Ultrawide band multifrequency high-field EMR technique: A methodology for increasing spectroscopic information

High-frequency/high-field EPR spectroscopy of the high-spin ferrous ion in hexaaqua complexes

Electron paramagnetic resonance (EPR) at conventional magnetic fields and microwave frequencies, respectively, B0 < or = 1.5 T, nu < or = 35 GHz, has been widely applied to odd electron-number (S = 1/2) transition metal complexes. This technique is less successfully applied to high-spin systems that have even electron configurations, e.g. Fe2+ (S = 2). The recently developed technique of high-frequency and high-field EPR (HFEPR), employing swept fields up to 25 T combined with multiple, sub-THz frequencies readily allows observation of EPR transitions in such high-spin systems. A parallel spectroscopic technique is frequency-domain magnetic resonance spectroscopy (FDMRS), in which the frequency is swept while at zero, or at discrete applied magnetic fields. We describe here the application of HFEPR and FDMRS to two simple high-spin (HS) ferrous (Fe2+) salts: ferrous perchlorate hydrate, [Fe(H2O)6](ClO4)2 and (NH4)2[Fe(H2O)6](SO4)2, historically known as ferrous ammonium sulfate. Both compounds contain hexaaquairon(II). The resulting spectra were analyzed using a spin Hamiltonian for S = 2 to yield highly accurate spin-Hamiltonian parameters. The complexes were also studied by powder DC magnetic susceptibility and zero-field Mössbauer effect spectroscopy for corroboration of magnetic resonance results. In the case of [Fe(H2O)6](ClO4)2, all the magnetic techniques were in excellent agreement and gave as consensus values: D = 11.2(2) cm(-1), E = 0.70(1) cm(-1). For (NH4)2[Fe(H2O)6](SO4)2, FDMRS and HFEPR gave D = 14.94(2) cm(-1), E = 3.778(2) cm(-1). We conclude that the spin-Hamiltonian parameters for the perchlorate best represent those for the isolated hexaaquairon(II) complex. To have established electronic parameters for the fundamentally important [Fe(H2O)6]2+ ion will be of use for future studies on biologically relevant systems containing high-spin Fe2+.

High-field/high-frequency EPR of paramagnetic functional centers in Cu2+- and Fe3+-modified polycrystalline Pb[Zr(x)Ti(1-x)]O3 ferroelectrics

Copper(II)- and iron(III)-modified Pb[Zr(x)Ti(1-x)]O3 ferroelectrics were investigated by means of high-field/high-frequency EPR. The results obtained suggest that Cu2+ and Fe3+ both substitute as acceptor centers for [Zr,Ti]4+. Whereas for the iron-doped system the charge compensating oxygen vacancies (V(O)**) lead to the formation of charged (Fe'(Ti-)V(O)**)* defect associates, no such associates have been observed for the copper-modified system. As regards the morphotropic phase boundary, the model of a mesoscopic mixing of the pure-member phases has been refined to a picture in which a nanoscale composition distribution prevails.

Electron paramagnetic resonance shift in II1-xMnxVI diluted magnetic semiconductors in the presence of strong exchange coupling

Electron paramagnetic resonance (EPR) has been investigated in two II1-xMnxVI alloys--Cd1-xMnxSe and Cd1-xMnxS--for a series of high Mn concentrations and at low temperatures T, i.e., under conditions where the spin subsystems in these materials are strongly coupled. We have observed a very significant shift of the resonance field from the EPR position of Mn2+ ions that increases with increasing x and with decreasing T. Furthermore, the use of multiple frequencies has allowed us to attribute the observed shift to an internal field that originates from the spin sublattice within the II1-xMnxVI host.

A Cu-Zn-Cu-Zn heterometallomacrocycle shows significant antiferromagnetic coupling between paramagnetic centres mediated by diamagnetic metal

An unprecedented antiferromagnetic exchange mediated by two -O-Zn-O- bridges, with singlet-triplet splitting J= 35.0 cm-1, was observed between two copper centers separated by 5.7062(9)A in the heterometallomacrocyclic diethanolamine (H2L) complex [Cu2Zn2(NH3)2Br2(HL)4]Br2.CH3OH.

Synthesis, Characterization, and Physicochemical Properties of Manganese(III) and Manganese(V)−Oxo Corrolazines

The structural and physicochemical properties of the manganese-corrolazine (Cz) complexes (TBP8Cz)Mn(V)O (1) and (TBP8Cz)Mn(III) (2) (TBP = p-tert-butylphenyl) have been determined. Recrystallization of 2 from toluene/MeOH resulted in the crystal structure of (TBP8Cz)Mn(III)(CH3OH) (2 x MeOH). The packing diagram of 2 x MeOH reveals hydrogen bonds between MeOH axial ligands and meso N atoms of adjacent molecules. Solution binding studies of 2 with different axial ligands (Cl-, Et3PO, and Ph3PO) reveal strong binding, corroborating the preference of the Mn(III) ion for a five-coordinate environment. High-frequency and field electron paramagnetic resonance (HFEPR) spectroscopy of solid 2 x MeOH shows that 2 x MeOH is best described as a high-spin (S = 2) Mn(III) complex with zero-field splitting parameters typical of corroles. Structural information on 1 was obtained through an X-ray absorption near-edge structure (XANES)/extended X-ray absorption fine structure (EXAFS) study and compared to XANES/EXAFS data for 2 x MeOH. The XANES data for 1 shows an intense pre-edge transition characteristic of a high-valent metal-oxo species, and a best fit of the EXAFS data gives a short Mn-O bond distance of 1.56 A, confirming the structure of the metal-oxo unit in 1. Detailed spectroelectrochemical studies of 1 and 2 were performed revealing multiple reversible redox processes for both complexes, including a relatively low potential for the Mn(V) --> Mn(IV) process in 1 (near 0.0 V vs saturated calomel reference electrode). Chemical reduction of 1 results in the formation of a Mn(III)Mn(IV)(mu-O) dimer as characterized by electron paramagnetic resonance spectroscopy.

