Nanosized hollow Cu(I)/Cu(II)-metal-organic frameworks for highly efficient catalysis in azide-alkyne cycloaddition...

The development of efficient heterogeneous catalysts for copper-catalyzed azide-alkyne cycloaddition (CuAAC) is crucial for overcoming the limitations of homogeneous systems, such as poor recyclability and catalyst leaching. In this study, we report the synthesis of nanosized hollow Cu(I)/Cu(II)-metal-organic frameworks (Nano-hollow Cu(I)/Cu(II)-MOF). The paddlewheel Cu(II) sites persist as robust structural nodes, while partial reduction engenders catalytically active planar tetranuclear Cu(I) sites strategically embedded within the framework. This unique architecture endowed Nano-hollow Cu(I)/Cu(II)-MOF with remarkable catalytic activity, achieving a turnover frequency (TOF) of 456.9 h^-1 over 10 min for the CuAAC reaction between phenylacetylene and benzyl azide with a TOF 1.66-fold higher than the nanosphere control, underscoring the hollow structure's advantages. Moreover, the catalyst also demonstrated exceptional recyclability, maintaining a nearly constant catalytic conversion rate over 6 cycles. Theoretical calculations unveiled a dual‑copper synergistic mechanism, wherein coordinated charge redistribution mediated by both copper sites significantly lowered the activation barrier. The alkyne dehydrogenation is identified as the rate-determining step with a theoretical overpotential of 0.82 eV, which was pivotal for enhancing reaction efficiency. Beyond conventional catalysis, the Nano-hollow Cu(I)/Cu(II)-MOF exhibited versatile applicability, including rapid fluorescent molecule synthesis with a rate constant (k_Flu) of 0.1 min^-1 and efficient DNA-templated ligation reactions with an apparent rate constant (k_obs) of 0.21 s^-1.