Miguel Julve

Spin Crossover in a Catenane Supramolecular System

The compound [Fe(tvp)2(NCS)2] � CH3OH, where tvp is 1,2-di-(4-pyridyl)-ethylene, has been synthesized and characterized by x-ray single-crystal diffraction. It consists of two perpendicular, two-dimensional networks organized in parallel stacks of sheets made up of edge-shared [Fe(II)]4 rhombuses. The fully interlocked networks define large square channels in the [001] direction. Variable-temperature magnetic susceptibility measurements and M�ssbauer studies reveal that this compound shows low-spin to high-spin crossover behavior in the temperature range from 100 to 250 kelvin. The combined structural and magnetic characterization of this kind of compound is fundamental for the interpretation of the mechanism leading to the spin crossover, which is important in the development of electronic devices such as molecular switches.

Coordination chemistry of N,N′-bis(coordinating group substituted)oxamides: a rational design of nuclearity tailored polynuclear complexes

Abstract The coordinating properties of N , N ′-bis(coordinating group substituted)oxamides have been thoroughly investigated both in aqueous solution and in the solid state. The easy cis – trans isomerization equilibria that they exhibit together with the great variety of N , N ′-substituents which can be used to play on the overall charge, complexing ability and polarity, make them very suitable ligands in designing homo- and heterometallic species. The knowledge of their complex formation in aqueous solution by potentiometry and using the hydrogen ion concentration as a probe, allowed us to settle the basis of a rational design of oxamidate-containing polynuclear species whose nuclearity can be easily tuned. Concerning their electronic properties, the strong basicity of the deprotonated amide–nitrogen atoms stabilizes high oxidation states of late first-row transition metal ions. Finally, one of the most appealing aspects of this type of ligands is the remarkable efficiency they exhibit to mediate strong antiferromagnetic interactions between paramagnetic centres when acting as bridges but in order to maintain the present work within a rational length, we have omitted this last point which deserves a further review.

Crystal structure and magnetic properties of the flexible self-assembled two-dimensional square network complex [Cu2(mal)2(H2O)2(4,4′-bpy)] (H2mal=malonic acid and 4,4′-bpy=4,4′-bipyridine)

Abstract The copper(II) complex [Cu 2 (mal) 2 (H 2 O) 2 (4,4′-bpy)] ( 1 ) (H 2 mal=malonic acid and 4,4′-bpy=4,4′-bipyridine) has been prepared and its structure determined by single crystal X-diffraction methods. Compound 1 has a two-dimensional square grid network structure. The square grids are stacked parallel but in a staggered manner on each other along the c -axis, with an interlayer separation of 3.850(1) A. Each layer contains a large cavity of 15.784(1)×15.784(1) A with each edge shared by one malonate group and one 4,4′-bpy ligand and a small planar square of 4.644(1)×4.644(1) A with Cu(II) ions and malonate groups at each corner and side, respectively. Each copper atom is in a distorted square-pyramidal surrounding with three carboxylate-oxygen atoms from two malonate groups and one nitrogen atom from a 4,4′-bpy ligand building the equatorial plane and a water molecule in the axial position. Each 4,4′-bpy molecule exhibits the bis-monodentate bridging mode whereas the malonate simultaneously adopts the bidentate (at one copper atom) and monodentate (at the adjacent copper atom) coordination modes. The bridging carboxylato exhibits the anti – syn coordination mode. The magnetic properties of 1 have been investigated in the temperature range 1.9–300 K and they correspond to a dominant ferromagnetic coupling through bridging malonato within the small square defined by four copper(II) ions ( J =+12.4(1) cm −1 ) and much weaker intrasheet antiferromagnetic coupling between copper(II) ions through bridging 4,4′-bpy ( j eff =−0.052(1) cm −1 ).

