trans-Dichloro(stilbazole)(methyl n-alkyl sulfoxide)platinum(II) complexes: the first examples of metallomesogens containing chiral centres directly bonded to the metal Francesco Paolo Fanizzi,* a Vincenzo Alicino, a Cosimo Cardellicchio, b Paolo Tortorella b and Jonathan P. Rourke* c a Dipartimento Farmaco-Chimico, Università di Bari, via E. Orabona 4, I-70125, Bari, Italy. E-mail: fanizzi@farmchim.uniba.it b CNR Centro di Studio sulle Metodologie Innovative di Sintesi Organica, Dipartimento di Chimica, Università di Bari, via G. Amendola 173, I-70126, Bari, Italy c Department of Chemistry, University of Warwick, Coventry, UK CV4 7AL. E-mail: j.rourke@warwick.ac.uk Received (in Cambridge, UK) 21st January 2000, Accepted 8th March 2000 Published on the Web 31st March 2000 The first examples of enantiomerically pure metallomeso- gens containing the chiral centre directly bonded to the metal are reported; the chiral complexes show different phase behaviour to the racemic modifications. There are many examples of chiral liquid crystals. Since the properties that chirality can confer to liquid crystals, such as ferroelectricity, are very attractive for technological applica- tions, interest in chiral liquid crystals is more than purely academic. Whilst there has been an upsurge of interest in metal containing liquid crystals (metallomesogens) over the last two decades, 1–3 there have been relatively few examples of chiral metallomesogens. Most of these metallomesogens have relied on a chiral centre remote from the central core of the molecule to induce the asymmetry. 4–8 To date, no-one has fully investigated the effect of directly bonding a chiral centre to a metal within the metallomesogen. Early examples include a family of platinum stilbazole alkene complexes which proved to be unresolvable, 9,10 and a family of ruthenium bipyridyl compounds, where no attempt was made to separate enantio- mers. 11 Sulfoxides are widely used as ligands in platinum chemistry, normally bonding through the sulfur. Chiral sulfoxides with two different substituents are configurationally stable, and can be resolved into enantiomers. 12,13 Thus chiral sulfoxides, with two different alkyl chains, present themselves as ideal ligands for platinum based metallomesogens. By using a stilbazole ligand to complete the coordination sphere of the platinum, we can prepare direct analogues of the alkene complexes reported earlier. 9,10 It has been shown that such a highly unsymmetrical structure can be very beneficial, with reductions in the melting and clearing points of up to 100 K. Enantiomerically pure methyl(n-alkyl)sulfoxides are availa- ble 14 and the methodology for the synthesis of appropriate platinum complexes is well understood. Crucially, the chirality at the sulfur has been shown to be configurationally stable on coordination to platinum with no racemisation observed even on ligand displacement. 15 Thus, using both racemic and enantio- merically pure sulfoxides we were able to isolate the required neutral complexes in a simple two step procedure, in effectively quantitative yield (Scheme 1).† Recrystallization of the prod- ucts in diethyl ether–chloroform gave analytically pure samples which were analysed by NMR,‡ elemental analysis,§ polari- metry,¶ DSC and hot-stage polarised optical microscopy (Table 1). All the new complexes reported here showed mesogenic behaviour at accessible temperatures. The phase behaviour of the new complexes is listed in Table 1 and summarised in Fig. 1. Thus it can be seen that all the complexes showed a melting transition into a mesophase at around 70 °C and cleared into the isotropic between 120 and 135 °C. The most significant observation is the difference between the thermal behaviour of the racemic and chiral forms. The racemic compounds clearly exhibit two mesophases: a highly Scheme 1 Table 1 Thermal behaviour of compounds 3 Racemic T/°C DH/J g 21 Chiral T/°C DH/J g 21 3a K–SmG 69.9 25.9 K–SmF 87.2 31.6 SmG–SmF 72.5 25.7 K–I 128.3 38.6 SmF–I 124.5 101.3 3b K–SmG 80.9 20.8 K–KA 75.4 5.4 SmG–SmF 84.4 20.0 KA–SmF 92.6 24.6 SmF–I 126.1 76.7 SmF–I 129.3 40.9 3c K–SmG 74.8 17.6 K–KA 72.3 2.3 SmG–SmF 85.1 17.4 KA–SmF 84.3 25.4 SmF–I 118.6 63.3 SmF–I 121.7 28.5 Fig. 1 Phase behaviour of compounds 3. This journal is © The Royal Society of Chemistry 2000 DOI: 10.1039/b000594k Chem. Commun., 2000, 673–674 673