Ayuk M. Ako

Heterometallic [Mn5‐Ln4] Single‐Molecule Magnets with High Anisotropy Barriers

The reaction of [Mn6O2(Piv)(10)(4-Me-py)(2.5)(PivH)(1.5)] (1) (py: pyridine, Piv: pivilate) with N-methyldiethanolamine (mdeaH2) and Ln(NO3)3 x 6 H2O in MeCN leads to a series of nonanuclear compounds [Mn5Ln4(O)6(mdea)2(mdeaH)2(Piv)6(NO3)4(H2O)2]2 MeCN (Ln=Tb(III) (2), Dy(III) (3), Ho(III) (4), Y(III) (5)). Single-crystal X-ray diffraction shows that compounds 2-5 are isostructural, with the central core composed of two distorted {Mn(IV)Mn(III)Ln2O4} cubanes sharing a Mn(IV) vertex, representing a new heterometallic 3d-4f motif for this class of ligand. The four new compounds display single-molecule magnet (SMM) behaviour, which is modulated by the lanthanide ion used. Moreover, the values found for Delta(eff) and tau(o) for 3 of 38.6 K and 3.0 x 10(-9) s respectively reveal that the complex 3 exhibits the highest energy barrier recorded so far for 3d-4f SMMs. The slow relaxation of the magnetisation for 3 was confirmed by mu-SQUID measurements on an oriented single crystal and the observation of M versus H hysteresis loops below 1.9 K.

Structural motifs and topological representation of Mn coordination clusters.

Polynuclear coordination clusters have become of particular interest in recent times as a result of their relevance to bioinorganic chemistry and to the special area of molecule-based magnetic materials where cluster compounds behave as single-molecule magnets (SMMs). In this review we have focused on describing Mn coordination cluster complexes. Adopting our topological approach for the description of coordination clusters we present a means of classifying the structural motifs found in manganese clusters which range in nuclearity from 5 to 84, as well as some representative heterometallic Mn-M (M = K, Na, Ca, Sr, Ln) cluster complexes that have been reported. This sheds new light on the classification of the types of core structure accessible which, in turn, provides a useful means for developing the so-far missing magneto-structural correlation algorithm for these finite 0-D systems (212 references).

A Family of 3d-4f Octa-Nuclear [MnIII4LnIII4] Wheels (Ln = Sm, Gd, Tb, Dy, Ho, Er, and Y): Synthesis, Structure, and Magnetism

We present the syntheses, crystal structures, and magnetochemical characterizations for a family of isostructural [Mn(4)Ln(4)] compounds (Ln = Sm, Gd, Tb, Dy, Ho, Er, and Y). They were prepared from the reactions of formic acid, propionic acid, N-n-butyl-diethanolamine, manganese perchlorate, and lanthanide nitrates under the addition of triethylamine in MeOH. The compounds possess an intriguing hetero-octanuclear wheel structure with four Mn(III) and four Ln(III) ions alternatively arranged in a saddle-like ring, where formate ions act as key carboxylate bridges. In the lattice, the molecules stack into columns in a quasi-hexagonal arrangement. Direct current (dc) magnetic susceptibility measurements indicated the depopulation of the Stark components at low temperature and/or very weak antiferromagnetic interactions between magnetic centers. The zero-field alternating current (ac) susceptibility studies revealed that the compounds containing Sm, Tb, and Dy showed frequency-dependent out-of-phase signals, indicating they are single-molecule magnets (SMMs). Magnetization versus applied dc field sweeps on a single crystal of the Dy compound down to 40 mK exhibited hysteresis depending on temperatures and field sweeping rates, further confirming that the Dy compound is a SMM. The magnetization dynamics of the Sm and Y compounds investigated under dc fields revealed that the relaxation of the Sm compound is considered to be dominated by the two-phonon (Orbach) process while the Y compound displays a multiple relaxation process.

