Amido complexes early-transition-metal

As noted in the introductory section to amido complexes, early-transition-metal-amido complexes have been used as catalysts for olefin polymerization, and they have been used as catalysts or precatalysts for alkyne and olefin hydroamination, olefin metathesis, - ... 

The 1,2-addition of C—H bonds across metal-heteroatom bonds has been reported for two different classes of complexes early transition metal d° complexes with imido ligands and late transition metal complexes with amido, hydroxo, and aryloxo ligands (Scheme 11.36). These transformations are potentially related to o-bond metathesis reactions discussed above however, the presence of a lone pair on the heteroatom that receives the activated hydrogen may impart important differences. 

Two principle strategies have been employed for the synthesis of siloxide-containing molecular precursors. The first involves a silanolysis, or condensation, reaction of the Si - OH groups with a metal amido, alkyl, hahde, or alkoxide complex. The second method involves salt metathesis reactions of an alkali metal siloxide with a metal hahde. Much of our work has been focused on formation of tris(tert-butoxy)siloxide derivatives of the early transition metals and main group elements. The largely imexplored regions of the periodic table include the lanthanides and later transition metals. 

The extensive chemistry of amido complexes, and, more particularly, of alkylamido complexes, reveals that the planar form is almost invariably found, along with bridging amides (221). Much attention has been paid to the synthesis of metal amido complexes of early transition metals, lanthanides and actinides. The amido group, particularly where it is bulky, confers unusual low coordination numbers on the metals and can also produce materials with considerable kinetic stability toward attack by nucleophiles (42, 67). However, the relevance of this extensive and fascinating chemistry to nitrogen fixation is somewhat problematic. 

Dinitrogen Activation by Early Transition Metal-Amido Phosphine Complexes... 

The formation of alkyne oligomers that are concomitantly formed in the hydroamination reactions catalyzed by the thorium complexes indicates that two possible different complexes can be considered as active, conceivably with inter-conversion causing the occurrence of the two parallel processes. The discernment between these two most probable mechanistic pathways to find the key organometallic intermediate, responsible for the hydroamination process, was achieved by kinetic and thermodynamic studies (Scheme 5). The first pathway proposed the insertion of an alkyne into a metal-imido (M=N) bond, as observed for early transition metal complexes [101]. The second pathway suggested the insertion of an alkyne into a metal-amido bond, as found in some lanthanide compounds [39, 58, 84, 85]... 

Other kinds of nonmetallocene complexes based on early transition metals (mainly Ti and Zr) containing dibenzyl-chelated diamido dipyrrole [19] or tetra-amido tetrapyrrole... 

The reaction chemistry of late-transition-metal-amido complexes resembles that of organometallic complexes more than that of early-transition-metal amides. Thus, the chemistry of this class of amido complex is presented first. Several reviews of the chemistry of late-metal amido complexes have been published. -... 

Early-transition-metal amides are extremely reactive toward protic adds and are therefore useful precursors to a variety of M-X complexes (in which X is an anionic ligand less basic than an amide) by reaction of the amido complex with H-X (Equation 4.13). Common exchange processes that favor elimination of amine involve the reaction with alcohols, amides, amidines, and guanidines. However, weakly acidic hydrocarbons, such as cyclopentadiene derivatives, also react to form Cp complexes (Equation 4.14). ... 

Half-sandwich terminal early transition metal imido amido complexes have come under considerable scrutiny in recent years, in part due to their participation in C-H bond activation [30] and cycloaddition reactions [31]. According to NMR evidence, many of them are involved in dynamic processes. Compounds of this sort are exemplified by Nb(V) and Ta(V) complexes [(Ti5-C5H5)NbX N(2,6-Me2C6H) ], where X = Cl, NH(2,6-Mc2C6H3), Me [32] and [(Ti5-C5Me5)TaNR2X N(2,6-Me2C6H) ], where R = Me, Pr X = Cl, Me [33]. 

Dimetallic elimination reactions leading to metal-metal bond formation are the amine or alkane eliminations that result from the condensation of a late transition metal hydrido complex with an early transition metal amido or alkyl complex, respectively. Examples of this method are Selegue s synthesis of the first Ti-Fe and Ti-Ru complexes 8a,b [8, 9] and the reaction of [Zr(CH2Ph)4) with [CoH(CO)4], although only spectroscopic evidence was provided for compound 18 (Scheme 4.3) [20]. 

Early transition metal, lanthanide and actinide alkoxy and amido complexes are common, and they often are stable because of the interaction between the filled p orbital of the O or N atom of the ligand and an empty d metal orbital. The alkoxy and aryloxy ligands play a crucial role in the catalytic properties of group 5-7 metal-alkylidene and metal-alkylidyne complexes for the metathesis of simple, double and triple bonds. - On the other hand, the behavior of late transition-metal alkoxy and amido complexes is less known. Many of them are stable, however, in spite of the possible repulsion between the filled d orbital and the p orbital of the heteroatom. The metal-heteroatom bonds are robust, and the main characteristic of these is that they are strongly polar and possess a significant ionic character. They exhibit nucleophilic reactivity and sometimes form strong bonds to proton donors (they even deprotonate relatively weak acids). 

Amine activatitMi pathway has been well studied in catalysis by lanthanides, early transition metals, and alkali metals. In metal amide chemistry of late transition metals, there are mainly two pathways to synthesize metal amide complexes applicable under hydroamination conditions [54], One is oxidative addition of amines to produce a metal amide species bearing hydride (Scheme 8a). The other gives a metal amide species by deprotonation of an amine metal intermediate derived from the coordination of amines to metal center, and it often occurs as ammonium salt elimination by the second amine molecule (Scheme 8b). Although the latter type of amido metal species is rather limited in hydroamination by late transition metals, it is often proposed in the mechanism of palladium-catalyzed oxidative amination reaction, which terminates the catalytic cycle by p-hydride elimination [26]. Hydroamination through aminometallation with metal amide species demands at least two coordination sites on metal, one for amine coordination and another for C-C multiple bond coordination. Accordingly, there is a marked difference between the hydroamination via C-C multiple bond activation, which demands one coordination site on metal, and via amine activation. 

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