High-spin Mn4 and Mn10 molecules: large spin changes with structure in mixed-valence MnII4MnIII6 clusters with azide and alkoxide-based ligands

The use has been explored of both azide (N3-) and alkoxide-containing groups such as the anions of 2-(hydroxymethyl)pyridine (hmpH), 2,6-pyridinedimethanol (pdmH2), 1,1,1-tris(hydroxymethyl)ethane (thmeH3) and triethanolamine (teaH3) in Mn cluster chemistry. The 1:1:1:1 reactions of hmpH, NaN3 and NEt3 with Mn(ClO4)(2).6H 2O or Mn(NO3)2.H2O in MeCN/MeOH afford [MnII4MnIII6O4(N3)4(hmp)12](X)2 [X=ClO4- (1), N3- (2)]. The [Mn10(mu4-O) 4(mu3-N3)4]14+ core of the cation has a tetra-face-capped octahedral topology, with a central MnIII6 octahedron, whose eight faces are bridged by four mu 3-N3- and four mu 4-O2- ions, the latter also bridging to four extrinsic MnII atoms. The core has Td symmetry, but the complete [MnII4MnIII6O4(N3)4(hmp)12]2+ cation has rare T symmetry, which is crystallographically imposed. A similar reaction of Mn(ClO4) (2).6H2O with one equiv each of NaN3, thmeH3, pdmH2, and NEt3 in MeCN/MeOH led to [MnII4MnIII6O2(N3)6(pdmH)4(thme)4] (3). Complex 3 is at the same oxidation level as 1/2 but its core is structurally different, consisting of two edge-fused [MnII2MnIII4(mu4-O)]14+ octahedra. Replacement of thmeH3 with teaH3 in this reaction gave instead [MnII2MnIII2(N3)4(pdmH)2(teaH)2] (4), containing a planar Mn 4 rhombus. Variable-temperature, solid-state dc and ac magnetization studies were carried out on 1-4 in the 5.0-300 K range. Complexes 1 and 2 are completely ferromagnetically coupled with a resulting S=22 ground state, one of the highest yet reported. Fits of dc magnetization vs field (H) and temperature (T) data by matrix diagonalization gave S=22, g=2.00, and D approximately 0.0 cm(-1) (D is the axial zero-field splitting parameter). In contrast, the data for 3 revealed dominant antiferromagnetic interactions and a resulting S=0 ground state. Complex 4 contains weakly ferromagnetically coupled Mn atoms, leading to an S=9 ground-state and low-lying excited states, and exhibits out-of-phase ac susceptibility signals characteristic of a single-molecule magnet. Theoretical values of the exchange constants in 1 obtained with density functional theory and ZILSH calculations were in good agreement with experimental values. The combined work demonstrates the synthetic usefulness of alcohol-based chelates and azido ligands when used together, and the synthesis in the present work of two "isomeric" MnIII6MnII4 cores that differ in spin by a remarkable 22 units.     

Manganese clusters derived from a 2,6-diacetylpyridine dioximato ligand: structure and magnetic study

Reactions of 2,6-diacetylpyridine dioxime (dapdoH?) with Mn(NO?)? or Mn(SO?CF?)? under a variety of conditions or co-ligands yield compounds with the formula [Mn?O?(OMe)?(dapdo)?(dapdoH)?](X)? in which X = NO?? (1) or SO?CF?? (2), [Mn?O?(dapdo)?(NO?)?]·H?O (3) and [Mn(dapdoH?)(N?)?](n) (4). Compounds 1, 3 and 4 were structurally characterized and equivalent structures for 1 and 2 were inferred from spectroscopic and analytical results. Compounds 1 and 2 consist of hexanuclear Mn?(II)Mn?(III) complexes whereas 3 consists of an octanuclear Mn?(II)Mn?(III) cluster in which the manganese atoms exhibit a rare bicapped elongated octahedral topology. Compound 4 consists of a 1D system bridged by double end-on azido ligands. Variable temperature magnetic studies were performed between 2-300 K, confirming the ground state S = 5 for 1 and 2, S = 0 for 3 and ferromagnetic response for 4.     

Mn7O5(OR)2(O2CPh)9(terpy)] (R = Me, CH2Ph) complexes with a fused cubane/butterfly core and an S = 6 ground-state spin

