Versatility in the binding of 2-pyrazinecarboxylate with iron. Synthesis, structure and magnetic properties of iron(II) and iron(III) complexes

The synthesis and characterization of two new iron(II) complexes, [Fe(pca)2(py)2].py (1) and {[Fe(pca)2(H2O)].H2O}n (2) and one new iron(III) complex, Na2{[Fe(pca)()]2O}.2H2O.2CH3CN (3) (pca- stands for 2-pyrazinecarboxylate), are reported. Complex 1 is obtained from the reaction of iron powder with 2-pyrazinecarboxylic acid. The reaction of Fe(ClO4)3.10H2O with Hpca in the presence of 3 equiv. of Bu4NOH yields 2, whereas the presence of NaOH yields 3. The molecular structure of 1 contains an iron(II) ion with a pseudo-octahedral environment resulting from the coordination of two pca- ligands in a bidentate chelating fashion and two pyridine molecules; pi-pi stacking interactions between pyridine and pyrazine rings lead to a one-dimensional chain. Complex 2 is an iron(II) coordination polymer with an infinite zig-zag motif and an Fe...Fe separation of 7.1 A. In 2, the pi-pi stacking interactions involving the pyrazine rings and the strong hydrogen bonds between the coordinated water molecule and the carboxylate oxygens of two pca- ligands result in a three-dimensional network structure. Complex 3 consists of an anionic micro-oxo-bridged diiron(III) core with two crystallographically distinct iron(iii) ions; the negative charge is compensated by two sodium cations. Complex 3 is assembled in a three dimensional network structure through coordination of Na(I) and hydrogen bond interactions. Temperature dependent magnetic susceptibility and M?ssbauer spectroscopic studies indicate that 1 and 2 have similar magnetic properties. Both complexes are paramagnetic above 12 K, whereas antiferromagnetic ordering is observed below 12 K. The magnetic properties of reveal strong intramolecular antiferromagnetic interactions between the two iron(III) ions with a J value of -221 cm(-1); no long range intermolecular magnetic coupling is observed between 295 and 4.2 K.     

Iron(II) complexes with tetradentate bis(aminophenolate) ligands: synthesis and characterization, solution behavior, and reactivity with O(2)

Tetradentate bis(aminophenolate) ligands H(2)salan(X) and H(2)bapen(X) (where X refers to the para-phenolate substituent = H, Me, F, Cl) react with [Fe{N(SiMe(3))(2)}(2)] to form iron(II) complexes, which in the presence of suitable donor ligands L (L = pyridine or THF) can be isolated as the complexes [Fe(salan(X))(L)(2)] and [Fe(bapen(X))(L)(2)]. In the absence of donor ligands, either mononuclear complexes, for example, [Fe(salan(tBu,tBu))], or dinuclear complexes of the type [Fe(salan(X))](2) are obtained. The dynamic coordination behavior in solution of the complexes [Fe(salan(F))(L)(2)] and [Fe(bapen(F))(L)(2)] has been investigated by VT (1)H and (19)F NMR spectroscopy, which has revealed equilibria between isomers with different ligand coordination topologies cis-α, cis-β and trans. Exposure of the iron(II) salan(X) complexes to O(2) results in the formation of oxo-bridged iron(III) complexes of the type [{Fe(salan(X))}(2)(μ-O)] or [{Fe(salan(X))(L)}(2)(μ-O)]. The lack of catalytic activity of the iron(II) salan and bapen complexes in the oxidation of cyclohexane with H(2)O(2) as the oxidant is attributed to the rapid formation of stable and catalytically inactive oxo-bridged iron(III) complexes.     

mer-[Fe(pcq)(CN)3]-: a novel cyanide-containing building block and its application to assembling cyanide-bridged trinuclear FeIII2MnII complexes [pcq- = 8-(pyridine-2-carboxamido)quinoline anion

A new cyanide-containing building block K[Fe(pcq)(CN)(3)] [1; pcq(-) = 8-(pyridine-2-carboxamido)quinoline anion] containing a low-spin Fe(III) center with three cyanide groups in a meridional arrangement has been successfully designed and synthesized. Three cyanide-bridged trinuclear Fe(III)(2)Mn(II) complexes, [Fe(pcq)(CN)(3)](2)[Mn(CH(3)OH)(2)(H(2)O)(2)].2H(2)O (2), [Fe(pcq)(CN)(3)](2)[Mn(bipy)(2)].CH(3)OH.2H(2)O (3), and [Fe(pcq)(CN)(3)](2)[Mn(phen)(2)].CH(3)OH.2H(2)O (4), have been synthesized and structurally characterized. The magnetic susceptibilities of the three heterometallic complexes have been investigated.     

