Reaction behavior of molybdocene dichloride towards ribonucleosides and ribonucleoside monophosphates: Rare example of sugar coordination

The coordination behavior of Cp₂Mo²⁺ towards the ribonucleosides and ribonucleoside monophosphates uridine, adenosine, cytidine, guanosine, 5′-UMP, 5′-AMP, 5′-CMP and 5′-GMP has been studied in solution in the range 4 ? pD ? 9 using NMR spectroscopy. The ribonucleosides were found to bind Cp₂Mo²⁺ exclusively through the ribose moiety giving rise to the chelate complexes [Cp₂Mo(urd-O2′,O3′)], [Cp₂Mo(ade-O2′,O3′)], [Cp₂Mo(cyd-O2′,O3′)], and [Cp₂Mo(gua-O2′,O3′)]. The ribonucleotides form three types of complex with Cp₂Mo²⁺ in neutral solution, namely N,PO-macrochelates, PO,O3′-coordinated species as well as O2′,O3′-chelates, while at pD 9 only sugar coordination is observed.     

Synthesis and derivatization of halflanthanidocene aryl(alk)oxide complexes

The reaction of halflanthanidocene aryloxides Cp^R′Ln(OAr^tBu,R)₂ (Ln = Y, La, Lu; Cp^R′ = C₅Me₅, C₄Me₄H; R = H, Me) and halflanthanidocene alkoxides [(C₅Me₅)Ln(OCH₂CMe₃)₂]₂ (Ln = Y, Lu) with trimethylaluminum (TMA) was investigated. Monomeric Cp^R′Ln(OAr^tBu,R)₂, derived from the ortho-tBu-substituted OC₆H₂tBu₂-2,6-R-4 (R = H, Me) ligands, form mono(tetramethylaluminate) complexes Cp^R′Ln(OAr^tBu,R)(AlMe₄) for the smaller lanthanide metal centers yttrium and lutetium. Such an [aryloxide] → [aluminate] ligand exchange was not observed at the larger lanthanum metal center. The mobility of the tetramethylaluminate ligands of complexes Cp^R′Ln(OAr^tBu,R)(AlMe₄) (Ln = Y, Lu) was examined by variable-temperature (VT) ¹H NMR spectroscopy, revealing two signals for bridging and terminal methyl groups at lower temperatures. The treatment of complexes Cp^R′Ln(OAr^tBu,R)(AlMe₄) with donor solvent d₈-THF gave Cp^R′Ln(OAr^tBu,R)(Me)(d₈-THF)₂ (Ln = Y, Lu) with terminal methyl groups, according to a donor-induced aluminate cleavage reaction. Dimeric [(C₅Me₅)Ln(OCH₂CMe₃)₂]₂ (Ln = Y, Lu) was synthesized from (C₅Me₅)Ln(NiPr₂)₂(THF) and reacted with two equivalents of TMA per Ln center to yield monomeric bis(TMA) adduct complexes (C₅Me₅)Ln(OCH₂CMe₃)₂(AlMe₃)₂(Ln = Y, Lu). VT NMR spectroscopic studies confirmed a high mobility of the Ln(μ-OCH₂CMe₃)(μ-Me)AlMe₂ moieties at an ambient temperature. Both bis(TMA) adduct complexes were characterized by X-ray structure analysis.     

Intramolecular exchange of coordinated and dangling phosphines in pentacarbonyl group 6 complexes of 1,1,2-tris(diphenylphosphino)ethane

