A photoresponsive palladium complex of an azopyridyl-triazole ligand: light-controlled solubility drives catalytic activity in the Suzuki coupling reaction

Herein, the design and synthesis of a click-derived Pd-complex merged with a photoswitchable azobenzene unit is presented. While in the trans-form of the switch the complex showed limited solubility, the photogenerated cis-form rendered the molecule soluble in polar solvents. This light-controllable solubility was exploited to affect the catalytic activity in the Suzuki coupling reaction. The effect of the substrate and catalyst concentration and light intensity on the proceeding and outcome of the reaction was studied. Dehalogenation of the aryl iodide starting material was found to be a major side reaction; however, its occurrence was dependent on the applied light intensity.

NMR spectra were acquired on a Varian 500 NMR spectrometer, running at 500 and 126 MHz for 1 H and 13 C, respectively, and on a Varian 300 NMR spectrometer, running at 300 and 75 MHz for 1 H and 13 C, respectively. NMR spectrometers were operated at 30°C, otherwise noted.  mmol) in pyridine (60 mL) was added a solution of NaOH (336 mg in 2 mL water). The mixture was stirred for 24 h at rt. The solvent was removed in vacuo, the residue was dissolved in ethyl acetate (60 mL) and washed with water (2x30 mL). The organic phase was dried (MgSO 4 ), filtered, and the solvent was evaporated under reduced pressure. The residue was purified by silica gel chromatography using gradient elution (hexanes to hexane-ethyl acetate 20%) to obtain an orange-coloured product (1.54 g, 71%
Note that in

Irradiation experiment to generate cis-5-Pd
Trans-5-Pd (~ 2 mg) was dissolved in DMF-d7 and the sample was irradiated for 2 h at rt and the 1 H NMR (500 MHz) spectra of the resulting mixture was recorded at 21°C ( Figure S1). No darkening of the solution was observed during irradiation, which indicates the lack of metal loss.

X-Ray Crystallography
Intensity data were collected on a Rigaku RAXIS-RAPID II diffractometer (using graphite monochromator; Mo-Kα radiation,  = 0.71075Å) at 153 and 143K in case of crystals 4 and 5-Pd, respectively. Crystals of crystals 5 and 5-Pd were measured in a loop. Crystal Clear 2 (developed by Rigaku Company) software were used for data collection and refinement.
Numerical absorption corrections 3 were applied to the data. The structures were solved by direct methods. Anisotropic full-matrix least-squares refinements were performed on F 2 for all nonhydrogen atoms. Hydrogen atoms bonded to C atoms were placed in calculated positions and refined in a riding-model approximation. The computer programs used for the structure solution, refinement and analysis of the structures were Shelx 4,5 , Wingx 6 , Platon 7 and Olex2 8 . Program Mercury 9 was used for the graphical representation. Details of crystallographic data, data collection and refinement for crystal crystals 4 and 5-Pd are collected in Table S2.  intensities of 2720 reflections were greater than 2(I). Completeness to θ = 0.980.
A numerical absorption correction was applied to the data (the minimum and maximum transmission factors were 0.834433 and 0.947750).
The structure was solved by direct methods (and subsequent difference syntheses).
Hydrogen atomic positions were calculated from assumed geometries. Hydrogen atoms were included in structure factor calculations but they were not refined. The isotropic displacement parameters of the hydrogen atoms were approximated from the U(eq) value of the atom they were bonded to. Hydrogen atomic positions were calculated from assumed geometries. Hydrogen atoms were included in structure factor calculations but they were not refined. The isotropic displacement S14 parameters of the hydrogen atoms were approximated from the U(eq) value of the atom they were bonded to.