On, off and intermediate coordination of a bridgehead triarylamine donor in tripodal complexes: towards the tuning of coordinative bond distance

A series of first row transition metal complexes of the tripodal ligand 2,2',2"-nitrilotribenzoic acid H3L has been prepared and characterised by X-ray crystallography: Mononuclear [M(L)]- species [Cu(H2O)4]3[Cu(L)(H2O)]6.25H2O (2), [Co(H2O)6][Co(L)(H2O)].8H2O (4), [Zn(H2O)6][Zn(L)(H2O)].8H2O (5) and a neutral [M(L)] complex [Fe(III)2(L)(H2O)3].5H2O (8) are formed as well as dimeric [M(L)]2 2- species (HNEt3)2[Cu(L)]2.2CH3CN (1), (HNEt3)3[Ni(L)]2(ClO4).H2O (3), (HNEt3)2[Fe(II)(L)]2.2CH3CN (6) and (HNEt3)2[Fe(III)2(L)2(mu-O)](7). The complexes display a unique variation in the M-N distance (2.09 A for Cu(II) to 3.29 A for Fe(III)) to the bridgehead triphenylamine donor and are classified into compounds with "On","Off" and "Intermediate" N-coordination. The trigonal-bipyramidal coordination polyhedron changes towards tetrahedral in the intermediate and octahedral in the Off-state. The M-N distance of individual complexes is reversibly tuned by external chemical input such as changes of metal ion oxidation state (Fe(II)/Fe(III)) or variation of the axial coligand as a consequence of solvent or pH variation. Possible reasons for the exceptional tolerance of the M-N bond to distance variations are discussed under consideration of gas phase DFT calculations of [Zn(L)]-.     

Investigations into the metal species of the homogeneous iron(III) catalyzed Michael addition reactions

An investigation into the species formed in the first step of the solvent free homogeneous Michael reaction of alpha,beta-unsaturated ketones with 2-oxocyclopentanecarboxylate (1) is presented. This reaction is catalyzed by FeCl(3).6H(2)O (2) and Fe(ClO(4))(3).9H(2)O (3). EXAFS, XANES, Raman and UV-Vis studies were carried out to explain the experimentally found higher catalytic activity of Fe(ClO(4))(3).9H(2)O (3) compared to FeCl(3).6H(2)O (2). A very intense pre-edge peak is found for a 1.6 mol% solution of FeCl(3).6H(2)O (2) in 1, suggesting a tetrachloroferrate(III) compound to be present in this solution. This is proved by UV-Vis and Raman spectroscopy. The counterion of this anionic complex is an octahedral [Fe(III)(1-H)(2)(H2O2)](+) complex with two deprotonated 2-oxocyclopentanecarboxylate (1) as the chelating ligand, (1-H)(-), as suggested by the examination of the XANES region, the obtained coordination numbers from the EXAFS analysis and by UV-Vis and Raman spectroscopies. In summary, the anion-cation species [Fe(III)Cl(4)](-)[Fe(III)(-H)(2)(H2O2)](+) is formed with FeCl(3).6H(2)O (2), whereas in the case of Fe(ClO(4))(3).9H(2)O (3) XAFS, Raman and UV-Vis investigations suggest the presence of a complex of the form [Fe(III)(1-H)(2)(H2O2)](+)[ClO(4)](-). The obtained results are discussed to explain the reduced catalytic activity of FeCl(3).6H(2)O (2) in comparison to Fe(ClO(4))(3).9H(2)O (3).     

