Mutual control of axial and equatorial ligands: model studies with [Ni]-bacteriochlorophyll-a

Modification of the metal's electronic environment by ligand association and dissociation in metalloenzymes is considered cardinal to their catalytic activity. We have recently presented a novel system that utilizes the bacteriochlorophyll (BChl) macrocycle as a ligand and reporter. This system allows for charge mobilization in the equatorial plane and experimental estimate of changes in the electronic charge density around the metal with no modification of the metal's chemical environment. The unique spectroscopy, electrochemistry and coordination chemistry of [Ni]-bacteriochlorophyll ([Ni]-BChl) enable us to follow directly fine details and steps involved in the function of the metal redox center. This approach is utilized here whereby electro-chemical reduction of [Ni]-BChl to the monoanion [Ni]-BChl(-) results in reversible dissociation of biologically relevant axial ligands. Similar ligand dissociation was previously detected upon photoexcitation of [Ni]-BChl (Musewald, C.; Hartwich, G.; Lossau, H.; Gilch, P.; Pollinger-Dammer, F.; Scheer, H.; Michel-Beyerle, M. E. J. Phys. Chem. B 1999, 103, 7055-7060 and Noy, D.; Yerushalmi, R.; Brumfeld, V.; Ashur, I.; Baldridge, K. K.; Scheer, H.; Scherz, A. J. Am. Chem. Soc. 2000, 122, 3937-3944). The electrochemical measurements and quantum mechanical (QM) calculations performed here for the neutral, singly reduced, monoligated, and singly reduced, monoligated [Ni]-BChl suggest the following: (a) Electroreduction, although resulting in a pi anion [Ni]-BChl(-) radical, causes electron density migration to the [Ni]-BChl core. (b) Reduction of nonligated [Ni]-BChl does not change the macrocycle conformation, whereas axial ligation results in a dramatic expansion of the metal core and a flattening of the highly ruffled macrocycle conformation. (c) In both the monoanion and singly excited [Ni]-BChl ([Ni]-BChl*), the frontier singly occupied molecular orbital (SOMO) has a small but nonnegligible metal character. Finally, (d) computationally, we found that a reduction of [Ni]-BChl*imidazole results in a weaker metal-axial ligand bond. Yet, it remains weakly bound in the gas phase. The experimentally observed ligand dissociation is accounted for computationally when solvation is considered. On the basis of the experimental observations and QM calculations, we propose a mechanism whereby alterations in the equatorial pi system and modulation of sigma bonding between the axial ligands and the metal core are mutually correlated. Such a mechanism highlights the dynamic role of axial ligands in regulating the activity of metal centers such as factor F430 (F430), a nickel-based coenzyme that is essential in methanogenic archea.     

Physicochemical Studies of Demetalation of Light-harvesting Bacteriochlorophyll Isomers Purified from Green Sulfur Photosynthetic Bacteria

Demetalation kinetics of bacteriochlorophylls (BChls) c, d and e from green sulfur photosynthetic bacteria were studied under weakly acidic conditions. Demetalation rate constants of BChl e possessing a formyl group at the 7-position were significantly smaller than those of BChls c and d , which had a methyl group at this position. The activation energy of demetalation of 3¹ R -8,12-diethyl([E,E])-BChl e was 1.5-times larger than that of 3¹ R -[E,E]-BChl c . ¹⁵N-labeled 3¹ R -[E,E]-BChls c and e were purified from cells of green sulfur bacteria grown in a medium containing ¹⁵NH₄Cl, and their ¹⁵N NMR spectra were measured. The chemical shifts of N₂₁, N₂₂ and N₂₃ atoms of 3¹ R -[E,E]-BChl e were lower-field shifted than those of 3¹ R -[E,E]-BChl c , respectively, and especially the difference in chemical shifts of N₂₂ was significantly large. These results suggest that the electron-withdrawing formyl group at the 7-position of BChl e affected an electronic state of the chlorin macrocycle and caused BChl e to be more tolerant for removal of the central magnesium compared with BChls c and d .     

