Synthesis, Crystal Structure, and Spectroscopic and Magnetic Properties of a New [Co 4 O(OOCNC 9 H 18 ) 6 ] Cluster Peter Jaitner,* ,† Jaume Veciana, ‡,§ Christian Sporer, † Holger Kopacka, † Klaus Wurst, † and Daniel Ruiz-Molina § Institut fu ¨ r Allgemeine, Anorganische und Theoretische Chemie, Universita ¨ t Innsbruck, Innrain 52a, A-6020 Innsbruck, Austria, and Institut de Cie ` ncia de Materials de Barcelona (CSIC), Campus de la UAB, E-08193 Cerdanyola, Spain Received April 14, 2000 Summary: The synthesis, X-ray structure, and NMR ( 1 H, 13 C, 17 O) and magnetic characterization of the complexes [Co(CO) 2 (tmpo)] (1) and [Co 4 O(OOCNC 9 H 18 ) 6 ](2) are reported. Reaction of Co 2 (CO) 8 in the presence of a 4-fold excess of tmpo yields in a few minutes the blue carba- mato complex 2 in >80% yield. The reaction possibly takes place by a cascade mechanism with the complex [Co(CO) 2 tmpo] (1) as an intermediate and the nitroxyl radical acting as an oxidizing agent. Introduction The nitroxyl radical 2,2,6,6-tetramethylpiperidin-1- oxyl (tmpo) has been successfully used (a) in the design of new organic magnetic materials, 1 (b) as a paramag- netic ligand to study magnetic interactions of organic radicals with metal ions, 2 and (c) as a building block in the preparation of new π-extended organic donor mol- ecules exhibiting conducting and magnetic properties. 3 We have also shown that tmpo is a valuable spin scavenger of metal carbonyl radicals. 4 For instance, photolysis of Co 2 (CO) 8 promotes the fission of the Co- Co bond, generating the radical Co(CO) 4 • , which reacts with an equimolar amount of the tmpo group to yield [Co(CO) 2 (tmpo)] (1). 4b The molecular structure of com- plex 1 was determined by single-crystal X-ray diffrac- tion, showing that the Co I atom is in a planar coordi- nation with the two carbonyls and the O-N group of the tmpo ligand. No addition of CO, PR 3 , or H 2 to 1 could be effected, since the two methyl groups of the coordi- nated tmpo protect the Co atom from the upper and lower side of the planar molecular arrangement. In this paper we report the synthesis and spectro- scopic and magnetic characterization of the new cluster [Co 4 (µ 4 -O)(OOCNC 9 H 18 ) 6 ](2). 5 Surprisingly, reaction of [Co 2 (CO) 8 ] with a 4-fold excess of tmpo promoted the generation of the blue carbamato complex 2 in a few minutes. Even though the mechanism of formation of this complex is not completely understood, this synthetic methodology opens a new route to clusters of the type [M 4 (µ 4 -O)] 6+ . Such clusters, with a four-coordinate oxy- gen atom occupying the center of a tetrahedron of di- valent metal ions, is a classic structure in inorganic chemistry. 6 The first such complexes reported were the acetate derivatives ([M 4 O(Ac) 6 ]) of beryllium 7 and zinc. 8 Later, similar complexes of Zn, 9 Mn, 10 Fe, 10 and Cr, 11 in which the acetate groups are replaced by other bridging ligands, were prepared. In the case of cobalt, the first cluster with a [M 4 (µ 4 - O)] 6+ core was obtained in 1966 by pyrolysis of anhy- drous cobalt pivalate under vacuum. 12 Since then, only a few more complexes, [{Ir(aet) 3 } 4 Co 4 O]Br 6 ] 13 (where aet ) 2-aminoethanethiolate), [Co 4 O(O 2 CCCo 3 (CO) 9 ) 6 ], 14 and, recently, [Co 4 O(DPhF) 6 ] 10 (where DPhF ) N,N′-diphen- ylformamidinate), have been synthesized. Complexes containing such oxo bridging groups are of interest in chemistry, 15 biochemistry, 16 magnetochemistry, 17 and catalysis. 18 * To whom correspondence should be addressed. Fax: +43 512/ 5072934. E-mail: peter.jaitner@uibk.ac.at. † Universita ¨ t Innsbruck. ‡ Fax: +34 93/5805729. E-mail: vecianaj@icmab.es. § Institut de Cie `ncia de Materials de Barcelona (CSIC). (1) (a) Gatteschi, D.; Kahn. O.; Miller, J. S.; Palacio, F. Molecular Magnetic Materials; Kluwer Academic: Dordrecht, The Netherlands, 1991. (b) Kahn, O. Molecular Magnetism; VCH: Weinheim, Germany, 1993. (c) Miller, J. S.; Epstein, A. J. Angew. Chem., Int. Ed. Engl. 1995, 73, 30. (2) (a) Lim, Y. Y.; Drago, R. S. Inorg. Chem. 1972, 11, 1334. (b) Richman, R. M.; Kuechler, T. C.; Tanner, S. P.; Drago, R. S. J. Am. Chem. Soc. 1977, 99, 1055. (c) Drago. R. S.; Kuechler, T. C.; Kroeger, M. Inorg. Chem. 1979, 18, 2337. (d) Dickman, M. H.; Doedens, R. J. Inorg. Chem. 1981, 20, 2067. (e) Richardson, R. F.; Kreilick, R. W. J. Am. Chem. Soc. 1977, 99, 8183. (f) Anderson, O. P.; Kuechler, T. C. Inorg. Chem. 1980, 19, 1417. (3) Fujiara, H.; Kobayashi, H. Chem. Commun. 1999, 2417. (4) (a) Jaitner, P.; Huber. 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