Propylene complexes

Figure 4.79 displays the optimized structures of secondary-Cp (IIsec) and primary-Cp(IIPri) complexes, and Table 4.43 includes geometrical and charge parameters of these propylene complexes for comparison with those of the corresponding ethylene complex in Table 4.42. The IIsec complex can be seen to have smaller Ti—Cp metal-alkene separation (by 0.1 A) and other evidence of tighter metal-alkene binding than that in the IIpri complex, in accordance with the donor-acceptor stabilizations discussed above. 

Figure 4.79 Optimized structures of the alternative propylene complexes IIsec and npri in insertion reactions (4.107a) and (4.107b).

Table 4.43. Skeletal geometries and atomic charges of the alternative secondary-Cp (Hsec) andprimary-Cp (IIpri) propylene complexes, as well as of the transition state (IIpri ) and actual product (IIIpri) of the model propylene-polymerization reaction (4.107) cf Figs. 4.79 and4.80...

The stability of the propylene complexes of various silver salts is very dependent upon the nature of the anion. The data of Table III in conjunction with the recorded (223) stability of the propylene complex of AgNOs establish that the order of increasing stability is NOg < CIO 4- <... 

Recently an increasing number of complexes have been prepared containing ligands which are not electron-pair donors. Typical of these are the ethylene and propylene complexes with Pd, Pt, Cu, Ag and Hg . The bonds involved have been discussed by Chatt and by Dewar (1951). 

Although group 5 organometallic systems have been found to be of relevance in transition-metal catalyzed hydroboration reactions, structurally authenticated group 5 boryl complexes remain relatively few in number. Smith and co-workers, for example, have probed the mechanisms for the formation of niobium and tantalum mono- and bis(boryls) from propylene complex precursors, with concomitant formation of propyl boronate esters [31,32]. Of particular interest from a structural viewpoint are the relative merits of alternative bonding descriptions for metal(V) boryl bis(hydrides) as borohydride complexes or as mono(hydride) a-borane systems [31-34]. 

Again, the stability of the complexes were found to vary with the nature of the anion so) for the propylene complexes. 

Hydrometalation of alkenes with Mo and W hydrides occurs on addition of PPhj to cationic hydridoalkene complexes such as [( j -Cp)2Mo(C2H4)H] and [(i ACp)2W(C2H4)H] and with the corresponding propylene complexes ... 

Table 5. Metal—Hydride Propylene Complexation, Hydrogen Insertion Activation, )8-Hydride Elimination Activation, and )6-Methyl Elimination Energies (kcal/mol)...

For the model catalysts 1 of Scheme 2 the olefin n-complexes with branched alkyls are more stable than with the linear ones [13d]. This electronic preference is strongly affected by the steric bulk on the catalyst due to an interaction b etween the alkyl group and the catalyst substituents [13d]. Thus, for the most bulky real catalysts the n-complexes with linear aUcyl have lower energy. This is true for both ethylene and propylene complexes. A similar effect was observed for both, the diimine and anilinotropone systems [13c,d, 28]. The presence of the steric bulk also affects the olefin complexation energies, and the relative stability of ethylene and propylene complexes. This has been discussed in details in [13d]. 

Pig. 26. Propylene complexed on active site. The plane of propylene is parallel to the RTiCL plane. 

Studies of catalyst decomposition in the presence of substrate have mostly focused on ethylene. In particular, it has been demonstrated that ethylene can induce the degradation of methylidene complex 19 to produce propylene as the main volatile organic byproduct [3, 39]. The proposed mechanism for this degradation involves the ruthenacyclobutane intermediate (20) undergoing a P-hydride elimination to form a ruthenium allyl-hydride species (21), which subsequently affords the propylene complex (22) upon reductive elimination (Scheme 11.8). 

The cleavage of a-allyl groups by protonic acids is often more complex than removal of simple alkyl groups. Green (52, 112, 113) has shown that ff-allyl derivatives of Fe, Mn, Mo, and W are protonated to cationic propylene complexes by anhydrous HCl. 

The C2-symmetric Zr metallocene catalyst (top) and a highly schematic view of propylene complexing to it. The middle two structures use the same face of the propylene, and lead to the same tacticity because of the C2 symmetry of the catalyst. The bottom two structures use the opposite faceof the olefin. The adverse steric interaction of the CH3 with the aromatic ring disfavors these structures. 

A key step in metal-induced olefin polymerization has the olefin tt face complexing to the metal center. The two faces of the propylene double bond are enantiotopic. Isotactic polypropylene forms when only one face of the propylene monomer consistently reacts to make polymer. Thus, a chiral catalyst is needed to distinguish enantiotopic faces of an olefin. But, how do we ensure that only one face reacts It is a complicated problem, because when an olefin like propylene complexes to a metal center in a typical chiral environment, not only will both faces complex to some extent, but many orientations are possible for each complex. This leads to many different reaction rates, and a mixture of Stereochemistries. A key to the solution, then, was to develop a catalyst that is chiral but not asymmetric. In particular, the C2-symmetric metallocene shown below was prepared. The metal is chirotopic but non-... 

As mentioned in Sect. 2.4, complex 8 is prepared by selective hydrogenation of the fluorenyl moiety of the parent highly syndiotactic-specific metallocene molecule 6. Its crystal structure is presented in Fig. 23. Complex 8 fulfills all symmetry requirements that one would expect a priori from a would-be syndiotactic-specific precatalyst molecule, nevertheless, after its activation with MAO and its exposure to propylene, complex 8 produces polypropylene chains with perfectly atactic microstructure [28, 30]. 

Computational work indicates the probable structures for the key intermediate propylene complexes in the two classes of catalyst. In the chiral isotactic catalyst, 12.10, the methyl group tends to be located as shown in Fig. 12.2 (upper), so that successive propylenes enter with the same chiralities and bind via the same face (re in the figure). In the achiral syndiotactic catalyst, 12.11, in contrast, successive propylenes enter with opposite chiralities and bind via alternating faces (re then si). 

Generation of the 16-electron fragment Tp Rh(CNCH2CMe3) (Tp =[HB(3,5-dimethylpyrazolyl)3]) t> the presence of cyclopropane resulted in C-H activation of the hydrocarbon. The cyclopropyl hydride complex rearranged in benzene solvent to the metallacyclobutane complex Tp Rh(CNCH2CMe3)(CH2CH2CH2). Thermolysis of the rhodacyclobutane complex produced an 77 -propylene complex (Scheme 38). ... 

---