Synthesis and coordination chemistry of an alkyne functionalised bis(pyrazolyl)methane ligand

The alkyne functionalised bidentate N-donor ligand (2-propargyloxyphenyl)bis(pyrazolyl)methane was prepared in high yield from the reaction of (2-hydroxyphenyl)bis(pyrazolyl)methane with propargyl bromide in the presence of base. A series of transition-metal complexes including [MCl2] (M=Cu, Co, Ni, Zn, Pt), [M2](NO3)2 (M=Cu, Co, Ni, Zn), [Ag]NO3 and [Pd(dppe)](OTf)2 were prepared and characterised by spectroscopic techniques. In addition, ligand as well as the Co(II) and Zn(II) complexes [CoCl2]2, [ZnCl2] were structurally characterized by single-crystal X-ray diffraction. The organometallic gold(I) and platinum(II) acetylide complexes [Pz2CH(C6H(4)-2-OCH2C[triple bond, length as m-dash]CAuPPh3)] and trans-[{Pz2CHC6H(4)-2-OCH2C[triple bond, length as m-dash]C}2Pt(PPh3)2] were prepared from and [AuCl(PPh3)] and trans-[PtCl2(PPh3)2], respectively. Treatment of these complexes with [Pd(OTf)2(dppe)] or [Cu(MeCN)4]PF6 results in formation of the cationic, mixed-metal complexes, which were isolated (Pt/Pd, Au/Pt) or detected by electrospray mass spectrometry (Au/Cu, Pt/Cu).     

Control of redox transitions and oxygen species binding in Mn centers by biologically significant ligands; model studies with [Mn]-bacteriochlorophyll a

Mn-superoxide dismutase (Mn-SOD), which protects the cell from the toxic potential of superoxide radicals (O(2)(-*)), is the only type of SOD which resides in eukaryotic mitochondria. Up-to-date, the exact catalytic mechanism of the enzyme and the relationship between substrate moieties and the ligands within the active site microenvironment are still not resolved. Here, we set out to explore the possible involvement of hydroperoxyl radicals ((*)OOH) in the catalytic dismutaion by following the interplay of Mn(III)/Mn(II) redox transitions, ligands binding, and evolution or consumption of superoxide radical, using a new model system. The model system encompassed an Mn atom chelated by a bacteriochlorophyll allomer macrocycle (BChl) in aerated aprotic media that contain residual water. The redox states of the Mn ion were monitored by the Q(y) electronic transitions at 774 and 825 nm for [Mn(II)]- and [Mn(III)]-BChl, respectively (Geskes, C.; Hartwich, G.; Scheer, H.; Mantele, W.; Heinze, J. J. Am. Chem. Soc. 1995, 117, 7776) and confirmed by electron spin resonance spectroscopy. Evolution of (*)OOH radicals was monitored by the ESR spin-trap technique using 5,5-dimethyl-1-pyrroline-N-oxide (DMPO). The experimental data suggest that the [Mn]-BChl forms a (HO(-))[Mn(III)]-BChl(OOH) complex upon solvation. Spectrophotometeric titrations with tetrabutylamonnium acetate (TBAA) and 1-methylimidazole (1-MeIm) together with ESI-MS measurements indicated the formation of a 1:1 complex with [Mn]-BChl for both ligands. The coordination of ligands at low concentrations to [Mn(III)]-BChl induced a release of a (*)OOH radical and a [Mn(III)]-BChl --> [Mn(II)]-BChl transition at higher concentrations. The estimated equilibrium constants for the total redox reaction ( )()are 1.9 x 10(4) +/- 1 x 10(3) M(-)(1) and 12.3 +/- 0.6 M(-)(1) for TBAA and 1-MeIm, respectively. The profound difference between the equilibrium constants agrees with the suggested key role of the ligand's basicity in the process. A direct interaction of superoxide radicals with [Mn(III)]-BChl in a KO(2) acetonitrile (AN) solution also resulted in [Mn(III)]-BChl --> [Mn(II)]-BChl transition. Cumulatively, our data show that the Mn(III) center encourages the protonation of the O(2)(-)(*) radical in an aprotic environment containing residual water molecules, while promoting its oxidation in the presence of basic ligands. Similar coordination and stabilization of the (*)OOH radical by the Mn center may be key steps in the enzymatic dismutation of superoxide radicals by Mn-SOD.     

