Single Molecule Magnets: Problems and Progress

Single-molecule magnets (SMMs) are molecules that can be magnetized in a magnetic field and which. show slow relaxation when the magnetic field is removed. The magnetization occurs because there is a large energy barrier between the spin-up and the spin-down states. The magnetic order is within the molecules (single domain). The requirements for SMM behavior are (1) a very high spin state and (2) a 
large magnetic anisotropy. In contrast to classical nanomagnets obtained by a top-down approach SMMs 
represent a bottom-up approach. The potential applications of SMMs are vast. These include information 
processing, data storage, quantum computing, spintronics, biomedical applications (like MRI contrast 
agents) or magnetic refrigeration. One of the early compounds to have been studied as an SMM is the 
dodecanuclear mixed-valent manganese cage [MnIII/IV  I2O12(CH3C02)16(H2O)4]1. This compound has an 
S=10 ground state and shows slow relaxation of the magnetization. Further, this compound shows 
hysteresis; evidence of quantum tunneling was also observed. Subsequently, several multinuclear 
transition metal complexes have been investigated for their SMM behavior. More recently there have 
been examples of3d-4fcomplexes that show SMM behavior. 

In this talk we will present various synthetic strategies that are useful for assembling compounds that can 
be potential SMMs. In general the requirement that SMMs should have large ground state spin values 
suggests that multinuclear transition metal/lanthanide/transition metal-lanthanide complexes should be 
prepared. The strategies for assembling such complexes can be varied and use of multi-topic ligands is usually practiced. In addition, a philosophical divergence in terms of strategy can arise with an approach 
allowing nature to pick the best possible ensemble from a reaction milieu. On the other hand, reliable 
architectures can be ensured by specific design of Iigands. The merits of these widely different 
methodologies will be presented with examples taken from the literature and from our work. 

References:
1.	Sessoli, R.; Tsai, H.-L.; Schake, A. R.; Wang, S.; Vincent, J. B.; Folting, K.; Gatteschi, D.; Christou, G.; 
Hendrickson, D. N. J. Am. Chern. Soc. 1993, 115, 1804-16 
2.	Chandrasekhar, V.; Pandian, B. M.; Vittal, J. J.; Clerac, R. 1norg. Chern. 2009, 48, 1149-57