Two new heptanuclear Mn clusters, [Mn7O5(OMe)2(O2CPh)9(terpy)] (1) and [Mn7O5(OCH2Ph)2(O2CPh)9(terpy)] (2), were prepared from the partial alcoholysis of the trinuclear complex [Mn3O(O2CPh)6(py)2(H2O)] (3) in the presence of terpy (terpy = 2,2':6',2' '-terpyridine). Complexes 1 and 2 crystallize in the triclinic P and the orthorhombic Pbca space groups, respectively. The clusters are both mixed valent, containing three Mn oxidation states: MnIV, 5MnIII, and MnII. The Mn ions are held together by nine doubly bridging benzoates, four mu3-O2- ions, one mu5-O2- ion, and either two mu-MeO- (1) or two mu-PhCH2O- (2) groups. The single terpy chelate in each complex is attached to the MnII ion. The core topology is novel and very unusual, comprising a cubane and a butterfly unit fused by sharing a MnIII and the mu5-O2- ion. Solid-state dc and ac magnetic susceptibility studies establish that complexes 1 and 2 both possess an S = 6 ground-state spin. Fits of variable-temperature and -field magnetization data gave S = 6, g = 1.88, and D = -0.21 cm-1 for 1 and S = 6, g = 1.86, and D = -0.18 cm-1 for 2. Single-crystal magnetization vs dc field scans down to 0.1 K for 2 show only very little hysteresis at 0.1 K.     

An oximate-based hexanuclear mixed-valence Mn(III)4Mn(II)2 edge-sharing bitetrahedral core with an St = 5 spin ground state

The synthesis, structures and magnetic properties of two hexanuclear Mn(6) clusters are reported: Mn(6)(mu(4)-O)(2)(dapdo)(2)(dapdoH)(4)(mu(2)-OH)(2)](ClO(4))(2).6MeCN (.6MeCN) and [Mn(6)(mu(4)-O)(2)(dapdo)(2)(dapdoH)(4)(mu(2)-OCH(3))(2)](ClO(4))(2).2Et(2)O (.2Et(2)O) [dapdo(2-) is the dianion of 2,6-diacetylpyridine dioxime and dapdoH(-) is the monoanion of the aforesaid dioxime ligand]. Both complexes are mixed-valent with two Mn(II) and four Mn(III) atoms disposed in an edge-sharing bitetrahedral core. Both complexes and display the same [Mn(III)(4)Mn(II)(2)(mu(4)-O)(2)(mu(2)-OR)(2)](10+) core in which R = H for and R = Me for . The [Mn(III)(4)Mn(II)(2)] core is rather uncommon compared to the reported [Mn(III)(2)Mn(II)(4)] core in the literature. DC magnetic susceptibility measurements on and reveal the presence of competing exchange interactions resulting in an S(t) = 5 ground spin state. The magnetic behavior of the compounds indicates antiferromagnetic coupling between the manganese(iii) centers, whereas the coupling between the manganese(iii) and manganese(ii) is weakly antiferromagnetic or ferromagnetic depending on the bridging environments. Finally the interaction between the manganese(ii) centers from the two fused tetrahedra is weakly ferromagnetic in nature stabilizing S(t) = 5 ground spin state in compounds and .     

New structural types and different oxidation levels in the family of Mn6-oxime single-molecule magnets

The synthesis and magnetic properties of three new members of a family of salicyaldoxime based [Mn6] single-molecule magnets possessing new structural types, core topologies and Mn oxidation state distributions are reported. The isostructural complexes [MnIII6O2(R-sao)6(X)2(EtOH)6] (R = Et, X = Br (1); R = Me, X = I (2)) exhibit single-molecule magnet behaviour with spin Hamiltonian parameters S = 12, g = 1.98 and D = -0.36 cm(-1) in both cases. The hexametallic cluster [MnIII4MnIV2O2(OMe)(4-)(Et-sao)6(MeOH)2].MeOH (3.MeOH) possesses a planar rod-like topology and a mixed valent [MnIV4MnIII2] core, which is unprecedented in this family of [Mn6] SMMs.     

From pseudo to true C(3) symmetry: magnetic anisotropy enhanced by site-specific ligand substitution in two Mn(15)-carboxylate clusters

Two new mixed-valence manganese-carboxylate clusters, [MnIII9MnIV6(O2CPh)12(micro3-O)13(micro-O)4(micro-OMe)5(MeOH)4(H2O)5]2.1.5PhCO2H.MeOH.6H2O (1, PhCO2H = benzoic acid) and [MnIII9MnIV6(O2CCh)12(micro3-O)13(micro-O)4(micro-OMe)5(MeOH)3(H2O)6].0.5MeOH.2.5H2O (2, ChCO2H= cyclohexanecarboxylic acid) contain new disklike Mn15 cores. Both 1 and 2 can be synthesized by the conventional manganese redox reaction (MnO4- oxidizing Mn2+) in methanol solution. 2 can be also synthesized via the site-specific ligand substitution reaction from 1. 1 crystallizes in the triclinic space group P, whereas 2 crystallizes in the trigonal space group P. Magnetic study shows that both 1 and 2 have the same ground spin states ST = 2. Compared to the silence of the out-of-phase ac susceptibility of 1, 2 shows clearly slow magnetic relaxation behavior above 1.8 K due to the dramatically enhanced axial magnetic anisotropy (D = -0.89 and -1.58 cm-1 for 1 and 2, respectively, which was obtained by fitting the plots of M vs H/T with the program ANISOFIT 2.0).     

