Cluster compounds kinetics

The higher catalytic activity of the cluster compound [Pd4(dppm)4(H2)](BPh4)2 [21] (20 in Scheme 4.12) in DMF with respect to less coordinating solvents (e.g., THF, acetone, acetonitrile), combined with a kinetic analysis, led to the mechanism depicted in Scheme 4.12. Initially, 20 dissociates into the less sterically demanding d9-d9 solvento-dimer 21, which is the active catalyst An alkyne molecule then inserts into the Pd-Pd bond to yield 22 and, after migratory insertion into the Pd-H bond, the d9-d9 intermediate 23 forms. Now, H2 can oxidatively add to 23 giving rise to 24 which, upon reductive elimination, results in the formation of the alkene and regenerates 21. 

Dialkylzinc derivatives are inert towards conjugated enones (e.g. 181) in hydrocarbon or ethereal solvents. The discovery that a conjugate addition can be promoted by Cu(I) salts in the presence of suitable ligands L (e.g. sulphonamide 182) opened a new route to zinc enolates (e.g. 183), and hence to the development of three-component protocols, such as the tandem 1,4-addition/aldol addition process outlined in equation 92186. If the addition of the aldehyde is carried out at —78 °C, the single adduct 184 is formed, among four possible diastereomeric products. The presence of sulphonamide is fundamental in terms of reaction kinetics its role is supposed to be in binding both Cu(I) and Zn(II) and forming a mixed metal cluster compound which acts as the true 1,4-addition catalyst. 

SPR also offers the unique possibility to study the kinetics of the membrane solute interaction if a dilution series of the compound under investigation is injected into the system. Abdiche (2004) clustered compounds with the same apparent partition coefficient Kd according to their binding kinetics and differentiated mainly between fast off and slow off rate drugs. 

Trifluoromethyl)phenylcopper was found to be an octamer by consideration of the kinetics of its decomposition, and by cryoscopy and vapor pressure osmometry in benzene solution 36). Its F NMR spectrum in ether at room temperature is a sharp singlet. Consequently, the suggested structure is a central copper cube with equivalent bridging benzotri-fluoride groups. The initial decomposition product, Cu8( n-CF3CgH4)e, is considered to be a Cu(0)—Cu(I) octanuclear cluster compound 36). For the octameric m-(trifluoromethyl)phenylcopper, the tetrameric ortho isomer, and pentafluorophenylcopper tetramer, the F NMR spectra were found to vary with temperature. The changes are not considered to involve important structural alterations, but only variations in solvent complexes and rotamer populations 32, 37). The spectra also... 

Ford used HP-IR to investigate an acyliron migratory insertion intermediate formed by flash photolysis. Thus, flash photolysis of (7 -Cp)Fe(C0)2C(0)GH3 affords coordinatively unsaturated ( -Gp)Fe(C0)C(0)CH3. Trapping of the latter with CO in the reverse reaction was studied, and the second-order rate constant could be determined for this reaction under the high CO pressures employed. Variable-temperature studies allowed calculation of activation parameters for methyl migration. Iron cluster compounds have been studied for the carbonylation of methanol to methyl formate. Consistent kinetics and a first-order dependence on cluster concentration confirmed the HP-IR results which showed that the cluster remained intact through the catalytic process. 

Examples of fluxional behaviour may also be found in organometallic compounds containing more than one metal atom in the molecule, for example (C5H5)(CO)2Mo(C7H,)Fe(CO)3 " and the cluster compound Ru3(C8H8)(CO)4, whose limited solubility unfortunately precludes satisfactory determination of kinetic parameters. " ... 

The kinetics and thermodynamics of the exchange of halide ions (X) in cluster compounds of the type [MgCyXe have been reported for M = Mo and W. Replacement of chloride ion (X ) by bromide ion involves aquation in the rate-determining step followed by rapid bromide-ion anation of the aqua-intermediate. The activation energies for the forward and reverse reactions are both reported to be ca. 18 kcal mol. The molybdenum clusters are about ten times as reactive as the analogous tungsten clusters. 

As discussed in Sect. 2.4, the reaction of carbonyl metal clusters with N-oxide trimethylamine may often be a convenient method for synthesizing cluster compounds via ligand substitution. Kinetic studies of the substitution of CO-ligands by phosphines and arsines in group-18 trimetal dodecarbonyls... 

All double-cluster compounds show inhibition potency with IC50 values in the submicromolar range. The only ontlier is GB-155, documenting the need for a cobalt bis(dicarbollide) full sandwich for the effective binding to HIV PR. Compound GB-162 with one cobalt bis(dicarbollide) moiety replaced for an 11-vertex dicarbollide ion [7,8-C2B9H12]- exhibits a slightly weaker inhibitory activity but different, noncompetitive inhibition mode. However, a rigorous comparison of the inhibition kinetic constants is possible only for the compounds that exhibit a competitive mode of inhibition. 

In 94, dppee acts as a chelating ligand whereas it adopts a bridging mode in 95. Molecular mechanical calculations using MM2 parameters indicate that 95 is probably kinetically favored whereas 94 is thermodynamically favored. This is reflected by the experimental yields (10% for 94 and 50% for 95). Table 13 contains OsOs bond length ranges and OsP bond lengths of these and related cluster compounds. 

C5H5)Mo(C4H6)2] has been monitored by NMR spectroscopy.NMR spectroscopy has been used to follow the reaction in Scheme 5. The Bu elimation kinetics from [Mo(E)(NBu Ar)3], E = O, S, Se, Te, to give [Mo(E)(NAr)(Bu Ar)2] have been determined using NMR spectroscopy. Mo NMR spectroscopy has been used to follow the substitution reactions of Mo-Cu-(Ag,Au)-Se cluster compounds. and P NMR spectroscopy has been used to follow the reduction of a-[S2Moi8062] by phosphines. ... 

Magnetic field effects on the reaction kinetics or yields of photochemical reactions in the condensed phase have been studied [20-23]. They have proved powerful for verifying the mechanism of photochemical reactions including triplet states. Previously, we obtained photogenerated triplet biradicals of donor-acceptor linked compounds, and found that the lifetimes of the biradicals were remarkably extended in the presence of magnetic fields up to 1T [24]. It has been reported that Cgo and its derivatives form optically transparent microscopic clusters in mixed solvents [25,26]. The clustering behavior of fullerene (C o) is mainly associated with the strong three-dimensional hydrophobic interactions between the C o units. Photoinduced... 

A wide range of thermochemical properties can be measured, including not only proton affinity or gas-phase basicity, but also electron affinity, ionization energy, gas-phase acidity and cation affinity Entropy changes upon attachment of an ion to a molecule are also accessible and provide information on both the nature of the bonding and fragmentation mechanisms in cluster ions, especially in biological compounds. Thermochemical determinations by the kinetic method also provide very useful structural information e.g., two-electron three-center bond has been observed in the gas phase by means of the kinetic method. " In the last years, the kinetic method has been also applied to characterize chiral ions in the gas phase. 

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