Field‐Induced Hysteresis and Quantum Tunneling of the Magnetization in a Mononuclear Manganese(III) Complex

High-nuclearity complexes of transition-metal ions have been of special interest during the last two decades owing to the possibility of observing slow magnetic relaxation effects at the molecular level. These molecular nanomagnets have potential applications as new high-density magnetic memories and quantum-computing devices in the field of molecular spintronics. The first example of a discrete molecule exhibiting hysteresis and quantum tunneling of the magnetization was the mixed-valent dodecanuclear manganese(III,IV) complex [Mn12O12(CH3CO2)16(H2O)4]. [3] Since then, a plethora of both homoand heterovalent, manganese-based molecular nanomagnets of varying metal oxidation states (i.e., Mn, Mn and/or Mn) have been reported, with nuclearities from up to [Mn84] down to the smaller [Mn III 2] species. [4] However, to our knowledge, there are no examples of mononuclear manganese complexes exhibiting the slow magnetic relaxation effects typical of molecular nanomagnets, referred to as single-ion magnet (SIMs). This is somewhat puzzling, since several SIMs of other highly anisotropic first-row transition metals (i.e., Co and Fe) have been recently reported which has rekindled the debate in the field of singlemolecular magnetism. The six-coordinated octahedral high-spin d Mn ion (S = 2) has an orbitally degenerate Eg ground electronic term that is split by the Jahn–Teller effect into A1g and B1g orbital singlet low-lying states. Owing to the large mixing between them, second-order spin-orbit coupling (SOC) effects are ultimately responsible for the occurrence of a large axial magnetic anisotropy whose sign depends on the ground state, that is, on the nature of the axial tetragonal distortion. For an axially elongated octahedral Mn environment, negative D values are expected that can potentially lead to a large energy barrier for the magnetization reversal between the two lowest MS = 2 states. To provide this type of geometry and obtain manganese(III)-based SIMs, planar tetradentate chelating ligands with strong donor groups are a well-suited choice. Herein, we report a complete study on the synthesis, structural characterization, spectroscopic and magnetic properties, and theoretical calculations of Ph4P[Mn(opbaCl2)(py)2] (1) [H4opbaCl2 = N,N’-3,4-dichloro-o-phenylenebis(oxamic acid), py = pyridine, and Ph4P + = tetraphenylphosphonium cation]. Complex 1 is the first example of a mononuclear manganese(III) complex exhibiting a field-induced slow magnetic relaxation behavior, thus increasing the number of first-row transition-metal-ion SIMs. Complex 1 was obtained as well-formed deep brown cubic prisms by slow evaporation of a methanol/pyridine (1:4 v/v) solution of its tetramethylammonium salt in the presence of an excess of Ph4PCl (see Supporting Information). It crystallizes in the P21/c space group of the monoclinic system (Table S1, Supporting Information). The crystal structure of 1 consists of mononuclear manganese(III) complex anions, [Mn(opbaCl2)(py)2] (Figure 1), which are well separated from each other due to the presence of the bulky tetraphenylphosphonium countercations (Figure S1, Supporting Information). The manganese atom of 1 has a tetragonally elongated octahedral coordination geometry which is typical of the Jahn–Teller distorted d Mn ion. The equatorial plane is formed by two amidate nitrogen and two carboxylate oxygen atoms from the opbaCl2 ligand, while the axial positions are occupied by two pyridine nitrogen atoms (Figure 1a). The planar opbaCl2 ligand adopts a tetradentate coordination

Structural versatility of the malonate ligand as a tool for crystal engineering in the design of molecular magnets

