Butadiene complexes structure

The hydrogenation of butadiene is structure-sensitive on Pd and Rh but lacks particle-size dependence in the case of platinum. The strong complexation of the diene to atoms of low coordination number is a possible explanation for this phenomenon where it occurs37,38. 

Table 5 Synthetic Routes and Some Structural Data of Metal 1,4-Disubstituted Tetraaza- 1,3-butadiene Complexes... 

Molecular orbital calculations by Hofmann (162) indicated that an if-allyl anion complex with a noncomplexed butadiene group is more stable than an -butadiene complex, in which the allyl anion portion of the seven-carbon ring is not bonded to the metal. We have now been able to confirm this result by an X-ray structure analysis (see Fig. 8) on [Ph4As][C7H7Fe(CO)3] (163). 

A variety of new ligand designs and ligand combinations were used in attempts to mimic some properties of the ubiquitous bent metallocene environment at the early metal centers consequently, some of these systems were used in the further development of butadiene zirconium chemistry. The pyridine based chelate zirconium dichloride complex 43 cleanly formed the butadiene complex 44 upon treatment with butadiene-magnesium. Its structure shows that the C4H6 is arranged perpendicular to the chelate ligand plane. Complex 44 inserts one equivalent of an alkene or alkyne to form the metallacyclic 7i-allyl system 4545 (Scheme 13). 

The general reaction model for the allylnickel complex-catalyzed 1,4-polymerization of butadiene is outlined in [26]. From the starting / -allylnickel(II) complex, which has a quasi-planar structure, two structurally different butadiene complexes are formed as the actual catalysts by successive ligand or anion substitution a monoligand allylnickel(II) complex, which may also contain the anion X instead of the neutral ligand L, with an coordinated butadiene, and a ligand-free complex with an t/ -cis coordinated butadiene. The concentration of these complexes, which is also limited by the double- bond coordination from the growing chain, and their reactivity determine the catalytic activity. 

For each of the butenylnickel(II) complexes, and for the butadiene complexes as well, an anti-syn equilibrium has to be assumed, with the syn configuration having the thermodynamically more stable structure (Xg/s 10 -10 ) [36, 60]. The rate of anti-syn isomerization has proved to be strongly dependent on stmcture. In the bis(ligand)-butenylnickel(II) complexes the isomerization rate is very low = 10 s ) [66], but in the ligand-free Ci2-allylnickel(II) complexes the anti-syn isomerization is accelerated considerably by the coordination of the next double bond, so that it is completed instantaneously even at -70 °C [60]. 

Instead of the coordination of the next double bond in the growing chain, one hard ligand or anion can coordinate to the nickel, in the axial position either above or below the plane of the complex, and give the necessary energetic support for the insertion reaction with the // -c/ -coordinated butadiene see Structure 3 in Scheme 6. 

Using the parent zirconocene-butadiene complex as a representative example, a typical bonding situation in these types of molecules is presented in Scheme 48. For 297, equilibration between the s-trans and the s-cis isomers occurs with a barrier of 23 kcal mol 1 at 283 K. The 72-olefin complex is believed to be a high-energy intermediate on the interconversion reaction surface. Significantly, structural data indicates that the s-cis complexes are best described as Zr(iv) compounds with a er2, ir ligand.158,175 The dynamic NMR measurements have also been extended to ansa-zirconocene and hafnocene butadiene complexes.176 Moreover, photoelectron spectroscopy has been used to determine the relative energetics of the two isomers for // -metallocenes.177... 

AB, ABA, ABC and ABCBA block copolymers have been prepared In this way Involving styrene and butadiene as monomers with mono- or dlfunctlonal anionic Initiators. More complex structures may also be made by titrating the living cationic block copolymers with dlfunctlonal reagents such as the dlsodlum salt of resorcinol (Equation 7). 

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