MAGNETIC materials of mesoscopic dimensions (a few to many thousands of atoms) may exhibit novel and useful properties such as giant magnetostriction, magnetoresistivity and magnetocaloric effects1–4. Such materials also allow one to study the transition from molecular to bulk-like magnetic behaviour. One approach for preparing mesoscopic magnetic materials is to fragment bulk ferromagnets; a more controllable method is to take a 'bottom-up' approach, using chemistry to grow well defined clusters of metal ions5,6. Lis7 has described a twelve-ion manganese cluster in which eight of the Mn ions are in the +3 oxidation state (spin S=2) and four are in the +4 state (S=3/2). These ions are magnetically coupled to give an S=10 ground state8, giving rise to unusual magnetic relaxation properties8,9. Here we report that the magnetization of the Mn12 cluster is highly anisotropic and that the magnetization relaxation time becomes very long below a temperature of 4 K, giving rise to pronounced hysteresis. This behaviour is not, however, strictly analogous to that of a bulk ferromagnet, in which magnetization hysteresis results from the motion of domain walls. In principle, a bistable magnetic unit of this sort could act as a data storage device.

Magnetic bistability in a metal-ion cluster

http://yaghi.berkeley.edu/pdfPublications/04MOFs.pdf

Metal-organic frameworks: a new class of porous materials

The purpose of this critical review is to give a representative and comprehensive overview of the arising developments in the field of magnetic metal–organic frameworks, in particular those containing cobalt(II). We examine the diversity of magnetic exchange interactions between nearest-neighbour moment carriers, covering from dimers to oligomers and discuss their implications in infinite chains, layers and networks, having a variety of topologies. We progress to the different forms of short-range magnetic ordering, giving rise to single-molecule-magnets and single-chain-magnets, to long-range ordering of two- and three-dimensional networks (323 references).

Magnetic metal-organic frameworks.

Basic concepts of computer topological analysis of crystal structures realized in the current version of the program package ToposPro are considered. Applications of the ToposPro methods to various classes of chemical compounds—coordination polymers, molecular crystals, supramolecular ensembles, inorganic ionic compounds, intermetallics, fast-ion conductors, microporous materials—are illustrated by many examples. It is shown that chemically and crystallographically different structures can be automatically treated in a similar way with the ToposPro approaches.

http://pubs.acs.org/doi/pdf/10.1021/cg500498k

Applied Topological Analysis of Crystal Structures with the Program Package ToposPro

/pdf/lanthanide-single-molecule-magnets-efv9mucvem.pdf

Lanthanide single-molecule magnets.

The syntheses and electrochemical and magnetochemical properties of [Mn 12 O 12 (O 2 CPh) 16 (H 2 O) 4 ] (3), its solvate 3.PhCOOH-CH 2 Cl 2 , and [Mn 12 O 12 (O 2 CMe) 16 (H 2 O) 4 ].MeCOOH-3H 2 O (4) are reported. Complex 3 can be prepared either by reaction of Mn(OAc) 2 .4H 4 O, benzoic acid, and NBu n 4 MnO 4 in pyridine or by reaction of PhCOOH with complex 4 slurried in CH 2 Cl 2 .

High-spin molecules: [Mn12O12(O2CR)16(H2O)4]

High-Spin Molecules: Unusual Magnetic Susceptibility Relaxation Effects in [Mn12O12(O2CEt)16(H2O)3] (S = 9) and the One-Electron Reduction Product (PPh4)[Mn12O12(O2CEt)16(H2O)4] (S = 19/2)

Crystal Engineering: Toward Intersecting Channels from a Neutral Network with a bcu‐Type Topology

The reaction of MnCl2·4H2O (1 equiv), salicyladoxime (H2salox) (1 equiv), and NaN3 (1 equiv) in MeOH with NEt4OH (1 equiv) gave (NEt4)3[Mn5O(salox)3(N3)6Cl2] (2) in ∼14% yield. Ferromagnetic exchange between the five Mn ions in 2 leads to an S = 11 ground state and is established as a new single-molecule magnet with an anisotropy energy, Ueff, of 40.3 K by frequency dependent out-of-phase alternating current susceptibility signals and temperature and sweep rate dependent hysteresis loops.

