Search

Two mononuclear nickel(II) complexes of the formula [Ni(terpyCOOH)2](ClO4)2∙4H2O (1) and [Ni(terpyepy)2](ClO4)2 MeOH (2) [terpyCOOH = 4′-carboxyl-2,2′:6′,2″-terpyridine and terpyepy = 4′-[(2-pyridin-4-yl)ethynyl]-2,2′:6′,2″-terpyridine] have been prepared and their structures determined by single-crystal X-ray diffraction. Complexes 1 and 2 are mononuclear compounds, where the nickel(II) ions are six-coordinate by the six nitrogen atoms from two tridentate terpy moieties. The mean values of the equatorial Ni-N bond distances [2.11(1) and 2.12(1) Å for Ni(1) at 1 and 2, respectively, are somewhat longer than the axial ones [2.008(6) and 2.003(6) Å (1)/2.000(1) and 1.999(1) Å (2)]. The values of the shortest intermolecular nickel–nickel separation are 9.422(1) (1) and 8.901(1) Å (2). Variable-temperature (1.9–200 K) direct current (dc) magnetic susceptibility measurements on polycrystalline samples of 1 and 2 reveal a Curie law behavior in the high-temperature range, which corresponds to magnetically isolated spin triplets, the downturn of the χMT product at lower temperatures being due to zero-field splitting effects (D). Values of D equal to −6.0 (1) and −4.7 cm−1 (2) were obtained through the joint analysis of the magnetic susceptibility data and the field dependence of the magnetization. These results from magnetometry were supported by theoretical calculations. Alternating current (ac) magnetic susceptibility measurements of 1 and 2 in the temperature range 2.0–5.5 K show the occurrence of incipient out-phase signals under applied dc fields, a phenomenon that is characteristic of field-induced Single-Molecule Magnet (SMM) behavior, which herein concerns the 2 mononuclear nickel(II) complexes. This slow relaxation of the magnetization in 1 and 2 has its origin in the axial compression of the octahedral surrounding at their nickel(II) ions that leads to negative values of D. A combination of an Orbach and a direct mechanism accounts for the field-dependent relation phenomena in 1 and 2.

A new mixed-valence one-dimensional coordination polymer of formula {[CoII(MeOH)2][(μ-NC)2CoIII(dmphen)(CN)2]2}n·2nH2O (1) was obtained by reacting the Ph4P[CoII(dmphen)(CN)3] metalloligand (dmphen = 2,9-dimethyl-1,10-phenanthroline and Ph4P+ = tetraphenylphosphonium ion) with cobalt(II) acetate tetrahydrate. The structural analysis shows the formation of a neutral 4,2-ribbon-like chain of vertex-sharing cyanido-bridged {CoIII2CoII2} squares in which the metalloligand underwent an oxidation process and a reorganization to form {CoIII(dmphen)(CN)4}− linkers that coordinate to the [CoII(MeOH)2]2+ units through single cyanido ligands. Both cobalt(II) and Co(III) cations are six-coordinated in distorted octahedral environments. The shortest intrachain distance between the paramagnetic cobalt(II) ions is 7.36 Å, a value which is shorter than the shortest interchain one (10.36 Å). Variable-temperature (1.9–300 K) static (dc) magnetic measurements for 1 indicate the occurrence of magnetically isolated high-spin cobalt(II) ions with a D value of +67.0 cm−1. Dynamic alternating current (ac) magnetic measurements between 2.0–13 K reveal that 1 exhibits slow magnetic relaxation under non-zero applied dc fields, being thus a new example of field-induced SIM with easy-plane magnetic anisotropy. Theoretical calculations by CASSCF/NEVPT2 on 1 support the results from magnetometry. The relaxation of the magnetization occurs in the ground state under external dc fields through a two-phonon Raman process and one intra-Kramers mechanism.

Materials that demonstrate long-range magnetic order are synonymous with information storage. Here, the authors report the effect of pressure on two mononuclear rhenium compounds that exhibit long-range magnetic order under ambient conditions via a spin canting mechanism, whereTcis proportional to pressure.

