A Chiral and Polar Single-Molecule Magnet: Synthesis, Structure, and Tracking of Its Formation Using Mass Spectrometry.

Abstract

The solvothermal reaction of cobalt(II) sulfate with S, S-1,2- bis(1-methyl-1 H-benzo[ d]imidazol-2-yl)ethane-1,2-diol, (H2L), neutralized with triethylamine (Et3N) in a mixture of methanol and water (2:1), resulted in triangular red crystals of [CoII7(L)3(SO4)3(OH)2(H2O)9]·4H2O·3CH3OH (Co7). It is formed of chiral and polar clusters crystallizing in the R3 space group. Co7 consists of apex-shared asymmetric dicubane units where all of the metals adopt an octahedral coordination and the three ligands wrap diagonally around the unit. One end of the cluster is bonded by six water molecules and the other end by three monodentate sulfates. The head-to-tail packing through extended H-bonds leads to polar chains. The ligand has lost two protons, adopts a cis-conformation, and is coordinated to five metals around the waist of the dicubane. Electrospray ionization mass spectrometry (ESI-MS) of solutions of the reaction as a function of time reveals the possible step-by-step assembly process of the cluster: the initial product [CoII(HL)(SO4)]2- combines with CoSO4, forming [CoII2(HL)(SO4)2]2-, and then, upon addition of Et3N, dimerizes through a [OH]- bridge to [CoII4(HL)2(OH)(CH3OH)2(SO4)3]- followed by capture of one Co2+ and one CoSO4 to form [CoII6(L)2(OH)(CH3O)(SO4)4]2- before eventually binding to CoL to form [CoII7(L)3(OH)2(SO4)4]2-. These results allow us to propose a possible process for the formation of Co7, which is a good example for chiral multidentate chelating ligand-controlled assembly of clusters. Magnetization measurements as a function of the temperature, field, and ac-frequency reveal ferromagnetic coupled moments and single-molecule magnetism (SMM).