Tetra-2,3-pyrazinoporphyrazines with externally appended thienyl rings: synthesis, UV-visible spectra, electrochemical behavior, and photoactivity for the generation of singlet oxygen

A series of pyrazinoporphyrazine macrocycles carrying externally appended 2-thienyl rings, represented as [Th(8)TPyzPzM], where Th(8)TPyzPz = tetrakis-2,3-[5,6-di(2-thienyl)pyrazino]porphyrazinato anion and M = Mg(II)(H(2)O), Zn(II), Co(II), Cu(II), or 2H(1), were prepared and isolated as solid air-stable hydrated species. All of the compounds, completely insoluble in water, were characterized by their UV-visible spectra and electrochemical behavior in solutions of dimethylformamide (DMF), dimethyl sulfoxide, and pyridine. Molecular aggregation occurs at concentrations of ca. 10(-4) M, but monomers are formed in more dilute solutions of 10(-5) M or less. The examined octathienyl compounds [Th(8)TPyzPzM] behave as electron-deficient macrocycles, and UV-visible spectral measurements provide useful information about how the peripheral thienyl rings influence the electronic distribution over the entire macrocyclic framework. Cyclic voltammetric and spectroelectrochemical data confirm the easier reducibility of the compounds as compared to the related phthalocyanine analogues, and the overall redox behavior and thermodynamic potentials for the four stepwise one-electron reductions of the compounds are similar to those of the earlier examined octapyridinated analogues [Py(8)TPyzPzM]. Quantum yields (Φ(Δ)) for the generation of singlet oxygen, (1)O(2), the cytotoxic agent active in photodynamic therapy (PDT), and fluorescence quantum yields (Φ(F)) were measured for the Zn(II) and Mg(II) complexes, [Th(8)TPyzPzZn] and [Th(8)TPyzPzMg(H(2)O)], and the data were compared to those of corresponding octapyridino macrocycles [Py(8)TPyzPzZn] and [Py(8)TPyzPzMg(H(2)O)] and their related octacations [(2-Mepy)(8)TPyzPzZn](8+) and [(2-Mepy)(8)TPyzPzMg(H(2)O)](8+). These measurements were carried out in DMF and in DMF preacidified with HCl (ca. 10(-4) M). All of the examined Zn(II) compounds behave as excellent photosensitizers (Φ(Δ) = 0.4-0.6) both in DMF and DMF/HCl solutions, whereas noticeable fluorescence activity (Φ(F) = 0.36-0.43) in DMF/HCl solutions is shown by the Mg(II) derivatives; these data might provide perspectives for applications in PDT (Zn(II)) and imaging response and diagnosis (Mg(II)).     

Self-assembly of [Et,Et]-bacteriochlorophyll cF on highly oriented pyrolytic graphite revealed by scanning tunneling microscopy

The chlorosomal light-harvesting antennae of green phototrophic bacteria consist of large supramolecular aggregates of bacteriochlorophyll c (BChl c). The supramolecular structure of (3(1)-R/S)-BChl c on highly oriented pyrolytic graphite (HOPG) and molybdenum disulfide (MoS2) has been investigated by scanning tunneling microscopy (STM). On MoS2, we observed single BChl c molecules, dimers or tetramers, depending on the polarity of the solvent. On HOPG, we observed extensive self-assembly of the dimers and tetramers. We propose C=O...H-O...Mg bonding networks for the observed dimer chains, in agreement with former ultraviolet-visible and infrared spectroscopic work. The BChl c moieties in the tetramers are probably linked by four C=O...H-O hydrogen bonds to form a circle and further stabilized by Mg...O-H bondings to underlying BChl c layers. The tetramers form highly ordered, distinct chains and extended two-dimensional networks. We investigated semisynthetic chlorins for comparison by STM but observed that only BChl c self-assembles to well-structured large aggregates on HOPG. The results on the synthetic chlorins support our structure proposition.     

Tetrakis(thiadiazole)porphyrazines. 8. Singlet oxygen production, fluorescence response and liposomal incorporation of tetrakis(thiadiazole)porphyrazine macrocycles [TTDPzM] (M = Mg(II)(H2O), Zn(II), Al(III)Cl, Ga(III)Cl, Cd(II), Cu(II), 2H(I))