Molecular magnetism has made a long journey, from the fundamental studies on through-ligand electron exchange magnetic interactions in dinuclear metal complexes with extended organic bridges to the more recent exploration of their electron spin transport and quantum coherence properties. Such a field has witnessed a renaissance of dinuclear metallacyclic systems as new experimental and theoretical models for single-molecule spintronics and quantum computing, due to the intercrossing between molecular magnetism and metallosupramolecular chemistry. The present review reports a state-of-the-art overview as well as future perspectives on the use of oxamato-based dicopper(II) metallacyclophanes as promising candidates to make multifunctional and multiresponsive, single-molecule magnetic (nano)devices for the physical implementation of quantum information processing (QIP). They incorporate molecular magnetic couplers, transformers, and wires, controlling and facilitating the spin communication, as well as molecular magnetic rectifiers, transistors, and switches, exhibiting a bistable (ON/OFF) spin behavior under external stimuli (chemical, electronic, or photonic). Special focus is placed on the extensive research work done by Professor Francesc Lloret, an outstanding chemist, excellent teacher, best friend, and colleague, in recognition of his invaluable contributions to molecular magnetism on the occasion of his 65th birthday.

The assembly of [Co2III(μ-2,5-dpp)(CN)8]2− anions and [MII(CH3OH)2(DMSO)2]2+ cations resulted into the formation of two heterobimetallic 1D coordination polymers of formula [MII(CH3OH)2(DMSO)2(μ-NC)2Co2III(μ-2,5-dpp)(CN)6]n·4nCH3OH [M = CoII (1)/FeII (2) and 2,5-dpp = 2,5-bis(2-pyridyl)pyrazine. The [Co2III(μ-2,5-dpp)(CN)8]2− metalloligand coordinates the paramagnetic [MII(CH3OH)2(DMSO)2]2+ complex cations, in a bis-monodentate fashion, to give rise to neutral heterobimetallic chains. Cryomagnetic dc (1.9–300 K) and ac (2.0–13 K) magnetic measurements for 1 and 2 show the presence of Co(II)HS (1) and Fe(II)HS (2) ions (HS – high-spin), respectively, with D values of +53.7(5) (1) and −5.1(3) cm−1 (2) and slow magnetic relaxation for 1, this compound being a new example of SIM with transversal magnetic anisotropy. Low-temperature Q-band EPR study of 1 confirms that D value is positive, which reveals the occurrence of a strong asymmetry in the g-tensors and allows a rough estimation of the E/D ratio, whereas 2 is EPR silent. Theoretical calculations by CASSCF/NEVPT2 on 1 and 2 support the results from magnetometry and EPR. The analysis of the ac magnetic measurements of 1 shows that the relaxation of M takes place in the ground state under external magnetic dc fields through dominant Raman and direct spin-phonon processes.

The structures of the series of N,N′-1,n-phenylenebis(oxamic acid ethyl ester) molecules with n= 2 (H2Et2opba, 1), 3 (H2Et2mpba, 2), and 4 (H2Et2ppba, 3) have been determined by single-crystal X-ray diffraction (XRD) methods. Density functional (DF) calculations have been performed on the simplest model system N-phenyloxamic acid methyl ester (HMepma). Compounds 1–3have either folded (H2Et2opba), bent (H2Et2mpba), or linear (H2Et2ppba) almost planar (periplanar) molecular configurations with the two oxalamide moieties being slightly tilted up and down, respectively, with respect to the benzene ring. The energy calculations as a function of the torsion angle () around the N(amide)–C(benzene) bond for HMepma reveal that the minimum energy synand antiperiplanar conformations of the carboxamide functions are more stable than the corresponding synand antiplanar ones (= 0 and 180°) by 0.18 and 0.13 kcal mol−1, respectively. The calculated values for the synand antiperiplanar minimized conformers of HMepma are 16.0 and 200.0°, respectively, in reasonable agreement with the experimental values for 1–3[= 39.0(4) and 225.0(3) (H2Et2opba), 32.6(5) (H2Et2mpba), and 34.7(2)° (H2Et2ppba)]. This situation likely minimizes the forced repulsive interactions between the amide hydrogen and the nearest benzene hydrogen atoms while it maximizes the attractive interactions between the carbonyl amide oxygen and the nearest benzene hydrogen atoms, which are then implicated in a relatively weak, intramolecular C–H(benzene)OC(amide) hydrogen bond [d(HO) = 2.45(2)–2.57(2) Å]. A supramolecular aggregation of molecules into either a duplex (H2Et2opba) or a brick-wall sheet (H2Et2ppba) occurs for 1and 3, respectively, through moderately strong, intermolecular N–H(amide)OC(amide) hydrogen bonds [d′(HO) = 2.17(2)–2.37(2) Å]. By contrast, moderately weak, intermolecular N–H(amide)OC(ester) hydrogen bonds between the H2Et2mpba molecules are involved in 2to give a meso-helical chain with a unique hydrogen-bonded oxalamide acid ester dimeric unit. The energy calculations as a function of the intermolecular N–H(amide)OC(ester) hydrogen bond distance (d′) for the {HMepma}2dimer show an energy minimum at 2.37 Å, in excellent agreement with the experimental value of 2[d′(HO) = 2.42(4) Å]. The calculated value of the hydrogen bond energy for {HMepma}2(EHB= 4.83 kcal mol−1) is consistent with a partially covalent nature of the interaction between the amide hydrogen and the carbonyl ester oxygen atoms, as confirmed by the existence of a significant electron density delocalization within the resulting four-center H2O2diamond core. [ABSTRACT FROM AUTHOR]

