Showing papers in "European Journal of Inorganic Chemistry in 2000"

Second Coordination Sphere Water Molecules and Relaxivity of Gadolinium(III) Complexes: Implications for MRI Contrast Agents

Abstract: Dipolar interaction between the metal ion and proximate water molecules represents an efficient mechanism for solvent relaxation in Gd3+ complexes currently employed as MRI contrast agents. Besides inner sphere (metal bound) and outer sphere hydration molecules, a well-defined second coordination shell may provide an additional mechanism for paramagnetic relaxation leading to a strong enhancement of the relaxivity of the complexes. Through a careful choice of hydrogen-bond-acceptor groups on the ligand we may: (1) promote the formation of a strong interaction; (2) increase the number of water molecules in the second hydration shell; (3) decrease their average distance from the paramagnetic metal center. These possibilities have been explored by considering complexes bearing phosphinate, phosphonate and carboxoamide pendant arms, by exploiting the formation of ion-pairs with cationic substrates and inclusion compounds of these adducts with -cyclodextrin. Finally, the contribution of this relaxation mechanism to the relaxivity of the commercially available MRI contrast agents is discussed and the NMRD data reevaluated and compared with crystallographic data.β

Microporous Titanosilicates and other Novel Mixed Octahedral‐Tetrahedral Framework Oxides

Abstract: Stable microporous materials, such as zeolites, are extremely important for applications in catalysis, adsorption, ion-exchange, and separation. In this review we describe a new class of stable microporous materials that involves novel mixed octhaedral-tetrahedral framework oxides. The archetypal material is based on titanosilicates, although there is tremendous scope for introducing many other transition metals. These materials not only have potential novel applications in the fields normally associated with zeolites but also possible applications in the areas of optoelectronics, nonlinear optics, batteries, magnetic materials and sensors.

Synthesis and Properties of Dinuclear Complexes with a Photochromic Bridge: An Intervalence Electron Transfer Switching “On” and “Off”

Abstract: Two cyclometallated Ru(bpy)2(pp) units (bpy = 2,2′-bipyridine; pp = 2-phenylpyridine) were grafted via ethynyl spacers onto the photochromic cores of the norbornadiene or dithienylethene families, to give the bimetallic ruthenium(II) complexes 1 and 2a, respectively. Two aspects of these compounds were studied: (i) photoisomerization (norbornadiene to quadricyclane for 1, open form to closed form of the dithienylethene core for 2a), and (ii) intervalence transitions in the mixed-valence ruthenium(II−III) state, with determination of the metal−metal electronic coupling (Vab) by Hush’s equation. Moderate metal−metal electronic coupling (0.068 eV) was found for 1, but 1 cannot be isomerized into the quadricyclane form. On the other hand, 2a can be reversibly photoisomerized to the closed form 2b. An iodine(III) reagent, the triflate salt of bis(pyridyl)phenyliodonium, was used for the oxidation of 2a and 2b. During these oxidations, an intervalence band due to intramolecular electron transfer between ruthenium(II) and ruthenium(III) was detected for 2b, but not for 2a. This switching “on” or “off” of the intervalence transition is rationalized with extended Huckel calculations. Finally, the molecular orbital calculation explains the observed tendency of the closed form 2b to reopen upon oxidation.

Reactivity of Copper(I) Complexes Towards Dioxygen

Abstract: Efforts to model the reactions of copper enzymes with dioxygen with low molecular weight complexes have been quite successful during recent years. Guided by Nature, intriguing model systems for the active sites of such proteins have been developed. In the course of these investigations, it has been discovered that a whole variety of copper−dioxygen adducts form when simple copper(I) complexes are oxidized with dioxygen. The course of these reactions depends on the temperature, the ligands present, and the solvent. The effects of these parameters (especially the effect of ligand modification) on the reactivity of copper(I) complexes towards dioxygen are discussed in detail in this review.

