The use of high magnetic field and high frequency in an unconventional spectrometer has provided very informative EPR spectra of a manganese(III) octahedral complex for the first time. The parameters of the spin Hamiltonian operator are in fair agreement with those calculated with ligand‐field theory. High‐frequency EPR is thus a powerful tool for the structural investigation of complexes that contain metal ions with integer spins.
Compound 5 is dynamic and shows a rapid flipping of the nonplanar metallacyclic five-membered ring, thereby interconverting the isoenergetic envelope conformations. The internal organometallic ion pair of 6 features an analogous automerization reaction, albeit with a substantially increased activation barrier (T,,,,,,,,, (Cp) = room temperature). The group in Amsterdam also added B(C,Fs), to 1,l-bis(cyclopentadieny1)zirconaindane [15] with opening of the Zr-C(spi) bond. According to NMR spectroscopy we assume a specific product geometry that at the same time conformationally favors the interaction of the zirconium atom with the CH2[B] moiety (6(I3C) = 14.6) and one of the ortho-fluorine substituents (6(I9F) =-180.2), which are both located in the major plane of the bent metallocene unit: M. Schreuder Goedheijt, dissertation, Vrije Universiteit
Two novel polynuclear manganese(II,III) complexes have been synthesized by exploiting controlled methanolysis. A one-pot reaction of MnCl(2), NaOMe, dibenzoylmethane (Hdbm), and O(2) in anhydrous methanol, followed by recrystallization from MeOH/CHCl(3) mixtures, afforded the alkoxomanganese complexes [Mn(7)(OMe)(12)(dbm)(6)].CHCl(3).14MeOH (2) and [Mn(2)(OMe)(2)(dbm)(4)] (3). Complex 2 crystallizes in trigonal space group R&thremacr; with a = 14.439(2) Å, alpha = 86.34(1) degrees, and Z = 1. Complex 3 crystallizes in triclinic space group P&onemacr; with a = 9.612(1) Å, b = 10.740(1) Å, c = 13.168(1) Å, alpha = 80.39(1) degrees, beta = 87.66(1) degrees, gamma = 83.57(1) degrees, and Z = 1. The solid-state structure of 2 comprises a [Mn(6)(OMe)(12)(dbm)(6)] "crown" with crystallographically imposed 6-fold symmetry plus a central manganese ion. The layered Mn/O core mimics a fragment of the manganese oxide mineral lithiophorite. Conductivity measurements confirmed the nonionic character of 2 and suggested a mixed-valence Mn(II)(3)Mn(III)(4) formulation. The metrical parameters of the core were analyzed with the aid of bond-valence sum calculations. The central ion is essentially a valence-trapped Mn(II) ion, whereas the average Mn-O distances for the manganese ions of the "crown" are consistent with the presence of two Mn(II) and four Mn(III) ions. However, (1)H NMR spectra in solution strongly support valence localization and suggest that the observed solid-state structure may be a result of static disorder effects. Magnetic susceptibility vs T and magnetization vs field data at low temperature are consistent with an S = (17)/(2) ground state. Complex 3 is a symmetric alkoxo-bridged dimer. The two high-spin Mn(III) ions are antiferromagnetically coupled with J = 0.28(4) cm(-)(1), g = 1.983(2), and D = -2.5(4) cm(-)(1).