High-Frequency and -Field EPR Investigation of a Manganese(III) N-Confused Porphyrin Complex, [Mn(NCTPP)(py)2]

We report the first high-frequency and -field electron paramagnetic resonance (HFEPR) study of a Mn(III) N-confused porphyrin (NCP) complex (NCP is also known as inverted porphyrin or 2-aza-21-carbaporphyrin). We have found a striking variation in the electronic properties of the S = 2 Mn(III) ion coordinated by NCP compared to other Mn(III) porphyrinoid complexes. Thus, inversion of a single pyrrole ring greatly changes the equatorial ligand field exerted and leads to large magnitudes of both the axial and rhombic zero-field splitting [respectively, D = -3.084(3) cm(-1), E = -0.608(3) cm(-1)], which are unprecedented in other Mn(III) porphyrinoids.

ESR spectrometer with a loop-gap resonator for cw and time resolved studies in a superconducting magnet

The design and performance of an electron spin resonance spectrometer operating at 3 and 9 GHz microwave frequencies combined with a 9-T superconducting magnet are described. The probehead contains a compact two-loop, one gap resonator, and is inside the variable temperature insert of the magnet enabling measurements in the 0-9T magnetic field and 1.5-400 K temperature range. The spectrometer allows studies on systems where resonance occurs at fields far above the g approximately 2 paramagnetic condition such as in antiferromagnets. The low quality factor of the resonator allows time resolved experiments such as, e.g., longitudinally detected ESR. We demonstrate the performance of the spectrometer on the NaNiO2 antiferromagnet, the MgB2 superconductor, and the RbC60 conducting alkaline fulleride polymer.

The Satellite-Shaped Co-15 Polyoxotungstate, [Co6(H2O)30{Co9Cl2(OH)3(H2O)9(β-SiW8O31)3}]5-

The 15-cobalt-substituted polyoxotungstate [Co(6)(H(2)O)(30){Co(9)Cl(2)(OH)(3)(H(2)O)(9)(beta-SiW(8)O(31))(3)}](5-) (1) has been characterized by single-crystal XRD, elemental analysis, IR, electrochemistry, magnetic measurements, and EPR. Single-crystal X-ray analysis was carried out on Na(5)[Co(6)(H(2)O)(30){Co(9)Cl(2)(OH)(3)(H(2)O)(9)(beta-SiW(8)O(31))(3)}].37H(2)O, which crystallizes in the hexagonal system, space group P6(3)/m, with a = 19.8754(17) A, b = 19.8754(17) A, c = 22.344(4) A, alpha= 90 degrees, beta = 90 degrees, gamma = 120 degrees, and Z = 2. The trimeric polyanion 1 has a core of nine Co(II) ions encapsulated by three unprecedented (beta-SiW(8)O(31)) fragments and two Cl(-) ligands. This central assembly {Co(9)Cl(2)(OH)(3)(H(2)O)(9)(beta-SiW(8)O(31))(3)}(17-) is surrounded by six antenna-like Co(II)(H(2)O)(5) groups resulting in the satellite-like structure 1. Synthesis of 1 is accomplished in a simple one-pot procedure by interaction of Co(II) ions with [gamma-SiW(10)O(36)](8-) in aqueous, acidic NaCl medium (pH 5.4). Polyanion 1 was studied by cyclic voltammetry as a function of pH. The current intensity of its Co(II) centers was compared with that of free Co(II) in solution. Our results suggest that 1 keeps its integrity in solution. Magnetic susceptibility results show the presence of both antiferro- and ferromagnetic coupling within the (Co(II))(9) core. A fully anisotropic Ising model has been employed to describe the exchange-coupling and yields g = 2.42 +/- 0.01, J(1) = 17.0 +/- 1.5 cm(-1), and J(2) = -13 +/- 1 cm(-(1). Variable frequency EPR studies reveal an anisotropic Kramer's doublet.

High-frequency, high-field EPR; magnetic susceptibility; and X-ray studies on a ferromagnetic heterometallic complex of diethanolamine (H2L), [Cu4(NH3)4(HL)4][CdBr4]Br2.3dmf.H2O

The novel heterometallic complex [Cu(4)(NH(3))(4)(HL)(4)][CdBr(4)]Br(2).3dmf.H(2)O has been prepared in the reaction of zerovalent copper with cadmium oxide in the air-exposed solution of ammonium bromide and diethanolamine (H(2)L) in dimethylformamide (dmf). The compound is monoclinic, with space group P2(1)/c, a = 14.876(3) A, b = 33.018(6) A, c = 11.437(2) A, beta = 108.182(3)(o), and Z = 4. The crystal lattice consists of [Cu(4)(NH(3))(4)(HL)(4)](4+) cations, [CdBr(4)](2)(-), Br(-) anions, and uncoordinated dmf and water molecules. In the cation, four independent Cu atoms occupy vertexes of a distorted tetrahedron with bridged Cu...Cu distances in the range 3.127(2)-3.333(3) A and other Cu...Cu separations being 3.445(3)-3.503(2) A. The magnetic susceptibility and the EPR spectra were measured over the temperature ranges 1.8-300 and 3-300 K, respectively. The magnetic moment was found to increase with decreasing temperature to reach a maximum of 2.60 muB per one copper atom at ca. 10 K and was found, subsequently, to diminish slightly at lower temperatures owing to zero-field and Zeeman splitting of the S = 2 ground state. The temperature dependence of the magnetic susceptibility was fitted to the spin Hamiltonian H = J(ab)S(a)S(b) + J(bc)S(b)S(c) + J(cd)S(c)S(d) + J(ad)S(a)S(d) + J(ac)S(a)S(c) + J(bd)S(b)S(d) with the exchange integrals J(ab) = J(bc) = J(cd) = J(ad) = -65(3) cm(-1) and J(ac) = J(bd) = +1(3) cm(-1). High-field, high-frequency (95-380 GHz) EPR spectra due to an S = 2 ground state were simulated with g(x) = 2.138(1), g(y)) = 2.142(1), g(z) = 2.067(1), D = -0.3529(3) cm(-1), and E = -0.0469(8) cm(-1). Calculations based on the X-ray structure indicate a negligible contribution of the magnetic dipole-dipole interactions to the zfs parameters D and E. A discussion of the isotropic and anisotropic exchange interactions and their effect on the zfs parameters is also given.