Axial ligand and spin-state influence on the formation and reactivity of hydroperoxo-iron(III) porphyrin complexes

The present study focuses on the formation and reactivity of hydroperoxo-iron(III) porphyrin complexes formed in the [Fe(III)(tpfpp)X]/H(2)O(2)/HOO(-) system (TPFPP=5,10,15,20-tetrakis(pentafluorophenyl)-21H,23H-porphyrin; X=Cl(-) or CF(3) SO(3)(-)) in acetonitrile under basic conditions at -15 °C. Depending on the selected reaction conditions and the active form of the catalyst, the formation of high-spin [Fe(III)(tpfpp)(OOH)] and low-spin [Fe(III)(tpfpp)(OH)(OOH)] could be observed with the application of a low-temperature rapid-scan UV/Vis spectroscopic technique. Axial ligation and the spin state of the iron(III) center control the mode of O-O bond cleavage in the corresponding hydroperoxo porphyrin species. A mechanistic changeover from homo- to heterolytic O-O bond cleavage is observed for high- [Fe(III)(tpfpp)(OOH)] and low-spin [Fe(III)(tpfpp)(OH)(OOH)] complexes, respectively. In contrast to other iron(III) hydroperoxo complexes with electron-rich porphyrin ligands, electron-deficient [Fe(III)(tpfpp)(OH)(OOH)] was stable under relatively mild conditions and could therefore be investigated directly in the oxygenation reactions of selected organic substrates. The very low reactivity of [Fe(III)(tpfpp)(OH)(OOH)] towards organic substrates implied that the ferric hydroperoxo intermediate must be a very sluggish oxidant compared with the iron(IV)-oxo porphyrin π-cation radical intermediate in the catalytic oxygenation reactions of cytochrome P450.     

Molecular [(fe(3))-(fe(3))] and [(fe(4))-(fe(4))] coordination cluster pairs as single or composite arrays

The synthesis of molecular cluster pairs is a challenge for coordination chemists due to the potential applications of these species in molecular spintronics or quantum computing. The ligand H(4)L, 1,3-bis-(3-oxo-3-(2-hydroxyphenyl)-propionyl)-2-methoxybenzene, has been successfully used to obtain a series of such complexes using the basic Fe(III) trinuclear carboxylates as starting materials. Synthetic control has allowed the isolation of the two molecular cluster pairs that form the composite [Fe(4)O(2)(PhCO(2))(6)(H(2)L)(pz)](2)[Fe(3)O(PhCO(2))(5)(py)(H(2)L)](2) (1). The dimers of trinuclear units, [Fe(3)O(PhCO(2))(5)(H(2)O)(H(2)L)](2) (2) and [Fe(3)O(o-MePhCO(2))(5)(H(2)L)(py)](2) (3), and the dimers of tetranuclear units, [Fe(4)O(2)(PhCO(2))(6)(H(2)L)(pz)](2) (4) and [Fe(4)O(2)(o-MePhCO(2))(6)(H(2)L)(pz)](2) (5), are presented here. The magnetic properties of the reported aggregates show that they are pairs of semi-independent clusters weakly interacting magnetically as required for two-qubit quantum gates.     

Synthesis, characterization, and reactivity of ferrous and ferric oxo/peroxo pivalate complexes in relation to Gif-type oxygenation of substrates

