Magnetic Properties of High-Nuclearity Fex-oxo (x = 7, 22, 24) Clusters Analyzed by a Multipronged Experimental, Computational, and Magnetostructural Correlation Approach.

The synthesis, structure, and magnetic properties of three related iron(III)-oxo clusters are reported, [Fe7O3(O2CPh)9(mda)3(H2O)] (1), [Fe22O14(OH)3(O2CMe)21(mda)6](ClO4)2 (2), and [Fe24O15(OH)4(OEt)(O2CMe)21(mda)7](ClO4)2 (3), where mdaH2 is N-methyldiethanolamine. 1 was prepared from the reaction of [Fe3O(O2CPh)6(H2O)3](NO3) with mdaH2 in a 1:2 ratio in MeCN, whereas 2 and 3 were prepared from the reaction of FeCl3/NaO2CMe/mdaH2 in a 2:∼13:2 ratio and FeCl3/NaO2CMe/mdaH2/pyridine in a 2:∼13:2:25 ratio, respectively, both in EtOH. The core of 1 consists of a central octahedral FeIII ion held within a nonplanar Fe6 loop by three µ3-O2- and three µ2-RO- arms from the three mda2- chelates. The cores of the cations of 2 and 3 consist of an A:B:A three-layer topology, in which a central Fe6 (2) or Fe8 (3) layer B is sandwiched between two Fe8 layers A. The A layers structurally resemble 1 with the additional Fe added at the center to retain virtual C3 symmetry. The central Fe6 layer B of 2 consists of a {Fe4(µ4-O)2(µ3-OH)2}6+ cubane with an Fe on either side attached to cubane O2- ions, whereas that of 3 has the same cubane but with an {Fe3(µ3-O)(µ-OH)} unit attached on one side and a single Fe on the other. Variable-temperature dc and ac magnetic susceptibility studies revealed dominant antiferromagnetic coupling in all complexes leading to ground-state spins of S = 5/2 for 1 and S = 0 for 2 and 3. All Fe2 pairwise exchange parameters (Jij) for 1-3 were estimated by two independent methods: density functional theory (DFT) calculations using broken symmetry methods and a magnetostructural correlation previously developed for high-nuclearity FeIII/O complexes. The two approaches gave satisfyingly similar Jij values, and the latter allowed rationalization of the experimental ground states by identification of the spin frustration effects operative and the resultant relative spin vector alignments at each FeIII ion.

High-nuclearity, mixed-valence Mn17, Mn18 and {Mn62}n complexes from the use of triethanolamine.

The use of both azide and triethanolamine, with or without the presence of carboxylate groups, has provided new Mn(17), Mn(18) and {Mn(62)}(n) complexes with aesthetically-pleasing cage, layered, and linked-chain-type structures; two are also new single-molecule magnets.

Spin maximization from S = 11 to S = 16 in Mn(7) disk-like clusters: spin frustration effects and their computational rationalization.

The use has been explored in Mn cluster chemistry of N(3)(-) or Cl(-) in combination with N-methyldiethanolamine (mdaH(2)) or triethanolamine (teaH(3)). The reactions of Mn(ClO(4))(2).6H(2)O, NEt(3), NaN(3), and either mdaH(2) or teaH(3) (1:2:1:2) in DMF/MeOH afford {[Na(MeOH)(3)][Mn(7)(N(3))(6)(mda)(6)]}(n) (1) and {Na[Mn(7)(N(3))(6)(teaH)(6)]}(n) (2), respectively, whereas the 2:1:1 reaction of MnCl(2).4H(2)O, mdaH(2), and NEt(3) in MeCN gives (NHEt(3))[Mn(7)Cl(6)(mda)(6)] (3). Similar reactions using NBu(n)(4)N(3) in place of NaN(3) gave (NHEt(3))[Mn(7)(N(3))(6)(mda)(6)] (4) and (NHEt(3))[Mn(7)(N(3))(6)(teaH)(6)] (5). The Mn(7) anions consist of a Mn(6) hexagon of alternating Mn(II) and Mn(III) ions surrounding a central Mn(II) ion. The remaining ligation is by six bridging and chelating mda(2-) or teaH(2-) groups, and either six terminal N(3)(-) (1, 2, 4, 5) or Cl(-) (3) ions. Each bridging mda(2-) or teaH(2-) ligand contains both mu- and mu(3)-O atoms, resulting in a similar, near-planar [Mn(7)(mu(3)-OR)(6)(mu-OR)(6)](5+) core for all three complexes. The Mn(7) anions of 1 and 2 are connected via Na(+) cations to yield one-dimensional zigzag chains and three-dimensional windmill-like "hexagons-of-hexagons", respectively. In contrast, the Mn(7) anion of 3 forms a strong hydrogen-bond between the NHEt(3)(+) cation and a terminal Cl(-) ion giving a discrete ion-pair. Variable-temperature, solid-state direct current (dc) and alternating current (ac) magnetization studies were carried out in the 5.0-300 K range. Fits of dc magnetization versus field (H) and temperature (T) data by matrix diagonalization gave S = 11, g = 1.95, D = -0.15 cm(-1) for 1, S = 16, g = 1.95, D = -0.02 cm(-1) for 2, and S = 11, g = 1.92, D = -0.13 cm(-1) for 3 (D is the axial zero-field splitting parameter). Complexes 4 and 5 were also found to possess S = 11 and S = 16 ground states, respectively. The different ground states of 1 and 2 were rationalized on the basis of the sign and magnitude of the various Mn(2) exchange parameters obtained from density functional theory (DFT) calculations. This analysis confirmed the presence of spin frustration effects, with the ground states being determined by the relative magnitude of the two weakest interactions. The combined results demonstrate the usefulness of N-based dipodal and tripodal alkoxide-based chelates as a route to structurally and magnetically interesting Mn clusters.