The photoactivity for the generation of singlet oxygen, (1)O(2), the key cytotoxic agent in the anticancer treatment known as photodynamic therapy (PDT), and the fluorescence response of the highly electron-deficient tetrakis(thiadiazole)porphyrazines of formula [TTDPzM] (M = Mg(II)(H(2)O), Zn(II), Al(III)Cl, Ga(III)Cl, Cd(II), Cu(II), 2H(I)) were examined (c ? 10(-5) M) in dimethylformamide (DMF) and/or in DMF preacidified with HCl (DMF/HCl; [HCl] = 1-4 × 10(-4) M). The singlet oxygen quantum yield (Φ(Δ)) of all the compounds was determined by using a widely employed procedure based on the selective oxidation of the 1,3-diphenylisobenzofuran (DPBF), modified in part as reported. The list of the Φ(Δ) values indicates excellent photosensitizing properties for the series of compounds carrying "closed shell" metal ions, with values measured in DMF/HCl respectful of the "heavy atom effect" for the first four lighter centers, increasing in the order Mg(II) < Al(III) < Zn(II) < Ga(III). Data of Φ(Δ) concerning the unmetalated species [TTDPzH(2)], present in solution in the form of the corresponding anion [TTDPz](2-), and the Cd(II) and Cu(II) complexes are also presented and discussed. Extensive discussion is also developed on the fluorescence quantum yield values Φ(F), with data on the Mg(II) and Al(III) compounds in DMF/HCl (0.44 and 0.53, respectively) indicative of promising perspectives for applications in fluorescence imaging techniques. The Φ(F) data of the studied porphyrazine series, Φ(F)(Pz), correlate linearly with those of the homologous phthalocyaninato complexes, Φ(F)(Pc), suggesting a closely similar behaviour between the two classes of compounds. The incorporation of [TTDPzZn] into liposomes was successfully achieved following the detergent depletion method (DDM) from a mixed micellar solution by means of gel-filtration. Retention of [TTDPzZn] (~40%) in its photoactive monomeric form into liposomes is proved by absorption and fluorescence spectra, this proposing the Zn(II) complex as a promising candidate for use in PDT.     

STRUCTURE-ACTIVITY RELATIONSHIP OF PORPHINES FOR PHOTOINACTIVATION OF BACTERIA

Abstract— The antibacterial photodynamic effects of uncharged ( o -tetrahydroxyphenyl porphine [THPP], m -THPP and p -THPP), cationic (5,10,15,20-tetra[4- N -methylpyridyllporphine [TMPyP]) and anionic (5,10,15,20-tetra[4-sulfonatophenyl porphine] [TPPS₄]) porphines on Staphylococcus aureus and Escherichia coli bacteria inactivation were examined. The results show that uncharged porphines provoked antibacterial photodynamic activity on S. aureus, and also on E. coli in the presence of the membrane-disorganizing peptide polymixin B nonapeptide (PMNP). The TMPyP compound was highly photoactive toward gram-positive bacteria but only marginally effective on gram-negative cells, whereas TPPS₄ showed no activity on either gram-positive or gram-negative bacteria. The photoactivity of TMPyP is due to the electrostatic attraction between the positively charged sensitizer molecule and the negatively charged membrane of the gram-positive target cells. For TPPS₄, the inactivity toward gram-positive bacteria is due to electrostatic repulsion between the charged sensitizer molecule and the cell membrane. For gram-negative bacteria, the inactivity is conceivably due to preferential (electrostatic) binding to the positively charged PMNP, which is an adjuvant for membrane disorganization, but has no effect on cell viability. For hydrophobic sensitizers, the photoactivity depends on the state of aggregation. The extent of deaggregation of the different THPP isomers was determined by fluorescence measurements of bound sensitizers and could be positively correlated with their photoinactivation capacity. We conclude that the structure-activity relationships of these porphines are affected by their net charge and by aggregation.     

Organometallic oligomers based on 1,8-Diisocyano-p-menthane (dmb): syntheses and characterization of the [[M(diphos)(dmb)]BF4]n and [[Pd2(diphos)2(dmb)](ClO4)2]n materials (M = Cu, Ag; diphos = dppe, dppp)