This study examines structural features and aspects of reactivity of Gif-type reagents, which depend on O2/Zn to mediate oxidation of hydrocarbons. The reagents investigated derive from the use of iron complexes with the anion of the weak carboxylic acid Me3CCO2H (pivalic acid (PivH)) in pyridine/PivH. In these solutions, the known compound [Fe3O(O2CCMe3)6(py)3] is reduced by Zn to generate yellow-green [FeII(O2CCMe3)2(py)4], which readily reverts to [Fe3O(O2CCMe3)6(py)3], and eventually to [Fe3O(O2CCMe3)6(py)3]+, upon exposure to dioxygen. All three species are equally well suited to mediate Gif-like oxygenation of substrates supported by O2/Zn. [FeIII3O(O2CCMe3)6(L)3]+ (L = H2O, py) is converted by H2O2 to afford the hexairon(III) peroxo compounds [Fe6(O2)(O)2(O2CCMe3)12(L)2] (L = Me3CCO2H, py), which feature a [Fe6(eta 2-mu 4-O2)(mu 3-O)2] core previously documented in the closely related [Fe6(O2)(O)2(O2CPh)12(H2O)2]. A similar peroxo species, [Fe6(O2)(O)2(O2CCMe3)2(O2CCF3)10(H2O)2], is obtained upon replacing all pivalate ligands by trifluoroacetate groups with the exception of those pivalates that bridge between the two [Fe3O(O2CCF3)5(H2O)]2+ units. The structure of the [Fe6(O2)(O)2] core in these peroxo species is found to range from a recliner to a butterfly-type conformation. Reduction of [Fe6(O2)(O)2(O2CCMe3)12(HO2CCMe3)2] with NaBH4 generates [Na2Fe4(O)2(O2CCMe3)10(L)(L')] (L = CH3CN, L' = Me2CO; L = L' = Me3CCO2H), which feature a [Na2Fe4(O)2] core possessing a bent butterfly conformation of the [Fe4(O)2] unit. Oxidation of the same peroxo complex by CeIV or NOBF4 regenerates the oxo-bridged [Fe3O(O2CCMe3)6(solv)3]+ (solv = EtOH, H2O, thf). Employment of the sterically encumbered 2-Me-5-Etpyridine provides the tetrairon compound [Fe4(O)2(O2CCMe3)8(2-Me-5-Etpy)2], which can be readily transformed upon treatment with H2O2 to the asymmetric peroxo complex [Fe6(O2)(O)2(O2CCMe3)12(2-Me-5-Etpy)2]. The peroxo-containing complexes oxidize both cis-stilbene and adamantane in either benzene or py/PivH, but only under forceful conditions and at very low yields. The low reactivity and high selectivity (tert/sec = 8) obtained in the oxidation of adamantane suggests that the present type of peroxo species is not directly involved in catalytic Gif-type oxygenations of adamantane.     

Catalase-peroxidase activity of iron(III)-TAML activators of hydrogen peroxide

Exceptionally high peroxidase-like and catalase-like activities of iron(III)-TAML activators of H 2O 2 ( 1: Tetra-Amidato-Macrocyclic-Ligand Fe (III) complexes [ F e{1,2-X 2C 6H 2-4,5-( NCOCMe 2 NCO) 2CR 2}(OH 2)] (-)) are reported from pH 6-12.4 and 25-45 degrees C. Oxidation of the cyclometalated 2-phenylpyridine organometallic complex, [Ru (II)( o-C 6H 4py)(phen) 2]PF 6 ( 2) or "ruthenium dye", occurs via the equation [ Ru II ] + 1/2 H 2 O 2 + H +-->(Fe III - TAML) [ Ru III ] + H 2 O, following a simple rate law rate = k obs (per)[ 1][H 2O 2], that is, the rate is independent of the concentration of 2 at all pHs and temperatures studied. The kinetics of the catalase-like activity (H 2 O 2 -->(Fe III - TAML) H 2 O + 1/2 O 2) obeys a similar rate law: rate = k obs (cat)[ 1][H 2O 2]). The rate constants, k obs (per) and k obs (cat), are strongly and similarly pH dependent, with a maximum around pH 10. Both bell-shaped pH profiles are quantitatively accounted for in terms of a common mechanism based on the known speciation of 1 and H 2O 2 in this pH range. Complexes 1 exist as axial diaqua species [FeL(H 2O) 2] (-) ( 1 aqua) which are deprotonated to afford [FeL(OH)(H 2O)] (2-) ( 1 OH) at pH 9-10. The pathways 1 aqua + H 2O 2 ( k 1), 1 OH + H 2O 2 ( k 2), and 1 OH + HO 2 (-) ( k 4) afford one or more oxidized Fe-TAML species that further rapidly oxidize the dye (peroxidase-like activity) or a second H 2O 2 molecule (catalase-like activity). This mechanism is supported by the observations that (i) the catalase-like activity of 1 is controllably retarded by addition of reducing agents into solution and (ii) second order kinetics in H 2O 2 has been observed when the rate of O 2 evolution was monitored in the presence of added reducing agents. The performances of the 1 complexes in catalyzing H 2O 2 oxidations are shown to compare favorably with the peroxidases further establishing Fe (III)-TAML activators as miniaturized enzyme replicas with the potential to greatly expand the technological utility of hydrogen peroxide.     