Dipolar and Non‐Dipolar Pyridine and Bipyridine Metal Complexes for Nonlinear Optics

Abstract: Metal complexes of pyridine and bipyridine-type ligands represent an important class of nonlinear optical (NLO) materials: (i) They can incorporate a variety of functionalised ligands with a wide range of metals which can give rise to tuneable NLO properties; (ii) they are generally associated with intense low-lying charge-transfer excitations such as metal to ligand charge-transfer (MLCT) or intraligand charge-transfer (ILCT); (iii) bipyridyl ligands are also good building blocks that allow the construction of octupolar complexes with a defined geometry and symmetry. This review describes the recent developments in the design of such dipolar and octupolar chromophores for second-order nonlinear optics.

Phosphaalkenes with Inverse Electron Density

Abstract: This review gives an account of the syntheses, structures, and properties of phosphaalkenes (RP)-P-1=(CRR3)-R-2 with an inverse electron distribution at the P-C double bond (Pdelta-Cdelta+). The chemical reactivity of compounds of this type is novel and quite surprising compared to the chemistry of "classically polarized" phosphaalkenes (Pdelta+Cdelta-).

Two Examples of Novel and Unusual Double-Layered, Two-Dimensional CuII Compounds with Bridging 1,3-Bis(1,2,4-triazol-1-yl)propane

Abstract: The synthesis and characterisation of two new polymeric CuII complexes is described, i.e. {[Cu(btp)2(CH3CN)(H2O)](CF3SO3)2}n (1) and {[Cu(btp)2(CH3CN)2](ClO4)2}n (2), in which btp = [1,3-bis(1,2,4-triazol-1-yl)propane]. Compound 1 crystallizes in space group P21/c with a = 11.9337(15) A, b = 20.108(6) A, c = 12.748(6) A, β = 92.247(14)°, and Z = 4. Compound 2 crystallizes in space group Pna21 with a = 18.770(8) A, b = 12.648(8) A, and c = 12.019(8) A. The structures refined to R1 values of 0.0683 for 1 and 0.0846 for 2. In both structures the CuII ions are linked by the bridging ligands, resulting in two-dimensional networks. Two such curved layers are arranged on top of each other with center-to-center of layer distances of 2.12 A in 1 and 1.98 A in 2. Such double layers are separated from each other by 10.05 A in 1 and 9.385 A in 2. The space between the double layers is occupied with interstitial anions. No significant interaction between CuII ions is observed by EPR and magnetic susceptibility measurements. The compounds form a new class of a lattice engineered system held together by the CuII ions. – The coordination geometry of the copper ions is distorted octahedral, with the equatorial plane formed by the N4 nitrogens of the four triazole groups and the axial sites occupied by solvent molecules; acetonitrile and water in structure 1 and two acetonitrile molecules in structure 2. The two structures are related by a group–subgroup relationship, which appears to be the first such case in supramolecular chemistry. – The Cu–N vibrations in the FIR region are found at 274 cm–1 for 1, and at 276 cm–1 for 2. The ligand-field maxima are observed at about 16·103 cm–1, with a shoulder at about 12·103 cm–1. The νCN stretching vibrations of the acetonitrile molecules are found at 2303 and 2261 cm–1 for 1, and at 2313, 2294, 2278, and 2260 cm–1 for 2.

Fluorescent Chemosensors Containing Polyamine Receptors

Abstract: Chemosensors have attracted interest in many different scientific fields, such as environmental chemistry, medicine, and the processing and storage of information. These molecular-scale devices have the advantage of working on the same spatial scale as the chemical structures that are responsible for macroscopic behaviour observed in the environment or those associated with health problems. Moreover, they allow the construction of molecular-scale devices for information storage. In this review, we describe a family of chemosensors based on a polyamine receptor and a fluorescent signalling unit. Polyamine receptors are water-soluble ambidentate receptors; they are able to coordinate either metal ions, when sufficient deprotonated amino groups are available, or anionic species, when there are sufficient protonated amino groups. On the other hand, the use of fluorescent signalling units confers the advantage of an immediate visual response/signal.