A new strategy to synthesize organometallic oligomers is presented and consists of using the title diisocyanide and chelated metal fragments with bis(diphenylphosphine)alkanes. The title materials are synthesized by reacting the [M(dppe)(BF4)] and [M2(dppp)2](BF4)2 complexes (M = Cu, Ag; dppe = bis(diphenylphosphino)ethane, dppp = bis(diphenylphosphino)propane) with dmb and the Pd2-bonded d9-d9 Pd2(dmb)2Cl2 dimer with dppe or dppp. The model compounds [M(diphos)(CN-t-Bu)2]BF4 (M = Cu, Ag) and [Pd2(diphos)2(CN-t-Bu)2](ClO4)2 (diphos = dppe, dppp) have been prepared and characterized as well for comparison purposes. Three of the model compounds were also characterized by X-ray crystallography to establish the diphosphine chelating behavior. The materials are amorphous and have been characterized from the measurements of the intrinsic viscosity, DSC, TGA, and XRD, as well as their capacity for making stand-alone films. The intrinsic viscosity data indicate that the Cu and Pd2 materials are oligomeric in solution (approximately 8-9 units), while the Ag materials are smaller. For [[Cu(dppe)(dmb)]BF4]n, a glass transition is reproducibly observed at about 82 degrees C (DeltaCp = 0.43 J/(g deg)), which suggests that these materials are polymeric in the solid state. The Cu and Ag species are luminescent in the solid state at room temperature exhibiting lambda(max) and tau(e) (emission lifetime) around 480-550 nm and 18-48 micros, respectively, while the Pd2 species are not luminescent under these conditions. During the course of this study, the unsaturated [M2(dppp)2](BF4)2 starting materials (M = Cu, Ag) were prepared, one of which (M = Ag) was characterized by crystallography. The bridging behavior of the dppp ligand in this case contrasts with the chelating behavior seen for the saturated [Cu(dppp)(CN-t-Bu)2]BF4 complex.     

Primary Fragmentation Pathways of Gas Phase [M(Uracil−H)(Uracil)]+ Complexes (M=Zn,Cu, Ni,Co, Fe,Mn, Cd,Pd , Mg,Ca, Sr,Ba, and Pb): Loss of Uracil versus HNCO

Complexes formed between metal dications, the conjugate base of uracil, and uracil are investigated by sustained off‐resonance irradiation collision‐induced dissociation (SORI‐CID) in a Fourier transform ion cyclotron resonance (FTICR) mass spectrometer. Positive‐ion electrospray spectra show that [M(Ura?H)(Ura)]⁺ (M=Zn, Cu, Ni, Co, Fe, Mn, Cd, Pd, Mg, Ca, Sr, Ba, or Pb) is the most abundant ion even at low concentrations of uracil. SORI‐CID experiments show that the main primary decomposition pathway for all [M(Ura?H)(Ura)]⁺, except where M=Ca, Sr, Ba, or Pb, is the loss of HNCO. Under the same SORI‐CID conditions, when M is Ca, Sr, Ba, or Pb, [M(Ura?H)(Ura)]⁺ are shown to lose a molecule of uracil. Similar results were observed under infrared multiple‐photon dissociation excitation conditions, except that [Ca(Ura?H)(Ura)]⁺ was found to lose HNCO as the primary fragmentation product. The binding energies between neutral uracil and [M(Ura?H)]⁺ (M=Zn, Cu, Ni, Fe, Cd, Pd ,Mg, Ca, Sr Ba, or Pb) are calculated by means of electronic‐structure calculations. The differences in the uracil binding energies between complexes which lose uracil and those which lose HNCO are consistent with the experimentally observed differences in fragmentation pathways. A size dependence in the binding energies suggests that the interaction between uracil and [M(Ura?H)]⁺ is ion–dipole complexation and the experimental evidence presented supports this.     

Infrared multiphoton spectra from metal dication complexes in the gas phase

Infrared multiphoton spectra have been recorded for the first time from metal dication complexes held in an ion trap. The photofragmentation of [M(pyridine)4]2+ complexes has been observed in the range 920-1090 cm(-1) and for M=Cu2+, Mg2+ and Zn2+. The narrow absorption features are identified as vibrational modes of the pyridine molecule and comparisons with calculations suggest that the Mg2+ and Zn2+ ion complexes have D2d (compressed tetrahedron) structures, and that the Cu2+ complex is probably square-planar (D4h).     

