A Fourth Isolated Oxidation Level of the [Mn₁₂O₁₂(O₂CR)₁₆(H₂O)₄] Family of Single-Molecule Magnets

The Mn12 family of single-molecule magnets (SMMs) has been extended to a fourth isolated member. [Mn12O12(O2CR)16(H2O)4] (1) exhibits three quasi-reversible one-electron-reduction processes at significantly higher potentials than [Mn12O12(O2CMe)16(H2O)4]. This has allowed the previous generation and isolation of the one- and two-electron-reduced versions of 1 to now be extended to the three-electron-reduced complex. For cation consistency and better comparisons, the complete series of complexes has been prepared with NPrn4+ counterions. Thus, complex 1 was treated with 1, 2, and 3 equiv of NPrn4I, and this led to the successful isolation of (NPrn4)[Mn12O12(O2CCHCl2)16(H2O)4] (2), (NPrn4)2[Mn12O12(O2CCHCl2)16(H2O)4] (3), and (NPrn4)3[Mn12O12(O2CCHCl2)16(H2O)4] (4), respectively. Another three-electron-reduced analogue (NMe4)3[Mn12O12(O2CCHCl2)16(H2O)4] (5) was prepared by the addition of 3 equiv of NMe4I to 1. Direct current magnetization data were collected on dried microcrystalline samples of 2−5 and were fit by matrix diagonalization methods to give S = 19/2, D = −0.35 cm-1, and g = 1.95 for 2; S = 10, D = −0.28 cm-1, and g = 1.98 for 3; S = 17/2, D = −0.25 cm-1, and g = 1.91 for 4; and S = 17/2, D = −0.23 cm-1, and g = 1.90 for 5, where D is the axial zero-field splitting parameter. Thus, the [Mn12]3- complexes 4 and 5 possess significantly decreased absolute magnitudes of both S and D as a result of the three-electron addition to 1, which has S = 10 and D = −0.45 cm-1. The D value of the series 1−4/5 shows a monotonic decrease with electron addition that is consistent with the progressive loss of MnIII ions, which are the primary source of the molecular anisotropy. Nevertheless, when studied by ac susceptibility techniques, the [Mn12]3- complexes still exhibit frequency-dependent out-of-phase susceptibility signals at ≤2.5 K, indicating them to be single-molecule magnets (SMMs), albeit at lower temperatures compared with 1 (6−8 K range), 2 (4−6 K range), and 3 (2−4 K range); the shifts to lower temperatures reflect the decreasing S and D values upon successive reduction and hence the decreasing energy barrier to magnetization relaxation. Thus, the [Mn12]3- complexes represent a fourth isolated oxidation level of the Mn12 family of SMMs, by far the largest range of oxidation levels yet encountered within single-molecule magnetism

‘Old’ Clusters with New Function: Oxidation Catalysis by High Oxidation State Manganese and Cerium/Manganese Clusters Using O₂ Gas

The family of polynuclear manganese clusters of formula [Mn12O12(O2CR)16(H2O)4] (R = Et, Ph, etc.) has been investigated in great detail over the years for their ability to function as single-molecule magnets (SMMs), but they have not been employed as oxidation catalysts. In the present report, the ability is described of these clusters to act as catalysts in the selective oxidation of benzyl alcohol to benzaldehyde using molecular O2 as the primary oxidant and the nitroxyl radical TEMPO as a cocatalyst. A systematic investigation of Mn clusters varied in their R group, oxidation state, and size was conducted in order to realize the electronic requirements that will lead to the best catalytic activity. The best reactivity (>99%) was obtained when the catalyst was the mixed-metal cluster [CeMn6O9(O2CMe)9(NO3)(H2O)2], which contains Ce4+Mn4+6 ions; in this case, lower loadings of catalysts (cluster and TEMPO) are required and the reaction can proceed even without a solvent. In addition, it has been demonstrated that the high efficiency can be only achieved when both high oxidation Ce4+ and Mn4+ ions are present within the same cluster

Semiempirical Magnetostructural Correlation for High-Nuclearity Mn^III-Oxo Complexes: Accommodation of Different Relative Jahn–Teller Axis Orientations