Highly Lewis Acidic Bifunctional Organoboranes

Abstract: Chelating Lewis acids have not been developed to the same degree as multidentate Lewis bases. Nonetheless, such compounds have attracted interest due to their potential for the enhanced activation of basic substrates and the selective binding of anions. Organodiboryl compounds form a class of bidentate Lewis acids which have a long, but relatively underdeveloped history. Many examples exist where donor groups on boron serve to stabilize the Lewis acid centers. More recently, advances in the chemistry of diboryls with highly Lewis acidic boron centers substituted with perfluoroaryl groups have been made. In particular, compounds of general formula (F5C6)2B−linker−B(C6F5)2 have been prepared and their chemistry examined. In this Microreview, we survey the classes of bifunctional boron Lewis acids known, including their synthesis, properties and anion binding chemistry. Particular emphasis is placed on the role these Lewis acids play in olefin polymerization catalyst generation from simple metallocene precursors.

FeIII‐Hydroperoxo and Peroxo Complexes with Aminopyridyl Ligands and the Resonance Raman Spectroscopic Identification of the Fe−O and O−O Stretching Modes

Abstract: Nonheme Fe(III)-hydroperoxo and Fe(III)-peroxo complexes with aminopyridyl-type ligands have been prepared and characterized by UV/Vis, EPR, mass and Resonance Raman (RR) spectroscopy. The Fe(III)(OOH) species are low-spin and exhibit a deep purple color due to the ligand-to-metal charge transfer (LMCT) hand centered at ca. 550 nm. The RR spectra of the Fe(III)(OOH) complexes display two bands at ca. 620 and 800 cm-1 that are assigned to the respective Fe-O and O-O stretching modes on the basis of the characteristic H/D and 16O/18O frequency shifts. Upon deprotonation, Fe(III)(O2) species are obtained which possess a high-spin configuration of nearly axial symmetry and a LMCT transition in the near infrared (ca. 750 nm). The frequencies of the Fe-O and O-O stretching modes at ca. 465 and 820 cm-1, as well as their respective 16O/18O shifts of -16 and -45 cm-1, indicate an ?2 coordination geometry for the Fe(III)(O2) complex.

Rhodium and Iridium β-Diiminate Complexes – Olefin Hydrogenation Step by Step

Abstract: The bulky β-diiminate ligands [(2,6-C6H3X2)NC(Me)CHC(Me)N(2,6-C6H3X2)]– (X = Me, LMe; X = Cl, LCl) have been found to be effective in stabilizing low coordination numbers (CN) in Rh and Ir complexes. The 14- complex LMeRh(COE) (COE = cyclooctene) has a three-coordinate T-shaped Rh environment and is nonagostic. Coordinative unsaturation is avoided by incorporation of a small ligand (e.g. N2, MeCN, olefins), by the intramolecular coordination of a chlorine atom in LClRh(COE), or by an agostic interaction in LMeRh(norbornene). In solution at room temperature, LMeRh(COE) undergoes rapid isomerization according to the allyl hydride mechanism; the corresponding 2,3-dimethylbutene complex actually prefers the allyl hydride structure. Rhodium(I) complexes of LMe and LCl catalyze olefin hydrogenation; hydrogenation of 2,3-dimethylbutene has been shown to be preceded by isomerization. The shielding properties of the bulky β-diiminate ligands allow direct observation of a number of reactive intermediates or their iridium analogues, including an olefin–dihydrogen complex (with Rh) and an olefin dihydride (with Ir). These observations, together with calculations on simple model systems, provide us with snapshots of a plausible hydrogenation cycle. Remarkably, hydrogenation according to this cycle appears to follow a 14-e/16-e path, in contrast to the more usual 16-e/18-e paths.

