Cyanide-Bridged [Fe8M6] Clusters Displaying Single-Molecule Magnetism (M = Ni) and Electron-Transfer-Coupled Spin Transitions (M = Co)

Kiyotaka MITSUMOTO, Emiko OSHIRO, Hiroyuki NISHIKAWA, Takuya SHIGA, Yasuhisa YAMAMURA, Kazuya SAITO and Hiroki OSHIO

Cyanide bridged metal complexes of [Fe8M6(m-CN)14(CN)10 (tp)8(HL)10(MeCN)2](PF6)4.nMeCN.mH2O (HL = 3-(2-pyridyl)-5-[4-(diphenylamino)phenyl]-1H-pyrazole), tp- = hydrotris(pyrazolylborate), 1: M = Ni with n = 11 and m = 7, and 2: M = Co with n = 14 and m = 5) were prepared. 1 and 2 are isomorphous to each other with the monoclinic space group of P21/n. 1 and 2 have tetradecanuclear cores composed with cyanide bridged eight low-spin (LS) FeIII and six high-spin (HS) MII ions (M = Ni and Co), resulting in a crown like core structure.  Magnetic susceptibility measurements revealed that intramolecular ferro- and antiferro-magnetic interactions are operative in 1 and fresh sample of 2, respectively. Ac magnetic susceptibility measurements of 1 showed frequency dependent in- and out-of-phase signals, characteristic of single molecule magnets (SMM), while the desolvated 2 showed thermal and photo induced intramolecular electron transfer coupled spin transition (ETCST) between the spin states of [(LS-FeII)3(LS-FeIII)5(HS-CoII)3(LS-CoIII)3] and [(LS-FeIII)8(HS-CoII)6].
(Chem. Eur. J., 17, 9612 (2011))

Multiple Bistability and Tristability with Dual Spin-State Conversions in [Fe(dpp)2][Ni(mnt)2]2.MeNO2

Masayuko NIHEI, Hitotaka TAHIRA, Nobukazu TAKAHASHI, Yusuke OTAKE, Yasuhisa YAMAMURA, Kazuya SAITO and Hiroki OSHIO

The multi-component system of [Fe(dpp)2][Ni(mnt)2]2.MeNO2 (dpp = 2,6-di(pyrazol-1-yl)pyridine and mnt = maleonitriledithiolate) was prepared by the reaction of [Fe(dpp)2](BF4)2 with (Bu2N)[Ni(mnt)2] in MeNO2. Variable temperature X-ray structural analyses, magnetic susceptibility, and heat capacity measurements confirmed that [Fe(dpp)2][Ni(mnt)2]2.EMeNO2 undergoes multiple spin state conversions both on the cationic and anionic components. The asymmetric unit in the crystal contains one [Fe(dpp)2]2+ cation, two [Ni(mnt)2]- anions ([Ni1]- and [Ni2]-), and one solvent molecule. Magnetic susceptibility measurements revealed that a paramagnetic state in the hightemperature region (HT phase) was abruptly converted to a diamagnetic low-temperature (LT) phase below 180 K as the temperature was lowered from 270 K. As the temperature was raised from 125 to 270 K, successive phase transitions occurred to the HT phase via intermediate phases (IM1, IM2, and IM3) at 175.5 K, 186.5 K, 194.0 K, and 244.0 K, respectively. In the HT phase [Fe(dpp)2]2+ is in the high-spin state, and each [Ni1]- and [Ni2]- moiety is arranged in monomeric form with an S = 1/2 spin ground state. In the LT phase [Fe(dpp)2]2+ is in the low-spin state and the nickel moieties are dimerized and diamagnetic. In the IM1 and IM2 phases the iron(II) sites are partially in the HS state and both [Ni]- moieties are dimeric, as suggested by 57Fe Moassbauer measurements. In the IM3 phase, [Fe(dpp)2]2+complex is in the HS state and the anions exist in both their monomeric ([Ni1]-) and dimeric ([Ni2]-) forms. Rapid thermal quenching from 300 to 5 K yielded a metastable HS phase, which relaxed to the LT phase via the IM1 phase as the temperature was raised to 150 K. A partial light induced spin transition on the iron site was observed at 5 K.
(J. Am. Chem. Soc., 132, 3553 (2010))

Heat Capacity of a Layered Molecule-Based Ferrimagnet [MnII(S-pnH)(H2O)][MnIII(CN)6].2H2O

