[C3N2H5]B7O10F2: Demonstrating a New Hydrogen-Bond-Induced Intralayer Charge-Balanced Structure Model with Large Birefringence.

The development of anisotropic materials, particularly within the borate family, is a key focus for achieving large birefringence in short-wave ultraviolet optical applications. Layered structures comprising interconnected planar fluorooxoborate units have emerged as an effective structural model. Recently, the incorporation of planar cations into fluorooxoborates has proven to be an effective strategy for enhancing optical anisotropy. In previously reported structures, these cations and fluorooxoborate anions typically form independent and alternating layers to maintain charge balance. Here, we report a novel compound in which the planar [C3N2H5] cation is integrated into the fluorooxoborate layer via hydrogen bonding, resulting in a unique single-layer packing arrangement. To our knowledge, this compound represents the first instance of a structure containing a new [B7O12F2] fundamental building block. Owing to the above structural innovations, the compound exhibits a remarkably high birefringence of 0.223 at 546 nm. This work establishes a new structural model wherein planar cations and fluorooxoborate anions are unified within a single layer, offering a promising strategy for enhancing the anisotropy of fluorooxoborates.