Ian J. Hewitt

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.

Spin Chirality in a Molecular Dysprosium Triangle: The Archetype of the Noncollinear Ising Model

Single crystal magnetic studies combined with a theoretical analysis show that cancellation of the magnetic moments in the trinuclear Dy3+ cluster [Dy{3}(mu{3}-OH)2L3Cl(H2O){5}]Cl{3}, resulting in a nonmagnetic ground doublet, originates from the noncollinearity of the single-ion easy axes of magnetization of the Dy3+ ions that lie in the plane of the triangle at 120 degrees one from each other. This gives rise to a peculiar chiral nature of the ground nonmagnetic doublet and to slow relaxation of the magnetization with abrupt accelerations at the crossings of the discrete energy levels.

Opening up a dysprosium triangle by ligand oximation

A simple trinuclear dysprosium complex shows complex slow relaxation of the magnetisation.

A series of new structural models for the OEC in photosystem II

A new series of MMn(II-III)(4) clusters (M = Na, Ca) has been structurally characterised and their relevance to understanding the oxygen evolving centre of photosystem II is discussed.

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.

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.

A [Mn18Dy] SMM resulting from the targeted replacement of the central MnII in the S = 83/2 [Mn19]-aggregate with DyIII

Anisotropy can be introduced into the [Mn(19)]-aggregate, which currently has the highest known spin ground state of S = 83/2, by the targeted replacement of the central Mn(II) cation with Dy(III) leading to a [Mn(18)Dy] complex with the same core topology showing slow relaxation of the magnetisation.

High-nuclearity 3d–4f [FeIII5LnIII8] complexes: synthesis, structure and magnetic properties

The isostructural tridecanuclear mixed-metal [FeIII5LnIII8] (Ln = Pr, Nd, Gd) aggregates are the largest [Fe-Ln] clusters reported to date and the magnetic properties suggest a ferrimagnetic arrangement in [Fe5Pr8] and [Fe5Nd8], whereas for [Fe5Gd8] ferromagnetic interactions are dominant leading to a large spin ground state.