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 MII(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)], MII[OC6H3-2-CHN(n-C8H17)-4-NNC6H4-4-CO2(n-C8H17)]2 [M = Ni (9), Pd (10)], MII[OC6H3-2-CHN(n-C8H17)-4-NNC6H4-4-C6H4-4-O(n-C7H15)]2 [M = Ni (14), Pd (15)], and MII[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 NiII complex of a salen-like ligand (17). In solution, broadened 1H NMR signals indicate distortions from square planar geometry for the bis-chelate NiII 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 λmax of this band lies in the range of 409−434 nm in dichloromethane, and replacing NiII with PdII causes small blue-shifts. Dichroic ratios measured in various liquid crystal hosts show complexation-induced increases with NiII, but using PdII has a detrimental effect.