The {FeIII[FeIII(L1)2]3} star-type single-molecule magnet

Abstract

Star-shaped complex {Fe^III[Fe^III(L¹)₂]₃} (3) was synthesized starting from N-methyldiethanolamine H₂L¹ (1) and ferric chloride in the presence of sodium hydride. For 3, two different high-spin iron(III) ion sites were confirmed by Mössbauer spectroscopy at 77 K. Single-crystal X-ray structure determination revealed that 3 crystallizes with four molecules of chloroform, but, with only three molecules of dichloromethane. The unit cell of 3·4CHCl₃ contains the enantiomers (Δ)-[(S,S)(R,R)(R,R)] and (Λ)-[(R,R)(S,S)(S,S)], whereas in case of 3·3CH₂Cl₂ four independent molecules, forming pairs of the enantiomers [Λ-(R,R)(R,R)(R,R)]-3 and [Δ-(S,S)(S,S)(S,S)]-3, were observed in the unit cell. According to SQUID measurements, the antiferromagnetic intramolecular coupling of the iron(III) ions in 3 results in a S = 10/2 ground state multiplet. The anisotropy is of the easy-axis type. EPR measurements enabled an accurate determination of the ligand-field splitting parameters. The ferric star 3 is a single-molecule magnet (SMM) and shows hysteretic magnetization characteristics below a blocking temperature of about 1.2 K. However, weak intermolecular couplings, mediated in a chainlike fashion via solvent molecules, have a strong influence on the magnetic properties. Scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS) were used to determine the structural and electronic properties of star-type tetranuclear iron(III) complex 3. The molecules were deposited onto highly ordered pyrolytic graphite (HOPG). Small, regular molecule clusters, two-dimensional monolayers as well as separated single molecules were observed. In our STS measurements we found a rather large contrast at the expected locations of the metal centers of the molecules. This direct addressing of the metal centers was confirmed by DFT calculations.