Protonation of a Hydride Complex

A very common and convenient method of preparation of H2 complexes is the addition of H+ to a hydride or polyhydride complex [Eq. (3.10)]. In most cases the 

The attack of the proton is generally directly at the M-H bond, Le., M is not protonated and then later transfers H+ to the hydride.74,109 An initial hydrogen bonding interaction may facilitate the protonation (see Section 7.3 for kinetic studies), which usually gives the H2 complex as the product even when the dihydride may be the thermodynamically favored product [Eq. (3.11)].167 

Normally the protonation is carried out at low temperatures to give a [M-H2]+ complex, but on warming there is sometimes rearrangement to a dihydride or equilibrium mixture. Occasionally the product is unstable toward loss of H2 and coordination of anion or solvent (S) if the electronics and thermodynamics of the system do not favor H2 binding [Eq. (3.12)]. 

The stability of H2 complexes prepared by protonation thus varies greatly Some are stable only below room temperature and cannot be isolated as solids and 

Needless to say, complexes formed by protonation, especially where HA is a strong acid, are readily deprotonated, even by bases as weak as diethyl ether, and are highly sensitive to solvent media and trace water. These properties relate in large measure to the high acidity of certain H2 complexes, which can have pK as low as -6, e.g., when generated from triflic acid (see Chapter 9). 

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Although most metal-dihydrogen chemistry is carried out in organic solvents, aqueous systems have been found that have potential applications in biomimetic and green chemistry, for example, chemoselective hydrogenation catalysis.31 Protonation of a hydride complex by acids is often used13,24 and is widely applicable because it does not require an unsaturated precursor that may not be available. An example is shown in Scheme 5.3. 

The binding of dihydrogen to a metal center can lead to highly acidic -H2 complexes. The most acidic of these complexes are generated by protonation of a hydride complex by strong acid. A few of these complexes have been prepared... 

The Pt2(pop)4 " oxidizes the alcohol and is converted to Pt2(pop)4H2 . This species can be photoconverted back to Pt2(pop)4 ", or it can undergo a known reaction with water to form dihydrogen and Pt2(pop)4(H20)2 " (135). Thus, at high pH, the catalyst is deactivated hy deprotonation of the pop ligand U 77), and at low pH, the catalyst is deactivated by protonation of the hydride complex to form the inactive aqua complex. Both of these reactions reduce the efficiency of the catalytic reaction, and at pH 3, these two side reactions are minimized, allowing for the greatest numbers of turnovers. 

Several synthetic routes to H2 complexes are available. The simplest method is reaction of H2 gas with a coordinatively unsaturated complex such as W(CO)3(PR3)2, the original method, as described in Chapter 2. This is analogous to the well-studied reversible addition of H2 to Vaska s complex, IrCl(COXPPh3)2, except that for the latter OA to the dihydride occurs. A second, very common method of preparation is protonation of metal hydride complexes ... 

The higher kinetic acidity of H2 complexes requires that the reverse reaction, protonation of a metal hydride, occur at H rather than at M, for which there is ample evidence. Actually protonation at the hydride is misleading because it is really the M-H bond that is protonated to form M-j/2-H2, as pointed out in a review by Kuhlman that addresses site selectivity of protonation of hydride-halide complexes, MH(X).64 Formal protonation of a hydride ligand would give M-ff -H2, which is not known to be stable. Proton transfer to halide ligands is quite rare because an add with a lower than the coordinated HX produced is necessary. One example is protonation of trens-PtHX(P Bu3)2 with triflic acid, which gives trans-[Pffl( f2-H2)(P Bu3)2][OTf] for X = H and an unstable spedes claimed to be... 

In order to show a meaningful kinetic preference for protonation of a hydride ligand, the dihydrogen complex carmot be the sole product. Also, if a dihydride complex is the only observable product of protonation, it is still possible that the initial protonation site was the hydride ligand. 

