Synthesis of Zirconium Boron-Oxo Clusters With Tunable Third-Order Nonlinear Optical Response

Due to the weak coordination ability with metals and the easy self-condensation of arylboronic acids, the development of organic-inorganic hybrid metal boron-oxo clusters is still challenging. In this study, by leveraging the in situ condensation reaction between boron sources (e.g., trimethyl borate and arylboronic acids) and dioxime, a series of dioxime-modified zirconium boron-oxo clusters (Zr-BOCs) was synthesized via a pyrazole-thermal condition. In these structures, dioximes not only induce the polymerization of boric acid into clusters but also chelate Zr ions, enabling electronic coupling between B-O and Zr-O moieties and resulting in a nearly planar cluster core Zr₂B₈O₁₀. Taking advantage of the open Zr sites and the boron atom hybridization change from sp² to sp³, we sequentially introduced aromatic carboxylic acids and arylboronic acids to controllably modulate both the planarity and the electronic structure of the Zr₂B₈O₁₀ core. In particular, the Zr-O quadrilateral ring in BOC-13 (Zr₂(Dmg)₄(CH₃OB₄O₅)₂(BA)₂) demonstrates aromaticity revealed by DFT calculations. Its conjugation with planar B-O clusters extends electron delocalization from a p-p-π to a d-p-π system, significantly enhancing third-order nonlinear optical (NLO) properties and achieving a minimum normalized transmittance (T_min) of 0.20. The present work establishes a modular synthesis strategy for organic-inorganic hybrid boron-oxo clusters with a d-p-π delocalization system.