The previous development of a magnetostructural correlation (MSC) for polynuclear FeIII/oxo clusters has now been extended to one for polynuclear MnIII/oxo clusters. A semiempirical model estimating each pairwise Mn2 exchange constant (Jij) from the Mn–O bond lengths and Mn–O–Mn angles has been formulated based on the angular overlap model. The extra complication, compared with the FeIII/oxo MSC, of different relative orientations of the Jahn–Teller distortion axes typical of high-spin MnIII in near-octahedral geometry was accommodated by developing a separate MSC variant for each possible situation. The final coefficients of the three MSC variants were determined by using reliable crystal structure data and experimentally determined Jij values from the literature. The estimated JMSC values from the new MnIII/oxo MSC have been employed to successfully rationalize the magnetic properties of a number of MnIII clusters in the nuclearity range Mn3–Mn10. These properties include relative spin vector alignments in the ground state, the presence of spin frustration effects, and the resulting overall ground state spin. In addition, the JMSC values can be used to simulate the direct-current magnetic susceptibility versus temperature data and provide realistic input values for fits of these data to minimize false-fit problems. A protocol for the use of the new MSC is also reported

New Mixed-Valent Mn Clusters from the Use of <i>N</i>,<i>N</i>,<i>N′</i>,<i>N′</i>-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 [Mn2IIMnIII(O2CMe)2(edteH2)2]­(ClO4) (1), [MnII2MnIII2(edteH2)2(hmp)2Cl2]­(MnIICl4) (2), [MnIII6O2(O2CBut)6(edteH)2(N3)2] (3), [Na2MnIII8MnII2O4(OMe)2(O2CEt)6(edte)2(N3)6] (4), and (NEt4)2[Mn8IIIMn2IIO4(OH)2-(O2CEt)6(edte)2(N3)6]­(5), where edteH4 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/edteH4 reaction system. The core of 1 consists of a linear MnIIMnIIIMnII unit, whereas that of 2 is a planar Mn4 rhombus within a [MnII2MnIII2(μ3-OR)2] incomplete-dicubane unit. The core of 3 comprises a central [MnIII4(OR)2] incomplete-dicubane on either side of which is edge-fused a triangular [MnIII3(μ3-O)] unit. The cores of 4 and 5 are similar and consist of a central [MnII2MnIII2(μ3-OR)2] incomplete-dicubane on either side of which is edge-fused a distorted [MnIIMnIII3(μ3-O)2(μ3-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

Carboxylate-Free Mn^III₂Ln^III₂ (Ln = Lanthanide) and Mn^III₂Y^III₂ Complexes from the Use of (2-Hydroxymethyl)pyridine: Analysis of Spin Frustration Effects

The initial employment of 2-(hydroxymethyl)pyridine for the synthesis of Mn/Ln (Ln = lanthanide) and Mn/Y clusters, in the absence of an ancillary organic ligand, has afforded a family of tetranuclear [MnIII2MIII2(OH)2(NO3)4(hmp)4(H2O)4](NO3)2 (M = Dy, 1; Tb, 2; Gd, 3; Y; 4) anionic compounds. 1–4 possess a planar butterfly (or rhombus) core and are rare examples of carboxylate-free Mn/Ln and Mn/Y clusters. Variable-temperature dc and ac studies established that 1 and 2, which contain highly anisotropic LnIII atoms, exhibit slow relaxation of their magnetization vector. Fitting of the obtained magnetization (M) versus field (H) and temperature (T) data for 3 by matrix diagonalization and including only axial anisotropy (zero-field splitting, ZFS) showed the ground state to be S = 3. Complex 4 has an S = 0 ground state. Fitting of the magnetic susceptibility data collected in the 5–300 K range for 3 and 4 to the appropriate van Vleck equations revealed, as expected, extremely weak antiferromagnetic interactions between the paramagnetic ions; for 3, J1 = −0.16(2) cm–1 and J2 = −0.12(1) cm–1 for the MnIII···MnIII and MnIII···GdIII interactions, respectively. The S = 3 ground state of 3 has been rationalized on the basis of the spin frustration pattern in the molecule. For 4, J = −0.75(3) cm–1 for the MnIII···MnIII interaction. Spin frustration effects in 3 have been quantitatively analyzed for all possible combinations of sign of J1 and J2

