Graham N. Newton

Three-way switching in a cyanide-bridged [CoFe] chain

Bistable compounds that exist in two interchangeable phases under identical conditions can act as switches under external stimuli. Among such switchable materials, coordination complexes have energy levels (or phases) that are determined by the electronic states of their constituent metal ions and ligands. They can exhibit multiple bistabilities and hold promise in the search for multifaceted materials that display different properties in different phases, accessible through the application of contrasting external stimuli. Molecular systems that exhibit both thermo- and photoinduced magnetic bistabilities are excellent candidates for such systems. Here we describe a cyanide-bridged [CoFe] one-dimensional chiral coordination polymer that displays both magnetic and electric bistabilities in the same temperature range. Both the electric and magnetic switching probably arise from the same electron-transfer coupled spin-transition phenomenon, which enables the reversible conversion between an insulating diamagnetic phase and either a semiconducting paramagnetic (thermoinduced) or a type of ferromagnetic single-chain magnet (photoinduced) state.

Programmable spin-state switching in a mixed-valence spin-crossover iron grid

Photo-switchable systems, such as discrete spin-crossover complexes and bulk iron-cobalt Prussian blue analogues, exhibit, at a given temperature, a bistability between low- and high-spin states, allowing the storage of binary data. Grouping different bistable chromophores in a molecular framework was postulated to generate a complex that could be site-selectively excited to access multiple electronic states under identical conditions. Here we report the synthesis and the thermal and light-induced phase transitions of a tetranuclear iron(II) grid-like complex and its two-electron oxidized equivalent. The heterovalent grid is thermally inactive but the spin states of its constituent metal ions are selectively switched using different laser stimuli, allowing the molecule to exist in three discrete phases. Site-selective photo-excitation, herein enabling one molecule to process ternary data, may have major ramifications in the development of future molecular memory storage technologies.

Undecanuclear mixed-valence 3d–4f bimetallic clusters

Two undecanuclear 3d-4f clusters with the general formula {Mn(III)(4)Mn(IV)Ln(III)(6)}, where Ln = Gd or Tb, were synthesized, with both showing large spin ground states, and the Tb species acting as a single molecule magnet.

Ferromagnetically coupled chiral cyanide-bridged {Ni6Fe4} cages

Enantiomeric, ferromagnetically coupled decanuclear {Ni₆Fe₄} cages with adamantane-like cores were synthesized around templating tetraethylammonium cations, as shown by crystallographic analysis and CSI-MS, and their homochiral nature was confirmed by circular dichroism measurements.

Redox‐Controlled Magnetic {Mn13} Keggin Systems

Polyoxometalates (POMs) have been widely reported in recent years. These molecular metal oxides, or polyanions, are most commonly constructed of tungsten, molybdenum, or vanadium ions in their highest oxidation state, bridged by oxide ligands to form clusters which can range in size from low-nuclearity building blocks to large-scale protein-like superstructures. An archetypical POM structural motif is the {XM12O40} n species (X = P, Si...) known as the Keggin anion and Keggin structures have been successfully shown to act as catalysts among other potential applications. POMs are inorganic materials that can be functionalized through their combination with organic ligands and/or the introduction of paramagnetic heterometal ions which leads to magnetic heterometallic POMs. In addition, there are a few studies of related species consisting exclusively of late first-row transition-metal ions such as some mixed-valence manganese Keggin-related clusters described by Lampropoulos et al, the uncapped {Fe13} cluster reported by Bino et al., and the reverse-Keggin structures presented by Baskar et al. To the best of our knowledge, there are no other examples of POM-type complexes consisting exclusively of open-shell transition metals, and so far their physical properties have been barely investigated. In contrast, transition-metal oxide materials are widely used and their properties such as magnetic ordering, semiand superconductivity, giant magnetoresistance, and ferroelectricity are much studied. Their electronic properties can be understood by their band structures and changed to show the desired characteristics. 11] Replication or improvement of metal oxide properties in discrete molecules can be extremely difficult. However, molecular metal clusters can possess wellseparated energy levels and their characteristic electronic structures can be altered to show, for example, valence tautomerism, multi-bistability with spin crossover, and singlemolecule-magnetic (SMM) behavior by tuning the frontier orbitals and the electronic interactions between the metal centers. The band filling in solids is readily controlled in their syntheses by altering the ratio of the constituent elements which drastically changes the physical properties. The question arises whether the chemist can synthesize metal oxide clusters displaying controllable redox states which can perturb the physical properties. Herein, a polyoxometalate-type cluster was synthesized by using exclusively first-row transition-metal ions in combination with organic capping ligands. In the resultant system the spin state and magnetic properties were tuned without substantial change to the molecular structure, and its SMM behavior was perturbed through manipulation of the cluster oxidation state. Herein, the synthesis, magnetic properties, and redox behavior of three mixed-valence {Mn13} Keggintype clusters are reported. The one-pot reaction of Mn(NO3)2·6 H2O with 2,6-bis[N(2-hydroxyethyl)iminomethyl]-4-methylphenol (H3bemp) [16] in methanol yielded a tridecanuclear cluster [Mn12Mn O6(OH)2(OMe)4(bemp)6](NO3)4·10MeOH·6H2O (1(NO3)4) (Figure 1 and Figure S1 in the Supporting Information). Subsequent high-yielding crystallization led to the formation of dark brown square blocks of 1(NO3)4, the counterions of which were exchanged to yield 1(PF6)4 from a solution of 1(NO3)4 and NH4PF6 in methanol. The core structure and physical behavior of 1(PF6)4 were identical to 1(NO3)4. Dark brown platelets of 1(PF6)4 were then dissolved in methanol with one or two equivalents of [Fe(bpy)3](PF6)3 (bpy = 2,2’-bipyridine) to yield dark brown rhombic crystals of oxidized 1(PF6)5 and 1(PF6)6, respectively. Cyclic voltammetry (CV) measurements conducted on 1(NO3)4 (1 mm) in N,N-dimethylformamide (DMF) showed four quasireversible waves at 0.02, 0.18, 0.50, and 0.73 V versus the saturated calomel electrode (SCE), corresponding to four one-electron redox processes of 1/1, 1/1, 1/ 1, and 1/1, respectively (Figure 2). Approximating the complex as a trinuclear redox-active system, we calculated comproportionation constants of 560, 2.6 10, and 5.5 10 for the reduced 1 (MnMn11Mn ), the native species 1 (Mn12Mn ), and the oxidation product 1 (Mn11Mn IV 2), [*] Dr. G. N. Newton, S. Yamashita, K. Hasumi, J. Matsuno, N. Yoshida, Dr. M. Nihei, Dr. T. Shiga, Prof. Dr. H. Oshio Graduate School of Pure and Applied Sciences University of Tsukuba Tennodai 1-1-1, Tsukuba 305-8571 (Japan) Fax: (+ 81)29-853-4238 E-mail: oshio@chem.tsukuba.ac.jp

