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While polycarboxylates and hydroxyl-acid complexes have long been known to be photoactive, simple carboxylate complexes which lack a significant LMCT band are not typically strongly photoactive. Hence, it was somewhat surprising that a series of reports demonstrated that materials synthesized from iron(III) and polysaccharides such as alginate (poly[guluronan-co-mannuronan]) or pectate (poly[galacturonan]) formed photoresponsive materials that convert from hydrogels to sols under the influence of visible light. These materials have numerous potential applications in areas such as photopatternable materials, materials for controlled drug delivery, and tissue engineering. Despite the near-identity of the functional units in the polysaccharide ligands, the reactivity of iron(III) hydrogels can depend on the configuration of some chiral centers in the sugar units and in the case of alginate the guluronate to mannuronate block composition, as well as pH. Here, using temperature- and field-dependent transmission Mössbauer spectroscopy, we show that the dominant iron compound detected for both the alginate and pectate gels displays features typical of a polymeric (Fe

The coordination of iron(III) to the marine amphiphilic marinobactin and aquachelin siderophores, as well as to petrobactin, an unusual 3,4-dihydroxybenzoyl siderophore is reported. Potentiometric titrations were performed on the apo siderophore to determine the ligand pK(a) values, as well as the complex formed with addition of 1 equiv of Fe(III). The log K(ML) values for Fe(III)-marinobactin-E and Fe(III)-aquachelin-C are 31.80 and 31.4, respectively, consistent with the similar coordination environment in each complex, while log K(ML) for Fe(III)-petrobactin is estimated to be about 43. The pK(a) of the beta-hydroxyaspartyl hydroxyl group was determined to be 10.8 by (1)H NMR titration. (13)C NMR and IR spectroscopy were used to investigate Ga(III) coordination to the marinobactins. The coordination-induced shifts (CIS) in the (13)C NMR spectrum of Ga(III)-marinobactin-C compared to apo-marinobactin-C indicates that the hydroxamate groups are coordinated to Ga(III); however, the lack of CISs for the carbons of the beta-hydroxyamide group suggests this moiety is not coordinated in the Ga(III) complex. Differences in the IR spectrum of Ga(III)-marinobactin-C and Fe(III)-marinobactin-C in the 1600-1700 cm(-1) region also corroborates Fe(III) is coordinated to the beta-hydroxyamide moiety, whereas Ga(III) is not coordinated.

New binuclear complexes of Co(II), Zn(II), and Ni(II) derived from a diatopic heteroscorpionate ligand, (4-carboxyphenyl)bis(3,5-dimethylpyrazolyl)methane (L4c), have been synthesized and characterized by X-ray diffraction, ESI-MS, IR, UV-vis spectroscopy, and magnetic susceptibility. These building blocks have been subsequently used for the construction of higher order metallosupramolecular architectures.

Complexes of Cu(II) derived from two new nonfacially coordinating heteroscorpionate ligands, (4-carboxyphenyl)bis(3,5-dimethylpyrazolyl)methane (L4c) and (3-carboxyphenyl)bis(3,5-dimethylpyrazolyl)methane (L3c), have been synthesized and characterized by X-ray diffraction, ESI-MS, IR, and UV-vis spectroscopy, electrochemistry, and magnetic susceptibility. The use of these new complexes as building blocks for the construction of supramolecular architectures is discussed.

UV photolysis of the ferric aerobactin complex results in decarboxylation of the alpha-hydroxy carboxylic acid group of the central citrate moiety of aerobactin. The structure determination of the photooxidized ligand shows that decarboxylation occurs at the citrate moiety forming a 3-ketoglutarate moiety. Proton and carbon-13 NMR establish the presence of keto and enol tautomers of the apo-photoproduct, with the enol form prevailing in water. The photoproduct retains the ability to coordinate iron(III). The values of the ligand protonation constants, the pKa of the Fe(III)-ligand complex, and the Fe(III) stability constant of the photoproduct of aerobactin are all close to those of aerobactin. CD spectroscopy suggests that the chirality of the ferric complexes of aerobactin and its photoproduct are similar. Like aerobactin, the photoproduct promotes iron acquisition by the source bacterium, Vibrio sp. DS40M5.

