Cyclodextrin phosphanes as first and second coordination sphere cavitands

The binding properties of two alpha-cyclodextrins, each containing two C(5)-linked "CH(2)PPh(2)" units, L 1 (A,D-substituted) and L 2 (A,C-substituted), have been investigated. Both ligands readily form transition-metal chelate complexes in which the metal centres are immobilised at the cavity entrance. Although diphosphane L 1 displays a marked tendency to behave only as a trans-spanning ligand, the ligand possesses a certain degree of flexibility, for example, allowing the stabilisation of a trigonal silver(I) complex in which the bite angle drops to 143 degrees. Another feature of L 1 concerns its ability to function as an hemilabile ligand. Together with four methoxy groups anchored onto the primary face, the two P(III) centres of L 1 form a circularly arranged P(2)O(4) 12-electron donor set able to complex an Ag(+) ion in a dynamic way, each of the four oxygen atoms coordinating successively to the silver ion. Furthermore, the particular structures of L 1 and L 2, characterised by the presence of P(III) units lying close to the cavity entrance, lead upon complexation to complexes whereby the first coordination sphere is partly entrapped in the cyclodextrin. Thus, when treated with metal chlorides, both ligands systematically produce complexes in which the Mbond;Cl unit is maintained inside the cyclodextrin through weak Cl.H-5 interactions. The chelate complex [Ag(L 1)]BF(4) reacts with acetonitrile in excess to afford a mixture of two equilibrating complexes, [Ag(acetonitrile)(L 1)]BF(4) and [Ag(acetonitrile)(2)(L 1)]BF(4), whose coordinated nitriles lie inside the cyclodextrin cavity. The inner-cavity ligands can be substituted by a benzonitrile molecule. The present study provides the first identification of an [Ag(acetonitrile)(2)(phosphane)(2)](+) ion. The unexpected stabilisation of this species probably rests on a cavity effect, the cyclodextrin walls favouring recombination of the complex after facile dissociation of the nitrile ligands.     

Crown-ether functionalised second coordination sphere palladium catalysts by molecular imprinting

Functionalisation of the second coordination sphere of a molecularly imprinted Pd complex was achieved by localising within the polymeric cavity a crown-ether receptor capable of altering the catalytic activity of the reactive site.     

Aromatic garlands,as new foldamers,to mimic protein secondary structure

Proteins modulate the majority of all biological functions and are composed of highly organized secondary structural elements such as helices, turns, and sheets. Many of these functions are affected by a small number of key structural element, protein–protein interactions. Their mimicry by peptide and non-peptide scaffolds has become a major focus of contemporary research. This paper examines oligomeric system as new foldamers, which either reproduce the local topography of the helix, or project appropriately functionality in a similar manner to residues of an alpha-helix.     

Simulation of metal ion coordination sphere in crystals with fluorite structure

Using quantum-chemical methods, it has been found that the structure of fluorite coincides with the symmetry and coordination number of central calcium atom in the (O_h)-Ca₇F₁₄ cluster. The Ca–F and F–F interatomic distances in the cluster are 3–4% shorter than in the crystal. The symmetry of the (O_h)–(ScCa₆F₁₄)⁺ cluster mimicking the cationic defect after the β-decay of ⁴⁵Ca does not correspond to the energy minimum. The increase in the cation charge from 1.79 to 2.80 a. u. reduces the radius of the first coordination sphere by 0.14 Å. For ytterbium dihalides, the bond lengths Yb–F 2.344, Yb–Cl 2.897 Å and the cation charges 1.81, 1.64 a. u., respectively, have been found.

     

Effect of the second coordination sphere on the electron transition energies in ZnS-Mn

The influence of the second coordination sphere on the energy of electron transitions in ZnS-Mn is investigated using the crystal field theory. The obtained perturbation effect is small and may be usually neglected.

---

Einfluß der zweiten Koordinationssphäre auf die Energie der Elektronenübergänge in ZnS-Mn (Kurze Mitteilung)

Zusammenfassung Der Einfluß der zweiten Koordinationssphäre auf die Energie der Elektronenübergänge in ZnS-Mn wird mit Hilfe der Kristallfeldtheorie untersucht. Es ergibt sich, daß der Störeffekt klein ist und gewöhnlich vernachlässigt werden kann.

