Adsorption of [Mn₁₉] Aggregates with S = 83/2 onto HOPG Surfaces

Abstract

A structural analogue of the previously reported record-high-spin (S = 83/2) mixed-valence [Mn₁₉] coordination cluster [Mn^III₁₂Mn^II₇(μ₄-O)₈(μ₃-η¹-N₃)₈(HL^Me)₁₂(MeCN)₆]Cl₂·10MeOH·MeCN (1) {H₃L^Me = 2,6-bis(hydroxymethyl)-4-methylphenol} was prepared in which the methyl group at the 4-position on the phenyl ring was replaced by a methoxy group to give the compound [Et₃NH]₂[Mn^III₁₂Mn^II₇(μ₄-O)₈(μ₃-η¹-N₃)_7.4(μ₃-Cl)_0.6(HL^OMe)₁₂(MeOH)₆]Cl₄·14MeOH (2). Single-crystal X-ray diffraction analysis of 2 confirms that the molecular structure motif found for 1, comprising two supertetrahedral {Mn^III₆Mn^II₄} units sharing a common Mn^II vertex, was retained. Magnetic susceptibility measurements on 2 show that the ferromagnetic spin structure, which results in the maximum S_T = 83/2 spin ground state for 1, is largely robust with respect to the change in ligand substituent in 2. Molecules of compounds 1 and 2 were deposited onto highly oriented pyrolytic graphite (HOPG) substrates and were investigated by scanning tunneling microscopy (STM) andcurrent imaging tunneling spectroscopy (CITS). By usingextremely dilute solutions (10^–9 mol L^–1), one-dimensionalarrangements as well as isolated single molecules of [1]Cl₂·10MeOH·MeCN were observed with STM topography, generally at the step edges or at defects of the HOPG surfaces, while small groups of molecules of 2 were observed. The size of the individual molecules was consistent with the X-ray crystallographic data. In the CITS images of the single molecules, a strong tunneling current contrast was observed at the positions of the metal ions.