Structure, Electrochemistry, and Magnetism of the Iron(III)-Substituted Keggin Dimer, [Fe6(OH)3(A-α-GeW9O34(OH)3)2]11-

The iron(III)-substituted tungstogermanate [Fe6(OH)3(A-alpha-GeWO34(OH)3)2]11- (1) has been synthesized and characterized by IR, elemental analysis, SQUID magnetometry, electron paramagnetic resonance (EPR), and electrochemistry. Single-crystal X-ray analysis was carried out on Cs4Na7[Fe6(OH)3(A-alpha-GeW9O34(OH)3)2] x 30H2O, which crystallizes in the monoclinic system, space group C2/m, with a = 36.981(4) A, b = 16.5759(15) A, c = 16.0678(15) A, beta = 95.311(3) degrees, and Z = 4. Polyanion 1 consists of two (A-alpha-GeW9O34) Keggin moieties linked via six Fe3+ ions, leading to a double-sandwich structure. The equivalent iron centers represent a trigonal prismatic Fe6 fragment, resulting in virtual D3h symmetry for 1. Electrochemistry studies revealed that 1 is stable in solution from pH 3 to at least pH 7. In pH = 3 media the reduction of the six Fe3+ centers was featured by a single voltammetric wave for most supporting electrolytes used. In that case, whatever the scan rate from 1000 mV x s(-1) down to 2 mV x s(-1), no splitting of the single Fe-wave of 1 was observed. The acetate medium induced a partial splitting of the wave, and this separation is enhanced with increasing pH. Remarkable efficiency of 1 in the electrocatalytic reduction of nitrite, nitric oxide, and nitrate is demonstrated. Magnetic susceptibility (chi) measurements indicate a diamagnetic (S(T) = 0) ground state, with an average J = -12 cm(-1) and g = 2.00. EPR studies confirm that the ground state is indeed diamagnetic, since the EPR signal intensity steadily decreases without any line broadening as the temperature is lowered and becomes unobservable below about 50 K. The signal is a single broad peak at all frequencies (90-370 GHz), ascribed to the thermally accessible excited states. Its g(iso) is 1.992 51, as expected for a high-spin Fe3+-containing species, and supports the chi data analysis.

Synthesis, Crystal Structure, and High-Precision High-Frequency and -Field Electron Paramagnetic Resonance Investigation of a Manganese(III) Complex: [Mn(dbm)2(py)2](ClO4)

The complex [Mn(dbm)(2)(py)(2)](ClO(4)) (dbm = anion of 1,3-diphenyl-1,3-propanedione (dibenzoylmethane), py = pyridine) was synthesized and characterized by X-ray crystallography. It has tetragonally distorted geometry with the axial positions occupied by the py ligands and the equatorial positions by the dbm ligands. This mononuclear complex of high-spin Mn(III) (3d(4), S = 2) was studied by high-frequency and -field electron paramagnetic resonance (HFEPR) both as a solid powder and in frozen dichloromethane solution. Very high quality HFEPR spectra were recorded over a wide range of frequencies. The complete dataset of resonant magnetic fields versus transition energies was analyzed using automated fitting software. This analysis yielded the following spin Hamiltonian parameters (energies in cm(-1)): D = -4.504(2), E = -0.425(1), B(4)(0) = -1.8(4) x 10(-4), B(4)(2) = 7(3) x 10(-4), B(4)(4) = 48(4) x 10(-4), g(x) = 1.993(1), g(y) = 1.994(1), and g(z) = 1.983(1), where the B(4)(n) values represent fourth-order zero-field splitting terms that are generally very difficult to extract, even from single-crystal measurements. The results here demonstrate the applicability of HFEPR at high-precision measurements, even for powder samples. The zero-field splitting parameters determined here for [Mn(dbm)(2)(py)(2)](+) are placed into the context of those determined for other mononuclear complexes of Mn(III).

Pseudooctahedral Complexes of Vanadium(III): Electronic Structure Investigation by Magnetic and Electronic Spectroscopy