The reaction of [Fe(pic)3] with hydrogen peroxide: a UV-visible and EPR spectroscopic study (Hpic = picolinic acid)

The Gif family of catalysts, based on an iron salt and O2 or H2O2 in pyridine, allows the oxygenation of cyclic saturated hydrocarbons to ketones and alcohols under mild conditions. The reaction between [Fe(pic)3] and hydrogen peroxide in pyridine under GoAgg(III)(Fe(III)/Hpic catalyst) conditions was investigated by UV-visible spectrophotometry. Reactions were monitored at 430 and 520 nm over periods ranging from a few minutes to several hours at 20 degrees C. A number of kinetically stable intermediates were detected, and their relevance to the processes involved in the assembly of the active GoAgg(III) catalyst was determined by measuring the kinetics in the presence and absence of cyclohexane. EPR measurements at 110 K using hydrogen peroxide and t-BuOOH as oxidants were used to further probe these intermediates. Our results indicate that in wet pyridine [Fe(pic)3] undergoes reversible dissociation of one picolinate ligand, establishing an equilibrium with [Fe(pic)2(py)(OH)]. Addition of aqueous hydrogen peroxide rapidly generates the high-spin complex [Fe(pic)2(py)(eta1-OOH)] from the labilised hydroxy species. Subsequently the hydroperoxy species undergoes homolysis of the Fe-O bond, generating HOO. and [Fe(pic)2(py)2], the active oxygenation catalyst.     

Synthesis and characterization of pyridine amide cation radical complexes of iron: stabilization due to coordination with low-spin iron(III) center

We reported the synthesis and characterization of peptide complexes of low-spin iron(III) [Fe(bpb)(py)2][ClO4] (1) and Na[Fe(bpb)(CN)2] (2) [H2bpb = 1,2-bis(pyridine-2-carboxamido)benzene; py = pyridine], where iron is coordinated to four nitrogen donors in the equatorial plane with two amide nitrogen anions and two pyridine nitrogen donors (Ray, M.; Mukherjee, R.; Richardson, J. F.; Buchanan, R. M. J. Chem. Soc., Dalton Trans. 1993, 2451). Chemical oxidation of 2 and a new low-spin iron(III) complex Na[Fe(Me6bpb)(CN)2].H2O (4) [synthesized from a new iron(III) complex [Fe(Me6bpb)(py)2][ClO4] (3) (S = 1/2)] [H2Me6bpb = 1,2-bis(3,5-dimethylpyridine2-carboxamido)-4,5-dimethylbenzene) by (NH4)2Ce(NO3)6 afforded isolation of two novel complexes [Fe(bpb)-(CN)2] (5) and [Fe(Me6bpb)(CN)2].H2O (6). All the complexes have been characterized by physicochemical techniques. While 1-4 are brown/green, 5 and 6 are violet/bluish violet. The collective evidence from infrared, electronic, M?ssbauer, and 1H NMR spectroscopies, from temperature-dependent magnetic susceptibility data, and from cyclic voltammetric studies provides unambiguous evidence that 5 and 6 are low-spin iron(III) ligand cation radical complexes rather than iron(IV) complexes. Cyclic voltammetric studies on isolated oxidized complexes 5 and 6 display identical behavior (a metal-centered reduction and a ligand-centered oxidation) to that observed for complexes 2 and 4, respectively. The M?ssbauer data for 6 are almost identical with those of the parent compound 4, providing compelling evidence that oxidation has occurred at the ligand in a site remote from the iron atom. Strong antiferromagnetic coupling (-2J > or = 450 cm(-1)) of the S = 1/2 iron atom with the S = 1/2 ligand pi-cation radical leads to an effectively S = 0 ground state of 5 and 6. The oxidized complexes display 1H NMR spectra (in CDCl3 solution), characteristic of diamagnetic species.     