The synthesis of ferro- and ferri-magnetic systems with a tunable Tc and three-dimensional (3-D) ordering from molecular precursors implying transition metal ions is one of the active branches of molecular inorganic chemistry. The nature of the interactions between the transition metal ions (or transition metal ions and radicals) is not so easy to grasp by synthetic chemists working in this field since it may be either electrostatic (orbital) or magnetic (mainly dipolar). Therefore, the systems fulfilling the necessary requirements to present the expected magnetic properties are not so easy to design on paper and realize in the beaker. In this work we show how the design of one-, two- and three-dimensional materials can strongly benefit from the use of crystal engineering techniques, which can give rise to structures of different shapes, and how these differences can give rise to different properties. We will focus on the networks constructed by assembling malonate ligands and metal centres. The idea of using malonate (dianion of propanedioic acid, H2mal) is that it can give rise to different coordination modes with the metal ions it binds. Extended magnetic networks of dimensionalities one (1-D), two (2-D) and three (3-D) can be chemically constructed from malonate-bridged metallic complexes. These coordination polymers behave as ferro-, ferri- or canted antiferro-magnets. We are currently trying to obtain analogous compounds using magnetically anisotropic ions, such as cobalt(II), in order to explore how structural differences influence the magnetic properties. In this case the control of the spatial arrangement of the magnetic building blocks is of paramount importance in determining the strength of the magnetic interaction. The possibility of controlling the shape of the networks depends on the coordination bond between the metal ion and the ligands and on supramolecular interactions such as stacking interactions or hydrogen bonding.

Malonate-based copper(II) coordination compounds: ferromagnetic coupling controlled by dicarboxylates

Studies on structural and magnetic properties of polynuclear transition metal complexes, aimed at understanding the structural and chemical factors governing electronic exchange coupling mediated by multiatom bridging ligands, are of continuing interest to design new molecular materials exhibiting unusual magnetic, optical and electrical properties, bound to their molecular nature. Looking at potentially flexible bridging ligands, the malonate group seems a suitable candidate. The occurrence of two carboxylate groups in the 1,3 positions allows this ligand to adopt simultaneously chelating bidentate and different carboxylato bridging modes (syn–syn, anti–anti and syn–anti trough one or two carboxylate groups) In the course of our research we have structurally and magnetically characterized several carboxylato bridged copper(II) complexes. In the present study we start describing briefly the structure and the magnetic behaviour of the compounds, subsequently we analyze the magneto-structural correlations concluding that the parameter that governs, in first order, the magnetic interaction between metal centres is the relative position of the carboxylato bridge of the malonate respect to the copper(II) ions: equatorial–equatorial (strong interaction), equatorial–apical (weak interaction) and apical–apical (negligible interaction). Inside this division another parameters become important such as β (angle between copper(II) basal planes) in the equatorial–equatorial, or the distortion t in the equatorial–apical.

Highly Selective Chemical Sensing in a Luminescent Nanoporous Magnet

Among the wide variety of properties of interest that a given material can exhibit, luminescence is attracting an increasing attention due to its potential application in optical devices for lighting equipment and optical storage, [ 1a − c] optical switching, [ 1d ,e] and sensing. [ 1f − i ] At this respect, many scientists, working in the multidisciplinary fi eld of the materials science, have directed their efforts to the obtention of luminescent materials with potential sensing applications. For instance, sensitive and selective detection of gas and vapor phase analytes can result specially interesting because of the variety of applications that can be found in many different fi elds. A key principle concerning the luminescent chemosensors [ 2 ] is that they must be able to detect differences between small molecules, [ 2 , 3 ] and sequentially implement a recognition– transduction protocol. [ 2b ] In this sense, the remarkable shape selectivity of a class of highly porous materials, the so-called metal-organic frameworks (MOFs) [ 4 ] which have already shown applications in different fi elds (gas storage and separation, molecular recognition and catalysis, molecular electronics and spintronics, molecular photonics, etc) [ 4–6 ] has converted them in excellent candidates for the fabrication of chemical sensors. [ 2 , 3 ] The key point responsible for the high potential success of MOFs as chemo-sensors is the exceptional tunability of their structures and properties.

