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PubMed2003

Five- to six-coordination in (nitrosyl)iron(II) porphyrinates: effects of binding the sixth ligand.

We report structural and spectroscopic data for a series of six-coordinate (nitrosyl)iron(II) porphyrinates. The structures of three tetraphenylporphyrin complexes [Fe(TPP)(NO)(L)], where L = 4-(dimethylamino)pyridine, 1-methylimidazole, 4-methylpiperidine, are reported here to a high degree of precision and allow observation of several previously unobserved structural features. The tight range of bonding parameters for the [FeNO] moiety for these three complexes suggests a canonical representation for six-coordinate systems (Fe-N(p) = 2.007 A, Fe-N(NO) = 1.753 A, angle FeNO = 138.5 degrees ). Comparison of these data with those obtained previously for five-coordinate systems allows the precise determination of the structural effects of binding a sixth ligand. These include lengthening of the Fe-N(NO) bond and a decrease in the Fe-N-O angle. Several other aspects of the geometry of these systems are also discussed, including the first examples of off-axis tilting of a nitrosyl ligand in a six-coordinate [FeNO](7) heme system. We also report the first examples of Mössbauer studies for these complexes. Measurements have been made in several applied magnetic fields as well as in zero field. The spectra differ from those of their five-coordinate analogues. To obtain reasonable fits to applied magnetic field data, rotation of the electrical field gradient is required, consistent with differing g-tensor orientations in the five- vs six-coordinate species.

Crystallography, X-Ray↗
PubMed2003

Cu(II)Gly2HisGly oxidation by H2O2/ascorbic acid to the CuIII complex and its subsequent decay to alkene peptides.

Protonation and stability constants for Gly2HisGly and its Cu(II) complexes (beta(mhl)), determined at 25.0 degrees C and mu = 0.10 M (NaClO(4)), have values of log beta(011) = 7.90, log beta(021) = 14.51, log beta(031) = 17.55, log beta(101) = 7.82, log beta(1-21) = -0.80, and log beta(1-31) = -12.7 (where the subscripts refer to the number of metal ions, protons, and ligands, respectively). The reaction of CuII(H(-2)Gly(2)HisGly)- with l-ascorbic acid and H(2)O(2) at p[H(+)] 6.6 rapidly generates Cu(III)(H(-2)Gly(2)HisGly) with lambda(max) at 260 and 396 nm, which is separated chromatographically. The Cu(III) complex decomposes to give alkene peptide isomers of glycylglycyl-alpha,beta-dehydrohistidylglycine that are separated chromatographically and characterized. These alkene peptide species are present as geometric isomers with imidazole tautomers that have distinct spectral and chemical characteristics. The proposed major isomeric form (94%), Z-N(tau)-H, has a hydrogen on the tau nitrogen of the imidazole ring and three conjugated double bonds. It has absorption bands at 295 and 360 nm at p[H+] 6.6 in the absence of copper. The proposed minor isomeric form (6%) is E-N(pi)-H with a hydrogen on the pi nitrogen of the imidazole ring. This isomer has four conjugated double bonds and strongly binds Cu(II) to give a complex with intense absorption bands at 434 and 460 nm.

Alkenes↗
PubMed2003

Synthesis, X-ray diffraction structures, spectroscopic properties, and in vitro antitumor activity of isomeric (1H-1,2,4-triazole)Ru(III) complexes.

