Hydroformylation: a versatile tool for the synthesis of new β-formyl-metalloporphyrins

Formyl derivatives of protoporphyrin-IX dimethyl ester metal complexes were obtained via hydroformylation reactions, catalysed by rhodium-triphenylphosphine complexes. The regioselectivity of the reaction is remarkably dependent on the metal centre of the porphyrin, yielding 100% of the branched aldehyde with zinc(II) complexes and 75% with the nickel(II). The NMR characterisation of the new compounds was carried out after their derivatisation into acetals.     

The pH-controlled hydrothermal synthesis and crystal structures of two zinc N,N'-piperazinebis(methylenephosphonate) frameworks

An exploration of the reactions of N,N'-piperazinebis(methylenephosphonic acid), H4L, with zinc salts has led to the isolation of two new framework zinc phosphonates. ZnLH2.H2O (I) is isostructural with previously reported manganese(II) and cobalt(II) analogues, and consists of infinite 'zinc phosphate' chains bridged into three dimensions via the organic moieties. The resulting framework encloses large channels in which the loosely bound H2O resides. The H2O is lost reversibly at around 160 degree C, without framework collapse. Zn2L (II) has a novel framework structure, prepared at an initial pH > 7, which consists of two-dimensional 'zinc phosphate' sheets, comprising both four- and eight-membered -Zn-O-P- rings, which are also linked into three dimensions via the organic groups. In both cases, the zinc centre is tetrahedral; in I coordination is by oxygen atoms from four different phosphonate groups, whereas in II the additional deprotonation of the ligand allows coordination via three oxygen atoms plus the amine nitrogen atom.     

Synthesis and Spectral Investigation of Some Transition Metal Complexes Containing Pentadentate Macroacyclic <Emphasis Type="Italic">NNNNN</Emphasis>-donor Schiff Base Ligands

A new Schiff base, 2,6-diacetylpyridine bis(2-hydrazinobenzothiazole) (DPHB), has been designed, and synthesized by the condensation of 2,6-diacetylpyridine with 2-hydrazinobenzothiazole, and structurally characterized. Copper(II), cobalt(II), nickel(II), manganese(II), zinc(II), cadmium(II) and oxovanadium(IV) complexes of DPHB have been synthesized for the first time. Their structures have been elucidated on the basis of elemental analyses, conductance measurements, magnetic properties, spectral (i.r., ¹H-n.m.r., u.v.–vis., e.p.r. and FAB-mass) and thermal studies. The complexes exhibit an octahedral geometry around the metal centre. The conductance data of all the complexes suggest them to be 1:1 electrolytes. The X-band e.p.r. spectra of the copper(II) and oxovanadium(IV) complexes in the polycrystalline state at room (300 K) and liquid nitrogen temperature (77 K) were recorded and their salient features are reported. Thermal stabilities of the manganese(II) and zinc(II) complexes have been studied.     

Anion control of isomerism, crystal packing and binding properties in a mononuclear zinc complex

The coordination chemistry of the tetradentate pyridyl N-donor ligand cis-3,5-bis-[2-pyridinyleneamin]-trans-hydroxycyclohexane (DDOP) has been investigated with zinc(II) nitrate and triflate. The resulting complexes, [Zn(DDOP)(H₂O)(NO₃)](NO₃) (1), and [Zn(DDOP)(H₂O)(OTf)](OTf) (2) differ not only in their counterions, but also the arrangement of the axial ligands and their solid state hydrogen bonded networks. Isothermal titration calorimetry was used to assess the difference in binding properties exhibited by the two zinc complexes at physiological pH in an aqueous environment. A series of coordinating amino acids were found to preferentially bind to the mononuclear zinc triflate (1) complex over the corresponding nitrate (2) assembly, with histidine exhibiting a two centre binding mode.     

