C-H complexes

The transformation of RhCl(PH3)2(HC=CH) to RhCl(PH3)2(C=CH2) has been calculated (MP2) to be exothermic by 7.8 kcal.mol"1. The intraligand 1,2-hydrogen shift mechanism found in the Ru11 system is not relevant to the present rhodium case. Starting from a T 2 C=C complex, both systems give a metal-( T]2 C-H) species in a subsequent step. In the case of the d6 Ru system this ri2 C-H complex is an intermediate. In contrast, the T 2 C-H coordinated state is a transition state in the d8 Rh1 system, the oxidative addition being a very facile process. 

The acetylene coordinates trans to the least o electron donor group, chlorine. Coordination of the C-H bond is a less favorable alternative to coordination of the n system. The o C-H complex is 17.1 kcal.mol 1 less stable than the rc-alkyne complex (Figure 5). From this c C-H intermediate the 1,2 shift is possible with a relatively small activation barrier (+15.5 kcaLmol 1) to yield the vinylidene complex. However this mechanism is in contradiction with the labeling experiment. 

Pd-catalysed chelate-directed acetoxylation of meta -substituted arenes has been studied.61 Many substituted groups are tolerated by this process and the reaction shows a high degree of regioselectivity for the less sterically hindered ortfto-position. For example, 2-(3-nitrophenyl)pyridine forms 2-(2-acetoxy-3-nitrophenyl)pyridine. Finally, density functional calculations62 on the palladium acetate-promoted cyclomet-allation of dimethylbenzylamine suggest that reaction occurs via an agostic C-H complex rather than a Wheland intermediate. An intramolecular H-transfer to a coordinated acetate via a six-membered transition state follows. 

Recently, Nickel and Fleischer reported a Raman scattering study of as-grown state-of-the-art ZnO. A number of Raman lines were detected, which were ascribed to LVMs of N-H and C-H complexes with n=l,2,3. °... 

Diamond CVD on group VIII transition metals (Cr, Mn, Fe, Co, and Ni) has been investigated in search of relevant substrates for diamond heteroepitaxy. The material properties are listed in Table 9.1 [166]. See Appendix E for more information. These transition metals can dissolve carbon and hydrogen to form a surface layer of metal (M)-C-H complex. This seems to be common to many metal substrates, including Pt and Ir, as will be seen later. 

In certain specimens, there were diamond crystals that were partially embedded in Pt near the peripheral region of the substrate, as seen in Figure 12.5. The diamond CVD was done for 5h using the standard growth conditions. It appears that the diamond nucleation took place beneath the Pt surface. It is likely that the nucleation site should be the diamond powders that were buried in Pt during the ultrasonic treatment. It was also inferred that a Pt-C-H complex formation plays an important role for the diamond nucleation, as depicted in Figure 12.6. 

The mechanism of diamond formation by the LPSSS method using metal catalyst is considered to be similar to that of heteroepitaxial growth of diamond on Pt(lll) and Ir(lOO) described in Sections 12.1 and 12.2, because a metal-C H complex seems to be involved in the nucleation process. An advantage of the LPSSS method in view of engineering is that substrates with complex shapes could be coated with diamond. 

Tomasik, P. and Schilling, C.H., Complexes of starch with inorganic guests, Adv. Carbohydr. Chem. Biochem., 53, 263, 1998a. 

Figure 4.6. Effect of C H ring size on exo C-H bond orientation in M(ri"-C H ) complexes.

BD in C-H complexes. The C-H bond simply becomes a better a donor to M when the carbon in 5b becomes more electron-rich and BD effects are inconsequential. ... 

Perhaps the strongest evidence for the formation of an alkane complex comes from the low temperature NMR experiments with CpRe(CO)3. Irradiation at —80°C in cyclopentane results in the formation of CpRe(CO)2(c-pentane), for which a resonance for theCH2 bound to the metal is observed in the NMR spectrum at 5 —2.32, consistent with either a symmetrical binding of both hydrogens ( 7 -H,H) or a rapid interchange between two cr-C-H ( -C,H) complexes. Irradiation in pentane solvent leads to the formation of three distinct species, assigned as coordination of rhenium to the methyl, Q-methylene, and central methylene of pentane (Equation (7)). ... 

Theoretical calculations comparing the reactivity of H - H and C - H bonds towards organometallic complexes confirm these different reactivities. For instance, Goddard et al have found barriers for oxidative additions of H2, CH4 and CH3CH3 to palladium of 5.1, 30.5 and 38.6 kcal/mol, respectively [35b]. If the energetic relationship between transition states is preserved for molecular complexes, a "molecular C - H" complex must be much weaker than a "molecular H - H" complex. 

The coordinate catalysts are also capable of coordination polymerization of dienes with isolated bonds. Atypical example of this reaction is the polymerization of 1.5-hexadiene in the presence of TiCl -Al (C H ) complex. One obtains in this case pol5mier with repetitive cyclopentanone rings and interconnected via methyl bridges of cis form in positions 1 and 3 ... 

MO calculation was run on the aldehyde 2a-c / H complexes as a model of 2a-c / Lewis acid complexes. 

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