A variety of physical methods has been used to probe the non-Kramers, S = 1, V(III) ion in two types of pseudooctahedral complexes: V(acac)(3), where acac = anion of 2,4-pentanedione, and VX(3)(thf)(3), where thf = tetrahydrofuran and X = Cl and Br. These methods include tunable frequency and high-field electron paramagnetic resonance (HFEPR) spectroscopy (using frequencies of approximately 95-700 GHz and fields up to 25 T) in conjunction with electronic absorption, magnetic circular dichroism (MCD), and variable-temperature variable-field MCD (VTVH-MCD) spectroscopies. Variable-temperature magnetic susceptibility and field-dependent magnetization measurements were also performed. All measurements were conducted on complexes in the solid state (powder or mull samples). The field versus sub-THz wave quantum energy dependence of observed HFEPR resonances yielded the following spin Hamiltonian parameters for V(acac)(3): D = +7.470(1) cm(-1); E = +1.916(1) cm(-1); g(x) = 1.833(4); g(y) = 1.72(2); g(z) = 2.03(2). For VCl(3)(thf)(3), HFEPR detected a single zero-field transition at 15.8 cm(-1) (474 GHz), which was insufficient to determine the complete set of spin Hamiltonian parameters. For VBr(3)(thf)(3), however, a particularly rich data set was obtained using tunable-frequency HFEPR, and analysis of this data set gave the folowing: D = -16.162(6) cm(-1); E = -3.694(4) cm(-1); g(x) = 1.86(1); g(y) = 1.90(1); g(z) = 1.710(4). Analysis of the VTVH-MCD data gave spin Hamiltonian parameters in good agreement with those determined by HFEPR for both V(acac)(3) and VBr(3)(thf)(3) and in rough agreement with the estimate for VCl(3)(thf)(3) (D approximately 10 cm(-1), |E/D| approximately 0.18), together with the finding that the value of D is negative for both thf complexes. The electronic structures of these V(III) complexes are discussed in terms of their molecular structures and the electronic transitions observed by electronic absorption and MCD spectroscopies.

Sandwich-type germanotungstates: structure and magnetic properties of the dimeric polyoxoanions [M4(H2O)2(GeW9O34)2]12 - (M = Mn2+, Cu2+, Zn2+, Cd2+)

The novel dimeric germanotungstates [M(4)(H(2)O)(2)(GeW(9)O(34))(2)](12)(-) (M = Mn(2+), Cu(2+), Zn(2+), Cd(2+)) have been synthesized and characterized by IR spectroscopy, elemental analysis, magnetic measurements, and (183)W-NMR spectroscopy. X-ray single-crystal analyses were carried out on Na(12)[Mn(4)(H(2)O)(2)(GeW(9)O(34))(2)].38H(2)O (Na(12)()-1), which crystallizes in the monoclinic system, space group P2(1)/n, with a = 13.0419(8) A, b = 17.8422(10) A, c = 21.1626(12) A, beta = 93.3120(10) degrees, and Z = 2; Na(11)Cs(2)[Cu(4)(H(2)O)(2)(GeW(9)O(34))(2)]Cl.31H(2)O (Na(11)()Cs-2) crystallizes in the triclinic system, space group P, with a = 12.2338(17) A, b = 12.3833(17) A, c = 15.449(2) A, alpha = 100.041(2) degrees, beta = 97.034(2) degrees, gamma = 101.153(2) degrees, and Z = 1; Na(12)[Zn(4)(H(2)O)(2)(GeW(9)O(34))(2)].32H(2)O (Na(12)()-3) crystallizes in the triclinic system, space group P, with a = 11.589(3) A, b = 12.811(3) A, c = 17.221(4) A, alpha = 97.828(6) degrees, beta = 106.169(6) degrees, gamma = 112.113(5) degrees, and Z = 1; Na(12)[Cd(4)(H(2)O)(2)(GeW(9)O(34))(2)].32.2H(2)O (Na(12)()-4) crystallizes also in the triclinic system, space group P, with a = 11.6923(17) A, b = 12.8464(18) A, c = 17.616(2) A, alpha = 98.149(3) degrees, beta = 105.677(3) degrees, gamma = 112.233(2) degrees, and Z = 1. The polyanions consist of two lacunary B-alpha-[GeW(9)O(34)](10)(-) Keggin moieties linked via a rhomblike M(4)O(16) (M = Mn, Cu, Zn, Cd) group leading to a sandwich-type structure. (183)W-NMR studies of the diamagnetic Zn and Cd derivatives indicate that the solid-state polyoxoanion structures are preserved in solution. EPR measurements on Na(12)()-1 at frequencies up to 188 GHz and temperatures down to 4 K yield a single, exchange-narrowed peak, at g(iso) = 1.9949, typical of Mn systems, and an upper limit of |D| = 20.0 mT; its magnetization studies still await further theoretical treatment. Detailed EPR studies on Na(11)()Cs-2 over temperatures down to 2 K and variable frequencies yield g( parallel ) = 2.4303 and g( perpendicular ) = 2.0567 and A( parallel ) = 4.4 mT (delocalized over the Cu(4) framework), with |D| = 12.1 mT. Magnetization studies in addition yield the exchange parameters J(1) = -11 and J(2) = -82 cm(-)(1), in agreement with the EPR studies.

Definitive spectroscopic determination of zero-field splitting in high-spin cobalt(II)

A high-spin Co(II) complex (3d(7), S = 3/2), Co(PPh(3))(2)Cl(2) (Ph = phenyl), has been investigated in the solid state by both high-frequency and -field electron paramagnetic resonance (HFEPR) and by variable-temperature, variable-field magnetic circular dichroism (VTVH-MCD). In HFEPR spectroscopy, the combination of variable sub-THz frequencies generated by backward wave oscillators (150-700 GHz, corresponding to energy 5-23 cm(-1)) and high magnetic fields (0-25 T) constitutes a novel experimental technique allowing accurate determination of a complete set of spin Hamiltonian parameters for this complex: D = -14.76(2) cm(-1), E = 1.141(8) cm(-1), g(x) = 2.166(4), g(y) = 2.170(4), g(z) = 2.240(5). Independent VTVH-MCD studies on multiple absorption bands of the complex yield D = -14(3) cm(-1), E = 0.96(20) cm(-1) (absolute value of E/D = 0.08(2)), g(x) = 2.15(5), g(y) = 2.16(4), and g(z) = 2.17(3). This very good agreement between HFEPR and MCD indicates that there is no inherent discrepancy between these two quite different experimental techniques. Thus, depending on the nature of the sample, either can be reliably used to determine zero-field splitting parameters in high-spin Co(II), with the HFEPR being more accurate but VTVH-MCD being more sensitive.

