While catalysis is highly dependent on the electronic structure of the catalyst, the understanding of catalytic performance affected by electron spin regulation remains challenging and rare. Herein, we have developed a facile strategy to the manipulation of the cobalt spin state over covalent organic frameworks (COFs), COF-367-Co, by simply changing the oxidation state of Co centered in the porphyrin. Density functional theory (DFT) calculations together with experimental results confirm that CoII and CoIII are embedded in COF-367 with S = 1/2 and 0 spin ground states, respectively. Remarkably, photocatalytic CO2 reduction results indicate that COF-367-CoIII exhibits favorable activity and significantly enhanced selectivity to HCOOH, accordingly much reduced activity and selectivity to CO and CH4, in sharp contrast to COF-367-CoII. The results highlight that the spin-state transition of cobalt greatly regulates photocatalytic performance. Theoretical calculations further disclose that the presence of CoIII in COF-367-Co is preferable to the formation of HCOOH but detrimental to its further conversion, which clearly accounts for its distinctly different photocatalysis over COF-367-CoII. To the best of our knowledge, this is the first report on regulating photocatalysis by spin state manipulation in COFs.

The general synthesis and control of the coordination environment of single-atom catalysts (SACs) remains ag reat challenge.H erein, ag eneral host-guest cooperative protection strategy has been developed to construct SACs by introducing polypyrrole (PPy) into abimetallic metal-organic framework. As an example,the introduction of Mg 2+ in MgNi-MOF-74 extends the distance between adjacent Ni atoms;t he PPy guests serve as Nsource to stabilizethe isolated Ni atoms during pyrolysis.Asaresult, aseries of single-atom Ni catalysts (named Ni SA -N x -C) with different Nc oordination numbers have been fabricated by controlling the pyrolysis temperature. Significantly,t he Ni SA -N 2 -C catalyst, with the lowest N coordination number,a chieves high CO Faradaic efficiency (98 %) and turnover frequency (1622 h À1 ), far superior to those of Ni SA -N 3 -C and Ni SA -N 4 -C,inelectrocatalytic CO 2 reduction. Theoretical calculations reveal that the low Nc oordination number of single-atom Ni sites in Ni SA -N 2 -C is favorable to the formation of COOH* intermediate and thus accounts for its superior activity.Single-atom catalysts (SACs), featured with metal atoms dispersed at atomic level, can achieve amaximal utilization of metal atoms and have demonstrated excellent catalytic performance for various reactions. [1] Combining the merits of both heterogeneous and homogenous catalysts,SACshave been regarded as akind of unique heterogeneous catalysts to serve as an ideal model for the exploration of structureproperty relationships during reaction processes. [2] Theatomically dispersed metal atoms of SACs are stabilized by the support through coordination interaction to minimize their surface energy.T herefore,t he local coordination environ-ment usually plays asignificant role for both catalytic activity and selectivity of SACs.A lthough much work has been devoted in this field, to our knowledge,accurate control over the coordination environment of SACs at an atomic level remains agreat challenge and has been rarely investigated. [3] Metal-organic frameworks (MOFs), ac lass of crystalline porous materials with well-defined structures and diverse compositions,n ot only exhibit great potentials for applications in diverse fields, [4] but also are promising precursors to produce desired materials for energy and catalysis. [5] Particularly,MOFs have recently emerged as an ideal platform for the construction of SACs,a st he targeted metal atoms in MOFs can be rationally and spatially separated in an atomically dispersed form. [6] More specifically,b ecause of the precisely designable and tailorable structures and components in MOFs,i ti sc onvenient to realize coordination environment regulation of SACs based on MOFs. [3b,c] While great progress has been achieved, the currently available strategies towards MOF-based SACs are limited to MOFs involving abundant Na toms,s uch as ZIFs [3b,c, 6c,d, 7] and porphyrinic MOFs. [8] Given that Na toms are not involved in most MOFs,itishighly desired to develop amore general approach for the ...