Dubravka Matković-Čalogović

Mercury(II) complexes of heterocyclic thiones. Part 1. Preparation of 1:2 complexes of mercury(II) halides and pseudohalides with 3, 4, 5, 6-tetrahydropyrimidine-2-thione. X-ray, thermal analysis and NMR studies

A series of complexes HgX_2(H_4pymtH)_2 (X = Cl, Br, I, SCN, CN ; H_4pymtH = 3, 4, 5, 6-tetrahydropyrimidine-2-thione) has been obtained by the reaction of H_4pymtH with mercury(II) halides and pseudohalides in the 2:1 molar ratio. X-ray diffraction studies revealed tetrahedral coordination of mercury with S-bound H4pymtH. The exception is Hg(CN)_2(H_4pymtH)_2 where the coordination is 2+2 with two strongly bound CN� {; ; ; ; ; ligands and weaker Hg� {; ; ; ; ; S bonds with H4pymtH. One- and twodimensional 1^H and 13^C NMR measurements in dimethylsulfoxide solution confirmed the complexation of mercury to sulphur. The greatest complexation effects on chemical shifts were detected for the C-2, C-5 and H-1, 3 atoms i.e. two, four and five bonds away from mercury atom. The complexation effects in Hg(SCN)_2(H_4pymtH)_2 and Hg(CN)_2(H_4pymtH)_2 are in agreement with the strongest intramolecular H-bonding in the former and the weakest Hg-S bonds in the latter as compared to other complexes here.

Mercury(II) compounds with 1,3-imidazole-2-thione and its 1-methyl analogue. Preparative and NMR spectroscopic studies. The crystal structures of di-μ-iodo-bis[iodo(1,3-imidazolium-2-thiolato-S)mercury(II)], bis[bromo(1,3-imidazolium-2-thiolato-S)]mercury(II) and bis[μ-(1-N-methyl-1,3-imidazole-2-thiolato-S)]mercury(II)

Abstract A series of mercury(II) compounds of the empirical formulae HgX2L, and HgX2L2 (X=Cl−, Br−, I−, SCN−; L=imtH2, meimtH; imtH2=1,3-imidazole-2-thione, meimtH=1-methyl-1,3-imidazole-2-thione) has been obtained by the reaction of mercury(II) salts and 1,3-imidazole-2-thione and 1-methyl-1,3-imidazole-2-thione, respectively. Mercury(II) acetate yields HgL2 complexes where L=imtH−, meimt−. The isolated compounds have been characterised by elemental chemical analysis, IR and 1H and 13C NMR spectroscopy. Complexation effects on chemical shifts in 1H and 13C spectra were shown to be a reliable probe for distinguishing HgX2L and HgX2L2 complexes. In the former molecules the thione carbon (C-2) is shielded up to 3.0 ppm and the thioamide protons (NH) are deshielded up to 0.5 ppm, as compared to the corresponding atoms in the latter molecules. In all complexes the 13C complexation shift at C-2 decreases with decreasing electronegativity of the halogen atom (X), indicating the corresponding increase in π-character of the C-2–S bond. The crystal structures of HgI2(imtH2), HgBr2(imtH2)2 and Hg(meimt)2 have been determined by X-ray diffractometry and revealed S-bound imtH2 in the iodo and bromo complex, while in Hg(meimt)2 the ligand acts as bridging with stronger S and weaker N bonds.

The first example of coexistence of the ketoamino–enolimino forms of diamine Schiff base naphthaldimine parts: the crystal and molecular structure of N,N′-bis(1-naphthaldimine)-o-phenylenediamine chloroform (1/1) solvate at 200 K

Abstract The N,N′-bis(1-naphthaldimine)-o-phenylenediamine chloroform (1/1) solvate was prepared from 2-hydroxy-1-naphthaldehyde and 1,2-phenylenediamine in a 2:1 molar ratio and characterized in solution and in the solid state using X-ray single crystal diffractometry, IR and NMR spectroscopy. The structure consists of three structural fragments: two planar, but not coplanar naphthaldimine moieties linked by phenyl ring that derived from o-phenylenediamine. The naphthaldimine fragments contain, along with naphthalene rings, six-membered pseudoaromatic chelate rings, formed by O H⋯N or N H⋯O intramolecular hydrogen bonds that retain planarity of the aldimine fragments. This structure is the first example of the coexistence of two different hydrogen bond types within Schiff base molecule, i.e. the one naphthaldimine part exists in the enolimino form (O H⋯N hydrogen bond of 2.5487(17) A), while the other exists in the ketoamino form (N H⋯O hydrogen bond of 2.5706(19) A). The different hydrogen bond types are related to the different π-electron density distribution within two parts. The amino hydrogen atom forms three-center intramolecular hydrogen bond, which includes the N H⋯O hydrogen bond of 2.5706(19) A within the ketoamino part and also the N H⋯N bond of 2.7238(18) A linking two naphthaldimine fragments of the molecule. IR and 1H and 13C NMR spectral data are consistent with revealed molecular structure in the solid state.

