DOTP versus DOTA as Ligands for Lanthanide Cations: Novel Structurally Characterized CeIV and CeIII Cyclen-Based Complexes and Clusters in Aqueous Solutions

The coordination and redox chemistry of aqueous Ce^IV/III macrocyclic compounds were studied by using the ligands DOTA and DOTP (1,4,7,10-tetraazacyclododecane-1,4,7,10-tetrayl)tetraacetic acid and 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetra(methylene phosphonic acid), respectively). The hydrolysis tendency of the tetravalent cation in the presence of DOTA is shown to result in the formation of a highly ordered, fluorite-like [Ce^IV₆(O)₄(OH)₄(H₂O)₈(DOTAH)₄] oxo-hydroxo structure both in solution and in the solid state. The lifetime of the analogous species formed in the presence of DOTP was found to be much shorter. Spectroscopic measurements of the latter suggest its similarity to the former. Its gradual decomposition in solution leads to the accumulation of the in-cage complexes [Ce^IVDOTP] and [Ce^IIIDOTP(H₂O)], which were crystallographically characterized in this study. The redox energetics and spectroscopic characteristics for the transition between these two in-cage complexes in aqueous solutions were studied as well. Together with the crystallographic structures of the above-mentioned species, the in-cage [Ce^IVDOTA(H₂O)] complex structure is presented herein for the first time. An elaborative analysis of the X-ray crystallographic structural data obtained for the in-cage complexes studied herein and similar structures published previously suggests that hard-bonding cyclen-derived ligands are, counter-intuitively, better suited for encapsulating, and perhaps kinetically stabilize softer cations than harder ones with DOTP, marked as a possible adequate chelator for the study of the aqueous properties of Ln^II and Ac^III cations.     

Titanium-Oxo Clusters with Bi- and Tridentate Organic Ligands: Gradual Evolution of the Structures from Small to Big

Homometallic titanium oxo clusters are one of the most important groups of metal oxo clusters, with more than 300 examples characterized by X-ray structure analyses. Most of them are uncharged and are obtained by partial hydrolysis and condensation of titanium alkoxo derivatives. The cluster cores, ranging from 3 to >50 titanium atoms, are stabilized by organic ligands. Apart from residual OR groups, carboxylato and phosphonato ligands are most frequent. The article critically reviews and categorizes the known structures and works out basic construction principles by comparing the different cluster types.     

Expanded Mercaptocalixarenes: A New Kind of Macrocyclic Ligands for Stabilization of Polynuclear Thiolate Clusters

The syntheses and properties of expanded 4-tert-butyl-mercaptocalix[4]arenes, in which the methylene linkers are replaced by −CH₂NRCH₂− or −CH₂NRCH₂− and −CH₂NRCH₂CH₂CH₂NRCH₂− units, are described. The new macrocycles were obtained in a step-wise manner, utilizing fully protected, i. e. S-alkylated, derivatives of the oxidation-sensitive thiophenols in the cyclisation steps. Reductive cleavage of the macrobicyclic or macrotricyclic intermediates ( 6 , 7 , 11 ) afforded the free thiophenols (H₄ 8 , H₄ 9 , and H₄ 12 ) in preparative yields as their hydrochloride salts. The protected proligands can exist in two conformations, resembling the “cone” and “1,3-alternate” conformations found for the parent calix[4]arenes. The free macrocycles do not show conformational isomerism, but are readily oxidized forming intramolecular disulfide linkages. Preliminary complexation experiments show that these expanded mercaptocalixarenes can serve as supporting ligands for tetranuclear thiolato clusters.     

From Cluster to Polymer: Ligand Cone Angle Controlled Syntheses and Structures of Copper(I) Alkynyl Complexes

Copper(I) alkynyl complexes have attracted tremendous attention in structural studies, as luminescent materials, and in catalysis, and homoleptic complexes have been reported to form polymers or large clusters. Herein, six unprecedented structures of Cu^I alkynyl complexes and a procedure to measure the cone angles of alkynyl ligands based on the crystal structures of these complexes are reported. An increase of the alkynyl cone angle in the complexes leads to a modulation of the structures from polymeric [((PhC≡CC≡C)Cu)₂(NH₃)]_∞, to a large cluster [(TripC≡CC≡C)Cu]₂₀(MeCN)₄, to a relatively small cluster [(TripC≡C)Cu]₈ (Trip=2,4,6‐iPr₃‐C₆H₂). The complexes exhibit yellow‐to‐red phosphorescence at ambient temperature in the solid state and the luminescence behavior of the Cu₂₀ cluster is sensitive to acetonitrile.     

