A short desymmetrization protocol for the coordination environment in bis-salphen scaffolds

A remarkable short preparation of desymmetrized bis-salphen scaffolds is presented. The protocol consists of an hydroxide-mediated hydrolysis of Lewis acidic bis-Zn(salphen) complexes yielding C(s)-symmetric diimine/amine salts that can be selectively transformed into bis-salphens with dissymmetric substitution patterns within each salphen unit under mild conditions. These isolated nonsymmetrical bis-salphen derivatives do not show signs of imine scrambling or decomposition due to a metal template effect. A possible rationale is provided for the formation and isolation of one of the intermediate bis-phenolate salts, and the hypothesis involves H-bond directed hydrolysis of the nearest located imine bond across the bis-salphen scaffold.     

Zn(II)-salphen complexes as versatile building blocks for the construction of supramolecular box assemblies

Zn(II)-salphen complexes are readily accessible and interesting supramolecular building blocks with a large structural diversity. Higher-order supramolecular assemblies, such as molecular boxes based on a bis-Zn(II)-salphen building block and various ditopic bipyridine ligands, have been constructed by means of supramolecular, coordinative Zn(II)-N(pyr) interactions. The use of bipyridine ligands of differing sizes enables the construction of structures with predefined box diameters. The features of the 2:2 box assemblies were investigated in detail by (variable temperature) NMR spectroscopy, UV-visible spectroscopy, NMR titrations, and X-ray crystallographic studies. The spectroscopic studies reveal a high association constant for the Zn(II)-salphen-pyridyl motif, which lies in the range 10(5)-10(6) M(-1). The strong interaction between the Zn(II) center and pyridine donors was supported by PM3 calculations that showed a relatively high Lewis acid character of the metal center in the salphen complex. Titration curves monitored by UV-visible show a cooperative effect between the two bipyridine ligands upon complexation to the bis-Zn(II) template, suggesting the formation of 2:2 complexes. The crystal structures of two supramolecular boxes have been determined. In both examples such a 2:2 assembly is present in the solid state, and the box size is different because they consist of different building blocks. Interestingly, the box assemblies line up in the solid state to form porous channels that are potentially useful in a number of applications.     

Shape‐Persistent (Pt‐salphen)2 Phosphorescent Coordination Frameworks: Structural Insights and Selective Perturbations

The development of molecular frameworks derived from binuclear platinum(II) aromatic Schiff base (salphen) complexes and their supramolecular chemistry have been undertaken. A series of axially rotating (Pt‐salphen)₂ luminophores, tethered in a cofacial manner by a rigid linker (xanthene, 1 ; dibenzofuran, 2 ; biphenylene, 3 ), was synthesized in which the O(salphen) groups are potentially amenable for guest‐binding. The molecular structures of 1 and 3 have been determined by X‐ray crystallography, revealing intra‐ and intermolecular π‐stacking interactions, as well as contrasting syn ( 1 ) and anti ( 3 ) configurations, for the (Pt‐salphen)₂ moiety. All complexes are luminescent in solution at room temperature. Their photophysical and solvatochromic properties have been examined, and the emissions are assigned to mixed triplet O(p)/Pt(d)→π*(diimine) excited states. The red‐shifted fluid emissions and lower quantum yields of 1 and 3 , relative to 2 , are ascribed to enhanced intramolecular π‐stacking interactions. Photophysical changes and selective responses to metal ions (particularly Pb²⁺) have been investigated by using various spectroscopic methods and DFT calculations, and through comparative studies with control complexes. A plausible binding mechanism is proposed based on occupation of the O(salphen)‐binding cavity, which induces perturbation of intramolecular π–π interactions, and hence the self‐quenching and emission properties, of the (Pt‐salphen)₂ unit.     

Extremely strong self-assembly of a bimetallic salen complex visualized at the single-molecule level

A bis-Zn(salphen) structure shows extremely strong self-assembly both in solution as well as at the solid-liquid interface as evidenced by scanning tunneling microscopy, competitive UV-vis and fluorescence titrations, dynamic light scattering, and transmission electron microscopy. Density functional theory analysis on the Zn(2) complex rationalizes the very high stability of the self-assembled structures provoked by unusual oligomeric (Zn-O)(n) coordination motifs within the assembly. This coordination mode is strikingly different when compared with mononuclear Zn(salphen) analogues that form dimeric structures having a typical Zn(2)O(2) central unit. The high stability of the multinuclear structure therefore holds great promise for the development of stable self-assembled monolayers with potential for new opto-electronic materials.     

