Transition metal hydrides proton donor

Since the nature of the hydride chemical shifts, particularly in transition metal hydride complexes, is not simple [32], there is no reliable correlation between Sh and the enthalpy of dihydrogen bonding. Nevertheless, the chemical shifts of hydride resonances and their changes with temperature and the concentration of proton-donor components, for example, can be used to obtain the energy parameters for dihydrogen bonding in solution. As earlier, the enthalpy (A/f°) and entropy (AS°) values can be obtained on the basis of equilibrium constants determined at different temperatures. Let us demonstrate some examples of such determinations. 

Table 7.1 lists energies of dihydrogen bonding and the H- H distances that have been calcnlated for transition metal hydride systems in the gas phase. As shown, the transition metal complexes calcnlated have different ligand environments and interact with different proton donors. 

Quantitative treatment of rate constants for the hydride attack, / hp, the metal protonation, A fpm. and the exchange process in the framework of Scheme 10.7 have resulted in A hp = 2.7 x 10 M Vs and A fpm = 2.8 x 10 M /s. Thus, the hydride protonation occurs faster by a factor of 10. In earlier chapters we have shown that transition metal hydrides form dihydrogen bonds in the presence of proton donors. Now, based on the principle of microscopic reversibility, one can suggest that proton transfer to a hydridic hydrogen actually occurs via a dihydrogen bond. 

Orlova, G., Scheiner, S., Kar, T., Activation and cleavage of H-R bonds through intermolecular H...H bonding upon reaction of proton donors HR with 18-electron transition metal hydrides. J. Phys. Chem. A 1999, 103, 514-520. 

HCo(CO) acts in the hydroformylation reaction more as a hydride rather than a proton donor. This behavior strongly contrasts the acidic properties of all the mentioned carbonyl hydride species, since in solution, and especially in water, they function as strong acids, with sometimes even very low pK values. The discrepancy that the very same transition metal hydrides may act as either hydride or proton sources, gave rise to controversial discussions in the literature for many years. Thus, HCo(CO) not only provides the prototypical example for the ambivalence of the TM-H bond, but also an early example of TMH complexes in homogeneous catalysis. 

In view of the acidity of hydride complexes, one might expect them to form hydrogen-bonded intennediates during proton-transfer reactions. However, it is relatively uncommon for neutral hydride complexes to form hydrogen bonds M-H A because most M-H bonds are polarized as M(8+)-H(8-), not as M(8-)-H(8+). (The need to reverse the observed polarization during deprotonation is a major cause of the low kinetic acidity of transition metal hydrides, mentioned previously.) The first M-H Ahydrogen bond from a neutral hydride has just been reported CpM(CO)3H (M = Mo and W) serves as a hydrogen bond donor to (octyl)3P=0 and even to pyridine, apparently because there is M(8-)-H(8+) polarization in its M-H bond. °... 

Another route to productive CT photochemistry can involve proton transfer reactions between A and D +, which are favored in the redox pair arising from the enhanced acidity of D + (relative to D) and basicity of A (relative to A). Numerous examples of these reactions exist in the organic literature [240-241], and such a pathway should be particularly important for transition-metal hydrides with significantly enhanced acidities of their (metastable) cation radicals [242]. Thus irradiation of the EDA complex of fumaronitrile (as acceptor) with the hydridic donor (CP2M0H2) [35] leads to CT hydro-metallation. 

Before venturing into the more compHcated case of polyhydrides, there is one point that should be made about transition metal hydrides in general. The term hydride is sometimes misleading. To many chemists, this term implies an H ionicaUy bound to metal. This term may be used with relative inpunity for main group hydrides such as NaH (Na H ). When applied to transition metal complexes, however, this term is sometimes a misnomer. In terms of their reactivity, transition metal hydrides can function as hydride (H ) donors, proton (H ) donors or be quite covalent in their bonding. 

Ionic hydrogenations of C=C and C=0 bonds were reported prior to the development of ionic hydrogenations mediated or catalyzed by transition metals. Tri-fluoroacetic acid (CF3C02H) as the proton donor and triethylsilane (HSiEt3) as the hydride donor are most commonly used, though a variety of other acids and several other hydride donors have also been shown to be effective. A review [1] by Kursanov et al. of the applications of ionic hydrogenations in organic synthe-... 

In the transition metal-catalyzed reactions described above, the addition of a small quantity of base dramatically increases the reaction rate [17-21]. A more elegant approach is to include a basic site into the catalysts, as is depicted in Scheme 20.13. Noyori and others proposed a mechanism for reactions catalyzed with these 16-electron ruthenium complexes (30) that involves a six-membered transition state (31) [48-50]. The basic nitrogen atom of the ligand abstracts the hydroxyl proton from the hydrogen donor (16) and, in a concerted manner, a hydride shift takes place from the a-position of the alcohol to ruthenium (a), re-... 

Sterically hindered alkenes can be hydrogenated at —50°C using triflic acid and a hydride donor.483 In addition to Et3SiH, transition-metal carbonyl hydrides such as HM(CO)3Cp (M = W, Mo, Os) and HMn(CO)5 (M = Mn, Re) are suitable hydride donors. Alkenes that form tertiary carbocation on protonation are hydrogenated in high yields (90-100%), whereas yields for styrenes are lower (50-60%). Alkynes are converted to the corresponding saturated hydrocarbon by using HW(CO)3Cp in combination with triflic acid.484... 

In the presence of transition metal complexes, such as those of Ir3+ (Scheme 24), Rh3+ and Ru3+, the proton sponge 1 behaves as a hydride donor from one of its methyl groups51,186. The methyleneimmonium cation 157 thus formed undergoes immediate cyclization to a l,l,3-trimethyl-2,3-dihydroperimidinium salt 158. 

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