Metal hydride complexes formation

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)]. 

Electrochemical reductions of CO2 at a number of metal electrodes have been reported [12, 65, 66]. CO has been identified as the principal product for Ag and Au electrodes in aqueous bicarbonate solutions at current densities of 5.5 mA cm [67]. Different mechanisms for the formation of CO on metal electrodes have been proposed. It has been demonstrated for Au electrodes that the rate of CO production is proportional to the partial pressure of CO2. This is similar to the results observed for the formation of CO2 adducts of homogeneous catalysts discussed earlier. There are also a number of spectroscopic studies of CO2 bound to metal surfaces [68-70], and the formation of strongly bound CO from CO2 on Pt electrodes [71]. These results are consistent with the mechanism proposed for the reduction of CO2 to CO by homogeneous complexes described earlier and shown in Sch. 2. Alternative mechanistic pathways for the formation of CO on metal electrodes have proposed the formation of M—COOH species by (1) insertion of CO2 into M—H bonds on the surface or (2) by outer-sphere electron transfer to CO2 followed by protonation to form a COOH radical and then adsorption of the neutral radical [12]. Certainly, protonation of adsorbed CO2 by a proton on the surface or in solution would be reasonable. However, insertion of CO2 into a surface hydride would seem unlikely based on precedents in homogeneous catalysis. CO2 insertion into transition metal hydrides complexes invariably leads to formation of formate complexes in which C—H bonds rather than O—H bonds have been formed, as discussed in the next section. 

The active catalytic species appears to involve a metal hydride complex, as added H2 enhances the reaction rate. Additionally, H4Ru4(CO)12 is observed during the course of the reaction and may also be used as an effective entry into the catalytic system. The oligomerization mechanism may proceed via a nucleophilic attack of the amine on a metal-activated Si-H bond, resulting in Si-N bond formation. 

The appropriate NMR experiment to determine whether the carbon dioxide-hydride complex is formed in equilibrium amounts in solution does not appear to have been done, but recent work on a similar system seems to support this hypothesis. Direct formation of the formate complex [(HCOO)-Ru(PPhMe2)4]+ was achieved, and when this complex is dissolved in CD2C12 at 30°C the NMR spectrum shows a broad hydride resonance centered at x = 17.4, indicating the presence of a metal-hydride complex (134), with the C02 possibly coordinated to the ruthenium. 

Rhenium is an element known for its abundance of metal hydride complexes spanning a variety of oxidation states, for example, from Re(I), Re(CO)sH to Re(VII), ReH92 . However, despite the recent interest in light-driven H2 formation from different substrates (such as water) there are few recent photochemical studies of Re hydride complexes. 

After a metal hydride complex was prepared from LAH and quinine (1 1), irradiation of a mixture of the resulting solution containing the above chiral hydride agent and the enamide (133) led to the formation of two optically active lactams 158 [6%, [a]D —63° (c = 0.48, CHC13)] and 155 [ 13%, [a]D — 102° (c = 0.44, CHC13)] with 37% optical purity. Reduction of the lactam 155 with LAH furnished (—)-xylopinine (20) in 48% chemical yield. 

The ability to catalyze certain reactions of molecular hydrogen homogeneously in solution has been demonstrated for many transition metal ions and complexes (34)—among them complexes of Cu Cu Ag Hg Hgi, Col, Coll, pdii, Ptii, Rhi, Rh i, Ru i Ruiii, and Ir. In each case it appears that H2 is split by the catalyst with the formation of a reactive transition metal hydride complex (which may or may not be detected) as an intermediate. Three distinct mechanisms by which this can occur have been recognized (34), which are exemplified by the following reactions. 

L, used in this mechanism, is a ligand which can stabilize the intermediate palladium complexes and satisfy a coordination number of the palladium whatever it is. L, for example, can be carbon monoxide, phosphines, solvents, or another molecule of palladium. Formation of hydride complexes by the oxidative addition of hydrogen chloride or hydrogen to a metal complex is well known (9, 27), as is formation of alkyl metal complexes by addition of metal hydrides to olefins. 

Carbon monoxide insertions into metal-hydrogen bonds have been elusive. The first direct formation of a metal-coordinated formyl group from a metal-hydride complex and carbon monoxide was observed with the hydride of octaethylporphyrinatorhodium(III), which reacts as follows with carbon monoxide at atmospheric pressure in benzene ... 

Kinetic studies made on [Pd(PP2)(PEt )[(BF )2> and reported elsewhere (45), indicate that the rate of catalysis is first order in CO2, first order in catalyst, and first order in acid at low acid concentrations. These results are consistent with the mechanism shown in Scheme 2. In comparison with Scheme 1, two important features should be noted. First in Scheme 2, the formation of a coordinatively unsaturated metal hydride complex is necessary for CO2 insertion to occur. A priori there is no way of knowing whether or not the generation of a coordinatively unsaturated metal hydride will be required for catalysis since evidence exists for both associative and dissociative pathways for CO2 insertion into metal hydride and metal carbon bonds (20-25). This is the reason that complexes of the types... 

This reaction also demonstrates the in situ formation of reactive metal-hydride complexes. 

In most cases, the immediate product of a-hydrogen elimination is not observed. Instead, reductive elimination of a hydride and an alkyl group to form an alkane frequently occius (Equation 10.30). In other cases, a carbene forms from a dialkyl complex of a d metal that cannot accommodate the additional valency required to form an alkylidene and a hydride ligand. In these cases, an alternative four-center pathway involving the transition state shown in Equation 10.31 that does not involve formation of a metal-hydride complex is followed. When the valency of the metal allows either pathway to occur, it is difficult to distinguish between the a-elimination and a-hydrogen abstraction pathways. ... 

Certain low-valent early transition metal complexes catalyze the dimerization of ethylene and propylene selectively to 1-butene and 2,3-dimethyl-l-butene. The regioselec-tivity of this dimerization of propene signals a different mechanism than the insertion and elimination mechanism presented in the previous section. The formation of 1-butene occurs selectively because of the absence of a persistent metal hydride complex that isomerizes this olefin to the more stable 2-butene. 

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