A Schiff base expanded porphyrin macrocycle that acts as a versatile binucleating ligand for late first-row transition metals

The coordination chemistry of the Schiff base polypyrrolic octaaza macrocycle 1 toward late first-row transition metals was investigated. Binuclear complexes with the divalent cations Ni(II), Cu(II), and Zn(II) and with the monovalent cation Cu(I) were prepared and characterized. Air oxidation of the Cu(I) ions in the latter complex to their divalent oxidation state resulted in a change in the coordination mode relative to the macrocycle.     

Silica-supported alkylidene complexes: their preparation, characterization and reactivity, especially towards olefins

When silica is employed as a bidentate ligand to bis(alkyl) complexes of the early first-row transition metals, rare terminal alkylidene fragments are readily formed and stabilized. These materials have the empirical formula (SiO)₂M=CHEMe₃, where M is Ti, V, or Cr and E is C or Si, incorporating two covalent interactions with oxygens of the silica surface which serve to immobilize the grafted complexes. The silica-supported alkylidene complexes undergo several typical reactions, including electrophilic cleavage with H⁺ and Br₂, metathetical exchange with acetone and styrene, and addition of a silane. Reaction with ethylene leads to oligomerization/polymerization, for which the material with M=Cr is particularly effective. The mechanism of initiation of polymerization, as well as the influence of the silica support (fumed versus precipitated) on the composition of the active sites, is discussed.     

The effect of metal cations on the nature of the first electronic transition of liquid water as studied by attenuated total reflection far-ultraviolet spectroscopy

The first electronic transition (?←X?) of liquid water was studied from the perspective of the hydration of cations by analyzing the attenuated total reflection far-ultraviolet (ATR-FUV) spectra of the Group I, II, and XIII metal nitrate electrolyte solutions. The ?←X? transition energies of 1 M electrolyte solutions are higher (Li(+): 8.024 eV and Cs(+): 8.013 eV) than that of pure water (8.010 eV) and linearly correlate with the Gibbs energies of hydration of the cations. The increases in the ?←X? transition energies are mostly attributable to the hydrogen bond formation energies of water molecules in the ground state induced by the presence of the cations. The deviation from the linear relation was observed for the high charge density cations, H(+), Li(+), and Be(2+), which reflects that the electronic energies in the excited states are also perturbed. Quantum chemical calculations show that the ?←X? transition energies of the water-cation complexes depend on the hydration structures of the cations. The calculated ?←X? transition energies of the water molecules hydrating high charge density cations spread more widely than those of the low charge density cations. The calculated transition energy spreads of the water-cation complexes directly correlate with the widths of the ?←X? transition bands measured by ATR-FUV spectroscopy.     

Encapsulation of tetraazamacrocyclic complexes of cobalt(II), copper(II) and oxovanadium(IV) in zeolite-Y and their use as catalysts for the oxidation of styrene

Encapsulation of tetraazamacrocyclic complexes of Co(II), Cu(II) and V(IV) into zeolite-Y has been accomplished, and the resulting materials were used as heterogeneous catalysts for aerobic oxidation of styrene. The materials were prepared by a ship-in-a-bottle method, in which the transition metal cations were first ion-exchanged into zeolite-Y and then reacted with ethylenediamine, followed by acetylacetone. The pure tetraazamacrocyclic complexes were characterized by FTIR, solid UV–Vis and elemental analysis. The structural integrity throughout the immobilization procedure, the successful immobilization of the macrocyclic complexes, and the loadings of metal ions and macrocyclic ligands were determined by characterization techniques such as FTIR, diffuse reflection UV–Vis, inductively coupled plasma atomic emission spectroscopy, scanning electron microscopy, TG/DTA and powder X-ray diffraction. Compared with their homogeneous analogues, the catalytic properties of the encapsulated macrocyclic complexes in the oxidation of styrene with air were investigated. The immobilized complexes proved to be active catalysts and could be reused without significant loss in activity.     

