Octavia A. Blackburn

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)(OC(6)H(3)-2-CHNR-4-N═NC(6)H(4)-4-CO(2)Et)(2) [M = Ni; R = n-Bu (3c), n-C(6)H(13) (3d), n-C(8)H(17) (3e), n-C(12)H(25) (3f), Ph (3g), OH (3h), C(6)H(4)-4-CO(2)Et (3i). M = Pd; R = i-Pr (4a), t-Bu (4b), n-Bu (4c), n-C(6)H(13) (4d), n-C(8)H(17) (4e), n-C(12)H(25) (4f), Ph (4g)], M(II)[OC(6)H(3)-2-CHN(n-C(8)H(17))-4-N═NC(6)H(4)-4-CO(2)(n-C(8)H(17))](2) [M = Ni (9), Pd (10)], M(II)[OC(6)H(3)-2-CHN(n-C(8)H(17))-4-N═NC(6)H(4)-4-C(6)H(4)-4-O(n-C(7)H(15))](2) [M = Ni (14), Pd (15)], and M(II)[OC(6)H(3)-2-CHN(CMe(2))-4-N═NC(6)H(4)-4-CO(2)Et](2) [M = Ni (17), Pd (18); the CMe(2) groups are connected]. These compounds have been characterized by using various physical techniques including (1)H 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 (1)H NMR signals indicate distortions from square planar geometry for the bis-chelate Ni(II) complexes. Electronic absorption spectroscopy and ZINDO_S (Zerners 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.

1,4-Di-n-heptyloxy-2,5-dinitrobenzene

The complete molecule of the title compound, C20H32N2O6, is generated by crystallographic inversion symmetry. The two mutually trans nitro substituents are hence in fully eclipsed conformation and also twisted by 43.2 (2)° with respect to the phenyl ring plane. The benzene-connected portions of the alkoxy substituents lie almost coplanar with the ring [C—O—C—C torsion angle = 2.0 (2)°]. In the crystal, weak C—H⋯O interactions link the molecules.

1,4-Di-n-hept­yloxy-2,5-dinitro­benzene

The complete molecule of the title compound, C20H32N2O6, is generated by crystallographic inversion symmetry. The two mutually trans nitro substituents are hence in fully eclipsed conformation and also twisted by 43.2 (2)° with respect to the phenyl ring plane. The benzene-connected portions of the alkoxy substituents lie almost coplanar with the ring [C—O—C—C torsion angle = 2.0 (2)°]. In the crystal, weak C—H⋯O interactions link the molecules.

1,4-Di(n-heptoxy)-2,5-dinitrobenzene

The complete molecule of the title compound, C20H32N2O6, is generated by crystallographic inversion symmetry. The two mutually trans nitro substituents are hence in fully eclipsed conformation and also twisted by 43.2 (2)° with respect to the phenyl ring plane. The benzene-connected portions of the alkoxy substituents lie almost coplanar with the ring [C—O—C—C torsion angle = 2.0 (2)°]. In the crystal, weak C—H⋯O interactions link the molecules.