Construction Of Shielded Metal-Organic Cages For Photo-Driven Reduction

In the enzymatic catalysis process,natural oxidoreductases encapsulate specific substrates and alter thermodynamics to activate redox inert substrates for efficient conversion of target products under mild conditions.Developing new catalyst that echo the structure and catalytic ability of enzymes is a major research area in green synthetic chemistry.Among which,the shielded metal-organic supramolecular is considered as an excellent model for enzyme simulation,due to the well defined cavities and plentiful activation sites.Remimiscent of natural enzymes,the metal-organic cages that self-assembled by coordination between organic ligands and metal ions has achieved successful application of molecular recognition and efficient catalytic effect via the modification of the reactivity of substrates through non-covalent interactions.Meanwhile,the shielded cavity can also modify and optimize the inner reaction pathways,especially for selectively catalytic conversion.By exploring the structure-activity relationship of the metal-organic supramolecular system,the diffusion process of electron donors and substrates into microenvironment catalysts,and the thermodynamic activation of substrates,we aimed to reveal the application of shielded supramolecular catalytic systems in photo-driven reduction reactions and the mechanism of the biomimetic catalysis.（1）Two iron metal-organic helicals（H1 and H2）are constructed with different ligands containing hydrazide groups and alkenyl groups separately.Both of them can encapsulate fluorescein to obtain supramolecular photocatalysis H1F and H2F,and realize the combination of photocatalytic H₂S splitting and nitroaromatic hydrogenation.Due to the hydrazide groups on the helical edges of H1F acting as hydrogen binding sites to active nitroaromatic substrates,the active hydrogen is utilized more efficienty in H1F system than H2F system even with weaker proton reduction capability.Also,the kinetics experiments of photocatalytic process verifies the simulation of enzyme catalytic behaviour.（2）A shielded metal-organic cage is used to modify the electron donation kinetics to optimize the pathways of stepwise electron transfers in nitro reduction reactions for highly selective generation of target products.The carefully orchestrated cobalt octahedron（H3）with robust openings and cavity can encapsulate fluorescein molecule to form the supramolecular photocatalytic system H3F.Taking advantage of the pseudo-intramolecular photoinduced electron transfer in the microenvironment fast enough,diffusion coefficients of electron donors into microenvironments are modified to control the electron injection kinetics and the selectivity of the azo and amino products from the reduction of nitroarenes.The faster electron injection kinetics into the microenvironment gives the amino products,whereas the slower electron injection kinetics endows the formation of azo products.Of the reaction mixtures contain different nitroarenes,the differences on the diffusion coefficients and on the inclusion free energy changes of the two different nitroarenes are found to govern the selectivity of asymmetric azo compounds over symmetric ones.（3）A cobalt-based molecular octahedron（H4）with hydrogen-bonding triggers is constructed and used to encapsule and further activate hydrazine substrates,mimicking the catalytic domain of natural Mo Fe proteins.Reminiscent of the catalytic properties of enzymes,the microenvironment shielding approach facilitates a significant anodic shift of the redox potential of hydrazine and a decreasing（about 70 k J/mol）of free energy change for initial electron transfer process in hydrazine reduction,making the ammonia formation proceeding under mild conditions.Also,H4 can encapsulate fluorescein molecule to form the supramolecular photocatalytic system H4F.By the introducing of light driven mode,the free energy change for initial electron transfer process in hydrazine reduction is further decreased.And the reduction of N₂H₄ exhibits inherent high selectivity and an ammonia conversion rate of approximately 1530 nmol/min,comparable to that of natural nitrogenase Mo Fe proteins with strong reductants.The new strategy realizes the boloenzyme simulation on thermodynamic activation,calling for the attract innovation of bioinspired photocatalysts.