We wanted to quantify the kinetic preference for protonation of a hydride ligand over a metal in a case where a dihydride complex was the sole observable product. We decided to search for a dihydrogen intermediate in the protonation of CpW(CO)2(PMej)H (3), known eventually to form the cationic dihydride [CpW(CO)2(PMej)H2] (4) [44]. Both 3 and 4 had been fully characterised. The hydride complex 3 was first synthesised in 1970 and identified by H and P NMR and by IR spectroscopy. It exists in solution in both cis (3c) and trans (3t) forms which interconvert rapidly on the NMR timescale (eq 23) [45, 46]. The structure of the cationic dihydride 4 has been determined by x-ray diffraction [44]. The hydride ligands of 4 interconvert rapidly at room temperature, and two inequivalent hydride resonances are not observed until the temperature is lowered to -112 °C [47]. 

This is the first example of a proton transfer process to a hydride complex with a second-order dependence. Theoretical calculations indicate that the role of the HX molecules is the formation of W-H H-Cl- H-Cl adducts that convert into W-Cl, H2 and HCl2 in the rate-determining state through hydrogen complexes as transition states. 

Iron hydride complexes can be synthesized by many routes. Some typical methods are listed in Scheme 2. Protonation of an anionic iron complex or substitution of hydride for one electron donor ligands, such as halides, affords hydride complexes. NaBH4 and L1A1H4 are generally used as the hydride source for the latter transformation. Oxidative addition of H2 and E-H to a low valent and unsaturated iron complex gives a hydride complex. Furthermore, p-hydride abstraction from an alkyl iron complex affords a hydride complex with olefin coordination. The last two reactions are frequently involved in catalytic cycles. 

Wakatsuki et al. (4) proposed vinyl complex, 5, and presented DFT results supporting isomerization to a vinylidene hydride as the rate determining step. Our results indicate that the rate determining step involves H-OH bond breaking and that protonation of a bound alkyne is the rate determining step in this... 

A similar involvement of palladium hydride, palladium alkyl, and palladium acyl complexes as intermediates in the catalytic cycle of the Pd-catalyzed hydroxycarbonylation of alkenes was reported for the aqueous-phase analogs. The cationic hydride PdH(TPPTS)3]+ was formed via the reduction of the Pd11 complex with CO and H20 to [Pd(TPPTS)3] and subsequent protonation in the acidic medium. The reaction of the hydride complex with ethene produced two new compounds, [Pd(Et)(TPPTS)3]+ and Pd(Et)(solvent)(TPPTS)2]+. The sample containing the mixture of palladium alkyl complexes reacted readily with CO to afford trans-[Pd(C(Q)Et)(TPPTS)2]+.665... 

The reduction of protons is one of the most fundamental chemical redox reactions. Transition metal-catalyzed proton reduction was reviewed in 1992.6 The search for molecular electrocatalysts for this reaction is important for dihydrogen production, and also for the electrosynthesis of metal hydride complexes that are active intermediates in a number of electrocatalytic systems. 

Several metallophthalocyanines have been reported to be active toward the electroreduction of C02 in aqueous electrolyte especially when immobilized on an electrode surface.125-127 CoPc and, to a lesser extent, NiPc appear to be the most active phthalocyanine complexes in this respect. Several techniques have been used for their immobilization.128,129 In a typical experiment, controlled potential electrolysis conducted with such modified electrodes at —1.0 vs. SCE (pH 5) leads to CO as the major reduction product (rj = 60%) besides H2, although another study indicates that HCOO is mainly obtained.129 It has been more recently shown that the reduction selectivity is improved when the CoPc is incorporated in a polyvinyl pyridine membrane (ratio of CO to H2 around 6 at pH 5). This was ascribed to the nature of the membrane which is coordinative and weakly basic. The microenvironment around CoPc provided by partially protonated pyridine species was suggested to be important.130,131 The mechanism of C02 reduction on CoPc is thought to involve the initial formation of a hydride derivative followed by its reduction associated with the insertion of C02.128... 

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