The Construction Of Photoactive Metal-Organic Frameworks For C-H Bond Activation

As a classical kind of multi-functional porous coordination polymer,metal-organic frameworks are a periodic network crystalline structure,being formed by the coordination self-assembly of metal nodes and organic functional ligands.Due to its superior properties,it has broad application prospects in the field of heterogeneous photocatalytic organic synthesis.Based on the long-range ordered structural advantages,metal-organic frameworks introducing different photoactive ligands and photocatalytic sites exhibit diverse and functional catalytic tendency.In this paper,multifunctional organic functional ligands and different metal salts were selected to construct photoactive metal-organic frameworks with different dimensions,possessing good structural stability and excellent photocatalytic performance.（1）In this work,we prepared a three-dimensional metal-organic framework,namely Mn-AQ by consisting of functional anthraquinone ligand H₂AQ and metal manganese salts,acting as heterogeneous catalysts for visible light-induced redox mediated hydrogen atom transfer to activate C（sp³）-H bond.The 3D Mn-AQ is conducive to the promotion of charge transport,and the rigid frame structure and one-dimensional pore structure are conducive to the stability of alkoxy radicals,avoiding the self-quenching.Under visible light irradiation,the excited Mn-AQ activates the fatty alcohol to form alkoxyl radicals via proton-coupled electron transfer event.Alkoxyl radicals can abstract the hydrogen atoms of C（sp³）-H bond to form alkyl radicals,which add to the electron-deficient olefins and obtain the alkylation products.Therefore,photoactive Mn-AQ as visible-light-induced redox HAT activates the alkylation of inner C（sp³）-H in an efficient and regionally selective way.（2）Based on the above work,we prepared a two-dimensional layered bifunctional metal-organic framework,namely Co-AQ,constructed by using the functionalized anthraquinone ligand H₂AQ and cobalt ions.The framework acts as a multifunctional photocatalytic platform for the selective oxidation of benzylic C（sp³）-H bond through hydrogen atom transfer and energy transfer events.The regular pore three-dimensional framework was constructed throughπ–πinteraction between the anthraquinone moieties from the adjacent layers,beneficial to confining the substrate molecules and stabilizing the free radical intermediates to promote the reaction and avoid the self-quenching or dimerization of anthraquinone.Under visible light irradiation,the excited bifunctional Co-AQ activates the benzylic C（sp³）-H bond to form benzylic carbon radicals through hydrogen atom transfer event.At the same time,an energy transfer event occurs between the excited bifunctional Co-AQ and oxygen to obtain singlet oxygen species,completing oxidation of benzylic radicals.Therefore,the careful decoration of anthraquinone fragments within metal–organic frameworks would represent a promising approach to orderly integrate both functions of the photocatalyst anthraquinone,which parallelly absorbs two photons as visible light-mediated HAT/En T event to achieve selective oxidation of benzylic C（sp³）-H bond.