Silvio Decurtins

A polymeric two-dimensional mixed-metal network. Crystal structure and magnetic properties of {[P(Ph)4][MnCr(ox)3]}

Abstract The mixed-metal ferromagnet {[P(Ph) 4 ][MnCr(ox) 3 ]} n , where Ph is phenyl and ox is oxalate, has been prepared and a two-dimensional network structure, extended by Mn(II)-ox-Cr(III) bridges, has been determined from single crystal X-ray data. Crystal data: space group R 3 c , a = b =18.783(3), c =57.283(24) A, α=β=90, γ=120°, Z =24 (C 30 H 20 O 12 PCrMn). The magnetic susceptibility data obey the Curie-Weiss law in the temperature range 260–20 K with a positive Weiss constant of 10.5 K. The temperature dependence of the molar magnetization exhibits a magnetic phase transition at T c =5.9 K. The structure is discussed in relation to the strategy for preparing molecular based ferromagnets and, in addition, it is a solution to the question of the dimensionality of the [MM′(ox) 3 ] n network, which in principle can extend two- or three-dimensionally to the crystal lattice. The optical absorption spectra of the single crystals are assigned to the ‘CrO 6 ’ chromophores. Their polarization patterns reflect the electric dipole selection rules for D 3 symmetry. A strong site selective luminescence from the chromium(III) 2 E states is observed at low temperature and the system may be suitable for studying energy transfer mechanisms.

MULTIFUNCTIONAL COORDINATION COMPOUNDS: DESIGN AND PROPERTIES

Abstract Cleverly designed molecular building blocks provide chemists with the tools of a powerful molecular-scale construction set. They enable them to engineer materials that possess a predictable order and useful solid-state properties. Hence, it is in the realm of supramolecular chemistry to follow a strategy for synthesizing materials which combine a selected set of properties, for instance from the areas of magnetism, photophysics and electronics. As a possible approach, host/guest solids which are based on extended anionic, homo- and bi-metallic oxalato-bridged transition-metal compounds with two- and three-dimensional connectivities are investigated. In particular, we report herein in detail about their structural properties and their multifunctional characteristics in the area of molecular magnetism and photophysics.

Trimethylsilyl-Terminated Oligo(phenylene ethynylene)s: An Approach to Single-Molecule Junctions with Covalent Au–C σ-Bonds

A new and efficient approach using cleaving of trimethylsilyl groups to create covalent Au-C anchoring sites has been developed for single-molecule junction conductance measurements. Employing the mechanically controllable break junction (MCBJ) technique in liquid, we demonstrate the formation of highly conducting single molecular junctions of several OPE derivatives. The created junctions are mechanically stable and exhibit conductances around one order of magnitude higher than those of their dithiol analogues. Extended assembly and reaction times lead to oligomerization. Combined STM imaging and gap-mode Raman experiments provide structure evidence to support the formation of covalent Au-C contacts and further oligomerization.

Cyclic Conductance Switching in Networks of Redox-Active Molecular Junctions

Redox-active dithiolated tetrathiafulvalene derivatives (TTFdT) were inserted in two-dimensional nanoparticle arrays to build interlinked networks of molecular junctions. Upon oxidation of the TTFdT to the dication state, we observed a conductance increase of the networks by up to 1 order of magnitude. Successive oxidation and reduction cycles demonstrated a clear switching behavior of the molecular junction conductance. These results show the potential of interlinked nanoparticle arrays as chemical sensors.

Nickel(II)-, cobalt(II)-, copper(II)-, and zinc(II)-phthalate and 1-methylimidazole coordination compounds: synthesis, crystal structures and magnetic properties

