Dihydride-dihydrogen rearrangement

D. Special Topics Dihydride-Dihydrogen Rearrangement in the Transition-Metal Polyhydride Complexes... 

The high kinetic acidity of complexes is illustrated by the system in eq 20 [40,41], For Ru and Os, the dihydrides exist as a mixture of cis and trans in an equilibrium ratio of 9 1 for Fe, the cis form is the only thermodynamically stable isomer. For Ru and Os, deprotonation of the hydride/dihydrogen cation led only to the thermally unstable trans dihydride, which rearranged to the cis... 

Interestingly, some systems, such as (1) itself, show a tautomeric equilibrium between an H2 complex and a classical dihydride form others show a stretched H2. In the first case we have a double minimum on the potential energy surface (PES) (H2 and dihydride) and in the other a single minimum. The difference appears to lie in the motion of the heavy ligands that produces a barrier to the dihydrogen/dihydride tautomerism and where there are no such heavy atom rearrangements, the barrier disappears and a stretched H2 becomes possible. 

As recounted, these studies demonstrate that two of the three expected intermediates in asymmetric hydrogenation may be directly observed, but the expected dihydride is too fleeting. There are two further experiments which are pertinent to this issue. A related diphosphine-iridium alkene complex reacts with dihydrogen at low temperatures and a series of alkene dihydrides are observed prior to the formation of the expected alkyl hydride. Based on the H-NMR chemical shifts of the respective Ir-H species, the initial addition (or to be more correct the initially observed species) possesses H trans to alkene and H trans to phosphine only at higher temperatures does this rearrange to the expected H trans to amide and H trans to phosphine structure (Fig. 9a) [36]. A more directly relevant experiment involves para-enriched hydrogen, and in the illustrated case a transient dihydride is observed. A problem is that the spectral characteristics are not entirely in accord with expectations for the proposed structure (the supposed trans-P-Rh-H coupHng is 4 Hz rather than ca. 120 Hz), but the presence of some transient Rh dihydride is definitive based on the evi-... 

Many metal hydrides protonate to give H2 complexes, but kinetic protonation can take place on M-H to give an M-(H2) complex, even when protonation at the metal is thermodynamically favoured. Protonation of [FeH(dppe)Cp ] (Cp = pentamethylcyclopentadienyl) gave the dihydrogen complex at — 80 °C, followed by rearrangement to the dihydride at 25 Kinetic protonation by... 

In the vast majority of studies on the protonation of hydride complexes, the kinetic site was the hydride ligand. This has been proven for many cases in which the thermodynamics of protonation favour a dihydride complex or a mixture of dihydride and dihydrogen complexes. In the case of CpW(CO)2(PMe3)H we have shown that hydride protonation (as monitored by exchange) is faster than protonation at the metal the intermediate dihydrogen complex rearranges too quickly to be observed [46]. 

Many metal hydrides can be protonated to give dihydrogen complexes [1]. In some cases, kinetic protonation takes place on an M-H bond of a complex to give an M-(H2) complex, even when the thermodynamically most favored protonation site is the metal itself An early example was protonation of CpFeH(dppe), where the dihydrogen complex was formed at low temperature, followed by rearrangement to the dihydride on warming [lb]. The more facile kinetic proto-... 

A few systems form an equilibrium mixture of the dihydride and tl -H2 complexes and their interconversion occurs on the NMR time scale. The process may not be exceptionally fast because the formation of M(H)2 from M(H2) requires an additional coordination site on M and some rearrangement of the geometry of the other ligands on M. Stopped-flow and NMR studies have found that the dihydrogen complex formed by W(CO)j(PR3)2 converts to the dihydride with a rate constant in the range of 10 to 20 s" at 25°C for R = cyclohexyl and isopropyl. 

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