Complexes of nitrilotrimethylphosphonic acid with cobalt, nickel, copper and zinc

Substances of the types MH₄ntmp, Mg₃[M(Hntmp)]₂, M₂H₂ntmp and Mg[M₂(Hntmp)]₂, where M = Co, Ni, Cu, Zn and H₆ntmp = N[CH₂PO(OH)₂]₃ were prepared. The sodium and cesium salts of the [Co(Hntmp)]³⁻ complexes were also prepared. The IR and electronic spectra and the experimental magnetic susceptibilities indicate that these are high-spin complexes. The coordination surroundings of the central atom consist of a highly distorted octahedron of the ligand oxygen atoms. The nitrogen atom is not coordinated to the central atom.     

Synthesis and physicochemical properties of metallobacteriochlorins

Access to metallobacteriochlorins is essential for investigation of a wide variety of fundamental photochemical processes, yet relatively few synthetic metallobacteriochlorins have been prepared. Members of a set of synthetic bacteriochlorins bearing 0-4 carbonyl groups (1, 2, or 4 carboethoxy substituents, or an annulated imide moiety) were examined under two conditions: (i) standard conditions for zincation of porphyrins [Zn(OAc)(2)·2H(2)O in N,N-dimethylformamide (DMF) at 60-80 °C], and (ii) treatment in tetrahydrofuran (THF) with a strong base [e.g., NaH or lithium diisopropylamide (LDA)] followed by a metal reagent MX(n). Zincation of bacteriochlorins that bear 2-4 carbonyl groups proceeded under the former method whereas those with 0-2 carbonyl groups proceeded with NaH or LDA/THF followed by Zn(OTf)(2). The scope of metalation (via NaH or LDA in THF) is as follows: (a) for bacteriochlorins that bear two electron-releasing aryl groups, M = Cu, Zn, Pd, and InCl (but not Mg, Al, Ni, Sn, or Au); (b) for bacteriochlorins that bear two carboethoxy groups, M = Ni, Cu, Zn, Pd, Cd, InCl, and Sn (but not Mg, Al, or Au); and (c) a bacteriochlorin with four carboethoxy groups was metalated with Mg (other metals were not examined). Altogether, 15 metallobacteriochlorins were isolated and characterized. Single-crystal X-ray analysis of 8,8,18,18-tetramethylbacteriochlorin reveals the core geometry provided by the four nitrogen atoms is rectangular; the difference in length of the two sides is ～0.08 ?. Electronic characteristics of (metal-free) bacteriochlorins were probed through electrochemical measurements along with density functional theory calculation of the energies of the frontier molecular orbitals. The photophysical properties (fluorescence yields, triplet yields, singlet and triplet excited-state lifetimes) of the zinc bacteriochlorins are generally similar to those of the metal-free analogues, and to those of the native chromophores bacteriochlorophyll a and bacteriopheophytin a. The availability of diverse metallobacteriochlorins should prove useful in a variety of fundamental photochemical studies and applications.     

Metal-ion-coordinating properties of the dinucleotide 2'-deoxyguanylyl(5'-->3')-2'-deoxy-5'-guanylate (d(pGpG)3-): isomeric equilibria including macrochelated complexes relevant for nucleic acids

The interaction between divalent metal ions and nucleic acids is well known, yet knowledge about the strength of binding of labile metal ions at the various sites is very scarce. We have therefore studied the stabilities of complexes formed between the nucleic acid model d(pGpG) and the essential metal ions Mg2+ and Zn2+ as well as with the generally toxic ions Cd2+ and Pb2+ by potentiometric pH titrations; all four ions are of relevance in ribozyme chemistry. A comparison of the present results with earlier data obtained for M(pUpU)- complexes allows the conclusion that phosphate-bound Mg2+ and Cd2+ form macrochelates by interaction with N7, whereas the also phosphate-coordinated Pb2+ forms a 10-membered chelate with the neighboring phosphate diester bridge. Zn2+ forms both types of chelates with formation degrees of about 91% and 2.4% for Zn[d(pGpG)]cl/N7 and Zn[d(pGpG)]-cl/PO, respectively; the open form with Zn2+ bound only to the terminal phosphate group, Zn[d(pGpG)]-op, amounts to about 6.8 %. The various intramolecular equilibria have also been quantified for the other metal ions. Zn2+, Cu2+, and Cd2+ also form macrochelates in the monoprotonated M[H;d(pGpG)] species (the proton being at the terminal phosphate group), that is, the metal ion at N7 interacts to some extent with the P(O)2(OH)- group. Thus, this study demonstrates that the coordinating properties of the various metal ions toward a pGpG unit in a nucleic acid differ: Mg2+, Zn2+, and Cd2+ have a significant tendency to bridge the distance between N7 and the phosphate group of a (d)GMP unit, although to various extents, whereas Pb2+ (and possibly Ca2+) prefer a pure phosphate coordination.     