A longitudinally detected high-field ESR spectrometer for the measurement of spin-lattice relaxation times

We describe a high-field longitudinally detected electron spin resonance (LOD-ESR) spectrometer operating at 35 and 75 GHz. The lack of resonant microwave circuits facilitates operation at different microwave frequencies without changing the probehead. A very low noise radio frequency detection compensates partially the resulting low sensitivity. The major elements of the LOD-ESR spectrometer are commercially available and may be adapted to usual high frequency spectrometers. The instrument allows field and frequency dependent spin lattice relaxation time (T1) studies. T1 in the range of 2-80 ns can be determined from the phase sensitively detected LOD-ESR spectra. We demonstrate the performance of the apparatus by the measurement of T1 in the normal state of RbC60, an electrically conducting alkaline fulleride polymer.

A continuous-wave and pulsed electron spin resonance spectrometer operating at 275 GHz

An electron paramagnetic resonance (EPR) spectrometer is described which allows for continuous-wave and pulsed EPR experiments at 275 GHz (wavelength 1.1 mm). The related magnetic field of 9.9 T for g approximately 2 is supplied by a superconducting solenoid. The microwave bridge employs quasi-optical as well as conventional waveguide components. A cylindrical, single-mode cavity provides a high filling factor and a high sensitivity for EPR detection. Even with the available microwave power of 1 mW incident at the cavity a high microwave magnetic field B1 is obtained of about 0.1 mT which permits pi/2-pulses as short as 100 ns. The performance of the spectrometer is illustrated with the help of spectra taken with several samples.

Structure and Magnetism of the Tetra-Copper(II)-Substituted Heteropolyanion [Cu4K2(H2O)8(α-AsW9O33)2]8-

The novel heteropolyanion [Cu(4)K(2)(H(2)O)(8)(alpha-AsW(9)O(33))(2)](8)(-) (1) has been synthesized and characterized by IR spectroscopy, elemental analysis, and magnetic studies. Single-crystal X-ray analysis was carried out on [K(7)Na[Cu(4)K(2)(H(2)O)(6)(alpha-AsW(9)O(33))(2)].5.5H(2)O](n)(K(7)Na-1), which crystallizes in the tetragonal system, space group P42(1)m, with a = 16.705(4) A, b = 16.705(4) A, c = 13.956(5) A, and Z = 2. Interaction of the lacunary [alpha-AsW(9)O(33)](9)(-) with Cu(2+) ions in neutral, aqueous medium leads to the formation of the dimeric polyoxoanion 1 in high yield. Polyanion 1 consists of two alpha-AsW(9)O(33) units joined by a cyclic arrangement of four Cu(2+) and two K(+) ions, resulting in a structure with C(2)(v)() symmetry. All copper ions have one terminal water molecule, resulting in square-pyramidal coordination geometry. Three of the copper ions are adjacent to each other and connected via two micro(3)-oxo bridges. EPR studies on K(7)Na-1 and also on Na(9)[Cu(3)Na(3)(H(2)O)(9)(alpha-AsW(9)O(33))(2)].26H(2)O (Na(9)-2) over 2-300 K yielded g values that are consistent with a square-pyramidal coordination around the copper(II) ions in 1 and 2. No hyperfine structure was observed due to the presence of strong spin exchange, but fine structure was observed for the excited (S(T) = 3/2) state of Na(9)-2 and the ground state (S(T) = 1) of K(7)Na-1. The zero-field (D) parameters have also been determined for these states, constituting a rare case wherein one observes EPR from both the ground and the excited states. Magnetic susceptibility data show that Na(9)-2 has antiferromagnetically coupled Cu(2+) ions, with J = -1.36 +/- 0.01 cm(-)(1), while K(7)Na-1 has both ferromagnetically and antiferromagnetically coupled Cu(2+) ions (J(1) = 2.78 +/- 0.13 cm(-)(1), J(2) = -1.35 +/- 0.02 cm(-)(1), and J(3) = -2.24 +/- 0.06 cm(-)(1)), and the ground-state total spins are S(T) = 1/2 in Na(9)-2 and S(T) = 1 in K(7)Na-1.

High-frequency and -field EPR spectroscopy of tris(2,4-pentanedionato)manganese(III): investigation of solid-state versus solution Jahn-Teller effects

High-frequency and -field electron paramagnetic resonance (HFEPR) spectroscopy of a classical coordination complex, Mn(acac)(3) (Hacac = 2,4-pentanedione), has been performed on both solid powder and frozen solution (in CH(2)Cl(2)/toluene, 3:2 v/v) samples. Parallel mode detection X-band EPR spectra exhibiting resolved (55)Mn hyperfine coupling were additionally obtained for frozen solutions. Magnetic susceptibility and field-dependent magnetization measurements were also made on powder samples. Analysis of the entire EPR data set for the frozen solution allowed extraction of the relevant spin Hamiltonian parameters: D = -4.52(2); |E| = 0.25(2) cm(-1); g(iso) = 1.99(1). The somewhat lower quality solid-state HFEPR data and the magnetic measurements confirmed these parameters. These parameters are compared to those for other complexes of Mn(III) and to previous studies on Mn(acac)(3) using X-ray crystallography, solution electronic absorption spectroscopy, and powder magnetic susceptibility. Crystal structures have been reported for Mn(acac)(3) and show tetragonal distortion, as expected for this Jahn-Teller ion (Mn(3+), 3d(4)). However, in one case, the molecule exhibits axial compression and, in another, axial elongation. The current HFEPR studies clearly show the negative sign of D, which corresponds to an axial (tetragonal) elongation in frozen solution. The correspondence among solution and solid-state HFEPR data, solid-state magnetic measurements, and an HFEPR study by others on a related complex indicates that the form of Mn(acac)(3) studied here exhibits axial elongation in all cases. Such tetragonal elongation has been found for Mn(3+) and Cr(2+) complexes with homoleptic pseudooctahedral geometry as well as for Mn(3+) in square pyramidal geometry. This taken together with the results obtained here for Mn(acac)(3) in frozen solution indicates that axial elongation could be considered the "natural" form of Jahn-Teller distortion for octahedral high-spin 3d(4) ions. The previous electronic absorption data together with current HFEPR and magnetic data allow estimation of ligand-field parameters for Mn(acac)(3).

