Hydride complexes Acidity

A ruthenium porphyrin hydride complex was lirst prepared by protonation of the dianion, [Ru(TTP) in THF using benzoic acid or water as the proton source. The diamagnetic complex, formulated as the anionic Ru(If) hydride Ru(TTP)(H )(THF)l , showed by H NMR spectroscopy that the two faces of the porphyrin were not equivalent, and the hydride resonance appeared dramatically shifted upheld to —57.04 ppm. The hydride ligand in the osmium analogue resonates at —66.06 ppm. Reaction of [Ru(TTP)(H)(THF)j with excess benzoic-acid led to loss of the hydride ligand and formation of Ru(TTP)(THF)2. 

These transition-metal catalysts contain electronically coupled hydridic and acidic hydrogen atoms that are transferred to a polar unsaturated species under mild conditions. The first such catalyst was Shvo s diruthenium hydride complex reported in the mid 1980s [41 14], Noyori and Ikatiya developed chiral ruthenium catalysts showing excellent enantioselectivity in the hydrogenation of ketones [45,46]. 

It is not clear whether the X anion remains ligated to the palladium(II) center. For example, for acetic acid, the palladium hydride was initially postulated as being HPd(OAc)L ,377,378 but more recently as HPdL +.367 To date, none of these complexes has been characterized.367 Oxidative addition of acetic acid or formic acid to a palladium(O) complex in DMF affords a cationic palladium hydride /ruw.v-I IPd(PPh3)2(DMF)+, with an acetate or a formate counter-anion. Both reactions are reversible and involve an unfavorable equilibrium so that a large excess of acid is required for the quantitative formation of the palladium hydride complex.379 This allows us to conclude that the catalytic reactions initiated by reaction of palladium(O) and acetic acid (or formic acid) proceed via a cationic palladium hydride trans-HPdfPPtHWDMF)"1", when they are performed in DMF.379... 

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 separation of rhodium hydride complex from a stream comprising catalyst and high molecular weight aldehyde condensation products using a carboxylic acid functionalized ion exchange resin in illustrated schematically in Figure 2.12. 

However, since the goal of this work was the synthesis of alcohols from olefins via hydrohydroxymethylation (75, 76), little attention was given to developing a shift-catalyst per se. Pettit has recently reexamined some of this work and shown that, by careful control of the pH of the reaction mixture, systems based on either Fe(CO)5 or Cr(CO)6 can be developed for the production of either formic acid or methanol from carbon monoxide and water (77, 78). Each of these latter systems involves the formation of metal hydride complexes consequently, molecular hydrogen is also produced according to the shift reaction [Eq. (16)]. 

The formation of cationic nickel hydride complexes by the oxidative addition of Brdnsted acids (HY) to zero-valent nickel phosphine or phosphite complexes (method C,) has already been discussed in Section II. Interesting in this connection is a recent H NMR study of the reaction of bis[tri(o-tolyl)phosphite]nickelethylene and trifluoroacetic acid which leads to the formation of a square-planar bis[tri(o-tolyl)phosphite] hydridonickel trifluoroacetate (30) (see below) having a cis arrangement of the phosphite ligands (82). 

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. 

As shown in Figure 1, the next step in the catalytic cycle of carbon dioxide hydrogenation is either reductive elimination of formic acid from the transition-metal formate hydride complex or CT-bond metathesis between the transition-metal formate complex and dihydrogen molecule. In this section, we will discuss the reductive elimination process. Activation barriers and reaction energies for different reactions of this type are collected in Table 3. 

It is known that transition metal hydrides exhibit acidic properties and it has been reported that in some cases complexes containing a group IV metal ligand react with weak bases ... 

Until there is a sufficient excess of ethene over [PdH(TPPTS)3] their fast reaction ensures that aU palladium is found in form of tratts-[Pd C(CO)Et (TPPTS)2]. However, at low olefin concentrations (e.g. in biphasic systems with less water-soluble olefins) [PdH(TPPTS)3] can accumulate and through its equihbrium with [Pd(TPPTS)3] (eq. 5.5) can be reduced to metallic palladium. This is why the hydroxycarbonylation of olefins proceeds optimally in the presence of Brpnsted acid cocatalyts with a weekly coordinating anion. Under optimised conditions hydrocarboxylation of propene was catalyzed by PdC + TPPTS with a TOE = 2507 h and l = 57/43 (120 °C, 50 bar CO, [P]/[Pd] = 4, P-CH3C6H4SO3H) [38], In neutral or basic solutions, or in the presence of strongly coordinatmg anions the initial hydride complex cannot be formed, furthermore, the fourth coordination site in the alkyl- and acylpaUadium intermediates may be strongly occupied, therefore no catalysis takes place. 

A robust and highly active catalyst for water-phase, acid-catalyzed THs of carbonyl compounds at pH 2.0-3.0 at 70 °C was disclosed by Ogo and coworkers [60]. The water-soluble hydride complex [Cp lr(bipy)H] (72, Cp = Tl -CsMes, bipy = 2,2 -bipyridine) was synthesized from the reaction of [Cp lr(bipy)(H20)] (73) with HCOOX (X = H or Na) in H2O under controlled pH conditions (2.0 < pH < 6.0, 25 °C). The pH control is pivotal in avoiding protonation of the hydrido ligand of 72 below pH ca. 1.0 and deprotonation of the aquo ligand of 73 above pH ca. 6.0. The rate of the reaction is heavily dependent on the pH of the solution, the reaction temperature, and the concentration of HCOOH. High TOFs of the acid-catalyzed transfer hydrogenations at pH 2.0-3.0, ranging from 150 to 525 h, were observed for a variety of linear and cyclic ketones, as summarized in Table 4.5. 

The dimerization of functional alkenes such as acrylates and acrylonitrile represents an attractive route to obtain bifunctional compounds such as dicarboxylates and diamine, respectively. The head-to-tail dimerizahon of acrylates and vinyl ketones was catalyzed by an iridium hydride complex generated in situ from [IrCl(cod)]2 and alcohols in the presence of P(OMe)3 and Na2C03 [26]. The reaction of butyl acrylate 51 in the presence of [IrCl(cod)]2 in 1-butanol led to a head-to-tail dimer, 2-methyl-2-pentenedioic acid dibutyl ester (53%), along with butyl propionate (35%) which is formed by hydrogen transfer from 1-butanol. In order to avoid... 

One of the important properties of dihydrogen ligands, particularly in charged transition metal complexes, is their ability to nndergo heterolytic cleavage [9]. In addition, protonation of transition metal hydrides with acids is a common method for preparation of transition metal dihydrogen complexes ... 

The dianion [Ni6(CO)12]2- is stable to hydrolysis in alkaline aqueous solution however, in acidic conditions (pH = 3-6) it is converted quantitatively into the interstitial hydride complex [Ni12(CO)21H4 B]"- (n = 2,3).5 6 The complex readily reacts in solution with carbon monoxide according to the following degradation-condensation equilibrium 1... 

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