Nickel(II) and Palladium(II) Complexes of Azobenzene-Containing Ligands as Dichroic Dyes

A large series of complexes has been synthesized with two chelating, Schiff base azobenzene derivatives connected linearly by coordination to a central nickel(II) or palladium(II) ion. These compounds have the general formulas M^II(OC₆H₃-2-CHNR-4-NNC₆H₄-4-CO₂Et)₂ [M = Ni; R = n-Bu (3c), n-C₆H₁₃ (3d), n-C₈H₁₇ (3e), n-C₁₂H₂₅ (3f), Ph (3g), OH (3h), C₆H₄-4-CO₂Et (3i). M = Pd; R = i-Pr (4a), t-Bu (4b), n-Bu (4c), n-C₆H₁₃ (4d), n-C₈H₁₇ (4e), n-C₁₂H₂₅ (4f), Ph (4g)], M^II[OC₆H₃-2-CHN(n-C₈H₁₇)-4-NNC₆H₄-4-CO₂(n-C₈H₁₇)]₂ [M = Ni (9), Pd (10)], M^II[OC₆H₃-2-CHN(n-C₈H₁₇)-4-NNC₆H₄-4-C₆H₄-4-O(n-C₇H₁₅)]₂ [M = Ni (14), Pd (15)], and M^II[OC₆H₃-2-CHN(CMe₂)-4-NNC₆H₄-4-CO₂Et]₂ [M = Ni (17), Pd (18); the CMe₂ groups are connected]. These compounds have been characterized by using various physical techniques including ¹H NMR spectroscopy and matrix-assisted laser desorption/ionization (MALDI) mass spectrometry. Single-crystal X-ray structures have been obtained for two pro-ligands and five complexes (3e, 4e, 14, 15, and 17). The latter always show a strictly square planar arrangement about the metal center, except for the Ni^II complex of a salen-like ligand (17). In solution, broadened ¹H NMR signals indicate distortions from square planar geometry for the bis-chelate Ni^II complexes. Electronic absorption spectroscopy and ZINDO_S (Zerner's intermediate neglect of differential overlap) and TD-DFT (time-dependent density functional theory) calculations show that the lowest energy transition has metal-to-ligand charge-transfer character. The λ_max of this band lies in the range of 409−434 nm in dichloromethane, and replacing Ni^II with Pd^II causes small blue-shifts. Dichroic ratios measured in various liquid crystal hosts show complexation-induced increases with Ni^II, but using Pd^II has a detrimental effect.