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The preparation of a binuclear Ni(II) complex with a pentacoordinate environment using a cryptand organic ligand and the imidazolate bridge is reported. The coordination sphere is close to trigonal bipyramidal (tbp) for one Ni(II) and to square pyramidal (spy) for the other. The use of the imidazolate bridge that undergoes π-π stacking with two benzene rings of the chelating ligand induces steric hindrance that stabilizes the pentacoordinate environment. Magnetic measurements together with theoretical studies of the spin states energy levels allow fitting the data and reveal a large Ising-type anisotropy and a weak anti-ferromagnetic exchange coupling between the metal ions. The magnitude and the nature of the magnetic anisotropy and the difference in anisotropy between the two metal ions are rationalized using wave-function-based calculations. We show that a slight distortion of the coordination sphere of Ni(II) from spy to tbp leads to an Ising-type anisotropy. Broken-symmetry density functional calculations rationalize the weak anti-ferromagnetic exchange coupling through the imidazolate bridge.

The preparations of related mononuclear and binuclear Co(II) complexes with a quasi-identical local C3v symmetry using a cryptand organic ligand are reported. The mononuclear complex behaves as a single molecule magnet (SMM). A relatively weak antiferromagnetic exchange coupling (J) of the same order of magnitude as the local magnetic anisotropy (D) is determined experimentally and theoretically for the binuclear complex. The weak magnitude of the antiferromagnetic exchange coupling, analyzed using a combination of broken-symmetry density functional theory and wave function based calculations, is ascribed to the weak overlap between the singly occupied orbitals because of the local C3v symmetry of the Co(II) ions; the organic ligand was found to contribute to the exchange coupling as the azido bridge that directly links the Co(II) ions. Calculation of the energy and wave functions of the spin states for the binuclear complex, in the general case, allows analysis of the effect of the |J/D| ratio on the mag...

To address the question of quantum mixing in the ground state of pseudo-octahedral catecholato-ferric complexes, advantage is taken from the thermal spin crossover of [(TPA)Fe(R-Cat)]X complexes to sound the electronic structures in the two spin isomers. EPR measurements were performed with ferric compounds possessing variable charge transfer interaction between the iron and dioxolene centers. The spectra of low-spin species observed either in the solid state or in frozen solutions were analyzed. For the set of complexes in glasses, the LS signal in the g = ge region is shown to be pseudo-axial with g⊥ > g∥. Interestingly, the anisotropy is markedly reduced when the dioxolene varies from DNC2− (4,5-dinitrocatecholate) to Cat2− (catecholate) and then DBC2− (3,5-di-tert-butylcatecholate), that is a sequence corresponding to an increasing FeII-semiquinonate character in the quantum mixing with the FeIII-catecholate configuration. The first analysis based on the Griffith model fails to reproduce the experimental g⊥ and g∥ values as a function of a reduced parameter deriving from the axial distortion of crystal field and spin–orbit coupling constant. In contrast, a g tensor written as a linear combination of g tensors associated with the two LS configurations (FeIIICat and FeIISQ) allows the experimental data to be adjusted. The electronic delocalization parameter determined by this simple approach confirms the expectation of a large quantum mixing correlated to a reduced anisotropy for the more electron donating catecholate.

The spin-dependent geometries, energetics, and electronic structures of the [FeO]2+ motif are investigated on the basis of DFT and perturbative CASPT2 ab initio calculations. The synthetic complex [(L52)FeO]2+ exhibits an S = 1 ground state with a low-lying (≈2000 cm–1) competing quintet and an open-shell singlet as the second excited state (≈11000 cm–1 above the ground state). By using the geometry optimization information, it was shown from a valence bond reading of the variational wavefunctions that a FeIII–O· picture dominates and that the number of d electrons is almost five in the model system [Fe(NH3)5O]2+. Analogy with the spin-crossover transition phenomenon is suggested, as the ground and low-lying quintet states can be seen as resulting from a doublet quartet transition of a Fe d5 ion. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)

With the aid of HClO{sub 4} as proton and BPh{sub 4}{sup -} as electron source, the Fe complex of a pentadentate pyridyl-appended cyclam ligands can activate dioxygen to yield the corresponding oxoiron(IV) complex (see scheme; TMC-py=1-(2'-pyridylmethyl)-4,8,11-trimethyl-1,4,8,11-tetraazacyclotetradecane; C gray, Fe magneta, N blue, O red). This transformation is proposed to occur via a hydroperoxoiron(III) intermediate.

The first example of a microcrystalline powder of a synthetic low-spin (LS) mononuclear Fe(III)(OOH) intermediate has been obtained by the precipitation of the [Fe(III)(L(5) (2))(OOH)](2+) complex at low temperature. The high purity of this thermally unstable powder is revealed by magnetic susceptibility measurements. EPR studies on this complex, in the solid state and also in frozen solution, are reported and reveal the coexistence of two related Fe(III)(OOH) species in both states. We also present a theoretical analysis of the g tensor for LS Fe(III) complexes, based on new perturbation equations. These simple equations provide distortion-energy parameters that are in good agreement with those obtained by a full-diagonalization calculation.

Two new mononuclear FeII complexes, [(L52aH)FeII](PF6)2 (1-(PF6)2) and [(L52a)FeII]BPh4 (2-(BPh4)) have been synthesized with the new aminopyridyl ligand bearing a pivaloylamido arm L52aH (2,2-dimethyl-N-[6-({[2-(methyl-pyridin-2-ylmethyl-amino)-ethyl]-pyridin-2-ylmethyl-amino}-methyl)-pyridin-2-yl]-propionamide), or its deprotonated form L52a-. The structures of the ferrous complexes have been determined by X-ray analysis. The mononuclear FeII is in a pseudo-octahedral environment in both complexes, the six positions around the metal center being occupied by five nitrogen atoms and one oxygen atom from the ligand. Whatever the protonation state of the amide function, the structures are very similar, the FeII being 6-fold coordinated by the two amines, three pyridines, and the oxygen atom from the ligand. These two complexes exhibit an acid/base equilibrium in solution that has been studied by UV-vis spectroscopy and cyclic voltammetry in acetonitrile. The reactivity of 1-(PF6)2 with H2O2 in methanol affords the formation of a new low-spin FeIII(OOH) intermediate in which the oxygen atom is retained in the coordination sphere of the metal.

A new, dinuclear high spin manganese(III) complex with bridging phenoxy and methoxy groups. Structure and magnetic properties

The GGGTH sequence has been proposed to be the minimal sequence involved in the binding of a fifth Cu(II) ion in addition to the octarepeat region of the prion protein (PrP) which binds four Cu(II) ions. Coordination of Cu(II) by the N- and C-protected Ac-GGGTH-NH(2) pentapeptide (P(5)) was investigated by using potentiometric titration, electrospray ionization mass spectrometry, UV-vis spectroscopy, electron paramagnetic resonance (EPR) spectroscopy and cyclic voltammetry experiments. Four different Cu(II) complexes were identified and characterized as a function of pH. The Cu(II) binding mode switches from NO(3) to N(4) for pH values ranging from 6.0 to 10.0. Quasi-reversible reduction of the [Cu(II)(P(5))H(-2)] complex formed at pH 6.7 occurs at E (1/2)=0.04 V versus Ag/AgCl, whereas reversible oxidation of the [Cu(II)(P(5))H(-3)](-) complex formed at pH 10.0 occurs at E (1/2)=0.66 V versus Ag/AgCl. Comparison of our EPR data with those of the rSHaPrP(90-231) (Burns et al. in Biochemistry 42:6794-6803, 2003) strongly suggests an N(3)O binding mode at physiological pH for the fifth Cu(II) site in the protein.