Three ruthenium(III) complexes containing 1H-1,2,4-triazole (Htrz), viz., (H(2)trz)[cis-RuCl(4)(Htrz)(2)], 1, (H(2)trz)[trans-RuCl(4)(Htrz)(2)], 2, and (Ph(3)PCH(2)Ph)[trans-RuCl(4)(Htrz)(2)], 3, have been synthesized by reaction between RuCl(3) and excess of the triazole in 2.38 M HCl (1 and 2), while 3 was obtained by metathesis of 2 and [Ph(3)PCH(2)Ph]Cl in water. The products were characterized by IR, UV-vis, electrospray mass spectrometry, cyclic voltammetry, and X-ray crystallography (1 and 3). X-ray diffraction study revealed cis and trans arrangements of the triazole ligands in 1 and 3, correspondingly, and unprecedented monodentate coordination of the triazole through N2 and stabilization of its 4H tautomeric form, which is the disfavored one for the free triazole. The cytotoxicity of 1 and 2 has been assayed in three human carcinoma cell lines SW480, HT29 (colon carcinoma), and SK-BR-3 (mammary carcinoma). Both compounds exhibit antiproliferative activity in vitro. Time-dependent response of all three lines to 1 and 2 and a structure-activity relationship, i.e., higher activity of the trans-isomer 2 than that of cis-species 1, have been observed.

Antineoplastic Agents↗
PubMed2003

Slow electron transfer rates for fluorinated cobalt porphyrins: electronic and conformational factors modulating metalloporphyrin ET.

The electron transfer (ET) properties of a series of closely related cobalt porphyrins, [2,3,7,8,12,13,17,18-octafluoro-5,10,15,20-tetrakis(pentafluorophenyl)porphyrinato]cobalt, CoF(28)TPP, [2,3,7,8,12,13,17,18-octafluoro-5,10,15,20-tetraphenyl)porphyrinato]cobalt, CoF(8)TPP, 5,10,15,20-tetrakis(pentafluorophenyl)porphyrinato]cobalt, CoF(20)TPP, and [5,10,15,20-tetraphenylporphyrinato]cobalt, CoTPP, were investigated by cyclic voltammetry, cyclic voltammetric digital simulation, in situ UV-vis and IR spectroelectrochemistry, kinetic ET studies, bulk electrolysis, (19)F NMR spectroscopy, X-ray crystallography, and molecular modeling. In benzonitrile containing 0.1 M tetrabutylammonium hexafluorophosphate (TBAPF(6)) as supporting electrolyte, the ET rate constants for the Co(2+/3+) redox couples were found to be strongly substituent dependent; the heterogeneous ET rate constant (k(el)) varied by a factor of 10(4), and the ET self-exchange rate constants (k(ex)) varied over 7 orders of magnitude for the compounds studied. The remaining observed ring oxidation and metal and ring reduction events exhibited nearly identical k(el) values for all compounds. UV-vis and IR spectroelectrochemistry, bulk electrolysis, and (19)F NMR spectroscopic studies support attribution of different ET rates to widely varying inner sphere reorganization energies (lambda(i)) for these closely related compounds. Structural and semiempirical (PM3) studies indicate that the divergent kinetic behavior of CoTPP, CoF(8)TPP, CoF(20)TPP, and CoF(28)TPP first oxidations arises mainly from large nuclear reorganization energies primarily associated with core contraction and dilation. Taken together, these studies provide rational design principles for modulating ET rate constants in cobalt porphyrins over an even larger range and provide strategies for similar manipulation of ET rates in other porphyrin-based systems: substituents that lower C-C, C-N, and N-M vibrational frequencies or minimize porphyrin orbital overlap with the metal-centered orbital undergoing a change in electron population will increase k(ET). The heme ruffling apparent in electron transfer proteins such as cytochrome c is interpreted as nature's exploitation of this design strategy.

Cobalt↗
PubMed2005

Oxomolybdenum tetrathiolates with sterically encumbering ligands: modeling the effect of a protein matrix on electronic structure and reduction potentials.

