Topological Magnetic Isomers DOI: 10.1002/ange.201203309 Crystal-to-Crystal Transformation of Magnets Based on Heptacyanomolybdate(III) involving Dramatic Changes in Coordination Mode and Ordering Temperature** Qing-Lun Wang, Heather Southerland, Jian-Rong Li, AndreyV. Prosvirin, Hanhua Zhao, and Kim R. Dunbar* Crystal-to-crystal transformation is an interesting phenom- enon with potential applications in molecular devices such as switches and sensors. [1] The structural changes typically involve minor movements of atoms in the crystal prompted by changes in metal coordination number, condensation reactions, rearrangement of bonds, or the removal or exchange of solvents. [1] It is challenging, however, to obtain high-quality crystals after solid-state transformations, espe- cially those involving the breaking and formation of chemical bonds. [2–5] Some rare examples are single-crystal transforma- tions of coordination polymers that change their dimension- ality, such as 1D to 2D, 1D to 3D, and 2D to 3D. [3] as well as 0D compounds to 1D and 2D coordination polymers. [4] Although various crystal-to-crystal transformations have been reported in recent years, very few examples involve drastic changes in the magnetic properties. [4a, 5] In the course of our research into the properties of the heptacyanomolybdate(III) anion we isolated the 3D material {[Mn(dpop)] 3 [Mn(dpop)(H 2 O)][Mo(CN) 7 ] 2 ·13.5 H 2 O} n (1; dpop = 2,13-dimethyl-3,6,9,12,18-pentaazabicyclo[12.3.1]- octadeca-1(18),2,12,14,16-pentaene), which undergoes a re- markable single-crystal to single-crystal transformation to afford {[Mn(dpop)] 2 [Mo(CN) 7 ]·2 H 2 O} n (2) by partial loss of solvent molecules with concomitant rearrangement of the binding mode of the heptacyanomolybdate linker. The alterations in the structure results in dramatic changes in the coercive field and ordering temperature of the magnet. Compound 1 was prepared by slow diffusion of solutions of K 4 [Mo III (CN) 7 ] and [Mn II (dpop)]Cl 2 in the dark under a nitrogen atmosphere; the related phase, 2, was prepared by subjecting crystals of 1 to a dynamic vacuum at 125 8C for 8 h and further pumping using the “outgas” function of a gas adsorption analyzer for 5 h at 150 8C. Crystals of 1 belong to the rhombohedral space group R  3. The crystallographically independent unit consists of a {[Mn III - (dpop)] 3 [Mn II (dpop)(H 2 O)][Mo(CN) 7 ] 2 } fragment (Fig- ure 1a) and interstitial water molecules. It can be seen from the asymmetric unit that both the Mo III and Mn II ions are heptacoordinate with slightly distorted pentagonal bipyrami- dal geometries. There are two types of Mn II ions in the structure: the Mn1 center is surrounded by five nitrogen atoms of the macrocyclic dpop ligand in the equatorial positions as well as one bridging CN  ion from the [Mo(CN) 7 ] 4 ion and one H 2 O molecule in the axial positions. This unit acts as a terminal capping group. In contrast, the Mn2, Mn3, and Mn4 centers are all bridged by two CN  groups in the axial positions and function as m-CN bridging units. The Mn N bond distances range from 2.21 to 2.35 , which are very close to the literature values for other Figure 1. a) The basic structural unit {[Mn(dpop)] 3 [Mn(dpop)(H 2 O)] [Mo(CN) 7 ] 2 } in 1 (H atoms are omitted for clarity). b) The 3D architecture of 1 viewed along the c direction. c) The basic structural unit {[Mn(dpop)] 2 [Mo(CN) 7 ]} in 2 (H atoms are omitted for clarity). d) The 3D architecture of 2 viewed along the c direction. e) Crystal-to- crystal transformation from 1 to 2. [*] Dr. Q.-L. Wang, H. Southerland, Dr. J.-R. Li, Dr. A.V. Prosvirin, Dr. H. Zhao, Prof. Dr. K. R. Dunbar Department of Chemistry, Texas A&M University College Station, TX 77842 (USA) E-mail: dunbar@mail.chem.tamu.edu Homepage: http://www.chem.tamu.edu/rgroup/dunbar/ Dr. Q.-L. Wang Department of Chemistry, Nankai University Tianjin, 300071 (P.R. China) [**] This work was supported by the U.S. Department of Energy (DE- FG02-02ER45999) and the National Natural Science Foundation of China (no. 21071085). Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/anie.201203309. A ngewandte Chemi e 1 Angew. Chem. 2012, 124,1–5  2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim These are not the final page numbers! Ü Ü