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-(PCP^CH2-iPr)-(R)-(CO)₂] (R = CH₃, CH₂CH₃, CH₂CH₂CH₃, CH₂CH₂CH₂CH₃) are accessible in good yields (69-78%) by reduction of [Fe-(PCP^CH2-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-(PCP^CH2-iPr)-(CH₂CH₂CH₃)-(CO)₂] was treated with CO and CNtBu affording the acyl complexes [Fe-(PCP^CH2-iPr)-((CO)-CH₂CH₂CH₃)-(CO)₂] and [Fe-(PCP^CH2-iPr)-(CO)-CH₂CH₂CH₃)-(CO)-(CNtBu)] in 98 and 91% yields. Upon treatment of [Fe-(PCP^CH2-iPr)-((CO)-CH₂CH₂CH₃)-(CO)₂] with pinacolborane (HBpin) the hydride complex [Fe-(PCP^CH2-iPr)-(H)-(CO)₂] is obtained. When [Fe-(PCP^CH2-iPr)-(CH₂CH₂CH₃)-(CO)₂] is reacted with an excess (4 equiv) of HBpin and SiH₂Ph₂, complexes [Fe-(PCP^CH2-iPr)-(κ² H,H-H₂BPin)-(CO)] and [Fe-(PCP^CH2-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-(PCP^CH2-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.