The effect of sterically encumbering ligands on the electronic structure of oxomolybdenum tetrathiolate complexes was determined using a combination of electronic absorption and magnetic circular dichroism spectroscopies, complimented by DFT bonding calculations, to understand geometric and electronic structure contributions to reduction potentials. These complexes are rudimentary models for a redox-active metalloenzyme active site in a protein matrix and allow for detailed spectroscopic probing of specific oxomolybdenum-thiolate interactions that are directly relevant to Mo-S(cysteine) bonding in pyranopterin molybdenum enzymes. Data are presented for three para-substituted oxomolybdenum tetrathiolate complexes ([PPh4][MoO(p-SPhCONHCH3)4], [PPh4][MoO(p-SPhCONHC(CH2O(CH2)2CN)3)4], and [PPh4][MoO(p-SPhCONHC(CH2O(CH2)2COOCH2CH3)3)4]). The Mo(V/IV) reduction potentials of the complexes in DMF are -1213, -1251, and -1247 mV, respectively. The remarkably similar electronic absorption and magnetic circular dichroism spectra of these complexes establish that the observed reduction potential differences are not a result of significant changes in the electronic structure of the [MoOS4]- cores as a function of the larger ligand size. We provide evidence that these reduction potential differences result from the driving force for a substantial reorganization of the O-Mo-S-C dihedral angle upon reduction, which decreases electron donation from the thiolate sulfurs to the reduced molybdenum center. The energy barrier to favorable O-Mo-S-C geometries results in a reorganizational energy increase, relative to [MoO(SPh)4](-/2-), that correlates with ligand size. The inherent flexible nature of oxomolybdenum-thiolate bonds indicate that thiolate ligand geometry, which controls Mo-S covalency, could affect the redox processes of monooxomolybdenum centers in pyranopterin molybdenum enzymes.

Circular Dichroism↗
PubMed2005

Synthesis and properties of cobalt(III) complexes of 4-substituted pyridine-capped dioxocyclams.

Cobalt(III) acetate and cyanide complexes of a series of 5,12-dioxocyclams capped across the 1,8-position by 4-substituted pyridines or pyrazine were synthesized and fully characterized. Both the spectroscopic and structural parameters for these complexes were remarkably insensitive to the electronic nature of the capping group, which ranged from the pi-accepting pyrazine group to the sigma-donating 4-[(dimethylamino)phenyl]pyridyl group. All of the complexes underwent an irreversible, one-electron reduction [Co(III)-->Co(II)] at potentials ranging from -0.95 V vs saturated calomel electrode (SCE) for the pyrazine-capped cobalt acetate complex to -1.36 V vs SCE for the pyridine-capped cobalt cyanide complexes. Pyridine-capped cobalt(III) cyanide complex underwent reaction with Rh2(OAc)4 and ruthenium(II) phthalocyanine[bis(benzonitrile)] to form tetrametallic and trimetallic complexes through coordination bridging by the cyanide nitrogen lone pair. These complexes represent two quite different structural types for cyanide-bridged polymetallics. Complex has a relatively long (2.192 A) cyanide N-to-Rh bond, and the CN-Rh bond angle (157.6 degrees) is strongly distorted from linear. In contrast, complex has a substantially shortened cyanide N-to-Ru bond (2.017 A) and an almost linear arrangement along the entire bridging axis of the molecule.

Cobalt↗
PubMed2006

Mono-, di-, and oligonuclear complexes of Cu(II) ions and p-hydroquinone ligands: syntheses, electrochemical properties, and magnetic behavior.

Four highly soluble square-planar Cu(II) and Ni(II) complexes of siloxy-salens (2SiCu, 2SiNi) and hydroxy-salens (2Cu, 2Ni) have been synthesized. An X-ray crystal structure analysis was performed on 2SiCu, 2SiNi, and 2Ni. The compounds have been investigated by cyclic voltammetry, UV-vis-NIR spectroelectrochemistry, and EPR spectroscopy. According to these results, the monooxidized species [2SiCu]+ and [2SiNi]+ are to be classified as Robin-Day class II and III systems, respectively. Magnetic measurements on the dinuclear (PMDTA)Cu(II) complex 1Cu2 x (PF6)2 with deprotonated 1,4-dihydroxy-2,5-bis(pyrazol-1-yl)-benzene (1) linker revealed antiferromagnetic coupling between the two Cu(II) ions thereby resulting in an isolated dimer compound. Coordination polymers [1Cu]n(H2O)(2n) of Cu(II) ions and bridging p-hydroquinone linkers were obtained from CuSO4 x 5 H2O and 1,4-dihydroxy-2,5-bis(pyrazol-1-yl)benzene. X-ray crystallography revealed linear chains running along the crystallographic a-direction and stacked along the b-axis. Within these chains, the Cu(II) ions are coordinated by two pyrazolyl nitrogen atoms and two p-hydroquinone oxygen atoms in a square-planar fashion.