The linkage isomers, (OC)₅M[κ¹-PPh₂ CH₂CH(PPh₂)₂] 1 and (OC)₅M[κ¹-PPh₂ CH(PPh₂)CH₂PPh₂] 2 (M = Cr, Mo and W) exist in equilibrium at room temperature. Equilibrium constants for 1Cr ? 2Cr, 1Mo ? 2Mo and 1W ? 2W at 25 °C in CDCl₃ are 2.61, 5.0 and 4.74, respectively. Enthalpy favors the forward reaction (ΔH = −13.5, −12 and −12.2 kJ mol⁻¹, respectively) while entropy favors the reverse reaction (ΔS = −37.6, −28 and −28.2 J K⁻¹ mol⁻¹, respectively). Isomerization is much faster than chelation with 1Mo ? 2Mo ? 1W ? 2W > 1Cr ? 2Cr. Enthalpies of activation for 1Cr ? 2Cr and 1W ? 2W are 119.0 and 92.6 kJ mol⁻¹, respectively, and entropies of activation are 1.4 and −28.2 J K⁻¹ mol⁻¹, respectively. Isomerization is 10⁴ times faster for these complexes than for (OC)₅M[κ¹-PPh₂CH₂CH₂P(p-tolyl)₂]. A novel mechanism is proposed to account for the rate differences. The X-ray crystal structure of 2W shows that the phosphorus atom of the short phosphine arm lies very close to a carbon atom of the W(CO)₄ equatorial plane (3.40 Å) which could allow “through-space” coupling, accounting in part for the observation of long-range J_PC and J_PW coupling. The X-ray structure of (OC)₅W[κ¹-PPh₂ C(CH₂)PPh₂] 5W has been determined for comparison to 2W.     

Unusual bond activation processes in the reaction of group 4 cyclopentadienyl alkyne complexes with azobenzene

The alkyne complexes [Cp′₂M(L)(η²-Me₃SiC₂SiMe₃)] (Cp′ = substituted or unsubstituted cyclopentadienyl; M = Ti, Zr, Hf; L = Py, THF) can serve as metallocene precursors by substitution of the alkyne molecule with other ligands. The reactions of the unsubstituted cyclopentadienyl complexes [Cp₂Zr(THF)(η²-Me₃SiC₂SiMe₃)] (1) and [Cp₂Ti(η²-Me₃SiC₂SiMe₃)] (2) with azobenzene were investigated. In the first case the diazene complex [Cp₂Zr(THF)(N₂Ph₂)] (3) was obtained by alkyne exchange. In the reaction of the titanium complex 2 a NN bond cleavage of azobenzene and a C-H activation of the cyclopentadienyl ligand were observed and the dinuclear imido bridged compound 4 was formed. This mixed valence complex is bridged additionally by a cyclopentadienyl ligand in a η¹:η⁵-coordination mode which is very unusual for titanium complexes. The molecular structures of both compounds were confirmed by X-ray crystallography and compared to former structural data shown in literature.     

A joint computational and experimental study of a novel dioxomolybdenum(VI) complex bearing chiral N,N-dimethyllactamide ligand

A new cis-dioxomolybdenum complex MoO₂(DMLA)₂ (DMLA = N,N-dimethyllactamide) has been synthesized and characterized by X-ray crystallography, H NMR and IR spectroscopies and electronic structure calculations at DFT/B3LYP level. This compound (chemical formula C₁₀H₂₀MoO₆N₂) crystallizes in the orthorhombic space group P2₁2₁2₁ with Z = 4, a = 6.9357(2) ?, b = 11.8761(4) ?, c = 17.7251(5), V = 1460.00(8) ?³ and renders a slightly distorted octahedral structure with two long Mo-O bonds (2.253(3) ? and 2.257(3) ?) trans to each of the MoO groups and with two short Mo-O bonds of 1.942(3)4 ? cis to them. The MoO bond length are 1.715(3) and 1.704(3) ?). Each lactamide ligand is bidentate; they are coordinated in their deprotonated form with the carbonyl oxygen occupying a position trans to the MoO moiety while the deprotonated hydroxyl oxygen is located cis to them. Structural characterization is complemented by DFT/B3LYP calculations.     

Hydrothermal synthesis, structure characterization and catalytic property of a new 1-D chain built on bi-capped Keggin type mix-valence molybdenum compound: (NH4)[Mo6Mo6O36(AsO4)Mo(MoO)]

The synthesis and structure of a new 1-D molybdenum-arsenic compound based on the bi-capped Keggin anion [Mo^VI₆Mo^V₆O₃₆(AsO₄)(Mo^VO)₂]⁻ have been reported, and its catalytic property has been examined. The title compound was characterized by IR, TG and X-ray diffraction analysis. Single crystal X-ray diffraction shows that it crystallizes in cubic crystal system, space group Pn-3m with cell dimension: a = 11.749(2) Å, V = 1622.0(5) Å³, Z = 2. Its structure has a 1-D infinite chain, in which the bi-capped Keggin anions are connected by sharing one terminal oxygen atom from the caps. The compound shows a moderate styrene conversion (48%), the major product for the oxidation of styrene is benzaldehyde (85.2%).     

