Mesogens based on palladium orthometallated complexes with carboxylato bridges: tuning and shaping a non-planar molecule

The binuclear cyclopalladated compounds Pd₂(μ-OH)₂(Lⁿ)₂] (1) derived from imines HLⁿ = p-C_nH_2n + 1O-C₆H₄-CHN-C₆H₄-OC_nH_2n + 1-p (n = 6,10) react with carboxylic acids to give the derivatives Pd₂(μ-ox)₂(Lⁿ)₂] (2) with a planar core for oxalic acid, and Pd₂(μ-OOCR)₂(Lⁿ)₂] (3-7) compounds with a non-planar ridge tent structure for other RCOOH acids: (3) R = C_mH_2m + 1 (m = 1, 3, 5, 7, 9, 11, 13, 15, 17); (4) R = CH₂(OCH₂CH₂)_pOCH₃ (p = 1, 2); (5) R = CH₂-C₆H₄-OC_qH_2q + 1-p (q = 2, 4, 6, 8, 10, 12); (6) R = C₆H₄-OC_rH_2r + 1-p (r = 4, 10); (7) R = C*H(OH)CH₃. The acids used were designed to explore the effect on the thermal properties of the compounds prepared of systematic variations in the type of carboxylato ligand, which induce structure, packing, and polarity changes, and in the length of the carboxylato chain. Most of the complexes prepared, even when far from planar, show liquid crystal behavior and display nematic, smectic A and smectic C phases.