Copper↗
PubMed2006

One-pot synthesis of metal primary phosphine complexes from O=PCl2R or PCl2R. Isolation and characterization of primary alkylphosphine complexes of a metalloporphyrin.

Treatment of [Ru(II)(Por)(CO)] [Por = porphyrinato(2-)] and O=PCl(2)R [R = Ad (adamantyl), Bu(t), Bu(sec)] or PCl2Mes (Mes = mesityl) with LiAlH4 afforded primary alkyl- and arylphosphine complexes [Ru(II)(Por)(PH2R)2], which have been isolated in pure form and characterized by 1H NMR, 31P NMR, IR, and UV-vis spectroscopy and mass spectrometry. The structures of [Ru(II)(TTP)(PH2Ad)2] and [Ru(II)(F20-TPP)(PH2Mes)2] were determined by X-ray crystallography.

Chlorine↗
PubMed2006

BF2 complex of fluorinated dipyrrolyldiketone: a new class of efficient receptor for acetate anions.

The beta-fluorinated derivative (2b) of the 1,3-dipyrrolyl-1,3-propanedione BF2 complex has been prepared from 3,4-difluoropyrrole and malonyl chloride, followed by treatment with BF3.OEt2. Despite the simple, acyclic, and neutral structure, 2b exhibits efficient 1:1 binding for anions in CH2Cl2 using the bridging CH and pyrrole NH as interaction sites. The binding constant (Ka) of 2b for acetate (CH3CO(2-)), associating more effectively than anions such as F-, Cl-, Br-, H2PO(4-), and HSO(4-), is estimated to be 9.6 x 10(5) M(-1), approximately 9 times larger than that of the beta-H derivative 2a (1.1 x 10(5) M(-1)). The UV-vis and fluorescence spectral changes of 2b elucidate the effective recognition of an amino acid, such as phenylalanine, in the anionic form; this is also supported by CD spectral changes with mirror images by L- and D-isomers. Furthermore, in the solid state, BF2 complex 2b provides Cl- -bridged supramolecular networks and, in sharp contrast, deprotonated "anionic" self-assembled structures by F- binding.

Acetates↗
PubMed2006

A rare and highly oxidized Mo(2)5.5+ unit stabilized by oxo anions and supported by formamidinate bridges.

A series of tetranuclear compounds consisting of two {Mo2[(m-CF3C6H4)NC(H)N(m-CF3C6H4)]3}n+ moieties linked by two OH- or two O2- ions has been characterized. Abbreviating the dimolybdenum plus three spectator bridging ligands as [Mo2], the following three compounds have been made-[Mo2](mu-OH)2[Mo2] (1), [Mo2](mu-O)2[Mo2] (2), and {[Mo2](mu-O)2[Mo2]}SbF6 (3). Compound 1, which is diamagnetic and contains quadruply bonded Mo2(4+) units, is converted to diamagnetic 2 by oxidation with O2. Compound 2, which has Mo2(5+) units, is oxidized by NOSbF6 to 3 which has a rare Mo2(5.5+) core and an odd electron delocalized over the two dimolybdenum units.

Electron Spin Resonance Spectroscopy↗
PubMed2006

Mononitrosyl tris(thiolate) iron complex [Fe(NO)(SPh)3]- and dinitrosyl iron complex [(EtS)2Fe(NO)2]-: formation pathway of dinitrosyl iron complexes (DNICs) from nitrosylation of biomimetic rubredoxin [Fe(SR)4]2-/1- (R = Ph, Et).