Ni5, Ni8, and Ni10 clusters with 2,6-diacetylpyridine-dioxime as a ligand

In the present work, novel coordination possibilities for the system dapdoH(2)/Ni(II) (dapdoH(2) = 2,6-diacetylpyridine-dioxime) have been explored. Depending on the starting reagents and solution conditions, several clusters with nuclearities ranging from Ni(5) to Ni(10) were achieved and structurally characterized, namely, [Ni(5)(R-COO)(2)(dapdo)(2)(dapdoH)(2)(N(CN)(2))(2)(MeOH)(2)] in which R-COO(-) = benzoate (1) or 3-chlorobenzoate (2), [Ni(8)(dapdo)(4)(NO(3))(4)(OH)(4)(MeOH)(4)] (3), and [Ni(10)(dapdo)(8)(N(CN)(2))(2)(MeO)(MeOH)](NO(3)) (4). For the first time, pentadentate coordination for the dapdo(2-) ligand has been established. All compounds show a combination of square-planar and octahedrally coordinated nickel atoms. According to the Ni(2)(sp)Ni(3)(Oh) (1 and 2), Ni(4)(sp)Ni(4)(Oh) (3), and Ni(4)(sp)Ni(6)(Oh) (4) environments, these systems magnetically behave as trimer, tetramer, and hexanuclear clusters, respectively. dc magnetic measurements in the 2-300 K range of temperature reveal antiferromagnetic coupling for all compounds, and the correlation of the superexchange interaction with the torsion angles involving the oximato bridges is experimentally confirmed.     

A family of Mn16 single-molecule magnets from a reductive aggregation route

The synthesis and magnetic properties of three isostructural hexadecametallic manganese clusters [Mn(16)O(16)(OMe)(6)(O(2)CCH(2)Ph)(16)(MeOH)(6)] (1), [Mn(16)O(16)(OMe)(6)(O(2)CCH(2)Cl)(16)(MeOH)(6)] (2), and [Mn(16)O(16)(OMe)(6)(O(2)CCH(2)Br)(16)(MeOH)(6)] (3) are reported. The complexes were prepared by a reductive aggregation reaction involving phenylacetic acid, chloroacetic acid or bromoacetic acid, and NBu(n)()(4)MnO(4) in MeOH. Complex 1 crystallizes in the monoclinic space group C2/c and consists of 6 Mn(IV) and 10 Mn(III) ions held together by 14 mu(3)-O(2)(-), 2 mu-O(2)(-), 4 mu-MeO(-), and 2 mu-O(2)CCH(2)Ph(-) groups. The remaining 14 mu-O(2)CCH(2)Ph(-) ligands, 2 mu-MeO(-) groups, and 6 terminal MeOH molecules constitute the peripheral ligation in the complex. Variable-temperature, solid-state dc magnetic susceptibility measurements on 1-3 in the temperature range 5.0-300 K reveal that all three complexes are dominated by intramolecular antiferromagnetic exchange interactions. Low-lying excited states preclude an exact determination of the spin ground state for 1-3 by magnetization measurements. Alternating current susceptibility measurements at zero dc field in the temperature range 1.8-10 K and a 3.5 G ac field oscillating at frequencies in the 5-1488 Hz range display, at temperatures below 3 K, a nonzero, frequency-dependent chi(M)"signal for complexes 1-3, with the peak maxima lying at temperatures less than 1.8 K. Single-crystal magnetization versus dc field scans down to 0.04 K for complex 1 show hysteresis behavior at <1 K, establishing 1 as a new member of the SMM family. No clear steps characteristic of quantum tunneling of magnetization (QTM) were observed in the hysteresis loops.     

Single-molecule magnets: a reductive aggregation route to new types of Mn12 complexes

Three dodecanuclear Mn clusters [Mn12O10(OMe)3(OH)(O2CC6H3F2)16(MeOH)2].8MeOH (1), [Mn12O10(OMe)4(O2CBu(t))16(MeOH)2] (2), and [Mn12O12(O2CBu(t))16(MeOH)4] (3) synthesized by reductive aggregation reactions are reported. Clusters 1 and 2 possess a central alkoxide-bridged planar Mn4 topology, whereas 3 is a new high-symmetry member of the normal Mn12 family. Complexes 1 and 2 crystallize in the monoclinic space groups C2/c and P2(1)/n, respectively. Both consist of four Mn(IV) and eight Mn(III) ions held together by 10 mu3-O2- ions, and either (i) one mu-OH- and three mu-MeO- groups for 1 or (ii) four mu-MeO- groups for 2. Complex 3 crystallizes in the orthorhombic space group Aba2 and possesses the normal Mn12 structure but with terminal MeOH molecules. The cyclic voltammogram (CV) of 1 exhibits no reversible redox processes. Variable-temperature, solid-state dc and ac magnetic susceptibility measurements on 1 and 2 reveal that they possess S = 5 and 9 ground states, respectively. In addition, ac susceptibility measurements on complex 1 in a zero dc field in the temperature range 1.8-10 K and in a 3.5 G ac field oscillating at frequencies in the 5-1488 Hz range display a nonzero frequency-dependent out-of-phase (chi(M)') signal at temperatures below 3 K, with the peak maxima lying at temperatures below 1.8 K. For complex 2, two frequency dependent chi(M)' signals are seen, one in the higher temperature range of 3-5 K and a second at lower temperatures with its peak maxima at temperatures below 1.8 K. Single-crystal magnetization vs dc field scans down to 0.04 K for 1.8MeOH and 2 show hysteresis behavior at <1 K, confirming that both complexes are new examples of SMMs.     

