Many transition metal clusters have been shown to exhibit single-molecule magnet behavior, the property which arises from bistable magnetic ground states of the coupled metal centers. Traditionally, most clusters are constructed from multiple metal centers connected through diamagnetic bridging ligands, the example of which is the first single-molecule magnet Mn12O12(O2CCH3)16. The limitation of such clusters is the weak magnetic interaction through superexchange mechanism, which is easily overcome by thermal energy. Recent attempts have focused on strengthening the magnetic interaction by utilizing the direct exchange mechanism, which can be realized in either radical bridging ligands or direct metal-metal orbital overlap. We will describe the synthesis of tetranuclear clusters M4(NPtBu3)4 (M = Co, Ni, Cu), where four metal centers are engaged in direct metal-metal bonding interactions. In these clusters, each metal center is coordinated by two phosphinimidinate ligands. The cationic clusters [M4(NPtBu3)4]+ can also be synthesized, while retaining the low-coordinate environment and short metal-metal contacts. We will show that the combination of direct metal-metal interactions and low-coordinate environment guarantee a large total spin ground state of the clusters, equivalent to ferromagnetic interaction, which persists even to 300 K. Moreover, magnetization measurements and HF-EPR spectroscopy reveal an unprecedentedly large magnetic anisotropy. Single-molecule magnet behavior will be discussed.

Tetranuclear transition metal clusters with direct metal-metal interactions: Synthesis, electronic structure, and magnetism

2019 Fall National Meeting

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