Anion‐Directed Self‐Assembly of Flexible Ligands into Anion‐Specific and Highly Symmetrical Organic Solids

Template-induced association of molecular species represents one of the main approaches in the control of supramolecular assembling. This strategy can be used in the design of porous materials of importance for inclusion (host–guest) chemistry and coordination polymers, in which the tunable size and nature of pores is a great advantage over their inorganic analogues. Most numerous in this class of compounds are the MOFs (metal–organic frameworks) and, to a minor extent, COFs (covalent organic frameworks). Porous organic architectures, in which the tectons are linked through noncovalent interactions, are very rare and still represent an undeveloped area of research. Both neutral and cationic templates have been known for a long time and are widely used in different fields of synthetic chemistry. However, anionic templates are rarely used in supramolecular chemistry because of the properties of anions, such as low charge to radius ratio, various geometries, high solvation energies, and pH-dependent charge. Templatedirected processes that are anion specific can lead us to the always challenging development of new selective systems with industrial, ecological, and biomedical applications. Systems that show great oxoanion selectivity are of special interest in the area of nuclear and toxic waste management. Herein we report a solid-state study of two supramolecular complexes assembled by an anion-templated reaction of a flexible ligand L (Scheme 1) and anions with trigonal planar (NO3 , 1; Figure 1), and tetrahedral geometry (SO4 2 ; 2) in methanol. The anion organizes folding of three podands to achieve hydrogen bond saturation, resulting in formation of a pseudomacrocyclic host PMH (Figure 2). The PMH assembly is highly anion specific, and the occurrence of the above-mentioned complexes in systems with high concentrations of competing anions has been explored. Expansion of 1 and 2 through hydrogen bonds leads to a highly symmetrical organic solid with voids and channels habited by counterions and more than 300 disordered solvent molecules per unit cell

Dynamic Molecular Recognition in Solid State for Separating Mixtures of Isomeric Dicarboxylic Acids

Molecular recognition emerges from non-covalent interactions and is of paramount importance for understanding of biological processes, ranging from enzymatic activity to DNA base pairing, as well as in the design of functional supramolecular systems, for example, molecular motors, sensors, ion receptors, or systems used in waste management. In the specific area of selective anion binding, numerous anion receptors (hosts) and sensors have been developed. The study of anion binding has traditionally been performed in solution where the host often experiences conformational freedom to form complexes with a wide range of guests. However, selectivity in separation has usually been achieved only upon crystallization, emphasizing the importance of intermolecular interactions in rigid crystal environment which lock the conformation of the host giving rise to its selectivity. In this context, recent advances in chemical reactivity achieved using mechanochemistry indicate that the concepts of supramolecular chemistry, such as templating, may be applicable also to solvent-free reactions. Mechanochemical reactivity can be highly dynamic and has thus far been employed for solid-state differentiation between enantiomers, supramolecular metathesis reactions, and for thermodynamic product selection. Although these reactions show specific interaction patterns between molecules comprising their respective solid phases, the possibility of selective binding and separation of target guest molecules from solid mixtures is, besides the pioneering studies by Etter and Caira, still an unexplored area. Here we focus on recognition and separation of isomeric or geometrically similar dicarboxylic acids (Scheme 1) from either their solid or solution mixtures using principles of supramolecular chemistry. The chosen acids belong to a class of guests of high biological and industrial relevance, and a considerable effort has been put into developing their sensors and receptors. Typically, the receptor for each dicarboxylate had to be meticulously designed because of the specific geometry of each acid molecule and their differing physicochemical properties. The importance of separation of the maleic/fumaric acid (H2mal/H2fum) stereoisomeric pair is not only related to the specific diastereomer recognition, but also arises from their conflicting biochemical behavior and abundant use of H2fum in food and pharmaceutical industry. We show here that the flexible polyamine receptor L (Scheme 1) discriminates among H2mal/H2fum diastereomers, succinic acid (H2suc), and three isomers of benzenedicarboxylic acid, by adapting its conformation and finally forming different solid hydrogenbonded (HB) frameworks. Regardless of whether the recognition takes place in the solid state by milling or by crystallization from solution, the resulting supramolecular complexes are the same and the selectivity bias of L towards the guest acids is fully retained. Milling improved yields to quantitative and almost eliminated the use of solvent. L proved to be an exceptional receptor for H2mal, also on the gram scale, excluding it from solid mixtures with even five other acids or from mixtures where there is a large surplus of a competing acid. Reacting L and H2mal in methanol (MeOH) or ethanol (EtOH) solutions yielded isoskeletal solvated solids, 1a (Table 1 and Section S.2 in the Supporting Information), Scheme 1. Dicarboxylic acids and the polyamine host L. The host binds anions as a cation (HL) resulting from protonation of the central amino group.

