Molybdenum complexes with O,N,S donor ligands as models for active sites in oxotransferases and hydroxylases

A series of homologous mononuclear dioxomolybdenum complexes were prepared and fully characterized with structurally related thiosemicarbazone ligands supplying a tridentate O,N,S donor set to the central metal atom. The ligands are derived from the prototype 2-hydroxybenzaldehyde-4-triphenylmethylthiosemicarbazone (H₂L). Within this series the crystal structures of 11 complex compounds [MoO₂(LRⁿ)(dmf)] and [MoO₂(LRⁿ)(MeOH)] were determined showing characteristic differences in the gross structural properties of the central metal core. From the variation of substituents in this ligand library the influences of electronic ligand effects on the spectroscopic, electrochemical, and functional properties of these biomimetic model complexes for molybdenum-containing oxotransferases are reported.     

The synthesis, crystal and molecular structures of two saccharinate-metal complexes with dipyridylamine (dipyr) as a co-ligand

The complexes [Cd(dipyr)₂(sac)(H₂O)] sac·H₂O 1 and [Hg(dipyr)(sac)₂] 2, where dipyr = dipyridylamine and sac = saccharinate, have been synthesised, and fully characterised by single-crystal X-ray diffraction at 120 K. The geometry around Cd in 1 is approximately octahedral, with the metal coordinated by two bidentate dipyr ligands, one N-bonded sac and one H₂O molecule; the second sac forms the counter-ion, and there is also a water of crystallisation. An extensive H-bonded network is formed. In the anhydrous Hg complex 2, the metal has approximately tetrahedral geometry, with coordination from a bidentate dipyr ligand and two N-bonded sac groups. H-bonding interactions are again extensive, even without the presence of H₂O molecules in the structure, leading to chains along the a-axis.     

Supramolecular architectures constructed on the skeleton of Silver nitrate, 4,4′-bipyridine and triphenylphosphine/1,2-bis(diphenylphosphino)ethane: Synthesis, structural characterization and non-covalent interactions

In the presence of PPh₃ and Ph₂PCH₂CH₂PPh₂ (dppe); AgNO₃ and 4,4′-bipyridine (bpy) react to form [Ag(PPh₃)(bpy)_1/2(ONO₂)]_n (1) and [Ag(dppe)_1/2(bpy)_1/2(ONO₂)]_n (2). The compositions of these complexes are supported by their elemental analysis. The structures are determined by spectroscopic (IR, ¹H NMR, UV-Vis) and X-ray crystallographic data. A zig-zag ladder type structure observed in 1 changes into straight ladder type structure in 2 in which [Ag(dppe)_1/2(ONO₂)]_n chains are bridged by bpy with Ag?Ag distance of 11.48 Å. However in 1, the two zig-zag chains are linked via π-π interactions at 3.82 Å between phenyl rings of terminal PPh₃ ligands. Both complexes luminesce in DMSO solution. Photoluminescence of a representative complex 2 is also measured.     

Self-assembled synthesis, characterization and antimicrobial activity of zinc(II) salicylaldimine complexes

One-pot metal promoted reactions between salicylaldehyde and 4-methyl-1,3-phenylenediamine in the presence of metal salts acting as template agents yield zinc(II) salicylaldimine complexes containing N,N′-bis(salicylidene)-4-methyl-1,3-phenylenediamine (H₂L) as a result of the [2 + 1] Schiff base condensation. The complexes of formula [Zn(HL)Cl(H₂O)₂] · C₂H₅OH and [Zn(H₂L)₂Cl(NO₃)(H₂O)] · CH₃OH were characterized as powder solids and in solution by spectroscopic methods (IR, ¹H and ¹³C NMR, FAB-MS, ESI-MS, UV-Vis), thermogravimetric and elemental analysis, potentiometry, and tested for antimicrobial activity against Staphylococcusaureus in a minimum inhibitory concentration (MIC) experiment. In these two powder solid species, the salicylaldimine, formed in a self-assembly process, acts in two different coordination modes: as monodeprotonated bidentate chelator with an N,O donor set or as a neutral monodentate using exclusively oxygen as the donor atom without involving the nitrogen atoms in the coordination. However, crystals of these two complexes are isomorphous, with 1:2 metal:ligand stoichiometry, and display the latter, relatively rare coordination pattern. In solution, the presence of a 1:1 complex of monodeprotonated state is only detected. The complexes exhibit antimicrobial activity against S.aureus.     