It is shown in model complexes designed to mimic the binding site of zinc-thiolate proteins that a single hydrogen bond between an amide N-H and a Zn-coordinated thiolate reduces its reactivity toward electrophiles by up to 2 orders of magnitude. In addition, we show that a single N-H...S hydrogen bond is sufficient to achieve near 100% regiospecificity of reaction between a strong, and hence inherently indiscriminate, alkylating agent like trimethyloxonium tetraflouroborate and a single sulfur in a dithiolate construct. The importance of these results in understanding how systems such as the zinc fingers of the GATA family and the E. coli DNA repair protein Ada, which share the same pseudotetrahedral structure and tetracysteinyl ligation around the zinc, can fulfill such widely divergent (structural vs reactive) roles and how specificity of reaction in such multi-thiolate containing systems can be achieved is discussed.

Four new Mo(VI)-dioxo complexes of a family of N2X heteroscorpionate ligands are reported which, together with data already available for (TpR)-, provide a unique example of a comprehensive set of isostructural, isoelectronic complexes differing only in one biologically relevant donor atom. A study of these complexes allows for a direct comparison of structural, spectroscopic, and oxygen atom transfer reactivity properties of the Mo(VI)-dioxo center (of relevance to various families of molybdoenzymes) as a function of donor atom identity.

Deconvoluting the different contributions of thiolate and ene-1,2-dithiolate donors to the underlying electronic structure of the Mo site in sulfite oxidase (SO) has proven to be a difficult task. One way in which these differences might be illuminated is by selectively substituting Se for S in model complexes which possess multiple sulfur donor ligand environments. Here we report the synthesis and structures of new oxo-Mo(V) complexes as effective models for the one-electron reduced active site of SO. We have used the tridentate heteroscorpionate ligand (2-dimethylethanethiol)bis(3,5-dimethylpyrazolyl)methane (L3SH) in order to model the constrained cysteinyl sulfur (S(Cys)) ligand environment observed in the crystal structure of the enzyme, and benzene-1,2-dithiol (bdt) as a mimic of the ene-1,2-dithiolate chelate. [(L3S)MoO(bdt)] and [(L3S)MoO(SPh)(2)] have been structurally characterized by X-ray crystallography, and as such, [(L3S)MoO(bdt)] is only the second known model compound that closely approximates the active site structure of reduced forms of SO. Additionally, benzenethiol (SPh) and benzeneselenol (SePh) have been used to perturb the equatorial ligand environment of [(L3S)MoO(bdt)].) This has provided much needed insight into the electronic structure of the one-electron reduced SO site and has allowed for increased understanding of the individual roles played by these different thiolate donors in the oxidative half-reaction of the enzyme. Interestingly, the EPR spectra of [(L3S)MoO(bdt)], [(L3S)MoO(SPh)(2)], and [(L3S)MoO(SePh)(2)] closely resemble that of both high pH (hpH) and low pH (lpH) SO, except for the fact that the magnitude of g(1) is found to be consistently higher in the model spectra compared to that of the enzyme. It is suggested that this derives from an increase in Mo-S covalency in the models relative to hpH and lpH SO.

