Zirconium complexes Spectroscopy

As shown in Scheme 94, the fj -styrene zirconium complex Cp Zr (f/ -PhCHCH2)[MeC(NPr )2] also served as starting material in the synthesis of alkyl-substituted 1,3-diene complexes of (pentamethylcyclopentadienyl)zirco-nium amidinates. NMR spectroscopy as well as single-crystal X-ray analyses of these complexes revealed that they are best described by the Zr(lV) a, n-metallacyclopent-3-ene limiting resonance form rather than as Zr(ll) f/ -diene complexes. ... 

The first organometallic zirconium complexes to be prepared all contained Zr( 1V) and were reported almost contemporaneously. The reaction of Zr(TPP)-(OAc)2 with RLi or RMgBr produced the dialkyl complexes Zr(TPP)R2 (R = Me, Et, n-Bu or Ph), characterized by. spectroscopy. The development of the chlorozirconium complexes opened up the chemistry further, with... 

Evidence for hydride species as the active catalysts comes from comparing the previously described non-hydride catalyst precursors with corresponding hydride complexes. For example, CpCp Zr[Si(SiMe3)3]Me and [CpCp ZrH2]2 react with a large excess of phenylsilane at comparable rates. Also, under the same conditions these two zirconium complexes produce polysilanes that have identical molecular weight properties. The reaction of CpCp Zr[Si(SiMe3)3]Me with 2 equivalents of phenylsilane allowed observation (by NMR spectroscopy) of initial conversion to... 

Casey was able to prepare related zirconocene alkenyl complexes according to Scheme 8.18. Alkene coordination was established by a number of NMR techniques. While zwitterionic compounds 38 allowed the determination of the alkene dissociation energy, AG = 10.5 kcal mol , very similar to that of 35. Thermally more stable complexes were obtained by protonation of 37 with [HNMePh2][B(C5F5)4[. Dynamic NMR spectroscopy and line shape analysis allowed the measurement of the barriers of alkene dissociation (AG = 10.7 and 11.1 kcal mol ), as well as for the site epimerisation ( chain skipping ) at the zirconium center (AG = 14.4 kcal mol" ) (Scheme 8.19) [77]. 

Reaction of achiral biscyclopentadienyl zirconium-( /2-acyl-C,0) complexes, such as la and lb, with sodium hexamethyldisilazide generates the di-hapto enolates 2a and 2b91. Enolate 2b is believed to adopt a Z geomeLry, only one isomer was observable by H-NMR spectroscopy (d.r. >98 2). 

In systems completely free of chloride, complex formation between the titanium or zirconium compound and the trialkylaluminum was not detected by either NMR or UV spectroscopy 191), or by cryoscopy (189). Such systems were thus assumed to be inactive for polymerization. However, NMR-spectroscopic studies in the presence of ethylene revealed a weak but lasting insertion of ethylene, with formation of polyethylene. 

Addition of thiocyanate ions to chloride or perchlorate solntions of zirconium and hafnium yields complexes containing from one to eight isothiocyanate groups per metal atom. These systems are of interest because of the importance of thiocyanate complexes in the extraction and separation of the elements. IR spectroscopy indicates that M-N bonds are present in the violet (Zr) and pink (Hf) complexes [NEt4]2[M(NCS)6] analogous complexes have been obtained with alkali metal cations. In the presence of pyridine, the dodecahedral Zr(bipy)2(NCS)4 complex is produced see Ammonia N-donor Ligands). 

Reactivity studies reveal that the bis(indenyl)zirconium sandwich complexes serve as isolable, modular precursors to the rich chemistry of divalent zirconocene. Addition of 1 atm of carbon monoxide generates the bis(indenyl)-zirconocene dicarbonyl complex, 307, while treatment with N,N-dimethylaminopyridine results in C-H activation to form the zirconocene pyridyl hydride complex 308. Crystallographic characterization of both complexes as well as multinuclear NMR spectroscopy establishes that the more familiar 77s, 77s hapticity for the indenyl ligands has been restored. Alkyne coupling reactions are also observed as addition of 2-butyne to 302 allows observation of the alkyne complex, 208, and ultimately the zirconacyclopentadiene 309 (Scheme 50).104... 

Neutral Ti(CO)6 is an extremely unstable compound which decomposed even below -220 °C, as shown by matrix isolation spectroscopy [165]. The much more stable phosphine derivatives Ti(CO)3(dmpe)2, Ti(CO)5(dmpe), Ti(CO)5(PMe3)2, Ti(CO)4(PMe3)3 have been isolated [166-168]. In contrast, the dianionic salt [Ti(CO)6] (53) is thermally much more stable and decomposes only above 200 C. Complex 53 was obtained by reductive carbonylation of Ti(CO)3(dmpe)2 by alkali metal naphthalenides in the presence of cryptand [169]. Carbonylation of 79 also produces 53 [170]. The naph-thalenide-assisted reductive carbonylation of the zirconium tetrachloride afforded the zirconium analog [Zr(CO)6] (54) [171], which was also derived by carbonylation of the tris(diene) dianion 45 [150]. One anion [R3Sn] effectively stabilizes Ti(CO)e as an air stable monoanionic salt, [R3SnTi(CO)J [172]. 

We have previously shown that the mononuclear zirconium hydride complexes 1 activate, under very mild conditions, the C-H bond of alkanes, including methane [7], The mechanism involves a four center intermediate, as proposed earlier for electrophilic activation of C-H bonds by group 3, 4 and lanthanides d° complexes [8], Given the similarities of the energies of dissociation of C-H and Si-H bonds, it is not surprising at ail that activation of Si-H bonds occurs with 1. Reactions of H/D exchange, followed by in situ IR spectroscopy, reveal that all types of silanes are activated, i.e. primary, secondary and even tertiary silanes [9],... 

Some physical properties and structural features of the normal tetramandelate have been reported. The solubility of the 1 4 mandelato complex in 2 Jlf perchloric acid was determined to be 7.8 x 10" mole/ liter (444), and was found to fall slowly with increasing pH to a minimum of 4 X 10 at pH 3.1, after which it then rose. The change in solubility was accompanied by a change in composition which involved the formation of metal oxo species. In 1958, R. W. Stromatt (540) concluded on the basis of infrared spectroscopy, that the tetramandelates exist as discrete 8-coordinate molecular species. This would seem to be supported by the fact (24) that an organic-soluble species can be extracted from aqueous solutions containing (1.0 to 8) x 10 mole/liter zirconium(IV) in 1 M perchloric acid with an isopentyl alcohol solution of p-bromo-mandelic acid. The normal tetramandelate precipitated from aqueous solution at the usual concentration conditions, however, is very insoluble... 

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