Hydride complexes Bonding

Finally, selective hydrogenation of the olefinic bond in mesityl oxide is conducted over a fixed-bed catalyst in either the Hquid or vapor phase. In the hquid phase the reaction takes place at 150°C and 0.69 MPa, in the vapor phase the reaction can be conducted at atmospheric pressure and temperatures of 150—170°C. The reaction is highly exothermic and yields 8.37 kJ/mol (65). To prevent temperature mnaways and obtain high selectivity, the conversion per pass is limited in the Hquid phase, and in the vapor phase inert gases often are used to dilute the reactants. The catalysts employed in both vapor- and Hquid-phase processes include nickel (66—76), palladium (77—79), copper (80,81), and rhodium hydride complexes (82). Complete conversion of mesityl oxide can be obtained at selectivities of 95—98%. 

Figure 2.68 Bond lengths in two 5-coordinate rhodium hydride complexes with bulky tertiary...

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). 

One family of porphyrin complexes that will be treated in the review, even though they do not contain metal-carbon bonds, are metalloporphyrin hydride and dihydrogen complexes. As in classical organometallic chemistry, hydride complexes play key roles in some reactions involving porphyrins, and the discovery of dihydrogen complexes and their relationship to metal hydrides has been an important advance in the last decade. 

However, the significant key difference for rhodium arises from the chemistry of the Rh(ll) dimer, [Rh(Por)]2, which exhibits a relatively low Rh—Rh bond strength. It undergoes homolytic dissociation and exists in equilibrium with the monomer, Rh(Por)- (Eq, (15)). The rhodium dimer can also exist in equilibrium with the hydride Rh(Por)H (Eq. (16)), and thus the hydride complex can exhibit the chemistry of the dimer, driven by formation of the Rh(Por)- monomer formed as in Eqs. (15) and (16). 

The chemistry of [Rh(OEP)h in benzene is dominated by Rh—Rh bond homolysis to give the reactive Rh(Il) radical Rh(OEP)-. This contrasts with the reactivity of fRh(OEP)] in pyridine, which promotes disproportionation via the formation of the thermodynamically favorable Rh(IlI). ct complex [RhjOEPKpy) ] together with the Rh(l) anion, Rh(OEP)J The hydride complex Rh(OEP)H shows NMR chemical shift changes in pyridine consistent with coordination of pyridine, forming Rh(OEP)H(py). Overall, solutions of Rh(OEP)l in pyridine behave as an equimolar mixture of [Rh(OEP)(py ) and (Rh(OEP). For example, reaction... 

Abstract Organic syntheses catalyzed by iron complexes have attracted considerable attention because iron is an abundant, inexpensive, and environmentally benign metal. It has been documented that various iron hydride complexes play important roles in catalytic cycles such as hydrogenation, hydrosilylation, hydro-boration, hydrogen generation, and element-element bond formation. This chapter summarizes the recent developments, mainly from 2000 to 2009, of iron catalysts involving hydride ligand(s) and the role of Fe-H species in catalytic cycles. 

A Fe-H bond is generally polarized as Fe -H because H is more electronegative than Fe. However, iron hydride complexes impart much less negative charge to the hydride than early transition-metal hydride complexes. 

The proposed mechanism for Fe-catalyzed 1,4-hydroboration is shown in Scheme 28. The FeCl2 is initially reduced by magnesium and then the 1,3-diene coordinates to the iron center (I II). The oxidative addition of the B-D bond of pinacolborane-tfi to II yields the iron hydride complex III. This species III undergoes a migratory insertion of the coordinated 1,3-diene into either the Fe-B bond to produce 7i-allyl hydride complex IV or the Fe-D bond to produce 7i-allyl boryl complex V. The ti-c rearrangement takes place (IV VI, V VII). Subsequently, reductive elimination to give the C-D bond from VI or to give the C-B bond from VII yields the deuterated hydroboration product and reinstalls an intermediate II to complete the catalytic cycle. However, up to date it has not been possible to confirm which pathway is correct. 

The proposed catalytic cycle is shown in Scheme 31. Hence, FeCl2 is reduced by magnesium and subsequently coordinates both to the 1,3-diene and a-olefin (I III). The oxidative coupling of the coordinated 1,3-diene and a-olefin yields the allyl alkyl iron(II) complex IV. Subsequently, the 7i-a rearrangement takes place (IV V). The syn-p-hydride elimination (Hz) gives the hydride complex VI from which the C-Hz bond in the 1,4-addition product is formed via reductive elimination with regeneration of the active species II to complete the catalytic cycle. Deuteration experiments support this mechanistic scenario (Scheme 32). 

The catalytic cycle, which is supported by stoichiometric and labeling experiments, is shown in Scheme 38. Loss of 2 equiv. of N2 from 5 affords the active species a. Reaction of a with the 1,6-enyne gives the metallacycle complex b. Subsequently, b reacts with H2 to give the alkenyl hydride complex c or the alkyl hydride complex d. Finally, reductive elimination constructs the C-H bond in the cyclization product and regenerates intermediate a to complete the catalytic cycle. 

The isolated iron hydride complexes introduced in this chapter are listed in Table 12, where the hydride chemical shifts in the HNMR spectra and the Fe-H bond distances are summarized. 

In the presence of H2, perhydrocarbyl surface complexes loose their ligands through the hydrogenolysis of their metal carbon bonds to generate putative hydride complexes, which further react with the neighbouring surface ligands, the adjacent siloxane bridges (Eqs. 8-9) [46,47]. 

Both Ni and Pd reactions are proposed to proceed via the general catalytic pathway shown in Scheme 8.1. Following the oxidative addition of a carbon-halogen bond to a coordinatively unsaturated zero valent metal centre (invariably formed in situ), displacement of the halide ligand by alkoxide and subsequent P-hydride elimination affords a Ni(II)/Pd(ll) aryl-hydride complex, which reductively eliminates the dehalogenated product and regenerates M(0)(NHC). ... 

Pd, or Ni (Scheme 5-3). First, P-H oxidative addition of PH3 or hydroxymethyl-substituted derivatives gives a phosphido hydride complex. P-C bond formation was then suggested to occur in two possible pathways. In one, formaldehyde insertion into the M-H bond gives a hydroxymethyl complex, which undergoes P-C reductive elimination to give the product. Alternatively, nucleophilic attack of the phosphido group on formaldehyde gives a zwitterionic species, followed by proton transfer to form the O-H bond [7]. 

After formation of Pd(0) from the Pd(II) precursor, oxidative addition of the P-H bond could give a hydride complex. Insertion of the alkyne into either the Pd-P or Pd-H bond, followed by reductive eUmination, gives the product Consistent with this proposal, treatment of Pt(PEt3)3 with PH(0)(0Et)2 gave the P-H oxidative addition product 14, which reacted with phenylacetylene to give primarily (>99 1) the Markovnikov alkenylphosphonate (Scheme 5-18, Eq. 2). 

---