Two manganese complexes, (py2(NMe)2)MnIICl2 (1) and [(py2(NMe)2)MnIIIF2]+ (2), are here described with the macrocyclic ligand py2(NMe)2 (py2(NMe)2 = N,N‘-dimethyl-2,11-diaza[3,3](2,6)pyridinophane). For both, the crystal structure is reported. The UV−visible spectrum of 2 exhibits a very broad near-infrared (NIR) band corresponding to the transition between the two eg-type orbitals split by the Jahn−Teller effect. A negative D value of ca. −4 cm-1 was estimated by high-field and high-frequency electron paramagnetic resonance (HF-EPR) spectroscopy, which was consistent with symmetry considerations. Density functional theory (DFT) calculations on 2 support the 5B1 electronic ground state predicted from the X-ray structure. Moreover, to explain the large value of the D parameter, a spin triplet first excited spin state was postulated to occur at low energy. This was confirmed by the DFT calculations.

Obtention and crystal structure of guanfacine (guaH) together with synthesis and crystal structure of its copper complex [CuII(gua)2] · DMF were reported. The free molecule guaH exhibits one tautomeric form (B) in contrast to the form (A) which was reported in the Merck index. In the copper(II) complex, the anionic form gua− exhibits the third tautomeric form (C). This complex is characterized by a CuN2O2 coordination. The EPR spectrum is in agreement with a Cu(II) ion in a square planar configuration.

Four iron(II) complexes, bearing hexa-, penta- or tetra-azadentate ligands, whose redox potentials (for the FeIII/FeII couple) vary from +280 to +1023 mV (versus saturated calomel electrode), were compared as catalysts for the oxidation of various substrates by H2O2. These complexes were bad catalysts for alkene epoxidation or alkane hydroxylation, whereas those exhibiting the highest redox potentials were interesting catalysts for the hydroxylation of aromatic compounds. For most of them, addition of an appropriate reducing agent, such as hydroquinones, PhSH or tetrahydropterins, in the medium, led to spectacular increases of the yields of hydroxylation of aromatic compounds such as anisole, toluene, ethylbenzene, benzene and naphthalene (with final yields based on H2O2 up to 69, 33, 18, 40 and 47%, respectively). Such systems catalyzed the transfer of an oxygen atom from H2O2, as shown by experiments using H2 18 O 2 and H2 18 O , into anisole with yields based on starting H2O2 up to 69%.

The anisotropic g and hyperfine tensors of the Mn di-micro-oxo complex, [Mn(2)(III,IV)O(2)(phen)(4)](PF(6))(3).CH(3)CN, were derived by single-crystal EPR measurements at X- and Q-band frequencies. This is the first simulation of EPR parameters from single-crystal EPR spectra for multinuclear Mn complexes, which are of importance in several metalloenzymes; one of them is the oxygen-evolving complex in photosystem II (PS II). Single-crystal [Mn(2)(III,IV)O(2)(phen)(4)](PF(6))(3).CH(3)CN EPR spectra showed distinct resolved (55)Mn hyperfine lines in all crystal orientations, unlike single-crystal EPR spectra of other Mn(2)(III,IV) di-micro-oxo bridged complexes. We measured the EPR spectra in the crystal ab- and bc-planes, and from these spectra we obtained the EPR spectra of the complex along the unique a-, b-, and c-axes of the crystal. The crystal orientation was determined by X-ray diffraction and single-crystal EXAFS (Extended X-ray Absorption Fine Structure) measurements. In this complex, the three crystallographic axes, a, b, and c, are parallel or nearly parallel to the principal molecular axes of Mn(2)(III,IV)O(2)(phen)(4) as shown in the crystallographic data by Stebler et al. (Inorg. Chem. 1986, 25, 4743). This direct relation together with the resolved hyperfine lines significantly simplified the simulation of single-crystal spectra in the three principal directions due to the reduction of free parameters and, thus, allowed us to define the magnetic g and A tensors of the molecule with a high degree of reliability. These parameters were subsequently used to generate the solution EPR spectra at both X- and Q-bands with excellent agreement. The anisotropic g and hyperfine tensors determined by the simulation of the X- and Q-band single-crystal and solution EPR spectra are as follows: g(x) = 1.9887, g(y) = 1.9957, g(z) = 1.9775, and hyperfine coupling constants are A(III)(x) = |171| G, A(III)(y) = |176| G, A(III)(z) = |129| G, A(IV)(x) = |77| G, A(IV)(y) = |74| G, A(IV)(z) = |80| G.

A series of dinickel(II) complexes with the 24-membered macrocyclic hexaazadithiophenol ligand H 2 L M e was prepared and examined. The doubly deprotonated form (L M e ) 2 - forms complexes of the type [(L M e )Ni I I 2(μ-L')] n + with a bioctahedral N 3 Ni I I (m-SR) 2 (μ-L')Ni I I N 3 core and an overall calixarene-like structure. The bridging coordination site L' is accessible for a wide range of exogenous coligands. In this study L'=NO 3 -, NO 2 -, N 3 -, N 2 H 4 , pyrazolate (pz), pyridazine (pydz), phthalazine (phtz), and benzoate (OBz). Crystallographic studies reveal that each substrate binds in a distinct fashion to the [(L M e )Ni 2 ] 2 + portion: NO 2 -, N 2 H 4 , pz, pydz, and phtz form μ 1 , 2 -bridges, whereas NO 3 -, N 3 -, and OBz - are μ 1 , 3 -bridging. These distinctive binding motifs and the fact that some of the coligands adopt unusual conformations is discussed in terms of complementary host-guest interactions and the size and form of the binding pocket of the [(L M e )Ni 2 ] 2 + fragment. UV/Vis and electrochemical studies reveal that the solid-state structures are retained in the solution state. The relative stabilities of the complexes indicate that the [(L M e )Ni 2 ] 2 + fragment binds anionic coligands preferentially over neutral ones and strong-field ligands over weak-field ligands. Secondary van der Waals interactions also contribute to the stability of the complexes. Intramolecular ferromagnetic exchange interactions are present in the nitrito-, pyridazine-, and the benzoato-bridged complexes where J=+6.7, +3.5, and +5.8 cm - 1 (H= -2JS 1 S 2 , S 1 =S 2 =1) as indicated by magnetic susceptibility data taken from 300 to 2 K. In contrast, the azido bridge in [(L M e )Ni 2 (μ 1 , 3 -N 3 )] + results in an antiferromagnetic exchange interaction J=-46.7 cm - 1 . An explanation for this difference is qualitatively discussed in terms of bonding differences.

