Metallic molecular crystals containing chiral or racemic guest molecules{ Lee Martin, a Peter Day,* a Hiroki Akutsu, b Jun-ichi Yamada, b Shin’ichi Nakatsuji, b William Clegg, c Ross W. Harrington, c Peter N. Horton, d Michael B. Hursthouse, d Paul McMillan e and Steven Firth e Received 22nd June 2007, Accepted 24th July 2007 First published as an Advance Article on the web 6th August 2007 DOI: 10.1039/b709558a Metallic charge-transfer salts of BEDT-TTF containing both racemic ( R/ S)- and chiral ( S)- sec-phenethyl alcohol as guest molecules have been crystallised and their structures and physical properties are compared. Electron transport through chiral crystals has the potential to generate new physical properties. One approach to synthesising such crystals is to incorporate chiral guest molecules. From this point of view charge-transfer salts of BEDT-TTF [bis(ethylene- dithio)tetrathiafulvalene] are an attractive target. The electronic ground state in such compounds is determined by the packing arrangement of the donor molecules. 1 Superconducting, metallic, semiconducting or insulating behaviour has been observed in salts of this type and small changes to the structure have been shown to lead to large changes in the physical properties. 2 The conducting donor layers are separated by layers of anions whose shape and charge dictate the packing arrangement of the donor layers via van der Waals interactions and hydrogen bonding. It has been shown previously in the series (BEDT-TTF) 4 [(A)M(C 2 O 4 ) 3 ]?G 3 (A = H 3 O + , NH 4 + ; M = Fe, Cr, Ga, Al, Co, Ge; G = PhCN, PhNO 2 , PhBr, PhCl, pyridine) that neutral guest molecules, G, can be included in the anion layer by growing crystals in different solvents and the included guest molecules can lead to different donor packing modes and drastically different electrical conductivities. 4 To date none of these included solvents have been chiral and in this communication we demonstrate for the first time the effects observed on the structure and conducting properties of two charge-transfer salts differing only in whether they contain racemic or chiral guest molecules. Crystals have been grown from either racemic (R/S)- (I) or chiral (S)-sec-phenethyl alcohol (II).{ The structures were determined as a-b0-(BEDT-TTF) 4 [(NH 4 )Fe(C 2 O 4 ) 3 ]?(X)-C 6 H 5 - COH(H)CH 3 (X = R/S for (I) and S for (II)).§ Compounds I and II follow the same structural arrangement previously observed in this series of BEDT-TTF–tris(oxalato)metallate salts mentioned above, 3,4 with alternating layers of donor cations and anion layers of tris(oxalato)metallate/H 3 O + with guest solvent molecules sitting inside the hexagonal honeycomb cavities (Fig. 1). Alternate anion layers each consist exclusively of D or L enantiomers of the tris(oxalato)metallate anion. The superconducting salts in this series contain only BEDT-TTF cations packed in a b0 arrange- ment. Compounds I and II are designated a-b0 compounds since alternate cation layers are packed in a and b0 arrangements (Fig. 2). 5 The asymmetric unit of I contains four crystallographically inequivalent BEDT-TTF donor molecules (two in a layers and two in b0 layers) and crystallises in the space group P1 ¯ , whilst II contains eight crystallographically inequivalent BEDT-TTF donor molecules (four in a layers and four in b0 layers) and crystallises in the space group P1. There are several S … S close contacts below the sum of the van der Waals radii ( ,3.6 A ˚ ). The S … S contacts in I range from 3.28 to 3.60 A ˚ within the a layer and 3.29 to 3.57 A ˚ in the b0 layer, whilst in II they range from 3.28 to 3.56 A ˚ within the a layer and 3.27 to 3.58 A ˚ in the b0 layer. The Raman spectra of I and II are identical (Fig. 3), containing two peaks in the region of the totally symmetric CLC vibrations at 1468 ¡ 2 and 1494 ¡ 2 cm 21 corresponding to n 3 and n 4 vibra- tions of BEDT-TTF. From the correlation established between stretching frequency and cation charge, 6 a charge of 0.5 + is calculated from the Raman data. These charges agree with those derived from the method of Guionneau, Kepert et al., 7 who correlated the charge on BEDT-TTF molecules with bond lengths in the crystal structure. All of the BEDT-TTF molecules have calculated charges of 0.5 + (¡0.1 + ). For the formula a-b0-(BEDT-TTF) 4 [(NH 4 )Fe(C 2 O 4 ) 3 ]?C 6 H 5 COH(H)CH 3 the four BEDT-TTF molecules have a combined charge of 2 + which is in a Davy-Faraday Research Laboratory, The Royal Institution of Great Britain, 21 Albemarle Street, London, UK W1X 4BS. E-mail: pday@ri.ac.uk; Fax: +44 (0)20 7670 2958; Tel: +44 (0)20 7409 2992 b Graduate School of Material Science, University of Hyogo, Kamigori- cho, Ako-gun, Hyogo, 678-1297, Japan c School of Natural Sciences (Chemistry), Newcastle University, Newcastle upon Tyne, UK NE1 7RU d Department of Chemistry, University of Southampton, Highfield, Southampton, UK SO17 1BJ e Department of Chemistry, Materials Chemistry Centre, Christopher Ingold Laboratories, University College London, 20 Gordon Street, UK W1CH 0AJ { CCDC reference numbers 651824 and 651825. For crystallographic data in CIF or other electronic format see DOI: 10.1039/b709558a Fig. 1 The layered structure of I and II projected along the c axis. COMMUNICATION www.rsc.org/crystengcomm | CrystEngComm This journal is ß The Royal Society of Chemistry 2007 CrystEngComm, 2007, 9, 865–867 | 865 Published on 06 August 2007. Downloaded by University College London on 07/06/2015 08:12:33. 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