Application of Supramolecular Bidentate Hybrid Ligands in Asymmetric Hydroformylation

In this study we report a novel class of supramolecular bidentate hybrid ligands in which the two inequivalent phosphorus units and pyridine moieties are covalently attached to a chiral scaffold and the supramolecular interactions are used as a second handle to control the coordination sphere around the transition‐metal centre. The coordination chemistry of these ligands was investigated under hydroformylation conditions by high‐pressure NMR and IR spectroscopy, revealing the formation of a single active species in which the phosphane ligand is in the axial position and the phosphoramidite adopts the equatorial position. These ligands were applied in the asymmetric Rh‐catalysed hydroformylation of styrene and para‐substituted analogues. In these hydroformylation reactions, modification of the electronic and steric properties of the zinc(II)‐templates appear to have a significant influence on the activity and selectivity of the catalysis. In particular, zinc(II)‐templates bearing more electron‐withdrawing substituents led to an increase in enantioselectivity.     

Deprotonation of zinc(II)-water and zinc(II)-alcohol and nucleophilicity of the resultant zinc(II) hydroxide and zinc(II) alkoxide in double-functionalized complexes: theoretical studies on models for hydrolytic zinc enzymes

The factors governing the deprotonation ability of zinc(II)-water and zinc(II)-alcohol and nucleophilicity of the resultant zinc(II) hydroxide and zinc(II) alkoxide as complex models for zinc enzymes have been investigated through Hartree-Fock and density-functional theory methods with the 6-311++G(d,p) basis set. Our calculations showed that in these double-functionalized complexes (i.e., zinc complexes having both a zinc(II)-alcohol motif and a zinc(II)-water motif) zinc(II)-alcohol is preferred in deprotonation over zinc(II)-water (i.e., zinc(II)-alcohol has a much lower pK(a) than zinc-coordinated water in the same molecule). Natural bond orbital analysis revealed that zinc(II) alkoxides are more nucleophilic than their respective counterparts zinc(II) hydroxides. The analysis of the transition state in the transformation reaction from zinc(II) hydroxide species to zinc(II) alkoxide species indicates that zinc(II) alkoxides are the preferred deprotonated species not only thermodynamically but also kinetically. Further examination of the proposed mechanisms of the zinc(II) alkoxide-promoted transesterification path and the zinc(II) hydroxide-promoted hydrolysis path revealed the structures of the intermediates and energy diagrams in the reactions. These results, entitled double-functionalized complexes, for the first time, put a firm theoretical foundation of why the zinc(II)-alcoholic OH is a better model for hydrolytic zinc enzymes (having both stronger acidity and better nucleophilicity).     

Kronenthioetherkomplexe des Blei(II), Zink(II) und Cadmium (II) und Cadmium(II). Die Kristallstrukturen von [PbL2(ClO4)2] und [ZnL2](ClO4)2 · CH3CN (L = 1,4,7 - Trithiacyclononan)

Crown Thioether Complexes of Lead (II), Zinc(II), and Cadmium (II). Crystal Structures of [PbL₂(ClO₄)₂] and [ZnL₂](ClO₄)₂ · CH₃CN (L = 1,4,7 - Trithiacyclononane) The reaction of 1,4,7-trithiacyclononane (L) with the perchlorate salts of lead(II) and zinc(II) in CH₃CN (2:1) affords colorless crystals of [PbL₂(ClO₄)₂] and [ZnL₂](ClO₄)₂ · CH₃CN, respectively, The crystal structures have been determined. The Pb^II centre is coordinated to six sulfur atoms (the average distance Pb? S is 3.076 Å) and two oxygen atoms, one of each ClO₄^? anion (monodentate ClO₄^?). A distorted square antiprismatic polyhedron is thus generated. In [ZnL₂](ClO₄)₂ · CH₃CN the zinc(II) centre is octahedrally surrounded by six sulphur atoms (average distance Zn? S = 2.494 Å); the ClO₄^? anions are not coordinated. For[CdL₂](ClO₄)₂ · H₂O an analogous structure is proposed.     

