Enhancing the Quantum Properties of Manganese–Lanthanide Single‐Molecule Magnets: Observation of Quantum Tunneling Steps in the Hysteresis Loops of a {Mn12Gd} Cluster

Abstract

Single-molecule magnets (SMMs) are individual molecules that function as single-domain nanoscale magnetic particles. A SMM derives its properties from a combination of a high-spin ground state (S) and an easy axis type of magnetoanisotropy (negative zero-field splitting parameter,D), which results in a significant energy barrier to the reversal of the magnetization vector. Such species display both classical magnetization hysteresis, quantum tunneling of magnetization (QTM), and quantum phase interference. Thus, SMMs represent a molecular (“bottom-up”) route to nanoscale magnetism, with potential technological applications in information storage and spintronics at the molecular level, and use as quantum bits (qubits) in quantum computation by exploiting the QTM through the anisotropy barrier. The upper limit to the barrier (U) is given by S jD j or (S-1/4) jD j for integer and half-integer S, respectively. In practice, QTM through upper regions of the barrier makes the true or the effective barrier (Ueff) lower than that of U. Ideally, the QTM can be observed and studied in magnetization vs. DC (direct current) field hysteresis loops, appearing as distinct step-like features at periodic field values, at which levels on either side of the anisotropy barrier to relaxation are in resonance. The steps are thus field positions at which the magnetization relaxation rate increases owing to the onset of QTM. Such steps are a diagnostic signature of resonant QTM, and have been clearly seen only for a few classes of compounds, such as manganese, iron, and nickel SMMs. 7, 8] The most fruitful source of SMMs is the manganese carboxylate chemistry. The prototype was the [Mn12O12(O2CR)16(H2O)4] family, [2,4, 9] and a number of others have since been discovered; almost all have been transition metal clusters, and the vast majority of them have been manganese clusters containing at least some manganese(III) ions. As the search for new SMMs expanded, several groups explored mixed transition metal/lanthanide (Ln) compounds, and particularly Mn–Ln ones, as an attractive area; these efforts were greatly stimulated by the Cu2Tb2 SMM reported by Matsumoto and co-workers. The strategy is obviously to take advantage of the lanthanide ion s significant spin, and/or its large anisotropy, as reflected in a largeD value, to generate SMMs distinctly different from the homometallic ones. Indeed, there are now several Mn–Ln SMMs, including Mn11Ln4, [11] Mn11Gd2, [12] Mn5Ln4, [13a] and Mn6Dy6 [13b] . Many of them have exhibited magnetization hysteresis loops, but unfortunately none of them have displayed resolved QTM steps in these loops. Thus, the incorporation of lanthanide ions has led to a degradation of the quantum properties, as reflected in the QTM steps. The likeliest reason for the degradation of the quantum properties is the step broadening owing to the low-lying excited states resulting from very weak exchange interactions involving the 4f metal ion(s). Herein we report a new structural type in mixed Mn–Ln SMMs having a {Mn12Gd} 38+ core, in which clear QTM steps have been observed in the hysteresis loops of a mixed 3d–4f SMM for the first time. As a result, the D value of a 3d–4f SMM can be measured directly for the first time from the hysteresis data, that is, from magnetic field separation between the steps. The reaction of Mn(O2CPh)2, nBu4NMnO4, Gd(NO3)3, and PhCO2H in a 4:1:4:32 molar ratio in nitromethane produced a dark brown solution, which upon filtration and slow evaporation of the solvent resulted in crystals of [Mn12GdO9(O2CPh)18(O2CH)(NO3)(HO2CPh)] (1) in 40% yield. The structure of 1 consists of a {MnMn11} 35+ cluster with a central Gd ion (Figure 1). The {Mn12Gd} 38+ core is held together by seven m4-O 2 and two m3-O 2 ions. Peripheral ligation is provided by a m4-, three m3-, fourteen m-benzoate groups, a m3-formate group, a chelating NO3 on Mn12, and a terminal benzoic acid on Mn5. The formate probably comes from oxidation of nitromethane by the highly oxidizing MnO4 reagent. The metal oxidation states and the protonation levels of O ions were established by bond valence sum (BVS) calculations and the observation of manganese(III) Jahn–Teller (JT) elongation axes (Figure S1). All manganese atoms are six-coordinate, whereas the gadolinium [*] Dr. T. C. Stamatatos, Prof. Dr. G. Christou Department of Chemistry, University of Florida Gainesville, FL 32611-7200 (USA) Fax: (+1)352-392-8757 E-mail: christou@chem.ufl.edu