High nuclearity single-molecule magnets: a mixed-valence Mn26 cluster containing the di-2-pyridylketone diolate dianion

The employment of the dianion (dpkd(2-)) of the gem-diol form of di-2-pyridylketone (dpk) as a tetradentate chelate in manganese chemistry is reported, and the synthesis, crystal structure, and magnetochemical characterization of [Mn26O16(OMe)12(dpkd)12(MeOH)6](OH)6 x solv (3 x solv) are described. The reaction of Mn(ClO4)2 x 6 H2O, dpk, NaOMe, and NEt3 (2:1:4:2) in MeCN/MeOH affords complex 3, which possesses a rare metal topology and is mixed-valence (4 Mn(II), 22 Mn(III)). The complicated [Mn26(mu4-O)10(mu3-O)6(mu3-OMe)12(mu-OR)12](18+) core of 3 consists of an internal Mn(III)16 cage of adjacent Mn4 tetrahedra surrounded by an external Mn(II)4Mn(III)6 shell. The latter is held together by the alkoxide arms of twelve eta(1):eta(2):eta(1):eta(1):mu3 dpkd(2-) groups. Variable-temperature, solid-state direct current (dc), and alternating current (ac) magnetization studies were carried out on 3 in the 1.8-300 K range. Complex 3 is predominantly antiferromagnetically coupled with a resulting S = 6 ground state, a conclusion confirmed by the in-phase (chi'(M)) ac susceptibility data. The observation of out-of-phase (chi'(M)) ac susceptibility signals suggested that 3 might be a single-molecule magnet, and this was confirmed by single-crystal magnetization vs dc field sweeps that exhibited hysteresis, the diagnostic property of a magnet. Combined ac chi'(M) and magnetization decay vs time data collected below 1.1 K were used to construct an Arrhenius plot; the fit of the thermally activated region above approximately 0.1 K gave U(eff) = 30 K, where U(eff) is the effective relaxation barrier. At lower temperatures, the complex exhibits temperature-independent relaxation, characteristic of ground-state quantum tunneling of magnetization between the lowest-lying M(s) = +/-6 levels. The combined work demonstrates the ligating flexibility of dipyridyl-diolate chelates and their usefulness in the synthesis of polynuclear Mn(x) clusters with interesting magnetic properties, without requiring the co-presence of carboxylate ligands.     

High-nuclearity Ce/Mn and Th/Mn cluster chemistry: preparation of complexes with [Ce4Mn10O10(OMe)6]18+ and [Th6Mn10O22(OH)2]18+ cores

The syntheses, structures, and magnetic properties are reported of the mixed-metal complexes [Ce4Mn10O10(OMe)6(O2CPh)16(NO3)2(MeOH)2(H2O)2] (1) and [Th6Mn10O22(OH)2(O2CPh)16-(NO3)2(H2O)8] (2), which were both prepared by the reaction of (NBun4)[Mn4O2(O2CPh)9(H2O)] (3) with a source of the heterometal in MeCN/MeOH. Complexes 1 and 2 crystallize in the monoclinic space group C2/c and the triclinic space group P, respectively. Complex 1 consists of 10 MnIII, 2 CeIII, and 2 CeIV atoms and possesses a very unusual tubular [Ce4Mn10O10(OMe)6]18+ core. Complex 2 consists of 10 MnIV and 6 ThIV atoms and possesses a [Th6Mn10O22(OH)2]18+ core with the metal atoms arranged in layers with a 2:3:6:3:2 pattern. Peripheral ligation around the cores is provided by 16 bridging benzoates, 2 chelating nitrates, and either (i) 2 each of terminal H2O and MeOH groups in 1 or (ii) 8 terminal H2O groups in 2. Complex 1 is the largest mixed-metal Ce/Mn cluster and the first 3d/4f cluster with mixed-valency in its lanthanide component, while complex 2 is the first Th/Mn cluster and the largest mixed transition metal/actinide cluster to date. Solid-state dc and ac magnetic susceptibility measurements on 1 and 2 establish that they possess S = 4 and 3 ground states, respectively. Ac susceptibility studies on 1 revealed nonzero frequency-dependent out-of-phase (chiM' ') signals at temperatures below 3 K; complex 2 displays no chiM' ' signals. However, single-crystal magnetization vs dc field scans at variable temperatures and variable sweep-rates down to 0.04 K on 1 revealed no noticeable hysteresis loops, except very minor ones at 0.04 K assignable to weak intermolecular interactions propagated by hydrogen bonds involving CeIII-bound ligands. Complex 1 is thus concluded not to be a single-molecule magnet (SMM), and the combined results thus represent a caveat against taking such ac signals as sufficient proof of a SMM.     