We have previously demonstrated that the complex [(L1O)MoOCl(2)], where L1OH = (2-hydroxy-3-tert-butyl-5-methylphenyl)bis(3,5-dimethylpyrazolyl)methane, exists as both cis and trans isomers (Kail, B.; Nemykin, V. N.; Davie, S. R.; Carrano, C. J.; Hammes, B. S.; Basu, P. Inorg. Chem. 2002, 41, 1281-1291). Here, the cis isomer is defined as the geometry with the heteroatom in the equatorial position, and the trans isomer is designated as the geometry with the heteroatom positioned trans to the terminal oxo group. The trans isomer represents the thermodynamically more stable geometry as indicated by its spontaneous formation from the cis isomer. In this report, we show that for complexes of [(LO)MoOCl(2)], where LOH is the sterically less restrictive (2-hydroxyphenyl)bis(3,5-dimethylpyrazolyl)methane, only the trans isomer could be isolated, while in the corresponding thiolate containing ligand (2-dimethylethanethiol)bis(3,5-dimethylpyrazolyl)methane (L3SH) only the cis isomer could be observed. In addition, we have isolated and structurally characterized the complex [(L1O)MoO(OPh)(Cl)], a rare example of a species possessing both cis and trans phenolates. Using DFT calculations, we have investigated the origins of the differences in stability between the cis and trans isomers in these complexes and suggest that they are related to the trans influence of the oxo-group. Crystal data for [(LO)MoOCl(2)] (1) include that it crystallizes in the triclinic space group P(-)1 with cell dimensions a = 8.9607 (12) A, b = 10.596 (4) A, c = 13.2998 (13) A, alpha = 98.03 (2) degrees, beta = 103.21 (2) degrees, gamma = 110.05(2) degrees, and Z = 2. [(L1O)MoO(OPh)Cl].2CH(2)Cl(2) (2.2CH(2)Cl(2)) crystallizes in the triclinic space group P(-)1 with cell dimensions a = 12.2740 (5) A, b = 13.0403 (5) A, c = 13.6141 (6) A, alpha = 65.799 (2) degrees, beta = 64.487 (2) degrees, gamma = 65.750 (2) degrees, and Z = 2. [(L3S)Mo(O)Cl(2)] (3) crystallizes in the orthorhombic space group Pna2(1), with cell dimensions a = 13.2213 (13) A, b = 8.817 (2) A, c = 15.649 (4) A, and Z = 4. The implications of these results on the function of mononuclear molybdoenzymes such as sulfite oxidase, and the DMSO reductase, are discussed.

Nine new pseudotetrahedral Zn(II) complexes of the heteroscorpionate ligands (3-tert-butyl-2-hydroxy(or thio)-5-methylphenyl)bis(3,5-dimethylpyrazolyl)methane, L1OH or L2SH, have been prepared and characterized (in most cases crystallographically). Complexes isolated include [(L1O)ZnCl], [(L1O)ZnI], [(L1OH)ZnI(2)], [(L1O)ZnCH(3)], [(L1O)ZnOAc], [(L1O)ZnSPh], [(L1O)ZnSBz], [(L2S)ZnCH(3)], [(L2S)ZnSPh], and [(L2S)(2)Zn]. In conjunction with the widely studied tris(pyrazolyl)borates this new series of heteroscorpionates provides a set of isolobal and isoelectronic ligands differing in donor set, i.e. N(3), N(2)O, and N(2)S. Preliminary reactivity studies with HX species, MeI, or trimethyl phosphate suggest differences between the three sets of ligands.

The synthesis and characterization of the tridentate “heteroscorpionate” mixed functionality ligand, (2-thiophenyl)bis(pyrazolyl)methane, L1, is reported. This ligand was used to synthesize the complexes, [Fe(L1)2][ClO4], [Co(L1)2][BPh4], and [V(L1)2][BPh4]. X-ray crystallographic analysis of the latter two complexes gave the following structural parameters: [V(L1)2][BPh4]·MeCN·iPr2O, C58H57N9B1O1S2V1, monoclinic, a = 16.197(2) A, b = 19.281(2) A, c = 17.985(2) A, β = 102.343(9)°, space group P21/c; [Co(L1)2][BPh4]·CH2Cl2, C51H42N8B1Cl2Co1S2, triclinic, a = 12.751(1) A, b = 12.830(1) A, c = 14.740(1) A, α = 98.351(7)°, β = 98.122(6)°, γ = 97.023(8)°, space group P1. An example of the use of this new class of “heteroscorpionate” ligands for systematic comparisons of electronic properties of metal complexes with identical topology that vary only in the nature of one of the donor atoms is given.

The mixed functionality pyrazole/phenol ligand (2-hydroxyphenyl)bis(pyrazolyl)methane, L1OH, has been used to prepare a series of linear trimetallic systems with the general structural motif [M3(L1...