Two neutral complexes [(L634M)Fe2Cl4]*2H2O*2CHCl3 (1) and [(L634M)Mn2Cl4]*CH3CN (2) have been synthesized {L634M = N,N,N′,N′-tetrakis[(6-methyl-2-pyridyl)methyl]propane-1,3-diamine} and their molecular structures established by X-ray crystallography. Both structures are similar, with each metal center in a trigonal-bipyramidal environment. No magnetic coupling is observed between the metal centers. UV/Vis spectra and cyclic voltammograms were recorded in CH2Cl2 and CH3CN solutions. Both complexes are stable in CH2Cl2, whereas only 2 is stable in CH3CN. On the contrary, 1 is in equilibrium with another FeII species in CH3CN. When this last solution is aerated, monocrystals of the neutral linear tetranuclear complex [(L634M)Fe4(μ-O)3Cl6]*2CH3CN (3) can be isolated. Its structure is unusual with two FeIII ions pentacoordinate and the two others tetracoordinate with only chloro ligands and oxo bridges. The magnetic properties reveal that two consecutive metal centers are strongly antiferromagnetically coupled. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)

Mononuclear FeIV intermediates have been generated in MeOH upon reaction of sodium hypochlorite or hypobromite with a ferrous complex bearing the pentadentate ligands N-methyl-N,N′,N′-tris(2-pyridylmethyl)ethane-1,2-diamine (L52) or N-methyl-N,N′,N′-tris(2-pyridylmethyl)propane-1,3-diamine (L53). These highly unstable green complexes are characterized by an absorption band at ca. 750 nm. Mossbauer data indicate that the iron center is low spin (S = 1) with an axial electronic structure, allowing identification of the mononuclear FeIV complexes as [L5FeIVO]2+ or [L5FeIVOCH3]3+. In acetonitrile/dichloromethane solutions, the L53FeIV system exhibits very selective activities toward hydroxylation of cyclohexane or epoxidation of cyclooctene and cis-stilbene. Computational studies performed on [L5FeIVO]2+ and on the model compound [(NH3)5FeIVO]2+ reveal that the ground state possesses two unpaired electrons in the two π* orbitals, as known for O2. Electronic spectra computed by time-dependent DFT exhibit only one band in the visible region that is essentially due to d-d transitions. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)

Three new FeIII complexes have been synthesized with the tridentate ligand salEen (salEen=N-ethyl-N-(2-aminoethyl)salicylaldiminate). Two of them are neutral dinuclear complexes and their structures have been resolved. Their formulae are [Fe(salEen)(Cl)(μ-CH3O)]2 and [Fe(salEen)(DBC)]2 (DBC=3,5 di-tert-butylcatecholate), respectively. These complexes exhibit a ‘diamond-core’ geometry with the ferric ions antiferromagnetically coupled by the bridging oxygens of the methoxy or DBC ligands, respectively. The last complex is a mononuclear one of formula [Fe(HsalEen)2Cl2]Cl, with the protonated ligand acting as a bidentate one. This compound is actually a mixture of two complexes with the same formula but with different coordination of the HsalEen fragments.

A new mu-phenoxo-bis-mu-acetato di-Mn(II) complex using the BpmpH ligand was isolated as a perchlorate salt (BpmpH = 2,6-bis[bis(2-pyridylmethyl)aminomethyl]-4-methyl-phenol). The X-ray structure has been solved showing that the two Mn(II) ions are in a distorted octahedral environment. Investigation of the variation of the molar magnetic susceptibility upon temperature reveals an antiferromagnetic exchange interaction between the two high-spin Mn(II) ions. Fitting of the experimental data led to g = 1.99 and J = 9.6 cm(-1) (H(HDvV) = JS(A).S(B)). EPR spectra recorded on a powder sample of [(Bpmp)Mn(2)(mu-OAc)(2)](ClO(4)).0.5H(2)O at X-band between 4.3 K and room temperature and at Q-band between 5 and 298 K are presented. A new method based on a scrupulous examination of the variation upon temperature of these experimental spectra is developed here to first assign the transitions to the relevant spin states and second to determine the associated spin parameters. This approach is compared to the deconvolution process that has been previously applied to dinuclear Mn(II) complexes or metalloenzyme active sites. Crystallographic data is as follows: triclinic, space group P one macro, a = 10.154(2) A, b = 12.0454(2) A, c = 17.743(4) A, alpha = 101.69(3) degrees, beta = 93.62(3) degrees, gamma = 94.67(3) degrees, Z = 2.

The ligand N,N′-bis(6-methyl-2-pyridylmethyl)-N,N′-bis(2-pyridylmethyl)ethane-1,2-diamine (L622M) has allowed to prepare the new FeII complex [(L622M)FeCl2] (1), and to compare its structural and spectroscopic characteristics with [(L622M)FeCl]PF6 (2). The molecular structure of 1, resolved by X-ray diffraction, exhibits the ligand tetracoordinated with the two non-methylated pyridine rings coordinated. As shown by UV/Vis spectroscopy and cyclic voltammetry, upon dissolution in methanol or acetonitrile, one chloride ion is released and replaced by a pyridine group. Therefore, complexes 1 and 2 adopt identical structures in solution, i.e. [(L622M)FeCl+. However, upon oxidation complex 1 gives several ferric complexes with the ligand L622M pentacoordinated, or tetracoordinated with two chloride ions. This peculiar behaviour is due to the presence of chloride ions in solution in the case of 1. Upon reaction with H2O2 in methanol, complex 2 leads to the formation of low-spin [(L622M)FeIII(OOH)2+, whereas it is not observed in the case of 1. Based on the presence or not of chloride ions in solution, a mechanism is proposed for the reactivity of complexes 1 and 2 toward H2O2. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003)

Menkes disease is a lethal genetic disorder of copper transport in humans. Its current treatment is the administration of copper–l-histidine complex. However, this therapy is only effective in some cases and when started early in life. In order to develop an improved treatment, copper(II) mixed ligand amino acid complexes were studied because these complexes play an important role in the physiological copper pathway. A promising strategy is the administration of l-histidine–copper(II)–l-glutamine complex, which has been reported to be the predominant low molecular-weight copper(II) complex in human blood serum. Before the biopharmaceutical studies of l-histidine–copper(II)–l-glutamine complex, a physicochemical study of this ternary system must be performed. The stoichiometry and the stability of l-histidine–copper(II)–l-glutamine relative to copper(II)–l-histidine and copper(II)–l-glutamine were established at pH 7 in solution by polarography and UV–visible spectroscopy. Nevertheless, it has bee...

