Ligand steric hindrance

By introducing substituents into cyclopentadienyl ligands, steric hindrance takes place which might reduce the number of ligands bonded to the metal. The synthesis of Cp Ln consists of the reaction... 

The equihbritim constants K, K2-..K are stepwise formation constants, or stepwise stability constants, which indicate the extent of formation of difierent species corresponding to a particular step. The values of stepwise stabihty constants for a complex formation reaction mosdy decrease successively (Xi > K2 > K3 >. .. > K ). A steady decrease in the values of these constants is due to a decrease in coordinated water l%ands that are available to fresh ligands for replacement. Other than this, the decrease in the abihty of metal ions with progressive intake of ligands, steric hindrance and columbic factors also contributes to a steady decrease in successive stepwise stability constant values. 

Research on rac-LA polymerization has been much more dynamic and successful, especially in the past decade. Suppression or even elimination of the segmental exchange turned out to be possible by applying metal alkoxide initiators with bulky ligands. Steric hindrance at the active center decreases the transesterification rate due to an increase in entropy of activation, while the propagation rate remains constant. ... 

Besides these direct applications of model compounds for the identification of surface sites, surface groups, or surface complexes, there are a variety of compounds available today which provide potential to mimic certain aspects of oxide surfaces at a molecular level. These include extended oxide structures (models of oxide layers or silicate cages), (CpM)nOm (models of M-0 arrangements on oxide surfaces, defect sites), and complexes with bulky alkoxide ligands (steric hindrance at surfaces). The usefulness of these materials as model compounds has not been fully exploited. 

The three water ligands located at meridional positions of the J ,J -DBFOX/Ph aqua complexes may be replaced by another molecule of DBFOX/Ph ligand if steric hindrance is negligible. Based on molecular model inspection, the hetero-chiral enantiomer S,S-DBFOX/Ph looks like a candidate to replace the water ligands to form the heterochiral meso-2 l complex J ,J -DBFOX/Ph-S,S-DBFOX/... 

The selectivity decreases with increasing amide size. This may be due to steric hindrance which prevents the chiral ligand from approaching the reaction site or may reflect a change in the reaction mechanism going from an SN1 reaction (A-acylimine 2 as intermediate) to an SN2 displacement of benzotriazole11. 

Although the actual reaction mechanism of hydrosilation is not very clear, it is very well established that the important variables include the catalyst type and concentration, structure of the olefinic compound, reaction temperature and the solvent. used 1,4, J). Chloroplatinic acid (H2PtCl6 6 H20) is the most frequently used catalyst, usually in the form of a solution in isopropyl alcohol mixed with a polar solvent, such as diglyme or tetrahydrofuran S2). Other catalysts include rhodium, palladium, ruthenium, nickel and cobalt complexes as well as various organic peroxides, UV and y radiation. The efficiency of the catalyst used usually depends on many factors, including ligands on the platinum, the type and nature of the silane (or siloxane) and the olefinic compound used. For example in the chloroplatinic acid catalyzed hydrosilation of olefinic compounds, the reactivity is often observed to be proportional to the electron density on the alkene. Steric hindrance usually decreases the rate of... 

The first complexes of a-keto ylides and group 5 early transition metals have only recently been obtained by reaction of Nb(III) derivatives [[NbCl3(dme) (R C=CR")] with 25 (R = thiazolyl) (Scheme 16). The chelation of the ylide occurs through an N,0-coordination to the metal center and in presence of MeLi a deprotonation of a phenyl ring takes place with the loss of alkyne, leading to the formation of a new orfho-metallated binuclear compound 32. The two ylides involved in the complexation behave as tridentate anionic ligands and are mutually in a trans disposition in order to minimize the steric hindrance [71,72]. Another binuclear niobium complex 33 has been obtained from 25 (R = Me, Ph) with this time an 0-coordinated a-keto ylide [68]. 

No cases of cis/trans isomerism have yet been reported among organo-cobalt(III) complexes, even among compounds not covered by this review. Complexes such as [R2Co(bipy)2] all have the cis configuration steric hindrance would be greatly increased by the coplanarity of the two bipyridyl ligands in the trans isomer 123). 

Salvadori et al. [62] tested the same strategy but derived the bis(oxazohne) ligands in such a way that they minimized the steric hindrance at the bridging methylene carbon (structure 53 in Scheme 25). The polymer was used affording enantiomeric excesses superior to 90% and was reused at least five times with almost no loss in enantioselectivity or activity. 

The limitations of the system with regard to substrates and oxidants was attributed to the strong electron-withdrawing character of the perfluorinated chains and the lower steric hindrance in the position adjacent to phenols, in marked contrast to the ferf-butyl groups present in Jacobsen s catalyst, hi view of this, a second generation of fluorinated salen ligands le and If was... 

The Pd-catalysed Heck reaction performed with thiourea as the Ugand exhibit good activities for some catalysts. As for carbene ligands [104], steric hindrance improves catalytic results. Thus, thioureas wearing bulky substituents afford the formation of air- and moisture-stable Pd complexes [105]. For example, the catalyst obtained with 2mol% Pd(dba)2 and Ar,M -dimesitylene-ethylene thiourea (Scheme 24) was still active even after 2 months in an air atmosphere. 

The steric environment of COP-X 46 and 47a around the catalytic palladium site mainly differs in a Ph (47a) and an i-Pr group (46) next to the coordinating N-site and the type and distance of the spectator ligand. While the distance of the two sandwich ligands differs only slightly between COP and 47a (3.4 A vs. 3.3 A), oxidation of the ferrocene to a ferrocenium species is expected to shorten this distance further. Overall, the steric hindrance to access the Pd-center is more distinct for 47a. These steric effects are capable to explain the higher ee obtained with 47a. 

The case of the disalicylate, 1-methyl trimethylene disalicylate, is interesting. Because of steric hindrance it is unlikely that the two salicylate ligands can chelate to one calcium atom. In theory the disalicylate... 

In a similar fashion, the homoleptic complex [Pd(ITmt) ] lb readily reacts with O2 to form the corresponding peroxo-complex 2b (Scheme 10.1). This complex, npon exposure to CO, leads to the peroxo-carbonate complex 3b [10]. Under the same reaction conditions, the formation of 3a does not occur, presumably due to the larger steric hindrance of the Mes ligand. 

Cationic Complexes with P-Bonded 2-Pyridylphosphines as N-Donor Ligands toward Rhodium. Electrical Charge vs. Steric Hindrance on the Conformational Control. Inorganic Chemistry, 45(17), 6628-6636. 

The percentage of linear product increases greatly by replacing CO with the much bulkier phosphine ligands. Due to the increased steric hindrance the catalyst shows a distinct preference for the n- over the tw-isomer. Tkatchenko (1991) has reported a detailed analysis of this system in terms of the correlation between catalyst performance (activity and selectivity) and detailed structure. 

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