Wheel-Shaped Mn 16 Single-Molecule Magnets Sonali J. Shah, † Christopher M. Ramsey, § Katie J. Heroux, † James R. O’Brien, ‡ Antonio G. DiPasquale, † Arnold L. Rheingold, † Enrique del Barco, § and David N. Hendrickson* ,† Department of Chemistry and Biochemistry, UniVersity of California at San Diego, La Jolla, California 92093, Quantum Design, San Diego, California 92121, and Department of Physics, UniVersity of Central Florida, Orlando, Florida 32765 Received February 12, 2008 The syntheses, structures, and magnetic properties of two new single-stranded hexadecanuclear manganese wheels [Mn 16 (CH 3 COO) 8 (CH 3 CH 2 CH 2 COO) 8 (teaH) 12 ] · 10 MeCN (1 · 10 MeCN) and [Mn 16 ((CH 3 ) 2 CHCOO) 16 (teaH) 12 ] · 4CHCl 3 (2 · 4CHCl 3 ), where teaH 2- is the dianion of triethanolamine, are reported. 1 crystallizes in the tetragonal I4 1 /a space group [a ) b ) 33.519(4) Å and c ) 16.659(2) Å]. 2 crystallizes in the monoclinic C2/c space group [a ) 21.473(5), b ) 26.819(6), c ) 35.186(7), and  ) 93.447(5)°]. Both complexes consist of 8 Mn(II) and 8 Mn(III) ions alternating in a wheel-shaped topology with 12 monoprotonated triethanolamine ligands. Variable-temperature direct current (DC) magnetic susceptibility data were collected in 1 T, 0.1 and 0.01 T fields, and in the 1.8-300 K temperature range for 1 and 2. Variable-temperature variable-field DC magnetic susceptibility data were obtained in the 1.8-10 K and 0.1-5 T ranges and least-squares fitting of these reduced magnetization versus H/T data indicates a S ) 13 ground-state for 1 and 2. Single-crystal magnetization hysteresis measurements were performed in a 0.04-1 K temperature range for complex 2. Hysteresis loops were observed that showed a temperature dependence, which indicates that 2 exhibits magnetization relaxation and is a SMM. Both 1 and 2 show frequency- dependent out-of-phase signals in the AC susceptibility measurements, collected in a temperature range of 1.8-5 K and in the frequency range of 50-10 000 Hz. Extrapolation of the in-phase component of the AC susceptibility data to 0 K indicates an S ) 12 ground state for 1 and an S ) 11 ground-state for 2. Complex 1 has the highest-spin ground state reported to date for a single-stranded manganese wheel and is likely to be an SMM based on a frequency-dependent out-of-phase signal in the AC susceptibility. The AC susceptibility as well as magnetization hysteresis data for 2 confirm that this species is an SMM. Introduction Single-molecule magnets (SMMs) have attracted much attention in recent years. SMMs are polynuclear metal complexes that have a large spin (S) ground state, with a negative axial zero-field splitting parameter D that leads to an appreciable potential energy barrier for magnetization reversal. SMMs are found to show magnetization relaxation, a property of classical magnets, as well as quantum effects such as quantum tunneling of magnetization 1 and quantum phase interference. 2 In 1993, the polynuclear manganese cluster [Mn 12 O 12 (CH 3 COO) 16 (H 2 O) 4 ] · 2 CH 3 COOH · 4H 2 O was reported to be the first single molecule magnet by Sessoli et al. 3 The synthesis of this complex was first reported by T. Lis in 1980. 4 This dodecanuclear manganese cluster was found to have an S ) 10 ground state and shows magnetiza- tion hysteresis below 4 K. 3 Since 1993, many polynuclear transition-metal complexes have been found to show SMM behavior. A vast majority of SMMs have been synthesized using transition metals such as iron, manganese, and nickel, with * To whom correspondence should be addressed. E-mail: dhendrickson@ucsd.edu. † University of California at San Diego. ‡ Quantum Design. § University of Central Florida. (1) (a) Gatteschi, D.; Sessoli, R. Angew. Chem., Int. Ed. 2003, 42, 3. (b) Christou, G.; Gatteschi, D.; Hendrickson, D. N.; Sessoli, R. MRS Bull. 2000, 25, 66. (2) Wernsdorfer, W.; Sessoli, R. Science 1999, 284, 133. (3) (a) Sessoli, R.; Tsai, H.-L.; Schake, A. R.; Wang, S.; Vincent, J. B.; Folting, K.; Gatteschi, D.; Christou, G.; Hendrickson, D. N. J. Am. Chem. Soc. 1993, 115, 1804. (b) Sessoli, R.; Gatteschi, D.; Caneschi, A.; Novak, M. A. Nature 1993, 365, 141. (4) Lis, T. Acta Crystallogr., Sect. B 1980, 36, 2042. Inorg. Chem. 2008, 47, 6245-6253 10.1021/ic800277j CCC: $40.75  2008 American Chemical Society Inorganic Chemistry, Vol. 47, No. 14, 2008 6245 Published on Web 06/21/2008