     

Aromatic δ-peptides: design, synthesis and structural studies of helical, quinoline-derived oligoamide foldamers

Oligoamides of 8-amino-4-isobutoxy-2-quinolinecarboxylic acid were designed and synthesized, and their helical structures were characterized in the solid state by single crystal X-ray diffraction, and in solution by ¹H NMR. The monomer methyl 4-isobutoxy-8-nitro-2-quinolinecarboxylate is easily prepared in three steps from 2-nitroaninile and dimethyl acetylene dicarboxylate. Successive hydrogenations of nitro groups, saponifications of esters and couplings of amines and acids via the acid chlorides gave a dimer, tetramer, hexamer, octamer, and decamer in a convergent fashion. The oligomers were shown to adopt a bent conformation stabilized by intramolecular hydrogen bonds between amide hydrogens and adjacent quinoline nitrogens. In the solid, the dimer adopts a planar crescent shape and the octamer a helical conformation. All NMR data are consistent with similar conformations in solution. The helices are apparently remarkably stable. Some of them remain helical even at 120°C in deuterated DMSO. The structural studies confirm the predictions made by computer and demonstrate the high potency of the design principles.     

Functionalization of white phosphorus in the coordination sphere of transition metal complexes

This review highlights the stoichiometric functionalization of both white phosphorus and naked P_n fragments derived from the metal-mediated demolition of the P₄ tetrahedron. In a first section, the alkylation of P_n ligands is discussed giving specific examples such as: (i) the electrophilic alkylation of η³-P₃ or, μ,η³-P₃ ligands: (ii) the transfer of a methyl group from molybdenum to η⁵-P₅ ligands to yield a norbornadiene-like μ₃,η⁴:η¹:η¹-MeP₇ ligand; (iii) the formation of P-C or P-H bonds mediated by rhodium and iron complexes; (iv) the use of ammonium salts to transfer an alkyl to polyphosphido clusters. Different methods to functionalise white phosphorus or other P_n ligands, including the cyclization of acrolein with diphosphenes and the insertion of CO or carbenes across P-P, P-M bonds, and P-E bonds (E = S, Se), are illustrated in appropriate sections. Finally, the last part of the article, reports on the astounding coupling of alkynes and phosphalkynes with P_n ligands which is a versatile, not yet completely explored, method to form an unprecedented variety of carbon-phosphorus heterocycles.     

Using the outer coordination sphere to tune the strength of metal extractants

A series of 3-substituted salicylaldoximes has been used to demonstrate the importance of outer-sphere interactions on the efficacy of solvent extractants that are used to produce approximately one-quarter of the world's copper. The distribution coefficient for extraction of copper by 5-tert-butyl-3-X-salicylaldoximes (X = H, Me, (t)Bu, NO(2), Cl, Br, OMe) varies by more than two orders of magnitude. X-ray structure determinations of preorganized free ligand dimers (10 new structures are reported) indicate that substituents with a hydrogen-bond acceptor atom attached to the 3-carbon atom, ortho to the phenolic oxygen, buttress the intermolecular hydrogen bond from the oximic proton. Density functional theory calculations demonstrate that this hydrogen-bond buttressing is maintained in copper(II) complexes and contributes significantly to their relative stabilities in energy-minimized gas-phase structures. A remarkable correlation between the order of the calculated enthalpies of formation of the copper complexes in the gas phase and the observed strength of the ligands as copper solvent extractants is ascribed to the low solvation energies of species in the water-immiscible phase and/or the similarities of the solvation enthalpies of the preorganized ligand dimers and their copper(II) complexes.     

Porphyrazines peripherally functionalized with hybrid ligands as molecular scaffolds for bimetallic metal-ion coordination

We report the synthesis and physical characterization of a new family of peripherally functionalized porphyrazine (pz) compounds, denoted 1[M1, M2], where metal ion M1 is incorporated into the pz core and metal ion M2 is bound to a salicylidene/picolinamide "hybrid" chelate built onto two nitrogen atoms attached to the pz periphery. The complexes 1[MnCl, Cu], 1[VO, Cu], and 1[Cu, Cu] have been prepared, and crystal structures show 1[MnCl, Cu] and 1[VO, Cu] to be isostructural. These complexes have been subjected to electron paramagnetic resonance and temperature-dependent magnetic susceptibility measurements. The variation of the ligand-mediated exchange splittings (delta) in these complexes is striking: delta/k(B) values for 1[MnCl, Cu] and 1[VO, Cu] are 22 and 40 K, respectively, while delta/k(B) for 1[Cu, Cu] is only 1 K. These coupling results are explained in terms of the relative orientation of the M1 and M2 orbitals and reflect the fact that the ligand set of M2 in the periphery is rotated in-plane by 45 degrees relative to the effectively coplanar pz ligand set of M1. The exchange couplings are essentially the same as those we determined for the Schiff base porphyrazines (pzs). Thus, the hybrid ligand has eliminated the dimerization found to occur when Cu(II) is bound to the periphery of bis(picolinamido) pzs and has created a more robust ligand system than the Schiff base pzs while retaining the ability they show to promote spin coupling between M1 and M2.     