Analytical applications of triethylenetetraminehexa-acetic acid: Part 4. amperometric titrations of metal ions

The behaviour of metal ions during titration with triethylenetetraminehexaacetic acid (TTHA) in several supporting electrolytes was investigated by d.c. and square-wave polarography. The composition of the complexes (M₂L or ML) formed during titration is reported as well as the [M] : [L] ratio corresponding to the end-point of amperometric titration. The optimal conditions (by d.c. and s.w. detection) are given for titrations of Cu(II), Ni(II), Cd(II), Pb(II) and Zn(II) based on the reduction current of metal ions, as well as for titrations of Cu(II), Cd(II), Pb(II), Zn(II), Al(III), Ca(II) and Mg(II) based on the anodic current of TTHA. Application of the anodic current of TTHA permits determinations of polarographically inactive metals and is valuable in analysis of mixtures.     

Tetra-2,3-pyrazinoporphyrazines with externally appended pyridine rings. 2. Metal complexes of tetrakis-2,3-[5,6-di(2-pyridyl)pyrazino]porphyrazine: linear and nonlinear optical properties and electrochemical behavior

A series of metal complexes of tetrakis-2,3-[5,6-di(2-pyridyl)pyrazino]porphyrazine, [Py(8)TPyzPzH(2)], having the general formula [Py(8)TPyzPzM].xH(2)O (M = Mg(II)(H(2)O), Mn(II), Co(II), Cu(II), Zn(II); x = 3-8) were synthesized by reaction of the free-base macrocycle with the appropriate metal acetate in pyridine or dimethyl sulfoxide under mild conditions. Clathrated water and retained pyridine molecules for the Mn(II) and Co(II) species are easily eliminated by heating under vacuum, the water molecules being recovered by exposure of the unsolvated macrocycles to air. Magnetic susceptibility measurements and EPR spectra of the materials in the solid state provide basic information on the spin state of the Cu(II), Co(II), and Mn(II) species. Colloidal solutions caused by molecular aggregation are formed in nondonor solvents (CH(2)Cl(2), CHCl(3)), a moderately basic solvent (pyridine), and an acidic solvent (CH(3)COOH), with the extent of aggregation depending on the specific solvent and the central metal ion. UV-vis spectral monitoring of the solutions after preparation indicates that disaggregation systematically occurs as a function of time leading ultimately to the formation of clear solutions containing the monomeric form of the porphyrazine. Cyclic voltammetry and thin-layer spectroelectrochemistry show that each compound with an electroinactive metal ion undergoes four reversible one-electron reductions, leading to formation of the negatively charged species [Py(8)TPyzPzM](n-) (n = 1 - 4). The stepwise uptake of four electrons is consistent with a ring-centered reduction, but in the case of the cobalt complex a metal-centered (Co(II) --> Co(I)) reduction occurs in the first process and only three additional reductions are observed. No oxidations are observed in pyridine or CH(2)Cl(2) containing 0.1 M tetrabuthylammonium perchlorate (TBAP). The nonlinear optical properties (NLO) of the species [Py(8)TPyzPzM] (M = 2H(I), Cu(II), Zn(II), Mg(II)(H(2)O)) have also been examined with nanosecond pulses at 532 nm in dimethyl sulfoxide solution. Reverse saturable absorption is shown by all of the [Py(8)TPyzPzM] species, which exhibit distinct behavior depending on the nature of M and extent of aggregation.     

