Crystal structures and magnetic properties of complexes of M(NO 3 ) 2 ; M = Mn II , Co II , Ni II , and Cu II , with pyridine ligands carrying an aminoxyl radical Zhicheng Zhu, Satoru Karasawa, Noboru Koga * Graduate School of Pharmaceutical Sciences, Kyushu University, 3-1-1 Maidashi, Higashi-Ku, Fukuoka 812-8582, Japan Received 5 October 2004; accepted 16 November 2004 Available online 5 May 2005 Abstract Four complexes of M(NO 3 ) 2 (4NOPy-OMe) 2 ,(4NOPy-OMe = 4-(N-tert-butyloxylamino)-2-(methoxymethylenyl)pyridine, and M = Mn II , 1; Co II , 2; Ni II , 3; Cu II , 4), were prepared and fully characterized. X-ray single crystal analysis reveals that four complexes are isostructural. The molecular structures are distorted octahedral in which the methoxy oxygen atoms coordinate to the metal ion by trans-configuration while the pyridyl nitrogen atoms and the nitrate oxygen atoms coordinate by cis-configuration. The magnetic properties of all complexes were investigated by SQUID magneto/susceptometry. Temperature dependence of the molar magnetic susceptibilities in the temperature range of 2–300 K indicated that the magnetic coupling between aminoxyl radicals and metal ion was antiferromagnetic in the complex 1 and were ferromagnetic in the complexes 2–4. The quantitative analysis based on the spin Hamiltonian, H = 2J(S 1 S M + S M S 2 ) yielded the best fit as J/k B = 13.4 ± 0.1 K, g = 1.94 ± 0.002, and h = 0.78 ± 0.02 K for the complex 1, J/k B = 48.7 ± 2.1 K, g = 2.07 ± 0.02, and h = 2.83 ± 0.41 K for the complex 3 (the data in the temperature range 300–50 K were used), and J/k B = 57.0 ± 1.2 K, g = 2.002 ± 0.004, and h = 9.8 ± 0.1 K for the complex 4. Ó 2005 Elsevier Ltd. All rights reserved. Keywords: Nitroxyl radical; Magnetic interaction; Complex; Manganese; Cobalt; Nickel; Copper 1. Introduction In the field of the molecule-based magnet, a low dimensional complex having a large magnetic anisot- ropy has been noted as new type of material [1]. For the construction of a single molecule magnet (SMM) with a zero dimension, we proposed to use the hetero- spin system consisting of the paramagnetic metal ion and the organic spins [2,3]. Actually, 1:4 complexes of CoX 2 , X = SCN  and OCN  , with 4-(N-tert-buty- loxylamino)-pyridine, 4NOPy, exhibited the magnetic behavior of SMM with relatively large activation bar- rier, D/k B , for flipping the spin, D/k B = 30 and 50 K, respectively [3]. In this SMM using a heterospin system, the pyridine-aminoxyl compound, 4NOPy, plays important role as a binding ligand for metal ion and an organic spin source. This time, to improve our SMM and develop our heterospin system to one- and two-dimensional assembly, a bidentate ligand carrying an aminoxyl radical was considered. For the purpose, 4-(N-tert-butyloxylamino)-2-(methoxymethylenyl)pyridine, 4NOPy-OMe, was designed and prepared. This 4NOPy- OMe ligated with a various metal ions to form low dimensional magnetic complexes through intermolecu- lar interaction. In preliminary experiments of the 1:2 complexes of CoX 2 , X = Cl  [4] and SCN  [5], with 4NOPy-OMe, their crystals showed magnetic behaviors with the structural and the local metal ion anisotropy. In order to understand the magnetic coupling between the metal ion and aminoxyl radical quantitatively, in this work, the molecular structures of the complexes of 0277-5387/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.poly.2005.03.026 * Corresponding author. Tel./fax: +926426590. E-mail address: koga@fc.phar.kyushu-u.ac.jp (N. Koga). www.elsevier.com/locate/poly Polyhedron 24 (2005) 2102–2107