Chou-Fu Sheu

Magnetostructural relationship in the spin-crossover complex t-{Fe(abpt)2[N(CN)2]2}: polymorphism and disorder phenomenon.

t-{Fe(abpt)(2)[N(CN)(2)](2)} [abpt = 4-amino-3,5-bis(pyridin-2-yl)-1,2,4-triazole] is an intriguing spin-crossover system that crystallizes in two polymorphs. Polymorph A is paramagnetic; its crystal structure consists of a single molecule located at the center of inversion symmetry. Polymorph B, on the other hand, exhibits a rather complicated two-step-like spin transition; its crystal structure consists of two symmetry-independent molecules. The crystal structure of polymorph B has been derived in the different spin states: above the high-temperature step (300 K), between the two steps (90 K), below the incomplete low-temperature step (50 K), in the light-induced metastable state (15 K), in the thermally quenched metastable state (15 K), and after relaxation from the quenched state (15 K). The correlation between the structure and magnetic properties is precisely established, allowing the complicated magnetic behavior of polymorph B to be well understood. A unique order-disorder phase transition, resulting in a modulation of the metastable state structures, is detected for the first time on such spin-transition compounds. The modulation of the structure originates from a particular ordering of the dicyanamide ligand at one of the two Fe sites.

Structure and Electronic Configuration of an Iron(II) Complex in a LIESST State: A Pump and Probe Method

Two polymorphs of mononuclear six-coordinate iron(II) spin-crossover complex trans-[Fe(tzpy)(2)(NCS)(2)] (tzpy = 3-(2-pyridyl)[1,2,3]triazolo[1,5-a]pyridine) (1) were isolated and structurally characterized. According to the thermally dependent magnetic measurements, polymorph A undergoes a gradual spin transition from a paramagnetic high-spin state ((5)T(2), S = 2, HS-1) above 200 K to a diamagnetic low-spin state ((1)A(1), S = 0, LS-1) below 120 K, whereas polymorph B shows an abrupt spin transition with T(1/2) at 102 K. Molecular and crystal structures of polymorph A in the HS-1 and LS-1 states were studied at 300 and 40 K, respectively. Significant differences in Fe-N distances and coordination geometries of Fe were found between the two spin states, as expected. Light-induced excited spin state trapping (LIESST) was observed upon irradiating the crystal with 532 nm laser light at 40 K, whereupon a metastable high-spin state (HS-2) was formed; the molecular and crystal structure of this metastable state were investigated by a pump and probe method because of its relatively fast relaxation. The electronic configuration of the Fe center in the HS-1, LS-1, and LIESST (HS-2) states were further confirmed by Fe K- and L-edge absorption spectroscopy. In addition, the C[triple bond]N stretching frequency on the ligand can also be followed through the spin transition. The excitation and relaxation process concerning such metastable state were followed by the C[triple bond]N stretching frequency and magnetic susceptibility measurements in the temperature ranges 15-55 K and 5-80 K, respectively. The structure and electronic configuration of the LIESST state of polymorph A were firmly established by X-ray diffraction, X-ray absorption, infrared absorption, and magnetic measurements. A single-crystal-to-single-crystal transition through irradiation was demonstrated. The changes in structure and electronic configuration as a result of the spin transition are believed to occur concurrently.

A long-lived photo-induced metastable state of linkage isomerization accompanied with a spin transition.

In addition to the generally observed LIESST phenomenon, polymorph D of trans-[Fe(II)(abpt)(2)(NCS)(2)] exhibits a long-lived photo-induced metastable state through linkage isomerization accompanied with a spin crossover transition, which is stable up to 108 K.

The photo-induced commensurate modulated structure in site-selective spin crossover complex trans-[Fe(abpt)2(NCS)2]

The photo-induced superstructure of polymorph C of trans-[Fe(abpt)(2)(NCS)(2)] (abpt = 4-amino-3,5-bis(pyridin-2-yl)-1,2,4-triazole) is discovered as a commensurate modulated structure by single-crystal X-ray diffraction under irradiation. The crystal structure at 25 K before the photo-irradiation composed of two crystallographically independent iron molecules, one of which exhibits high spin (HS) state and the other at low spin (LS) state. Under green laser light (λ = 532 nm) irradiation, the LS molecule (Fe1) is found to be excited to a metastable HS state and gives rise to a commensurate tripled superstructure along the c axis. In addition, it is confirmed that this modulation persists until the HS → LS relaxation temperature beyond 52 K. Our structural findings suggest that the structural modulation and the site-selective LS → HS excitation are highly correlated.

Crystal engineering from a 1D chain to a 3D coordination polymer accompanied by a dramatic change in magnetic properties.

A unique structural transformation in the crystalline state assisted by coordination substitution is induced during a dehydration process. A 1D chain coordination polymer is irreversibly converted to a 3D interpenetrated network accompanied by a change in magnetic properties from a paramagnetic material to a spin crossover system.

