Zirconium complexes examples

Perhaps chemists will be able to mimic nature without duplicating the iron-sulfur-molybdenum structure. For example, a zirconium complex with tetramethyl cyclopentadiene can bind dinitrogen in a manner that breaks the NON bond, as shown below. Treatment with hydrogen gas results in formation of small amounts of ammonia. Although the yields are too low to make this a viable commercial process, researchers hope to make the process more efficient through chemical modifications and changes in conditions. 

Certain half-sandwich phenoxides have been shown to be highly active olefin polymerization catalysts. For example, the zirconium complex (60) polymerizes ethylene with an activity of 1,220 gmmol-1 h-1 bar-1.181 A similar titanium complex (61) displays an activity of 560gmmol ll bar 1 at 60°C.182-189 Comparable activities were also recorded for the copolymerization of ethylene with 1-butene and 1-hexene. 

Example The high-resolution spectrum in the molecular ion range of a zirconium complex is typified by the isotopic pattern of zirconium and chlorine (Fig. 3.22). Zr represents the most abundant isotope of zirconium which is accompanied by Zr, r, Zr and Zr, all of them having considerable abun-... 

Both enolate species 2a and 2b are readily alkylated by iodomethane to provide the corresponding acyl complexes 3a and 3b, respectively (yield of 3b is not reported)91. No examples of the generation of zirconium-acyl enolates from chiral biscyclopentadienyl zirconium complexes has yet been reported. The reported preparation of the enantiomerically pure complex 4 indicates that chiral zirconium-acyl enolate species may be accessible92. 

For tri-n-octylphosphine oxide, for example, the shift in the P=0 stretching frequency from its value for the free extractant is greater for the hafnium complex (85 cm-1) than for the zirconium complex (70 cm-1).297... 

Many stable metal complexes of arynes are known but in most of their reactions of synthetic interest, the yields are poor. For example, thermolysis of titanocene (Cp2TiPh2) at 80-100 C gives rise to a tita-nium-benzyne complex which reacts with molecular nitrogen to afford aniline with low efficiency.29 However, procedures are available for in situ generation of zirconium complexes (14) and for their coupling reactions to synthesize functionalized aromatic compounds in preparatively useful yields (Scheme I).30 Whether such complexes should be regarded as ir-bonded benzynes or o-bonded o-phenylenes, remains a debatable point.31... 

Although the ability of zirconium complexes to generate radicals selectively was noted early [165, 166], not many radical reactions mediated by zirconium complexes are known [167]. Oshima et al. reported the only examples of zirconium-catalyzed radical reactions. 

Some seemingly simple zirconium salts are best regarded as essentially covalent molecules or as complexes examples are the carboxylates Zr(OCOR)4, the tetraace-tylacetonate, the oxalate, and the nitrate. Like its Ti analogue, the last of these is made by heating the initial solid adduct of N205 and N204 obtained in the reaction... 

Specific examples of alkene insertions are the reactions of cationic hydrides [Cp2ZrH(L)]+ to give metal alkyls which may either be stabilized by agostic interactions, as in (21-XLII), or free of such interactions (21-XLIII), depending on L.180 The insertion of isobutene into M—C bonds of electron-deficient zirconium complexes was found to be reversible.109... 

Notably, phospholes with controlled regioselectivity can also be obtained from zirconium complexes using silylated alkynes. For example, sequential treatment of Schwartz reagent 167 with 2-butyne, MeLi, and silylated alkynes... 

Along with the development of chiral Lewis acid catalysts, a chiral trialkanolamine (42) has been used to prepare the catalyst (43) (Eq. 19). By use of this zirconium complex as a catalyst, enantioselective addition of the azide to meso epoxides was achieved [20a]. Thus, the oxirane ring was opened by /-PrMe2SiN3 to give the adduct (44) with high enantioselectivity (Eq. 20). In another example, a diamide ligand (45), which behaves as a tetradentate ligand, was used to achieve a similar reaction (Eqs 21 and 22) [20b]. 

The ansa-bridged homobinuclear titanium and heterobinuclear titanium-zirconium complexes have been reported. For more examples of this type of complexes, see Sections 4.05.4.1.1.(i).(g) and 4.05.4.1.3.(i)... 

Here is an example from the McMurry flexibilene synthesis quoted in chapter 1. An alkyne 127 with a protected aldehyde group reacts with Cp2ZrHCl to give a vinyl zirconium complex 128 which is coupled to a palladium-allyl complex in the next step. The double bond so produced is present with the same E configuration in the final product. It is marked 129 with an arrow in the diagram.28... 

Other chemists would argue that this is a load of rubbish and that both are examples of stereospecificity. We should be concerned with the mechanistic element of the reaction. We want to know how the zirconium complex reacts, and this is what the stereospecificity refers to delivery of hydrogen and zirconium to the same side of a multiple bond. The first group of chemists would probably come back with the argument that in order to probe the mechanism in the first place you need two diastereomers of starting material and how can we be sure that the reaction with the triple bond goes by the same mechanism - and so on. Perhaps we should mention that Schwartz himself describes the addition across a triple bond as stereospecific.14... 

Various titanium or zirconium complexes have been shown to catalyze the addition of allyl(tri-M-butyl)tin [29,30,31] or allyltrimethylsilane [32,33] to aldehydes, giving good enantioselectivities and some asymmetric amplification. In all these examples the chiral auxiliary is derived from (R)- or (S)-BINOL. 

Zirconium complexes, generated in situ by addition of HZrCp2Cl to alkenes, can be animated with <9-(mesitylenesulfonyl)hydroxylamine an example is shown in Eq. 41.116 When the initial hydrozirconation is not regioselective, as with styrene, mixtures of amines are formed. A reaction that permits animation at the tertiary carbon in a similar substrate is discussed below (Eq. 49). 

However, the trend in the phosphorus resonances of the isostructural titanium and zirconium complexes strikes us as odd. We know that the chemical shift moves upfield, not downfleld, as one moves down the triad. In our two examples, the trend is reversed. The only logical explanation would seem that the larger zirconium atom favours the cr-complex over the 7 -complex, and thus the downfleld shift seen in the transition from titanium to zirconium reflects the extent of the 7 -complex present in the true structure of this complex. 

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