Synthesis and properties of (±)-trans-(η2-alkene)(4-alkyloxy-4′-stilbazole)dichloroplatinum: a remarkable family of low-melting metallomesogens

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Abstract
A novel series of low-melting metallomesogens (metal-containing liquid crystals) has been prepared and characterised. Reaction of Zeise's salt K[PtCl3(η2-CH2 CH2)] with the 4-n-alkyloxy-4′-stilbazole ligands (L = CnH2n+1OC6H4CHCHC5H4N; n= 3–12) yielded trans-[PtCl2(η2-CH2 CH2)L]1, which react with alk-1-enes (CH2 CHCmH2m+1; m= 1–8, 10 or 12) to give trans-[PtCl2(CH2 CHCmH2m+1)L]2. The complexes where (m + n)ca. 8 are generally liquid crystalline (smectic A mesophases); those with shorter tails [(m + n)= 8 to ca. 13] show monotropic mesophases, while those with longer tails [(m + n) > ca. 13] show enantiotropic behaviour. The melting temperatures are lower than would be anticipated for metallomesogens of this size, reflecting the non-centrosymmetric molecular structures which make the crystal packing less easy. A 70 : 30 mixture of the complexes 2(n= 12, m= 3; K → SA at 61 °C) and (n= 12, m= 6; K → SA at 70 °C) shows the transition into the mesophase, K → SA, at 45 °C, an unusually low temperature. The mesophase to isotropic (SA→ I) transition temperatures remain rather constant at 92 ± 4 °C for all compositions.