Aqueous Speciation of ansa- and non-ansa- Substituted [Cp2Mo(μ-OH)]2[OTs]2

Titration curves were measured for three molybdocene dimers, [Cp₂Mo(μ-OH)]₂[OTs]₂ (4), [Mo(μ-OH)]₂[OTs]₂ (4′; Cp′ = C₅H₄CH₃), and ansa-[C₂Me₄Cp₂Mo(μ-OH)]₂[OTs]₂ (4^a), and for two monomeric molybdocene complexes, Cp₂MoO (6) and Cp₂MoCl₂ (1). The titration curves for 4, 6, and 1 were identical and showed three equivalence points each. The titration curve for 4′ was also similar in appearance but the equivalence points were shifted higher by ∼0.3, as expected for the more electron-rich Mo center in this molecule. The titration curve for the ansa-[C₂Me₄Cp₂Mo(μ-OH)]₂[OTs]₂ complex showed only two equivalence points. Two of the equivalence points observed in the titration of 4, 6, and 1 were previously reported in potentiometric measurements of aqueous solutions of Cp₂MoCl₂ and were attributed to the Cp₂Mo(OH₂)²⁺ species (pK_a = 5.5 ± 0.1 and 8.3 ± 0.2). The third equivalence point (pK_a = 2.2 ± 0.2) is assigned to protonation/deprotonation of the [Cp₂Mo(μ-OH)]₂[OTs]₂/[Cp₂Mo(μ-OH₂)(μ-OH)MoCp₂]³⁺ dimer. A new equilibrium scheme is proposed for the aquated molybdocenes to provide a more complete picture of the aqueous speciation of the non-ansa molybdocene complexes, specifically by accounting for the third acidic proton in the titration curves and by describing the hydrolysis of Cp₂MoO. Although the titration curve of the ansa-[C₂Me₄Cp₂Mo(μ-OH)]₂[OTs]₂ complex is different from that of [Cp₂Mo(μ-OH)]₂[OTs]₂, ¹H NMR data suggest that the aqueous speciation of the ansa-[C₂Me₄Cp₂Mo(μ-OH)]₂[OTs]₂ complex is analogous to that of the non-ansa molybdocenes.     

New molybdenum(V) complexes based on the {Mo2O4} structural core with esters or anions of malonic and succinic acid

