Proton transfer rates between metal

DR. STEPHEN NEUMANN (Eastman Kodak Co.) In your observation of the proton transfer rates between the metal base and the amine base, do you have any feeling for whether the slower transfer is unique to the metal bases or whether it has to do more with the bulk of the overall base participating in the proton transfer For instance, would a sterically hindered amine show rates similar to the metal bases. 

The pJCj values are now available for many hydride complexes. Extensive tables have been compiled recently by Bullock and by Tilset. The rate of proton transfer to and from transition metals is rather slow (see below), so it is often possible to detect separate NMR signals for M-H and M , and tiius to determine the position of proton transfer equilibria between hydride complexes (M-H) and bases (B), or metal bases (M") and organic acids (HA). The pX values in Table 3.1 have been obtained in acetonitrile, an excellent solvent for acid-base chemistry because it solvates cations well enough to minimize ion pair formation it is both a weak acid and a weak base, with a very low autoprotolysis constant (ion product). ... 

The general features discussed so far can explain the complexity of these reactions alone. However, thermodynamic and kinetic couplings between the redox steps, the complex equilibria of the metal ion and/or the proton transfer reactions of the substrate(s) lead to further complications and composite concentration dependencies of the reaction rate. The speciation in these systems is determined by the absolute concentrations and the concentration ratios of the reactants as well as by the pH which is often controlled separately using appropriately selected buffers. Perhaps, the most intriguing task is to identify the active form of the catalyst which can be a minor, undetectable species. When the protolytic and complex-formation reactions are relatively fast, they can be handled as rapidly established pre-equilibria (thermodynamic coupling), but in any other case kinetic coupling between the redox reactions and other steps needs to be considered in the interpretation of the kinetics and mechanism of the autoxidation process. This may require the use of comprehensive evaluation techniques. 

Olefins can only be polymerized by metal halides if a third substance, the co-catalyst, is present. The function of this is to provide the cation which starts the carbonium ion chain reaction. In most systems the catalyst is not used up, but at any rate part of the cocatalyst molecule is necessarily incorporated in the polymer. Whereas the initiation and propagation of cationic polymerizations are now fairly well understood, termination and transfer reactions are still obscure. A distinction is made between true kinetic termination reactions in which the propagating ion is destroyed, and transfer reactions in which only the molecular chain is broken off. It is shown that the kinetic termination may take place by several different types of reaction, and that in some systems there is no termination at all. Since the molecular weight is generally quite low, transfer must be dominant. According to the circumstances many different types of transfer are possible, including proton transfer, hydride ion transfer, and transfer reactions involving monomer, catalyst, or solvent. 

The -catalyzed hydride transfer from BNAH to Q is known to proceed via a -promoted ET from BNAH to Q, followed by a proton transfer from the resulting BNAH" to the Q" /(Mg ) complex and the subsequent fast ET from BNA to QH /Mg (117, 143). The change in the type of reaction depending on the Lewis acidity of the metal ion can be explained well by the ET mechanism in Scheme 23. The initial rate-determining ET from BNAH to Q results in the formation of radical ion pair (BNAH + and Q ), where Q forms 1 1 and 1 2 complexes with Sc. This result is followed by fast radical couphng between Q and BNAH to give the zwitterionic intermediate that is eventually... 

The rate of proton transfer has been measured for a number of metal hydride/organic amine combinations. The rates appear to follow Marcus behavior see Marcus Treatment), in which the rate goes up with driving force (equation 21, where ab is the rate of proton transfer between AH and B , and Kxr is the equilibrium constant for the proton transfer). Proton transfer appears to be the slow step in the process, rather than slow electron transfer followed by fast H atom transfer, because the rates show an isotope effect. For example, in the self-exchange of [CpM(H,D)(CO)3]/[CpM(CO)3] , kn/ko is 3.6, 3.7, and 3.7 for Cr, Mo, and W. There seems to be a good relation between thermodynamic acidity and kinetic... 

The rate constants for protonation at the metal of (6-dimethylamino-fulvene)M(CO)3 increase in the order Cr < Mo < W quantitative results are given but they appear unreliable. Proton transfer between protonated and unprotonated forms of (arene)Cr(CO)j is fast on the NMR time scale and occurs by rate-determining proton removal by the conjugate base ([FSOj] ) of the acid employed" (FSOjH). 

When, however, the rates of electron and proton transfer are comparable and coupled to each other, intermediate slopes are obtained (between -0.5 and -1.0) [76-78]. Accordingly, the measured Marcus slopes of-0.72 and -0.71 support the assignment of a concerted PCET process for the oxidation of the substituted phenols by the Cu metal complexes. Additionally, KIEs of 1.21 to 1.56 are also consistent with an asynchronous PCET process in which there is some proton motion but a large ET component in the transition state. 

There is an increase in the importance of electrophilic catalysis by zinc cation relative to acetic acid for deprotonation of the a-carbonyl carbons of hydroxyace-tone, a substrate which provides a second stabilizing chelate interaction between the hydroxy group at the substrate and the metal dication that is expressed at transition state for proton transfer [19]. For example, the third-order rate constants kx for the Zn +-assisted acetate-ion-promoted deprotonation of the a-CHs and a-CH20H groups of hydroxyacetone are 32-fold and 770-fold larger, respectively, than the corresponding second-order rate constants kAco for proton transfer to acetate anion assisted by solvent water that is present at 55 M (Scheme 1.12). This shows that Zn + stabilizes the transition state for proton transfer from the a-CHs... 

As demonstrated in this chapter, there have always been the fundamental mechanistic questions in oxidation of C-H bonds whether the rate-determining step is ET, PCET, one-step HAT, or one-step hydride transfer. When the ET step is thermodynamically feasible, ET occurs first, followed by proton transfer for the overall HAT reactions, and the HAT step is followed by subsequent rapid ET for the overall hydride transfer reactions. In such a case, ET products, that is, radical cations of electron donors and radical anions of electron acceptors, can be detected as the intermediates in the overall HAT and hydride transfer reactions. The ET process can be coupled by proton transfer and also by hydrogen bonding or by binding of metal ions to the radical anions produced by ET to control the ET process. The borderline between a sequential PCET pathway and a one-step HAT pathway has been related to the borderline between the outer-sphere and inner-sphere ET pathways. In HAT reactions, the proton is provided by radical cations of electron donors because the acidity is significantly enhanced by the one-electron oxidation of electron donors. An electron and a proton are transferred by a one-step pathway or a sequential pathway depending on the types of electron donors and acceptors. When proton is provided externally, ET from an electron donor that has no proton to be transferred to an electron acceptor (A) is coupled with protonation of A -, when the one-electron reduction and protonation of A occur simultaneously. The mechanistic discussion described in this chapter will provide useful guide to control oxidation of C-H bonds. 

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