The design of novel high-spin molecules represents a major goal of current research in the field of nanoscale materials. In particular, the synthesis of large transition-metal ion clusters has provided excellent materials for the observation of unusual physical properties, such as quantum tunneling of the magnetization and magnetic hysteresis effects of purely molecular origin. 1 Unfortunately, large molecular clusters are almost invariably obtained in a serendipitous manner from self-assembly reactions and general strategies for the synthesis of large clusters with preordained structures and properties have not yet been developed. We recently suggested that host-guest interactions are among the available tools for a fine-tuning of the structure and properties of magnetic clusters. 2a We now show that the magnetic properties of high-nuclearity clusters may be varied to a very large extent by acting on the electronic structure of the constituent metal ions. By substituting iron(III) with manganese(III) in the core of the cyclic cluster [NaFe 6 (OMe) 12 (dbm) 6 ] + (1) (Hdbm ) dibenzoylmethane), 2b,c we were able to switch from a S ) 0 to a S ) 12 ground state. Changing the number of d electrons on the metal centers may therefore represent a powerful tool for modulating the properties of single-molecule magnets.The complex [NaMn 6 (OMe) 12 (dbm) 6 ] + (2) was synthesized from manganese(II) dichloride by simultaneous oxidation and methoxide-promoted aggregation. A one-pot reaction involving MnCl 2 (1 equiv), dibenzoylmethane (1 equiv), NaOMe (4 equiv), and dioxygen in anhydrous methanol, followed by recrystallization from CHCl 3 /MeOH mixtures in the presence of NaBPh 4 , afforded black-brown crystals of [2]BPh 4 . 3 A single-crystal X-ray investigation at 188 K 4 pointed to a cyclic structure with approximate S 6 point-group symmetry but crystallographic C i symmetry ( Figure 1). The [Mn 6 (OMe) 12 ] ring displays a 12-metallacrown-6 structure and acts as a host for an alkali-metal ion, 2 which has a trigonally distorted octahedral environment. Although the average nearest-neighbor Mn‚‚‚Mn distance [3.21(2) Å] compares well with that observed in 1 [3.194(9) Å], the coordination environment of the metal ions shows important differences. 5 A tetragonal elongation of the coordination polyhedra is evident along the O8-Mn1-O2, O11-Mn2-O6, and O10-Mn3-O4′ directions, which are roughly perpendicular to each other as a consequence of edgesharing between MnO 6 octahedra (Figure 2). Axial bonds involve oxygen atoms from dbm and µ 3 -OMe ligands and have unequal lengths [average: 2.06(2) and 2.23(4) Å, respectively]. Two trans equatorial sites are occupied by µ 2 -OMe ligands [average Mn-O: 2.00(3) Å]. The remaining oxygen of dbm and a µ 3 -OMe ligand (involved in an axial bond with an adjacent metal ion) act as in-plane donors with much shorter Mn-O distances [average: 1.910(8) Å]. The observed coordination geometry of the metal Cornia, A.; Fabretti, A. C.; Malavasi, W.; Schenetti, L.; Caneschi, A.; Gatteschi, D. Inorg. Ch...
A multitechnique approach has allowed the first experimental determination of single-ion anisotropies in a large iron(III)-oxo cluster, namely [NaFe6(OCH3)12(pmdbm)6ClO4 (1) in which Hpmdbm = 1,3-bis(4-methoxyphenyl)-1,3-propanedione. High-frequency EPR (HF-EPR). bulk susceptibility measurements, and high-field cantilever torque magnetometry (HF-CTM) have been applied to iron-doped samples of an isomorphous hexagallium(III) cluster [NaGa6(OCH3)12-(pmdbm)6]ClO4, whose synthesis and X-ray structure are also presented. HF-EPR at 240 GHz and susceptibility data have shown that the iron(III) ions have a hard-axis type anisotropy with DFe = 0.43(1) cm(-1) and EFe = 0.066(3) cm(-1) in the zero-field splitting (ZFS) Hamiltonian H = DFe[S2(z) - S(S + 1)/3] + Fe[S2(x) - S2(y)]. HF-CTM at 0.4 K has then been used to establish the orientation of the ZFS tensors with respect to the unique molecular axis of the cluster, Z. The hard magnetic axes of the iron(III) ions are found to be almost perpendicular to Z, so that the anisotropic components projected onto Z are negative, DFe(ZZ)= -0.164(4) cm(-1). Due to the dominant antiferromagnetic coupling, a negative DFe(ZZ) value determines a hard-axis molecular anisotropy in 1, as experimentally observed. By adding point-dipolar interactions between iron(III) spins, the calculated ZFS parameter of the triplet state, D1 = 4.70(9) cm(-1), is in excellent agreement with that determined by inelastic neutron scattering experiments at 2 K, D1 = 4.57(2) cm(-1). Iron-doped samples of a structurally related compound, the dimer [Ga2(OCH3)2(dbm)4] (Hdbm = dibenzoylmethane), have also been investigated by HF-EPR at 525 GHz. The single-ion anisotropy is of the hard-axis type as well, but the DFe parameter is significantly larger [DFe = 0.770(3) cm(-1). EFe = 0.090(3) cm(-1)]. We conclude that, although the ZFS tensors depend very unpredictably on the coordination environment of the metal ions, single-ion terms can contribute significantly to the magnetic anisotropy of iron(III)-oxo clusters, which are currently investigated as single-molecule magnets.
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