Columnar organization in mesogenic cis- and trans-[MX2(CNR)2] complexes (M = Pd, Pt)

Palladium and platinum(II) complexes [MX2{C=NC6H2(3,4,5-OCnH2n+1)(3)}(2)] (X = Cl, Pr, I; n = 4, 6, 8, 10, 12) have been prepared. The free isocyanides are not liquid crystals, but most of their complexes [MX2{C equivalent to NC6H2(3,4, 5-OCnH2n+1)(3)}(2)] (M = Pd, Pt; X = Cl, Br, I) show hexagonal columnar phases. For M = Pd and X = Cl and Pr, an isomerization is observed going from the condensed phases to the solution. In the mesophase all the complexes seem to be cis for X = Cl and Pr and trans for X = I. Their melting points depend mostly on the peripheral chains and are little sensitive to the configuration cis or trans or to the metal (Pd or Pt). The clearing points depend more on the core interactions and show noticeable variations (cis > trans; Pt > Pd). The reasons for this behavior are discussed.