Synthesis, Crystal Structure, Magnetic Properties and 57Fe Mössbauer Spectroscopy of the New Trinuclear [Fe3(4‐(2′‐hydroxyethyl)‐1,2,4‐triazole)6(H2O)6](CF3SO3)6 Spin Crossover Compound

Abstract: Hyetrz = 4-(2prime-hydroxyethyl)-1,2,4-triazole represents the first structurally characterized iron(II) spin crossover compound for which the structure has been solved above and below room temperature in both spin states. The compound crystallizes in the trigonal system, space group R3, a = 12.763(1) b= 67.144(1), V = 1684.3(1) 3, Z = 6 at 120K. At 330K the space group is retained and a = 13.0183(3) b=67.376(3), V = 1805.8(1) 3. The molecular structure consists of trinuclear FeII entities linked together by an unprecedented three-dimensional network of hydrogen bonds. This compound shows a gradual spin crossover behavior centered around room temperature, which has been followed by temperature-dependent magnetic susceptibility measurements as well as by 57Fe M ssbauer spectroscopy. The nature of the spin crossover behavior is discussed on the basis of the structural features.

Methyl(2-phosphanylphenolato[P,O])nickel(II) Complexes – Synthesis, Structure, and Activity as Ethene Oligomerization Catalysts

Abstract: Reactions of various substituted 2-phosphanylphenols 1a–f with half-molar amounts of cis-[MeNi(μ-OMe)(PMe3)]2 have been found to yield square-planar methyl(2-phosphanylphenolato)(trimethylphosphane)nickel(II) complexes 2a–f. 2JPP coupling constants of 305–316 Hz at low temperature indicate a trans-configuration for the products, while broad 31P-NMR signals at room temperature can be attributed to rapid dissociation of PMe3. Reaction with excess 1 gave rigid bis(2-phosphanylphenolato)nickel(II) complexes as exemplified by 3e, whereas addition of PMe3 to 2a led to the pentacoordinate methyl(2-phosphanylphenolato)bis(trimethylphosphane)nickel(II) complex 4a. Higher yields of 4a and 4d were obtained by reactions of 1a and 1d with Me2Ni(PMe3)3. Single-crystal X-ray diffraction analyses of 3e and 4a have revealed the structures as square-planar trans-bis- and trigonal-bipyramidal mono(2-phosphanylphenolato)nickel(II) P1O-chelate complexes, respectively. The methylnickel complexes 2 and 4 have been found to be effective one-component catalysts for the oligomerization of ethene. High conversions (> 96%) were achieved with the P-basic derivatives 2e, 2f, and 4d bearing one or two branched alkyl groups (isopropyl, tert-butyl), whereas the diphenylphosphanyl derivatives were less active; 4d gave shorter oligomers than 2e or 2f.

Microwave Assisted Rapid Incorporation of Ruthenium into Lacunary Keggin-Type Polyoxotungstates: One-step Synthesis, 99Ru, 183W NMR Characterization and Catalytic Activity of [PW11O39RuII(DMSO)]5-

Abstract: Microwave irradiation for 15 minutes allowed the highly selective hydrothermal synthesis of the diamagnetic, air-stable oxidation catalyst [PW11O39RuII(DMSO)]5–, as confirmed by 1H, 31P, 99Ru and 183W NMR techniques.

New Insights into the Mechanism of Hydroperoxide Activation by Investigation of Dynamic Processes in the Coordination Sphere of Seven-Coordinated Molybdenum Peroxo Complexes

Abstract: Seven-coordinated molybdenum oxobisperoxo complexes with chelate nitrogen donors like pyrazolylpyridines are catalysts for the epoxidation of olefins. An NMR spectroscopic and quantumchemical study on the fluxionality of the chelate ligand proves that during this process partial ligand dissociation takes place. This gave rise to a detailed theoretical study on the activation of CH3OOH at the model complex (NH3)2MoO(O2)2 including dissociation of one of the ammonia ligands and proton transfer from the hydroperoxide to one of the peroxo ligands.