1H NMR spectra on the paramagnetic CoII and NiII, ML2 “sandwich” complexes of the “heteroscorpionate” ligands (2-hydroxyphenyl)bis (pyrazolyl)methane and its derivatives revealed the presence of isomeric, cis ↔ trans equilibria of these complexes in solution. The presence of some of the cis isomer in solution at equilibrium has been exploited to synthesize new heterometallic trinuclear species of the type [M1(L1O)2M2(L1O)2M1][BF4], where M1 (octahedral) and M2 (tetrahedral) are Zn2+, Cu2+, Ni2+, and Co2+. Rearrangements during the synthesis of some of these heterometallic species provided an interesting experimental confirmation of the site preferences (octahedral vs tetrahedral) for the first-row transition metals as predicted on the basis of crystal field stabilization energies. X-ray crystallographic characterization of many of the complexes reported in this study gives the following structural parameters: [Co2(L2O)2Cl2]·MeCN, C36H41N9Cl2Co2O2, monoclinic, a = 10.742(1) A, b = 21.387(2) A, c = 16.890(...

The heterobimetallic complex 1, [LV(mgr;-O)(mgr;-OAc)(2)MnL] (where L = hydridotris(pyrazolyl)borate), has been synthesized and characterized. X-ray crystal structural analysis of 1 gave the following parameters: C(24)H(29)B(2)N(13)O(5)VMn, Cmc2(1), a = 13.364(2) Å, b = 17.383(3) Å, c = 14.132(3) Å, Z = 4, V = 3289.9(16) Å(3). An analysis of the structure, optical and resonance Raman spectroscopies, and magnetic measurements indicates that a V(IV)=O.Mn(II) valence formulation is the best description of the oxo-bridged core in 1, although other resonance forms must also contribute.

The syntheses of a new class of potentially biomimetic, tripodal, mixed functionality ligands, (2-hydroxyphenyl)bis(pyrazolyl)methane (L1), (2-hydroxyphenyl)bis(3,5-dimethylpyrazolyl)methane (L2), and (2-hydroxyphenyl)bis(3-isopropylpyrazolyl)methane (L3), are described. These ligands have been used to prepare the cobalt(II) complexes [Co(L1)2]·2.5MeOH·1.5H2O, 1·2.5MeOH·1.5H2O; [Co(L2)2]·0.5H2O, 2·0.5H2O; [Co(L3)Cl2], 3; and [Co3(μ3-OH)2(μ-L3)2(L3)(H2O)]BF4, 4. X-ray structural analysis of 1−4 gave the following structural parameters: 1, C30H34N8CoO6, triclinic, a = 8.286(2) A, b = 10.053(3) A, c = 10.562(2) A, α = 109.66(2)°, β = 96.57(1)°, γ = 91.11(3)°, space group P1, Z = 1; 2, C38H38N8CoO6, triclinic, a = 8.746(5) A, b = 10.900(5) A, c = 11.399(8) A, α = 70.82(4)°, β = 75.13(5)°, γ = 81.46(4)°, space group P1, Z = 1; 3, C20H26N4Cl4CoO, monoclinic, a = 10.934(2) A, b = 12.404(2) A, c = 19.267(3) A, β = 98.33(1)°, space group P21/n, Z = 4; 4, C58H70N12BCo3F4O9, monoclinic, a = 14.798(5) A, b = 23.65...

The preparation of a new class of mixed functionality biomimetic ligand, (2-hydroxyphenyl)bis(pyrazolyl)methane, L1, (2-hydroxyphenyl)bis(3,5-dimethylpyrazolyl)methane, L2, and (2-hydroxyphenyl)bis(3-isopropylpyrazolyl)methane, L3, is described. These ligands were used to synthesize the nickel(II) complexes, [Ni(L1)2], 1·MeCN; [Ni(L2)2], 2·0.5MeOH and 2·2H2O; [Ni(μ-L2)2NiCl2], 3·2H2O and 3·2CH2Cl2; [Ni3(μ3-Cl)2(μ-L3)2(L3)(MeOH)]Cl, 4·MeOH·4H2O and 4·MeOH·1.42iPr2O; and [Ni3(μ3-OH)2(μ-L3)3][BF4], 5 and 5·2Me2CO. X-ray crystal analysis of 1−5 gave the following structural parameters: 1·MeCN, C28H25N9NiO2, triclinic, a = 10.600(3) A, b = 11.371(3) A, c = 12.194(3) A, α = 71.17(2)°, β = 94.69(2)°, γ = 83.04(2)°, space group P1, Z = 2; 2·2H2O, C34H42N8NiO4, monoclinic, a = 9.037(2) A, b = 19.272(3) A, c = 19.643(3) A, β = 94.69(2)°, space group C2/c, Z = 4; 3·2CH2Cl2, C36H42N8Cl6Ni2O2, monoclinic, a = 13.891(4) A, b = 18.188(7) A, c = 15.012(4) A, β = 92.70(2)°, space group C2/c, Z = 4; 4·MeOH·1.42iPr2O, C67...