A series of malonato complexes of molybdenum(V) was prepared by reacting (PyH)₅[MoOCl₄(H₂O)]₃Cl₂ or (PyH)_n[MoOBr₄]_n with malonic acid (H₂mal) or a half-neutralized acid, hydrogen malonate (Hmal⁻), at ambient conditions: (PyH)₃[Mo₂O₄Cl₄(μ₂-Hmal)] · CH₃CN (1), (PyH)₃[Mo₂O₄Br₄(μ₂-Hmal)] · CH₃CN (2), (PyH)₂[Mo₂O₄Cl(η²-mal)(μ₂-Hmal)Py] (3), (3,5-LutH)₂(H₃O) [Mo₂O₄(η²-mal)₂(μ₂-Hmal)] (4), (PyH)[Mo₂O₄Cl₂(μ₂-Memal)Py₂] (5), (3,5-LutH)[Mo₂O₄Cl₂(μ₂-Memal)(3,5-Lut)₂] (6), (PyH)[Mo₂O₄Cl₂(μ₂-Etmal)Py₂] (7), (3,5-LutH)[Mo₂O₄Cl₂(μ₂-Prmal)(3,5-Lut)₂] (8) and [{Mo₂O₄(μ₂-Memal)Py₂}₂(μ₂-OCH₃)₂] (9) (where Py = pyridine, C₅H₅N; PyH⁺ = pyridinium cation, C₅H₅NH⁺; 3,5-Lut = 3,5-lutidine, C₇H₉N; 3,5-LutH⁺ = 3,5-lutidinium cation, C₇H₉NH⁺; mal²⁻ = malonate, ⁻OOCCH₂COO⁻; Memal⁻ = monomethyl malonate, ⁻OOCCH₂COOCH₃; Etmal⁻ = monoethyl malonate, ⁻OOCCH₂COOC₂H₅ and Prmal⁻ = monopropyl malonate, ⁻OOCCH₂COOC₃H₇). The complex anions of compounds 1-8 have a common structural feature: a dinuclear, singly metal-metal bonded {Mo₂O₄}²⁺ core with the carboxylate moiety of the malonato ligand coordinated in a syn-syn bidentate bridging manner to the pair of metal atoms. The remaining four coordination sites of the {Mo₂O₄}²⁺ core are occupied with halides in 1 and 2, with halides/pyridine ligands in 5-8, with a pair of bidentate malonate ions in 4 and with the combination of all in 3. The neutral molecules of 9 consist of two {Mo₂O₄}²⁺ cores linked with a pair of methoxide ions into a chain-like, tetranuclear cluster. An esterification of malonic acid was observed to take place in the reaction mixtures containing alcohols. Solvothermal reactions with malonic acid carried out at 115 °C produced anionic acetato complexes as found in (PyH)[Mo₂O₄Cl₂(μ₂-OOCCH₃)Py₂] · Py (10), (PyH)[Mo₂O₄Cl₂(μ₂-OOCCH₃)Py₂] (11), (3,5-LutH)[Mo₂O₄Cl₂(μ₂-OOCCH₃)(3,5-Lut)₂] (12) and (4-MePyH)₃[Mo₂O₄Cl₂(μ₂-OOCCH₃)(4-MePy)₂]₂Cl (13) (4-MePy = 4-methylpyridine, C₆H₇N). The acetate coordinated in the syn-syn bidentate bridging mode in all. Reactions of (PyH)₅[MoOCl₄(H₂O)]₃Cl₂ with succinic acid (H₂suc) at ambient conditions resulted in a complex with a half-neutralized acid, (PyH)[Mo₂O₄Cl₂(μ₂-Hsuc)Py₂] · Py (14) (Hsuc⁻ = hydrogen succinate, ⁻OOC(CH₂)₂COOH), while those carried out at 115 °C in a tetranuclear succinato complex, (4-MePyH)₂[{Mo₂O₄Cl₂(4-MePy)₂}₂(μ₄-suc)] (15) (suc²⁻ = succinate, ⁻OOC(CH₂)₂COO⁻). The tetranuclear anion of 15 consists of two {Mo₂O₄}²⁺ cores covalently linked with a tetradentate succinato ligand. The compounds were fully characterized by infrared vibrational spectroscopy, elemental analyses and X-ray diffraction studies.     

Binding of copper(II) to thyrotropin-releasing hormone (TRH) and its analogs

Spectroscopy (UV-Vis, ¹H NMR, ESR) and electrochemistry revealed details of the structure of the Cu(II)-TRH (pyroglutamyl-histidyl-prolyl amide) complex. The ¹H NMR spectrum of TRH has been assigned. NMR spectra of TRH in the presence of Cu(II) showed that Cu(II) initially binds TRH through the imidazole. TRH analogs, pGlu-His-Pro-OH, pGlu-(1-Me)His-Pro-amide, pGlu-His-(3,4-dehydro)Pro-amide, pGlu-His-OH, pGlu-Glu-Pro-amide, and pGlu-Phe-Pro-amide provided comparison data. The stoichiometry of the major Cu(II)-TRH complex at pH 7.45 and greater is 1:1. The conditional formation constant (in pH 9.84 borate with 12.0 mM tartrate) for the formation of the complex is above 10⁵ M⁻¹. The coordination starts from the 1-N of the histidyl imidazole, and then proceeds along the backbone involving the deprotonated pGlu-His amide and the lactam nitrogen of the pGlu residue. The fourth equatorial donor is an oxygen donor from water. Hydroxide begins to replace the water before the pH reaches 11. Minority species with stoichiometry of Cu-(TRH)_x (x = 2-4) probably exist at pH lower than 8.0. In non-buffered aqueous solutions, TRH acts as a monodentate ligand and forms a Cu(II)-(TRH)₄ complex through imidazole nitrogens. All the His-containing analogs behave like TRH in terms of the above properties.     