Tridecanuclear [MnIII5LnIII8] Complexes Derived from N-tButyl-diethanolamine: Synthesis, Structures, and Magnetic Properties

The synthesis, structures and magnetic properties of a family of heterometallic [Mn(III)(5)Ln(III)(8)(mu(3)-OH)(12)(L(2))(4)(piv)(12)(NO(3))(4)(OAc)(4)](-) (Ln = Pr, 2; Nd, 3; Sm, 4; Gd, 5; Tb, 6) aggregates are reported. The complexes were obtained from the direct reaction of N-(t)butyldiethanolamine (H(2)L(2)) with Mn(OAc)(2) x 4 H(2)O and Ln(NO(3))(3) x 6 H(2)O in the presence of pivalic acid (pivH) in MeCN under ambient conditions. Compounds 2-6 are isomorphous and crystallize in the monoclinic space group P2(1)/n with four molecules in the unit cell. The complexes have a centrosymmetric tridecanuclear anionic core consisting of two distorted inner heterometallic [Mn(III)Ln(III)(3)(mu(3)-OH)(4)](8+) cubane subunits sharing a common Mn vertex flanked by four edge-sharing heterometallic [Mn(III)Ln(III)(2)(mu(3)-OH)(4)](5+) defect cubane units. Complexes 2-6 are the first high-nuclearity 3d-4f aggregates reported to date using (t)Bu-deaH(2) as ligand. These compounds show no evidence of slow relaxation behavior above 1.8 K, which appears to be the consequence of the very weak or non-existent magnetic interactions between the Mn(III) and Ln(III) ions resulting from the particular angles at the bridging oxygens.

Molecular magnets containing wheel motifs.

An overview of some compounds with structures that can be regarded as derived from an {Fe(7)(mu(3)-OR)(6)(mu-OR)(6)} motif is presented. Many of these compounds act as single-molecule magnets. In addition, the results of a comprehensive study on the single-crystal structures, magnetism, and spin structures for five new iron compounds with structural components related to this are given. This serves to illustrate the methodology that can be used to unravel the complicated magnetic behavior of such systems. Thus, the alkoxo- and phenoxo-bridged multinuclear iron complexes, [Fe(II)(7)(Hbmsae)(6)(OMe)(6)]Cl(2) x 6 H(2)O (6), [Fe(II)Fe(III)(6)(bmsae)(6)(mu(3)-OMe)(6)]Cl(2) x 23 H(2)O (7), [NaFe(III)(6)(bmsae)(6)(mu(3)-OMe)(6)]Cl x 30 H(2)O (8), [Fe(III)(3)(bmsae)(3)Cl(2)(MeOH)(OMe)] x MeOH x Et(2)O (9), and [Fe(III)(2)(Hbmsae)(2)(OMe)(2)Cl(2)][Fe(3)(bmsae)(3)(OMe)Cl(2)(MeOH)](2) x 1.5 MeOH (10), were prepared by the reactions of iron sources with Schiff base ligands (H(2)bmsae = 5-bromo-3-methoxysalicylideneaminoethanol). Heptanuclear complexes of 6 and 7 have wheel structures in which six iron(II) and six iron(III) ions locate on the rim with the central iron(II) ions, respectively, while 8 also has a wheel structure with a diamagnetic sodium ion in the center. 9 is a trinuclear ferric complex with an incomplete cubic structure. In 10, di- and trinuclear complexes exist in the crystal, where the trinuclear complex has an incomplete cubic structure similar to that of 9. Magnetic susceptibility measurements revealed that 6 has an S = 10 spin ground state, while the antiferromagnetic interactions dominate in 7-10.

Family of MnIII2Ln2(μ4-O) Compounds: Syntheses, Structures, and Magnetic Properties

An isostructural family of tetranuclear aggregates [Mn(III)(2)Ln(2)(O)(Piv)(2)(hep)(4)(NO(3))(4)].MeCN (where Ln = Y(III) (1), Pr(III) (2), Nd(III) (3), Gd(III) (4), Tb(III) (5), Dy(III) (6), Ho(III) (7), and Yb(III) (8)) is reported. They were obtained from the reactions of 2-(2-hydroxyethyl)pyridine (hepH) with a preformed hexanuclear manganese complex, [Mn(6)], and the respective lanthanide salt. The complexes are isomorphous and represent a new heterometallic 3d-4f complex type for this class of ligand. The structural core of 1-8 consists of a distorted Mn(2)Ln(2) tetrahedron with the four metal centers linking through a mu(4)-O(2-) bridging atom. The magnetic properties of all complexes were investigated by variable temperature magnetic susceptibility and magnetization measurements. The magnetic data of all compounds suggest that antiferromagnetic interactions are present between adjacent paramagnetic centers. Complexes 5-7 containing highly anisotropic lanthanide ions (Tb, Dy, and Ho) show slow relaxation of their magnetization.

