Positive Zero-field Splitting Research Articles

Nickel(II) and iron(II) mononuclear complexes with 1-methylimidazole-2-aldoximate: New building units for molecule-assembled magnetic materials

A new class of mononuclear metal complexes with 1-methylimidazole-2-aldoximate (miao) has been synthesized and characterized: trans-Ni II(Cl) 2(Hmiao) 2 ( 1), trans-Ni II(miao) 2(py) 2 ( 2), NO- trans-Ni II(miao) 2(phen) ( 3), and NO- trans-Fe II(miao) 2(phen) ( 4). The crystal structures of 2, 3, and 4 have been determined by single-crystal X-ray crystallography. Compound 1 having the protonated miao ligand (i.e., Hmiao) is a precursor for synthesizing 2 and 3. Compound 2 is an octahedral Ni II complex surrounded by two miao bidentate ligands and two monodentate ligands of pyridine in a trans-arrangement. Compound 3 is a cis-type octahedral Ni II complex with two miao ligands and a bidentate ligand of 1,10-phenanthroline, in which the ligand arrangement around Ni II center is found in an NO- trans form. Compound 4 is an isostructural Fe II derivative of 3. Compounds 1, 2, and 3 exhibit paramagnetic nature with an S = 1 spin and a positive zero-field splitting, among which it for 3 is overlapped with intermolecular ferromagnetic interaction ( zJ/ k B = +0.16 K). Compound 4 is diamagnetic due to the existence of low-spin Fe II ion.

Large spin differences in structurally related Fe6 molecular clusters and their magnetostructural explanation.

The syntheses, crystal structures, and magnetic characterizations of three new hexanuclear iron(III) compounds are reported. Known [Fe(6)O(2)(OH)(2)(O(2)CBu(t))(10)(hep)(2)] (1) is converted to new [Fe(6)O(2)(OH)(O(2)CBu(t))(9)(hep)(4)] (3) when treated with an excess of 2-(2-hydroxyethyl)-pyridine (hepH). Similarly, the new compound [Fe(6)O(2)(OH)(2)(O(2)CPh)(10)(hep)(2)] (2), obtained from the reaction of [Fe(3)O(O(2)CPh)(6)(H(2)O)(3)] with hepH, is converted to [Fe(6)O(2)(OH)(O(2)CPh)(9)(hep)(4)] (4) when treated with an excess of hepH. This can be reversed by recrystallization from MeCN. The cores of the four Fe(6) complexes all comprise two triangular [Fe(3)(mu(3)-O)(O(2)CR)(3)(hep)](+3) units connected at two of their apices by two sets of bridging ligands. However, 1 and 2 differ slightly from 3 and 4 in the precise way the two Fe(3) units are linked together. In 1 and 2, the two sets of bridging ligands are identical, consisting of one mu-hydroxo and two mu-carboxylate groups bridging each Fe(2) pair, i.e., a (mu-OH(-))(mu-O(2)CR(-))(2) set. In contrast, 3 and 4 have two different sets of bridging ligands, a (mu-OH(-))(mu-O(2)CR(-))(2) set as in 1 and 2, and a (mu-OR(-))(2)(mu-O(2)CR(-)) set, where RO(-) refers to the alkoxide arm of the hep(-) chelate. Variable-field and -temperature dc magnetization measurements establish that 1 and 2 have S = 5 ground states and significant and positive zero-field splitting parameters (D), whereas 3 and 4 have S = 0 ground states. This dramatic difference of 10 unpaired electrons in the ground state S values for near-isomeric compounds demonstrates an acute sensitivity of the magnetic properties to small structural changes. The factors leading to this have been quantitatively analyzed. The semiempirical method ZILSH, based on unrestricted molecular orbital calculations, was used to obtain initial estimates of the Fe(2) pairwise exchange interaction constants (J). These calculated values were then improved by fitting the experimental susceptibility versus T data, using a genetic algorithm approach. The final J values were then employed to rationalize the observed magnetic properties as a function of the core topologies and the presence of spin frustration effects. The large difference in ground state spin value was identified as resulting from a single structural difference between the two types of complexes, the different relative dispositions (cis vs trans) of two frustrated exchange pathways. In addition, use of the structural information and corresponding J values allowed a magnetostructural correlation to be established between the J values and both the Fe-O bond distances and the Fe-O-Fe angles at the bridging ligands.

