Abstract:Conditional pseudo-first-order rate constant k 0 increased linearly with the k /k ϭ 12.0 Ϯ 0.3. Ϫ1 2 concentration of NaY, while it decreased drastically with the concentration of NaX. Some plausible mechanisms were examined, and the following mechanism was proposed.
“…An increase of the temperature to the maximum possible for the solvents used indicated the presence of a dynamic process with free sulfide, as expected for the rate constant found for its interchange (see below). 2D 1 H NOESY experiments have already indicated that this process includes the interchange of the two platinum-bonded methyl groups via a turnstile twist process that stops when the less labile phosphine derivative is considered or when the temperature is decreased. 5a, Nevertheless, even when the temperature is decreased, no better 1 H NMR signals are obtained; Figure is representative for the spectra obtained for these situations. 1 Variable-temperature 1 H NMR in the SMe 2 region for the compound dFMe in the presence of free dimethyl sulfide ligand.…”
Section: Resultsmentioning
confidence: 97%
“…Furthermore, the introduction of very good σ donors on the coordination sphere should lead to an increase in the metal electron density that could produce an increase of its lability . Consequently, it is not strange that the introduction of Pt−C bonds in Pt(IV) t 2g 6 complexes leads to an important lability increase in their substitution reactions, and that they tend to operate through a dissociatively activated mechanism …”
Section: Introductionmentioning
confidence: 99%
“…We have been interested in the last few years on the mechanisms of formation and substitution reactions of cyclometalated platinum and palladium complexes with a variety of imines . Dimethyl sulfide substitution by phosphines on cyclometalated Pt(IV) complexes of the type shown in eq 1 have already been studied 5a …”
Section: Introductionmentioning
confidence: 99%
“…Nevertheless, a certain electronic tuning related to the nature of the halogen, X, ligand and the presence of electron-withdrawing substituents affecting the aromatic metalated carbon has been found for the activation entropy values. Namely, a lesser degree of dissociation to go to the transition state was observed when the cyclometalated compound contained a maximum of electron-withdrawing elements 5a…”
The substitution reactions of sulfide by phosphines on Pt(IV) complexes having a cyclometalated imine ligand, two methyl groups in a cis geometrical arrangement, and a halogen and a sulfide as ligands, [Pt (Me)(2)X(C-N)(SR(2))], have been studied as a function of temperature, solvent, and electronic and steric characteristics of the phosphines, sulfides, X, and C-N. In most of these cases, a limiting dissociative mechanism has been found, where the dissociation of the sulfide ligand corresponds to the rate-determining step. The intermediate species formed behaves as a true pentacoordinated Pt(IV) compound in a steady-state concentration only for the systems with SMe(2); for the bulkier SEt(2) and SBzl(2) leaving ligands the rate constants and activation parameters show an important degree of solvent dependence, which correlates with the ability of the solvent to form hydrogen bonds. The X-ray crystal structure of one of the dibenzyl sulfide complexes has been determined, and the geometrical arrangement of the ligands has been determined by NOE NMR measurements at low temperature. The nature of the solvent, imine, sulfide, and halogen ligands produces differences in the reaction rates, which can be quantified very well by the corresponding DeltaS values that move from +48 to -90 J K(-1) mol(-1). The reaction on [Pt(Me)(2)F(C(5)CF(4)CHNCH(2)Ph) (SMe(2))] has been found to take place via a mechanism that depends strongly on the bulkiness of the substituting phosphine. While for PCy(3) the reaction is dissociative, for smaller entering ligands the first associatively activated substitution mechanisms on organometallic Pt(IV) complexes have been established with values of DeltaH and DeltaS in the 28-44 kJ mol(-1) and -120 to -83 J K(-1) mol(-1) ranges. Important intramolecular hydrogen bonding in the starting material can be held responsible for this difference with the remaining systems.
