Alternative coordination modes in palladium(II)-diimino-bispyridine complexes with an axially chiral biphenyl backbone

Abstract

The chiral biphenyl-bridged diimino-bispyridine ligandsN,N′-(6,6′-dimethylbiphenyl-2,2′-diyl)bis(2-pyridylmethyl)-diimine(1)and N,N′-(6,6′-dimethylbiphenyl-2,2′-diyl)bis[(6-methyl-2-pyridyl)methyl]diimine (2) react with Pd(COD)Cl₂ to give, depending on the reaction conditions, either mono- or binuclear PdCl₂ complexes. In the binuclear complex 1-(PdCl₂)₂, the Pd nuclei are held at a distance of 3.37 Å by the ligand backbone. N,N′-(6,6′-dimethylbiphenyl-2,2′-diyl)bis[(5-methyl-2-furyl)methyl]diimine (3), with furyl instead of pyridyl rings, gives mononuclear, C₂-symmetric complexes only. Reactions of [Pd(NCCH₃)₄]²⁺(BF₄^–)₂ with ligand 1 or 3 give the C₂-symmetric cations [1-Pd]²⁺or [3-Pd(NCCH₃)₂]²⁺ respectively, as their BF₄^– salts. Solid-state structures of the chloride complexes 1-PdCl₂, 1-(PdCl₂)₂ and 3-PdCl₂, and of the complex cations [1-Pd]²⁺ and [3-Pd(NCCCH₃)2]²⁺ with tetradentate and bidentate ligand coordination, respectively, all show square-planar coordination, with some distortion toward tedrahedral geometry due to the twisted biaryl-backbone. Preliminary observations on norbornene polymerization with the catalysts 1-PdCl₂/MAO, 2-PdCl₂/MAO and [(3-Pd(NCCH₃)₂]²⁺ suggest that a certain degree of stereoregularity of the polymers is induced by these chiral catalysts.