William M. Reiff

Molecular/Organic Ferromagnets

Quantitative bulk ferromagnetic behavior has been established for the molecular/organic solid [FeIII(C5Me5)2]�+[TCNE]�-. Above 16 K the dominant magnetic interactions are along a 1-D chain and, near Tc, 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 D2d 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 [FeIII(C5Me5)2]�+[TCNE]�-. Additionally this model suggests that the NiIII and CrIII 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 [FeIII(C5Me5)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.

Ferromagnetic properties of one-dimensional decamethylferrocenium tetracyanoethylenide (1 : 1): [Fe(η5-C5Me5)2]˙+[TCNE]˙–

[Fe(C5Me5)2]˙+[TCNE]˙– has been characterized by magnetic susceptibility to possess dominant ferromagnetic interactions; its structure has been determined by X-ray crystallography.

Interactions Of Porphyrins With Purified DNA And More Highly Organized Structures

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.

Consequences of a linear two-coordinate geometry for the orbital magnetism and Jahn-Teller distortion behavior of the high spin iron(II) complex Fe[N(t-Bu)2]2.

Mossbauer, EPR, magnetic susceptibility, and DFT studies of the unusual two-coordinate iron(II) amide Fe[N(t-Bu)(2)](2) show that it retains a linear N-Fe-N framework due to the nonbonding delta nature of the (xy, x(2)-y(2)) orbitals. The resulting near-degenerate ground state gives rise to a large magnetic moment and a remarkably large internal hyperfine field. The results confirm that extraordinary orbital magnetic effects can arise in linear transition metal complexes in which orbital degeneracies are not broken by Jahn-Teller or Renner-Teller distortions.

Direct Spectroscopic Observation of Large Quenching of First Order Orbital Angular Momentum with Bending in Monomeric, Two-Coordinate Fe(II) Primary Amido Complexes and the Profound Magnetic Effects of the Absence of Jahn- and Renner-Teller Distortions in Rigorously Linear Coordination

The monomeric iron(II) amido derivatives Fe{N(H)Ar*}(2) (1), Ar* = C(6)H(3)-2,6-(C(6)H(2)-2,4,6-Pr(i)(3))(2), and Fe{N(H)Ar(#)}(2) (2), Ar(#) = C(6)H(3)-2,6-(C(6)H(2)-2,4,6-Me(3))(2), were synthesized and studied in order to determine the effects of geometric changes on their unusual magnetic properties. The compounds, which are the first stable homoleptic primary amides of iron(II), were obtained by the transamination of Fe{N(SiMe(3))(2)}(2), with HN(SiMe(3))(2) elimination, by the primary amines H(2)NAr* or H(2)NAr(#). X-ray crystallography showed that they have either strictly linear (1) or bent (2, N-Fe-N = 140.9(2) degrees ) iron coordination. Variable temperature magnetization and applied magnetic field Mossbauer spectroscopy studies revealed a very large dependence of the magnetic properties on the metal coordination geometry. At ambient temperature, the linear 1 displayed an effective magnetic moment in the range 7.0-7.50 mu(B), consistent with essentially free ion magnetism. There is a very high internal orbital field component, H(L) approximately 170 T which is only exceeded by a H(L) approximately 203 T of Fe{C(SiMe(3))(3)}(2). In contrast, the strongly bent 2 displayed a significantly lower mu(eff) value in the range 5.25-5.80 mu(B) at ambient temperature and a much lower orbital field H(L) value of 116 T. The data for the two amido complexes demonstrate a very large quenching of the orbital magnetic moment upon bending the linear geometry. In addition, a strong correlation of H(L) with overall formal symmetry is confirmed. ESR spectroscopy supports the existence of large orbital magnetic moments in 1 and 2, and DFT calculations provide good agreement with the physical data.

Synthesis, Structural, and Magnetic Characterization of Linear and Bent Geometry Cobalt(II) and Nickel(II) Amido Complexes: Evidence of Very Large Spin–Orbit Coupling Effects in Rigorously Linear Coordinated Co2+