Compatibility of nitrilotriacetate andN-methylisatin-β-amidinohydrazone in the coordination sphere of transition metal ions

Summary A study of the competition of nitrilotriacetate (NTA) andN-methylisatin--amidinohydrazone (-MIAG) in the coordination sphere of transition metal ions has been carried out by calculating the reproportionation constant which relates the stability of the mixed-ligand complex to those of the complexes formed by each ligand independently. NTA and -MIAG are incompatible ligands, and the mixed-ligand complexes formed are less stable than either of the parent complexes. This is also supported by the change in free energy occurring during mixed-ligand complex formation. All these studies have been carried out at 30±0.5°C and =0.1 M NaClO₄ in 50% v/v aqueous dioxan medium.     

Energetics of the first and second coordination spheres of transition metal ions

For complexes of transition metals (manganese, iron, cobalt, nickel) with monodentate ligands, equilibrium metal-ligand distances and ligand bond energies in the first and second coordination spheres have been calculated by the CNDO method. Some effects of ligand bond energies in different coordination spheres are analyzed. These effects significantly differ between the first and second coordination spheres. In the first sphere, the ligand bond energy is mainly determined by the nature of the central ion and the type of donor atom of the ligand, but weakly depends on the structure of the ligand. Conversely, in the second coordination sphere, the ligand bond energy weakly depends on the nature of the central ion and the type of donor atom, but considerably depends on the structure of the ligands in the first coordination sphere. In the second coordination sphere, ligand binding is determined by ligand interactions with both the central ion and the ligands of the first sphere. In the general case, when strong specific interactions between ligands are absent, the energetics of the second sphere is determined by the size of the inner-spheric ligands, which may be considered to be a specific steric effect. V. I. Vernadskii Institute of Geochemistry and Analytical Chemistry. Translated fromZhurnal Strukturnoi Khimii, Vol. 36, No. 2, pp. 370–374, March–April, 1995. Translated from L. Smolina     

Influence of second coordination sphere hydroxyl groups on the reactivity of copper(I) complexes

We report the enhanced reactivity of hydroxyl substituted CuN(3)(+) derivatives, where N(3) = tris(picolinyl)methane (tripic) and related derivatives, upon deprotonation of the O-H functionality. The work capitalizes on new methodology for incorporating hydroxyl groups into the second coordination sphere of copper centers. The key synthetic methodology relies on Pd-catalyzed coupling reactions of dilithiated 6-methyl-2-pyridone with bromopyridyl derivatives. These building blocks allow the preparation of tridentate N(3) ligands with OH and OMe substituents flanking the fourth coordination site of a tetrahedral complex. Coupling of these tridendate ligands gives the corresponding hydroxy- and methoxy-functionalized bistripodal ligands. [Cu[bis(2-methylpyrid-6-yl)(2-hydroxypyrid-6-yl)methane](NCMe)](+) ([Cu(2H)(NCMe)](+)) oxidizes readily in air to afford the mixed valence Cu(1.5) dimer ([Cu(2)(2)(2)](+)). Formation of [Cu(2)(2)(2)](+) is accelerated in the presence of base and can be reversed with a combination of decamethylferrocene and acid. The reactivity of [Cu(2H)(NCMe)](+) with dioxygen requires deprotonation of the hydroxyl substituent: neither [Cu(tripic)(NCMe)](+) nor the methoxy-derivatives displayed comparable reactivity. A related mixed valence dimer formed upon oxidation of the dicopper(I) complex of a tetrahydroxy bis(tridentate) ligand, [Cu(2)(6H(4))(NCMe)(2)](2+). The dicopper(I) complex of the analogous tetramethoxy N(6)-ligand, [Cu(2)(5)(NCMe)(2)](2+), instead reversibly binds O(2). Deprotonation of [Cu(2H)(CO)](+) and [Cu(2H)(NCMe)](+) afforded the neutral derivatives Cu(2)(CO) and Cu(2)(2)(2), respectively. The dicopper(I) derivative Cu(2)(2)(2) can be reoxidized, reprotonated, and carbonylated. The silver(I) complex, [Ag(2H)(NCMe)]BF(4), forms an analogous neutral dimer (Ag(2)(2)(2)) upon deprotonation of the hydroxyl group. The structures of ligand 2H, [Cu(2)(5)(NCMe)(2)](+), [Cu(2)(2)(2)](+), [Cu(2)(6H(2))](+), [Ag(2H)(NCMe)]BF(4), and Ag(2)(2)(2) were confirmed by single crystal X-ray diffraction.     

