Direct Observation of Single-Molecule Magnets Organized on Gold Surfaces

A dodecamanganese(III,IV) cluster, [Mn12O12(OAc)16(H2O)4]·4H2O·2AcOH, is considered to be the prototype of a class of materials referred to as single-mol. magnets (SMMs), whose magnetic behavior is strongly reminiscent of bulk magnets. A method to deposit SMMs on a gold film and to observe them directly using scanning tunneling microscopy (STM) was demonstrated. To obtain suitably derivatized Mn12-type clusters, 16-sulfanylhexadecanoate ligands (L') were introduced around the Mn12 core by exploiting a well-known ligand-exchange reaction. However, treatment of I·4H2O·2AcOH with HL' in toluene or CH2Cl2 afforded intractable solids. The use of the corresponding acetyl-protected acid HL yielded a fully-substituted deriv. [Mn12O12(L)16(H2O)4] (II) which is highly sol. in org. solvents. Deposition of the nanoclusters on a Au(111) surface was carried out by incubating gold substrates in dild. solns. of compd. II in THF together with aq. NH4OH. STM anal. showed complete coverage of the gold substrate by a disordered layer of round particles with an apparent lateral size of ∼5.8 nm, which agreed with the size of compd. II obtained from mol. modeling studies (5.0-5.6 nm).