Synthesis and Characterization of a Series of Edge-Sharing Octahedral-Tetrahedral-Octahedral Linear Trinuclear Complexes [M(3)(L1O)(4)](2+), Where M = Mn(2+), Co(2+), Ni(2+), Cu(2+), and Zn(2+) and L1OH Is the "Heteroscorpionate" Ligand (2-Hydroxyphenyl)bis(pyrazolyl)methane.

The mixed functionality pyrazole/phenol ligand (2-hydroxyphenyl)bis(pyrazolyl)methane, L1OH, has been used to prepare a series of linear trimetallic systems with the general structural motif [M(3)(L1O)(4)](2+), where M = Mn(2+), Co(2+), Ni(2+), Cu(2+), and Zn(2+). Each of these complexes has been structurally characterized by X-ray crystallography giving the following structural parameters: [Zn(3)(L1O)(4)][BF(4)](2).H(2)O, C(52)H(44)N(16)B(2)F(8)O(5)Zn(3), monoclinic, a = 18.572(4) Å, b = 22.400(5) Å, c = 15.921(3), beta = 112.439(8) degrees, space group C2/c, Z = 4; [Cu(3)(L1O)(4)] [BF(4)](2).2MeCN, C(56)H(44)N(18)B(2)Cu(3)F(8)O(4), monoclinic, a = 40.574(2) Å, b = 16.701(1) Å, c = 19.841(2) Å, beta = 111.388(5) degrees, space group C2/c, Z = 8; [Ni(3)(L1O)(4)][ClO(4)](2).MeCN.0.5H(2)O, C(54)H(44)N(17)Cl(2)Ni(3)O(12.5), monoclinic, a = 12.324(4) Å, b = 26.537(2) Å, c = 18.829(3) Å, beta = 102.78(1) degrees, space group C2/c, Z = 4; [Co(3)(L1O)(4)][BF(4)](2).MeCN, C(54)H(44)N(17)B(2)Co(3)F(8)O(4), monoclinic, a = 12.395(2) Å, b = 26.483(3) Å, c = 18.703(4) Å, beta = 103.22(2) degrees, space group C2/c, Z = 4; [Mn(3)(L1O)(4)(MeCN)][ClO(4)](2).1.4MeCN, C(56.68)H(44)N(18.34)Cl(2)Mn(3)O(12), orthorhombic, a = 15.471(2) Å, b = 17.364(2) Å, c = 24.216(2) Å, space group Pbcn, Z = 4. For Zn(2+), Cu(2+), Ni(2+), and Co(2+) the central metal atom of the linear trimetallic [M(3)(L1O)(4)](2+) unit is four coordinate and has a pseudotetrahedral geometry with a dihedral angle, omega, between the two M(central)O(2)M(terminal) planes of 79.9 degrees (Zn), 61.2 degrees (Co), 60.4 degrees (Ni), and 46.8 degrees (Cu). The central Mn(2+) atom of [Mn(3)(L1O)(4)(MeCN)][ClO(4)](2).1.4MeCN is five-coordinate, with a trigonal bipyramidal stereochemistry, the result of an equatorially coordinated MeCN solvent molecule. Variable-temperature magnetic data indicate that the Ni, Cu, and Mn complexes display modest antiferromagnetic coupling between the metal centers, while the Co derivative is strongly ferromagnetically coupled.

Vanadium Hydrotris(pyrazolyl)borate Complexes of Diphenyl Phosphate. Heterometallic Complexes of the [LV{(PhO)(2)PO(2)}(3)](-) Fragment.

The synthesis and characterization of nine trinuclear, mixed metal complexes of the type [LV{&mgr;-(PhO)(2)PO(2)}(3)](2)M, where the vanadium is in the 3+ oxidation state, L = hydrotris(pyrazolyl)borate, and M = Ba(II) (1), Ca(II) (2), Mg(II) (3), Mn(II) (4), Co(II) (5), Ni(II) (6), Fe(II) (7), 2 Na(I) (8), Al(III) (9), and La(III) (10), are described. X-ray crystal structural analysis of 1, 3, and 4 gave the following parameters: 1, C(92)H(84)B(2)N(12)O(24)P(6)Cl(4)BaV(2), C2/c, a = 26.730(5) Å, b = 15.521(3) Å, c = 27.648(6) Å, beta = 100.53(3) degrees, Z = 4; 3, C(94)H(80)B(2)N(14)O(24)P(6)MgV(2), P&onemacr;, a = 13.410(3) Å, b = 14.179(3) Å, c = 15.694(3) Å, alpha = 112.22(3) degrees, beta = 101.31(3) degrees, gamma = 106.15(3) degrees, Z = 1; 4, C(111)H(80)B(2)N(12)O(28)P(6)MnV(2), R&thremacr;c, a = 18.670(3) Å, b = 18.671(3) Å, c = 61.114(12) Å, Z = 6. Magnetic measurements indicate that compounds containing an integral spin central ion display moderate antiferromagnetic coupling while those with half-integral spins give more complex behavior.