Syuma YASUZUKA, Yasuhisa YAMAMURA, Wakako KANEKO, Masaaki OHBA, Susumu KITAGAWA and Kazuya SAITO

The thermodynamic investigation of the layered molecule-based ferrimagnet [MnII(S-pnH)(H2O)][MnIII(CN)6].2H2O reveals its ferrimagnetic ordering at Tc = 20.8 K and the low-dimensional short-range ordering well above Tc. Below Tc, Cmag varies as exp(-D/kB) with D/kB = 10.8 K. The likely origin of the activated behavior lies in the solitary excitations related to the single-ion anisotropy of the MnIII ion.
(J. Phys. Soc. Jpn., 78, 065001 (2009))

Spin Crossover Phenomenon Accompanying Order-Disorder Phase Transition in the Ligand of [FeII(DAPP)(abpt)](ClO4)2 Compound [DAPP = Bis(3-aminopropyl)(2-pyridylmethyl)amine, abpt = 4-Amino-3,5-bis(pyridin-2-yl)-1,2,4-triazole)] and Its Successive Self-Grinding Effect

Yuji MIYAZAKI, Tadahiro NAKAMOTO, Satoaki IKEUCHI, Kazuya SAITO, Akira INABA, Michio SORAI, Takeo TOJO, Tooru ATAKE, Galina S. MATOUZENKO, Samir ZEIN, and Serguei A. BORSHCH

The spin crossover phenomenon of the recently described spin crossover complex [FeII(DAPP)(abpt)](ClO4)2 [DAPP = bis(3-aminopropyl)(2-pyridylmethyl)amine, abpt = 4-amino-3,5-bis(pyridin-2-yl)-1,2,4-triazole] accompanying an order-disorder phase transition of the ligand was investigated by adiabatic heat capacity calorimetry, far-IR, IR, and Raman spectroscopies, and normal vibrational mode calculation. A large heat capacity peak due to the spin crossover transition was observed at Ttrs = 185.61 K. The transition enthalpy and entropy amounted to DeltatrsH = 15.44 kJ mol-1 and DeltatrsS = 83.74 J K-1 mol-1, respectively. The transition entropy is larger than the expected value 60.66 J K-1 mol-1, which is contributed from the spin multiplicity (R ln 5; R: the gas constant), disordering of the carbon atom of the six-membered metallocycle in the DAPP ligand, and one of the two perchlorate anions (2R ln 2), and change of the normal vibrational modes between the high-spin (HS) and low-spin (LS) states (35.75 J K-1 mol-1). The remaining entropy would be ascribed to changes of the lattice vibrations and molecular librations between the HS and LS states. Furthermore, [Fe(DAPP)(abpt)](ClO4)2 crystals disintegrated and became smaller crystallites whenever they experienced the phase transition. This may be regarded as a successive self-grinding effect, evidenced by adiabatic calorimetry, DSC, magnetic susceptibility, and microscope observation. The relationship between the crystal size and the physical quantities is discussed.
(J. Phys. Chem. B, 111, 12508 (2007))

Magnetic-Field Dependent Heat Capacity of the Single-Molecule Magnet, [Mn12O12(O2CEt)16(H2O)2]


Accurate heat capacities of the single-molecule magnet [Mn12O12(O2CEt)16(H2O)3] were measured from 0.3 to 311 K by adiabatic calorimetry without an external magnetic field. Heat-capacity anomalies were separated by assuming several contributions including lattice vibration, magnetic anisotropy, and hyperfine splitting. Among them, a tiny thermal anomaly between 1 and 2 K is attributable to the presence of Jahn-Teller isomers. The heat capacities of the polycrystalline sample were also measured with applied magnetic fields from 0 to 9 T in the 2-20 K temperature region by the relaxation method. With an applied magnetic field of up to 2 T, a steplike heat-capacity anomaly was observed around the blocking temperature TB = 3.5 K. The magnitude of the anomaly reached a maximum at 0.7 T. With a further increase in the magnetic field, the step was decreasing, and finally it disappeared above 3 T. The step at TB under 0.7 T can be roughly accounted for by assuming that a conversion between the up-spin and down-spin states is allowed above TB by phonon-assisted quantum tunneling, while it is less effective below TB. Excess heat capacity under a magnetic field revealed a large heat-capacity hump around 14 K and 2 T, which would be attributed to a thermal excitation from the S = 9 ground state to the spin manifold with different S values, where S is the total spin quantum number.
(Inorg. Chem., 40, 6632 (2001))

See also Functional Crystal with Supramolecular Architecture

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