Redox-active dithiafulvenyldiphenylphosphine as a mono- or bidentate ligand: intramolecular coupling reaction in the coordination sphere of a metal carbonyl fragment

The coordinating ability of dithiafulvenyldiphenylphosphine (P-DTF) has been investigated with cis-W(CO)4(piperi-dine)2. As shown by the metal carbonyl complexes obtained, this redox-active vinylphosphine can act as a monodentate (P) and as a bidentate (P,S) ligand. Oxidation of cis-M(CO)4(P-DTF)2, M = Mo and W, leads to the carbon-carbon bond formation between the two coordinated dithiafulvenyldiphenylphosphines. This chemical coupling of the dithiafulvenyl cores in the coordination sphere of M(CO)4 (M = Mo, W) fragment has been studied in the presence of various oxidizing agents. The use of (BrC6H4)3NSbCl6 or AgBF4 induces the formation of a five-membered metallacycle with a vinylogous TTF backbone while DDQ leads to a six-membered metallacycle. The syntheses, crystal structures, and electrochemical properties of the complexes obtained are described.     

Artificial oligopeptide scaffolds for stoichiometric metal binding

Two artificial peptides with pendant pyridine or bipyridine ligands have been synthesized and incorporated into oligomeric strands that are analogous to peptide nucleic acid. Spectrophotometric titrations with Cu(2+) and Fe(2+) show that the oligomers bind stoichiometric quantities of transition metals based on the number of pendant ligands. The identities of the titration products are confirmed by high resolution mass spectrometry. In the case of the bipyridine tripeptides, the titration stoichiometry and mass spectra indicate that the metal ions form interstrand cross-links between two oligopeptides, creating duplex structures linked exclusively by metal ions. Calculated molecular structures of the metalated oligopeptides and duplexes indicate that the peptide backbone acts as a scaffold for the directed assembly of metal ions. Electron paramagnetic resonance spectroscopy of the Cu-containing molecules have varying degrees of electronic interaction based on their charge and supramolecular structure. Cyclic voltammetry of the Fe(2+)- and Cu(2+)-linked bpy oligopeptide duplexes shows that they possess unique electrochemical signatures based on the redox reactivity of the metal complex.     

Nuclear magnetic resonance relaxometry as a spectroscopic probe of the coordination sphere of a paramagnetic metal bound to a humic acid mixture

Protons on water molecules are strongly affected by paramagnetic ions. Since the acid-base properties of water facilitate rapid proton exchange, a single proton nuclear magnetic resonance (NMR) signal is seen in aqueous solutions of paramagnetic ions. Proton relaxation times are significantly affected by paramagnetic species and the readily detectable single signal serves as a powerful amplifier of the information contained concerning the protons in the paramagnetic environment. Where water molecules coordinated to free paramagnetic ions and to metal complexes of ligands that form non-labile (on the NMR time scale) complexes, the effects on water in the two environments can be distinguished. This can provide information on the nature of the ligand binding sites. The example of Cu²⁺ bound to the Laurentian humic acid mixture reported here using convenient low field NMR relaxometers shows that the information can enrich our understanding of complexation and speciation in the presence of complex mixture ligands characteristic of natural water systems. In this case, the data underline the role of aggregation and conformation in defining the complexation sites.     

Oligocholate foldamers as carriers for hydrophilic molecules across lipid bilayers

Three cholate foldamers were synthesized by the click reaction between an azide-functionalized cholate trimer and different dialkynyl linkers. The foldamers were labeled with pyrene groups at the ends for their conformational study. The linkers between the two tricholate fragments were found to strongly influence the conformation of the foldamers in solution, as well as their ability to transport hydrophilic molecules across lipid bilayers. The folding of the oligocholates in mixed organic solvents was studied by fluorescence and UV/Vis spectroscopy. Although these molecules could not fold permanently in lipid bilayers, they were found to translocate carboxyfluorescein readily across by a carrier-based mechanism. The transport is proposed to happen when the oligocholates adopt transiently folded structures with a hydrophobic exterior and a hydrophilic internal cavity. The transport rate strongly depended on the structure of the oligocholates and was sensitive even to the change of a single bond in a foldamer 3000 Da in MW. Better folded oligocholates in solution gave slower transport in the membranes.