High frequency and field EPR spectroscopy of Mn(III) complexes in frozen solutions

We have performed high-frequency and -field electron paramagnetic resonance (HFEPR) experiments on two complexes of high-spin Mn(III) (3d(4),S=2): mesotetrasulfonato-porphyrinatomanganese(III) (Mn(TSP)) and [(R,R)-(-)-N,N(')-bis(3,5-di-tert-butylsalicylidene)-1,2-cyclohexanediaminomanganese(III)] (Mn(salen)). The main aim of this work was to qualitatively and quantitatively characterize the conditions suitable for HFEPR of high-spin transition metal complexes in frozen solutions, and compare them with experiments performed on solid samples. Mn(TSP) is a porphyrin complex soluble in water, in contrast to most metalloporphyrins. Mn(salen), often referred to as Jacobsen's catalyst, is a complex widely used in organic synthesis for alkene epoxidation, and is soluble in organic solvents. High-quality HFEPR signals were observed for solid state Mn(TSP), as has been previously shown for many Mn(III) complexes. The present study is, however, the first to report high-quality HFEPR spectra of a Mn(III) complex in frozen aqueous solution. Analysis of the data yielded the following spin Hamiltonian parameters: S=2; D=-3.16+/-0.02 cm(-1), E=0, and isotropic g=2.00(2). No X-band EPR signals were observed for Mn(TSP), which is a consequence of this being a rigorously axial spin system. Mn(salen), in contrast, did not give good quality HFEPR spectra in the solid state, but high-quality HFEPR spectra were recorded in frozen organic solutions. Analysis of the data yielded the following spin Hamiltonian parameters: S=2; D=-2.47+/-0.02 cm(-1), |E|=0.17+/-0.01 cm(-1), and isotropic g=2.00(2). These values differ from those reported using X-band parallel mode EPR [J. Am. Chem. Soc. 123 (2001) 5710], as discussed in the text. Therefore, a comparison between HFEPR and parallel-mode X-band spectroscopy is made. Finally, the concentration sensitivity aspect of HFEPR spectroscopy is also discussed.

EPR spectra from "EPR-silent" species: high-frequency and high-field EPR spectroscopy of pseudotetrahedral complexes of nickel(II)

High-frequency and high-field electron paramagnetic resonance (HFEPR) spectroscopy (using frequencies of approximately 90-550 GHz and fields up to approximately 15 T) has been used to probe the non-Kramers, S = 1, Ni(2+) ion in a series of pseudotetrahedral complexes of general formula NiL(2)X(2), where L = PPh(3) (Ph = phenyl) and X = Cl, Br, and I. Analysis based on full-matrix solutions to the spin Hamiltonian for an S = 1 system gave zero-field splitting parameters: D = +13.20(5) cm(-1), /E/ = 1.85(5) cm(-1), g(x) = g(y) = g(z) = 2.20(5) for Ni(PPh(3))(2)Cl(2). These values are in good agreement with those obtained by powder magnetic susceptibility and field-dependent magnetization measurements and with earlier, single-crystal magnetic susceptibility measurements. For Ni(PPh(3))(2)Br(2), HFEPR suggested /D/ = 4.5(5) cm(-1), /E/ = 1.5(5) cm(-1), g(x) = g(y) = 2.2(1), and g(z) = 2.0(1), which are in agreement with concurrent magnetic measurements, but do not agree with previous single-crystal work. The previous studies were performed on a minor crystal form, while the present study was performed on the major form, and apparently the electronic parameters differ greatly between the two. HFEPR of Ni(PPh(3))(2)I(2) was unsuccessful; however, magnetic susceptibility measurements indicated /D/ = 27.9(1) cm(-1), /E/ = 4.7(1), g(x) = 1.95(5), g(y) = 2.00(5), and g(z) = 2.11(5). This magnitude of the zero-field splitting ( approximately 840 GHz) is too large for successful detection of resonances, even for current HFEPR spectrometers. The electronic structure of these complexes is discussed in terms of their molecular structure and previous electronic absorption spectroscopic studies. This analysis, which involved fitting of experimental data to ligand-field parameters, shows that the halo ligands act as strong pi-donors, while the triphenylphosphane ligands are pi-acceptors.

High-frequency and -field electron paramagnetic resonance of high-spin manganese(III) in tetrapyrrole complexes

