Manganese (II) Coordination Complexes Involving Nitronyl Nitroxide Radicals Mohammed Fettouhi,* ,² Mazen Khaled, ² Abdel Waheed, ² Ste ´ phane Golhen, ‡ Lahce ` ne Ouahab,* ,‡ Jean-Pascal Sutter, § and Olivier Kahn § Chemistry Department, King Fahd University of Petroleum and Minerals, Dhahran 31261, Saudi Arabia, Laboratoire de Chimie du Solide et Inorganique Mole ´culaire, UMR 6511, Universite ´ de Rennes 1, Campus de Beaulieu, 35042, Rennes Cedex, France, and Laboratoire des Sciences Mole ´culaires, ICMCB, UPR CNRS no 9048, 33608 Pessac, France ReceiVed March 11, 1999 Three new complexes involving nitronyl nitroxide and PhCOO - or N 3 - ligands have been synthesized and structurally and magnetically characterized. These compounds are formulated as Mn(L) 4 (X) 2 ‚nH 2 O, L ) 2-(p- pyridyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (PNN) or 2-(p-pyridyl)-4,4,5,5-tetramethylimidazoline-1- oxyl (PN) and X ) PhCOO - or N 3 - . Compound A [Mn(PNN) 2 (PhCOO) 2 (H 2 O) 2 ] presents the following structural parameters: triclinic, space group P1 h (No. 2), a ) 6.859(1) Å, b ) 11.271(2) Å, c ) 13.978(6) Å, R) 88.88(2)°,  ) 89.38(2)°, γ ) 78.28(2)°, Z ) 1. The complex has a centrosymmetric distorted octahedral geometry in which the manganese ion is bound to two radical ligands through the nitrogen atom of the pyridine rings, two benzoate groups, and two water molecules. The two compounds B [Mn(PNN) 4 (N 3 ) 2 ] and C [Mn(PN) 4 (N 3 ) 2 ] are isostructural with the following structural parameters: B; triclinic, space group P1 h (No. 2), a ) 7.177(4) Å, b ) 13.767(3) Å, c ) 13.928(4) Å, R) 90.20(2)°,  ) 102.94(4)°, γ ) 91.86(4)°, Z ) 1; C, triclinic, space group P1 h (No. 2), a ) 7.004(2) Å, b ) 13.885(1) Å, c ) 14.036(2) Å, R) 90.34(1)°,  ) 101.42(2)°, γ ) 92.92(1)°, Z ) 1. They adopt a centrosymmetric tetragonally distorted octahedral geometry in which the manganese ion is bound to four radical ligands through the nitrogen atom of the pyridine rings, and the azido groups occupy the apical positions. For all three compounds A, B, and C intramolecular ferromagnetic interactions between the Mn(II) ion and the nitronyl nitroxide radicals are observed, but at very low temperatures, intermolecular antiferromagnetic interactions dominate. Introduction During the past two decades, the field of molecular magnets has attracted scientists from different horizons. Their main objectives are on one hand the chemical design of molecular assemblies that exhibit a spontaneous magnetization and on the other hand the rationalization of the magnetostructural correla- tions. 1 Among the ligands capable of building extended 1D, 2D, and 3D networks in association with open-shell metal ions, great attention was devoted separately to both nitronyl nitroxides 2-4 and acetate, carboxylate, and azido anions. 1,5-15 Apart from the early solution ESR studies of some metal-pyridyl nitronyl nitroxide complexes, 16,17 the coordination chemistry of such ligands was initiated with the aim of building chainlike species. However, in most cases the NO groups of the radical do not coordinate even strongly electrophilic metal ions. 18,19 Mean- while, the particular azido anion has emerged as a versatile ligand, generally leading to high-nuclearity systems with metals such as Mn(II), 5-12 Cu(II), 13,14 and Ni(II). 15 It stands furthermore as a good superexchange pathway, both antiferromagnetic in µ 1,3 -N 3 (end-to-end) coordination and ferromagnetic in µ 1,1 -N 3 (end-on) coordination. In the latter case ferromagnetic coupling has been interpreted by the spin polarization concept. 20 The most important examples of these systems are those exhibiting a spontaneous magnetization between 16 and 40 K. 5 The majority ² King Fahd University of Petroleum and Minerals. ‡ Universite ´ de Rennes 1. § ICMCB. (1) Kahn, O. Molecular Magnetism; VCH: New York, 1993. (2) Inoue, K.; Hayamizu, T.; Iwamura, H.; Hashizume, D.; Ohashi, Y. J. Am. Chem. Soc. 1996, 118, 1803. (3) Caneschi, A.; Gatteschi, D.; Sessoli, R.; Rey, P. 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