Synthetic models of the reduced active site of superoxide reductase

We report the synthesis, structural and spectroscopic characterization, and magnetic and electrochemical studies of a series of iron(II) complexes of the pyridyl-appended diazacyclooctane ligand L(8)py(2), including several that model the square-pyramidal [Fe(II)(N(his))(4)(S(cys))] structure of the reduced active site of the non-heme iron enzyme superoxide reductase. Combination of L(8)py(2) with FeCl(2) provides [L(8)py(2)FeCl(2)] (1), which contains a trigonal-prismatic hexacoordinate iron(II) center, whereas a parallel reaction using [Fe(H(2)O)(6)](BF(4))(2) provides [L(8)py(2)Fe(FBF(3))]BF(4) (2), a novel BF(4)(-)-ligated square-pyramidal iron(II) complex. Substitution of the BF(4)(-) ligand in 2 with formate or acetate ions affords distorted pentacoordinate [L(8)py(2)Fe(O(2)CH)]BF(4) (3) and [L(8)py(2)Fe(O(2)CCH(3))]BF(4) (4), respectively. Models of the superoxide reductase active site are prepared upon reaction of 2 with sodium salts of aromatic and aliphatic thiolates. These model complexes include [L(8)py(2)Fe(SC(6)H(4)-p-CH(3))]BF(4) (5), [L(8)py(2)Fe(SC(6)H(4)-m-CH(3))]BF(4) (6), and [L(8)py(2)Fe(SC(6)H(11))]BF(4) (7). X-ray crystallographic studies confirm that the iron(II)-thiolate complexes model the square-pyramidal geometry and N(4)S donor set of the reduced active site of superoxide reductase. The iron(II)-thiolate complexes are high spin (S = 2), and their solutions are yellow in color because of multiple charge-transfer transitions that occur between 300 and 425 nm. The ambient temperature cyclic voltammograms of the iron(II)-thiolate complexes contain irreversible oxidation waves with anodic peak potentials that correlate with the relative electron donating abilities of the thiolate ligands. This electrochemical irreversibility is attributed to the bimolecular generation of disulfides from the electrochemically generated iron(III)-thiolate species.     

Molecular, supramolecular and solution structures of peroxovanadium complexes with ON and O3N donor set ligands: two new types of cationic-anionic peroxovanadium(V) peroxovanadates(V)

The complexes, [VO(O(2))(pa)(2)]ClO(4).3H(2)O (1), [VO(O(2))(pa)(2)][VO(O(2))(2)(pa)].3H(2)O (2), [VO(O(2))(pa)(2)][VO(O(2))(ada)].2H(2)O (3) and [VO(O(2))(pa)(pca)].H(2)O (4)[pa = picolinamide, ada = carbamoylmethyliminodiacetate(2-) and pca = 2-pyrazinecarboxylate(1-)], were synthesized. 2 and 3 are new types of peroxovanadium complexes: monoperoxovanadium diperoxovanadate (2) and monoperoxovanadium monoperoxovanadate (3). The complexes were characterized by chemical analysis and IR spectroscopy, and 1, 3 and 4 also by X-ray analysis. The structure of 1 is disordered, with alternating positions of the oxo and peroxo ligands. The peroxo oxygen atoms, O(p), in 1 are involved in weak hydrogen bonds with water molecules and close intramolecular C-HO...(p) bonds [d(HO(p)) approximately 2.0 A]. The supramolecular structure of 1 is formed by a network of hydrogen bonds and strong attractive intermolecular pi-pi interactions between the pyridine rings. The supramolecular architecture in 4 is constructed by (N,O)-H...O hydrogen bonds between the neutral complex molecules and water of crystallization. The peroxo oxygen atoms in 4 form intramolecular C-H...O(p) bonds [d(H...O(p))= 2.303 A]. The pa and pca ligands are ON coordinated via the oxygen atoms of the C(NH(2))=O and COO(-) groups, respectively, and nitrogen atoms of the heterocyclic rings, and ada as a tetradentate O(3)N ligand. The thermal analysis of 4 showed that the loss of water of crystallization and the active oxygen release (T(min)/ degrees C 82, T(max)/degrees C 165) are, under given conditions, individual processes separated by the temperature interval 90-132 degrees C. The solution structures and stability were studied by UV-VIS and (51)V NMR spectroscopies.     

Phenylthiyl radical complexes of gallium(III), iron(III), and cobalt(III) and comparison with their phenoxyl analogues.

