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Predicting Cocrystallization Based on Heterodimer Energies: The Case of N,N′‑Diphenylureas and Triphenylphosphine Oxide

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posted on 2015-10-07, 00:00 authored by Marina A. Solomos, Cameron Mohammadi, Jessica H. Urbelis, Elizabeth S. Koch, Rochelle Osborne, Claire C. Usala, Jennifer A. Swift
Diarylureas frequently assemble into structures with one-dimensional H-bonded chain motifs. Herein, we examine the ability of triphenylphosphine oxide (TPPO) to disrupt the H-bonding motif in 14 different meta-substituted N,N′-diphenylureas (mXPU) and form cocrystals; 1:1 mXPU:TPPO cocrystals were obtained in 9 of 14 cases examined (64% success rate). Cocrystals adopt five different lattice types, all of which show unsymmetrical H-bonded [R21(6)] dimers between the urea hydrogens and the phosphine oxygen. Heterodimer (mXPU···TPPO) and homodimer (mXPU···mXPU) interaction energies, ΔEint, calculated using density functional theory at the B3LYP/6-31G­(d,p) level were used to rationalize the experimental results. A clear trend was observed in which cocrystals were experimentally realized only in cases in which the differences in heterodimer versus homodimer energy, ΔΔEint, were greater than ∼5.3–6 kcal/mol. Although calculated interaction energies are a simplified measure of the system thermodynamics, these results suggest that the relative ΔΔEint between heterodimers and homodimers is a good predictor of cocrystal formation in this system.

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