Nitrosylation of the biomimetic reduced- and oxidized-form rubredoxin [Fe(SR)4]2-/1- (R = Ph, Et) in a 1:1 stoichiometry led to the formation of the extremely air- and light-sensitive mononitrosyl tris(thiolate) iron complexes (MNICs) [Fe(NO)(SR)3]- along with byproducts [SR]- or (RS)2. Transformation of [Fe(NO)(SR)3]- into dinitrosyl iron complexes (DNICs) [(RS)2Fe(NO)2]- and Roussin's red ester [Fe2(mu-SR)2(NO)4] occurs rapidly under addition of 1 equiv of NO(g) and [NO]+, respectively. Obviously, the mononitrosyl tris(thiolate) complex [Fe(NO)(SR)3]- acts as an intermediate when the biomimetic oxidized- and reduced-form rubredoxin [Fe(SR)4]2-/1- exposed to NO(g) were modified to form dinitrosyl iron complexes [(RS)2Fe(NO)2]-. Presumably, NO binding to the electron-deficient [Fe(III)(SR)4]- and [Fe(III)(NO)(SR)3]- complexes triggers reductive elimination of dialkyl/diphenyl disulfide, while binding of NO radical to the reduced-form [Fe(II)(SR)4]2- induces the thiolate-ligand elimination. Protonation of [Fe(NO)(SEt)3]- yielding [Fe(NO)(SPh)3]- by adding 3 equiv of thiophenol and transformation of [Fe(NO)(SPh)3]- to [Fe(NO)(SEt)3]- in the presence of 3 equiv of [SEt]-, respectively, demonstrated that complexes [Fe(NO)(SPh)3]- and [Fe(NO)(SEt)3]- are chemically interconvertible. Mononitrosyl tris(thiolate) iron complex [Fe(NO)(SPh)3]- and dinitrosyl iron complex [(EtS)2Fe(NO)2]- were isolated and characterized by X-ray diffraction. The mean NO bond distances of 1.181(7) A (or 1.191(7) A) in complex [(EtS)2Fe(NO)2]- are nearly at the upper end of the 1.178(3)-1.160(6) A for the anionic {Fe(NO)2}9 DNICs, while the mean FeN(O) distances of 1.674(6) A (or 1.679(6) A) exactly fall in the range of 1.695(3)-1.661(4) A for the anionic {Fe(NO)2}9 DNICs.

Iron↗
PubMed2000

Magnetic interactions in the high-spin iron(III) oxooctaethylchlorinato derivative [Fe(oxoOEC)(Cl)] and its pi-cation radical [Fe(oxoOEC.)(Cl)]SbCl6.

The preparation and characterization of the beta-oxochlorin derivative [3,3,7,8,12,13,17,18-octaethyl-(3H)-porphin-2-onato(2-)]iron(III) chloride, [Fe(oxoOEC)(Cl)], and its pi-cation radical derivative [Fe(oxoOEC.)(Cl)]SbCl6 is described. Both compounds have been characterized by single-crystal X-ray structure determinations, IR, UV/vis/near-IR, and Mössbauer spectroscopies, and temperature-dependent magnetic susceptibility measurements. The macrocycles of [Fe(oxoOEC)(Cl)] and [Fe(oxoOEC.)(Cl)]SbCl6 are both saddled, and [Fe(oxoOEC.)(Cl)]-SbCl6 is slightly ruffled as well. [Fe(oxoOEC)(Cl)] shows a laterally shifted dimeric unit in the solid state, with a mean plane separation of 3.39 A and a lateral shift of 7.39 A. Crystal data for [Fe(oxoOEC)(Cl)]: triclinic, space group P1, Z = 2, a = 9.174(2) A, b = 13.522(3) A, c = 14.838(3) A, alpha = 95.79(3) degrees, beta = 101.46(2) degrees, gamma = 104.84(3) degrees. Upon oxidation, the inter-ring geometric parameters increase; the mean plane separation and the lateral shift of the dimeric unit of [Fe(oxoOEC.)(Cl)]SbCl6 are 4.82 and 8.79 A, respectively. Crystal data for [Fe(oxoOEC.)(Cl)]SbCl6: monoclinic, space group Cc, Z = 4, a = 19.8419(13) A, b = 10.027(2) A, c = 22.417(4) A, beta = 96.13(2) degrees. A broad near-IR absorption band appears at 1415 nm for the pi-cation radical, [Fe(oxoOEC.)(Cl)]SbCl6. Zero-field Mössbauer measurements at 4.2 K for both [Fe(oxoOEC)(Cl)] and [Fe(oxoOEC.)(Cl)]SbCl6 confirmed that the oxidation state of the iron atom did not change upon chemical oxidation. Solid-state magnetic susceptibility measurements for [Fe(oxoOEC.)(Cl)]SbCl6 resulted in a large temperature dependence of the magnetic moment that can best be fit with a model that includes a zero-field splitting parameter of D = 6 cm-1, antiferromagnetic intermolecular iron-iron coupling (2JFe-Fe = -0.14 cm-1), antiferromagnetic intramolecular iron-radical coupling (2JFe-r = -76 cm-1), and antiferromagnetic radical-radical coupling (2Jr-r = -13 cm-1).