The synthesis and characterization of two linear trinuclear V(III) complexes are described: 1, [LV(μ-(PhO)2PO2)3-V-(μ-(PhO)2PO2)3VL]PF6, and 2, LV(μ-(PhO)2PO2)2(μ-OH)V(μ-(PhO)2PO2)2(μ-OH)VL]ClO4, ...

The syntheses of five mixed ligand vanadium(III) phosphate/phosphonate complexes are described: dinuclear I, [LVCl{μ-(PhO)2PO2}]2; II, [LVCl{μ-Ph2PO2}]2; and III, [LVCl{μ-PhP(H)O2}]2; along with mononuclear IV, [LV{(PhO)2POS}2DMF]; and V, [LV{(PhO)2PO2}]2·H2O where L = hydrotris(pyrazolyl)borate. X-ray crystal structure analysis of I, II, IV, and V gave the following parameters: I, C12H40B2Cl2N12O8P2V2, P1, a = 9.949(4)A, b = 11.732(4) A, c = 12.729(3) A, α = 110.37(1)°, β = 106.78(1)°, γ = 104.36(1)°, Z = 1, V = 1228.8(7) A3; II, C46H98B2Cl10N12O4P2V2, P1, a = 11.801(2) A, b = 12.086(1) A, c = 12.955(2) A, α = 81.99(1)°, β = 65.51(1)°, γ = 62.60(1)°, Z = 1, V = 1491.1(5) A3; IV, C36H37BN7O7P2S2V, P1, a = 11.003(2) A, b = 11.816(2) A, c = 17.167(5) A, α = 98.68(2)°, β = 101.81(2)°, γ = 102.59(2)°, Z = 2, V = 2087.0(6) A3; V, C69.5H65B2N15O8P4V2, C2/c, a = 12.360(2) A, b = 26.362(4) A, c = 23.254(2) A, β = 91.24(8)°, Z = 4, V = 7575(2) A3. Magnetic measurements indicate little or no interaction betwee...

Vanadium(III) and -(V) alkolo complexes of the ligands hydridotripyrazolylborate, L, and hydridotris(3,5-dimethylpyrazolyl)borate, L * , have been prepared and characterized by X-ray diffraction, 1 H, 13 C, and 51 V NMR, electrochemistry, optical spectroscopy, and EXAFS. Two general classes of V(V) complexes have been synthesized with the general formulas LVO(OR) 2 and LVO(OR)Cl. The latter complexes contain the shortest V-OR bonds yet reported (1.706-1.754 A) and show that a vanadium-alkoxide bond could account for the short vanadium «light atom» interaction seen in the EXAFS of the enzyme, bromoperoxidase

Vanadium(V) complexes of catechol and several polydentate analogs have been prepared and characterized in solution by optical and [sup 51]V NMR spectroscopies as well by solution thermodynamic measurements. The vanadium(V) catecholate species displayed relatively sharp [sup 51]V NMR signals in the range [minus]164 to [minus]268 ppm but in general were susceptible to decomposition by internal redox reactions yielding V(IV) and quinone, particularly in aqueous solution. The complexes of 1,5,10-tris(2,3-dihydroxybenzoyl)-1,5,10-triazadecane, and its sulfonated analog (3,4-LICAM and LICAMS) are an exception to this generalization and were quite stable. The sulfonated V(V)-LICAMS complex undergoes reversible protonations in aqueous solutions similar to those seen in the analogous Fe[sup 3+] complexes. The proton dependent formation constant for the reaction, K[sup *] = [V[sup V]LICAMS]/[VO[sub 2][sup +]][LICAMS][H[sup +]] has been measured, yielding a log K[sub ML] of 24.2 (1) at 0.5 M in KNO[sub 3] at 25[degrees]C and pH 5.50.

