Interaction dependent anisotropic thermal expansion of a hydrogen bonded cocrystal

A binary cocrystal of 2,4,6-triethyl-1,3,5-benzenetricarboxylic acid (TEBH₃) and trans-4,4′-bispyridylethylene (BPE), 1, exhibits a volumetric thermal expansion (α_{V, 298K}) of 162(19) [10⁻⁶ K⁻¹] between 118 ─ 298 K. The axial thermal expansion coefficients are α_{1(010), 298K} = 15(9) [10⁻⁶ K⁻¹], α_{2(100), 298K} = 17(5) [10⁻⁶ K⁻¹], and α_{3(001), 298K} = 113(3) [10⁻⁶ K⁻¹] respectively. The structure of 1 is a typical 2―fold interpenetrated hydrogen bonded corrugated square lattice (sql) net, sustained by strong O–H⋯O (along the b axis) and O–H⋯N (along the a-axis) hydrogen bonds. The 2‒fold interpenetrated sql nets get stacked into layers along the 001 direction sustained by weak C–H⋯O hydrogen bonds. The α_{1(010), 298K} and α_{2(100), 298K} directed along the plane of the sql net, are attributed to the presence of strong O–H⋯O and O–H⋯N hydrogen bonds which restrict expansion along the plane of the sql net. The major principle axis of expansion is directed along the direction of the stacking of the interpenetrated nets linked by weak C–H⋯O hydrogen bonds resulting into α_{3(001), 298K} being ∼ 7 times higher than α_{1(010), 298K} and α_{2(100), 298K}. Between 118 K and 208 K a shortening of the strong O–H⋯N hydrogen bond is observed along with a negligible increase in O–H⋯O which accounts for the biaxial NTE exhibited by 1 between 118 K - 208 K along the a and the b axes respectively.