Autocatalytic demetalation of a Zn(salphen) complex provoked by unprotected N-heterocycles

The interaction of Zn(salphen) complex with biologically important structures such as (benz)imidazoles and purine has been studied and revealed in the case of unprotected purine and (benz)imidazole derivatives a demetalated product, whereas for structurally related 1-methyl-(benz)imidazole the formation of 1 : 1 coordination complexes was evidenced by NMR, MS and X-ray crystallography.     

Coordination chemistry of a pi-extended, rigid and redox-active tetrathiafulvalene-fused Schiff-base ligand

A pi-extended, redox-active tetradentate tetrathiafulvalene-fused salphen [salphen = N,N'-phenylenebis(salicylideneimine)] compound (L) was prepared via a direct Schiff-base condensation of the corresponding diamine-tetrathiafulvalene (TTF) precursor with salicylaldehyde. Its chelating coordination ability has been demonstrated by the formation of the corresponding transition metal complexes in the presence of M(OAc)2.nH2O (M = Co(II), Ni(II), Cu(II)) and FeCl3.6H2O. Three complexes have been characterized by single-crystal X-ray diffraction analysis showing that the TTF-salphen ligand coordinates to the metal ions in a planar mode through the nitrogen and oxygen atoms in a N2O2 cis-configuration. In the case of Fe(III), a dinuclear oxo-bridged Fe(III) complex is formed. These paramagnetic complexes are promising building blocks for the construction of dual functional materials due to their unique structural features (planarity and rigidity) as well as their inherent redox properties.     

Comparison of the spectroscopic properties of π-conjugated, fused salphen triads embedded with Zn-homo-, Ni-homo-, and Ni/Zn-heteronuclei

Stepwise condensation reactions of 2,6-dihydroxynaphthalene-1,5-dicarbaldehyde and a phenylenediamine with concomitant binding of metal ions afforded a trinuclear complex of a fully π-conjugated, fused salphen ligand. By changing the synthetic pathway, we obtained a series of homo- and heteronuclear complexes containing selected combinations of nickel(II) and zinc(II) ions. Comparison of the trinuclear complexes' spectroscopic features with those of analogous dinuclear complexes revealed that the absorption spectrum of each trinuclear complex is composed of a salphen-centered absorption at 400 nm and a naphthalene-centered absorption around 500-600 nm, suggesting that the π-conjugated system is divided into several compartments, each of which independently undergoes electronic excitation. Molecular orbital calculations revealed that the formal fusion of the salphen moieties increases the highest occupied molecular orbital (HOMO) level by ～0.4 eV, which in turn causes the low-energy absorption observed in the spectra. In contrast, interorbital interactions mediated by the N(2)O(2) metal coordination site are small, even though this site is bridged by an o-phenylene linkage. These results suggest that the coordination site effectively breaks electronic communication between the compartments, which in turn affect various spectroscopic properties of the π-conjugated metallo-polysalphens.     

Copper(II) complexes with derivatives of salen and tetrahydrosalen: a spectroscopic, electrochemical and structural study