Racemic nickel(II) pyridine-2-aldoximate complexes capped by 2,2 ′-bipyridine and 1,10-phenanthroline: syntheses, structures and magnetic properties

Two Ni(II) pyridine-2-aldoximate complexes, Ni(pao) 2(bpy) ( 1) and Ni(pao) 2(phen) ( 2) (pao −=pyridine-2-aldoximate, bpy=2,2 ′-bipyridine, phen=1,10-phenanthroline), were synthesized via the deprotonation of NiCl 2(Hpao) 2 in methanol followed by the addition of bidentate ligands of 2,2 ′-bipyridine and 1,10-phenanthroline. Crystallization in CHCl 3 gave block-type crystals of 1 and 2 in high yields. The mononuclear structure surrounded by three bidentate ligands, i.e., two pao − and one bpy or phen, was revealed by X-ray crystallography: 1 crystallizes in monoclinic space group P2 1/ c with cell dimensions of a=13.457(3) Å, b=14.493(3) Å, c=19.104(4) Å, β=108.681(3)°, Z=4, and 2 crystallizes in monoclinic space group P2 1/ n with cell dimensions of a=14.235(5) Å, b=12.018(4) Å, c=20.696(7) Å, β=110.304(4)°, Z=4. 1 and 2 each have two oximate groups (pao −), with an NO- trans arrangement around the Ni II ion. Complexes 1 and 2 are racemic, namely, each molecule has a chiral center of Δ or Λ, thereby forming NO- trans-Δ and -Λ geometries in the solid state. Magnetic measurements revealed a paramagnetic S=1 spin state with a positive zero-field splitting parameter.

EPR spectroscopy of [Fe2O2(5-Et3-TPA)2]3+: electronic origin of the unique spin-Hamiltonian parameters of the Fe2(III,IV)O2 diamond core.

The electronic origins of the magnetic signatures of [Fe(2)O(2)(5-Et(3)-TPA)(2)](ClO(4))(3), where 5-Et(3)-TPA = tris(5-ethyl-2-pyridylmethyl)amine, were investigated by density functional calculations. These signatures consist of a near-axial EPR spectrum, anisotropic superhyperfine broadening upon (17)O substitution in the Fe(2)O(2) core, and an unusually large, positive zero-field splitting parameter, D = 38 +/- 3 cm(-1). Density functional calculations identify the anisotropic (17)O superhyperfine broadening to be due to a preponderance of oxo 2p density perpendicular to the plane of the Fe(2)O(2) core in the three singly occupied molecular orbitals of the S = (3)/(2) ground state. The near-axial g-matrix arises from DeltaS = 0 spin-orbit mixing between the singly and doubly occupied d(pi) orbitals of the iron d-manifold. The large D is due to DeltaS = +/-1 spin-orbit mixing with low-lying d(pi) excited states. These experimental observables reflect the dominance of iron-oxo (rather than Fe-Fe) bonding in the Fe(2)O(2) core, and define the low-lying valence orbitals responsible for reactivity.

Synthesis and magnetic properties of 1:1 salts containing the bis(η6-hexamethylbenzene)cobalt cation

The salts [Co(η6-C6Me6)2]+[A]–(A = BPh4, NbF6, PF6 or SbF6) have been prepared from anhydrous CoCl2. Solid-state magnetic susceptibility measurements on these salts indicate that they all obey the Curie–Weiss law in the temperature range 30–300 K with µeff= 2.7, 2.8, 2.8 and 2.6 µB with corresponding θ values of –1.4, 0.2, –4.1 and 0.4 K for the BPh4–, NbF6–, PF6– and SbF6– salts respectively. Below 20 K, the magnetic susceptibility for all the salts deviate substantially from the Curie–Weiss law. The experimental susceptibility data for the compounds [Co(η6-C6Me6)2]+[BPh4]– and [Co(η6-C6Me6)2]+[NbF6]– can be fitted to a model in which the 3A2g triplet ground state assigned to the [Co(η6-C6Me6)2]+ moiety exhibits a positive zero-field splitting of 17.8 and 20.5 cm–1 respectively. In contrast, the magnetic susceptibilities for [Co(η6-C6Me6)2]+[A]–(A = PF6 or SbF6) cannot be fitted to this model in the range 2–30 K due to more significant intermolecular antiferromagnetic spin–spin interactions. All the salts were EPR silent at between 4 and 300 K both in the solid state and in frozen solution. Reaction of [Co(η6-C6Me6)2]+[PF6]– with either [NBu4][tcnq](tcnq = tetracyano-p-quinodimethane) or [NBu4][C3{C(CN)2}3][C3{C(CN)2}3= tris(dicyano-methylene)cyclopropane] gave insoluble paramagnetic precipitates with µeff= 4.1 and 4.5 µB and θ values of –12.9 and –3.5 K respectively.

Mössbauer and electron paramagnetic resonance studies of horseradish peroxidase and its catalytic intermediates.

