Initial use of di-2-pyridyl ketone oxime in chromium carboxylate chemistry: Triangular fCr III 3 ðl 3 -OÞg 7þ compounds and unexpected formation of a carboxylate-free dichromium(II,II) complex Konstantina V. Pringouri a , Catherine P. Raptopoulou b , Albert Escuer c , Theocharis C. Stamatatos a, * a Department of Chemistry, University of Patras, 265 04 Patras, Greece b Institute of Materials Science, NCSR ‘‘Demokritos’’, 153 10 Aghia Paraskevi Attikis, Greece c Departament de Quimica Inorganica, Universitat de Barcelona, Diagonal 647, Barcelona 08028, Spain Received 12 June 2006; received in revised form 24 July 2006; accepted 24 July 2006 Available online 3 August 2006 Inorganic Chemistry – The Next Generation. Abstract A systematic investigation of the [Cr 3 O(O 2 CCMe 3 ) 6 (H 2 O) 3 ](O 2 CCMe 3 )/(py) 2 CNOH [(py) 2 CNOH = di-2-pyridyl ketone oxime] reac- tion system is described, involving the determination of the influence of the Cr III :(py) 2 CNOH ratio, the temperature and the presence of counterions on the identity of the reaction products. As a consequence, complexes [Cr 2 {(py) 2 CNO} 4 ] Æ 2H 2 O(1 Æ 2H 2 O), [Cr 3 OCl(O 2 CC- Me 3 ) 4 {(py) 2 CNO} 2 ] Æ 2Me 2 CO (2 Æ 2Me 2 CO) and [Cr 3 O(O 2 CCMe 3 ) 4 {(py) 2 CNO} 2 (H 2 O)](NO 3 ) Æ H 2 O Æ 0.5Me 2 CO (3 Æ H 2 O Æ 0.5Me 2 CO) have been isolated and structurally characterized by single-crystal X-ray studies. (py) 2 CNO  behaves as a N,N 0 -bidentate chelating and/or N,N 0 ,O-tridentate bridging ligand binding through one 2-pyridyl and the oximate nitrogen atoms, and/or the oximate oxygen atom. 1 is a relatively stable air-stable dinuclear Cr(II) complex with no metal–metal bonding arising, from the ligand-assisted reduction of the ‘basic carboxylate’ Cr(III) triangle. The structures of 2 and 3 consist of similar neutral and cationic triangles of Cr III ions, respec- tively, centered by a triply bridging oxo ligand and with two edge-bridging oximate groups from the two 2.1110 (py) 2 CNO  ligands. Variable-temperature magnetic susceptibility studies and EPR data for 2 reveal an antiferromagnetically-coupled system with an S T = 3/2 ground state. Solid-state IR and ligand field spectra of 2 are briefly discussed in terms of its structure. Ó 2006 Elsevier B.V. All rights reserved. Keywords: Dinuclear chromium(II) complexes; Chromium(III) triangles; Di-2-pyridyl ketone oximate complexes; EPR spectra; Magnetic properties 1. Introduction There is currently a renewed interest in the coordination chemistry of oximes [1]. The research efforts are driven by a number of considerations. These include the solution of pure chemical problems [2–4], the desire to provide useful bioinorganic models (oximes may be considered to be rea- sonable models for the biologically significant imidazole donor group of the amino acid histidine) [5], the design of Ca 2+ - and Ba 2+ -selective receptors based on site-selec- tive transmetallation of multinuclear polyoxime–zinc(II) complexes [6], the development of new oxygen activation catalysts based on nickel(II)–polyoximate complexes [7], the application of metal ion/oxime systems as simple and efficient catalysts for the hydrolysis of organonitriles [8] 0020-1693/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.ica.2006.07.063 * Corresponding author. Tel.: +30 2610 996019; fax: +30 2610 997118. E-mail address: thstama@chemistry.upatras.gr (T.C. Stamatatos). www.elsevier.com/locate/ica Inorganica Chimica Acta 360 (2007) 69–83