Synthesis and properties of (+/-)-trans-([small eta]2-alkene)(4-alkyloxy-4[prime or minute]-stilbazole)dichloroplatinum: a remarkable family of low-melting metallomesogens

A novel series of low-melting metallomesogens (metal-containing liquid crystals) has been prepared and characterised. Reaction of Zeise{'}s salt K[PtCl([small eta]-CH[double bond{,} length half m-dash]CH)] with the 4--alkyloxy-4[prime or minute]-stilbazole ligands (L = CHOCHCH[double bond{,} length half m-dash]CHCHN; = 3-12) yielded -[PtCl([small eta]-CH[double bond{,} length half m-dash]CH)L]{,} which react with alk-1-enes (CH[double bond{,} length half m-dash]CHCH; = 1-8{,} 10 or 12) to give -[PtCl(CH[double bond{,} length half m-dash]CHCH)L]. The complexes where ()[gt-or-equal] 8 are generally liquid crystalline (smectic A mesophases); those with shorter tails [()= 8 to 13] show monotropic mesophases{,} while those with longer tails [() > 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 (= 12{,} = 3; K [rightward arrow] S at 61 [degree]C) and (= 12{,} = 6; K [rightward arrow] S at 70 [degree]C) shows the transition into the mesophase{,} K [rightward arrow] S{,} at 45 [degree]C{,} an unusually low temperature. The mesophase to isotropic (S[rightward arrow] I) transition temperatures remain rather constant at 92 +/- 4 [degree]C for all compositions.