A series of vanadium(III) and -(IV) complexes of the type [L 2 V]BPh 4 , [LVCl 2 DMF], [LVOCIDMF], and [LVO(acac)] where L is either the anion of hydridotris(pyrazolyl)borate, HB(pz) 3 , or hydridotris(3,5-dimethylpyrazolyl)borate, HB(Me 2 pz) 3 , have been synthesized and characterized by elemental analysis, molar conductance, electrochemistry, magnetic, UV-vis, IR, and ESR techniques. In all cases, the metal ion is in a distorted octahedral environment

Vibrioferrin (VF) is a member of the carboxylate class of siderophores originally isolated from Vibrio parahaemolyticus, an enteropathogenic estuarine bacterium often associated with seafood-borne gastroenteritis. Recently we have also isolated this siderophore from several species of Marinobacter, which are closely associated or "symbiotic" with toxic, bloom-forming dinoflagellates such as Gymnodinium catenatum. We have measured the overall metal-ligand binding constant for iron-vibrioferrin (FeVF) as 10(24.02(5)) making vibrioferrin one of the weakest iron chelators of any known marine siderophore. FeVF is also shown to be considerably more sensitive to photolysis under relatively low illumination conditions than other photoactive siderophores leading primarily to a monodecarboxylated photoproduct that has no significant affinity for Fe(III). The consequences that these features have on bacterial-algal interactions with potential importance to understanding the origin and sustenance of harmful algal blooms are discussed.

Vanadium (3,5-dimethylpyrazolyl) borate complexes with a series of phenolate ligands have been prepared and characterized. The single-crystal X-ray structure of the p-bromophenol derivative tris[(3,5-dimethylpyrazolylborato]bis(p-bromophenoxy)oxovanadium(V) has been determined by standard methods and refined to an unweighted R factor of 0.052. The dark green triclinic crystals belong to space group P1

A series of ferric phenolate complexes with Schiff base or methylamine substitutents ortho to the phenolic oxygen have been investigated. These include complexes with N,N'-ethylenebis((o-hydroxyphenyl)glycine), N-[2-(o-salicylideneamino)ethyl](o-hydroxyphenyl)glycine, and N 1 ,N 4 -disalicylidenetriethylenetetramine

A series of monooxo-Mo(IV,V) and dioxo-Mo(VI) complexes of the "soft" tripodal ligand, sodium tris(mercaptoimidazolyl)borate (NaTm(Me)), have been synthesized as potential oxygen atom transfer (OAT) models for sulfite oxidase. Complexes have been characterized by X-ray crystallography, cyclic voltammetry, and EPR, where appropriate. Oxygen atom transfer kinetics of Tm(Me)MoO(2)Cl, both stoichiometric and catalytic, have been studied by a combination of UV-vis and (31)P NMR spectroscopies under a variety of conditions. OAT rates are consistent with previously established relationships between redox potential/reactivity and mechanistic studies. The analysis of these complexes as potential structural and functional analogues of relevance to molybdoenzymes is further discussed.

Both dioxo Mo(VI) and mono-oxo Mo(V) complexes of a sterically restrictive N2O heteroscorpionate ligand are found to exist as cis and trans isomers. The thermodynamically stable isomer differs for the two oxidation states, but in each case, we have isolated the kinetically labile isomer and followed its isomerization to the thermodynamically stable form. The Mo(VI) complex is more stable in the cis geometry and isomerizes more than 6 times faster than the Mo(V) complex, which prefers the trans geometry. In OAT reactions with PPh3, the trans isomer of the dioxo-Mo(VI) reacts approximately 20 times faster than the cis isomer. Thus, there are both oxidation state and donor atom dependent differences in isomeric stability and reactivity that could have significant functional implications for molybdoenzymes such as DMSO reductase.

