Hui-Lien Tsai

Manganese carboxylate clusters: From structural aesthetics to single-molecule magnets

Abstract An overview is provided of some recent developments in manganese carboxylatecluster chemistry. A variety of synthetic methodologies are described together with the structuresof the resultant products, which span metal nuclearities of 4 to 18 and oxidation states of II to IV,including mixed-valency. The spins of the ground states of these products are often large andsometimes abnormally large, and in certain cases when this is coupled to a sufficiently largemagnetoanisotropy, the clusters function as single-molecule magnets i.e., they can be magnetizedby an external magnetic field below a critical temperature. These complexes display hysteresis inmagnetization vs . magnetic field studies, and clear evidence for quantum tunnelling ofmagnetization. Such results establish single-molecule magnetism as a new magnetic phenomenon,and one that holds great promise for next-century technological applications.

Single-Molecule Magnets: Characterization of Complexes Exhibiting Out-of-Phase AC Susceptibility Signals

Abstract The structures and characterization are described of complexes that exhibit an out-of-phase AC magnetic susceptibility x m” The dodecanuclear complexes [Mn12O12(O2CR)16(H2O)4]z contain 8MnIII, 4MnIV or MnII, 7MnIII, 4MnIV and possess ground state spins of S = 9 or 10 (z = 0) or S = 19/2 (z = −1). The distorted-cubane complexes [Mn4O3X(O2CR)3(dbm)3] contain 3MnIII, MnIV and have been prepared with a variety of X groups; in every case, the ground state is S = 9/2. The anion [Mn11O10X6(O2CPh)11(H2O)4]2− (9MnIII, 2MnIV; X = NCO−, N3 −) contains two [Mn4O3X] distorted-cubanes linked by a [Mn3O4]+ unit; the ground state is S = 5. The cation [V4O2(O2CEt)7(bpy)2]+ (4VIII) has a [V4O2]8+ butterfly core and a S = 3 ground state. All these complexes exhibit out-of-phase AC magnetic susceptibility signals at low temperatures (<8 K). These signals are indicative of slow relaxation of the magnetization, unable to keep in phase with the oscillating AC magnetic field, and are taken as evidence for single-molecul...

[Mn3O(O2CPh)6(py)2]2(4,4′-bpy) and [Mn9O7(O2CC6H4-p-OMe)13 (4,4′-bpy)]2: new multinuclear manganese complexes

Abstract The synthesis, X-ray crystal structures and magnetic properties of two new types of dimers of multinuclear manganese aggregates are described. [Mn 3 O(O 2 CPh) 6 (py) 2 ] 2 (4,4′-bpy) is prepared from the reaction of the trinuclear starting material [Mn 3 O(O 2 CPh) 6 (py) 2 (H 2 O)] with 4,4′-bipyridine. This compound can be described as a dimer of trinuclear units and has been characterized crystallographically as part of a unit cell which also contains a monomeric, trinuclear species. Dimers of enneanuclear manganese complexes linked by two bifunctional units (4,4′-bipyridine or trans -1,2-bis(4-pyridylethene)) may be prepared from the tetranuclear starting material, (su0)Bu 4 N)[Mn 4 O 2 -(O 2 CR) 9 (H 2 O)], where R = Ph or C 6 H 4 - p -OMe. The new octadecanuclear complex [Mn 9 O 7 (O 2 CC 6 H 4 - p -OMe) 13 (4,4′-bpy)] 2 has been structurally characterized. This complex shows magnetic behavior which can be fit by considering only magnetic interactions within the individual Mn 9 O 7 units.

Variation in the electron count and ground state of [Mn12O12(O2CR)16(H2O)4](R = Me or Ph) by metal substitution and redox changes: preparation and properties of [Mn8Fe4O12(O2CMe)16(H2O)4]·4H2O·2MeCO2H and [NPrn4][Mn12O12(O2CPh)16(H2O)4]·H2O

As a means of varying the electron count of complexes [Mn12O12(O2CR)16(H2O)4](1; R = Me or Ph), the aggregates [Mn8Fe4O12(O2CMe)16(H2O)4]2 and [NPrn4][Mn12O12(O2CPh)16(H2O)4]3 have been prepared; magnetochemical measurements confirm that the unusual of properties 1 can be significantly altered by ‘conversion’ to 2 and 3.

