METALLOMESOGENS - SYNTHESIS AND MESOMORPHIC PROPERTIES OF 2-HYDROXY-4-N-ALKOXY-4'-N-ALKYLAZOBENZENES PALLADIUM(II) COMPLEXES

The synthesis and characterization of o-hydroxyazobenzene mesogens [HL(n)(m)] which form the two different homologous series HL(n)(12) (2-hydroxy-4-n-dodecyloxy-4'-n-C(n)H2n+1-azobenzenes, n = 1-4, 6, 8) and HL1(m) (2-hydroxy-4-n-C(n)H2n+1-O-4'-methylazobenzenes, m = 7, 9, 12, 14) are reported. They form N and/or S(C) mesophases at quite low temperatures. These ligands react with [Pd(PhCN)2Cl2] or Pd[(Azoxy-6)Pd(MeCN)2]BF4 giving rise to the corresponding [L(n)(m)]2Pd and {(Azoxy-6)Pd[L(n)(m)]} species [Azoxy-6 is the deprotonated form of 4,4'-bis(hexyloxy)azoxybenzene]. The [L(n)(m)]2Pd compounds are not mesogenic. On the contrary, the cyclopalladated mixed-ligand complexes, {(Azoxy-6)Pd[L(n)(m)]}, are prevalently nematogenic as optical, DSC, and X-ray analysis evidence. The {(Azoxy-6)Pd[L(n)(12)]} compounds show enantiotropic nematic (n = 1), monotropic nematic (n = 2-4, 6) and monotropic nematic and S(c) mesophases (n = 8). The {(Azoxy-6)Pd[L1(m)]} species from monotropic (m = 7, 9) and enantiotropic (m = 12, 14) nematic mesophases. A comparison between thermal behavior displayed by [L(n)(m)]2Pd and {(Azoxy-6)Pd[L(n)(m)]} complexes seems to explain the role played by the respective rigid molecular cores. The electronic and/or structural modifications induced by the azoxy group are discussed.