Magnetic properties of tunicate blood cells. II. Ascidia ceratodes.

The magnetic properties of intact blood cells of the tunicate Ascidia ceratodes have been measured up to 50 kOe with a SQUID susceptometer. Analysis of total metal contents by plasma emission spectroscopy and V(IV) content by epr indicates that approximately 5% of the accumulated vanadium is +4 vanadyl ion. Measured values of the magnetic moment Mp at different values of the applied magnetic field H over the temperature range T = 2-100 K depend on the magnitude of the field indicating magnetic anisotropy of the ground state. The slope of the Mp vs. H/T curve at high temperature is significantly higher than expected from electron spin S = 1 per vanadium(III) ion. The model that fits these data best is a dimer with one V(III) S = 1 ion ferromagnetically coupled to a second V(III) S = 1 ion, with spin-coupling constant J = 3.5 cm-1, and 5% of the total vanadium content in the form of a V(IV) S = 1/2 ion. Since vanadium in A. ceratodes is known to reside in at least three different types of blood cell, the excellent fit indicates that the metal is stored predominantly as a dimer regardless of blood cell type. Ferromagnetic coupling implies that the two vanadium ions in the dimer are connected by an unprotonated mu-oxo bridge.

Mössbauer spectroscopy of iron-ovotransferrin: a crystal field interpretation.

Mössbauer spectra from frozen solutions of ovotransferrin were recorded in a variety of applied external magnetic fields and at various temperatures in a small applied field. The results were fitted to a simple model for the electronic structure at the iron site. This model requires admixtures of the free ion 6S and 4P states, indicating a weak cubic crystal field. Possible implications of this model regarding the binding site are discussed.

Magnetic properties of tunicate blood cells. I. Ascidia nigra.

The magnetic properties of intact and freeze-dried blood cells of the tunicate Ascidia nigra and of model vanadium(III) and (IV) compounds as polycrystalline solids and in aqueous solution have been measured up to 50 kOe with a SQUID susceptometer. Corrections for the samples' diamagnetism were extracted from the temperature dependence of the data without any further assumptions. For vanadium(IV), measured values of the magnetic moment at different values of the applied magnetic field over the temperature range 2-100 K obey a Brillouin function with spin 1/2. For vanadium(III), the magnetic moment data did not obey a Brillouin function and were analyzed in terms of a spin Hamiltonian with S = 1. Measurements on both whole and freeze-dried blood samples give consistent results with vanadium(III) the predominant species. These results are discussed in terms of the mechanisms of vanadium accumulation and the use of vanadium oxidation states as criteria of ascidian taxonomy.

A Mössbauer spectroscopic investigation of the redox behaviour of the molybdenum-iron protein from Klebsiella pneumoniae nitrogenase. Mechanistic and structural implications.

The redox properties of the nitrogenase Mo-Fe protein from Klebsiella pneumoniae have been monitored by 57Fe Mössbauer spectroscopy between -460 and -160mV (relative to the normal hydrogen electrode). Two redox processes associated with the atoms of the protein were observed. One at -216mV (pH 8.7) was associated with the Fe-Mo cofactor centres in the protein and allowed identification of the Mössbauer parameters of the oxidized form of these centres. The other redox process at -340mV (pH 8.7) was associated with species M5 [Smith & Lang (1974) Biochem. J. 137, 169-180]. This latter redox process may be involved in enzyme turnover. The oxidized form of species M5 interacts magnetically with species M4. The structural implications of the data have been considered in relation to other published data. It is concluded that an unequivocal assignment of the M4 and M5 atoms to Fe-S cluster types is not yet possible.

High magnetic field Mössbauer studies of deoxymyoglobin, deoxyhemoglobin, and synthetic analogues.

