Rafael Ruiz-García

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

Ferromagnetic Coupling by Spin Polarization in a Trinuclear Copper(II) Metallacyclophane with a Triangular Cage-Like Structure

A series of trinuclear copper(II) complexes of general formula A(6)[Cu(3)L(2)] x nH(2)O [L = benzene-1,3,5-tris(oxamate); A = Li(+) (n = 8), 1a; Na(+) (n = 11.5), 1b; and K(+) (n = 8.5), 1c] have been synthesized, and they have been structurally and magnetically characterized. X-ray diffraction on single crystals of 1c shows the presence of three square-planar copper(II)-bis(oxamato) moieties which are connected by a double benzene-1,3,5-triyl skeleton to give a unique metallacyclophane-type triangular cage. The copper basal planes are virtually orthogonal to the two benzene rings, which adopt an almost perfect face-to-face alignment. Complexes 1a-c exhibit a quartet (S = 3/2) ground spin state resulting from the moderate ferromagnetic coupling (J values in the range of +7.3 to +16.5 cm(-1)) between the three Cu(II) ions across the two benzene-1,3,5-tris(amidate) bridges [H = -J(S(1) x S(2) + S(2) x S(3) + S(3) x S(1)) with S(1) = S(2) = S(3) = S(Cu) = 1/2]. Density functional theory calculations on the S = 3/2 Cu(II)(3) ground spin state of 1c support the occurrence of a spin polarization mechanism for the propagation of the exchange interaction, as evidenced by the sign alternation of the spin density in the 1,3,5-substituted benzene spacers.

Dicopper(II) metallacyclophanes as multifunctional magnetic devices: a joint experimental and computational study.

Metallosupramolecular complexes constitute an important advance in the emerging fields of molecular spintronics and quantum computation and a useful platform in the development of active components of spintronic circuits and quantum computers for applications in information processing and storage. The external control of chemical reactivity (electro- and photochemical) and physical properties (electronic and magnetic) in metallosupramolecular complexes is a current challenge in supramolecular coordination chemistry, which lies at the interface of several other supramolecular disciplines, including electro-, photo-, and magnetochemistry. The specific control of current flow or spin delocalization through a molecular assembly in response to one or many input signals leads to the concept of developing a molecule-based spintronics that can be viewed as a potential alternative to the classical molecule-based electronics. A great variety of factors can influence over these electronically or magnetically coupled, metallosupramolecular complexes in a reversible manner, electronic or photonic external stimuli being the most promising ones. The response ability of the metal centers and/or the organic bridging ligands to the application of an electric field or light irradiation, together with the geometrical features that allow the precise positioning in space of substituent groups, make these metal-organic systems particularly suitable to build highly integrated molecular spintronic circuits. In this Account, we describe the chemistry and physics of dinuclear copper(II) metallacyclophanes with oxamato-containing dinucleating ligands featuring redox- and photoactive aromatic spacers. Our recent works on dicopper(II) metallacyclophanes and earlier ones on related organic cyclophanes are now compared in a critical manner. Special focus is placed on the ligand design as well as in the combination of experimental and computational methods to demonstrate the multifunctionality nature of these metallosupramolecular complexes. This new class of oxamato-based dicopper(II) metallacyclophanes affords an excellent synthetic and theoretical set of models for both chemical and physical fundamental studies on redox- and photo-triggered, long-distance electron exchange phenomena, which are two major topics in molecular magnetism and molecular electronics. Apart from their use as ground tests for the fundamental research on the relative importance of the spin delocalization and spin polarization mechanisms of the electron exchange interaction through extended π-conjugated aromatic ligands in polymetallic complexes, oxamato-based dicopper(II) metallacyclophanes possessing spin-containing electro- and chromophores at the metal and/or the ligand counterparts emerge as potentially active (magnetic and electronic) molecular components to build a metal-based spintronic circuit. They are thus unique examples of multifunctional magnetic complexes to get single-molecule spintronic devices by controlling and allowing the spin communication, when serving as molecular magnetic couplers and wires, or by exhibiting bistable spin behavior, when acting as molecular magnetic rectifiers and switches. Oxamato-based dicopper(II) metallacyclophanes also emerge as potential candidates for the study of coherent electron transport through single molecules, both experimentally and theoretically. The results presented herein, which are a first step in the metallosupramolecular approach to molecular spintronics, intend to attract the attention of physicists and materials scientists with a large expertice in the manipulation and measurement of single-molecule electron transport properties, as well as in the processing and addressing of molecules on different supports.