Synthesis and X-ray Crystal Structures of (C5HiPr4)Ln(BH4)2(THF)(Ln = Nd and Sm), Versatile Precursors for Polymerization Catalysts

Abstract: The new half metallocenes [(C5HiPr4)Ln(BH4)2(THF)] [Ln = Sm (1) and Nd (2)], and [(C5HiPr4)U(BH4)3] (3) have been synthesized. The crystal structures of 1 and 2 and of the metallocenes [(C5HiPr4)2Ln(BH4)] [Ln = Sm (4) and Nd (5)] have been determined. The substitution of the BH4 groups of 1 and 2 by a nitrogen-based ligand is possible. In the presence of butyllithium, these complexes show an activity in isoprene and styrene polymerization.

Sulfonamide‐Functionalized Gadolinium DTPA Complexes as Possible Contrast Agents for MRI: A Relaxometric Investigation

Abstract: A novel Gd-DTPA derivative with a built-in sulfonamide (SA) was synthesized as a contrast agent for MRI. The complex was designed to selectively target the enzyme carbonic anhydrase. It is shown that the longitudinal relaxation rates of aqueous solutions of Gd-DTPA-SA in the presence of carbonic anhydrase increase significantly. The binding constant is determined to be 15,000 ± 5,000 M–1. This value ensures substantial formation of the carbonic anhydrase adduct at imaging concentrations of Gd-DTPA-SA. The complex interacts with erythrocytes, presumably due to a high affinity for the carbonic anhydrase present on the outer surface of the latter. This takes place even though the enzyme has a low abundance and is easily saturated by small amounts of Gd-DTPA-SA. The interaction of Gd-DTPA-SA with serum proteins is negligibly small. Therefore, the complex could potentially be tested as a selective contrast agent for compartments outside the blood pool.

Electrochemical and Spectroscopic Properties of Cyclometallated and Non-Cyclometallated Ruthenium(II) Complexes Containing Sterically Hindering Ligands of the Phenanthroline and Terpyridine Families

Abstract: Two series of cyclometallated and noncyclometallated ruthenium(II) complexes incorporating mono- or disubstituted 1,10-phenanthroline- and 2,2′:6′,2′′-terpyridine-type ligands have been synthesized and characterized. An X-ray crystal structure for one of the complexes, Ru(ttpy)(mapH)-(Cl)(PF6), has been obtained (mapH = 2-p-anisyl-1,10-phenanthroline; ttpy = 4′-tolyl-2,2′:6′,2′′-terpyridine). Distinct electrochemical and photophysical properties have been observed for the two series: a remarkable feature is the observation of relatively long-lived MLCT excited states (from 70 to 106 ns at room temperature in CH3CN) for three of the cyclometallated complexes. A discussion is given on the role of factors like sigma donation by the cyclometallating ligands, interligand steric hindrance and interligand π-π interactions that affect the electrochemical and spectroscopic properties.

Aluminium Coordinations in Zeolite Mordenite By 27Al Multiple Quantum MAS NMR Spectroscopy

Abstract: 27Al 3Q MAS NMR spectroscopy has been applied to study the coordination state of the species giving the 30 ppm Al NMR signal in the 27Al MAS NMR spectrum of activated mordenite materials. From the 27Al 3Q MAS NMR measurements it is evident that the broad peak at 30 ppm in the 27Al NMR spectrum of the mordenite calcined at temperatures up to 600 °C comes mainly from the distorted four-coordinated Al. By simulation a quadrupolar coupling constant of 5.8 MHz was estimated for the distorted tetrahedral Al. For samples calcined at 650 and 700 °C, a small amount of pentacoordinated Al emerges. The majority of the signal, however, arises from distorted tetrahedral Al. A two-step calcination results in a significant contribution of the pentacoordinated Al to the signal at 30 ppm. From the simulated line-shape, a quadrupole coupling constant of 6.2 MHz is obtained for the latter signal. These data show that during the calcination of the mordenite, the coordination environment at the Al centre gradually becomes distorted to give rise to the shoulder at 30 ppm. With the increase of the calcination temperature, pentacoordinated Al species form

Control of Chemo‐ and Regioselectivity in the Palladium‐Catalyzed Telomerization of Butadiene with Methanol − Catalysis and Mechanism

