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.