Naphthyridine-imidazole hybrid ligands for the construction of multinuclear architecture

Reaction of 2-imidazolyl-5,7-dimethyl-1,8-naphthyridine (L¹) with [Rh(COD)Cl]₂ (COD = 1,5-cyclooctadiene) affords the dinuclear complex [Rh(COD)Cl]₂(μ-L¹) (1). Elimination of chloride from the metal coordination sphere leads to a self-assembled tetranuclear macrocycle [Rh(COD)L¹]₄[ClO₄]₄ (2). A subtle alteration in the ligand framework results in the polymeric chain compound {Rh(COD)(L²)}_n(PF₆)_n (3) (L² = 2-imidazolyl-3-phenyl-1,8-naphthyridine). In all these complexes, the imidazole nitrogen and one of the naphthyridine nitrogen (away from the imidazole substituent) bind the metal. The ‘parallel’ and ‘perpendicular’ dispositions of nitrogens are observed in these compounds contributing to different Rh···Rh separations. The L¹ ligand adopts planar configuration, whereas the naphthyridine-imidazole rings deviate from planarity in L² yielding a polymeric structure. The extent of deviation is less in the polymeric structure {Mo₂(OAc)₄(L²)}_n (4) in which the ligand exhibits weak axial interactions to the metal.     

Preparation of new tetraaza macrocyclic nickel(II) and copper(II) complexes bearing N-propionic methyl ester groups

A 14-membered tetraaza macrocycle, 2,13-bis(2-carbomethoxyethyl)-5,16-dimethyl-2,6,13,17-tetraazatricyclo[16.4.0.^1.180^7.12]docosane (L²) bearing two N-CH₂CH₂COOMe groups, and its nickel(II) and copper(II) complexes have been prepared and characterized. The nickel(II) and copper(II) complexes of 2-(2-carbomethoxyethyl)-5,16-dimethyl-2,6,13,17-tetraazatricyclo[16.4.0.^1.180^7.12]docosane (L³) containing one N-CH₂CH₂COOMe group have also been prepared. The crystal structure of [NiL²](ClO₄)₂ shows that the complex has a slightly distorted trans-octahedral coordination geometry with two relatively short axial Ni-O (N-CH₂CH₂COOMe group) bonds (2.136(3) Å). In various solvents, however, a considerable proportion of [NiL²]²⁺ exists as a square-planar form, in which the functional pendant arms are not involved in coordination. The proportion of the square-planar isomer varies with solvents in the order of nitromethane ? acetonitrile < H₂O < DMF ? DMSO. In the case of [CuL²](ClO₄)₂, only one N-CH₂CH₂COOMe group is involved in coordination. The N-CH₂CH₂COOMe group of [NiL³](ClO₄)₂ is not directly involved in coordination even in the solid state, though the functional group of [CuL³](ClO₄)₂ is coordinated to the metal ion.     

Extraction properties for alkali metal picrates of macrocyclic trinuclear (p-cymene)ruthenium(II) and (pentamethylcyclopentadienyl)rhodium(III) complexes

The solvent extraction properties of macrocyclic trinuclear organometallic complexes, [(p-cymene)Ru(pyO₂)]₃ and [Cp^∗Rh(pyO₂)]₃, for Li⁺, Na⁺, and K⁺ picrates have been investigated in a dichloromethane-water system at 25 °C. The extraction rates of the alkali metal picrates with these macrocyclic complex ligands are unusually slow; the shaking times required to attain equilibrium are at least 1 h for [(p-cymene)Ru(pyO₂)]₃ and 20-40 h for [Cp^∗Rh(pyO₂)]₃. From analysis of the equilibrium data, the extraction constants (K_ex = [ML⁺A⁻]_o/[M⁺][L]_o[A⁻]; M⁺ = alkali metal ion, L = macrocyclic ligand, A⁻ = picrate ion, o = organic phase) have been determined. The log K_ex value varies in the sequences, Li⁺ (5.72) > Na⁺ (4.50) > K⁺ (2.88) for [(p-cymene)Ru(pyO₂)]₃ and Li⁺ (4.79) > Na⁺ (2.70) ≈ K⁺ (2.69) for [Cp^∗Rh(pyO₂)]₃. The K_ex values of 6,6-dibenzyl-14-crown-4 (DBz14C4), which is one of the best Li⁺-selective crown ethers, have also been determined for comparison. It is revealed that [Cp^∗Rh(pyO₂)]₃ is much superior to DBz14C4 both in the extractability for Li⁺ and the selectivity for Li⁺ over Na⁺.     