Crystal structures of DMSO reductases isolated from two different sources and the crystal structure of related trimethylamine-N-oxide reductase indicate that the angle between the terminal oxo atom on the molybdenum and the serinato oxygen varies significantly. To understand the significance of this angular variation, we have synthesized two isomeric compounds of the heteroscorpionato ligand (L1OH) (cis- and trans-(L1O)Mo(V)OCl(2)), where the phenolic oxygen mimics the serinato oxygen donor. Density functional and semiempirical calculations indicate that the trans isomer is more stable than the cis. The lower stability of the cis isomer can be attributed to two factors. First, a strong antibonding interaction between the phenolic oxygen with molybdenum d(xy) orbital raises the energy of this orbital. Second, the strong trans influence of the terminal oxo group in the trans isomer places the phenol ring, and hence the bulky tertiary butyl group, in a less sterically hindered position. In solution, the cis isomer spontaneously converts to the thermodynamically favorable trans isomer. This geometric transformation follows a first-order process, with an enthalpy of activation of 20 kcal/mol and an entropy of activation of -9 cal/mol K. Computational analysis at the semiempirical level supports a twist mechanism as the most favorable pathway for the geometric transformation. The twist mechanism is further supported by detailed mass spectral data collected in the presence of excess tetraalkylammonium salts. Both the cis and trans isomers exhibit well-defined one-electron couples due to the reduction of molybdenum(V) to molybdenum(IV), with the cis isomer being more difficult to reduce. Both isomers also exhibit oxidative couples because of the oxidation of molybdenum(V) to molybdenum(VI), with the cis isomer being easier to oxidize. This electrochemical behavior is consistent with a higher-energy redox orbital in the cis isomer, which has been observed computationally. Collectively, this investigation demonstrates that by changing the O(t)-Mo-O(p) angle, the reduction potential can be modulated. This geometrically controlled modulation may play a gating role in the electron-transfer process during the regeneration steps in the catalytic cycle.

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

Six new Ni(II) complexes of the "heteroscorpionate" ligand (3-tert-butyl-2-hydroxy-5-methylphenyl)bis(3,5-dimethylpyrazolyl)methane, L1OH, have been prepared and characterized (in most cases crystallographically). These complexes include: tetracoordinate [Ni(L1O)Cl]; pentacoordinate [Ni(L1O)(acac)] and [Ni(L1O)(OAc)]; and hexacoordinate [Ni(L1O)(acac)(pz)], [Ni(L1O)(OAc)(MeOH)], and [Ni(L1O)(2)]. This second generation of heteroscorpionate ligand supports a tetrahedral environment for Ni(II) but is not a tetrahedral enforcer. Thus the tetra- and pentacoordinate species readily add additional ligands to produce 5- and 6-coordinate complexes. The formation of the undesirable, coordinatively saturated, [Ni(L1O)(2)], "sandwich" complex is only a minor interference in most of these reactions unlike the situation seen with the previous generation of ligand. This family of heteroscorpionates shows promise as platforms for biomimetic studies which are already underway.

A series of zinc complexes using a new tripodal, N(2)S, heteroscorpionate ligand (L3SH) that is isostructural and isoelectronic with the well-known N(3) trispyrazolylborates have been methylated in solution and the coordination properties of the resulting thioether examined. This system models the reactivity of zinc-containing enzymes involved in alkyl group transfers such as the DNA repair protein Ada from E. coli, or farnasyl transferase where it has been shown that the thioether resulting from alkyl group transfer remains in the coordination sphere of the zinc. The following complexes have been structurally characterized: [(L3S)ZnI] (1), [(L3SCH(3))ZnI(2)] (2), [(L3SCH(3))ZnI]BF(4) (3), [(L3SCH(3))Zn-mu-bis-acetato-mu-hydroxo-Zn(L3SCH(3))]BF(4) (5), [(L3SCH(3))ZnSPh(F5)]ClO(4) (7), and [(L3SCH(3))(2)Zn](BF(4))(2) (8). Complexes 3, 4, 5, 7, and 8 all display thioether coordination. Thus in the absence of superior anionic ligands, thioethers are reasonably good donors to zinc in either a tetrahedral or octahedral geometry. The methylation of the complex [(L3S)ZnSPh(F5)], which contains two different thiols, produces a single product, 7, where only the aliphatic thiol has been alkylated. This observation validates the suggestion that reactivity in enzymes with multiple zinc-bound thiols could be controlled by differences in thiol pK(a) (Hammes, B. S.; Warthen, C. R.; Crans, D.; Carrano, C. J. J. Biol. Inorg. Chem. 2000, 6, 82. Compound 7 is also of interest in that it resembles the metal ion-binding site of the blue copper protein, azurin.