Redox-active ligands as a challenge for electronic structure methods

To improve the catalytic activity of 3d transition metal catalysts, redox-active ligands are a promising tool. These ligands influence the oxidation state of the metal center as well as the ground spin-state and make the experimental determination of both properties challenging. Therefore, first-principles calculations, in particular employing density functional theory with a proper choice of exchange-correlation (xc) functional, are crucial. Common xc functionals were tested on a simple class of metal complexes: homoleptic, octahedral tris(diimine) iron(II) complexes. The spin-state energy splittings for most of these complexes showed the expected linear dependence on the amount of exact exchange included in the xc functionals. Even though varying redox-activity affects the electronic structure of the complexes considerably, the sensitivity of the spin-state energetics to the exact exchange admixture is surprisingly small. For iron(II) complexes with highly redox-active ligands and for a broad range of ligands in the reduced tris(diimine) iron(I) complexes, self-consistent field convergence to local minima was observed, which differ from the global minimum in the redox state of the ligand. This may also result in convergence to a molecular structure that corresponds to an energetically higher-lying local minimum. One criterion to detect such behavior is a change in the sign of the slope for the dependence of the spin-state energy splittings on the amount of exact exchange. We discuss possible protocols for dealing with such artifacts in cases in which a large number of calculations makes checking by hand unfeasible.     

Matrix-assisted laser desorption/ionization studies on transition metal complexes of benzimidazole thiosemicarbazones

The transition metal (M=Fe, Co, Ni, Cu, Zn, Cd and Hg) complexes of 2- acetylbenzimidazolethiosemicarbazone (L(1)) and 1-methyl 2-acetylbenzimidazole-thiosemicarbazone (L(2)) are analyzed by MALDI using HCCA, THP, MMNPD and DMN as the matrices. All the MALDI spectra are clean without any contribution from the complex ions resulted by multiple proton addition/removal. All the complexes, except Cu, irrespective of the matrix used, show 1:2 complex ions wherein two ligands (neutral or deprotonated) complex with the metal ion depending on the nature and stable oxidation state of the central metal ion viz., [M + 2L - 2H](+) ion for Fe and Co complexes (+3 oxidation state) and [M + 2L - H](+) ion for Ni, Zn, Cd and Hg (+2 oxidation state). The Cu complex show 1:1 complex ion corresponding to [2M + 2L - 2H](+) ions. When HCCA is used as a matrix, the complex ions due to ligand exchange by matrix are also found, and this process is relatively more if a neutral ligand is bound to the metal ion in the original complex ion. The type of complex ions found under MALDI experiments are similar to those found under ESI experiments. However, the complex ions due to reduction of Cu are found only in the MALDI analysis of Cu complexes.     

Synthesis,spectroscopic, DNA cleavage and antibacterial activity of binuclear schiff base complexes

The binuclear Schiff base complexes are formed newly using different transition metals at their stable oxidation state as Cu(II), Ni(II), and VO(II). 3,3′,4,4′-tetraminobiphenyl and 2-aminobenzaldehyde were condensed to form a new Schiff base ligand having an two N₄ group responsible for better chelating to the metal centers. The ligand and their complexes have been established by analytical, spectral and electrochemical data. The interaction studies of the complexes with CT-DNA were carried out using cyclic voltammetry, viscosity measurements and fluorescence spectroscopy. The free ligand and their metal complexes were screened for their antimicrobial activities against the following species: Klebsiella pneumoniae, Escherichia coli and Staphylococcus aureus. A comparative study of minimum inhibitory concentration (MIC) values of the Schiff base and its complexes indicate that the metal complexes exhibit higher antibacterial activity than the free ligand.     