Dioxomolybdenum(VI) and dioxotungsten(VI) complexes chelated with the ONO tridentate hydrazone ligand: synthesis, structure and catalytic epoxidation activity

Synthesis of the dioxomolybdenum(VI) complexes [MoO2(L3OMe)(EtOH)] (1), [MoO2(L4OMe)(EtOH)] (2) and [MoO2(LH)(EtOH)] (3) and dioxotungsten(VI) complexes [WO2(L3OMe)(EtOH)] (4), [WO2(L4OMe)(EtOH)] (5) and [WO2(LH)]n (6a) was carried out using [MO2(C5H7O2)2] (M = Mo or W) and the corresponding aroylhydrazone ligand H2LR (3-methoxysalicylaldehyde 4-hydroxybenzhydrazone (H2L3OMe), 4-methoxysalicylaldehyde 4-hydroxybenzhydrazone (H2L4OMe), or salicylaldehyde 4-hydroxybenzhydrazone (H2LH) in ethanol. Compounds obtained upon heating of the mononuclear complexes in acetonitrile or dichloromethane, [MO2(LR)]n (1a–6a) or [MoO2(L3OMe)]2 (1b), respectively, were also investigated. Crystal and molecular structures of the mononuclear 1, 2 and 3, polynuclear 1a·MeCN and dinuclear 1b complexes were determined by the single crystal X-ray diffraction method. Powder X-ray diffraction showed isostructurality of 1 and 4, and 2 and 5. The complexes were further characterized by elemental analysis, IR spectroscopy, TG and DSC analyses, and one- and two-dimensional NMR spectroscopy. The catalytic performances of 1–5 and 6a were investigated for epoxidation of cyclooctene using aqueous tert-butyl hydroperoxide (TBHP) as the oxidant.

Crystallographic Snapshots of Tyrosine Phenol-lyase Show That Substrate Strain Plays a Role in C–C Bond Cleavage

The key step in the enzymatic reaction catalyzed by tyrosine phenol-lyase (TPL) is reversible cleavage of the Cβ–Cγ bond of l-tyrosine. Here, we present X-ray structures for two enzymatic states that form just before and after the cleavage of the carbon–carbon bond. As for most other pyridoxal 5′-phosphate-dependent enzymes, the first state, a quinonoid intermediate, is central for the catalysis. We captured this relatively unstable intermediate in the crystalline state by introducing substitutions Y71F or F448H in Citrobacter freundii TPL and briefly soaking crystals of the mutant enzymes with a substrate 3-fluoro-l-tyrosine followed by flash-cooling. The X-ray structures, determined at ∼2.0 Å resolution, reveal two quinonoid geometries: “relaxed” in the open and “tense” in the closed state of the active site. The “tense” state is characterized by changes in enzyme contacts made with the substrate’s phenolic moiety, which result in significantly strained conformation at Cβ and Cγ positions. We also captured, at 2.25 Å resolution, the X-ray structure for the state just after the substrate’s Cβ–Cγ bond cleavage by preparing the ternary complex between TPL, alanine quinonoid and pyridine N-oxide, which mimics the α-aminoacrylate intermediate with bound phenol. In this state, the enzyme–ligand contacts remain almost exactly the same as in the “tense” quinonoid, indicating that the strain induced by the closure of the active site facilitates elimination of phenol. Taken together, structural observations demonstrate that the enzyme serves not only to stabilize the transition state but also to destabilize the ground state.

Insights Into the Catalytic Mechanism of Tyrosine Phenol-Lyase from X-Ray Structures of Quinonoid Intermediates.

