FULL PAPER New Chelating Nitroxide Free Radical Ligands For Heterospin-Magnetic Engineering Kira E. Vostrikova, [a] Elie Belorizky, [b] Jacques Pe ´caut, [a] and Paul Rey* [a] Keywords: Nitronyl nitroxides / Coordination chemistry / Magnetic properties / Radicals / Chelates / Chelates Polychelating nitroxide ligands derived from N-methyl- of their transition metal complexes, the magnetic properties of which are compared to those of previously reported ethylenediamine and piperazine have been prepared by reactions of 2-bromomethyl nitronyl nitroxide with the related species. In particular, the properties of a nickel(II) derivative of a pentadentate tris(nitroxide), the ground spin- corresponding polyamines. Two of these ligands have been structurally and magnetically characterized, along with some state of which is a quadruplet, are discussed in detail. The popularity of nitroxide free radicals stems from their culty, we have investigated the coordination properties of a few nitronyl nitroxides in which the substituent bears a do- high stability and the well-studied, sophisticated organic chemistry. [1-3] Thus, ease of chemical modification allows nor such as a nitrogen atom, which enforces the coordi- nation of the nitroxyl oxygen atom through the chelate ef- the synthesis of nitroxides specially designed for use in fi- elds as different as spin labelling [4] [5] and materials chemis- fect. [12,14,25] The results of these preliminary investigations have prompted us to develop a more general method, which try. [6-9] In recent years, there has been increasing interest in the incorporation of nitroxide spin carriers into large should allow the design of paramagnetic ligands suitable for developing the field of molecular nitroxide-based magnetic homo- [9-11] and heterospin assemblies [6-8,12-15] of ex- change-coupled species. coordination compounds. We report herein on the preparation of a reactive nitro- Coordination compounds incorporating nitroxide ligands are especially attractive since ligation through the nitroxyl xide synthon, 1 (Figure 1), which couples with N-methyl- ethylenediamine to afford tri- (2) and tetranitroxide (3) li- oxygen atom ensures significant magnetic interactions. Moreover, overlap of magnetic orbitals of different types, d gands, and with piperazine to give a biradical (4). The mag- netic properties of these polychelating ligands have been in- and p, may result in special situations where these orbitals are orthogonal and the interaction is ferromagnetic. [6] [7] vestigated and two of them (3, 4) have been structurally characterized. In a preliminary step, and for comparison Among the various types of nitroxides, nitronyl nitro- xides [16] [17] have attracted special interest because of their with previously reported species, complexes of metal hexa- fluoroacetylacetonates 6 {4[Mn(hfac) 2 ] 2 }, 7 {4[Co- delocalized electronic structures and their bridging proper- ties, which have been exploited in the construction of ex- (hfac) 2 ] 2 }, and 8 {4[Ni(hfac) 2 ] 2 } have been prepared and their structural and magnetic properties have been investi- tended structures made up of interacting organic and met- allic spin carriers. The coordination chemistry of nitroxide gated. Finally, we report the synthesis and magnetic be- havior of a pentacoordinate nickel(II) complex, 5 free radicals is, however, highly dependent on the weakly basic character of ligands of this type, which precludes co- {2[Ni(ClO 4 ) 2 ] · H 2 O}, which illustrates the potential of li- gands of this type in the design of new molecular mag- ordination to metal ions other than those bearing electron- withdrawing groups. [18-21] Enhancement of the Lewis acid netic materials. properties of the metal center may easily be achieved, but most of the coordination sites would then be occupied by ancillary ligands and structures of high dimensionality Results and Discussion would not be accessible, other than for high-spin polyra- dicals. [8] Consequently, most of the extended species de- Although the yield of 1 was rather low (ca. 30%), the scribed to date are one-dimensional alternating arrays of one-pot synthesis from the readily available 2,3-dimethyl- metal ions and nitroxide ligands which are ordered at low 2,3-dinitrobutane described here affords samples of the temperature. [22-24] In an attempt to circumvent this diffi- compound from which the amino derivatives can con- veniently be prepared. Synthesis of a similar synthon, the chloro compound analogous to 1, has been described pre- [a] De ´partement de Recherche Fondamentale sur la Matie `re Con- dense ´e, Service de Chimie Inorganique et Biologique, Labora- viously. [26] This chloro compound was used in the prep- toire de Chimie de Coordination (Unite ´ de Recherche Associe ´e aration of 2-(phenyloxymethyl)imidazoline-N,N'-dioxyl de- au CNRS No. 1194), rivatives, but it is unreactive toward amines. CEA-Grenoble, F-38054 Grenoble Cedex 09, France E-mail: rey@drfmc.ceng.cea.fr The synthesis of metal complexes did not present any [b] Laboratoire de Spectrome ´trie Physique (CNRS-UMR 5588), problems; the chelating nature of these ligands ensures co- Universite ´ Joseph Fourier Grenoble 1, B. P. 87, F-38402 Saint Martin d'He `res Cedex, France ordination of the nitroxyl groups to the metal center, as Eur. J. Inorg. Chem. 1999, 1181-1187  WILEY-VCH Verlag GmbH, D-69451 Weinheim, 1999 1434-1948/99/0707-1181 $ 17.50+.50/0 1181