Palladium complexes of multidentate pyrazolylmethyl pyridine ligands: Synthesis, structures and phenylacetylene polymerization

The compounds, 2,6-bis(3,5-dimethylpyrazol-1-ylmethyl)pyridine (_MeN̂N̂N) (L1) and 2,6-bis(3,5-ditertbutylpyrazol-1-ylmethyl)pyridine (_tBuN̂N̂N) (L2), react with either [Pd(NCMe)₂Cl₂] or [Pd(COD)ClMe] to form the mononuclear palladium complexes [Pd(_MeN̂N̂N)Cl₂] (1), [Pd(_MeN̂N̂N)ClMe] (2), [Pd(_tBuN̂N̂N)Cl₂] (3) and [Pd(_tBuN̂N̂N)ClMe] (4). Reactions of 1, 2 and 4 with the halide abstractor, NaBAr₄ (Ar = 3,5-(CF₃)₂C₆H₃), led to the formation of stable tridentate cationic species [Pd(_MeN̂N̂N)Cl]⁺(5), [Pd(_MeN̂N̂N)Me]⁺ (6) and [Pd(_tBuN̂N̂N)Cl]⁺ (7) respectively. The analogous carbonyl linker cationic species [Pd{(3,5-Me₂pz-CO)₂-py}Cl]⁺ (9) and [Pd{(3,5-^tBu₂pz-CO)₂-py}Cl]⁺ (10), prepared by halide abstraction of the neutral complexes [Pd{(3,5-Me₂pz-CO)₂-py}Cl₂] and [Pd{(3,5-^tBu₂pz-CO)₂-py}Cl₂] by NaBAr₄, were however less stable with t_1/2 of 14 and 2 days respectively. Attempts to crystallize 1 and 3 from the mother liquor resulted in the isolation of the salts [Pd(_MeN̂N̂N)Cl]₂[Pd₂Cl₆] (11) and [Pd(_tBuN̂N̂N)Cl]₂[Pd₂Cl₆] (12). Although when complexes 1-4 were reacted with modified methylaluminoxane (MMAO) or NaBAr₄, no active catalysts for ethylene oligomerization or polymerization were formed, activation with silver triflate (AgOTf) produced active catalysts that oligomerized and polymerized phenylacetylene to a mixture of cis-transoidal and trans-cisoidal polyphenylacetylene.