Three new gadolinium(III) complexes with carboxylate ligands of formula [Gd2(ac)6(H2O)4]·2H2O (1), [Gd2(ac)2(fum)2(H2O)4]n(2) and [Gd2(ox)(fum)2(H2O)4]n·4nH2O (3) (ac = acetate, fum = fumarate and ox = oxalate) have been prepared and their structures determined by X-ray diffraction on single crystals. The structure of 1is made up of discrete centrosymmetric di-µ-oxo(carboxylate acetate)digadolinium(III) units with an intramolecular GdGd separation of 4.1589(3) . Each gadolinium atom in 1is nine-coordinated with two water molecules and seven carboxylate-oxygen atoms from four acetate ligands building a monocapped square antiprism environment. Compound 2exhibits a sheet-like structure, the repeating intralayer motif being a rectangle of di-µ-oxo(carboxylate acetate)digadolinium(III) units [GdGd separation of 3.8659(10) ] where the edges are defined by bridging bis-bidentate and tetrakis-monodentate fumarate groups. Each gadolinium atom in 2is nine-coordinated with two water molecules and seven carboxylate oxygens from two acetate and three fumarate ligands building a distorted monocapped square antiprism. Compound 3has a three-dimensional structure where chains of double oxo(carboxylate fumarate)-bridged gadolinium(III) ions are interlinked by bis-bidentate fumarate to afford layers which are further interconnected through bis-bidentate oxalate ligands. The corresponding values of the GdGd separation are 4.5816(1), 8.2292(2) and 6.2989(1) . Each gadolinium atom in 3is ten-coordinated with two water molecules and eight oxygen atoms from four fumarate ligands and one oxalate group building a distorted bicapped square antiprism. Variable-temperature magnetic susceptibility measurements show the occurrence of weak but significant ferro- (1and 3) and antiferromagnetic (2) interactions through the double oxo(carboxylate acetate) (1and 2), double syn-syncarboxylate fumarate (2) and double oxo(carboxylate fumarate) and bis-bidentate oxalate (3) bridges. Monte Carlo calculations were performed in the case of 3to simulate its magnetic data and to substantiate the best-fit values of the magnetic couplings through the different exchange pathways involved. [ABSTRACT FROM AUTHOR]

