Hierarchical Assembly of a {MnII15MnIII4} Brucite Disc: Step-by-Step Formation and Ferrimagnetism.

Yong-Kai Deng, HaiFeng Su, Jia-Heng Xu, Wen-Guang Wang, Mohamedally Kurmoo, Shuichao Lin, Yuan-Zhi Tan, Jiong Jia, Di Sun, Lan-Sun Zheng,

Journal of the American Chemical Society, January 18, 2016

In search of functional molecular materials and the study of their formation mechanism, we report the elucidation of a hierarchical step-by-step formation from monomer (Mn) to heptamer (Mn7) to nonadecamer (Mn19) satisfying the relation 1+n6n, where n is the ring number of the Brucite structure using high-resolution electrospray ionization mass spectrometry (HRESI-MS). Three in-termediate clusters, Mn10, Mn12 and Mn14, were identified. Furthermore, the Mn19 disc remains intact when dissolved in acetoni-trile with a well-resolved general formula of [Mn19(L)x(OH)y(N3)36-x-y]2+ (x = 18, 17, 16; y = 8, 7, 6; HL = 1-(hydroxymethyl)-3,5-dimethylpyrazole) indicating progressive exchange of N3- for OH-. The high symmetry (R-3) Mn19 crystal structure consists of a well-ordered discotic motif where the peripheral organic ligands form a double calix housing the anions and solvent molecules. From the formula and valence bond sums the charge state is mixed-valent, [MnII15MnIII4]. Its magnetic properties and electrochem-istry have been studied. It behaves as a ferrimagnet below 40 K and has a coercive field of 2.7 kOe at 1.8 K, which can be possible by either weak exchange between clusters through the anions and solvents or through dipolar interaction through space as con-firmed by the lack of ordering in frozen CH3CN. The moment of nearly 50 NµB suggests MnII-MnII and MnIII-MnIII are ferromag-netically coupled while MnII-MnIII is antiferromagnetic which is likely if the MnIII are centrally placed in the cluster. This compound displays the rare occurrence of magnetic ordering from non-connected high-spin molecules.

Pubmed Link: 26780000