Covalent linkage of [Mn4O2(O2CPh)6(dbm)2] into a dimer and a one-dimensional polymer (dbmH = dibenzoylmethane)

Methods have been developed for covalently linking [Mn4O2(O2CPh)6(dbm)2]2(dbmH = dibenzoylmethane) units to give a dimer and a polymer; treatment of 2 with trans-1,2-bis(4-pyridyl)ethene (bpe) or 4,4′-bipyridine (4,4′-bpy) yields [Mn4O2(O2CPh)6(dbm)2(bpe)]24 and [Mn4O2(O2CPh)6(dbm)2(4,4′-bpy)]n5, respectively.

High Spin Molecules: Unusual Magnetic Susceptibility Relaxation Behavior of a Dodecanuclear Manganese Aggregate in Two Oxidation States

Abstract The mixed valence dodecanuclear complex [Mn12O12(O2CEt) 16(H2O)3 may be reduced by one electron to give (PPh4)[Mn12O12(O2CEt)16(H2O)4. Reduction of the complex occurs at one MnIII ion to give an unusual trapped-valence MnIIMnIII 7MnIV 4 complex. This formulation is supported by X-ray crystallography at both low temperature and room temperature. DC (2.0–4.0K, 20–50 kG fields) and AC (2.0–30 K, 1.0 and 0.0050G fields, 25–1000 Hz frequencies) magnetic susceptibility measurements indicate ground states of S=9 and S=19/2 for the neutral and reduced species, respectively. (PPh4)[Mn12O122(O2CEt)16(H2O)4 exhibits an out-of-phase component of the AC magnetic susceptibility (X M) which has a frequency dependent maximum. This is a very unusual property for a discrete molecular species to display. The neutral analog shows two peaks in the X M vs. T plot. DC measurements also show the presence of prominent hysteresis loops, and field-cooled and zero-field-cooled magnetization data are not superimposed at low ...

High Spin Molecules: A Structural and Magnetic Comparison of High Nuclearity Manganese Carboxylate Aggregates

Abstract A structural and magnetic comparison is presented for a number of high nuclearity, higher oxidation state manganese complexes with similar ligation environments. The complexes considered are hepta-, octa-, ennea-, deca-, and dodecanuclear and contain only MnIII, except for the dodecanuclear complexes, which are mixed valence (MnIII 8MnIV 4). The ground state spins of these complexes range from very low to very high, and while the dodecanuclear complexes exhibit unusual AC magnetic susceptibility behavior, none of the other complexes show such properties. Magnetic properties, as well as structural and preparative information, are discussed.

Superparamagnetic-like properties of the valence-trapped MnIIMnIII7MnIV4 anion in the salt (PPh4)[Mn12O12(O2CEt)16(H2O)4]

Reduction of the 8MnIII,4MnIV complex [Mn12O12(O2CEt)16(H2O)3] with PPh4l in CH2Cl2 leads to isolation of (PPh4)[Mn12O12(O2CEt)16(H2O)4](3) whose crystal structure shows that the added electron is localized on a formerly MnIII ion to give a trapped-valence MnII,7MnIII,4MnIV situation; DC and AC magnetic susceptibility studies show that 3 has an S= 19/2 ground state and that it displays a superparamagnet-like non-zero, frequency-dependent, out-of-phase response in the AC susceptibility behaviour that is extremely unusual for a molecular species and unique for an ionic species.

High nuclearity molecular species exhibiting spin frustration : fusion of two MnIII4O2 butterfly complexes to yield an intermediate spin ground state MnIII7O4 complex

The preparation and characterization of (NEt4)[Mn7O4(OAc)10(dbm)4]·3CH2Cl2·2C6H14(1·3CH2Cl2·2C6H14)(H dbm = dibenzoylmethane)are reported; the MnIII7O4 core of 1 consists of two Mn4O2 butterfly units fused together by sharing of one wing-tip manganese atom and it is shown using magnetization data that complex 1 has either an S= 3 or S= 4 ground state.

High spin molecules: (NBun4)2[Mn8O4(O2CPh)12(Et2mal)2(H2O)2], a mixed-valence managenese(II/III) aggregate with dicarboxylate ligation, an unusual linked-butterfly structure, and an S= 3 ground state

Treatment of (NBun4)[Mn4O2(O2CPh)9(H2O)] with Na2(Et2mal) in MeCN leads to a nuclearity and oxidation level change to give (NBun4)2[Mn8O4(O2CPh)12(Et2mal)2(H2O)2]1; the crystal structure and magnetochemical studies show the core of the anion to have a unique topology consisting of two linked [Mn4O2] butterfly units and to possess an S = 3 ground state, respectively.