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

Treatment of iron POCOP-pincer hydride complexes cis-[2,6-(iPr2PO)2C6H3]­Fe­(H)­(PMe3)2 (1-H), [2,6-(iPr2PO)2C6H3]­Fe­(H)­(PMe3)­(CO) (2-H, trans H/CO; 2′-H, cis H/CO), and cis-[2,6-(iPr2PO)2C6H3]­Fe­(H)­(CO)2 (3-H) with HBF4·Et2O in CD3CN/THF-d8 results in a rapid evolution of H2. 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]+[BF4]− and Gomberg’s dimer, which supports a single electron transfer pathway. Cationic complexes {[2,6-(iPr2PO)2C6H3]­Fe­(CO)­(PMe3)­(CH3CN)}+[BF4]− (2+-BF4, trans CO/CH3CN) and cis-{[2,6-(iPr2PO)2C6H3]­Fe­(CO)2(CH3CN)}+[BF4]− (3+-BF4) are prepared from protonation of 2-H (or 2′-H) and 3-H with HBF4·Et2O, respectively. Both compounds react with H2 with the aid of iPr2NEt to yield neutral hydride complexes and [iPr2N­(H)­Et]+[BF4]−. In addition, they catalyze the hydrosilylation of benzaldehyde and acetophenone with (EtO)3SiH 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.