Light induced modulated structure of the spin crossover compound {Fe(abpt)2[N(CN)2]2}

{Fe(abpt)2[N(CN)2]2} is an intriguing spin crossover system which crystallizes in two separate phases. Phase A has already been investigated; its crystal structure consists of a single molecule sitting on a center of symmetry. Phase B is the subject of the present structural analysis and presents two independent molecules in the asymmetric unit. We have observed a unique order-disorder phase transition at very low temperature induced either by laser light irradiation or through rapid thermal quenching from room temperature. The corresponding metastable phase exhibits an incommensurate modulation of the crystal structure, disappearing at the HS to LS relaxation temperature (52 K). This modulation is most probably related to a particular ordering of the dicyanamide ligands.

Structural characterization and LIESST properties of spin crossover complex trans-[Fe(abpt)2(NCS)2] polymorph D (abpt = 4-amino-3,5-bis(pyridin-2-yl)-1,2,4-triazole)

A polymorph D of Complex trans-[Fe(abpt)2(NCS)2] was synthesized and structurally characterized. It crystallize in a monoclinic space group P21/c with cell parameter a = 10.803(1), b = 15.926(1), c = 17.451(2) A, β = 106.81(1)o, V = 2874.1(4) A and Z=1 at 293 K. The asymmetric unit contains two unique iron sites (Fe and Fe’); each is located at the crystallographic inversion centers. The main difference in these two molecules is the planarity of the abpt ligand; the dihedral angle between the triazole ring and the uncoordinated pyridine ring is 6.2 and 20.1o for Fe and Fe’ complex respectively. The result of temperature-dependent FTIR spectra indicates that Fe undergoes a gradual spin crossover transition in the temperature range of 200 ~ 100 K ( Tc = 145 K), while the Fe’ remains at high spin state in the temperature range studied. This shows a definite correlation between the spin crossover character and the planarity of the ligand in this complex; this applies to the known crystal structures of polymorph A and B. The closer to the planar of the ligand abpt is, the easier a spin crossover complex will be. The single crystal diffraction data at 90 K were also studied. Comparing to the structure at 293 K, the Fe-Navg distances change from 2.155(3) to 1.976(3) A at site Fe and 2.163(2) to 2.181(2) A at site Fe’, it is consistent with the FTIR measurement. Below 50 K, the Fe site can be excited from low spin to high spin state using a green light laser (532 nm), the light induce exited spin state strapping (LIESST) process was monitored by the CN stretching frequency in the range of 1900-2200 cm.

Charge density studies of 3d metal (Ni/Cu) complexes with a non-innocent ligand

High-resolution X-ray diffraction experiments and atom-specific X-ray absorption experiments are applied to investigate a series of square planar complexes with the non-innocent ligand of maleonitriledithiolate (mnt), [S2C2(CN)2]z-, containing M-S bonds. Four complexes of (PyH)z[M(mnt)2]z-, where M = Ni or Cu, z = 2 or 1 and PyH+ = C5NH6+, were studied in order to clarify whether such one-electron oxidation-reduction, [M(mnt)2]2-/[M(mnt)2]1-, is taking place at the metal or the ligand site. Combining the techniques of metal K-, L-edge and S K-edge X-ray absorption spectroscopy with high-resolution X-ray charge density studies, it is unambiguously demonstrated that the electron redox reaction is ligand based and metal based for Ni and Cu pairs, respectively. The bonding characters in terms of topological properties associated with the bond critical points are compared between the oxidized form [ML]- and the reduced form [ML]2-. In the case of Ni complexes, the formal oxidation state of Ni remains as Ni2+ and each mnt ligand carries a 2- charge in [Ni(mnt)2]2-, but only one of the ligands is formally oxidized in [Ni(mnt)2]1-. In contrast, in the case of Cu complexes, the mnt remains as 2- in both complexes, but the formal oxidation states of the metal are Cu2+ and Cu3+. Bond characterizations and d-orbital populations will be presented. The complementary results of XAS, XRD and DFT calculations will be discussed. The conclusion on the redox reactions in these complexes can be firmly established.

Photo-induced disorder-to-incommensurate order phase transition in an Fe(II) spin crossover complex

KDP (KH2PO4) is a typical ferroelectrics, in which the proton distribution changes through the phase transition. We measured scattering intensity from the single crystal by using neutron 4-circle diffractometer (FONDER) installed at JRR3M in JARERI, Tokai. The crystal structures at different temperatures between 10 and 300K are refined. The Debye-Waller factors are determined carefully. Though the transition temperature, the factors for K, P and O change without noticeable anomaly and approach the zero temperature values. However, the proton’s factor along the double-well axis shows discontinuous behavior indicating the modification of the hydrogen bonding. The figure shows U11 vs. temperature; U11 of the split atom H changes continuously. The temperature dependence is compared with a quantum model of the structural phase transition. The phase transition mechanism seems to be not a typical orderdisorder or displacive type but a quantum type transition