Salen type complexes, CuL, the corresponding tetrahydrosalen type complexes, Cu[H₄]L, and N,N′-dimethylated tetrahydrosalen type complexes, Cu[H₂Me₂]L, were investigated using cyclic voltammetry, and electronic and ESR spectroscopy. In addition, the analogous copper(II) complexes with a derivative of the tetradentate ligand ‘salphen’ [salphen=H₂salphen=N,N′-disalicylidene-1,2-diaminobenzene] were studied. Solutions of CuL, Cu[H₄]L and Cu[H₂Me₂]L are air-stable at ambient temperature, except for the complex Cu(^tBu, Me)[H₄]salphen [H₂(^tBu, Me)[H₄]salphen=N,N′-bis(2-hydroxy-3-tert-butyl-5-methylbenzyl)-1,2-diaminobenzene]. Cu(^tBu, Me)[H₄]salphen interacts with dioxygen and the ligand is oxidatively dehydrogenated (–CH₂–NH–→–C=N–) to form Cu(^tBu, Me)[H₂]salphen and finally, in the presence of base, Cu(^tBu, Me)salphen. X-ray structure analysis of Cu(^tBu, Me)[H₂Me₂]salen confirms a slightly tetrahedrally distorted planar geometry of the CuN₂O₂ coordination core. The complexes were subjected to spectrophotometric titration with pyridine, to determine the equilibrium constants for adduct formation. It was found that the metal center in the complexes studied is only of weak Lewis acidity. In dichlormethane, the oxidation Cu(II)/Cu(III) is quasireversible for the CuL type complexes, but irreversible for the Cu[H₄]L and Cu[H₂Me₂]L type. A poorly defined wave was observed for the irreversible reduction Cu(II)/Cu(I) at potentials less than −1.0 V. The ESR spectra of CuL at both 77 K and room temperature reveal that very well resolved lines can be attributed to the interaction of an unpaired electron spin with the copper nuclear spin, ¹⁴N donor nuclei and to a distant interaction with two equivalent protons [A^Cu(iso)≈253 MHz, A^N(iso)≈43 MHz, A^N(iso)≈20 MHz]. These protons are attached to the carbon atoms adjacent to the ¹⁴N nuclei. In contrast to CuL, the number of lines in the spectra of the complexes Cu[H₄]L and Cu[H₂Me₂]L is greatly reduced. At room temperature, only a quintet with a considerably smaller nitrogen shf splitting constant [A^N(iso)≈27 MHz] is observed. Both factors, planarity and conjugation, are thus essential for the observation of distant hydrogen shf splitting in CuL. Due to the C=N bond hydrogenation, the coordination polyhedra of the complexes Cu[H₄]L and Cu[H₂Me₂]L is more flexible and more sensitive to ligand modification than that of CuL. The electron-withdrawing effect of the phenyl ring of the phenylenediamine bridge is reflected in a reduction of the copper hyperfine coupling constants in Cu(^tBu, Me)[H₄]salphen and Cu(^tBu, Me)[H₂Me₂]salphen complexes [A^Cu(iso)≈215 MHz].     

A highly selective colorimetric aqueous sensor for mercury

A new colorimetric mercury sensor is reported based on binding to terpyridine derivatives. It is able to selectively detect Hg II ions over a number of environmentally relevant ions including Ca II, Pb II, Zn II, Cd II, Ni II, Cu II, and others. The response time upon exposure to Hg II is instantaneous. By the "naked eye," the detection limit of Hg II is 2 ppm (25 microM) in solution. With a spectrometer, this detection limit is increased down to 2 ppb (25 nM), which is the current EPA standard for drinking water. The significant problem of mercury poisoning requires new methods of detection that are sensitive and selective. Here we report a new simple system that takes advantage of the unique optical properties generated by terpyiridine-Hg complexes.     

Rigid bis-zinc(II) salphen building blocks for the formation of template-assisted bidentate ligands and their application in catalysis

The template-induced formation of chelating bidentate ligands by the selective self-assembly of two monodentate pyridyl phosphorus ligands on a rigid bis-zinc(II) salphen template with two identical binding sites was studied. Using UV-vis, NMR-spectroscopy and X-ray analysis the formed structures were unambiguously proven. The application of these templated bidentate ligands in transition metal catalysis showed, in most cases, typical bidentate character. Compared to previous work based on a more flexible bis-zinc(II) porphyrin template, the current catalytic data suggest that the rigidity of the template is not an important factor for the improvement of the regio- and enantioselectivity under the applied reaction conditions.     