Three new coordination polymers [M(Pht)(1-MeIm)2]n (where M=Cu (1), Zn (2), Co (3); Pht2−=dianion of o-phthalic acid; 1-MeIm=1-methylimidazole) and two compounds [M(1-MeIm)6](HPht)2 · 2H2O (M=Co (4), Ni (5)) have been synthesized and characterized by X-ray crystallography. The structures of 1–3 (2 is isostructural to 3) consist of [M(1-MeIm)2] building units connected by 1,6-bridging phthalate ions to form infinite chains. In complex 1, each copper(II) center adopts a square coordination mode of N2O2 type by two O atoms from different phthalate ions and two N atoms of 1-MeIm, whereas in 3 two independent metal atoms are tetrahedrally (N2O2) coordinated to a pair of Pht ligands and a pair of 1-MeIm molecules. There are only van der Waals interactions between the chains in 1, while the three-dimensional network in 3 is assembled by C–H⋯O contacts. In contrast to polymers 1–3 the structures of 4 and 5 (complexes are also isostructural) are made up of the [M(1-MeIm)6]2+ cation, two hydrogen phthalate anions (HPht−) and two H2O solvate molecules. The coordination around each metal(II) atom is octahedral with six nitrogen atoms of 1-MeIm. Extended hydrogen bonding networks embracing the solvate water molecules and a phthalate residue as well as the weak C–H⋯O interactions stabilize the three-dimensional structures. Magnetic studies clearly show that the magnetic ions do not interact with each other. Furthermore, in compound 4 we have another example of a highly anisotropic Co2+ ion with a rhombic g-tensor and large zero-field-splitting. The complexes were also characterized by IR and 1H NMR spectroscopy, thermogravimetric analysis, and all data are discussed in the terms of known structures.

Synthesis, structure and properties of {M4O4} cubanes containing nickel(II) and cobalt(II)

A survey of the crystal structures containing simple {M4O4} cubane units is reported. It shows that the average M-M distance in these complexes is relatively constant for a given metal ion M. The structures are all distorted from the idealised cube to a T(d) structure, and most show a further distortion which, however, usually maintains some elements of symmetry. A system for classifying the different types of ligand in these complexes is proposed. Two new cubanes of cobalt(II) and nickel(II) with the ligand (R,R)-bis-1,2-(1-methylbenzimidazol-2-yl)ethane-1,2-diol, (R,R)- or its enantiomer have been isolated and the crystal structure of the cobalt(II) complex confirms the cubane structure. Electronic, CD and (1)H NMR spectra and magnetic susceptibility data are reported. The magnetic data for these and other compounds in the literature are discussed in terms of the structural parameters.

Imidazole-Annulated Tetrathiafulvalenes Exhibiting pH-Tuneable Intramolecular Charge Transfer and Redox Properties

In order to study the electronic interactions in donor-acceptor ensembles as a function of pH, an efficient synthetic route to three imidazole-annulated tetrathiafulvalene (TTF) derivatives 1-3 is reported. Their electronic absorption spectra, in view of photoinduced intramolecular charge transfer, and their electrochemical behavior were investigated, and pK(a) values for the two protonation processes on the acceptor unit were determined in organic solvents by photometric titration. The influence of the TTF moiety on these values is discussed.

Pronounced Electrochemical Amphotericity of a Fused Donor-Acceptor Compound : A Planar Merge of TTF with a TCNQ-Type Bithienoquinoxaline

Narrowing the gap: A compactly fused π‐conjugated molecule combines a high‐lying HOMO with a low‐lying LUMO (Eox−Ered=0.52 eV) and a fairly low‐lying LUMO+1 on the bridging unit, giving rise to strong optical charge‐transfer transitions. A facile electron transfer has been observed by EPR and NMR spectroscopies.

Polyfunctional two- (2D) and three- (3D) dimensional oxalate bridged bimetallic magnets

We report major results concerning polyfunctional two- (2D) and three- (3D) dimensional oxalate bridged bimetallic magnets. As a consequence of their specific organisation they are composed of an anionic sub-lattice and a cationic counter-part. These bimetallic polymers can accommodate various counter-cations possessing specific physical properties in addition to the magnetic ones resulting from the interactions between the metallic ions in the anionic sub-lattice. Thus, molecular magnets possessing paramagnetic, conductive and optical properties are presented in this review.

Tunneling, remanence, and frustration in dysprosium-based endohedral single-molecule magnets

Paramagnetic atoms inside nanometer sized fullerenes realize robust, and chemically protected, spin systems. Changing the stoichiometry of the endohedral clusters results in a variety of magnetic ground states, as it is demonstrated for DynSc3-nN@C-80 (n = 1,2,3). All three exhibit distinct hysteresis and qualify as single-molecule magnets. In zero field the magnetization of n = 1 decays via quantum tunneling, while ferromagnetic coupling of the individual dysprosium moments results in remanence for Dy2ScN@C-80 and in a frustrated ground state for n = 3. The latter ground state turns out to be one of the simplest realizations of a frustrated, ferromagnetically coupled, system.