Zirconium complexes bimetallic

Figure 7.4. Crystal structure of the 1,1-bimetallic complex of boron and zirconium 22. Adapted by the authors.

No formation of bimetallic complexes is observed on MoO(OPr )4 dissolution in the alcohol solutions of zirconium isopropoxide, due presumably to the high stability of the structure of the latter. A very unusual complex of Zr3Mo,024(OPri)12( PrOH)4 composition precipitates slowly from solutions of isopropoxides in hexane subjected in advance to evacuation to dryness and redissolution repeated three times. The structure of the complex obtained is very close to that of the zirconium methoxide hydrolysis product, Zr O OMe) (Fig. 5.1 c) [901]. The formation of a complex very rich in oxoligands is presumably due to the trend of ZrCOPhV PrOH to form oxocomplexes on desolvation (see Section 12.12). 

Bimetallic Transition Metal-Zirconocene Complexes from Zirconium Hydrides... 

Interestingly, Hoveyda and coworkers observed a second-order dependence of the reaction rate on the concentration of zirconium in these reactions, suggesting that the zirconacyclopentane is formed from a bimetallic alkene-zirconate complex such as A in Fig. 1 [21]. This finding suggests that olefin alkylations and substitutions occur via reaction of a nucleophilic alkene unit [23]. 

Figure 4 shows the remarkable structural similarity between the bimetallic carbene (1 2) and alkoxy complexes formed from diverse paths 1,2 addition of Zr-H to a carbonyl bound to tungsten (eq. 1) and 1,1 addition of Re-H to a zirconium-bound acetyl (eq. 2). 

Stable zirconium, platinum, molybdenum, and tungsten complexes of cyclooctyne, a zirconium complex of cydoocta-5-enyne, and a bimetallic molybdenum complex of cyclocta-3,7-dienyne have been discussed in earlier reviews.28 More recently, two stable zirconocene complexes of cycloocta-trienyne (275 and 276) have been prepared101 by /3-hydride elimination from 274 in the presence of PMe2R [Eq. (45)]. 

IrsMo) prepared from the bimetallic carbonyl complex was somewhat more active for the -butane reaction at 488 K than a pure iridium catalyst, but high ethane selectivity was retained (1.43). The IraMoa complex gave a less active catalyst, with a lower value of S2 (1.08). The addition of zirconium to platinum (Pt7sZr25/C) lowered the rate of ethane hydrogenolysis about 20-fold, and raised the activation energy. ... 

In all cases there was a small amount of the cyclic carbonate formed. It was observed that the monometallic complex Zr(L )(0 Pr)2 was inactive for the polymerisation. This suggests that the bimetallic nature of the complex is important, which has previously been shown for zinc(ii) complexes. Ko has also screened a series of benzotriazole phenolate zirconium complexes. Figure 8.10. Zr(L)2(0 Pr)2, where R=H was by far the most active (pC02 = 600 psi, T= 100 °C, 16 h) giving an 87% conversion of CHO, with a 99% selectivity to polymer with 90% carbonate linkages. Although these results are inferior to the more established systems they do show promise and this is an area of future investigation. 

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