Breaking steric hindrance restriction: compressed metal bonds realizing deep-level orbital delocalization.

The introduction of large functional groups is a well-established strategy to enhance the stability and value of chemicals. However, accompanying steric hindrance effects can significantly impede the reaction progress, presenting a critical dilemma in synthetic design. Herein, a vacancy-constrained strategy was proposed for breaking steric hindrance restriction by realizing the deep-level delocalization of a catalyst with compressed metal bonds. In detail, the delocalized deep-level orbit exhibits strong penetrating capability that overcomes the steric hindrance effect of large functional groups by enhancing the charge-transfer process between the active sites and substrates. By taking bismuth atoms supported on defective CeO2 as an example, an 'O4-Bi-Bi-O4' ensemble was constructed by confining two adjacent Bi atoms in the vacancy of CeO2 nanosheets, where the Bi-Bi bond was strongly constrained from 3.10 to 3.05 Å. Such a functional Bi2/CeO2 catalyst exhibits enhanced photocatalytic efficiency in overcoming the steric hindrance from large functional groups, which is three times and two times higher than that of Bi single-atom catalyst and Bi cluster catalyst, respectively. This work proposes an applicable way of overcoming steric hindrance effects in the synthesis of high-value-added chemicals.