Amino acid transformations catalyzed by a number of pyridoxal 5′-phosphate (PLP)-dependent enzymes involve abstraction of the Cα proton from an external aldimine formed between a substrate and the cofactor leading to the formation of a quinonoid intermediate. Despite the key role played by the quinonoid intermediates in the catalysis by PLP-dependent enzymes, limited accurate information is available about their structures. We trapped the quinonoid intermediates of Citrobacter freundii tyrosine phenol-lyase with l-alanine and l-methionine in the crystalline state and determined their structures at 1.9- and 1.95-Å resolution, respectively, by cryo-crystallography. The data reveal a network of protein-PLP-substrate interactions that stabilize the planar geometry of the quinonoid intermediate. In both structures the protein subunits are found in two conformations, open and closed, uncovering the mechanism by which binding of the substrate and restructuring of the active site during its closure protect the quinonoid intermediate from the solvent and bring catalytically important residues into positions suitable for the abstraction of phenol during the β-elimination of l-tyrosine. In addition, the structural data indicate a mechanism for alanine racemization involving two bases, Lys-257 and a water molecule. These two bases are connected by a hydrogen bonding system allowing internal transfer of the Cα proton.

STRUCTURAL CHARACTERISTICS OF N,N'-BIS(SALICYLIDENE)-2,6-PYRIDINEDIAMINE

Abstract The Schiff base N , N ′-bis(salicylidene)-2,6-pyridinediamine has been synthesized and characterized in the solid state and in solution using X-ray analysis, IR, UV/Vis and NMR spectroscopy. Crystal data: C 19 H 15 N 3 O 2 , M r =317.34, monoclinic, space group P2 1 /c, a =19.313(5), b =5.854(2), c =14.957(6) A, β =110.45(2)°, V =1584.4(9) A 3 , Z =4, R =0.049, R w =0.095 for 3323 independent reflections with I >2 σ ( I ). There are two intramolecular hydrogen bonds O–H⋯N between the hydroxyl and imino groups of 2.564(3) and 2.633(3) A. The enolimino form is found in the solid state and is also the predominant tautomeric form in solution. A tendency of interconversion to ketoamine has been observed only in rather polar solvents, such as methanol and methanol/water mixtures and has been found to be very low: tautomeric constants K t =[ketoamine]/[enolimine] amount to 0.02 and 0.03 in methanol and methanol/water 4/1, respectively.© 1997 Elsevier Science B.V.

Preparation and characterization of the 1:1 adducts of mercury(II) halides with N-benzyl- and N-p-tolyl-2-oxo-1-naphthylideneamine. The crystal and molecular structures of two isostructural di-μ-halo-bis[halo(N-benzyl-2-oxo-1-naphthylideneamine)mercury(II)] adducts (halo=chloro, bromo)

Abstract A series of adducts of the type HgX2(C18H15NO) [X=Cl−, Br−, I−; C18H15NO=N-benzyl-2-oxo-1-naphthylideneamine (bznapH), N-p-tolyl-2-oxo-1-naphthylideneamine (tolnapH)] was obtained by refluxing the solution of the corresponding mercury(II) salt and the related naphthylideneamine in absolute ethanol in a 1:1 molar ratio. The adducts di-μ-chloro-bis[chloro(N-benzyl-2-oxo-1-naphthylideneamine)mercury(II)], HgCl2(bznapH) (1) and di-μ-bromo-bis[bromo(N-benzyl-2-oxo-1-naphthylideneamine)mercury(II)], HgBr2(bznapH) (2) are isostructural with the effective coordination of mercury being 2+2. The HgX2 moieties are retained in the adducts and contain two covalently linked chlorine [2.321(3) and 2.299(3) A in 1] or bromine atoms [2.446(1) and 2.422(1) A in 2]. These moieties are linked mutually by additional contacts of bridging halogen atoms [Hg⋯μX 3.189(2) and 3.298(1) A in 1 and 2, respectively], forming non-planar dimeric Hg2X4 units. Each Schiff-base ligand has a strong contact with mercury through the oxygen atom [Hg⋯O 2.392(5) and 2.410(5) A in 1 and 2, respectively]. The Schiff-base ligand in 1 and 2 exists as the ketoamino (quinoid) tautomeric structure with an intramolecular hydrogen bond of the NH⋯O type [2.58(1) and 2.59(1) A in 1 and 2, respectively]. On the contrary, in DMSO solution, due to dissociation of adducts 1–6, the enolimino (benzenoid) tautomeric form of the Schiff bases was established by NMR spectroscopy.