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