Trapping an elusive intermediate in manganese-oxo cubane chemistry

A new member of the Mn-oxo cubane core complex family [Mn2III,2IV4O4L6] (1), where L = (p-MeOPh)2PO2-, has been synthesized and characterized. Compound 1 possesses structurally inequivalent MnIII and MnIV with clear valence electron localization in the crystal phase, quite unlike the structurally equivalent sites, tetrahedral core symmetry, and delocalized valence of its analogue where L = Ph2PO2-. Compound 1 exhibits appreciable positive shifts (0.1-0.3 V) of both the oxidation and reduction electrochemical potentials, attributable to the remote electron donating p-MeO groups. Reduction of 1 by methanol yields a novel core complex [MnIII4O2(OMe)2(MeOH)[(p-MeOPh)2PO2]6] (2). Each MnIII of 2 is tetragonally distorted due to the Jahn-Teller effect with elongated Mn-O bonds that are directed at the two micro3-MeO bridges and neither of the two micro3-oxos. These electronically driven distortions provide a structural rationale for the greater basicity of the former sites and explain why 2 of the 4 corner oxos are preferentially reduced to water molecules.     

Single-molecule magnets: a family of MnIII/CeIV complexes with a [Mn8CeO8]12+ core

Four heterometallic, enneanuclear Mn8Ce clusters [Mn8CeO8(O2CMe)12(H2O)4] (4), [Mn8CeO8(O2CMe)12(py)4] (5), [Mn8CeO8(O2CPh)12(MeCN)4] [Mn8CeO8(O2CPh)12(dioxane)4] (6), and [Mn8CeO8(O2CCHPh2)12(H2O)4] (7) have been prepared by various methods. Their cores are essentially isostructural and comprise a nonplanar, saddlelike [MnIII8O8]8+ loop containing a central CeIV ion attached to the eight micro3-O2- ions. Peripheral ligation around the [Mn8CeO8]12+ core is provided by eight micro- and four micro3-O2CR- groups. Terminal ligation on four MnIII atoms is provided by H2O in 4 and 7, pyridine in 5, and MeCN/dioxane in 6. Solid-state magnetic susceptibility studies, fits of dc magnetization vs field and temperature data, and in-phase ac susceptibility studies in a zero dc field have established that complexes 4, 5, and 7 possess S=16, S=4 or 5, and S=6+/-1 spin ground states, respectively, but in all cases there are very low-lying excited states. The large variation in the ground-state spins for this isostructural family is rationalized as due to a combination of weak exchange interactions between the constituent MnIII atoms, and the presence of both nearest-neighbor and next-nearest-interactions of comparable magnitudes. Magnetization vs applied dc field sweeps on single crystals of 4.4H2O and 7.4H2O.3MeCN.2CH2Cl2 down to 0.04 K have established that these two complexes are new single-molecule magnets (SMMs). The former also shows an exchange-bias, a perturbation of its single-molecule properties from very weak intermolecular interactions mediated by hydrogen-bonding interactions with lattice-water molecules of crystallization.     

New valence-sandwich [Mn(II)4Mn(III)4Mn(II)4] aggregate showing single-molecule magnet behavior

The reaction of manganese(II) acetate, 1,1,1-tris(hydroxymethyl)methane (H3thme), and triethylamine in methanol leads to the formation of [Mn12O2(OMe)2(thme)4(OAc)10(H2O)4].2MeOH. The [Mn(III)4Mn(II)8] core consists of a central [Mn(III)4O6] rhombus sandwiched by two [Mn(II)4O7] fragments. Frequency-dependent ac susceptibility and hysteresis loops in the magnetization indicate single-molecule magnet behavior with a pure quantum-tunneling regime of relaxation below 0.2 K.     

Mn12O12(OMe)2(O2CPh)16(H2O)2]2- single-molecule magnets and other manganese compounds from a reductive aggregation procedure

