Previous studies have demonstrated the promising flame-retardant potential of phenylphosphonic acid (PPOA) [1]. The carbon-rich aromatic structure of PPOA enhances gas-phase inhibition, and the oxygen atoms around its phosphorus center promote condensed-phase char formation, making it an effective dual-action phosphorus flame retardant for polymeric systems [2]. In this study, to obtain a novel precursor for coordination chemistry, a new cation–anion compound was synthesized via salt formation between PPOA and 2-aminopyrimidine. The NMR spectra confirmed the structure: a single resonance at 13.4 ppm in the 31P{1H} NMR and the expected signals (one triplet, two multiplets, one doublet, and a broad resonance) in the 1H NMR spectrum. The integration ratios of the doublet, triplet, and two multiplets are consistent with a 1:1 stoichiometry between the 2-aminopyrimidinium cation and the [C6H5P(O)(OH)(O)]– anion. The 13C{1H} NMR spectrum revealed the carbon atoms of the phenyl ring as doublets (1J, 2J, 3J, and 4J), while those of the 2-aminopyrimidinium cation appeared as three singlets. The mass spectrum exhibited a base peak at m/z 96, attributed to protonated 2-aminopyrimidine, and a prominent peak at m/z 157, corresponding to a fragment with the elemental composition C6H6O3P. Furthermore, peaks were observed at m/z 253 and 411, which are attributed to adducts formed by the combination of the C6H6O3P and C4H6N3 fragments, and two C6H6O3P fragments with one C4H7N3 fragment, respectively. This mass spectrometric pattern provides further evidence for the salt-type nature of the compound. The title salt provides multiple potential coordination sites through the phosphonate oxygen atoms and the nitrogen donors of the 2-aminopyrimidinium cation. This structural feature makes it a promising precursor for the synthesis of novel metal-organic complexes, with potential future applications in catalysis or materials science. Scheme 1: Proposed synthesis route. The proton is depicted on the NH2 group for simplicity; the exact protonation site (e.g. on NH2 or ring nitrogen atoms) is not unequivocally determined. References [1] H. Rong, Y. Guo, Z. Chen, et al., Polym. Degrad. Stab. 2023, 216, 110471. [2] Y. Guo, H. Rong, Y. Yu, et al., J. Appl. Polym. Sci. 2022, 139, e53100.

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