RESUMEN
The preparation and physical characterization are reported for the single-molecule magnet salts [M(Cp')(2)](n)()[Mn(12)O(12)(O(2)CC(6)F(5))(16)(H(2)O)(4)] (M = Fe, n = 1, Cp' = C(5)Me(5) (2a), C(5)H(5) (2b); M = Co, n = 1, Cp' = C(5)Me(5) (2c), C(5)H(5) (2d); M = Fe, n = 2, Cp' = C(5)Me(5) (2e), C(5)H(5) (2f)) to investigate the effects of paramagnetic cations on the magnetization relaxation behavior of [Mn(12)]- anionic single-molecule magnets. Complex 2a.2H(2)O crystallizes in the orthorhombic space group Aba2, with cell dimensions at 173 K of a = 25.6292(2) A, b = 25.4201(3) A, c = 29.1915(2) A, and Z = 4. Complex 2c.2CH(2)Cl(2).C(6)H(14) crystallizes in the monoclinic space group P2(1)/c, with cell dimensions at 173 K of a = 17.8332(6) A, b = 26.2661(9) A, c = 36.0781(11) A, beta = 92.8907(3) degrees, and Z = 4. These two salts consist of either paramagnetic [Fe(C(5)Me(5))(2)]+ cations or diamagnetic [Co(C(5)Me(5))(2)]+ cations, and [Mn(12)O(12)(O(2)CC(6)F(5))(16)(H(2)O)(4)]- anions. The structures of the anions in the two salts are similar, consisting of a central Mn(4)O(4) cubane moiety, surrounded by a nonplanar ring of eight Mn atoms that are bridged by and connected to the cube via mu(3)-O(2)- ions. The oxidation states of four Mn sites out of eight outer Mn ions in complex 2a were assigned to be +2.75 from the valence bond sum analysis although the disordering of bridging carboxylates prevents more precise determination. On the other hand in complex 2c, one Mn site out of eight outer Mn ions was identified as a Mn(II) ion, accommodating the "extra" electron; this was deduced by a valence bond sum analysis. Thus, the anion in complex 2c has a Mn(II)(1)Mn(III)(7)Mn(IV)(4) oxidation state description. The Jahn-Teller axes of the Mn(III) ions in both anions are roughly aligned in one direction. All complexes studied exhibit a single out-of-phase ac magnetic susceptibility (chi"(M)) signal in the 4.6-4.8 K range for complexes 2a-2d and in the 2.8-2.9 K range for complexes 2e and 2f at 1 kHz ac frequency. The temperature of the chi"(M) peaks is frequency dependent, as expected for single-molecule magnets. From Arrhenius plots of the frequency dependence of the temperature of the chi"(M) maxima, the effective energy barriers U(eff) for changing spin from "up" to spin "down" were estimated to be 50-54 K for complexes 2a-2d and 27-28 K for complexes 2e and 2f. The least-squares fits of the reduced magnetization data indicate that both complexes 2a and 2d have ground states of S = (21)/(2). High-frequency EPR spectra were recorded for complex 2a at frequencies of 217, 327, and 434 GHz in the 4.5-30 K range. The observed transition fields were least-squares fit to give g = 1.91, D = -0.35 cm(-1), and B(4)(0) = -3.6 x 10(-7) cm(-1) for the S = (21)/(2) ground state. The effective energy barrier U(eff) is slightly lower than U estimated from D, which is consistent with the thermally assisted tunneling model. Magnetization hysteresis loops were observed for complexes 2a and 2c. Although 2a was oriented in a different manner as expected by strong magnetic field, both complexes show clear hysteresis loops with some steps on them, indicating that the effect of the magnetic cation on the magnetization relaxation of the anionic [Mn(12)]- complex is rather small. An 11% (57)Fe enriched complex 2b was studied by means of Mössbauer spectroscopy down to as low as 1.7 K. Slow paramagnetic relaxation broadening and magnetic hyperfine splitting were evident in the low-temperature spectra, indicating that the iron atoms feel a growing magnetic field owing to slow magnetization reversal in the [Mn(12)]- anions.
RESUMEN
1,2-Diaminoethane (en) and FeCl3 give (enH2) [FeCl5(H2O)] (1) in concentrated HCl, extending the aquapentachloroferrate(III) series. For 1: C2H12N2Cl5OFe, orthorhombic, P2(1)2(1)2(1), a = 14.531(6) A, b = 10.772(4) A, c = 6.888(2) A, Z = 4. Diazabicyclo[2.2.2]octane dihydrochloride (DABCO-2HCl) and FeCl3 in concentrated HCl form a tetrachloroferrate(III) derivative whose subsequent ethanol treatment (restricted water access) results in the formation of a compound of composition (DABCOH2)2 [FeCl4(H2O)2]Cl3 (2). This contains the trans-[FeCl4(H2O)2](-) anion, in which the trans-Fe-O distances are 2.049(4) A. For 2: C12H32N4Cl7O2Fe, orthorhombic, Pnma, a = 16.378(3) A, b = 7.3323(6) A, c = 19.431(3) A, Z = 4. A combination of 57Fe Mössbauer spectroscopy and ac susceptibility data confirm uncanted 3D antiferromagnetic ground states with T(Néel) approximately 3.4 K for (enH2)[FeCl5(H2O)] and approximately 2.0 K for [DABCOH2]2[FeCl4(H2O)2]Cl3.
