Crystals of isomeric tritolylamines: embrace motifs in crystals, and their thermochemical properties

Abstract

Molecules NAr₃ with planar stereochemistry at N and canted Ar rings can form the intermolecular sixfold aryl embrace (6AE). The presence of 3-methyl groups on the aryl groups enhances the stabilisation of the 6AE, by contributing to the EF motifs which comprise it. Analysis of the crystal packing of four tritolylamines, N,N,N-tris(4-methylphenyl)amine [444TTA], N-(3-methylphenyl)-N,N-bis(4-methylphenyl)amine [344TTA], N,N-bis-(3-methylphenyl)-N-(4-methylphenyl)amine [334TTA] and N,N,N-tris(3-methylphenyl)amine [333TTA], reveals the presence of (333TTA)₂ dimers maintained by an excellent 6AE, an unprecedented almost linear chain of repeated 6AE in 334TTA _(cryst), and poorly formed embrace motifs in 344TTA _(cryst), and 444TTA _(cryst). Calculated intermolecular potentials for the actual or possible 6AE embraces formed by 333TTA, 334TTA and 444TTA confirm the stabilisation of the 6AE by 3-methyl groups, and it is estimated that each such methyl group augments the stabilisation of the underlying 6AE embrace by ca 5 kJ mol⁻¹. The dominant dimerisation that occurs in crystalline 333TTA is reflected in the phase change enthalpies for the four isomers. The enthalpy of sublimation of 333TTA (51 kJ mol⁻¹) is ca 45 kJ mol⁻¹ less than the enthalpies of sublimation of 334TTA (97 kJ mol⁻¹), 344TTA (98 kJ mol⁻¹) and 444TTA (93 kJ mol⁻¹), which leads to the hypothesis that (333TTA)₂ dimers occur in the gas phase at ambient pressure. Mass spectrometric attempts to detect (333TTA)_2(g), as [(333TTA)₂]⁺, were unsuccessful, possibly due to dissociation under high vacuum. The unknown crystal packing of tris(3,5-dimethylphenyl)amine is predicted to be dominated by linear chains of methyl-stabilised 6AE.