Synthesis and Structures of Nickel and Palladium Salicylaldiminato 1,3,5-Triaza-7-phosphaadamantane (PTA) Complexes

The synthesis of nickel(II) and palladium(II) salicylaldiminato complexes incorporating the water-soluble phosphine
1,3,5-triaza-7-phosphaadamantane(PTA) has been achieved employing two preparative routes. Reaction of the
original ethylene polymerization catalyst developed by Grubbs and co-workers (Organometallics 1998, 17, 3149),
(salicylaldiminato)Ni(Ph)PPh3, with PTA using a homogeneous methanol/toluene solvent system resulted in the
formation of the PTA analogues in good yields. Alternatively, complexes of this type may be synthesized via a
direct approach utilizing (tmeda)M(CH3)2 (M = Ni, Pd), the corresponding salicylaldimine, and PTA. Yields by this
method were generally near quantitative. The complexes were characterized in solution by 1H/13C/31P NMR
spectroscopy and in the solid-state by X-ray crystallography. All derivatives exhibited square-planar geometry with
the bulky isopropyl groups on the aniline being perpendicular to the plane formed by the metal center and its four
ligands. Such orientation of these sterically encumbering groups is responsible for polymer chain growth during
olefin polymerization in favor of chain termination via β-hydride elimination. Polymerization reactions were attempted
using the nickel−PTA complexes in a biphasic toluene/water mixture in an effort to initiate ethylene polymerization
by trapping the dissociated phosphine ligand in the water layer, thereby eliminating the need for a phosphine
scavenger. Unfortunately, because of the strong binding ability of the small, donating phosphine(PTA) as compared
to PPh3, phosphine dissociation did not occur at a temperature where the complexes are thermally stable.