Thallium(III) complexes of the metalloligands [Pt2(μ-S)2(PPh3)4] and [Pt2(μ-Se)2(PPh3)4]

Reactions of [Pt₂(μ-S)₂(PPh₃)₄] with the diarylthallium(III) bromides Ar₂TlBr [Ar = Ph and p-ClC₆H₄] in methanol gave good yields of the thallium(III) adducts [Pt₂(μ-S)₂(PPh₃)₄TlAr₂]⁺, isolated as their salts. The corresponding selenide complex [Pt₂(μ-Se)₂(PPh₃)₄TlPh₂]BPh₄ was similarly synthesised from [Pt₂(μ-Se)₂(PPh₃)₄], Ph₂TlBr and NaBPh₄. The reaction of [Pt₂(μ-S)₂(PPh₃)₄] with PhTlBr₂ gave [Pt₂(μ-S)₂(PPh₃)₄TlBrPh]⁺, while reaction with TlBr₃ gave the dibromothallium(III) adduct [Pt₂(μ-S)₂(PPh₃)₄TlBr₂]⁺[TlBr₄]⁻. The latter complex is a rare example of a thallium(III) dihalide complex stabilised solely by sulfur donor ligands. X-ray crystal structure determinations on the complexes [Pt₂(μ-S)₂(PPh₃)₄TlPh₂]BPh₄, [Pt₂(μ-S)₂(PPh₃)₄TlBrPh]BPh₄ and [Pt₂(μ-S)₂(PPh₃)₄TlBr₂][TlBr₄] reveal a greater interaction between the thallium(III) centre and the two sulfide ligands on stepwise replacement of Ph by Br, as indicated by shorter Tl-S and Pt?Tl distances, and an increasing S-Tl-S bond angle. Investigations of the ESI MS fragmentation behaviour of the thallium(III) complexes are reported.     

Water soluble dinuclear complexes with a hexaaza macrocyclic ligand bearing four acetohydrazide groups

A potentially 18 donor macrocyclic ligand bearing four acetohydrazide pendant arms has been used to synthesize nine new dinuclear metal complexes with Co(II), Ni(II), Cu(II), Zn(II) and Cd(II). All compounds have been characterized by elemental analysis, MALDI-TOF-MS and FAB spectrometry, and by IR, UV-Vis and NMR spectroscopy. The system is a very good candidate for in situ recognition effects by MALDI-TOF-MS spectrometry and absorption spectroscopy. The presence of four acetohydrazide groups in L allows the synthesis of new three-dimensional self-assembling ditopic supramolecular compounds.     

New coordination polymer based on salpn Schiff base and azide bridging ligands: Synthesis and structural characterization of {Na[Co(μ-salpn)(μ1,1-N3)2]}n (H2salpn = N,N′-bis(salicylidene)-1,3-diaminopropane)

One-pot reaction of cobalt(II) nitrate hexahydrate Co(NO₃)₂ · 6H₂O with H₂salpn (N,N′-bis(salicylidene)-1,3-diaminopropane) in presence of a large excess of sodium azide (NaN₃) gives the new Co(III) compound {Na[Co^III(μ-salpn)(μ_1,1-N₃)₂]}_n (1), which was characterized by single crystal X-ray diffraction analysis. The crystal structure shows polymeric 1D complex generated by the hexadentate Schiff base salpn²⁻ and two crystallographically different azide ligands. The two nitrogen atoms of the salpn ligand are bonded to the cobalt(III) ion while each phenoxo oxygen atom is bonded to the same Co(III) ion and to two equivalent sodium ions. Each azide ligand acts with the end-on bridging coordination mode between Co(III) and Na(I) ions. The Co(III) ion adopts a distorted octahedral geometry arising from two oxygen and two nitrogen atoms of the salpn ligand and from two nitrogen atoms of the two crystallographically different azide ligands in trans positions. Such [Co(salpn)(N₃)₂] entities are connected each other by sodium ions through four oxygen atoms of two equivalent Schiff base ligands and two nitrogen atom of the two different azide ligands to generate the 1D structure of 1.     

