Notes Bull. Korean Chem. Soc. 2004, Vol. 25, No. 6 917 Preparation and Magnetic Properties of a Neutral Octadecanuclear Manganese Complex [Mn II 4 Mn III 14 (O) 14 (O 2 CMe) 18 (hmp) 4 (hmpH) 2 (H 2 O) 2 ] Akhilesh K. Gupta, Seokwon Yoon, † Myoung Soo Lah, ‡ and Jinkwon Kim * Department of Chemistry, Kongju National University, 182 Shinkwan, Kongju, Chungnam 314-701, Korea † Department of Physics, The Catholic University of Korea, 43-1 Yoggok, Puchon 420-743, Korea ‡ Department of Chemistry, Hanyang University, 1271 Sa-1-dong, Ansan, Gyunggi 425-791, Korea Received January 15, 2004 Key Words : Manganese complex, Metal cluster, Magnetic properties The synthesis of a magnetic molecule having unusually large spin value and large negative anisotropy value is an area of intensive current research, because it is the prereq- uisite for developing the emerging class of single-molecule magnets (SMMs). SMMs, nanometer-size single-domain magnetic clusters, have been found to display intramolecular magnetic hysteresis loop due to very slow magnetic relaxation below their blocking temperature as well as steps in the hysteresis loop assigned to the presence of quantum tunneling of the magnetization. 1 Since the dodecanulear manganese cluster with the composition [Mn12O12(O2CMe)16- (H2O)4] (Mn12ac) had been discovered as an single- molecule magnet, 2,3 many efforts have been made to achieve larger cluster compounds showing SMM behaviors. 4 Espe- cially, manganese carboxylate cluster chemistry has proved to be a rich source of a variety of polynuclear species. 5,6 Specific examples of SMMs except Mn12ac include the tetranuclear cubane [Mn IV Mn III 3O3X] 6+ core 7 and [Fe4(sae)4- (MeOH)4] (sae = 2-salicylidene-amino-1-ethanol), 8 the octa- nuclear Fe(III) cluster [Fe8O2(OH)12(tacn)6] 8+ (tacn = tetra- azacyclononane), 9 and the tetranuclear butterfly complex [V4O2(O2CR)7(L)2]n+ (L=bipyridine or picolinate). 10 Recently, a new family of manganese-based SMMs, such as [Mn7- (OH)3Cl3(hmp)9] 2+ and [Mn12O8X4(O2CPh)8L8], has been designed by using of hmp-bridging ligand (hmpH = 2- hydroxymethylpyridine). 11,12 On the other hand, oxidation of Mn(II) by MnO4 - in CH2Cl2 leads to Mn II 2Mn III 2 chain complex instead of Mn12 cluster. 13 Thus we have been trying new oxidation reaction of Mn(II) by MnO4 - in a presence of hmpH and obtained a new octadecanuclear mixed-valent Mn cluster of formula [Mn18O14(O2CMe)18- (hmp)4(hmpH)2(H2O)2] (1). The synthesis of 1 was achieved by reaction of an aqueous slurry of one equivalent of Mn(O2CCH3)2·4H2O in methyl- ene chloride, 2.5 equivalents of hmpH, 0.5 equivalents of NBu4 n MnO4 in a presence of carboxylic acid. NBu4 n MnO4 oxidizes Mn II to Mn III in presence of water and carboxylic acid. An ORTEP diagram of 1 with atom labeling scheme is displayed in Figure 1. The centrosymmetric complex 1 consists of [Mn18(μ3-O)10(μ4-O)4] core with peripheral chelation provided by eighteen acetate ligands and six hmp - ligands, and two terminal water molecules. On the basis of Jahn-Teller distortions and bond valence sum calculations, the seven manganese atoms (Mn1-Mn7) in a crystallographi- cally asymmetric unit were assigned to Mn III and Mn8 and Mn9 to Mn II . The elongated axial Mn III -O distances (2.144(4)-2.456(4) Å) are significantly longer than the other bonds (1.854(3)-1.977(4) Å). The Mn II -O bond distances are in a range from 2.104(4) Å to 2.267(3) Å. As shown in the side view, ten manganese atoms from Mn1 to Mn5 and their * Corresponding Author. Phone: +82-41-850-8496; Fax: +82-41- 850-8479; e-mail: jkim@kongju.ac.kr Figure 1. (a) ORTEP diagram of 1 with atom numbering scheme. (b) A side view emphasizing the planarity of [Mn10O6] unit.