DALTON FULL PAPER J. Chem. Soc., Dalton Trans., 1999, 2923–2926 2923 Novel square-planar cyclic tetranuclear copper(II) complex containing oximate bridges. Synthesis, crystal structure and magnetic properties of tetrakis[diaqua(-1,3-dimethylviolurato)- copper(II)] tetraperchlorate dihydrate Enrique Colacio,* a Carlos López-Magaña, b Vickie McKee c and Antonio Romerosa* b a Departamento de Química Inorgánica, Facultad de Ciencias, Universidad de Granada, 18071 Granada, Spain b Departamento de Química Inorgánica, Facultad de Ciencias Experimentales, Universidad de Almería, 04071 Almería, Spain c Chemistry Department, The Queen’s University of Belfast, Belfast, UK BT9 5AG Received 19th May 1999, Accepted 13th July 1999 The tetranuclear complex [Cu 4 (L) 4 (H 2 O) 8 ][ClO 4 ] 4 2H 2 O (HL = 1,3-dimethylvioluric acid, i.e. 1,3-dimethyl-5- (hydroxyimino)-2,4,6(1H,3H,5H)-pyrimidinetrione) has been prepared and its crystal structure determined by X-ray diffraction methods. The structure consists of a tetranuclear cation [Cu 4 (L) 4 (H 2 O) 8 ] 4+ , four non-co-ordinated perchlorate anions and two water molecules located on a 4-fold rotation-inversion axis. Within each cation the four copper() ions, linked through four bis(bidentate) L - ligands, are arranged at the vertices of a strict square-planar parallelogram with edge and diagonal Cu  Cu distances of 4.764(3) and 6.738(4) Å, respectively. Each oxime group acts as a bridge between adjacent copper ions with an anti conformation and each copper is co-ordinated by two L - ligands. The copper atoms exhibit a CuO 5 N elongated distorted octahedral co-ordination environment, with three oxygen atoms and one nitrogen atom of the L - ligands at equatorial positions and the water oxygens at axial positions. From variable-temperature magnetic susceptibility measurements the compound was found to exhibit a very strong antiferromagnetic exchange interaction through the oximate bridges with J = -349 cm -1 [-J(S Cu S CuA + S CuA S CuC + S CuC S CuB + S CuB S Cu )], leading to complete spin coupling at 70 K. Tetranuclear d-block metal complexes are of current interest in bioinorganic modelling, multielectron transfer, catalytic and new extended materials, as well as in magnetochemical investi- gations. 1 Owing to their relative ease of synthesis, and of the design of many tetranucleating chelating ligands for use in bio- inorganic model and magnetochemical studies, there are many examples of Cu 4 clusters, which exhibit a large diversity of structural types. 1 Among them, structurally and magnetically characterized planar cyclic structures with no diagonal bridging groups are rather scarce 2–5 and, as far as we know, those adopt- ing a strict square-planar structure have not been reported up to now. Polydentate ligands containing oxime groups are known to form a variety of hetero- and homo-polynuclear complexes, in which the oximate bridging group generally mediates very strong magnetic exchange interactions between metal ions. 6–14 Most of them are dinuclear and trinuclear species and, with the exception of two recently reported linear tetranuclear 11,12 and one cubic octanuclear especies, 13 to the best of our knowledge no other examples of structurally and magnetically character- ized genuine higher nuclearity oximate-bridged complexes exist. In this paper we report the synthesis, crystal structure and magnetic properties of [Cu 4 (L) 4 (H 2 O) 8 ][ClO 4 ] 4 2H 2 O (HL = 1,3- dimethylvioluric acid, i.e. 1,3-dimethyl-5-(hydroxyimino)- 2,4,6(1H,3H,5H)-pyrimidinetrione), a tetranuclear copper() complex with a square planar ring of copper() atoms and bis- (bidentate) bridging 1,3-dimethylviolurate ligands, which can be considered as a new example of metallacrowns. Experimental Physical measurements Elemental analyses were carried out at the Technical Services of the University of Granada on a Fisons-Carlo Erba model EA 1108 analyzer. The IR spectra were recorded on a Perkin- Elmer 983 G spectrometer using KBr pellets. Variable- temperature magnetic susceptibility data were collected on powdered samples of the compounds with use of a SQUID- based sample magnetometer on a Quantum Design Model MPMS instrument. Data were corrected for the diamagnetism of the ligands using Pascal’s constants. Variable-temperature EPR spectra were measured with a Bruker ER 200D spec- trometer equipped with a nitrogen cryostat. CAUTION: perchlorate salts are potentially explosive and should only be handled in small quantities. All reagents were of analytical grade and used without further purification. Preparation of compounds [Cu 4 (L) 4 (H 2 O) 8 ][ClO 4 ] 4 2H 2 O. 6-Chloro-1,3-dimethyluracil (1 g, 5.70 mmol) was added to a stirred solution of 1,5-diamino- 3-hydroxypentane (0.33 g, 2.85 mmol) in 20 ml of ethanol con- taining 8.6 mmol of triethylamine and the resulting solution refluxed for two hours. A white precipitate formed, which was filtered off, washed with ethanol and air dried. To a stirred suspension of this white precipitate (0.5 g, 1.27 mmol) in 50 ml of water were added NaNO 2 (0.175 g, 2.54 mmol), CuCl 2 (0.34 g, 2.54 mmol) and 3 mmol of glacial acetic acid. After stirring overnight a brown solid precipitated, which was filtered off, washed with water (3 × 5 ml), 3 ml of ethanol and air dried. Its elemental analysis does not match for any formulae of a pure compound. All attempts to obtain a pure compound were unsuccessful. The brown solid (0.5 g) was added to a solution of Cu(ClO 4 ) 2 6H 2 O (1.11 g, 3 mmol) in 50 ml of ethanol– acetonitrile–water (1 : 1 : 0.25) and the resulting suspension refluxed. After 1 h the suspension had all dissolved and after 2 h of reflux the hot solution was filtered. The deep brown solution, kept at room temperature, provided brown crystals of the tetra- nuclear compound (0.67 g, yield 30% based on 6-chloro-1,3-