Treatment of [M(H2Li)] with U(acac)4 in refluxing pyridine led to the formation of the trinuclear complexes [{MLi(py)x}2U] [L1 = N,N′-bis(3-hydroxysalicylidene)-2,2-dimethyl-1,3-propanediamine and M = Ni, Cu or Zn; L2 = N,N′-bis(3-hydroxysalicylidene)-1,3-propanediamine and M = Cu or Zn; L3 = N,N′-bis(3-hydroxysalicylidene)-2-methyl-1,2-propanediamine and M = Ni, Cu or Zn; x = 0 or 1]. The dinuclear compounds [ML3(py)U(acac)2] (M = Cu, Zn) were isolated from the reaction of [M(H2L3)] and U(acac)4 in pyridine at 60 °C. The crystal structures of the trinuclear complexes are built up by two orthogonal MLi(py)x units which are linked to the central uranium ion by the two pairs of oxygen atoms of the Schiff base ligand; the U(IV) ion is found in the same dodecahedral configuration but the Cu(II) ion coordination geometry and the Cu⋯U distance are different by passing from L1 or L2 to L3, due to the shortening of the diimino chain of L3. These geometrical parameters seem to have a great influence on the magnetic behaviour of the complexes since the Cu–U coupling in [{CuLi(py)x}2U] (i = 1, 2) is ferromagnetic while it is antiferromagnetic in [{CuL3(py)x}2U]. In the compounds [{CuL3(py)x}2U] and [CuL3(py)U(acac)2], the Cu coordination and the Cu⋯U distance are very similar, and both exhibit an antiferromagnetic interaction.

study demonstrates that resin and fatty acids are able to extract copper(II) ions from verdigris (copper acetate) and verditer (basic copper carbonate). Ligand exchange reactions of basic copper carbonate with fatty acids and resin acids are much slower than is the case with copper acetate. The browning of paint layers is closely correlated with the relative ease of copper extraction; the copper diffuses in the form of fatty acid or resin carboxylic acid complexes. These complexes are formed in the painting layer during grinding of the pigment with binding media containing oleoresin acids, as well as being formed at the interface of this layer and organic upper layers, such as varnishes.

Attempts to prepare heterobimetallic complexes in which 3d and uranium magnetic ions are associated by means of the Schiff bases H(2)L(i) derived from 2-hydroxybenzaldehyde or 2-hydroxy-3-methoxybenzaldehyde were unsuccessful because of ligand transfer reactions between [ML(i)] (M=Co, Ni, Cu) and UCl(4) that led to the mononuclear Schiff base complexes of uranium [UL(i)Cl(2)]. The crystal structure of [UL(3)Cl(2)(py)(2)] [L(3)=N,N'-bis(3-methoxysalicylidene)-ethylenediamine; py=pyridine] was determined. The hexadentate Schiff base ligand N,N'-bis(3-hydroxysalicylidene)-2,2-dimethyl-1,3-propanediamine (L) was useful for the synthesis of novel trinuclear complexes of the general formula [[ML(py)](2)U] (M=Co, Ni, Zn) or [[CuL(py)]M'[CuL]] (M'=U, Th, Zr) by reaction of [M(H(2)L)] with [M'(acac)(4)] (acac=MeCOCHCOMe). The crystal structures of the Co(2)U, Ni(2)U, Zn(2)U, Cu(2)U, and Cu(2)Th complexes show that the two ML fragments are orthogonal, being linked to the central actinide ion by the two pairs of oxygen atoms of the Schiff base ligand. In each compound, the UO(8) core exhibits the same dodecahedral geometry, and the three metals are linear. The magnetic study indicated that the two Cu(2+) ions are not coupled in the Cu(2)Zr and Cu(2)Th compounds. The magnetic behavior of the Co(2)U, Ni(2)U, and Cu(2)U complexes was compared with that of the Zn(2)U derivative, in which the paramagnetic 3d ion was replaced with the diamagnetic Zn(2+) ion. A weak antiferromagnetic coupling was observed between the Ni(2+) and the U(4+) ions, while a ferromagnetic interaction was revealed between the Cu(2+) and U(4+) ions.

The compound [L1(H2O)Fe(μ-O)Fe(OH2)L1](ClO4)4·2H2O [1(ClO4)4·2H2O] [L1 = N,N′-bis(1-methylimidazolyl-2-methyl)-N,N′-bismethyl-1,2-ethanediamine] was synthesized. It is characterized by a linear Fe−(μ-O)−Fe motif with an Fe···Fe distance of 3.584(1) A. The measurement of the magnetic susceptibility as a function of temperature indicated a strong antiferromagnetic coupling between the two high-spin FeIII ions (J = −223 cm−1 with H = −J·S1·S2). In the solid state, the symmetric stretching vibration mode of the Fe−(μ-O)−Fe core unit was detected at 347 cm−1, in agreement with a straight Fe−(μ-O)−Fe angle. Upon addition of up to one equivalent of triethylamine to an acetonitrile solution of 1, conversion to the [L1(H2O)Fe(μ-O)Fe(OH)L1]3+ complex 2 was observed. This last species was isolated as a perchlorate salt [2(ClO4)3·H2O]. Susceptibility measurements and Raman and UV/Vis investigations on the powder and/or an acetonitrile solution confirmed the presence of an intramolecular hydrogen bond between the coordinated water molecule and the hydroxide group leading to a bent [Fe(μ-O)Fe]4+ core structure.

Nonheme Fe(III)-hydroperoxo and Fe(III)-peroxo complexes with aminopyridyl-type ligands have been prepared and characterized by UV/Vis, EPR, mass and Resonance Raman (RR) spectroscopy. The Fe(III)(OOH) species are low-spin and exhibit a deep purple color due to the ligand-to-metal charge transfer (LMCT) hand centered at ca. 550 nm. The RR spectra of the Fe(III)(OOH) complexes display two bands at ca. 620 and 800 cm-1 that are assigned to the respective Fe-O and O-O stretching modes on the basis of the characteristic H/D and 16O/18O frequency shifts. Upon deprotonation, Fe(III)(O2) species are obtained which possess a high-spin configuration of nearly axial symmetry and a LMCT transition in the near infrared (ca. 750 nm). The frequencies of the Fe-O and O-O stretching modes at ca. 465 and 820 cm-1, as well as their respective 16O/18O shifts of -16 and -45 cm-1, indicate an ?2 coordination geometry for the Fe(III)(O2) complex.

Four new Fe(III) catecholate complexes, [(bispicMe2en)FeIII(DBC)]+, [(bispicCl2Me2en)FeIII(DBC)]+, [(trispicMeen)FeIII(DBC)]+, and [(BQPA)FeIII(DBC)]+, which all contain aminopyridine ligands, were synthesized. The structure of [(bispicMe2en)FeIII(DBC)]+ was determined by X-ray diffraction. It crystallizes in the triclinic space group P1 with a = 10.666(3) A, b = 13.467(5) A, c = 17.685(2) A, alpha = 93.46(2) degrees, beta = 93.68(2) degrees, gamma = 109.0(3) degrees, V = 2387.4 A3, and Z = 2. All of these complexes were found to be active toward oxidation of catechol by O2 in DMF at 20 degrees C to afford intradiol cleavage products. The catechol was quantitatively oxidized, mainly (90%) into 3,5-di-tert-butyl-5-(carboxymethyl)-2-furanone. Reaction rates were measured, and for the first three (topologically similar) complexes, a correlation of the second-order kinetic constants k with the optical parameters of the two LMCT O(DBC)--Fe(III) bands was found. In particular, k increases with the epsilon max of the charge-transfer bands. The k value of the complex [(BQPA)FeIII(DBC)]+, containing a tripodal ligand, is smaller than expected on the basis of these correlations. This discrepancy could be related to steric hindrance induced by the BQPA ligand. However, the much lower activity of the bispicen-Fe(III)-type complexes compared to that of the [(TPA)FeIII(DBC)]+ complex synthesized by Jang et al. (J. Am. Chem. Soc. 1991, 113, 9200-9204), despite similar epsilon max values, shows that a knowledge of optical and NMR parameters values is not sufficient to explain the dioxygenase activity rate. In their study of protocatechuate 3,4-dioxygenase, Orville et al. (Biochemistry 1997, 36, 10052-10066) suggested that asymmetric chelation of the catecholate to Fe(III) is of great importance in the efficiency of the intradiol dioxygenase reaction. Indeed, a comparison of the X-ray structures of [(TPA)FeIII(DBC)]+ and [(bispicMe2en)FeIII(DBC)]+ shows that the Fe(III)-O bonds differ by 0.019 A in the former and are identical in the latter. Asymmetry could also play a role in the model complexes. An alternative explanation is the possible existence of a low-spin state for [(TPA)FeIII(DBC)]+, as recently identified in [(TPA)FeIII(cat)]+ by Simaan et al.

