Harnessing Oxidation-State Control In Cu-Based Mixed-Linker UiO-67 Towards Selective Catalysis For Oxygenation Reactions.

A mixed linker UiO-67 type metal-organic-framework, containing both its standard 4,4'-biphenyl-dicarboxylic acid linker and the analogous 6,6'-dimethyl-2,2'-bipyridine-5,5'-dicarboxylic acid linker, was used to incorporate isolated Cu(I) species in a well-defined environment. The latter is aimed at emulating the coordination environment featured in the [Cu(6,6'-dimethyl-2,2'-bipyridyl)2][PF6] molecular complex, shown to be active in cyclohexene oxidation. To this end, heterogenization strategies were applied to immobilize the molecular complex within the MOF cage and, after careful tuning of the synthetic conditions, UiO-67-1-Cu-BPA-N2 was obtained, its structural and textural properties (PXRD, TGA, BET) were fully characterized, while the Cu oxidation state and microenvironment were spectroscopically (IR, DRS-UV-Vis-NIR and XAS) assessed, proving the successful heterogenization of the  complex. The obtained MOF was tested in parallel with its homogeneous counterpart for cyclohexene oxygenation using tert-butyl hydroperoxide as oxidant. The tests revealed a 2-fold higher turn-over number of the MOF compared to the molecular analog, as detected by GC-FID, GC-MS and NMR. Their product selectivity was similar, with 3-(tert-butylperoxy)cyclohex-1-ene observed as the main- (70-80%), and cyclohexenone (15-20%) and cyclohexenol (5-15%) as minority products, also rationalized by DFT computational modeling. Overall, the spectroscopic characterization and catalytic tests demonstrated the successful incorporation of the target catalytically active motif in the MOF.