Phosphine-Iminopyridines as Platforms for Catalytic Hydrofunctionalization of Alkenes

A series
of phosphine-diimine ligands were synthesized by the condensation
of 2-(diphenylphosphino)­aniline (PNH2) with a variety of
formyl and ketopyridines. Condensation of PNH2 with acetyl-
and benzoylpyridine yielded the Ph2P­(C6H4)­NC­(R)­(C5H4N), respectively
abbreviated PNMepy and PNPhpy. With ferrous
halides, PNPhpy gave the complexes FeX2(PNPhpy) (X = Cl, Br). Condensation of pyridine carboxaldehyde
and its 6-methyl derivatives with PNH2 was achieved using
a ferrous template, affording low-spin complexes [Fe­(PNHpyR)2]2+ (R = H, Me). Dicarbonyls
Fe­(PNRpy)­(CO)2 were produced by treating PNMepy with Fe­(benzylideneacetone)­(CO)3 and reduction
of FeX2(PNPhpy) with NaBEt3H under
a CO atmosphere. Cyclic voltammetric studies show that the [FeL3(CO)2]0/– and [FeL3(CO)2]+/0 couples are similar for a range of
tridentate ligands, but the PNPhpy system uniquely sustains
two one-electron reductions. Treatment of Fe­(PNPhpy)­X2 with NaBEt3H gave active catalysts for the hydroboration
of 1-octene with pinacolborane. Similarly, these catalysts proved
active for the addition of diphenylsilane, but not HSiMe­(OSiMe3)2, to 1-octene and vinylsilanes. Evidence is presented
that catalysis occurs via iron hydride complexes of intact PNPhpy.