FULL PAPER Trinuclear Cu II Complexes Containing Peripheral Ketonic Oxygen Bridges and a μ 3 -OH Core: Syntheses, Crystal Structures, Spectroscopic and Magnetic Properties Mau Sinha Ray, [a] Shouvik Chattopadhyay, [a] Michael G. B. Drew, [b] Albert Figuerola, [c] Joan Ribas, [c] Carmen Diaz,* [c] and Ashutosh Ghosh* [a] Keywords: Copper trimers / Schiff bases / X-ray structures / Magnetic properties Four new trinuclear copper(II) complexes, [(CuL 1 ) 3 (μ 3 - OH)](ClO 4 ) 2 ·H 2 O(1), [(CuL 2 ) 3 (μ 3 -OH)](ClO 4 ) 2 (2), [(CuL 3 ) 3 - (μ 3 -OH)](ClO 4 ) 2 ·H 2 O(3), and [(CuL 4 ) 3 (μ 3 -OH)](ClO 4 ) 2 ·H 2 O (4), where HL 1 = 8-amino-4,7,7-trimethyl-5-azaoct-3-en-2- one, HL 2 = 7-amino-4-methyl-5-azaoct-3-en-2-one, HL 3 = 7- (ethylamino)-4-methyl-5-azahept-3-en-2-one, and HL 4 = 4- methyl-7-(methylamino)-5-azahept-3-en-2-one, have been derived from the four tridentate Schiff bases (HL 1 , HL 2 , HL 3 , and HL 4 ) and structurally characterized by X-ray crystal- lography. For all compounds, the cationic part is trinuclear with a Cu 3 OH core held by three carbonyl oxygen bridges between each pair of copper(II) atoms. The copper atoms are five-coordinate with a distorted square-pyramidal geometry; the equatorial plane consists of the bridging oxygen atom of the central OH group together with three atoms (N, N, O) from one ligand whereas an oxygen atom of a second ligand Introduction Cyclic trinuclear metal complexes [1] are of interest be- cause these systems can be regarded as geometrically spin- frustrated and offer the opportunity to test magnetic ex- change models. [2] In fact, geometrically frustrated antiferro- magnetic compounds have attracted much attention over the past few years because of their propensity to adopt un- usual, even exotic magnetic ground states, which remain po- orly understood. [3] μ-Hydroxo or μ-oxo ions have generally been observed as central bridging ligands in cyclic trinuclear compounds. The existence of the M 3 O core held by the peripheral bridg- ing ligands is well documented in the chemistry of iron(/ ) and chromium(). [4] There are only a few reports with [a] Department of Chemistry, University College of Science, Uni- versity of Calcutta, 92 A. P. C. Road, Kolkata 700009, India E-mail: ghosh_59@yahoo.com [b] School of Chemistry, The University of Reading, P. O. Box 224, Whiteknights, Reading RG6 6AD, UK [c] Departament de Química Inorgànica, Universitat de Barcelona, Marti i Franques 1–11, 08028 Barcelona, Spain E-mail: carme.diaz@qi.ub.es Supporting information for this article is available on the WWW under http://www.eurjic.org or from the author. © 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim DOI: 10.1002/ejic.200500289 Eur. J. Inorg. Chem. 2005, 4562–4571 4562 occupies the axial position. Magnetic measurements have been performed in the 2–300 K temperature range. The ex- perimental data could be satisfactorily reproduced by using an isotropic exchange model, H =–J(S 1 S 2 +S 2 S 3 +S 1 S 3 ) yield- ing as best-fit parameters: J = –66.7 and g = 2.19 for 1, J = –36.6 and g = 2.20 for 2, J = –24.5 and g = 2.20 for 3, and J = –14.9 and g = 2.05 for 4. EPR spectra at low temperature show the existence of spin frustration in complexes 3 and 4, but it has not been possible to carry out calculations of the antisymmetric exchange parameter, G, from magnetic data. In frozen methanolic solution, at 4 K, hyperfine splitting in all complexes and spin frustration in complex 4 seem to be confirmed. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005) a Cu 3 O(H) core of which the majority contain oxime-oxi- mate-, [5] N,N-pyrazole- [6] , or N,N-triazole [7] -type peripheral ligands. Examples of cyclic trinuclear complexes with ke- tonic oxygen bridges are, however, very rare. To the best of our knowledge, in the literature only three such complexes containing Schiff-base ligands, for example, 7-amino-4- methyl-5-azahept-3-en-2-one (AMAH), [8] 8-amino-4- methyl-5-azaoct-3-en-2-one (AMAO), [9] and 7-(dimeth- ylamino)-4-methyl-5-azahept-3-en-2-one (AE) [10] are known. The first complex having an AMAH ligand was isolated accidentally during the preparation of the ternary complex, having pyridine as the other ligand. All three com- plexes were synthesized conveniently by adding triethyl- amine to the respective reaction mixtures. In order to inves- tigate if the existence of these complexes is an exception or the formation of such trinuclear Cu II complexes is a general phenomenon for this type of ligand, we synthesized four very similar tridentate N,N,O donor Schiff-base ligands de- rived from C- or N-substituted diamines and 2,4-pentane- dione (Scheme 1, HL 1 –HL 4 ) to obtain the desired com- plexes. Another objective of the synthesis of these com- pounds is to study their magnetic properties, especially the existence of spin frustration, which has not been detected in the other three reported complexes. [8–10] This paper re-