Two new mononuclear iron complexes, [(LBzl2)Fe(II)Cl2]·H2O and [(LBzl2) Fe(III)Cl2]·PF6, (LBzl2=N,N′-bisbenzyl-N,N′-bis(2-pyridylmethyl)-ethane-1,2-diamine) have been synthesised in view of generating complexes to mimic the active site of methane monooxygenase. Their structures have been determined by X-ray analysis. In both species, the iron atom shows a pseudo-octahedral coordination with two pyridine nitrogen atoms in axial positions and two amine nitrogen atoms in the equatorial plane. Two other equatorial positions are occupied by chloride ions. The coordination bond lengths clearly indicate the sensitivity of the ligand to the oxidation state of the iron. Thus, the FeN and FeCl bond distances in Fe(II) complex are more elongated than corresponding distances in Fe(III). A statistical examination of the bond distances of hexacoordinated Fe(II) and Fe(III) complexes using the Cambridge Structural Database provides evidence to relate both complexes to their spin state. The redox potential of the Fe(III)/Fe(II) couple was determined by cyclic voltammetry. The UV–Vis spectra are dominated by charge transfer transitions. The X-band EPR spectrum of [(LBzl2) FeCl2]·PF6 is characteristic of an S=5/2 species with an unusual zero-field splitting.

A new ferric complex, [(bispic(PA)2en)Fe(III)Cl2]·[PF6]3 (bispic(PA)2en=N,N′-bis(2-pyridylmethyl)-N,N′-bis(5-trimethylammoniumpentyl)-1,2-diaminoethane) has been synthesized, and its structure has been determined by X-ray crystallography. The cation adopts a cis-α-dichloro configuration with the six-coordinated iron(III) in a pseudo octahedral geometry. Its relative catecholato adduct [(bispic(PA)2en)Fe(III)(DBC)]·[PF6]3 has been synthesized and found to be active toward oxidation of catechol by O2 to afford intradiol cleavage products.

The new disymmetric ligand N,N,N′ -tris(2-pyridylmethyl)- N′ -(2-salicylideneethyl)ethane-1,2-diamine (LH) has been synthesized in the search of a novel type of manganese complex to mimic the active site of the water-oxidizing enzyme in photosystem II. The complex [Mn(II)L]ClO 4 has been obtained and characterized by X-ray diffraction techniques. It crystallizes in the monoclinic space group Pn with the following unit cell parameters: a =10.164(3), b =10.122(4), c =14.166(5) A, β =93.48(2)° and Z =2. The manganese ion is heptacoordinated with the coordination being achieved by only one ligand; it is bonded to the oxygen atom of the phenolate group in an axial position, the imino and the three pyridine nitrogen atoms in an equatorial position and the two amine atoms in a pseudo-axial position. The coordination polyhedron is best described as a distorted monocapped trigonal prism. This structure was compared with the seven-coordinated Mn(II) complexes deposited in the Cambridge Structured Database (CSD). The redox potential of the Mn(III)/Mn(II) couple was determined by cyclic voltammetry.

The synthesis and characterization of three complexes with a potent nonsteroidal anti-inflammatory drug niflumic acid {2-[3-(trifluoromethyl)phenyl]aminonicotinic acid} with formula [Cu(niflumato)2L] (L = H2O, DMSO = dimethylsulfoxide, DMF = N,N-dimethylformamide) were investigated. The crystal and molecular structure of the {Cu(niflumato)2(DMSO)}2 was reported. Crystallographic data are as follows: monoclinic system, space group P2(1)/n, Z = 2, a = 11.1318(8), b = 17.513(2), c = 15.336(1) A, beta = 103.316(8) degrees, V = 2909.4(4) A3. The structure was refined to R = 0.030 and wR = 0.037 for 3702 reflections with Isigma (I). It consists of centrosymmetric binuclear units with the Cu-Cui (symmetry code i: 1-x, -y, 1-z) distance between two centrosymmetrically related ions of 2.6272(5) A. Each Cu(II) ion in [Cu2(DMSO)2(mu-niflumato)4] is coordinated to an apical dimethylsulfoxide O atom on the one hand and to the equatorial carbonyl and carboxylic O atoms of two crystallographically independent niflumate moieties and their centrosymmetric counterparts on the other hand. In spite of the low-temperature (190 K) crystal measurements, one L-CF3 grouping exhibits some disorder. The biological activities of these complexes were compared to that of niflumic acid. Niflumic acid and its various copper complexes significantly inhibited polymorphonuclear leukocyte (PMNL) oxidative metabolism, as assessed by chemiluminescence and O2- generation measurement. This effect was dose-dependent. All copper complexes exerted a similar inhibiting effect which was always significantly higher than that exerted by the parent drug.

MnIII(bpy)Cl3H2O (Goodwin, H. A.; Sylva, R. N. Aust. J. Chem. 1965, 18, 1743) crystallizes in the triclinic space group P1, with a = 6.627(1) A, b = 9.991(7) A, c = 10.116(4) A, α = 84.38(4)°, β = 73.47(3)°, γ = 78.48(3)°, V = 629 A,3 and Z = 4. The MnIII ion has an octahedral surrounding distorted by a strong Jahn−Teller effect. Two chloride ions and the water molecule are involved into a network of hydrogen bonds. A careful study of aqueous solutions of MnIIILCl3H2O (L = bipyridine or phenanthroline) at different pH demonstrated the formation by condensation and disproportionation of [L2MnIIIO2MnIVL2](ClO4)3 (Cooper, S. R.; Calvin, M. J. Am. Chem. Soc. 1977, 99, 6623), [MnIV3O4L4(H2O)2](ClO4)4 (Sarneski, J. E.; et al. J. Am. Chem. Soc. 1990, 112, 7255) and [Mn4O6(bpy)6](ClO4)4 (Philouze, C.; et al. J. Am. Chem. Soc. 1994, 116, 8557). Condensation of MnLCl3H2O has been studied theoretically using partial charge models. It was possible to identify the mononuclear MnIII and MnIV species which are able to ...