Photoswitching of the antiferromagnetic coupling in an oxamato-based dicopper(II) anthracenophane

Thermally reversible photomagnetic (ON/OFF) switching behavior has been observed in a dinuclear oxamatocopper(II) anthracenophane upon UV light irradiation and heating; the two Cu(II) ions (S(Cu) = 1/2) that are antiferromagnetically coupled in the dicopper(II) metallacyclic precursor (ON state) become uncoupled in the corresponding [4+4] photocycloaddition product (OFF state), as substantiated from both experimental and theoretical studies.

Ordered mesoporous silicas as host for the incorporation and aggregation of octanuclear nickel(II) single-molecule magnets: a bottom-up approach to new magnetic nanocomposite materials

Silica-based mesoporous materials have been employed as the support host for a suitably designed small octanuclear nickel(II) guest complex with a moderately anisotropic S = 4 ground spin state (D = −0.23 cm−1), which behaves as a single-molecule magnet at low temperature (TB = 3.0 K). Both unimodal MCM-41 and bimodal UVM-7 porous silica provide appropriate template conditions for the incorporation and aggregation of the Ni8 complex precursor into larger complex aggregates, showing slow relaxation of the magnetization at higher blocking temperatures than the crystalline material. By playing with the initial complex vs. silica concentration, two series of samples with varying complex loading amounts have been obtained. The degree of aggregation varies, largely depending on the silica used, being higher for the bimodal UVM-7 silica series. The mesophasic and porous nature of the Ni8 adsorbed silica samples has been verified from XRD and TEM images. N2 adsorption–desorption isotherms show that incorporation initiates inside the small intra-particle mesopores while subsequent aggregation occurs at the external particle surface (close to the mesopore entrances). Both DC and AC magnetic susceptibility measurements have demonstrated the occurrence of such a unique silica-mediated surface aggregation process of cationic Ni8 molecules into oligomeric [Ni8]x aggregates of large spin values (S = 4x) and high blocking temperatures (TB = 4.5–10.5 K). The existence of a wide distribution of aggregates with different conformation and association degree (size distribution) and the presence of weak interactions between the aggregates leads to an exotic spin glass magnetic behavior for this family of host–guest hybrid nanocomposite materials.

From metal to ligand electroactivity in nickel(II) oxamato complexes

The locus of oxidation in square-planar nickel(ii) oxamato complexes can be continuously shifted from the metal to the ligand by an appropriate choice of electron-donating substituents on the aromatic moiety of the ligand.

Topological control in the hydrogen bond-directed self-assembly of ortho-, meta-, and para-phenylene-substituted dioxamic acid diethyl esters

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 1–3 have 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 (ϕ) around the N(amide)–C(benzene) bond for HMepma reveal that the minimum energy syn and anti periplanar conformations of the carboxamide functions are more stable than the corresponding syn and anti planar ones (ϕ = 0 and 180°) by 0.18 and 0.13 kcal mol−1, respectively. The calculated ϕ values for the syn and anti periplanar minimized conformers of HMepma are 16.0 and 200.0°, respectively, in reasonable agreement with the experimental values for 1–3 [ϕ = 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(H⋯O) = 2.45(2)–2.57(2) A]. A supramolecular aggregation of molecules into either a duplex (H2Et2opba) or a brick-wall sheet (H2Et2ppba) occurs for 1 and 3, respectively, through moderately strong, intermolecular N–H(amide)⋯OC(amide) hydrogen bonds [d′(H⋯O) = 2.17(2)–2.37(2) A]. By contrast, moderately weak, intermolecular N–H(amide)⋯OC(ester) hydrogen bonds between the H2Et2mpba molecules are involved in 2 to 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}2 dimer show an energy minimum at 2.37 A, in excellent agreement with the experimental value of 2 [d′(H⋯O) = 2.42(4) A]. 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 H2O2 diamond core.

