Journal of Organometallic Chemistry 637 – 639 (2001) 251 – 257 www.elsevier.com/locate/jorganchem Ferrocene as a ferromagnetic coupler. Synthesis and characterization of a ferrocene bridged polychlorotriphenylmethyl diradical O. Elsner, D. Ruiz-Molina, I. Ratera, J. Vidal-Gancedo, C. Rovira, J. Veciana * Institut de Cie `ncia dels Materials de Barcelona (CSIC), Campus Uniersitari de Bellaterra, 08193 Cerdanyola, Spain Received 2 January 2001; received in revised form 29 January 2001; accepted 29 January 2001 Abstract A polychlorotriphenylmethyl diradical connected by a 1,1-ferrocenylendivinylene bridge has been synthesized and character- ized. ESR frozen solution experiments down to helium temperature showed that the organometallic unit acts as a ferromagnetic coupler. This fact was supported by ZINDO/1 semiempirical calculations, which showed that the two singly occupied molecular orbitals (SOMOs) are non disjoint in addition to be almost degenerated. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Ferromagnetism; Organic radical; Ferrocene; ESR 1. Introduction The synthesis and characterization of high-spin macromolecules with purely organic spin-carrier centers have attracted great interest [1]. One of the basic tools to realize such high-spin macromolecules is the m -phen- ylene bridge since it acts as a ferromagnetic coupler when it connects two open-shell units. Indeed, since the Schlenk hydrocarbon, first prepared in 1915 [2], high- spin alignment in the ground state has been successfully demonstrated for several 1,3-phenylene connected car- benes [3] and radicals based on triarylmethyl [4], nitro- gen-centered [5] and aminoxyl units [6]. Efforts to increase the number of the aligned spins using 1,3- phenylene couplers have been disturbed by the presence of spin defects and/or bond distortions, which have been shown to affect the ferromagnetic exchange cou- pling through m -phenylenes [7]. To overcome such problems, other complementary approaches such as the use of -conjugated linear polymers bearing pendant radical groups [8], -conjugated polycyclic polymers with alternative coupling pathways [9], or the use of diamagnetic metal ions as ferromagnetic couplers [10], have been used. Indeed, in the case of -conjugated linear polymers bearing pendant radical groups, the radical units are -conjugated with the backbone so ferromagnetic interactions take place. The main advan- tage of these polyradicals is that in spite of the great number of spins accumulated along the polymeric backbone, the ferromagnetic interaction is not sensitive to the spin defects that are unavoidable for macro- molecular polyradicals. The main drawback is the weakness of the ferromagnetic interactions obtained with this approach. -Conjugated polycyclic polymers have also been shown to minimize the interruption of the exchange coupling. Following this approach, a macrocyclic polyradical with an average S =10 below 5 K has been reported [9]. Alternatively, when diamag- netic metal ions are coordinated by radical ligands, those may be assembled so that ferromagnetic spin alignments are obtained. For instance, superexchange through orthogonal magnetic orbitals has been ob- served in a gadolinium complex octahedrally coordi- nated by three semiquinone radicals [10]. In our group, we focussed our attention in the devel- opment and study of new coupling units able to pro- mote ferromagnetic interactions between pure organic radicals. Metallocenes were excellent candidates to be prospected as magnetic couplers not only because of their rich chemistry but also because they are electroac- * Corresponding author. Tel.: +34-93-580-1853; fax: +34-93-580- 5729. E-mail address: vecianaj@icmab.es (J. Veciana). 0022-328X/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved. PII:S0022-328X(01)00912-3