High-field and -frequency electron paramagnetic resonance (HFEPR) spectroscopy has been used to study three complexes of high spin Manganese(III), 3d4, S = 2. The complexes studied were tetraphenylporphyrinatomanganese(III) chloride (MnTPPCI), phthalocyanatomanganese(III) chloride (MnPcCl), and (8,12-diethyl-2,3,7,13,17,18-hexamethylcorrolato)manganese(III) (MnCor). We demonstrate the ability to obtain both field-oriented (single-crystal like) spectra and true powder pattern HFEPR spectra of solid samples. The latter are obtained by immobilizing the powder, either in an n-eicosane mull or KBr pellet. We can also obtain frozen solution HFEPR spectra with good signal-to-noise, and yielding the expected true powder pattern. Frozen solution spectra are described for MnTPPCl in 2:3 (v/v) toluene/CH2Cl2 solution and for MnCor in neat pyridine (py) solution. All of the HFEPR spectra have been fully analyzed using spectral simulation software and a complete set of spin Hamiltonian parameters has been determined for each complex in each medium. Both porphyrinic complexes (MnTPPCl and MnPcCl) are rigorously axial systems, with similar axial zero-field splitting (zfs): D approximately -2.3 cm(-1), and g values quite close to 2.00. In contrast, the corrole complex, MnCor, exhibits slightly larger magnitude, rhombic zfs: D approximtely -2.6 cm(-1), absolute value(E) approximately 0.015 cm(-1), also with g values quite close to 2.00. These results are discussed in terms of the molecular structures of these complexes and their electronic structure. We propose that there is a significant mixing of the triplet (S = 1) excited state with the quintet (S= 2) ground state in Mn(III) complexes with porphyrinic ligands, which is even more pronounced for corroles.

EPR study of substrate binding to the Mn(II) active site of the bacterial antibiotic resistance enzyme FosA: a better way to examine Mn(II)

FosA is a manganese metalloglutathione transferase that confers resistance to the broad-spectrum antibiotic fosfomycin, (1R,2S)-epoxypropylphosphonic acid. The reaction catalyzed by FosA involves the attack by glutathione on fosfomycin to yield the product 1-(S-glutathionyl)-2-hydroxypropylphosphonic acid. The enzyme is a dimer of 16 kDa subunits, each of which harbors one mononuclear Mn(II) site. The coordination environment of the Mn(II) in the FosA x Mn(2+) complex is composed of a glutamate and two histidine ligands and three water molecules. Here we report EPR spectroscopic studies on FosA, in which EPR spectra were obtained at 35 GHz and 2 K using dispersion-detection rapid-passage techniques. This approach provides an absorption envelope line shape, in contrast to the conventional (slow-passage) derivative line shape, and is a more reliable way to collect spectra from Mn(II) centers with large zero-field splitting. We obtain excellent spectra of FosA bound with substrate, substrate analogue phosphate ion, and product, whereas these states cannot be studied by X-band, slow-passage methods. Simulation of the EPR spectra shows that binding of substrate or analogue causes a profound change in the electronic parameters of the Mn(II) ion. The axial zero-field splitting changes from [D] = 0.06 cm(-1) for substrate-free enzyme to 0.23 cm(-1) for fosfomycin-bound enzyme, 0.28 (1) cm(-1) for FosA with phosphate, and 0.27 (1) cm(-1) with product. Such a large zero-field splitting is uncommon for Mn(II). A simple ligand field analysis of this change indicates that binding of the phosphonate/phosphate group of substrate or analogue changes the electronic energy levels of the Mn(II) 3d orbitals by several thousand cm(-1), indicative of a significant change in the Mn(II) coordination sphere. Comparison with related enzymes (glyoxalase I and MnSOD) suggests that the change in the coordination environment on substrate binding may correspond to loss of the glutamate ligand.

Mixed-valence tetranuclear manganese single-molecule magnets

The preparations, X-ray structures, and detailed physical characterizations are presented for two new mixed-valence tetranuclear manganese complexes that function as single-molecule magnets (SMM's): [Mn4(hmp)6Br2(H2O)2]Br2-4H2O (2) and [Mn4(6-me-hmp)6Cl4]-4H2O (3), where hmp(-) is the anion of 2-hydroxymethylpyridine and 6-me-hmp(-) is the anion of 6-methyl-2-hydroxymethylpyridine. Complex 2-4H2O crystallizes in the space group P2(1)/c, with cell dimensions at -160 degrees C of a = 10.907(0) A, b = 15.788(0) A, c = 13.941(0) A, beta = 101.21(0) degrees, and Z = 2. The cation lies on an inversion center and consists of a planar Mn4 rhombus that is mixed-valence, Mn2(III)Mn2(II). The hmp(-) ligands function as bidentate ligands and as the only bridging ligands in 2-4H2O. Complex 3-4H2O crystallizes in the monoclinic space group C2/c, with cell dimensions at -160 degrees C of a = 17.0852(4) A, b = 20.8781(5) A, c = 14.835(3) A, beta = 90.5485(8) degrees, and Z = 4. This neutral complex also has a mixed-valence Mn2(III)Mn2(II) composition and is best described as having four manganese ions arranged in a bent chain. A mu2-oxygen atom of the 6-me-hmp(-) anion bridges between the manganese ions; the Cl(-) ligands are terminal. Variable-field magnetization and high-frequency and -field EPR (HFEPR) data indicate that complex 2-4H2O has a S = 9 ground state whereas complex 3.4H(2)O has S = 0 ground state. Fine structure patterns are seen in the HFEPR spectra, and in the case of 2.4H(2)O it was possible to simulate the fine structure assuming S = 9 with the parameters g = 1.999, axial zero-field splitting of D/k(B) = -0.498 K, quartic longitudinal zero-field splitting of B4(omicron)/k(B) = 1.72 x 10(-5) K, and rhombic zero-field splitting of E/k(B) = 0.124 K. Complex 2-4H2O exhibits a frequency-dependent out-of-phase AC magnetic susceptibility signal, clearly indicating that this complex functions as a SMM. The AC susceptibility data for complex 2-4H2O were measured in the 0.05-4.0 K range and when fit to the Arrhenius law, gave an activation energy of DeltaE = 15.8 K for the reversal of magnetization. This DeltaE value is to be compared to the potential-energy barrier height of U/k(B) = absolute value DSz(2) = 40.3 K calculated for 2-4H2O.