“…An increase of the temperature to the maximum possible for the solvents used indicated the presence of a dynamic process with free sulfide, as expected for the rate constant found for its interchange (see below). 2D 1 H NOESY experiments have already indicated that this process includes the interchange of the two platinum-bonded methyl groups via a turnstile twist process that stops when the less labile phosphine derivative is considered or when the temperature is decreased. 5a, Nevertheless, even when the temperature is decreased, no better 1 H NMR signals are obtained; Figure is representative for the spectra obtained for these situations. 1 Variable-temperature 1 H NMR in the SMe 2 region for the compound dFMe in the presence of free dimethyl sulfide ligand.…”
Section: Resultsmentioning
confidence: 97%
“…Furthermore, the introduction of very good σ donors on the coordination sphere should lead to an increase in the metal electron density that could produce an increase of its lability . Consequently, it is not strange that the introduction of Pt−C bonds in Pt(IV) t 2g 6 complexes leads to an important lability increase in their substitution reactions, and that they tend to operate through a dissociatively activated mechanism …”
Section: Introductionmentioning
confidence: 99%
“…We have been interested in the last few years on the mechanisms of formation and substitution reactions of cyclometalated platinum and palladium complexes with a variety of imines . Dimethyl sulfide substitution by phosphines on cyclometalated Pt(IV) complexes of the type shown in eq 1 have already been studied 5a …”
Section: Introductionmentioning
confidence: 99%
“…Nevertheless, a certain electronic tuning related to the nature of the halogen, X, ligand and the presence of electron-withdrawing substituents affecting the aromatic metalated carbon has been found for the activation entropy values. Namely, a lesser degree of dissociation to go to the transition state was observed when the cyclometalated compound contained a maximum of electron-withdrawing elements 5a…”
The substitution reactions of sulfide by phosphines on Pt(IV) complexes having a cyclometalated imine ligand, two methyl groups in a cis geometrical arrangement, and a halogen and a sulfide as ligands, [Pt (Me)(2)X(C-N)(SR(2))], have been studied as a function of temperature, solvent, and electronic and steric characteristics of the phosphines, sulfides, X, and C-N. In most of these cases, a limiting dissociative mechanism has been found, where the dissociation of the sulfide ligand corresponds to the rate-determining step. The intermediate species formed behaves as a true pentacoordinated Pt(IV) compound in a steady-state concentration only for the systems with SMe(2); for the bulkier SEt(2) and SBzl(2) leaving ligands the rate constants and activation parameters show an important degree of solvent dependence, which correlates with the ability of the solvent to form hydrogen bonds. The X-ray crystal structure of one of the dibenzyl sulfide complexes has been determined, and the geometrical arrangement of the ligands has been determined by NOE NMR measurements at low temperature. The nature of the solvent, imine, sulfide, and halogen ligands produces differences in the reaction rates, which can be quantified very well by the corresponding DeltaS values that move from +48 to -90 J K(-1) mol(-1). The reaction on [Pt(Me)(2)F(C(5)CF(4)CHNCH(2)Ph) (SMe(2))] has been found to take place via a mechanism that depends strongly on the bulkiness of the substituting phosphine. While for PCy(3) the reaction is dissociative, for smaller entering ligands the first associatively activated substitution mechanisms on organometallic Pt(IV) complexes have been established with values of DeltaH and DeltaS in the 28-44 kJ mol(-1) and -120 to -83 J K(-1) mol(-1) ranges. Important intramolecular hydrogen bonding in the starting material can be held responsible for this difference with the remaining systems.
“…In fact, the interchange of the methyl groups in oxidative addition reactions of MeI in this type of complexes has already suggested its fast substitution reactivity. Recent kinetic studies have also confirmed that dimethyl sulfide substitution reaction rates are quite fast 1 …”
The substitution reactions of SMe 2 by phosphines (PMePh 2 , PEtPh 2 , PPh 3 , P(4-MeC 6 H 4 ) 3 , P(3-MeC 6 H 4 ) 3 , PCy 3 ) on Pt IV complexes having a cyclometalated imine ligand, two methyl groups in a cis-geometrical arrangement, a halogen, and a dimethyl sulfide as ligands, [Pt(CkN)(CH 3 ) 2 (X)(SMe 2 )], have been studied as a function of temperature, solvent, and electronic and steric characteristics of the phosphines and the X and CkN ligands. In all cases, a limiting dissociative mechanism has been found, where the dissociation of the SMe 2 ligand corresponds to the rate-determining step. The pentacoordinated species formed behaves as a true pentacoordinated Pt IV compound in a steady-state concentration, given the solvent independence of the rate constant. The X-ray crystal structures of two of the dimethyl sulfide complexes and a derivative of the pentacoordinate intermediate have been determined. Differences in the individual rate constants for the entrance of the phosphine ligand can only be estimated as reactivity ratios. In all cases an effect of the phosphine size is detected, indicating that an associative step takes place from the pentacoordinated intermediate. The nature of the CkN imine and X ligands produces differences in the dimethyl sulfide dissociation reactions rates, which can be quantified by the corresponding ∆S q values (72, 64, 48, 31, and 78 J K -1 mol -1 for CkN/X being C
Full understanding of a chemical change requires the identity and nature of intermediates formed between reactants and products to be identified and knowledge of the processes of bond making and breaking and of electron transfer to be established. Inorganic reaction mechanisms are thus of intrinsic interest. Moreover, they underpin our understanding of key environmental, biological, medical and industrial processes. This review of the 1998 literature does not, except for studies of particular importance or interest, cover homogeneous catalysis or redox processes involving organic substrates, organic reactions of the heavier p-block elements, or fluxional, photochemical, electrochemical, solid-state or heterogeneous processes.A new journal, Inorg. React. Mech., and a special issue of Transition Met. Chem. are devoted to inorganic mechanisms. Activation volumes in solution, models for electron transfer, the use of density functional theory and electron-transfer reactions of polyoxymetalates have been reviewed and the application of genetic-algorithm and non-linear optimization methods to the treatment of kinetic data described. Stoppedflow EXAFS has been used to study short-lived intermediates. Studies of processes on the femtosecond timescaleare noted.
Redox reactionsA series of papers on electron transfer in proteinsand a review by Marcus on theory and experiment in electron-transfer reactions have appeared in a special issue of J. Electroanal. Chem.
Long range electron transferDifferent sites on cytochrome bc are associated with oxidation and reduction, with the Fe-S protein component of cytochrome bc acting as an electron shuttle between †Dedicated to the memory of Professor Bob Hay, an esteemed friend and colleague.
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