The complexes M(II){N(H)Ar(Pr(i)(6))}(2) (M = Co, 1 or Ni, 2; Ar(Pr(i)(6)) = C(6)H(3)-2,6(C(6)H(2)-2,4,6-Pr(i)(3))(2)), which have rigorously linear, N-M-N = 180°, metal coordination, and M(II){N(H)Ar(Me(6))}(2) (M = Co, 3 or Ni, 4; Ar(Me(6)) = C(6)H(3)-2,6(C(6)H(2)-2,4,6-Me(3))(2)), which have bent, N-Co-N = 144.1(4)°, and N-Ni-N = 154.60(14)°, metal coordination, were synthesized and characterized to study the effects of the metal coordination geometries on their magnetic properties. The magnetometry studies show that the linear cobalt(II) species 1 has a very high ambient temperature moment of about 6.2 μ(B) (cf. spin only value = 3.87 μ(B)) whereas the bent cobalt species 3 had a lower μ(B) value of about 4.7 μ(B). In contrast, both the linear and the bent nickel complexes 2 and 4 have magnetic moments near 3.0 μ(B) at ambient temperatures, which is close to the spin only value of 2.83 μ(B). The studies suggest that in the linear cobalt species 1 there is a very strong enhanced spin orbital coupling which leads to magnetic moments that broach the free ion value of 6.63 μ(B) probably as a result of the relatively weak ligand field and its rigorously linear coordination. For the linear nickel species 2, however, the expected strong first order orbital angular momentum contribution does not occur (cf. free ion value 5.6 μ(B)) possibly because of π bonding effects involving the nitrogen p orbitals and the d(xz) and d(yz) orbitals (whose degeneracy is lifted in the C(2h) local symmetry of the Ni{N(H)C(ipso)}(2) array) which quench the orbital angular momentum.

Topochemical lithium insertion into Fe2(MoO4)3: Structure and magnetism of Li2Fe2(MoO4)3

Abstract Powder X-ray diffraction, variable temperature magnetic susceptibility, and zero-field Mossbauer spectroscopy measurements were used to characterize the new phase Li2Fe2(MoO4)3. This material is obtained simply by the mixing of solutions of lithium iodide in acetonitrile with solid Fe2(MoO4)3 at ambient temperature. The reaction is entirely reversible using bromine as an oxidant. Li2Fe2(MoO4)3 possesses the high-temperature orthorhombic ferric molybdate structure and Guinier photographs were completely indexed in space group Pnca with cell constants a = 9.3483(5), b = 12.8974(9), and c = 9.4941(6) A versus the monoclinic ( P2 1 a ) Fe2(MoO4)3 precursor phase. Chemical analytical data, room-temperature magnetic susceptibility and Mossbauer spectroscopy indicate essentially complete stoichiometric reduction of the latter compound. Magnetic hyperfine splitting of the zero field Mossbauer spectrum below 12.5 K indicates a three-dimensional magnetically ordered state which susceptibility results show to be weakly ferromagnetic owing to probable canting of antiferromagnetically coupled lattices. Above ∼40 K, the material obeys a Curie-Weiss law whose parameters are C = 3.51 emu/mole, μeff = 5.30β and ϑ = −19 K.

The oxidation state of iron in some BaFeS phases: A Mössbauer and electrical resistivity investigation of Ba2FeS3, Ba7Fe6S14, Ba6Fe8S15, BaFe2S3, and Ba9Fe16S32

Abstract Mossbauer spectroscopy, electrical resistivity, and magnetic susceptibility results are used in conjunction with crystal structure information to characterize the oxidation state of iron in five phases formed in the BaFeS system. The compounds have as a common feature FeS4 tetrahedra which articulate by edge and corner sharing into infinite chains or columns. In Ba2FeS3 and Ba7Fe6S14 iron is divalent in the first compound and in the latter the ratio of Fe (II) Fe (III) is 2:1 as expected by stoichiometry. The electrons are localized and Fe(II) and Fe(III) are in definite locations in the trinuclear [ Fe 3 S 6 S 2 2 ] unit. Delocalization of electrons occurs in Ba6Fe8S15, BaFe2S3, and Ba9Fe16S32 and these compounds have low electrical resistivities and display only one quadrupole doublet in the room temperature Mossbauer spectrum. The isomer shift values of 0.2 mm/sec and 0.6 mm/sec are diagnostic of high spin Fe(III) and Fe(II), respectively, when they are in tetrahedral coordination with sulfur; intermediate values are found when electron delocalization occurs.

Linear Chain Ferromagnetic Compounds – Recent Progress

Since our observation that the kinetically stable 1-D 1:1 phase of Fe(C5Me5)+ 5(TCNQ)− exhibits metamagnetic behavior (TN = 2.55°K; HC=1.5 Koe) we have pursued the synthesis of compounds with a fer...

A dinuclear iron(II) complex, [(TPyA)FeII(THBQ2–)FeII(TPyA)](BF4)2 [TPyA = tris(2-pyridylmethyl)amine; THBQ2– = 2,3,5,6-tetrahydroxy-1,4-benzoquinonate] exhibiting both spin crossover with hysteresis and ferromagnetic exchange

Dinuclear [(TPyA)FeII(THBQ(2-))FeII(TPyA)](BF4)2 (1) possesses hydrogen bonding interactions that form a 1-D chain, and pi-pi interactions between the 1-D chains that give rise to a 2-D supramolecular-layered structure, inducing hysteresis in the spin crossover behavior; 1 has shown spin crossover behavior around 250 K with thermal hysteresis and ferromagnetic interactions at low temperature.