Abstract: The palladium-catalyzed telomerization reaction of butadiene with methanol has been examined with the aim of controlling the chemoselectivity (telomerization vs. dimerization products) and regioselectivity (linear vs. branched telomerization product) of the reaction. We have shown that the reaction temperature, ligand-to-metal ratio and ratio of substrates exert a large influence on the selectivity of the reaction. Selectivities of up to 97% for the desired linear telomerization product 1 can be achieved below 50 °C by employing both low PPh3/Pd and butadiene/methanol ratios. Mono(phosphane)palladium(0)−diallyl ether complexes, Ar3P−Pd[(CH2=CHCH2)2O] (5), serve as new catalysts for the reaction. In order to gain a mechanistic understanding of the observed selectivity effects, we synthesized the phosphane(octadienyl)palladium(II) complexes 7a and 9a as model compounds for key reaction intermediates and examined their stoichiometric reactions with the methoxide nucleophile. Based on our results, we propose an extension of the known telomerization mechanism that accounts for the observed selectivity effects.

Syntheses and Carbonyliridium Complexes of Unsymmetrically Substituted Fluorous Trialkylphosphanes: Precision Tuning of Electronic Properties, Including Insulation of the Perfluoroalkyl Groups

Abstract: Reactions of iodides I(CH2)mRf8 [m = 2−4; Rf8 = (CF2)7CF3] and LiPH2 · DME (−45 °C, THF) give the primary phosphanes PH2(CH2)mRf8 (7−9; 48−76%). Radical-initiated reactions (100 °C) of 7 and H2C=CHCH2Rf8, 8 and H2C=CHRf8 or H2C=CHCH2CH2Rf8, and 9 and H2C=CHCH2Rf8, give the title phosphanes P[(CH2)mRf8]2[(CH2)m′Rf8] [m/m′ = 2/3 (10), 3/2 (11), 3/4 (12), 4/3 (13); 70−76%]. The symmetrically substituted phosphane P[(CH2)mRf8]3 (m = 5, 6) is similarly prepared from PH3 and H2C=CHCH2CH2CH2Rf8, analogously to previously reported homologs [m = 2 (2), 3 (4), 4 (5)]. Reactions of 10−13 and 2, 4, 5, and 6 with [Ir(COD)Cl]2 and CO give trans-Ir(CO)(Cl)(PR2R′)2 (70−83%). The IR νCO values show a monotonic decrease with increasing numbers of CH2 groups. Phosphanes 13, 5, and 6 have the most CH2 groups, and give νCO values 10, 7, and 4 cm−1 higher than the unfluorinated phosphane P[(CH2)7CH3]3. Hence, 6 provides nearly complete insulation of the iridium center from the electronegative perfluoroalkyl groups. Analogous rhodium derivatives of 4 and 5 are also described.

Selective and Environmentally Benign Aerobic Catalytic Oxidation of Alcohols by a Molybdenum‐Copper System

Abstract: The oxidation of activated primary and secondary alcohols to the corresponding aldehydes and ketones can be carried out with molecular oxygen, in the presence of the bimetallic molybdenum–copper system MoO2(acac)2–Cu(NO3)2 as catalyst.

Mesoporous Silicas Modified with Dioxomolybdenum(VI) Complexes: Synthesis and Catalysis

Abstract: The dioxomolybdenum(VI) fragment MoO2X2 has been confined within the ordered mesopores of pure siliceous hexagonal MCM-41 and cubic MCM-48 molecular sieves either by direct grafting (solvent impregnation) with MoO2X2(THF)2 or by using a spacer ligand [L = NC(CH2)2Si(OEt)3]. The materials have been characterised by elemental analysis, powder X-ray diffraction, N2 adsorption, IR spectroscopy, and magic-angle spinning NMR spectroscopy (13C, 29Si). All catalysts, homogeneous and heterogenised, are active in the epoxidation of cyclooctene with tert-butyl hydroperoxide.