Electrospray ionization studies of transition-metal complexes of 2-acetylbenzimidazolethiosemicarbazone using collision-induced dissociation and ion-molecule reactions

The complexes of transition-metal ions (M2+, where M = Fe, Co, Ni, Cu, Zn, Cd, and Hg) with 2-acetylbenzimidazolethiosemicarbazone (L) are studied under electrospray ionization (ESI) conditions. The ESI mass spectra of Fe and Co complexes showed the complex ions corresponding to [M+2L-2H]+, and those of Ni and Zn complexes showed [M+2L-H]+ ions, wherein the metal/ligand ratio is 1:2 and the oxidation state of the central metal ion is +3 in the case of Fe and Co and +2 in the case of Ni and Zn. The Cd and Cu complexes showed preferentially 1:1 complex ions, i.e., [M+L-H]+ or [M+L+Cl]+, whereas Hg formed both 1:1 and 1:2 complex ions. During formation of the above complex ions one or two ligands are deprotonated after keto-enol tautomerism, depending on the nature and oxidation state of central metal ion. The structures and coordination numbers of the metal ions in the complex ions were studied by their collision-induced dissociation spectra and ion-molecule reactions with acetonitrile or propylamine in the collision cell. Based on these results it is concluded that Fe, Co, Ni and Zn form stable octahedral complexes, whereas tetrahedral or square planar complexes are formed preferentially for other metals. In addition, the Cu complex showed a [2L+2Cu-3H]+ ion with a Cu-Cu bond.     

Synthesis, characterization, and electrochemical studies of beta,beta'-fused metallocenoporphyrins

Beta,beta'-Fused monoruthenocenylporphyrins, Cp*Ru(III)[1,2-[M(II)-5,10,15,20-tetrakis(3,5-di-tert-butylphenyl)-porphyrinato]-3-methyl-cyclopentadienide] (M = Ni (20), Cu (21), Zn (22)), and bisferrocenoporphyrins, Fe(II) bis[1,2-[M(II)-5,10,15,20-tetraphenylporphyrinato]-3-methyl-cyclopentadienide] (M = Ni (24), Cu (25), Zn (26)), were synthesized and characterized. A novel synthetic approach to beta,beta'-fused porphyrins through Pd(0)-catalyzed [3 + 2] cycloaddition was implemented in this work. UV-vis spectra of these compounds show largely broadened and red-shifted bands (relative to their precursors) indicating potential electronic communication between the attached organometallic moiety and the porphyrin core. The electrochemistry of these molecules suggests significant electronic interactions between the metallocene and metalloporphyrin in molecules 20 and 24. The crystal structure of the bisferrocenoporphyrin 26, Fe(II) bis[1,2-[Zn(II)-5,10,15,20-tetraphenylporphyrinato]-3-methyl-cyclopentadienide], was determined: [Cp2Fe[ZnTPP(THF)]2][Cp2Fe[ZnTPP(THF)ZnTPP(MeOH)]].3MeOH.6THF, M = 3804.35, monoclinic, space group P21/c, a = 33.327(5) A, b = 19.145(3) A, c = 29.603(5) A, beta = 106.309(2) degrees , V = 18128(5) A3, Z = 4. In this molecule, one porphyrin ring is rotated by about 72 degrees with respect to the other in the 5-fold axis of the Cp ring.     

Carboxyphenyl metalloporphyrins as photosensitizers of semiconductor film electrodes. A study of the effect of different central metals

Free-base (P), Zn(II) (P(Zn)), Cu(II) (P(Cu)), Pd(II) (P(Pd)), Ni(II) (P(Ni)), and Co(II) (P(Co)) 5-(4-carboxyphenyl)-10,15,20-tris(4-methylphenyl) porphyrins were designed and synthesized to be employed as spectral senzitizers in photoelectrochemical cells. The dyes were studied adsorbed on SnO(2) nanocrystalline semiconductor and also in Langmuir-Blodgett film ITO electrodes in order to disclose the effect of molecular packing on the studied properties. Electron injection yields were obtained by fluorescence quenching analysis comparing with the dyes adsorbed on a SiO(2) nanocrystalline insulator. Back electron-transfer kinetics were measured by using laser flash photolysis. The unmetallized and metallized molecules have different singlet state energies, fluorescence quantum yields, and redox properties. The quantum yields of sensitized photocurrent generation are shown to be highly dependent on the identity of the central metal. It is shown that P(Ni) and P(Co) do not present a photoelectric effect. The other porhyrins present reproducible photocurrent, P(Pd) being the one that gives the highest quantum yield even in closely packet ITO/LB films. Photocurrent quantum yields increase as the dye ground-state oxidation potential becomes more anodic, which is in agreement with the observation, obtained by laser flash photolysis, that back electron-transfer kinetics decrease with the increase in the driving force for the recombination process. This effect could be exploited as a design element in the development of new and better sensitizers for high-efficiency solar cells involving porphyrins and related dyes.