Three novel Re(iv) mononuclear complexes of formulae NBu4[ReCl5(pyz)] (1), NH2Me2[ReCl5(pyz)] (2) and NH4[ReCl5(pyz)]·0.75H2O (3), (pyz being pyrazine; NBu4+ = tetra-n-butylammonium cation, NH2Me2+ = dimethylammonium cation and NH4+ = ammonium cation), were synthesized by ligand substitution reaction from [ReCl6]2− anion and pyrazine in N,N-dimethylformamide (DMF). In addition, two new heterobimetallic compounds, the salt namely [ReCl5(pyz)]2[Ni(cyclam)] (4) (cyclam = 1,4,8,11-tetraazacyclotetradecane) and the heterotrinuclear [{ReCl5(μ-pyz)}2Cu(DMF)4] (5) complex, were prepared by using as precursor 1 and 3, respectively. Compounds 1–5 were characterized by single-crystal X-ray diffraction. 1–3 are made up of discrete [ReCl5(pyz)]− anions and NBu4+ (1), NH2Me2+ (2) and NH4+ (3) cations. [ReCl5(pyz)]− unit interacts toward Ni(ii) (4) and Cu(ii) (5) metal ions through different modes. In 4 two [ReCl5(pyz)]− anions weakly interact with a [Ni(cyclam)]2+ cation through chloro atoms, while compound 5 is a heterotrinuclear pyrazine-bridged ReIV2CuII complex made up of [ReCl5(pyz)]− anions and [Cu(DMF)4]2+ cations. The magnetic properties of 1–5 were investigated in the temperature range 1.9–300 K. The magnetic behaviour of 1 is that of a magnetically diluted Re(iv) complex with a large value of zero-field splitting of the ground state (|2D| is ca. 18.8(1) cm−1), whereas those of 2 and 3 are typical of antiferromagnetically coupled systems exhibiting susceptibility maxima at 10 (2) and 12 K (3). Compound 4 shows antiferromagnetic interactions between Re(iv) metal ions, Ni(ii) being diamagnetic (because of its square-planar geometry), while 5 exhibits a ferromagnetic coupling between Re(iv) and Cu(ii) metal ions across the pyrazine bridges with a JReCu value of +11.8(1) cm−1. [ABSTRACT FROM AUTHOR]

The aim and scope of this review is to show the general validity of the ‘complex-as-ligand’ approach for the rational design of metallosupramolecular assemblies of increasing structural and magnetic complexity. This is illustrated herein on the basis of our recent studies on oxamato complexes with transition metal ions looking for the limits of the research avenue opened by Kahn's pioneering research twenty years ago. The use as building blocks of mono-, di- and trinuclear metal complexes with a novel family of aromatic polyoxamato ligands allowed us to move further in the coordination chemistry-based approach to high-nuclearity coordination compounds and high-dimensionality coordination polymers. In order to do so, we have taken advantage of the new developments of metallosupramolecular chemistry and in particular, of the molecular-programmed self-assembly methods that exploit the coordination preferences of metal ions and specifically tailored ligands. The judicious choice of the oxamato metal building block (substitution pattern and steric requirements of the bridging ligand, as well as the electronic configuration and magnetic anisotropy of the metal ion) allowed us to control the overall structure and magnetic properties of the final multidimensional nD products (n = 0–3). These species exhibit interesting magnetic properties which are brand-new targets in the field of molecular magnetism, such as single-molecule or single-chain magnets, and the well-known class of molecule-based magnets. This unique family of molecule-based magnetic materials expands on the reported examples of nD species with cyanide and related oxalato and dithiooxalato analogues. Moreover, the development of new oxamato metal building blocks with potential photo or redox activity at the aromatic ligand counterpart will provide us with addressable, multifunctional molecular materials for future applications in molecular electronics and nanotechnology. [ABSTRACT FROM AUTHOR]

DFT calculations reveal the unusual ferromagnetic exchange observed in an oxo-centered MnIII triangle may originate from a combination of the ‘non-planarity’ of the bridging oxime ligands and the non-parallel alignment of the Jahn–Teller axes. [ABSTRACT FROM AUTHOR]

The reaction in water of Cu(OH)2 with 2,2′-bipyridine (bipy) and (NH4)2HPO4 in a 4 : 4 : 2 molar ratio under an inert atmosphere leads to the formation of a tetranuclear copper(ii) complex of formula {[(H2O)2Cu4(bipy)4(μ4–PO4)2(μ2–OH)]·0.5HPO4·15.5H2O}, 1, with butterfly topology. The structure of the tetranuclear core in 1 consists of four crystallographically unique copper(ii) ions in approximate square-pyramidal geometry with each coordinated to a bipy ligand and interacting through two μ4–O,O′,O″-phosphate bridges. Additional bridging between Cu(3) and Cu(4) is provided by a hydroxide ligand, whereas two water molecules cap the Cu(1) and Cu(2) square pyramids to yield a N2O3 chromophore at each copper atom. Adjacent tetranuclear units align in anti-parallel fashion where proximate metal-bound water molecules interact with each other through both intra- and inter-molecular H-bonding to link two such clusters. These pairs then further associate through π⋯π interactions between bipy ligands to form a 2D sheet with neighbouring sheets separated by H-bonded lattice water molecules, which form a 2D H-bonded network. Variable-temperature magnetic susceptibility measurements performed upon 1 reveal net intramolecular ferromagnetic coupling between the copper(ii) ions and this is supported and rationalized by a DFT study. [ABSTRACT FROM AUTHOR]