Bis(terpyridyl)‐Ruthenium(II) Units Attached to Polyazacycloalkanes as Sensing Fluorescent Receptors For Transition Metal Ions

Abstract: A synthetic strategy has been devised for the preparation of new compounds in which terpyridyl fragments are linked to 1,4,8,11-tetraazacyclotetradecane (cyclam). Reaction of excess cyclam with 4′-[(4-bromomethyl)phenyl]-2,2′:2″,6′-terpyridine afforded the ligand 1-[4′-p-tolyl-(2,2′:6′,2″-terpyridyl)]-1,4,8,11-tetraazacyclotetradecane (L1) in which the tetraaza macrocycle was covalently attached to one benzyl-terpyridyl fragment. Under similar conditions reaction of cyclam with excess 4′-[(4-bromomethyl)phenyl]-2,2′:2″,6′-terpyridine gave the tetra substituted cyclam derivative 1,4,8,11-[4′-p-tolyl-(2,2′:6′,2″-terpyridyl)]-1,4,8,11-tetraazacyclotetradecane (L2). The multidentate ligand L2 was crystallographically characterised by single-crystal X-ray diffraction techniques. Reaction of L1 with [Ru(mtpy)Cl3] gave the heteroleptic ruthenium(II) complex [Ru(L1)(mtpy)][PF6]2 (mtpy = 4′-methyl-2,2′:6′2″-terpyridine). The fluorescent intensity of the metallo-receptor [Ru(L1)(mtpy)][PF6]2 was quenched selectively in the presence of copper(II) in an aqueous environment. To gain insight into the nature of this interaction, potentiometric titrations on [Ru(L1)(mtpy)]2+ in the presence of Cu2+ were carried out. The quenching of the emission intensity was associated with the presence of the copper {Cu[Ru(L1)(mtpy)]}4+ complex in solution. The receptor 1-[4′-methyl-(2,2′:6′,2″-terpyridyl)]-1,4,8,11-tetraazacyclotetradecane (L3), in which the cyclam fragment is separated from the terpyriyl by a methylene group, has also been synthesised by reaction of 4′-bromomethyl-2,2′:2″,6′-terpyridine and cyclam. With L3 as starting material, the ruthenium complex [Ru(L3)(mtpy)]2+ was prepared in order to evaluate the effect that the nature of the spacer has on the quenching of the fluorescence upon addition of Cu2+.

Synthesis and Structure of Siloxy‐Substituted ZnO Aggregates Having (ZnO)n (n = 2, 4) and Zn3O4 Cores

Abstract: Convenient syntheses and X-ray crystallographic characterizations of the first bis(trimethylsilyl)amido-, methyl-, and iodozinc triorganosiloxide aggregates 1−5 are described. They are accessible by the simple reaction of ZnR′2 [R = Me, N(SiMe3)2] with the respective silanols R3SiOH (R = Me, Et, iPr), which affords the dimeric [(Me3Si)2NZnOSiR3]2 (1a: R = iPr; 1b: R = Et), trinuclear [(MeZn)2Zn(OSiiPr3)4] (2a), {[(Me3Si)2NZn]2Zn(OSiR3)4} (2b: R = Et; 2c: R = Me), and tetranuclear heterocubanes [MeZnOSiR3]4 (3a: R = Me; 3b: R = Et), respectively. The latter were oxidized with four equivalents of elemental iodine to form the tetraiodo derivatives [IZnOSiR3]4 (4a: R = Me; 4b: R = Et) in 82 and 88% yield, respectively. Due to the higher polarity of the Zn−I vs. Zn−C σ-bond, the Zn−O distances of the almost regular Zn4O4 core in 4a are 2−6 pm shorter than those observed in the less Lewis-acidic cluster 3b. However, the Zn−O distances in 3b and 4a are ca. 10−15 pm longer than those in 1a, 2a, and 2c, due to different coordination numbers at Zn and the effects of ring strain. Remarkably, the iodo derivatives 4a,b undergo dissociation in THF to give the respective dimeric THF solvates [IZn(THF)OSiR3]2 (5a: R = Me; 5b: R = Et), whereas the Zn4O4 cores in 3a and 3b are retained even in aprotic polar solvents.