N,N′-Ethylenediaminobis(benzylphosphonic acids) as a potent class of chelators for metal ions

Aminophosphonates were found to be effective metal ion chelators. Ethylenediamine (EN) itself binds strongly to a series of metal ions forming efficient five-membered chelate ring using two nitrogen donors. Combination of EN with phosphonate function results in the family of low molecular chelating agents. Introduction of a pyridine moiety into EN-phosphonate structure results in the very effective ligands involving a four-nitrogen donor set to coordinate the metal ion. In this work, the potentiometric and spectroscopic data for two analogues 2,2′-(ethylenedi-imino)bis(3-pyridylphosphonic-acid) - L² and 2,2′-(ethylenedi-imino)bis(2-pyridyl-phosphonic-acid) - L³, comprising EN, two phosphonates and two pyridines with Cu(II), Ni(II) and Zn(II) ions are presented showing very high efficacy of one of the ligands studied.     

Metal-binding ability of histidine-containing peptidehydroxamic acids: Imidazole versus hydroxamate coordination

Three new peptidehydroxamic acids (l-alanyl-l-histidinehydroxamic acid, l-Ala-l-HisNHOH, l-alanyl-l-alanyl-l-histidinehydroxamic acid, l-Ala-l-Ala-l-HisNHOH and l-histidyl-l-alaninehydroxamic acid, l-His-l-AlaNHOH) were synthesized and their complexation with Cu(II), Ni(II) and Zn(II) were studied by pH-potentiometric, UV-Vis, CD, ¹H NMR, EPR and ESI-MS methods. Each of the studied peptide derivatives involves one side-chain imidazole unit and the effect of this group on the metal binding of the hydroxamic moiety is evaluated in the paper. The obtained results are compared to those of the complexes of some histidine-containing di- or tripeptides and also to those of hydroxamic derivatives of aliphatic peptides.A competition between the hydroxamate and imidazole functions occurs in all systems, but the extent differs from metal to metal, from ligand to ligand and depends very much on the pH. The imidazole was found to play the most determinant role in the Cu(II) complexes, somewhat less in the Ni(II)-containing ones, while (except the case of l-Ala-l-HisNHOH) negligible role was found in the Zn(II)-complexes. Common feature of the Ni(II)- and especially Cu(II)-containing systems is that if an imidazole-N is displaced by a hydroxamate, imidazole-bridged di- and polynuclear complexes are formed.     

Synthesis and structure of heteroleptic ytterbium (II) tetrahydroborate complexes

The synthesis and characterization of (Tp^tBu,Me)Yb(BH₄)(THF)_n (n = 0, 3; n = 1, 4) complexes are reported. The compounds represent rare examples of lanthanide (II) tetrahydroborate complexes. The X-ray crystal structure of complex 4 has been determined and it shows a monomeric, formally seven coordinate ytterbium center, bearing one κ³ bonded Tp^tBu,Me ligand, a tetrahydroborate ligand and a coordinated THF molecule. The tetrahydroborate ligand binds in a κ³ fashion, via three bridging hydrogen atoms. IR spectroscopy data are consistent with the solid-state structure and the corresponding BD₄ analog of 4 shows the expected IR isotope shifts. The ¹H NMR spectra of 3 and 4 shows one set of resonances each for the BH₄ and the pyrazolylborate ligands indicating dynamic solution behavior. For complex 3, although X-ray quality crystals could not be obtained, the IR and NMR data are consistent with its formulation as the solvent-free analog of complex 4 with κ³-bonded BH₄ ligand.