Irreversible metalation, acetonitriles

We have applied Eq. 29 to a series of Cp and Tp Group 6 metal carbonyl hydrides in acetonitrile, mostly on the basis of irreversible metal-hydride oxidation... 

The question of which pathway is preferred was very recently addressed for several diimine-chelated platinum complexes (93). It was convincingly shown for dimethyl complexes chelated by a variety of diimines that the metal is the kinetic site of protonation. In the system under investigation, acetonitrile was used as the trapping ligand L (see Fig. 1) which reacted with the methane complex B to form the elimination product C and also reacted with the five-coordinate alkyl hydride species D to form the stable six-coordinate complex E (93). An increase in the concentration of acetonitrile led to increased yields of the methyl (hydrido)platinum(IV) complex E relative to the platinum(II) product C. It was concluded that the equilibration between the species D and B and the irreversible and associative1 reactions of these species with acetonitrile occur at comparable rates such that the kinetic product of the protonation is more efficiently trapped at higher acetonitrile concentrations. Thus, in these systems protonation occurs preferentially at platinum and, by the principle of microscopic reversibility, this indicates that C-H activation with these systems occurs preferentially via oxidative addition (93). 

Adsorption of a specific probe molecule on a catalyst induces changes in the vibrational spectra of surface groups and the adsorbed molecules used to characterize the nature and strength of the basic sites. The analysis of IR spectra of surface species formed by adsorption of probe molecules (e.g., CO, CO2, SO2, pyrrole, chloroform, acetonitrile, alcohols, thiols, boric acid trimethyl ether, acetylenes, ammonia, and pyridine) was reviewed critically by Lavalley (50), who concluded that there is no universally suitable probe molecule for the characterization of basic sites. This limitation results because most of the probe molecules interact with surface sites to form strongly bound complexes, which can cause irreversible changes of the surface. In this section, we review work with some of the probe molecules that are commonly used for characterizing alkaline earth metal oxides. 

The authors assumed that the precipitation of these metal sulfides was controlled by the hydrolysis reaction of TAA promoted by proton in the acidic conditions. However, the hydrolysis of TAA observed in acidic and alkaline ranges is a much slower process than observed in the precipitation of these metal sulfides (7-12), and it may not be accelerated by consumption of S2 ions because of its irreversible nature. In addition, the reaction virtually finished with a great part of the starting metal ions and TAA left unreacted. Also, it has already been verified that the probability of direct reaction of TAA with metal ions is zero or at least negligible from its strong dependence of pH in reactivity (7). Thus, it seems reasonable to consider that the main path is the release of S2 ions from TAA according to the following reaction scheme with the production of acetonitrile (7,13) as found in the reaction of TAA with Cd2+ ions in an alkaline media ... 

Similarly to the low chemical reactivity of (simple) alkylsilanes devoid of functional groups, the electrochemical reactivity of simple alkylsilanes is quite low. Klingler and Kochi measured the oxidation potentials of tetraalkyl derivatives of group-14-metal compounds by using cyclic voltammetry3. These compounds exhibit an irreversible anodic peak in acetonitrile. The oxidation potential (7 p) decreases in the order of Si>Ge>Sn>Pb as illustrated in Table 1. This order is the same as that of the gas-phase ionization potentials (7p). The absence of steric effects on the correlation of Ev with 7p indicates that the electron transfer should take place by an outer-sphere mechanism. Since tetraalkylsilane has an extremely high oxidation potential (>2.5 V), it is generally difficult to oxidize such alkylsilanes anodically. 

Data were obtained in acetonitrile solution containing 0.1 mol dm-3 Bu"NBF4 as supporting electrolyte. Solutions were 3 x 10"3 mol dm-3 in compound and potentials were determined with reference to SCE at 21 1°C at 50 mV s"1 scan rate. The CVs of [28], [29] and [31] consisted of a main current wave (reversible for [30] and [32] and EC mechanism for [28], [29] and [31]) corresponding to the Fc+/Fc couple and minor current waves (irreversible or quasi-reversible) from the oxidation of the amino groups. p, represents the anodic current peak potential of the Fc+/Fc couple. "Anodic shifts of the anodic peak potential of the Fc+/Fc couple produced by the presence of metal cations (1 or 2 equiv added as their perchlorate salts). For [28], [29] and [31], after addition of cations, the current waves from the respective amino groups disappeared and that of the Fc+/Fc couple became reversible. Obtained in methanol, instant oxidation by silver cations. 

Thus, methyl hydrogen atoms cannot easily closely approach surface metal atoms even through vibrationally excited states as shown schematically at the left in the figure. However, acetonitrile molecules bound near step or kink sites can have methyl hydrogen atoms in positions from which there can be a facile close approach of these hydrogen atoms to the surface metal atoms. This geometric or stereochemical feature explains the reactivity (irreversible C—H bond-breaking processes) observed for acetonitrile on the stepped, stepped-kinked, and super-stepped (110) nickel surfaces. 

The only electrochemical study available on palladium derivatives reports that Pd(acac)2 in acetonitrile undergoes an irreversible two-electron reduction to palladium metal Ep = -0.97 V, vs. SCE)23 . 

Metal units both in the core and in the periphery are present in dendrimers 27 and 28 [107]. In 27, oxidation of the core occurs in acetonitrile in a irreversible process, as often observed for encapsulated redox units. This process is followed, at more positive potential, by a single reversible wave due to the six Ru-based moieties, indicating that there is no interaction between the peripheral units. In 28, the behavior of the Ru-based units is essentially the same as in 27, but no oxidation due to the Co-based core was evidenced. Interestingly, also chemical treatment with chlorine was ineffective in this regard. 

In acetonitrile solution, it undergoes a quasireversible one-electron reduction (E° = — 1.99 V, at — 40 °C) as well as an irreversible one-electron oxidation (Ep= + 0.18 V, at — 40 °C). Such redox changes are attributed to the reduction of the central tungsten atom and to the oxidation of one outer molybdenum atom, respectively. At variance with the previously discussed thiolate bridged trimetal clusters (which undergo a chemically reversible two-electron oxidation), the occurrence of an irreversible one-electron oxidation is attributed to the inability of the central W(VI) to form metal-metal bonds with the outer Mo(I) moieties [74]. 

---