Three hexadentate, asymmetric pendent arm macrocycles containing a 1,4,7-triazacyclononane-1,4-diacetate backbone and a third, N-bound phenolate or thiophenolate arm have been synthesized. In [L(1)](3)(-) the third arm is 3,5-di-tert-butyl-2-hydroxybenzyl, in [L(2)](3)(-) it is 2-mercaptobenzyl, and in [L(3)](3)(-) it is 3,5-di-tert-butyl-2-mercaptobenzyl. With trivalent metal ions these ligands form very stable neutral mononuclear complexes [M(III)L(1)] (M = Ga, Fe, Co), [M(III)L(2)] (M = Ga, Fe, Co), and [M(III)L(3)] (M = Ga, Co) where the gallium and cobalt complexes possess an S = 0 and the iron complexes an S = (5)/(2) ground state. Complexes [CoL(1)].CH(3)OH.1.5H(2)O, [CoL(3)].1.17H(2)O, [FeL(1)].H(2)O, and [FeL(2)] have been characterized by X-ray crystallography. Cyclic voltammetry shows that all three [M(III)L(1)] complexes undergo a reversible, ligand-based, one-electron oxidation generating the monocations [M(III)L(1)(*)](+) which contain a coordinated phenoxyl radical as was unambiguously established by their electronic absorption, EPR, and M?ssbauer spectra. In contrast, [M(III)L(2)] complexes in CH(3)CN solution undergo an irreversible one-electron oxidation where the putative thiyl radical monocationic intermediates dimerize with S-S bond formation yielding dinuclear disulfide species [M(III)L(2)-L(2)M(III)](2+). [GaL(3)] behaves similarly despite the steric bulk of two tertiary butyl groups at the 3,5-positions of the thiophenolate, but [Co(III)L(3)] in CH(2)Cl(2) at -20 to -61 degrees C displays a reversible one-electron oxidation yielding a relatively stable monocation [Co(III)L(3)(*)](+). Its electronic spectrum displays intense transitions in the visible at 509 nm (epsilon = 2.6 x 10(3) M(-)(1) cm(-)(1)) and 670sh, 784 (1.03 x 10(3)) typical of a phenylthiyl radical. The EPR spectrum of this species at 90 K proves the thiyl radical to be coordinated to a diamagnetic cobalt(III) ion (g(iso) = 2.0226; A(iso)((59)Co) = 10.7 G).     

Synthesis, structure and catalase-like activity of dimanganese(III) complexes of 1,5-bis(X-salicylidenamino)pentan-3-ol (X = 3- and 5-methyl). Influence of phenyl-ring substituents on catalytic activity

The diMn(III) complexes [Mn2(5-Me-salpentO)(mu-MeO)(mu-AcO)(H2O)Br] (1) and [Mn2(3-Me-salpentO)(mu-MeO)(mu-AcO)(MeOH)2]Br (2), where salpentOH = 1,5-bis(salicylidenamino)pentan-3-ol, were synthesised and structurally characterized. The two complexes include a bis(micro-alkoxo)(micro-acetato) triply-bridged diMn(III) core with an Mn...Mn separation of 2.93-2.94 A, the structure of which is retained upon dissolution. Complexes 1 and 2 show catalytic activity toward disproportionation of H2O2, with first-order dependence on the catalyst, and saturation kinetics on [H2O2], in methanol and DMF. In DMF, the two complexes are able to disproportionate at least 1500 eq. of H2O2 without significant decomposition, while in methanol, they rapidly lose activity with formation of a non-coupled Mn(II) species. Electrospray ionisation mass spectrometry, EPR and UV/vis spectroscopy used to monitor the reaction suggest that the major active form of the catalyst occurs in the Mn2(III) oxidation state during cycling. The correlation between log(k(cat)) and the redox potentials of 1, 2 and analogous complexes of other X-salpentOH derivatives indicates that, in this series, the oxidation of the catalyst is probably the rate-limiting step in the catalytic cycle. It is also noted that formation of the catalyst-peroxide adduct is more sensitive to steric effects in DMF than in methanol. Overall, kinetics and spectroscopic studies of H2O2 dismutation by these complexes converge at a catalytic cycle that involves the Mn2(III) and Mn2(IV) oxidation states.     

Functional mimic of dioxygen-activating centers in non-heme diiron enzymes: mechanistic implications of paramagnetic intermediates in the reactions between diiron(II) complexes and dioxygen

