Equilibrium Studies of the Binding of Unsaturated Fatty Acid Methyl Esters to Palladium(II)

Cationic palladium(II)−olefin complexes [CpPd(PR3)(olefin)]BF4 (PR3 = PPh3, PMePh2,
PEt3, PMe3; olefin = methyl oleate (18:1), methyl linoleate (18:2), methyl linolenate (18:3),
cis-3-hexene, 1,4-pentadiene) have been synthesized and characterized. (For 18:1, 18:2, and
18:3, the first number represents the chain length while the second indicates the number of
double bonds). The diene (1,4-pentadiene and 18:2) and triene (18:3) ligands also form
binuclear complexes {[CpPd(PR3)]2(olefin)}2+ containing two coordinated [CpPd(PR3)]+ units.
The crystal structure of {[CpPd(PMe3)]2(1,4-pentadiene)}(BF4)2 (5b) was determined by X-ray
crystallography. In all of the complexes, 1H and 13C{1H} NMR spectra establish the presence
of the η2-coordinated fatty acid methyl ester and alkene ligands. Equilibrium constants for
the formation of the [CpPd(PR3)(olefin)]+ complexes with 18:1, 18:2, 18:3, and cis-3-hexene
show that the binding abilities of the large unsaturated fatty acid methyl esters to [CpPd(PR3)]+ are similar to those of simple cis-olefins, such as cis-3-hexene and cis-2-butene. The
equilibrium constants for the binding of [CpPd(PR3)]+ to 18:1 and cis-3-hexene increase with
the increasing electron-donating ability and decreasing size of the phosphines:  PPh3 <
PMePh2 < PEt3 < PMe3.