In plants, solar energy is used to extract electrons from water, producing atmospheric oxygen. This is conducted by Photosystem II, where a redox ”triad” consisting of chlorophyll, a tyrosine, and a manganese cluster, governs an essential part of the process. Photooxidation of the chlorophylls produces electron transfer from the tyrosine, which forms a radical. The radical and the manganese cluster together extract electrons from water, providing the biosphere with an unlimited electron source. As a partial model for this system we constructed a ruthenium(II) complex with a covalently attached tyrosine, where the photooxidized ruthenium was rereduced by the tyrosine. In this study we show that the tyrosyl radical, which gives a transient EPR signal under illumination, can oxidize a manganese complex. The dinuclear manganese complex, which initially is in the Mn(III)/(III) state, is oxidized by the photogenerated tyrosyl radical to the Mn(III)/(IV) state. The redox potentials in our system are comparable t...

The Mn4 complex which is involved in water oxidation in photosystem II is known to exhibit three types of EPR signals in the S2 state, one of the five redox states of the enzyme cycle: a multiline signal (spin 1/2), signals at g > 5 (spin 5/2), and a signal at g = 4.1 (spin value 3/2 or 5/2). The multiline and g = 4.1 signals are those the most readily observed. The relative proportions of the g = 4.1 signal and of the multiline signal are affected by many biochemical treatments including the substitution of Ca2+and Cl- which are two essential cofactors for O2 evolution. The state responsible for the multiline signal can also be converted, reversibly, to that responsible for the g = 4.1 signal upon the absorption of near-IR light at around 150 K. These infrared-induced effects are confined to the Mn4 cluster, and no other redox change occurs in the enzyme. Here, we have used the IR-induced photochemistry of the Mn4 cluster to measure the changes in magnetization occurring upon interconversion of the stat...

The synthesis and structural characterization of the novel [MnIII,IV2O2(N,Nbispicen)2](ClO4)3·CH3CN complex employing the tetradentate ligand N,Nbispicen = N,N-bis(2-pyridylmethyl)-1,2-diaminoethane are reported. Magnetic properties were determined and show that the ground state is a spin doublet. This can be quantitatively interpreted by antiferromagnetic coupling between a Mn(III) high spin and a Mn(IV) (J = −316 cm−1). The 16 line solution EPR spectrum exhibits an unusual splitting in the low field resonances. The following rhombic tensors were needed to simulate the EPR spectrum: |A1x|= 160 10−4 cm−1, |A1y| = 144 10−4 cm−1, |A1z|= 109 10−4 cm−1, |A2x|= 69 10−4 cm−1, |A2y| = 72 10−4 cm−1, |A2z| = 75 10−4 cm−1, gx = 2.001, gy = 1.996, gz = 1.984. The classical ligand field theory of local [gIII] and [gIV] tensors implemented with the first order perturbation theory to describe the properties of the pair does not result in a satisfying description of the [g1/2] tensor unless a large reduction in the spin-orbit constant is invoked. A simplified version of second-order perturbation theory leads to effects in qualitative agreement with experiment but weak as expected from the large |J| value. The magnitude of these effects depends, however, on the anisotropy effects on each Mn ion. It is suggested that determination of the anisotropy of the magnetic properties of the monomeric Mn(III) and Mn(IV) moieties would be a valuable goal for a future study of these mixed valent dimanganese-di-μ-oxo complexes. The complex exhibits two quasi-reversible waves in the cyclic voltammogram, one at E1/2 = 0.18 V vs SCE for the III/IV █ III/III couple and the other at E1/2 = 0.98 V vs SCE for the III/IV █ IV/IV couple. The UV-Vis spectra of the three redox states have been recorded spectroelectrochemically.

The complex [L(H2O)Fe(μ-O)Fe(OH)L](ClO4)3·H2O, where L = N,N‘-dimethyl-N,N‘-bis(2-pyridylmethyl)ethane-1,2-diamine, was synthesized. It crystallizes in the orthorhombic space group P212121 with a = 13.283(5) A, b = 16.050(9) A, c = 20.050(9) A, V = 4476(6) A3, and Z = 4. It presents the hydrogen-bonded [(H2O)Fe(μ-O)Fe(OH)]3+ core unit characterized by an Fe−O−Fe angle of 137.5(2)° and an Fe−Fe distance of 3.396(1) A. The measurement of the magnetic susceptibility as a function of the temperature indicated an antiferromagnetic coupling between the two high-spin Fe(III) ions J = −184 cm-1 (H = −JS1·S2). In the solid state the symmetric stretching vibration was observed at 438 cm-1. Upon dissolution in dry acetonitrile, this vibration was no longer detected and an intense band was observed at 600 cm-1. This frequency can be correlated with an Fe−O−Fe angle of 111°. This value suggests that the species which exists in these conditions is the protonated diamond core [Fe(μ-O)(μ-OH)Fe]3+, analogous to that ident...

The species LFe III Cl ( 1 ) was synthesized where L 2− is the dianion N , N ′-bis(4-methyl-6-tert-butyl-2-methyl-phenolato)- N , N ′-bismethyl1,2-diaminoethane. It crystallizes in the triclinic space group P -1 with a = 14.704(6), b = 17.421(7), c = 17.328(8) A , α = 89.45(8) , β = 129.76(9), γ = 102.71(9)°, V = 3277(2) A 3 and Z = 2 . The molecule has approximately a square pyramidal structure. In the presence of DBCH 2 (3,5-di-tert-butylcatechol), this complex gives LFe III (DBSQ) ( 2 ), a stable Fe(III)-semiquinonato complex (DBSQ − stands for the 3,5-di-tert-butyl- o -benzosemiquinone monoanion). It crystallizes in the monoclinic space group C 2/ c with a = 36.24(2), b = 10.438(5), c = 23.928(12) A , β = 115.31(5)°, V = 8183(7) A 3 and Z = 8 . This X-ray study allows the structure of the DBSQ − monoanion complexed to Fe(III) to be compared with that of the DBC 2− dianion complexed to Fe(III) as found in several complexes already described (see for instance H.G. Jang, D.D. Cox and L. Que, Jr., J. Am. Chem. Soc., 113 (1991) 9200–9204). A clear alternation is found in CC bond lengths in the DBSQ − monoanion. Magnetic coupling between the S Fe = 5 2 electronic spin on Fe(III) and the S R = 1 2 electronic spin on the DBSQ − anion radical has been deduced from the behavior of the magnetic susceptibility as a function of temperature. It has been found antiferromagnetic with J = −206 cm −1 (with the notation H = − JS Fc S R ). This is a weaker coupling than that found in other analogous complexes. Mossbauer spectroscopy confirms the S = 2 nature of the ground state. Analysis of the 57 Fe hyperfine coupling parameters prove that the earlier description of the electronic structure of 2 is correct. It is possible to reduce 2 to get [LFe III (DBC)] − ( E 0 = −0.3 V/SCE in AcN). This species has UV-Vis and EPR properties typical of this type of complex. In the absence of protons, [LFe III (DBC)] − is stable under pure O 2 but the addition of protons results in its oxidation to form 2 . The insensitivity of [LFe III (DBC)] − to O 2 is compatible with the idea that the system has to go through an Fe(II) (DBSQ) form by internal electron transfer to allow the attack of the aromatic ring by O 2 (see L. Que and R.Y.N. Ho, Chem. Rev., 96 (1996) 2607–2624, for a review). Observation of the lowest energy LMCT band in [LFe III (DBC)] − at 620 nm (2.00 eV, AcN) suggests that the Fe(II) (DBSQ) form is indeed at high energy ( ΔG 0 ) above the catechol form. This contributes from Marcus theory to a high activation energy. In intradiol dioxygenases the decoordination of one of the tyrosine ligands (D.H. Ohlendorf, A.M. Orville and J.D. Lipscomb, J. Mol. Biol., 244 (1994) 586–608) is certainly important for the deprotonation of the catechol substrate but could also accelerate the formation of the postulated Fe(II)-semiquinonato intermediate. The stability of 2 under O 2 also demonstrates the importance of the Fe(II) oxidation state in order to lead to a peroxo form of the oxygen adduct. The insensitivity of [LFe III (DBC)] − to O 2 argues against the view of the dioxygenase activity as the result of an electrophilic attack of the catecholato by O 2 in model complexes.

