Synthesis, Characterization and Reactivity of Alkyl Iron(II) PCP Pincer Complexes.

The syntheses and reactivity of a series of alkyl iron PCP pincer complexes is described. The alkyl complexes of the type [Fe-(PCPCH2-iPr)-(R)-(CO)2] (R = CH3, CH2CH3, CH2CH2CH3, CH2CH2CH2CH3) are accessible in good yields (69-78%) by reduction of [Fe-(PCPCH2-iPr)-(Br)-(CO)2] with sodium followed by oxidative addition of alkyl halides RX (X = Br or I). As alkyl complexes are typically prone to undergo migratory insertion reactions in the presence of potential strong ligands, [Fe-(PCPCH2-iPr)-(CH2CH2CH3)-(CO)2] was treated with CO and CNtBu affording the acyl complexes [Fe-(PCPCH2-iPr)-((CO)-CH2CH2CH3)-(CO)2] and [Fe-(PCPCH2-iPr)-(CO)-CH2CH2CH3)-(CO)-(CNtBu)] in 98 and 91% yields. Upon treatment of [Fe-(PCPCH2-iPr)-((CO)-CH2CH2CH3)-(CO)2] with pinacolborane (HBpin) the hydride complex [Fe-(PCPCH2-iPr)-(H)-(CO)2] is obtained. When [Fe-(PCPCH2-iPr)-(CH2CH2CH3)-(CO)2] is reacted with an excess (4 equiv) of HBpin and SiH2Ph2, complexes [Fe-(PCPCH2-iPr)-(κ2 H,H-H2BPin)-(CO)] and [Fe-(PCPCH2-iPr)-(SiHPh2)-(CO)], respectively, is formed. The first is a very labile complex readily liberating HBpin thereby undergoing decomposition to intractable materials. The latter is an unusually stable 16e- complex. Finally, in a preliminary reactivity study [Fe-(PCPCH2-iPr)-(CO)2(CH2CH2CH3)] is demonstrated to be an active precatalyst for the hydroboration of nitriles with HBPin. The hydroboration reaction requires no additives and proceeds with a catalyst loading of 2 mol % at 60 °C. The reduction of nitriles and subsequent hydrolysis provide ready access to aminomethylene units in organic molecules.