Unusual Structural Types in Manganese Cluster Chemistry from the Use of <i>N</i>,<i>N</i>,<i>N</i>‘,<i>N</i>‘-Tetrakis(2-hydroxyethyl)ethylenediamine: Mn₈, Mn₁₂, and Mn₂₀ Clusters

The syntheses, crystal structures, and magnetochemical characterization are reported for three new mixed-valent Mn clusters [Mn8O3(OH)(OMe)(O2CPh)7(edte)(edteH2)](O2CPh) (1), [Mn12O4(OH)2(edte)4Cl6(H2O)2] (2), and [Mn20O8(OH)4(O2CMe)6(edte)6](ClO4)2 (3) (edteH4 = (HOCH2CH2)2NCH2CH2N(CH2CH2OH)2 = N,N,N‘,N‘-tetrakis(2-hydroxyethyl)ethylenediamine). The reaction of edteH4 with Mn(O2CPh)2, MnCl2, or Mn(O2CMe)2 gives 1, 2, and 3, respectively, which all possess unprecedented core topologies. The core of 1 comprises two edge-sharing [Mn4O4] cubanes connected to an additional Mn ion by a μ3-OH- ion and two alkoxide arms of edteH22-. The core of 2 consists of a [Mn12(μ4-O)4]24+ unit with S4 symmetry. The core of 3 consists of six fused [Mn4O4] cubanes in a 3 × 2 arrangement and linked to three additional Mn atoms at both ends. Variable-temperature, solid-state dc and ac magnetization (M) studies were carried out on complexes 1−3 in the 5.0−300 K range. Fitting of the obtained M/NμB vs H/T data by matrix diagonalization and including only axial zero-field splitting (ZFS) gave ground-state spin (S) and axial ZFS parameter (D) of S = 8, D = −0.30 cm-1 for 1, S = 7, D = −0.16 cm-1 for 2, and S = 8, D = −0.16 cm-1 for 3. The combined work demonstrates that four hydroxyethyl arms on an ethylenediamine backbone can generate novel Mn structural types not accessible with other alcohol-based ligands

A New N,N,O Chelate for Transition Metal Chemistry: Fe₅ and Fe₆ Clusters from the Use of 6-Hydroxymethyl-2,2‘-bipyridine (hmbpH)

The initial use of the anion of 6-hydroxymethyl-2,2‘-bipyridine (hmbpH) as a chelate in coordination chemistry is described. The syntheses, crystal structures, and magnetochemical characterization are reported of four new iron(III) clusters [Fe5O2(OH)(O2CMe)5(hmbp)3](ClO4)2 (1) and [Fe6O2(OH)2(O2CR)6(hmbp)4](NO3)2 (R = Ph (2), Me (3), But (4); hmbpH = 6-hydroxymethyl-2,2‘-bipyridine). The reaction of Fe(ClO4)3, hmbpH, and sodium acetate in a 1:1:∼4 ratio in EtOH gave 1, and the reaction between [Fe3O(O2CR)6(H2O)3](NO3) (R = Ph, Me, But) and hmbpH in a 1:1 ratio in MeCN gave 2−4, respectively. The core of 1 consists of a [Fe4(μ3-O)2]8+ butterfly unit to which is attached a fifth Fe atom by bridging O atoms. The core of 2−4 also consists of a [Fe4(μ3-O)2]8+ butterfly unit to which are attached an Fe atom on either side by bridging O atoms. Variable-temperature (T) and -field (H) solid-state DC and AC magnetization (M) studies were carried out on complexes 1−4 in the 5.0−300 K range. Fitting of the data revealed that 1 has an S = 5/2 ground state spin whereas 2−4 possess an S = 5 ground state. Fitting of the M/NμB vs H/T data by matrix diagonalization and including only axial zero-field splitting (ZFS) gave values of the axial ZFS parameter |D| of 0.75, 0.36, 0.46, and 0.36 cm-1 for 1−4, respectively

A New N,N,O Chelate for Transition Metal Chemistry: Fe₅ and Fe₆ Clusters from the Use of 6-Hydroxymethyl-2,2‘-bipyridine (hmbpH)