The dinuclear species [(trispicMeen)(ClFeOFeCl)-O-III-Cl-III(trispicMeen)]Cl(OH)(H2O)(7) (1) (trispicMeen = N,N,N'-tris-(2-pyridylmethyl)-N'-methylethane- 1,2-diamine) was synthesized. It crystallizes in the monoclinic space group C2/c with a = 33.87(2) Angstrom, b = 17.42(2) Angstrom, c = 23.41(5) Angstrom, beta = 132.88(5)degrees, V = 10 121(25) Angstrom(3), and Z = 8. It contains an almost linear unit (Fe-O-Fe angle = 177.4(7)degrees). The potentially pentadentate Ligand is in fact only tetracoordinated with one pyridine not bound to the metal ion. The octahedral coordination of Fe(Ln) is completed by one chloride ion. The structure of [(bispicMeen)ClFeOFeCl(bispicMeen)]Cl-2 . CH3COCH3 . 2H(2)O (2) (bispicMeen = N,N'-bis(2-pyridylmethyl)-N'-methylethane-1,2-diamine) was also determined. It crystallizes in the monoclinic space group C2/c with a 11.124(4) Angstrom, b = 22.769(9) Angstrom, c = 15.874(6) Angstrom, beta = 97.79(4)degrees, V = 3984(3) Angstrom(3), and Z = 4. The main difference from 1 is that, in 2, the Fe-O-Fe unit is bent with an FeOFe angle 152.3(3)degrees. In cyclic voltammetry, 1 exhibits two reduction peaks at -0.230 and -0.960 V/SCE. They correspond respectively to the reduction to the (FeFeIII)-Fe-II and (FeFeIII)-Fe-II states. Cyclic voltammetry shows that the mixed-valent form [(trispicMeen)(ClFeOFeCl)-O-II-Cl-III(trispicMeen)](+) (E degrees = -0.175 V/SCE) is in equilibrium with another species (E degrees = +0.065 V/SCE) proposed to be [(trispicMeen)(FeOFeCl)-O-II-Cl-III(trispicMeen)](2+) in which a chloride ion has been displaced by the originally unbound pyridine. The equilibrium constant was estimated to be 90 M(-1), and the rate of the recombination of chloride to the [(trispicMeen)(FeOFeCl)-O-III-Cl-III(trispicMeen)](3+) complex was found equal to 3 x 10(5) M(-1) s(-1). Controlled potential electrolysis of an acetonitrile solution of 1 allowed the preparation of the mixed-valent Fe(II)-O-Fe(III) form which gives an almost isotropic EPR signal similar to that already observed with ore-bridged model compounds (Holt, et al. Inorg. Chem. 1993, 32, 5844. Hartman, et al. J. Am. Chem. Sec. 1987, 109, 7387) but different from the rhombic one observed in the mixed valent form of MMO. The mixed-valent forms slowly disproportionate to a mixture of (FeFeIII)-Fe-III and Fe(II) forms. The mixed-valent forms could be generated by radiolysis at 77 K, and an EPR study of the mixed-valent forms obtained by this procedure demonstrated that these species could not be protonated. Radiolysis of 1 at 77 K afforded the EPR spectrum of [(trispicMeen)(ClFeOFeCl)-O-II-Cl-III(trispicMeen)](+); upon annealing at 200 K, the solution gave an EPR spectrum very similar to that observed on the electrochemically reduced solution. This is in agreement with the observation of substitution of a chloride ligand. The mixed-valent form was not detected with the analogous complexes of the bispicen family: [(bispicMeen)ClFeOFeCl(bispicMeen)]Cl-2 . CH3COCH3 . 2H(2)O (2) and [(bispicMe(2)en)ClFeOFeCl(bispicMe(2)en)]Cl-2 (3). Upon reduction, these complexes quickly (at CV time scale) decompose to mononuclear species.

gamma-Ray irradiation at liquid nitrogen temperature of a dimethylformamide solution of the tetranuclear complex [(Mn4O6)-O-IV(bipy)(6)](4+) (bipy = 2,2'-bipyridine) allowed the generation of the first mixed-valence tetranuclear system containing Mn-III and Mn-IV ions and exhibiting a S = 1/2 ground state. The X-band EPR spectrum of this tetranuclear system has been obtained. Simulations have been undertaken and the Mn hyperfine coupling tensors determined clearly show a (MnMn3IV)-Mn-III, composition for the EPR active species. A general approach for the analysis of the isotropic components of the Mn hyperfine tensors is presented in detail. This allowed the determination of the spin projection value for each Mn site. A three J coupling scheme assuming that the linear topology of the starting compound remains is able to reproduce these spin projection values if and only if the Mn-III ion is located at a terminal position in a N4O2 environment. The EPR signal of this [Mn4O6(bipy)(6)](3+) species is compared with the multiline signal observed in the S-2 state of the photosynthetic Oxygen Evolving Complex.

The near-IR transition associated with valence delocalization in the class III mixed-valence dimer [Fe(2)(OH)(3)(tmtacn)(2)](2+) is studied using variable-temperature (VT) electronic absorption and resonance Raman (RR) spectroscopies to gain insight into the properties of electron delocalization in this dimer. Laser excitation into this absorption band leads to dominant resonance Raman enhancement of totally-symmetric [Fe(2)(OH)(3)](2+) core vibrational modes at 316 and 124 cm(-)(1), descriptions of which are calculated from a normal coordinate analysis. Vibronic analysis of the near-IR resonance Raman excitation profiles and VT-absorption bandshapes using an anharmonic excited-state model provides a description of the geometric distortions accompanying this excitation. The excited-state distortion is dominated by expansion of the [Fe(2)(OH)(3)](2+) core along the Fe.Fe axis, reflecting the significant Fe-Fe sigma --> sigma character of this transition. The ground-state sigma-interaction between the two metals has been identified as the orbital pathway for valence delocalization, and the sigma --> sigma distortion analysis is used to quantify the structural dependence of the electronic-coupling matrix element, H(AB), associated with this pathway. The dominant role of totally-symmetric nuclear coordinates in the absorption and RR spectroscopies of [Fe(2)(OH)(3)(tmtacn)(2)](2+) is also discussed in relation to the Q(-) vibrational coordinate and the vibronic spectroscopies of other class II and class III mixed-valence dimers. It is shown that intensity contributions from the Q(-) coordinate to the absorption and RR spectra of [Fe(2)(OH)(3)(tmtacn)(2)](2+) are small relative to those of the totally-symmetric coordinates due to the inefficient change-in-curvature mechanism by which the Q(-) coordinate gains intensity, compared to the efficient excited-state displacement mechanism allowed for totally-symmetric coordinates. This is in contrast with the dominance of the Q(-) coordinate over other totally-symmetric coordinates observed in intervalence transfer (IT) absorption and RR spectroscopies of class II mixed-valence complexes.