Simple Preparations of the Anhydrous and Solvent-Free Uranyl and Cerium(IV) Triflates UO2(OTf)2 and Ce(OTf)4 − Crystal Structures of UO2(OTf)2(py)3 and [{UO2(py)4}2(μ-O)][OTf]2

Abstract: Treatment of UO3 with pure triflic acid TfOH at 110 °C or with boiling triflic anhydride TfOTf afforded [UO2(OTf)2] (1) in high yields. The latter was also prepared by the reaction of UO3 with TfOH in water, or by dehydration of [UO2(OTf)2(H2O)n] in boiling TfOTf. Anhydrous [Ce(OTf)4] (2) was similarly obtained from the commercially hydrated compound. X-ray analysis revealed that in [UO2(OTf)2(py)3] (3), the triflate ligands are monodentate whereas they are dissociated in [{UO2(py)4}2(μ-O)][OTf]2 (4).

Structure and Dynamic Behavior of (η3‐Allyl)bromodicarbonylmolybdenum(II) Complexes Containing Polydentate 2‐Pyridylphosphanes or Their Oxides as Chelating Ligands: Occurrence of Three Fluxional Processes

Abstract: The pseudo-octahedral complexes [Mo(η3-allyl)Br(CO)2(PPynPh3–n-P,N)] (Py = 2-pyridyl; n = 2, 3) and [Mo(η3-allyl)Br(CO)2(OPPymPh3–m-O,N)] (m = 1, 2, 3) undergo three different dynamic processes in solution, depending on the chelating ligand. The complexes containing PPhPy2 and PPy3 as chelating P,N-donors undergo a novel “pivoted double switch” mechanism which scrambles two equatorial coordination sites with racemization, while maintaining the identity of the phosphorus atom trans to the allyl ligand. The complexes containing 2-pyridylphosphane oxides as chelating O,N-donors undergo a nondissociative intramolecular trigonal-twist (or turnstile) rearrangement which maintains the identity of the phosphane oxide oxygen atom coordinated in an equatorial position. All the complexes containing uncoordinated pyridyls undergo a slow dissociative exchange of pyridyls without coordination site scrambling. The structure of the complex [Mo(η3-allyl)Br(CO)2(OPPy3-O,N)] has been determined by X-ray diffraction.

Syntheses and Structures of Silver and Copper Coordination Polymers with 4,4′-Azopyridine − Effect of Counter Anions and π−π Interactions on the Network Systems

Abstract: The syntheses and structures of [Ag(azpy)](NO3)⋅(H2O)⋅(CH3OH) (1), [Cu(azpy)2](ClO4) (2), [Cu(azpy)2(H2O)2](NO3)2⋅(H2O)2 (3) and [Cu(azpy)2(H2O)2](ClO4)2⋅(C2H5OH)2 (4), where azpy = trans-4,4′-azopyridine, are reported to test the dependence of network topology on counter anions and π−π interactions. Complex 1 contains pairs of linear chains of alternating AgI ions and azpy ligands, resembling a ladder structure in which the ligands form π−π interactions (with a distance of 3.53 A) and AgAg interactions (with a distance of 3.30 A). Complex 2 exists as a 3D adamantoid polymer in which each CuI center is coordinated to four azpy ligands in a tetrahedral geometry, the ligands are in turn coordinated to four other CuI centers. The cavities are filled with four other adamantoid units forming five interpenetrating networks. Complexes 3 and 4 are composed of two-dimensional sheets in which the CuII atom is coordinated in an octahedral geometry to two water molecules and four ligands. The ligands are in turn coordinated to four other CuII centers. While the sheets in complex 3 display inclined 2D interpenetration, the sheets in complex 4 are all parallel and isolated, with an intersheet separation of ca. 4.7 A.