Protonated N-heterocycle-templated supramolecular frameworks built of triangular molybdenum(IV) clusters

A series of porous supramolecular complexes (Hoxine)₂ · [Mo₃O₄(C₂O₄)₃(H₂O)₃] · 5H₂O (1),(Hphen)₂ · [Mo₃O₄(C₂O₄)₃(H₂)₃]  · 0.5C₂H₅OH · 7H₂O (2), H₂bpy · [Mo₃O₄(C₂O₄)₃(H₂O)₃] · 2.5H₂O (3), H₂TTD · [Mo₃O₄(C₂O₄)₃(H₂O)₃] · C₂H₅OH · 3H₂O (4), (oxine = 8-hydroxyquinoline, phen = 1,10-phenanthroline, bpy = 4,4′-bipyridine, TTD = triethylene diamine) have been prepared and characterized by single-crystal X-ray crystallography, elemental analysis and infrared spectroscopy. Self-assembly of [Mo₃O₄(C₂O₄)₃(H₂O)₃]²⁻ directed by H-bonding association between the coordination water molecules and oxalate groups forms 2-D host H-bonded single layer in 1, double layer in 2 and 3, and undulated layer in 4 depending on the nature of the guest protonated N-heterocycles. Unlike cis-Hoxine⁺ or Hphen⁺ that employs lattice water molecules H-bonded to them to interconnect the host layers, trans-H₂bpy²⁺ or H₂TTD²⁺ acts a linker between the neighboring host layers to form 3-D supramolecular frameworks with channeled structures wherein the guest protonated cations are located.     

One-dimensional azido bridge [1-ethyl-2-(phenylazo)imidazole]manganese(II): Structure of low bridge angle system and magnetism

The reaction of 1-ethyl-2-(phenylazo)imidazole (PaiEt), NaN₃ and Mn(OAc)₂ · 4H₂O at 1:2:1 molar ratio has isolated azido bridge 1-D coordination polymer. In monomeric unit of the complex, MnN₆ distorted octahedral coordination is achieved by two EO-N₃, two EE-N₃ and N,N′-chelation of PaiEt. Bridge angle Mn-(EO)N₃-Mn is 98.55(10)°. Variable temperature (300-2 K) magnetic measurements determine J values −13.63 ± 0.10 and 2.31 ± 0.15 cm⁻¹ and support antiferromagnetic (EE-N₃) and ferromagnetic (EO-N₃) interaction, respectively. Maximum susceptibility (χ_M = 0.0251 cm³ mol⁻¹) is observed at 43 K.     

Influence of substituents on the structures of hybrid complexes constructed from tetranuclear copper(I) 1,2,4-triazolate clusters and octamolybdates

By changing the substituents on 1,2,4-triazole ring, six novel organic-inorganic hybrid complexes constructed from tetranuclear copper(I) 1,2,4-triazolate clusters and octamolybdates, [{Cu₄(L)_x}Mo₈O₂₆] (L = 3,5-diamino-1,2,4-triazole (datrz) and x = 4 for 1; L = 3-amino-1,2,4-triazole (3atrz) and x = 4 for 2; L = 3,5-dimethyl-1,2,4-triazole (dmtrz) and x = 4 for 3; L = 3,5-dimethyl-4-amino-1,2,4-triazole (dmatrz) and x = 6 for 4; L = 3,5-diethyl-4-amino-1,2,4-triazole (deatrz) and x = 4 for 5; L = 3,5-di(n-propyl)-4-amino-1,2,4-triazole (dpatrz) and x = 3 for 6), were obtained. The tetranuclear Cu(I) cluster in compound 1 acts as charge-compensating unit, which is the first polynuclear metal 1,2,4-triazole structure only with N1, N2 bridging mode. Compounds 2, 4, 5 and 6 are of polymeric 1D chains and 3 is of a 2D layer structure. In 2, three distinct Cu(I)-coordination geometries, distorted tetrahedral, T-shaped and V-shaped linear Cu(I), are observed in the same structure. The first extended hybrid structure constructed by δ-octamolybdates is founded in 4. A novel [Mo₈O₂₆]⁴⁻ anion is found in 5, which contains only three crystallographically independent Mo atoms. In compounds 5 and 6, terminal oxo groups of octamolybdate cluster act as μ₃-oxo bridges to link the copper(I) coordination complexes; such an unusual linking manner is unique in the coordination chemistry of octamolybdates with transition metal fragments. The influences of substituent on the structures of the tetranuclear units are also discussed in details.     