Algorithms↗
PubMed2001

Raman spectroscopic and electrochemical characterization of myoglobin thin film: implication of the role of histidine 64 for fast heterogeneous electron transfer.

Myoglobin (Mb) thin films formed on various substrates have been characterized by using Raman spectroscopy, reflectance absorbance FT-IR, UV-vis absorption spectroscopy, and electrochemical methods. Raman spectra were obtained upon excitation within the Soret band as well as alpha-beta bands. The spin state marker bands observed from the Mb film in the 1550-1630 cm(-)(1) region (excitation at 514.5 nm) are approximately 20 cm(-)(1) higher than those of aqueous metMb having the high spin state. The 1210 cm(-)(1) band (methine bridge C-H vibration) also shifts to 1240 cm(-)(1) upon the formation of the film. These results indicate that the heme iron of myoglobin in the film is the ferric low-spin state, and the iron atom is pulled to the heme plane. A comparison of the Raman spectra of the Mb film with that of an Mb-imidazole derivative leads to the conclusion that the distal histidine is responsible for the change in the spectral characteristics. The escape of water from the sixth position upon the formation of the Mb film may result in a conformational change at the heme distal pocket: the histidine residue at the E7 helical position (H64) moves toward the central iron and is coordinated with it through the N on the imidazole ring. These structural features facilitate the fast electron transfer between the thin protein film and the electrode. Distal histidine may serve as an electron-transfer pathway as it does in cytochrome c.

Animals↗
PubMed2001

Coordination sphere flexibility of active-site models for Fe-only hydrogenase: studies in intra- and intermolecular diatomic ligand exchange.

A series of dinuclear complexes, (mu-SRS)Fe(2)(CO)(6) (R = -CH(2)CH(2)-, -CH(2)CH(2)CH(2)-, -CH(2)-C(6)H(4)-CH(2)-; edt, pdt, and o-xyldt, respectively) has been examined for specific characteristics that might relate to structural similarity with the active site of Fe-only hydrogenases. Variable-temperature proton NMR studies display the fluxionality of the iron-dithiocyclohexane unit in (mu-pdt)Fe(2)(CO)(6) while in the (mu-o-xyldt)Fe(2)(CO)(6) compound, the bridge is fixed. Temperature-dependent (13)C NMR spectral studies establish intramolecular CO site exchange localized on discrete Fe(CO)(3) units in all complexes, which is influenced by steric effects of the mu-SRS unit. Kinetic studies of intermolecular CO/CN(-) ligand-exchange reactions establish associative or I(a) mechanisms in sequential steps to form the dicyano dianion, (mu-SRS)[Fe(CO)(2)(CN)](2)(=) with 100% selectivity. Theoretical calculations (DFT) of transition states in the intramolecular site-exchange processes lead to a rationale for the interesting cooperativity in the CN(-)/CO intermolecular ligand-exchange process. The hinge motion of the three light atom S-to-S bridge is related to a possible heterolytic H(2) activation/production process in the enzyme.

Binding Sites↗