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

Acta Crystallographica Section B Structural Science Crystal Engineering and Materials, vol. 79, pp. 148–156

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 (φ chiral ≃ −129° and 58°) of the two molecules in the asymmetric unit. Each of these molecules comprises planar biphenyl moieties (φ biphenyl = 0°). Using temperature-dependent single-crystal X-ray diffraction experiments we show that the compound undergoes a phase transition below T ∼ 124 K that is characterized by a commensurate modulation wavevector, q = δ(101), δ = ½. The (3+1)-dimensional modulated structure at T = 100 K suggests that the phase transition drives the biphenyl moieties towards noncoplanar conformations with significant variation of internal torsion angle (φ max biphenyl ≤ 20°). These intramolecular rotations lead to dimerization of the molecular stacks that are described predominantly by distortions in intermolecular tilts (θ max ≤ 20°) and small variations in intermolecular distances (Δ d max ≃ 0.05 Å) between biphenyl molecules. Atypical of modulated structures and superstructures of biphenyl and other polyphenyls, the rotations of individual molecules are asymmetric (Δφ biphenyl ≈ 5°) while φ biphenyl of 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.

Authors 10

1. Somnath Dey Aachen Institute of Crystallography

   Indian Institute of Science Education and Research Kolkata · RWTH Aachen University

   Affiliation as printed

   Department of Chemical Sciences, Indian Institute of Science Education and Research (IISER) Kolkata, Mohanpur 741246, India

   Institute of Crystallography, RWTH Aachen University, Jägerstraße 17-19, 52066 Aachen, Germany

2. Supriya Sasmal

   Indian Institute of Science Education and Research Kolkata

   Affiliation as printed

   Department of Chemical Sciences, Indian Institute of Science Education and Research (IISER) Kolkata, Mohanpur 741246, India

3. Saikat Mondal

   Indian Institute of Science Education and Research Kolkata

   Affiliation as printed

   Department of Chemical Sciences, Indian Institute of Science Education and Research (IISER) Kolkata, Mohanpur 741246, India

4. Santosh Kumar

   Indian Institute of Science Education and Research Kolkata

   Affiliation as printed

   Department of Chemical Sciences, Indian Institute of Science Education and Research (IISER) Kolkata, Mohanpur 741246, India

5. Rituparno Chowdhury

   Indian Institute of Science Education and Research Kolkata

   Affiliation as printed

   Department of Chemical Sciences, Indian Institute of Science Education and Research (IISER) Kolkata, Mohanpur 741246, India

6. Debashrita Sarkar

   Indian Institute of Science Education and Research Kolkata

   Affiliation as printed

   Department of Chemical Sciences, Indian Institute of Science Education and Research (IISER) Kolkata, Mohanpur 741246, India

7. Chilla Malla Reddy

   Indian Institute of Science Education and Research Kolkata

   Affiliation as printed

   Department of Chemical Sciences, Indian Institute of Science Education and Research (IISER) Kolkata, Mohanpur 741246, India

8. Lars Peters Aachen Institute of Crystallography

   RWTH Aachen University

   Affiliation as printed

   Institute of Crystallography, RWTH Aachen University, Jägerstraße 17-19, 52066 Aachen, Germany

9. Georg Roth Aachen Institute of Crystallography

   RWTH Aachen University

   Affiliation as printed

   Institute of Crystallography, RWTH Aachen University, Jägerstraße 17-19, 52066 Aachen, Germany

10. Debasish Haldar

    Indian Institute of Science Education and Research Kolkata

    Affiliation as printed

    Department of Chemical Sciences, Indian Institute of Science Education and Research (IISER) Kolkata, Mohanpur 741246, India