Molecular structure and spectroscopic studies on novel complexes of coumarin-3-carboxylic acid with Ni(II), Co(II), Zn(II) and Mn(II) ions based on density functional theory

Novel Ni(II), Co(II), Zn(II) and Mn(II) complexes of coumarin-3-carboxylic acid (HCCA) were studied at experimental and theoretical levels. The complexes were characterised by elemental analyses, FT-IR, (1)H NMR, (13)C NMR and UV-Vis spectroscopy and by magnetic susceptibility measurements. The binding modes of the ligand and the spin states of the metal complexes were established by means of molecular modelling of the complexes studied and calculation of their IR, NMR and absorption spectra at DFT(TDDFT)/B3LYP level. The experimental and calculated data verified high spin Ni(II), Co(II) and Mn(II) complexes and a bidentate binding through the carboxylic oxygen atoms (CCA2). The model calculations predicted pseudo octahedral trans-[M(CCA2)(2)(H(2)O)(2)] structures for the Zn(II), Ni(II) and Co(II) complexes and a binuclear [Mn(2)(CCA2)(4)(H(2)O)(2)] structure. Experimental and calculated (1)H, (13)C NMR, IR and UV-Vis data were used to distinguish the two possible bidentate binding modes (CCA1 and CCA2) as well as mononuclear and binuclear structures of the metal complexes.     

DNA binding and biological activity of some platinum(II) intercalating compounds containing methyl-substituted 1,10-phenanthrolines

This study documents the first detailed investigation into the relationship between molecular structure and biological activity of platinum(II) complexes containing methylated derivatives of 1,10-phenanthroline (phen). A series of square planar platinum(II) compounds incorporating methylated derivatives of phen, 4-methyl-1,10-phenanthroline (4-Mephen), 5-methyl-1,10-phenanthroline (5-Mephen), 4,7-dimethyl-1,10-phenanthroline (4,7-Me2phen), 5,6-dimethyl-1,10-phenanthroline (5,6-Me2phen) and 3,4,7,8-tetramethyl-1,10-phenanthroline (3,4,7,8-Me4phen) were synthesised and the relationship between their structure and biological activity investigated. The biological activity of these compounds was quantified using the in vitro cytotoxicity assay against the L1210 Murine leukaemia cell line. Large variation in cytotoxicities with different methylation was observed. The 5- and 5,6-methylated derivatives of phen displayed a greater biological activity, with IC50 values of 2.8 +/- 0.8 microM and 1.5 +/- 0.3 microM respectively, compared with the phen compound, with an IC50 value of 9.7 +/- 0.3 microM, while all the others were inactive with IC50 values over 50 microM. Binding constants were determined using circular dichroism spectroscopy (CD) and induced circular dichroism (ICD). ICD was used to highlight any differences in the spectra. Viscometry studies and linear dichroism (LD) experiments indicate that the platinum(II) complexes intercalate although for [Pt(en)(4-Mephen)]Cl2 and [Pt(en)(4,7-Me2phen)]Cl2 this mode of binding appears to be concentration dependent. The binding of the platinum(II) complexes to the oligonucleotide d(GTCGAC)2 was studied using two-dimensional 1H NMR spectroscopy. The addition of each metal complex to the hexamer d(GTCGAC)2 produced upfield shifts of the metal complex resonances, characteristic of intercalation. Through the observation of NOE cross-peaks, two-dimensional NMR studies provided insight into the site and groove preferences of these compounds when binding to DNA.     

Interaction of Cu(II) and Cd(II) ions with DNA from Spirulina platensis

Equilibrium dialysis and atomic absorption analysis were used to obtain adsorption isotherms and determine the stoichiometric binding constants of Cu(II) and Cd(II) ions to DNA from Spirulina platensis in solutions. The stoichiometric constants of Cu(II) and Cd(II) ions with DNA from S. platensis in 3 mM NaCI are 15.56⋅10⁴ and 14.40⋅10⁴, respectively. Effect of ionic strength and DNA GC content on binding constants of Cu(II)- and Cd(II)-DNA complexes were studied out. It was showed that the binding constants of Cu(II)- and Cd(II)-DNA complexes decrease with increase of ionic strength. The empirical dependences of logK on the GC content has been derived for Cd(II)- and Cu(II)-DNA complexes.     

