Hydrido carbonyl systems

Table III. Selected SCF and CAS SCF Populations for Hydrido Carbonyl Systems...

An excess of ligand, including CO, will often inhibit isomerisation. HCo(CO)4, an unstable hydrido-carbonyl complex, belongs to the examples of catalysts also active in an atmosphere of CO. This is the only homogeneous catalyst being commercially applied, albeit primarily for its hydroformylation activity. Higher alkenes are available as their terminal isomers or as mixtures of internal isomers and the latter, the cheaper product, is mainly converted to aldehydes/alcohols by hydroformylation technology. Later we will see that the isomerisation reaction also plays a pivotal role in this system. Since 1990 several catalysts based on rhodium, platinum and palladium have been discovered that will also hydroformylate internal products to terminal aldehydes. 

This latter reaction is exemplified by the reaction of [Fe(PF3)5] with cyclopentadiene, which, in contrast with the analogous iron carbonyl system, gives the stable hydrido complex [Fe(f/5-C5H5)(PF3)2H]. 

Another salient feature of these hydrido metal carbonyl systems is the underestimation, at the SCF level, of the d - back donation which we have already mentioned. This is not peculiar to the hydrido carbonyl complexes and has already been noted in many other carbonyl complexes (7-10,25,26). It is best evidenced by the orbital occupation number of the d orbitals (Table V) which range between 1.91 and 1.94e, with the noticeable exception of the HCr(CO)4(NO) system... 

Prior to the conversion of the alkene, the Rh/TPPTS catalyst system has to be pre-formed (formation of the Rh-hydrido-carbonyl complex) applying a specific chemical procedure is necessary to form the active Rh(l) species, mostly starting from Rh(lll) precursors. In general this is achieved under hydroformylation conditions. Thus, treatment of the rhodium precursor with syn gas in the absence of alkene for a couple of hours will transform it into a Rh-hydrido-carbonyl complex. 

Most researchers currently agree that the hydrido mechanism is more common than the alkoxycarbonyl path in the alkoxycarbonylation of alkenes with palladium systems. However, carbalkoxy complexes are putative intermediates in carbonylation reactions giving succinates and polyketone diesters, with metals like Co, Rh, or Pd.137... 

However, when a less active olefin (e.g., diisobutylene or cyclohexene) or a liganded system (Bu3P/Co = 2/1,80 atm CO/H2, 190°C) was used, the hydrido species, e.g., HCo(CO)3PBu3, predominated throughout the reaction. The author concluded that in slower systems, initial interaction of the olefin with the hydrido species HCo(CO)3L could be the ratedetermining step. These results are complementary to those discussed (vide supra) for the rhodium carbonyl catalysis. 

Nitroarenes are reduced to anilines (>85%) under the influence of metal carbonyl complexes. In a two-phase system, the complex hydridoiron complex [HFe,(CO)u]2-is produced from tri-iron dodecacarbonyl at the interface between the organic phase and the basic aqueous phase [7], The generation of the active hydridoiron complex is catalysed by a range of quaternary ammonium salts and an analogous hydrido-manganese complex is obtained from dimanganese decacarbonyl under similar conditions [8], Virtually no reduction occurs in the absence of the quaternary ammonium salt, and the reduction is also suppressed by the presence of carbon monoxide [9], In contrast, dicobalt octacarbonyl reacts with quaternary ammonium fluorides to form complexes which do not reduce nitroarenes. 

In an earlier report, Maitlis et al. showed that 1 could be easily converted into a hydrido complex [Cp lrHCl]2 (2) under ambient conditions by treatment with alcohol and a weak base (Scheme 5.1) [19], probably accompanied by the formation of carbonyl compounds. This fact means that the hydrogen atom in an alcohol can be rapidly transferred to the iridium center in the form of a hydride but then, if the hydride on the iridium could be re-transferred to another hydrogen acceptor, a new catalytic system using alcohols as substrates might be realized. In fact, a wide variety of Cp Ir complex-catalyzed hydrogen transfer systems using alcohols as substrates, and based on the above hypothesis, have been reported to date [20]. 

The scheme reduces to its most simple form when carbon monoxide is the only ligand present in the system, because equilibria of mixed ligand/carbon monoxide complexes do not occur. The kinetics of the hydroformylation reaction using hydrido rhodium carbonyl as the catalyst was studied by Marko [20]. For 1-pen-tene the rate expression found is ... 

An interesting feature of hydrido transition metal-PF3 complexes is that apart from a few dinuclear systems (Section VI) only mononuclear systems are so far known and there is as yet no corresponding chemistry analogous to that of polynuclear carbonyl hydrido compounds. The trifluorophosphine metal hydrido compounds are usually highly acidic and can readily form metallate ions such as [M(PF3)m]x and [MH(PF3) r. 

Reaction schemes for hydrogenation of dienes using dihydride catalysts usually follow the basic hydride or unsaturated pathways of Scheme 1 species 2 will be, however, a hydrido cr-alkenyl or a 7r-allyl intermediate [see equation (f) below]. Transfer of the second hydrogen yields a monoene product if diene coordination is preferred over monoene coordination, selective reduction of diene to monoene can occur. Some Rh (and Ir ) dihydride catalysts that effect such catalysis are listed in Table 2, although systems based on the Cr and Fe carbonyl have been studied most (see also 14.3.3.5) . 

In general, trivalent phosphorus compounds, arsines, stibenes and several amines improve the thermal stability of hydrido metal-carbonyl complexes because of superior o-donor and weaker it-acceptor properties [18]. This feature enhances the electron density at the metal center, and hence the metal-CO bond is strengthened as a result of enhanced electron backdonation. However, the special effect of a ligand on the activity and selectivity may be entirely different from one metal to another, and therefore conclusions should be drawn only in close relation to the metal that is used. Only some selected observations will be detailed here, showing the uniqueness of each catalytic system. 

In the Graham system, irradiation of the carbonyl iridium complex 1 extrudes a CO molecule and leads presumably to the coordinatively unsaturated intermediate 2 which then inserts into the CH bonds of cyclohexane or neopentane giving the compounds 3. Similarly, Bergman found that the iridium dihydride 4 loses H2 upon irradiation when the reaction takes place in cyclohexane or neopentane, the hydrido alkyl compounds 6 are formed, resulting from the oxidative addition of a CH bond to the intermediate 5. 

Due to the low oxidation state of the metal in carbonyliron complexes and ferrates, these species can be applied for the reduction of various carbonyl compounds. Initially, these reagents have been applied in stoichiometric amounts. First examples describe the hydrogenation of a,p-unsaturated carbonyl compounds by carbonyl(hydrido)ferrate complexes to give saturated carbonyl compounds or saturated alcohols. Low valent iron species for the reduction of carbonyl compounds and imines can also be generated in situ from iron(II) chloride and lithium powder in the presence of 4,4 -di-rert-butylbiphenyl. Catalytic versions have been developed subsequently. Thus, pentacarbonyliron functions as a precatalyst for the hydrogenation of aldehydes and ketones in the presence of a tertiary amine as solvent (Scheme 4-322). The catalytically active system probably consists of (tetracarbonyl)(hydrido)ferrate and the protonated amine. ... 

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