A pentanuclear manganese single-molecule magnet with a large anisotropy.

We have calculated the electronic structures of five different manganese-oxo dimer complexes using density functional methods combined with the broken symmetry and spin projection concepts. The number of carboxylate, oxo, and peroxo bridging ligands was varied, and the terminal ligands were triazacyclononane (TACN). The formal Mn oxidation states varied from Mn(III)(2) and Mn(III)Mn(IV) to Mn(IV)(2). These complexes have been synthesized and their X-ray structures and magnetic properties measured previously. We have calculated the Heisenberg spin coupling parameters J and resonance delocalization parameters B for all of these systems. Despite the very small energy differences involved, there is a good correspondence between calculated and experimental Heisenberg J parameters. We have analyzed potential changes in the calculated effective Heisenberg coupling J(eff) for the mixed-valence Mn(III)Mn(IV) complexes when partial or complete delocalization due to the B parameter is taken into account. These changes depend also on the energy of the relevant intervalence band. Surprisingly, in the two mixed-valence systems studied, the high spin S = (5)/(2) state lies below S = (7)/(2). This is consistent with spin coupling between Mn with site spins S(1) = 1, S(2) = (3)/(2), corresponding to intermediate spin Mn(I) and Mn(II) respectively, instead of the coupling expected from the formal oxidation states, S(1) = 2, S(2) = (3)/(2) from high spin Mn(III) and Mn(IV). The spin and charge distributions in the broken symmetry ground states are also consistent with intermediate spin S(1) = 1, S(2) = (3)/(2). The calculated charge distributions show strong metal-ligand covalency. In fact, as the formal oxidation states of the Mn sites increase, the net Mn charges generally show a slow decrease, consistent with a very strong ligand --> metal charge transfer, particularly from m-oxo or m-peroxo ligands. TACN is a better donor ligand than carboxylate, even when calculated on a per donor atom basis. The ligand atom charge transfer order is peroxo >/= oxo >> TACN > acetate. The TACN > acetate ordering is expected from the spectrochemical series, but the strong charge transfer and strong metal-ligand covalency of peroxo and oxo ligands with the Mn sites cannot be simply related to their positions in the spectrochemical series. In the Mn(IV)(2)(m-O)(2)(m-O(2))(TACN)(2), each peroxo oxygen has a small charge (-0.3), much less than found for each m-O atom (-0.7). The high-spin S = 3 state lies quite low in energy, 8 kcal/mol from our calculations and about 4 kcal/mol based on the experimental Heisenberg spin coupling parameters. Potential molecular oxygen dissociation pathways involving a spin S = 1 state are discussed. Effective ligand field diagrams are constructed from the calculated energy levels which display the competition between spin polarization splitting and the ligand field t(2g)-e(g) splitting and allow comparisons of electronic structure among different complexes. The electronic structure and spin coupling of these complexes was also compared to the corresponding "core-only" complexes where both TACN ligands were removed, yielding a far weaker ligand field. There is a strong ferromagnetic shift in the "core-only" complexes compared with the complete TACN complexes, also showing the effects of a weaker ligand field.

Hui Lien Tsai

Papers

High-spin molecules: [Mn12O12(O2CR)16(H2O)4]

High-Spin Molecules: Unusual Magnetic Susceptibility Relaxation Effects in [Mn12O12(O2CEt)16(H2O)3] (S = 9) and the One-Electron Reduction Product (PPh4)[Mn12O12(O2CEt)16(H2O)4] (S = 19/2)

Crystal Engineering: Toward Intersecting Channels from a Neutral Network with a bcu‐Type Topology

A pentanuclear manganese single-molecule magnet with a large anisotropy.

Density Functional Calculations of Electronic Structure, Charge Distribution, and Spin Coupling in Manganese-Oxo Dimer Complexes