Bao Li

Solvent-Induced Transformation of Single Crystals of a Spin-Crossover (SCO) Compound to Single Crystals with Two Distinct SCO Centers

The long-sought-after crystal structure of Fe(tpa)(NCS)(2) (1, tpa = tris(2-pyridylmethyl)amine), an otherwise well-studied spin-crossover (SCO) complex, has been obtained, and its one-step, incomplete spin transition was correlated to its solid-state structures at different temperatures. Upon exposure to methanol vapor, single-crystal-to-single-crystal transformation of 1 to a new SCO compound, 2, formulated as {[Fe(tpa)(NCS)(2)] x [Fe(tpa)(NCS)(2) x CH(3)OH]}, occurs with a dramatic color change from yellow to red. Crystallographic studies revealed that the asymmetric unit of the structure of 2 contains two independent Fe(II) centers. Studies by magnetic measurements and Mossbauer spectroscopy revealed a two-step complete spin transition for compound 2, between LS-LS and HS-HS, via an unambiguous intermediate LS-HS phase; the two SCO centers of disparate spin states were resolved crystallographically. That a significant portion of the original crystal structure is maintained indicates that the present approach is a more subtle means of altering the properties associated with SCO phenomenon than by changing counteranions or crystallization using different solvents. Furthermore, the dramatic changes in crystal structure and SCO behaviors triggered by mere solvent sorption suggest that this approach is rather efficient in modifying and hopefully fine-tuning and optimizing properties of SCO compounds. Coupled with the aforementioned gentleness and subtlety, the present approach of heterogeneously introducing perturbations to pre-existing supramolecular arrays of SCO units is more conducive to systematic studies aiming at the discovery of new SCO systems and phenomenon toward their ultimate materials applications.

Thermally Induced and Photoinduced Valence Tautomerism in a Two-Dimensional Coordination Polymer

Herein reported is the first two-dimensional coordination polymer capable of undergoing thermally induced and photoinduced valence tautomeric transitions.

Making Spin-Crossover Crystals by Successive Polymorphic Transformations

The complex [Fe(tpa)(NCS)(2)] showed successive polymorphic transformations (polymorphs II-IV) that produced crystals with distinct changes in color, crystal structure, and spin-crossover behavior.

Pressure Effects on a Spin-Crossover Monomeric Compound Fe(pmea)(SCN)(2) (pmea = bis (2-pyridyl)methyl -2-(2-pyridyl)ethylamine)

Spin-crossover (SCO) compounds are a sort of bistable material whose electronic and magnetic properties can be tuned by external physical stimuli, such as heat, light, and pressure. The title SCO compound [Fe(pmea)(NCS)(2)] (1; pmea = bis[(2-pyridyl)methyl]-2-(2-pyridyl)ethylamine) undergoes spin transition in such a way that it is an ideal candidate to investigate pressure effects on the SCO behavior. First, the spin transition is complete and abrupt so that the pressure-dependent spin transition should be remarkable. Second, the T(1/2) value under ambient pressure is 184 K, which guarantees that the SCO temperature under various pressures does not exceed that restrained by high-pressure devices. The magnetic data of compound 1 under different external pressures were analyzed through a known method, as reported by Gutlich, which gave an interaction parameter Gamma of 264(5) cm(-1) and a volume change DeltaV degrees (HL) of 32(3) A(3) molecule(-1) [HL represents a high-spin (HS) <--> low-spin (LS) transition], respectively. Meanwhile, the calculated entropy change DeltaS degrees (HL)(T) at 1 bar is 59.79 J mol(-1) K(-1), which is a typical value that drives the spin transition from a LS to HS state. The pressure effects on the SCO behavior of compound 1 reported here may provide information for a deep understanding of the correlation between pressure and spin transition.

Four Coordination Polymers Based on Identical Eight-Connected Heptanuclear Clusters: Spin Canting, Spin Glass, Antiferromagnetism, and Gas Adsorption

Four 3D coordination polymers, [Co7(OH)4(H2O)2(ina)4(ip)3]·10H2O (1·10H2O, ina = isonicotinate, ip = isophthalate), [Ni7(OH)4(H2O)2(ina)4(ip)3]·10H2O (2·10H2O), [Co7(OH)4(H2O)2(ina)4(pip)3]·5H2O (3·5H2O, pip = 5-phenyl-isophthalate), and [Ni7(OH)4(H2O)2(ina)4(pip)3]·5H2O (4·5H2O), respectively, were hydrothermally synthesized. They crystallized in the orthorhombic space group Pba2 for 1·10H2O and 2·10H2O and monoclinic space group P2/n for 3·5H2O and 4·5H2O, respectively, and were constructed with the identical 8-connected heptanuclear {M7(OH)4} (M = Co(II) or Ni(II)) clusters, possessing uninodal hexagonal primitive net with the point symbol {3(6)·4(18)·5(3)·6}. The four coordination polymers showed dominant antiferromangetic properties, in which 1·10H2O shows spin-canted behavior and 2·10H2O exhibits the coexistence of spin canting and spin glass. Meanwhile, the activated polymers 1 and 2 possessed permanent porosity, displaying relatively large H2 uptake capacity (77 K, 1 atm) of 114 and 133 cm(3) g(-1), and CO2 uptake capacity (273 K, 1 atm) of 65.8 and 73.3 cm(3) g(-1), for 1 and 2, respectively.

