Search results for "Effects"
showing 10 items of 2634 documents
Early lanthanide organometallic hydrides: electronic and steric control of the stability
2000
Abstract Steric and electronic effects on the stability of early lanthanide, neodymium and samarium, organometallic hydrides are discussed. The instability of (tmp)2SmH (tmp=tetramethylphospholyl) is attributed to electronic factors and the low stability of (tBuC5H4)2SmH is related to steric reasons. The analogous neodymium hydride (tBuC5H4)2NdH, could not be obtained, whereas the more hindered triethylborohydride was formed. Access to bisphospholyl bridged heterobimetallic ruthenium–lanthanide hydrides is also related to the size of the lanthanide atom. The bimetallic structures are accessible for lanthanides of ionic radii smaller than 1 A. The role of organometallic hydrides versus alkyl…
The oligomerisation of 3-hydroxy-1-alkynes with palladium(II) diketonates and phosphorus ligands as the catalytic system
1993
Abstract 3-Methylhex-1-yne-3-ol has been oligomerised by use of Pd(II) acetylacetonate with 31 different phosphorus ligands as catalyst, yielding a dimer (2,4-disubstituted but-1-en-3-yne) and a linear trimer (1,4,6-trisubstituted hexa-1,3-dien-5-yne) as the two main products. By input-output relations a variation of the diketonate and the P ligand, as weil as an alteration of the phosphorus/palladium ratio, has been connected with product ratios as dependent variables. Phosphines produced three association steps up to a cone angle of 170°; the third step representing a stop complex. For the first association the activity of the catalytic system and the portion of the 2,4-dimer increased wi…
An unprecedented, bridged dihydrogen complex of a cofacial metallodiporphyrin and its relevance to the bimolecular reductive elimination of hydrogen
1990
Recently, the authors characterized the first metalloporphyrin dihydrogen complex, Os(OEP)(H{sub 2}), and proposed that a similar ruthenium porphyrin dihydrogen complex, Ru(OEP)(H{sub 2}), is involved as an intermediate in the catalytic H/D isotopic exchange between water and hydrogen. They now report that treatment of a metal-metal-bonded cofacial ruthenium porphyrin dimer with a sterically bulky ligand in the presence of hydrogen gas has yielded the first known complex containing a dihydrogen ligand bound between two metals. Such a bridged dihydrogen complex is proposed as an intermediate in the bimolecular elimination of dihydrogen from two metalloporphyrin hydrides.
A density functional theory study of uranium(VI) nitrate monoamide complexes.
2011
Density functional theory calculations were performed on uranyl complexed with nitrate and monoamide ligands (L) [UO(2)(NO(3))(2)·2L]. The obtained results show that the complex stability is mainly governed by two factors: (i) the maximization of the polarizability of the coordinating ligand and (ii) the minimization of the steric hindrance effects. Furthermore, the electrostatic interaction between ligands and uranium(vi) was found to be a crucial parameter for the complex stability. These results pave the way to the definition of (quantitative) property/structure relationships for the in silico screening of monoamide ligands with improved extraction efficiency of uranium(vi) in nitrate ac…
Synthesis and Structure of a Potassium Potassiochromate: A Bis-Chromium(II) Molecule Held Together by Near-Square-Planar Potassium−Ligand Bridges
2010
No Cr-Cr bonding is found in a new type of mixed-metal ate complex having two coordinatively unsaturated but sterically saturated bisamido-monoalkyl Cr(II) groups linked via an unusual near-square-planar-coordinated K atom in the moiety of the ate, while the cationic moiety is a separated iris-tmeda solvated second potassium atom.
Unexpected self-assembly of a homochiral metallosupramolecular M4L4 catenane
2014
Two enantiomerically pure 9,9'-spirobifluorene-based bis(pyridine) ligands 1 and 2 were prepared to study their self-assembly behavior upon coordination to cis-protected palladium(II) ions. Whereas the sterically more demanding ligand, 2, gave rise to the expected dinuclear metallosupramolecular M2L2 rhombi, the sterically less demanding ligand, 1, acts as a template to give rise to a homochiral metallosupramolecular M4L4 catenane.
Synthesis of Sterically Hindered Chiral 1,4-Diols from Different Lignan-Based Backbones
2013
Methods for synthetic modifications of the natural dibenzylbutyrolactone lignan hydroxymatairesinol into chiral 1,4-diols with different lignan-derived backbones have been developed. A stepwise procedure, involving alkylation and oxidation, was shown to be successful and several highly substituted 1,4-diols were prepared. Some substituted butyrolactones resisted alkylation and led to the formation unusually stable hemiketals (butyrolactols). The formation of stable hemiketals was investigated in detail, showing that different backbone structures influence the formation and reactivity of the hemiketals.
Spectroscopic and conformational properties of size-fractions separated from a lignite humic acid
2007
Abstract A lignite humic acid (HA) was fractionated by preparative high performance size-exclusion chromatography (HPSEC) in seven different size-fractions. The size-fractions were characterized by cross polarization (CP) magic angle spinning (MAS) 13 C NMR spectroscopy and a further analytical HPSEC elution under UV and fluorescence detection. The alkyl hydrophobic components mainly distributed in the largest molecular-size-fraction, whereas the amount of oxidized carbons increased with decreasing size of fractions. Cross polarization time ( T CH ) and proton spin-lattice relaxation time in the rotating frame ( T 1 ρ (H)) were measured from variable contact time (VCT) experiments. The bulk…
Compensation of steric demand by cation–π interactions, cobaltocenium cations as guests in tetraurea calix[4]arene dimers
2003
The affinities of ferrocene (2) and the cobaltocenium cation (3+), which have roughly the same size and differ in their charge, towards the inner cavity of the dimeric capsule formed by tetraurea calix[4]arene (1) were studied in C2D4Cl2 solutions. While 3+, which occupies more than 75% of the internal volume of the dimer, is readily encapsulated this is not the case for 2. This is probably due to cation-pi interactions, which operate only between 3+ and the aromatic rings of the calix[4]arene dimer. We found that the affinity of the cobaltocenium cation is higher than that of the tropylium cation (4+) and is only 2-3 times less than that of the tetraethylammoniun cation (5+). From the vari…
Kinetico-mechanistic studies of the acidolysis of Rh-C bonds in monocyclometallated dirhodium(II) acetato complexes; influence of electronic and ster…
2011
The protonation/demetallation reaction, in CD(3)COOD, of phosphine monometallated triacetato-bridged dirhodium(ii) complexes of general formulae [Rh(2)(μ-OOCCH(3))(3){(RC(5)CH(3))P(RC(6)H(4))(2)}(2)] has been studied from a kinetico-mechanistic perspective. The process has been monitored via the disappearance of the nuclear magnetic resonance signals of the protons present in the non-metallated ortho positions of the phosphine ligand and taking advantage of the relatively fast back metallation process that follows the acidolysis reaction, the sequence behaving as an overall equilibrium reaction. The process has a high associative character with important entropy demands, increasing both for…