Homogeneous Hydrogenation and Isomerization of 1‑Octene Catalyzed by Nickel(II) Complexes with Bidentate Diarylphosphane Ligands

A systematic library of 24 nickel­(II)
complexes with bidentate diphosphane ligands was synthesized, and
the solid-state structures of five of them were determined with X-ray
crystallography. The compounds C1–C3 are common P2NiIIX2-type complexes,
while C4 contains a unique [P2NiII(NH3)­(OAc)]+ square-planar structure with a
P2NO donor set and C5 constitutes a rare [(P2NiII)2(μ-OH)2]2+ dinuclear compound. The catalytic activity of all complexes
was tested in the hydrogenation and/or isomerization of 1-octene in
a CH2Cl2/CH3OH reaction medium. Catalyst
precursors bearing ligands with o-alkoxy aryl rings
selectively hydrogentate 1-octene to n-octane, while
catalytic systems comprising ligands without the o-alkoxy functionality selectively isomerize the substrate to a mixture
of internal alkenes, mostly cis- and trans-2-octene. The conversion is enhanced by equipping the ligand aryl
rings with electron-donating alkoxy groups, by increasing the steric
bulk of the backbone and/or the aryl rings, by employing relatively
noncoordinating anions, and by adding a base as the cocatalyst. Using
the compound [Ni­(L3X)­I2] as the catalyst precursor and
upon application of standard hydrogenation conditions, full conversion
of the substrate was achieved in 1 h to isomerization products only
(TON = 1940). When a catalytic amount of the base is added, a similar
result is obtained even in the absence of H2. A maximum
TON of 4500 in 1 h with 96% selectivity for n-octane
was achieved by employing [Ni­(oMeO-L3X)­(NH3)­(OAc)]­PF6 as the catalyst precursor.