RESUMEN
The origins of the extraordinarily large internal hyperfine field (62.4 T) for the three-dimensional (weak) ferromagnetically ordered ground state of alpha-Fe(OETAP) are discussed semiquantitatively in terms of existing physical theory. In particular, the classical Fermi-contact contribution to the internal field is found to be highly enhanced by a very large orbital contribution and a significant dipolar term of the same sign. A rationale for the unexpected ordering of this S = 1 non-Kramers system is also presented.
RESUMEN
The synthesis and single-crystal structure of a new one-to-one charge-transfer salt, derived from decamethylferrocene and 2,3-dicyano-1,4-naphthoquinone, are described. [Fe(Cp*)2][DCNQ] crystallizes in the orthorhombic space group Pbca, with a = 17.3149(5) A, b = 14.6862(4) A, c = 21.0507(6) A, and Z = 8. Magnetization vs temperature data obtained in 100 G suggest that the compound exhibits dominant one-dimensional ferromagnetic coupling and that it subsequently undergoes an antiferromagnetic phase transition below TN approximately 4 K. Results of magnetization vs applied field experiments show that the compound is a metamagnet with a critical field of approximately 3 kG at 1.8 K. In the nominally antiferromagnetic state, apparent canting of the moments gives rise to a small amount of hysteresis. This picture is supported by ac susceptibility data. The 57Fe Mössbauer spectrum exhibits the expected decamethylferrocenium unresolved quadrupole doublet (delta = 0.53 mm/s) at 77 K and magnetic hyperfine splitting, Hint = 37.9 T, corresponding to long-range magnetic order at 1.63 K.
RESUMEN
Quantitative bulk ferromagnetic behavior has been established for the molecular/organic solid [Fe(III)(C(5)Me(5))(2)].(+)[TCNE].(-). Above 16 K the dominant magnetic interactions are along a 1-D chain and, near T(c), 3-D bulk effects as evidenced by the value of the critical exponents dominate the susceptibility. The extended McConnell model was developed and provides the synthetic chemist with guidance for making new molecular materials to study cooperative magnetic coupling in systems. Assuming the electron-transfer excitation arises from the POMO, ferromagnetic coupling by the McConnell mechanism requires stable radicals (neutral, cations/anions, or ions with small diamagnetic counterions) with a non-half-filled POMO. The lowest excited state formed via virtual charge transfer (retro or forward) must also have the same spin multiplicity and mix with the ground state. These requirements limit the structure of a radical to D(2d) or C>/=(3) symmetry where symmetry breaking distortions do not occur. Intrinsic doubly and triply degenerate orbitals are not necessary and accidental degeneracies suffice. To achieve bulk ferromagnetism, ferromagnetic coupling must be established throughout the solid and a microscopic model has been discussed. These requirements are met by [Fe(III)(C(5)Me(5))(2)].(+)[TCNE].(-). Additionally this model suggests that the Ni(III) and Cr(III) analogs should be antiferromagnetic and ferrimagnetic, respectively, as preliminary data suggest. Additional studies are necessary to test and further develop the consequences of these concepts. Some molecular/organic solids comprised of linear chains of alternating metallocenium donors (D) and cyanocarbon acceptors (A) with spin state S = 1/2 (...D.(+)A.(-)D.(+)A.(-)...) exhibit cooperative magnetic phenomena, that is, ferro-, antiferro-, ferri-, and metamagnetism. For [Fe(III)(C(5)Me(5))(2)].(+)[TCNE](-). (Me = methyl; TCNE = tetracyanoethylene), bulk ferromagnetic behavior is observed below the Curie temperature of 4.8 K. A model of configuration mixing of the lowest charge-transfer excited state with the ground state was developed to understand the magnetic coupling as a function of electron configuration and direction of charge transfer. This model predicts that ferromagnetic coupling requires stable radicals with a non-half-filled degenerate valence orbital and a charge-transfer excited state with the same spin multiplicity that mixes with the ground state. Ferromagnetic coupling must dominate in all directions to achieve a bulk ferromagnet. Thus, the primary, secondary, and tertiary structures are crucial considerations for the design of molecular/organic ferromagnets.
RESUMEN
Studies of the solution properties of gold(III)tetrakis(4-N-methylpyridyl) porphine and its DNA binding characteristics have been conducted utilizing uv/vis absorption spectroscopy, circular dichroism (CD), Mossbauer spectroscopy, and temperature-jump relaxation techniques. These studies indicate that over the concentration range considered this water soluble gold(III) porphyrin does not aggregate, binds axial ligands only weakly with a preference for soft Lewis bases, and is capable of intercalation into nucleic acids of appropriate base pair content. The interaction of this and several other porphyrins with the synthetic polynucleotide poly(dA-dC).poly(dT-dG) has been studied. Spectroscopic signatures for intercalation were found for those derivatives not having axial ligands. Intercalation into chromatin in vitro can also occur with those porphyrins and metalloporphyrins which do not have axial ligands. Finally, studies utilizing microinjection techniques indicate that once within the cell, tetrakis(4-N-methylpyridyl)porphine tends to localize in the nucleus.
