Isomerization of Cyclooctadiene to Cyclooctyne with a Zinc/Zirconium Heterobimetallic Complex

Reaction of a zinc/zirconium heterobimetallic complex with 1,5‐cyclooctadiene (1,5‐COD) results in slow isomerization to 1,3‐cyclooctadiene (1,3‐COD), along with the formation of a new complex that includes a cyclooctyne ligand bridging two metal centers. While analogous magnesium/zirconium and aluminum/zirconium heterobimetallic complexes are competent for the catalytic isomerization of 1,5‐COD to 1,3‐COD, only in the case of the zinc species is the cyclooctyne adduct observed.     

Synthesis and Crystal Structure of a Dinuclear Titanium(IV) Complex containing an Allopyranosidato Ligand

The reaction of methyl‐4,6‐O‐benzylidene‐α‐D ‐allopyranoside (α‐MeBAH₂) with trichlorocyclopentadienyltitanium [CpTiCl₃] and triethylamine in dichloromethane led to the dinuclear complex [(CpTiCl₂)₂‐μ‐(α‐MeAG)] ( 1 ). 1 was characterized by elemental analysis, ¹H NMR spectroscopy as well as single crystal X‐ray diffraction.     

Complex Formation of Crown Ethers with the Cucurbit[6]uril–Spermidine and Cucurbit[6]uril–Spermine Complex in Aqueous Solution

Alkyl amines are able to form complexes with either crown ethers or cyclodextrins or cucurbit[6]uril. The same is known for polyamines such as spermidine and spermine. However, the simultaneous formation of such polyamines with crown ethers and cucurbit[6]uril has not been studied. The ability of polyamines such as spermidine and spermine to form mixed complexes with different ligands, e.g. crown ethers and cucurbit[6]uril has been studied in aqueous solution using pH-metric and calorimetric titrations. The thermodynamic data of reaction between crown ethers with spermidine, spermine and their cucurbit[6]uril complexes have been determined. The presence of cucurbit[6]uril on the polyamines has no important influence upon the reaction of these amines with crown ethers. The reactions between polyamines, cucurbit[6]uril and crown ethers are simple examples for the self organization of molecules due to specific interactions. Received in final form: 26 January 2005     

Tribo-Excited Chemical Reaction Using an EuIII Complex with a Stacked Anthracene Framework

A spin-selective tribo-chemical reaction using a dinuclear lanthanide complex is demonstrated for the first time. The dinuclear complex is composed of two Eu^III ions, hexafluoroacetylacetonato ligands, and anthracene-based phosphine oxide bridges. Single-crystal analysis revealed a face-to-face-type anthracene dimer structure in the dinuclear Eu^III complex. Mechanical stimulus on the dinuclear Eu^III complex induced selective formation of oxidized anthracene. The tribo-chemical reaction is based on a characteristic energy-transfer pathway for the selective formation of an excited triplet state.     

Stabilization of a Discrete Lanthanide(II) Hydrido Complex by a Bulky Hydrotris(pyrazolyl)borate Ligand

Divalent and solvent-free : the ytterbium hydrido complex 1 was obtained by the hydrogenolysis of [(Tp^tBu,Me)Yb(CH₂SiMe₃)(thf)]. The steric demand of the bulky hydrotris(3-tert-butyl-5-methylpyrazolyl)borate ligand, Tp^tBu,Me, is sufficient to stabilize the dimer, yet facile room-temperature reactions with amines, alkynes, diynes, and CO indicate a rich chemistry of 1 .     

Complex formation of inorganic salts withβ-cyclodextrin

The interactions between some alkali halides, perchlorates and sulfates and-cyclodextrin (cycloheptaamylose, CD) have been investigated by solubility, spectrophotometric, thermoanalytical and preparative methods. The main conclusions are as follows. (i) The most pronounced interaction occurs with the anions; in dilute solutions this can be characterized as the formation of 1 : 1 complexes. In more concentrated solutions ternary associates and more complicated ones (involving the cations as well) are also formed. (ii) Changes in the activity coefficients or of the activity of water may also have some role in the phenomena, but it cannot be dominant: differences according to the individual properties of the anions are much more pronounced than to those of the cations or to changes of the ionic strength. (iii) Regarding the type of complex formation, real inclusion and outer sphere interactions (via hydrogen bonds) are also probable.     

An Alumanylyttrium Complex with an Absorption due to a Transition from the Al−Y Bond to an Unoccupied d-Orbital

The reaction between a dialkyl-substituted alumanyl anion and [Y(CH₂SiMe₃)₂(thf)₃][BPh₄] resulted in the formation of (dialkylalumanyl)yttrium complex 2 , which exhibits the first 2-center–2-electron (2 c-2 e) Al−Y bond. The ¹H and ¹³C NMR spectra of 2 together with an X-ray crystallographic analysis indicated a C_2v symmetrical structure. DFT calculations on 2 revealed that its LUMO consists of overlapping 3 p- and 4 d-orbitals of the Al and Y atoms, respectively, and that the HOMO–LUMO gap is narrow. The UV/Vis spectrum of 2 exhibited a visible absorption at 432 nm, which suggests that the strong σ-donating and π-accepting character of the three-coordinate dialkylalumanyl ligand generates a colored d⁰-complex that does not contain any π-electrons.     