Spin maximization: switching of the usual S = 11 state of Mn(II)4Mn(III)3 disklike complexes to the maximum S = 16.

The S = 11 ground states of the Mn 7 family of mixed-valence complexes with a metal-centered hexagonal topology have been found by density functional theory calculations to arise by spin frustration involving small differences in the magnitudes of the two weakest interactions controlling the alignment of the central spin. Targeted structural perturbation has allowed a complex with the central spin flipped to be discovered, which thus possesses the maximum S = 16 ground state.

"Switching on" the properties of single-molecule magnetism in triangular manganese(III) complexes.

The reaction between oxide-centered, triangular [MnIII3O(O2CR)6(py)3](ClO4) (R = Me (1), Et (2), Ph (3)) compounds and methyl 2-pyridyl ketone oxime (mpkoH) affords a new family of Mn/carboxylato/oximato complexes, [MnIII3O(O2CR)3(mpko)3](ClO4) [R = Me (4), Et (5), and Ph (6)]. As in 1-3, the cations of 4-6 contain an [MnIII3(mu3-O)]7+ triangular core, but with each Mn2 edge now bridged by an eta1:eta1:mu-RCO2- and an eta1:eta1:eta1:mu-mpko- group. The tridentate binding mode of the latter causes a buckling of the formerly planar [MnIII3(mu3-O)]7+ core, resulting in a relative twisting of the three MnIII octahedra and the central O2- ion now lying approximately 0.3 A above the Mn3 plane. This structural distortion leads to ferromagnetic exchange interactions within the molecule and a resulting S = 6 ground state. Fits of dc magnetization data for 4-6 collected in the 1.8-10.0 K and 10-70 kG ranges confirmed S = 6 ground states, and gave the following D and g values: -0.34 cm(-1) and 1.92 for 4, -0.34 cm(-1) and 1.93 for 5, and -0.35 cm(-1) and 1.99 for 6, where D is the axial zero-field splitting (anisotropy) parameter. Complexes 4-6 all exhibit frequency-dependent out-of-phase (chi" M) ac susceptibility signals suggesting them possibly to be single-molecule magnets (SMMs). Relaxation rate vs T data for complex 4 down to 1.8 K obtained from the chi" M vs T studies were supplemented with rate vs T data measured to 0.04 K via magnetization vs time decay studies, and these were used to construct Arrhenius plots from which was obtained the effective barrier to relaxation (Ueff) of 10.9 K. Magnetization vs dc field sweeps on single-crystals of 4.3CH2Cl2 displayed hysteresis loops exhibiting steps due to quantum tunneling of magnetization (QTM). The loops were essentially temperature-independent below approximately 0.3 K, indicating only ground-state QTM between the lowest-lying Ms = +/-6 levels. Complexes 4-6 are thus confirmed as the first triangular SMMs. High-frequency EPR spectra of single crystals of 4.3CH2Cl2 have provided precise spin Hamiltonian parameters, giving D = -0.3 cm(-1), B40 = -3 x 10(-5) cm(-1), and g = 2.00. The spectra also suggest a significant transverse anisotropy of E > or = 0.015 cm(-1). The combined work demonstrates the feasibility that structural distortions of a magnetic core imposed by peripheral ligands can "switch on" the properties of an SMM.