Self-assembly, metal binding ability, and magnetic properties of dinickel(II) and dicobalt(II) triple mesocates

Two metallacyclic complexes of general formula Na8[MII2L3]·xH2O [M = Ni (4) and Co (5) with x = 15 (4) and 17 (5)] have been self-assembled in aqueous solution from N,N′-1,3-phenylenebis(oxamic acid) (H4L) and M2+ ions in a ligand/metal molar ratio of 3 : 2 in the presence of NaOH acting as base. X-Ray structural analyses of 4 and 5 show triple-stranded, dinuclear anions of the meso-helicate-type (so-called mesocates) with C3h molecular symmetry. The two octahedral metal–tris(oxamate) moieties of opposite chiralities (Δ,Λ form) are connected by three m-phenylene spacers at intermetallic distances of 6.822(2) (4) and 6.868(2) A (5) to give a metallacryptand core. In the crystal lattice, the binding of these heterochiral dinickel(II) and dicobalt(II) triple mesocates to sodium(I) ions leads to oxamato-bridged heterobimetallic three-dimensional open-frameworks with a hexagonal diamond architecture having small pores of 17.566(4) (4) and 17.640(2) A (5) in diameter where the crystallization water molecules and the sodium(I) countercations are hosted. Variable temperature (2.0–300 K) magnetic susceptibility measurements reveal relatively anisotropic S = 2 NiII2 (4) and S = 3 CoII2 (5) ground states resulting from the moderate to weak intramolecular ferromagnetic coupling between the two high-spin NiII (SNi = 1) or CoII (SCo = 3/2) ions across the m-phenylenediamidate bridges [J = +3.6 (4) and +1.1 cm−1 (5); H = −JS1·S2]. A simple molecular orbital analysis of the electron exchange interaction identifies the π-type pathways of the meta-substituted phenylene spacers involving the dz2 and dx2−y2 pairs of magnetic orbitals of the two trigonally distorted octahedral high-spin MII ions (M = Ni and Co) as responsible for the overall ferromagnetic coupling observed in 4 and 5 in agreement with a spin polarization mechanism. The decrease of the overall ferromagnetic coupling from 4 to 5 is in turn explained by the additional antiferromagnetic exchange contribution involving the dxy pair of magnetic orbitals of the two trigonally distorted octahedral high-spin CoII ions across the σ-type pathway of the meta-substituted phenylene spacers.

Dicopper(II) anthraquinophanes as multielectron reservoirs for oxidation and reduction: a joint experimental and theoretical study.

Two new dinuclear copper(II) metallacyclophanes with 1,4-disubstituted 9,10-anthraquinonebis(oxamate) bridging ligands are reported that can reversibly take and release electrons at the redox-active ligand and metal sites, respectively, to give the corresponding mono- and bis(semiquinonate and/or catecholate) Cu(II)2 species and mixed-valent Cu(II)/Cu(III) and high-valent Cu(III)2 ones. Density functional calculations allow us to give further insights on the dual ligand- and metal-based character of the redox processes in this novel family of antiferromagnetically coupled di- copper(II) anthraquinophanes. This unique ability for charge storage could be the basis for the development of new kinds of molecular spintronic devices, referred to as molecular magnetic capacitors (MMCs).

Magneto-structural correlations in asymmetric oxalato-bridged dicopper(II) complexes with polymethyl-substituted pyrazole ligands

Abstract Two oxalato-bridged dinuclear copper(II) complexes, [{Cu(Hdmpz)3}2(μ-ox)](ClO4)2·2H2O (1) and [{Cu(Htmpz)3}2(μ-ox)](ClO4)2·2H2O (2) (Hdmpz = 3,5-dimethyl-1H-pyrazole and Htmpz = 3,4,5-trimethyl-1H-pyrazole), have been synthesized and structurally and magnetically characterized. The crystal structures of 1 and 2 consist of asymmetric bis-bidentate μ-oxalatodicopper(II) complex cations with two short [Cu–O = 1.976(2) (1) and 1.973(2) Å (2)] and two long copper–oxygen bonds [Cu–O = 2.122(2) (1) and 2.110(2) Å (2)]. The environment at each CuII ion in 1 and 2 is closer to the trigonal bipyramidal geometry than to the square pyramidal [τ = 0.633 (1) and 0.711 (2)]. The magnetic properties of 1 and 2 show moderate antiferromagnetic coupling between the CuII ions through the oxalato bridge [J = −129 (1) and 161 cm−1 (2); H = −JS1 · S2 with S1 = S2 = SCu = ½]. A thorough magneto-structural analysis has been carried out for the family of asymmetric μ-oxalatodicopper(II) complexes with different terminal ligands to determine the influence on the sign and strength of the magnetic coupling of subtle structural factors, such as the trigonal distortion of the metal geometry and the lengthening or the asymmetry of the metal-oxalato bond distances.