A new synthetic procedure has been developed in Mn cluster chemistry involving reductive aggregation of permanganate (MnO4-) ions in MeOH in the presence of benzoic acid, and the first products from its use are described. The reductive aggregation of NBu(n)4MnO4 in MeOH/benzoic acid gave the new 4Mn(IV), 8Mn(III) anion [Mn12O12(OMe)2(O2CPh)16(H2O)2]2-, which was isolated as a mixture of two crystal forms (NBu(n)4)2[Mn12O12(OMe)2(O2CPh)16(H2O)2].2H2O.4CH2Cl2 (1a) and (NBu(n)4)2[Mn12O12(OMe)2(O2CPh)16(H2O)2].2H2O.CH2Cl2 (1b). The anion of 1 contains a central [Mn(IV)4(mu3-O)2(mu-O)2(mu-OMe)2]6+ unit surrounded by a nonplanar ring of eight Mn(III) atoms that are connected to the central Mn4 unit by eight bridging mu3-O2- ions. This compound is very similar to the well-known [Mn12O12(O2CR)16(H2O)4] complexes (hereafter called "normal Mn12"), with the main difference being the structure of the central cores. Longer reaction times (approximately 2 weeks) led to isolation of polymeric [Mn(OMe)(O2CPh)2]n2, which contains a linear chain of repeating [Mn(III)(mu-O2CPh)2(mu-OMe)Mn(III)] units. The chains are parallel to each other and interact weakly through pi-stacking between the benzoate rings. When KMnO4 was used instead of NBu(n)4MnO4, two types of compounds were obtained, [Mn12O12(O2CPh)16(H2O)4] (3), a normal Mn12 complex, and [Mn4O2(O2CPh)8(MeOH)4].2MeOH (4.2MeOH), a new member of the Mn4 butterfly family. The cyclic voltammogram of 1 exhibits three irreversible processes, two reductions and one oxidation. One-electron reduction of 1 by treatment with 1 equiv of I- in CH2Cl2 gave (NBu(n)4[Mn12O12(O2CPh)16(H2O)3].6CH2Cl2 (5.6CH2Cl2), a normal Mn12 complex in a one-electron reduced state. The variable-temperature magnetic properties of 1, 2, and 5 were studied by both direct current (dc) and alternating current (ac) magnetic susceptibility measurements. Variable-temperature dc magnetic susceptibility studies revealed that (i) complex 1 possesses an S = 6 ground state, (ii) complex 2 contains antiferromagnetically coupled chains, and (iii) complex 5 is a typical [Mn12]- cluster with an S = 19/2 ground state. Variable-temperature ac susceptibility measurements suggested that 5 and both isomeric forms of 1 (1a,b) are single-molecule magnets (SMMs). This was confirmed by the observation of hysteresis loops in magnetization vs dc field scans. In addition, 1a,b, like normal Mn12 clusters, display both faster and slower relaxing magnetization dynamics that are assigned to the presence of Jahn-Teller isomerism.     

High-spin Mn wheels

The syntheses, structures, and magnetic properties of the complexes [MnIV4MnIII10MnII2O2(OCH3)12(tmp)8(O2CCH3)10].3Et2O (1.3Et2O), [MnIV2MnIII18MnII2O6(OCH3)14(O2CCH3)16(tmp)8(HIm)2].2CH3OH (2.2CH3OH), and [MnIV2MnIII18MnII2O6(OCH3)14(O2CCH3)16(Br-mp)8(HIm)2].2C6H14.5CH3OH (3.2C6H14.5CH3OH) are reported. The unusual wheel-like complexes were prepared by the treatment of [Mn3O(O2CCH3)6(HIm)3](O2CCH3) (HIm = imidazole) with 1,1,1-tris-(hydroxymethyl)propane (H3tmp) (1 and 2) or 2-(bromomethyl)-2-(hydroxymethyl)-1,3-propanediol (Br-mpH3) (3) in the presence of sodium methoxide (NaOCH3, 2, and 3) in CH3OH. Complex 1.3Et2O crystallizes in the triclinic space group P, while complexes 2.2CH3OH and 3.2C6H14.5CH3OH crystallize in the orthorhombic space group Pbca. Direct current magnetic susceptibility data, collected for 1-3 in the respective 1.8-300 K and 0.1-7 T temperature and magnetic-field ranges, afford spin ground-state values of S = 14 +/- 1 for complex 1 and S = 9 +/- 1 for complexes 2 and 3. Alternating current susceptibility measurements performed on all three complexes in the 1.8-10 K temperature range in a 3.5 G oscillating field at frequencies between 50 and 1000 Hz reveal out-of-phase chi"M signals below approximately 3 K. Single-crystal hysteresis loop and relaxation measurements confirm single-molecule magnetism behavior.     

New mixed-valent Mn clusters from the use of N,N,N',N'-tetrakis(2-hydroxyethyl)ethylenediamine (edteH4): Mn3, Mn4, Mn6, and Mn10

