Ancillary hydride ligands

Aromatics. A new generation of homogeneous arene hydrogenation catalysts was developed by Rothwell and coworkers. These are hydride derivatives of niobium and tantalum with bulky ancillary aryloxide ligands (see, i.e., 43) exhibiting high regio- and stereoselectivities 467... 

The X-ray structure of the latter complex (Fig. 5) shows that it contains a [HRh(/A-H)2RhH] core. Both complexes may be considered classical rhodium hydride species, although they are of a less common variety in that the ancillary amine ligands are purely room temperature, the complex of 4 is fluctional whereas that of 3 is rigid. 

The steric effects of the ancillary ligands on reductive elimination are also significant, even when the elimination involves a small hydride ligand. As shown in Table 8.2, the rate constant for reductive elimination of benzene from Cp Rh(PR3)(Ph)(H) increased with increasing cone angle of the phosphine ligand. More information on the steric properties of ancillary ligands on the rates of reductive eliminations to form C-C, C-N, and C-0 bonds is presented in subsequent sections of this chapter. 

The first hydride complex investigated by the neutron diffraction technique was K2[ReH9] containing the terminal M—H linkage. Many examples of complexes containing the terminal hydride ligands are now known for virtually all d-block transition elements. Binary transition-metal hydrides are rather few and the majority are stabilized by carbonyl, phosphine, or other ancillary ligands. 

Another class of heteroleptic alkyl complexes contains TT-donating ancillary ligands such as RU[N(Si(CH2)3)2]3 (R = 4)- The hydride... 

Our attempts to prepare chromium hydrides and to evaluate their role in polymerization catalysis eventually led to the isolation of a series of alkyls and hydrides lacking any ancillary ligands besides the cyclopentadienyl moiety (see below).[6] Reduced to the essence of alkyls, these complexes provided another piece of evidence in the growing case against polymerization activity of divalent chromium none of the alkyls even reacted with ethylene. The hydride underwent one insertion and stopped at the stage of an ethyl group. 

When less bulky ancillary ligands are used /3-hydride elimination leads to the formation of Q-olefins. As a consequence iminopyridine complexes are typically much less active than the diimine catalysts and afford lower-molecular-weight PE.321-324 For example, MAO/(122) polymerizes ethylene to branched oligomers with Mn < 600, and 240 branches per 1,000 carbons.325 Complex (123), is highly active for ethylene polymerization (820gmmol 1 h bar ).326 As with the diimine systems, reduction in the steric bulk of the ligand substituents results in reduced activity and lower-molecular-weight products. 

Insertion of COz into a metal-hydride bond normally requires the prior dissociation of an ancillary ligand to generate a coordinatively unsaturated complex, because C02 coordination to the metal usually precedes the formal insertion (Scheme 17.3, lower pathway). Ah initio calculations [59] support this mechanism for the insertion of C02 into the Ru-H bond of RuH2(PH3)4, a model for the catalyst RuH2(PMe3)4. However, it is theoretically possible for C02 insertion to take place without prior C02 coordination (Scheme 17.3, upper pathway) [60, 61]. The... 

In the transformation of a 1-alkyne to a vinylidene in the coordination sphere of a transition metal, the migrating hydrogen atom plays a key role. Usually, ancillary ligands on the metal are only spectators and do contribute to small modifications of the bonding properties of the metal fragment. However, if a hydride is present as a ligand to the transition metal center, it may interfere with the alkyne to vinylidene transformation. This may open up new selective and efficient routes to vinylidene complexes. 

Upon discovery of this mechanism, new catalysts have been developed, now presenting alkylidene ligands in the metal coordination sphere, such as [(=SiO) Ta(=CH Bu)Np2 and [(=SiO)Mo(=NAr)(=CH Bu)Np] [43, 88]. Table 11.4 presents results obtained with several catalysts prepared by SOMC. Although [(=SiO) Ta(CH3)3Cp (=SiOSi=)] is not active in alkane metathesis (the tantalum site would not be as electrophilic as required) [18], results obtained with [(=SiO)Mo(=NAr) (=CH Bu)Np] show that ancillary ligands are not always detrimental to catalytic activity this species is as good a catalyst as tantalum hydrides. Tungsten hydrides supported on alumina or siHca-alumina are the best systems reported so far for alkane metathesis. The major difference among Ta, Mo and W catalysts is the selectivity to methane, which is 0.1% for Mo and less than 3% for W-based catalysts supported on alumina, whereas it is at least 9.5% for tantalum catalysts. This... 

The first direct observation of equilibrium between an acidic H2 complex and a corresponding hydride complex with a protonated ancillary ligand is shown in Eq. (16) (70). 

Another class of heteroleptic alkyl complexes contains 7t-donating ancillary ligands such as RU[N(Si(CH3)3)2]3 (R = CH3, H, BH ). The hydride species can be converted into the methyl species via reaction with BuLi and CH3Br. The methyl compound has exhibited insertion chemistry with small molecules including aldehydes, ketones, nitriles, and isocyanides (206). Stable metallacyde compounds are also known, ie,... 

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