Designing Copper(I) Iodide Clusters with Tunable D-π-A Ligands for Photocatalytic Thiocyanation

Five donor-π-acceptor ligands (D-π-A ligand: π-conjugated ligand possessing electron donor-acceptor hybrid characteristics) and six copper(I) iodide compounds [Cu^I_xI_x(L_m)_x] (x = 2, 4, L_m = L₁-L₅) are prepared and evaluated as photocatalysts for thiocyanation reactions of (hetero)arenes. The results indicate that the combination of thiophene as a bridge and 9,9-dimethylfluorene as an electron-donating group causes the maximum wavelength red shift and increases absorbance. With its unique cubic conformation, the tetranuclear [Cu^I₄I₄(L₄)₄] cluster demonstrates good performance on photocatalysis in connection with its good light-harvesting efficiency and remarkable electron-transfer capability. The compound PC5 is applied as a catalyst for the photocatalytic thiocyanation of anilines, phenols, and heteroarenes to aryl thiocyanates by using NH₄SCN as the SCN^- source. The reactions proceed efficiently under optimized conditions. A total of 31 examples show that our method has a wide scope of applicability for thiocyanation, with products yielded up to 85%. The experimental study reveals that a single-electron transfer (SET) process is involved in the light-induced photoactivation of the [Cu^I₄I₄] cluster, generating anilinium and thiocyanato radicals that initiate the subsequent free radical reaction mechanism. The synthetic strategy was studied on a gram scale, showing the potential of the practicality of this method for the preparation of aryl thiocyanates.