Rational design, synthesis and evaluation of first generation inhibitors of the Giardia lamblia fructose-1,6-biphosphate aldolase

Inhibitors of the Giardia lamblia fructose 1,6-bisphosphate aldolase (GlFBPA), which transforms fructose 1,6-bisphosphate (FBP) to dihydroxyacetone phosphate and glyceraldehyde 3-phosphate, were designed based on 3-hydroxy-2-pyridone and 1,2-dihydroxypyridine scaffolds that position two negatively charged tetrahedral groups for interaction with substrate phosphate binding residues, a hydrogen bond donor to the catalytic Asp83, and a Zn²⁺ binding group. The inhibition activities for the GlFBPA catalyzed reaction of FBP of the prepared alkyl phosphonate/phosphate substituted 3-hydroxy-2-pyridinones and a dihydroxypyridine were determined. The 3-hydroxy-2-pyridone inhibitor 8 was found to bind to GlFBPA with an affinity (K_i = 14 μM) that is comparable to that of FBP (K_m = 2 μM) or its inert analog TBP (K_i = 1 μM). The X-ray structure of the GlFBPA-inhibitor 8 complex (2.3 Å) shows that 8 binds to the active site in the manner predicted by in silico docking with the exception of coordination with Zn²⁺. The observed distances and orientation of the pyridone ring O=C-C-OH relative to Zn²⁺ are not consistent with a strong interaction. To determine if Zn²⁺coordination occurs in the GlFBPA-inhibitor 8 complex in solution, EXAFS spectra were measured. A four coordinate geometry comprised of the three enzyme histidine ligands and an oxygen atom from the pyridone ring O=C-C-OH was indicated. Analysis of the Zn²⁺ coordination geometries in recently reported structures of class II FBPAs suggests that strong Zn²⁺ coordination is reserved for the enediolate-like transition state, accounting for minimal contribution of Zn²⁺ coordination to binding of 8 to GlFBPA.     

Structural comparison of (pentamethylcyclopentadienyl)iridium(III) azido complexes containing potentially N,S-bidentate N-heterocyclic thiolates: 2- or 8-quinolinethiolate and benzimidazole-2-thiolate

The crystal structures of mononuclear (azido)(pentamethylcyclopentadienyl)iridium(III) complexes bearing 2- or 8-quinolinethiolate (n-Sqn⁻), [Cp^∗Ir(N₃)(n-Sqn)] {n = 2 (1) or 8 (2); Cp^∗ = η⁵-C₅Me₅} have been determined by X-ray analysis. The 2-Sqn complex, 1, acquires severe steric strains in the four-membered κ²N,S chelate ring, while the 8-Sqn isomer, 2, forms a strain-free five-membered planar κ²N,S chelate ring. It has also been revealed that the corresponding benzimidazole-2-thiolate (Hbimt⁻) complex, which was obtained similarly to the above n-Sqn complexes from [Cp^∗Ir(N₃)₂]₂ and Na(Hbimt), takes an unsymmetrical dinuclear structure bridged by two Hbimt⁻ ligands with different bonding modes, [Cp^∗Ir(N₃){μ(S:N¹)-Hbimt}{μ(S:S)-Hbimt}Ir(N₃)Cp^∗] · MeOH (3).     