The hydrothermal reaction of 2-cyanopyrimidine and either CoCl2·6H2O or FeCl2·4H2O affords 2D isostructural coordination polymers [M2(χ-pymca)3]OH·H2O ((M = CoII (1) and FeII (2) pymca = pyrimidine-2-carboxylato). The bisdidentate ligand (pymca) that can be considered an intermediate between bipyrimidine and oxalato is generated in situ from the hydrolysis of 2-cyanopyrimidine. The structure of 1 and 2 consists of heterochiral (6,3) honeycomb layers, crystal water molecules, and OH- anions, the latter playing a template and balancing charge role in the structure. Both compounds exhibit antiferromagnetic interactions between metal ions through the pyrimidine-2-carboxylate bridging ligand. Compound 1 is a spin-canted antiferromagnet leading to weak ferromagnetism at Tc < 10 K with a coercitive field of 580 Oe, whereas compound 2 is an antiferromagnet with TN = 21 K. Fit of the variable-temperature magnetic susceptibility data of 2 to the empirical equation for a regular honeycomb with S = 1 derived from Monte Carlo simulations leads to the following parameters: J = -4.57(2) cm-1 and g = 2.300(4). Density functional calculations have been used to explain the magnetic coupling in 2. [ABSTRACT FROM AUTHOR]

The isomorphous coordination polymers {µ-Au(CN)2]2[(M(NH3)2)2(µ-bpym)]}[Au(CN)2]2 (M = Co(ii) (1), Ni(ii) (2), Cu(ii) (3)) have been prepared from the reaction of 2 equiv. M(NO3)2·nH2O (M = Cu(ii), n = 3; M = Ni(ii) and Co(ii), n = 6) with 1 equiv. of bipyrimidine (bpym) in aqueous ammonia and then with an aqueous solution containing 1 equiv. of K[Au(CN)2]. The structures of these complexes are made of bpym bridged centrosymmetric dinuclear [M(NH3)2(µ-bpym)M(NH3)2] units connected by [Au(CN)2]− anions to four other dinuclear units giving rise to a cationic 2D (4,4) rectangular grid network, its charge being balanced by two non-coordinated [Au(CN)2]−. The layers are stacked in such a way that the ammonia coordinated molecules are interdigitated and aligned above and below one sheet with cavities in neighbouring sheets, giving rise to an ABAB⋯ repeat pattern of layers. Gold atoms of bridging and non-bridging dicyanoaurate anions are involved in short aurophilic interactions (Au1–Au2 distances in the range 3.12–3.14 Å), leading to a chain of gold atoms running along the a direction. Neighbouring gold chains are further connected by weaker aurophilic interactions (Au1–Au1 distances in the range 3.43–3.49 Å), affording a honeycomb-like 2D network of gold atoms. The (4,4) rectangular sheets and (6,3) honeycomb sheets share the Au2 atoms, leading to a unique 3D network. Magnetic measurements clearly show the existence of antiferromagnetic exchange coupling between the metal ions with susceptibility maxima at 17 K (1), 22 K (2), and 17 K (3). The data of 1 were analyzed through a full Hamiltonian involving spin–orbit coupling, axial distortion, Zeeman interactions and magnetic exchange coupling between CoII, and the best fit gives J = −9.23 cm−1, κ = 0.99, λ = −142 cm−1, Δ = −562 cm−1. For 2 and 3, magnetic data were fitted to the theoretical equations derived from the isotropic Hamiltonian: H = −JS1S2. The best fit parameters were g = 2.050(1), J = −17.51(1) and P = 0.01(2) for 2 and g = 2.068(5), J = −20.07(8) and P = 0.015(4) for 3, respectively (P takes into account the amount of paramagnetic impurity). In order to explain the weak magnetic interaction between copper(ii) ions mediated by the bipyrimidine bridging ligand in 3, we have carried out electronic structure calculations based on the density functional theory (DFT). [ABSTRACT FROM AUTHOR]

This theoretical study discusses the interplay of the magnetic anisotropy and magnetic exchange interaction of two Mn6 complexes and suggests that large magnetic anisotropy is not favoured by a high spin state of the ground state. [ABSTRACT FROM AUTHOR]