Synthetic Routes to Methylpalladium(II) and Dimethylpalladium(II) Chemistry and the Synthesis of New Nitrogen Donor Ligand Systems

Convenient and widely applicable synthetic routes to methylhalogenopalladium(II), PdXMe(L2), and dimethylpalladium(II) complexes, PdMe2(L2), have been developed, including complexes of triphenylphosphine and a wide range of bidentate nitrogen donor ligands. These routes involve either the generation of Pd Me species at low temperature from methyllithium reagents and trans-PdCl2(SMe2)2 followed by addition of ligand PdIMe(22'-bipyridyl) being synthesized through the oxidative-addition reactivity of Pd2(dba)3(CHCl3) or the facile synthesis of complexes with the reagents [PdIMe(SMe2)]2 and [PdMe2(pyridazine)] in organic solvents at ambient temperature. These reagents are particularly suitable for ligands sensitive to MeLi reagents and [PdIMe(SMe2)]2 is also a suitable substrate for the synthesis of chloro and bromo complexes PdXMe(L2) including PPh3 complexes. Several new nitrogen donor bidentate ligands are described containing 1-methylimidazol-2-y1(mim) and pyridin-2-y1 (py) groups in (mim)2C=CH2 and (py)(mim)C=CH2 as relatives of planar ligands such as 22'-bipyridyl and mim py and pyrazol-1-yl (pz) groups in (py)(mim)CH2 (py)(mim)CHMe (mim)2CH2 (mim)2CHMe and (pz)(mim)CH2 as relatives of ligands such as (py)2CH2 and (pz)2CH2. Methylpalladium(II) complexes of unsymmetrical bidentate ligands exhibit isomerism; e.g. isomers of PdIMe[(py)(mim)C=O] occur in the ratio 9:1 where the dominant isomer has the pyridine ring trans to methyl. The ligands with methane bridges e.g. (pz)(py)CH2 ethane bridges e.g. (pz)2CHMe and propane bridges e.g. (py)CMe2 form complexes PdMe2(L2) and PdIMe(L2) that exhibit variable-temperature NMR spectra indicating boat-to-boat inversion of the chelate ring but complexes of (mim)2CHMe and (py)(mim)CHMe appear to adopt only the conformation with the methyl group axial and adjacent to palladium."