Bis-(thiosemicarbazonato) Zn(II) complexes as building blocks for construction of supramolecular catalysts

In this paper we report the application of bis-(thiosemicarbazonato) Zn(II) complexes as building blocks in the construction of supramolecular transition metal assemblies. We investigated their coordination behaviour towards pyridylphosphine molecules and found these systems comparable to those based on Zn(porphyrin) and Zn(salphen) complexes. Additionally, catalytic experiments and an in situ high-pressure FTIR study of the supramolecular rhodium hydroformylation catalysts, assembled using the bis-(thiosemicarbazonato) Zn(II) complexes, demonstrate their applicability in supramolecular catalysis and their potential for application in other areas of supramolecular chemistry.     

Metal complexes of porphycene,corrphycene, and hemiporphycene: stability and coordination chemistry

Porphyrin (P), porphycene (Pc), corrphycene (Cn), and hemiporphycene (Hpc) represent a series of well defined "4-N in" constitutional porphyrin isomers. These isomers, in the form of their octaethyl derivatives, represent a congruent set of porphyrinoids whose properties can be compared. In this study we report how variations in electronic structure and nitrogen-core size in the free-base forms of these four systems are reflected in the properties of their corresponding metal complexes. Specifically, the effects that these differences have on the axial ligation properties of the Zn(II), Mg(II), Ni(II), and Co(II) complexes of P, Pc, Cn, and Hpc in toluene using pyridine as the axial ligand are detailed. Also reported are the relative stabilities of these complexes under acidic conditions. It is shown that for the zinc, magnesium, and cobalt complexes, there are distinct differences in the ability to maintain four-, five-, or six-coordinate geometries in the presence of similar concentrations of pyridine. By contrast, no apparent differences in axial ligand binding affinity are seen for the four nickel complexes. Little difference in stability was likewise seen when these same complexes were subject to acid-mediated demetallation, with all four falling into stability class II, according to the accepted porphyrin stability ranking system. High stabilities were also seen in the case of the cobalt complexes, with the Pc and Cn complexes being of stability class III and the P and Hpc derivatives falling into stability class II. The Zn(II) and Mg(II) complexes were all far less stable than the corresponding Ni(II) and Co(II) complexes. In this case, semiquantitative analyses of the rate of acid-induced decomposition revealed the following stability sequence P>Cn>Hpc>Pc for both the Zn(II) and Mg(II) complexes. Single-crystal X-ray diffraction structures were solved for the Zn(II), Mg(II), and Ni(II) complexes of the octaethyl derivatives of Hpc, Cn, and Pc as well as a Co(II) octamethylcorrphycene and are reported as part of this study. These solid-state structures confirm four-coordinate species for the Ni(II) complexes, four- and five-coordinate species for the Mg(II) and Zn(II) complexes, and a six-coordinate species for the lone Co(II) complex.     

Synthesis of Cyclic Carbonates Catalysed by Chromium and Aluminium Salphen Complexes

Chromium and aluminium salphen complexes have been found to display remarkable catalytic activity in the synthesis of cyclic carbonates from a range of epoxides and carbon dioxide. The Al(salphen) complex is more reactive towards terminal epoxides at ambient temperature and pressure, whereas the Cr(salphen) complex exhibits higher catalytic activity towards more challenging internal epoxides at elevated temperature and pressure.     

Bis(oxazoline) Lewis acid catalyzed aldol reactions of pyridine N-oxide aldehydes--synthesis of optically active 2-(1-hydroxyalkyl)pyridine derivatives: development, scope, and total synthesis of an indolizine alkaloid

A new, short, and simplified procedure for the synthesis of optically active pyridine derivatives from pro-chiral pyridine-N-oxides is presented. The catalytic and asymmetric Mukaiyama aldol reaction between ketene silyl acetals and 1-oxypyridine-2-carbaldehyde derivatives catalyzed by chiral copper(II)-bis(oxazoline) complexes gave optically active 2-(hydroxyalkyl)- and 2-(anti-1,2-dihydroxyalkyl)pyridine derivatives in good yields and diastereoselectivities, and in excellent enantioselectivities-up to 99 % enantiomeric excess. As a synthetic application of the developed method, a full account for the asymmetric total synthesis of a nonnatural indolizine alkaloid is provided.     