Two tetracarboxylate diiron(II) complexes, [Fe(2)(mu-O(2)CAr(Tol))(2)(O(2)CAr(Tol))(2)(C(5)H(5)N)(2)] (1a) and [Fe(2)(mu-O(2)CAr(Tol))(4)(4-(t)BuC(5)H(4)N)(2)] (2a), where Ar(Tol)CO(2)(-) = 2,6-di(p-tolyl)benzoate, react with O(2) in CH(2)Cl(2) at -78 degrees C to afford dark green intermediates 1b (lambda(max) congruent with 660 nm; epsilon = 1600 M(-1) cm(-1)) and 2b (lambda(max) congruent with 670 nm; epsilon = 1700 M(-1) cm(-1)), respectively. Upon warming to room temperature, the solutions turn yellow, ultimately converting to isolable diiron(III) compounds [Fe(2)(mu-OH)(2)(mu-O(2)CAr(Tol))(2)(O(2)CAr(Tol))(2)L(2)] (L = C(5)H(5)N (1c), 4-(t)BuC(5)H(4)N (2c)). EPR and M?ssbauer spectroscopic studies revealed the presence of equimolar amounts of valence-delocalized Fe(II)Fe(III) and valence-trapped Fe(III)Fe(IV) species as major components of solution 2b. The spectroscopic and reactivity properties of the Fe(III)Fe(IV) species are similar to those of the intermediate X in the RNR-R2 catalytic cycle. EPR kinetic studies revealed that the processes leading to the formation of these two distinctive paramagnetic components are coupled to one another. A mechanism for this reaction is proposed and compared with those of other synthetic and biological systems, in which electron transfer occurs from a low-valent starting material to putative high-valent dioxygen adduct(s).     

From one-dimensional chain to pentanuclear molecule. Magnetism of cyano-bridged Fe(III)-Ni(II) complexes

Two cyano-bridged Ni(II)-Fe(III) complexes [(H(3)O)[Ni(H(2)L)](2)[Fe(CN)(6)](2).[Fe(CN)(6)].6H(2)O](n) (1) and [K(18-C-6)(H(2)O)(2)][Ni(H(2)L)](2)[Fe(CN)(6)](3).4(18-C-6).20H(2)O (2) (L = 3,10-bis(2-aminoethyl)-1,3,6,8,10,12-hexaazacyclotetradecane, 18-C-6 = 18-crown-6-ether) have been synthesized and characterized structurally and magnetically. Complex 1 has a zigzag one-dimensional structure, in which two trans-CN(-) ligands of each [Fe(CN)(6)](3)(-) link two trans-[Ni(H(2)L)](4+) groups, and in turn, each trans-[Ni(H(2)L)](4+) links two [Fe(CN)(6)](3)(-) in a trans fashion. Complex 2 is composed of cyano-bridged pentanuclear molecules with moieties connected by the trans-CN(-) ligands of [Fe(CN)(6)](3)(-). Magnetic studies show the existence of ferromagnetic Ni(II)-Fe(III) interactions in both complexes. The intermetallic magnetic coupling constant of both complexes was analyzed by using an approximate model on the basis of the structural features.     

Racemic monoperoxovanadium(V) complexes with achiral OO and ON donor set heteroligands: synthesis, crystal structure and stereochemistry of [NH3(CH2)2NH3][VO(O2)(ox)(pic)].2H2O and [NH3(CH2)2NH3][VO(O2)(ox)(pca)

Monoperoxovanadium(V) complexes, [NH3(CH2)2NH3][VO(O2)(ox)(pic)].2H2O (1) and [NH3(CH2)2NH3][VO(O2)(ox)(pca)] (2) [NH3(CH2)2NH3 = ethane-1,2-diammonium(2+), ox=oxalate(2-), pic=pyridine-2-carboxylate(1-), pca=pyrazine-2-carboxylate(1-)], were synthesized and characterized by X-ray analysis, IR and Raman spectroscopies. The five equatorial positions of the pentagonal bipyramid around the vanadium atoms are occupied by the eta2-peroxo ligand, two oxygen atoms of the ox, and the nitrogen atom of the pic or pca ligands, respectively. The oxo ligand and the oxygen atom of pic or pca are in the axial positions. Networks of X-HO (X=C, N or O) hydrogen bonds, and pi-pi interactions between aromatic rings in and anion-pi interactions in , determine the molecular packings and build up the supramolecular architecture. Three stereochemical rules for occupation of the donor sites in two-heteroligand [VO(O2)(L1)(L2)] complexes (L1, L2 are bidentate neutral or differently charged anionic heteroligands providing an OO, NN or ON donor set) are discussed. and crystallize as racemic compounds. The 51V NMR spectra proved that the parent complex anions of and partially decompose on dissolution in water to the monoperoxo-ox, -pic or -pca complexes.