Protonation Behavior of a Tetrahydrido Molybdenum(IV) Complex with Organic and Inorganic Acids

The reaction of [MoH₄(depe)₂] ( 1 ) (depe=1,2-bis(diethylphosphino)ethane) with different proton sources (HP^tBu₃⁺, C₆H₅COOH, C₆F₅COOH, H₂SO₄) was investigated. Whereas complete conversion of 1 into its protonated form is observed in the presence of the phosphonium salt, [MoH₅(depe)₂]⁺ only transiently forms upon treatment with the other acids. Further reactivity of the pentahydride complex is indeed noticed with the conjugated base, typically resulting in the formation of neutral (C₆F₅COOH, H₂SO₄) or cationic (PhCOOH) molybdenum dihydride complexes. The compounds were characterized by NMR spectroscopy and X-ray crystallographic studies were performed when suitable crystals were obtained.     

Reversible Fixation of Ethylene on a SmII Calix-Pyrrole Complex

Reversible ethylene fixation in lanthanide chemistry is demonstrated by the Sm^II derivatives [R₈-calix-pyrrole)(Et₂O)Sm{Li(thf)₂}{Li(μ₃-OCH=CH₂)}] (R=Et, {−(CH₂)₅−}_0.5), which react with ethylene to afford the corresponding dinuclear complexes (see picture).     

Mild and Complete Carbonyl Ligand Scission on a Mononuclear Transition Metal Complex

The complete scission of the carbon–oxygen bond of carbon monoxide, while frequently observed on bulk metals and with bimetallic and cluster transition metal complexes, is unknown with monometallic systems. Reaction of a zerovalent iron bis(borylene) complex with a cyclic (alkyl)(amino)carbene revealed a highly selective intramolecular cleavage of the C?O bond of a carbonyl ligand at room temperature, leading to the formation of a highly unusual iron complex containing a base‐stabilized (bora)alkylideneborane ligand. DFT investigation of the reaction mechanism suggested that the two Lewis acidic borylene boron atoms cooperate to cleave the C?O multiple bond.     

Electronic Structure and Hydrogen Bonding in a Dicobalt(III) Werner Complex

The bonding of the O-O group in the dicobalt cation 1a [(NH₃)₆Co₂(μ-O₂)(μ-OH)(μ-NH₂)]³⁺ was studied by DFT methods (ADF program) and the bridging O₂ ligand was characterized as superoxide(O₂⁻). In this complex, three bridging ligands connect the two cobalt atoms, forcing a cis conformation of the Co-O-O-Co atoms. A comparison was made with [(NH₃)₁₀Co₂(μ-O₂)]⁵⁺, 2a, where a trans arrangement is observed. Superoxide binds more strongly to the dicobalt(III) fragment in 2a than in 1a, both as a result of weaker Pauli repulsion and stronger covalent interaction. It was found that in 1a the electronic structure with one unpaired electron, where cobalt is formally Co(III), d⁶, and O₂ carries one negative charge gives rise to the most stable structure, compared to possibilities with three and five unpaired electrons. The hydrogen bonds in the crystal were analyzed and the interactions between one water molecule or one nitrate ion studied in more detail.     

DNA-Targeting RuII-Polypyridyl Complex with a Long-Lived Intraligand Excited State as a Potential Photodynamic Therapy Agent

Subtle ligand modifications on Ru^II-polypyridyl complexes may result in different excited-state characteristics, which provides the opportunity to tune their photo-physicochemical properties and subsequently change their biological functions. Here, a DNA-targeting Ru^II-polypyridyl complex (named Ru1 ) with highly photosensitizing ³IL (intraligand) excited state was designed based on a classical DNA-intercalator [Ru(bpy)₂(dppz)] ⋅ 2 PF₆ by incorporation of the dppz (dipyrido[3,2-a:2′,3′-c]phenazine) ligand tethered with a pyrenyl group, which has four orders of magnitude higher potency than the model complex [Ru(bpy)₂(dppz)] ⋅ 2 PF₆ upon light irradiation. This study provides a facile strategy for the design of organelle-targeting Ru^II-polypyridyl complexes with dramatically improved photobiological activity.     

Enhanced Hydrogen Evolution in Neutral Water Catalyzed by a Cobalt Complex with a Softer Polypyridyl Ligand

To explore the structure–function relationships of cobalt complexes in the catalytic hydrogen evolution reaction (HER), we studied the substitution of a tertiary amine with a softer pyridine group and the inclusion of a conjugated bpy unit in a Co complex with a new pentadentate ligand, 6‐[6‐(1,1‐di‐pyridin‐2‐yl‐ethyl)‐pyridin‐2‐ylmethyl]‐[2,2′]bipyridinyl (Py3Me‐Bpy). These modifications resulted in significantly improved stability and activity in both electro‐ and photocatalytic HER in neutral water. [Co(Py3Me‐Bpy)(OH₂)](PF₆)₂ catalyzes the electrolytic HER at ?1.3 V (vs. SHE) for 20 hours with a turnover number (TON) of 266 300, and photolytic HER for two days with a TON of 15 000 in pH 7 aqueous solutions. The softer ligand scaffold possibly provides increased stability towards the intermediate Co^I species. DFT calculations demonstrate that HER occurs through a general electron transfer/proton transfer/electron transfer/proton transfer pathway, with H₂ released from the protonation of Co^II?H species.