High-frequency and field EPR investigation of (8,12-diethyl-2,3,7,13,17,18-hexamethylcorrolato)manganese(III)

High-field and frequency electron paramagnetic resonance (HFEPR) of solid (8,12-diethyl-2,3,7,13,17,18-hexamethylcorrolato)manganese(III), 1, shows that in the solid state it is well described as an S = 2 (high-spin) Mn(III) complex of a trianionic ligand, [Mn(III)C(3)(-)], just as Mn(III) porphyrins are described as [Mn(III)P(2)(-)](+). Comparison among the structural data and spin Hamiltonian parameters reported for 1, Mn(III) porphyrins, and a different Mn(III) corrole, [(tpfc)Mn(OPPh(3))], previously studied by HFEPR (Bendix, J.; Gray, H. B.; Golubkov, G.; Gross, Z. J. Chem. Soc., Chem. Commun. 2000, 1957-1958), shows that despite the molecular asymmetry of the corrole macrocycle, the electronic structure of the Mn(III) ion is roughly axial. However, in corroles, the S = 1 (intermediate-spin) state is much lower in energy than in porphyrins, regardless of axial ligand. HFEPR of 1 measured at 4.2 K in pyridine solution shows that the S = 2 [Mn(III)C(3)(-)] system is maintained, with slight changes in electronic parameters that are likely the consequence of axial pyridine ligand coordination. The present result is the first example of the detection by HFEPR of a Mn(III) complex in solution. Over a period of hours in pyridine solution at ambient temperature, however, the S = 2 Mn(III) spectrum gradually disappears leaving a signal with g = 2 and (55)Mn hyperfine splitting. Analysis of this signal, also observable by conventional EPR, leads to its assignment to a manganese species that could arise from decomposition of the original complex. The low-temperature S = 2 [Mn(III)C(3)(-)] state is in contrast to that at room temperature, which is described as a S = 1 system deriving from antiferromagnetic coupling between an S = (3/2) Mn(II) ion and a corrole-centered radical cation: [Mn(II)C(*)(2-)] (Licoccia, S.; Morgante, E.; Paolesse, R.; Polizio, F.; Senge, M. O.; Tondello, E.; Boschi, T. Inorg. Chem. 1997, 36, 1564-1570). This temperature-dependent valence state isomerization has been observed for other metallotetrapyrroles.

Pulsed EPR spectroscopy: biological applications

Pulsed electron paramagnetic resonance (EPR) methods such as ESEEM, PELDOR, relaxation time measurements, transient EPR, high-field/high-frequency EPR, and pulsed ENDOR, have been used successfully to investigate the local structure and dynamics of paramagnetic centers in biological samples. These methods allow different contributions to the EPR spectra to be distinguished and can help unravel complicated EPR spectra consisting of overlapping resonance lines, as are often found in disordered protein samples. The basic principles, specific potentials, technical requirements, and limitations of these advanced EPR techniques will be reviewed together with recent applications to metal centers, organic radicals, and spin labels in proteins.

Single-Ion versus Dipolar Origin of the Magnetic Anisotropy in Iron(III)-Oxo Clusters: A Case Study

A multitechnique approach has allowed the first experimental determination of single-ion anisotropies in a large iron(III)-oxo cluster, namely [NaFe6(OCH3)12(pmdbm)6ClO4 (1) in which Hpmdbm = 1,3-bis(4-methoxyphenyl)-1,3-propanedione. High-frequency EPR (HF-EPR). bulk susceptibility measurements, and high-field cantilever torque magnetometry (HF-CTM) have been applied to iron-doped samples of an isomorphous hexagallium(III) cluster [NaGa6(OCH3)12-(pmdbm)6]ClO4, whose synthesis and X-ray structure are also presented. HF-EPR at 240 GHz and susceptibility data have shown that the iron(III) ions have a hard-axis type anisotropy with DFe = 0.43(1) cm(-1) and EFe = 0.066(3) cm(-1) in the zero-field splitting (ZFS) Hamiltonian H = DFe[S2(z) - S(S + 1)/3] + Fe[S2(x) - S2(y)]. HF-CTM at 0.4 K has then been used to establish the orientation of the ZFS tensors with respect to the unique molecular axis of the cluster, Z. The hard magnetic axes of the iron(III) ions are found to be almost perpendicular to Z, so that the anisotropic components projected onto Z are negative, DFe(ZZ)= -0.164(4) cm(-1). Due to the dominant antiferromagnetic coupling, a negative DFe(ZZ) value determines a hard-axis molecular anisotropy in 1, as experimentally observed. By adding point-dipolar interactions between iron(III) spins, the calculated ZFS parameter of the triplet state, D1 = 4.70(9) cm(-1), is in excellent agreement with that determined by inelastic neutron scattering experiments at 2 K, D1 = 4.57(2) cm(-1). Iron-doped samples of a structurally related compound, the dimer [Ga2(OCH3)2(dbm)4] (Hdbm = dibenzoylmethane), have also been investigated by HF-EPR at 525 GHz. The single-ion anisotropy is of the hard-axis type as well, but the DFe parameter is significantly larger [DFe = 0.770(3) cm(-1). EFe = 0.090(3) cm(-1)]. We conclude that, although the ZFS tensors depend very unpredictably on the coordination environment of the metal ions, single-ion terms can contribute significantly to the magnetic anisotropy of iron(III)-oxo clusters, which are currently investigated as single-molecule magnets.

Multifrequency EPR spectra of molecular oxygen in solid Air

Multifrequency EPR spectra in the 94 to 550 GHz range were performed on solid air samples condensed at 5 K in the waveguide of a single pass probe. The spectra of molecular oxygen were observed and interpreted in the frame of the spin Hamiltonian model as axial S = 1 spectra with a zero field splitting parameter D = 3.572(3) cm(-1). The result of this study is relevant in the field of high field-high frequency EPR application in which solid air O(2) is a common paramagnetic impurity. Copyright 2000 Academic Press.