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 efficient synthetic methods. All products have been fully characterized in solid state and solution. Solid-state structures of complexes have been resolved by a single crystal or powder X-ray diffraction (PXRD) and solid-state NMR spectroscopy. Their electronic structures have been rationalized by the quantum-chemical calculations. The structural features of complexes and their stability in solutions have been evaluated by 1H and 31P NMR spectroscopy along with the ESI-MS spectrometry. The absorption and emission properties of complexes have been studied, and the nature of their electronic transitions in the low energy region has been examined by TD-DFT calculations. Significant fluorescence observed for the complexes with 4-dimethylamino-4′-nitroazobenzene ligand at room temperature in solid state and/or in the solution makes them interesting for potential applications as photoactive units in optical devices.