Zirconium-based catalysts

Department of Chemistry, University of Bath, Claverton Down, Bath BA2 7AY, UK Email mj205 bath.ac.uk 

One of the earliest examples of the use of zirconium in catalysis is the area of Ziegler-Natta polymerisation, the catalysts employed are typically based on a family of Cp2ZrCl2 metallocene complexes. With judicious choice of the cyclopentadienyl ligand either atactic (bridged Cp-derived systems), 

Sustainable Catalysis With Non-endangered Metals, Part 1 

Whilst the polymerisation of propene is not truly sustainable it illustrates the importance zirconium catalysts have in the area of polymer science. The reader is referred to the following references for a comprehensive review of zirconium in Ziegler-Natta polymerisation. This chapter will focus on the use of zirconium in sustainable polymer application (polylactide or polycarbonates), its use in the homogeneous catalysis for the activation of carbon dioxide and other small molecules and finally detailing some examples of the use of zirconia in heterogeneous catalysis. 

Industrially this process is catalysed with tin(ii) octanoate and there is a desire to move away from tin and use less-toxic metals or even metal-free conditions. The polymer itself is used in a diverse range of applications from commodity plastics (packaging) to high-end biological uses (tissue engineering, sutures and stents). In the mechanism initially the ester coordinates to the Lewis-acidic metal centre, the alkoxide then attacks the carbonyl carbon and the lactide ring opens and new alkoxide is formed and 

---

With diphosphanes recently Stephan et al. reported an intriguing Al and P based macrocyclic structure [37]. A zirconium based catalyst precursor first was employed in the catalytic dehydrocoupling of the primary bidentate phosphane to give the tetraphosphane 6, (Scheme 4). The function of 6 as a molecular building block has been demonstrated by its reaction with MMe3(M = Al, Ga). Although, the gallium derivative 7 has not been... 

Cyclohexyl- and phenylmethylsilanes do not polymerize, but give dimer. With titanium-based catalysts the value of n is about 10 and does not vary very much with R or experimental conditions with zirconium based catalysts, n can be as high as 20. 

Waymouth and coworkers reported a unique system where the unbridged bis(2-phenylindenyl)zirconium-based catalysts (129) gave elastomeric, isotactic-atactic stereoblock polypropylene, controlled by rotation of the 2-phenylindenyl as shown in Scheme VII [173]. 

Collins et al. reported in 1995 that catalysts based on hafnium are desirable for the production of elastomeric polypropylene in that they polymerize propylene to a high molecular weight polymer and are indefinitely stable under typical polymerization conditions [8], Based on the theory that hafnium as a catalytic center leads to a significant increase of molecular weight in propene polymerization compared with the zirconium-based catalyst, Rieger et al. searched for hafnocene systems to obtain polymers with new properties. 

The discovery of a zirconium-based catalyst able to promote polyolefin depolymerization encourages the search for more electrophilic catalytic systems that could be obtained either by changing the metal center or the inorganic support. 

Representative metal complexes employed for the catalytic asymmetric Strecker reaction are summarized in Figure 4.2. Aluminum-, titanium-, lanthanoid-, and zirconium-based catalysts are highly efficient. Direct one-pot synthesis starting from aldehydes, and amines is reported using the Zr complex described in Figure 4.2. ... 

As regards a comparison of the relative effectiveness of titanium-and analogous zirconium-based catalysts in syndiospecific styrene polymerisation, the latter in general are less active than the former. Usually, polymer yields are lower, and a higher polymerisation temperature and reaction time as well as higher [Al(Me)01/transition metal compound ratios are required. Among the few zirconium compounds examined, only tetrabenzylzirconium activated with methylaluminoxane has relatively higher syndiospecific activity [10,48,56],... 

Group 4 bis(amidate)bis(amido) complexes have also been identified as precatalysts for the more challenging hydroamination of alkenes. The majority of investigations in this field focus on the intramolecular cychzation of aminoalkenes with zirconium-based catalysts. [64e] Neutral group 4 bis(amidate) zirconium amido or imido complexes are efficient precatalysts for the intramolecular cychzation of primary amines to form pyrrolidine and piperidine products (Scheme 12). The monomeric imido complex can be generated by reaction of the bis(amido) complex with 2,6-dimethylaniline and trapped with triphenylphosphine oxide. [64e] The bis(amido) and imido complexes... 

The zirconium-based catalyst 5 -136/MAO is highly syndioselective ([r] > 99%, Tp = 0 C) and apparently provides the highest-melting syndiotactic polypropylene known The unannealed Tm for the polypropylene is 164 °C, and the annealed Tm is 174 °C. Stereoerrors have not been detected in this polymer by NMR, but pentads presumably attributable to site epimerization (single m mistakes) can be found if polymerizations are conducted in dilute propylene at elevated temperatures. 

While dibutyltin dilaurate has been the most commonly used catalyst over the years, it is being replaced by safer alternatives such as bismuth and zinc carboxylates. Zirconium-based catalysts are preferred in waterbased two-component systems because of their better selectivity for isocyanate-hydroxyl reactions over isocyanate-water reactions. 

Titanium and zirconium based catalysts oligomerize ethylene into a mixture of a-olefin ranging from C4 to C30 with a Schulz—Flory type distribution. The content of linear a-olefin is nearly 100% provided conversion is low enough (a high concentration of ethylene has to be maintained during the whole reaction). 

Furthermore, the formation of primary amides from earbojq lic acids and ammonium carbamates has also been investigated, again requiring a titanium or zirconium based catalyst. The eonditions required to carry out this transformation are forcing in comparison with the acid and amine counterpart, where the earbamate is present in exeess and high temperatures of at least 100 °C are required. Under these eonditions it was seen that... 

More recently, neutral zirconium-based catalysts capable of performing reactions with both primary and secondary amines in intra- [55-57] and intermolecular [57, 58] reactions were reported. The imido mechanism is obviously impossible, and an insertion mechanism, similar to the lanthanide-like mechanism shown in Scheme 2 was proposed [55]. The isolation of an insertion intermediate in an intermolecular alkyne hydroamination reaction is compelling evidence in favor of the insertion mechanism [58]. 

---