The syntheses, crystal structures, and magnetochemical characterization are reported for the new mixed-valent Mn clusters [Mn(2)(II)Mn(III)(O(2)CMe)(2)(edteH(2))(2)](ClO(4)) (1), [Mn(II)(2)Mn(III)(2)(edteH(2))(2)(hmp)(2)Cl(2)](Mn(II)Cl(4)) (2), [Mn(III)(6)O(2)(O(2)CBu(t))(6)(edteH)(2)(N(3))(2)] (3), [Na(2)Mn(III)(8)Mn(II)(2)O(4)(OMe)(2)(O(2)CEt)(6)(edte)(2)(N(3))(6)] (4), and (NEt(4))(2)[Mn(8)(III)Mn(2)(II)O(4)(OH)(2)-(O(2)CEt)(6)(edte)(2)(N(3))(6)](5), where edteH(4) is N,N,N',N'-tetrakis-(2-hydroxyethyl)ethylenediamine and hmpH is 2-(hydroxymethyl)pyridine. 1-5 resulted from a systematic exploration of the effect of different Mn sources, carboxylates, the presence of azide, and other conditions, on the Mn/edteH(4) reaction system. The core of 1 consists of a linear Mn(II)Mn(III)Mn(II) unit, whereas that of 2 is a planar Mn(4) rhombus within a [Mn(II)(2)Mn(III)(2)(μ(3)-OR)(2)] incomplete-dicubane unit. The core of 3 comprises a central [Mn(III)(4)(OR)(2)] incomplete-dicubane on either side of which is edge-fused a triangular [Mn(III)(3)(μ(3)-O)] unit. The cores of 4 and 5 are similar and consist of a central [Mn(II)(2)Mn(III)(2)(μ(3)-OR)(2)] incomplete-dicubane on either side of which is edge-fused a distorted [Mn(II)Mn(III)(3)(μ(3)-O)(2)(μ(3)-OR)(2)] cubane unit. Variable-temperature, solid-state direct current (dc) and alternating current (ac) magnetization studies were carried out on 1-5 in the 5.0-300 K range, and they established the complexes to have ground state spin values of S = 3 for 1, S = 9 for 2, and S = 4 for 3. The study of 3 provided an interesting caveat of potential pitfalls from particularly low-lying excited states. For 4 and 5, the ground state is in the S = 0-4 range, but its identification is precluded by a high density of low-lying excited states.     

New Fe4, Fe6, and Fe8 clusters of iron(III) from the use of 2-pyridyl alcohols: structural, magnetic, and computational characterization

The syntheses, crystal structures, magnetochemical characterization, and theoretical calculations are reported for three new iron clusters [Fe 6O 2(NO3) 4(hmp) 8(H 2O) 2](NO3)2 (1), [Fe4(N3)6(hmp)6] (2), and [Fe8O3(OMe)(pdm)4(pdmH) 4(MeOH)2](ClO4)5 (3) (hmpH=2-(hydroxymethyl)pyridine; pdmH2=2,6-pyridinedimethanol). The reaction of hmpH with iron(III) sources such as Fe(NO3) 3.9H2O in the presence of NEt 3 gave 1, whereas 2 was obtained from a similar reaction by adding an excess of NaN3. Complex 3 was obtained in good yield from the reaction of pdmH 2 with Fe(ClO4)3.6H2O in MeOH in the presence of an organic base. The complexes all possess extremely rare or novel core topologies. The core of 1 comprises two oxide-centered [Fe3(mu3-O)](7+) triangular units linked together at two of their apexes by two sets of alkoxide arms of hmp(-) ligands. Complex 2 contains a zigzag array of four Fe (III) atoms within an [Fe4(mu-OR) 6](6+) core, with the azide groups all bound terminally. Finally, complex 3 contains a central [Fe 4(mu4-O)](10+) tetrahedron linked to two oxide-centered [Fe3(mu3-O)](7+) triangular units. Variable-temperature, solid-state dc and ac magnetization studies were carried out on complexes 1-3 in the 5.0-300 K range. Fitting of the obtained magnetization versus field (H) and temperature (T) data by matrix diagonalization and including only axial anisotropy (zero-field splitting, ZFS) established that 1 possesses an S=3 ground-state spin, with g=2.08, and D=-0.44 cm(-1). The magnetic susceptibility data for 2 up to 300 K were fit by matrix diagonalization and gave J1=-9.2 cm(-1), J2=-12.5 cm(-1), and g=2.079, where J 1 and J 2 are the outer and middle nearest-neighbor exchange interactions, respectively. Thus, the interactions between the Fe(III) centers are all antiferromagnetic, giving an S=0 ground state for 2. Similarly, complex 3 was found to have an S=0 ground state. Theoretically computed values of the exchange constants in 2 were obtained with DFT calculations and the ZILSH method and were in good agreement with the values obtained from the experimental data. Exchange constants obtained with ZILSH for 3 successfully rationalized the experimental S = 0 ground state. The combined work demonstrates the ligating flexibility of pyridyl-alcohol chelates and their usefulness in the synthesis of new polynuclear Fex clusters without requiring the copresence of carboxylate ligands.     

