Hydrides protonation

Scheme 3.4 The heterolytic splitting of dihydrogen at Ru(ll) to give a hydridic-protonic bond, as proposed by Chu et al. [55] in the mechanism of the homogeneous hydrogenation of carbon dioxide.

Scheme 20.13 Concerted hydride-proton transfer mechanism.

Clearly, in the related catalysts containing just simple bidentate phosphines, dipyridines, or bis-oxazolines the concerted, heterolytic transfer cannot take place in the same way, unless we invoke an alkoxide or other anion as the proton-receiving moiety. In Figure 4.28 we have presented a simplified scheme for the hydride/proton mechanism for hydrogen transfer using an external base. 

The intermediacy of ion-neutral complexes is neither restricted to even-electron fragmentations nor to complexes that consist of a neutral molecule and an ion. hi addition, radical-ion complexes and radical ion-neutral complexes occur that may dissociate to yield the respective fragments or can even reversibly interconvert by hydride, proton or hydrogen radical shifts. Many examples are known from aliphatic alcohols, [180-183] alkylphenylethers, [184-187] and thioethers. [188]... 

It is difficult to correlate the kyi/k values with the operation of a hydride, proton or H atom transfer. The temperature dependence of the kinetic isotope effect, which is now easier to measure accurately, is more diagnostic of mechanism but has been applied mainly to organic systems. ... 

Ru dihydride [in the absence of (CF3)2CHOH], and the Ti,min (RuH H) is a minimal relaxation time of the hydride ligand that is involved in dihydrogen bonding. The Ti min CRuH- H) value is calculated from eq. (4.11) as 0.0894 s. This time is remarkably shorter than that in the individual dihydride (0.178 s), due to an additional hydride-proton dipolar coupling. Then this additional relaxation rate, 1/ T i min (RuH- H), governed by the single hydride-proton dipolar contact, is expressed as... 

In previous sections we have shown clearly that intramolecular dihydrogen bonds X-H- H-Y, with X and Y representing various chemical elements, can exist in both the solid state and in solution. In addition, the bonds can be a critical factor in the control of molecular conformational states or effects on rapid and reversible hydride-proton exchanges related to the process shown in Scheme 5.1, or the well-known H-D isotope exchanges in similar subsystems [23]. Such bonds could also play an important role in the stabilization of transition states, appearing as a reaction coordinate in many transformations. This is particularly... 

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. 

In fact, if free H+ attacks a hydride, proton transfer should be suppressed by the addition of X ions. However, NBn4Bp4, for example, does not affect the protonation rates of cw-[FeH2PP3] by the action of HBF4 in THE solutions. This is good evidence for the parallel attack of the molecnlar form HX and the ion pair nnder conditions when ion pairs react more slowly than HX [5]. 

This complex is a starting material to make complexes with novel hydride-proton interactions.16 The mer isomer has been used to make heterobimetallic clusters.17... 

FeFe-enzyme - proton or hydrogen substrate binding and also the hydride-proton reaction exclusively occurs at the iron distal to the [4Fe-4S] cluster, suggesting that mononuclear iron complexes might also be viable catalysts. Consequently, Ott and coworkers have synthesized and characterized some stable pentacoordinated Fe(II) complexes with five ligands that nicely mimic the native ones and exhibit an open coordination site [163, 164]. This approach avoids the formation of the less reactive bridging hydrides that are found in the dinuclear complexes [153]. Catalytic H2 formation from weak acids at low overpotentials with promising TOF and catalyst stability could be demonstrated [164]. 

Chlorohydridobis(tricyclohexylphosphine)nickel is a yellow-brown solid. It is thermally stable at ambient temperature but reacts with air. It is very soluble in benzene, tetrahydrofuran, and dichloromethane and is soluble in diethyl ether and petroleum ether. Carbon tetrachloride, carbon disulfide, and chloroform decompose the complex. The infrared spectrum shows a sharp v(Ni—H) band at 1916 cm-1 (KBr disk and Nujol mull). The high-field H nmr spectrum in benzene solution has a triplet (1 2 1) at t34.6 (TMS) with JpH 73.5 Hz. The splitting is caused by the coupling of the hydride proton with two equivalent 31P nuclei. This is consistent with a trans square-planar configuration. 

The resultant charged species can then take part in hydride, proton, and electron transfer equilibria in the solution. Intermediate oxidation products, which have not had time to engage in the bimolecular equilibria, thus have shorter lengths of conjugation. The behavior of oxidized polyfurfuryl in solution may prove to be a useful model for the conductivity of insoluble conductive polymers. 

These data suggest that the initial step in the reaction of ozone with the Si-H bond is the reversible formation of a silicon-ozone complex. This cannot be the rate step since p would have to be positive (nucleophilic attack) and no primary isotope effect would be predicted. To eliminate the statistical factor for the di- and trihydrosilanes, attack by ozone on the hydridic proton from within the complex must be much more favorable than direct encounter and reaction with uncomplexed ozone and Si-H. 

The BH3 adduct of 2-aminopyridine seems to be very stable to internal hydroboration and polymerisation compared to borane pyridine. We attribute the extra stability to hydride-proton interactions these together with those of fluorine-proton were reported for other amine borane compounds16 (Figure 5). The MM models for the two boron complexes shown in Figure 6, present atomic distances... 

Figure 5 Atomic distances A between hydrides-protons or fluorine-proton in amine boranes where weak bonding interactions are proposed16...

Figure 9 MM models of the two conformers for N-BH3 adduct of the 2-aminopyridine N-silylated showing hydride-proton interactions...

Besides traces of residual solvent, the presence of three types of compounds was detected. In addition to unreacted ds-3-hexene (C6H12), the most important product was Ci0H22 or C10H20 which most likely was formed by f-butylation of c -3-hexene followed by rapid hydridation, Addition of f-Bu to ds-3-hexene produces a secondary carbenium ion which may rearrange to form a tertiary carbenium ion prior to hydridation. Significantly, the absence of C12H26 indicates the absence of ethylation and surest addition of r-Bu to 1,4-unsaturations followed by rapid termination by hydridation. Proton elimination might have also occurred but could not be definitively established. 

Figure 6 summarizes reactions anticipated to occur during the grafting of growing polystyrene cations onto polybutadiene The growing polystyrene cation chain may terminate by hydridation, proton elimination, ethylation and alkylation of poly -butadiene on the basis of model experiments the latter event is expected to be followed by rapid hydridation to yield poly(butadiene-g-styrene). 

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