Influence of N‐Alkyl Substituents and Counterions on the Structural and Mesomorphic Properties of Guanidinium Salts: Experiment and Quantum Chemical Calculations

A series of N‐4‐(4′‐alkoxybiphenyl)‐N′,N′,N”,N“‐tetramethylguanidinium salts was synthesized with varying alkoxy chain lengths and additional N‐alkyl substituents, each with a number of different counterions. X‐ray crystal‐structure analyses of 1b I , 1b PF₆ , 2a I , and 4a I reveal bilayer structures in the solid state and, for the 1b and 1b PF₆ salts, a hydrogen‐bond‐type connectivity between the guanidinium N‐H group and the anion is found. For the N‐alkyl homologues 2a I and 4a I the anion is still oriented close to the head group, although at a larger distance. Ion pairs are present also in solution, as demonstrated by ¹H NMR: the N‐H chemical shift shows a good linear correlation with the radius, and hence the hardness, of the anion. The intramolecular conformational flexibility of 1b I , 2b I , 3b I, and 4b I was studied by temperature‐dependent ¹H NMR spectroscopy and discrete activation barriers were determined for rotations about each of the three C? N partial double bonds of the guanidinium core. The relative heights of the individual barriers change between the N‐H and the N‐alkylguanidinium salts. A fourth barrier is observed for the rotation about the N? biphenyl bond. DFT calculations of charge densities show that the positive charge resides primarily on the central carbon atom. Rotational barriers were calculated for N′‐substituted 2‐amino‐1,3‐dimethylimidazolidinium cations as models, and are in qualitatively good agreement with the NMR data. Mesomorphic properties were studied by differential‐scanning calorimetry, polarizing optical microscopy, and X‐ray diffraction (WAXS/SAXS). All liquid‐crystalline guanidinium salts exhibit smectic A mesophases. Clearing temperatures show a linear correlation with the anionic radius. Substitution of the N‐H group with methyl, ethyl, or propyl results in decreasing mesophase widths and a concomitant shrinkage of the layer spacings.