activation of Dihydrogen and Si lanes by Cationic Iron Bis(phosphinite) Pincer Complexes

Treatment of iron POCOP-pincer hydride complexes cis-[2,6-((Pr2PO)-Pr-i)(2)C6H3]Fe(H)(PMe3)(2) (1-H), [2,6-((Pr2PO)-Pr-i)(2)C6H3]Fe(H)(PMe3)(CO) (2-H, trans H/CO; 2'-H, cis H/CO), and cis-[2,6-((Pr2PO)-Pr-i)(2)C6H3]Fe(H)(CO)(2) (3-H) with HBF4 center dot Et2O in CD3CN/THF-d(8) results in a rapid evolution of H-2. Except for the reaction of 1-H, which leads to decomposition of the pincer structure, all other hydrides are converted cleanly to acetonitrile-trapped cationic complexes. Protonation of these hydrides with the weaker acids CF3CO2H and HCO2H establishes the basicity order of 1-H > 2-H > 2'-H > 3-H, with 3-H bearing the least basic hydride ligand. An alternative method of abstracting hydride by [Ph3C](+) [BF4](-) gives complicated products; the reaction of 2-H generates two pincer products, [HPMe3](+)[BF3](-) and Gombergs dimer, which supports a single electron transfer pathway. Cationic complexes {[2,6-((Pr2PO)-Pr-i)(2)C6H3]Fe(CO)(PMe3)(CH3CN)}(+) [BF4](-) (2(+)-BF4, trans CO/CH3CN) and cis-{[2,6-((Pr2PO)-Pr-i)(2)C6H3]Fe(CO)(2)(CH3CN)}(+)[BF4](-) (3(+)-BF4) are prepared from protonation of 2-H (or 2'-H) and 3-H with HBF4 center dot Et2O, respectively. Both compounds react with H2 with the aid of (Pr2NEt)-Pr-i to yield neutral hydride complexes and [(Pr2N)-Pr-i(H)Et](+)[BF4](-). In addition, they catalyze the hydrosilylation of benzaldehyde and acetophenone with (EtO)(3)SiH and show higher catalytic activity than the neutral hydrides 2-H/2'-H and 3-H. The mechanism for the formation of 2(+)-BF4 and the X-ray structure of 2(+)-BF4 are also described.