Can Co(II) or Cd(II) substitute for Zn(II) in zinc fingers?

Zinc finger domains consist of sequences of amino acids containing cysteine and histidine residues tetrahedrally coordinated to a zinc ion. The role of zinc in a DNA binding finger was considered purely structural due to the absence of redox chemistry in zinc. However, whether other metals e.g. Co(II) or Cd(II) can substitute Zn(II) is not settled. For an answer the detailed interaction of Co(II) and Cd(II) with cysteine methylester and histidine methylester has been investigated as a model for the zinc core in zinc fingers. The study was extended to different temperatures to evaluate the thermodynamic parameters associated with these interactions. The results suggest that zinc has a unique role.     

Kinetically controlled separation of cadmium(II) from zinc(II) with dithizone in the presence of nitrilotriacetic acid.

The extraction rates of cadmium(II) and zinc(II) with dithizone (H2dz) in the presence of nitrilotriacetic acid (NTA) were measured, and the possible kinetic separation of cadmium(II) from zinc(II) was investigated. Upon the addition of NTA, the difference in the extraction rate between cadmium(II) and zinc(II) became large. Based on the observed rate constant under the condition [NTA] = 1 x 10(-2) mol dm-3, [H2dz]org = 1 x 10(-3) mol dm-3, and pH = 7.0, the shaking time required for the quantitative separation of cadmium(II) from zinc(II) was calculated to be between 326 and 995 s. The experimental results agreed with the prediction, and the quantitative separation of cadmium(II) from zinc(II) was performed within the above-mentioned range of shaking times.     

Photoluminescent columnar zinc(II) bimetallomesogen of tridentate [ONO]-donor Schiff base ligand

A new zinc(II) bimetallomesogenic complex, [Zn₂L₂], of tridentate [ONO]-donor Schiff base ligand (L = N-(2-hydroxyethyl)-4-hexadecyloxysalicylaldimines) was synthesised and their mesomorphic and photoluminescence properties were investigated. The compounds were characterised by Fourier transform infrared spectroscopy (FTIR), ¹H and ¹³C nuclear magnetic resonance (NMR), ultraviolet-visible spectroscopy (UV-Vis) spectroscopy, elemental analyses and fast atom bombardment (FAB) mass spectrometry. The mesomorphic behaviour of the complex was investigated by polarised optical microscopy, differential scanning calorimetry and X-ray diffraction (XRD) study. A rectangular or oblique columnar mesophase is conjectured on the basis of powder X-ray diffraction (PXRD) study. The complex is found to be blue light emitter in solution, in solid and in condensed states with broad emission maxima at ～427–464 nm. The density functional theory (DFT) calculations revealed a distorted square planar structure around each zinc(II) centre in the dinuclear framework. Time-dependent DFT spectral correlative study was undertaken to account for the electronic transition.     

Effect of Varying Alkyl Chain Length on Thermal Decomposition Temperature of Zinc(II) Xanthates and its Impact on Curing of Epoxy Resin

Linear and branched zinc(II) xanthates with varying alkyl chain length were synthesized and characterized by ¹H NMR, ¹³C NMR, and IR spectroscopy, as well as elemental analysis. Zinc sulfide as the final decomposition product upon thermal annealing of zinc(II) xanthates was confirmed by XRD analysis. Cure time for epoxy resin composite at various temperatures was analyzed employing zinc(II) xanthates (5 % mass) as latent cure catalysts. XRD investigation of the cured epoxy resin including zinc(II) xanthates upon thermal annealing revealed the presence of ZnS in‐situ in the composite matrix, indicating the in‐situ thermal decomposition of zinc(II) xanthates as probable mechanism for curing. Thermogravimetric analysis was performed to investigate the thermal decomposition temperature trend of zinc(II) xanthates. A parallel trend was observed correlating the thermal decomposition temperature trend of zinc(II) xanthates and the order of curing catalytic efficiency utilizing zinc(II) xanthates. In the case of linear alkylzinc(II) xanthates with an increase in the alkyl chain length, both thermal decomposition temperature and the cure time were enhanced. In contrast, in case of branched alkyl chain zinc(II) xanthates with increasing alkyl chain length show decreasing thermal decomposition temperature as well as cure time.     

