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

The compounds, 2,6-bis(3,5-dimethylpyrazol-1-ylmethyl)pyridine (MeNˆNˆN) (L1) and 2,6-bis(3,5-ditertbutylpyrazol-1-ylmethyl)pyridine (tBuNˆNˆN) (L2), react with either [Pd(NCMe)2Cl2] or [Pd(COD)ClMe] to form the mononuclear palladium complexes [Pd(MeNˆNˆN)Cl2] (1), [Pd(MeNˆNˆN)ClMe] (2), [Pd(tBuNˆNˆN)Cl2] (3) and [Pd(tBuNˆNˆN)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(MeNˆNˆN)Cl]+(5), [Pd(MeNˆNˆN)Me]+ (6) and [Pd(tBuNˆNˆN)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 t1/2 of 14 and 2 days respectively. Attempts to crystallize 1 and 3 from the mother liquor resulted in the isolation of the salts [Pd(MeNˆNˆN)Cl]2[Pd2Cl6] (11) and [Pd(tBuNˆNˆN)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.

Graphical abstractBis(pyrazolylmethyl) pyridine ligands react with [PdCl2(NCMe)2] or [Pd(COD)ClMe)] to form mononuclear palladium complexes in which the ligands bind in a bidentate fashion with a dangling pyrazolyl unit; but on removal of a chloride from these complexes the dangling arm coordinates to form tridentately bound palladium complexes. These tridentatley bound palladium complexes, however, can open up and catalyze the polymerization of phenylacetylene to polyphenylacetylene.Figure optionsDownload full-size imageDownload as PowerPoint slide