Solvent-controlled synthesis of Ti-based porphyrinic metal-organic frameworks for the selective photocatalytic oxidation of amines.

The photocatalytic activity of metal-organic frameworks (MOFs) can be managed by the milieu of synthesis. Herein, N,N'-dimethylacetamide (DMA) and N,N'-diethylformamide (DEF) were employed as solvents for the synthesis of two Ti-based porphyrinic MOFs, namely Ti-PMOF-DMA and Ti-PMOF-DEF, from tetrabutyl orthotitanate and 4,4',4'',4'''-(porphine-5,10,15,20-tetrayl)tetrakis(benzoic acid). Notably, both DMA and DEF were adsorbed onto the Ti-oxo clusters of the two MOFs to shape their properties. Ti-PMOF-DMA was observed with better optoelectronic response and charge transfer than Ti-PMOF-DEF. Moreover, Ti-PMOF-DMA owned a larger pore volume than Ti-PMOF-DEF, imparting more accessible sites to benzyl amines. Ti-PMOF-DMA exhibited better activity in selective photocatalytic aerobic oxidation of benzylamine than Ti-PMOF-DEF. Irradiated by red light-emitting diodes, outstanding results for selective conversion of  benzyl amines to imines over Ti-PMOF-DMA were attained. Superoxide radical anion, generated by the electron transfer from porphyrin via Ti-oxo clusters to dioxygen, turned out to be the primary reactive oxygen species. There was generality towards aerobic oxidation of amines to imines and considerable stability for Ti-PMOF-DMA. This work provides a new perspective on the altering MOFs to enhance photocatalytic organic transformations.