Universal Anticancer Cu(DTC)2 Discriminates between Thiols and Zinc(II) Thiolates Oxidatively

Aerobic organisms must rely on abundant intracellular thiols to reductively protect various vital functional units, especially ubiquitous zinc(II) thiolate sites of proteins, from deleterious oxidations resulting from oxidizing environments. Disclosed here is the first well‐defined model study for reactions between zinc(II) thiolate complexes and copper(II) complexes. Among all the studied ligands of copper(II), diethyldithiocarbamate (DTC) displays a unique redox‐tuning ability that enables copper(II) to resist the reduction by thiols while retaining its ability to oxidize zinc(II) thiolates to form disulfides. This work proves for the first time that it is possible to develop oxidants to discriminate between thiols and zinc(II) thiolates, alluding to a new chemical principle for how oxidants, especially universal anticancer Cu(DTC)₂, might circumvent the intracellular reductive defense around certain zinc(II) thiolate sites of proteins to kill malignant cells.     

Decoupling optical and potentiometric band gaps in pi-conjugated materials

Syntheses, optical spectroscopy, potentiometric studies, and electronic structural calculations are reported for two classes of conjugated (porphinato)metal oligomers that feature a meso-to-meso ethyne-bridged linkage topology. One set of these systems, bis[(5,5'-10,20-bis[3,5-bis(3,3-dimethyl-1-butyloxy)phenyl]porphinato)zinc(II)]ethyne (DD), 5,15-bis[[5'-10',20'-bis[3,5-di(3,3-dimethyl-1-butyloxy)phenyl]porphinato)zinc(II)]ethynyl]-10,20-bis[3,5-di(9-methoxy-1,4,7-trioxanonyl)phenyl]porphinato)zinc(II) (DDD), and 5,15-bis[[15' '-(5'-10',20'-bis[3,5-bis(3,3-dimethyl-1-butyloxy)phenyl]porphinato)zinc(II)]-[(5' '-10' ',20' '-bis[3,5-di(9-methoxy-1,4,7-trioxanonyl)phenyl]porphinato)zinc(II)]ethyne]ethynyl]-10,20-bis[3,5-di(9-methoxy-1,4,7-trioxanonyl)phenyl]porphinato)zinc(II) (DDDDD), constitute highly soluble analogues of previously studied examples of this structural motif having simple 10,20-diaryl substituents, while a corresponding set of conjugated oligomers, [(5-10,20-bis[3,5-bis(3,3-dimethyl-1-butyloxy)phenyl]porphinato)zinc(II)]-[(5'-15'-ethynyl-10',20'-bis[10,20-bis(heptafluoropropyl)porphinato)zinc(II)]ethyne (DA), 5,15-bis[[5'-10',20'-bis[3,5-di(3,3-dimethyl-1-butyloxy)phenyl]porphinato)zinc(II)]ethynyl]-10,20-bis(heptafluoropropyl)porphinato]zinc(II) (DAD), and 5,15-bis[[15' '-(5'-10',20'-bis[3,5-bis(3,3-dimethyl-1-butyloxy)phenyl]porphinato)zinc(II)]-[(5' '-(10' ',20' '-bis(heptafluoropropyl)porphinato)zinc(II)]ethyne]ethynyl]-10,20-bis[3,5-di(9-methoxy-1,4,7-trioxanonyl)phenyl]porphinato)zinc(II) (DADAD), features alternating electron-rich and electron-poor (porphinato)zinc(II) units. Electrooptic and computational data for these species demonstrate that it is possible to engineer conjugated oligomeric structures that possess highly delocalized singlet (S1) excited states yet manifest apparent one-electron oxidation and reduction potentials (E1/20/+ and E1/2-/0 values) that are essentially invariant with respect to those elucidated for their constituent monomeric precursors.     

