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Oxidative addition of Me1 to [PdMe2(bpy)] (bpy = 2,2'-bipyridine) occurs by the S N~ mechanism. Evidence includes the observation of second-order kinetics in acetone solvent, with a large negative value for the entropy of activation, and the observation of a cationic species, [PdMe3(bpy)(CD3CN)]+I-, in CD3CN solvent. The reaction occurs more slowly than the analogous reaction of [PtMe2(bpy)], but the same mechanism operah. Reductive elimination from [PdIMe,(bpy)] to give ethane and [PdIMeCbpy)] follows good firsborder kinetics, occurs more rapidly in polar solvents, and is strongly retarded by added iodide. These observations are interpreted in terms of a mechanism that involves preliminary ionization of iodide followed by reductive elimination from the cation [PdMe,(bpy)]+. Studies by differential scanning calorimetry allow an estimate of the Pd-C bond energy of-130 kJ mo1-l to be obtained, and this value is considerably higher than the activation energy for reductive elimination of ethane from [PdIMe,(bpy)]. The reductive elimination step is therefore concerted, and possible mechanisms, which may involve direct CC coupling or CC coupling after an agostic CHPd interaction, are discussed. This work is relevant to catalytic CC coupling reactions using palladium complex catalysts.

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Several mononuclear Ru(II) dyads possessing 1,10-phenanthroline-appended pyrenylethynylene ligands were synthesized, characterized, and evaluated for their potential in photobiological applications such as photodynamic therapy (PDT). These complexes interact with DNA via intercalation and photocleave DNA in vitro at submicromolar concentrations when irradiated with visible light (λ irr g 400 nm). Such properties are remarkably sensitive to the position of the ethynylpyrenyl substituent on the 1,10-phenanthroline ring, with 3-substitution showing the strongest binding under all conditions and causing the most deleterious DNA damage. Both dyads photocleave DNA under hypoxic conditions, and this photoactivity translates well to cytotoxicity and photocytotoxicity models using human leukemia cells, where the 5-and 3-substituted dyads show photocytotoxicity at 5-10 μM and 10-20 μM, respectively, with minimal, or essentially no, dark toxicity at these concentrations. This lack of dark cytotoxicity at concentrations where significant photoactivity is observed emphasizes that agents with strong intercalating units, previously thought to be too toxic for phototherapeutic applications, should not be excluded from the arsenal of potential photochemotherapeutic agents under investigation.

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