One-pot syntheses of alkenyl-phosphonio complexes of ruthenium(II), rhodium(III) and iridium(III) bearing p-cymene or pentamethylcyclopentadienyl groups

Reaction of [(p-cymene)RuCl₂(PPh₃)] (1) or [Cp^∗MCl₂(PPh₃)] (Cp^∗ = C₅Me₅) (3a: M = Rh; 4a: M = Ir) with 1-alkynes and PPh₃ were carried out in the presence of KPF₆, generating the corresponding alkenyl-phosphonio complexes, [(p-cymene)RuCl(PPh₃){CHCR(PPh₃)}](PF₆) (2a: R = Ph; 2b: R = p-tolyl) or [Cp^∗MCl(PPh₃){CHCPh(PPh₃)}](PF₆) (5: M = Rh; 6: M = Ir). Similar reactions of complexes [Cp^∗RhCl₂(L¹)] (3a: L¹ = PPh₃; 3c: L¹ = P(OMe)₃) with L² (L² = PPh₃, PMePh₂, P(OMe)₃) gave [Cp^∗RhCl(L¹)(L²)](PF₆) (7bb: L¹ = L² = PMePh₂; 7ca: L¹ = P(OMe)₃, L² = PPh₃; 7cc: L¹ = L² = P(OMe)₃). Alkenyl-phosphonio complex 5 was treated with P(OMe)₃ or 2,6-xylyl isocyanide, affording [Cp^∗RhCl(L){CHCPh(PPh₃)}](PF₆) (8a: L = P(OMe)₃; 8b: L = 2,6-xylNC). X-ray structural analyses of 2a, 6 and 8a revealed that the phosphonium moiety bonded to the C_β atom of the alkenyl group are E configuration.     

Synthesis, structure and redox properties of an unexpected trinuclear copper(II) complex with aspartame: [Cu(apm)2Cu(μ-N,O:O′-apm)2(H2O)Cu(apm)2(H2O)] · 5H2O

Structural, magnetic and spectroscopic data of a new trinuclear copper(II) complex with the ligand aspartame (apm) are described. [Cu(apm)₂Cu(μ-N,O:O′-apm)₂(H₂O)Cu(apm)₂(H₂O)] · 5H₂O crystallizes in the triclinic system, space group P1 (#1) with a = 7.3300(1) Å, b = 15.6840(1) Å, c = 21.5280(1) Å, α = 93.02(1)°, β = 93.21(1)°, γ = 92.66(1)° and Z = 1. Aspartame coordinates to Cu(II) through the carboxylate and β-amino groups. The carboxylate groups of the two central ligands act as bidentate bridges in a syn-anti conformation while the carboxylate groups of the four peripheral ligands are monodentate in a syn conformation. The central copper ion is in a distorted square pyramidal geometry with the apical position being occupied by one oxygen atom of the water molecule. The two terminal copper(II) atoms are coordinated to the ligands in the same position but their coordination sphere differs from each other due to the fact that one copper atom has a water molecule in an apical position leading to an octahedral coordination sphere while the other copper atom is exclusively coordinated to aspartame ligands forming a distorted square pyramidal coordination sphere. Thermal analysis is consistent with the X-ray structure. EPR spectra and CV curves indicate a rupture of the trinuclear framework when this complex is dissolved in ethanol or DMF, forming a mononuclear species, with a tetragonal structure.     

P NMR and Raman spectroscopic and voltammetric studies on the formation and conversion processes of Keggin-type molybdotungstophosphate(V) and -arsenate(V) complexes in aqueous-organic solvents

The formation conditions of Keggin-type molybdotungstophosphate(V) (PW_xMo_12 − xO₄₀ ³⁻ (PW_xMo_12 − x), x=0-12) and -arsenate(V) (AsW_xMo_12 − xO₄₀ ³⁻ (AsW_xMo_12 − x)) complexes in aqueous-organic solvents were investigated with ³¹P NMR, Raman spectroscopy, and cyclic voltammetry. The conversion processes of the PMo₁₂ and the PW₁₂ anions into the PW_xMo_12 − x anions were also examined and compared with those of the AsMo₁₂ and AsW₁₂ anions into the AsW_xMo_12 − x anions. The mean vibration frequencies of the PW_xMo_12 − x and AsW_xMo_12 − x (x=1-11) complexes were calculated in IR and Raman spectra from those of the PMo₁₂ and PW₁₂, and AsMo₁₂ and AsW₁₂ complexes, respectively.