Metal-hydride bonds protonation

Molybdenum and tungsten carbonyl hydride complexes were shown (Eqs. (16), (17), (22), (23), (24) see Schemes 7.5 and 7.7) to function as hydride donors in the presence of acids. Tungsten dihydrides are capable of carrying out stoichiometric ionic hydrogenation of aldehydes and ketones (Eq. (28)). These stoichiometric reactions provided evidence that the proton and hydride transfer steps necessary for a catalytic cycle were viable, but closing of the cycle requires that the metal hydride bonds be regenerated from reaction with H2. 

Formate production stems from similar metal-C02 intermediate species that yield CO as a product. Formate can be formed by the protonation of metal-C02 complexes through intermediates that have not been determined experimentally, namely the metallocarboxylate intermediate described above. A proposed mechanism for formate production by transition metal complexes also involves a metal hydride intermediate, where C02 actually inserts into the metal hydride bond to form the metallocarboxylate intermediate [9]. 

The first site of protonation in a dinitrogen complex, based on mechanistic studies of complexes, must be the dinitrogen itself, yielding the diazenido species (N2H). If protonation occurs at the metal then reaction proceeds no further, or results in the loss of coordinated dinitro-gen. The formal insertion of dinitrogen into a metal-hydride bond (a popular proposal in the early chemical nitrogen-fixation literature) is unknown. 

Additional M-HNO complexes were more recently well characterized in nonaqueous solutions.65 Among them, the first porphyrin derivative, [Run(TTP)(l-MeIm)HNO], was obtained similarly as in reaction 7.30.65a The [ReI(CO)3(PPh3)2HNO] complex was prepared by two-electron oxidation of the hydroxylamine precursor with Pb(Ac)4.65b A new alternative synthetic route for the latter Re1 complex proceeds by direct insertion of NO + into the metal-hydride bond in ReI(H)(CO)2(PPh3)2.65c Finally, cA,ira 5-ReICl(CO)2(PR3)2HNO (R = Ph, Cy) have been obtained through similar protonation reactions as in Equation 7.29.65d... 

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. 

It is probable that the negative charge induced by these three electrons on FeMoco is compensated by protonation to form metal hydrides. In model hydride complexes two hydride ions can readily form an 17-bonded H2 molecule that becomes labilized on addition of the third proton and can then dissociate, leaving a site at which N2 can bind (104). This biomimetic chemistry satisfyingly rationalizes the observed obligatory evolution of one H2 molecule for every N2 molecule reduced by the enzyme, and also the observation that H2 is a competitive inhibitor of N2 reduction by the enzyme. The bound N2 molecule could then be further reduced by a further series of electron and proton additions as shown in Fig. 9. The chemistry of such transformations has been extensively studied with model complexes (15, 105). 

The isopropenyl-benzonitrile units were then released from the phosphine group via cleavage of the P-C bond to afford osmapyrroles, whose metal centers were protonated to furnish the desired azabutadienylosmium(iv) hydride complexes 180. 

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