A geometrically constraining bis(benzimidazole) ligand and its nearly tetrahedral complexes with Fe(II) and Mn(II)

2,2'-Bis[2-(1-propylbenzimidazol-2-yl)]biphenyl), 4, and its bis complexes with Fe(II) and Mn(II) have been prepared and characterized structurally and spectroscopically. Ligand 4 adopts an open, "trans" conformation in the solid state with the benzimidazole (BzIm) groups on opposite sides of the biphenyl unit. In its complexes with metal ions, a "cis" conformation is observed, and 4 behaves as a geometrically constraining bidentate ligand with four planar groups connected by three "hinges". Reaction of 4 with Fe(II) or Mn(II) yielded isomorphous crystals (space group Pnn2) of Fe(II)(4)2.(ClO4)2 and Mn(II)(4)2.(ClO4)2, in which the M(II)(4)2 cations exhibit distorted-tetrahedral coordination geometries (N-M-N angles, 109 +/- 11 degrees ) enforced by rigid, chiral nine-membered M(4) rings in the twist-boat-boat conformation. Individually, the cations show R,R or S,S stereochemistry, and the crystals are racemates. Mn(II)(4)2.(ClO4)2 exhibits a quasi-reversible Mn(II) --> Mn(III) oxidation at E(1/2) = 0.64 V; the corresponding Fe(II) --> Fe(III) oxidation occurs at E(1/2) = 1.76 V. The electrochemical stability of the Fe(III) oxidation state in this system suggests the possibility of isolating an unusual pseudotetrahedral Fe(III)N(BzIm)(4) species. Ultraviolet spectra of the iron and manganese complexes are dominated by absorptions of the ligand 4 blue-shifted by approximately 2000-3000 cm(-1). Ligand-field absorptions were observed for the Fe(II) complex; those for the Mn(II) complex were obscured by tailing ultraviolet absorptions. Electron paramagnetic resonance and magnetic susceptibility measurements are consistent with a high-spin Mn(II) complex, while for the Fe(II) complex, the falloff of the magnetic moment with decreasing temperature is indicative of zero-field splitting with D approximately 4 cm(-1).     

Fine tuning of the oxidation locus, and electron transfer, in nickel complexes of pro-radical ligands

A large number of complexes of the first-row transition metals with non-innocent ligands has been characterized in the last few years. The localization of the oxidation site in such complexes can lead to discrepancies when electrons can be removed either from the metal center (leading to an M((n+1)+) closed-shell ligand) or from the ligand (leading to an M(n+) open-shell ligand). The influence of the ligand field on the oxidation site in square-planar nickel complexes of redox-active ligands is explored herein. The tetradentate ligands employed herein incorporate two di-tert-butylphenolate (pro-phenoxyl) moieties and one orthophenylenediamine spacer. The links between the spacer and both phenolates are either two imines ([Ni(L1)]), two amidates ([Ni(L3)]2-), or one amidate and one imine ([Ni(L2)]-). The structure of each nickel(II) complex is presented. In the noncoordinating solvent CH2Cl2, the one-electron-oxidized forms are ligand-radical species with a contribution from a singly occupied d orbital of the nickel. In the presence of an exogenous ligand, such as pyridine, a Ni(III) closed-shell ligand form is favored: axial ligation, which stabilizes the trivalent nickel in its octahedral geometry, induces an electron transfer from the metal(II) center to the radical ligand. The affinity of pyridine for the phenoxylnickel(II) species is correlated to the N-donor ability of the linkers.     

Influence of Cations on the Formation of Cobalt(II) Complexes with Thiocyanate Ions in Solutions of Nonionic Micelles

The influence of additives of alkali, alkaline-earth, and several transition metal cations, protonated amines, and quaternary ammonium on the state of the tetrahedral cobalt(II) thiocyanate complex is studied in an aqueous solution of the nonionogenic surfactant Triton X-100. It is shown that alkali and alkaline-earth metal cations and compounds containing protonated primary amino groups favor the formation of additional amounts of the micellar-bonded [Co(NCS)₄]^2– complex anion. This fact is explained by the interaction of these cations with the oxyethylene chains of the nonionogenic surfactant as was observed in the crown ether coordination. This provides the formation and transfer into micelles of additional amounts of their associates with [Co(NCS)₄]^2–. The Mn²⁺, Ni²⁺, and Cd²⁺ cations decompose cobalt tetrathiocyanate due to the formation of their own complexes with the ligand. This effect is not observed in the case of the quaternary ammonium compounds, which is explained by their incapability of coordinating the oxyethylene chains of the nonionogenic surfactant.     

