Complexes of thiophene derivatives as potential metallomesogens

Abstract

This study involves the synthesis and structural characterization of new metal complexes of thiophene derivatives that have (potential) liquid crystalline properties. Thiophene has been selected because of its stability and versatility in lending itself to synthetic modification and hence forms links in chain structures for rod-like metallomesogens. Thiophene, when compared with 1,4-disubstituted benzene units, can change considerably the polarity, polarizability and also the geometry of the compounds, altering the types of mesophases, phase transition temperatures, dielectric constants and other properties of mesogens. The reactions of a series of 5-alkyl-2-thiophenedithiocarboxylates with nickel(II) chloride formed two types of complexes, blue mononuclear nickel(II) complexes with two terminal dithiocarboxylate ligands, [Ni(S2CTR)2] (T = 2,5-disubstituted thiophene) and violet mononuclear nickel(II) complexes with perthio- and dithiocarboxylate ligands, [Ni(S3CTR)(S2CTR)] (R = alkyl groups). The blue monomers are preferred for the shorter alkyl chains (C4 and C6), and the violet compounds for the longer chain lengths (C8, C12 and C16) in the alkylthiophene complexes. In addition to the above series, [Ni(S2CTCH3)2], was prepared in a one-pot reaction and it was possible to isolate both the blue and violet products. The thermal properties of the complexes were studied by using differential scanning calorimetry (DSC) and polarizing optical microscopy (POM). Nickel complexes of the violet type with longer alkyl chains showed liquid crystalline properties. Zinc(II) complexes analogous to nickel(II) complexes prepared similarly. A crystal structure determination of one of those complexes revealed the fusion of two monomers to give a dimeric structure with bridging sulfur atoms, [Zn2 (ƒÝ-S2CTR) 2 (S2CTR) 2]. Although an irregular melting pattern was observed, the complexes did not show any liquid crystalline properties. In an attempt to extend the study towards organometallic compounds, complexes of the type [Re(CO) 4 (S2CTR)] or [Re(CO) 4 (S2CTTR)] (T = 2,5-disubstituted thiophene, TT = 2,5-disubstituted bithiophene; R = H, CH3, C14H29) were synthesized and characterized by IR and NMR spectroscopy. Further characterization of [Re(CO) 4 (S2CTTH)] by single crystal X-ray diffraction confirmed the molecular structure of the complexes. These compounds showed sharp single melting points. Fischer-type carbene complexes of manganese(I) with octahedral coordination of the type [MnMeCp(CO) 2{C(OEtTR)}] or [MnMeCp(CO) 2{C(OEtTTR)}] (R = H, C6H13, C12H25, C16H33) were synthesized and characterized by IR, NMR and mass spectrometry. Thermal properties of the complexes were studied by using thermogravimetric analysis (TGA). All the organometallic rhenium(I) and Fischer-type carbene complexes of manganese(I) showed weight loss upon heating due to decomposition. Therefore it can be assumed that these complexes are not suitable as liquid crystals.