The initial use of the anion of 6-hydroxymethyl-2,2‘-bipyridine (hmbpH) as a chelate in coordination chemistry is described. The syntheses, crystal structures, and magnetochemical characterization are reported of four new iron(III) clusters [Fe5O2(OH)(O2CMe)5(hmbp)3](ClO4)2 (1) and [Fe6O2(OH)2(O2CR)6(hmbp)4](NO3)2 (R = Ph (2), Me (3), But (4); hmbpH = 6-hydroxymethyl-2,2‘-bipyridine). The reaction of Fe(ClO4)3, hmbpH, and sodium acetate in a 1:1:∼4 ratio in EtOH gave 1, and the reaction between [Fe3O(O2CR)6(H2O)3](NO3) (R = Ph, Me, But) and hmbpH in a 1:1 ratio in MeCN gave 2−4, respectively. The core of 1 consists of a [Fe4(μ3-O)2]8+ butterfly unit to which is attached a fifth Fe atom by bridging O atoms. The core of 2−4 also consists of a [Fe4(μ3-O)2]8+ butterfly unit to which are attached an Fe atom on either side by bridging O atoms. Variable-temperature (T) and -field (H) solid-state DC and AC magnetization (M) studies were carried out on complexes 1−4 in the 5.0−300 K range. Fitting of the data revealed that 1 has an S = 5/2 ground state spin whereas 2−4 possess an S = 5 ground state. Fitting of the M/NμB vs H/T data by matrix diagonalization and including only axial zero-field splitting (ZFS) gave values of the axial ZFS parameter |D| of 0.75, 0.36, 0.46, and 0.36 cm-1 for 1−4, respectively

Carboxylate-Free Mn^III₂Ln^III₂ (Ln = Lanthanide) and Mn^III₂Y^III₂ Complexes from the Use of (2-Hydroxymethyl)pyridine: Analysis of Spin Frustration Effects

The initial employment of 2-(hydroxymethyl)pyridine for the synthesis of Mn/Ln (Ln = lanthanide) and Mn/Y clusters, in the absence of an ancillary organic ligand, has afforded a family of tetranuclear [MnIII2MIII2(OH)2(NO3)4(hmp)4(H2O)4](NO3)2 (M = Dy, 1; Tb, 2; Gd, 3; Y; 4) anionic compounds. 1–4 possess a planar butterfly (or rhombus) core and are rare examples of carboxylate-free Mn/Ln and Mn/Y clusters. Variable-temperature dc and ac studies established that 1 and 2, which contain highly anisotropic LnIII atoms, exhibit slow relaxation of their magnetization vector. Fitting of the obtained magnetization (M) versus field (H) and temperature (T) data for 3 by matrix diagonalization and including only axial anisotropy (zero-field splitting, ZFS) showed the ground state to be S = 3. Complex 4 has an S = 0 ground state. Fitting of the magnetic susceptibility data collected in the 5–300 K range for 3 and 4 to the appropriate van Vleck equations revealed, as expected, extremely weak antiferromagnetic interactions between the paramagnetic ions; for 3, J1 = −0.16(2) cm–1 and J2 = −0.12(1) cm–1 for the MnIII···MnIII and MnIII···GdIII interactions, respectively. The S = 3 ground state of 3 has been rationalized on the basis of the spin frustration pattern in the molecule. For 4, J = −0.75(3) cm–1 for the MnIII···MnIII interaction. Spin frustration effects in 3 have been quantitatively analyzed for all possible combinations of sign of J1 and J2

Largest Mixed Transition Metal/Actinide Cluster: A Bimetallic Mn/Th Complex with a [Mn₁₀Th₆O₂₂(OH)₂]¹⁸⁺ Core

A high-nuclearity mixed transition metal/actinide complex has been prepared from the reaction of a MnIII4 complex with Th(NO3)4 in MeCN/MeOH. The complex [Th6Mn10O22(OH)2(O2CPh)16(NO3)2(H2O)8] is the largest such complex to date and the first Th/Mn species. It is rich in oxide groups, which stabilize all of the metals in the high ThIV and MnIV oxidation levels. Magnetic characterization establishes that the complex has an S = 3 ground-state spin value