A three-dimensional complex with a one-dimensional cobalt-hydroxyl chain based on planar nonanuclear clusters showing spin-canted antiferromagnetism.

A new three-dimensional magnetic network, [Co9(OH)6(C42H24O18P3N3)2(H2O)8] (1), has been successfully prepared by utilizing the flexible hexacarboxylate ligand derived from cyclotriphosphazene, which had been characterized by single-crystal X-ray diffraction and magnetic measurement. This compound consists of one-dimensional (1D) cobalt-hydroxyl chains based on planar nonanuclear clusters, which are located in the b-c plane to form the nearly two-dimensional cobalt layer. Magnetic measurements reveal that 1 shows spin-canted antiferromagnetism with spin-glass behavior. These results suggest that reasonable design and choice of large carboxylate ligand based on a specific scaffold could be effective for the construction of magnetic materials based on a novel 1D magnetic chain.

An iron(II) incomplete spin-crossover compound: pressure effects and Mössbauer spectroscopy study

A one-dimensional iron(II) spin-crossover compound [Fe(3py-im)(2)(NCS)(2)].7H(2)O (1) (3py-im = 2,4,5-tris(4-pyridly)-imidazole) has been solvothermally synthesized and structurally characterized. Compound 1 crystallizes in the monoclinic space group P2/c with a = 11.9078(2), b = 9.9474(1), c = 17.7290(3) and beta = 102.361(2) degrees at 105 K. Studies on the variable-temperature magnetic susceptibilities and Mössbauer spectra suggest that compound undergoes incomplete spin transition behaviour. Pressure effects on the transition behaviour have also been investigated, the thermal-induced spin transition becomes more gradual and the critical temperature shifts towards slightly higher temperature range when external pressure increases. However, the spin transition can not be completed by applying external pressure even as high as 0.79 GPa.

Unidirectional Charge Transfer in Di-cobalt Valence Tautomeric Compound Finely Tuned by Ancillary Ligand

A dinuclear valence tautomeric compound containing a cationic structure with crystallographically distinguishable hs-Co(II) and ls-Co(III) centers undergoes unidirectional charge transfer.

Lanthanide coordination polymers with hexa-carboxylate ligands derived from cyclotriphosphazene as bridging linkers: synthesis, thermal and luminescent properties

Solvothermal reactions of two ligands with different geometries, derived from cyclotriphosphazene, hexakis(4-carboxylato-phenoxy)cyclotriphosphazene (H6L1) and hexakis(3-carboxylato-phenoxy)cyclotriphosphazene (H6L2) with Eu(NO3)3·6H2O in H2O/DMF under similar synthesis conditions produced three new compounds, namely, {[Eu2(L1)(H2O)4]·(H2O)4·(DMA)2}n (1), {[Eu2(L1)(H2O)8]·(H2O)2·DMA}n (2), and {[Eu2(L2)(H2O)3(DMF)]·(H2O)2·(DMA)}n (3). Compounds 1 and 2 display a 2D layer crystal structure with a distinct topological network incorporating the extended hexa-carboxylate ligand L1; in contrast, 3 has a 1D crystal structure with the highly distorted hexa-carboxylate ligand L2. In these three compounds, the ligands L1 and L2 are fully deprotonated, whose six extended carboxyl arms connect six different/same metallic nodes to generate metal–organic frameworks. The luminescence properties of three compounds have been studied at room temperature in relative detail.

Template controlled synthesis of cluster-based porous coordination polymers: crystal structure, magnetism and adsorption

Two distinct template mediated coordination polymers, [Co3(ina)4(N3)2(CH3OH)2] with tri-nuclear nodes (1) and [Co8(OH)(ina)8(N3)8·X] with dual tetra-nuclear cobalt nodes (2), have been synthesized under hydrothermal conditions by utilizing the template agent of pentaerythritol. 1 shows a 2D layer comprised of linear tri-nuclear CoII clusters, in which the central cobalt ion is inter-linked with two terminal ones via mixed bridges as syn–syn carboxylates and end-on azide ions. In contrast, compound 2 exhibits a three-dimensional framework based on two kinds of six-connected tetra-nuclear cobalt clusters: square-planar node and cuboidal node. Both 1 and 2 might exhibit spin-canted magnetism. In addition, the activated sample of 2 exhibits the sorption ability of H2 and CO2 molecules.