Synthesis and photophysical properties of multinuclear zinc-salophen complexes: enhancement of fluorescence by fluorene termini

Incorporation of fluorene groups into the salicylidene moiety significantly enhances the luminescence of a number of multinuclear alkynylated Zn(II)-salophen complexes. Preparation of these complexes was achieved by a synthetic strategy with facile handling of the reactants, simple purification of the products, and one-pot reaction process. Two synthetic methods are used for the preparation of different types of multinuclear salophen complexes. The introduction of a bis- or a tris-salicylaldehyde as a bridging unit in the presence of various alkynyl substituted monoimines in the reaction mixture containing zinc acetate resulted in the preparation of di- and tri-nuclear Zn(II)-salophen complexes of type 1, respectively. For a different type, treatment of tetraminobenzene with various arylethynyl-substituted salicylaldehyde afforded dinuclear Zn(II) alkynylated salophen complexes of type 2 with a different structure. The photophysical behaviors of these multinuclear metal salophen complexes were investigated. Particularly, the dinuclear complex 9b of type 1 having ethynylfluorene groups in salophen moieties and dialkoxyl groups in the bridging moiety exhibits higher quantum efficiency than that of other complexes in this report. In addition, the bis-Zn(II) alkynylated salophen complex 11e bearing nitrogen donor groups displays more red-shifted pattern than those with other functional substituents both in absorption and emission spectra.     

Eilatin as a bridging ligand in ruthenium(II) complexes: synthesis,crystal structures,absorption spectra,and electrochemical properties

The potential of the heptacyclic aromatic alkaloid eilatin (1), that features two nonequivalent binding sites, to serve as a bridging ligand is reported. The nonequivalency of the binding sites allowed the selective synthesis of both mono- and dinuclear complexes. The mononuclear Ru(II) complexes [Ru(dmbpy)(2)(eilatin)](2+) (2) and [Ru(tmbpy)(2)(eilatin)](2+) (3) in which eilatin selectively binds "head-on" were synthesized and employed as building blocks in the synthesis of the dinuclear complexes [[Ru(dmbpy)(2)](2)(mu-eilatin)](4+) (4) and [[Ru(tmbpy)(2)](2)(mu-eilatin)](4+) (5). Complete structure elucidation of the complexes in solution was accomplished by 1D and 2D NMR techniques. The X-ray structures of the mononuclear complex 3 and of the two dinuclear complexes 4 and 5 were solved, and absorption spectra and electrochemical properties of the complexes were explored. Both dinuclear complexes formed as racemic mixtures in a 3:1 diastereoisomeric ratio, the major isomer being the heterochiral one (Delta Lambda/Lambda Delta) as revealed by crystallography. The mononuclear complexes feature an exceptionally low energy MLCT band around 600 nm that shifted to over 700 nm upon the binding of the second Ru(II) center. The mononuclear complexes show one reversible oxidation and several reversible reduction waves, the first two reductions being substantially anodically shifted in comparison with [Ru(bpy)(3)](2+), attributed to the reduction of eilatin, and consistent with its low lying pi* orbital. The dinuclear complexes follow the same reduction trend, exhibiting several reversible reduction waves, and two reversible well-resolved metal centered oxidations due to the nonequivalent binding sites and to a significant metal-metal interaction mediated by the bridging eilatin.     

Access to multinuclear salen complexes using olefin metathesis

The use of olefin metathesis as a construction tool for multimetallic salen-based structures is described. The approach involves mono- and diallyl-functionalized metallosalen complexes that can be directly coupled by metathesis leading to dimetallic species or mixtures of linear and cyclic oligomers. The metathesis of bis-allyl Ni(salen) complexes has been studied in detail. At high concentration it is possible to selectively obtain di-Ni species rather than heavier oligomers while under dilute conditions cyclic rather than linear oligomers are preferentially obtained. A mono-allyl Zn(salphen) complex was efficiently coupled using metathesis to give the di-Zn(salphen) product, which was subsequently transmetalated with a variety of metals to yield dimetallic salens of potential catalytic interest. Finally, a tetranuclear Zn(4) macrocycle was also prepared using buildings blocks obtained by metathesis from commercially available precursors. The methods described herein allow for the facile construction of multi-centered Schiff base complexes of catalytic or supramolecular interest.