We report Mössbauer and EPR measurements on horseradish peroxidase in the native state and the reaction intermediates with peroxide and chlorite. A detailed analysis of the electronic state of the heme iron is given, and comparisons are drawn with related systems. The native enzyme is high-spin ferric and thus has three Kramers doublets. The unusual magnetic properties of the ground doublet and the large energy of the second, (E2-E1)/k approximately equal to 41 K, and third doublet, (E3-E1)/k greater than or equal to 170 K, can be modeled with a quartet admixture of approximately 11% to the spin sextet. All evidence suggests a ferryl, OFeIV, state of the heme iron in compounds I and II and related complexes. The small isomer shift, delta Fe approximately equal to 0.06 mm/s, the (positive) quadrupole splitting, delta EQ approximately equal to 1.4 mm/s, the spin S = 1, and the large positive zero field splitting, D/k approximately equal to 35 K, are all characteristic of the ferryl state. In the green compound I the iron weakly couples to a porphyrin radical with spin S' = 1/2. A phenomenological model with a weak exchange interaction S . J . S', magnitude of less than or equal to 0.1 D, reproduces all Mössbauer and EPR data of compound I, but the structural origin of the exchange and its apparent distribution require further study. Reaction of horseradish peroxidase with chlorite leads to compound X with delta Fe = 0.07 mm/s and delta EQ = 1.53 mm/s, values that are closest to those of compound II. The diamagnetism of compound III and its Mössbauer parameters delta Fe = 0.23 mm/s and delta EQ = -2.31 mm/s at 4.2 K clearly identify it as an oxyheme adduct.

Effect of Uniaxial Stress on the Zero-Field Splitting ofCr3+in Alums

The effect of uniaxial stress on the zero-field splitting of ${\mathrm{Cr}}^{3+}$ in five alum crystals is reported. In conjunction with uniaxial stress electron-spin-resonance measurements at room temperature are capable of determining the sign of the zero-field-splitting parameter $2D$ in alum crystals. This method provides a means of determining the sign of $D$ in those alums which undergo crystallographic or phase transitions at low temperature. The positive zero-field splitting in the three investigated $\ensuremath{\alpha}$-alums is reduced by an externally applied uniaxial stress along the trigonal axis, and the negative zero-field splitting in the two $\ensuremath{\beta}$-alums is increased by the same stress. These results are in agreement with McGarvey's model of a ${\mathrm{Cr}}^{3+}$ complex in which the zero-field splitting is positive or negative depending on whether the complex is trigonally extended or compressed. If the association is made that $\ensuremath{\alpha}$-alums have a positive $D$ value, and $\ensuremath{\beta}$-alums have a negative $D$ value, a result which seems to be generally valid, the effect of stress on the chromium spectrum in alums can be used to differentiate between $\ensuremath{\alpha}$ and $\ensuremath{\beta}$ types. Stress-induced changes in the zero-field splitting were used to calculate the ${C}_{33}$ spin-lattice coefficient for the five alums.

Electronic Structure and Dynamic Jahn-Teller Effect of Cobaltocene from EPR and Optical Studies

In order to determine quantitative information about chemical bonding in metallocenes, we investigated the electronic structure of the cobaltocene molecule by EPR and magnetic susceptibility measurements at 4.2°K and by optical spectroscopy at 77°K. Diamagnetic ruthenocene, weakly paramagnetic nickelocene, and paramagnetic cobaltocene single crystals as well as polycrystalline ferrocene served as host systems. From the poorly resolved optical spectra of pure cobaltocene, approximate ligand field parameters were determined. The magnetic properties (g tensor, cobalt hfs tensor) of the lowest Kramers doublet are explained in terms of the relative magnitudes of (a) spin-orbit coupling, (b) static orthorhombic distortion, and (c) vibronic coupling (dynamic Jahn-Teller effect) in the orbitally degenerate 2E1g ground state. From the analysis of the EPR data of cobaltocene-doped ruthenocene, we conclude that covalency effects and vibronic interactions (``Ham effect'') are of comparable importance resulting in a drastic modification of the magnetic parameters compared to a free Co2+ ion in a static crystal field. In agreement with earlier qualitative and semiquantitative predictions, considerable covalency of the singly occupied e1g* orbital (42%± 5% ligand, 58%± 5% cobalt 3d character) was found. The strong change of the EPR parameters going from the ruthenocene to the ferrocene host lattice originates mainly in a strongly enhanced static orthorhombic splitting parameter in the tighter packed ferrocene environment. In cobaltocene single crystal, magnetic dipole-dipole interactions broaden the EPR lines beyond detection even at 2°K. Nickelocene, an S =1 case with a large positive zero field splitting, behaves as a pseudodiamagnet at liquid helium temperature; exchange interactions with the cobaltocene dopant cause significant modifications of the g values but leaves the cobalt hfs tensor almost unaffected.