Aging and Ball-Milling as Low-Energy and Environmentally Friendly Methods for the Synthesis of Pd(II) Photosensitizers

Simple and efficient solid-state synthetic methods, aging and ball milling (liquid-assisted grinding, LAG), have been employed to achieve reactions of triphenylphosphine (tpp) and 1,1′-bis(diphenylphosphino)ferrocene (dppf) with dicyclopalladated azobenzenes (DMF)PdCl(μ-R1C6H3N═NC6H3R2)PdCl(DMF), R1 = H, OCH3, N(CH3)2 and R2 = H, NO2. For the first time the aging processes have been applied in the formation of Pd(II) heteroleptic complexes. Both synthetic procedures lead to four types of tetra- or pentacoordinated complexes (tpp)PdCl{(μ-Cl)(μ-R1C6H3N═NC6H3R2)}Pd(tpp) 1A–3A, (tpp)2PdCl(μ-R1C6H3N═NC6H3R2)PdCl(tpp)2 1B–3B, PdCl{(μ-Cl)(μ-R1C6H3N═NC6H3R2)}Pd(dppf) 1C–3C, and (dppf)PdCl(μ-R1C6H3N═NC6H3R2)PdCl(dppf) 1D–3D, in which azobenzenes simultaneously act as monodentate C- and bidentate C,N-donors (A, B, and C complexes) or, for the first time, only as double C-donors (D complexes). Although their formation requires complex intramolecular transformations, aging and ball milling have been proved to be effi...