Isolation and characterization of stable iron(i) sulfide complexes

The first examples of iron(I) sulfide complexes are presented, in contrast with the +2 and +3 oxidation states that are well-known in synthetic and biological systems. Spectroscopic and computational studies show a high-spin d(7) configuration at the metal. Alkali metal cations play a key role in supporting the unusually low oxidation state.     

Fluorescent and electroactive cyclic assemblies from perylene tetracarboxylic acid bisimide ligands and metal phosphane triflates

Tetraaryloxy-substituted perylene tetracarboxylic acid bisimides with one or two 4-pyridyl receptor substituents at the imide functionality were synthesized and employed in transition metal directed self-assembly with Pd(II) and Pt(II) phosphane triflates. Upon mixing of the components, quantitative formation of functional molecular square-type complexes containing four dye molecules and model complexes of a 2:1 (perylene bisimide ligand:transition metal ion) stoichiometry was observed. The isolated metallosupramolecular squares were characterized by 1H and 31P [1H] NMR spectroscopy as well as conventional electrospray ionization (ESI) and ESI-FTICR mass spectrometry, which gave evidence for the structure and the high stability of these giant cyclic dye assemblies (molecular weight (3a) 8172, Pt-Pt corner diagonal ca. 3.4 nm). Studies of the optical absorption and fluorescence properties and the electrochemistry and spectroelectrochemistry of both the perylene bisimide ligands and the perylene bisimide metal complexes show that Pt(II) coordination does not interfere with the optical and electrochemical properties of the perylene bisimide ligands; this gives squares with high fluorescence quantum yields (phiF (3a)=0.88) and three fully reversible redox couples. The latter could be unambiguously related to quantitative formation of perylene bisimide radical cations (E1/2 = +0.93 V vs. Fc/Fc+), radical anions (E1/2= - 1.01 V vs. Fc/Fc+), and dianions (E1/2 = -1.14 V vs. Fc/Fc+); these redox reactions change the charge state of the cyclic assembly from +12 to zero. In contrast, Pd(II) coordination influenced the electrochemical properties of the assembly because of an irreversible palladium reduction at E1/2= -1.15 V versus Fc/Fc+. Finally, dynamic ligand exchange processes between different metallosupramolecular assemblies were investigated by multinuclear NMR and electrospray mass spectrometry. These studies confirmed the reversible nature of the pyridine-Pt(II)/Pd(II) coordination process.     

Three-coordinate metal centers in extended transition metal oxides

The n = 3 Ruddlesden-Popper phase Sr3LaFe1.5Co1.5O10+/-delta is capable of sustaining O contents as low as O7.5 with a mean metal oxidation state of +2 and three coordination at the central site in the trilayer of originally octahedral transition metal sites. The shortening of the axial bonds to the flanking octahedral layers stabilizes the low oxidation state and consequent unusual low coordination number of the Fe2+ and Co2+ cations within the extended structure.     

Manganese(II): the black sheep of the organometallic family

The organometallic chemistry of manganese in the +2 oxidation state is distinct from the organometallic chemistry of a 'typical' transition metal due to a significant ionic contribution to the manganese(II)-carbon bonds. The reduced influence of covalency and the 18-electron rule result in organomanganese(II) cyclopentadienyl, alkyl and aryl complexes possessing reactivity and structural diversity that is unique in organotransition metal chemistry. Recently, this unusual reactivity has resulted in a range of novel applications in selective organometallic and organic synthesis, and polymerization catalysis. This tutorial review summarizes key milestones in the development of manganese(II) organometallics and discusses how some of their current synthetic applications have evolved from many fascinating fundamental studies in the area.     

Binding and Ionophoric Properties of Polythioamide Compounds. Ag(I) and Hg(II) Selectivities

The synthesis of tetrathiolactams and related di- and tetrathioamide compounds is described. The formation constants of their heavy-metal complexes are determined by using the strong UV absorption of the thioamide chromophore. Extraction and transport abilities of tetrathioamide ionophores show selectivities for Ag(I) and Hg(II) cations over alkali, alkaline-earth or other heavy metal cations including transition metals such as Co(II).     

