Hydride as Proton Acceptor

We recentlyf came to the conclusion that the large dominance of isomer Ib over Ic is due to the stabilization elfects of intramolecular hydrogen-bonds involving metal hydrides as proton acceptors from N H bonds. In solution, the occurrence of this type of interaction has been unambiguously demonstrated by evaluating the contribution to the relaxation of Hy and He arising from the N-H moiety. 

HYDRIDES OF THE ELEMENTS IN GROUP 4A ACTING AS PROTON ACCEPTORS AND PROTON DONORS VERY WEAK DIHYDROGEN BONDS... 

R.H. Crabtree et al., A new intermolecular interaction Unconventional hydrogen bonds with element-hydride bonds as proton acceptor. Acc. Chem. Res. 29, 348-354 (1996)... 

Crabtree. R.H. Siegbahn. P.E.M. Eisenstein, O. Rheingold. A.L. A new intermolecular interaction Unconventional hydrogen bonds with element-hydride bonds as proton acceptor. Acc. Chem. Res. 1996. 29. 348. Morris, R.H. 1995 Alcan award lecture—New intermediates in the homolytic and heterolytic splitting of dihydrogen. Can. J Chem. 1996. 74, 1907. 

R.H. Crabtree, O. Eisenstein, A.L. Rheingold et al. - A New Intermolecular Interaction Unconventional Hydrogen Bonds with Element-Hydride Bonds as Proton Acceptor, Acc. Chem. Res. 29,348,1996. 

Epstein, L. M., Shubina, E. S., Bakhmutova, E. V. et al. 1998. Unusual hydrogen bonds with a hydride atom in boron hydrides acting as proton acceptor. Spectroscopic and theoretical studies. Inorg. Chem. 37 3013-3017. 

This reaction is due to the very strong basic property of the hydride ion H" which behaves as a powerful proton acceptor and is therefore strongly basic, i.e. 

All conjugates are hydrogen bonded in the homoconjugates the hydrogen may be considered to be equally divided between the donors B (a) or the acceptors A (b), in the hetroconjugates as depicted in the scheme BH+ is a weak proton donor (a) and A a weak proton acceptor (b). In fact, in the preceding conjugates we have to deal with a proton bond in contrast with a hydride bond as exists in some less common compounds or a complex ion such... 

Since many of the transformations undergone by metabolites involve changes in oxidation state, it is understandable that cofactors have been developed to act as electron acceptors/ donors. One of the most important is that based on NAD/NADP. NAD+ can accept what is essentially two electrons and a proton (a hydride ion) from a substrate such as ethanol in a reaction catalysed by alcohol dehydrogenase, to give the oxidized product, acetaldehyde and the reduced cofactor NADH plus a proton (Figure 5.2). Whereas redox reactions on metal centres usually involve only electron transfers, many oxidation/reduction reactions in intermediary metabolism, as in the case above, involve not only electron transfer but... 

This analogy is plausible on energetic grounds, since the decreased base strength of the proton acceptor should be approximately compensated by the increased acid strength of the proton donor. In view of the different species involved, however, it is reasonable to expect appreciable differences in the configurations of the transition states and hence in the activation barriers for the two paths. Therefore, the failure to observe an acid-catalyzed exchange reaction cannot be taken as conclusive evidence in favor of the alternative (hydride ion) mechanism. 

As pointed out earlier, the principal requirement for an active catalyst for the heterolytic splitting of hydrogen is the presence of two suitably disposed functional groups—a metal atom to combine with the hydride ion and a base ( B) to act as a proton acceptor. In line with the evidence for the presence of a ferrous complex in hydrogenase, Rittenberg (18) has suggested the following model for the active site of the enzyme. 

Thus we tend to favor the mechanism outlined in Reactions 12 and 13 (followed by Reactions 7 and 8). The mechanisms as presented do not indicate how the N-formyl product is formed although formation of a Ru-CO-N moiety at some stage seems essential metal-assisted hydride shifts are a possibility. An alternative role of the attacking piperidine in Reaction 12 or 17 could be that of a proton acceptor as discussed by others (1, 13). For example, a plausible scheme would be the following (writing R2 for C5H10)... 

Very recently, various DHB complexes were analyzed [39].12 The complexes of ammonia and hydronium ions were included in this analysis, in addition to the complexes with acetylene and methane, and their derivatives. Generally, in such complexes, lithium hydride and berylium hydride (and its fluorine derivative) act as the Lewis bases (proton acceptors) while hydronium ion, ammonia ion, methane, acetylene, and their simple derivatives act as the proton donors. Therefore, it was possible to investigate the wide spectrum of DHB interactions, starting from those that possess the covalent character and extending to the systems that are difficult to classify as DHBs (since they rather possess the characteristics of the van der Waals interactions). Figure 12.8 displays the relationship between H—H distance and the electron density at H—H BCP.13 One can observe the H—H distances close to 1 A, (as for the covalent bond lengths) and also the distances of about 2.2—2.5 A, typical for the van der Waals contacts. This also holds for the pc-values - of the order of 0.1 a.u. as for the covalent bonds and much smaller values as for the HBs and weaker interactions. 

DHB is a special type of the HB where the negatively charged hydrogen plays a role of the proton acceptor (Lewis base). Additionally, DHB may be treated as the class of hydride bond. In the other words, DHB possesses characteristics of both the hydrogen and hydride bonds. 

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