Transition metal-hydride complexes from solvent

The M-H bonds of transition-metal hydride complexes may be cleaved heterolyti-cally (H, H transfer) or homolytically (H transfer). AG for the transfer in Equation 1.1 is readily quantified by pKj measurements (see Chapter 3). Analogous measurements for H transfer, or hydricities , are difficult because the loss of generates a vacant coordination site. However, AG for Equation 1.2 can be determined indirectly, from electrochemical and pJG measurements in the appropriate solvent [1, 2], and we can thus compare the hydricities of various hydride complexes (see Chapter 3). The lowest values of AG. (corresponding to the complexes most eager to transfer H ) are found for second- and third-row transition metals [3], which is why those (relatively expensive) metals are good donors and effective catalysts for reactions like ionic hydrogenation [5-10],... 

All reactions and sample preparations are carried out in an inert-atmosphere enclosure under dry nitrogen. Solvents and reagents are dried in the following manner. Benzene, tetrahydrofuran, and n-pentane are freshly distilled from lithium aluminum hydride pyridine is distilled over barium oxide and tetramethylethylenediamine is distilled over calcium hydride. Solvents used in preparing nmr and infrared samples are degassed by a freeze-thaw technique. Nmr spectra are obtained with torch-sealed nmr tubes. The commercial transition metal carbonyl complexes are recrystallized and vacuum-dried before use. Glassware is routinely flame-dried. 

Figure 10.10 Energy profiles of proton transfer to a hydride ligand of a transition metal complex in solution AEi = + 3 to 4kcal/mol, AE2 = — 5 to — 7 kcal/mol, A 3= + 10 to 14 kcal/mol, and A 4 = —7 kcal/mol the energy is a function of the proton-hydride distance, varying from an initial state (2.5 A) to the final product (0.9 A) conversion of the intimate ion pair to the solvent-separated ion pair is shown as a function of the H+- O" distance. (Reproduced with permission from ref. 29.)...

ZIEGLER CATALYST. A type of stereospedfre catalyst, usually a chemical complex derived from a transition metal halide and a metal hydride oi a metal alkyl. The transition metal may be any of those in gioups IV to VIII of the periodic table the hydride or alkyl metals are those of groups I, II. and III. Typical, titanium chloride is added to aluminum alkyl in a hydrocarbon solvent to form a dispersion or precipitate of ilie catalyst complex. These catalysts usually operate at atmospheric pressure and are... 

The transition metal complex-catalyzed formation of 1,3-dioxepanes from vinyl ethers has also been described. For example, reaction of allyl vinyl ether 157 with a nonhydridic ruthenium complex at higher temperatures and without any solvent produced 1,3-dioxepane 159 whereas, the use of a hydridic ruthenium complex resulted in the formation of vinyl ether 158 by double-bond isomerization (Scheme 43). It was suggested that cyclic acetal formation proceeds via a 7i-allyl-hydrido transient complex, which undergoes nucleophilic attack of the OH group at the coordinated Jt-allyl <2004SL1203>. 

Deviating from the route via nucleophilic attack of the carbanion at the carbon atom of a CO ligand and then reaction of the acylmetallate with an electrophile are those methods which involve (a) addition of the carbanion to the carbon atom of a carbyne ligand, (b) displacement of halides from transition-metal carbonyl halides by cyclohepta-trienyllithium, or derivatives thereof, followed by hydride abstraction or (c) substitution of a coordinated solvent from a metal-carbonyl complex (see also reaction of LiR with carbene complexes). 

HPLC grade nonpolar solvents, toluene and dichloromethane, were purchased from Aldrich. Toluene was distilled over sodium/benzophenone, and dichloromethane was distilled over calcium hydride. All solvents were stored under nitrogen, over molecular sieves. Water content in dried solvents was typically less than 1 mM. Potassium salts of transition-metal-substituted heteropoly complexes were prepared according to the methods published previously. The IR, UWVIS spectra, and Cyclic Voltammograms agreed... 

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 reported oxidative additions of weakly acidic X-H bonds are urJikely to occur by protonation of the metal center and collapse of the anion and cation. Most of these reactions were conducted in nonpolar solvents in which substrates like water, alcohols, and amines have very high values. Instead, these reactions are more likely to occur by initial coordination of the reactants to the metal center to generate a transient aqua, alcohol, or amine complex, which subsequently undergoes insertion of the metal into the X-H bond. The acidity of the X-H bond does appear to promote the reaction by this pathway. Thus, the oxidative addition of aniline ° is more common than the oxidative addition of alkylamines and ammonia. - In many cases, the product from the coordination of amine is more stable than the oxidative addition product because amines are basic and are common ligands for transition metals. Thus, the product from the coordination of a basic amine or ammonia can be more stable than the amido hydride complex that would form by oxidative addition (Equation 7.20). ... 

Electrocatalysis by transition metal complexes is elegantly illustrated by the work of DuBois and colleagues.2 In the absence of CO2, the palladium (II) complex undergoes two-electron reduction, but when CO2 is present, the one-electron reduction product binds CO2 (DMF as solvent) (Figure 5.9). With added acid, carbon monoxide is produced catalytically from carbon dioxide. This chemistry likely involves the protonated carbon dioxide adduct (hydroxycarbonyl complex) shown in Figures 5.9 and 5.10. Catalytic turnover numbers greater than 100 have been reported for this and related compounds. Some hydrogen is produced in parallel, evidently via a hydride complex. 

The last step, the release of acetaldehyde, can be interpreted as a reductive elimination to give the hydrate of acetaldehyde (Eqs. (9.19) and (9.20)) where Eq. (9.19) represents merely the completion of the complex ligation sphere at the central Pd by a solvent molecule. A reductive elimination is a common reaction of group 8 metal compounds. For this case, it has been first proposed in [20]. Keith etal. [21] derived such a pathway but chloride assisted from theoretical considerations. The barrier heights of the transition states for other pathways, for example, P-hydride elimination, were found to be too high. The route according to Eq. (9.20) would also be valid for chloride-free Pd compounds. 

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