Mössbauer spectra of deoxymyoglobin, deoxyhemoglobin, and the synthetic analogues, iron (II) 2-methylimidazole meso-tetraphenylporphyrin, and iron (II) 1,2-dimethylimidazole meso-tetraphenylporphyrin have been observed in high magnetic fields and over a wide range of temperature. At temperatures greater than 20 K all materials exhibit remarkably similar spectra, with anisotropic internal magnetic fields decreasing as 1/T. All have negative quadrupole interaction, and both this and the magnetic anisotropy imply that the orbital of the odd electron is prolate in the ground quintet, with little unquenched orbital angular momentum. At 4.2 K the spectra differ, suggesting different detailed structure within the quintet. In contrast to the proteins, the 2-methyl model exhibits spectra at 4.2 K which imply that the lowest spin state has high susceptibility in a single direction.

Mössbauer investigation of deoxymyoglobin in a high magnetic field. Orientation of the electric field gradient and magnetic tensors.

We have examined the Mössbauer spectra of deosymyoglobin in a 6 T magnetic field in the temperature range 4.2-195 K. Spectra were fitted by the least-squares method using a phenomenological model in which the internal magnetic hyperfine field was assumed to be related to the applied field by a temperature dependent tensor õmega. The results indicate that õmega has axial symmetry and a principal axis system aligned with that of the electric field gradient (efg). The principal component of the latter is negative and lies on the symmetry axis of õmega. Our fits indicate that of efg asymmetry parameter is eta = 0.7, with no appreciable temperature dependence. Both axial and transverse components of õmega have the expected 1/T temperature dependence for T greater than 20 K. Our experimentally determined value of eta, combined with published single crystal zero-field measurements, constrains the efg-heme relative orientation to two possibilities. In neither of these is an efg principal axis near to the heme normal.

Mössbauer spectroscopic study of compound ES of cytochrome c peroxidase.

Mössbauer spectra of Compound ES of cytochrome c peroxidase have been observed over a range of temperature and applied magnetic field. These have been interpreted in terms of a crystal field model of the iron site in which the iron is assumed to be in the Fe(IV) state with unpaired spin S = 1. Detailed least-squares fitting of the spectra fitting of the spectra indicates that both the electric field gradient choice of a single parameter, the axial crystal field, the magnetic properties are well reproduced. The model also provides the observed positive sign for the electric field gradient interaction, but overestimates its magnitude. This apparent discrepnancy may be caused by the presence of significant electronic charge in filled bonding orbitals, a feature which is in keeping with expected covalent charge compensation of the extreme oxidation state. There is no evidence in the Mössbauer spectra of interaction between the iron and the ESR-visible free radical. This suggests they are well separated.

Low temperature photodissociation studies of ferrous hemoglobin and myoglobin complexes by Mössbauer spectroscopy.

57Fe-enriched complexes of hemoglobin and myoglobin with CO and O2 were photodissociated at 4.2 degrees K, and the resulting spectra were compared with those of the deoxy forms. Differences in both quadrupole splitting and isomer shift were noted for each protein, the photoproducts having smaller isomer shift and larger quadrupole splitting than the deoxy forms. The photoproducts of HbCO and HbO2 had narrow absorption lines, indicating a well-defined iron environment. The corresponding myoglobin species had broader absorption lines, as did both deoxy forms. The weak absorption lines of photodissociated NO complexes appeared to be wide, possibly indicating magnetic interaction with the unpaired electron of the nearby NO.

Mossbauer effect studies in the fungus Phycomyces.

Mossbauer spectra of 57- Fe have been observed from different parts (mycelia, spores, sporangiophores) of the fungus Phycomyces blakesleeanus grown in an agar medium isotopically enriched with 57- Fe. The spectra indicate that the iron within Phycomyces exists primarily in two chemical states: one which is the same as that of the iron in the growth medium and the other in the form of ferritin, an iron-storage protein. The amount of iron in the former state is observed to decrease relative to the amount of iron in the latter state in going from mycelia to the sporangiophores to the sporangia themselves. Thus, the conversion of iron from the chemical state of the nutrient to ferritin has been monitored for different parts of the phycomyces. In addition, our spectra indicate that at low temperatures the iron atoms clustered within a ferritin molecule are antiferromagnetically coupled. The size of these clusters is inferred from their superparamagnetic behavior at low tempertures and comparison with horse ferritin indicates that the phycomyces ferritin iron clusters are smaller by a factor of two.