Bond valence sums in coordination chemistry. The calculation of the oxidation state of samarium in complexes containing samarium bonded only to oxygen

A simple method is presented for calculating the oxidation state of Sm in complexes where Sm is bonded only to O ligands. A total of 88 SmO(n)() fragments with n = 4-12 were retrieved from the Cambridge Structural Database and were analyzed using the bond valence sum (BVS) method. New R(0) values for Sm(II)-O of 2.116(21) A and for Sm(III)-O of 2.055(13) A were derived. The average R(0) value of 2.086 A gives a good approximation of the oxidation state of the Sm ion, either +2 or +3, from the observed distances without any assumptions. The Sm-O distances for +2 and +3 complexes with coordination numbers of 4-11 are tabulated and reflect the requirement that the BVS must equal the oxidation state. The distances for CN = 12 were not included because of problems with the reported crystal structures. Several X-ray structure determinations where the BVS and the oxidation state did not agree are discussed.     

Free Aluminylenes: An Emerging Class of Compounds

Aluminylenes (R−Al) are aluminium analogs of carbenes. In contrast to isolable carbenes, aluminylenes are extremely rare species. In the past years, pioneered by Schnöckel, Roesky and Power, a few free aluminylenes and their complexes have been reported. Such compounds have the aluminium atom in the oxidation state +I, which contrasts with classical organoaluminium derivatives that contain the element in the +III oxidation state. Aluminylenes, either in their free state or in the coordination sphere of a Lewis base, are capable of coordinating to transition metals and activating inert chemical bonds. Free aluminylenes are emerging as potent synthetic platforms for unusual aluminium species.     

Radical ion reactivity—I : Application of the dewar-zimmerman rules to certain reactions of radical anions and cations

The application of the Dewar-Zimmerman rules to the interaction between radical cations, derived from 4n + 2 systems, shows that a nucleophile orbital interacting suprafacially with the ion should correspond to an antiaromatic transition state and hence to a less favored pathway relative to competing ones, e.g. electron transfer. An antarafacial interaction would on the other hand correspond to an aromatic transition state and be energetically favorable. Both types of interaction are generally possible for the same reagent, but since the suprafacial one for geometric reasons ensues and thus predominates in the early stage of the reaction, the net result should be an anomalously low reactivity of radical cations vs attack by nucleophiles.By calibration against known, qualitative reactivity data for perylene radical cation it is strongly indicated that halide ions belong to a class of reagents which do not react nucleophilically with radical cations but instead undergo electron transfer oxidation or do not react at all. This type of reaction is discussed in some detail. and several mechanisms involving radical cation/halide ion combination as a critical step can either be ruled out or considered open for reinvestigation.The same idea can be applied to the reaction between radical anions and electrophiles. Accordingly, the protonation of radical anions derived from 4n+2 aromatic systems is remarkably slow, as compared to that of analogous carbanions.     

The mechanism of the (bispidine)copper(II)-catalyzed aziridination of styrene: a combined experimental and theoretical study

Experimental and DFT-based computational results on the aziridination mechanism and the catalytic activity of (bispidine)copper(I) and -copper(II) complexes are reported and discussed (bispidine=tetra- or pentadentate 3,7-diazabicyclo[3.1.1]nonane derivative with two or three aromatic N donors in addition to the two tertiary amines). There is a correlation between the redox potential of the copper(II/I) couple and the activity of the catalyst. The most active catalyst studied, which has the most positive redox potential among all (bispidine)copper(II) complexes, performs 180 turnovers in 30 min. A detailed hybrid density functional theory (DFT) study provides insight into the structure, spin state, and stability of reactive intermediates and transition states, the oxidation state of the copper center, and the denticity of the nitrene source. Among the possible pathways for the formation of the aziridine product, the stepwise formation of the two N-C bonds is shown to be preferred, which also follows from experimental results. Although the triplet state of the catalytically active copper nitrene is lowest in energy, the two possible spin states of the radical intermediate are practically degenerate, and there is a spin crossover at this stage because the triplet energy barrier to the singlet product is exceedingly high.