Tetrafluoroborate (BF₄^-) is a common counterion for active pharmaceutical ingredient (API) or its intermediates. Counterion selection can affect crystal morphology, solid stability and downstream chemical reactions. During the salt metathesis study, the presence of elevated levels of fluoride was observed and the source was hypothesized as BF₄^- degradation. Excess fluoride in the presence of acid has the propensity to generate hydrofluoric acid in situ which can damage glass-lined vessels commonly used in large scale pharmaceutical manufacturing. This finding led to evaluation of BF₄^-stability in both process solvent conditions as well as analytical diluents as any degradation in the analytical standard preparation would lead to inaccurate analysis. Herein, we systematically investigated 1) the stability of BF₄^-ion chromatography (IC) standard and 2) the stability of counterion BF₄^- under process-like conditions.
To investigate long-term stability of BF₄^- IC standard, NaBF₄ was aged in water or 50:50%V/V Acetonitrile(ACN):Water at 4°C or room temperature (RT) up to 18 days. In addition, NaBF₄ was subjected to conditions which resemble the Fischer esterification, 80°C for 2hr or overnight at RT in various solvents to elucidate its solvent and temperature dependence. Anion analysis via IC was implemented to quantitate fluoride and BF4^-.
BF₄^- IC standard stored in 100% water showed significant fluoride formation over time, and higher amounts were observed when stored at room temperature vs 4°C. Interestingly, when stored in 50:50%V/V ACN:Water, significantly less fluoride was generated regardless of storage temperature. When evaluating mock reaction conditions, 2hr at 80 °C in water showed the highest fluoride generation, followed by samples in isopropyl alcohol. However, water with varying amount of ACN greatly suppressed fluoride generation even at elevated temperature. This finding highlights the importance of understanding the stability of counterion and IC standard during the counterion definition and process development.

Wenjun Liu

Merck & Co Inc

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