Zn2+'s ability to alter the distribution of Cu2+ among the available binding sites of Aβ(1-16)-polyethylenglycol-ylated peptide: implications in Alzheimer's disease

The formation of mixed copper(II) and zinc(II) complexes with Aβ(1-16)-PEG has been investigated. The peptide fragment forms stable mixed metal complexes at physiological pH in which the His13/His14 dyad is the zinc(II)'s preferred binding site, while copper(II) coordination occurs at the N-terminus also involving the His6 imidazole. Copper(II) is prevented by zinc(II) excess from the binding to the two His residues, His13 and His14. As the latter binding mode has been recently invoked to explain the redox activity of the copper-Aβ complex, the formation of ternary metal complexes may justify the recently proposed protective role of zinc(II) in Alzheimer's disease. Therefore, the reported results suggest that zinc(II) competes with copper for Aβ binding and inhibits copper-mediated Aβ redox chemistry.     

The synthesis and fluorescent properties of analogues of the zinc(II) specific fluorophore zinquin ester

The synthesis and fluorescent properties in the absence and presence of zinc(II) of a range of 2-substituted derivatives of N-(6-methoxy-2-methyl-8-quinolyl)-4-methylbenzenesulfonamide are described. These analogues formed complexes with zinc(II) as indicated by a bathochromic shift in their UV/vis spectra. Analogues with isobutenyl and isobutyl side chains at the 2-position formed fluorescent complexes whose fluorescence was stronger than that of the 2-methyl-containing parent. These derivatives were converted, via conversion to the phenol with boron tribromide and reaction with ethyl bromoacetate, to systems with ester-containing side chains analogous to zinquin ester, a specific cellular fluorophore for zinc(II). All of these ester derivatives formed complexes with zinc(II) resulting in a bathochomic shift in their UV/vis spectra. Compounds with isobutyl, isobutenyl, and styryl side chains exhibited increased fluorescence compared to that of zinquin ester in the presence of zinc(II). The compound with the 2-isobutyl side chain was more selective in its fluorescence for zinc(II) over cadmium(II) compared to zinquin ester.     

Ion-pair formation of a copper(II)-ammine complex with an anionic surfactant and the recovery of copper(II) from ammonia medium by the surfactant-gel extraction method.

The extraction and separation of copper(II), zinc(II), cobalt(II), and cadmium(II) were investigated. Both copper(II) and zinc(II) formed ammine-complexes, while cadmium(II) and cobalt(II) formed hydroxide precipitates in an ammonia medium. By the addition of sodium dodecylsulfate (SDS), a copper(II) complex formed an ion-pair (copper-ammine-DS), which was extracted into the SDS phase. However, a zinc(II) complex did not form an ion-pair, and was soluble in water. Copper(II) ion was recovered by stripping (back-extraction) after the addition of hydrochloric acid. This method was applied to the separation of copper(II) in a brass alloy.     

Balance between fluorescence enhancement and association affinity in fluorescent heteroditopic indicators for imaging zinc ion in living cells