Synthesis and characterisation of a lanthanide-capped dodecavanadate cage

The synthesis of a series of discrete lanthanide-capped polyoxovanadate cages is presented along with magnetic and electrochemical measurements which reveal a redox active dodecavanadate cluster with potential as a new functional building unit in polyoxovanadate chemistry.

Contrasting Magnetism of [MnIII4] and [MnII2MnIII2] Squares

Two tetranuclear manganese distorted square-shaped clusters, [Mn(III)(4)(L1)(4)(mu(2)-OMe)(4)].2.5H(2)O (1) and [Mn(II)(2)Mn(III)(2)(L2)(4)(H(2)O)(2)](PF(6))(2).CHCl(3).CH(3)OH.1.5H(2)O (2) (H(2)L1 = 2-[3-(2-hydroxyphenyl)-1H-pyrazol-5-yl]-6-pyridinecarboxylic acid methyl ester; H(2)L2 = 2-[3-(2-hydroxyphenyl)-1H-pyrazol-5-yl]-6-pyridinecarboxylic acid ethyl ester), exhibit antiferromagnetic and ferromagnetic interactions between neighboring manganese ions, respectively.

Exploring a series of isostructural dodecanuclear mixed Ni:Co clusters: toward the control of elemental composition using pH and stoichiometry.

The compositional parameter space in the formation of polynuclear clusters is probed in the synthesis of a series of dodecanuclear coordination clusters of Ni(II) and Co(II) with isostructural D(3h)-symmetric frameworks. At the core of their construction are a carbonate template and the directing ligands cis,trans-1,3,5-triaminocyclohexane and acetate at contrasting pH values. The pH and stoichiometric dependence has been mapped, and analysis by electrospray mass spectrometry reveals the cluster cores in solution. In two specific cases, site-specific occupations are eluded to by analysis of the magnetic properties, and we discuss the possibility of controlling the molecular composition of mixed metal polynuclear clusters.

Controlled reactivity tuning of metal-functionalized vanadium oxide clusters.

Controlling the assembly and functionalization of molecular metal oxides [Mx Oy ](n-) (M=Mo, W, V) allows the targeted design of functional molecular materials. While general methods exist that enable the predetermined functionalization of tungstates and molybdates, no such routes are available for molecular vanadium oxides. Controlled design of polyoxovanadates, however, would provide highly active materials for energy conversion, (photo-) catalysis, molecular magnetism, and materials science. To this end, a new approach has been developed that allows the reactivity tuning of vanadium oxide clusters by selective metal functionalization. Organic, hydrogen-bonding cations, for example, dimethylammonium are used as molecular placeholders to block metal binding sites within vanadate cluster shells. Stepwise replacement of the placeholder cations with reactive metal cations gives mono- and difunctionalized clusters. Initial reactivity studies illustrate the tunability of the magnetic, redox, and catalytic activity.

Heat capacity reveals the physics of a frustrated spin tube.

We report on theoretical and experimental results concerning the low-temperature specific heat of the frustrated spin-tube material [(CuCl(2)tachH(3)Cl]Cl(2) (tach denotes 1,3,5-triaminocyclohexane). This substance turns out to be an unusually perfect spin-tube system which allows to study the physics of quasi-one-dimensional antiferromagnetic structures in rather general terms. An analysis of the specific-heat data demonstrates that at low enough temperatures the system exhibits a Tomonaga-Luttinger liquid behavior corresponding to an effective spin-3/2 antiferromagnetic Heisenberg chain with short-range exchange interactions. On the other hand, around 2 K the composite spin structure of the chain is revealed through a Schottky-type peak in the specific heat. We argue that the dominating contribution to the peak originates from gapped magnon-type excitations related to the internal degrees of freedom of the rung spins.