Hard–Soft Acid–Base Principle Drives Rational Synthesis of Super-Dense Rare-Earth-Based Diatomic Sites

Diatomic catalysts (DACs) hold inherent superiority in atomic economy and synergistic catalysis for complex multi-intermediate reactions. However, several fundamental challenges persist, including typically low diatomic loading (<2 wt %), ambiguous diatomic site identification, and most critically, the lack of universal design principles for rational synthesis. Herein, we present a general synthetic strategy guided by the hard–soft acid–base (HSAB) principle, successfully yielding 14 rare-earth (RE)-based DACs with record-high metal loadings (12.8–30.7 wt %, 3.2–5.4 at. %) and corresponding atomic site densities exceeding 1.12 × 1021 sites g–1. Through an advanced deep learning-powered diatomic recognition method, we unambiguously identify the heterodiatomic configurations with consistently high pairing ratios (60.5%–70.3%) across the DACs. Mechanistic studies combining experimental and theoretical analyses disclose that the strategic incorporation of soft-base phosphorus in the synthesis effectively diminishes coordination dynamics between hard-acid metals and hard-base nitrogen ligands via the HSAB principle-driven antibonding interactions, thus achieving the superdense diatomic sites. Significantly, the high-loading DACs demonstrate superior electrocatalytic nitrate reduction performance, exhibiting up to a 2.7-fold enhancement in ammonia yield rates over their low-loading counterparts. This work establishes a general coordination chemistry-based design principle for rational construction of advanced diatomic catalysts.