The synthesis, characterization and melting properties of thiophene-containing dithiocarboxylate complexes of nickel(II) Mary S. Thomas a , Marile ´ Landman b, * , Simon Lotz b, * , Elizabeth M. van der Merwe c a Department of Chemistry, University of Limpopo, Turfloop Campus, P. Bag X 1106, Sovenga 0727, South Africa b Department of Chemistry, University of Pretoria, Pretoria 0002, South Africa c Department of Chemistry, University of South Africa, P. O. Box 392, Pretoria 0003, South Africa Received 16 February 2007; accepted 22 March 2007 Available online 12 April 2007 Abstract The reactions of a series of 5-alkyl-2-thiophenedithiocarboxylates with nickel(II) chloride afforded two types of complexes, blue nick- el(II) complexes with two terminal dithiocarboxylate ligands, [Ni(S 2 CTR) 2 ] and violet nickel(II) complexes with perthio- and dithiocarb- oxylate ligands, [Ni(S 2 CTR)(S 3 CTR)] (where T = 2,5-disubstituted thiophene, R = C n H 2n+1 , n = 4, 6, 8, 12, 16). The blue monomers are preferred for the shorter chains (C 4 and C 6 ) and the violet compounds form exclusively for the longer chains (C 8 ,C 12 , and C 16 ) in the alkylthiophene complexes. In addition to the above series, [Ni(S 2 CTCH 3 ) 2 ], was prepared in a one-pot reaction in THF and both the blue and violet products were isolated. It was possible to convert the blue complexes [Ni(S 2 CTR) 2 ] (R = butyl, hexyl) into the corresponding violet complexes [Ni(S 2 CTR)(S 3 CTR)] after stirring in THF solutions for prolonged periods of time. Liquid-crystalline properties of these complexes were examined by DSC and POM. The violet complexes with C 8 and C 12 alkyl chains showed liquid-crystalline properties. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: Nickel; Thiophene; Liquid crystal; Dithiocarboxylate ligands 1. Introduction An important class of nickel(II) complexes are those with sulfur donor-atoms such as thiols, dithiolene and dithiocarboxylate as ligands [1,2]. Earlier literature (1963–1975) has revealed nickel(II) complexes displaying a variety of coordination numbers and many different coor- dination modes for thiol, dithiocarboxylate, dithiocarba- mate and trithiocarboxylate ligands [1]. Square-planar, tetrahedral, 5-coordinate, and octahedral geometries are known for nickel(II) with dithiocarboxylate ligands. It has been widely demonstrated that square-planar or square-pyramidal coordination leads to the appearance of liquid-crystalline properties, whereas tetrahedral geometry, it was assumed, inhibits mesophase formation. This assumption has recently been refuted [3]. Examples of com- plexes with two bidentate dithiocarboxylate or dithiocarba- mate ligands surrounding one nickel(II) metal have been documented before and a crystal structure determination of [Ni{S 2 CPh} 2 ] revealed a flat, square-planar mononu- clear nickel(II) complex [4]. Nickel(II) ions in complexes containing different coor- dination modes and structural features must not be under- estimated for potential applications as liquid-crystalline materials [5,6]. The first mesogenic nickel(II) complexes were those with dithiolene ligands and were studied by Giroud and Mu ¨ ller-Westerhoff in [7]. The nickel(II) com- plexes displayed enantiotropic mesophases, which were nematic for the shorter chains and smectic for the longer alkyl chains. In addition to the calamitic (rod-like) metall- omesogens, Ohta and co-workers reported disc-shaped 0277-5387/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.poly.2007.03.056 * Corresponding authors. Tel.: +27 12 420 2527; fax: +27 12 3625297. E-mail address: marile.landman@up.ac.za (M. Landman). www.elsevier.com/locate/poly Polyhedron 26 (2007) 3471–3477