Enhancing single molecule magnet parameters. Synthesis, crystal structures and magnetic properties of mixed-valent Mn4 SMMs

A novel synthetic route is reported for the synthesis of three dicubane complexes [MnII2MnIII2(bdea)2(bdeaH)2(O2CCMe3)4] (1), [MnII2MnIII2(bdea)2(bdeaH)2(O2CPh)4] (2), and [MnII2MnIII2(teaH)2(teaH2)2(O2CPh)2](O2CPh)2·0.7MeCN·0.3EtOH (3), obtained from the reaction of N-butyldiethanolamine (bdeaH2, for 1 and 2) or triethanolamine (teaH3, for 3) with Mn(OAc)2·4H2O as manganese source and [M3O(O2CR)6](O2CR) (R = CMe3 for 1, and Ph for 2 and 3 and M: Fe or Mn) compounds as carboxylate source. Complex 1 crystallises in the monoclinic space group P21/c (a = 13.2125(7) A, b = 10.9602(6) A, c = 22.2329(11) A, β = 95.053(1)°, V = 3235.9(3) A3, with Z = 2). Complex 2 crystallises in the monoclinic space group P21/n (a = 16.6894(14) A, b = 8.7360(7) A, c = 21.3378(18) A, β = 92.949(2)°, V = 3106.9(4) A3, with Z = 2). Complex 3 crystallises in the monoclinic space group C2/c (a = 30.242(3) A, b = 8.6710(6) A, c = 26.618(3) A, β = 119.915(9)°, V = 6050.0(10) A3, with Z = 4). The X-ray crystallographic structure analyses revealed that 1–3 are essentially isostructural, each consisting of a planar mixed-valent “butterfly” core lying about an inversion centre and comprising two outer “wingtip” heptacoordinate MnII ions and two inner hexacoordinate MnIII. Magnetic properties have been studied for all three complexes using combined ac and dc measurements, revealing single-molecule magnet (SMM) behaviour for 1–3, with an S = 9 spin ground state. A detailed analysis of the slow relaxation of the magnetisation has been performed to characterise their SMM behaviour.

Magnetic coordination clusters and networks: synthesis and topological description

With the discovery of the phenomenon of single-molecule magnetism, coordination chemists have turned their attention to synthesizing cluster aggregates of paramagnetic ions. This has led to a plethora of coordination clusters with various topologies and diverse magnetic properties. In this paper, we present ways of describing and understanding such compounds as well as outlining a new approach, which we have recently developed, to describing cluster topology. Our approach is based upon and pays tribute to the huge contribution made to coordination chemistry through the development of the Schläfli symbols for describing architectures. To illustrate the developments that are taking place in modern coordination chemistry, we start with some basic definitions of relevance to what follows. Then we describe approaches to discovering new magnetically interesting 3d/4f clusters, assigning their topological descriptions. Finally, we show how the concepts behind the construction of metal–organic frameworks can be extended to using clusters as nodes in the frameworks to give super metal–organic frameworks.

An Undecanuclear Fe III Single-Molecule Magnet

The undecanuclear complex in [Fe(III)(11)(mu(4)-O)(3)(mu(3)-O)(4)(L)(3)(mu-O(2)CCMe(3))(12)]Cl.5MeCN has been synthesized and characterized by single-crystal X-ray crystallography. Mossbauer spectroscopy and bulk susceptibility studies reveal that the complex has an S = (13)/(2) spin ground state and exhibits single-molecule-magnet behavior.

New heterometallic [Mn(III)(4)Ln(III)(4)] wheels incorporating formate ligands.

Two heteroctanuclear wheel complexes with an eight-membered saddle-like ring of [Mn(III)(4)Ln(III)(4)] (Ln = Dy, Gd), prepared by employing formates as key carboxylate bridges, exhibit antiferromagnetic interactions between Mn(III) and Ln(III) centers, and the wheel containing anisotropic Dy(III) ions shows SMM behaviour.