The compound [Mn2III,IVO2(bisimMe2en)2](ClO4)3·H2O (bisimMe2en = N,N‘-dimethyl-N,N‘-bis(imidazol-4-ylmethyl)ethane-1,2-diamine) was synthesized. It crystallizes in the monoclinic space group C2/c with a = 18.139(8) A, b = 12.694(5) A, c = 17.694(8) A, β = 107.5(6)°, V = 3885(6) A3, and Z = 4. The cation [Mn2III,IVO2(bisimMe2en)2]3+ contains a di-manganese di-μ-oxo unit. The Mn−Mn axis is a C2 axis. The Mn−imidazole distances for MnIII or MnIV are distinct: respectively, 2.208(9) and 2.004(8) A. The ground state is a |SA2,SB3/2,S1/2〉 state separated by 420 cm-1 from the S = 3/2 state. The EPR spectrum has been simulated with |A1x,y| = 160.10-4 cm-1, |A1z| = 136.10-4 cm-1, |A2x,y| = 72.10-4 cm-1, |A2z| = 71.10-4 cm-1, gxy = 1.997, gz = 1.993. The ESEEM spectrum is reported. The 2H nuclei corresponding to the exchangeable hydrogen atoms of imidazole groups in D2O have been detected at 2.4 MHz at 3480 G. In cyclic voltammetry, [Mn2III,IVO2(bisimMe2en)2]3+ presents in oxidation two reversible waves at 1.04 an...

The tetranuclear species [Mn[sup IV] [sub 4]O[sub 6] (bpy)[sub 6]] (ClO[sub 4])[sub 4][center dot]H[sub 2]O was isolated from an aqueous nitric acid solution (pH = 2) of Mn[sup III] (bpy) Cl[sub 3](H[sub 2]O) upon addition of NaClO[sub 4]. It crystallizes in the triclinic space group P[bar l] with a = 20.213(7) [angstrom], b = 13.533(7) [angstrom], c = 13.411(8) [angstrom], [alpha] = 112.01(8)[degree], [beta] = 96.72(11)[degree], [gamma] = 100.34(12)[degree], V = 3276.9 [angstrom][sup 3], and Z = 2. The cation has a nonrectilinear chain structure [(bpy)[sub 2]Mn[sup IV[sub a]]O[sub 2]Mn[sup IV[sub b]](bpy)O[sub 2]Mn[sup IV[sub c]](bpy)O[sub 2]Mn[sup IV[sub d]](bpy)[sub 2]][sup 4+]. The metal-metal distances are Mn[sub a]-Mn[sub b] = 2.746(5) [angstrom], Mn[sub b]-Mn[sub c] = 2.760(5) [angstrom], Mn[sub c]-Mn[sub d] = 2.735(4) [angstrom], Mn[sub a]-Mn[sub c] = 4.899(10) [angstrom], Mn[sub b]-Mn[sub d] = 4.897(6) [angstrom], and Mn[sub a]-Mn[sub d] = 6.419(11) [angstrom]. The variable temperature magnetic susceptibility data for [Mn[sup IV[sub 4]]O[sub 6](bpy)[sub 6]](ClO[sub 4])[sub 4][center dot]H[sub 2]O in the range 12-294 K were fit using the spin Hamiltonian H[sub S] = J[sub ab]S[sub a]S[sub b] -J[sub bc]S[sub b]S[sub c] - J[sub cd]S[sub c]S[sub d], with J[sub ab] = J[sub cd] = -176 cm[sup [minus]1] and J[sub bc] = [minus]268 cm[supmore » [minus]1]. 55 refs., 11 figs., 3 tabs.« less

The origin of the pair-delocalized ground state of spin S=2, observed in chemically symmetric mixed-valence [Fe 3 S 4 ] 0 cores present in proteins and synthetic models, is analyzed in the framework of an effective-Hamiltonian model, comprising terms for excess-electron transfer (leading to double-exchange coupling of the paramagnetic Fe(III) cores), vibronic coupling (trapping the excess electron), and antiferromagnetic exchange. The basic mechanisms underlying the inhomogeneous electron distributions in trinuclear mixed-valence clusters with paramagnetic ion cores are illustrated in a simple model with electronic structure d 1 -d 1 -d 2

In a modelling effort to mimic the degradation products of haemoglobin, iron(III) proto- and deuteroporphyrin aggregates were synthesised using the corresponding iron porphyrin grafted on agarose gel. It is demonstrated, using resonance-Raman and EPR spectroscopy and magnetic measurements, that the major molecular species of the aggregates is an unexpected monomeric high-spin hydroxoiron(III) porphyrin, whereas the expected µ-oxo-bridged dimer is a minor species.

The synthesis and structural characterization of the novel [MnIII,IV2O2(N,Nbispicen)2](ClO4)3·CH3CN complex employing the tetradentate ligand N,Nbispicen = N,N-bis(2-pyridylmethyl)-1,2-diaminoethane are reported. Magnetic properties were determined and show that the ground state is a spin doublet. This can be quantitatively interpreted by antiferromagnetic coupling between a Mn(III) high spin and a Mn(IV) (J = −316 cm−1). The 16 line solution EPR spectrum exhibits an unusual splitting in the low field resonances. The following rhombic tensors were needed to simulate the EPR spectrum: |A1x|= 160 10−4 cm−1, |A1y| = 144 10−4 cm−1, |A1z|= 109 10−4 cm−1, |A2x|= 69 10−4 cm−1, |A2y| = 72 10−4 cm−1, |A2z| = 75 10−4 cm−1, gx = 2.001, gy = 1.996, gz = 1.984. The classical ligand field theory of local [gIII] and [gIV] tensors implemented with the first order perturbation theory to describe the properties of the pair does not result in a satisfying description of the [g1/2] tensor unless a large reduction in the spin-orbit constant is invoked. A simplified version of second-order perturbation theory leads to effects in qualitative agreement with experiment but weak as expected from the large |J| value. The magnitude of these effects depends, however, on the anisotropy effects on each Mn ion. It is suggested that determination of the anisotropy of the magnetic properties of the monomeric Mn(III) and Mn(IV) moieties would be a valuable goal for a future study of these mixed valent dimanganese-di-μ-oxo complexes. The complex exhibits two quasi-reversible waves in the cyclic voltammogram, one at E1/2 = 0.18 V vs SCE for the III/IV █ III/III couple and the other at E1/2 = 0.98 V vs SCE for the III/IV █ IV/IV couple. The UV-Vis spectra of the three redox states have been recorded spectroelectrochemically.