A stable oxoverdazyl free radical: Structural and magnetic characterization M. John Plater a, * , Steven Kemp a , Eugenio Coronado b , Carlos J. Go ´ mez-Garcı ´a b , Ross W. Harrington c , W. Clegg c a Department of Chemistry, University of Aberdeen, Meston Walk, Aberdeen, Scotland AB24 3UE, UK b Instituto de Ciencia Molecular, Universidad de Valencia, Dr. Moliner 50, 46100 Burjasot, Spain c School of Natural Sciences (Chemistry), University of Newcastle, Newcastle upon Tyne, NE1 7RU, UK Received 12 January 2006; accepted 14 February 2006 Available online 6 March 2006 Abstract The structure and magnetic properties (susceptibility and ESR) of the stable oxoverdazyl free radical 6-(4-acetamidophenyl)-1,4,5,6- tetrahydro-2,4-dimethyl-1,2,4,5-tetrazin-3(2H)-one are presented. The crystal structure consists of chains of parallel planar molecules running along the b-axis. These chains are formed by dimers with a ring-over-bond overlap and a significant offset between dimers, although with similar inter- and intradimer distances. The susceptibility measurements show that this compound is an S = 1/2 paramag- net with weak antiferromagnetic interactions. The magnetic susceptibility can be very well reproduced with an antiferromagnetic regular chain model with g = 2.012(1) and an intrachain exchange parameter J = À1.54(1) K = À1.07(1) cm À1 . Solid-state Q-band ESR spectra confirm the presence of an unpaired electron in the oxoverdazyl radical and the thermal behavior observed in the susceptibility measure- ments. Solution ESR spectrum show a very complicated spectrum with up to 13 hyperfine lines, some of them further splitted into three or more superhyperfine lines. A very satisfactory simulation of the shape, intensity and position of all the lines in this spectrum could be obtained with hyperfine coupling constants of 6.5 and 5.4 G for the two groups of two equivalent N atoms and 5.3 G for the six H atoms of the two methyl groups bonded to the verdazyl ring. Ó 2006 Elsevier Ltd. All rights reserved. Keywords: Oxoverdazyl; Radical; Antiferromagnetic; Susceptibility; Eelectron spin resonance 1. Introduction Much attention has been focussed on stable organic rad- icals in the last few years, specially after the discovery of the first purely organic ferromagnets in 1989 [1]. Unfortunately, most of the well-known families of organic radicals are very unstable [2], and among the few that are stable enough, most of them present bulky groups that, although stabilising the radicals, also preclude the desirable magnetic interactions in the solids. Among the very few families of organic radicals fulfiling these two requirements (stability and absence of bulky groups) the most important ones are the nitroxide and the verdazyl ones [3]. Although the nitroxide family has received much attention [4] because it provided the first examples of purely organic radicals, in recent years the extremely high stability and synthetic versatility [3] of many verdazyl radicals has rendered this family of stable organic radicals as the most promising and appealing alternatives to the nitroxide radicals. Thus, verdazyl radicals are the only family of organic radicals that are, generally, air- and mois- ture-stable without bearing bulky substituents [3]. Due to these advantages, verdazyl radicals, extensively studied by Neugebauer [5] and others [6,7], have been used to prepare different materials with various interesting magnetic proper- ties such as weak ferromagnetism [8–10], ferromagnetism [11,12], anti-ferromagnetism [13], spin frustration [14] and spin-peierls transitions [15,16]. The magnetic properties of 0277-5387/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.poly.2006.02.007 * Corresponding author. Tel.: +44 1224 272927; fax: +44 1224 272921. E-mail address: m.j.plater@abdn.ac.uk (M.J. Plater). www.elsevier.com/locate/poly Polyhedron 25 (2006) 2433–2438