Slow diffusion of an ethanolic solution of copper(ii) into an aqueous solution of partially deprotonated malonodihydroxamic acid (H4mdh) affords the pentanuclear complex of formula {[K(H2O) 2]2[Cu5(mmh)4]}n (1) (H3mmh = malonomonohydroxamic acid) which is formed by 12-MC-4 metallacrown units with the fifth copper(ii) ion being placed at the center of the square metallacrown unit. The dianionic pentacopper(ii) planar entity interacts with diaquapotassium(i) counterions through the carboxylate-oxygen atoms resulting in a neutral three-dimensional structure. Magnetic susceptibility measurements in the temperature range 1.9-300 K for 1 show the occurrence of relatively strong antiferromagnetic interactions within the pentanuclear copper(ii) core leading to two degenerated low-lying spin doublets at T < 50 K. X-band EPR spectra at low temperatures give additional support to the ground spin doublet nature. The topology of the pentanuclear copper(ii) unit of 1 together with good ability to mediate magnetic interactions between the copper(ii) ions of the diatomic N-O(oximato) and monoatomic O(oximato) bridges which connect the metal centers within the pentanuclear core account for this magnetic behaviour. © 2011 The Royal Society of Chemistry and the Centre National de la Recherche Scientifique.

A.B.L. thanks the Xunta de Galicia for a postdoctoral-fellowship ‘‘Á ngeles Alvariñ o’’. Funding for this work was provided by the Spanish Ministerio de Ciencia e Innovació n through projects MAT2010-16891,MAT2007-60660, CTQ2010-15364, ‘‘Molecular Nanoscience’’ (Consolider-Ingenio CSD2007-00010) and ‘‘Factoría de Cristalización’’ (Consolider-Ingenio2010, CSD2006-00015). J.P. thanks the NANOMAC structural project from the ACIISI for a post-doctoral research contract.

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