Tuning the Functional Properties of Metal Complexes Containing Polytopic Heteroaromatic Nitrogen Ligands

The preparation, characterization, and optimization of the functional properties of mono‐ and polynuclear coordination complexes containing heteroaromatic nitrogen ligands are discussed here, taking the advantage of numerous studies performed in our laboratories on exploring a variety of different metal ions and polytopic ligands. We highlight how very minor changes in connectivity, composition, and polarity of the molecular entities employed in the self‐assembly steps may significantly affect the structural, thermal, sorptive, magnetic, and mesomorphic behavior of the resulting materials. Examples from three different classes are included: 1) pyrazolate‐based polynuclear coordination compounds, 2) homoleptic and heteroleptic coordination polymers, and 3) 2,2′‐bipyridine metal‐based liquid crystals.     

A Study of Formation Equilibria of the Complexes of Some Metal Cations with Polyoxa Macrocyclic Ligands

Abstract

The complexes formed by the Na⁺, K⁺, Rb⁺, Ca²⁺, UO²⁺ ₂, and Ag⁺ cations with the macrocyclic polyethers 18-crown-6, benzo-15-crown-5, and dicyclohexy1-18-crown-6 are investigated. The stability constants of these complexes have been determined potentiometrically in (90% vol.) ethanol-water solutions at 25[ddot]C and an ionic strength μ= 0.1 (achieved with tetrabuty lammonium perchlorate). The stablity of the investigated complexes was interpreted in terms of “caging” the metal cation into the cavity of the macrocyclic ligand, an effect which depends on the ratio of the diameter of the complexed cation over the diameter of the cavity of the complexing ligand.     

Molecular Organometallic Chemistry on Surfaces: Reactivity of Metal Carbonyls on Metal Oxides

Metal carbonyls react on metal oxide surfaces to give a wide range of structures analogous to those of known compounds. The reactions leading to formation of surface-bound metal carbonyls are explained by known molecular organometallic chemistry and the functional group chemistry of the surfaces. The reaction classes include formation of acid-base adducts as the oxygen of a carbonyl group donates an electron pair to a Lewis acidic center; nucleophilic attack at CO ligands by basic surface hydroxyl groups or O^2? ions; ion-pair formation by deprotonation of hydrido carbonyls to give carbonylate ions; interaction of bifunctional complexes with surface acid-base pair sites such as [Mg^2⊕O^2?]; and oxidative addition of surface hydroxyl groups to metal clusters. The reactions of surface-bound organometallic species include redox condensation and cluster formation on basic surfaces (paralleling the reactions in basic solution) as well as oxidation of mononuclear metal complexes and oxidative fragmentation of metal clusters by reaction with surface hydroxyl groups. Most supported metal carbonyls are unstable at high temperatures, but some, including osmium carbonyl cluster anions on the basic MgO surface, are strongly stabilized in the presence of CO and are precursors of catalysts for CO hydrogenation at 550 K.     

1,3-Diketone Calix[4]arene Derivatives—A New Type of Versatile Ligands for Metal Complexes and Nanoparticles

The present review is aimed at highlighting outlooks for cyclophanic 1,3-diketones as a new type of versatile ligands and building blocks of the nanomaterial for sensing and bioimaging. Thus, the main synthetic routes for achieving the structural diversity of cyclophanic 1,3-diketones are discussed. The structural diversity is demonstrated by variation of both cyclophanic backbones (calix[4]arene, calix[4]resorcinarene and thiacalix[4]arene) and embedding of different substituents onto lower or upper macrocyclic rims. The structural features of the cyclophanic 1,3-diketones are correlated with their ability to form lanthanide complexes exhibiting both lanthanide-centered luminescence and magnetic relaxivity parameters convenient for contrast effect in magnetic resonance imaging (MRI). The revealed structure–property relationships and the applicability of facile one-pot transformation of the complexes to hydrophilic nanoparticles demonstrates the advantages of 1,3-diketone calix[4]arene ligands and their complexes in developing of nanomaterials for sensing and bioimaging.