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 PN^Mepy and PN^Phpy. With ferrous halides, PN^Phpy gave the complexes FeX₂(PN^Phpy) (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(PN^Hpy^R)₂]²⁺ (R = H, Me). Dicarbonyls Fe(PN^Rpy)(CO)₂ were produced by treating PN^Mepy with Fe(benzylideneacetone)(CO)₃ and reduction of FeX₂(PN^Phpy) 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 PN^Phpy system uniquely sustains two one-electron reductions. Treatment of Fe(PN^Phpy)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 PN^Phpy.