A fluorescent heteroditopic indicator for the zinc(II) ion possesses two different zinc(II) binding sites. The sequential coordination of zinc(II) at the two sites can be transmitted into distinct fluorescence changes. In the heteroditopic ligand system that our group developed, the formations of mono- and dizinc(II) complexes along an increasing gradient of zinc(II) concentration lead to fluorescence enhancement and an emission bathochromic shift, respectively. The extents of these two changes determine the sensitivity and, ultimately, the effectiveness of the heteroditopic indicator in quantifying zinc(II) ion over a large concentration range. In this work, a strategy to increase the degree of fluorescence enhancement upon the formation of the monozinc(II) complex of a heteroditopic ligand under simulated physiological conditions is demonstrated. Fluorination of the pyridyl groups in the pentadentate N,N,N'-tris(pyridylmethyl)ethyleneamino group reduces the apparent pK(a) value of the high-affinity site, which increases the degree of fluorescence enhancement as the monozinc(II) complex is forming. However, fluorination impairs the coordination strength of the high-affinity zinc(II) binding site, which in the triply fluorinated ligand reduces the binding strength to the level of the low-affinity 2,2'-bipyridyl. The potential of the reported ligands in imaging zinc(II) ion in living cells was evaluated. The subcellular localization properties of two ligands in five organelles were characterized. Both benefits and deficiencies of these ligands were revealed, which provides directions for the near future in this line of research.     

Design and photophysical properties of zinc(II) porphyrin-containing dendrons linked to a central artificial special pair

The click chemistry synthesis and photophysical properties, notably photo-induced energy and electron transfers between the central core and the peripheral chromophores of a series of artificial special pair-dendron systems (dendron = G1, G2, G3; Gx = zinc(II) tetra-meso-arylporphyrin-containing polyimides) built upon a central core of dimethylxanthenebis(metal(II) porphyrin) (metal = zinc, copper), are reported. The dendrons act as singlet and triplet energy acceptors or donors, depending on the dendrimeric systems. The presence of the paramagnetic d(9) copper(II) in the dendrimers promotes singlet-triplet energy transfer from the zinc(II) tetra-meso-arylporphyrin to the bis(copper(II) porphyrin) unit and slow triplet-triplet energy transfer from the central bis(copper(II) porphyrin) fragment to the peripheral zinc(II) tetra-meso-arylporphyrin. If bis(zinc(II) porphyrin) is the central core, evidence for chain folding is observed; this is unambiguously demonstrated by the presence of triplet-triplet energy transfer in the heterobimetallic systems, a process that can only occur at short distances.     

Syntheses and crystal structures of nickel(II), copper(II), and zinc(II) complexes with a biphenyl-bridged bis(pyrrole-2-yl-methyleneamine) ligand

The reactions of nickel(II), copper(II), and zinc(II) acetate salts with a potentially tetradentate biphenyl-bridged bis(pyrrole-2-yl-methyleneamine) ligand yielded three complexes with different coordination geometries. X-ray crystal structural analysis reveals that in the nickel(II) complex each nickel is five-coordinate, distorted trigonal bipyramid. In the copper(II) complex, each copper is four-coordinate, between square planar and tetrahedral. In the zinc(II) complex, each zinc is four-coordinate with a distorted tetrahedral geometry and the molar ratio of the zinc and ligand is 1 : 2.     

Bis­(saccharinato‐κN)zinc(II) com­plexes with N,N′‐bidentate 2‐amino­methyl­pyridine and 2‐amino­ethylpyridine

The structures of trans‐bis[2‐(amino­methyl)­pyridine‐κ²N,N′]­bis­(saccharinato‐κN)­zinc(II), [Zn(C₇H₄NO₃S)₂(C₆H₈N₂)₂], (I), and [2‐(amino­ethyl)­pyridine‐κ²N,N′]bis­(saccharinato‐κN)­zinc(II), [Zn(C₇H₄NO₃S)₂(C₇H₁₀N₂)], (II), exhibit octa‐ and tetrahedrally coordinated Zn^II atoms, respectively. The di­amine ligands behave as N,N′‐bidentate ligands, while saccharinate (sac) is coordinated through the N atom. In (I), the complex lies about an inversion centre with the Zn atom disordered and displaced by 0.256 (2) Å from a centre of symmetry towards a sac N atom. The crystal structure of (I) is stabilized by N—H⋯O hydrogen bonds and the crystal packing of (II) is determined by hydrogen bonding as well as weak π–π stacking interactions between the sac ligands.