Please use this identifier to cite or link to this item: https://doi.org/10.1021/ja8029416
Title: Structure, magnetism, and theoretical study of a mixed-valence (Co3Co4III)-Co-II heptanuclear wheel: Lack of SMM behavior despite negative magnetic anisotropy
Authors: Chibotaru, Liviu F
Ungur, Liviu 
Aronica, Christophe
Elmoll, Hani
Pilet, Guillaume
Luneau, Dominique
Keywords: Science & Technology
Physical Sciences
Chemistry, Multidisciplinary
Chemistry
SINGLE-MOLECULE MAGNETS
HIGH-SPIN MOLECULES
GROUND-STATE
AB-INITIO
ELECTRONIC-STRUCTURE
EXCHANGE INTERACTION
PERTURBATION-THEORY
LIGAND-FIELD
IRON CLUSTER
COMPLEXES
Issue Date: 17-Sep-2008
Publisher: AMER CHEMICAL SOC
Citation: Chibotaru, Liviu F, Ungur, Liviu, Aronica, Christophe, Elmoll, Hani, Pilet, Guillaume, Luneau, Dominique (2008-09-17). Structure, magnetism, and theoretical study of a mixed-valence (Co3Co4III)-Co-II heptanuclear wheel: Lack of SMM behavior despite negative magnetic anisotropy. JOURNAL OF THE AMERICAN CHEMICAL SOCIETY 130 (37) : 12445-12455. ScholarBank@NUS Repository. https://doi.org/10.1021/ja8029416
Abstract: A mixed-valence Co(II)/Co(III) heptanuclear wheel [CoII3CoIII4(L)6(MeO)6] (LH2 = 1,1,1-trifluoro-7-hydroxy-4-methyl-5-aza-hept-3-en-2-one) has been synthesized and its crystal structure determined using single-crystal X-ray diffraction. The valence state of each cobalt ion was established by bond valence sum calculations. Studies of the temperature dependence of the magnetic susceptibility and the field dependence of the magnetization evidence ferromagnetic interactions within the compound. In order to understand the magnetic properties of this Co7 wheel, we performed ab initio calculations for each cobalt fragment at the CASSCF/CASPT2 level, including spin-orbit coupling effects within the SO-RASSI approach. The four Co(III) ions were found to be diamagnetic and to give a significant temperature-independent paramagnetic contribution to the susceptibility. The spin-orbit coupling on the three Co(II) sites leads to separations of ∼200 cm-1 between the ground and excited Kramers doublets, placing the Co7 wheel into a weak-exchange limit in which the lowest electronic states are adequately described by the anisotropic exchange interaction between the lowest Kramers doublets on Co(II) sites. Simulation of the exchange interaction was done within the Lines model, keeping the fully ab initio treatment of magnetic anisotropy effects on individual cobalt fragments using a recently developed methodology. A good description of the susceptibility and magnetization was obtained for nearest-neighbor (J1) and next-nearest-neighbor (J2) exchange parameters (1.5 and 5.5 cm-1, respectively). The strong ferromagnetic interaction between distant cobalt ions arises as a result of low electron-promotion energies in the exchange bridges containing Co(III) ions. The calculations showed a large value of the magnetization along the main magnetic axis (10.1μB), which is a combined effect of the ferromagnetic exchange interaction and negative magnetic anisotropy on the two marginal Co(II) sites. The lack of single-molecule magnet behavior in [CoII3CoIII4(L)6(MeO)6] is explained by relatively large matrix elements of transverse magnetic moments between states of maximal magnetization of the ground Kramers doublet, evidenced by ab initio calculations, and the associated large tunneling rates between these states in the presence of dipolar transverse magnetic fields in the crystal. © 2008 American Chemical Society.
Source Title: JOURNAL OF THE AMERICAN CHEMICAL SOCIETY
URI: https://scholarbank.nus.edu.sg/handle/10635/228696
ISSN: 00027863
15205126
DOI: 10.1021/ja8029416
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