Novel unsymmetrical PCP′ pincer ligands and their palladium(II) complexes

Synthetic routes towards novel PCP′ pincer ligands were devised. Ligand 1-(Pr2iPOCH2)-3-(Bu2tPCH2)(C6H4) is prepared in a three step synthesis from 1,3-benzenedimethanol and 1-(Pr2iPO)-3-(Bu2tPCH2)(C6H4) is accessible in three steps from 3-hydroxybenzylalcohol. Both their palladium(II) complexes are prepared in good yields but are distinctly different since [PdCl{(C6H3)(OPPr2i)-2-(CH2PBu2t)-6}] possesses two five-membered palladacycles, whereas [PdCl{(C6H3)(CH2PBu2t)-2-(CH2OPPr2i)-6}] is unusual for a pincer complex in that it contains both five- and six-membered palladacycles. Both compounds also represent the first examples of pincer complexes where one donor is a phosphinite and the other is a phosphine. The X-ray structures of these complexes were solved and are discussed. The data reveal that an increase in the metallacycle ring-size leads to changes in bond lengths, but more importantly to significant increases in the bond angles.