Cyanide-bridged Fe(III)–Co(II) bis double zigzag chains with a slow relaxation of the magnetisation

Reaction of [Fe(III)(bipy)(CN)4]- with fully solvated M(II) cations [M = Co (1) and Mn (2)] produces the isostructural bis double zigzag chains [[Fe(III)(bipy)(CN)4]2M(II)(H2O)] x MeCN x (1/2)H2O; 1 exhibits intrachain ferromagnetic and interchain antiferromagnetic couplings, slow magnetic relaxation and hysteresis effects.

Synthesis, crystal structure and magnetic properties of two-dimensional malonato-bridged cobalt(II) and nickel(II) compounds

Two isostructural malonato-bridged complexes of formula {[M(H2O)2][M(mal)2(H2O)2]}n [M = Co(II) (1), Ni(II) (2); H2mal = malonic acid] have been synthesised and characterized by X-ray diffraction. Their structure consists of corrugated layers of trans-diaquabismalonatemetalate(II) and trans-diaquametal(II) units bridged by carboxylate–malonate groups in the anti–syn conformation. Two crystallographycally independent metal atoms occur in 1 and 2. The malonate anion acts simultaneously as a bidentate and bis-monodentate ligand. Variable-temperature (1.9–295 K) magnetic susceptibility measurements indicate the occurrence of weak antiferro- (1) and ferromagnetic (2) interactions between the cobalt(II) (1) and nickel(II) ions (2) through the anti–syn caboxylate–malonate bridge. A brief discussion on the structural diversity and crystal engineering possibilities of the malonate complexes with divalent first-row transition metal ions other than copper(II) is carried out.

2,2′-Bipyrimidine (bipym)-bridged dinuclear complexes. Part 4. Synthesis, crystal structure and magnetic properties of [CO2(H2O)8(bipym)][NO3]4, [CO2(H2O)8(bipym)][SO4]2·2H2O and [CO2(bipym)3(NCS)4]

Three new dinuclear cobalt(II) complexes [Co2(H2O)8(bipym)][NO3]41, [Co2(H2O)8(bipym)]-[SO4]2·2H2O 2 and [Co2(bipym)3(NCS)4]3(bipym = 2,2′-bipyrimidine) have been synthesised and their crystal structures determined by X-ray diffraction methods. Crystals of 1 and 3 are triclinic, space group P, Z= 1 with a= 7.511(1), b= 8.844(2), c= 9.514(1)A, α= 79.67(1), β= 88.54(1) and γ= 82.46(1)° for 1 and a= 9.045(2), b= 9.149(2), c= 11.621(2)A, α= 74.73(2), β= 80.67(2) and γ= 61.17(1)° for 3. Crystals of 2 are monoclmic, space group P21/c with a= 8.115(1), b= 11.596(2), c= 11.823(3)A, β= 91.57(2)° and Z= 2. The structures of 1 and 2 are made up of dinuclear cations [Co2(H2O)8(bipym)]4+ with nitrate counter ions for 1 and water of crystallization and sulfate groups for 2. The structure of 3 consists of dimeric neutral [Co2(bipym)3(NCS)4] units. Bipyrimidine bridges the cobalt atoms in this series of complexes in a bis(chelating) fashion and a crystallographically imposed inversion centre is located halfway between its two pyrimidyl rings. It is present also as a terminal ligand in 3. The two equivalent cobalt atoms of 1and 2 are each bound to four water oxygens and two cis nitrogens of bipym in a slightly distorted octahedral environment. Each cobalt atom in 3 is bound to six nitrogen atoms belonging to two thiocyanate groups in cis position and to two bipym ligands, one terminal and the other bridging. The intramolecular metal–metal separation varies between 5.761(1) and 5.942(2)A. The magnetic properties of 1–3 have been investigated in the temperature range 4.2–300 K. They all exhibit antiferromagnetic exchange with susceptibility maxima at 13.0–16.4 K. Fits of the magnetic data through the Lines model and the simpler lsing spin-only formalism are compared.