Four copper(II) pyrazolido complexes derived from reactions of 3{5}-substituted pyrazoles with CuF(2) or Cu(OH)(2).

Treatment of CuF(2) with 2 equiv of 3{5}-[pyrid-2-yl]pyrazole (Hpz(Py)), 3{5}-phenylpyrazole (Hpz(Ph)) or 3{5}-[4-fluorophenyl]pyrazole (Hpz(PhF)) in MeOH, followed by evaporation to dryness and recrystallisation of the solid residues, allows solvated crystals of [{Cu(micro-pz(Py))(pz(Py))}(2)] (1), [{Cu(micro-pz(Ph))(2)}(4)] (2) and [Cu(4)F(2)(micro(4)-F)(micro-pz(PhF))(5)(Hpz(PhF))(4)] (3) to be isolated in moderate-to-good yields. Similar reactions of these three pyrazoles with Cu(OH)(2) in refluxing MeOH respectively afford 1, 2 and [Cu(pz(PhF))(2)(Hpz(PhF))(2)] (4) in ca. 10% yield. Crystalline 1 x 1/2H(2)O x 2CHCl(3) contains two independent dinuclear molecules with a puckered di-(1,2-pyrazolido) bridge motif, linked by a bridging, hydrogen-bonding water molecule. Compound 2 x 1/2C(5)H(12), containing cyclic, square tetranuclear complex molecules, is the first homoleptic divalent metal pyrazolide to have a discrete molecular rather than polymeric structure, for a metal other than Pd or Pt. The two independent complex molecules in 3 x 3/4CH(2)Cl(2) x Hpz(PhF) contain a unique tetrahedral [Cu(4)(micro(4)-F)](7+) core, three of whose edges are spanned by bridging pyrazolido groups. Magnetic data show that the copper centres in 1 and 3 are antiferromagnetically coupled, but that dried bulk samples of 2 do not retain their molecular structure.

Initial example of a triangular single-molecule magnet from ligand-induced structural distortion of a [MnIII3O]7+ complex.

The reaction of [Mn3O(O2CR)6(py)3](ClO4) (R = Me, Et) with methyl 2-pyridyl ketone oxime (mpkoH) in a 1:3 molar ratio in MeOH/MeCN leads to [Mn3O(O2CR)3(mpko)3](ClO4) in 80-90% isolated yield. Ferromagnetic exchange interactions between the three MnIII ions in the nonplanar [MnIII3O]7+ triangular core lead to a spin ground state of S = 6; single-crystal studies reveal the temperature and sweep rate dependent hysteresis loops expected for a single-molecule magnet.

Direct determination of the anisotropy and exchange splittings in the dimeric single-molecule magnet [Mn4O3Cl4(O2CEt)3(py)3]2.8MeCN by inelastic neutron scattering.

Energy splittings resulting from anisotropy and exchange interactions in the dimer of single-molecule magnets [Mn4O3Cl4(O2CEt)3(py)3]2.8MeCN are determined for both an undeuterated and a partially deuterated sample using inelastic neutron scattering. The antiferromagnetic (AF) exchange coupling between the two Mn4 subunits strongly depends on their separation. The Cl...Cl distance between the two subunits can be modified either by exchanging the solvent of crystallization or by deuteration of the C-H...Cl hydrogen bonds. The exchange of acetonitrile for n-hexane leads to a five times greater shortening of the Cl...Cl separation than does full deuteration of all the hydrogen bonds. As a result, the AF exchange coupling constants between the subunits are 0.0073(4) and 0.0103(9) meV in the samples with acetonitrile and n-hexane solvent molecules, respectively, in the crystal structure. On the other hand, the effect of C-H...Cl deuteration on the AF exchange coupling is not detectable within the experimental accuracy of INS.

High-nuclearity homometallic iron and nickel clusters: Fe22 and Ni24 complexes from the use of N-methyldiethanolamine.

The use of N-methyldiethanolamine (mdaH2) in reactions with Fe(III) and Ni(II) sources has led to Fe22 and Ni24 products; the clusters are the highest and second-highest, respectively, homometallic clusters for these metals to date, and possess S = 0 and S = 6 ground states, respectively.