Intermediate ligand helds

To account for stereochemical results for the epoxidation of allyl alcohols, a slightly different intermediate has been proposed as shown in Fig. 6.9.16 The authors propose an intermediate (A) analogous to the intermediate in peracid oxidations. A small molecule of alcohol or water is coordinated to Ti with deprotonation and another is coordinatively ligated to Ti without deprotonation to achieve a pentacoordinated ligand sphere. During epoxidation, the allyl alcohol substrate is held in position by a hydrogen bond. 

At 24 °C and 15-60 bar ethylene, [Rh(Me)(0H)(H20)Cn] catalyzed the slow polymerization of ethylene [4], Propylene, methyl acrylate and methyl methacrylate did not react. After 90 days under 60 bar CH2=CH2 (the pressure was held constant throughout) the product was low molecular weight polyethylene with Mw =5100 and a polydispersity index of 1.6. This is certainly not a practical catalyst for ethylene polymerization (TOP 1 in a day), nevertheless the formation and further reactions of the various intermediates can be followed conveniently which may provide ideas for further catalyst design. For example, during such investigations it was established, that only the monohydroxo-monoaqua complex was a catalyst for this reaction, both [Rh(Me)3Cn] and [Rh(Me)(H20)2Cn] were found completely ineffective. The lack of catalytic activity of [Rh(Me)3Cn] is understandable since there is no free coordination site for ethylene. Such a coordination site can be provided by water dissociation from [Rh(Me)(OH)(H20)Cn] and [Rh(Me)(H20)2Cn] and the rate of this exchange is probably the lowest step of the overall reaction.The hydroxy ligand facilitates the dissociation of H2O and this leads to a slow catalysis of ethene polymerization. 

The constrained proximity of the two metal centres, held together by the tripod ligand, should increase the probability of an intramolecular hydride transfer. Thus a mechanism can be proposed (Figure 6) in which bimetal cooperativity, via an intramolecular hydride transfer, facilitates the elimination of aldehyde from the acyl intermediate. 

For iron(IlI)-porphyrinato complexes, strong-held ligands lead to low-spin (5 = 2) complexes. A pair of identical weak-held ligands, such as tetrahydro-furan, leads to intermediate-spin (5 = ) species. Five-coordinate species are, with few exceptions, high-spin (5 = f), with all hve 3d electrons in separate orbitals. Spin equilibria 5 = i 5 = f and 5 = 15 = i are not unusual. Specihc examples of these spin systems are given in Table 4.4. Higher oxidation states are found in some other hemoproteins. Fe(V)-porphyrin systems actually occur as Fe(IV)-porphyrin cation radical species, and Fe(I)-porphyrin systems exist as Fe(II)-porphyrin anion radical species. 

Species (1) and (4) in the scheme represent coordinatively unsaturated Ti(IV) complexes that are formally dP 16felectron pseudotetrahedral species species (2) represents the interacting catalyst/cocatalyst combination, while intermediate species (3) is shown with the monomer coordinated at an a molecular orbital with the three non-Cp ligands and the transition metal occupying a common equatorial plane. The growing chain is held between two lateral coordination sites accommodating an unidentified non-Cp anion (R ) and the monomer. 

---