Pyrazole‐Protected Heteronuclear Rare‐Earth Boron‐Oxo Cluster Cage With Tunable Luminescence

Rare-earth boron-oxo clusters, which combine the structural diversity of borates with the unique chemical properties of rare-earth ions, represent a promising platform for advanced functional materials. Herein, we report the synthesis and characterization of a series of heteronuclear rare-earth boron-oxo clusters (BOCs), [RE₃B₉(μ₃-O)₁₂Pz₆HPz₆Cl₆] (BOC-9, Pz = pyrazolate, HPz = pyrazole), [RE₃B₉(μ₃-O)₁₂Pz₇HPz₆(MeO)₂Cl₃] (BOC-10), and [RE(NO₃)₆RE₅B₁₈(μ₃-O)₂₄Pz₁₈]n (BOC-11), where RE = Dy³ ⁺/Y³ ⁺. Structural evolution from discrete to 2-dimensional (2D) architectures was achieved through systematic variation of boron sources and rare-earth salts. These clusters exhibit a well-defined stability gradient correlated with increasing organic ligands and structural dimensionality. Spectroscopic characterizations uncover two distinct photoluminescence emission mechanisms in the boron-oxo based functional materials. Dy³ ⁺-based BOCs exhibit typical f-f transition emissions, which are effectively sensitized by the ligand-to-metal charge transfer (LMCT) process, whereas closed-shell Y³ ⁺-based BOCs show exclusive photoluminescence derived from the ligand-to-boron charge transfer pathway. Dynamic B─N bond cleavage, as evidenced by electrospray ionization mass spectrometry (ESI-MS), enables isolation of cluster cores, providing mechanistic insights into structural rearrangements. This work establishes design principles for next-generation luminescent materials based on rare-earth boron-oxo clusters.