Cationic mu-oxo V(III) dimers of the type [V2OL4Cl2]2+ (L = 1,10-phenanthroline, 3,4,7,8-tetramethyl-1,10-phenanthroline, 4,7-diphenyl-1,10-phenanthroline; or 2,2'-bipyridine) are shown to interact very strongly with DNA and to lead ultimately to its degradation. Spectroscopic binding studies, electrophoreses, DNA melting temperature experiments, and other tests on the parent 1,10-phenthroline complex all yield results consistent with tight binding. However, the exact nature of the binding--i.e., intercalative, groove binding, electrostatic, or covalent--remains unclear. Resonance Raman spectroscopy is found to be a powerful method for studying the interaction of these mu-oxo V(III) dimers with DNA and shows that in frozen aqueous solution, the parent complex [V2O(phen)4Cl2]2+ undergoes initial aquation, followed by the reaction of the aquated species with the DNA. Once the V(III) dimer is bound to the DNA, redox takes place, leading to the formation of alkaline-sensitive lesions. Hydrogen peroxide is implicated as a partner in this redox event, based on the effects of the enzymes SOD and catalase.

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

Rhizoferrin is a member of a new class of siderophores (microbial iron transport compounds) based on carboxylate and hydroxy donor groups rather than the commonly encountered hydroxamates and catecholates. We have studied the coordination chemistry of rhizoferrin (Rf), as a representative of this group, with Fe(3+), Rh(3+), Cr(3+), Al(3+), Ga(3+), VO(2+), and Cu(2+). The metal complexes have been studied by UV-vis, CD, NMR, and EPR spectroscopies and mass spectrometry. The formation constants for the iron complex have also been measured and yield a log K(LFe) of 25.3. The Rh and Cr rhizoferrin complexes are unusual in that they appear to adopt a chirality about the metal center that is the opposite of the native iron analog. Several of the alternative metal ion complexes are found to have biological activity toward Morganella morganii in a plate type assay.

Tetranuclear V(III) complexes, [HB(pz)(3)](4)V(4)(mgr;-C(6)H(5)OPO(3))(4) (I), its acetonitrile solvate (I.4CH(3)CN), and [HB(pz)(3)](4)V(4)(mgr;-O(2)NC(6)H(4)OPO(3))(4).4C(7)H(8).H(2)O (II), and tetranuclear vanadyl complexes, (t-Bupz)(4)V(4)O(4)(mgr;-C(6)H(5)PO(3))(4).2H(2)O (III) and (t-Bupz)(5)V(4)O(4)(mgr;-C(6)H(5)PO(3))(4).4CH(3)CN.0.6 H(2)O (IV), have been prepared and characterized by spectroscopic, magnetic, and electrochemical methods (pz = pyrazole, t-Bupz = tert-butylpyrazole). The use of organic solvents and bulky organic groups as ancillary ligands leads to formation of neutral species instead of the anionic clusters commonly found in the hydrothermal synthesis of vanadium organophosphate/phosphonate systems. Complexes I.4CH(3)CN and IV have also been characterized by single-crystal X-ray diffraction. Crystal data: I.4CH(3)CN, triclinic, Ponemacr;, a = 15.495(3) Å, b = 17.000(3) Å, c = 17.949(4) Å, alpha = 89.17(3) degrees, beta = 86.00(3) degrees, gamma = 78.60(3) degrees, Z = 2; IV, triclinic, Ponemacr;, a = 15.541(3) Å, b = 16.340(2) Å, c = 19.069(5) Å, alpha = 83.58(2) degrees, beta = 79.67(2) degrees, gamma = 63.68(1) degrees, Z = 2. Both are closed clusters, the core structure of the first consisting of a cubane-like arrangement of metal octahedra and phosphate tetrahedra and the core structure of the second consisting of a distorted, collapsed variant of the first. Unlike other vanadium phosphate clusters, these compounds form in the absence of a central, templating agent. As such they represent the simplest form of a closed cluster in which steric forces and cluster connectivity requirements play the primary role in organizing the cluster framework.

Two isomers (cis and trans) of (L1O)MoOCl2 have been synthesized and separated using the heteroscorpinate ligand L1OH, which possesses an N2O donor set. These geometric isomers differ in the orient...

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