Coordination chemistry of the soluble metal oxide analogue [Mo5O13(OCH3)4(NO)]3- with manganese carbonyl species

The reactions of neutral or cationic manganese carbonyl species towards the oxo-nitrosyl complex [Na(MeOH)[Mo(5)O(13)(OCH(3))(4)(NO)]](2-) have been investigated in various conditions. This system provides an unique opportunity for probing the basic reactions involved in the preparation of solid oxide-supported heterogeneous catalysts, that is, mobility of transition-metal species at the surface and dissolution-precipitation of the support. Under nitrogen and in the dark, the reaction of in situ generated fac-[Mn(CO)(3)](+) species with (nBu(4)N)(2)[Na(MeOH)-[Mo(5)O(13)(OMe)(4)(NO)]] in MeOH yields (nBu(4)N)(2)[Mn(CO)(3)(H(2)O)[Mo(5)O(13)(OMe)(4)(NO)]] at room temperature, while (nBu(4)N)(3)[Na[Mo(5)O(13)(OMe)(4)(NO)](2)[Mn(CO)(3)](2)] is obtained under reflux. The former transforms into the latter under reflux in methanol in the presence of sodium bromide; this involves the migration of the fac-[Mn(CO)(3)](+) moiety from a basal kappa(2)O coordination site to a lateral kappa(3)O site. Oxidation and decarbonylation of manganese carbonyl species as well as degradation of the oxonitrosyl starting material and reaggregation of oxo(methoxo)molybdenum fragments occur in non-deareated MeOH, and both (nBu(4)N)(4)[Mn(H(2)O)(2)[Mo(5)O(16)(OMe)(2)](2)[Mn(CO)(3)](2)] and (nBu(4)N)(4)[Mn(H(2)O)(2)[Mo(5)O(13)(OMe)(4)(NO)](2)] as well as (nBu(4)N)(2)[MnBr[Mo(5)O(13)(OMe)(4)(NO)]] have been obtained in this way. The rhenium analogue (nBu(4)N)(2)[Re(CO)(3)(H(2)O)[Mo(5)O(13)(OMe)(4)(NO)]] has also been synthesized. The crystal structures of (nBu(4)N)(2)[Re(CO)(3)(H(2)O)[Mo(5)O(13)(OMe)(4)(NO)]], (nBu(4)N)(3)[Na[Mo(5)O(13)(OMe)(4)(NO)](2)[Mn(CO)(3)](2)], (nBu(4)N)(4)[Mn(H(2)O)(2)[Mo(5)O(16)(OMe)(2)](2)[Mn(CO)(3)](2)], (nBu(4)N)(4)[Mn(H(2)O)(2)[Mo(5)O(13)(OMe)(4)(NO)](2)] and (nBu(4)N)(2)[MnBr[Mo(5)O(13)(OMe)(4)(NO)]] have been determined.     

Mixed-valence MnIIIMnIV clusters [Mn7O8(O2SePh)8(O2CMe)(H2O)] and [Mn7O8(O2SePh)9(H2O)]: single-chain magnets exhibiting quantum tunneling of magnetization

The syntheses, structures, and magnetic properties of two new Mn7 complexes containing phenylseleninate ligands are reported. [Mn7O8(O2SePh)8(O2CMe)(H2O)] (1) and [Mn7O8(O2SePh)9(H2O)] (2) were both prepared by the reaction of 18 equiv of benzeneseleninic acid (PhSeO2H) with [Mn12O12(O2CMe)16(H2O)4] in MeCN. Complex 1 x 6MeCN crystallizes in the triclinic space group P, and complex 2 x 2CH2Cl2 crystallizes in the monoclinic space group P2(1)/m. Both compounds possess an unprecedented [Mn7O8]9+ core comprising a central [MnIII3(micro3-O)4]+ unit attached to [MnIV2(micro-O)2]4+ and [MnIV2(micro-O)(micro3-O)]4+ units on either side. In each cluster, the PhSeO2- groups function as bridging ligands between adjacent Mn centers. The structure reveals strong Se.O intermolecular contacts between Mn7 units to give a one-dimensional chain structure, with weak interchain interactions. Solid-state DC magnetic susceptibility measurements of complexes 1 and 2 reveal that they have very similar properties, and detailed studies on 1 by AC susceptibility measurements confirm an S = 2 ground-state spin value. In addition, out-of-phase AC signals are observed, suggesting slow magnetization relaxation. Magnetization versus DC field sweeps down to 0.04 K reveals hysteresis loops, but the temperature dependence of the coercivity is not what is expected of a single-molecule magnet. Instead, the behavior is due to single-chain magnetism, albeit with weak antiferromagnetic interactions between the chains, with the barrier to relaxation arising from a combination of molecular anisotropy and ferromagnetic intermolecular exchange interactions mediated by the Se...O contacts. An Arrhenius plot was constructed from the magnetization versus time decay data. The thermally activated region at > 0.5 K gave an effective relaxation barrier (Ueff) of 14.2 K. Below approximately 0.1 K, the relaxation is independent of temperature, which is characteristic of magnetization quantum tunneling through the anisotropy barrier. These Mn7 compounds are thus the first single-chain magnets to comprise polynuclear metal clusters and also the first for which the temperature-independent relaxation characteristic of tunneling has been identified. The work also emphasizes that out-of-phase AC signals for ostensibly molecular compounds are not sufficient proof by themselves of a single-molecule magnet.