Asymmetric rotations and dimerization driven by normal to modulated phase transition in 4-biphenylcarboxy coupledL-phenylalaninate

Abstract

Amongst the derivatives of 4-biphenylcarboxylic acid and amino acid esters, the crystal structure of 4-biphenylcarboxy-(L)-phenylalaninate is unusual owing to its monoclinic symmetry within a pseudo-orthorhombic crystal system. The distortion is described by a disparate rotational property around the chiral centers (φchiral≃ −129° and 58°) of the two molecules in the asymmetric unit. Each of these molecules comprises planar biphenyl moieties (φbiphenyl= 0°). Using temperature-dependent single-crystal X-ray diffraction experiments we show that the compound undergoes a phase transition belowT∼ 124 K that is characterized by a commensurate modulation wavevector,q= δ(101), δ = ½. The (3+1)-dimensional modulated structure atT= 100 K suggests that the phase transition drives the biphenyl moieties towards noncoplanar conformations with significant variation of internal torsion angle (φmaxbiphenyl≤ 20°). These intramolecular rotations lead to dimerization of the molecular stacks that are described predominantly by distortions in intermolecular tilts (θmax≤ 20°) and small variations in intermolecular distances (Δdmax≃ 0.05 Å) between biphenyl molecules. Atypical of modulated structures and superstructures of biphenyl and other polyphenyls, the rotations of individual molecules are asymmetric (Δφbiphenyl≈ 5°) while φbiphenylof one independent molecule is two to four times larger than the other. Crystal-chemical analysis and phase relations in superspace suggest multiple competing factors involving intramolecular steric factors, intermolecular H—C...C—H contacts and weak C—H...O hydrogen bonds that govern the distinctively unequal torsional properties of the molecules.