Hafnium derivatives

Recently, dithio-diolate ligands have been employed for construction of group 4 metal catalysts for the ROP of lactide. These metal dithiolate complexes form mononuclear species of the type [(OSSO)M(OR)2] with an octahedrally coordinated metal center. These fluxional compounds acted as highly active catalysts in the ROP of L- and rac-lactide. Hafnium complexes were also introduced as initiators for the ROP of L-lactide and rac-lactide (vide infra) in very limited cases. To our knowledge, the hafnium derivative 146 displayed the highest activity among the group 4 catalysts reported to date (complete conversion of 300 equiv. of... 

Triphenylsilyl zirconium and hafnium derivatives have been prepared from the silyl lithium species. Triphenylsilyl bis(cyclopentadienyl) zirconium chloride undergoes carbonyl insertion under pressure of carbon monoxide (100 psi) to give the corresponding acyl zirconium species which, upon treatment with anhydrous hydrogen chloride in a benzene matrix at —196 °C and warming to room temperature, gives rise to formyl triphenylsilane55. 

Normally, nitration of deactivated compounds (and therefore polynitration of toluene) is carried out using aggressive nitric acid - oleum mixtures. Dinitration of toluene with mixed acids produces a 4 1 ratio of 2,4- and 2,6-dinitrotoluenes, from which the former is isolated for manufacture of toluenediisocyanate (TDI) and toluenediamine, both of which are used in the manufacture of polyurethanes. Zirconium and hafnium derivatives catalyse nitration of o-nitrotoluene, but ratios of 2,4- 2,6-dinitrotoluene are modest (66 34).12 Dinitration of toluene using Claycop (copper nitrate on K10 clay), acetic anhydride and nitric acid in the presence of carbon tetrachloride produced dinitrotoluenes in a yield of 85% with a ratio of 2,4- 2,6-dinitrotoluene of 9 1.13 This method, however, requires a large excess of nitric acid, the use of an unacceptable solvent and long reaction times. The direct nitration of toluene to 2,4-dinitrotoluene using nitric acid over a zeolite P catalyst, with azeotropic removal of water, is reported to give a 2,4 2,6 ratio of 14, but full results are yet to be published.14... 

Preparation. Both the title zirconium catalyst (1), and the corresponding hafnium derivative (2) have been synthesized according to Scheme 1. Addition of 2 equiv of 2-bomen-2-yllithium (4) (generated via the Shapiro reaction from (-b)-camphor (3)) to ethyl formate gives bis(2-bomen-2-yl)methanol (5). Cycliza-tion of alcohol (5) is carried out with Potassium Hydrogen Sulfate at 100-150 °C to afford dibornacyclopentadiene (6). Deprotonation of (6) with n-Butyllithium followed by reaction with Zir-conium(IV) Chloride affords (1). Reaction with HfCl4 affords (2). 

This chapter describes the use of Hf-centered Lewis acids in organic synthesis. Although the last two decades have witnessed an explosive growth of the use of titanium-centered Lewis acids [la] and then their zireonium eounterparts [lb], the corresponding status of the hafnium derivatives remains limited, presumably because of less ready availability, and also the well-known similarity to Zr because of the lanthanide contraction [2]. Increasing data on the uniqueness of hafnium-centered Lewis acids are, however, becoming available. 

Some commercially available hafnium derivatives are listed in Table 1. HfC is the major starting material for the different hafnium derivatives. Cp2HfCl2 is the hafnium complex that has found most widespread utility in carbohydrate synthesis [3] and, to some extent, in polymerization chemistry [4], The corresponding Cp (CsMes) complex is also available. Hf(OTf)4 was recently prepared from HfCU and CF3SO3H [5]. 

Compound (CsH4SiMe2NBut)TiCl2 has been synthesized and used as pre-catalyst for ethylene polymerization. The activities and the properties of the polymers have been compared to similar zirconium and hafnium derivatives.720 The consequences of anion-cation interactions on the activity of GGG group 4 metal complexes in olefin polymerizations have been explored for a series of zirconocene derivatives as well as the cationic species [(C5Me4SiMe2NBut)TiMe]+ with the sterically congested tris(perfluorobiphenyl)fluoroaluminate as the counteranion.721 The co-polymerization of ethylene and 1-butene by (CsMe iMe Bu TiC in the presence of... 

New catalysts based on Titanium and Hafnium derivatives co-supported on magnesium chloride are actractlng more and more Interest for the preparation of olefin polymers and. In particular, of high density polyethylene (HDPE). 

And other heuristic guides could kick in to give plausible, but never deductively derived, predictions . (So, for example, Paneth in the 1920s used simple reasoning based on the Periodic Table to predict that then undiscovered hafnium would occur in the same ores as zirconium. This is nonetheless a considerably looser notion of prediction than applies to standard cases from physics. See Scerri, 1994). 

The isomer shifts in hafnium Mossbauer isotopes usually are of the order of some percent of the line width. Boolchand et al. [168] observed a relatively large isomer shift of -1-0.19 0.06 mm s between cyclopentadienyl hafnium dichloride (Hf(Cp)2Cl2) and Hf metal. From a comparison with Os(Cp)2 and Os-metal, a value of 5 r ) ( Hf) = —0.37 10 fm has been derived, which implies a shrinking of the nuclear radius in the excited 2 state. Figure 7.37 shows some typical spectra for Hf in various hafnium compounds (from [168]). 

A somewhat surprising group of coordination compounds consists of the volatile heavy metal nitrates, such as those of copper, zinc, mercury, titanium, zirconium and hafnium. The structures of some of these, in the gaseous state, have been determined thus Cu(N03)2 contains two bidentate, almost planar, staggered nitrato groups. Some derivatives of metal nitrates have also been found to be volatile for example, Fe(N03)3 N204 and Al(N03)3 2MeCN.39... 

Thermolysis of tin and lead alkoxozirconates leads to the formation of metals. The mass-spectral data indicate the presence ofbarium and aluminium derivatives in the gas phase, but no preparative data are accessible for them. The major application of zirconium and hafnium alkoxides lies now in the sol-gel technology of zirconate-titanate and solid solutions Zr02-Y203 (see Section 10.3), Except in the synthesis of oxide materials, the alkoxides of zirconium and hafnium are traditionally used in the polymer chemistry, where they are applied as the components in catalysts [1278, 1269] and as additives to polymers, improving their characteristics [825, 1403] and so on. Already in 1930s Meerwein has proposed the use of zirconium alkoxides for the reduction of aldehydes intoprimary alcohols (Meerwein-Schmidt reaction) [1420],... 

The corresponding hafnium compound behaves similarly. Attempts to make the titanium derivative using this method failed. Primarily titanium is reduced, as indicated by the color change of the solution from green to brown. Variation of the reaction conditions did not provide better results, in all cases only bi(undecamethylcyclohexasilanyl) could be identified among the reaction products. 

There has been no recent comprehensive review of this area, although a book on the organometallic chemistry of titanium, zirconium, and hafnium deals, in part, with some of the hydride derivatives (1). In the present review, the first part of the discussion reflects the fact that much of the early work on organotitanium hydrides was, often unknowingly at the time, interwoven with attempts to prepare titanocene, Cp2Ti (Cp = tj3-C5H5). Subsequent sections deal with similar compounds containing an additional metal (e.g., aluminum), miscellaneous titanium hydride compounds, and a summary of the main properties of the above species. 

The change to substituted allyl ligands influences the 48 49 equilibrium in the corresponding hafnium systems.53 Silver et al. have used the specific deprotonation at related methylallyl derivatives to generate the conjugated diene ligand in the coordination sphere of the metal. 

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